Investigation of the Durability of a Water Electrolysis Cell with Iridium Fiber Anode Against Voltage Fluctuations Simulating Wind Power
Authors: Nagatomo Shiko , Hutapea Yasir Arafat , Ito Kohei , Hayashi Akari 
Published: December 2024

Introduction

Water electrolysis is the core technology for successfully transferring renewable energy to hydrogen energy. Our group have been focusing on wind power and PEM water electrolyzer. For aiming at hydrogen production through the water electrolysis directly using fluctuating wind power, the durability against voltage fluctuations has been studied1,2. So far, we have developed an accelerated potential fluctuation protocol based on voltage fluctuations of the actual wind power located in Hibikinada, Japan. Long-term durability of a water electrolysis cell equivalent to 320 days was evaluated using a membrane electrode assembly (MEA) with IrO2 anode and Pt/KB cathode. As a result, we have found that the current loss during the durability test is mostly recovered after the rest time, which enhances the removal of generated gasses remained in catalyst layers. On the other hand, a certain amount of irreversible current loss has also been found. That was caused by the partially peeled off catalyst layer due to increase in the internal pressure within the catalyst layer by remained gases.
Besides the durability study, development of new anode catalysts has been tried in our group. Iridium fibers are one of our developed catalysts. Since the requirement of high porosity in the catalyst layers is expected for avoiding the stagnation of gases based on our durability study, adapting the Iridium fiber catalyst is aimed in order to improve the durability of a water electrolysis cell in this study.

Experimental

Iridium fibers were synthesized by electrospinning a solution containing IrCl3∙xH2O, ethanol, dimethylformamide, and polyvinylpyrrolidone with a slightly modified heat treatments from the original method3. When the electrospun Iridium fiber precursor was heated at 500°C for 2 hours with heating rate of 3°C/min in the air atmosphere, Iridium fibers shown in Figure 1 were obtained. After the ball-milling process, Iridium fibers were spray printed within the area of 1 cm2 square to get 0.5 g Ir /cm2 on Nafion117 membrane as an anode. Commercially available 46% Pt/KB was similarly spray printed on the other side to get 0.3 g Pt/cm2. The resulting MEA was assembled into a JARI holder, and the electrochemical properties were evaluated by flowing 5 cc/min of water at 80°C to the anode and cathode. A durability test was performed using an accelerated potential fluctuation protocol with an upper potential limit of 2.3 V developed in our previous study1. In this protocol, one set of voltage fluctuation corresponds to 48 hours, and then 80 sets were performed. AC impedance and I-V performance were evaluated after every set, and each overvoltage was separately analyzed.

Results and Discussion

When the current at 2.0 V was monitored during the durability test, the large current drop was observed from the 1st set to the 5th set during the durability test. Based on the separated overvoltage analysis, the activation overvoltage and diffusion overvoltage increased simultaneously. Rather slow drop was observed for a standard MEA with IrO2 in our previous study2. The result suggests that generated gas is more easily stagnated in the catalysts layer with Iridium fiber anode, which is not our expectation. Therefore, the anode structure was evaluated by FIB-SEM. As a result, inter-fiber pores were covered by extra Nafion ionomer, leading to reduced mass transfer within the catalyst layer. However, the current loss was mostly recovered after the 100 h rest time after 40 sets and 80 sets of potential fluctuations. The recovery was 95% and 93% after 40 sets and 80 sets, respectively, for a standard MEA. In contrast, for MEA with Ir fibers, the complete recovery of the current was obtained, which suggesting higher resistance to the internal pressure derived by gas stagnation within the catalyst layer.

References

Y. Honsho et. al., J. Power Sources, 564, pp.232826, 2023.

T. Hoshii et. al., ECS Trans., 112(4), pp.451, 2023.

Y.-B. Cho et. al., ACS Appl. Mater. Interfaces, 10, pp.541, 2018.

Figure 1