EXPERIMENTAL TEST AND MODELLING OF PERFORMANCE DEGRADATION OF HT-PEM FUEL CELLS FOR USE IN MICRO-CHP SYSTEMS

Fuel cells based Micro Combined Heat and Power (micro-CHP) systems maintain good efficiencies for small size plants and at partial load making them suitable for domestic applications. High Temperature Polymer Electrolyte Membrane (HT-PEM) fuel cells are a promising technology for micro-CHP system, especially thanks to their high CO tolerance that allows the use of fuels other than hydrogen. Nevertheless, cost, performance and degradation issues are still to be overcome to fully achieve commercialization. Regarding performance degradation, operating conditions strongly affect their durability. The purpose of this research is to determine how the degradation issues for HT-PEM fuel cells, installed in micro-CHP systems, can be handled to make the system become suitable for long term operation, in terms of performance. To do so, a specific methodology has been developed for experimental testing and modelling of performance degradation of HT-PEM fuel cells for use on micro-CHP systems. Data of 8 Membrane Electrode Assemblies (MEAs) operated with different accelerated ageing tests have been collected and compared with literature. In order to assess the cell performance, Polarization Curves (PC), Electro Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) have been recorded during the ageing tests. As expected, voltage degradation was strongly influenced by operating conditions. The voltage decay rate at 200 mA/cm2 was found to be about 30 µV/h for constant load operation, 34 µV/h for triangular load cycle ([0.01 – 0.5]V), 45 µV/h for triangular load cycle with permanence at OCV ( [0 – 0.5]V - 2s OCV), 81 µV/h for Start/Stop cycles, while for constant permanence at OCV a wide variability of the degradation rate has been encountered. In order to analyse the nano-morphological evolution of the catalyst layer on large portions of the MEA, Small Angle X-Ray Scattering (SAXS) has been carried out. The SAXS results showed a mean size increase of the platinum nanoparticles up to 130% when the MEA is subjected to load cycles. SAXS data has then been compared with data obtained by Transmission Electron Microscopy (TEM) analysis confirming the catalyst particles growing trend. The experimental data collected during this research activity allowed to identify the performance degradation of the fuel cell over different load conditions and allowed to infer a performance degradation model that has then been implemented in a CHP system process simulation model. Stack degradation has been shown to be still an issue that hampers the full exploitation of the technology, but, in CHP configuration, heat production can partially compensate electrical energy loss due to degradation and its detrimental effect can be mitigated choosing some operational control strategies and increasing the size of the stack, even if this affects system cost. Finally, the model can be a valuable tool for conducting sensitivity analysis and find optimal size of system components taking into account the system performance degradation over time. In the future this model could be upgraded introducing different degradation behaviour for each operational condition encountered during the lifetime of the CHP-system.