Cation-exchange and oxygen vacancies triggered capacity in hierarchical α-Ni1−xCuxMoO4@CC flexible electrodes for energy-storage applications

Abstract

Development of hierarchical nano/microarchitectures and cation-doping strategies to enhance the electrochemical activity of supercapacitors has been well established. However, generation of oxygen vacancies (OVs) during the metal cation-exchange process could lead to the intrinsic modification of electrochemistry while supplying more electro-active sites for better electrochemical results. Understanding of the concept of OVs and their influence on electrochemistry is rare but more significant. Herein, we prepared α-NiMoO₄@carbon cloth (CC) and α-Ni_1−xCu_xMoO₄@CC (x = 1, 3, 5, and 7 mol%) flexible electrodes by a facile hydrothermal method. Without any calcination, the pristine and Cu-containing flexible electrodes preserved their pure phase forms, and a comparative electrochemical study was carried out. During the metal cation-exchange process (Ni is replaced by Cu), the host lattice was distorted, and OVs were generated, which helped to minimize the electrochemical barriers and obtain plenty of electroactive sites. Utilizing the hierarchical urchin-like morphology, the synergistic effects of foreign Cu, other composite metal atoms, and developed OVs, the α-Ni_0.95Cu_0.05MoO₄@CC electrode demonstrated a higher specific capacity of ∼342 C g⁻¹ at 1 A g⁻¹ and better rate capability (∼58%) than their counterparts. Besides, a flexible hybrid supercapacitor (FHS) device (α-Ni_0.95Cu_0.05MoO₄@CC//activated carbon@CC) was assembled, and it delivered a superior capacitance of ∼299 F g⁻¹ and remarkable cyclability (91% retention after 10 000 cycles). Furthermore, the FHS device showed maximum energy density (83 W h kg⁻¹) and power density (10.5 kW kg⁻¹) and was successfully tested in portable electronics for energy-storage applications.