Characterization of a high pressure alkaline electrolyser with respect to product gas purity

Abstract

Zero Emission Fuels B.V. (ZEF) is developing a microplant, which ultimately provides a pathway to produce a hydrocarbon in a sustainable manner. This microplant consists of several subsystems. One of which is an alkaline water electrolyser, in which water is split into oxygen and hydrogen under specific operating conditions.
As a mixture of the product gasses can be highly explosive, gas crossover in alkaline water electrolysis (AEC) systems is a point of concern in terms of safety. This explorative study aims to identify the main routes for crossover in AEC systems, as well as characterizing the ZEF AEC system. This characterization has been accomplished by studying the effect of process conditions and system geometry on gas crossover.

Based on a literature study, gas crossover is found to be the result of diffusion as well as convection. Both of these phenomena occur at the interconnects between the sides of the system, which is over the diaphragms and over the pressure equalization tube (PET) in case of the ZEF AEC system.

Gas measurements where conducted during over 194 hours of experiments. From this it was concluded that hydrogen crossover is the main point of concern in terms of safety. Operating pressure and temperature both very clearly affect hydrogen crossover. The effect of current density is also significant but obscured by the fact that temperature is an uncontrollable variable in the ZEF AEC system. From the steady-state data a window for safe operations has been framed.
Upon start-up of the system a spike in oxygen-side hydrogen concentration is consistently measured, which is a result of crossover by diffusion during system off-time. This spike in concentration also poses a threat in terms of safety.
Three different pressure equalization tubes have been tested to investigate the influence of interconnect geometry on gas crossover.

In order to identify the main route for hydrogen crossover a CSTR model of the oxygen side gas phase was developed, in which temperature and pressure effects were taken into account. The model showed that hydrogen crossover by diffusion could be held accountable for ~8% of the total amount of crossed-over hydrogen, which is in line with reports in literature for a mixed electrolyte cycle AEC system. The model also showed that crossover by solely diffusion through the PET is negligible compared to through the diaphragm.

From this, it was concluded that the ZEF AEC system behaves more like a system with mixed electrolyte cycles with respect to gas crossover levels. Recommendations are made in order to reduce hydrogen crossover, and enlarge the window for safe operation as a result.