Research On Integrated Material Devices For Multidimensional Energy Conversion And Storage

With the advantages of high power density,long cycle life and excellent safety performance,supercapacitors have occupied an essential position in civil applications such as smart grids,consumer electronics and new energy vehicles,as well as in military applications such as special vehicles,nuclear submarines and space vehicles.However,the lower energy density(about 10 Wh kg^-1 for current commercial use)is still the main factor restricting supercapacitors from realizing more comprehensive applications.The specific capacitance of the electrode materials is positively proportional to the energy density,and improving the activity and charge transport kinetics of electrode materials to obtain better electrochemical performance is the key to increasing energy density.Based on this,this dissertation focuses on transition metal cobalt-based phosphide/alloy compounds and copper selenide thermoelectric materials with thermoelectric properties and combines morphology modulation,doping design and composite structure to prepare positive/negative electrode materials with high specific capacity,excellent multiplicity performance,and long cycling stability.Asymmetric/symmetric supercapacitor devices with excellent energy storage performance are constructed to obtain supercapacitor devices with high energy density.Furthermore,the concept of the thermoelectric enhancement effect is introduced to systematically study the changes of different thermoelectric materials and properties on the electrochemical performance of supercapacitors,and the influence mechanism of the thermoelectric enhancement effect is explored and analyzed.The specific research contents are as follows:1.Transition metal phosphide-skutterudite Co P₃ thermoelectric material with a unique structural formation that exhibits favorable electronic properties and thermoelectric properties for obtaining high energy density supercapacitors.Herein,novel skutterudite Ni-Co P₃ nanosheets are constructed by etching and coprecipitating at room temperature and subsequent low-temperature phosphorization reaction.Benefiting from the pleasant thermoelectric properties,as well as the enhanced conductivity and the more electroactive sites brought about by incorporating Ni to adjust the electronic structure,the Ni-Co P₃ electrode with a battery-type demonstrates a high specific capacity of 775 m Ah g^-1(0.7 m A h cm^-2)and exceptional cycling stability.The asymmetric supercapacitor device is fabricated by employing the Ni-Co P₃ cathode and the activated carbon（AC）anode,exhibiting a remarkably high energy density of 89.6Wh kg^-1 at 796 W kg^-1 and excellent stability of 94%after 7000 cycles.2.In order to further explore the influence of thermoelectric properties on the specific capacity,the Sn and Te co-doped transition metal alloy compound-skutterudite Co Sb₃ thermoelectric material(Co Sb_2.8Sn_0.005Te_0.195)is constructed through melting and annealing methods,which has better thermoelectric properties than of Ni-Co P₃thermoelectric materials.When it is also used as the cathode of a supercapacitor,it demonstrated a high specific capacity of 1357 m Ah g^-1(at 1 A g^-1).Even when the current density is increased to 20 A g^-1,42%of the initial capacity is maintained.The advantageous performance of the supercapacitor cathode based on Co Sb_2.8Sn_0.005Te_0.195is attributed to the better thermoelectric properties,the more redox active sites and the improved electrical conductivity.Benefiting from the above advantages,the asymmetric supercapacitors device assembled based on Co Sb_2.8Sn_0.005Te_0.195 cathode and AC anode exhibited an energy density as high as 113 Wh kg^-1,and a capacity retention rate of 85%（after 7000 cycles）.3.To confirm the beneficial effect of thermoelectric properties to enhance the specific capacity in the anode,which is as important as the cathode,the mixed-valence Cu_1.85Se/MWCNTs anode with fine thermoelectric properties is obtained by using the multi-walled carbon nanotubes（MWCNTs）composite and salt stripping strategy.Meanwhile,combining the advantages of the rich oxidative active sites of mixed valence and the enhanced conductivity brought by the MWCNTs composite,the Cu_1.85Se/MWCNTs anode exhibits a specific capacity of up to 435 m Ah g^-1 at 2 A g^-1,which is much higher than that of the pure Cu_1.85Se anode of 247.8 m Ah g^-1.Ex-situ X-ray Powder Diffraction（XRD）shows that the introduction of MWCNTs can improve the reversibility via chemical interactions and accelerate the electron transfer in the Cu_1.85Se/MWCNTs.Notably,the assembled symmetric supercapacitor device expresses a high energy density of 41.4 Wh kg^-1,and the capacity remains 83%even after 8000charge/discharge cycles.4.In view of the phenomenon that cathode and anode with thermoelectric properties exhibit the superior electrochemical performances,the thermoelectric effect is further introduced to investigate the ways and rules of the influence of thermoelectric properties on the specific capacity of supercapacitors.It is shown that electrodes with thermoelectric augmentation effect caused by temperature difference will greatly enhance the specific capacity of supercapacitors.In the presence of a temperature difference（0～180 K）,electrochemical reaction processes at the interface between the thermoelectric electrode and the electrolyte are accelerated through thermally driven charge carriers,thereby improving the electrochemical performance.Compared to that without thermoelectric effect（ΔT=0 K）,the specific capacity of n-type Cu_0.3Ag_1.7Se（anode）electrodes with thermoelectric effect（ΔT=40 K）is increased to 141%,which became 208%when the thermoelectric effect is further enhanced as temperature difference increasing toΔT=100 K.Interestingly,the electrochemical performance only be improved under appropriate conditions,that is when the movement directions of holes in p-type electrodes and electrons in n-type electrodes are opposite and the same as those of electrons in electrolytes,respectively.Furthermore,electrodes with different conduction types and various thermoelectric properties,such as n/p-type bismuth tellurides and n/p-type half-Heusler thermoelectric materials are also studied,confirming the universality of this phenomenon in thermoelectric electrodes.This work provides a universal route for the thermoelectric effect to promote the energy density of supercapacitors used in high temperature difference environments.