Catalyst Dissolution Analysis in PEM Water Electrolyzers during Intermittent Operation

Polymer electrolyte membrane (PEM) electrolysis is widely considered a promising technology for energy storage and clean hydrogen production, due to the advantages it has over state-of-the-art alkaline water electrolysis.1 Typically, PEM electrolysers can operate at higher current densities than their alkaline counterparts, have a more robust design, and due to the zero-gap design, exhibit smaller Ohmic losses. Furthermore, PEM electrolysers can yield hydrogen at elevated pressures, eliminating the need for compression. Nevertheless, a series of challenges hinder the large-scale commercial implementation of PEM electrolysers. One of the most critical drawbacks is the need for noble metal electrocatalysts, especially at the anode of the cell, such as iridium or ruthenium, to catalyze the oxygen evolution reaction (OER) (1).2H2O → O2 + 4H+ + 4e- E° = 1.23 V vs NHE (1)These metals are used due to their high catalytic activity and long-term stability in the corrosive acidic environment of the PEM cells and their replacement by more available materials is difficult. Over the years, the main efforts of the scientific community towards solving this challenge were focused on increasing the noble metal dispersion, by using various high surface area catalyst supports, using core-shell catalysts, with the aim to minimize the amount of noble metal, modifying the morphology of the electrode, to improve mass transport limitations and electric contact between components and maximize noble metal utilization.2-4 Antimony-doped tin oxide has been reported as a stable OER catalyst support, offering sufficient electronic conductivity at the same, in order to minimize Ohmic losses.5 Furthermore, recent reports indicate that SnO2 does not only act as a catalyst support but takes part in catalysis.6 Building upon literature reports, we prepared a series of Sn/Ir OER catalysts by a two-step sodium borohydride reduction method, in which Sn nanoparticles were deposited over the surface of iridium black particles, decorating it. Catalysts with varying Sn content were incorporated in membrane electrode assemblies (MEAs) and their cell performance was evaluated in a single cell PEM electrolyser. We found that catalysts with up to 20 wt% tin content exhibited an improved cell performance compared to MEAs with similar iridium black loading. We modeled Ohmic, kinetic and mass transport overpotential contributions to experimentally acquired polarization curves and identified that the better cell performance arises mainly from the higher intrinsic catalytic activity, reflected by a higher apparent exchange current density of Sn/Ir OER catalysts compared to Ir black catalysts. The Sn/Ir MEAs also displayed lower mass transport overpotentials, which indicate improved transport in the catalyst layer. This can be due to a more porous structure and less agglomeration of iridium particles induced by the presence of tin over their surface.Interestingly, this performance improvement was only observed for iridium black catalysts. We prepared a series of IrO2-SnO2 catalysts by solution combustion synthesis method, which is known to yield high surface area disperse oxide materials. Comparing the performance of these catalysts to commercial iridium oxide catalysts in the PEM electrolysis cell, the MEAs with SnO2-containing catalysts displayed worse cell performance than MEAs with commercial iridium oxide. Modelling the overpotential contributions showed a lower apparent exchange current density, indicating a lower intrinsic catalytic activity of IrO2-SnO2 catalysts, as well as a higher Ohmic resistance, due to SnO2 addition. These observations indicate the importance of iridium oxidation state for harnessing the promotion effect of tin addition and give important insights into designing more efficient OER catalysts for PEM electrolysis cells.

To achieve widespread commercialization of proton exchange membrane (PEM) water electrolyzers, the optimization of iridium (Ir) utilization is crucial. Traditional full-cell-based approaches are time-consuming and labor-intensive. In this work, the feasibility of using a gas diffusion electrode (GDE) half-cell as an alternative to full-cell setups for accelerated investigation of Ir-oxide-containing anode catalyst layers (CLs) is scrutinized. Using CLs composed of Ir oxides of different intrinsic oxygen evolution reaction (OER) activity as a probe, we show that a GDE can successfully reveal the differences in the performance of the CLs. Comparison of the results obtained in the GDE to those from rotating disk electrode (RDE) and full-cell membrane electrode assembly (MEA) measurements indicate that GDE data can closely mimic both setups. However, essential discrepancies are observed between GDE and MEA, which are linked to differences in the catalyst layer | membrane interface and the presence of liquid electrolyte in the GDE setup. Our findings reveal that even though the direct comparison of the OER performance to full-cell measurements is still partially hampered, GDE half-cell setups can already be used for fundamental assessments and accelerated screening of electrocatalysts and CLs at relevant current densities up to 1.5 A cm−2.

Anion exchange membrane water electrolysis (AEMWE) is a promising technology to produce hydrogen from low-cost, renewable power sources. Recently, the efficiency and durability of AEMWE have improved significantly due to advances in the anion exchange polymers and catalysts. To achieve performances and lifetimes competitive with proton exchange membrane or liquid alkaline electrolyzers, however, improvements in the integration of materials into the membrane electrode assembly (MEA) are needed. In particular, the integration of the oxygen evolution reaction (OER) catalyst, ionomer, and transport layer in the anode catalyst layer has significant impacts on catalyst utilization and voltage losses due to the transport of gases, hydroxide ions, and electrons within the anode. This study investigates the effects of the properties of the OER catalyst and the catalyst layer morphology on performance. Using cross-sectional electron microscopy and in-plane conductivity measurements for four PGM-free catalysts, we determine the catalyst layer thickness, uniformity, and electronic conductivity and further use a transmission line model to relate these properties to the catalyst layer resistance and utilization. We find that increased loading is beneficial for catalysts with high electronic conductivity and uniform catalyst layers, resulting in up to 55% increase in current density at 2 V due to decreased kinetic and catalyst layer resistance losses, while for catalysts with lower conductivity and/or less uniform catalyst layers, there is minimal impact. This work provides important insights into the role of catalyst layer properties beyond intrinsic catalyst activity in AEMWE performance.

The usage of scarce and expensive iridium as a catalyst for the acidic oxygen evolution reaction (OER) represents a significant barrier to up-scaling the deployment of proton exchange membrane water electrolysers (PEM-WEs). Accordingly, strategies such as nanostructuring catalysts to increase the electrochemical surface area (e.g. nanoparticles, single-atom catalysts, nanoclusters) as well as investigating mixed metal alloys and oxides (e.g. perovskites, pyrochlores) have been developed with promising half-cell performances. One further approach that has recently gained attention is the deployment of catalyst-supports whereby a high surface area support is used to scaffold the dispersed iridium catalyst.1 Analogous to the selection of a catalyst, the catalyst support must withstand the highly corrosive and oxidising conditions of a PEM-WE anode. Thus, metal oxides such as titanium dioxide are commonly deployed as catalyst-supports for acidic OER. The catalyst-support moiety must also be sufficiently conductive when integrated into the PEM-WE catalyst layers. Otherwise, the cell voltages required to generate sufficient activity will render the device impractical. Accordingly, strategies such as doping, or the addition of conductive layers between the catalyst and support have been investigated.2 In this study we investigate a unique support morphology that affords promising OER activity and stability in acidic electrolyte with high iridium mass activities. Specifically, we deploy a hollow TiO2 catalyst support,3 coated with a Au/Pd interlayer to provide electrical conductivity. The OER activity and stability is initially screened in a rotating disk electrode (RDE) configuration as a function of thermal treatments (reducing vs oxidising) as well as with at various Au/Pd (1 – 5 wt%) and Ir loadings (35 – 50 wt%). Our highest performing catalyst have a mass activity of around 480 A/gIr at 1.60 V vs RHE. The catalysts are subsequently investigated in a membrane electrode assembly (MEA) and subject to 10k accelerated stress test stability measurements. To understand the role of the support morphology, Au/Pd interlayer and iridium loadings on electrochemical performance, we also characterise the synthesised catalysts by (scanning) transmission electron microscopy (STEM/TEM), X-ray diffraction (XRD), X-ray photoelectron microscopy (XPS) and electrical conductivity measurements. Collectively, we report the critical role of the Au/Pd interlayer to enable such high electrochemical performance. We also show that the hollow structure of the TiO2 support may be enhancing mass transport in the catalyst layers. References 1 A. S. Pushkarev, I. V Pushkareva and D. G. Bessarabov, Energy & Fuels, 2022, 36, 6631–6625.2 Y. N. Regmi, E. Tzanetopoulos, G. Zeng, X. Peng, D. I. Kushner, T. A. Kistler, L. A. King and N. Danilovic, ACS Catal., 2020, 10, 13125–13135.3 U. S. Jonnalagadda, X. Su and J. J. Kwan, Ultrason. Sonochem., 2021, 73, 105530.

To achieve carbon neutrality, extensive research is ongoing to produce green hydrogen via water electrolysis combined with renewable energy sources. Understanding and developing catalysts for membrane electrode assembly (MEA) type water electrolysis systems, such as proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE), are critical steps toward this goal. Therefore, this presentation will focus on catalyst development for PEMWE and AEMWE, with emphasis on cost-effective catalysts.For oxygen evolution reaction (OER) catalysis in PEMWE, the primary challenge is to minimize iridium (Ir) usage for sustainable green hydrogen production. Layered monoclinic iridium nickel oxide (IrNiOx) platelets were synthesized using a molten salt method and employed for the OER. These platelets exhibited high OER activity with reduced Ni dissolution in acidic conditions. When applied in MEA, they demonstrated enhanced interconnectivity within the catalyst layer, promoting electron transfer. Even at low Ir loading (0.2 mgIr cm-2), the platelets showed good performance with an initial cell voltage of 1.70 V at 1 A cm-2 and minimal degradation over 100 hours. This highlights the effectiveness of incorporating transition metals into Ir oxide to reduce Ir usage in PEMWE.For oxygen evolution reaction (OER) catalysis in AEMWE, attention is focused on nickel iron (NiFe) hydroxide catalysts for their high activity and stability in alkaline conditions. However, the lack of initial electrical conductivity of as-prepared NiFe LDH limits its potential as an electrocatalyst. To address this issue, a monolayer structuring approach was proposed. This method improved mass transport to allow a high energy conversion efficiency of 72.6% with exceptional stability over 50 hours at 1 A cm-2. Additionally, lack of initial electrical conductivity hinders determination of electrochemically active surface area (ECSA) of NiFe LDH using conventional double layer capacitance method. Use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface was thus investigated. This allowed the estimation of ECSA and intrinsic activity of NiFe LDH, validating the methodology through rigorous catalyst loading and support studies.For hydrogen evolution reaction (HER) catalysis in AEMWE, replacing platinum (Pt) with Ni-based alloys containing molybdenum (Mo) species has shown promise. Dynamic active sites of Ni catalysts with Mo species require innovative approaches to maintain initial performance, warranting material innovation or careful manipulation of operating protocol. The approaches in which this is made possible is shortly presented, together with the innovations that allows probing of such phenomenon.

Proton Exchange Membrane Water Electrolysis (PEMWE) is considered one of the most promising form of green hydrogen production that could lead to a successful decarbonization of industrial processes, with a potential of at least 10% reduction of CO2 emissions of the total abatement needed by 2050 [1]. Since the oxygen evolution reaction (OER) catalyst on the anode side is dominated by Iridium (Ir)-based materials, scarce and expensive, reducing the loading to 0.1 - 0.2 mgIr/cm2 is critical to ensure cost-effectiveness in large-scale PEMWE deployment for hydrogen production. As outlined by the U.S. Department of Energy (DOE), the target Ir content in electrodes should be reduced to less than 0.5 mg/cm2 by 2026 [2].To achieve low Ir loadings, Smoltek Hydrogen’s aim is to significantly increase the available surface area of the Porous Transport Layers (PTLs) by growing vertically its patented Carbon NanoFiber (CNFs) structure [3], protected with a corrosion-resistant and conductive layer and electrodeposited with OER catalyst (Fig. 1a), creating a unique Porous Transport Electrode (PTE). With this electrodeposition method, we obtain a thin continuous Ir layer with a strong adhesion along the nanostructures, a high electrical conductivity [4] reducing contact resistance between the catalyst layer and PTL substrates, and a maximum utilization of the catalyst.This work will concentrate on the successful reduction of Ir loading to 0.1 mg/cm2 in the PTE, demonstrating excellent mass activity in Half-Cell tests (Fig. 1c). The polarization curves of electrodes with low Ir loading obtained in Full-Cell exhibited superior performance compared to cells with significantly higher Ir loading. The aim is now to reach constant current durability tests (Fig. 1d) for 2000 hours or more on PEMWE operation using 0.1 mg/cm2 Ir loaded PTEs, with minimum degradation and overcoming mass transport limitations, particularly at higher current densities. References IEA (2021), Net Zero by 2050, IEA, Paris, https:/www.iea.org/reports/net-zero-by-2050https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysisXin Wen. Enhancing Efficiency and Durability of PEM Water Electrolysis with Low Iridium Loading through Nanofiber-Modified Porous Transport Electrodes, 2024, ECS Meeting s. MA2024-01Krivina et al. Adv.Mater., 2022, 34, 2203033. Figure 1

Proton exchange membrane (PEM) water electrolysis offers significant potential for sustainable production of green hydrogen due to its high current density, superior hydrogen purity, and excellent conversion efficiency. However, widespread industrial implementation remains challenging because state-of-the-art oxygen evolution reaction (OER) catalysts heavily rely on iridium, a precious metal with limited global supply and high costs.1–4 Therefore, considerable research efforts have been dedicated to developing new catalysts requiring lower iridium loadings. Although PEM electrolysis typically operates at elevated temperatures (50-80°C),1 most studies assessing catalyst activity and stability have been performed at room temperature, and the limited available temperature-dependent studies primarily focus on alkaline environments.5,6 In this work, we have systematically investigated the temperature-dependent activity and stability of two representative Ir-based catalysts: a commercial IrO₂ catalyst and a supported core-shell Ir@IrOₓ catalyst. Using a high-temperature rotating-disk electrode (RDE) setup, electrochemical assessments of the activity and stability of the catalysts were performed across a range of operating temperatures from 20°C up to 80°C. We investigated how the increased operating temperature affects the OER activities of these two catalysts, observing not only enhanced kinetics with the increased working temperatures but also altered reaction mechanisms, as indicated by the Tafel analysis of the OER polarization curves. Furthermore, by employing an accelerated stress test protocol, we evaluated the stability of the two catalysts under varied temperatures. Comprehensive in-situ and post-mortem characterization techniques, including in-situ XRD, XPS (Figure 1), TEM, and ICP, were employed to investigate the structural changes of the catalysts during the stability tests, which revealed the accelerated degradation at higher working temperatures, but also the surprisingly high OER stability of the core-shell Ir@IrOx structure under high working temperatures.Figure 1: Comparison the Ir 4f XPS spectra of the sample before and after the stability test at 60 °C.References Carmo, M., Fritz, D. L., Mergel, J. & Stolten, D. A comprehensive review on PEM water electrolysis. Int J Hydrogen Energy 38, 4901–4934 (2013). Wang, C. & Feng, L. Recent advances and perspectives of Ir-based anode catalysts in PEM water electrolysis. Energy Advances 3, 14–29 (2024). Kibsgaard, J. & Chorkendorff, I. Considerations for the scaling-up of water splitting catalysts. Nature Energy 2019 4:6 4, 430–433 (2019). Reier, T., Oezaslan, M. & Strasser, P. Electrocatalytic oxygen evolution reaction (OER) on Ru, Ir, and pt catalysts: A comparative study of nanoparticles and bulk materials. ACS Catal 2, 1765–1772 (2012). Shi, G. et al. Temperature Dependence of Oxygen Evolution Reaction Activity in Alkaline Solution at Ni-Co Oxide Catalysts with Amorphous/Crystalline Surfaces. ACS Catal 12, 14209–14219 (2022). Lyu, X., Li, J., Yang, J. & Serov, A. Significance of slight ambient temperature variation on the electrocatalyst performance toward oxygen evolution reaction. J Environ Chem Eng 11, 111492 (2023). Figure 1

Membrane electrode assemblies (MEA) and specifically the anode catalyst are of particular interest in PEM water electrolysis (WE) research as they are considered key components for the capital expenditure and operating expenditure of a PEMWE system (1). An effective method to understand the operation of the MEA is to apply diagnostic tools during the operation of the electrolysis cell. Previous studies in this regard have been mainly limited to the total cell voltage. However, the individual electrode behavior cannot be investigated in the standard setup. To improve the MEA design, the individual electrode behavior is crucial. This can be captured by using a reference electrode (RE).In this work, a salt bridge is used for the first time as RE concept for polarization curves and electrochemical impedance spectroscopy (EIS). The implementation is shown in figure 1.a. Via a porous transport layer impregnated with Nafion, the reference electrode can access the ionic potential of the catalyst layer under investigation. This isolates the kinetic overvoltage from other cell voltage contributions such as ohmic membrane losses, see figure 1.b. The experiments in this study include polarization curves with conventional materials and operating conditions. In addition, EIS is performed to test the applicability of the measurement method in RE operation. Exemplary results are shown in Figure 1.c.In addition, a previous analysis of RE from fuel and electrolytic cells is presented and a classification by electrode type and positioning is made. The key results are subsequently summarized. First, a distinction is made according to the principle behavior of the electrode or its possible configurations. The principal electrode behavior includes the dynamic hydrogen electrode (DHE), the quasireference electrodes, external REs and other configurations. Concrete concepts follow from this. For example, the DHE can be implemented with two platinum wires. By applying a current in the microampere range, the hydrogen evolution reaction takes place in the presence of water. The platinum wire in hydrogen atmosphere becomes the RE (2). Quasireference electrodes are very commonly used with ionic liquids. They are often metal wires (3). A platinum wire is used in a recent work for recording individual electrochemical impedance spectra of anode and cathode (4). Any type of RE can be used as an external RE, such as the silver-silver chloride electrode. Another possibility is to create a free-standing catalyst strip by laser ablation of the MEA. This strip can be used as a reversible electrode to measure the voltage difference with the active catalyst layer participating in the reaction (5).In addition to the behavior of the electrode, a subdivision is made into the position of the RE. In order to separate the potential of one of the two electrodes from the total potential, the measurement can be made as close as possible to the electrode under investigation. This can be implemented by a direct measurement in the catalyst layer (6) or a special geometry of the active area (7). Alternatively, the potential can be measured at the membrane and a correction of the ohmic membrane losses can be used to infer the electrode potential to be investigated. Here, too, two implementations are possible. On the one hand, the RE can be positioned between two membrane halves (4). On the other hand, the membrane can be contacted outside the active area (8).One requirement chosen for the presented concept is the applicability of the RE independent of the geometry of the MEA. Furthermore, the RE should be insensitive to misalignment of the catalyst layers. The exact alignment of the anodic and cathodic catalyst layers is challenging and even small deviations lead to a shift of the potential in the membrane (8). For the two reasons mentioned above, the concept of salt bridge with external RE is used in the direct approach at the electrode.References The national hydrogen strategy (Juni 2020).M. V. Lauritzen, P. He, A. P. Young, S. Knights, V. Colbow and P. Beattie, Journal of New Materials for Electrochemical Systems, 10(3), 143–145 (2007).G. Inzelt, A. Lewenstam, F. Scholz and F. G. K. Baucke, Editors, Handbook of reference electrodes, Springer, Berlin (2013).A. Hartig-Weiß, M. Bernt, A. Siebel and H. A. Gasteiger, J. Electrochem. Soc., 168(11), 114511 (2021).D. Gerteisen, J Appl Electrochem, 37(12), 1447–1454 (2007).E. Brightman, J. Dodwell, N. van Dijk and G. Hinds, Electrochemistry Communications, 52, 1–4 (2015).A. A. Kulikovsky and P. Berg, J. Electrochem. Soc., 162(8), F843-F848 (2015).G. Li and P. G. Pickup, Electrochimica Acta, 49(24), 4119–4126 (2004). Figure 1

Hydrogen plays a significant role in the transition of the global energy system towards achieving net-zero emissions. According to the “Hydrogen for Net-Zero” report by Hydrogen Council and McKinsey & Company, the demand for clean hydrogen could reach 140 metric tons (MT) by 2030, with a further increase to 660 MT by 2050. For future high-volume production of green hydrogen, Proton Exchange Membrane Water Electrolysis (PEMWE) is a crucial technology due to its advantages of high-purity hydrogen, large operational current densities, and great energy conversion efficiency [1]. However, the use of precious metal-based catalysts for the sluggish oxygen evolution reaction (OER) on the anode side hinders the high-volume manufacturability of PEMWE. To ensure cost-effectiveness in large-scale PEMWE deployment for hydrogen production, developing strategies to reduce Iridium loading to 0.1 - 0.2 mg Ir/cm2 without compromising overall performance is essential. The challenge of reducing Ir loading also involves improving or maintaining the electrical contact between the catalyst layers and the substrate [2-3].Smoltek Hydrogen has made significant strides in achieving low iridium loading through our innovative Porous Transport Electrode (PTE) concept. This approach involves modifying a Porous Transport Layer (PTL) by growing vertically aligned Carbon Nanofibers (CNFs) using our in-house patented technology, efficiently increasing surface area [4]. Following this modification, a protective layer of corrosion-resistant and conductive material (e.g., Platinum) is applied to the CNFs. Subsequently, an OER catalyst layer is deposited utilizing a three-electrode cathodic electrodeposition method (Fig. 1a and 1b). The electrodeposited Ir catalyst exhibits strong adhesion to the CNF PTEs, resulting in a high electrical conductivity [5].In this work, we successfully reduced Ir loadings to 0.1 mg/cm2 in the PTE, demonstrating excellent mass activity in the half-cell tests (Fig. 1c). In the PEMWE tests, the polarization curves of electrodes with low Ir loading exhibited superior performance compared to cells with significantly higher Ir loading (Fig. 1d). The next step of our research involves conducting constant current durability tests for over 600 hours on PEMWE operation using low Ir loaded PTEs. The aim is to minimize degradation and overcome mass transport limitations, particularly at higher current densities.These unique PTEs developed by Smoltek Hydrogen, with CNF technology, maximize the electrode surface area and pave a new way to increase the utilization ratio of catalyst surface at a low loading amount, which makes it a competitive anode material in today’s PEMWE market. References https://www.iea.org/reports/hydrogen-2156#dashboardMöckl et al. J. Electrochem. Soc., 2022, 169, 064505.Bernt et al. Chem. Ing. Tech., 2020, 92, 31-39.https://www.smoltek.com/applications/electrolyzers/Krivina et al. Adv.Mater., 2022, 34, 2203033.https://publications.jrc.ec.europa.eu/repository/handle/JRC104045 Figure 1

Water electrolysis has the potential to become a key element in coupling the electricity, mobility, heating and chemical sector via power-to-liquids or power-to-gas in a future sustainable energy system [1]. While proton exchange membrane (PEM) water electrolysis offer several advantages over traditional electrolysis technology, catalyst durability at low catalyst loading remains a barrier for industrial commercialization [2].Fundamental understanding of degradation mechanisms is crucial to elucidate the key parameters for high performance and durability of low-catalyst-loading electrodes. In this work, electron microscopy methods, including transmission electron microscopy/scanning transmission electron microscopy (TEM/STEM) and X-ray energy dispersive spectroscopy (XEDS) are developed to quantify the degradation of iridium- and platinum-based electrocatalysts in membrane electrode assemblies (MEAs).This work aims for a comprehensive understanding of the degradation of PEM water electrolyzer with a low catalyst loading of 0.3 mg cm−2 Pt on the cathode and 0.08 mg cm−2 Ir on the anode. A long-term electrolysis operation of ~4500 h at 1.8 A cm−2 and 2.76 MPa differential pressure was performed. Degradation studies on such low catalyst loading and at high operating current are still lacking in this field. For the first time, the mechanism of cathode degradation is proposed using established physical models from two aspects: (1) direct Pt dissolution from nanoparticles due to Gibbs-Thomson relation [3] and (2) transient Pt dissolution due to rapid Pt oxide reduction [4] during start/stop of operation. Furthermore, iridium dissolution and subsequent re-deposition on the cathode constitute a major portion (42%) of the anode catalyst loss [5]. As Ir dissolves and migrates through the membrane, Pt-Ir precipitates are formed in the membrane [5] . The overall picture of degradation mechanisms and the distribution of platinum and iridium across the MEA are provided in Figure 1 [5]. In summary, quantitative analysis from microscopic imaging (TEM/STEM) and chemical analysis (XEDS) is an effective tool to characterize catalyst morphology, dissolution, and re-distribution in degraded MEAs. Quantitative results are helpful defining the target of research to improve the performance and durability of PEM electrolyzers.Figure 1. Schematic diagram depicting the mechanism leading to the formation of Pt-Ir precipitates and Ir band in the membrane, adapted from [5]. The distribution of Pt and Ir at different regions of the MEA is shown as weight percent. Irn+ is used to universally represent ionic Ir species.

In times of an increasing demand for energy production through renewable but fluctuating energy sources, such as wind or solar energy, hydrogen as an energy carrier becomes more and more important. Proton-exchange membrane water electrolysis (PEM-WE) is a suitable and already quite advanced technique for sustainable production of hydrogen.1 However, coupling a PEM-WE with intermittent renewable energy sources will induce frequent current interrupts of the PEM-WE system. These events can potentially lead to rapid degradation of the membrane electrode assemblies (MEAs) and hence, a thorough understanding of the underlying mechanisms is crucial to assess the stability and lifetime of a PEM-WE and to choose appropriate operating conditions. In this work, we present a test protocol involving operation at high (3 Acm-2 geo) and low (0.1 Acm-2 geo) current density, alternating with current interrupts during which the system remains at the open circuit voltage (OCV). Previous studies in our lab revealed that the permeation of hydrogen through the membrane into the anode compartment during extended OCV periods can cause the reduction of IrOx 2, the most commonly used anode catalyst for the oxygen evolution reaction (OER) owing to its decent activity and high stability. During a subsequent start-up of the PEM-WE, metallic Ir is oxidized to a hydrous Ir-oxide. The transformation of the catalyst surface was probed by cyclic voltammetry (CV) during the degradation test. While the initial CV (Fig. 1, black curve) typical for crystalline IrOx is essentially featureless, CVs recorded after ten current-interrupt cycles revealed the formation of hydrogen under-potential-deposition (H-UPD) features (region 1, blue curve), which are characteristic for metallic Ir electrodes.3 The redox-features evolving at ≈0.8 V are characteristic of an amorphous, hydrous Ir-oxide (region 2).4 The appearance of these hydrous Ir-oxide features indicates a change in hydration state as well as in surface chemistry, which is known to affect both the OER activity and the stability of IrOx.5 Amorphous hydrous Ir-oxide exhibits higher OER activity but lower stability compared to crystalline thermally grown IrOx. Interestingly, the polarization curve recorded directly after IrOx reduction during an OCV period shows a lower cell voltage (i.e., improved OER activity), thus supporting the formation of a hydrous Ir-oxide. However, since this hydrous oxide is less stable, a rapid decay of cell performance over an extended number of OCV cycles due to Ir dissolution/re-precipitation occurs. In summary, this study will provide a better understanding of the MEA degradation mechanism occurring over an extended number of OCV cycles, which could result in a PEM-WE system when operated with intermittent renewable energy sources. This implies that hybridization strategies are required to maximize PEM-WE durability. In addition, PEM-WE load cycles to OCV may serve as an accelerated aging test. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A Comprehensive Review on Pem Water Electrolysis. International Journal of Hydrogen Energy 2013, 38, 4901-4934.Weiß, A.; Bernt, M.; Siebel, A.; Rheinländer, P. J.; Gasteiger, H. A., ECS Meet. 232 2017, Abstr. # I01F-1648.Woods, R., Hydrogen Adsorption on Platinum, Iridium and Rhodium Electrodes at Reduced Temperatures and the Determination of Real Surface Area. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1974, 49, 217-226.Pickup, P. G.; Birss, V. I., A Model for Anodic Hydrous Oxide Growth at Iridium. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 220, 83-100.Reier, T.; Teschner, D.; Lunkenbein, T.; Bergmann, A.; Selve, S.; Kraehnert, R.; Schlögl, R.; Strasser, P., Electrocatalytic Oxygen Evolution on Iridium Oxide: Uncovering Catalyst-Substrate Interactions and Active Iridium Oxide Species. Journal of The Electrochemical Society 2014, 161, F876-F882. Figure 1 Cyclic Voltammograms (CVs) recorded at 50mV/s during the accelerated degradation before cycling (black) and after 10 cycles (blue) at 80 °C, ambient pressure and 5 mL min-1 H2O (anode)/ 50 mL min-1 H2 (cathode) for an MEA with ~1.6 mgIr cm-²MEA anode and ~0.3 mgPt cm-²MEA cathode loading using a Nafion® 212 (~50 µm) membrane Figure 1

Water electrolysis is a crucial technology for large-scale hydrogen generation, that is required for the transition to a fossil fuel-free energy system. Even though water electrolysis systems are already deployed in a limited capacity, the technology is largely constrained to liquid alkaline electrolysis. Proton exchange membrane (PEM) electrolysis could pose an alternative but it is still hindered by high investment costs, in-part due to its reliance on scarce noble-metal catalysts. Alternative structural designs of the anode catalyst layer (CL) could reduce Iridium loading of the whole system and thus accelerate its wide-spread application. In fuel cell research, it was already reported that a multi-layer design with varied ionomer content enhances performance of the CL and in-turn lowers required catalyst loading.[1,2,3]In this study, a gradient design for ionomer content is employed for anode CLs for the application in PEM water electrolysis. CLs are coated in a stacked multi-layer design via ultrasonic spray coating of Iridium/Nafion® suspensions. The target ionomer loadings (10 and 30 wt.%) are confirmed by a combination of thermo-gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The coating process yields homogenously loaded reference CLs as well as through-plane graded CLs with the desired ionomer loading. Additionally, scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) shows equal CL thickness across all ionomer loadings and spatially differentiated Nafion® content throughout the CLs thickness. Electrochemical characterizations are carried out in an electrolysis-adapted gas diffusion electrode (GDE) half-cell setup to allow for quick examination and prototyping of produced layers. Differences in electrochemical performance of these layers can be observed with one gradient design CL showing reduced overpotential across all current densities compared to the homogenous CLs. In-particular a high ionomer loading near the membrane improves performance, most likely due to increased proton conduction to the membrane and higher available pore volume near the porous transport layer.Further analysis is performed on electrochemical studies to deepen the understanding of the role of ionomer in anodic PEM water electrolysis CLs. Particularly, the investigation of protonic and electric conductivity through the CL is of interest, because this parameter is most influenced by ionomer loading.[4]

Oxygen evolution reaction (OER) catalyst stability metricsderivedfrom aqueous model systems (AMSs) prove valuable only if they aretransferable to technical membrane electrode assembly (MEA) settings.Currently, there is consensus that stability data derived from ubiquitousrotating disk electrode (RDE)-based investigations substantially overestimatematerial degradation mainly due to the nonideal inertness of catalyst-backingelectrode materials as well as bubble shielding of the catalyst byevolved oxygen. Despite the independently developed understandingof these two processes, their interplay and relative impact on intrinsicand operational material stability have not yet been established.Herein, we employ an inverted RDE-based approach coupled with onlinegas chromatographic quantification that exploits buoyancy and anodehydrophilicity existing under operating acidic OER conditions andexcludes the influence of bubble retention on the surface of the catalyst.This approach thus allows us to dissect the degradation process occurringduring the RDE-based OER stability studies. We demonstrate that thestability discrepancy between galvanostatic nanoparticle (NP)-basedRDE and MEA data does not originate from the accumulation of bubblesin the catalyst layer during water oxidation but from the utilizationof corrosion-prone substrate materials in the AMS. Moreover, we providemechanistic insights into the degradation process and devise experimentalmeasures to mitigate or circumvent RDE-related limitations when thetechnique is to be applied to an OER catalyst stability assessment.These findings should facilitate the transferability between AMS andMEA approaches and promote further development of the latter.

Proton exchange membrane (PEM) water electrolysis (WE) and fuel cells (FCs) stand out as promising technologies for hydrogen generation and utilization, leveraging renewable and sustainable energy sources. A pivotal element in both PEMFC and PEMWE is the membrane-electrode assembly (MEA), comprising catalyst layers (CLs) and a membrane. These CLs facilitate electrochemical reactions at the anode and cathode, crucial for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in PEMWE and PEMFC. The catalyst ink for CLs consists of electrocatalyst, ionomer dispersion, and additional solvents. Among these components, the choice of solvent significantly influences PEMFC and PEMWE performance, shaping the catalyst ink's properties. In this investigation, we formulated ionomer dispersions for CLs to enhance performance in PEMWE and PEMFC. We scrutinized the effects of solvents in terms of solvation energy, thermogravimetric analysis (TGA), particle size distribution (PSD), and electrochemical assessment. Acknowledgments This research was supported in part by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520) and by 2021 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.

Green hydrogen production through renewable energy-based water electrolysis is the most efficient technology for decarbonization, and demand has recently been rapidly increasing. Addressing this surge in demand requires the rapid development of reliable large-scale manufacturing methods for PEMWE membrane-electrode assemblies (MEAs).For reliable large-scale manufacturing, it is necessary to optimize electrode components, including catalysts, binders, solvents, and additives, as well as their dispersion methods. Particularly, ensuring the homogeneity and stability of the electrode slurry is crucial.In this study, we enhanced the stability of the electrode slurry to twice and five times that of the existing standard by selecting an appropriate dispersion method for the metal-oxide oxygen evolution reaction (OER) catalyst and by adjusting the OER catalyst (both support and particle size control), respectively. Additionally, we secured stable electrode coating characteristics by controlling the solvent composition and additives, based on the rheology of the slurry. By optimizing the composition, formulation, and preparation method of the electrode slurry to form an optimal catalyst layer, we achieved a 20% improvement in performance compared to commercial MEA products.