High Temperature Polymer Electrolyte Membrane Research Articles

The Grotthuss mechanism for bifunctional proton transfer in poly(benzimidazole).

Poly(benzimidazole) (PBI) has received considerable attention as an effective high-temperature polymer electrolyte membrane for fuel cells. In this work, the Grotthuss mechanism for bifunctional proton transfer in PBI membranes was studied using density functional theory and transition state theory. This study focused on the reaction paths and kinetics for bifunctional proton transfer scenarios in neutral ([PBI]2), single (H+[PBI]2) and double-protonated (H2+[PBI]2) dimers. The theoretical results showed that the energy barriers and strength for H-bonds are sensitive to the local dielectric environment. For [PBI]2 with ε = 1, the uphill potential energy curve is attributed to extraordinarily strong ion-pair H-bonds in the transition structure, regarded as a ‘dipolar energy trap’. For ε = 23, the ion-pair charges are partially neutralized, leading to a reduction in the electrostatic attraction in the transition structure. The dipolar energy trap appears to prohibit interconversion between the precursor, transition and proton-transferred structures, which rules out the possibility for [PBI]2 to be involved in the Grotthuss mechanism. For H+[PBI]2 and H2+[PBI]2 with ε = 1, the interconversion involves a low energy barrier, and the increase in the energy barrier for ε = 23 can be attributed to an increase in the strength of the protonated H-bonds in the transition structure: the local dielectric environment enhances the donor–acceptor interaction of the protonated H-bonds. Analysis of the rate constants confirmed that the quantum effect is not negligible for the N–H+ … N H-bond especially at low temperatures. Agreement between the theoretical and experimental data leads to the conclusion that the concerted bifunctional proton transfer in H2+[PBI]2 in a high local dielectric environment is ‘the rate-determining scenario’. Therefore, a low local dielectric environment can be one of the required conditions for effective proton conduction in acid-doped PBI membranes. These theoretical results provide insights into the Grotthuss mechanism, which can be used as guidelines for understanding the fundamentals of proton transfers in other bifunctional H-bond systems.

High-Efficiency Combined Heat and Power through a High-Temperature Polymer Electrolyte Membrane Fuel Cell and Gas Turbine Hybrid System

High-temperature proton-exchange membrane fuel cells are a promising technology for distributed power generation thanks to their high-power density, high efficiency, low emissions, fast start-up, and excellent dynamic characteristics, together with their high tolerance to CO poisoning (i.e., CO in the feed up to 3%). In this paper, we present an innovative, simple, and efficient hybrid high-temperature proton-exchange membrane fuel cell gas turbine combined heat and power system whose fuel processor relies on partial oxidation. Moreover, we demonstrate that the state-of-the-art fuel processors based on steam reformation may not be the optimal choice for high-temperature proton-exchange membrane fuel cells’ power plants. Through steady-state modeling, we determine the optimal operating conditions and the performance of the proposed innovative power plant. The results show that the proposed hybrid combined heat and power system achieves an electrical efficiency close to 50% and total efficiency of over 85%, while a state-of-the-art system based on steam reformation has an electrical efficiency lower than 45%. The proposed innovative plant consists of a regenerative scheme with a limited power ratio between the turbine and fuel cell and limited optimal compression ratio. Therefore, micro-gas turbines are the most fitting type of turbomachinery for the hybrid system.

Investigation of Different Electrochemical Corrosion Treatments on Bipolar Plates for High and Low Temperature Polymer Electrolyte Membrane Fuel Cell Application

Bipolar plates (BPP) play an important role in a fuel cell system.1 They take on several tasks, such as distributing the reactants homogeneously over the active membrane-electrode-assembly surface, electrical and thermal conductivity and preventing leakages.1-2 The aim of this present research is to investigate the corrosion behaviour of BPP materials for high and low temperature (HT and LT) polymer electrolyte membrane fuel cell (PEMFC) applications. For this study BPP materials were investigated by different electrochemical corrosion methods using a three-electrode cell setup shown in figure 1. The three-electrode cell setup was utilised for bipolar plates corrosion at the DLR institute before.3 As working electrode a BPP sample with an active area of 1 cm2, as counter electrode a carbon-rod and as reference electrode a reversible hydrogen electrode (RHE) was used. To investigate the electrochemical stability of the BPP for use in HT- and LT-PEMFC potentiostatic conditions such as open circuit, anodic and cathodic potentials were recorded in conc. (~14.6 M) phosphoric acid (PA) and 0.5 M sulfuric acid, respectively. Cyclic voltammograms (CV) were plotted before and after the above mentioned potentiostatic treatments to evaluate the oxidation resistance behaviour of the BPP. Due to stability limitations of the set-up, the electrochemical measurements were carried out at room temperature. Therefore BPP for use in HT-PEMFC were chemical aged at 160 °C in conc. PA before electrochemical corrosion treatments to achieve good agreement with HT-PEMFC conditions. Figure 2 shows an example of the CVs of pristine, after chemical and electrochemical aged BPP for use in HT-PEMFC. Typical quinone/hydroquinone peaks can be observed between 0.6 and 0.7 VRHE. The double layer capacitance is increasing after each aging step which can be explained by the oxidation of carbon surface leading to BPP corrosion. Further investigations with methods like scanning electron microscopy, confocal microscopy, µ-computed tomography and titration will be shown to correlate surface area changes and acid uptake with corrosion measurement results. References Hermann, A.; Chaudhuri, T.; Spagnol, P., Bipolar plates for PEM fuel cells: A review. Int. J. Hydrogen Energy 2005, 30, 1297-1302. Antunes, R. A.; de Oliveira, M. C. L.; Ett, G.; Ett, V., Carbon materials in composite bipolar plates for polymer electrolyte membrane fuel cells: A review of the main challenges to improve electrical performance. J. Power Sources 2011, 196 (6), 2945-2961. Pilinski, N.; Käding, C.; Dushina, A.; Hickmann, T.; Dyck, A.; Wagner, P., Investigation of Corrosion Methods for Bipolar Plates for High Temperature Polymer Electrolyte Membrane Fuel Cell Application. Energies (Open Access Journal) 2020, 13 (1), 235. Figure 1

(Invited) Reaction Kinetics and Gas Permeability Properties of Thin Film Ionomers for High-Temperature Polymer Electrolyte Membrane Systems

Polymer electrolyte membranes and binders are important materials to the performance and efficiency of numerous electrochemical processes such as electrolyzers and fuel cells. This talk presents the impetus behind the re-emergence of high-temperature polymer electrolyte membrane (HT-PEM) fuel cells. New polymer architectures based upon polycation-acid anion interactions have resulted in superior HT-PEMs in terms of ionic conductivity and stability over the classic phosphoric acid imbibed polybenzimidazole (PBI). Despite the advent of more functional membranes, gas reactant transport and reaction kinetic limitations in electrode layers still stymie the power density of HT-PEM fuel cells. To address these problems, our lab has leveraged high-throughput experimental methods to study the electrochemical properties of thin film high-temperature polymer electrolytes. These methods include a new class of interdigitated electrodes featuring nanoscale electrocatalysts from block copolymer templating. Furthermore, the new HT-PEMs and binders developed in our lab are effective for carrying out challenging hydrogen separations from reforming effluents.

High-Temperature PEM Fuel Cells: Towards Decreasing the Pt Loadings

The commercialization of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) has been limited by (1) their considerable platinum (Pt) loadings, and (2) durability, compared to low temperature PEMFCs (LT-PEMFC). However, the elevated temperature in HT-PEMFC (140 – 160 °C) ensures resilience towards catalytic poisoning from CO (<3%) and H2S (<20 ppm) as fuel impurities and SO2 (<40 ppm) as air pollution. This enables the use of methanol, via reforming, as an affordable and environmentally benign fuel.[1] These temperature-driven advantages have been the force behind the development of the HT-PEMFC technology.We have been optimizing methods and materials to enhance Pt utilization and cell lifetime. Recent results showed cell peak performance >400 mW/cm2 using Pt loadings as low as 0.1 mgPt/cm2, however, with lab scale MEAs operating with pure hydrogen. [1,2] Herein we present efforts to decrease Pt loading while retaining or improving the cell performance on industrial scale (humidified reformate feed). These efforts can be split in two different approaches: (1) the MEA materials optimizations, and (2) MEA fabrication process optimizations. The former focused on studying the MEA onset voltage after the 24 – 48 h cell activation interval, versus the optimal MEA Pt loadings. Efforts to decrease the Pt amounts were expanded to comparing the performances of pure Pt and Pt/Co (Cobalt) -alloyed catalysts. The Pt/Co catalyst was superior towards the oxygen reduction reaction (ORR) as compared to its monometallic counterpart. Additionally, the Pt loading in the MEA was lowered by diluting the catalyst with pristine carbon support particles. The goal was to homogeneously distribute Pt towards the bulk of the catalytic layer (CL) so the phosphoric acid (PA) front, originating from the pressed PA-doped polybenzimidazole (PBI) membrane, does not significantly flood Pt nanoparticles. Increased mass activity was demonstrated for cells in which the Pt/C catalyst was “diluted” by additional carbon particles. The effect is explained by adaptation to a dynamic flooding effect. Furthermore, we demonstrate that a pressure increase of the in-going gases (1.5 BarA) enhanced the operating voltage and the cell showed 500 mW/cm2 at 900 mA/cm2. This entails that for the same performance, the Pt MEA content could be reduced.Additionally, the MEA fabrication processes were optimized with the focus on fine-tuning the (1) hot-pressing conditions and (2) the thermal treatment of the electrodes. The parameter changes affected the cell performance favorably, both qualitatively and quantitatively. By optimizing the MEA fabrication processes, the cells exhibited positive activation trends and up to 50 mV voltage increase.Several demonstration projects have been made for automotive and stationary applications to showcase a new generation of economically more efficient MEAs.

Experimental Analysis on the Influence of Operating Profiles on High Temperature Polymer Electrolyte Membrane Fuel Cells

The Energy System lab at the University of Trieste has carried out a study to investigate the reduction in performance of high temperature polymer electrolyte membrane (HTPEM) fuel cell membrane electrode assemblies (MEAs) when subjected to different ageing tests. In this study, start and stop cycles, load cycles, open circuit voltage (OCV) permanence and constant load profile were considered. Polarization curves (PC) together with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) data were recorded during the ageing tests to assess the fuel cell performance. In this paper, experimental data are presented to confirm the test methodology previously proposed by the authors and to quantitatively correlate the performance degradation to the operational profiles. It was demonstrated that OCV condition, start and stop and load cycling increase degradation of the MEAs with respect to constant load operation. As expected, the OCV is the operational condition that influences performance degradation the most. Finally, the MEAs were analyzed with synchrotron small angle X-ray scattering (SAXS) technique at the Austrian SAXS beamline at Elettra-Sincrotrone Trieste to analyze the nano-morphological catalyst evolution. As for the catalyst morphology evolution, the ex situ SAXS methodology proposed by the authors is confirmed in its ability to assess the catalyst nanoparticles aggregation.

Investigation of Different Electrochemical Corrosion Treatments on Bipolar Plates for High and Low Temperature Polymer Electrolyte Membrane Fuel Cell Application

This present research focusses on the corrosion behavior of bipolar plate (BPP) materials for high and low temperature (HT and LT) polymer electrolyte membrane fuel cell (PEMFC) applications. BPP materials were investigated by different electrochemical corrosion methods using a three-electrode cell setup. Potentiostatic conditions were recorded in concentrated phosphoric acid (PA) and diluted sulfuric acid, respectively, to investigate the stability of BPP for use in HT- and LT-PEMFC. Cyclic voltammograms (CV) were plotted before and after the potentiostatic treatments to evaluate the oxidation resistance behavior of the BPP. Further investigations with methods like scanning electron microscopy (SEM), confocal microscopy, µ-computed tomography (µ-CT) and titration will be shown to correlate surface area changes with corrosion measurement results.

Ionic Liquid in Phosphoric Acid-Doped Polybenzimidazole (PA-PBI) as Electrolyte Membranes for PEM Fuel Cells: A Review.

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.

A Low-Temperature Carbon Encapsulation Strategy for Stable and Poisoning-Tolerant Electrocatalysts.

Carbon encapsulation is an effective strategy for enhancing the durability of Pt-based electrocatalysts for the oxygen reduction reaction (ORR). However, high-temperature treatment is not only energy-intensive but also unavoidably leads to possible aggregation. Herein, a low-temperature polymeric carbon encapsulation strategy (≈150°C) is reported to encase Pt nanoparticles in thin and amorphous carbonaceous layers. Benefiting from the physical confinement effect and enhanced antioxidant property induced by the surface carbon species, significantly improved stabilities can be achieved for polymeric carbon species encapsulated Pt nanoparticles (Pt@C/C). Particularly, a better antipoisoning capability toward CO, SOx , and POx is observed in the case of Pt@C/C. To minimize the thickness of the catalyst layer and reduce the mass transfer resistance, the high mass loading Pt@C/C (40 wt%) is prepared and applied to high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). At 160°C, a peak power density of 662mW cm-2 is achieved with 40% Pt@C/C cathode in H2 -O2 HT-PEMFCs, which is superior to that with 40% Pt/C cathode. The facile strategy provides guidance for the synthesis of highly durable carbon encapsulated noble metal electrocatalysts toward ORR.

A Review of Recent Developments and Advanced Applications of High-Temperature Polymer Electrolyte Membranes for PEM Fuel Cells

This review summarizes the current status, operating principles, and recent advances in high-temperature polymer electrolyte membranes (HT-PEMs), with a particular focus on the recent developments, technical challenges, and commercial prospects of the HT-PEM fuel cells. A detailed review of the most recent research activities has been covered by this work, with a major focus on the state-of-the-art concepts describing the proton conductivity and degradation mechanisms of HT-PEMs. In addition, the fuel cell performance and the lifetime of HT-PEM fuel cells as a function of operating conditions have been discussed. In addition, the review highlights the important outcomes found in the recent literature about the HT-PEM fuel cell. The main objectives of this review paper are as follows: (1) the latest development of the HT-PEMs, primarily based on polybenzimidazole membranes and (2) the latest development of the fuel cell performance and the lifetime of the HT-PEMs.

Nitrogen-functionalized carbon-supported Pt catalysts implemented in high-temperature polymer electrolyte membrane fuel cell

Nitrogen-modified carbon supports were investigated in the catalyst layer of a high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) with respect to fuel cell performance, kinetic, and transport-related voltage losses. Catalysts with different amounts of nitrogen functional groups (NHx) were synthesized and characterized by elemental analysis, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The interaction between the nitrogen-containing functional groups and the phosphate ions was investigated in RDE experiments. The experiments showed that in the case of the nitrogen-functionalized catalysts, the activity is reduced by a factor of up to 2 compared with a commercial Pt/C catalyst in 0.1 M HClO4 + 0.05 M H3PO4. The interaction was further confirmed by membrane electrode assembly (MEA) testing. Impedance spectra and distribution of relaxation times (DRT) analysis were used to pinpoint the voltage losses of the fuel cell. We show that a low amount of nitrogen-containing functional groups improves performance; this can be attributed to the improved phosphoric acid distribution in the catalyst layer. This method can successfully control the acid distribution but must be fine-tuned for extended cell operation.

New ionic liquid-based polyoxadiazole electrolytes for hydrogen middle- and high-temperature fuel cells

New polyoxadiazole-based supported ionic liquid membranes were designed and elaborated for the first time to be used as a middle- and high-temperature polymer electrolyte membrane for fuel cell application. The proposed membrane molecular design combined hydroxyl-functionalized sulfonated polyoxadiazole, hydrophilic polyvinylpyrrolidone and specific protic ionic liquid, thus allowing elaboration of membranes with remarkable physical-chemical characteristics. It is found that the fine tuning of the membranes properties is possible by varying the ratio of polyoxadiazole and polyvinylpyrrolidone. The obtained membranes reveal high thermal stability in dynamic and static modes (up to 350 °C), good oxidative stability, sufficient mechanical properties (Young's modulus of 0.3–21.4 MPa and elongation at break from of 6 to 174%), high degree of ionic liquid impregnation (up to 297%), stable and relatively high conductivity (up to 1.3 mS∙cm−1) in a wide temperature range (from 40 to 120 °С) and under dry conditions. All these properties make the elaborated membranes very attractive for middle- and high-temperature fuel cell application.

Boosting the catalytic efficiency of platinum nanoparticles supported on pristine carbon nanotubes: Synergistic effects of conducting polymers

An advanced energy conversion catalyst comprising pristine carbon-nanotubes and conductive polyaniline polymer successfully synthesized and characterized for high temperature and non-humidified polymer electrolyte membrane fuel cells. The fabricated polyaniline-based catalyst shown a good distribution of platinum nanoparticles (Pt-NPs), and displayed an electrochemical surface area of 645 cm2/mgPt. The catalyst indicated remarkable catalytic activity towards the oxygen reduction reaction (ORR). The overpotential of ORR, the open-circuit voltage, and the power density of the polyaniline-based catalyst declined by a 30 mV, increased by a 50 mV, and enhanced by 26.5%, respectively, when compared to the polybenzimidazole-based catalyst. The impedance measurements have emphasized the improved catalytic activity and the enhanced fuel cell performance of the polyaniline-based catalyst, showing a remarkable reduction of 45% in the charge transfer resistance. This study revealed the advantages of using polyaniline as an anchor to improve the Pt-catalytic activity, opening the door for the wide applications of hydrogen energy.

Facile synthesis and properties of poly(ether ketone cardo)s bearing heterocycle groups for high temperature polymer electrolyte membrane fuel cells

Novel high temperature polymer electrolyte membranes (HT-PEMs) are fabricated from commercial and low-cost poly (ether ketone cardo) (PEK-Cardo). Rather than normal chloromethylation and quaternization procedure to introduce basic groups into poly (arylene ether)s, three different long side-chain basic groups are grafted into PEK-Cardo through a facile one-step lactamization between the cardo phenolphthalein type moiety in PEK-Cardo and amino compounds (i.e. 1-(3-aminopropyl) imidazole (APIm), 1-(3-aminopropyl) pyrrolidin-2-one (APy) and 3-(dimethylamino)-1-propylamine). The obtained lactamized PEK-Cardo membranes exhibit excellent phosphoric acid doping capability and the physicochemical properties are optimized by adjusting the chemical structure and grafting degree of side-chain basic groups. Fuel cell results demonstrate a big potential for HT-PEM fuel cell applications. Thus, this proposal provides a new, straightforward and low-cost method to prepare high-performance HT-PEMs.

Enhanced performance of high temperature polymer electrolyte membrane fuel cell using a novel dual catalyst layer structured cathode

BackgroundHigh cost is a main barrier of commercialization for high temperature polymer electrolyte membrane fuel cell (HT-PEMFC). Thus, enhancing the efficiency of platinum (Pt) utilization and improving the performance of HT-PEMFC is expected to promote its commercialization. MethodsA novel cathode with dual catalyst layer (DCL) structure is designed for enhancing the performance of HT-PEMFC. The catalyst is mainly distributed in outer CL (next to membrane) to concentrate Pt, while a small amount of catalyst is used in inner CL (close to gas diffusion layer). High-Pt-content catalysts (40, 50 wt% Pt/C) and low-Pt-content catalysts (10, 20 wt% Pt/C) are utilized in outer and inner CLs, respectively, to maintain the same thicknesses of two CLs. Significant findings2/4-DCL gas diffusion electrode (GDE) with an outer CL consists of 40 wt% Pt/C and an inner CL consists of 20 wt% Pt/C shows the highest performance among prepared DCL structured GDEs, which also shows a significantly higher performance than the conventional single CL structured GDEs due to the novel structure of DCL. Finally, a good durability of 2/4-DCL GDE indicates the feasibility in practical HT-PEMFC.

Polybenzimidazoles via polyamidation: A more environmentally safe process to proton conducting membrane for hydrogen HT-PEM fuel cell

The article presents a new environmentally safe synthetic approach towards durable polybenzimidazole proton-conducting membrane via polyamidation at ambient conditions and subsequent thermal heterocyclization of films cast from reaction solution followed by phosphoric acid doping. The method avoids exhausts, acid waste and inorganic salts, compared with the existing methods and technologies, allowing to minimize the negative environmental impact related to the process. The membrane possesses an excellent proton conductivity of 105 mS cm−1 and good mechanical properties and has been successfully tested in high temperature polymer electrolyte membrane fuel cells. An extremely low hydrogen crossover through the membrane of 0.040 mA cm−2 at 180 °C has been registered compared with quite high 4–5 mA cm−2 for Celazole® m-PBI.

Electrochemical Properties of High-Temperature Polymer Electrolyte Thin Films

Electrode binders based on ionomer chemistries have a profound impact on the performance of polymer electrolyte membrane (PEM) fuel cells. High temperature PEM fuel cells (HT-PEMFCs) typically use phosphoric acid (H3PO4) imbibed polymers or polymers without ionic moieties as the binder (e.g., PTFE). The former materials hinder gas reactant transport and compromise electrode kinetics, while the latter materials suffer from severe ohmic penalties in the electrode layers. The ionomer binder limitations for HT-PEMFCs stymie power density and lead to poor utilization of the costly platinum catalyst. This talk presents our work probing the electrochemical properties of high-temperature ionomer thin films. The polymer electrolytes investigated are phosphoric acid imbibed polycations as well as phosphonated polymers. Thin films are examined because they mimic the structure found in electrode layers and they can be interrogated with interdigitated electrode (IDE) arrays featuring nanoscale electrocatalysts. The IDE platform is useful for drawing definitive conclusions about the electrochemical behavior of ionomer binder materials without being muddled by the complexity of bulk electrodes. Our work has shown that phosphonated ionomers display better hydrogen diffusivity and hydrogen oxidation reaction kinetics than acid imbibed polycations. The phosphonated ionomers can match the anhydrous proton conductivity of acid imbibed polymers provided they contain electron withdrawing moieties in the polymer. The final part of the talk presents Machine Learning (ML) tools (e.g., support vector regression, density-based clustering, and self-organized maps) that use data from IDEs and single-cell polarization. The ML bridges materials properties to device performance and is poised to accelerate the timeline for realizi

Effect of catalyst layer microstructures on performance and stability for high temperature polymer electrolyte membrane fuel cells

The microstructures of catalyst layers (CLs) provide the paths for phosphoric acid (PA) invasion and decide the amount and distribution of PA in CLs, which is essential to improve the performance and stability of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). In this work, the CLs with different microstructures are constructed and the effects of Pt loading on the performance and degradation of HT-PEMFCs are studied. The results show the CLs with flat surface slow down the process of PA invasion and well-developed pore structures promote the redistribution of PA, which results in low mass transfer resistance. Therefore, the peak power density of HT-PEMFC based on CLs prepared by ultrasonic-spraying is 1.4 times than that by screen-printing, while the performance degradation is only 11% after accelerated stress test of 30,000 cycles with the Pt loading of 0.5 mg cm−2. Distribution of relaxation times analysis is used to assist the electrochemical impedance spectroscopy to further distinguish the different physicochemical process within cells. The result reveals that mass transfer is affected greatly by the effects of microstructures and Pt loadings, and gets deterioration gradually with the invasion of PA into CLs, which not only makes Pt particle growth but decreases kinetics of oxygen reduction reaction.

A Review on Ionic Liquids-Based Membranes for Middle and High Temperature Polymer Electrolyte Membrane Fuel Cells (PEM FCs).

Today, the use of polymer electrolyte membranes (PEMs) possessing ionic liquids (ILs) in middle and high temperature polymer electrolyte membrane fuel cells (MT-PEMFCs and HT-PEMFCs) have been increased. ILs are the organic salts, and they are typically liquid at the temperature lower than 100 °C with high conductivity and thermal stability. The membranes containing ILs can conduct protons through the PEMs at elevated temperatures (more than 80 °C), unlike the Nafion-based membranes. A wide range of ILs have been identified, including chiral ILs, bio-ILs, basic ILs, energetic ILs, metallic ILs, and neutral ILs, that, from among them, functionalized ionic liquids (FILs) include a lot of ion exchange groups in their structure that improve and accelerate proton conduction through the polymeric membrane. In spite of positive features of using ILs, the leaching of ILs from the membranes during the operation of fuel cell is the main downside of these organic salts, which leads to reducing the performance of the membranes; however, there are some ways to diminish leaching from the membranes. The aim of this review is to provide an overview of these issues by evaluating key studies that have been undertaken in the last years in order to present objective and comprehensive updated information that presents the progress that has been made in this field. Significant information regarding the utilization of ILs in MT-PEMFCs and HT-PEMFCs, ILs structure, properties, and synthesis is given. Moreover, leaching of ILs as a challenging demerit and the possible methods to tackle this problem are approached in this paper. The present review will be of interest to chemists, electrochemists, environmentalists, and any other researchers working on sustainable energy production field.

Effects of Impurities on Pre-Doped and Post-Doped Membranes for High Temperature PEM Fuel Cell Stacks

In this paper, we experimentally investigated two high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) stacks for their response to the presence of reformate impurities in an anode gas stream. The investigation was aimed at characterizing the effects of reformate impurities at the stack level, including in humidified conditions and identifying fault features for diagnosis purposes. Two HT-PEMFC stacks of 37 cells each with active areas of 165 cm2 were used with one stack containing a pre-doped membrane with a woven gas diffusion layer (GDL) and the other containing a post-doped membrane with non-woven GDL. Polarization curves and galvanostatic electrochemical impedance spectroscopy (EIS) were used for characterization. We found that both N2 dilution and impurities in the anode feed affected mainly the charge transfer losses, especially on the anode side. We also found that humidification alleviated the poisoning effects of the impurities in the stack with pre-doped membrane electrode assemblies (MEA) and woven GDL but had detrimental effects on the stack with post-doped MEAs and non-woven GDL. We demonstrated that pure and dry hydrogen operation at the end of the tests resulted in significant recovery of the performance losses due to impurities for both stacks even after the humidified reformate operation. This implies that there was only limited acid loss during the test period of around 150 h for each stack.