Durability of Water Electrolysis Cells Against the Long-Term Voltage Fluctuation Simulating Wind Power
Authors: Hoshii Taisei , Sasaki Kazunari , Hayashi Akari 
Published: February 2024

Introduction

Water electrolysis is an important technology for efficiently combining the renewable energy with hydrogen energy. For directly using the renewable energy to produce hydrogen and oxygen through water electrolysis, the long-term durability against the voltage fluctuation of the renewable energy is necessary. We have particularly focused on the voltage fluctuation of the wind power. Typical 24-h wind power voltage fluctuations were obtained from one of three wind lenses of a 10-kW Muti-Lens Turbine® installed in Hibikinada, Japan, and applied to a single PEM water electrolysis cell after reducing the actual voltage to one-hundredth of the original value. When 20-d voltage fluctuations were applied, no degradation of the water electrolysis cell was observed if 1-h rest time was included after every 48-h voltage fluctuations for removing stagnated gases.
Therefore, we have recently designed two types of voltage fluctuation protocols with different upper potential (2.0 V and 2.3 V) based on actual wind voltage fluctuations and evaluated durability of water electrolysis cells equivalent to 160 days.1 In both cases, reversible and irreversible current losses were observed. Reversible performance degradation was found to be most likely due to the gas stagnation and the reversible change of the oxidation state of iridium oxide catalysts. On the other hand, a few percent of the irreversible loss was found, which was mostly due to physical changes of the anode catalyst layer.
In this study, to understand the irreversible loss more deeply, further long-term durability was evaluated and deterioration mechanisms were investigated.

Experimental

Water electrolytic cells were prepared by spray printing 46.3% Pt/KB and IrO2 for the cathode and the anode, respectively, on a Nafion 117 membrane. Voltage fluctuations with an upper limit of 2.3 V, shown in Fig. 1, were applied to a water electrolysis cell after accelerating the voltage fluctuation five times. Repeating the voltage fluctuation 228 times was defined as one set, corresponding to 48 h of actual operation. A total of 160 sets, corresponding to 320 d of actual operation, were performed in order to evaluate the durability of water electrolysis cells. The potential was applied to the anode, while maintaining the cathode at ~0 V.
Electrochemical measurements of AC impedance and I-V performance were done at the beginning and after each set. Each overvoltage was separately analyzed. CV measurements of the anode side were also performed at the beginning and after 160 sets.
For material characterizations, FIB-SEM observation followed by three-dimensional reconstruction were performed to evaluate the anode catalyst layer.

Result and discussion

A similar trend of decreasing and recovering the current to our previous study1 was confirmed in the extended voltage fluctuations. On the other hand, the recovery speed of the current was found to be slowed down. This may be related to increase in hydrophilicity of the IrO2 catalyst surface, which was highly oxidized by cycling to 2.3 V. Owing to increased hydrophilicity, pores of the catalyst layer were mostly filled with water and then the limited space was available for gas transfer. Although the oxidation state reversibly changed, its speed was probably slow.
Regarding to the irreversible current loss, it was more pronounced. Figure 2 shows a cross-sectional image of the anode catalyst layer after the test equivalent to 320 d. An increase in the thickness of the anode, a part of which peeled off, was confirmed. This is most likely due to stagnated gas has increased the pressure within the anode catalyst layer. Since the gas stagnation is related to the initial pore structure of the anode catalyst layer, three-dimensional reconstruction of the anode structure using hundreds of FIB-SEM images was performed and analyzed in detail while comparing to the structure in our previous study. As a result, the water electrolysis cell used in this study had a relatively dense structure, which might be related to the amount of Nafion ionomer added in the anode catalyst layer, probably led to more gas stagnation, leading to an irreversible change of the anode structure.

Reference

(1) Y. Honsho et. al., J Power Sources 564, 232826, 2023.

Figure 1