2 August 2005: Less platinum for cheaper fuel cells

A platinum-free electrode that could help reduce the costs of polymer electrolyte fuel cells (PEFCs), the most promising electrical-power-generating alternative to burning fossil fuels, is being developed by Japanese chemists. 

’Limited supplies of expensive platinum (Pt) could in future prohibit widespread use of PEFCs in electric vehicles and for domestic power generation’, said Jun Maruyama of Osaka’s Municipal Technical College, ’So there is a demand for catalysts that function with far less Pt.’

PEFCs generate electricity by oxidising hydrogen to protons at the anode and reducing oxygen to water at the cathode. The two electrodes are separated by a special polymer membrane that is permeable to protons, completing the circuit, but the all important reactions at the electrodes require expensive precious-metal catalysts like platinum to work efficiently. ’Finding a replacement for precious metals, particularly at the cathode, is one of the Holy Grails for PEFC fuel cell developers,’ said Dan Brett from the department of chemical engineering at Imperial College, London. ’It is particularly important for the cathode since the oxygen reduction is so much slower than hydrogen oxidation, so lots more Pt catalyst is required,’ Brett told Chemistry World.

Maruyama’s solution is to take the oxygen reducing enzyme called catalase and carbonise it onto the oxygen electrode. Although this destroys the catalyst’s fragile protein structure, it doesn’t affect the catalytic centres, which consist of chelated iron molecules called haem. In effect, the carbonisation process produces a highly porous carbon-based matrix with haem catalytic centres homogeneously distributed throughout. In the presence of fluoro-sulphonic acid, this produces a highly active catalyst for oxygen reduction. The new electrode even works in a PEFC-type fuel cell, though performance is currently inferior to the usual Pt-based fuel cells. Unperturbed, Maruyama is bullish about the chances for improvements. ’We haven’t optimised the catalase carbonisation process yet, or how we layer the catalyst onto the bare electrode. This should boost fuel-cell performance,’ he said. ’Also, catalase is produced by all air-breathing organisms, so advanced biotechnology can help cut costs by improving catalase extraction procedures.’

Brett is less concerned about possible future shortages of platinum for fuel cells, and more about its toughness as an oxygen-reduction catalyst. ’In terms of outright performance, non-precious metal catalysts are unlikely to be better than Pt, etc in PEFCs,’ he said. ’The capital cost of the Pt is no longer a burning issue - the Pt can, after all, be reclaimed at end of the fuel-cell’s life.’

The important point is the durability of the materials, says Brett. ’The problem with Pt is that it can easily be poisoned and it sinters, increasing its particle size and reducing fuel-cell performance,’ he said. ’This is precisely where a tough non precious metal-based oxygen-reducing catalyst like Maruyama’s could cut the mustard.’ Lionel Milgrom