Search

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

not Available.