This paper presents an experimental characterization of a high temperature proton exchange membrane fuel cell (HT-PEMFC) short stack carried out by means of impedance spectroscopy. Selected operating parameters; temperature, stoichiometry and reactant compositions were varied to investigate their effects on a reformate-operated stack. Polarization curves were also recorded to complement the impedance analysis of the researched phenomena. An equivalent circuit model was used to estimate the

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... rent resistances at varying parameters. It showed a significantly higher low frequency resistance at lower stoichiometry. Both anode and cathode stoichiometric ratio had significant effects on the stack performance during the dry hydrogen and reformate operation modes. In both cases the effects faded away when sufficient mass transport was achieved, which took place at λ anode = 1.3 for dry hydrogen, λ anode = 1.6 for reformate operation and λ cathode = 4. The work also compared dry hydrogen, steam reforming and autothermal reforming gas feeds at 160 • C and showed appreciably lower performance in the case of autothermal reforming at the same stoichiometry, mainly attributable to mass transport related issues. In a CO poisoning analysis the stack showed good tolerance to concentration up to 1% CO in the fuel stream. Temperature is a crucial parameter for HT-PEMFCs, as it is the parameter that gives them the edge over their lower temperature counter parts in terms of improved tolerance to impurities, easier water managements and simpler system design. However, its increase also enhances stack degradation and material challenges, by increasing mechanical and thermal stresses [33, 45] . Like temperature, stiochiometry also depends on conflicting considerations; optimization of fuel utilization without causing significant mass transport losses. Galbiati et al. [13] showed that stoichiometry also affects water transport, where increasing air stoichiometry reduces water transport from cathode, because it enhances water removal on that side, resulting in lower anode exhaust water flow and higher hydrogen stoichiometry increases water transport from cathode leading to higher water flux in the anode exhaust. These effects like other parametric characterizations have been extensively studied both by model simulations and single cell testing for fundamental understanding of degradation mechanisms [3, 12, 19, 36, 37] . This work extends the study of parametric characterization to stack level in order to understand the fundamentals at a step closer to real life applications. It investigates the effects of temperature, stoichiometry and CO poisoning in an HT-PEMFC short stack. Characterization was done by means of electrochemical impedance spectroscopy (EIS) and polarization curves under varying operating conditions. Impedance spectroscopy is a strong electrochemical characterization tool that can be used in-situ to obtain several valuable information about a device's electrochemical operation. However, its interpretation by means of EC model fitting, though quick, can be highly ambiguous. Several equivalent circuit (EC) models can fit the same impedance data, and therefore, when choosing an EC model one should have a good knowledge of the behaviour of the tested device. Also, how the different phenomena in the fuel cells and its components contribute to the impedance spectra are not entirely clear. In the case of HT-PEMFCs several EC models based on different combinations and modifications of Randles' circuit (electrolyte resistance in series with the parallel combination of the double-layer capacitance and a charge transfer resistance) have been proposed [5, 18, 34, 39] . Below is a brief literature review of the interpretation of impedance spectra and the corresponding EC model elements. Impedance measurements of real fuel cell systems have a positive high frequency intercept and up to three arcs, depending on the extent of the mass transport effects. It is widely agreed that the high frequency intercept represents the sum of ohmic resistances of the cell components such as, the membrane, catalyst layer, gas diffusion layer, flow plates, and contact resistances of component connections [13, 41, 45] . Even though, it is difficult to discriminate among the different contributions to the ohmic resistance, its changes during fuel cell operation can be attributed to the changes in proton conductivity [18, 45] . Proton conduction takes place both in the membrane electrolyte and the electrolyte in the catalyst layer by means of proton hopping mechanism along anion chains formed from self-ionization and self-dehydration of H 3 PO 4 [9, 45]. Kondratenko et al. [18] distinguish between the contributions of the membrane resistance and electrolyte resistances between catalyst sites in the active layer using an EC model with a transmission line. However, in the current work a simpler EC model is used and changes in ohmic resistance are attributed to the overall variations in conductivity. The EC model fit of the impedance arc at high frequency gives high frequency resistance (R HF ). associate this exclusively to the structural features of the membrane electrode assembly (MEA). Yuan et al. [41] argue that the impedance of the full fuel cell almost equals the cathode impedance due to the fast hydrogen oxidation reaction in H 2 /O 2 (air) operation [18] . Other authors distinguish between anode and cathode faradaic resistances, with the high frequency arc being the anode resistance and the intermediate frequency arc cathode resistance [5, 25] . According to Mench et al. [25] changes in low and intermediate frequency resistance provide insight into degradation mechanisms in the cathode catalyst layer, with the intermediate frequency time constant attributable to the charge transfers and lower frequency ones to mass transport. To avoid these ambiguities, in this work the analyses of the fitted resistances are not discriminated between anode and cathode, but rather into frequency ranges as high frequency resistance (R HF ), intermediate frequency resistances (R IF ) and low frequency resistance (R LF ). To the knowledge of the authors there is no standardized way of representing impedance spectra and fitted data to date. In literature different representations of fuel cell impedance can be found, including, no units, Ohms (Ω) and Ωcm 2 (impedance normalized with respect to unit cell area) [14, 20, 29, 30] . Multiplying impedance and fitted EC model data with the active single cell area makes it easier to