Vanadium boride bears fuel cell hopes

New technology promises twice the capacity of petrol

by Claudia Flavell-While -- 28/7/2008

Developing better fuel cells for car is a critical task

ENGINEERS IN THE US have developed a novel kind of fuel cell that can deliver twice the amount of energy as a

conventional petrol engine.

According to Stuart Licht, professor of chemistry at the University of Massachusetts in Boston, the vanadium boride

(VB2)/air fuel cell stores more energy than petrol and has a capacity that is an order of magnitude higher than that of

lithium-ion batteries.

The new VB2 system has a practical capacity of 5 kWh/l, compared with 2.7 kWh/l for petrol and 0.5 kWh/l for a lithium-

ion battery, they say.

Crucial to this comparison is the concept of practical capacity – that is actually delivered capacity including voltage losses

and all limitations from energy and system mass, air cathode size and other cell components. “For example, the

practical energy of a small, portable commercial zinc air cell exceeds 18% of the intrinsic energy capacity, and can be

higher in an optimized, large fuel cell configuration,” Licht explains. “The relative practical capacity of the VB2/air cell

can be estimated as similar to that of the well studied zinc-air system (electrolytes and cathodes are similar).” This gives

the vanadium boride fuel has a practical lower limit of 18% of its intrinsic 27 kWh L-1, for an estimated vanadium boride

air practical storage capacity of 5 kWh L-1, he adds.

The new fuel cell works similar to a zinc-air fuel cell in that the vanadium boride cell reacts to oxygen brought in via the

cathode with the anode to produce electricity. Much like in a zinc-air cell, the reaction is irreversible; spent anodes need

to be replaced and chemically regenerated (a process Licht and his team could be achieved through a solar-powered

magnesium reaction). The cell’s vanadium boride anode is coated in zirconia to prevent boride corrosion, which would

reduce the cell’s electrochemical capacity and could potentially damage the fuel cell

Licht and his team will now work to further optimise the technology by reducing the size of the vanadium boride

particles used, and using liquid rather than solid magnesium to regenerate the anodes.

Their research was published in the 28 July issue of the journal Chemical Communications.