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

Ranjan Kumar Dash

doi:10.17918/etd-867

On-board hydrogen storage is one of the major hurdles for success of hydrogen economy. Hydrogen storage using physisorption technique demands highly porous materials. Carbide derived carbons (CDC), a new class of porous carbons produced by thermo chemical etching of metal atoms from carbides were selected as a method for producing highly porous material for hydrogen storage. In order to synthesize tunable nanoporous carbon and to establish a structure-property relation between initial metal carbide and resultant nanoporous carbon, CDCs were synthesized from four metal carbides, two that have uniform carbon to carbon distance in the lattice structure (ZrC, TiC and SiC) and one that has a non-uniform carbon distribution in the lattice (B4C). It was shown that a uniform distribution of carbon atoms in the carbide is important for obtaining a narrow pore size distribution (PSD). CDC derived from B4C had a relatively broad PSD and contained mesopores even at the lowest synthesis temperature, while the CDC produced from SiC maintained a narrow PSD even at the synthesis temperature of 1200oC. CDC produced from ZrC and TiC has a narrow PSD at low synthesis temperature and pores gets wider at higher temperatures. Comparison of CDCs produced from ZrC, TiC and B4C shows that CDCs produced from ZrC and TiC show a lower degree of ordering than that from B4C at high temperatures. Unlike CDCs produced from ZrC and TiC, the PSD of CDCs from B4C does not change appreciably in the 600-1200oC range. CDCs produced from ZrC and TiC can have both narrowly distributed micropores (pores smaller than 2 nm) and mesopores (pores larger than 2 nm), depending on synthesis temperature. In this work, it is demonstrated that porosity of CDC can be fine tuned with a high accuracy by using different starting carbides and varying the synthesis temperatures. This is very important in many applications of porous carbon, especially for gas storage. CDC from ZrC, TiC, B4C and SiC resulted in a family of high surface area nanoporous carbons (300 to 2300 m2/g) of average pore size ranging from 0.5 to 1.4 nm, A systematic study on hydrogen storage capacity of CDCs and comparison with other materials including metal organic framework (MOF-5), single (SWCNT) and multiwalled carbon nanotubes (MWCNT) dispelled the popular myth that hydrogen physisorption is directly proportional to SSA, and provided guidance for optimal design by showing that a large volume of small open pores is the key to high hydrogen uptake at cryogenic temperatures and ambient pressure. Values up to 3 wt.% and 30 kg/m3 have been demonstrated in CDCs; these are twice what can be stored in MOF-5 and several times higher than has been achieved in SWCNT or MWCNT at 77 K and atmospheric pressure. The isosteric heat of hydrogen adsorption up to 11 kJ/mole of CDCs is higher than that reported for activated carbon, CNT and MOF.

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

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

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