Voltage cycling is one of the most damaging stressors for automotive PEMFC. Understanding of the effects of voltage cycling on performance degradation is crucial to improve PEMFC durability for automotive applications. This study focuses on the interaction between upper potential limit (UPL) and lower potential limit (LPL) on the stability of PEMFC. A well-defined peak of degradation rate is observed when the LPL is ∼0.8 V with UPL of 1.35 V. A mathematical model was developed to understand the observed relationship between degradation rate and lower potential. Modeling results suggest that when cycling to a lower potential of ∼0.8 V, almost all dissolved Pt migrate from the catalyst layer to the membrane with negligible re-deposition, resulting in a peak of degradation rate at ∼0.8 V. The amount of Pt in the membrane (PITM) measured at end of life (EOL) samples correlates with degradation rates and is in agreement with modeling results.An automotive fuel cell needs to withstand thousands of operational hours. During normal operation of the vehicle, while load cycles occur, there will be fast dynamic change of cathode potential in the range of ∼0.6-0.95 V. During startup and shutdown, the cathode potential can be higher than 1.35 V. 1 It is well known that Pt is electrochemically unstable during voltage transitions, especially when the potential goes above 1.0 V. 2,3 The reaction of Pt to form ions, Eq. 1, is a typical failure mode during voltage cycling and it has been extensively studied by many groups. 4-10 Some researchers have shown that Pt will go into ion form during an anodic potential sweep. 11-13 It is also known that Pt can be oxidized to PtO or PtO 2 at the potentials above 1.15 V. [11][12][13] The reactions are shown as below Eq. 2 and 3.During the cathodic sweeps, Pt oxide can be electrochemically dissolved by Eq. 4 or chemically dissolved by Eq. 5. 11,14,15 Repeated anodic and cathodic voltage sweeps will accelerate Pt dissolution. As a result, permanent Pt loss could occur and ultimately lead to the fuel cell stack decay. The dissolution of Pt is further exacerbated with higher upper potentials. Imai et al. carried out in-situ measurements of Pt oxide at different potentials in aqueous media. 16 They observed place exchange phenomenon when the potential was higher than 1.4 V, suggesting Pt could suffer more severe dissolution at such upper potentials. Therefore, it is of great interest to study the impact of parameters in voltage cycling on Pt durability.The Pt durability can be dependent on many factors, including voltage window (upper potential, lower potential), operation conditions (relative humidity (RH), temperature, gas pressure, etc). The impact of RH and temperature has been investigated and discussed by some publications from Bi et al. 17,18 The importance of upper potential on platinum dissolution during voltage cycles has also been studied by many groups. Wang et al. conducted experiments to measure the equilibrium concentration of dissolved Pt at different upper potentials in acidic s...

Fuel cell startup from freezing temperatures is a requirement for automotive applications as many countries experience cold climate. It has been shown at Ballard Power Systems that an optimum MEA water content is necessary at startup in order to achieve fast freeze startup with no performance loss. In this paper we present stack models and designs for obtaining an appropriate MEA water content at startup by maximizing water migration from MEA to plate channels due to temperature gradients during natural cooling. Using this concept no external power is needed after fuel cell shut down from wet operation to achieve the intended MEA water content at startup. Experimental results for the MEA water content along the stack after shut down and natural cooling and for the stack performance during startup from freezing temperatures are presented for the new stack design.

A model is presented of a polymer electrolyte fuel cell including slow transient effects of liquid water accumulation and evaporation in gas diffusion electrodes ͑GDEs͒ and gas channels. The model is reduced dimensionally, coupling a one-dimensional ͑1D͒ model of gas and coolant channel flow to 1D models of transport through the membrane electrode assembly ͑MEA͒ and bipolar plates. An asymptotic reduction of the two-phase flow to a sharp interface model is used, in which phase change occurs at a front that evolves in time. The asymptotic reduction is based on an immobile water fraction in the GDE and a large capillary pressure. The water content in the membrane and channels is also tracked in time. Gas and thermal transport are taken to be at quasi-steady state on the time scale of liquid accumulation. The model is fit to Ballard Mk9 cells and validated against experimental measurements of both steady-state and transient MEA water content distributions along the length of the channel. Predictions of slow cyclovoltammograms are presented based on the model. Polymer electrolyte membrane ͑PEM͒ fuel cells are complex electrochemical devices that display transient behavior and hysteresis on a wide range of time scales. We present a model that captures the slow transient behavior associated to the accumulation of liquid water within the membrane electrode assembly ͑MEA͒ and gas flow fields of PEM fuel cells. Liquid water is widely known to play a critical role in overall cell performance, influencing the protonic conductivity of the Nafion membrane, the transportation of oxygen to reaction sites within the catalyst layers, 1 and the transportation of gases within the flow fields. Moreover, the time scale for liquid water buildup, on the order of tens of minutes, has an important impact on the dynamic loading of fuel cells typical for driving cycles. 2,3 Indeed, any predictive model designed for the optimization of automotive and other nonstationary applications of PEM fuel cells must resolve the transients and hysteresis on these time scales.There is substantial literature on computational fluid dynamicsbased models for PEM fuel cells that include two-phase effects ͑see for example, Ref. 4-9, and references therein͒. These models resolve various subcomponents of PEM fuel cells, or even the full cell. More recent work has included transient, two-phase effects; however, they either consider analysis of a single component 10,11 or present a full three-dimensional simulation without rigorous reduction, 12 for which the computational requirements are unsuitable for optimization schemes requiring extensive parameter testing or for upscaling to stack-level codes. Models that use analytical reductions of the subscale processes within the fuel cell, particularly the 1 + one-dimensional ͑1D͒ models, which exploit the nearly 1000:1 aspect ratio of the MEA thickness vs the along-the-channel distance of PEM unit cells, are well suited for these tasks. Several authors have proposed 1 + 1D models, 13-17 which couple 1D through-plane ...

This paper describes a novel method to determine the membrane electrode assembly ͑MEA͒ resistance and electrode diffusion ͑MRED͒ coefficient for a fuel cell ͑the MRED method͒ under in situ conditions. It is shown that the MRED method allows the determination of ͑i͒ the ohmic resistance of an MEA and ͑ii͒ the mass-transport coefficient of the electrodes. The method is based on the galvanostatic discharge of a fuel cell with interrupted reactant supply. Application of the method to the cathode of a polymer electrolyte membrane fuel cell is demonstrated, and the experimental results are analyzed using a theoretical model based on a simple one-dimensional diffusion process using Fick's law. Comparison of the experimental O 2 mass-transport coefficient with theoretical values indicates that diffusion in the active layer is mass-transport limiting.To meet the power density, reliability, and cost requirements that enable a widespread use of fuel cells, cost reduction and increased power output remain key challenges. To reduce development time and cost, a rapid and convenient means for the optimization of fuel cell designs is needed. This requires foremost a sufficient understanding of the structure-property-performance relationships for the fuel cell and its components. In particular, the dependence of cell voltage and cell voltage degradation on the physicochemical and structural properties of the components of the fuel cell must be better understood. Due to the complexity of the heat-and masstransport processes occurring in fuel cells, there is typically a multitude of parameters to be determined. Therefore, the challenge is the identification of those critical parameters that which have the highest impact on fuel cell performance. This can be done through either theoretical or empirical models, which can then be used as design tools to optimize fuel cell designs more quickly. For the development of these design tools, accurate measurement tools are needed to provide reliable data to establish empirical correlations, to provide model inputs, or to validate model predictions.In this study, we present a novel method for determination of the cell resistance and the effective diffusion coefficient for a fuel cell cathode, which can be used as a measurement tool to provide valuable experimental data for the investigation of the structureperformance relationships for fuel cell electrodes. The membrane electrode assembly ͑MEA͒ resistance and electrode diffusion ͑MRED͒ method allows the investigation of the effect of operating conditions as well as structural properties of the electrodes on performance and can thus support the development of improved design tools for fuel cells.Generally, the cell voltage losses can be kinetic, ohmic, and mass-transport related; 1 therefore, one of the design challenges is to optimize electrode structures so that these cell voltage losses are minimized. Especially on the oxidant side mass-transport losses associated with transport of reactants and products to and from the active catalyst site...

The functionality of fuel cell vehicles is continuously being improved by several automakers and efforts are being made to demonstrate performance, durability, efficiency and freeze start capability comparable to conventional vehicles. Recent results demonstrating acceptable operational performance, efficiency, freeze start capability and durability are shown and discussed. The main barrier preventing fuel cells from being competitive with conventional ICE is the high cost of the stack and system. For the stack, the key cost driver is catalyst loading. Requirements for future catalysts are presented, demonstrating the challenges and technology gaps following from a reduction of catalyst loadings as well as the use of Pt/Co based catalysts.