Localised electrochemical impedance spectroscopy investigation of polymer electrolyte membrane fuel cells using Print circuit board based interference-free system

•Interference-free localised electrochemical impedance spectroscopy diagnostic system.•Segment gas diffusion layer does not be needed in a localised impedance measurement.•Anode inlet gas relative humidity is essential in the increase of catalyst utilisation.•The optimum reaction site shifts as the current density increases.•Cathode outlet suffers mass transport problem even at a medium current density.