Tracking the Oxygen Dynamics of Solid-Liquid Electrochemical Interfaces by Correlative In Situ Synchrotron Spectroscopies.

Oxygen-involved electrocatalytic processes, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), are central to a series of advanced modern energy and conversion technologies, such as water electrolyzers, fuel cells, and CO reduction or N fixation devices. A comprehensive and in-depth understanding of the charge transfer and energy conversion process that ubiquitously occurs over solid-liquid electrochemical interfaces during oxygen electrocatalysis is crucial for understanding the key essence of oxygen-related electrochemistry. The huge challenges for dynamic studies over solid-liquid interfaces during oxygen electrocatalysis lie in the all-embracing electrochemical processes of the catalytic reactions, associated with both structural and reactive intermediates evolution on the electrode surface, and in the significant influence of the aqueous environments of electrolytes used. Hence, overcoming these challenges intrinsically calls for a great cooperation of multiple cutting-edge in situ technologies. Synchrotron radiation (SR) X-ray absorption fine structure (SR-XAFS) spectroscopy is highly sensitive to the local atomic structure of nanomaterials, and SR-based Fourier transform infrared (SR-FTIR) spectroscopy features unique molecular fingerprint identification to determine active species on the surface of electrodes. One can imagine that the correlative in situ SR-XAFS/FTIR spectroscopic investigations will potentially provide sufficient, reliable, and complementary information at the atomic/molecular level to depict vivid and comprehensive "dynamic movies" of solid-liquid electrochemical interfaces during oxygen electrocatalysis, which will help effectively promote/simplify the complicated screening process of advanced oxygen electrocatalysts for efficient high-energy-density energy systems.In this Account, starting with some fundamentals of SR-based spectroscopic technologies, tips for obtaining high-quality SR-XAFS and SR-FTIR spectroscopy results during the electrocatalytic process are comprehensively specified. Subsequently, the latest research achievements of dynamic investigations mainly from our group based on in situ SR-XAFS and/or SR-FTIR spectroscopies will be systematically scrutinized and properly emphasized in detail, where the currently attractive metal-organic-framework (MOF) nanomaterials and single-atom catalysts (SACs) are selected as the main object of research. Moreover, the vital contributions of correlative in situ SR-XAFS/FTIR studies on new discoveries of the dynamic evolution of solid-liquid interfaces during oxygen electrocatalysis are highlighted. In particular, our pioneering research found that the potential-dependent dynamically coupled oxygen formed in the precatalytic stage was a very useful promoter in SACs to promote efficient OER kinetics under acidic conditions. In addition, the in situ generated metastable Ni-N centers with more structural degrees of freedom in SACs could potentially facilitate the fast 4e ORR kinetics. This Account is anticipated to stimulate broad interest in dynamic explorations in various catalytic processes of interest in the material science and electrochemistry communities using correlative SR-based technologies.