Exploring precursor effect in in-situ reconstruction of Bi metal organic-frameworks during CO₂ electroreduction reaction to tailor formate selectivity towards 100%

Bi-based metal-organic frameworks (Bi-MOFs) as efficient catalysts for the CO₂ electroreduction reaction (CRR) have been extensively investigated. It has been established that Bi-MOFs usually undergo in-situ transformation during the CRR, ultimately converting into Bi nanostructures. However, the effect of the MOF precursors on the structure of the derived Bi catalysts and consequently on their electrocatalytic properties has not been clearly elucidated. In this work, we prepared four variants of a novel Bi-MOF with different morphologies and dimensions, and explored the effects of the precursors on the resulting Bi catalysts and their CRR performance. A precursor-dependent in-situ transformation and its profound impact on tuning the CO₂-formate FE towards ∼100% have been revealed. Among the four examined variants, Bi-MOF with nanocube-like morphology (BiMOF-NC) is transformed into thin and flat Bi nanosheets capable of successfully enveloping the fibers of carbon paper (working electrode), while the other three variants with increased size and dodecahedral/octahedral geometry have been transformed into Bi nanoflowers, exposing a fair portion of the carbon fibers in the working electrode. Furthermore, the transformation rate was found to be dependent on the precursors and BiMOF-NC exhibited the highest rate of reconstruction compared with other samples. Consequently, the Bi catalyst derived from BiMOF-NC delivered a selectivity for the formate product of nearly 100% at −1.2 V (vs. reversible hydrogen electrode (RHE)) and of over 95% in a wide potential range due to efficient suppression of the competing hydrogen evolution reaction (HER) over carbon fibers. The in-situ transformed BiMOF-NC also demonstrated equivalently high formate selectivity at large current densities (≽100 mA cm⁻²) in flow cell reactors as that in H-type reactors, underscoring its potential for industrial applications. This work highlights the necessity of finely tuning the structural features of MOF precursors to achieve optimized electrocatalytic performance.