Enhanced stability of high energy aqueous capacitor based on redox-active nanomaterials and electrolyte

The incorporation of redox-active species can deliver high-energy-density aqueous electrochemical capacitors by simultaneously enhancing capacitance and voltage. Building on a previously reported double-redox capacitor combining a YP50F porous carbon functionalized with Bi 2 S 3 nanorods (NRs) as the negative electrode, a pristine YP50F positive electrode, and a 1M NaI electrolyte, in this work we investigated the effect of reducing Bi 2 S 3 particle size and increasing its loading. Incorporating 15 % wt. Bi 2 S 3 nanoparticles (NPs) delivered a capacitance of 235 F g −1 at 0.5 A g −1 and 193 F g −1 at 10 A g −1 , outperforming the device with a 10 % wt. NPs loading and the NRs-based hybrid at high rates. The energy output of the full cell (18.3 Wh kg −1 ) surpassed other aqueous devices using carbon-based or other bismuth-based anodes. Despite this improvement, the device lost 20 % of its initial capacitance after only 60 charge-discharge cycles. Electrolyte buffering and pre-iodination of the YP50F in the positive electrode improved the stability, yielding 100 % capacitance retention after 1000 cycles. Post-mortem ex situ XPS analysis revealed that these treatments suppress iodate/periodate formation and prevents oxidation of Bi-S species to sulfates, mitigating corrosion and precipitation, and securing long-term stability. • Redox-active species increase the energy density of aqueous capacitors. • Bi 2 S 3 nanoparticles and aqueous NaI solution create a double-redox capacitor. • The energy of the device is close to commercial capacitors with organic electrolytes. • Strategies to improve the device stability have been studied. • Post-mortem ex-situ XPS analyses explain the reasons behind the improved stability.