Towards Low Iridium Loading in Proton-Exchange Membrane Water Electrolysis: Full-Cell Performance of IrOx@TiO2 Core-Shell Particles in Anode Catalyst Layers
Authors: Finger Selina , Hoffmeister Darius , Ma Tien Ching , Freiberg Anna T.S. , Thiele Simon , Pham Chuyen , Hutzler Andreas 
Published: February 2024

Hydrogen as a promising energy carrier to bridge the gap between supply and demand of renewable energies can be produced by proton-exchange membrane water electrolysis (PEMWE). However, the scarcity of iridium limits the widespread and large-scale implementation of PEMWE. Thus, the state-of-the-art iridium loading for the oxygen evolution reaction (OER) of ~1-2 mgIr·cm-2 must be distinctly reduced.1, 2 A decrease in loading involves a reduction of the catalyst layer (CL) thickness. The reduction is, however, only possible down to a certain threshold (≤ 0.5 mgIr·cm-2), below which the CL shows cracks leading to incomplete connection of the catalyst particles.1, 3 To overcome this issue, we developed a novel catalyst structure with reduced iridium content (< 50 wt% Ir). The synthesized catalyst exhibits a core-shell structure consisting of an insulating TiO2 core (d ≈ 1 µm) coated with iridium oxide (IrOx@TiO2). This optimized structure enables a reduction of the loading while maintaining the thickness and, thus, the electrical connection within the CL.
In two related contributions, we demonstrate the suitability of the catalyst by a comprehensive analysis: The first contribution mainly focuses on structural optimization, activity, and stability investigations of the catalyst powder. In this contribution, we optimize the catalyst layer aiming towards high catalyst utilization and in-plane conductivity at iridium loadings below 0.5 mgIr·cm-2. PEMWE full-cell performance is investigated in catalyst-coated membrane (CCM) configuration and the structure and thickness of the CL are characterized by SEM cross-sections. The results of the full-cell tests with our catalyst in comparison to a state-of-the-art IrO2/TiO2 catalyst are shown in Fig. 1. The polarization curve of our catalyst exhibits high OER activity and performance at low loadings (Graph A: 1.76 V @ 1.5 A·cm-2). In comparison to the state-of-the-art catalyst (Graph B), our catalyst outperforms the commercial catalyst in terms of activity. This becomes evident when comparing the HFR-free polarization curves in the low current density region. It could be assigned to a higher catalyst utilization due to the maintained CL thickness or, more likely, to a higher activity of the catalyst itself.1 The performance difference between our and the state-of-the-art catalyst in higher current density regions mainly stems from ohmic losses in the electrode as can be seen from the high frequency resistance (HFR) in Fig. 1. These ohmic losses, however, can supposedly be assigned to the material itself and not to a decreased in-plane conductivity due to a disconnection of parts of the CL.
In conclusion, we successfully synthesized an IrOx@TiO2 catalyst with a core-shell structure. This catalyst enables the fabrication of catalyst layers with decreased loadings below 0.5 mgIr·cm-2 while maintaining the catalyst utilization demonstrated in full-cell testing. Therefore, our newly developed catalyst contributes to a scale-up of the PEMWE technology by drastically reducing the demand for the scarce element iridium.
Figure 1: Polarization curves, HFR-free polarization curves and HFR values at 80°C (ambient pressure, 100 mlH2O·min-1 on anode) of CCMs fabricated with different anode catalyst types and loading. A: in-house core-shell catalyst with 0.2 mgIr·cm-2 and B: commercial catalyst with 0.3 mgIr·cm-2 (IrO2/TiO2 with 75 wt% Ir, Umicore).
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Figure 1