Integration boundary condition that is probably of circular dependency (dye-sensitized solar cell)

I tried to model the ionic exchange in a dye-sensitized solar cell. The concentration of cations in the electrolyte follows a Boltzmann distribution:

Nc = N0 * exp(-Q/kT). N0 is a constant.

That is, Nc is a variable dependent on N0.

To fulfill ionic conservation, one of the non-local boundary conditions is:

∫Ω Nc*dΩ = N0*Ω , where Ω is the volume of the cell.

That is, the integration of Nc over the entire domain = N0 * volume of the domain.

My question is, how can I fulfill it in Comsol? I know how to add an integration coupling variable (intop1), but what to do next?

Thanks.