The various methods that have been reported for the synthesis of nanostructured carbons are reviewed, emphasizing sol-gel processing, soft-templating and hydrothermal procedures. Hybrid materials can be obtained by incorporation of nanosized carbon components, such as carbon nanotubes or graphene-derived materials. In addition, the carbon surface chemistry can be tuned by functionalization with surface groups and/or by doping with heteroatoms, in order to suit specific applications. Nanostructured porous carbons can play a decisive role towards the development of efficient and cost-effective electrochemical devices for energy conversion and storage. In this article, we focus on novel electrocatalysts for fuel cells and electrolysers, and electrodes for electric double layer capacitors.

Introduction

Activated carbons are widely used as adsorbents and in other industrial applications, due to their large surface areas and high stability, both in acidic and basic media. They are obtained from carbonaceous precursors (such as peat, coal, wood, coconut shell) by carbonization (pyrolysis in the absence of oxygen) and subsequent partial gasification (with steam, carbon dioxide, air, or their mixtures), or by carbonization in the presence of substances that minimize the formation of tars (zinc chloride, phosphoric acid, potassium hydroxide) followed by washing [1,2]. These top-down production routes (named “physical” and “chemical” activation, respectively) lead to a well-developed porous structure with pores of different sizes, which are classified, according to their widths, as micropores (< 2 nm), mesopores (2–۵۰ nm) and macropores (> 50 nm) [3]. The high adsorption capacity of activated carbons stems from their large micropore volumes, which may range from 0.15 to 0.50 cm3 g−۱ . Unless special activation procedures are used, the volume of mesopores is relatively low (< 0.10 cm3 g−۱ ), and they account for < 5% of the adsorption capacity. Macropores are not generated during activation; they are already present in the carbon precursors, thus facilitating the access of reactants during the activation procedure. The volume of macropores in activated carbons can range from 0.20 to 0.50 cm3 g−۱ . Due to the large dimensions of the macropores, their surface area (and adsorption capacity) is negligible (< 2 m2 g−۱ ); however, their presence is essential for providing access of the adsorptives to the inner pores where adsorption occurs [2]. The different types of pores are arranged in a hierarchical pattern, with the micropores branching off from the mesopores, and these from the macropores, which in turn open out to the external surface of the particles [1], as shown in Fig. 1. However, the high temperature processes used in the production of activated carbons cannot achieve the fine control of pore size and pore size distribution needed for more sophisticated applications.