Origin of the Selective Electroreduction of Carbon Dioxide to Formate by Chalcogen Modified Copper

Ditopic ligand effects on solution structure and redox chemistry in discrete [Cu12S6] clusters with labile Cu-S bonds

Copper chalcogenide nanoclusters (Cu-S/Se/Te NCs) are a broad and diverse class of atomically precise nanomaterials that have historically been studied for potential applications in luminescent devices and sensors, and for their beautiful, mineral-like crystal structures. By the "cluster-surface" analogy, Cu-S/Se NCs are prime candidates for the development of nanoscale multimetallic catalysts with atomic precision. However, the majority of studies conducted to date have focused exclusively on their solid-state structures and physical properties, leaving open questions as to their solution stability, dynamics, and reactivity. Herein, we report the first detailed interrogation of solution structure, dynamics, electrochemistry, and decomposition of Cu-S NCs. Specifically, we report the detailed NMR spectroscopy, diffusion-ordered spectroscopy, MALDI mass spectrometry, electrochemical and stoichiometric redox reactivity studies, and DFT studies of a series of [Cu12S6] clusters with labile Cu-S bonds supported by monodentate phosphines and ditopic bis(diphenylphosphino)alkane ligands PPh2R (R = Et, -(CH2)5-, -(CH2)8-). We find that the ligand binding topology dictates the extent of speciation in solution, with complete stability being afforded by the longer octane chelate in dppo (1,8-bis(diphenylphosphino)octane) according to 1H and DOSY NMR and MALDI-MS studies. Furthermore, a combined electrochemical and computational investigation of [Cu12S6(dppo)4] reveals that the intact [Cu12S6] core undergoes a quasireversible one-electron oxidation at mild applied potentials ([Cu12S6]0/+: -0.50 V vs. Fc0/+). In contrast, prolonged air exposure or treatment with chemical oxidants results in cluster degradation with S atom extrusion as phosphine sulfide byproducts. This work adds critical new dimensions to the stabilization and study of atomically precise metal chalcogenide NCs with labile M-S/Se bonds, and demonstrates both progress and challenges in controlling the solution behaviour and redox chemistry of phosphine-supported copper chalcogenide nanoclusters.

Rapid nucleation and optimal surface-ligand interaction stabilize wurtzite MnSe

Non-native structures (NNS) differ in discrete translational symmetry from the bulk ground state native structure (NS). To explore the extent of deconvolution of various factors relevant to the stabilization of the wurtzite/NNS of MnSe via a heat-up method, we performed experiments using various ligands (oleic acid, oleylamine, octadecylamine, stearic acid, and octadecene), solvents (tetraethylene glycol and octadecene), and precursor salts (manganese chloride and manganese acetate). Experiments suggest that oleic acid in the presence of tetraethylene glycol and oleylamine in the presence of octadecene stabilize wurtzite/NNS. Further, density functional theory (DFT) computations explore the interaction between the functional groups in ligands and the most exposed surfaces of wurtzite/NNS and rocksalt/NS polymorphs. Computations suggest that the interactions between relevant surface facets with carboxylic acid and the double bond functional groups suppress the phase transformation from NNS to NS. In addition, the ionizability of the precursor salt also determines the rate of formation of the metal-ligand complex and the rate of nucleation. Consequently, the formation rate of the Mn-ligand complex is expected to be greater in the case of chloride salt than acetate salt because the chloride salt has higher ionizability in ethylene glycol. From the above, we conclude that the kinetics of the wurtzite/NNS to rocksalt/NS phase transformation depends mainly on two factors: (1) nucleation/growth kinetics which is controlled by the ionizability of the precursor salt, solvent, and stability of the metal-ligand complex, and (2) the activation energy barrier of the NNS to NS conversion which is controlled by surface energy minimization with the ligand.

Anionic Coordination Control in Building Cu-Based Electrocatalytic Materials for CO2 Reduction Reaction

Renewable energy-driven conversion of CO2 to value-added fuels and chemicals via electrochemical CO2 reduction reaction (CO2RR) technology is regarded as a promising strategy with substantial environmental and economic benefits to achieve carbon neutrality. Because of its sluggish kinetics and complex reaction paths, developing robust catalytic materials with exceptional selectivity to the targeted products is one of the core issues, especially for extensively concerned Cu-based materials. Manipulating Cu species by anionic coordination is identified as an effective way to improve electrocatalytic performance, in terms of modulating active sites and regulating structural reconstruction. This review elaborates on recent discoveries and progress of Cu-based CO2RR catalytic materials enhanced by anionic coordination control, regarding reaction paths, functional mechanisms, and roles of different non-metallic anions in catalysis. Finally, the review concludes with some personal insights and provides challenges and perspectives on the utilization of this strategy to build desirable electrocatalysts.

Local CO2 reservoir layer promotes rapid and selective electrochemical CO2 reduction

Electrochemical CO2 reduction reaction in aqueous electrolytes is a promising route to produce added-value chemicals and decrease carbon emissions. However, even in Gas-Diffusion Electrode devices, low aqueous CO2 solubility limits catalysis rate and selectivity. Here, we demonstrate that when assembled over a heterogeneous electrocatalyst, a film of nitrile-modified Metal-Organic Framework (MOF) acts as a remarkable CO2-solvation layer that increases its local concentration by ~27-fold compared to bulk electrolyte, reaching 0.82 M. When mounted on a Bi catalyst in a Gas Diffusion Electrode, the MOF drastically improves CO2-to-HCOOH conversion, reaching above 90% selectivity and partial HCOOH currents of 166 mA/cm2 (at -0.9 V vs RHE). The MOF also facilitates catalysis through stabilization of reaction intermediates, as identified by operando infrared spectroscopy and Density Functional Theory. Hence, the presented strategy provides new molecular means to enhance heterogeneous electrochemical CO2 reduction reaction, leading it closer to the requirements for practical implementation.

Aligned InS Nanorods for Efficient Electrocatalytic Carbon Dioxide Reduction

Electrochemical CO₂ reduction technology can combine renewable energy sources with carbon capture and storage to convert CO₂ into industrial chemicals. However, the catalytic activity under high current density and long-term electrocatalysis process may deteriorate due to agglomeration, catalytic polymerization, element dissolution, and phase change of active substances. Here, we report a scalable and facile method to fabricate aligned InS nanorods by chemical dealloying. The resulting aligned InS nanorods exhibit a remarkable CO₂RR activity for selective formate production at a wide potential window, achieving over 90% faradic efficiencies from -0.5 to -1.0 V vs reversible hydrogen electrode (RHE) under gas diffusion cell, as well as continuously long-term operation without deterioration. In situ electrochemical Raman spectroscopy measurements reveal that the *OCHO* species (Bidentate adsorption) are the intermediates that occurred in the reaction of CO₂ reduction to formate. Meanwhile, the presence of sulfur can accelerate the activation of H₂O to react with CO₂, promoting the formation of *OCHO* intermediates on the catalyst surface. Significantly, through additional coupling anodic methanol oxidation reaction (MOR), the unusual two-electrode electrolytic system allows highly energy-efficient and value-added formate manufacturing, thereby reducing energy consumption.

Modeling Operando Electrochemical CO2 Reduction

Since the seminal works on the application of density functional theory and the computational hydrogen electrode to electrochemical CO₂ reduction (eCO₂R) and hydrogen evolution (HER), the modeling of both reactions has quickly evolved for the last two decades. Formulation of thermodynamic and kinetic linear scaling relationships for key intermediates on crystalline materials have led to the definition of activity volcano plots, overpotential diagrams, and full exploitation of these theoretical outcomes at laboratory scale. However, recent studies hint at the role of morphological changes and short-lived intermediates in ruling the catalytic performance under operating conditions, further raising the bar for the modeling of electrocatalytic systems. Here, we highlight some novel methodological approaches employed to address eCO₂R and HER reactions. Moving from the atomic scale to the bulk electrolyte, we first show how ab initio and machine learning methodologies can partially reproduce surface reconstruction under operation, thus identifying active sites and reaction mechanisms if coupled with microkinetic modeling. Later, we introduce the potential of density functional theory and machine learning to interpret data from Operando spectroelectrochemical techniques, such as Raman spectroscopy and extended X-ray absorption fine structure characterization. Next, we review the role of electrolyte and mass transport effects. Finally, we suggest further challenges for computational modeling in the near future as well as our perspective on the directions to follow.

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.

Recent Advances in Catalyst Structure and Composition Engineering Strategies for Regulating CO2 Electrochemical Reduction

Electrochemical CO₂ reduction has been recognized as a promising solution in tackling energy- and environment-related challenges of human society. In the past few years, the rapid development of advanced electrocatalysts has significantly improved the efficiency of this reaction and accelerated the practical applications of this technology. Herein, representative catalyst structures and composition engineering strategies in regulating the CO₂ reduction selectivity and activity toward various products including carbon monoxide, formate, methane, methanol, ethylene, and ethanol are summarized. An overview of in situ/operando characterizations and advanced computational modeling in deepening the understanding of the reaction mechanisms and accelerating catalyst design are also provided. To conclude, future challenges and opportunities in this research field are discussed.

Highly Ethylene-Selective Electrocatalytic CO2 Reduction Enabled by Isolated Cu-S Motifs in Metal-Organic Framework Based Precatalysts

Copper-based materials are efficient electrocatalysts for the conversion of CO₂ to C₂₊ products, and most these materials are reconstructed in situ to regenerate active species. It is a challenge to precisely design precatalysts to obtain active sites for the CO₂ reduction reaction (CO₂ RR). Herein, we develop a strategy based on local sulfur doping of a Cu-based metal-organic framework precatalyst, in which the stable Cu-S motif is dispersed in the framework of HKUST-1 (S-HKUST-1). The precatalyst exhibits a high ethylene selectivity in an H-type cell with a maximum faradaic efficiency (FE) of 60.0 %, and delivers a current density of 400 mA cm^-2 with an ethylene FE up to 57.2 % in a flow cell. Operando X-ray absorption results demonstrate that Cu^δ+ species stabilized by the Cu-S motif exist in S-HKUST-1 during CO₂ RR. Density functional theory calculations indicate the partially oxidized Cu^δ+ at the Cu/Cu_x S_y interface is favorable for coupling of the *CO intermediate due to the modest distance between coupling sites and optimized adsorption energy.

Dynamic Restructuring of Cu-Doped SnS2 Nanoflowers for Highly Selective Electrochemical CO2 Reduction to Formate

With ever-increasing energy consumption and continuous rise in atmospheric CO₂ concentration, electrochemical reduction of CO₂ into chemicals/fuels is becoming a promising yet challenging solution. Sn-based materials are identified as attractive electrocatalysts for the CO₂ reduction reaction (CO₂ RR) to formate but suffer from insufficient selectivity and activity, especially at large cathodic current densities. Herein, we demonstrate that Cu-doped SnS₂ nanoflowers can undergo in situ dynamic restructuring to generate catalytically active S-doped Cu/Sn alloy for highly selective electrochemical CO₂ RR to formate over a wide potential window. Theoretical thermodynamic analysis of reaction energetics indicates that the optimal electronic structure of the Sn active site can be regulated by both S-doping and Cu-alloying to favor formate formation, while the CO and H₂ pathways will be suppressed. Our findings provide a rational strategy for electronic modulation of metal active site(s) for the design of active and selective electrocatalysts towards CO₂ RR.

CuS-Decorated GaN Nanowires on Silicon Photocathodes for Converting CO2 Mixture Gas to HCOOH

Hybrid materials consisting of semiconductors and cocatalysts have been widely used for photoelectrochemical (PEC) conversion of CO₂ gas to value-added chemicals such as formic acid (HCOOH). To date, however, the rational design of catalytic architecture enabling the reduction of real CO₂ gas to chemical has remained a grand challenge. Here, we report a unique photocathode consisting of CuS-decorated GaN nanowires (NWs) integrated on planar silicon (Si) for the conversion of H₂S-containing CO₂ mixture gas to HCOOH. It was discovered that H₂S impurity in the modeled industrial CO₂ gas could lead to the spontaneous transformation of Cu to CuS NPs, which resulted in significantly increased faradaic efficiency of HCOOH generation. The CuS/GaN/Si photocathode exhibited superior faradaic efficiency of HCOOH = 70.2% and partial current density = 7.07 mA/cm² at -1.0 V_RHE under AM1.5G 1 sun illumination. To our knowledge, this is the first demonstration that impurity mixed in the CO₂ gas can enhance, rather than degrade, the performance of the PEC CO₂ reduction reaction.

Catalytic Cooperation between a Copper Oxide Electrocatalyst and a Microbial Community for Microbial Electrosynthesis

Electrocatalytic metals and microorganisms can be combined for CO₂ conversion in microbial electrosynthesis (MES). However, a systematic investigation on the nature of interactions between metals and MES is still lacking. To investigate this nature, we integrated a copper electrocatalyst, converting CO₂ to formate, with microorganisms, converting CO₂ to acetate. A co-catalytic (i. e. metabolic) relationship was evident, as up to 140 mg L^-1 of formate was produced solely by copper oxide, while formate was also evidently produced by copper and consumed by microorganisms producing acetate. Due to non-metabolic interactions, current density decreased by over 4 times, though acetate yield increased by 3.3 times. Despite the antimicrobial role of copper, biofilm formation was possible on a pure copper surface. Overall, we show for the first time that a CO₂ -reducing copper electrocatalyst can be combined with MES under biological conditions, resulting in metabolic and non-metabolic interactions.

Electrocatalysis for CO2 conversion: from fundamentals to value-added products

This timely and comprehensive review mainly summarizes advances in heterogeneous electroreduction of CO₂: from fundamentals to value-added products.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H₂ splitting by a cooperative mechanism with metal nanoparticles.

Electrocatalytic CO2 Reduction over Cu3P Nanoparticles Generated via a Molecular Precursor Route

The design of nanoparticles (NPs) with tailored morphologies and finely tuned electronic and physical properties has become a key strategy for controlling selectivity and improving conversion efficiency in a variety of important electrocatalytic transformations. Transition metal phosphide NPs, in particular, have emerged as a versatile class of catalytic materials due to their multifunctional active sites and composition- and phase-dependent properties. Access to targeted transition metal phosphide NPs with controlled features is necessary to tune the catalytic activity. To this end, we have established a solution-synthesis route utilizing a molecular precursor containing M-P bonds to generate solid metal phosphide NPs with controlled stoichiometry and morphology. We expand here the application of molecular precursors in metal phosphide NP synthesis to include the preparation of phase-pure Cu3P NPs from the thermal decomposition of [Cu(H)(PPh3)]6. The mechanism of [Cu(H)(PPh3)]6 decomposition and subsequent formation of Cu3P was investigated through modification of the reaction parameters. Identification and optimization of the critical reaction parameters (i.e., time, temperature, and oleylamine concentration) enabled the synthesis of phase-pure 9-11 nm Cu3P NPs. To probe the multifunctionality of this materials system, Cu3P NPs were investigated as an electrocatalyst for CO2 reduction. At low overpotential (-0.30 V versus RHE) in 0.1 M KHCO3 electrolyte, Cu3P-modified carbon paper electrodes produced formate (HCOO-) at a maximum Faradaic efficiency of 8%.

Spontaneously Formed CuSx Catalysts for Selective and Stable Electrochemical Reduction of Industrial CO2 Gas to Formate

The electrochemical CO₂ reduction in aqueous media is a promising method for both the mitigation of climate changes and the generation of value-added fuels. Although many researchers have demonstrated selective and stable catalysts for electrochemical reduction of pure CO₂ gas, the conversion of industrial CO₂ gas has been limited. Here, we fabricated the copper sulfide catalysts (CuS_x), which were spontaneously formed by dipping a Cu foil into a laboratory-prepared industrial CO₂-purged 0.1 M KHCO₃ electrolyte. Because industrial CO₂ contains H₂S gas, sulfur species dissolved in the electrolyte can easily react with the Cu foil. As the concentration of dissolved sulfur species increased, the reaction between the Cu foil and sulfur enhanced. As a result, the average size and surface density of CuS_x nanoparticles (NPs) increased to 133.2 ± 33.1 nm and 86.2 ± 3.3%, respectively. Because of the larger amount of sulfur content and the enlarged electrochemical surface area of CuS_x NPs, the Faradaic efficiency (FE) of formate was improved from 22.7 to 72.0% at -0.6 V_RHE. Additionally, CuS_x catalysts showed excellent stability in reducing industrial CO₂ to formate. The change in FE was hardly observed even after long-term (72 h) operation. This study experimentally demonstrated that spontaneously formed CuS_x catalysts are efficient and stable for reducing the industrial CO₂ gas to formate.

Au/Pb Interface Allows the Methane Formation Pathway in Carbon Dioxide Electroreduction

The electrochemical conversion of carbon dioxide (CO₂) to high-value chemicals is an attractive approach to create an artificial carbon cycle. Tuning the activity and product selectivity while maintaining long-term stability, however, remains a significant challenge. Here, we study a series of Au–Pb bimetallic electrocatalysts with different Au/Pb interfaces, generating carbon monoxide (CO), formic acid (HCOOH), and methane (CH₄) as CO₂ reduction products. The formation of CH₄ is significant because it has only been observed on very few Cu-free electrodes. The maximum CH₄ formation rate of 0.33 mA cm^–2 was achieved when the most Au/Pb interfaces were present. In situ Raman spectroelectrochemical studies confirmed the stability of the Pb native substoichiometric oxide under the reduction conditions on the Au–Pb catalyst, which seems to be a major contributor to CH₄ formation. Density functional theory simulations showed that without Au, the reaction would get stuck on the COOH intermediate, and without O, the reaction would not evolve further than the CHOH intermediate. In addition, they confirmed that the Au/Pb bimetallic interface (together with the subsurface oxygen in the model) possesses a moderate binding strength for the key intermediates, which is indeed necessary for the CH₄ pathway. Overall, this study demonstrates how bimetallic nanoparticles can be employed to overcome scaling relations in the CO₂ reduction reaction.

Electrochemically Driven Cation Exchange Enables the Rational Design of Active CO2 Reduction Electrocatalysts

Metal oxides or sulfides are considered to be one of the most promising CO₂ reduction reaction (CO₂ RR) precatalysts, owing to their electrochemical conversion in situ into highly active electrocatalytic species. However, further improvement of the performance requires new tools to gain fine control over the composition of the active species and its structural features [e.g., grain boundaries (GBs) and undercoordinated sites (USs)], directly from a predesigned template material. Herein, we describe a novel electrochemically driven cation exchange (ED-CE) method that enables the conversion of a predesigned CoS₂ template into a CO₂ RR catalyst, Cu₂ S. By means of ED-CE, the final Cu₂ S catalyst inherits the original 3 D morphology of CoS₂ , and preserves its high density of GBs. Additionally, the catalyst's phase structure, composition, and density of USs were precisely tuned, thus enabling rational design of active CO₂ RR sites. The obtained Cu₂ S catalyst achieved a CO₂ -to-formate Faradaic efficiency of over 87 % and a record high activity (among reported Cu-based catalysts). Hence, this study opens the way for utilization of ED-CE reactions to design advanced electrocatalysts.

Hierarchical heterostructure of SnO2 confined on CuS nanosheets for efficient electrocatalytic CO2 reduction

The direct electroreduction of CO2 to ratio-tunable syngas (CO + H2) is an appealing solution to provide important feedstocks for many industrial processes. However, low-cost, Earth-abundant yet efficient and stable electrocatalysts for composition-adjustable syngas have still not been realized for practical applications. Herein, new hierarchical 0D/2D heterostructures of SnO2 nanoparticles (NPs) confined on CuS nanosheets (NSs) were designed to enable CO2 electroreduction to a wide-range syngas (CO/H2: 0.11-3.86) with high faradaic efficiency (>85%), remarkable turnover frequency (96.12 h-1) and excellent durability (over 24 h). Detailed experimental characterization studies together with theoretical calculations manifest that the ascendant catalytic performance is not only attributed to the heterostructure of ultrasmall SnO2 NPs homogeneously confined on ultrathin CuS NSs, which endows the maximum exposure of active sites and faster charge transfer, but is also accounted by the strong interaction between well-defined SnO2 and CuS interfaces, which modulated reaction free-energies of reaction intermediates and hence improved the activity of CO2 electroreduction to highly ratio-tunable syngas. This work provides a better understanding and a new strategy for intermediate regulation by interface engineering of hereostructures for CO2 reduction and beyond.

Photoelectrochemical solar fuels from carbon dioxide, water and sunlight

It is science, not magic. Solar fuels can be obtained from sunlight, water and carbon dioxide.

Promoting electrocatalytic CO2 reduction to formate via sulfur-boosting water activation on indium surfaces

Electrocatalytic reduction of CO₂ to fuels and chemicals is one of the most attractive routes for CO₂ utilization. Current catalysts suffer from low faradaic efficiency of a CO₂-reduction product at high current density (or reaction rate). Here, we report that a sulfur-doped indium catalyst exhibits high faradaic efficiency of formate (>85%) in a broad range of current density (25–100 mA cm⁻²) for electrocatalytic CO₂ reduction in aqueous media. The formation rate of formate reaches 1449 μmol h⁻¹ cm⁻² with 93% faradaic efficiency, the highest value reported to date. Our studies suggest that sulfur accelerates CO₂ reduction by a unique mechanism. Sulfur enhances the activation of water, forming hydrogen species that can readily react with CO₂ to produce formate. The promoting effect of chalcogen modifiers can be extended to other metal catalysts. This work offers a simple and useful strategy for designing both active and selective electrocatalysts for CO₂ reduction.

CO₂ conversion to liquid fuels provides an appealing means to remove the greenhouse gas, although it is challenging to find materials that are both active and selective. Here, authors show sulfur-doped indium to be a highly active and selective electrocatalyst that transforms CO₂ into formate.