Effect of Electrolyte Anions on the Activity of Iridium Oxide Catalysts for Water Electrolysis
Authors: Cao Weijie , Uchiyama Tomoki , Thakur Neha , Fujita Mitsuharu , Arima Ikkei , Watanabe Toshiki , Yamamoto Kentaro , Takami Tsuyoshi , Matsunaga Toshiyuki , Kuroda Yoshiyuki , Mitsushima Shigenori , Uchimoto Yoshiharu 
Published: February 2024

Developing environmentally friendly and renewable energy sources is essential due to the environmental and energy security issues caused by traditional combustion of fossil fuel-based energy sources. Therefore, as a clean and sustainable energy carrier, hydrogen has attracted great attention in recent years. In this regard, water electrolysis utilizing renewable energy sources to produce hydrogen is considered one of the most promising technologies. It is believed elucidation of the reaction mechanism of highly active iridium oxides is promising for the further development of cost-efficient PEM water electrolysis catalysts. The properties of the MEA could be affected by many parameters such as the catalysts, operating temperature, and pressure1,2. Another important factor is the cation impurities that can be found in the circulating water, which could severely degrade the MEA performance during the operation of a PEM water electrolyze. However, there are less reports on anion impurities such as SO4
2- and PO4
3-. Therefore, in this study, the relationship between electrolyte anions and OER activity was investigated for IrOx catalysts with different crystal structure. By using operando X-ray absorption spectroscopy (XAS), we investigated the electronic state of Ir and O under operation potential.

IrOx with different degree of crystallinity (SA3.5 and SA58) were provided by Tanaka Kikinzoku Kogyo K.K., Chemical & Refining Company and named after their BET surface areas. These two samples were then tested for electrochemical properties in different concentrations of phosphoric acid and perchloric acid, respectively. The electrochemical measurements were performed using a standard three-electrode cell connected to an MPG-205-NUC system (Bio-Logic). The cyclic voltammograms (CV) were scanned from 0.1 V to 1.2 V vs. RHE at 50 mV s-1. The electrochemical activities of the catalysts were subsequently investigated using linear sweep voltammetry (LSV) in the range of 1.1 to 1.7 V vs RHE at 10 mV s−1. operando XAS were collected using a home-made flow-type cell, where Pt wire and RHE worked as counter and reference electrodes, respectively. The catalyst ink was coated onto an Nafion membrane to prepare both the X-ray window and working electrode. The electrolyte was circulated at the flow rate of 100 mL min-1.

The catalytic performance of IrOx was investigated at different types and content of electrolyte under H3PO4 and HClO4. Figure 1 shows the CVs of SA3.5 and SA58 under different types and content of electrolyte. In the presence of phosphate anions, SA3.5 showed a tendency to decrease Ir redox between 0.6 and 1.1 V with increasing electrolyte concentration. On the other hand, there were no obvious changes in SA58. It was found that IrOx catalysts have an great effect of specific adsorption of anions and low symmetry of monoclinic phase catalysts (SA 3.5) are more susceptible to anion poisoning. Figure 2 shows the result of examining the poisoning mechanism using soft X-ray XAS. O K-edge results present the formation of µ2-O (O-O) bond at around 528.7 eV, which was previously observed by Nong, H. N. et. al. through operando O-K edge measurements during OER process3. The peak intensity at 528.7 eV of the amorphous samples was in the same order as the OER activities. According to potential dependence change of the pre-edge peak, under the presence of phosphate anions, the hole formation to Ir-O hybrid orbitals was suppressed in SA3.5. This result indicates that the anion adsorbs to the active site, poisoning the catalyst surface and inhibiting the reaction.

Acknowledgements

This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

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Nong, H. N.; Falling, L. J.; Bergmann, A.; et al. Nature
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Figure 1