Sustainable seaweed for supercapacitors

Chemicals extracted from edible seaweed, algae, could be used as precursors for components of high-performance, carbon-based superconductors, lithium-ion batteries and fuel cells, according to research reported at the American Chemical Society national meeting recently.

"Carbon-based materials are the most versatile materials used in the field of energy storage and conversion," suggests Dongjiang Yang of Qingdao University, China. "We wanted to produce carbon-based materials via a really green pathway. Given the renewability of seaweed, we chose seaweed extract as a precursor and template to synthesize hierarchical porous carbon materials." Yang explains that the research could lead to sustainable alternatives for a wide range of applications in energy storage and catalysis, for instance, that side-steps the need for rare elements, such as precious metals.

Traditional carbon materials, such as graphite, have been essential to creating the current energy landscape. But to make the leap to the next generation of lithium-ion batteries and other storage devices, an even better material is needed, preferably one that can be sustainably sourced, Yang says.

Seaweed is abundant across the oceans and Yang, while at Griffith University in Australia, reasoned that it could be a useful and sustainable source of materials. He worked with colleagues at Qingdao University and at Los Alamos National Laboratory in the USA to make porous carbon nanofibers from a seaweed extract. Chelating cobalt ions to the alginate molecules led to nanofibers with what he describes as an "egg-box" structure. This is key to the material's stability and how the team could control synthesis of the materials, Yang explains.

In preliminary tests, the researchers demonstrated how the seaweed-derived material has a large reversible capacity of 625 milliampere hours per gram (mAhg-1), which is considerably more than the 372 mAhg-1 capacity of traditional graphite anodes for lithium-ion batteries. Such a capacity might extend the range of future electric cars if the cathode counterpoint material can be developed to have equivalent quality. The team also showed that their egg-box fibers could function as well as commercial platinum-based catalysts in a fuel cell and have better long-term stability. In addition, the same materials have high capacitance as superconductor materials at 197 Farads per gram, which could be applied in zinc-air batteries and supercapacitors.

Initial work carried out in 2015 has been extended significantly and building on the same egg-box structure, the team has managed to suppress defects in their cathodes for lithium-ion batteries that would otherwise block the flow of lithium ions. Recently, they have also developed an approach using red algae-derived carrageenan and iron to make a porous sulfur-doped carbon aerogel with an ultra-high surface area. The structure could be a good candidate to use in lithium-sulfur batteries and supercapacitors. The team's work has now evolved into a general strategy based on the "egg-box" structure for fabricating transition metal oxides microfibers with different architectures, Yang says.

"We plan to fabricate high performance energy materials directly derived from seaweed, such as Laminaria japonica (brown alga), Eucheuma (red alga), and Enteromorpha prolifera (green alga)," Yang told Materials Today. "We also plan to develop full algal energy storage devices, such as batteries and supercapacitors, composed of seaweed-based electrodes, seaweed separator membranes or seaweed macromolecular solid electrolyte."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".