Cobalt Cyclotetraphosphate (Co₂P₄O₁₂): A New High-Performance Electrode Material for Supercapacitors

Recently, transition metal phosphates (TMPs) are demonstrated as promising candidates for supercapacitors due to their good conductivity and long-term stability. Herein, we have inrtoduced cobalt cyclotetraphosphate (Co₂P₄O₁₂) as novel supercapacitive electrode material, which deliver a capacitance of 437 F/g with good stability over 3000 cycles (around 90%). To display the practical relevance, we have fabricated asymmetric device using activated carbon and Co₂P₄O₁₂ as negative and positive electrodes, respecitvely, which can be operated up to 1.4 V. The introduction of redox-active moieties (such as potassium iodide (KI)) in parent KOH solution enhances the capacitance from 120 F/g to 156 F/g for Co₂P₄O₁₂//AC cell with extended voltage window (1.8 V). Owing to the additional voltage and advancement in the capacitance, the cell delivers battery-level energy of 70 Wh/kg at power density of 2.3 kW/kg. It should be emphasized that even at high power (7.6 kW/kg), the cell maintained very good specific energy of 48 Wh/kg, suggesting excellent rate performance. The asymmetric device further showed long cycling stability with negligible loss and excellent Coulombic efficiency over 5000 cycles. Thus, metal cyclotetraphosphate can be employed as a promising electrode material in energy storage systems.