Polymer Materials Offer New Forms of Energy Storage

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-base d fuel cells.

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.{{Pause=2}}

The polymer electrolyte-base d 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 reactant s and products as well as good ionic and electronic conductivity. Therefore, the catalyst layer should have high porosity and large electrochemicaly 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 requirement s 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 degrees sellsious.{{Pause=2}}

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-base d 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.{{Pause=2}}

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-base d 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.

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