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.
In this study, a commercial IrO2/TiO2 catalyst (75 wt% Ir, named “Benchmark”) for the oxygen evolution reaction (OER) is compared to a newly developed IrO(OH)x/TiO2 catalyst (45 wt% Ir, named “P2X”). Due to its lower Ir packing density and higher OER activity vs the Benchmark catalyst (440 vs 12 A gIr −1 at 1.43 ViR-free), the P2X catalyst shows an improved PEM (proton exchange membrane) water electrolyzer performance at ≈9 times reduced Ir loading, however, only if a platinum-coated porous transport layer (PTL) at the anode is used. While the performance of membrane electrode assemblies (MEAs) with the Benchmark catalyst is unaffected when using an untreated titanium PTL, MEAs with the P2X catalyst perform poorly, which can be attributed to a contact resistance at the anode/PTL interface due to the low electrical conductivity of the P2X catalyst (0.7 S cm−1) vs the Benchmark catalyst (416 S cm−1) and the passivation of the Ti-PTL. A heat treatment procedure is used to transform the amorphous IrO(OH)x of the P2X catalyst into crystalline IrOx and, hence, increases its electrical conductivity. The optimum temperature for heat treatment to maximize electrical conductivity, OER activity and MEA performance will be evaluated.
One of the building blocks to transition to a fully renewable energy supply is the utilization of hydrogen as a replacement of fossil fuels and as a chemical energy storage/carrier medium. This requires the economical and sustainable generation of hydrogen by water electrolysis, whereby proton exchange membrane (PEM) water electrolyzers would enable much higher power densities compared to conventional electrolyzers based on liquid alkaline electrolytes [1]. However, one of the short-comings of PEM water electrolyzers (PEMWEs) is the need for expensive and resource-limited iridium based catalysts for the oxygen evolution reaction (OER), so that the large-scale global deployment of PEMWEs would require a substantial reduction of the iridium loading from currently ~1-2 mgIr/cm2 elelctrode to below ~0.05 mgIr/cm2 elelctrode [2]. In this contribution, we will discuss the technical challenge to reduce the iridium loading using currently employed iridium catalysts, which is related to the high iridium packing density in the electrode (in units of gIr/cm3 electrode), so that for iridium loadings below ~0.4 mgIr/cm2 the electrode becomes too thin to allow for a homogenous electrode with sufficient in-plane electrical conductivity [3]. We will then present a catalyst concept that results in much lower iridium packing densities and that thus enables lower iridium loadings [4]. While such a catalyst exhibits a lower electrical conductivity than a currently employed benchmark catalyst, this drawback can be mitigated by utilizing porous transport layers at the anode that have a highly conductive coating [4]. The long-term stability of this novel type of iridium based OER catalyst will be examined in a 30 cm2 active area short-stack over ~3700 h; comparing the evolution of the OER mass activity and of the high frequency resistance corrected cell voltage with that of a benchmark catalyst that is evaluated in the same short-stack, which allows for mechanistic insights into the observed degradation rates [5]. References: [1] A. Buttler, H. Spliethoff; "Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review"; Renewable and Sustainable Energy Reviews 82 (2018) 2440. [2] M. Bernt, A. Weiß, M. Fathi Tovini, H. El-Sayed, C. Schramm, J. Schröter, C. Gebauer, H. A. Gasteiger; "Current Challenges in Catalyst Development for PEM Water Electrolyzers"; Chem. Ing. Tech. 92 (2020) 31. [3] M. Bernt, A. Siebel, H. A. Gasteiger; "Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings"; J. Electrochem. Soc. 165 (2018) F305. [4] M. Bernt, C. Schramm, J. Schröter, C. Gebauer, J. Byrknes, C. Eickes, H. A. Gasteiger; "Effect of the IrOx Conductivity on the Anode Electrode/Porous Transport Layer Interfacial Resistance in PEM Water Electrolyzers"; J. Electrochem. Soc. 168 (2021) 084513. [5] M. Möckl, M. Ernst, M. Kornherr, F. Allebrod, M. Bernt, J. Byrknes, C. Eickes, C. Gebauer, A. Moskovtseva, H. A. Gasteiger; "Durability investigation and benchmarking of a novel iridium catalyst in a PEM water electrolyzer at low iridium loading"; manuscript to be submitted. Acknowledgements: This work was conducted within the framework of the Kopernikus P2X project funded by the German Federal Ministry of Education and Research (BMBF).
Electrochemical impedance spectroscopy (EIS) is a powerful and versatile tool to investigate interfaces in batteries. In order to disentangle the anode and cathode contributions from the full-cell impedance, a reference electrode (RE) is required. In the field of batteries based on liquid electrolytes, the concept of a RE has become a widespread tool for the EIS analysis of small-scale cells [1]. However, there are only very few reports on the use of a three-electrode setup with a reference electrode for all-solid-state batteries (ASSBs) [2,3], which is due to the complexity of integrating a RE with a suitable geometry in the typical ASSB test cells that are based on a compressed electrolyte pellet (further on referred to as bulk-type ASSB cells), since for artifact-free single-electrode impedance spectra, the RE should be placed between the electrodes and should be thin compared to the thickness of the pellet. In contrast to the widely used bulk-type ASSB cells, a recently available alternative construction is offered by the use of free-standing separator sheets based on a solid electrolyte / polymer binder composite (further on referred to as sheet-type ASSB cells),[4,5] in which case a micro-RE can be placed between two separator sheets, in analogy to the micro-RE concept used with batteries based on liquid electrolytes [1]. In this study, we use sheet-type separators based on a composite consisting of Li6PS5Cl (LPSCl) solid electrolyte and a hydrogenated nitrile butadiene rubber (HNBR) binder to build ASSB pouch cells that include a gold wire micro-RE (µ-GWRE). We show that upon in-situ lithiation of the µ-GWRE a stable reference potential is obtained and that artifact-free single-electrode impedance spectra can be obtained, analogous to what we had found previously for a µ-GWRE in a lithium ion battery with liquid electrolyte.[1] Figure 1 shows both half-cell impedance spectra of an InLi | separator sheet | Li cell. The sum of both half-cell impedances (blue) is identical to the full-cell impedance (green) and now impedance loops or other common artefacts are observed for the InLi (black) and the Li (red) electrodes, indicating the viability of this setup to determine single-electrode impedances. Since the InLi electrode is commonly used as counter electrode (CE) for ASSB testing cells, we will also use this setup to investigate the potential stability of InLi alloys and their impedance evolution upon lithiation and delithiation. Acknowledgements: This work was carried out as part of the research project “Industrialisierbarkeit Festkörperelektrolytzellen”, funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy. References: [1] Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind and H. A. Gasteiger, J. Electrochem. Soc., 163 (10) A2265-A2272 (2016). [2] Dougas, Y. Dupraz, E. Quemin, T. Koc, and J.-M. Tarascon, J. Electrochem. Soc., 168 (9), 090508 (2021). [3] J. Nam, K. H. Park, D. Y. Oh, W. H. An and Y. S. Jung, J. Mater. Chem., 6, 14867-14875 (2018). [4] Riphaus, P. Strobl, B. Stiaszny, T. Zinkevich, M. Yavuz, J. Schnell, S. Indris, H. A. Gasteiger and S. Sedlmaier, J. Electrochem. Soc., 165 (16) A3993-A3999 (2018). [5] Sakuda, K. Kuratani, M. Yamamoto, M. Takahasi, T. Takeuchi and H. Kobayashi, J. Electrochem. Soc., 164 (12) A2474-A2478 (2017). Figure 1: Impedance Spectra of an InLi | separator sheet | Li pouch cell (4 cm2 electrode area) with a µ-GWRE recorded at open circuit voltage (OCV) with a voltage amplitude of 10 mV between 100 kHz and 100 mHz. The half-cell impedance spectra of the InLi working electrode (WE) and of the Li counter electrode (CE) are displayed in black and red, respectively. The sum of the individual half-cell impedance spectra (blue) matches with the full-cell impedance spectrum (green). The separator sheet is a » 400 µm thick composite of LPSCl and HNBR and no impedance loops are observed, demonstrating the successful implementation of the RE. Figure 1
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