In situ Spectroscopy: Delineating the mechanistic understanding of electrochemical energy reactions

The development of in situ spectroscopy methods has enabled detailed studies of the surface chemistry and structures of electrodes and/or electrocatalysts under active electrochemical conditions, providing real-time insights into reaction pathways at the electrode–electrolyte interface, which is mandatory for understanding electrochemical processes in energy devices. Key challenges in understanding the high electrochemical selectivity and activity of catalysts for energy reactions include measuring reaction kinetics, detecting changes in the chemical environment, identifying reaction intermediates, and linking material properties to device performance. This review examines the advanced utilities of various in situ and operando spectroscopic methods, such as Fourier transform infrared, Raman, X-ray absorption, and X-ray photoelectron spectroscopy, in the study of rechargeable lithium-ion batteries, supercapacitors, water-splitting (O₂ and H₂ evolution), and hybrid electrolysis with small molecule oxidation into hydrogen fuel and value-added chemical production. Emphasizing the significance of the various in situ/operando methods in optimizing catalyst design and improving energy storage and conversion efficiency and durability, we provide a systematic assessment of their roles in addressing major challenges in energy material research, summarizing their operational mechanisms, benefits, and limitations, and delivering guidance for future experimental strategies.