Surface Electrochemical Reactions Research Articles

Effects of Cr, Zr, V, Mn, Fe, and Co to the hydride properties of Laves phase-related body-centered-cubic solid solution alloys

Chemical composition modifications of a Laves phase-related BCC solid solution base alloy, Ti15.6Zr2.1V44Cr11.2Mn6.9Fe2.7Co1.4Ni15.7Al0.3, were investigated in order to study the function of each constituent element on the structural, gaseous phase and electrochemical hydrogen storage properties of these alloys. In general, removal of Fe and decrease in V-content in exchange for higher Ni-content were found to improve both the electrochemical capacity and high-rate dischargeability, which are related to the decrease in C14-content and increase in TiNi-content. However, total elimination of the C14 phase by removal of Zr resulted in a reduced discharge capacity, a prolonged activation period, and a less catalytic surface for electrochemical reaction. Besides the BCC and C14 phases, the TiNi phase was also found in every alloy in this study, contributing positively to the bulk diffusion of hydrogen while hindering the surface electrochemical reaction.

Facile Synthesis of Mesoporous MoS2‐TiO2 Nanofibers for Ultrastable Lithium Ion Battery Anodes

AbstractTiO2 hybrid nanocomposites are promising electrode materials for lithium‐ion batteries because of the large specific capacity, excellent rate capability, and high cycling stability. We have developed an effective facile incipient wetness impregnation method that can be used to fabricate novel mesoporous MoS2–TiO2 fibrous nanocomposites. The MoS2–TiO2 nanocomposites exhibited notable electrochemical performance with a high capacity of 120 mA h g−1 at a rate of 40 C and long‐term, stable capacity of 124 mA h g−1 with a capacity retention of 75.2 % after 1000 cycles. These results suggest that the structure of mesoporous TiO2 nanofibers composited with a thin, uniform, well‐dispersed MoS2 layer improve the rate performance and cycling stability. Such good performance may be derived from the dispersed MoS2 layer, which accelerates the surface electrochemical reactions and offers a stable structure.

Electrochemical Kinetics Investigation of Li4Ti5O12/Reduced Graphene Oxide Nanocomposite Using Voltammetric Charge Analysis

The electrochemical kinetic properties of a Li4Ti5O12 (LTO)/reduced graphene oxide (RGO) nanocomposite were systematically investigated using cyclic voltammetry (CV). Since there are several factors that influence the electrochemical kinetics of LTO depending on the potential scan rate, the kinetic properties of LTO in the nanocomposite were evaluated by analyzing the scan rate dependence of the voltammetric charge, as measured by CV. When we formed a nanocomposite of LTO and electrically conductive RGO, the number of surface electrochemical reaction sites increased and that of the bulk electrochemical reaction sites decreased. This led to an improvement in the kinetics related to Li+ ion diffusion in LTO. The range of scan rates for which the voltammetric charge was decreased by the kinetic limitations ascribed to diffusion was reduced because of the decrease in the effective diffusion length and the increase in the diffusion area, compared to commercial LTO particles. Thus, unlike previous studies, we report that the improved rate capability of the composite with RGO is ascribed to enhanced diffusion properties in the bulk electrochemical reaction sites rather than an increase in the electrical conductivity, which is a surface property.

Effects of free cyanide and cuprous cyanide on the flotation of gold and silver bearing pyrite

At gold and silver mineral processing plants, cyanide species are always present in the process water recycled to flotation circuits, despite the cyanide destruction process. The effect of cyanide, in particular, cuprous cyanide on gold and silver flotation has not been well understood. In the present study, free cyanide and cuprous cyanide species were isolated and their effects on the flotation of a pyritic ore were evaluated. It was found that free cyanide depressed gold and silver flotation through their carrier, pyrite. Cuprous cyanide mainly in the form of Cu(CN)32− depressed pyrite flotation similarly as free cyanide. Electrochemical studies including open circuit potential (OCP), cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques were carried out to understand the underpinning depression mechanism of cyanide species on pyrite flotation using xanthate as collector. It was found that all surface electrochemical reactions were inhibited by either free cyanide or cuprous cyanide. The surface layer as a result of xanthate adsorption on pyrite was completely removed in the presence of these cyanide species, which was suggested to contribute to the hydrophilic pyrite surface.

Spectroelectrochemical analysis of conduction band edge shift in nanoporous TiO2 electrodes

In this work, conduction band edge (Vfb) shift in the nanostructure TiO2 electrodes was investigated in detail using photoelectrochemical methods. The density of conduction band at a given applied potential was affected by the electrochemical reaction on the nano-TiO2 surface and spectroelectrochemistry techniques could determinate Vfb from the measured absorbance as a function of applied potential for the TiO2 electrodes. It is indicated that the increased thickness of TiO2 electrodes led to the positive shift of conduction band edge (Vfb), which resulted in the increased reaction rate of electrochemical reaction in nanostructure TiO2 electrode. The N and Mg2+ incorporation into the TiO2 lattice resulted in the negative conduction band edge, and an analytical dependence of the photoelectrochemical methods on the TiO2 surface performance found that the negative shift of Vfb could be attributed to the substitutional dopant into TiO2 lattice to decrease the electrochemical surface reaction in the photoelectrodes.

Amphiphilic carbonaceous material-based hierarchical porous carbon aerogels for supercapacitors

Novel hierarchical porous carbon aerogels (PCAs) derived from amphiphilic carbonaceous material (ACM) have been mass-prepared via a facile solvent exchange induced self-assembly process and subsequent carbonization and KOH activation. The resulting products are stacked up by highly interconnected carbon nanoparticles with a certain amount of micropores and mesopores, which aggregate to build a three-dimensional macroporous architecture. The hierarchical porous structure facilitates fast ion transportation inside the electrode simultaneously preserving efficient ion surface electrochemical reactions. Capacitive and rate performances were evaluated by fabricating symmetric capacitors with both aqueous and organic electrolytes. The PCA-0.5 and PCA-1.0 electrodes exhibit superior specific capacitances of 261.2 and 227.9 F g−1 at a current density of 0.05 A g−1 in 6 M KOH electrolyte, and still remain 145.5 and 175.4 F g−1 as the current density increases to 100 A g−1, respectively. Remarkably, the PCA-0.5 and PCA-1.0 electrodes show stable cycle durability with a slight capacitance loss of 8.2 and 11.2 % after 5000 cycles at 1 A g−1, respectively. Furthermore, in organic electrolyte system, the PCA-1.0 electrode manifests an outstanding capacitance of 155.4 F g−1 at a current density of 0.05 A g−1. The encouraging results demonstrate that PCAs are a sort of promising and competitive supercapacitors electrode materials.

Influence of gas ambient on charge writing at the LaAlO₃/SrTiO₃ heterointerface.

We investigated the influences of charge writing on the surface work function (W) and sheet resistance (R) of the LaAlO3/SrTiO3 (LAO/STO) heterointerface in several gas environments: H2(2%)/N2(98%), air, N2, and O2. The decrease in W and R due to charge writing was much larger in air (ΔW = -0.45 eV and ΔR = -40 kΩ/S) than in O2 (ΔW = -0.21 eV and ΔR = -19 kΩ/S). The reduced R could be maintained more than 100 h in H2/N2. Such distinct behaviors were quantitatively discussed, based on the proposed charge-writing mechanisms. Such analyses showed how several processes, such as carrier transfer via surface adsorbates, surface redox, electronic state modification, and electrochemical surface reactions, contributed to charge writing in each gas.

Multi-resistive reduced graphene oxide diode with reversible surface electrochemical reaction induced carrier control.

The extended application of graphene-based electronic devices requires a bandgap opening in order to realize the targeted device functionality. Since the bandgap tuning of pristine graphene is limited to 360 meV, the chemical modification of graphene is considered essential to achieve a large bandgap opening at the expense of electrical properties degradation. Reduced graphene oxide (RGO) has attracted significant interest for fabricating graphene-based semiconductors since it has several advantages over other forms of chemically modified graphene; such as tunable bandgap opening, decent electrical properties, and easy synthesis. Because of the reduced bonding nature of RGO, the role of metastable oxygen in the RGO matrix is recently highlighted and it may offer emerging ionic devices. In this study, we show that multi-resistivity RGO/n-Si diodes can be obtained by controlling the RGO thickness at a nanometer scale. This is made possible by (1) a metastable lattice-oxygen drift within bulk RGO and (2) electrochemical ambient hydroxyl (OH) formation at the RGO surface. The effect demonstrated in a p-RGO/n-Si heterojunction diode is equivalent to electrochemically driven reversible electronic manipulation and therefore provides an important basis for the application of O bistability in RGO for chemical sensors and electrocatalysis.

Role of solution conductivity in reaction induced charge auto-electrophoresis

Catalytic bimetallic Janus particles swim by a bipolar electrochemical propulsion mechanism that results from electroosmotic fluid slip around the particle surface. The flow is driven by electrical body forces which are generated from a coupling of a reaction-induced electric field and net charge in the diffuse layer surrounding the particle. This paper presents simulations, scaling, and physical descriptions of the experimentally observed trend that the swimming speed decays rapidly with increasing solution conductivity. The simulations solve the full Poisson-Nernst-Planck-Stokes equations with multiple ionic species, a cylindrical particle in an infinite fluid, and nonlinear Butler-Volmer boundary conditions to represent the electrochemical surface reactions. The speed of bimetallic particles is reduced in high-conductivity solutions because of reductions in the induced electric field in the diffuse layer near the rod, the total reaction rate, and the magnitude of the rod zeta potential. This work suggests that the auto-electrophoretic mechanism is inherently susceptible to speed reductions in higher ionic strength solutions.

Electrochemical, Catalytic and O2-Permeation Studies of Iron-Doped Barium Zirconates for Membrane Reactor Applications

Recently, interest in solid-oxide electrolysis cells (SOECs) with platinum or nickel cermet electrodes capable of reducing combustion effluents to synthesis gas has driven the search for mixed ionic-electronic conductors with suitable catalytic activity for reduction of carbonaceous species. There remains a need for discovering chemically stable electrode materials combining electronic and oxygen-ionic conductivities with suitable catalytic activity for oxidation or reduction. Our research group has focused upon identifying ambivalent transition-metal dopants capable of imparting both electrochemical and catalytic properties to perovskite electrolyte materials. In this study, the authors present a combined electrochemical, catalytic and permeation investigation of iron-doped barium zirconium perovskite in the context of the electrochemical reduction of combustion gas (CO2, H2O) to synthesis gas (CO, H2).Powders of BaZr0.90Fe0.10O3-δ (BZF10) were synthesized via solid-state reaction and characterized by synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS). The powders were pressed and sintered into 13mm dense button cells, and a porous Pt electrode and a dense Pt blocking electrode were attached to either face of the pellet. The resulting system was used to conduct electrical conductivity relaxation (ECR), electrochemical impedance spectroscopy (EIS), and reaction analysis over a wide range of dry O2, CO2, H2O, and humidified CO, H2 partial pressures at temperatures of 600 – 800oC. Analysis of equilibrium electrical conductivities (σ∞) vs. temperature and partial pressures provides critical insight into the underlying electrochemical mechanisms for surface reaction. Non-linear regression of conductivity vs. time data allows deduction of surface electrochemical reaction rates and solid-state diffusivities of charge carriers, and probing of surface oxygen-acceptor/donor kinetics. BZF10 was verified as an electrochemical stable, mixed ionic-electronic conducting ceramic suitable for SOEC application. The σ∞ vs. O2 partial pressure indicates a high oxygen-ion acceptor activity for Ba0.9Fe0.1ZrO3+δ. Equivalent experiments conducted over a range of CO2 partial pressures (pCO2) indicate a net decrease in σ∞ with respect to pCO2, coinciding with up to a 16% conversion of CO2 to CO and O2 at steady-state. Reduction of the Fe-doped perovskite suggests the formation of a CO3 -2 intermediate during the overall conversion of CO2 to O2 and probing of CO2 reduction activity indicates significant catalytic activity, which is attributed to the oxygen acceptor-donor activity of the Fe-doped perovskite. Experimental results indicate that the material behaves as a mixed oxygen ionic-electronic conductor, with significant catalytic activity for CO2 reduction.

Dynamic electrochemical impedance spectroscopy, for electrocatalytic reactions

Dynamic electrochemical impedance spectroscopy with the ac signal applied while sweeping the potential, is a powerful method to study reaction mechanisms of surface electrochemical reactions. The ability to reliably measure low frequencies while sweeping is key to its successful implementation. Numerical modeling of a simple adsorption reaction is used to decide how fast the potential sweep can be at a given frequency. Careful baseline subtraction is crucial to achieve accurate results. Hardware and software implementing this method are described. With attention to issues of resolution and using the baseline correction algorithm described, accurate results can be obtained down to about 1Hz under practical conditions. Sample experimental data is validated using the Kramers-Kronig transform and a consistency test involving the voltammogram slope and the low frequency data.

TiO2 Nanotube Arrays Modified with Bi Nanoparticles for Enhancing Photoelectrochemical Oxidation of Organics

Well aligned TiO2 nanotube arrays are used as photoelectrochemical sensor for organic compounds detection due to their excellent photocatalytic performance. The high background photocurrent and low current response to trace addition of organics restrain the detection performance of the photoelectrochemical sensor. Deposition of Bi nanoparticles onto the nanotube arrays by photocatalytic reduction of Bi3+ ions is found to decrease the background photocurrent and increase the current response effectively at the same time. Scanning electron microscope, high resolution transmission electronmicroscope and X-ray photoelectron spectrometer are used to characterize the structure, morphology and component of the Bi/TiO2 nanotube arrays. The concentration of Bi3+ ions and deposition parameters are optimized to achieve the best detecting performance. When glucose is used as target organics, the photoelectrochemical sensor can achieve a sensitivity of 0.3 μA/μM with a linear range of 0 to 1500 μM. The detection limit is calculated by triple signal noise ratio to be 4.5 μM. The mechanism of enhancing oxidation of organics is ascribed to the electrons transfer and the surface electrochemical reactions during the photoelectrochemical process.

Effect of Electronic Wiring on the Electrochemical Reaction Sites in Manganese Oxide with Pseudocapacitive Behavior

The effect of electronic wiring on the electrochemical reaction sites in manganese oxide (MnO2) was studied by electrochemical impedance spectroscopic (EIS) investigation of MnO2/multiwall carbon nanotube (MWCNT) composites with and without added conducting agents. The results helped in confirming the effects of the electrical conductivity of the composite on its electrochemical properties. The electronic wiring in MnO2 influences not only the electron transfer reactions at the surface electrochemical reaction sites but also Na+ ion diffusion in the bulk electrochemical reaction sites. Electronic wiring plays a more important role in deciding the rate capability of MnO2 than does the electrical conductivity of the electrode.

Comparison of electrochromic properties of Ni1−xO in lithium and lithium-free aprotic electrolytes: From Ni1−xO pigment coatings to flexible electrochromic devices

Electrochromic nickel oxide (Ni1−xO) pigment powder which was made from nickel acetate reacting with H2O2/urea solution followed by heat treatment of xerogels at 400°C (24h) was milled with zirconia beads in pure water or water with added nickel oxyhydroxy precipitate acting simultaneously as the dispersant and the coating binder. The corresponding pigment particle suspensions were spin coated on FTO glass and flexible ITO-PET foils, respectively, and cured at 150°C. The optical properties of the deposited pigment coatings on FTO glass substrates were determined with an UV VIS spectrometer, providing total direct transmittance (TT), total diffuse transmittance (DT) and haze (in %). Haze increased with the coating thickness but did not exceed 4%. SEM micrographs of the deposited pigment coatings revealed that they consisted of agglomerated Ni1−xO pigment with voids.Potential cycling of Ni1−xO pigment coatings and Ni1−xO thin films was performed in 1M LiClO4/PC and 0.1M TBA+ triflate/PC electrolytes, revealing that the pigment coatings exhibited electrochemical and electrochromic activity also in lithium free (aprotic) electrolyte, while variations of the oxidation and reduction reaction peaks at various scan rates confirmed the presence of surface electrochemical reactions. The near-grazing incidence angle reflection–absorption spectroscopic technique (IR RA) was employed for identifying surface and inner-grain Ni–O stretching modes, the former being responsible for observed electrochemical properties, which was also demonstrated from the ex situ IR RA measurements performed for coatings charged/discharged in 1M LiClO4/PC and 0.1M TBA+ triflate/PC electrolytes. The results disclosed in this study suggest that the electrochromic effect was not in direct correlation with the de-insertion/insertion of small ions into the NiO (bunsenite) grains but rather the pigment coatings behave similarly to nickel oxide electrochemical capacitors. The electrochromic response of a flexible EC devices made either of PEDOT (self-standing foil) or WO3 and Ni1−xO pigment coatings glued with PMMA based electrolytes without and with lithium salt were recorded for the demonstration of the possible practical application of wet deposited Ni1−xO pigment coatings made as “electrochromic” paints.

Modeling dendrite growth during lithium electrodeposition at sub-ambient temperature

Increased propensity for dendritic lithium electrodeposition during sub-ambient temperature operation has been widely reported in lithium battery systems, yet is not fully understood. In the present paper, a mathematical model is developed to quantify the dendritic growth rate during lithium electrodeposition at sub-ambient temperature. This model builds on a diffusion–reaction framework presented recently by Akolkar [J. Power Sources 232 (2013) 23–28]. Using a steady-state diffusion model with a concentration-dependent diffusion coefficient, the lithium-ion concentration depletion in the stagnant Nernst diffusion boundary layer near the lithium surface is modeled. A surface electrochemical reaction model is then employed to correlate the lithium concentration depletion to the dendrite growth rate. Temperature effects on the lithium-ion transport and its electrochemical surface reaction are incorporated in the model via an Arrhenius-type temperature-dependence of the diffusion coefficient and the apparent charge transfer coefficient. It is shown that lowering the system temperature has the effect of increasing the lithium-ion diffusion resistance and decreasing the surface film thickness – conditions favorable for the formation of dendrites during lithium electrodeposition.

A rapid analytical assessment tool for three dimensional electrode microstructural networks with geometric sensitivity

Electrochemical fin theory is applied to the microstructural analysis of Sr2Fe1.5Mo0.5O6−δ (SFM), a redox stable solid oxide fuel cell (SOFC) electrode. The electrode microstructure is imaged by X-ray nanotomography, then partitioned into a network of resistive components with distinct geometric characteristics. The network is analyzed using an analytical electrochemical fin network model. The resulting predictions of electrode performance are compared to predictions made using three-dimensional finite element simulations of charge transport with surface electrochemical reactions in the imaged microstructure. For a representative subvolume of the structure, the electrochemical fin and finite element models provide comparable predictions. Analysis of larger representative volume elements extracted from the X-ray nanotomography data demonstrates good agreement with experimental measurements of the electrodes analyzed. Finally, advantages of applying the analytical electrochemical fin network models to real microstructures are addressed, particularly with respect to significant reduction in memory requirements and computational time. The use of the electrochemical fin theory is able to rapidly analyze real microstructures with microstructural details that are comparable to finite element and lattice Boltzmann methods, but at volume sizes that finite element and lattice Boltzmann methods were not able to perform due to limits in memory and computational time.

Percolation Theory in Solid Oxide Fuel Cell Composite Electrodes with a Mixed Electronic and Ionic Conductor

Percolation theory is generalized to predict the effective properties of specific solid oxide fuel cell composite electrodes, which consist of a pure ion conducting material (e.g., YSZ or GDC) and a mixed electron and ion conducting material (e.g., LSCF, LSCM or CeO2). The investigated properties include the probabilities of an LSCF particle belonging to the electron and ion conducting paths, percolated three-phase-boundary electrochemical reaction sites, which are based on different assumptions, the exposed LSCF surface electrochemical reaction sites and the revised expressions for the inter-particle ionic conductivities among LSCF and YSZ materials. The effects of the microstructure parameters, such as the volume fraction of the LSCF material, the particle size distributions of both the LSCF and YSZ materials (i.e., the mean particle radii and the non-dimensional standard deviations, which represent the particle size distributions) and the porosity are studied. Finally, all of the calculated results are presented in non-dimensional forms to provide generality for practical application. Based on these results, the relevant properties can be easily evaluated, and the microstructure parameters and intrinsic properties of each material are specified.

Highly ordered mesoporous phenol–formaldehyde carbon as supercapacitor electrode material

Highly ordered, body-centered cubic mesoporous phenol–formaldehyde carbon C-FDU-16 is prepared by a facile evaporation induced self-assembly strategy under a basic aqueous condition. Afterwards, in order to increase the active site of surface electrochemical reactions and promote the wettability in aqueous electrolyte, a simple chemical surface modification is carried out on the C-FDU-16 by nitric acid treatment. The C-FDU-16 and the acid-modified C-FDU-16 (N-FDU-16) are used as the active materials for supercapacitors. Although C-FDU-16 has unique 3D mesoporous network combined with high specific surface area, it do not show well supercapacitor properties for its bad wettability with the aqueous electrolyte. However, the N-FDU-16 with acid treatment exhibits the largest specific capacitance of 219 F g−1 at a current density of 0.1 A g−1 in 1 M H2SO4 electrolyte which is 7 times larger than that of the C-FDU-16 and it also shows good cycling stability as well. Therefore, based on the above investigations, ordered mesoporous phenol–formaldehyde carbon such as C-FDU-16 can be a potential candidate for supercapacitors after the simply acid treatment.

Tailoring nanostructured catalysts for electrochemical energy conversion systems

AbstractThis review covers topics related to the synthesis of nanoparticles, the anodic and cathodic electrochemical reactions and low temperature electrochemical energy devices. The thermodynamic aspects of nucleation and growth of nanoparticles are discussed. Different methods of chemical synthesis such as w/o microemulsion, Bönnemann, polyol and carbonyl are presented. How the electrochemical reactions take place on the surface of the catalytic nanoparticles and the importance of the substrate is put in evidence. The use of nanomaterials in low temperature energy devices such as H2/O2 polymer electrolyte or proton exchange membrane fuel cell (PEMFC) and micro-direct methanol fuel cell (μDMFC), as well as recent progress and durability, is discussed. Special attention is given to the novel laminar flow fuel cell (LFFC). This review starts with the genesis of catalytic nanoparticles, continues with the surface electrochemical reactions that occur on them, and finally it discusses their application in electrochemical energy devices such as low temperature fuel cells or Li-air batteries.

Electrochemically induced charge transfer in platinum–silicon heterosystems

AbstractElectrochemical cyclovoltammetry experiments were carried out on the platinum surface of a platinum–silicon thin‐film device in sulfuric acid. The internal electronic barrier of the device allows the separation of ground state and excited state charge transfer events released by the electrochemical surface reaction. The traces of the device current show similar features as the cyclovoltammogram and can be discussed in terms of small deviations from the equilibrium state of the thin platinum film. The device current plotted as a function of the electrode potential shows a well‐defined shift of the baseline when hydrogen is adsorbed. This baseline shift may be originated by an adsorbate‐induced change of the chemical potential of the platinum film, thus influencing the internal field in the device (up to values of 23 mV).magnified imagePlot of the electrochemical current and the device current as functions of the electrode potential.