Revision of the NaBO2–H2O phase diagram for optimized yield in the H2 generation through NaBH4 hydrolysis

Abstract

The binary phase diagram NaBO 2-H 2 O at ambient pressure, which defines the different phase equilibria that could be formed between borates, end-products of NaBH 4 hydrolysis, has been reviewed. Five different solid borates phases have been identified: NaBO 2 •4H 2 O (Na[B(OH) 4 ]•2H 2 O), NaBO 2 •2H 2 O (Na[B(OH) 4 ]), NaBO 2 •2/3H 2 O (Na 3 [B 3 O 4 (OH) 4 ]), NaBO 2 •1/3H 2 O (Na 3 [B 3 O 5 (OH) 2 ]) and NaBO 2 (Na 3 [B 3 O 6 ]), and their thermal stabilities have been studied. The boundaries of the different Liquid+Solid equilibria for the temperature range from-10 to 80 °C have been determined, confirming literature data at low temperature (20 °C ~ 50 °C). Moreover the following eutectic transformation, Liq. → Ice + NaBO 2 •4H 2 O, occurring at-7 °C, has been determined by DSC. The Liquid-Vapour domain has been studied by ebullioscopy. The invariant transformation Liq. → Vap. + NaBO 2 •2/3H 2 O has been estimated at 131.6 °C. This knowledge is paramount in the field of hydrogen storage through NaBH 4 hydrolysis, in which borate compounds were obtained as hydrolysis reaction products. As a consequence, the authors propose a comparison with previous NaBO 2-H 2 O binary phase diagrams and its consequence related to hydrogen storage through NaBH 4 hydrolysis.

A hybrid hydrolytic hydrogen storage system based on catalyst-coated hollow glass microspheres

“…The pressure in the proposed system is one order of magnitude smaller than in conventional hydrogen storage pressure vessels. This is also supported by Andrieux et al (2012), who revised the phase diagram of NaBO 2 -H 2 O in 2012 and found that at 250 ± 2°C, the hydrated metaborate consists only of pure NaBO 2 and vapour [59]. The temperature dependence of the vapour pressure of NaBH 4 or NaBO 2 solution is not known to the author, but from the results of Marrero-Alfonso et al (2007) and Andrieux (2012) [47,58], where hydrate formation and dehydration of NaBO 2 were investigated, it can be inferred that, at given temperature, it is much higher than of pure water.…”

“…There, the so-called water excess factor x was introduced, which is commonly set to 2, when the hydrolysis is carried out near RT. [59]. Considering these facts, the hydrolysis reaction equation used in the following study is assumed as…”

Boron-based hydrides for chemical hydrogen storage

“…This is due to the hydration of the borate by-products, which are in the forms NaBO 2 Á2H 2 O and NaBO 2 Á4H 2 O [35,36]. The first issue is the low net GHSC of the system NaBH 4 -H 2 O.…”

“…The first issue is the low net GHSC of the system NaBH 4 -H 2 O. This is due to the hydration of the borate by-products, which are in the forms NaBO 2 Á2H 2 O and NaBO 2 Á4H 2 O [35,36]. For example, the GHSC of NaBH 4 -H 2 O is 7.3 wt% when 4 mol H 2 O per mol NaBH 4 are required for the hydrolysis reaction (Eq.…”

Thermal evolution of solid-liquid equilibria in the quaternary system: NaBH4–NaBO2–NaOH–H2O

“…To increase the energy density of this H 2 generator system, the NaBH 4 concentration has to be maximized. For this, one of the main challenges remains to increase the NaBH 4 concentration of alkaline aqueous solution without drawbacks due to the by-products crystallization, NaBO 2 .yH 2 O, with the pseudo hydration degree y = 0, 1/3, 2/3, 2 and 4 [5]. Sodium hydroxide is added to the solution to limit the self-decomposition of NaBH 4 , thus stabilizing the system.…”