This git project contains code solving PNP equations. We rely on the model of crystal with honeycomb grain structure. This model is defined in [Tautschnig M. P., Harrison N. M., Finnis M. W. A model for time-dependent grain boundary diffusion of ions and electrons through a film or scale, with an application to alumina //Acta Materialia. – 2017. – Т. 132. – С. 503-516.].