Investigation of Fe-Based Integrated Electrodes for Water Oxidation in Neutral and Alkaline Solutions

We report here an iron-integrated transparent electrode by a scalable one-pot hydrothermal reaction and its applications as an active electrocatalyst or as a functional electrode substrate for the oxygen evolution reaction (OER). Characterization of the electrode reveals that it is a well-defined, compact β-FeOOH film whose thickness, photoelectric properties, and catalytic performance are controllable by varying synthetic conditions. The direct-growth strategy is beneficial in improving its electrical conductivity compared to that of catalyst-powder-coated electrodes. The integrated electrode exhibits an onset overpotential of 383 mV toward OER and a Tafel slope of 36 mV dec–1 at pH 7, which are impressive results for an iron-based catalyst. Calcination of β-FeOOH at 650 °C gives α-Fe2O3, which allows for a direct comparison of catalysis performance between the two electrodes and for a comparison by density functional theory. The iron-integrated electrode can also serve as an excellent electrode substrate for stepwise fabrication of a high-performance NiFe-based electrocatalyst for OER in alkaline media. In this line, the β-FeOOH film electrode is first calcined at a low temperature of 215 °C, which produces amorphous Fe2O3. Consequently, the amorphous nature of this electrode substrate favors anodic deposition of Ni with a maximized Ni–Fe contact, resulting in a high-performance NiFe electrocatalyst.