Scientists claim that harmful algal blooms, or HABs, can be converted into a material which may be used as high-capacity and low-cost electrodes for sodium-ion, or Na-ion, batteries.

The researchers, led by environmental engineer Dr Da Deng at Wayne State University in Detroit, found this out after collecting some of the toxic HABs from Lake Erie’s seasonal algal bloom in August 2015. Calling it a “trash-to-treasure” approach, Deng says their findings could be promising for mitigating HABs to overcome their environmental threats and providing “green” electrodes for reversible sodium storage in Na-ion batteries.

In a study published in the journal of Environmental Science & Technology, the researchers explain that the conversion process addresses two problems at once. In addition to mitigating the HAB problem in freshwater lakes, the technology also provides a useful electrode material for Na-ion batteries which cost significantly less than lithium-ion, or Li-ion, batteries.

Harmful algal blooms, caused by cyanobacteria or blue-green algae, severely threaten humans, livestock and wildlife, which could lead to illness and even death, Deng said in an interview with Phys.org. “The Toledo water crisis in 2014 caused by HABs in Lake Erie is a vivid example of their powerful and destructive impact. The existing technologies to mitigate HABs are considered a passive technology and have certain limitations. It would significantly and broadly impact our society and environment if alternative technologies could be developed to convert the HABs into functional high-value products,” Deng explained.

Compared to Li-ion batteries, Na-ion battery technology is still in its infancy stage, Deng said. One of the challenges in developing Na-ion batteries is to find a reliable electrode material such as hard carbon. While hard carbon is most often derived from petroleum, it can also be made from biomass.

In the new study, the team shows that HABs can easily be converted into hard carbon by simple heat treatment, without the need for purification or other additional processes. For their investigation, the researchers heated the HABs at temperatures of 700 to 1000 degrees Celsius in argon gas. Afterwards, they made the electrodes out of a mixture of 80 per cent hard carbon derived from algae, 10 per cent carbon black and 10 per cent binder.

The team left this mixture to dry overnight before assembling it into coin cells with sodium foil as the counter electrode.

When put to a test, the electrodes started out with a high capacity of up to 440 milliamp hours per gram, or mAh/g, but suffered from an irreversible capacity loss after the first cycle, decreasing the capacity to about 230 mAh/g. However, the researchers found that the electrodes have good capacity retention after the second cycle and onward. They also observed that some performance factors, such as capacity and stability, depend on the temperature at which the algae was heated.

According to Deng, they will try to address the issue of first-cycle irreversible capacity loss in Na-ion batteries in their next research. He said the team is also interested in developing methods for the large-scale harvesting of HABs and studying their implications on the ecosystem.

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