Researchers Develop Vanadium Boride Air Cell; Twice the Practical Energy Capacity of Gasoline
25 July 2008
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| Energy capacity comparison of gasoline, hydrogen and electrochemical energy sources. Shading superimposed on solid colors indicates practical capacities. Click to enlarge. Source: Licht 2008. |
A team of researchers led by Dr. Stuart Licht at the University of Massachusetts, Boston, has developed a vanadium boride (VB2)/air cell—a new renewable electrochemical energy system which stores more energy than gasoline and has an order of magnitude higher capacity than lithium-ion batteries. A report on their work is published in the 28 July issue of the journal Chemical Communications.
The energy capacity gap between gasoline and electrochemical storage systems has been a fundamental barrier to more widespread use of electric drive vehicles. Gasoline has a practical energy storage capacity of about 2.7 kWh/Liter, Li-ion about 0.5 kWh/L. Zinc-air cells, another high-capacity electrochemical system, have a practical capacity of about 1.75 Wh/L according to Licht and his colleagues. The new VB2 system has a practical capacity of 5 kWh/L, they calculate.
Practical, compared to intrinsic, energy electrochemical capacity is limited by the delivered energy and system mass, incorporates all voltage losses, air cathode size, and all other cell components. For example, the practical energy of a small, portable commercial zinc air cell exceeds 18% of the intrinsic energy capacity, and can be higher in an optimized, large fuel cell configuration. The relative practical capacity of the VB2/air cell can be estimated as similar to that of the well studied Zn/air system (electrolytes and cathodes are similar). Based on this analog, the practical vanadium boride fuel has a lower limit of 18% of its intrinsic 27 kWh L-1, for an estimated vanadium boride air practical storage capacity of 5 kWh L-1.
—Licht 2008
As in a zinc air cell, the vanadium boride cell reacts oxygen brought in via the cathode with the anode to produce electricity. And also as in a zinc-air cell, the reaction is irreversible; spent anodes need to be replaced in a “refueling” operation and chemically regenerated. (Earlier post.) The vanadium boride cells combine a conventional air cathode with a zirconia-stabilized vanadium boride anode.
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| Optimization of the vanadium boride air cell anode capacity as a function of the indicated anode composition, capacity, and discharge load conditions. Click to enlarge. Source: Licht 2008. |
The researchers used the zirconia coating to avoid issues such as boride corrosion, which can result in “not only a chemical loss of the electrochemical capacity, but evolved hydrogen is flammable, and the evolved gas can swell or even crack a cell.” Zirconia is highly stable and maintains effective charge transfer during boride anodic discharge.
The researchers overcame a series of impediments to the effective discharge of the vanadium boride fuel cell and showed experimentally that they could realize substantial capacity of VB2. (See plot at right.)
For regeneration of the anodes, Licht and his team proposed a solar photochemical pathway based on Mg reduction of the fuel cell discharge products.
The large volumetric capacity of the fuel cell, and the pathway for a renewable (solar) energy recharge, are positive attributes of this novel vanadium boride air cell. Systems aspects will continue to be analyzed and optimized. Liquid (higher temperature, solar driven), rather than solid, Mg, should facilitate the recharge formation of VB2...The discharge studies indicate that sub micron particle size VB2, as available following high energy ball milling, can further improve anodic kinetics and coulombic efficiency.
—Licht 2008
This material was based on work supported in part by the United States National Science Foundation, with research support to Stuart Licht while working at the Foundation.
Resources
Stuart Licht, Huiming Wu, Xingwen Yu and Yufei Wang (2008) Renewable highest capacity VB2/air energy storage. Chem. Commun., 2008, 3257-3259 doi: 10.1039/b807929c
I have a problem with the energy storage density of gasoline. Gasoline has about 43000 kJ of energy per kg, which equals 57333 kJ per liter if a density of 0.75 kg/L is assumed. 57333 kJ equals 15.926 kWh, not anywhere close to the 2.7 kWh reported. Perhaps they're also (erroneously) considering tank-to-wheel energy conversion efficiencies? If so, it's an apples-to-oranges comparison.
Posted by: TDIMeister | 25 July 2008 at 08:49 AM
That's pretty cool. If the cell could be regenerated by simply plugging it in to an electricity source, with good efficiency, then redox cells could have a very bright future.
Posted by: rob | 25 July 2008 at 08:58 AM
Perhaps it's a total propulsion system volume they are using in their calculation (gas tank, engine, transmission, driveline) If so, it would be better if they would clearly state that.
As long as the electric system uses the same basis (battery, motor, etc), it could still be apples-to-apples - I don't know enough about it to know - again - it would help if they were explicit.
Posted by: shane | 25 July 2008 at 09:08 AM
TDIMeister,
43000 KJ/Kg = 32250 KJ/litre @ 0.75 Kg/litre
32250 KJ/litre = 8.958 KW-hour
assuming a 30% conversion efficiency from chemical to mechanical energy, then 8.958 KWh X 0.3
is approximately 2.7 KW-hour
Posted by: Jorge | 25 July 2008 at 09:10 AM
My guess is they are using a "total propulsion system" volume (gas tank, engine, transmission, driveline). If so, it would be best to clearly explain that.
As long as they calculate the electric volume the same way (motor, motor controller, battery, etc) it would be apples-to-apples.
Posted by: shane | 25 July 2008 at 09:10 AM
Ok
Thanks Jorge
Posted by: shane | 25 July 2008 at 09:13 AM
Its a big disappointment that this vanadium boride cell isn't electrically rechargable. What you have then, is a fuel cell rather than a battery. As you need electrical rechargability for true hybrid operation, one would need to supplement vanadium boride cells with a lithium ion battery for regenerative braking.
Posted by: Alex Kovnat | 25 July 2008 at 09:19 AM
Is this the answer for aircraft?
Posted by: OldNeil | 25 July 2008 at 09:36 AM
rob:
they need to replace the anodes for recharge - probably using a scheme like the Zinc-air cassettes. The spent anodes have to be physically removed and replaced with fresh material. They indicate they can use some kind of Mg reduction of cell discharge to facilitate this. Though no specifics.
Posted by: gr | 25 July 2008 at 09:40 AM
43000kJ/kg * 0.75 kg/L = 35000 kJ/L
/ 3600 = 9.7 kWh/L
which is the energy capacity on the graph.
Posted by: DavidJ | 25 July 2008 at 09:42 AM
OldNeil:
Very good question.
If this type of fuel cell can ever effectively supply 2x the practical energy density over current turbo-fuel engines system, it could eventually become a competitve alternate power source to drive high efficiency electric prop aircraft.
Noise level and vibration should be a lot less. Electric motors are inherently safer and much more efficient than ICE and turbines.
The spent cells parts/modules could be quickly replaced with freshly recharge-regenerated modules at regular planned stops.
A multiple cells, on-board system, could supply the high level of safety required. A few spare regenerated modules could extend the aircraft range and/or used where facilities do not exist.
Posted by: HarveyD | 25 July 2008 at 10:10 AM
David:
Right, but a gasoline engine is only about 20% efficient.
So if you are talking about practical rather than intrensic energy content, then you have to take that into consideration.
Posted by: bbm | 25 July 2008 at 10:16 AM
OldNeil:
Very good question.
If this type of fuel cell can ever effectively supply 2x the practical energy density over current turbo-fuel engines system, it could eventually become a competitve alternate power source to drive high efficiency electric prop aircraft.
Noise level and vibration should be a lot less. Electric motors are inherently safer and much more efficient than ICE and turbines.
The spent cells parts/modules could be quickly replaced with freshly recharge-regenerated modules at regular planned stops.
A multiple cells, on-board system, could supply the high level of safety required. A few spare regenerated modules could extend the aircraft range and/or used where facilities do not exist.
Posted by: HarveyD | 25 July 2008 at 10:18 AM
For air transport, weight energy density will be more important than volumetric energy density.
Posted by: bbm | 25 July 2008 at 10:21 AM
Whoops, divided the kg>L conversion instead of multiplied. The kWh density quoted for gasoline is still way off, unless as presumed they're considering conversion and drivetrain losses.
As for the quoted figures for the different battery types, they do not seem to include losses as the gasoline figure seems to, and they're quite optimistic at that based on the data I have seen.
Posted by: TDIMeister | 25 July 2008 at 10:29 AM
From the paper itself: http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b807929c&JournalCode=CC
----
Fig. 1 Energy capacity comparison of gasoline (petrol), hydrogen and
electrochemical energy sources. The intrinsic energy of gasoline yields a
maximum practical efficiency of 30%due to Carnot and friction losses. Air
fuel cells do not have this Carnot inefficiency, and have practical capacities
instead constrained by overpotential losses, and the requisite volume of the
air anode and all other cell components. Volumetric capacity of liquid H2
is constrained by its density of 0.0708 kg L1
. Shaded superimposed on
solid, colors, compare intrinsic and practical capacities.
---
So their quoted numbers for gasoline do account for the efficiency of the engine. I find this disingenuous. Batteries also have losses in charging, discharging, in the power electronics and in the motor itself. Again, apples to oranges. I would have expected a more rigorous peer-review.
Posted by: TDIMeister | 25 July 2008 at 10:39 AM
as a range extender that would be used occasionally in a 40m plug-in ev it would be brilliant. I hope the anode cassettes are stable is not used for a long time, and that they can be started and stopped with no problems.
A ICE range extender suffers from storing the gasoline/diesel for a long time.. it will go bad, and the pollution equipment needs to warm up to be effective. Refueling with a cassette has to be safer than dealing with flammable liquids.
The only question is cost in mass usage and the weight/energy.. if it is lighter than gasoline or diesel it will be revolutionary.
Posted by: Herm | 25 July 2008 at 10:54 AM
This was very exciting news until shane, TDIMeister and Jorge added some reality.
Now I'm really confused
So -
We need to know the kWh/liter and the kWh/lb of the ANODES alone to compare with gas and hydrogen?
And the kWh/liter and the kWh/lb of the rest of the cell and motor to compare with an ICE and fuel tank?
Posted by: ToppaTom | 25 July 2008 at 10:59 AM
If it's not rechargeable, then that brings up serious infrastructure issues. If might be a better solution for aircraft, if the fuel cell doesn't weigh too much.
Posted by: Jim | 25 July 2008 at 11:27 AM
"So their quoted numbers for gasoline do account for the efficiency of the engine. I find this disingenuous. Batteries also have losses in charging, discharging, in the power electronics and in the motor itself. Again, apples to oranges. I would have expected a more rigorous peer-review."
these losses are nowhere near what they are for gasoline (roughly 80% of the energy lost). it's not unusual for power electronics to be 90%+ efficient.
Posted by: eric | 25 July 2008 at 11:44 AM
TDIMeister: So their quoted numbers for gasoline do account for the efficiency of the engine. I find this disingenuous.
Well, crap, if you have some better way of getting the energy out of gasoline than burning it and sustaining the Carnot inefficiency, why not spit it out?
As it is, this paper is comparing the electricity you would get from an ICE-powered generator to the electricity you'll get out of a "practical" VB2 storage system.
Batteries also have losses in charging, discharging, in the power electronics and in the motor itself.
Maybe you should read the paper, instead of misinterpreting the pretty pictures? There are no motors or power electronics here. This is all intrinsic vs. practical energy storage density. From page two:
"Based on this analog, the practical vanadium boride fuel has a lower limit of 18% of its intrinsic 27 kWh/L, for an estimated vanadium boride air practical storage capacity of 5 kWh/L."
Gee! It looks a lot like they are applying practical efficiencies even to the VB2, and this gives a number that is larger than you get when you run the gasoline through a ICE.
And if you check the graphic, you'll even notice that the distinction between "intrinsic" and "practical" is also explicit. You may also see that even in the 100% efficient case, VB2 looks better than gasoline.
I would have expected a more rigorous peer-review.
Physician, heal thyself?
Posted by: mdf | 25 July 2008 at 11:49 AM
I suspect the limiting factor in how much this will help improve transportation will be the availability of the minerals in high quantities.
However, where is the excitement? 10x lithium batteries! Amazing!
2x gasoline taking into account unavoidable losses turning gasoline into motive power, incredible!
And to top it all off, they think it may be possible to recharge using a solar photothermal process that doesn't require PV cells! Astounding!
Obviously it will take a lot of R&D to know if it will be practial and cost effective, but what's with the blather about apples and oranges?
Posted by: Grant | 25 July 2008 at 12:08 PM
What is the possibility of developing a 'kit' to retrofit existing vehicles. The abandonment of useable platforms is basically wasteful. The development of mass/fleet retro-programs to train mechanics and fold in new technologies[which also should be upgradeable] should be a given.[only] When worn out, new vehicles should be cycled in.
Conversion could become an important industry.
Posted by: sigmund Rosen | 25 July 2008 at 01:11 PM
OldNeil/HarveyD-
Interesting thought; one thing I'd throw in there is that Zn-Air (which this is admittedly not) is terrible at low temps, and if this has similar limitations (which would be a reasonable, but by no means proven assumption), that would certainly add to the complexity of the system.
Posted by: KZ | 25 July 2008 at 01:44 PM
Regarding retrofits, many of the vehicles today are way oversized to begin with, so replacing the power source seems like throwing good money after bad. A smaller and lighter vehicle will be much less expensive to retrofit and operate.
I have seen electric retrofit kits based upon lead-acid batteries. You essentially unbolt the engine from the clutch and keep the clutch, transmission and drive axles. The motor is designed to mount directly to it from there.
The hard part after that as we all know is the question of batteries.
Posted by: JackRussell | 25 July 2008 at 01:47 PM