Spin-Orbit Coupling-Dominated Catalytic Activity of Two-Dimensional Bismuth toward CO2 Electroreduction: Not the Thinner the Better.

Developing two-dimensional (2D) catalysts that are as thin as possible to increase the density of active sites seems to always be desirable for the pursuit of ideal catalytic performance. In this work, by taking CO2 electroreduction on 2D bismuth (Bi) as an example, we theoretically demonstrated that it is not always "the thinner the better" for 2D electrocatalysts. Because of the existence of a sizable band gap, the Bi monolayer exhibits lower catalytic activity than thicker metallic Bi nanosheets. Remarkably, because of the considerable stabilization energy contributed by the spin-orbit coupling effect, the Bi bilayer shows the best catalytic performance among all studied Bi nanosheets. By constructing a microkinetic model of CO2 conversion to HCOOH, we provided an explicit demonstration of the thickness-dependent activity of Bi nanosheets. Our results significantly deepen our understating of the fundamental factors governing the activity of 2D catalysts.