TGA/MaxRes Used for Studies on Hydrogen Storage

The production of energy from renewable sources is usually subject to fluctuations and requires intermediate storage systems. Hydrogen is a particularly promising possibility. One field of research is focused on developing ways to make hydrogen safe to use by storing it in suitable solids. Borohydrides show great potential. Hydrogen can be regenerated through controlled reaction with water. This produces hydrates of different composition. The following article shows how TGA can be used to study the stability ranges of the hydrates. This information is important for optimum process control and for obtaining a high yield of hydrogen.

Introduction

Renewable energies such as solar energy, wind energy or biomass are crucial to solving the energy crisis and providing a sustainable supply of energy. 

A major difficulty with renewable forms of energy is that they are not continuously available (e.g. the day and night cycle with solar energy). To bridge gaps in supply, renewable energy forms have to be somehow temporarily stored. Hydrogen is a particularly promising candidate for this purpose. It can be generated electrolytically from water and converted to electricity as needed in a fuel cell thereby producing water once again. Hydrogen is thus a form of energy that can be stored and transported.

Hydrogen can in fact be stored and transported as a gas, as a liquid or bound in a solid. Due to safety regulations, its storage as a gas or liquid is rather difficult and expensive. 

For this reason, research is focused on the storage of hydrogen in solids, for example as metal or complex hydrides [1]. The latter includes sodium borohydride, NaBH₄ . This material can store up to 10.6 mass% hydrogen. Hydrogen is regenerated from the hydride through the controlled addition of water according to the following equation [2, 3]: NaBH₄ + (2 + x) H₂O -> NaBO₂ . x H₂O + 4 H₂ where x = 0, 2, 4

The percentage yield of hydrogen depends on the degree of hydration x of the metaborate produced. For example, the hydrates bind additional water molecules that are then no longer available to generate hydrogen. The maximum hydrogen capacity is 10.6% if the anhydride is formed (x = 0), but only 5.5% if the tetrahydrate (x = 4) is formed. These calculations refer to the hydrogen that is produced from the sum of the reactants (sodium borohydride and water, see Table 1). For mobile applications in particular, the weight of water that has to be transported is important.

The aim of our research is to determine the different hydrate steps of the metaborate using thermogravimetric analysis (TGA) and in particular the MaxRes method. This information is important for defining the experimental conditions to ensure that only hydrates with relatively low degrees of hydration occur when hydrogen is generated from the sodium borohydride. 

Summary and Conclusions

The well-known hydrates of NaBO₂ were identified and their thermal stability ranges characterized using the MaxRes method. In addition, two previously unknown hydrates were found. Since borates can form metastable states, it can be assumed that the event-controlled dehydration (MaxRes method) favors the formation of such states. Knowledge of the stability ranges of the different hydrates helps one to optimize the hydrolysis of sodium borohydride. When sodium borohydride is used as a storage medium to generate hydrogen through reaction with water, the degree of hydration of the resulting NaBO2 should be as low as possible.

_{TGA/MaxRes used for Studies on Hydrogen Storage | Thermal Analysis Application No. UC 315 | Application published in METTLER TOLEDO Thermal Analysis UserCom 31}