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Yoshinobu Tanaka - One of the best experts on this subject based on the ideXlab platform.
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Mass transport in a boundary layer and in an ion Exchange Membrane: Mechanism of concentration polarization and water dissociation
Russian Journal of Electrochemistry, 2012Co-Authors: Yoshinobu TanakaAbstract:In an unforced flowing NaCl solution in bulk, gravitational or electro convection supplies ions from bulk toward the Membrane surface through a boundary layer. In a boundary layer formed on an anion Exchange Membrane, the convection converts to migration and diffusion and carries an electric current. In a boundary layer formed on a cation Exchange Membrane, the convection converts to migration and carry an electric current. In a forced flowing solution in bulk, the boundary layer thickness is reduced and gravitation or electro convection is disappeared. An electric current is carried by diffusion and migration on the anion Exchange Membrane and by migration on the cation Exchange Membrane. Ion transport in a boundary layer on the cation Exchange Membrane immersed in a NaCl solution is more restricted comparing to the phenomenon on the anion Exchange Membrane. This is due to lower counter-ion mobility in the boundary layer and the restricted water dissociation reaction in the Membrane. The water dissociation reaction is generated in an ion Exchange Membrane and promoted due to the increased forward reaction rate constant. However, the current efficiency for the water dissociation reaction is generally low. The intensity of the water dissociation is more suppressed in the strong acid cation Exchange Membrane comparing to the phenomenon in the strong base anion Exchange Membrane due to lower forward reaction rate constant in the cation Exchange Membrane. In the strong acid cation Exchange Membrane, the intensity of electric potential is larger than the values in the strong base anion Exchange Membrane. Accordingly, the stronger repulsive force is developed between ion Exchange groups (SO _3 ^• groups) and co-ions (OH^− ions) in the cation Exchange Membrane, and the water dissociation reaction is suppressed. In the strong base anion Exchange Membrane, the repulsive force between ion Exchange groups (N^+(CH_3)_3 groups) and co-ions (H^+ ions) is relatively low, and the water dissociation reaction is not suppressed. Violent water dissociation is generated in metallic hydroxides precipitated on the desalting surface of the cation Exchange Membrane. This phenomenon is caused by a catalytic effect of metallic hydroxides. Such violent water dissociation does not occur on the anion Exchange Membrane.
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Acceleration of water dissociation generated in an ion Exchange Membrane
Journal of Membrane Science, 2007Co-Authors: Yoshinobu TanakaAbstract:Abstract Intensity of water dissociation generated in an anion Exchange Membrane is stronger than that in a cation Exchange Membrane. Violent water dissociation occurs when metallic hydroxides such as Mg(OH) 2 or Fe(OH) 3 are precipitated on the cation Exchange Membrane. When the metallic hydroxides are precipitated on the anion Exchange Membrane, the violent water dissociation does not occur. The accelerated water dissociation occurs by the auto-catalytic reaction caused by the functional groups in the cation Exchange Membrane (sulfonic acid groups) and in the anion Exchange Membrane (quaternary ammonium or quaternary pyridinium groups) or metallic hydroxides precipitated on the cation Exchange Membrane. In electrodialysis of seawater, Fe(OH) 3 suspending in the feeding seawater precipitates on ion Exchange Membranes and causes violent water dissociation on cation Exchange Membranes. In order to prevent scale troubles caused by the water dissociation in electrodialyzers operating in salt-manufacturing plants, cation Exchange Membranes served for 6 years or more should be replaced with new ones. Washing the surface of the cation Exchange Membrane by applying ultrasonic waves prevents the water dissociation.
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concentration polarization in ion Exchange Membrane electrodialysis the events arising in a flowing solution in a desalting cell
Journal of Membrane Science, 2003Co-Authors: Yoshinobu TanakaAbstract:Abstract Using the NaCl concentration distribution observed by measuring the refraction index in a boundary layer in an unforced flowing state in a desalting cell, ionic fluxes, current density, solution velocity and potential gradient in the boundary layer were evaluated. Solution velocity was divided into the terms of electro-osmosis, concentration-osmosis and natural convection. The greater part of the solution velocity was due to natural convection, which is a horizontal component of ascending flow produced by the decrease in the solution density near the Membrane surface in the boundary layer. Ionic fluxes and current density were divided into the terms of diffusion, migration and convection. Na+ ion transport in the boundary layer on the desalting surface of a cation-Exchange Membrane was suppressed due to the lower mobility of Na+ ions. Furthermore, water dissociation was strongly restricted on the surface of the cation-Exchange Membrane. Accordingly, Na+ ion transport on the surface of the cation-Exchange Membrane placed in a diluted NaCl solution was promoted at the limiting current density by the increase of boundary layer thickness, the solution velocity in the boundary layer and the intensity of NaCl concentration oscillation on the Membrane surface. In this circumstance, ionic transport due to the convection fluxes, which do not carry an electrical current, converted to migration fluxes and carried an electrical current. In this instance, diffusion fluxes did not support an electrical current because of lower mobility of Na+ ions comparing that of Cl− ions. On the other hand, Cl− ion transport in the boundary layer on the desalting surface of the anion-Exchange Membrane was not suppressed because of the greater mobility of Cl− ions. Furthermore, water dissociation was not strongly restricted on the anion-Exchange Membrane. Accordingly, phenomena like the increase of the boundary layer thickness and the solution velocity observed in the boundary layer on the cation-Exchange Membrane did not occur on the anion-Exchange Membrane. On the anion-Exchange Membrane, convection fluxes were converted to diffusion fluxes and carry an electrical current. In this instance, migration fluxes did not support an electrical current because the contribution of diffusion fluxes is sufficiently effective. The potential gradient was divided into the terms of ohmic potential and diffusion potential. The ohmic potential gradient on the cation-Exchange Membrane was positive, and that on the anion-Exchange Membrane was negative. The diffusion potential gradient was positive on both the cation-Exchange Membrane and the anion-Exchange Membrane. By integrating the potential gradient distribution in a boundary layer, a current density versus voltage drop curve was obtained.
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Water dissociation in ion-Exchange Membrane electrodialysis
Journal of Membrane Science, 2002Co-Authors: Yoshinobu TanakaAbstract:When an electrical current larger than the limiting current density passes across an ion-Exchange Membrane, H + ions and OH − ions are generated and transported in a water dissociation layer formed between the ion-Exchange Membrane and the boundary layer. Water dissociation reactions consist of a forward reaction and a reverse reaction. The forward reaction rate increases along with the increase of electrical potential difference in the water dissociation layer. From this, we introduced equations expressing pH changes in the concentrating and desalting side of the Membrane; electrical current efficiency and the forward reaction rate constant of the water dissociation reaction; thickness of the water dissociation layer; and concentration distribution of H + and OH − ions, electrical resistance, electrical potential difference and electrical potential gradient in the water dissociation layer. The reasonability of values calculated using these equations was confirmed by electrodialysis of several electrolyte solutions. The intensity of water dissociation on an anion-Exchange Membrane was generally greater than on a cation-Exchange Membrane. This phenomenon is explained by the fact that the hydrophilicity of the anion-Exchange Membrane is less than that of the cation-Exchange Membrane. When a cation-Exchange Membrane was electrodialyzed in a MgCl2 or a NiCl2 solution, violent water dissociation occurred. This phenomenon is due to the hydrophilic effect of Mg(OH)2 or Ni(OH)2 crystals formed in the water dissociation layer. © 2002 Elsevier Science B.V. All rights reserved.
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Concentration polarization in ion Exchange Membrane electrodialysis
Journal of Membrane Science, 1991Co-Authors: Yoshinobu TanakaAbstract:The distribution of NaCl concentration in the boundary layer formed on the desalting and concentrating surfaces of an ion Exchange Membrane was estimated theoretically and compared with values obtained by the Schlieren diagonal method. p]In the desalting chamber, if the convective flow rate (the flow rate component perpendicular to the Membrane surface) on the desalting surface of the Membrane exceeds a critical value, the NaCl concentration distribution in the boundary layer becomes uniform and takes on the value for the bulk solution. However, if the convective flow rate drops even slightly below the critical value, the NaCl concentration in the boundary layer decreases considerably at the Membrane/ solution interface. p]In the concentrating chamber, the NaCl concentration in the boundary layer rises on the surface of a cation Exchange Membrane and falls on the surface of an anion Exchange Membrane. This is due to the diffusion constant of Cl− ions being larger than that of Na+ ions.
Joong Hee Lee - One of the best experts on this subject based on the ideXlab platform.
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Polymer Membranes for high temperature proton Exchange Membrane fuel cell: Recent advances and challenges
Progress in Polymer Science, 2011Co-Authors: Saswata Bose, Thi Xuan Hien Nguyen, Nam-hoon Kim, Kin-tak Lau, Tapas Kuila, Joong Hee LeeAbstract:Proton-Exchange Membrane fuel cells (PEMFCs) are considered to be a promising technology for efficient power generation in the 21st century. Currently, high temperature proton Exchange Membrane fuel cells (HT-PEMFC) offer several advantages, such as high proton conductivity, low permeability to fuel, low electro-osmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. Owing to the aforementioned features, high temperature proton Exchange Membrane fuel cells have been utilized more widely compared to low temperature proton Exchange Membrane fuel cells, which contain certain limitations, such as carbon monoxide poisoning, heat management, water leaching, etc. This review examines the inspiration for HT-PEMFC development, the technological constraints, and recent advances. Various classes of polymers, such as sulfonated hydrocarbon polymers, acid–base polymers and blend polymers, have been analyzed to fulfill the key requirements of high temperature operation of proton Exchange Membrane fuel cells (PEMFC). The effect of inorganic additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, Membrane fabrication and physicochemical characterizations is provided. The proton conductivity and cell performance of the polymeric Membranes can be improved by high temperature treatment. The mechanical and water retention properties have shown significant improvement., However, there is scope for further research from the perspective of achieving improvements in certain areas, such as optimizing the thermal and chemical stability of the polymer, acid management, and the integral interface between the electrode and Membrane.
Tong Zhang - One of the best experts on this subject based on the ideXlab platform.
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the reactant starvation of the proton Exchange Membrane fuel cells for vehicular applications a review
Energy Conversion and Management, 2019Co-Authors: Huicui Chen, Xin Zhao, Tong ZhangAbstract:Abstract The short service life of fuel cell is a key problem that restricts the commercialization of fuel cell vehicles. Many scholars have found that gas starvation is one of the most important causes of the proton Exchange Membrane fuel cell lifetime decay, which leads to a series of severe consequences such as carbon support corrosion, cell reversal and output performance degradation. However, accurate diagnosis and effective mitigation of fuel cell gas starvation are not achieved currently. Gas starvation is a condition that the reaction gas of proton Exchange Membrane fuel cell working in the sub-stoichiometric state. In this paper, we will study the causes, severe consequences, diagnostic methods and mitigation measures of the gas starvation in proton Exchange Membrane fuel cells through previous literature review. This research is aim to provide guidance to the diagnose methods, to optimize the system control strategy and structure design and to contribute to the studies which are focus on prolong the proton Exchange Membrane fuel cell lifetime.
Sui Sheng - One of the best experts on this subject based on the ideXlab platform.
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Development status of reversible proton Exchange Membrane fuel cells
Chinese Journal of Power Sources, 2004Co-Authors: Sui ShengAbstract:Reversible proton Exchange Membrane fuel cell (RPEMFC) is an electrochemical cell working both as the water electrolyzer (charging) and fuel cell (discharging). Promoted by the improvement of proton Exchange Membrane fuel cell (PEMFC) technology, RPEMFCs are attracting much attention. Its working principle was described, and development status in electro catalysis, Membrane electrode assembly (MEA) preparation, performance, and application were reviewed. One key issue is the development of the bifunctional electrodes for oxygen reduction and evolution, but its highly activity and long-term stability of electro catalysts on the dual modes are under solving. Due to its theoretical energy density (as high as 3 600 Wh/kg), reliability and long lifetime, RPEMFCs have the potential to be well in excess of the energy storage systems of secondary battery, especially in weight-sensitive application, such as solar-powered aircraft, and space shuttle.
Zhang Hu - One of the best experts on this subject based on the ideXlab platform.
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Research on proton Exchange Membrane fuel cell
Chinese Journal of Power Sources, 2015Co-Authors: Zhang HuAbstract:Proton Exchange Membrane fuel cell(PEMFC) has an extensive application respective in EV, portable electronic device, stationary power plant and special power with the advantages of high energy conversion efficiency and quick startup at ambient temperature. The technology and mechanism of PEMFC was researched, and its structure defects were analyzed. It is concluded that to research novel catalysts with high activity and excellent stability is very important for the future fuel cell.
