NSRRC Activity Report 2023

030 NSRRC ACTIVITY REPORT 2023 Activating Dynamic Atomic Configuration for Single-Site Electrocatalysts in Electrochemical CO 2 Reduction A paramount indicator of the atomic surface charge is used to evaluate the intrinsic activities for single-atomic electrocatalysts. C opper atoms dispersed at the atomic level provide an intriguing system that exhibits distinctive electrocatalytic activity in CO 2 reduction reactions (CO 2 RR). 1 However, the exact structure of these copper single-atom sites, their reactivity, and particularly their dynamic atomic configuration during reactions remain unresolved and are the subject of intense debate. Various reactive configurations have been suggested in the literature, including copper coordinated by four nitrogen atoms on a graphene-like carbon support, copper phthalocyanine complexes, and small copper clusters. Notably, despite the similarity in four nitrogen-coordinated sites, there persists a disparity in resulting product profiles. For example, single-atom copper enclosed in nitrogen-doped porous carbon has demonstrated heightened activity and selectivity toward acetone. This is mainly attributed to the environment created by four pyrrolic nitrogen atoms, facilitating lower-energy CO 2 activation and C–C coupling. By contrast, other studies have indicated carbon monoxide as the exclusive product from CO 2 reduction through a similar four nitrogen-coordinated copper site. The production of CH 3 OH and CH 4 relies on an appropriate binding affinity of surface-bonded CO. This affinity facilitates further reduction rather than subsequent desorption of CO from the catalyst, which leads to CO generation. Unlike C1 products, the formation of the C–C bond in C–C coupling is thought to take place on the catalytic surface. However, it seems Fig. 1 : (a) Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy image of N–Cu single-atom catalyst (SAC). (b) Cu K-edge near-edge X-ray absorption fine structure (XANES) spectra of N–Cu SAC (blue dotted line) with references of Cu foil (orange), Cu 2 O (blue), CuO (green). (c) R-space extended X-ray absorption fine structure (EXAFS) spectra of Cu K-edge for N–Cu SAC (with fitting result) and references. (d) Schematic of liquid electrochemical transmission electron microscopy (TEM) and corresponding electrochemical chip. (e) Dark field scanning TEM images at truly calibrated potentials vs RHE with various durations from 0 to 160 s. (f) Magnified images of selected area in (e). [Reproduced from Ref. 2]