US and China Researchers Show New Approach to Hydrogen Storage; Applied Electric Field Can Significantly Improve H2 Storage and Reversibility Properties of Polarizable Substrates
03 February 2010
| An applied electric field polarizes hydrogen molecules and the substrate, inducing hydrogen absorption with good thermodynamics and kinetics. Image courtesy of Qian Wang, Ph.D./VCU. Click to enlarge. |
Using density functional theory, a team of researchers from Virginia Commonwealth University; Peking University in Beijing; and the Chinese Academy of Science in Shanghai has shown that an applied electric field can substantially improve the hydrogen storage properties of polarizable substrates. The new approach could make the synthesis of hydrogen fuel storage materials less complicated and improve the thermodynamics and reversibility of the system.
They demonstrated their new concept by adsorbing a layer of hydrogen molecules on a number of nanomaterials. When one layer of H2 molecules is adsorbed on a BN sheet, the binding energy per H2 molecule increases from 0.03 eV/H2 in the field-free case to 0.14 eV/H2 in the presence of an electric field of 0.045 au (atomic units). The corresponding gravimetric density of 7.5 wt% is consistent with the 6 wt% system target set by Department of Energy for 2010.
They found that the strength of the electric field can be reduced if the substrate is more polarizable. For example, a hydrogen adsorption energy of 0.14 eV/H2 can be achieved by applying an electric field of 0.03 au on an AlN substrate, 0.006 au on a silsesquioxane molecule, and 0.007 au on a silsesquioxane sheet.
They concluded that such an application of an electric field to a polarizable substrate provides a novel way to store hydrogen; once the applied electric field is removed, the stored H2 molecules can be easily released, thus making storage reversible with fast kinetics. They also showed that materials with rich low-coordinated nonmetal anions are highly polarizable and can serve as a guide in the design of new hydrogen storage materials.
The research is published online in the Early Edition of the Proceedings of the National Academy of Sciences.
Using an external electric field as another variable in our search for such a material will bring a hydrogen economy closer to reality. This is a paradigm shift in the approach to store hydrogen. Thus far, the efforts have been on how to modify the composition of the storage material. Here we show that an applied electric field can do the same thing as doped metal ions.
More importantly, it avoids many problems associated with doping metal ions such as clustering of metal atoms, poisoning of metal ions by other gases, and a complicated synthesis process. In addition, once the electric field is removed, hydrogen desorbs, making the process reversible with fast kinetics under ambient conditions.
This work will help researchers create an entirely new way to store hydrogen and find materials that are most suitable. The challenge now is to find materials that are easily polarizable under an applied electric field. This will reduce the strength of the electric field needed for efficient hydrogen storage.
—Puru Jena, Ph.D., distinguished professor in the VCU Department of Physics
The research is based on a 1992 published polarization theory by Jena, the late B.K. Rao, a former professor of physics at VCU, and their student, J. Niu.
This work is supported by grants from the National Natural Science Foundation of China, the Foundation of National Laboratory for Infrared Physics, the National Grand Fundamental Research 973 Program of China, the US National Science Foundation and the US Department of Energy.
Resources
J. Zhou, Q. Wang, Q. Sun, P. Jena, and X. S. Chen (2010) Electric field enhanced hydrogen storage on polarizable materials substrates. PNAS doi: 10.1073/pnas.0905571107
As far as I understand, you need an electric field to keep the hydrogen stored. If your energy supply fails, then what? Will all stored hydrogen be released instantly?
Posted by: Arne | 03 February 2010 at 03:21 AM
That's a very important question, Anne. I suppose that no energy expenditure is required to maintain adsorption of the H2, just the electric field that needs to be maintained via a potential difference between the + and the - pole, so the battery would not be drained as the result of maintaining storage of H2, like in a capacitor, though there may be a problem of eventual leakage of electric field that would let a capacitor to self-discharge over a significant amount of time. Perhaps a rectifier circuitry can be built in to prevent the reverse discharge of the electric field in the storage tank back toward the battery, even when the battery is discharged.
Additionally, all pressurize tanks will have safety discharge valve when the pressure builts up above a safe level. Since the lost of electrical field will be a gradual process, the H2 leakage will be a slow and gradual process and will not exceed the flow capability of the safety discharge valve. Likely, the leakage of H2 will be slow enough and the H2 will rise fast enough away from the vehicle that an explosion hazard won't be likely. All garages for H2-vehicles should have safety vent for preventing built-up of H2.
Posted by: Roger Pham | 03 February 2010 at 12:09 PM
If your energy supply fails, then what? Will all stored hydrogen be released instantly?
Well, not "all" of it. This article says the electric field 'improves' the hydrogen storage properties, so we can assume even without the electric field the material has the capacity to store some/much/most of the hydrogen on its own.
Posted by: ai_vin | 03 February 2010 at 09:21 PM
Thanks for the responses.
Roger, I understand that maintaining an electric field does not cost energy. What I was wondering: in case of an accident, when the hydrogen storage tank is punctured, I can imagine the field leaking away. And I am not so sure that the hydrogen is then released slowly as you suggest.
Encasing this material in a pressurized tank for safety reasons will negate one of the big advantages of this type this type of hydrogen storage, which is that you don't need such a tank which is bulky and heavy.
ai_vin, the article says it 'substantially improves' the hydrogen storage. I took that as meaning that the amount that remains stored after removal of the electric field is negligible.
Posted by: Arne | 03 February 2010 at 11:29 PM
Good point, Anne. Disruption in the insulation due to shock or other causes can cause current to break through the insulation causing rapid lost of electric field. This leads to catastrophic failure in the capacitors, and in the presence of H2 leakage and air, fire may break out.
Much testing will be needed!
Posted by: Roger Pham | 04 February 2010 at 11:15 AM
A thought,
If the field were used in a higher pressure tank only during filling, a lower pumping pressure may be required which would allow energy and pump and infrastructure cost savings. Then turn off or let the field drain. Similar approach may7 be of benifit in ground storage tanks with possible extra reserve tank capacity for bleed or blow discharge.
Then what about a piggy back or leapfrog idea that pumped into progressively heavier tanks that looked like empty capacitors . The H2 appearing to rush into the next tank, that then switched off and rushes into the next etc.
Posted by: Arnold | 04 February 2010 at 05:38 PM
Good idea, Arnold, a pumpless pressurizing device, no less. But, I suspect that you will have to add electrical energy to the tank continously as you are filling it, as its capacitance will be altered with more H2 adsorbed, requiring higher and higher input voltage. Then, the electrical charge from the tank can be turned into useful work as the tank is drained of the H2, like a capacitor. If the tank is not allowed to drain the H2 away, but the electrical field is removed, however, the pressure will build up inside the tank. The mechanical energy of this pressure built up must come from the electrical energy put into the tank earlier, so you won't get as much electrical energy returned as you put into the tank originally when you drain the capacitor electrically. The voltage across the capacitor will drop precipitously as the pressure rises up inside the tank such that the electrical energy obtained will be far less. This voltage drop as the pressure builds up will reduce the tendency for break-thru current across the insulation and helps to reduce the risk of catastrophic discharge of the capacitor.
The physics of this is quite intriguing, and the non-mechanical-purely-electrical H2 pumping device as Arnold envisioned can help reduce the maintenance cost of mechanical H2 pumps due to wear and tear and friction loss.
Posted by: Roger Pham | 04 February 2010 at 07:56 PM
Then, again, I may be wrong. The mechanical energy of the H2 pressure built up may come from environmental heat. The adsorbed H2 has very little molecular kinetic energy since the H2 is not moving. When the H2 molecule is released, it must absorb kinetic energy from its surrounding in order to be turned into a gas, according the Ideal Gas Law. The desorbtion of the H2 will cause significant cooling of the tank, and pressure cannot build up inside the tank until the tank can absorb heat from the outside. If the tank is significantly insulated, then catastrophic built up of H2 pressure cannot happen. However, the converse of this is that when the H2 is converted from a gaseous state to the adsorbed state, it must give up a lot of molecular kinetic energy that will heat up the tank considerably, requiring vigorous cooling of the tank. I'd better do some more reading into this before any further speculation!
Posted by: Roger Pham | 04 February 2010 at 08:07 PM