Lowering the hydrogen desorption temperature of NH3BH3 through B-group substitutions

Abstract

We present ab initio results for substitutions intended to lower the hydrogen desorption temperature of NH₃BH₃, a promising hydrogen-storage material. Substitutions have already been investigated with significant success recently, but in all cases a less electronegative element is substituted for the protic hydrogen in the NH₃ group of NH₃BH₃. We propose a different route, substituting the hydridic hydrogen in the BH₃ group with a more electronegative element. To keep the gravimetric density high, we focus on the second period elements C, N, O, and F, all with higher electronegativities compared to H. In addition, we investigate Cu and S as possible substitutions. Our main results include Bader charge analyses, hydrogen binding energies, and kinetic barriers for the hydrogen release reaction in the gas phase as well as in the solid. While the different substituents show varying effects on the kinetic barrier and desorption temperature, we identify Cu as a very promising substituent, which lowers the reaction barrier by approximately 38% compared to pure NH₃BH₃ resulting in a significantly reduced hydrogen desorption temperature.