Bipolar plates (BPP) are one of the essential components of electrolyzer stacks for the generation of pure hydrogen by water electrolysis. In order to replace conventional and expensive titanium based BPP, stainless steel becomes more and more important against the background of increasing production volume due to cost and functional reasons. On the other hand, some requirements exist, which limit the use of stainless steel BPP in electrolyzers. Firstly, due to harsh corrosive conditions, the application of the stainless steel BPP requires the protection against corrosion. Secondly, the electrical conductivity of BPP must remain constantly low over the entire service life.

The expected increasing demand of hydrogen in the upcoming years requires adequate electrolysis capacities in order to be able to serve the market. One technology for mass production of hydrogen is the alkaline polymer membrane water electrolysis (AEM-WE). Heart of AEM-WE are the catalysts, which enable the decomposition of water at low energy input. Therefore, the availability of sufficient and suitable catalysts is a limiting factor for high H₂ production volumes.

Objective of the R&D project “BipolarPilot” was the economic mass production of bipolar plates, which form the heart of fuel cells and represent around 30% of the total system costs. Together with its research partner The hydrogen and fuel cell center ZBT GmbH, KCS Europe has both developed appropriate PVD coatings as well as an inline PVD pilot coating system in order to enable mass production of cheap and corrosion stable stainless steel Bipolar plates.

While batch coaters are coating units with only one cycle-time limiting coating chamber, inline systems are coating units that enable a continuous processing and coating with short cycle times. Thus, the pilot coating unit has been designed with a future high demand for bipolar plates in mind.

Introducing hydrogen-based energy production, storage, and conversion technologies implies materials withstanding hydrogen's specific, reactive behavior. Durability-related issues such as hydrogen embrittlement in structural components or corrosion-induced phenomena in fuel cell or electrolyzer technologies are of significant interest in transitioning to green and sustainable energy supplies. In more detail, the stimulated interaction of hydrogen with the topmost micrometers and the microstructural features of materials will play a predominant role. Therefore, using physical vapor deposited (PVD) coating materials to protect and functionalize material surfaces will be critical in diverse future applications.

Degradation mechanisms related to hydrogen occur at multiple length scales, involving different strategies for exposure or treatments. These strategies are typically divided into the two hydrogen-related material research worlds: (i) electrochemical degradation setups and (ii) and non-electrochemical treatments as a collective term for pressure/gas-related setups. Nevertheless, the broad field of hydrogen-related applications is increasingly merging. Consequently, the different degradation strategies primarily focus on electrochemical test setups as they are highly versatile.

Therefore, in this study, the interaction of hydrogen with well-known protective coating materials such as TiN, CrN, ZrN, or TiAlN and AlCrN is described by an electrochemical Devanathan-Stachurski permeation setup. Different sets of these ceramic coating materials have been deposited on ferritic steel sheets through sputter and arc-evaporation technologies with varying deposition parameters. Subsequently, the coatings have been electrochemically loaded, and parameters such as diffusion coefficients, permeability, or permeation reduction factors are estimated. These results are correlated with the coatings’ microstructural appearance before and after hydrogen testing using a set of diverse high-resolution techniques such as SEM, TEM, XRD, and micro-mechanical testing methods.

Keywords: Protective coatings; Hydrogen; Electrochemical testing; PVD; Nitrides;