Controlled Synthesis Of Porous Nickel-based Electrodes For Electrochemical Energy Storage

Electrode materials play a key role in affecting the overall performances of supercapacitor and water splitting. At present, the electrode materials are still limited by its poor electrical conductivity, small specific surface area and poor structure stability, resulting in insufficient energy density and stability for supercapacitor and large voltage and low efficiency for water splitting, which still cannot meet the requirements of practical production and hinder the further development of supercapacitor and hydrogen production by electrolysis of water. Herein, several porous nickel-based materials were successfully prepared through a simple electrochemical deposition. The as-prepared electrodes with large specific surface, good electrical conductivity and stably porous structure, are expected to replace traditional carbon-based materials to become very promising electrode materials. The electrochemical test results show that the porous nickel-base electrode materials exhibited high specific capacitance, fast charge-discharge capability and performance stability. Therefore, the assembled asymmetric supercapacitors displayed improved energy densities with favorable power densities and long-term stability. In addition, the porous nickel-base electrodes also exhibited excellent catalytic performance for the electrolysis of hydrogen and oxygen evolution reactions. Particularly, the assembled overall water splitting device could effectively reduce the electrolysis voltage. Detailed works are displayed as follows:?1? A new polypyrrole shell@3D-Ni core structured electrode was successfully prepared to solve the defects of polypyrrole, such as the low real capacitance and unstable structure. In this rational design electrode, not only nickel metal effectively improve the conductivity of polypyrrole, but 3D porous structure can be conducive to the accumulation and rapid transport of electrolyte ions, which can effectively improve the specific capacitance and cycle stability of polypyrrole. Moreover, the symmetric supercapacitor based on the PPy@3D-Ni electrode also exhibited a high specific energy?21.2 Wh kg-1? and outstanding cycling life.?2? Different from polypyrrole depositing on 3D Ni as film structure, NixCo2x?OH?6x as layered nanosheets decorating on dual 3D Ni frameworks was constructed, in which Nix Co2x?OH?6x shows layered structure morphology with numbers of nanosheets in favour of ion diffusion and largely diminishing “dead mass” of the active materials. Consequently, Nix Co2x?OH?6x@Ni exhibited excellent electrochemical performance for supercapacitor with high specific capacitance, high-current capacitive behaviour and excellent cycling stability. Meanwhile, the assembled asymmetric supercapacitor further showed impressive energy density?44.2 Wh kg-1? and outstanding long-term stability. Furthermore, the Ni_xCo_2x?OH?_6x@Ni also exhibited superior OER electrocatalytic performance with small onset overpotential, low Tafel slope and prominent electrochemical durability.?3? 3D chuzzle-like Ni@PPy@Mn O2 and 3D cochleate-like Ni@MnO₂@PPy based on 3D Ni metal framework were prepared by simply changing the order of electrodeposition. As expected, both the two electrodes exhibited excellent electrochemical performance within different stable voltage window in a neutral aqueous electrolyte. Notably, Ni@MnO₂@PPy cathode had a more negative voltage window from-0.7 V to +0.1 V?vs. SCE?. Furthermore, an asymmetric supercapacitor was firstly assembled by serving Ni@PPy@MnO₂ as positive electrode and Ni@MnO₂@PPy as negative electrode, which had a large work voltage of 1.3¹.5 V and achieved an energy density as high as 59.8 Wh kg-1.?4? Porous perchlorate-doped PPy was controllably grown on nickel nanotube arrays?NiNTAs@PPy?. A bottom-up synthesis of supercapacitor electrodes from molecular level?perchlorate doping? to nanometer scale?porous wave-superpositional PPy nanocoating? to micron level?free-standing Ni nanotube array film? was carried out to obtain remarkable pseudo-capacitance and enhanced cycling stability as well as widened and stable potential window up to 1.5 V. The symmetric and asymmetric supercapacitors were assembled based on NiNTAs@PPy film, both of which afforded the maximal specific energy density about 50.0 Wh kg-1.?5? A simple and readily scalable method for the fabrication of new 3D porous Ni/NiP and Ni/NiS electrocatalytic electrodes was developed and demonstrated. The formation of surface oxides/hydroxides on phosphides/sulfides is believed to be contributed to their bifunctional catalytic activity. In addition, the free-standing 3D architecture, strong physical integration, rapid charge transfer capability and structure exerting electronic effect collectively are responsible for highly active and stable OER and HER catalytic activity of these new electrodes. These attributes led to excellent overall water splitting performance requiring a cell voltage of 1.61 V to deliver a 10 mA cm-2 and achieving nearly 100 % Faradaic efficiency.