La—Ni system porous anode in an alkaline fuel cell

La—Ni system alloys in fine powder form were used to make the hydrogen electrode for an alkaline fuel cell. LaNi₅ electrodes were found to have the highest electrode performance among the La—Ni system electrodes. Using LaNi₅ powder of mean particle size 3.4 μm for the electrode, the limiting current density was 180 mA/cm². However mixing the same size powder (mean particle size 3.4 μm) with powder of mean particle size 40.5 μm in a ratio of 4:1, the limiting current density of the new electrode was 250 mA/cm². The electrode was found to have both relatively fine pores in the range 0.01–0.2 μm and relatively large pores in the ranges 0.5–1 μm and 4–5 μm. Thus, it was found that not only relatively fine pores but relatively large pores had an important effect upon the LaNi₅ electrode characteristics.     

A mathematical model for platinum/ PTFE/porous nickel fuel cell oxygen diffusion electrodes

This paper describes the cylindrical agglomerate model for oxygen/alkali gas diffusion electrodes fabricated from platinum, PTFE and porous nickel. Corrections for the increase in hydroxyl ion concentration with increasing current density have been made to the original model of Brown and Horve. Changes in performance by variation of the bulk structural parameters, e.g. agglomerate radius, porosity and tortuosity, have been studied. Theoretical modes of electrode decay have been explored.List of symbols Transfer coefficient - C Concentration of O₂ in elec trolyte mol cm^–3 - C _i Concentration of O₂ atr = R mol cm^–3 - C _o Concentration of O₂ in electrolyte atr = mol cm^–3 - Diffusion coefficient of O₂ in KOH cm² sec^–1 - Film thickness cm - E Overpotential of the electrode V - F Faraday's constant - i Electrode current density A cm^–2 - i _a Current per agglomerate A - I ₁(Z) First order Bessel function - I ₀(Z) Zero order Bessel function - j Local current density A cm^–2 - j _o Exchange current density A cm^–2 - L Agglomerate length (catalyst thickness) cm - N Number of electrons in rate determining step - N _a Number of agglomerates per cm² of electrode - Potential drop along ag glomerate V - _L Potential drop at L_a V - r Radial direction - R Radius of agglomerate cm - R _o Gas constant - Density of platinum g cm^–3 - S _g Surface area per gram cm² g^–1 - Solubility coefficient of O₂ mol cm^–3 - _m Electrolyte conductivity (ohm cm)^–1 - T Absolute temperature °K - _a Axial tortuosity - Porosity of platinum in the agglomerate - _r Aadial tortuosity of the agglomerate - W Catalyst loading g cm^–2 - x Axial direction     

Scanning electron microscopic studies of porous carbon electrodes used in alkaline fuel cells

Multilayer, polytetrafluoroethylene (PTFE)-bonded gas diffusion-type electrodes were prepared by the rolling method. Changing the electrode structure and manufacturing method improved alkaline fuel cell performance. Activated carbon or carbon black was used as the support material, with platinum as a catalyst and nickel screen as the backing material. Double-layer electrodes possessed both active and diffusion layers on the backing layer. However, the single-layer electrodes had only the active layer on the backing layer. The electrodes were prepared by using different PTFE contents and platinum loadings. In this study the surface photographs of the electrodes were taken with a scanning electron microscope. Elemental analyses of the surface elements were performed by energy dispersive X-ray spectroscopy (EDXS). Electrodes having activated carbon on their surfaces were observed to possess a nonuniform and porous structure. These electrodes showed better performance than electrodes having carbon black, which presented a uniform and nonporous structure.     

Ag/PAMAM树形分子纳米复合材料对PTFE膜性能的影响

以第4.0代端氨基聚酰胺-胺型树形分子(G4.0-NH2PAMAM)为模板制备Ag/PAMAM纳米复合材料(Ag·DENPs), 利用了Ag·DENPs与聚四氟乙烯(PTFE)膜表面的氢键作用,通过超声沉积的方法制备出了PTFE-Ag/PAMAM复合膜.通过研究沉积时间对Ag·DENPs在PTFE膜表面沉积量的影响,得到饱和沉积时间是60min,沉积量约为2.32mg.通过UV-vis光谱和SEM对其化学组成和微观形貌进行了表征.复合膜的水接触角为105.3°,较纯PTFE膜降低了23.6°,显著改善PTFE膜表面亲水性.复合膜对水和牛血清白蛋白的通量比纯PTFE膜低,但傅立叶变换衰减全反射红外光谱(FRR)为88.2%,比纯PTFE膜的35.4%高出了52.8%,且对大肠杆菌及金黄色葡萄球菌具有抑菌作用.表明Ag·DENPs的引入可以改善PTFE的抗污染性及抑菌性.     

Influence of mass transport on the performance of carbon gas-diffusion air electrodes in alkaline solution

The effect of mass transport on the electrochemical behaviour of carbon gas-diffusion air electrodes in alkaline solution was investigated on the basis of ΔE(i) curves. These curves are obtained by subtraction of potential values for an electrode operating with airE _air(i) from potential values for the same electrode operating with pure oxygenE _oxygen(i) at the same current densityi. Three different regions on these curves connected with different modes of mass transport are recognized. A model of the gas-diffusion air electrode which takes into account the diffusion of the gas, diffusion of the dissolved gas, electrochemical reaction and IR drop is used to explain the experimental results.     

Effect of a GDL based on carbon paper or carbon cloth on PEM fuel cell performance

A commercially available GDL based on carbon paper or carbon cloth as a macroporous substrate was characterized by various physical and electrochemical measurements: mercury porosimetry, surface morphology analysis, contact angle measurement, water permeation measurement, polarization techniques, and ac-impedance spectroscopy. SGL 10BB based on carbon paper demonstrated dual pore size distribution and high water flow resistance owing to less permeable macroporous substrate, and more hydrophobic and compact microporous layer, as compared to ELAT-LT-1400 W based on carbon cloth. The membrane-electrode-assembly fabricated using SGL 10BB showed an improved fuel cell performance when air was used as an oxidant. The ac-impedance response indicated that a microporous layer which has high volume of micropores and more hydrophobic property allows oxygen to readily diffuse towards the catalyst layer due to effective water removal from the catalyst layer to the gas flow channel.     

Oxidative conversion of CH4 on Ni and Ag electrode-catalysts in molten carbonate fuel cell reactor

A fuel cell system using molten carbonates of potassium and lithium as electrolyte was applied to the oxidative conversion of methane over Ni and Ag electrodes. A possibility of cogeneration of valuable chemicals, like C₂-hydrocarbons, and electricity in such a system was demonstrated. With CO ₃ ^2– ions (oxygen) transported electrochemically, the rate of formation of C₂-hydrocarbons and selectivity for them on the Ag electrode were found to be greater than those with oxygen premixed to the gase phase.     

Influence of fabrication procedure on the electrochemical performance of Ag/AgCl reference electrodes

The influence of several parameters in the preparation procedure of thermal–electrolytic Ag/AgCl electrodes on the resulting electrode performance has been studied. In particular, we report the effect on electrode performance of subtle variations in the preparation of silver oxide paste used for electrode manufacture, in thermal annealing conditions employed and in the procedure for electrochemically converting a fraction of the electrode from silver to silver chloride. Scanning electron microscopy and electrochemical impedance spectroscopy have been used to study the characteristics of the electrodes produced. This work reveals a correlation between the electrochemical behaviour and surface physical characteristics – in particular electrode porosity. The outputs of this study have positive implications for improving the accuracy and comparability of primary pH measurement.     

Effect of graphite felt and activated carbon fiber felt on performance of freshwater sediment microbial fuel cell

BACKGROUND: Sediment microbial fuel cells (SMFCs) could be used as power sources and one type of new technology for the removal of organic matters in sediments. Various types of materials have been used as electrodes. Nevertheless, there is still room to improve electrode materials and enhance their effect on the performance of SMFCs. In this work, performances of SMFCs with activated carbon fiber felt (ACFF) and with nitric acid‐treated ACFF were compared with graphite felt (GF) materials. RESULTS: The maximum power density of the SMFC with ACFF electrode was the highest (33.5 ± 1.5 mW m^?2). Nitric acid‐treated GF electrode slightly increased the maximum power density of SMFC, while the nitric acid treated‐ACFF resulted in significant decline in the maximum power density of SMFC. The maximum power density further increased to 74.5 ± 7.5 mW m^?2 in SMFC using GF cathode and ACFF anode. CONCLUSIONS: ACFF as anode can enhance the transport of electrons from the oxidation of organic matter in the sediment, while the output power was found to reduce in SMFC with ACFF cathode. Further efforts are needed to study the formation conditions of the biocathode and new electrode modification technology. Copyright © 2012 Society of Chemical Industry     

Simulation of influences of layer thicknesses in an alkaline fuel cell

A computational simulation was conducted by using a one-dimensional isothermal model for an alkaline fuel cell (AFC) single cell to investigate influences of the thicknesses of the separator, catalyst layer, and gas-diffusion layer in an AFC. The cell polarizations were predicted at various thicknesses and their influences were also analysed. Thickening the separator layer decreased the limiting current density and increased the slope of the ohmic polarization region. Investigation on the thickness of the anode catalyst layer showed that the optimum thickness varied between 0.04–0.15 mm according to the cell voltage. The thickness of the cathode catalyst layer significantly influenced the cell performance. Also, a limitation of thickness effect in the cathode catalyst layer was observed. This limitation was considered to be caused by the mass transfer resistance of the electrolyte.     

Investigation of oxygen reduction on activated carbon electrodes in alkaline solution

Oxygen reduction has been studied on gas diffusion electrodes made from various kinds of activated carbon materials. The potentiodynamic method was used to study the electrochemical behavior of the reduction reaction, while nitrogen gas adsorption and powder X-ray diffraction were used to determine the pore size distribution and the crystal structure of the carbon material, respectively. The relationship between the catalytic activity of activated carbon and the surface content of edge orientation was discussed. The specific catalytic activity of activated carbon is determined by the percentage of the edge orientation on the surface. The higher the content of the edge plane, the lower the oxygen reduction overpotential. The reduction overpotential is also related to the effective surface area, which determines the local current density.     

Effect of compression on the performance of a HT-PEM fuel cell

This paper shows by thorough electrochemical investigation that (1) the performances of high-temperature polymer electrolyte fuel cell membrane electrode assemblies of three suppliers are differently affected by compressive forces. (2) Membrane thickness reduction by compressive pressure takes place less than expected. (3) A contact pressure cycling experiment is a useful tool to distinguish the impact of compression on the contact resistances bipolar plate/gas diffusion layer (GDL) and GDL/catalytic layer. A detailed visual insight into the structural effects of compressive forces on membrane and gas diffusion electrode (GDE) is obtained by micro-computed X-ray tomography (μ-CT). μ-CT imaging confirms that membrane and GDEs undergo severe mechanical stress resulting in performance differences. Irreversible GDL deformation behavior and pinhole formation by GDL fiber penetration into the membrane could be observed.     

Effect of cerium oxide nanoparticles coating on the electrodes of benthic microbial fuel cell

Benthic microbial fuel cell is a power source for low-power devices. For enhancement of power, Cerium oxide (CeO₂) nanoparticles (NPs) were coated on anode and cathode electrodes and compared separately. CeO₂ NPs were synthesized by hydrothermal method and characterized by Dynamic light scattering. Polyvinylidene fluoride and graphite powder were used as a conductive matrix for binding CeO₂ NPs. Coated electrodes were characterized by physical and electrochemical analysis. Maximum power densities generated by CeO₂ coated cathode and anode were 60 and 43 mW/m³ respectively; whereas conductive matrix only produced 14 mW/m³. Results demonstrated that CeO₂ at cathode performed better than at anode. NPs show their effectiveness as an oxygen reduction reaction catalyst in the sea water.     

Effect of carbon paper substrate of the gas diffusion layer on the performance of proton exchange membrane fuel cell

Gas diffusion layers (GDLs) were fabricated using non-woven carbon paper substrates with various thicknesses developed by Hollingsworth & Vose Co. Highly consistent carbon slurry containing Pureblack carbon and vapor grown carbon fiber (3:1 ratio) with 25 wt.% Teflon was prepared by using a dispersion agent, Novec-7300 in isopropyl alcohol. Micro-porous layer was coated by using a fully automated Coatema coating tool with a uniform carbon loading of 2.6-3 mg cm⁻² using carbon slurry. The surface morphology, contact angle and pore size distribution of the GDLs were examined using SEM, Goniometer and Hg Porosimeter, respectively. Various cathode GDLs assembled into MEAs were evaluated in a single cell PEMFC under various operating relative humidity (RH) conditions using H₂/O₂ and H₂/air as reactants. The peak power density of the single cell using the optimum carbon paper substrate thickness was about 1400 and 700 mW cm⁻² with H₂/O₂ and H₂/air at 60% RH, respectively. It was found that the pore diameter as well as the corresponding pore volumes of the GDLs played a key role in exhibiting the optimum fuel cell performance.     

Study of the performance of secondary alkaline pasted zinc electrodes

Calcium zincate was prepared by a chemical coprecipitation method and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical performance of pasted zinc electrodes with bismuth and calcium additives was investigated by the charge–discharge method. The addition of metallic bismuth powder improves the discharge performance of zinc electrodes due to the formation of an electronic conduction matrix. The calcium-containing zinc electrodes showed higher discharge capacity, less shape change and longer cycle lifetime. Moreover, zinc electrodes using calcium zincate as active material show better electrochemical performance than those with the physical mixture of zinc oxide and calcium hydroxide.     

Electrolytic studies on the system Ag/Ag2O/AgO in alkaline chloride solutions

Electrolytic silver oxide, formed by the constant current anodization of silver sinters in KOH electrolyte, has been found to be higher in AgO content when prepared in the presence of KCl than when prepared in pure electrolyte. The former AgO is considerably more stable in alkaline solution, seemingly due to the presence of residual AgCl. The formation of unstable oxy-chlorides at potentials more anodic than that for AgO formation results in the isolation of the AgO from the unoxidized Ag in the case of Ag foil electrodes, leading to suppression of the open circuit potential.     

Effect of unequal load of carbon xerogel in electrodes on the electrochemical performance of asymmetric supercapacitors

This paper investigates the electrochemical performance of asymmetric supercapacitors in an environmentally friendly aqueous electrolyte (1.0 mol L^?1 sodium sulfate solution). The asymmetric configuration is based on the use of a highly porous carbon xerogel as active material in both the positive and negative electrodes, but the carbon xerogel loading in each electrode has been substantially modified. This configuration leads to an increase in the operational voltage window up to values of 1.8 V and consequently to a higher specific capacitance (200 F g^?1) and energy density (~25 Wh kg^?1). Four different mass ratios were employed (1, 1.5, 2 and 3), and the electrochemical response of the cells was evaluated by means of cyclic voltammetry, galvanostatic charge–discharge and impedance spectroscopy. The results demonstrate that the optimal carbon mass ratio in the electrodes is about 2.0 because in these conditions the devices are able to operate with a maximum cell voltage of 1.8 V and with a high electrical efficiency.     

Modelling the 3D microstructure and performance of solid oxide fuel cell electrodes: Computational parameters

In this paper, the computational parameters for a 3D model for solid oxide fuel cell (SOFC) electrodes developed to link the microstructure of the electrode to its performance are investigated. The 3D microstructure model, which is based on Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials. Once formed, the synthetic electrodes are discretized into voxels (small cubes) of equal sizes from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three-phase boundary (TPB) length. Transport phenomena and electrochemical reactions taking place within the electrode are modelled so that the performance of the synthetic electrode can be predicted. The degree of microstructure discretization required to obtain reliable microstructural analysis is found to be related to the particle sizes used for generating the structure; the particle diameter should be at least 20–40 times greater than the edge length of a voxel. The structure should also contain at least 25³ discrete volumes which are called volume-of-fluid (VOF) units for the purpose of transport and electrochemical modelling. To adequately represent the electrode microstructure, the characterized volume of the electrode should be equivalent to a cube having a minimum length of 7.5 times the particle diameter. Using the modelling approach, the impacts of microstructural parameters on the electrochemical performance of the electrodes are illustrated on synthetic electrodes.     

The oxygen,oxygen/sulphur dioxide and oxygen/carbon dioxide electrodes in molten lithium sulphate—potassium sulphate eutectic

Open-ciruit potential, potentiostatic and voltammetric measurements of the O₂, O₂/SO₂ and O₂/CO₂ electrodes have been made in molten Li₂SO₄K₂SO₄ eutectic at 625°C. In the absence of SO₂ the overall reaction of the O₂ electrode is the reduction of O₂ to O^2? through the formation of O^2?₂ ions. In the presence of SO₂ the overall reaction is the reduction of SO₂ and O₂ to SO^2?₄, probably through the formation of SO^2?₃. The overall reaction of the O₂/CO₂ electrode is similar to that of the same electrode in molten carbonates, ie the reduction of CO₂ and O₂ to CO^2?₃. The transfer coefficients and exchange currents have been estimated for these reactions on platinum and gold.