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Organic redox-active materials, combined with high-surface-area carbonaceous substrates, form sustainable and low-cost composites with greatly enhanced electrochemical charge storage capacities. The electrochemical capacitive behavior of a composite electrode containing tetraphenylporphyrin sulfonate (TPPS), Chemically polymerized luminol (CpLum), and carbon nanotubes (TPPS-CpLum-CNT) was studied and compared with individual TPPS-CNT and CpLum-CNT composites. The dual-layer TPPS-CpLum had a combined contribution to the electrochemical charge storage, which led to an increased volumetric capacitance over the bare CNT and individual TPPS-CNT and CpLum-CNT composites. The synergistic interactions in the composite enabled faster charge storage kinetics and great stability. Spectroscopic analyses revealed that TPPS and CpLum interact electronically through noncovalent π-π and van der Waals bonds, which facilitates the transfer of electrons during charge and discharge. The synergy in charge storage was confirmed by density functional theory computational analysis, which suggested favorable physisorption and interfacial electronic interactions for TPPS adsorbed to a CpLum-carbon substrate. The combined insights from experimental and computational characterizations show that superimposing redox-active organic layers can be an effective and sustainable approach to design and engineer the surface of carbonaceous materials for capacitive charge storage.

The pressing concerns of environmental sustainability and growing needs of clean energy have raised the demands of carbon and organic based energy storage materials to a higher level. Redox-active organic-carbon composites electrodes are emerging to be enablers for high-performance, high power and long-lasting energy storage solutions, especially for electrochemical capacitors (EC). This review discusses the electrochemical redox active organic compounds and their composites with various carbonaceous materials focusing on capacitive performance. Starting with the most common conducting polymers, we expand the scope to other emerging redox active molecules, compounds and polymers as well as common carbonaceous substrates in composite electrodes, including graphene, carbon nanotube and activated carbon. We then discuss the first-principles computational studies pertaining to the interactions between the components in the composites. The fabrication methodologies for the composites with thin organic coatings are presented with their merits and shortcomings. The capacitive performances and features of the redox active organic-carbon composite electrodes are then summarized. Finally, we offer some perspectives and future directions to achieve a fundamental understanding and to better design organic-carbon composite electrodes for ECs.

Since the beginning of this century, considerable research efforts have focused on solid supercapacitors that leverage hydroxide (OH−) ion conducting polymer electrolytes as a high performance, safe energy storage technology, capable of lightweight and flexible architectures. Here, we present an overview of the state-of-the-art solid supercapacitors enabled by OH− ion conducting polymer electrolytes. We found that progress regarding OH− ion conducting polymer electrolytes is slow compared with others such as proton conducting electrolytes. Furthermore, their role in the capacitance, rate capability, and long-term reliability of solid supercapacitors is unclear. This has resulted in many demonstrations of materials that are excellent in OH− media, but there is no clear road map moving forward due to the limited availability of viable polymer electrolyte chemistry. In this review, we briefly introduce the fundamentals of supercapacitors, and the mechanisms for OH− ion conduction in a polymer matrix while identifying several important relationships between the properties of the polymer electrolyte and the performance of the solid supercapacitor. We categorized OH− ion conducting polymer electrolytes into two types: anion exchange membranes and alkaline polymer electrolytes based on composition and synthesis. The performance and shortcomings of solid supercapacitors enabled by these classes of electrolytes are discussed. Some perspectives are offered to address the areas of improvement. Finally, we propose key research directions to facilitate robust contributions for advanced solid supercapacitors based on OH− ion conducting polymer electrolytes.

Electrode materials composed of carbon nanotubes (CNTs) modified by a Keggin-type polyoxometalate (molybdovanadogermanic anions, GeMoV), electrochemically active chemically polymerized polyluminol (CpLum), and polydiallyldimethylammonium (PDDA) polycations were developed via a layer-by-layer (LbL) deposition process. The composite electrodes, with 5–6 nm inorganic–organic coatings, demonstrated a uniquely combined electrochemical redox behavior, enhanced charge storage capacity, high rate performance and good cycling stability. The synergistic interaction among the three components resulted in fast kinetic capacitive contribution over the respective single component modified CNT electrodes. The surfaces of the composites were investigated using X-ray photoelectron spectroscopy and revealed that the CpLum and PDDA polymers bonded with GeMoV via N+O− electrostatic interactions. Within the inorganic–organic layers, the GeMoV anions exhibited fast and highly reversible electrochemical redox activities in both higher and lower potential regions. Both PDDA and CpLum organic layers acted as anchors for the GeMoV anion on the CNT surface. CpLum also contributed to the redox processes in charge storage while improving the overall reaction kinetics.

Phosphate functionalized carbon nanomaterials have attracted significant attention because of their potential applications in energy storage applications. Herein we report a facile one-pot method to prepare water dispersible phosphate functionalized reduced graphene oxide and demonstrate the potential of the novel materials for energy storage applications. The synthesis method shows promise to promote a wider adoption of reduced graphene oxide for high performance applications.

A NiCo2S4 nanoparticles composite, in-situ grown on rice husk hierarchical porous carbon, is synthesized through a facile one-step hydrothermal method for energy storage. The NiCo2S4 nanoparticles with a size of 20 ± 10 nm are homogeneously distributed on the carbon matrix, leading to a high material utilization and high capacity, significantly outperformed the pure NiCo2S4. The chemical bonds of C-O-Co and C-O-Ni between the NiCo2S4 nanoparticles and the carbon matrix not only result in a superior power density, but also improve the cycling stability. An asymmetrical supercapacitor is assembled using the composite material and commercial YP-50 activated carbon and shows high energy density, high power density and excellent cycling stability. The hydrothermal method can be extended to synthesize other composite materials for energy storage devices.

Electrochemically active polyluminol was synthesized using an oxidative chemical polymerization reaction. The chemically polymerized luminol (CpLum) exhibited reversible oxidation and reduction reactions with fast kinetics. From Fourier transform infrared spectroscopy analyses, the polymerization occurred by the conversion of primary amine groups into secondary amine groups. The polymer contained benzoid and quinoid segments which were confirmed by UV–visible spectroscopy and X-ray photoelectron spectroscopy. Thermal analysis revealed the semi-crystalline nature of CpLum, with two thermal events at 99 °C and 152 °C corresponding to a glass transition and a melting temperature. The interesting features and electrochemical activities may lead to applications in energy storage.

An ultra-high rate solid-state electrochemical capacitor (EC) is studied for 60 Hz AC line-filtering applications. The combination of vertically-oriented graphene nanosheet electrodes and a hydroxide ion-conducting tetraethylammonium hydroxide-polyacrylamide polymer electrolyte enabled an interdigitated planar EC with capacitive behaviour at rates up to 1000 V s−1 and a response time of less than 1 ms. This performance level was maintained for over 3 weeks without capacitor packaging. The EC responded capacitively at 120 Hz at a temperature of 120 °C, demonstrating outstanding high-temperature resiliency. Thermal and chemical characterizations of the polymer electrolyte revealed that electrochemical changes caused by high-temperature are reversible. Two competing factors were identified that govern solid-state capacitor performance at elevated temperatures.

Solid-state, thin, and flexible electrochemical capacitors (ECs) are promising power sources for wearable electronics such as smart textiles and medical sensors. One of the key enablers for safe and high performance solid electrical double layer capacitors (EDLCs) is aqueous-based neutral pH polymer electrolytes (NPPEs) [1-2]. NPPEs containing chloride [3-5] or sulfate salts [6-8] as ion conductors, have been demonstrated high ionic conductivities with wide cell voltage window (>1.5 V, beyond the typical limit of aqueous-based systems). While nitrate solution can also offer similarly wide potential window (demonstrated in liquid electrolytes [9]), their polymer electrolytes may offer several additional advantages: (i) better compatibility with wide-range of polymers from its chaotropic nature, (ii) good thermal stability from deep eutectic temperature with water, and (iii) good water-retaining ability from hygroscopic nature that allows for higher retention than sulfates while maintaining better structural integrity than chlorides. In this study, we aim to: (i) develop a class of high performance LiNO3 based NPPEs, (ii) investigate the underlying material characteristics of the NPPE that support good electrochemical performance, and (iii) demonstrate its application in solid EDLC devices using carbon-based electrodes. The polymer electrolytes utilizing either polyacrylamide or poly(vinyl alcohol) host with various amount of LiNO3 have been systematically studied for their ionic conductivities and performance in solid capacitive devices. While increasing the salt content can lead to higher ionic conductivity, the mechanical properties may be compromised from excessive water absorption. The optimized electrolytes exhibited a high ionic conductivity (>20 mS cm-1) at ambient, relatively high conductivity retention at sub-zero temperatures, and long shelf-life (>30 days). These electrolytes maintained well-hydrated ions and would enable solid-state double layer capacitors, without any separator. References: [1] K. Fic et al., "Novel insight into neutral medium as electrolyte for high-voltage supercapacitors," Energy & Env. Sci., 2, 2012. [2] C. Zhong et al., "A review of electrolyte materials and compositions for electrochemical supercapacitors," Chem. Soc. Rev., 44, 2015. [3] G. Wang et al., “LiCl/PVA Gel Electrolyte Stabilizes Vanadium Oxide Nanowire Electrodes for Pseudocapacitors,” ACS Nano, 6, 2012. [4] X. Peng et al., “A zwitterionic gel electrolyte for efficient solid-state supercapacitors,” Nat. Comm., 7, 2016. [5] A. Virya and K. Lian, “Polyacrylamide-lithium chloride polymer electrolyte and its applications in electrochemical capacitors,” Electrochem. Comm., 74, 2016 [6] N. Batisse and E. Raymond- Piñero, “A self-standing hydrogel neutral electrolyte for high voltage and safe flexible supercapacitors,” J. Power Sources, 348, 2017. [7] A. Virya et al., "Na2SO4-polyacrylamide electrolytes and enabled solid-state electrochemical capacitors," Batteries & Supercaps, 2019. [8] T. Gu and B. Wei, “High-performance all-solid-state asymmetric stretchable supercapacitors based on wrinkled MnO2/CNT and Fe2O3/CNT macrofilms,” J. Mater. Chem. A, 4, 2016. [9] K. Fic et al., “Comparative operando study of degradation mechanisms in carbon-based electrochemical capacitors with Li2SO4 and LiNO3 electrolytes,” Carbon, 120, 2017.

The rising demand for clean energy and environmental sustainability has led to the growing interest in redox-active organic-carbon composite electrodes for high power and long-lasting energy storage, especially for electrochemical capacitors (EC)[1]. Nitrogen containing organic redox active materials such as the conducting polymer polyluminol chemically polymerized (CpLum) on multiwalled carbon nanotubes (CNT) showed increased capacitive charge storage properties in the composite (CpLum-CNT)[2]. With merely a few nm CpLum, the composite electrodes exhibited reversible faradaic redox reactions and stored c.a. 3x higher volumetric charge than that of bare CNT. Predictions using DFT showed π interactions between the polymers and the CNT substrate. These interactions are thought to have stabilized the composite and contributed to the strong electrochemical performance. In addition to CNT substrates, activated carbons (ACs) offer high specific surface areas, diverse pore size distributions and native surface functionalities owing to their sources. Biomass-derived ACs can serve as low cost, sustainable alternative substrates from feedstocks as pinecone and waste tea [3]–[6]. In this work, we investigate the effects of different surface chemistry, porosity and graphitization on CNT and porous activated carbon surfaces towards the redox activity of the CpLum-carbon composite. The surface functionalities of interest include hydroxyl and carboxyl groups owing to their known contributions toward surface wettability and pseudocapacitance while being present on naturally derived ACs. Varying porosity and graphitization also offer insight into the effects of differing specific surface areas, pore sizes, wettability, and conductivity on the CpLum-carbon composites. Figure 1 a. shows a schematic of the groups of interest and graphitization to investigate the surface interaction of luminol on CNT and Figure 1 b. shows the redox active behavior of CpLum-CNT. The insights from these studies will be used to engineer the surface of carbons such as CNTs and ACs to improve the interfacial properties for redox active materials. References [1] J. N’Diaye, R. Bagchi, J. Y. Howe, and K. Lian, “Redox Active Organic-Carbon Composites for Capacitive Electrodes: A Review,” Sustainable Chemistry, vol. 2, no. 3, pp. 407–440, 2021, doi: 10.3390/suschem2030024. [2] J. N’Diaye, J. Hyun Chang, and K. Lian, “The Capacitive Behavior of Polyluminol on Carbon Nanotubes Electrodes,” ChemElectroChem, vol. 6, no. 21, pp. 5454–5461, Oct. 2019, doi: 10.1002/celc.201901473. [3] M. Genovese and K. Lian, “Polyoxometalate modified pine cone biochar carbon for supercapacitor electrodes,” Journal of Materials Chemistry A, vol. 5, no. 8, pp. 3939–3947, 2017, doi: 10.1039/c6ta10382k. [4] M. Y. Bhat, N. Yadav, and S. A. Hashmi, “Pinecone-derived porous activated carbon for high performance all-solid-state electrical double layer capacitors fabricated with flexible gel polymer electrolytes,” Electrochimica Acta, vol. 304, pp. 94–108, 2019, doi: 10.1016/j.electacta.2019.02.092. [5] H. Eom, J. Kim, I. Nam, and S. Bae, “Recycling Black Tea Waste Biomass as Activated Porous Carbon for Long Life Cycle Supercapacitor Electrodes,” Materials, vol. 14, no. 21, p. 6592, Nov. 2021, doi: 10.3390/ma14216592. [6] Z. Zhu and Z. Xu, “The rational design of biomass-derived carbon materials towards next-generation energy storage: A review,” Renewable and Sustainable Energy Reviews, vol. 134, no. September, p. 110308, 2020, doi: 10.1016/j.rser.2020.110308. Figure 1

The effects of electrode material loading and operating temperature on solid-state electrochemical double layer capacitors (EDLCs) with Na2SO4-polyacrylamide-DMSO electrolyte were studied. Two types of solid EDLC cells, using carbon nanotube (CNT) or activated carbon (AC), with very different surface areas and pore structures were compared to reveal the limitations in designing solid capacitive devices. Based on the gravimetric capacitance values, the utilizable portion of the electrode for EDLC can be estimated. Although increasing carbon loading leads to higher capacitance, there are two possible adverse effects especially at low temperatures. A high loading and thick electrode may reduce the penetration of viscous polymer electrolyte precursor solutions and may increase diffusion limitation leading to lower material utilization. These phenomena are more aggravated at faster rates and on micropore-rich materials. The results from this work can be used to quantify the effective utilization of the materials at different temperatures and the insights can be added to the guiding principles for designing and developing solid EDLCs.

A polymer electrolyte was developed by blending lithium (LiPAA) with non-ionic polyacrylamide (PAM). The LiPAA-PAM showed synergic effect and achieved an ionic conductivity of 13.8 ± 2.4 mS cm−1, higher than previously developed neutral pH polymer electrolytes. Chemical and structural characterizations of the LiPAA-PAM films revealed a stable homogeneous amorphous structure. The performance of double layer capacitors using LiPAA-PAM electrolyte system was demonstrated using YP-50F activated carbon electrodes. These solid cells demonstrated wide voltage window (1.5 V), good cycle life (>10,000 cycles), and excellent rate capability (up to 500 mV s−1 in cyclic voltammetry). Keywords: Solid supercapacitor, Polymer electrolytes, Neutral pH electrolytes, All-polymer, Polyelectrolytes

A phosphoric acid (H3PO4)-polyvinyl alcohol (PVA) electrolyte was demonstrated as a gate dielectric for electrolyte-gated field-effect transistors (EGFETs). These devices exhibited high performance with sub 1 V operation, a high ON/OFF ratio >105 and a low subthreshold swing of 90 mV/decade. The results show the strong viability of proton conducting polymer electrolytes as gate dielectrics which open the door for further development of low-power EGFETs.

The increasing demand for wearable electronics has driven the development of supercapacitor electrode materials toward enhanced energy density, while being mechanically strong, flexible, as well as environmentally friendly and low-cost. Taking advantage of faradaic reaction of quinone groups in natural lignin that is covalently bound to the high-strength cellulose nanofibrils, the fabrication of a novel class of mechanically strong and flexible thin film electrodes with high energy storage performance is reported. The electrodes were made by growing polyaniline (PANI) on flexible films composed of lignin-containing cellulose nanofibrils (LCNF) and reduced graphene oxide (rGO) nanosheets at various loading levels. The highest specific capacitance was observed for the LCNF/rGO/PANI electrode with 20 wt% rGO nanosheets (475 F g-1 at 10 mV s-1 and 733 F g-1 at 1 mV s-1 ), which represented a 68 % improvement as compared to a similar electrode made without lignin. In addition, the LCNF/rGO(20)/PANI electrode demonstrated high rate performance and cycle life (87 % after 5000 cycles). These results indicated that LCNF functioned as an electrochemically active multifunctional component to impart the composite electrode with mechanical strength and flexibility and enhanced overall energy storage performance. LCNF/rGO(20)/PANI electrode was further integrated in a flexible supercapacitor device, revealing the excellent promise of LCNF for fabrication of advanced flexible electrodes with reduced cost and environmental footprint and enhanced mechanical and energy storage performances.

This study explores the conduction of hydroxide (OH−) ions in solid alkaline polymer electrolytes, and their contribution to the performance of solid-state electric double layer capacitors (EDLCs). The conductivity of tetraethylammonium hydroxide (TEAOH) polyacrylamide (PAM) and the capacitance of a solid-state EDLC based on this solid polymer electrolyte were investigated under three controlled relative humidity (RH) conditions (15, 45, and 75% RH). These properties are found to be highly dependent on the hydration structure of OH− ions that evolved in the different environments. To identify the role of hydration and ion structure in solid alkaline polymer electrolytes on device performance, dielectric analyses on the impedance spectra of TEAOH-PAM were utilized and revealed an interesting connection between the frequency dependent capacitive behaviour of the device and the ion response in the polymer electrolyte. The capacitance of EDLCs is closely related to the accumulation of ions at electrode/electrolyte interface, while the high frequency time constant for EDLCs corresponds to the transition of ion motion from vibration (energy dissipating) to translation (energy storing). Based on dielectric analysis and correlating the hydration of TEAOH-PAM with the activation energies of ionic conductivity in the three environments, we propose a framework to explain the changes of OH− ion transport in the solid-state. Under 15% RH, the segmental motion of the polymer chains facilitates the transport of TEAOH ions bound as 4:1 H2O:TEAOH crystal hydrates in the solid phase. As hydration rises under 45% RH, the segmental motion coordinates the conduction of 7.5:1 H2O:TEAOH crystal hydrates in the liquid phase. Under 75% RH, TEAOH-PAM is well hydrated and the conduction mechanism is dominated by liquid-like OH− ion-hopping of fully dissociated ions (hydration number 16.6:1). These results demonstrate the interplay of segmental motion and OH− ion-hopping in solid alkaline polymer electrolytes. The methods and findings of this study can be used to design solid aqueous polymer electrolytes with predictable long-term behaviour for inherently safe, thin, and lightweight electrochemical devices.

Chemically polymerized fuchsin (CpF) and electrochemically polymerized fuchsin (EpF) were developed to modify multi-walled carbon nanotubes (MWCNT) as composite electrodes for electrochemical capacitors (EC). The cyclic voltammograms (CV) of both modified electrodes exhibited multiple highly reversible redox features indicating pseudocapacitive behavior. The capacitance was ca. 2x greater than that of bare MWCNT electrode. While all electrodes exhibited excellent cycle life, CpF-MWCNT performed even better than its electropolymerized counterpart. Scanning electron microscopy (SEM) images showed the deposition of a 1-nanometer thin layer on MWCNT, with good adhesion. The chemical structure and thermal analyses confirmed that H 2 O 2 is an effective reactant to polymerize fuchsin, and the resulting polymer is semi-crystalline and stable for EC applications.

Two of the most promising current trends in supercapacitor research, (i) the development of biomass based carbon electrodes, and (ii) the transition to solid thin flexible form factors via polymer electrolytes are combined and investigated. A high surface area (3312 m2 g−1) mesoporous activated carbon was synthesized from chitosan biomass and showed excellent capacitive behaviour in a range of acidic, neutral, and alkaline liquid electrolytes. The performance in the neutral Li2SO4 electrolyte system was particularly promising with the chitosan AC electrodes showing a high capacitance (264 F g−1) similar to the values in acidic and alkaline electrolytes but with a much larger 1.8 V potential window. The chitosan AC also proved compatible with a series of solid polymer electrolytes through a detailed comparison in which solid-state chitosan supercapacitor devices were shown to closely mimic the capacitance and high rate performance of their liquid counterparts. This is an important finding as it demonstrates that high surface area, intricately porous activated carbon networks can still be readily accessible to solid electrolytes. Combining the chitosan AC with a Li2SO4-polyacrylamide (PAM) solid electrolyte enabled the fabrication of ultra-thin (

Proton-conducting polymer electrolytes consisting of borotungstic acid (BWA) and polyacrylamide (PAM) were developed. The addition of BWA to PAM progressively improved the electrolyte conductivity. The as-fabricated BWA-PAM electrolyte exhibited a high ionic conductivity of ca. 27 mS cm −1 , but suffered from low shelf life due to poor water retention. With the modification of 10 wt% phosphoric acid (H 3 PO 4 ), a longer shelf life and improved ionic conductivity of ca. 30 mS cm −1 were achieved. A ternary electrolyte composition of 75BWA-PAM + 10% H 3 PO 4 was identified to have a high conductivity and good shelf life. At BWA content greater than 75 wt%, the addition of H 3 PO 4 led to an undesirable phase separation within the electrolyte. The ternary 75BWA-PAM+ 10% H 3 PO 4 electrolyte was used to assemble solid cells with CNT-graphite electrodes. The cells exhibited 11.2 mF cm −2 at 50 mV s −1 and high rate performance at 1 V s –1 . The cells also retained ≥90% of their initial capacitance and maintained ca. −85° phase angle after 10,000 charging-discharging cycles at 6 A g −1 .

A novel asymmetrical-bipolar electrochemical capacitor system leveraging the contributions of a Zn-CNT asymmetrical electrode and a KOH-H2SO4 dual-pH electrolyte was developed. The positive and negative electrodes operated in electrolytes with different pH, exploiting the maximum potential of both electrodes, which led to a cell voltage of 2.4 V. The potential tracking of both electrodes revealed that the Zn negative electrode could maintain a potential at −1.2 V, while the CNT positive electrode can be charged to +1.2 V without significant irreversible reactions. A bipolar ion exchange membrane has effectively separated the acid and alkaline from neutralization, which resulted in stable performance of the device with capacitance retention of 94% and coulombic efficiency of 99% over 10,000 cycles. This asymmetrical-bipolar design overcomes the thermodynamic limit of water decomposition, opening a new avenue towards high energy and high power density aqueous-based ECs.

Li2SO4–polyacryalamide (PAM) neutral pH polymer electrolytes with different salt:polymer molar ratios (5000:1 and 10000:1) were characterized for their ionic conductivity and material properties. T...

The effect of SiO2 additives on the conductivity and longevity of an alkaline tetraethylammonium hydroxide (TEAOH)-poly(acrylamide) (PAM) polymer electrolyte was investigated. Electrochemical impedance spectroscopy (EIS) and Raman microscopy studies were performed for TEAOH-PAM with micro-sized (mSiO2) or nano-sized (nSiO2) additives under highly hydrated and under ambient conditions. At a high relative humidity (RH) of 75%, nSiO2 significantly increased the ionic conductivity of OH-, achieving 25 mS cm-1, while mSiO2 had little influence (10 mS cm-1). Further investigation at lower RH (45%) revealed that dehydration of TEAOH led to crystallization and lower conductivity of the polymer electrolytes. The degree and rate of crystallization in the different systems varied greatly: mSiO2 accelerated the process while nSiO2 delayed it. Using characteristic signatures obtained from Raman microscopy, a correlation between the ionic conductivity and the structural differences among these systems has been established and an explanation for the impact of the SiO2 additives has been proposed.

Battery-capacitor asymmetrical systems, using Zn as battery-type negative electrode and CNT as capacitor-type positive electrode are developed with both aqueous and solid polymer based electrolytes. The devices combine the merits of both electrodes: The Zn electrode contributed 1.6 V cell voltage, while the CNT electrode results in fast and reversible capacitive behavior. An all-solid asymmetrical system is demonstrated using a PVA-TEAOH polymer electrolyte and exhibit high power capability and long cycle life. The good capacitive behavior, excellent stability upon 10,000 cycles of charge and discharge, and high cell voltage suggest a promising system for thin film solid energy storage applications.

A tetraphenylporphyrin sulfonate-carbon nanotube (TPPS-CNT) composite was developed and characterized for capacitive charge storage. Density functional theory (DFT) simulations suggested that the adsorption of TPPS on CNT was energetically favored with a strong contribution of the sulfonate groups. The composite electrodes, with a

This study focused on the design and fabricating of a new type of hydroxyl anion conducting membranes employing the interpenetrating polymer network (IPN) comprising poly (vinyl alcohol) (PVA) and linear structured poly bis(2-chloroethyl) ether-1,3-bis [3-(dimethylamino)propyl] urea copolymer (PUB). The membranes are synthesized through blending assisted by a simple chemical cross-linking process. Various characterizations are conducted including Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), gravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS) and AC impedance. Results revealed that by simply tuning the mass fraction of PUB (the content of PUB changed from 20%–50%) in the membrane, the OH – conductivity (10 − 2 S cm − 1 at 80 °C, fully hydrated membranes) with high extensibility (at break in the range of 200–400%) and tensile stress (at break around 15–30 MPa, ∼ 50% relative humidity) are achieved. XPS analysis reveals that a slight degradation occurs when the membrane is exposed to exceedingly tough conditions such as 8 M KOH at 80 °C for 240 h, but the OH – conductivity is changed insignificantly. When assembled in a real H 2 /O 2 alkaline fuel cell, the initial peak power densities in the range of 5.7–28.6 mW cm − 2 and an open circuit potential reaching to 1.0 V are obtained for MEAs fabricated with these membranes at 25 °C.

A neutral polymer electrolyte comprised of lithium sulfate (Li2SO4) and polyacrylamide (PAM) was developed. The Li2SO4-PAM electrolyte film shows an ionic conductivity up to 10mScm−1 in 45%RH conditions. Solid double layer capacitors were demonstrated using CNT-graphite electrodes and Li2SO4-PAM solid electrolytes. The voltage window of the solid cell was about 2.0V, identical to that of a Li2SO4 liquid cell used as baseline. The demonstrated voltage window is significantly larger than that reported for proton- or hydroxyl-conducting electrolytes, suggesting that the Li2SO4-PAM electrolyte is a promising system for high energy density supercapacitors. The solid device also demonstrated excellent rate capability (up to 5Vs−1) and good cycle life (beyond 10,000charge/discharge cycles). Keywords: Solid supercapacitor, Polymer electrolyte, Neutral pH electrolyte, Wide voltage window

Pine cones were used as a biomass template for the synthesis of activated carbons with high specific surface area (up to 2450 m2 g−1) and a pore structure optimized for the adsorption of redox active polyoxometalate (POM) clusters. We have found that POM adsorption is highly favored within a carbon matrix possessing pore diameters in the 1–2 nm range. These large micropores are big enough to accommodate the large POM cluster, while still being small enough to effectively trap and hold the molecule. Pine cone activated carbon with this optimal pore arrangement demonstrated ultra-high loading of the PMo12O403− (PMo12) molecule resulting in carbon–POM hybrid materials consisting of over 55 wt% PMo12. This large POM loading imparted tremendous redox activity to the already large double layer capacity of the carbon substrate, leading to a high areal capacitance of 1.19 F cm−2 for the hybrid material, close to 2.5 times larger than for unmodified carbon. We have also demonstrated that a mixed molecular modifier combining multiple POM chemistries can be adsorbed onto the activated carbon substrate to create a more ideally capacitive charge storage profile. These results demonstrate a promising method for the design of high performance yet cost effective hybrid energy storage electrodes.

A new hydroxide (OH−) ion-conducting polymer electrolyte comprised of tetraethylammonium hydroxide (TEAOH) and polyacrylamide (PAM) was developed. This electrolyte exhibits excellent ionic conductivity greater than 10 mS cm−1 at room temperature and stable shelf-life over an 80 day exposure in various environments. Solid electrochemical double layer capacitors (EDLC) were fabricated and compared to their liquid counterparts. While all EDLC devices showed similar capacitance, the solid EDLC devices outperformed the liquid devices in cycle-life and offered additional advantages such as safety, light weight, and a flexible form factor. Based on structural, thermal, and chemical analyses, this excellent performance can be attributed to a stable amorphous structure and the higher degree of hydration of the polymer electrolyte promoted by a slight hydrolysis between TEAOH and PAM.

A neutral polymer electrolyte containing lithium chloride (LiCl) and polyacrylamide (PAM) was developed. The LiCl-PAM electrolyte film had an amorphous structure and an ionic conductivity > 10 mS cm−1. The addition of LiCl to the polyacrylamide did not alter the chemical bonding of PAM. Symmetric double layer capacitors (EDLC) were constructed using CNT-graphite electrodes. The solid EDLC retained approximately 85% of the capacitance achieved with a baseline cell in a LiCl aqueous solution. The solid EDLC devices demonstrated a wide voltage window (1.5 V), good cycle life (>10,000 cycles), and excellent rate capability (up to 5 V s−1). Keywords: Solid supercapacitors, Neutral pH electrolyte, Polymer electrolyte

Proton conducting polymer electrolytes employed in many electrochemical pseudocapacitors are typically based on aqueous systems, which limit the operating potential and temperature range of these energy storage devices. In this study, we developed non-aqueous polymer electrolytes based on protic ionic liquids (PIL). We relied on cationic substitution to obtain proton conducting yet environmentally benign non-fluorinated ionic liquids. By developing binary systems of PILs with different cations, eutectic compositions of PILs with drastically lowered melting points were demonstrated. Through thermal analyses of these binary systems, phase diagrams were constructed which allowed us to obtain eutectic binary PIL mixtures exhibiting a liquidus range from − 70 °C to 150 °C. These eutectic PIL mixtures were incorporated into polymer systems to develop non-aqueous thin film proton-conducting polymer electrolytes for solid pseudocapacitors. The proton conductivity of the eutectic ionic liquids was observed in the polymer electrolyte systems and promoted pseudocapacitive behavior in solid and liquid capacitor cells.

Polymer electrolytes are key enablers in solid flexible devices for sensing, ac-line filtering and energy storage. While most of the reports in literature are focusing on demonstration of their important functionalities and promising performance in pristine devices, little attention has been given to the long term service life and shelf life of the devices or the failure mechanisms of the polymer electrolytes. In this talk we will discuss these issues through a few case studies to show the influential factors on the properties of the polymer electrolytes and the longevity of the solid devices.

A novel all-solid dual-pH electrochemical capacitor was developed. The device leverages a bi-polar ion exchange membrane (IEM) to separate an acidic polymer electrolyte from an alkaline one. Due to the selective ion permeability of the cation and anion sides of the bi-polar IEM, acid-base neutralization is minimized, so that the potential of both sides can be stabilized. As a result, the device reaches a high cell voltage of 1.9 V, surpassing the thermodynamic water decomposition window. This is the first time that the dual-pH electrolyte is demonstrated in an all-solid device. These findings are especially important towards the development of high voltage aqueous energy storage device for future wearable electronics.

Dimethyl sulfoxide (DMSO) was added to an aqueous-based Na2SO4-polyacrylamide (PAM) electrolyte to improve its low temperature performance. The addition of DMSO significantly enhanced the ionic conductivity of Na2SO4-PAM binary electrolyte at sub-zero temperatures, while virtually maintained their excellent properties at and above ambient temperature. The improvement in low temperatures was correlated to the lower melting point and smaller crystal hydrates at freezing point. Solid capacitor cells using carbon nanotube electrodes with the ternary Na2SO4-PAM-DMSO electrolytes were assembled and compared against the binary Na2SO4-PAM baseline. Under ambient conditions, both cells showed similarly wide 1.8 V window and excellent rate capability. While the cells with binary electrolytes showed pure resistive behaviour at -20 °C, those with ternary electrolytes showed significant improvement in their capacitive voltammogram profile and impedance spectra. The results showed that DMSO can be effective in widening the operating temperature of neutral pH polymer electrolytes and their enabled solid capacitive devices.

The development of neutral pH polymer electrolytes has enabled high-performance solid-state, thin, and flexible electrochemical capacitors (ECs) to provide power for future consumer electronics and Internet-of-Thing devices. Notwithstanding their promising prospect, there is still some lack of understandings or disconnections from fundamental science to practical applications of these electrolytes. In this review, we provide an overview of state-of-the-art studies on ECs with neutral pH electrolytes in both liquid and solid configurations. Starting from the fundamental studies on the voltage window and ion conduction of salt species in liquid solution to polymer electrolytes, key considerations in developing neutral pH polymer electrolytes are discussed. The performance of the polymer electrolytes along with their enabled solid symmetric and asymmetric EC devices, as well as some enhanced functionalities are presented. The future directions for research on neutral pH polymer electrolytes are proposed, expected to provide reference for further enriching the fundamental knowledge and improving the device performances.