Electrocatalytic oxidation of 5‐hydroxymethylfurfural for sustainable 2,5‐furandicarboxylic acid production—From mechanism to catalysts design

Abstract

Catalytic conversion of biomass‐based platform chemicals is one of the significant approaches to utilize renewable biomass resources. 2,5‐Furandicarboxylic acid (FDCA), obtained by an electrocatalytic oxidation of 5‐hydroxymethylfurfural (HMF), has attracted extensive attention due to the potential of replacing terephthalic acid to synthesize high‐performance polymeric materials for commercialization. In the present work, the pH‐dependent reaction pathways and factors influencing the degree of functional group oxidation are first discussed. Then the reaction mechanism of HMF oxidation is further elucidated using the representative examples. In addition, the emerging catalyst design strategies (defects, interface engineering) used in HMF oxidation are generalized, and structure–activity relationships between the abovementioned strategies and catalysts performance are analyzed. Furthermore, cathode pairing reactions, such as hydrogen evolution reaction, CO₂ reduction reaction (CO₂RR), oxygen reduction reaction, and thermodynamically favorable organic reactions to lower the cell voltage of the electrolysis system, are discussed. Finally, the challenges and prospects of the electrochemical oxidation of HMF for FDCA are presented, focusing on deeply investigated reaction mechanism, coupling reaction, reactor design, and downstream product separation/purification.

INTRODUCTION

In an era of heavy reliance on fossil fuel–based energy and chemicals, the increasing concerns on the drastic depletion of fossil fuels and carbon emissions are key motivations to explore renewable sources and pursue sustainable technologies.^1–4 Biomass is an abundant and sustainable resource formed from photosynthesis, which makes it a unique feedstock and a clean energy source to meet the ambitious goals of fossil fuel–free future.⁵ It is of great economic potential and social benefit to develop high value-added products from biomass. 5-Hydroxymethylfurfural (HMF), as one of the most important renewable platform chemicals, is formed by the dehydration of hexose-based biomass such as fructose and glucose.⁶ Functional groups, such as aldehyde groups (–CHO), hydroxyl groups (–OH), and furan rings, endow the versatility of HMF, which is considered a key bridge connecting biomass resources with future energy. Specifically, it can be transformed into commercial chemicals for industry, agriculture, and medicine through a series of reactions, such as hydrolysis, polymerization, hydrogenation, and redox.^7,8 In addition, high-quality bio-based platform chemicals, such as 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 2,5-diformylfuran (DFF), 5-formyl-2-furancarboxylic acid (FFCA), and 2,5-furandicarboxylic acid (FDCA), which are used as precursors or intermediates in chemical synthesis, polymer production, and pharmaceutical production, can be prepared by catalytic oxidation of aldehyde and hydroxyl groups of HMF at different positions and degrees.^7,9 Noticeably, one of the most promising derivatives is FDCA, an important monomer with aromatic rings with a planar and rigid structure for the production of bio-based polymers such as polyamides, polyesters, and polyclones.¹⁰ Most attractively, bio-based FDCA is expected to replace terephthalic acid, a petroleum-based organic synthesis intermediate used in the synthesis of polyethylene terephthalate.^11,12

In view of the promising prospects and significant impact of FDCA on the production of bio-based polymers, how to obtain high value-added FDCA through HMF has drawn considerable attention in recent years. As early as 1876, the first work of using thermochemical catalysis to produce FDCA was reported.^13,14 However, this approach requires high temperature (>100°C), high O₂ pressure (0.3–2.0 MPa), and precious metals with the limited FDCA yield, which utilizes the chemical potential as the driving force.^14–16 Compared with the traditional thermal catalytic oxidation, electrocatalytic HMF oxidation reaction (HMFOR) to prepare FDCA has been proposed and developed as a new organic synthesis technology due to its inherent advantages.^17–22 Specifically, electrocatalytic oxidation can achieve HMF transformation through electric drive under ambient conditions (normal temperature and pressure).¹⁷ Second, HMF electrooxidation can be carried out with water as oxygen source without adding oxidant or organic solvent to trigger the reaction.¹⁹ Most importantly, the yield of FDCA is maintained above 90% in most cases even with non-noble metal-based catalysts.¹⁸ Moreover, the yield, conversion, and Faraday efficiency of FDCA can be easily realized by the adjustment of electrolyte pH, the design and construction of catalyst, and the adjustment of point solution parameters.²⁰ What is more attractive is that one can couple the thermodynamically favorable HMF oxidation with different reduction reactions, which can not only enrich high value-added products but also greatly improve the energy efficiency.^21,22

Until recent years, electrooxidation of HMF into FDCA, a sustainable and economic technology in-line with the two-carbon target as it enables efficient conversion of biomass at mild conditions, has attracted significant attention of researchers. Although updated progresses on the electrochemical oxidation of HMF to FDCA are continuously reported, few systematic reviews concentrated on fundamentals, influence factors of pathway, mechanism, electrocatalysts design strategies, and related couple reactions. Herein, this review aims to provide an overview of recent advances in FDCA fabrication by selective HMF oxidation. The reaction mechanism and pH-mediated reaction pathways will be thoroughly analyzed. Furthermore, catalyst design strategies involving defect construction and surface interface engineering as well as performance analysis are discussed in depth. Subsequently, several important reactions that can be coupled to HMF oxidation are highlighted, including hydrogen evolution reaction (HER), CO₂RR, oxygen reduction reaction (ORR), and organics reduction reactions. Finally, the potential prospects of the electrochemical oxidation of HMF to FDCA are proposed, covering high-precision characterization technology, the improvement of catalyst performance, and possible practical application challenges.

REACTION PATHWAY AND MECHANISM OF HMFOR

The process of selective oxidation of HMF to FDCA typically involves six electrons transfer in three-step reaction,^23,24 which, in most cases, electrode reaction equations under alkaline electrolyte are summarized as follows: 1 $$$\begin{equation}{\rm{Anode}}:{\rm{HMF}} + 6{\rm{O}}{{\rm{H}}^ - } \to {\rm{FDCA}} + 4{{\rm{H}}_2}{\rm{O}} + 6{{\rm{e}}^ - }\end{equation}$$$2 $$$\begin{equation}{\rm{Cathode}}:6{{\rm{H}}_2}{\rm{O}} + 6{{\rm{e}}^ - } \to 6{\rm{O}}{{\rm{H}}^ - } + 3{\rm{H}}2\end{equation}$$$3 $$$\begin{equation}{\rm{Overall}}:{\rm{HMF}} + 2{{\rm{H}}_2}{\rm{O}} \to {\rm{FDCA}} + 3{{\rm{H}}_2}\end{equation}$$$

Each step contains two electron exchanges with two different pathways, which are dependent on the pre-oxidization of functional group (alcohol or aldehyde group). Generally speaking, path І involves the preferential oxidation of the aldehyde group in HMF to the intermediate of HMFCA. However, the preferential oxidation of the hydroxyl group leads to the formation of DFF, which belongs to path ІІ. Later on, the two reaction intermediates consume hydroxide together to form FFCA, which is further oxidized to produce FDCA (Figure 1).

[IMAGE OMITTED. SEE PDF]

Reaction pathway

According to high-performance liquid chromatography (HPLC) results from the literature, it was found that the oxidation path of HMF was dependent on pH.^21,25–28 Under high pH values (pH ≥ 13), HMF oxidation is apt to begin with aldehyde group to form HMFCA, whereas path II is the primary route at weak alkali environment (pH < 13). Combining HPLC and electrochemistry-coupled attenuated total reflection infrared (EC-ATR-IR) spectroscopy results, it was demonstrated that in 1 M KOH, no peaks of DFF in HPLC and ATR-FTIR spectra were observed, manifesting that electrocatalytic HMF oxidation on Ni_xB preferentially followed the HMFCA pathway (Figure 2A,B).²¹ However, the IR spectra of HMF and DFF exhibit high similarity, which makes it complicated to determine the HMF oxidation pathway by this method. Theoretically, product detection results combined with more refined in situ detection techniques can help to track and analyze the reaction intermediates to clarify the reaction pathway more accurately. Along this line, Wang et al.²⁶ explored the electrooxidation upgrade pathway of HMF using in situ sum frequency generation (SFG) vibrational spectroscopy, a second-order nonlinear spectral technique with intrinsic interfacial selectivity, which can capture interfacial molecular information that is difficult to obtain by other spectral methods. As shown in Figure 2C,D, under electrolysis at different potentials and durations with strong alkaline environment (in 1.0 M KOH with 10 mM HMF), only the signal of the intermediate HMFCA was detected, and these in situ SFG results directly confirmed that HMF electrocatalytic oxidation proceeds via the HMFCA pathway. The above studies both verify the path І is favorable under strong alkaline conditions (pH ≥ 13). As a matter of fact, the aldehyde oxidation of HMF is inhibited to form a DFF intermediate.²⁹ As reported by Duan's group, the experimental studies manifested that the pH of the electrolyte played a crucial role in the selectivity of HMFOR. As exhibited in Figure 2E,F, DFF and HMFCA were the primary product at neutral pH and alkaline medium, respectively.²⁷ The results revealed that the neutral medium was more conducive to the preferential oxidation of hydroxyl groups to obtain highly selective aldehydes than the alkaline medium (path II).²⁷ Moreover, ex situ product detection methods combined with advanced first-principles calculations were also used to investigate the reaction pathway of HMF oxidation. For example, according to the HPLC results, Wang et al. found that the dominant path of HMF oxidation changed from path II (pH = 9) to path I (pH = 13.8) with the increase of pH value (Figure 2H–K). The theoretical calculation showed that the aldehyde group had higher reactivity when pH was 13.8, which further corroborated the experimental results and clarified the pH-determination of the HMF oxidation pathway.²⁸ Another thermodynamic calculation result showed that the activation energy of the rate determining step (RDS) (FFCA* to form FDCA*) of path II (0.8 eV) was slightly lower than that of the RDS (oxidation of HMF*, forming HMFCA* via TSA-1) of path I (1.2 eV) (Figure 2G). It implies that the oxidation of HMF on Ni(NS)@CP is more inclined to DFF pathway, which is different from most reported understanding of preferentially difficult oxidation.^30–32 Despite this, small amounts of HMFCA and DFF can be detected experimentally simultaneously, suggesting that the electrooxidation reaction (EOR) of HMF to FDCA can actually go through both I and II pathways, and hydroxymethyl and aldehyde groups process considerable reactivity under the environment of pH 13. Nevertheless, there is no report on in situ characterization to monitor the reaction intermediates in weak alkaline media in real time. Hence, in situ technologies are expected to be applied in a wide range of pH electrolytes to verify the pH dependence of the reaction pathway more accurately and rigorously.

[IMAGE OMITTED. SEE PDF]

In addition to the pH-dependent pathway, the oxidation degrees of functional groups of HMF are also affected by the electrode materials. Choi et al.³³ found that MnO_x anode could oxidize hydroxyl and aldehyde groups of HMF to carboxyl groups to form FDCA. However, when Pt was used as the anode under the same conditions, the reaction only stayed at the step of selective oxidation of hydroxyl groups to form DFF resulting in no formation of FDCA. The above results indicate that the electrode material affects the electrooxidation pathway and oxidation degree of HMF, which may be related to the differences in the selectivity of various electrodes to functional groups and tolerance character of the catalyst. In subsequent investigations, researchers proposed the functional group preferential oxidation differentiation in HMF on different electrode surfaces. Through electrochemical characterization and product distribution detection, Wang et al.²⁸ found that the oxidation trends of aldehyde groups and hydroxyl groups on the surface of Co₃O₄ and NiO electrodes were inconsistent (Figure 2L,M). To elucidate the cause of this phenomenon, density functional theory (DFT) calculation was conducted. The results showed that on the surface of Co₃O₄ electrode, the hydration barrier (1.67 eV) in oxidation path II was much higher than that in dehydration barrier (0.75 eV) in path I (Figure 2N). The DFT calculation revealed that the aldehyde group was more easily oxidized on Co₃O₄ than the hydroxyl group. In addition, the adsorption energies of hydroxyl and aldehyde groups on HMF were calculated, and it was found that the adsorption capacity of hydroxyl groups (−1.15 eV) on the surface of NiO was stronger than that of aldehyde group (−0.86 eV), indicating that the adsorption capacity of hydroxyl group was superior to that of aldehyde group (−0.86 eV). The results of product distribution and theoretical calculation confirm that the catalyst plays a significant part in the oxidation degree of functional groups of HMF and synergistically exploits their different properties to improve the conversion performance of HMF oxidation to FDCA. In addition, Lee et al. reported an excellent reactivity of NiOOH and Cu(OH)₂ to alcohol and aldehyde on HMF, respectively,³⁴ which further proved the selective preferential oxidation of functional groups by catalysts. In general, the conversion of HMF oxidation to FDCA is affected by a combination of pH and electrocatalysts. When the reaction is triggered in a strong alkali medium, the design and construction of the catalyst become the primary task to improve the yield of FDCA.

The reaction mechanism of HMF oxidation

In addition to the aforementioned factors affecting the oxidation degree of pathways and functional groups, the entanglement studies of the redox properties of electrode material surface and oxidation reaction mechanism are also crucial for the in-depth understanding of HMF electrocatalytic oxidation to FDCA.

Indeed, the oxidation reactions of small organic molecules, such as alcohols, aldehydes, amines, and urea, in essence, take the nucleophilic groups with active hydrogen (such as hydroxyl, aldehyde, and amino) as reaction sites in the biomass oxidation reaction, which is classified as “nucleophilic oxidation reaction” (NOR).³⁶ HMF as a nucleophile is involved in complex six-electron transfer reactions (electrochemical dehydrogenation or spontaneous dehydrogenation) in electrocatalytic oxidation.³⁷ Like alcohols, HMFOR is divided into direct and indirect oxidation pathways, pending on whether the application of additional voltage can directly drive the oxidation of substrates.³⁸

Direct oxidation usually shows a clear potential-dependent oxidation (PD oxidation) character, as the oxidation of substrate molecules is driven by the applied voltage rather than dominated by chemical redox–mediator pairs, which displays more positive than that of indirect oxidation.^39–41 The direct oxidation process involves the adsorption of substrate molecules and OH⁻ on the electrode surface, followed by products formation and desorption.²⁹ The mechanism of direct oxidation is that the protons in substrate molecules are captured by OH⁻ adsorbed on the electrode surface, along with de-electronation.^42,43 Naturally, different oxidation pathways confer upon matching reaction mechanisms (Figure 3). Specifically, under weak alkaline environment, the hydration process of the aldehyde group is inhibited,²⁹ and the hydroxymethyl group of HMF, absorbed on electrode surface, will preferentially trigger the deprotonation of the C–H bond and the O–H bond through the activation of OH⁻, resulting in the production of DFF intermediate. The aldehyde group then undergoes a nucleophilic addition reaction with H₂O to form a geminal diol. The C–H and O–H bonds of the geminal diol are subsequently activated by OH⁻ and deprotonated to form FFCA. After that, the two steps of nucleophilic addition and deprotonation of the aldehyde group are repeated to generate FDCA (Figure 3A). In a strong alkaline environment (pH ≥ 13), the aldehyde group will preferentially adsorb on the catalyst surface, and it will be converted into geminal diol by addition with H₂O under alkaline catalysis.⁴⁴ The electrocatalytic dehydrogenation will then be triggered under OH⁻ activation to form the HMFCA intermediate. Later on, the hydroxymethyl group is deprotonated to form FFCA. Finally, the nucleophilic addition and dehydrogenation steps are repeated to form FDCA (Figure 3B). In a nutshell, the application of voltage will promote the adsorption of OH⁻ on electrode, and adsorption hydroxyl group (OH*) will be formed by electrons losing from OH⁻. Thus, in the presence of high concentrations of OH⁻, HMF molecules will be immediately oxidized on OH*-rich surfaces.

[IMAGE OMITTED. SEE PDF]

Indirect oxidation, as another well-established mechanism, was originally proposed by Fleischmann et al. through the establishment of oxidation of alcohols, amines, and aldehydes on nickel-based materials.^45,46 It is defined as the oxidation of an organic molecule that is not directly driven by an applied potential, in which the catalyst actually acts as a redox mediator to drive the chemical process oxidation of the substrate (non-electrochemical oxidation). In particular, indirect oxidation contains single-electron transfer two-step continuous processes that involve categories of heterogeneous (Figure 4A) and homogeneous (Figure 4B) redox routes. Taking the typical heterogeneous catalysis as an example, first, the initial catalyst underwent surface reconstruction and is oxidized to the high-state medium under the applied voltage (Mⁿ⁺ → Mⁿ⁺¹ (MOOH/(MOH)O)). Subsequently, the resulting oxidized medium (Mⁿ⁺¹) will complete the non-electrochemical oxidation of nucleophile/substrate (process b) through the transfer of hydride or hydrogen atoms of the nucleophile (spontaneously HMF dehydrogenation), accompanied by a chemical reduction of Mⁿ⁺¹ to the original valence state (Mⁿ⁺) (process a: Mⁿ⁺¹→Mⁿ⁺) (Figure 4A). Therefore, the oxidation potential of the nucleophile is highly consistent with that of Mⁿ⁺.^36,41 In recent studies, it is generally accepted that the mechanism of HMF oxidation catalyzed by nickel-based materials is in accordance with “electrochemical–chemical” (E–C). Namely, the oxidation process of HMF is accompanied by the redox of Ni²⁺/Ni^δ+, including the self-reconfiguration dehydrogenation of the catalyst and the spontaneous dehydrogenation of the nucleophile HMF (processes a and b in Figure 4A), which are the reaction steps of NOR.^21,36,47 Furthermore, the theoretical calculation reveals that the oxidizing mediator can be regarded as the active species, in which the lattice oxygen or adsorbed oxygen is the active site.³⁶ In order to undergo dehydrogenation, reactive oxygen species activate and clave the C–H/O–H bond by harvesting protons or electrons in the nucleophile, thereby promoting the process of NOR. Accordingly, one may enhance the intrinsic activity of the catalyst by decreasing the oxidation potential from the mediator to the reactive oxygen species/lattice oxygen and ultimately promote the indirect oxidation rate of the substrate molecules.

[IMAGE OMITTED. SEE PDF]

As mentioned in Section 2.1, the strongly alkaline environment (pH ≥ 13) is more conducive to the preferential adsorption and oxidation of aldehyde groups of HMF to form HMFCA (path I).^26,28 Under such conditions, it is favorable for the medium oxidation at low potential, which further support that high hydroxide concentration is conducive to the potential dependent indirect oxidation of HMF. Conversely, in weak alkaline electrolytes (pH ≤ 13), the medium prefers to bond with HMF rather than the hydroxides provided by the environment, thus hindering the electrochemical oxidation of the catalyst as an intermediate medium. As a result, the hydroxyl group in HMF is preferentially adsorbed on the catalyst over the aldehyde group, and HMF is directly oxidized through the DFF intermediate path.⁴⁸

The oxidation mechanism of substrates can be preliminarily evaluated by linear voltammetry scanning curves,⁴⁹ cyclic voltammetry (CV) curves, and multipotential step curves.^27,28,47,50 From Figure 5A, it can be seen that HMF oxidation goes through both direct and indirect oxidation mechanisms. Specifically speaking, the volcano plots exhibited direct oxidation for the HMFOR process at voltages of 1.4 and 1.55 V versus RHE due to the competitive adsorption of OH* and HMF (Figure 5B).²⁸ The indirect oxidation mechanism was investigated by the measurement of the multipotential step. Co₃O₄ was first oxidized at 1.5 V versus RHE potential (enriched high-priced Co⁴⁺ species). When the applied potential was switched to the open-circuit potential (OCP), a reduction current (blue line) appeared, indicating a transition from the high-valence Co⁴⁺ species to the Co³⁺ species (Figure 5C). However, before switching to OCP, the reduction current disappeared when 50 mM HMF was injected into the electrolyte (red line), indicating that the oxidation of HMF molecules chemically reduced the high-price Co⁴⁺ species, which conformed to an indirect oxidation process at 1.5 V versus RHE.²⁸

[IMAGE OMITTED. SEE PDF]

Choi et al. discovered that the oxidation behaviors for alcohol-containing species (HMF, HMFCA) were different from species containing only one aldehyde (DFF, FFCA) in electrolyte at pH value of 13. For HMF, a shoulder peak at ∼0.5 V versus Ag/AgCl was associated with the oxidation of Ni(OH)₂ to NiOOH, which can be ascribed to the first step of indirect oxidation (Figure 5D). Another oxidation peak (∼0.69 V vs. Ag/AgCl), as a second oxidation feature in CV, was related to PD oxidation, the rate of which was driven by the applied voltage without chemical oxidation contribution (Figure 5D).⁴¹ Analogously, HMFCA exhibited two oxidation peaks of indirect and PD oxidation (Figure 5E). In contrast, DFF and FFCA only showed enhanced features of the indirect oxidation of Ni(OH)₂ oxidation peak (Figure 5F,G). In the reverse scan, there was absolutely no cathodic peak for the reduction of NiOOH to Ni(OH)₂, which demonstrated that DFF and FFCA were mainly oxidized through an indirect oxidation mechanism (non-electrochemical consumption of Ni³⁺, the second step of indirect oxidation) (Figure 5F,G).⁵⁰ In addition, unlike DFF and FFCA, the NiOOH reduction peak of HMF and HMFCA decreased in the reverse scan, implying that NiOOH was partially reduced to Ni(OH)₂ by electrochemical reduction (ECR). Namely, the indirect process (the second step: reactants oxidation) was not sufficient to completely consume Ni³⁺ (Figure 5D,E).^41,51 Taken all these results together, the oxidation mechanism can be preliminarily recognized by CV feature peaks.

In 1.0 M PBS (pH = 13), from the CV curve of NiO (Figure 5H), it can be seen that 0.8–1.1 V versus RHE belonged to the OH* adsorption region (OH⁻ → OH* + e⁻).^52,53 In addition, the broad peak centered at 1.36 V, which enhanced with decreasing pH, was identified as the water dissociation process (H₂O → OH* + H⁺ + e⁻).^54,55 More importantly, the oxidation position of HMF was the same as the dissociation of water, indicating that the main active species of HMFOR on NiO is OH* produced by water dissociation rather than OH⁻ oxidation.²⁷ In addition, no signal of Ni³⁺ species was observed from ex situ Raman spectra of Ru₁–NiO after electrocatalytic reactions,⁵⁶ suggesting that the surface oxidation of Ru₁–NiO at neutral pH is negligible (Figure 5I). In combination with previous works,^36,57 the authors proposed that HMFOR on Ru₁–NiO is mainly controlled by the oxidation pathway involving OH*, rather than the E–C mechanism.

To more precisely identify and understand the reaction mechanism of HMFOR, in situ Raman spectra were conducted by Du's group.⁵⁸ They found that the dynamic transition of Ni(OH)₂ and NiOOH was potentially dependent during oxygen evolution reaction (OER) process.^58,59 Under HMFOR conditions, only a tiny peak was observed at ∼448 cm⁻¹ during potential cycling at 1.0–1.55–1.0 V (Figure 5J), indicating the formation of Ni(OH)₂ (453 and 523 cm⁻¹). However, no signals related to the phase transition of Ni(OH)₂, NiOOH, and Ni(OH)₂ were observed. Combined with similar in situ Raman spectroscopy results from other groups,^21,26,60 it was suggested that NiOOH was rapidly consumed by HMF and reduced to Ni(OH)₂, coupling with chemical HMF oxidation (E–C mechanism). In other words, during HMFOR process, Ni(OH)₂ was first electro-oxidized to NiOOH and then caused a fast-spontaneous reaction in terms of the interaction between NiOOH and HMF, which is independent of the applied potential (indirect oxidation) (Figure 5K).

From the above discussion, it can be seen that OH⁻ plays a crucial role in both direct and indirect oxidations. The diffusion of OH⁻ will be limited with insufficient OH⁻ concentration, thereby reducing the OH* supply and resulting in slow aldehyde group oxidation (oxygen species mediator). In addition, high alkalinity increases the adsorption capacity of OH⁻ on the electrode surface, thus directly promoting the dehydrogenation step to direct or indirect oxidation of substrate. Therefore, it is favorable for the oxidation of HMF in a strong alkaline environment.

DESIGN STRATEGIES FOR ELECTROCATALYSTS

Electrocatalytic water splitting is a promising and efficient technology to obtain green hydrogen energy, whereas the wide application is severely restricted by the thermodynamics and slow kinetics of the anodic OER. Choosing a more thermodynamically favorable HMFOR to replace OER can not only improve the hydrogen evolution efficiency but also prepare high value-added chemicals, providing a feasible way to improve energy utilization and conversion efficiency. The improvement of electrocatalyst is undoubtedly an important propellant to break through the bottleneck of electrocatalytic oxidation. So far, much effort has been devoted to the construction of transition metal catalysts for HMF oxidation, including borides,^21,61 nitrides,⁶² phosphides,^63,64 oxides,⁶⁵ chalcogenides,^66,67 and hydroxides,^43,68,69 due to their accessible regulation of electronic structure and high earth abundance. However, currently explored electrocatalysts for HMF oxidation are still far from satisfactory for practical applications. In an attempt to improve catalytic performance, several emerging design and construction strategies have been implemented, such as doping,⁷⁰ microstructure design,⁷¹ interface engineering,⁷² strain,⁷³ and defect engineering. In this section, performance improvement strategies including interface and defect engineering will be highlighted for HMFOR. In addition, the HMFOR performances of recently reported electrocatalysts are summarized in Table 1.

TABLE 1 The electrocatalytic performances of recently reported metal-based electrocatalysts for 5-hydroxymethylfurfural (HMF) oxidation

Catalyst	CHMF (mmol/L)	Product	Electrolyte	J (mA/cm2)/E (VRHE)	Conversion (%)	Yield (%)	FE (%)	Stability (cycles/VRHE)	Ref.
NixB	10	FDCA	1.0 mol/L KOH	100/1.45	100	98.5	100	NA	21
NiSe@NiOx	10	FDCA	1.0 mol/L KOH	50/1.35a	100	99	99	6/1.423	66
Ni3N@C	10	FDCA	1.0 mol/L KOH	50/1.38	100a	98a	99a	6/1.45	26
Ni(NS)/CP	5	FDCA	0.1 mol/L KOH	Onset/1.33	99.7	99.4	NA	3/1.36	35
CoOOH	50	FDCA	1.0 mol/L KOH	20/1.213	100	100	99	4/1.423	101
NiCoFe LDHs	10	FDCA	1.0 mol/L NaOH	10/1.51a	95.5	84.9	90	10/1.54	43
NiCo2O4	5	FDCA	1.0 mol/L KOH	10/1.47a	99.6	90.4	87.5	3/1.5	23
NixSey	50	FDCA	1.0 mol/L KOH	50/1.37	NA	99.3	98.9	6/NA	102
CoNiP	10	FDCA	1.0 mol/L KOH	20/1.46	NA	85.8	87.2	8/1.5	103
NiCoS	10	FDCA	1.0 mol/L KOH	Onset/1.2	99.1	97.1	96.4	5/1.45	104
E-CoAl-LDH	10	FDCA	1.0 mol/L KOH	10/1.30	100a	99.5a	99.4	7/1.52	105
d-NiFe LDH	10	FDCA	1.0 mol/L KOH	10/1.47a	97.3	96.8	84.5	10/1.48	83
CoFe LDH@NiFe LDH	10	FDCA	1.0 mol/L KOH	10/1.311	100	100	99.8	NA	106
NiVCo-LDHs	10	FDCA	1.0 mol/L KOH	100/1.37	100a	99.7	97	10/1.376	107
NiCu NTs	20	FDCA	1.0 mol/L KOH	136/1.424	100	99	96.4	5/1.424	108
Cu(OH)2/CF	10	FDCA	1.0 mol/L KOH	20/1.58a	100	98.7	100	6/1.45a	109
Ru–NiO	50	DFF	1.0 mol/L PBS	10/1.283	72.4	42.5	43.3	NA	27
Ir–Co3O4	50	FDCA	1.0 mol/L KOH	Onset/1.15	100a	98	98	6/1.42	110
Mo–Ni0.85Se	10	FDCA	1.0 mol/L KOH	50/1.39a	100	99	99	6/1.4	90
Pt–Ni(OH)2	50	FDCA	1.0 mol/L KOH	10/1.39a	100a	96a	98.7	NA	60
Ce–CoP	10	FDCA	1.0 mol/L KOH	10/1.45	100	98	96.4	4/NA	85
Co–Ni3S2	50	FDCA	1.0 mol/L KOH	Onset/0.9	100	100	99.1	11/1.45	84
Mo–Co3O4	50	FDCA	1.0 mol/L KOH	30/1.39	NA	95	92	4/NA	111
MoO2–FeP	10	FDCA	1.0 mol/L KOH	10/1.359	100	98.6	97.8	10/1.424	32
Ni(OH)2–NiFeP	10	FDCA	1.0 mol/L KOH	Onset/1.22	100	99	94	3/1.55	58
Ni2P–NiCoP	5	FDCA	0.1 mol/L KOH	10/1.32	100	98.1	97.6	6/1.45	63
NiO–Co3O4	10	FDCA	1.0 mol/L KOH	50/1.37a	100a	98	96	6/1.45	87
NiSx–Ni2P	10	FDCA	1.0 mol/L KOH	10/1.34a	100	98.5	95.1	5/1.46	95
Co–CoSx	5	FDCA	0.1 mol/L KOH	10/1.29	100	96	93.5	4/1.4	99
FeP–NiMoP2	10	FDCA	1.0 mol/L KOH	10/1.333	100	99.2	99	10/1.404	64
PbO–CuO	50	FDCA	1.0 mol/L KOH	Onset/1.14	99.5	96.2	93.7	6/NA	112
MoS2–Ni3S2	50	FDCA	1.0 mol/L KOH	10/1.44	NA	95a	95a	5/1.7	113
CoP–CoOOH	150	FDCA	1.0 mol/L KOH	Onset/1.02	98.3	96.3	96.3	5/1.42	96

Defects

Crystal defect refers to the periodic repeated arrangement of particles deviating from the crystal structure.⁷⁴ It is well known that crystal defects are usually classified into point, line, surface, and bulk defects.^75,76 Among them, point defects, including vacancy, gap, and impurity defects, have a critical impact on the functional properties of materials and are widely used in various application scenarios of catalysis.^76,77 Here, we mainly focus on the few typical point defect catalysts and their important roles in the improvement of catalytic performance. Ultraviolet laser with the advantages of narrow pulse width, short wavelength, fast speed, large output energy, and high peak power shows great potential in generating oxygen vacancies (V_o) in materials. Based on this, Tao's group constructed an oxygen-rich vacancy nano-nickel oxide (V_o-NiO) catalyst by laser ablation strategy to effectively oxidize HMF to FDCA.⁷⁸ The existence of oxygen vacancies regulated the electron distribution state of Ni atoms, which was conducive to the pre-oxidation of Ni²⁺ to facilitate the formation of NiOOH on the surface, cooperating with V_o-NiO to promote the catalytic oxidation of HMF with an advanced onset potential of 200 mV (Figure 6A). Furthermore, theoretical calculation results showed that oxygen vacancy enhanced the adsorption strength of HMF on the NiO surface, resulting in ameliorative HMFOR kinetics (Figure 6B).⁷⁸ However, the surface adsorption energy of catalysts has not been reported as one of the descriptors of HMF oxidation activity until Mu et al. proposed the importance of descriptors for catalyst design. They demonstrated that the adsorption energy and catalytic activity of metal oxides (NiO, Co₃O₄, CuO, Fe₂O₃, and Mn₂O₃) to HMF by DFT calculation and experiment were consistent with the relationship of near volcanic diagram (Figure 6C).⁷⁹ It is widely recognized in various catalytic fields that the volcano shape diagram can provide intuitive insight into the reaction and reveal the activity trend of different catalysts.^80–82 Therefore, Mu's team proposed for the first time that the adsorption energy of HMF on the catalyst surface is a potential activity descriptor for the HMFOR and further clarified the rationality of the adsorption energy of HMF as a descriptor.⁷⁹

[IMAGE OMITTED. SEE PDF]

In addition to monomial transition metal oxides,⁸⁶ high-entropy oxides (HEOs) have also stimulated research enthusiasm. Wang's group synthesized oxygen-enriched vacancies of P-HEOs ((FeCrCoNiCu)₃O₄) nanosheets by low-temperature plasma method, showing higher specific surface area and intrinsic activity than C-HEOs prepared by high-temperature calcination method, which greatly improved the performance of HMFOR.¹⁸ In addition, the construction of cationic vacancies has emerged as an effective strategy for tailoring the electronic configuration and coordination environment of metal sites, thereby promoting electrocatalytic performance.⁸⁷ For example, Wang et al. successfully prepared carbon paper–supported NiFe-layered double hydroxide (LDH) rich in cation defects (d-NiFe LDH/CP) for catalytic HMF oxidation to FDCA via hydrothermal and alkaline etching methods.⁸³ Characterization results showed that the electron density of d-NiFe LDH could be effectively increased by the implantation of cationic vacancies, which was beneficial to improve the electrocatalytic performance (Figure 6D). Heteroatom doping, an important and well-established means of forming point defects, has been widely used in electrocatalysis. Wang et al. synthesized an efficient and low-cost Co-doped Ni₃S₂@NF electrocatalyst for HMF oxidation via a simple hydrothermal method. The results supported that the successful introduction of Co resulted in the lattice distortion and crystallinity reduction of Ni₃S₂, tuning the local environment of electrons.⁸⁴ Moreover, the microstructure of the sample was well regulated, which significantly increased the ECSA (Figure 6E), endowing the catalyst with an ultralow onset potential (0.9 V vs. RHE) and excellent cycling durability in a 10 ml electrolyte containing 1.0 M KOH and 10 × 10⁻³ M HMF at a constant potential of 1.45 V (Figure 6F). In situ Raman spectroscopy studies showed that the surface of Co_0.4NiS@NF is quickly reconstructed into nickel (oxy) hydroxide, which was the actual active site of HMFOR.⁴¹ The peak of the progressive NiCo(oxy)hydroxides disappeared within 140 s after HMF injection (Figure 6G), indicating that HMFOR occurred quickly.^88,89 Li et al. designed and fabricated porous Ce-doped CoP (Ce–CoP) nanosheets, which integrated elemental doping, vacancy defects, and 2D nanostructures through a deep eutectic solvent (DES) method for the first time (Figure 6H,I).⁸⁵ On the one hand, the DFT calculation results revealed that Ce doping and phosphorus vacancies adjusted the behavior of HMF adsorption, which contributed to a low reaction barrier, facilitating reaction kinetics. On the other hand, the two-dimensional charge difference isosurface (red: electron-rich area, blue: deficient area) illustrated the electron redistribution of CoP (Figure 6J). The formation of P vacancies led to the transfer of electrons from Ce to P vacancies. The resulting electron consumption on Ce atoms helped to reduce the adsorption energy of HMF, thereby promoting the electrocatalytic oxidation. Consequently, Ce–CoP exhibits up to 98% yield of FDCA and FE of 96.4%, surpassing most reported electrocatalysts.^62,90–92 In summary, defect engineering contributes greatly to the performance improvement in HMFOR reaction. First, the vacancy can be used as the active site to adsorb OH*. Second, the charge distribution of the material will be regulated to increase the reaction kinetics. And finally, the conductivity will be improved, producing promoted electron transfer and optimized catalytic performance.

Interface engineering

Interfacial electronic regulation of electrocatalysts is an effective strategy to enhance the catalytic activity of the catalysts, which is conducive to accelerating the electron transfer and regulating the adsorption capacity of species, thus improving the reaction kinetics.^93,94 Take nanostructured NiO–Co₃O₄, for example, the hetero-interface formed by NiO and Co₃O₄ induced the formation of cationic vacancies, which enhanced the oxidation state of Ni species, promoting HMF oxidation activities.⁸⁷ In addition to create vacancies, Mu et al. reported that NiS_x/Ni₂P (SNP-2) heterogeneous with abundant interfaces reduced the adsorption energy of HMF at SPN-2 surface (Figure 7A) and strengthened the interaction between formyl and hydroxymethyl groups in HMF and catalyst (Figure 7B), endowing superb HMF oxidation performance (Figure 7C).⁹⁵ In addition, Tao's group investigated CoP–CoOOH heterojunction for biomass-upgrading.⁹⁶ The Mott–Schottky plots combined with HRTEM data demonstrated the formation of heterojunctions and the flow of electrons (Figure 7D). Specifically, electrons will be transferred from CoOOH (n-type semiconductor) at low Fermi level (E_F) to CoP (p-type semiconductor), resulting in electron-deficient CoOOH generation and electron redistribution.^97,98 Moreover, XAS characterization and DFT calculations further confirmed that holes on CoOOH formed by electron transfer promoted the adsorption and oxidation of HMF (Figure 7E,F). Apart from compounds, heterojunctions can exist in the form of metal–compound and metal–metal as well.⁴⁸ For example, Li et al. fabricated Co–CoS_x heterojunctions embedded in yolk shell polyhedron (Figure 7G), exhibiting enhanced HMFOR activity with a low voltage of 1.29 V versus RHE to provide the current density of 10 mA/cm² (Figure 7H). The DFT calculation revealed that the construction of Co–CoS₂ heterojunction induced d-electron spin polarization, resulting in favorable p–d couple between substrate/intermediate and catalyst. Furthermore, the comparison of limiting potentials plot demonstrates the suppression of competing OER over Co–CoS₂ heterojunction, affording a higher HMFOR selectivity.⁹⁹ Although it is more accessible to introduce carbon materials to construct heterojunctions, the preparation process is usually complicated and involves the emission of organic solvents. Under the careful investigation of Mu et al., an N, S, O-doped carbon (NSOC) ternary heterogeneous interface material (Co₉S₈–Ni₃S₂@NSOC) was prepared by a DES method (Figure 7I). In addition, the heat treatment process creates many vacancies (N, O, and S) in the material (Figure 7J). Experimental and calculation investigations verified that the formations of heterojunction contributed to electrons transfer from Ni₃S₂ to Co₉S₈ and NSOC, making the easier generation of the Ni³⁺ active species, thus enhancing the activity of HMF oxidation (Figure 7K).¹⁰⁰

[IMAGE OMITTED. SEE PDF]

COUPLING REACTIONS

The process of electrocatalysis involves the coupling of two half-reactions, anodic oxidation, and cathodic reduction. A typical example is an electrolysis of water to produce green hydrogen, which consists of cathodic HER and anodic OER. However, the efficiency of water splitting is hindered by the high activation barrier of OER. Therefore, selecting a thermodynamically favorable HMF oxidation inverse coupling hydrogen production system provides a feasible way to improve the energy conversion efficiency and obtain high value-added products. In addition, according to the demand of value-added products, the pairing of CO₂,¹¹⁴ N₂ reduction,¹¹⁵ and organic molecules electroreduction reactions (ERRs) and EORs is of great significance for the development of the catalysis field.

Coupling hydrogen evolution reaction (HER)

Hydrogen energy, as a sustainable energy source with high mass-energy density and zero carbon emission, is helping to solve the energy crisis.^116–118 Electrocatalytic water splitting is a promising and efficient technology to obtain high-purity hydrogen.¹¹⁹ However, the anodic OER with slow kinetic seriously restricts the wide application. Therefore, the selection of HMFORs to replace OER has attracted much attention. Shao's group reported an ultrathin LDH nanosheet array with abundant oxygen vacancies (E-CoAl-LDH-NSA), which exhibited a low voltage for HMF oxidation as well as excellent stability with high FE% (>90%) for both FDCA and H₂ production in dual-electrode cells (Figure 8A,B).¹⁰⁵ In addition, the increased bandgap for E-CoAl-LDH-NSA reveal an enhanced electric conductivity originating from the oxygen vacancies implantation (Figure 8C). Subsequently, on the basis of previous study, they designed and prepared the CoNiP integrated electrode (CoNiP-NIE) for both HMFOR and HER.¹⁰³ DFT calculations combined with in situ characterization indicate that CoNiP can effectively improve the conductivity of the electrode and accelerate the desorption of FDCA molecules on the electrode surface (Figure 8D). The optimized catalyst in the coupling reaction showed a cell voltage (1.46 V) of 300 mV lower than that of water splitting (1.76 V) (Figure 8E) and delivers high-evolution H₂ rate (41.2 L/(h m²)) and high FDCA yield (85.5 g/(h m²)) in alkaline electrolyte (Figure 8F). Excitingly, single-metal ultrathin nickel hydroxide nanosheets (Ni(OH)₂/NF),¹⁹ Ni₂P nanoparticle arrays (Ni₂P NPA/NF),¹²⁰ and copper-doped nickel nanotubes (NiCu NTs) have all been shown to possess excellent bifunctional catalytic activity of electrocatalytic HMF oxidation and hydrogen evolution.¹⁰⁸ Furthermore, the porous carbon-coated MoO₂–FeP heterojunction (MoO₂–FeP@C) with abundant active interface exhibits a low overpotential of 103 mV at 10 mA/cm² for HER (Figure 8G–I).³² The enhanced activity is attributed to the electron transfer from MoO₂ to FeP at the interface (Figure 8J), where electron accumulation on FeP helps to optimize the H₂O and H* adsorption energies for HER, whereas hole accumulation on MoO₂ is beneficial for improving HMFOR activity.³²

[IMAGE OMITTED. SEE PDF]

Coupling CO₂ reduction reaction (CO₂RR)

ECR of CO₂ into value-added chemicals is an alternative approach to alleviate global warming.^121,122 Different transfer electron numbers of CO₂RR involve different reaction intermediates and products, such as C₁ product (CO and HCOOH) obtained by two-electron reduction.¹²³ The adsorption/desorption ability of CO₂, possible intermediates, and products on the electrode surface is dominating parameters to determine the product yield, which is affected by the geometry and electronic structure of catalyst, the electrolyte, and the applied potential.^124,125 To improve the economics of CO₂RR and electrolyzer efficiency of paired reaction, seeking kinetically favorable anode reactions to replace OER provides a promising route for sustainable production of high value-added chemicals and carbon-based fuels. Currently, only a few successful examples of anode replacement reactions have been reported, such as methanol,¹²⁶ glycerol,¹²⁷ glucose,¹²⁷ urea,¹²⁸ and HMF.²² Impressively, when HMFOR was paired with CO₂RR for electrolysis, the cell voltage decreased significantly under strong acidic conditions. The reported strategy used to improve the efficiency of the reforming reaction was to employ porous PdO_x/ZIF-8 (8-PZ) (Figure 9A) as cathode and PdO as anode pairings for CO₂RR and HMFOR, respectively.²² The good adsorption performance of porous ZIF-8 for CO₂ (Figure 9B) and the high catalytic activity of PdO enabled the coupled reaction to obtain a satisfactory yield of high value-added products, showing a lower cell voltage (1.06 V) than that of pairing with OER (1.77 V) (Figure 9C). However, in the electrochemical environment, the mechanistic details of this reaction is still rare, and only very few works are reported.

[IMAGE OMITTED. SEE PDF]

Coupling oxygen reduction reaction (ORR)

FDCA, an important near-market chemical product, is expected to replace terephthalic acid in polyester production.¹²⁹ A sustainable and energy-saving electrocatalytic FDCA process has always been a hot spot in fuel cell research. It has attracted much attention due to the high energy conversion efficiency and environmentally friendly feature. Fuel cell technology involves two important chemical reactions, namely, the fuel oxidation reaction at the anode and the ORR at the cathode.^94,130 Theoretically, the utilization of high-performance bifunctional catalysts plays a key role in lowering reaction energy barrier to promote the efficiency of the paired reactions. To this end, Zhang et al. designed a direct HMF fuel cell combining ORR with organic synthesis, employing PtNiS_x as the catalyst.²² It was found that the close interaction and interfacial effect between Pt and NiS_x nanoparticles endowed the catalyst with good ORR and HMFOR activities (Figure 9D). The fuel cell with HMF exhibits a discharge efficiency of 2.12 mW/cm² and a current density of 6.8 mA/cm² at 60°C (Figure 9E). Additionally, the formation rate of valued FDCA (rather than CO₂) was approximated to 98% at the anode (Figure 9F). This pioneering work made an important contribution to the development of direct fuel cells.

Coupling 4-nitrobenzyl alcohol (4-NBA) reduction

Upgrading cheap organics to valuable chemicals is important for sustainable production.¹³² Generally, active hydrogen/oxygen species formed by electron transfer precede the formation of H₂/O₂, thus requiring lower energy for organic ERR/EOR, which is driven by electrons rather than chemical reactions. Among them, the electrooxidation of HMF to valuable FDCA is considered an alternative reaction to replace OER with slow kinetics.¹²⁰ The key challenges are to design catalysts with high selectivity/efficiency, low cost, and wide potential windows.¹³³ More importantly, the coupling of the less-electron-transferred reduction reaction with the multiple-electron-transferred oxidation reaction in the paired electrolysis system will result in a mismatch of optimal current densities for the reactions. To overcome this challenge, given the lower energy requirements and better matching of reactive electron/proton numbers, ERR is more suitable for pairing with HMFOR (a six-electron–proton reaction) than HER. For instance, electroreduction of 4-nitrobenzyl alcohol (4-NBA) to 4-aminobenyl alcohol, a significant drug intermediate, involves six-electron–proton process.¹³⁴ Based on this, Fu et al. applied an electrolytic cell integrated with FeP–MoP/FF heterojunctions (FF, namely, Fe foam) cathode and FeP–NiMoP₂/FNF heterojunctions (FNF namely FeNi foam) anode to reduce 4-NBA and oxidize HMF system for paired electrolysis.⁶⁴ Due to the small interference for electrode materials and the matching reaction kinetics of 4-NBA and HMF, the paired electrolysis system exhibited a low voltage of 1.594 V at a current density of 100 mA/cm² (Figure 9G), as well as a high Faradaic efficiency (≥99%). In addition, the electrolyzer can be powered by the solar cell at a voltage of 1.420 V (Figure 9H). More importantly, isotopic labeling and ¹H-NMR results confirmed that the H* species used for 4-NBA ERR were derived from water (Figure 9I).^135,136 The OH* radical generated in the first step of water dissociation was identified as the active species of HMFOR by electron spin resonance (Figure 9J).¹³⁷ The enhanced electronic conductivity and electron transfer in heterojunctions, as well as the well-defined reaction pathways and their pairing advantages, provide guidance for the sustainable synthesis of various value-added chemicals via paired electrocatalysis.

Coupling nitrophenol hydrogenation

Apart from the abovementioned pairing reactions of dual electrodes to generate valuable chemical species and electron-matching, pairing electrolysis with hydrogenation also plays an important role in boosting efficiency of green synthesis. Sun's group investigated an organic paired electrolysis system with NiB_x as the bifunctional electrode, coupling HMFOR at anode and p-nitrophenol (p-NP) at cathode to enable electronic economy and the production of high value-added FDCA and 4-aminophenol (4-AP).¹³⁸ This paired reaction system with six-electron transfer for two half-reactions avoids the problem of optimal current density mismatch, giving a high HMF conversion of 99% at a low cell voltage of 1.4 V in 1 M KOH.¹³⁸ However, the reaction mechanism involved in the H/O species produced by the activation and dissociation of water remains indistinct. To further clarify the reaction mechanism and active site, Shi et al. conducted a systematic research on integrating HMFOR and 4-nitrophenol (4-NP) hydrogenation using Cu(OH)₂ as a model catalyst.¹⁰⁹ A series of characterization results show that the oxidation of HMF on Cu(OH)₂ goes through an indirect oxidation mechanism (E–C mechanism), in which the CuOOH active species generated by the electrochemical activation of Cu(OH)₂ in the first step are the main catalytic sites for the oxidation of HMF. In the presence of NaBH₄, 4-NP is deprotonated to form 4-nitrophenolate ions and adsorbed on the surface of the catalyst. The electrons generated by the anodic reaction are transferred to the surface of the catalyst through the external circuit and attack 4-nitrophenolate ions together with protons (H⁺), resulting in the formation of 4-AP and desorption from the surface of Cu(OH)₂ (Figure 9K).^139,140 The paired electrocatalytic reaction shows both reactants are completely converted under the synergistic catalysis of the catalysts (Figure 9L,M).

Overall, the pairing of HMFOR with an organic/inorganic reduction reaction presents a strategy for the simultaneous production of two value-added chemicals on two electrodes, which is beneficial to facilitate the development of green and sustainable chemical synthesis. However, there is still a lot of work that needs to be done to develop efficient and cost-effective bifunctional catalysts and to clearly elucidate reaction mechanisms. Moreover, it is crucial to seek rate-paired coupled reactions for large-scale production economy and improvement of energy utilization efficiency.

CONCLUSION AND OUTLOOK

Electrocatalytic conversion of potential bio-based platform compound (HMF) into high value-added product (FDCA) is promising approach for the development of economic and social benefits. The electrochemical oxidation of HMF to FDCA, which possesses lower standard potential (0.3 V) than OER (1.23 V), has received extensive attention to replace OER.¹⁴¹

In this paper, the latest progress in the preparation of FDCA by electrochemical oxidation of hydroxymethyl furfural is reviewed. First, to understand the reaction process, we analyzed and summarized the pH-dependent mediated reaction pathway of HMFOR. To gain a deeper understanding of the reaction, we elucidate the reaction mechanism, that is, direct oxidation and indirect oxidation so to provide guidance for the identification and resolution of the electrochemical oxidation of NOR. Furthermore, catalyst design strategies involving two categories, defect and interface engineering (focusing on defects and interfaces in non-noble metal materials), are discussed in depth. In addition, according to the reported thermodynamic data,^141,142 HMFOR is thermodynamically more favorable than OER. Therefore, the integrated system of proper cathodic coupling reduction reactions (inorganic/organic) are discussed.

Most research studies focused on HMF oxidation in the laboratory environment, making a great progress to the improvement of the activity, selectivity, and stability of catalysts. However, the problem of technical blindness still exists. To achieve sustainable production goals and maintain a balance between green chemistry and economic effects, more efforts should be devoted to reaction engineering, catalysts’ and reactors’ design, and the separation/purification of products.

First of all, it is necessary to have deep understanding of the reaction mechanism. In situ characterizations can be applied to clarify the pathways in the weak alkaline environment. At the same time, combined with synchrotron radiation and theoretical calculations, the electronic structure, the exact active center of catalysts, and the changes of intermediates during the reaction process can be analyzed in detail.

Second, compared with precious metals, non-precious metals have excellent stability and improved catalytic activity, which is certainly worthy of exploration and attention. Despite the similarity between HMFOR and OER, NiFe LDHs, the most effective catalytic materials for OER, are not well suitable for HMFOR, possibly due to the surface adsorption of HMF on Fe sites. Based on this, modification strategies involving compositional differences, such as doping, may not match HMFOR for doping elements suitable for OER. Therefore, it is necessary to further combine experiments and theoretical calculations to propose an exclusive set of descriptors to evaluate the feasibility of designing novel catalytic materials, and to provide guidance for reducing the number of trials and errors in obtaining high-performance catalysts.

Third, the design of the reactor needs to be emphasized to improve the reaction efficiency. Most of the reported experimental-scale HMF oxidations are usually carried out in H-type electrolytic cells that are separated by Nafion membrane, which belongs to the batch reactor oxidation reaction, suffering from the problems of unstable product quality, inconvenient operation, and low economic effect. The continuous flow electrochemical reactor designed and processed by digital control system can effectively overcome the above shortcomings, accelerate the mass transfer and diffusion rate, and is conducive to continuous and large-scale production.

Fourth, the balance between HMF degradation and catalysts’ reactivity deserves attention. The higher pH environment with improved conductivity is conducive to the hydration of aldehyde groups in HMF with sufficient OH⁻ ions, thus promoting the HMFOR process. However, high pH value leads to self-degradation of HMF (with a degradation rate of 10% per hour in 1.0 M KOH).^3,38,143,144 Hence, it is still a delicate issue to balance the reactivity and degradation of HMF under alkaline conditions.

Fifth, more efforts are still needed to explore more coupling reactions between anode and cathode with matching reaction efficiency for large-scale production of high value-added chemicals/fuels. At present, the cathodic reactions of HMF pairing involve CO₂RR, ORR, and organic reactions with favorable thermodynamics, and so forth, but the reaction mechanism and the unexplored pairing reactions need further optimization. In addition, the activity and electron economy of bifunctional catalysts require further evolution.

Sixth, the separation and purification of FDCA cannot be ignored. In the aqueous phase, dissolved by-products of side reactions or competing reactions reduce the selectivity of FDCA. Boosting purity of FDCA obtained by crystallization and solid–liquid separation is inevitable. The design of the separation and recovery subsystem still needs to integrate the high-purity separation/purification technology for cost reduction, energy-saving, and less environmental impact to achieve a competitive and economical system to produce FDCA.

In conclusion, this paper reviews the pathway and mechanism of HMF oxidation, discusses applicable catalyst design strategies and principles, and pairs reactions. It provides a reference guide for the design of advanced catalysts and the innovation of reaction engineering in the field of electrochemical biomass conversion.

ACKNOWLEDGMENTS

This work was financially supported by “Tianfu Emei” Science and Technology Innovation Leader Program in Sichuan Province, University of Electronic Science and Technology of China Talent Start-up Funds (A1098 5310 2360 1208), Scientific Research Foundation (Y030212059003045), the China Postdoctoral Science Foundation (2021TQ0059 and 2022M710610), and National Natural Science Foundation of China (21464015, 21472235).

CONFLICT OF INTEREST

The authors declare no conflict of interest.

References

Chu S, Cui Y, Liu N. The path towards sustainable energy. Nat Mater. 2017;16(1):16‐22.

Zhang X, Wilson K, Lee AF. Heterogeneously catalyzed hydrothermal processing of C₅–C₆ sugars. Chem Rev. 2016;116(19): [eLocator: 12328].

Giannakoudakis DA, Colmenares JC, Tsiplakides D, Triantafyllidis KS. Nanoengineered electrodes for biomass‐derived 5‐hydroxymethylfurfural electrocatalytic oxidation to 2,5‐furandicarboxylic acid. ACS Sustain Chem Eng. 2021;9(5):1970‐1993.

Li W, Zhao L, Jiang X, Chen Z, Zhang Y, Wang S. Confinement engineering of electrocatalyst surfaces and interfaces. Adv Funct Mater. 2022(46): [eLocator: 2207727].

Gallezot P. Conversion of biomass to selected chemical products. Chem Soc Rev. 2012;41(4):1538‐1558.

Zakrzewska ME, Bogel‐Łukasik E, Bogel‐Łukasik R. Ionic liquid‐mediated formation of 5‐hydroxymethylfurfural—a promising biomass‐derived building block. Chem Rev. 2011;111(2):397‐417.

Su T, Zhao D, Wang Y, Lü H, Varma RS, Len C. Innovative protocols in the catalytic oxidation of 5‐hydroxymethylfurfural. ChemSusChem. 2021;14(1):266‐280.

Tuck CO, Pérez E, Horváth IT, Sheldon RA, Poliakoff M. Valorization of Biomass: deriving More Value from Waste. Science. 2012;337(6095):695‐659.

Román‐Leshkov Y, Chheda JN, Dumesic JA. Phase modifiers promote efficient production of hydroxymethylfurfural from fructose. Science. 2006;312(5782):1933‐1937.

Jacquel N, Saint‐Loup R, Pascault J‐P, Rousseau A, Fenouillot F. Bio‐based alternatives in the synthesis of aliphatic–aromatic polyesters dedicated to biodegradable film applications. Polymer. 2015;59:234‐242.

Liguori F, Barbaro P, Calisi NJC. Continuous‐flow oxidation of HMF to FDCA by resin‐supported platinum catalysts in neat water. ChemSusChem. 2019;12(12):2558‐2563.

Mishra DK, Lee HJ, Kim J, et al. MnCo₂O₄ spinel supported ruthenium catalyst for air‐oxidation of HMF to FDCA under aqueous phase and base‐free conditions. Green Chem. 2017;19(7):1619‐1623.

Fittig R, Heinzelmann HJCB. Production of 2, 5‐furandicarboxylic acid by the reaction of fuming hydrobromic acid with mucic acid under pressure. Chem Ber. 1876;9:1198.

Chen C, Wang L, Zhu B, et al. 2,5‐Furandicarboxylic acid production via catalytic oxidation of 5‐hydroxymethylfurfural: catalysts, processes and reaction mechanism. J Energy Chem. 2021;54:528‐554.

Zhang Z, Deng K. Recent advances in the catalytic synthesis of 2,5‐furandicarboxylic acid and its derivatives. ACS Catal. 2015;5(11):6529‐6544.

Han G, Jin Y‐H, Burgess RA, Dickenson NE, Cao X‐M, Sun Y. Visible‐light‐driven valorization of biomass intermediates integrated with h₂ production catalyzed by Ultrathin Ni/CdS nanosheets. J Am Chem Soc. 2017;139(44):15584‐15587.

Rezaee S, Shahrokhian S. Facile synthesis of petal‐like NiCo/NiO‐CoO/nanoporous carbon composite based on mixed‐metallic MOFs and their application for electrocatalytic oxidation of methanol. Appl Catal B – Environ. 2019;244:802‐813.

Gu K, Wang D, Xie C, et al. Defect‐rich high‐entropy oxide nanosheets for efficient 5‐hydroxymethylfurfural electrooxidation. Angew Chem Int Ed. 2021;60(37):20253‐20258.

Zhang J, Gong W, Yin H, et al. Situ growth of ultrathin Ni(OH)₂ nanosheets as catalyst for electrocatalytic oxidation reactions. ChemSusChem. 2021;14(14):2935‐2942.

Ma C, Fang P, Mei T‐S. Recent advances in C–H functionalization using electrochemical transition metal catalysis. ACS Catal. 2018;8(8):7179‐7189.

Barwe S, Weidner J, Cychy S, et al. Electrocatalytic oxidation of 5‐(hydroxymethyl)furfural using high‐surface‐area nickel boride. Angew Chem Int Ed. 2018;57(35):11460‐11464.

Bi J, Zhu Q, Guo W, et al. Simultaneous CO₂ reduction and 5‐hydroxymethylfurfural oxidation to value‐added products by electrocatalysis. ACS Sustain Chem Eng. 2022;10(24):8043‐8050.

Kang MJ, Park H, Jegal J, Hwang SY, Kang YS, Cha HG. Electrocatalysis of 5‐hydroxymethylfurfural at cobalt based spinel catalysts with filamentous nanoarchitecture in alkaline media. Appl Catal B – Environ. 2019;242:85‐91.

Latsuzbaia R, Bisselink R, Anastasopol A, et al. Continuous electrochemical oxidation of biomass derived 5‐(hydroxymethyl)furfural into 2,5‐furandicarboxylic acid. J Appl Electrochem. 2018;48(6):611‐626.

Lu Y, Dong C‐L, Huang Y‐C, et al. Identifying the geometric site dependence of spinel oxides for the electrooxidation of 5‐hydroxymethylfurfural. Angew Chem Int Ed. 2020;59(43):19215‐19221.

Zhang N, Zou Y, Tao L, et al. Electrochemical oxidation of 5‐hydroxymethylfurfural on nickel nitride/carbon nanosheets: reaction pathway determined by in situ sum frequency generation vibrational spectroscopy. Angew Chem Int Ed. 2019;58(44):15895‐15903.

Ge R, Wang Y, Li Z, et al. Selective electrooxidation of biomass‐derived alcohols to aldehydes in a neutral medium: promoted water dissociation over a nickel‐oxide‐supported ruthenium single‐atom catalyst. Angew Chem Int Ed. 2022;61(19): [eLocator: e202200211].

Lu Y, Liu T, Huang Y‐C, et al. Integrated catalytic sites for highly efficient electrochemical oxidation of the aldehyde and hydroxyl groups in 5‐hydroxymethylfurfural. ACS Catal. 2022;12(7):4242‐4251.

Lu Y, Liu T, Dong C‐L, et al. Tailoring competitive adsorption sites by oxygen‐vacancy on cobalt oxides to enhance the electrooxidation of biomass. Adv Mater. 2022;34(2): [eLocator: 2107185].

Vuyyuru KR, Strasser P. Oxidation of biomass derived 5‐hydroxymethylfurfural using heterogeneous and electrochemical catalysis. Catal Today. 2012;195(1):144‐154.

Chadderdon DJ, Xin L, Qi J, et al. Electrocatalytic oxidation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid on supported Au and Pd bimetallic nanoparticles. Green Chem. 2014;16(8):3778‐3786.

Yang G, Jiao Y, Yan H, et al. Interfacial engineering of MoO₂‐FeP heterojunction for highly efficient hydrogen evolution coupled with biomass electrooxidation. Adv Mater. 2020;32(17): [eLocator: 2000455].

Kubota SR, Choi K‐S. Electrochemical oxidation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid (FDCA) in acidic media enabling spontaneous FDCA separation. ChemSusChem. 2018;11(13):2138‐2145.

Woo J, Moon BC, Lee U, et al. Collaborative electrochemical oxidation of the alcohol and aldehyde groups of 5‐hydroxymethylfurfural by NiOOH and Cu(OH)₂ for superior 2,5‐furandicarboxylic acid production. ACS Catal. 2022;12(7):4078‐4091.

Lu X, Wu KH, Zhang B, et al. Highly efficient electro‐reforming of 5‐hydroxymethylfurfural on vertically oriented nickel nanosheet/carbon hybrid catalysts: structure–function relationships. Angew Chem Int Ed. 2021;60(26):14528‐14535.

Chen W, Xie C, Wang Y, et al. Activity origins and design principles of nickel‐based catalysts for nucleophile electrooxidation. Chem. 2020;6(11):2974‐2993.

Wise CF, Mayer JM. Electrochemically determined O–H bond dissociation free energies of NiO electrodes predict proton‐coupled electron transfer reactivity. J Am Chem Soc. 2019;141(38):14971‐14975.

Yang Y, Mu T. Electrochemical oxidation of biomass derived 5‐hydroxymethylfurfural (HMF): pathway, mechanism, catalysts and coupling reactions. Green Chem. 2021;23(12):4228‐4254.

Chen S, Perathoner S, Ampelli C, Mebrahtu C, Su D, Centi G. Electrocatalytic synthesis of ammonia at room temperature and atmospheric pressure from water and nitrogen on a carbon‐nanotube‐based electrocatalyst. Angew Chem Int Ed. 2017;56(10):2699‐2703.

Houache MSE, Hughes K, Ahmed A, et al. Electrochemical valorization of glycerol on Ni‐rich bimetallic NiPd nanoparticles: insight into product selectivity using in situ polarization modulation infrared‐reflection absorption spectroscopy. ACS Sustain Chem Eng. 2019;7(17):14425‐14434.

Bender MT, Lam YC, Hammes‐Schiffer S, Choi K‐S. Unraveling two pathways for electrochemical alcohol and aldehyde oxidation on NiOOH. J Am Chem Soc. 2020;142(51):21538‐21547.

Davis SE, Zope BN, Davis RJ. On the mechanism of selective oxidation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid over supported Pt and Au catalysts. Green Chem. 2012;14(1):143‐147.

Zhang M, Liu Y, Liu B, Chen Z, Xu H, Yan K. Trimetallic NiCoFe‐layered double hydroxides nanosheets efficient for oxygen evolution and highly selective oxidation of biomass‐derived 5‐hydroxymethylfurfural. ACS Catal. 2020;10(9):5179‐5189.

Zhou H, Li Z, Xu S‐M, et al. Selectively upgrading lignin derivatives to carboxylates through electrochemical oxidative C(OH)–C bond cleavage by a Mn‐doped cobalt oxyhydroxide catalyst. Angew Chem Int Ed. 2021;60(16):8976‐8782.

Fleischmann M, Korinek K, Pletcher D. The oxidation of organic compounds at a nickel anode in alkaline solution. J Electroanal Chem and Inter Electrochem. 1971;31(1):39‐49.

Fleischmann M, Korinek K, Pletcher D. The kinetics and mechanism of the oxidation of amines and alcohols at oxide‐covered nickel, silver, copper, and cobalt electrodes. J Chem Soc, Perkin Trans. 1972;2(10):1396‐1403.

Deng X, Xu G‐Y, Zhang Y‐J, et al. Understanding the roles of electrogenerated Co³⁺ and Co⁴⁺ in selectivity‐tuned 5‐hydroxymethylfurfural oxidation. Angew Chem Int Ed. 2021;60(37):20535‐42052.

Heidary N, Kornienko N. Electrochemical biomass valorization on gold‐metal oxide nanoscale heterojunctions enables investigation of both catalyst and reaction dynamics with operando surface‐enhanced Raman spectroscopy. Chem Sci. 2020;11(7):1798‐1806.

Taitt BJ, Nam D‐H, Choi K‐S. A comparative study of Nickel, Cobalt, and iron oxyhydroxide anodes for the electrochemical oxidation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid. ACS Catal. 2019;9(1):660‐670.

Bender MT, Choi K‐S. Electrochemical oxidation of HMF via hydrogen atom transfer and hydride transfer on NiOOH and the impact of NiOOH composition. ChemSusChem. 2022;15(13): [eLocator: e202200675].

Bender MT, Warburton RE, Hammes‐Schiffer S, Choi K‐S. Understanding hydrogen atom and hydride transfer processes during electrochemical alcohol and aldehyde oxidation. ACS Catal. 2021;11(24):15110‐15124.

Subbaraman R, Tripkovic D, Chang K‐C, et al. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat Mater. 2012;11(6):550‐557.

Mao B, Sun P, Jiang Y, et al. Identifying the transfer kinetics of adsorbed hydroxyl as a descriptor of alkaline hydrogen evolution reaction. Angew Chem Int Ed. 2020;59(35):15232‐15237.

Shao Y, Xiao X, Zhu Y‐P, Ma T‐Y. Single‐crystal cobalt phosphate nanosheets for biomimetic oxygen evolution in neutral electrolytes. Angew Chem Int Ed. 2019;58(41):14599‐14604.

Wang N, Cao Z, Zheng X, et al. Hydration‐effect‐promoting Ni–Fe oxyhydroxide catalysts for neutral water oxidation. Adv Mater. 2020;32(8): [eLocator: 1906806].

Sun H, Chen L, Lian Y, et al. Topotactically transformed polygonal mesopores on ternary layered double hydroxides exposing under‐coordinated metal centers for accelerated water dissociation. Adv Mater. 2020;32(52): [eLocator: 2006784].

Heidary N, Chartrand D, Guiet A, Kornienko N. Rational incorporation of defects within metal–organic frameworks generates highly active electrocatalytic sites. Chem Sci. 2021;12(21):7324‐7333.

Luo R, Li Y, Xing L, et al. A dynamic Ni(OH)₂‐NiOOH/NiFeP heterojunction enabling high‐performance E‐upgrading of hydroxymethylfurfural. Appl Catal B. 2022;311: [eLocator: 121357].

Herranz J, Durst J, Fabbri E, et al. Interfacial effects on the catalysis of the hydrogen evolution, oxygen evolution and CO₂‐reduction reactions for (co‐)electrolyzer development. Nano Energy. 2016;29:4‐28.

Zhou B, Li Y, Zou Y, et al. Platinum modulates redox properties and 5‐hydroxymethylfurfural adsorption kinetics of Ni(OH)₂ for biomass upgrading. Angew Chem Int Ed. 2021;60(42):22908‐22914.

Song X, Liu X, Wang H, Guo Y, Wang Y. Improved performance of nickel boride by phosphorus doping as an efficient electrocatalyst for the oxidation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid. Ind Eng Chem Res. 2020;59(39):17348‐17356.

Li S, Sun X, Yao Z, et al. Biomass valorization via paired electrosynthesis over vanadium nitride‐based electrocatalysts. Adv Funct Mater. 2019;29(42): [eLocator: 1904780].

Zhou M, Chen J, Li Y. CoP nanorods anchored on Ni2P‐NiCoP nanosheets with abundant heterogeneous interfaces boosting the electrocatalytic oxidation of 5‐hydroxymethyl‐furfural. Catal Sci Technol. 2022;12(13):4288‐4297.

Yang G, Jiao Y, Yan H, et al. Unraveling the mechanism for paired electrocatalysis of organics with water as a feedstock. Nat Commun. 2022;13(1):3125.

Wang C, Bongard H‐J, Weidenthaler C, Wu Y, Schüth F. Design and application of a high‐surface‐area mesoporous δ‐MnO₂ electrocatalyst for biomass oxidative valorization. Chem Mater. 2022;34(7):3123‐3132.

Gao L, Liu Z, Ma J, et al. NiSe@NiOx core‐shell nanowires as a non‐precious electrocatalyst for upgrading 5‐hydroxymethylfurfural into 2,5‐furandicarboxylic acid. Appl Catal B. 2020;261: [eLocator: 118235].

Yang G, Mu X, Jin Y, et al. In‐situ chemical corrosive nickel foam as high‐efficient electrocatalyst for 5‐hydroxymethylfurfural oxidation. Appl Surf Sci. 2022;594: [eLocator: 153432].

Zheng R, Zhao C, Xiong J, et al. Construction of a hierarchically structured, NiCo–Cu‐based trifunctional electrocatalyst for efficient overall water splitting and 5‐hydroxymethylfurfural oxidation. Sustainable Energy Fuels. 2021;5(16):4023‐4031.

Liu B, Xu S, Zhang M, et al. Electrochemical upgrading of biomass‐derived 5‐hydroxymethylfurfural and furfural over oxygen vacancy‐rich NiCoMn‐layered double hydroxides nanosheets. Green Chem. 2021;23(11):4034‐4043.

Jiang X, Yue X, Li Y, et al. Anion‐cation‐dual doped tremella‐like nickel phosphides for electrocatalytic water oxidation. Chem Eng J. 2021;426: [eLocator: 130718].

Zhang B, Shan J, Wang W, Tsiakaras P, Li Y. Oxygen vacancy and core‐shell heterojunction engineering of anemone‐like CoP@CoOOH bifunctional electrocatalyst for efficient overall water splitting. Small. Mar 2022;18(12): [eLocator: e2106012].

Qian G, Chen J, Yu T, Liu J, Luo L, Yin S. Three‐phase heterojunction NiMo‐based nano‐needle for water splitting at industrial alkaline condition. Nanomicro Lett. 2021;14:1‐15.

Zhou Z, Xie Y‐n, Sun L, et al. Strain‐induced in situ formation of NiOOH species on Co–Co bond for selective electrooxidation of 5‐hydroxymethylfurfural and efficient hydrogen production. Appl Catal B. 2022;305: [eLocator: 121072].

Li W, Chen Z, Jiang X, Jiang J, Zhang Y. Recent advances in the design of single‐atom electrocatalysts by defect engineering. Front Chem. 2022;10: [eLocator: 1011597].

Xie C, Yan D, Chen W, et al. Insight into the design of defect electrocatalysts: from electronic structure to adsorption energy. Mater Today. 2019;31:47‐68.

Xie C, Yan D, Li H, et al. Defect chemistry in heterogeneous catalysis: recognition, understanding, and utilization. ACS Catal. 2020;10(19):11082‐11098.

Li W, Wang D, Zhang Y, et al. Defect engineering for fuel‐cell electrocatalysts. Adv Mater. 2020;32(19): [eLocator: e1907879].

Wang H, Zhang J, Tao S. Nickel oxide nanoparticles with oxygen vacancies for boosting biomass‐upgrading. Chem Eng J. 2022;444: [eLocator: 136693].

Yang Y, Xu D, Zhang B, Xue Z, Mu T. Substrate molecule adsorption energy: an activity descriptor for electrochemical oxidation of 5‐Hydroxymethylfurfural (HMF). Chem Eng J. 2022;433: [eLocator: 133842].

Nørskov JK, Rossmeisl J, Logadottir A, et al. Origin of the overpotential for oxygen reduction at a fuel‐cell cathode. J Phys Chem B. 2004;108(46):17886‐17892.

Nørskov JK, Bligaard T, Logadottir A, et al. Trends in the exchange current for hydrogen evolution. J Electrochem Soc. 2005;152(3):J23.

Abild‐Pedersen F, Greeley J, Studt F, et al. Scaling properties of adsorption energies for hydrogen‐containing molecules on transition‐metal surfaces. Phys Rev Lett. 2007;99(1): [eLocator: 016105].

Qi Y‐F, Wang K‐Y, Sun Y, Wang J, Wang C. Engineering the electronic structure of NiFe layered double hydroxide nanosheet array by implanting cationic vacancies for efficient electrochemical conversion of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid. ACS Sustain Chem Eng. 2021;10(1):645‐654.

Sun Y, Wang J, Qi Y, Li W, Wang C. Efficient electrooxidation of 5‐hydroxymethylfurfural using co‐doped Ni₃S₂ catalyst: promising for h₂ production under industrial‐level current density. Adv Sci (Weinh). 2022;9(17): [eLocator: e2200957].

Bi J, Ying H, Xu H, et al. Phosphorus vacancy‐engineered Ce‐doped CoP nanosheets for the electrocatalytic oxidation of 5‐hydroxymethylfurfural. Chem Commun (Camb). 2022;58(56):7817‐7820.

Zhong R, Wang Q, Du L, et al. Ultrathin polycrystalline Co₃O₄ nanosheets with enriched oxygen vacancies for efficient electrochemical oxygen evolution and 5‐hydroxymethylfurfural oxidation. Appl Surf Sci. 2022;584: [eLocator: 152553].

Lu Y, Dong C‐L, Huang Y‐C, et al. Hierarchically nanostructured NiO‐Co₃O₄ with rich interface defects for the electro‐oxidation of 5‐hydroxymethylfurfural. Sci China Chem. 2020;63(7):980‐986.

Huang H, Yu C, Han X, et al. Ni, Co hydroxide triggers electrocatalytic production of high‐purity benzoic acid over 400 mA cm⁻². Energy Environ Sci. 2020;13(12):4990‐4999.

Bai L, Lee S, Hu X. Spectroscopic and electrokinetic evidence for a bifunctional mechanism of the oxygen evolution reaction. Angew Chem Int Ed. 2021;60(6):3095‐3103.

Yang C, Wang C, Zhou L, et al. Refining d‐band center in Ni_0.85Se by Mo doping: a strategy for boosting hydrogen generation via coupling electrocatalytic oxidation 5‐hydroxymethylfurfural. Chem Eng J. 2021;422: [eLocator: 130125].

Wang W, Wang M. Nitrogen modulated NiMoO₄ with enhanced activity for the electrochemical oxidation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid. Catal Sci Technol. 2021;11(22):7326‐7330.

Wang H, Li C, An J, Zhuang Y, Tao S. Surface reconstruction of NiCoP for enhanced biomass upgrading. J Mater Chem A. 2021;9(34):18421‐18430.

Li W, Zhao L, Jiang X, Chen Z, Zhang Y, Wang S. Confinement engineering of electrocatalyst surfaces and interfaces. Adv Funct Mater. 2022;32(46): [eLocator: 2207727].

Li W, Wang D, Liu T, et al. Doping‐modulated strain enhancing the phosphate tolerance on PtFe alloys for high‐temperature proton exchange membrane fuel cells. Adv Funct Mater. 2022;32(8): [eLocator: 2109244].

Zhang B, Fu H, Mu T. Hierarchical NiS_x/Ni₂P nanotube arrays with abundant interfaces for efficient electrocatalytic oxidation of 5‐hydroxymethylfurfural. Green Chem. 2022;24(2):877‐884.

Wang H, Zhou Y, Tao S. CoP‐CoOOH heterojunction with modulating interfacial electronic structure: a robust biomass‐upgrading electrocatalyst. Appl Catal B. 2022;315: [eLocator: 121588].

Lin Y‐X, Zhang S‐N, Xue Z‐H, et al. Boosting selective nitrogen reduction to ammonia on electron‐deficient copper nanoparticles. Nat Comm. 2019;10(1):4380.

Liu Y‐X, Wang H‐H, Zhao T‐J, et al. Schottky barrier induced coupled interface of electron‐rich N‐doped carbon and electron‐deficient Cu: in‐built Lewis acid‐base pairs for highly efficient CO₂ fixation. J Am Chem Soc. 2019;141(1):38‐41.

Chen J, Wang Y, Zhou M, Li Y. Boosting the electro‐oxidation of 5‐hydroxymethyl‐furfural on a Co–CoS_x heterojunction by intensified spin polarization. Chem Sci. 2022;13(16):4647‐4653.

Zhang Y, Xue Z, Zhao X, Zhang B, Mu T. Controllable and facile preparation of Co₉S₈–Ni₃S₂ heterostructures embedded with N,S,O‐tri‐doped carbon for electrocatalytic oxidation of 5‐hydroxymethylfurfural. Green Chem. 2022;24(4):1721‐1731.

Zhang R, Jiang S, Rao Y, Chen S, Yue Q, Kang Y. Electrochemical biomass upgrading on CoOOH nanosheets in a hybrid water electrolyzer. Green Chem. 2021;23(6):2525‐2530.

Zhong Y, Ren R‐Q, Wang J‐B, Peng Y‐Y, Li Q, Fan Y‐M. Grass‐like Ni_xSe_y nanowire arrays shelled with NiFe LDH nanosheets as a 3D hierarchical core–shell electrocatalyst for efficient upgrading of biomass‐derived 5‐hydroxymethylfurfural and furfural. Catal Sci Technol. 2022;12(1):201‐211.

Song Y, Xie W, Song Y, et al. Bifunctional integrated electrode for high‐efficient hydrogen production coupled with 5‐hydroxymethylfurfural oxidation. Appl Catal B. 2022;312: [eLocator: 121400].

Zhao Z, Guo T, Luo X, et al. Bimetallic sites and coordination effects: electronic structure engineering of NiCo‐based sulfide for 5‐hydroxymethylfurfural electrooxidation. Catal Sci Technol. 2022;12(12):3817‐3825.

Song Y, Li Z, Fan K, et al. Ultrathin layered double hydroxides nanosheets array towards efficient electrooxidation of 5‐hydroxymethylfurfural coupled with hydrogen generation. Appl Catal B. 2021;299: [eLocator: 120669].

Xie Y, Zhou Z, Yang N, Zhao G. An overall reaction integrated with highly selective oxidation of 5‐hydroxymethylfurfural and efficient hydrogen evolution. Adv Funct Mater. 2021;31(34): [eLocator: 2102886].

Gao L, Wen X, Liu S, et al. Nickel‐vanadium‐cobalt ternary layered double hydroxide for efficient electrocatalytic upgrading of 5‐hydroxymethylfurfural to 2,5‐furancarboxylic acid at low potential. J Mater Chem A. 2022;10(39):21135‐21141.

Zheng L, Zhao Y, Xu P, Lv Z, Shi X, Zheng H. Biomass upgrading coupled with H₂ production via a nonprecious and versatile Cu‐doped nickel nanotube electrocatalyst. J Mater Chem A. 2022;10(18):10181‐10191.

Pang X, Bai H, Zhao H, Fan W, Shi W. Efficient electrocatalytic oxidation of 5‐hydroxymethylfurfural coupled with 4‐nitrophenol hydrogenation in a water system. ACS Catal. 2022;12(2):1545‐1557.

Lu Y, Liu T, Dong C‐L, et al. Tuning the selective adsorption site of biomass on Co₃O₄ by Ir single atoms for electrosynthesis. Adv Mater. 2021;33(8): [eLocator: 2007056].

Xia B, Wang G, Cui S, et al. High‐valance molybdenum doped Co₃O₄ nanowires: origin of the superior activity for 5‐hydroxymethyl‐furfural oxidation. Chinese Chem Lett. 2022: [eLocator: 107810].

Zhou P, Lv X, Tao S, et al. Heterogeneous‐interface‐enhanced adsorption of organic and hydroxyl for biomass electrooxidation. Adv Mater. 2022;34(42): [eLocator: 2204089].

Yang S, Guo Y, Zhao Y, et al. Construction of synergistic Ni₃S₂‐MoS₂ nanoheterojunctions on Ni foam as bifunctional electrocatalyst for hydrogen evolution integrated with biomass valorization. Small. 2022;18(24): [eLocator: 2201306].

Choi S, Balamurugan M, Lee K‐G, et al. Mechanistic investigation of biomass oxidation using nickel oxide nanoparticles in a CO₂‐saturated electrolyte for paired electrolysis. J Phy Chem Lett. 2020;11(8):2941‐2948.

Xu G‐R, Batmunkh M, Donne S, et al. Ruthenium(iii) polyethyleneimine complexes for bifunctional ammonia production and biomass upgrading. J Mater Chem A. 2019;7(44):25433‐25440.

Ma Y, Chen M, Geng H, et al. Synergistically tuning electronic structure of porous β‐Mo₂C spheres by Co doping and Mo‐vacancies defect engineering for optimizing hydrogen evolution reaction activity. Adv Funct Mater. 2020;30(19): [eLocator: 2000561].

Xie X, Yu R, Xue N, et al. P doped molybdenum dioxide on Mo foil with high electrocatalytic activity for the hydrogen evolution reaction. J Mate Chem A. 2016;4(5):1647‐1652.

Yan D, Mebrahtu C, Wang S, Palkovits R. Innovative electrochemical strategies for hydrogen production: from electricity input to electricity output. Angew Chem Int Ed. 2022. [DOI: https://dx.doi.org/10.1002/anie.202214333]

Yan J, Li L, Ji Y, et al. Nitrogen‐promoted molybdenum dioxide nanosheets for electrochemical hydrogen generation. J Mater Chem A. 2018;6(26):12532‐12540.

You B, Jiang N, Liu X, Sun Y. Simultaneous H₂ generation and biomass upgrading in water by an efficient noble‐metal‐free bifunctional electrocatalyst. Angew Chem Int Ed. 2016;55(34):9913‐9917.

Davis SJ, Caldeira K, Matthews HD. Future CO₂ emissions and climate change from existing energy infrastructure. Science. 2010;329(5997):1330‐1333.

Medina‐Ramos J, DiMeglio JL, Rosenthal J. Efficient reduction of CO₂ to CO with high current density using in situ or Ex situ prepared bi‐based materials. J Am Chem Soc. 2014;136(23):8361‐8367.

Seh ZW, Kibsgaard J, Dickens CF, Chorkendorff I, Nørskov JK, Jaramillo TF. Combining theory and experiment in electrocatalysis: insights into materials design. Science. 2017;355(6321): [eLocator: eaad4998].

Nitopi S, Bertheussen E, Scott SB, et al. Progress and perspectives of electrochemical CO₂ reduction on copper in aqueous electrolyte. Chem Rev. 2019;119(12):7610‐7672.

Zheng Y, Vasileff A, Zhou X, Jiao Y, Jaroniec M, Qiao S‐Z. Understanding the roadmap for electrochemical reduction of CO₂ to multi‐carbon oxygenates and hydrocarbons on copper‐based catalysts. J Am Chem Soc. 2019;141(19):7646‐7659.

Wei X, Li Y, Chen L, Shi J. Formic acid electro‐synthesis by concurrent cathodic CO₂ reduction and anodic CH₃OH oxidation. Angew Chem Int Ed. 2021;60(6):3148‐3155.

Verma S, Lu S, Kenis PJA. Co‐electrolysis of CO₂ and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption. Nat Energy. 2019;4(6):466‐474.

Wang L, Zhu Y, Wen Y, et al. Regulating the local charge distribution of Ni active sites for the urea oxidation reaction. Angew Chem Int Ed. 2021;60(19):10577‐10582.

You B, Liu X, Jiang N, Sun Y. A general strategy for decoupled hydrogen production from water splitting by integrating oxidative biomass valorization. J Am Chem Soc. 2016;138(41):13639‐13646.

Martins CA, Ibrahim OA, Pei P, Kjeang E. “Bleaching” glycerol in a microfluidic fuel cell to produce high power density at minimal cost. Chem Commun. 2018;54(2):192‐195.

Wang J, Zhang X, Wang G, Zhang Y, Zhang H. Sustainable 2,5‐furandicarboxylic synthesis by a direct 5‐hydroxymethylfurfural fuel cell based on a bifunctional PtNiSx catalyst. Chem Commun. 2020;56(88):13611‐13514.

Wu X, Fan X, Xie S, et al. Solar energy‐driven lignin‐first approach to full utilization of lignocellulosic biomass under mild conditions. Nat Catal. 2018;1(10):772‐780.

Sherbo RS, Delima RS, Chiykowski VA, MacLeod BP, Berlinguette CP. Complete electron economy by pairing electrolysis with hydrogenation. Nat Catal. 2018;1(7):501‐507.

Chong X, Liu C, Huang Y, Huang C, Zhang B. Potential‐tuned selective electrosynthesis of azoxy‐, azo‐ and amino‐aromatics over a CoP nanosheet cathode. Natl Sci Rev. 2019;7(2):285‐295.

Zhao Y, Liu C, Wang C, Chong X, Zhang B. Sulfur vacancy‐promoted highly selective electrosynthesis of functionalized aminoarenes via transfer hydrogenation of nitroarenes with H₂O over a CO₃S_4‐x nanosheet cathode. CCS Chem. 2021;3(1):507‐515.

Gemoets HPL, Laudadio G, Verstraete K, Hessel V, Noël T. A modular flow design for the meta‐Selective C–H arylation of anilines. Angew Chem Int Ed. 2017;56(25):7161‐7165.

Ding L, Li M, Zhao Y, et al. The vital role of surface Brönsted acid/base sites for the photocatalytic formation of free ·OH radicals. Appl Catal B. 2020;266: [eLocator: 118634].

Zhang P, Sheng X, Chen X, et al. Paired electrocatalytic oxygenation and hydrogenation of organic substrates with water as the oxygen and hydrogen source. Angew Chem Int Ed. 2019;58(27):9155‐9159.

Rajkumar C, Veerakumar P, Chen S‐M, Thirumalraj B, Lin K‐C. Ultrathin sulfur‐doped graphitic carbon nitride nanosheets as metal‐free catalyst for electrochemical sensing and catalytic removal of 4‐nitrophenol. ACS Sustainable Chem Eng. 2018;6(12):16021‐16031.

Strachan J, Barnett C, Masters AF, Maschmeyer T. 4‐Nitrophenol reduction: probing the putative mechanism of the model reaction. ACS Catal. 2020;10(10):5516‐5521.

Verevkin SP, Emel'yanenko VN, Stepurko EN, Ralys RV, Zaitsau DH, Stark A. Biomass‐derived platform chemicals: thermodynamic studies on the conversion of 5‐hydroxymethylfurfural into bulk intermediates. Ind Eng Chem Res. 2009;48(22):10087‐10093.

Kwon Y, Schouten KJP, van der Waal JC, de Jong E, Koper MTM. Electrocatalytic conversion of furanic compounds. ACS Catal. 2016;6(10):6704‐6717.

Patel P, Schwartz D, Wang X, Lin R, Ajao O, Seifitokaldani A. Technoeconomic and life‐cycle assessment for electrocatalytic production of furandicarboxylic acid. ACS Sustainable Chem Eng. 2022;10(13):4206‐4217.

Cai M, Zhang Y, Zhao Y, Liu Q, Li Y, Li G. Two‐dimensional metal–organic framework nanosheets for highly efficient electrocatalytic biomass 5‐(hydroxymethyl)furfural (HMF) valorization. J Mater Chem A. 2020;8(39):20386‐20392.