An illustrative example hydrogen concentration sensor includes a plurality of electrically conductive plates. A hydrogen evolving electrode assembly in a first location between two of the plates is configured to generate hydrogen. A detection electrode assembly in a second location between two of the plates is configured to provide an indication of a concentration of hydrogen in a fluid of interest. A plurality of isolating layers includes a first isolating layer at the first location between two of the plates and a second isolating layer at the second location between two of the plates. The first and second isolating layers each include a sealant that secures the two plates together and seals a perimeter around the electrode assembly at the corresponding location.