NOVEMBER 2011 | VOLUME 14 | NUMBER 11514

RESEARCH NEWS

A new study has shown that alginates taken from

fast-growing brown algae could offer extended energy

storage capabilities for a new generation of lithium ion

batteries using environmentally friendly manufacturing

technologies. Such cheap, lightweight, and improved

batteries could benefit a range of applications, such as

electrical cars, computers, and cell phones.

The researchers, from the Georgia Institute of Technology

and Clemson University in the United States, whose work

was published in Science Express [Kovalenko et al. Science

(2011) doi:10.1126/science.1209150], showed that the

algae, which are produced in stalks as long as 60 meters

in large oceanic clusters, can provide more energy storage

and output than the two standard types of commercial

electrodes; graphite-based and silicon-based.

They initially looked for a natural replacement

binder in aquatic plants that grow in salt water with

high concentration of ions. The binder is crucial in

suspending the silicon or graphite particles that

interact with the electrolyte and produce power.

Lithium ion batteries transfer lithium ions between

two electrodes through a liquid electrolyte, so the

easier the lithium ions can enter the electrodes during

charge and discharge, then the greater the capacity of

the battery.

The low-cost alginate-nanoSi-electrode can be

extracted from seaweed using a straightforward soda-

based process that produces a uniform material. From

there, the anodes can be developed using a water-

based slurry to suspend either the silicon or graphite

nanoparticles. Such electrodes have the advantage of

being compatible with existing methods of production

and can therefore be integrated into standard designs

for batteries.

The alginate needs to deal with decomposition

occurring when the lithium ion electrolyte forms

a solid electrolyte interface (SEI) on the surface of

the anode, hampering the potential of high-energy

silicon anodes. The SEI has to be stable to allow

the lithium ions to pass through it as well as to

restrict the flow of fresh electrolyte. For graphite

particles, the SEI remains stable as the volume

does not change; the alginate also manages to bind

silicon nanoparticles to each other as well as the

anode, and coat the silicon nanoparticles themselves

to offer a rigorous support for the SEI, therefore

stopping any degradation.

As researcher Gleb Yushin points out “The carboxylic

groups in alginate actively interact with ions from

water and the intracellular environment, protecting the

cell from an excess of toxic chemicals. We utilize these

uniformly distributed carboxylic groups to improve the

performance of battery electrodes.”

Algae can be produced on salt water or waste water

land and do not use valuable agricultural land, and

also need less area to produce the same amount of

biomass as regular crops. The team expects such use

of plant matter to increase and believe this should

be prioritized to help achieve greater sustainability,

especially as alginates are already extensively used in

the paper, pharmaceutical, biotechnological, dental,

and food industries.

Laurie Donaldson

Algae, for cheaper batteriesENERGY

SEM Aliginate-nanoSi-electrode. Courtesy of Gleb Yushin.

Lights up time for graphene devicesCARBON

Graphene has been touted as the natural successor to silicon in microelectronics devices. Indeed, experimental and theoretical results suggest that it is has many properties that will make it the perfect material for building a new generation of transistors, chemical sensors, composites, nanoeletromechanical (NEMS) devices and optoelectronics components that might operate at 10 gigabits per second. However, there is an obstacle visible on the roadmap: graphene-based photodetectors demonstrate a poor response when compared to conventional semiconductor devices.Now, UK researchers have combined graphene with plasmonic nanostructures to boost their photodetector sensitivity twentyfold [Novoselov et al., Nature Commun (2011) doi:10.1038/ncomms1464] The research also suggests that it is possible to achieve wavelength and polarization selectivity by tweaking the nanoscopic geometry of the materials.

Geim and Novoselov and colleagues at Manchester and the University of Cambridge explain that graphene-based photodetectors ought to have excellent characteristics in terms of quantum efficiency and reaction time and indeed they do. But they absorb light only inefficiently and it is difficult to extract electrons from the critical p-n junctions in any such device.In order to circumvent this latter obstacle to the development of graphene-based photodetectors, Geim and colleagues have focused on incorporating plasmonic nanostructures close to the junctions. The team explains that these plasmonic structures can absorb photons, producing plasmonic oscillations, which in turn boosts the local electric field guiding the electromagnetic energy to the p-n junction.With this in mind, the team therefore prepared graphene flakes using their Nobel-winning micromechanical exfoliation technique to peel off monoatomic layers of carbon from

an adhesive surface. Raman spectroscopy and optical contrast techniques proved that the necessary templated layouts were produced. They then created various nanostructures at the p-n terminals of these graphene layouts. They tested the photo response of the devices using several low-intensity lasers coupled to a microscope to scan the points of illumination.The researchers were able to measure the local photovoltage and photocurrent response. Their theoretical calculations agreed with the responses, however, they were unable to make a direct quantitative comparison between the theoretical field enhancement and the photovoltaic signals obtained. Nevertheless, they point out that the qualitative correspondence between theory and experiment is sufficient and it “proves the viability of the concept of using field amplification by plasmonic nanostructures for light harvesting in graphene-based photonic devices,” they conclude.

David Bradley

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