(Digital Presentation) Solving the Mystery of Ohmic Resistance in Zero-Gap Alkaline Water Electrolyzers Using Electrochemical Impedance Spectroscopy and Polarization Curves

Abstract

Hydrogen can be produced using renewable energy sources through alkaline water electrolysis (AWE). Alkaline water electrolyzers are made of inexpensive materials such as nickel perforated electrodes and the Zirfon porous diaphragm. However, one of the bottlenecks of this technology is the high internal ohmic resistance compared to other technologies, which implies that the technology is traditionally operated at low current densities. To decrease the ohmic resistance advanced AWE electrolyzers use a zero-gap configuration. However, even with such a zero-gap configuration the ohmic resistance remains relatively high.An analysis of polarization data on zero-gap alkaline electrolyzers with the commonly used Zirfon diaphragm suggests that the ohmic area resistances are significantly higher than what would be expected based on the properties of the diaphragm supplier [1]. In this study we use Electrochemical Impedance Spectroscopy (EIS) to further investigate this phenomenon [2], since EIS enables the separation of ohmic and charge transfer resistances.EIS in combination with polarization measurements enables us to determine electrodes overpotentials, diaphragm ohmic losses and bubbles resistance and to estimate key parameters such as the reversible cell potential, the Tafel slopes and the exchange current densities of hydrogen and oxygen evolution reactions. Impedance data were fitted with an equivalent electric circuit, which consist of an ohmic resistance connected in series with two components, each including a resistance and a constant phase element (CPE) in parallel. Besides, the polarization measurements were fitted to a given correlation [1] using MATLAB as shown in Figure 1.It was found that the ohmic resistance determined with EIS (~0.48 Ω.cm²) is lower than the fitted area resistance from the I-V curve (0.68 Ω.cm²), but is still higher than what would be expected based on the diaphragm properties for Zirfon. Obtained values for Tafel slopes and exchange current densities, both by EIS and I-V curves, are comparable to typical values from the literature. The resistance does not increase with current density suggesting that an increase of bubble volume with current density does not play a significant role in the resistance. Therefore, it still remains unclear what the origin is of the increased ohmic resistance.[1] M.T. de Groot, A.W. Vreman. Ohmic resistance in zero gap alkaline electrolysis with a Zirfon diaphragm. Electrochimica Acta, 2021, 369, 137684. https://doi.org/10.1016/j.electacta.2020.137684[2] Rodríguez, J.; Palmas, S.; Sánchez-Molina, M.; Amores, E.; Mais, L.; Campana, R. Simple and Precise Approach for Determination of Ohmic Contribution of Diaphragms in Alkaline Water Electrolysis. Membranes, 2019, 9, 129. https://doi.org/10.3390/membranes9100129 Figure 1