Influence of Atomic Surface Structure on the Activity of Ag for the Electrochemical Reduction of CO2 to CO

Sulfidation-Induced Surface Local Electronic and Atomic Structures in a Silver Catalyst Enables Silicon Photocathode for Selective and Efficient Photoelectrochemical CO2 Reduction

Converting CO2 to value-added chemicals through a photoelectrochemical (PEC) system is a creative approach toward renewable energy utilization and storage. However, the rational design of appropriate catalysts while being effectively integrated with semiconductor photoelectrodes remains a considerable challenge for achieving single-carbon products with high efficiency. Herein, we demonstrate a novel sulfidation-induced strategy for in situ grown sulfide-derived Ag nanowires on a Si photocathode (denoted as SD-Ag/Si) based on the standard crystalline Si solar cells. Such an exquisite design of the SD-Ag/Si photocathode not only provides a large electrochemically active surface area but also endows abundant active sites of Ag2S/Ag interfaces and high-index Ag facets for PEC CO production. The optimized SD-Ag/Si photocathode displays an ideal CO Faradic efficiency of 95.2% and an onset potential of +0.26 V versus the reversible hydrogen electrode, ascribed to the sulfidation-induced synergistic effect of the surface atomic arrangement and electronic structure in Ag catalysts that promote charge transfer, facilitate CO2 adsorption and activation, and suppress hydrogen evolution reaction. This sulfidation-induced strategy represents a scalable approach for designing high-performance catalysts for electrochemical and PEC devices with efficient CO2 utilization.

Data-driven discovery of electrocatalysts for CO2 reduction using active motifs-based machine learning

The electrochemical carbon dioxide reduction reaction (CO2RR) is an attractive approach for mitigating CO2 emissions and generating value-added products. Consequently, discovery of promising CO2RR catalysts has become a crucial task, and machine learning (ML) has been utilized to accelerate catalyst discovery. However, current ML approaches are limited to exploring narrow chemical spaces and provide only fragmentary catalytic activity, even though CO2RR produces various chemicals. Here, by merging pre-developed ML model and a CO2RR selectivity map, we establish high-throughput virtual screening strategy to suggest active and selective catalysts for CO2RR without being limited to a database. Further, this strategy can provide guidance on stoichiometry and morphology of the catalyst to researchers. We predict the activity and selectivity of 465 metallic catalysts toward four expected reaction products. During this process, we discover previously unreported and promising behavior of Cu-Ga and Cu-Pd alloys. These findings are then validated through experimental methods.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.

Al-Doped Octahedral Cu2O Nanocrystal for Electrocatalytic CO2 Reduction to Produce Ethylene

Ethylene is an ideal CO2 product in an electrocatalytic CO2 reduction reaction (CO2RR) with high economic value. This paper synthesised Al-doped octahedral Cu2O (Al-Cu2O) nanocrystal by a simple wet chemical method. The selectivity of CO2RR products was improved by doping Al onto the surface of octahedral Cu2O. The Al-Cu2O was used as an efficient electrocatalyst for CO2RR with selective ethylene production. The Al-Cu2O exhibited a high % Faradic efficiency (FEC2H4) of 44.9% at -1.23 V (vs. RHE) in CO2 saturated 0.1 M KHCO3 electrolyte. Charge transfer from the Al atom to the Cu atom occurs after Al doping in Cu2O, optimizing the electronic structure and facilitating CO2RR to ethylene production. The DFT calculation showed that the Al-Cu2O catalyst could effectively reduce the adsorption energy of the *CHCOH intermediate and promote the mass transfer of charges, thus improving the FEC2H4. After Al doping into Cu2O, the center of d orbitals shift positively, which makes the d-band closer to the Fermi level. Furthermore, the density of electronic states increases due to the interaction between Cu atoms and intermediates, thus accelerating the electrochemical CO2 reduction process. This work proved that the metal doping strategy can effectively improve the catalytic properties of Cu2O, thus providing a useful way for CO2 cycling and green production of C2H4.

Unravelling the Effect of Activators used in The Synthesis of Biomass-Derived Carbon Electrocatalysts on the Electrocatalytic Performance for CO2 Reduction

N-doped carbon materials can be efficient and cost-effective catalysts for the electrochemical CO2 reduction reaction (CO2 RR). Activators are often used in the synthesis process to increase the specific surface area and porosity of these carbon materials. However, owing to the diversity of activators and the differences in physicochemical properties that these activators induce, the influence of activators used for the synthesis of N-doped carbon catalysts on their electrochemical performance is unclear. In this study, a series of bagasse-derived N-doped carbon catalysts is prepared with the assistance of different activators to understand the correlation between activators, physicochemical properties, and electrocatalytic performance for the CO2 RR. The properties of N-doped carbon catalysts, such as N-doping content, microstructure, and degree of graphitization, are found to be highly dependent on the type of activator applied in the synthesis procedure. Moreover, the overall CO2 RR performance of the synthesized electrocatalysts is not determined only by the N-doping level and the configuration of the N-dopant, but rather by the overall surface chemistry, where the porosity and the degree of graphitization are jointly responsible for significant differences in CO2 RR performance.

The importance of a charge transfer descriptor for screening potential CO2 reduction electrocatalysts

It has been over twenty years since the linear scaling of reaction intermediate adsorption energies started to coin the fields of heterogeneous and electrocatalysis as a blessing and a curse at the same time. It has established the possibility to construct activity volcano plots as a function of a single or two readily accessible adsorption energies as descriptors, but also limited the maximal catalytic conversion rate. In this work, it is found that these established adsorption energy-based descriptor spaces are not applicable to electrochemistry, because they are lacking an important additional dimension, the potential of zero charge. This extra dimension arises from the interaction of the electric double layer with reaction intermediates which does not scale with adsorption energies. At the example of the electrochemical reduction of CO₂ it is shown that the addition of this descriptor breaks the scaling relations, opening up a huge chemical space that is readily accessible via potential of zero charge-based material design. The potential of zero charge also explains product selectivity trends of electrochemical CO₂ reduction in close agreement with reported experimental data highlighting its importance for electrocatalyst design.

The Facet Dependence of CO2 Electroreduction Selectivity on a Pd3Au Bimetallic Catalyst: A DFT Study

The electrochemical carbon dioxide reduction reaction (CO2RR) has emerged as a promising approach to addressing global energy and environmental challenges. Alloys are of particular importance in these applications due to their unique chemical and physical properties. In this study, the possible mechanism of the C1 products from the electrochemical reduction of CO2 on four different surfaces of Pd3Au alloy bimetallic catalysts is predicted using the density functional theory. The differences in the number of d-band electrons and the charge distribution and morphology of the different surfaces result in differing catalytic activity and selectivity on the same surface. On different surfaces, Pd3Au alloy bimetallic catalysts have different potential limiting steps in CO2RR, resulting in differing selectivity. The Pd3Au (100) surface has a good selectivity for HER, indicating that the increase in the net charge on the surface of the alloy improves the selectivity for HER. The Pd3Au (211) surface, with a step structure, shows a good selectivity for methanol production from CO2RR. In addition, an electronic structure analysis shows that the selectivity of the reactions involved in the conversion of adsorbates is determined by the difference between the center of the d-band on the top of the catalyst, where the reactant and the product are located. The results of this study may provide some theoretical basis for designing and developing more efficient and selective CO2 reduction catalysts.

Atomically Dispersed Cu Catalysts on Sulfide-Derived Defective Ag Nanowires for Electrochemical CO2 Reduction

Single-atom catalysts (SACs) have shown potential for achieving an efficient electrochemical CO₂ reduction reaction (CO2RR) despite challenges in their synthesis. Here, Ag₂S/Ag nanowires provide initial anchoring sites for Cu SACs (Cu/Ag₂S/Ag), then Cu/Ag(S) was synthesized by an electrochemical treatment resulting in complete sulfur removal, i.e., Cu SACs on a defective Ag surface. The CO2RR Faradaic efficiency (FE_CO2RR) of Cu/Ag(S) reaches 93.0% at a CO2RR partial current density (j_CO2RR) of 2.9 mA/cm² under -1.0 V vs RHE, which outperforms sulfur-removed Ag₂S/Ag without Cu SACs (Ag(S), 78.5% FE_CO2RR with 1.8 mA/cm²j_CO2RR). At -1.4 V vs RHE, both FE_CO2RR and j_CO2RR over Cu/Ag(S) reached 78.6% and 6.1 mA/cm², which tripled those over Ag(S), respectively. As revealed by in situ and ex situ characterizations together with theoretical calculations, the interacted Cu SACs and their neighboring defective Ag surface increase microstrain and downshift the d-band center of Cu/Ag(S), thus lowering the energy barrier by ∼0.5 eV for *CO formation, which accounts for the improved CO2RR activity and selectivity toward related products such as CO and C₂₊ products.

Heterogenization of Molecular Electrocatalytic Active Sites through Reticular Chemistry

The electrochemical conversion of small molecules, such as CO₂ , O₂ , and H₂ O, has received significant attention as a potential engine for sustainable life. Metal-organic frameworks (MOFs) are a promising class of electrocatalytic materials for such processes. An attractive aspect of utilizing this class of materials as electrocatalysts is that well-known molecular active sites can be introduced to well-defined crystalline heterogeneous catalytic systems with high tunability. This review offers strategic insights into recent studies on MOF-based electrocatalysts by discussing the notable active sites that have been utilized in both homogeneous and heterogeneous catalysts, while highlighting instances where such active sites have been introduced into MOFs. In addition, material design principles enabling the integration of electrochemically active components with the MOF platform are outlined. Viewpoints on the viability of MOFs as an alternative to currently used electrocatalysts are also discussed. Finally, the future direction of MOF-based electrocatalysis research is established.

Toward economical application of carbon capture and utilization technology with near-zero carbon emission

Carbon capture and utilization technology has been studied for its practical ability to reduce CO₂ emissions and enable economical chemical production. The main challenge of this technology is that a large amount of thermal energy must be provided to supply high-purity CO₂ and purify the product. Herein, we propose a new concept called reaction swing absorption, which produces synthesis gas (syngas) with net-zero CO₂ emission through direct electrochemical CO₂ reduction in a newly proposed amine solution, triethylamine. Experimental investigations show high CO₂ absorption rates (>84%) of triethylamine from low CO₂ concentrated flue gas. In addition, the CO Faradaic efficiency in a triethylamine supplied membrane electrode assembly electrolyzer is approximately 30% (@-200 mA cm^-2), twice higher than those in conventional alkanolamine solvents. Based on the experimental results and rigorous process modeling, we reveal that reaction swing absorption produces high pressure syngas at a reasonable cost with negligible CO₂ emissions. This system provides a fundamental solution for the CO₂ crossover and low system stability of electrochemical CO₂ reduction.

A Happy Get-Together - Probing Electrochemical Interfaces by Non-Linear Vibrational Spectroscopy

Electrochemical interfaces are key structures in energy storage and catalysis. Hence, a molecular understanding of the active sites at these interfaces, their solvation, the structure of adsorbates, and the formation of solid-electrolyte interfaces are crucial for an in-depth mechanistic understanding of their function. Vibrational sum-frequency generation (VSFG) spectroscopy has emerged as an operando spectroscopic technique to monitor complex electrochemical interfaces due to its intrinsic interface sensitivity and chemical specificity. Thus, this review discusses the happy get-together between VSFG spectroscopy and electrochemical interfaces. Methodological approaches for answering core issues associated with the behavior of adsorbates on electrodes, the structure of solvent adlayers, the transient formation of reaction intermediates, and the emergence of solid electrolyte interphase in battery research are assessed to provide a critical inventory of highly promising avenues to bring optical spectroscopy to use in modern material research in energy conversion and storage.

Ordered Ag Nanoneedle Arrays with Enhanced Electrocatalytic CO2 Reduction via Structure-Induced Inhibition of Hydrogen Evolution

Silver is an attractive catalyst for converting CO₂ into CO. However, the high CO₂ activation barrier and the hydrogen evolution side reaction seriously limit its practical application and industrial perspective. Here, an ordered Ag nanoneedle array (Ag-NNAs) was prepared by template-assisted vacuum thermal-evaporation for CO₂ electroreduction into CO. The nanoneedle array structure induces a strong local electric field at the tips, which not only reduces the activation barrier for CO₂ electroreduction but also increases the energy barrier for the hydrogen evolution reaction (HER). Moreover, the array structure endows a high surface hydrophobicity, which can regulate the adsorption of water molecules at the interface and thus dynamically inhibit the competitive HER. As a result, the optimal Ag-NNAs exhibits 91.4% Faradaic efficiency (FE) of CO for over 700 min at -1.0 V vs RHE. This work provides a new concept for the application of nanoneedle array structures in electrocatalytic CO₂ reduction reactions.

Boosted ammonium production by single cobalt atom catalysts with high Faradic efficiencies

Efficient n = O bond activation is crucial for the catalytic reduction of nitrogen compounds, which is highly affected by the construction of active centers. In this study, n = O bond activation was achieved by a single-atom catalyst (SAC) with phosphorus anchored on a Co active center to form intermediate N-species for further hydrogenation and reduction. Unique phosphorus-doped discontinuous active sites exhibit better n = O activation performance than conventional N-cooperated single-atom sites, with a high Faradic efficiency of 92.0% and a maximum ammonia yield rate of 433.3 μg NH4·h^-1·cm^-2. This approach of constructing environmental sites through heteroatom modification significantly improves atom efficiency and will guide the design of future functional SACs with wide-ranging applications.

Accelerating CO2 Electroreduction to Multicarbon Products via Synergistic Electric-Thermal Field on Copper Nanoneedles

Electrochemical CO₂ reduction is a promising way to mitigate CO₂ emissions and close the anthropogenic carbon cycle. Among products from CO₂RR, multicarbon chemicals, such as ethylene and ethanol with high energy density, are more valuable. However, the selectivity and reaction rate of C₂ production are unsatisfactory due to the sluggish thermodynamics and kinetics of C-C coupling. The electric field and thermal field have been studied and utilized to promote catalytic reactions, as they can regulate the thermodynamic and kinetic barriers of reactions. Either raising the potential or heating the electrolyte can enhance C-C coupling, but these come at the cost of increasing side reactions, such as the hydrogen evolution reaction. Here, we present a generic strategy to enhance the local electric field and temperature simultaneously and dramatically improve the electric-thermal synergy desired in electrocatalysis. A conformal coating of ∼5 nm of polytetrafluoroethylene significantly improves the catalytic ability of copper nanoneedles (∼7-fold electric field and ∼40 K temperature enhancement at the tips compared with bare copper nanoneedles experimentally), resulting in an improved C₂ Faradaic efficiency of over 86% at a partial current density of more than 250 mA cm^-2 and a record-high C₂ turnover frequency of 11.5 ± 0.3 s^-1 Cu site^-1. Combined with its low cost and scalability, the electric-thermal strategy for a state-of-the-art catalyst not only offers new insight into improving activity and selectivity of value-added C₂ products as we demonstrated but also inspires advances in efficiency and/or selectivity of other valuable electro-/photocatalysis such as hydrogen evolution, nitrogen reduction, and hydrogen peroxide electrosynthesis.

Strategies for Breaking Molecular Scaling Relationships for the Electrochemical CO2 Reduction Reaction

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion...

In Silico Design of Metal-Free Hydrophosphate Catalysts for Hydrogenation of CO2 to Formate

CO2 reduction by H2 using metal-free catalysts is highly challenging. Frustrated Lewis pairs (FLPs) have been considered potential metal-free catalysts for this reaction. However, most FLPs are unstable, which limits...

Improving the intrinsic activity of electrocatalysts for sustainable energy conversion: where are we and where can we go?

As we are in the midst of a climate crisis, there is an urgent need to transition to the sustainable production of fuels and chemicals. A promising strategy towards this transition is to use renewable energy for the electrochemical conversion of abundant molecules present in the earth's atmosphere such as H₂O, O₂, N₂ and CO₂, to synthetic fuels and chemicals. A cornerstone to this strategy is the development of earth abundant electrocatalysts with high intrinsic activity towards the desired products. In this perspective, we discuss the importance and challenges involved in the estimation of intrinsic activity both from the experimental and theoretical front. Through a thorough analysis of published data, we find that only modest improvements in intrinsic activity of electrocatalysts have been achieved in the past two decades which necessitates the need for a paradigm shift in electrocatalyst design. To this end, we highlight opportunities offered by tuning three components of the electrochemical environment: cations, buffering anions and the electrolyte pH. These components can significantly alter catalytic activity as demonstrated using several examples, and bring us a step closer towards complete system level optimization of electrochemical routes to sustainable energy conversion.

Implicit Solvation Methods for Catalysis at Electrified Interfaces

Implicit solvation is an effective, highly coarse-grained approach in atomic-scale simulations to account for a surrounding liquid electrolyte on the level of a continuous polarizable medium. Originating in molecular chemistry with finite solutes, implicit solvation techniques are now increasingly used in the context of first-principles modeling of electrochemistry and electrocatalysis at extended (often metallic) electrodes. The prevalent ansatz to model the latter electrodes and the reactive surface chemistry at them through slabs in periodic boundary condition supercells brings its specific challenges. Foremost this concerns the difficulty of describing the entire double layer forming at the electrified solid–liquid interface (SLI) within supercell sizes tractable by commonly employed density functional theory (DFT). We review liquid solvation methodology from this specific application angle, highlighting in particular its use in the widespread ab initio thermodynamics approach to surface catalysis. Notably, implicit solvation can be employed to mimic a polarization of the electrode’s electronic density under the applied potential and the concomitant capacitive charging of the entire double layer beyond the limitations of the employed DFT supercell. Most critical for continuing advances of this effective methodology for the SLI context is the lack of pertinent (experimental or high-level theoretical) reference data needed for parametrization.

Electrochemical Reduction of CO2 to HCOOH over Copper Catalysts

Although great progress has been made in the field of electrochemical CO₂ reduction reaction (eCO₂RR), inducing product selectivity is still difficult. We herein report that a thiocyanate ion (SCN^-) switched the product selectivity of copper catalysts for eCO₂RR in an H-cell. A cuprous thiocyanate-derived Cu catalyst was found to exhibit excellent HCOOH selectivity (faradaic efficiency = 70-88%) over a wide potential range (-0.66 to -0.95 V vs RHE). Furthermore, it was revealed that the formation of CO and C₂H₄ over commercial copper electrodes could be dramatically suppressed with the presence of SCN^-, switching to HCOOH. Density functional theory calculations disclosed that SCN^- made the formation of HCOO* easier than COOH* on Cu (211), facilitating the HCOOH generation. Our results provide a new insight into eCO₂RR and will be helpful in the development of cheap electrocatalysts for specific utilization.

Guiding the Catalytic Properties of Copper for Electrochemical CO2 Reduction by Metal Atom Decoration

Tuning bimetallic effects is a promising strategy to guide catalytic properties. However, the nature of these effects can be difficult to assess and compare due to the convolution with other factors such as the catalyst surface structure and morphology and differences in testing environments. Here, we investigate the impact of atomic-scale bimetallic effects on the electrochemical CO2 reduction performance of Cu-based catalysts by leveraging a systematic approach that unifies protocols for materials synthesis and testing and enables accurate comparisons of intrinsic catalytic activity and selectivity. We used the same physical vapor deposition method to epitaxially grow Cu(100) films decorated with a small amount of noble or base metal atoms and a combination of experimental characterization and first-principles calculations to evaluate their physicochemical and catalytic properties. The results indicate that the metal atoms segregate to under-coordinated Cu sites during physical vapor deposition, suppressing CO reduction to oxygenates and hydrocarbons and promoting competing pathways to CO, formate, and hydrogen. Leveraging these insights, we rationalize bimetallic design principles to improve catalytic selectivity for CO2 reduction to CO, formate, oxygenates, or hydrocarbons. Our study provides one of the most extensive studies on Cu bimetallics for CO2 reduction, establishing a systematic approach that is broadly applicable to research in catalyst discovery.

Mapping Composition-Selectivity Relationships of Supported Sub-10 nm Cu-Ag Nanocrystals for High-Rate CO2 Electroreduction

Colloidal Cu-Ag nanocrystals measuring less than 10 nm across are promising candidates for integration in hybrid CO₂ reduction reaction (CO₂RR) interfaces, especially in the context of tandem catalysis and selective multicarbon (C₂-C₃) product formation. In this work, we vary the synthetic-ligand/copper molar ratio from 0.1 to 1.0 and the silver/copper atomic ratio from 0 to 0.7 and study the variations in the nanocrystals' size distribution, morphology and reactivity at rates of ≥100 mA cm^-2 in a gas-fed recycle electrolyzer operating under neutral to mildly basic conditions (0.1-1.0 M KHCO₃). High-resolution electron microscopy and spectroscopy are used in order to characterize the morphology of sub-10 nm Cu-Ag nanodimers and core-shells and to elucidate trends in Ag coverage and surface composition. It is shown that Cu-Ag nanocrystals can be densely dispersed onto a carbon black support without the need for immediate ligand removal or binder addition, which considerably facilitates their application. Although CO₂RR product distribution remains an intricate function of time, (kinetic) overpotential and processing conditions, we nevertheless conclude that the ratio of oxygenates to hydrocarbons (which depends primarily on the initial dispersion of the nanocrystals and their composition) rises 3-fold at moderate Ag atom % relative to Cu NCs-based electrodes. Finally, the merits of this particular Cu-Ag/C system and the recycling reactor employed are utilized to obtain maximum C₂-C₃ partial current densities of 92-140 mA cm^-2 at -1.15 V_RHE and liquid product concentrations in excess of 0.05 wt % in 1 M KHCO₃ after short electrolysis periods.

Chemical Modifications of Ag Catalyst Surfaces with Imidazolium Ionomers Modulate H2 Evolution Rates during Electrochemical CO2 Reduction

Bridging polymer design with catalyst surface science is a promising direction for tuning and optimizing electrochemical reactors that could impact long-term goals in energy and sustainability. Particularly, the interaction between inorganic catalyst surfaces and organic-based ionomers provides an avenue to both steer reaction selectivity and promote activity. Here, we studied the role of imidazolium-based ionomers for electrocatalytic CO₂ reduction to CO (CO₂R) on Ag surfaces and found that they produce no effect on CO₂R activity yet strongly promote the competing hydrogen evolution reaction (HER). By examining the dependence of HER and CO₂R rates on concentrations of CO₂ and HCO₃^-, we developed a kinetic model that attributes HER promotion to intrinsic promotion of HCO₃^- reduction by imidazolium ionomers. We also show that varying the ionomer structure by changing substituents on the imidazolium ring modulates the HER promotion. This ionomer-structure dependence was analyzed via Taft steric parameters and density functional theory calculations, which suggest that steric bulk from functionalities on the imidazolium ring reduces access of the ionomer to both HCO₃^- and the Ag surface, thus limiting the promotional effect. Our results help develop design rules for ionomer-catalyst interactions in CO₂R and motivate further work into precisely uncovering the interplay between primary and secondary coordination in determining electrocatalytic behavior.

Regulation of functional groups on graphene quantum dots directs selective CO2 to CH4 conversion

A catalyst system with dedicated selectivity toward a single hydrocarbon or oxygenate product is essential to enable the industrial application of electrochemical conversion of CO₂ to high-value chemicals. Cu is the only known metal catalyst that can convert CO₂ to high-order hydrocarbons and oxygenates. However, the Cu-based catalysts suffer from diverse selectivity. Here, we report that the functionalized graphene quantum dots can direct CO₂ to CH₄ conversion with simultaneous high selectivity and production rate. The electron-donating groups facilitate the yield of CH₄ from CO₂ electro-reduction while electron-withdrawing groups suppress CO₂ electro-reduction. The yield of CH₄ on electron-donating group functionalized graphene quantum dots is positively correlated to the electron-donating ability and content of electron-donating group. The graphene quantum dots functionalized by either -OH or -NH₂ functional group could achieve Faradaic efficiency of 70.0% for CH₄ at -200 mA cm^-2 partial current density of CH₄. The superior yield of CH₄ on electron-donating group- over the electron-withdrawing group-functionalized graphene quantum dots possibly originates from the maintenance of higher charge density of potential active sites (neighboring C or N) and the interaction between the electron-donating group and key intermediates. This work provides insight into the design of active carbon catalysts at the molecular scale for the CO₂ electro-reduction.

Designing electrode materials for the electrochemical reduction of carbon dioxide

Electrochemical reduction of carbon dioxide is a viable alternative for reducing fossil fuel consumption and reducing atmospheric CO₂ levels. Although, a wide variety of materials have been studied for electrochemical reduction of CO₂, the selective and efficient reduction of CO₂ is still not accomplished. Complex reaction mechanisms and the competing hydrogen evolution reaction further complicates the efficiency of materials. An extensive understanding of reaction mechanism is hence essential in designing an ideal electrocatalyst material. Therefore, in this review article we discuss the materials explored in the last decade with focus on their catalytic mechanism and methods to enhance their catalytic activity.

Gold-like activity copper-like selectivity of heteroatomic transition metal carbides for electrocatalytic carbon dioxide reduction reaction

An overarching challenge of the electrochemical carbon dioxide reduction reaction (eCO₂RR) is finding an earth-abundant, highly active catalyst that selectively produces hydrocarbons at relatively low overpotentials. Here, we report the eCO₂RR performance of two-dimensional transition metal carbide class of materials. Our results indicate a maximum methane (CH₄) current density of −421.63 mA/cm² and a CH₄ faradic efficiency of 82.7% ± 2% for di-tungsten carbide (W₂C) nanoflakes in a hybrid electrolyte of 3 M potassium hydroxide and 2 M choline-chloride. Powered by a triple junction photovoltaic cell, we demonstrate a flow electrolyzer that uses humidified CO₂ to produce CH₄ in a 700-h process under one sun illumination with a CO₂RR energy efficiency of about 62.3% and a solar-to-fuel efficiency of 20.7%. Density functional theory calculations reveal that dissociation of water, chemisorption of CO₂ and cleavage of the C-O bond—the most energy consuming elementary steps in other catalysts such as copper—become nearly spontaneous at the W₂C surface. This results in instantaneous formation of adsorbed CO—an important reaction intermediate—and an unlimited source of protons near the tungsten surface sites that are the main reasons for the observed superior activity, selectivity, and small potential.

It is of high interests to develop new catalysts for selective CO2 electroreduction. Here the authors investigate two-dimensional transition metal carbides for CO2 to methane conversion with superior activity, selectivity and low overpotentials.

Enhancing the CO2-to-CO Conversion from 2D Silver Nanoprisms via Superstructure Assembly

The electrochemical reduction of CO₂ in a highly selective and efficient manner is a crucial step toward its reuse for the production of chemicals and fuels. Nanostructured Ag catalysts have been found to be effective candidates for the conversion of CO₂-to-CO. However, the ambiguous determination of the intrinsic CO₂ activity and the maximization of the density of exposed active sites have greatly limited the use of Ag toward the realization of practical electrocatalytic devices. Here, we report a superstructure design strategy prepared by the self-assembly of two-dimensional Ag nanoprisms for maximizing the exposure of active edge ribs. The vertically stacked Ag nanoprisms allow the exposure of >95% of the edge sites, resulting in an enhanced selectivity and activity toward the production of CO from CO₂ with an overpotential of 152 mV. The Ag superstructures also demonstrate a selectivity of over 90% for 100 h together with a current retention of ≈94% at -600 mV versus the reversible hydrogen electrode and a partial energy efficiency for CO production of 70.5%. Our electrochemical measurements on individual Ag nanoprisms with various edge-to-basal plane ratios and the Ag superstructures led to the identification of the edge ribs as the active sites thanks to the ≈400 mV decrease in the onset potential compared to that of the Ag (111) basal planes and a turnover frequency of 9.2 × 10^-3 ± 1.9 × 10^-3 s^-1 at 0 V overpotential.

Engineering Ag-Nx Single-Atom Sites on Porous Concave N-Doped Carbon for Boosting CO2 Electroreduction

The electrochemical CO₂ reduction reaction (CO₂RR) offers an environmentally benign pathway for renewable energy conversion and further regulation of the environmental CO₂ concentration to achieve carbon cycling. However, developing desired electrocatalysts with high CO Faradaic efficiency (FE_CO) at an ultralow overpotential remains a grand challenge. Herein, we report an effective CO₂RR electrocatalyst that features Ag single-atom coordinated with three nitrogen atoms (Ag₁-N₃) anchored on porous concave N-doped carbon (Ag₁-N₃/PCNC), which is identified by X-ray absorption spectroscopy. Ag₁-N₃/PCNC shows a low CO₂RR onset potential of -0.24 V, high maximum FE_CO of 95% at -0.37 V, and high CO partial current density of 7.6 mA cm^-2 at -0.55 V, exceeding most of the previous Ag electrocatalysts. The in situ infrared absorption spectra technique proves that Ag₁-N₃ single-atom sites have sole linear-adsorbed CO and can easily desorb *CO species to achieve the highest CO selectivity in comparison with the corresponding counterparts. This work provides significant inspiration on boosting CO₂RR by tuning the active center at an atomic level to achieve a specific absorption with an intermediate.