While the most efficient electro-catalyst in electric car fuel cells for constant-speed motorway driving is platinum, for stop-start motoring such as around towns and cities, platinum dissolves, which reduces its efficiency.
For years this has impeded platinum’s application to fuel cells for vehicles but research in the US could provide fresh hope and solve the problem. Scientists at the Department of Energy’s Brookhaven National Laboratory in New York have discovered that by imitating the environment of a fuel cell in lab conditions, platinum electrocatalysts remain intact when gold clusters are added to them.
After being put through five days of accelerated tests that simulated stop-start driving conditions, researchers found that as much as 45 per cent of platinum cathode electrocatalysts were lost after 30,000 oxidation-reduction cycles.
Under the same test conditions but with the addition of gold clusters, the loss was ‘almost nothing to notice, or negligible’, according to Radoslav Adzic, one of the four Brookhaven researchers behind the project.
‘The gold clusters protected the platinum from being oxidised,’ he said. ‘Gold exchanges electronic properties with platinum and it covers the sides that are exposed to dissolution.’
A hydrogen-oxygen fuel cell converts hydrogen and oxygen into water and, as part of the process, produces electricity. Platinum electrocatalysts speed up oxidation and reduction reactions. When hydrogen is oxidised, it releases electrons and forms hydrogen ions. The released electrons supply current to the electric motor.
Meanwhile, oxygen is reduced by gaining electrons and when it reacts with hydrogen ions it produces water — the only by-product of hydrogen-oxygen fuel cells.
The researchers developed a unique procedure for placing gold clusters around the electrocatalysts. They replaced a single layer of copper with gold on carbon-supported platinum particles, measuring three to five nm. After being subjected to several sweeps of 1.2V, the gold layer morphed into 3D clusters.
The team used x-ray probes to observe the new catalyst undergoing electrochemical processes and verified that there was less oxidation of platinum. It was also possible to further study the structure of the resulting platinum electrocatalyst with gold clusters, which helped the team understand the effects of the clusters.
While the team is still trying to fully determine what makes this method work, it estimates the resulting particle architecture is about 10 per cent gold and 90 per cent platinum.
One of the main issues with platinum is cost. For years scientists have been studying ways to reduce the amount of platinum being used in catalysts — or, perhaps, replace it with another catalyst.
Using a precious metal like gold might not seem an ideal way to decrease the price of fuel cells but gold is half the price of platinum. And while the new electrode composition will not decrease the price of fuel cells, it will not increase it either.
Adzic said his team took the cost of platinum into consideration when conducting its research. ‘We’ve made a lot of effort to reduce platinum down to its lowest possible level,’ he said. ‘We only used a single monolayer in our experiments but we could use much less for a real fuel cell catalyst.’
This idea will be put to the test at Los Alamos National Laboratory for Hydrogen and Fuel Cell Research in New Mexico, where the Brookhaven team is now testing its vamped-up catalysts with real fuel cells.
For years stop-start motoring has proved a no-go area for the use of platinum electrocatalysts in car fuels cells. US researchers believe they have solved the problem.