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FUEL CELLS

DOI: 10.1615/AtoZ.f.fuel_cells

The discoverer of the fuel cell was W. R. Grove who in 1839 reported the working of a hydrogen-oxygen system in dilute sulphuric acid.

A fuel cell is an electrochemical device which converts the "chemical" energy of a fuel into electrical energy. The "chemical" energy is associated with the free-energy change of an electrochemical reaction from which one can write.

where n is the number of electrons transferred, F is the Faraday Constant and is the reversible potential of the cell (see Faraday's Law).

A fuel cell will, in principle, continuously supply direct electrical energy when continuously supplied with fuel and an "oxidant", usually oxygen. The simplest of fuel cells is the H2 – O2 system. In this, hydrogen gas is oxidized at the anode and oxygen is reduced at the cathode to form water overall

The performance of fuel cells is best illustrated in its cell potential versus current density response (see Figure 1). This behavior is largely determined by the kinetics of the electrode reactions and the internal resistance of the cell. The major kinetic factor is associated with the oxygen electrode, which requires much more negative potentials than the reversible potential of –1.23 V (w.r.t. reversible hydrogen electrode) to drive the reaction at significant current densities. With catalytic electrodes based on dispersed platinum on carbon, the loss of voltage can be limited to values of 0.4 V at current densities around 0.1 A cm−2.

Schematic cell potential vs. current behavior for hydrogen-oxygen fuel cell.

Figure 1. Schematic cell potential vs. current behavior for hydrogen-oxygen fuel cell.

Hydrogen is the most common fuel utilized in most fuel cells although other fuel such as hydrazine, methane and methanol are possible. There are several types of fuel cell, which can be classified according to the "electrolyte"—alkaline (usually KOH), acid (mainly phosphoric acid), molten carbonate (mixture of Li and K), solid oxide (typically yttria-stabilized zirconia) and solid polymer (proton-conducting membrane).

Alkaline fuel cells usually utilize potassium hydroxide at a concentration of 30%, which is the approximate optimum value for maximum conductivity. The two half cell reactions are

Alkaline fuel cells operate at low temperatures, typically 60-80°C, and at low pressures (1-2 bar).

The phosphoric acid fuel cell is the most technically advanced for large scale power generation. The electrolyte is a thin layer of ortho-phosphoric acid absorbed into a solid matrix (SiC/carbon composite). The electrodes are teflon bonded porous carbon structures using dispersed platinum based electrocatalysts. Operating temperatures are around 200°C which serves to improve the kinetics of oxygen reduction. Operating pressures are typically 4.5—5 bar for air/hydrogen systems, hydrogen being supplied usually by the reforming of methane, the primary fuel. This, or similar primary fuels, is the usual source of hydrogen for fuel cells.

Molten carbonate fuel cells, which can operate with either carbon monoxide or hydrogen as fuel, use an electrolyte immobilized in a porous inorganic matrix. Temperature of operation is 650°C and the anode is a porous nickel or Ni/Cr alloy and the cathode is porous NiO. Solid oxide fuel cells operate at 1000°C and are based on ceramic oxide electrolytes.

The solid polymer fuel cell is based on a solid polymeric proton conducting membrane as the electrolyte sandwiched between two platinum-catalyzed porous carbon electrodes. Operating temperatures are approximately 100°C at their highest, otherwise damage to the membrane is risked. Applications are in space, underwater, standby power and transportation. The principle fuel used is hydrogen although methanol powered cells are attracting significant interest in view of the fuels superior energy density.

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