The invention relates to a sensor fuel cell that can be activated by a first substance (O2) in its environment. The sensor fuel cell includes a catalytically active anode, a cathode that has a cathode surface at least partially exposed to the environment, and a proton-conductive membrane located between the anode and the cathode so as to convey protons through from the anode to the cathode. An anode surface of the anode is at least partially exposed to the environment for access of at least one second substance (H2) from the environment to the anode. Such a disposition enables access of a first reactant in the form for example of oxygen from the ambient air to the cathode, and additionally access of a second reactant in the form for example of hydrogen from the ambient air to the free surface of the anode.