Electrochemical Capacitors Based On Functionalized Poly(Aryl Ether Sulfone) Membranes

With the growing concern about energy and environmental issues in the21st century, one ofthe greatest challenges is doubtlessly energy storage and conversion. Research anddevelopment of renewable energy sources and novel energy storage device have been widelycarried out. Electrochemical capacitors (ECs), also referred as supercapacitor orultracapacitors, have ignited significant worldwide investigation as one of the mostpromising energy storage devices. In recent years, ECs with high power density, long cyclelife and environment-friendly features have played a unique role that bridges the gapbetween high energy batteries and high power conventional electrostatic capacitors.Separator is a non-active component, but plays a critical role in EC. Separator can separatethe two electrodes to prevent physical contact and electronic short circuit ensuring the safeoperation of EC. Therefore, favorable thermal stability, mechanical strength, chemical andelectrochemical stability of the EC separator are the necessary conditions. In addition, theseparators should enable free ionic transport of electrolyte to provide EC excellentelectrochemical performance. In summary, high-performance separator materials is animportant guarantee for outstanding-performance EC.Poly(aryl ether sulfone)(PAES), a kind of amorphous superior engineering thermoplasticwith electrical isolation, is one of the most ideal matrix polymer for membranes. Attributedto the aromatic structure, PAES resin performs excellent thermal stability, mechanicalproperties, good acid-base resistance and outstanding flame-retardant porperty. At the sametime, the flexible ether bonds in the molecular chain of PAES provide the good toughness,ductility and processing performance. However, the strong hydrophobic character limits theapplication of PAES in EC based on aqueous electrolyte. Considering the excellentperformance of PAES, the different preparation process of PAES membranes or thestructural modification of the PAES copolymers should be carried out to meet the requirements of the separator and improve the electrochemical performance of EC. Basedon the above point of view, a series of PAES membranes for the aqueous EC have beenprepared by proceeding from membrane preparation technique and molecular structuredesign of PAES in the thesis. The structures and properties of membranes and theperformance of ECs fabricated with the resultant membranes were investigatedsystematically.First of all, a series of porous membranes were prepared with the industrial-grade polymerpoly(ether sulfone)(PES) as the substrate material by the method of immersion-precipitationphase transformation. The prepared PES porous membranes could provide the transmissionchannels for the free ions of electrolyte. The EC single cells were then fabricated with thePES porous membranes as separators and activated carbon as active materials of electrodesin6M KOH aqueous electrolyte. The influence of casting solution composition (theconcentration of PES in NMP and the contents of non-solvent additive PVP (8000)) andpreparation process of porous membranes (volatilization time in the air and the compositionof precipitation bath) on the properties of the PES porous membranes and EC single cellswere investigated systemically. As the concentration of PES in NMP increased, the porosityand ionic conductivity of PES porous membrane and the performance of EC were alldecreased. The porosity and ionic conductivity of PES porous membrane and theperformance of EC were enhanced at first and reduced latter with the increasing of thecontents of PVP in the casting solution. The volatilization time had no significant influenceon the porosity and ionic conductivities of the PES porous membranes, as well as theperformance of EC single cells. With the increasing of the content of ethanol (EtOH) inprecipitation bath (EtOH≤75vol.%), the ionic conductivities of PES porous membraneswere affected only a little, so was the performance of the EC single cells fabricated with theresultant membranes. After comprehensive considerations, the optimized preparationcondition of the PES porous separators for the EC was that the concentration of PES in NMPwas15wt%, the content of PVP was8wt%, the volatilization time in the air was30s andthe precipitation bath was deionized water. The EC single cell fabricated with the PESporous membrane prepared by the optimized condition exhibited a coulombic efficiency of95%after5000charging-discharging cycles at0.5A g-1, which demonstrated a good cyclelife of the EC single cell and excellent comprehensive performance of the PES porousmembrane.High porosity of PES porous membranes could result in good electrochemical propertiesto provide good capacitive performance for the EC single cell. However, the seriously decreased mechanical properties of PES porous membranes with the high porosity wouldcause the potential safety hazard for the EC single cells. Therefore, flow casting method wasemployed to prepared PAES dense membrane in order to improve the mechanical propertiesensuring the safe operation of EC cells. Nevertheless, PES have no ability to transmitelectrolyte ions due to the hydrophobic property. A series of quaternary ammoniumfunctionalized poly(aryl ether sulfone) copolymers (PAES-Q-x) with the ability of ionicconductivity were synthesized as the substrate material of separators with2-methylhydroquinone (HQ-Me) and4,4-dichlorodiphenylsulfone (DCDPS). The influence of thedegree of quaternization of PAES-Q-x on the properties of dense membranes and the ECsingle cells were studied. As the degree of quternization increased, the ionic conducntivitywas elevated to2.02×10-3S cm-1at room temperature. The specific capacitance (Csp) of theEC (EDLC-Q-x) was increased as well. The EC single cells exhibited good electric double-layer performance with a top Cspof92.79F g-1and a maximun energy density (Ecell) of2.61W h kg-1at a current density of0.1A g-1. The large coulombic efficiency (~95%) after5000cycles at current density of0.5A g-1indicated a good cycle life, which demonstrated thatthe PAES-Q-x provided a good electrochemical stability, which predicted that the PAES-Q-x dense membranes with excellent mechanical properties and thermal stability exhibitedpotential applications in EC field.In order to meet the rapid growing demand for portable electronic devices (e.g. mobilephones and wearable electronics), development of high performance and reliable solid-stateelectrochemical capacitors with light-weight, small-size and safety is of great importance.Solid-state polymer electrolyte (SPE), the critical role in EC, can influence the performanceof the solid-state EC. The PAES-Q-x membranes could provide excellent comprehensiveproperties, but they could not contain enough free ions and ion carrier with such lowelectrolyte uptake so that the PAES-Q-x membranes could not acted as SPE. Consequently,a novel series of PAES-Q and PVP composite membranes were prepared by a solutionblending method to form the SPE and then the solid-state EC single cells were fabricated inChapter5. The SPE (PAES-Q/PVP-40%/KOH) with40%PVP showed an ionic conductivityof41.05×10-3S cm-1and the solid-state EC single cell (S-EDLC-PVP-40%) exhibitedoutstanding electric double-layer performance with a Cspof140.85F g-1and an Ecellof4.81W h kg-1at a current density of0.1A g-1. The high coulombic efficiency of S-EDLC-PVP-40%(97%) indicated an excellent cycle life, which demonstrated superior electrochemicalproperties of the PAES-Q/PVP composite membranes. Even though the electrolyte uptake was enhanced, the energy density of the EC remaineda lower level. In order to improve the energy density of the ECs, the redox additive (m-phenylenediamine, MPD) was introduced in2M KOH aqueous with PAES-Q/PVP-40%toform a series of redox SPE to provide extra pseudocapacitance for the EC single cells. TheEC single cell (EC-MPD-5) provided a superior electrochemical performance with a Cspof203.0F g-1(0.1A g-1) increasing by108.8%compared to EC with no MPD (EC-KOH), andthe Ecellof EC-MPD-5was as high as7.05W h kg-1much better than that of EC-KOH (3.38W h kg-1).