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Thijs de Groot, Eindhoven University of Technology

Breaking down the performance of alkaline water electrolyzers

Abstract

Although the hype around green hydrogen has ended and the molecule is no longer seen as the solution to everything, green hydrogen still has an important role to play in our future energy system as a raw material for the chemical industry. Of the different water electrolysis technologies, alkaline water electrolysis is the most mature and has the advantage that it does not require noble or rare earth metals. Yet, a weakness of the technology is that it is operated at much lower current densities than the competing PEM technology, implying that larger and heavier electrolyzers are needed to make the same amount of hydrogen.

Why is this the case? Is it because of the membrane? The anode? The cathode? Cell design? Bubbles? This is not so easy to say, because we typically only measure the cell potential, which does not include a breakdown into the individual contributors. To understand this behavior we started by fitting of current-voltage curves [1], which suggested that the ohmic resistance in the electrolyzer was much higher than expected. Yet, it was unclear what was causing this. More recently we started using electrochemical impedance spectroscopy (EIS) to separate ohmic and kinetic overpotentials and found a combination of kinetic and bubble effects [2]. The latter can be reduced by optimizing the electrode geometry [3].

Our work shows that it should be possible to design alkaline water electrolyzers that can efficiently operate at much higher current densities, especially if relatively thin membranes are used. However, an attention point for these advanced electrolyzers will be gas crossover, which determines the operational window of the electrolyzer. This can be managed by choosing the right operating conditions and optimizing the cell design [4].

 

[1]       M.T. de Groot, A.W. Vreman, Ohmic resistance in zero gap alkaline electrolysis with a Zirfon diaphragm, Electrochim. Acta. 369 (2021). https://doi.org/10.1016/j.electacta.2020.137684.

[2]       R. Lira Garcia Barros, M.H.G. Kelleners, L. van Bemmel, T.V.N. van der Leegte, J. van der Schaaf, M.T. de Groot, Elucidating the increased ohmic resistances in zero-gap alkaline water electrolysis, Electrochim. Acta. 507 (2024) 145161. https://doi.org/10.1016/j.electacta.2024.145161.

[3]       R. Lira Garcia Barros, J. Scholl, I. Hoedemakers, X.L. Liang, K. Skadell, J. van der Schaaf, M.T. de Groot, Impact of nickel electrode geometry on the electrochemical performance and bubble dynamics of a zero-gap alkaline electrolyzer, J. Power Sources. 630 (2025) 236116. https://doi.org/10.1016/j.jpowsour.2024.236116.

[4]       R. Lira Garcia Barros, J.T. Kraakman, C. Sebregts, J. van der Schaaf, M.T. de Groot, Impact of an electrode-diaphragm gap on diffusive hydrogen crossover in alkaline water electrolysis, Int. J. Hydrogen Energy. 49 (2024) 886–896. https://doi.org/10.1016/j.ijhydene.2023.09.280.

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