Toward low cost grid scale energy storage: supercapacitors based on up-cycled industrial mill scale waste

Mill scale is a waste product from the steel industry available cheaply in tonne quantities and consisting of various iron oxides. The supercapacitive behaviour of mill scale directly from the steel plant, and after various cheap and scalable physical and chemical treatments, has been studied in electrodes formed by spraying mill-scale containing suspensions onto large area current collectors. Half-cell and full cell supercapacitors in cheap, non-toxic aqueous sodium sulphite electrolyte were investigated by cyclic voltammetry, charge-discharge and electrochemical impedance spectroscopy, and delivered a capacitance of up to 92 F g-1 at a scan rate of 5 mV s-1, which was maintained at more than 80% after 5,000 cycles. The approximate costs of commercial and mill scale-based supercapacitors were compared, and showed that while mill scale absolute capacitances were lower than more expensive laboratory synthesised metal oxides, the cost per kilo-watt performance was competitive, especially for very large grid scale storage applications.