Electrochemiluminescence Discloses Active Intermediates in Oxygen Evolution Reactions of Metal-Organic Framework

In situ disclosure about the complex four-electron transfer of anodic oxygen evolution reaction (OER) is crucial to further optimization of electrocatalysts. However, tracing the formation and evolution of unstable intermediates remains challenging. Here, electrochemiluminescence (ECL) analysis and imaging technique are developed to in situ monitor the important intermediate hydrogen peroxide intermediate (OOH*) on transition-metal-based 7,7,8,8-tetracyanoquinodimethane (TM─TCNQ) metal-organic framework materials as OER catalysts. Thanks to the highly selective ECL probe L-012 to OOH*, the ECL signal evolution (e.g., onset ECL potential and maximal ECL intensity) is bridged with the generation and accumulation of OOH*, allowing for convenient and quick judgment to the rate-determining step of OER. Due to the spatial and temporal resolution of ECL microscopy, it is found that the phase I of Cu─TCNQ shows better catalytic activity. The COMSOL simulation also shows the heterogeneous current distribution during the OER reaction, in agreement with ECL imaging results. Finally, the ECL approach enables to compare the OER performance of three TM─TCNQ, well explaining the free energy profiles for different reaction intermediates. Therefore, the proposed ECL analysis and microscopy provide an alternative way to understand the detailed elemental steps in OER, thus further indicating the direction for high-performance catalysts.