Sodium tetrahydroborate, a material storing and generating hydrogen (for fuel cells)

Sodium tetrahydroborate (borohydride) NaBH ₄ is a common, well known chemical because very largely used as reducing agent in e.g. organic chemistry. It has been discovered in the 1950s and one of its inventors has been awarded the Nobel Prize. Prof. H.C. Brown (1912-2004), inventor with Prof. H.I. Schlesinger of NaBH ₄, was awarded in Chemistry in 1979 “for the development of use of boron-containing compounds into important reagents in organic synthesis” [Nobel]. NaBH ₄ is today used in some industrial applications: e.g. bleaching agent in paper manufacture or reducing agent in pharmaceutical industry. Besides these applications, since the late 1990s, NaBH ₄ is establishing itself as a potential, promising energy/hydrogen carrier. An energy carrier because it can be used as fuel (as an alkaline aqueous solution) of direct liquid-feed fuel cell: it is the principle of the direct borohydride fuel (DBFC) [Demirci, 2007]. A hydrogen carrier because it stores atomic hydrogen and it can release molecular hydrogen H ₂, which is an energy carrier for fuel cells [Demirci, 2008a]. In other words, NaBH ₄ has become, in less than ten years, a promising compound in the energy field.

Depletion of fossil fuels resources and environmental issues (like e.g. global warming) favour the emergence of alternative, renewable fuels. H ₂ is one of them. It is considered as a promising alternative to fossil fuels but the development of a hydrogen economy (that is, production, storage, distribution and consumption) is today quite difficult, some technological problems facing it. Hydrogen storage is one of the problems. Six different methods of storage are currently being considered: high pressure, liquid H ₂, adsorption on porous materials, adsorption on metals, chemical hydrides, and oxidation of reactive metals (e.g. Li or Na) with water [Schlapbach, 2001].

Chemical hydrides, like e.g. tetrahydroborates, offer a safe alternative for hydrogen storage and gravimetric hydrogen storage capacities up to 10 wt%. NaBH ₄ is one of them. It is the most investigated of the tetrahydroborates because it is recognised as being non-flammable and stable under anhydrous medium up to 300 °C. Furthermore it is commercially available and it has a storage capacity of 10.7 wt% [Wee, 2006]. Moreover the stored hydrogen can be released without generating carbon dioxide, the main greenhouse gas at the origin of global warming and climate change. Note that, however, H ₂ generated from NaBH ₄ cannot be regarded as a renewable energy carrier because boron is a limited resource.

Hydrolysis of NaBH ₄ generates H ₂ according to the following reaction: NaBH ₄ + 2 H ₂O à NaBO ₂ + 4 H ₂. For this reaction, the storing medium is the couple (NaBH ₄ + 2 H ₂O) that stores 10.8 wt% of H ₂; half of hydrogen is provided by water [Demirci, 2008a]. This reaction is the ideal, i.e. stoichiometric, one. This remark is essential since in reality the hydrolysis reaction needs an excess of water and is as follows: NaBH ₄ + (2 + x) H ₂O à NaBO ₂. xH ₂O + 4 H ₂. This excess of water (i.e. x) is detrimental to the gravimetric hydrogen storage capacities of the couple (NaBH ₄ + 2 H ₂O); typically if x is equal to 2 or 4, the capacity is then of 7.3 or 5.5 wt%, respectively. In the experimental conditions of the hydrolysis, the stable form of the reaction product, sodium metaborate NaBO ₂, is the hydrated form NaBO ₂. xH ₂O and that’s why the reaction requires an excess of water. Typically it has been showed that the excess of water varied from 2 to 4 at temperatures below 100 °C [Marrero, 2007]. Excess of water is besides necessary because, at 25 °C, the solubility of NaBO ₂, i.e. 16 g per 100 g of water, is inferior to that of NaBH ₄, i.e. 55 g per 100 g of water and that implies that the concentration of NaBH ₄ should be below 16 g per 100 g of water to keep the liquid state of NaBO ₂ (it is essential to avoid NaBO ₂ crystallisation) [Kojima, 2002]. Accordingly the gravimetric hydrogen storage capacity is only of about 3 wt%. This value is lower than the claimed 10.8 wt%. Investigations are in progress in order to improve the storage capacity of the couple (NaBH ₄ + 2 H ₂O). Several solutions are being investigated: - Utilization of water stream generated by a fuel cell; as water is not stored anymore, this solution would improve the storage capacity to values up to 10 wt% [Marrero, 2007]; - Utilization/storage of NaBH ₄ in its solid state, water being provided in controlled amount for generating the required hydrogen [Liu, 2008a]; - Gel of NaBH ₄ by using a polymer as a super absorbent, water being supplied in controlled amount [Liu, 2008b].

Otherwise, it is interesting to note that hydrogen from NaBH ₄ can be recovered by thermolysis [Züttel, 2007]. The main drawback of this process is the high temperatures that are required for generating hydrogen, what is therefore a drawback for any mobile, portable application.

Besides the issue briefly discussed above (that is, this one related to the effective hydrogen storage capacity), some other issues do not enable NaBH ₄ of being competitive. The first issue is cost. NaBH ₄ is an expensive material and investigations are in progress for proposing cost-effective production way while it is thought that its cost should decrease if NaBH ₄ is mass-produced [Wee, 2006]. The second issue is the recycling of the reaction by-products, which are the hydrated metaborates. These compounds can be used for other purposes but the main challenge is today to recycle them into NaBH ₄ and this should lead to lower the costs [Demirci, 2008a]. Globally these issues primarily aim to reduce the cost of NaBH ₄ as well as not to deplete the boron resources.

The third issue is the catalyst. Its utilization is crucial for generating hydrogen at rates matching the requirements of fuel cells. The catalyst has to be effective, durable and viable. Lots of papers are devoted to the catalyst; it is certainly one of the most studied topics that are related to NaBH ₄. Ruthenium is one of the most performing metals, followed by platinum [Amendola, 2000; Brown, 1962; Demirci, 2008b; Demirci, 2008c]. However because of their expensiveness, alternative metals are deeply being sought. The transition metals known to be the most reactive were tested and, among e.g. the metals Fe, Ni or Rh, Co showed promising catalytic performances [Jeong, 2005; Patel, 2007]. Furthermore Co has attracted attention due to its lower cost than the noble metals. Today Co is the most investigated metal for catalysing hydrogen generation through hydrolysis of NaBH ₄. Globally the efforts are being focused on improving the reactivity of Co by improving the methods of catalyst preparation and, above all, by synthesising cobalt-boron compounds. Indeed it is rather unanimously recognised and accepted that the active phase of the Co-based catalyst is a compound like Co _xB _y [Patel, 2007; Walter, 2008].

The aspects and issues discussed above represent the state-of-the-art of the utilisation of NaBH ₄ as hydrogen storage material. Investigations are evidently in progress and many questions have still to be answered. The following points need to be highlighted; for example: - Hydrolysis reaction mechanisms; - Active phase (reactive sites) of Co-based catalysts; - Long-term stability and durability of the catalysts; - Nature of the reaction intermediates; - Involvement of the hydroxide ions in the hydrolysis reaction; - Recycling of the reaction by-products; - Effective hydrogen storage capacity in application; - Conception and long-term testing of the storage system coupled with a fuel cell; - Safety of the storage system as well as the whole fuel cell-storage system; - Etc.

In conclusion, it seems obvious that NaBH ₄ is among the promising solutions for storing hydrogen. However given the state-of-the-art, one has to recognise that it is far from an eventual commercialisation. This has been well noticed by the US Department of Energy. In 2007, they have recommended a no-go for sodium borohydride for on-board vehicular applications. The decision has been clearly based on experimental data and the state-of-the art. The experts working for the US DOE unanimously recommended the no-go because the fact that sodium borohydride meets the capacity targets is surrealistic. Nevertheless the US DOE has only consideres automotive applications and not portable ones. We believe that this material has a potential in portable and niche applications and in that sense R&D should not be given up.

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