AC Impedance Spectra Research Articles

Preparation and Testing of Metal/Ce0.80Gd0.20O1.90 (Metal: Ni, Pd, Pt, Rh, Ru) Co-Impregnated La0.20Sr0.25Ca0.45TiO3 Anode Microstructures for Solid Oxide Fuel Cells

La0.20Sr0.25Ca0.45TiO3 (LSCTA-) is a novel mixed ionic and electronic conductor (MIEC) material which can act as a potential replacement Solid Oxide Fuel Cell (SOFC) anode ‘backbone’ microstructure, for the current state-of-the-art Ni-based cermet. By impregnating this ‘backbone’ with electrocatalytically active coatings of metal oxides and metallic particles, it is possible to create high performance SOFC anodes which offer improved redox stability and tolerance to non-optimal fuel gases. Here, we present short-term test data for SOFC containing LSCTA- anode ‘backbones’ impregnated with a variety of catalyst systems including: Ni/CGO, Pd/CGO, Pt/CGO, Rh/CGO and Ru/CGO. Electrolyte-supported SOFC containing Ni/CGO impregnated anodes showed large reductions in Area Specific Resistance (ASR), in comparison to previous generation research (0.55 Ω cm2 versus 1.2 Ω cm2, respectively). Exchange of the Ni component, for Pd and Rh, led to much lower ASR of 0.39 Ω cm2 and 0.41 Ω cm2 (in 97% H2:3% H2O, at 900°C and 0.8 V), respectively. Equivalent circuit fitting of AC impedance spectra revealed the absence of an anode charge transfer process for the Rh/CGO catalyst system above 875°C, in comparison to all other systems, identifying this system as a potential replacement for the Ni-based cermet.

Fabrication and electrochemical properties of Li1.3Al0.3Ti1.7(PO4)3 solid electrolytes by sol-gel method

Sodium super ionic conductor (NASICON)-type solid electrolytes, Li1.3Al0.3Ti1.7(PO4)3 (LATP), have high lithium ion conductivity and chemical/electrochemical stability. In this study, we proposed a new sol-gel route to synthesize LATP precursor powder. The LATP powder was prepared from lithium nitrate (LiNO3), aluminum phosphate (AlPO4), ammonium phosphate (NH3PO4), and titanium isopropoxide (Ti-(OCH(CH3)2)4). The LATP electrolyte with high relative density (99%) and lithium ion conductivity (4.2 × 10−4 S/cm at 30 °C) could be fabricated by sintering the precursor powder at 1000 °C for 6 h. XRD analysis results revealed that the electrolyte sample sintered at 1000 °C for 6 h was impurity-free single phase LATP. FE-SEM observations showed that the grain size and density increased with increasing sintering temperature. AC impedance spectra and SEM observations suggest that the enhanced lithium ion conductivity was due to an increase in grain size and density of the LATP electrolyte.

Research on the Properties of Solid Oxide Fuel Cells Based on Composite Cathode of YBaCo1.4Cu0.6-O5+δ-xSDC.

The new type of cathode material YBaCo1.4Cu0.6-O5+δ-xSDC (x = 20, 30, 40, 50 wt.%) was synthesized by sol-gel method, electrolyte material of Ce0.8Sm0.2O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) were synthesized by glycine combustion, composite cathode materials were prepared by mixing the two materials evenly. The performance was analyzed using the ac impedance spectrum and SEM, the best performance was obtained with the ratio of x = 30%. After 2 hour sintering at 950 °C, YBCC-30%SDC cathode polarization resistance are 0.027 Ω · cm2, 0.043 Ω · cm2 and 0.074 Ω · cm2 at 850 °C, 800 °C, 750 °C, respectively. The performance of single battery YBCC-30SDC/LSGM/SDC/NiO-SDC was tested and the power density reached its maximum of 662 mW/cm2 at 850 °C.

Pyroelectric properties and AC impedance study of bismuth ferrite (BiFeO3) ceramics

The pyroelectric properties of bismuth ferrite (BiFeO3) ceramics has been investigated over temperatures between 25 °C and 180 °C. AC impedance spectra have been measured over a wide frequency range from 2 mHz to 100 kHz. The results have been analyzed with respect to permittivity, conductivity, impedance and electric modulus. Based on an equivalent circuit model the bulk resistance Rbulk has been obtained from extrapolation of their Cole-Cole plots. It has been found that Rbulk decreases rapidly with increasing temperature. Origins of the observed results were discussed.

Large strain response in (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xSr0.8Bi0.1▫0.1Ti0.8Zr0.2O2.95 lead-free piezoelectric ceramics

A series of novel lead-free (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xSr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95 (SBTZ) ceramics were prepared via a traditional solid state sintering method. The ferroelectric, dielectric, AC impedance and fatigue properties were systematically studied. The introduction of VO and VA decreased the stability of ferroelectric phase, facilitated field-induced relaxor to ferroelectric transition and resulted in pinched and sloped P-E curves. The dielectric properties indicated that the relaxor phase of prepared ceramics was gradually dominated and the temperature of phase transition (TF-R) went downwards to room temperature with the increase of SBTZ. Therefore, a giant unipolar strain of 0.494% with a normalized strain d33* of 706 pm/V was obtained at x = 0.06. The AC impedance spectra of x = 0.06 ceramics revealed that its capacitance mechanism was derived from bulk effect. In addition, the stable fatigue resistance properties implied that the prepared ceramics were promising candidates for strain sensors and actuators.

Analysis of interfaces in Bornite (Cu5FeS4) fabricated Schottky diode using impedance spectroscopy method and its photosensitive behavior

In this report, we have synthesized Bornite (Cu5FeS4) material by hydrothermal synthesis technique. The interface characteristics of Al/Cu5FeS4/FTO Schottky barrier diode (SBD) are investigated by using ac impedance spectroscopy (IS) analysis (under dark condition) and dc current-voltage (I–V) measurements (under dark and light both condition). IS is a powerful tool to identify the interface regions of SBDs. Ac impedance spectra of Al/Cu5FeS4 SBD are recorded in the frequency range 40 Hz-20 MHz during dc bias scanning from -0.6 V to 0.6 V under dark condition. The diode parameter including ideality factor and barrier height is calculated from the conventional I–V measurement based on thermionic emission (TE) theory. Space charge limited current (SCLC) theory has been employed to further exemplify the improved performance of Cu5FeS4 based SBD, which points out that the carrier mobility is enhanced ∼2-fold after irradiation of light.

The Research of Copper Ion Doped Lanthanum Silicate Electrolyte

La9.33Si6-xCuxO26-x powders with high purity were prepared. Using XRD, IR, SEM and ac impedance spectrum, crystal structure, surface microstructure and conductivity properties were respectively characterized. The results showed that the La9.33Si6-xCuxO26-x grains embrace the crystal structure of p63/m apatite and the optimum sintering temperature of La9.33Si6-xCuxO26-x electrolyte was 1500 ℃. The ac impedance spectrum test for the La9.33 Si6 xCuxO26 - x (x = 0.1, 0.2, 0.1, 0.2, 0.5) in different temperature conductivity showed that La9.33Si6-xCuxO26-x had the highest ionic conductivity which value was up to 0.0735 S·cm-1 when the doping content was 0.3.

Enhanced ionic conductivity in LAGP/LATP composite electrolyte**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0100100), the National Natural Science Foundation of China (Grant Nos. 52315206 and 51502334), Fund from Beijing Municipal Science & Technology Commission, China (Grant No. D171100005517001).

Nasicon materials (sodium superionic conductors) such as Li1.5Al0.5Ge1.5(PO4)3 (LAGP) and Li1.4Al0.4Ti1.6(PO4)3 (LATP) have been considered as important solid electrolytes due to their high ionic conductivity and chemical stability. Compared to LAGP, LATP has higher bulk conductivity around 10−3 S/cm at room temperature; however, the apparent grain boundary conductivity is almost two orders of magnitude lower than the bulk, while LAGP has similar bulk and grain boundary conductivity around the order of 10−4 S/cm. To make full use of the advantages of the two electrolytes, pure phase Li1.5Al0.5Ge1.5(PO4)3 and Li1.4Al0.4Ti1.6(PO4)3 were synthesized through solid state reaction, a series of composite electrolytes consisting of LAGP and LATP with different weight ratios were designed. XRD and variable temperature AC impedance spectra were carried out to clarify the crystal structure and the ion transport properties of the composite electrolytes. The results indicate that the composite electrolyte with the LATP/LAGP weight ratio of 80:20 achieved the highest bulk conductivity which shall be due to the formation of solid solution phase Li1.42Al0.42Ge0.3Ti1.28(PO4)3, while the highest grain boundary conductivity appeared at the LATP/LAGP weight ratio of 20:80 which may be due to the excellent interfacial phase between Li1+xAlxGeyTi2−x−y(PO4)3/LATP. All the composite electrolytes demonstrated higher total conductivity than the pure LAGP and LATP, which highlights the importance of heterogeneous interface on regulating the ion transport properties.

Ionic and electronic conductivity of solid electrolyte Li0.5La0.5TiO3 doped with LiO2-SiO2-B2O3 glass

The LiO2-SiO2-B2O3 (LBS) glass doped solid electrolyte Li0.5La0.5TiO3 (LLTO) ceramics were prepared by solid-state reaction method. The results show that the samples present mainly tetragonal perovskite structure and grain size increase with glass content increases. The frequency dependent AC conductivity shows two plateaus that correspond to the bulk and grain boundary responses associated with two semi-circles observed in AC impedance spectrum. The maximum bulk ionic conductivity appeared at 1.0 wt% glass addition calculated by equivalent circuit at a value of 1.12 × 10−3 S/cm, which is nearly 1.7 times compared to pure LLTO at room temperature. The overall electronic conductivity is about 10−9 S/cm calculated from a long time leakage current measurement, which is 6 orders of magnitude lower than the ionic conductivity σbulk. These results would be helpful for us to adjust and control the conductivity of glass added solid electrolyte composite for use of all-solid-state lithium batteries.

Corrosion Resistance Property of Mild Steel in Simulated Concrete Pore Solution Prepared in Well Water by Using an Aqueous Extract of Turmeric

Corrosion resistance property of mild steel immersed in simulated concrete pore solution (SCPS) prepared in well water has been evaluated by weight loss method, in the absence and presence of an aqueous extract of Turmeric (Curcuma longa L), The main constituent of the plant extract is curcumin It is observed that in the presence of an aqueous extract of turmeric corrosion inhibition efficiency increases. The mechanistic aspects of corrosion inhibition have been studied by weight loss method, polarisation study and AC impedance spectra. In the presence of the Turmeric extract, the inhibition efficiency increases, linear polarisation resistance value increases; corrosion current decreases; charge transfer resistance value increases and double layer capacitance decreases. Thus it is observed that in the presence of turmeric inhibitor the corrosion resistance of mild steel incorporated in concrete structure increases. Thus the corrosion resistance of mild steel incorporated in concrete structure can be improved by addition of an aqueous extract of turmeric.

Ag@Sr2TiO4/Bi5O7I Heterostructured Composite for Solar-Driven Photoelectrochemical Analysis

Plasmonic Ag nanoparticles were photodeposited on the surface of Sr2TiO4 and Bi5O7I to fabricate a photoelectrode for photoelectrochemical measurements. The Ag@Sr2TiO4/Bi5O7I heterostructured composite exhibited extended visible-light absorption, surface plasmonic resonance, improved photocurrent response, decreased film resistance, and increased donor density. The semiconducting properties of these samples, including the conduction and valence band levels, flatband potential, and donor density, were obtained from AC impedance spectra, a Mott–Schottky analysis, and a linear current–potential scan under the illumination of a xenon lamp with an AM1.5 filter to simulate AM 1.5-type solar irradiation. The Mott–Schottky analysis clearly displayed the formation of a p–n junction at the interface of the Ag@Sr2TiO4/Bi5O7I composite, which is advantageous to the electron transfer. Thanks to its interfacial p–n junction and the surface plasmonic resonance effect of Ag on the Ag@Sr2TiO4/Bi5O7I composite, this photoelectrode can serve as a potential candidate for solar-driven photoelectrochemical applications.

Development of disk-type solid oxide electrolysis cell for CO2 reduction in an active carbon recycling energy system

CO2 recycling method characterized by the application of an Active Carbon Recycling Energy System (ACRES) to an iron- making process (iACRES) was studied. In this system, pure iron is produced from the reduction of iron oxide using regenerated CO by high temperature CO2 electrolysis with the use of a disk-type solid oxide electrolysis cell (SOEC) and powered by a high temperature gas-cooled reactor (HTGR) generating electricity and thermal energy.Thus, the focus of this research is to further improve the performance of disk-type SOEC by developing its structure and determining the most suitable electrode and electrolyte materials, and the appropriate fabrication method. In this present study, new cathode material such as Ni-SDC (SDC: samarium-doped ceria) was checked and tested for the SOEC. The electrochemical performance of the cells were evaluated at 800 ~ 900 °C by analyzing current-voltage characteristics, AC impedance spectra, and CO production data. Results showed CO production rates close to theoretical values corresponding to current density values observed. The performances of the SOEC samples were investigated for their potential use in iACRES, and the SOEC with Ni- SDC cathode showed the most enhanced electrochemical performance for iACRES.The results showed that iACRES is available for utilization of the high temperature heat at about 850 ~ 950 °C provided by HTGR in this system. This would mean utilization of CO2 emission-free thermal energy, and eventual expansion of use of nuclear energy beyond electrical power generation.

Comprehensive Understanding of Electrode Processes in PEMFCs Exposed to Various Pollutants Using Spatial AC-Impedance Spectroscopy

The success of recent deployments of automotive and stationary sources of energy utilizing proton exchange membrane fuel cells (PEMFCs) relies on a high performance under a broad range of environmental conditions. However, PEMFCs using Pt-based catalysts as electrode materials were shown to be very sensitive to many fuel and air contaminants, which typically originate from natural as well as anthropogenic emission sources [1]. The knowledge of the electrochemical behavior of impurities and their intermediates on Pt is a principal requirement for understanding their impacts on the oxygen reduction reaction (ORR), fuel cell performance, mitigation and recovery procedures. AC-impedance is a powerful in-situ technique for determination and characterization of different processes in electrochemical systems. In this work, PEMFC electrode reactions occurring under exposure to various airborne contaminants (SO2, NO2, VOCs) were studied by spatial AC-impedance, which gives locally resolved data over the electrode area, while a single cell only provides average values of measured parameters. The experimental work was performed using a GRandalytics test station, a segmented cell system and a commercially available 100 cm2 membrane/electrode assembly (MEA) [2]. The MEA was operated under galvanostatic control of the total cell current. Many airborne impurities including inorganic and organic compounds (SO2, NO2, propylene, acetylene, acetonitrile, isopropanol, benzene, naphthalene, bromomethane, methyl methacrylate, etc) were studied. The electrochemical properties of the selected compounds are typically potential dependent. This should be taken into account when considering the ORR which occurs in parallel with contaminant chemical and electrochemical transformations. The effects of the studied pollutants on fuel cell impedance can be separated into three major groups: 1) the impurity does not significantly affect charge and mass transfer resistances, 2) the impurity increases charge and mass transfer resistances and, 3) the impurity leads to the appearance of a low frequency inductance. The first type of behavior was found when the cathode was poisoned by isopropanol and SO2 (at a steady state condition) (Fig. 1 a and c). Rotating ring/disk electrode (RRDE) data showed that isopropanol and SO2 (at a sulfur coverage below 0.3) do not significantly change the ORR mechanism on Pt, which proceeds via a 4-electron pathway [3, 4]. Contaminant adsorption on Pt decreases the electrochemical surface area (ECSA) and the exchange current density (i0) but it does not noticeably affect intrinsic Pt properties and ORR kinetics. AC-impedance is not sensitive to i0 variations at high current operation and it does not detect any changes. However, a decrease in ECSA and i0lead to a performance loss. An increase in impedance response was observed for the majority of the studied contaminants and it is connected with an increase in the Tafel slope, a decrease in the proton conductivity and possible diffusion limitations due to foreign species adsorption on Pt (Fig. 1 b). RRDE studies clearly demonstrated that these contaminants cause an increase in the Tafel slope for ORR and noticeable H2O2 production, indicating a shift in O2 reduction to combined 4- and 2-electron mechanisms [3]. This observation implies that the adsorbed contaminant not only decreases the ECSA but also alters the Pt intrinsic activity due to strong electronic interactions. The first and the second AC-impedance behavior groups are usually observed at potentials which are favorable to contaminant oxidation via electrochemical and chemical pathways. A low-frequency inductance in AC-impedance spectra indicates electrochemical reactions involving adsorbed contaminant species and proceeding with the formation of intermediates on the Pt surface. Moreover, such an inductance means that the current signal follows a perturbation with a phase delay due to the slow relaxation of adsorbate coverage compared to the ORR [5]. The inductive loop was detected for PEMFCs exposed to C2H2, SO2, CH3CN and naphthalene at potentials where the electroreduction of contaminants can occur (Fig. 1 c). A detailed discussion of AC-impedance data in connection to ORR mechanisms, contaminant reactions and poisoning propagation along the cathode channel length will be presented. In addition, anode and cathode poisoning processes will be compared. ACKNOWLEDGEMENTS The author gratefully acknowledges funding from ONR (N00014-12-1-0496) and DOE EERE (DE-EE0000467), and Hawaiian Electric Company support. The author thanks G. Randolf for system operation support and A. Kulikovsky for discussion. REFERENCES Y. Zhai, J. St-Pierre, M. Angelo, ECS Trans., 50(2), 635 (2012).T.V. Reshetenko, K. Artyushkova, J. St-Pierre, J. Power Sources, 342, 135 (2017).J. St-Pierre et al., in DOE Hydrogen and Fuel Cells Program - 2014 Annual Progress Report, DOE/GO-102014-4504, November 2014, pp. V-133-V-139.Y. Garsany, O. Baturina, K. Swider-Lyons, J. Electrochem. Soc., 154, B670 (2007).D.A. Harrington, B.E. Conway, Electrochim. Acta, 32, 1703 (1987) Figure 1

Simulation of HT-PEMFC AC Impedance Spectra: Relaxation Impedance and Identification of Oxygen Reduction Reaction Mechanism

High Temperature Polymer Electrolyte Membrane Fuel Cells, (HT-PEMFCs), are one of the most promising alternatives of clean power production by converting chemical energy to electrical energy. Even though the operating principles of an HT-PEMFC are understood, the overall system’s behavior is determined by a number of strongly coupled processes, each proceeding at a different rate.In a HT-PEMFC the elusive multistep Oxygen Reduction Reaction, (ORR), taking place at the cathodic electrode takes a major fraction of the open circuit potential. Thus, in order to optimize fuel cell performance by minimizing ORR power losses, an in depth understanding of the various processes and their interactions is necessary. One of the most powerful electrochemical characterization methods is the Electrochemical Impedance Spectroscopy technique, (EIS). It is a very powerful and sensitive in situ technique that has the ability to separate potential losses corresponding to different processes which proceed at different rates, (have different timescales), [1]. Our research revealed that ORR impedance spectrum consists of two arcs, a high frequency arc attributed to the charge transfer reactions across the interface and a low frequency arc due to the hysteresis phenomena between the oxygenated intermediate species coverage and the current dynamic response, (ORR relaxation impedance), [2]. Thus, by simulating the AC impedance spectra through the development of an appropriate physical model, valuable information on the electrochemical and chemical process at the electrode can be drawn. In order to demystify the multistep ORR kinetics a 1-D dynamic mathematical model has been developed in order to simulate AC impedance spectra at the low current linear activation region while at the same time a Monte Carlo type algorithm was used to estimate the values of the respective kinetic constants by simultaneously fitting polarization-dc bias and the corresponding galvanostatic AC Spectra. The model incorporates a three-step proposed mechanism for the ORR as well as a detailed macroscopic description of mass and charge transfer across the various parts of the Membrane Electrode Assembly, (MEA). The low current activation regime of operation was chosen because there the ORR overpotential dominates the AC spectra, the effect of the finite electrolyte conductivity and mass transport limitations are minimized and the adsorbed intermediate relaxation impedance is prominent while it diminishes at higher current densities as can be seen in Figure 1. As a result, the cathodic overpotential is constant throughout the cathodic electrode which may be well considered to operate under ideal kinetic conditions allowing thus an analytic expression for the HT-PEMFC impedance to be developed and used in the identification of the ORR kinetics. As far as validation of our approach is concerned, synthetic AC Impedance Spectra with known kinetic constants where produced and utilized by the Monte Carlo fitting algorithm at the low current operation regime. Figure 2 shows the histograms of the fitting algorithm for the OH intermediate reduction kinetic constant, by employing different number of bias points (I-V) and their respective AC spectra the algorithm successfully converges to the set value for k3+ , (k3+ =2*10-6 mol/(m2s)). By decoupling the various processes of the simulated AC impedance spectra, information was successfully extracted on the various kinetic and equilibrium constants related with the interaction of the reacting gases in the catalytic layer and the electrochemical interface. [1] O. Antoine, Y. Bultel, and R. Durand, Journal of Electroanalytical Chemistry, vol. 499, no. 1, pp. 85–94, Feb.2001. [2] X. Yuan, H. Wang, J. Colinsun and J. Zhang. 2007. International Journal of Hydrogen Energy, Volume:32, 4365-4380. This work has been conducted and supported within the framework of DeMStack, (DEMSTACK 325368, FCH-JU-2012-1). Figure 1

Structural, optical and dielectric studies of Er substituted zinc ferrite nanospheres

The cationic distributions among tetrahedral and octahedral sites in spinel ferrites have a profound influence on their properties. Many studies were reported with various transition metal (TM) and rare earth (RE) cations distribution. We report the synthesis, structural, morphology, lattice vibrational, optical and dielectric properties of heavy RE cation (Er) substituted ZnFe2O4 as a function of different experimental parameters. The coprecipitated and calcined ZnFe2-xErxO4 (x = 0, 0.2, 0.4, 0.6 & 0.8) compounds crystallizes in cubic spinel structure and show narrow particle size distribution due to surfactant PEG. Particle size reduces (27-16 nm) with enhance of Er concentration (x = 0 to 0.8) in the compounds. The EDAX spectra of the samples incorporation Er3+ ions into the lattice and the cubic lattice parameter enhances with Er3+ concentration. The lattice vibrational spectra reveal that the particles surface were free from surfactant PEG. Optical excitation studies show that energy band gap (Eg) of compounds reduces (1.85–2.00 eV) with enhance of Er concentration. The temperature, as well as frequency dependent dielectric constant (εrʹ) of ZnFe2-xErxO4, shows enhanced εrʹ with Er concentration up to x = 0.4 and then reduces with further enhancing of Er concentration. The dielectric loss factor (εrʹʹ) show similar variation with Er concentration. Further, both εrʹ and εrʹʹ were reduced with enhancing of field frequency. In addition, both εrʹ and εrʹʹ were enhanced with temperature and showed exponential change at low frequencies and high temperature and is attributed magnetic transition at around 525 K. The interface polarization mechanism is the predominant one in the present samples. The ac impedance spectra show two semicircles and each semicircle results from dielectric relaxation due to either particle (grain) or particles interface (grain boundary) polarization. The ac impedance reduces with enhancing of temperature. From the above studies, it is concluded that ZnFe1.6Er0.4O4 composition was best for high εrʹ and εrʹʹ values at low frequencies. All samples could be used in high-frequency application.

Electro analytical studies on indium incorporated SnSb alloy anode for Li-ion batteries

Preparation of indium doped tin antimony (SnSb) alloy is achieved with simple co-precipitation technique. Structural analysis from XRD and Raman spectra shows the phase purity, crystallinity of the synthesized alloys. From XRD data calculated crystallite sizes are 35nm for SnSb:In and 21nm for pristine. SEM-Morphological images show spherical nature of the synthesized particles. Electrical conductivity analyses carried out via AC impedance spectra for both the samples at the frequency range of 100kHz to 100mHz reveal the improved bulk conductivity of doped SnSb. Better electrochemical lithiation and de-lithiation behavior of SnSb:In than SnSb is observed from cyclic voltammogram in organic and aqueous electrolytes. From cyclic voltammetry analysis the reversible nature and the ability to provide stable capacitance over higher number of cycles are observed which is desirable for a potential anode material to be used in Li-ion batteries.

A Comparative Study on Sintering Behaviour of Low and High Density Pellets of Ni-YSZ by Electrochemical Impedance Spectroscopy

In this study, a systematic investigation on in-situ sintering behavior of Ni-YSZ (50: 50wt. %) pellets of density of 4.2 (low density) and 4.9 g/cm3 (high density) in ambient and oxygen environment by impedance spectroscopy is presented. X-ray diffraction indicated the formation of cubic phases of NiO and YSZ. The low density pellet sintered for 16 h showed low content of monoclinic phase when compared to high density pellet. The microstructure of the high density pellet revealed finer and homogenous distribution of Ni in YSZ matrix due to longer sintering duration when compared with the low density pellet. AC impedance spectra were recorded for both low and high density pellets during sintering in ambient and oxygen environment in the temperature range 873-1173 K. The results indicate that for both the pellets, the impedance values decreased when sintering temperature increased from 873 to 1173 K in both ambient and oxygen environment. However, the impedance was low while sintering in oxygen atmosphere than in ambient. Besides these observation, impedance of the high density pellet was much lower than that of the low density pellet at all sintering temperature in both ambient and oxygen atmosphere. While the impedance decreased with increasing sintering temperature, the capacitance increased slowly in both the ambient and oxygen atmosphere. The change in the impedance behavior due to grain interior and grain boundaries is explained in relation with the microstructural changes that occur during sintering in different environments.

Charge transfer resistance of IB and VIB family electrodes in 1 M Na2SO4

The ac impedance spectra of IB and VIB family metals (Cu, Ag, Au, Cr, Mo and W) in 1M Na2SO4 were investigated by electrochemical impedance spectroscopy. Charge transfer resistances of the metals were obtained by simulation of the impedance spectra with Randle’s equivalent circuit. It is found that the charge transfer resistances of IB and VIB family metals display periodic properties. Metals with large standard electrode potentials display large charge transfer resistances. VIB family metals display larger charge transfer resistance compared with IB family metals. Our work provides basic understanding of the electrochemical performance of metal electrodes in the charge transfer process enabling the design and development of high-power and high-rate electrochemical devices.

Ni-SDC Based Cermets for Direct Dry Reforming of Methane on SOFC Anode

Solid oxide fuel cells operated with direct internal reforming of carbon-containing fuels (DIR-SOFC) can achieve high energy conversion efficiency because heat generated with electrochemical reaction can be effectively used with endothermic reforming reaction [1]. Up to now, many researchers have studied DIR-SOFC with steam reforming of hydrocarbons, but recently it was reported that a DIR-SOFC system with dry reforming of methane (CH4 + CO2 → 2H2 + 2CO) showed higher electric efficiency than conventional one with direct steam reforming of methane [2]. Moreover, DIR-SOFC with dry reforming of methane enables us to utilize biogas, mixture of methane and carbon dioxide, as fuel, contributing to reducing greenhouse gas emissions. However, when applying dry reforming reaction, carbon formation, which can cause significant deterioration of nickel in the anode, is more prone to occur than when operating on steam and methane fuel. Therefore, developing new anode materials with high coking resistance is desirable for practical use of DIR-SOFC with dry reforming of methane. Ceria-based oxides are widely used as anode components for DIR-SOFC due to their high O2- and e- conductivity in reducing atmosphere. This mixed ionic electronic conduction can expand the reaction sites over the surface of electrode, and it is also expected that carbon removal reaction is more likely to proceed. So in this study, Ni-Ce0.8Sm0.2O2- δ (samarium doped ceria; abbreviated as SDC) was applied as anode material for DIR-SOFC with dry reforming of methane. In addition, CaO addition to Ni-SDC anode was examined, aiming at enhancing coking resistance and CO2 reactivity by basicity of the additive. SDC powder was prepared by co-precipitation method, and then it was mixed with NiO at a volumetric ratio of Ni-metal:SDC = 60:40, followed by calcination at 1300°C for 5 h in air. The obtained cermet was screen-printed with polyethylene glycol on YSZ disk (20 mm in diameter, 0.5 mm in thickness), and then calcined in air to form porous Ni-SDC electrode. CaO-modified Ni-SDC anode was fabricated by infiltrating Ca(NO3)2 solution on the Ni-SDC anode, followed by calcination at 1200°C for 2 h in air. Power generation performance of the cells was evaluated by current-voltage characteristics at 1000, 900, 800, 700°C with VersaSTAT3 (Ametek Inc.), feeding 0.3% humidified H2 or the mixture of CH4 and CO2 on anode side and pure O2 on cathode side. In addition, stability tests on direct dry reforming operation were conducted under a constant current load. Also, alternative current impedance spectroscopy in humidified H2 flow was conducted under open circuit condition before and after the power generation tests to evaluate the durability of the cells. Figure 1 shows I-V characteristics of the cells with Ni-SDC, CaO-modified Ni-SDC and Ni-YSZ anode, in hydrogen operation and direct dry reforming operation at 1000°C. Ni-SDC based anodes showed higher performance than conventional Ni-YSZ, with both hydrogen and dry reforming fuel. CaO-modified Ni-SDC showed slightly lower performance than pristine Ni-SDC possibly because CaO is an insulator and may have lowered the conductivity of the anode. In Figure 1(b), Ni-YSZ displayed lower open circuit voltage and steep I-V curve, which indicates catalytic activity of Ni-YSZ was insufficient to promote dry reforming and thus leading to low H2 partial pressure and high O2 partial pressure in the anode. On the other hand, for Ni-SDC based anodes, the open circuit voltage was higher and the slope of I-V curve was as small as that with hydrogen fuel. For Ni-SDC and CaO-modified Ni-SDC anodes, overvoltages were separated into ohmic resistance, electrode reaction overvoltage and gas diffusion overvoltage from AC impedance spectra. Although CaO-modified anode showed almost no difference before and after power generation test, pristine Ni-SDC anode, on the other hand, showed larger ohmic resistance and gas diffusion overvoltage after power generation, which probably due to carbon deposition on anode.

Investigation of Solid Oxide Electrolysis Cell Electrodes for Methane Synthesis

Electrochemical methane synthesis from CO2 and water is one of the means of storing intermittent renewable energy. It also attracts attentions because it can contribute to the fuel cycle which does not rely on fossil sources. Methane-synthesis solid oxide electrolysis cells (MS-SOEC) can be a candidate technology for such processes. The concept is shown in Figure 1. A novel point of this concept is that CO methanation reaction is integrated into well-known co-electrolysis. The methanation is favoured at relatively low temperature (below 800oC) from the equilibrium viewpoint. It is reported that exothermic nature of the reaction leads to smaller heat loss when combined with endothermic electrolysis, which results in higher round-trip efficiency for energy storage[1,2]. However, the number of experimental researches on MS-SOEC is very limited, and investigation of suitable materials for MS-SOEC is necessary to realize this concept. In this study, several electrodes were tested for MS-SOEC application. Because anode polarization resistance is dominant in SOEC operation[3], anode materials play an important role. Perovskite-type oxide-based materials are commonly used for SOEC anodes. Among them, LSM (La0.6Sr0.4MnO3-δ) and LSCF-SDC composite (LSCF: La0.6Sr0.4Co0.2Fe0.8O3-δ, SDC: Ce0.8Sm0.2O1.9, LSCF 50wt%) were compared. Electrolyte and cathode were fixed in YSZ (8mol% yttria stabilized zirconia) and Ni-SDC composite (Ni 60wt%), respectively. Electrode pastes were screen-printed on both sides of YSZ electrolyte discs and calcined for 5 hours at designated temperatures. For LSCF-SDC anode, an interlayer of SDC was placed between the electrolyte and the anode using spin coating technique to prevent them to react. Before measurements, NiO in the cathode composite was reduced by hydrogen flow at 1000oC. In MS-SOEC test, 30 mL min-1 air was fed to the anode side while cathode feed gas was CO2(10)/H2O(5)/H2(10)/N2(7) mixture. Figures in parentheses indicate gas flow rate in mL min-1. Hydrogen was added to the cathode gas to stabilize the cell performance. This is a possible condition in industrial operations, because part of the cathode outlet gas can be recycled and mixed into feed stream. Cell operating temperature was set to 800oC. AC impedance measurements were conducted for frequency range of 106~0.1 Hz with amplitude of 10 mV RMS. Current-voltage characteristics were also measured. Cathode outlet gas composition was checked by micro gas chromatograph for open circuit and 1.0 A cm-2 galvanostatic conditions. With the aim of improving cathode performance, typical cermet of Ni-YSZ was modified by ruthenium impregnation. Ruthenium is expected to suppress nickel oxidation which is likely to occur in oxidizing atmosphere, and also has high activity for methanation reaction. Therefore, it is a promising candidate for MS-SOEC cathode additives. Ruthenium nitrate solution was dropped on calcined cathode of NiO-YSZ|YSZ|LSM cell and dried at room temperature. Ruthenium amount was set to be 5% of cathode weight. Then the cell was kept in nitrogen atmosphere during temperature rising, and at 1000oC, ruthenium precursor was reduced along with NiO by hydrogen flow. AC impedance spectra for cathode were taken in three-electrode configuration at 800 and 600oC, then the data were analyzed by introducing Δ∂Z' spectra[4]. In this experiment, gas conditions were the same as the previous tests. In all tests, progression of co-electrolysis and methanation were confirmed by gas analysis. According to AC impedance spectra of Ni-SDC|YSZ|LSM and Ni-SDC|YSZ|SDC|LSCF-SDC cells, significantly small non-ohmic resistance was observed with LSCF-SDC anode. Galvanostatic test results in Figure 2 indicate that cell performance was more stable with LSCF-SDC. Impedance data of the cells with or without ruthenium were compared by calculating Δ∂Z' value. The resultant Δ∂Z' spectra showed peaks around 1~10 Hz. This implies that electrochemical activity was enhanced by ruthenium addition.