Direct borohydride fuel cells (DBFCs) are a subcategory of

alkaline fuel cell In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...

s which are directly fed by

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula (sometimes written as ). It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodi ...

or potassium borohydride as a fuel and either air/oxygen or hydrogen peroxide as the oxidant. DBFCs are relatively new types of fuel cells which are currently in the developmental stage and are attractive due to their high operating potential in relation to other type of fuel cells. Recently, DBFCs that rival proton-exchange membrane fuel cells (PEMFCs) in peak power but operating at double the voltage have been reported.

Chemistry

Sodium borohydride has been used with more conventional hydrogen

fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...

systems as a means of storing hydrogen. The hydrogen can be regenerated for a fuel cell by

catalytic Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...

decomposition Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is ess ...

of the borohydride with water, including successful hydration with synthetic urine: :NaBH₄ + 2H₂O → NaBO₂ + 4H₂ Direct borohydride fuel cells decompose and

oxidize Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

the borohydride directly, side-stepping

hydrogen production Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Article in press. Most hydrogen is ''gray hydrogen'' made through steam methane reforming. In this process, ...

and even producing slightly higher energy yields: :Cathode: 2O₂ + 4H₂O + 8e⁻ → 8OH⁻ (E⁰ = +0.4 V) :Anode: NaBH₄ + 8OH⁻ → NaBO₂ + 6H₂O + 8e⁻ (E⁰ = -1.24 V) ::Total E⁰ = +1.64V The simplified reaction is: :NaBH₄ + 2O₂ → NaBO₂ + 2H₂O + Electricity The working temperature of a direct sodium borohydride fuel cell is 70 °C (158 °F).

Advantages

DBFCs could be produced more cheaply than a traditional fuel cell because they do not need expensive

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

catalysts. In addition, they have a higher

power density Power density, defined as the amount of power (the time rate of energy transfer) per unit volume, is a critical parameter used across a spectrum of scientific and engineering disciplines. This metric, typically denoted in watts per cubic meter ...

. The high operating voltage of a DBFC reduces the number of cells (in a

series circuit Terminal (electronics), Two-terminal components and electrical networks can be connected in series or parallel. The resulting electrical network will have two terminals, and itself can participate in a series or parallel Topology (electrical ci ...

) needed in a stack to achieve a desired rated voltage and thus reduces the stack costs considerably.

Disadvantages

Unfortunately, DBFCs do produce some hydrogen from a side reaction of NaBH₄ with water heated by the fuel cell. This hydrogen can either be piped out to the exhaust or piped to a conventional hydrogen fuel cell. Either fuel cell will produce water, and the water can be recycled to allow for higher concentrations of NaBH₄. More importantly, the process of creating electricity via a DBFC is not easily reversible. For example, after sodium borohydride (NaBH₄) has released its hydrogen and has been oxidized, the product is NaBO₂ (

sodium metaborate Sodium metaborate is a chemical compound of sodium, boron, and oxygen with formula . However, the metaborate ion is trimeric in the anhydrous solid, therefore a more correct formula is or . The formula can be written also as to highlight the re ...

). Sodium metaborate might be hydrogenated back into sodium borohydride fuel by several different techniques, some of which might theoretically require nothing more than water and

electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

or heat. However, these techniques are still in active development. As of June 30, 2010, many patents claiming to effectively achieve the conversion of sodium metaborate to sodium borohydride have been investigated but none have been confirmed—the current efficiency of "boron hydride recycling" seems to be well below 1% which is unsuitable for recharging a vehicle.

Cost

Mass production projected prices for the fuel are as low as US$5/kg, rivalling the cost of hydrocarbon fuels.

References

External links