Optimisation of Anode Construction for PEM Water Electrolysis

Abstract

Transformation of energy sector to carbon dioxide neutrality is one of the main goals of governments around the globe. This is closely connected with propagation of Hydrogen Economy scheme, where excess electric energy from renewable sources is stored in the form of hydrogen. Hydrogen is a valuable commodity and promising energy vector with numerous applications not only directly in industry, but also as a fuel for mobility sector. Water electrolyser with polymer electrolyte membrane (PEMWE) represents a key component for hydrogen production within Hydrogen Economy scheme. Among its principal advantages is use of water as a feed, option of operation at elevated pressure, flexibility of operation suitable for pairing with renewable electric sources and very high power to volume ratio, critical for delocalised hydrogen production.Despite these outstanding properties of PEMWE, several shortcomings still limit its application on industrial scale. The main constraint is a high loading of precious metals, especially on the anode, where IrO2 is the state-of-the-art catalyst and anode itself is a component most susceptible to degradation due to highly oxidative, acidic environment. In general, the anode consists of unsupported catalyst nanoparticles based on IrO2 and Ti porous transport layer (PTL), which is often coated with Pt, Au or Ir to prevent its passivation during electrolyser operation. The absence of catalyst support and microporous layer (MPL) on the top of Ti PTL decreases utilisation of catalyst, leading to the use of high catalyst loadings. Despite the significant progress in development of catalyst supports and Ti PTLs with gradient porosity in recent years, the issue of high Ir content and PTL without precious meatal coating passivation is still far from being solved. This presents a bottleneck for further PEMWE application.The goal of this work is to optimise anodic PTL for PEMWE in an efficient and economically feasible way, avoiding coating of PTL by precious metals and decreasing catalyst loading by its better utilisation. This was achieved by preparation of microporous layer on the top of commercially available PTLs, based on Ti felts and sintered Ti. From the materials potentially applicable for MPL, TiH2 was chosen due to its good electron conductivity and material compatibility with PTL. TiH2 is inherently thermodynamically unstable in presence of water and O2, forming non-stoichiometric Ti oxides with significantly lower electron conductivity. However, the rate of TiH2 oxidation at PEMWE operating conditions and impact of this process on cell Ohmic resistance was not yet investigated.MPL based on TiH2 powder was deposited on the surface of Ti PTLs by means of ultrasonically assisted spray deposition from ink. TiH2 was bonded in MPL by ionomer based on perfluorinated-sulfonated polymer. Stability of treated PTLs with and without MPL was investigated by single cell tests performed with in-house made membrane-electrode assembly in catalyst-coated membrane arrangement, consisting of commercial state-of-the-art materials at temperature of 80 °C and constant voltage of 2 V for 120 h. During single cell operation, its performance was periodically investigated by polarisation curves and impedance spectroscopy, performed at selected voltages. Impact of MPL of PEMWE performance is discussed in term of cell performance, Ohmic resistance and polarisation resistances.This study was supported by the Grant Agency of the Czech Republic under project no. GC20–06422J.