CO2 Reduction Reaction Selectivity Research Articles

(Invited) Defect Engineering and Surface Probing of Few-Layer MoS2 As Photocatalyst for CO2 Reduction to Solar Fuels

(Invited) Photocatalytic CO2 conversion to hydrocarbon fuels, which makes solar energy harvesting and CO2 reduction reaction (CO2RR) simultaneously, is a killing two birds with one stone approach to solving the energy and environmental problems. However, challenges are the low photon-to-fuel conversion efficiency of the photocatalysts and lack of the product selectivity. Recently, 2D-layered nanomaterials with defect engineering, e.g. the carbon interstitial-doped SnS2 nanosheets [Nature Comm. 9, 169 (2018)], have shown promise for CO2RR. Here, I will present a different case, MoS2 with vacancies controlled by plasma. Productivity and selectivity of CO2RR were found to be dependent strongly with the different Mo/S ratios of the MoS2 of single to few layers. Orders-of-magnitude enhancement in productivity and selectivity of C2 product over 97% has been obtained. The role and interplay of the defects and the hosting materials, and their effects on the adsorption of CO2 and subsequent CO2RR, studied by the near-ambient pressure X-ray photoemission spectroscopy, along with density functional theory will be discussed.

Metal-Ligand Cooperativity via Exchange Coupling Promotes Iron- Catalyzed Electrochemical CO2 Reduction at Low Overpotentials.

Biological and heterogeneous catalysts for the electrochemical CO2 reduction reaction (CO2RR) often exhibit a high degree of electronic delocalization that serves to minimize overpotential and maximize selectivity over the hydrogen evolution reaction (HER). Here, we report a molecular iron(II) system that captures this design concept in a homogeneous setting through the use of a redox non-innocent terpyridine-based pentapyridine ligand (tpyPY2Me). As a result of strong metal-ligand exchange coupling between the Fe(II) center and ligand, [Fe(tpyPY2Me)]2+ exhibits redox behavior at potentials 640 mV more positive than the isostructural [Zn(tpyPY2Me)]2+ analog containing the redox-inactive Zn(II) ion. This shift in redox potential is attributed to the requirement for both an open-shell metal ion and a redox non-innocent ligand. The metal-ligand cooperativity in [Fe(tpyPY2Me)]2+ drives the electrochemical reduction of CO2 to CO at low overpotentials with high selectivity for CO2RR (>90%) and turnover frequencies of 100 000 s-1 with no degradation over 20 h. The decrease in the thermodynamic barrier engendered by this coupling also enables homogeneous CO2 reduction catalysis in water without compromising selectivity or rates. Synthesis of the two-electron reduction product, [Fe(tpyPY2Me)]0, and characterization by X-ray crystallography, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), variable temperature NMR, and density functional theory (DFT) calculations, support assignment of an open-shell singlet electronic structure that maintains a formal Fe(II) oxidation state with a doubly reduced ligand system. This work provides a starting point for the design of systems that exploit metal-ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned through secondary metal-dependent interactions.

Boosting Activity and Selectivity of CO2 Electroreduction by Pre-Hydridizing Pd Nanocubes.

The electrochemical CO2 reduction reaction (CO2 RR) to syngas represents a promising solution to mitigate CO2 emissions and manufacture value-added chemicals. Palladium (Pd) has been identified as a potential candidate for syngas production via CO2 RR due to its transformation to Pd hydride under CO2 RR conditions, however, the pre-hydridized effect on the catalytic properties of Pd-based electrocatalysts has not been investigated. Herein, pre-hydridized Pd nanocubes (PdH0.40 ) supported on carbon black (PdH0.40 NCs/C) are directly prepared from a chemical reduction method. Compared with Pd nanocubes (Pd NCs/C), PdH0.40 NCs/C presented an enhanced CO2 RR performance due to its less cathodic phase transformation revealed by the in situ X-ray absorption spectroscopy. Density functional theory calculations revealed different binding energies of key reaction intermediates on PdH0.40 NCs/C and Pd NCs/C. Study of the size effect further suggests that NCs of smaller sizes show higher activity due to their more abundant active sites (edge and corner sites) for CO2 RR. The pre-hydridization and reduced NC size together lead to significantly improved activity and selectivity of CO2 RR.

Lead-free perovskite Cs2AgBiBr6@g-C3N4 Z-scheme system for improving CH4 production in photocatalytic CO2 reduction

Solar-energy-driven CO2 conversion into value-added chemical fuels holds great potential renewable energy generation. However, most of the photocatalysts facilitate a two-electron reduction process producing CO, hard for the eight-electron CH4 production pathways which can stockpile more solar energy for further utilization. Herein, we developed an in situ assembly strategy to fabricate the lead-free perovskite Cs2AgBiBr6@g-C3N4 Z-scheme system in toluene and the Cs2AgBiBr6@g-C3N4 type-II heterojunction structure in CH2Cl2. By combining the reducing ability of the conduction band of g-C3N4 and the oxidizing ability of the valence band of Cs2AgBiBr6 perovskite, this Z-scheme system exhibits superior CH4 production in photocatalytic CO2 reduction, in contrast to the high CO selectivity for the heterojunction photocatalysts, which is 10-fold and 16-fold higher than that of pure g-C3N4 and pure CABB, respectively. The stability (four consecutive photocatalytic cycles in solvent methanol as sacrificial reagent without obvious decrease of efficiency) and the mechanism (the prominent activity and the high CH4 production selectivity boosted by Z-scheme system) were demonstrated. In this work, the first report for constructing lead-free halide perovskite Z-scheme and type-II photocatalytic systems for the regulation products selectivity of CO2 reduction reaction provides a promising strategy for the fabrication other types of inorganic/organic heterojunction systems.

Single transition metal atoms on nitrogen-doped carbon for CO2 electrocatalytic reduction: CO production or further CO reduction?

Electrochemical CO2 reduction reaction (CO2RR) is an effectivestrategy for CO2 conversion and clean fuel generation. Diverse nitrogen-doped carbon-supported transition metal (M-N-C) have been proved as favorable catalysis materials for CO2RR, especially for CO production. Herein, density functional theory is performed to investigate the CO2RR mechanisms over M-Nx-C with twelve types of transition metal centers. Fe-, Co-, Ni-, Cu-, Rh-, Pd-, Ir-, and Pt-Nx embedded on graphene with double vacancies are relatively stable structures contributing to electrocatalysis operation. The adsorption energies of COOH and CO, which are the reaction intermediates during CO2RR, are linearly related and can be used as a descriptor for CO production. Moderate adsorption strengths of CO and COOH are favorablefor CO generation with low limiting potential. Co-N4, Ni-N1, Pd-N1, Pt-N1, and Rh-N4 are the most active sites for CO2RR by thermodynamics analysis. However, the discussion on competition between CO2RR and hydrogen evolution reaction (HER) indicates that these sites show low selectivity. Considering both CO2RR selectivity and activity, Ni-N4 site is proved to be the most effective site for CO production among all the M-Nx sites. It is also noted that Fe-N4 is more favorable to further reduction towards *CO with the products of CH3OH and CH4.

Unveiling hydrocerussite as an electrochemically stable active phase for efficient carbon dioxide electroreduction to formate

For most metal-containing CO2 reduction reaction (CO2RR) electrocatalysts, the unavoidable self-reduction to zero-valence metal will promote hydrogen evolution, hence lowering the CO2RR selectivity. Thus it is challenging to design a stable phase with resistance to electrochemical self-reduction as well as high CO2RR activity. Herein, we report a scenario to develop hydrocerussite as a stable and active electrocatalyst via in situ conversion of a complex precursor, tannin-lead(II) (TA-Pb) complex. A comprehensive characterization reveals the in situ transformation of TA-Pb to cerussite (PbCO3), and sequentially to hydrocerussite (Pb3(CO3)2(OH)2), which finally serves as a stable and active phase under CO2RR condition. Both experiments and theoretical calculations confirm the high activity and selectivity over hydrocerussite. This work not only offers a new approach of enhancing the selectivity in CO2RR by suppressing the self-reduction of electrode materials, but also provides a strategy for studying the reaction mechanism and active phases of electrocatalysts.

CO2 thermoreduction to methanol on the MoS2 supported single Co atom catalyst: A DFT study

Single-atom catalysts have attracted wide attention due to the maximum usage of single atom and the great potential to achieve high activity and selectivity for CO2 reduction reaction (CO2RR). CO2RR on the MoS2 supported single cobalt atom (Co/MoS2) monolayer is performed by using the first-principles simulation, and it is found that cobalt atom prefers to dispersedly anchor on MoS2 support as single atom. According to the transition state calculation for the various possible reaction pathways together with the reaction rate constants calculation, the preferable CO2RR pathway is the reverse water gas conversion (RWGS) and CO hydrogenation pathway with the rate-limiting step of CO hydrogenation into formyl (HCO). The entire reaction pathway can be summarized as *CO2 → *CO → *CHO → *CH2O → *CH2OH and *CH3O → CH3OH. The electronic structures analysis indicates that Co adatom induced gap states play an important role for CO2 activation and reduction. Therefore, the single cobalt atom supported on MoS2 monolayer is an efficient single atom catalyst for CO2 activation and conversion, and the current work may shed light on the development of single atom catalysts to CO2 reduction.

(Invited) Mechanism of Formic Acid Synthesis in Low pH During CO2 Reduction via Electrified Plasma/Liquid Interface

The current mechanism for the CO2 reduction reaction (CO2RR) via electrified plasma/liquid interface (electrochemical discharge) does not explain the favorable formic acid synthesis in aqueous acid media, bringing imprecision on the theoretical predictions of CO2RR selectivity in acid electrolytes. For this reason, this work proposes three CO2RR mechanisms toward formic acid, one of them being a long-stand established in the radiolysis field and passes to analyze the cases employing numerical simulation of their respective chemical dynamic models. With it, it is possible to identify two routes substantially contributing to the global formic acid production, namely the disproportionation-like and radical cross-combination reactions related to the last step. Despite studied here as two separate mechanisms, these two routes combined serve to propose an assertive mechanism of CO2RR in acid media.

Hexagonal Zn Nanoplates Enclosed by Zn(100) and Zn(002) Facets for Highly Selective CO2 Electroreduction to CO.

Electrochemical reduction of CO2 to carbon-neutral fuels is a promising strategy for renewable energy conversion and storage. However, developing earth-abundant and cost-effective electrocatalysts with high catalytic activity and desirable selectivity for the target fuel is still challenging and imperative. Herein, hexagonal Zn nanoplates (H-Zn-NPs) enclosed by Zn(100) and Zn(002) facets were successfully synthesized and studied for their feasibility toward the CO2 reduction reaction (CO2RR). Compared with similarly sized Zn nanoparticles (S-Zn-NPs), the H-Zn-NPs exhibit remarkably enhanced current density, together with an improved CO faradaic efficiency (FE) of over 85% in a wide potential window, where a maximum FE of 94.2% is achieved. The enhancement in the CO2RR performance benefits from the substantial catalytically active sites introduced by the special architecture of H-Zn-NPs. Density functional theory calculations reveal that the exposed Zn(100) facets and edge sites on H-Zn-NPs are energetically favorable for CO2RR to CO, which directly result in an enhanced CO2RR performance. This study undoubtedly provides a straightforward approach to controlling the catalytic activity and selectivity of CO2RR through tuning the shape of Zn-based catalysts so as to maximize the percentage of exposed Zn(100) facets.

Cu nanowire bridged Bi nanosheet arrays for efficient electrochemical CO2 reduction toward formate

Abstract Electrocatalytic reduction of CO2 can not only reduce its concentration in atmosphere but also produces useful fuels and value-added chemicals like CO, CH4, and HCOOH, which has been considered as a promising means to resolve both environmental and energy problems. Recent studies showed that bismuth (Bi) in aqueous solution can electrocatalytically reduce CO2 to formic acid. However, individual Bi when used as electrocatalyst still suffers from low efficiency of electrocatalytic CO2 reduction reaction (CO2RR) to formate. Considering that copper (Cu) has good conductivity and feature of empty half 4s level that can behave as an electron donor or acceptor, here we synthesized Cu nanowires (NWs) bridged Bi nanosheets (Cu NWs-Bi NSs) arrays grown on carbon cloth through facile electrochemical routes. The as-obtained electrocatalysts showed larger partial current density and better selectivity for CO2RR to fomate than pristine Bi NSs. The enhanced electrocatalytic activity and improved selectivity could be attributed to improved charge transfer and electronic effect incurred by Cu NWs. The present work provides a new strategy for designing nanostructured electrocatalysts for highly efficient CO2RR.

(Invited) Defect Engineering and Surface Probing of Few-Layer MoS2 As Photocatalyst for CO2 Reduction to Solar Fuels

Photocatalytic CO2 conversion to hydrocarbon fuels, which makes solar energy harvesting and CO2 reduction reaction (CO2RR) simultaneously, is a killing two birds with one stone approach to solving the energy and environmental problems. However, challenges are the low photon-to-fuel conversion efficiency of the photocatalysts and lack of the product selectivity. Recently, 2D-layered nanomaterials with defect engineering, e.g. the carbon interstitial-doped SnS2 nanosheets [Nature Comm. 9, 169 (2018)], have shown promise for CO2RR. Here, I will present a different case, MoS2 with vacancies controlled by plasma. Productivity and selectivity of CO2RR were found to be dependent strongly with the different Mo/S ratios of the MoS2 of single to few layers. Orders of magnitude enhancement in productivity and selectivity of C2 product over 97% have been obtained. The role and interplay of the defects and the hosting materials, and their effects on the adsorption of CO2 and subsequent CO2RR, studied by the near-ambient pressure X-ray photoemission spectroscopy, along with density functional theory will be discussed.

Selectivity roadmap for electrochemical CO2 reduction on copper-based alloy catalysts

Due to the complex reaction network of the electrochemical CO2 reduction reaction (CRR), developing highly selective electrocatalysts for desired products remains a major challenge. In this study, a series of Cu-based single atom alloys (M@Cu) with multiple active sites are modelled to investigate their CRR selectivity trends by evaluating various adsorption configurations and energetics. The hydrogen (H) and oxygen (O) affinity of the secondary metals in the M@Cu model catalysts are found to be effective descriptors in determining CRR selectivity. The observed product grouping offers valid theoretical elucidation for available reports of CRR selectivity trends for Cu-based alloy catalysts. It also provides further mechanistic insight into the CRR product selectivity for an extensive range of Cu-based bimetallic materials. The selectivity trend based on the intrinsic catalyst properties provides a rational design strategy for highly selective CRR electrocatalysts.

Polyoxometalate-based electron transfer modulation for efficient electrocatalytic carbon dioxide reduction.

The electrocatalytic carbon dioxide (CO2) reduction reaction (CO2RR) involves a variety of electron transfer pathways, resulting in poor reaction selectivity, limiting its use to meet future energy requirements. Polyoxometalates (POMs) can both store and release multiple electrons in the electrochemical process, and this is expected to be an ideal “electron switch” to match with catalytically active species, realize electron transfer modulation and promote the activity and selectivity of the electrocatalytic CO2RR. Herein, we report a series of new POM-based manganese-carbonyl (MnL) composite CO2 reduction electrocatalysts, whereby SiW12–MnL exhibits the most remarkable activity and selectivity for CO2RR to CO, resulting in an increase in the faradaic efficiency (FE) from 65% (MnL) to a record-value of 95% in aqueous electrolyte. A series of control electrochemical experiments, photoluminescence spectroscopy (PL), transient photovoltage (TPV) experiments, and density functional theory (DFT) calculations revealed that POMs act as electronic regulators to control the electron transfer process from POM to MnL units during the electrochemical reaction, enhancing the selectivity of the CO2RR to CO and depressing the competitive hydrogen evolution reaction (HER). This work demonstrates the significance of electron transfer modulation in the CO2RR and suggests a new idea for the design of efficient electrocatalysts towards CO2RR.

Why do RuO2 electrodes catalyze electrochemical CO2 reduction to methanol rather than methane or perhaps neither of those?

The electrochemical CO2 reduction reaction (CO2RR) on RuO2 and RuO2-based electrodes has been shown experimentally to produce high yields of methanol, formic acid and/or hydrogen while methane formation is not detected. This CO2RR selectivity on RuO2 is in stark contrast to copper metal electrodes that produce methane and hydrogen in the highest yields whereas methanol is only formed in trace amounts. Density functional theory calculations on RuO2(110) where only adsorption free energies of intermediate species are considered, i.e. solvent effects and energy barriers are not included, predict however, that the overpotential and the potential limiting step for both methanol and methane are the same. In this work, we use both ab initio molecular dynamics simulations at room temperature and total energy calculations to improve the model system and methodology by including both explicit solvation effects and calculations of proton–electron transfer energy barriers to elucidate the reaction mechanism towards several CO2RR products: methanol, methane, formic acid, CO and methanediol, as well as for the competing H2 evolution. We observe a significant difference in energy barriers towards methane and methanol, where a substantially larger energy barrier is calculated towards methane formation than towards methanol formation, explaining why methanol has been detected experimentally but not methane. Furthermore, the calculations show why RuO2 also catalyzes the CO2RR towards formic acid and not CO(g) and methanediol, in agreement with experimental results. However, our calculations predict RuO2 to be much more selective towards H2 formation than for the CO2RR at any applied potential. Only when a large overpotential of around −1 V is applied, can both formic acid and methanol be evolved, but low faradaic efficiency is predicted because of the more facile H2 formation.

Electrochemical CO2 Reduction to C1 Products on Single Nickel/Cobalt/Iron-Doped Graphitic Carbon Nitride: A DFT Study.

Electrocatalytic CO2 reduction reaction (CRR) is one of the most promising strategies to convert greenhouse gases to energy sources. Herein, the CRR was applied towards making C1 products (CO, HCOOH, CH3 OH, and CH4 ) on g-C3 N4 frameworks with single Ni, Co, and Fe introduction; this process was investigated by density functional theory. The structures of the electrocatalysts, CO2 adsorption configurations, and CO2 reduction mechanisms were systematically studied. Results showed that the single Ni, Co, and Fe located from the corner of the g-C3 N4 cavity to the center. Analyses of the adsorption configurations and electronic structures suggested that CO2 could be chemically adsorbed on Co-C3 N4 and Fe-C3 N4 , but physically adsorbed on Ni-C3 N4 . The H2 evolution reaction (HER), as a suppression of CRR, was investigated, and results showed that Ni-C3 N4 , Co-C3 N4 , and Fe-C3 N4 exhibited more CRR selectivity than HER. CRR proceeded via COOH and OCHO as initial protonation intermediates on Ni-C3 N4 and Co/Fe-C3 N4 , respectively, which resulted in different C1 products along quite different reaction pathways. Compared with Ni-C3 N4 and Fe-C3 N4 , Co-C3 N4 had more favorable CRR activity and selectivity for CH3 OH production with unique rate-limiting steps and lower limiting potential.

Cu3N Nanocubes for Selective Electrochemical Reduction of CO2 to Ethylene.

Understanding the Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) under ambient conditions is both fundamentally interesting and technologically important for selective CO2RR to hydrocarbons. Current Cu catalysts studied for the CO2RR can show high activity but tend to yield a mixture of different hydrocarbons, posing a serious challenge on using any of these catalysts for selective CO2RR. Here, we report a new perovskite-type copper(I) nitride (Cu3N) nanocube (NC) catalyst for selective CO2RR. The 25 nm Cu3N NCs show high CO2RR selectivity and stability to ethylene (C2H4) at -1.6 V (vs reversible hydrogen electrode (RHE)) with the Faradaic efficiency of 60%, mass activity of 34 A/g, and C2H4/CH4 molar ratio of >2000. More detailed electrochemicalcharacterization, X-ray photon spectroscopy, and density functional theory calculations suggest that the high CO2RR selectivity is likely a result of (100) Cu(I) stabilization by the Cu3N structure, which favors CO-CHO coupling on the (100) Cu3N surface, leading to selective formation of C2H4. Our study presents a good example of utilizing metal nitrides as highly efficient nanocatalysts for selective CO2RR to hydrocarbons that will be important for sustainable chemistry/energy applications.

Three-dimensional porous SnO2@NC framework for excellent energy conversion and storage

SnO2-based materials are deemed to be attractive electrodes for lithium/sodium ion batteries (LIBs and SIBs) and electrocatalytic CO2 reduction reaction (CRR) because of high energy density and large abundance. However, the practical application of the SnO2-based materials is prevented by low electrical conductivity and large volume change. Herein, we construct a three-dimensional (3D) porous network with SnO2 nanoparticles into N-doped carbon (namely P–SnO2@NC) synthesized by freeze drying followed by a pyrolyzation process. In the composite, the 3D hierarchical framework can facilitate the ion penetration and gas diffusion. In addition, the NC network can optimize the conductivity of the material and suppress the electrode material to fall off from the electrode. Therefore, the electrode delivers excellent electrochemical properties with high capacities of 510 mA h g−1 after 1000 cycles for LIBs and 497 mA h g−1 after 500 cycles for SIBs. Furthermore, the electrode shows high selectivity for CRR with a large coulombic efficiency (CE) of 52.7% for HCOOH at 0.6 V.

High-Rate Electrochemical Reduction of CO2 to C2-3 Products Under Neutral, Aqueous Condition, Tracing the Evolution of Phase and Morphology of Cu2o Nanocubes Towards Rational Catalyst Design

The need of a sustainable society to advance towards a circular economy, in which there is a balance between the emission and capture of anthropogenic gases like carbon dioxide (CO2), is of utmost scientific and technological importance to ensure the increase or rather preservation of the current prosperity for future generations. The growing global population and increase in worldwide prosperity is tightly connected to a rise in power consumption. Traditionally, this energy demand is supplied by the combustion of fossil fuels, resulting in growing emission of greenhouse gases. In this, CO2 is a key issue which climatic influences and general mitigation is rigorously disused not only in scientific field, but in politics as well. Here, the electrochemical CO2 reduction reaction (CO2RR) is posing as one potential technology, to address questions of power storage for renewable energies and sustainable usage of natural resources. The CO2RR shows a diverse spectrum of products ranging from liquid fuels, as ethanol and propanol, to gaseous building blocks for the chemical industry, as ethylene and CO. The selectivity of this reaction is highly dependent on the nature of the catalyst and is always competing with the hydrogen evolution reaction (HER), due to the aqueous conditions. Recently, very selective and active catalysts have been developed for the production of CO and formic acid. Both compounds are comparably easy to produce, as the reduction only involves the transfer of 2 electrons. Unfortunately, the production of valuable hydrocarbons is much more difficult and suffers from losses in selectivity due to the highly complex reaction mechanism. So far, only copper showed acceptable production rates for hydrocarbons, owing to the favorable binding energies of intermediates. Recent studies have been focused on fundamental parameters to control the selectivity of this complex reaction on copper for C2+ species, ranging from catalyst design to electrolyte selection. On the catalyst side, many effects as the presence of (100) facets1, grain boundaries2 and oxides3 have been suggested to be beneficial, whereas influences of buffer capacity and local electric field brought additional advantages by choice of the electrolyte. While promising results were shown, many studies only focused on low-current density, which are far from an industrial application. Here, at least 200 mA cm-2 are needed for a functional electrolyzer. This poses as a discrepancy between research and application and raises the question if the results from fundamental studies can be transferred to full size electrolyzers.4 For this, we are presenting a cubic Cu2O catalyst, which we investigate in a comprehensive study, moving from initial tests in an H-Cell towards high currents on a Gas Diffusion Electrode (GDE) in a Flow-Cell setup. To deconvolute the parameters dictating CO2RR selectivity, we used X-Ray-Diffraction (XRD), quasi in-situ X-Ray Photon Spectroscopy (XPS) and operando X-ray Absorption Spectroscopy (XAS) to trace phase changes during reaction, in addition to morphological investigations by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). This complementary analysis depicts a highly dynamic system, in which the reduction of oxidized Cu progresses from the surface towards deeper layers, resulting in a purely metallic, defect-rich material. We further focus on performance tests of our Cu2O catalyst at high current density of up to 700 mA cm-2 in a flow-electrolyzer. By varying system parameters as mass loading and nafion content we observe strong changes in selectivity and activity during CO2RR, which we correlate to accessibility of the active copper sites and issues of mass transport. We further discuss the role of surface pH at high current density by varying electrolyte concentration and therefore buffer capacity. Our study suggests distinct differences between CO2RR in electrochemical cells for fundamental studies at low currents compared to tests at high currents in a flow-electrolyzer. Furthermore, we show how recent results from literature translate to performance at high current density and comment on the importance of electrode preparation. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39-47.A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. L. Stephens, M. W. Kanan and I. Chorkendorff, Journal of the American Chemical Society, 2015, 137, 9808-9811.H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser and B. R. Cuenya, Nature Communications, 2016, 7, 12123.T. Burdyny and W. A. Smith, Energy & Environmental Science, 2019, DOI: 10.1039/c8ee03134g.

Black reduced porous SnO2 nanosheets for CO2 electroreduction with high formate selectivity and low overpotential

The great bottleneck that lies in CO2 reduction reaction (CO2RR) electrocatalysts is to simultaneously enhance their conductivity and density of active sites. Herein, we developed a black reduced porous SnO2 nanosheets electrocatalyst that enabled possessing both metallic conductivity and high density of active sites via vacancy engineering. The black reduced porous SnO2 nanosheets showed high activity and selectivity for CO2RR to formate, with maximum Faradaic efficiency (FE) of 92.4% at the low overpotential of 0.51 V and stable FE of 90 ± 2% in the large potential range from -0.6 to -1.1 V vs. reversible hydrogen electrode (RHE). Density functional theory (DFT) calculations indicated that the introduction of oxygen vacancy increased carrier density and lowered the adsorption of HCOO- * and HCOOH by 0.29 and 0.17 eV, respectively, accounting for the improved CO2RR to formate performance.

Rational design of carbon-based metal-free catalysts for electrochemical carbon dioxide reduction: A review

Abstract Electrochemical CO2 reduction to chemicals or fuels presents one of the most promising strategies for managing the global carbon balance, which yet poses a significant challenge due to lack of efficient and durable electrocatalyst as well as the understanding of the CO2 reduction reaction (CO2RR) mechanism. Benefiting from the large surface area, high electrical conductivity, and tunable structure, carbon-based metal-free materials (CMs) have been extensively studied as cost-effective electrocatalysts for CO2RR. The development of CMs with low cost, high activity and durability for CO2RR has been considered as one of the most active and competitive directions in electrochemistry and material science. In this review article, some up-to-date strategies in improving the CO2RR performance on CMs are summarized. Specifically, the approaches to optimize the adsorption of CO2RR intermediates, such as tuning the physical and electronic structure are introduced, which can enhance the electrocatalytic activity of CMs effectively. Finally, some design strategies are proposed to prepare CMs with high activity and selectivity for CO2RR.