The phase behaviours in mixed G + tetra-n-butylphosphonium borohydride and G + tetra-n-butylammonium hydroxide hydrates (where G = H2, N2)

Abstract : The use of hydrogen hydrates stabilized by tetra-n-butylphosphonium borohydride (TBPBH4) and tetra-n-butylammonium hydroxide (TBAOH) as media for hydrogen storage was investigated using High Pressure Differential Scanning Calorimetry (HP-DSC) and gas volumetric analysis. In this study the TBPBH4 was obtained by reacting sodium borohydride with tetra-n-butylphosphonium bromide (TBPB). The TBPBH4 solutions were found to form hydrates at low temperature, and HP-DSC was used for determining the dissociation (p, T) points of mixed N2 + TBPBH4 and H2 + TBPBH4 hydrates at various salt concentrations. The results show that the new additive has very efficient stabilizing effect on the hydrate structure, the dissociation temperatures ranging from 280 to 295K, depending on the gas, in a pressure range of 0 to 40 MPa. The amount of chemically stored hydrogen was found to be 1.26 % in mass, using volumetric analysis of the H2 released by hydrolysis of the TBPBH4 compound in presence of acetic acid. The influence of the gas nature on the hydrate stability was investigated. The higher stabilization effect of nitrogen comparing to hydrogen was discovered. The possibility of gas enclathration and stabilizing effect of hydrogen into TBAOH - H2O hydrates was investigated as well. The formation of two hydrates with high dissociation temperatures were found in this system. Analysis of the p-T data of the TBAOH + water + H2 system revealed the quite promising hydrogen absorption properties of one of these hydrates.