Understanding Structure Differences of Iridium Oxides Depends on Loading in Proton Exchange Membrane Electrolyzers
Authors: Kwon Obeen , Chabot Florian , Korzeniewski Carol , Boettcher Shannon , Morimoto Yu , Zenyuk Iryna 
Published: December 2024

Proton exchange membrane water electrolyzers (PEMWEs) have drawn attention among various hydrogen production technologies due to their zero-emission, high conversion efficiency, high hydrogen purity (>99.99%), and quick response under dynamic operation.1,2 With these auspicious characteristics in PEMWEs, they are believed to produce 40 % of green hydrogen by 2050 among all electrolyzer technologies. Nonetheless, many bottlenecks within PEMWEs still hinder their broader commercialization. Iridium oxide is the most widely used oxygen evolution reaction (OER) electrocatalyst at the anode. Moreover, this catalyst occupies the biggest portion (26–47%) of the overall cost of PEMWE systems, mostly due to the scarcity of iridium.3 Commercial iridium oxide catalysts are generally two types depending on the material structure: amorphous IrOx and crystalline IrO2. The trade-off relationship exists in catalytic activity vs. stability between amorphous and crystalline IrOx catalysts.4–6 To overcome these problems, fundamental studies of iridium oxide need to be conducted to reduce catalyst loading while maintaining high performance.

Herein, we report the new iridium oxide catalysts (amorphous IrOx, crystalline IrO2, Ishifuku Metal) for the first time. These new iridium oxide catalysts are physically and electrochemically characterized. To be specific, we confirm the distinct characteristics of each catalyst using scanning electron microscopy (SEM), focused ion beam SEM (FIB-SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) analysis with nitrogen adsorption isotherm, Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). As a result, the crystalline IrO2 has a unique morphology with grown needles onto catalyst particles thereby the BET specific surface area of crystalline IrO2 (~110 m2 g-1) is over two times higher than amorphous IrOx (~50 m2 g-1). Electrochemical performance is evaluated for varied catalyst loadings of membrane electrode assembly (MEA). The catalyst loadings (0.10, 0.26, 0.35, and 0.85 mgIrOx cm-2) for both anode catalysts were prepared. The CV results of amorphous IrOx exhibit unique redox peaks under specific voltage (~0.8 and ~1.2 V vs. RHE), which indicates the transition of oxidation state. These peaks develop with CV cycling and become fully developed after 100 cycles, indicating that the surface of the catalyst takes some time to activate. On the other hand, crystalline IrO2 shows no specific redox peaks but rather has a double-layer capacitance under lower voltage (<0.5 V vs. RHE). Figure 1 shows 1.92 V (amorphous) and 1.94 V (crystalline) at 5 A cm-2 for 0.85 mg cm-2 loading, respectively. Both catalysts showed relatively low Tafel slopes of 43.8 mV dec-1 (amorphous IrOx) and 53.7 mV dec-1 (crystalline IrO2). This presentation will correlate the electrochemical performance described here to the structural properties of these two types of IrOx catalysts.

Figure 1. Polarization curve for prepared iridium oxide catalysts (amorphous IrOx and crystalline IrO2). Both cells have anode loadings of 0.85 mg cm-2, cell temperature at 60 °C, 10 sccm for H2O flow rate at the anode, and dry for the cathode.

References

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Figure 1