## Georgia Tech ultra-thin hollow nanocages could significantly reduce platinum use in fuel cell electrodes

##### 24 July 2015

A team led by researchers at Georgia Tech has developed a new fabrication technique to produce platinum-based hollow nanocages with ultra-thin walls that could significantly reduce the amount of the costly metal needed to provide catalytic activity.

Use of these nanocage structures in fuel cell electrodes could increase the utilization efficiency of the platinum electrocatalyst by a factor of as much as seven, potentially changing the economic viability of the fuel cells. The work also involved researchers at the University of Wisconsin-Madison; Oak Ridge National Laboratory; Arizona State University; and Xiamen University in China. The process is described in a paper in the journal Science.

 A) Mass and (B) specific activities of the catalysts at 0.9 VRHE. (C) Mass activities (at 0.9 VRHE) and (D) specific ECSAs of the catalysts before and after accelerated durability test. The color scheme applies to all panels. Source: Zhang et al. Lei Zhang et al. Science 2015; 349:412-416 Click to enlarge.

In catalytic applications, only the surface layers of platinum contribute to the chemical reaction, leading researchers to develop new structures designed to maximize the amount of platinum exposed to reactants.

One strategy to increase the utilization efficiency (UE) of platinum group metals (PGMs) is to increase the proportion of atoms exposed on the surface (the dispersion) by reducing particle size. For example, the UE of platinum (Pt) atoms can be increased from 9.5 to 26% by reducing the edge length of a Pt cube from 11.7 to 3.9 nm. Despite the extensive use of this strategy, it has been difficult to optimize the specific activity of such small nanocrystals (NCs) by engineering their surface structure through facet-controlled synthesis. Such NCs also tend to sinter (form larger particles), detach from the support, or both during operation.

An alternative strategy is to use nanoframes—open nanostructures comprising multiple ridges as thin as a few nanometers. Each ridge of a nanoframe can be considered as a linear aggregate of NCs. Essentially, all the PGMs can be prepared as nanoframes by using methods that involve the selective removal of a sacrificial component: for example, the more reactive metal in alloyed NCs or the NC serving as a template for the site-selected deposition of the PGM. … this method still faces challenges in selecting the exposed crystal facet with which to control their catalytic activity and selectivity.

A different strategy for increasing the UE of a PGM is to assemble the metal atoms into nanosheets. For such a system consisting of four atomic layers, the UE could in principle reach 50%, but the use of PGM nanosheets as catalysts encounters several drawbacks: (i) the top and bottom surfaces of a sheet must be capped by ligands; (ii) the metal atoms can only assume a hexagonal lattice, corresponding to one type of facet only; and (iii) it is challenging to deposit and expose individual nanosheets on a catalytic support. An alternative to this strategy is to deposit the PGM conformally as sub-nanometer-thick shells of only a few atomic layers on the surfaces of NC templates made of another metal.

—Zhang et al.

The Georgia Tech-led team took that last approach. Their technique uses a solution-based method to grow platinum layers on palladium nanocrystal templates. The palladium is then etched away to leave behind nanocages approximately 20 nanometers in diameter, with between three and six atom-thin layers of platinum. When conducted appropriately, the researchers said, the facets presented on the surface of the template can be well preserved during the Pt coating and Pd etching processes so as to engineer the activity and/or selectivity of the catalyst.

In the work described in the paper, the researchers demonstrated the concept by coating the surfaces of Pd nanoscale cubes and octahedra with four atomic layers of Pt, followed by selective removal of the Pd templates. (The shape, cubic or octahedral, controls the surface structure, thus engineering the catalytic activity.)

We can get the catalytic activity we need by using only a small fraction of the platinum that had been required before. We have made hollow nanocages of platinum with walls as thin as a few atomic layers because we don’t want to waste any material in the bulk that does not contribute to the catalytic activity.

We can control the process so well that we have layer-by-layer deposition, creating one layer, two layers or three layers of platinum. We can also control the arrangement of atoms on the surface so their catalytic activity can be engineered to fit different types of reactions.

—Younan Xia, a professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University

Hollow platinum structures have been made before, but not with walls this thin, Xia said. Earlier work produced shells with wall thicknesses of approximately five nanometers. The new process can produce shell walls less than one nanometer thick. With both the inner layer and outer layer of the porous nanocages contributing to the catalytic activity, the new structures can use up to two-thirds of the platinum atoms in an ultra-thin three-layer shell. Some palladium remains mixed with the platinum in the structures.

Durability testing showed that Pt octahedral nanocages delivered the best performance, with the ORR mass activity only reduced by 36% after 10,000 cycles, still showing 3.4-fold enhancement relative to the pristine Pt/C. The ECSAs of the cubic and octahedral nanocages only dropped by 13 and 6% after 5000 cycles and by 32 and 23% after 10,000 cycles, respectively.

Contributing to the experimental work done at Georgia Tech, researchers at Arizona State University and Oak Ridge National Laboratory used their specialized microscopy facilities to map the nanocage structures. Researchers at the University of Wisconsin-Madison modeled the system to help understand etching of palladium from the core while preserving the platinum shell.

Researchers have explored alternatives to platinum, but none of the alternatives so far has provided the equivalent amount of catalytic activity in such a small mass, Xia noted.

Other authors in the paper include Professor Manos Mavrikakis and researchers Luke Roling and Jeffrey Herron from the University of Wisconsin-Madison, Miaofang Chi from Oak Ridge National Laboratory, Professor Jingyue Liu from Arizona State University, Professor Zhaoxiong Xie from Xiamen University, and Lei Zhang, Xue Wang, Sang-Il Choi, Madeleine Vara and Jinho Park, from Georgia Tech.

Resources

• Lei Zhang, Luke T. Roling, Xue Wang, Madeline Vara, Miaofang Chi, Jingyue Liu, Sang-Il Choi, Jinho Park, Jeffrey A. Herron, Zhaoxiong Xie, Manos Mavrikakis, and Younan Xia (2015) “Platinum-based nanocages with subnanometer-thick walls and well-defined, controllable facets” Science 349 (6246), 412-416 doi: 10.1126/science.aab0801

Combusting fuel is starting to seem really old fashioned, as well as inefficient and polluting.....

Yes, more and more people (and posters) are starting to think so. Unfortunately, the majority still believe that burning cheaper highly polluting fossil and bio fuels are justified and are not ready to pay the price for cleaner environment.

Dave, until they fix the problems with below zero cold and make this stuff cheap enough its not going to happen. I would say there is more than a decade of patience before people jump in to get an electric or even hydrogen anything.

I am VERY interested in a BOLT for my daily commute but not even sure it will live up to my expectations when the Chicago Hawk comes visiting and the wind chill is 20 to 80 below.

I was looking at the tiny Nisan LEAF but with such a limited range that gets real bad when its cold I dropped that idea.
I thought for sure Lit Motors C-1 would make it but its WAY WAY overdue and the price.. MAN 24k? I can get a cheap car for 1/2 that and use the other 1/2 to buy gas for the life of the car.

Elio motors has an interesting ride (gas engine) but I am not sure its safe enough on a major highway in this town.

So What is there really as far as affordable that can go 80 to 100 miles a DAY winter spring summer or fall?
So far, and I have been looking NOTHING reasonable.

A TESLA S-40D would do the job but it is no longer available. You may have to wait another 2 to 3 years for the new TESLA small SUV

D:
This is about fuel cells, which are around 50% efficient in turning the hydrogen into electricity.
That means there is quite a bit of waste heat, which can be used to both keep the battery, and fuel cell cars still need one around as large as for a hybrid, at optimum temperature and to keep the occupants warm.

Range is reduced in very cold weather, just as it is in your petrol car, but since fuel cell cars have a longer range than even the Tesla S the problem is not a serious one.

They will work fine in the cold.

I should have said that: 'they work fine in the cold' not that 'they will work fine in the cold' as in real life we already know that they do, with the Toyota Highlander FCEV and the Hyundai i35 FCEV both doing the job with no problems.

@ Davemart
I can't agree with your efficiency statement. That cannot even be reached with steam reforming of NG which would be the cheapest and most efficient but still below overall 40% and still contributing to pollution.
Using REs would involve electrolysis losses, H2 compression losses, H2 pump losses, storage losses, battery losses and the rel. low effieciency of FCs. The total adds up to overall losses > 70%. In the end effect, the remaining efficiency is somewhere in the area of a run-of-the-mill gas hog. A BEV achieves more than 90%.

@yoatman:

The efficiencies I was referring to are in the car, hydrogen to electricity and the drive train, which is where the cold reduces the range of BEVs below acceptable levels.

Your comments on the very different subject of overall efficiency are also without foundation.

Here is ORNL on well to wheels efficiency and GHG emissions:
http://www.greencarcongress.com/2014/09/20140924-onrl.html

Note my comments that ORNL screwed up the efficiency of hydrogen fuel cell vehicles by taking out reforming losses twice, but that aside ICE is far less efficient than hydrogen and fuel cells, which is one of the reasons aside from their being producible from renewables that Toyota, the maker of about the most efficient ICE engines in mass production, is developing FC cars.

They know a bit about the efficiencies of both ICE and fuel cells.

So What is there really as far as affordable that can go 80 to 100 miles a DAY winter spring summer or fall?
So far, and I have been looking NOTHING reasonable.

There are no FC vehicles available in Chicago.
If there were they would certainly not meet your affordability requirements.
Even if they were available in your area and were affordable there is no refueling infrastructure in your locale. These three issues are not expected to change in the short to midterm time range (0-5 years.)

Tesla has 10 models of BEVs that meet all but your affordability requirements. An extended range BMW i3 also falls into the same category.

In the next 1-3 years Tesla, Nissan, and Chevrolet will likely have options for you if you consider a median priced new car affordable.

The 2016 Volt should meet your requirements if you consider slightly below a median priced new car affordable.

If that is beyond your pain point then a used Chevy Volt (12-20K$) might be a viable option for you. I have not read the full paper. Is there a reason for using palladium nanocrystal templates and not for example silica? Made in China, TESLA's S-40, S-60 and S-85 almost exact copies, at less than half price, may be the low cost solution that many posters have been waiting for. Unfortunately, those extended range high performance BEVs will not be imported in USA but will probably be exported to neighboring countries by 2017 or so. Real worldwide free trade may be the only way to get lower cost EV batteries and extended range BEVs. The real Tesla Model 3, with 200 miles of range at$35,000 before incentives, will probably be the most successful new car introduction of modern times. The Model X's entire first year production run is already sold out with \$5,000 deposits, most of them to current Model S drivers.

I've been in the car business a long time and I can't remember ever hearing of a car that sold out 30,000 units without a single test drive.

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