No abstract

High efficiencies, wide operation range and rapid response time have motivated the recent interest in proton exchange membrane (PEM) electrolysis for hydrogen generation with surplus electricity. However, degradation at high current densities and the associated mechanism has not been thoroughly explored so far. In this work, membrane electrode assemblies (MEA) from different suppliers are aged in a commercial PEM electrolyzer (2.5 N m 3 H 2 h -1 ), operating up to 4 A cm -2 for more than 750 h. In all cases, the cell voltage (E cell ) decreases during the testing period. Interestingly, the cells with Ir-black anodes exhibit the highest performance with the lowest precious metal loading (1 mg cm -2 ). Electrochemical impedance spectroscopy (EIS) shows a progressive decrease in the specific exchange current, while the ohmic resistance decreases when doubling the nominal current density. This effect translates into an enhancement of cell efficiency at high current densities. However, Ir concurrently leaches out and diffuses into the membrane. No decrease in membrane thickness is observed at the end of the tests. High current densities do not lead to lowering the performance of the PEM electrolyzer over time, although MEA components degrade, in particular the anode.

Cost reduction and high efficiency are the mayor challenges for sustainable H2 production via proton exchange membrane (PEM) electrolysis. Titanium-based components such as bipolar plates (BPP) have the largest contribution to the capital cost. This work proposes the use of stainless steel BPPs coated with Nb and Ti by magnetron sputtering physical vapor deposition (PVD) and vacuum plasma spraying (VPS), respectively. The physical properties of the coatings are thoroughly characterized by scanning electron, atomic force microscopies (SEM, AFM); and X-ray diffraction, photoelectron spectroscopies (XRD, XPS). The Ti coating (50 μm) protects the stainless steel substrate against corrosion, while a 50-fold thinner layer of Nb decreases the contact resistance by almost one order of magnitude. The Nb/Ti-coated stainless steel bipolar BPPs endure the harsh environment of the anode for more than 1000 h of operation under nominal conditions, showing a potential use in PEM electrolyzers for large-scale H2 production from renewables.

A three-dimensional (3D), two-phase numerical model was developed and presented as a useful tool for investigating oxygen bubble propagation in porous transport layers (PTLs) (otherwise known as gas diffusion layers (GDLs)) of polymer electrolyte membrane (PEM) electrolyzers. The volume-of-fluid (VoF) technique was employed to simulate the liquid-gas interface movement through liquid-saturated porous media designed to be representative of PEM electrolyzer PTLs. The circulation of the liquid within the channel and the porous domain was included in the model. Bubble propagation patterns and bulk saturations for porous material representations of commonly used PTLs were determined as a function of time leading up to the moment of breakthrough. Previously conducted experimental microfluidic investigations were used for model validation, and it was found that the numerical results were in good agreement with the numerical predictions. The validated model was used to calculate pressure variations in bubbles during propagation, and the highest threshold capillary pressure corresponding to a critical throat was introduced as a means to measure the efficacy of oxygen bubble removal. The information obtained from the developed numerical tool can be used for designing and evaluating PTL microstructures for next generation electrolyzer materials.

Given its rapid response to fluctuating currents and wide operation range, proton exchange membrane (PEM) water electrolysis is utmost suitable for generation of hydrogen from renewable power. However, it is still hindered by the high cost of the stack components compared to those used in the alkaline technology. In particular, the titanium bipolar plates (BPP) are an issue and the replacement of this metal by stainless steel is a challenge, due to the highly corrosive environment inside PEM electrolyzer stack. Herein, we coat stainless steel BPPs with 50-60 µm Ti and 1.5 µm Pt coatings by vacuum plasma spraying (VPS) and magnetron sputtering physical vapor deposition (PVD), respectively. The BPPs are evaluated at constant 1 A cm-2 for more than 1000 h. The thermally sprayed Ti coatings fully protect the stainless steel substrate during this period of time, and the Pt surface modification allows achieving a cell performance comparable to the baseline.