Rapid and controllable in-situ self-assembly of main-group metal nanofilms for highly efficient CO₂ electroreduction to liquid fuel in flow cells

The electrocatalytic reduction of CO₂ is a promising pathway to generate renewable fuels and chemicals. However, its advancement is impeded by the absence of electrocatalysts with both high selectivity and stability. Here, we present a scalable in-situ thermal evaporation technique for synthesizing series of Bi, In, and Sn nanofilms on carbon felt (CF) substrates with a high-aspect-ratio structure. The resulting main-group metal nanofilms exhibit a homogeneously distributed and highly exposed catalyst surface with ample active sites, thereby promoting mass transport and ad-/desorption of reaction intermediates. Benefiting from the unique fractal morphology, the Bi nanofilms deposited on CF exhibit optimal catalytic activities for CO₂ electroreduction among the designed metal nanofilms electrodes, with the highest Faradaic efficiency of 96.9% for formate production at −1.3 V vs. reversible hydrogen electrode (RHE) in H-cell. Under an industrially relevant current density of 221.4 mA·cm⁻² in flow cells, the Bi nanofilms retain a high Faradaic efficiency of 81.7% at −1.1 V (vs. RHE) and a good long-term stability for formate production. Furthermore, a techno-economic analysis (TEA) model shows the potential commercial viability of electrocatalytic CO₂ conversion into formate using the Bi nanofilms catalyst. Our results offer a green and convenient approach for in-situ fabrication of stable and inexpensive thin-film catalysts with a fractal structure applicable to various industrial settings.