Potential Step Method Research Articles

An investigation on hydrogen storage kinetics of nanocrystalline and amorphous Mg 2Ni 1− xCo x ( x = 0–0.4) alloy prepared by melt spinning

In order to improve the hydrogen storage kinetics of the Mg 2Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg 2Ni 1− x Co x ( x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys. The structures of the as-cast and spun alloys are characterized by XRD, SEM and TEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage kinetics of the as-spun alloys is tested by an automatic galvanostatic system. The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The electrochemical impedance spectrums (EIS) and the Tafel polarization curves are plotted by an electrochemical workstation. The results show that the substitution of Co for Ni notably enhances the glass forming ability of the Mg 2Ni-type alloy. Furthermore, the substitution of Co for Ni, instead of changing major phase Mg 2Ni, leads to forming secondary phases MgCo 2 and Mg. Both the melt spinning treatment and Co substitution significantly improve the hydrogen absorption and desorption kinetics. The high rate discharge ability, the hydrogen diffusion coefficient and the limiting current density of the alloys significantly increase with raising both the spinning rate and the amount of Co substitution.

Investigation on electrochemical performances of melt-spun nanocrystalline and amorphous Mg 2Ni 1− xMn x ( x = 0–0.4) electrode alloys

In order to enhance the glass forming ability of the Mn 2Ni-type electrode alloy, Ni in the Mg 2Ni compound is partially substituted by Mn. The nanocrystalline and amorphous Mg 2Ni-type alloys with nominal compositions of Mg 2Ni 1− x Mn x ( x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are successfully obtained. The microstructures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were tested by an automatic galvanostatic system. The electrochemical impedance spectra (EIS) are plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The results show that no amorphous structure is detected in the as-spun Mn-free alloy, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure. The amorphization degree of the alloy increases with rising spinning rate, suggesting that the substitution of Mn for Ni facilitates the glass formation in the Mg 2Ni-type alloy. The substitution of Mn for Ni markedly improves the electrochemical hydrogen storage performances of the Mg 2Ni-type alloy, enhancing both the discharge capacity and the electrochemical cycle stability. Furthermore, the high rate dischargeability (HRD), electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with increasing amount of Mn substitution.

Enhanced electrochemical characteristics of as-spun nanocrystalline and amorphous Mg 2Ni-type alloy electrodes

The nanocrystalline and amorphous Mg 2Ni-type electrode alloys with nominal compositions of Mg 2Ni 1− x Mn x ( x = 0, 0.1, 0.2, 0.3, 0.4) are synthesized by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are obtained. The structures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical characteristics as well as the electrochemical impedance spectrums (EIS) of the as-spun alloy electrodes are measured. The hydrogen diffusion coefficients ( D) in the alloys were obtained by virtue of potential-step method. The results show that the as-spun ( x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun ( x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni evidently intensifies the glass forming ability of the Mg 2Ni-type alloy. The substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving the discharge capacity and the electrochemical cycle stability. Furthermore, the high rate discharge ability (HRD), the electrochemical impedance spectrum (EIS) and the potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with rising the amount of Mn substitution.

Hydrogen diffusion behavior in α-solid solution phase of LmNi 3.55Co 0.75Mn 0.4Al 0.3 metal hydride electrode—Effect of temperature

The hydrogen diffusion coefficient in an LmNi 3.55Co 0.75Mn 0.4Al 0.3 alloy electrode was evaluated by potential-step method as a function of initial hydrogen concentrations in the bulk of alloy electrode ( C H < 0.1 H/M) and temperature (293–333 K). The hydrogen diffusion coefficient was found to decrease linearly with the increase of C H at all temperatures. However, the slope of this linear dependence was found to reduce with the increasing temperature. The partial interaction Gibbs energy of dissolved hydrogen ( Δ G ¯ int ) revealed a strong influence of temperature on the stability of α-solid solution phase in the alloy electrode. Namely, the Δ G ¯ int gradually increased −25.6 to −2.5 kJ mol −1 with increasing temperature, showing a linear dependence. It is found that the Einstein diffusion coefficient of hydrogen increased also with increasing temperature, and that the apparent energy of activation of hydrogen diffusion is 27.9 kJ mol −1. A novel viewpoint in understanding of the hydrogen diffusion kinetic in hydrogen storage alloys is discussed in view of the thermodynamic stability of the resulting hydrides.

The effect of CeO2 on Pt/CeO2/CNT catalyst for CO electrooxidation

AbstractPt/CeO2/CNT catalysts were prepared by adsorbing Pt nanoparticles on the supports of CNTs coated with CeO2. The electrocatalytic performances in respect to the electrooxidation of chemisorbed CO were tested using potential step and stripping voltammetry methods under variable sweep rate and temperature conditions. At 10 mV s–1, the CO stripping voltammogram exhibited the peak splitting phenomenon. The oxidation charge and the peak potential of the two voltammetric peaks changed regularly with the number of Pt and CeO2 neighbours, the sweep rate, and the temperature. We considered that the low potential peak originated from the reaction of COads with hydroxyl groups on CeO2 adjacent to Pt sites, while the high potential peak came from the reaction of COads with hydroxyl groups produced on pure Pt. Furthermore, the experimental results of the peak potential against the logarithm of the sweep rate and the logarithm of the current maximum time against the step potential were plotted and intersecting lines with different slopes in high and low potential regions in the plot were observed. The lines intersected at lower potentials on the Pt/CeO2/CNT electrode than on the Pt/CNT electrode, which was attributed to the contribution of hydroxyl groups on CeO2.

Effects of Temperature and Relative Humidity on Oxygen Permeation in Nafion® and Sulfonated Poly(Arylene Ether Sulfone)

Oxygen permeation in Nafion®117 and sulfonated poly(arylene ether sulfone) (BPSH-50) were investigated at different temperatures and relative humidities by a potential step method using a Pt microelectrode. The diffusion coefficient increased, while the solubility decreased with an increased in temperature and relative humidity for both samples. Consequently, the permeability increased with an increased in temperature and relative humidity for both samples. The permeability in BPSH-50 was approximately four times lower than that in Nafion®117, due to a smaller diffusion coefficient and a lower solubility in BPSH-50 than those in Nafion®117. Mass transport of oxygen in these membranes was correlated with microstructural changes of the membranes.

Fast test for the durability of PEM fuel cell catalysts

ETek Pt/C catalyst was used as standard materials to develop a new test protocol for fast screening durable catalyst for PEM fuel cells. Potential step (Pstep) method with the upper potential of 1.4V and the potential-static (Pstat) holding at 1.4 V or 1.2V are used to degrade the catalyst. The degradation in the electrochemical surface area (ESA) for Pt/C under Pstep conditions is greatly accelerated as compared with that under other conditions. The durability of Pt/Vulcan and Pt/CNT were studied using the new protocol with the electrochemical stressing of Pstep(1.4V/0.6V), which provided the same results as those tested using conventional protocols: Pt/CNT is more durable than Pt/Vulcan. This confirms that the new protocol works well in screening catalyst in terms of durability. The new protocol can differentiate the durability of electrocatalysts by shortening the test time to several hours.

Influence of surface morphology on methanol oxidation at a glassy carbon-supported pt catalyst

Platinum supported on glassy carbon (GC) was used as a model system for studying the influence of the surface morphology of a Pt catalyst on methanol oxidation in alkaline and acidic solutions. Platinum was deposited by the potential step method on GC samples from H2SO4 + H2PtCl6 solution under the same conditions with loadings from 10 to 80 mg cm-2. AFM and STM images of the GC/Pt electrodes showed that the Pt was deposited in the form of 3D agglomerates composed of spherical particles. Longer deposition times resulted in increased growth of Pt forms and a decrease in the specific area of the Pt. The real surface area of Pt increased with loading but the changes were almost negligible at higher loadings. Nevertheless, both the specific and mass activity of platinum supported on glassy carbon for methanol oxidation in acidic and in alkaline solutions exhibit a volcanic dependence with respect to the platinum loading. The increase in the activity can be explained by the increasing the particle size with the loading and thus an increase in the contiguous Pt sites available for adsorption and decomposition of methanol. However, the decrease in the activity of the catalyst with further increase of loading and particle size after reaching the maximum is related to the decrease of active sites available for methanol adsorption and their accessibility as a result of more close proximity and pronounced coalescence of the Pt particles.

Electrochromic Properties of Pd-capped Mg-Ni Switchable Mirror Thin Films

Switchable mirrors composing magnesium-nickel thin films with a palladium top layer are prepared by dc magnetron sputtering. Electrochromic properties of the switchable mirrors are investigated by means of various electrochemical methods. Electrochromic switching conditions influenced by the composition of the Mg–Ni alloy thin film and the thickness of palladium and Mg–Ni layers are investigated to obtain optimal switching behavior and transparent films using cyclic voltammetry and optical measurements. The values of the diffusion coefficients of hydrogen in Mg2Ni, Mg4Ni, and Mg6Ni films are estimated using a potential-step method.

Influence of Poly(ethylene oxide) on the Process of Copper Electrodeposition onto p-Si(100)

In this study we examined the influence of the illumination intensity and the presence of poly(ethylene oxide) (PEO) as an additive for the process of copper electrodeposition onto p-Si(100). The study was carried out by means of cyclic voltammetry (CV) and the potential step method from which the corresponding nucleation and growth mechanism (NGM) were determined. Both methods were performed under illumination for the electron's photogeneration. Likewise, a morphologic analysis of the deposits obtained at different potential values by means of atomic force microscopy (AFM) was carried out. In the first stage, Mott−Schottky measurements were taken to characterize the energetic of the semiconductor/electrolyte interface. The CV results indicated that the presence of PEO inhibits the electrochemical reaction of oxide formation on the surface of the semiconductor. This allows a decrease in the overpotential associated with the electrodeposition process. The analysis of the j/t transients shows that in the ab...

Electrodeposition of Pt onto RuO2(110) Single-Crystal Surface

The electrodeposition of Pt on RuO2(110) from acid solutions of several Pt complexes was studied using programmed potential step or potential sweep methods. The RuO2(110) single-crystal surface was obtained by gas-phase oxidation of Ru(0001). The electrodeposition process is characterized by a large crystallization overpotential and three-dimensional growth from a Pt adlayer. The mismatch between the RuO2(110) and Pt lattices is the likely origin of that overpotential. The nucleation is instantaneous, as verified by potential step experiments. The process starts with depositing a 0.25 ML of Pt, with Pt atoms arranged in a c(2 × 2) array, which is followed by the growth of Pt islands and three-dimensional clusters as in the Stranski−Krastanov growth mode. Density functional theory calculations were used to help in elucidating atomically resolved electrochemical scanning tunneling microscopy (ECSTM) images of the initial stages of Pt deposition. A Pt adlayer on RuO2(110) has lower catalytic activity for the...

마이크로전극에 의한 니켈수소전지용 수산화니켈 입자의 전기화학적 거동

본 논문은 마이크로전극 측정시스템을 사용하여 니켈수소전지의 전극 소재로 사용되고 있는 수산화니켈의 단일 입자에 대해 전기화학적 평가를 수행 하였다. 즉 Carbon fiber 마이크로전극을 수산화니켈 입자 한개 위에 전기적인 접촉을 이루도록 조정하고 전기화학적 평가를 수행하였다. Cyclic Voltammetry 실험 결과 수산화니켈의 산화 환원 반응과 산소 발생 반응(OER)이 명확하게 분리 되고 있음을 확인하였으며, 전위주사속도를 증가 시킬 경우 환원 전하량은 주사 속도에 의존하지 않고 거의 일정한 수치를 보여 주고 있으나, 산화 전하량은 환원 전하량 보다 크고 주사속도 구간에서 부반응인 산소발생이 증가하고 있음을 확인했다. 그리고 Calvanostat에 의한 정전류 충방전 실험의 결과 수산화니켈 단일 입자의 방전용량은 이론용량 289 mAh/g에 근접한 수치(약 250 mAh/g)를 보여 주었으며 또한 Potential Step에 의해 단일 입자내의 수소이온 확산계수(<TEX>$D_{app}=3{\sim}4{\times}10^{-9}\;cm^2/s$</TEX>)가 얻어졌다. Electrochemical studies were performed for a single particle of nickel hydroxide for the cathode of Ni-MH batteries. A carbon fiber microelectrode was manipulated to make electrical contact with an alloy particle, and electrochemical experiments were performed. As a result of cyclic voltammetry, the oxidation/reduction and oxygen evolution reaction (OER) are clearly separated for a single particle. The total cathodic charge (Qred) is practically constant for the scan rate investigated, indicating that the whole particle has reacted. The total anodic charge(Qox) was larger than that of reduction reaction, and the magnitude of oxygen evolution taking place as a side reaction was enhanced at lower scan rates. As a result of galvanostatic charge and discharge measurement, the discharge capacity of single particle was found to be 250 mAh/g, value being very close to the theoretical capacity (289 mAh/g). The apparent proton diffusion coefficient(Dapp) using potential step method inside the nickel hydroxide was found to range within <TEX>$3{\sim}4{\times}10^{-9}\;cm^2/s$</TEX>.

Electrocrystallized Ag nanoparticle on functional multi-walled carbon nanotube surfaces for hydrazine oxidation

4-Aminobenzoic acid was covalently grafted on multi-walled carbon nanotubes (MWNTs) by amine cation radical formation in the electrooxidation process of the amino-containing compound. Then, silver (Ag) nanoparticles were electrocrystallized on 4-aminobenzoic acid monolayer-grafted MWNTs by a potential-step method. The structure and nature of the resulting Ag/MWNT composites were characterized by transmission electron microscopy and X-ray diffraction. The electrocatalytic properties of the Ag/MWNT electrode for hydrazine oxidation have been investigated by cyclic voltammetry, high electrocatalytic activity of the Ag/MWNT electrode can be observed. This may be attributed to the small particle size of the silver particles. The results imply that the Ag/MWNT composites have a good application potential in fuel cells.

Electrochromic Properties of Bismuth Oxide Thin Films Prepared by Reactive Pulsed Laser Deposition

The electrochromic behaviors of the tetragonal Bi_2O_3 films deposited by pulsed laser deposition at 300℃were investigated by potential step and cyclic voltammetry coupled with an in situ charge-coupled device (CCD) spectrophotometer.The time-resolved absorption spectrum determined by potential step method indicated that the coloration time for Bi_2O_3 film was estimated to be 30 s.In addition,the electrochromic properties determined by cycfic voltammetry showed that this film could exhibit a significantly reversible variation between transparent and dark brown more than 50 times on switching between 3.5 and 0.7 V.The color efficiency at 630 nm was estimated to be 15 cm~2·C~(-1).

Effects of stannous ions on the electrochemical performance of the alkaline zinc electrode

The effect of stannous ions as an electrolyte additive on the electrodeposition characteristics of Zn was investigated by chronoamperometry, the potential-step method and cyclic voltammetry. The chronoamperometry measurements showed that the addition of stannous ions inhibited the dendritic growth of Zn deposits. SEM observation also revealed that the Zn deposit was in the form of compact cylinders with rounded tops that consist of many small crystallites, rather than the classical dendrites with side branches. The inhibition effect of Sn2+ on the very initial electrocrystallization of Zn was not a substrate effect. The 0.17 V difference in reduction potential between Zn and Sn resulted in the codeposition of Sn and Zn. An interruption effect was proposed to illuminate the inhibition effect of Sn2+ on the formation of Zn dendrites. Furthermore, Sn2+ additive was found to suppress the corrosion reaction of Zn by 87% in the Sn2+-containing zincate electrolyte, comparing to that in the blank zincate electrolyte.

Potential-step coulometry of d-glucose using a novel FAD-dependent glucose dehydrogenase

This paper describes the construction and characterization of a batch-type coulometric system for the detection of D-glucose using a novel FAD-dependent glucose dehydrogenase. In order to overcome the problem of interferents, such as ascorbate and urate, a potential-step method was proposed to separate the electrolysis reactions of interferents and D-glucose by selecting a mediator possessing an appropriate formal potential. The rapid oxidative consumption of the interferents proceeded in the first step, whereas the mediator and glucose remained reduced. In the second step, the mediator was immediately oxidized, and subsequent bioelectrocatalytic oxidation of D-glucose occurred with the aid of aldose 1-epimerase. In this study, potassium octacyanomolybdate (IV) with a formal potential of 0.6 V vs. Ag|AgCl was chosen as a mediator, and the first and second electrolysis potentials were set at 0.4 V and 0.8 V, respectively, by considering the heterogeneous electron-transfer kinetics and the potential window. The background-corrected response in charge corresponded to 99+/-2 % efficiency in terms of the amount of D-glucose.

Effects of dopants on percolation behaviors and gas sensing characteristics of polyaniline film

Effects of dopants on the correlation between the polyaniline (PAn) film resistance ( R) and the reduction charge ( Q) injected to the PAn film was investigated in dry acetonitrile solutions during electrochemical reduction by using the double potential step method. The R– Q correlation behaves as S-type curves, leading to the determination of the critical reduction charge ( Q c). The latter represents the reduction charge required for the formation of a continuous partially reduced phase in the PAn film. It was observed that the PAn film doped with sodium dodecylbenzene sulphate (SDBS) yielded a smaller Q c than that doped with perchlorate, when the PAn films were electrochemically reduced under given conditions. The resistance of the pre-doped PAn film will increase significantly when the film is injected with reduction charge more than Q c. Hence, a smaller Q c means that the film can respond to very light reduction (or dedoping), being indicative of better sensing ability toward alkaline and reducing gases. This was confirmed by the increased sensitivity of the PAn/SDBS sensor toward 100 ppm NH 3 vapor, compared with the PAn/ClO 4 − sensor.

An improved structure model to explain variations of electric properties of polyaniline film in the reduction process by using double potential step technique

The double potential step method was used to study the electric properties of polyaniline (PAn) film in acidic aqueous solutions. The resistance and capacitance of bulk film changed with prolonging of the reduction time and/or shifting of reduction potential in the negative direction. And also a shoulder appeared in the cathodic chronoamperometric curve indicate the two-stage reduction process of PAn. These experimental results have been satisfactorily explained by using an improved heterogeneous model, which takes the merits of both the percolation theory and the electrochemistry stimulated conformational relaxation model. This new model overcomes some difficulties that the previous two models may meet, and especially can quantitative explain the change of electric properties of PAn film in the reduction process.

Diffusion layer approximation under transient conditions

First, a theoretical expression of diffusion layer thickness is derived for simple (one-step) electrochemical reactions with reversible electron-transfer processes investigated by LSV under semi-infinite linear diffusion conditions in the electrolytic solution. The above expression is compared to previous results in electrochemical literature. In particular, it is shown that Padé-approximants used in previous works fail to predict the potential dependence of diffusion layer thickness over the whole potential domain. More generally, the change of diffusion layer thickness with respect to time is derived in this article in terms of reaction rate and mass-transport conditions, irrespective of the interfacial reaction mechanism and the electron-transfer kinetics. The formulation applies to one-dimensional diffusion processes eventually coupled with homogeneous chemical reactions, using electrochemical techniques such as potential and current step methods, linear sweep voltammetry (LSV), etc. Finally, experimental investigation of Fe III/Fe II redox couple by LSV is presented as an illustration of the theoretical derivation in this work.

Rate laws for reductive stripping of NO adlayers at single-crystal platinum electrodes as deduced from transient experiments

The electrochemical reduction of NO adlayers on Pt(111) and Pt(110) surfaces has been investigated using the potential step method. The resulting current transients were interpreted in terms of a kinetic model for the first- and the second-order reactions in the mean-field approximation with strong lateral interactions. Under electrochemical conditions and in the limit of saturated NO coverage, considerable repulsive interactions exist between the reacting species, having a significant effect on the chronoamperometric response of the systems examined. Accordingly, the current transients for NO reduction on Pt(111) show a hyperbolic (t−1) decay, which indicates (i) first-order kinetics and (ii) strong lateral interactions. For NO reduction on Pt(111) surface, the experimentally assessed lateral interaction between two neighboring NO species varies between ca. 0.03 and 0.05eV. The current transients for the reductive stripping of the saturated NO adlayers on the Pt(110) surface (with atop NO as the reactive species) show a potential dependent maximum, occurring at high coverages, which indicates (i) second-order kinetics and (ii) significant lateral interactions. The second-order kinetics observed in the limit of high NO coverage was attributed to the necessity of having a neighboring free site, required for breaking the N–O bond. For the reduction of the (nearly) saturated NO adlayers on Pt(111) surface, the apparent first-order kinetics may be tentatively explained by the availability of the neighboring free site at all coverages of NO, as suggested by the much lower saturation coverage on Pt(111) (ca. 0.5ML) compared to the saturation coverage on Pt(110) (ca. 1ML).