Nanoporous carbons derived from binary carbides and their optimization for hydrogen storage

Abstract

Introduction and Objective The transition from an energy economy currently dominated by fossil fuels to more environmentally friendly hydrogen requires a material that can store hydrogen effectively. Many types of materials have been tried, including carbon nanotubes, activated carbons, metal-organic framework compounds, metal hydrides, and clathrates, but none of these meets the target. Physisorption route of hydrogen storage relies on weak van der Waals bonding, and as the minimum potential energy between the surface of the storage material and hydrogen is at a distance of approximately one molecular size of hydrogen (~0.41 nm), extremely small pores are necessary for maximizing physisorption of hydrogen. Thus, the challenge for higher hydrogen storage using physisorption technique lies on synthesizing highly porous materials with narrowly distributed small pores. As most synthesis routes for porous carbon result in larger pores with wider pore size distributions (PSD), the challenge lies in the development of a synthesis route leading to narrowly distributed nanoporous carbons with controlled pore size. Carbide derived carbon (CDC) provides the possibility of producing small pores. The primary objective of this work is to synthesize tunable nanoporous carbons from binary metal carbides and establish a relation between the structure of the initial metal carbide and the properties of the resultant carbon. As there is lack of understanding of the effect of porosity, the other main objective of this work is to determine its effect on hydrogen sorption and optimize CDC for achieving the maximum hydrogen uptake. Unlike synthesis of most porous carbons which involves thermal decomposition of organic materials, CDC production is based on selective thermo-chemical etching of the metal(s) or metalloid from a rigid metal carbide lattice. The general reaction involved in synthesis of carbon from metal carbides can be written as: MaCb(s) + (c/2)Cl2(g) → aMClc(g) + bC(s), (1), where M represents a metal or metalloid.