Time Course of Polymer Electrolyte Electrochemical Cells (PEEC) Hydrogen Evolution Properties with MnIrOx Oxygen Evolution Catalyst

Hydrogen production using water electrolysis in polymer electrolyte electrochemical cells (PEEC) is an attractive technique for renewable energy storage. The PEEC is known to be relatively strong for input energy fluctuation, thus, it is a suitable technique for fluctuating renewable energy. The electrochemical catalysts of PEEC are Pt for hydrogen evolution in the cathode and IrOx for oxygen evolution in the anode. Since the Ir of IrOx for oxygen evolution is one of the precious metals, reducing the use of Ir is strictly desired. One of the Ir amount reduction methods is the use of the MnIrOx catalyst. The MnIrOx electrochemical oxygen evolution catalyst was demonstrated [1], and the MnIrOx catalyst performance is similar to the conventional IrOx catalyst. Although the MnO₂ catalyst dissolves into the electrolyte in pH around 2 over 1.8 V [2], the stability of MnIrOx against the voltage was not studied well. The stable voltage or current, and the step rectangular-like voltage applications were applied for the stability evaluation in this study.<br/>The electrode was 5 cm² area, Pt/C conventional cathode catalyst, MnIrOx (Ir 0.08%) anode catalyst with cation exchange membrane were used for the evaluation. When the stable voltage was applied, the current density decreased. The current density decreasing for the applied voltage of 2.0 V was approaching 15% of the initial value after 400 hrs operation. The voltage increase with constant current operation was not the same as the current decrease with constant voltage operation and was less than 10% for 400 hrs applied. These results show that the degradation processes were different either the voltage or current applied.<br/>For the rectangular-like voltage applied, the high and the low voltages were 2.0 and 1.43 V, and 2.0 and 0.05 V. The voltage applied time was 30 sec each. The current density at the high voltage applied was rapidly decreased at the voltage of 2.0 V high and 0.05 V low without the prevent reverse current. In the case of reverse voltage prevention, the current density decreasing rate became milder. The current density became much milder at the voltages of 2.0 V high and 1.43 V low. From these results, the reverse current affects the current decreasing drastically.<br/>The constant current operation, not the constant voltage operation, and the reverse current prevention effectively prevent property degradation.<br/>Part of this work is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).<br/>[1] Ailong Li et al., Science 384 (2024) 666.<br/>[2] Ailong Li, et al., Angew. Chem. Int. Ed. 58 (2019) 5054.