UK, Saudi team shows hydrocarbon wax is a viable, safe medium for on-board hydrogen storage
20 October 2016
Researchers at the universities of Oxford, Cambridge and Cardiff in the UK, and the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia have shown that benign, readily-available heavy alkane hydrocarbon wax is capable of rapidly releasing large amounts of hydrogen—sufficient to meet the 7 wt% target set by the US DOE—through microwave-assisted catalytic decomposition.
This discovery, reported in an open-access paper in Scientific Reports, offers a new material and system for safe and efficient hydrogen storage and could facilitate its application in a hydrogen fuel cell vehicle. Hydrocarbon wax is the major product of the low temperature Fischer-Tropsch synthesis process from syngas and is currently thermally “cracked” to produce various fuels.
The F-T wax can be manufactured using renewable energy and any carbon-containing resources including biomass, CO2, natural gas, coal—and the resulting carbon residue from the hydrogen-depleted wax.
Despite strenuous efforts over the past decades covering a vast range of hydrogen storage materials, no single material has met simultaneously the critical requirements for a viable hydrogen storage and hydrogen releasing material suitable for use in HFCVs and other fuel cell applications.
… Chemical, complex and metallic hydrides all present significant difficulties of pyrophoricity, accidental hydrogen release (from accidental reaction with atmospheric moisture) and heat management problems, together with troublesome changes in particle morphology in the hydrogen charging/discharging processes. Similarly, rechargeable organic liquids—used to store hydrogen in a liquid carrier form—must be handled with great care as several react violently with strong oxidants, have fire and explosion hazards and, in certain cases, carry toxicity concerns from occupational exposure. Similar safety issues and concerns surround the potential use of ammonia as an on-board hydrogen storage material.
Important efforts have been made previously to liberate pure hydrogen from the catalytic decomposition of the lightest alkane, methane. Against this backdrop, we sought to investigate heavy alkane hydrocarbon waxes as cheap, safe, readily producible and widely accessible hydrogen storage materials. We reasoned that such materials—if suitably activated to rapidly release hydrogen, or hydrogen-rich mixtures—could exhibit many desirable features and yield hydrogen gravimetric densities approaching a theoretical value of ca. 14 wt%.
—Gonzalez-Cortes et al.
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| (a) Gravimetric and volumetric densities of various hydrogen storage materials and options. Inset shows the hydrogen storage density of alkanes as a function of the number of carbon atoms. (b) Dependence of the Gibbs free energy with temperature for the deep dehydrogenation reaction (or hydrogen formation reactions) of various straight-chain alkanes (i.e. CH4; n-C5H12, n-C10H22, n-C15H32 and n-C20H42). Inset shows the dependence of the standard enthalpy and entropy for the hydrogen formation reactions with carbon numbers of various lineal alkanes. Gonzalez-Cortes et al. Click to enlarge. |
Dehydrogenating a hydrocarbon wax so as to release hydrogen effectively and rapidly while minimizing unwanted by-products is a challenge. The research team developed highly selective catalysts that, with the assistance of microwave irradiation, can extract hydrogen from hydrocarbons instantly through a non-oxidative dehydrogenation process.
The researchers selected a representative wax, C26H54, and found that around 7 wt% hydrogen is rapidly produced by microwave radiation-assisted catalysis involving ruthenium nanoparticles on a carbon support (CS), intimately dispersed within paraffin wax (PW).
A thermodynamic analysis showed that the deep dehydrogenation reactions necessary for efficient hydrogen formation reactions become more favorable with increasing reaction temperature and with increasing number of carbon atoms in the lineal alkane.
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| Schematic representation of one scenario for the decarbonization of a transportation fuel economy. Gonzalez-Cortes et al. Click to enlarge. |
Clearly, considerable engineering work is needed to adapt this laboratory discovery to large-scale hydrogen storage applications. However, we believe that the storage of hydrogen in, and rapid evolution from, paraffin wax could usher in a new and attractive path towards a decarbonized, hydrogen economy.
—Gonzalez-Cortes et al.
Resources
S. Gonzalez-Cortes, D. R. Slocombe, T. Xiao, A. Aldawsari, B. Yao, V. L. Kuznetsov, E. Liberti, A. I. Kirkland, M. S. Alkinani, H. A. Al-Megren, J. M. Thomas & P. P. Edwards (2016) “Wax: A benign hydrogen-storage material that rapidly releases H2-rich gases through microwave-assisted catalytic decomposition” Scientific Reports 6, Article number: 35315 doi: 10.1038/srep35315
Looks like Saudi Arabia just found a way to market their ultra-heavy crude as a "green" energy source: crack it for hydrogen.
It might even work, too, as long as you buried the carbon.
Posted by: Engineer-Poet | 20 October 2016 at 06:04 PM
Presumably you would get the wax into the car by heating it slightly and pumping it as a gloop?
For comparison, the CF tank in the Mirai is around 5.7% hydrogen by weight:
http://www.greencarcongress.com/2014/11/20141118-mirai.html
So this is a worthwhile increase in energy density, but not of a different order.
More importantly though the hassle and expense of making a complex CF tank would be avoided, and the packaging would be a lot easier, as CF tanks have to be cylindrical.
At 33kwh/kg and 7% by weight, we are coming out to around 2.3kwh/kg of the total pack weight.
At the ~50% efficiency a PEM fuel cell in a car typically runs at, that is going to work out to over 1000Wh/kg to use, even after adding weight for the microwave etc.
Batteries are no where remotely near that.
More earth abundant catalysts would be nice though.
Posted by: Davemart | 21 October 2016 at 12:29 AM
Well Dave, not only would you have to heat the wax slightly to pump it into the car, if I'm reading this right you'd need a way to get the leftover carbon out. I think a better way might be to "plug" the wax in as a cartridge which could be pulled out when dehydrogenated with the carbon safely sealed inside. Of course such an idea only really works for the larger vehicles fuel cells would be best for.
Posted by: ai_vin | 21 October 2016 at 09:35 AM
Hi Al.
Good point about the residues.
Cartridges would be heavy though, and perhaps the first users would be scooters and such.
Here is the original paper if you are interested:
http://www.nature.com/articles/srep35315
Posted by: Davemart | 21 October 2016 at 10:51 AM
Good points, E-P, Davemart and ai vin,
IMHO, a Plug-in FCV (PFCV) would be best to take advantage of this technology. Imagine a PFCV having about 40-mi of plug-in range, and about 300-mi on H2 range. So, the fuel for the FC will come from wax-filled cartridges that can be purchased and swapped at any gasoline stations. No H2 refill infrastructure needed, and will be much lighter than a long-range BEV and much quicker refills at most gasoline station.
Those without a charging receptacle where they park will still be able to enjoy their PFCV without having to plug-in, though just have to swap out those wax cartridges on weekly basis instead of on monthly basis.
Note that a PFCV would not have to be any more expensive nor any heavier than a FCV, nor having any less internal space than a FCV, due to the reduction in size of the FC stack to 1/2 when bigger and more powerful Li-ion battery pack is used.
In other words, the power density of a FC system and a Li-ion battery pack @ 6-7 C, are comparable.
Right now, the $ per kW of Li-ion battery is much much less than the $ per kW of PEM FC stack, so a PFCV will cost MUCH LESS than a FCV of comparable power. Though with mass production, the $ per kW of FC stack is expect to be comparable with the $ per kW of Li-ion battery at 6-7 C discharge power.
Posted by: Roger Pham | 21 October 2016 at 01:45 PM
To borrow a line from this article: "Clearly, considerable engineering work is needed to adapt this laboratory discovery to large-scale hydrogen storage applications." As such I don't see the first users being private vehicle owners (in other words, not cars or scooters). Long distance truckers might be a better fit. The fuel tanks for such trucks are located where they could be placed and removed by a forklift; http://www.injuryrelief.com/wp-content/uploads/2015/09/18-wheeler-fuel-tank.jpg and can have a standard size and shape.
Posted by: ai_vin | 21 October 2016 at 03:58 PM
The first "privately owned" users might be pickup drivers. If you've ever seen an auxiliary tank in the bed, under the rear window, of one you'll get how such a cartridge could be slid in and out.
Posted by: ai_vin | 21 October 2016 at 04:14 PM
Now there's an idea I hadn't considered - a range extender for plug-ins. Thanks Roger, with that I am much more optimistic about this for personal vehicle use.
Posted by: ai_vin | 21 October 2016 at 10:05 PM
Hi guys.
The first use would not be in transport at all, but in things like stationary storage where swapping would not be a major issue.
I'm thinking of applications like this:
'Toshiba Corporation announced that H2One, Toshiba’s hydrogen-based autonomous energy supply system, which integrates renewable energy generation and uses hydrogen as a fuel for power generation, has entered operation in the Phase-2 building of the Henn na Hotel, at the Huis Ten Bosch theme park in Nagasaki, Kyushu.
H2One integrates a photovoltaic power generation system with batteries for storing output power; a hydrogen-producing water electrolysis unit; solid state hydrogen storage—a MmNi5 (Mischmetal-nickel) alloy); and a hydrogen fuel cell unit. (Mischmetal is an alloy of rare earth elements.)
MmNi5 alloys belong to a class known as AB5-type alloys. AB5 alloys combine a hydride forming metal A, usually a rare earth metal (La, Ce, Nd, Pr, Y or Mischmetal), with a non-hydride forming element—nickel. The nickel can be doped with other metals such as Co, Sn or Al to improve materials stability or to adjust equilibrium hydrogen pressure and temperature required for its charging discharging with hydrogen.
One commercially available MmNi5 alloy from Sigma-Aldrich offers hydrogen storage capacity of 1.5-1.6 wt.% @25 °C—not at all sufficient for on-board vehicle storage, but not a problem for a stationary system.'
http://www.greencarcongress.com/2016/03/20160321-toshiba.html
Posted by: Davemart | 22 October 2016 at 01:30 AM
Roger's 300 mile pack would weigh 70kgs or so, a pretty chunky size needing robotic handling.
It would also need quite a slot in the vehicle, as well as standardisation.
Pumping stuff would be far preferable, even if you had to pump out the used fuel first.
Perhaps small balls of hydrocarbon wax, used and put in another compartment?
I have seen similar ideas before, although I can't track down my bookmark.
Posted by: Davemart | 22 October 2016 at 06:35 AM
Dave, I get what you are saying about stationary storage and agree but one of the selling points this article makes is wax stores more H2 per weight and volume than some of the other options. Neither weight nor volume is all that important in stationary storage. It may be good to have those is this app but as this site is Green Car Congress of course we are going to think in terms of transportation apps.
Posted by: ai_vin | 22 October 2016 at 08:44 AM
BTW, which would be more of an issue, swapping out a cartridge for your home or car? In the case of cars everybody is going to go to a service station where they would likely have the special equipment needed to make the swap. For home use you can't expect every home owner to keep such equipment on hand, but if this is just for backup power he would not be swapping out a cartridge all that often.
Posted by: ai_vin | 22 October 2016 at 08:55 AM
Why do you need Hydrogen? Burn it. Implement a clean external combustion Rankine engine.
Such wasteful research (pun).
Posted by: Dr. Strange Love | 22 October 2016 at 09:57 AM
Why hydrogen instead of DME or methanol? Sounds no brainer.
Posted by: Darius | 22 October 2016 at 01:02 PM
IIUC the goal is to have ZEVs. Burning DME or MeOH in an engine generates CO2 and will always have some criteria pollutants. Reforming MeOH to H2 creates the problem of CO, which poisons some catalysts (and many people).
Posted by: Engineer-Poet | 22 October 2016 at 01:12 PM
Ok. So now we have all this coal tar to get rid of after dehydrogenation. What next? Microwaves aren't free either.
Posted by: Dr. Strange Love | 22 October 2016 at 03:31 PM
The wax can be pumped, but the left-over carbon is probably pure carbon powder at best. Which can not be pumped.
Though, there will be a lot of "intermediate" dehydrated leftovers, which are sticky, toxic, and not easy to handle. In addition, also volatile carbohydrates are formed that may poison the FC or escape to the atmosphere.
Though I love the idea of storing renewable electricity in wax, which can be stored for years, and is nontoxic, I doubt this will work for fuelcell vehicles within a reasonable time.
I see a lot of applications, but for cars I bet on batteries and compressed H2.
Posted by: Alain | 24 October 2016 at 09:01 AM
The carbon leftover can be mixed into a low viscosity liquid ("slippery water" with polyethylene oxide) and fed into a bladder in tank.
Posted by: philippe | 27 October 2016 at 05:13 PM