Water management represents one of the main challenges in the design and operation of Polymer Electrolyte Fuel Cells (PEFCs). Besides performance, the water level also affects the durability of the cell. Understanding the degradation processes is of vital importance for extending durability of PEFCs by suitable mitigation strategies. In this work, the degradation processes related to operation with fully-and nonhumidified gas streams were locally studied. The differences were analyzed using in-situ diagnostic tools, such as segmented cell for local current density measurements, during a 300 h test operating under constant conditions, in combination with local post-test analysis, i.e. SEM/EDX and XPS. The results showed the deep impact of the RH on homogeneity during the degradation process due to the fact that different water distribution influences the chemical environment. Under nonhumidified gas streams, the cathode inlet region exhibited increased degradation, whereas with fully humidified gases the bottom of the cell had the higher performance losses. The degradation and the degree of reversibility produced by Pt dissolution, PTFE defluorination, and contaminants such as silicon (Si) and nickel (Ni) were locally evaluated.

Adequate water management represents one of the main challenges in the design and operation of polymer electrolyte membrane fuel cells. In this work, the influence of inlet gas humidification on cell performance is investigated by in-situ current density measurements obtained using the segmented cell approach. Particular attention is paid to the combined effect of cell temperature and relative humidity of the anode and cathode feed streams. When operated at 80 • C and low humidity conditions, the cell is seen to undergo a severe voltage decline that is not observed at 60 • C. The analysis shows that the variation with temperature of the water uptake rate of the gaseous streams plays a key role in determining the observed differences in performance stability. In the case of 60 • C operation, the water uptake rate of the cathode stream at 50% inlet relative humidity is roughly 30% of its value at 80 • C at the same humidification level, resulting in a significantly lower drying capacity. A simple balance of water model, able to explain the observed cell behavior, is finally presented and discussed. Energy demand has become one of the most serious concerns of modern society due to the problems related with greenhouse gas emissions and the depletion of fossil fuels. In this context, hydrogen is expected to play an important role as future energy vector, with polymer electrolyte membrane fuel cells (PEMFCs) being the leading candidates to provide efficient and clean electric energy conversion during the XXI century. Recently, significant progress has been made toward meeting the challenging cost and performance targets required for the widespread use of PEMFCs, specifically in the automotive industry. 1The state-of-the-art of polymer electrolyte membrane fuel cell technology is based on perfluorosulfonic acid (PFSA) polymer membranes operating at a typical temperature between 60• C and 80 • C. 2 Since the ionic conductivity of PFSA membranes depends on the water content of the membrane, 3,4 water management is one of the most important issues for successful operation, high performance, and good durability of PEMFCs. Excess inlet gas humidification as well as condensation processes within the cell are likely to produce the accumulation of liquid water in the porous electrodes and gas diffusion media (effect known as flooding), thereby decreasing cell performance. On the other hand, an insufficient level of gas humidification lowers the ionic conductivity of the membrane and also results in a performance reduction.Numerous studies have investigated the operation of PEMFC under dry conditions in order to simplify operation. 5,6 Early work to demonstrate stable performance for PEMFC using dry or slightly humidified gases has been reported by Büchi et al. 5 Strategies for operating polymer electrolyte fuel cells include also the reduction of humidification of both reactant gases 7-10 or the dry operation of the cathode 11,12 or anode 13 sides. In the last decade, a wide variety of diagnostic and visualization tools have b...

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Water management represents one of the main challenges in the design and operation of PEMFCs. The influence of inlet gas humidification on cell performance is analyzed using in-situ diagnostic tools, such as cyclic voltammetry and segmented cell current density measurements, supported by post-mortem ex-situ investigations. Particular attention is paid to the effect of low humidity conditions in both cathode and anode, under which the cell is observed to suffer severe voltage decline. A simple onedimensional water balance model is proposed to contribute to the understanding of the various operation regimes observed in PEMFCs under medium-to-low humidification conditions.

Water management represents one of the main challenges for the design and operation of polymer electrolyte membrane fuel cells (PEMFC), the amount of water in the cell affects the performances and degradation processes. Commonly, the main goal is try to reduce the humidification in order to simplifying the humidification system and enhancing the performance stability of the cell. However, insufficient water content can lead to increased degradation and performance loss. These negative effects depend strongly on water distribution [1] in the cell and can therefore be highly non-uniform [2]. The relation between low humidification and degradation is complex and not yet fully understood. To bring more insight into this, the differences in the local conditions of the cell must be taken into account when examining performance and degradation. Thus, in this work, particular attention is paid to the local analysis of performance and degradation of cells operated under low or high humidity conditions at cathode and/or anode. In order to study locally the degradation influence of inlet gas humidification on cell performance all the experiments were performaned with the DLR- PCB segmented cell current density measurements, other global effect were analyzed using in-situ diagnostic tools, such as cyclic voltammetry. The changes observed locally in the current densities distributions were investigated locally by post-mortem ex-situ investigations by XPS and SEM/EDX to determine changes in the chemical composition of the different layers during the experiment.  [1] D. G. Sanchez and P. L. Garcia-Ybarra, Int. J. Hydrogen Energy, 37, 7279 (2012). [2] D. G. Sanchez, D. G. Diaz, R. Hiesgen, I. Wehl and K. A. Friedrich, J. Electroanal. Chem., 649, 219 (2010).