Alkaline polymer electrolyte (APE) is the core component of modern alkaline hydrogen and oxygen fuel cells, and its single ion conductor nature makes the “electrode/APE” interfaces different from the conventional “electrode/solution” interfaces in terms of ion distribution, electrical double layer structure and polarization behavior. Due to the complexity of the APE and the associated solid–solid interfaces, fundamental investigations are challenging and deeper understanding of the structures and properties of such interfaces is in the infant stage. In this work, we aim to investigate the double layer structure from the aspects of differential capacitance curve and potential of zero charge (PZC) at the electrode/QAPPT (quaternary ammonia poly(Nmethyl-piperidine-co-p-terphenyl) interface. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and microelectrode-based immersion techniques were employed. The differential capacitance curves of Pt/QAPPT interfaces exhibited an asymmetric U-shaped feature with a minimum at the potential which is consistent with the PZTC measured by the immersion method. The capacitance raised less quickly on the negative than the positive sides of the PZTC. These results reflect the characteristics of the single ion conductor and role of alkaline polyelectrolytes in modifying the double layer structure of the electrode/APE interfaces.