Amorphous carbon nitride (a-CNX) thin films possessing different nitrogen contents (0.12 ≤ x ≤ 0.30) have been synthetized by the DC cathodic reactive magnetron sputtering technique on transparent glass/indium tin oxide (ITO) substrates. They were characterized ex-situ by using two microscopy techniques: scanning electron microscopy (SEM) and atomic force microscopy (AFM), a spectroscopic technique: X-ray photoelectron spectroscopy (XPS), and in solution by using electrochemical methods: cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results obtained with these methods have permitted to highlight why a-CN0.30, the a-CNx sample with the highest nitrogen content among those investigated in this work, behaves both as a dielectrical material (low electronic conductivity) and as an ideally polarizable interface (high polarization resistance). For this purpose, the role of both the ionic strength and the overpotential were investigated in blocking electrode conditions, i.e. only in the presence of the supporting electrolyte. It was observed that the interfacial capacitance can be strongly electrically modulated with the ionic charge concentration and with the applied overpotential. The understanding of the behaviour of a-CNx materials presents a great challenge for their integration in new microfluidic transistor type devices in view of the elaboration of polarizable interface flow-field effect transistors (PI-FFETs).