Lithiumair batteries go viral

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.Ageing iron nanoparticles feel the strainRusting occurs much more rapidly on thenanoscale compared to the macroscale,scientists have discovered. This is causedby strain present in nanoparticles due totheir small size.Iron and iron oxide nanoparticles arebeing utilized in an ever increasing numberof applications; from targeted drug delivery,to cancer treatment, to catalysis in watertreatment, 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 toaddress.Understanding the reactivity and ageingof nanomaterials is of vital importance fortheir use in environmental applications orinside the body, Pratt told Materials Today.For example, Fe and Fe oxide nanoparticlesshow great promise in removing toxicmetals and chemicals from contaminatedwater and soil systems. However, if [overtime] a particle reacts and degrades toomuch, its performance will be compro-mised and it may even rerelease a capturedcontaminant back into the environment.For the study, published in Nature Materi-als [Pratt, et al., Nat. Mater. (2013),doi:10.1038/nmat3785], the team trackedhow natural oxidation progresses in cubiciron nanoparticles. This was done over sev-eral years using state-of-the-art aberration-corrected transmission electron micro-scopes. After several months, we noticedsome interesting effects that could onlyarise because of the nanoscale size of theparticles and their cubic geometry, Prattsays. The centers of each of the cubes faceswere oxidizing faster than the regionsnearer the corners. The nanoparticles werealso observed to be rusting several magni-tudes faster that bulk iron crystals. Wepostulated that this was due to strainintroduced because of confinement at thenanoscale.To prove this theory, the scientistsmapped the distribution of strain on theindividual iron oxide molecules within theparticles. We found the strain is greatest inthe center of the nanoparticles faces and. . that the atoms in the oxide are further awayfrom each other than would normally befound in the bulk material, he says. Theatoms in the oxide shell are being pulledapart because of the need for the oxidedomains on the six faces of the cubic coreto stay connected. This opens up the atomiclattice which promotes the inward move-ment of oxygen ions and outward move-ment of iron ions. This acts to speed up theoxidation process, meaning the nanoparti-cles were fully oxidized in just two years.Currently, the team is studying silver andcopper nanoparticles to see if they behavethe same. We are also looking at waysto engineer nanoparticle properties toimprove performance or to mitigate someof their more damaging effects, Pratt adds.Nina NotmanLithiumair batteries go viralPower density could be boosted consider-ably if air is added to the recipe for recharge-able lithium batteries make long-distanceelectric vehicles a much more viable propo-sition. But first, materials scientists need todevelop better materials for such lithiumair batteries ones that can undergo manymore chargedischarge cycles than currentexperimental systems. Now, researchers atMassachusetts Institute of Technology haveused genetically modified viruses to makenanowire electrodes.Writing in Nature Communications,Dahyun Oh and colleagues explain howthey use the virus M13 to sequester manga-nese from solution to sculpt wires just80 nm in diameter for use in a lithiumairelectrode. The viral approach makes a roughspiky wiry surface which greatly increasesthe surface area of the wire relative to othermanufacturing 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 themanner in which an abalone assimilatescalcium ions from seawater to grow itsshell.Aside from giving rise to a high surface tovolume ratio, the viral approach to makingnanowires avoids the energy-intensive,high temperature approaches of conven-tional electronics manufacturing as wellas precluding the need for toxic solvents,working as it does in room temperaturewater. Moreover, rather generating isolatedwires, the viral method produces three-dimensional structure of cross-linked wires,which make for a more stable electrode.In order to activate their electrodes, theteam dopes the nanowires with palladiumto boost conductivity and to facilitate thenecessary catalytic processes that mustoccur during charge and discharge. Theamount of noble metal dopant required ismuch lower than was reported by othergroups for their electrode materials, againby virtue of the biological underpinning ofthe fabrication.The team suggests that a lithiumairbattery using their electrode materialsmight have a density more than doublethat of the best conventional lithium ionbatteries. However, the team admits thatmore work is now required to make thisviral approach a commercial viablemethod for electrode manufacture. In par-ticular, they have demonstrated proof ofprinciple with the material for chargedis-charge cycles but a commercial batteryneeds to be able to operate over thousandsof cycles.The team also points out that whileviruses have been used in the laboratory,it would be most likely that a more conven-tional fabrication process that emulated theviral approach would be developed in acommercial manufacturing system, theviruses are simply the pathfinders.David BradleyMaterials Today Volume 17, Number 1 January/February 2014 NEWSA cuboid iron nanoparticle after six monthsexposure to the environment. Credit: Amish Shahand Roland Kroger.9


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