Surface Charge Transfer Reaction Research Articles

Reaction Kinetics of Photoelectrochemical CO2 Reduction on a CuBi2O4-Based Photocathode.

The CO2 reduction reaction (CO2RR) is an essential step in natural photosynthesis and artificial photosynthesis to provide carbohydrate foods and hydrocarbon energy in the carbon-neutral cycle. However, the current solar conversion efficiencies and/or product selectivity of the CO2RR are very sluggish due to its complicated multiple-step charge transfer reactions. Here, we systematically investigate the charge transfer reaction rate during CO2 reduction on CuBi2O4 photocathodes, where the surface is modified with 3-aminopropyltriethoxysilane (APTES). We discover that the surface amine group increases the charge separation rate, significantly enhancing the surface charge transfer reaction rate. However, the surface acidity has less influence on the first-order reaction, indicating that a rate-determining step (RDS) exists in the early stage of the photoelectrochemical cell (PEC) processes. Moreover, the intensity-modulated photocurrent spectroscopy (IMPS) confirms that both surface charge transfer and the recombination rate on APTES-coated CuBi2O4 are larger than bare CuBi2O4 while possessing comparable charge transfer efficiencies. Overall, the surface charge transfer reactions under the PEC condition require designing more effective nanostructured photoelectrodes and powerful characterization methods to intrinsically increase the charge separation and transfer rate while reducing the recombination rate.

Ni(OH)2-CuxO-TiO2 nanocomposite for the enhanced H2 production under solar light: The mechanistic pathway

The synergistic effect of Ni(OH)2 and CuxO on TiO2 enhanced the H2 production 4 times higher than bare TiO2 at the optimum loading of 1.0 wt% Ni(OH)2 and 0.5 wt% CuxO (1.0 wt% Ni(OH)2-CT) under natural sunlight. CuxO-TiO2 heterojunctions improved the visible activity and charge transfer efficiency of TiO2. The layered morphology of non-precious Ni(OH)2 cocatalyst with intercalated H2O molecules enhanced H2 production by improving the surface charge transfer reaction with additional formation of H+ ions through the redox reactions. Based on the characterization and experimental results, the double charge transfer and direct Z-scheme mechanisms were proposed for the charge carrier pathway in CuxO-TiO2 heterojunction. On the whole, the improved visible light response, efficient interfacial charge transfer and production of additional excited free electrons/H+ ions enhanced the overall H2 production on Ni(OH)2-CT, which was also confirmed with various experimental and characterization results.

Kinetic Influence of Surface Charge Transfer Reactions Preceded by Non‐Electrochemical Processes on the Response in Cyclic Voltammetry

AbstractA general analytical explicit expression for the response of a surface charge transfer preceded by a non‐electrochemical process has been deduced. This solution is applicable to any electrochemical technique independent of the rates of both reactions. Interesting simplified cases are obtained for very common experimental situations in which the rates of electrochemical and non‐electrochemical steps are similar. These expressions have been applied to cyclic voltammetry in order to analyze the kinetic influences on the current‐potential response and to propose procedures for determining the rate and equilibrium constants of the mechanism. Moreover, it has been obtained an “apparent” rate constant of the overall process that reduces to a pure non‐electrochemical rate‐limiting process or to a pure electrochemical rate limiting one, under the right conditions. Experimental verification of the theoretical framework has been done with the analysis of the host‐guest complexation between a surface confined electroactive ferrocene moiety and a solution soluble cyclodextrin in a water‐ethanol media. From the dynamic analysis of the influence of the scan rate on the cyclic voltammetry curves, values have been obtained for the binding constant ( ) and rate constants for the inclusion complex formation and dissociation showing the great stability of the electro‐inactive ferrocene‐cyclodextrin complex.

Evaluation of the Influence of Nickel Addition on LST-CGO Based Solid Oxide Fuel Cell Anodes Performance

Ni/YSZ composite materials have shown the tremendous success over past years for application as SOFC anodes. One of the biggest advantages of Ni/YSZ SOFC anode is its ability of direct utilization of various fuel types, i.e. H2/H2O, CO/CO2 and hydrocarbons. It is, however, well known that operation of those anodes upon various fuels could cause various types of cell degradation. Therefore, the replacement of Ni cermet with alternative ceramic materials is essential to develop long-term operating SOFC technology. In the present contribution, we combine modeling and experimental study of electrochemical hydrogen oxidation at an alternative perovskite based mixed-conducting SOFC anode with and without an addition of nickel. Composite electrodes were produced by conventional wet ceramic processing (screen printing – spraying) and sintering on YSZ electrolytes (La0.1Sr0.9TiO3-α-Ce1-xGdxO2-α | YSZ) with different compositions and microstructure, and were electrochemically characterized using symmetrical button-cells configuration. An elementary kinetic model was developed and applied to explore the performance of LST based SOFC anode. A detailed multi-step heterogeneous chemical and electrochemical reaction mechanism was established taking into account transport of ions in all ionic phases, and gas transport in channel and porous media. It was found that heterogeneous chemistry at LST surface has capacitive behavior that alters the impedance spectra. In addition, surface charge-transfer reaction, which describes partial oxygen ionization, caused impedance feature and is rate-limiting at high temperature. However, if nickel is added to composite perovskite anodes the impact of LST surface chemistry is diminished leading to an improvement of cell performance.

Surface Chemistry, Charge Transfer, and Transport in La0.1Sr0.9TiO3-Alpha-Based Solid Oxide Fuel Cell Anodes: Modeling and Experimental Study

Ni/YSZ composite materials have shown the tremendous success over past years for application as SOFC anodes. One of the biggest advantages of Ni/YSZ SOFC anode is its ability of direct utilization of various fuel types, i.e. H2/H2O, CO/CO2 and hydrocarbons. It is, however, well known that operation of those anodes upon various fuels could cause various types of cell degradation. Therefore, the replacement of Ni cermet with alternative ceramic materials is essential to develop long-term operating SOFC technology. In the present contribution, we combine modeling and experimental study of electrochemical hydrogen oxidation at an alternative perovskite based mixed-conducting SOFC anode. Composite electrodes were produced by conventional wet ceramic processing (screen printing – spraying) and sintering on YSZ electrolytes (La0.1Sr0.9TiO3-α-Ce1-xGdxO2-α | YSZ) with different compositions and microstructure, and were electrochemically characterized using symmetrical button-cells configuration. An elementary kinetic model was developed and applied to explore the performance of LST based SOFC anode. A detailed multi‐step heterogeneous chemical and electrochemical reaction mechanism was established taking into account transport of ions in all ionic phases, and gas transport in channel and porous media. It was found that heterogeneous chemistry at LST surface has capacitive behavior that alters the impedance spectra. In addition, surface charge-transfer reaction, which describes partial oxygen ionization, caused impedance feature and is rate-limiting at high temperature. The gas transport in the supply chamber (gas conversion) is significant only at moderate temperatures.

Towards understanding surface chemistry and electrochemistry of La0.1Sr0.9TiO3-α based solid oxide fuel cell anodes

In the present contribution, we combine modeling and experimental study of electrochemical hydrogen oxidation at an alternative perovskite based mixed-conducting SOFC anode. Composite electrodes were produced by conventional wet ceramic processing (screen printing – spraying) and sintering on YSZ electrolytes (La0.1Sr0.9TiO3-α-Ce1-xGdxO2-α|YSZ) with different compositions and microstructure, and were electrochemically characterized using symmetrical button-cells configuration. An elementary kinetic model was developed and applied to explore the performance of LST based SOFC anode. A detailed multi-step heterogeneous chemical and electrochemical reaction mechanism was established taking into account transport of ions in all ionic phases, and gas transport in channel and porous media. It was found that heterogeneous chemistry at LST surface has capacitive behavior that alters the impedance spectra. In addition, surface charge-transfer reaction, which describes partial oxygen ionization, caused impedance feature and is rate-limiting at high temperature. The gas transport in the supply chamber (gas conversion) is significant only at moderate temperatures.

Thermally driven metastable solid-solution Li0.5FePO4 in nanosized particles and its phase separation behaviors

Nanosized LiFePO4 particles easily show a fast electrochemical response that can be achieved via a non-equilibrium pathway. To understand this intriguing phase transition behavior in nanosized LiFePO4 particles, the metastable solid-solution phase was prepared by thermal treatment with a chemically delithiated nanosized Li0.5FePO4 sample. Thermal treatment makes all the nanosized particles transform easily to the metastable solid-solution phase because of the large thermal energy while an electrochemical reaction does not. The phase separation behavior of the metastable solid-solution sample (Li0.5FePO4) was investigated under various kinetic conditions to understand critical factors affecting the phase separation behavior of nanosized LiFePO4 particles. The main findings in this study are as follows. The first finding is that the depressed phase separation behavior of the metastable phase may originate from the nanoparticle effect, in which the formation of a second phase inside a nanosized particle is not energetically favored because of the large interfacial energy. Therefore, phase separation in nanosized particles occurs between particles rather than inside a particle. If there was no over-potential, such as in the relaxed pellet experiment or in the relaxed electrode experiment in the electrolyte, the metastable phase was quite stable showing no phase separation behavior even though efficient pathways for lithium ions and electrons were well developed. The second finding is that the phase separation behavior of the metastable phase actually depends on the over-potential. Under open circuit voltage (OCV) conditions, the metastable phase started to exhibit a slight structural change during a long relaxation time, about ten days. The slow change of the metastable phase may be due to the low driving force, less than 10 mV, which comes from the energetic difference between the two-phase state and the metastable phase. This indicates that the phase separation behavior may require a large over-potential. When a large over-potential was applied using an external current, phase separation of the metastable phase was achieved, indicating that the phase separation behavior may be related to activation processes. Furthermore, the requirement for a large over-potential indirectly shows that the spinodal decomposition is depressed in nanosized particles. Considering that phase separation in nanosized particles occurs between particles, the surface charge transfer reaction can be a limited reaction for achieving phase separation because it is an activated process and governed by the over-potential. Considering the understanding obtained from the phase separation behavior of the metastable phase, the phase transition behavior of nanosized LiFePO4 particles during charging/discharging can proceed via the metastable phase because there is no spinodal decomposition behavior in nanosized particles and the metastable phase is quite stable.

An analytical method for dye-sensitized solar cell geometric design

Novel fabrication techniques have enabled increasingly controlled microstructural geometries in electrochemical energy devices, including dye-sensitized solar cells. An analytical tool, based on an electrochemical analogy to thermal fin analysis, has been developed to support the design of such microstructures. An adapted version of this tool is presented for analyzing the performance of microstructural geometry in dye-sensitized solar cells. This electrochemical fin model is capable of describing experimentally observed performance gains associated with specific microstructural geometries. The assessment of structures based on nanorod and nanoparticle architectures is addressed, and insights into trade-offs between surface charge transfer reactions and diffusive transport in solid photoelectrode phases are discussed.

Ozone Formation from Illuminated Titanium Dioxide Surfaces

The nitrogen oxides (NO(x)) decomposition on illuminated TiO(2) surfaces has been widely studied, but little is known about the subsequent formation of non-nitrogen containing products. In this study, TiO(2) coated glass surfaces and TiO(2) films with nitrate anions (either premixed with TiO(2) as KNO(3) or deposited from gaseous NO(x)) are irradiated with broad-band light. Upon irradiation, detected gas phase products include NO(2), HNO(2), and O(3). To the best of our knowledge, this is the first study that reveals the production of O(3) from TiO(2) surfaces. By surface charge transfer reactions, nitrate anions are oxidized into nitrate radicals and their photochemistry (almost in the visible) leads to O(3) formation, enhancing the oxidizing power of these surfaces.

Changes in Surface Stress of Platinum Electrode in Acidic and Alkaline Sulfate or Alkaline Fluoride Solutions

The changes in surface stress of a platinum electrode in acidic and alkaline sulfate or alkaline fluoride solutions were measured by the bending-beam method in connection with the surface oxygenation reaction of platinum. The surface stress exhibited a maximum in the electrical double layer potential region of platinum. The potential of the surface stress maximum obtained in the anodic potential sweep showed the dependence of −40 mV/pH in acidic sulfate solution and of −90 mV/pH in alkaline sulfate and fluoride solutions, respectively, indicating the strong specific adsorption of ion on platinum in alkaline solutions. It was found that the derivative of surface stress with electrode potential showed a maximum in the potential range corresponding to the onset of the surface oxygenation reaction for formation of and deviated upward from the surface charge density estimated from the cyclic voltammogram. The potential at which the derivative of surface stress with electrode potential showed a maximum, gave a dependence of −68 mV/pH over a wide pH range, irrespective of the anionic species (sulfate or fluoride ions). It has been pointed out that the difference between the derivative of surface stress with electrode potential and the surface charge density increases when the surface charge transfer reaction of an adsorbed ion such as surface oxygenation reaction takes place on a noble metal. © 2003 The Electrochemical Society. All rights reserved.

Experimental evidence of vibrational selectivity in surface charge-transfer reactions

Surface (charge-transfer) reactions from polyatomic molecules adsorbed on metals have been investigated by infrared radiation. Examples of laser catalysis on adsorbed SF6, C3H3N (ACN) and CH3I on different surfaces are reported in which clear catalytic products were detected and analyzed. A detailed study of ACN adsorbed on Cu was carried out. Thus, the electron emission and CN− yield produced by laser irradiation of acrylonitrile (ACN) adsorbed on a polycrystalline Cu surface were investigated over the 930–953 cm−1 wavenumber range. Both emissions show a clear wavelength dependence with a≈20 cm−1 redshift with repsect to the gas-phase ACN spectrum. The laser catalysis CN− yield is discussed in light with both substrate and adsorbate mediated mechanisms, which seem to be controlling this (charge-transfer) surface reaction. Emphasis is made on the observed vibrational selectivity for this surface reaction.

Impedance of metal hydride electrodes. Rate of phase transformation limited by nucleation and growth mechanisms

A mathematical model for the electrochemical impedance spectroscopy of a metal hydride electrode was developed. Ac impedance data of phase transformation were derived by considering a nucleation and growth mechanism based on the theory developed by Johnson–Mehl–Avramy. Different mechanisms such as grain edge and grain boundary preferential nucleation sites are discussed. Global Nyquist plots of the metal hydride electrode are obtained by adding a surface charge transfer reaction and a double-layer capacitance to the model.

Study by cyclic voltammetry of a reversible surface charge transfer reaction when the reactant diffuses to the electrode

A general theoretical equation for the I - E curve of cyclic voltammetry for a reversible surface charge transfer reaction has been established at low scan rates. It is considered that the substrate diffuses to the hanging mercury drop electrode and is adsorbed during the potential sweep. The molecules of both oxidized and reduced species are assumed to be strongly adsorbed on the electrode surface, following a Langmuir isotherm, and to form a non-compact monolayer. From this equation, the theoretical expressions for cathodic and anodic peak currents have been obtained. The equations for the I - E curve, the peak potentials and the peak currents when the film of adsorbed molecules is compact before reduction or oxidation in the negative or positive scan have also been deduced. The validity of the equations obtained has been confirmed by means of an experimental study of the benzo( c)cinnoline—dihydrobenzo( c)cinnoline system in a basic aqueous organic medium at low scan rates and substrate concentrations.

Study of the impedance of the water—1,2-dichloroethane interface: Influence of the picrate ion transfer

Impedance measurements of the water—1,2-dichloroethane (1,2-DCE) interface in the presence of supporting electrolytes in both phases and picrate ions in the organic phase have been carried out using ac voltammetric and ac bridge methods. The analysis of the impedance data has been performed according to a simple electrical equivalent circuit. An improved method for the evaluation of these model components has been applied. It has been found that the capacity of the interface has a higher value than that calculated from the Gouy—Chapman theory. This can be caused by the surface charge transfer reactions or by the limited application of Gouy—Chapman theory in the system studied, considered by other authors. The interface capacity found in the presence of picrate ions in the organic phase has higher values than that observed in the presence of supporting electrolytes only.