For more than a century, generating electricity was an inherently polluting exercise, involving the burning of coal and gas to heat water, which in turn, powers steam turbines. In more recent decades, we’ve seen a slow transition away from fossil fuels and toward cleaner modes of electricity generation, such as hydro, wind and solar power. Energy harvesting technologies, which convert various forms of ambient or ‘waste’ energy into electricity, are increasingly playing a role too. In a paper published in Nano Energy [DOI: 10.1016/j.nanoen.2022.107378], Chinese researchers report on their latest contribution to this effort – an evaporation-driven generator made from sugarcane which, they say, greatly outperforms similar devices in the literature.
Pristine sugarcane is a very porous material, with a structure that comprises bundles of vertical, open microchannels – some 100 µm-wide and others 20 µm-wide – aligned along its length. The inner surface of these microchannels is rich in hydroxyl groups, making them hydrophilic. This combination of chemical and structural properties allows water and nutrients to travel throughout the plant via highly-efficient capillary action. It’s also the basis for the generator proposed by the team from Yangzhou University. They started by removing the sugar from inside the cane, by immersing small pieces of it in deionized water. After freeze-drying, the canes were immersed in a heated solution of citric acid to enhance the hydrophilicity of their microchannels. The authors note that their “chemically modified sugarcane exhibits about 7-fold faster water absorption when compared to the sugarcane that is not chemically treated.” The canes then underwent another freeze-drying step. This removed any remaining water, and slightly reduced the diameter of the larger microchannels. To complete the generator, two stainless steel mesh electrodes were attached to the cane – one each on the upper and lower surfaces – using waterproof tape. The sugarcane-based evaporative-driven generator (SEDG) was then immersed in various solutions, and the open-circuit potential (VOC) measured. The source of this electrical output is an electrokinetic potential generated by asymmetric wetting due to water evaporation. In deionized water, the SEDG had a maximum VOC of 322 mV; five times higher than that measured for unmodified sugarcane. However, both VOC and the short-circuit current were higher in alkaline and acidic solutions. The SEDG displayed its optimal output (VOC = 470 mV, or ~38 % higher) when immersed in seawater. This is about twice the output of an equivalent wood-based generator. The authors write that this use of seawater as a “…feedstock…cannot be achieved using other systems reported in the literature.” The SEDG was also found to be fairly stable over time – even after five days of immersion in seawater, no obvious structural changes occurred within the cane. By way of a proof-of-concept, the team connected three generators in series, producing an output of 1020 mV. In addition, a five-SEDG array was used to charge a capacitor, which in turn was used to successfully power a household humidity and temperature sensor. — Haitao Li, Xuan Li, Xiangming Li, Huan Wang, Jiangchao Huang, Siew Kheng Boong, Hiang Kwee Lee, Jie Han, Rong Guo. “Utilizing sugarcane as green transpiration-driven generator for efficient electricity harvesting from seawater,” Nano Energy 99 (2022) 107378. DOI: 10.1016/j.nanoen.2022.107378