Theoretical and Experimental Analysis of the Cathode-Vapour-Feed PEM-Electrolyser for Space Applications
Authors: Heizmann Sören , Manfletti Chiara 
Published: December 2024

Water electrolysis is experiencing growing interest due to its importance in the transition to a carbon-neutral transportation sector and even in spaceflight it can play a vital role. Here, its typical application has been the oxygen generation for the life support of astronauts onboard a spacecraft or the International Space Station. However, in recent years the application of electrolysis for a new form of spacecraft propulsion is receiving increasing attention, as well. This technology is called Water Electrolysis Propulsion (WEP).

So far highly toxic and expensive propellants have been used for in-space propulsion, like hydrazine-derivatives and dinitrogen tetroxide. However, with a constantly growing number of satellite-launches per year, the spaceflight industry is searching for more inexpensive solutions which do not entail the corresponding safety hazards and engineering challenges of these conventional propellants. WEP is currently considered to be one of the most promising solutions for these issues. Within such a system a satellite is filled on ground with pure water. Once launched into space an electrolyser, powered by the satellite’s solar panels, is used to split the water into gaseous hydrogen and oxygen which are stored in intermediate storage tanks at pressures of up to 100 bar. Subsequently the gases can be combusted in a rocket engine to generate thrust and to propel the spacecraft for various propulsive needs.

One of the core components of such a system is the electrolyser. In order to be competitive, an electrolyser type has to be found, which is lightweight, able to operate in zero-gravity and is able to pressurize the gases without the need for additional mechanical pumps. Currently the most promising electrolyser type is the so-called Cathode-Vapour-Feed (CVF) PEM electrolyser. Here a conventional PEM electrolyser is operated in cathode feed and modified by integrating a second membrane (Water Feed Barrier) between the electrolysis membrane and the water inlet. In order for the electrolyser to operate, the water has to diffuse through this Water Feed Barrier (WFB) and is subsequently present in a vapour state at the electrolysis membrane. Therefore, no phase separators are needed and the electrolyser is able to operate independently and unaffected by gravity. Furthermore, a low voltage is applied on the Water Feed Barrier slightly exceeding the Nernst-Voltage. Hence the WFB is effectively acting as an electrochemical pump which allows the generation of the gases at higher pressures than the water inlet pressure.

This conceptual change is however introducing several unconventional mechanisms and phenomena that have to be considered during the design of such a device. However, previous research on the CVF technology has been very limited due to its niche application so far and most of these phenomena and mechanisms remain unanalysed. This paper is aiming to contribute towards the closure of this knowledge gap.

The working principle and the theory behind these additional phenomena appearing in the CVF electrolyser are presented. These are the significantly affected mass transport of the water towards the anode side of the electrolysis membrane, the gas diffusion through both membranes and its effect on the electrolyser’s performance, as well as the mutual interaction between the applied voltages on the three electrodes. Furthermore, many researchers have used the same membrane type for the WFB as for the electrolysis membrane, although it serves a different purpose and is exposed to different operating conditions. Therefore, special attention will be devoted to the determination of the optimal membrane type for the WFB since barely any research has been conducted on this topic. In addition, a proof-of-concept electrolyser has been built and tested in a parameter study to validate the theoretical considerations. The experimental findings on the effect of membrane selection for the WFB, distance between the membranes and impact of flow field topology are presented and discussed. Therefore, the paper contributes towards a more targeted development of space electrolysers in the future.

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