Bismuthene for highly efficient carbon dioxide electroreduction reaction

Global warming and energy crises necessitate efficient methods for CO2 conversion. Electrochemical CO2 reduction reaction (CO2RR) offers a promising approach to produce valuable products like formic acid (HCOOH) or formate (HCOO−). While various metals have been explored as CO2RR electrocatalysts, they often suffer from high cost, low selectivity, and poor durability. Bismuth (Bi), a low-toxicity, cost-effective, and stable alternative, has shown potential but existing Bi-based catalysts demonstrate limited performance. This study focuses on the synthesis and application of a novel, highly efficient Bi-based catalyst: bismuthene, a free-standing two-dimensional (2D) Bi monolayer. Previous studies have only demonstrated bismuthene on templates, while this research aims to achieve a stable, free-standing structure and investigate its catalytic performance in CO2RR for formate production. The expectation is that the unique properties of the 2D structure will significantly enhance the catalytic efficiency compared to bulk Bi or other Bi nanostructures.

Extensive research has investigated various bulk metals (Pb, Hg, In, Cd, Sn, Co) as CO2RR electrocatalysts for formate production. However, these materials often exhibit limitations such as high cost, large overpotentials, limited availability, poor selectivity, and poor durability, hindering large-scale application. Recent efforts have focused on earth-abundant and chemically stable metals. Bismuth, with its low toxicity, cost-effectiveness, and higher stability, is a promising alternative. Yet, previously reported Bi-based electrocatalysts for CO2RR typically require large overpotentials or exhibit low current densities. Different Bi nanostructures have been synthesized, including single atoms, spherical nanoparticles, nanobelts, thin films, and nanowires, all showing limited CO2RR catalytic performance for HCOO− production. While 2D multilayered Bi nanosheets have been synthesized, these also display limited CO2RR performance. The synthesis of unsupported, stable free-standing Bismuthene, although theoretically predicted, remained elusive until this study.

This research employed a simple, scalable wet chemical method to synthesize free-standing hexagonal Bismuthene. Bismuth(III) chloride (BiCl3) served as the Bi precursor, and NaBH4 acted as the reductant. The synthesis yielded Bi nanosheets (BiNSs) with varying thicknesses, from monolayer Bismuthene to multilayers. Transmission electron microscopy (TEM), atomic force microscopy (AFM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to characterize the synthesized BiNSs. The thickness of the bismuthene was confirmed to be around 0.65 nm through multiple characterization techniques. The electrochemical performance of the BiNSs for CO2RR was evaluated using a three-electrode system in a CO2-saturated 0.5 M KHCO3 solution. Linear sweep voltammetry (LSV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) were employed to assess the catalytic activity, selectivity, and durability. Gas chromatography (GC) and nuclear magnetic resonance (NMR) were used to quantify the gaseous (H2) and liquid (HCOO−) products. To enhance the catalytic performance, Bismuthene nanosheets were mixed with inert carbon black (BP2000) to prevent compact stacking and increase the electrochemically active surface area (ECSA). Density functional theory (DFT) calculations were performed to investigate the CO2RR mechanism on both Bismuthene (111) and thicker BiNSs (011) facets. The calculations focused on determining the adsorption energies and free energies of reaction intermediates (OCHO*, COOH*, CO*, OH*) involved in CO2RR and HER, providing insights into the catalytic performance differences. The ECSA were estimated from the scan-rate dependence of cyclic voltammetric stripping. A detailed analysis of the calibration of the products from CO2RR, with particular emphasis on liquid products HCOO− using NMR, was performed to ensure the reliability of the experimental results.

The study successfully synthesized stable, free-standing hexagonal Bismuthene using a simple, scalable wet-chemical method. Characterization confirmed its single-atom layer thickness (0.65 nm) and (111) facet exposure. Electrocatalytic CO2RR performance was found to be highly dependent on BiNS thickness, with Bismuthene exhibiting significantly superior activity compared to thicker BiNSs. Bismuthene demonstrated a high Faradaic efficiency of approximately 99% for formate (HCOO−) production at -580 mV vs. RHE, with an exceptionally low onset overpotential of <90 mV. Remarkable long-term durability was observed, with no performance decay after 75 hours of continuous operation and even after annealing at 400 °C. The Tafel slope for HCOO− formation on Bismuthene (87.6 mV dec−1) was significantly lower than that of thicker BiNSs, indicating faster electron transfer kinetics. Incorporating 3 wt% carbon black (BP2000) further improved the CO2RR performance of Bismuthene by mitigating the compact stacking of the nanosheets and increasing ECSA. DFT calculations revealed that the compressive strain in the Bismuthene (111) facet weakens adsorption, leading to enhanced selectivity for formate. In contrast, the (011) facet in thicker BiNSs strongly binds reaction intermediates, promoting strong adsorption and possibly catalyst poisoning, which is in agreement with the CO2-TPD results. The DFT calculations also predicted that the HER activity of thick BiNSs is intrinsically higher than that of Bismuthene, which was then verified experimentally. The experiments demonstrated that Bismuthene outperforms other reported catalysts for CO2RR to formate in terms of Faradaic efficiency, durability, and selectivity. Overall, the results demonstrate that the single-atom thick Bi sheet, or bismuthene, is a superior and highly durable catalyst for CO2RR with excellent selectivity towards formate.

The findings demonstrate that Bismuthene exhibits superior electrocatalytic activity for CO2RR to formate compared to thicker BiNSs. This superior performance is attributed to the unique properties of the atomically thin, (111) facet-exposed structure. The compressive strain in the Bismuthene lattice weakens the adsorption of reaction intermediates, preventing catalyst poisoning and promoting high selectivity for formate. This is consistent with DFT calculations showing the (011) facet in thicker BiNSs bind strongly with reaction intermediates which is likely why they are less active and stable. The remarkable stability of Bismuthene highlights the potential for long-term CO2RR applications. The successful integration of Bismuthene with carbon black further enhances the catalytic performance, highlighting a practical pathway for scaling up the technology. The observed superior performance of Bismuthene in CO2RR opens up exciting opportunities for developing advanced electrocatalysts for CO2 conversion, offering a potential solution for tackling climate change and energy challenges. These results contribute significantly to the ongoing search for highly efficient and sustainable CO2 conversion strategies.

This study successfully synthesized and characterized free-standing Bismuthene for the first time, demonstrating its exceptional performance as a CO2RR electrocatalyst. Bismuthene exhibited high Faradaic efficiency, low overpotential, and remarkable durability, outperforming other Bi-based catalysts. DFT calculations revealed the crucial role of compressive strain in enhancing catalytic activity and selectivity. The synthesis method is simple and scalable, paving the way for large-scale applications. Future research could explore further optimization of Bismuthene's structure and investigate its performance in different CO2RR environments. This research presents a significant advancement toward efficient and sustainable CO2 conversion into valuable chemicals.

While the study demonstrates exceptional performance of Bismuthene in CO2RR, some limitations exist. The synthesis method, although scalable, requires further optimization to improve yield and control over the size and morphology of the nanosheets. The long-term durability testing was performed under specific conditions; further investigation is needed to assess its performance under varying operational parameters and potential degradation pathways. Additionally, while this study focuses on formate, the mechanism and performance of bismuthene towards other CO2RR products remains to be explored.