Reduced graphene oxide‐based materials for electrochemical energy conversion reactions

Abstract

There have been ever‐growing demands to develop advanced electrocatalysts for renewable energy conversion over the past decade. As a promising platform for advanced electrocatalysts, reduced graphene oxide (rGO) has attracted substantial research interests in a variety of electrochemical energy conversion reactions. Its versatile utility is mainly attributed to unique physical and chemical properties, such as high specific surface area, tunable electronic structure, and the feasibility of structural modification and functionalization. Here, a comprehensive discussion is provided upon recent advances in the material preparation, characterization, and the catalytic activity of rGO‐based electrocatalysts for various electrochemical energy conversion reactions (water splitting, CO₂ reduction reaction, N₂ reduction reaction, and O₂ reduction reaction). Major advantages of rGO and the related challenges for enhancing their catalytic performance are addressed.

1 INTRODUCTION

Substituting conventional fossil fuel-based energy systems as a renewable and sustainable energy source is highly desirable due to the ever-growing issues of energy shortage and environmental impact. Electrochemical energy conversion systems, such as H₂ evolution reaction (HER), O₂ evolution reaction (OER), CO₂ reduction reaction (CO₂RR), N₂ reduction reaction (NRR), and O₂ reduction reaction (ORR), have been extensively explored as the promising approaches for constructing an environmental-benign energy circulation. Using electricity from renewable sources, most probably solar energy, the electrochemical process can be driven to convert universal feedstocks, for example, H₂O, CO₂, O₂, and N₂, to value-added fuels and chemicals. In this way, the electricity from renewable source can be stored in chemical bonds and readily extracted to sustainable electrical energy via fuel cell systems at any time. Thus, the development of state-of-the-art electrocatalysts is of prime importance to enhance the kinetics, efficiency, and selectivity of each electrochemical energy conversion reactions.

Recently, substantial research have been conducted on developing earth-abundant element based electrocatalysts with high activity, selectivity, and long-term stability. Transition metals and their compounds, such as transition metal oxides, hydroxides, chalcogenides, phosphides, carbides, and borides, have been intensively investigated as a substitute of noble metal catalysts. Albeit tremendous effort and attention, the performance of these electrocatalysts for energy conversion is still unsatisfactory compared to noble-metal–catalysts. Motivated by this challenge, extensive efforts have been taken to make breakthroughs for the development of efficient and earth-abundant electrocatalysts.

Graphene and its derivatives can be one promising solution as conducting scaffold for hybrid materials with the aforementioned electrocatalysts owing to their exotic structure and unique physical properties. The derivatives indicate the graphene obtained through various synthetic methods, and thus have various physical properties in lateral sizes, morphology, number of layers, and defect inside layers. The methods employed for graphene production play an important role in deciding the final properties. However, due to restricted cost-effectiveness and scalability, particular methods such as mechanical exfoliation or chemical vapor deposition (CVD) inevitably limited the usage of graphene toward fundamental research and academic applications. Away from aforementioned synthetic methods, reduction of graphene oxide (GO) is widely adopted for scalable production of graphene. GO is typically made by oxidizing pristine graphite and sequentially applying strong physical energy such as ultrasonication or stirring in the liquid medium, allowing the bulk production with high yield and reduced costs. Therefore, GO has a high density of defect and a disruption in sp²-bonding network. GO should be reduced, making reduced graphene oxide (rGO), to bring back the π network, which is the representative characteristic of graphene. Reduction techniques can be employed in various way as chemical, thermal, and electrochemical process.

Apart from efficiency and scalability, this approach has several advantages in applying rGO, as a conductive scaffold, to electrocatalysts, as shown in Figure . Despite the oxidation of pristine graphite introduces defective sites which are indelible even after the reduction process, these defect sites rather give rGO the possibility to decorate with various functional groups. The synthetic pathway of rGO enables the construction of composite materials via various chemical redox routes because the dispersion of GO is easily accomplished in a various range of solvents, facilitating the hybridization with electroactive materials. Such composites can be used as it was or further reduced to taking advantage of the conducting properties of rGO.

Fig. 1. An illustration of engineering strategies and electrochemical applications for rGO-based catalysts. NP, nanoparticle; rGO, reduced graphene oxide [Color figure can be viewed at wileyonlinelibrary.com]

In recent years, a number of publications have been reported about the application of rGO in electrochemical energy conversion reactions (Figure ). Our recent literature survey (according to Web of Science, August 2019), related to rGO based electrocatalysts, shows tremendous interest in water based-conversion reaction such as HER, OER, and ORR using rGO as electron mediator. It is noteworthy that ORR-related publications are saturated, while publications of water splitting show ever-growing publications. CO₂RR and NRR research are still in their infancy, with the majority of studies focusing on investigating the relatively facile reactions of HER, OER, or ORR. Recently, a variety of review articles on CO₂RR have been published that utilize electroactive materials with the support of rGO and spotlight their fundamental properties, attributed to the unique properties and structures of rGO. Even the research of the rGO based electrocatalysts for NRR have now begun to grow. For the rGO based NRR electrocatalysts, the research has just begun to grow, and now, even in the around half of 2019, has twice the number of publications as last year. These high degrees of interests obviously indicate that rGO-based electrocatalysts for electrochemical energy conversion are an emerging technology which has versatile potential for sustainable and renewable energy innovation.

Fig. 2. Publications per year on electrochemical applications on rGO-based catalysts since 2010. The publication search was performed using the Science Citation Index Expanded database of Web of Science provided by Thomson Reuters. CO2RR, CO2 reduction reaction; NRR, N2 reduction reaction; ORR, O2 reduction reaction; rGO, reduced graphene oxide [Color figure can be viewed at wileyonlinelibrary.com]

In this review, we describe the various properties of the rGO-based electrocatalysts in terms of each electrochemical conversion reactions. Some synthetic strategies for the electroactive materials supported on rGO are introduced and their performances are highlighted in terms of catalytic activity, selectivity, and stability of the rGO-based electrocatalysts, including the electronic properties and spatial arrangements. A major objective of this review is to provide the comprehensive accounts of recent research (Figure ) and the systematic knowledge to promote further investigations in this research field.

2 WATER SPLITTING

To achieve water splitting, the electrochemical voltage at least 1.23 V is required between H₂ evolving cathode and O₂ evolving anode. Due to the absence of light absorbing materials in electrochemical water splitting electrodes, the electrodes for water splitting are unable to generate electrons above 1.23 V vs reversible hydrogen electrode (RHE), independently. For initiating the water-splitting reaction, external bias should be exerted to adjust the Fermi level appropriately with respect to the redox potential of H₂/H⁺ and O₂/H₂O. With the assistance of external bias, the electrons (holes) in the electrocatalysts can be transferred to the electrode/electrolyte interface to drive HER (OER) reaction. However, the practical requirement of external bias for water electrolysis is greater than the theoretical requirement of 1.23 V vs RHE due to the kinetic barriers that are commonly occurred during multistep reactions. On the surface of most electrodes, the intermediate species are emerged during electrolysis, inducing a large energy barrier for charge carriers to overcome. An additional bias, referred as overpotential, is essential to drive HER or OER over the kinetically rate-determining step. These kinetic activation energies could be reduced by the use of appropriate electrocatalysts. There are numerous types of catalysts for water splitting, such as noble metals, 2D materials, and transition metal-based compounds. This following section mainly focuses on the electroactive materials supported on rGO for efficient water splitting reaction. Furthermore, Tables and summarize the main catalytic performance of recent rGO-based electrocatalysts for water splitting that are discussed in this chapter.

1 Summary of rGO-based electrocatalysts for H₂ evolution reaction from recent literature

Electrocatalysts	Overpotential (V @ 10 mA/cm2)	Tafel slope (mV/dec)	Electrolyte	Reference
MoS2/rGO	−0.154	41	0.5M H2SO4	Li et al
MoS2-rGO	−0.104	63	0.5M H2SO4	Chatti et al
MoS2 QDs/rGO	−0.064	63	0.5M H2SO4	Li et al
MoSe2/NrGO	−0.106	57	0.5M H2SO4	Zhuang et al
FeP/rGO/FeP	−0.101	55.2	0.5M H2SO4	Liu et al
N-doped Carbon/CoP/N-doped rGO	−0.135	59.3	0.5M H2SO4	Ma et al
P-doped rGO/NiCoP	−0.169	58.3	1M KOH	Dong et al
Co2P@NPrGO	−0.103	58	0.5M H2SO4	Zhuang et al
Ru2P/rGO	−0.022 −0.013	29 56	0.5M H2SO4 1M KOH	Liu et al
TaC-rGO	−0.167	58	0.5M H2SO4	He et al
Fe3W3C NRs/rGO	−0.057	50	0.5M H2SO4	He et al
Mo2N-Mo2C/Holey rGO	−0.157 −0.154	55 68	0.5M H2SO4 1M KOH	Yan et al

Abbreviations: NrGO, nitrogen-doped reduced graphene oxide; rGO, reduced graphene oxide.

2 Summary of rGO-based electrocatalysts for O₂ evolution reaction from recent literature

Electrocatalysts	Overpotential (V @ 10 mA/cm2)	Tafel slope (mV/dec)	Electrolyte	Reference
Co1-xNixS2/N-doped rGO aerogel	0.330	47	1M KOH	Han et al
3D FeNiS2 NS/rGO	0.200	40	1M KOH	Jiang et al
FeSe2@CoSe2/rGO	0.260	36	1M KOH	Zhu et al
P-doped rGO/NiCoP	0.281	60.1	1M KOH	Dong et al
CoP/rGO	0.320	66	1M KOH	Jiao et al
Ni2P@C/rGO	0.285	44	1M KOH	Wang et al
Ni3B/rGO	0.290	88.4	1M KOH	Arivu et al

Abbreviations: rGO, reduced graphene oxide.

2.1 Transition metal sulfides/rGO-based electrocatalysts

The structures of transition metal sulfides can be divided into two obviously different class, 2D MeS₂ (Me = Mo, W) and non-2D M_xS_y (M = Co, Fe, Ni, etc). Especially, the 2D transition metal dichalcogenides (TMDs) materials have been considered as a promising candidate for substituting noble metal catalysts for HER. Each layer of TMDs, the general formula of MeS₂ consists of atomically stacked S-Me-S elements and combined by the van der Waals forces. Since the study of the Norkov's group in 2005, MoS₂ has been spotlighted as a promising electrocatalyst for HER. Several studies reported that the near-zero value of H₂ adsorption free energy (ΔG_H) for the edge site of MoS₂ was predicted which comparable to Pt. However, there are major shortcomings of TMD-based electrocatalysts, such as the limited number of active edge sites, electrical conductivity, and specific surface area. Considering these factors into account, designing TMD-based electrocatalysts with the materials with high conductivity and specific surface area is desirable, which facilitates the exposure of active edge sites of TMD materials. Therefore, the TMD/rGO hybrid electrocatalysts have been widely investigated with a large specific surface area, a high density of edge sites, an enhanced conductivity. Li et al reported a MoS₂/rGO hybrid electrocatalyst via solvothermal method for the first time. MoS₂/rGO hybrid materials exhibited that the edge-exposed few-layer MoS₂ nanosheets (NSs) are stacked onto rGO. On the contrary, the MoS₂ particles grown freely without the support of GO were aggregated to relatively large particles. The MoS₂/rGO hybrid catalysts showed superior HER activity with the Tafel slope of 41 mV/dec. Chatti et al reported a novel approach to prepare vertically aligned and interlayer expanded MoS₂ sheets on well-conducting rGO support via the facile microwave synthesis. X-ray diffraction, Raman, and X-ray photoelectron spectroscopy (XPS) analyses were conducted to confirm the reduction of (NH₄)₂MoS₄ into MoS₂ during microwave synthesis. By using Bragg's law, the interlayer spacing of (002) planes of synthesized MoS₂ is calculated as 9.5 Å, which highly exceeds the reported value of 6.3 Å for pristine MoS₂. Substantially enhanced d₀₀₂ is attributed to the intercalation effect by oxidized DMF. When the microwave was treated on (NH₄)₂MoS₄ and GO precursors together, as shown in Figure A, the resultant morphology was quite different and the d₀₀₂ of MoS₂ was even higher (as high as 10.2 Å) for MoS₂/rGO composites synthesized at the temperature of 200°C and 240°C. The transmission electron microscopic images in Figure B shows a uniform distribution of MoS₂ on rGO. The top-view image on the MoS₂/rGO composites provides side-on visualizations of few-nm-thick MoS₂ NSs, most of which are vertically aligned on the rGO support. The high-resolution transmission electron microscopy (TEM) image further reveals the defective structure of the MoS₂ NSs, composed of less than five layers with the interlayer spacing of 10 Å. The scanning electron microscope (SEM) and TEM images in Figure C,E,F clearly showed the significant difference in morphology between the microwave-synthesized MoS₂ and MoS₂/rGO composites, indicating that the existence of GO plays a crucial role in mediating the formation of well-distributed MoS₂ nanostructures. It is speculated that the C–O functionalities on the surface of GO act as nucleation sites for the growth of MoS₂. Linear sweep voltammetry (LSV) measurements in Ar-saturated 0.5M H₂SO₄ were conducted for all samples to evaluate the HER performance (Figure D). The optimized MoS₂/rGO composite electrocatalysts, synthesized at 240°C, exhibited the impressive electrocatalytic performance, such as overpotential (required to reach 10 mA/cm²) of 104 mV, mass-weighed HER current density of 31 A/g and mass-weighed exchange current density of 1.0 A/g with the Tafel slope of 63 mV/dec. The advantageous electrocatalytic properties of these MoS₂/rGO composites attributed not only to the superior conductivity of rGO but the presence of functional groups on GO which can serve as the uniform nucleation sites for the MoS₂ precursor.

Fig. 3. A, Putative scheme of MoS2 growth on rGO under microwave conditions. B, XRD patterns of microwave-synthesized rGO (gray), MoS2 (teal), and MoS2-rGO composites. rGO and MoS2 were synthesized at 200°C, and MoS2-rGO at 200°C (blue), 240°C (red), and 300°C (wine). Vertical black lines show tabulated positions and relative intensities of the diffraction reflections for MoS2 (JCPDS#37-1492). C, TEM and HRTEM image of the microwave-synthesized MoS2-rGO240 composite. D, Polarization curves for the HER (scan rate, 0.005 V/s) in Ar-saturated 0.5M H2SO4 obtained for microwave-synthesized: (1) rGO, (2) MoS2, (3) MoS2-rGO300, (4) MoS2-rGO200, and (5) MoS2-rGO240; (6) data for 20 wt% Pt/C are shown as a reference. Currents are normalized to the electrode surface area (left axis) or mass of the material (right axis); potentials are corrected for ohmic losses. Inset shows a long-term chronoamperogram measured using MoS2-rGO240 at η = 0.12 V. E, SEM image of MoS2 synthesized using a microwave method at 200°C. (A-F) Reprinted with permission from Hinnemann et al Copyright 2017 American Chemical Society. HER, H2 evolution reaction; HRTEM, high-resolution transmission electron microscopy; rGO, reduced graphene oxide; SEM, scanning electron microscope; TEM, transmission electron microscopy; XRD, X-ray diffraction [Color figure can be viewed at wileyonlinelibrary.com]

Compared to HER process of the two-electron transfer kinetics, OER process is more complicated, as it goes through sluggish four-electron transfer kinetics. In addition, several reports have demonstrated that transition metal sulfides are unstable under OER conditions. Therefore, transition metal sulfides are easily oxidized to their corresponding metal oxides/(oxy)hydroxides, named as autoxidation, especially under operation in strong alkaline electrolyte. Autoxidation during the OER process has both strength and weakness. The autoxidized surface could be highly active sites to accelerate the kinetics of OER. However, it could also dissolve electroactive catalysts and consequently hamper conductivity and stability. Up to now, various non-2D transition metal sulfides (eg, Co_xS_y, Ni_xS_y, Fe_xS_y, etc) have been widely investigated with the support of rGO. Han et al revealed the origin of the improved OER performance and stability of a Co_1-xNi_xS₂/N doped-rGO (CNS–NGA) hybrid. According to density functional theory (DFT) calculations, the coupling between the CNS and NGA template facilitated the metal-semiconductor heterojunction at the anchored Co atoms on a pyridinic N site of NGA. As Co atoms anchored on the N-rGO layer, band bending is induced at the edge of conduction band minimum and valence band maximum of NGA. In this band structure, the heterojunction plays a role as an electron sink for surplus electrons of OH⁻ ions on its surface, which considerably improve the OER activity of CNS-NGA hybrid. Consequently, all these features together supported the CNS–NGA as an efficient and robust OER catalyst in alkaline electrolyte. Jiang et al proposed a 3D FeNiS₂ NSs/rGO hybrids to overcome the abovementioned drawbacks (Figure A). As expected, the FeNiS₂ NS/rGO hybrids exhibited a much better OER performance in terms of small overpotential at 10 mA/cm² (200 mV) than those of independent FeNiS₂ NS (410 mV) and simple mixture of FeNiS₂ NS + rGO (260 mV) (Figure C-E). Furthermore, the FeNiS₂ NS/rGO hybrids showed superior long-term stability which retains the OER performance over 50 hours. On the contrary, apparent degradation for a mixture of FeNiS₂ NS + rGO was observed after 1 hour measurement. Even if slight autoxidation occurred for the FeNiS₂ NS/rGO hybrids after 5000 cycles of LSV measurements, TEM and XPS characterization confirmed that the raw morphology and hybrid structure of the FeNiS₂ NS/rGO hybrids retained well, while the FeNiS₂ NS suffered the severe compositional and structural changes by autoxidation. The excellent OER performance as well as good stability of the FeNiS₂ NS/rGO hybrids were attributed to the support of rGO scaffolds. The rGO scaffolds facilitate a rapid electron transport between the domains, and enable the good dispersibility of the FeNiS₂ NS to expose a large number of active sites.

Fig. 4. A, Illustration of the optimization strategy for the OER performance of FeNiS2 NS by in situ rGO hybridization. B, HADDF image and the corresponding element mapping images of Fe (purple), Ni (green), S (yellow), C (cyan), and mix-and-match of all elements in the products. C, D, LSV of the FeNiS2 NS/rGO, FeNiS2 NS + rGO, RuO2, and FeNiS2 NS with corresponding Tafel slope plots in 1.0M KOH. E, The required potentials to achieve a current density of 10 mA/cm2 for different electrocatalysts in 0.1M and 1.0M KOH. A-E, Reprinted from Chatti et al, Copyright 2018, with permission from Elsevier. HADDF, high-angle annular dark-field; LSV, linear sweep voltammetry; OER, O2 evolution reaction; rGO, reduced graphene oxide [Color figure can be viewed at wileyonlinelibrary.com]

2.2 Transition metal phosphides/rGO-based electrocatalysts

Transition metal phosphides (TMPs) were also found to be very active materials for water splitting. The origin of the TMPs superior catalytic activities primarily owing to the electronic properties of phosphorus atoms. Since high electronegativity of phosphorus, P atoms in TMPs attract the electrons of the adjacent transition metals. The P atoms with a negative charge could attract the positively charged protons. Liu and Rodriguez first reported the activity of Ni₂P as HER catalyst in acidic media by DFT calculations. They found that the (001) plane of Ni₂P actually had the potential for the HER, even higher than that of Pt/C as well as the pure metallic Ni surface. Moreover, an excellent OER performance was discovered for TMPs in alkaline media. These OER activity is attributed to the metal oxide shell by the autoxidation of TMPs. Motivated by these results, substantial interests have been focused on the TMP-based electrocatalysts. Hybridizing TMPs with rGO could improve the conductivity of TMPs and enhance the electrocatalytic activity of TMPs by modifying the electronic structures. Liu et al demonstrated Fe₂P nanoparticles (NPs)/rGO/Fe₂P NPs (Fe₂P@rGO) nanowall arrays on a Ti plate via one-step electrodeposition followed by a low-temperature phosphidation process. The Fe₂P@rGO nanowall arrays exhibited a minimal overpotential of 101 mV at 10 mA/cm² and a low Tafel slope of 55.2 mV/dec. These outstanding HER performance originated from widely opened surface area to expose the Fe₂P catalyst and the conductive rGO which enable the rapid electron transfer of the supported Fe₂P NPs. Ma et al prepared an N-doped carbon coated CoP NPs on N-doped graphene (CoP@NC-NG) by the decomposition of polyaniline (PANI). The nitrogen sources from PANI act as nucleation sites for Co ions and N-dopant for N-doped carbon and N-GO. The N-doped rGO, the conductive support material, inhibited the agglomeration of the CoP NPs (Figure A,B,E). The N-doped carbon shell, converted from PANI, successfully covered the CoP NPs to protect the CoP from autoxidation. The CoP@NC-NG electrocatalysts showed the outstanding HER activity (Figure C and D) with a low overpotential of 135 mV at 10 mA/cm² and a Tafel slope of 59.3 mV/dec in 0.5M H₂SO₄. Surprisingly, when the SCN⁻ ions, the deactivator of the metal centered active sites, are applied in acidic electrolyte, the additional overpotential of 47 mV was measured to obtain 10 mA/cm² for CoP NPs, while only additional 5 mV was increased for CoP@NC-NG (Figure F and G). These results clearly showed that the N-doped carbon materials could be efficient HER active sites as well as the conductive scaffolds for electroactive materials.

Fig. 5. A, Illustration of the preparation procedure for the CoP@NC-NG composite. B, TEM and HRTEM images of CoP@NC-NG. C, Polarization curves for HER in 0.5M H2SO4 at a glassy carbon electrode modified with rGO, NC-NG, CoP, CoP-G, CoP@NC, CoP@NC-NG, and Pt-C (20 wt%), respectively. D, Tafel plots of different catalysts derived from polarization curves. E, F, HER performances for the catalysts CoP@NC-NG and CoP with and without the addition of KSCN in 0.5M H2SO4. A-G, Reprinted from Jiang et al, Copyright 2018, with permission from Wiley-VCH. HRTEM, high-resolution transmission electron microscopy; KSCN, potassium thiocyanate; PANI, polyaniline; RHE, reversible hydrogen electrode; TEM, transmission electron microscopy [Color figure can be viewed at wileyonlinelibrary.com]

Dong et al demonstrated the hierarchical bimetallic NiCoP hollow spheres encapsulated in P-doped rGO (PrGO) with the excellent electrochemistry performances. The Ni-Co bimetallic hollow spheres hybridized with rGO were synthesized via hydrothermal method using the Ni-Co based glycerates. As shown in Figure A, The solid-state phosphorization process was followed to make the PrGO encapsulated Ni-Co phosphides hollow microspheres (PrGO/NiCoP). SEM was utilized to characterize the microstructure and morphology of PrGO/NiCoP composites. Figure B and C showed that the crumpled PrGO NSs organize the 3D conductive network, uniformly covering the NiCoP hollow microspheres. Figure D provided clear evidence of the hollow structure through the broken part. Energy dispersive spectroscopy elemental mapping of Ni, Co, P, C, and N are illustrated in Figure E. It can be observed that Ni and Co are concentrated around the hollow microspheres. However, the P element is uniformly distributed over the entire region, suggesting that the phosphorus is not only bound to Ni-Co hollow microspheres but also doped into rGO NSs. Due to the unique morphology and synergistic effect of bimetallic composition, the PrGO/NiCoP (Ni/Co, 1:1) electrocatalysts showed the outstanding HER and OER properties. The PrGO/NiCoP showed superior HER performance with the overpotential of 106 mV at the current density of 10 mA/cm² and the Tafel slope of 58.3 mV/dec in 1M KOH electrolyte (Figure F). For the OER activity, as shown in Figure G, the PrGO/NiCoP exhibited the small overpotential of 281.3 mV at the current density of 10 mA/cm² and the low Tafel slope of 60.1 mV/dec. Furthermore, overall water splitting application was demonstrated using the nickel foams coated with PrGO/NiCoP as cathode and anode (Figure H). The overall water splitting cell showed superior electrocatalytic activity with a cell voltage of 1.56 V to reach the current density of 10 mA/cm². In addition, no noticeable degradation was observed in both LSV curve and chronoamperometry test for 10 hours (Figure I and J). Clearly, constructing hierarchical heterostructure with TMPs and rGO could facilitate the exposure of active sites and accelerate the reaction kinetics of both HER and OER for overall water splitting.

Fig. 6. A, Schematic diagram of formation process. B-D, SEM images of PrGO/NiCoP. E, SEM image and elements mapping. F, LSV curves for HER of Ni2P, CoP, NiCoP, PrGO/NiCoP, and commercial Pt/C catalyst (20 wt%). G, LSV curves for OER of Ni2P, CoP, NiCoP, PrGO/NiCoP, and commercial RuO2. H, The photograph of two-electrode system for overall water splitting and the H2 and O2 bubbles on electrodes during testing. I, LSV curves of initial test and after 10 hours test. J, Chronoamperometry curves for overall water splitting at 1.56 V. A-J, Reprinted from Liu et al, Copyright 2019, with permission from Elsevier. HER, H2 evolution reaction; LSV, linear sweep voltammetry; OER, O2 evolution reaction; SEM, scanning electron microscope [Color figure can be viewed at wileyonlinelibrary.com]

2.3 Transition metal carbides/rGO-based electrocatalysts

Transition metal carbides (TMCs) are spotlighted due to several superiorities, such as noble-metal-like electronic structures, wide pH operation window, earth abundance, and remarkable HER activity. The d orbitals of parent metal atoms are broadened by the introduction of s and p orbitals of C atoms, resulting in the d-band state similar to that of Pt. As the vital element for the synthesis of TMCs and the typical byproduct, carbon allotropes are widely used as supporting materials for HER. He et al demonstrated the 2D TaC-rGO hybrid electrocatalysts via a simple in situ method. The 2D TaC-rGO hybrid was prepared by mixing K₂TaF₂ with GO dispersion. TaF₇^2- ions were attached onto the surface of GO NSs, exploiting their oxygen-containing functional group. The mixture was settled down by continuous heat treatment and further freeze-dried. The TaO₂F phase was created at an intermediate temperature of 800°C. When the temperature increased to 1200°C, the TaO₂F fully converted to 2D TaC on the rGO NSs. The 2D TaC-rGO exhibited exceptional HER performance with a small overpotential of 167 mV at a current density of 10 mA/cm² and a low Tafel slope of 58 mV/dec. Interestingly, the HER activity of bulk TaC/rGO is highly enhanced compared to bulk TaC, indicating the enhancement in charge transfer kinetics with the help of conductive rGO. He et al reported another study with TMCs/rGO system. He et al also reported an rGO induced bimetallic carbide nanorods (Fe₃W₃C NRs/rGO) via two-step synthesis approach. As illustrated in Figure A, Fe(WO₄) NRs were grown on rGO NSs by hydrothermal method. Subsequent pseudomorphic carbonization of Fe(WO₄) NRs was conducted under 900°C. The SEM image in Figure B showed that the F₃W₃C NRs were grown on both side of rGO, constructing a sandwich-like structure. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Fe₃W₃C NRs/rGO exhibited the 1D Fe₃W₃C NRs with the thickness of ~10 nm (Figure C). The HER activity of Fe₃W₃C NRs/rGO was examined with reference to commercial Pt/C catalyst. The Fe₃W₃C NRs/rGO electrocatalysts exhibited superb HER activity with the small overpotential of 57 mV at current density of 10 mA/cm², showing only 20 mV higher value than Pt/C catalyst (Figure D), and a low Tafel slope of 50 mV/dec (Figure E). Furthermore, Fe₃W₃C NRs/rGO showed electrochemical stability over a wide range of pH solutions such as 0.5M H₂SO₄, 0.5M K₂SO₄, and 1M KOH (Figure F-H). According to DFT calculations, the hydrogen adsorption the Gibbs free energy ΔG_H for the catalysts and Pt reference was displayed in Figure I. A highly positive ΔG_H of 1.93 eV for rGO was shown, indicating weak chemical interaction between H* and rGO. On the contrary, strong chemical interaction between H* and Fe₃W₃C NRs was suggested with a bit negative ΔG_H of −0.49 eV for Fe₃W₃C NRs, which is also undesirable for the electrochemical HER. After coupling the Fe₃W₃C NRs with rGO, The Fe₃W₃C NRs/rGO hybrid exhibited intermediate bond strength for H* with the near zero value of ΔG_H. Such enhancement is clearly attributed to the redistribution of accumulated charges at the interface by the aid of conductive rGO. The DFT calculations indicate that the exceptional HER activity of Fe₃W₃C NRs/RGO primarily due to the synergistic effect of chemical and electronic couplings between Fe₃W₃C NRs and RGO.

Fig. 7. A, Schematic illustration of the synthesis of Fe3W3C NRs/rGO. B, The representative SEM image of Fe3W3C NRs/rGO. C, HADDF-STEM image of Fe3W3C NRs/rGO. D, Polarization curves of bare rGO, WC NRs/rGO, Fe3W3C NRs/rGO, and Pt/C in 0.5M H2SO4 solution. E, Tafel plots of WC NRs/rGO, Fe3W3C NRs/rGO, and Pt/C. F, G, Polarization curves of Fe3W3C NRs/rGO before (black) and after 10 000 cycles (red) between −0.15 and 0.15 V (vs RHE) in 0.5M H2SO4, 1M KOH, 0.5M K2SO4 solutions, respectively. I, DFT-calculated free energy diagrams for the electrochemical reduction of H+ at the equilibrium potential for Pt, rGO, Fe3W3C, WC, Fe3W3C@rGO, and WC@rGO heterostructure. A-I, Reprinted from Gao et al, Copyright 2019, with permission from Elesvier. DFT, density functional theory; HADDF, high-angle annular dark-field imaging; rGO, reduced graphene oxide; RHE, reversible hydrogen electrode; SEM, scanning electron microscope; STEM, scanning transmission electron microscope [Color figure can be viewed at wileyonlinelibrary.com]

2.4 Molecular catalyst/rGO-based electrocatalysts

Molecular catalysts have been widely explored as active OER catalysts, while their HER activity is rarely reported. Effective OER electrocatalysts should accelerate the kinetics in terms of the turnover frequency (TOF) at the low overpotential. The molecular catalysts are very consistent with this requirement, since the molecular active sites can be maximally exposed to reaction intermediates and their electrocatalytic activity can be systematically tuned with well-defined manner. However, molecular catalysts alone require specific solvents to maintain their activity and their recovery from reaction solvent is very difficult. Anchoring the molecular catalysts onto carbon-based templates would be one promising approach to overcome these shortcomings because carbon-based templates can provide a high surface area, excellent electrical conductivity, and numerous surface functional sites to immobilize molecular catalysts with covalent bonds. Wang et al reported Co-based molecular OER electrocatalyst which is anchored on the heteroatom-doped rGO. Electrochemical analysis revealed that the heteroatoms (S, N, and O atoms) in rGO play a crucial role as the binding sites of Co²⁺ ions. Among them, a C─S═O configuration exhibited significantly enhanced TOFs for Co sites, showing a higher TOF than an IrO₂ catalyst. In contrast to conventional Co₃O₄ where dual Co sites are known as the OER active sites, single Co ions with terminal oxo ligands, formed after a successive conversion of Co²⁺→Co³⁺→Co⁴⁺ coupled with HO⁻ (H+) transfer, could drive the OER. It was shown that A side-on hydroperoxo ligand on the Co⁴⁺ active sites enabled evolution of dioxygen.

3 ELECTROCHEMICAL CO₂ REDUCTION

By the excessive emission of CO₂ by the massive consumption of fossil fuels, the increased concentration of CO₂ in the atmosphere has caused a serious environmental issues, mainly global warming. As a solution, carbon capture and utilization processes that capture CO₂ and convert it into valuable carbon-based products have been attracted great attentions. Among different processes for conversion of CO₂, electrochemical reduction of CO₂ has attracted much attention as promising ways to produce useful chemicals and fuels because it can reduce CO₂ by direct input of electrical energy under ambient condition. Major challenges are the low energy efficiency due to the large overpotential for CO₂RR, and the poor product selectivity because there is a possibility of evolving unwanted byproducts, particularly hydrogen. To increase the efficiency and the selectivity for the CO₂RR, various kinds of catalysts are needed. rGO has many advantages in terms of efficiency, not only because of the unique electrical and physical properties but also rGO is a support material that can be combined with many kinds of electroactive materials for the CO₂RR. Therefore many studies have been conducted with rGO-based catalysts for the electrochemical reduction of CO₂. In this section, recent research of rGO-based electrocatalysts on electrochemical reduction of CO₂ are focused, which is summarized in Table .

3 Summary of electrochemical CO₂ reduction over various rGO-based catalysts from recent literature

Electrocatalysts	Potential	Products (FE%)	Electrolyte	Reference
Cu-rGO	−0.4 V vs RHE	CO (21.7), CH4 (8.6), Liquid products (46.2)	0.1M NaHCO3	Hossain et al
Cu nanoparticles on rGO	−1.0 V vs Ag/AgCl	CO (~46)	0.1M KHCO3	Cao et al
Cu nanoparticles/pyridinic-N rich rGO	−0.8 V vs RHE	Formate (62)	0.5M KHCO3	Li et al
Cu2O/rGO	−0.9 V vs Ag/AgCl	C2H5OH (9.93)	0.5M NaHCO3	Geioushy et al
Pd0.5-In0.5/3D-rGO	−1.4 V vs Ag/AgCl	Formate (84.5)	0.5M KHCO3	He et al
Au-OLA/rGO	−0.7 V vs RHE	CO (75)	0.1M KHCO3	Zhao et al
Ni-N-rGO	−0.8 V vs RHE	CO (97)	0.5M KHCO3	Jeong et al
In2O3 nanobelts-rGO	−1.2 V vs RHE	Formate (84.6)	0.1M KHCO3	Zhang et al
Ag2S/N-S-doped rGO	−0.76 V vs RHE	CO (87.4)	0.1M KHCO3	Zeng et al
SnS2 nanosheets/rGO	−1.4 V vs Ag/AgCl	Formate (84.5)	0.5M NaHCO3	Li et al
Phen-Cu complex/rGO	−0.9 V vs RHE	CO (75)	0.1M KHCO3	Wang et al
Co porphyrin/rGO	−0.65 V vs RHE	CO (~90)	0.5M NaHCO3	Zhu et al
Fe porphyrin/rGO	−0.54 V vs RHE	CO (98.7)	0.1M KHCO3	Choi et al
Pt@Adenine-rGO	−0.3 V vs Ag/AgCl	CH3OH (85)	0.1M KNO3	Alinajafi et al

Abbreviations: FE, Faradaic efficiency; OLA, oleylamine; RHE, reversible hydrogen electrode; rGO, reduced graphene oxide.

3.1 Metals/rGO-based electrocatalysts

Among the various metal elements, copper has been widely studied for electrochemical reduction of CO₂ because Cu has a low surface affinity to CO and enables the production of hydrocarbon. Numerous studies have reported the production of CO, CH₄, methanol, and C2 hydrocarbon in electrochemical reduction of CO₂ using Cu electrodes. Furthermore, it is reported that when Cu NPs combined with rGO, CO₂RR performance is dramatically improved. Alves et al demonstrated that rGO is a promising support material for nanosized catalysts. They synthesized Cu NPs decorated RGO catalysts by coreduction of Cu²⁺ and GO using hydrazine solution as reducing agent. They found that Cu NPs were uniformly distributed on rGO sheets even though there were no additional passivating agents, such as organic molecules or polymers for stabilization of copper particles. And they mentioned that the rGO act as stabilizing support for Cu NPs to be 20 to 40 nm sizes, by preventing the agglomeration of Cu NPs.

In addition, Hossain et al synthesized Cu NPs/rGO nanocomposites on Cu foil with various proportions through a facile electrochemical reduction method, while controlling the concentration of Cu and GO precursors. The SEM images in Figure A and B are the morphology of the synthesized Cu NPs with the absence of GO, and the Cu/rGO nanocomposite, respectively. This images showed that large grain-sized Cu particles were formed in the absence of GO, but the Cu NPs with an average size of ~10 nm were uniformly formed on the rGO, demonstrating the role of rGO as stabilizing support. To evaluate the electrocatalytic activity of the Cu/rGO nanocomposite for electrochemical reduction of CO₂ reaction, LSV and electrochemical impedance spectroscopy were measured comparing with bare Cu substrate, CuNPs, and rGO as shown in Figure C. In contrast to other electrodes, the Cu/rGO nanocomposite exhibited a much higher current density and lower onset potential. In addition, the lowest charge-transfer resistance (355.40 Ω/cm²) was obtained on Cu/rGO nanocomposite. In Figure D, the optimized proportion of Cu/rGO nanocomposite exhibited the excellent Faradaic efficiency (FE) for electrochemical reduction of CO₂ for, 76.6% at a relatively low potential of −0.4 V vs RHE in during bulk electrolysis of CO₂. The electrode was founded to be highly stable during electrolysis. They demonstrated that the superior electrocatalytic activity and stability of the Cu/rGO nanocomposites for electrochemical reduction of CO₂ were attributed to the uniformly distributed small Cu NPs on the rGO and their synergistic coupling effect.

Fig. 8. A, B, SEM image of the formed Cu NPs and the Cu/rGO nanocomposite on a Cu substrate. C, LSV curves of the bare Cu substrate, rGO, Cu NPs, and Cu/rGO nanocomposite electrodes. D, The Faradaic efficiency of the formed products at the different applied potentials over the 6 hours. A-D, Reprinted from Schrader et al, Copyright 2017, with permission from Springer Nature. E, Schematic illustration of amine modification on the rGO-Au composite. F, HRTEM images of ultrasmall Au NPs. G, The Faradaic efficiency at various potentials for rGO-Au and Au-amine catalysts. E-G, Reprinted from Zhang et al, Copyright 2018, with permission from WILEY-VCH. H, Formation of the single-atom nickel- and nitrogen-doped rGO sheets (Ni-N-rGO) (gray, carbon; red, oxygen; blue, nitrogen; green, nickel). F, HAADF-STEM image of single-atom nickel sites of Ni-N-rGO. J, LSV curves measured in Ar-saturated (dotted line) or CO2-saturated (solid line) 0.5M KHCO3 electrolytes. K, Faradaic efficiency of CO production (gray bar) and current density (based on geometric surface area) vs applied potential (black dot, total current; red dot, CO; blue dot, H2). H-K, Reprinted from Geioushy et al, Copyright 2018, with permission from WILEY-VCH. EDA, ethylenediamine; HA, hexylamine; HRTEM, high-resolution transmission electron microscopy; LSV, linear sweep voltammetry; NP, nanoparticle; OLA, oleylamine; PA, propylamine; PEI, polyethyleneimine; rGO, reduced graphene oxide; RHE, reversible hydrogen electrode; SEM, scanning electron microscope [Color figure can be viewed at wileyonlinelibrary.com]

It is reported that rGO acts as a stabilizing support not only on Cu but also on other elements. Noble metals, such as Au, Ag, Pd, were known as the catalysts which could selectively produce CO in electrochemical reduction of CO₂. Saquib and Halder reported that Au NPs on defective rGO could reduce the overpotential of CO₂RR to CO product. They observed that Au NPs with a diameter of ~5 nm were uniformly distributed on rGO support, and confirmed the synergistic effect of rGO as support material for enhancement on activity of electrochemical reduction of CO₂ via Raman, Fourier-transform infrared spectroscopy and electrochemically active surface area. The defect sites on rGO enabled the facile desorption of CO from Au surfaces by rapid electron transfer between rGO and Au.

Furthermore, Zhao et al fabricated the ultra-small Au NPs on rGO by surfactant-free wet chemistry method and modified the surface with amine to control the electrocatalytic activity of ultra-small Au NPs (Figure E). The defect sites of rGO played an important role as the adsorption and nucleation sites for ultra-small Au NPs, and their strong interactions with Au atoms lead to the stabilization of Au NPs. In Figure F, The Au NPs showed a size distribution of 2.4 nm on average and narrow dispersion of 0.5 nm, and exhibited the preferential (111) plane of fcc-Au. Then, the synthesized Au/rGO composite was modified with various amine structure to tune the electrocatalytic properties, as shown in Figure G. The synthesized catalysts exhibited excellent properties, good FEs (59%-75%) for the electrochemical reduction of CO₂ to CO at moderate overpotentials (450–600 mV).

rGO can be also good candidate materials for the synthesis of single-atom catalysts. To synthesize single-atom catalysts, it is very important to stabilize single metal atoms due to their high surface energy. For electrochemical application, the supporting materials have to be electrically conductive. In this situation, rGO can be a promising candidate substrate to achieve electroactive single atom catalysts. Nam group reported a new strategy to synthesize single-atom nickel sites over N-doped rGO sheets using tris(2-benzimidazolylmethyl)amine (NTB) ligand for stabilization. First, NTB ligands were adsorbed onto the GO sheets, resulting in self-assembly of the GO sheets. Since NTB ligands strongly interact with GO sheets and can also ligate with transition-metal ions, NTB ligands can be used as binders between the Ni ions and the GO sheets, resulting in stabilizing the metal atoms uniformly over the GO sheets. Then, the Ni(NTB)-GO complex thermally reduced to Ni-N-rGO complex by annealing at 800°C under argon condition, as shown in Figure H. The structure of the Ni-N-rGO complex was investigated by HAADF-STEM image. Ni atoms on the GO support were observed as brighter dots in Figure I because high-angle scattered electrons are sensitive to atomic number. The catalytic activity of Ni-N-rGO complex for electrochemical reduction of CO₂ reaction was studied in comparison with Ni-N-C complex without rGO supports. As shown in Figure J, Ni-N-rGO complex exhibited a higher catalytic activity than that of Ni-N-C complex, showing a more positive onset potential of −0.6 V vs RHE at the current density of −5 mA/cm² under CO₂. Also, FE for CO product is 70% at −0.4 V vs RHE near the onset potential (Figure K). As the applied potential increased, the FE also increased and reached its maximum of 97% at −0.8 V vs RHE. At −1.0 V vs RHE, high current density of −42 mA/cm² is observed. The excellent catalytic activity is attributed to high surface area of 2D rGO structure and high specific activity of single atom Ni sites. Consequently, the rGO sheets in CO₂RR electrocatalysts not only facilitate the synthesis of nanoparticular or even single-atom catalysts but enable the surface stabilization in a variety of studies.

3.2 Transition metal oxides and sulfides/rGO-based electrocatalysts

Not only for metal catalysts, but rGO is known to have also good synergistic effects with other materials such as oxides or sulfides. Zhang et al reported that the chemical coupling interaction between rGO and In₂O₃ nanobelts highly improves the electrocatalytic activity for CO₂ reduction to formate. The porous In₂O₃ nanobelts-rGO hybrid was fabricated by a two-step method (Figure A) of the hydrothermal method for producing In(OH)₃-rGO hybrid, and the followed annealing process. Then, to investigate the chemical coupling interaction, they also synthesized porous In₂O₃ nanobelts and rGO by physical mixing (denoted as In₂O₃/rGO) for comparison with In₂O₃-rGO hybrid. By X-ray absorption near edge structure and XPS, they found that there is strong chemical interaction between In₂O₃-rGO hybrids, but negligible interaction between In₂O₃/rGO hybrids. As shown in Figure B and C, In₂O₃-rGO hybrid exhibited the higher catalytic activity than In₂O₃/rGO mixture with the larger current density and the better selectivity for formate and CO. Especially, the maximum FE for the major product, formate, reached 84.6% at −1.2 V for In₂O₃-rGO hybrid catalyst. The charge redistribution and the Gibbs free energy were investigated through the DFT calculations (Figure D). The DFT calculations clearly showed that the major factor of enhancement in electrocatalytic activity is attributed to the chemical coupling interaction between rGO and In₂O₃.

Fig. 9. A, Schematic illustration shows the preparation procedure for the In2O3-rGO hybrid. B, Faradaic efficiencies of CO and formate for In2O3-rGO (blue), In2O3/rGO (olive green), and In2O3/C (red) catalysts. C, Current density of formate based on the electrode's geometric surface area. D, The differential charge diagram of In2O3-rGO hybrid catalyst. Yellow represents the electron accumulation area; blue represents the electron loss area. A-D, Reprinted with permission from Blakemore et al, Copyright 2019, American Chemical Society. E, The rGO-TEPA CC biocathode fabrication and configuration. Reproduced from Hossain et al with permission from The Royal Society of Chemistry. F, Plot of the concentration of methanol produced and Faradaic efficiency vs the applied potential at pH 2.0. Reprinted from Neaţu et al, Copyright 2018, with permission from Elsevier. rGO, reduced graphene oxide; RHE, reversible hydrogen electrode; TEPA, tetraethylene pentamine [Color figure can be viewed at wileyonlinelibrary.com]

Also, it is reported that the electrocatalytic activity for CO₂ reduction to ethanol is improved when Cu₂O is combined with rGO support material. The high electrical conductivity of rGO increased the available surface area of Cu₂O, resulting in the larger current density. Furthermore, enhanced electron mobility facilitated the reduction of *CH_xO adsorbed species to *C₂H_xO₂ intermediate, which is an important process for producing ethanol from electrochemical reduction of CO₂.

3.3 Molecular catalysts/rGO-based electrocatalysts

Molecular catalysts, specifically, transition metal complexes have been investigated to improve the catalytic activity and selectivity for electrochemical CO₂ reduction. Because their molecular structures can be accurately tailored, high selectivity for the desirable products can be achieved on molecular catalysts for electrochemical CO₂RR. Recently, Wang et al reported that a 1,10-phenanthroline-Cu complex on a mesostructured rGO matrix can be active and selective for CO₂RR over HER in an aqueous solution. The phen-Cu complex, made of 1,10-phenanthroline ligands and Cu²⁺ ions, was synthesized from an assembly of CuCl₂ and phen in a solvent mixture of CH3OH/CH2Cl2 and immobilized on the rGO substrate. In electrolysis for CO₂RR, phen-Cu complex/rGO exhibited the highest value of FE as 75% for CO, at −0.9 V vs RHE. Furthermore, through in situ infrared spectroelectrochemical investigation, it is found that the Cu complex can be reversibly heterogenized near the rGO surface, which causes an increase of electron density in the complex under the catalytic condition, leading to efficient catalytic activity for CO₂RR. Also, Zhu et al synthesized Cobalt porphyrin/rGO composites for electrochemical reduction of CO₂ to CO. The high FE for CO (~90%) was achieved on the potential range between −0.8 and −0.6 V vs RHE on the CoP/nitrogen-doped rGO (NrGO) composites. Through the DFT calculation and experimental results, it is found that N atoms doped rGO have the ability to tune the electronic structure of the cobalt porphyrins, resulting in increasing the electron density around Co atoms. Because of the increased electron density at Co atoms, Co sites were more nucleophilic and tend to bind CO₂ more strongly, which lead to enhance the electrocatalytic performance for electrochemical CO₂ reduction to CO. In the work, it is demonstrated that the rGO substrate can act as a media to tune the electronic structure and catalytic activity of the immobilized molecular catalysts.

3.4 Biomaterials/rGO-based electrocatalysts

Recently, microbes are used for electrochemical reduction of CO₂ at the cathode of a bioelectrochemical reactor, which called microbial electrosynthesis (MES). One of the main challenges to achieve highly productive MES is sluggish electron transfer from cathode to microbes. rGO is an effective support material for the MES due to its the high electronic conductivity, low charge-transfer resistance.

Chen et al reported that the rGO functionalized carbon cloth cathodes with tetraethylene pentamine (rGO-TEPA) can produce a higher amount of acetate from electrochemical reduction of CO₂. Bacterial cells generally have a negative surface charge, which makes them difficult to electrostatically interact with rGO, which also has negative charges.

To solve this problem, the positively charged TEPA was applied on the surface of rGO, enabling electrostatic interaction, as shown in Figure E. As a result, the rGO-TEPA successfully improved the electron transfer rate from the cathode to the microbial catalysts, and this system could produce large amounts of acetate by electrochemical reduction of CO₂.

In another case, rGO can be functionalized with biochemical for electrochemical reduction of CO₂. Alinajafi et al demonstrated Pt@Adenine functionalized rGO catalysts on glassy carbon electrode for reduction of CO₂ to methanol. The adenine on rGO was used as the active catalyst, and Pt was used for the production of hydrogen atoms to transfer to the reduction of CO₂. As a result (Figure F), this electrode exhibited high FE to methanol from CO₂ in electrochemical reduction even at low voltage.

4 ELECTROCHEMICAL N₂ REDUCTION

Ammonia (NH₃) is one of the most essential chemicals to produce fertilizers, pharmaceuticals, and also can be a clean energy carrier due to its the characteristics of high energy density. Although the availability of nitrogen from the atmosphere is unlimited, due to the inertness and stability of N₂, it is very difficult to fix at ambient temperature and pressure. Thus, at present, the ammonia production depends primarily on the Haber-Bosch process that operates under high pressure and high temperature, which leads to significant energy consumption and enormous CO₂ emissions. Therefore it is highly required to develop an energy-effective and sustainable process for NH₃ production.

Electrochemical reduction of N₂ to NH₃ at ambient conditions is one possible solution for energy-saving and convenient process for NH₃ production, however, this process is limited by the sluggish NRR kinetics and low FE. Accordingly, efficient catalysts are necessary to accelerate the NRR kinetics and promote the FE. The recent progress in the field of rGO-based NRR electrocatalysts is presented in the next section.

4.1 Noble metal/rGO-based electrocatalysts

Recently, noble metal electrocatalysts, such as Au, Ru, Pd, have been largely used in NRR, but the high costs and low abundance greatly restrict their large-scale applications. Therefore, introduction of transition metals can be a promising way to reduce the amount of noble metal as well as improve the catalytic activity. To improve the performance of NRR, Shi et al synthesized ultrafine Pd_0.2Cu_0.8 heterogeneous amorphous nanoclusters on rGO as shown in Figure A. The Pd_0.2Cu_0.8/rGO nanocomposites were fabricated via coreduction of GO, Cu, and Pd precursors using tannic acid and NaBH₄ mixed reductants, resulting in metal clusters stabilized on rGO sheets by π-π interactions between the rGO and tannic acid. The Pd_0.2Cu_0.8 nanoclusters exhibited an average particle size of 1.7 nm. The electrochemical NRR properties of Pd_0.2Cu_0.8/rGO were measured in N₂ saturated 0.1M KOH electrolytes. The nanocomposites show an N₂ yield of 2.8 μg/h∙mg at −0.2 V vs RHE at room temperature, which is much higher than that of compared composites (Figure B). The superior performances of the Pd_0.2Cu_0.8/rGO nanocomposite are attributed to rGO substrate that enabled to maximize the exposure of active sites and high utilization ratio of catalysts by dispersion of ultrafine nanoclusters. In addition, rGO also provided a continuous pathway for electron transportation, decreasing the electron transport resistance and facilitating the catalytic reaction kinetics.

Fig. 10. A, Schematic illustration of the preparation of PdxCu1-x/rGO. B, Catalytic activities of the bimetallic Pd0.2Cu0.8/rGO composites, monometallic Pd/rGO and Cu/rGO counterparts, and Pd, Cu single metal for NRR at −0.2 V vs RHE under room temperature and atmospheric pressure. A, B, Reprinted from Zhu et al, Copyright 2018, with permission from WILEY-VCH. C, HADDF-STEM elemental mapping images of CoO QD/rGO catalysts. D, Faradaic efficiencies and NH3 yields of CoO QD/rGO catalysts in N2-saturated solution at various potentials. E, NH3 yields of CoO QD/rGO, controlled CoO NP/rGO, and bare rGO in N2-saturated solution at −0.6 V. C-E, Reproduced from Hou et al with permission from the Royal Society of Chemistry. F, SEM image of the MoS2/rGO hybrid. G, NH3 yields and Faradaic efficiencies of MoS2/rGO on CPE at a series of potentials. H, Amount of produced NH3 for MoS2/rGO on CPE after charging at −0.45 V for 2 hours at various electrodes. I, Free-energy profiles of the NRR at edge sites of MoS2/rGO (red/pink dotted line) and MoS2 (blue dotted line), respectively. The asterisk denotes the adsorption site. F-I, Reproduced from Guo et al with permission from the Royal Society of Chemistry. CPE, constant phase element; HADDF, high-angle annular dark-field; rGO, reduced graphene oxide; RHE, reversible hydrogen electrode; SEM, scanning electron microscope; STEM, scanning transmission electron microscope [Color figure can be viewed at wileyonlinelibrary.com]

4.2 Transition metal oxides and sulfides/rGO-based electrocatalysts

Also, a variety of earth-abundant transition metal-based catalysts, such as oxides, sulfides has emerged as promising economical catalysts to replace the noble metal catalysts for the NRR. Oxide materials have the advantages of easy synthesis and tunable catalytic activity, but the activity for NRR is still poor due to the low electrical conductivity and limited active sites. To address these shortcomings, recent research has been conducted using transition metal oxide catalysts in combination with rGO support materials. Through the DFT calculations, Chu et al examined the possibility of CoO as an active catalyst for NRR and found that CoO possessed poor HER activity but favorable NRR activity. They prepared CoO quantum dots loaded on rGO catalysts for proof-of-concept experiments, using a novel self-propagating combustion method. Figure C shows that the STEM images of uniformly distributed CoO quantum dots on rGO with elemental mapping. The electrochemical NRR performances of CoO/rGO loaded on carbon paper were measured in N₂-saturated 0.1M Na₂SO₄ electrolytes. At −0.6 V vs RHE (Figure D), the CoO/rGO loaded carbon paper achieved the high NH₃ yield and FE values of 21.5 μg/h∙mg and 8.3%, respectively. These are comparable to or exceeding those of most of the reported NRR electrocatalysts. The size effect of CoO on NRR performance is investigated while comparing CoO quantum dots/rGO catalyst with CoO NPs/rGO catalyst. As shown in Figure E, the CoO NPs/rGO catalysts exhibited remarkably lower performance than CoO quantum dots/rGO, underlining the importance of small-sized CoO quantum dots for enhancing the NRR activity of CoO.

The other transition metal oxide materials also used for NRR. Wang et al reported that the CuO NPs supported on rGO nanocomposite could be used as an efficient and robust NRR catalysts. Synthesized CuO/rGO nanocomposites by a microwave-assisted solvothermal method exhibited a high NH₃ yield of 0.18 nmol/s∙cm² and FE of 3.9% far outperforming the bare CuO or rGO alone. The Nyquist plot revealed that the CuO/rGO exhibited a smaller charge transfer resistance than bare CuO NPs, advocating the improved conductivity by the rGO support to enhance the NRR kinetics. Also, the enlarged active surface area was verified by double-layer capacitance measurement. They demonstrated that the prominent catalytic and structural stability of CuO/rGO come from the uniformly distributed small-sized CuO NPs on rGO, and the strong CuO-rGO interaction. Similarly, Mn₃O₄, TiO₂, and Cr₂O₃ NPs have also been reported to combine with rGO, resulting in high improvements in electrochemical reduction of N₂ reaction into NH₃.

MoS₂, one of the layered sulfide materials with low-cost and nontoxicity, also used as electrocatalysts for NRR. A study has demonstrated that a MoS₂ NS array is also active for NRR, but it suffers from relatively low electrical conductivity with an FE of 1.17%. However, Li et al demonstrated that greatly boosted electrocatalytic N₂ reduction to NH₃ by using MoS₂ NSs-rGO hybrid under ambient conditions. Figure F shows the SEM image of the MoS₂/rGO hybrid, in which MoS₂ NSs are uniformly decorated on rGO sheets. They deposited the hybrid catalyst on a carbon paper electrode and measured the catalytic activity of NRR. In Figure G, the MoS₂/rGO hybrid catalyst exhibited a high FE of 4.58% and a high NH₃ yield of 24.82 μg/h∙mg at −0.45 V vs RHE, comparable to the NRR activity of most aqueous-based NRR electrocatalysts (Figure H). The excellent catalytic activity of MoS₂/rGO indicates that the construction of hybrid materials with rGO is an effective strategy to boost the activity of MoS₂ for NRR. They also performed DFT calculations to investigate the catalytic mechanism of the NRR at sites of MoS₂/rGO, as shown in Figure I. DFT calculations suggest that the potential determining step of the NRR process is hydrogenation of *NHNH₂ to form *NH₂NH₂ with an energy barrier of 0.49 eV. So far, we have explored a variety of electroactive materials that combine with rGO to improve the catalytic activity for the NRR. Although the electrochemical NRR has not been actively studied yet, the combination of various catalysts with rGO is one possible way to improve the catalytic activity for NRR.

5 ELECTROCHEMICAL O₂ REDUCTION

Fuel cells have been recognized as a promising technology for clean and efficient energy conversion, which directly convert the chemical energy into electricity. The main challenge for large-scale commercialization of fuel cell is to improve the kinetics of ORR, which is intrinsically sluggish reaction kinetics. Pt have been considered as the best catalyst for the ORR, as well as in most of the chemical reactions. However, due to its excessive costs, scarcity and self-poisoning effect, Pt is not suitable for large-scale applications in fuel cells. Therefore, it is important to discover low-cost and efficient electrocatalysts to replace Pt catalysts for ORR. As was effective for the abovementioned reactions, rGO is recognized as a promising material to build innovative heterogeneous ORR catalysts, due to its excellent electrical conductivity and low costs. In this section, the recent development of rGO based heterogeneous catalysts are introduced for efficient ORR, followed by summarized in Table .

4 Summary of electrochemical O₂ reduction over various rGO-based catalysts from recent literature

Electrocatalysts	Limiting current density (mA/cm2)	Half-wave potential (V vs RHE)	Onset potential (V vs RHE)	Tafel slope (mV/dec)	Electrolyte	Reference
Co NPs on Co-N/rGO	6.085	0.848		47	0.1M KOH	Zhao et al
Fe2Ni1 NPs/N-rGO	…	0.90	0.95	68	0.1M KOH	Nguyen et al
N-Holey Carbon/rGO	6.1	0.85	1.0	…	0.1M KOH	Sun et al
Fe-N-CNT/rGO	4.43	0.79	0.93	83	0.1M KOH	Zheng et al
Ru SA/N-rGO	4.81	0.75	0.89	134	0.1M HClO4	Zhang et al
HfP-rGO NS	…	…	0.97	212	0.1M KOH	Meganathan et al
HfS2-rGO NS	…	…	0.95	199	0.1M KOH	Meganathan et al
WS3-x/rGO-HU	…	0.74	0.84	67	0.1M KOH	Tan et al
Co-doped ZnO NPs/rGO	3.94	0.74	0.90	…	0.1M KOH	Sun et al
CFCOSMs/rGO	…	…	0.85	90.6	0.1M KOH	Wang et al
SmMn2O5/N-rGO	5.55	0.84	…	52	0.1M KOH	Yu et al
FeS/rGO	…	0.85	1.0	69	0.1M KOH	Gautam et al

Abbreviations: rGO, reduced graphene oxide; RHE, reversible hydrogen electrode.

5.1 Metal NPs/rGO-based electrocatalysts

rGO support material is an ideal platform for both the growth and stabilization of metallic NPs, due to the combination of its excellent electrical conductivity, high surface area, and good stability. The metal NPs on rGO have the capacity for high catalytic activity due to an improvement of charge transfer. Zhao et al fabricated Co NPs on Co- and N-codoped rGO for efficient ORR catalysts. They synthesized the catalysts through pyrolysis of the homogeneous mixture of Prussian blue analogues, GO and graphitic carbon nitride with the presence of silica as hard templates. Figure A shows the morphology structure of Co@Co-N/rGO, which is rough sheets with Co NPs. The synthesized catalysts exhibited excellent catalytic activity for ORR in O₂-saturated 0.1M KOH electrolyte, as shown in Figure B and C. The linear polarization curves of a catalyst-loaded glassy carbon electrode show that the half-wave potential of Co@Co-N/rGO catalysts is 0.848 V, which is slightly higher than that of Pt/C catalyst. Furthermore, the catalysts exhibited a remarkable limiting current density. In the Tafel plot, the Co@Co-N/rGO displays a slope of only 47 mV/dec, which is highly comparable to that of Pt/C, representing that the surface kinetics of Co@Co-N/rGO is similar to that proceeding on Pt/C catalyst. The synergistic effects of Co NPs and rGO sheets led to the excellent ORR catalytic activities.

Fig. 11. A, TEM image and illustration of Co@Co-N/rGO catalysts. B, C, LSV curves and the corresponding Tafel plots of different samples determined in O2-saturated 0.1M KOH aqueous electrolyte at a rotation speed of 1600 rpm with a sweep rate of 10 mV/s. A-C, Reprinted from Wang et al, Copyright 2019, with permission from Elsevier. D, FESEM image of Co0.38Zn0.62O NPs@rGO. E, LSV curves of the different catalyst samples modified RDE in the O2-saturated 0.1M KOH solution at a scan rate of 10 mV/s and the rotating speed of 1600 rpm. D, E, Reprinted from Xiao et al, Copyright 2017, with permission from Elsevier. F, The charge density difference of SMO@rGO, SMO@Pyrr-NrGO, and SMO@Pyri-NrGO. The accumulation and loss of charge are represented by yellow and blue regions (isosurface value: 0.02 e/Å), respectively. G, The half-wave potential and Tafel's slope of SMO, NrGO, SMO/NrGO-2, SMO@rGO-2, SMO@NrGO-2, and Pt/C. F, G, Reprinted with permission from Zhao et al, Copyright 2019, American Chemical Society. FESEM, field emission scanning electron microscope; LSV, linear sweep voltammetry; NP, nanoparticle; NrGO, nitrogen-doped reduced graphene oxide; TEM, transmission electron microscopy [Color figure can be viewed at wileyonlinelibrary.com]

Alloyed NPs can improve electrical conductivity and chemical stability as compared to their monometal counterparts. Most recently, Fe₂Ni₁ alloyed NPs anchored rGO sheets has been reported as efficient ORR catalysts. Nguyen et al fabricated Fe₂Ni₁ NPs/rGO using facile and cheap hydrothermal method followed by a calcination step. The Fe₂Ni₁ NPs are densely distributed on the rGO surface without aggregation, which was expected to enlarge the electroactive sites of catalysts and promote the interfacial charge transfer ability. This stabilization is attributed to the intimate interaction between NPs and rGO sheets because the detachment and reaggregation of NPs are prevented. The evaluation of the electrocatalytic activity of Fe₂Ni₁ NPs/rGO for ORR indicated an excellent onset potential (0.95 V) and half-wave potential (0.80 V). The enhancement of catalytic activity came from the improved active site numbers and lessened strain behavior of catalyst by the synergistic effects from the Fe₂Ni₁ NPs and rGO.

As interest in single-atom catalysts increases, so does interest in rGO, as a versatile platform to support them. Recently, Zhang et al synthesized atomically dispersed Ru single atom catalysts on rGO sheet for ORR in acidic medium. The Ru/rGO catalyst was prepared by facile annealing using GO containing trace amounts of Ru salts as the precursor under an NH₃ condition. The majority of Ru was observed as uniformly distributed with small size (~1 Å), except for a minority of Ru aggregated with sizes of 1 to 2 nm. The Ru/rGO catalyst exhibited excellent ORR catalytic activity, representing onset and half-wave potentials of 0.89 and 0.75 V vs RHE. In 0.1M HClO₄, which is more positive than that of Pt/C catalyst. From the NPs to the single-atom catalyst, the properties of rGO that can be combined with various types of catalysts seem to have more potential in the study of oxygen reduction reactions.

5.2 Transition metal oxides and sulfides/rGO-based electrocatalysts

Groups of transition metal compounds that have electrochemical activity for HER and OER are often effective in the ORR reaction in many cases. Transition metal oxides are frequently used in ORR reactions with the help of rGO despite the disadvantages of their low conductivity and deficient active sites. Sun et al synthesized the ultrafine Co-doped ZnO NPs on rGO sheets via a simple solution refluxing and annealing treatment. The morphology of the Co-ZnO NPs/rGO were characterized by SEM as shown in Figure D. The Co-doped ZnO NPs were tightly anchored on the rGO sheets with the sizes of 5 nm, which is expected to obtain excellent charge transfer. The hybrid catalysts exhibited good electrocatalytic activity for ORR, which is better than those of ZnO, CoO, rGO, ZnO@rGO, and CoO@rGO in O₂-saturated 0.1M KOH electrolyte, in Figure E. The excellent catalytic activity is ascribed to the hierarchical structure of porous Co-ZnO NPs, offering larger surface area and efficient utilization of catalytic active sites. Ternary ceramic oxides combined with rGO have been used as catalytic materials for ORR. The ternary ceramic oxides/rGO composite catalysts not only promote the interfacial electron transfer but also improve the ORR performance. Yu et al reported the highly efficient ORR catalytic activities with SmMn₂O₅@NrGO hybrid nanocomposites. Chemical interactions between Mn and N (inside the rGO) were investigated by charge density difference study as shown in Figure F. The charge density difference results revealed that the electron transfer from the rGO substrate to Mn activated the catalytically inert Mn⁴⁺ sites by accumulated electrons, and this phenomenon could be affected to the catalytic activity of the hybrid composites. In addition, electrons could be replenished through the rGO substrate when it consumed in the ORR, which helps the charge transfer process speed up. For this reason, the hybrid catalysts SmMn₂O₅@NrGO exhibited an excellent half-wave potential of 0.84 V, which is comparable to the Pt/C catalyst, as shown in Figure G. Also, CoMn₂O₄ NPs on NrGO sheets reported to exhibit higher ORR catalytic activity. There are abundant electroactive sites on uniformly dispersed CoMn₂O₄ NPs in sizes of 10 nm on rGO sheets via hydrothermal synthesis, such as Co3⁺/Co²⁺, Mn³⁺/Mn²⁺, Mn⁴⁺/Mn³⁺. These sites facilitate electron transport and improved electrochemical activity for ORR, resulting in low onset potential of 0.69 V and the current density 1.57 mA/cm² at 0.4 V.

Transition metal sulfides, the attractive materials especially for water splitting, are also effective to ORR when combined with rGO. Gautam et al reported that a FeS anchored rGO hybrid nanocomposites exhibited a remarkably improved catalytic performance for ORR. Through a facile hydrothermal technique, they uniformly synthesized FeS NPs on the whole surface of the rGO sheet with the size of 5 to 10 nm. TEM images revealed that the highly crystalline FeS NPs were synthesized, which is attributed to the role of rGO sheet for preventing the aggregation of the FeS NPs. The catalytic activities of the FeS/rGO nanocomposites for ORR were measured in O₂-saturated 0.1M KOH solution, representing superb electrocatalytic performance with the half-wave potential of 0.845 V and the onset potential of 1.0 V. The other metal sulfides, HfS₂ NSs were also reported to have the outstanding catalytic activities for ORR in interaction with the rGO sheets. Meganathan et al synthesized HfS₂/rGO NS structures, where the HfS₂ NS was closely packed with rGO. The fabricated HfS₂/rGO NSs are electrochemically tested for the ORR in 0.1M KOH. The HfS₂/rGO electrocatalysts initiated its oxygen reduction reaction at 0.97 V, and the maximum current density was 0.612 mA/cm². They demonstrated that the synergistic effect between the catalytic NSs and rGO sheets also can improve the catalytic activity for ORR as like anchoring the catalytic NPs on the rGO sheets.

6 CONCLUSION

We have reviewed the latest progress in the field of rGO-based electrocatalysts for various energy conversion reactions, such as water splitting (HER and OER), CO₂ reduction (CO₂RR), N₂ reduction (NRR), and O₂ reduction (ORR). The academic interest in these research fields has surged in recent years. From the results of substantial research, rGO have demonstrated widespread practical applications, facilitating its scalability, native defect sites, and superior conductivity. However, there are still several challenges to be improved. Most studies simply suggest that the origin of enhanced catalytic activity for rGO-based electrocatalysts is attributed to the good dispersion of electroactive materials and the improved conductivity by the aid of rGO. There are still few in-depth investigations for reaction mechanisms of rGO-based electrocatalysts. More fundamental characterization should be given to identify the explicit catalytic mechanism and unravel the synergistic effect of rGO-based hybrid materials. Combined computational and in situ spectroscopic study can be powerful tools to interpret phenomenal results of hybrid materials. In addition, this precise mechanistic study of rGO-based hybrid electrocatalysts could provide key insight to design and develop rGO-based electrocatalyst with novel structure and excellent catalytic activities for various energy conversion reaction.

Furthermore, the stability of the rGO-based electrocatalyst is one of the important issues for the practical applications. Most of the rGO-based electrocatalyst, reported to date, are in powdery form with a few tens of operation stability, far below the industrial requirement. One drawback of powdery electrocatalyst is that it must be casted on conductive electrodes using binding polymers, which unavoidably increase the resistance and screen the electroactive sites. Also, casted electrocatalyst could be readily peeled off during robust reaction or long-term operations due to the poor adhesion between powdery electrocatalyst and substrate. Therefore, there is a need to develop an electrode in which the catalyst grows directly on the substrate or a free-standing catalyst on its own. In this way, adhesion of rGO-based electrocatalyst and substrates can be stronger without binding polymer. The intimate contact guarantees enhanced reaction kinetics and long-term stability of rGO-based electrocatalyst. Further progress in these fascinating field will certainly advance the rGO-based technology to industrial reality, which would revolutionize the current energy-related issues in a sustainable and renewable way.

ACKNOWLEDGEMENTS

This study was supported by Korea Hydro & Nuclear Power Co., Ltd. (No.: 2018-Tech-21) as well as the National Research Foundation of Korea (NRF) grant funded by the Korea government MSIT (2019M3E6A1064763).