Selective CO2 Electroreduction to Formate on Polypyrrole-Modified Oxygen Vacancy-Rich Bi2O3 Nanosheet Precatalysts by Local Microenvironment Modulation

Challenges remain in the development of highly efficient catalysts for selective electrochemical transformation of carbon dioxide (CO₂) to high-valued hydrocarbons. In this study, oxygen vacancy-rich Bi₂O₃ nanosheets coated with polypyrrole (Bi₂O₃@PPy NSs) are designed and synthesized, as precatalysts for selective electrocatalytic CO₂reduction to formate. Systematic material characterization demonstrated that Bi₂O₃@PPy precatalyst can evolve intoBi₂O₂CO₃@PPy nanosheets with rich oxygen vacancies (Bi₂O₂CO₃@PPy NSs) via electrolyte-mediated conversion and function as the real active catalyst for CO₂ reduction reaction electrocatalysis. Coating catalyst with a PPy shell can modulate the interfacial microenvironment of active sites, which work in coordination with rich oxygen vacancies in Bi₂O₂CO₃ and efficiently mediate directional selective CO₂ reduction toward formate formation. With the fine-tuning of interfacial microenvironment, the optimized Bi₂O₃@PPy-2 NSs derived Bi₂O₂CO₃@PPy-2 NSs exhibit a maximum Faradaic efficiency of 95.8% at −0.8 V (versus. reversible hydrogen electrode) for formate production. This work might shed some light on designing advanced catalysts toward selective electrocatalytic CO₂ reduction through local microenvironment engineering.     

One-Step Phase Separation for Core-Shell Carbon@Indium Oxide@Bismuth Microspheres with Enhanced Activity for CO2 Electroreduction to Formate

It is a substantial challenge to construct electrocatalysts with high activity, good selectivity, and long-term stability for electrocatalytic reduction of carbon dioxide to formic acid. Herein, bismuth and indium species are innovatively integrated into a uniform heterogeneous spherical structure by a neoteric quasi-microemulsion method, and a novel C@In₂O₃@Bi₅₀ core-shell structure is constructed through a subsequent one-step phase separation strategy due to melting point difference and Kirkendall effect with the nano-limiting effect of the carbon structure. This core-shell C@In₂O₃@Bi₅₀ catalyst can selectively reduce CO₂ to formate with high selectivity (≈90% faradaic efficiency), large partial current density (24.53 mA cm⁻² at −1.36 V), and long-term stability (up to 14.5 h), superior to most of the Bi-based catalysts. The hybrid Bi/In₂O₃ interfaces of core-shell C@In₂O₃@Bi will stabilize the key intermediate HCOO* and suppress CO poisoning, benefiting the CO₂RR selectivity and stability, while the internal cavity of core-shell structure will improve the reaction kinetics because of the large specific surface area and the enhancement of ion shuttle and electron transfer. Furthermore, the nano-limited domain effect of outmost carbon prevent active components from oxidation and agglomeration, helpful for stabilizing the catalyst. This work offers valuable insights into core-shell structure engineering to promote practical CO₂ conversion technology.     

Engineering Single Cu Sites into Covalent Organic Framework for Selective Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into value-added chemicals is a promising route but remains challenging due to poor product selectivity. Covalent organic frameworks (COFs) as an emerging class of porous materials are considered as promising candidates for photocatalysis. Incorporating metallic sites into COF is a successful strategy to realize high photocatalytic activities. Herein, 2,2′-bipyridine-based COF bearing non-noble single Cu sites is fabricated by chelating coordination of dipyridyl units for photocatalytic CO₂ reduction. The coordinated single Cu sites not only significantly enhance light harvesting and accelerate electron–hole separation but also provide adsorption and activation sites for CO₂ molecules. As a proof of concept, the Cu-Bpy-COF as a representative catalyst exhibits superior photocatalytic activity for reducing CO₂ to CO and CH₄ without photosensitizer, and impressively, the product selectivity of CO and CH₄ can be readily modulated only by changing reaction media. Experimental and theoretical results reveal the crucial role of single Cu sites in promoting photoinduced charge separation and solvent effect in regulating product selectivity, which provides an important sight onto the design of COF photocatalysts for selective CO₂ photoreduction.     

Ni‐Nanocluster Modified Black TiO2 with Dual Active Sites for Selective Photocatalytic CO2 Reduction

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO₂ molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni‐nanocluster loaded black TiO₂ (Ni/TiO_2[Vo]) with built‐in dual active sites for selective photocatalytic CO₂ conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO₂ binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO_2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO₂ (P‐25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.     

Mesoporous PdAg Nanospheres for Stable Electrochemical CO2 Reduction to Formate

Palladium is a promising material for electrochemical CO₂ reduction to formate with high Faradaic efficiency near the equilibrium potential. It unfortunately suffers from problematic operation stability due to CO poisoning on surface. Here, it is demonstrated that alloying is an effective strategy to alleviate this problem. Mesoporous PdAg nanospheres with uniform size and composition are prepared from the co-reduction of palladium and silver precursors in aqueous solution using dioctadecyldimethylammonium chloride as the structure-directing agent. The best candidate can initiate CO₂ reduction at zero overpotential and achieve high formate selectivity close to 100% and great stability even at <-0.2 V versus reversible hydrogen electrode. The high selectivity and stability are believed to result from the electronic coupling between Pd and Ag, which lowers the d-band center of Pd and thereby significantly enhances its CO tolerance, as evidenced by both electrochemical analysis and theoretical simulations.     

Highly Active,Durable Ultrathin MoTe2 Layers for the Electroreduction of CO2 to CH4

The electroreduction of CO₂ to CH₄ is a highly desirable, challenging research topic. In this study, an electrocatalytic system comprising ultrathin MoTe₂ layers and an ionic liquid electrolyte for the reduction of CO₂ to methane is reported, efficiently affording methane with a faradaic efficiency of 83 ± 3% (similar to the best Cu‐based catalysts reported thus far) and a durable activity of greater than 45 h at a relatively high current density of 25.6 mA cm^?2 (?1.0 V_RHE). The results obtained can facilitate research on the design of other transition‐metal dichalcogenide electrocatalysts for the reduction of CO₂ to valuable fuels.     

Hybridization of Defective Tin Disulfide Nanosheets and Silver Nanowires Enables Efficient Electrochemical Reduction of CO2 into Formate and Syngas

Integrating the defect engineering and conductivity promotion represents a promising way to improve the performance of CO₂ electrochemical reduction. Herein, the hybridized composite of defective SnS₂ nanosheets and Ag nanowires is developed as an efficient catalyst for the production of formate and syngas toward CO₂ electrochemical reduction. The Schottky barrier in Ag‐SnS₂ hybrid nanosheets is negligible due to the similar Fermi level of SnS₂ nanosheets and Ag nanowires. Accordingly, the free electrons of Ag nanowires participate in the electronic transport of SnS₂ nanosheets, and thus give rise to a 5.5‐fold larger carrier density of Ag‐SnS₂ hybrid nanosheets than that of SnS₂ nanosheets. In CO₂ electrochemical reduction, the Ag‐SnS₂ hybrid nanosheets display 38.8 mA cm^?2 of geometrical current density at –1.0 V vs reversible hydrogen electrode, including 23.3 mA cm^?2 for formate and 15.5 mA cm^?2 for syngas with the CO/H₂ ratio of 1:1. A mechanistic study reveals that the abundant defect sites and carrier density not only promote the conductivity of the electrocatalyst, but also increase the binding strength for CO₂, which account for the efficient CO₂ reduction.     

Regulation of Electrocatalytic Behavior by Axial Oxygen Enhances the Catalytic Activity of CoN4 Sites for CO2 Reduction

Recent studies have found that the existence of oxygen around the active sites may be essential for efficient electrochemical CO₂-to-CO conversion. Hence, this work proposes the modulation of oxygen coordination and investigates the as-induced catalytic behavior in CO₂RR. It designs and synthesizes conjugated phthalocyanine frameworks catalysts (CPF-Co) with abundant CoN₄ centers as an active source, and subsequently modifies the electronic structure of CPF-Co by introducing graphene oxide (GO) with oxygen-rich functional groups. A systematic study reveals that the axial coordination between oxygen and the catalytic sites could form an optimized O-CoN₄ structure to break the electron distribution symmetry of Co, thus reducing the energy barrier to the activation of CO₂ to COOH*. Meanwhile, by adjusting the content of oxygen, the proper supports can also facilitate the charge transfer efficiency between the matrix layer and the catalytic sites. The optimized CPF-Co@LGO exhibits a high TOF value (2.81 s⁻¹), CO selectivity (97.6%) as well as stability (24 h) at 21 mA cm⁻² current density. This work reveals the modulation of oxygen during CO₂RR and provides a novel strategy for the design of efficient electrocatalysts, which may inspire new exploration and principles for CO₂RR.     

Morphological and Compositional Design of Pd–Cu Bimetallic Nanocatalysts with Controllable Product Selectivity toward CO2 Electroreduction

Electrochemical conversion of carbon dioxide (electrochemical reduction of carbon dioxide) to value‐added products is a promising way to solve CO₂ emission problems. This paper describes a facile one‐pot approach to synthesize palladium–copper (Pd–Cu) bimetallic catalysts with different structures. Highly efficient performance and tunable product distributions are achieved due to a coordinative function of both enriched low‐coordinated sites and composition effects. The concave rhombic dodecahedral Cu₃Pd (CRD‐Cu₃Pd) decreases the onset potential for methane (CH₄) by 200 mV and shows a sevenfold CH₄ current density at ?1.2 V (vs reversible hydrogen electrode) compared to Cu foil. The flower‐like Pd₃Cu (FL‐Pd₃Cu) exhibits high faradaic efficiency toward CO in a wide potential range from ?0.7 to ?1.3 V, and reaches a fourfold CO current density at ?1.3 V compared to commercial Pd black. Tafel plots and density functional theory calculations suggest that both the introduction of high‐index facets and alloying contribute to the enhanced CH₄ current of CRD‐Cu₃Pd, while the alloy effect is responsible for high CO selectivity of FL‐Pd₃Cu.     

Engineering Catalytically Active Sites by Sculpting Artificial Edges on MoS2 Basal Plane for Dinitrogen Reduction at a Low Overpotential

Engineering catalytically active sites have been a challenge so far and often relies on optimization of synthesis routes, which can at most provide quantitative enhancement of active facets, however, cannot provide control over choosing orientation, geometry and spatial distribution of the active sites. Artificially sculpting catalytically active sites via laser-etching technique can provide a new prospect in this field and offer a new species of nanocatalyst for achieving superior selectivity and attaining maximum yield via absolute control over defining their location and geometry of every active site at a nanoscale precision. In this work, a controlled protocol of artificial surface engineering is shown by focused laser irradiation on pristine MoS₂ flakes, which are confirmed as catalytic sites by electrodeposition of AuNPs. The preferential Au deposited catalytic sites are found to be electrochemically active for nitrogen adsorption and its subsequent reduction due to the S-vacancies rather than Mo-vacancy, as advocated by DFT analysis. The catalytic performance of Au-NR/MoS₂ shows a high yield rate of ammonia (11.43 × 10⁻⁸ mol s⁻¹ cm⁻²) at a potential as low as −0.1 V versus RHE and a notable Faradaic efficiency of 13.79% during the electrochemical nitrogen reduction in 0.1 m HCl.     

High-Dispersive Pd Nanoparticles on Hierarchical N-Doped Carbon Nanocages to Boost Electrochemical CO2 Reduction to Formate at Low Potential

Electrochemical CO₂ reduction reaction (CO₂RR) to value-added chemicals/fuels is an effective strategy to achieve the carbon neutral. Palladium is the only metal to selectively produce formate via CO₂RR at near-zero potentials. To reduce cost and improve activity, the high-dispersive Pd nanoparticles on hierarchical N-doped carbon nanocages (Pd/hNCNCs) are constructed by regulating pH in microwave-assisted ethylene glycol reduction. The optimal catalyst exhibits high formate Faradaic efficiency of >95% within −0.05–0.30 V and delivers an ultrahigh formate partial current density of 10.3 mA cm⁻² at the low potential of −0.25 V. The high performance of Pd/hNCNCs is attributed to the small size of uniform Pd nanoparticles, the optimized intermediates adsorption/desorption on modified Pd by N-doped support, and the promoted mass/charge transfer kinetics arising from the hierarchical structure of hNCNCs. This study sheds light on the rational design of high-efficient electrocatalysts for advanced energy conversion.     

Nanostructure Engineering of Sn-Based Catalysts for Efficient Electrochemical CO2 Reduction

Excessive anthropogenic CO₂ emission has caused a series of ecological and environmental issues, which threatens mankind's sustainable development. Mimicking the natural photosynthesis process (i.e., artificial photosynthesis) by electrochemically converting CO₂ into value-added products is a promising way to alleviate CO₂ emission and relieve the dependence on fossil fuels. Recently, Sn-based catalysts have attracted increasing research attentions due to the merits of low price, abundance, non-toxicity, and environmental benignancy. In this review, the paradigm of nanostructure engineering for efficient electrochemical CO₂ reduction (ECO₂R) on Sn-based catalysts is systematically summarized. First, the nanostructure engineering of size, composition, atomic structure, morphology, defect, surficial modification, catalyst/substrate interface, and single-atom structure, are systematically discussed. The influence of nanostructure engineering on the electronic structure and adsorption property of intermediates, as well as the performance of Sn-based catalysts for ECO₂R are highlighted. Second, the potential chemical state changes and the role of surface hydroxides on Sn-based catalysts during ECO₂R are introduced. Third, the challenges and opportunities of Sn-based catalysts for ECO₂R are proposed. It is expected that this review inspires the further development of highly efficient Sn-based catalysts, meanwhile offer protocols for the investigation of Sn-based catalysts.     

Synergistic Catalysis over Iron‐Nitrogen Sites Anchored with Cobalt Phthalocyanine for Efficient CO2 Electroreduction

Simultaneously achieving high Faradaic efficiency, current density, and stability at low overpotentials is essential for industrial applications of electrochemical CO₂ reduction reaction (CO₂RR). However, great challenges still remain in this catalytic process. Herein, a synergistic catalysis strategy is presented to improve CO₂RR performance by anchoring Fe‐N sites with cobalt phthalocyanine (denoted as CoPc©Fe‐N‐C). The potential window of CO Faradaic efficiency above 90% is significantly broadened from 0.18 V over Fe‐N‐C alone to 0.71 V over CoPc©Fe‐N‐C while the onset potential of CO₂RR over both catalysts is as low as ?0.13 V versus reversible hydrogen electrode. What is more, the maximum CO current density is increased ten times with significantly enhanced stability. Density functional theory calculations suggest that anchored cobalt phthalocyanine promotes the CO desorption and suppresses the competitive hydrogen evolution reaction over Fe‐N sites, while the *COOH formation remains almost unchanged, thus demonstrating unprecedented synergistic effect toward CO₂RR.     

Phase-Inversion Induced 3D Electrode for Direct Acidic Electroreduction CO2 to Formic acid

Direct electrochemical CO₂ reduction to formic acid (FA) instead of formate is a challenging task due to the high acidity of FA and competitive hydrogen evolution reaction. Herein, 3D porous electrode (TDPE) is prepared by a simple phase inversion method, which can electrochemically reduce CO₂ to FA in acidic conditions. Owing to interconnected channels, high porosity, and appropriate wettability, TDPE not only improves mass transport, but also realizes pH gradient to build higher local pH micro-environment under acidic conditions for CO₂ reduction compared with planar electrode and gas diffusion electrode. Kinetic isotopic effect experiments demonstrate that the proton transfer becomes the rate-determining step at the pH of 1.8; however, not significant in neutral solution, suggesting that the proton is aiding the overall kinetics. Maximum FA Faradaic efficiency of 89.2% has been reached at pH 2.7 in a flow cell, generating FA concentration of 0.1 m . Integrating catalyst and gas–liquid partition layer into a single electrode structure by phase inversion method paves a facile avenue for direct production of FA by electrochemical CO₂ reduction.     

Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2

Here, the hydrogen evolution reaction (HER) activities at the edge and basal‐plane sites of monolayer molybdenum disulfide (MoS₂) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected‐area lithography. Reaction windows are opened by e‐beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)‐covered monolayer MoS₂ triangles. The HER properties of MoS₂ edge sites are obtained by subtraction of the activity of the basal‐plane sites from results containing both basal‐plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10^?4 s^?1, found for 1T′‐, 2H‐MoS₂ edges, and 1T′‐, 2H‐MoS₂ basal planes, respectively. Edge sites of both 2H‐ and 1T′‐MoS₂ are proved to have comparable activities to platinum (≈1–10 s^?1). When fitted into the HER volcano plot, the MoS₂ active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition‐metal dichalcogenides (TMDs) and metal catalysts.