Limitations of aqueous model systems in the stability assessment of electrocatalysts for oxygen reactions in fuel cell and electrolysers

Cost and stability remain the greatest technical barriers to sustainably commercialize low temperature fuel cells and electrolysers. To tackle this problem, numerous advanced electrocatalysts have been proposed and tested in aqueous model systems. There are, however, increasing and evident concerns regarding the value of stability data coming from such studies. Hence, we anticipate that finding new approaches to assess degradation will be a major undertaking in the electrocatalysis research in the next years. Specifically, existing differences between fundamental and actual systems have to be addressed first: (a) electrode architecture; (b) electrolyte; (c) reactant and product transport; and (d) operating conditions. In this perspective, we discuss their influence on the stability of electrocatalysts using the challenging oxygen reduction and oxygen evolution reactions as illustrative cases

Fuel Cell Catalyst Layer Evaluation using a Gas Diffusion Electrode Half-Cell: Oxygen Reduction Reaction on Fe-N-C in Alkaline Media

Anion exchange membrane fuel cells (AEMFC) are a promising technology to allow the application of non-precious metal catalysts. While many of such catalysts have been identified in numerous recent fundamental research studies, reports evaluating these catalysts in realistic AEMFC catalyst layers together with stability assessments are rare. In the present work we show that fast and reliable evaluation and optimization of Fe-N-C-based oxygen reduction reaction (ORR) catalyst layers can be achieved using a gas diffusion electrode (GDE) half-cell approach. To set a benchmark in such measurements, a commercial Pajarito Powder Fe-N-C catalyst and commercial AemionTM ionomer are used. It is demonstrated that the ORR currents can be increased significantly by fine-tuning of the ionomer activation time. Furthermore, the optimized Fe-N-C-based catalyst layer shows very high stability with no observable performance deterioration after 5000 cycles in the 0.6 – 1.0 V vs. RHE potential window

Comparison of methods to determine electrocatalysts’ surface area in gas diffusion electrode setups: a case study on Pt/C and PtRu/C

In recent years, gas diffusion electrode (GDE) half-cell setups have attracted increasing attention, bridging the gap between fundamental and applied fuel cell research. They allow quick and reliable evaluation of fuel cell catalyst layers and provide a unique possibility to screen different electrocatalysts at close to real experimental conditions. However, benchmarking electrocatalysts’ intrinsic activity and stability is impossible without knowing their electrochemical active surface area (ECSA). In this work, we compare and contrast three methods for the determination of the ECSA: (a) underpotential deposition of hydrogen (H _upd ); (b) CO-stripping; and (c) underpotential deposition of copper (Cu _upd ) in acidic and alkaline electrolytes, using representative electrocatalysts for fuel cell applications (Pt and PtRu-alloys supported on carbon). We demonstrate that, while all methods can be used in GDE setups, CO-stripping is the most convenient and reliable. Additionally, the application of Cu _upd offers the possibility to derive the atomic surface ratio in PtRu-alloy catalysts. By discussing the advantages of each method, we hope to guide future research in accurately determining surface area and, hence, the intrinsic performance of realistic catalyst layers

Evaluating Electrocatalysts at Relevant Currents in a Half-Cell: The Impact of Pt Loading on Oxygen Reduction Reaction

In this work gas diffusion electrode (GDE) half-cells experiments are proposed as powerful tool in fuel cell catalyst layer evaluation as it is possible to transfer the advantages of fundamental methods like thin-film rotating disk electrode (TF-RDE) such as good comparability of results, dedicated elimination of undesired parameters etc. to relevant potential ranges for fuel cell applications without mass transport limitations. With the developed setup and electrochemical protocol, first experiments on different Pt/C loadings confirm excellent reproducibility. Thereby mass-specific current densities up to 30 A mg(Pt)(-1) at 0.6 V vs. RHE are achieved. From a methodological perspective, good comparability to single cell measurements is obtained after theoretical corrections for temperature and concentration effects. In comparison to previous studies with GDE half-cells, polarization curves without severe mass transport limitations are recorded in a broad potential window. All these achievements indicate that the proposed method can be an efficient tool to bridge the gap between TF-RDE and single cell experiments by providing fast and dedicated insights into the effects of catalyst layers on oxygen reduction reaction performance. This method will enable straightforward and efficient optimization of catalyst layer composition and structure, especially for novel catalysts, thereby contributing to the performance enhancements of fuel cells with reduced Pt loading. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0911915jes

Platinum Dissolution in Realistic Fuel Cell Catalyst Layers

Pt dissolution has already been intensively studied in aqueous model systems and many mechanistic insights were gained. Nevertheless, transfer of new knowledge to real‐world fuel cell systems is still a significant challenge. To close this gap, we present a novel in‐situ method combining a gas diffusion electrode (GDE) half‐cell with inductively coupled plasma mass spectrometry (ICP‐MS). With this setup, Pt dissolution in realistic catalyst layers and the transport of dissolved Pt species through Nafion membranes are evaluated directly. We observe that (i) specific Pt dissolution is increasing significantly with decreasing Pt loading, (ii) in comparison to experiments on aqueous model systems with flow cells, the measured dissolution in GDE experiments is considerably lower and, (iii) by adding a membrane onto the catalyst layer, Pt dissolution is reduced even further. All these phenomena are attributed to the varying mass transport conditions of dissolved Pt species, influencing re‐deposition and equilibrium potential

On the effect of anion exchange ionomer binders in bipolar electrode membrane interface water electrolysis

Bipolar membranejelectrode interface water electrolyzers (BPEMWE) were found to outperform a protonexchange membrane (PEM) water electrolyzer reference in a similar membrane electrode assembly(MEA) design based on individual porous transport electrodes (PTE) and a free-standing membrane. Wepresent a detailed study on bipolar interfaces between anion exchange ionomer (AEI) based anodecatalyst layers in direct contact with a PEM aiming to unravel influences of local pH, the water splittingbipolar interface and catalyst layer structure. It is conventionally accepted that AEIs used in anionexchange- and bipolar membrane water electrolysis conduct hydroxide anions and ensure a high pHenvironment in the catalyst layer. We have investigated the effect of different ionomers on the local pHat a metal surface and found a strong correlation with the pH of the surrounding solution rather than theionomer type. Thus, solely the use of an AEI cannot maintain high pH. A study on BPEMWEs revealedstrong indications for the co-existence of a water dissociating bipolar interface, and an acidic oxygenevolution mechanism. The superior performance compared to a PTE-based PEM water electrolyzerseems to stem from reduced contact resistances due to adhesive effects between the oppositelycharged polymers. Our study shows that the bipolar approach can be utilized to make PTE-basedelectrolyzers competitive to commonly employed catalyst coated membranes