Current State of the Knowledge and Needs

A fuel cell is an electrochemical system that generates electricity by converting chemical energy into electrical energy. In general form, a fuel cell consists of an anode and a cathode separated by an electrolyte, which acts as an ion carrier. An oxidant is fed to the cathode to supply oxygen, while a fuel is fed to the anode to supply hydrogen. When hydrogen is used as fuel, it is electro-oxidised in the anode producing electrons and protons according to Eq.18.1.

Electrons are transported through an external circuit to the cathode while protons go through the electrolyte. Finally, electrons and protons are recombined together with O2 in the cathode to give H2O (Eq. 18.2).

One of the best known and currently most promising fuel cells is the ProtonExchange Membrane (PEM) fuel cell (Fig. 18.1), which has been proposed as an automotive power source due to its relatively light weight, low operating temperature (60°-100°C), high power density, relatively quick start-up, and rapid response to varying loads (Mehta et al. 2003). Also PEM fuel cells are considered as alternatives to replace batteries in portable electronic equipment such as laptop computers (Jung et al. 1998), cellular phones (Hockaday et al. 1999), and handheld devices. Significant investment in the fuel cell technology is taking place resulting in considerable progress regarding their reliability, increased power density, improved manufacturing technologies, and materials cost reduction. Nevertheless, to meet the fuel cell's full application potential, there is still significant work to be done, since further cost reduction is required.

Fig. 18.1. Proton-Exchange Membrane (PEM) fuel cell

For a fuel cell to be effective, strong acidic or alkaline solutions and high temperatures and pressures are needed. Most fuel cells use platinum as catalyst (Adzic 1998; Markovic et al. 2002), which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anodic chamber (Markovic et al. 2002). In PEM fuel cells, the type of fuel used dictates the appropriate type of anode catalyst needed. In this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO concentration of only several parts per million, due to the strong chemisorption force of CO onto the anode catalyst (Mehta et al. 2003). For anode catalyst materials, platinum-based binary and tertiary platinum/ruthenium-based alloys seem to offer the best performance when CO poisoning is of concern (Toda et al. 1999; Neergat et al. 2001; Markovic et al. 2002).

Current fuel cell cathode catalysts are also made of platinum and platinum-based alloys, which exhibit higher catalytic activity for the oxygen electroreduction reaction (ORR) in pure acid electrolytes than pure platinum (Toda et al. 1999; Neergat et al. 2001). The enhanced electrocatalysis can be explained by an electronic factor, i.e., the change of the d-band vacancy in Pt upon alloying and/or by geometric effects such as the Pt coordination number and Pt-Pt distance (Toda et al. 1999). Both effects should enhance the reaction rate for oxygen adsorption and breaking of the O-O bond during the reduction reaction (Toda et al. 1999). Although, the ORR is enhanced by platinum-based catalysts, still the oxygen kinetics is significantly slow. The exchange current density of oxygen electroreduction on platinum is only about 1.0 nA/cm2 in acidic electrolyte, which is six orders of magnitude lower than that of hydrogen oxidation (Markovic et al. 2002). This sluggish oxygen reduction reaction on the Pt catalyst causes a large overpotential at the cathode (Adzic 1998), requiring a much larger amount of Pt in the cathode than in the anode to achieve the kinetics requirements. In addition, when platinum-based catalysts are considered, further platinum reduction is needed due to its near-term availability and high cost. Hence, there is a need to enhance the oxygen electroreduction kinetics for which new or improved cathode catalysts must be designed. Numerous studies have been aimed toward reducing the amount of platinum required in current fuel cells where platinum-based nanoparticles are considered due to their unique properties.

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