Dynamics of the voltage response of PEM water electrolyzer cells: modeling principles and effect on active power

Abstract

Hydrogen produced by water electrolysis using electricity from renewable sources will be a key enabler of the fossil-free economy. As the share of renewable electricity, especially from wind and solar power, will increase, so will the intermittency in the availability of energy. This fluctuation will require a shift of paradigm of water electrolyzer operation from the present maximization of operation hours to more flexible operation following the renewable electricity generation. As a result, water electrolyzers will increasingly operate at reduced loads.

The power supply of a water electrolyzer is controlled with power electronic rectifiers responsible for converting the alternating current (AC) of the electric grid into direct current (DC) required for the electrochemical reactions. Industrial rectifiers, in particular the ones based on thyristor bridges that are widely used for their cost-effectiveness, cause additional AC ripple in the rectified current, thereby reducing the power quality. The amplitude of this ripple is higher at lower loads. Ripple current is known to increase the power consumption of water electrolysis without affecting the hydrogen production rate, thus making power quality a significant aspect to consider when assessing the cost structure of industrial water electrolyzers. Taking the fluctuating power supply for water electrolyzers into consideration requires dynamic modeling of the system, i.e., analysis of the electrolyzer voltage response as a function of the frequency and amplitude of the supplied current in addition to its mean value.

The goal of this doctoral dissertation is twofold: The first aim is to experimentally investigate the additional power consumption caused by current ripple for a proton exchange membrane (PEM) water electrolyzer cell. The measurements support previous findings regarding the additional power consumption caused by current ripple and the observations that the hydrogen production is dependent on the mean of the current only. In addition, the results reveal a linearization of voltage response above a cell-specific frequency threshold, which enables the usage of the impedance spectrum of the cell for voltage response modeling regardless of the amplitude of the current ripple. This finding leads to a simplified method of simulating ripple-induced additional power consumption for arbitrary current waveforms in the frequency domain using only the impedance spectrum measured by electrochemical impedance spectroscopy (EIS), given that all the significant frequency components are above the frequency threshold for linearization. Nonlinear modeling of the voltage response is needed only when operating below the threshold.

The second goal of this dissertation is to bridge the gap in theory between the highfrequency linear voltage response and the static polarization curve. Principles of dynamic voltage modeling are developed for water electrolyzer cells, extending in frequency of the AC ripple from sub-hertz to thousands of hertz and its amplitudes from 0% to 100% of the supplied DC current. Computational tools are established in MATLABR environment enabling parametrization of these models and simplifying their usage for predicting voltage response for arbitrary current waveforms. Furthermore, an experimental procedure is developed that allows fitting of half-cell activation overpotential parameters of the model by using full-cell dynamic measurements, providing a potentially useful tool for cell diagnostics.