Structural Design of Electrocatalytic Electrodes of Zn-Ni-S/Ni-Co-P Microspheres to Improve Stability and Activity for Supercapacitors and Water Splitting

Abstract

Transition-metal sulfides (TMSs) and transition-metal phosphites (TMPOs) have been recommended for promising electrocatalytic materials used for supercapacitors (SCs) and water splitting. Nevertheless, the application of only TMSs or TMPOs is limited due to their slow reaction kinetics and drastic volume expansion during electrochemical processes. Designing a stable structure by combining TMSs with TMPOs was expected to improve their performance. A Zn-Ni-S/Ni-Co-P (ZNS/NCP) composite was synthesized by a hydrothermal method. SEM, XRD, XPS, BET, and electrochemical tests were used to study the morphologies, phases, capacitive performance, and electrocatalytic activities. It is shown that ZNS/NCP composites were composed of NiS33.3Zn32.3, Co11(HPO3)8(OH)6, and Ni11(HPO3)8(OH)6 with a microsphere structure, were closely bound with nickel substrates, and had a large specific surface area, which can provide abundant active sites and shorten the electron/ion diffusion distance. The specific capacitance of the ZNS/NCP electrode was 6.94F/cm2 at 8 mA/cm2 and had an excellent cycle stability. The asymmetric supercapacitor assembled with Fe2O3-IrO2 as an anode and ZNS/NCP as a cathode had a power density of 5.95 mW/cm2, with an energy density of 0.52 mWh/cm2. It also had an excellent stability. The overpotential values of ZNS/NCP, ZNS, NCP, and NF were 126, 185, 154, and 324 mV for HER and 230, 270, 330, and 470 mV for OER at 10 mA/cm2, respectively. If the ZNS/NCP electrode was used both as a cathode and anode for water splitting, an excellent performance could be obtained. The required voltages for ZNS/NCP||ZNS/NCP, ZNS||ZNS, NCP||NCP, and NF||NF were 1.46, 1.68, 1.66, and 1.81 V at 10 mA/cm2, respectively.