Juhyung Choi, Sungin Kim, Ji Yong Choi, Sejin Park, Kwanghwi Je, Shikai Liu, Jiahong Jiang, Shaojinlin Yang, Christopher J. Pollock, Rafael Guzman-Soriano, Kathryn Bairley, Bayu I. Z. Ahmad, Phillip J. Milner, Erik H. Thiede, Yun Jeong Hwang, Jihye Park, Yao Yang
Understanding dynamic catalyst evolution, particularly Cu-based single-atom catalysts, faces tremendous challenges of tracking rapid and nanoscale evolution and uncontrolled catalyst reoxidation during post-reaction air exposure. Although ex situ / in situ studies have indirectly indicated the structural reconstruction of single-atom catalysts, direct probing of single-atom catalyst evolution requires time-resolved nanoscale operando methods. Here, we present direct experimental evidence of dynamic evolution from single-atom catalysts to Cu nanostructures rich in active nanograins, based on a conductive metal–organic framework-based Cu single-atom catalyst (Cu-SAC). Operando synchrotron-based high-energy-resolution X-ray spectroscopy and IR absorption spectroscopy quantitatively tracked the structural and molecular fingerprints during single-atom-to-nanograin evolution. Cu-SAC supported on nanocarbon (Cu-SAC-NC) with nearly 100% metallic Cu nanograins achieved a 5-fold increase in multicarbon Faradaic efficiency (C 2+ FE), relative to the Cu-SAC control group with less than half metallic Cu nanograins. Cu-SAC-NC, with superior electronic conductivity provided by the nanocarbon, facilitated the formation of dense copper carbonyl (Cu–CO) intermediates, leading to a larger fraction of active metallic Cu nanograins for effective C–C coupling and significantly enhanced C 2+ selectivity. Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) directly captured real-time movies of dynamic structure evolution from isolated Cu single atoms to metallic Cu nanograins under the CO 2 RR. Operando electrochemical four-dimensional (4D) STEM reveals the complex polycrystalline Cu nanostructures rich in metallic nanograin boundaries, serving as catalytically active sites. This study paves the way for the design of a new generation of single-atom catalysts based on their operando active structures instead of pristine structures.