Environmentally Benign Humic Acid for Potassium-Ion Hybrid Capacitors

Potassium-ion hybrid capacitors (PIHCs), which are assembled by a capacitor-type electrode with a battery-type electrode, have stimulated great attention owing to their integrated advantages including high energy density and power output. Currently, most of the recorded PIHCs are based on inorganic components, which bring about environmental and resource problems. Herein, to circumvent these issues, humic acid (HA), which features resource abundance, cost-effectiveness, and environmental benignity, is explored as anode material for K+ storage. Benefitting from the abundance of oxygen-containing functional groups in the HA skeleton, HA presents both a satisfactory capacity (164.1 mAh g–1 at 20 mA g–1) and desirable cyclability (70.2 mAh g–1 after 1000 cycles at 50 mA g–1). Meanwhile, the storage mechanism analysis based on electrochemical technologies and ex situ FTIR spectrum reveals that the surface-driven capacitive mechanisms dominate the storage of K+ through the reversible transformation between C═O and C-O-K functionalities. Cooperating with activated carbon (AC) derived from pear wood charcoal, the fabricated hybrid capacitor (HA//AC) presents a remarkably high energy density and power density (79.1 Wh kg–1/186.94 kW kg–1). This work not only furnishes a new possibility to develop green organic materials for alkali metal-ion storage but also promotes the advancement of PIHCs.