Evaluation of GaN and In0.2Ga0.8N Semiconductors as Potentiometric Anion Selective Electrodes

The response of potentiometric anion selective electrodes consisting of undoped GaN or In_0.2Ga_0.8N films grown on Al₂O₃ (sapphire) was measured in electrolyte solutions of F^?, NO₃^?, Cl^?, SCN^?, ClO₄^? or Br^? anions at concentrations ranging from 10^?6 to 10^?1 M. The slope of the linear regions varied between ?32.8 and ?51.9 mV/decade for the GaN electrode and between ?31.0 and ?72.0 mV/decade for the In_0.2Ga_0.8N electrode. The drift of the GaN electrode reached 1.57 mV/day in KNO₃ solutions, whereas the drift of the In_0.2Ga_0.8N electrode could not be evaluated due to large drops in the slope of its linear range over time. Both electrodes were sensitive to pH variations over the pH range from 12.8 to 1.3. The GaN electrode surface could be electrochemically etched under anodic polarization; however, both GaN and In_0.2Ga_0.8N electrodes remained chemically stable and mechanically intact under open circuit conditions even after prolonged use.     

In-depth analysis of InAlN/GaN HEMT heterostructure after annealing using angle-resolved X-ray photoelectron spectroscopy

During high-electron-mobility transistor elaboration process, a thermal treatment of In_0.2Al_0.8N (InAlN) barrier layer is performed in order to improve electrical performances. We showed previously that In_0.2Al_0.8N/GaN heterostructures, annealed at 850°C under O₂ partial pressure, present a specific in-depth organization. Angle-resolved X-ray photoelectron spectroscopy is a powerful tool to precisely determine the spatial localization and relative position of the different interfaces, from InAlN until buried GaN layer. The proposed in-depth model of the stack evidences (1) an Al-rich surface oxide with embedded N₂ molecules, (2) an interlayer of InAlN_<1 governed by nitrogen lattice defects, (3) a stable In_0.2Al_0.8N matrix, and finally (4) the GaN buffer layer underneath.     

Study of transition metal oxide doped LaGaO3 as electrode materials for LSGM-based solid oxide fuel cells

Transition metal oxide doped lanthanum gallates, La_0.9Sr_0.1Ga_0.8M_0.2O₃ (where M=Co, Mn, Cr, Fe, or V), are studied as mixed ionic-electronic conductors (MIECs) for electrode applications. The electrochemical properties of these materials in air and in H₂ are characterized using impedance spectroscopy, open cell voltage measurement, and gas permeation measurement. Three single cells based on La_0.9Sr_0.1Ga_0.8 Mg_0.2O₃ (LSGM) electrolyte (1.13 to 1.65 mm thick) but with different electrode materials are studied under identical conditions to characterize the effectiveness of the lanthanum gallate-based MIECs for electrode applications. At 800 °C, a single cell using La_0.9Sr_0.1- Ga_0.8Co_0.2O₃ as the cathode and La_0.9Sr_0.1Ga_0.8Mn_0.2O₃ as the anode shows a maximum power density of 88 mW/cm², which is better than that of a cell using Pt as both electrodes (20 mW/cm²) and that of a cell using La_0.6Sr_0.4CoO₃ (LSC) as the cathode and CeO₂-Ni as the anode (61 mW/cm²) under identical conditions. The performance of LSGM-based fuel cells with MIEC electrodes may be further improved by reducing the electrolyte thickness and by optimizing the microstructures of the electrodes through processing. Received: 9 January 1998 / Accepted: 1 May 1998     

Characterization of Electrode/Electrolyte Interface of ECIS Devices

Electric cell‐substrate impedance sensing requires low electrode/electrolyte interface impedance for effective biomedical and biophysical applications. Thus a complete understanding of physical processes involved in the formation of an electric double layer is required to design a low interface impedance device. This paper presents the numerical simulation of the impedance for the electrode/electrolyte interface of three‐electrode devices along with the practical realization for the effective workout of impedance sensing devices. The three‐electrode based impedance sensing devices along with phosphate buffered saline as electrolyte is simulated using COMSOL Multiphysics to evaluate the impedance of the electrode/electrolyte interface. Microfabrication technology is used to realize three‐electrode impedance sensing devices with diverse configuration which are used to measure the electrode/electrolyte interface impedance. The measured impedance data were then compared with the COMSOL simulated results and it is found that both the data sets fitted well with less than 5 % RSE. The results obtained from simulation and experiments indicate that the impedance due to double layer diffusion dominates in the low frequency region up to few kHz whereas electrolytic bulk resistance plays a major role in the higher frequency range. The experimental impedance data were further interpreted by electrochemical impedance spectroscopy analysis software to model the equivalent circuit of the electrochemical system.     

Behavior of manganite electrodes in contact with LSGM electrolyte: the nature of low electrochemical activity

The electrochemical cells with electrodes based on La_0.8Sr_0.2MnO₃ (LSM) and supporting solid electrolytes La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 (LSGM) and Ce_0.80Sm_0.20O_1.90 (SDC) were studied comparatively. Characteristics of LSM electrodes and composite electrodes comprising a mixture of LSM and electrolytes of different origins [LSGM, SDC, and Zr_0.82Sc_0.18O_1.91 (SSZ) in the mass ratio of 1:1] were analyzed. It was shown that: 1) the electrode polarization conductivity and the ohmic resistance of the cells with the LSM–LSGM composite electrodes on the LSGM and SDC electrolytes had very similar values, while they were largely different from all the other electrodes, 2) the electrochemical activity of the electrodes on the SDC electrolyte was much higher than on the LSGM electrolyte, and 3) the ohmic resistance of the cells with the SDC electrolyte corresponded to the electrolyte resistance, whereas, the ohmic resistance of the cells with the LSGM electrolyte was much larger than the electrolyte resistance. The obtained results are due to the interaction between the LSM and LSM-containing electrodes with the LSGM electrolyte during sintering, leading to the formation of a product with a very low conductivity.     

Electrocatalytic properties of La0.9Sr0.1MnO3-based electrodes for oxygen reduction

Electrochemical reduction of oxygen at the interface between a La_0.9Sr_0.1MnO₃ (LSM)-based electrode and an electrolyte, either yttria-stabilized-zirconia (YSZ) or La_0.8Sr_0.2Ga_0.9Mg_0.1O₃ (LSGM), has been investigated using DC polarization, impedance spectroscopy, and potential step methods at temperatures from 1053 to 1173 K. Results show that the mechanism of oxygen reduction at an LSM/electrolyte interface changes with the type of electrolyte. At an LSM/YSZ interface, the apparent cathodic charge transfer coefficient is about 1 at high temperatures, implying that the rate-determining step (r.d.s.) is the diffusion of partially reduced oxygen species, while at an LSM/LSGM interface the cathodic charge transfer coefficient is about 0.5, implying that the r.d.s. is the donation of electrons to atomic oxygen. The relaxation behavior of the LSM/electrolyte interfaces displays an even more dramatic dependence on the type of electrolyte. Under cathodic polarization, the current passing through an LSM/YSZ interface increases with time whereas that through an LSM/LSGM interface decreases with time, further confirming that it is the triple phase boundaries (TPBs), rather than the surface of the LSM or the LSM/gas interface, that dominate the electrode kinetics when LSM is used as an electrode. Electronic Publication     

Electrochemical evaluation of La2NiO4+δ -based composite electrodes screen-printed on Ce0.8Sm0.2O1.9 electrolyte

La₂NiO_4+δ , 60 wt.% La₂NiO_4+δ –40 wt.% La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ , and 60 wt.% La₂NiO_4+δ –40 wt.% Ce_0.8Sm_0.2O_1.9 electrodes were prepared from fine powders on dense Ce_0.8Sm_0.2O_1.9 electrolyte substrates by screen-printing technique. Electrochemical impedance spectroscopy and chronopotentiometry techniques were employed to evaluate the electrochemical properties of the composite electrodes in comparison with the La₂NiO_4+δ electrode. For the three electrodes, main electrode processes were resolved to be charge-transfer at the electrode/electrolyte interface and oxygen exchange on the electrode surface. The contribution of the surface oxygen exchange process was detected to be dominant for the overall electrode polarization. The addition of Ce_0.8Sm_0.2O_1.9 into La₂NiO_4+δ was favorable for the charge transfer process whereas it was undesired for the surface oxygen exchange process. On comparison, adding La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ into La₂NiO_4+δ was found to benefit both the two electrode processes. The La₂NiO_4+δ -La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ composite electrode showed optimum electrochemical properties among the three electrodes. At 800 °C, the composite electrode achieved a polarization resistance of 0.20 Ω cm², an overpotential of 45 mV at a current density of 200 mA cm^?2, together with an exchange current density of ～200 mA cm^?2.     

Generation of high incident photon to current conversion efficiency by semiconductor sensitization of microcrystals of In2S3 on wide bandgap In2O3 pellets

Generation of anodic photocurrent on In₂S₃|In₂O₃ electrodes was explained from the viewpoint of semiconductor sensitization. A very high incident photon to current conversion efficiency of 80% was achieved at an In₂S₃|In₂O₃ electrode in a polysulfide electrolyte.     

Characteristics of the La0.6Sr0.4Fe0.8Co0.2O3 − δ electrode in contact with the lanthanum gallate-based electrolyte

Lowering the working temperature of solid oxide fuel cells (SOFCs) is the main trend in their development, which requires selection of materials for electrolyte and electrodes. A highly conducting lanthanum gallate-based electrolyte is a promising material for creating medium-temperature SOFCs. The electrochemical characteristics of the La_0.6Sr_0.4Fe_0.8Co_0.2O_{3 ? δ} cathode that contacted with the La_0.88Sr_0.12Ga_0.82Mg_0.18O_2.85 electrolyte subject to electrode formation temperatures have been investigated. It was found that at optimum bake-on temperatures of 1200–1250°C, the cathode polarization resistance at 800°C was ～0.08 Ohm cm², which is comparable to the world’s best achievements.     

Mechanism of anodic oxidation of tungsten in neutral sulphate-fluoride solutions

The anodic oxidation of tungsten has been studied in 1 M Na₂SO₄ solutions containing 0–0.25 M NaF. Steady-state currents measured in the passivation and passivity ranges increase significantly with increasing fluoride concentration, indicating enhanced dissolution of the oxide film. The electrochemical impedance response is dominated by the processes in the barrier layer and at its interface with the electrolyte. The presence of a pseudo-inductive loop in the impedance spectra at intermediate frequencies indicates point defect interaction during film growth and dissolution processes. A kinetic model including the recombination reaction between oppositely charged point defects at the film/solution interface as well as a kinetic scheme for tungsten dissolution through the film mediated by cation vacancies is proposed. It is found to reproduce satisfactorily the steady-state currents and the impedance spectra in the potential range 0.2–2 V. Such a model for the conduction mechanism in the barrier layer is believed to be an essential part of a modelling approach to the formation of a nanoporous overlayer on tungsten in fluoride-containing solutions.     

Electrochemical oxidation of catechol at poly(indole-5-carboxylic acid) electrode

In this work we examined the electrochemical properties of poly(indole-5-carboxylic acid), PIn₅COOH. The polymer was produced by electrochemical polymerisation using cyclic voltammetry (CV). It was shown that PIn₅COOH is electroactive in aqueous solutions showing two redox processes in acidic solution and one redox process in solutions with pH > 4. The oxidation of catechol (CT) on Pt/In₅COOH modified electrodes was investigated by cyclic voltammetry (CV) and rotating disc electrode (RDE) voltammetry. It was established that CT was oxidised only after the oxidation of polymer film was initiated and that polymer significantly enhanced the oxidation and reduction peak currents in comparison with bare Pt electrode. The variation of peak currents (i _pa, i _pc) as a function of CT concentration was found to be linear up to 6 mM. Experiments with a rotating disk electrode show that the oxidation reaction of catechol occures not only at the polymer/electrolyte interface but also in the polymer film.     

Studies on the effects of coated Li2CO3 on lithium electrode

Although a lithium metal anode has a high energy density compared with a carbon insertion anode, the poor rechargeability prevents the practical use of anode materials. A lithium electrode coated with Li₂CO₃ was prepared as a negative electrode to enhance cycleability through the control of the solid electrolyte interface (SEI) layer formation in Li secondary batteries. The electrochemical characteristics of the SEI layer were examined using chronopotentiometry (CP) and impedance spectroscopy. The Li₂CO₃-SEI layer prevents electrolyte decomposition reaction and has low interface resistance. In addition, the lithium ion diffusion in the SEI layer of the uncoated and the Li₂CO₃-coated electrode was evaluated using chronoamperometry (CA).     

Peculiarities of the adsorption behavior of adamantanol-1 on a mechanically renewable silver electrode

Using the method of electrochemical measurements on electrodes with mechanically renewable surface, the adsorption behavior of adamantanol-1 (AdOH) on the Ag electrode interface with solutions of a surface-inactive electrolyte (NaF) is studied. Based on the results of impedance and voltammetric measurements, it is shown that the kinetics of AdOH adsorption in the ideal polarizability potential range (near the zero charge potential) is described within the framework of the mechanism of the quasichemical reaction Ag(H₂O)_ads + AdOH = Ag(AdOH)_ads + H₂O on metal surfaces with energy uniformity. A phenomenological model is proposed that makes it possible to consistently describe the temporal effects on renewable Ag electrodes in the potential range of its initial oxidation in solutions containing AdOH.     

Properties of Ag/AgCl electrodes fabricated with IC-compatible technologies

The purpose of this work is to fabricate and characterize Ag/AgCl electrodes made on a silicon chip at the wafer level with integrated circuit-compatible fabrication techniques. Such electrodes are useful as reference electrodes in several kinds of chemical sensors. Two types of electrode were investigated. The first type uses an evaporated AgCl layer that is patterned with lift-off photolithography. The second type is formed by exposing a selected part of the silver substrate to a KCrO₃Cl solution. Both types of electrode give the thermodynamically expected potential response to variations of Cl⁻ ion concentration. The potential generated by the KCrO₃Cl-formed electrodes was more stable, however. Auger electron spectroscopy depth profiles indicate that immersion in a KCrO₃Cl solution produces a thin layer of AgCl on top of a layer of AgO. The low electronic resistance of AgO then reduces the measured series resistance of the KCrO₃Cl-formed electrodes. Impedance plane plots and the impedance as a function of frequency were measured for both types of electrode, and the impedance of the evaporated AgCl electrodes was indeed considerably higher. The impedance measurements could be successfully modelled by assuming a Randles equivalent circuit for the AgCl/electrolyte interface. For the KCrO₃Cl-formed electrodes, the impedance was modified by the porosity these electrodes manifested.     

A combined μ-mercury reference electrode/Au counter-electrode system for microelectrochemical applications

Different types of mercury-based μ-reference electrodes (Hg/Hg₂SO₄/Na₂SO₄, Hg/Hg₂(CH₃COO)₂/NaCOOCH₃) have been developed following the concept of agar-based μ-reference electrodes. Mercury was electrochemically deposited onto a gold wire to form an amalgam. The corresponding mercury salt was formed electrochemically at the surface. This electrode can be inserted into a capillary that is filled with the electrolyte of interest. To simplify the handling of this μ-reference electrode, to reduce diffusion and to avoid leakage, the electrolyte was immobilised with agar. A 250-nm-thick gold layer on the outer surface of the capillary of the reference electrode served as counter-electrode. The electrochemical behaviour of reference electrodes and counter-electrodes were proven by micro-polarisation curves, electrochemical impedance spectroscopy, potential transients and cyclic voltammetry.     

Chemical interaction and diffusion on interface cathode/electrolyte of SOFC

The high-temperature solid oxide fuel cell (SOFC) is suited for the environmentally acceptable and efficient conversion of chemical into electric energy. A prerequisite for introducing this technology on the market is the controlled formation of the interface between electrodes and the electrolyte. In the case of using an electrolyte based on LaGaO₃ the formation of third phases and the diffusion of individual metallic cations from and to the electrolyte was investigated with the aid of point analyses on micrographs of the environment of the interface using quantitative EDS analysis. In case of an anode of Ni-CeO₂ cermet the mixed oxide SrLaGa₃O₇ is formed and, in addition, a relatively pronounced transport of La from the electrolyte into the CeO₂ phase was observed. A relatively strong diffusion of Mn and an even stronger diffusion of Co into the electrolyte took place between the cathode of, e.g., La_0.75Sr_0.2Mn_0.8Co_0.2O₃ and the La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ electrolyte, whereas a weak transport of Ga to the cathode was identified.     

Mechanism and kinetics of electrode processes in systems comprising solid polyaluminate electrolyte

Cathodic reduction of organic semiconductors (charge-transfer complexes and radical-ion salts) at interfaces in Na(Hg)/β-Al₂O₃/organic semiconductor systems is studied by inversion voltammetry and chronopotentiometry. Formation of transition layer at the organic semiconductor/solid electrolyte interface is revealed. The mechanism of the charge transfer complex and radical-ion salt cathodic reduction depends on the potential scan rate; the cathodic process at nonmetal electrodes occurs under the conditions of double injection of electronic and ionic charge carriers to electrode bulk.     

Effect of 15-crown-5 on the charge transfer resistance at the polymer electrolyte/modified Li-electrode interface

The effect of 15-crown-5, which is applied immediately to pure and modified surface of a lithium electrode, on the charge transfer resistance at the electrode/polymer electrolyte interface is studied. The polymer electrolyte consists of a 1: 1 mixture of oligourethan dimethacrylate and polypropylene glycol monomethacrylate (20 wt %), an initiator (azobisisobutyronitrile) (2 wt %), and a 1 M LiClO₄ solution in gamma-butyrolactone (78 wt %). The conductivity of this gel electrolyte is 3 × 10^?3 S cm^?1. The temperature dependence of the impedance of the Li/gel electrolyte/Li electrochemical cells is measured for electrodes of four types. The activation energies for the charge transfer at the Li/electrolyte interface are calculated. It is found that, after treating the test lithium electrodes with 15-crown-5, the charge transfer resistance decreases, and in the case of the modified lithium surface, the activation energy for the process decreases by 1.8 times.     

The constant phase element reveals 2D phase transitions in adsorbate layers at the electrode/electrolyte interfaces

Using one of the most understood and well-characterized electrochemical systems, Pt(111) surface in contact with H₂SO₄, we provide evidences that specific adsorption, 2D phase transitions in the adsorbate layers and, in general, structural effects in the double layer are largely responsible for the so-called frequency dispersion of the double layer. The results also show promise that parameters of the constant phase element (which is used in impedance spectroscopy to account for the frequency dispersion) obtained as a function of the electrode potential can be reasonably used to detect 2D phase transitions at the electrode/electrolyte interfaces. This would provide a better insight into the interface, increasing the impact of measurements made by electrochemical impedance spectroscopy.     

Electrochemical properties of nanoporous carbon electrodes in various nonaqueous electrolytes

Electrical double layer and electrochemical characteristics at the nanoporous carbon|acetonitrile interface with additions of Et₄NBF₄, Et₃MeNBF₄, EtMe₃NBF₄, LiClO₄, and LiBF₄ have been studied by cyclic voltammetry and impedance spectroscopy methods. A value of zero charge potential, dependent on the structure of the cations as well as on the composition of the anions, the region of ideal polarizability, and other characteristics has been established. Analysis of the complex plane plots shows that the nanoporous carbon|acetonitrile+0.1 M electrolyte (Et₄NBF₄, Et₃MeNBF₄, or EtMe₃NBF₄) interface can be simulated by the equivalent circuit, in which the two parallel conduction parts in the solid and liquid phases are interconnected by the double layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer process or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte composition and on the electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. In the region of moderate a.c. frequencies, the modified Randles-like equivalent circuit has been used for simulation of the complex plane plots. In the region of negative surface charge densities, the intercalation process of Li⁺ ions from LiClO₄ and LiBF₄ solutions into the surface film is possible and these data can be simulated using the modified Ho et al. model or Meyer et al. model. Electronic Publication