Analysis of Shunt Currents and Associated Corrosion of Bipolar Plates in PEM Fuel Cells

Proton exchange membrane (PEM) fuel cells are being developed for future NASA missions.  These fuel cells use a bipolar construction, with internal manifolds to conduct product water and coolant water.  The wetted surfaces of the manifolds present large voltage gradients across the product water and coolant water passages, which can induce water electrolysis in the manifolds of full-scale stacks.  As such, shunt currents lead to parasitic power losses and corrosion of the fuel cell metal surfaces; therefore, it is important to understand and characterize the effects of shunt currents.   The circuit analog models provide an approximation of the shunt currents, limited by the estimation of the charge transfer resistance. As such, the methods should be used cautiously.

Shunt currents are influenced by cell voltage, cell number, manifold ohmic resistance, product water/coolant purity, and the charge transfer resistance of bipolar plate surfaces to electrolysis.  In this work, electrochemical characterizations were performed to estimate these characteristics under relevant operating conditions.  A numerical solution of the shunt currents is presented, and the distribution of corrosion and hydrogen evolution are described.  The same calculations would apply to PEM electrolyzers used for hydrogen and oxygen production. With some adjustment, similar calculations may also be applied to other electrochemical system with common electrolyte channels, such as flow-batteries and other devices where an electrolyte manifold experiences voltage gradients.