Researchers Demonstrate 7 wt% Hydrogen Storage Capacity in Carbon Nanotubes
28 January 2008
Researchers at the Stanford Synchrotron Radiation Laboratory and FYSIKUM at Stockholm University have demonstrated that specific carbon nanotubes can have a hydrogen storage capacity of more than 7 wt% through the formation of reversible C-H bonds—i.e., through chemisorption, rather than physisorption.
The team found that the maximal degree of nanotube hydrogenation depends on the nanotube diameter, and for the diameter values around 2.0 nm, nanotube-hydrogen complexes with close to 100% hydrogenation exist and are stable at room temperature. They reported on their work in a paper in the journal Nano Letters.
Carbon nanotubes have attracted a great deal of interest as possible storage media for hydrogen on-board vehicles, using both physisorption and chemisorption. Recent work has suggested that while physisorption-based storage is limited to about 1 wt% capacity, chemisorption—by saturating the C-C double bonds in the nanotube walls and forming C-H bonds on the single-walled carbon nanotube (SWCN) surfaces—theoretically can provide a hydrogen storage capacity up to 7.7 wt %.
In exploring actual capacities experimentally, the researchers found that most of the C-H bonds formed on the nanotube surface dissociate in the temperature range between 200° C and 300° C and that hydrogen desorption is mainly controlled by reaction kinetics due to large activation barriers for H2 formation from stable H pairs adsorbed on the SWCN.
In the study, the researchers used in situ atomic hydrogen treatment of the nanotube films to separate the study of hydrogenation capacity from the process of molecular hydrogen dissociation. In other words, the current study shows the capacity of the nanotubes, but doesn’t test mechanisms that could be deployed in a vehicle for forming those C-H bonds. They do, however, suggest a possible mechanism.
To be able to use hydrogen chemisorption in SWCN as a hydrogen storage mechanism for technological applications, we need to find a viable way to form H-SWCN complexes. A possible way to do this is to use the spillover process. In this case, the H2 molecules dissociate at the surface of catalyst nanoparticles deposited on the nanotube surface and H radicals spill over from the catalyst to the surface of the nanotubes and form C-H bonds. It has been shown that the presence of Pt nanoparticles inside nanotube material gives a 5-fold increase of the hydrogen uptake. This experimental result directly indicates that a spillover process exists for SWCN covered with a catalyst. We can thereby envisage that by choosing nonbundled nanotubes with appropriate diameter distribution, optimizing the nanostructure of deposited catalyst, and by choosing appropriate hydrogenation temperature to enhance H species diffusion along nanotube surface we can use the spillover process to form the H-SWCN complexes without compromising the hydrogen weight capacity of the material.
Resources
Anton Nikitin, et. al. Hydrogen Storage in Carbon Nanotubes through the Formation of Stable C-H Bonds. Nano Lett., 8 (1), 162 -167, 2008. 10.1021/nl072325k S1530-6984(07)02325-9
New carbon nanotube hydrogen storage results surpass Freedom Car requirements (Nanowerk)
Ok, so you need a tank full of expensive nanotube material to store the expensively produced hydrogen and then, you need to somehow - expensively - heat a portion of the tank to 200-300 degC to desorb the fuel for the expensive fuel cell.
Did I mention this was expensive? IFF you're going to use a gaseous fuel in a mobile applications at all, compressed or adsorbed natural gas (CNG/ANG) makes a whole lot more sense. Also, I suspect renewable methane can be produced from cellulosic (non-woody) feedstock much more easily - read: at much lower cost - than hydrogen from any renewable source.
That said, the nanotube storage technology presented in this article might prove useful in stationary applications that specifically call for hydrogen, e.g. certain petrochemical processes. However, it will have to demonstrate significant advantages over LH2 and CH2G storage.
Posted by: Rafael Seidl | 28 January 2008 at 04:22 AM
I agree Rafael. If we can't first figure out how to make stationary hydrogen power generation make sense why are we wasting time on mobile applications. If we focused as much on inexpensive carbon fiber manufacturing process we could go 100% battery/ultra capacitor electric. Although, we have not mastered electricity production/storage we are much further down the pike than hydrogen.
Posted by: s dogood | 28 January 2008 at 05:45 AM
At those temperatures (200-300C) couldn't you perform pyrolysis on the fly?
Why not use those carbon nanotubes in an advanced battery?
I just don't understand the fascination with hydrogen. If we want to use H as our energy carrier we've had an excellent storage option for years in the form of aluminum.
Posted by: GreenPlease | 28 January 2008 at 05:53 AM
s dogwood & greenplease:
A USA university group has already developed an ultra battery using nanotubes electrodes-anodes giving 10X the performance over current NimH or 700 to 1000 W/Kg. The charge-discharge rates will probably be much higher too.
This battery could be mass produced within 3 or 4 years and make PHEV-100 Km and BEV-500+ KM viable.
This is the type of ESSU required for the future e-car economy. USA should actively support development and mass production facilities (with a few $$ bilion) as soon as possible.
Otherwise, price will be a problem, specially in the first 5+ years or until mass production goes worldwide.
Posted by: Harvey D | 28 January 2008 at 07:17 AM
GM said years ago that they were going to explore the home CHP market for fuel cells and then enter the fuel cell car business. It was one of the many press releases that they put out that made little sense before they lost so much money.
It seemed reasonable that you would go into the home market for small fuel cells that could reform natural gas, produce electricity and heat your home, if you were GE, but not GM.
Posted by: sjc | 28 January 2008 at 08:24 AM
Calculate the weight of hydrogen needed for the 300 mile trip that most manufacturers seem to believe the magic trip length people want. Then calculate the actual weight of nanotubes needed to store that hydrogen at 7%wt. And people complain about the weight of batteries in an EV.
Reminded of the old documentary Connections. The relationship of R&D to useful technologies is not always a straight line. Even if the tech is not viable, it may come up with usefull applications in other fields. Hopefully at least.
Posted by: aym | 28 January 2008 at 10:07 AM
GreenPlease:
"A USA university group has already developed an ultra battery using nanotubes electrodes-anodes giving 10X the performance over current NimH or 700 to 1000 W/Kg. The charge-discharge rates will probably be much higher too.
This battery could be mass produced within 3 or 4 years and make PHEV-100 Km and BEV-500+ KM viable...."
GreenPlease, please learn more about Electrical Engineering. 1000 W/Kg will only help you accelerate faster and run down your battery 10X faster.
What is really needed for range is 10X greater kJ/kg (Specific Energy). That will not come as easy as the power due to chemical energy limits.
HEV will be around for a long time.
Posted by: GdB | 28 January 2008 at 01:02 PM
To be fair, 40kg of nanotubes would hold about 2.8kg of H2. 40kg is practically nothing for a 1,000kg car.
In a car that gets 45mpg(UK) 1.4 kg used in in ICE would get you about 110 miles. And that's with a normal cheapo IC engine.
For most commuting purposes this would be entirely adequate. Longer trips could use gasoline.
Rafael, I take your point about woody biomass providing gas cheaper, but the big question is as always, does it scale.
As I see it, industrially generated hydrogen from high temp sulphur-iodine process or household scale electrolysis from renewable night surplus electricity are the only things that will truly scale far enough.
Most other solutions impose too high a cost on the environment/food provision.
That and PHEV of course, but I still see range issues for a great many people. Its possible that high % mass hydrogen storage systems such as this could allow H2 ICE to work in tandem with PHEV.
The big issues are capital cost and life expectancy of the storage.
Andy
Posted by: Andy | 28 January 2008 at 01:28 PM
Posted by: Reality Czech | 28 January 2008 at 01:50 PM
@ Andy -
biogas is produced from non-woody cellulosic biomass in so-called anaerobic digesters. Woody feedstocks contain lignin, a hard-to-digest natural glue that gums up the works.
You can find digesters on every livestock farm, where they process copious quantities of slurry and manure. Other farms use them to deal with excess grass, corn stover or other vegetable matter. Wet feedstocks are no problem and neither is scaling up. The residue contains all the trace minerals from the feed and can be used as fertilizer, thus making biogas a sustainable technology.
However, raw biogas is a mixture of 50-70% methane, 25-45% CO2 and ~5% H2S, O2 and/or N2 (the exact composition depends on the feedstock and process parameters). Often, the gas produced is vented or flared. Larger farms do sometimes use it to produce electricity on site, feeding any excess into the grid. However, the sulfur in the gas means special engines and engine oils have to be used.
To upgrade the product, the undesired gases can be scrubbed out using an amine wash, which is admittedly expensive but perfectly feasible at industrial scales. Indeed, the problem so far has been that amine scrubbing is rarely economical at the level of individual farms. Transporting wet feedstock or installing small-diameter short-range pipeline systems for raw biogas is expensive and/or fraught with legal issues. Alternative scrubbing technologies are not quite as efficient but they do allow individual farmers to produce odor-free biomethane with few impurities.
http://www.epa.gov/agstar/pdf/conf07/mcdonald.pdf
Biomethane can be fed directly into the existing NG distribution pipelines. Indeed, biogas production is the *only* way to turn cellulosic waste streams into a transportation fuel at industrial scales - albeit an awkward one, given the downstream CNG/ANG requirements. Nevertheless, Austria is now one of a number of countries currently looking to biogas as a way to gradually diversify its on-road transportation sector away from crude oil. However, its NG vehicle fleet is still quite small compared to those in e.g. Italy, Germany and Argentina.
http://www.boku.ac.at/oega/tagung/2003/03_Walla.pdf
http://biopact.com/2007/11/salzburg-ag-opens-biomethane-gas.html
Posted by: Rafael Seidl | 28 January 2008 at 03:27 PM
@ Reality Czech -
a) using fossil NG instead of fuels refined from fossil crude oil emits less CO2 per km driven. It therefore represents an improvement on the status quo. Carbon capture is infeasible in mobile applications, though it could theoretically be applied to H2 production from fossil NG. Relative to the vast amounts of CO2 currently released by burning fossil fuels, flaring associated gas and tropical deforestation, this would make little sense until and unless H2 production levels become very large.
Driving around on domestically produced biogas would be carbon-neutral in the sense that the quantity of CO2 emitted at the tailpipe is absorbed by the next year's feedstock crop. Unlike fossil fuel consumption and forest fires, this is a closed CO2 cycle with a short timescale. The impact on the climate is therefore minimal because the Northern and Southern hemispheres do not exchange a lot of gas and, there is less solar heat to be trapped in winter. That means permanent CO2 sequestration is essentially a moot point for sustainable biofuels like biomethane.
Since feedstock production for biomethane doesn't involve artificial fertilizer inputs, the CO2 emission overheads are in planting, harvesting, transportation to the digester on the premises and gas, compression in the upgrade step. In principle, a fraction of the gas produced could be used to avoid fossil fuel inputs but afaik, agricultural machinery still runs on diesel rather than CNG/ANG.
b) Available fossil NG supplies do differ from country to country. For historical reasons, Austria happens to be a distribution hub for Russian gas whereas the neighboring Czech Republic has no major feed pipeline. In due course, energy policy will shift from national governments to the EU, which will insist on cross-border backbone networks and increased market liquidity (even if producers continue to insist on long-term contracts covering price and quantity).
In addition, there are concrete plans for a major new NG pipeline (Nabucco) between Northern Iraq and Austria (acting as the gateway) by way of Turkey. Down the road, this could be extended to Iran and/or Qatar, two countries with vast NG reserves. The plans are still on hold because no-one knows how long the problem of Kurdish terrorism in Turkey, the civil war in the rest of Iraq and the nuclear-related tensions with Iran will take to resolve themselves.
Italy, Germany, the Netherlands, the UK and Norway all have significant domestic gas reserves, though all are having to plan ahead as yields from many existing fields are already declining. Some are building LNG terminals to gain access to remote sources, while others are investing in GTL plants.
The upshot of all this is that CNG/ANG vehicles don't make sense everywhere, but they clearly do in some countries. If European farms were to produce large amounts of pipeline-grade biomethane, the picture would change considerably for e.g. France, Spain, Poland and Romania.
Posted by: Rafael Seidl | 28 January 2008 at 04:14 PM
Not presuming to speak for anyone else, but Rafael says:
This is only true if the NG use is not simply taken from somewhere else and the oil shifted to the former users of NG. Given the depleting supplies of NG in the N. American markets, the potential for improvement is questionable.If we're going to convert transportation to a new energy medium, the one to use is electricity.
False premise; tests of e.g. switchgrass production has required nitrate fertilization to achieve maximum yields. This means nitrous oxide emissions.Posted by: Engineer-Poet | 28 January 2008 at 07:55 PM
The supply of natural gas in North America will have to be addressed soon. The U.S. still produces more than 80% of the natural gas that we use, but the same can not be said of oil.
"The combustion of natural gas emits almost 30 percent less carbon dioxide than oil, and just under 45 percent less carbon dioxide than coal."
http://naturalgas.org/environment/naturalgas.asp
So, natural gas emits less CO2 than oil or coal and if we synthesize methane using biomass, it will be emitting CO2 that the plants take in during growth.
Posted by: sjc | 28 January 2008 at 08:22 PM
Engineer-Poet:
I believe the NG currently flared of in countries like Quatar, Kuwait and the like, plus the flares on a lot of offshore platforms can be used without have to supply those flare fires with (refined) oil... Some of these flare-offs could easily power all the transportation needs of a country the size of austria. The primary problem is transportation of the NG - which simply was not economically feasible for the last century. The price for crude would need to rise a bit (150-200 USD/gal) to make NG/PG transportation by ship or pipeline feasible. The price of NG / PG is tied to the price of crude...
In addition to the pipeline Rafael was mentioning, Russia is also scrambling to built a sencond (southern) pipeline to western europe, with less issues.
You also missed the point of where biogas reactors (fermenters) currently get their feedstock. Unlike Ethanol production, no vegetables are actually planted to feed exclusive Bio-Gas fermenters. Instead, all the otherwise unused biomaterial is being use, thus you cann't attribute any fertilizer use directly to the production of Biogas.
Of course, the efficiency of using Methane in mobile applications still lacks any decent levels, being limited by the carnot process limits in internal combustion engines (35-40% peak efficiency in diesel-like ICEs). Instead of trying to make hydrogen fuel cell, it would make much more sense to develop current industrial production SOFCs for direct methane conversion (you can actually go and buy such a SOFC today in europe to heat & power your house) for mobile appliactions (SOFC are too brittle currently, afaik).
H2 simply doesn't scale, has a stream of low-efficiency conversions attached to it and is way to costly.
The only feasible business plan having to do with H2 fuel cells is either being in space technology (NASA's big pockets), or to extract other government (= public) money. I'ven't seen a single company with a solid business plan revolving around H2 fuel cell, able to show that private venture capital is not entirely wasted on them....
Posted by: realarms | 28 January 2008 at 11:53 PM
I have seen articles on this site showing 5kw SOFC APUs made by Delphi and others for mobile applications. I realize that these are only 5kw and are APUs and not mobile electricity sources for cars, but it sounds like it could be something in our futures.
Posted by: sjc | 29 January 2008 at 09:53 AM
100 kg of tubes would ho;d 7 kg h2 and thus about 140 kwh. A battery that big would weight ... more then a ton and wou;d require at least 3 days to recharge on 110 outlet...
Posted by: wintermane | 30 January 2008 at 12:02 AM