( Invited ) Analysis of Varying Alkaline Exchange Membrane Water Electrolyzer Configurations via Edge-Type Reference Electrode Experiments and Numerical Simulation

The operation of electrolyzers in alkaline conditions allows for the use of less expensive electrocatalyst (e.g. nickel alloys), porous transport layers, and bipolar plates (e.g. nickel and stainless steel) [1]. In the past decade, remarkable strides in polymer synthesis and stabilization techniques have enabled the fabrication of alkaline exchange membranes (AEMs) and ionomers that are stable at high pH and above 80 °C, and have ionic conductivities rivalling standard PEM materials. As a result, several commercial AEMs and ionomers are now available, e.g., Ionomr Aemion (hexamethyl-p-terphenyl poly(benzimidazolium)) and Versogen PiperION. AEM water electrolyzers (AEMWEs) using these AEMs have demonstrated remarkable power and current densities, e.g., 2 A/cm2 at 1.8 V using 1 M KOH [2] and 1 A/cm2 at 1.8 V using pure water and PGM-free catalysts [3], as well as remarkable stability, e.g., degradation rates of 13 μV/h over 8,900 h at a current density of 0.6 A/cm2 [4]. Research however is still needed for the advancement of durable AEMWEs with low-platinum group metal (PGM) and PGM-free electrodes that are capable of operating over a wide range of conditions, e.g., alkalinity, temperature, pressure and low flow rate. Advanced AEMWE cells have several features that make their analysis more challenging than state-of-the-art proton exchange membrane water electrolyzers (PEMWEs), e.g., 1) operation with a liquid electrolyte feed; 2) use of active porous transport layers (PTLs); and, 3) the sluggishness of both anodic and cathodic reactions. These features enable novel AEMWE materials and cell architectures but also require advanced characterization tools and numerical models to determine: a) the role of liquid and polymer electrolytes in transporting charge and promoting reactions; b) whether a catalyst layer is required; c) the fate of counter-ions in the liquid electrolyte; and d) the distribution of voltage losses within the cell. This presentation will highlight the research advancements in our research group on the development of experimental characterization techniques and numerical models for AEMWE analysis. First, experimental results from two AEMWE cell configurations will be discussed. The first configuration is composed of two Ni single-serpentine flow fields in contact with two Ni felt porous transport layers (PTLs), and a catalyst coated membrane (CCM) with a 1 mg/cm2 IrOx anode and a 0.1 mg/cm2 Pt/C cathode deposited on an Aemion membrane. In the second configuration, the anode catalyst layer is removed and the Ni felt PTL is replaced with an electrochemically active stainless steel fiber felt containing Ni-Fe nanoparticles. The performance of the two cell configurations are characterized at 80 oC, 1 atm and 1 M KOH feed on both sides by polarization scans using an integrated edge-type reference electrode [5] to separate anode and cathode losses, as well as by impedance spectroscopy to obtain the high-frequency resistance of the cell. Experimental results show that a performance of 1 A/cm2 at 1.8 V using 1 M KOH can be achieved even without an anode catalyst layer and a 0.1 mg/cm2 Pt/C cathode. Furthermore, anode and cathode catalyst layer losses have a significantly different behaviour, with cathode losses surpassing anode losses at high current density (see figure). To provide a more in-depth analysis of the physical processes occurring inside the cell, an AEMWE numerical model developed in our open-source software OpenFCST is presented and used to study ion transport in both liquid and polymer electrolyte, as well as to analyze the electrochemical reaction distribution inside the catalyst layers and PTLs. The proposed model is first validated by reproducing overall and half-cell voltage losses, as well as high-frequency resistance experimental values (see figure). Then, it is used to provide a voltage breakdown separating individual electrode kinetics, charge and mass transport losses, as well as predicting the reaction distribution in each electrode (see figure). Interestingly, at high current density, cathode ohmic losses are shown to be significant. Comparing the reaction distribution of cells with and without anode CL, it can be observed that the ACL has a much higher reaction rate, while the PTL is less active and has a more uniform reaction distribution (see figure). The concentration distribution of ions in solution and polymer electrolyte will also be discussed. Finally, the model will be used to discuss the effect of electrode misalignment in edge-type reference electrode measurements. References: [1] Du et al. Chemical Reviews, 122 (13):11169-11896, 2022. [2] Fortin et al. Journal of Power Sources, 451:227814, 2020. [3] Li et al., Nature Energy, 5:378-385, 2020. [4] Molero-Gonzalez et al. Journal of Power Sources Advances 19:100109, 2023. [5] Liu, Mouallem et al. International Journal of Hydrogen Energy, 184, 151887, 2025. Figure 1