Oxygen vacancies meet partial S substitution: an effective strategy to achieve obvious synergistic effects and adjustable electrochemical behavior in NiFe-LDH for enhanced OER and capacitive performance

Abstract

A bifunctional electrode (oxygen evolution reaction (OER) and capacitive properties) with high electrochemical activity is crucial for both supercapacitors and water splitting devices. Herein, a S and oxygen vacancy co-doped nickel-iron layered double hydroxide (NiFe-LDHS) electrode was delicately fabricated through combining acid etching and sulfidation strategies. The strong synergy between S and oxygen vacancies was firmly confirmed in this work, which increased the intrinsic activity of the electrode via a double internal polarization field path. Furthermore, the impact of S doping on the bifunctionality of the electrode was systematically studied. The characterization and measurement results demonstrated that the sample after S doping exhibited adjustable electrochemical behavior, thereby improving the bifunctional activity of the electrode further. Thus, the NiFe-LDHS electrode showed outstanding catalytic and energy storage properties. Specifically, the OER overpotential was only 224 mV at 10 mA cm⁻², and the stability in an alkaline electrolyte could reach 32 h at 50 mA cm⁻². In terms of capacitive performance, the NiFe-LDHS electrode showed a high areal capacitance of 6.33 F cm⁻². The assembled NiFe-LDHS//AC asymmetric supercapacitor (ASC) with 2.0 V cell voltage displayed a high energy density of 0.279 mW h cm⁻² and long cycling stability. These results indicated the versatile applicability of the NiFe-LDHS electrode in both water splitting devices and supercapacitors and revealed promising prospects for advancing the construction of high-performance bifunctional electrode materials.