Electric Double Layer Capacitance Research Articles

Ionoelastomers at Electrified Interfaces: Differential Electric Double-Layer Capacitances of Cross-Linked Polymeric Ions and Mobile Counterions

Ionoelastomers at Electrified Interfaces: Differential Electric Double-Layer Capacitances of Cross-Linked Polymeric Ions and Mobile Counterions

Biomass activated carbon supported Keggin-type polyoxometalate as an electrode material for high-performance acidic aqueous supercapacitors

Activated carbon (SSAC-OP) with micropore and mesopore was prepared by using soybean straw as a carbon source. Redox-active H3PMo12O40 (PMo12) was introduced into SSAC-OP by solution impregnation to obtain composite PMo12/SSAC-OP. The effect of loading amount of PMo12 on the supercapacitive performance of the composite was investigated. BET and EDS results confirmed that PMo12 is adsorbed in the micropores of 1–2 nm and mesopores of SSAC-OP, as well as on the carbon wall of SSAC-OP. Meanwhile, SSAC-OP with positive charge is more firmly connected with PMo12 anions via the effect of electrostatic attraction. CV and GCD test results proved that the composite materials have both electric double layer capacitance and pseudocapacitance characteristics. The CV curve of 10-PMo12/SSAC-OP obtained from SSAC-OP impregnating the initial concentration of PMo12 solution of 10 mM shows four pairs of obvious reversible redox peaks, which indicates that the protonation of PMo12 anion in the reduction process is sufficient. The composite 10-PMo12/SSAC-OP has a specific capacitance as high as 397.02F g−1 at scan rate of 5 mV s−1. The assembled symmetric supercapacitor delivers an energy density of 23.90 Wh kg−1 under the power density of 1016.05 W kg−1, with 97.96 % capacitance retention after 10,000 cycles at 3 A g−1.

Inspired by game theory: A spatial confinement strategy for narrow-pore distribution porous carbons toward high-performance supercapacitors

This work suggests how understanding the Gibbs free energy evolution mechanism recorded for solid activators at high temperatures can provide a simple and general route to targeted tuning in preparing porous carbons. We show that based on a novel spatial confinement strategy, it is likely to target-produce pine sawdust (PS)-derived porous carbons characterized by higher specific surface area than analogous conventionally porous carbons but similar average pore size and pore size distribution (PSD). Notwithstanding the increased specific surface area and total pore volume of PS-derived porous carbon prepared under spatial confinement forces compared to conventional porous carbon, the pore structures recorded for PS-derived porous carbon obtained under spatial confinement forces arise from narrow pores, which leads to a sharp growth in specific capacitance over a range. Specifically, the texture properties can be tuned by various spatial confinement forces, bypassing the activation process tuning, template, sequential activation, and trial-and-error methods. Notably, PC-40-850 achieved higher specific capacitance values. Encouragingly, PC-40-850 offered excellent cycle stability accompanied by great energy density values (e.g., 29.31 Wh/kg upon 499.99 W/kg). Specifically, PC-40-850 realized a surface area-normalized electrical double-layer capacitance of 65.93 mF/m2. Excellent manufacturability and variable narrow-pore distribution permit the spatial confinement strategy to produce numerous advanced and multistage porous carbons for matching specific applications.

Effect of simple cubic, face-centered cubic, and body-centered cubic-electrodes on the electric double layer capacitance of supercapacitors

The continuum theory has been used to analyze the polarization, ion crowding, and electrostatic forces of the electric double layer in the electrode materials having simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC) morphologies. The study manifests the effect of thickness of electrodes, electrode’s particle size, and porosity on electric double-layer specific capacitance (EDLC). Electrochemical interference and the specific capacitance depend on the packing factor. The larger particle size decreases the specific capacitance, but porosity increases due to more surface area. Due to symmetry, SC, BCC, and FCC morphologies have 1, 3, and 5 spheres in a unit cell. The number of unit cells is varied from 1 to 100 in model 1 to analyze the effect of electrode thickness. Model 2 has three unit cells to understand the effect of porosity, and only pore lengths are varied. The critical thickness of the electrodes is the integer multiples of 1.71 μm in all the morphologies. The Stern layer-specific capacitance is 167.6 μF cm−2 in all cases. The EDLC in BCC is around 5.6–7.6 μF cm−2 in the steady state that is intermediate between SC and FCC morphologies. The more dense packing of carbon particles in a unit cell increases the energy storage capabilities of electrodes. The average electrode permittivity slightly decreases due to the combined effect of the high electric field, status of polarization, and electrode particle size. The least optical transmission of electrodes is 98.35%.

Ultrafast Fabrication of H2SO4, LiCl, and Li2SO4 Gel Electrolyte Supercapacitors with Reduced Graphene Oxide (rGO)-LiMnOx Electrodes Processed Using Atmospheric-Pressure Plasma Jet.

Pastes containing reduced graphene oxide (rGO) and LiCl-Mn(NO3)2·4H2O are screen-printed on a carbon cloth substrate and then calcined using a nitrogen atmospheric-pressure plasma jet (APPJ) for conversion into rGO-LiMnOx nanocomposites. The APPJ processing time is within 300 s. RGO-LiMnOx on carbon cloth is used to sandwich H2SO4, LiCl, or Li2SO4 gel electrolytes to form hybrid supercapacitors (HSCs). The areal capacitance, energy density, and cycling stability of the HSCs are evaluated using electrochemical measurement. The HSC utilizing the Li2SO4 gel electrolyte exhibits enhanced electrode-electrolyte interface reactions and increased effective surface area due to its high pseudocapacitance (PC) ratio and lithium ion migration rate. As a result, it demonstrates the highest areal capacitance and energy density. The coupling of charges generated by embedded lithium ions with the electric double-layer capacitance (EDLC) further contributed to the significant overall capacitance enhancement. Conversely, the HSC with the H2SO4 gel electrolyte exhibits better cycling stability. Our findings shed light on the interplay between gel electrolytes and electrode materials, offering insights into the design and optimization of high-performance HSCs.

Modelling and Optimization of the Differential Surface Capacitance in Electrolyte-Gated Graphene Field-Effect Transistors

Graphene field-effect transistors (GFETs) are a promising avenue for the detection of biomarkers, allowing to envision improvements in diagnostic approaches for different types of cancer and other diseases. To enable such applications, the detection metrics of the devices must be optimized, including their sensitivity, selectivity, and stability. In particular, the capacitance of the device is one of the key determinants of the sensitivity of the electrical current of graphene to nearby changes in electrical potentials. In this presentation, we present our approach to optimize the surface capacitance of electrolyte-gated GFETs, by investigating theoretically and/or experimentally the effect of specific design parameters, including the graphene area, the removal of the backgate, and the passivation of electrodes. On the theoretical side, a mathematical representation of the device capacitance was established as an equivalent circuit of capacitors modelling for the quantum capacitance of graphene, the backgate capacitance, and the capacitance of electrical double layers in the electrolyte. On the experimental side, microchips of GFET arrays were assembled using microfabrication techniques on two types of substrates: a standard Si++/SiO2 wafer in which the doped silicon layer is used as a backgate, and a pure non-conducting borosilicate glass wafer in comparison. The capacitance of the devices was then measured using a custom parallel electrical prober by stepping the applied voltage in the solution and analyzing the resulting current response through a standard RC direct current model. We will present our analyses by comparing the relative impact of electrode passivation, backgate usage, and graphene area on the capacitance of devices. These results will enable a rational design of GFET devices with improved sensitivity for their use in applications such as biological, chemical, and gas sensing.

An Exploration of Graphene-Water Interface Electrochemical Circuit Model

Graphene has been considered a good candidate for sensor material due to its large surface-to-volume ratio and high conductivity. For instance, high-sensitivity solvent sensors can be achieved through liquid-gated graphene field effect transistors (GFET). In such kind of sensors, ions and other molecules in the test solution will be chemically adsorbed by the graphene sheet. It results in changes of the channel surface potential and conductance. However, the measured potential difference are affected by electrical double-layer capacitance in solution, and quantum capacitance in graphene. Studies have reported the quantum capacitance of graphene in an ionic or aqueous liquid, while the interface between graphene and deionized (DI) water hasn’t been fully explored yet. To address this point, in this work, we measured the electrochemical impedance spectroscopy (EIS) of graphene electrodes in deionized water and proposed an electrical circuit model for the electrochemical cell. The meaning of each element of the circuit model was also discussed.In our experiment, single-layer graphene was grown by chemical vapor deposition (CVD) on copper foil and transferred to Si/SiO2 substrate with gold contacts. Our test solution, DI water, was confined on the graphene sensing layer by polydimethylsiloxane (PDMS). This electrochemical cell was measured with BioLogic SP-150 potentiostat by 2-probe EIS method, with graphene as working electrode (WE) and Ag/AgCl electrode placed in DI water as counter electrode (CE). Another electrochemical cell with gold pad WE was also prepared as the control group.The electrochemical impedance of the water-graphene cell was plotted in a Nyquist plot and showed two semicircles. This observation is different from a regular electrochemical cell, whose impedance shows a semicircle, corresponding to the interface RC impedance, and a low-frequency 45° straight line, corresponding to molecular mass transport limitation. The two semicircles in our water-graphene cell impedance indicate that the equivalent circuit model is based on two parallel RC circuits, corresponding to the interface and solution impedance. By modifying the biasing potential and adding supporting electrolyte, we observed the response of the impedance and identified that the high-frequency semicircle is related to solution impedance, and the low-frequency semicircle is related to interface impedance, which is consistent with the reaction rate of both parts. To make more precise fittings, an additional Warburg element was introduced into the circuit model for the effect of mass-transfer limitation. We proposed a model based on an RC parallel circuit in series with a Randles circuit. Comparing with the 2-RC model, the fitting residual of the proposed model improved by 13%. The RWC parallel circuit was fitted to the high-frequency semicircle, which corresponds to the solution impedance. This is different from the commonly used Randles model, where the Warburg element is included in interface impedance. This difference arises from the lack of supporting electrolytes. When there are supporting electrolytes in an electrochemical cell, the double-layer capacitor at the electrode-solution interface is mainly contributed by supporting electrolytes. While using DI water as the solution in our case, the charged species contributes to both faradic current and non-faradic current, which means that the mass-transport limitation affects both the charge transfer resistor and the double-layer capacitor. Hence, the Warburg element for mass-transport limitation can be extracted from the interface RC parallel circuit and put into the solution impedance when there is no supporting electrolyte.After the circuit model was decided and the elements were identified, we fit the electrochemical impedance curves of both the water-graphene cell and the control group, whose WE was replaced by a gold pad. The fitting result shows that the bulk resistor and capacitor, Rb and Cb, values are unrelated to the WE material, which again supports our circuit model. While the WE surface capacitance Cs of the graphene-water electrochemical cell is 0.0606 F/m2, which is 1/65 of that of the control group. To verify the reason for the capacitance difference, we considered the effect of the graphene quantum capacitance. The theoretic value of the quantum capacitance in single-layer graphene was found to be 0.0752 F/m2 under our setup, which is only 18.8% smaller than our experimental value. Hence, we concluded that SLG surface capacitance was dominated by quantum capacitance. Figure 1

Construction of high-entropy oxy/hydroxides with dense nanosheet array structures on FeCoNiCrMo electrode for efficient oxygen evolution reaction

The high-entropy alloy (HEA) films prepared by magnetron sputtering on 3D substrates have excellent ability to release bubbles, good electrical conductivity, and abundant active sites, which lead to excellent performance in oxygen evolution reaction. HEA films with various elements were prepared on different substrates. In general, oxygen evolution reaction abilities of HEA film electrodes are related to morphologies of catalyst and types of active sites. Notably, nanostructures with regular array could help to increase the surface area of electrocatalysts and expose active sites. Also, chloride salt solution of transition metals can promote the formation of high-entropy oxy/hydroxides due to oxygen absorption oxidation reaction and displacement reaction on the surface of HEA film electrodes. And electrochemical oxidation at suitable high potential can also produce high-entropy oxy/hydroxides. In this work, dense nanosheet array structures of high-entropy oxy/hydroxides were constructed on the surface of FeCoNiCrMo HEA film electrodes preprocessed in NiCl2 solution and reconstructed by electrochemical oxidation in KOH solution further. The FeCoNiCrMo HEA film electrode with dense nanosheet array structures has excellent OER performance with the overpotential of 250 mV, Tafel slope of 41.5 mV dec−1, and electric double layer capacitance of 4.3 mF cm−2.

Rapid and Sensitive Pathogen Detection in Serum for Early Sepsis Diagnostics Using an Electrical-Double-Layer Gated Field-Effect Transistor-Based Sensor Array

Sepsis is a major medical inflammatory response to bloodstream infection caused bypathogens. Over 30 million infections occur due to sepsis in a year throughout the globe.With over 6 million deaths, sepsis treatment demands timely diagnosis and medicalinterventions to prevent large outspread of the infection. Rapid and sensitive detection ofinfectious bacteria plays an important role in disease control. However, fast and accuratediagnosis remains highly challenging for many hospitalized patients. Among variousbacterial diseases, Escherichia coli, (E. coli) a gram-negative bacterium, is the mostfrequently and extensively researched and has become a global concern of food safetyproblems. Traditional cell culture methods are considered as most accurate method forbacterial diagnosis. But some microorganisms like Bartonella spp, Treponema pallidum, etc.are difficult to grow on a culture plate, while some other virus species are unculturable, soeventually the traditional cell culture may take several days to a week for the diagnosticresults. Hence, over several years, different novel methods have been developed, such asoptical methods, electrochemical methods, molecular detection by PCR etc. Even thoughseveral techniques exist, concerns have been raised regarding their sensitivity, specificity etc.In addition, various fluorescence-based methods, and Surface Plasmon resonance (SPR)-based sensors have also been reported for bacterial detection. However, high cost, largeturnaround time, expensive instrumentation, and requirement of skilled personnel are some ofthe disadvantages of these existing techniques.FET biosensors have previously been used in different diagnosis areas. Nowadays, severalFET-based biosensors have been developed, which primarily concentrate on improving theoverall sensitivity of FET devices. However, the charge screening effect observed in aphysiological environment, and additional sample processing increase the complexity of thesensor platform. To overcome this hurdle, an electrical double layer (EDL) gated FETbiosensor has been proposed and explored in the detection of a variety of biological entities.Here, we proposed an EDL-FET sensor platform for the detection of Escherichia coliO157:H7 strain in blood serum. A single-step surface functionalization strategy was used forimmobilizing receptor probes as ssDNA. The probes were designed and immobilized over adisposable chip containing a pair of gold electrodes. A handheld reader which consists of anenhancement mode N-channel MOSFETs was connected to the laptop through a USB port tomeasure the sensor signals. Pulsed gate voltage was applied to the input electrode and whichmodulated the EDL capacitance and potential change eventually resulting in the drain currentof the MOSFET. The probe was immobilized on the gate electrode and tested with differentconcentrations of cDNA spiked in TE buffer. The probe-target binding was observed throughfluorescence change and electrically by measuring the change in drain current. In addition,surface potential change was also examined by performing KPFM test. Similarly, the BioFETdetections were validated for E. coli O157 variants by spiking with short and long DNAtarget sequences simultaneously in serum. The test results confirm selectivity by testingvarious non-complementary sequences, which ensures the potential of this BioFET as apoint-of-care sensing device. Figure 1

(Invited) Bridging the Gap between the Physics and Chemistry of Electric Double Layer

Classical models of electric double layer (EDL) are mostly concerned with ion distributions and their responses to the surface potential that can be regulated by changing either the voltage or the extent of surface ionization. With the help of adjustable parameters, classical EDL models are able to describe diverse physical phenomena including EDL capacitance and ion transport in good agreement with experimental measurements. However, major challenges remain in understanding chemical reactions within EDL important for the rational design and optimization of electrochemical processes such as energy storage and electrocatalysis. In this talk, I will present a molecular-thermodynamic model to predict the charging behavior of ionizable surfaces under various solution conditions. If a solid is non-conductive, the surface reactions can be reliably described with a Langmuir-type model coupled with liquid-state methods to account for the local thermodynamic non-ideality. When a solid is conductive, however, the electronic properties of different surface sites are intrinsically coupled with each other leading to highly correlated surface reactions. While the brute-force prediction of chemical equilibrium at an electrified surface is computationally prohibitive, a combination of the first-principles calculations and coarse-grained modeling provides complemental information that may be useful for diverse electrochemical applications.

Simple Fabrication Method for Solution-gated One-piece Transistors for Biosensing Applications

INTRODUCTIONSolution-gated field-effect transistors (FETs) with silicon or other semiconductive materials as channels can specifically and selectively detect ions and biomolecules related to biological functions by chemically modifying the gate electrode surface, and these are known as biologically coupled FETs (Bio-FETs) [1]. Since Bio-FETs can directly detect the charges of ions and biomolecules, they do not require to label fluorescent dyes and to induce redox reactions based on enzymes. Therefore, Bio-FETs are expected to be used as in vitro diagnostic devices for a label-free monitoring of health conditions in daily life.Recently, indium tin oxide (ITO) has been widely used as an electrode material for flat panel displays owing to its transparency and conductivity. Our laboratory previously reported that solution-gate FETs fabricated by one-step sputtering provided a steep subthreshold slope (SS) owing to the absence of the interfaces among source/channel/drain electrodes and the large electric double-layer capacitance caused by direct contact between electrolyte solutions and the channel surface [2]. While this technique was simple, it utilized the diffusion of atoms during sputtering between a modified mask and substrates and was also dependent on the mask size, which limited the miniaturization of the transistor. On the other hand, the conventional fabrication methods of solution-gated FETs facilitate the miniaturization and the precise control of device structure by use of photolithography, but such methods inevitably produce impurities at the source/channel/drain electrode interfaces and may cause the degradation of device performances.In this study, we propose a new method to fabricate solution-gated ITO thin-film channel FETs without interfaces among the source/channel/drain electrodes, which facilitates to miniaturize and control the device structure. A conductive ITO thin film with a thickness of about 100 nm is deposited on a glass substrate by sputtering. Then, the ITO film is protected by photoresist and then the channel area is exposed by photolithography. The exposed area is etched to 20 nm or less thickness, which induces semiconductive characteristics. This method enables the fabrication of the desired solution-gated ITO thin-film channel FET. Furthermore, we evaluate its electrical characteristics and examines its potential for biosensor applications.EXPERIMENTALFirst, photoresist was patterned on a glass substrate by photolithography. ITO thin film (100 nm thickness) was deposited on the patterned area by sputtering, and the photoresist was removed using a solvent. Next, photolithography was used again to deposit the resist film so that the channel area was exposed. Etching was performed by dropping 0.1 M hydrochloric acid on the exposed channel area (Figure). Etching time was controlled by applying a voltage between the source and drain electrodes and measuring the change in current over time. A semiconductor parameter analyzer was used to measure the current during etching, the gate voltage (VG)-drain current (ID) transfer characteristics, and the pH responsivity of the fabricated devices. The film thickness was measured by a white interferometer-equipped laser microscope (Keyence).RESULTS and DISCUSSIONBy controlling the current during etching, the current value was decreased rapidly at the end of the etching. This may be because the thickness of ITO film was thinner and reached about 20 nm, which resulted in the semiconductive characteristics. Then, the etching was stopped to obtain the solution-gated ITO thin-film channel FET. The fundamental electrical characteristics such as VG-ID transfer curves were investigated, and the fabricated device was found to operate as the solution-gated FET. It was also found that the devices showed the Nernst response for the change in pH and a steep subthreshold slope of approximately 90 mV/decade (pH 7.41). Furthermore, the ITO channel thickness was measured by laser microscopy; as a result, it was about 10–20 nm.CONCLUSIONS In this study, we proposed the new method to fabricate the solution-gated ITO thin-film channel FET. The device actually worked as a transistor, and it showed good sensitivity to potential. This method enabled the solution-gated FETs to be produced by the simple method. Thus, the solution-gated ITO thin-film channel FET is applied to biosensors, which is highly sensitive and affordable to detect biomolecules in the future.REFERENCES[1] Sakata, T. ACS Omega, 4, 11852 (2019).[2] Sakata, T.; Nishitani, S.; Saito, A.; Fukasawa, Y. ACS Appl. Mater. Interfaces, 13, 38569 (2021). Figure 1

Core-shell Fe3O4@carbon aerogels assembled aqueous zinc ion hybrid capacitor with ultra-wide voltage window

The unsatisfactory energy density seriously hinders the practical application of supercapacitors. Herein, core-shell Fe3O4@carbon aerogels (Fe3O4@CAs) with high specific capacity are synthesized by the hydrothermal self-assembly method, which can contribute both electric double-layer capacitance (EDLC) and pseudo-capacitance under the synergistic mechanism of Fe3O4 and CAs. The zinc ion hybrid supercapacitor (ZHSC) is constructed using Fe3O4@CAs, zinc foil, and 2 M ZnSO4 as the cathode, anode, and electrolyte, respectively. The assembled ZHSC exhibits an ultra-wide voltage window (0–2.1 V in the aqueous electrolyte) and high specific capacitance (342.1 F g−1 at a current density of 0.5 A g−1) characteristics, successfully achieving an attractive energy density (284.7 Wh kg−1 at a power density of 210 W kg−1). This study provides a highly universal hydrothermal self-assembly method for the composite preparation of Fe3O4 and various carbon materials.

A quantitative evaluation of the large pore-size effect on the electric double-layer capacitance for high voltage by 19F-NMR

Electric double-layer capacitors (EDLCs) afford long cycle life and high-power density by dielectric ion adsorption and desorption on the surface of porous materials. However, high potential charges/discharges lower the capacitance of EDLCs. In this study, we quantitatively evaluated the large pore-size effect of activated carbons (ACs) on the electric double layer capacitances at various voltages based on a hierarchical domain model from a structural point of view. ACs with highly developed microporous and mesoporous (2–4 nm) structures were prepared using KOH activation. The AC with small amount of mesopores showed a specific capacitance of 30.35 F/g at a voltage of 2.7 V, where it significantly lowered to 20.93 F/g at a high voltage of 3.3 V. Meanwhile, the highly microporous AC with abundant mesopores had a higher specific capacitance (40.77 F/g at 2.7 V to 38.22 F/g at 3.3 V). 19F-NMR results indicate that the amounts of solvated ions in the mesopores significantly increased in all the ACs after charging at a higher voltage. The mitigation of the high-voltage capacitance drop might occur because of the mesopore structure that functions as a buffer space for the electric double layer.

Symbiotic study of N-bridged metal complexes as electrocatalysts for hydrogen evolution reaction

Design and fabrication of non-noble metal catalysts for the extensive generation of H2 (hydrogen) gas by water splitting is the upsurging field aiming towards the sustainable environment and need the future clean and green energy. However, scheming and understanding the basic principle regulating the activity of the catalyst is still inexplicable. Although Pt is currently the material of choice owing to its high electrocatalytic activities but need for its high loading and high cost is an intriguing issue needing amicable solution. The hybrid structures of heteroatom-based transition metal complexes are more effective and are cost effective electrocatalysts for hydrogen production. In this proposed work, the electrocatalytic behaviour of alkali (Li+, Na+, K+) and alkaline metal (Ca2+, Sr2+, Ba2+) was figured out along with their synthetic procedure and characterization. Owing to their high complexing ability, ring formation, and bidentate nature, substituted naphthoic acid and aminoguanidine were chosen as bridging ligands. The thermodynamic stability and best catalytic behaviour of synthesized alkali and alkaline earth metals were compared and presented based on electrochemical studies. Linear sweep voltammetry (LSV) curves display excellent activity and Tafel slope was found in the range of 6.38–40 mVdec−1 in 0.5 M H2SO4. The electrochemical impedance spectroscopy (EIS) test was accomplished to recognize the mechanism of HER, and charge transfer resistance was less which indicates that composites are favourable for the hydrogen evolution. The electrochemical surface area (ECSA) was figured by studying the electric double layer capacitance (C dl) and it was found to be 0.377 μFcm2 - 0.143 μFcm2. More prominent ECSA values implies, the target complexes have enhanced electrochemically active/dynamic sites and better hydrogen evolution performance. According to the results, high-performing catalytically active sites are found to be Li [1NA-AMG] metal ions, thus showing a potential method for electrocatalyst engineering. Furthermore, in a volcano plot, the position of the Li [1NA-AMG] metal ions is found to be close to the apex with near thermoneutral catalytic activity. Based on the results, we successfully designed an electrocatalyst as a prospective candidate for hydrogen evolution reaction.

Directed morphology engineering of 2D MoS2 nanosheets to 1D nanoscrolls with enhanced hydrogen evolution and specific capacitance

1D-molybdenum disulfide (MoS2) nanoscrolls displayed enhanced electrochemical properties compared to 2D-MoS2 nanosheet counterparts. Rolling of nanosheets is the main fabrication route to nanoscrolls. However, owing to the conflict between chemical stability and multiple bending, the morphology transition from nanosheets to nanoscrolls is quite challenging. Herein we describe a reversible morphology transition from nanosheets to nanoscrolls by utilizing non-covalent interactions between MoS2 nanosheets and phenothiazine based organic dye. Interestingly, nanoscrolls can easily be opened back into nanosheets by destroying the non-covalent interactions with organic solvents. The prepared nanoscrolls exhibited enhanced electrochemical properties than nanosheets. Compared to nanosheets, nanoscrolls exhibited comparatively lower overpotential with a Tafel slope of 141 mV dec-1 and high specific capacitance of 1868 F g−1. Hydrogen evolution by the Volmer-Heyrovsky mechanism being superior for the nanoscrolls is envisaged by the relatively increased availability of Hads sites at MoS2 edges induced by scrolling. Whereas the high specific capacitance value of nanoscrolls is ascribed to the enhanced electrical double-layer capacitance mediated charge storage, which arises due to the synergistic effect of both scrolled structure and the electron-rich phenothiazine-based dye.

Flexible solid-state supercapacitor based on ternary nanocomposites of reduced graphene oxide and ruthenium oxide nanoparticles bridged by polyaniline nanofibers

The rising demand for wearable electronics has driven research effort tremendously into flexible and compact supercapacitor materials furnished with high energy density, while retaining high-power density, cyclic stability, and durability. In this regard, we, herein, embedded ruthenium oxide nanoparticles (RuO2 NPs) and polyaniline nanofibers (PAni NFs) in the interplanar spaces of reduced graphene oxide (rGO) to prevent their re-stacking to maximize its electric double layer capacitance. It also integrates the pseudo-capacitance of RuO2 and PAni to the ternary nanocomposites (NCs), while the PAni NFs act as interconnecting conducting transmission channel between RuO2 and rGO to regulate charge-transfer kinetics within the system. The resultant ternary NCs display maximum areal capacitance of 1.66 F·cm−2 at a current density of 2 mA·cm−2. A flexible symmetric solid-state device (FSSSD), obtained by assembling the ternary NCs based electrode, attains a specific capacitance of 677 mF·cm−2 with 87 % coulombic efficiency at 2 mA·cm−2 and faster charge transport characteristics (Rct = 5.5 Ω). The device demonstrates maximum energy density of 60.18 μW·h·cm−2 at a power density of 0.8023 mW·cm−2. The functionality of the device is confirmed by turning on a red LED for up to 180 s with three FSSSDs connected in series.

Gamma-induced interconnected networks in microporous activated carbons from palm petiole under NaNO3 oxidizing environment towards high-performance electric double layer capacitors (EDLCs)

Activated carbons (ACs) were developed from palm petiole via a new eco-friendly method composed of highly diluted H2SO4 hydrothermal carbonization and low-concentration KOH-activating pyrolysis followed by gamma-induced surface modification under NaNO3 oxidizing environment. The prepared graphitic carbons were subsequently used as an active material for supercapacitor electrodes. The physiochemical properties of the ACs were characterized using field emission scanning electron microscope–energy dispersive X-ray spectroscopy, N2 adsorption/desorption isotherms with Brunauer–Emmett–Teller surface area analysis, Fourier transform infrared spectroscopy, X-ray diffraction and Raman spectroscopy. The electrochemical performance of the fabricated electrodes was investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Even treated with extremely low H2SO4 concentration and small KOH:hydrochar ratio, the maximum SBET of 1365 m2 g−1 for an AC was obtained after gamma irradiation. This was attributed to radiation-induced interconnected network formation generating micropores within the material structure. The supercapacitor electrodes exhibited electric double-layer capacitance giving the highest specific capacitance of 309 F g−1 as well as excellent cycle stability within 10,000 cycles. The promising results strongly ensure high possibility of the eco-friendly method application in supercapacitor material production.

Highly Localized Charges of Confined Electrical Double‐Layers Inside 0.7‐nm Layered Channels

AbstractA confined electrical double‐layer (EDL) inside nanoporous electrodes has a large capacitance and deviates from traditional ones. Unfortunately, its capacitive mechanism is still unclear. Herein, expanded vertical graphene/diamond (EVG/D) films with regular and ordered 0.7‐nm layered channels are designed and synthesized to serve as an ideal model for understanding confined EDL. A clear overall picture of confined EDL is provided at an atomic resolution with the aid of in situ electrochemical Raman spectroscopy, electrochemical quartz crystal microbalance (EQCM), and density functional theory (DFT) calculations combined with three‐dimension reference interaction site method (3D‐RISM). It is especially interesting that the induced charges in electrode hosts are highly localized with a density far higher than that on a traditional EDL and even close to those of ion batteries. It is proposed that such a high localization of induced charges plays an essential role in the high energy storage efficiency of confined EDL capacitance. This work not only provides a previously unexplored way to refine the mechanism of confined EDL, but also further lays the foundation for understanding the functions of nanoporous or layered materials in electrochemical energy storage.

Biochar-ZnO/polyaniline composite in energy storage application: Synthesis, characterization and electrochemical analysis

Carbon materials displaying electrical double layer capacitance are widely used in the electrochemical energy storage devices. To enhance the electrochemical performance, compositing carbon with transition metal oxides and conducting polymers have been very much appreciated. A novel and effective approach to prepare a high energy density and high specific capacitance composite electrode material from a biomass is presented in the work. Pyrolysis of biomass at 500 °C in the nitrogen atmosphere yielded a biochar (BC) with well-developed porosity, surface functionality and suitable morphological characteristics. Anchoring ZnO nanoparticles on BC and subsequently coating it with a conducting polymeric material, polyaniline (PANI) obtained the hierarchical BC-ZnO/PANI composite. Both BC-ZnO and BC-ZnO/PANI composites have been characterized by FTIR, UV–Visible, XRD, SEM and TEM studies. The characteristic signals at 2θ values 18.5°, 28.4°, 24.0° and 31.4° in XRD correspond to wurtzite structure of ZnO nanoparticles. ZnO rod like structural morphology was confirmed by SEM analysis. The maximum specific capacitance was found to be 110.0 F/g for BC-ZnO and 198.0 F/g for BC-ZnO/PANI. The ternary composite displayed low solution resistance and charge transfer resistance as evident from the Nyquist plots. The results proved that PANI coating is a promising methodology in the fabrication of electrode materials for energy storage applications.

Performance evaluation of novel ml-scale microbial fuel cells using different polymeric hollow-fiber membranes

A novel-designed milliliter-scale MFC was fabricated using three different polymeric hollow fiber membranes of Polyethersulfone (PES), Polyacrilonitryl (PAN), and Polyvinyldenfluoride (PVDF). PES, with a maximum power density of 629 mW/m2, a current density of 1333 mA/m2, and a coulombic efficiency (CE) of 4.22 %, achieved the best performance among the others. After that, the PAN membrane gained power and current densities of 450 mW/m2 and 847 mA/m2, and the PVDF membrane obtained the lowest power and current density of 63 mW/m2 and 187 mA/m2. For all the MFCs, the ohmic resistance incorporates only 2.5 to 5 % of internal resistance due to the small thickness of hollow fibers and the short distance between the electrodes. In the case of the PES membrane, the presence of the sulfonic group resulted in superior proton conductivity (128.10 ± 0.37 mS/cm) and reduced ohmic resistance (24.86 ± 0.07 Ω) compared to the others. Moreover, the charge transfer impedances of both electrodes for PES were nearly half of the values for PAN and about one-fifth of the values for PVDF. The increased electrical double layer capacitance nearby the electrodes of PES reveals more convenient growth of anodic exo-electrogenic bacteria and better accomplishment of cathodic reaction for this membrane. The COD removal efficiencies of 11.48 %, 10.55 %, and 6.48 % have been attained for PES, PAN, and PVDF, respectively.