Scanning Photoelectrochemical (Cell) Microscopy for <i>In Situ</i> Measurements of Photo(electro)catalysis

Photo(electro)catalysis promises energy-efficient and selective generation of chemical products. However, rational catalyst design is constrained by an incomplete understanding of the underlying mechanisms, necessitating in situ, surface-sensitive, and spatially localized characterization techniques. Scanning photoelectrochemical (cell) microscopy (SPECM/SPECCM) utilizes ultramicroelectrodes near the catalyst surface to spatially map structure-dependent activity, charge-transfer dynamics, and active site generation. By directly correlating local structure with catalytic behavior, critical insights into the factors governing catalytic activity, stability, and selectivity are gained, guiding the rational design of high-performance photo(electro)catalysts. In this review, we first introduce scanning electrochemical techniques followed by a detailed description of common methods of light integration, underscoring their individual strengths and limitations. We then highlight the ways SPECM and SPECCM have been utilized to gain information on structure-dependent activity in anisotropic materials, charge-transfer in heterostructures, and photogenerated active site formation. We conclude with an outlook on potential research directions to further advance the mechanistic insights obtainable through scanning photoelectrochemical techniques.