Yoshinori Nishiki

Apparent diffusion coefficients for electroactive anions in coatings of protonated poly(4-vinylpiridine) on graphite electrodes

Abstract Chronoamperometry, chronocoulometry and chronopotentiometry were applied to measure apparent diffusion coefficients, D app , for Mo(CN) 8 4− , W(CN) 8 4− , Fe(CN) 6 3− and Fe(CN) 6 4− bound electrostatically within coatings of protonated poly(4-vinylpyridine) on graphite electrodes. The relative advantages of the three experimental techniques are discussed. Values of D app for the incorporated anions showed a strong decrease as the anion concentration within the coating increased until a steady value was reached at anion concentrations near 1 M . Some of the factors that may contribute to this behavior are discussed.

Effect of gas evolution on current distribution and ohmic resistance in a vertical cell under forced convection conditions

The volume fraction of gas bubbles in a vertical cell with a separator was evaluated on the basis of the Bruggemann equation by taking into account the increase in velocity of the rising gas bubbles when fresh solution without gas bubbles is supplied to the bottom of the cell at constant velocity. This enhancement of the velocity results from an increase in the volume of gases evolving at the working electrode. The following three cases for overpotential at the working electrode were considered: no overpotential, overpotential of the linear type and of the Butler-Volmer type. The volume fraction, εh, at the top of the cell was expressed as a function of the dimensionless height of the cell and kinetic parameters. The total cell resistance can be expressed by {(2/5εh)[1−εh)−3/2−1+εhD;]+µ}ϱ1d1/wh, where ϱ1 is the resistivity of the solution without gas bubbles,d1 the interelectrode distance,w the cell width,h the cell height and μ the parameter involving overpotential and resistance of the separator. It was found that there is an optimum value of the interelectrode distance. The optimum value is about a quarter of the value for the case of constant gas rise velocity, which corresponds to a closed system.

Secondary current distribution in a two-dimensional model cell composed of an electrode with an open part

A two-dimensional model for industrial production-type cells in which electrodes have holes for releasing gas bubbles to the back side of the electrodes and a separator located between the working- and counter-electrodes is proposed in conjunction with some geometrical parameters of the electrode and the cell. The primary current distribution in this model was calculated for a series of values of the parameters by the finite element method. The current distribution in the cell with the separator is quite different from that without the separator. Variations of the ohmic potential drop with the parameters reveal that the cell resistance is determined not only by the interelectrode distance but also by the per cent open area and in some cases by the superficial surface area. The partitions of the total current into the currents on the front, the back and the intermediate sides of the working-electrode are obtained as functions of the per cent open area and the superficial surface area. These results may be useful for estimating the performance of the electrode.

Current distribution in a two-dimensional narrow gap cell composed of a gas evolving electrode with an open part

On the basis of the observation of gas bubbles evolved by electrolysis, a two-dimensional vertical model cell composed of electrodes with open parts for releasing gas bubbles to the back side is proposed. The model cell consists of two layers. One layer forms a bubble curtain with a maximum volume fraction of gas bubbles in the vicinity of the working electrode with open parts. The other. being located out of the bubble layer, is a convection layer with a small volume fraction distributed in the vertical direction under forced convection conditions. The cell resistance and the current distribution were computed by the finite element method when resistivity in the back side varied in the vertical direction along the cell. The following three cases for overpotential were considered: no overpotential, overpotential of the linear type and overpotential of the Butler-Volmer type. It was found that the cell resistance was determined not only by the interelectrode gap but also by the percentage of open area and in some cases by the superficial surface area. The cell resistance varied only slightly with the distribution of the bubble layer in the back side.

Variations of cell resistance and current distribution with current feeder configurations

Two models of current feeder configurations for resistive electrodes are presented, one in which the feeders were connected at the same end of a pair of electrodes of a unit cell and one in which connection was made at opposite ends. Expressions for the cell resistance and the current distribution were derived for these two current feeder configurations on the assumption of a linear type of overpotential. The cell resistance in the ‘opposite ends’ configuration was larger than that for the ‘same ends’ arrangement. Conversely, the current distribution in the former was more uniform than that in the latter. The relation between the total cell resistance and the number of current feeders,n, was obtained. An increase inn led to a decrease in the resistive loss of the electrodes by an amount corresponding to 1/n2, irrespective of current feeder configuration, when the resistance of the electrode was not so great as that of the solution.

Approximate evaluation of cell resistance with resistive electrodes

The resistance of a cell composed of a resistive anode and a conductive cathode has been theoretically calculated by solving the two-dimensional Laplace equation for the solution phase where the anode has an overpotential of a linear type. Conditions were evaluated under which the cell resistance could be obtained on the assumption that the direction of the current flowing in solution was perpendicular to the electrode. These were not strongly dependent on the ratio of the interelectrode distance to the height of the electrodes. The main parameters which determine the conditions were the Wagner number and the ratio of the electrode resistance to the solution resistance.

Estimation of optimum anode geometry in chlor-alkali membrane cells

In order to explore the dependence of cell voltage on electrode geometries, precious metal oxide anodes (DSA®) with the following three kinds of geometries were constructed: an assembled strip with a parallel array (louvre type), a circularly perforated plate and a flattened mesh. Voltages in the membrane cell of a laboratory scale were measured with chlorine gas evolution in NaCl solution. The cell voltages exhibited a linear relation to a unit cell characteristic dimension as well as to the square of the per cent open area of the anode, as predicted from the theoretical analysis of the primary current distribution in the two-dimensional rectangular model cell. Therefore, small sizes of the iterative pattern unit reduced cell voltages. Slopes of the linear relation gave the increase in solution resistivity caused by the gas bubbles, indicating the presence of a bubble curtain around the anode.

Electrolysis of (CH3)4NF•5HF Melt with Boron-doped Diamond Anode

Introduction Although electrolytic synthesis of (CF3)3N was conducted by Simons method, this process had a few disadvantages. One of them was an operation at temperatures less than 0 degree Celsius. Use of a room temperature molten salt (CH3)4NF·mHF as electrolyte enables to solve this problem. Recently, electrolysis of the (CH3)4NF·mHF melt using the Ni anode has been developed for electrolytic synthesis of (CF3)3N. In this case, electrolysis was stopped for shorter duration because of formation of NiF2 film on the Ni anode. Hence, it is necessary to develop a new anode for electrolytic production of (CF3)3N from only the (CH3)4NF·mHF melt. Since Boron-doped diamond (BDD) has a high electric conductivity and stability of structure such as diamond, it is expected that BDD electrode can be used as a new anode material in this process. Hence, the performance of a BDD anode for electrolytic production of (CF3)3N was investigated in the (CH3)4NF·mHF melt. Experimental A room temperature molten salt of (CH3)4NF·5HF was used as an electrolyte. Electrochemical measurements and electrolysis were carried out in a three-electrode cell. Anode potential was determined galvanostatically in the range of current density from the 5 mA cm to 100 mA cm in the (CH3)4NF·5HF melt with BDD and Ni electrodes. Electrolysis of the (CH3)4NF·5HF melt was conducted at 40 or 100 mA cm for 200h at room temperature with BDD electrode. The anode gas was analyzed by gas chromatography mass spectroscopy and gas chromatography. The anode surface after electrolysis was also analyzed by X-ray diffraction (XRD). Result and discussion Fig. 1 shows the galvanostatic polarization curves of BDD electrode and Ni electrode in the (CH3)4NF·5HF. In the Ni electrode, anode potential was larger even at low current density. On the other hand, anode potential increased slightly with increasing current density and no anode effect took place at 100 mA cm in the BDD electrode. The compositions of anode gas evolved at the BDD anode in electrolysis of the (CH3)4NF·5HF melt at room temperature are shown in Table 1. The anode gas was composed of CF4, NF3, CHF3, CH2F2, C2F6, (CF3)3N, (CF3)2CHF2N, CF3(CHF2)2N, and (CHF2)3N. The ratios of (CF3)3N and (CF3)2CHF2N increased with increasing current density. This result suggests that electrolytic fluorination may be promoted at high current density. XRD patterns of the BDD anodes before and after electrolysis are shown in Fig. 2. Both XRD patterns have (1 1 1) peak and (2 2 0) peak. They are assigned to BDD. The structure of BDD was maintained after electrolysis. From these results, the electrolytic production of (CF3)3N from (CH3)4NF·5HF melt using the BDD anode is an appropriate process because the electrolytic conductivity of BDD anode surface is kept higher during electrolysis.

Computation of primary current distribution and cell resistance at model electrodes with circular perforations

A model of a gas-evolving production-type cell with a circularly perforated anode is described. A unit of the model was composed of a disk cathode, a separator and a ring anode in turn. These were located in a cylindrical cell filled with solution. Primary current and potential distributions in the unit cell were computed by solving the Laplace equation in cylindrical coordinates by the finite element method. Geometric parameters determining the distributions were primarily the interelectrode distance and the percentage open area. Current distribution in the open part was larger than that in a rectangular cell with the same geometric parameters because of the cylindrically concentrated supply of the current from the inner part of the ring and the back side of the anode. The unit cell resistance was evaluated as a function of the geometric parameters. It exhibited a linear variation of the interelectrode distance and the square of the percentage open area. There was, however, a slight dependence of the percentage open area on the unit cell resistance and hence it is concluded that circularly perforated electrodes provide higher performance than louvre-type electrodes.

New Catalysts for a Zero-Gap Type Gas-Diffusion Electrode in Chlor-Alkali Membrane Process

Introduction It is expected that the cell voltage in chlor-alkali membrane processes could be greatly reduced by using an oxygen gas-diffusion cathode instead of a conventional hydrogen evolving cathode. To realize this new process, several kinds of electrolysis methods and gas-diffusion electrodes (GDE) have been proposed. A zero-gap type GDE with a two-compartment cell, as schematically drawn in Fig.1, has an advantage to attain good electrolytic performance because the electrodes are able to be placed close to the membrane. It also has a merit for constructing large scale cells because there is no requirement to construct a gas chamber separated from a caustic liquid chamber . It has been confirmed by using the Ag catalyst in a small-sized cell that the durable life time at 4kA m is over 5 years (2.45 V as an averaged cell voltage). However, the cell voltage gradually increased in long-term operation, which could be strongly related to the agglomeration of the Ag catalyst particles. This study deals with an excellent electrolytic performance of Ag and Pd binary catalysts that improve the catalytic activity and also inhibit agglomeration of the catalyst particles.