Electrode Reaction Resistance Research Articles

Performance Analysis Method for PCFC Considering Leak Current and Application to 1000hr Endurance Test

We estimate the internal leakage current in PCFC mode, and report the results of a study on a performance analysis method that takes account of the leakage current. As results, the leak current increased with the supplied hydrogen, oxygen, and the water vapor concentration on the fuel electrode side, and the leak current tended to decrease only with the water vapor concentration on the air electrode side. Also, the higher the temperature, the more remarkable the change was observed in leakage current. Moreover, it was clarified that the cathode overvoltage greatly increases as the temperature decreases. From the result of 1000hr endurance test, it was revealed that the decrease output voltage was increase in the fuel electrode reaction resistance.

Performance Analysis Method for PCFC Considering Leak Current and Application to 1000hr Endurance Test

We estimate the internal leakage current in PCFC mode, and report the results of a study on a performance analysis method that takes account of the leakage current. As results, the leak current increased with the supplied hydrogen, oxygen, and the steam concentration on the fuel electrode side, and the leak current tended to decrease only with the steam on the air electrode side. Also, the higher the temperature, the more remarkable the change was observed in leakage current. Moreover, it was clarified that the cathode overvoltage greatly increases as the temperature decreases. From the result of 1000hr endurance test, it was revealed that the decrease output voltage was increase in the fuel electrode reaction resistance.

Electrochemical verification of the redox mechanism of FeS2 in a rechargeable lithium battery

The redox mechanism of micro-sized FeS2 particles in a rechargeable lithium battery is studied by galvanostatic cycling and ac-impedance analysis. It is shown that FeS2 is irreversibly reduced on the first discharge, turning Li/FeS2 cell into a combination of Li/FeS and Li/S chemistries, as suggested by two distinct discharge plateaus at ∼1.5 and 2.0V, respectively. The first discharge consists of an irreversible conversion of FeS2 to Li2FeS2 intermediate and its subsequent reduction to metallic Fe and Li2S. The first discharge suffers an initial voltage delay, suggesting that the discharge progresses in a thermodynamic non-equilibrium condition. The initial voltage delay can be attributed to the large grain boundary resistance (GBR) of pristine FeS2 particles, which impedes the nucleation of a new solid Li2FeS2 phase causing high polarization. Ac-impedance spectra of the FeS2 cathode are composed of a semicircle and a straight sloping line, representative of an electrode reaction resistance (Rer) and a Li+ adsorption impedance, respectively. The Rer is found to decrease progressively during the first discharge and reaches a plateau when the cell is charged above 2.5V vs. Li/Li+, being consistent with the model that FeS2 is irreversibly reduced during the first discharge and that the Li2S/Li2Sn redox couple is formed in recharge. It is indicated that Li/FeS2 batteries face the same problems as Li/S batteries, such as the dissolution of lithium polysulfide, the formation of a redox shuttle, and the loss of sulfur active material.

Design of New Power Source Based on Micro-SOFC for Mobile Electronic Devices

Low operating temperature micro solid oxide fuel cell (micro-SOFC) systems which utilize liquid fuels are attractive for high energy efficiency and density. Long time operation is possible for micro-SOFCs by refueling and it is not restricted by battery charge. Hence, it is expected to alternative power source for compact mobile electronic devices which consume a large amount of power. In previous, diverse types of micro-SOFCs focused on its performance was reported and a maximum power density of 1 W/cm2 at 500oC was achieved. However, they have many problems of durability and repeatability for practical use. Additionally, it is necessary to consider how to heat up micro-SOFC cells to operating temperature, compatibility with other electronic circuits and thermally self-sustained condition. In this study, micro-SOFC system was proposed in order to adopt the micro-SOFCs into a power source of compact mobile electric devices. We designed the micro-SOFC package with thermal insulation and heat exchanger. Its insulation performance was experimentally confirmed and power generation test with micro heater in the micro-SOFC package was performed. Micro-SOFC packages require thermal insulation to keep the surface temperature below about 85oC, which is the maximum operating temperature of other electronic circuits. We applied vacuum insulation widely used for thermal insulation in MEMS devices to the package. The heat of the exhaust gas was recovered by the liquid fuel through heat exchanger. From the calculation of temperature distribution by the finite element method, surface temperature of the package was kept from 57 to 88oC when operating temperature is 500oC. As long as the liquid is evaporated, temperature of the heat exchanger is kept constant. Therefore, heat conduction from high temperature glass flow channels to vacuum insulator was prevented and low surface temperature was achieved. Applying micro-SOFC system as independent power sources, thermally self-sustained condition is required, i.e. micro-SOFC packages should keep operating temperature more than 300oC only by the generated heat from reaction of fuel cells during power generation. For thermally self-sustained condition, the amount of heat generated during power generation needs to exceed that of heat loss from surface of the package and transferred heat by exhaust gas. We calculated thermally self-sustained condition of a micro-SOFC system with 4 W output fueled by methanol and water that has reformer and heat exchanger from exhaust gas. From the result, generated heat from reaction of fuel cells was 4.8W, heat loss from surface was 1.5 W and transferred heat by exhaust gas was 4.6 W at 500oC. The result confirmed that transferred heat by exhaust gas was dominated in this system. Hence, recovering the heat of exhaust gas with the gas-liquid heat exchanger was essential to achieve thermally self-sustained condition. Micro-SOFC cells and micro heater were fabricated in general MEMS technology. Power generation test and electrochemical impedance spectroscopy of micro-SOFC (Pt-Pd | 15 mol% Y-doped BaZrO3 | Pt-Pd) by micro heater were measured in the temperature range of 200-330oC. A maximum power density of 1.12 mW/cm2 and open circuit voltage of 1.09 V at 330oC were obtained using wet H2 as fuel and air as the oxidant. As a result of impedance measurement, electrode reaction resistance was very high in cell total resistance. Therefore, the improvement of electrode will reduce cell resistance to a level for actual utility. We actually fabricated the micro-SOFC package installed vacuum structure and heat exchanger to evaluate cooling performance of the insulation package and performed power generation test in the package. In the case of heat exchanger filled with 50 vol% ethanol solution, surface temperature was 89.6oC when micro heater temperature was 500oC. In accordance with the result, it was possible to keep surface temperature of the package sufficiently low with heat exchanger. As a result of power generation test of micro-SOFC inside the package, a maximum power density of 1.27 mW/cm2 and open circuit voltage of 0.94 V at 300oC were obtained using wet 25% or 50% H2 as fuel and air as the oxidant. Additionally, surface temperature of the package was kept under 80oC. Therefore I achieved power generation in compact micro-SOFC system with thermal insulation. The micro-SOFC package was designed for applying micro-SOFCs to power source of mobile electronic devices in this study. As a result of calculation, the package installed vacuum insulation and heat exchanger could achieve thermally self-sustained condition and low surface temperature. Power generation test of micro-SOFC inside the prototype package was achieved and the package kept low surface temperature. Hence, this study presented that applicability of micro-SOFC system to power source for compact mobile electronic devices.

Duty Cycling Influences Current Generation in Multi-Anode Environmental Microbial Fuel Cells

Improving microbial fuel cell (MFC) performance continues to be the subject of research, yet the role of operating conditions, specifically duty cycling, on MFC performance has been modestly addressed. We present a series of studies in which we use a 15-anode environmental MFC to explore how duty cycling (variations in the time an anode is connected) influences cumulative charge, current, and microbial composition. The data reveal particular switching intervals that result in the greatest time-normalized current. When disconnection times are sufficiently short, there is a striking decrease in current due to an increase in the overall electrode reaction resistance. This was observed over a number of whole cell potentials. Based on these results, we posit that replenishment of depleted electron donors within the biofilm and surrounding diffusion layer is necessary for maximum charge transfer, and that proton flux may be not limiting in the highly buffered aqueous phases that are common among environmental MFCs. Surprisingly, microbial diversity analyses found no discernible difference in gross community composition among duty cycling treatments, suggesting that duty cycling itself has little or no effect. Such duty cycling experiments are valuable in determining which factors govern performance of bioelectrochemical systems and might also be used to optimize field-deployed systems.

Modification of LiFePo<sub>4</sub> by Citric Acid Coating and Nb<sup>5+</sup> Doping

In order to improve the electrochemical performance of LiFePO4, Li0.99Nb0.01FePO4/C composite materials were synthesized with citric acid coating and Nb2O5 doping. The physical chemistry and electrochemical performances of Li0.99Nb0.01FePO4/C were investigated by X-Ray diffraction (XRD) and Transmission electron microscope (TEM). The results show that Li0.99Nb0.01FePO4/C has smooth charge-discharge voltage platform, with first capacity of 151.6 mAh•g-1 (0.1C) and no obvious capacity fading after 16 cycles. The materials have favorable high rate discharge performances, with the first capacity of 131.6 mAh•g-1 at 0.5C, 119.8 mAh•g-1 at 1.0C and 106.2 mAh•g-1 at 2.0C. Equivalent circuits analysis shows that the impedance, especially electrode reaction resistance of Li0.99Nb0.01FePO4/C are significantly reduced compared with the pure LiFePO4 and this helps to improve the electric conductivity, discharge capacity and cycle performance. The diffusion coefficient of Li+ in Li0.99Nb0.01FePO4/C is 2.51×10-12 cm2•s-1 increased two orders of magnitude in comparison with the pure LiFePO4. Carbon-coating makes a more remarkable contribution to lithium diffusion than Nb5+ ion doping.

Heat treatment effect on electrochemical properties of spinel Li 4Ti 5O 12

Anode material Li 4Ti 5O 12 was prepared at 800°C by a solid-state reaction, followed by heat-treatment at 600°C for different times (0, 2, 8, and 12 h). The effects of heat-treatment time on the particle morphology, rate-capability, and electrode kinetic process of the Li 4Ti 5O 12 electrode, and on the lithium ion diffusion coefficient inside the Li 4Ti 5O 12 electrode were investigated. Proper heat treatment could smoothen the particle surface of Li 4Ti 5O 12 particles and increase the rate-capability of the electrode. Overlong heat treatment might cause particle aggregation and hence result in a poor electrode kinetic process. A sample with 8 h of heat treatment showed the best rate-capability and the lowest electrode reaction resistance. Heat treatment for 2-8 h does not significantly change the lithium ion diffusion coefficient inside the Li 4Ti 5O 12 electrode, whereas, 12-h treatment results in a lower lithium ion diffusion coefficient.

Estimation of State-of-Charge of Lead-Acid Batteries Using Electrochemistry Theorem

There are several methods to estimate the residual capacity of lead-acid batteries. Most of these methods are not able to precisely predict the amount of remaining capacity due to the battery aging effect. This paper presents a method using the electrochemistry theorem to analyze the electrode reaction resistance effect and to estimate the internal energy loss and the electrolyte specific gravity in batteries. Through a series of calculations, the state-of-charge of a lead-acid battery can be estimated. To validate the proposed algorithm, some experiments are conducted and the measured data is applied to the proposed method and two other methods for comparison. The experiment results prove that the proposed algorithm can accurately compute the state-of-charge of lead-acid batteries to a certain extend and can be used in real-time controls.

Ni/Ceria Cermet as Anode of Reduced-Temperature Solid Oxide Fuel Cells

Anode characteristics of a cermet on the electrolyte were studied by a three-probe ac impedance method combined with dc polarization, instead of the conventional current-voltage curve measurement. The experimentally obtained Cole-Cole plots of ac impedance showed a shift with added dc voltage. Both bulk resistance and electrode reaction resistance increased with increasing anodic dc current. The shift of bulk resistance is explained by the electronic conduction of the electrolyte. The increase of electrode reaction resistance is thought to be a result of discrepancy between the ionic and the external current, differing from Ni/yttria-stabilized zirconia anode. An interface resistance lower than 0.1 Ω/cm2 was obtained below 700°C by using a coprecipitation method to prepare the anode precursor and cofiring it with the electrolyte to improve the contact. © 2002 The Electrochemical Society. All rights reserved.

The direct electrochemistry of folic acid at a 2-mercaptobenzothiazole self-assembled gold electrode

A pair of well-defined redox waves of folic acid at a 2-mercaptobenzothiazole self-assembled gold electrode (MBT/SAM/Au) was achieved. Folic acid can bind strongly to MBT/SAM/Au and form a closely packed monolayer, whose average electron transfer rate is 0.085 s −1 with a two-electron/two proton transfer, a maximum surface coverage of 2.8×10 −10 mol cm −2, an adsorption equilibrium constant of (4.0±0.2)×10 5 l mol −1, and an electrode reaction resistance of 12489 Ω. The oxidation peak currents achieved on MBT/SAM/Au vary linearly with the logarithm of the content of folic acid in the range of 8.0×10 −9–1.0×10 −6 mol l −1. The detection limit is 4.0×10 −9 mol l −1. This can be used to determine folic acid.

Fabrication and Electrical Properties of YSZ Thin-Film Electrolyte Spin-Coated on Porous La0.6Sr0.4Co0.2Fe0.8O3 Substrate from Polymeric Precursor Solution.

Deposition of dense 8mol% Y2O3-stabilized ZrO2 (YSZ) electrolyte thin-films with and without Ce0.9Gd0.1O1.95 (CGO) interlayer by spin coating of polymeric precursor solutions on a porous La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) substrate and the electrical properties of the constructed thin-film cell were investigated. Anisotropic crystal growth in the YSZ thin films annealed at 1150°C was confirmed by XRD and SEM. The high-temperature chemical reaction between YSZ and LSCF was depressed by coating a CGO interlayer on the porous LSCF substrate before coating YSZ thin film. The impedance measurements of the electrochemical cell constructed by the LSCF electrode-supported YSZ thin-film electrolyte with CGO interlayer showed a lower electrode reaction resistance.

Single‐Chamber Solid Oxide Fuel Cells at Intermediate Temperatures with Various Hydrocarbon‐Air Mixtures

The performance of a single‐chamber solid oxide fuel cell (SOFC) was studied between 350 and 900°C in flowing mixtures of methane, ethane, propane, or liquefied petroleum gas and air with a fuel/air volume ratio of one, where their oxidation proceeded safely without explosion. Among all tested electrode materials, cermet and oxide functioned best as the anode and cathode, respectively, in various gas mixtures. A cell constructed from a electrolyte with the two electrodes generated >900 mV in a methane‐air mixture between 600 and 800°C and in an ethane‐air mixture between 450 and 650°C. A small electrode reaction resistance resulted in increasing power density with decreasing electrolyte thickness. The peak power density at 450°C increased from 34 to with decreasing electrolyte thickness from 0.50 to 0.18 mm. The working mechanism of the single‐chamber SOFC at different temperatures was also studied by measuring the catalytic activities of the two electrodes for partial oxidation of the hydrocarbons. © 2000 The Electrochemical Society. All rights reserved.

A low-operating-temperature solid oxide fuel cell in hydrocarbon-Air mixtures

The performance of a single-chamber solid oxide fuel cell was studied using a ceria-based solid electrolyte at temperatures below 773 kelvin. Electromotive forces of approximately 900 millivolts were generated from the cell in a flowing mixture of ethane or propane and air, where the solid electrolyte functioned as a purely ionic conductor. The electrode-reaction resistance was negligibly small in the total internal resistances of the cell. The resulting peak power density reached 403 and 101 milliwatts per square centimeter at 773 and 623 kelvin, respectively.

Solid electrolyte interphase (SEI) electrode—V. the formation and properties of the solid electrolyte interphase on calcium in thionyl chloride solutions

The electrochemical behaviour of calcium in neutral thionyl chloride solutions containing FeCl 3, AlCl 3, CaCl 2 and CaO was investigated using pulse galvanostatic and ac techniques. In all systems the calcium electrode is completely covered by a passivating layer consisting of CaCl 2 which serves as a SEI. The SEI is probably an anionic conductor. The ionic transport number ( t i) of the SEI under OCV conditions was calculated from the ratio: OCV/thermodynamic EMF. It is about 0.9 for AlCl 3-based electrolytes and about 0.8 for FeCl 3-based electrolytes. Immediately after laoding the cell at about 10mA cm t–2 its OCV reaches 3.7 V indicating that t i reaches unity and the OCV of the cell equals the thermodynamic value. The electrode reaction resistance and the apparent thickness of the SEI varies with type and concentration of the electrolyte and with storage time. When an aged electrode is anodically dissolved, the reaction resistance drops by three orders of magnitude, and the SEI-resistivity drops by two orders of magnitude. The mechanism responsible for the ”diode” behaviour of the calcium-SEI electrode in thionyl chloride is discussed.