The Relationship between Shunt Currents and Edge Corrosion in Flow Batteries

Abstract

Shunt currents occur in electrochemical reactors like flow batteries, electrolyzers, and fuel cells where many bipolar cells that are connected in series electrically contact a mobile electrolyte through one or more common fluid distribution manifolds. Shunt currents reduce energy efficiency, and can cause unwanted side reactions including corrosion and gas generation. Equivalent-circuit models have been widely used to examine shunt currents in multi-cell electrochemical reactors. However, a detailed investigation of the interesting electrochemical processes occurring at the edges of the active areas has not been presented. In this work, the generation of shunt currents and their tendency to drive corrosion at the edges of positive electrodes in the most positive cells in a reactor stack are investigated with a comprehensive numerical model. An analytical model based on the penetration of current into a semi-infinite electrode, that can be used in conjunction with traditional equivalent-circuit models to assess the tendency for shunt currents to drive corrosion, is developed and compared to the numerical model. The models provided here can be used to set requirements on maximum allowable port currents in order to achieve a particular durability goal. Shunt currents are an important source of inefficiency in electrochemical reactors like flow batteries, electrolyzers, and fuel cells where many bipolar cells are connected electrically in series and contact a mobile electrolyte through one or more common fluid distribution manifolds. Figure 1 shows a flow-battery stack with arrows added to illustrate how one of the two electrolyte streams enters through a single inlet manifold, distributes to the individual cells, and recombines and leaves through an exit manifold. The other electrolyte traces a similar path through the stack in a second set of inlet and exit manifolds. The fluid manifolds act as ionic short circuits between cells in the reactor stack. The shunt currents that flow through the manifolds continuously discharge the reactants and can drive parasitic reactions including corrosion that hastens battery failure and gas generation that represents a safety hazard. Shunt currents are a particularly acute concern in typical flow batteries because very conductive electrolytes circulate through the reactors. Thus, minimizing the deleterious effects of shunt currents is a primary concern of stack designers. This can be accomplished by making the liquid paths outside of the active area long and narrow to increase ionic resistance at the cost of increasing the pumping work associated with circulation. While shunt currents can be minimized in this way, they cannot be completely eliminated. Therefore, understanding how shunt currents emanate and terminate at the edges of the active areas is critical to the successful operation of bipolar electrochemical reactors. Figure 2 is a picture of a portion of the junction between the active area of a bipolar plate and its surrounding plastic frame. This picture was taken duri...

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