Abstract
Platinum group metal‐free catalysts have attracted significant attention due to the high cost and scarcity of iridium for the oxygen evolution reaction (OER) in acidic media. However, the fundamental understanding of catalyst formation and active species under acidic conditions remains limited. In this work, iron–manganese oxide catalysts based on were synthesized via a hydrothermal approach followed by calcination. The as‐prepared materials consist of poorly crystalline Fe
2
O
3
/FeOOH and MnO
2
phases that transform into a disordered mixed‐metal oxide upon thermal treatment. Raman spectroscopy supported by quantum‐chemical calculations indicates the formation of a solid solution in which Mn induces lattice distortion and partial disruption of Fe─O─Fe bonding within the hematite structure. The highest OER activity and stability are obtained after calcination at 350 °C, corresponding to an optimum degree of structural disorder for adsorption processes. Operando Raman spectroscopy reveals potential‐dependent spectral changes, including the emergence of a band at ≈450 cm
−1
and enhanced intensity at ≈610 cm
−1
, indicating the formation of high‐valent surface species during OER operation. These findings suggest cooperative Fe–Mn interactions and a possible role for substitution‐induced structural disorder in promoting OER activity.