Electrooxidation of hydrogen at Pt/carbon nanotube catalysts for hydrogen–air fuel cell

A possibility for application of the method of thin-layer rotating disk electrode (RDE) for investigation of kinetics of hydrogen electrooxidation on highly dispersed platinum catalysts formed on the carbon nanotubes (CNT) is studied. It is shown that the polarization curves of hydrogen oxidation on the studied catalysts approach the calculated curves for the diffusion overpotential of hydrogen reaction both in the acidic and alkaline electrolytes. This is the evidence, on the one hand, for a high activity of proposed catalysts in the hydrogen oxidation reaction and, on the other hand, for incorrect use of the Koutecky–Levich equation for calculating the kinetic currents in the case under consideration. The characteristics of hydrogen–oxygen fuel cell (FC) with anode based of synthesized 40Pt/CNT catalysts are highly comparative with the characteristics of FC containing commercial 60Pt catalyst (HiSPEC 9100) on the anode.     

Tungsten electrochemistry in alkaline solutions—Anodic dissolution and oxygen reduction reaction

Oxygen reduction reaction (ORR) was investigated in alkaline solution on tungsten electrode subjected to a previous anodic dissolution. The rotating disk cyclic voltammetry and rotating disc chronoamperometry were used. Both unsupported and potassium pechlorate and sulfate supported solutions were examined. The most striking feature of recorded ORR curves is the large difference of ORR overpotential during anodic and cathodic sweep. This was attributed to the formation of tungsten oxide on the surface. It was demonstrated that electrode pretreatment as well as the electrolyte composition greatly affects ORR electrochemistry on tungsten electrode, and the influence of sulfates is discussed.     

玻碳电极上DTAB对氧还原反应的促进作用

用旋转圆盘玻碳电极研究了阳离子表面活性剂十二烷基三甲基溴化铵(DTAB)对氧还原反应的影响. 结果表明, DTAB明显提高了玻碳电极对氧还原的电催化活性. 通过对氧还原电流与旋转速度的关系以及动力学电流与电位的Tafel关系分析, 发现DTAB提高了玻碳电极对氧还原反应电荷传递步骤的传递系数, 因此加快了氧还原的动力学过程.     

微带电极上同时受扩散、化学反应、电极反应速率控制的伏安方程

运用积分变换的方法推导了超微带电极上同时受扩散、化学反应、电化学反应动力学控制的伏安关系,得到了前行化学反应、平行化学反应和后行化学反应的准可逆伏安方程,并列出了计算所得的典型伏安曲线。     

Limiting current to a rotating disk electrode modified with an electroactive polymeric film in the presence of a redox pair in the adjacent solution volume

Expressions for the limiting current to a rotating disk electrode modified by a conductive polymeric film are obtained for different localization of a redox process that involves redox species in the bulk of the adjacent solution. For an electrode reaction on the modifying film surface, it is shown that the dependence of the limiting current on the bulk concentration of reacting species is described by a curve with saturation. Such a limitation does not arise for a reaction on the electrode substrate. This can serve as a criterion in location of the reaction zone. In addition, other possible prospects of the use of a rotating disk in studying the charge transfer in electroactive polymeric films are discussed.     

Kinetics and mechanism of oxygen electroreduction in acidic and neutral solutions on carbon black XC-72R modified by products of pyrolysis of cobalt 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin

Cathodic oxygen reduction on the XC-72R carbon black modified by the products of pyrolysis of cobalt 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin (CoTMPP) (XC-72M) was studied in acidic and neutral electrolytes. Formation of new active centers on XC-72M is confirmed by voltammetric curves (specific charge density grows as compared to the XC-72R carbon black by 2–2.5 times) using the methods of rotating disk electrode (a shift in half-wave potential E _1/2 by 600 mV) and rotating ring-disk electrode (the fraction of the direct reaction increases to 70%). Herewith, the $\frac{{\partial E_{1/2} }} {{\partial pH}}$\frac{{\partial E_{1/2} }} {{\partial pH}} value in the range of pH 0.3–8.5 is −60 mV. It is shown that proton necessarily participates in the slow stage of the first electron transfer for the further occurrence of the direct reaction to water. At a transition from acidic solutions to neutral ones, the polarization curves converge for XC-72M and XC-72R, which is due to a decrease in the concentration of proton in the solution and variation of the mechanism of the oxygen reduction slow stage.     

Application de la methode polarovoltrique : Interprétation du dosage de L'acide maléique par des bases organiques en milieu n,n-diméthylformamide

The determination of the voltammetric curves, I=f(E), of maleic acid and of a basic titrant in N,N-dimethylformamide using 3 electrodes allowed interpretation of the polarovoltric titration curves of the neutralization of this acid by sodium methylate and tetrabutylammonium hydroxide. The different variations of the potential between 2 polarized platinum electrodes observed in such a neutralization reaction were identified and a notation was developed so that the origin of the observed potential variations could be established. The signal E_c^H+→S, which is related to the initial acidity, is given by the cathode and corresponds to a jump from the potential of the H⁺ reduction curves to that of the solvent reduction curves. The signal E_A^S→OH- characteristic of the second acidity, is given by the anode and is related to a jump from the potential of the solvent oxidation curves to that of the titrant oxidation curves.The effect of the use of unsymmetrical polarized electrodes (a rotating filament electrode and a large-surface, stationary electrode) on the shape of the titration curve was also examined: a rotating cathode gave rise to a curve resembling an inverted U, whereas a rotating anode gave rise to a curve resembling a deformed M.     

The electrochemical reaction mechanism of selenocystine on selenium-gold film modified glassy carbon electrode

A simple and selective voltammetric method based on selenium-gold film modified glassy carbon electrode has been developed for investigating electrochemical reaction mechanism of selenocystine. With N₂ saturated, redox reactions between selenocystine (SeC) and selenocysteine (SeCys) were judged to be two simple electron-transfer processes. With air saturated, the reduction reaction was diagnosed to be EC catalytic reaction (the chemical oxidation reaction of the SeCys by O₂ (C) following the electron-transfer reaction (E)) and oxidation reaction is a simple electron-transfer process. With pure O₂ saturated, only reduction peak was observed and the reaction was judged to be EC catalytic reaction. The electron-transfer numbers of redox reaction were calculated to be 2 by chronocoulometry and rotating disk electrode.     

聚苯胺薄膜修饰电极对抗坏血酸的电催化氧化

本文表明聚苯胺(PAn)薄膜修饰电极对水溶液中的抗坏血酸(AH_2)在较宽的pH范围和较宽的浓度范围内均有良好的电催化氧化作用, 为EC平行催化过程。利用旋转圆盘电极(RDE)进行了催化过程动力学分析, 求出了催化反应动力学参数。在抗坏血酸浓度10~(-2)～10~(-6) mol·L~(-1)范围内, 催化峰电流与AH_2浓度均成良好的线性关系, 且PAn薄膜修饰电极具有很好的稳定性, 有应用分析抗坏血酸的意义。     

玻炭汞膜电极多阶半微分溶出曲线的理论方程式

本文建立了汞膜电极阳极溶出多阶半微分电分析法理论的基本数学模型,借助电子计算机,直接描绘出其理论曲线,并给出有关特征参数,计算结果与实验验证表明,本文所推导的理论曲线方程式是正确的。     

镁在DMF中电沉积机理的研究

自本世纪初始,人们一直试图在非水体系中电沉积镁,迄今尚未见到令人满意的报道。此外,有关镁在非水体系中,在固态电极上电沉积机理的研究工作也未见系统报道。本工作首先确证了在高氯酸镁的DMF溶液中、在光亮铂电极上能电沉积出金属镁。在此基础上初步研究了这一电沉积过程的动力学特征,提出了在该体系中镁电沉积的机理。     

Oscillatory behavior in the electrochemical oxidation of formic acid on Pt(100): rotation and temperature effects

We investigated the oscillatory behavior in the kinetics of formic acid electrooxidation on Pt(100) in 1 mM HClO₄ solution. We studied the effect of different experimental parameters on the oscillatory behavior, viz. defined HCOOH mass-transport to the electrode surface by using the rotating disk electrode technique, the temperature of the supporting electrolyte, and the nature of anions. We suggest that the interdependence of the reaction steps during HCOOH oxidation, the adsorption of anions and the competition for adsorption sites among the reaction partners and intermediates lead to complex non-linear kinetics. It was evident that once the individual reactions in the dual path mechanism reach steady state the oscillations vanish. These conditions can be reached either by enhanced formic acid reaction rates induced by electrode rotation or by increased temperature. Under specific conditions of anion and formic acid concentration, relaxational oscillations can be transformed into mixed-mode oscillations.     

Mechanism and reaction rate of the karl-fischer titration reaction: Part III. Rotating ring-disk electrode measurements— comparison with the aqueous system

A platinum disk-platinum ring electrode was used to investigate the oxidation of sulfur dioxide by iodine and triiodide in aqueous solutions. Contrary to methanolic solutions, where the monomethyl sulfite ion is the only oxidizable species, in aqueous solutions both the hydrogen sulfite ion and the sulfite ion can be oxidized. The reaction rate was generally so high, that the method for measurements of homogeneous second order reactions had to be used. At pH values >5, the reaction proceeded too fast to be measured reliably. In a solution “diluted” with ethanol (50% of weight), however, the reaction rate was within the range where a rotating ring-disk electrode can be applied to measure fast homogeneous reactions. At very low pH values both the first order calculation technique and the second order method could be used. The results with both methods were in fair agreement.     

Electrochemical Properties of Carbon Black AD-100 and AD-100 Promoted with Pyropolymer of Cobalt Tetra(p-methoxyphenyl)porphyrin

Carbon black AD-100: initial, activated, and promoted with pyropolymer of cobalt tetra(p-methoxyphenyl)porphyrin is characterized by a complex of electrochemical (floating electrode, rotating disk electrode, rotating ring-disk electrode, electrochemical impedance) and structural (standard porosimetry, BET) methods of investigation. Procedures for the AD-100 activation and promotion and the preparation of thin layers of the material to be studied and deposited on disk electrodes are described. The effect of the activation and promotion of carbon black on the surface and electrocatalytic activity of materials under study in the reduction of oxygen and hydrogen peroxide is shown. The ratio of constants of oxygen reduction directly to water and through intermediate formation of hydrogen peroxide is determined. A path for the oxygen reduction is discussed.     

Pt/C催化剂氧还原反应的交流阻抗动态研究

本文通过RDE和EIS联合技术、等效电路模型,研究了酸性体系中商业Pt/C催化剂ORR行为. 研究发现Pt/C动态界面包括两个彼此独立的过程：1）Pt表面原有PtO还原至Pt过程,2）ORR促进新PtO形成过程,为催化材料稳定性及活化性提供了关键依据;并发现动态界面促进多孔电极重构以及与传输匹配过程.在高过电位下,ORR的高反应速率可通过增加催化材料憎水性予以改善. 上述研究结果可对ORR的直流电化学研究进行有效补充,并提供建模基础.     

Voltammetric determination of reducing agents and dyestuffs in textile printing pastes

Summary Basic research concerning voltammetry in highly viscous, paste-like systems is presented. Using model compounds for dyestuffs and reducing agents, respectively, the principal applicability of electrochemical techniques for direct quantitative determination of electroactive compounds in such media is proved. The influence of non-Newtonian viscosic properties of the thickeners used (polygalactomannanes, carboxymethylcellulose) on current voltage curves obtained with rotating electrodes is discussed. An increase of the anodic measuring range due to electrode surface modification by the products of the electrode reaction of hydroxymethanesulphinate and by the properties of the thickeners is evaluated.     

Development of rotating liquid membrane disk electrode and rotating liquid membrane ring-liquid membrane disk electrode and evaluation of characteristics of ion transfer reactions at the rotating aqueous/organic solution interface.

A liquid membrane disk electrode, LMDE, and a liquid membrane ring-liquid membrane disk electrode, LMRE-LMDE, were developed by placing a gelled polyvinyl chloride thin membrane impregnated with 2-nitrophenyl octyl ether, NPOE-LM, on the surface of a glassy carbon, GC, disk or ring electrode. The voltammogram for the ion transfer at the interface between an aqueous solution, W, and NPOE-LM was recorded by setting the developed electrode in W and rotating at a rate, omega, between 0 and 4000 rpm. The sensitivity of the ion-transfer current at the WINPOE-LM interface, I, was enhanced to be more than 100 times better than that at the WINPOE (solution) interface when LMDE was rotated at omega higher than 200 rpm. The reversibility of the ion transfer reaction could be evaluated based on the dependence of I on omega of LMDE, and the reaction product at LMDE could be identified at LMRE when the rotating LMRE-LMDE system was adopted.     

Ultrathin layer convolution

Ultrathin layers are defined as thin layers which are sufficiently thin that no concentration gradients are established within the layer on the time scale of a voltammetric measurement. Mathematically, ultrathin layers are characterized by ordinary differential equations in time. These equations are simpler to solve than the space and time dependent partial differential equations which describe all other electrode geometries. In this paper, a method is presented which capitalizes on the mathematical simplicity of the ultrathin layer to model any arbitrary, parameterizable electrode geometry. Laplace transforms are used to find an integral relationship between the current response of the modeled geometry and the ultrathin layer current. The integral relationship can be evaluated either analytically or numerically. Any voltammetric perturbation, under either Nernstian or mass transport-controlled conditions, can be modeled. The method is demonstrated for both planar and spherical electrodes. Cyclic voltammetric responses are modeled numerically and potential step responses are modeled analytically. It is also shown that for cyclic voltammetric perturbations, the current-voltage curves for the following systems have the same functional form. That is, the curves are identical within known multiplicative constants. The functionally equivalent responses are for (1) radial diffusion to a point electrode (the polarographic curve), (2) convective transport to a rotating disk, (3) the integral of the ultrathin layer response, and (4) the convolution or semi-integration of the response for linear diffusion to a planar electrode.     

A contribution to the theory of D.C. polarographic current-voltage curves: Electrode reactions with three parallel charge transfer processes

Equations of polarographic current-voltage curves are derived for electrode reactions with three parallel charge transfer processes coupled by two chemical reactions on the basis of the staeady-state assumption in the reaction layer. If the rate constants of the charge transfer processes are sufficiently different from one another, the current-voltage curve consists of three separated waves and the equation for each wave has the same form as for a simple electrode reaction with one pathway.     

Electrochemical studies on glassy carbon electrodes: II. Oxygen reduction in solutions of high pH (pH>10)

On the basis of measurements using rotating disc, rotating ring disc and stationary glassy carbon electrodes, together with polarization curves, reaction order and stoichiometric number determinations, a mechanism for the reduction of oxygen in aqueous solution at pH>10 is proposed. This involves an initial electron transfer, followed by rate-determining surface migration of O₂^? ions to active sites on the electrode surface. Differences between our interpretation of the experimental results and the conclusions of other investigators are discussed.