A Tandem (Bi2O3 → Bimet) Catalyst for Highly Efficient ec-CO2 Conversion into Formate: Operando Raman Spectroscopic Evidence for a Reaction Pathway Change

Customizing catalyst surface/interface structures for electrochemical CO2 reduction

Electrochemical CO2 reduction reaction (CO2RR) provides a promising route to converting CO2 into value-added chemicals and to neutralizing the greenhouse gas emission. For the industrial application of CO2RR, high-performance electrocatalysts featuring high activities and selectivities are essential. It has been demonstrated that customizing the catalyst surface/interface structures allows for high-precision control over the microenvironment for catalysis as well as the adsorption/desorption behaviors of key reaction intermediates in CO2RR, thereby elevating the activity, selectivity and stability of the electrocatalysts. In this paper, we review the progress in customizing the surface/interface structures for CO2RR electrocatalysts (including atomic-site catalysts, metal catalysts, and metal/oxide catalysts). From the perspectives of coordination engineering, atomic interface design, surface modification, and hetero-interface construction, we delineate the resulting specific alterations in surface/interface structures, and their effect on the CO2RR process. At the end of this review, we present a brief discussion and outlook on the current challenges and future directions for achieving high-efficiency CO2RR via surface/interface engineering.

Face-Dependent Reconstruction of Bi-Oxyiodides toward Selective Growth of (BiO)2 CO3 Edge Side to Maximize CO2 Conversion Efficiency

Chemical reconstruction of bismuth oxyiodides using bicarbonates is tried to selectively grow (BiO)2 CO3 edge side. Orthorhombic o-Bi5 O7 I undergoes a total reconstruction process by its phase transformation into tetragonal (BiO)2 CO3 (BOC-o) to form a well-aligned nanosheet array with maximally exposing CO3 2- moiety at the edge side. The post-reconstruction BOC-o catalyst achieved 100 % Faradaic efficiency at -0.86 V vs. RHE for CO2 -to-formate conversion. However, another conservative reconstruction of tetragonal t-BiOI into tetragonal (BiO)2 CO3 (BOC-t) exposed majorly a less reactive [BiO]+ layer. At low overpotential regions, the catalytic cycle of BOC-o begins with the initial conversion of the CO3 2- moiety into formate at the [-OBi-(CO3 )-BiO-] site, but at high overpotential regions, the [BiO]+ layer undergoes reduction to metallic Bi and multi-catalytic species proceed with CO2 reduction. Otherwise, the deactivation of Bi+ site by an organic molecule switched on another catalysis of proton reduction, preventing CO2 reduction.

Correlating the valence state of a Cu-based electrocatalyst for CO2 reduction to C2

In this study, a facile ligand-protected strategy for preparing Cu@Cu2O and CuO nanoparticles is presented. The electrocatalyst efficacy of the CuO variant, particularly for CO2 reduction to multi-carbon products (C2+), is significant, boasting faradaic efficiencies (FEs) surpassing 85% and a current density peak at 340 mA cm-2. This exceptional performance markedly exceeds that of the Cu@Cu2O electrocatalyst. This observed enhancement in the electrosynthesis efficiency of C2+ is attributed to the abundant Cu0 active sites, which originate from the in situ electroreduction of CuO.

A conductive catecholate-based framework coordinated with unsaturated bismuth boosts CO2 electroreduction to formate

Bismuth-based metal-organic frameworks (Bi-MOFs) have received attention in electrochemical CO₂-to-formate conversion. However, the low conductivity and saturated coordination of Bi-MOFs usually lead to poor performance, which severely limits their widespread application. Herein, a conductive catecholate-based framework with Bi-enriched sites (HHTP, 2,3,6,7,10,11-hexahydroxytriphenylene) is constructed and the zigzagging corrugated topology of Bi-HHTP is first unraveled via single-crystal X-ray diffraction. Bi-HHTP possesses excellent electrical conductivity (1.65 S m^-1) and unsaturated coordination Bi sites are confirmed by electron paramagnetic resonance spectroscopy. Bi-HHTP exhibited an outstanding performance for selective formate production of 95% with a maximum turnover frequency of 576 h^-1 in a flow cell, which surpassed most of the previously reported Bi-MOFs. Significantly, the structure of Bi-HHTP could be well maintained after catalysis. In situ attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirms that the key intermediate is *COOH species. Density functional theory (DFT) calculations reveal that the rate-determining step is *COOH species generation, which is consistent with the in situ ATR-FTIR results. DFT calculations confirmed that the unsaturated coordination Bi sites acted as active sites for electrochemical CO₂-to-formate conversion. This work provides new insights into the rational design of conductive, stable, and active Bi-MOFs to improve their performance towards electrochemical CO₂ reduction.

One-Step Electrochemical Dealloying of 3D Bi-Continuous Micro-Nanoporous Bismuth Electrodes and CO2RR Performance

The rapid development of electrochemical CO₂ reduction offers a promising route to convert intermittent renewable energy into products of high value-added fuels or chemical feedstocks. However, low faradaic efficiency, low current density, and a narrow potential range still limit the large-scale application of CO₂RR electrocatalysts. Herein, monolith 3D bi-continuous nanoporous bismuth (np-Bi) electrodes are fabricated via a simple one-step electrochemical dealloying strategy from Pb-Bi binary alloy. The unique bi-continuous porous structure ensures highly effective charge transfer; meanwhile, the controllable millimeter-sized geometric porous structure enables easy catalyst adjustment to expose highly suitable surface curvatures with abundant reactive sites. This results in a high selectivity of 92.6% and superior potential window (400 mV, selectivity > 88%) for the electrochemical reduction of carbon dioxide to formate. Our scalable strategy provides a feasible pathway for mass-producing high-performance and versatile CO₂ electrocatalysts.

From Traditional to New Benchmark Catalysts for CO2 Electroreduction

In the last century, conventional strategies pursued to reduce or convert CO₂ have shown limitations and, consequently, have been pushing the development of innovative routes. Among them, great efforts have been made in the field of heterogeneous electrochemical CO₂ conversion, which boasts the use of mild operative conditions, compatibility with renewable energy sources, and high versatility from an industrial point of view. Indeed, since the pioneering studies of Hori and co-workers, a wide range of electrocatalysts have been designed. Starting from the performances achieved using traditional bulk metal electrodes, advanced nanostructured and multi-phase materials are currently being studied with the main goal of overcoming the high overpotentials usually required for the obtainment of reduction products in substantial amounts. This review reports the most relevant examples of metal-based, nanostructured electrocatalysts proposed in the literature during the last 40 years. Moreover, the benchmark materials are identified and the most promising strategies towards the selective conversion to high-added-value chemicals with superior productivities are highlighted.

Nanosecond Laser Confined Bismuth Moiety with Tunable Structures on Graphene for Carbon Dioxide Reduction

Substrate-supported catalysts with atomically dispersed metal centers are promising for driving the carbon dioxide reduction reaction (CO₂RR) to produce value-added chemicals; however, regulating the size of exposed catalysts and optimizing their coordination chemistry remain challenging. In this study, we have devised a simple and versatile high-energy pulsed laser method for the enrichment of a Bi "single atom" (SA) with a controlled first coordination sphere on a time scale of nanoseconds. We identify the mechanistic bifurcation routes over a Bi SA that selectively produce either formate or syngas when bound to C or N atoms, respectively. In particular, C-stabilized Bi (Bi-C) exhibits a maximum formate partial current density of -29.3 mA cm^-2 alongside a TOF value of 2.64 s^-1 at -1.05 V vs RHE, representing one of the best SA-based candidates for CO₂-to-formate conversion. Our results demonstrate that the switchable selectivity arises from the different coupling states and metal-support interactions between the central Bi atom and adjacent atoms, which modify the hybridizations between the Bi center and *OCHO/*COOH intermediates, alter the energy barriers of the rate-determining steps, and ultimately trigger the branched reaction pathways after CO₂ adsorption. This work demonstrates a practical and universal ultrafast laser approach to a wide range of metal-substrate materials for tailoring the fine structures and catalytic properties of the supported catalysts and provides atomic-level insights into the mechanisms of the CO₂RR on ligand-modified Bi SAs, with potential applications in various fields.

Promoting water dissociation for efficient solar driven CO2 electroreduction via improving hydroxyl adsorption

Exploring efficient electrocatalysts with fundamental understanding of the reaction mechanism is imperative in CO₂ electroreduction. However, the impact of sluggish water dissociation as proton source and the surface species in reaction are still unclear. Herein, we report a strategy of promoting protonation in CO₂ electroreduction by implementing oxygen vacancy engineering on Bi₂O₂CO₃ over which high Faradaic efficiency of formate (above 90%) and large partial current density (162 mA cm^-2) are achieved. Systematic study reveals that the production rate of formate is mainly hampered by water dissociation, while the introduction of oxygen vacancy accelerates water dissociation kinetics by strengthening hydroxyl adsorption and reduces the energetic span of CO₂ electroreduction. Moreover, CO₃* involved in formate formation as the key surface species is clearly identified by electron spin resonance measurements and designed in situ Raman spectroscopy study combined with isotopic labelling. Coupled with photovoltaic device, the solar to formate energy conversion efficiency reaches as high as 13.3%.

Ag-induced Phase Transition of Bi2 O3 Nanofibers for Enhanced Energy Conversion Efficiency towards Formate in CO2 Electroreduction

Bi-based electrocatalysts have been widely investigated in the CO₂ reduction reaction (CO₂ RR) for the formation of formate. However, it remains a challenge to achieve high Faradaic efficiency (FE) and industrial current densities at low overpotentials for obtaining both high formate productivity and energy efficiency (EE). Herein, we report an Ag-Bi₂ O₃ hybrid nanofiber (Ag-Bi₂ O₃ ) for highly efficient electrochemical reduction of CO₂ to formate. Ag-Bi₂ O₃ exhibits a formate FE of >90% for current densities from -10 to -250 mA ⋅ cm^-2 and attains a yield rate of 11.7 mmol ⋅ s^-1  ⋅ m^-2 at -250 mA ⋅ cm^-2 . Moreover, Ag-Bi₂ O₃ increased the EE (52.7%) by nearly 10% compared to a Bi₂ O₃ only counterpart. Structural characterization and in-situ Raman results suggest that the presence of Ag induced the conversion of Bi₂ O₃ from a monoclinic phase (α-Bi₂ O₃ ) to a metastable tetragonal phase (β-Bi₂ O₃ ) and accelerated the formation of active metallic Bi at low overpotentials (at > -0.3 V), which together contributes to the highly efficient formate formation.

A Nanocomposite of Bismuth Clusters and Bi2 O2 CO3 Sheets for Highly Efficient Electrocatalytic Reduction of CO2 to Formate

The renewable-electricity-driven CO₂ reduction to formic acid would contribute to establishing a carbon-neutral society. The current catalyst suffers from limited activity and stability under high selectivity and the ambiguous nature of active sites. Herein, we report a powerful Bi₂ S₃ -derived catalyst that demonstrates a current density of 2.0 A cm^-2 with a formate Faradaic efficiency of 93 % at -0.95 V versus the reversible hydrogen electrode. The energy conversion efficiency and single-pass yield of formate reach 80 % and 67 %, respectively, and the durability reaches 100 h at an industrial-relevant current density. Pure formic acid with a concentration of 3.5 mol L^-1 has been produced continuously. Our operando spectroscopic and theoretical studies reveal the dynamic evolution of the catalyst into a nanocomposite composed of Bi⁰ clusters and Bi₂ O₂ CO₃ nanosheets and the pivotal role of Bi⁰ -Bi₂ O₂ CO₃ interface in CO₂ activation and conversion.

Driving up the Electrocatalytic Performance for Carbon Dioxide Conversion through Interface Tuning in Graphene Oxide-Bismuth Oxide Nanocomposites

The integration of graphene oxide (GO) into nanostructured Bi₂O₃ electrocatalysts for CO₂ reduction (CO₂RR) brings up remarkable improvements in terms of performance toward formic acid (HCOOH) production. The GO scaffold is able to facilitate electron transfers toward the active Bi₂O₃ phase, amending for the high metal oxide (MO) intrinsic electric resistance, resulting in activation of the CO₂ with smaller overpotential. Herein, the structure of the GO-MO nanocomposite is tailored according to two synthetic protocols, giving rise to two different nanostructures, one featuring reduced GO (rGO) supporting Bi@Bi₂O₃ core-shell nanoparticles (NP) and the other GO supporting fully oxidized Bi₂O₃ NP. The two structures differentiate in terms of electrocatalytic behavior, suggesting the importance of constructing a suitable interface between the nanocarbon and the MO, as well as between MO and metal.

The Critical Role of Initial/Operando Oxygen Loading in General Bismuth-Based Catalysts for Electroreduction of Carbon Dioxide

Operando reconstruction of solid catalyst into a distinct active state frequently occurs during electrocatalytic processes. The correlation between initial and operando states, if ever existing, is critical for the understanding and precise design of a catalytic system. Inspired by recently established intermediate metallic state of Bi-based catalysts during electrocatalytic carbon dioxide reduction (CO₂RR), here we investigate a series of Bi oxide catalysts (Bi, Bi₂O₃, BiO₂) and demonstrate that the operando surface/subsurface oxygen loading, positively correlated to the initial oxygen content, plays a critical role in determining Bi-based CO₂RR performance. Higher initial oxygen loading indicates a better electrocatalytic efficiency. Further analysis shows that this conclusion generally applies to all Bi-based electrocatalysts reported up to date. Following this principle, cost-effective BiO₂ nanocrystals demonstrated the highest formate Faradaic efficiency (FE) and current density compared to Bi/Bi₂O₃, further allowing a pair-electrolysis system with 800 mA/cm² current density and an overall 175% FE for formate production.

Novel Ni foam catalysts for sustainable nitrate to ammonia electroreduction

Electrochemical nitrate reduction (NO₃^-RR) is considered a promising approach to remove environmentally harmful nitrate from wastewater while simultaneously producing ammonia, a product with high value. An important consideration is the choice of catalyst, which is required not only to accelerate NO₃^-RR but also to direct the product selectivity of the electrolysis toward ammonia production. To this end, we demonstrate the fabrication of novel Ni foam catalysts produced through a dynamic hydrogen bubble template assisted electrodeposition process. The resulting foam morphology of the catalyst is demonstrated to crucially govern its overall electrocatalytic performance. More than 95% Faradaic efficiency of ammonia production was achieved in the low potential range from -0.1 to -0.3 V vs. RHE. Hydrogen was found to be the only by-product of the nitrate reduction. Intriguingly, no other nitrogen containing products (e.g., NO,N₂O, or N₂) formed during electrolysis, thus indicating a 100% selective (nitrate→ammonia) conversion. Therefore, this novel Ni foam catalyst is a highly promising candidate for truly selective (nitrate→ammonia) electroreduction and a promising alternative to mature copper-based NO₃^-RR benchmark catalysts. Excellent catalytic performance of the novel Ni foam catalyst was also observed in screening experiments under conditions mimicking those in wastewater treatment.

Phase Separation Induced Binary Core-Shell Alloy Nanoparticles Embedded in Carbon Sheets for Magnesium Storage

Magnesium-ion batteries (MIBs) have aroused widespread interest in large-scale applications due to their low cost, high volumetric capacity, and safety. However, magnesium (Mg) metals are incompatible with conventional electrolytes, making it difficult to plate and strip reversibly. Therefore, developing novel Mg²⁺ host anodes remains a huge challenge. Herein, we present a rational design and fabrication of binary Bi@Sn alloy nanoparticles embedded in carbon sheets (Bi@Sn-C) as a superior anode for MIBs employing phase separation during the annealing of bimetallic MOFs. The Bi@Sn-C simultaneously integrates the nanostructure design and multi-element coordination strategies which is favorable to improve the overall structural stability and Mg²⁺ diffusion kinetics. Benefiting from the aforementioned features, the Bi@Sn-C electrodes deliver good cycling stability of 214 mA h g^-1 at 100 mA g^-1 after 100 cycles and rate capability with 200 mA h g^-1 at 500 mA g^-1. And when using all-phenyl complex with lithium chloride (LiCl-APC) dual-salt electrolyte, the electrochemical performance of Bi@Sn-C is further optimized and shows enhanced rate performance (238 mA h g^-1 at 500 mA g^-1) and reversible capacity (308 mA h g^-1 at 100 mA g^-1 after 100 cycles). This novel strategy holds great promise for designing efficient alloy electrode materials for MIBs.

MOF-Transformed In2O3-x@C Nanocorn Electrocatalyst for Efficient CO2 Reduction to HCOOH

For electrochemical CO₂ reduction to HCOOH, an ongoing challenge is to design energy efficient electrocatalysts that can deliver a high HCOOH current density (J_HCOOH) at a low overpotential. Indium oxide is good HCOOH production catalyst but with low conductivity. In this work, we report a unique corn design of In₂O_3-x@C nanocatalyst, wherein In₂O_3-x nanocube as the fine grains dispersed uniformly on the carbon nanorod cob, resulting in the enhanced conductivity. Excellent performance is achieved with 84% Faradaic efficiency (FE) and 11 mA cm^-2 J_HCOOH at a low potential of - 0.4 V versus RHE. At the current density of 100 mA cm^-2, the applied potential remained stable for more than 120 h with the FE above 90%. Density functional theory calculations reveal that the abundant oxygen vacancy in In₂O_3-x has exposed more In³⁺ sites with activated electroactivity, which facilitates the formation of HCOO* intermediate. Operando X-ray absorption spectroscopy also confirms In³⁺ as the active site and the key intermediate of HCOO* during the process of CO₂ reduction to HCOOH.

Enhanced Electrocatalytic CO2 Reduction of Bismuth Nanosheets with Introducing Surface Bismuth Subcarbonate

The electrocatalytic CO2 reduction reaction (CO2RR) into hydrocarbon products is one of the most promising approaches for CO2 utilization in modern society. However, the application of CO2RR requires optimizing state-of-the-art catalysts as well as elucidating the catalytic interface formation mechanism. In this study, a flower-like nano-structured Bi catalyst is prepared by a facile pulse current electrodeposition method wherein the morphologies could be accurately controlled. Interestingly, nano-structured Bi is inclined to generate Bi2O2CO3 in the air and form a stable Bi2O2CO3@Bi interface, which could enhance the CO2 adsorption and conversion. In-situ Raman spectroscopy analysis also proves the existence of Bi2O2CO3 on the electrode surface. In a practical CO2 reduction test by a flow-cell reactor, the Bi2O2CO3@Bi electrode delivers a high faradaic efficiency of the CO2 to formate/formic acid (~90%) at −1.07 V vs. reversible hydrogen electrode (RHE) with no obvious decay during more than a 10 h continuous test. The introducing surface Bi2O2CO3 in nano-structured Bi supports a promising strategy as well as facile access to prepare improved CO2RR electrocatalysts.

Bi2O3 Nanosheets Grown on Carbon Nanofiber with Inherent Hydrophobicity for High-Performance CO2 Electroreduction in a Wide Potential Window

The ever-increasing concern for adverse climate changes has propelled worldwide research on the reduction of CO₂ emission. In this regard, CO₂ electroreduction (CER) to formate is one of the promising approaches to converting CO₂ to a useful product. However, to achieve a high production rate of formate, the existing catalysts for CER fall short of expectation in maintaining the high formate selectivity and activity over a wide potential window. Through this study, we report that Bi₂O₃ nanosheets (NSs) grown on carbon nanofiber (CNF) with inherent hydrophobicity achieve a peak formate current density of 102.1 mA cm^-2 and high formate Faradaic efficiency of >93% over a very wide potential window of 1000 mV. To the best of our knowledge, this outperforms all the relevant achievements reported so far. In addition, the Bi₂O₃ NSs on CNF demonstrate a good antiflooding capability when operating in a flow cell system and can deliver a current density of 300 mA cm^-2. Molecular dynamics simulations indicate that the hydrophobic carbon surface can repel water molecules to form a robust solid-liquid-gas triple-phase boundary and a concentrated CO₂ layer; both can boost CER activity with the local high concentration of CO₂ and through inhibiting the hydrogen evolution reaction (HER) by reducing proton contacts. This water-repelling effect also increases the local pH at the catalyst surface, thus inhibiting HER further. More significantly, the concept and methodology of this hydrophobic engineering could be broadly applicable to other formate-producing materials from CER.

Highly efficient electrochemical reduction of carbon dioxide to formate on Sn modified Bi2O3 heterostructure

In this work, Sn species are deposited onto the surface of a Bi₂O₃ material by a facile disproportionated reaction and the prepared catalyst shows a superior electrocatalytic performance towards CO₂ reduction. The deposition of Sn atoms can donate electrons to the Bi₂O₃ material and increase its electrical conductivity. The Sn_M-Bi₂O₃ catalyst with the optimal Sn content delivers a high faradaic efficiency of 95.8% at -1.0 V for formate production. In addition, the partial current density of formate can reach 41.8 mA cm^-2. The Sn_M-Bi₂O₃ catalyst also exhibits superior stability towards long-term electrolysis. The modification of Sn species not only helps to stabilize the reaction intermediate but also inhibits the hydrogen evolution reaction (HER) pathway, achieving the synergetic enhancement of catalytic activity.

Suppression of the Hydrogen Evolution Reaction Is the Key: Selective Electrosynthesis of Formate from CO2 over Porous In55Cu45 Catalysts

Direct electrosynthesis of formate through CO₂ electroreduction (denoted CO₂RR) is currently attracting great attention because formate is a highly valuable commodity chemical that is already used in a wide range of applications (e.g., formic acid fuel cells, tanning, rubber production, preservatives, and antibacterial agents). Herein, we demonstrate highly selective production of formate through CO₂RR from a CO₂-saturated aqueous bicarbonate solution using a porous In₅₅Cu₄₅ alloy as the electrocatalyst. This novel high-surface-area material was produced by means of an electrodeposition process utilizing the dynamic hydrogen bubble template approach. Faradaic efficiencies (FEs) of formate production (FE_formate) never fell below 90% within a relatively broad potential window of approximately 400 mV, ranging from -0.8 to -1.2 V vs the reversible hydrogen electrode (RHE). A maximum FE_formate of 96.8%, corresponding to a partial current density of j_formate = -8.9 mA cm^-2, was yielded at -1.0 V vs RHE. The experimental findings suggested a CO₂RR mechanism involving stabilization of the HCOO* intermediate on the In₅₅Cu₄₅ alloy surface in combination with effective suppression of the parasitic hydrogen evolution reaction. What makes this CO₂RR alloy catalyst particularly valuable is its stability against degradation and chemical poisoning. An almost constant formate efficiency of ∼94% was maintained in an extended 30 h electrolysis experiment, whereas pure In film catalysts (the reference benchmark system) showed a pronounced decrease in formate efficiency from 82% to 50% under similar experimental conditions. The identical location scanning electron microscopy approach was applied to demonstrate the structural stability of the applied In₅₅Cu₄₅ alloy foam catalysts at various length scales. We demonstrate that the proposed catalyst concept could be transferred to technically relevant support materials (e.g., carbon cloth gas diffusion electrode) without altering its excellent figures of merit.