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Figure from:A Review of Current Trends on Polyvinyl Alcohol (PVA)-Based Solid Polymer Electrolytes

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Table 1. Ionic conductivity of some polymer electrolytes based on PVA and their corresponding conducting salts.

Table 1 Ionic conductivity of some polymer electrolytes based on PVA and their corresponding conducting salts.

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Figure 1. Ragone plot comparing main energy storage devices of fuel cells, batteries, supercapacitors and capacitors [2].
Figure 1. Ragone plot comparing main energy storage devices of fuel cells, batteries, supercapacitors and capacitors [2].
Figure 2. Schematic summary of the article.
Figure 2. Schematic summary of the article.
Figure 3. Schematic illustration of a typical charged EDLC [20,21]. SCs are used in applications, where storing or releasing a large amount of energy is required. Nowadays, the SCs are used primarily in Hybrid Electric Vehicles (HEV), Elec- tric Vehicles (EV) and Fuel Cell Vehicles (FCV) such as passenger cars, trains, and trolley- buses. Other areas of SCs use are electronic devices such as Uninterruptible Power Sup- plies (UPS), flexible devices such as handphones, and volatile memory backups in PCs. Similarly, supercapacitors can be used to increase the efficiency of hybrid electric vehicles in several ways [17,18]. SCs store energy by the accumulated oppositely charged ions from the electrolyte on the high surface area of the two electrodes when a voltage is applied. The positively charged ions move and align towards the negative electrode, while the negatively charged ions align towards the positive electrode, thereby producing a double layer charge, as shown in Figure 3. The applied voltage is always less than the required voltage capable of causing a chemical reaction. Hence, no chemical changes occur in the system. Due to the complete physical mechanism, the SCs are very stable and maintain a very high cycle life of up to 10° [19,20].
Figure 3. Schematic illustration of a typical charged EDLC [20,21]. SCs are used in applications, where storing or releasing a large amount of energy is required. Nowadays, the SCs are used primarily in Hybrid Electric Vehicles (HEV), Elec- tric Vehicles (EV) and Fuel Cell Vehicles (FCV) such as passenger cars, trains, and trolley- buses. Other areas of SCs use are electronic devices such as Uninterruptible Power Sup- plies (UPS), flexible devices such as handphones, and volatile memory backups in PCs. Similarly, supercapacitors can be used to increase the efficiency of hybrid electric vehicles in several ways [17,18]. SCs store energy by the accumulated oppositely charged ions from the electrolyte on the high surface area of the two electrodes when a voltage is applied. The positively charged ions move and align towards the negative electrode, while the negatively charged ions align towards the positive electrode, thereby producing a double layer charge, as shown in Figure 3. The applied voltage is always less than the required voltage capable of causing a chemical reaction. Hence, no chemical changes occur in the system. Due to the complete physical mechanism, the SCs are very stable and maintain a very high cycle life of up to 10° [19,20].
Figure 4. Block diagram for types of supercapacitors.
Figure 4. Block diagram for types of supercapacitors.
Electrical double-layer capacitor Electrostatic capacitor Fundamentally, a supercapacitor is a special type of capacitor, which is different from the classical electrostatic capacitors, as shown in Figure 5. It can be distinguished in sev- eral ways, such as the charge storage mechanism, the electrolyte, the electrode material and the cell structure [17].
Electrical double-layer capacitor Electrostatic capacitor Fundamentally, a supercapacitor is a special type of capacitor, which is different from the classical electrostatic capacitors, as shown in Figure 5. It can be distinguished in sev- eral ways, such as the charge storage mechanism, the electrolyte, the electrode material and the cell structure [17].
Figure 5. Schematic diagram of (A) an electrostatic capacitor, (B) an electric double-layer capacitor and (C) a pseudocapacitor [17].
Figure 5. Schematic diagram of (A) an electrostatic capacitor, (B) an electric double-layer capacitor and (C) a pseudocapacitor [17].
Figure 6. The three major constituents of a supercapacitor. A supercapacitor consists of three main components, namely, electrodes, separators, and electrolytes, as illustrated in Figure 6. The electrode is an electrical conductor that contacts the electrolyte. Carbon material (mostly porous) is reported to be suitable mate- rial for use as a supercapacitor electrode owing to several advantages such as chemical and physical stability [7,8]. Among supercapacitor carbon electrode materials widely ex- plored are activated carbon (AC), carbon nanotubes (CNTs) and graphene [3]. Nowadays, porous carbon is recognized as the most suitable leading electrode material for superca- pacitors due to its excellent characteristic including large surface area, high electrical con- ductivity and good physical and chemical stability [22,23]. Similarly, an ordered pore in the carbon electrode materials can effectively combine the typical roles of most pores/channels, which serve as the ion-buffering reservoirs (macro-pores), fast ion trans- porting channels (mesopores) and ion-storage sites (micro-pores) [24]. For a perfect car- bon electrode, a large surface area can provide enough electrode/electrolyte interfaces for ion or charge storage [25].
Figure 6. The three major constituents of a supercapacitor. A supercapacitor consists of three main components, namely, electrodes, separators, and electrolytes, as illustrated in Figure 6. The electrode is an electrical conductor that contacts the electrolyte. Carbon material (mostly porous) is reported to be suitable mate- rial for use as a supercapacitor electrode owing to several advantages such as chemical and physical stability [7,8]. Among supercapacitor carbon electrode materials widely ex- plored are activated carbon (AC), carbon nanotubes (CNTs) and graphene [3]. Nowadays, porous carbon is recognized as the most suitable leading electrode material for superca- pacitors due to its excellent characteristic including large surface area, high electrical con- ductivity and good physical and chemical stability [22,23]. Similarly, an ordered pore in the carbon electrode materials can effectively combine the typical roles of most pores/channels, which serve as the ion-buffering reservoirs (macro-pores), fast ion trans- porting channels (mesopores) and ion-storage sites (micro-pores) [24]. For a perfect car- bon electrode, a large surface area can provide enough electrode/electrolyte interfaces for ion or charge storage [25].
Figure 7. Li-ion battery charge—discharge process [31].
Figure 7. Li-ion battery charge—discharge process [31].
Figure 8. Block diagram for types of Li-ion batteries. manganese and manganese spinel batteries. This type of battery’s technology was first uncovered in the 1980s. The first commercial lithium-ion cells were created by Moli En- ergy in 1996, utilizing LCO as the cathode material. LCO batteries are safer than other types of LiBs due to their high-temperature stability [32,35]. They can therefore be utilized in power tools, electric motorbikes, and other applications, and are widely found in med- ical equipment and devices. Electric cars and laptop computers can both be powered by LCOs. In lithium iron phosphate (LIP) batteries, phosphate is employed as the cathode. They are more thermally stable and safe thanks to their low resistance characteristics. Other benefits include longevity and durability; fully charged batteries can be stored with- out affecting their overall charge life. LIP batteries are typically the most economical choice when the long battery life is taken into account. On the other hand, the lithium- phosphate battery has less energy than conventional lithium batteries, due to its lower voltage. Because of this, these batteries are commonly used in electric motorcycles and other applications that need high levels of safety and a long lifecycle [33].
Figure 8. Block diagram for types of Li-ion batteries. manganese and manganese spinel batteries. This type of battery’s technology was first uncovered in the 1980s. The first commercial lithium-ion cells were created by Moli En- ergy in 1996, utilizing LCO as the cathode material. LCO batteries are safer than other types of LiBs due to their high-temperature stability [32,35]. They can therefore be utilized in power tools, electric motorbikes, and other applications, and are widely found in med- ical equipment and devices. Electric cars and laptop computers can both be powered by LCOs. In lithium iron phosphate (LIP) batteries, phosphate is employed as the cathode. They are more thermally stable and safe thanks to their low resistance characteristics. Other benefits include longevity and durability; fully charged batteries can be stored with- out affecting their overall charge life. LIP batteries are typically the most economical choice when the long battery life is taken into account. On the other hand, the lithium- phosphate battery has less energy than conventional lithium batteries, due to its lower voltage. Because of this, these batteries are commonly used in electric motorcycles and other applications that need high levels of safety and a long lifecycle [33].
Figure 9. Type of electrolytes used for supercapacitors. Figure 9 summarized the classification of electrolytes into liquid and solid electro- ytes [4,12,38]. Generally, liquid electrolytes are found to have higher conductivity, and the use of liquid electrolytes has significant limitations and drawbacks that include envi- ronmental unfriendliness, leakage problems, low cycle life, long charging time, and low potential window [39]. Hence, the improvement of new sorts of supercapacitors, for ex- ample, adaptable, solid-state and miniaturized scale supercapacitors, additionally depend profoundly on new electrolytes, for example, solid electrolytes.
Figure 9. Type of electrolytes used for supercapacitors. Figure 9 summarized the classification of electrolytes into liquid and solid electro- ytes [4,12,38]. Generally, liquid electrolytes are found to have higher conductivity, and the use of liquid electrolytes has significant limitations and drawbacks that include envi- ronmental unfriendliness, leakage problems, low cycle life, long charging time, and low potential window [39]. Hence, the improvement of new sorts of supercapacitors, for ex- ample, adaptable, solid-state and miniaturized scale supercapacitors, additionally depend profoundly on new electrolytes, for example, solid electrolytes.
Figure 10. Chemical structures of important polymers mostly applied for solid electrolytes (a) PS (b) PVP, (c) PVC, (d) PVDF, (e) PMMA, () PVA, (g) PCL and (h) PEO [27]. Intensive studies are using various polymers as host materials in SPEs due to the simplicity of their synthesis, high conductivity, cost efficiency, and lightweight. Some of the polymers that have been mostly used in solid electrolytes are: polystyrene (PS), poly(ethylene oxide) (PEO), Polyvinyl alcohol (PVA), Poly(3-caprolactone) (PCL), poly (methyl methacrylate) (PMMA), Poly(vinyl chloride) (PVC), Poly(vinylpyrrolidone) (PVP), Chitosan (CS) and poly (vinylidene fluoride) (PVDF) [27]. Figure 10 presents the chemical structures of such important polymers.
Figure 10. Chemical structures of important polymers mostly applied for solid electrolytes (a) PS (b) PVP, (c) PVC, (d) PVDF, (e) PMMA, () PVA, (g) PCL and (h) PEO [27]. Intensive studies are using various polymers as host materials in SPEs due to the simplicity of their synthesis, high conductivity, cost efficiency, and lightweight. Some of the polymers that have been mostly used in solid electrolytes are: polystyrene (PS), poly(ethylene oxide) (PEO), Polyvinyl alcohol (PVA), Poly(3-caprolactone) (PCL), poly (methyl methacrylate) (PMMA), Poly(vinyl chloride) (PVC), Poly(vinylpyrrolidone) (PVP), Chitosan (CS) and poly (vinylidene fluoride) (PVDF) [27]. Figure 10 presents the chemical structures of such important polymers.
Figure 11. Possible structure for the synthesized PVA-K2COs composite electrolyte [66]. The previous literature reveals that electrical transport properties in a polymer occur through the amorphous phase rather than crystalline. In view of this concept, it appears more appropriate to select a polymer host that is predominantly amorphous, such as PVA, which has a very high amorphous content at room temperature [17]. In addition, PVA is a lightweight and transparent polymer that has a lower reactivity toward the lithium- metal-based anode, thereby providing scope for the improvement of electrode—electrolyte interfacial stability. These expected advantages provided substantial motivation to mod- ify PVA, which is predominantly insulating with only electronic transport, into an ion- ically conducting system. In addition to combined high mechanical strength and temper- ature resistance, the O-H groups in PVA, a source of H-bonding, are capable of construct- ing polymer complexes as shown in Figure 11, where PVA forms a complex with K2COs salt [66].
Figure 11. Possible structure for the synthesized PVA-K2COs composite electrolyte [66]. The previous literature reveals that electrical transport properties in a polymer occur through the amorphous phase rather than crystalline. In view of this concept, it appears more appropriate to select a polymer host that is predominantly amorphous, such as PVA, which has a very high amorphous content at room temperature [17]. In addition, PVA is a lightweight and transparent polymer that has a lower reactivity toward the lithium- metal-based anode, thereby providing scope for the improvement of electrode—electrolyte interfacial stability. These expected advantages provided substantial motivation to mod- ify PVA, which is predominantly insulating with only electronic transport, into an ion- ically conducting system. In addition to combined high mechanical strength and temper- ature resistance, the O-H groups in PVA, a source of H-bonding, are capable of construct- ing polymer complexes as shown in Figure 11, where PVA forms a complex with K2COs salt [66].
Figure 12. Possible structure of the synthesized PVA-K2CO3-SiOz composite PE [75].
Figure 12. Possible structure of the synthesized PVA-K2CO3-SiOz composite PE [75].
Figure 13. Schematic diagram of lithium-ion conduction mechanism of polymer-based polymer electrolyte [80]. In order to develop SPEs with improved ionic conductivity, the polymer should not only dissolve the conducting salt but also be able to couple with ions in the salt. The polar groups in the polymer such as —OH should be effective building blocks for dissolving any conducting salts. The major research on all-SPEs nowadays is focused on PVA and other polymers and their derivatives. This is because the polar group in the PVA is coor- dinated with the ion in the salt through Coulombic interaction, initiating the anion and cation of the salts to dissociate. When an electric field is applied, the movement of cations and anions from one coordination point to another along the polymer segments from one segment to another will initiate [79]. A previous study has reported the ion transport mechanism of polymer electrolytes using Lit as a conducting ion, as shown in Figure 13 [80].
Figure 13. Schematic diagram of lithium-ion conduction mechanism of polymer-based polymer electrolyte [80]. In order to develop SPEs with improved ionic conductivity, the polymer should not only dissolve the conducting salt but also be able to couple with ions in the salt. The polar groups in the polymer such as —OH should be effective building blocks for dissolving any conducting salts. The major research on all-SPEs nowadays is focused on PVA and other polymers and their derivatives. This is because the polar group in the PVA is coor- dinated with the ion in the salt through Coulombic interaction, initiating the anion and cation of the salts to dissociate. When an electric field is applied, the movement of cations and anions from one coordination point to another along the polymer segments from one segment to another will initiate [79]. A previous study has reported the ion transport mechanism of polymer electrolytes using Lit as a conducting ion, as shown in Figure 13 [80].
oil Ca = The movement of mobile charge carriers (or ions) which dissociate from the com- plexes causes ionic hopping in polymer electrolytes. Some basic conditions must be satis- fied to create the mechanism for ionic conduction through hopping, such as the availabil- ity of mobile ions for migration in addition to the availability of vacant sites for the ions to occupy. In addition, for good conduction to happen, the vacant and involved destina- tions of the electrolyte should have equivalent potential energies and a low activation en- ergy boundary for bouncing instigated by thermal vibrations. Low activation energy fo1 bond breaking is fundamental for a high bouncing rate since particle transport in electro- ytes is a thermally activated jumping process [49,84]. A frequently reported mechanism is portrayed for Li* ion transport in ether-based SPE in Figure 14 [77]. It observed that the ionic conduction process is linked to either the main chain segmental motion or Lit ion hopping process for polyether-based SPEs [85]. Figure 14. Ion transport by hopping and segmental motion in SPE [85]. ity of mobile ions for migration in addition to the availability of vacant sites for the ions where ks is the Boltzmann constant and T is the absolute temperature. However, owing to the link between dielectric constant (¢’) and charge carriers, an increase in ¢' is the same as an increase in charge carrier concentrations. This is because ¢’ is linked to the ratio of the capacitance (C) to the capacitance of the cell (Co) (e’ = C/Co), whereas capacitance is associated with the amount of stored charge (C = Q/V) in which Q is the total charge and V is applied voltage [82]. It can be observed from Equation (4) that the concentration of charge carrier (n) can be increased by the increase in ¢' [83]. Therefore, it can be seen that in the polymer electrolyte, the increment of the concentration of the movable ions and the migration speed of the ions can increase the conductivity of the ions.
oil Ca = The movement of mobile charge carriers (or ions) which dissociate from the com- plexes causes ionic hopping in polymer electrolytes. Some basic conditions must be satis- fied to create the mechanism for ionic conduction through hopping, such as the availabil- ity of mobile ions for migration in addition to the availability of vacant sites for the ions to occupy. In addition, for good conduction to happen, the vacant and involved destina- tions of the electrolyte should have equivalent potential energies and a low activation en- ergy boundary for bouncing instigated by thermal vibrations. Low activation energy fo1 bond breaking is fundamental for a high bouncing rate since particle transport in electro- ytes is a thermally activated jumping process [49,84]. A frequently reported mechanism is portrayed for Li* ion transport in ether-based SPE in Figure 14 [77]. It observed that the ionic conduction process is linked to either the main chain segmental motion or Lit ion hopping process for polyether-based SPEs [85]. Figure 14. Ion transport by hopping and segmental motion in SPE [85]. ity of mobile ions for migration in addition to the availability of vacant sites for the ions where ks is the Boltzmann constant and T is the absolute temperature. However, owing to the link between dielectric constant (¢’) and charge carriers, an increase in ¢' is the same as an increase in charge carrier concentrations. This is because ¢’ is linked to the ratio of the capacitance (C) to the capacitance of the cell (Co) (e’ = C/Co), whereas capacitance is associated with the amount of stored charge (C = Q/V) in which Q is the total charge and V is applied voltage [82]. It can be observed from Equation (4) that the concentration of charge carrier (n) can be increased by the increase in ¢' [83]. Therefore, it can be seen that in the polymer electrolyte, the increment of the concentration of the movable ions and the migration speed of the ions can increase the conductivity of the ions.
Figure 15. The schematic diagram of the PVA-BC-KOH electrolyte membrane structure [54]. Consequently, Polu and R. Kumar has reported that the conductivity of PVA-based SPE is generally initiated by two conductive mechanisms: the transfer of ions along the polymer molecular chain through the dissociation and combination process between ions and —OH of the polymer matrix [39]. Another major factor is the conducting salt providing more ion transfer channels due to the swelling structure where the movement of ions in- creased at higher content [41]. The two mechanisms depend on the crystalline-to-amor- phous ratio, which might be decreased by the addition of the conducting salt. The increase in amorphous region and free volume could support local PVA chain segmental motion. Hence, the ions migration increased at a higher salt amount where more ions will migrate at the same time, and the conductivity improved. There are many hydroxyl groups that exist on the surface of the PVA chain, so the hydrogen bond should be easily formed. The proposed schematic structure and ion movement model of PVA-salt SPE is shown in Fig- ure 15 [54].
Figure 15. The schematic diagram of the PVA-BC-KOH electrolyte membrane structure [54]. Consequently, Polu and R. Kumar has reported that the conductivity of PVA-based SPE is generally initiated by two conductive mechanisms: the transfer of ions along the polymer molecular chain through the dissociation and combination process between ions and —OH of the polymer matrix [39]. Another major factor is the conducting salt providing more ion transfer channels due to the swelling structure where the movement of ions in- creased at higher content [41]. The two mechanisms depend on the crystalline-to-amor- phous ratio, which might be decreased by the addition of the conducting salt. The increase in amorphous region and free volume could support local PVA chain segmental motion. Hence, the ions migration increased at a higher salt amount where more ions will migrate at the same time, and the conductivity improved. There are many hydroxyl groups that exist on the surface of the PVA chain, so the hydrogen bond should be easily formed. The proposed schematic structure and ion movement model of PVA-salt SPE is shown in Fig- ure 15 [54].
Figure 16. Cole—Cole plots of (a) P-10, P-20, P-30, P-40 and P-50 films; and (b) P-40-5, P-40-10, P-40- 15, and P-40-20 films [88]. The basis of impedance spectroscopy is to investigate and study the bulk and inter- facial resistances of any electrolyte where the Rv will be determined for ionic conductivity study. Hence, many authors reported the impedance analysis. For instance, the imped- ance spectroscopic of PVA-based SPEs was studied by Wang and co-authors [88]. The Nyquist impedance plots of PVA-based electrolytes incorporated with LiTFSI in differen wt.% and PVA incorporated with 40LiTFSI + EMITFSI in different wt.% are presented in Figure 16a,b, respectively. All the samples displayed a characteristic impedance pattern of polymer electrolyte, with a single semi-circular curve at high frequencies, and a spiked ine at low frequencies [89]. Apparently, the circular arc gradually shrinks with increasing LiTFSI content up to 40 wt.% and then enlarges, which signifies that the bulk resistance o the electrolytes decreases and increases, suggesting an increase or decrease in ionic con- ductivity of the electrolytes. This characteristic behavior was ascribed to the presence o the high amount of mobile Li* ions that are available due to the presence of a high conten of conducting salts to coordinate with PVA and jump from one side of the polymer matrix to another. It was further found that with the incorporation of EMITFSI-conducting salts, the circular arc continues to shrink, owing to the dissolution of the salt in the PVA matrix, which largely acts as a plasticizer that makes the polymer chains more flexible and the amorphous domains expand, resulting in an increase in ion mobility ji [88].
Figure 16. Cole—Cole plots of (a) P-10, P-20, P-30, P-40 and P-50 films; and (b) P-40-5, P-40-10, P-40- 15, and P-40-20 films [88]. The basis of impedance spectroscopy is to investigate and study the bulk and inter- facial resistances of any electrolyte where the Rv will be determined for ionic conductivity study. Hence, many authors reported the impedance analysis. For instance, the imped- ance spectroscopic of PVA-based SPEs was studied by Wang and co-authors [88]. The Nyquist impedance plots of PVA-based electrolytes incorporated with LiTFSI in differen wt.% and PVA incorporated with 40LiTFSI + EMITFSI in different wt.% are presented in Figure 16a,b, respectively. All the samples displayed a characteristic impedance pattern of polymer electrolyte, with a single semi-circular curve at high frequencies, and a spiked ine at low frequencies [89]. Apparently, the circular arc gradually shrinks with increasing LiTFSI content up to 40 wt.% and then enlarges, which signifies that the bulk resistance o the electrolytes decreases and increases, suggesting an increase or decrease in ionic con- ductivity of the electrolytes. This characteristic behavior was ascribed to the presence o the high amount of mobile Li* ions that are available due to the presence of a high conten of conducting salts to coordinate with PVA and jump from one side of the polymer matrix to another. It was further found that with the incorporation of EMITFSI-conducting salts, the circular arc continues to shrink, owing to the dissolution of the salt in the PVA matrix, which largely acts as a plasticizer that makes the polymer chains more flexible and the amorphous domains expand, resulting in an increase in ion mobility ji [88].
Figure 17. Cole—Cole plot for pure PVA and PVA-K2COs SPEs [66].
Figure 17. Cole—Cole plot for pure PVA and PVA-K2COs SPEs [66].
Figure 18. The impedance spectra of the PVA-BC-KOH electrolyte membranes with different con- tents of BC filler; the inset of b for the high-frequency area [54].
Figure 18. The impedance spectra of the PVA-BC-KOH electrolyte membranes with different con- tents of BC filler; the inset of b for the high-frequency area [54].
Figure 19. Variation of conductivity of chitosan—PVA on (a) NH4NOs and (b) EC concentrations at room temperature [69].
Figure 19. Variation of conductivity of chitosan—PVA on (a) NH4NOs and (b) EC concentrations at room temperature [69].
Figure 20. Effect of salt concentration on the ionic conductivity of PVA-K2COs at ambient tempera- ture [66]. In their work, Bashir and co-authors reported the ionic conductivity of the synthe- sized polymer electrolyte based on PVA incorporated with K2COs composites, and the result is shown in Figure 20 [66]. From the figure, the ionic conductivity of the electrolyte was reported to increase with an increase in K2COs, where the ionic conductivity attained was at a maximum at 30 wt.%, then slowly decreased with the increase in wt.% amount confirming that the ionic conductivity of the electrolyte improved with the appropriate doping amount of K2COs. This is due to the amount and mobility of mobile charge carriers that have reached the optimum level in this conductive polymer electrolyte. Furthermore, the strong conducting effect of K2xCOs aids the softening of the PVA backbone and hence the polymer matrix with highly flexible polymer chains. Moreover, the higher flexibility of polymer chains is reported to improve the mobility of polymer segments and assists the ionic transport in the polymer complexes [93]. Likewise, a conducting salt such as K2COs can decrease the Ts of polymer electrolytes [66].
Figure 20. Effect of salt concentration on the ionic conductivity of PVA-K2COs at ambient tempera- ture [66]. In their work, Bashir and co-authors reported the ionic conductivity of the synthe- sized polymer electrolyte based on PVA incorporated with K2COs composites, and the result is shown in Figure 20 [66]. From the figure, the ionic conductivity of the electrolyte was reported to increase with an increase in K2COs, where the ionic conductivity attained was at a maximum at 30 wt.%, then slowly decreased with the increase in wt.% amount confirming that the ionic conductivity of the electrolyte improved with the appropriate doping amount of K2COs. This is due to the amount and mobility of mobile charge carriers that have reached the optimum level in this conductive polymer electrolyte. Furthermore, the strong conducting effect of K2xCOs aids the softening of the PVA backbone and hence the polymer matrix with highly flexible polymer chains. Moreover, the higher flexibility of polymer chains is reported to improve the mobility of polymer segments and assists the ionic transport in the polymer complexes [93]. Likewise, a conducting salt such as K2COs can decrease the Ts of polymer electrolytes [66].
Figure 21. The conductivity of the PVA-based electrolytes with different amounts of BC [54]. Likewise, Fan and co-authors studied and reported the results of ionic conductivity obtained through Equation (10) for all PVA-BC-KOH PEs with different amount of BC. The authors observed that when the amount of BC is lower than 16 wt %, the resultant ionic conductivity value increased as presented in Figure 21. It was established that the ionic conductivity greatly depends on the amount of BC added, where the highest ionic conductivity was reached when the BC amount is 16 wt %. However, the ionic conductiv- ity decreases when the amount of BC goes beyond the optimum (16 wt %) [54].
Figure 21. The conductivity of the PVA-based electrolytes with different amounts of BC [54]. Likewise, Fan and co-authors studied and reported the results of ionic conductivity obtained through Equation (10) for all PVA-BC-KOH PEs with different amount of BC. The authors observed that when the amount of BC is lower than 16 wt %, the resultant ionic conductivity value increased as presented in Figure 21. It was established that the ionic conductivity greatly depends on the amount of BC added, where the highest ionic conductivity was reached when the BC amount is 16 wt %. However, the ionic conductiv- ity decreases when the amount of BC goes beyond the optimum (16 wt %) [54].
7.5. Potential Window Linear sweep voltammetry (LSV) is an electroanalytical chemistry applied to study the electrochemical stability window (ESW) of SCs or potential energy pioneered by Hey- rovsky, in 1959 [105]. The experimental system for LSV utilizes a potentiostat in a three- or two-e ectrode setup to deliver a potential to a solution and monitor its change in cur- rent. The electrode setup usually consists of a working electrode, a counter electrode, and a reference electrode. The potentiostat brings the potentials over the electrode setup. A potentia time gra , E, is delivered through the working electrode. The slope of the potential against ph is called the scan rate and can range from mV/s to 1,000,000 V/s. The working electrode is one of the electrodes at which the redox reactions occur, and the processes that hap pen at this electrode are the ones being observed. The auxiliary counter electrode is the one at which a process opposite from the one taking place at the working electrode occurs, which is not monitored [106,107].
7.5. Potential Window Linear sweep voltammetry (LSV) is an electroanalytical chemistry applied to study the electrochemical stability window (ESW) of SCs or potential energy pioneered by Hey- rovsky, in 1959 [105]. The experimental system for LSV utilizes a potentiostat in a three- or two-e ectrode setup to deliver a potential to a solution and monitor its change in cur- rent. The electrode setup usually consists of a working electrode, a counter electrode, and a reference electrode. The potentiostat brings the potentials over the electrode setup. A potentia time gra , E, is delivered through the working electrode. The slope of the potential against ph is called the scan rate and can range from mV/s to 1,000,000 V/s. The working electrode is one of the electrodes at which the redox reactions occur, and the processes that hap pen at this electrode are the ones being observed. The auxiliary counter electrode is the one at which a process opposite from the one taking place at the working electrode occurs, which is not monitored [106,107].
Figure 22. ESW for EDLC cell with PVA-LiCLOs-TiO: electrolyte [90]. The ESW is the main parameter to be examined in an electrochemical device such as EDLC that explains the maximum operational potential of energy stored in the device. In their study, the performance of a polymer electrolyte based on PVA incorporated with LiCLOs-TiO2 in terms of electrochemical behavior, the charge storage at separate inter- faces in the anodic and cathodic regions was evaluated and reported by Lim and co-au- thors, and the result is presented in Figure 22. The results designate the ESW results at a scan rate of 10 mVs". From Figure 22, the prepared electrolytes based on PVA-LiCLOs- TiOz show highly stable potential energy where the electrolytes do not undergo any chem- ical transformation within the potential region [90].
Figure 22. ESW for EDLC cell with PVA-LiCLOs-TiO: electrolyte [90]. The ESW is the main parameter to be examined in an electrochemical device such as EDLC that explains the maximum operational potential of energy stored in the device. In their study, the performance of a polymer electrolyte based on PVA incorporated with LiCLOs-TiO2 in terms of electrochemical behavior, the charge storage at separate inter- faces in the anodic and cathodic regions was evaluated and reported by Lim and co-au- thors, and the result is presented in Figure 22. The results designate the ESW results at a scan rate of 10 mVs". From Figure 22, the prepared electrolytes based on PVA-LiCLOs- TiOz show highly stable potential energy where the electrolytes do not undergo any chem- ical transformation within the potential region [90].
Figure 23. LSV plots of the synthesized PE PKO (pure PVA) and PVA-K2COs composites [66]. Similarly, the ESW of the PEs based on PVA-K2COs was studied using LSV and re- ported by Bashir and co-authors, and the result is depicted in Figure 23 (and b) [66]. The authors found that the current remains nearly continuous with increasing applied voltage until the potential reaches a certain limit (Vmax), which subsequently increases. The sudden increase in the current has been considered as the upper limit of the electrolyte’s stability range. The results show that no electrochemical decomposition occurs when the voltages are below 1.4 V for all the electrolytes, which are wide enough to enable safe application in any energy storage devices such as EDLC (as shown in Figure 23a). The range of elec- trochemical stable windows increases with increasing the K2COs content until 30 wt.% and reached 2.7 V (as shown in Figure 23b). The synthesized PEs in this study have a substantial value of potential window that is a sufficient working voltage range for super- capacitor applications. As the authors reported, the improved electrochemical windows of the prepared PEs could be attributed to the unique properties of the conducting salts used [108].
Figure 23. LSV plots of the synthesized PE PKO (pure PVA) and PVA-K2COs composites [66]. Similarly, the ESW of the PEs based on PVA-K2COs was studied using LSV and re- ported by Bashir and co-authors, and the result is depicted in Figure 23 (and b) [66]. The authors found that the current remains nearly continuous with increasing applied voltage until the potential reaches a certain limit (Vmax), which subsequently increases. The sudden increase in the current has been considered as the upper limit of the electrolyte’s stability range. The results show that no electrochemical decomposition occurs when the voltages are below 1.4 V for all the electrolytes, which are wide enough to enable safe application in any energy storage devices such as EDLC (as shown in Figure 23a). The range of elec- trochemical stable windows increases with increasing the K2COs content until 30 wt.% and reached 2.7 V (as shown in Figure 23b). The synthesized PEs in this study have a substantial value of potential window that is a sufficient working voltage range for super- capacitor applications. As the authors reported, the improved electrochemical windows of the prepared PEs could be attributed to the unique properties of the conducting salts used [108].
Figure 24. LSV for the PVA-based electrolyte [103].
Figure 24. LSV for the PVA-based electrolyte [103].
Figure 25. LSV plots of the synthesized PVA (a) un-plasticized and (b) plasticized PEs [75]. In another study by Abdulkadir et al., the ESW of the polymer electrolyte based on plasticized PVA-K2COs3 was examined at a 5 mV s" scan rate was reported previously, and the result was depicted in Figure 25a,b [75]. The authors observed that the plasticized electrolyte exhibited a wider electrochemical stability window than un-plasticized elec- trolyte and the authors attributed this to the inclusion of silica particles that enhanced the electrochemical stability of the electrolyte. Consequently, the aut addition of SiOz to the PEs considerably increased the potential sized electrolyte. According to a reported study, electrochemica hors concluded that the window of the synthe- stability may be influ- enced owing to the influence of the silica and the dielectric constant of the host polymer, which would give a higher concentration of charge carriers [110]. Previous study has con- firmed that SiO2 is a known ingredient that increases the amorphous phase of the electro- lyte, which is attributed to the creation of mobile defects as well as an increase in the flow of the material possibly at the interface region [75].
Figure 25. LSV plots of the synthesized PVA (a) un-plasticized and (b) plasticized PEs [75]. In another study by Abdulkadir et al., the ESW of the polymer electrolyte based on plasticized PVA-K2COs3 was examined at a 5 mV s" scan rate was reported previously, and the result was depicted in Figure 25a,b [75]. The authors observed that the plasticized electrolyte exhibited a wider electrochemical stability window than un-plasticized elec- trolyte and the authors attributed this to the inclusion of silica particles that enhanced the electrochemical stability of the electrolyte. Consequently, the aut addition of SiOz to the PEs considerably increased the potential sized electrolyte. According to a reported study, electrochemica hors concluded that the window of the synthe- stability may be influ- enced owing to the influence of the silica and the dielectric constant of the host polymer, which would give a higher concentration of charge carriers [110]. Previous study has con- firmed that SiO2 is a known ingredient that increases the amorphous phase of the electro- lyte, which is attributed to the creation of mobile defects as well as an increase in the flow of the material possibly at the interface region [75].
Table 2. Potential window of some polymer electrolytes based on PVA and their corresponding conducting salts. 7.6. Dielectric Studies
Table 2. Potential window of some polymer electrolytes based on PVA and their corresponding conducting salts. 7.6. Dielectric Studies
Figure 26. Dielectric constant as a function of frequency at various temperatures of (a) pure MC- PVA polymer blend and (b) MC-PVA doped with NH4NOs [114].
Figure 26. Dielectric constant as a function of frequency at various temperatures of (a) pure MC- PVA polymer blend and (b) MC-PVA doped with NH4NOs [114].
Figure 27. The plots of (a) ér and (b) €i against frequency for PVA-K2COs electrolytes [113]. 8. Thermodynamic Effects and Ion Transport Models for Polymer Electrolytes
Figure 27. The plots of (a) ér and (b) €i against frequency for PVA-K2COs electrolytes [113]. 8. Thermodynamic Effects and Ion Transport Models for Polymer Electrolytes
Figure 28. Conductivity versus temperature of PEs (PVA-K2COs) electrolytes [66].
Figure 28. Conductivity versus temperature of PEs (PVA-K2COs) electrolytes [66].
Table 3. Summary of the ionic conductivities with different conducting salts and solvents based on polymer blend electrolytes. 9.3. Mixed Salt System
Table 3. Summary of the ionic conductivities with different conducting salts and solvents based on polymer blend electrolytes. 9.3. Mixed Salt System
Figure 29. Variation of conductivity of chitosan—PVA on (a) NHsNOs and (b) EC concentrations at room temperature [69].
Figure 29. Variation of conductivity of chitosan—PVA on (a) NHsNOs and (b) EC concentrations at room temperature [69].
Figure 30. FTIR spectra of pure PVA (PKO), PK10, PK20, PK30, PK40 and PK50 polymer electrolytes [66]. Similar presence of y, Bashir and co-authors reported polymer electrolytes based on PVA in th K2COs salt [66]. The electrolyte was prepared using solution casting techniqu by incorporating different wt.% (0-50 wt.%) of the conducting salt into the polymer hos‘ The authors by FTIR analysis. As the previous studies reported the addition of the conductin; salt into the firmed confirmed a complex formation between the polymer and the salt, as con polymer host, the cation of the metal from the salt would coordinate with th polar groups in the host polymer matrix, which resulted in the complexation [88]. Thi type o f inter: action will influence the local structure of the polymer backbone, and certai infrared active modes of vibration will be affected (Figure 30). FTIR spectroscopy is use to ana trolyte polym er ele yze the interactions and complexations between atoms or ions in PVA-K2COs elec composites. These interactions can induce changes in the vibrational modes of th ctrolyte. Consequently, the changes in the peak intensities and shifting ar important in order to establish the complex formation between the host polymer an« K2COs [146]. The increased intensities of the peak and shifting in the spectra of the elec trolytes confirmed successful formation of PVA-K2COs composites [66].
Figure 30. FTIR spectra of pure PVA (PKO), PK10, PK20, PK30, PK40 and PK50 polymer electrolytes [66]. Similar presence of y, Bashir and co-authors reported polymer electrolytes based on PVA in th K2COs salt [66]. The electrolyte was prepared using solution casting techniqu by incorporating different wt.% (0-50 wt.%) of the conducting salt into the polymer hos‘ The authors by FTIR analysis. As the previous studies reported the addition of the conductin; salt into the firmed confirmed a complex formation between the polymer and the salt, as con polymer host, the cation of the metal from the salt would coordinate with th polar groups in the host polymer matrix, which resulted in the complexation [88]. Thi type o f inter: action will influence the local structure of the polymer backbone, and certai infrared active modes of vibration will be affected (Figure 30). FTIR spectroscopy is use to ana trolyte polym er ele yze the interactions and complexations between atoms or ions in PVA-K2COs elec composites. These interactions can induce changes in the vibrational modes of th ctrolyte. Consequently, the changes in the peak intensities and shifting ar important in order to establish the complex formation between the host polymer an« K2COs [146]. The increased intensities of the peak and shifting in the spectra of the elec trolytes confirmed successful formation of PVA-K2COs composites [66].
Figure 31. Morphological images of different electrolytes: (a) pure PVA, (b) P-40, (c) P-40-5, (d) P- 40-10, (e) P-40-15 and (f) cross-section of P-40-10 [88]. High-performing PVA-based Li-Ion-conducting SPEs films for EDLC applications was reported by Wang and co-authors [88]. The electrolyte was prepared by adding lith- ium bis (trifluoromethane) sulfonamide (LiTFSI) and 1-ethyl-3 methyl imidazolium bis (trifluoro methyl sulfonyl) imide (EMITFSI) into the PVA matrix by means of the solution casting process. The electrolyte was characterized for their structural and electrochemical performance. The morphologies of the prepared electrolyte are denoted in Figure 31. The authors observed that the electrolyte exhibited a homogeneous and compact wrinkled texture due to the disordered structure of the polymer matrix with the addition of the salts, which confirmed the formation of composites. Similarly, it was found that the PVA- salt electrolyte is capable of coordinating and transporting Li* ions, and it has a relatively wide electrochemical stability window, which is suitable for EDLC applications. The elec- trolyte was applied in EDLCs and was found to have a specific capacitance of 101 F g" and an energy density of 10.3 Wh kg-|, implying that the specific capacitance and energy density retentions are as high as 94.4% and 98.1%, respectively. This demonstrates that PVA-LiTFSI+ EMITFSI electrolyte is a favorable electrolyte candidate for electronic device applications.
Figure 31. Morphological images of different electrolytes: (a) pure PVA, (b) P-40, (c) P-40-5, (d) P- 40-10, (e) P-40-15 and (f) cross-section of P-40-10 [88]. High-performing PVA-based Li-Ion-conducting SPEs films for EDLC applications was reported by Wang and co-authors [88]. The electrolyte was prepared by adding lith- ium bis (trifluoromethane) sulfonamide (LiTFSI) and 1-ethyl-3 methyl imidazolium bis (trifluoro methyl sulfonyl) imide (EMITFSI) into the PVA matrix by means of the solution casting process. The electrolyte was characterized for their structural and electrochemical performance. The morphologies of the prepared electrolyte are denoted in Figure 31. The authors observed that the electrolyte exhibited a homogeneous and compact wrinkled texture due to the disordered structure of the polymer matrix with the addition of the salts, which confirmed the formation of composites. Similarly, it was found that the PVA- salt electrolyte is capable of coordinating and transporting Li* ions, and it has a relatively wide electrochemical stability window, which is suitable for EDLC applications. The elec- trolyte was applied in EDLCs and was found to have a specific capacitance of 101 F g" and an energy density of 10.3 Wh kg-|, implying that the specific capacitance and energy density retentions are as high as 94.4% and 98.1%, respectively. This demonstrates that PVA-LiTFSI+ EMITFSI electrolyte is a favorable electrolyte candidate for electronic device applications.
Figure 32. X-ray diffraction patterns of pure Mg(NOs)2 and PVA:Mg(NOs)2 complexes [39].
Figure 32. X-ray diffraction patterns of pure Mg(NOs)2 and PVA:Mg(NOs)2 complexes [39].
Figure 33. Digital photographs of (A) PVA electrolyte, (B) flexibility of PVA electrolyte by bending in circular arc and (C) PVA electrolyte stretched to check its elastic ability [70].
Figure 33. Digital photographs of (A) PVA electrolyte, (B) flexibility of PVA electrolyte by bending in circular arc and (C) PVA electrolyte stretched to check its elastic ability [70].
Figure 34. (A) The CV curves of fabricated supercapacitor with the developed electrolytes based on PVA, CMC and PEO at a scan rate of 100 mV s", (B) Galvanostatic charge—discharge curves at a current density of 2 A g™ [70]. Cyclic voltammetry (CV) curves using the developed e nV s within an optimized operating potential window is upercapacitor device with PVA electrolyte exhibits a good ectrolyte at a scan rate of 100 presented in Figure 34A. The operating potential window. urthermore, the supercapacitor device with PVA electrolyte demonstrates a higher area inder the CV curves with symmetry along the cathodic and anodic directions signifying ood supercapacitive features. Similarly, Figure 34B further reveals the charge—discharge urves of the supercapacitor using PVA-based electrolytes at a constant current density of A g. The author established that the PVA electrolyte-based supercapacitor shows good erformance such as much large discharge time and nearly symmetric behavior of charge— lischarge curve, as compared to the supercapacitor device with other electrolytes. The etter electrochemical performance of PVA electrolyte-based supercapacitor is due to the igher ionic conductivity, better compatibility and excellent mechanical integrity as com- ared to the other electrolytes [70].
Figure 34. (A) The CV curves of fabricated supercapacitor with the developed electrolytes based on PVA, CMC and PEO at a scan rate of 100 mV s", (B) Galvanostatic charge—discharge curves at a current density of 2 A g™ [70]. Cyclic voltammetry (CV) curves using the developed e nV s within an optimized operating potential window is upercapacitor device with PVA electrolyte exhibits a good ectrolyte at a scan rate of 100 presented in Figure 34A. The operating potential window. urthermore, the supercapacitor device with PVA electrolyte demonstrates a higher area inder the CV curves with symmetry along the cathodic and anodic directions signifying ood supercapacitive features. Similarly, Figure 34B further reveals the charge—discharge urves of the supercapacitor using PVA-based electrolytes at a constant current density of A g. The author established that the PVA electrolyte-based supercapacitor shows good erformance such as much large discharge time and nearly symmetric behavior of charge— lischarge curve, as compared to the supercapacitor device with other electrolytes. The etter electrochemical performance of PVA electrolyte-based supercapacitor is due to the igher ionic conductivity, better compatibility and excellent mechanical integrity as com- ared to the other electrolytes [70].
Figure 35. Electric field vs. conductivity of PVA-based electrolyte at different temperatures [99]. x In another study, nanocomposite SPEs based on PVA with different combinations of nano ferroelectric fillers for application in LIB were prepared using the solution casting technique, as reported by Sunitha and co-authors [99]. The authors found that the addition of nanofillers to PVA increases the DC conductivity of the SPEs. Similarly, the structural change induced in the electrolyte due to the addition of nanofillers was investigated using XRD techniques, and it was found that the amorphosity of the sample increased as the combination of nanofillers was added to the polymer matrix. The effect of temperature on conductivity was studied in the range of 298-323 K while changing the electric fields from 0 to 100 Vcm-1. It was established that the ionic conductivity of the electrolyte increased non-linearly with the electric field, as depicted in Figure 35.
Figure 35. Electric field vs. conductivity of PVA-based electrolyte at different temperatures [99]. x In another study, nanocomposite SPEs based on PVA with different combinations of nano ferroelectric fillers for application in LIB were prepared using the solution casting technique, as reported by Sunitha and co-authors [99]. The authors found that the addition of nanofillers to PVA increases the DC conductivity of the SPEs. Similarly, the structural change induced in the electrolyte due to the addition of nanofillers was investigated using XRD techniques, and it was found that the amorphosity of the sample increased as the combination of nanofillers was added to the polymer matrix. The effect of temperature on conductivity was studied in the range of 298-323 K while changing the electric fields from 0 to 100 Vcm-1. It was established that the ionic conductivity of the electrolyte increased non-linearly with the electric field, as depicted in Figure 35.
Figure 36. SEM images of pure chitosan—PVA films at (a) surface and (b) cross-section [69]. “ “ “ tl Transport studies of PVA-chitosan polymer electrolyte for application in proton bat- teries was reported by Kadir and co-authors [69]. The solution casting technique was used and prepared the electrolytes system. The authors blended 36 wt.% of PVA with 24 wt.% chitosan and doped with 40 wt.% NHNOs as a source of ions. Based on the SEM surface and cross-section micrographs result of chitosan—PVA blended electrolyte (shown in Fig- ure 36), the film displays a smooth and homogeneous surface indicating that both poly- mers are miscible with each other.
Figure 36. SEM images of pure chitosan—PVA films at (a) surface and (b) cross-section [69]. “ “ “ tl Transport studies of PVA-chitosan polymer electrolyte for application in proton bat- teries was reported by Kadir and co-authors [69]. The solution casting technique was used and prepared the electrolytes system. The authors blended 36 wt.% of PVA with 24 wt.% chitosan and doped with 40 wt.% NHNOs as a source of ions. Based on the SEM surface and cross-section micrographs result of chitosan—PVA blended electrolyte (shown in Fig- ure 36), the film displays a smooth and homogeneous surface indicating that both poly- mers are miscible with each other.
Figure 37. The schematic diagram for the preparation of PVA/PVP by blending and chemical cross- linking [128]. PVA/PVP blended electrolytes. The physicochemical properties of the samples were in- vestigated by FTIR, TG, and SEM analyses. The membrane showed flawless alkaline strength without losing its integrity, even upon exposure to high concentrations of KOH solution and at a higher temperature. Morphological analysis by SEM revealed a highly ordered micro-void structure that was homogeneously distributed on the surface of the membrane, which conveyed the membrane with good electrolyte (KOH) retention ability. FTIR spectra confirmed that the formation of a good complex might be ascribed to the presence of excess free KOH in the polymer matrix in addition to KOH bound to the pol- ymers. The electrolyte was applied in proton-conducting batteries application where its electrochemical performance of the electrolytes was found to be significantly dependent on the concentration of KOH and the PVP content. Virtually continuous, highly stable ionic conductivity while maintaining mechanical integrity was retained at room temper- ature for more than one month [128].
Figure 37. The schematic diagram for the preparation of PVA/PVP by blending and chemical cross- linking [128]. PVA/PVP blended electrolytes. The physicochemical properties of the samples were in- vestigated by FTIR, TG, and SEM analyses. The membrane showed flawless alkaline strength without losing its integrity, even upon exposure to high concentrations of KOH solution and at a higher temperature. Morphological analysis by SEM revealed a highly ordered micro-void structure that was homogeneously distributed on the surface of the membrane, which conveyed the membrane with good electrolyte (KOH) retention ability. FTIR spectra confirmed that the formation of a good complex might be ascribed to the presence of excess free KOH in the polymer matrix in addition to KOH bound to the pol- ymers. The electrolyte was applied in proton-conducting batteries application where its electrochemical performance of the electrolytes was found to be significantly dependent on the concentration of KOH and the PVP content. Virtually continuous, highly stable ionic conductivity while maintaining mechanical integrity was retained at room temper- ature for more than one month [128].