Fuelled by fungus

Research at Oxford University could lead to an enzyme extracted from fungus being used as a cheap, effective catalyst in fuel cells that could replace batteries in domestic appliances.

Use of low-temperature fuel cells could help reduce the billions of spent batteries dumped every year, but the catalysts in current fuel cells use platinum, which is expensive and rare.

Laccase, an enzyme extracted from white-rot fungi, has been proposed as a catalyst for the oxygen reduction reaction since the 1960s, but has not yet successfully been used in fuel cells.

Dr Chris Blanford is undertaking a five-year project to use this ‘catalyst that grows on trees’ to help make cathodes that are longer lasting, thermally stable and cheap.

‘Based on our initial work, we find that laccase works at ambient temperatures better than a platinum catalyst and in a less harshly acidic environment,’ said Blanford. ‘It does this without precious metals — it just has four copper molecules.’

Blanford’s initial goal is to keep the laccase cathode stable for the lifetime of a normal fuel cell cathode. So far his team has got it to work for about three months, producing modest amounts of power.

The next step will be to scale up the technology, increasing the surface-to-volume ratio using a three-dimensional electrode structure, while making sure that there is a good flow of fuel (such as hydrogen or methanol) and oxygen molecules.

‘The densities we’re getting are less than a milliwatt a square centimetre on a flat electrode,’ said Blanford. ‘If we can put some of this surface area on to the inside of the material, then we can scale up the power output.

‘It’s going to take a lot of optimisation. We need to make sure the channels aren’t so small that the oxygen can’t get in. If we make the electrode material too small, there will be too much resistance for the electrons to flow through the material in and out of the laccase.’

Project partner Guildford-based Mast Carbon is supplying prototype structured carbon materials (carbons) made using a patented process that controls the pore size distribution within the material.

‘They can engineer carbons with 2-50 nanometre mesopores,’ said Blanford. ‘We’re using enzyme-sized ones that will act as pockets for the enzymes to sit in with larger micrometer pores connecting them up for gas fuel flow.’

In terms of oxygen molecules produced per second, laccase is still about 10 times more expensive than platinum, despite its abundance in nature. Blanford is working with molecular plant pathologist Prof Sarah Gurr, who is developing genetic modifications that could improve laccase’s catalytic ability, or make specific groups on the surface of the protein so that it adheres better to the electrode surface.

‘The main problem is that proteins and surfaces tend to be a bit finicky,’ said Blanford. ‘There are two competing bodies of research: one, like ours, is trying to get proteins to stick to surfaces in a particular way; the other is trying to stop proteins getting onto surfaces, for example preventing bacterial build-up on medical equipment, such as a catheter, or stopping fouling in sewage treatment.

‘I hope a spin-off of our research could be a more general understanding of protein attachment to well-defined surfaces.’

Blanford plans to quantify how best to use laccase as a catalyst material using a quartz crystal microbalance, which will allow the researchers to see how different modification techniques and different conditions for operations affect how the catalyst is performing and its failure mechanism.

‘Our research is a unique approach to studying enzyme attachment where we’re weighing these individual proteins on an electrochemically-active surface to see how they respond to the changing conditions,’ he said.

By 2013, Blanford’s key goal is to have a cathode material for methanol or hydrogen fuel cells. Ideally, his team will have produced a laccase cathode fuel cell with the same format and lifetime as everyday batteries, which is around 200 charge and discharge cycles.

‘We want replacement devices that will be competitive with a rechargeable battery, producing 2,000-2,500 milliamp/hour, and with the same size and shape as lithium-style batteries, but running off liquid or gas fuels rather than stored charge,’ said Blanford.

Berenice Baker