Heterogeneous and homogenous catalysts for hydrogen generation by hydrolysis of aqueous sodium borohydride (NaBH₄) solutions

Abstract

It is clear that in order to satisfy global energy demands whilst maintaining sustainable levels of atmospheric greenhouse gases, alternative energy sources are required. Due to its high chemical energy density and the benign by-product of its combustion reactions, hydrogen is one of the most promising of these. However, methods of hydrogen storage such as gas compression or liquefaction are not suitable for portable or automotive applications due to their low hydrogen storage densities. Accordingly, much research activity has been focused on finding higher density hydrogen storage methods. One such method is to generate hydrogen via the hydrolysis of aqueous sodium borohydride (NaBH 4 ) solutions, and this has been heavily studied since the turn of the century due to its high theoretical hydrogen storage capacity (10.8 wt%) and relatively safe operation in comparison to other chemical hydrides. This makes it very attractive for use as a hydrogen generator, in particular for portable applications. Major factors affecting the hydrolysis reaction of aqueous NaBH 4 include the performance of the catalyst, reaction temperature, NaBH 4 concentration, stabilizer concentration, and the volume of the reaction solution. Catalysts based on noble metals, in particular ruthenium (Ru) and platinum (Pt), have been shown to be particularly efficient at rapid generation of hydrogen from aqueous NaBH 4 solutions. However, given the scarcity and expense of such metals, a transition metal-based catalyst would be a desirable alternative, and thus much work has been conducted using cobalt (Co) and nickel (Ni)-based materials to attempt to source a practical option. "Metal free" NaBH 4 hydrolysis can also be achieved by the addition of aqueous acids such as hydrochloric acid (HCl) to solid NaBH 4 . This review summarizes the various catalysts which have been reported in the literature for the hydrolysis of NaBH 4 .

The kinetics of lightweight solid-state hydrogen storage materials: A review

“…The hydrogen capacity can be controlled by varying the alloy combination and compound ratio. However, some of the alloy compounds encounter kinetic difficulties, and their hydrogen storage capacity is relatively low in the range of 1.4 wt% to 3.6 wt% H 2 for portable applications [90,117]. Asano et al [118] reported that the hydrogen sorption kinetics and the thermal stability of metal hydrides are related to the crystal lattice structure of the alloy because the interstitial sites of the crystal lattice can be occupied by hydrogen atoms.…”

About the Technological Readiness of the H2 Generation by Hydrolysis of B(−N)−H Compounds

“…(1)] is spontaneous and exothermic (240±5 kJ mol −1 ); however, the kinetics is severely deteriorated when the pH value is increased because of the formation of basic byproducts (B(OH) 3 /B(OH) 4 − at p K a =9.24) . Total hydrolysis requires then the use of an accelerator, either an acid or more broadly a metal‐based catalyst . Hence, NaBH 4 , the x +4 mol of H 2 O [Eq.…”

“…[46] Total hydrolysis requires then the use of an accelerator,e ither an acid or more broadly am etal-based catalyst. [47][48][49][50] Hence, NaBH 4 ,t he x + 4mol of H 2 O[ Eq. (3)],t he accelerator, the borate byproducts,a nd the product H 2 have to be separately considered to gain ac omprehensive understandingo ft he reaction:…”

Sodium borohydride for the near-future energy: a ''rough diamond'' for Turkey

“…16 Hydrolysis of NaBH 4 and concomitant H 2 release can be controlled with the help of a catalyst. 17 The reaction temperature is mostly set between 10 and 40…”