Fuel Cells and Batteries

Many developments in fuel cell technology have taken place over the last two decades as a result of concerns over the efficiency of and the environmental problems associated with existing energy converters. The nanostructured tita-nates and TiO2 have been considered as supports for the electrocatalyst of fuel oxidation as shown in Figure 5.4 a. Early studies of palladium nanoparticles deposited on the surface of titanate nanotubes demonstrated the feasibility of nanostructured titanates for the oxidation of methanol in liquid fuel cells.44 Further improvements in catalyst performance have been achieved by increasing the electrical conductivity of the elongated Pd/TiO2 nanorods by

Figure 5.4 Electron transport processes in a direct borohydride fuel cell: a) the principle and b) a cyclic voltammogram for the oxidation of borohydride ion using an Au/TiNT electrode (adapted from ref. 47).

carbon-coating via calcination of nanotubes coated with poly(ethylene glycol) at 600 °C45

Recent studies have also showed that the simple addition of titanate nano-tubes to the standard Pt/C catalyst, by mixing in slurry followed by drying on the electrode surface, results in an enhancement of catalyst performance. This is due to the structural water in the nanotubes and an increase in tolerance to CO poisoning, as a result of stimulating CO desorption from the catalyst surface.46 Gold deposited onto titanate nanotubes has also demonstrated a performance comparable to commercial Au/C catalysts for the anodic oxidation of NaBH4 in a direct borohydride fuel cell47 (see Figure 5.4 b). The electrical charge during oxidation of the borohydride ion per unit mass of gold was approximately 8300mCcm2mg_1 and 3900mCcm2mg~1 for Au/TiNT and Au/C electro-catalysts, respectively (see Table 5.2).

Titanate nanotubes have been used not only for enhanced fuel cell catalyst efficiency, but also for improving proton conductive membrane performance. It has been shown that the addition of titanate nanotubes (up to 15 wt%) to the Nafion membrane, enhanced proton exchange conductivity at elevated temperatures (130 °C) due to water retention in the nanotubular titanates.48 Such a composite membrane can be cast from a mixed slurry of nanotubes and 5% Nafion solution (DuPont), redissolved in dimethylsulfoxide.48

Applications using anodic TiO2 nanotube array electrodes in fuel cell technology have not yet been realised, probably due to the relatively low surface area of nanotubes. In combination with other nanostructures, however, such an array can be used for structural reinforcement.

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