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Supercapacitor electrode slurry is prepared for mass production by mixing activated carbon powder, conductive agent, and binder, which is then deposited on a substrate using the doctor-blade method. Polyvinylidene fluoride (PVDF) and 1-methyl-2-pyrrolidone (NMP) are used as binder and solvent, respectively, to form the electrode slurry on a metal substrate. In this study, ethyl cellulose (EC) is evaluated as a binder to prepare an electrode on an indium-tin-oxide (ITO) substrate obtaining transparent supercapacitors. Terpineol and isopropyl alcohol (IPA) are compared as suitable solvents for the EC binder. When terpineol is employed as a solvent, the conductive agent is uniformly deposited around the activated carbon powder. An electrode prepared using EC and terpineol exhibits slightly lower specific capacitance and rate performance than that using conventional PVDF and NMP. However, the electrode prepared using EC and terpineol securely adheres to the electrode components, resulting in a robust electrode. In contrast, an electrode prepared using EC and IPA exhibits high charge transfer resistance at the interface of the electrode/electrolyte, leading to a low specific capacitance and rate performance. Thus, ecofriendly EC and terpineol can substitute the conventional PVDF and NMP for depositing activated carbon powder on an ITO substrate, while improving the specific capacitance of manufactured electrodes.

Composites were prepared, through hot pressing, using carbon materials with different pore size distributions as additives for commercial Bi0.5Sb1.5Te3 thermoelectric material (BST, p-type). Thermoelectric properties of the composites were measured in a temperature range of 298‒473 K. Thermal conductivity of the composites, especially lattice thermal conductivity, was effectively decreased due to the mesoporous properties of the incorporated carbon additives. The electrical conductivity of the composites slightly decreased due to the electron scattering at the interface between the carbon material and the commercial BST matrix. The composite with 0.2 vol.% mesoporous carbon powder (36% mesoporosity) exhibited a figure of merit value approximately 10.7% higher than that of commercial BST without additives. This behavior resulted in 116% improved output power in the composite block-based single element compared with a bare BST thermoelectric block. The enhanced figure of merit was attributed to the effective reduction of lattice thermal conductivity by acoustic phonons scattering at the interface between the BST matrix and the mesoporous carbon as well as at the pore surfaces within the mesoporous carbon. By utilizing mesoporous carbon materials used in this study, the shortcomings and economic difficulties of the composite process with low dimensional carbon additives (carbon nanotubes, graphene, and nanodiamond) can be overcome for extensive practical applications. Mesoporous carbon powder with a tailored porosity distribution revealed the validity of bulk-type carbon additives to enhance the figure of merit of commercial thermoelectric materials.

Porous carbon has found commercial applications as a filter material based on the sorption ability of its pores. The pore size and surface properties of the carbon can be varied depending on the type of particles to be filtered. Here, mesoporous carbon was induced through the pyrolysis of polyvinylidene fluoride (PVDF) to fabricate a porous material for microparticle filtration. Because removal of the constituent fluorine at elevated temperature leaves small-sized micropores, the PVDF precursor mainly generates micropores during pyrolysis. To suppress the micropore evolution mechanism, the PVDF precursor was defluorinated before the heat treatment using 1,8-Diazabicyclo[5.4.0]undec-7-ene(DBU) and then pyrolyzed. The suppressed evolution of the micropores during carbon synthesis leads to a lower specific surface area, suggesting low adsorption capacity. The polytetrafluoroethylene (PTFE) was mixed with the PVDF precursor to induce mesoporosity. The PVDF precursor mixed with the PTFE enhanced the surface area since the PTFE could be removed, leaving mesopores after pyrolysis. The effect of the defluorination process on the porosity was investigated by varying the ratio of DBU to vinylidene fluoride unit (1, 5, 10, 20) in the precursor solution. With higher DBU content in the precursor, the micropore evolution was reduced with a lower specific surface area. The porous carbons synthesized from the precursor with a high DBU amount (DBU/vinylidene fluoride unit = 5, 10, 20) were almost entirely composed of mesopores. In addition, the higher DBU content reduced the hydrophilicity of the synthesized carbon. In summary, to separate and absorb relatively large impurities, the mesoporous carbon should be synthesized using a mixture of PVDF and PTFE precursor with an appropriate amount of DBU for a higher specific surface area.

Coprecipitation effortlessly fabricated nickel hexacyanoferrate (NiHCF) with outstanding rate capability and stability for aqueous batteries. Citrate-aided coprecipitation decelerated the crystallization, assembling cubic-shaped powder based on separation between nucleation and growth. This study revealed that coprecipitation temperature determined the electrochemical performance. With lower temperatures, smaller particles with more water were formed by predominant nucleation, resulting in low crystallinity and capacity of 58 mAh g-1 . Expanded surface area reduced electrode/electrolyte interface charge-transfer resistance and showed excellent rate capability (79 % of initial capacity at 100 C-rate). However, poor cyclability was obtained. At elevated temperatures, nuclei growth and dehydration occurred, and thus highly crystalline large particles were formed. In turn, NiHCF delivered excellent capacity of 76 mAh g-1 at 1 C-rate but exhibited inferior rate performance because of longer diffusional path. Meanwhile, normal coprecipitation at 70 °C induced irregular-shaped tiny particles, presenting 93 % retention of initial capacity at 100 C-rate.

Nickel hexacyanoferrate (NiHCF) has a three-dimensional open framework structure, excellent long-cycling stability and rate performance as a cathode for aqueous sodium-ion batteries. However, the specific capacity of NiHCF is lower than that of present cathodes for aqueous batteries. A sodium-ion electrolyte was explored to achieve optimum capacity with NiHCF. Powder-type NiHCF was fabricated by coprecipitation with the atomic composition K0.065Ni1.44Fe(CN)6·4.4H2O. The presence of Fe vacancies in the atomic composition is attributable to the inclusion of coordinating and zeolitic water during coprecipitation. Two sodium-ion electrolytes, 1 M Na2SO4 and 1 M NaNO3, were employed to analyze the electrochemical behavior of the NiHCF electrode. Identical redox potentials to 0.58 V (vs. NHE) were measured in both electrolytes. However, a lower overpotential was observed in NaNO3 compared to Na2SO4 as a result of the smaller interfacial charge transfer resistance. The lower charge transfer resistance in the NaNO3 solution produced a higher specific capacity of 57 mAh g-1 (1 C-rate) and the superior capacity retention of 46.6% at 20 C-rate. The anion in the aqueous electrolyte changed the charge transfer resistance at the electrode/electrolyte interface, confirming the electrolyte anion has a crucial effect on the charge capacity and rate performance of NiHCF.

A transparent energy storage device is an essential component for transparent electronics. The increasing demand for high-power devices stimulates the development of transparent supercapacitors with high power density. A transparent electrode for such supercapacitors can be assembled via the electrophoretic deposition of an active material powder with a binder onto a transparent substrate. The properties of the binder critically influence the electrochemical behavior and performance of the resulting electrode. Ethyl cellulose (EC) is known as an eco-friendly, transparent, flexible, and inexpensive material. Here, we fabricated an electrode film with EC binder via electrophoretic deposition on an indium tin oxide (ITO) substrate instead of using the conventional polytetrafluoroethylene (PTFE) binder. The assembled electrodes with EC and PTFE were compared to investigate the feasibility of EC as a binder from different perspectives, including homogeneity, wettability, electrochemical behavior, and mechanical stability. The EC enabled the formation of a homogeneous film composed of smaller particles and with a higher specific capacitance compared with films prepared with PTFE. The annealing improved the adhesion strength of the EC because of its glass transition; however, its hydrophobic nature limited utilization of the active material for charge storage. Subsequent electrochemical activation improved the wettability of the electrode, resulting in an increased capacitance of 60 F g−1. Furthermore, even with the lower wettability of EC compared with that of PTFE, better rate performance was possible with the EC electrode. The increased mechanical stability after the annealing process ensured an excellent cycle life of 95 % capacitance retention for 15,000 cycles.

We report the synthesis of manganese-doped nickel cobalt oxide (Mn-doped NiCo2O4) nanoparticles (NPs) by an efficient hydrothermal and subsequent calcination route. The material exhibits a homogeneous distribution of the Mn dopant and a battery-type behavior when tested as a supercapacitor electrode material. Mn-doped NiCo2O4 NPs show an excellent specific capacity of 417 C g-1 at a scan rate of 10 mV s-1 and 204.3 C g-1 at a current density of 1 A g-1 in a standard three-electrode configuration, ca. 152-466% higher than that of pristine NiCo2O4 or MnCo2O4. In addition, Mn-doped NiCo2O4 NPs showed an excellent capacitance retention of 99% after 1000 charge-discharge cycles at a current density of 2 A g-1. The symmetric solid-state supercapacitor device assembled using this material delivered an energy density of 0.87 μW h cm-2 at a power density of 25 μW h cm-2 and 0.39 μW h cm-2 at a high power density of 500 μW h cm-2. The cost-effective synthesis and high electrochemical performance suggest that Mn-doped NiCo2O4 is a promising material for supercapacitors.

Easy shapeable highly porous and robust three dimensional (3D) nano-carbon architectures (3D NCA) are crucial for the practical applications of electrochemical energy storage devices. Here, a facile easy shapeable nitrogen-doped graphene coated 3D NCA exhibiting an ultra-high specific surface area, remarkable robustness, and excellent aqueous wettability is reported. A 3D single-walled carbon nanotube (SWCNT) hydrogel composed of isolated SWCNTs is first prepared, and then a thin polydopamine (pDA) layer is uniformly coated onto the fabricated 3D SWCNT hydrogel via an in situ polymerization of dopamine. A nitrogen-doped graphene-coated 3D NCA is obtained via pyrolysis of the pDA-coated 3D NCA. By decorating this highly porous nitrogen-doped 3D NCA onto helical micro carbon fibers, a highly stretchable (∼100% strain) wire-type supercapacitor (WTSC) is fabricated. The areal specific power and energy density of the WTSC are determined to be 2.59 mW cm−2 and 1.1 μWh cm−2, respectively. These values are remarkably larger than those previously reported WTSCs. Moreover, our WTSC maintains more than 91% of its capacitance after 10,000 stretch-release cycles at tensile strains of up to 50%. The combination of the easy shapeable, robust and highly porous nitrogen-doped 3D NCA paves a new way for the development of high-performance wearable textile-based energy devices.

Although mesoporous carbon materials with hierarchical nanostructures have been produced by the synthesis of hybrid nanoparticles with a silicon dioxide (SiO2) core and a shell of resorcinol formaldehyde resin, it still remains a challenge to effectively tune the pore size distribution. Among a series of phenol derivatives, 3-aminophenol was found to exhibit not only excellent tunability of the size and low roughness of the sphere surface but also high pyrolysis yields in the synthesis of carbon nano/microspheres. Here, we report that mesoporous carbon networks with a bimodal pore size distribution in their hierarchical nanostructure were prepared by 3-aminophenol and formaldehyde polymerization on the SiO2 cores. In particular, the systematic control of the ratio of carbon precursors and silica nanoparticles provides a better control of the microstructure in hybrid nanoparticles with a shell of variable thickness composed of well-defined 3-aminophenol and formaldehyde resins, resulting in the tunability of their pore size distribution.

Highly microporous carbon obtained by KOH etching of carbohydrates exhibited enhanced specific capacitance due to the increased adsorption of electrolyte ions on its large surface, which renders it a promising electrode material. However, the KOH-activated carbon electrode did not achieve its optimum charge capacity in organic electrolytes due to the limited accessibility of the electrolyte ions to the micropores, which hindered the adsorption of ions. The electrode performance was enhanced by enlarging the micropores of KOH-activated carbon to mesopores via reactivation in a stream of CO2, which allowed the mesopore/micropore ratio to be increased without compromising the originally high specific surface area. The extended amount of mesopores increased the charge capacity of the electrode by enabling the large organic electrolyte ions to access the porous surface, as compared to untreated KOH-activated carbon.

A critical bottleneck in the development of aqueous electrochemical energy storage systems is the lack of viable complete cell designs. We report a metal-free, bipolar pouch cell designed with carbon black/polyethylene composite film (CBPE) current collectors as a practical cell architecture. The light-weight, corrosion-resistant CBPE provides stable operation in a variety of aqueous electrolytes over a ∼2.5 V potential range. Because CBPE is heat-sealable, it serves simultaneously as both the pouch cell packaging and seal in addition to its use as a current collector. Although this non-metallic composite has a low electrical conductivity relative to metal foils, current travels only a short distance in the through-plane direction of the current collector in the bipolar cell configuration. This shorter path length lowers the effective electrical resistance, making the design suitable for high-power applications. We test the cell architecture using an aqueous ZnBr2 battery chemistry and incorporate tetrabutylammonium cations to improve the intrinsic low Coulombic efficiency and fast self-discharge of non-flow ZnBr2 cells. These devices demonstrate a cell-level energy density of 50 W h L−1 at a 10C rate (0.5 kW L−1), with less than 1% capacity loss over 500 cycles. A large-area (>6 cm2) 4-cell stack is built to illustrate that the pouch cells are scalable to practical dimensions and stackable without sacrificing performance. The device operates in the range of 6–7 V and has an internal self-balancing mechanism that prevents any individual cell in the stack from overcharging. The results thus demonstrate both a conceptually new cell architecture that is broadly applicable to many aqueous electrolyte chemistries and a specific high-performance example thereof.

Activated carbon, with its porous morphology and extremely high surface area, maintains the exclusive position as an electrode material in supercapacitors owing to its low manufacturing cost. Although its surface area can be boosted using a chemical etchant to create nanoscale pores, the use of chemicals requires a post-washing of the material to eliminate impurities. Herein, the activation of polyvinylidene chloride (PVDC) resin using ZnO chemical is described to prepare porous activated carbon materials for use as supercapacitor electrodes. During heat-treatment of a 1:1 mass ratio of PVDC resin:ZnO at 950 °C, activation and templating processes consecutively take place to produce porous carbon. Between 140 °C and 600 °C, ZnCl2 formed from the conversion of ZnO chemically activates the carbon with creating micropores. Above 800 °C, unreacted ZnO from the initial activation is reduced to Zn upon oxidation of the carbon with the additional micropore creation. Above 907 °C the Zn evaporates to leave activated carbon with no impurities. Through this process, the sites initially occupied by ZnO would turn to the pores by templating. With a rationally-designed ZnO ratio, porous carbon can be produced without washing. The activated carbon exhibits a high quinone content that reacts with H+ ions, with a high specific capacitance of 219 F g−1 in 1 M H2SO4 based on pseudocapacitance. However, the rate performance of this material is 55% due to the slow kinetics of the charge transfer reaction. On the contrary, a high quaternary-N content increases the rate capability of the material in 6 M KOH, where the double-layer mostly contributes toward charge storage.

The mechanism of chemical activation by potassium hydroxide (KOH) was studied in highly controlled non-graphitic carbon structures to explore the constituent reactions and the related pore formation processes involved in producing highly microporous activated materials. For this purpose, nongraphitic carbon was activated independently with intermediate species of either metallic potassium (K) or potassium carbonate (K2CO3) reduced from KOH (activating agent). Structural and morphological changes during activation were probed ex situ using X-ray diffractometry and Brunauer–Emmett–Teller (BET) nitrogen gas adsorption. Reduced K and K2CO3 disordered the stacking graphene layers to different extents. While micropore features were induced upon K evaporation following infiltration, the existing micropores were expanded into mesopores by K2CO3 gasification. Exclusive activation with K or K2CO3 induced ultra-small micropores, as measured using cyclic voltammetry. This work explains why activation using a KOH solution develops the preferable porous texture for use in many devices by creating open microporous structures as a result of the synergistic activation of both K and K2CO3.

Spinel-layered composites of Li1.5MnTiO4+δ were studied for their use as high-energy, low-cost, and environmentally benign cathode materials. The bulk particles showed an attractive specific capacity of up to 250 mAh g−1 at C/10. To improve the performance of this cathode at a high C-rate, a spinel-layered Li1.5MnTiO4+δ nanorod was successfully synthesized using a β-MnO2 nanorod template. The nanorod, which had an average diameter of 200 nm and a length of 1 μm, showed specific capacity as high as the bulk particle at C/10. However, owing to a one-dimensional nanostructure with a large effective contact area for Li+ diffusion, the nanorod sample exhibited enhanced capacities 11% (170 mAh g−1) and 167% higher (80 mAh g−1) at 1C and 10C rates, respectively, compared to the bulk particles. Moreover, both samples showed good cycle stability and capacity retention of over 85% after 100 cycles at 1C.

The performance of redox-enhanced electrochemical capacitors (redox ECs) is substantially improved when oxidized catholyte (bromide) and reduced anolyte (viologen) are retained within the porous electrodes through reversible counterion-induced solid complexation. Investigation of the mechanism illustrates design principles and identifies pentyl viologen/bromide (PV/Br) as a new high-performance electrolyte. The symmetric PV/Br redox EC produces a specific energy of 48.5 W·h/kgdry at 0.5 A/gdry (0.44 kW/kgdry) with 99.7% Coulombic efficiency, maintains stability over 10 000 cycles, and functions identically when operated with reversed polarity.

In order to improve the performance of thermoelectric materials, nanoinclusions are often used to enhance phonon scattering. In this study, activated carbon, which is porous and thus has a large surface area, was incorporated in GeTe-based materials to cause increased boundary phonon scattering. Carbon dispersed in grain boundaries resulted in improved thermal properties without significant deterioration in electrical properties. Due to the extrinsic addition of activated carbon, the lattice thermal conductivity decreased by 13.8% on average. A maximum dimensionless figure of merit of 1.66 was achieved at 723 K for the Ge0.9Sb0.1Te composition with additional activated carbon.

Amorphous MnO2 (a-MnO2) that exhibits a high theoretical specific capacitance can be conveniently grown via electrodeposition as a pseudocapacitor electrode. However, the electrodeposited a-MnO2 exhibits lower electrochemical stability than a-MnO2 synthesized using other routes. We investigate the influence of the electrodeposition conditions on the evolution of the properties of a-MnO2 during electrodeposition and their corresponding effect on the electrochemical stability. MnO2 is electrodeposited at potentiostatic and galvanostatic mode. The different surface morphology/chemistry of a-MnO2 is obtained by applying high and low potentials or current. The applied potential or current determines the surface morphology based on a predominance between nucleation and growth of a-MnO2. The fine surface enables a higher specific capacitance because of the enlarged specific surface area but also leads to lower rate performance by limiting ion transport through the smaller microstructure. The opposite results are observed in a coarse MnO2 deposit. When the applied potential or current is high, the manganese has a lower average valence number. The lifetime of a-MnO2 is degraded by the irreversible reaction between Mn4+/Mn7+ and oxygen evolution. Particularly, finer MnO2 is less stable than coarse one, which can arise from multiple high-energy connecting sites. A narrow potential window without Mn4+/Mn7+ interconversion promotes long-term stability.

Eumelanins are extended heterogeneous biopolymers composed of molecular subunits with ambiguous macromolecular topology. Here, an electrochemical fingerprinting technique is described, which suggests that natural eumelanin pigments contain indole-based tetramers that are arranged into porphyrin-like domains. Spectroscopy and density functional theory calculations suggest that sodium ions undergo occupancy-dependent stepwise insertion into the core of porphyrin-like tetramers in natural eumelanins at discrete potentials.

We report a new electrochemical capacitor with an aqueous KI-KOH electrolyte that exhibits a higher specific energy and power than the state-of-the-art nonaqueous electrochemical capacitors. In addition to electrical double layer capacitance, redox reactions in this device contribute to charge storage at both positive and negative electrodes via a catholyte of IOx -/I- couple and a redox couple of H2O/Had on the negative electrode, respectively. Here, we, for the first time, report utilizing IOx -/I- redox couple for the positive electrode, which pins the positive electrode potential to be 0.4-0.5 V vs. Ag/AgCl. With the positive electrode potential pinned, we can polarize the cell to 1.6 V without breaking down the aqueous electrolyte so that the negative electrode potential could reach -1.1 V vs. Ag/AgCl in the basic electrolyte, greatly enhancing energy storage. Both mass spectrometry and Raman spectrometry confirm the formation of IO3 - ions (+5) from I- (-1) after charging. Based on the total mass of electrodes and electrolyte in a practically relevant cell configuration, the device exhibits a maximum specific energy of 7.1 Wh/kg, operates between -20 to 50 °C, provides a maximum specific power of 6222 W/kg, and has a stable cycling life with 93% retention of the peak specific energy after 14,000 cycles.

Biodegradable electronics represents an attractive and emerging paradigm in medical devices by harnessing simultaneous advantages afforded by electronically active systems and obviating issues with chronic implants. Integrating practical energy sources that are compatible with the envisioned operation of transient devices is an unmet challenge for biodegradable electronics. Although high-performance energy storage systems offer a feasible solution, toxic materials and electrolytes present regulatory hurdles for use in temporary medical devices. Aqueous sodium-ion charge storage devices combined with biocompatible electrodes are ideal components to power next-generation biodegradable electronics. Here, we report the use of biologically derived organic electrodes composed of melanin pigments for use in energy storage devices. Melanins of natural (derived from Sepia officinalis) and synthetic origin are evaluated as anode materials in aqueous sodium-ion storage devices. Na(+)-loaded melanin anodes exhibit specific capacities of 30.4 ± 1.6 mAhg(-1). Full cells composed of natural melanin anodes and λ-MnO2 cathodes exhibit an initial potential of 1.03 ± 0.06 V with a maximum specific capacity of 16.1 ± 0.8 mAhg(-1). Natural melanin anodes exhibit higher specific capacities compared with synthetic melanins due to a combination of beneficial chemical, electrical, and physical properties exhibited by the former. Taken together, these results suggest that melanin pigments may serve as a naturally occurring biologically derived charge storage material to power certain types of medical devices.

The relationship between electrolyte solution pH and the local retention of electrochemically-generated hydrogen under extreme bias conditions for highly receptive activated carbons was studied for the first time. Hydrogen was generated through electrolysis on the surface of custom-made activated carbon electrodes in aqueous solutions with a range of pH (approximately 2–10). The carbon's ability to retain this generated hydrogen was analyzed via a galvanostatic test protocol, while the pH was monitored in situ. A rise in the basicity and corresponding Pourbaix shift was observed for tests performed in intermediate pH solutions, in contrast to that observed for strongly acidic or alkaline solutions. For the three intermediate pH solutions, an increased hydrogen evolution overvoltage was observed and was correlated to an increased hydrogen storage efficiency. These results clearly demonstrate the importance of pH control to extract maximum degree of hydrogen storage in idealized carbons, and show that the optimal pH for this effect is in the range of approximately 4–8. Furthermore, these high surface area carbons were found to have over 300 mAh g−1 of energy storage capacity when charged with hydrogen in the proper pH range.

We report here a rapid process to synthesize microporous carbon from nongraphitic carbon source through microwave-assisted exfoliation in a non-inert environment. A ternary compound of potassium-nongraphitic carbon-tetrahydrofuran (THF) was prepared and then heating by microwave irradiation for 2 min widens the interstices between adjacent graphene layers in ternary compound, inducing microporous texture with a large amount of ultramicropores. Exploiting microwave heating allows for efficient and rapid synthesis of activated carbon compared with commonly used chemical activation process. After microwave irradiation on nongraphitic carbon, the constituent stacked graphene layers were broken and the specific surface area of 563 m2 g−1 was developed. The feasibility of an electrode material for supercapacitor was estimated by cyclic voltammetry and galvanostatic charge/discharge cycling. The specific areal capacitance reveals as high as 16.4 μF cm−2 in 1 M NaNO3 aqueous solution, which is significantly larger than values found in traditional activated carbons made for use in electrochemical double layer capacitors. Without restrictive processing conditions of chemical activation, microporous structure carbon can be efficiently and rapidly synthesized via microwave irradiation for possible electrochemical capacitor electrode.

SUMMARY We compare battery performance simulations from a commercial lithium-ion battery modeling software package against manufacturer performance specifications and laboratory tests to assess model validity. A set of commercially manufactured spiral wound lithium-ion cells were electrochemically tested and then disassembled and physically characterized. The Battery Design Studio® (BDS) software was then used to create a mathematical model of each battery, and discharge simulations at constant C-rates ranging from C/5 to 2C were compared against laboratory tests and manufacturer performance specifications. Results indicate that BDS predictions of total energy delivered under our constant C-rate battery discharge tests are within 6.5% of laboratory measurements for a full discharge and within 2.8% when a 60% state of charge window is considered. Average discrepancy is substantially lower. In all cases, the discrepancy in simulated vs. manufacturer specifications or laboratory results of energy and capacity delivered was comparable to the discrepancy between manufacturer specifications and laboratory results. Results suggest that BDS can provide sufficient accuracy in discharge performance simulations for many applications. Copyright © 2012 John Wiley & Sons, Ltd.

An approach to making large format economical energy storage devices based on a sodium-interactive set of electrodes in a neutral pH aqueous electrolyte is described. The economics of materials and manufacturing are examined, followed by a description of an asymmetric/hybrid device that has λ-MnO 2 positive electrode material and low cost activated carbon as the negative electrode material. Data presented include materials characterization of the active materials, cyclic voltammetry, galvanostatic charge/discharge cycling, and application-specific performance of an 80 V, 2.4 kW h pack. The results indicate that this set of electrochemical couples is stable, low cost, requires minimal battery management control electronics, and therefore has potential for use in stationary applications where device energy density is not a concern.

The electrokinetics of electrochemical capacitor electrodes composed of either pyrolyzed dextrose or chemically activated pyrolyzed dextrose during progressive steps in the production process were studied under dynamic charging condition by using cyclic voltammetry. Two different sizes and species of aqueous electrolyte anions, SO42− and NO3−, were used to probe the relationship between functional species dimension and electrode morphology. Cyclic voltammetry data collected with different scan rates demonstrate the appearance of slow charging process in the non-activated pyrolyzed carbons because of their ultramicroporous properties. The KOH activated material had significant micro-pores, and demonstrated electrokinetics of an ideal electric double layer. The activation on the carbon pyrolyzed at lower temperature is assumed to develop cylindrical pores, while that on the carbon pyrolyzed at higher temperature evolves slit-shaped pores.

Electrochemical double-layer capacitors exhibit high power and long cycle life but have low specific energy compared with batteries, limiting applications. Redox-enhanced capacitors increase specific energy by using redox-active electrolytes that are oxidized at the positive electrode and reduced at the negative electrode during charging. Here we report characteristics of several redox electrolytes to illustrate operational/self-discharge mechanisms and the design rules for high performance. We discover a methyl viologen (MV)/bromide electrolyte that delivers a high specific energy of ∼14 Wh kg−1 based on the mass of electrodes and electrolyte, without the use of an ion-selective membrane separator. Substituting heptyl viologen for MV increases stability, with no degradation over 20,000 cycles. Self-discharge is low, due to adsorption of the redox couples in the charged state to the activated carbon, and comparable to cells with inert electrolyte. An electrochemical model reproduces experiments and predicts that 30–50 Wh kg−1 is possible with optimization., The energy density of electrochemical capacitors can be increased by using a redox-active electrolyte, but such capacitors often suffer from significant self-discharge and low operating voltage. Here, the authors report a new redox-active aqueous electrolyte to effectively tackle the problems.

In the present work, the mechanism of charging/discharging at the amorphous manganese oxide electrode was investigated in 0.1 M Na2SO4 solution with respect to amount of hydrates and valence (oxidation) states of manganese using a.c.-impedance spectroscopy, anodic current transient technique and cyclic voltammetry. For this purpose, first the amorphous manganese oxide film was potentiostatically electrodeposited, followed

Electrochemical storage systems that utilize divalent cations such as Mg2+ can improve the volumetric charge storage capacities compared to those that use monovalent ions. Here, a cathode based on naturally derived melanin pigments is used in secondary Mg2+ batteries. Redox active catechol groups in melanins permit efficient and reversible exchange of divalent Mg2+ cations to preserve charge storage capacity in biopolymer cathodes for more than 500 cycles.

Research in electric double layer capacitors (EDLCs) and rechargeable batteries is converging, producing devices known as pseudocapacitors. Pseudocapacitors store energy electrochemically, utilizing fast and reversible faradaic redox reactions at the interface between high surface area electrodes and an electrolyte. The result is a technology that delivers a long cycle life, high specific power, and moderate specific energy. One frontier in pseudocapacitor research is redox-active electrolytes, which replace the traditional solid redox-active materials with soluble redox couples. Here we have identified viologen bromide salts as promising aqueous redox-active electrolytes for redox-EDLCs.[i] During charging, Br- is oxidized to Br3 - at the positive electrode and the viologen dication (V2+) is reduced to the stable monocation radical (V+•) at the negative electrode (Figure 1). The system shows unusually high coulombic efficiency and low self-discharge rates for an aqueous redox-supercapacitor. This was initially attributed to strong adsorption of the Br3 - and V+• to the activated carbon electrodes, but we now present a more detailed analysis that confirms the behavior is attributable to two electroprecipitation mechanisms. In these mechanisms, each ion acts as a charge-storing redox couple at one electrode an as a complexing agent at the other electrode. The processes are highly reversible and cells show negligible capacity fade even after 20,000 cycles. The devices use conventional activated carbon electrodes, and because crossover is not a concern and self-discharge is suppressed, a simple cellulose separator is sufficient and ion-selective membranes are not required. Heptyl viologen devices achieve specific energy densities of 11 Wh/kg (normalized to the mass of both electrodes and electrolyte), significantly higher than commercially available EDLCs (Figure 2). Using methyl viologen increased specific energy to 14 Wh/kg, but at the expense of cycling stability. By studying the operating mechanisms and synthesizing a large series of viologens with different alky substituents we have now identified viologens that deliver both the stability of heptyl viologen and the specific energy of methyl viologen. Finally, a simple electrochemical device model was developed to understand the behavior of the system and predict the maximum theoretically achievable performance (Figure 3). The results model experimental data well and suggest that significant performance improvement is possible, making this technology an exciting candidate for applications like automotive engine start-stop and grid-scale energy storage. [i] S.-E. Chun, B. Evanko, X. Wang, D. Vonlanthen, X. Ji, G. D. Stucky, and S. W. Boettcher, “Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge,” Nat. Commun., vol. 6, p. 7818, 2015. Figure 1

Novel nanoporous nitrogen-enriched carbon materials were prepared through a simple carbonization procedure of well-defined block copolymer precursors containing the source of carbon, i.e., polyacrylonitrile (PAN), and a sacrificial block, i.e., poly(n-butyl acrylate) (PBA). The preparation of nitrogen-enriched nanocarbons with hierarchical pore structure was enabled by the high fidelity preservation of the initial phase-separated nanostructure between two polymer blocks upon carbonization. Supercapacitors fabricated from the prepared carbons exhibited unusually high capacitance per unit surface area (>30 μF/cm(2)) which was attributed to the pseudocapacitance resulting from the high nitrogen content originating from the PAN precursor. Electrochemical availability of the nitrogen species was also evident from the results of oxygen reduction experiments. The hierarchical pore structure and the high nitrogen content in such materials make them particularly promising for use in supercapacitor and electrocatalyst applications.

Flexible biodegradable electronics have the potential to serve as the centerpiece for temporary electronically active medical implants. Biodegradable electronics may exhibit many advantages over traditional chronic implants. Two important long-term goals for biodegradable electronics are (1) supplying sufficient power and (2) reducing the invasiveness of device deployment. Edible electronic devices are capable of addressing both challenges. Here, we introduce electrochemical electronic power sources that are compatible with non-invasive deployment strategies and are composed entirely of edible materials and naturally occurring precursors that are consumed in common diets. The current sources developed herein are powered by onboard sodium ion electrochemical cells. Potentials up to 0.6 V and currents in the range of 5-20 μA can be generated routinely. These devices could serve as an enabling platform technology for edible electronics used in non-invasive sensing and stimulation of tissues within the human body.

The importance of precursor structure on the activation process step was investigated for carbons derived from carbohydrates, a consistent and abundant precursor stream. Precursor (nonactivated) carbons were derived from anhydrous α-D-glucose precursor by pyrolysis in Ar atmosphere at temperatures ranging from 500 to 1000°C. These carbons were then characterized and chemically (KOH) activated at 800°C. The electrochemical and physicochemical characteristics of the KOH-activated carbons were studied during and after synthesis using thermogravimetric analysis, X-ray diffractometry (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy, nitrogen gas adsorption method, and cyclic voltammetry. Both XRD and HRTEM results on the pyrolyzed carbons reveal that more graphene layers align in a parallel way with higher precursor carbonization temperatures. Chemical activation on the carbon prepared at lower temperatures achieved surface areas greater than 2900 m 2 g -1 , and specific capacitance numbers in excess of 180 F g -1 in 1 M Na 2 SO 4 solution. Relationships between electrochemical and physical properties are discussed.

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