Fundamental and practical aspects of CO2 sensors based of nasicon electrolytes

Potentiometric CO₂ sensors based on the Na⁺ conducting solid electrolyte Nasicon have been investigated. The sensor arrangement may be described as chemical sensor of type III, $$( - )Pt(or Au), Na_{0.9} CoO_2 |Nasicon|Na_2 CO_3 , Pt(or Au) ( + ),$$ , with Na_0.9CoO₂ as reference electrode and Na₂CO₃ as auxiliary or sensing electrode. It is shown that major problems are related to the magnitude of the voltage and long-term stability of this type of sensor. The measured EMFs are generally reduced compared to the values calculated from literature data of standard Gibbs energies of formation. This observation is ascribed to reactions at the interfaces and competing surface reactions. The interfacial processes may occur at both electrolyte / sensing and electrolyte / reference electrode, and represent charged capacitors. It has also been observed that the slope of the measured EMF as a function of the CO₂ partial pressure is reversed in some cases. This phenomenon may be related to the possible reduction of Na₂O which is dissolved in Nasicon during the formation of Na₂CO₃ upon an increase of the CO₂ partial pressure.     

Electrochemical gas detection based on dynamic measurements

Dynamic measurements are described for the solid state electrochemical detection of CO₂. The sensor device was built up in the same way as a type III potentiometric sensor based on the galvanic cell Pt / Na_xCoO₂ / Na-β/β″-Alumina / Na₂CO₃ / Pt, CO₂, O₂. A periodically varying voltage was applied which results in a current changing with time as a result of the alternating cell reaction. The I/U characteristics of this process depends strongly on the specific dynamics of the sensor, i.e., the kinetics and thermodynamics of the cell reaction and also on the capacitive behaviour. In order to separate these different processes, measurements were carried out under various experimental conditions. The current-time dependence was investigated as a function of the CO₂ partial pressure, temperature and both amplitude and frequency of the applied voltage. It could be seen, that the effect of each process on the sensor performance is increased or decreased by the selection of appropriate sensor parameters. Kinetic, thermodynamic and capacitive responses are generally altogether involved in the sensor response.     

Sodium superionic conductor sputter-deposited coatings

Thin sodium superionic conductor (Nasicon) coatings are deposited on rotating substrates by co-sputtering in the reactive mode of a Zr-Si and a Na₃PO₄ target. The influence of the discharge current and of the target-to-substrate distance is investigated owing to the targeted Na₃Zr₂Si₂PO₁₂ stoichiometry. A thermo-structural analysis shows that the amorphous as-deposited coating of convenient composition crystallises around 700 °C in the monoclinic structure. Electrical measurements performed at room temperature after various annealing treatments indicate a ionic conductivity of about 2·10⁻³ S·cm⁻¹, consistent with that of bulk Nasicon. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

Can electron paramagnetic resonance measurements predict the electrical sensitivity of SnO<Subscript>2</Subscript>-based film?

Paramagnetic singly ionized oxygen vacancies V_o and chemisorbed Sn⁴⁺-O₂⁻ species were detected by electron paramagnetic resonance measurements on SnO₂ and transition metal (Pt, Ru)-doped SnO₂ thin film that had been reduced with CO at different temperatures and then brought into contact with oxygen. The amounts of the two paramagnetic species were evaluated and are discussed as a function of the film annealing temperature in air, the reduction temperature under CO, and the type and concentration of the doping transition element. Also the structural properties of the film were identified through glancing incidence X-ray diffraction analysis. Measurements of the electrical sensitivityS(S=R _air/R _CO, whereR _air andR _CO are the resistance under air and under CO(800 ppm)/air respectively) show that the trend of the sensitivity values vs. the reduction temperature with CO could be predicted by the parallel trend of the number of Sn⁴⁺−O₂⁻ centers.     

Reference electrode formation in potentiometric CO2 sensors with alkali ion conducting alumosilicate glasses as solid electrolytes

Potentiometric CO₂ gas sensors with Li conducting glasses/glass ceramics of the system Li₂O-Al₂O₃-SiO₂ (different nominal composition) as solid electrolytes have been investigated. Li₂CO₃ was used as CO₂ and O₂ sensitive auxiliary electrode. During the sensor test measurements, the CO₂ partial pressure was varied between 1×10⁻³ and 1×10⁻¹ bar at a constant O₂ partial pressure of 2.1×10⁻¹ bar whereas N₂ was used as carrier gas. Comparative measurements were accomplished with sensors comprising Na and K ion conducting glasses. A metastable reference electrode was formed at the contact zone between the Au metal electrode and the former Li glasses of definite nominal composition by crystallization processes taking place, which lead to stable, reproducible CO₂ dependent EMF signals for more than 90d. The thermodynamically expected EMF difference and the observed EMF difference agree quite well between 500 and 600 °C. At 600 °C, the drift of sensors with glasses as solid electrolytes and direct Au glass/glass ceramics contact as reference electrode amounts typically 0.32 mV/d (p(CO₂)=1×10⁻³ bar, p(O₂)=2.1×10⁻¹ bar at the measuring electrode), if a metastable multiphase equilibrium is formed. At identical partial pressures of CO₂ and O₂, the signal reproducibility of these sensors with different solid electrolyte glasses of the same nominal composition lies within 30 mV at 600 °C. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrochemical promotion of catalyst surfaces deposited on ionic and mixed conductors

The effect of non-Faradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP) was investigated on Pt films deposited on Y₂O₃-stabilized-ZrO₂ (YSZ), an O²⁻ conductor, TiO₂, a mixed conductor, and Nafion 117 solid polymer electrolyte (SPE), a H⁺ conductor and also on Pd films deposited on YSZ and β″-Al₂O₃ a Na⁺ conductor. Four catalytic systems were investigated, i.e. C₂H₆ oxidation on Pt/YSZ, C₂H₄ oxidation on Pd/YSZ and Pd/β″-Al₂O₃, C₂H₄ oxidation on Pt/TiO₂ and H₂ oxidation on Pt/Nafion 117 in contact with 0.1 M aqueous KOH solution. In all cases pronounced and reversible non-Faradaic electrochemical modification of catalytic rates was observed with catalytic rate enhancement up to 2000% and Faradaic efficiency values up to 5000. All reactions investigated exhibit a pronounced electrophobic behaviour which is due to the weakening of chemisorptive oxygen bond at high catalyst potentials. Ethane oxidation, however, also exhibits electrophilic behaviour at low potentials due to weakened binding of carbonaceous species on the surface. The general features of the phenomenon are similar for all four cases presented here showing that the NEMCA effect is a general, electrochemically induced, promoting catalytic phenomenon not depending on the reaction and the type of supporting electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

快离子导体中的钠沉积

采用电子探针(EPMA)定点轰击法研究了β-,β″-Al₂O₃和Nasicon(Na₃Zr₂Si₂PO₁₂)的钠沉积效应。按照递减离子源的离子迁移和电解沉积机理导出了钠沉积动力学方程式,可以较好地说明快离子导体在电子束轰击下钠计数率随时间的变化。在β-Al₂O₃单晶内有丰富的可迁移钠离子源,曲线上升的延续时间较长;而对于Na_关键词：     

Potentiometric CO2 - sensors with ion - conducting glasses as solid electrolytes

Alkali-ion conducting glasses/glass ceramics of the system Me₂O-A1₂O₃-SiO₂ (Me=Li, Na) were applied as solid electrolytes in potentiometric gas sensors to detect CO₂ in the presence of O₂ at increased temperatures. The corresponding Me-Carbonates were utilized as auxiliary electrodes. Sensors using the direct Au-glass contact as a kind of reference electrode (type I), as well as symmetrical sensors with carbonate phase at the reference and measuring electrode (type II - for comparative measurements) were manufactured. By applying Au as electrode metal, the theoretically expected EMF difference and the observed EMF difference of both sensor types agree quite well with the expected values according to the Nernst equation between 500 and 600 °C (over four orders of magnitude of CO₂ partial pressure (10⁻⁵ – 10⁻¹ bar) at constant O₂ partial pressure (2.1×10⁻¹ bar)). A long time stability of 120 days for sensors of type I with Li glasses has been observed, although evaporation of carbonate phase (Li₂CO₃) was detected under the conditions of sensor application. Sensors of type I (with Li₂CO₃) show thermodynamically unexpected cross-sensitivities to H₂O. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrodeposition of nanostructured Pt films from lyotropic liquid crystalline phases on α-Al2O3 supported dense Pd membranes

Using the lyotropic liquid crystalline templating strategy, the nanostructured platinum film was electrochemically deposited on the α-Al₂O₃ supported dense palladium membrane. The XRD and TEM results of the Pt film revealed a hexagonal array of cylindrical pores with a uniform pore diameter of ca. 3.8 nm and a pore-to-pore separation of ca. 7.6 nm. The structure parameters of the Pt film were almost the same as those of the hexagonal liquid crystalline template. Based on SEM observations, the Pt film was featureless, smooth, and tightly adherent to the dense Pd membrane. The specific surface area of the Pt film, measured by using cyclic voltammetry, was ca. 13.8 m² g⁻¹, which was in accord with the theoretical value of 14.5 m² g⁻¹ for a perfect hexagonal nanostructure with the same structure parameters. By combining the dense Pd membrane for selective permeation to hydrogen with the Pt film of high specific surface area for catalysis, the as-synthesized two-layer film will be a promising catalytic membrane to intensify hydrogen-related reaction processes.     

Angular distribution of the desorption of CO2 produced on Pt(111) surface at low temperatures

A new CO₂ formation process was observed in the CO oxidation over Pt(111) surface below 200 K. The desorption flux of the product CO₂, which is formed from the interaction between chemisorbed CO and adsorbed oxygen molecules O₂^2? (a), showed a very sharp angular distribution along the surface normal.     

Composite effect of solid electrolytes based on alkali carbonates and sulfates

To improve both the thermal stability and the electrical conductivity, alkali ion conducting carbonates and sulfates with admixtures of alkaline earth compounds as well as insulating materials such as γ-Al₂O₃ were prepared and characterized by conductivity measurements, X-ray studies, DSC and SEM/EDX. The addition of the divalent carbonates and sulphates causes two effects. On the one hand side, the alkali earth compounds can be dissolved depending on the ionic radius to a more or less large extent, which is accompanied by an increase in the electrical conductivity. On the other hand side, a two phase mixture consisting of an excess of dopant is formed with an enhancement in the conductivity and improvement of the mechanical stability. This phenomenon, known as composite effect, could be observed in the following systems Na₂CO₃-BaCO₃, Na₂CO₃-SrCO₃, Na₂SO₄-BaSO₄, K₂SO₄-BaSO₄, Na₂SO₄-γ-Al₂0₃, K₂SO₄-γ-Al₂O₃.     

Room temperature response of SnO2-electrode modified solid state electrochemical CO2 sensors

A thin film solid state electrochemical gas sensor has been investigated for CO₂ detection based on the cell reaction: Na⁺+OH+CO₂=NaHCO₃. The galvanic cell arrangement is Au | Na_xCoO_2−δ (ref.) | NASICON | Au, SnO₂ where the right hand electrode is in contact with CO₂ and O₂ in a humid atmosphere. The response has been compared to results obtained with a conventional pellet type sensor. Furthermore, both devices have been exposed to CO and humidity. Strong cross-sensitivities were observed leading to large changes in the emf in both cases. The response to moisture is reversible and fast with a response time of about 1 min according to a fast surface reaction of H₂O with SnO₂. The presence of CO leads to a signal change with a high response time and a very slow reverse reaction. However, the response to CO₂ is not influenced by the presence of CO or H₂O with regard to the signal height and response time. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Compatibility of field emitter studies of oscillating surface reactions with single crystal measurements: catalytic CO oxidation on Pt

Kinetic oscillations in catalytic CO oxidation on Pt have been studied on large (millimeter size) single crystal planes of Pt as well as on a Pt field emitter tip that exposes different crystal facets of nanometer size. In order to examine the compatibility of results from the two types of experiments, the regions of different dynamical behavior (bifurcation diagram) have been mapped out in p_CO,T-parameter space using a field electron microscope (FEM) and a field ion microscope (FIM). The comparison with the results of single crystal measurements shows that in the case of applied electrostatic fields less than 5 V nm⁻¹ (FEM), the field-induced effects are negligible, but they are significant for fields exceeding 12 V nm⁻¹ (FIM). The field-induced shift of the bifurcation diagram toward lower p_CO values, observed with FIM, is explained in terms of a field-modified interaction of CO and O₂ with Pt studied here with field ion appearance energy spectroscopy. With coadsorbed lithium (submonolayer coverage), the existence range for rate oscillations is shifted toward higher p_CO values. This shift is attributed to a redistribution of the electron density at the surface induced by alkali metal co-adsorption.     

Electrochemical promotion of CH4 oxidation on Pd

The effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion (EP) was used to promote the methane oxidation reaction to CO₂ and H₂O over Pd polycrystalline films interfaced with yttria-stabilized zirconia in galvanic cells of the type: CH₄, O₂, CO₂, Pd/YSZ/Au, CH₄, O₂, CO₂ It was found that by applying positive potentials or currents and thus, supplying O²⁻ onto the catalyst surface, up to 90-fold increases in CH₄ oxidation catalytic rate can be obtained. The induced changes in catalytic rate were two orders of magnitude higher than the corresponding rate of ion transfer to the catalyst-electrode surface, i.e. faradaic efficiency Λ values above 100 can be attained. The reaction exhibits electrophobic behavior under the experimental conditions of the investigation. The results can be rationalized on the basis of the theoretical considerations invoked to explain NEMCA behavior, i.e. the effect of changing work function on chemisorptive bond strengths of catalytically active electron donor or acceptor adsorbates. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Composition depth profiles of Bi3.15Nd0.85Ti3O12 thin films studied by X-ray photoelectron spectroscopy

In the present work, X-ray photoelectron spectroscopy (XPS) was used to investigate the composition depth profiles of Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) ferroelectric thin film, which was prepared on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by chemical solution deposition (CSD). It is shown that there are three distinct regions formed in BNT film, which are surface layer, bulk film and interface layer. The surface of film is found to consist of one outermost Bi-rich region. High resolution spectra of the O 1s peak in the surface can be decomposed into two components of metallic oxide oxygen and surface adsorbed oxygen. The distribution of component elements is nearly uniform within the bulk film. In the bulk film, high resolution XPS spectra of O 1s, Bi 4f, Nd 3d, Ti 2p are in agreement with the element chemical states of the BNT system. The interfacial layer is formed through the interdiffusion between the BNT film and Pt electrode. In addition, the Ar⁺-ion sputtering changes lots of Bi³⁺ ions into Bi⁰ due to weak Bi-O bond and high etching energy.     

The mechanism of formation of the hydroperoxyl radical in the CF<Subscript>3</Subscript>COOH + <Superscript>3</Superscript>O<Subscript>2</Subscript> system: a quantum-chemical study

Ab initio quantum-chemical calculations of the (CF₃CO₂H₂⁺…³O₂) and (CF₃CO₂⁻…³O₂) complexes were performed by the MP2 method. It was found that these complexes were characterized by low complex formation energies, of 2.97 and 1.72 kcal/mol, respectively. According to the MP2(full)/6-311++G(d, p) calculation data, the bridge stabilization of oxygen by linking with both the CF₃CO₂H₂⁺ cation and CF₃CO₂⁻ anion is much more favorable energetically. A study of the potential energy surface of the joint molecular system (CF₃CO₂H₂⁺…³O₂…CF₃CO₂⁻) shows that proton experiences activationless transfer from the cation to the ³O₂ molecule accompanied by electron transfer from the CF₃COO⁻ anion. An analysis of spin density distribution shows that two radicals are stabilized in the (CF₃CO₂….OOH….O=C(OH)CF₃) complex in the triplet state observed on the potential energy surface.     

Influence of the thickness of sputter-deposited platinum films on the electrochemical promotion of propane combustion

8 to 120 nm-thick Pt coatings were sputter-deposited on Yttria-Stabilised Zirconia (YSZ) membranes, 17 mm in diameter, by magnetron sputtering of a Pt target at low pressure (0.3 Pa). The catalytic activity of propane combustion under open-circuit conditions is first measured near to the stoichiometry (0.2%C₃H₈/1%O₂) and shows that the coatings present a rather high catalytic activity. Close-circuit measurements were finally performed at 337 °C and 400 °C. They show that rather high faradaic efficiencies in the range 10⁶ can be reached as soon as the Pt film is thick enough to allow a bias of its whole area. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

CO2/ SOx-sensors with different β“-aluminas as solid electrolytes

Sensors with solid electrolytes provide the possibility of correct and fast measurements of partial pressures of various gases. By modification of the solid electrolyte, sensors with specific performances may be manufactured. Layers of Na⁺-, Li⁺-, Ca²⁺- and Sr²⁺-β“-alumina on top of polycrystalline α-alumina substrates were produced by an in-situ formation process and were used as solid electrolytes for CO₂ and SO_x sensors. Carbonates and sulphates were applied as measuring electrodes and oxidic mixtures of SiO₂ and silicates were used as reference electrodes. The different performance of these sensors was investigated over a wide temperature range and the results were compared with theoretical data. Different solid electrolyte / electrode combinations were applied, which all showed different characteristic cross sensitivities against water and organic components. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

STM observation of the origin of electrochemical promotion on metal catalyst-electrodes interfaced with YSZ and β″-Al2O3

Scanning tunneling microscopy (STM) was used to investigate the surfaces of Pt(111) single crystals interfaced with YSZ and β″-Al₂O₃ at atmospheric pressure. In both cases the STM imaged the reversible electrochemically controlled dosing (backspillover) of O²⁻ species and of Na⁺ species on Pt(111) surface respectively, which both form a (12 × 12) hexagonal structure on the Pt(111) surface. On the mechanistic side, the STM has confirmed the backspillover mechanism of electrochemical promotion and metal support interactions.     

Evaluation of the electrical capacitance of M,O2/O2− interfaces (M=Pt, Au, Pd, In2O3; O2− = zirconia based electrolyte) exhibiting constant phase element behavior

The behavior of the electrode systems M,O₂/O² (M = porous Pd, Pt, A and dense In₂O₃; O²⁻ = ZrO₂-based single-crystal solid electrolyte) was studied by means of impedance measurements. The examination of the Pt,O₂/O²⁻ electrode system showed that the constant phase element (CPE) can be attributed to a nonuniform distribution of current at the electrode surface. It was observed that the CPE parameters n and B in the expression Y_CPE = B (jω)ⁿ may be related by B=(C_dl)ⁿ (R_Ω)_n-1, where C_dl is the double layer capacitance and R_Ω the resistance of the electrolyte in the cell. Then, C_dl of the electrode - electrolyte interface could be determined. The specific C_dl of the oxidized noble metals and india electrodes is nearly one order of magnitude lower than C_dl of the electrodes in the metallic state. The C_dl value of all the electrodes studied depends little or is independent of temperature and oxygen pressure. It is concluded that the Helmholtz model of double layer structure does not contradict the C_dl behavior.