Tailored metallacarboranes as mediators for boosting the stability of carbon-based aqueous supercapacitors

Expanding the operating voltage of aqueous-based electrolytes by using neutral electrolytes and advanced cell designs is a promising strategy for the development of greener and safer supercapacitors. However, solvent decomposition and the oxidation of carbon electrodes are issues that still need to be resolved. Herein, we propose a novel protection strategy for stabilizing aqueous electrolytes at high voltages by using metallacarboranes with tuned redox potentials specifically selected for matching those of the electrolyte decomposition. Such metallacarboranes are strongly adsorbed in the micropores of conventional activated carbons without compromising their capacitance or their power capabilities. As a proof of concept, supercapacitors with optimized electrode weight ratios in 0.5 M Na2SO4 were constructed using a highly stable commercial activated carbon with the aim of operating them at 2.2 V. While this device malfunctioned after several hundreds of cycles, the addition of small amounts of the Na[Co(C2B9Cl2H9)2] metallacarborane (redox pair at −0.98 V vs. Ag/AgCl) dramatically increased its durability. The supercapacitor prepared using 0.15 mmol g−1 of Na[Co(C2B9Cl2H9)2] retained 80% of its original capacitance and an energy density of 10.67 W h kg−1 at 1 kW kg−1 after 5000 cycles at 2.2 V. This strategy has the potential to be extended to different electrolytes, enabling the development of more durable supercapacitors that operate at voltages close to those of organic electrolytes while using safer and greener aqueous electrolytes.