The capacitance of the diffuse layer of electric double layer of electrodes in molten salts

Similarly to aqueous electrolytes, the electric double layer of electrodes in molten salts is assumed to be composed of compact and diffuse layers. The charge density of the compact layer, formed as a monolayer of specifically adsorbed anions (primary ionic shell), is calculated as the difference between the charge of the primary ionic shell and the charge removed by the exchange current density. The centre of the specifically adsorbed anions create the inner Helmholtz plane (iHp). The counterions to the specifically adsorbed anions in the primary ionic shell, take place in the numerous neighbouring holes, introduced into the molten salt structure by the melting process, and being subjected to thermal motion, create the diffuse layer. The electrostatically adsorbed metal cations form the outer Helmholtz plane (oHp) with the value of the inner potential equal φ₂. Using the Boltzman and Poissone equations, the equation for the capacitance of the diffuse layer of the metallic electrode in molten salt is derived and tested on some literature experimental results.     

All solid electric double layer capacitors based on Nafion ionomer

The aim of the present work is to demonstrate that an efficient all solid electric double layer capacitor (EDLC) may be realised with electrolyte membrane and carbon based electrodes prepared by using a Nafion ionomer solution. Polymer membrane was prepared by a casting method. Electrodes were prepared with two overlapped layers formed of a carbon-Nafion layer and a carbon paper substrate. Three different electrolyte separators in capacitor configuration have been tested and compared: (1) a commercial Nafion 115 membrane (N115), (2) a membrane prepared by casting the Nafion 1100 solution (NRG50) and (3) a porous glass fibre matrix impregnated with a 1 M H₂SO₄ solution (FVH2SO4). The membrane and electrodes assemblies (MEA) had thickness of 0.6-0.8 mm and geometric area of 4 cm². The EDLCs characteristics have been studied by conductivity, cyclic voltammetry and DC charge-discharge methods. Proton conductivities of 5.7×10⁻² and 3.1×10⁻² S cm⁻¹ have been measured at room temperature for the N115 and the NRG50, respectively. Specific capacity of 13.2 F g⁻¹ has been obtained by capacitor utilising the cast Nafion membrane. This value is 70% of specific capacity obtained from the capacitor using sulphuric acid and about 140% of that using Nafion 115.     

Spatio-temporal charge behavior in electric double layer capacitors observed by pulsed electro acoustic method

The use of the pulsed electro acoustic (PEA) method allowed us to perform the direct observations of spatio-temporal charge distributions in EDLCs based on polarizable nanoporous carbonaceous electrode. The negative charge density became the maximum, about −173 C/m³ at the region where was near to anode layer in EDLCs for case of dc voltage 2.5 V, while the positively charged density became the maximum, about 70.7 C/m³ at the region where it was located around the cathode layer. The behavior of negative and positive charges measured by the PEA method reflected the hetero-charges distribution in EDLCs. The performance of the sample was found to be better in terms of the capacitance (C_s) with a maximum value of 55.4 F/g than general EDLCs which use organic electrolyte.     

Electrochemical characterization of new electric double layer capacitor with polymer hydrogel electrolyte

A new electric double layer capacitor (EDLC) was constructed by using polymer hydrogel electrolyte prepared from crosslinked potassium poly(acrylate) and KOH aqueous solution, and its electrochemical characteristics were investigated by cyclic voltammetry and charge-discharge cycle tests, compared with a case of the cell using only a KOH aqueous solution as an electrolyte. As a result, the cell with the polymer hydrogel electrolyte was found to exhibit higher capacitance than that with the KOH aqueous solution and excellent high-rate dischargeability. The impedance spectroscopic measurements suggested that the higher capacitance could be ascribed to the pseudocapacitance. These results indicate the potential applicability of the polymer hydrogel electrolyte to EDLCs as an electrolyte with good performance.     

Imaging of DNA microarray with scanning electrochemical microscopy

DNA duplex regions of a spot on a DNA microarray were successfully visualized by scanning electrochemical microscopy (SECM) coupled with ferrocenylnaphthalene diimide as an electrochemically active DNA hybrid indicator. Correlation of a normalized steady-state current with the normalized tip-substrate distance suggested that the tip current was generated by direct contact of the DNA duplex layer on the DNA spot bound to this ferrocenylnaphthalene diimide with the tip (not meaning the direct physical touching of the surface). Single nucleotide polymorphisms (SNPs) of Arg or Pro type of cancer suppression gene p53 were detected by using this system.     

Study on synthesis of low surface area activated carbons using multi-step activation for use in electric double layer capacitor

Low surface area activated carbon derived from compact mesocarbon microbeads (MCMB2010) was synthesized using a lower amount of KOH (1:1 weight ratio of KOH to MCMB) than normally used followed by electrochemical activation. The specific capacitance of the activated carbon heat treated at between 650 and 900 °C was increased up to ca. 118 F/cc (half cell base, 750 °C-heat treated sample) after electrochemical activation, even with a low surface area carbon (<50 m²/g). The morphology of low surface area activated MCMB determined by FE-SEM showed a smooth carbon surface without pores. The charge/discharge profiles were similar to those of conventional activated carbon. The specific capacitance of the activated samples increased with increasing heat treatment up to 850 °C after electrochemical activation. However, it was lower for the sample heat treated at 900 °C.     

Imaging decorated platinum single crystal electrodes by scanning electrochemical microscopy

Platinum single crystal electrodes, Pt(h k l), represent ideal materials where studying surface sensitive reactions such as oxygen reduction reaction (ORR). Moreover, there is a great interest in testing carbon supported electrocatalyts mixed with Nafion^® ionomer in order to directly evaluate catalysts under practical fuel cell conditions. Thus, we provide a first imaging attempt by scanning electrochemical microscopy (SECM) to locally evaluate the electrocatalytic activity during ORR on a Pt(1 1 1) single crystal electrode decorated with spots of commercial carbon supported platinum nanoparticles entrapped in Nafion^®. Both electrocatalysts present the same chemical composition and then, total surface area, particle size and crystallographic orientation at the electrode surface are the effects studied. Our SECM images prove that the peroxide pathway can also be considered a relevant reaction route on platinum electrodes. We agree with some recent reports pointing the Nafion^® content and the three-dimensional surface electrode area as key factors to control for achieving a proper evaluation of the apparent number of electrons exchanged during ORR.     

Electrochemical properties of novel ionic liquids for electric double layer capacitor applications

An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF₄⁻) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF₃SO₂)₂N⁻] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF₄) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF₄ is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0 V) and high ionic conductivity (4.8 mS cm⁻¹ at 25 °C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF₄ as the electrolyte. This EDLC (working voltage ∼2.5 V) has both, a higher capacity above room temperature and a better charge-discharge cycle durability at 100 °C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.     

Activated carbon fiber cloths as electrodes for high performance electric double layer capacitors

Activated carbon fiber cloth (ACFC) electrodes with high double layer capacitance and good rate capability were prepared from polyacrylonitrile (PAN) fabrics by optimizing the carbonization temperature prior to CO₂ activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the ACFCs. Moderate carbonization at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the ACFCs in 6 mol L⁻¹ KOH aqueous solution can be as high as 208 F g⁻¹. It remains 129 F g⁻¹ as the current density increases to 10 000 mA g⁻¹.     

Visualisation of the local electrochemical activity of thermal sprayed anti-corrosion coatings using scanning electrochemical microscopy

Scanning electrochemical microscopy was used to study the electrochemical activity of anti-corrosion coatings formed from Inconel 625, a Ni-Cr-Mo alloy commonly used in engineering applications. The coatings were formed using a high velocity oxygen fuel thermal spraying technique. Upon spraying the alloy onto mild steel substrates, clear splat boundaries were formed at the interface between adjacent droplets as they cooled on the substrate surface. Scanning electrochemical microscopy in the feedback mode, employing ferrocenemethanol as redox mediator, was used to study the local electrochemical activity of samples of the wrought alloy, the sintered alloy and the thermal sprayed coating. The wrought and sintered materials showed responses typical of that expected for a purely insulating material. However, SECM approach curve data showed that the electrochemical activity of the thermal sprayed material was higher than that of the bulk alloy. Local variations in the coating's electrochemical activity were then visualised using SECM imaging, which appear to be related to the splat boundaries formed during the thermal spray process.     

Electrodeposition of Ti from TiCl4 in the ionic liquid l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide at room temperature: study on phase formation by in situ electrochemical scanning tunneling microscopy

Titanium was electrodeposited from a nominal 0.24 M TiCl₄ in l-methyl-3-butyl-imidazolium bis (trifluoro methyl sulfone) imide ([BMIm]BTA) at room temperature on a Au(1 1 1) substrate. The process of electrodeposition was studied by cyclic voltammetry, chrono amperometry and in situ scanning tunneling microscopy (STM). In a first step TiCl₄ is reacted to TiCl₂, which is subsequently reduced to metallic Ti. Two dimensional (2D) clusters form preferentially on the terraces in the under potential deposition range. 2D clusters presumably of TiCl₃ precipitates grow and coalesce to cover the whole substrate with a 2D film at a substrate potential below −1.1 V versus ferricenium/ferrocene ([Fc]⁺/[Fc]) redox couple. At a potential of −1.8 V a dense layer of three dimensional (3D) clusters of titanium of 1-2 nm thickness is formed. The features of the I-U tunneling spectra and the relative reduction of the effective tunneling barrier by 0.8 eV with respect to gold clearly indicate the metallic character of Ti deposits. Observation of circular holes on the Au(1 1 1) substrate after dissolution of the deposited Ti indicates the formation of Au-Ti surface alloying.     

Characterization of coating systems by scanning electrochemical microscopy: Surface topology and blistering

Operation of the scanning electrochemical microscope used in feedback mode over a coated metal allows changes in the state of the coating surface to be monitored during immersion in aqueous electrolytes. This paper reports changes in the coating induced by specific anions in the electrolyte in situ during immersion. Significant surface roughening is observed for immersion times shorter than 1 day when the electrolyte contains chloride ions. This effect is also observed when the oxygen dissolved in the electrolytic phase is employed as redox mediator for SECM imaging. The coated system exposed to chloride-free electrolytes containing sulphate or nitrate maintains a featureless topography within the same time scale. The observed features are due to the nucleation and growth of blisters at the metal/coating interface induced by chloride ions in the environment. The implication is that ionic migration occurs simultaneously with the absorption of water by the coating already from the beginning of exposure to the aqueous environment. The unique role of chloride ions compared with sulphate or nitrate ions towards coating performance has been established at a very early stage following immersion of the sample.     

Studying the localized deposition of Ag nanoparticles on self-assembled monolayers by scanning electrochemical microscopy (SECM)

The local deposition of Ag nanoparticles (NPs) on ω-mercaptoalkanoic acid, HS(CH₂)_nCO₂H, (n = 2, 10) self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. We found that the presence of a SAM had a pronounced effect on Ag deposition. Experiments were conducted by applying different potentials to an Au(1 1 1) substrate either in the presence of a constant concentration of Ag⁺ ions in solution (bulk deposition) or by generating a flux of Ag⁺ from an Ag microelectrode that was positioned close to the Au(1 1 1) substrate (SECM deposition). SECM was used for generating a controlled flux of silver ions by anodic dissolution of an Ag microelectrode close to the SAMs modified Au(1 1 1). We found that the shape of the NPs was affected by the length of the carbon-chain of the SAM. Tetrahedral NPs were obtained on bare Au(1 1 1) surfaces while rod like and cubic Ag NPs were deposited onto 3-mercaptopropanoic acid (MPA) and 11-mercaptoundecanoic acid (MUA) SAMs, respectively. The size and shape of the deposited NPs were influenced by the deposition potential.We conclude that the shape and distribution of locally deposited Ag NPs on Au(1 1 1) can be controlled by modification of the substrate with a SAM and through controlling the Ag⁺ flux generated by SECM.     

Morphological reason for enhancement of electrochemical double layer capacitances of various acetylene blacks by electrochemical polarization

Enhancement of electrochemical capacitance and morphological variations of various acetylene blacks caused by electrochemical polarization are presented. Acetylene blacks of different mean particle diameters were modified by air-oxidation and heat treatment to diversify the morphologies of the acetylene blacks before electrochemical polarization. The various acetylene blacks were electrochemically oxidized at 1.6 V (vs. Ag/AgCl) for 10 s and the polarization step was repeated until the capacitance values did not change any longer. These polarization steps enhanced the capacitances of the acetylene blacks and the specific enhancement factors range from 2 to 5.5. Such an enhancement is strongly related to morphological modification as revealed by transmission electron microscopic observations. The electrochemical polarization resulted in formation of tiny graphene sheets on the wide graphitic carbon surfaces, which were most responsible for the observed capacitive enhancement. Although the pseudo-capacitance increased after polarization by forming oxygenated species on the surfaces, its contribution to the total capacitance was less than 10%. The mechanism of the formation of the tiny graphene sheets during the electrochemical oxidation is described schematically.     

The use of scanning electrochemical microscopy for the characterisation of patinas on copper alloys

The scanning electrochemical microscopy (SECM) was employed in the feedback mode for the visualisation of the local changes of morphology and reactivity, occurring on the surface of a quaternary copper-based alloy, due to exposure to environment. First, samples artificially aged by exposure to leaching acid rain were tested. The layer of corrosion products (“patina”) was investigated by performing SECM scans and approach curves, and the information provided by this electrochemical technique was compared with SEM–EDX–Raman characterisation. Successively, to highlight the early stages of localised corrosion processes, in situ examination of the surface exposed to an acidic environment was performed. The results show that SECM is a powerful characterisation tool of the deterioration process and is able to map the precursor sites of corrosion on the bronze surface.     

Electrochemical characteristics of electric double layer capacitor using sulfonated polypropylene separator impregnated with polymer hydrogel electrolyte

Sulfonated polypropylene separators impregnated with the polymer hydrogel electrolyte were used in electric double layer capacitors (EDLCs). The electrochemical properties of the EDLC with the polymer hydrogel electrolyte were investigated by cyclic voltammetry and charge-discharge cycle tests and compared with a KOH aqueous electrolyte. Furthermore, effects of KOH concentration and temperature on capacitance of the EDLC were studied. As a result, it was found that the capacitance of the EDLC with the polymer hydrogel electrolyte was higher than that with a KOH aqueous solution in the wide range of KOH concentration and temperature.     

Understanding the effects of electrode meso-macropore structure and solvent polarity on electric double layer capacitors based on a continuum model

The structures of electrode meso-macropore and the solvent polarity are the crucial factors dominating the performance of the electric double layer capacitors (EDLCs), but their impacts are usually tangled and difficult to decouple and quantitate. Here the effects of electrode meso-macropore structure and solvent polarity on the specific capacitance of an EDLC are quantitatively investigated using a steady-state continuum model. The simulation results indicate the specific capacitances are significantly affected by the meso-macropore surface structure. The specific capacitances significantly decrease for both convex surface structures but obviously increase for both concave surface structures, with the increase of curvature radius from 1 to 20 nm. As for solvents, the polar solvent with high saturated dielectric permittivity improves the capacitance performance. Moreover, the electrode meso-macropore structure is of more concern compared with solvent polarity when aiming at enhancing the specific capacitance. These results provide fundamentals for the rational design of porous electrodes and polar electrolytes for EDLCs.     

Sulfonated poly(ether ether ketone) membranes for electric double layer capacitors

Sulfonated poly(ether ether ketone) (S-PEEK) with different degree of sulfonation (DS) has been prepared and evaluated as a proton conducting membrane for electric double layer capacitor (EDLC). The polymer electrolytes prepared with S-PEEK membrane exhibited ionic conductivities about 1.2 × 10⁻³-4.5 × 10⁻³ S cm⁻¹ at room temperature, which depended on both soaking solvent and degree of sulfonation. The quasi-solid-state EDLCs consisted of activated carbon electrodes and S-PEEK membrane were assembled, and their electrochemical characteristics were studied by cyclic voltammetry and charge-discharge cycle tests. The effect of DS on the electrochemical performances of EDLCs has been investigated.     

Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode

Herein, the redox competition mode of scanning electrochemical microscopy (RC-SECM) has been applied to in situ monitor the local reactivity arising from a circular holiday operated in a painted metal. The metal-coating system consisted of a carbon steel substrate coated with an epoxy-polyamine polymeric film containing glass flakes as pigment. The present work demonstrates the possibility to use RC-SECM for investigation of the corrosion reactions occurring when protective coatings are applied on a metal surface without the addition of a redox mediator to the experimental system. Oxygen reduction was employed to monitor the reactive metal-polymer system, though the onset of a redox competition for this redox species between the SECM tip and the bare metal inside the holiday could be found in certain conditions. Whether attack predominated inside the holiday or the system became non-reactive depended on the composition of the test electrolyte, with borate ions acting as corrosion inhibitor.