Predicting Shunt Currents in Stacks of Bipolar Plate Cells with Conducting Manifolds

Abstract

A method is presented for predicting shunt currents in stacks of bipolar plate cells with conducting manifolds. The method is based on the requirement that the potential drop through the solution in a manifold be large enough to force current to leave the solution and to enter the conducting manifold. The current that leaves the solution in the manifold enters the conducting manifold at the anode end of the stack and returns to the solution at the cathode end. This could cause catastrophic failure of a manifold.Shunt currents occur in stacks of bipolar plate electrolyzers because of the common electrolytic solutions. Previous work (1, 2) has shown how to determine shunt currents for electrolyzers that have nonconducting manifolds. The purpose of this paper is to present a method for determining shunt currents in electrolyzers with conducting manifolds. It is important to account for shunt currents in large stacks of bipolar plate cells with conducting manifolds because of the possibility of producing unwanted species in the manifolds (hydrogen gas in the chlorine gas/anolyte manifolds of chlor-alkali cells, e.g.) and the much worse possibility of failure of the manifolds due to dissolution. Conducting (metal) manifolds are often selected over other piping materials for economic reasons. For example, in the case of the membrane chlor-alkali process, titanium and nickel are highly corrosion resistant to the anolyte and catholyte, respectively. Plastic (i.e., PTFE) lined steel pipe is often used for the piping of this process. Unfortunately, a typical manifold does not have standard lined pipe dimensions; consequently, fabrication of a manifold with a plastic liner would be difficult.Figures 1 and 2 show pictorially and schematically, respectively, how shunt current can enter a conductor in an electrolytic solution. As shown in the figures, current enters a brine solution by oxidation of chloride ions to form primarily chlorine gas, travels in the solution to a platinum foil, enters the foil by reduction of water to form hydrogen gas, and leaves the foil by oxidation of chloride ions. Finally, the current leaves the cell by reduction of water.If the Pt foil in Fig. 2 were replaced with a commercially pure Ti foil, anodic current would cause dissolution of the titanium, which would occur at the end of the foil nearest the cathode. If the solution in Fig. 2. were changed to a sodium hydroxide solution and the foil changed to nickel, shunt current might leave the foil by oxidation of hydroxyl ions to form oxygen which would be an impurity gas in the hydrogen of a chlor-alkali cell. Consequently, it is desirable to be able to predict the location and extent to which shunt currents enter and leave conducting manifolds in electrolyzers. This can be done in a worst case sense by a simple extension of a previously published model (1, 2).* Electrochemical Society Active Member. Figure 3 is a circuit analog model for shunt currents in a stack of bipolar cells with conducting manifolds. The circuit includes current pa...

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