Unraveling Structural Evolution of Cobalt-Based Metal-Organic Frameworks for Boosting Nitrate Electroreduction to Ammonia.

Metal-organic frameworks (MOFs) have been widely studied for various complex electrocatalytic reactions, such as nitrate reduction reaction (NORR) to ammonia. Currently, their real active sites are controversial due to the inevitable structure transformations of MOFs during the catalytic process, limiting the rational design of effective electrocatalysts. Here, we clarified the structural evolution of zeolitic imidazole framework-67 (ZIF-67) with Co-N units and Co(hexahydroxytriphenylene) (Co-HHTP) with Co-O units on carbon paper (CP) toward enhancing NORR. Both ZIF-67/CP and Co-HHTP/CP achieve NH Faradaic efficiencies more than 95% within the wide potential range of -0.2 to -1.0 V versus reversible hydrogen electrode (RHE). At -1.0 V versus RHE, they deliver NH yield rates of 87.41 and 79.11 mg h cm, respectively, which outperform the most reported MOF-based electrocatalysts. Combining with in-situ Raman and ex-situ X-ray diffraction analysis, we verified the faster transformation of ZIF-67 into Co(OH) compared to Co-HHTP. The newly generated Co(OH) was recognized as the catalytic species and presented more favorable hydrogenation as elucidated by in-situ Fourier transform infrared and differential electrochemical mass spectroscopy. This work offers insightful understanding on the active phase of MOFs for designing reasonable active units toward different electrochemical reactions.