With the rapid development of modern society, the excessive burning of fossil fuels has led to huge amounts of CO₂ emissions, breaking the ecological carbon cycle and causing the growing greenhouse effect (Singh et al., 2016; Xiao et al., 2017; Garza et al., 2018; Birdja et al., 2019; Li et al., 2020). Renewably powered CO₂ conversion to value-added fuels or chemical raw materials, such as CO, HCOO⁻, C₂H₄, C₂H₅OH, etc., is becoming an important way to maintain energy and environmental sustainability (Kortlever et al., 2015; Wang et al., 2021a; Wang et al., 2021b; Wang et al., 2022). For example, the electroreduction of CO₂ using renewable electricity has attracted great research attention (Wang et al., 2021c). Among the commonly reported CO₂R products, formic acid or formate stands out as a promising liquid chemical due to its high energy value in the techno-economic analysis and high volumetric mass density (53.4 g L⁻¹) for easy storage and transport (Yoo et al., 2016; Chi et al., 2021).

Conventionally, p block metals such as Bi, Pb, and In have appropriate *OCHO binding energy, favoring the CO₂–to–formate conversion. However, the electrical conductivity of these metals is not satisfactory, causing a large potential loss at high current densities. Cu is naturally abundant and has good electrical conductivity, possible for practical use (Huang et al., 2018). Cu-based materials are widely investigated as electrocatalysts for CO₂R to multi-carbon (C₂₊) production (Mistry et al., 2016a; Mistry et al., 2016b; Gao et al., 2019; Zaza et al., 2022). However, due to the modest binding to H, C, and O (Qin et al., 2019; Zheng et al., 2021; Zhang et al., 2022), Cu shows poor selectivity to one specific product which leads to an increased product separation cost.

Bimetallic catalysts have been reported to increase the activity and selectivity of electrocatalytic CO₂R by taking the advantage of both metals. Zeng et al. improved the formate production by fabricating single atom Pb anchored Cu catalysts (Zheng et al., 2021); Thomas J. Meyer et al. enhanced selectivity for methane production by forming Cu-Pd bimetal alloys (Zhang et al., 2015); Douglas R. MacFarlane et al. improved the electrocatalytic reduction of CO₂ to CO by loading Au on Cu (Chen et al., 2017); Wang et al. (2010) prepared novel silver-coated nanoporous copper composite electrocatalysts for CO₂R to produce dimethyl carbonate. As for formate production, Bi is extensively studied as a promising CO₂R–to–formate catalyst due to its abundance on Earth, low cost, and environmental-benign properties. Jiang and collaborators reported that a Bi nanostructured catalyst electrochemically reduced by BiOCl nanosheets obtained 92% FE_HCOOH at −1.5 V vs. SCE at room temperature (Zhang et al., 2014); Zhong et al. achieved over 95% formate selectivity with ultra-long stability of more than 100 days (Li et al., 2021); Li et al. (2019) achieved nearly 100% formate selectivity using Bi/Bi₂O₃ with abundant grain boundaries as catalysts. Yet the Bi-based catalysts still require a high potential to conduct electrocatalytic reduction of CO₂ (Tian et al., 2021).

Herein, we present the large-area fabrication of Cu _1-x Bi _x (x = 0.1, 0.2, 0.25) catalysts using controllable thermal evaporation. We evaluated the CO₂R and also the competing hydrogen evolution reaction (HER) performance in flow cells in CO₂-saturated and Ar-saturated electrolytes. We obtained a formate selectivity of 95% and a cathode energy efficiency of 70% at −0.72 V vs. RHE with Cu_0.8Bi_0.2. The CO₂R can proceed stably and efficiently at a current density of 200 mA cm⁻² over 24 h. We conclude that the bimetallic Cu_0.8Bi_0.2 improves formate selectivity and enhances the CO₂R activity and cathodic energy efficiency, which may offer new perspectives for future design and synthesis of bimetallic CO₂R catalysts.

We prepared a series of Cu _1-x Bi _x (x = 0.1, 0.2, 0.25) bimetallic materials, pure Cu, and pure Bi samples on a large area by thermal evaporation. As shown in Figures 1A,B and Supplementary Figure S1, the as-prepared Cu _1-x Bi _x (x = 0.1, 0.2, 0.25) catalysts were all tightly wrapped on the PTFE fibers and formed uniformly distributed Cu and Bi nanoparticles with a size of 100–200 nm. It can be seen from Figure 2A and Supplementary Figure S1 that with the increase of Bi content, the particles gradually form nanocrystals, and the particle size of Cu _1-x Bi _x (x = 0.1, 0.2, 0.25) is about 100–200 nm, and the Cu-Bi bimetallic distribution is uniform. The particle sizes of Bi and Cu are 200 nm and 50–100 nm, respectively. Then we measured the element distribution using SEM-EDX. As shown in the mapping spectrum, Cu and Bi elements were uniformly distributed on the catalyst surfaces (Figures 2B,C; Supplementary Figure S2). Through SEM-EDX and XPS analysis, the element ratio of our prepared Cu-Bi was 0.8:0.2 (Supplementary Figure S3).

The chemical states of the synthesized Cu, Cu_0.8Bi_0.2, and Bi catalysts were studied by the high-resolution XPS spectra of Bi 4f and Cu 2p, respectively (Figures 3A,B). We observed that the strong peaks at 158.7 and 164.0 eV corresponded to Bi³⁺ 4f_7/2 and Bi³⁺ 4f_5/2, and the peaks at 157.0 eV, 162.3 eV corresponded to Bi⁰ 4f_7/2 and Bi⁰ 4f_5/2. From the high-resolution XPS spectrum of Cu 2p, the peaks at 932.0 and 951.8 eV corresponded to Cu^{0, 1+}, while the peaks at 933.9 and 953.6 eV were consistent with Cu²⁺. It should be explained that both Cu and Bi are easily oxidized in air, the positively charged Bi³⁺ and Cu²⁺ are detected, probably because the oxidation occurred during the storage of the sample in air. Compared with Cu, the d-band center of Cu_0.8Bi_0.2 moved more positively after the introduction of Bi species (Figure 3C). As reported, the positively shifted d-band center likely increases the electron donation from the catalysts to the adsorbed *OCHO intermediate, strengthening the *OCHO surface binding (Xin et al., 2014; Zhang et al., 2018). This is consistent with our electrochemical CO₂R test that Cu-Bi catalysts show improved performance.

The CO₂R electrochemical performance of the Cu _1-x Bi _x (x = 0.1, 0.2, 0.25), pure Cu, and pure Bi catalysts were tested in a flow cell with a three-electrode system (Figure 4). The anode was Ni, the reference electrode was an Ag/AgCl electrode, and the electrolyte was 1 M KOH solution (pH = 14). From the linear sweep voltammetry (LSV), we can intuitively see that the reaction overpotential of Cu-Bi bimetal was reduced significantly (Figure 5A; Supplementary Figure S4). As shown in Figure 3B, the overpotential (520 mV) of the Cu_0.8Bi_0.2 catalyst is significantly smaller than that of the Bi catalyst (660 mV) at the same current density (100 mA cm⁻²). The overpotential for Cu is the best (400 mV), but the product selectivity is poor (Figure 5B). The cathodic current density on Cu, Cu_0.8Bi_0.2, and Bi electrodes was largely reduced when N₂ was passing through (Figure 5A). In the presence of N₂, the current density started to increase slowly at −0.70 V vs. RHE, which was mainly caused by the hydrogen evolution reaction (HER). It is also clear that HER on the Cu electrode is worse than CO₂R on the Bi electrode, indicating that the introduction of Bi into Cu can suppress HER In the presence of CO₂, which is more beneficial to CO₂R.

To investigate the product selectivity of CO₂R with Cu_0.8Bi_0.2, Cu, and Bi catalysts, chronopotentiometry tests were performed at current densities of 100, 200, and 300 mA cm⁻², respectively (Supplementary Figure S5). The products were quantitatively analyzed by gas chromatography (GC), ion chromatography (IC), and nuclear magnetic resonance (NMR). As shown in Figures 5C-E, and Supplementary Figure S6, the Cu_0.8Bi_0.2 catalyst achieved over 90% selectivity to formate at all current densities, in particular, the selectivity for formate reached 95% at a current density of 100 mA cm⁻², with a cathode energy efficiency reaching about 70%. Although the single-metal Bi catalyst has a formate selectivity close to that of Cu_0.8Bi_0.2, its cathode energy efficiency is much lower than that of Cu_0.8Bi_0.2 at all current densities due to its high reaction potential. In the case of the Cu catalyst, the formate selectivity is very low. Considering both the selective and energy efficiency, Cu_0.8Bi_0.2 outperforms Bi and Cu in the electroreduction of CO₂ to formate.

As shown in Figure 5E, we found that the introduction of Bi into Cu can significantly reduce HER, which is consistent with the LSV results under N₂ environment. To explain the reason for the excellent performance of Cu_0.8Bi_0.2 catalysts, we performed EIS tests on the three samples. As is shown in Figure 5F, Cu_0.8Bi_0.2 shows a smaller semicircle diameter than Bi in the impedance spectrum, suggesting that the charge transfer resistance of Cu_0.8Bi_0.2 is lower than that of Bi, ensuring a faster electron transfer during the reaction. The conductivity of Cu is 58.13953 S/m, and that of Bi is 0.95238 S/m. The conductivity of Cu is significantly better than that of Bi. Therefore, we analyze that the loading of Cu metal increases the conductivity of the material and reduces the charge transfer resistance, also the HER is greatly suppressed in the presence of CO₂, as a result, Cu_0.8Bi_0.2 can reduce the reaction overpotential and maintain high CO₂R catalytic activity and selectivity.

To verify the stability of the Cu_0.8Bi_0.2 catalyst, we performed the stability test at a current density of 200 mA cm⁻², and SEM characterization of the reacted sample was performed. It can be seen from Figure 6 that during the reaction process of the 24 h CO₂ reduction, the reaction potential did not change significantly, and the selectivity of HCOO⁻ was maintained above 90%. We also carried out SEM, SEM-EDX, and XRD analyses of the sample after 24 h of reaction. From Supplementary Figure S7, we can observe that the morphology and structure of the Cu_0.8Bi_0.2 bimetallic catalyst did not change after the reaction. The XRD characterizations also showed that the catalyst did not change significantly after the reaction, indicating that good stability with Cu_0.8Bi_0.2 for CO₂R.

In conclusion, we developed a simple, controllable, and large-area preparation method for the synthesis of Cu_0.8Bi_0.2 catalysts. Cu_0.8Bi_0.2 exhibited excellent formate selectivity and cathode energy efficiency under all current densities. Specifically, it exhibited a formate selectivity of 95% and a cathode energy efficiency of 70% at a potential of −0.72 V vs. reversible hydrogen electrode and maintained the CO₂R durability for over 24 h at a current density of 200 mA cm⁻². The excellent catalytic performance of Cu_0.8Bi_0.2 is attributed to the following factors: 1) CuBi alloy likely has a favorable work function to improve CO₂ adsorption for formate production; 2) CuBi alloy improves electron transport. We expect that the bimetal Cu_0.8Bi_0.2 electrocatalyst may offer a material foundation for the improved catalytic CO₂R to formate conversion.