  (Photo)electrocatalysts capture sunlight and use it to drive chemical
reactions such as water splitting to produce H2. A major factor limiting
photocatalyst development is their large heterogeneity which spatially
modulates reactivity and precludes establishing robust structure-function
relationships. To make such links requires simultaneously probing of the
electrochemical environment at microscopic length scales (nm to um) and broad
timescales (ns to s). Here, we address this challenge by developing and
applying in-situ steady-state and transient optical microscopies to directly
map and correlate local electrochemical activity with hole lifetimes, oxygen
vacancy concentration and the photoelectrodes crystal structure. Using this
combined approach alongside spatially resolved X-Ray absorption measurements,
we study microstructural and point defects in prototypical hematite (Fe2O3)
photoanodes. We demonstrate that regions of Fe2O3, adjacent to microstructural
cracks have a better photoelectrochemical response and reduced back electron
recombination due to an optimal oxide vacancy concentration, with the film
thickness and carbon impurities also dramatically influencing activity in a
complex manner. Our work highlights the importance of microscopic mapping to
understand activity and the impact of defects in even, seemingly, homogeneous
solid-state metal oxide photoelectrodes.
