The energy analysis of a Proton Exchange Membrane (PEM) electrolyser cell under fluctuating current density

Abstract

To reduce carbon emissions worldwide, scientists and engineers are trying to find a new energy carrier alternative to fossil fuel. Hydrogen has become a competitive candidate, owing to its high energy density, light weight, and easy transport. Nowadays, many countries are planning to build a hydrogen economy path that contains production, storage, transportation, and utilization to achieve the objective of carbon zero in 2050. Among all the methods to produce hydrogen, water electrolysis is a promising option since it can be powered by renewable energies to produce green hydrogen. Alkaline (ALK) water electrolysis and proton exchange membrane (PEM) water electrolysis technologies have been commercialised on the market. Compared with ALK water electrolyser, PEM water electrolyser has the advantage of operating with high current and high efficiency, and fast response to dynamic control. The fluctuation in renewable energy supply will lead to variable electricity that drives the PEM water electrolyser. Most of the published research is focused on the improvement of materials of cell components or simulation of the reaction progress, but little literature studied the cell behaviour experimentally when connecting to a fluctuating power supply. Our research aims to observe the behaviour of a PEM water electrolyser under the fluctuating current densities and analyse the cell energy balance condition regarding the optimum operation in energy saving. A PEM water electrolyser stack containing two cells was tested in this research. Current-voltage characteristic curve (IV curves) with different scan rates were measured for the characterization of the electrolyser. The result reveals the relationship between the cell voltage, temperature, and hydrogen production rate. To further explore the impact of external conditions on the system behaviour, cell temperature (by adjusting the pump speed) and operation mode with various current step changes were controlled as variables in the experiment. Electrolyser cell voltage and temperature at transient and steady states under different operating conditions were analysed and compared. The experiment results showed that under various operation modes (Glossary Experiment series for various operation conditions), a lower voltage can be generated at the same current, following this order: continuously decreasing current operation > directly increasing current from 0 A to the target current with high temperature (low water flow ii rate) > continuously increasing current operation > directly increasing current from 0 A to the target current with low temperature (high water flow rate). The dominant factor is not only the temperature but also other factors that need to be investigated in the future, such as membrane dynamics and bubble formation. The fluctuating control mode was done by adjusting the current to various values. In this experiment, the system voltage and temperature were monitored during the process from the transient state to steady state. The result implied that decreasing current from a high value to a low value can reduce the voltage to a high extent, and thus save energy. The comparison of hydrogen production rate with different experimental conditions revealed that hydrogen production is only influenced by input current rather than current control mode. However, current control mode can determine the cell voltage, which has dominant influence on the energy efficiency. Therefore, the decreasing current operation maximizes energy efficiency. This research provides insights into the optimum operation mode for industrial hydrogen production. With the proper operation mode, more energy can be saved, especially at the electrolyser start-up stage.