Measurement of electrode overpotentials for direct hydrocarbon conversion fuel cells

Cathodic and anodic overpotentials were measured using current interruption and AC impedance spectroscopy for two separate solid oxide fuel cells (SOFCs). The fuel cells used yttria-stabilized zirconia (YSZ) as the electrolyte, strontium-doped lanthanum manganite (LSM) as the cathode, and a porous YSZ layer impregnated with copper and ceria as the anode. The Cu/CeO₂/YSZ anode is active for the direct conversion of hydrocarbon fuels. Overpotentials measured using both current interruption and impedance spectroscopy for the fuel cell operating at 700 °C on both hydrogen and n-butane fuels are reported. In addition to providing the first electrode overpotential measurements for direct conversion fuel cells with Cu-based anodes, the results demonstrate that there may be significant uncertainties in measurements of electrode overpotentials for systems where there is a large difference between the characteristic frequencies of the anode and cathode processes and/or complex electrode kinetics.     

Electrochemical performance of ceria-gadolinia electrolyte based direct carbon fuel cells

A direct carbon fuel cell offers a high efficiency alternative to traditional coal fired electrical power plants. In this paper, the electrochemical performance of electrolyte supported button cells with Gd₂O₃-doped CeO₂ (CGO) electrolyte is reported over the temperature range 600 to 800 °C with solid carbon as a fuel and He/CO₂ as the purge gases in the fuel chamber. The electrochemical characterisation of the cells was carried out by the Galvanostatic Current Interruption (GCI) technique and measuring V-I and P-I curves. Power densities over 50 mWcm^-2 have been demonstrated using carbon black as the fuel. Results indicate that at low temperatures around 600 °C, the direct electrochemical oxidation of carbon takes place. However, at higher temperatures (800 °C) both direct electrochemical oxidation and the reverse Boudouard reaction take place leading to some loss in fuel cell thermodynamic efficiency and reduced fuel utilisation due to the in-situ production of CO. In order to avoid reverse Boudouard reaction whilst maximising performance, an operating temperature of around 700 °C appears optimal. Further, the electrochemical performance of fuel cells has been compared for graphite and carbon black fuels. It was found that graphitic carbon fuel is electrochemically less reactive than relatively amorphous carbon black fuel in the DCFC when tested under similar conditions.     

电化学阻抗谱在本科实验教学中的应用

电化学阻抗谱是常用的一种电化学测试技术,该方法具有频率范围广、对体系扰动小的特点,是研究电极过程动力学、电极表面现象以及测定固体电解质导电率的重要工具。本文以固体氧化物燃料电池的电化学阻抗谱的测试为例设计了一套在线分析电化学阻抗的应用课程。介绍了电化学阻抗谱的原理,数据采集方法和数据处理方法,阐述了电化学阻抗谱技术在本科实验教学中的可行性和重要性。     

Oxygen-ion conducting electrolyte materials for solid oxide fuel cells

Oxygen-ion conducting solid electrolyte systems have been reviewed with specific emphasis on their use in solid oxide fuel cells. The relationships between phase assemblage, electrolyte stability and ionic conductivity have been discussed. The role of parameters such as sintering temperature and atmosphere which influence the segregation of impurities, present in the starting ceramic powders, at grain boundaries and at the external surface of the electrolyte compacts has been emphasised. The stability of various electrolyte materials in contact with other fuel cell components and in fuel environments has been discussed in detail. The ageing behaviour at fuel cell operating temperatures has been described. Data on ionic conductivity, mechanical and thermal properties have been presented for a number of electrolyte materials.     

Novel solid acid fuel cell based on a superprotonic conductor Tl3H(SO4)2

We have fabricated a fuel cell based on a superprotonic conductor, a Tl₃H(SO₄)₂ crystal, and have measured the electrical properties of this fuel cell. It is found that the open-circuit voltage in the fuel cell based on the Tl₃H(SO₄)₂ crystal increases by supplying H₂ fuel gas and typically becomes 0.83 V. Moreover, we have observed that the cell voltage decreases with increasing current density, as observed in fuel cells such as proton exchange membrane fuel cell, solid oxide fuel cell, etc. These results indicate that it is possible to use the Tl₃H(SO₄)₂ crystal as the electrolyte of a solid acid fuel cell. In addition, we suggest that the selection of the electrode and the preparation of the very thin electrolyte are extremely important to achieve high-efficiency of power generation of this fuel cell.     

Three-electrode solid-state fuel cell for evaluating working electrodes

Development of a commercial solid-state fuel cell depends on identification of suitable catalytic electrodes to replace platinum. A three-electrode test cell for electrode evaluation is reported. The solid protonic electrolyte used was dodecamolybdophosphoric acid, H₃Mo₁₂PO₄₀·29H₂O, and a thin platinum wire inserted into the electrolyte served as the third electrode. Reproducibility and insensitivity to third-electrode position were demonstrated. The third electrode measures separately the anode and cathode interfacial resistances, thus providing a direct measure of the relative catalytic activity of a given test electrode. Application of the technique is illustrated.     

Impurities and solid electrolyte failure

A thermodynamically based model is used to assess the effects of the formation of a layer of lowered ionic conductivity and increased electronic conductivity on the applied voltage at which solid electrolyte failure is to be expected. Such a layer can result from electrolyte contamination with electrode impurities. It is found that when a significant electrolyte resistivity increase is produced, electrolyte failure can be initiated by a mechanism that deposits sodium internally in the solid electrolyte. The results stress the necessity of operating solid electrolyte cells, such as Na/S, in a manner that minimizes electrode contamination.     

Cathode overpotential investigation by means of “Built-in” potential electrode

The objective of the present work is the development of a “built-in” potential electrode method for direct measurements of the cathode and anode overpotentials and the corresponding interface resistances of solid oxide fuel cells (SOFC). The studies were performed on a yttria-stabilised zirconia (YSZ) electrolyte-supported SOFC using La_0.8Sr_0.2MnO₃ as cathode, GDC as protecting layer and Ni-ScSZ cermet as anode. The mesh potential electrode was placed inside the YSZ membrane near the cathode side. Using the combination of the I-U and the impedance measurements with the built-in potential electrode technique, the temperature dependencies of the electrodes and electrolyte contributions to the total cell resistance were determined.     

Engineering of solid state ionic devices

Solid state ionic devices such as high performance batteries, fuel and electrolysis cells, electrochromic devices, chemical sensors, thermoelectric converters or photogalvanic solar cells are of tremendous practical interest in view of our energy and environmental needs. The challenges are the achievement of higher energy and power densities, longer lifetimes, cheaper materials, lower cost, improved sensitivity and higher stability. The engineering of new devices is based on the better fundamental understanding of materials for galvanic cells and their interaction in order to approach solutions more systematically than in the past. The fundamental aspects of the generation of voltages and electrical currents are compiled and analysed in view of the materials requirements. Conflicts exist in forming chemically stable interfaces of functionally different electrolyte and electrode materials, achieving simultaneously high energy and power densities in view of low conductivities of chemically stable materials, fast chemical diffusion in electrodes which should have a wide range of non-stoichiometry for delivering and absorbing the mobile ionic species, practical problems of using less expensive polycrystalline materials which have high intergranular resistances and finally reaching both ionic and electronic equilibria at the electrolyte - electrode interfaces at low temperatures. The engineering of new or improved solid state ionic devices is commonly based on individual materials considerations and their interaction in galvanic cells. Simultaneously high ionic conductivity and chemical stability may be reached by designing structures of poly-ions of the non-conducting components with the conducting species in-between. The chemical stability may be based on kinetic restrictions for sufficiently long periods of time of operation of the devices. Electrodes should not be made of metallic conductors but of electronic semi-conductors with fast enhancement of the diffusion of ions by internal electrical fields. Device considerations are based on the development of single element arrangements (SEAs) which incorporate the electrodes into the electrolyte in the case of fuel and electrolysis cells. The electronic conductivity is generated by the applied gas partial pressures or the applied voltage. The same simplification may be applied for electrochromic systems which consist of a single active layer instead of the conventional three galvanic cell materials. A new design of active chemical sensors probing the environment by the magnitude of the applied voltage or current may overcome the limitations of cross sensitivities and interfacial reactions which allows the simultaneous detection of several species by a single galvanic cell. The paper has been prepared for presentation at the International Conference on Ionic Devices — 2003, Anna University, Nov. 28–30, 2003, Chennai, India.     

Electrochemical impedance spectroscopic study of the lithium storage mechanism in commercial molybdenum disulfide

The first lithiation and delithiation processes of commercial molybdenum disulfide (MoS₂) electrode as anode material for lithium-ion batteries were studied by electrochemical impedance spectroscopy (EIS). It is found that the typical EIS is composed of four parts, namely, high-frequency semicircle, middle-frequency semicircle, low-frequency short sloping line, and low-frequency arc in the Nyquist diagram, and they can be attributed to the solid electrolyte interphase (SEI) film and ionic resistance in pores, charge transfer step, solid state diffusion process, and phase transformation, respectively. An equivalent circuit that includes elements related to the SEI film and charge transfer process, in addition to phase transformation, is proposed to simulate the experimental EIS data. The change of kinetic parameters for lithiation and delithiation of MoS₂ electrode as a function of potential in the first charge–discharge cycle is analyzed, and the reason for the rapid degradation in capacity of the MoS₂ electrode when cycled between 3.00 and 0.01 V is discussed in detail.     

SOFC characteristics along the flow path

Solid oxide fuel cells (SOFCs) in metallic housings were integrally and locally characterised. The tests were performed in counter flow operation for hydrogen concentrations from 2% to 100%, to identify concentration limitations and to optimise fuel utilisation. Cell characterisations were performed by spatially resolved electrochemical impedance spectroscopy (EIS), current density/voltage (i–V) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilisation. The fuel utilisation was calculated from the local current densities and compared to the values measured by gas chromatography. Both sets of results showed good agreement. At low hydrogen inlet concentrations the voltage at the fuel outlet drops to values that might be harmful for the stability of the anode since reoxidation of nickel can occur. The impedances obtained by local EIS did not show an overall coherent dependency on the hydrogen concentration. EIS under load revealed two distinct domains: in the range of hydrogen concentrations of 2–10% H₂ the impedance decreased significantly with increasing hydrogen content whereas at higher hydrogen contents the impedance was hardly affected. This indicates significant concentration and diffusion overpotential at low hydrogen concentrations. The local data showed differing behaviour in the middle of the cell compared to the fuel outlet. Leakage at the sealing could be identified as a possible reason. As an additional method of investigation, the voltage drop over the contact resistance of the cathode side was measured. Temperature measurements show that local temperatures differ significantly depending on the load applied to the cell. This observation emphasizes the importance of a thermal management adapted to the characteristics on operation conditions of the cells, particularly when the stack itself has only a low mass.     

Solid hydrocarbon conversion in a fuel cell with molten carbonate electrolyte

A new architecture of a molten carbonate electrolyte fuel cell is considered. The architecture ensures efficient implementing the processes of direct electrochemical solid hydrocarbon oxidation. Experimental results demonstrate the possibility of achieving high specific characteristics of the fuel cell (current density and specific power values) by using graphite, various types of coal, and plastic as the dispersive fuel. The effect of the organic fuel composition on the energy parameters of the electrochemical cell is illustrated. It is shown that the fuel oxidation rate and the achievable maximal values of the specific power increase with the relative amount of hydrogen. It is concluded that application of the proposed ideology is promising for creating real energy devices with direct electrochemical oxidation of solid hydrocarbons.     

Overview on grain-boundary and transport problems in solid oxide fuel cell

An overview is given on the present state of development of fuel cells based on solid oxygen ion conductors. The performance is compared to other types of fuel cells. The employed electrolyte and electrode materials and current problems are described. The possibilities of reduced temperature and the reduction of internal resistances is discussed. Paper presented at the 97th Xiangshan Science Conference on New Solid State Fuel Cells, Xiangshan, Beijing, China, June 14–17, 1998.     

On the reversibility of anode supported proton conducting solid oxide cells

Reversible proton conducting solid oxide cells (SOCs) off a highly efficient route to matching supply from intermittent, renewable resources, with power demand by consumers. The cells would store excess electrical energy as chemical fuel during times of peak production, and operate in reverse during times of peak demand. In this study we examine the operation of anode supported proton conducting SOCs in electrolysis mode. The required overpotential for a given current density decreases with increasing humidity at the anode and increasing temperature. All of the V-I curves show distinct curvature. The electrode polarization resistance increases and electrolyte ohmic resistance decreases with increasing current density. This is accompanied by a deviation below the theoretical rate of hydrogen production. We interpret these changes as resulting from deviation away from pure proton conduction in the cell with increasing polarization.     

The Electrochemical Stability of Starch Carbon as an Important Property in the Construction of a Lithium-Ion Cell

This paper shows use of starch-based carbon (CSC) and graphene as the anode electrode for lithium-ion cell. To describe electrochemical stability of the half-cell system and kinetic parameters of charging process in different temperatures, electrochemical impedance spectroscopy (EIS) measurement was adopted. It has been shown that smaller resistances are observed for CSC. Additionally, Bode plots show high electrochemical stability at higher temperatures. The activation energy for the SEI (solid–electrolyte interface) layer, charge transfer, and electrolyte were in the ranges of 24.06–25.33, 68.18–118.55, and 13.84–15.22 kJ mol⁻¹, respectively. Moreover, the activation energy of most processes is smaller for CSC, which means that this electrode could serve as an eco-friendly biodegradable lithium-ion cell element.     

仪器分析技术在直接甲醇燃料电池研究中的应用

介绍了仪器分析技术在直接甲醇燃料电池电催化剂及其载体结构,电池反应动力学、产物分析、固体聚合物电解质膜以及电池性能评价等各方面的应用及研究进展。     

Apatite type lanthanum silicate and composite anode half cells

Apatite type rare earth silicates are being extensively studied as electrolyte material for intermediate temperature solid oxide fuel cells (SOFC). In this paper we presents results on synthesis of Al and/or Fe-doped ATLS, the design of compatible anode materials, thermal expansion properties and co-sintering of half-cells from expansion matched materials using the advanced pulsed electric current sintering (PECS) technique. The issues related to the co-sintering of half cells have been addressed successfully by the combined use of nano powders and PECS.     

Interrelation of anode and cathode processes in electrochemical carbon oxidation in a fuel cell with molten carbonate electrolyte

The object to be investigated is a fuel cell with a free molten carbonate electrolyte, which ensures direct electrochemical oxidation of solid hydrocarbons. The polarization characteristics of anode and cathode fuel cell assemblies, and also composition and gas release rate of gaseous products of anode reactions are studied. It is shown that the maximum voltages in the open cell circuit are obtained when the oxygen-carbon dioxide ratio in the cathode gas mixture corresponds to stoichiometric reaction coefficients that ensure replenishment of ions in electrolyte. However, the maximum current density values were obtained with a low carbon dioxide content. It is found that at high current values, anode potential fluctuations are observed. It is shown that carbon monoxide is the product of anode processes, along with carbon dioxide. The carbon monoxide content grows with temperature. The carbon dioxide content grows with increasing current in the fuel cell and with growing carbon dioxide content in cathode gases. The release rate of carbon oxidation products nonlinearly depends on the current value in the fuel cell. It is concluded that there is interrelation between the mass-exchange processes in the fuel cell, which is determined by the balance between cathode gas incoming into the reaction zone, the number of molecules generated during fuel oxidation, molecule dissolution and diffusion into the cathode region, and also the amount of gas released in the form of bubbles.     

Effect of Nafion dispersion solvent on the interfacial properties between the membrane and the electrode of a polymer electrolyte membrane-based fuel cell

A new Nafion binder solution was prepared using a different organic solvent, dimethylacetamide (DMAc), and applied to a polymer electrolyte membrane-based fuel cell. Wide angle X-ray diffraction (WAXD), electrochemical impedance spectroscopy (EIS), and polarization of the fuel cell were carried out to determine the crystallinity of the Nafion binder film, the cell resistance, and the fuel cell performance. This new Nafion binder film, which was created using a homemade Nafion solution containing DMAc, dissolved slower than a recast Nafion film that was made using a commercial Nafion solution in methanol (2 M). It was found that the slow dissolution of the homemade Nafion binder film was due to a more highly developed crystalline morphology, which can lead to good structural integrity in the catalyst layer for long-term operation of the fuel cell. The micellar structure of Nafion in the commercial Nafion binder solution is broken by new organic solvent, which leads to higher physical chain entanglement between the Nafion membrane and the Nafion binder during preparation of the membrane/electrode assembly (MEA), thereby improving the interfacial stability between the membrane and the electrode and providing long-term stability of the fuel cell.