Method and device for determining the redox state of a reaction surface coated with a catalyst material

The invention relates to a method and device for determining the redox state of an anode, which is coated with a catalyst material or consisting thereof, associated with a high temperature fuel cell or a reaction surface of a reformer. According to the invention, at least one first resonator of a piezoelectric sensor device is arranged in the anode gas flow of the high temperature fuel cell or in the gas flow of the reformer. The first resonator is provided with a coating which is oxidizable or reducible in the gas flow. The device also comprises a device for measuring at least one modification of the resonance behaviour of the first resonator. The detected measuring value acts as an indicator of the redox state of the anode of the high temperature fuel cell or reaction surface of the reformer.