High Performance Ionic Capacitive Energy Storage and Harvesting Devices

Open Access
Author:
Zhou, Yue
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 02, 2015
Committee Members:
  • Qiming Zhang, Dissertation Advisor
  • Qiming Zhang, Committee Chair
  • Shizhuo Yin, Committee Member
  • Noel Christopher Giebink, Committee Member
  • Qing Wang, Committee Member
Keywords:
  • Energy storage
  • Energy harvesting
  • supercapacitor
Abstract:
Due to the deep concerns of environmental issues and the consumption of fossil fuels, such as petroleum, natural gas and coal, as well as the accelerated greenhouse effect, the renewable energy sources e.g., wind, solar and hydroelectricity have attracted enormous interests. The large fluctuations of these renewable energy sources in power output have brought the vigorous development of the area of energy storage system. Supercapacitors, which bridge the gap of power and energy between batteries and dielectric capacitors, have developed fast in the last decades among these energy storage devices. Although batteries can store a large quantity of energy, they release energy in a slow rate, resulting in a very low power density with a limited lifetime. On the other hand, dielectric capacitors can be charged and discharged at high rate and hence possess very high power density, but their energy density is low. With relatively high power, mid-high energy density and long cycle lifetime, supercapacitors are attractive for many applications, such as in grid scale renewable energy storage and in hybrid electric vehicles where high energy, high power and reasonable lifetime are all required. However, the current commercial supercapacitor product still suffers from the low energy density (less than 10 Wh/kg) and low power density (1 kW/kg). Hence, it is highly desired to further improve electrochemcial performance of supercapacitors cells for the advanced and wide applications. In this dissertation, different supercapacitor cells are introduced to improve the performance by several strategies such as controlling the nanomorphology of electrodes and optimizing the cell configuration. The tortuous ion transport pathways formed in activated carbon, which is widely used as electrodes in the current commerical products, have influenced the power denisty of the cell. To overcome this drawback, the aligned carbon nanotubes (A-CNTs) were investigated in this dissetation due to the superior electrical conductivity and parallel ion pathways of electrodes. Meanwhile, to achieve high volumetric energy and power densities of the cells, a unique mechanical densification method was developed to allow the density of A-CNTs to be tuned precisely over a broad range from 1% volume fraction (Vf) to 40% Vf while preserving the straight ion pathway between A-CNTs. As a result, the supercapacitors fabricated from 40% volumetric fraction (Vf) of A-CNTs as the electrodes with the thickness of 0.8 mm exhibit a power density of 25 kW L-1 (50 kW kg-1), which is much higher than that of the A-CNTs electrodes with similar thickness fabricated by other methods and that of activated carbon electrodes. Pseudocapacitive materials, such as conducitng polymers and transition metal oxides, can be incorporated into the electrode to increase the specific capacitance because the whole bulk (not only the surface for pure carbon electrode) of pseudocapacitive material has involved the electrochemical energy storage. Poly(ethylenedioxythiophene) (PEDOT) was studied as the pseudocapacitive materials in this dissertation. The conformal coating of PEDOT on A-CNTs can exhibit long cycle life compared with pure PEDOT or PEDOT coated on random CNTs since the A-CNTs can provide a mechanical structure to absorb the large volume change of PEDOT during the charge and discharge processes. The unique mechanical densification method was also used to densify the composite to 5% to improve the volumetric performance of the cell. Symmetric supercapacitors using the 5% compacted A-CNTs coated with 10nm thickness PEDOT as electrodes, as well as using BMI-BF4/PC as electrolyte can achieve the highest volumetric specific capacitance of 92.79 F cm-3. Meanwhile, the QV curve based on CV curve or galvanostatic curve was firstly introduced. This new investigation method can be used to evaluate the cell energy loss and coulombic efficiency better compared with the conventional one. The energy density and maximum power density of supercapacitor cells are strongly dependent with the operation potential which is limited by the electrochemical window of the interface between electrode and electrolyte. In order to expand the electrochemical window of the supercapacitor cell, the asymmetric configuration was introduced in this dissertation. One electrode can be pseudocapacitive materials and the other one is carbon materials so that the asymmetric configuration can make full use of the different windows and the advantages of two kinds of materials to expand the operation potential and enhance the performance of the cell. An asymmetric supercapacitor with high electrochemical performance has been developed with conformal coating of PEDOT on A-CNTs as one electrode and the ultra-high density A-CNTs as the other one in 2 M BMIBF4/ PC electrolyte. The positive and negative electrodes materials are individually tailored and work synergistically together in the asymmetric cell configuration so that the cell can be operated under the high operation voltage of 4 V to achieve energy and power densities with a long cycle life. Moreover, the EIS of each electrode is modeled by equivalent circuit elements to describe quantitatively the functions of pseudocapacitor and carbon electrode directly. The EIS of the asymmetric cell is also simulated based on the parameters from two electrodes which demonstrate the optimized asymmetric design. In order to improve the electrochemical performance of asymmetric cell further, activated graphene electrode with the higher specific area (more than 3000 m2/g) were used in the high power side to replace the A-CNTs. The aligned microwave exfoliated graphite oxide electrode, fabricated via a self-assembly process, shows high specific gravimetric and volumetric capacitance for the high power electrode. As a result of complementary tailoring of the asymmetric electrodes, the layered device exhibits a wide 4V electrochemical window, and the highest power and energy densities reported thus far for carbon-based supercapacitors, 149 kW L-1 and 113 Wh L-1 in volumetric performance and 233 kW kg-1 and 177 Wh kg-1 in gravimetric performance, respectively. Finally, the mechanical and thermal energy harvesting devices based on high performance supercapacitor cells have been investigated due to the high specific capacitance and strong dependence with the pressure or temperature. For the mechanical energy harvesting devices based on supercapacitor cell, 40 mV open circuit potential change was obtained, resulting in the high energy harvested of 0.2 J/cc. For the thermal energy harvesting devices, 0.26 J/cc energy harvested per increasing/decreasing temperature cycle was achieved. The large energy harvested per cycle based on supercapacitor cells have surpassed all other energy harvesting devices based on pyroelectric and piezoelectric materials.