Microstructure optimization and electrochemical behavior of in-situ growth Ramsdellite-MnO2@NCA-LDH@CC for supercapacitors and oxygen evolution reaction catalysts

Supercapacitors are electrochemical energy storage devices with great potential applications. Meanwhile, the oxygen evolution reaction (OER) determines the efficiency of some electrochemical energy conversions. This study aims at constructing, exploring, and optimizing Ramsdellite-MnO2@NiCoAl-LDH@CC (R-MNCA@CC) composites. The effect of microstructure and Al role on the performance is investigated when R-MNCA@CC was used as supercapacitor electrode material and OER catalyst. Coral-like R-MNCA@CC in-situ growth composites were synthesized by a two-step hydrothermal method. R-MNCA@CC-2 (molar ratio of Ni:Co:Al is 1:1:1) performs the best with the largest specific capacitance, 1,742 F/g at 1 A/g, increased by 797% and 1,489% compared to that of NiCoAl-LDH and Ramsdellite-MnO2. The capacitance retention rate of the R-MNCA@CC-2//AC@CC supercapacitor is 80.1% after 5,000 cycles at 0.8 A/g. The overpotential for driving an OER to reach 10 m/cm2 is only 276 mV, which is lower than that of commercial IrO2 (300 mV). Noteworthy, we propose a view that is "competing to trigger redox reaction" of electrochemical active sites in LDH during electrochemical processes derived from a discrepancy between theory and experimental results.