Water electrolysis emerges as key technology for the long-term storage of renewable energies, such as wind and solar power, which are characterized by an intermittent availability.[1] Hereby, proton exchange membrane (PEM) electrolyzers have the greatest potential due to low ohmic losses, low kinetic overpotentials, fast dynamic behavior and large partial load range.[2] Electrocatalytic efficiency losses and stability problems in PEM electrolyzers are primarily related to the anode, where water is oxidized to O₂ (oxygen evolution reaction, OER). For this reaction, Ir oxide is the benchmark catalyst, combining high activity and stability.[3] Unfortunately, Ir is a very scarce element with an abundance below that of Pt.[2] Hence, one of the biggest challenges in the context of PEM electrolyzers is to provide improved anode catalysts which offer high catalytic activity and stability at minimal noble metal content.

One strategy to lower the Ir content in PEM electrolyzer anode catalysts is the dispersion of the active component creating a large surface area to bulk ratio. However, this approach alone is not expected to be sufficient due to the low abundance of Ir. Thus, the intrinsic activity and stability of Ir oxide need to be optimized additionally. Unfortunately, the materials properties determining the activity and stability of Ir oxide OER catalysts remain uncertain, impeding a purposeful optimization.

Here, we present Ir-Ni mixed oxide thin-film model OER catalysts, which allow for a continuous tuning of the OER performance as a function of the Ni content. Optimizing the Ni content within this system, the Ir mass-based OER activity was increased by an unprecedented factor of 20 compared to an Ir oxide reference sample. In order to identify the materials properties determining the OER activity and stability, the materials properties were assessed comprehensively while the OER performance was tuned via the Ni content. The catalysts stability under reaction conditions was precisely monitored by ICP-MS coupled to an electrochemical scanning flow cell and the activity was determined by RDE measurements. The materials properties were examined by a wide array of spectroscopic, microscopic and scattering techniques such as XPS, SEM, SAED and XAS in conjunction with DFT calculations.

References

[1] H. Dau et al., ChemCatChem 2010, 2, 724
[2] M. Carmo et al., Int. J. Hydrogen Energy 2013, 38, 4901
[3] T. Reier et al., ACS Catal 2012, 2, 1765