The increasing requirement for renewable energy places high-temperature proton exchange membrane fuel cells (HT-PEMFCs) on the forefront of “green” energy-generating power devices. Compared to certain PEMFC technologies, HT-PEMFCs possess faster electrode kinetics, high tolerance to fuel poisons and impurities, no humidification requirements, simplified cooling and system design.1 Herein we present optimization strategies of membrane electrode assemblies (MEAs) for HT-PEMFCs, focusing mainly on the (1) component characterization, (2) MEA fabrication, and (3) testing protocols. A selection of gas diffusion electrodes (GDEs) was tested, as well as the presence/absence of a microporous layer (MPL) on the cell performance. The catalyst layer (CL) has been modified in the terms of fabrication methods, catalyst type, platinum (Pt) content, as well as a variety of binders and additives. The PEM optimizations focused on varying and testing: (1) the molecular weight (Mw) of polybenzimidazole (PBI) for membrane casting, (2) PEM thickness, and its (3) acid doping levels (ADL). The MEAs were then fabricated at altering hot-pressing conditions and their performances were compared. The MEA testing focused on activation, humidity levels, high current operations, elevated temperature and pressure operations, as well as long-term stationary and dynamic durability protocol studies. The material fabrication optimizations resulted in higher onset and peak-of-life voltages, as well as longer cell durability. Together with optimized cell testing, we have achieved power densities of more than 0.8 W/cm2, with stationary durability of more than 13,000 hours at 0.3 A/cm2 current density, with the degradation rate of 4 μV/h. Originally the dynamic durability was demonstrated with more than 290 start-stop cycles showing the performance degradation of up to 100 µV/cycle, however, new tests show degradation rates of less than 46 µV/cycle. The performance and durability that we have demonstrated here position HT-PEMFCs as matured technology that can enter the mass market as a commercial and reliable electrochemical power device, to answer the ever-increasing societal energy demands in the age of climate change. Figure 1. Performance of Blue World Technologies MEAs: (A) from different production batches showing small performance scatter, and (B) i-V and power curves at elevated pressures. Reference: 1. O. Jensen, D. Aili, H. A. Hjuler, Q. Li, High Temperature Polymer Electrolyte Membrane Fuel Cells - Approaches, Status and Perspective, ISBN 978-3-319-17081-7; DOI 10.1007/978-3-319-17082-4, Springer International Publishing, New York, 2015. Figure 1

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The vanadium redox flow battery (VRFB) is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy.1 Perfluorinated-based ion exchange membranes such as Nafion are the most widely used membranes in VRFB due to their high proton conductivity. However, the extremely high cost and low ion selectivity of Nafion have limited their further application in VRFB. Among the aromatic hydrocarbon-based membranes that combine both high ion selectivity and high proton conductivity, polybenzimidazole- based membranes have served as one of the most promising alternatives to Nafion due to their excellent chemical and mechanical stability and low cost.2 In the present work m-PBI membranes of varying thickness and modified PBI were prepared, and their properties and VRFB performances were compared. The voltage efficiency (VE) increased with decreasing membrane thickness because of the decreasing ohmic resistance. The excellent current efficiency (CE) of VRFBs using meta-PBI based membranes is attributed to the negligible vanadium crossover observed in ex-situ permeability tests. The stability of the membranes after long term cycling is one of the most challenging problems in large-scale VRB applications. The durability of PBI membrane in VRFB cell testing and the capacity decay was measured by operating the cells for 100 cycles under a high current density of 200 mA cm−2. Fig 1: Flow battery efficiencies using electrodes: 2.5 mm SGL, CR = 20%, membrane: PBI 7 µm at current density of 200 mA/cm2, the electrolyte solution V(IV):V(III) (50:50) in 2.0 M H2SO4 and 0.05 M H3PO4 . The chemical stability of PBI membranes was analyzed after immersion of the PBI samples in various highly oxidizing and reducing sulfuric acid based vanadium and cerium solutions. The membranes stayed unchanged for more than 4 months. This result suggests that the PBI membranes demonstrate excellent ex-situ chemical stability in the harsh acid, oxidizing and reducing conditions. References: 1) Noh, M. Jung, D. Henkensmeier, S. W. Nam and Y. Kwon, ACS Appl. Mater. Interfaces, 9 (2017) 36799-36809. 2) Chen, H. Qi, T. Sun, C. Yan, Y. He, C. Kang, Z. Yuan, X. Li, J. Membr. Sci., 586 (2019) 202-210. Figure 1

The work presented here focuses on recent results obtained on degradation of PBI membranes used in membrane electrode assemblies (MEAs) for high temperature polymer electrolyte fuel cells (HTPEM) that operates in the temperature range 140-180 °C. The testing is done at 160 °C using either pure hydrogen or reformate of different compositions. Pure platinum versus PtCo alloys have been studied. The latest developments show that it is possible to reduce the Pt loading, increase the platinum utilization and achieve a long lifetime. We have shown that cells based on a thermally cured membrane proved a degradation rate of as little as 0.5 μV/h over an extended period of time. This is, to the authors’ knowledge, lower than what is ever reported for HTPEM. The degradation mechanisms over time have been studied using SEM and TEM characterization tools. It has been shown that the Pt nanocatalyst particles grow from approx. 3 to 9 nm over 17,000 hours on the cathode side, while the anode is far less affected. The durability of HTPEM can now be considered similar to low temperature PEM as shown in Fig. 1. Here, more than 15,000 hours is demonstrated in single cells at 300 mA/cm2. The degradation rate is around 4 μV/h for approx. 13,000 hours. The high operating temperature makes it possible to make commercial fuel cell systems using methanol (methanol-water mixtures) as fuel. The HTPEM cells can tolerate CO impurities up to more than 3 vol-% without significant losses. Several demonstration projects have been made, including a Fiat 500 equipped with a 5 kW methanol based fuel cell system. This car is in every day operation at a catering company in Denmark. Another project is a street sweeper with the same type and size of system as shown in Fig. 2. Figure 1

High temperature PEM fuel cell based on phosphoric acid doped polybenzimidazole have reached a quite mature state of development. However, durability is still a critical issue and the key to improving it is a better understanding of the degradation mechanisms. It can be expected that the cells have most, if not all, degradation modes of the low temperature PEM fuel cell (corrosion, particle growth, membrane thinning etc.), but additionally there are issues with the phosphoric acid that the membrane is filled with to provide ionic conductivity at high temperature. In terms of degradation, one could argue that the cell combines the challenges of the low temperature PEM fuel cell and the phosphoric acid fuel cell. Nevertheless, lifetimes on the order of 10.000 hours are demonstrated by several groups [1] In the present study, a large number of cells manufactured by Danish Power Systems were tested with hydrogen in three in-house built multichannel test rigs over several years. The working temperature, the oxidant and fuel flow rates and the current load were varied between 160 and 200 °C, stoichiometry up to about 10 and current densities between 200 and 800 mA cm-2, respectively. The cells were characterized repeatedly by polarization curves and/or electrochemical impedance spectroscopy. After test, selected cells were subjected to post mortem analyses by means of cross section examination by microscopy, X-ray diffraction for platinum particle sized and acid titration. All the major degradation mechanisms were seen (catalyst particle growth, membrane thinning and acid loss). The natural question to answer is which mechanisms dominate under which conditions. It was a general trend that degradation rates increased significantly with temperature and with current density. The temperature of the cell is normally measured outside the gas diffusion layers and even outside the channel plates. Since the produced heat is liberated within less than a hundred micrometers (membrane and catalyst layers) and since the transport away from that area is driven by the temperature gradient, one must expect a significantly higher temperature in the center of the cell than the temperature measured in the channel plates or end plates of the test cell. At higher current densities, the heat evolution rate is higher and thus a higher central temperature is expected. The temperature was measured inside the membrane, but the temperatures could not alone explain the increased degradation rate at higher current densities. A stronger effect seems to origin from the reactant flow rates. Some test series were carried out at high flow rates to mimic an air-cooled stack and in those cases, degradation was significant even at moderate current densities. A combination of acid loss and acid dehydration is suggested as decisive, although dehydration should at the same time limit acid evaporation. The presentation will review the findings of the temperature/load/flow matrix studies and suggest conclusions on the main factors limiting durability of high temperature PEM fuel cells.   [1] M. T. Dalsgaard Jakobsen, J. O. Jensen, L. N. Cleemann and Q. Li. Chapter 22. Durability Issues and Status of PBI Based Fuel Cells. In Q. Li, D. Aili, H. A. Hjuler and J. O. Jensen (eds). High Temperature Polymer Electrolyte Membrane Fuel Cells- Approaches, Status and Perspectives. Springer-Verlag 2016 Figure 1