Positive Valent Metal Sites in Electrochemical CO2 Reduction Reaction

The discovery of high-performance catalysts for the electrochemical CO₂ reduction reaction ( CO₂RR) has faced an enormous challenge for years. The lack of cognition about the surface active structures or centers of catalysts in complex conditions limits the development of advanced catalysts for CO₂RR. Recently, the positive valent metal sites (PVMS) are demonstrated as a kind of potential active sites, which can facilitate carbon dioxide (CO₂) activation and conversation but are always unstable under reduction potentials. Many advanced technologies in theory and experiment have been utilized to understand and develop excellent catalysts with PVMS for CO₂RR. Here, we present an introduction of some typical catalysts with PVMS in CO₂RR and give some understanding of the activity and stability for these related catalysts.     

Electrochemical CO2 reduction reaction on cost-effective oxide-derived copper and transition metal–nitrogen–carbon catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) either to generate multicarbon (C₂₊) or single carbon (C₁) value-added products provides an effective and promising approach to mitigate the high CO₂ concentration in the atmosphere and promote energy storage. However, cost-effectiveness of catalytic materials limits practical application of this technology in the short term. Herein, we summarize and discuss recent and advanced works on cost-effective oxide-derived copper catalysts for the generation of C₂₊ products (hydrocarbons and alcohols) and transition metal–nitrogen–doped carbon electrocatalytic materials for C₁ compounds production from CO₂RR. We think they represent suitable electrocatalyst candidates for scaling up electrochemical CO₂ conversion. This short review may provide inspiration for the future design and development of innovative active, cost-effective, selective and stable electrocatalysts with improved properties for either the production of C₂₊ (alcohols, hydrocarbons) or carbon monoxide from CO₂RR.     

Recent advances in single atom catalysts for the electrochemical carbon dioxide reduction reaction

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels. Single atom catalysts (SACs) are encouraging to catalyze the electrochemical CO₂RR due to the tunable electronic structure of the central metal atoms, which can regulate the adsorption energy of reactants and reaction intermediates. Moreover, SACs form a bridge between homogeneous and heterogeneous catalysts, providing an ideal platform to explore the reaction mechanism of electrochemical reactions. In this review, we first discuss the strategies for promoting the CO₂RR performance, including suppression of the hydrogen evolution reaction (HER), generation of C₁ products and formation of C₂₊ products. Then, we summarize the recent developments in regulating the structure of SACs toward the CO₂RR based on the above aspects. Finally, several issues regarding the development of SACs for the CO₂RR are raised and possible solutions are provided.

The electrochemical carbon dioxide reduction reaction (CO₂RR) offers a promising solution to mitigate carbon emission and at the same time generate valuable carbonaceous chemicals/fuels.     

Low-coordination Nanocrystalline Copper-based Catalysts through Theory-guided Electrochemical Restructuring for Selective CO2 Reduction to Ethylene

Revealing the dynamic reconstruction process and tailoring advanced copper (Cu) catalysts is of paramount significance for promoting the conversion of CO₂ into ethylene (C₂H₄), paving the way for carbon neutralization and facilitating renewable energy storage. In this study, we initially employed density functional theory (DFT) and molecular dynamics (MD) simulations to elucidate the restructuring behavior of a catalyst under electrochemical conditions and delineated its restructuring patterns. Leveraging insights into this restructuring behavior, we devised an efficient, low-coordination copper-based catalyst. The resulting synthesized catalyst demonstrated an impressive Faradaic efficiency (FE) exceeding 70 % for ethylene generation at a current density of 800 mA cm⁻². Furthermore, it showed robust stability, maintaining consistent performance for 230 hours at a cell voltage of 3.5 V in a full-cell system. Our research not only deepens the understanding of the active sites involved in designing efficient carbon dioxide reduction reaction (CO₂RR) catalysts but also advances CO₂ electrolysis technologies for industrial application.     

Effect of pore diameter and length on electrochemical CO2 reduction reaction at nanoporous gold catalysts

In this work, we employ differential electrochemical mass spectrometry (DEMS) to track the real-time evolution of CO at nanoporous gold (NpAu) catalysts with varying pore parameters (diameter and length) during the electrochemical CO₂ reduction reaction (CO₂RR). We show that due to the increase in the local pH with increasing catalyst roughness, NpAu catalysts suppress the bicarbonate-mediated hydrogen evolution reaction (HER) compared to a flat Au electrode. Additionally, the geometric current density for CO₂RR increases with the roughness of NpAu catalysts, which we attribute to the increased availability of active sites at NpAu catalysts. Together, the enhancement of CO₂RR and the suppression of competing HER results in a drastic increase in the faradaic selectivity for CO₂RR with increasing pore length and decreasing pore diameter, reaching near 100% faradaic efficiency for CO in the most extreme case. Interestingly, unlike the geometric current density, the specific current density for CO₂RR has a more complicated relation with the roughness of the NpAu catalysts. We show that this is due to the presence of ohmic drop effects along the length of the porous channels. These ohmic drop effects render the pores partially electrocatalytically inactive and hence, they play an important role in tuning the CO₂RR activity on nanoporous catalysts.

In this work, we employ differential electrochemical mass spectrometry (DEMS) to track the real-time evolution of CO at nanoporous gold (NpAu) catalysts with varying pore parameters (diameter and length) during the electrochemical CO₂ reduction reaction (CO₂RR).     

Electrochemical Reduction of Carbon Dioxide to Ethanol: An Approach to Transforming Greenhouse Gas to Fuel Source

Converting carbon dioxide (CO₂) into high-value fuels or chemicals is considered as a promising way to utilize CO₂ and alleviate the excessive greenhouse gas emission. Among multiple catalysis approaches, electrochemical reduction of CO₂ to ethanol has an important prospect due to the high energy density and widely applications of ethanol. In recent years, many electrocatalysts for CO₂ reduce reaction (CO₂RR) have shown promising catalytic activity for ethanol production. In this review, we will introduce the recent progress in this field. The basic principles and electrochemical performances of CO₂RR are reviewed at first. Then, several categories of active electrocatalysts for CO₂RR to ethanol are summarized, including the discussion of reaction mechanism and catalytic sites. Finally, several possible strategies are proposed, providing guidance for future design and preparation of high-performance catalysts.     

Key factors for designing single-atom metal-nitrogen-carbon catalysts for electrochemical CO2 reduction

The electrochemical CO₂ reduction (CO₂RR) is a sustainable approach to mitigate the increased CO₂ emissions and simultaneously produce value-added chemicals and fuels. Metal-nitrogen-carbon (M-N-C) based single-atom catalysts (SACs) have emerged as promising electrocatalysts for CO₂RR with high activity, selectivity, and stability. To design efficient SACs for CO₂RR, the key influence factors need to be understood. Here, we summarize recent achievements on M-N-C SACs for CO₂RR and highlight the significance of the key constituting factors, metal sites, the coordination environment, and the substrates, for achieving high CO₂RR performance. The perspective views and guidelines are provided for the future direction of developing M-N-C SACs as CO₂RR catalysts.     

Enhancing CO2 Electroreduction to Methane with a Cobalt Phthalocyanine and Zinc–Nitrogen–Carbon Tandem Catalyst

Developing copper-free catalysts for CO₂ conversion into hydrocarbons and oxygenates is highly desirable for electrochemical CO₂ reduction reaction (CO₂RR). Herein, we report a cobalt phthalocyanine (CoPc) and zinc–nitrogen–carbon (Zn-N-C) tandem catalyst for CO₂RR to CH₄. This tandem catalyst shows a more than 100 times enhancement of the CH₄/CO production rate ratio compared with CoPc or Zn-N-C alone. Density functional theory (DFT) calculations and electrochemical CO reduction reaction results suggest that CO₂ is first reduced into CO over CoPc and then CO diffuses onto Zn-N-C for further conversion into CH₄ over Zn-N₄ site, decoupling complicated CO₂RR pathway on single active site into a two-step tandem reaction. Moreover, mechanistic analysis indicates that CoPc not only generates CO but also enhances the availability of *H over adjacent N sites in Zn-N₄, which is the key to achieve the high CH₄ production rate and understand the intriguing electrocatalytic behavior which is distinctive to copper-based tandem catalysts.     

Electrochemical CO2 Activation and Valorization on Metallic Copper and Carbon-Embedded N-Coordinated Single Metal MNC Catalysts

The electrochemical reductive valorization of CO₂, referred to as the CO2RR, is an emerging approach for the conversion of CO₂-containing feeds into valuable carbonaceous fuels and chemicals, with potential contributions to carbon capture and use (CCU) for reducing greenhouse gas emissions. Copper surfaces and graphene-embedded, N-coordinated single metal atom (MNC) catalysts exhibit distinctive reactivity, attracting attention as efficient electrocatalysts for CO2RR. This review offers a comparative analysis of CO2RR on copper surfaces and MNC catalysts, highlighting their unique characteristics in terms of CO₂ activation, C₁/C₂₍₊₎ product formation, and the competing hydrogen evolution pathway. The assessment underscores the significance of understanding structure–activity relationships to optimize catalyst design for efficient and selective CO2RR. Examining detailed reaction mechanisms and structure-selectivity patterns, the analysis explores recent insights into changes in the chemical catalyst states, atomic motif rearrangements, and fractal agglomeration, providing essential kinetic information from advanced in/ex situ microscopy/spectroscopy techniques. At the end, this review addresses future challenges and solutions related to today's disconnect between our current molecular understanding of structure–activity-selectivity relations in CO2RR and the relevant factors controlling the performance of CO₂ electrolyzers over longer times, with larger electrode sizes, and at higher current densities.     

Transition Metal Nitrides as Promising Catalyst Supports for Tuning CO/H2 Syngas Production from Electrochemical CO2 Reduction

The electrochemical carbon dioxide reduction reaction (CO₂RR) to produce synthesis gas (syngas) with tunable CO/H₂ ratios has been studied by supporting Pd catalysts on transition metal nitride (TMN) substrates. Combining experimental measurements and density functional theory (DFT) calculations, Pd‐modified niobium nitride (Pd/NbN) is found to generate much higher CO and H₂ partial current densities and greater CO Faradaic efficiency than Pd‐modified vanadium nitride (Pd/VN) and commercial Pd/C catalysts. In‐situ X‐ray diffraction identifies the formation of PdH in Pd/NbN and Pd/C under CO₂RR conditions, whereas the Pd in Pd/VN is not fully transformed into the active PdH phase. DFT calculations show that the stabilized *HOCO and weakened *CO intermediates on PdH/NbN are critical to achieving higher CO₂RR activity. This work suggests that NbN is a promising substrate to modify Pd, resulting in an enhanced electrochemical conversion of CO₂ to syngas with a potential reduction in precious metal loading.     

Recent advances in carbon-based materials for electrochemical CO2 reduction reaction

The increase of atmospheric CO₂ concentration has caused many environmental issues. Electrochemical CO₂ reduction reaction（CO₂RR） has been considered as a promising strategy to mitigate these challenges. The electrocatalysts with a low overpotential, high Faradaic efficiency, and excellent selectivity are of great significance for the CO₂RR. Carbon-based materials including metal-free carbon catalysts and metal-based carbon catalysts have shown great p...     

The atomic-level regulation of single-atom site catalysts for the electrochemical CO2 reduction reaction

The electrochemical CO₂ reduction reaction (CO₂RR) is viewed as a promising way to remove the greenhouse gas CO₂ from the atmosphere and convert it into useful industrial products such as methane, methanol, formate, ethanol, and so forth. Single-atom site catalysts (SACs) featuring maximum theoretical atom utilization and a unique electronic structure and coordination environment have emerged as promising candidates for use in the CO₂RR. The electronic properties and atomic structures of the central metal sites in SACs will be changed significantly once the types or coordination environments of the central metal sites are altered, which appears to provide new routes for engineering SACs for CO₂ electrocatalysis. Therefore, it is of great importance to discuss the structural regulation of SACs at the atomic level and their influence on CO₂RR activity and selectivity. Despite substantial efforts being made to fabricate various SACs, the principles of regulating the intrinsic electrocatalytic performances of the single-atom sites still needs to be sufficiently emphasized. In this perspective article, we present the latest progress relating to the synthesis and catalytic performance of SACs for the electrochemical CO₂RR. We summarize the atomic-level regulation of SACs for the electrochemical CO₂RR from five aspects: the regulation of the central metal atoms, the coordination environments, the interface of single metal complex sites, multi-atom active sites, and other ingenious strategies to improve the performance of SACs. We highlight synthesis strategies and structural design approaches for SACs with unique geometric structures and discuss how the structure affects the catalytic properties.

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising way to remove CO₂ and convert it into useful industrial products. Single-atom site catalysts provide opportunities to regulate the active sites of CO₂RR catalysts at the atomic level.     

Halogen-Incorporated Sn Catalysts for Selective Electrochemical CO2 Reduction to Formate

Electrochemically reducing CO₂ to valuable fuels or feedstocks is recognized as a promising strategy to simultaneously tackle the crises of fossil fuel shortage and carbon emission. Sn-based catalysts have been widely studied for electrochemical CO₂ reduction reaction (CO₂RR) to make formic acid/formate, which unfortunately still suffer from low activity, selectivity and stability. In this work, halogen (F, Cl, Br or I) was introduced into the Sn catalyst by a facile hydrolysis method. The presence of halogen was confirmed by a collection of ex situ and in situ characterizations, which rendered a more positive valence state of Sn in halogen-incorporated Sn catalyst as compared to unmodified Sn under cathodic potentials in CO₂RR and therefore tuned the adsorption strength of the key intermediate (*OCHO) toward formate formation. As a result, the halogen-incorporated Sn catalyst exhibited greatly enhanced catalytic performance in electrochemical CO₂RR to produce formate.     

Design and Preparation of Electrocatalysts by Electrodeposition for CO2 Reduction

With the increasing emission of carbon dioxide (CO₂), the conversion and utilization of CO₂ have become a topic of increasing concern. Electrochemical CO₂ reduction reaction (CO₂RR) is an attractive and sustainable approach for solving energy and environmental problems. Design of efficient catalysts is crucial for achieving highly efficient CO₂RR. Different methods to prepare catalysts have been reported and used. Among them, electrodeposition is one of the common approaches, which has some obvious advantages, such as requiring simple equipment, environmentally benign. Especially, it can direct deposit catalysts on different substrates to prepare electrodes for CO₂RR. In this review, we discuss recent advances in design and preparation of the catalysts by electrodeposition and their applications in CO₂RR. Furthermore, the perspective of this promising area is also discussed.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single‐Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen‐doped carbon supported single‐atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single‐atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm^?2) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single‐atom configurations for the H₂ and CO evolution. The results present a useful case on how non‐precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single-Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen-doped carbon supported single-atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single-atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm⁻²) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single-atom configurations for the H₂ and CO evolution. The results present a useful case on how non-precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

Time-Resolved Product Observation for CO2 Electroreduction Using Synchronised Electrochemistry-Mass Spectrometry with Soft Ionisation (sEC-MS-SI)

The mechanistic understanding of electrochemical CO₂ reduction reaction (CO₂RR) requires a rapid and accurate characterisation of product distribution to unravel the activity and selectivity, which is yet hampered by the lack of advanced correlative approaches. Here, we present the time-resolved identification of CO₂RR products by using the synchronised electrochemistry-mass spectrometry (sEC-MS). Transients in product formation can be readily captured in relation to electrochemical conditions. Moreover, a soft ionisation (SI) strategy is developed in MS for the direct observation of CO, immune to the interference of CO₂ fragments. With the sEC-MS-SI, the kinetic information, such as Tafel slopes and onset potentials, for a myriad of CO₂RR products are revealed and we show the hysteresis seen for the evolution of some species may originate from the potential-driven changes in surface coverage of intermediates. This work provides a real-time picture of the dynamic formation of CO₂RR products.     

Selective CO2 Electroreduction to Ethylene and Multicarbon Alcohols via Electrolyte‐Driven Nanostructuring

Production of multicarbon products (C₂₊) from CO₂ electroreduction reaction (CO₂RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO₂RR catalysts that are highly selective for C₂₊ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO₂ into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO₃ solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm^?2 for C₂₊ products at ?0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C₂₊ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu⁺ species, and the adsorbed halides.     

Liquid Fluxional Ga Single Atom Catalysts for Efficient Electrochemical CO2 Reduction

Precise design and tuning of the micro-atomic structure of single atom catalysts (SACs) can help efficiently adapt complex catalytic systems. Herein, we inventively found that when the active center of the main group element gallium (Ga) is downsized to the atomic level, whose characteristic has significant differences from conventional bulk and rigid Ga catalysts. The Ga SACs with a P, S atomic coordination environment display specific flow properties, showing CO products with FE of ≈92 % at −0.3 V vs. RHE in electrochemical CO₂ reduction (CO₂RR). Theoretical simulations demonstrate that the adaptive dynamic transition of Ga optimizes the adsorption energy of the *COOH intermediate and renews the active sites in time, leading to excellent CO₂RR selectivity and stability. This liquid single atom catalysts system with dynamic interfaces lays the foundation for future exploration of synthesis and catalysis.     

In Situ Bismuth Nanosheet Assembly for Highly Selective Electrocatalytic CO2 Reduction to Formate

The reduction of carbon dioxide (CO₂) into value-added fuels using an electrochemical method has been regarded as a compelling sustainable energy conversion technology. However, high-performance electrocatalysts for CO₂ reduction reaction (CO₂RR) with high formate selectivity and good stability need to be improved. Earth-abundant Bi has been demonstrated to be active for CO₂RR to formate. Herein, we fabricated an extremely active and selective bismuth nanosheet (Bi-NSs) assembly via an in situ electrochemical transformation of (BiO)₂CO₃ nanostructures. The as-prepared material exhibits high activity and selectivity for CO₂RR to formate, with nearly 94% faradaic efficiency at −1.03 V (versus reversible hydrogen electrode (vs. RHE)) and stable selectivity (>90%) in a large potential window ranging from −0.83 to −1.18 V (vs. RHE) and excellent durability during 12 h continuous electrolysis. In addition, the Bi-NSs based CO₂RR/methanol oxidation reaction (CO₂RR/MOR) electrolytic system for overall CO₂ splitting was constructed, evidencing the feasibility of its practical implementation.