关键词: Zinc-ion capacitors, Carbon materials, Activated carbon, Methylene blue, Redox-active electrolyte

Abstract: Zinc-ion capacitors have gained significant interest because of the exceptional capacity of zinc, the safety of aqueous electrolytes, and the low cost of carbon materials. However, the low capacity of carbon materials greatly limits the energy density of the capacitors. Structural modification or introduction of heteroatom doping to carbon materials sacrifices their volumetric density and cycling stability. Here, we introduce soluble methylene blue (MB) into the electrolyte of the carbon electrode to enhance the capacity by utilizing its redox reactions at the interface. Adding MB increases the electrode capacity to 118 mA·h·g^-1, 3.8 times higher compared to that without MB. Importantly, AC electrode with MB delivers an ultrahigh cycling stability with a retention of 90% after 12000 cycles. In situ and ex situ characterization indicates that MB undergoes reversible structural changes during the processes. Theoretical calculations further demonstrate that the reaction of MB discharging process involves two electrons and two protons, going through a radical intermediate state. Assembled Zn//AC capacitor with MB achieves a maximum capacity of 137 mA·h·g^-1, 3.4 times higher compared to the capacitor without MB. Additionally, the capacitor with MB exhibits an energy density of 105.8 W·h·kg^-1, three times higher than that without MB. Moreover, the capacitor exhibits outstanding cycling stability, retaining 92% of the capacity even after 8000 cycles. Our results demonstrate that MB can effectively promote the energy density of the capacitor without sacrificing its cycling stability.

Key words: Zinc-ion capacitors, Carbon materials, Activated carbon, Methylene blue, Redox-active electrolyte