Direct borohydride fuel cells (DBFCs) are a subcategory of

alkaline fuel cell The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, Francis Thomas Bacon, is one of the most developed fuel cell technologies. Alkaline fuel cells consume hydrogen and pure oxygen, to produce potable wate ...

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 Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds appli ...

potassium borohydride Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example cyanoborohydride or cyanotrihydr ...

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 could potentially be used in more conventional hydrogen

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

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

catalytic Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

decomposition Decomposition or rot 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 e ...

of the borohydride: :NaBH₄ + 2H₂O → NaBO₂ + 4H₂ Direct borohydride fuel cells decompose and

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

the borohydride directly, side-stepping

hydrogen production Hydrogen production is the family of industrial methods for generating hydrogen gas. As of 2020, the majority of hydrogen (∼95%) is produced from fossil fuels by steam reforming of natural gas and other light hydrocarbons, partial oxidation of h ...

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 with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...

catalysts. In addition, they have a higher

power density Power density is the amount of power (time rate of energy transfer) per unit volume. In energy transformers including batteries, fuel cells, motors, power supply units etc., power density refers to a volume, where it is often called volume p ...

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

series circuit 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. Whether a two-terminal "object" is an ...

) 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 ...

). Sodium metaborate might be

hydrogenated Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic co ...

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 that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...

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