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 behavior 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, nontoxic 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 5000 cycles. The approximate costs of commercial and mill scale-based supercapacitors were compared, and showed that although mill scale laboratory synthesized metal oxides, the cost per kilo-watt performance can be competitive, especially for very large grid scale storage applications.