State-of-the-art membranes for DIFC suffer from degradation in isopropanol/acetone/water mixtures and high organic cross-over. In this study, cross-linked SFS–OPBI membranes are presented as a tuneable material to address these challenges.

Efficient storage technologies are a major challenge for the use of hydrogen in industrial processes or power generation using fuel cells. Solutions such as compressed hydrogen and liquid hydrogen frequently demand high energy inputs for processes such as compression and/or cooling, which has led to a growing interest in liquid-organic-hydrogen-carriers (LOHCs). [1] This solution offers the possibility of utilizing the existing infrastructure for liquid fuels, including easy and safe storage opportunities. However, traditional LOHCs require an energy-intensive thermocatalytic process to free the chemically bound hydrogen. In contrast, electrochemical LOHCs (EC-LOHCs) offer a viable alternative, exhibiting analogous properties while enabling the release of hydrogen "on demand" through the application of an electrical potential. This offers advantageous opportunities regarding dynamic operation, system simplicity and cost efficiency. One possible EC-LOHC couple under investigation is Isopropanol/Acetone. The hydrogen-rich isopropanol molecule can be converted to the hydrogen-lean acetone in a polymer electrolyte membrane electrode assembly (PEM-MEA) cell setup, resulting in the release of hydrogen. In general, the isopropanol oxidation reaction (IOR) is facilitated in a low potential range (0.18 V vs. RHE) by a PtRu/C catalyst on the anode side. [2] On the cathode side the hydrogen evolution reaction (HER) is catalyzed by Pt/C. The formed acetone can be effectively rehydrogenated to isopropanol, which closes the overall cycle and allows repeated charging and discharging of the EC-LOHC couple. The released hydrogen can either be used directly in industrial processes or fed in a fuel cell for power generation. Focusing on the isopropanol dehydrogenation reaction, this contribution addresses the severe performance decay rates, which have been previously observed in related studies. [2][3] It can be hypothesized that the acetone that has formed blocks the active centers of the catalyst, which is why the activity of the system can be restored by removing the product. This is already an indication that certain material properties may be limiting the full potential of the system. However, this transient behavior also necessitates different measurement procedures and operation protocols than those currently used in typical electrolyzer tests to thoroughly investigate and optimize the system’s behavior. The present study focuses on the characterization of the performance decay in order to decouple the contribution of different components in the cell setup. By implementing “wash cycles”, the accumulated acetone can be removed to retrieve pristine electrode conditions for each measurement point. This technique accounts for the transient system behavior, while traditional electrochemical measurement methods assume steady state conditions. Based on this approach, an investigation of not only different porous media and flowfields, but also operating temperature and flowrate can reveal the potential for enhanced and stable performance by evaluating the decay patterns. References [1] D. Teichmann, W. Arlt and P. Wasserscheid, International Journal of Hydrogen Energy 2012 , 37, 18118–18132. [2] P. Khanipour, F. D. Speck, I. Mangoufis-Giasin, K. J. J. Mayrhofer, S. Cherevko, and I. Katsounaros, ACS Applied Materials & Interfaces 2020 12 (30), 33670-33678. [3] D. Venus, A. Marth, S. Riess, A.T.S. Freiberg, M. Brodt, M. Wensing, P. Wasserscheid and S. Thiele, Advanced Energy Materials 2024 , 2403824

In this study, we vary the 1-propanol/water and ionomer/carbon (I/C) ratios in catalyst inks with a high total solid content of 10 wt%. The influence of the ratios on the inks is analyzed via the processing properties, while the resulting electrodes are characterized using electrochemical hydrogen fuel cell tests. The 1-propanol/water ratio variation showed a stronger effect on the rheological and wetting properties and thus the coating qualities for low I/C inks than for high I/C inks. Different void volume fractions and electrode thicknesses are observed for the various ionomer contents and alcohol concentrations for the coated electrodes. In some cases, primarily in low I/C inks, significant differences in proton conduction and mass transport properties are observed by detailed electrochemical analysis. For some ink formulations these differences counterbalance each other, resulting in minimal net differences in the polarization curves. As a result, the interplay between ionomer concentration and alcohol content is shown to be non-straightforward and significant for optimizing the formulations of high solid weight content inks.

Using the isopropanol–acetone redox couple as an electrochemical liquid organic hydrogen carrier (EC-LOHC) enables environmentally benign and cost-effective hydrogen storage, eliminating the need for high-pressure or cryogenic hydrogen containers. In this system, hydrogen is chemically stored in isopropanol. An Acetone Electrochemical Hydrogenation Unit (EHU) generates electric power while spontaneously reducing acetone with hydrogen to isopropanol in fuel cell mode, thereby storing hydrogen in chemical form. Conversely, hydrogen is released from isopropanol to form acetone at low cell potential (<200 mV vs. RHE at 60 °C) in an Isopropanol Electrochemical Dehydrogenation Unit (EDU), operating as an electrolyzer. The high selectivity of isopropanol oxidation to acetone at low cell potential enables CO₂-free hydrogen release.¹ Despite its potential, the EDU suffers from significant performance decay during operation.² While the root cause of this current decay remains unclear, it has been suggested that mass transport limitations may play a key role in the oxidation of isopropanol.³ In both EDU and EHU, the reactants and products are in liquid form, and diffusion through the swollen ionomer in the catalyst layer may introduce substantial resistance. In addition, current decay may result from the slow desorption of acetone from electroactive PtRu sites, as reported in various studies.³⁻⁵ However, no systematic study has been conducted to identify the dominant cause of performance degradation in EDU. This contribution investigates the extent of mass transport limitations in both EDU and EHU configurations. A series of PtRu/C electrodes with varying catalyst layer thickness, carbon dilution, and loading were prepared with a constant ionomer-to-carbon (I/C) ratio. These electrodes were designed to introduce a controlled physical barrier to the transport of reactants, products, or protons. Each set of electrodes was examined using polarization curves, electrochemical impedance spectroscopy, and crossover measurements in a 5 cm² Scribner cell at 60 °C with 1 M organic concentration. By comparing the results, we analyzed the influence of catalyst layer thickness, dilution, and loading on the performance of EDU and EHU, aiming to identify the main cause of performance decay and determine the optimal catalyst layer composition. Additionally, the feasibility of a reversible cell system using a single MEA for both EDU and EHU was explored. P. Khanipour, F. D. Speck, I. Mangoufis-Giasin, K. J. J. Mayrhofer, S. Cherevko, and I.Katsounaros, ACS Applied Materials & Interfaces 2020 12 (30), 33670-33678 M. Brodt, K. Müller, J. Kerres, I. Katsounaros, K. Mayrhofer, P. Preuster, P. Wasserscheid and S. Thiele, Energy Technol. 2021, 9, 2100164 D.Cao, S.H. Bergens, Journal of Power Sources, Volume 124, Issue 1, 2003, Pages 12-17, A.Santasalo, T. Kallio and K. Kontturi, Platinum Metals Review, Volume 53, Issue 2, Apr 2009, p. 58 - 66 D. Venus, M. Valeske, M. Brodt, P. Wasserscheid and S. Thiele, Electrochemistry Communications 169 (2024) 107823

A three-electrode setup facilitates the investigation of half-cell reactions. The reaction at the Working Electrode (WE) is the reaction of interest, and the potential at the WE is measured with respect to the Reference Electrode (RE). Current flows between the Counter Electrode (CE) and the WE. In such experiments, the CE completes the circuit by supplying or consuming electrons. Since the RE is nonpolarizable, the WE can be investigated without influence from the other electrodes masking its actual behavior in a three-electrode setup. In a traditional H 2 Proton Exchange Membrane Fuel Cell (PEMFC), the anode acts as a RE, allowing investigators to study the cathode reaction, i.e. Oxygen Reduction Reaction (ORR), by setting up the RE and the CE at the anode. In such a configuration, the cell potential, measured as the difference between the WE and the CE, is the potential at the WE with respect to the RE set up by the oxidation of H 2 gas at the anode. Since the Hydrogen Oxidation Reaction (HOR) is kinetically uninhibited, investigating ORR and the cathode behavior becomes possible in an H 2 PEMFC. In the case of electrochemical cells involving two kinetically inhibited reactions at the anode and the cathode, since both electrodes have considerable overpotentials, investigating a specific electrode behavior in a full cell becomes cumbersome [1] . This challenge has been mainly addressed in the context of Direct Alcohol Fuel Cells (DAFCs). Two sets of solutions have been proposed to solve this problem. In the first approach, a platinum wire is embedded within the membrane of the Membrane Electrode Assembly (MEA) to set up an RE [2] . In the second approach, a salt bridge connects the membrane to a Dynamic Hydrogen Electrode (DHE) [3] . Both methods require precise alignment of the RE and alter the actual MEA's structure [4] . In this contribution, we propose a simple solution to investigate two complex electrochemical half-cell reactions happening at the anode and cathode of a full cell, as in the case of DAFCs. We achieve this through the decal transfer of 3 electrodes onto a Nafion membrane. The third electrode is transferred at a distance sufficiently large enough to allow the successful setting up of a non-invasive RE to investigate the two other electrodes in their pristine operation conditions. We show the versatility of this innovative technique in exploring unchartered territories in DAFCs, including but not limited to Direct Isopropanol Fuel Cells, wherein, in contrast to other DAFCs, no CO2 is produced. References J. A. Nogueira, K. Krischer and H. Varela, Chemphyschem : a European journal of chemical physics and physical chemistry, 20(22), 3081–3088 (2019). H. Kuhn, B. Andreaus, A. Wokaun and G. G. Scherer, Electrochimica Acta, 51(8-9), 1622–1628 (2006). S. B. Adler, J. Electrochem. Soc., 149(5), E166 (2002). S. Adler, Solid State Ionics, 134(1-2), 35–42 (2000).

Catalyst degradation in the cathode electrode for H2-PEM (hydrogen proton-exchange membrane) fuel cells is a crucial topic to tackle to achieve high durability and efficiency. Despite ongoing research, a concurrently fast, easy-to-adapt, and effective recovery protocol is still missing. In this study, we report a fast and easy-to-adapt recovery protocol that significantly mitigates the negative effects associated with catalyst degradation for the cathode electrode in H2-PEMFCs. Following accelerated stress tests (AST) of 30,000-cycles, membrane-electrode assemblies (MEAs) using our new recovery protocol exhibit remarkable higher end-of-life performance compared to similar MEAs subjected to the same AST but utilizing the DOE-defined recovery protocol. The end-of-life differences for the new recovery protocol are over 100 % increase in power density at 0.6 V and around 26 % increase at peak power density. By analyzing performance, the Tafel slope, the electrochemical surface area (ECSA), and impedance data, the improvements are traced back to better catalyst recovery and thus improved performance at end-of-life.

In this work we show that, when creating ionomer dispersions, in addition to the solvent composition, it is critically important to also consider the ionomer's solvent exposure history as this has an impact on its final dispersion state. By variation of the solvent addition sequence to the ionomer, the retainment of properties gained from previous solvent exposure states is illustrated. The order of solvent addition to the ionomer has a direct impact on the dispersions' final properties, such as the aggregate size and viscosity. Variations in dispersion properties subsequently affect catalyst inks, which in turn can lead to different coating qualities and distinct electrochemical behaviors. Notably, in fuel cell experiments, differences in the mass transport properties are observed, especially among electrodes with higher ionomer content, either due to different Knudsen diffusion processes in the catalyst layer or local O2 diffusion through the ionomer films.

The intermittent nature of renewable energy sources is the key obstacle to decarbonizing global energy consumption, rendering cheap and efficient energy storage solutions pivotal. Hydrogen gas has extremely low volumetric energy density at atmospheric pressure, and storing renewable energy in compressed or cryogenic hydrogen requires compression and subsequent expansion, in addition to expensive high-pressure tanks, making it costly and inefficient energetically [1] . Organic couples that can be reversibly converted between hydrogen-rich and hydrogen-lean forms by redox reactions in electrochemical cells are categorized as electrochemically active Liquid Organic Hydrogen Carriers (EC-LOHCs) [2] . This concept allows facile handling of energy-dense liquids and low-temperature operation. The Isopropanol (IPA)-Acetone (ACE) couple is an EC-LOHC, wherein ACE can be electrochemically hydrogenated to IPA and then IPA converted back to ACE in a Proton Exchange Membrane (PEM) Direct Isopropanol Fuel Cell (DIFC). In a DIFC, the IPA oxidation is coupled with the Oxygen Reduction Reaction (ORR) to draw electric power [3] . The established state-of-the-art polymer used in PEM electrochemical cells is Nafion, a brand name for a perfluorinated sulfonic acid (PFSA) polymer developed by Chemours. However, IPA and ACE liquid solutions compromise the integrity of Nafion-based Membrane electrode assemblies (MEAs) at concentrations as low as 2M [4] and an 8M 1:1 mixture of IPA and ACE completely dissolves Nafion [5] . The feasibility of DIFC is dictated by the interplay between ionic conductivity, physical and chemical stability of the PEM in the IPA-ACE mixture, and the crossover of IPA and ACE through the PEM. In this contribution, we critically analyze Nafion’s compatibility in DIFC application and show that while Nafion offers excellent ionic conductivity, it fails to maintain physical and chemical stability in IPA-ACE mixtures. Furthermore, the parasitic cross-over of IPA and ACE through the Nafion membrane negatively impacts the OCV of a DIFC and hinders the ORR at the cathode. This contribution proposes using the novel SFS-OPBI ionic cross-linked, in-house-developed polymer in a DIFC. The versatility of the novel polymer is in tailoring its composition to suit the specific concentration of the fuel used. We show that the extent of crosslinking directly correlates with the ionic conductivity, physical and chemical stability, and cross-over of IPA and ACE through the membrane. Owing to the strides made by tailored cross-linking, we show that the ionic crosslinked SFS-OPBI polymer outperforms Nafion at low catalyst loadings in a Direct Isopropanol Fuel Cell. References J. Reynolds, D. Ali, J. Njuguna and F. Amadhe, GEET, 3 (2024). J. Cho, B. Kim, S. Venkateshalu, D. Y. Chung, K. Lee and S.-I. Choi, J. Am. Chem. Soc., 145(31), 16951–16965 (2023). M. Brodt, K. Müller, J. Kerres, I. Katsounaros, K. Mayrhofer, P. Preuster, P. Wasserscheid and S. Thiele, Energy Technology, 9(9), 2100164 (2021). D. Cao and S. H. Bergens, J. Power Sources, 124(1), 12–17 (2003). S. Auffarth, W. Dafinger, J. Mehler, V. Ardizzon, P. Preuster, P. Wasserscheid, S. Thiele and J. Kerres, J. Mater. Chem. A, 10(33), 17208–17216 (2022).

Electrochemical energy storage in organic compounds has gained increasing interest recently. In particular, ketones and their secondary alcohols can serve as an electrochemical liquid organic hydrogen carrier (EC-LOHC) due to the possibility of selective oxidation of the hydrogen-rich counterpart. This study examines the possibility of hydrogenating ketones (acetone, 2-butanone, 2-pentanone) in alkaline media (KOH) and shows the influence of aliphatic side-chain length on the surface coverage of the polycrystalline platinum catalyst in an H-cell. This is done by performing linear sweep voltammetry (LSV) and cyclovoltammetry (CV). LSVs reveal similar onset potentials for the ketone reduction reaction, and the influence of onsetting hydrogen evolution reaction and its dependency on surface coverage in the voltage range below 0 V vs. RHE. Furthermore, it reports increased faradaic efficiency (FE) in alkaline media in comparison to acidic media.

The degradation of catalyst activity at the oxygen reduction reaction (ORR) cathode limits the performance and lifetime of low-temperature hydrogen proton-exchange-membrane (PEM) fuel cells, especially at low platinum (Pt) loadings [1]. Degradation mechanisms include Pt dissolution, Ostwald ripening, Pt oxide formation, carbon corrosion, and the poisoning and deactivation of active catalyst sites from impurities in air or adsorption of sulfur-containing species from the ionomer. Pt oxides are thermodynamically favorable at potentials higher than ~0.7 V vs SHE, and their formation can hinder the ORR [2]. Similarly, ionomer restructuring in the catalyst layer during operation can lead to the adsorption of sulfates and sulfonates onto active catalyst sites that hinder the ORR [3]. Significant research has been aimed at designing and manufacturing more robust catalysts, through e.g., heat treatments, Pt-alloying, and the use of novel geometries. Recovery protocols can also be implemented in order to periodically clean and refresh the catalyst layer. In this way, some of the reversible lost activity is thus regained. However, there is no widely-accepted standard recovery protocol. Generally, the reactivation of the cathode electrode requires reductive potentials (e.g, < 0.4 V) to reduce Pt oxidizes and reductive conditions as well as high humidity/water flushing to remove poisons [4,5]. In this contribution we report a new recovery protocol that uses an applied reductive potential and a slight differential pressure to pump hydrogen from the anode to the cathode, which along with transported water is designed to reduce oxides and wash out contaminants. Results are presented from a study using in-house manufactured 25 cm 2 MEAs with commercial Pt/C catalysts and GORE® membranes and 3M TM ionomer. Following 30,000 cycle accelerated stress tests, fuel cell performance, along with insights from electronic impedance spectroscopy and detailed electrochemical analysis are provided before and after the new recovery protocol. A comparison is made with the DOE-defined recovery protocol [6]. Overall, we show our new recovery protocol to be fast, easy to implement, and effective. [1] G. P. Keeley, S. Cherevko, and K. J. J. Mayrhofer, ChemElectroChem , vol. 3, no. 1, pp. 51–54, 2016, doi: 10.1002/celc.201500425. [2] J. Mitzel, Q. Zhang, P. Gazdzicki, and K. A. Friedrich, Journal of Power Sources , vol. 488, p. 229375, 2021, doi: 10.1016/j.jpowsour.2020.229375. [3] Q. Zhang, M. Schulze, P. Gazdzicki, and K. A. Friedrich, Energies , vol. 17, no. 4, p. 774, 2024, doi: 10.3390/en17040774. [4] F. Guillet, M. Chatenet, A. Paul, L. Svecova, and L. Dubau Ind. Chem. Mater. , vol. 2, no. 1, pp. 118–131, 2024, doi: 10.1039/D3IM00085K. [5] S. Jomori, K. Komatsubara, N. Nonoyama, M. Kato, and T. Yoshida, J. Electrochem. Soc. , vol. 160, no. 9, F1067-F1073, 2013, doi: 10.1149/2.103309jes. [6] Energy.gov, U.S. DRIVE, Fuel Cell Technical Team Roadmap . Available: https:// www.energy.gov / eere/ vehicles/ us- drive

During steady-state operation, the proton conduction profile and the concentration profiles of the reactants and products transported through catalyst layers are non-uniform in the in-plane and through-plane directions. It is, therefore, a reasonable hypothesis that the optimal arrangement of the constituents of the catalyst layers should also be non-uniform. One way to address the non-uniformity is through graded catalyst layers. This study elucidates the state-of-the-art for graded catalyst layers, which so far were primarily investigated for proton exchange membrane fuel cells (PEMFCs). We identify the most impactful types of gradients in the PEMFC cathode and highlight studies displaying their merits in terms of better conversion efficiencies and longer lifetimes. Furthermore, two critical issues that have received little attention so far are emphasized: on the one hand, industrially relevant manufacturing techniques must be developed and implemented. On the other hand, suitable techniques are needed to identify and characterize the gradients. In this study, guidance to navigate both of these challenges is offered.

Molecular hydrogen has exceptionally high gravimetric energy density but low volumetric energy density at ambient conditions. This discrepancy requires technical measures to increase its volumetric energy density for widespread hydrogen applications. Compressed hydrogen and liquified hydrogen are the most established hydrogen storage technologies. In addition, thermocatalytic liquid organic hydrogen carriers (LOHC) have drawn significant attention in the last decade. [1] LOHC couples consist of a hydrogen-lean and a hydrogen-rich form that can be reversibly converted into each other on demand. So far, only a few approaches adapted this hydrogen storage concept to electrochemistry. However, there is strong interest in electrochemical liquid organic hydrogen carriers (EC-LOHC) as electrochemical processes offer significant advantages like flexible operation, device size, scalability, and system simplicity. [2] This contribution demonstrates the concept of acetone/isopropanol as an electrochemical liquid organic hydrogen carrier focusing on the electrochemical dehydrogenation of isopropanol in a polymer electrolyte membrane electrode assembly (PEM-MEA) setup. The isopropanol oxidation at a PtRu electrode shows two distinct oxidation peaks at 0.18 V vs. RHE and 0.75 V vs. RHE, whereby the first oxidation peak is associated with a selective oxidation of isopropanol to acetone, electrons and protons. [3] The produced protons travel from the PtRu anode through the PEM and recombine with the electrons at the Pt cathode to form hydrogen. Therewith, this configuration is an electrolysis cell that produces hydrogen out of isopropanol upon polarization above the reversible cell potential. The study investigates the influence of acetone/isopropanol concentrations, temperature, and flow rates on the I-V characteristics of the electrochemical dehydrogenation of isopropanol at voltages below 0.35 V, revealing its possible potential as EC-LOHC. References [1] D. Teichmann, W. Arlt and P. Wasserscheid, International Journal of Hydrogen Energy 2012 , 37, 18118–18132. [2] A.J. Bard, M. Stratmann, Encyclopedia of Electrochemistry Volume 5 Electrochemical Engineering , Wiley, 2007 , 4. [3] P. Khanipour, F. D. Speck, I. Mangoufis-Giasin, K. J. J. Mayrhofer, S. Cherevko, and I. Katsounaros, ACS Applied Materials & Interfaces 2020 12 (30), 33670-33678. Figure 1

Liquid organic hydrogen carriers (LOHC) offer a promising option to store and release hydrogen on demand within existing infrastructure. The direct isopropanol fuel cell (DIFC) uses the electrochemical acetone/isopropanol LOHC couple and combines the advantages of high fuel energy density at ambient conditions with CO2-free direct electricity production. Like other alcohol fuel cells, the DIFC combines two kinetically slow reactions, the isopropanol oxidation reaction (IOR) and the oxygen reduction reaction (ORR), requiring considerable overpotentials to drive the reactions. Accordingly, deconvoluting kinetic characteristics in the full cell is difficult. Therefore, this work uses the electrolytic electrochemical dehydrogenation unit (EDU), consisting of the IOR and the kinetically fast hydrogen evolution reaction in acidic media. This EDU then serves as an IOR full-cell model to get insights on the DIFC. Correspondingly, the demonstrated work is a comparison study investigating in-house fabricated catalyst-coated membrane electrode assemblies as hydrogen fuel cells, DIFC, and EDU. It investigates characteristic features of the DIFC and demonstrates how the acetone and isopropanol crossover affect the cathode of the DIFC.

Grading electrodes is a promising approach to reduce ohmic and mass transport-related voltage losses in proton exchange membrane fuel cells. In graded electrodes, the ionomer and/or catalyst are spatially non-uniformly distributed to optimize performance over a wide operating range. In this study, through-plane ionomer gradients were fabricated with a simple wet-layer deposition technique that can readily be adapted in a roll-to-roll process. The presence of ionomer gradients was verified using scanning transmission electron microscopy and the profiles were found to be continuous despite using only two coating steps. Single cell tests revealed that the gradients outperform conventional electrodes and maintain the optimal performance for different relative humidities. These improvements were traced back to enhanced mass transport and protonic conduction properties identified by detailed electrochemical analysis. This manufacturing approach for electrodes offers an accessible toolbox to produce versatile and specialized multi-layered catalyst layers for different applications. (c) 2024 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open accessarticle distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY,https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

Liquid organic hydrogen carrier (LOHC) systems offer a promising way to store hydrogen using the existing infrastructure for liquid fuels. While LOHC hydrogenation and dehydrogenation processes have so far mainly been investigated using thermocatalytic processes, this work explores the concept of a low-temperature (<80 degrees C) electrochemical acetone/isopropanol LOHC cycle and indicates its potential benefits for a future hydrogen economy. This electrochemical liquid organic hydrogen carrier (EC-LOHC) system builds on low-cost chemicals with low ecotoxicology. In this study, the influence of temperature and fuel concentrations on the polarization curves of the electrochemical hydrogenation and dehydrogenation units in a small, single-cell set-up is investigated using proton exchange membrane fuel cell components. Based on the experimental results, efficiencies are determined for a power-to-power cycle that can be competitive to mature hydrogen storage technologies, such as liquid and compressed hydrogen storage. Finally, material-related challenges are discussed, encouraging future research in this new field of hydrogen storage.

Molecular hydrogen is a gas at ambient conditions and requires compression or cryogenic liquefaction for storage and transport, leading to significant losses in efficiency. Therefore, liquid hydrogen carriers are desirable as they are storable without losses. One such fuel that has shown promise recently is 2-propanol (isopropanol), which is converted with high selectivity to acetone in a fuel cell. [1],[2],[3] Power densities over 200 mW/cm 2 have been reported for direct isopropanol fuel cells (DIFCs), which rivals the best direct methanol fuel cells (DMFCs), and unlike methanol, isopropanol is non-toxic and it is not fully oxidized to CO 2. [4] The literature has demonstrated many advantages of DIFCs, yet significant challenges remain. [5] Continuous operation at high current densities can be limited due to anodic catalyst layer network effects: slow removal of acetone from active sites and out of the catalyst layer. Additionally, perfluorinated sulfonic acid (PFSA) ionomers are not fully chemically compatible with 2-propanol and acetone. Operation at higher temperatures with a vaporized 2-propanol feed partially mitigates both of these disadvantages, yet the high energy cost of vaporization reduces the overall efficiency of the system. Future research should focus on stable, high-power operation with a liquid 2-propanol feed. This contribution chronicles the progress that has been made and discusses the current state-of-the-art and outlook of direct electrification with DIFCs. Recent advances in liquid-fed DIFCs are presented, including design of membrane-electrode-assemblies and operation strategies in full cells. Finally, the concept of using the 2-propanol/acetone couple as an electrochemical liquid organic hydrogen carrier is briefly introduced: a reversible hydrogenation and dehydrogenation cycle for storing and releasing molecular hydrogen on demand. References [1] D. Cao, S. H. Bergens, J. Power Sources 2003 , 124 , 12–17. [2] Z. Qi, A. Kaufman, J. Power Sources 2003 , 118 , 54–60. [3] M. L. Perry, Z. Yang, J. Electrochem. Soc. 2019 , 166 , F3268-F3276. [4] P. Hauenstein, D. Seeberger, P. Wasserscheid, S. Thiele, Electrochemistry Communications 2020 , 118 , 106786. [5] M. Brodt, K. Müller, J. Kerres, I. Katsounaros, K. Mayrhofer, P. Preuster, P. Wasserscheid, S. Thiele, Energy Technol. 2021 , 9 , 2100164.

Ohmic and mass transport overpotentials remain investigation and optimization areas for Proton Exchange Membrane Fuel Cell (PEMFC) applications. One potential optimization approach involves the preferential distribution of the chemical substituents spatially in the catalyst layer. This would ultimately result in graded catalyst layers (GCLs). The spatial grading directions can thereby be in the in plane and/or through plane direction(s). Chemical constituents often graded in both directions include the binder and the catalyst[1, 2]. The ionomer, an essential constituent for PEMFC applications has also been a key focus area for many research institutes and groups. Novel optimization approaches include finding new ionomers with low equivalent weights (EWs). Typically, in a PEMFC, the highest proton conduction rate is at the membrane interface and decreases towards the GDL[3, 4]. Hence, previous studies with gradients containing high ionomer contents close to the membrane and lower ones at the GDL interface resulted in improved performances. The higher ionomer content at the membrane interface resulted in better protonic conductions, and the bigger void spaces in the GDL interface resulted in improved mass transport properties[1, 5]. Though precise electrochemical analysis is lacking, it is commonly agreed that the graded layers have lower mass transport and ohmic resistances. However, the studies encountered in the literature are almost exclusively based on high EW ionomers, such as Nafion. In contrast, low EW ionomers, which have higher protonic conductivity and water uptake capabilities have not yet been a focus of researchers. Additionally, the few reports encountered in the literature focus solely on GCLs while often disregarding other contributing effects to the overall cell voltage such as the equivalent weight. Low EW ionomers offer a wider operation range for PEMFCs in terms of relative humidities. By combining the low EW ionomers with graded catalyst layers, the operation range becomes even more flexible, and the limits are further stretched. In this study, we push the operating range limits of low EW graded catalyst layers and compare them to non-graded ones. We investigate different manufacturing techniques and use suitable scalable coating methods for the manufacturing of the electrodes. Afterwards, suitable in situ and ex situ analysis techniques are used in detail to trace down the improvements for the gradients. Among the key findings, we notice that potential improvements for low EW ionomer graded catalyst layers become apparent at considerable low relative humidities. The improvements are visible in the kinetic, ohmic, and mass transport regions. References: [1] L. Xing et al., “Membrane electrode assemblies for PEM fuel cells: A review of functional graded design and optimization,” Energy, vol. 177, pp. 445–464, 2019, doi: 10.1016/j.energy.2019.04.084. [2] S. Ebrahimi, B. Ghorbani, and K. Vijayaraghavan, “Optimization of catalyst distribution along PEMFC channel through a numerical two-phase model and genetic algorithm,” Renewable Energy, vol. 113, pp. 846–854, 2017, doi: 10.1016/j.renene.2017.06.067. [3] T. Reshetenko and A. Kulikovsky, “Impedance Spectroscopy Study of the PEM Fuel Cell Cathode with Nonuniform Nafion Loading,” J. Electrochem. Soc., vol. 164, no. 11, E3016-E3021, 2017, doi: 10.1149/2.0041711jes. [4] D. Gerteisen, “Impact of Inhomogeneous Catalyst Layer Properties on Impedance Spectra of Polymer Electrolyte Membrane Fuel Cells,” J. Electrochem. Soc., vol. 162, no. 14, F1431-F1438, 2015, doi: 10.1149/2.0511514jes. [5] Z. Xie et al., “Functionally Graded Cathode Catalyst Layers for Polymer Electrolyte Fuel Cells,” J. Electrochem. Soc., vol. 152, no. 6, A1171, 2005, doi: 10.1149/1.1904990.

The 2‐propanol fuel cell has been shown to hold several key advantages over the more established methanol fuel cell, including a comparably high real open‐circuit voltage, reduced fuel crossover through a Nafion membrane and a benign toxicological fuel profile. In addition, while the highly selective partial oxidation of 2‐propanol to acetone in a fuel cell (rather than the more typical complete combustion of organic fuels to CO 2 ) has been viewed as a disadvantage in the past, recent work has shown that the 2‐propanol/acetone couple is compatible with traditional hydrocarbon liquid organic hydrogen carrier (LOHC) systems though transfer hydrogenation. With this approach, a disadvantage of hydrogen LOHC logistics—the steep energy cost of dehydrogenation that must be provided during energy‐lean times—can be largely avoided. This LOHC compatibility along with the potential for high fuel‐cell performance could place the 2‐propanol fuel cell (also referred to as the direct isopropanol fuel cell or DIFC) in a position to enable a hydrogen energy economy while avoiding the drawbacks of molecular hydrogen transport and storage. In this Review, the purpose is to ascertain the state‐of‐the‐art of DIFCs—an understudied yet promising research area with unique advantages and challenges.

The impact of polyvinylidene fluoride (PVDF) as a binder component on the durability of Pt/C cathodes in a proton exchange membrane fuel cell membrane-electrode-assembly (MEA) during a carbon corrosion accelerated stress test (AST) was examined using electrochemical fuel cell data and visual inspection/analysis of the cathode morphology via electron-microscopy. Electrospun nanofiber cathode mat MEAs with a Nafion®/PVDF or Nafion/poly(acrylic acid) (PAA) binder or a slurry cathode MEA with neat Nafion or a Nafion/PVDF binder were investigated. The presence of PVDF had profound effects on the structure and chemical/electrochemical properties of a fuel cell cathode; its hydrophobic property slowed the rate of carbon loss and its robust mechanical properties added strength to the binder. Thus, the extent of carbon loss during an AST was inversely proportional to the PVDF content of the binder and there was no observable cathode thinning nor any change in cathode porosity after the AST, when the cathode binder contained at least 50 wt% PVDF. In terms of long-term durability, these beneficial structural effects outweighed the lower Nafion/PVDF binder conductivity and the associated lower initial power output of a Nafion/PVDF cathode MEA. For hydrophilic slurry and nanofiber cathodes with neat Nafion or Nafion/PAA fibers, low power after the carbon corrosion AST was due to greater carbon losses, cathode thinning and the collapse of cathode pores, which dominated MEA performance even though the initial cathode ECSA and mass activity were high for these two MEAs.