Mitigation of Performance Degradation of PEM Water Electrolyzers by Power Distribution Control of Fluctuating Renewable Energy

Abstract

Introduction Hydrogen production by water electrolysis using renewable energy such as wind and solar power generation (i.e., green hydrogen production) has attracted considerable attention as a key technology towards decarbonization. Proton exchange membrane (PEM) water electrolyzers are often used in combination with renewable energy source since they show fast response to input power fluctuation. However, lifetime of PEM electrolyzer needs to be improved for reducing hydrogen production cost. Recently we reported a durability analysis of PEM electrolyzers under wind power voltage fluctuations, where the reversible and irreversible voltage losses were analyzed in terms of gas bubble transport and catalyst degradation [1]. We also developed a novel control strategy called the power distribution control (PDC) that controls the power converters so that the input fluctuating power is decomposed to several fluctuating patterns that less degrade the electrolyzers than the input pattern [2]. The purpose of this study is to validate the effect of the PDC on mitigation of performance degradation of PEM electrolyzers under wind power fluctuations. Experimental method A membrane electrode assembly (MEA) with an active area of 1 cm2 was prepared using Nafion 117 membrane, IrO2 anode catalyst and Pt/KB cathode catalyst. The catalyst loading for anode and cathode was 0.5 mg IrO2/cm2 and 0.3 mg Pt/cm2, respectively. A PEM water electrolysis cell was prepared by assembling the MEA into a cell holder. The cell temperature was kept at 80°C during the durability test.Figure 1a) and 1b) show a schematic system diagram for performing the PDC and the two voltage fluctuation patterns (A and B) used for the durability test, respectively. As in our previous study [1], actual 24-h voltage fluctuation of wind power turbine was used as the primary data. The pattern A was prepared from the primary data by (i) scaling down the maximum voltage by 1/100 and setting the minimum voltage higher than the standard potential of water electrolysis and (ii) accelerating the time scale by 5 times. The pattern B was prepared by applying the PDC, that is, converting the pattern A into an alternative sequence of a high-operating rate pattern and a low-operating rate pattern as described in Ref. [2]. Here we set the average voltage of the high-operating rate pattern (~2.4 V) and the low-operating rate pattern (~1.7 V) as the same value with the voltage of the pattern A (~2.05 V) for each time, ensuring that we model the situation in which the input fluctuating power is distributed between two series of electrolyzers as shown in Fig. 1a). We performed 240-h durability tests and analyzed performance degradation of the PEM cell for the pattern A and B, respectively. Results and Discussion Figure 2 summarizes the performance degradation of the PEM cell during the 240-h durability test. The cell current density at the cell voltage of 2.1 V showed a steep decrease during the initial ~30 h for both pattern A and B. For 30 – 240 h, the current density continuously decreased for the pattern A (−0.68 mAcm-2/h), while no distinguishable decrease was observed for the pattern B (+0.06 mAcm-2/h). Therefore we successfully demonstrated that performance degradation of the PEM electrolyzers can be effectively suppressed by the PDC when using actual wind power voltage fluctuation as the input power.To further investigate the performance degradation behavior, the increase in overpotential components of the PEM cell was analyzed. Figure 2b) shows the comparison of overpotential components before and after the 240-h test. For the pattern A, the diffusion overpotential (E diff) showed a significant increase after the test for the current density higher than 1.0 mAcm-2. In contrast, for the pattern B, increase in E diff was suppressed and confirmed only for > 1.4 Acm-2. Increase in the activation overpotential (E act) was small compared to increase in E diff for both pattern A and B. Furthermore, we confirmed that the performance degradation confirmed in this study was reversible, that is, the current density was almost recovered within 24-h rest time after the test. These results are consistent with our previous study [1] in that the reversible degradation is related to gas bubble transport, that is, the increase in E diff. Therefore it was shown that the PDC suppresses reversible degradation, which is important for mitigating irreversible degradation during long-term operation. Our results demonstrate that PDC is a promising technique for improving lifetime of PEM electrolyzers operating under fluctuating renewable energy.