Electrochemical characterization of cell components for proton exchange membrane electrolyzers

Kurzfassung

Energy from renewable sources are in a quest to replace fossil fuels. In Europe it is expected an electricity surplus increasing originated by the seasonality of the energy production from renewable sources, creating a potential market for long-term storage. Hydrogen, when produced by water electrolysis, is one of the most promising energy carrier for the large scale storage of energy, mobility, de-carbonization of refineries and chemistry industries. Proton exchange membrane (PEM) electrolysis is the most suitable technology given its high level of maturity and hydrogen purity, low footprint and wide range of operation. Yet, is still expensive, mainly due to the stack materials and components such as gas diffusion layers (GDL) that are made of titanium coated with precious metals, and therefore compromising 43 % of the costs in a stack. In this regard, stainless steel GDLs would preferable to reduce the stack cost and thus the price of hydrogen produced by PEM electrolysis. However, stainless steel corrodes severely in the acidic environment of the PEM electrolyzer. In this work, it was investigated the performance and durability of low cost stainless steel GDL meshes coated with porous layers of Ti and Nb by vacuum plasma spraying. Coated and uncoated Ti meshes were used for comparison purposes. The morphology and elemental composition of the GDLs was analyzed by scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDX), respectively. The high purity Ti and Nb layers covered the substrate uniformly. The electrochemical characterization was carried out in 4 cm2 active area PEM electrolyzer. The polarization curve showed a reduction in overpotential of ca. 320 mV at 2 A∙cm-2 with the Ti/Nb-coated stainless steel mesh, compared to the uncoated Ti mesh. This result means an improvement efficiency of ca. 10 %, which can hardly be achieved with more active anode catalysts and thinner membranes. Electrochemical impedance spectroscopy showed an almost complete elimination of mass transport losses at low and high current densities for all coated GDLs when compared to uncoated ones. Furthermore, a 435 h test at constant current density of 2 A∙cm-2 was carried out for Ti/Nb-coated stainless steel mesh. The cell potential rapidly increased over time, mainly due to technical difficulties with the electrolyzer setup, which needs to be improved in the future for long-term tests. The deionized water ion exchange resin was analyzed in the end of the test by X-ray photonelectron spectroscopy (XPS) and no traces of iron, chromium or other possible corrosion products from the stainless steel were detected. The Ti/Nb coatings protected the stainless steel mesh GDLs against corrosion and improved substantially the cell performance at high current densities. These coated GDLs can potentially reduce the capital cost of PEM electrolyzers for large-scale hydrogen production from energy from renewable sources.