Controllable Preparation And Electrochemical Properties Of Nickel-Cobalt Bimetallic Selenide Based Active Materials For Supercapacitors

In recent years,electrochemical energy storage devices such as batteries and electrochemical capacitors have attracted extensive attention with their wide application in mobile tools and vehicles.Among plenty of energy storage devices,supercapacitors（SCs）play an important role in this research field due to their high-power density,excellent cycle stability and good rate performance.At present,the electrode material in commercial SCs is mainly activated carbon（AC）.However,AC electrode can only produce electric double-layer capacitance,and its specific capacitance is usually less than300 F g^–1,which seriously limits the application of SCs in the field requiring large energy density.Therefore,the development of novel electrode materials has attracted the common attention of academia and industry.Transition bimetallic selenides are considered to be one of the candidate materials for high-performance SCs due to their variable oxidation state,good conductivity and high theoretical specific capacity.However,according to the currently published research,the preparation methods of transition bimetallic selenides are complex,and the conditions are harsh.Since the properties of the materials are closedly related to its micro-nano structures and morphologies,it is difficult to produce on a large scale with high repetition rate.Therefore,according to the characteristics of the pseudocapacitance of the Ni Se and Co Se,the transition bimetallic Ni-Co selenides and their composites are designed and fabricated through a simple electrochemical method.Their phase composition,microstructure and supercapacitor performance were systematically characterized and studied.The main contents of this dissertation are as follows:（1）Three types of bimetallic selenides（Ni Co Se₂,Ni₂Co Se₄,and Ni Co₂Se₄）were prepared by one-step electrodeposition on nickel foam（NF）at room temperature and directly used as electrode materials for SCs.It is found that different redox reaction mechanisms of Ni and Co ions make Ni Co Se₂,Ni₂Co Se₄,and Ni Co₂Se₄ electrodes show different electrochemical properties.That is,Ni₂Co Se₄ with high Ni/Co ratio often shows high specific capacity at low current density,but its cycle stability and rate performance are relatively poor;Ni Co₂Se₄ electrode with high Co/Ni ratio shows good rate performance and cycle stability,but its specific capacity is low.Ni in Ni₂Co Se₄ and Co in Ni Co₂Se₄ are usually in a lower valence state,which may have a more direct contribution to the pseudocapacitance of bimetallic selenides.Compared with Ni₂Co Se₄ and Ni Co₂Se₄,both Ni/Co elements are in low valence state in Ni Co Se₂ electrode,which makes Ni Co Se₂not only have high specific capacity,but also have satisfactory cycle stability and rate performance.Therefore,the Ni Co Se₂ electrode with the best electrochemical performance in the three-electrode system test and the AC electrode were further assembled into an asymmetric supercapacitor（ASC）to test its practical availability.The results showed that the Ni Co Se₂//AC ASC possess of high energy density(41.7 Wh kg^–1,800 W kg^–1)and good cycle stability（83.1%,10000 cycles）.（2）Polyethylene glycol（PEG）was selected as an additive to improve the bottom-up filling behavior of Ni Co Se₂（NCSe）nanostructures while maintaining its roughness characteristics,thereby achieving the purpose of controlling the morphology of NCSe.The influence of the amount of PEG on the morphology,crystal form and electrochemical performance of NCSe electrodes was studied.Experiment results indicated that NCSe nanostructure prepared with 10%（w/v）PEG concentration offers an improved specific capacity of 480.5 m Ah g^–1 at current density of 2 A g^–1(325.5 m Ah g^–1 of NCSe without PEG assisting at the same current density).Additionally,the asymmetric supercapacitor（ASC）device constructed with NCSe prepared under the assist of PEG as a positive electrode displayed a high specific energy of 44.5 Wh kg^–1 as well as a long cycling life（92.1%capacity retention after 10000 cycles）.（3）The core/shell structured Ni Te@Ni Co Se₂ composite with a 3D hierarchical architecture has been reasonably defined by coating large theoretical capacity Ni Co Se₂on Ni Te nanorods using a two-step process.Density functional theory（DFT）calculations indicated that the Ni Te has excellent conductivity and Ni Co Se₂ possesses strong affinity for OH^–,which provide additional theoretical assistance for the improved electrochemical performance of Ni Te@Ni Co Se₂.Through reasonably adjusting the content of Ni Te and Ni Co Se₂,the electrochemical properties of Ni Te@Ni Co Se₂ can be enhanced effectively.Thanks to the above advantages,the Ni Te@Ni Co Se₂ electrode exhibited a high specific capacity of 560.6 m Ah g^–1 at 1 A g^–1 and good rate performance(84.9%,from 1 to 20 A g^–1).In addition,the Ni Te@Ni Co Se₂//AC asymmetric supercapacitor（ASC）delivered a maximum energy density of 59.8 Wh kg^–1 at 800 W kg^–1.Importantly,the outstanding durableness with 96.6%capacity retention after 10000 cycles manifests the good stability of the core/shell structure of the Ni Te@Ni Co Se₂ electrode.（4）CNTs/Ni Co Se₂ composites were designed and prepared on hydrophilic treated carbon cloth（ACC）according to the advantages,disadvantages and structural characteristics of carbon（CC）,carbon nanotubes（CNTs）and Ni Co Se₂.CNTs/Ni Co Se₂composite electrode material prepared by co-electrodeposition method has good mechanical adhesion with ACC,so the active material is not easy to peel off from the ACC surface in the long cycle process,which makes the material have excellent cycle stability.Moreover,due to the good synergy between CNTs and Ni Co Se₂,the composite electrode material shows high specific capacity(251.8 m Ah g^–1,1 A g^–1)and good rate properties(71.9%,from 1 to 20 A g^–1).It is worth noting that CNTs/Ni Co Se₂//AC flexible all-solid-state ASC could reach a high voltage window of 1.7 V and provide an energy density of up to 112.2 Wh kg^–1.In addition,the flexible all-solid-state ASC could be prepared in a large area（5×6 cm²）.Using three CNTs/Ni Co Se₂//AC flexible all-solid-state ASCs in series,the LED array with voltage greater than 3.7 V could be easily light.（5）Sodium ions and nitrogen ions（Na-N-ACC）were introduced into ACC by electrodeposition followed by hydrothermal process,and Ni Co Se₂ materials with network crosslinking structure were further grown on the surface of Na-N-ACC surface through electrodeposition.It is found that the existence of sodium and nitrogen atoms in the composite electrode material increases the defects of the material,effectively improves the oxygen reduction voltage and conductivity of the electrode,and widens the working voltage window of the electrode material.In addition,the Ni Co Se₂/Na-N-ACC electrode still delivered a high specific capacity of 282.2 m Ah g^–1,at 1 A g^–1.Meanwhile,the assembled Ni Co Se₂/Na-N-ACC//AC all-solid-state ASC could work stably at a high voltage window of 2.0 V,and its energy density is as high as 139.5 Wh kg^–1 at the power density of 1000 W kg^–1.