New catalyst could replace platinum in fuel cells

The finding, detailed in Scientific Reports, is said to be a step toward eliminating what industry regards as the largest obstacle to large-scale commercialisation of fuel cell technology.

Manufacturers have been testing and developing different forms of fuel cells for more than a decade but the high cost and insufficiencies of platinum catalysts have been problematic.

‘The catalysts are more stable than platinum catalysts and tolerate carbon monoxide poisoning and methanol crossover,’ said Liming Dai, the Kent Hale Smith Professor of macromolecular science and engineering at Case Western Reserve and one of the report’s authors.

In their initial tests, a cathode coated with one form of catalyst—graphene nanoparticles edged with iodine—proved more efficient in the oxygen reduction reaction, generating 33 per cent more current than a commercial cathode coated with platinum generated.

Like a battery, a fuel cell converts chemical energy into electrical energy. It works by removing an electron from a fuel, usually hydrogen or methanol mixed with water, at the cell’s anode, creating a current. Hydrogen ions produced then pass through a membrane to the cathode. Here, oxygen molecules from the air are split and reduced by the addition of electrons and combined with the hydrogen ions to form water and heat.

A better, cheaper catalyst than scarce and costly platinum is required if hydrogen fuel cells and direct methanol fuel cells are to become realistic alternatives to fossil fuels, the authors said in a statement.

The technology to make alternative catalysts builds on a simple and cheap industrial process several of the researchers developed to make graphene sheets.

Inside a ball miller the researchers broke graphite down into single-layer graphene nanoparticles. While the canister turned, they injected chlorine, bromine or iodine gas to produce different catalysts.

In each case, gas molecules replaced carbon atoms along the zigzag edges of nanoplatelets created by milling. Not only were the edges then favourable to binding with oxygen molecules, but the bond strength between the two oxygen atoms weakened. The weaker the oxygen bonds became, the more efficiently the oxygen was reduced and converted to water at the cathode.

In testing, a cathode coated with iodine-edged nanoplatelets performed best. A cathode coated with bromine-edged nanoparticles generated seven per cent less current than the commercial cathode coated with platinum, the chlorine-edged nanoplatelets 40 per cent less.

In a test of durability, electrodes coated with the nanoplatelets maintained 85.6 per cent to 87.4 per cent of their initial current after 10,000 cycles while the platinum electrodes maintained 62.5 per cent.

Carbon monoxide was added to replicate the poisoning that many scientists blame for the poor performance of platinum at the cathode. The performance of the graphene-based catalysts was unaffected.

When methanol was added to replicate methanol crossover from the anode to cathode in direct methanol fuel cells, the current density of the platinum catalyst dropped sharply. Again, the graphene-based catalysts were unaffected.

‘This initial research proves such catalysts work better than platinum,’ said research leader Jong-Beom Baek, Ulsan National Institute of Science and Technology, South Korea. ‘We are working now to optimize the materials.’