The generation of hydrogen by (solar-driven) water splitting requires highly active and earth-abundant catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In nature, the OER is catalyzed in the process of photosynthesis by active CaMn₄O₅-clusters embedded in the photosystem II.^[1] For artificial photosynthesis, Mn-based compounds are on the focus of intensive research aiming at an efficient, low-cost and environmentally benign OER catalyst. Among this class of materials, bixbyite type α-Mn₂O₃ has attracted attention due to its promising OER activity in alkaline solutions.^[2] In literature, thick and porous α-Mn₂O₃ films were commonly prepared.^[2] Such morphology of electrodes, however, complicates benchmarking of OER catalysts due to the incomparable electrochemical active surface area (ECSA) of the electrodes.

Herein, we report α-Mn₂O₃ electrodes fabricated using galvanostatic electrodeposition. A detailed analysis of the specific activity (current density per ECSA) of α-Mn₂O₃ provides a direct comparison of the intrinsic activity with other OER catalysts.^[3] Cyclic voltammetry measurements in 1 M KOH showed a strong dependency of the OER activity on the deposited amount of α-Mn₂O₃. The required overpotential to achieve a current density of j = 10 mA/cm² decreases from 590 and 340 mV with increasing film thicknesses of up to ~ 1 µm. Differential capacity (C_d) measurements suggest that the improvement of activity is attributed to the enlarged ECSA of the porous and electrolyte permeable films. The specific activity was calculated to be j_ECSA ≈ 0.25 mA/cm²_real at an overpotential of η = 350 mV, using the ECSA estimated from the differential capacity of the electrodes. This value is rather low compared to highly active NiFeO_x (j_ECSA ≈ 15 mA/cm²_real, η = 350 mV) prepared and measured in this work as a reference catalyst.^{[3, 4]} The α-Mn₂O₃ electrodes were found to be stable when applying a potential < 1.7 V vs RHE in 1 M KOH.

Despite the rather low specific activity of α-Mn₂O₃, nanoporous films with a thickness of ~ 1 µm are showing high activities due to their large ECSA and their apparently high conductivity. As such, these properties make α-Mn₂O₃ attractive for the use in electrolyzers or photovoltaic hybrid systems for water splitting.

[1] P. E. M. Siegbahn et al., Acc. Chem. Res. 2009, 42, 1871−1880

[2] A. Ramírez et al., J. Phys Chem.C 2014, 118, 14073-14081

[3] C. C. L. McCrory et al., J. Am. Chem. Soc. 2015, 137, 4347-4357

[4] M. D. Merrill et al., J. Phys. Chem. C 2008, 112, 3655-3666