Controlled Synthesis Of Nickel Ferrite Transition Bimetallic Oxide And Its Pseudocapacitive Property

In the era of a fast growing global economy,the ever-increasing consumption of fossil fuels and the associated environmental deterioration have stimulated an urgent and strong demand for green and renewable energy sources,such as solar and wind power.These renewable energy sources are intermittent,and require the help of energy storage devices to regulate the power fluctuations for smart grid applications.In addition,the exploding markets for portable electronics and electric vehicles have also attracted tremendous interest in developing various energy storage devices as the power supply.Among the emerging energy storage technologies,supercapacitors（SCs）,owing to their high power density,long cycling life and fast recharge capability,has become one of most promising candidates for efficient energy storage devices,and attracted extensive attention in recent years.The capacitance and charge storage of SCs are closely related to electrode materials,so the research and development of electrode materials with high capacitance is the current research hotspot.Transition metal oxides are emerging as promising electrode materials for SCs due to their high theoretical capacitance and abundance in nature.Metal oxides such as MnO₂,NiO,Co₃O₄ and Fe₂O₃ with high surface areas have been intensely used as pseudocapacitive electrode materials.However,some single oxides suffer from low electrical conductivity and poor cycling performance due to their agglomeration after several hundred cycles and phase changes.Recently multi cation oxides have become more attractive as a pseudocapacitive material,compared with the single oxides,due to their high electrical conductivity and excellent electrochemical performance.The mixed binary transition metal oxide with spinel structure（AB₂O₄）is emerging as a new class of pseudocapacitive materials for high performance SCs.In AB₂O₄,A,B and O represent two different transition metal cations（Mn,Fe,Ni,Co,Zn,Mg,etc.）and divalent anions of oxygen,respectively.As the most promising candidates,transition bimetallic oxides have received considerable attention because of their low cost,natural abundance and environmental compatibility.In particular,bimetallic nickel ferrite（NiFe₂O₄）has recently been investigated as a high-performance electrode material for SCs due to its superior electrical conductivity and electrochemical performance compared to those of nickel oxide（NiO）and iron oxide（Fe₂O₃）.Based on the above analysis,we carried out the following work:1.Hierarchical NiFe₂O₄@MnO₂ core-shell nanosheet arrays（NSAs）were synthesized on Ni foam as an integrated electrode for supercapacitors,using a facile two-step hydrothermal method followed by calcination treatment.The NiFe₂O₄ nanosheets were designed as the core and the ultrathin MnO₂nanoflakes as the shell,creating a unique 3D hierarchical electrode on Ni foam.The composite electrode exhibited a remarkable electrochemical performance with a high specific capacitance of 1391 F g^-1 at a current density of 2 mA cm^-2and long cycling stability at a high current density of 10 mA cm^-2（only 11.4%loss after 3000 cycles）.Additionally,an asymmetric supercapacitor（ASC）device was fabricated with NiFe₂O₄@MnO₂ composite as the positive electrode material and activated carbon（AC）as the negative one.The ASC device exhibited a high energy density(45.2 W h kg^-1)at a power density of 174 W kg^-1,and excellent cycling stability over 3000 cycles with 92.5%capacitance retention.The remarkable electrochemical performance demonstrated its great potential as a promising candidate for high-performance SCs.2.We demonstrated a facile one-step hydrothermal method to synthesize a3D hierarchical core-shell NiFe₂O₄@NiFe₂O₄ nanosheet arrays（NSAs）on Ni foam for supercapacitor applications.The unique additive/binder-free NiFe₂O₄@NiFe₂O₄ composite electrode possessed a large specific surface area and exhibited a high specific capacitance of 1452.6 F g^-1(5 m A cm^-2)and excellent cycling stability with 93.0%retention of initial capacitance at 10 mA cm^-2 over 3000 cycles.In order to achieve high energy and power densities,a high-voltage ASC was constructed utilizing NiFe₂O₄@NiFe₂O₄ NSAs as the positive electrode and AC as the negative electrode.The optimized ASC showed extraordinary performances with a high energy density of 33.6 W h kg^-1 at a power density of 367.3 W kg^-1 and an excellent cycling stability of 95.3%capacitance retention over 3000 cycles.The impressive results presented here paved the way for its applications in high energy density storage systems.3.A supercapacitor-battery hybrid energy storage device,combining with NiFe₂O₄/rGO as negative electrode and 3D rGO as positive electrode,had been designed and fabricated.The 3D rGO was synthesized by using foam nickel as the“sacrificial template”and NiFe₂O₄/rGO was fabricated by NiFe₂O₄ egg-shell microspheres on the surface of 3D rGO.The Ni Fe₂O₄/rGO composite showed a high reversible specific capacity exceeding 4000 mA h g^-1 at 100 mA g^-1 and remaining at 2000 mA h g^-1 at 800 m A g^-1,as well as excellent rate capability and improved cycle stability.Meanwhile the 3D rGO positive electrode also displayed great electrochemical performance.With these two electrode materials,the hybrid supercapacitor NiFe₂O₄/rGO//3D rGO demonstrated an ultrahigh energy density of 275.7 W h kg^-1(power density of 362.9 W kg^-1),which also remained of 165.7 W h kg^-1 even at high power density of 3809.1 W kg^-1.Furthermore,the energy density of the hybrid supercapacitor was comparable to lithium ion batteries,and the power density also reached that of symmetric supercapacitors,indicating that the hybrid supercapacitor could be a very promising novel energy storage system for fast and efficient energy storage in the future.