EES Blog

US scientists have designed an ultra-thin, flexible battery with the highest charge capacity reported for thin film cells. The battery can also be charged at a lower voltage than lithium ion batteries.

Flexible batteries offer advantages over more rigid systems as they can be incorporated into many modern devices, from powering up limb prostheses to detection systems for cracks and strains in concrete structures, for example. But electrochemical energy sources for the batteries are plagued by toxicity, the risk of explosive flammability and physical bulkiness.

Daniel Lowy from FlexEl, a company in Maryland that develops rechargeable batteries made from thin films, and Martin Peckerar from the University of Maryland, and colleagues, have developed a thin galvanic cell that is safe to use and non-toxic because it doesn’t corrode in electrolyte media.

The team made the cathode by blending RuO₂·nH₂O nanoparticles and activated carbon with a supporting electrolyte made of zinc and ammonium chloride, and a perfluorinated polymer binder. The resulting paste was spread out on a flexible and conductive current collector (a graphite film). A thin zinc sheet served as the anode and the whole assembly was packaged between sheets of flexible plastic.

The flexible power source is made of a zinc anode on top of a RuO2.nH2O/activated carbon cathode surrounded by packaging and sealing materials

‘While the usefulness of RuO₂ as an electrode material in supercapacitors is well documented, its use in galvanic cells has almost been neglected because of its perceived high cost,’ says Lowy.

To test the corrosion resistance of the cell, the team used different electrolytes of varying pH. They found that using a moderately acidic electrolyte prevented the electrode materials and package sealing materials dissolving, while enabling the electroactive reactions to proceed close to equilibrium. When they tested the life cycle of the battery, they found that the cells undergo over 400 charge-discharge cycles. ‘We can maintain up to 85 per cent of the capacity over 300 cycles. Over the next 100 cycles, the capacity decays gradually to 20 per cent of its initial value,’ says Lowy.

‘The galvanic cell’s electrochemical performance exceeds that of lithium ion batteries,’ says Mojtaba Mirzaeian, an expert in electrochemical energy storage at the University of Strathclyde. He finds it interesting that using a ruthenium oxide-based cathode can be applied to developing a hybrid energy storage and power supply system.

‘The mechanical flexibility and malleability of our cell will have a positive impact on miniaturisation. Imagine, for example, an ultra-thin battery moulded to the case of a laptop! Clearly, this will enable more room for the electronic payload,’ adds Lowy.

Carl Saxton

Read the Energy & Environmental Science article:

A novel high energy density flexible galvanic cell
Martin Peckerar, Zeynep Dilli, Mahsa Dornajafi, Neil Goldsman, Yves Ngu, Robert B. Proctor, Benjamin J. Krupsaw and Daniel A. Lowy
Energy Environ. Sci., 2011, DOI: 10.1039/c1ee01075a

Nanoporous black silicon photocathode for H₂ production by photoelectrochemical water splitting

Jihun Oh, Todd G. Deutsch, Hao-Chih Yuan and Howard M. Branz
Energy and Environmental Science, 2011, C1EE01124C

Water splitting, a technique whereby hydrogen is extracted from water, could be a potential revolution in the direct conversion of solar energy into clean and storable fuel. Even though the photoelectrochemical (PEC) splitting of water at a semiconductor/electrolyte interface has drawn much attention as a viable method to produce H₂, there have been some major limitations impeding the progress of this technology. However, scientists from the USA have developed a nanoporous black silicon photocathode which dramatically improves H₂ production in such a system.

Although silicon, as an abundant and well-used semiconductor, is promising for use as the photocathode in a PEC system, it has an inherent drawback in its planar ‘wafer’ form.  Around 25% of incident photons are reflected away from its planar surface, which means it does not make full use of the solar energy that falls upon it. Jihun Oh and coworkers at the National Renewable Energy Laboratory, Colorado, have tackled this issue by creating a nanostructured silicon photocathode which exhibits less than 5% reflectance due to the unique optical properties of the nanoporous surface. In addition to its impressive anti-reflective performance, the nanostructured photocathode also improves H₂ production efficiency and should improve corrosion resistance.

This work provides an excellent example of how manipulation of the nanostructure of a semiconductor surface can improve its performance as a photocathode, and will hopefully guide future researchers in the design of advanced PEC systems.

To read the full article, click here.

April’s issue of Energy & Environmental Science is now published online – take a look at this great issue today!

Here are just a few highlights:

Perspective
New conjugated polymers for plastic solar cells
David Gendron and Mario Leclerc
Energy Environ. Sci., 2011, 4, 1225

Minireview
Carbon-free energy: a review of ammonia- and hydrazine-based electrochemical fuel cells
Neil V. Rees and Richard G. Compton
Energy Environ. Sci., 2011, 4, 1255

HOT Communication
Nano-structured textiles as high-performance aqueous cathodes for microbial fuel cells
Xing Xie, Mauro Pasta, Liangbing Hu, Yuan Yang, James McDonough, Judy Cha, Craig S. Criddle and Yi Cui
Energy Environ. Sci., 2011, 4, 1293

The front cover features the Perspective by Stanislaus Wong and colleagues on one-dimensional noble metal electrocatalysts.

This month’s inside cover highlights the work by Matthew Eisaman et al. where they use bipolar membrane electrodialysis for CO2 separation.

Scientists in the US have developed a membrane-free, solvent extraction method to remove salt from seawater that works at low temperatures.

Access to clean, fresh water is a necessity. Unfortunately, supply is becoming over-stretched and there is a struggle to meet demand. As a result, the development of desalination technology (the conversion of salt water to fresh water) has become increasingly important.

Current desalination techniques require large amounts of energy or membranes that need to be changed constantly as they become blocked. Although significant advances have been made in these areas, Gang Chen and colleagues from the Massachusetts Institute of Technology, Cambridge, have gone a step further and removed the need for a membrane entirely.

The team used decanoic acid as a solvent to mix with the water. ‘Upon slight heating, our solvent dissolves the water out, leaving salts and impurities behind. Then, upon cooling, the mixture separates into two layers by gravity, releasing pure water. Unlike reverse osmosis, this method does not use expensive membranes and unlike evaporation processes, does not need heating to high temperatures,’ explains Chen. The process was shown to be effective at temperatures as low as 40 degrees Celsius and the recovered water met the salinity standards set by the World Health Organisation and the US Environmental Protection Agency.

Adel Sharif, an expert in water engineering and director of the Centre for Osmosis Research and Applications at the University of Surrey, UK, believes that further research is needed in areas such as scalability and practicality, but believes that the concept has promise. ‘The proposed desalination process has the potential for low environmental impact, since it uses low grade heat, and for low capital and operating costs,’ he says.

Chen believes that the work opens up a new field of research in desalination. ‘Being a simple, inexpensive process, directional solvent extraction also bears tremendous commercial potential in the desalination of seawater, clean-up of industrial waste water, treatment of water produced from oil and gas wells and other such uses,’ he concludes.

Rebecca Brodie

Read the Energy & Environmental Science article in full:

Very low temperature membrane-free desalination by directional solvent extraction
Anurag Bajpayee, Tengfei Luo, Andrew Muto and Gang Chen
Energy Environ. Sci., 2011, DOI: 10.1039/c1ee01027a

Plants may help to reduce carbon dioxide in the atmosphere when dead as well as alive, say scientists from Spain.

Carbon capture – the removal of CO₂ from waste gases (such as from power plants) – is an important method for reducing CO₂ emissions. One such strategy is the use of porous solids, such as zeolites and porous carbons, to absorb CO₂ into their pores. However, these materials exhibit poor uptake capacities (about 3mmol CO₂/g) and have complex and costly syntheses.

Now, Antonio Fuertes and his group at the National Institute of Carbon, Oviedo, have made a porous carbon material that performs better than other currently available ones, using a simple and inexpensive process. The major difference in this work, however, is that the raw material is sawdust.

The two step synthesis involves hydrothermal carbonisation of the sawdust, creating a hydrochar, which is then activated using potassium hydroxide. The KOH treatment creates pores in the sawdust structure by oxidation of carbon and carbon gasification from K₂CO₃ decomposition. These pores are responsible for the material’s uptake capabilities, bestowing it with a capacity as high as 4.8mmol CO₂/g. In addition, Fuertes’ material has good selectivity for CO₂ over N₂, fast adsorption rates and can be easily regenerated.

‘This type of carbonaceous material gives rise to an activated carbon that possesses textural properties that are appropriate for CO₂ capture,’ says Fuertes. ‘What’s more, the fabrication process is not complex and the raw material is abundant and widely available.’

Peter Styring, an expert in carbon capture technologies at the University of Sheffield says that the material has advantages over the currently most popular class of materials. ‘They’re comparable in terms of performance [to alkanolamines], but in terms of their engineering capabilities, these are superior,’ he explains. ‘With the alkanolamines, you get problems with corrosion, evaporation and degradation.’

Fuertes says that there is more work to be done before this technology can be commercialised, including investigations into scaling up. For now, he is focusing on other materials, such as nitrogen-doped carbons.

Yuandi Li

Read the Energy & Environmental Science article:
Sustainable porous carbons with a superior performance for CO2 capture
Marta Sevilla and Antonio B. Fuertes
Energy Environ. Sci., 2011, DOI: 10.1039/c0ee00784f

Scientists in China have made a reflective coating with a structure that mimics the underside of a poplar tree leaf. The coating could be used on the outside of buildings to counteract the heating effect of carbon dioxide emissions, reducing the energy needed to cool the building from the inside.

Yanlin Song and colleagues from the Chinese Academy of Sciences, Beijing, mimicked the structure of the leaf’s lower surface using polymers spun into reflective films consisting of long, hollow uniform fibres.

The underside of the poplar leaf is better at reflecting light than the top. This is because of the ‘cool roof’ effect, in which a layer of hairs on the underside reflects the light, so that less heat penetrates the leaf. The leaf turns over in strong sunlight to reveal the underside and as the light is being reflected rather than absorbed, the leaf appears white. ‘Normally, the poplar tree looks green, but sometimes in the summer, the tree shows a white cast,’ says Song.

The poplar leaf's hair structure (left) and the reflective coating with a magnified image showing one of the hollow fibres (right)

The team discovered that controlling the film thickness and making the cross section of the fibres as similar to the leaf hair as possible is the key to high reflectivity. They tested their films by coating them onto the compound diarylethene, which changes from red to colourless in the presence of visible light – the structure changes from a closed ring to an open ring. They found that the coating stopped the diarylethene changing colour, and had the additional benefit of being hydrophobic.

‘The reflectance and waterproof nature of the coatings make them ideal candidates for a number of building situations,’ says Robert Lamb, an expert on surface science. ‘Improving the durability of such delicate interfaces with the environment will be the major hurdle, but the alternative of sticking poplar leaves to our roofs to achieve the same effect is really not an option!’

Song says that his team will continue to develop highly reflective materials, widening the wavelength at which they function, to eventually be used to improve the efficiency of lighting.

Holly Sheahan

Read the journal article in full:

Highly reflective superhydrophobic white coating inspired by poplar leaf hairs toward an effective cool roof
Changqing Ye, Mingzhu Li, Junping Hu, Qunfeng Cheng, Lei Jiang and Yanlin Song
Energy Environ. Sci., 2011, DOI: 10.1039/c0ee00686f

UK scientists have analysed the chemistry taking place in storage ponds at nuclear power sites, such as Sellafield, to come up with a way to remove radioactive waste as nuclear regulatory bodies are pressing on the nuclear industry to clean up the ponds.

Storage ponds are used to store spent Magnox rods, which are uranium fuel rods covered by a magnesium-aluminium alloy cladding. The rods contain large amounts of fission products, which are highly reactive. The ponds are maintained to minimise corrosion of the rods, but the cladding corrodes in water, creating fine particle sludge. ‘The sludge in one of these ponds is estimated to contain tonnes of fuel debris including considerable quantities of plutonium,’ says Stephen Parry from the University of Manchester.

Parry, together with his colleagues, made a model of Magnox storage pond liquor to study how plutonium interacts with the corroded Magnox sludge to find a way of removing the plutonium before the ponds are emptied.

A magnified image of nuclear storage pond sludge showing brucite crystals, which sequester plutonium, making it difficult to remove from the mixture

‘One potential problem is the risk that disturbing the sludge will release fine, plutonium-containing particles in the effluent from the ponds. Pond effluents are treated before discharge into the sea under authorisation, but we need to be sure that the treatment process will effectively remove plutonium from the effluents before we can start to empty them,‘ explains Parry.

The team found that a low carbonate concentration, high CMS concentration and high polyelectrolyte concentration resulted in almost all of the plutonium being filtered.

Read the rest of the Chemistry World article…

Or view the Energy & Environmental Science paper:

Plutonium behaviour in nuclear fuel storage pond effluents
Stephen A. Parry, Luke O’Brien, Andy S. Fellerman, Christopher J. Eaves, Neil B. Milestone, Nicholas D. Bryan and Francis R. Livens
Energy Environ. Sci., 2011, DOI: 10.1039/c0ee00390e