Stability and Oer Activity of IrOx in PEM Water Electrolysis
Authors: Rheinländer Philipp J. , Bernt Maximilian , Incedag Yasin , Gasteiger Hubert A. 
Published: February 2020

Renewable H2 production is a prerequisite for successfully establishing fuel cell-based electromobility and hydrogen infrastructure. In this respect, proton exchange membrane water electrolysis (PEM-WE) has attracted much interest, not least due to the enormous power densities possible in these devices (1). Research is mainly focused on the anode side, where the oxygen evolution reaction (OER) takes place, causing the vast majority of kinetic losses.
OER catalysts of choice are usually iridium oxide (IrOx) based, owing to its decent activity and acceptable stability (2,3). Recent studies showed a strong dependence of the OER activity and dissolution resistance of IrOx on its surface morphology and hydration state, which were controlled by the calcination temperature during the synthesis. They found that crystalline, thermal IrO2is the most stable but least active species (4,5).
TGA-analysis from our lab reveals that IrOx can easily be reduced to metallic Ir in dilute H2 at a typical PEM-WE operation temperature of 80 °C. This is also observed for IrOx based membrane electrode assemblies (MEAs) held at OCV in a PEM-WE, where crossover H2 from the cathode side reduces the surface of the IrOx catalyst at the anode within hours, as evident from the formation of H-UPD features in the cyclic voltammogram (see Figure 1b, red vs. blue CV). At the same time, polarization curves after this reduction show significantly decreased cell voltage at slightly reduced Tafel slopes (Figure 1a, red vs. blue curves), corresponding to an increased OER activity. However, a subsequent CV following these polarization curves (see Figure 1b, black CV) reveals that the H-UPD features have disappeared again and that the catalyst surface properties have transformed to a state closer to the less crystalline, hydrous IrOx reported in ref. 4.
This suggests that partial IrOx reduction and re-oxidation into (hydrous) IrOx can occur during cycles of extended OCV periods and electrolyzer operation. In analogy to voltage cycling degradation observed in fuel cells, the here described reduction/oxidation cycles might also lead to iridium dissolution. Therefore, we will study the effect of transient operation conditions in PEM-WEs on the OER activity, surface properties, and stability of IrOx based anodes. We will also provide a systematic analysis of OER kinetic parameters such as exchange current density and activation energy as well as their dependence on relative humidity for a commercial iridium oxide catalyst.

References

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(3) E. Fabbri, A. Habereder, K. Waltar, R. Kötz, and T. J. Schmidt, Catal. Sci. Technol., 4, 3800 (2014).
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Figure 1a. PEM-WE polarization curves recorded at 80 °C under dynamic O2 (anode)/H2 (cathode) at 147 kPaabs and 200 % RH (inlet). Blue curves were taken after a 12 h conditioning at 1 Acm-2 while red curves signify the status after a 15 h in-situ reduction of the anode catalyst in an H2 atmosphere. Hollow circles and crosses are subsequent repetitions.

Figure 1b. Anode CVs recorded under dry N2 at 100mVs-1. Blue CV: before the polarization curves in blue, red and black CVs: before and after the polarization curves in red.
The anode was loaded with 0.66 mgIrcm-2 of a commercial IrOx/TiO2 using 12 wt-% of ionomer in the catalyst layer, cathode was loaded with Pt/C at 0.35 mgPtcm-2 and a Nafion® XL membrane was used. MEAs with 5 cm2 active area were tested in a house-made single cell hardware with gold-plated titanium flowfields using porous Ti sheets and carbon fiber paper as GDLs on the anode and cathode side, respectively.

Acknowledgements:

P. J. Rheinländer would like to acknowledge financial support from Greenerity GmbH. M. Bernt would like to acknowledge funding from the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies).

Figure 1