The durability and reliability of fuel cell products need to be improved. The lack of early diagnosis and failure-prevention techniques is one of the limiting factors. This thesis presents a non-invasive method for the early diagnosis of flooding, dehydration and low fuel stoichiometry (three common failure modes). The method is based on micro sensing electrodes (SE) that are placed at appropriate locations in a single cell. These electrodes have a characteristic potential response to each of the fa ilure modes, which enables detection prior to overall fuel cell failure. The specific features in the measured responses (or combinations thereof) can be used to discern between different failure modes, and initiate corrective actions.

This thesis also reports on the separation of anodic and cathodic potentials in a working fuel cell via reference electrodes maintained at constant conditions. The reference electrodes consisted of four platinized platinum electrode wires and two patches of the same catalyst layer used in the anode. All the reference electrodes were unaffected by the operating conditions of the fuel cell and those with patches provided the most stable potentials.

Individual anodic and cathodic overpotentials (activation, ohmic, concentration and mass transport) were obtained in a segmented and un-segmented fuel cell for the first time. An array of reference electrodes and gases with different diffusion coefficients were used to discern the different overpotentials. The results show that the anodic overpotentials cannot be ignored, even if the conditions are changed at the cathode only. The oxygen concentration has an effect on the anode and in particular hydrogen oxidation and proton flux.

Under dry conditions the current in-plane gradients are very large and the heat generation profiles are affected, creating an uneven temperature distribution in the catalyst layer due to concurrent effects of the half-cell reactions and the water vaporization.

The combination of reference electrodes and multi-component gas analysis, allows the measurement and calculation of kinetic and diffusion parameters that can be used for modeling and to understand the behavior of different layers of a fuel cell.