General Current Distribution Problems
Start with the following suggestions if you encounter difficulty solving a stationary problem or finding consistent initial values for a time-dependent problem.
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Make sure that the potential levels are boot-strapped somewhere in the model, preferably by grounding one electrode.
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Consider using a Stationary with Initialization or a Time-dependent with Initialization study. Both these studies will use a Current Distribution Initialization study step as a first step to solve for the potentials only. If you run into problems solving for the second step in this study you may have to change the Current distribution type setting to Secondary on the Current Distribution Initialization study step node, and also review the Initial Values as described in the next bullet. Alternatively you can add a second Current Distribution Initialization study step node, set to Secondary, after the first (Primary) Current Distribution Initialization study step in the study sequence.
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If you are not using a (Primary) Current Distribution Initialization study step, review the Initial Values for the potentials. Suitable initial potential values can usually be derived making a “potential walk” through the geometry, starting at the grounded boundary expected to deviate the least from equilibrium (usually the one with the fastest kinetics, for instance the hydrogen electrode in a PEM fuel cell) and assuming zero overpotentials for the main electrode reactions.
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Switch to Linearized Butler–Volmer kinetics while troubleshooting. This can be useful to help achieve a solution for a model that does not solve with nonlinear kinetics, thereby indicating suitable initial values for the nonlinear problem.
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If your model contains porous electrodes, try refining the mesh resolution in these domains, especially toward the electrolyte boundaries.
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Electrochemistry Interfaces
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Specifying Initial Values and Inspecting and Troubleshooting Meshes in the COMSOL Multiphysics Reference Manual