Impedance study of formic acid oxidation on platinum electrodes

The electrooxidation of formic acid on polycrystalline platinum electrodes in sulphuric acid is studied by a dynamic impedance method. Impedance spectra in the frequency range 0.7 Hz to 20 kHz, assembled from ac voltammograms, are interpreted in terms of a mechanism with three adsorbed species, although one of these may be free sites. The spectra show evidence for incipient oscillations in two potential regions, and observed oscillations were related to these spectra. The zeroes of the interfacial impedance were directly extracted from suitable equivalent circuits in distinct potential regions where oscillations may be triggered. The observation of only real zeroes of the interfacial impedance eliminates the possibility of pure potentiostatic oscillations arising from chemical reasons alone, and therefore potential is an essential variable.     

A catalytic study of formic acid oxidation on preferentially oriented platinum electrodes

The oxidation of formic acid was examined by cyclic voltammetry and chronoamperometry in order to determine the rate of catalytic activity (reaction turnover) as a function of surface crystallography on preferentially oriented (electrochemically modified) platinum electrodes. The resulting turnover rates indicated a maximum fourfold current enhancement for an approximately 60% (111)-oriented surface versus a polycrystalline surface, suggesting that preferentially oriented electrodes are of potential practical significance.     

Activating and deactivating mass transport effects in methanol and formic acid oxidation on platinum electrodes

Transient increases in rotation rate at an RDE increase the methanol oxidation current, even though in slower experiments the current decreases with increasing rotation rate as usually reported. Methanol oxidation on smooth polycrystalline platinum rotating disk electrodes in sulfuric acid electrolyte was studied by RDE voltammetry and hydrodynamic impedance spectroscopy combined with cyclic voltammetry. A positive low-frequency real part in the hydrodynamic admittance spectra for the main oxidation peak was used to predict that a transient increase in rotation rate would increase the current, as was observed. In contrast, slow scan rate voltammograms showed a decrease in current with increasing rotation rate. The transient current increase was explained by enhanced production of soluble intermediates, while increased production of adsorbed CO poisoning explained the slower inhibition. Comparative experiments for formic acid oxidation showed increasing current with rotation rate in both hydrodynamic admittance spectra and slow-scan voltammograms.     

Kinetics of formic acid oxidation in advanced porous electrodes

The electrochemical oxidation of formic acid HCOOH at porous PTFE-bonded platinum-carbon electrodes in acidic electrolyte has been studied using the AC-impedance method. A reaction sequence based on known details of this process was transferred into an equivalent circuit which was used to simulate successfully the measured impedance data. The results confirm earlier data obtained with smooth platinum electrodes and allow, for the first time, a correlation between the electrode overpotential and the contribution of the various reaction steps. Further improvements of the porous electrode based on these results are suggested.     

甲酸在钛基纳米多孔网状铂电极上的电化学氧化

将水热法和电解过程相结合，制备出一种新型的钛基纳米多孔网状铂电极(nanoPt)，它具有特殊的网络状结构及巨大的表面积. 循环伏安曲线表明,在碱性溶液中，甲酸在nanoPt上的氧化电流密度远远大于在多晶Pt上的电流密度，其氧化峰电流几乎是后者的100倍；恒电位电解时，甲酸在nanoPt上的氧化电流开始时随时间的增加而增加，随后达到稳定，其稳定电流也明显高于多晶Pt；在nanoPt上甲酸的非线性现象出现的电流密度范围要比在多晶铂上的宽. 多次循环扫描和长时间电解实验表明. 这种钛基nanoPt结构稳定，催化剂颗粒不易脱落，有可能作为一种新型的燃料电池阳极材料而得到应用.     

FTIR spectroelectrochemical investigation of the electrocatalytic oxidation of ascorbic acid at platinum electrodes in acid medium

The electrochemical oxidation of ascorbic acid (AA) at a platinum electrode has been studied in 0.1 M HClO₄. In situ infrared reflectance spectroscopy (SPAIRS and SNIFTIRS techniques) was used to investigate the reaction intermediates. The identification of the different electroreactive species and adsorbed intermediates allowed us to postulate a reaction mechanism for the transformation of AA into dehydroascorbic acid (DHA).     

非水体系中铂微盘电极上甲酸的电催化氧化行为

采用循环伏安法,以铂微盘为工作电极,铂片为辅助电极,饱和甘汞电极为参比电极,研究了甲酸在含有四丁基高氯酸铵(TBAP)电解质的乙腈有机溶液中的电氧化行为,并探讨了扫描速度、底物浓度和温度等因素对甲酸电化学氧化行为的影响.结果表明该反应是受扩散控制的不可逆反应.     

Photocurrent multiplication at n-GaN electrodes in formic acid solutions

Photocurrent multiplication (‘photocurrent doubling’) effects were studied at n-GaN electrodes in 1 M H₂SO₄ solutions containing formic acid. The photocurrent multiplication factor f was measured as a function of different parameters. It was found that f increases with increasing formic acid concentration, reaching a limiting value at high concentrations. Surprisingly, this limiting value for f was found to depend upon the extent of photoanodic etching prior to performing the photocurrent multiplication measurements: it increased from about 1.05 for ‘new’ electrodes to a maximum of 1.3 for etched electrodes. This suggests that somehow, intrinsic surface states are involved in the photo-oxidation of formic acid, the concentration of which changes due to etching. Further, it appeared that n-GaN dissolves photoanodically in 1 M H₂SO₄ solutions containing formic acid, even at very high concentrations. A mechanism, accounting for all of the experimental findings was proposed.     

The oxidation of carbon monoxide at platinum and gold metallized membrane electrodes

The oxidation of carbon monoxide at gold platinum membrane electrodes is discussed and it is shown that the electrochemical characteristics of these electrodes are similar to massive electrodes. At platinum the short time response is determined by the oxidation of a surface monolayer of carbon monoxide by a reactant pair mechanism occuring at the edges of growing islands of an oxidised platinum species whilst at gold, the process is diffusion controlled. On both metals the longer timescale response is complicated by poisoning of the surface by a reaction intermediate but it is also shown that a gold anode may be reactivated by potential cycling. The relevance of these results to the construction of an analytical device is discussed.     

The influence of UPD-lead on the adsorption of formaldehyde,formic acid and methanol on Pt in acid solution

Using fast linear sweep voltammetry, the adsorption behaviour of formaldehyde, formic acid and methanol was studied on an electrochemically modified (100)-like Pt electrode without or covered with UPD-lead. It was found that both the lead inhibits the formation of strongly bound intermediates and the intermediates inhibit the deposition of lead. This is explained by a mutual hindrance after the “third body” concept. This cannot explain, however, the electrocatalytic effect of UPD-lead on the oxidation of the above fuels since this effect has its maximum at θ_Pb > 0.5. Therefore, we postulate that UPD-lead catalyzes the direct oxidation of formic acid and formaldehyde at potentials between 0.2 and 0.7 V. On the other hand, the direct oxidation of methanol on smooth Pt occurs at potentials more positive than 0.6 V where UPD-lead is already anodically dissolved. Therefore, only a small catalytic effect of UPD-lead on methanol oxidation is observed.     

The influence of lead upd on the oxidation of glucose at platinum electrodes an electrochemical quartz crystal microbalance study

The oxidation of glucose at Pt electrodes with upd deposits of Pb in acid media has been investigated using the electrochemical quartz crystal microbalance (EQCM). Glucose alone has a small effect on the mass response but changes in upd Pb coverage have a large effect. This allows changes in upd Pb coverage to be followed easily despite the fact that the associated current is largely obscured in the cyclic voltammogram. Cyclic voltammetry at low glucose concentrations shows a multiple peak structure in the double layer region of potential at low upd coverages that is charged to the more familiar single peak as upd coverage increases. Mass responses also reveal some blockage of Pb upd by glucose or species derived from it during cyclic voltammetry. Data from mass transients show that the deliberate formation of poisons prior to addition of Pb²⁺ ions to the electrolyte results in a substantial suppression of upd coverage especially between −0.1 and 0.1 V (sce). Finally, mass transients accompanying injections of Pb²⁺ in the presence of glucose in the electrolyte reveal the suppression of adsorption of Pb²⁺ cations on an oxidised Pt surface by species derived from glucose.     

Investigation of formic acid oxidation on Ti/IrO2 electrodes

A model has been proposed according to which the voltammetric charge involved in the Ti/IrO₂ electrodes is due to two contributions: a faradaic contribution due to surface redox activities at the IrO₂ coating and a non-faradaic contribution due to the charging of electrical double layer (). The later has been proposed as a tool for the estimation of the relative surface area of the Ti/IrO₂ electrodes.Differential electrochemical mass spectrometry (DEMS) measurements using H₂¹⁸O has demonstrated that we are dealing with an active electrode in which the surface redox couple IrO₃/IrO₂ acts as mediator in the oxidation of formic acid (FA).From the voltammetric measurements using different IrO₂ loading and FA concentrations, the kinetic parameters of FA oxidation via the surface redox couple IrO₃/IrO₂ have been determined.Finally a model has been proposed considering that FA oxidation at Ti/IrO₂ anodes is controlled by mass transfer. The good agreement between the experimental results and the model indicates that the surface reaction between FA and the electrogenerated IrO₃ is a fast reaction.     

Electrochemical oxidation of borohydride on platinum electrodes: The influence of thiourea in direct fuel cells

The electrochemical behaviour of sodium borohydride on a platinum electrode in the absence and presence of thiourea (TU) was investigated by cyclic voltammetry. In the absence of thiourea, several overlapping peaks associated with the hydrolysis of BH₄⁻ appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 to −0.50 V versus Ag/AgCl. As a consequence of secondary reactions, the borohydride oxidation in 3 M NaOH solution shows a four to six-electron process, according to its concentration, in direct fuel cells. A conveyable TU/NaBH₄ concentration ratio of 0.6 inhibits the delivery of hydrogen simultaneously with catalytic hydrolysis of BH₄⁻. Thus, the coulombic efficiency in direct fuel cell discharge was increased showing an about eight-electron process for the oxidation of BH₄⁻.     

钛基纳米多孔钯电极的制备及对甲酸氧化的电活性

采用水热法,通过改变溶液组成制备了5种不同的钛基纳米多孔钯电极(Pd/PEG、Pd-EDTA/PEG、Pd-EDTA/HCHO、Pd/EG和Pd/HCHO)。扫描电镜图(SEM)分析表明,加入络合剂乙二胺四乙酸(EDTA)后,钯颗粒均匀,粒径明显减小,仅有60 nm左右。利用循环伏安法研究了甲酸在这些电极上的电催化氧化,发现在1.0 mol/L NaOH+0.5 mol/L HCOOH溶液中,加入EDTA且以甲醛作还原剂的电极(Pd-EDTA/HCHO)对甲酸氧化电流密度达132.00 mA/cm2,甲酸氧化的起始电位为-0.85 V,表明电催化活性优于其他电极。同时研究了Pd-EDTA/HCHO电极对不同浓度甲酸电催化氧化,结果表明,在一定甲酸浓度范围内,甲酸氧化的阳极电流密度随浓度的增加而增大。     

Impedance study of methanol oxidation on platinum electrodes

Methanol oxidation on Pt electrodes is studied by ac voltammetry. Data from voltammograms at frequencies from 0.5 Hz to 20 kHz are assembled into electrochemical impedance spectra and analysed using equivalent circuits. Inductive behavior and negative relaxation times are attributed to nucleation and growth behavior. The rate-determining step is proposed to be the reaction of adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites.     

On the anodic oxidation of cyclohexene on platinized platinum electrodes

Cyclic voltammetry experiments performed in acid solutions of cyclohexene on platinized platinum anodes show that platinum oxide must play a role in the anodic oxidation of the hydrocarbon through a mechanistic scheme of the type proposed by Gilman[10]. This fact would explain results obtained with other unsaturated hydrocarbons     

Adsorption,oxidation and effect of CO on hydrogen adsorption at smooth rhodium electrodes in acid solution

The adsorption of carbon monoxide was studied under steady state conditions on smooth rhodium electrodes in 0.5 M H₂SO₄ at room temperature. The coverage with carbon monoxide is equal to saturation coverage between 0.1 and 0.5 V vs a hydrogen electrode in the same solution. Above 0.5 V it decrease to 0 within 0.2 V. The oxidation of chemisorbed carbon monoxide at constant current is accompanied by the formation of the oxygen layer above 0.6 V. A correction for the contribution of the oxygen layer formation to the anodic charge has to be applied in the determination of the CO_ad coverage even at small current densities.The coverage with H_ad was determined as a function of potential at constant coverage with preadsorbed carbon monoxide from anodic charging curves. These curves were taken at a sufficiently small current density so that the equilibrium of the Volmer reaction remained practically established. The coverage of CO_ad, computed from the charge for the oxidation of CO_ad, agreed well with that obtained from the decrease of the length of the hydrogen branch of the charging curves. The isotherms of hydrogen adsorption are of the Temkin type at CO_ad coverages above 0.4. The characteristic parameters of the Temkin isotherm were determined as a function of the coverage with CO_ad.     

Electrooxidation of formic acid at platinum nanoclusters electrodeposited on PEDOT coated carbon paper electrode

Nanoclusters of Pt were electrochemically deposited on a conducting polymer, namely, poly(3,4-ethylenedioxythiophene) (PEDOT), which was also electrochemically deposited on carbon paper current collector. PEDOT facilitated uniform distribution of Pt nanoclusters, when compared with Pt electrodeposition on bare carbon paper substrate. Spectroscopy data indicated absence of any interaction between PEDOT and Pt. The electrochemically active surface area as measured from carbon monoxide adsorption followed by its oxidation was several times greater for Pt–PEDOT/C electrode in comparison with Pt/C electrode. The catalytic activity of Pt–PEDOT/C electrode for electrooxidation of formic acid was significantly greater than that of Pt/C electrode. Amperometry data suggested that the electrodes were stable for continuous oxidation of HCOOH.     

The influence of electrolyte purity on electrochemical measurements using platinum electrodes

The influence of electrolyte purity on electrochemical processes at low-area platinum electrodes has been studied by a fast potential-step method and by cyclic voltammetry. Pre-purification of the electrolyte had no significant influence on the metal surface area available for H adsorption determined immediately after an electrochemical cleaning step. Maintaining the electrode potential within the ‘double-layer’ region of platinum (at 0.5 V vs rhe) for relatively long periods of time (1–30 min) results in the progressive suppression of H adsorption, owing to the adsorption of impurities blocking active Pt sites. All samples of electrolyte, irrespective of the degree of pre-purification, exhibited this effect. In the determination of current-time transients during the first few hundred milliseconds of the methanol electro-oxidation reaction, impurities in the untreated electrolyte contribute to the measured anodic currents at Pt electrodes working in the potential range of 0.4–0.6 V. The anomalous effects observed within this potential range can be largely eliminated by pre-electrolysis of the H₂SO₄ electrolyte for several days using Pt wire gauze electrodes at 2.1 V.