Introduction
Hydrogen peroxide (H2O2) is a versatile and green oxidant with vital applications across diverse industries, such as pulp and paper, textiles, chemical synthesis, wastewater treatment, electronics, and healthcare.[1] Recognized as one of the world's 100 most important chemicals,[2] H2O2 has garnered recent interest as a potential alternative energy carrier to hydrogen (H2), particularly for fuel cell applications due to its easy storability as an aqueous solution.[3] While a H2O2 fuel cell produces a theoretical output voltage of 1.09 V, slightly lower than that of a conventional H2 fuel cell (1.23 V), the energy density of H2O2 (2.1 M J kg−1 for 60% aqueous H2O2) remains comparable to compressed H2 (3.5 M J kg−1).[4] Compared to other liquid fuels such as methanol, H2O2 offers a more favorable zero-emission alternative, fostering the transition toward carbon-neutral energy sources. The global market demand for H2O2 is predicted to reach 5.7 million tons by 2027.[5]
Currently, ≈95% of global H2O2 production relies on the energy-intensive multistage anthraquinone (AQ) oxidation process, which consumes significant volume of solvents and generates substantial wastewater effluents.[1b] Due to efficiency considerations, the AQ process is typically centralized, resulting in the production of highly concentrated H2O2 (up to 70 wt%) to minimize transportation and distribution costs. However, many applications only require low-concentration H2O2 solutions (1–10 wt%), necessitating dilution at the point of use.[6] A more desirable approach would be decentralized, on-site production of H2O2, achieved through the direct synthesis of H2O2 from oxygen (O2) and H2 using Pd or bimetallic Au-Pd catalysts. However, safety concerns related to the explosive nature of the H2/O2 gas mixture impedes the implementation of this process.[7] Artificial photosynthesis (AP) presents an appealing alternative for on-site H2O2 synthesis. This technique utilizes a semiconductor photocatalyst to harness solar energy and drive the direct conversion of water (H2O) and O2 into H2O2 with minimal waste generation. By eliminating the need for fossil fuels and minimizing its environmental footprint, AP offers a safe, cost-effective, eco-friendly, and sustainable approach for H2O2 production.
Heterogeneous AP utilizes two main configurations: powder suspension (PS) and photoelectrochemical (PEC) systems. In PS systems, photocatalyst particles are suspended in an aqueous solution, while PEC systems involve depositing these particles onto a conductive substrate to form photoelectrodes, with either one or both electrodes being photoactive. Both PS and PEC systems harness light-induced electron transfer reactions initiated by photocatalyst excitation, but their mechanisms for charge separation and transport differ significantly. While PEC systems spatially separate oxidation and reduction reactions on distinct electrodes, PS systems facilitate the redox reactions on the same photocatalyst particle surface, increasing the likelihood of charge recombination. As a result, optimizing photocatalyst performance may necessitate different strategies for PS and PEC systems. Our previous research demonstrated the contrasting effects of particle size on BiVO4 performance in photocatalytic PS and PEC water oxidation.[8] Smaller particles enhanced charge transport and charge collection efficiency within BiVO4 photoelectrode, whereas larger particles benefited PS systems through improved charge separation facilitated by greater band bending and better crystallinity. Our recent review article further elaborated on the similarities and differences between these systems.[9]
To date, significant advancements have been made in developing organic semiconductors such as carbon nitride (C3N4) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole)/1-[3-(Methoxycarbonyl)propyl]-1-phenyl-[6.6]C61 (PFBT/PCBM) polymer dots for H2O2 generation with commendable production yields of up to 3.76 mM h−1.[10] However, their susceptibility to degradation due to poor chemical stability, particularly against hydroxyl radical (•OH) produced from H2O2 decomposition, remains a significant challenge limiting their practical applications.[11] Consequently, inorganic metal oxide semiconductors, which generally offer relatively higher intrinsic chemical stability, are favored.[12] Among the various metal oxides available for solar-driven H2O2 generation, such as titanium dioxide (TiO2),[13] tungsten oxide (WO3),[14] and zinc oxide (ZnO),[15] monoclinic scheelite bismuth vanadate (BiVO4) stands out as a highly promising photocatalyst. Monoclinic scheelite BiVO4 possesses a narrow bandgap of ≈2.4 eV, enabling efficient utilization of the solar spectrum and possessing suitable band structures for two-electron O2 reduction and H2O oxidation essential for selective H2O2 production. Furthermore, BiVO4 is predicted to facilitate easier charge extraction due to its much lighter effective masses of photogenerated carriers compared to other oxides such as TiO2 and In2O3.[16] Nevertheless, the performance of BiVO4 for solar H2O2 generation is greatly hindered by its intrinsic shortcomings such as poor carrier mobility and poor charge separation.[17] Additionally, the easy decomposition or disproportionation of H2O2 presents a challenge, resulting in typically low H2O2 yields by bare BiVO4. To address these challenges, numerous approaches, including material engineering and design (i.e., surface modification, crystal facet engineering, heterojunction formation, and doping) and optimization of reaction conditions, have been developed to enhance the visible-light-driven H2O2 generation by BiVO4-based materials.
While several reviews on solar-driven H2O2 production have been published, they predominantly focus on the development of photocatalysts for either PEC[18] or PS systems.[10c,19] In contrast, reviews by Zeng et al.[5] and Qu et al.[20] provide insights into photocatalyst development for both PS and PEC systems, emphasizing reaction pathways and material engineering strategies, respectively. Although these reviews examine a broad range of photocatalysts, a comprehensive review that systematically compares the two main configurations of heterogeneous AP systems for H2O2 production, particularly focusing on a specific photocatalyst, is still lacking. A thorough understanding of the similarities and differences in the strategies employed to enhance the catalytic performance of the same material in these two distinct systems would be particularly valuable. Over the past decade, notable advancements have been made in the use of BiVO4-based materials for solar H2O2 production (Figure 1), underscoring the need for a detailed review of the strategies developed to enhance the performance of BiVO4-based materials in both PS and PEC systems.
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In this review, we elucidate the fundamental principles of H2O2 production in BiVO4-based PS and PEC systems, dissecting the distinct reaction mechanisms. We assess the unique characteristics and challenges associated with each system, emphasizing the critical role of tailored material design and reaction optimization. Additionally, we provide a comprehensive overview of performance evaluation methods and strategies for optimizing H2O2 generation. By outlining the working principles and comparing the efficacy of both approaches, this review aims to inspire further research efforts toward the development of BiVO4-based photocatalysts for sustainable solar-driven H2O2 production under visible light. While the paper primarily focuses on BiVO4-based materials, the established principles governing H2O2 production, along with the detailed performance evaluation methodologies and material design strategies outlined here can serve as valuable frameworks for researchers exploring alternative photocatalysts for AP systems.
Fundamentals of Solar-Driven H2O2 Production over BiVO4
When a semiconductor photocatalyst absorbs photons with energy greater than its bandgap, electrons are excited to the conduction band (CB), creating vacancies known as holes in the valence band (VB). These photogenerated electron–hole pairs drive the production of solar H2O2 on the surface of the photocatalyst via O2 reduction and/or H2O oxidation reactions (ORR and WOR, respectively).[5,20] This section delves into the reaction pathways for H2O2 production using BiVO4 in PS and PEC systems.
Powder Suspension (PS) System
For PS system, the primary redox reactions involve a direct two-electron reduction of O2 to H2O2 (Equation 1) and a four-electron oxidation of H2O to O2 (Equation 2). This indicates that H2O2 can be generated from O2 and H2O with an overall four-electron transfer (Equation 3). However, undesirable side reactions can occur, including the ORR competing with one-electron reduction pathways for reactive oxygen species (ROS) generation (e.g., •O2− and •OOH) and H2O reduction for H2 evolution. These side reactions reduce the semiconductor's selectivity for H2O2 production. Although H2O2 can also be formed through an indirect stepwise one-electron ORR, where •O2− formed from O2 reduction is further reduced to H2O2, the selectively for H2O2 production is lower compared to the direct two-electron pathway. Among metal oxides materials, BiVO4, characterized by a CB with a weak reducing potential at ≈ +0.02 V versus NHE, shows promise in inhibiting these major side reactions, as depicted in Figure 2a. This makes BiVO4 a highly favorable material for achieving selective and efficient H2O2 generation in the PS system.
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Nevertheless, the photocatalytic H2O2 production process is hampered by H2O2 decomposition, induced by photogenerated charge carriers in the presence of transition metals or organic compounds. H2O2 can either be reduced by the photogenerated electrons to form •OH (Equation 4), oxidized by the photogenerated holes to form •O2− (Equation 5), or disproportionate to form H2O and O2.[21] Additionally, H2O2 can easily decompose in the presence of ultraviolet light irradiation or heat stimuluses.[22]
With semiconductor particles freely suspended in an aqueous medium, both reduction and oxidation reactions take place simultaneously on the surface of the same particle in the PS system. As photogenerated electrons and holes have to diffuse from the bulk to the surface, they become susceptible to recombination through radiative or nonradiative processes occurring in the bulk, at the interface, and at the surface (Figure 2b).[23] These processes diminish the number of usable charge carriers for redox reactions and deteriorate the photocatalytic performance of a semiconductor, highlighting the significance of charge separation in the PS system.
Photoelectrochemical (PEC) System
In contrast, a PEC system utilizes photoelectrode(s) made by immobilizing the photocatalyst on conducting substrate(s) such as fluorine-doped tin oxide-coated glass (FTO). Photogenerated electrons are drawn from the photoanode to the counter electrode with the assistance of an external bias, enabling redox reactions to occur independently at each electrode. This eliminates the necessity for efficient charge separation within the photocatalyst. However, photogenerated electrons have to transport through the photocatalyst particles to the back substrate for effective collection and transfer of electrons through the external circuit, whereas holes move to the interface between the photoanode and the electrolyte for oxidation reaction (Figure 2c). This indicates charge transport as a more critical factor in the PEC system compared to charge separation efficiency.[8]
Similar to the PS system, H2O2 can be synthesized through cathodic direct two-electron ORR (Equation 1) with H2O oxidized at the anode (Equation 2). However, PEC offers another attractive route: anodic two-electron WOR (Equation 6). This approach is highly favored as it can be coupled with H2 generation from H2O reduction (Equation 7) at the cathode, resulting in the simultaneous production of H2O2 and H2 (Equation 8)—a highly sought-after low-carbon energy carrier.[24]
Nonetheless, the anodic two-electron WOR for H2O2 production has two major challenges. First, the WOR to O2 is thermodynamically favored over H2O2 production due to a lower thermodynamic barrier. Second, the produced H2O2 is susceptible to further oxidation back to O2 on the anode (Equation 1). Suppressing these competitive reactions is imperative in achieving high H2O2 production selectivity and accumulation concentration.
As an n-type semiconductor with a suitable VB potential for H2O2/H2O oxidation (depicted in Figure 2a), BiVO4 stands out as an ideal photoanode material for PEC H2O2 production via the two-electron WOR pathway. While its function for water oxidation to O2 has been extensively studied,[25] its potential for H2O2 production is scantly explored. Shi et al. assessed various metal oxides for their ability to electrochemically produce H2O2 through anodic H2O oxidation.[24b] Density functional theory (DFT) calculations and experimental measurements determined an onset potential sequence of WO3 < BiVO4 < SnO2 < TiO2, and BiVO4 was experimentally validated as the most promising anode candidate for electrochemical and PEC water oxidation to form H2O2. Under dark conditions, BiVO4 produced the highest Faraday efficiency for H2O2 production (FE(H2O2)) of 70% at 3.1 V versus RHE (Figure 3a) and the highest amount of H2O2 (Figure 3b). When light was introduced, optimizing the electrolyte and BiVO4 thickness boosted the FE(H2O2) to 98% and decreased its onset potential from 2.2 V under dark conditions to ≈1.1 V (Figure 3c). This aligns with other research screening different metal oxides (i.e., CoO, WO3, La2O3, Nb2O5, Al2O3, TiO2, ZrO2, V2O5, Bi2O3, and BiVO4) for electrochemical H2O2 production, where BiVO4 was identified as one of the best-performing anode materials.[24a]
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Alternatively, coupling anodic two-electron WOR with cathodic two-electron ORR leads to a more energetically efficient two-electron dual-channel pathway for H2O2 production (Equation 9).
Leveraging on BiVO4’s favorable CB and VB potentials which straddle the redox potentials of H2O2 (as illustrated in Figure 2a), a PEC system with a BiVO4 photoanode holds promise for achieving the highly coveted dual-side H2O2 production at both the photoanode and cathode. This configuration eliminates the need for an external bias, offering a more efficient and potentially simpler system for H2O2 production.
Powder Suspension (PS) System
Table 1 presents an overview of the reported photocatalytic performance of BiVO4-based materials for H2O2 generation, particularly in a PS system. While BiVO4’s band potentials are suitable for driving photocatalytic H2O2 production from two-electron ORR and four-electron WOR, its bare form exhibits a negligible H2O2 production rate, often below 1 µM h−1. This limitation is primarily attributed to inherent material constraints, such as rapid charge recombination, slow hole mobility, and lack of surface active sites for ORR. However, significant advancements have been achieved over the past decade, with modified BiVO4-based materials demonstrating improvements of 1–3 orders of magnitude compared to bare BiVO4. This section delves into various modification methods and reaction conditions used in the PS system, along with mechanisms behind this performance enhancement. To establish a common ground for understanding material development, we first describe the experimental characterization methods used to evaluate the H2O2 generation in PS systems.
Table 1 Summary of photocatalytic H2O2 production via a PS system using BiVO4-based materials dispersed in an O2-saturated aqueous solution.
| Photocatalyst | Faceted BiVO4 | Experimental Conditions | H2O2 Production Rate [µM h−1]a) | Enhancement in H2O2 Production Rate Compared to Unmodified BiVO4b) | AQY [%] at 420 nm | Ref | |||
| Light | Sacrificial Reagent | Buffer Solution | Temperature Control [°C] | ||||||
| Au/BiVO4 | No | λ > 420 nm | No | – | 25 | 4 | 8 | 0.24 | [26] |
| Pd/BiVO4 | Yes | λ > 420 nm | Yes | – | Ice bath | 170 | 3400 | – | [27] |
| Phosphate ion coated Pd/BiVO4 | Yes | λ > 420 nm | Yes | – | Ice bath | 300 | 6000 | – | [27] |
| Cu@Au core–shell /BiVO4 | Yes | λ = 420 nm | Yes | – | – | 30 | 91 | 0.88 | [28] |
| AuPd/BiVO4 | Yes | λ = 420 nm | No | citrate buffer (pH 3) | 20 | 1145 | 2 | 11.38 | [29] |
| NiS/BiVO4 | Yes | λ = 420 nm | No | citrate buffer | – | 488 | 87 | 4.8 | [30] |
| CoOx/BiVO4 | Yes | λ > 420 nm | Yes | – | – | 2 | 2 | – | [31] |
| CoOx/Mo-doped BiVO4/Pd | Yes | AM 1.5G | No | phosphate buffer (pH 7.4) | 12±0.5 | 1425 | 14250 | 5.8 | [32] |
| CoOx/Mo-doped BiVO4/ core–shell AgPd | Yes | AM 1.5G | No | – | 12±0.5 | 9700 | – | 13.1 | [33] |
| Pd/Y-doped BiVO4 | No | AM 1.5G | Yes | phosphate buffer (pH 7) | 5±0.5 | 15 | 4 | – | [34] |
| Au/Gd-doped BiVO4 | Yes | λ = 420 nm | Yes | – | 20 | 1035 | 200 | 7.44 | [35] |
| C3N4/BiVO4/Au | Yes | λ = 420 nm | No | citrate buffer (pH 3) | 20 | 676 | 151 | 6.7 | [36] |
| NiCo2O4/BiVO4 | No | λ > 420 nm | No | – | – | 390 | 16 | 3.27 | [37] |
| Au/BiVO4 QD/PTA | No | λ > 420 nm | No | – | 25 | 219 | 11 | – | [38] |
| MnOOH/BiVO4/Cu2O | Yes | λ = 420 nm | No | phosphate buffer | 20 | 112 | 16 | – | [39] |
| TpPa-1-COF/BiVO4 | Yes | λ > 420 nm | No | – | – | 1205 | 4 | 7.2 | [40] |
Evaluation of Photocatalytic Performance for H2O2 Generation
In photocatalytic studies using PS system, material performance is evaluated by activity, stability, and solar conversion efficiency. For H2O2 generation, the reactor setup involves dispersing the photocatalyst particles in an O2-saturated aqueous solution under light irradiation. The temperature is typically controlled at room temperature or lower to minimize the thermal decomposition of H2O2. Since H2O oxidation to O2 is slow, alcohols such as methanol or ethanol can be added as hole scavengers (also termed as sacrificial reagents) to minimize charge recombination and promote H2O2 production. Photocatalytic activity is evaluated by the amount of H2O2 produced upon light exposure. The produced H2O2, typically in the liquid phase, can be detected and quantified using various analytical techniques (Table 2). These techniques often involve oxidizing or reducing H2O2 to create a colored compound with specific light absorption or emission wavelengths. Ultraviolet–visible (UV–vis) spectroscopy is commonly employed in tandem with titrimetric or colorimetric techniques due to its high accuracy and reduced susceptibility to human error. This allows for precise quantification based on the absolute absorbance values of the colored compound.
Table 2 Summary of analytical techniques reported in the literature for quantifying H2O2 in the liquid phase using BiVO4-based materials.
| Analytical Technique | Description | Pros | Cons | Ref |
| Titrimetric |
|
High accuracy | Labor intensive | [26] |
| Colorimetric + Spectrophotometric |
|
High accuracy and sensitivity | Linear calibration is required to obtain a standard curve | [22, 27, 41] |
|
[28, 29, 36] | |||
|
[37, 42] | |||
|
[43] | |||
|
[44] | |||
| Titrimetric + Spectrophotometric |
|
High accuracy and sensitivity | Linear calibration is required to obtain a standard curve | [31] |
| HPLC colorimetric |
|
Rapid and selective | High cost and equipment requirements | [32, 33] |
The amount of H2O2 produced and accumulated over a specific reaction time (i.e., production) is typically reported in concentration units such as µmol l−1 or µM. However, comparing studies solely based on H2O2 concentration can be challenging due to varying experimental conditions such as solvent volume, mass of photocatalyst, and reaction duration. To address this, calculating the production rate or productivity as µmol g−1 l−1 h−1 eliminates these variables and serve as a more suitable metric for comparison. It is important to note that high productivity does not always correspond to high production, as they are inversely correlated. Productivity may decrease over time while total H2O2 production increases.[21]
Photocatalytic H2O2 production involves two competing pathways: H2O2 formation and decomposition. In an O2-saturated solution, the formation rate (kf) is assumed to be constant (zero-order kinetics), while the decomposition rate (kd) is proportional to existing H2O2 concentration (first-order kinetics).[45] The rate constant for H2O2 (kf) is an important parameter as a higher kf signifies a faster rate of H2O2 production by the photocatalyst.
Equation (10) expresses the relationship between H2O2 concentration ([H2O2]), the rate constants kf and kd, and the illumination time (t), and allows for the estimation of kf based on the known kd value. kd can be determined by a first-order reaction kinetic model through a dedicated H2O2 decomposition experiment. The experiment involves exposing a known amount of commercial H2O2 solution to the studied photocatalyst under light irradiation and monitoring the decrease in H2O2 concentration over time.
The stability of a photocatalyst in the PS system is commonly assessed by recycling the material over a series of activity tests. Typically, the spent photocatalyst is recovered from one activity test and reused for subsequent tests under identical experimental conditions, repeated multiple times. One main challenge lies in fully retrieving the dispersed photocatalyst, leading to potential material loss. While there is no single standardized metric for comparing stability in PS systems, a stable photocatalyst should maintain comparable activity levels over multiple cycles. Additionally, characterization techniques such as electron microscopy (e.g., scanning electron microscopy (SEM) and transmission electron microscopy (TEM)), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) can be used alongside recycling tests for a more comprehensive evaluation of a photocatalyst's stability. Characterization of the photocatalyst before and after an activity test enables detection of potential changes in its physicochemical properties.
Considering that light scattering commonly occurs in PS systems, the efficiency of the photocatalyst to convert absorbed photons into desired chemical products can be evaluated by apparent quantum yield or efficiency (AQY/AQE) and solar-to-chemical conversion (SCC) efficiency. AQY/AQE measures the ratio of the number of electrons involved in a reaction under a particular wavelength irradiation to the total incident photons, while SCC efficiency refers to the effectiveness of the photosystem in converting solar energy into chemical energy. For H2O2 production, the AQY/AQE (Equation 11) and solar-to-H2O2 (STH) (Equation 12) for photocatalytic H2O2 production can be calculated as follows:
Based on the band structure of BiVO4 (Section 2.1.), the photocatalytic production of H2O2 in a PS system theoretically involves four-electron oxidation of H2O to O2 and two-electron reduction of O2 to H2O2 reactions. While the oxidative half-reaction can be validated using AgNO3 as the electron scavenger, confirming the two-electron ORR is more intricate due to competing one-electron reactions generating reactive oxygen species such as •O2− and •OOH. The following two techniques can verify the two-electron ORR half-reaction pathway:
- i)Electron Paramagnetic Resonance (EPR) analysis: This technique detects the generation of •O2− and •OOH radicals using a radical scavenger such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The absence of these radicals suggests a two-electron ORR pathway.
- ii)Koutecky–Levich plot analysis: This method utilizes a rotating disk electrode to determine the number of electrons (n) involved in ORR. The linear region of the plot follows the equation:
In this analysis, the slope of the Koutecky–Levich plot (S) corresponds to B−1. Therefore, the number of electrons (n) can be estimated using the following equation:
Strategies for Enhanced Photocatalytic H2O2 Production
One commonly employed strategy to improve photocatalyst performance involves loading a cocatalyst onto the photocatalyst surface. These cocatalysts provide active sites and tune interfacial energetics by promoting charge separation and enhancing surface reaction kinetics for the targeted reactions. Hirakawa and coworkers were the first to successfully demonstrate an all-inorganic system for photocatalytic H2O2 synthesis using Au-loaded BiVO4. Their work achieved significant enhancement in H2O2 generation directly from H2O and O2 (40.2 µM H2O2 after 10 h of visible-light irradiation), compared to negligible amounts (<5.0 µM) with bare BiVO4.[26] Notably, among various metals tested (Ag, Pd, Pt, Co, and Ni), only Au effectively promoted H2O2 formation on BiVO4 (Figure 4a). While H2O oxidation is suggested as the rate-determining step, the selectivity toward the two-electron reduction pathway is attributed to BiVO4’s CB potential being more positive than the potential for one-electron reduction of O2, therefore, suppressing the undesired one-electron pathway (Figure 4b).
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In contrast, a study conducted by Fuku et al. suggests Pd as the most effective metal cocatalyst for promoting H2O2 production from O2, using methanol as an electron donor and BiVO4 as the photocatalyst.[27] Most metal cocatalysts, apart from Ir and Mn, improved H2O2 yield compared to bare BiVO4, with Pd-loaded BiVO4 showing a remarkable 9.3-fold improvement over Au-loaded BiVO4 (Figure 4c). This superior performance was ascribed to Pd's lower overpotential for O2 electrochemical reduction than Au, and its higher selectivity for the two-electron reduction of O2 compared to Pt. Interestingly, the selective deposition of Pd on the {010} facets of BiVO4 (via photodeposition) led to the best performance (Figure 4d). This aligns with the seminal work by Li et al., where {010} and {110} facets were identified as reduction and oxidation functional sites of BiVO4, respectively, due to spatial separation of photogenerated electrons and holes on the two facets.[46] Given that Pd is the reduction cocatalyst that promotes O2 reduction to H2O2, its deposition on the electron-accumulated {010} facet is strategically beneficial for H2O2 production. Such a rational loading of cocatalyst on the right crystal facet of a photocatalyst, that is, reduction cocatalyst on the {010} facets and oxidation cocatalyst on the {110} facets, has been widely reported to enhance the photocatalytic activities of BiVO4.[47] It is noted that majority of the subsequent studies of photocatalytic H2O2 production from BiVO4 have focused on the synergistic effects of crystal facet engineering and cocatalyst loading. Dual-faceted BiVO4 with well-defined {010} and {110} surfaces are adopted to leverage their inherent charge separation properties and enhance overall photocatalytic activity.
Although Au is a widely recognized metal cocatalyst, Wang et al. identified two main limitations that affect the performance of Au-modified BiVO4 (illustrated in Figure 5a): i) inefficient transfer of photogenerated electrons associated with built-in potential (qVD) arising from upward band bending of BiVO4, and ii) reduced catalytic activity for the two-electron O2 reduction to H2O2.[28] This reduction in activity is attributed to an increased negative density on Au upon its contact with BiVO4, leading to weaker adsorption of O2 and HOO* intermediates on the metal surface. To circumvent these limitations, they demonstrated the use of nanostructured Cu@Au core–shell particles as a more efficient metal cocatalyst compared to single-metal cocatalysts (i.e., Au or Cu) for photocatalytic H2O2 generation from the faceted BiVO4. Owing to the lower work function of Cu (ФCu = 4.7 eV) compared to Au (ФCu = 5.1 eV), an ohmic contact can be formed between Cu and BiVO4, facilitating efficient electron transfer. This catalyst design reduces negative charge accumulation on Au, resulting in positive charge accumulation (Figure 5b), which was suggested to be beneficial for generating stronger adsorption of O2 and HOO* on the Au surface. Experimentally, Cu@Au/BiVO4 with the Cu@Au cocatalyst selectively formed on the {010} facets of BiVO4 and an optimized Cu:Au molar ratio of 1:1 showed the highest H2O2 production (91.1 µM under 3 h visible-light irradiation in an aqueous methanol solution; Figure 5c). The superior performance of Cu@Au/BiVO4 was postulated to follow the reaction mechanism depicted in Figure 5d. After photoexcitation, the photogenerated electrons and holes in BiVO4 spatially separate to the {010} and {110} facets, respectively. The ohmic contact between Cu and BiVO4 on the {010} facets enables rapid electron transfer from BiVO4 to the Cu nanocore and subsequently to the catalytically active Au site, initiating the two-electron reduction of O2.
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Alloying Au with Pd presents an alternative strategy to surpass the limitations of single metal cocatalysts s and achieve enhanced photocatalytic performance. This method addresses the distinctive shortcomings of each metal cocatalyst: Au suffers from weak O2 binding, while Pd tends to favor the four-electron reduction of O2 to H2O over the two-electron reduction of O2 to H2O2.[48] Using a photodeposition technique, Shi et al. successfully fabricated an AuPd alloy nanocatalyst with uniform atom dispersion, primarily supported on the {010} facet of BiVO4, as illustrated in Figure 6a.[29] While modification of the dual-faceted BiVO4 with either Au or Pd improved the photocatalytic H2O2 performance compared to the bare material, incorporation of AuPd alloy nanocatalysts led to a more significant enhancement (Figure 6b). The AuPd-BiVO4 catalyst with an optimized Au:Pd mass ratio of 19:1 achieved the highest H2O2 generation of 2.29 mM after 2 h of visible light irradiation in citrate buffer solution. Analysis of rate constants for H2O2 formation (kf) and decomposition (kd) revealed that as Pd content in the AuPd alloy increased, the kd increased, while the kf displays a volcanic trend, peaking at an Au:Pd mass ratio of 19:1 (Figure 6c). This mass ratio, leading to the best performing AuPd alloy supported on BiVO4, is attributed to the uniform dispersion of Au and Pd atoms. Further increasing the Au:Pd mass ratio beyond its optimal ratio leads to the formation of Pd atom clusters (Figure 6d), which are detrimental to the photocatalytic H2O2 reaction.
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The discussion thus far has identified noble metal such as Au and Pd as promising cocatalysts for photocatalytic H2O2 production. Notably, Shi et al. demonstrated the potential of nickel sulfide (NiS) as an effective, low-cost, non-precious metal alternative cocatalyst to enhance the photocatalytic H2O2 production of BiVO4.[30] The NiS cocatalyst was selectively deposited on the {010} facet of BiVO4, which is the electron-rich, using a sulfur-mediated photodeposition method. In this process, Ni2+ ions are electrostatically adsorbed onto the electron-rich {010} facet of BiVO4 under visible-light illumination, followed by in situ NiS formation through the reaction between the enriched Ni2+ and surrounding S molecules. The optimal NiS/BiVO4 photocatalyst (10 wt% NiS) achieved an H2O2 production concentration of 975 µM, which is 87 times higher than that of unmodified BiVO4 (11.25 µM), with an AQY of 4.8% under 420 nm irradiation. The NiS cocatalyst was found to enhance O2 adsorption on the BiVO4 surface. The selective loading on the {010} facet facilitates the effective capture of photogenerated electrons, thereby promoting the reduction of adsorbed O2 molecules to H2O2.
Apart from loading ORR cocatalyst, deposition of WOR cocatalyst can also improve the photocatalytic H2O2 production performance of BiVO4. Xie et al. showed that the H2O2 yield of dual-faceted BiVO4 can be enhanced by selectively loading CoOx (a known cocatalyst for WOR) onto the {110} reduction functional facet.[31] Interestingly, the degree of enhancement was uncovered to be dependent on the relative exposure ratio of {010}/{110} facets in BiVO4. P-BiVO4, with a {010} dominant facet, exhibited a 79.2% increase, while {110}-dominant T-BiVO4 showed a 42.9% increase (Figure 7a,b). The superior performance of CoOx/P-BiVO4 was ascribed to the synergistic effects of a larger built-in electric field in P-BiVO4 compared to T-BiVO4 and a greater degree of band bending at the interface upon heterojunction formation with CoOx. These factors result in a larger space charge layer at the {110} facets of CoOx/P-BiVO4 compared to CoOx/T-BiVO4 (Figure 7c), ultimately leading to enhanced charge separation and photoactivity in CoOx/P-BiVO4.
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Liu et al. recently reported a breakthrough in photocatalytic H2O2 production, achieving the highest efficiency among inorganic photocatalysts. By strategically loading dual cocatalysts onto specific facets of Mo-doped BiVO4 (Mo:BiVO4), they greatly enhanced the material's performance.[32,33] Molybdenum was doped into the BiVO4 crystal structure by replacing the V sites, thereby increasing the bulk conductivity of the material. In their initial study, selective deposition of Pd cocatalyst on the {010} facets of Mo:BiVO4 (Mo:BiVO4/Pd) significantly enhanced the H2O2 production yield (53.7-fold compared to bare Mo:BiVO4; 4.1 µM for bare Mo:BiVO4 and 220.1 µM for Mo:BiVO4/Pd after 1 h of simulated light irradiation). The subsequent loading of CoOx cocatalyst onto the {110} facets resulted in a remarkable 347.6-fold enhancement (Figure 8a). Specifically, CoOx/Mo:BiVO4/Pd achieved a H2O2 production rate of 1425 µM h−1, surpassing the photocatalytic H2O2 production performance of previously reported inorganic photocatalysts by an order of magnitude, and without the presence of any sacrificial reagent.[32] The photocatalytic H2O2 production reaction was proved to proceed mainly through the two-electron ORR. Beyond the improved surface kinetics for O2 evolution and H2O2 selectivity facilitated by CoOx and Pd as WOR and ORR cocatalysts, respectively, the coloading of these cocatalysts on selective BiVO4 facets was experimentally probed using transient absorption spectroscopy (TAS) to enhance charge separation within the material, as illustrated in Figure 8b. The presence of cocatalysts not only facilitates the accumulation of photogenerated holes in Mo:BiVO4 and promotes the transfer of free/shallowly trapped photogenerated electrons to Pd, but also activates and transfer the deeply trapped electrons to Pd for subsequent surface reactions.
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In a subsequent study, Liu et al. further improved the performance by replacing Pd with an AgPd core/shell structure cocatalyst, which was obtained by stepwise photoreduction of Ag+ and PdCl42− (Figure 8c).[33] The Ag core was reported to lower the Schottky barrier at the BiVO4/cocatalyst interface, as it has a lower work function (4.3 eV) compared to Pd (5.6 eV), while retaining the Pd shell to promote two-electron ORR to H2O2. Such a reduction in the Schottky barrier at the {010} facets through the construction of a BiVO4/Ag junction enhances electron migration and charge separation, as evidenced by TAS measurement (Figure 8d). The optimized CoOx/Mo:BiVO4/AgPd composite achieved a record-breaking H2O2 production rate of 9.7 mM h−1 with an AQY of 13.1% at 420 nm, setting a new benchmark for inorganic semiconductors.
Rare earth elements such as yttrium (Y) and gadolinium (Gd) have also been investigated as effective dopants for BiVO4 in photocatalytic H2O2 production. Due to the similar ionic radii of Y3+ (0.90 Å), Gd (1.05 Å), and Bi3+ (1.03 Å), Y3+ or Gd3+ can substitute for Bi3+ in the BiVO4 lattice.[34,35] Dai et al. demonstrated the feasibility of doping BiVO4 with Y3+ using a hydrothermal method, resulting in a fourfold increase in the H2O2 production rate for the optimized Pd/Y-doped BiVO4 (114 µmol g−1 h−1) compared to undoped Pd/BiVO4 (26 µmol g−1 h−1) under simulated sunlight (AM 1.5) irradiation.[34] DFT calculations suggested that Y doping enhances O2 adsorption on the BiVO4 surface, while experimental results revealed that it induces monoclinic/tetragonal heterojunction formation, which further promotes charge separation and enhances photocatalytic activity for H2O2 production. On the other hand, Gd-doped BiVO4 was also successfully fabricated by Li et al. via hydrothermal approach.[35] Au was selectively deposited on the {010} facets of the resulting Gd-doped BiVO4 as a cocatalyst. The optimized Au/Gd-doped BiVO4 demonstrated significantly enhanced photocatalytic performance, producing 2.07 mM of H2O2 with an AQY of 7.44% within 2 h under visible-light irradiation, which is a 1.39-fold increase in activity compared to the undoped Au/BiVO4. Gd doping was found to inhibit H2O2 decomposition and lower the Fermi energy level of BiVO4 to better match that of Au. This adjustment reduces the Schottky barrier, facilitating faster charge transfer and improving H2O2 production.
The formation of a heterojunction with another photocatalyst exhibiting appropriate band alignment presents an alternative strategy to mitigate the intrinsic limitation of the high charge recombination rate in bare BiVO4. Shi et al. showcased the formation of a Type II heterojunction by coupling BiVO4 with C3N4, where photogenerated electrons migrate from C3N4 to BiVO4, and holes move in the opposite direction (Figure 9a).[36] Strategically, presynthesized ultrathin C3N4 was electrostatically attracted primarily on the hole-rich {110} facets of BiVO4 to promote efficient transfer of photogenerated holes, whereas Au nanoparticles were photodeposited onto the electron-rich {010} facets to facilitate the two-electron ORR necessary for H2O2 generation. The resulting C3N4/BiVO4/Au composite exhibited superior H2O2 production, achieving 1351.78 µM in 2h, which is 2.65- and 150.5-fold higher than that of Au/BiVO4 and BiVO4, respectively. The incorporation of C3N4 functions as a photocatalytic H2O2 decomposition suppresser due to its weaker valence band oxidation potential, which limits its capacity to oxidize H2O2 to O2.
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The step-scheme (S-scheme) heterojunction has also been identified as an effective method to enhance the photocatalytic performance of BiVO4 for H2O2 production. In an S-scheme heterojunction, BiVO4 is typically paired with a photocatalyst that has a higher CB to compensate for BiVO4’s relatively weak reducing potential. When these materials come into contact, electrons spontaneously transfer from the photocatalyst with the higher CB to BiVO4 until equilibrium is reached at the Fermi level. This process leads to band bending and charge redistribution, creating an internal electric field (IEF) at the interface. Under irradiation, this IEF drives photogenerated electrons from BiVO4 to recombine with photogenerated holes from the other photocatalyst, resulting in the retention of electrons in the CB of the latter and holes in the VB of BiVO4 (Figure 9b). This S-scheme heterojunction effectively promotes interfacial charge transfer, enhances charge separation, and preserves energetic charge carriers. BiVO4 has been successfully integrated into S-scheme heterojunctions with NiCo2O4,[37] Cu2O,[39] perylenetetracarboxylic acid (PTA),[38] and TpPa-1-covalent organic framework (COF)[40] to improve photocatalytic H2O2 generation. Due to the higher reducing potential of the electrons in the additional photocatalyst that serves as the reduction site, H2O2 generation in these S-scheme heterojunctions occurs via an indirect stepwise one-electron transfer reaction (O2 + e− → •O2− + e− + 2H+ → H2O2) during the ORR, while H2O is oxidized to O2 in the oxidation half-reaction on BiVO4. Of note, the TpPa-1-COF/BiVO4 and Cu2O/BiVO4 S-scheme heterojunctions underscores the importance of crystal facet engineering in the rational design of highly effective hybrid photocatalysts.[39,40] The COF and Cu2O were intentionally grown primarily on the electron-rich {010} facets of BiVO4 (as shown in Figure 9c,d, respectively) to facilitate directional interfacial charge transfer. These composites exhibited superior photoactivity, which was attributed to enhanced charge separation compared to their counterparts formed randomly.
Another strategy to promote H2O2 generation involves the inclusion of phosphate (PO43−) into the reaction system as an H2O2 stabilizer. Phosphoric acid (H3PO4) is generally employed to stabilize commercially available H2O2 and inhibit its decomposition.[49] The interaction between H2O2 and H3PO4 was examined by Shiraishi et al. using Raman spectroscopy and ab initio calculations, as illustrated in Figure 10a.[50] Their findings suggest that H3PO4 forms a stabilized H2O2-H2PO4− bidentate complex through hydrogen bonding. The efficacy of PO43− as a H2O2 stabilizer in the photocatalytic H2O2 generation system was demonstrated by Liu et al., in which they reported a lower cumulative H2O2 yield in pure water compared to a phosphate buffer solution using CoOx/Mo:BiVO4/Pd (Figure 10b,c).[32] However, PO43− was also found to react with the CoOx cocatalyst, transforming it into Co-Pi and leading to a drastic performance decline during cyclic photocatalytic H2O2 generation reactions with phosphate solution (Figure 10b). On the other hand, Fuku et al. showcased improved BiVO4 photoactivity with partial phosphate (PO43−) coating onto the Pd nanoparticles, achieving an optimal H2O2 yield of ≈600 µM.[27] This represents a marked increase from the 340 µM yield obtained with uncoated Pd-loaded BiVO4 under identical experimental conditions (2 h irradiation, 0.1 wt% Pd). The superior H2O2 production performance of the PO43−-coated Pd-loaded BiVO4 was attributed to its ability to effectively inhibit H2O2 degradation, as depicted in Figure 10d.
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Photoelectrochemical (PEC) System
Table 3 outlines the performance of BiVO4-based photoelectrodes reported to date for PEC H2O2 generation. These photoelectrodes, typically employing n-type BiVO4 as the light absorber, function primarily as photoanodes, catalyzing H2O2 production through two-electron water oxidation. Notably, there have been significant advancements in the material's PEC H2O2 generation performance in less than a decade, with a FE exceeding 85% attained at applied biases lower than the theoretical electrolysis voltage (+1.77 V versus RHE).[43a,c] Moreover, BiVO4-based materials have demonstrated the potential to generate H2O2 concurrently on both the photoanode and cathode without an external bias. These advancements have been made possible through various strategies, including material engineering and reaction conditions optimization, which will be elaborated upon in this section. To establish a thorough understanding of photoelectrode development, we will first provide an overview of the experimental characterization methods used to assess the performance of PEC systems for H2O2 generation.
Table 3 Summary of PEC H2O2 production using BiVO4-based materials as photoanodes.
| Photoanode | Cathode | Experimental Conditions | Reaction Type | H2O2 Production Rate [µmol min−1 cm−2] | FE(H2O2) [%] | ABPE [%]a) | Ref | ||||
| Light | Electrolyte | Temperature Control [°C] | Membrane | Applied Bias [V versus RHE] | |||||||
| WO3/BiVO4 | Pt | λ > 420 nm |
2M KHCO3 (pH 7.7), CO2 bubbling |
Ice bath (< 5°C) |
Yes | 1.5 | Anodic Oxidation | 0.18b) | 54 | 2.2 | [22] |
| WO3/BiVO4 | Au | AM 1.5G |
2M KHCO3 (pH 7.8), O2 and CO2 bubbling |
Ice bath | Both | Bias-free | Dual-sided | 1.12b) |
Anode: ≈50 Cathode: ≈90 |
– | [41b] |
|
WO3/BiVO4/Al2O3 (MOD) |
Pt | λ > 420 nm | 2M KHCO3 (pH 7.7) |
Ice bath (< 5°C) |
Yes | 1.5 | Anodic Oxidation | 0.45b) | 79 | 1.5 | [41a] |
|
WO3/BiVO4/Al2O3 (CVD) |
Pt | AM 1.5G |
2M KHCO3 (pH 7.0–7.8) CO2 bubbling |
Ice bath | Yes | 0.8 | Anodic Oxidation | 0.86b) | 80 | 2.57 | [41c] |
| Biomass-derived carbon (WSoy/GnP-CP) | AM 1.5G |
Anode: 2M KHCO3 (pH 7.9), CO2 bubbling Cathode: 2M KHCO3 (pH 8.6), O2 bubbling |
Ice bath (0–5°C) |
Yes | Bias-free | Dual-sided | 0.58b) |
Anode: 60 Cathode: 44 |
– | [41c] | |
| SnO2-x /BiVO4 | Pt | AM 1.5G | 1M NaHCO3 (pH 8.3) | – | Yes | 1.23 | Anodic Oxidation | 0.825 | 86 | – | [43a] |
| ZnO/BiVO4 | Pt | AM 1.5G | 2M KHCO3 (pH 8.3) | – | Yes | 1.4 | Anodic Oxidation | 0.222 | ≈39.7 | – | [44a] |
| BiVO4-Air/V | Pt | AM 1.5G | 1M NaHCO3 (pH 8.3) | – | Yes | 1.23 | Anodic Oxidation | – | ≈58.4 | – | [51] |
| O-BiVO4 | Pt | AM 1.5G | 2M KHCO3 (pH 8.3) | – | Yes | 1.6 | Anodic Oxidation | 0.04 | 50.2 | – | [52] |
| N-Ovac-BiVO4 | Pt | AM 1.5G | 1M NaHCO3 (pH 8.3) | – | Yes | 1.6 | Anodic Oxidation | 0.15b) | 81.2 | 1.1 | [43b] |
| PTFE/BiVO4 | Pt | AM 1.5G | 1M NaHCO3 (pH 8.3) | – | Yes | 1.23 | Anodic Oxidation | 0.05b) | 85 | – | [43c] |
| Mo-doped BiVO4 | Pt | AM 1.5G | 1M NaHCO3 (pH ≈ 7.8) | – | Yes | 1.0 | Anodic Oxidation | 0.13 | 35 | – | [53] |
| AQ-modified Carbon | AM 1.5G | 1M NaHCO3 (pH ≈ 7.8) | – | Yes | 1.0 | Dual-sided | 0.45 |
Anode: ≈40 Cathode: ≈100 |
– | ||
| AM 1.5G | 1M NaHCO3 (pH ≈ 7.8) | – | Yes | Bias-free | Dual-sided | 0.11 |
Anode: ≈25 Cathode: ≈100 |
– | |||
|
Phosphate-treated Mo-doped BiVO4 |
AQ-modified Carbon | AM 1.5G | 1M NaHCO3 (pH ≈ 7.8) | – | Yes | 1.0 | Dual-sided | 0.66 |
Anode: ≈48 Cathode: ≈100 |
– | [53] |
| AM 1.5G | 1M NaHCO3 (pH ≈ 7.8) | – | Yes | Bias-free | Dual-sided | 0.16 |
Anode: ≈43 Cathode: ≈100 |
– | |||
| WO3(NN/NH)/Mo-doped BiVO4 | Pt | AM 1.5G |
2M KHCO3, CO2 bubbling |
Ice bath | Yes | 0.8 | Anodic Oxidation | 1.15 | – | – | [54] |
| Co2+ and PO43− ions modified Mo-doped BiVO4 | Pt | AM 1.5G |
Anode: 2M KHCO3 (pH ≈ 8.3), CO2 bubbling Cathode: 0.2M Na2SO4 (pH ≈ 7) |
Ice bath (5°C) |
Yes | 1.7 | Anodic Oxidation | 0.23 | 26 | – | [41d] |
| Gd-doped BiVO4 | Carbon | AM 1.5G | 2M KHCO3 (pH ≈ 8.3) | – | No | 2.6 | Anodic Oxidation | 1.1 | ≈99.5 | – | [55] |
| (010)-dominant BiVO4 | Pt | AM 1.5G | 1M NaHCO3 (pH 8.3) | – | Yes | 0.93 | Anodic Oxidation | – | ≈70 | 0.94 | [43e] |
| BiVO4 | Graphite | λ = 420 nm | Carbon quantum dots aqueous solution | Ice bath (< 5°C) | No | 1.23 | Anodic Oxidation | 0.33 | 93.5 | – | [43d] |
| BiVO4 | Carbon | AM 1.5G |
Anode: 2M KHCO3 (pH 8.3), O2 bubbling Cathode: 1M Na2SO4 (pH 8.3), O2 bubbling |
Ice bath | Yes | 1.5c) | Dual-sided | 2.42 | – | – | [56] |
|
Tap water, O2 bubbling |
Ice bath | Yes | 1.5c) | Dual-sided | 0.342 | – | – | ||||
|
Distilled water, O2 bubbling |
Ice bath | Yes | 1.5c) | Dual-sided | 0.225 | – | – | ||||
|
Anode: 2M KHCO3 (pH ≈ 8.3), O2 bubbling Cathode: 1M Na2SO4 (pH ≈ 8.3), O2 bubbling |
Ice bath | Yes | Bias-free | Dual-sided | 0.48 |
Anode: 53 Cathode: 92 |
– |
Evaluation of PEC Performance for H2O2 Generation
The performance of a photocatalyst for PEC H2O2 generation can be quantified and analyzed through three key parameters: activity, selectivity, and stability. Electrocatalytic activity is typically determined using either cyclic voltammetry (CV) or linear sweep voltammetry (LSV) to obtain the current–voltage (J–V) curve under both dark and illuminated conditions. For a detailed exploration of proper experimental protocols for voltammetric measurements and assessment, a comprehensive review paper by Shi et al. is highly recommended.[57] A one- or two-compartment PEC configuration can be employed with a BiVO4-based photoanode, a counter electrode such as a Pt wire, and a reference electrode such as Ag/AgCl. However, a two-compartment configuration equipped with an ion-exchange membrane (i.e., Nafion) is generally preferred. This configuration minimizes the cathodic degradation of H2O2 generated at the photoanode during the WOR.[18b] The electrolyte is commonly cooled in an ice bath (below 5 °C) to avoid further thermal decomposition of H2O2.
The onset potential, overpotential, and photocurrent density serve as crucial parameters for comparing the electrocatalytic activity of photoelectrodes fabricated with various photocatalysts. Due to the thermodynamically unfavorable nature of two-electron WOR for H2O2, the J–V onset might be attributed to the evolution of O2 via four-electron WOR. To ensure precise measurement of the onset potential for H2O2 generation, Shi et al. suggested defining it as the voltage at which the H2O2 concentration reaches 1 ppm.[24b] Conversely, the onset potential can also be evaluated as the voltage where the current density toward H2O2 (J(H2O2)) is 1 mA cm−2. J(H2O2) is determined by multiplying the overall current density with the voltage-dependent Faraday efficiency (FE(H2O2)).[57] The overpotential for H2O2 generation (ƞ(H2O2)) signifies the additional voltage required to drive this anodic reaction beyond the thermodynamic limit (1.77 V versus RHE), and can then be calculated using the following equation:
Material performance can also be evaluated by analyzing the current densities generated at a constant voltage. While some levels of H2O2 generation can occur electrochemically in the dark, light irradiation of the photoanode readily lowers the onset potential, improves the current density, and increases FE(H2O2).
The detection and quantification of generated H2O2, typically in the liquid phase, employ methodologies similar to those documented for PS system, are summarized in Table 2. However, disparities in experimental setups, such as the photoelectrode's irradiation area and measurement time, necessitates careful consideration when comparing H2O2 production performance across studies. For a meaningful comparison, the reported amount of H2O2 generated should be normalized by both the irradiation area and reaction duration (i.e., µmol min−1 cm−2). The presence of the competing O2 evolution reaction, where generated O2 can exist in either the gaseous or dissolved phase, can be verified and quantified using gas chromatography equipped with a thermal conductivity (TCD) detector or an O2 sensor.
Faraday efficiency (FE) serves as a key performance metric to evaluate the selectivity of a PEC system toward a desired product. It measures the ratio between the actual amount of product generated and the theoretically amount expected based on the measured current and the assumption of perfect efficiency. The FE for H2O2, O2, and H2 production can be calculated using the following equation:[57]
In addition to FE, solar energy conversion efficiency, or half-cell applied-bias photon-to-current efficiency (ABPE), can also be used to assess the efficiency of a photoanode. For BiVO4-based photoanode generating H2O2 and O2 as the oxidation products, ABPE can be determined using the following equation:
Chronoamperometry is the primary technique used to assess photoelectrode stability. This technique involves measuring the photocurrent density over an extended period under a constant applied bias. A stable photoelectrode exhibits minimal degradation, as evidenced by a steady photocurrent density maintained over several hours to days. However, there is currently no standardized metric for comparing photoanode stability for water oxidation.
Strategies for Enhanced PEC H2O2 Production
Bias-Assisted, Anodic H2O2 Generation
Sayama et al. have been at the forefront of designing WO3/BiVO4 heterojunctions photoanodes for electrochemical and PEC H2O2 production applications,[22,24a,41a–c] with Fuku and Sayama achieving a landmark demonstration of H2O2 generation through H2O oxidation in a PEC system. They utilized a WO3/BiVO4 photoanode alongside concurrent H2 evolution on a Pt cathode (as depicted in Figure 11a), accomplishing this feat at a significantly lower applied bias (<1 V versus RHE) than the theoretical electrolysis voltage (+1.77 V versus RHE) under simulated solar light.[22] The WO3/BiVO4 photoanode was designed by coating BiVO4 on a WO3 underlayer to facilitate the transfer of photogenerated electrons from BiVO4 to the FTO substrate (Figure 11b). Meanwhile, photogenerated holes on the BiVO4 surface oxidized H2O to H2O2. The choice of KHCO3 electrolyte proved to be a key factor for driving oxidative H2O2 generation on the WO3/BiVO4 photoanode. Compared to other electrolytes (i.e., K2SO4, phosphate buffer, and H3BO3), KHCO3 displayed the highest photocurrent generation across a wide applied bias range (Figure 11c), and H2O2 production was detected exclusively in the electrolyte (Figure 11d). Through optimization of the HCO3− concentration at 2 M under CO2 bubbling, a yield of ≈2 mM of H2O2 was achieved under visible-light irradiation (λ > 420 nm) and at 1.5 V versus RHE, despite a maximum FE(H2O2) of merely 54%. O2 was reported as the only oxidative byproduct at the photoanode. The beneficial role of the KHCO3 electrolyte was attributed to the formation of percarbonate intermediates, such as HCO4− and C2O62−, resulting from the oxidation of HCO3− by the photogenerated holes in BiVO4. Subsequent hydrolysis of these percarbonate species by H2O led to the formation of H2O2 and HCO3−. High concentrations of HCO3− were found to not only promote H2O2 formation, but also aid in inhibiting H2O2 degradation at the photoanode.[22,41b]
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The low FE(H2O2) observed for the WO3/BiVO4 photoanode could be attributed to the undesired oxidative degradation of generated H2O2 to O2. To address this issue, the WO3/BiVO4 photoanode surface was strategically modified with various amorphous, mesoporous metal oxide layers (MeOx) deposited using the metal–organic decomposition (MOD) method (Figure 12a).[41a] Compared to the unmodified WO3/BiVO4 photoanode, all MeOx-coated photoanodes exhibited enhanced H2O2 generation performance, with the exception of CoOx (Figure 12b). The observed enhancement followed the trend: CoOx << SiO2 < TiO2 < ZrO2 < Al2O3. Notably, the FE(H2O2) of WO3/BiVO4 photoanode almost doubled upon the addition of an Al2O3 surface coating. The subpar performance of CoOx in H2O2 generation was attributed to its propensity to catalyze either O2 evolution or H2O2 decomposition reactions. In a 2M KHCO3 electrolyte, the FE(H2O2) was enhanced from ≈54% for the unmodified WO3/BiVO4 photoanode to 79% for the Al2O3-coated WO3/BiVO4 photoanode (denoted as WO3/BiVO4/Al2O3(MOD)). Additionally, the H2O2 yield at 50C increased from ≈1300 µM for the unmodified photoanode to ≈2500 µM for the WO3/BiVO4/Al2O3(MOD) photoanode. The Al2O3 layer deposited on the BiVO4 surface is speculated to function as a passivation layer, potentially exerting three effects: i) preventing direct contact between BiVO4 and the generated H2O2 that diffuses into the electrolyte, thereby suppressing its oxidative degradation, ii) inhibiting direct O2 evolution from four-electron WOR by covering the active sites, and iii) enriching the local KHCO3 concentration around the photoanode due to the acid-base adsorption between HCO3− anions (weak base) and the mildly acidic Al2O3 surface.
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However, one main drawback of the MOD method was the formation of a thick mesoporous Al2O3 layer on the WO3/BiVO4 photoanode. This excessively thick Al2O3 layer could potentially disrupt electrical contact between the BiVO4 and the electrolyte solution, as substantiated by the decreased photocurrent density generated by WO3/BiVO4/Al2O3(MOD) compared to the unmodified WO3/BiVO4 photoanode (Figure 12c). Additionally, the ABPE also exhibited a decline, decreasing from 2.2% for the unmodified WO3/BiVO4 photoanode to 1.5% for the WO3/BiVO4/Al2O3(MOD) photoanode (Figure 12d). Chemical vapor deposition (CVD) was subsequently demonstrated as an alternative fabrication method capable to depositing a thin Al2O3 layer on the WO3/BiVO4 photoanode.[41c] The obtained WO3/BiVO4/Al2O3(CVD) not only enhanced the FE(H2O2) to 80% using a 2M KHCO3 electrolyte solution but also maintained its photocurrent generation ability (Figure 12c), leading to an improved ABPE with a maximum value of 2.57% (Figure 12d). Nevertheless, the presence of an Al2O3 layer (regardless of the fabrication methods) consistently enhanced the H2O2 yield with respect to the unmodified WO3/BiVO4 photoanode, as shown in Figure 12e. Analysis of the FE(H2O2) for all photoanodes over time (Figure 12f) revealed a gradual decrease due to the oxidative degradation of H2O2 into O2. However, the rate of decrease for Al2O3-modified photoanodes was found to be slower than that of the unmodified WO3/BiVO4 photoanode. This indicates that the Al2O3 layer inhibits the oxidative degradation of generated H2O2 into O2, possibly by blocking H2O2 interaction with the WO3 underlayer (Figure 12g).
In contrast, Zhang et al. utilized an oxygen-deficient SnO2 coating (SnO2-x) as a passivation layer on the BiVO4 photoanode, effectively modulating the surface reaction kinetics to attain highly selective water oxidation for H2O2 generation.[43a] While the SnO2-x/BiVO4 configuration exhibited enhanced photocurrent density compared to bare BiVO4 and SnO2/BiVO4 photoanodes, it showed an apparent negative shift in the onset potential (Figure 13a). Quantitative analysis of oxidation products indicated that the SnO2-x/BiVO4 photoanode yielded a negligible amount of O2, while BiVO4 and SnO2/BiVO4 photoanodes produced 2.628 and 0.773 µmol of O2, respectively (Figure 13b), implying near-complete suppression of O2 evolution (from both water oxidation and H2O2 decomposition) by the SnO2-x/BiVO4 photoanode. Additionally, the presence of an SnO2 or SnO2-x overlayer on the BiVO4 photoanode induced the formation of •OH, originating from one-electron WOR (H2O → •OH + H+ + e−), potentially facilitating H2O2 generation. Surface photovoltage and flat band potential measurements indicated that the addition of an SnO2-x overlayer on BiVO4 promoted hole migration to the surface, thus kinetically favoring H2O2 evolution with high selectivity due to reduced band bending (Figure 13c). Consequently, surface modification of the BiVO4 photoanode with an SnO2-x overlayer regulates the surface reactions, shifting from the competing hole oxidation reactions of H2O2 production and O2 evolution to favor H2O2 and •OH generation while also suppressing H2O2 decomposition. The SnO2-x/BiVO4 photoanode exhibited an average FE(H2O2) of 86% over a wide potential range of 0.6–2.1 V versus RHE. At 1.23 V versus RHE, it achieved an average H2O2 evolution rate of 0.825 µmol min−1 cm−2 under AM 1.5G illumination, corresponding to an ABPE of ≈5.6%, surpassing reported values.
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Ng and his team uncovered zinc oxide (ZnO) as another effective passivator for enhancing the selectivity and activity of BiVO4 in PEC H2O2 production.[44a] A homogeneous layer of ZnO on the BiVO4 photoanode demonstrably improved the FE(H2O2) within the potential range of 1.0–2.0 V versus RHE under AM 1.5G irradiation, (Figure 14a). The highest FE(H2O2) of ≈39.7% was achieved at 1.4 V versus RHE, representing an approximately 1.5-fold increase compared to the unmodified BiVO4 photoanode. Notably, both unmodified and ZnO-coated BiVO4 photoanodes produced O2 as the only byproduct. Moreover, the ZnO-coated BiVO4 photoanode exhibited higher photocurrent densities than the unmodified counterpart, accompanied by an evident negative shift in the onset potential (Figure 14b). The thin ZnO passivation layer on the BiVO4 photoanode was revealed to flatten the band bending and induce a positive shift in the quasi-Fermi level within the depletion layer at the semiconductor/electrolyte interface, as evidenced by Mott–Schottky, open-circuit potentials, and photoelectrochemical impedance spectra measurements. These combined effects promote selective H2O2 production while impeding the competing O2 evolution reaction, as illustrated in Figure 14c. Additionally, the ZnO overlayer suppresses H2O2 decomposition and serves as holes reservoir under photoexcitation, facilitating expedited charge extraction from BiVO4 for the water oxidation reaction.
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Building upon the established effectiveness of surface passivation in enhancing BiVO4’s PEC water oxidation for H2O2 generation, Wang et al. identified intrinsic oxygen vacancies (Ovac) on the material's surface as a barrier to selective H2O2 formation.[51] While Ovac introduction improves electron conductivity and density in n-type BiVO4,[58] their roles as traps and recombination sites remain controversial.[59] This controversy likely stems from the existence of two types of Ovac—bulk and surface—with distinct functionalities.[60] Wang et al. calculated the Gibbs free energy changes (ΔG) of water oxidation intermediates (OH*, O*, and OOH*) on the most stable (001), (101), and (111) surfaces of BiVO4 with and without surface Ovac. Their calculations (Figure 15a) demonstrated that presence of Ovac significantly lowers the activation energy for O2 evolution but creates a higher barrier for H2O2 formation. These results align with their experimental data, where BiVO4 photoanodes with reduced surface Ovac exhibit superior PEC water oxidative H2O2 generation. Surface Ovac reduction was achieved by post-treatment with air annealing (BiVO4-Air) and V2O5-assisted air annealing (BiVO4-Air/V) as evidenced by weaken EPR signals (Figure 15b). Notably, V2O5-rich annealing effectively eliminated the surface Ovac, as indicated by the negligible EPR signal. Both BiVO4-Air and BiVO4-Air/V photoanodes displayed enhanced photocurrent densities and more cathodic onset potentials compared to the unmodified counterpart. Despite exhibiting a slightly lower photocurrent density (potentially due to lower carrier density), BiVO4-Air/V achieved the highest average FE(H2O2) of 58.4% within the 0.8–1.8 V versus RHE potential range under AM 1.5G irradiation (Figure 15c). A total amount of 27.7 µmol H2O2 was produced after 5 h of illumination at 1.23 V versus RHE. In situ Raman analysis using 1M NaHCO3 electrolyte identified distinct intermediate species: OOH* for the unmodified photoanode (Figure 15d) and OH* for the BiVO4-Air/V (Figure 15e), further supporting the notion that reduced surface Ovac steers the water oxidation pathway toward H2O2 formation instead of O2. Similarly, a more recent study by Qu et al. reported a twofold increase in WOR selectivity for H2O2 with decreased Ovac on a BiVO4 photoanode, attributing this improvement to flattened band bending, a positively shifted quasi-Fermi level, and suppressed H2O2 decomposition.[52] The passivation of Ovac was accomplished through thermal treatment of the photoanode in a pressurized Parr reactor filled with O2, resulting in the formation of an O-BiVO4 photoanode.
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Surface microenvironment modification of BiVO4 photoanodes presents a valuable approach to kinetically control of competitive reactions during WOR on the electrode. Specifically, increasing photoanode surface hydrophobicity can inhibit the release of O2 while promoting H2O2 desorption. Wan et al. demonstrated this by post-treating an oxygen-vacancy-enriched BiVO4 (Ovac-BVO) photoanode, obtained via photoetching, under N2 atmosphere at elevated temperature to modulate surface wettability.[43b] This N2 treatment induced the formation of N2 molecules and N atoms binding to Ovac sites on BiVO4. While the Ovac sites were claimed to enhance charge separation, the N species decreased the surface wettability (i.e., increase hydrophobicity) of the as-obtained N2-treated Ovac-BVO photoanode (N-Ovac-BVO) without altering its intrinsic electronic properties (band structure and band bending). The N-Ovac-BVO photoanode with the lowest surface wettability was achieved at a calcination temperature of 350 °C (denoted as N-Ovac-BVO-350). Compared to the unmodified BiVO4 photoanode (BVO), N-Ovac-BVO-350 exhibited ≈1.3 and 4.1 times enhancement in photocurrent density (Figure 16a) and average FE(H2O2) (Figure 16b), respectively, under applied biases of 0.6–1.9 V versus RHE. The H2O2 production was revealed to be inversely correlated with surface wettability (Figure 16c). As illustrated in Figure 16d, the superior PEC H2O2 generation performance of N-Ovac-BVO-350 can be attributed to its poor surface wettability, which effectively increases the surface tension of O2 bubbles. This inhibits O2 release while promoting H2O2 separation from the photoanode surface before competitive reactions reach equilibrium, thereby kinetically enhancing H2O2 production. Nonetheless, H2O2 accumulated nonlinearly with reaction time for both BVO and N-Ovac-BVO-350 (Figure 16e), indicating that WOR kinetics for H2O2 evolution do not follow a first-order charge transfer mechanism. The initial fast reaction rate likely involves competing four-electron and two-electron WOR for O2 and H2O2 generation, respectively, whereas the subsequent slow process relates to competing H2O2 generation and decomposition reactions (Figure 16f). Illuminating the N-Ovac-BVO-350 photoanode for 2 h under simulated light and an applied bias of 1.6 V versus RHE yielded a total H2O2 concentration of 458 µM, corresponding to a production rate of 11.45 µmol h−1.
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Similarly, Ou et al. employed surface microenvironment engineering to kinetically favor H2O2 production over O2 evolution. They achieved this by coating the BiVO4 photoanode with a thin hydrophobic polytetrafluoroethylene (PTFE) (Figure 17a).[43c] This polymer coating is found to confine O2 gas near active sites, potentially favoring H2O2 generation thermodynamically through shifting of *OH intermediates. As depicted in the volcano plot for two-electron WOR as a function of the *OH binding energy (ΔG*OH), particularly within a solvation model, the highest H2O2 activity can be obtained when Bi sites are surrounded by four O2 molecules (Figure 17b). However, the inherent hydrophobicity of PTFE lowers the electrochemical surface area, leading to subpar photocurrent densities for the PTFE-coated BiVO4 photoanodes (PTFE/BVO) compared to the unmodified BiVO4 photoanode (BVO) (Figure 17c). Nevertheless, all PTFE/BVO photoanodes exhibited significant enhanced FE(H2O2) compared to BVO across a wide applied bias range of 0.6 to 2.1 V versus RHE (Figure 17d). Specifically, the highest FE(H2O2) of 85% was exhibited by the 10PTFE/BVO photoanode, corresponding to a fourfold increase over the BVO photoanode. A final H2O2 concentration of 150 µM was obtained at 1.23 V versus RHE under AM 1.5G illumination for 2 h.
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Doping presents another viable strategy for enhancing the PEC performance of BiVO4. Typically, this process involves substituting V5+ sites with hexavalent metal ions such as Mo6+ and W6+ to increase donor density, thereby enhancing electrical conductivity.[61] Jeon et al. investigated the impact of various dopants (Mo, W, and Cr) on the PEC H2O2 production performance of BiVO4 photoanodes.[53] As shown in Figure 18a, the introduction of these metal dopants enhanced BiVO4’s photocurrent generation. This improvement is attributed to the higher electrical conductivity and lower charge transfer resistance observed in BiVO4 upon doping, as evidenced by Mott–Schottky measurements (Figure 18b) and Nyquist plots (Figure 18c). Nevertheless, while Mo-doping improved H2O2 production and FE(H2O2), these parameters were compromised with W and Cr doping (Figure 18a). The superior performance of Mo as a dopant for enhancing H2O2 generation efficiency in BiVO4 photoanodes stems from its ability to inhibit H2O2 decomposition. This is supported by the observation that Mo-doped BiVO4 (Mo-BVO) exhibited the least amount of H2O2 decomposition relative to the total charge passing through the PEC cell (–Δ[H2O2]/QT) during the irradiation duration (3 h) compared to unmodified BiVO4 (BVO), W-doped BiVO4 (W-BVO), and Cr-doped BiVO4 (Cr-BVO) photoanodes (Figure 18d).
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Utilizing a WO3/Mo-doped BiVO4 heterojunction photoanode, Shi et al. introduced a unique method to enhance its light absorption and charge carrier dynamics by modulating the WO3 core surface before Mo-doped BiVO4 shell formation. In particular, exposing the preformed WO3 nanohelices (NHs) on an FTO substrate to a flame surface treatment in a reducing environment created an epitaxial layer of 1D oriented WO3 nanoneedles (NNs), as illustrated in Figure 19a.[54] The resulting carrot-shaped WO3 NN/NH nanostructure exhibited improved PEC performance compared to the WO3 NH counterpart, attributed to changes in optical and electronic properties (i.e., band gap energy, flat band potential, and VB position). Subsequently, the WO3 NN/NH nanostructure was employed as a scaffold to form a heterojunction by coating the surface with Mo-doped BiVO4, forming the core–shell WO/BVO photoanode. The obtained WO/BVO photoanode demonstrated activity for both H2O2 generation and O2 evolution in the respective KHCO3 and phosphate buffer electrolytes (Figure 19b). For H2O2 production, ≈414 µmol cm−2 of H2O2 was detected after illuminating the WO/BVO photoanode in 2 M KHCO3 for 6 h using a solar simulator at 0.8 V versus RHE. This corresponds to an H2O2 generation rate of 1.15 µmol min−1 cm−2.
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Yang et al. demonstrated that co-deposition of Co2+ and PO43− ions (CPMB) onto the Mo-doped BiVO4 (MB) surface constructively improves their activity for PEC water oxidative H2O2 generation via •OH radical intermediates.[41d] While modifying MB with either Co2+ or PO43− ions alone (denoted as CMB and PMB, respectively) improved photocurrent density generation compared to unmodified BiVO4 (BVO) and MB photoanodes, a synergistic effects was realized with the co-deposition of both ions, resulting in further enhancement (Figure 20a). At 1.7 V versus RHE, the H2O2 production activity trend followed the order: BVO < MB < CMB < PMB < CPMB (Figure 20b). The CPMB photoanode achieved an optimal H2O2 evolution rate of 0.23 µmol min−1 cm−2 with a FE(H2O2) of 26% (Figure 20c). In situ EPR spectroscopy, revealed the distinct roles of Co2+ and PO43− ions in contributing to the improved H2O2 evolution activity of CPMB (Figure 20d). While PO43− ions promote •OH formation from H2O dissociation and H2O2 desorption from the photocatalyst surface, Co2+ ions facilitate the •OH conversion for H2O2 and O2 production. Importantly, Co2+ ion plays a vital role in improving the kinetics of both H2O2 production and O2 evolution, supported by CMB showing a comparable FE(H2O2) to MB and BVO despite a higher H2O2 yield.
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Molybdenum remains the most prevalent and effective dopant for BiVO4. However, a recent study from Baek et al. explored gadolinium (Gd), a rare earth element, as a dopant to reduce the overpotential for water oxidative H2O2 production in BiVO4 photoanodes, while also improving its stability.[55] Gd was chosen due to its elemental stability, good electrical conductivity, and strong oxygen affinity compared to bismuth (Bi). This stronger oxygen affinity is anticipated to inhibit VO43− anion dissolution in BiVO4, a critical stability issue under both dark and illuminated conditions.[62] According to the Bi-V Pourbaix diagram calculated by Toma et al., BiVO4 is stable across a wide pH range of 1–11, but V dissolution into VO4− ions is predicted to occur near the water oxidation potential. This dissolution is accelerated by illumination and anodic biasing to 1.23 V versus RHE.[62b] Baek et al.’s DFT calculations revealed that doping a low concentration of Gd can activate several inactive BiVO4 facets (i.e., (011) and (101)) for H2O2 production, as illustrated in Figure 21a. The calculations also predicted a decrease in the overpotential required for H2O2 production on different facets of Gd-doped BiVO4 compared to bare BiVO4 (Figure 21b). Additionally, the estimated increase in the energy barrier for VO43− dissolution by 0.65 eV suggests improved stability for Gd-doped BiVO4. Experimental results corroborated these theoretical findings. With an optimal Gd doping concentration of 6%, the overpotential for H2O2 production reduced by ≈110 mV compared to pristine BiVO4 under dark conditions. Under 1 sun illumination, the Gd-doped BiVO4 photoanode portrayed a negatively shifted photocurrent onset, higher photocurrent densities, higher FE(H2O2), and an improved H2O2 production rate (Figure 21c,d). The Gd-doped BiVO4 photoanode accomplished an FE(H2O2) of ≈99.5% at 2.6 V versus RHE. The improved stability resulting from Gd doping is evident from the sustained photocurrent densities exhibited for the Gd-doped BiVO4 photoanodes compared to the pristine BiVO4 in the stability test (Figure 21e).
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Crystal facet engineering, particularly the fabrication of BiVO4 with exposed (010) and (110) facets, has emerged as a promising strategy to improve charge separation due to the differing band energy levels between the facets. Park et al. provided new insights into how crystal facet engineering can be used to manipulate interfacial energetics (i.e., band bending and quasi-Fermi level) at the BiVO4/electrolyte interface, thereby regulating the product selectivity for H2O2 or O2 during water oxidation in a PEC system.[43e] Three BiVO4 photoanodes with varying ratios of exposed (010) and (110) facets were synthesized using a controlled hydrothermal method (Figure 22a–c). The morphologies of the BiVO4 microcrystals on each photoanode are schematically depicted in Figure 22d. The photoanodes are designated as (010), (010)/(110), and (110) based on their dominant exposed facets. Ultraviolet photoelectron spectroscopy (UPS) analysis revealed significant differences in the band energy levels between the three BiVO4 photoanodes (Figure 22e). Specifically, the work function and VB maxima shifted away from the vacuum level with increasing (010) facet exposure. This indicates a decreasing Fermi level and less upward band bending. The weaker band bending leads to a more anodic hole quasi-Fermi level, which was also attested with the positive shift of flat-band potential observed in Mott–Schottky plots (Figure 22f). A more anodic hole quasi-Fermi level was hypothesized to increase the oxidation potential of holes to favor water oxidation at a higher potential level, for instances water oxidative H2O2 production at 1.77 V versus RHE is preferred over water oxidative O2 evolution at 1.23 V versus RHE. Such hypothesis is testified with improved photocurrent densities and a negative shift in onset potentials for BiVO4 photoanodes with increasing (010) facet (Figure 22g). The average FE(H2O2) was calculated to enhance from 11% for the (110) photoanode to 70% for the (010) photoanode at applied bias ranging from 0.6 to 1.8 V versus RHE under AM 1.5G illumination (Figure 22h). Using the (010) photoanode, the ABPE for PEC water oxidation to H2O2 was found to be 30 times higher compared to O2 evolution.
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While the focus thus far has been on the design and development of BiVO4 photoanodes for enhanced PEC water oxidative H2O2 generation, electrolyte selection presents another approach to influence catalytic activity and selectivity as it plays a crucial role in charge migration and mass diffusion within the system. Concentrated HCO3− electrolyte is commonly acknowledged as the most effective reaction medium for directly producing H2O2 from water oxidation. However, recent studies indicate that HCO3– can deplete the generated H2O2 through unwanted reactions.[63]
Wang et al. highlighted the potential of carbon quantum dots (CQDs) aqueous solution as an alternative electrolyte.[43d] They prepared three CQDs aqueous solutions (3-CQDs, 4-CQDs, and 6-CQDs) using different linear aliphatic amino acids as precursors. Interestingly, illuminating the BiVO4 photoanode with a 420 nm LED lamp in the CQDs solutions enhanced photocurrent densities across a wide range of applied biases compared to the traditional KHCO3 electrolyte (Figure 23a). Furthermore, the CQDs solutions yielded H2O2 production amounts surpassing KHCO3 (Figure 23b). The 6-CQDs solution demonstrated the best performance, producing H2O2 at an average rate of 0.33 µmol min−1 cm−2 with an FE(H2O2) of 93.5% at 1.23 V versus RHE. The superior performances of the CQDs solutions can be attributed to several factors: i) improved charge separation due to the formation of a dynamic heterojunction between the BiVO4 substrate and CQDs particles (Figure 23c); ii) the hydrophilic surface of CQDs allows water molecule confinement, facilitating water oxidation to H2O2 via •OH radicals formation; and iii) efficient inhibition of H2O2 decomposition. Figure 23d illustrates the proposed mechanism of PEC water oxidation to H2O2 using a BiVO4 photoanode with a CQDs solution. Due to an accumulation of photogenerated holes at the positively charged BiVO4 surface, negatively charged CQDs particles can be attracted via electrostatic attraction. Subsequently, they are oxidized by the holes and released back into the solution to drive water oxidation for H2O2 production before restoring to their original states. This adsorption and desorption process of CQDs on the BiVO4 surface continues until an equilibrium is achieved.
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Bias-Free, Dual-Sided H2O2 Generation
When utilizing BiVO4 as the photoanode, an external bias is necessary to induce H2 generation on the counter electrode. This is because the CB potential of BiVO4 is more positive than the standard reduction potential for converting H2O to H2 (Equation 7). Conversely, the standard reduction potential for reducing O2 to H2O2 (Equation 1) is more positive than the CB potential of BiVO4, indicating the feasibility of H2O2 production at the counter electrode even in the absence of an external bias. Sayama's team demonstrated this concept by assembling a PEC system with an Au cathode, a WO3/BiVO4 photoanode, and a 2 M aqueous KHCO3 solution electrolyte.[41b] Under simulated solar light and without an external bias, this system showcased H2O2 generation on both electrodes (Figure 24a). While H2O is oxidized to H2O2 at the anode, the cathode simultaneously produces H2O2 through O2 reduction, realizing a dual-sided H2O2 generation process. In a two-compartment cell, calculations revealed FE(H2O2) of ≈50% and ≈90% for the photoanode and cathode, respectively. Conspicuously, comparable H2O2 production was yielded in a one-compartment cell (Figure 24b), indicating that the ion-exchange membrane is dispensable. The researchers further demonstrated light-triggered simultaneous H2O2 generation using single photocatalyst sheet, which consisted of one half coated with WO3/BiVO4 and the other half coated with Au on an FTO substrate (Figure 24c). Exposing this sheet to simulated sunlight for 60 min in 2 M KHCO3 electrolyte reportedly generated 130 µM of H2O2. The team went on to showcase another bias-free PEC system consisting of a WO3/BiVO4/Al2O3(CVD) photoanode and a biomass-derived carbon (WSoy/GnP-CP) cathode. This combination simultaneously generated H2O2 with FE(H2O2) values of 60% and 44%, respectively.[41c] The combination also showed an open-circuit voltage of ≈0.30 V and a short-circuit current density of 0.7 mA.
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Shi et al. underlined such a bias-free, unassisted PEC system to be a light-driven fuel cell that utilizes light to trigger H2O2 production at both electrodes in the presence of water and oxygen, while concurrently generating electricity.[56] The simplified reaction can be expressed as follows:
Assuming that a BiVO4 photoanode absorbs all the photons with energies greater than its bandgap (2.4 eV) and all the photogenerated charge carriers are fully utilized for H2O2 generation (i.e., 100% internal quantum efficiency and 100% Faraday efficiency), a theoretical maximum H2O2 production rate of 4.6 µmol min−1 cm−2 can be attained under 1 sun irradiation. The team proceeded to investigate the effects of electrolyte composition and O2 purging on H2O2 production activity of both the BiVO4 photoanode and the carbon-supported carbon paper cathode (C-cathode) in their bias-free PEC system. Consistent with previously studies,[22,24a] the BiVO4 photoanode presented superior H2O2 production performance in a 2 M KHCO3 electrolyte compared to a 1 M Na2SO4 electrolyte, achieving a maximum FE(H2O2) of 95% at 1.7 V versus RHE. This can be ascribed to the role of HCO3− anion as a catalyst in H2O2 production. The valence band (VB) potential of BiVO4 (≈ +2.4 V versus NHE)[41b] is thermodynamically sufficient for driving HCO3− oxidation to HCO4− (Equation 20). Subsequently, HCO4− acts as a mediator, oxidizing H2O to H2O2 while regenerating HCO3− (Equation 21).
On the contrary, the cathode exhibited the opposite trend, with a higher FE(H2O2) observed in the 1 M Na2SO4 electrolyte compared to the 2 M KHCO3 electrolyte. This discrepancy might be attributed to the abundance of electrons at the cathode surface, which can drive Equation (20) in the reverse direction. The resulting higher concentration of HCO3− leads to the consumption of H2O2 through the reverse process of Equation (21), ultimately leading to lower H2O2 production when KHCO3 is used.
While O2 serves as a crucial reactant for H2O2 production via the ORR at the cathode, it is an undesirable product of the oxygen evolution reaction at the photoanode. Interestingly, O2 purging was found to be essential to promote H2O2 production at both the photoanode and cathode. As shown in Figure 25a, O2 purging has an insignificant impact on the H2O2 yield in the absence of mechanical stirring, as O2 diffusion between the electrodes is slow. However, when stirring is introduced, H2O2 production with O2 purging continues to rise, whereas it declines without it. This decline occurs because, under stirring and without O2 purging, O2 produced at the photoanode is transported to the cathode and consumed by the ORR. This, in turn, promotes the consumption of H2O2 at the photoanode to replenish O2 for the OER. In contrast to the photoanode, O2 purging consistently increases the H2O2 yield at the cathode regardless of stirring (Figure 25b). This is because O2 acts as the essential reactant for H2O2 production.
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Building upon the optimal operating conditions identified, the researchers constructed a PEC system for obtaining dual-sided H2O2 production on both electrodes. This system comprised a BiVO4 photoanode with 2 M KHCO3 electrolyte, a C-cathode with 1 M Na2SO4 electrolyte, separated by a membrane, and employed O2 purging on both sides (Figure 25c). H2O2 production was achieved using this system under both biased and unbiased conditions. Under an external bias of 1.5 V applied between the two electrodes, the optimized PEC setup produced ≈2610 ppm and 1530 ppm of H2O2 at the cathode and photoanode, respectively, after a 5 h reaction. This corresponds to a net production rate of 2.42 µmol min−1 cm−2. Interestingly, the study also demonstrated the potential of this setup to generate water disinfection-grade H2O2 using both tap water and distilled water. The system produced a total of 497 ppm and 319 ppm H2O2 using tap water and distilled water, respectively. In the absence of an external bias, the optimized system was still able to produce H2O2 at a rate of 0.48 µmol min−1 cm−2 (0.18 µmol min−1 cm−2 for photoanode and 0.30 µmol min−1 cm−2 for cathode) (Figure 25d). The system had an open-circuit voltage of 0.61 V and a short-circuit current density of 1.09 mA cm−2 (Figure 25e). The maximum output power density was reported as 0.194 mW cm−2.
Jeon et al. discovered that anthraquinone-modified single-walled carbon nanotube (AQ-CNT) is a promising material for achieving highly selective cathodic ORR to produce H2O2.[53] They fabricated an AQ-CNT/C cathode using carbon paper as the substrate for the AQ-CNTs. When paired with a Mo-doped BiVO4 photoanode (Mo-BVO as previously described in Figure 18), this cathode achieved near 100% FE(H2O2) across a broad bias range (0.75 to 2 V versus RHE) with a negligible amount of H2 being detected. However, the Mo-BVO photoanode itself only achieved a concurrent FE(H2O2) of 20–40% (Figure 26a), with noticeable amounts of O2 produced. Post-surface treatment of the Mo-BVO photoanode with phosphate (P-Mo-BVO) effectively boosted its FE(H2O2) to 40–50%. This treatment also improved photocurrent generation and slowed down H2O2 decomposition. In addition, the P-Mo-BVO photoanode was highly durable, maintaining 90% of its photocurrent over a 100-h reaction period. The higher stability of P-Mo-BVO with respect to Mo-BVO is further evident in time-profiled photocurrent generation experiments (Figure 26b), conducted at 1 V versus RHE with an AQ-CNT/C cathode. Clearly, the P-Mo-BVO||AQ-CNT/C configuration depicted a steadier photocurrent than that of the Mo-BVO||AQ-CNT/C over a 5 h reaction. Under bias-free conditions, the P-Mo-BVO||AQ-CNT/C system could generate the highest short-circuit current density of ≈0.19 mA under AM 1.5G light irradiation (Figure 26c). Furthermore, this system displayed steady photocurrent generation and H2O2 production on both electrodes under bias-free conditions for 5 h (Figure 26d). The net H2O2 production rate was 0.16 µmol min−1 cm−2, corresponding to a solar-to-H2O2 conversion efficiency of 0.27%.
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Conclusion and Outlook
Artificial photosynthesis presents a promising avenue for H2O2 production owing to its safety, economic viability, environmentally benignity, and long-term sustainability. It utilizes water, oxygen, and solar energy as primary resources, offering a clean and renewable approach. In this review, we evaluate the recent advancements of BiVO4-based materials in this field, with a focus on their applications in both PS and PEC systems, which are pivotal methods in artificial photosynthesis. A comprehensive analysis is provided, encompassing fundamental principles, performance assessment methodologies, photocatalyst and photoelectrode development, and optimization of reaction conditions specific to each system.
Despite BiVO4’s promising properties, its performance is hindered by inherent limitations such as poor charge carrier separation and mobility. To mitigate these challenges, various material design strategies have been explored, including doping, heterojunction formation, cocatalyst addition, crystal facet engineering, and surface passivation. These strategies are applied in both PS and PEC systems to enhance BiVO4’s photocatalytic activity for H2O2 generation. However, their implementation differs between the two systems, as detailed in Table 4. While doping and heterojunction formation are common strategies for both, other approaches are specifically tailored to meet the unique requirements of each system, highlighting the nuanced strategies necessary to optimize BiVO4’s efficiency for H2O2 production.
Table 4 Comparison of strategies for material modifications and reaction conditions in H2O2 production using PS and PEC systems.
| Category | Strategy/Condition | Powder Suspension (PS) | Photoelectrochemical (PEC) |
| Material Modification Strategies | Doping | Enhances bulk conductivity and material stability | Enhances bulk conductivity and material stability |
| Heterojunction | Improves charge separation by incorporating another photocatalyst | Improves charge separation by incorporating another photocatalyst | |
| ORR and WOR cocatalysts | Provides active sites and enhances surface reaction kinetics | Not applicable | |
| Crystal facet Engineering | Enables spatial charge separation on different facets | Not applicable | |
| Surface passivation | Not applicable | Optimizes BiVO4/electrolyte interfacial reaction kinetics and selectivity for H2O2 production | |
| Reaction Conditions | O2 atmosphere | Required | Not required |
| CO2 bubbling | Not required | Required (beneficial for anode reaction) | |
| Temperature control | Preferred (room temperature or lower) | Not necessary | |
| Sacrificial reagent | Preferred | Not necessary | |
| Reaction medium | Pure water | Bicarbonate solution (HCO3−) |
The design of BiVO4-based particles for H2O2 generation in PS systems primarily focuses on enhancing charge separation and increasing the number of surface active sites. Crystal facet engineering is prominently used to facilitate the spatial separation of photogenerated electrons and holes on the respective {010} and {110} facets of BiVO4 particles. Cocatalysts are incorporated on the surface of BiVO4 particle to provide active sites to promote the ORR and WOR. In contrast, the development of BiVO4-based photoanodes for H2O2 generation in PEC systems prioritizes improving the BiVO4/electrolyte interfacial reaction kinetics and enhancing selectivity of WOR toward the two-electron pathway for H2O2 generation. This is achieved through various surface passivation approaches, such as introducing a passivation layer and tuning of the photoanode's hydrophobicity. Each strategy targets distinct material aspects, including charge separation, surface reaction kinetics, and selectivity. The most successful approach often involves a carefully integration combination of different modification methods to leverage on their synergistic effects and maximize H2O2 production efficiency.
Moreover, each system requires distinct optimization of reaction conditions for H2O2 generation (see Table 4). The PS reaction typically operates under O2-saturated conditions as O2 serves as the reactant for H2O2 production via the two-electron ORR. The temperature of the reaction medium is typically controlled at room temperature or lower to minimize the thermal decomposition of H2O2. Sacrificial reagents such as methanol or ethanol are also added to expedite hole consumption, minimize charge recombination, and promote H2O2 production. In contrast, electrolytes containing bicarbonate ion (HCO3−) are essential for facilitating the direct production of H2O2 from PEC water oxidation, with CO2 bubbling of the electrolyte being beneficial for the operation of BiVO4-based photoanodes.
Significant progress has been made in the development of BiVO4-based materials for H2O2 generation via PS and PEC systems over the past decade. However, this field is still in its nascent stages, and achieving practical applications will require substantial further advancements. Figure 27 outlines the key challenges that need be addressed to develop highly efficient BiVO4-based photocatalysts for sustainable H2O2 production. These challenges are elaborated upon below:
- 1)Development of Alternative Cocatalysts: The current reliance on noble metals such as Pd and Au as cocatalysts in PS systems presents a significant hurdle due to their scarcity and hefty cost. For long-term sustainability and widespread adoption, it is imperative to explore earth-abundant and inexpensive alternatives. Additionally, optimizing three-phase mass transfer is crucial for efficient H2O2 production via the two-electron ORR pathway. This entails enhancing O2 adsorption onto the catalyst surface and minimizing H2O2 decomposition by facilitating its desorption to the liquid phase once formed.
- 2)Minimization of Sacrificial Reagent Use: The utilization of sacrificial reagents such as methanol and ethanol in PS systems, while effective in promoting photogenerated electrons for the two-electron ORR and improving H2O2 generation, introduces drawbacks. Their use should be minimized due to the potential generation of toxic by-products and challenges associated with separation of the H2O2 generated. Alternative strategies for suppressing charge carrier recombination are necessary.
- 3)Enhancement of BiVO4 Stability: Understanding of stability of BiVO4 in PS and PEC H2O2 generation is critical for developing efficient and durable solar-driven H₂O₂ production technologies. While extensive research has explored the chemical and photochemical corrosion mechanisms of BiVO4, particularly regarding V dissolution during water splitting, studies specifically addressing its stability in H2O2 generation are limited. Gaining insights into these photocorrosion mechanisms is essential for devising strategies to enhance BiVO4’s stability and optimize its performance in H₂O₂ production.
- 4)Understanding and Controlling Competing Reactions: Solar-driven H2O2 production involves competing reactions between H2O2 formation and decomposition. Current research primarily focuses on demonstrating increased H2O2 accumulation without a thorough understanding of how different photocatalyst and photoelectrode modification strategies influence reaction mechanisms. Utilizing advanced in-situ characterization techniques and theoretical calculations can provide valuable insights into the reaction pathways, guiding future advancements.
- 5)Achieving Higher H2O2 Yields: The current yields of H2O2 in both PS and PEC systems are insufficient for commercial viability. Future research should prioritize achieving H2O2 concentrations of at least several tens of millimoles per liter (mmol L−1) for practicality, particularly in small-scale and on-site applications such as disinfection and bleaching. By focusing on improving H2O2 yields to meet these concentration targets, researchers can pave way for broader adoption of these technologies.
- 6)Standardization of Methodologies and Performance Reporting: The current disparity in methodologies and performance reporting benchmarks makes it challenging to compare results across different research groups for photocatalytic PS and PEC H2O2 production studies. Standardization in these aspects is crucial for facilitating easy comparability of their reported performance. Standardized protocols can also serve as a valuable guide for new researchers, enhancing the reliability of research outcomes, ultimately accelerating the growth of this field.
- 7)Innovation in Reactor Design: While batch reactors are currently employed in H2O2 production via PS and PEC systems, further studies into flow reactors hold promise due to their superior mass transport capabilities. Flow reactors can improve the interaction between reactants and photocatalyst, potentially reducing H2O2 accumulation near the photocatalyst surface and minimizing its decomposition. Exploring the potential of flow reactors could lead to advancements in H2O2 production efficiency and process understanding.
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Addressing these challenges is essential for advancing the technology toward industrial implementation. This review paper is expected to guide the development of high-performance photocatalysts, particularly BiVO4-based materials, specifically tailored for PS and PEC systems. By providing a comprehensive overview of the field, it offers a valuable framework for researchers exploring other potential photocatalysts for solar-driven H2O2 generation.
Acknowledgements
The authors acknowledge the funding support from the Central Research Fund from the Agency for Science, Technology and Research (A*STAR).
Conflict of Interest
The authors declare no conflict of interest.
a) C. W. Jones, Applications of Hydrogen Peroxide and Derivatives, Royal Society of Chemistry, London 1999;
b) J. M. Campos‐Martin, G. Blanco‐Brieva, J. L. Fierro, Angew. Chem., Int. Ed. 2006, 45, 6962;
c) J. Liu, Y. Zou, B. Jin, K. Zhang, J. H. Park, ACS Energy Lett. 2019, 4, 3018.
R. L. Myers, The 100 Most Important Chemical Compounds: a Reference Guide, Bloomsbury Publishing USA, New York 2007.
a) S. Fukuzumi, Y. Yamada, ChemElectroChem 2016, 3, 1978;
b) Y. Yamada, Y. Fukunishi, S.‐i. Yamazaki, S. Fukuzumi, Chem. Commun. 2010, 46, 7334;
c) L. An, T. Zhao, X. Yan, X. Zhou, P. Tan, Sci. Bull. 2015, 60, 55;
d) S. Fukuzumi, Y. Yamada, K. D. Karlin, Electrochim. Acta 2012, 82, 493.
a) S. Fukuzumi, Joule 2017, 1, 689;
b) K. Mase, M. Yoneda, Y. Yamada, S. Fukuzumi, Nat. Commun. 2016, 7, [eLocator: 11470].
X. Zeng, Y. Liu, X. Hu, X. Zhang, Green Chem. 2021, 23, 1466.
R. J. Lewis, G. J. Hutchings, ChemCatChem 2019, 11, 298.
G. Gao, Y. Tian, X. Gong, Z. Pan, K. Yang, B. Zong, Chin. J. Catal. 2020, 41, 1039.
H. L. Tan, R. Amal, Y. H. Ng, ACS Appl. Mater. Interfaces 2016, 8, [eLocator: 28607].
H. Wu, H. L. Tan, C. Y. Toe, J. Scott, L. Wang, R. Amal, Y. H. Ng, Adv. Mater. 2020, 32, [eLocator: 1904717].
a) Z. Zeng, X. Quan, H. Yu, S. Chen, S. Zhang, J. Catal. 2019, 375, 361;
b) S. Wang, B. Cai, H. Tian, Angew. Chem. 2022, 134, [eLocator: e202202733];
c) W. Yu, C. Hu, L. Bai, N. Tian, Y. Zhang, H. Huang, Nano Energy 2022, 104, [eLocator: 107906].
a) J. Xiao, Q. Han, Y. Xie, J. Yang, Q. Su, Y. Chen, H. Cao, Environ. Sci. Technol. 2017, 51, [eLocator: 13380];
b) L. Liu, M.‐Y. Gao, H. Yang, X. Wang, X. Li, A. I. Cooper, J. Am. Chem. Soc. 2021, 143, [eLocator: 19287].
a) D. Bae, B. Seger, P. C. Vesborg, O. Hansen, I. Chorkendorff, Chem. Soc. Rev. 2017, 46, 1933;
b) Y. Kuang, Q. Jia, G. Ma, T. Hisatomi, T. Minegishi, H. Nishiyama, M. Nakabayashi, N. Shibata, T. Yamada, A. Kudo, Nat. Energy 2016, 2, 1.
a) F. Shiraishi, T. Nakasako, Z. Hua, J. Phys. Chem. A 2003, 107, [eLocator: 11072];
b) Y. Shiraishi, S. Kanazawa, D. Tsukamoto, A. Shiro, Y. Sugano, T. Hirai, ACS Catal. 2013, 3, 2222;
c) T. Baran, S. Wojtyła, A. Minguzzi, S. Rondinini, A. Vertova, Appl. Catal., B 2019, 244, 303.
a) Y. Wang, Y. Wang, J. Zhao, M. Chen, X. Huang, Y. Xu, Appl. Catal., B 2021, 284, [eLocator: 119691];
b) F. Ye, T. Wang, X. Quan, H. Yu, S. Chen, Chem. Eng. J. 2020, 389, [eLocator: 123427];
c) Y. Wang, Y. Wang, J. Zhao, Y. Xu, Appl. Catal., B 2021, 299, [eLocator: 120676].
a) X. Meng, P. Zong, L. Wang, F. Yang, W. Hou, S. Zhang, B. Li, Z. Guo, S. Liu, G. Zuo, Catal. Commun. 2020, 134, [eLocator: 105860];
b) Y. Zhang, J. Qiu, B. Zhu, M. Fedin, B. Cheng, J. Yu, L. Zhang, Chem. Eng. J. 2022, 444, [eLocator: 136584];
c) Y. Liu, J. Han, W. Qiu, W. Gao, Appl. Surf. Sci. 2012, 263, 389.
a) Y. Park, K. J. McDonald, K.‐S. Choi, Chem. Soc. Rev. 2013, 42, 2321;
b) Z. Zhao, Z. Li, Z. Zou, Phys. Chem. Chem. Phys. 2011, 13, 4746.
a) F. F. Abdi, R. Van De Krol, J. Phys. Chem. C 2012, 116, 9398;
b) F. F. Abdi, T. J. Savenije, M. M. May, B. Dam, R. Van De Krol, J. Phys. Chem. Lett. 2013, 4, 2752.
a) Y. Sun, L. Han, P. Strasser, Chem. Soc. Rev. 2020, 49, 6605;
b) Y. Xue, Y. Wang, Z. Pan, K. Sayama, Angew. Chem. Int. Ed. Engl. 2021, 60, [eLocator: 10469].
a) A. Torres‐Pinto, M. J. Sampaio, C. G. Silva, J. L. Faria, A. MT Silva, Catalysts 2019, 9, 990;
b) H. Hou, X. Zeng, X. Zhang, Angew. Chem., Int. Ed. 2020, 59, [eLocator: 17356];
c) Y. Ding, S. Maitra, S. Halder, C. Wang, R. Zheng, T. Barakat, S. Roy, L.‐H. Chen, B.‐L. Su, Matter 2022, 5, 2119.
S. Qu, H. Wu, Y. H. Ng, Adv. Energy Mater. 2023, 13, [eLocator: 2301047].
T. Freese, J. T. Meijer, B. L. Feringa, S. B. Beil, Nat. Catal. 2023, 6, 553.
K. Fuku, K. Sayama, Chem. Commun. 2016, 52, 5406.
F. E. Osterloh, Chem. Soc. Rev. 2013, 42, 2294.
a) K. Fuku, Y. Miyase, Y. Miseki, T. Gunji, K. Sayama, ChemistrySelect 2016, 1, 5721;
b) X. Shi, S. Siahrostami, G. L. Li, Y. Zhang, P. Chakthranont, F. Studt, T. F. Jaramillo, X. Zheng, J. K. Norskov, Nat. Commun. 2017, 8, 701.
a) H. L. Tan, R. Amal, Y. H. Ng, J. Mater. Chem. A 2017, 5, [eLocator: 16498];
b) J. H. Kim, J. S. Lee, Adv. Mater. 2019, 31, [eLocator: 1806938].
H. Hirakawa, S. Shiota, Y. Shiraishi, H. Sakamoto, S. Ichikawa, T. Hirai, ACS Catal. 2016, 6, 4976.
K. Fuku, R. Takioka, K. Iwamura, M. Todoroki, K. Sayama, N. Ikenaga, Appl. Catal., B 2020, 272.
K. Wang, M. Wang, J. Yu, D. Liao, H. Shi, X. Wang, H. Yu, ACS Appl. Nano Mater. 2021, 4, [eLocator: 13158].
H. Shi, Y. Li, K. Wang, S. Li, X. Wang, P. Wang, F. Chen, H. Yu, Chem. Eng. J. 2022, 443, [eLocator: 136429].
H. Shi, S. Li, M. Wang, X. Yin, J. Huang, W. Qi, X. Wang, P. Wang, F. Chen, H. Yu, Catal. Sci. Technol. 2023, 13, 3884.
Z. Xie, H. L. Tan, H. Wu, R. Amal, J. Scott, Y. H. Ng, Mater. Today Energy 2022, 26.
T. Liu, Z. Pan, J. J. M. Vequizo, K. Kato, B. Wu, A. Yamakata, K. Katayama, B. Chen, C. Chu, K. Domen, Nat. Commun. 2022, 13, 1034.
T. Liu, Z. Pan, K. Kato, J. J. M. Vequizo, R. Yanagi, X. Zheng, W. Yu, A. Yamakata, B. Chen, S. Hu, Nat. Commun. 2022, 13, 7783.
D. Dai, X. Bao, Q. Zhang, Z. Wang, Z. Zheng, Y. Liu, H. Cheng, Y. Dai, B. Huang, P. Wang, Chemistry 2023, 29, [eLocator: e202203765].
Y. Li, Z. Liu, W. Qi, H. Shi, K. Wang, X. Wang, P. Wang, F. Chen, H. Yu, Appl. Surf. Sci. 2024, 656, [eLocator: 159664].
H. Shi, Y. Li, X. Wang, H. Yu, J. Yu, Appl. Catal., B 2021, 297.
L.‐W. Wei, S.‐H. Liu, H. P. Wang, ACS Appl Nano Mater 2022, 5, [eLocator: 15378].
T. Liang, Y. Li, S. Xu, Y. Yao, R. Xu, J. Bian, Z. Zhang, L. Jing, Chin. J. Catal. 2024, 62, 277.
J. Huang, H. Shi, X. Wang, P. Wang, F. Chen, H. Yu, Catal. Sci. Technol. 2024, 14, 2514.
Z. Tong, D.‐K. Ma, J. Wu, X. Hu, N. Jiang, X. Yang, T. Peng, Chem. Eng. J. 2024, 496, [eLocator: 154262].
a) K. Fuku, Y. Miyase, Y. Miseki, T. Gunji, K. Sayama, RSC Adv. 2017, 7, [eLocator: 47619];
b) K. Fuku, Y. Miyase, Y. Miseki, T. Funaki, T. Gunji, K. Sayama, Chem. Asian J. 2017, 12, 1111;
c) Y. Miyase, S. Takasugi, S. Iguchi, Y. Miseki, T. Gunji, K. Sasaki, E. Fujita, K. Sayama, Sustain. Energy Fuels 2018, 2, 1621;
d) L. Yang, H. Chen, Y. Xu, R. Qian, Q. Chen, Y. Fang, Chem. Eng. Sci. 2022, 251.
A. N. Baga, G. A. Johnson, N. B. Nazhat, R. A. Saadalla‐Nazhat, Anal. Chim. Acta 1988, 204, 349.
a) K. Zhang, J. Liu, L. Wang, B. Jin, X. Yang, S. Zhang, J. H. Park, J. Am. Chem. Soc. 2020, 142, 8641;
b) S. Wan, C. Dong, J. Jin, J. Li, Q. Zhong, K. Zhang, J. H. Park, ACS Energy Lett. 2022, 7, 3024;
c) M. Ou, M. Geng, X. Fang, W. Shao, F. Bai, S. Wan, C. Ye, Y. Wu, Y. Chen, Adv. Sci. 2023, 10, [eLocator: 2300169];
d) N. Wang, Y.‐S. Zhang, D.‐D. Wei, H.‐M. Duan, L.‐M. Mo, H.‐Y. Wang, J. Mater. Chem. A 2023, 11, 4162;
e) K. Zhang, Y. Lu, Q. Zou, J. Jin, Y. Cho, Y. Wang, Y. Zhang, J. H. Park, ACS Energy Lett. 2021, 6, 4071.
a) S. Qu, H. Wu, Y. H. Ng, Small 2023, 19, [eLocator: e2300347];
b) T. M. Gill, X. Zheng, Chem. Mater. 2020, 32, 6285.
a) L. Zheng, H. Su, J. Zhang, L. S. Walekar, H. V. Molamahmood, B. Zhou, M. Long, Y. H. Hu, Appl. Catal., B 2018, 239, 475;
b) S. Zhao, X. Zhao, H. Zhang, J. Li, Y. Zhu, Nano Energy 2017, 35, 405.
R. Li, F. Zhang, D. Wang, J. Yang, M. Li, J. Zhu, X. Zhou, H. Han, C. Li, Nat. Commun. 2013, 4, 1432.
a) R. Li, H. Han, F. Zhang, D. Wang, C. Li, Energy Environ. Sci. 2014, 7, 1369;
b) J. Zhu, S. Pang, T. Dittrich, Y. Gao, W. Nie, J. Cui, R. Chen, H. An, F. Fan, C. Li, Nano Lett. 2017, 17, 6735;
c) D. Li, R. Chen, P. Wang, Z. Li, J. Zhu, F. Fan, J. Shi, C. Li, ChemCatChem 2019, 11, 3763;
d) Y. Qi, Y. Zhao, Y. Gao, D. Li, Z. Li, F. Zhang, C. Li, Joule 2018, 2, 2393.
M. Ledendecker, E. Pizzutilo, G. Malta, G. V. Fortunato, K. J. Mayrhofer, G. J. Hutchings, S. J. Freakley, ACS Catal. 2020, 10, 5928.
a) P. C. Wegner, Hydrogen Peroxide Stabilizer and Resulting Product and Applications, Google Patents, United States, 2004.
b) N.‐H. Kim, J.‐H. Lim, S.‐Y. Kim, E.‐G. Chang, Mater. Lett. 2003, 57, 4601.
Y. Shiraishi, Y. Ueda, A. Soramoto, S. Hinokuma, T. Hirai, Nat. Commun. 2020, 11, 3386.
L. Wang, Y. Lu, N. Han, C. Dong, C. Lin, S. Lu, Y. Min, K. Zhang, Small 2021, 17, [eLocator: 2100400].
S. Qu, H. Wu, Y. H. Ng, ACS Catal. 2024, 14, 5297.
T. H. Jeon, H. Kim, H.‐i. Kim, W. Choi, Energy Environ. Sci. 2020, 13, 1730.
X. Shi, L. Cai, I. Y. Choi, M. Ma, K. Zhang, J. Zhao, J. K. Kim, J. K. Kim, X. Zheng, J. H. Park, J. Mater. Chem. A 2018, 6, [eLocator: 19542].
J. H. Baek, T. M. Gill, H. Abroshan, S. Park, X. Shi, J. Nørskov, H. S. Jung, S. Siahrostami, X. Zheng, ACS Energy Lett. 2019, 4, 720.
X. Shi, Y. Zhang, S. Siahrostami, X. Zheng, Adv. Energy Mater. 2018, 8.
X. Shi, S. Back, T. M. Gill, S. Siahrostami, X. Zheng, Chem. 2021, 7, 38.
a) W. Zhang, L. Song, J. Cen, M. Liu, J. Phys. Chem. C 2019, 123, [eLocator: 20730];
b) S. Wang, P. Chen, Y. Bai, J. H. Yun, G. Liu, L. Wang, Adv. Mater. 2018, 30, [eLocator: 1800486];
c) S. Wang, T. He, P. Chen, A. Du, K. Ostrikov, W. Huang, L. Wang, Adv. Mater. 2020, 32, [eLocator: 2001385].
a) W. Qiu, S. Xiao, J. Ke, Z. Wang, S. Tang, K. Zhang, W. Qian, Y. Huang, D. Huang, Y. Tong, Angew. Chem. 2019, 131, [eLocator: 19263];
b) S. Selim, E. Pastor, M. Garcia‐Tecedor, M. R. Morris, L. Francas, M. Sachs, B. Moss, S. Corby, C. A. Mesa, S. Gimenez, J. Am. Chem. Soc. 2019, 141, [eLocator: 18791].
W. Wang, P. J. Strohbeen, D. Lee, C. Zhou, J. K. Kawasaki, K.‐S. Choi, M. Liu, G. Galli, Chem. Mater. 2020, 32, 2899.
a) H. Ye, J. Lee, J. S. Jang, A. J. Bard, J. Phys. Chem. C 2010, 114, [eLocator: 13322];
b) M. Li, L. Zhao, L. Guo, Int. J. Hydrogen Energy 2010, 35, 7127;
c) H. S. Park, K. E. Kweon, H. Ye, E. Paek, G. S. Hwang, A. J. Bard, J. Phys. Chem. C 2011, 115, [eLocator: 17870];
d) H. W. Jeong, T. H. Jeon, J. S. Jang, W. Choi, H. Park, J. Phys. Chem. C 2013, 117, 9104.
a) R. Lei, Y. Tang, W. Qiu, S. Yan, X. Tian, Q. Wang, Q. Chen, Z. Wang, W. Qian, Q. Xu, Nano Lett. 2023, 23, [eLocator: 11785];
b) F. M. Toma, J. K. Cooper, V. Kunzelmann, M. T. McDowell, J. Yu, D. M. Larson, N. J. Borys, C. Abelyan, J. W. Beeman, K. M. Yu, Nat. Commun. 2016, 7, [eLocator: 12012];
c) X. Yao, X. Zhao, J. Hu, H. Xie, D. Wang, X. Cao, Z. Zhang, Y. Huang, Z. Chen, T. Sritharan, iScience 2019, 19, 976;
d) S. Zhang, M. Rohloff, O. Kasian, A. M. Mingers, K. J. Mayrhofer, A. Fischer, C. Scheu, S. Cherevko, J. Phys. Chem. C 2019, 123, [eLocator: 23410].
a) D. E. Richardson, H. Yao, K. M. Frank, D. A. Bennett, J. Am. Chem. Soc. 2000, 122, 1729;
b) X. Yang, Y. Duan, J. Wang, H. Wang, H. Liu, D. L. Sedlak, Environ. Sci. Technol. Lett. 2019, 6, 781;
c) C. Dong, Y. Yang, X. Hu, Y. Cho, G. Jang, Y. Ao, L. Wang, J. Shen, J. H. Park, K. Zhang, Nat. Commun. 2022, 13, 4982.
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Abstract
Solar hydrogen peroxide (H2O2) production has garnered increased research interest owing to its safety, cost‐effectiveness, environmental friendliness, and sustainability. The synthesis of H2O2 relies mainly on renewable resources such as water, oxygen, and solar energy, resulting in minimal waste. Bismuth vanadate (BiVO4) stands out among various oxide semiconductors for selective H2O2 production under visible light via direct two‐electron oxygen reduction reaction (ORR) and two‐electron water oxidation reaction (WOR) pathways. Significant advancements have been achieved using BiVO4‐based materials in solar H2O2 production over the last decade. This review explores advancements in BiVO4‐based photocatalysts for H2O2 production, focusing on photocatalytic powder suspension (PS) and photoelectrochemical (PEC) systems, representing the main approaches for heterogenous artificial photosynthesis. An overview of fundamental principles, performance assessment methodologies, photocatalyst and photoelectrode development, and optimization of reaction conditions is provided. While diverse strategies, such as heterojunction, doping, crystal facet engineering, cocatalyst loading, and surface passivation, have proven effective in enhancing H2O2 generation, this review offers insights into their similar and distinct implementations within the PS and PEC systems. The challenges and future prospects in this field are also discussed to facilitate the rational design of high‐performing BiVO4‐based photocatalysts and photoelectrodes for H2O2 generation under visible light.
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; Chai, Casandra Hui Teng 2
; Heng, Jerry Zhi Xiong 2
; Thi, Quyen Vu 1
; Wu, Xuelian 3
; Ng, Yun Hau 4
; Ye, Enyi 2
1 Institute of Sustainability for Chemicals, Energy and Environment (ISCE2), Agency for Science, Technology and Research (A*STAR), Singapore, Singapore
2 Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), Singapore, Singapore
3 School of Mechanical Engineering, Chengdu University, Chengdu, China
4 Chemical Engineering Program, Physical Science and Engineering (PSE) Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia