Hydrogen safety aspects related to high-pressure polymer electrolyte membrane water electrolysis

Polymer electrolyte membrane (PEM) water electrolysis has demonstrated its potentialities in terms of cell efficiency (energy consumption ca. 4.0–4.2 kW/Nm3 H2) and gas purity (>99.99% H2). Current research activities are aimed at increasing operating pressure up to several hundred bars for direct storage of hydrogen in pressurized vessels. Compared to atmospheric pressure electrolysis, high-pressure operation yields additional problems, especially with regard to safety considerations. In particular the rate of gases (H2 and O2) cross-permeation across the membrane and their water solubility both increase with pressure. As a result, gas purity is affected in both anodic and cathodic circuits, and this can lead to the formation of explosive gas mixtures. To prevent such risks, two different solutions, reported in this communication, have been investigated: first, the chemical modification of the solid polymer electrolyte, in order to reduce cross-permeation phenomena; second, the use of catalytic H2/O2 recombiners to maintain H2 levels in O2 and O2 levels in H2 at values compatible with safety requirements.