Next Generation of Improved High Temperature Membrane Electrode Assembly for Aviation

Challenges and opportunities in modelling of proton exchange membrane fuel cells (PEMFC)

Recently, high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have attracted much attention due to their tolerance to carbon monoxide and simple water management in the system level. HT-PEMFCs mostly use polybenzimidazole membrane doped with phosphoric acid (PA) as electrolyte. Therefore, uniform distribution of PA through a whole MEA plays a critical role in determining fuel cell performance. Distribution of PA in a MEA does not only depend only on properties of membrane such as a doping level of PA in membrane but also on gas diffusion layer(GDE) characteristics, especially the structure of the catalyst layer (CL). At present, the CL for HT-PEMFCs is typically composed of platinum catalyst and a polymeric binder. The polymeric binder maintaining the integrity of CL has an influence on mechanical stability of CL and the PA distribution within MEA. There have been many studies to explore the characteristics of different binders such as polytetrafluoroethylene(PTFE), polybenzimidazole(PBI) and polyvinylidenefluoride(PVDF). As previously described by Huaneng Su et al., PVDF has a good affinity with PA which leads to quick distribution of PA, on the other hand hydrophobic PTFE reduce PA leaching.[1] In this study, dual structured CLs with two different binders(PVDF and PTFE) by controlling the contents in catalyst layer are fabricated to investigate the influence of the wettability of CL surface in terms of fuel cell perfoemance. The properties of these CL were evaluated by structure characterization and electrochemical analysis. At ambient pressure and 150oC, the GDEs with PTFE binder exhibit the highest power density (383mW/cm2) with H2/air which may result from highly hydrophobic CL, leading to improved mass transport. It was suggested that the change in ratio of two binders with different wettabilities respectively had an impact on small arc on EIS. Keywords :high temperature proton exchange membrane fuel cell, catalyst layer, Membrane electrode assembly, poly(2,5-benzimidazole), hydrophobicity, Pt loading, gas diffusion electrode References S. Huaneng, J. Power Sources, 246, 63 (2014).

Experimental study of the potential degradation due to the polarization curve of a high temperature proton exchange membrane fuel cell

Recently, the polymer electrolyte membrane fuel cells (PEMFCs) have attracted the great attention as energy conversion devices for powertrain for the fuel cell electric vehicle and a residential combined heat and power system due to the zero-emission of harmful gas and high energy efficiency. To overcome of the drawbacks of perfluorosulfonic acid (PFSA) membrane and Pt poisoning by CO, the high-temperature PEMFC (HT-PEMFC) has been developing over decades, which has significant advantages as compared to low-temperature PEMFC (LT-PEMFC). For example, it has a much better fuel tolerance, enhanced the reaction kinetics and a simple water management [1,2]. However, the practical power density of membrane electrode assembly (MEA) for HT-PEMFC is much lower than that of LT-PEMFC because of the huge activation loss by acid poisoning [3–5]. Thus, a large amount of Pt is required in the electrode to ensure the sufficient power and long-term durability, which is a critical issue for HT-PEMFC. Therefore, one of the most critical issue for the HT-PEMFC is the decreasing the Pt amount in the electrode without sacrificing the MEA performance [5]. In addition, the fabrication of electrode using the advanced supported catalyst is the very important topic for revealing the improved catalytic activity into performance of MEA [2,5]. In this study, various formulation parameters for making the catalyst slurry using ball-milling were investigated to obtain the smooth electrode for MEA. The parameters are the presence of a viscosity agent, the solid content, and the amount of binder for the MEA of HT-PEMFC. When the viscosity agent is added in the 2.5 wt.% in the catalyst slurry, the slurry is very suitable for making a coated electrode on the gas diffusion layer (GDL). On the contrary, while when the amount of viscosity agent is increased over than 5 wt.%, the electrode is not completely dried. The optimum solid content of catalyst in the slurry was determined in the range of 12 wt.% because if the amount of catalyst exceeded the 14wt.%, the electrode is peeled off severely after drying. The open circuit voltage and performance of MEA under HT-PEMFC condition is depended significantly on the amount of polyvinylidene fluoride (PVDF) in the catalyst slurry. The voltage at a very small current density at 0.008A/cm2 of MEAs with different amount of PVDF was decreased from 0.775 to 0.735 V by increasing the 10wt.% to 30 wt.% of PVDF, which provide the increase of the activation overpotential by poisoning of phosphoric acid on the Pt catalyst. However, the performance of MEA at 0.6A/cm2 was maximized as 0.44 V at the 30wt.% of PVDF in the catalyst slurry, which suggests that the Ohmic overpotential decreased with increasing the PVDF amount. In summary, for maximizing the MEA performance under HT-PEMFC at 150℃, the various conditions for the manufulation of electrode was invesitgated. The amount of PVDF has a significant effect on the performance of MEA. [1] J. Zhang, Y. Xiang, S. Lu, and S. P. Jiang, Adv. Sustainable Syst. 2, 1700184 (2018). [2] D. J. You, D. H. Kim, J. R. De Liled, C. Li, S. G Lee, J. M. Kim, and C. Pak, Appl. Catal. A: General 562, 250–257 (2018). [3] R. E. Rosli, A. B. Sulong, W. R. W. Daud, M. A. Zulkifley, T. Husaini, M. I. Rosli, E. H. Majlan, and M. A. Haque, Int. J. Hydrogen Energy 42, 9293–9314 (2017). [4] A. Chandan, M. Hattenberger, A. El-kharouf, S. Du, A. Dhir, V. Self, B. G. Pollet, A. Ingram, and W. Bujalski, J. Power Sources 231, 264–278 (2013). [5] S.-W. Choi, J. O. Park, C. Pak, K.H. Choi, J.-C. Lee, and H. Chang, Polymers 5, 77–111 (2013).

High Temperature Proton Exchange Membrane Fuel Cells (HT-PEMFC) can reject very small quantities of phosphoric acid due to the ageing of their electrolyte. Also, as a preventive measure, the tests performed with this technology are often not pressure-regulated to prevent the outlet valve opening from clogging in case of acid leakage. In this configuration, atmospheric pressure variations lead to pressure changes in both fuel cell anode and cathode compartments, these pressure variations themselves generating cell voltage variations. The work presented in this article introduces a readjustment method for the fuel cell voltage according to the variations of the operating conditions. This readjustment is based on the realization of a design of experiments to characterize the voltage sensitivity to the ambient pressure and temperature of a commercial membrane electrode assembly (MEA, BASF Celtec®-P 1100W type). An empirical model is identified from these experimental data obtained. It is then used to generate the compensating coefficients to smooth the impact of variations in operating conditions. Finally, the readjustment method is applied and discussed to the fuel cell voltage for a test example.

Membrane electrode assemblies (MEAs) are critical components in influencing the electrochemical performance of high-temperature proton exchange membrane fuel cells (HT-PEMFCs). MEA manufacturing processes are mainly divided into the catalyst-coated membrane (CCM) and the catalyst-coated substrate (CCS) methods. For conventional HT-PEMFCs based on phosphoric acid-doped polybenzimidazole (PBI) membranes, the wetting surface and extreme swelling of the PA-doped PBI membranes make the CCM method difficult to apply to the fabrication of MEAs. In this study, by taking advantage of the dry surface and low swelling of a CsH5(PO4)2-doped PBI membrane, an MEA fabricated by the CCM method was compared with an MEA made by the CCS method. Under each temperature condition, the peak power density of the CCM-MEA was higher than that of the CCS-MEA. Furthermore, under humidified gas conditions, an enhancement in the peak power densities was observed for both MEAs, which was attributed to the increase in the conductivity of the electrolyte membrane. The CCM-MEA exhibited a peak power density of 647 mW cm-2 at 200 °C, which was ~16% higher than that of the CCS-MEA. Electrochemical impedance spectroscopy results showed that the CCM-MEA had lower ohmic resistance, which implied that it had better contact between the membrane and catalyst layer.

The development of reliable power sources is important for the continuous operation of various electric equipment in unmanned aircraft and the field environment. Currently electric power delivery to such systems is mainly by battery packs. An alternative is to use fuel cells (FCs). FCs are electrochemical devices that are used to convert chemical energy of fuels such as hydrogen and methanol to electricity. Methanol is an attractive fuel because it is liquid at ambient temperature, has a much higher energy density than hydrogen and low reforming temperature (220–300 °C). Thus, integration of methanol steam reformers (MSRs) with FCs makes it possible to continuously produce electricity. The key challenge in such power system is the development of fuel cells which can be effectively operated at compatible temperature range of MSR, i.e., 220–300 °C in order to increase synergetic heat integration and system reliability. Herein, the latest development of high temperature polymer electrolyte membrane fuel cells (HT‐PEMFCs) is critically reviewed. The prospect of the integrated HT‐PEMFCs‐MSR as a reliable and compact power source is discussed. The results indicate that phosphoric acid doped polybenzimidazole (PA/PBI) membranes with in situ formed phosphosilicate nanocluster proton carriers show the technical feasibility of the development of HT‐PEMFCs at temperatures of 200–300 °C.

Increasing world energy demand and the rapid depletion of fossil fuels has initiated explorations for sustainable and green energy sources. High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are viewed as promising materials in fuel cell technology due to several advantages, namely improved kinetic of both electrodes, higher tolerance for carbon monoxide (CO) and low crossover and wastage. Recent technology developments showed phosphoric acid-doped polybenzimidazole (PA-PBI) membranes most suitable for the production of polymer electrolyte membrane fuel cells (PEMFCs). However, drawbacks caused by leaching and condensation on the phosphate groups hindered the application of the PA-PBI membranes. By phosphate anion adsorption on Pt catalyst layers, a higher volume of liquid phosphoric acid on the electrolyte–electrode interface and within the electrodes inhibits or even stops gas movement and impedes electron reactions as the phosphoric acid level grows. Therefore, doping techniques have been extensively explored, and recently ionic liquids (ILs) were introduced as new doping materials to prepare the PA-PBI membranes. Hence, this paper provides a review on the use of ionic liquid material in PA-PBI membranes for HT-PEMFC applications. The effect of the ionic liquid preparation technique on PA-PBI membranes will be highlighted and discussed on the basis of its characterization and performance in HT-PEMFC applications.

High-temperature proton exchange membrane fuel cells (HT-PEMFC) directly convert hydrogen and oxygen to produce electric power at a temperature significantly higher than conventional low-temperature fuel cells. This achievement is due to the use of a phosphoric acid-doped polybenzimidazole membrane that can safely operate up to 200 °C. PBI-based HT-PEMFCs suffer severe performance limitations, despite the expectation that a higher operating temperature should positively impact both fuel cell efficiency and power density, e.g., improved ORR electrocatalyst activity or absence of liquid water flooding. These limitations must be overcome to comply with the requirements in mobility and stationary applications. In this work a systematic analysis of an HT-PEMFC is performed by means of electrochemical impedance spectroscopy (EIS), aiming to individuate the contributions of components, isolate physical phenomena, and understand the role of the operating conditions. The EIS analysis indicates that increases in both the charge transfer and mass transport impedances in the spectrum are negatively impacted by air humidification and consistently introduce a loss in performance. These findings suggest that water vapor reduces phosphoric acid density, which in turn leads to liquid flooding of the catalyst layers and increases the poisoning of the electrocatalyst by phosphoric acid anions, thus hindering performance.

Stainless steels for low and high temperature proton exchange membrane fuel cell present several advantages over graphitic materials. The fabrication of bipolar plates from these kinds of alloys could reduce weight of the stack and provide high mechanical stability. Test results show that bipolar plates fabricated from standard V2A or V4A steel can withstand the corrosive conditions in a low-temperature (50 {degree sign}C) proton exchange membrane fuel cell. For water cooled applications with an operating temperature around 80 {degree sign}C and thus resulting heavily corrosive conditions, two high-alloyed steel specimens have proven to be suitable. Further analyses show variations in corrosion resistance after pre-treating steel samples with e.g. physicochemical surface treatments, electro-polishing, thermal annealing and corrosion protective coating solution. The high temperature PEMFC test condition with 85% phosphoric acid and 160 {degree sign}C operation temperature leads to visible corrosion traces on all specimens except one.

Three-dimensional non-isothermal modeling of a phosphoric acid-doped polybenzimidazole (PBI) membrane fuel cell

Investigation of the effect of graphitized carbon nanotube catalyst support for high temperature PEM fuel cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are interesting alternatives to fossil fuel-based technologies for power generation. Their higher operating temperature compared to classic low temperature (LT) PEMFCs is highly beneficial regarding the total system complexity and weight, especially for applications which cannot involve bulky/heavy cooling systems, like aeronautics. However, at the current state-of-art of the technology, the membrane electrode assembly performance in HT-PEMFC does not reach that of LT-PEMFCs and needs to be improved; this requires better catalyst material, catalyst layer structure and membrane doping [1], [2]. A given catalyst performance is largely influenced by phosphoric acid electrolyte poisoning [3], [4] and some studies have already evaluated such poisoning level on various catalysts toward oxygen reduction reaction [5], [6]. The current study further addresses this issue and specifically aims to understand the impact of various parameters which should impact the catalyst activity in H3PO4 electrolytes (loading, nano-particle size and shape, chemistry of the catalyst particles (Pt vs PtNi), density of aggregates, nature of the carbon support) and to evaluate the poisoning effect whether at low potential (anode) or at high potential (cathode).The effect of different parameters was unveiled thanks to a catalyst library (Figure a), analyzed comparatively in 1 M HClO4 and 1 M H3PO4 electrolytes at room temperature with a classic rotating disk electrode set-up and a gas diffusion electrode set-up. Pseudo CO-Stripping voltammetry, Hupd and CO-stripping voltammetry enable to shed light on the poisoning of the Pt surfaces: the electrochemical surface area (ECSA) is divided by a factor around 2 in H3PO4 electrolyte compared to HClO4 and varies according to the catalyst properties. The impact on hydrogen oxidation reaction is then noticeable. The poisoning at high potential (> 0.6 VvsRHE) was mainly evaluated thanks to the ORR activity (Figure b): the Pt nanoparticle size/shape, their loading on the carbon support and alloying with Ni do impact their ORR activity. All the catalysts exhibit significantly lower activity in H3PO4 than in HClO4 (by a factor ca 10). The mechanisms of H3PO4-induced poisoning and potential strategies to mitigate it will be detailed.[1] S. S. Araya et al., “A comprehensive review of PBI-based high temperature PEM fuel cells,” Int J Hydrogen Energy, vol. 41, no. 46, pp. 21310–21344, Dec. 2016, doi: 10.1016/j.ijhydene.2016.09.024.[2] R. E. Rosli et al., “A review of high-temperature proton exchange membrane fuel cell (HT-PEMFC) system,” Int J Hydrogen Energy, vol. 42, no. 14, pp. 9293–9314, Apr. 2017, doi: 10.1016/j.ijhydene.2016.06.211.[3] B. F. Gomes et al., “Effect of phosphoric acid purity on the electrochemically active surface area of Pt-based electrodes,” Journal of Electroanalytical Chemistry, vol. 918, Aug. 2022, doi: 10.1016/j.jelechem.2022.116450.[4] N. Sugishima et al., “Phosphorous Acid Impurities in Phosphoric Acid Fuel Cell Electrolytes: I . Voltammetric Study of Impurity Formation,” J Electrochem Soc, vol. 141, no. 12, pp. 3325–3331, Dec. 1994, doi: 10.1149/1.2059334.[5] Q. He et al, “Influence of phosphate anion adsorption on the kinetics of oxygen electroreduction on low index Pt(hkl) single crystals,” Physical Chemistry Chemical Physics, vol. 12, no. 39, pp. 12544–12555, Oct. 2010, doi: 10.1039/c0cp00433b.[6] K. ‐L. Hsueh et al, “Effects of Phosphoric Acid Concentration on Oxygen Reduction Kinetics at Platinum,” J Electrochem Soc, vol. 131, no. 4, pp. 823–828, Apr. 1984, doi: 10.1149/1.2115707. Figure 1

Introduction of polymeric ionic liquids containing quaternary ammonium groups to construct high-temperature proton exchange membranes with high proton conductivity and stability

<p indent="0mm">The gradual depletion of fossil energy sources and the excessive emission of greenhouse gases have significantly contributed to global warming, profoundly impacting the natural environment. Proton exchange membrane fuel cells (PEMFCs), specifically hydrogen (H₂) fuel cells, offer a promising solution by converting chemical energy directly into electrical energy through electrochemical reactions. PEMFCs boast numerous advantages, including zero emissions, high power density, rapid response times, and a simple structure, effectively mitigating the effects of the energy crisis and environmental degradation. Based on their operating temperatures, PEMFCs are categorized into low-temperature PEMFCs (LT-PEMFCs) and high-temperature PEMFCs (HT-PEMFCs). In LT-PEMFCs, operating at lower temperatures in the presence of water, water serves as the proton transport medium. Conversely, in HT-PEMFCs, proton conductivity relies on a different mechanism. Nafion, which is highly dependent on a hydrated condition, exhibits a dramatic drop in conductivity when the operating temperature exceeds 100°C. Polybenzimidazole (PBI) membranes, known for their excellent thermal and chemical stability, have garnered significant attention for high-temperature PEM applications, providing better proton conductivity under anhydrous conditions. However, large-scale hydrogen production technologies often fail to meet purity requirements due to high associated costs, with less than 5% of the globally produced H₂ coming from electrolytic water. For instance, most available H₂ is derived from methane steam reforming, introducing gaseous impurities such as carbon monoxide (CO), sulfuretted hydrogen (H₂S), and ammonia (NH₃) when used as fuel. These impurities can significantly degrade the performance of the electrode catalyst. The presence of trace amounts of CO in H₂ fuel poisons the catalyst during the reaction process. When reformate H₂ is supplied in a PEMFC, these concentrations CO can cause CO-poisoning and activity degradation. For the commercial Pt anodes, it is noted that the acknowledged maximum CO poisoning limits are 10 and <sc>100 ppm,</sc> respectively. A typical method for overcoming CO poisoning is the further purification of the hydrogen fuel after the reforming process, which can minimize the CO concentration. However, even for the CO concentration limits to <sc>0.2 ppm,</sc> the efficiency of PEMFCs is considerably reduced. Furthermore, the purification cycles required to achieve such low CO concentrations prohibitively increase the costs. Additionally, the release of phosphoric acid (PA) during HT-PEMFC operation can lead to phosphoric acid poisoning, reducing catalyst activity. The catalysts used in PEMFCs, primarily based on precious metals like platinum (Pt), exhibit superior electrocatalytic properties but are susceptible to toxicity and have limited stability. Pt, with its unique d-band structure of 5d electron orbitals, is the most efficient electrocatalysts for PEMFCs. The hydrogen oxidation reaction (HOR) rate on Pt-based nanoparticles is very fast, allowing the Pt load of the anode catalyst to be as low as <sc>0.05 mg cm⁻².</sc> However, the oxygen reduction reaction (ORR) at the cathode is about five orders of magnitude slower than the HOR at the anode, requiring a higher Pt load <sc>(~0.4 mg cm⁻²)</sc> to maintain performance. While Pt-based catalysts are highly efficient for the HOR at the anode, their significantly slower kinetics during the ORR at the cathode require increased Pt loadings. This discrepancy contributes to reduced catalyst stability and higher costs, thereby hindering widespread commercial adoption. Therefore, it is imperative to design and improve Pt-based catalysts to enhance the utilization rate and anti-poisoning ability of Pt. This paper reviews the current application status of Pt-based catalysts. It begins by describing the toxicity challenges faced by PEMFCs, focusing on the design of Pt-based catalysts to improve anti-toxicity. Finally, the development direction of Pt-based catalysts in PEMFCs is discussed and prospected.