Highly Concentrated, Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices

Solution-based techniques are considered as a promising strategy for scalable fabrication of flexible electronics owing to their low-cost and high processing speed. The key to the success of these techniques is dominated by the ink formulation of active nanomaterials. This work successfully prepares a highly concentrated two dimensional (2D) crystal ink comprised of ultrathin nickel hydroxide (Ni(OH)₂) nanosheets with an average lateral size of 34 nm. The maximum concentration of Ni(OH)₂ nanosheets in water without adding any additives reaches as high as 50 mg mL⁻¹, which can be printed on arbitrary substrates to form Ni(OH)₂ thin films. As a proof-of-concept application, Ni(OH)₂ nanosheet ink is coated on commercialized carbon fiber yarns to fabricate wearable energy storage devices. The thus-fabricated hybrid supercapacitors exhibit excellent flexibility with a capacitance retention of 96% after 5000 bending–unbending cycles, and good weavability with a high volumetric capacitance of 36.3 F cm⁻³ at a current density of 0.4 A cm⁻³, and an energy density of 11.3 mWh cm⁻³ at a power density of 0.3 W cm⁻³. As a demonstration of practical application, a red light emitting diode can be lighted up by three hybrid devices connected in series.