Abstract: The present invention relates to a sensor device (10) for a fuel cell system (100) for determining a purging parameter (SP) for controlling a purging process of the fuel cell system (100), comprising a first flow channel (20) for arranging in an anode feed section (122) of an anode section (120) of a fuel cell stack (110) and a second flow channel (130) for arranging in a recirculation section (126) of the anode section (120) of the fuel cell stack (110), which are separated from each other, at least in sections, by means of a gas-tight membrane (40), wherein the membrane (40) is designed to be permeable for protons and has an electrode section (42, 44) on both sides, as well as comprising a measuring device (50) for determining a fuel concentration difference between the first flow channel (20) and the second flow channel (30) as a purging parameter (SP) based on an electrical voltage between the two electrode sections (42, 44).