Thin protective titanium suboxide coatings for next-generation porous transport layers of PEM electrolysers – PROTIS

Titanium suboxides (TinO2n-1, also called substoichiometric titanium) are an oxygen-deficient type of titanium compound exhibiting high electrical conductivity and chemical stability in a large pH range. These properties make titanium suboxides suitable as protective coatings for metallic components in proton exchange membrane water electrolysers (PEMWE), in particular for porous transport layers (PTL) and bipolar plates (BBP). Notably in the anode (oxygen)
compartment, high potentials and an oxidizing environment provide favourable conditions for corrosion and oxidation of metals, leading to pollution of the catalyst coated membrane (CCM), and/or undesired increase of interfacial contact resistances. State-of-the-art PEMWE stacks employ titanium PTLs with noble metal coatings. In the present project, we will replaced the expensive titanium PTLs by more cost-effective 316L stainless steel (SS). To avoid corrosion, we will protect the SS anode PTL with thin titanium suboxide protective coatings. This approach can significantly reduce the capital expenditures (CAPEX) of the PEMWE stack while maintaining a long-term durability.
Two deposition techniques will be utilized: physical vapour deposition (PVD) and atomic layer deposition (ALD). The former will protect solely the outermost surface of the PTL that is in direct contact with the strongly acidic conditions of the CCM (interfacial protection), while the latter will cover the entire internal PTL surface in a conformal way (pore walls included, integral protection), which has never been demonstrated before. A strong focus will be made, at a fundamental level, on the understanding of how the depositions parameters affects surface overage, nanostructure and (nano)porosity of coatings (WP1). For that purpose, extensive high-resolution electron microscopy investigations (HAADF-STEM), with an emphasis on cross-sectional imaging, will be carried out and complemented by analysis of chemical composition and crystalline structures (Raman spectroscopy, XRD, XPS, EDS, EELS…) (WP2).
To mimic the environment of PEMWE anodes, weeks-long corrosion measurements will be performed ex situ in diluted H2SO4 solutions containing fluoride anions. This provides a basis for investigating the widely understudied relationships between the microstructural features of the coating (coverage, porosity, thickness) and their protective properties under various experimental conditions (pH, potential, time…) (WP3).
Stainless steel PTLs with coatings of different coverage and microstructure and promising barrier properties will be selected from ex situ corrosion experiments. Those will be implemented in operando PEMWE testing, in order to evaluate their influence on the performance and durability of the electrolysis process (WP4). Specific accelerated stress tests will be defined to perform ageing measurements of coated PTLs in “practical” durations of up to two months (compared to the expected operation lifetime of > 50000 hours for an industrial PEMWE system). Post-mortem analysis after dismantling of the PEMWE will enable to assess the coating nanostructure after operation (with the tools of WP2) and the possible trapping of corrosion products in its core. The potential pollution of the CCM by metal elements from the 316L PTL will also be assessed by wavelength dispersive X-ray spectroscopy as an indicator of the coating performance. Based on the results of these four WPs, the efficiency of interfacial protection (PVD) and integral protection (ALD) will be compared. Conclusions will be drawn on the possibility of using titanium oxide as cost-effective coating on stainless steel PTL in industrial PEMWE systems.