Electrochemical solid electrolyte gas sensors — hydrocarbon and NOx analysis in exhaust gases

For the in situ measurement of the free oxygen concentration and the equilibrium oxygen partial pressure oxygen sensors based on zirconia solid electrolytes are widely used in order to monitor and control technical high temperature processes. Similarly combustibles (HC, CO) and NO_x can be determined in non equilibrated oxygen containing gas mixtures of exhausts by mixed potential sensors and amperometric solid electrolyte sensors. It is expected that their long-term stability is similar to that of oxygen sensors. In both cases the electrode material with the desired electrochemical and catalytic properties is the key component. Different electrode materials made of perovskites (La_1-xSr_xCr_1-yGa_yO_3-δ) and composites (Au/Metal oxide) were investigated in different combustibles including CO, C₃H_6/8, C₇H₈ and CH₄. The response behaviour of mixed potential sensors is determined by the catalytic activity of the measuring electrode, which is closely connected with the defect structure and depends on the measuring conditions. Furthermore the electrode response can be understood by electrokinetic data. Gas symmetrical mixed potential sensors with electrodes made of Au/Nb₂O₅ composites show maximum sensitivity. By using Ptreference electrodes without equilibrium behaviour the sensors are applicable in lean and rich mixtures as well. In the amperometric sensor mode the consecutive determination of oxygen and NO_x or combustibles at two working electrodes is possible. The catalytic activity of the oxygen pumping electrode should be low in order to avoid the decomposition of NO and HC respectively. Alternatively, the electrochemical reduction of NO can be performed at a single working electrode, made of materials with improved NO selectivity, without the previous reduction of oxygen. Paper presented at th 8th EuroConference on Ionics, Ixia, Rhodos, Greece, Sept. 15–21, 2002.     

Oxygen electrodes of zirconia electrolytes: fundamentals and application to analysis of oxygen containing gases

The kinetics of the oxygen exchange reaction and the reduction of NO at La_0.8Sr_0.2CoO₃−, La_0.8Sr_0.2MnO₃− and Ag-electrodes on stabilized zirconia (8mol% Y₂0₃=YSZ) has been studied by means of electrochemical methods (impedance, I-U characteristics). For La_0.8Sr_0.2CoO₃ electrodes the oxygen exchange was found to proceed via the bulk of the electrode with a rate limiting oxygen exchange at the electrode surface. Electrodes based on La_0.8Sr_0.2MnO₃ change their electrode characteristics with the applied potential. At low cathodic polarization the electrode reaction is limited to the three-phase boundary electrode/YSZ/gas. At high cathodic potentials oxygen vacancies are created and consequently additional oxygen is exchanged via the electrode bulk. Furthermore, a significant NO reduction was observed which indicate a reaction with the oxygen vacancies at the electrode surface. For Ag a rate limiting transport of oxygen atoms through the bulk of the electrode was found. As a consequence the oxygen concentration at the electrode surface remains nearly constant. In this context, the observed inactivity for the NO reduction of Ag-electrodes may be explained. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Switching effect at a platinum electrode on stabilized zirconia during oxygen reduction in the presence of NO

A galvanic cell based upon the use of stabilized zirconia as solid oxygen ion conductor has been used to measure cathodic reduction currents at a porous platinum electrode in both nitrogen-oxygen gas mixtures with and without small amounts of NO (up to 5450 ppm). Adding small amounts of NO to the N₂/O₂ mixture induced a considerable cathodic current peak at the working electrode in the first moment after addition. After interruption of the NO exposure, the opposite effect, a high current pulse in anodic direction, was observed. The switching effect is reproducible and its magnitude depends on the concentrations of oxygen and nitrogen monoxide in the gas. As the main contribution to the current results from the reduction of the excess oxygen in the gas, it must be concluded that the presence of NO strongly affects the steady state, in particular the adsorbed oxygen at the electrode/electrolyte interface. These experimental results are interpreted in terms of a reversible change of the interface which may be due to a reconstruction of the platinum surface in the presence of NO and corresponding drastic change in the amount of adsorbed oxygen at the platinum surface. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Nasicon sensor for NO detection in the automotive exhaust

Development of a sensor where both electrodes are exposed to the same gas offers simple design with no need for a separate reference gas with a complicated sealing system. Such sensors are still in research stage. The NASICON electrolyte, which has high ionic conductivity at the temperature of the exhaust gas mixture, allows such sensors to work without a heater, making a simple and miniaturised construction technique. Using two asymmetric electrodes, where each electrode has a different catalytic activity at a given temperature, can result in anemf, which is a function of partial pressure of NO. A NASICON sensor with SmFeO₃ as a sensing electrode and Au as an oxygen electrode was calibrated as a NO sensor and tested in oxidising atmosphere in simulated exhaust gases. The sensor responded quickly with reproducibleemf values. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Behaviour of a dense In2O3/ YSZ electrode in oxygen containing atmospheres

A study of electrical and electrochemical properties of a dense In₂O₃ electrode in contact with a single crystal YSZ electrolyte was carried out by d.c. and a.c. methods. As a result, it was found that dense In₂O₃ electrodes have high electrical conductivity but very low electrochemical activity. In a vicinity of the equilibrium potential and under the anodic polarisation, the rate of Faraday reaction at the In₂O₃ electrodes was as low as to consider the electrode a blocking one. The blocking properties of the In₂O₃ electrodes were used to measure the hole conductivity of the YSZ electrolyte in the temperature range between 795 to 1163 K and oxygen partial pressure from 1 to 10⁵ Pa. A comparison with the literature data confirmed that the dense In₂O₃ electrode blockes the ionic transfer through the YSZ. A set of experiments indicated that the oxygen exchange between the indium oxide surface presented to the oxygen containing gaseous phase and this phase is very poor. A route of the electrode process at O₂, In₂O₃ / YSZ electrode was proposed a limiting stage of which is the discharge of the oxygen ions to the atomic oxygen adsorbed on the electrode surface: $$O_0 ^x \left( {In_2 O_3 } \right)_s = V_0 ^{ \bullet \bullet } \left( {In_2 O_3 } \right)_s + O_{ad} \left( {In_2 O_3 } \right)_s + 2e'\left( {In_2 O_3 } \right).$$ The polarisation resistance decreases when platinum or the praseodymium oxide is deposited on the surface of the In₂O₃ electrodes. The cathodic polarisation also increases the electrochemical activity of the electrodes. Both the establishment of the steady state of the electrode under polarisation and the recovery of the equilibrium state by the electrode are very long processes, which are probably related to the diffusion mechanism by which the stoichiometry of the indium oxide is changed.     

Microscopic observation of oxygen reaction pathway on high temperature electrode materials

Attempts have been made to develop experimental techniques for obtaining microscopic information on high temperature electrodes under current flow. Several techniques are proposed to investigate the geometrical distribution of the series and parallel processes inside the electrode layer. Based on the local equilibrium concept, the contribution of the series processes were studied by probing the local oxygen potential. A microprobe oxygen sensor was developed to measure the oxygen potential profile on a model electrode of (La,Sr)MnO₃ on YSZ. Use of alternative methods that detect the valence state of the cations in the electrode material were also proposed. As an ex-situ technique, active sites for gas–solid oxygen exchange reaction were investigated by using isotope exchange and SIMS (secondary ion mass spectrometry) analysis. The three-dimensional distribution of the isotope ratio was obtained by SIMS with imaging capability. A non-uniform distribution of the reaction rate on the electrode surface was visualized for the electrode material with and without current flow. By the imaging technique, an extraordinary fast oxygen exchange site was found on (La,Sr)₂CoO₄/(La,Sr)CoO₃ boundaries.     

Electrocatalytic properties of glassy carbon electrodes modified with hydroxy derivatives of 9,10-anthraquinone for oxygen reduction reaction

The electrochemical and electrocatalytic behavior of glassy carbon electrodes modified by one mono and four dihydroxy derivatives of anthra-9,10-quinone compounds have been investigated by cyclic voltammetric technique. The stability of the modified electrodes was ascertained in acidic and neutral media. The surface morphology of modified electrode was characterized by scanning electron microscope. The influence of pH on the electrochemical and electrocatalytic behavior was studied and pH?6.0 or 7.0 was chosen as the optimum working pH by comparing the shift in oxygen reduction potential. The anthraquinone-adsorbed glassy carbon electrodes possess excellent electrocatalytic ability for oxygen reduction with overpotential ranging from 388 to 547?mV lower than that at a plain glassy carbon electrode. Hydrodynamic volatammetric studies were performed to determine the heterogeneous rate constants for the reduction of O₂ at the surface of the modified electrodes, mass specific activity of the anthraquinones used, and the apparent diffusion coefficient of O₂ in buffered aqueous O₂-saturated solutions.     

Possibilities of NOx and CHx determination using galvanic cells with perovskite-electrodes on YSZ

Perovskite-type mixed oxides with the formula La_1−xA_xMe_1−yB_yO_3±δ (x=0.01...0.2; y=0.01...0.5; A=Ca, Sr; Me=Cr, Mn, Fe, Co; B=Mg, Ga) were investigated with the aim to use the oxides as electrode materials for galvanic cells with Y₂O₃-stabilized ZrO₂ solid electrolytes (YSZ). The catalytic activity of the oxides for the oxidation, reduction and decomposition of gas components was varied by changing the transition metal and by partial substitution of lanthanum and the metal. The behaviour of different substituted lanthanum chromite/YSZ and manganite/YSZ electrodes in the presence of nitric oxides (NO_x) and combustible components (CH_x) in gases containing oxygen was investigated. Various electrode combinations were tested in gas-symmetrical cells. For the NO determination cells with a catalytically active Pt electrode and a La_0.8Sr_0.2MnO₃ electrode with a low catalytic activity for the decomposition of NO were used. The results show the possible quantitative measurement at low and constant p(O₂) on the basis of a calibration. For the determination of C₃H₆ a CH_x-sensitive Au and an O₂-sensitive La_0.99Sr_0.01CrO₃ electrode were combined in a potentiometric thick film sensor. In gases containing oxygen with small amounts of C₃H₆ the sensor provided a utilizable voltage in dependence of the C₃H₆ concentration. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Oxygen reduction on lanthanum strontium manganites investigated by the use of cone shaped electrodes

The oxygen reduction kinetics has been investigated by impedance spectroscopy on two different cone shaped electrodes of La_0.85Sr_0.15MnO₃ in contact with either YSZ or CGO electrolyte pellets. The contact area has been varied by varying the mechanical load on the electrode on both polarized and unpolarized electrodes. The experiments showed the activation of the electrode observed after a cathodic polarization to be caused by the creation of an active zone on the electrolyte close to the triple phase boundary. As a result of this, the oxygen reduction on polarized electrodes probably takes place by several parallel routes making a kinetic investigation complicated. Impedance analysis of unpolarized electrodes at different oxygen partial pressures showed a charge transfer process at high frequencies involving oxygen defects and a mass transport limited reaction at lower frequencies. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Electrocatalysis of oxygen reduction reaction on Nafion/platinum/gas diffusion layer electrode for PEM fuel cell

In this work a new membrane electrode based on Pt-coated Nafion membrane was fabricated. Chemical deposition process was used to coat platinum on Nafion 117 membrane and then Pt-coated Nafion membrane was hot pressed on gas diffusion layer (GDL) to make new membrane electrode. The electrochemical and chemical studies of the Pt-coated Nafions were investigated by electrochemical techniques, X-ray diffraction and scanning electron microscopy. The electrochemical results indicated that as the concentration of H₂PtCl₆ increased, the oxygen reduction reaction rate increased until the concentration was reached where the reduction reaction was limited by the problem of mass transport. The electrochemical results for oxygen reduction reaction showed that the new electrode which prepared by plating Nafion membrane with 0.06 M H₂PtCl₆ in electroless plating solution, has a higher performance than other electrodes. The XRD results showed that the average platinum particle size of the best sample was about 3 nm. The loading of platinum for this electrode was 0.153 mg cm⁻².     

Nanostructured copper particles-incorporated Nafion-modified electrode for oxygen reduction

The electrocatalytic activity of nanostructured copper particles (represented as Cu_nano) incorporated Nafion (Nf) film-coated glassy carbon (GC) electrode (GC/Nf/Cu_nano) towards oxygen reduction was investigated in oxygenated 0.1 M phosphate buffer (pH 7.2). The electrodeposited Cu_nano in Nf film was characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activity of Cu_nano at the modified electrode towards oxygen reduction was studied using cyclic voltammetry technique. The molecular oxygen reduction at the GC/Nf/Cu_nano-modified electrode started at a more positive potential than at a bare GC electrode. A possible reaction mechanism was proposed in which oxygen reduction may proceed through two-step two-electron processes at the GC/Nf/Cu_nano electrode. The GC/Nf/Cunano electrode shows higher stability for oxygen reduction in neutral solution and the electrode may find applications in fuel cells.     

Electrode reactions of La0.8Sr0.2MnO3±δ-electrodes on stabilized zirconia with oxygen and the nitrogen oxides NO and NO2

The kinetics for the electrode reactions with oxygen and with NO and NO₂ in the presence of oxygen has been studied for La_0.8Sr_0.2MnO_3±δ-electrodes on stabilized zirconia (8 mol% Y₂O₃=YSZ) in the temperature range between 500°C and 900°C for oxygen partial pressures between 1 kPa and 20 kPa by means of electrochemical methods (impedance, I-U characteristics) and temperature programmed desorption (TPD). For oxygen reduction below 900°C a mechanism is proposed which describes the formation of peroxidic ions at the electrode surface and a subsequent rate-determining electron transfer at the three-phase-boundary. At temperatures below 650°C the electrode reaction between NO and NO₂ is much faster than the oxygen reduction. The results for the NO₂-reduction to NO can be explained by a two-step mechanism consisting of a fast one-electron transfer to adsorbed NO₂ at the electrode surface and a subsequent rate-determining transfer of the second electron to NO₂ at the three-phase-boundary. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994     

Gold-composite electrodes for hydrocarbon sensors based on YSZ solid electrolyte

In potentiometric zirconia based sensors gold electrodes show a high sensitivity for hydrocarbons (HC's) when the measurements are carried out in non equilibrated oxygen containing gas mixtures at temperatures <700 °C. This behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To modify the electrode behaviour composites consisting of gold and gallium oxide were investigated. Gold pastes with different amount of Ga₂O₃ were prepared and screen printed on YSZ pellets. After sintering at defined temperatures between 900 and 950 °C the cells were tested regarding the electrode behaviour in a C₃H₆, O₂ gas mixture using a platinum air reference electrode. These composite electrodes show as compared with pure gold an enhanced sensitivity at low propylene concentrations and a time-independent characteristic at high concentrations of C₃H₆. The optimal composition is found to be at 20 mass-% Ga₂O₃. This electrode can be treated in reducing gases at temperatures 850 °C without changing its characteristics. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

X-ray photoelectron spectroscopy of TiO2 and other titanate electrodes and various standard Titanium oxide materials: Surface compositional changes of the TiO2 electrode during photoelectrolysis

Surface compositional changes were observed for TiO₂ single crystal electrodes used for photoelectrolysis of water. Surface stoichiometries of several types of TiO₂, SrTiO₃ and BaTiO₃ electrodes were characterized by XPS and compared with a variety of titanium, titanium oxide and titanium hydride standard materials. Reduction of the electrode surface in a hydrogen atmosphere results in an oxygen deficient surface composition. Photoelectrolysis at current densities of $\text{10–15}\text{mA}\text{cm}{}^2$ for periods up to 8 h appears to return the electrode surface to a nearly stoichiometric oxygen-to-metal ratio. Reduction of the titanium oxide surfaces was also observed by exposure to an argon ion beam. Analysis of the electrode surface by a combination of XPS and ion-sputter profiling was still possible by simultaneous analysis of standard materials.     

Effect of electrode microstructure on gas-phase diffusion in solid oxide fuel cells

The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873–1273 K and oxygen partial pressure (P_O2) of 0.01–1 atm.Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073–1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P_O2). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz.From the results of a comparison of electrode performances with different electrode microstructures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse.     

Top electrode effects on resistive switching behavior in CuO thin films

Top electrode (TE) material on the resistive switching behavior of (TE)/CuO/SnO₂:F/Si substrate has been studied. We investigated the switching properties of CuO films deposited by sol-gel process. Two kinds of top electrode (TE) material on the resistive switching behaviors have been studied. The nonpolar and bipolar resistive switching phenomenon was observed in CuO thin films with different top electrodes. The filamentary mechanism was used to explain the two kinds of resistive switching behaviors. For the Pt/CuO/ATO device, it showed the nonpolar resistive switching where conducting path is formed and disappear due to the oxygen vacancy. For the Cu/CuO/ATO device, the resistance reduction is due to the existing Cu to form conduction Cu-rich pathways. An opposite bias takes the existing Cu back to the Cu electrode to its high-resistance state. CuO thin films are also observed by XRD, AFM and XPS.     

SERS investigation of interfacial water at a silver electrode in acetonitrile solutions

Surface-enhanced Raman scattering from a silver electrode in solution of 0.1 M LiClO₄ in acetonitrile has been analyzed as a function of applied potential. Three ν(O-H) bands associated with the interfacial water and two ν(O-H) bands associated with the OH⁻ ion species were observed depending on the electrode potential. The band at 3487 cm⁻¹ is favored at relatively positive potentials and assigned to H₂O molecules interacting with the electrode surface via the oxygen atoms. Another band at 3586 cm⁻¹ appears in a wider potential region and is assigned to the H₂O molecules with one or both of the hydrogen atoms facing the electrode surface. Additionally, evidence for the possible surface ion pair, Li⁺OH⁻, which is closely associated with H₂O molecules and the quasi-crystalline form of LiOH are also presented in this paper.     

Chemical modifications of au-electrodes on YSZ and their influence on the non-Nernstian behaviour

The interface between gold electrodes and the solid electrolyte Zr_0.86Y_0.14O_1.93 (YSZ) was chemically modified in several ways. Nb₂O₅ was obtained in two states: as an X-ray amorphous surface layer after vacuum evaporation and dissolved in YSZ after annealing at 1150 °C. This was confirmed by X-ray diffraction with grazing incidence and Bragg-Brentano technique. On the metal side the modification was made by potentiostatic deposition of the electronically conducting perovskite La_1-xSr_xCrO₃ and subsequent annealing at 1000 °C. The influence of these modifications on the electrode potential in gases containing oxygen and propylene at temperatures ≤700 °C was investigated. The known sensitivity of the Au electrode to propylene is nearly maintained after both Nb₂O₅ modifications of the solid electrolyte. After modification with the chromite layer, the sensitivity to propylene disappeared completely in gas mixtures with excess oxygen. Strong differences in the sensitivity caused by chemical modifications can possibly be utilized in hydrocarbon sensors. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrode materials for potentiometric hydrogen sensors

Due to their relatively high sensitivity, improved long-term stability, possibilities for miniaturization and low cost products, mixed potential solid electrolyte sensors can be competitive for the in situ measurement of hydrogen trace concentrations in oxygen containing gases. Their response behavior in non-equilibrated oxygen containing gas mixtures is mainly determined by the catalytic activity of the measuring electrode and depends strongly on preparation and measuring conditions. In this work the sensitivity of electrodes made of composites (Au/MeO) has been investigated in hydrogen containing gases in the concentration range φ(H₂) = 0…800 vol.-ppm using a two-chamber setup with Pt-air reference. Electrodes made of Au/Nb₂O₅ composites show the highest sensitivities of up to 20 mV/vol.-ppm at φ(H₂) = 10 vol.-ppm and the lowest catalytic activity for hydrogen oxidation. Selected composite materials were tested additionally in self-heated solid electrolyte sensors with both electrodes exposed to the same atmosphere (gas-symmetrical sensor).     

Electrochemical promotion of NO reduction by C3H6 and CO on Rh/YSZ catalyst — Electrodes

The selective catalytic reduction of NO by propylene or CO in the presence of excess oxygen is a system of great technological importance. The effect of Electrochemical Promotion (or Non-faradaic Electrochemical Modification of Catalytic Activity — NEMCA) was used to promote this reaction (C₃H₆ or CO/NO/O₂) on Rh/YSZ catalyst-electrodes. It was found that both the catalytic activity and the selectivity of the Rh catalyst-electrode is affected dramatically upon varying its potential with respect to a Au pseudoreference electrode. Catalytic rate enhancements up to 15000% and 6000% were observed in the case of NO reduction by propylene, while the product selectivity to N₂ production is affected significantly (up to 200%) upon positive potential application. Remarkable promotion of the catalytic activity was also observed in the case of NO reduction by CO, since up to 20-fold increases both in catalytic rates and in NO conversion were obtained under NEMCA conditions. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.