Measurement of Electrolyte Gradient in an Operating Fuel Cell

Abstract

Reference mercury, mercuric oxide electrodes have been developed for monitoring the concentration of potassium hydroxide in hydrogen-oxygen fuel cells operating at 27 ~ and 70~The distribution of electrolyte in the matrix of an operating fuel cell can be measured by imbedding appropriate reference electrodes in various parts of the fuel cell matrix. Knowledge of the concentration distribution in a fuel cell is important for understanding the operation of a cell and for designing fuel cells. Mercury, mercuric oxide electrodes have been shown to be reversible to hydroxyl ions (1, 2) at concentrations less than 0.85M. These electrodes will be useful for measuring the distribution of potassium hydroxide in fuel cells provided the electrodes can be shown to respond to hydroxyl ions at the higher concentrations encountered in an operating fuel cell. ExperimentalFuel cells.--Two types of hydrogen-oxygen fuel cells were used in the development of reference electrodes. One was a small poly(methylmethacrylate) cell similar to the cell described by Bone (3). A circular area of matrix 2.5 cm in diameter was exposed to gases. This cell was run at room temperature without temperature control, and at 70~ in an air bath. A second cell, which is described in ref.(4), was made from stainless steel plates separated by polytetrafluoroethylene gaskets. The surface of the matrix exposed to gases in this cell was 5.0 x 5.0 cm. The larger cell was heated externally by two electric heaters, and its temperature was regulated by a temperature-control unit connected to a thermocouple within the cell. The pattern of gas flow in each cell was determined by the position of the ports through which gases entered and left the cell. These positions are shown in Fig. 1. The catalytic electrodes were Cyanamid AB1 electrodes (5). The gel membrane was made from crosslinked poly(vinyl alcohol) and contained filler (6). The matrix asbestos was Fuel Cell Asbestos supplied by Johns-Manville.Calibration electrodes.--The ability of mercury, mercuric oxide electrodes to respond to changes in hydroxyl ion concentration at high concentrations of potassium hydroxide was determined in a series of experiments which made use of relatively large electrodes, referred to as "calibration electrodes." The calibration electrodes were made from 2.4 mm diameter glass tubes, one end of which was covered with a poly(vinyl alcohol) membrane (unfilled) which allowed transport of water and ions, but retained solid materials within the tube. A slurry of mercury and mercuric oxide (prepared by triturating with a solution of potassium hydroxide having the concentration of the standard solution in which the electrode was to be used) was placed in the electrode tube together with a platinum wire that had been gold plated and then amalgamated.Electrodes were made with potasisum hydroxide solutions containing 3.25, 4.67, 6.00, 6.56, 7.40, 8.85, and 10.58 moles of potassium hydroxide per kg of water. These electrodes were compared in the cell Hg, HgO KOH(Cz) : :KOH(C2)HgO, Hg [1] at 2...

Steady‐State Composition Profiles in Mixed Molten Salt Electrochemical Devices: I . Lithium/Sulfur Battery Analogs

“…Even the profile calculated numerically with unequal equivalent volumes, and with the transference number given by Eq. [16], differs by less than 5% from the analytical solution for equal equivalent volumes. The simple analytical solution (Eq.…”

“…3 but with the transference number given by Eq. [16] and the different equivalent volumes of LiC1 and KC1. A' and B' are composition profiles for initial compositions of 0.2 and 0.5 mole fraction KC1, respectively, at a current density of 0.15 A cm -2.…”

Composition Gradients in Molten Salt Binary Mixtures during Electrolysis at High Current Density

“…Equation [1] together w i t h Eq. [2] can be n umerically solved (13) for the cases where D and (or) tKNO~ are composition dependent. The computation gives the entire composition profile between the two electrodes at any time and the time dependences of concentration at both electrodes.…”

“…These transient concentration profiles during electrolysis were calculated by numerical solution of Eq. [1] and [2], with Eq. [3] for the emf at the end of electrolysis and during the relaxation.…”

“…Electrolyte composition gradients in aqueous fuel cells and electrolyzers have been observed (2,3) and explained in terms of electrode reactions, diffusion, and migration (4)(5)(6). Previously derived equations for (one dimensional) diffusion and migration in molten salt binary mixtures during current flow predicted the establishment of concentration gradients in the electrolytes of molten salt batteries and fuel cells (7).…”