Resolving Microscale Selectivity of Electrochemical CO₂ Reduction Using Hybrid Dual-Probe Scanning Electrochemical...

The electrochemical CO₂ reduction reaction (CO₂RR) involves multiple reaction pathways leading to a variety of C₁ or C₂₊ products, while competing with the parasitic hydrogen evolution reaction (HER). As a result, CO₂RR 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 CO₂RR 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 CO₂RR. Au and Pt nanoelectrodes embedded within the SECCM tip showed a high collection efficiency (CE) of 98.2 ± 2.1% for FcMeOH₂ 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 CO₂RR 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 H₂. Using the redox cycling of formate, we map the microscale heterogeneity in formate selectivity of Ag-Bi₂O₃ 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 CO₂RR.