The role of diffusion of ionic species and of interdiffusion in the solid state in electrolytic deposition processes from molten salts

Technical applications of electrocoating from molten salts involve the study of electrochemical reactions which are under the control of diffusion of electroactive species. Chemical reactions and ionic transport in the melt have previously been examined. Now the role of interdiffusion in the solid state is considered in the electrolytic deposition processes for coating metal and alloy substrates. The rate of diffusion into the substrate is the rate-controlling process. The study of the kinetics of incorporation by pulse electrochemical techniques is described.It is shown that the classical treatment used to describe the diffusion-controlled phenomena has to be modified to take account of the volume change of the electrode due to the metal incorporation. A mathematical analysis is presented to include the perturbation resulting from the bojndary motion which occurs during electrolysis at constant potential or at constant current (galvanodiffusion). Examples of application of this treatment in recent studies of metalliding reactions are given and discussed in relation to the experimental case of aluminiding iron.This paper was presented at a workshop on the electrodeposition of refractory metals, held at Imperial College, London, in July 1985.     

Localized electrodeposition of praseodymium oxide on boron-doped diamond

The deposition of praseodymium oxide (PrO₂) films on diamond electrodes is of interest for electrochemical applications, because of the catalytic and permselective properties of this material. In this work, PrO₂ was deposited on commercial boron-doped diamond (BDD) electrodes from solutions of praseodymium nitrate in hydrogen peroxide, where a local increase of pH at the electrode surface is used to precipitate hydrated praseodymium oxide at the solution-electrode interface. For particular deposition conditions, it is observed that pronounced lateral heterogeneities are seen in the observed deposition rate, arising from lateral fluctuations in the electrochemical activity of the diamond electrodes used. It is also observed that the use of scanning electrochemical techniques enables oxide deposition with good spatial control to be achieved.     

锂离子电池介尺度电化学反应非均匀性

锂离子电池是重要的一类能量存储与转化装置。本文从化工角度上出发,将电池视为一类“特殊的”化工反应器,其中也存在着“三传一反”：电子传输、离子传输、热量传递和电化学反应。电极是这些传递与反应过程发生的主要场所;在不同空间与时间尺度上,电极中的传递与反应过程具有不均匀性,进而导致电池材料的利用与失效的差异。本文进一步讨论了研究电极反应不均匀性的必要性,并从准二维电极模拟计算与多尺度表征分析技术两个角度,对不均匀电极反应过程的传质、传热过程、电化学反应过程、电极结构演变规律、电极失效机制等方面的研究进行综述,对未来高性能、长寿命和具备快充性能的电池开发具有一定的指导意义。     

Electrochemical performance of low-temperature solid oxide fuel cells running on syngas from pyrolytic urban sludge

Low temperature solid oxide fuel cells (SOFCs) that efficiently utilize widely available hydrocarbon resources are highly desirable for cost reduction and durability purposes. In this work, SOFCs consisting of highly ionic conductive ceria-carbonate composite electrolytes and lithiated transition metal oxide symmetric electrodes are assembled and their electrochemical performances at reduced temperature (≤650 °C) are investigated using syngas fuel (44.65% H₂, 10.19% CH_4, 2.01% CO and the balanced CO₂) derived from pyrolytic urban sludge. The cell gives a peak power output of 127 mW cm^?2 at 600 °C and shows a relatively stable operation for 11 hours under constant voltage operational conditions. Though the composite electrode presents a moderately high polarization resistance toward CH₄ and CO oxidation and the electrochemical performance is highly correlated with the microstructure of ceria-carbonate electrolyte, it is interesting to see that a higher concentration of methane is obtained after the fuel cell reaction, which may suggest an alternative approach to realize the power and chemical co-generation within such a SOFC reactor. Finally, the symmetric electrode shows high resistance toward carbon deposition, possibly due to its high alkaline nature.     

Platinum oxidation and its effect on concanavalin A adsorption

Oxidation of platinum electrode by chemical and electrochemical means was investigated through electrochemical impedance spectroscopy and cyclic voltammetry. It was possible to observe that concanavalin A readily adsorbs on platinum surface. The degree of adsorption is found to be related to oxidation of the electrode surface. It is argued that the protein may have a strong affinity for hydrated oxide layer on the metal surface, similarly as it has affinity for carbohydrates. Using an equivalent circuit as a model for the interface, it was possible to obtain the thickness of the oxide layer. These results were compared with an electron tunneling model across the oxide layer and showed reasonable agreement. There are differences between the thickness of the oxides prepared chemically and electrochemically, which reflect over the adsorption of the protein.     

Nitric oxide reduction at noble metal electrodes: a voltammetric study in acid solution

Features of the electrochemical reduction of nitric oxide on platinum, palladium, rhodium and ruthenium in aqueous perchloric acid solutions (0.33–1.0 M) are compared. The results from voltammetric studies (ie linear potential sweep and rotating disc electrode) using the bulk metal electrodes are described and compared with residual current voltage plots in acid electrolyte alone. In general, three nitric oxide reduction peaks are observed on the metals. The most anodic peak, at ca E = 0.15 V vs sce is attributed to the one-electron reduction of nitric oxide to an adsorbed NOH intermediate on a bare metal surface (ie one free of oxides or adsorbed hydrogen). The other two peaks occur in potential regions where adsorbed hydrogen is present on the metal surface (ca E = 0.0 and −0.20 V, respectively). The co-adsorbed hydrogen complicates the analysis and precludes an unambiguous interpretation of these two peaks. However, they apparently reflect nitric oxide reduction to nitrogen, hydroxylamine and/or ammonia. In a cathodic scan on the rhodium electrode, a current plateau is seen instead of the first (most anodic) peak, a probable consequence of oxide film formation with subsequent chemical complications. On the ruthenium electrode the first two (most anodic) peaks are not observed probably due to a relatively stable oxide layer. Reaction selectivities at metal black gas diffusion cathodes operating in an electrogenerative (ie galvanic) mode with perchloric acid electrolyte are compared with the voltammetric results at the corresponding bulk electrodes. Dinitrogen formation is observed on the platinum and rhodium black electrodes as suggested from voltammetric results. A series-parallel reaction sequence is proposed to explain the results. Limitations of using simple voltammetric techniques for predicting behavior of large scale preparative electrochemical reactors are discussed.     

The effects of coumarin on the electrodeposition of nickel

Systematic variations in the thickness of the diffusion boundary layer over a rough surface are an essential factor in producing the levelling action of metals electrodeposited in the presence of levelling additives. Information on the behaviour of these additives at an electrode at which metal is being deposited and their effects on metal deposition under conditions of known diffusion-layer thickness is therefore valuable for understanding the mechanism of levelling. Experiments are described which show that the behaviour of additives in the electrodeposition of metals can be studied under conditions of known rates of transport of additive to the electrode using a rotating disk electrode.

In the deposition of nickel from a Watts-type solution, coumarin reduces the current efficiency for nickel deposition. The current efficiency for hydrogen evolution is increased, the coumarin encouraging evolution from a few points on the electrode.

The analysis of products has confirmed that reduction to melilotic acid is the principal cathodic reaction of coumarin but there is a small amount of a second product. Coulomb balances show that this is a product involving the addition of many more than two electrons to each coumarin molecule.

The diffusion coefficient for coumarin in the Watts nickel solution has been measured and used to calculate rates of transport of coumarin to the cathode under various conditions. These have been compared with measured rates of coumarin consumption. The comparison has shown conditions where the rate of consumption is transport controlled and where it is electrochemically controlled.

The levelling power of coumarin on a rough surface has been derived from the results. This shows maxima in levelling power as a function of coumarin concentration resulting from the change over from transport to electrochemical control of the rate of coumarin consumption.     

Review of Computational Studies of NCM Cathode Materials for Li-ion Batteries

Lithium-ion based rechargeable batteries are considered among the most promising battery technologies because of the high energy- and power-densities of these electrochemical devices. Computational studies on lithium ion batteries (LIBs) facilitate rationalization and prediction of many important experimentally observed properties, including atomic structure, thermal stability, electronic structure, ion diffusion pathways, equilibrium cell voltage, electrochemical activity, and surface behavior of electrode materials. In recent years, Ni, Co and Mn-based (NCM) layered transition metal oxide positive electrode materials (LiNi_1-x-yCo_xMn_yO₂) have shown tremendous promise for high-energy density LIBs, and these NCM-based batteries are effectively commercialized. Here, we present an overview of recent theoretical work performed using first principles density functional theory on these layered cathode materials. This short review focuses on recent computational efforts of popular NCMs with increasing Ni content, ranging from NCM333 to NCM811.     

The electrochemical study of a chromium plating bath. I. Reactions leading to solution-free species

The electrode reactions occurring at C, Cr, Cu and Ni cathodes in the standard electroplating solution for chromium, 200 g dm^?3 CrO₃ and 2 g dm^?3 H₂SO₄, have been reinvestigated. This paper emphasises the study of those reactions which lead to solution-free products; these occur at potentials positive to that necessary for film or metal deposition. For each cathode, theI-E curve shows a wave or peak for the reduction of chromium (VI) to chromium(III) and it is demonstrated that this process is mass transport controlled, apparently by the transport of sulphate ions to the electrode surface. At Cr, Ni and Cu, the potential of the chromium(VI)/chromium(III) couple is determined by the removal of an insulating oxide film which forms spontaneously when the metal is immersed in the bath. In certain conditions it is also possible to define a well formed wave for proton reduction.     

A voltammetric and nanogravimetric study of Te underpotential deposition on Pt in perchloric acid medium

This work describes the study of Te underpotential deposition on Pt in acid media using cyclic voltammetry, rotating ring-disc electrode and electrochemical quartz crystal microbalance techniques. The voltammetric results indicate the presence of two dissolution peaks in the positive scan with a total charge density of 420 μC cm⁻². These phenomena are attributed to the deposition of one Te monolayer with the occupancy of two active Pt sites by each ad-atom. This is confirmed by rotating ring-disc electrode results. The electrochemical quartz crystal microbalance (EQCM) experiments yielded the small mass variation of −32 ng cm⁻² (while the theoretical one is −140.4 ng cm⁻² for a complete Te monolayer). This low value can be attributed to the simultaneous adsorption of water, perchlorate anions and the formation of platinum oxide.     

Electrochemical behavior of a novel palladium pentacyanonitrosylferrate modified aluminum electrode

Novel inorganic film modified electrodes have been prepared by chemical deposition of a thin palladium pentacyanonitrosylferrate (PdPCNF) film on the surface of aluminum substrate. The modification process including the electroless deposition of metallic palladium on the aluminum electrode surface from PdCl₂+25% ammonia solution and chemical derivatization of deposited palladium to the PdPCNF film in 0.1 M Na₂[Fe(CN)₅NO]+0.5 M KNO₃+HNO₃ solution (pH 1.5-2.5), are described. The aluminum-based modified electrodes exhibit, one pair of well-defined voltammetric peaks which correspond to the Fe(III)/Fe(II) transition in complex structure. The effect of pH, ammonium, alkali metal and alkaline earth metal cations of supporting electrolyte on the electrochemical characteristics of the modified electrode was studied in detail. Diffusion coefficients of hydrated ammonium and alkali metal cations in the film (D), transfer coefficient (α) and transfer rate constant for electron (k_s), were determined. The high stability of this modified electrode makes it attractive in practical application.     

Modeling of a Tubular‐SOFC: The Effect of the Thermal Radiation of Fuel Components and CO Participating in the Electrochemical Process

A mathematical model based on first principles is developed to study the effect of heat and electrochemical phenomena on a tubul solid oxide fuel cell (SOFC). The model accounts fordiffusion, inherent impedance, transport (momentum, heat and mass transfer) processes, internal reforming/shifting reaction, electrochemical processes, and potential losses (activation, concentration, and ohmic losses). Thermal radiation of fuel gaseous components is considered in detail in this work in contrast to other reported work in the literature. The effect of thermal radiation on SOFC performance is shown by comparing with a model without this factor. Simulation results indicate that at higher inlet fuel flow pressures and also larger SOFC lengths the effect of thermal radiation on SOFC temperature becomes more significant. In this study, the H₂ and CO oxidation is also studied and the effect of CO oxidation on SOFC performance is reported. The results show that the model which accounts for the electrochemical reaction ofCO results in better SOFC performance than other reported models. This work also reveals that at low inlet fuel flow pressures the CO and H₂ electrochemical reactions are competitive and significantly dependent on the CO/H₂ ratio inside the triple phase boundary.     

Modelling mass transport in solid oxide fuel cell anodes: a case for a multidimensional dusty gas-based model

In this work the mass transport phenomena taking place in the fuel channel and the porous electrode of the anode of planar solid oxide fuel cells (SOFCs) are discussed. A comprehensive review of SOFC mass transport models in the literature is given and a new multidimensional, multicomponent, isothermal, dynamic model of the mass transport phenomena taking place in the fuel channel and the porous electrode of the anode of planar SOFCs is presented. The model can be used to predict species composition profiles and is based on the dusty-gas model (DGM) [Mason, E.A., Malinauskas, A.P., 1983. Gas Transport in Porous Media: The Dusty-Gas Model: Elsevier; Jackson, R., 1977. Transport in Porous Catalysts: Elsevier], which is considered to be the most accurate of the existing mass transfer models in porous media [Suwanwarangkul, R., Croiset, E., Fowler, M.W., Douglas, P.L., Entchev, E., Douglas, M.A., 2003. Performance comparison of Fick's, dusty-gas and Stefan-Maxwell models to predict the concentration overpotential of a SOFC anode. Journal of Power Sources 122, 9-18]. Our two-dimensional DGM is validated using experimental data [Yakabe, H., Hishinuma, M., Uratani, M., Matsuzaki, Y., Yasuda, I., 2000. Evaluation and modeling of performance of anode-supported solid oxide fuel cell. Journal of Power Sources 86, 423-431] and it is tested against a two-dimensional Stefan-Maxwell model (SMM) and against one-dimensional models (Fick's model, SMM and DGM) reported in the literature. It is shown that a detailed model is essential for the accurate prediction of concentration overpotentials especially at high fuel utilisation conditions, which are typical operating conditions for fuel cells [Hernández-Pacheco, E., Singh, D., Hutton, P.N., Patel, N., Mann, M.D., 2004. A macro-level model for determining the performance characteristics of solid oxide fuel cells. Journal of Power Sources 138, 174-186].     

Electrochemical fabrication and electrocatalytic characteristics studies of gold nanopillar array electrode (AuNPE) for development of a novel electrochemical sensor

Gold nanopillar array electrodes were prepared by electrochemical deposition of gold into the nanopores of anodic aluminum oxide membrane placed onto the gold thin film electrode surface, which was in advance modified with cysteamine self-assembled monolayer as an anchoring layer. The Au nanopillar electrode is electrochemically stable and consists of highly dense, upstanding pillars assembled on the cysteamine monolayer. The structural morphology and chemical composition of the nanoarray electrode was characterized by field emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Electrochemical measurements indicate that the Au nanopillar electrode possesses high electrocatalytic activities in the reduction of hydrogen peroxide and molecular oxygen, especially in glucose oxidation due to its higher electroactive surface area. The electro-oxidation studies of several electroactive neurotransmitters demonstrate that the nanopillar electrode can be utilized as a promising material for the construction of novel electrochemical sensor.     

Preparation and characterization of ZrO2/Ti electrode for waste water purification system

We have investigated the effects of the etching method of a Ti substrate for a metal oxide electrode on the electrochemical characteristics of the electrode. The preparation method and electrochemical characterization of zirconium oxide films on etched Ti substrate has been also studied. The HCl etching was developed a fine and homogeneous roughness on the Ti substrate. Fabrication and material properties of the metal oxide electrode, which is known to be so effective to generate ozone and sodium hypochlorite (NaOCl) as power oxidant, were studied. A proper metal oxide material focus zirconium oxide through reference paper. A coating method to enhance the fabrication reproducibility of the zirconium oxide electrode was used dip-coating method by zirconium oxychloride. Zirconium oxide films on the Ti substrate were analyzed by SEM, XPS and cyclic voltammetry.     

A computational laboratory on the role of mass transport contributions in electrochemical systems: Copper deposition

This paper presents a computational laboratory that describes the ionic transport of chemical species in an electrochemical process. The system is modeled in 1D using a kinetic model type Butler–Volmer coupled with mass balance equations, i.e. Nernst–Planck formalism. This laboratory is intended to be a practical learning tool to study the deposition of chemical species, e.g. Cu²⁺, subject to the typical mass transfer mechanisms, i.e. diffusion, migration and convection. Sensitivity analyses are used to analyze the effect of each mass transport phenomena over the process reaction rate. The material showed in this paper is a section (laboratory) of two third-year courses in the Nanotechnology and Chemical Engineering undergraduate programs at the University of Waterloo. The pedagogical goals, learning experiences and students’ comments of this laboratory are presented in this work.     

Amorphous carbon nitride thin films for electrochemical electrode: Effect of molecular structure and substrate materials

We report on the effect of molecular structure and substrate material on amorphous carbon nitride (a-CN:H) electrode properties including film adhesion to the substrate and electrochemical properties. Films were prepared by neutral beam enhanced chemical vapor deposition on different substrate materials (p-type Si, Cu, Ti, and Pt) below room temperature. When depositing on Si, doping nitrogen into carbon improved the electrochemical properties despite weak adhesion to the substrate. Nitrogen in a-CN:H formed two different bonding configurations: incorporation into aromatic carbon rings and hydrogen nitride by infrared (IR) spectroscopy. Therefore, delocalization of π bonds by incorporation of nitrogen affected the electrochemical improvement of the a-CN:H electrode. For samples deposited on a different metal substrate, the adhesion to substrate increased as a function of decreasing oxygen concentration on the metal substrate surface; the Pt substrate performed well with no delamination in our evaluation. The electrochemical properties were improved only in the case of deposition on Pt. Moreover, Pt surface modification by hydrogen beam was also effective; consequently, the electrochemical property of the a-CN:H electrode was superior to the graphite electrode with high temperature annealing. The observed increases in IR spectra of aromatic clusters were in line with the electrochemical improvements of a-CN:H.     

Novel electrochemical measurements on direct electro-deoxidation of solid TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> in molten calcium chloride medium

A solid metal oxide cathode undergoes significant chemical changes during the molten salt electro-deoxidation process. The changes in the chemical composition lead to changes in the electrical resistivity and potential of the electrode. Two novel electrochemical techniques, based on these two parameters, have been employed to study the electro-deoxidation of solid TiO₂ and ZrO₂ in molten calcium chloride at 900 °C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO₂ electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO₂ electrode, on the other hand, developed very high resistance midway in the electro-deoxidation, and could not be reduced completely. The resistance measurements give strong indication that the electron-transfer reactions taking place at the cathode determine the rate and efficiency of the electro-deoxidation process to a great extent. The low-current galvanostatic electro-deoxidation of TiO₂ electrodes, in conjunction with a graphite pseudo reference electrode to monitor the half cell potentials, showed that the metal oxide passes through two stages during the electrolysis; a high current, low resistant stage 1, where Ca²⁺ ions are inserted to the metal oxide cathode to produce different intermediate compounds and stage 2 where electro-deoxidation of the cathode take place continuously. Removal of oxygen, from the cathode, in stage 1 of the electro-deoxidation is considered to be insignificant. The anodic and cathodic voltages in this stage remained more or less stable at ~1.4 V and ~−1 V, respectively. When the oxygen ions in the melt were depleted at the end of this stage, both the anode and cathode potentials were increased in the anodic direction and this behaviour suggested that the graphite pseudo reference electrode was changed from a C/CO electrode in stage 1 to a Ca²⁺/Ca electrode in stage 2.     

Novel one-step synthesis for 3-D four-pointed micro-star of quaternary metal (Mo–V–Mn–Ag) oxide electrode for asymmetric supercapacitor

An intention of the present work is to synthesize a quaternary metal oxide by a simple and cost-effective method. MoVMnAg-oxide@Ni-foam is synthesised by one-step hydrothermal method. The as-deposited MoVMnAg-oxide sample is systematically examined through XRD, FESEM, EDS-mapping, and TEM analysis. The electrochemical performance of an MoVMnAg@Ni-foam electrode is tested using CV, GCD, and EIS techniques. MoVMnAg-oxide@Ni-foam has a considerable high areal capacitance of 651 mFcm^?2 with 0.13 mWhcm^?2 energy at 1.8 mWcm^?2 power density in 1 M KOH electrolyte calculated from GCD curves. Also, the electrode shows a diffusion coefficient of 1.52 × 10^?7 cm²s^?1 along with 91 % of diffusive-controlled contribution and a b-value of 0.51, which depicts faradaic charge storage mechanism. An assembled asymmetric supercapacitor device (MoVMnAg@Ni-foam//AC) delivers an areal capacitance of 312 mFcm^?2 with 0.37 mWcm^?2 power density at 1 mAcm^?2 current density within 0 – 1.5 V voltage window. The asymmetric device showed cyclability and coulombic efficiency of 80.3% and 95% respectively measured up to 10,000 GCD cycles. These results demonstrate the deposition of quaternary metal oxide directly on Ni-foam showing highly competitive electrochemical performance so that they can be utilized in energy storage applications.