Controlled Hydride Formation on Titanium Porous Transport Layer for Application in PEM Water Electrolysis
Authors: Bautkinova Tereza , Utsch Nikolai , Bystron Tomas , Shviro Meital , Bouzek Karel 
Published: July 2022

Titanium belongs to a group of valve metals that are known to possess a surface layer of passive oxide that protects the material from corrosion. Thanks to this attribute, titanium can withstand even harsh conditions prevailing in the anode compartment of PEM water electrolyzer. This makes titanium, together with its high electrical conductivity, a suitable material for the anode porous transport layer (PTL) and the bipolar plates in PEM water electrolyzers. However, too extensive passivation leads to an increase of contact resistance between the PTL and catalyst layer and consequently decreases the energy efficiency. This is the reason why titanium PTL is usually covered with a thin coating of precious metal (such as iridium or platinum) [1]. This thin coating hinders further titanium oxidation during electrolyzer operation and mediates good electrical contact with the catalyst particles. Recently, a much cheaper alternative approach based on titanium surface etching in acid has been introduced [2]. Titanium hydride that forms during this treatment has been shown to inhibit the passive layer growth, thus decreasing the contact resistance and improving the energy efficiency of the electrolysis.

This work aims to further develop the ideas presented in reference [2] and investigate the role of titanium hydride during PEM water electrolysis. The hydridation treatment was done on pieces of titanium foil and consists of two steps. The first step is etching in boiling 35% HCl. This step removes the native oxide layer, increases the surface roughness, and introduces some amount of hydrogen into the crystal lattice of the metal. The second step involves cathodic polarization of the etched samples allowing more pronounced hydridation.

By changing the condition of the cathodic polarization process, it is possible to tune the hydrogen uptake. The effects of polarization time, current density, temperature and concentration of electrolyte were studied with respect to the amount of the hydride layer formed, its thickness and penetration into the bulk phase. Treated titanium foils were characterized by various physicochemical techniques. X-ray diffractometry was used for hydride quantification, further analysis was done by scanning electron microscopy and cross-sectional metallography. Electrochemical properties of samples were studied by impedance spectroscopy and linear sweep voltammetry.

The hydridation procedure was then used for titanium PTLs treatment which were then tested at the anode side of the PEM water electrolyser. Significant increase of current density was observed for etched and polarized Ti PTLs in comparison with pristine Ti PTL (Figure A). This can be attributed to significantly decreased contact resistance of treated PTLs (Figure B). A further performance increase was achieved when PTLs were treated prior to iridium coating (Figure A, B). It is thus possible to conclude, that the hydridation represents a promising approach to reduce precious metals demands in PEM water electrolysis construction.

Financial support by the Czech Science Foundation within the framework of the project No.: 20-06422J is gratefully acknowlegded.

Liu, C., et al., Exploring the Interface of Skin-Layered Titanium Fibers for Electrochemical Water Splitting. Advanced Energy Materials, 2021. 11(8): p. 2002926.

Bystron, T., et al., Enhancing PEM water electrolysis efficiency by reducing the extent of Ti gas diffusion layer passivation. Journal of Applied Electrochemistry, 2018. 48(6): p. 713-723.

Figure 1