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New composite MOF traps oxygen selectively; potential use for energy applications such as fuel cells

A research team has created a composite of a MOF and a helper molecule in which the two work in concert to separate oxygen from other gases simply and cheaply. The results, reported in Advanced Materials, might help with a wide variety of applications, including making pure oxygen for fuel cells, using that oxygen in a fuel cell, removing oxygen in food packaging, making oxygen sensors, or for other industrial processes. The technique might also be used with gases other than oxygen as well by switching out the helper molecule.

Currently, industry uses a common process called cryogenic distillation to separate oxygen from other gases. It is costly and uses a lot of energy to chill gases. Also, it can’t be used for specialty applications like sensors or getting the last bit of oxygen out of food packaging.

A great oxygen separator would be easy to prepare and use, be inexpensive and be reusable. MOFs, or metal-organic frameworks, are materials containing lots of pores that can suck up gases like sponges suck up water. They have potential in nuclear fuel separation and in lightweight dehumidifiers.

But of the thousands of MOFs out there, less than a handful absorb molecular oxygen. And those MOFs chemically react with oxygen, forming oxides that render the material unusable.

When we first worked with MOFs for oxygen separation, we could only use the MOFs a few times. We thought maybe there’s a better way to do it.

—PNNL materials scientist Praveen Thallapally

The new tack for Thallapally and colleagues at PNNL involved using a second molecule to mediate the oxygen separation—a helper molecule would be attracted to but chemically uninterested in the MOF. Instead, the helper would react with oxygen to separate it from other gases.

They chose a MOF called MIL-101 that is known for its high surface area and lack of reactivity.

MOFs that react with oxygen need to be handled carefully in the laboratory, but MIL-101 is stable at ambient temperatures and in the open atmosphere of a lab. For their helper molecule, they tried ferrocene, an inexpensive iron-containing molecule.

The scientists made a composite of MIL-101 and ferrocene by mixing them and heating them up. Initial tests showed that MIL-101 took up more than its weight in ferrocene and at the same time lost surface area. This indicated that ferrocene was taking up space within the MOF’s pores, where they need to be to snag the oxygen.

Then the team sent gases through the black composite material. The material bound up a large percentage of oxygen, but almost none of the added nitrogen, argon or carbon dioxide. The material behaved this way whether the gases went through individually or as a mix, showing that the composite could in fact separate oxygen from the others.

Additional analysis showed that heating caused ferrocene to decompose in the pores to nanometer-sized clusters, which made iron available to react with oxygen. This reaction formed a stable mineral known as maghemite, all within the MOF pores. Maghemite could be removed from the MOF to use the MOF again.

Future research will explore other combinations of MOF and helper molecules.

In addition to PNNL, participating researchers were from and used analytical instruments at two Office of Science User Facilities, the Environmental Molecular Sciences Laboratory at PNNL and the Advanced Photon Source at Argonne National Laboratory, as well as the University of Amsterdam. This work was supported by the Department of Energy Office of Science.

Resources

  • Zhang, W., Banerjee, D., Liu, J., Schaef, H. T., Crum, J. V., Fernandez, C. A., Kukkadapu, R. K., Nie, Z., Nune, S. K., Motkuri, R. K., Chapman, K. W., Engelhard, M. H., Hayes, J. C., Silvers, K. L., Krishna, R., McGrail, B. P., Liu, J. and Thallapally, P. K. (2016), Redox-Active Metal–Organic Composites for Highly Selective Oxygen Separation Applications. Adv. Mater. doi: 10.1002/adma.201600259

Comments

Davemart

This technology may greatly reduce one of the biggest losses in fuel cells, increasing system efficiency by a fair chunk and reducing the complexity of the engineering.

Amusingly, this is just as applicable to lithium/air batteries, so those who want to knock fuel cells at every chance and promote batteries as the sole solution are faced with the uncomfortable fact that in some respects the same technology enables both.

HarveyD

Wonder if this technology could be scaled and optimized for use in board FCEVs, to separate H2 and Oxygen from the air (or water), to feed the vehicle FC or fixed FCs?

If so, FCs could become excellent safe e-energy generators?

Batteries and super caps could be used as power boasters and for energy recovery?

Davemart

Harvey:

It sounds scalable.
That does not prove it is scalable though, so we will have to wait and see!

gryf

Davemart,

Your idea that the PNNL MOF/Ferrocene composite as an Oxygen Reduction Reaction (OARR) catalyst for Lithium/Air batteries is an excellent idea.
Ferrocene and its derivatives are widely studied in Li-ion batteries.Research Check out
http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b01224 by Guihua Yu at John Goodenough Lab at UT Austin or "A chemistry and material perspective on lithium redox flow batteries towards high-density electrical energy storage" https://www.researchgate.net/profile/Yu_Ding10/publication/280966588_A_chemistry_and_material_perspective_on_lithium_redox_flow_batteries_towards_high-density_electrical_energy_storage/links/55d33ee008aec1b0429f32ce.pdf.

Arnold

Davemart,

Honestly?

A lithium air cell / battery is closer to a fuel cell - it consumes or converts it's energy source.

It is not a secondary battery as per rechargable liPO any) etc.

It is more like a a fuel cell in that the fuel is consumed and not at all rechargeable in its usual form
.
Having said that of course the Hydrogen fuel cell is easily reversible in concept.

All the above are very closely related in principle if you 'get one you get the others' yes?
They do however markedly differ in simplicity, cost and efficiency.
It is sensible to concentrate on those matters.

Do you have any idea how f*in boring your schoolboy pettiness's sound?

That is my criticism /comment.

FACT:
Get this. Your arguments are as irrational as they are self contradicting and repetitive.
Any sympathy the reader feels is put against the wall by the sheer pettiness of attitude. I don't come here to hear people's personal bitching.


I get it you like fuel cells.

FFS I like vegemite. (Let's not go there!)

But I prefer the GG's and irony and humour if it is in good taste.

https://en.wikipedia.org/wiki/Gimcrack



Roger Pham

Good point, Davemart. If FC can be made significantly more efficient, to approach the efficiency of Li-ion batteries, then this would allow reduction on the fuel tank and making FCEV's even more practical. It would help counter arguments from BEV supporters regarding the lower efficiency of FCEV's.

If O2 can be easily separated from the air, then this would give a boost to diesel vehicles as well. Imagine that a diesel engine can be made to run using pure O2 + CO2 + steam + diesel fuel without any nitrogen. This would eliminate any NOx production without expensive emission control equipment. This is known as Oxy combustion and has been proposed as a way for power plants to control NOx emission.
A high degree of cooled EGR is done with added O2 prior to compression of the working fluid, then diesel fuel is injected to start combustion. Even better, diesel fuel is aerosolized when injected together with some pure O2, as with the Orbital type of injector, and is introduced into the combustion chamber before top dead center. This will give very rapid, very high temperature combustion, and complete combustion that potentially can be free of carbon monoxide and hydrocarbon and particulate matters WITHOUT any further oxidation catalyst, NOR any expensive Diesel Particulate Filter (DPF).
Then, low-cost diesel will rival gasoline engine for the automotive market.

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