New highly stable fuel-cell catalyst gets strength from its nano core

November 10, 2010
New highly stable fuel-cell catalyst gets strength from its nano core
This high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image shows a bright shell on a relatively darker nanoparticle, signifying the formation of a core/shell structure — a platinum monolayer shell on a palladium nanoparticle core.

( -- Stop-and-go driving can wear on your nerves, but it really does a number on the precious platinum that drives reactions in automotive fuel cells. Before large fleets of fuel-cell-powered vehicles can hit the road, scientists will have to find a way to protect the platinum, the most expensive component of fuel-cell technology, and to reduce the amount needed to make catalytically active electrodes.

Now, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have developed a new electrocatalyst that uses a single layer of platinum and minimizes its wear and tear while maintaining high levels of reactivity during tests that mimic stop-and-go driving. The research - described online in , International Edition, and identified by the journal as a "very important paper" - may greatly enhance the practicality of fuel-cell vehicles and may also be applicable for improving the performance of other metallic catalysts.

The newly designed catalysts are composed of a single layer of platinum over a palladium (or palladium-gold alloy) nanoparticle core. Their structural characterization was performed at Brookhaven's Center for Functional and the .

"Our studies of the structure and activity of this - and comparisons with platinum-carbon catalysts currently in use - illustrate that the palladium core 'protects' the fine layer of platinum surrounding the particles, enabling it to maintain reactivity for a much longer period of time," explained Brookhaven Lab chemist Radoslav Adzic, who leads the research team.

In conventional fuel-cell catalysts, the oxidation and reduction cycling - triggered by changes in voltage that occur during stop-and-go driving - damages the platinum. Over time, the platinum dissolves, causing irreversible damage to the fuel cell.

In the new catalyst, palladium from the core is more reactive than platinum in these oxidation and reduction reactions. Stability tests simulating fuel cell voltage cycling revealed that, after 100,000 potential cycles, a significant amount of palladium had been oxidized, dissolved, and migrated away from the cathode. In the membrane between the cathode and anode, the dissolved palladium ions were reduced by hydrogen diffusing from the anode to form a "band," or dots.

In contrast, platinum was almost unaffected, except for a small contraction of the platinum monolayer. "This contraction of the platinum lattice makes the catalyst more active and the stability of the particles increases," Adzic said.

Reactivity of the platinum monolayer/palladium core catalyst also remained extremely high. It was reduced by merely 37 percent after 100,000 cycles.

Building on earlier work that illustrated how small amounts of gold can enhance catalytic activity, the scientists also developed a form of the platinum monolayer catalyst with a palladium-gold alloy core. The addition of gold further increased the stability of the electrocatalyst, which retained nearly 70 percent of reactivity after 200,000 cycles of testing.

"This indicates the excellent durability of this electrocatalyst, especially when compared with simpler platinum-carbon catalysts, which lose nearly 70 percent of their reactivity after much shorter cycling times. This level of activity and stability indicates that this is a practical catalyst. It exceeds the goal set by DOE for 2010-2015 and it can be used for automotive applications," Adzic said.

He noted that fuel cells made using the new catalyst would require only about 10 grams of platinum per car - and less than 20 grams of . Currently, in catalytic convertors used to treat exhaust gases, 5 to 10 grams of platinum is used. Since fuel-cell-powered cars would emit no exhaust gases, there would be no need for such catalytic converters, and therefore no net increase in the amount of platinum used.

"In addition to developing electrocatalysts for automotive fuel cell applications, these findings indicate the broad applicability of platinum monolayer catalysts and the possibility of extending this concept to catalysts based on other noble metals," Adzic said.

Explore further: Brookhaven Lab chemists receive patents for fuel-cell catalysts

More information: Scientific paper: Core-Protected Platinum Monolayer Shell High-Stability Electrocatalysts for Fuel-Cell Cathodes:

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not rated yet Nov 10, 2010
Couldn't the cc charge up a battery or an ultra capacitor to buffer the device from the bad effects of stop and go driving?
not rated yet Nov 10, 2010
just some simple arithmetic for ya: if you drove your car 4 times a day, 200,000 cycles would last you 136 years. Not bad, i would like to see a gas engine go for 200,000 miles in 1 mile, stop and go, driving.
not rated yet Nov 10, 2010

What I got was that the "cycles" that were mentioned had to do with each stop and go not how many times the vehicle was started up per day.
4 / 5 (1) Nov 10, 2010
I also understood that any (big) change in the production like stopping to traffic lights is one stop and go -instance. Therefore one trip through say a city could mean tens of cycles.

Low cycle life had been big issue to fuel cells, along with big needed amount of precious metals needed. So fuel cells have been expensive and you have had to replace them regularly, obviously an unworkable equation. But this new innovation seems to solve these two issues, making fuel cell cars much more realistic devices. So ICE has again more competition.
not rated yet Nov 11, 2010
in my commute i get into a stop and go situation where i literally stop probably 15-20 times before it smoothes out and then i stop at least 8 times in the parking structure if not just to turn a corner. I would estimate i stop probably 40 - 60 times on the way to work and 25 times on the way home in clear traffic... got to count those traffic lights and stopping to trun a corner.
hemitite - you read the article correctly.

Try counting these stops yourself and see what huge number you come up with... remember EVERY stop counts. so every corner turned.. every traffic light stopping in parking lots - heck parallel parking, and stopping at a gate to use your card or punch in a number. And do you inch closer to the car in front of you at lights, yep me too - got to count those.

with an average of 75 stops/cycles a day gives you 7.3 years on 200,000 stops - till you reach 70% effeciency. That is pretty darn good. --
not rated yet Nov 12, 2010

with an average of 75 stops/cycles a day gives you 7.3 years on 200,000 stops - till you reach 70% effeciency. That is pretty darn good. --

Not if you realize that it means your fuel consumption goes up by 1/3. If an ordinary car went from 30 mpg to 20 mpg, you would take it to the dealer to get it repaired.
not rated yet Nov 14, 2010
The loss of REACTIVITY not efficiency is what they are talking about. That means that the fuel cell will put out 1/3rd less power not 1/3rd less mileage. So you 100kW Fuel cell would put out 70kW at peak output.

Also cycles count as any major change in output.... climbing a hill to coasting down the other side to climbing the next one, or speeding up for passing on the highway to coasting back down to your normal speed.

All of this is sort of silly. They should always make fuel cell/battery hybrids. If you do fuel cell only operation you are planning the system to be less efficient (i.e. no Regen braking, and the average driver only use peak power 1-5% of the time). The vehicle should run on a battery with the fuel cell to recharging/maintaining the battery. This allows the fuel cell to be smaller (cheaper) and to run at a steadier output that is best for fuel cell operation.

It is things like this that make me think that the manufacturers don't really want it to work.

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