Exploring the Performance of Alkaline Water Electrolyzers Under Dynamic Operational Conditions
Authors: Abdelhaleem Ashraf , Kuroda Yoshiyuki , Kawakami Noriko , Souda Manabu , Mitsushima Shigenori 
Published: August 2024

Green hydrogen, produced through water electrolysis driven by Renewable Energy Sources (RES), is a compelling solution for storing renewable energy. Directly coupling RES with water electrolyzers enhances system efficiency and cost-effectiveness. Alkaline water electrolysis, a mature and cost-effective technology, faces challenges operating under fluctuating RES conditions using AC/DC convertor , which supply rippled DC. A comprehensive understanding of electrolyzer performance in dynamic conditions is crucial for developing durable, efficient systems.
In the present experimental study, we extensively explored the performance of alkaline water electrolyzers and the key factors influencing it under dynamic operational conditions. The electrolytic cell was constructed using a zero-gap configuration, comprising a NiCoOx-based anode deposited on a Ni-mesh and an Ln-doped RuOx cathode deposited on a fine mesh of Ni metal. The Zirfon Perl UTP500 membrane served as a separator between the two cell compartments [1]. A 7.0 Molar KOH electrolyte at a room temperature of approximately 30°C was employed. The cell underwent dynamic operational mode through the application of a square wave cell voltage, shown in Fig.1 (a), utilizing the NF BP4620 power supply. Square waves with different frequencies, specifically 1, 100, 1000, and 3000 Hz, were employed. Steady-state operations were conducted while applying various DC voltages, serving as a reference study. The resulting current and the actual cell voltage waveforms were monitored and recorded using a high-resolution oscilloscope (YOKOGAWA DL850E). The flow of H2 gas was measured using a film flow meter (HORIBA STEC, SF series VP-3). The apparent power, real power, and reactive power components were calculated using formulas from a previously published report [2]. Subsequently, the specific energy consumption was determined for each condition.
The applied cell voltages were categorized into two groups. In the first category, the maximum cell voltage (Vmax) was set at 2.0 V, a value sufficiently high and suitable for industrial cell operation when applied in a DC mode. Within this category, the minimum cell voltage (Vmin) took various values, namely 1.8, 1.6, and 1.4 V. In the second category, the average cell voltage was maintained at a constant 2.0 V, while the amplitude of the waveform was adjusted to 0.2 and 0.4 V by modifying both Vmax and Vmin. Figure 1(b) depicts the resulting current of the voltage waveform at 100 Hz, illustrating three different Vmin values. The figure reveals that at a high Vmin of 1.8V, the current consistently maintained positive values during the voltage cycle, indicating that the double-layer capacitance remained charged throughout the operational period. However, with Vmin set at 1.6V, a relatively small capacitive current emerged, suggesting partial discharge of the double-layer capacitance. Moreover, at a lower Vmin of 1.4V, the cell voltage became insufficient to drive water electrolysis in the current cell. Consequently, a relatively large negative (capacitive) current manifested during the Vmin period, denoting a deep discharge of the double-layer capacitance.
The obtained results revealed a profound influence of the charge and discharge status of the double-layer capacitance on the AWE cell's performance throughout the voltage cycles. Notably, when the double-layer capacitor remained fully charged during the voltage cycles, minimal impact of the frequency was observed on key performance parameters, including H2 flow rate that shown in figure 1 (c), specific energy consumption that shown in figure 1 (d), and power factor. However, under conditions of partial discharge within the cell voltage cycles, a noticeable correlation between the size and frequency of the voltage waveform and the AWE cell's performance emerged. Deep discharge of the double-layer capacitance led to a frequency-dependent reduction in H2 flow rate, attributed to incomplete charge/discharge cycles. Hence, the critical factor influencing the performance of the electrolytic cell during dynamic operation is the condition (charging/discharging) of the double-layer capacitance. These findings offer valuable insights into optimizing the dynamic operation of AWE cells for enhanced efficiency and performance.

Acknowledgments:

This research was partially supported by New Energy and Industrial Technology Development Organization (NEDO).

References

[1] A. Abdel Haleem, J. Huyan, K. Nagasawa, Y. Kuroda, Y. Nishiki, A. Kato, T. Nakai, T. Araki, S. Mitsushima, Effects of operation and shutdown parameters and electrode materials on the reverse current phenomenon in alkaline water analyzers, J. Power Sources, Vol 535, 2022, 231454.
[2] V. Ruuskanen, J. Koponen, A. Kosonen, M. Niemelä, J. Ahola, A. Hämäläinen, Power quality and reactive power of water electrolyzers supplied with thyristor converters, J. Power Sources, Vol. 459, 2020, 228075.

Figure 1