Bismuth-Based Bimetallic Catalysts for Electrochemical CO2 Reduction to Formate (https://doi.org/10.63386/620645)

Xinyue Hu ^1,a,* , Haiqian Yu ^2,b , Xi He^1,c

¹ School of Chemistry and Chemical Engineering, Anhui University of Technology,

Maanshan243032, Anhui Province, China

² School of Life Sciences, Huainan Normal University,

Huainan232038, Anhui Province, China

^ahuxinyue@ahut.edu.cn

^b3427064970@qq.com

^c1934241342@qq.com

*Corresponding author: Xinyue Hu

Email: huxinyue@ahut.edu.cn

^aAuthor Information

Full Name: Xinyue Hu

Highest Degree: Undergraduate student

Affiliation: School of Chemistry and Chemical Engineering, Anhui University of Technology

Email:huxinyue@ahut.edu.cn

Address: Ma’anshan, Anhui Province, China

Postal Code: 243032 (Note: Corrected standard postal code for Anhui University of Technology)

Author Biography

Name: Xinyue Hu

Gender: Female

Date of Birth: December 2004

Place of Origin: Anhui Province, China

Ethnicity: Han Chinese

Education: Undergraduate student

Research Focus: CO₂ electrocatalysis

^bAuthor Information

Full Name: Haiqian Yu

Highest Degree: Undergraduate student

Affiliation: School of Life Sciences, Huainan Normal University

Email: 3427064970@qq.com

Address: Huainan, Anhui Province, China

Postal Code: 232038 (Note: Corrected standard postal code for Huainan Normal University)

Author Biography

Name: Haiqian Yu

Gender: Female

Date of Birth: October 2003

Place of Origin: Anhui province, China

Ethnicity: Han Chinese

Education: Undergraduate student

Research Focus: CO₂ electrocatalysis

^cAuthor Information

Full Name: Xi He

Highest Degree: Undergraduate student

Affiliation: School of Chemistry and Chemical Engineering, Anhui University of Technology

Email:1934241342@qq.com

Address: Ma’anshan, Anhui Province, China

Postal Code: 243032 (Note: Corrected standard postal code for Anhui University of Technology)

Author Biography

Name: Xi He

Gender: Female

Date of Birth: May 2004

Place of Origin: Hubei Province, China

Ethnicity: Han Chinese

Education: Undergraduate student

Research Focus: CO₂ electrocatalysis

 

 

1. Introduction

The growing global energy demand driven by socioeconomic development has led to the unsustainable extraction and combustion of fossil fuels, causing both energy shortages and severe environmental impacts. Conventional fossil fuel combustion releases massive CO₂ emissions, contributing to rising atmospheric CO₂ levels and accelerating climate change through global warming and glacier melting (Fig. 1).^{[1, 2]} Addressing this crisis requires urgent CO₂ emission reduction strategies.^[3] At present, CO₂ emission reduction is mainly achieved through two ways: finding new green energy to replace traditional fossil energy and capturing and reusing CO_2.^[4]To date, multiple approaches for CO₂ conversion to value-added chemicals have been explored, including photochemistry, plasma catalysis, thermochemistry, and electrochemistry.^[5-8] However, significant limitations persist across these methods: Photocatalytic CO₂ reduction suffers from rapid electron-hole recombination and inefficient solar energy utilization. Plasma catalysis faces an inherent trade-off between CO₂ conversion efficiency and energy consumption, while thermochemical processes require Energy-intensive, high-temperature, and high-pressure conditions.^[9] In contrast, electrocatalytic CO₂ reduction (CO₂RR) offers distinct advantages, including precise reaction control, recyclable electrolytes, and compact system design. These attributes position electrochemical CO₂RR as a promising pathway for synthesizing value-added chemicals—a critical strategy for mitigating greenhouse effects and achieving carbon neutrality.^[10–12]

Fig. 1 Global CO₂ emissions and warming.Copyright GCP 2001-2025.

However, electrochemical CO₂RR faces four challenges:(1) The process requires high overpotential due to the inherent thermodynamic stability of CO₂, originating from its linear centrosymmetric structure and strong C=O bond energy (750 kJ mol^-1), necessitating a reduction potential of −1.49 V vs. RHE to form the critical *CO₂⁻ intermediate.^[13] (2) Product selectivity is inherently limited by aqueous-phase CO₂RR competing with the hydrogen evolution reaction (HER), while multi-electron transfer pathways generate diverse products (CO, HCOOH, CH₄, C₂H₄), complicating separation and reducing cost-effectiveness. (3) Limited solubility of CO₂ in aqueous electrolytes (34 mM under standard conditions) severely restricts mass transfer rates, limiting the current density to less than 100 mA cm^-2—far below industrial-scale requirements. (4) A critical systemic challenge lies in the energy-intensive oxygen evolution reaction (OER) at the anode, which consumes 94.5% of total input energy during HCOOH production yet yields low-value O_2.^[14] This inefficiency is compounded by OER-derived reactive oxygen species (ROS) that degrade battery separators, further escalating operational costs.^[15]

Addressing these challenges demands innovative catalysts to lower activation barriers, suppress HER, enhance CO₂ adsorption, and decouple or replace OER with thermodynamically favorable anodic processes. In this review, we summarize the recent progress of bismuth-based bimetallic catalysts in electrocatalytic CO₂RR, including material synthesis and performance evaluation. The fundamental understanding of active sites, reaction mechanisms, and structure-activity relationships is emphasized, with strategies for enhancing activity, such as morphology control, defect engineering, doping, and alloying to modulate electronic structures, being systematically explored, providing critical insights for advancing electrocatalytic performance. Finally, we present the challenges and future research directions of bismuth-based bimetallic catalysts in electrocatalytic CO₂RR.

2. Electrocatalytic CO₂ to formic acid

2.1 Reaction Mechanism of Electrochemical CO₂ Reduction to Formic Acid

The electrochemical CO₂RR involves multiple proton-coupled electron transfer (PCET) steps, where competitive reaction pathways dictate product selectivity.^[16] The rate-determining step is generally the first electron transfer to adsorbed CO₂ (*CO₂), forming the *CO₂ intermediate, with subsequent protonation steps governed by the electrocatalyst’s surface-bound atoms.^[17-19] Selectivity toward formate (HCOO⁻/HCOOH) or other products (e.g., CO, H₂, multi-carbon species) hinges on the relative binding strengths of key intermediates (*OCHO, *COOH, *CO, *H) on the electrode surface.^[20,21] Catalysts can be categorized into four groups based on product selectivity (Fig. 2):^[20,34,35] (1) HER-dominant transition metals (Fe,^[18] Ni,^[1] Pt^[2]), which favor hydrogen evolution due to low HER overpotential and moderate *H adsorption; (2) CO-selective metals (Zn,^[3] Ag,^[4] Au^[22]), which thermodynamically prefer *COOH adsorption to yield CO as the main product;^[20] (3) formate-selective p-block metals (In,^[23] Pb,^[24] Sn,^[25] Bi^[26]), which exhibit stronger *OCHO adsorption to promote formate formation;^[20,27,28] (4) hydrocarbon/alcohol-producing Cu-based catalysts, which stabilize *COOH and *CO intermediates for multi-carbon product synthesis.^[29-33] Herein, this review briefly summarizes the progress of research on the formation of bimetallic systems with bismuth for the generation of formic acid by the third group of p-block metals (aluminum, gallium, indium, tin, antimony, and lead) and looks ahead to the development of these catalysts because of their relatively high efficiencies.

Fig. 2 Partial periodic table showing main products of metal catalysts for catalyzed CO2RR to obtain different products. Four groups include metal catalysts for H₂ (gray), CO (yellow), formic acid (green), and hydrocarbons/alcohols (blue).^[35] Copyright 2024, Elsevier B.V. and Science Press.

2.2 Formic acid generation over Bi-based catalyst

Bi-based catalysts hold significant promise for CO₂RR due to their low toxicity, cost-effectiveness, and high hydrogen evolution reaction (HER) overpotential, which effectively suppresses hydrogen production.^[39-43] Their catalytic performance is attributed to two key characteristics:(1)efficient CO₂ Activation: The favorable standard reduction potential of Bi³⁺/Bi (0.308 V vs. RHE) enables strong adsorption and activation of CO₂ molecules, facilitating the initial steps of the reduction reaction.^[44-47] (2) HER Suppression: A high HER overpotential minimizes competitive H₂ evolution, ensuring preferential formate formation by directing reaction pathways toward CO₂ reduction.^[47-49]

Despite the advantages of Bi-based catalysts in CO₂ electroreduction, the nature of their active sites remains a subject of debate. While Bi-O structures are widely acknowledged as critical for CO₂RR,^{[40,43,50,51]} studies indicate that surface oxide layers can remain stable under reaction conditions without forming metal carbonates.^[52,53] Research has revealed distinct active site configurations across different catalyst systems, which can be categorized into three dominant viewpoints: bismuth metal (Bi⁰), bismuth oxide (Bi-O), and interfacial synergism.

Han et al. demonstrated that ultrathin Bi nanosheets (BiNS) derived from in-situ topotactic transformation of BiOI expose under-coordinated Bi atoms on the (001) plane of a single-crystalline rhombohedral structure(Fig. 3a).^[27] DFT calculations show that the Bi (001) surface stabilizes the *OCHO intermediate with a low free energy barrier (+0.49 eV for formation) and enables exothermic transformation to formate (−0.17 eV)(Fig. 3b). The high selectivity (~100% Faradaic efficiency) and activity (24 mA cm^-2 formate partial current density) arise from optimized *OCHO adsorption, facilitated by the electronic structure of BiNS, as evidenced by p-orbital density of states (DOS) indicating stronger binding near the Fermi level. The ultrathin structure enhances surface area and active site density, ensuring durability over 10 hours(Fig. 3c).

Deng et al. highlighted enhanced Bi-O structures in bismuth oxides (e.g., Bi₂O₃) as active sites ^[43] where crystalline Bi-O bonds promote higher CO₂ adsorption and stabilize the CO₂⁻• intermediate. The reaction’s rate-determining step shifts from initial electron transfer on metallic Bi to subsequent hydrogenation on Bi₂O₃, accompanied by a lower intermediate free energy. The Bi-O structure improves charge transfer efficiency (evidenced by smaller charge transfer resistance, R_ct, in electrochemical impedance spectroscopy)(Table 1), and the increased ratio of Bi-O to Bi-OH in Bi₂O₃ correlates with higher formate selectivity, confirming Bi-O bonds as key active sites.

Lin et al. identified the interface between 2-3 nm Bi clusters and Bi₂O₂CO₃ nanosheets in electrocatalytic nanocomposites as active sites.^[54] DFT reveals that interfacial Bi sites on Bi₂O₂CO₃ adjacent to Bi clusters exhibit optimized charge distribution (+1.55 Bader charge)(Fig. 3d), enhancing CO₂ adsorption (2.88 Å) and stabilizing *HCOO. This interface enables exothermic CO₂ activation (ΔG = −0.08 eV for *CO₂) and reduces hydrogenation barriers, outperforming single-phase materials to achieve an activity of 2.0 A cm^-2 and a selectivity of 93% FE.

Yuan et al. reconfigured Bi-MOFs into stable Bi₂O₂CO₃ via HCO₃⁻-mediated in-situ transformation ^[55], addressing long-standing stability issues in MOF electrocatalysts and providing a new synthetic pathway for constructing active sites. This approach not only overcomes material instability but also offers insights for designing next-generation CO₂RR systems by leveraging structural transformation to optimize active site formation.

Fig. 3 (a)Structural characterizations of the topotactically reduced BiNS SAED pattern of BiNS. (b)free-energy diagrams for HCOO⁻, CO, and H₂ formation on the Bi (001) plane. (c) Amperometric (i~t) stability of BiNS atη=680mV for 10 h.^[27] Copyright 2018, The Author(s). (d)Geometric configures of *CO2 species adsorbed on different sites.^[54] Copyright 2022, Wiley‐VCH GmbH.

Table 1: The corresponding impedance via fitting circuit for Bi and Bi2O3 at the potential of −0.9 V vs. RHE.^[43]Copyright 2019, American Chemical Society.

Catalysts	Bi	Bi2O3-1h	Bi2O3-3h	Bi2O3-5h	Bi2O3-7h
Rs(Ω)	22.08	22.96	22.94	22.65	26.93
Rct(Ω)	116.4	108.7	84.14	90.70	99.30

2.3 Limitations of Bi-based catalysts

The identity of active sites remains unresolved, with debates focusing on zero-valent Bi versus in situ-formed Bi₂O₂/Bi₂O₃ ^[43,56-60]. For example, Li et al. showed that in situ-generated metallic Bi from BiOI is essential for CO₂RR,^[27,61] while Wang et al. emphasized the role of Bi⁰-Bi₂O₂CO₃ heterostructures in carbonate electrolytes^[54,63] In copper-bismuth bimetallic systems, Li et al. revealed that electron-rich Bi nanosheets formed by electron transfer from Cu promote CO₂•− formation at lower potentials, with proton transfer from HCO₃⁻ as the rate-limiting step.^[58] The upshifted p-band center of electron-rich Bi enhances *OCHO adsorption, enabling high formate selectivity across universal pH electrolytes. Meanwhile, Deng et al. demonstrated that Bi-O structures in Bi₂O₃@C nanocomposites serve as active sites, with the carbon matrix enhancing conductivity to facilitate rapid CO₂ adsorption and *OCHO formation, achieving >93% formate Faradaic efficiency and 200 mA cm^-2 current density in flow cells.^[62] However, synthesis methods like electrochemical reduction of BiOI or Bi₂O₃ often introduce structural defects or heteroatom doping, complicating the mechanistic analysis of active sites.^[43,64-66]

3. Bi-Based Bimetallic Catalysts for CO₂RR

3.1 Bi-based bimetallic strategy

To address the persistent challenges of stability and selectivity in electrocatalysts, bimetallic Bi-based systems (Bi-M, M = Al, Ga, In, Sn, Sb, Pb) demonstrate unique synergistic effects through coordinated electronic structure regulation and defect engineering.^[67] In the Cu-Bi alloy system, Hoffman et al. revealed that lattice mismatch between heterogeneous metals (8.7% difference in lattice constants between Cu and Bi) induces the formation of high-density defective sites(Fig. 4), significantly enhancing the adsorption capacity of critical *OCHO intermediates during CO₂ reduction.^[68] When Bi content is optimized to 40–60 wt%, this system achieves stable formate Faraday efficiency, FE_HCOOH exceeding 90% across a wide potential window, representing a nearly twofold improvement over a monometallic catalyst. This performance enhancement originates from dynamically formed strain fields at bimetallic interfaces, which strengthen intermediate stabilization.

Fig. 4 X-ray diffraction patterns for electrodeposited pure Cu, Bi, and Cu-Bi films. Dotted lines and arrows are provided for the identification of corresponding reflections. Peaks labeled S indicate reflections originating from the substrate.^[68] Copyright 2019, Wiley-VCH.

Further investigations on Bi-Sn systems elucidate electronic redistribution through orbital hybridization mechanisms. Chen et al. demonstrated that p-d orbital hybridization of Sn induces electronic state reconstruction near the Fermi level, shifting electron cloud density toward p/d orbitals of Sn at Bi-Sn interfaces.^[69] This electronic restructuring weakens excessive binding between oxygen-terminated intermediates (*OCHO) and catalyst surfaces, achieving 96% FE_HCOOH at −1.1 V vs RHE while maintaining a high production rate of 0.74 mmol h^-1 cm^-2. X-ray photoelectron spectroscopy (XPS) analysis confirms a 0.45 eV positive shift in Bi 4f orbital binding energy upon Sn incorporation, indicating effective regulation of active sites’ electron affinity through interfacial charge transfer.

These findings collectively verify that precise control over crystallographic compatibility and electronic interactions in bimetallic components enables synergistic optimization of adsorption strength, selectivity, and durability. Such progress establishes a new paradigm for designing high-efficiency CO₂ reduction systems, where tailored atomic-scale synergies bridge the gap between fundamental mechanisms and practical catalytic performance.

3.2 Bi-Based Bimetallic Catalysts

3.2.1 In-Bi

Recent advances in electrocatalytic CO₂RR have demonstrated the remarkable efficacy of bimetallic doping strategies in enhancing formate production ^[70,71], with Bi and In emerging as particularly effective dopants. Feng et al. developed Bi-doped In₂O₃ nanocubes where the introduction of Bi accelerates surface reconstruction, modulates electron density at In sites, and induces controlled amorphization, collectively lowering the energy barrier for HCOOH* formation while suppressing competing HER.^[72] This optimized catalyst achieves exceptional performance, maintaining >97.6% Faradaic efficiency across an unprecedented 800 mV window in flow cells(Fig. 5a). Wang et al. confirmed through in situ Raman spectroscopy that interstitial doping in Bi₂O₃ stabilizes lattice oxygen by strengthening Bi-O bonds. Concurrently, with increasing In content, the lattice oxygen peak shifts to higher binding energy, while Bi peaks shift to lower binding energy, indicating that In incorporation reduces the covalency of Bi-O bonds(Fig. 5b)， which enables sustained >90% formate FE over 100 hours (Fig. 5c and d).^[73] Further advancing the field, Yang et al. designed an in-situ-grown In-doped Bi/Bi₂O₃ catalyst on carbon paper (Fig. 5f), where DFT calculations(Fig. 5e) show that In-induced electronic modulation stabilizes key *OOCH intermediates while suppressing *H adsorption, yielding 96.3% FE with excellent stability (Fig. 5g).^[74] These studies collectively establish that strategic doping creates synergistic effects through three universal mechanisms: electronic modulation of active sites, structural stabilization under operating conditions, and kinetic acceleration of key reaction steps. By elucidating both the structural transformations and underlying electronic interactions, this body of work provides crucial insights for designing next-generation CO₂RR catalysts, moving closer to the practical implementation of this sustainable technology.

Fig. 5 (a) FE of CO₂RR products.^[72] Copyright 2024, American Chemical Society. (b) Raman spectra of all In-Bi₂O₃-x samples. (c) FE_HCOOH. (d) Long-term stability of In-Bi₂O₃-0 and In-Bi₂O₃-100 at −0.9 V vs. RHE.^[74] Copyright 2025, Wiley-VCH GmbH. (e) DFT-optimized configurations of different intermediates, (i-iii) adsorbed on Bi(021)/Bi₂O₃(220), In-Bi(021)/Bi₂O₃(220), and In-Bi(021)/Bi₂O₃(220)-Ov. (f) Schematic illustration of the synthesis of In-Bi/Bi₂O₃/CP. (g)Faradaic efficiencies for generations of formate over Bi/Bi₂O₃/CP and In-Bi/Bi₂O₃/CP at various applied potentials. ^[73] Copyright 2024, Elsevier B.V.

3.2.2 Sn-Bi

Tin (Sn), a Group ⅣA metal adjacent to bismuth (Bi) in the periodic table, both are oxyphilic. These characteristics facilitate the protonation of the C atom in the *CO₂^– intermediate, promoting the formation of *OCHO and significantly enhancing the selectivity for formic acid (HCOOH).^[20] Leveraging this synergy, Wang et al. developed an innovative Bi-Sn bridged electrocatalyst (Bi₁-O-Sn₁@C), featuring oxygen-mediated electronic coupling between the two metals.^[75] In this unique architecture, the oxygen bridge enables directional electron transfer from Bi to Sn, precisely tuning the electronic structure of Sn to achieve optimal adsorption energy for the *OCHO intermediate—a critical determinant of formate production efficiency. The optimized Bi₁-O-Sn₁@C catalyst demonstrates exceptional performance: a formate Faradaic efficiency of 97.7% at −1.1 V vs. RHE and an outstanding current density of 310 mA cm^-2 at −1.5 V(Fig. 6a). In situ ATR-FTIR spectra (Fig. 6b) show a CO₂ adsorption peak at 2300-2400 cm^-1, with water vibration peaks at 1635 cm^-1 (δ(H-O-H)) and 3000-3600 cm^-1 (δ(O-H)). Characteristic peaks at 1275 and 1440 cm^-1, assigned to O-C-O vibrations in *OCHO, strengthen with increasing potential, confirming continuous formate production. Weak peaks at 1900-2100 cm^-1 indicate negligible *CO formation. Operando Raman measurements (Fig. 6c) display symmetric C-O stretching of adsorbed carbonate (CO₃²⁻) at 1012 and 1066 cm^-1, a C=O stretching peak at 1180 cm^-1, and O-(CH)-O vibrations of *OCHO at 1350 and 1426 cm^-1. The increasing intensity of the 1350 cm^-1 peak with cathode potential confirms *OCHO as the dominant intermediate in CO₂RR to formate.DFT calculations (Fig. 6d) show that Bi₁-O-Sn₁@C and Sn have lower energy barriers (ΔG) for *OCHO formation than Bi, SnO, and SnO₂. Bi₁-O-Sn₁@C exhibits optimal *OCHO adsorption, with double Sn-O coordination (ΔG = −0.02 eV) being more stable than double Bi-O (ΔG = 0.58 eV) or mixed Bi/Sn-O (ΔG = 0.41 eV), enabling moderate adsorption and low energy barriers on Sn sites. Mechanistic investigations integrating in situ ATR-FTIR, operando Raman spectroscopy, and density functional theory (DFT) calculations clarify that the Bi-O-Sn bridged structure serves as the active center for efficient CO₂-to-formate conversion. The oxygen bridge not only enables favorable electron redistribution but also stabilizes key reaction intermediates, explaining the catalyst’s superior activity and selectivity.

In a complementary study, Chen et al. designed nano-crumpled Sn-Bi bimetallic interface-rich materials via in situ precise electrodeposition tuning under CO₂ electrolysis conditions.^[76] Compared with Sn-Bi bulk alloys and pure Sn, this unique interface architecture achieves an optimal upshift of the Sn p-band center, coupled with moderate valence electron depletion. This electronic reconstruction weakens competitive Sn-C hybridization with *COOH intermediates while optimizing Sn-O hybridization for *OCHO/HCOO⁻, significantly enhancing formate production. Experimental results show a superior formate partial current density of up to 140 mA cm^-2, maintained with a Faradaic efficiency >90% across a wide potential window (−0.74 to −1.14 V vs. RHE) and a durability of 160 hours(Fig. 6e and j). Moreover, it further reveals that in situ overpotential electrodeposition during CO₂RR induces nano-crumpled morphologies, exposing abundant active Sn-Bi interfaces(Fig. 6i). Combined with DFT calculations, X-ray photoelectron spectroscopy (XPS), and in situ X-ray absorption spectroscopy (in situ XAS) (Fig. 6f,g, and h), the Sn-Bi bimetallic interface is confirmed to optimize intermediate adsorption: the upshifted Sn p-band center reduces *COOH binding strength while maintaining moderate *OCHO/HCOO^⁻ adsorption, lowering the thermodynamic barrier for HCOOH formation. This interface engineering strategy not only provides a new pathway to enhance current densities at low potentials for formate production but also offers a generalizable design concept for constructing highly active and stable interfaces in other bimetallic catalytic systems.

Fig. 6 (a) Faradaic efficiency and partial current density of HCOOH on Bi₁-O-Sn₁@C at different potentials;(b) in-situ ATR-FTIR spectra; (c) in-situ Raman spectra of the intermediate adsorption on Bi₁-O-Sn₁@C at different applied potentials in 0.5 m CO₂ saturated KHCO₃ aqueous solution;(d) The calculated free energy change diagram for CO₂ reductionto formate.^[75] Copyright 2023， Wiley‐VCH GmbH. (e) FE of formate；(f)Gibbs free energy profiles of CO and HCOOH production pathways on Sn-Bi bimetallic interface;(g)XPS spectra of Sn 3d spectrum and Bi 4f spectrum for Sn-Bi interface and alloy. Solid color lines represent the curve fittings;(h)XANES spectra of Bi L₃-edge;(i)SEM images of microflower structures; (j)The stability testing of Sn-Bi interface and alloy at −0.84 V vs. RHE.^[76]Copyright 2022, The Author(s).

3.2.3 Sb-Bi

The strategic integration of bismuth (Bi) into antimony (Sb) to form Sb-Bi alloys has emerged as an effective approach to enhance formate production in electrochemical CO₂ reduction. While initial studies demonstrated catalytic improvements, the underlying mechanism remained unclear until Sun et al. employed advanced in situ spectroscopic techniques to unravel the reaction pathway.^[77] Using precisely controlled co-sputtering to fabricate Sb-Bi alloy thin films, they established a direct correlation between Bi content and formate yield. In situ differential electrochemical mass spectrometry (DEMS) revealed that increasing Bi suppresses the competitive hydrogen evolution reaction (HER)(Fig. 7a), while combined in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and Raman spectroscopy showed that Bi incorporation shifts the dominant reaction intermediate from *COOH to *OCHO, with optimal *OCHO stabilization at intermediate Bi concentrations(Fig. 7b and c). This mechanistic insight clarified that pure Sb operates via a *COOH pathway, whereas Sb-Bi alloys favor the more efficient *OCHO pathway, with intermediate stability inversely related to Bi content.

Complementing these findings, Ma et al. developed a metal-organic framework (MOF)-templated strategy to synthesize carbon-supported Sb/Bi nanocomposites (Sb/Bi@C).^[78] Optimal Sb doping (≤3.1 wt%) dual-functionally suppressed *H formation and stabilized *OCHO, maximizing formate selectivity, while excessive Sb loading promoted HER(Fig. 7d). The Sb/Bi@C system exhibited superior structural stability and operational lifetime compared to Bi@C(Fig. 7e), attributed to Sb-induced morphological optimization of carbon-supported nanostructures. This approach addressed the low current density (<10 mA cm^-2) and short lifetime (<24 h) limitations of pure Bi catalysts by creating high-surface-area materials with abundant active sites.

Yang et al.^[79] further advanced this field by fabricating nanoporous Sb-Bi alloys via chemical dealloying, the partial current density of 734 mA cm^-2 and 59% cathodic energy efficiency in a flow cell(Fig. 7f). Experimental and DFT studies revealed that Sb-Bi interactions activate Sb sites by stabilizing *OCHO(Fig. 7g), weakening *COOH adsorption, and suppressing HER. Collectively, these studies establish bimetallic interaction engineering as a paradigm for transforming inert p-block metals into efficient CO₂ reduction catalysts, integrating mechanistic understanding with material design to enable sustainable formate production.

，

Fig. 7 (a)The HER onset potential of each catalyst derived from the DEMS analysis. (b) The in-situ ATR-SEIRAS spectra on the surface of Sb₂₅Bi₇₅ catalysts during LSV measurements in CO₂-saturated 0.5 M KHCO₃ from 0.3 to −1.3 V vs. RHE; (c) The in situ Raman spectra of Sb₂₅Bi₇₅ catalysts at different applied potentials from 0 to −1.2 V vs. RHE in CO₂-saturated 0.5 M KHCO₃ in an H-cell.^[77] Copyright 2025, The Royal Society of Chemistry. (d)Calculated free energy diagrams for CO₂RR to formate and HER on Bi (012) and BixSby (012). (e) 30 h long-term stability experiment of Bi@C and Sb_2.5/Bi@C at −1.4 V(vs. Ag/AgCl).^[78]Copyright, Wiley-VCH GmbH. (f) EEs of formate at various cathodic current densities;(g)The calculated Gibbs free energy diagrams for CO₂-to-HCOOH conversion on the Sb site of Sb and Sb₆Bi₂ slabs. The gold, purple, red, white, and brown spheres represent Sb, Bi, O, H, and C atoms, respectively.^[79]Copyright 2022, Elsevier B.V.

3.2.4 Pb-Bi

Recent advances in bimetallic Bi-Pb electrocatalysts demonstrate significant progress in CO₂-to-formate conversion through tailored structural and compositional engineering.^[80] In one approach, Farid et al. employed an electrochemical dealloying strategy to fabricate bicontinuous nanostructures by selectively dissolving Sn and Pb from Bi₅₀Pb₄₀Sn₁₀ precursors in acidic electrolytes.^[81] This process generated distinct morphologies: web-like Bi₈₅Pb₁₅ formed in 1.0 M H₂SO₄ at 0.3 V, and flake-like Bi₆₀Pb₄₀ produced in 1.0 M HCl at 0.2 V(Fig. 8a). These dealloyed structures exhibited substantially enhanced specific surface areas (10.293 m²/g for Bi₈₅Pb₁₅, 7.159 m²/g for Bi₆₀Pb₄₀) and reduced charge transfer resistance (228.28 Ω versus 4796.8 Ω in the parent alloy), facilitating efficient electron transport(Fig. 8b). Electrochemically, the web-like Bi₈₅Pb₁₅ achieved 96.5% formate Faradaic efficiency (FE) at −1.0 V vs. RHE with 16 mA cm^-2 current density, outperforming both its flake-like counterpart (83% FE, 9 mA cm^-2) and the original alloy(Fig. 8c). In situ Raman spectroscopy identified key reaction intermediates (*OCHO at 1400 cm^-1)(Fig. 8d), while DFT calculations confirmed Bi₈₅Pb₁₅ optimizes *OCHO formation energy (ΔG = 0.32 eV) and formate desorption energy (ΔG = 0.03 eV)(Fig. 8e). Alternatively, Zhang et al. synthesized a mixed-phase BiPb electrode via pulse-electrodeposition on copper foam,^[82] combining metallic Bi with Pb₇Bi₃ alloy phases into a unique flake-dendritic hybrid(Fig. 8f). This configuration achieved 91.86% formate FE at −0.96 V vs. RHE with 15.56 mA cm^-2 current density, surpassing single-metal electrodes. Remarkably, despite in situ transformation to Bi₂O₂CO₃ during extended 50-hour operation in CO₂-rich electrolyte, the electrode maintained >90% FE, demonstrating exceptional structural resilience(Fig. 8g). Tafel analysis (102.4 mV/dec) attributed this performance to synergistic electron transfer between Bi and Pb, accelerating the rate-determining CO₂⁻* formation step(Fig. 8h).

Collectively, these studies establish that bimetallic Bi-Pb systems leverage complementary strategies—structural engineering through dealloying and phase control via electrodeposition—to achieve high selectivity (>90% FE) and stability for CO₂-to-formate conversion. The consistent electronic synergy between Bi and Pb components, evidenced by interfacial charge transfer and optimized adsorption energetics, provides a unified design principle for next-generation electrocatalysts targeting sustainable carbon utilization.

Fig. 8 (a) Schematic for the synthesis of web-like Bi₈₅Pb₁₅ and flaky Bi₆₀Pb₄₀ using Bi₅₀Pb₄₀Sn₁₀ alloy foil via the electrochemically anodized/dealloyed method. (b) Nyquist plots of all the samples obtained under CO₂-saturated 0.2 MKHCO₃ (pH = 6.8) environment recorded in the frequency range of 0.01 KHz-50 000 Hz at a potential of −0.6 V vs. RHE. (c) Faradaic efficiency of reduction product obtained using web-like Bi₈₅Pb₁₅ (blue), flake-like Bi₆₀Pb₄₀ (red) and Bi₅₀Pb₄₀Sn₁₀ alloy foil (black) in 0.2 M KHCO₃ saturated with CO₂ at various potentials for a 1 h period of electrolysis (conducted twice for accuracy). (d) Demonstrated vibrational frequencies (cm^-1) for *COOH, *OCHO, and HCOOH species. (e)Calculated Gibbs free energy profiles for formic acid generation on Bi₈₅Pb₁₅, Bi₆₀Pb_40, and Bi₅₀Pb₅₀.^[81] Copyright 2025, Journal of Materials Chemistry.(f)Schematic illustration of the preparation procedure of the single Bi, single Pb, and BiPb electrodes. (g)The stability test of the BiPb electrode at −0.96 V vs. RHE. (h)Tafel plots of single Bi, single Pb, and BiPb electrodes.^[82] Copyright 2022, Elsevier B.V.

3.2.5 Al-Bi

Kuang et al. successfully synthesized Al_xBi_yO_z nanorods via a facile hydrothermal method, demonstrating their high efficiency as electrocatalysts for CO₂RR. The nanorods, fabricated through a simple hydrothermal approach (Fig. 9a), exhibit a homogeneous structure, as confirmed by elemental mapping images that reveal uniform distributions of Al, Bi, and O elements(Fig. 9b).^[83] Through systematic electrochemical characterization, it was found that the optimized catalyst demonstrated outstanding performance within a wide potential window ranging from −0.79 V to −1.29 V vs. RHE. Specifically, it maintained a Faradaic efficiency (FE) above 90% for CO₂ conversion. Notably, at −1.19 V vs. RHE, the catalyst reached a peak formate FE of 97.5% (Fig. 9c). Moreover, it showed excellent operational stability during prolonged electrolysis, with the stability lasting for 10 hours at −0.79 V vs. RHE (Fig. 9d).

The improved catalytic performance can be attributed to three key structural modifications resulting from Al incorporation. First, Al doping modulates the electronic structure of Bi₂O₃. Second, it increases the specific surface area, thereby providing more active sites for the reaction. Third, it leads to the preferential exposure of the (201) crystalline facets in Bi₂O₃. These combined effects synergistically enhance the catalyst’s ability to adsorb and activate CO₂, as evidenced by comprehensive material characterization and electrochemical analysis. Among these factors, the exposure of the (201) facet is particularly crucial, playing a decisive role in endowing the Al-modified Bi₂O₃ catalyst with exceptional CO₂RR activity.

Fig. 9 (a) Schematic illustration of the synthesis procedure for the Al_xBi_yO_z nanorods; (b) Elemental mapping images of the Al_xBi_yO_z nanorods; (c) Corresponding faradaic efficiencies of Al₂O₃ (left), R-Bi₂O₃ (middle), and Al_xBi_yO_z (right) at different potentials; (d) stability test results of the Al_xBi_yO_z nanorods at −0.79 V vs. RHE^[83]. Copyright 2021, Royal Society of Chemistry (RSC).

3.2.6 Ga-Bi

Electrodes/catalysts are prone to performance degradation due to sintering and poisoning in long-term use.^[84,85] Conventional catalysts loaded on rigid substrates (e.g., glassy carbon, carbon paper, etc.) are difficult and costly to repair.^[86]

Liquid metals (e.g., Ga and its alloys) have attracted much attention due to their excellent electrical conductivity and mobility ^[87,88] and have been applied in areas such as batteries ^[89], flexible intelligent devices ^[90], self-healing materials ^[94], and biological drug delivery.^[92] The unique properties of liquid metals also make them ideal candidates for catalysis and can be used to prepare nanocatalysts.^[93-96] Its mobility characteristics give it water-like fluidic properties, which can be used as a solvent to disperse catalytic particles. The free electrons in the liquid metal form a sea of electrons, providing abundant active sites for the reaction.^[97.98] More importantly, liquid metals produce electrohydrodynamic effects in the presence of an electric field, leading to deformation or turbulence.^[99] This property enables catalysts integrated in liquid metals to flexibly switch between liquid metal substrates and surfaces, providing the possibility of catalyst construction and repair.

To address stability challenges in electrocatalytic CO₂ reduction (CO₂RR), Hou et al. developed an innovative LM-based self-healing system by dispersing Bi₂O₃ precursors within a liquid Ga matrix.^[100] Under negative potentials, this system electrochemically reduces Bi₂O₃, drives surface polarization of Bi atoms via electrohydrodynamics, and forms activated bismuth nanostructures in situ. Using triangular potential scanning (TPS) generated two-dimensional bismuth nanosheets that achieved 93% formate Faradaic efficiency (FE) with industrial-grade current density at −0.9 V versus RHE(Fig. 10a). The LM carrier enables self-healing: mechanical stirring redisperses deactivated Bi₂O₂CO₃, while reapplied potential reconstructs the catalytic interface to fully restore activity, allowing repeatable regeneration after performance decline in 10-hour tests(Fig. 10b). TPS proved optimal for stable Bi loading, unlike pulse-on plating (POP) which caused disordered growth and high impedance(Fig. 10c). In situ XRD revealed the electrohydrodynamic evolution from dispersed Bi₂O₃/amorphous Ga to crystalline Bi with preferred (012) orientation(Fig. 10d).

Complementing this repair strategy, Guo et al. demonstrated inherent stability through a surface-enriched liquid Bi-Ga catalyst.^[101] Doping trace bismuth (740 ppm atomic) into gallium induced spontaneous surface enrichment upon cooling to 40°C, achieving over 400-fold Bi surface concentration (30 at%) without aggregation. This catalyst sustained >98% FE over 80 hours of continuous operation(Fig. 10e). Theoretical calculations revealed that liquid Bi surface sites provide energetically favorable pathways for *OCHO intermediate formation, outperforming solid Bi or Ga-containing sites(Fig. 10f).

Collectively, these advances showcase liquid metals as transformative platforms—leveraging strategies like electrohydrodynamic restructuring and spontaneous surface enrichment—to overcome the activity-stability trade-off in CO₂RR electrocatalysis using conventional rigid electrodes.

Fig. 10 (a)Faraday efficiency (FE) on TPS-derived LMelectrode from −1.2 V to −0.5 V; (c)Electrochemical impedance spectrum (EIS) before and after self-healing;(c)Three replicates of Faradaic efficiency healing after 10h of reaction;(d) In situ XRD patterns for revealing the pathway and phase transformation of Bi species by electrohydrodynamic evolution.^[100] Copyright 2022, The Author(s). (e) The 80h stability test of Bi-Ga(L) alloy at −1.30 V versus RHE.Insets show the internal resistance of the electrolyte over 80 h before and after the electrolysis, and the formate concentration during the electrolysis. (f)The maximum uphill free energy changes for formate production on three sites of Bi–Ga(L) alloy and two sites of Bi-Ga(S) alloy were examined for the electrochemical CO₂ reduction. The *OCHO formation serves as the limiting potential step for both Bi-site(L) and Bi-site(S), while the formate generation acts as the limiting potential step for Ga-site(L), GaBi-site(L), and Ga-site(S). Copyright 2024, Wiley‐VCH GmbH.

4. Summary and prospects

Although bismuth-based bimetallic catalysts exhibit remarkable potential in CO₂ electroreduction to formate, their practical implementation faces multifaceted challenges, which can be systematically categorized as follows:

a)Ambiguity of Active Sites: The nature of active species (zero-valent Bi⁰, in-situ formed Bi₂O₃/Bi₂O₂CO₃, or bimetallic interfaces) remains debated, lacking clear theoretical guidance for catalyst design. For instance, in-situ Raman and operando X-ray absorption spectroscopy (XAS) have revealed conflicting roles of Bi⁰ and Bi-O structures in intermediate adsorption, hindering rational catalyst engineering.

b)Long-term Stability Limitations: Potential-driven reduction of Bi³⁺ to Bi⁰ alters surface chemistry, while bimetallic systems often suffer from elemental segregation or sintering. For example, Pb-Bi alloys show activity decay after 24 h due to Bi leaching, highlighting the need for structural reinforcement.

c)Trade-off Between Selectivity and Activity: HER competes strongly in aqueous electrolytes, especially at low overpotentials. Current catalysts like Sb-Bi alloys achieve 95.8% formate Faradaic efficiency (FE) but struggle to maintain high current density (>200 mA cm^-2) without HER interference.

d)Industrial Scalability Hurdles: Nanostructured bimetallics (e.g., Bi-Sn core-shell nanowires) suffer from high synthesis costs (e.g., template-based methods) and low production yields (<1 g/h), restricting large-scale deployment.

1. a) Clarifying Active Sites: i) Use high-level DFT calculations with explicit solvent effects and dynamic surface restructuring to simulate CO₂ adsorption and intermediate reactions on Bi⁰, Bi₂O₃/Bi₂O₂CO₃, and bimetallic interfaces. This predicts their stability and reaction mechanisms under operational conditions.ii)Combine in-situ Raman, operando XAS, and in-situ XPS to monitor real-time vibrational modes, oxidation states, and surface compositions. Correlating these data resolves the roles of active species in CO₂
2. b) Enhancing Long-term Stability: i) Introduce surface ligands or redox-stable dopants (e.g., transition metals) to coordinate with Bi³⁺, inhibiting potential-driven reduction to Bi⁰ and preserving surface chemistry.
3. c) Balancing Selectivity and Activity: i) Optimize alloy composition (e.g., Sb-Bi) and nanostructure morphology (e.g., via templated synthesis) to enhance *OCHO stabilization and suppress HER. Introduce functional groups or dopants that specifically interact with CO₂ intermediates, lowering activation energy for CO₂ ii) Use electrolytes with high CO₂-complexing agents to increase local CO₂ concentration, and tune pH and applied potential to favor formate production over HER, balancing high FE and current density.
4. d) Overcoming Industrial Scalability Hurdles: i) Replace template-based approaches with scalable techniques (hydrothermal synthesis, electrodeposition, spray-drying) for controlled nanostructure growth on large-area substrates.ii) Optimize reaction parameters (temperature, time, reactant ratios) and adopt continuous-flow synthesis (e.g., continuous-flow hydrothermal reactors) to boost throughput and reduce batch-wise inefficiencies.

In the future, research on Bi-based bimetallic synthesis of formic acid in the field of electrocatalysis is expected to focus on the following areas:

a)Dynamic Mechanistic Elucidation: i) Track active site evolution via in-situ techniques (e.g., operando XAFS, in-situ TEM) to resolve Bi⁰/Bi-O synergies during CO₂RR.ii) Use DFT calculations to model electron transfer at bimetallic interfaces.

b)Advanced Material Design: i) Construct atomically dispersed interfaces via atomic layer deposition (ALD) or MOF templating, e.g., Fe single atoms on Bi₂O₂CO₃ nanosheets to enhance *OCHO stabilization.ii) Introduce defect engineering (e.g., oxygen vacancies in BiVO₄) to modulate the d-band center and suppress HER.

c)Stability Enhancement Strategies: i) Employ liquid metal alloys (e.g., Ga-Bi) for self-healing against sintering, or design core-shell structures (Bi@C) to prevent corrosion.ii) Develop non-aqueous electrolytes (e.g., ionic liquids) to suppress HER and widen the potential window for formate production.

d)Scalable Manufacturing and System Integration: i) Optimize hydrothermal batch synthesis for Bi-Sn-O-C catalysts to achieve industrial current densities (310 mA cm^-2) at reduced costs.ii) Integrate bimetallic catalysts with renewable energy-driven electrolyzers (e.g., solar-PV coupled flow cells) to build carbon-neutral systems, as demonstrated by Sn-Bi-MOFs achieving 13.7% solar-to-formate efficiency.

In conclusion, Bismuth-based bimetallic catalysts offer a promising pathway for CO₂ valorization, but resolving active site dynamics, enhancing stability, and scaling production is critical. By integrating in-situ characterization, atomistic design, and interdisciplinary system engineering, these materials can advance the carbon recycling economy, supporting global efforts toward carbon neutrality.