Mixed Potential Model Research Articles

Mixed potential type ppb-level acetaldehyde gas sensor based on stabilized zirconia electrolyte and a NiTiO3 sensing electrode

An attempt has been performed to develop a selective and sensitive acetaldehyde gas sensor based on yttria-stabilized zirconia (YSZ) solid electrolyte in conjunction with NiTiO3 sensing electrodes (SEs) calcined at different temperatures (800, 1000 and 1200 ℃). Among all the evaluated sensing devices, the sensor attached with NiTiO3-SE calcined at 1000 ℃ was found to have a maximum sensing response value of −43 mV to 50 ppm acetaldehyde and a lower detection limit of 200 ppb at an optimal working temperature of 575 ℃. The response and recovery times of the sensing device to 50 ppm acetaldehyde were 10 and 26 s, respectively. The present sensor also exhibited a segmentally linear relationship between the ΔV values and the logarithm of acetaldehyde concentration in the range of 0.2–1 ppm and 1–200 ppm at 575 ℃ and the sensitivities were −3 mV/decade and −26.5 mV/decade. Furthermore, the developed sensor had a good selectivity, reproducibility, moisture insensitivity, as well as a high temperature stability for 20 days. On basis of the polarization-curve evaluation of the present sensor, the good agreement between the observed values and the estimated values revealed that the sensing mechanism based on mixed-potential model has been confirmed.

Performance and DRT Analysis of Surface Decorated Spinel-Oxide Electrodes for Mixed-Potential Type Ammonia Gas Sensor

The solid-electrolyte based electrochemical gas sensors appeared as the most promising sensing technology for the in-situ quantification of exhaust pollutants. At present, high sensitivity and selectivity supplemented by suitable long-term stability is the bottleneck challenge for these technologies to commercialize. In present work, highly sensitive and ammonia (NH3) selective mixed-potential gas sensors were developed using surface decorated CuFe2O4(CFO) spinel oxide electrodes. The CFO–NiO composite SE delivered a maximum response of −62 mV to 80 ppm ammonia, supported by a sensitivity of -70 mV/dec. at 650 ℃. By a thorough analysis of response behavior and I–V characteristics, the sensing mechanism was found to be based upon the mixed-potential model complying to the reaction-rate limited Butler–Volmer NH3 oxidation kinetics. Finally, the DRT analysis of impedance spectra confirmed that the overall polarization resistance was invariable of the mass-transport processes and solely governed by the extent of interfacial redox reactions proceeding at the triple-phase boundaries (TPB), determining the electrode potential of the sensor. Moreover, the high sensitivity, selectivity, and excellent stability without a degradation over five months, validated the suitability of these sensors to be a potential candidate for in-situ ammonia quantifications in industrial and automotive applications.

Surface decorated spinel-oxide electrodes for mixed-potential ammonia sensor: Performance and DRT analysis

The mixed-potential gas sensors appeared as the most promising sensing technology for the in-situ quantification of exhaust pollutants due to their simple configuration, low-cost, and thermochemical stability. Presently, high sensitivity and selectivity supplemented by long-term stability is the bottleneck challenge for these sensors to commercialize. Herein, highly sensitive and ammonia (NH3) selective mixed-potential gas sensors were developed using surface decorated CuFe2O4 (CFO)–MOX (M = Sn, Ni, Zn) composite sensing electrodes (SE). The CFO–NiO SE enriched of the surface oxygen vacancies produced a maximum response of −62 mV to 80 ppm NH3, supported by excellent sensitivity at 650 ℃. The comprehensive analysis of the response behavior and current-voltage (I–V) characteristics verified the sensing mechanism to be based upon the mixed-potential model conforming to the reaction-rate limited Butler–Volmer NH3 oxidation kinetics. Finally, the distribution of relaxation times (DRT) analysis of impedance spectra confirmed that the overall polarization resistance was invariable of the mass-transport processes and solely governed by the extent of interfacial redox reactions proceeding at the triple-phase boundaries (TPB). Moreover, the high sensitivity, selectivity, and exceptional stability over five months substantiate the suitability of the presented sensor as a potential candidate for in-situ ammonia quantifications in industrial and automotive applications.

Synergistic enhancement in the sensing performance of a mixed-potential NH3 sensor using SnO2@CuFe2O4 sensing electrode

A mixed-potential type NH3 sensor equipped with CuFe2O4 and SnO2@CuFe2O4 sensing electrode is presented. The CuFe2O4 spinel-oxide and SnO2@CuFe2O4 composites were synthesized by a modified-Pechini route. The electrode materials were characterized for the physical properties by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS) analysis. It was found that the sensing characteristics were critically dependent on the extent of Triple-phase boundary (TPB) lengths and operating conditions of the sensor. Furthermore, the sensing performance of CuFe2O4 spinel-oxide was enhanced by compositing with SnO2 nanocrystals resulting in a synergistically enhanced response (ΔV) of −40 mV towards 80 ppm NH3, almost double and quadruple of the response of bare CuFe2O4 and SnO2 electrodes at 650 ℃, respectively. The sensor also displayed excellent stability towards oxygen and humidity variations, along with low cross-sensitivities towards interfering gases; e.g. NO, CO, CH4, and NO2. The complex impedance spectra (EIS) and dc polarization (I–V) measurements were performed for an insightful analysis of the sensing mechanism conforming to the mixed-potential model.

Controlling the mixed potential of polyelectrolyte-coated platinum electrodes for the potentiometric detection of hydrogen peroxide

The use of a Pt electrode coated with a layer of Nafion has been described in previous works as an attractive way to perform the potentiometric detection of hydrogen peroxide. Despite of the attractive features of this approach, the nature of the non-Nernstian response of this system was not properly addressed. In this work, using a mixed potential model, the open circuit potential of the Pt electrode is shown to be under kinetic control of the oxygen reduction reaction (ORR). It is proposed that hydrogen peroxide acts as an oxygenated species that blocks free sites on the Pt surface, interfering with the ORR. Therefore, the effect of the polyelectrolyte coating can be understood in terms of the modulation of the factors that affects the kinetics of the ORR, such as an increase of the H+ concentration, minimization of the effect of the spectator species, etc. Because of the complexity and the lack of models that accurately describe systems with practical applications, this work is not intended to provide a mechanistic but rather a phenomenological view on problem. A general framework to understand the factors that affect the potentiometric response is provided. Experimental evidence showing that the use of polyelectrolyte coatings are a powerful way to control the mixed potential open new ways for the development of robust and simple potentiometric sensors.

Effects of temperature and pH on the electrochemical behaviour of alloy 600 in simulated pressurized water reactor primary water

The effects of temperature and pH on the electrochemical properties of Alloy 600 were studied in oxygenated borate buffer solutions by the measurements of open circuit potential (OCP), anodic potentiostatic polarization, Mott-Schottky analysis (MSA), and electrochemical impedance spectroscopy (EIS). At each temperature and pH, the stabilized anodic passive current density was independent of the formation potential, and the slope of the M − S plot for the passive film formed at +0.3 VOCP (OCP + 0.3 V) is positive, indicating that the passive films formed on Alloy 600 have n-type semiconductor character. With increasing temperature (in each solution), the OCP and the impedance modulus decreased, while the passive current density and the defect density calculated from MSA increased, indicating a significant reduction in corrosion resistance. With the increase in pH value, the passive current density decreased, the absolute value of impedance increased slightly, revealing a slight increase in corrosion resistance. According to the optimization of Mixed Potential Model (MPM) on the experimentally measured EIS data for Alloy 600 at OCP and +0.3 VOCP, a series of fundamental parameters related to the micro-characteristic of passive film were extracted. In the barrier layer of the passive film, the concentration of cation interstitials is much higher than that of anion vacancies, indicating that the former was a dominant defect. With increasing temperature, the linear increase in the defect density and film thickness resulted in a linear increase of the anodic current density. However, with increasing pH, the decrease of defect density and the slight increase of film thickness caused a linear reduction in anodic current density. Elevated temperature has a more significant deterioration effect on the corrosion resistance of Alloy 600 than the increased pH value. Additionally, all the kinetics parameters describing the generation of cation interstitials and oxygen vacancies at the substrate/film were obtained.

YSZ-based solid electrolyte type sensor utilizing ZnMoO4 sensing electrode for fast detection of ppb-level H2S

The planar and compact type all-solid state electrochemical gas sensing device based on yttria-stabilized zirconia (YSZ) and Zinc molybdate (ZnMoO4) composite sensing electrode (SE) was proposed and fabricated to effectively detect ppb-level H2S at high temperature. The gas sensing property of different SEs (ZnMoO4, CoMoO4 and NiMoO4) is compared and explored. The sensor attached with ZnMoO4-SE processes the highest response value of −62.5 mV to 1 ppm H2S and lower detection limit of 5 ppb H2S at 500 °C. The response and recovery times of the sensor to 500 ppb H2S are 20 s and 58 s, respectively. The sensor also displays high sensitivity, good continuous reproducibility, selectivity and stability of humidity and high temperature at 500 °C. In addition, the sensing principle involving mixed potential model was proposed and verified using polarization curves.

High performance solid electrolyte-based NO2 sensor based on Co3V2O8 derived from metal-organic framework

In this paper, a well-performing solid electrolyte-based NO2 sensor was initially assembled by using Co3V2O8 derived from Co-V-MOF as sensitive electrode material and La10Si5.5Al0.5O26.75 as solid electrolyte. The sensor displayed excellent sensing behaviors towards NO2 by comparison with the sensor using Co3V2O8 fabricated from classical solid state method as electrode material at 575 °C. The sensor exhibited good response-recovery characteristics to different NO2 concentrations, with a sensitivity of 78.2 mV/decade in the NO2 concentration range of 50–500 ppm. The sensor also showed good repeatability, strong anti-interference ability to CH4, CO2, O2, NO and CO, as well as good long-term stability for 30 test days at 575 °C. In addition, the sensor had a low cross-sensitivity to water vapor in the relative humidity of 20%–98% at 575 °C. The good sensing behaviors could be attributed to the synergistic effect of highly disperse Co/V catalysis sites from Co-V-MOF as well as the larger three-phase boundary of Co3V2O8/ La10Si5.5Al0.5O26.75/NO2. The sensing mechanism of the sensor was investigated and it followed the mixed-potential model.

Studies on Pitting Corrosion of Al-Cu-Li Alloys Part III: Passivation Kinetics of AA2098-T851 Based on the Point Defect Model.

In this paper, the passivation kinetics of AA2098–T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098–T851 in NaHCO3 solution in a CO2 atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler–Volmer (B–V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface.

Solid state electrolyte type gas sensor using stabilized zirconia and MTiO3 (M: Zn, Co and Ni)-SE for detection of low concentration of SO2

This paper deals with solid state electrolyte type gas sensors based on mixed potential mechanism using yttria-stabilized zirconia (YSZ) as solid electrolyte and MTiO3 (M: Zn, Co and Ni) as sensing electrodes (SEs) for monitoring very low concentrations of SO2 in atmospheric environments. Among all the devices attached with different SEs, ZnTiO3-SE brought about the highest response value of −22.5 mV to 1 ppm SO2 at 600 ℃ and even down to 100 ppb SO2 with the response value of −2.5 mV. Meanwhile, the measured ΔV of the sensor was found to vary linearly as a function of the logarithm of SO2 concentration in the range of 100 ppb–2 ppm at 600 ℃ and the sensitivity (slop) was −23 mV/decade. In addition, the sensor coupled with ZnTiO3-SE gave negligible responses to other interfering gases such as C2H2, CH4, CO, NO2, NO, NH3. Furthermore, the present sensor possessed good repeatability, slightly affected by the change of oxygen concentration and relatively stability during 20 days of continuous high-temperature operation. Moreover, the polarization-curve measurements demonstrated that the good agreement between the observed sensitivity and the estimated value, confirming that the sensing mechanism was based on mixed-potential model.

The electrochemical properties of alloy 690 in simulated pressurized water reactor primary water: Effect of temperature

By anodic potentiostatic polarization and electrochemical impedance spectroscopy (EIS) measurements, the effect of temperature (200 °C, 250 °C, and 300 °C) on the electrochemical behavior of Alloy 690 was investigated in oxygenated borate buffer solution containing H3BO3 (2000 ppm B) + LiOH (2 ppm Li). The steady state passive current density was independent of the film formation potential at each temperature, indicating that the passive film is n-type in electronic character based on the predictions of the Point Defect Model (PDM). The passive current density increased and the absolute value of impedance decreased with increasing temperature, indicating a corresponding decrease in corrosion resistance. A mixed potential model (MPM), containing the PDM for describing the anodic passive dissolution of the alloy and the generalized Butler–Volmer equation for the cathodic oxygen reduction reaction was optimized on the experimental EIS data in order to extract model parameters, including kinetic parameters for the generation and annihilation of point defects in the barrier layer of the passive film. Metal (Cr,Ni,Fe) interstitials, which were found to have a higher concentration than oxygen vacancies, dominate the point defect structure of the barrier layer of the passive film at each temperature, thereby accounting for the n-type behavior of the passive film. With increasing temperature, the film thickness and defect density also increased, resulting in the decreased corrosion resistance of film. This indicates that the defect density of passive film plays a more important role on corrosion behavior of alloy than the film thickness.

High-response mixed-potential type planar YSZ-based NO2 sensor coupled with CoTiO3 sensing electrode

In this work, we developed a high-response mixed potential type yttria stabilized zirconia (YSZ) based planar gas sensing device utilizing CoTiO3 sensing electrode (SE) prepared via a facile sol-gel method to sensitively detect NO2 at an operating temperature of 650 °C. The present gas sensor coupled with 1200 ℃-sintered CoTiO3-SE exhibited high responses to NO2 in the concentration range of 500 ppb-200 ppm, as the value of the response was 130 mV to 100 ppm NO2. A linear relationship was observed between the ΔV of the sensor and the logarithmic scale of the NO2 concentration, which the sensitivity was 60 mV/decade. In addition, the sensor also displayed good reproducibility, the maximum response change of 5.4% and 16% in different coexisting gases and 98% high relative humidity, and response decrease of 6.9%–100 ppm NO2 after high temperature test for 20 days. Moreover, the sensing mechanism of sensor, which was based on mixed potential model, was confirmed using polarization curve measurements in air and different concentrations of NO2.

Mixed-potential type triethylamine sensor based on NASICON utilizing SmMO3 (M = Al, Cr, Co) sensing electrodes

The polycrystalline perovskite SmMO3 (M = Al, Cr, Co) sensing electrode was synthesized by the sol-gel method, and the NASICON-based sensors attached with SmMO3 were designed and fabricated for triethylamine (TEA) detection. This study investigated the effects of different M3+ ions in SmMO3 and various calcination temperatures of the sensing material on TEA sensing properties. The results reveal that compared with the other devices, the sensor attached with SmCrO3 sintered at 800 °C displays a higher response value (−108.2 mV) towards 100 ppm TEA, and that the sensor even detects 2 ppm TEA with an acceptable value (−6 mV). The sensor utilizing SmCrO3 (800 °C) also exhibits pretty swift response and recovery times of 2 and 22 s, respectively, toward 20 ppm TEA. A linear relationship exists between the response values and the logarithm of TEA concentrations, and the sensitivity is −64 mV/decade. At the same time, good repeatability, selectivity, and long-term stability of the sensor are also observed. Furthermore, the sensing mechanism related to the mixed potential model is demonstrated by the polarization curve measurements.

Large signal stability analysis for DC microgrid under droop control based on mixed potential theory

Brayton–Moser's mixed potential theory has been used to analyse the large disturbance stability of DC microgrid with constant power load (CPL), such as current-mode controlled BUCK/BOOST converter with CPL or DC system with multi-stage LC filters. However, the stability analysis of other topology of DC–DC converter or multiple converters worked under droop control is rarely involved. Based on this issue, this paper first establishes the mixed potential model of a single bidirectional BUCK/BOOST converter with CPL. Then, the paper gives the method for establishing the mixed potential model of a droop-controlled DC microgrid with multiple bidirectional BUCK/BOOST converters and CPL. Afterwards, the stability criterion of the analytic form of the system under large disturbance is obtained, which shows that the parameters such as system parameters, control parameters, droop coefficients, and CPL power value have the effects on the system's large signal stability. This paper gives and discusses the relationship between system parameters and stability. Thus, instead of calculating and simulating repeatedly, the stability criterion can judge and ensure the stability of the system under large disturbance in the design process. Simulation results demonstrate the correctness of the simple and easily implemented stability criterion.

Highly selective and stable mixed-potential type gas sensor based on stabilized zirconia and Cd2V2O7 sensing electrode for NH3 detection

In this work, the highly selective and stable mixed potential type stabilized zirconia (YSZ) based gas sensor using Cd2V2O7 sensing electrode (SE) synthesized via simple coprecipitation method was fabricated and developed for effective detection of NH3 at 650 °C. The as-prepared Cd2V2O7 sensing material was characterized by X-ray diffraction (XRD), Raman spectrum, X-ray photoelectron spectroscopy (XPS) and Field-emission scanning electron microscopy (FESEM). The results of gas sensing measurement indicated that the sensor attached with Cd2V2O7- SE displayed the response value of −68 mV and rapid response rate of 5 s to 100 ppm NH3 at 650 °C. The present device also exhibited the low detection limit of 1 ppm and the piecewise sensitivities of −6 and −66 mV/decade to NH3 in the concentration ranges of 1–10 ppm and 10–200 ppm, respectively. Moreover, the fabricated sensor showed good reproducibility, excellent selectivity, stability to oxygen concentration, relative humidity and 30 days continuous aging of high temperature at 650 °C. And the complex impedance and polarization curves were performed to explain the selectivity and verify the sensing mechanism involving mixed potential model.

The Mixed Potential Model for the Passive State and Hydrogen Evolution Reaction of AA2098-T851

Theoretical and experimental investigations for fundamental understanding of the passivation process and its kinetics could lead to the prediction of the electrochemical behavior of the third-generation Al-Li alloy 2098. In this paper, the kinetics of the passive film formation on AA2098-T851 in 0.1M NaHCO3 in a CO2 atmosphere at 25°C, at potentials with both metal dissolution and hydrogen evolution reaction (HER) was investigated. This was done by performing electrochemical impedance spectroscopy (EIS) on the passive layer after each potentiostic steps in the anodic direction that followed by stepping in the opposite direction. The optimization of the EIS data was done by applying the previously developed Mixed Potential Model (MPM) that considers the Point Defect Model (PDM) for dissolution and passivation of the metal along with the Butler-Volmer equation for the cathodic reaction. It was shown that the inequality of certain kinetic processes of passive layer formation leads to the irreversibility of the metallic passivation in opposite potential scanning directions. Also, considering that the hydrogen evolution reaction incorporates quantum mechanical tunneling of the charge carriers through the barrier oxide layer on the passive metal surface, a cathodic Tafel constant (βc ) was calculated. The estimated βc was in accord with the predicted value for the HER according to the slow discharge of protons mechanism.

YSZ-based sensor using Cr-Fe-based spinel-oxide electrodes for selective detection of CO

A selective carbon monoxide (CO) sensor was developed by the use of both of CuCrFeO4 and CoCrFeO4 as the sensing electrode (SE) for yttria-stabilized zirconia (YSZ)-based potentiometric sensor. The sensing-characteristic examinations of the YSZ-based sensors using each of spinel oxides as the single-SE sensor showed that CuCrFeO4-SE had the ability to detect CO, hydrocarbons and NOx gases, while CoCrFeO4-SE was sensitive to hydrocarbons and NOx gases. Thus, when both SEs were paired as a combined-SEs sensor, the resulting sensor could generate a selective response to CO at 450 °C under humid conditions. The sensor was also capable of detecting CO in the concentration range of 20–700 ppm. Its sensing mechanism that was examined via polarization-curve measurements was confirmed to be based on mixed-potential model. The CO response generated by the combined-SEs sensor was unaffected by the change of water vapor concentration in the range of 1.3–11.5 vol% H2O. Additionally, the sensing performance was stable during 13 days tested.

Tuning H2 Sensing Performance of Zirconia-based Sensor using ZrSiO4(+Au) Sensing-electrode

We are reporting here a potentiometric sensor by using yttria-stabilized zirconia (YSZ) as a solid electrolyte as well as zirconium silicate (ZrSiO4) added with Au as a sensing electrode (SE), for achieving sensitive and selective detection of hydrogen (H2). The addition of 10wt.%Au into ZrSiO4-SE increased the sensitivity toward 100ppm H2 at 500°C, while lower responses to other examined gases were observed. The ability to detect H2 selectively was elucidated via polarization-curve measurements in air and in N2 and confirmed via gas conversion test. It was found that the sensitive and selective H2 behavior was due to a lower catalytic activity against the gas-phase oxidation of H2 than that of C3H6, in combination with a higher catalytic activity to anodic reaction of H2 than that of C3H6. The good agreement between the observed sensitivity and the estimated value from the polarization-curve measurements confirmed that the sensing mechanism is based on mixed potential model. The sensor using ZrSiO4(+ 10wt.%Au)-SE demonstrated a linear relationship between sensitivity and H2 concentration on a logarithmic scale and was capable of detecting H2 concentration as low as 20ppm at 500°C under humid conditions.

The Irreversibility of the Passive State of Carbon Steel in the Alkaline Concrete Pore Solution under Simulated Anoxic Conditions

The commonly observed irreversibility of the passive state on carbon steel in alkaline solutions was examined in simulated concrete pore solution under anoxic conditions replicated by applying relatively negative film formation potentials, at which the cathodic process dominates the passive system. The fundamental source of this irreversibility was investigated by describing the kinetics of the passive system via a previously developed mixed potential model that combines the point defect model accounting for the anodic process and the Butler-Volmer equation accounting for the cathodic process. Electrochemical impedance spectroscopy, Mott-Schottky analysis and model optimization were performed at each potential when the potential was first stepped in the anodic direction and then in the cathodic direction. The experiment and optimization results demonstrate that the irreversibility of the passive state is closely associated with the discrepancy in the defect structure of the passive film upon opposite stepping directions of the film formation potential, and is essentially caused by the slower film formation and slower film dissolution during the cathodic potential stepping than those during the anodic potential stepping.

Physical principles for the calculation of equilibrium potential for co-electrolysis of steam and carbon dioxide in a Solid Oxide Electrolyzer Cell (SOEC)

This paper reports a comparative study on the calculation of equilibrium potential (also known as Open Circuit Voltage) of co-electrolysis of H2O and CO2 in a Solid Oxide Electrolyzer Cell. We start with the thorough examination of literature and investigation of physical principles governing the development of such a potential difference. Based on that review, we compare the use of three different models. A standard model refers to the use of Nernst equation for fuel electrode's reaction H2O/H2 or CO2/CO. An oxygen partial pressure model refers to the use of Nernst referring to oxygen partial pressure difference across the cell. A mixed potential model refers to the phenomena described by Fleming et al. as the Mixed Potential Theory (MPT), i.e. the observable potential difference is the superposition of all above potential differences. We conclude that the oxygen partial pressure model suffices for modern cells operated at temperature above 800°C and if the concentration of CO2 in the gas mixture is below 25%. Above this limit, MPT model is more accurate and thus recommended. The standard model exhibit large error in all cases and is not recommended. We also present the values of parameters, used in conjugation with MPT model, fitted to a cell with Ni-YSZ fuel electrode operated at 800°C.