Desaturation of polymer electrolyte fuel cells (PEFCs) is a critical operation step for providing cell cold-start performance by minimizing residual water in the gas diffusion layers (GDLs), flow field (FF) channels, catalyst layers and membrane after cell shutdown. In this work, transient liquid water removal processes in the FF channels and GDLs are visualized and quantified by subsecond in situ X-ray tomographic microscopy (XTM), and correlated to high frequency resistance (HFR) measurements of the cell. Time-resolved desaturation profiles are analyzed for three commercially available GDLs with representative substrate dimensions. The influence of different substrates on the GDL desaturation behavior is investigated with a cluster connectivity analysis and saturation-dependent effective diffusivities are determined by numerical simulations. Characteristic drying phases are identified for the HFR curves and confirmed with XTM imaging results, providing fundamental understanding of the desaturation dynamics in the PEFCs and enabling the optimization of GDL substrates and gas purge protocols accordingly.
X-ray tomographic microscopy (XTM) of liquid water in polymer electrolyte fuel cells (PEFC) has proven a valuable tool in order to improve understanding of water transport in the gas diffusion layer (GDL) [1-5]. So far, in-operando X-ray tomographic microscopy was restricted to constant operation conditions since the minimal XTM scan time of a few seconds [4] was not sufficient to capture the dynamics of the water distribution under transient operation. The presentation reports about the developments to reduce the scan time of in-operando XTM below a second at the TOMCAT beamline of the Swiss Light Source. First of all, this offers the possibility to study the evolution of the liquid water distribution in the GDL during transient PEFC operation conditions. Second, the shorter scan times reduce the X-ray dose to the cell and increase the number of XTM scans before radiation induced degradation biases the cell electrochemistry and consequently the water distribution [6]. The consequences of the reduced exposure time and lower number of angular steps on image quality are discussed and the temporal and spatial development of the water distribution in the channel and the GDL following a current density variation from 0.1 A/cm 2 to 1.0 A/cm 2 (see Figure 1) will be presented. Special focus will be put on channel vs. land variations, percolation paths of the liquid water and their stability over time. References [1] A. Schneider, et al., J. Power Sources, vol. 195 (2010), pp. 6349-6355. [2] P. Krüger, et al., J. Power Sources, vol. 196 (2011), pp. 5250-5255. [3] J. Eller, et al., J. Electrochem. Soc., vol. 158 (2011), pp. B963-B970 [4] J. Eller, et al., ECS Trans., vol. 41 (2011), pp. 387-394. [5] T. Rosén, et al., J. Electrochem. Soc., vol. 159 (2012), pp. F536-F544. [6] J. Eller, et al., J. Power Sources, vol. 245 (2014), pp. 796-800. Figure 1 : Left: XTM through-plane slice of a single channel cell about 60s after current jump from 0.1 to 1 A/cm 2 . Cathode flow field is on top and identified liquid water is highlighted in blue. Right: 3D rendering of the water in the cathode. The flow field is removed for visualization of the water accumulation under the ribs. Figure 1
In high temperature polymer electrolyte fuel cells, at high current densities, phosphoric acid (PA) migrates toward the anode and invades catalyst, microporous and gas diffusion layers (GDL). This work studies this PA redistribution using synchrotron based operando X-Ray tomographic microscopy (XTM) and electrochemical impedance spectroscopy (EIS) during a current cycling protocol. It is shown that under reformate conditions, during the first 2 minutes after a positive current step, the cell voltage increases due to better wetting of the anode catalyst layer (CL). From 2 to 20 minutes, the cell voltage drops due to increasing mass transport losses in the microporous layer (MPL) and the GDL. At the anode, cracks in MPL and CL, both with widths up to 150 μm, are flooded within 2 minutes after a current density increase. Acid flooding is only observed for MPL cracks that overlap with CL cracks. The CL cracks therefore act as injection points for the flooding of the MPL cracks and the gas diffusion layer. No change in the PA content of any of the cathodic porous components was observed.
transport in gas diffusion layers (GDLs) Polymer electrolyte fuel cells (PEFCs) Channel cross-flows in PEFCs Impact of dry and wet GDLs on cross-flows X-ray tomographic microscopy a b s t r a c t Three-dimensional direct numerical simulations were performed for investigating the flow in a serpentine channel and the under-laying porous gas diffusion layer (GDL) of a micro polymer electrolyte fuel cell (PEFC).The flow field comprised three straight sections and two U-turns.The geometry was acquired with high-resolution (2.9 μm) in situ X-ray tomographic measurements on an operating cell.Simulations considered the GDL under dry and partially saturated conditions, whereby saturation was established via electrochemically produced water.A lattice Boltzmann (LB) methodology was adopted for simulating the single-phase (gas) transport in the actual 3D channel and porous GDL geometry.The global pressure drop in the dry GDL was dominated by the turns in the gas channel, while the pressure drops were quite small along the straight channel sections.In the wet GDL case, however, the pressure drop was mainly dictated by the neck-shaped passages created by the large water clusters inside the channel.Owing to the water blockage, the local accumulated cross-flows along the serpentine channel length, when normalized by the inlet channel flow, were substantially higher in the wet GDL, reaching local values up to 45% compared to 18% for the dry GDL.The implications are that in an electrochemically operating cell, the GDL under the rib would receive more gas (and thus O 2 ).The creation of cross-flows through the porous GDL would enhance cell performance under the ribs since diffusion will not be the main driving mechanism for oxygen transport and water evaporation.The analysis indicated that the flow field, although designed as serpentine, behaved like half-interdigitated (with a rib of 1.5 mm, half-serpentine flow field, depending on the state of channel flooding).
The evolution of the liquid water distribution in the gas diffusion layers (GDL) of polymer electrolyte fuel cells (PEFC) during transient operation conditions was visualized by operando sub-second X-ray tomographic microscopy (XTM) at the TOMCAT beamline of the Swiss Light Source. The consequences of the reduced exposure time and lower number of angular steps on image quality are discussed and the temporal and spatial development of the water distribution in the channel and the GDL following a current density variation from 0.1 A/cm² to 1.0 A/cm² are presented. Special focus is put on channel vs. land variations, formation of liquid water percolation paths, their stability over time as well as the influence of the water saturation on diffusive gas transport.
Non-optimal oxygen transport in polymer electrolyte water electrolysis is expected to cause severe efficiency losses at high current density. In this study, we shed the first light on the complex fluid transport in PTL materials using operando X-ray tomographic microscopy.
Starting from subfreezing temperatures presents a challenge for PEFC since ice may form in the gas flow channel, gas diffusion layer and catalyst layer and restrict the gas transport. Under non-isothermal conditions, a start-up is successful if the cell temperature rises above 0°C before freezing occurs. Under isothermal conditions below 0°C cells operate for a certain time followed by a sudden drop in power. The distribution of liquid/solid water during an isothermal start at subfreezing temperatures is quantitatively mapped by means of X-ray tomographic microscopy to improve understanding the circumstances at the phase transition and its effects on gas diffusion. Evidence was found that the produced water is in supercooled state at -10 °C initially and the drop of power is associated with freezing of the water in the cell at a GDL saturation of 20 to 30%.
Abstract Channel‐to‐channel cross convection in serpentine flow fields of polymer electrolyte fuel cells (PEFC) can influence the overall cell performance. The effect strongly depends on the gas transport properties of the gas diffusion layer (GDL). For the first time measured anisotropic, compression dependent permeability and effective diffusivity of GDLs are used to quantify the influence of cross convection on the local current distribution and performance. A model was developed to examine different channel‐rib geometries and GDL characteristics. The results show that cross convection can significantly increase the current density and consequently the power density of PEFCs. A strong sensitivity to GDL compression, flow velocity and rib width was found. As an optimised case the GDL thickness under the rib was increased resulting in about 20% higher current densities. Precise knowledge of the GDL characteristics and its compression are key to understand channel‐to‐channel cross convection and optimise perfomance.
A series of differently cross‐linked FEP‐g‐polystyrene proton exchange membranes has been synthesized by the preirradiation grafting method [FEP: poly(tetrafluoroethylene‐co‐hexafluoropropylene)]. Divinylbenzene (DVB) and/or triallyl cyanurate (TAC) were used as cross‐linkers in the membranes. It was found that the physical properties of the membranes, such as water‐uptake and specific resistance, are strongly influenced by the nature of the cross‐linker. Generally it can be stated that DVB decreases water‐uptake and increases specific resistance; on the other hand TAC increases swelling and decreases specific resistance to values as low as 5.0 Ω cm at 60°C. The membranes were tested in fuel cells for stability and performance. It was found that thick (170 μm) DVB cross‐linked membranes showed stable operation for 1400 h at temperatures up to 80°C. The highest power density in the fuel cell was found for the DVB and TAC double‐cross‐linked membrane; it exceeded the value of a cell with a Nafion® 117 membrane by more than 60%.
