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Stress tests are developed for proton exchange membrane electrolyzers that utilize low catalyst loading, elevated potential, and frequent cycling with square- and triangle-waves to accelerate anode catalyst layer degradation during intermittent operation. Kinetics drive performance losses (ohmic/transport secondary) and are accompanied by decreasing exchange current density, decreasing cyclic voltammetric capacitance, and increasing polarization resistance. Decreased kinetics are likely due to a combination of iridium (Ir) migration into electrochemically inaccessible locations in the anode or membrane, Ir particle growth (supported by X-ray scattering), changes in the extent of the Ir oxidation state (supported by X-ray absorption spectroscopy), and anode catalyst layer reordering. Decreasing catalyst/transport layer contact and catalyst/membrane interfacial tearing may add contact resistances and account for increasing ohmic losses. Performance losses for low and moderate catalyst loading, as well as from accelerated and model wind/solar cycling protocols, were likewise dominated by kinetics but vary in severity. Accelerated cycling (1 cycle per minute) appears to reasonably accelerate relevant loss mechanisms and can be used to project electrolyzer lifetime from anode deterioration. Ongoing accelerated stress test development and studies into performance loss mechanisms will continue to be critical as electrolysis shifts to intermittent power and low-cost applications. [ABSTRACT FROM AUTHOR]

Capacity loss induced by the undesired transport of vanadium ions across the ion-exchange membrane (i.e. crossover) is one of the most critical issues associated with vanadium redox flow batteries. This work reports on the manufacturing and testing of an innovative barrier layer to mitigate crossover. The barrier layer conceptual design is described in detail in the patent application WO 2019/197917. The barrier was deposited directly onto Nafion® 212 using the Reactive Spray Deposition Technology, in which carbon-rich particles (∼4–10 nm in diameter) formed in the flame were deposited simultaneously with a mixture of 1100EW Nafion® and Vulcan® XC-72R (∼40 nm diameter) that was sprayed from air-assisted secondary nozzles. During cycles at fixed capacity, the presence of the barrier layer significantly reduced battery self-discharge; the average variation of battery state of charge compared to a reference cell with Nafion® 115 was reduced from 21% to 7%. Moreover, battery energy efficiency was increased by nearly 5%, indicating that the barrier layer does not significantly hinder proton transport. During cycles at 50 mA cm−2 with fixed cut-off voltages, the barrier layer exhibited stable operation, maintaining a coulombic efficiency around 99.4%. Additionally, the use of the barrier layer projects to a 30% reduction of stack-specific cost. [ABSTRACT FROM AUTHOR]

Reduction of platinum loading is one of the major challenges for the commercialization of proton exchange membrane fuel cells. The role of catalyst layer microstructure and ionomer content is critically important at ultra-low platinum loading due to enhanced oxygen transport resistance compared to high platinum loading. In the present work, catalyst-coated membranes with two types of carbon supports and various I/C ratios are fabricated using reactive spray deposition technology (RSDT) where low Pt loading (cathode 0.1 mg cm-2, anode 0.05 mg cm-2) catalyst layers are directly deposited on Nafion membranes. Similar performance is obtained at optimal I/C ratios for Pt/Ketjen Black (KB) CCMs and Pt/multi-wall carbon nanotube (MWNT) CCMs, with optimal I/C ratio being 1.0 and 0.5, respectively. Six types of polarization overpotentials are analyzed using a six-step method established in our previous work to elucidate the influence of catalyst layer microstructure and ionomer content on fuel cell performance. Since MWNT requires 50% less ionomer and are chemically more stable than KB, MWNT is a preferred support for ultra-low platinum loading electrodes. The six-step method connects catalyst layer properties with specifically targeted overpotentials and is thus a powerful tool in catalyst layer optimization. [ABSTRACT FROM AUTHOR]

Significant effort has been devoted to reduce the cathode platinum loading for proton exchange membrane fuel cells (PEMFCs). To achieve this, it is imperative to have a comprehensive understanding of the polarization behavior for the low-Pt-loading electrodes, and to reduce the polarization loss due to oxygen transport limitation. Herein, a systematic breakdown of six types of polarization sources is presented to elaborate the effect of cathode Pt loading and the catalyst layer fabrication process. Four modifications are applied to accommodate low cathode Pt loading. GORE PRIMEA catalyst-coated membranes (CCMs) were used as a baseline and tested with 0.4 and 0.1 mg cm-2 cathode Pt loading. A novel electrode fabrication method, reactive spray deposition technology (RSDT), was employed to fabricate 0.1 mg cm-2 Pt loading cathode using Ketjen black carbon as catalyst supports. Non-electrode concentration overpotential is determined by the cathode Pt loading and the type of diffusion medium, while cathode electrode concentration overpotential is determined by the ionomer thin film and ionomer/Pt interface which are dependent on the fabrication process. It is shown that the RSDT process can improve fuel cell performance at 0.1 mg cm-2 cathode Pt loading by reducing the cathode electrode concentration overpotential. [ABSTRACT FROM AUTHOR]

Iridium oxide is one of the most common anode catalysts in commercial proton exchange membrane (PEM) electrolyzers because of its strong mix of high activity and stability under oxygen evolution reaction (OER) conditions. Unfortunately, benchmarking iridium oxide OER catalysts has proven difficult since IrO2 cannot undergo proton underpotential deposition like platinum and other transition metal eletrocatalysts, making it difficult to estimate the electrochemically active surface area (ECSA), as well as OER specific and mass activity. In this work, we propose a method to calculate the ECSA of iridium oxide in an operating PEM electrolyzer. A universal constant, 596 (±21) µC/cm², was obtained from the correlation of pseudocapacitive charge and ECSA of iridium oxide. In the membrane electrode assembly (MEA), the calculated ECSA (1.81 (±0.065) m² over a 25-cm² geometric area) showed an iridium oxide catalyst utilization of ~93%. Additionally, the IrO2 OER specific and mass activities at 80°C, 1.6 V in an operating PEM electrolyzer were 0.401 (±0.014) mA/cm² and 132 mA/mg, respectively. [ABSTRACT FROM AUTHOR]

Catalysts are required for both the oxidation of water and the reduction of oxygen. Blended oxides of Ir and Ru are superior for water oxidation whereas mixtures of Pt and Ir perform better when both oxidation of water and reduction of oxygen are required on the same electrode. A strategy for rationing these elements is explored by the formation of a thin film using a dry, flame process. IrxPt1-xO2-y and IrxRu1-xO2-y were both deposited from the vapor phase, as thin films, onto substrates of glassy carbon, polypropylene, and quartz. Elemental analysis of the Ir-Pt electrode suggests a stoichiometry of Ir0.56Pt0.44O2-y. Bulk diffraction of the film shows two separate phases consisting of Pt metal and IrO2. The sample showed signs of spallation after 10 cycles when scanned between 1.2 and 1.7 V. A weak oxygen evolution current of 0.8-0.4 mA/mg was measured at 1.6 V. Elemental analysis of the Ru-Ir film suggests a ratio of Ru0.41Ir0.59O2-y. Phases of a homogeneous solid-solution of IrO2 and RuO2 and to a lesser extent Ru metal are shown by bulk X-ray diffraction. An exceptional oxygen evolution current of 25-40 mA/cm² was observed for the Ru0.41Ir0.59O2-y sample corresponding to a normalized mass activity of 400 mA/mg. [ABSTRACT FROM AUTHOR]