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

TL;DR

This study uses ab initio calculations to explore novel substitutions in NH3BH3, aiming to lower its hydrogen desorption temperature by replacing hydride hydrogens with more electronegative elements, notably finding Cu as a promising candidate.

Contribution

It introduces a new substitution strategy targeting hydride hydrogens with electronegative elements, expanding beyond previous approaches focused on NH3 group substitutions.

Findings

Cu substitution lowers hydrogen release barrier by ~37%

Different substituents variably affect desorption temperature

Cu significantly influences hydrogen desorption temperature

Abstract

We present ab initio results for substitutions in the promising hydrogen-storage material NH3BH3 to lower its hydrogen desorption temperature. 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 th ehydridic 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 electronegativity 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 thus desorption temperature - some overshoot our goal while others have little effect - we identify Cu as a very promising substituent, which lowers the reaction barrier by approximately 37% compared to NH3BH3 and thus significantly influences the hydrogen desorption temperature.