The p‐Orbital Delocalization of Main‐Group Metals to Boost CO 2 Electroreduction

Electron delocalization of robust high-nuclear bismuth-oxo clusters for promoted CO2 electroreduction

The integration of high activity, selectivity and stability in one electrocatalyst is highly desirable for electrochemical CO2 reduction (ECR), yet it is still a knotty issue. The unique electronic properties of high-nuclear clusters may bring about extraordinary catalytic performance; however, construction of a high-nuclear structure for ECR remains a challenging task. In this work, a family of calix[8]arene-protected bismuth-oxo clusters (BiOCs), including Bi4 (BiOC-1/2), Bi8Al (BiOC-3), Bi20 (BiOC-4), Bi24 (BiOC-5) and Bi40Mo2 (BiOC-6), were prepared and used as robust and efficient ECR catalysts. The Bi40Mo2 cluster in BiOC-6 is the largest metal-oxo cluster encapsulated by calix[8]arenes. As an electrocatalyst, BiOC-5 exhibited outstanding electrochemical stability and 97% Faraday efficiency for formate production at a low potential of -0.95 V vs. RHE, together with a high turnover frequency of up to 405.7 h-1. Theoretical calculations reveal that large-scale electron delocalization of BiOCs is achieved, which promotes structural stability and effectively decreases the energy barrier of rate-determining *OCHO generation. This work provides a new perspective for the design of stable high-nuclear clusters for efficient electrocatalytic CO2 conversion.

Electron-Rich Bi Nanosheets Promote CO2 ⋅- Formation for High-Performance and pH-Universal Electrocatalytic CO2 Reduction

Electrochemical CO₂ reduction reaction (CO₂ RR) to chemical fuels such as formate offers a promising pathway to carbon-neutral future, but its practical application is largely inhibited by the lack of effective activation of CO₂ molecules and pH-universal feasibility. Here, we report an electronic structure manipulation strategy to electron-rich Bi nanosheets, where electrons transfer from Cu donor to Bi acceptor in bimetallic Cu-Bi, enabling CO₂ RR towards formate with concurrent high activity, selectivity and stability in pH-universal (acidic, neutral and alkaline) electrolytes. Combined in situ Raman spectra and computational calculations unravel that electron-rich Bi promotes CO₂ ⋅^- formation to activate CO₂ molecules, and enhance the adsorption strength of *OCHO intermediate with an up-shifted p-band center, thus leading to its superior activity and selectivity of formate. Further integration of the robust electron-rich Bi nanosheets into III-V-based photovoltaic solar cell results in an unassisted artificial leaf with a high solar-to-formate (STF) efficiency of 13.7 %.

Electronic Perturbation of Copper Single-Atom CO2 Reduction Catalysts in a Molecular Way

Fine-tuning electronic structures of single-atom catalysts (SACs) plays a crucial role in harnessing their catalytic activities, yet challenges remain at a molecular scale in a controlled fashion. By tailoring the structure of graphdiyne (GDY) with electron-withdrawing/-donating groups, we show herein the electronic perturbation of Cu single-atom CO₂ reduction catalysts in a molecular way. The elaborately introduced functional groups (-F, -H and -OMe) can regulate the valance state of Cu^δ+ , which is found to be directly scaled with the selectivity of the electrochemical CO₂ -to-CH₄ conversion. An optimum CH₄ Faradaic efficiency of 72.3 % was achieved over the Cu SAC on the F-substituted GDY. In situ spectroscopic studies and theoretical calculations revealed that the positive Cu^δ+ centers adjusted by the electron-withdrawing group decrease the pK_a of adsorbed H₂ O, promoting the hydrogenation of intermediates toward the CH₄ production. Our strategy paves the way for precise electronic perturbation of SACs toward efficient electrocatalysis.

Rapid and Green Electric-Explosion Preparation of Spherical Indium Nanocrystals with Abundant Metal Defects for Highly-Selective CO2 Electroreduction

Electrochemical conversion of CO₂ into high-value-added chemicals has been considered a promising route to achieve carbon neutrality and mitigate the global greenhouse effect. However, the lack of highly efficient electrocatalysts has limited its practical application. Herein, we propose an ultrafast and green electric explosion method to batch-scale prepare spherical indium (In) nanocrystals (NCs) with abundant metal defects toward high selective electrocatalytic CO₂ reduction (CO₂RR) to HCOOH. During the electric explosion synthesis process, the Ar atmosphere plays a significant role in forming the spherical In NCs with abundant metal defects instead of highly crystalline In₂O₃ NCs formed under an air atmosphere. Analysis results reveal that the In NCs possess ultrafast catalytic kinetics and reduced onset potential, which is ascribed to the formation of rich metal defects serving as effective catalytic sites for converting CO₂ into HCOOH. This work provides a feasible strategy to massively produce efficient In-based electrocatalysts for electrocatalytic CO₂-to-formate conversion.

A Tin Oxide-Coated Copper Foam Hybridized with a Gas Diffusion Electrode for Efficient CO2 Reduction to Formate with a Current Density Exceeding 1 A cm-2

The electrochemical CO₂ reduction reaction (CO₂ RR) is a promising strategy for closing the carbon cycle. Increasing the current density (  J) for CO₂ RR products is a critical requirement for the social implementation of this technology. Herein, nanoscale tin-oxide-modified copper-oxide foam is hybridized with a carbon-based gas-diffusion electrode (GDE). Using the resultant electrode, the J_formate is increased to -1152 mA cm^-2 at -1.2 V versus RHE in 1 m KOH, which is the highest value for CO₂ -to-formate electrolysis. The formate faradaic efficiency (FE_formate ) reaches ≈99% at -0.6 V versus RHE. The achievement of ultra-high-rate formate production is attributable to the following factors: i) homogeneously-modified Sn atoms suppressing H₂ evolution and ii) the hydrophobic carbon nanoparticles on GDEs penetrating the macroporous structure of the foam causing the increase in the thickness of triple-phase interface. Additionally, the FE_formate remains at ≈70% under a high J of -1.0 A cm^-2 for more than 20 h.

Nitrogen-Doped Bismuth Nanosheet as an Efficient Electrocatalyst to CO2 Reduction for Production of Formate

Electrochemical CO₂ reduction (CO₂RR) to produce high value-added chemicals or fuels is a promising technology to address the greenhouse effect and energy challenges. Formate is a desirable product of CO₂RR with great economic value. Here, nitrogen-doped bismuth nanosheets (N-BiNSs) were prepared by a facile one-step method. The N-BiNSs were used as efficient electrocatalysts for CO₂RR with selective formate production. The N-BiNSs exhibited a high formate Faradic efficiency (FE_formate) of 95.25% at -0.95 V (vs. RHE) with a stable current density of 33.63 mA cm^-2 in 0.5 M KHCO₃. Moreover, the N-BiNSs for CO₂RR yielded a large current density (300 mA cm^-2) for formate production in a flow-cell measurement, achieving the commercial requirement. The FE_formate of 90% can maintain stability for 14 h of electrolysis. Nitrogen doping could induce charge transfer from the N atom to the Bi atom, thus modulating the electronic structure of N-Bi nanosheets. DFT results demonstrated the N-BiNSs reduced the adsorption energy of the *OCHO intermediate and promoted the mass transfer of charges, thereby improving the CO₂RR with high FE_formate. This study provides a valuable strategy to enhance the catalytic performance of bismuth-based catalysts for CO₂RR by using a nitrogen-doping strategy.

Modulating p-Orbital of Bismuth Nanosheet by Nickel Doping for Electrocatalytic Carbon Dioxide Reduction Reaction

Electrochemical reduction of CO₂ (CO₂ RR) to value-added chemicals is an effective way to harvest renewable energy and utilize carbon dioxide. However, the electrocatalysts for CO₂ RR suffer from insufficient activity and selectivity due to the limitation of CO₂ activation. In this work, a Ni-doped Bi nanosheet (Ni@Bi-NS) electrocatalyst is synthesized for the electrochemical reduction of CO₂ to HCOOH. Physicochemical characterization methods are extensively used to investigate the composition and structure of the materials. Electrochemical results reveal that for the production of HCOOH, the obtained Ni@Bi-NS exhibits an equivalent current density of 51.12 mA cm^-2 at -1.10 V, which is much higher than the pure Bi-NS (18.00 mA cm^-2 at -1.10 V). A high Faradaic efficiency over 92.0 % for HCOOH is achieved in a wide potential range from -0.80 to -1.10 V, and particularly, the highest efficiency of 98.4 % is achieved at -0.90 V. Both experimental and theoretical results reveal that the superior activity and selectivity are attributed to the doping effect of Ni on the Bi nanosheet. The density functional theory calculation reveals that upon doping, the charge is transferred from Ni to the adjacent Bi atoms, which shifts the p-orbital electronic density states towards the Fermi level. The resultant strong orbital hybridization between Bi and the π* orbitals of CO₂ facilitates the formation of *OCHO intermediates and favors its activation. This work provides an effective strategy to develop active and selective electrocatalysts for CO₂ RR by modulating the electronic density state.

Regulation of electrocatalytic activity by local microstructure: focusing on catalytic active zone

Traditional regulation methods of active sites have been successfully optimized the performance of electrocatalysts, but seem unable to achieve further breakthrough in the catalytic activity. Unlike the conventional viewpoint of focusing on single active site, the concept of local microstructure active zone is more comprehensive and new suits of methods to regulate reaction zone for electrocatalytic reactions are developed accordingly. The local microstructure active zone refers to the zone with high catalytic activity formed by the interaction between active atoms and neighboring coordination atoms as well as the surrounding environment. Instead of the traditional single active atom site, the active zone is more suitable for the actual electrochemical reaction process. According to this concept, the activity of the electrocatalysts can be coordinated by multiple active atoms. This strategy is beneficial to understand the relationship between material, structure and catalysis, which realizes the scientific design and synthesis of high performance electrocatalysts. This review provides the research progress of this strategy in electrocatalytic reactions, with the emphasis on their important applications in oxygen evolution reaction, urea oxidation reaction and carbon dioxide reduction.

Empower C1: Combination of Electrochemistry and Biology to Convert C1 Compounds

The idea to somehow combine electrical current and biological systems is not new. It was subject of research as well as of science fiction literature for decades. Nowadays, in times of limited resources and the need to capture greenhouse gases like CO₂, this combination gains increasing interest, since it might allow to use C1 compounds and highly oxidized compounds as substrate for microbial production by "activating" them with additional electrons. In this chapter, different possibilities to combine electrochemistry and biology to convert C1 compounds into useful products will be discussed. The chapter first shows electrochemical conversion of C1 compounds, allowing the use of the product as substrate for a subsequent biosynthesis in uncoupled systems, further leads to coupled systems of biology and electrochemical conversion, and finally reaches the discipline of bioelectrosynthesis, where electrical current and C1 compounds are directly converted by microorganisms or enzymes. This overview will give an idea about the potentials and challenges of combining electrochemistry and biology to convert C1 molecules.

Boron Dopant Induced Electron-Rich Bismuth for Electrochemical CO2 Reduction with High Solar Energy Conversion Efficiency

Electrochemical CO₂ reduction to formate offers a mild and feasible pathway for the utilization of CO₂ , and bismuth is a promising metal for its unique hydrogen evolution reaction inhibition. Reported works of Bi-based electrodes generally exhibit high selectivity while suffering from relatively narrow working potential range. From the perspective of electronic modification engineering, B-doped Bi is prepared by a facile chemical reduction method in this work. With B dopant, above 90% Faradaic efficiency for formate over a broad window of working potential of -0.6 to -1.2 V (vs. reversible hydrogen electrode) is achieved. In situ Raman spectroscopy, X-ray adsorption spectroscopy, and computational analysis demonstrate that the B dopant induces the formation of electron-rich bismuth, which is in favor of the formation of formate by fine-tuning the adsorption energy of *OCHO. Moreover, full-cell electrolysis system coupled with photovoltaic device is constructed and achieves the solar-to-formate conversion efficiency as high as 11.8%.

Polymer-Supported Liquid Layer Electrolyzer Enabled Electrochemical CO2 Reduction to CO with High Energy Efficiency

The electrochemical conversion of carbon dioxide (CO2 ) to carbon monoxide (CO) is a favorable approach to reduce CO2 emission while converting excess sustainable energy to important chemical feedstocks. At high current density (>100 mA cm-2 ), low energy efficiency (EE) and unaffordable cell cost limit the industrial application of conventional CO2 electrolyzers. Thus, a crucial and urgent task is to design a new type of CO2 electrolyzer that can work efficiently at high current density. Here we report a polymer-supported liquid layer (PSL) electrolyzer using polypropylene non-woven fabric as a separator between anode and cathode. Ag based cathode was fed with humid CO2 and potassium hydroxide was fed to earth-abundant NiFe-based anode. In this configuration, the PSL provided high-pH condition for the cathode reaction and reduced the cell resistance, achieving a high full cell EE over 66 % at 100 mA cm-2 .

A Supported Pd2 Dual-Atom Site Catalyst for Efficient Electrochemical CO2 Reduction

Dual-atom site catalysts (DACs) have emerged as a new frontier in heterogeneous catalysis because the synergistic effect between adjacent metal atoms can promote their catalytic activity while maintaining the advantages of single-atom site catalysts (SACs), like 100 % atomic utilization efficiency and excellent selectivity. Herein, a supported Pd₂ DAC was synthesized and used for electrochemical CO₂ reduction reaction (CO₂ RR) for the first time. The as-obtained Pd₂ DAC exhibited superior CO₂ RR catalytic performance with 98.2 % CO faradic efficiency at -0.85 V vs. RHE, far exceeding that of Pd₁ SAC, and coupled with long-term stability. The density functional theory (DFT) calculations revealed that the intrinsic reason for the superior activity of Pd₂ DAC toward CO₂ RR was the electron transfer between Pd atoms at the dimeric Pd sites. Thus, Pd₂ DAC possessed moderate adsorption strength of CO*, which was beneficial for CO production in CO₂ RR.

Controllable CO adsorption determines ethylene and methane productions from CO2 electroreduction

Among all CO₂ electroreduction products, methane (CH₄) and ethylene (C₂H₄) are two typical and valuable hydrocarbon products which are formed in two different pathways: hydrogenation and dimerization reactions of the same CO intermediate. Theoretical studies show that the adsorption configurations of CO intermediate determine the reaction pathways towards CH₄/C₂H₄. However, it is challenging to experimentally control the CO adsorption configurations at the catalyst surface, and thus the hydrocarbon selectivity is still limited. Herein, we seek to synthesize two well-defined copper nanocatalysts with controllable surface structures. The two model catalysts exhibit a high hydrocarbon selectivity toward either CH₄ (83%) or C₂H₄ (93%) under identical reduction conditions. Scanning transmission electron microscopy and X-ray absorption spectroscopy characterizations reveal the low-coordination Cu⁰ sites and local Cu⁰/Cu⁺ sites of the two catalysts, respectively. CO-temperature programed desorption, in-situ attenuated total reflection Fourier transform infrared spectroscopy and density functional theory studies unveil that the bridge-adsorbed CO (CO_B) on the low-coordination Cu⁰ sites is apt to be hydrogenated to CH₄, whereas the bridge-adsorbed CO plus linear-adsorbed CO (CO_B + CO_L) on the local Cu⁰/Cu⁺ sites are apt to be coupled to C₂H₄. Our findings pave a new way to design catalysts with controllable CO adsorption configurations for high hydrocarbon product selectivity.

A hydrolysis synthesis route for (001)/(102) coexposed BiOCl nanosheets with high visible light-driven catalytic performance

The (001)/(102) co-exposed BiOCl nanosheet shows good adsorption of cationic dyes and high visible light-driven catalytic performance.

Design of a Single-Atom Indiumδ+ -N4 Interface for Efficient Electroreduction of CO2 to Formate

Main-group element indium (In) is a promising electrocatalyst which triggers CO₂ reduction to formate, while the high overpotential and low Faradaic efficiency (FE) hinder its practical application. Herein, we rationally design a new In single-atom catalyst containing exclusive isolated In^δ+ -N₄ atomic interface sites for CO₂ electroreduction to formate with high efficiency. This catalyst exhibits an extremely large turnover frequency (TOF) up to 12500 h^-1 at -0.95 V versus the reversible hydrogen electrode (RHE), with a FE for formate of 96 % and current density of 8.87 mA cm^-2 at low potential of -0.65 V versus RHE. Our findings present a feasible strategy for the accurate regulation of main-group indium catalysts for CO₂ reduction at atomic scale.

Design of a Graphene Nitrene Two-Dimensional Catalyst Heterostructure Providing a Well-Defined Site Accommodating One to Three Metals, with Application to CO2 Reduction Electrocatalysis for the Two-Metal Case

Recently, the reduction of CO₂ to fuels has been the subject of numerous studies, but the selectivity and activity remain inadequate. Progress has been made on single-site two-dimensional catalysts based on graphene coupled to a metal and nitrogen for the CO₂ reduction reaction (CO₂RR); however, the product is usually CO, and the metal-N environment remains ambiguous. We report a novel two-dimensional graphene nitrene heterostructure (grafiN₆) providing well-defined active sites (N₆) that can bind one to three metals for the CO₂RR. We find that homobimetallic FeFe-grafiN₆ could reduce CO₂ to CH₄ at -0.61 V and to CH₃CH₂OH at -0.68 V versus reversible hydrogen electrode, with high product selectivity. Moreover, the heteronuclear FeCu-grafiN₆ system may be significantly less affected by hydrogen evolution reaction, while maintaining a low limiting potential (-0.68 V) for C1 and C2 mechanisms. Binding metals to one N₆ site but not the other could promote efficient electron transport facilitating some reaction steps. This framework for single or multiple metal sites might also provide unique catalytic sites for other catalytic processes.

Sulfur Atomically Doped Bismuth Nanobelt Driven by Electrochemical Self-Reconstruction for Boosted Electrocatalysis

Recent years have witnessed various in-depth research efforts on self-reconstruction behavior toward electrocatalysis. Tracking the phase transformation and evolution of true active sites is of great significance for the development of self-reconstructed electrocatalysts. Here, the optimized atomic sulfur-doped bismuth nanobelt (S-Bi) is fabricated via an electrochemical self-reconstruction evolved from Bi₂S₃. Advanced technologies have demonstrated that the nonmetallic S atoms have been doped into the lattice Bi frame, leading to the reconstruction of local electronic structure of Bi. The as-prepared S-Bi nanobelt exhibits a remarkable NH₃ generation rate of 10.28 μg h^-1 mg^-1 and Faradaic efficiency of 10.48%. Density functional theory calculations prove that the S doping can significantly lower the energy barrier of the rate-determining step and enlarge the N≡N bond for further dissociation toward N₂ fixation. This work not only establishes insights into the evolution process of electrochemically derived self-reconstruction but also unravels the root of the N₂ reduction reaction mechanism associated with the atomic nonmetal dopants.

Bismuthene for highly efficient carbon dioxide electroreduction reaction

Bismuth (Bi) has been known as a highly efficient electrocatalyst for CO2 reduction reaction. Stable free-standing two-dimensional Bi monolayer (Bismuthene) structures have been predicted theoretically, but never realized experimentally. Here, we show the first simple large-scale synthesis of free-standing Bismuthene, to our knowledge, and demonstrate its high electrocatalytic efficiency for formate (HCOO-) formation from CO2 reduction reaction. The catalytic performance is evident by the high Faradaic efficiency (99% at -580 mV vs. Reversible Hydrogen Electrode (RHE)), small onset overpotential (<90 mV) and high durability (no performance decay after 75 h and annealing at 400 °C). Density functional theory calculations show the structure-sensitivity of the CO2 reduction reaction over Bismuthene and thicker nanosheets, suggesting that selective formation of HCOO- indeed can proceed easily on Bismuthene (111) facet due to the unique compressive strain. This work paves the way for the extensive experimental investigation of Bismuthene in many different fields.

Electrochemical Transformation of Facet-Controlled BiOI into Mesoporous Bismuth Nanosheets for Selective Electrocatalytic Reduction of CO2 to Formic Acid

Mesoporous bismuth nanosheets are prepared through electrochemical transformation of (100)-facet exposed BiOI. Theoretical modeling and calculations are used to simulate the in situ morphological transformation of BiOI into Bi. Mesoporous Bi nanosheets show superior electrochemical CO₂ reduction performance. A faradaic efficiency of 95.9 % at -0.77 V_RHE for the conversion of CO₂ into formic acid, is achieved for the mesoporous Bi nanosheet catalyst compared with 93.8 % at -0.87 V_RHE for the smooth Bi nanosheets. Tafel analysis and DFT calculations indicate that the electrochemical CO₂ reduction on mesoporous Bi nanosheets is kinetically faster with a higher resistance to H₂ generation than that on smooth Bi(001) nanosheets. The CO₂ -to-HCOOH pathway is preferred through formation of an *OCHO intermediate on the (012) and (001) planes of Bi. The mesoporous structure induces a more accessible interaction with CO₂ , which makes a predominant contribution to the enhanced performance compared with the subsequent CO₂ activation on different facets of Bi.

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.