Effect of H3PO4 on the PbSO4/PbO2 electrode in H2SO4 solutions

The influence of H₃Po₄ small additions on the polarization behaviour of the PbSO₄/PbO₂ electrode, in H₂SO₄ solutions of different concentrations, was studied using the microelectrode technique and the coulometric method. The reference electrode was the Hg/Hg₂SO₄/H₂SO₄ electrode. The potentiogram analysis and the coulometric measurements show that the main effect of the H₃PO₄ is the drastic passivation of the electrode processes, within the potential range under study, ie the +800 mV to +1650 mV potential range. The Q_a and Q_c electricity amounts are of about 5–10 times smaller in the H₂SO₄ solution with H₃PO₄ addition, compared with the H₂SO₄ solution free of H₃PO₄ addition. A high increase of the oxygen evolution overvoltage could also be seen. Taking into account the ratio between the H₃PO₄ addition amount, in % (w/o) and the PbO₂ active mass per unit surface area, from the lead acid batteries, we can conclude that the small H₃PO₄ additions do not decrease the capacity of the active mass of the battery, but they hinder the insulator film formation between the active mass and the grid.     

Effect of molecular grafting on the pore size distribution and the double layer capacitance of activated carbon for electrochemical double layer capacitors

High specific surface carbon (Black Pearls 2000) has been modified by electroactive anthraquinone (AQ) groups by spontaneous reduction of the corresponding in situ generated diazonium derivative. A set of modified carbons with different AQ loadings have been synthesized to study the effect of the grafted groups on the pore texture and on the double layer capacitance of the modified carbons. Grafting of a small amount of AQ molecules caused a significant loss of the BET surface area and affected mainly the ultramicroporosity of the carbon powder. A correlation of the pore texture with the electrochemical data allows to conclude that pores smaller than 0.65 nm do not significantly contribute to the double layer capacitance in H₂SO₄ electrolyte and therefore are not efficiently accessible to the ions of this electrolyte. Above an AQ loading of 7 wt.%, both microporosity and mesoporosity are strongly affected but the decrease of double layer capacitance seems to be directly related to the coverage of the carbon surface by AQ molecules.     

Anion-adsorption-related frequency-dependent double layer capacitance of the platinum-group metals in the double layer region

The interfacial impedance for platinum-group metals in simple binary electrolytes, in their so-called double layer region, is generally assumed to be ideally capacitive. Using single crystalline platinum-group metals with various solutions we demonstrate that this is not necessarily the case when the anion of the solute is specifically adsorbed on the electrode. We attribute the frequency dependence of the interfacial capacitance to the relatively slow exchange of anions between the outer and inner Helmholtz planes.     

Studies of double layer capacitance and electron transfer at a gold electrode exposed to protein solutions

Electrochemical impedance spectroscopy (EIS) was used to investigate the electrochemical behaviour of a gold electrode exposed to proteins prepared in phosphate buffer. Exposure to solutions of human serum albumin (HSA) and immunoglobulin G (Ig.G) resulted in a decrease of the double layer capacitance (C_dl) and an increase in the charge transfer resistance (R_ct) at the gold electrode solution interface. The greatest capacitance decrease for both proteins was observed when exposure occurred at or more positive to the electrode open circuit potential (OCP). Exposure to Ig.G resulted in a greater decrease in capacitance as compared to HSA under identical conditions. These capacitance and charge transfer resistance variations were attributed to the formation of a proteinaceous layer on the electrode surface during exposure.     

Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l⁻¹ H₂SO₄ and 6 mol l⁻¹ KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m² g⁻¹. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m² g⁻¹, the higher BET surface area the higher specific capacitance (F g⁻¹) for both electrolytes. The surface capacitance (μF cm⁻²) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g⁻¹ and a relatively high surface capacitance about 16 μF cm⁻² measured in H₂SO₄ solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H₂SO₄ than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm⁻²). However, in the case of H₂SO₄ the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm⁻²). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm⁻².     

Effect of gelling on the open circuit potential against time transients of Pb/PbSO4 electrodes at various states of charge

The technique of open-circuit potential relaxation from a prior prepolarized state has been employed to study the kinetics of the Pb/PbSO₄ electrode in 4.81 m sulphuric acid. The significant features governing the relaxation have been quantitatively analysed to obtain a correlation between state-of-charge of the electrode and its potential-recovery time constant. The effect of immobilizing the electrolyte has also been included due to its relevance in making sealed maintenance-free batteries. The study provides a method for estimating the variation of exchange current density of the Pb/ PbSO₄ and hydrogen evolution reactions at various states-of-charge with and without added gelling agents.This paper is dedicated to the memory of the late Professor S. Sathyanarayana.     

Rational determination of exchange current density for hydrogen electrode reactions at carbon-supported Pt catalysts

The rotating disk electrode (RDE) is a useful technique for precise determination of exchange current density (j₀) in electrochemistry. For the study of powder catalysts, a common practice is to apply the powder onto an inert disk substrate (such as glassy carbon). However, this approach in its usual version will lead to wrong results for the exchange current density of hydrogen electrode reactions at carbon-supported Pt nanoparticles (Pt/C) because of the poor utilization of the loaded Pt nanoparticles. Our new approach is to dilute the Pt/C powder with a large amount of pristine carbon support to make the catalyst layer. In this way, all the catalyst particles in the catalyst layer have nearly the same and much enhanced mass transport so that rational exchange current density can be obtained. Using the new approach, the current density for hydrogen electrode reactions at Pt/C in 0.1 M perchloric acid at 25 °C is found to be 27.2 ± 3.5 mA/cm² with an apparent activation energy 43 kJ/mol. These results are in agreement with the j₀ estimation based on real fuel cell experiments.     

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.     

Role of surface chemistry on electric double layer capacitance of carbon materials

A large number of porous carbon materials with different properties in terms of porosity, surface chemistry and electrical conductivity, were prepared and systematically studied as electric double layer capacitors in aqueous medium with H₂SO₄ as electrolyte. The precursors used are an anthracite, general purpose carbon fibres and high performance carbon fibres, which were activated by KOH, NaOH, CO₂ and steam at different conditions. Among all of them, an activated anthracite with a BET surface area close to 1500 m²/g, presents the best performance, reaching a value of 320 F/g, using a three-electrode system. The results obtained for all the samples, agree with the well-known relationship between capacitance and porosity, and show that the CO-type oxygen groups have a positive contribution to the capacitance. A very good correlation between the specific capacitance and this type of oxygen groups has been found.     

Effect of temperature on sulphate adsorption/desorption by tricalcium silicate hydrates

Sulphate adsorption from internal sources was studied in hydrating systems containing C-S-H gel and gypsum with respect to delayed ettringite formation. The influence of C₃A addition on sulphate desorption was also investigated. Research indicates that C-S-H gel will adsorb sulphate faster at high temperature resulting in quick depletion of the gypsum phase in C-S-H - gypsum mixtures. Sulphate adsorbed at high temperature is desorbed more slowly than that adsorbed at normal temperature. Slower release of sulphate from an internal sulphate source may be a critical condition for delayed ettringite formation in high temperature cured Portland cement paste.     

Towards understanding the structure and capacitance of electrical double layer in ionic liquids

In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of the electrode potential, as prescribed by the mean-field theory and the charge conservation law at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected their remarkable overscreening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system, unlike it is often assumed, the double layer is not one layer thick. The overscreening effects, dominating near the potential of zero charge (p.z.c.), are suppressed by the high electrode polarizations, following the onset of the so-called ‘lattice saturation effect’. The maximum of the capacitance coincides with the p.z.c., but it is true only for this ‘symmetric’ system. If sizes of cations and anions are different the maximum will be shifted away from the p.z.c., and generally the shape of the capacitance curve could be more complicated.     

基于阴离子交换反应的NiMn-LDH电极电容性能提升研究

采用一步水热法在泡沫镍网上原位生长镍锰基层状双氢氧化物（NiMn-LDH）纳米片阵列电极,并通过氢氧化钾溶液中浸泡的方式提升电极的容量。采用SEM、XRD、TEM和XPS等手段对浸泡前后的电极材料进行表征。结果表明,在浸泡前后NiMn-LDH电极的形貌没有变化,但在电极材料内部发生了明显的CO₃^2-和OH^-的交换反应,降低体积较大的CO₃^2-在LDH层间的分布数量,使层内空间成为OH^-的“蓄水池”,缩短了电荷存储过程中OH^-的迁移距离,因此电容性能有了明显提升。电化学测试结果表明,在5 mA/cm²电流密度下,电极的比电容从18.0 F/g增加至766.6 F/g（1.69 F/cm²）。将该电极与活性炭组装的全固态不对称超级电容器在功率密度为900 W/kg时,可呈现的能量密度为35.9 W·h/kg,并且器件的循环稳定性良好。     

Time resolved second harmonic generation studies of double layer reorganization: sulfate adsorption and desorption on a polycrystalline silver electrode

The kinetics of the adsorption and desorption of non-specifically adsorbed anions on polycrystalline Ag electrodes biased within the limits of the ideally polarisable region have been investigated using time resolved second harmonic generation. The transient SH intensity directly correlates with the time and potential dependent surface charge density on the electrode. Attempts to fit the temporal data with existing theoretical models were unsuccessful and suggest the need for more detailed models.     

Electrosorption on activated biochar: effect of thermo-chemical activation treatment on the electric double layer capacitance

Biochar, a by-product of woody biomass pyrolysis, is investigated as a renewable and low-cost carbon-based electrode material for electric double layer (EDL) applications. To increase the surface area and porosity of the biochar chemical (7 M KOH) and thermal (at 675 and 1,000 °C, respectively) activation treatments are applied. The thermo-chemically activated biochar samples are investigated by a combination of physico-chemical surface characterization and electrochemical methods to reveal the relationship between the activation process variables, the resulting porous carbon structural features and EDL capacitance. For electrochemical testing, the activated biochar is sprayed onto Ni mesh current collectors with or without Nafion^® as binder. Based on cyclic voltammetry experiments in 0.1 M NaCl–0.1 M NaOH a maximum EDL capacitance of 167 F g^?1 is obtained for the activated biochar electrode prepared at 675 °C. The latter capacitance is about 50 times higher than the EDL capacitance of a Vulcan XC-72 electrode prepared and tested under identical conditions. The activated biochar electrodes show also promising galvanostatic charge/discharge behavior and electrical conductivities up to 0.058 S cm^?1 indicating suitability for EDL-type applications.     

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m²/g), do not seem to be effective for double layer capacitors.     

The chlorine/carbon electrode in a silver chloride melt in the temperature range 475–820°C—II: The double layer capacitance of a carbon electrode in the silver chloride melt

The double layer capacitance of a vitreous carbon electrode in a silver chloride melt has been measured as a function of potential at four different temperatures. The minimum capacitance is virtually constant from 475 to 750°C; it is relatively high ≈48 μF/cm², possibly the result of the polarisability of the silver ions. At temperatures above 750°C the minimum capacitance increases, and the potential range of minimum capacitance becomes very narrow. This confirms the theory of Graves and Inman[1,2] that the minimum capacitance is independent of temperature, and that the apparent increase is the result of intruding Faradaic processes.     

Some observations on recent theories of adsorption of organic compounds in the double layer

Contemporary theories of adsorption of neutral organic substances at electrodes are analysed. Attention is focussed on the theories of Bockris and co-workers and those of Frumkin and Damaskin. The respective merits and defects of both theories are presented.