Enhanced Electroreduction of Carbon Dioxide to Methanol Using Zinc Dendrites Pulse-Deposited on Silver Foam

Addressing the Carbonate Issue: Electrocatalysts for Acidic CO2 Reduction Reaction

Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy provides a promising route to CO2 conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO2 to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO2RR in acidic electrolytes offers a promising solution to address the "carbonate issue"; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO2RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO2RR. Subsequently, an in-depth analysis of recent advancements in acidic CO2RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device-level applications is summarized, aiming to develop high-performance acidic CO2RR systems. Finally, the existing challenges and future directions in the design of acidic CO2RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

Recent insights on the use of modified Zn-based catalysts in eCO2RR

Converting CO2 into valuable chemicals can provide a new path to mitigate the greenhouse effect, achieving the aim of "carbon neutrality" and "carbon peaking". Among numerous electrocatalysts, Zn-based materials are widely distributed and cheap, making them one of the most promising electrocatalyst materials to replace noble metal catalysts. Moreover, the Zn metal itself has a certain selectivity for CO. After appropriate modification, such as oxide derivatization, structural reorganization, reconstruction of the surfaces, heteroatom doping, and so on, the Zn-based electrocatalysts can expose more active sites and adjust the d-band center or electronic structure, and the FE and stability of them can be effectively improved, and they can even convert CO2 to multi-carbon products. This review aims to systematically describe the latest progresses of modified Zn-based electrocatalyst materials (including organic and inorganic materials) in the electrocatalytic carbon dioxide reduction reaction (eCO2RR). The applications of modified Zn-based catalysts in improving product selectivity, increasing current density and reducing the overpotential of the eCO2RR are reviewed. Moreover, this review describes the reasonable selection and good structural design of Zn-based catalysts, presents the characteristics of various modified zinc-based catalysts, and reveals the related catalytic mechanisms for the first time. Finally, the current status and development prospects of modified Zn-based catalysts in eCO2RR are summarized and discussed.

Direct Electroreduction of Carbonate to Formate

A cause of losses in energy and carbon conversion efficiencies during the electrochemical CO2 reduction reaction (eCO2RR) can be attributed to the formation of carbonates (CO32-), which is generally considered to be an electrochemically inert species. Herein, using in situ Raman spectroscopy, liquid chromatography, 1H nuclear magnetic resonance spectroscopy, 13C and deuterium isotope labeling, and density functional theory simulations, we show that carbonate intermediates are adsorbed on a copper electrode during eCO2RR in KHCO3 electrolyte from 0.2 to -1.0 VRHE. These intermediates can be reduced to formate at -0.4 VRHE and more negative potentials. This finding is supported by our observation of formate from the reduction of Cu2(CO3)(OH)2. Pulse electrolysis on a copper electrode immersed in a N2-purged K2CO3 electrolyte was also performed. We found that the carbonate anions therein could be first adsorbed at -0.05 VRHE and then directly reduced to formate at -0.5 VRHE (overpotential of 0.28 V) with a Faradaic efficiency of 0.61%. The nature of the active sites generating the adsorbed carbonate species and the mechanism for the pulse-enabled reduction of carbonate to formate were elucidated. Our findings reveal how carbonates are directly reduced to a high-value product such as formate and open a potential pathway to mitigate carbonate formation during eCO2RR.

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion

Electrocatalytic CO2 reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti3 C2 Tx MXene-regulated Ag-ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag-ZnO/Ti3 C2 Tx catalyst achieves an outstanding CO2 conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm-2 at -0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag-ZnO interfaces contributes the high selectivity of CO. The CO2 conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO2 reduction.

CO Electroreduction Mechanism on Single-Atom Zn (101) Surfaces: Pathway to C2 Products

Electrocatalytic reduction of carbon dioxide (CO₂RR) employs electricity to store renewable energy in the form of reduction products. The activity and selectivity of the reaction depend on the inherent properties of electrode materials. Single-atom alloys (SAAs) exhibit high atomic utilization efficiency and unique catalytic activity, making them promising alternatives to precious metal catalysts. In this study, density functional theory (DFT) was employed to predict stability and high catalytic activity of Cu/Zn (101) and Pd/Zn (101) catalysts in the electrochemical environment at the single-atom reaction site. The mechanism of C2 products (glyoxal, acetaldehyde, ethylene, and ethane) produced by electrochemical reduction on the surface was elucidated. The C-C coupling process occurs through the CO dimerization mechanism, and the formation of the *CHOCO intermediate proves beneficial, as it inhibits both HER and CO protonation. Furthermore, the synergistic effect between single atoms and Zn results in a distinct adsorption behavior of intermediates compared to traditional metals, giving SAAs unique selectivity towards the C2 mechanism. At lower voltages, the Zn (101) single-atom alloy demonstrates the most advantageous performance in generating ethane on the surface, while acetaldehyde and ethylene exhibit significant certain potential. These findings establish a theoretical foundation for the design of more efficient and selective carbon dioxide catalysts.

Upcycling air pollutants to fuels and chemicals via electrochemical reduction technology

The intensification of fossil fuel usage results in significant air pollution levels. Efforts have been put into developing efficient technologies capable of converting air pollution into valuable products, including fuels and valuable chemicals (e.g., CO₂ to hydrocarbon and syngas and NO_x to ammonia). Among the strategic efforts to mitigate the excessive concentration of CO₂ and NO_x pollutants in the atmosphere, the electrochemical reduction technology of CO₂ (CO₂RR) and NO_x (NO_xRR) emerges as one of the most promising approaches. It is even more attractive if CO₂RR and NO_xRR are paired with renewables to store intermittent electricity in the form of chemical feedstocks. This review provides an overview of the electrochemical reduction process to convert CO₂ to C₁ and/or C₂₊ chemicals and NO_x to ammonia (NH₃) with a focus on electrocatalysts, electrolytes, electrolyzer, and catalytic reactor designs toward highly selective electrochemical conversion of the desired products. While the attempts in these aspects are enormous, economic consideration and environmental feasibility for actual implementation are not comprehensively provided. We discuss CO₂RR and NO_xRR from the life cycle and techno-economic analyses to perceive the feasibility of the current achievements. The remaining challenges associated with the industrial implementation of electrochemical CO₂ and NO_x reduction are additionally provided.

Single-Atom Catalysts in Environmental Engineering: Progress, Outlook and Challenges

Recently, single-atom catalysts (SACs) have attracted wide attention in the field of environmental engineering. Compared with their nanoparticle counterparts, SACs possess high atomic efficiency, unique catalytic activity, and selectivity. This review summarizes recent studies on the environmental remediation applications of SACs in (1) gaseous: volatile organic compounds (VOCs) treatment, NO_x reduction, CO₂ reduction, and CO oxidation; (2) aqueous: Fenton-like advanced oxidation processes (AOPs), hydrodehalogenation, and nitrate/nitrite reduction. We present the treatment activities and reaction mechanisms of various SACs and propose challenges and future opportunities. We believe that this review will provide constructive inspiration and direction for future SAC research in environmental engineering.

Recent progress of theoretical studies on electro- and photo-chemical conversion of CO2 with single-atom catalysts

The CO₂ reduction reaction (CO₂RR) into chemical products is a promising and efficient way to combat the global warming issue and greenhouse effect. The viability of the CO₂RR critically rests with finding highly active and selective catalysts that can accomplish the desired chemical transformation. Single-atom catalysts (SACs) are ideal in fulfilling this goal due to the well-defined active sites and support-tunable electronic structure, and exhibit enhanced activity and high selectivity for the CO₂RR. In this review, we present the recent progress of quantum-theoretical studies on electro- and photo-chemical conversion of CO₂ with SACs and frameworks. Various calculated products of CO₂RR with SACs have been discussed, including CO, acids, alcohols, hydrocarbons and other organics. Meanwhile, the critical challenges and the pathway towards improving the efficiency of the CO₂RR have also been discussed.

Selective CO2 electroreduction to methanol via enhanced oxygen bonding

The reduction of carbon dioxide using electrochemical cells is an appealing technology to store renewable electricity in a chemical form. The preferential adsorption of oxygen over carbon atoms of intermediates could improve the methanol selectivity due to the retention of C-O bond. However, the adsorbent-surface interaction is mainly related to the d states of transition metals in catalysts, thus it is difficult to promote the formation of oxygen-bound intermediates without affecting the carbon affinity. This paper describes the construction of a molybdenum-based metal carbide catalyst that promotes the formation and adsorption of oxygen-bound intermediates, where the sp states in catalyst are enabled to participate in the bonding of intermediates. A high Faradaic efficiency of 80.4% for methanol is achieved at -1.1 V vs. the standard hydrogen electrode.

Electrocatalytic Reduction of CO2 to C1 Compounds by Zn-Based Monatomic Alloys: A DFT Calculation

Electrocatalytic reduction of carbon dioxide to produce usable products and fuels such as alkanes, alkenes, and alcohols, is a very promising strategy. Recent experiments have witnessed great advances in precisely controlling the synthesis of single atom alloys (SAAs), which exhibit unique catalytic properties different from alloys and nanoparticles. However, only certain precious metals, such as Pd or Au, can achieve this transformation. Here, the density functional theory (DFT) calculations were performed to show that Zn-based SAAs are promising electrocatalysts for the reduction of CO2 to C1 hydrocarbons. We assume that CO2 reduction in Zn-based SAAs follows a two-step continuous reaction: first Zn reduces CO2 to CO, and then newly generated CO is captured by M and further reduced to C1 products such as methane or methanol. This work screens seven stable alloys from 16 SAAs (M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, V, Mo, Ti, Cr). Among them, Pd@Zn (101) and Cu@Zn (101) are promising catalysts for CO2 reduction. The reaction mechanisms of these two SAAs are discussed in detail. Both of them convert CO2 into methane via the same pathway. They are reduced by the pathway: *CO2 → *COOH → *CO + H2O; *CO → *CHO → *CH2O → *CH3O → *O + CH4 → *OH + CH4 → H2O + CH4. However, their potential determination steps are different, i.e., *CO2 → *COOH (ΔG = 0.70 eV) for Cu@Zn (101) and *CO → *CHO (ΔG = 0.72 eV) for Pd@Zn, respectively. This suggests that Zn-based SAAs can reduce CO2 to methane with a small overpotential. The solvation effect is simulated by the implicit solvation model, and it is found that H2O is beneficial to CO2 reduction. These computational results show an effective monatomic material to form hydrocarbons, which can stimulate experimental efforts to explore the use of SAAs to catalyze CO2 electrochemical reduction to hydrocarbons.

Controlling C-C coupling in electrocatalytic reduction of CO2 over Cu1-xZnx/C

From the perspective of sustainable environment and economic value, the electroreduction of CO₂ to higher order multicarbon products is more coveted than that of C₁ products, owing to their higher energy densities and a wider applicability. However, the reduction process remains extremely challenging due to the bottleneck of C-C coupling over the catalyst surfaces, and therefore designing a suitable catalyst for efficient and selective electrocatalytic reduction of CO₂ is a need of the hour. With the target of producing C₃₊ products with higher selectivity, in this study we explored the nano-alloys of Cu_1-xZn_x as electrocatalysts for CO₂ reduction. The nano-alloy Cu_1-xZn_x synthesized from the corresponding bimetallic metal organic framework materials demonstrated a gradual enhancement in the selectivity of acetone upon CO₂ electroreduction with higher doping of Zn. The Cu_1-xZn_x alloy opened up a wide possibility of fine-tuning the electronic structure by shifting the position of the d-band centre and modulating the interaction with intermediate CO and thus enhanced the selectivity of desirable products, which might not have been accessible otherwise. The postulated molecular mechanism of CO₂ electroreduction involving the desorption of the poorly adsorbed intermediate CO due to the presence of Zn and spilling over of free CO to Cu sites in the nano-alloy Cu_1-xZn_x for further C-C coupling to yield acetone was corroborated by the first principles studies.

Energy conversion efficiency comparison of different aqueous and semi-aqueous CO2 electroreduction systems

An energy conversion efficiency index, that is independent of the anode reaction performance, is proposed for CO₂ reduction in aqueous and semi-aqueous systems. The energy conversion efficiency of CO₂ reduction under 107 typical conditions was calculated based on the derived formula. Notably, the resulting efficiency trends of the reduction products differed from their faradaic efficiency trends. When the products were CO, HCOOH, C₂H₄, and CH₄, the electrocatalysts with the higher energy conversion efficiencies were Au, Pd, Cu, and Pt, respectively. Based on the discussion on the overall energy conversion efficiency of all products, Pt should be a specific energetically advantageous catalyst for CO₂ reduction because the activation energy is negligibly small. Moreover, the energy conversion and faradaic efficiencies were discovered to not only depend on the electrocatalyst species, but also on the complexity of the reaction, including the number of reaction electrons. Our proposed method for evaluating the energy conversion efficiency of cathode reactions can potentially serve as a novel platform for comparing the CO₂ reduction efficiencies of different electroreduction systems.

Structure-Tailored Surface Oxide on Cu-Ga Intermetallics Enhances CO2 Reduction Selectivity to Methanol at Ultralow Potential

Electrochemical CO₂ reduction reaction (eCO₂ RR) is performed on two intermetallic compounds formed by copper and gallium metals (CuGa₂ and Cu₉ Ga₄ ). Among them, CuGa₂ selectively converts CO₂ to methanol with remarkable Faradaic efficiency of 77.26% at an extremely low potential of -0.3 V vs RHE. The high performance of CuGa₂ compared to Cu₉ Ga₄ is driven by its unique 2D structure, which retains surface and subsurface oxide species (Ga₂ O₃ ) even in the reduction atmosphere. The Ga₂ O₃ species is mapped by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) techniques and electrochemical measurements. The eCO₂ RR selectivity to methanol are decreased at higher potential due to the lattice expansion caused by the reduction of the Ga₂ O₃ , which is probed by in situ XAFS, quasi in situ powder X-ray diffraction, and ex situ XPS measurements. The mechanism of the formation of methanol is visualized by in situ infrared (IR) spectroscopy and the source of the carbon of methanol at the molecular level is confirmed from the isotope-labeling experiments in presence of ¹³ CO₂ . Finally, to minimize the mass transport limitations and improve the overall eCO₂ RR performance, a poly(tetrafluoroethylene)-based gas diffusion electrode is used in the flow cell configuration.

Surfactant-modified Zn nanosheets on carbon paper for electrochemical CO2 reduction to CO

We report a strategy that tunes the CO₂ and proton concentrations near the electrode-electrolyte interface using surfactant modification with various amounts (0.05, 0.8, 1.6, and 3.2 mg) of hexadecyl trimethyl ammonium bromide (CTAB). The positively charged group of CTAB favors CO₂ surface diffusion and inhibits excessive proton accumulation on Zn nanosheets on carbon paper. A CO faradaic efficiency of 95.6% and a total ampere density of -13.1 mA cm^-2 were obtained over the optimal CTAB-modified Zn electrode at -1.1 V with stability over 12 hours.

Hydrophobic Electrode Design for CO2 Electroreduction in a Microchannel Reactor

Microchannel reactor is a novel electrochemical device to intensify CO₂ mass transfer with large interfacial areas. However, if the catalyst inserted in the microchannel is wetted, CO₂ is required to diffuse across the liquid film to get access to reaction sites. In this paper, a hydrophobic polytetrafluoroethylene (PTFE)-doped Ag nanocatalyst on a Zn rod was synthesized through a facile galvanic replacement of 2Ag⁺+Zn → 2Ag + Zn²⁺. The catalyst layer, which was PTFE incorporated into the Ag matrix, was detected to distribute uniformly on the Zn substrate with a thickness of 77 μm. Consequently, the PTFE-doped electrode demonstrated enhanced activity with an optimal 96.19% CO faradaic efficiency (FE_CO) in the microchannel reactor. Typically, the catalyst could maintain over 90% FE_CO even at the current density of 24 mA cm^-2, which was nearly 30% higher than that of a similar catalyst without PTFE. In addition, influences of the concentration of PTFE and deposition time were also investigated, determining that 1 vol % of PTFE and 30 min of coating yielded best electrocatalytic efficiency. To achieve a further breakthrough of CO₂ mass transfer limitations, reactions were applied under relatively high pressures (3-15 bar) in a single-compartment high-pressure cell. The maximum CO partial current density (j_CO) can reach 106.76 mA cm^-2 at 9 bar.

Local environment-mediated efficient electrocatalysis of CO2 to CO on Zn nanosheets

Polytetrafluoroethylene-modified Zn nanosheets inhibit the hydrogen evolution reaction and then enhance the selectivity for electrochemical CO2-to-CO conversion.

Designing a Zn-Ag Catalyst Matrix and Electrolyzer System for CO2 Conversion to CO and Beyond

CO₂ emissions can be transformed into high-added-value commodities through CO₂ electrocatalysis; however, efficient low-cost electrocatalysts are needed for global scale-up. Inspired by other emerging technologies, the authors report the development of a gas diffusion electrode containing highly dispersed Ag sites in a low-cost Zn matrix. This catalyst shows unprecedented Ag mass activity for CO production: -614 mA cm^-2 at 0.17 mg of Ag. Subsequent electrolyte engineering demonstrates that halide anions can further improve stability and activity of the Zn-Ag catalyst, outperforming pure Ag and Au. Membrane electrode assemblies are constructed and coupled to a microbial process that converts the CO to acetate and ethanol. Combined, these concepts present pathways to design catalysts and systems for CO₂ conversion toward sought-after products.

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.

In(III) Metal-Organic Framework Incorporated with Enzyme-Mimicking Nickel Bis(dithiolene) Ligand for Highly Selective CO2 Electroreduction

Inspired by the exciting physical/chemical properties in metal-organic frameworks (MOFs) of the redox-active tetrathiafulvalene (TTF) ligands, nickel bis(dithiolene-dibenzoic acid), [Ni(C₂S₂(C₆H₄COOH)₂)₂], has been designed and developed as an inorganic analogue of the corresponding TTF-type donors (such as tetrathiafulvalene-tetrabenzoate, TTFTB), where a metal site (Ni) replaces the central C═C bond. In this work, [Ni(C₂S₂(C₆H₄COOH)₂)₂] and In³⁺ have been successfully assembled into a three-dimensional MOF, (Me₂NH₂⁺){In^III-[Ni(C₂S₂(C₆H₄COO)₂)₂]}·3DMF·1.5H₂O (1, DMF = N,N-dimethylformamide), with satisfying chemical and thermal stabilities. With the combination of reversible redox activity and unsaturated metal sites originated from [Ni(C₂S₂(C₆H₄COOH)₂)₂], 1 showed a significantly enhanced performance in electrocatalytic CO₂ reduction compared with the isomorphic MOF, (Me₂NH₂⁺)[In^III-(TTFTB)]·0.7C₂H₅OH·DMF (2, with TTFTB ligand). More importantly, by mimicking the active [NiS₄] sites of formate dehydrogenase and CO-dehydrogenase, a prominently higher conversion rate and Faradaic efficiency (FE), with FE_HCOO^- increasing from 54.7% to 89.6% (at -1.3 V vs RHE, j_HCOO^- = 36.0 mA cm^-2), were achieved in 1. Mechanistic investigations further confirm that [NiS₄] can serve as a CO₂ binding site and efficient catalytic center. This unprecedented effect of redox-active nickel dithiolene-based MOF catalysts on the performance of electroreduction of CO₂ provides an important strategy for designing stable and efficient crystalline enzyme-mimicking catalysts for the conversion of CO₂ into high-value chemical stocks.

Tandem Electrocatalytic CO2 Reduction with Efficient Intermediate Conversion over Pyramid-Textured Cu-Ag Catalysts

If combined with renewably generated electricity, electrochemical CO₂ reduction (E-CO₂R) could be used as a sustainable source of chemicals and fuels. Tandem catalysis approaches are attractive for providing the product selectivity, which would be required for commercial applications. Here, we demonstrate a two-step tandem electrocatalytic E-CO₂R with efficient conversion of the intermediate species. The catalyst scaffold is Si(100), which is etched to form a textured surface consisting of micron-sized pyramid structures with the {111} facets. Two metals are used in the electrocatalytic cascade: Ag is employed to perform a two-electron reduction of CO₂ to the intermediate CO, and Cu performs conversion to more reduced products. Using high-angle physical vapor deposition, we form separated, micron-scale areas of the two electrocatalysts on opposite sides of the pyramids, with their relative surface coverages being tunable with the deposition angle. Compared to the textured scaffolds with blanket Ag and Cu used as controls, bimetallic pyramid tandem catalysts have higher current densities and much lower faradic efficiencies (FE) for CO. These effects are due to efficient conversion of the CO formed on Ag to more reduced products on Cu. Methane is the main product to be enhanced by the cascade pathway: a bimetallic catalyst with approximately equal coverages of Ag and Cu produces methane with a FE of 62% at -1.1 V_RHE, corresponding to a partial current density of 12.7 mA cm^-2. We estimate an intermediate conversion yield for the CO intermediate of 80-90%, which is close to the mass-transport limited value predicted by reaction-diffusion simulations.

Effect of the Nanostructured Zn/Cu Electrocatalyst Morphology on the Electrochemical Reduction of CO2 to Value-Added Chemicals

Zn/Cu electrocatalysts were synthesized by the electrodeposition method with various bath compositions and deposition times. X-ray diffraction results confirmed the presence of (101) and (002) lattice structures for all the deposited Zn nanoparticles. However, a bulky (hexagonal) structure with particle size in the range of 1–10 μm was obtained from a high-Zn-concentration bath, whereas a fern-like dendritic structure was produced using a low Zn concentration. A larger particle size of Zn dendrites could also be obtained when Cu²⁺ ions were added to the high-Zn-concentration bath. The catalysts were tested in the electrochemical reduction of CO₂ (CO₂RR) using an H-cell type reactor under ambient conditions. Despite the different sizes/shapes, the CO₂RR products obtained on the nanostructured Zn catalysts depended largely on their morphologies. All the dendritic structures led to high CO production rates, while the bulky Zn structure produced formate as the major product, with limited amounts of gaseous CO and H₂. The highest CO/H₂ production rate ratio of 4.7 and a stable CO production rate of 3.55 μmol/min were obtained over the dendritic structure of the Zn/Cu–Na200 catalyst at −1.6 V vs. Ag/AgCl during 4 h CO₂RR. The dissolution and re-deposition of Zn nanoparticles occurred but did not affect the activity and selectivity in the CO₂RR of the electrodeposited Zn catalysts. The present results show the possibilities to enhance the activity and to control the selectivity of CO₂RR products on nanostructured Zn catalysts.

Graphdiyne/Graphene Heterostructure: A Universal 2D Scaffold Anchoring Monodispersed Transition-Metal Phthalocyanines for Selective and Durable CO2 Electroreduction

Electrochemical CO₂ reduction (CO₂R) is a sustainable way of producing carbon-neutral fuels, yet the efficiency is limited by its sluggish kinetics and complex reaction pathways. Developing active, selective, and stable CO₂R electrocatalysts is challenging and entails intelligent material structure design and tailoring. Here we show a graphdiyne/graphene (GDY/G) heterostructure as a 2D conductive scaffold to anchor monodispersed cobalt phthalocyanine (CoPc) and reduce CO₂ with an appreciable activity, selectivity, and durability. Advanced characterizations, e.g., synchrotron-based X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculation disclose that the strong electronic coupling between GDY and CoPc, together with the high surface area, abundant reactive centers, and electron conductivity provided by graphene, synergistically contribute to this distinguished electrocatalytic performance. Electrochemical measurements revealed a high FE_CO of 96% at a partial current density of 12 mA cm^-2 in a H-cell and an FE_CO of 97% at 100 mA cm^-2 in a liquid flow cell, along with a durability over 24 h. The per-site turnover frequency of CoPc reaches 37 s^-1 at -1.0 V vs RHE, outperforming most of the reported phthalocyanine- and porphyrin-based electrocatalysts. The usage of the GDY/G heterostructure as a scaffold can be further extended to other organometallic complexes beyond CoPc. Our findings lend credence to the prospect of the GDY/G hybrid contributing to the design of single-molecule dispersed CO₂R catalysts for sustainable energy conversion.

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.

Thermodynamically driven self-formation of Ag nanoparticles in Zn-embedded carbon nanofibers for efficient electrochemical CO2 reduction

The electrochemical CO₂ reduction reaction (CO₂RR), which converts CO₂ into value-added feedstocks and renewable fuels, has been increasingly studied as a next-generation energy and environmental solution. Here, we report that single-atom metal sites distributed around active materials can enhance the CO₂RR performance by controlling the Lewis acidity-based local CO₂ concentration. By utilizing the oxidation Gibbs free energy difference between silver (Ag), zinc (Zn), and carbon (C), we can produce Ag nanoparticle-embedded carbon nanofibers (CNFs) where Zn is atomically dispersed by a one-pot, self-forming thermal calcination process. The CO₂RR performance of AgZn–CNF was investigated by a flow cell with a gas diffusion electrode (GDE). Compared to Ag–CNFs without Zn species (53% at −0.85 V vs. RHE), the faradaic efficiency (FE) of carbon monoxide (CO) was approximately 20% higher in AgZn–CNF (75% at −0.82 V vs. RHE) with 1 M KOH electrolyte.

Ag nanoparticles in Zn-embedded carbon nanofiber were synthesized by a simple one-pot, self-forming strategy. Charged Zn single atoms act as Lewis acidic sites and improving the CO₂ reduction reaction performance of the Ag nanoparticles.

Selective CO2 reduction towards a single upgraded product: a minireview on multi-elemental copper-free electrocatalysts

Electrocatalytic conversion of the greenhouse gas carbon dioxide to liquid fuels or upgraded chemicals is a critical strategy to mitigate anthropogenic climate change. To this end, we urgently need high-performance CO₂ reduction catalysts.

Hierarchically Porous SnO2 Coupled Organic Carbon for CO2 Electroreduction

Converting CO₂ into value-added chemicals or fuels by electrochemical CO₂ reduction reaction (CO₂ RR) has aroused great interest, whereas designing highly active and selective electrocatalysts is still a challenge. Herein, a novel kind of electrochemical catalyst composed with SnO₂ and organic carbon (OC), named as SnO₂ /OC, was facilely constructed for CO₂ RR. The obtained SnO₂ /OC exhibits both high faradaic efficiency for formate (∼75 %) and carbon products (∼95 %) as well as excellent stability. High surface area with hierarchically porous structure and the homogeneous formation of Sn-O-C linkages in SnO₂ /OC jointly promote the adsorption and activation of CO₂ , as well as fast transport of reactants and products.

Bi@Sn Core-Shell Structure with Compressive Strain Boosts the Electroreduction of CO2 into Formic Acid

As a profitable product from CO2 electroreduction, HCOOH holds economic viability only when the selectivity is higher than 90% with current density (j) over -200.0 mA cm-2. Herein, Bi@Sn core-shell nanoparticles (Bi core and Sn shell, denoted as Bi@Sn NPs) are developed to boost the activity and selectivity of CO2 electroreduction into HCOOH. In an H-cell system with 0.5 m KHCO3 as electrolyte, Bi@Sn NPs exhibit a Faradaic efficiency for HCOOH (FEHCOOH) of 91% with partial j for HCOOH (j HCOOH) of -31.0 mA cm-2 at -1.1 V versus reversible hydrogen electrode. The potential application of Bi@Sn NPs is testified via chronopotentiometric measurements in the flow-cell system with 2.0 m KHCO3 electrolyte. Under this circumstance, Bi@Sn NPs achieve an FEHCOOH of 92% with an energy efficiency of 56% at steady-state j of -250.0 mA cm-2. Theoretical studies indicate that the energy barrier of the potential-limiting step for the formation of HCOOH is decreased owing to the compressive strain in the Sn shell, resulting in the enhanced catalytic performance.

Exploiting Two-Dimensional Bi2 O2 Se for Trace Oxygen Detection

We exploit a high-performing resistive-type trace oxygen sensor based on 2D high-mobility semiconducting Bi₂ O₂ Se nanoplates. Scanning tunneling microscopy combined with first-principle calculations confirms an amorphous Se atomic layer formed on the surface of 2D Bi₂ O₂ Se exposed to oxygen, which contributes to larger specific surface area and abundant active adsorption sites. Such 2D Bi₂ O₂ Se oxygen sensors have remarkable oxygen-adsorption induced variations of carrier density/mobility, and exhibit an ultrahigh sensitivity featuring minimum detection limit of 0.25 ppm, long-term stability, high durativity, and wide-range response to concentration up to 400 ppm at room temperature. 2D Bi₂ O₂ Se arrayed sensors integrated in parallel form are found to possess an oxygen detection minimum of sub-0.25 ppm ascribed to an enhanced signal-to-noise ratio. These advanced sensor characteristics involving ease integration show 2D Bi₂ O₂ Se is an ideal candidate for trace oxygen detection.

Electrode Materials Engineering in Electrocatalytic CO2 Reduction: Energy Input and Conversion Efficiency

Electrocatalytic CO₂ reduction (ECR) is a promising technology to simultaneously alleviate CO₂ -caused climate hazards and ever-increasing energy demands, as it can utilize CO₂ in the atmosphere to provide the required feedstocks for industrial production and daily life. In recent years, substantial progress in ECR systems has been achieved by the exploitation of various novel electrode materials. The anodic materials and cathodic catalysts that have, respectively, led to high-efficiency energy input and effective heterogenous catalytic conversion in ECR systems are comprehensively reviewed. Based on the differences in the nature of energy sources and the role of materials used at the anode, the fundamentals of ECR systems, including photo-anode-assisted ECR systems and bio-anode-assisted ECR systems, are explained in detail. Additionally, the cathodic reaction mechanisms and pathways of ECR are described along with a discussion of different design strategies for cathode catalysts to enhance conversion efficiency and selectivity. The emerging challenges and some perspective on both anode materials and cathodic catalysts are also outlined for better development of ECR systems.

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.

Synthesis of V-doped In2O3 Nanocrystals via Digestive-Ripening Process and Their Electrocatalytic Properties in CO2 Reduction Reaction

The development of synthetic methods for monodisperse nanomaterial is of great importance in science and technology related to nanomaterials. The strong demands to prepare exceptionally monodisperse nanocrystals have made digestive-ripening one of the most sought-after size-focusing processes. Although digestive-ripening processes have been demonstrated to produce various metals and semiconductors, their applicability to oxides has rarely been studied despite various unique properties and applications of oxide nanomaterials. In this work, we demonstrate the successful synthesis of monodisperse V-doped In₂O₃ nanocrystals via a modified digestive-ripening process. The nanocrystals have truncated octahedral shape faceted with eight (222) and six (220) planes. To the best of our knowledge, this is the first report on the digestive-ripening synthesis of highly symmetrical doped oxide nanocrystals. Moreover, V-doped In₂O₃ nanocrystals exhibit electrocatalytic activities for CO₂ electrochemical reduction and produce CH₃OH, which has not been attainable from previously reported electrocatalysts based on indium or indium oxide. This distinctive catalytic property of V-doped In₂O₃ is attributed to the presence of V-dopants in the In₂O₃ host. Our demonstration has important implications for both nanocrystal synthesis and electrocatalyst development.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.

Boron Phosphide Nanoparticles: A Nonmetal Catalyst for High-Selectivity Electrochemical Reduction of CO2 to CH3 OH

Electrocatalysis has emerged as an attractive way for artificial CO₂ fixation to CH₃ OH, but the design and development of metal-free electrocatalyst for highly selective CH₃ OH formation still remains a key challenge. Here, it is demonstrated that boron phosphide nanoparticles perform highly efficiently as a nonmetal electrocatalyst toward electrochemical reduction of CO₂ to CH₃ OH with high selectivity. In 0.1 m KHCO₃ , this catalyst achieves a high Faradaic efficiency of 92.0% for CH₃ OH at -0.5 V versus reversible hydrogen electrode. Density functional theory calculations reveal that B and P synergistically promote the binding and activation of CO₂ , and the rate-determining step for the CO₂ reduction reaction is dominated by *CO + *OH to *CO + *H₂ O process with free energy change of 1.36 eV. In addition, CO and CH₂ O products are difficultly generated on BP (111) surface, which is responsible for the high activity and selectivity of the CO₂ -to-CH₃ OH conversion process.