Crab shells provide a cheap and convenient template to make high performance carbon electrodes for energy storage and conversion, say Chinese scientists. 

Carbon materials have many potential applications, including as electrodes in supercapacitors and fuel cells. The pore structure is known to affect their physicochemical properties and is normally controlled by using a porous hard template such as zeolite or silica. But the process usually involves using hydrofluoric acid to remove the templates, which can be complex and costly. 

A research group from Fudan University, led by Yong-Yao Xia, has demonstrated that crab shell has a well aligned porous structure at the microscopic level. Exploiting this unique structure, they have generated porous carbon nanofibre arrays by combining the hard crab shell template with an established soft templating method. 'Biological templates are generally abundant, renewable, inexpensive and environmentally benign compared to artificial templates,' explains Xia. 

After burning the crab shell in air, the porous template mainly consists of calcium carbonate. Adding a soft copolymer template and resol precursor forms the carbon framework. Heating under nitrogen gas removes the soft template and the hard template can be dissolved in hydrochloric acid. 

'The crab shell hard template is not only easy to remove but also hierarchically porous,' says Rui Zhang, an expert in porous carbon materials at the Shanghai Institute of Technology. The templated carbon nanofibre arrays retain this hierarchical porosity, forming pores of three sizes. The largest result from the packing of nanofibres, the medium pores from voids between the nanofibres and the carbon nanofibres themselves contain the smallest pores. 

The pore structure is suitable for charge storage by ion adsorption/desorption as an electrode material for supercapacitors or platinum/palladium catalyst loading for fuel cell applications, says Xia. Aided by the large surface area and complex structure, Xia' material shows excellent results in both cases. 

Xia's team is now using crab shell to template other porous materials as well as investigating alternative shellfish templates. 

Erica Wise 

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