Resolving Microscale Selectivity of Electrochemical CO 2 Reduction Using Hybrid Dual-Probe Scanning Electrochemical Cell Microscopy

The electrochemical CO 2 reduction reaction (CO 2 RR) involves multiple reaction pathways leading to a variety of C 1 or C 2+ products, while competing with the parasitic hydrogen evolution reaction (HER). As a result, CO 2 RR electrocatalysts exhibit complex spatial heterogeneity in both activity and selectivity across their surfaces. Although scanning electrochemical cell microscopy (SECCM) has enabled nanoscale electrochemical measurements and provided the structure–activity relationships of various electrocatalytic systems, its application to CO 2 RR remains limited by the lack of accessible selectivity information. In this work, we introduce the integration of SECCM with scanning electrochemical microscopy (SECM) to investigate the local selectivity of the CO 2 RR. Au and Pt nanoelectrodes embedded within the SECCM tip showed a high collection efficiency (CE) of 98.2 ± 2.1% for FcMeOH 2 redox cycling due to the submicrometer-scale tip–substrate separation (ca. 554 nm for a 1.5 μm diameter wetted area). Substrate generation/tip collection (SG/TC) mode voltammetry during CO 2 RR on Au and Bi substrates reveals that redox cycling and fast gas exchange at the electrode–electrolyte–gas three-phase boundary play key roles in governing the electrochemical behavior at the tip for locally generated CO, formate, and H 2 . Using the redox cycling of formate, we map the microscale heterogeneity in formate selectivity of Ag–Bi 2 O 3 catalysts supported on glassy carbon as a function of applied potential. These results demonstrate that hybrid SECCM–SECM is a powerful platform for elucidating dynamic structure–activity–selectivity relationships in CO 2 RR.