This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.This work addresses current challenges in catalyst development for proton exchange membrane water electrolyzers (PEM-WEs). To reduce the amount of iridium at the oxygen anode to levels commensurate with large-scale application of PEM-WEs, high-structured catalysts with a low packing density are required. To allow an efficient development of such catalysts, activity and durability screening tests are essential. Rotating disk electrode measurements are suitable to determine catalyst activity, while accelerated stress tests on the MEA level are required to evaluate catalyst stability.
This study reveals the physical origin of the rapid performance decay when measuring the activity and durability of oxygen evolution reaction (OER) catalysts using the rotating disk electrode (RDE) technique or other half-cell test configurations with liquid electrolyte. By subjecting the electrochemical cell or the electrolyte to ultrasonication while conducting a typical RDE-based measurement of the OER performance of a polycrystalline iridium-disk electrode, we demonstrate that it is the accumulation of microscopic oxygen bubbles that is responsible for the rapid OER catalyst performance decay observed during RDE experiments.
Hydrogen production from renewable resources and its reconversion into electricity are two important pillars toward a more sustainable energy use. The efficiency and viability of these technologies heavily rely on active and stable electrocatalysts. Basic research to develop superior electrocatalysts is commonly performed in conventional electrochemical setups such as a rotating disk electrode (RDE) configuration or H-type electrochemical cells. These experiments are easy to set up; however, there is a large gap to real electrochemical conversion devices such as fuel cells or electrolyzers. To close this gap, gas diffusion electrode (GDE) setups were recently presented as a straightforward technique for testing fuel cell catalysts under more realistic conditions. Here, we demonstrate for the first time a GDE setup for measuring the oxygen evolution reaction (OER) of catalysts for proton exchange membrane water electrolyzers (PEMWEs). Using a commercially available benchmark IrO 2 catalyst deposited on a carbon gas diffusion layer (GDL), it is shown that key parameters such as the OER mass activity, the activation energy, and even reasonable estimates of the exchange current density can be extracted in a realistic range of catalyst loadings for PEMWEs. It is furthermore shown that the carbon-based GDL is not only suitable for activity determination but also short-term stability testing. Alternatively, the GDL can be replaced by Ti-based porous transport layers (PTLs) typically used in commercial PEMWEs. Here a simple preparation is shown involving the hot-pressing of a Nafion membrane onto a drop-cast glycerol-based ink on a Ti-PTL.
This study reveals the source of discrepancy between the lifetime of oxygen evolution reaction (OER) catalysts determined by rotating disk electrode (RDE) measurements vs that obtained in a membrane electrode assembly (MEA) in an electrolyzer. We show that the accumulation of microscopic oxygen bubbles in the pores of the electro-catalyst layer during the OER takes place in both RDE and MEA measurements. However, this accumulation was found to be much more significant in RDE measurements, where the shielding of almost all of the catalyst active sites by gas bubbles leads to rapid performance deterioration. This decrease in performance, albeit largely reversible, was found to also induce irreversible catalyst degradation, which could be avoided if the accumulation of microscopic bubbles is prevented. This type of artefact results in vastly under-estimated catalyst lifetimes obtained by RDE experiments, resulting in values that are orders of magnitude shorter than those obtained using MEA measurements, and a hypothesis for this discrepancy will be proposed. Therefore, electrochemical cells with liquid electrolytes are not reliable for OER catalyst lifetime determination. This was paper 236 presented at the Atlanta, Georgia, Meeting of the Society, October 13–17, 2019.
A 3D RuO/MnO/carbon nanofiber (CNF) composite has been prepared in this study by a facile two step microwave synthesis, as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). RuO nanoparticles with the mean size of 1.57 nm are uniformly distributed on MnO nano-rods grown on electrospun CNFs. The electrocatalytic activity of the composites are investigated towards ORR/OER under alkaline condition. The ternary RuO/MnO/CNF composite showed superior ORR activity in terms of onset potential (0.95 V versus RHE) and Tafel slope (121 mV dec) compared to its RuO/CNF and MnO/CNF counterparts. In the case of OER, the RuO/MnO/CNF exhibited 0.34 V over-potential value measured at 10 mA cm and 52 mV dec Tafel slope which are lower than those of the other synthesized samples and as compared to state of the art RuO and IrO type materials. RuO/MnO/CNF also exhibited higher specific capacity (9352 mAh [Formula: see text]) than CNF (1395 mAh [Formula: see text]), MnO/CNF (3108 mAh [Formula: see text]) and RuO/CNF (4859 mAh g ) as the cathode material in Na-O battery, which indicates the validity of the results in non-aqueous medium. Taking the benefit of RuO and MnO synergistic effect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V versus Na/Na by using RuO/MnO/CNF, which is 388 mV more cathodic compared with CNF.
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