Fuel cells have been known for a long time and still are under development. Several challenges to widespread implementation of fuel cell technology are needed, although novel inexpensive and long-lasting electrocatalyst materials are major factors in design and development. In all fuel cell technologies, independent of the fuel, operating temperature, and the type of material used as the cathode electrocatalyst, the overpotential for the reduction of oxygen at operating currents is significantly high due to the slow oxygen electrochemical kinetics. As a result of this sluggish kinetics, cell voltage is decreased and therefore fuel cells loss in efficiency. Current fuel cell cathode catalysts are carbon-supported Pt or Pt-based alloys nanoparticles. While noble metal cost is less critical for applications in which small power sources are needed, platinum costs and supplies constraint for large-scale applications. The high overpotential for oxygen reduction, however, is a long-standing problem and, so far, research on the fundamental processes of oxygen reduction and catalysis has not yielded a breakthrough. Thus, new alternatives for finding cheaper materials with lower overpotential are needed.
Recent advances in the application of nanostructured carbon-based materials have suggested the possibility of using carbon nanotubes as novel electrocatalyst supports. Studies have shown that Pt nanoparticles supported on carbon nanotubes display remarkably higher electrocatalytic activity toward the reduction of oxygen than Pt nanoparticles supported on carbon black, which would contribute to substantial cost reduction in PEM fuel cells. The finite size of nanoscale materials such as carbon nanotubes positively influences the thermodynamics and kinetics of oxygen reduction due to their length scale and specific properties. Overall, their unique characteristics encourage the use of nanosize catalyst materials instead of their bulk counterparts to enhance the oxygen electroreduction performance. To gain a better understanding of the catalyst surface and mechanistic aspects of oxygen reduction by MWCNT (Multi-wall Carbon Nanotube) supported Pt nanoparticles, the effect of nanoparticle size and external factors, such as temperature, needs to be investigated. Thus, detailed studies of different cluster morphologies are needed to understand the preferred atomic arrangements (shape) as function of external conditions and characteristics of the MWCNT support. Additionally, in order to understand the connection of microscopic information with the time evolution of the rates of oxygen reduction reactions, their dependence on pressure and temperature needs to be elucidated. These studies will help elucidate the effect of electrode potential when Pt nanocatalysts supported on MWCNT are used as cathode catalysts in fuel cell devices.
Researchers need to focus on understanding the unique surface and interfaces of nanocomposites and how they affect the energetics, kinetics, and thermodynamics of oxygen kinetics. For this purpose, a combined experimental-theoretical approach would be appropriate to provide a new platform for the design of more selective and impurity-tolerant catalysts for the fuel cell technology.
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