Power Distribution Control for Improving Lifetime of Proton Exchange Membrane Water Electrolyzers

There is a growing demand for improving lifetime of water electrolyzers in order to reduce the hydrogen production cost. Recently, degradation behaviors of proton exchange membrane (PEM) water electrolyzers operating under fluctuating power such as renewable power sources have been extensively studied. Degradation of electrolyzer appears as increase in the operating voltage (i.e., the overpotential). It was shown that the rate of voltage increase highly depends on the operation parameters such as the current density and the interval of current fluctuation [1,2]. However, it has not been clarified so far what kind of operation control can suppress the degradation of the electrolysis stacks. Here we have developed a novel control technology for improving lifetime of PEM water electrolyzers.<br/>First, we proposed a control strategy called the Power Distribution Control (PDC) for suppressing the degradation of water electrolyzers. The concept is to decompose a fluctuating power that highly degrades electrolyzers into the power patterns of low-degradation rates. For example, a PEM electrolysis cell operating under the current fluctuation between 2 and 1 A/cm² (mode A) degrade rapidly than that between 2 and 0 A/cm² (mode B) [1]. When two series of stacks (i.e., two strings) operate in mode A, it is possible to distribute the current by the PDC so that one string operates in mode B and the other at a constant current of 2 A/cm² (mode C). It is thus expected that degradation of the stacks can be suppressed by the power distribution. An example of the system architecture to perform the PSC is an array of electrolysis stacks with power converters such as DC/DC converters installed in each string.<br/>In order to experimentally validate the effect of PDC, a series of durability tests for a PEM electrolysis cell were performed. A membrane electrode assembly (MEA) composed of a PEM (Nafion 117), an anode catalyst layer (IrO₂), a cathode catalyst layer (Pt/C) and gas diffusion layers was prepared and used for the tests. The rate of cell voltage increase was 111 and 69 μV/h under the operation in mode A (reference) and in the sequence of mode B and C (simulation of PDC), respectively. This result suggests that the degradation rate was reduced by 38% as a result of PDC. As a result of the analysis of current-voltage characteristics, it was found that PDC is particularly effective in suppressing the increase in the activation overvoltage.<br/>Furthermore, the degradation mechanism has been investigated by analyzing the degraded sample using synchrotron radiation. Scanning X-ray fluorescence microscopy (SXFM) [3] was performed for obtaining elemental maps of the cross section of the catalyst coated membrane (CCM). An X-ray microbeam 1 μm in diameter was formed, and the distribution of Pt and Ir was obtained. While the distribution of Pt showed little change after the durability test, Ir was observed within the PEM and even near the interface between the PEM and the cathode. Then we performed microbeam X-ray absorption fine structure (XAFS) imaging and estimated the Pt and Ir valence state from the white-line height of Pt and Ir <i>L</i>_III-edge X-ray absorption near edge structure (XANES), respectively. It was found that Ir within the PEM is in a higher valence state (Ir^&gt;4+) than Ir at the anode (Ir⁴⁺), indicating that highly oxidized IrO_X dissolved and migrated in the PEM. This scenario is consistent with the Pourbaix diagram of Ir [4], where the stable phase changes between IrO₂ (pH = 0) and IrO₄^- (pH = 7) during the operation of PEM electrolyzers.<br/>Our results demonstrate that PDC is a promising technique for improving lifetime of PEM electrolyzers by suppressing degradation such as Ir dissolution.<br/><br/>[1] C. Rakousky <i>et al</i>., J. Power Sources <b>342</b>, 38 (2017).<br/>[2] S. H. Frensch <i>et al</i>., Int. J. Hydrogen Energy <b>44</b>, 29889 (2019).<br/>[3] A. Yoneyama <i>et al</i>., J. Instrum. <b>15</b>, P12029 (2020).<br/>[4] M. Pourbaix, “Atlas of Electrochemical Equilibria in Aqueous Solutions” (1974).