Glowing battery

The fluorescent dye BODIPY, boron-dipyrromethene, could be the ideal energy-storage material for rechargeable, liquid-based batteries according to researchers at the University at Buffalo. The compound has an unusually high capacity for storing electrons and participating in electron transfer processes, two characteristics of high-performance batteries that might be used in mobile devices, vehicles and even our homes.

"As the world becomes more reliant on alternative energy sources, one of the huge questions we have is, 'How do we store energy?' What happens when the sun goes down at night, or when the wind stops?" asks Timothy Cook. "All these energy sources are intermittent, so we need batteries that can store enough energy to power the average house." The team describes details of the dye-based battery of the future in the journal ChemSusChem [Cook et al. ChemSusChem (2016); DOI: 10.1002/cssc.201601104].

The fire risk associated with lithium-ion batteries is a perennial concern despite advances, there are repeated widespread incidents involving laptop computers and mobile phones that employ this power source. A dye-based battery would not be inflammatory if the casing is damaged, the contents would simply leak out as the two active species are held in separate reservoirs. Moreover, conventional batteries have severe energy-storage limitations. A redox flow battery however, could simply be made bigger to store more energy. For instance, a homeowner with solar panels on their roof could charge up a battery and release the electricity after nightfall. Similar, a utility company could "stockpile" wind energy for peak usage times. Scaling up lithium-ion batteries is plausible but not entirely feasible in terms of economics and safety.

A redox flow battery's effectiveness depends on the chemical properties of the fluids in each of its two tanks. "The library of molecules used in redox flow batteries is currently small but is expected to grow significantly in coming years," Cook explains. "Our research identifies BODIPY as a promising candidate." The team's experiments used a powdered BODIPY dye called PM 567 dissolved in liquid. A test battery was capable of going through its charge-discharge cycle 100 times without failing giving up to 2.3 volts, which would be sufficient for recharging portable gadgets as well as powering LED lighting. There are many other BODIPY dyes that might have greater still longevity and the potential to produce a higher voltage. The team saw voltage losses because they were using a test battery in a laboratory setting rather than a fully engineered device. "Once proper charge carriers are identified, optimization can take place to ensure that this maximum voltage is achieved," Cook adds.

"The next step is to continue to develop new and better charge carriers based on a few key factors," Cook told Materials Today. For instance, the team is using self-assembly techniques to construct molecules that can store more than one electron each, to increase the storage capacity of flow batteries even more and also focusing on making their molecules more soluble. "All increases to concentration mean that smaller volumes can be used," he adds. Another improvement will come from making the molecules large so that they cannot cross the membrane separator and so do not mix between the two sides of the battery.

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