Direct borohydride fuel cell
Encyclopedia
Direct borohydride fuel cells (DBFCs) are a subcategory of alkaline fuel cell
s which are directly fed by sodium borohydride
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.
systems as a means of storing hydrogen. The hydrogen can be regenerated for a fuel cell by catalytic decomposition
of the borohydride:
Direct borohydride fuel cells decompose and oxidize
the borohydride directly, side-stepping hydrogen production
and even producing slightly higher energy yields:
The simplified reaction is:
The working temperature of a direct sodium borohydride fuel cell is 70°C (158°F).
catalysts. In addition, they have a higher power density
.
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 might be hydrogenated
back into sodium borohydride fuel by several different techniques, some of which might theoretically require nothing more than water and electricity
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.
Alkaline fuel cell
The alkaline fuel cell , also known as the Bacon fuel cell after its British inventor, is one of the most developed fuel cell technologies. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. AFCs consume hydrogen and pure oxygen producing...
s which are directly fed by sodium borohydride
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
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.
Chemistry
Sodium borohydride could potentially be used in more conventional hydrogen fuel cellFuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
systems as a means of storing hydrogen. The hydrogen can be regenerated for a fuel cell by catalytic decomposition
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...
of the borohydride:
- NaBH4 + 2H2O → NaBO2 + 4H2
Direct borohydride fuel cells decompose and oxidize
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
the borohydride directly, side-stepping hydrogen production
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
and even producing slightly higher energy yields:
- Cathode: 2O2 + 4H2O + 8e− → 8OH− (E0 = +0.4V)
- 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 platinumPlatinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
catalysts. In addition, they have a higher power density
Power density
Power density is the amount of power per unit volume....
.
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 might be hydrogenated
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
back into sodium borohydride fuel by several different techniques, some of which might theoretically require nothing more than water and electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
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.