Thank you for your reply! As you suggested I've retried with a mesh with much lower aspect ratio (According to ANSYS-Meshing: 3,3063 - 7,15 ). Unfortunately the problem remains the same (at least in my opinion). I think the gradient near the edge, where Gas-Channel and GDL meet, has to be slightly higher than in the next cells, but not as high as it currently is. But maybe I am mistaken? I think the gradient in the first cell underneath the Gas-Channel is also odd. My guess would be that the edge is not well-defined for C_UDSI?
To further explain my problem I've added two photos, where you can clearly see that along the edge Gas-Channel <-> GDL current collector boundary, there is a much higher gradient, both in the cells underneath the channel and underneath the current collector right next to the edge, where the GDL meets the Gas-Channel.
If you can give me any advice why this is the case, I would much appreciate it! Other tips or suggestions are also very welcome!