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Engineer uses silver in fuel cells

By HENRY GENS | Wednesday, October 2, 2013

 

When it comes to the field of chemical engineering, silver may soon be worth more than platinum.

Suljo Linic, associate professor of chemical engineering at the University of Michigan, presented two major innovations making extensive use of silver in fuel and solar cell technology in the department of chemical and biomolecular engineering’s annual Thiele Lecture Tuesday. 

Linic began by outlining the need to approach problems in the field of chemical engineering by relying on theory and models to guide experimental design, rather than trial-and-error attempts which lead to only incremental improvements. 

Linic said throughout his career he has challenged himself to use this method in his work with chemical transformations.

“When I joined the University of Michigan, I defined a difficult challenge for myself –

and I think that this is a problem for most people who work in this field – which is to try and develop some models of heterogeneous catalysis,” Linic said. “I want to try and use these models to design materials that can perform certain chemical transformations, either with better efficiency than what we have now or some chemical conversion that we couldn’t execute before.”

Linic said he has worked to better understand the mechanism of electrochemical oxidation-reduction reactions in order to design improved fuel cells. He said the current state-of-the-art cells each employ pure platinum to produce an equilibrium potential of roughly 1.1 volts, which is not representative of the true effectiveness of the battery.

“The problem is that in the state-of-the-art proton exchange membrane fuel cells we use today we have to sacrifice a large amount of this potential in order to start moving current from the cell,” Linic said. “And the main reason you have to sacrifice this potential is that this reaction is very slow at high potential and hence you only start generating current after losing about 30 percent of the equilibrium potential.”

Linic said the solution he developed involves using silver, a much less expensive material, as a substitute for pure platinum, and then scaling up to account for the lower efficiency.

“We have been looking at a silver-based compound and we have known for a long time that silver can form this type of reaction at about 10 to 15 times lower rates than platinum. But what we like about silver is that it’s very cheap, about 50 times cheaper than platinum, and when forming silver nanoparticles it allows for a high density of active sites in the material.”

Linic said up until recently silver would have been useless in the acidic environment of fuel cells, but advances in new membrane technologies in the last decade allow for silver to be used in basic fuel cells without being prohibitively expensive. 

After extensive testing of a variety of silver-based candidate compounds, Linic found that a silver-cobalt alloy achieved the desired level of efficiency, he said. His team set a minimum threshold efficiency of 10 percent of pure platinum’s potential for a viable candidate.

“The silver-cobalt alloy actually performed significantly better than pure silver metal – it has a five to six time higher rate than pure monometallic silver, and it’s about half the performance of platinum, which is really good,” Linic said. “If you have a really cheap material you can get by on about 10 percent; this is a 50 percent performance, and it is as stable as platinum in phase.”

Linic next discussed using plasmonic nanostructures to enhance solar cell efficiency. Essentially, Linic explained, plasmonic nanostructures are nanoparticles of metal which are characterized by very small interaction with resonant photon excitation of localized surface plasmon resonance.

“The interesting thing is that the local surface plasmon resonance takes place at UV-Vis near-infrared light, which means it can be used very nicely for solar applications,” Linic said.

After noticing the potential for this application, Linic employed his expertise in silver chemistry to create improved solar cells using the metal as a plasmonic nanostructure

“We demonstrated that by combining silver nanoparticles with semiconductors we can enhance the photo-catalytic activity of the semiconductor,” Linic said.

Linic said these discoveries underscore the importance of understanding the theory in order to engineer significant technological advancements.