Northwestern-led team finds slightly imperfect graphene can serve as a highly selective proton separation membrane
18 March 2015
Researchers from Northwestern University, together with collaborators from Oak Ridge National Laboratory, the University of Virginia, the University of Minnesota, Pennsylvania State University and the University of Puerto Rico, have discovered that protons can transfer easily through graphene—conventionally thought to be unfit for proton transfer absent nanoscale holes or dopants—through rare, naturally occurring atomic defects.
In an open access paper published in the journal Nature Communications, the researchers reported that a slightly imperfect graphene membrane’s speed and selectivity are much better than that of conventional proton separation membranes, offering engineers a new and simpler mechanism for fuel cell design.
We found if you just dial the graphene back a little on perfection, you will get the membrane you want. Everyone always strives to make really pristine graphene, but our data show if you want to get protons through, you need less perfect graphene.
—Professor Franz M. Geiger, who led the research
A major challenge in fuel cell technology is efficiently separating protons from hydrogen. In the atomic world of an aqueous solution, protons are pretty big, and scientists have not believed that protons can be driven through a single layer of chemically perfect graphene at room temperature. (Graphene is a form of elemental carbon composed of a single flat sheet of carbon atoms arranged in a repeating hexagonal, or honeycomb, lattice.)
When Geiger and his colleagues studied graphene exposed to water, they found that protons were indeed moving through the graphene. Using advanced laser techniques, imaging methods and computer simulations, they discovered that naturally occurring defects in the graphene—where a carbon atom is missing—trigger a chemical merry-go-round where protons from water on one side of the membrane are shuttled to the other side in a few seconds. Their advanced computer simulations showed this occurs via a classic “bucket-line” mechanism first proposed in 1806.
The thinness of the atom-thick graphene makes it a quick trip for the protons, Geiger said. With conventional membranes, which are hundreds of nanometers thick, proton selection takes minutes—much too long to be practical.
Removing only a few carbon atoms results in others being highly reactive, which starts the proton shuttling process. Only protons go through the tiny holes, making the membrane very selective. (Conventional membranes are not very selective.)
| Proton transfer through the hydroxyl-terminate graphene quad-vacancy, as obtained from an unbiased ReaxFF reactive force field molecular dynamics simulation at T=300K. Hydrogen atoms specifically relevant to the proton transfer event are colored blue. Credit: Murali Raju, Penn State. |
From the SHG signal jump rates and the time required for 2D proton diffusion, we estimate that the presence of as few as a handful of atomic defects in a 1 μm2 area sample of single-layer graphene is sufficient to allow for the apparent unimpeded protonation and deprotonation of the interfacial silanol groups within 10 s. Yet, we caution that given the limited accuracy with which the defect density can be determined in large (mm)-scale graphene, aqueous protons may transfer across single-layer graphene not only along the path discussed here but also along others as well. The identification of low barriers specifically for water-assisted transfer of protons through OH-terminated atomic defects in graphene, and high barriers for oxygen-terminated defects could be an important step towards the preparation of zero-crossover proton-selective membranes.
—Achtyl et al.
Our results will not make a fuel cell tomorrow, but it provides a mechanism for engineers to design a proton separation membrane that is far less complicated than what people had thought before. All you need is slightly imperfect single-layer graphene.
—Franz Geiger
Resources
Jennifer L. Achtyl, Raymond R. Unocic, Lijun Xu, Yu Cai, Muralikrishna Raju, Weiwei Zhang, Robert L. Sacci, Ivan V. Vlassiouk, Pasquale F. Fulvio, Panchapakesan Ganesh, David J. Wesolowski, Sheng Dai, Adri C. T. van Duin, Matthew Neurock & Franz M. Geiger (2015) “Aqueous proton transfer across single-layer graphene” Nature Communications 6, Article number: 6539 doi: 10.1038/ncomms7539doi
Of course none of this will make the slightest difference to those who think that they have infallibly calculated that the only way to go is batteries.
When opinions do not change as the evidence develops and changes it is a sure sign that the position was not based on logic in the first place.
Posted by: Davemart | 18 March 2015 at 02:10 PM
To be more accurate, improvements in PEMs don't affect appraisals based on energy balances or overall economics.
This bears watching, though. The recent announcements of non-PM OER catalysts suggest that it may be possible to squeeze much of the capital cost out of the system. If you have low enough capital cost, you can afford to keep things on standby and operate them at low duty cycles. That is required (but not sufficient) for an RE-powered hydrogen economy to work.
Another requirement is really cheap energy to feed into storage. So long as the RE inputs require FITs to be economically viable, this is a guaranteed failure. Some uses like hydro-reforming biomass to make limited amounts of high-value materials are going to work out at much higher input costs than bulk energy.
I would not be surprised if gasified biomass into a steam-reformer is a more viable source of hydrogen for FCEVs than RE into electrolysis. Figure $50/ton for lignocellulose at 45% carbon, balance breaking down to H2O; that's $111/ton C. 1 atom carbon can liberate 4 atoms H from water; assuming 75% efficiency we'd get 3, or 1/4 ton H2 out of 1 ton C. Feedstock cost for the H2 would be about 45¢/kg.
1 billion tons biomass would contain 450 million tons C and produce 150 billion kg H2. At 60 vehicle-miles/kg you'd get 9 trillion vehicle-miles. I believe the USA logs about 3 trillion vehicle-miles per year in LDVs, so that pencils out.
Posted by: Engineer-Poet | 19 March 2015 at 01:28 PM
Hi EP:
Yeah, I pretty much loath some of the schemes such as the German idea of wind and solar surpluses into hydrogen, in places where it ain't very sunny.
Things look a bit different perhaps though if you combine a lot of partial solutions instead of looking for the big one which solves everything.
So maybe you have some PHEV FCEVs which don't use so much hydrogen, and charge during the day on solar installed at workplaces in parking lots, then use some biomass for the hydrogen.
Then in many regions you might have some overbuild of solar and wind, which gets electrolysed.
Plus of course even at a further cost in energy once you have the hydrogen, you can them maybe convert it to methanol etc and ship it anywhere in the world to make up some of the difference in places like Germany where renewable resources aren't too great.
Carbon capture was a technology I could not see much evidence for working, but technologies like zeolitic adsorbtion may be changing that to a realistic option:
http://www.sciencedaily.com/releases/2015/03/150303183327.htm
I'm purposely leaving out biggies like direct solar to hydrogen, as even without that and with relatively limited use of both of our preferred option of nuclear, it is starting to look to me as though we may be able to muddle through in a very messy way, with loads of partial and imperfect solutions.
My bottom line is that we have to go with the flow, and they just aren't building the numbers of nuclear reactors we would both like, which might largely avoid the need for hydrogen.
My view is that it is pretty darn difficult to use a very high percentage of renewables instead of nuclear with hydrogen, and impossible almost everywhere without.
Posted by: Davemart | 19 March 2015 at 03:03 PM
I should add that solar pv to hydrogen is a different ball game in most of the US to in Germany, and can help mediate the difference in sunshine in winter to summer, with overbuild not at stupid levels.
Posted by: Davemart | 19 March 2015 at 03:10 PM
The problem you get in discussion of such matters is that the Greens will mention 10 different things, but each one is capable of handling only 1-3% of the required work (or is commercially defunct, like wave power). In practice, you need one thing that serves at least 50% of need. For home heating, that used to be coal (now natural gas).
Does it make either economic or engineering sense to put a PHEV-class battery in an FCEV? Both batteries and H2 tanks are bulky.
You still have to find a place to put it. The recent legal issues arising from deep-well injection of used frack water doesn't make me sanguine about this.
Posted by: Engineer-Poet | 20 March 2015 at 05:18 AM
Using CO2 as refrigerant for Heat pumps in future 2B vehicle HVACs and in as many homes/offices to provide Hot water, heating and air conditioning could increase total energy efficiency while reducing GHG released in the atmosphere.
Secondly, it could partially solve the captured CO2 storage problem.
Posted by: HarveyD | 21 March 2015 at 12:06 PM