New aqueous energy storage devices comprising graphite cathodes, MXene anodes and concentrated sulfuric acid solutions

The newly emerging demand for ‘beyond-lithium’ electrochemical energy storage systems necessitates the development of alternative options in providing sustainable cost-effective storage capabilities. In pursuit of discovering such a solution, the intercalation of bisulfate anions into graphite in 17 ​M ​H₂SO₄ solutions has been revaluated. Although the insertion process of bisulfate into graphite was extensively studied many years ago, only poor electrochemical performance has been demonstrated. In this work, we discovered the superior performance of the graphite bisulfate system, associated with the electrodes’ fabrication method which presents a high energy density of more than 80 ​mW ​h/g and a surprising rate capability (75 ​mW ​h/g was obtained at 15 ​C) alongside impressive long-term stability of more than 1500 cycles with only 5% capacity fading. Potentiostatic intermittent titration technique followed by slow-scan-rate cyclic voltammetry (SSCV) was used to shed light on the bisulfate intercalation process. Combining the bisulfate intercalation into the graphite with a highly reversible proton insertion process into Ti₃C₂ MXene in such a concentrated acidic environment allows the development of a dual-ion device composed of graphite positive electrode (cathode) and MXene negative electrode (anode). This asymmetric system shows a high energy density of 35 ​mW ​h/g, good cyclability and an extended potential window of 1.5 ​V, demonstrating new opportunities for further developments of intercalation electrodes for large energy storage.