Nafion and modified-Nafion membranes

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Bull. Mater. Sci., Vol. 32, No. 3, June 2009, pp. 285–294. © Indian Academy of Sciences. Nafion and modified-Nafion membranes for polymer electrolyte fuel cells: An overview A K SAHU†, S PITCHUMANI†, P SRIDHAR† and A K SHUKLA* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India †Central Electrochemical Research Institute, Karaikudi 630 006, India Abstract. Polymer electrolyte fuel cells (PEFCs) employ membrane electrolytes for proton transport during the cell reaction. The membrane forms a key component of the PEFC and its performance is controlled by several physical parameters, viz. water up-take, ion-exchange capacity, proton conductivity and humidity. The article presents an overview on Nafion membranes highlighting their merits and demerits with efforts on modified-Nafion membranes. Keywords. Composite membrane; relative humidity; surface area; PEFC. 1. Introduction Energy security† is one of the key challenges facing the mankind. A substantial proportion of our energy is met through fossil fuels derived from ultimately finite reserves and thus cannot be sustained indefinitely in the long-term. The deleterious effects of excessive consump- tion of carbonaceous fuels on the economy and ecology of a large part of the world is already apparent and well known to be recounted here. There is also a lurking fear that, at present and projected discovery, production and consumption rates, world oil-supply will fail to meet the demand in the near future. In response to these problems, green energy and sustainable living movements are gain- ing popularity. In recent years, while solar energy, geothermal energy, wind energy and fusion power tech- nology have attracted attention, there is also increasing interest in hydrogen and its most efficient utilization in generating electrical energy. The latter is most appropri- ately achieved through fuel cells. A fuel cell is an electrochemical power source with advantages of both the combustion engine and the bat- tery. The difference between a battery and a fuel cell can be related to the definitions of a thermodynamic system and thermodynamic control volume (Mench et al 2008). In a thermodynamic system, no mass flux is permitted to cross the system boundaries like in a battery, while in a thermodynamic control volume, mass flux is permitted across the boundaries like in a fuel cell. Like a combus- tion engine, a fuel cell runs as long as it is provided fuel; and like a battery, fuel cells convert chemical energy *Author for correspondence (akshukla2006@gmail.com) †Energy security refers to various security measures that a given nation, or the global community as a whole, must carryout to maintain an adequate energy supply directly to electrical energy. As an electrochemical power source, fuel cells are not subject to the Carnot limitations of heat engines. As early as in 1839, Sir William Grove discovered fuel cells by reversing water electrolysis to generate electricity from hydrogen and oxygen using an acid-electrolyte fuel cell. Since then fuel cell technology has evolved substantially. Among the competing fuel cell technologies, polymer electrolyte fuel cells (PEFCs) are commercially most at- tractive owing to their quick start-up and ambient- temperature operations (Springer et al 1991; Gottesfeld and Zawodzinski 1997). PEFCs exhibit high-operational efficiencies with both specific and volumetric energy- densities comparable to internal-combustion engines while emitting no pollutants (Grant 2003). The operating principle of a PEFC is depicted in figure 1. At the interface between anode and the electrolyte, the fuel is converted into protons (H+) and electrons (e–), a process which is made possible by a catalyst that is typi- cally Pt-based. Polymer electrolyte membrane allows protons to flow through, but prevents electrons from passing through it. Electrons travel to the cathode through an external circuit producing electrical current and H+- ions (protons) pass through the membrane from anode to cathode, where they combine with oxygen molecules and electrons to form water. The half-cell reactions taking place in a PEFC are Anode: H2↔2H++2e–, (1) Cathode: 1/2O2+2H++2e–↔H2O, (2) Overall: H2 + 1/2O2 ↔ H2O ΔG° = –237 kJ/mol. (3) The Gibbs free energy change (ΔG°) of reaction (3) is related to the cell voltage by 285

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