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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.

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 .

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.

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.

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.

Hydroxide (OH − ion)-conducting polymer electrolytes are essential for enabling cost effective, solid-state energy storage devices such as electrochemical capacitors (ECs). Ultra-high rate ECs using OH − ion-conducting polymer electrolytes have been developed and characterized. These polymer electrolytes comprised of aqueous tetraethylammonium hydroxide (TEAOH) incorporated into poly(vinyl alcohol) (TEAOH-PVA), poly(ethylene oxide) (TEAOH-PEO), or poly(acrylic acid) (TEAOH-PAA). Solid EC cells enabled by TEAOH-PVA, TEOAH-PEO, and TEAOH-PAA were characterized by electrochemical impedance spectroscopy and cyclic voltammetry under ambient conditions, demonstrating clear differences in performance. Through XRD and DSC analyses, the effects of the polymer hosts on the OH − ion-transport mechanism was revealed. The differences in OH − ion-transport in the polymer electrolytes is likely a result of differences in crystallinity, hydrophilicity, and functional group electronegativity of the polymer hosts. TEAOH-PVA and TEAOH-PAA based solid EC cells achieved an ionic conductivity of 5 mS cm −1 and an ultra-high rate of 5000 V s −1 making them suitable for EC applications.

Imidazolium cations derived from ionic liquids were demonstrated as effective linker molecules for the layer-by-layer (LbL) deposition of polyoxometalates (POMs) to increase the charge storage of multi-walled carbon nanotube (MWCNT) electrodes. MWCNTs modified with GeMo12O40(4-) (GeMo12) via an imidazolium cation linker demonstrated highly reversible redox reactions and a capacitance of 84 F cm(-3), close to 4 times larger than bare CNT. Compared to CNT-GeMo12 composites fabricated with a conventional polyelectrolyte linker poly(diallyldimethylammonium chloride), (PDDA), the imidazolium cations resulted in lower POM loading, but higher conductivity and in turn superior performance at fast charge-discharge conditions. A polymerized imidazolium linker (PIL) was also synthesized based on the ethyl-vinyl-imidazolium monomer. CNT-GeMo12 composites fabricated with this PIL achieved high POM loading comparable to PDDA, while still maintaining the good conductivity and high rate capabilities shown by the monomer imidazolium units. The high conductivity imparted by the PIL is especially valuable for the fabrication of multilayer POM composites. Dual-layer GeMo12 O40(4-)-SiMo12O40(4-) (GeMo12-SiMo12) electrodes built with this PIL demonstrated a combined contribution of the individual POMs resulting in a capacitance of 191 F cm(-3), over nine times larger than bare MWCNT. The PIL dual layer composites also maintained 72% of this capacitance at a fast rate of 2 V s(-1), compared to just over 50% retention for similar electrodes fabricated with PDDA.

Ultra-fast solid electric double layer capacitors (EDLCs) have been developed in both sandwich and planar interdigitated configurations using vertically-oriented graphene nanosheet (VOGN) electrodes with a hydroxide ion-conducting tetraethylammonium hydroxide (TEAOH)–polyvinyl alcohol (PVA) polymer electrolyte. These solid-state EDLCs could be scanned at a rate of 1000 V s −1 in cyclic voltammetry and demonstrated response times of less than 1 ms. They retained high performance over 18 months of shelf storage and after 100,000 charge/discharge cycles with limited packaging, demonstrating the high stability of TEAOH–PVA electrolyte. The solid-state capacitors are capable of performing at elevated temperatures and have demonstrated a response time of 0.35 ms at 90 °C. Given their ultra-fast rate capability, excellent shelf-life and cycle life, and excellent temperature stability, these solid-state EDLCs are promising smaller and lighter alternatives to the bulky electrolytic capacitors now used for ac line-filtering.

A facile method to combine Keggin polyoxometalates (POMs) e.g. PMo12O403− (PMo12) and PW12O403− (PW12) in aqueous solutions has led to a unique electrochemical phenomenon, in which the POM mixtures demonstrate their own tunable redox activity, far different from the pure components. Through electrochemical and NMR analyses we have confirmed that this interesting behaviour arises from the spontaneous hydrolysis and recombination of PMo12 and PW12 ions forming complex blends of PMo12-xWx mixed addenda chemistries. While this process occurs spontaneously for the PMo12-PW12 system due to the ready hydrolysis, it can be induced in other POM mixtures such as SiMo12-SiW12 by adjusting the pH to stimulate hydrolysis. The highly tunable redox properties of the mixtures can be leveraged to design molecular coatings for a variety of applications. When deposited on nanocarbon, the overlapping redox features of the POM mixtures result in cyclic voltammograms with substantial pseudocapacitive currents which maintain relatively smooth charge/discharge profiles, ideal for supercapacitor electrodes.

A polymer electrolyte comprised of H5BW12O40 (BWA) and cross-linked polyvinyl alcohol (BWA–XLPVA) has been developed and characterized for solid supercapacitors. The performance of this polymer electrolyte was compared to that of a known polymer electrolyte based on H4SiW12O40 (SiWA). An enhanced proton conductivity was observed for BWA–XLPVA compared to its SiWA counterpart, especially under low humidity conditions (5% RH). Dielectric analyses revealed an increase of proton density and proton mobility in the BWA-based electrolyte. A solid-state 1H NMR study showed that all protons in the BWA-based electrolyte were hydrated in the low humidity environment. This indicated that BWA had more crystallized water content than SiWA, resulting in higher proton mobility in the PVA matrix. An in situ tracking of electrode potential in solid supercapacitors was utilized to identify the reactions and the factors limiting solid supercapacitor cell voltage for both BWA- and SiWA-based polymer electrolyte systems. A solid device leveraging the BWA-based polymer electrolyte achieved a cell voltage of 1.3 V, 0.2 V wider than that of a SiWA-based device.

Polymer electrolytes are key enablers for solid-state electrical double layer capacitors (EDLCs) that have thin and flexible form factors. Aqueous-based polymer electrolytes with neutral pH are interesting for their non-corrosiveness, intrinsically safe, and wider voltage window from high overpotential on water decomposition [1-3]. Previously, two neutral pH polymer electrolytes based on polyacrylamide (PAM) host and Li2SO4 [4] or Na2SO4 [5] as ion conductor were developed. These two systems demonstrated good chemical stability (shelf-life >30 days) and wide voltage window (1.9 V with activated carbon). Na2SO4-PAM showed higher ionic conductivity than Li2SO4-PAM under ambient conditions. Nonetheless, the effects of temperature and change in structure (e.g. freezing) to the electrochemical performance are still unclear. In this study, the performances of Li2SO4-PAM and Na2SO4-PAM at different temperatures were compared. The objectives were to (i) understand the ion conduction mechanisms within the polymeric structure, (ii) investigate the working temperature range of these electrolytes, and (iii) correlate the ion-conducting behaviour to their respective thermal properties, especially at the temperature below 0 °C. Metallic cells were constructed to study the electrochemical characteristics of the polymer electrolytes, while differential scanning calorimetry (DSC) was employed to characterize the thermal properties. From the activation energy of conduction above ambient temperatures, ion hopping was considered as the primary mechanism for ion movement in both electrolytes. Interestingly, although Li2SO4-PAM was able to maintain capacitive behaviour 2SO4-PAM underwent severe loss in capacitive behaviour and significant loss in ionic conductivity. By comparing the DSC curves against the respective ionic conductivity at low temperatures (Fig. 1), the amount of crystallized water involved during freezing/melting was deduced as the source of this behaviour. This study elucidated the structure-performance relationship of polymer electrolytes at the low temperature conditions. References: [1] K. Fic, G. Lota, M. Meller, and E. Frackowiak, “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] H.Y. Jin, Z.H. Peng, W.M. Tang, and H.L.W. Chan, “Controllable functionalized carbon fabric for high-performance all-carbon-based supercapacitors”, RSC Advances, 4, 2014 [4] A. Virya and K. Lian, “Li2SO4-polyacrylamide polymer electrolytes for 2.0 V solid symmetric supercapacitors,” Electrochem. Comm., 81, 2017 [5] A. Virya, J. Abella, A. Grindal, and K. Lian, “Na2SO4‐polyacrylamide electrolytes and enabled solid‐state electrochemical capacitors,” Batteries & Supercaps, 2019 Figure 1: Ionic conductivities at various low temperature overlaid with DSC curve for (a) Li2SO4-PAM and (b) Na2SO4-PAM Figure 1

Solid, thin and flexible supercapacitors have been investigated leveraging sustainable and low-cost biomass-based carbon electrodes and a series of solid polymer electrolytes. The performance of these solid flexible devices was systematically compared to commercial activated carbon (AC) and liquid electrolyte baseline. Solid-state devices especially chitosan AC enabled supercapacitors were shown to closely resemble the highly capacitive behavior and high rate performance of their liquid counterparts. This demonstrates that high surface area, intricately porous activated carbon networks can still be readily accessible to polymer electrolytes, which is important to the transition of supercapacitor devices from liquid to solid-state. These materials and systems represent simple, sustainable and cost-effective approaches for next-generation solid thin, flexible energy storage devices.

Modification of organic substrates with inorganic polyoxometalate (POM) clusters can be used to engineer nanocomposite materials with improved properties and diverse functionalities. This review will outline concepts and methodologies for fabricating POM based inorganic–organic composite materials with a special focus on the electrochemical functionality of these composites for energy storage applications. The strengths and limitations of three different fabrication techniques, chemisorption to a carbon surface, immobilization in a polymer matrix, and layer-by-layer self-assembly will be assessed. Furthermore, the latest developments in the use of POM nanocomposite materials in energy storage applications like electrochemical capacitors (ECs) and lithium ion batteries will be presented. This review will highlight the issues and challenges that need to be addressed to achieve inorganic–organic POM nanocomposites able to support high performance energy storage applications.

A thin film Mo oxide–nitride pseudocapacitive electrode was synthesized by electrodeposition of Mo oxide on Ti and a subsequent low-temperature (400 °C) thermal nitridation. Two nitridation environments, N2 and NH3, were used and the results were compared. Surface analyses of these nitrided films showed partial conversion of Mo oxide to nitrides, with a lower conversion percentage being the film produced in N2. However, the electrochemical analyses showed that the surface of the N2-treated film had better pseudocapacitive behaviors and outperformed that nitrided in NH3. Cycle life of the resultant N2-treated Mo oxide–nitride was also much improved over Mo oxide. A two-electrode cell using Mo oxide–nitride electrodes was demonstrated and showed high rate performance.

The effects of SiO 2 and TiO 2 nanofillers on a poly(ethylene oxide) (PEO)–1-ethyl-3-methylimidazolium hydrogensulfate (EMIHSO 4 ) electrolyte were studied and compared with respect to ionic conductivity, crystallinity, and dielectric properties over a temperature range from −10 °C to 80 °C. X-ray diffraction and differential scanning calorimetry were used to study the impact of fillers on the structure of the polymer electrolytes. Using an electrochemical capacitor model, impedance (complex capacitance) and dielectric analyses were performed to understand the ionic conduction process with and without fillers in both semi-crystalline and amorphous states. Despite their different nanostructures, both SiO 2 and TiO 2 promoted an amorphous structure in PEO–EMIHSO 4 and increased the ionic conductivity by a factor of two. While in the amorphous phase, the dielectric constant characteristic of the fillers (TiO 2 in this case) contributed to increased conductivity and cell capacitance. Combining complex capacitance and dielectric analyses is an effective approach to study solid electrochemical capacitors and to identify and explain the impact of different fillers on ionic conduction of polymer electrolytes.

A high performance exfoliated biochar carbon with a layered nanosheet structure was prepared from a low cost agricultural residue (corn cob) via a novel synthesis strategy involving biomass pre-treatment, nitrogen pyrolysis, and a high temperature thermal–chemical flash exfoliation. The exfoliation strategy resulted in porous carbon nanosheets with BET specific surface area of 543.7 m2 g−1, far higher than the 7.9 m2 g−1 of the natural biochar produced without any pre- or post-treatment modifications. The exfoliated material also showed increased oxygen functionality in the form of electrochemically active quinone and pyrone surface groups. This combination of high specific surface area and highly active surface functional groups resulted in very promising capacitive performance, demonstrating a high capacitance of 221 F g−1, over 100 times greater than the natural biochar. The exfoliated biochar electrodes fabricated without any conductive or organic additives showed outstanding high rate capability retaining 78% of their low rate capacitance at a fast 40 A g−1 discharge. This combination of high capacitance and fast charge–discharge capability distinguishes this material from most other high surface area activated carbons; in fact, the electrochemical behaviour more closely resembles that of designer nanomaterials such as graphene and carbon nanotubes. The biochar electrodes were also extremely durable showing only a 3% reduction in capacitance after 5000 successive potential cycles. The exfoliation strategy developed here could provide a novel route for the low cost production of high performance energy storage materials from a variety of waste biomass feedstocks.

The interactions of H2O and H2S monomers with Cu(111) in the absence and presence of an external electric field are studied using density functional theory. It is found that the adsorption is accompanied by a rippled pattern of the surface Cu atoms and electron accumulation on the surface Cu atoms surrounding the adsorption site. The response of the H2O/Cu(111) and H2S/Cu(111) interfaces to the external electric field is computed up to the field magnitude of 10(10) V m(-1). The results show that H2O rotates and translates much more with an electric field than H2S does. The extent of the surface deformation changes considerably with the applied electric field, which influences the translation pattern of the adsorbates. On the other hand, the rotation of the adsorbates is correlated to the dipole moment of the molecules and their adsorption energies.

H5BW12O40 (BWA), a heteropolyacid with B3+ as the central heteroatom, was developed as an electrolyte for solid supercapacitors. The structural properties of the synthesized BWA were investigated using FTIR and XRD and were compared to those of the known silicotungstic acid (H4SiW12O40, SiWA) electrolyte. Factors affecting the cell voltage of supercapacitors using liquid BWA and SiWA electrolytes were identified using an in situ electrode potential tracking method. The respective contributions of the electrodes and electrolyte were revealed and BWA showed a wider potential window than SiWA. Solid supercapacitors enabled by BWA- and SiWA-based polymer electrolytes were demonstrated and exhibited excellent rate performance. The solid device leveraging a BWA-based polymer electrolyte achieved a cell voltage of 1.6 V, better than 1.4 V achieved by the SiWA-based device.

A thin film vanadium oxide electrode was synthesized by a simple electroless deposition method. Surface and structural analyses revealed that the deposited oxide is a mixture of amorphous V 2 O 5 and VO 2 . Electrochemical characterizations of the synthesized vanadium oxide showed capacitive behavior with good cycle life. The electroless deposition of vanadium oxide is inexpensive, easy to process, and environmentally benign, offering a promising route for electrode development for electrochemical capacitors.

The modification of conductive organic supports with polyoxometalate (POM) metal oxide clusters is a promising approach for the design of nanocomposite electrodes for energy storage. The fast and reversible electron-transfer processes of these POM molecules make them particularly well suited for supercapacitor (SC) applications. This chapter will provide a detailed overview of POM-based composite SC electrodes, including an evaluation of the most promising fabrication methods: (1) chemisorption to a carbon surface, (2) immobilization in a conductive polymer matrix, and (3) layer-by-layer self-assembly. Furthermore, the role of these POM composite materials in enhancing the performance of SC devices will also be reviewed, from pioneering work to the latest developments. This will include an analysis of how the tunability of POM redox properties can be leveraged to move from faradaic SC electrodes using only a single POM chemistry to pseudocapacitive electrodes incorporating multiple different POM molecules.

Lithium, sodium, and potassium salts of silicotungstic acid were synthesized and characterized as aqueous neutral electrolytes for electrochemical supercapacitors. The acidity of the aqueous solution and the structure of the solid-state anion were examined to confirm the presence of SiW salts. Ionic conductivity and the electrochemical stability potential window were characterized and compared to a silicotungstic acid solution using metallic blocking electrodes. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the performance of carbon EDLC cells enabled by the neutral electrolytes and revealed a 1.5 V cell voltage and good cycle life. The similarities and differences among the three salts are explained based on the properties of cations in these neutral electrolytes.

Mo oxynitride was developed as pseudocapacitive electrode material for supercapacitors. A thin film Mo oxynitride was produced by first electrodeposition of Mo oxide on a Ti substrate followed by a low-temperature (400 oC) heat treatment in N2 environment. XPS analyses showed that the surface of the Mo oxynitride film was composed of a mixture of MoO3, MoO2 and less than 20 at.% Mo2N. However, the electrochemical behavior of the Mo oxynitride film was significantly different from that of Mo oxide but much similar to the behavior of pure Mo2N, a known pseudocapacitive material. The cycle life and stability of the Mo oxynitride were much improved over the Mo oxide. A two-electrode symmetric cell using the developed Mo oxynitride electrodes was demonstrated and showed high rate performance. An asymmetric cell using a Mo oxynitride as negative electrode and a carbon as positive electrode was also established and showed an extended voltage window and, thus, an increased energy density.

The interaction of water monomers with a gold surface is investigated using density functional theory (DFT) to develop a better understanding of the response of a water molecule to an imposed elect...