Lithium–air batteries go viral

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    Ageing iron nanoparticles feel the strainRusting occurs much more rapidly on the

    nanoscale compared to the macroscale,

    scientists have discovered. This is caused

    by strain present in nanoparticles due to

    their small size.

    Iron and iron oxide nanoparticles are

    being utilized in an ever increasing number

    of applications; from targeted drug delivery,

    to cancer treatment, to catalysis in water

    treatment, to use in clean fuel technologies.

    But little is known about how they age,

    something Andrew Pratt from the Univer-

    sity of York, UK, and his team were keen to

    address.

    Understanding the reactivity and ageing

    of nanomaterials is of vital importance for

    their use in environmental applications or

    inside the body, Pratt told Materials Today.

    For example, Fe and Fe oxide nanoparticles

    show great promise in removing toxic

    metals and chemicals from contaminated

    water and soil systems. However, if [over

    time] a particle reacts and degrades too

    much, its performance will be compro-

    mised and it may even rerelease a captured

    contaminant back into the environment.

    For the study, published in Nature Materi-

    als [Pratt, et al., Nat. Mater. (2013),

    doi:10.1038/nmat3785], the team tracked

    how natural oxidation progresses in cubic

    iron nanoparticles. This was done over sev-

    eral years using state-of-the-art aberration-

    corrected transmission electron micro-

    scopes. After several months, we noticed

    some interesting effects that could only

    arise because of the nanoscale size of the

    particles and their cubic geometry, Pratt

    says. The centers of each of the cubes faces

    were oxidizing faster than the regions

    nearer the corners. The nanoparticles were

    also observed to be rusting several magni-

    tudes faster that bulk iron crystals. We

    postulated that this was due to strain

    introduced because of confinement at the

    nanoscale.

    To prove this theory, the scientists

    mapped the distribution of strain on the

    individual iron oxide molecules within the

    particles. We found the strain is greatest in

    the center of the nanoparticles faces and. .

    that the atoms in the oxide are further away

    from each other than would normally be

    found in the bulk material, he says. The

    atoms in the oxide shell are being pulled

    apart because of the need for the oxide

    domains on the six faces of the cubic core

    to stay connected. This opens up the atomic

    lattice which promotes the inward move-

    ment of oxygen ions and outward move-

    ment of iron ions. This acts to speed up the

    oxidation process, meaning the nanoparti-

    cles were fully oxidized in just two years.

    Currently, the team is studying silver and

    copper nanoparticles to see if they behave

    the same. We are also looking at ways

    to engineer nanoparticle properties to

    improve performance or to mitigate some

    of their more damaging effects, Pratt adds.

    Nina Notman

    Lithiumair batteries go viralPower density could be boosted consider-

    ably if air is added to the recipe for recharge-

    able lithium batteries make long-distance

    electric vehicles a much more viable propo-

    sition. But first, materials scientists need to

    develop better materials for such lithium

    air batteries ones that can undergo many

    more chargedischarge cycles than current

    experimental systems. Now, researchers at

    Massachusetts Institute of Technology have

    used genetically modified viruses to make

    nanowire electrodes.

    Writing in Nature Communications,

    Dahyun Oh and colleagues explain how

    they use the virus M13 to sequester manga-

    nese from solution to sculpt wires just

    80 nm in diameter for use in a lithiumair

    electrode. The viral approach makes a rough

    spiky wiry surface which greatly increases

    the surface area of the wire relative to other

    manufacturing processes provide more sur-

    face for a given volume to charge and dis-

    charge [Oh, et al., Nat. Commun. 4 (2013)

    2756]. The growth process is not unlike the

    manner in which an abalone assimilates

    calcium ions from seawater to grow its

    shell.

    Aside from giving rise to a high surface to

    volume ratio, the viral approach to making

    nanowires avoids the energy-intensive,

    high temperature approaches of conven-

    tional electronics manufacturing as well

    as precluding the need for toxic solvents,

    working as it does in room temperature

    water. Moreover, rather generating isolated

    wires, the viral method produces three-

    dimensional structure of cross-linked wires,

    which make for a more stable electrode.

    In order to activate their electrodes, the

    team dopes the nanowires with palladium

    to boost conductivity and to facilitate the

    necessary catalytic processes that must

    occur during charge and discharge. The

    amount of noble metal dopant required is

    much lower than was reported by other

    groups for their electrode materials, again

    by virtue of the biological underpinning of

    the fabrication.

    The team suggests that a lithiumair

    battery using their electrode materials

    might have a density more than double

    that of the best conventional lithium ion

    batteries. However, the team admits that

    more work is now required to make this

    viral approach a commercial viable

    method for electrode manufacture. In par-

    ticular, they have demonstrated proof of

    principle with the material for chargedis-

    charge cycles but a commercial battery

    needs to be able to operate over thousands

    of cycles.

    The team also points out that while

    viruses have been used in the laboratory,

    it would be most likely that a more conven-

    tional fabrication process that emulated the

    viral approach would be developed in a

    commercial manufacturing system, the

    viruses are simply the pathfinders.

    David Bradley

    Materials Today Volume 17, Number 1 January/February 2014 NEWS

    A cuboid iron nanoparticle after six monthsexposure to the environment. Credit: Amish Shah

    and Roland Kroger.9

    http://dx.doi.org/10.1038/nnano.2013.230http://dx.doi.org/10.1016/j.mattod.2013.12.008http://dx.doi.org/10.1002/adma.201303304http://dx.doi.org/10.1002/adma.201303304http://dx.doi.org/10.1038/nature12610http://dx.doi.org/10.1038/ncomms3676http://dx.doi.org/10.1038/ncomms3676http://dx.doi.org/10.1038/nphys2753http://dx.doi.org/10.1038/nnano.2013.227http://dx.doi.org/10.1016/j.biomaterials.2013.10.021http://dx.doi.org/10.1016/j.biomaterials.2013.10.021http://dx.doi.org/10.1038/nmat3785

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