Electrochemical etching mediated enhanced supercapacitor performance of a binder-free Ni/Co/Mo carbonate hydroxide electrode material

Abstract

Transition metal carbonate hydroxides (TMCHs) possess impressive theoretical capacitance, rapid ion transfer ability, and minimal volume expansion during long charge–discharge cycles. Increasing the electrochemical active surface area/porosity of the material is expected to enhance the charge storage ability. This report highlights how the electrochemical etching of intercalated molybdate (MoO₄²⁻) anions can improve the charge storage ability of TMCHs via the reconstruction process. Herein, we have directly grown MoO₄²⁻ intercalated bimetallic cobalt/nickel carbonate hydroxide (C_XN_1−XMO) materials onto a redox inactive carbon cloth (CC) substrate via a hydrothermal route. The electrochemical activation of these materials results in the formation of bimetallic hydroxide/(oxy) hydroxide species. Meanwhile, the intercalated MoO₄²⁻ions have etched away from the material by leaving the pores inside, which is expected to increase the charge storage ability. Mainly, the material developed from C₇₅N₂₅MO (denoted as C₇₅N₂₅MO-A) exhibits an impressive specific capacitance (C_sp) of 2039 F g⁻¹ at 1 A g⁻¹ current density with 71% capacitive retention (∼99% coulombic efficiency) up to 5000 cycles. Furthermore, to unlock the importance of etching in the charge storage process, C₇₅N₂₅O was separately synthesized without a Mo source, showcasing inferior charge storage ability. The meticulous mechanistic investigations manifest that the etching of intercalated MoO₄²⁻ anions enhances the diffusion phenomenon within the C₇₅N₂₅MO-A material, while the charge storage is primarily attributed to the surface redox process in activated C₇₅N₂₅O-A. In addition, the etching process facilitates a good balance between diffusion and surface contribution, which is beneficial for better C_sp. Finally, a hybrid device C₇₅N₂₅MO-A (+)//AC (−) was fabricated, manifesting a maximum 1.5 V operational voltage window with a 17.30 W h kg⁻¹ energy density at a power density of 1510.55 W kg⁻¹. Furthermore, the device can retain 88% of its initial capacity up to 10 000 cycles, demonstrating the suitability of the material for real-world use.