High Temperature Polymer Electrolyte Membrane Fuel Cells

The concept of the high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is presented and the motivation for increasing the working temperature of the PEMFC given. Approaches towards high temperature operation, which is defined as 100–200 °C, are briefly outlined and similarities and differences as compared to the conventional PEM fuel cell as well as to the phosphoric acid fuel cell are discussed.

Aiming at intermediate temperature operation (100–150 °C), composite polymer electrolyte membranes consisting of perfluorosulfonic acid (PFSA) ionomer and inorganic fillers, particularly short-side chain perfluorosulfonic membranes, e.g., Aquivion^® membranes with an equivalent weight of 790–850 g eq⁻¹ and their composites with inorganic fillers represent a practical approach to advanced membrane materials. This chapter is devoted to an updated review of the methodologies and materials including their practical applications in direct alcohol fuel cells, water electrolysers, and automotive hydrogen fuel cells. An analysis of the basic operation mechanism of such materials is provided and the characteristic performances achieved under intermediate temperature operation are reviewed. The influence of the surface chemistry and acid–base characteristics of the inorganic fillers is also discussed.

Acid–base chemistry deals with proton transfer from an acid to a base and represents an effective approach to the development of proton conducting materials. The acidity difference (ΔpK _a) of the two components dictates the extent of proton transfer and therefore the ionicity and other properties of an acid–base system. Only an appropriate acidity matching allows for formation of extensive hydrogen bond networks which in turn promotes the proton dynamics and Grotthuss mechanism of the proton conductivity. To frame the hypothesis initial effort is made to compile information of acid–base systems including aqueous solutions, ionic liquids, solid crystals, acid-doped basic polymers, base-doped acidic polymers as well as inorganic solid acids. Upon further validation, the insight might open vision to avenues of material sciences in the field of proton conducting materials.

In this chapter, an overview is given about the scientific work done so far in the synthesis, characterization, and fuel cell application of acid–base blends from different polybenzimidazoles as the major blend component and different kinds of acidic polymers such as sulfonated and phosphonated polymers as the minor blend component. In these blends, ionical cross-links are formed by proton transfer from the acidic group to the polybenzimidazole imidazole-N, forming ionical cross-links by the electrostatic interaction between the acidic anions and the imidazolium cations, which leads to improvement of mechanical and chemical membrane stability, compared to pure polybenzimidazoles. The chapter is concluded by a short comparative study between differently cross-linked polybenzimidazoles in terms of their properties such as thermal stability, stability in Fenton’s Reagent, and proton conductivity when doped with phosphoric acid. The outcome of this study was that the properties of all the different polybenzimidazole blends were quite similar.

The chapter describes the development of aromatic poly(ether sulfone)s carrying main chain pyridine units as alternative to poly(benzimidazole) (PBI) polymer electrolytes for high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) applications operating at 180 °C. These polymeric materials present excellent thermal and oxidative stability both ex situ and in situ as well as high proton conductivities after doping with strong protic acids. The pathway from monomers’ design to polymerization conditions optimization and finally to membranes preparation and doping with phosphoric acid is analytically presented. Further structural and mechanical stabilization of such polyelectrolytes and their application in HT-PEMFCs operating above 180 °C even up to 220 °C has been achieved through cross-linking. The cross-linked materials’ superiority over linear analogues is depicted.

The chapter describes common methods for membrane characterization. While it does not go into theoretical depths, general explanations, limitations, practical notes, and critical comments are provided. Covered methods include determination of water uptake and acid doping level, measurement of proton conductivity, molecular weight analysis by viscosity and size exclusion chromatography, determination of solubility and gel content, filtration of polymer solutions, characterization of mechanical properties (tensile testing, compression and creep tests, dynamic mechanical analysis), permeability of hydrogen and methanol and electroosmotic drag of water as well as definition and control of humidity.

Recent progress in the synthesis of polybenzimidazole (PBI) derivatives is summarized for application as high temperature polymer electrolyte membrane in fuel cells. Various designs in the polymer structure are described aiming at improvement of the membrane performance. The ways to produce PBI derivatives containing different functional groups, segments, or blocks of other macromolecules are classified as main-chain modification, copolymerization, and side-chain grafting. The synthetic routes and associated characterization methods particularly with respect to the polymer structures are also addressed.

In high-temperature polymer electrolyte membranes, phosphoric acid is used as dopant for polybenzimidazole-type membranes to provide the protonic conductivity. In addition, phosphoric acid also serves as proton conductor in the porous electrodes in order to establish the three-phase boundary. In the first part of this chapter a short overview is given on the physico-chemical properties of (aqueous) phosphoric acid. In the second part the focus is on the adsorption of phosphoric acid as a protic electrolyte on polybenzimidazole-type polymers. Although polybenzimidazole-type membranes are routinely doped with phosphoric acid, few studies on the exact nature of the acid inside the membrane have been published. Experimental data from our institute and data compiled from literature indicate that the polymer chain is protonated by the acid and that the anions are bound by coulomb interactions. Additional electrolyte molecules can interact with the polymer chain by formation of H bonds or via H bonds with other H₃PO₄ molecules. It is demonstrated that the uptake of phosphoric acid can be described by a modified BET isotherm, assuming multilayer-like adsorption. The assumption of free phosphoric acid in the membrane at high doping levels is supported by Raman spectroscopy.

Casting of polybenzimidazole membranes by solvent evaporation from homogeneous solution followed by phosphoric acid doping is a widely used procedure for the preparation of proton conducting membranes for high-temperature polymer electrolyte membrane fuel cells. This contribution covers the membrane fabrication process from casting to acid doping and extends to a review of membrane characteristics and modifications. Furthermore, it briefly addresses membrane aspects in connection to fuel cell performance and durability.

The Polyphosphoric Acid (PPA) process has greatly impacted many aspects of phosphoric acid imbibed polybenzimidazole (PBI) films. The ability to cast the PBI solution directly from PPA not only improves manufacturability, but allows for various chemistries of PBI to be synthesized by overcoming some of the solubility limits seen in the traditional synthesis and imbibing process. PBIs prepared via the sol–gel PPA process exhibit high inherent viscosities (I.V.), good mechanical properties, and higher acid loadings and ionic conductivities than those produced by other methods. Extensive fuel cell testing has shown that PBI membranes are applicable to many fuel cell devices, and possess the durability in different simulated operating environments to remain at the forefront of high-temperature polymer membrane fuel cell applications.

A major issue of polybenzimidazole (PBI)-based membranes is the leaching loss of the doping acid during fuel cell operation. In this chapter, two approaches to improve the conductivity and its stability are presented by tailoring the polymer basicity: (a) modification of the polymer backbone by synthesis of new monomers and (b) fabrication of (nano)composite membranes with functionalized fillers. Chemical modification of the polymer backbone is likely the strategic choice for its significant effect, simplicity, reproducibility and cost.

One of the important factors that determine the polymer electrolyte membrane fuel cell (PEMFC) performance is the efficiency of proton transfer across the proton exchange membrane (PEM) from the anode to the cathode. A PEM with a lower thickness (L) and higher conductivity (σ) has a lower resistance (L/σ) and thus higher proton transport efficiency. However, a thin PEM may be mechanically weak and exhibit high gas crossover which lowers the open circuit voltage. Thus obtaining a PEM with low thickness and high mechanical strength without increasing the gas crossover rate or reducing the proton conductivity is important for obtaining a high performance PEMFC. The composite membranes fabricated using a high mechanical strength porous thin film such as porous poly(tetrafluoro ethylene) (PTFE) as a supporting material for reinforcement has been demonstrated as an effective approach to reach those targets for proton conducting membranes based on Nafion or polybenzimidazole (PBI) doped with H₃PO₄. In this chapter, we first briefly describe the current status of the Nafion/PTFE composite membranes and then report the PBI/PTFE composite membrane preparations, characterizations, and their application in high temperature PEMFCs (operating at 120–200 °C). Some new polyelectrolyte/fiber reinforced composite membranes for high temperature PEMFC applications such as PBI reinforced with phosphoric acid compatible crosslinked PBI-polybenzoxazine nano-fiber and highly conducting polyelectrolytes (i.e., quaternized polysulfone and poly(ether sulfone)/poly(vinyl pyrrolidone) blend) reinforced with porous PTFE are also discussed.

This chapter focuses on the review of PBI-based composite membranes. These are composite materials with inorganic nanoparticles, such as, hygroscopic oxides, heteropolyacids and their derivates, and pyrophosphate, ionic liquids, and even carbon-based materials. Information about the membrane preparation, physicochemical characterization, proton conductivity, and fuel cell results has been provided. In most of the cases, good results have been presented in the literature, improving key properties such as the mechanical and thermal properties, and more importantly, the proton conductivity, which has impacted positively on the fuel cell performance. However, continuous investigations must be done to improve the fuel cell performance for practical applications.

High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) need to improve their lifetime! Especially, catalysts and catalyst-layers (CL) can degrade fast and severely. It is therefore sensible to explore new opportunities to increase their durability on an operational and material-based level. In order to achieve this goal, it is furthermore necessary to establish reliable characterization methods for CLs. In this chapter, several methods are discussed which help to shed more light onto the nature of CLs. Techniques for reproducible and spatially resolved in situ measurements of the electrochemically active surface area (ECSA) are introduced. Moreover, two CL degradation mitigation strategies are discussed. The first strategy is based on intentionally increased CO partial pressures at the fuel electrode during fuel cell start-up and shut-down. The second approach utilizes graphitized carbon support, which is less vulnerable to start-/stop-induced carbon oxidation. Overall, this chapter tries to open a small window for new insights into properties and characterization of HT-PEMFC catalyst layers.

A high-temperature PEM electrode consists of a gas diffusion layer, a micro porous layer, and a catalyst layer, where electrochemical reactions take place. Requirements and properties of the electrode materials as well as their degradation mechanisms are discussed. Special attention is paid to the catalyst support materials. Techniques for manufacturing of gas diffusion electrodes and production of MEAs are introduced.

The HT-PEMFC systems are mostly developed for the combined heat and power (CHP) and auxiliary power unit (APU) generation applications because of their relatively simple system configuration and tolerance to a wide choice of fuels. To achieve the cost and durability targets that are needed for the commercialization of the HT-PEMFC systems, many developments have been made to enhance the performance of the HT-PEMFC MEAs. The efforts to enhance the cell voltage include using catalysts with high ORR activity, controlling properties of binders and optimizing amount of acid to increase the electrochemically active surface area in the catalyst layer. The lifetime of the MEA is reported to depend on the catalyst degradation and acid loss, and notable improvements on the durability of MEAs were achieved by using oxidation-resistant catalysts. In this chapter, the factors that determine the cell voltage and durability of MEAs for HT-PEMFC are discussed, and the design of low-cost durable MEAs for HT-PEMFC is suggested.

This chapter describes the most commonly used electrochemical measurement techniques to characterize HT-PEM membrane electrode assemblies (MEAs). Moreover, it will be shown what kind of information can be subtracted from each technique in order to provide a correct diagnosis of the HT-PEMFC behavior. Detailed descriptions of test procedures and methodology routines for each one of the electrochemical techniques are important for the comparison of data between different working groups is not always possible, thus leading to the need of standardized test protocols. The described test procedures and routines have already been verified in two European Projects. Micro-computed tomography X-ray technique has been introduced as a rather new post-mortem three-dimensional imaging method of the specimens under investigation. Therefore, μ-CT imaging enables the nondestructive characterization of usually hidden interfaces in HT-PEM MEAs that cannot be done by conventional microscopy techniques. MEA contact pressure plays an important role for degradation of MEA materials that can affect performance and lifetime of the HT-PEMFC. Thus, electrochemical and ex situ imaging techniques have jointly been used to investigate the effect of contact pressure increase as well as contact pressure cycling to verify the state-of-the-art of HT-PEM technology in different commercial MEAs. The importance to define long-term test routines will be discussed in the last section of this chapter. Long-term testing should reproduce real operation conditions and determine the issues to identify, understand, and minimize degradation mechanisms that limit the applicability of HT-PEM technology nowadays in systems ready for the market.

Modeling and simulation of all components of high temperature polymer electrolyte (HT-PEM) fuel cells are important tools to provide additional understanding of the operation behavior. The use of mathematical models is one possibility for analyzing species concentrations, temperature gradients, and pressure distributions for predicting the internal workings of HT-PEM fuel cells for different operating conditions and designs. This work reviews phosphoric acid fuel cell (PAFC) and HT-PEM fuel cell modeling and simulation activities since both technologies are very similar. The current state-of-the-art PAFC and HT-PEM fuel cell technology is overviewed. Selected literature is discussed and dedicated modeling equations listed. Next, electrolyte modeling and simulation possibilities are highlighted including the physicochemical properties of phosphoric acid (H₃PO₄), description of the vapor–liquid equilibrium (VLE), non-equilibrium effects at the interphase, and the coupling to electrochemistry and mass transport properties. Finally, numerical aspects are shortly presented, examples of practical implications given, and input parameters and experimental data for model validation listed.

This chapter is dedicated to graphite-composite based bipolar plates and gaskets for fuel cells. It describes different material configurations and processing methods for bipolar plates covering the range from conventional low temperature PEM technology at 80 ¯C to high temperature PEM technology at up to 180 ¯C. The chapter discusses bipolar plate manufacturing via two different routes, the resin method and the thermoplastic method as well as technical requirements for their application in fuel cells. Material characterization covers electric conductivity testing, conductivity mapping to measure homogeneity, and thermal analysis. Additionally, manufacturing aspects and technical requirements of gaskets are discussed. Very often the influence of gaskets on performance and life time of fuel cells is underestimated. This chapter presents material candidates and characterization data for a broad variety of gaskets.

Various system topologies are available when it comes to designing high temperature PEM fuel cell systems. Very simple system designs are possible using pure hydrogen, and more complex system designs present themselves when alternative fuels are desired, using reformer systems. The use of reformed fuels utilizes one of the main advantages of the high temperature PEM fuel cell: robustness to fuel quality and impurities. In order for such systems to provide efficient, robust, and reliable energy, proper control strategies are needed. The complexity and nonlinearity of many of the components in such systems allow the development of both simple linear and also advanced fuzzy logic and neural network controllers able to adapt system performance to the ever changing conditions which the systems operate in over their entire lifetime.

This chapter briefly reviews durability and stability issues with key materials and components for HT-PEMFCs, including the polymer membrane, the doping acid, the electrocatalyst, the catalyst support and bipolar plates. Degradation mechanisms and their dependence on fuel cell operating conditions are summarized as well. To date, lifetimes of this type of fuel cells of up to 18,000 h with degradation rates of around 5 μV/h at temperatures of 150–160 °C have been demonstrated using hydrogen and air under constant moderate load. However, the degradation rate increases by a factor 10 when the cell is exposed to start-up–shutdown or load cycling.

HTPEFC as a power source for propulsion in aviation is demonstrated with the flying platform Antares DLR H2, showing a number of advantages. Methanol is selected as a hydrogen carrier and a compact modular system layout with methanol steam reforming units upstream of the liquid-cooled HT-PEFC has been established. With an appropriate dimensioning of the fuel cell system, the flight endurance is expected to be between 25 and 50 h. Power peaks during takeoff, ground operation, or critical flight situations are planned to be optionally addressed by engaging a direct fuel cell battery hybrid. Challenges and further development are discussed.

The use of polymer electrolyte membranes for hydrogen separation and purification was reported many years ago, but has seen new growth in recent years with the development of new membrane materials. Electrochemical hydrogen pumping has the potential to be used in many applications such as hydrogen recirculation, fuel cell applications, compression, and electroanalytical characterization methods. Various types of polymer membranes, e.g., polybenzimidazoles, perfluorosulfonic acid-based membranes, and poly(ether ether ketones) have all been examined in hydrogen pumping. The type of polymer membrane used in the pump cell affects the temperature of operation and the overall performance and efficiency. This chapter discusses the electrochemistry behind electrochemical hydrogen pumping, various types of polymer membranes that have been tested, and potential applications and limitations for these devices.