Ultrathin nanosheets of Ni-Co-Fe layered double hydroxide as a novel binder-free electrode for fabrication of a high-energy and flexible asymmetric supercapacitor and water splitting

Discovering efficient energy conversion and storage technologies has become one of the grand challenges in an era where energy plays an essential role in our everyday lives. The demand for renewable and clean energy from the sun and the wind is rapidly increasing. However, their intermittent nature necessitates the development of efficient energy storage devices that can deliver energy at any time and place.[1] Batteries and supercapacitors are the two major energy storage devices, and the direct storage of solar energy into the chemical bonds (H2 + O2) is another potential solution. Supercapacitors with high power density, superior rate capability, rapid charging/discharging rate, long cycle life ( 100,000 cycles), and low maintenance costs have attracted a great deal of attention from both industry and academia.[2,3] On the other hand, the electrochemical splitting of water into hydrogen and oxygen is one of the most promising approaches for converting the energy harvested from renewable sources into clean fuels.[4] Therefore, the development of multifunctional energy harvesting and storage devices in which supercapacitive energy storage and electro-catalytic water splitting are integrated into a single device is of considerable interest. In this study we propose a fast, simple and scalable approach for the synthesis of Ni-Co-Fe layered double hydroxide (LDH) mesoporous nanoplatelets on a Ni foam substrate. Ni-Co-Fe LDH is a promising active electrode material for both electrochemical energy storage and electro-catalytic oxygen evolution reaction (OER). A supercapacitor based on Ni-Co-Fe LDH exhibits a high specific capacitance of 2460 F g-1 (2460 mF cm−2) at a discharge current density of 1.0 A g-1 (1.0 mA cm−2), with an excellent rate capability. An asymmetric supercapacitor based on Ni-Co-Fe LDH as the positive electrode and activated carbon as the negative electrode displays an operating potential window of 1.5 V, excellent cycle life, an ultrahigh specific energy of 57.5 Wh kg−1 and outstanding specific power of 14.95 kW kg-1. As an OER electro-catalyst, Ni-Co-Fe LDH exhibits remarkable performance with a low overpotential of 207 mV to afford 10 mA cm-2, much better than that of the most previously reported electro-catalysts. This active electrode material based on earth-abundant species opens new prospects for the design and fabrication of highly efficient multi-functional energy storage and conversion systems.