Carbon nanofibers

Abstract

Carbon nanofibers (CNF) are a synthetic graphite-like nanostructured carbon material, with advantageous properties for several application fields such as catalysis or composites. CNF can be produced by means of the catalytic decomposition of a carbon containing gas (such as methane or ethylene) over transition metals (like nickel, cobalt or iron). Some synthesis conditions, i.e. temperature, gas composition and space velocity, affect the nanofilaments morphology, their crystalline structure, ordering degree, porosity and electronic conductivity, which enables favoring either a certain group of properties or the other (e.g. porosity or crystallinity), but not all of them jointly. Their high electronic conductivity results of great importance for electronic applications, being tunable by means of an appropriate choice of the carbon nanofibers growth conditions. Carbon nanofibers can also be chemically treated with the aim of increasing the amount of surface oxygen groups or of developing a higher specific surface area. The functionalization with strong acids is effective to generate oxygen surface groups that, under certain conditions, can be useful in practical purposes. However, chemical activation of CNF does not entail significant advantages regarding the development of porosity. These CNF can be applied as catalyst support for several catalytic processes. This chapter focuses on the use of CNF as electrocatalyst support for fuel cells. These kinds of devices directly and efficiently convert the chemical energy of a fuel to electrical energy, which has caused an increasing interest in research and development in the last two decades. The electrocatalysts, based on platinum supported on carbon materials, are responsible of this energy conversion electrochemical process. The use of CNF as electrocatalytic support is advantageous in several aspects, improving the diffusion of reactants and products, contributing to the increase of the electronic conductivity, enhancing the metal-support interaction and improving the resistance to accelerated corrosion processes that simulate several thousand hours of fuel cell operation. In this chapter we will see the effect of substituting conventional carbon supports by CNF as support for Pt and PtRu on the electrocatalytic activity. The support properties seriously determine the catalyst activity and durability, finding that a compromise situation between the carbon ordering degree, the porous structure and surface chemistry is mandatory.