Various models leading to predictions of negative capacitance,
C, are briefly reviewed. Their relation to the nature of
electric control is discussed. We reconfirm that the calculated
double layer capacitance can be negative under σ-control
- an artificial construct that requires
uniform distribution of the electrode surface charge density, σ.
However, only the total charge C (or the average surface charge
density 
)
can be experimentally fixed in isolated cell
studies (q-control). For those σ where C becomes negative under
σ-control, the transition to q-control (i.e. relaxing the
lateral change density distribution, fixing its mean value to
σ) leads to instability of the uniform distribution and a
transition to a non-uniform phase. As an illustration, a
“membrane capacitor” model is discussed. This exactly solvable
model, allowing for both uniform and inhomogeneous relaxation of
the electrical double layer, helps to demonstrate both the onset
and some important features of the instability. Possibilities for
further development are discussed briefly.