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 the integrated advantages including both high power density and energy density. Currently, most of the recorded PIHCs are based on inorganic components, which brings 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 potassium ion storage. Benefitting from the abundance of oxygen contained functional groups in HA skeleton, HA presents a satisfactory capacity (164.1 mAh g-1 at 20 mA g-1 ) and a desirable cycling stability (70.2 mAh g-1 at 50 mA g-1 for 1000 cycles). Meanwhile, the storage mechanism analysis based on electrochemical technologies and ex-situ FTIR spectrum reveals that the surface-controlled capacitive behavior plays a dominant role through the reversible transformation between C=O and C-O-K functionalities. Cooperating with activated carbon (AC) derived from HA, the fabricated hybrid capacitor (HA//AC) presents a remarkably high energy and power density (79.1 Wh kg-1 /186.94 kW kg-1 ). This work provides a new possibility for the development of green organic materials for alkali metal ion storage, and promotes the advancement of PIHCs.