Negative real impedances

are sometimes associated with persistent current reductions

as potential increases; this might be characteristic of unstable

electrochemical systems e.g. when some of the following processes

might take place: a) potential-dependent adsorption/desorption of a

catalyst, b) electrostatic effects at low ionic strengths, as in the

reduction of anions at a negatively charged surface, or c) when the

active surface available for electron-transfer decreases as polarization

raises (for example, when the interface shows activity peaks

followed by passivity stabilization); in these cases, a part of the

polarization curve (or voltammogram if it is the case) has a negative

slope—the value of the impedance at zero frequency is related

to the slope of the steady-state polarization curve—then the impedance

measured in the potential range where this occurs is found

with a negative real part.

Reference: Xochitl D. Benetton, Navarro-Avila S., Carrera-Figueiras C. (2010) Electrochemical evaluation of Ti/TiO2-polyaniline anodes for microbial fuel cells using hypersaline microbial consortia for synthetic-wastewater treatment, Journal of New Materials for Electrochemical Systems, Vol.13., No. 1, p. 1-6. 

Hopefully this point will help to solve your doubts

Negative real impedances

are sometimes associated with persistent current reductions

as potential increases; this might be characteristic of unstable

electrochemical systems e.g. when some of the following processes

might take place: a) potential-dependent adsorption/desorption of a

catalyst, b) electrostatic effects at low ionic strengths, as in the

reduction of anions at a negatively charged surface, or c) when the

active surface available for electron-transfer decreases as polarization

raises (for example, when the interface shows activity peaks

followed by passivity stabilization); in these cases, a part of the

polarization curve (or voltammogram if it is the case) has a negative

slope—the value of the impedance at zero frequency is related

to the slope of the steady-state polarization curve—then the impedance

measured in the potential range where this occurs is found

with a negative real part.

Reference: Xochitl D. Benetton, Navarro-Avila S., Carrera-Figueiras C. (2010) Electrochemical evaluation of Ti/TiO2-polyaniline anodes for microbial fuel cells using hypersaline microbial consortia for synthetic-wastewater treatment, Journal of New Materials for Electrochemical Systems, Vol.13., No. 1, p. 1-6.