Polymers for Energy Storage and Conversion

Chapter 1

High Performance Polymer Hydrogel Based Materials for Fuel Cells

Yogeshwar Sahai and Jia Ma

Department of Materials Science & Engineering, The Ohio State University, Columbus OH USA

Abstract

In recent years, there has been extensive research on the development of high performance electrochemical devices which can generate and store energy at low cost. Fuel cells have been receiving attention due to its potential applicability as a good alternative power source. Polymer hydrogel electrolyte is prospective material to deliver high performance at low cost in fuel cells which use polymer membrane as electrolyte and separator. This chapter introduces structure and properties of polymer hydrogel with respect to its applications for low to intermediate temperature polymer electrolyte-based fuel cells.

Keywords: Fuel cell, polymer hydrogel, electrolyte

1.1 Introduction

A fuel cell is an electrochemical device that produces electrical energy via electrochemical reactions between the fuel and the oxidant. Unlike a battery, which stores a finite amount of energy, a fuel cell continues to produce energy as long as the oxidant and the fuel are fed into it. Energy generation from combustion in a heat engine is intrinsically inefficient and also causes environmental problems. On the contrary, a fuel cell is inherently energy efficient, environmentally friendly, and silent.

The polymer electrolyte-based fuel cell employs a polymer membrane as the electrolyte. Compared to other types of fuel cells, it is capable of achieving reasonably high power performance at relatively low working temperatures, and thus is considered a promising power supply for transport, stationary, and portable applications. The major component of a fuel cell is the membrane electrode assembly (MEA) which consists of solid polymer electrolyte membrane (either a cation exchange membrane (CEM) or an anion exchange membrane (AEM)) sandwiched between an anode and a cathode. An electrode generally consists of a catalyst layer and a diffusion layer. The catalyst layer must have facile transport of reactants and products as well as good ionic and electronic conductivity. Therefore, the catalyst layer should have high porosity and large electrochemically active surface area. The solid polymer electrolyte membrane should have good ionic conductivity and no electronic conductivity. For such an application, an ideal solid electrolyte membrane should fulfill a number of requirements including high ionic proton conductivity, long-term chemical and mechanical durability under heated and humidified conditions. A primary goal is to find stable polymer-based materials with ionic conductivities within the range of mS cm−1 at temperatures up to 100°C [1]. Ionic conductivity of many polymeric membranes, increases with its water content, and thus hydration is of significance to achieve high conductivity, especially at high temperatures. Perfluorinated ionomers, such as Nafion, with fluoroalkyl ether side chains and sulphonic acid end groups on polytetrafluoroethylene backbones, have been the most commonly used polymer electrolyte membrane so far. Nafion material is also used as an electrode binder which facilitates ionic conduction, provides mechanical support for catalyst particles, and enhances dispersion of catalyst particles in the catalyst layer. Nafion possesses many desirable properties as a polymer electrolyte, and yet it is very expensive and loses ionic conductivity if not sufficiently hydrated. For application in a polymer electrolyte-based fuel cell using methanol as the fuel or direct methanol fuel cell, solid polymer electrolyte membrane also needs to have low methanol permeability. However, Nafion membrane has relatively high methanol crossover.

Research has been going on in the development of high-performance, cost-effective polymer-based membrane electrolyte as an alternative to Nafion for use in polymer electrolyte-based fuel cells. Hydrogel polymer electrolyte has high potential for applications in fuel cells. This chapter introduces structure and properties of polymer hydrogel electrolyte with respect to its applications in fuel cells.

1.2 Hydrogel Electrolyte

Hydrogel is macromolecular network that is capable of trapping large amount of water or biological fluids [2]. The three-dimensional network of hydrogel is insoluble in the precursor solution due to the presence of chemical cross-links, and/or physical entanglements [3]. Chemical hydrogels are formed by covalent cross-linking reaction between the polymer and a cross-linking reagent, whereas physical hydrogels are stabilized by physical entanglements, electrostatic attractive forces, and hydrogen bonding. The schematic diagram of a chemical hydrogel with point cross-links and a physical hydrogel with multiple junction zones are shown in Figure 1 (a) and (b), respectively. In Figure 1(a) the solid lines represent polymer chains that constitute the hydrogel matrix. The solid dots in the cross-link points of the polymer helices represent the cross-links formed by chemical reaction between the polymer and cross-linker. In Figure 1(b) the solid lines represent the polymer helices aligning themselves laterally in extended junction zones.

Figure 1.1 Schematic diagram of (a) a chemical hydrogel with point cross-links, and (b) a physical hydrogel with multiple-junction zones [4].

Many hydrogels have been found to possess the ability of easy film making, good ionic conductivity, and ionic exchange property [5], which have led to their applications in many electrochemical energy devices. Gel electrolytes based on poly (ethylene oxide), poly (acrylonitrile), poly (methyl methacrylate), and poly (vinylidene fluoride) have been employed in solid-state lithium-ion batteries [6, 7]. One way of producing hydrogel membranes is entrapping of an aqueous solution of strong electrolyte with a polymeric matrix. It was found that polyacrylamide-based hydrogels doped with H3PO4 exhibited ionic conductivities in the range of 10−3–10−2 S·cm−1 at room temperature [8]. Another method to produce polymeric hydrogel membranes is introduction of copolymers based on highly conducting monomers. Hydrogel electrolyte membranes containing highly conducting sulpho group was prepared by radical copolymerization of sodium styrensulphonate or potassium sulphopropyl acrylate with acrylamide and acrylonitrile [9]. These membranes are capable of holding water at temperatures 70–90°C and have an ion exchange capacity of 0.8–1.4 mg-equiv/g.

A number of polymeric materials are employed to produce hydrogel electrolyte. This chapter does not intend to review all reported polymer hydrogel materials for fuel cell applications, but concentrates on poly(vinyl alcohol) and chitosan since they are cost-effective materials which have been intensively investigated for fuel cell applications.

1.3 Poly(vinyl alcohol) Hydrogel

Poly(vinyl alcohol) (PVA) discovered in 1924, is one of the most widely investigated polymers for hydrogels [10]. It is a cheap, non-toxic, and chemically stable synthetic polymer used in a wide range of industrial, commercial, medical, and food applications [11]. General chemical and physical properties of PVA are summarized in Table 1. PVA is prepared by hydrolysis or partial hydrolysis of polyvinyl acetate. Different length of the initial vinyl acetate polymer and the degree of hydrolysis under alkaline or acidic conditions yield PVA of differing physical properties.

Table 1.1 General chemical identity and physical properties of polyvinyl alcohol.

Pure PVA does not possess intrinsic protonic conductivity. However, several organic functional groups, such as sulfonate, hydroxyl, amine, carboxylate, phenolic, and quaternary ammonium salts can be incorporated into PVA to enhance its proton conductivity and hydrophilicity. Some of the sulfonating agents for modification of PVA are shown in Figure 2 [12]. Cross-linking is one way to modify the polymer properties, such as degree of swelling, thermal, chemical, and mechanical stability, methodology adopted to impart proton conductivity. For instance, cross-linking decreases water solubility or swelling, and yet excessive cross-linking leads to brittleness of polymer membrane. PVA can be cross-linked in a variety of ways like freezing, heat treatment, irradiation, and chemical treatment. Some typical examples are shown in Figure 3.

Figure 1.2 Chemical modification of PVA to introduce sulfonate groups [12].

Figure 1.3 Cross-linked PVA [12].

The –OH groups of PVA react with –CHO groups of certain aldehydes to form acetal or hemiacetal linkages under acidic conditions [13]. The resultant polymeric entity is water insoluble and gel like in nature. The cross-linking reaction between PVA and glutaraldehyde leading to the formation of PVA chemical hydrogel is schematically depicted in Figure 4. PVA chemical hydrogel was employed as electrode binders for an alkaline fuel cell using borohydride as the fuel or a direct borohydride fuel cell (DBFC) [14, 15]. As indicated in Figure 5, a high power density of PVA binder-based DBFC was achieved. The PVA chemical hydrogel in an inverted glass beaker is shown in Figure 6, where a Teflon-coated magnetic stirring bar that was used to mix solutions of PVA and glutaraldehyde is seen stuck within the hydrogel at the bottom of the beaker. This figure clearly shows the solid nature of PVA chemical hydrogel, and it also makes it easier to understand how the electrode materials are held within the hydrogel and bound to the carbon cloth substrate in the actual electrode while allowing transport of any water-soluble species such as ion, fuel or oxidant to the catalyst.

Figure 1.4 Reaction between glutaraldehyde and PVA leading to the formation of PVA chemical hydrogel.

Figure 1.5 Plots of cell voltage and power density versus current density for DBFCs with PVA chemical hydrogel binder-based electrodes at different operating cell temperatures [15].

Figure 1.6 A picture of PVA chemical hydrogel along with a Teflon-coated magnetic stirring bar in an inverted glass beaker.

In addition to the electrode binder, a PVA hydrogel was used to cast as electrolyte membrane by a solution casting method. Sahu et al. characterized the PVA membrane using different techniques [16]. The scanning electron micrograph reveals a smooth surface of the PVA membrane with no defects. The X-ray diffraction pattern of PVA membrane exhibits broad peaks at 2θ values of 11°, 20° and 41°, respectively. The broad peaks in the XRD pattern indicate a partially amorphous nature of the PVA membrane. The thermogravimetric analysis of acidic PVA membrane shows a weight loss of about 10 % in the temperature range between 30 and 150°C due to evaporation of surface and moderately bound water. The PVA membrane undergoes total thermal oxidation at temperature between 150 and 470°C due to the decomposition of its polymer chains. The midpoint ASTM glass transition temperature for PVA membrane is 108.26°C. The Young’s modulus and proportional limit stress values for PVA membrane are 3.24 and 0.977 MPa, respectively. The water uptake value for PVA membrane is about 1.3 g H2O/g PVA hydrogel membrane. A PVA hydrogel membrane was employed in a DBFC and delivered peak power densities comparable to the Nafion membrane (Figure 7) [17]. For membrane application in direct methanol fuel cells for which proton conductivity and methanol crossover are of significance, PVA membranes are becoming competitive with respect to the state of art Nafion membranes [12].

Figure 1.7 Curves of cell polarization and power density of a DBFC using a PVA hydrogel membrane electrolyte (PHME) or a Nafion membrane electrolyte (NME) with oxygen as the oxidant at 60°C [17].

1.3.1 Chitosan-based Hydrogel in Fuel Cells

Cost-effective and eco-friendly polymer electrolytes from renewable sources can become a promising substitute for synthetic polymers for use in fuel cells [18]. Chitosan (CS), a polysaccharide bioresource, has been attracting considerable interest as solid polymer electrolyte and binder material in low and intermediate temperature polymer electrolyte-based fuel cells. Chitosan (Figure 8) is a hydrophilic, inexpensive, biodegradable, and non-toxic natural polymer that is derived by deacetylation of chitin [poly(N-acetyl-d-glucosamine)] [19]. Chitin, which is present in the exoskeleton of arthropods, is the second most abundant natural biopolymer next to cellulose. Characteristics of CS are influenced by a number of parameters such as its molecular weight and degree of acetylation [19]. CS contains hydroxyl (–OH), primary amine (–NH2), and ether (C–O–C) groups, and due to the presence of these functional groups, CS has high water attracting capacity. The –OH and –NH2 functional groups in CS enable various chemical modification of CS to tailor it for specific applications [20–23]. Chemical modifications, such as sulfonation [24–26], phosphorylation [27–30], and quaternization [31, 32] possibly generate ion exchange sites and improve ionic conductivity. However these chemical modifications also increase swelling of chitosan and as a result have negative effect on its mechanical strength. CS membranes are normally cross-linked before being employed in fuel cells. Cross-linking is a common chemical modification to ensure good mechanical and chemical stability of CS. In cross-linked CS, polymer chains are interconnected by cross-linkers to form three dimensional networks. Main interactions forming the network are covalent or ionic bonds. In addition, some secondary interactions, such as hydrogen bridges and hydrophobic interactions, also occur in CS networks [33]. Dialdehydes, such as glyoxal and glutaraldehyde [34], diethylene glycol diglycidyl ether [35], and epichlorohydrin are used to form covalent linkage in CS chains [36]. CS dissolved in weak organic acid solution becomes polycationic which can form ionic cross-links with a number of cross-linking reagents, such as acids of sulfate ions, phosphate ions, and sulfosuccinic ions.

Figure 1.8 Structures of chitin and chitosan [18].

1.3.2 Chitosan Membrane for Polymer Electrolyte Membrane Fuel Cell

Various CS-based membranes, both anionic and cationic, have been extensively examined for fuel cell applications. Table 2 lists some properties of CS-based polymer membrane for applications in fuel cells. These CS-based membranes generally do not offer significant advantages over traditional Nafion membrane, as far as proton conductivity is concerned. In its dry state, CS has a very low electrical conductivity. However, CS can be used as a polymer matrix for ionic conduction. It was found that when solvated with lithium salt or proton donor salts such as ammonium salts [37, 38], ionic conductivity of CS membrane was enhanced. Due to intrinsic hydrophilic nature of CS, it is able to significantly reduce methanol crossover, which makes CS a suitable material for use in a DMFC. For instance, sulfuric acid cross-linked CS membrane was found to have methanol permeability almost three times lower than that for Nafion 117 membrane [39, 40].

Table 1.2 Summary of properties of chitosan-based membranes

Cross-linking reagent has impact on properties of CS membrane electrolyte. It was found that introducing sulfosuccinic acid as cross-linker in addition to glutaraldehyde improved proton conductivity, and yet also increased methanol permeability as compared to using glutaraldehyde alone. Chemical structure of sulfosuccinic acid and glutaraldehyde cross-linked CS is shown Figure 9. A peak power density of 41 mWcm−2 was achieved by a sulfosuccinic acid and glutaraldehyde cross-linked CS membrane, at 60°C in a DMFC [41]. Sulfonation of CS and subsequent cross-linking were found to enhance proton conduction and methanol resistance as compared to pure CS membrane [42].

Figure 1.9 Structure of cross-linked CS, I: amino and aldehyde groups reaction; II: ionic interaction of sulfosuccinic acid and CS [41].

In addition to proton conductivity, mechanical strength and shelf life of CS also need further improvement. CS is hydrophilic and thus has a high degree of swelling. An excessively high level of water uptake increases the fragility of the membrane, and makes it less durable in a fuel cell. Efforts have been made to improve properties of CS membrane, including chemical modification, formation of CS blend and composite. These methods improve some properties of CS with or without sacrificing the others. CS and poly(vinyl alcohol) (PVA) are miscible in one another, and are compatible for blending. A blend of phosphorylated CS and PVA membrane exhibited proton conductivity of the same order of magnitude as Nafion membrane [43]. To overcome the disadvantage of loss in mechanical strength in the wet state, CS is blended with tough polymer such as poly(vinyl pyrrolidone) (PVP). PVP, upon blending with CS followed by cross-linking with glutaraldehyde, forms a semi-interpenetrating network [44].

Mixing of negatively and positively charged polymer leads to the formation of a complex by ionic interaction. The resulting blend membrane may exhibit higher tensile strength, than homopolymers, possibly attributed to the electrostatic interactions and restriction in chain mobility. The weight ratio of polycation and polyanion is important to achieve optimized properties of blend membranes. Polymer complex of CS with a polyanion such as poly(acrylic acid) (PAA) [45], sodium alginate [46], and acrylic acid-2-acrylamido-2-methylpropane sulfonic acid ((P(AA-AMPS)) [47], poly(4-styrenesulfonic acid-co-maleic acid) (PSSA-MA) [48] demonstrates improved properties such as ionic conductivity, methanol resistance as compared to pure CS membrane.

Properties of CS membrane can also be improved by incorporating a vast variety of inorganic component to form composite membrane. Properties of CS composite membrane are highly influenced by pore size and content of inorganic material particles, and their hydrophilic/hydrophobic nature [49]. Embedding nonporous or porous inorganic fillers with proper structure and pore size within the membrane play an important role in suppressing methanol crossover because they can interfere with polymer chain packing and create a more tortuous diffusion path. In many cases, proton conductivity decreases with an increase in filler content due to the relatively low proton conductivity of the fillers themselves and their considerable dilution effect on the proton exchange groups in the original polymer matrix. For instance, it was found that the proton conductivity decreased with the incorporation of silica [50] or titanate nanotubes [51]. Thus, inorganic fillers are functionalized (including sulfonic, carboxylic, quaternary, and phosphorus groups) before embedding into CS membrane to reduce methanol permeability while simultaneously preserve or enhance proton conductivity [50, 52]. Functionalization also improves interfacial morphology of inorganic material and CS polymer [53–55], which contribute to the suppression of methanol crossover.

Solid superacids, such as metal oxide supported sulfate (MxOy–SO²-4) [56], heteropolyacid (HPA) [57, 58], are used to improve properties of CS membrane, due to their hygroscopic and proton conductive properties as well as good mechanical properties. The addition of stabilized silicotungstic acid enhanced proton conductivity of CS/PVA membrane, and with optimized content of stabilized silicotungstic acid, the conductivity of blend membrane was higher than that of Nafion, as shown in Figure 10 [59]. In addition to the benefit in proton conductivity, stabilized silicotungstic acid also serves to restrict methanol crossover.

Figure 1.10 Proton conductivity vs. temperature plot of Nafion 117, sulfosuccinic acid (SSA) cross-linked CS/PVA, and CS/PVA/stabilized silicotungstic acid (SWA) membranes [59].

As inexpensive synthetic polymer, sulfonated poly(aryl ether ketone) (SPAEK) is being investigated as an alternative to Nafion, due to their desirable durability [60]. Large content of sulfonic acid groups leads to undesirable high swelling and excessive methanol crossover. CS was used to modify SPAEK for reducing methanol crossover [61]. A multilayer film was constructed onto the surface of SPAEK membrane by layer-by-layer self-assembly of polycationic CS and negatively charged phosphotungstic acid by sequential electrostatic adsorption as illustrated in Figure 11 [62]. The ionic conductivities of CS/phosphotungstic acid modified SPAEK membranes were superior to the pristine SPAEK membrane and comparable or even slightly higher than Nafion 117 membrane.

Figure 1.11 Schematic representation of the fabrication of CS/phosphotungstic acid (PTA) multilayer films on SPAEK membrane [62].

CS-based polymer and Nafion have been used in combination for the purpose of enhancing methanol resistance of Nafion and ionic conductivity of CS [63]. A triple-layer composite membrane comprising Nafion 105 membrane with its both sides coated with glutaraldehyde/sulfosuccinic acid cross-linked CS was prepared [64]. Proton conductivity and methanol permeability measurements revealed a remarkably reduced methanol crossover and a higher conductivity for multi-layer membrane compared to Nafion 117.

Being a hydrophilic polymer, CS also has potential use alone or in combination with Nafion or other synthetic polymer, for intermediate temperature (>100°C) polymer electrolyte fuel cells [65, 66]. A microfluidic platform was developed for the synthesis of mono-dispersed CS based nanoparticles using anogelation with adenosine triphosphate [67]. As shown in Figure 12 (a), CS-adenosine triphosphate filled nanocomposites have higher conductivities in all ranges of temperature, which may be due to the creation of new proton transfer pathways. Moreover, the maximum power output of a fuel cell based on the CS-adenosine triphosphate filled Nafion membrane is about three times higher than that for recast Nafion at 120°C, as shown in Figure 12 (b).

Figure 1.12 (a) Size dependant proton conduction of the CS-adenosine triphosphate filled Nafion nanocomposite at 25°C and 120°C; (b) Polarization curves of hydrogen-oxygen single cells consisting of a Nafion nanocomposite with 2 wt.% of CS-adenosine triphosphate as well as, recast Nafion, at 40% R.H. and 120°C