基于金属氧化物材料的二氧化碳电催化还原

郝磊端，孙振宇

有机-无机复合材料国家重点实验室，北京化工大学化学工程学院，北京 100029

1 Introduction

Although the development of renewable energy has achieved significant progress in recent years, the largest portion of energy supply still relies on the combustion of fossil fuels, which causes rising CO2emission year by year1. The increasing CO2level in the atmosphere leads to global warming and a series of environmental problems. Electricity generated from solar, wind and water is a main form of renewable energy. However, storage and transportation of the renewable electricity remains a big challenge2. Making use of CO2as carbon source to produce value-added chemicals through electrochemical CO2reduction(ECR) is a promising strategy to mitigate the green-house effect and meanwhile, harness and store the renewable electricity3.Therefore, ECR has raised extensive research interests.

Various products including C1(e.g., CO, methane, formate,methanol), C2and C2+(e.g., ethylene, ethane, ethanol, propanol)products can be formed from ECR4–7. The main issues of ECR lie in how to lower the overpotential and improve the product selectivity. To remedy the challenges here, significant efforts have been devoted to the development of electrocatalysts,aiming to suppress the side reactions like hydrogen evolution reaction (HER) and improve the durability and efficiency (low potential, high current density, high Faradaic Efficiency (FE)) of the catalytic materials. Numerous electrocatalysts such as metals(e.g., Cu, Au, Ag, Sn, Bi)8–14, metal complexes (e.g., copper(II)phthalocyanine)15,16, carbon-based materials (e.g., graphene,carbon nanotubes)17–19have been applied to ECR. Despite the progresses that have been made, it is still highly desirable yet challenging to develop stable and efficient electrocatalysts towards practical electroreduction of CO2.

Using conductive metal oxides as electrocatalysts, metal oxides have been proved to be active for CO2electroreduction since 199020. It was also predicted from theoretical point of view that transition metal oxides are able to catalyze the electroreduction of CO2with high selectivity under low overpotentials, which provides a new way to produce liquid fuels from CO221. In addition, the special effect of metal oxides in improving the efficiency of ECR was revealed by a lot of recent reports22–25. With the presence of metal oxide species, the efficiency and selectivity of CO2electroreduction can be improved and adjusted compared with the corresponding pure metal electrocatalysts. Therefore, metal oxide-based materials as electrocatalysts for ECR have attracted more and more attention in recent years. Herein, we provide a review about recent progress in ECR using metal oxide-based materials as electrocatalysts. This review focuses on reports that involve metal oxides as catalytic active species for ECR, distinguishing from those only use metal oxides as precursors for preparing metallic electrocatalysts, aiming to gain insights about a) the role of metal oxides in enhancing the performance of ECR, b) design of stable metal oxide electrocatalysts, and c) the relationship between the structure and performance of metal oxide-based materials.

2 Mechanistic understanding of ECR

To date, multiple products including C1, C2, and C2+have been obtained from ECR. Designing suitable electrocatalysts for ECR is critical to achieve high efficiency and selectivity, which currently limits the practical application of ECR. Although the mechanism for different reaction pathways are not fully unraveled, it is widely accepted that the intermediate species forming from CO2activation and the subsequent proton-electron transfer would determine which product can be generated from ECR26. It has been reported that the surface roughness and presence of defects on the electrocatalysts are preferred for activation of CO2and formation of *CO2•−(* being a surface adsorbed species)27. In the subsequent steps, different intermediates can be formed, thus leading to different products.*COOH from the protonation of *CO2•−can yield CO28. The formation of *OCHO is key for the generation of formate while the formation of *CO is critical for further transformation to different products29. Methane is commonly formed from the hydrogenation of *CO to *CHO species and its subsequent proton-electron transfer30. *CO can also dimerize to *COCO,which is the most accepted rout for C―C bond formation to yield C2and C2+hydrocarbons and oxygenates products4,31,32.Electrocatalysts are critical in these steps as well-designed electrocatalysts are beneficial for stabilizing the intermediates and lowering the energy barrier to reach high energy efficiency,and the selectivity of the reaction can also be tuned through the different binding strengths of intermediates on the catalyst surface28. For example, if an electrocatalyst has a strong Hbinding, it will result in the competing HER33. Moreover,through designing the morphology and electronic characteristics of the catalysts, the local pH on the catalyst surface can be affected, which may result in high coverage of *CO for further C―C coupling to yield C2and C2+products34.

Therefore, through tuning the structure of the electrocatalysts,the binding of these intermediates to the catalyst surfaces can be adjusted. In the following parts, design of metal oxide-based materials using different strategies, including nanostructure engineering, metal doping, synergistic effect and so on, will be discussed, which highlights the effect of material structure on the activation of CO2and the reaction intermediates, thus performing different efficiency and product selectivity for ECR.

3 Metal oxide-based materials for ECR

The activity of metal oxides for ECR has been revealed using stable and conductive metal oxides. In recent years,with the development of instrumentation, especiallyin situcharacterization techniques, the role of metal oxide species in enhancing the performance of ECR has been recognized during the study of metallic electrocatalysts. More and more electrocatalysts involving metal oxides as catalytic active species have been designed and applied in ECR. The following parts will discuss some representative work based on different metal oxides.

3.1 Copper oxide-based materials for ECR

3.1.1 Copper oxides

Among different metals, copper and copper-based catalysts are the most promising and efficient materials for ECR,especially for yielding hydrocarbon products with multi carbons28.When bulk Cu was used in the electrocatalytic reduction of CO2,it requires high overpotential and causes side reactions such as HER35. To get higher ECR efficiency and selectivity towards the desired products, researchers have made tremendous efforts in fabrication and modification of Cu-based materials, including nanostructured engineering, surface modification, doping and so on36–38. It was found that Cu catalyst derived from electrochemical reduction of the pre-oxidized thick Cu2O layers possesses higher CO2reduction activity than polycrystalline Cu with a larger current density and lower overpotential39. This arises wide research interests in oxide-derived Cu catalysts and the mechanism behind its high ECR efficiency. Some reports suggested that subsurface oxygen in the oxide-derived Cu plays an important role in improving the catalytic performance of ECR through benefiting the CO2adsorption on catalyst surface and stabilizing the *CO intermediate for subsequent coupling40,41.While experimental and theoretical results from some researchers demonstrated that subsurface oxygen is not stable under the electrocatalytic reaction conditions with reduction potential, and it has very limited effect on the activity of the oxide-derived Cu catalysts42,43.

To gain direct evidence about the reason that oxide-derived Cu catalyst performed better than polycrystalline Cu, Cuenyaet al.designed Cu-based samples through plasma treatment22.Using O2plasma, Cu materials with copper oxide layers on the surface was prepared and used in ECR. With the optimal O2plasma treatment condition of 20 W for 2 min, the sample exhibited a remarkable performance for ethylene selectivity. At a potential of −0.9 Vversusreversible hydrogen electrode(RHE), 60% ethylene selectivity was obtained. It was previously reported that the morphology, for example roughness of the material, will benefit the stabilization of reaction intermediates39.This is also one of the reasons to explain why oxide-derived Cu catalysts have better performance. However, after further treatment of the O2plasma treated sample with H2plasma to reduce the oxide species, the roughness remained the same while the activity decreased about two times, which indicated the importance of the oxide layer for electrocatalytic activity.Combining a series of characterization techniques, includingin situX-ray absorption spectroscopy (XAS) and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS), it was proved that Cu2O exists in the O2plasma treated material during the electrocatalytic reaction and Cu+species is necessary for yielding ethylene from ECR with high efficiency. Through adjusting the plasma treatment conditions, which changed the surface roughness and species, the product selectivity can be tuned between methane and ethylene (Fig. 1).

Fig. 1 Relationship of hydrocarbon selectivity and plasma-treated Cu-foils. The insets show scanning electron microscopy (SEM) images of Cu foils with different plasma-treated conditions (500 nm scale bars).Adapted from Ref. 22. Copyright © 2016, Springer Nature.

The above work provides direct evidence that Cu+species from the oxide-derived catalyst can resist the electrocatalysis process and play a critical role in the selective reduction of CO2.To get more insights about the function of Cu2O on electrocatalytic reduction of CO2, theoretical calculations were conducted by Goddardet al.44. Using quantum mechanics calculations, they found that the high efficiency of ethylene selectivity was attributed to the synergistic effect of Cu+and Cu0.This was further supported by experimental results from Wuet al.45. Cu catalysts with different oxidation states (Cu(0) from asprepared Cu, Cu(I) from electrodeposited Cu, mixture of Cu(0)and Cu(I) from cyclic voltammetry (CV)-treated Cu) were prepared.In situsurface-enhanced infrared absorption spectroscopy (SEIRAS) was used to study the adsorbed CO intermediates on different catalysts. The results displayed that atop-adsorbed CO (COatop) and bridge-adsorbed CO (CObridge)were detected as the dominated intermediates on Cu(I) and Cu(0), respectively. While for the material with a mixture of Cu(0) and Cu(I), both COatopand CObridgewere observed.Different intermediates lead to different CO2reduction products distribution on Cu materials with different oxidation states, as shown in Fig. 2a. For ethylene production, the existence of both Cu(I) and Cu(0) is necessary to generate COatopand CObridgeintermediates (Fig. 2b), which favored CO dimerization to yield ethylene product over methane.

Similar study was carried out by Chuanget al.to investigate the effect of Cu oxidation states on CO2electrocatalytic reduction46. They found that cupric-like oxides or Cu2+will be deactivated due to the formation of a copper carbonate layer, thus showing no activity for the reduction of CO2. Partially reduced copper catalysts or cuprous oxides can activate CO2through a dissociative way of adsorption and generate *CO intermediate to further produce hydrocarbon products (Fig. 3).

Fig. 2 a) FEs of the major products over different Cu catalysts. b)Schematic illustration of the electrochemical CO2 reduction on catalyst surfaces with different Cu species.

Fig. 3 Schematic illustration of the catalyst activation(formation of CO)-deactivation (formation of CO3) on Cu with different oxidation states.

Although Cu+is critical in improving the efficiency and C2+hydrocarbon selectivity for ECR, it can be reduced to metallic copper under the cathodic potentials. To protect the cuprous copper from total reduction, Yuet al. developed Cu2O catalyst with nanocavities47. Using hydrochloric acid as corrosion reagent for the solid Cu2O nanoparticles, multihollow Cu2O containing nanocavities with uniform distribution of copper and oxygen was prepared (Fig. 4a–d). This material was used as electrocatalyst for ECR and showed an FE of C2+products as high as 75.2% at potential of −0.61 VversusRHE with a C2+-to-C1ratio of 7.2, which is several times higher than that of the solid Cu2O catalyst. In addition, the structure of the material remains stable after electrocatalysis and the multihollow Cu2O catalyst can produce multicarbon products from CO2reduction with steady FE for three hours. Finite-element method simulations revealed the confinement effect of the nanocavities for the carbon intermediates in CO2reduction. Operando Raman spectroscopy further proved that *CO intermediate was confined in the multihollow structure, and it can interact with the Cu+sites in Cu2O, thus protecting the Cu+from reduction. The confinement effect also benefits the subsequent C―C coupling,which brings high selectivity for C2+products (Schematic show in Fig. 4e).

Fig. 4 a) SEM image, b) Transmission electron microscopy (TEM)image and selected-area electron diffraction (SAED) analysis (inset),c) High-resolution transmission electron microscopy (HRTEM) image and d) Scanning TEM-EDS elemental mapping of the multihollow Cu2O. e) Schematic illustration of the confinement effect.

Through careful design of the electrocatalyst structure, the copper oxide active species can remain stable during the CO2electrocatalytic reduction reaction, assisting the production of multicarbon products from CO2reduction. Hanet al. designed Cu-Cu2O/Cu electrode usingin situelectrodeposition and electroreduction method48. As shown in Fig. 5, copper complex was deposited on the Cu electrode throughin situgrowth,followed by electroreduction to form the Cu-Cu2O/Cu electrode.Thisin situelectrode preparation method makes compact contact between the electrocatalyst and Cu substrate, which can reduce the charge transfer resistance and overpotential. The obtained 3D dendritic structure also favors the formation of multicarbon products. As a result, the overpotential for acetic acid and ethanol was 0.53 V and 0.48 V, respectively. Moreover, with the synergistic effect of Cu/Cu2O, the FE for C2product was as high as 80.7% with a current density of 11.5 mA·cm−2at −0.4 VversusRHE.

Fig. 5 a) Cu-complex growth on Cu substrate. b) Reduction of the Cu-complex and formation of Cu-Cu2O/Cu.

In a more recent work, oxygen-bearing copper catalyst with a formula of Cu4O was prepared by Yuet al.49. This material contains hierarchical pores in its structure and exhibits high selectivity for ethylene production from ECR. Compared with the oxygen free copper catalyst counterpart, Cu4O had a more positive onset potential of −0.54 V and a 26 times larger current density for ethylene formation. The structure of Cu4O can be retained after four hours of reaction.In situX-ray absorption fine structure (XAFS) characterization proved the presence of Cu-O coordination, which promoted the *CO intermediate adsorption on catalyst surface and its further dimerization to ethylene product.

3.1.2 Copper oxide-based composites

Metal-organic frameworks (MOFs), as a category of highly crystalline porous materials, have been applied in various fields.The porous structure with high CO2adsorption ability makes MOFs attractive materials for ECR. However, the low conductivity of MOFs and the stability issue cause drawbacks for using MOFs as electrocatalysts. In a recent report by Qiuet al., the advantages of MOFs were fully taken through Cu2O@MOF composite, which also made full use of the Cu2O species50. As shown in Fig. 6, Cu2O was partially oxidized to Cu2+and dissolved into the solution containing 1,3,5-benzenetricarboxylic acid (H3BTC) ligand. Within situetching and reconstruction, Cu2O was converted to Cu2O@CuMOF. The composite was used in electrocatalytic reduction of CO2. At a potential of −1.71 VversusRHE, Cu2O@CuMOF gave a hydrocarbon FE of 79.4% with a CH4/C2H4ratio of 3.89. As comparison, the pure CuMOF or Cu2O performed much lower activity and selectivity for hydrocarbon products due to the HER side reaction. The performance of the Cu2O@CuMOF composite was attributed to the synergistic effect of Cu2O and CuMOF, in which the CuMOF can promote the adsorption of CO2to increase its concentration and suppress the HER, while the Cu2O encapsulated inside of the CuMOF increases the conductivity of MOF for charge transfer to yield the hydrocarbon products.

Fig. 6 Schematic illustration of Cu2O@CuMOF preparation.

Besides MOFs, Cu2O was impregnated into multi wall carbon nanotubes (MWCNTs) to fabricate the Cu2O-MWCNTs composite51. Cu2O-MWCNTs with different amounts of Cu2O from 10% to 50% were used as electrocatalysts for CO2electroreduction. The study demonstrated that Cu2O-MWCNTs performed better catalytic activity than pure MWCNTs and with the optimal amount of Cu2O (30% (w)), the Cu2O-MWCNTs composite as electrocatalyst can convert CO2to methanol with an FE of 38% at a potential of −0.8 VversusAg/AgCl. The impregnation of Cu2O increased the active surface area of MWCNTs while MWCNTs provided more reaction sites and benefited the electron transfer. The cofunction of two components in the Cu2O-MWCNTs composites led to better performance for ECR and enhanced stability of the catalyst.

It was reported that the metal-oxide interface can enhance the electroreduction reaction of CO2by activate CO2and facilitate its further adsorption on the catalyst through the synergy between the metal and the oxide52–54. In the work of Leeet al.,Ag was incorporated into Cu2O55. After introducing Ag, the HER was suppressed with improvement of ethanol selectivity compared with the catalyst without Ag doping. The authors found that the Ag-Cu biphasic boundary plays a key role through comparing the two doping patterns, namely the phase-separated Ag-Cu2OPSand phase-blended Ag-Cu2OPB. Ag as electrocatalyst was known to favor the production of CO. In the CO-insertion mechanism, the CO needs to be inserted between the Cu and the intermediate (CH2*) on the Cu surface in order to form ethanol instead of ethylene. The Ag-Cu distance in the Ag-Cu2OPBwas more favorable for this insertion step, thus Ag-Cu2OPBshows higher ethanol selectivity than Ag-Cu2OPS. This work provides guidance on designing composite catalyst with a second metal component for electrocatalytic reduction of CO2.

3.1.3 Copper oxide-based hybrids with other metal oxides

Copper catalysts have shown great potential in production of value-added hydrocarbons from electroreduction of CO2.However, it is still challenging to achieve high selectivity for certain product with high activity and low overpotentials.Designing bimetallic catalysts with mixed-metal structures has been proved to be an effective approach to improve ECR performance of Cu-based catalysts56,57. For example, doping different amount of Zn to Cu was able to enhance the selectivity of ethanol over ethylene58. Besides, copper alloys such as Cu-Au, Cu-Sn, Cu-In and Cu-Pt were reported to show superior activity or selectivity in comparison to that of each single metal component59–63. Despite these progresses made on mixed-metal catalysts, there is fewer study about metal oxide mixtures,mainly due to their structural evolution and stability issue during the electrocatalysis procedure.

Fig. 7 a) Schematic illustration of CuxNiyOC preparation.b) SEM image of Cu3Ni-imidazole complex; c) SEM, d, e) TEM and f) HR-TEM images of Cu3NiOCs. g) Elemental mappings of Cu3NiOCs.

Hanet al. prepared Cu/Ni oxide composites (CuxNiyOCs)from the corresponding mixed-metal complex (Fig. 7a)64. TEM and SEM images showed that the metal oxide composite contains porous structure with similar morphology of the pristine complex, and the two metal oxides distribute uniformly in the material (Fig. 7b–g). Composites with different atomic ratios of Cu and Ni were fabricated and tested for CO2electroreduction.The experimental results demonstrated that CuxNiyOCs possessed higher catalytic efficiency than CuO or NiO. With an optimum ratio of 3 to 1, Cu3NiOC exhibited the best performance for formate production from reduction of CO2,which had a current density of 10.9 mA·cm−2. The FE of formate achieved 95.9% at a potential of –0.57 VversusRHE with an overpotential of 0.37 V. The structure of the material remained stable after the electroreduction reaction and the performance for formate production can keep steady for 25 h. Cu3NiOC was also suggested to be the best combination compared with Cu3FeOC and Cu3CoOC. The authors attribute the excellent performance of Cu3NiOC to the synergistic effect of the two oxides, as well as its porous structure, which provides high adsorption capacity of CO2, more active sites and large specific surface area for ECR.

Perovskite materials, with unique and adjustable electronic structures, have shown promising applications in photoelectric field65. Perovskite oxides have also been applied to electroreduction of CO2to CO, although high temperature was needed66. Xiaet al. reported the use of La2CuO4as electrocatalyst for CO2reduction under ambient conditions67.Evolution investigation for the surface structure of La2CuO4before and after electrocatalysis suggested the electronic change of the material. X-ray photoelectron spectroscopy (XPS) results revealed the difference of oxidation state for Cu and O after electrocatalysis (Fig. 8a,b), indicating partial reduction of the copper species in La2CuO4and the formation of metallic Cu.This structural change led to enhanced CO2adsorption and charge transfer compared with the original La2CuO4(Fig. 8c,d).As a result, this material displayed better performance than the corresponding CuO or H2reduced La2CuO4for electroreduction of CO2to methane, indicating the importance of thein situgenerated Cu/La2CuO4. Further theoretical study illustrated that the interface between thein situgenerated Cu and the La2CuO4changes the electronic structure of the material and brings the improved methane production from CO2electroreduction.

Fig. 8 a) Cu 2p and b) O 1s XPS spectra. c) CO2 adsorption isotherms. d) Electrochemical impedance spectroscopy (EIS) curves of the initial and cathodized La2CuO4 catalysts in the flow cell at a potential of −0.4 V vs. RHE in 1.0 mol·L−1 KOH.

3.1.4 Effect of copper oxide-based materials’morphology

For metallic copper catalysts, researchers have designed materials with various morphologies, including nanowires,nanocubes and dendritic structures, for application in ECR to adjust their performance in reaction rate and product selectivity36,38,68–70. Morphologies of the copper catalysts play important roles in their electrocatalytic performance for CO2reduction. The effect of catalyst morphology was also revealed in copper oxide materials. As above mentioned, the nanocavities in the material can help to reserve the active Cu+species47, and the dendritic and porous structure can increase the specific surface area, thus enhancing the catalytic activity of the catalysts48.In a recent work of Songet al., it was suggested that the morphology may be more important than people thought, and morphology itself can be critical for the ECR performance71. In their study, branched and cubic copper oxides both containing Cu+as the active species were used as electrocatalysts for CO2reduction. The branched catalyst exhibited much higher efficiency than the cubic one for ethylene production due to the high surface area and local pH caused by its branched morphology. For future design of copper oxide electrocatalysts,besides the composition, special attention should be paid to the morphology of the materials.

3.2 Tin oxide-based materials for ECR

3.2.1 Tin oxides

Sn-based electrodes have attracted significant research interests since Sn is active for ECR to produce valuable products such as formic acid, and it is abundant on earth considering economic issues for practical use. Although researchers have gained awareness that the Sn surface with different treatments has effect on the activity of the electrode, the role of tin oxide on the activity of Sn electrode remains unclear for a long time72. In 2012, Kananet al. reported that SnOxtook part in the electrocatalytic reduction of CO2on Sn electrode73. They compared the ECR catalytic efficiency of untreated Sn electrode,which has a native layer of SnOx, and the acid-etched Sn electrode with Sn0on surface. The electrocatalysis result showed that the acid-etched Sn electrode can catalyze the HER with no CO2reduction activity whereas the untreated Sn electrode exhibited activity for CO and formate formation with a low current density. With this result, an Sn/SnOxthin film catalyst was designed by electrodeposition and used in CO2electroreduction. It turned out that Sn/SnOxreached the current density and FE several times higher than the untreated Sn electrode, thus suggesting the important role of SnOxfor CO2reduction in Sn electrode. In a later work of Leiet al., they found that after exposing the acid-etched Sn electrode in air for some time, the activity for CO2electroreduction can be restored, and the performance was better than the untreated Sn electrode because of the surface roughness from the etching treatment23.This further proved the indispensable function of the SnOxlayer.

Zhouet al. explored the dependence of CO2electroreduction selectivity and activity on the thickness of the SnOxlayer in Sn gas diffusion electrode74. As shown in Fig. 9, at −1.2 VversusAg/AgCl, the FE of CO and formate from CO2reduction, as well as H2from the HRE, all varied with the thickness of the oxide layer. With an SnOxthickness of 3.5 nm, formate had the highest FE of 64%, while CO reached its largest FE when the thickness was 7.0 nm. With increasing thickness of the SnOxlayer, the HER was enhanced due to the formation of Sn clusters from the reduction of SnOx. Guanet al. further studied the effect of Sn species with different oxidation states on CO2electroreduction75.Using different methods, they prepared electrocatalysts with different distributions of surface Sn species. Among them, the electrocatalyst obtained from unipolar pulse electrodeposition(UPED) method displayed the best performance for formate production from ECR. Density functional theory (DFT)calculation results suggested that each Sn species (Sn2+, Sn4+and the Sn/SnOx) has a relation to the electrocatalysis performance related to lowering the overpotential, improving the selectivity and suppressing the HER, and a proper ratio of these species is important for the performance of the electrocatalyst.

With awareness of the important role of tin oxides for CO2electroreduction, researchers sought to gain insights in the mechanism behind it76. In this process of study, the role of hydroxyl groups on the electrode surface was discovered. Geet al. carried out DFT calculations using SnO monolayer as a model for SnOx77. It was found that water molecules can dissociate on SnO to form hydroxyl groups. The hydroxylated surface will facilitate CO2insertion into the OH* to form a bicarbonate intermediate HCO3*, which can be reduced to HCOO* or COOH*, leading to the production of formate or CO,respectively. Gonget al. conducted experimental study using Sn branches catalyst with different amounts of surface hydroxyl groups78. Catalytic performance test showed that with suitable surface Sn-OH concentration (5.9 mmol·cm−2), the FE of formate and C1product reached the highest. When the hydroxyl concentration was too high, it would occupy the active sites leading to low efficiency. In their study combiningin situattenuated total reflection surface enhanced infrared adsorption spectroscopy (ATR-SEIRAS) and DFT calculations, the authors conclude that Sn-OH will assist CO2adsorption to form H2CO3instead of HCO3*. H2CO3will further be reduced to HCOO*,thus producing formate product.

Fig. 9 Relationship of FE for different products and the thickness of SnOx layer.

These experimental results and theoretical investigations about Sn-based materials provide guidance for future design of tin oxides electrocatalysts with enhanced performance for ECR application.

3.2.2 Tin oxide-based composites

It has been reported that metal oxide interfaces can play important role in electrocatalysis52. SnO2was used for Pd nanosheets to regulate the CO2electroreduction product from CO or formate to methanol through creating Pd-O-Sn interfaces79. In the efforts of improving the electrocatalytic efficiency of tin oxide catalysts, adding a second metal species to adjust its performance for CO2electroreduction has been investigated.

It is known that different metals tend to form different products for the electroreduction of CO2. For example, noble metals like Au and Pd as electrocatalysts generally reduce CO2to CO, while Sn is more favorable for formate production80–82.In the work of Sunet al., they discovered that the product from ECR over tin oxide can be tuned through the synergistic effect of a core/shell Cu/SnO2structure83. Three kinds of materials,namely Cu nanoparticles, core/shell Cu/SnO2nanoparticles with SnO2thickness of 1.8 nm and 0.8 nm were prepared and supported on carbon to obtain electrocatalyst C-Cu, C-Cu/SnO2-1.8 and C-Cu/SnO2-0.8, respectively. In electroreduction of CO2,the product over C-Cu was mainly ethylene with an FE of 6.3%at −1.1 VversusRHE with a small amount of ethane. For CCu/SnO2-1.8, the major product was formate, which was similar to that of the acid-treated Sn foil. However, on the C-Cu/SnO2-0.8 catalyst with a thinner layer of SnO2shell, the major product was supplanted by CO with an FE of 93% at −0.7 VversusRHE.The performance of Cu/SnO2-0.8 catalyst was comparable to noble metal catalysts for CO2reduction to CO. DFT calculations demonstrated that it is energetically favorable for formation of formate on the 1.8 nm SnO2shell. When the shell thickness is 0.8 nm, the Cu and SnO2interacted to have synergistic effect,thus prefer the generation of CO. This work opens a new way to tune the ECR product selectivity through structure design of electrocatalysts.

In a later work of Wanget al., a comprehensives investigation was conducted to study the effect of different copper and tin oxide compositions on the product of CO2electroreduction84.Carbon nanotubes (CNT) were used as a support to anchor CuOy/SnOx. Sn atomic ratio varied from 0.01% to 30.2%. As shown in Fig. 10, with the increase of SnOx, the CO2reduction products were tuned from hydrocarbons to CO with the highest FE for CO achieved at an SnOxratio of 6.2%. When further increasing the ratio of SnOx, the major product was replaced by formate, and the highest FE for formate was obtained with an SnOxratio of 30.2%. The result in this work was in accordance with that of Sunet al. It was noteworthy that the catalyst with 30.2% SnOxdisplayed higher FE for formate than the pure SnOx,indicating the synergistic effect of Cu and SnOx, which benefits the production of formate.

Kanget al. studied lead tin oxides for oxalate production from CO2electroreduction85. Pb and Sn mixed oxides with different atomic ratios were prepared using carbon black as a support(Pb3Sn1Ox/C, Pb1Sn3Ox/C and PbSnO3/C). Electrocatalysis of CO2was carried out in propylene carbonate solvent with 0.1 mol·L−1tetrabutylammonium hexa-fluorophosphate, and the catalyst with varied Pb and Sn ratios performed different activities for oxalate formation. PbSnO3/C exhibited superiority over catalysts with other ratios and the PbO/C from aspects of onset potential, current density and FE for oxalate as well as product selectivity. At −1.9 VversusAg/Ag+, PbSnO3/C shows an FE of 85.1% and a current density of 2.0 mA·cm−2for oxalate production from CO2reduction. Further exploration on the material structure of PbSnO3/C demonstrated that with the optimum Pb/Sn ratio, Pb atoms were incorporated into the SnOxlattice and Pb oxide in the PbSnO3/C was partly reduced to metallic Pb during the electrocatalysis. This metal-oxide interaction could act as active sites for stabilization of the reaction intermediate and facilitating the formation of oxalate(Fig. 11a). In comparison to PbO/C, it cannot stabilize the intermediate effectively, thus larger overpotential was needed(Fig. 11b).

Fig. 10 Correlations between material structures and product distributions.

Fig. 11 Proposed mechanism for CO2 reduction over a) PbSnO3 and b) PbO.

Besides the above-mentioned metal-oxide interactions,Xiaoet al. designed a heterojunction electrocatalyst of Zn2SnO4/SnO2to make use of the heterojunction for improved electrocatalytic performance of the material86. The heterojunction electrocatalyst was obtained from annealing of the ZnSn(OH)6precursor. As shown in the SEM images in Fig.12a,b, Zn2SnO4/SnO2remained the microcube shape of its precursor, while possessing more cavities on the corner and edge sites (Fig. 12c). From the TEM images and EDS elemental mapping, it can be seen the uniform distribution of each element with close contact between the SnO2and Zn2SnO4planes (Fig.12d–f). This catalyst structure was proved to have excellent performance in the electrocatalysis test. At −1.08 VversusRHE,Zn2SnO4/SnO2could produce formate from CO2reduction with an FE of 77% and remains stable for 24 h. The formate current density of Zn2SnO4/SnO2was several times higher than that of single component Zn2SnO4or SnO2, although Zn2SnO4/SnO2had a lower surface area. Structural study further revealed that the heterojunction between Zn2SnO4and SnO2caused electron transfer in the material and the charge transfer during electrocatalysis was enhanced. Moreover, DFT calculations further suggested that the combination of Zn2SnO4and SnO2suppressed HER side reaction and improved the selectivity of formate production for CO2reduction.

3.2.3 Effect of tin oxide-based materials’ morphology

Fig. 12 a) SEM images of ZnSn(OH)6 precursor. b, c) SEM, d)TEM, e) HRTEM images and f) EDS mapping of Zn2SnO4/SnO2.

Fig. 13 a, b) HRTEM images and SAED pattern (inset in b) of the WIT SnO2 nanofibers. c, d) Scanning TEM images and line scan results (inset in d) of the WIT SnO2 nanofibers. e) Magnified TEM image of the WIT SnO2 nanofibers. FE for f) C1 products, g) HCOOH,h) CO and i) H2 over the WIT SnO2 electrode and the NP SnO2 electrode.

Liet al. prepared one-dimensional SnO2nanofiber using a electrospinning method87. The synthesized material has a wirein-tube (WIT) structure, as examined by TEM in Fig. 13a,b.SAED pattern exhibits the crystalline structure of the material.From the scanning TEM images and the line scan result, it can be seen the nanowire inside of the nanofiber, which contains nanoparticles and grain boundaries (Fig. 13c–e). This WIT structure brings the SnO2nanofiber with high BET surface area compared with the SnO2nanoparticles (NP). Fig. 13f–i demonstrates the performance of WIT SnO2and NP SnO2.Compared with NP SnO2, WIT SnO2as electrocatalyst performed higher FE of 63% for formate and over 90% for total C1product in a wide potential range while largely suppressed the HER. The excellent performance of WIT SnO2for production of C1product from CO2reduction was attributed to its nanoporous structure with small SnO2particle size and large amount of grain boundaries, which benefits for the activity of CO2reduction. In the later work of Louet al., the dimensions of SnO2particles were shortened to SnO2quantum dots (QDs), and the QDs were further grown and interconnected to form sub-2 nm SnO2quantum wires (QWs)88. The QWs were used as electrocatalysts in ECR and displayed a similar FE as the WIT SnO2nanofibers for C1product with a higher formate FE of over 80%. This work further illustrated the important role of the material morphology and grain boundaries for the performance of the electrocatalysts.

Zhanget al. fabricated 3D hierarchical structure from 2D SnO2nanosheets grown on carbon cloth (CC)89. SnO2/CC was obtained through a two-step process with the growth of SnS2on CC and subsequent calcination. The nanosheets morphology of SnS2was retained and a 3D structure of SnO2/CC with mesopores was formed with the CC support (Fig. 14). The performance of SnO2/CC as electrocatalyst was tested for electroreduction of CO2. The current density for formate reached 50 mA·cm−2at a potential of −1.6 VversusAg/AgCl. With a moderate overpotential of 0.88 V, the FE for formate was around 87%. The performance of SnO2/CC remain stable in a period of 24 h, indicating the stability of this material. Further characterization confirmed the unchanged mesoporous structure with partially reduce Sn after electrocatalysis. The nanosized SnO2nanosheets and the porous structure involved in the 3D structure SnO2/CC, which generates mor active sites with easy accessibility as well as improved mass and charge transfer, are responsible for the high efficiency of formate production from CO2reduction.

Fig. 14 a, b) SEM images of SnS2 nanosheets on CC. c, d)SnO2 nanosheets on CC.

3.3 Cobalt oxide-based materials for ECR

Co is abundant on earth, economical and environmentally friendly, making it a good candidate as catalysts. As a result, Cobased catalysts in both metallic and oxide forms have been widely used in different conversions90–92. However, Co-based materials show poor performance for electrocatalytic reduction of CO293. Xieet al. explored the effect of surface structure and oxidation state on metal catalysts from atomic-scale through fabricating four-atom-thick layers of pure Co metal and the corresponding partially oxidized counterpart94. For electroreduction of CO2to formate, the thin layers of Co atoms exhibited much higher activity than the bulk Co, and the thin layers with partially oxidized Co displayed further enhanced activity. Therefore, designing the morphology and oxidation sate of the material can significantly change its catalytic activity.

In a later work, they synthesized ultrathin Co3O4layers with different thicknesses95. Bulk Co3O4and the Co3O4layers with thickness of 1.72 nm and 3.51 nm, respectively were used as electrocatalysts for CO2reduction. At −0.88 Vversusthe saturated calomel electrode (SCE), Co3O4layers with a thickness of 1.72 nm exhibited a current density that was 20 times higher than the bulk Co3O4, and the FE for formate production was much larger on the 1.72 nm layer (64.3%) than on the 3.51 nm layer (51.2%). Further study demonstrated that the thinner layer of Co3O4has higher electrochemically active surface area(ECSA), thus more catalytic active sites. Moreover, the linear sweep voltammetric (LSV) study suggested that the thinner layer also has higher intrinsic activity. From the CO2adsorption isotherm in Fig. 15a, it can be seen that the CO2adsorption capacity was enhanced with thinner layer of Co3O4. In addition,DFT calculations indicated that thin layer of Co3O4has better electronic conductivity due to its increased and dispersed charge density. These factors enabled different catalytic mechanisms of the thin layer Co3O4and the bulk Co3O4, as confirmed by the Tafel slope in Fig. 15b. The charge transfer on thin layer Co3O4was faster to reduce CO2to CO2•−, which was the ratedetermining step for bulk Co3O4with high overpotentials (Fig.15c).

3.4 Bismuth oxide-based materials for ECR

Bi-based materials as electrocatalysts are able to convert CO2to formate with high selectivity, especially with some nanostructured Bi-based catalysts, and nearly 100% selectivity for formate can be achieved96–100. Bi oxides are used as templates to construct metallic Bi with desired nanostructures since the direct preparation is challenging due to the instability of Bi101. The active sites for the Bi-based catalysts are assumed to be the metallic Bi under the reduced potential, although residue oxygen species is inevitable. Fewer reports are about the use of Bi oxide as electrocatalysts for ECR and its mechanism.

Fig. 15 a) CO2 adsorption isotherms. b) Tafel plots of formate for the Co3O4 atomic layers with different thicknesses.c) Schematic illustration of CO2 electroreduction to formate over the Co3O4 atomic-layers.

Xiaet al. explored the effect of Bi oxide on electroreduction of CO2through developing Bi2O3material with enhanced Bi-O structure102. Bi-based materials with different amounts of Bi2O3were obtained through controlling the oxidation time of metallic Bi in the air. With oxidation time of 5 h, Bi2O3-5 h possessed the highest amount of Bi3+species. The electrocatalytic performance of metallic Bi and Bi2O3with different oxidation times were tested. As shown in Fig. 16, Bi oxide exhibited better performance for CO2reduction and formate production than the metallic Bi (Fig. 16a,b), and the FE of formate increased with increasing the amount of Bi2O3in the material (Fig. 16c). Bi2O3-5 h showed the best efficiency for formate production with an FE of 91%. The current density of 8 mA·cm−2at −0.9 VversusRHE was stable over a period of 24 hours (Fig. 16d).In situRaman spectra demonstrated that Bi2O3was reduce to Bi under the cathodic potential, but Bi―O remained during the electrocatalytic reaction, which was also confirmed by XPS characterization. Further mechanism study and DFT calculations suggested that the Bi―O structure was beneficial for CO2adsorption and activation, and the rate-determining step was changing from CO2•−formation to the following hydrogenation step.

Fig. 17 a) Preparation of Bi@C and Bi2O3@C catalysts.b–d) TEM images of Bi@C-800. e–g) TEM images of Bi2O3@C-800.

Fig. 18 Schematic illustration for the preparation of In2O3@C.

In view of the effect of Bi oxide, in a later work, they fabricated Bi2O3derived from Bi-MOF material (Bi2O3@C)103.Bi2O3@C was prepared through carbonization of Bi-MOF under Ar to get Bi@C and subsequent oxidation of Bi@C in the air at 200 °C (Fig. 17a). From the TEM images, the generation of Bi nanoparticles from carbonization and their oxidation to Bi2O3nanoparticles encapsulated in carbon can be observed (Fig. 17e–g). With a higher carbonization temperature at 800 °C,Bi2O3@C-800 had a higher oxidation degree than Bi2O3@C-600 that was carbonized at 600 °C. This led to a better electrocatalytic performance of Bi2O3@C-800. At −0.9 VversusRHE, the current density for formate from CO2reduction was 7.5 and 1.8 mA·cm−2over Bi2O3@C-800 and Bi2O3@C-600,respectively. Although Bi@C-800 possessed higher ECSA than Bi2O3@C-800, its efficiency for formate production was lower(FE of 81%versus92%), indicating the function of Bi―O in Bi2O3@C-800. The role of carbon matrix was recognized through the better catalytic performance of Bi2O3@C-800 compared with bare Bi2O3, which suggests the synergistic effect between Bi2O3and the carbon matrix. This synergy between Bi2O3and the carbon-based matrix was also revealed by Liuet al. who used nitrogen-doped graphene quantum dots to make composite with Bi2O3104. The carbon-based matrix can benefit the charge transfer during the reaction. In addition, it enhanced the adsorption and activation of CO2as well as the stabilization of reaction intermediate through the interaction with Bi2O3.

3.5 Indium oxide-based materials for ECR

In-based electrocatalysts, including metallic indium catalysts,composites of indium and other metals (e.g., Cu, Sn, Mo), have shown good activity for CO2electroreduction105–108. Fewer reports are about the application of In oxide in ECR109. Bocarslyet al. compared the electrocatalytic activity of In, In2O3and In(OH)3nanoparticles with bulk In for CO2reduction110. Both In2O3and In(OH)3nanoparticles exhibited better performance than bulk In whereas lower efficiency for formate production than In nanoparticles due to the low conductivity and amorphous structure of In(OH)3. But it also indicated that the oxidized In species are favorable for enhanced electrocatalytic performance.In a later work, Liuet al. used carbon black to grow In2O3nanoparticlesin situ, as shown in Fig. 18111. With the optimum carbon black content of 11.6% (w, mass fraction), the obtained In2O3@C as catalyst showed the best activity for formate production from CO2electroreduction. The current density for formate over In2O3@C was 4.9 times higher than that over In2O3.The excellent performance of In2O3@C was attributed to the high ECSA and the positive effect of carbon black, which makes the electron transfer in the rate-determining step much easier as confirmed by the Tafel slope.

3.6 Zinc oxide-based materials for ECR

Zn catalysts are promising in catalyzing the electroreduction of CO2to produce CO. Considering its low cost compared with noble metals, Zn catalysts are widely studied for electroreduction of CO2, and high efficiency for CO production has been achieved through tuning the morphology and surface structure of Zn catalysts with different nanostructures112–114.However, the influence of oxidized Zn on electrocatalysis of CO2reduction was seldom explored. Hwanget al. prepared Zn catalysts with different pretreatment methods to obtain reduced Zn under different bubble atmospheres115. As shown in Fig. 19a,after electrodeposition of the Zn foil, E-ZnO was reduced with Ar gas bubble or CO2gas bubble in KHCO3or KCl aqueous solutions. XPS characterization of the materials found that the ratio of oxidized Zn is different according to different reducing methods, and it increased in the order of Zn foil, RE-Zn-Ar, REZn-CO2and RE-Zn-CO2/KCl, which was the same trend of the FE for CO production from CO2reduction over those materials(Fig. 19b). The results indicated the role of oxidized Zn in the performance of electrocatalytic CO2reduction. In addition, the presence of oxidized Zn can suppress the carbon deposition, thus protecting the catalysts from deactivation (Fig. 19c). This work provides a viewpoint about the effect of oxidized Zn species on electrocatalytic performance of the material through experimental results, while it lacksin situcharacterization results and theoretical support.

Fig. 19 a) Schematic illustration for Zn-based electrocatalyst preparation. b) FE for CO production vs area ratio of oxidized Zn.c) Carbon atomic percentage of all samples before and after electrocatalysis.

In a later work of Cuenyaet al., the size effect of Zn nanoparticles on CO2electroreduction was investigated116.Usingin situXAFS spectroscopy, they observed the presence of Zn(OH)2clusters in the Zn nanoparticles, and Zn(OH)2remained stable during the electrocatalysis process. However, it is still unclear about the specific effect of oxidized Zn species on the reaction mechanism of CO2electroreduction. Zenget al.introduced oxygen vacancies with different amounts into ZnO through H2plasma treatment of the ZnO nanosheets117. The performance for CO2electroreduction was compared between pristine ZnO nanosheets, oxygen vacancies-rich (VO-rich),and oxygen vacancies-poor (VO-poor) ZnO nanosheets.Experimental results demonstrated that VO-rich ZnO nanosheets exhibited the best performance for CO production, and a current density of 16.1 mA·cm−2with an FE of 83% was achieved at−1.1 VversusRHE. Mechanism study revealed that the presence of oxygen vacancies assisted the activation of CO2and the formation of *COOH intermediate with lower Gibbs free energy.

The above reports suggest that regulating the electron properties of Zn catalysts, through the oxidized Zn species or oxygen vacancies, are effective ways to tune the activity of the Zn catalysts. However, further mechanism study is needed to clarify the role of Zn oxide in ECR.

3.7 Other metal oxide-based materials (ZrO2, Ga2O3,RuO2) for ECR

ZrO2was reported to be effective for CO2adsorption, makingit promising to act as electrocatalyst for CO2reduction118. Due to the low intrinsic electron-conductivity of ZrO2, it is barely studied for application in CO2electroreduction. Liet al. used Ndoped carbon as support to improve the electron-conductivity of the ZrO2composite and prepared ZrO2/N-C as electrocatalyst for CO2reduction119. ZrO2/N-C was obtained through pyrolysis of the precursors with sodium alginate as chelating reagent to stabilize the ZrO2nanoparticles. The morphology of the composite was influenced by the amount of sodium alginate and pyrolysis temperature. With two molar ratios of sodium alginate to Zr and pyrolysis temperature of 800 °C, the as-prepared ZrO2/N-C-2-800 displayed porous structure of the N-doped carbon with ZrO2nanoparticles distributed uniformly on it. For CO2reduction to CO, ZrO2/N-C-2-800 exhibited an FE of 64%with a current density of 2.6 mA·cm−2at −0.4 VversusRHE,which outperformed the material with other ratios of sodium alginate or different pyrolysis temperature as well as N-C-800 and ZrO2-800. The better performance of ZrO2/N-C-2-800 was attributed to its porous structure with small size of ZrO2particles as active sites, and their synergistic effect with the N-doped carbon to afford enhanced electron-conductivity. Despite the activity of ZrO2composite shown in this work, the current density for CO production is relatively low compared with other metals or metal oxides as electrocatalysts. Further exploration on ZrO2for its application in CO2electroreduction is still desirable yet challenging.

Table 1 Summary of representative metal oxide-based materials for ECR.

Gallium oxide is another metal oxide that has been studied for electrocatalytic reduction of CO2, considering that Ga has a similar outer shell electronic structure as other metals such as Sn,which was proved to be active for ECR. Sekimotoet al. prepared conductive single crystal Ga2O3and explored its catalytic activity for CO2electroreduction120. Although a small amount of Sn or Si exists as dopant in the structure of Ga2O3, the authors suggested that the Ga2O3itself was active for formate production from CO2reduction, and an FE over 80% was obtained at −2.0 VversusAg/AgCl. This work revealed the activity of Ga2O3as electrocatalyst for CO2reduction to formate, which is different from common product of CO formation over Ga2O3through other catalytic methods121,122, while the efficiency was low.

Ruthenium oxide was active for production of formate and methanol in different electrolytes as reported by Fujishimaet al. using RuO2deposited on boron-doped diamond as electrocatalyst, although the FE was low (40% and 7.7%for formate and methanol, respectively)123. Nørskovet al.conducted DFT calculations to reveal the electroreduction of CO2on RuO2(110) from a mechanistic point of view124.Through theoretic study, their results suggested that adsorbed formic acid (HCOOH*) was formed from reduction of CO2, and further reduction of HCOOH* could yield methane and methanol. This is a different pathway compared with CO2reduction on Cu where HCOOH will not be reduced and CO* is the key intermediate for further generation of different hydrocarbon products. The authors indicated that through the HCOOH* reduction pathway, it is promising to lower the overpotential of producing methanol and methane using RuO2as electrocatalyst for CO2reduction.

The representative metal oxide-based materials as electrocatalysts for ECR and the corresponding reaction conditions and performances were summarized in Table 1.

4 Conclusions

Various metal oxide-based materials are demonstrated to be active for electrocatalytic reduction of CO2. The value-added products from CO2electroreduction varied from C1to multi carbon products based on different metal oxides involved in the electrocatalysts. Copper oxide-based electrocatalysts can facilitate the C―C bonds coupling to yield hydrocarbons with multi carbons, which are very promising to produce high energy density fuels from CO2. Other metal oxides tend to generate C1products (e.g., methane, formate, CO and methanol) from ECR.Through regulating the structure of metal oxides, including changing the size of the metal oxide nanoparticles, varying the surface morphology, or introducing other metal species, the adsorption and activation of reactants can be adjusted, thus suppressing the side reactions such as HER and tuning the electrocatalytic products to a specific one with high selectivity.It is even possible to convert a non-catalytic material to catalytic active by surface structure engineering to modulate the electronic property of the material.

To make the ECR economically favorable and practicable,it is critical to lower the overpotential and improve the FE and current density for CO2electroreduction. To achieve these goals,careful design of the electrocatalysts and electrodes is a prerequisite. The important role of the metal oxide species in ECR has been demonstrated. However, metal oxides incline to be reduced to metallic state during CO2electroreduction due to the more negative redox potential applied. In this scenario,synthesis of stable materials that are capable to reserve the catalytic active species is highly desirable. In addition,increasing the electrochemical active surface area through introducing defects or modulating the morphology (e.g., surface roughness, grain boundaries) of the metal oxides is beneficial for enhanced current density and catalytic efficiency. Considering the low intrinsic conductivity of metal oxides, minimizing the contact resistance between the metal oxides and the support or substrates is highly necessary to assist the electron transfer and lower the overpotential during CO2reduction. Preparing electrocatalysts and electrodes usingin situstrategies are preferred to get close contact between the metal oxides and the conductive substrates.

For metal oxides as electrocatalysts for CO2reduction, it is inevitable to reduce some of the oxidized species to metallic state during electrocatalytic reaction. On the other hand, the metallic state metals are easy to be oxidized when exposing in ambient atmosphere. As a result, it may cause bias if characterizations of the materials are not operando. There is also debate on the active species for CO2reduction when metal oxides as electrocatalysts are involved. Taking copper oxide as an example, many researchers accepted that the presence of Cu+species is critical to produce ethylene from CO2reduction over oxide-derived copper catalysts. While it is also reported that copper oxide residual is only a very limited fraction as indicated from the18O labeling testing, and the oxide-derived copper can undergo reoxidation rapidly125. Therefore,in situcharacterizations are indispensable for determining the content and specific role of metal oxides in CO2electroreduction. With the development of instrumental techniques,in situcharacterizations are widely applied in mechanism study of ECR. Taking advantages of both the experimental results from those advanced techniques and the theoretic study, the mechanism of ECR using metal oxides as electrocatalysts will be unraveled more and more clear, thus providing fundamental guidance for future development of efficient metal oxide electrocatalysts.