Effect of Catalyst Layer Ionomer Content on Performance of Intermediate Temperature Proton Exchange Membrane Fuel Cells (IT-PEMFCs) under Reduced Humidity Conditions

Multi-scale revealing how real catalyst layer interfaces dominate the local oxygen transport resistance in ultra-low platinum PEMFC

In view of a catalyst layer (CL) with low-Pt causing higher local transport resistance of O2 (Rlocal), we propose a multi-study methodology that combines CO poisoning, the limiting current density method, and electrochemical impedance spectroscopy to reveal how real CL interfaces dominate Rlocal. Experimental results indicate that the ionomer is not evenly distributed on the catalyst surface, and the uniformity of ionomer distribution does not show a positive correlation with the ionomer content. When the ionomer coverage on the supported catalyst surface is below 20 %, the ECSA is only 10 m2·g-1, and the ionomer coverage on the supported catalyst surface reaches 60 %, the ECSA is close to 40 m2·g-1. The ECSA has a positive correlation with ionomer coverage. Because the ECSA is measured by CO poisoning, it can be inferred that the platinum contacted with ionomer can generate effective active sites. Furthermore, a more uniform distribution of ionomer can create additional proton transport channels and reduce the distance for oxygen transport from the catalyst layer bulk to the active sites. A higher ECSA and a shorter distance for oxygen transport will reduce the Rlocal, leading to better performance.

High-current density alkaline electrolyzers: The role of Nafion binder content in the catalyst coatings and techno-economic analysis

We report high-current density operating alkaline (water) electrolyzers (AELs) based on platinum on Vulcan (Pt/C) cathodes and stainless-steel anodes. By optimizing the binder (Nafion ionomer) and Pt mass loading (mPt) content in the catalysts coating at the cathode side, the AEL can operate at the following (current density, voltage, energy efficiency -based on the hydrogen higher heating value-) conditions (1.0 A cm-2, 1.68 V, 87.8%) (2.0 A cm-2, 1.85 V, 79.9%) (7.0 A cm-2, 2.38 V, 62.3%). The optimal amount of binder content (25 wt%) also ensures stable AEL performances, as proved through dedicated intermittent (ON-OFF) accelerated stress tests and continuous operation at 1 A cm-2, for which a nearly zero average voltage increase rate was measured over 335 h. The designed AELs can therefore reach proton-exchange membrane electrolyzer-like performance, without relying on the use of scarce anode catalysts, namely, iridium. Contrary to common opinions, our preliminary techno-economic analysis shows that the Pt/C cathode-enabled high-current density operation of single cell AELs can also reduce substantially the impact of capital expenditures (CAPEX) on the overall cost of the green hydrogen, leading CAPEX to operating expenses (OPEX) cost ratio <10% for single cell current densities ≥0.8 A cm-2. Thus, we estimate a hydrogen production cost as low as $2.06 kgH2 -1 for a 30 years-lifetime 1 MW-scale AEL plant using Pt/C cathodes with mPt of 150 μg cm-2 and operating at single cell current densities of 0.6-0.8 A cm-2. Thus, Pt/C cathodes enable the realization of AELs that can efficiently operate at high current densities, leading to low OPEX while even benefiting the CAPEX due to their superior plant compactness compared to traditional AELs.

The Influence Catalyst Layer Thickness on Resistance Contributions of PEMFC Determined by Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy is an important tool for fuel-cell analysis and monitoring. This study focuses on the low-AC frequencies (2–0.1 Hz) to show that the thickness of the catalyst layer significantly influences the overall resistance of the cell. By combining known models, a new equivalent circuit model was generated. The new model is able to simulate the impedance signal in the complete frequency spectrum of 105–10−2 Hz, usually used in experimental work on polymer electrolyte fuel cells (PEMFCs). The model was compared with experimental data and to an older model from the literature for verification. The electrochemical impedance spectra recorded on different MEAs with cathode catalyst layer thicknesses of approx. 5 and 12 µm show the appearance of a third semicircle in the low-frequency region that scales with current density. It has been shown that the ohmic resistance contribution (Rmt) of this third semicircle increases with the catalyst layer’s thickness. Furthermore, the electrolyte resistance is shown to decrease with increasing catalyst-layer thickness. The cause of this phenomenon was identified to be increased water retention by thicker catalyst layers.

Zr2N2O Coating-Improved Corrosion Resistance for the Anodic Dissolution Induced by Cathodic Transient Potential

Developing a corrosion-resistant and electrically conductive coating on metallic bipolar plates is essential to mitigate the performance degradation induced by the high cathodic transient potentials (CTPs) in the start-up/shut-down (SU/SD) processes of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a zirconium oxynitride (Zr₂N₂O) coating prepared by atomic layer deposition was used to improve the corrosion resistance of 304 stainless steel (304 SS) toward anodic dissolution at various CTPs. Triangular potential pulses were applied to the specimens to simulate potential variations at the cathode side of the PEMFCs at SU/SD stages. Results show that the Zr₂N₂O coating can provide effective protection at a CTP as positive as 1.1 V versus Ag/AgCl. At all CTPs examined, the peak current density ( i_peak) extracted from the pulse test of the coated specimen (Zr₂N₂O/SS) is 2 orders of magnitude lower than that of uncoated 304 SS, indicating that the presence of the Zr₂N₂O coating remarkably increases the corrosion resistance for the anodic dissolution induced by CTPs. More importantly, upon increasing the CTPs, 304 SS experiences severe intergranular corrosion after 4050 pulses, whereas Zr₂N₂O/SS shows slight pitting corrosion. The quite low i_peak and the mitigated corrosion morphologies of Zr₂N₂O/SS confirm that incorporating oxygen into the protective coating for achieving a high oxidation resistance is a feasible way to restrain the anodic dissolution caused by high CTPs. Analysis of the electron energy level diagrams of the passive film suggests a protective coating with a wider valence band contributing to the improved corrosion resistance toward the transpassive dissolution.