(Invited) Evaluation of α-Mn2O3 As Catalyst for the Oxygen Evolution Reaction in Photoelectrochemical Devices

Abstract

Solar powered electrolysis of water is a promising way to harvest and store solar energy in the form of large quantities of hydrogen. The photovoltage of semiconducting materials is used in photoelectrochemical cells or photovoltaic devices to provide the required thermodynamic potential for water splitting (1.23V) and the over-potentials for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) under illumination. In order to keep the electrochemical overvoltages as low as possible, highly active electrocatalysts are deposited onto the semiconducting absorbers. Since the catalytic layer could potentially shadow the semiconducting light absorber, crucially important is balancing catalytic activity with optical transparency. The development of low cost, but highly efficient, transparent and stable electro-catalysts is currently a challenging issue for solar water splitting devices. Among the electro-catalytic materials being pursued, manganese oxide is an especially promising material due to its low cost, high activity and stability in alkaline electrolytes. In this contribution, we have investigated electrodeposited and annealed α-Mn2O3 layers as catalysts for the OER in alkaline electrolytes (pH14) and evaluated their suitability for use in photoelectrochemical devices. The electrochemically deposited α-Mn2O3 layers are highly porous and we could vary the layer thickness over a wide range (nm to µm). In cyclo-voltammetry experiments, all samples show an anodic oxidation process before the OER starts. This can be explained by a partial oxidation of Mn3+ to Mn4+ as verified by in-line XPS studies. From current-voltage measurements and the specific capacity of these electrodes, we were able to determine the specific current densities involved in the OER (related to the effective accessible electrochemical surface area, ECSA) which dependence on the layer thickness. We found that the specific OER activity increases from ca. 0.01mA/cm2 (ECSA) for very thin layers to ca. 0.2 mA/cm2 (ECSA) for µm thick layers (at an overvoltage of 350mV). In order to investigate this effect in more detail we also prepared well-defined Mn2O3 layers with different thicknesses in the nm range by atomic layer deposition. First results are presented and compared to the electrochemically deposited materials. With respect to “benchmark” catalysts (e.g. Fe/Ni) the specific activity of the electrodeposited α-Mn2O3 is only moderate but high current densities can still be achieved at low over-potentials. This is attributed to the high electrochemically active surface areas of the electrodes (10mA/cm2 (electrode surface) at 340mV over-voltage). Long-term tests revealed high stability of the electrodes operated in pH14 electrolyte at 10mA/cm2. Together with the light transmission spectra of the α-Mn2O3-electrodes, in total, these data allows us to assess the applicability of Mn2O3 as a catalyst to combine with various light absorbing materials. As an example, the influence of Mn2O3 deposition on the photoelectrochemical performance of semiconducting BiVO4 and Fe2WO6 electrodes will be discussed in terms of charge carrier dynamics at the solid/electrolyte interface.