Electric Double Layer Charge Storage Research Articles

Attaining superior performance in an aqueous hybrid supercapacitor based on N-Doped highly porous carbon spheres and N-S dual doped Co3O4

Cobalt oxide (Co3O4) has seen a significant interest for its application as an electrode for supercapacitors, due to its cost-effectiveness, high theoretical capacitance and outstanding redox activity. However, Co3O4 exhibits poor electrical conductivity and cycle life restricting its high-power energy storage applications. High porosity carbons are materials of choice for applications in electric double layer charge storage but suffer from poor energy densities due to limited charge storage capabilities. Here we propose a hierarchical functionalization strategy to develop N, S co-doped Co3O4 and N doped high porosity carbon micro-spheres for efficient and durable hybrid supercapacitor. Benefiting from the structural eminence, improved electrical conductivity and high quantity of N content, the heteroatom enriched N, S co-doped Co3O4 and N doped porous carbon spheres demonstrates superior electrochemical performance. Moreover, hybrid supercapacitor cell with N, S co-doped Co3O4 as a cathode and N doped porous carbon spheres as anode deliver a high specific energy of 52 Wh kg-1 at power density of 1462 W kg-1 coupled with the capacity retention of 95 % after 5,000 charge-discharge cycles.Therefore, by improving both anode and cathode simultaneously can be considered an effective approach to enhance energy storage capabilities of hybrid supercapacitors while maintain exceptionally high-power densities.

The Effect of Modifications of Activated Carbon Materials on the Capacitive Performance: Surface, Microstructure, and Wettability

In this review, the efforts done by different research groups to enhance the performance of the electric double-layer capacitors (EDLCs), regarding the effect of the modification of activated carbon structures on the electrochemical properties, are summarized. Activated carbon materials with various porous textures, surface chemistry, and microstructure have been synthesized using several different techniques by different researchers. Micro-, meso-, and macroporous textures can be obtained through the activation/carbonization process using various activating agents. The surface chemistry of activated carbon materials can be modified via: (i) the carbonization of heteroatom-enriched compounds, (ii) post-treatment of carbon materials with reactive heteroatom sources, and (iii) activated carbon combined both with metal oxide materials dan conducting polymers to obtain composites. Intending to improve the EDLCs performance, the introduction of heteroatoms into an activated carbon matrix and composited activated carbon with either metal oxide materials or conducting polymers introduced a pseudo-capacitance effect, which is an additional contribution to the dominant double-layer capacitance. Such tricks offer high capacitance due to the presence of both electrical double layer charge storage mechanism and faradic charge transfer. The surface modification by attaching suitable heteroatoms such as phosphorus species increases the cell operating voltage, thereby improving the cell performance. To establish a detailed understanding of how one can modify the activated carbon structure regarding its porous textures, the surface chemistry, the wettability, and microstructure enable to enhance the performance of the EDLCs is discussed here in detail. This review discusses the basic key parameters which are considered to evaluate the performance of EDLCs such as cell capacitance, operating voltage, equivalent series resistance, power density, and energy density, and how these are affected by the modification of the activated carbon framework.

Facile synthesis of nickel oxalate@carbon as electrical double layer and its derived nickel oxide as pseudo-type supercapacitor electrodes

We report a surfactant-free, facile, and scalable synthesis of porous nickel oxalate (NiOX) and its derived cabbage-like nickel oxide (NiO) nanostructures. These cabbage-like NiO nanostructures are derived from NiOX by simple open atmosphere annealing for a prolong time at different temperatures. Structural data revealed the NiOX as NiC2O4·2H2O with defective layered carbon and NiO phase formation before and after annealing, respectively. Morphological changes have been observed using a scanning electron microscope, which revealed the effect of annealing in inducing the surface area, by surface modified phase formation. Further, surface area analysis corroborated the above finding confirming an increase in surface area after annealing. As a proof-of-concept application, these as-synthesized and annealed powders are used as electrode materials for super capacitor and electrochemically tested in a three-electrode configuration using KOH as electrolyte. It is estimated that the electrodes fabricated using as-synthesized NiOX, annealed at exhibit specific capacity (Cs) of 428 Cg−1 (@ 2 Ag−1) in a potential range of − 0.3 to + 0.4 V following the electrical double layer charge storage mechanism. After annealing, the Cs value of this material increased by twofold having the highest estimated value of 888 Cg−1 (0 to + 0.46 V) for 450 °C in comparison to 835 and 547 Cg−1 for 350 and 550 °C annealed samples at 3 Ag−1, respectively, following pseudo-capacitive mechanism. Further, a symmetric device was fabricated using 450 °C annealed sample which exhibit a maximum energy density of 26.1 Wh kg−1 and power density of 302.5 W kg−1 at a current density of 1 Ag−1. Moreover, a systematic study on the facile synthesis of NiOX powder and its further modification to tune the structure, morphology, electrochemical charge storage capacity, and working potential window for super capacitor application has been demonstrated.

High performance coin-cell and pouch-cell supercapacitors based on nitrogen-doped reduced graphene oxide electrodes with phenylenediamine-mediated organic electrolyte

Abstract Coin-cell (CR2023) and pouch-cell supercapacitors (SC) with high specific energy were fabricated using nitrogen-doped reduced graphene oxide (x-N-rGO) as active electrodes and para-phenylenediamine (PPD) in 1 M tetraethyl ammonium tetra-fluoroborate (TEABF4) in acetonitrile as a redox-media organic electrolyte. x-N-rGO with different contents of N-functional groups were synthesized through a hydrothermal reduction of graphene oxide using different concentrations of NH4OH. Electrochemical measurement results suggested that x-N-rGO could provide the enhancements of electrical double layer charge storage as well as redox charge capacity due to its high surface area and redox properties of x-N-rGO. To further improve specific capacitance and specific energy, PPD was introduced into 1 M TEABF4/ACN to amplify the redox reaction rate. The specific capacitance and specific energy significantly enhanced with increasing N content of x-N-rGO because N-functional groups can interact with PPD in electrolyte, resulting in an enhanced redox reaction rate as indicated by the occurrence of redox peaks in cyclic voltammograms. The coin-cell and pouch-cell SCs using 20-N-rGO with 0.2 M PPD + 1 M TEABF4/ACN exhibited the high specific capacitance of 563 and 340 F g−1 with high energy density 149.4 and 77.2 Wh·kg−1 at 1 A g−1, respectively. Hence, they could be a promising alternative for advanced energy storage applications.

Enhancement of the supercapacitive properties of laser deposited graphene-based electrodes through carbon nanotube loading and nitrogen doping.

Several technological routes are being investigated for improving the energy storage capability and power delivery of electrochemical capacitors. In this work, ternary hybrid electrodes composed of conducting graphene/reduced graphene oxide (rGO), which store charge mainly through electric double-layer mechanisms, covered by NiO nanostructures, for adding pseudocapacitance, were fabricated through a matrix assisted pulsed laser evaporation technique. The incorporation of multiwall carbon nanotubes (MWCNTs) provokes an increase of the porosity and thus, a substantial enhancement of the electrodes' capacitance (from 4 to 20 F cm-3 at 10 mV s-1). Volumetric capacitances of 34 F cm-3 were also obtained with electrodes containing just carbon nanotubes coated with NiO nanostructures. Moreover, the use of nitrogen containing precursors (ammonia, urea) for laser-induced N-doping of the nanocarbons also provokes a notable increase of the capacitance. Remarkably, N-containing groups in rGO-MWCNTs mainly add electric double layer charge storage, pointing to an increase of electrode porosity, whereas redox reactions contribute with a minor diffusion fraction. It was also observed that the loading of carbon nanotubes leads to an increase of diffusion-controlled charge storage mechanisms versus capacitive ones in rGO-based electrodes, the opposite effect being observed in graphene electrodes.

Deconvoluting Capacitive and Pseudo-Capacitive Contributions to Electrochemical Capacitor Electrode Behaviour

Generally the capacitive behaviour of the electrochemical capacitors are associated with electrical double layer at the surface of the electrode, and in some materials pseudo-capacitance, which results from faradaic processes. Several electrochemical methods have been developed for determining the contribution of different charge storage mechanisms such as via the electrical double layer and diffusion-limited processes. This includes using a cyclic voltammetry (CV) data at different scan rates to obtain the relationship between a voltammetric current and scan rate (v). In this method, it can be assumed that the total current response for a particular potential is a summation of the current associated with a capacitive processes and a current associated with a diffusion-limited process. While, the capacitive current is proportional to v, and the diffusion limited current is relative to v1/2 (Conway, et al. 1998). The cyclic voltammetry data also can be applied to determine the relationship between a voltammetric charge and scan rate. The charge attributed to the surface capacitive processes is assumed to be constant with scan rate while, the charge attributed to diffusion-limited processes varies with v-1/2 (Trasatti, et al. 1990). Both approaches have been used widely for characterization of the performance of electrochemical systems. However, these methods have some limitations as the relationship between current and v1/2 and also, between charge and v-1/2 is not always linear in a different range of scan rates. Step potential electrochemical spectroscopy (SPECS) also, has been used to effectively differentiate between different charge storage mechanisms. The SPECS method is based on applying a series of equal magnitude potential steps on a working electrode, with sufficient rest time to allow for equilibrium to be established for each step throughout an applied potential window. This slow scan rate enables an electrode to approach its maximum charge storage capabilities. More importantly, it allows separation of the charge storage mechanisms, such as electrical double layer charge storage and diffusion-limited processes. The aim of this study is to compare these three methods experimentally and also to discuss about their limitations and advantages for interpreting the current data and their ability to distinguish between different charge storage mechanisms. Moreover, the relationship between voltammetric current and scan rate has been modified by introducing the constant parameter of residual current. The modified relationship then, can be used over the entire range of potential scan rate. In this work, the combined CV and SPECS methods have been applied to electrolytic manganese dioxide (EMD) in 0.5 M K2SO4 between 0.0 – 0.8 V (vs SCE). Firstly, EMD electrode was cycled for 250 cycles at different scan rates ranged from 1 to 100 mV/s. Subsequently, the SPECS method was applied using 25 mV potential steps and 300 s equilibration time. Eventually, the contribution of capacitive and diffusion-limited processes were obtained using three techniques which were discussed here. The linear dependence of current to v1/2 and the linear dependence of capacity to v-1/2 have not been observed. However, it can be seen that there is a good agreement between the modified model of the scan rate dependence of current and experimental data. Liu, T. C., Pell, W. G., Conway, B. E., & Roberson, S. L. (1998). Behavior of molybdenum nitrides as materials for electrochemical capacitors comparison with ruthenium oxide. Journal of the Electrochemical Society, 145(6), 1882-1888. Ardizzone, S., Fregonara, G., & Trasatti, S. (1990). “Inner” and “outer” active surface of RuO2 electrodes. Electrochimica Acta, 35(1), 263-267.

Investigating the Electrode-Electrolyte Behaviour of Carbon Materials in Various Non-Aqueous Electrolytes

Charge storage in electrochemical capacitors has been shown to be via charge separation at the electrode-electrolyte interface (so called electrical double layer charge storage) and via pseudo-capacitance, which is often described as fast redox reactions through the same interface. Charge storage via both of these mechanisms depends heavily on the interaction between the electrode morphology (electrochemically active surface area, pore size distribution, etc.) and the solvated ion in the electrolyte. In this work the interaction between a series of carbon-based substrates and various electrolyte species has been investigated. Electrolytes examined include tetraethylammonium bis(trifluoromethane)sulfonimide (Et4NTFSI), 1-butyl-1-methylpyrrolidinium tetrafluoroborate (PYR14BF4), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), and tetraethylammonium tetrafluoroborate (Et4NBF4) in acetonitrile. The interaction between these electrolytes and various carbon-based substrates was examined using a combination of cyclic voltammetry (CV) and step potential electrochemical spectroscopy (SPECS). These techniques have enabled differentiation of the charge storage mechanisms based on ion size, solvated ion size and charge density, as well as substrate surface area and porosity.

Designing Pseudocapacitance for Nb2O5/Carbide-Derived Carbon Electrodes and Hybrid Devices

Composite structures for electrochemical energy storage are prepared on the basis of using the high-rate lithium ion insertion properties of Nb2O5. The Nb2O5 is anchored on reduced graphene oxide (rGO) by hydrothermal synthesis to improve the charge-transfer properties, and by controlling the surface charge, the resulting Nb2O5-rGO particles are attached to a high-surface-area carbide-derived carbon scaffold without blocking its exfoliated layers. The electrochemical results are analyzed using a recently published multiscale physics model that provides significant insights regarding charge storage kinetics. In particular, the composite electrode exhibits surface-confined charge storage at potentials of <1.7 V (vs Li/Li+), where faradaic processes dominate, and electrical double layer charge storage at potentials of >2.2 V. A hybrid device composed of the composite electrode with activated carbon as the positive electrode demonstrates increased energy density at power densities comparable to an activated carbon device, provided the hybrid device operates in the faradaic potential range.

Engineering the Pores of Biomass-Derived Carbon: Insights for Achieving Ultrahigh Stability at High Power in High-Energy Supercapacitors.

Electrochemical supercapacitors with high energy density are promising devices due to their simple construction and long-term cycling performance. The development of a supercapacitor based on electrical double-layer charge storage with high energy density that can preserve its cyclability at higher power presents an ongoing challenge. Herein, we provide insights to achieve a high energy density at high power with an ultrahigh stability in an electrical double-layer capacitor (EDLC) system by using carbon from a biomass precursor (cinnamon sticks) in a sodium ion-based organic electrolyte. Herein, we investigated the dependence of EDLC performance on structural, textural, and functional properties of porous carbon engineered by using various activation agents. The results demonstrate that the performance of EDLCs is not only dependent on their textural properties but also on their structural features and surface functionalities, as is evident from the electrochemical studies. The electrochemical results are highly promising and revealed that the porous carbon with poor textural properties has great potential to deliver high capacitance and outstanding stability over 300 000 cycles compared with porous carbon with good textural properties. A very low capacitance degradation of around 0.066 % per 1000 cycles, along with high energy density (≈71 Wh kg-1 ) and high power density, have been achieved. These results offer a new platform for the application of low-surface-area biomass-derived carbons in the design of highly stable high-energy supercapacitors.

The Effects of Duty Cycle on the Step Potential Electrochemical Spectroscopy (SPECS) and Electrochemical Impedance Spectroscopy (EIS) Analysis of Manganese Oxides for Electrochemical Capacitors

Step potential electrochemical Spectroscopy (SPECS) and electrochemical impedance spectroscopy (EIS) have been developed for characterising the performance of electrochemical capacitors. However the effects of the different duty cycle on the behaviour and analysis of electrochemical methods still need to be investigated. In SPECS method a small potential step is applied with a sufficient rest time to allow equilibrium between each step throughout an applied potential window. This small scan rate enables an electrode to reach to its maximum charge storage capabilities. More importantly, it allows separation of the charge storage mechanisms such as electrical double layer charge storage and diffusional processes. EIS method also can provide the different information about the electrochemical cell such as double layer capacitance, series resistance, charge transfer resistance and mass transport using a small AC excitation signal. However, the AC excitation signal can be large in magnitude for the electrochemical capacitors due to the cyclability over a large potential window. In this work, the combined SPECS and EIS methods have been applied to electrolytic manganese dioxide (EMD) in 0.5 M Na2SO4 between 0.0 – 0.8 V (vs SCE) at different duty cycles to examine the changes in charge storage mechanisms. Hence, the different scan rates and equilibrium times have been applied for the same electrode ink and electrolyte. Firstly, the same duty cycle with the same effective scan rate has been examined with increasing the step size and equilibrium time which is constituted of the scan rate of 10 mV in 120 s, 25 mV in 300 s and 50 mV in 600 s. Then, the longer duty cycle with slower discharge rate such as 25 mV in 10 min has been applied to compare the results of different duty cycles. So, nine different experiments have been run using the different step size of 10 mV, 25 mV and 50 mV and different equilibrium times ranging of 120 s, 300 s and 600 s. The outcome of this work indicates the systematic effects of different scan rate on the charge storage mechanisms. Also, it can be seen that, the average capacitance of the electrochemical cell from cycling voltammetry (CV), increased with decreasing the scan rate of the duty cycle.

Simulations and Interpretation of Three-Electrode Cyclic Voltammograms of Pseudocapacitive Electrodes

This study aims to provide physical interpretation of cyclic voltammograms obtained from pseudocapacitive electrodes using three-electrode systems. It presents numerical simulations based on a recent continuum model able to reproduce experimental CV curves obtained with different Nb2O5-based electrodes and LiClO4 in propylene carbonate as electrolyte. First, the respective contributions of faradaic and electric double layer charge storage mechanisms were clearly identified along with the associated faradaic and capacitive regimes. This was further illustrated by comparing CV curves for pseudocapacitive (Nb2O5) and EDLC (carbon) electrodes. Transition from the faradaic to the capacitive regime was caused by Li+ starvation at the electrode/electrolyte interface and the formation of ClO4− electric double layer. Moreover, the effects of electrode crystallinity on CV curves were reproduced and interpreted in terms of enhanced transport properties for lithium intercalation and electrode electrical conductivity. Finally, the electrode thickness featured an optimum corresponding to a compromise between accommodating large amounts of intercalated lithium and minimizing the potential drop across the electrode to drive faradaic reactions.

Electrocatalytic and supercapacitor performance of Phosphorous and Nitrogen co-doped Porous Carbons synthesized from Aminated Tannins

A phosphorus and nitrogen co-doped carbon material (PNDC) was synthesized from aminated tannin and polyphosphoric acid by a rapid and highly efficient microwave synthetic technique. X-ray photoelectron spectroscopy study was useful in the identification of nitrogen and phosphorous environments in a sp2 hybridized carbon lattice. The PNDC was found to be a porous material with a surface area of 433m2g−1. PNDC sample exhibited excellent thermal stability and the Raman spectroscopic studies were used for analyzing defects in the sp2 hybridized carbon lattice. This material has promising electrochemical applications, especially for catalyzing oxygen reduction reaction in fuel cells and for charge storage in supercapacitors. The oxygen reduction capability of PNDC was investigated in 0.1M KOH solution, and rotating disk and ring disk electrode studies were performed to identify the mechanism of oxygen reduction. The capacitative behavior of the PNDC was investigated in 6M KOH and specific capacitance was determined to be 161Fg−1 due to the electric double layer charge storage phenomenon.

Enhancing Faradaic Charge Storage Contribution in Hybrid Pseudocapacitors

This study aims to provide metrics and design rules for hybrid pseudocapacitors consisting of a transition metal oxide pseudocapacitive electrode and an inert carbon electrode with asymmetric and binary electrolyte. Hybrid pseudocapacitors are often limited by intercalation of Li+ ions in the pseudocapacitive electrode. Then, during cyclic voltammetry, only a portion of the potential window is dominated by faradaic current while the electric double layer (EDL) formation and dissolution dominate the rest. However, faradaic reactions result in a significantly larger current magnitude and capacitance than EDL charge storage. Hence, it is beneficial to extend the fraction of the potential window dominated by the faradaic current. This fraction increases with a combination of thinner pseudocapacitive electrode, faster intercalation, and slower scan rate. To study the interplay between these variables, a scaling analysis was performed to identify the relevant dimensionless similarity parameters governing Li+ transport and intercalation in the pseudocapacitive electrode. The fraction of the potential window dominated by faradaic reactions was a unique function of a dimensionless parameter Πf accounting for the respective contributions of the electrode thickness, Li+ diffusion coefficient, and scan rate. Above a critical value of Πf, the faradaic current dominated the entire potential window. Then, the device was no longer limited by intercalation in the electrode and the performance of the electrode was maximal.

Insight on the effect of surface modification by carbon materials on the Ionic Liquid Electric Double Layer Charge Storage properties

Development of new and better performing energy storage devices rely on the use of innovative materials. The combination of nanostructured materials with high specific area, such as graphene, and conventional electrode materials such as Glassy Carbon (GC) and the use of the composite electrode in ionic liquids (IL) are considered to be a promising strategy for improved energy storage devices. Following our previous studies of electrochemical interfaces involving 1-butyl-3-methylimidazolium (tris(pentafluoroethyl) trifluorophosphate) ([C4MIM][FAP]) ionic liquid and Hg, Au, Pt and GC we extended the search for better electrochemical performance by preparing GC electrode surfaces modified with different carbon materials (reduced graphene oxide, reduced graphite oxide and graphite). Cyclic voltammetry of these electrode surfaces in [C4MIM][FAP] ionic liquid shows a 100 fold increase in the capacitive current of the composite electrode when compared with plain GC electrode.

A Step Potential Electrochemical Spectroscopy Analysis of Electrochemical Capacitor Electrode Performance

Step potential electrochemical spectroscopy (SPECS) has been developed as an electrochemical method for application to electrochemical capacitors, in this case for an activated carbon electrode in 0.5M H2SO4. The method involves the application of a sequence of small potential steps over the potential window used by the electrode under study, followed by a relatively long rest time after each potential step. Modelling of the SPECS data has been used to extract the contributions made by electrical double layer charge storage and diffusional (redox) processes to the total electrode performance. Outcomes from the application of this approach highlight the dominant charge storage mechanisms at both high and low cycle rates, as well as allow differentiation of ‘capacitor-like’ or ‘battery-like’ electrode behaviour. The SPECS data has also been analysed so as to enable calculation of voltammograms at different scan rates for the electrode system. Finally, electrode performance data has also been presented and discussed.

Flowable Conducting Particle Networks in Redox-Active Electrolytes for Grid Energy Storage

This study reports a new hybrid approach toward achieving high volumetric energy and power densities in an electrochemical flow capacitor for grid energy storage. The electrochemical flow capacitor suffers from high self-discharge and low energy density because charge storage is limited to the available surface area (electric double layer charge storage). Here, we examine two carbon materials as conducting particles in a flow battery electrolyte containing the VO2+/VO2+ redox couple. Highly porous activated carbon spheres (CSs) and multi-walled carbon nanotubes (MWCNTs) are investigated as conducting particle networks that facilitate both faradaic and electric double layer charge storage. Charge storage contributions (electric double layer and faradaic) are distinguished for flow-electrodes composed of MWCNTs and activated CSs. A MWCNT flow-electrode based in a redox-active electrolyte containing the VO2+/VO2+ redox couple demonstrates 18% less self-discharge, 10 X more energy density, and 20 X greater power densities (at 20 mV s−1) than one based on a non-redox active electrolyte. Furthermore, a MWCNT redox-active flow electrode demonstrates 80% capacitance retention, and >95% coulombic efficiency over 100 cycles, indicating the feasibility of utilizing conducting networks with redox chemistries for grid energy storage.

Potential-induced electronic structure changes in supercapacitor electrodes observed by in operando soft X-ray spectroscopy.

The dynamic physiochemical response of a functioning graphene-based aerogel supercapacitor is monitored in operando by soft X-ray spectroscopy and interpreted through ab initio atomistic simulations. Unanticipated changes in the electronic structure of the electrode as a function of applied voltage bias indicate structural modifications across multiple length scales via independent pseudocapacitive and electric double layer charge storage channels.

Effects of structural disorder and surface chemistry on electric conductivity and capacitance of porous carbon electrodes.

This article reports on changes in electric double layer charge storage capacity as a function of surface chemistry and graphitic structure of porous carbon electrodes. By subjecting 20 nm to 2.0 μm sized carbide-derived carbons (CDCs) synthesized at 800 °C to high-temperature vacuum annealing at 700-1800 °C, we produce three-dimensional internal surface architectures with similar pore sizes and volumes but divergent surface chemistry and wall graphitization. Annealing increases carbon ordering and selectively removes functional groups, and both transformations affect conductivity and wettability. Contrary to an expected increase in gravimetric capacitance, we demonstrate no increases in charge storage despite increased conductivity and pore accessibility. At the same time, annealing improves the charge/discharge rates in EMIm-TFSI ionic liquid electrolyte. The annealing process eliminates faradaic reactions that limit the voltage window, but potentially accelerates catalytic breakdown of the ions themselves. We therefore corroborate the theory that surface groups and defects in the graphitic structure act as dopants that allow facile movement of ions into pores, improve screening in the superionic state, and affect the quantum capacitance contribution from the carbon structure.