A team at the University of Kentucky has converted the leftover waste from the bourbon distillation process into electrodes for supercapacitors that could store more energy than similarly sized commercial devices. The work, presented at the American Chemical Society’s meeting in Atlanta, Georgia on 24 March, is not yet published and is under review at a journal.
The project, which began under the leadership of environmental chemist Marcelo Guzman in the summer of 2000, used hydrothermal carbonisation – a process in which biomass is heated in aqueous solution to high temperatures under elevated pressure, to convert this distillery waste into carbon materials, which make good electrodes for energy storage devices.
‘In our process, the pressure and the water act as a reaction media … and that helps in the process of transformation into carbon materials,’ explains Josiel Barrios Cossio, a PhD student at the University of Kentucky who joined Guzman’s team after to moving to the US from Cuba in 2021.
For a proof-of-concept, the researchers made double-layer capacitors by sandwiching a liquid electrolyte between activated carbon electrodes made from bourbon waste. Their analysis indicated these coin-sized supercapacitors could store up to 48 watt hours per kilogram, which is similar to the performance of currently available devices.
The bourbon-making process begins with a blend of grains – to be considered ‘bourbon’ a whiskey must be at least 51% corn – then that mixture gets grinded, cooked, fermented and is finally distilled. When that process is over, a liquid leftover remains that is known as stillage. The mixture contains about 10% solids, and Barrios Cossio describes it as a yellow or brown sloppy ‘suspension’. In general, he says, the final volume of bourbon produced is six to 10 times that amount of stillage waste.
The number of bourbon distilleries has grown exponentially in Kentucky over the last 15 years. Data from the Kentucky Distillers’ Association shows there were 19 such establishments in the state in 2010, and current estimates indicate that number had grown to 125 by February of this year. As a chemistry student in Kentucky, Barrios Cossio says it is impossible not to notice the industry’s presence and culture in the state.
Kentucky distilleries typically sell their stillage to farmers as a livestock feed or a soil additive. But the explosion of bourbon distilleries across Kentucky has meant that this byproduct has significantly surpassed demand from farmers in the state.
To create their proof-of-concept device, Guzman’s team worked with several local bourbon distilleries. The researchers conducted hydrothermal carbonisation on stillage samples, and then performed the various heating and chemical processes to obtain activated carbon, which has a porous structure that makes it good as an electrode.
Through a separate partnership with a group at the Friedrich Schiller University in Germany, the researchers have also experimented with hybrid lithium-ion supercapacitors. ‘Together we built devices with one capacitor-type activated carbon electrode and one battery-type hard carbon electrode, which were both infused with lithium ions,’ Cossio tells Chemistry World. These stillage-derived supercapacitors stored between five and 25 times the energy per kilogram as conventional versions, he says.
‘Nobody had ever done this with bourbon stillage before, as far as I know,’ Cossio adds. He suggests that these findings have applications for the entire whiskey industry in the US, as well as for breweries and the nation’s ethanol production industry.
The group continues to work with local distilleries to expand the applications of its findings. The researchers also plan to study the energy storage mechanisms of their stillage-derived supercapacitors so that they can be scaled up for commercialisation. They also intend to pursue a life cycle analysis, as well as economic and technological feasibility studies to assess the sustainability of their concept.
‘Developing and optimising that strategy is critical for those industries to grow and thrive, because at some point you have to figure out what to do with the waste,’ Cossio states.
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