In Situ Transformation of Hybrid Bismuth Halide into Rhombohedral Bismuth for Electrochemical CO₂ Reduction to Formate

Abstract Bi‐based electrocatalysts have attracted high attention due to their high selectivity for formate, low cost, and high biocompatibility. Surface modification with halides can adjust the surface charge distribution of metal catalysts, thereby regulating the binding force of the intermediate. Organic‐inorganic hybrid bismuth halides provide an alternative, especially low dimensional structures. Herein, zero‐dimensional hybrid bismuth halides containing Bi 4 I 16 units (denoted as Bi 4 I 16 ) was recommended as pre‐catalyst due to the Bi ⋅ ⋅ ⋅ Bi spacing in Bi 4 I 16 is 4.760 Å, nearly equaling to the Bi ⋅ ⋅ ⋅ Bi spacing in rhombohedral Bi (4.750 Å). The equal spacing may be more beneficial for the electricity‐driven in situ conversion and rearrangement of Bi atoms in the catalytic process. As a contrast, zero‐dimensional bismuth halide containing Bi 2 I 9 units (denoted as Bi 2 I 9 ) with shorter Bi ⋅ ⋅ ⋅ Bi spacing (4.2415 Å) was prepared. The working electrode prepared by Bi 4 I 16 ink was measured for CO 2 RR, and the partial formate current density can reach 8.2 mA cm −2 at −1.1 V vs RHE. The Bi 4 I 16 catalyst delivers a maximum Faradaic Efficiency (FE, ~80 %) for formate at −0.86 V vs RHE and maintain a FE higher than 78.5 % after 16 h.