Plasma Electrolytically Surface-Engineered SS316L Bipolar Plates for PEM Fuel Cells

Abstract

The proton exchange membrane fuel cell (PEMFC) technology is one of the clean energy technologies with advantages like, quick start-up, low working temperature (60-90 oC), high specific energy and specific power, and high reliability. Stainless steel bipolar plates for PEMFC offer many advantages over conventional graphite and graphite composites. Some of them have superior mechanical properties over graphite, the possibility to fabricate in thin cross sections, and easy formation of flow fields by stamping, which is more economical when compared with the grooving of the graphite counterparts. Despite these advantages, metallic bipolar plates are prone to corrosion in fuel cell working conditions which is often overcome by oxide layer formation on the surface. But the interfacial ohmic loss between the metallic bipolar plate and membrane electrode assembly due to oxide formation decreases the overall performance of PEMFC. For combating these problems, there are different surface engineering techniques like chemical vapor deposition, physical vapor deposition, sputtering, thermal nitridation, etc. Among all methods, plasma electrolytic processing has advantages like less processing time, use of environmentally friendly electrolytes, economical process, easy scale-up for mass production and etc.The 316L stainless steel was surface-engineered using the cathodic plasma electrolytic process in two different aqueous electrolytes containing urea and acetic acid, which acted as a nitrogen and carbon source, respectively. The processed samples were characterized using a scanning electron microscope to study surface morphology and for qualitative chemical analysis. It revealed the diffusion of nitrogen and carbon into the surface in respective electrolytes. The electrochemical performance of as-received/bare 316L and surface-engineered samples were evaluated in a simulated fuel cell environment (half-cell conditions). The electrolyte comprised 0.5 M H2SO4 with 2 ppm HF at 80 oC, and it was purged with N2 or air to simulate anodic and cathodic conditions, respectively. The potentiodynamic polarization studies revealed similar active-passive behavior for bare 316L and surface-engineered samples. Different electrochemical parameters like corrosion current density (icorr), corrosion potential (Ecorr), and passive current density (ipassive) were deduced. The corrosion potentials for the bare sample were observed to have -0.306 V and -0.316 V in cathodic and anodic conditions, respectively. After surface modification, corrosion potentials shifted to more noble values, i.e., -0.234 V, -0.250 V for samples processed in urea solution and -0.219 V, -0.198 V for samples processed in the acetic acid solution for anodic and cathodic conditions, respectively. But the surface-modified samples showed higher passive current densities for both anodic and cathodic conditions than the bare sample, suggesting inferior passivation ability.Also, long-term potentiostatic studies for 8 hours were conducted to understand the samples' behavior. The current densities at the end of the test were -28.7 µA cm-2 (anodic condition) and 7.5 µA cm-2 (cathodic condition) for bare samples. While the samples processed in urea solution showed 27.4 and 9.0 µA cm-2, and the samples processed in acetic acid solution had -115.5 and 89.8 µA cm-2 of current densities in anodic and cathodic conditions, respectively. At different loads, the interfacial contact resistance measurements between the samples and the gas diffusion layer for all samples were determined. At a load of 140 N cm-2, the bare SS316L and samples processed in urea and acetic acid solution showed values of 118.82, 26.78, and 10.97 mΩ cm2, respectively. These studies suggest plasma electrolytically processed SS316L in acetic acid solution offers the least interfacial contact resistance of 10.97 mΩ cm2 and almost satisfies the Department of Energy target of 10 mΩ cm2 for metallic bipolar plates. A scale-up of coating in a 4×4 cm area and its performance in a single-cell setup will be highlighted.