Illuminating the Interface: In Situ Optical Insights Into Two-Electron ORR Pathways for H₂O₂ Electrosynthesis

Hydrogen peroxide (H₂O₂) is a prototypical green oxidant with wide applications. In pursuit of sustainable development, H₂O₂ electrosynthesis via the two-electron oxygen reduction reaction (2e^- ORR) under mild, green, and decentralized conditions has emerged as a promising alternative to the industrial anthraquinone process. H₂O₂ electrosynthesis is an interfacial electrochemical process in which appropriate surface active sites are critical for achieving high performance. To realize scalable H₂O₂ production via 2e^- ORR, in situ optical characterization is crucial for elucidating interfacial mechanisms and reaction principles, thereby facilitating further technological advances. This review is organized around the progressive enhancement of spatial resolution and accessible reaction information, and systematically summarizes in situ optical characterization methods for H₂O₂ electrosynthesis via 2e^- ORR, spanning one-dimensional (1D) spectroscopy, two-dimensional (2D) imaging, three-dimensional (3D) reaction-field imaging, and multimodal coupling that correlates optical, electrochemical, and structural information. It highlights the working principles, device configurations, and representative applications of these methods, and provides perspectives on future technological innovation and practical application of in situ optical characterization for advancing sustainable and economically viable H₂O₂ electrosynthesis in support of green industry and carbon neutrality.