Fe-N-C catalysts are considered an earth-abundant alternative to Pt in cathodes of anion exchange membrane fuel cells, although their stability still requires improvement for further commercialization. The degradation of Fe-N-C during both load cycles and start-stop events must be understood and mitigated to minimize system costs. Several approaches have recently been proposed to improve the durability of Fe active species during the oxygen reduction reaction in acidic media. On the other hand, knowledge of the degradation of Fe-N-C catalysts during start-stop events of anion exchange membrane fuel cells remains scarce. In this work, we use a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry (GDE-ICP-MS) to quantify the Fe dissolution rates in the potential range between 0.93 and 1.5 V-RHE. It is shown that Fe dissolution accelerates with increased anodic potential and temperature, while it is independent of the presence/absence of O-2. The onset potential of Fe dissolution at room temperature agrees with the reported onset potentials of carbon corrosion and denitrogenation, C and N being oxidized to gaseous COx and NOx species, respectively. This correlation supports that the electrochemical oxidation of the N-C matrix triggers the observed catalyst demetalation in these conditions. Using a set of ex situ physicochemical characterization techniques, including spectroscopy and microscopy, the various degrees of degradation under three sets of experimental conditions of interest (O-2-RT, O-2-HT, and Ar-HT, where RT = 22 degrees C and HT = 62 degrees C) are rationalized. Combining the GDE-ICP-MS technique and post-mortem analyses, this work provides detailed insights into the degradation pathways of various Fe, N, and C species during start-stop events, which may inspire the next generation of durable Fe-N-C catalysts for anion exchange membrane fuel cells.

The scanning gas diffusion electrode (S-GDE) half-cell is introduced as a new tool to improve the evaluation of electrodes used in electrochemical energy conversion technologies. It allows both fast screening and fundamental studies of real catalyst layers by applying coupled mass spectrometry techniques such as inductively coupled plasma mass spectrometry and online gas mass spectrometry. Hence, the proposed setup overcomes the limitations of aqueous model systems and full cell-level studies, bridging the gap between the two approaches. In this proof-of-concept work, standard fuel cell electrodes are investigated at elevated oxygen reduction reaction current densities, while dissolved Ptx+ ions in the electrolyte and gaseous CO2 in the outlet gas stream are detected to track platinum dissolution and carbon corrosion, respectively. Relevant current densities of up to 0.75 A cm(-2) are demonstrated. The electrochemically active surface area, oxygen reduction reaction activity, and Pt dissolution rates are quantified and benchmarked to the values obtained in the conventional stationary GDE half-cell. Moreover, it is found that Pt dissolution is suppressed when O2 is purged into the catalyst layer. Overall, this work demonstrates the feasibility of fast fuel cell electrode screening obtaining, complementary to electrochemical, mass spectrometry data necessary in fundamental studies on structure/performance relationships under actual reaction conditions. While Pt/C, in relevance to its fuel cell application, is used in this study, the proposed setup can be applied in water electrolysis, CO2 conversion, metal-air batteries, and other neighbor technologies.

The superior performance enhancement in catalyst research for proton exchange membrane fuel cells (PEMFC) revealed in the model rotating disc electrode (RDE) environment is rarely demonstrated in membrane electrode assemblies (MEA). The discrepancy is typically attributed to the difference in the chemical structure and morphology of the catalyst layer (CL), affecting the transport of reactants of the oxygen reduction reaction (ORR). In this study, the gas diffusion electrode (GDE) half-cell setup is used to focus on crucial aspects of CL development, especially on the activation and morphology of CLs, to gain a fundamental understanding of the development of PEMFCs. Adjusting the CL porosity by using different solvent compositions of water and isopropyl alcohol and implementing an activation method for low platinum content catalysts, we focus on understanding the contributions of macro-porosity and hydrophobicity in a liquid electrolyte-based system. We show that macro-porosity significantly influences the O-2 mass transport in the CL, demonstrating increased performance with higher porosities. Coupling inductively coupled plasma mass spectrometry (ICP-MS) with the GDE half-cell setup, we propose a method for qualitative estimation of water content in the CL. Lower macro-porosity shows higher platinum dissolution, attributed to improved mass transport of dissolved ions in aqueous media.

Understanding catalyst dissolution pathways in protonexchangemembrane water electrolyzers is paramount for developing mitigationstrategies aiming toward higher durability and lower catalyst loadings.To this end, Ir dissolution has been extensively studied using aqueousmodel systems but not in real devices. Aiming to bridge this knowledgegap, we use a metal-free water electrolysis setup to determine themass balance of the dissolved Ir in an electrolyzer when applyinga protocol mimicking intermittent operation. We find that the mainIr sinks are the cathode catalyst layer and the membrane, while Irdissolution into the water lines is significantly lower. Althoughreproducible estimation of Ir present in the membrane is challenging,quantification of Ir within the cathode is reliable and efficient.This new finding implies that tracking Ir present in the cathode canbe used to estimate anode catalyst stability, thus accelerating catalystdevelopment and operational parameters optimization.

Fuel cell catalyst layers contain an essential active catalyst, a support material for electron conductivity, ionomer for proton conductivity, and porosity for gas transport, which build up complex interfaces that determine the overall performance. Subtle variations in the processing of the catalyst layers can significantly alter the performance, which demands intensive research efforts, and requires considerable amount of time. In the last few years, gas diffusion electrode (GDE) half-cell setups have been introduced as a promising approach to speed up catalyst layer evaluation. Yet, advanced methods to thoroughly characterize transport phenomena within the catalyst layer have not been established for GDE half-cell setups. In the present work, we adapt electrochemical characterization methods, such as O (2) transport resistance and CO-displacement, which have been previously developed for single cell testing, to enable unique insights into catalyst layers' structure-performance relationships with the GDE method. Utilizing a commercial Pt/Vulcan catalyst as a test system, we identify the cause of mass transport limitations due to different ionomer contents. We show that an intermediate I/C ratio of 0.70, which forms a thin layer of ionomer, leads to an optimal performance for the Vulcan carbon support, due to an optimal compromise between O-2 and proton accessibility.

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 .

Although the performance of new catalyst materials for proton exchange membrane fuel cells (PEMFC) often show very promising results in ideal rotating disc electrode (RDE) environment, these results cannot always be transferred to applicable conditions in membrane electrode assemblies (MEAs). This is mainly attributed to differences in the catalyst layer (CL) structure, which hinders the transport of reactants and products for the oxygen reduction reaction (ORR). Therefore, the chemical structure and morphology of the CL have to be considered to improve the performance of these systems in fuel cells. However, due to the various possibilities in producing CLs, many experiments must be carried out until the optimum is reached. The interaction of Pt nanoparticles, carbon support, and ionomer creates a complex triple phase interface dependent on numerous parameters. Characteristics of the carbon support,[1] the Pt loading[2] or the ionomer content[3-5] leads to intensive research efforts and a considerable amount of time as many MEA experiments have to be performed for such studies.[6] To accelerate the screening of catalyst layers, the gas diffusion electrode (GDE) was recently proposed as a half-cell to study real catalyst layers.[7] As a proof of concept study, our group recently shown that catalyst layers with different Pt loading in GDE demonstrate similar ORR trends to MEA experiments.[7] Subsequently, GDEs from different laboratories were benchmarked with standardized electrochemical protocols obtaining reproducible results.[8] Most recently, advanced electrochemical characterization methods developed for MEA technique were established as a GDE method.[9] In the present work, we focus on main aspects of catalyst development, such as the morphology of catalyst layer formation and different catalyst materials, to gain fundamental insights into catalyst layer development. We tuned the porosity of real catalyst layers using different solvent compositions. The results show that low macroporosity leads to severe O2 mass transport limitations, whereas a better performance could be achieved for higher porosity. This allows a critical consideration of key parameters in MEA manufacturing, which has hot pressing as an indispensable step. This work demonstrates the importance of morphology and treatment of the catalyst layer and how substantially it affects the performance and mass transport in GDE setups to elucidate the optimal conditions for the accessibility of the triple phase interface. Literature [1] V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson and A. Kongkanand, ACS Energy Lett. 2018, 3, 618-621. [2] J. P. Owejan, J. E. Owejan and W. Gu, J. Electrochem. Soc. 2013, 160, F824. [3] E. Antolini, L. Giorgi, A. Pozio and E. Passalacqua, J. Power Sources 1999, 77, 136-142. [4] K.-H. Kim, K.-Y. Lee, H.-J. Kim, E. Cho, S.-Y. Lee, T.-H. Lim, S. P. Yoon, I. C. Hwang and J. H. Jang, Int. J. Hydrogen Energy 2010, 35, 2119-2126. [5] G. Sasikumar, J. W. Ihm and H. Ryu, J. Power Sources 2004, 132, 11-17. [6] K. Ehelebe, D. Escalera-López and S. Cherevko, Current Opinion in Electrochemistry 2021, 29, 100832. [7] K. Ehelebe, D. Seeberger, M. T. Y. Paul, S. Thiele, K. J. J. Mayrhofer and S. Cherevko, J. Electrochem. Soc. 2019, 166, F1259-F1268. [8] K. Ehelebe, N. Schmitt, G. Sievers, A. W. Jensen, A. Hrnjić, P. Collantes Jiménez, P. Kaiser, M. Geuß, Y.-P. Ku, P. Jovanovič, K. J. J. Mayrhofer, B. Etzold, N. Hodnik, M. Escudero-Escribano, M. Arenz and S. Cherevko, ACS Energy Lett. 2022, 7, 816-826. [9] P. Kaiser, V. Lloret Segura, K. Ehelebe and S. Cherevko, ECS Meeting Abstracts 2022, MA2022-01, 2071.

One of the most promising candidates to replace platinum group metal catalysts for oxygen reduction reaction (ORR) in fuel cells (FCs) is the sub-class of iron-nitrogen-carbon (Fe-N-C) catalysts. However, Fe-N-C materials considerably suffer from varied degradation mechanisms. [1-2] Compared to FC operating conditions, start/stop events where the cathodes experience anodic potential may be more damaging due to the carbon corrosion phenomenon. [3] The correlation between carbon corrosion and Fe dissolution rates has been reported in aqueous model systems in acidic media. [3] However, different carbon corrosion mechanisms are reported for acidic and alkaline media. [4] While anion-exchange membrane FCs (AEMFCs) have gained increased attention, studies on the effects of carbon corrosion on realistic AEMFC Fe-N-C catalyst layers are still missing. In this work, using a gas diffusion electrode (GDE) half-cell coupled with inductively coupled plasma mass spectrometry (ICP-MS), [5] we observed that the rate of Fe loss significantly accelerates with rising potential (E > 1.0 VRHE), commonly experienced during the start/stop events. Increased temperature intensifies the rate of Fe leaching during carbon corrosion (see Figure 1), while the gas atmosphere (Ar or O2) shows a negligible influence. On the contrary, the subsequent Fe deposition and the drop of ORR activity depend on the presence of O2 and the varied temperature. Combining in situ and post-mortem analyses, we report how carbon corrosion in alkaline media degrades Fe-N-C catalyst layers in various atmospheres and at different temperatures. These insights can contribute to rational designs of AEMFCs' start/stop protocol and more robust Fe-N-C materials. Figure 1. Fe-N-C demetallation during anodic potential holds (1.0 - 1.5 VRHE) in O2-saturated alkaline (0.1 M NaOH) environment at 22 ± 2 and 62 ± 2 ℃. (A) Potential profile. (B) The Fe dissolution rate normalized to catalyst loading. References: [1] Adabi, Horie, et al. Nat. Energy., 2021, 6.8: 834-843. [2] Speck, Florian D., et al. JACS Au, 2021, 1.8: 1086-1100. [3] Choi, Chang Hyuck, et al. Angew. Chem. Int. Ed., 2015, 54.43: 12753-12757. [4] Yi, Youngmi, et al. Catal. Today, 2017, 295: 32-40. [5] Ku, Yu-Ping, et al. J. Am. Chem. Soc., 2022, 144.22: 9753-9763. Figure 1

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.

The extremely promising activities of advanced fuel cell catalyst materials achieved in an ideal rotating disk electrode (RDE) environment can frequently not be transferred to technologically relevant membrane electrode assemblies (MEAs). This can mainly be ascribed to a non-optimal catalyst layer composition, which significantly affects the management of educts and products on the catalyst layer surface for the oxygen reduction reaction (ORR). Therefore, the composition of the catalyst layer has enormous potential to significantly improve the performance of these systems in MEA. However, it has to be optimized for each individual catalyst system. Currently, MEA experiments have to be employed for catalyst layer optimization.[1] However, such investigations are time consuming, expensive (large quantities of catalyst and extended test equipment needed) and do not allow independent investigation of either cathode or anode catalyst layer. In order to accelerate catalyst layer optimization screening, the gas diffusion electrode (GDE) half-cell setup for investigating realistic catalyst layers was recently proposed.[2] In a Pt loading study, it was previously shown that similar performance trends compared to MEA experiments can be achieved.[2] In the present work, we introduce advanced electrochemical characterization methods, such as Oxygen Transport Resistance [3, 4] and CO-Displacement [5] , which have been developed for MEA technique, to the GDE method. By using a commercial Pt on Vulcan catalyst system, we investigate the impact of Nafion loading on the electrochemical performance. The results show, that high ionomer loadings lead to severe O2 mass transport limitations, whereas for small loadings, lower ionomer coverage are measured. Both result in a significant performance loss at high current densities. Therefore, an intermediate ionomer loading which forms a thin layer of ionomer leads to an optimal performance for the Vulcan carbon support. This work demonstrates that advanced electrochemical methods can also be applied to GDE setups to shed light on the optimal composition of the triple phase interface of catalyst layers. This is an innovative step for the future to efficiently optimise catalyst systems and to gain fundamental insight into the understanding of catalyst layers. Literature [1] K. Ehelebe, D. Escalera-López, S. Cherevko, Current Opinion in Electrochemistry 2021, 29, 100832. [2] K. Ehelebe, D. Seeberger, M. T. Y. Paul, S. Thiele, K. J. J. Mayrhofer, S. Cherevko, Journal of The Electrochemical Society 2019, 166, F1259-F1268. [3] D. R. Baker, C. Wieser, K. C. Neyerlin, M. W. Murphy, ECS Transactions 2006, 3, 989-999. [4] D. R. Baker, D. A. Caulk, K. C. Neyerlin, M. W. Murphy, Journal of The Electrochemical Society 2009, 156, B991. [5] T. R. Garrick, T. E. Moylan, V. Yarlagadda, A. Kongkanand, Journal of The Electrochemical Society 2016, 164, F60-F64.

Sustainable development of the global energy sector requires a transition from fossil fuels to renewable energies. Considering the continuously increasing energy demand, effective utilization of intermittent output from the renewable sources will depend on the efficiency of energy storage and utilization processes. Low chemical complexity and high energy density and efficiency make hydrogen produced via proton exchange membrane water electrolysis (PEMWE) a prominent solution for the mentioned challenges. Acidic conditions and high potentials at the anode side of PEM water electrolyzers, where the oxygen evolution reaction (OER) takes place, demand for materials with high catalytic activity and corrosion stability. The state-of-the-art platinum and iridium (oxide) catalysts in the cathode and anode catalyst layers (CLs), respectively, demonstrate relatively good activity and stability during steady operation at low and moderate electrical loads. Indeed, it is anticipated that a significant decrease in the noble metal amount may be achieved without sacrificing the cell performance [1]. An intermittent operation of PEMWE, however, represents a considerable risk factor as both CLs may degrade with time. The extent of such degradation, especially at low catalyst loadings and high current densities, alternated with numerous off cycles is still not well understood and hence, difficult to predict and mitigate. Recent results from our group indicate a severe discrepancy between OER catalyst dissolution in aqueous model systems (AMS) and membrane electrode assemblies (MEA), with the main reasons being a suggested discrepancy between estimated and real pH in MEA and stabilization occurring over time [2]. In this work, CLs degradation during dynamic electrolyzer operation in a specially designed PEMWE test station was studied via ex-situ inductively coupled mass spectrometry analysis (ICP-MS) and its influence on the cell’s overall performance was analyzed. The S number, a new metric for OER catalyst lifetime estimation [3], was also used to compare catalyst stability properties within the two systems. References: Bernt, A. Siebel, H. Gasteiger, J. Electrochem. Soc. 165 (5), F305-F314 (2018) Knöppel, M. Möckl, D. Escalera-Lopez, K. Stojanovski, M. Bierling, T. Böhm, S, Thiele, M. Rzepka, S. Cherevko, Nat. Commun. 12, 2231 (2021) Geiger, O. Kasian, M. Ledendecker, E. Pizzutilo, A. M. Mingers, W. T. Fu, O. Diaz-Morales, Z. Li, T. Oellers, L. Fruchter, A. Ludwig, K. J. J. Mayrhofer, M. T. M. Koper, Serhiy Cherevko, Nat. Cat. 1, 508 (2018) Figure 1. Longterm stability of IrOx in AMS and MEA environment. a) Loading-normalized total dissolved iridium amount at current densities of 0.2 A mgIr -1 and 2 A mgIr -1 in AMS and MEA respectively; b) S-numbers calculated from the amount of dissolved iridium. Figure 1

Durability and degradation are in the focus of modern electrocatalysis research. Before moving to real applications, e.g. fuel cells in transportation or water electrolyzers for production of green hydrogen, novel electrocatalytic materials must prove acceptable stability, but “how to test the stability of electrocatalysts”? In the relatively mature proton exchange membrane fuel cell (PEMFC) research, stability is evaluated using various accelerated stress tests (ASTs). Unfortunately, even for the most studied Pt/C electrocatalysts, degradation processes like carbon corrosion and Pt dissolution that occur during common ASTs are not easily distinguishable [1]. Moreover, advanced electrocatalysts such as different shape-controlled Pt alloy nanostructures, showing promising stability in ASTs performed in model aqueous systems, are often rendered useless when moved to real applications [2]. Catalysts free of platinum-group-metals, e.g. FeNC, demonstrate different degradation extents if tested in oxygen or argon [3]. Iridium oxides, the state of the art oxygen evolution reaction (OER) electrocatalysts, are prone to dissolution in aqueous media but much more stable in solid electrolyte based electrolyzers [4]. These examples demonstrate the need for rethinking current approaches to test electrocatalyst stability. This work highlights our recent results on using coupled electrochemical techniques and tuned gas diffusion electrode (GDE) and membrane electrode assembly (MEA) cells in fuel cell and water electrolysis research. It shows that by hyphenating GDE with inductively coupled plasma mass spectrometry (ICP-MS) it is possible to investigate dissolution of electrocatalysts, such as Pt/C for PEMFC and Fe-N-C for anion exchange membrane fuel cells (AEMFC), in-operando at conditions closely resembling those in real devices [5, 6]. As another representative example, the use of model MEAs to address the discrepancy of Ir dissolution in aqueous and solid polymer electrolytes is given [7]. Based on these examples, new strategies to test and understand electrocatalysts’ degradation are discussed. References: [1] E. Pizzutilo et al., On the need of improved accelerated degradation protocols (ADPs): Examination of platinum dissolution and carbon corrosion in half-cell tests, J. Electrochem. Soc., 163 (2016) F1510-F1514. [2] K. Kodama et al., Challenges in applying highly active Pt-based nanostructured catalysts for oxygen reduction reactions to fuel cell vehicles, Nature Nanotechnology, 16 (2021) 140-147. [3] K. Kumar et al., On the influence of oxygen on the degradation of Fe-N-C catalysts, Angew. Chem. Int. Ed., 59 (2020) 3235-3243. [4] S. Geiger et al., The stability number as a metric for electrocatalyst stability benchmarking, Nature Catalysis, 1 (2018) 508-515. [5] K. Ehelebe et al., Platinum dissolution in realistic fuel cell catalyst layers, Angew. Chem. Int. Ed., 60 (2021) 8882-8888. [6] Y.-P. Ku et al., Oxygen reduction reaction causes iron leaching from Fe-N-C electrocatalysts, (2021) Submitted, DOI: 10.21203/rs.3.rs-1171081/v1. [7] J. Knöppel et al., On the limitations in assessing stability of oxygen evolution catalysts using aqueous model electrochemical cells, Nature Communications 12 (2021) 2231.

The electrochemical activity of modern Fe-N-C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells (FCs), however, also high durability and longevity must be demonstrated. Currently, a limited understanding of degradation pathways, especially under operando conditions, hinders the design and synthesis of simultaneously active and stable Fe-N-C electrocatalysts. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under conditions close to those in FCs, that is, with a porous catalyst layer (CL) and at current densities up to -125 mA·cm-2. Varying the rate of the oxygen reduction reaction (ORR), we show a remarkable linear correlation between the Faradaic charge passed through the electrode and the amount of Fe dissolved from the electrode. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe redox transitions in Fe species involved in the ORR, such as FeNxCy and Fe3C@N-C. Moreover, such a linear correlation allows the application of a simple metric─S-number─to report the material's stability. Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand a more advanced mechanistic understanding of Fe-N-C degradation in CLs.

Despite intensive investigations for unravelling the water splitting reaction, the catalyst behavior during the oxygen evolution reaction (OER) is still not fully understood. This is especially true under more demanding conditions like high potentials and high temperatures. Rotating disk electrode measurements show a gradual increase of OER current when increasing the temperature up to 80 degrees C. However, strong bubble formation at elevated temperatures makes in-situ characterization of the catalyst challenging. Here we utilize an in-situ electrochemical and heated flow cell, which aims at an efficient removal of bubbles from the catalyst surface and enables structural studies by X-ray absorption spectroscopy (XAS) at temperatures up to 80 degrees C. Changes in the Ir L-3-edge X-ray absorption near edge spectra (XANES) were observed with respect to the white line position and principal components related to structural changes were extracted. At temperatures of 60 degrees C and above, the white line position of XANES spectra reaches a steady state, which is possibly caused by an equilibrium of different Ir oxidation states. These findings provide first spectroscopic insights in the behavior of OER catalysts at elevated temperatures which are typical for industrial applications and rarely addressed until now.

ADVERTISEMENT RETURN TO ISSUEPREVEnergy FocusNEXTBenchmarking Fuel Cell Electrocatalysts Using Gas Diffusion Electrodes: Inter-lab Comparison and Best PracticesKonrad Ehelebe*Konrad EhelebeHelmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Cauerstrasse 1, 91058 Erlangen, GermanyDepartment of Chemical and Biological Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 1, 91058 Erlangen, Germany*[email protected]More by Konrad Ehelebehttps://orcid.org/0000-0001-9441-5642, Nicolai SchmittNicolai SchmittDepartment of Chemistry, Technical University of Darmstadt, Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Alarich-Weiss-Strasse 8, 64287 Darmstadt, GermanyMore by Nicolai Schmitthttps://orcid.org/0000-0001-8668-288X, Gustav SieversGustav SieversLeibniz Institute for Plasma Science and Technology, Felix-Hausdorff-Strasse 2, 17489 Greifswald, GermanyMore by Gustav Sievers, Anders W. JensenAnders W. JensenDepartment of Chemistry, Center for High Entropy Alloy Catalysis, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, DenmarkMore by Anders W. Jensen, Armin HrnjićArmin HrnjićDepartment of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, Ljubljana SI-1000, SloveniaUniversity of Nova Gorica, Vipavska 13, Nova Gorica SI-5000, SloveniaMore by Armin Hrnjić, Pablo Collantes JiménezPablo Collantes JiménezLeibniz Institute for Plasma Science and Technology, Felix-Hausdorff-Strasse 2, 17489 Greifswald, GermanyMore by Pablo Collantes Jiménezhttps://orcid.org/0000-0003-3381-154X, Pascal KaiserPascal KaiserHelmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Cauerstrasse 1, 91058 Erlangen, GermanyDepartment of Chemical and Biological Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 1, 91058 Erlangen, GermanyMore by Pascal Kaiserhttps://orcid.org/0000-0002-8438-8238, Moritz GeußMoritz GeußHelmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Cauerstrasse 1, 91058 Erlangen, GermanyDepartment of Chemical and Biological Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 1, 91058 Erlangen, GermanyMore by Moritz Geußhttps://orcid.org/0000-0003-3287-088X, Yu-Ping KuYu-Ping KuHelmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Cauerstrasse 1, 91058 Erlangen, GermanyDepartment of Chemical and Biological Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 1, 91058 Erlangen, GermanyMore by Yu-Ping Kuhttps://orcid.org/0000-0003-4234-3135, Primož JovanovičPrimož JovanovičDepartment of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, Ljubljana SI-1000, SloveniaMore by Primož Jovanovičhttps://orcid.org/0000-0003-2477-3895, Karl J. J. MayrhoferKarl J. J. MayrhoferHelmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Cauerstrasse 1, 91058 Erlangen, GermanyDepartment of Chemical and Biological Engineering, Friedrich-Alexander University Erlangen-Nürnberg, Cauerstrasse 1, 91058 Erlangen, GermanyMore by Karl J. J. Mayrhoferhttps://orcid.org/0000-0002-4248-0431, Bastian EtzoldBastian EtzoldDepartment of Chemistry, Technical University of Darmstadt, Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Alarich-Weiss-Strasse 8, 64287 Darmstadt, GermanyMore by Bastian Etzoldhttps://orcid.org/0000-0001-6530-4978, Nejc HodnikNejc HodnikDepartment of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, Ljubljana SI-1000, SloveniaUniversity of Nova Gorica, Vipavska 13, Nova Gorica SI-5000, SloveniaMore by Nejc Hodnikhttps://orcid.org/0000-0002-7113-9769, María Escudero-EscribanoMaría Escudero-EscribanoDepartment of Chemistry, Center for High Entropy Alloy Catalysis, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, DenmarkMore by María Escudero-Escribanohttps://orcid.org/0000-0002-6432-3015, Matthias ArenzMatthias ArenzDepartment of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern, Freiestrasse 3, 3012 Bern, SwitzerlandMore by Matthias Arenzhttps://orcid.org/0000-0001-9765-4315, and Serhiy Cherevko*Serhiy CherevkoHelmholtz-Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Forschungszentrum Jülich GmbH, Cauerstrasse 1, 91058 Erlangen, Germany*[email protected]More by Serhiy Cherevkohttps://orcid.org/0000-0002-7188-4857Cite this: ACS Energy Lett. 2022, 7, 2, 816–826Publication Date (Web):January 24, 2022Publication History Received6 December 2021Accepted11 January 2022Published online24 January 2022Published inissue 11 February 2022https://pubs.acs.org/doi/10.1021/acsenergylett.1c02659https://doi.org/10.1021/acsenergylett.1c02659newsACS PublicationsCopyright © Published 2022 by American Chemical Society. 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The lack of efficient and durable proton exchange membrane fuel cell electrocatalysts for the oxygen reduction reaction is still restraining the present hydrogen technology. Graphene-based carbon materials have emerged as a potential solution to replace the existing carbon black (CB) supports; however, their potential was never fully exploited as a commercial solution because of their more demanding properties. Here, a unique and industrially scalable synthesis of platinum-based electrocatalysts on graphene derivative (GD) supports is presented. With an innovative approach, highly homogeneous as well as high metal loaded platinum-alloy (up to 60 wt %) intermetallic catalysts on GDs are achieved. Accelerated degradation tests show enhanced durability when compared to the CB-supported analogues including the commercial benchmark. Additionally, in combination with X-ray photoelectron spectroscopy Auger characterization and Raman spectroscopy, a clear connection between the sp 2 content and structural defects in carbon materials with the catalyst durability is observed. Advanced gas diffusion electrode results show that the GD-supported catalysts exhibit excellent mass activities and possess the properties necessary to reach high currents if utilized correctly. We show record-high peak power densities in comparison to the prior best literature on platinum-based GD-supported materials which is promising information for future application.

In aqueous rotating disk electrode (RDE) measurements, advanced catalyst materials show tremendous performance improvements in comparison to commercial Pt/C catalysts towards oxygen reduction reaction (ORR). However, these promising improvements cannot (yet) be transferred to realistic membrane electrode assembly (MEA). [1] These discrepancies can be explained by non-ideal catalyst layer composition, leading to mass transport limitations of either oxygen (which is transported through the pores) or protons (via the ionomer) to the active centers of the catalyst material. Mass transport phenomena in real catalyst layers are still under debate and hold huge potential to significantly improve catalyst performance. However, mass transport in realistic fuel cell catalyst layers cannot be assessed with RDE experiments, as they are limited to low current densities and idealized catalyst layers on solid substrate. Therefore, MEA experiments are usually employed to evaluate those phenomena. Yet, those investigations are time consuming, expensive (large quantities of catalyst needed, extended test equipment) and do not allow segregated investigation of either cathode or anode catalyst layer. Furthermore, comparison of different MEA studies can be challenging due to varying operating conditions. Therefore, techniques combining the advantages of RDE, namely simplicity and comparability, with the realistic operating ranges of MEA are urgently required to retrieve the potential of catalyst layer optimization. In this presentation half-cells using gas diffusion electrodes (GDE) are proposed as a new powerful experimental tool to enable high mass transport catalyst screening in relevant potential ranges and realistic electrode structures. On the example of a Pt loading study we show, that current densities of up to 2 A/cm² can be achieved [2]. In contrary to other formerly used methods [3], it is thereby possible to overcome mass transport limitations at relevant fuel cell operating potentials. Additionally, we demonstrate good compliance with MEA experiments, proving the method´s suitability for catalyst evaluation in realistic fuel cell potential ranges. Besides activity, also stability of the electrocatalyst is affected significantly by the catalyst layer structure. We present here a novel method, where a GDE half-cell is coupled to an inductively coupled plasma mass spectrometer (ICP-MS) to gain deeper insights into dissolution of electrocatalysts in realistic electrode structures [4]. With this unique online tool, we could measure Pt dissolution in realistic catalyst layers for the first time reported in literature. We show, how mass transport of dissolved Pt species influences net Pt dissolution and how Pt dissolution can also be detected through Nafion membranes. Besides that, also the mobility of Pt-ions through Nafion membranes is further investigated. Literature [1] Ly, A., et al. J. Power Sources, 2020. 478: 228516 [2] Ehelebe, K., et al., J. Electrochem. Soc., 2019. 166(16): F1259-F1268, [3] Inaba, M., et al., Energy Environm. Sci., 2018. 11(4): 988-994, [4] Ehelebe, K. et al. submitted Figure 1

Abstract The electrochemical activity of modern Fe-N-C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells, however, also high durability and longevity must be demonstrated. Currently, design and synthesis of simultaneously active and stable platinum group metal-free electrocatalysts is hindered by a limited understanding of Fe-N-C degradation, especially under operando conditions. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under more realistic conditions, i.e. real catalyst layer and current densities up to 125 mA·cm-2. Varying the rate of oxygen reduction reaction, we show a remarkable correlation between Faradaic electrode charge and Fe dissolution. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe3+/Fe2+ redox transitions in coordinated Fe cations. Moreover, such linear correlation allows an introduction and use of a simple metric (stability number). Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand more advanced mechanistic understanding of Fe-N-C degradation in catalyst layers.

Cost and stability remain the greatest technical barriers to sustainably commercialize low-temperature fuel cells and electrolyzers. For tackling 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 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.

A fast and facile pulse combustion (PC) method that allows for the continuous production of multigram quantities of high-metal-loaded and highly uniform supported metallic nanoparticles (SMNPs) is presented. Namely, various metal on carbon (M/C) composites have been prepared by using only three feedstock components: water, metal-salt, and the supporting material. The present approach can be elegantly utilized also for numerous other applications in electrocatalysis, heterogeneous catalysis, and sensors. In this study, the PC-prepared M/C composites were used as metal precursors for the Pt NPs deposition using double passivation with the galvanic displacement method (DP method). Lastly, by using thin-film rotating disc electrode (TF-RDE) and gas-diffusion electrode (GDE) methodologies, we show that the synergistic effects of combining PC technology with the DP method enable production of superior intermetallic Pt-M electrocatalysts with an improved oxygen reduction reaction (ORR) performance when compared to a commercial Pt-Co electrocatalyst for proton exchange membrane fuel cells (PEMFCs) application.