Fuel cells

US scientists have demonstrated a significant boost to fuel cells that could also cut costs. By coating the cathode with a thin layer of platinum instead of using solid metal, efficiency is raised by ten percent and the use of expensive platinum can be reduced.

Platinum monolayer on Rh (111) 50 x 50 nm

Platinum monolayer on Rh (111) 50 x 50 nm

Fuel cells are being touted as a greener electricity source than power stations and other centralized systems. By providing electrical energy where it is needed they avoid the inefficient step of transporting the power from the source to the user. In addition, because they do not themselves burn fossil fuels, there are no pollutants at the site of use, whether that is a vehicle or domestic fuel cell, other than water and heat. In the case of future domestic fuel cells the “waste” heat would be recycled into the system to augment the heating function of a fuel cell.

“Anything that would reduce the cost of materials in low temperature fuel cells – a major fraction of that is the cost of noble metals, such as Pt – would certainly make the technology more and more competitive and thus bring fuel cells closer to being implemented, competitively, in our everyday lives,” Mavrikakis told Reactive Reports.

Manos  Mavrikakis
Manos Mavrikakis

However, fuel cells, by definition are not 100% efficient and researchers are continually looking for ways to reduce the energy losses that occur. “Inefficiency of the oxygen reduction reaction in a fuel cell, and at large current densities, is responsible for about 60% of the overall efficiency loss,” explains Manos Mavrikakis of the University of Wisconsin-Madison.

Mavrikakis and Ye Xu of the University of Wisconsin-Madison and their colleagues Junliang Zhang, Miomir Vukmirovic, and Radoslav Adzic at Brookhaven National Laboratory in Upton, New York, are investigating ways to reduce the amount of expensive platinum catalyst that is needed in fuel cells. They have experimented with decreased platinum content cathodes in which electrocatalytic activity is observed in the oxygen reduction reaction (ORR) at platinum monolayers supported on less pricey metals such as gold, palladium, ruthenium, and iridium.

Ye Xu
Ye Xu

The researchers found that their bimetallic cathodes worked even more effectively in the ORR than platinum alone, despite only containing a fractional amount of platinum in the monolayer. Adzic and Mavrikakis suggest that the efficiency boost coupled with the lower cost of a bimetallic compared with a pure platinum cathode points to a commercially viable alternative to current ORR fuel cell design. They add that their work also indicates how the electrocatalytic activity of fuel cells might be fine-tuned through an insightful combination of well-controlled experiments and first-principles calculations.

In terms of making hydrogen, the majority of today’s hydrogen comes from reforming of fossil fuels. However, new technologies are emerging that could potentially allow its competitive production from other sources, such as biomass reforming or water electrolysis using solar energy [see item #4 in this month’s issue].

Angew Chem Int Ed Engl, 2005, 44, 2-5; http://dx.doi.org/10.1002/anie.200462335