Branched/Crosslinked Sulfonated Polyimide Membranes for Polymer Electrolyte Fuel Cells

In this study, the low-humidity proton conductance and degradation mechanisms are theoretically investigated for Nafion®, which is the most frequently used proton exchange membrane. The proton conductance mechanism of Nafion membrane was first explored to reveal the cause for its high proton conductance under low-humidity conditions (Ref. 1). Nafion membrane gets an advantage in proton exchange membranes in fuel cells mainly due to its high low-humidity proton conductance. The actual proton conductance of Nafion at low humidity was explored by comparing to that of hydrocarbon multiblock sulfonated poly(arylene ether sulfone)s (SPE) membrane. Using long-range corrected density functional theory (LC-DFT) that one of the authors (TT) developed (Ref. 2), we calculated the proton dissociation energies from the sulfonic acid groups and investigated the subsequent proton conductance mechanism at low humidity. As a result, we found that these membranes have negligible difference in the proton dissociation ability inside the hydrated water clusters but show a remarkable difference in the conductivity of protonated water clusters in disconnected hydrogen bond networks. Note that low-humidity membranes are known to contain the regions of disconnected hydrogen bond networks (Ref. 3), in which Grotthuss mechanism assuming connected hydrogen bond networks does not proceed. Though vehicle mechanism assuming the diffusion of protonated water molecules (or clusters) seems to explain the proton conductance in the disconnected networks, we found that protonated water molecules and clusters need very large dissociation energies (about 400 kJ/mol). As an alternative, we suggested the relay mechanism (Fig. 1), in which protonated water clusters are relayed between the sulfonic acid groups through the doubly-hydrated structures. The calculated results showed that protonated water clusters require only about 100kJ/mol to dissociate in this relay mechanism. The vibrational spectra of hydrated Nafion membrane was also investigated to reveal the dependence of the proton conductance on humidity in further detail by a collaborative experimental and theoretical infrared (IR) spectroscopy study (Ref. 4). The experimental time-resolved attenuated total reflection Fourier transform IR spectroscopic results showed a unique IR peak intensity dependence on the hydration number. Based on the experimental results, we theoretically explored the dependence of the vibrational spectra of Nafion membrane on the hydration number by using LC-DFT. As a result, we found that this unique IR peak intensity dependence is correctly reproduced for the singly-hydrated but for the doubly-hydrated Nafion, in which protons are initially detached from the sulfonic acid groups in the geometry optimizations. This strongly supports the above-mentioned relay mechanism of the proton conductance. The degradation mechanism of Nafion membrane was also explored focusing on the H2O2-induced dissociation of the ether-linkages of the side chains. Many conventional studies on the degradation of membranes in fuel cells have assumed the principal mode of the degradation involves OH radicals. However, recent experimental studies have raised questions concerning this assumption. For example, a Fenton test, which is supposed to provide OH radicals, have revealed that the ratio of the degradation fragment molecules is considerably different from that of actual operating conditions (Ref. 5). Using LC-DFT, we found an H2O2-induced degradation mechanism, which is consistent with experimental results. In this mechanism, the ether-linkage of the side chains of Nafion is decomposed with a relatively low activation energy to produce (H2O)lHO3S-CF2-CF2-O-OH (l is the hydration number) as a key fragment. We also elucidated the subsequent decomposition reaction mechanisms, though these mechanism need metal ions to proceed. The vibrational spectra for the decomposed Nafion are found to be consistent with the experimental IR spectra. This mechanism is supposed to proceed together with OH radical-induced degradation mechanism. This work was supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cells (SPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. T. Tsuneda, R. K. Singh, K. Miyatake, and M. Watanabe, Chem. Phys. Lett., 608, 11 (2014).H. Iikura, T. Tsuneda, T. Yanai, and K. Hirao, J. Chem. Phys., 115, 3540 (2001).G. Gebel, Polymer, 41, 5829 (2000); Macromolecules, 46, 6057 (2013).R. K. Singh, K. Kunimatsu, K. Miyatake, and T. Tsuneda, Macromolecules, 49, 6621 (2016).T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y. Iriyama, T. Abe, and Z. Ogumi, J. Pow. Sour., 158, 1222 (2006). Figure 1. Schematic diagram for the proton conductance in Nafion membrane in fuel cells. Figure 1

Sulfonated polyimides with flexible aliphatic side chains for polymer electrolyte fuel cells

Proton exchange membrane fuel cell (PEMFC) is considered as a clean and high-efficient power source for portable and on-site applications. Perfluorosulfonic acid (PFSA) polymer is the most common used electrolyte membrane material in PEMFC due to its high proton conductivity and excellent long-term stability under fully hydrated condition. However, its low glass transition temperature limits the operating temperature of PEMFC below 80oC. High operating temperature results in lowered mechanical and dimensional stabilities of PFSA polymer though high temperature operation has many other advantages for PEMFC including higher catalytic and anti-poisoning activities of electrical catalysts, and better controlled water and heat management. Moreover, proton transport capability of PFSA polymer is sensitive to environmental humidity. Membrane dehydration at high temperature sharply decreases its proton conductivity. The required external humidification equipment complicates the design and operation of fuel cell systems and lowers system reliability and energy utilization efficiency, as well as increases their weight and size. Therefore, to develop proton exchange membrane which can be operated under high temperature and low humidity conditions has become a very active research topic. PEMFC has been stably operated under high temperature and low humidity conditions by adding inorganic fillers (e.g., zirconium phosphate, metal oxides, zeolites) into PFSA polymer to prepare polymer-inorganic composite membranes. However, main issues are the increase of proton transport resistance, the aggregation of inorganic filler particles and the appearance of phase separation for the above-mentioned composite membranes. Recently, we have developed and patented a new kind of high-temperature self-humidifying proton exchange membrane (i.e., confined structural composite membrane) by confining PFSA polymer into zeolite-coated porous substrate. Confined structural composite membrane harmonizes the structure and function of different membrane compositions to utilize water retention property of zeolite and avoid the formation of discontinuous phase within PFSA matrix. Confinement effect further improves the performance of structural composite membrane. New confined structural composite membrane can be stably operated up to 160oC without humidification. Self-humidifying hydrogen PEMFC and passive direct methanol fuel cell with new confined structural composite membrane output one order of magnitude higher maximum power densities than those with commercial Nafion 117 membrane. The effect of preparation parameters including zeolite type (e.g., silicate-1, NaZSM-5, HZSM-5, HY), coating thickness, coating morphology (e.g., single-layered, multi-layered, continuous, discontinuous) and PFSA precursor (e.g., solvent, incorporated material) on membrane physicochemical properties and fuel cell performance is also investigated to explore formation mechanisms of high thermal stability and self-humidifying property. On one hand, zeolite coating regulates water within confined structural composite membrane through adsorption of reaction generated water and catalytic formation of water. On the other hand, the confinement of PFSA polymer within zeolite-coated porous substrate limits the shrinkage and swelling of polymer caused by temperature and humidity changes, and induces the oriented rearrangement of polymer chains within confined space.

Hydrogen fuelled Proton Exchange Membrane Fuel Cells (PEMFCs) are being developed as next generation energy delivery devices to replace conventional combustion technology and enable the development of a hydrogen economy. The attraction of the technology is its ability to operate at higher efficiencies than traditional combustion technology, reducing overall harmful gas and particulate emissions. When hydrogen derived from renewable resources is used as the energy vector, an environmentally benign system is created. Despite the promise held by PEMFC technology, important materials design and system engineering challenges still exist, preventing commercialisation. From a materials perspective, proton exchange membranes capable of operating at or above 140oC need to be developed. This minimum operating temperature is necessary for specific applications in transportation, to reduce catalyst poisoning by carbon monoxide and to increase the system’s ability to dissipate heat. System engineering challenges are due to poor understanding of the water balance in the fuel cell, the lack of design metrics for fuel cell systems and a poor understanding of dynamic operation. This thesis explores each of the aforementioned challenges as follows: • A review of the latest developments in solid acid membranes for PEMFCs operating at 140oC is presented. The most promising areas for future advancements are identified, analysed and discussed and the concept of a thin film, inorganic membrane fuel cell is introduced. • Sol-gel synthesised zirconium phosphate membranes are investigated as an alternative to layered zirconium phosphate proton conductors. The materials show conductivity values similar to α-zirconium phosphate. The advantage of the solgel synthesis route is that the materials can be manufactured as thin film proton conducting membranes. • Sol-gel titanium phosphate membranes are synthesised and their proton conduction mechanism and chemical stability are investigated. The materials show stable conductivity values up to 0.0044 S.cm−1 at 100oC and 100% relative humidity. 31P MAS NMR was used to elucidate the functional groups and develop a model for the Grotthuss proton conduction mechanism. • The STR fuel cell, which can fully self humidify when operating with dry feeds, is characterised and demonstrated and design equations are developed. The STR design cedes only a small performance penalty to fully humidified designs and eases the requirement to develop PEMs capable of operating independent of water in conditions below 130oC. • Dynamic operation of the STR autohumidified fuel cell is investigated. Current ignition and extinction phenomena and multiple steady states are characterised. It is shown that through careful choice of the operating parameters the dynamic response of the autohumidified fuel cell can replicate that of hydrated feed systems. • A spatially resolved current density distribution and impedance analysis of a 20 cm2 autohumidified STR fuel cell system is undertaken. The analysis demonstrates that the STR fuel cell design reduces current density gradients in the fuel cell under reduced relative humidity operation and that the performance of the system is superior to a single channel serpentine system. Through its multi-scale approach, this thesis demonstrates the importance of considering the fuel cell as a system. New thermally and chemically stable fuel cell membranes which are capable of operating at temperatures above the glass transition temperature of current state of the art polymers are needed. Additionally, there is a requirement to operate fuel cells with atmospheric condition feed streams while maintaining the hydration of the membranes. Together, the sol-gel titanium phosphate membranes and autohumidified STR fuel cell developed fill this knowledge gap, leading to a closer realisation of a commercial fuel cell system.

Graphene oxide (GO) exhibits relatively high proton conduction even at low relative humidity (RH) and temperatures. Thus, multi-layered GO film (GO paper) is a promising electrolyte in various cells and batteries. We report the in-plane and through-plane proton conductivities of GO paper at low RH and room temperature. The conductivities were 10−5-10−4 S/cm at RH of 10–20%. We examined the performance of a graphene oxide fuel cell (GOFC) with GO paper as the electrolyte at low RH (<20%) and room temperature. We demonstrated that a simple membrane electrode assembly (MEA) based on Pt/GO/Pt system functioned as a H2/O2 fuel cell. The GOFC with Pt/C electrodes afforded better fuel cell performance than a MEA composed of Pt/C electrodes and Nafion electrolyte. GO paper composed of nano-sized GO (nanoGO) flakes enhanced the performance of the GOFC, particularly the current density. Furthermore, a composite of Fe phthalocyanine (PcFe) /reduced GO (rGO)/TiO2 could act as the Pt-free oxygen electrode in GOFC, although the fuel cell performance should be improved for practical applications. Our study suggests that a low cost GOFC without a Nafion electrolyte and noble metal electrodes could be fabricated in the near future.

The durability of proton exchange membranes (PEMs) is a critical factor influencing the reliability and longevity of PEM fuel cells (PEMFCs). In the quest for durable fuel cell systems, expanded polytetrafluoroethylene (ePTFE) reinforced ionomer membranes stand as a promising avenue for designing membrane electrode assembly (MEA) compared to plain perfluorinated sulfonic acid (PFSA) membranes. The incorporation of ePTFE reinforcement introduces a complex microstructure that influences the membrane's ability to withstand chemical attack, mechanical stress, and oxidative damage during operation1,2. Despite several recent studies demonstrating enhanced durability in accelerated stress testing (AST)3–5, the intricate relationship between the membrane's microstructure, ePTFE reinforcement, and chemo-mechanical degradation in fuel cell operation has not been explored. In the present work, the durability of MEAs composed with ePTFE-reinforced ionomer membranes of 15 µm nominal thickness in PEMFCs was investigated using a combined chemical and mechanical accelerated stress test (CM-AST). The chemo-mechanical degradation of the ePTFE-reinforced membranes was characterized using X-ray computed tomography (XCT). The cell exhibited two degradation stages under CM-AST. The initial phase of 25 AST cycles was marked by a notable decline in cell performance. Subsequently, in the second phase, the cell maintained consistent operation until the conclusion of testing at the 65th cycle. The cell degradation rate for the first stage was 62 µV h-1 and the overall degradation rate was 58 µV h-1. The hydrogen crossover current density slowly increased from 3.3 mA cm−2 to 5.1 mA cm-2 at 80 °C under atmospheric pressure. A comprehensive analysis of cells using XCT showed significant decay of the catalyst layers and membrane thinning. 3D visualization by XCT scans of the entire active surface area confirmed the absence of pinholes or cracks in the membrane that explained the observed low hydrogen crossover. Chemical degradation was observed in the ionomer-rich layers, while no signs of ePTFE reinforcement degradation were found. Therefore, the reinforcement layer plays a key role in mitigating the chemical and mechanical degradation by limiting the gas crossover rather than altering the degradation process. The results concluded that by optimizing the microstructure of ePTFE, reinforced membranes can pave the way for more reliable and durable PEMFC systems.Keywords: Composite ionomer membranes, ePTFE reinforcement, PEMFC, chemo-mechanical degradation, accelerated stress testing, X-ray computed tomography References (1) Liu, W.; Suzuki, T.; Mao, H.; Schmiedel, T. Development of Thin, Reinforced PEMFC Membranes through Understanding of Structure-Property-Performance Relationships. ECS Trans. 2013, 50 (2), 51–64. https://doi.org/10.1149/05002.0051ecst.(2) Shi, S.; Weber, A. Z.; Kusoglu, A. Structure/Property Relationship of Nafion XL Composite Membranes. Journal of Membrane Science 2016, 516, 123–134. https://doi.org/10.1016/j.memsci.2016.06.004.(3) Ramani, D.; Singh, Y.; White, R. T.; Wegener, M.; Orfino, F. P.; Dutta, M.; Kjeang, E. 4D in Situ Visualization of Mechanical Degradation Evolution in Reinforced Fuel Cell Membranes. International Journal of Hydrogen Energy 2020, 45 (16), 10089–10103. https://doi.org/10.1016/j.ijhydene.2020.02.013.(4) Sadeghi Alavijeh, A.; Bhattacharya, S.; Thomas, O.; Chuy, C.; Kjeang, E. A Rapid Mechanical Durability Test for Reinforced Fuel Cell Membranes. Journal of Power Sources Advances 2020, 2, 100010. https://doi.org/10.1016/j.powera.2020.100010.(5) Tang, H.; Pan, M.; Wang, F.; Shen, P. K.; Jiang, S. P. Highly Durable Proton Exchange Membranes for Low Temperature Fuel Cells. J. Phys. Chem. B 2007, 111 (30), 8684–8690. https://doi.org/10.1021/jp073136t.

Fuel Cells (FC) systems are characterized by high costs and complex auxiliary devices. For this reason, can be useful to have a FC Emulator (FCE) to replace a real FC system during the development of the power converter for the conditioning of the energy generated by the FC. The FCE must be able to reproduce the nonlinear output voltage-current (V-I) characteristic of the FC. The paper presents a 5 kW FCE power converter aimed to be driven by an optimized parametric model of a Proton Exchange Membrane (PEM) FC. Both the FC parametric model and the converter control are implemented with a Real-Time prototyping system (RT-LAB).

Perfluorosulfonic acid (PFSA) ionomers have been used extensively in proton exchange membranes due to their high proton conductivity and strong electrochemical stability. However, proton and anion conductivity through the PFSA membranes are contingent upon water uptake, but high water content in the membrane causes loss of long-term durability of the fuel cell system because of the water swelling-induced reduction in membrane’s mechanical strengths. Large body of literatures are available to address these issues, for example, hydroscopic nanoparticulate fillers have been used for self-hydration of the membrane, and aligned polytetrafluoroethylene (PTFE) has been employed to reinforce the tensile strengths of the PFSA ionomer. Unfortunately, there is yet no report to date that can resolve pareto fronts issues, i.e., simultaneously having high proton conductivity and high tensile property. Besides, the cost of PFSA membranes, e.g., Nafion, is still prohibitively high. Here, we report a new and low-cost hierarchical membrane with high proton/anion conductivity, low moisture/methanol uptake and super-high tensile strengths. The new membrane is a hybridized composite structure consisting of biaxially oriented nanoporous ultrahigh molecular weight polyethylene (UHMWPE) film coated with Nafion and polyvinyl alcohol (PVA) blend. The tensile strength of the composite membrane is at least 10 times higher than that of the pure Nafion membrane when fully humidified, and the proton and alkaline conductivity is comparable to that of the Nafion/PVA blend. The nanoporous UHMWPE simultaneously acted to provide the high tensile strengths, ion-conductivity selectivity and nanoconfinement to limit the water and methanol uptake. Thus the new hierarchical Nafion/PVA/UHMWPE membrane is potentially an ideal ion-exchange membrane for low temperature hydrogen and direct methanol fuel cells. The work is financially supported by the theme-based research project of the university grants committee of Hong Kong, Grant No. T23-601/17-R

In this paper a control-oriented PEM fuel cell system model and a repetitive controller design strategy for PEM fuel cell systems is discussed. A mathematical model of a Polymer Electrolyte Membrane (PEM) Fuel Cell system is presented and then used to design the controller intended to improve system performance. The PEM Fuel cell is tested on an available test station (PEM-FCT) and the data used for the modelling and subsequent controller development study. For model validation a comparison is made between the results from a MATLAB/Simulink model and the test data from the PEM-FCT. The model is then used for repetitive controller design for the fuel cell system. Simulation results show that the repetitive controller tracks the reference signal very closely, with little deviation from the desired value.

Solid polymer electrolyte membranes for fuel cell applications—a review

Fuel cells using biomaterials have the potential for environmentally friendly clean energy and have attracted a lot of interest. Moreover, biomaterials are expected to develop into in vivo electrical devices such as pacemakers with no side effects. Ion channels, which are membrane proteins, are known to have a fast ion transport capacity. Therefore, by using ion channels, the realization of fuel cell electrolytes with high-proton conductivity can be expected. In this study, we have fabricated a fuel cell using an ion channel electrolyte for the first time and investigated the electrical properties of the ion channel electrolyte. It was found that the fuel cell using the ion channel membrane shows a power density of 0.78 W/cm2 in the humidified condition. On the other hand, the power density of the fuel cell blocking the ion channel with the channel blocker drastically decreased. These results indicate that the fuel cell using the ion channel electrolyte operates through the existence of the ion channel and that the ion channel membrane can be used as the electrolyte of the fuel cell in humidified conditions. Furthermore, the proton conductivity of the ion channel electrolyte drastically increases above 85% relative humidity (RH) and becomes 2 × 10−2 S/m at 96% RH. This result indicates that the ion channel becomes active above 96%RH. In addition, it was deduced from the impedance analysis that the high proton conductivity of the ion channel electrolyte above 96% RH is caused by the activation of ion channels, which are closely related to the fractionalization of water molecule clusters. From these results, it was found that a fuel cell using the squid axon becomes a new fuel cell using the function of the ion channel above 96% RH.

The impeding ban on per- and polyfluoroalkyl substances (PFAS) prompted researchers to focus on hydrocarbon-based materials as constituents of next-generation proton exchange membranes (PEMs) for polymer electrolyte fuel cells (PEFCs). Here, we report on the fuel cell performance and durability of fluorine-lean PEMs prepared by the post-sulfonation of co-grafted α-methylstyrene (AMS) and 2-methylene glutaronitrile (MGN) monomers into preirradiated 12 µm polyvinylidene fluoride (PVDF) base film. The membranes were subjected to two distinctly different accelerated stress test (AST) protocols performed at open-circuit voltage (OCV): the US Department of Energy-similar chemical AST (90 °C, 30% relative humidity (RH), H2/air, 1 bara), developed originally for perfluoroalkylsulfonic acid (PFSA) membranes, and the high relative humidity AST (80 °C, 100% RH, H2/O2, 2.5 bara), designed for aromatic hydrocarbon membranes. We found that doping the grafted membranes with a metalated porphyrin antioxidant can simultaneously reduce membrane aging and improve fuel cell performance.

Ionomer membrane degradation in polymer electrolyte fuel cell (PEFC) becomes one of the most urgent and critical problems that decrease the longevity of a PEFC, thereby limiting its development with good efficiency and affordability 1. Adding ceria into the PEFC system has proven to be a promising and sustainable way to mitigate this chemical membrane degradation, as the cerium ions can quench the radicals before they attack the ionomer membrane 2. Understanding the distribution and movement of cerium ions within a fuel cell membrane is essential for improving the design of a PEFC system, in terms of its operating environment, material properties, boundary conditions, etc., whereas this has rarely been studied. Due to the experimental limitations of measuring cerium ions transport in an operating fuel cell, numerical simulation becomes a practical approach to quantitatively analyse cerium ions migration under certain conditions.In this study, we aim to provide: (1) a reliable and practical simulation approach to investigate 2D distribution of the cerium ions under specific conditions; and (2) a flexible strategy to analyse the migration of cerium ions attributed to a variety of factors. These techniques would support future research into the development of a promising fuel cell not only with longevity but also at a sustainable cost.From the simulation, we quantified many factors that have impacts on the migration of cerium ions. The diffusion coefficient of cerium ion with respect to the local water content, as well as how the ions are drugged by the local water diffusion, was obtained at first by performing molecular dynamics simulations. Then, by combining these results into a computational fluid dynamics simulation using finite volume method, transport of the cerium ions under certain conditions was resolved (see Figure 1).Along the direction of the path from anode to cathode side, the ion transport in the membrane is affected more substantially by the electric field rather than the water convection and its self-diffusion. Meanwhile in the membrane cross-section plane, it is mainly affected by the water diffusion and its self-diffusion. As the primary finding, the electric field has the greatest influence on the transport of cerium ions, while local distribution of the water content plays an important role when the electric potential difference is small. Credibility of these preliminary simulation results was confirmed by comparing against experimental observations at the similar fuel cell operating environments.Detailed settings of the material properties and results on the reactions of cerium ions to various fuel cell environments will be discussed in the presentation. Acknowledgement This study is support by New Energy and Industrial Technology Development Organisation (NEDO) and the integrated supercomputer system at Institute Fluid Science, Tohoku University. Reference [1] Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88 (4), 981–1007.[2] Wong, K. H.; Kjeang, E. In-Situ Modeling of Chemical Membrane Degradation and Mitigation in Ceria-Supported Fuel Cells. J. Electrochem. Soc. 2017, 164 (12), F1179–F1186. Figure 1

Interfacial active-sites p-n heterojunction SFT-WO3 for enhanced fuel cell performance at 400–500 °C

There are several obstacles to the commercialization of PEM fuel cells. One of the reasons is that the presence of carbon monoxide (CO) in the reformatted fuel, even at a very small scale, decreases the fuel cell performance. The aim of this paper is to investigate the effect of CO in reformatted fuel on PEM fuel cell performance. For this purpose, a steady state, one-dimensional and non-isothermal model is utilized to evaluate the PEM fuel cell performance with and without CO in the fuel stream. The governing equations which includes the conservation of mass, energy and species equations are solved in MATLAB software and validated by the available data in the literatures. The results indicate that when pure hydrogen is used as anode fuel the activation loss of the cathode is very large relative to the anode value; also, the maximum temperature occurs in the cathode catalyst layer. When reformatted fuel is applied as anode gas stream, activation loss and anode temperature increase by increasing the CO concentration in the reformatted fuel. As example, when CO concentration is over 50 ppm in the fuel stream, the activation loss and anode will be higher than the relevant amounts in cathode catalyst layer. Also it is observed that by increasing the fuel cell temperature and anode pressure, the CO effects on fuel cell performance are reduced.