Liquid asset

A former rocket fuel called hydrazine could be the new, green replacement for hydrogen in automotive industry fuel cells, according to Italy-based catalyst manufacturer Acta.

Scientists at the company demonstrated that a fuel cell using hydrazine (N2H4, a nitrogen-hydrogen compound) can deliver a peak power of 700mW/cm2, using platinum-free HYPERMEC catalysts. This exceeded by 40 per cent the 500mW/cm2 reported for Daihatsu power cells under the same operating conditions. According to Acta, this is sufficient enough improvement to allow hydrazine-fuelled cells to replace hydrogen-fuelled cells in existing cars and buses.

‘Hydrazine is the perfect fuel for fuel cells in that it is stored in a liquid form, yet it delivers the same sort of power output that a hydrogen fuel cell delivers, despite the fact that hydrazine itself is very difficult to handle,’ said Acta chief executive Toby Woolrych.

The fuel is highly toxic and flammable, but the company is optimistic about its potential as a future fuel after Daihatsu unveiled a new way of safely storing the volatile liquid fuel, which is usually industrially produced from ammonia.

‘Following this, a global car company customer, who we are not allowed to name, asked us whether we could match Daihatsu’s performance. We said “absolutely — we can actually do 40 per cent better”,’ said Woolrych.

In Daihatsu’s storage technology, the fuel tank is filled with a granulised polymer, which is embedded with a carbonyl group consisting of an oxygen atom double bonded to a carbon atom. Hydrazine hydrate reacts with the carbonyl group and bonds with the polymer, forming a solid called hydrazone, which can be stored.

Circulating warm water through the solid returns the hydrazone to the original carbonyl group and releases liquid hydrazine hydrate, which is then supplied to the fuel cell.

The fact that hydrazine is a fluid gives it an advantage over hydrogen, said Woolrych. ‘The best fuel cells use hydrogen gas, as hydrogen fuel cells are a well-known technology and deliver the highest power for the lowest temperature.

‘But the first problem is where do you get the hydrogen gas from? It is an utterly impractical fuel to transport and store. Second, you need a platinum catalyst to run a hydrogen fuel cell.’

The platinum required in existing hydrogen fuel cell technology is very costly; according to Acta, a standard hydrogen fuel cell contains up to $2,500 (£1,270) worth of highly corrosion-resistant platinum in the catalyst, which is necessary because of the cell’s acidity. But the hydrazine cell has an alkaline membrane, allowing cheaper metals, such as cobalt and nickel, to be used as an electrode catalyst instead.

Woolrych added: ‘Hydrazine is the only liquid where the reaction rate — the rate at which it decomposes — is comparable to that of hydrogen. Being liquid, it is also practical to store.’

Published reports put hydrazine’s electromotive force (the maximum potential difference between two electrodes of a galvanic or voltaic cell) at 1.56V, compared with 1.23V for hydrogen — a 27 per cent difference.

Once hydrazine can be stabilised and made more widely available in fuel cells, Acta believes that future cars should pose less of an environmental health problem.

‘Once hydrazine decomposes in a fuel cell, it releases nitrogen, which is in 80 per cent of air, and then the hydrogen mixes with oxygen and becomes water. So it really is zero-emission motoring,’ said Woolrych.