Characterizing Local O₂Diffusive Losses in GDLs of PEFCs Using Simplified Flow Field Patterns (“2D”,“1D”,“0D”)

Abstract

By means of advanced diagnostic tools -neutron imaging, pulsed gas method (helox or O 2 pulses during air operation) and electrochemical impedance spectroscopy (EIS) -three different flow fields are characterized in a differential (small-size cell operated at high stoichiometry) Polymer Electrolyte Fuel Cell (PEFC) to emulate the local behavior of a full-size cell. The channelrib structures are designed so as to obtain 2D, 1D and 0D gradients of O 2 concentration across the gas diffusion layer (GDL). The important bulk-diffusion losses (evaluated with pulsed helox) of the "2D" cell compared to the "1D" cell indicate the effect of lateral oxygen diffusion in the GDL. Non-bulk diffusion losses (Knudsen and/or thin film diffusion, evaluated with pulsed O 2 ) also play a non-negligible role on the overall diffusive losses. In particular, their increase in dry conditions for the "1D" and the "0D" cells might be a consequence of drying effects in the electrode. EIS spectra can be interpreted consistently with the results of the pulsed gas method: the increase of arc diameter can be correlated with the overall mass transport losses including both bulk and non-bulk diffusion losses.In order to increase the market attractiveness, the cost of the Polymer Electrolyte Fuel Cell (PEFC) must be reduced. This can be achieved by lowering the cost of the components and by enhancing the performance. The so-called flow field plate (also referred to as bipolar plate) is an essential component of the fuel cell used for the reactant distribution, the water removal, and the electrical and thermal collection. The cost of the flow field must be minimized (material, manufacturing and coating) and the influence on performance must be improved. In this context, the flow field local structure (meaning here the shape and the size of the channels and ribs) is a crucial set of parameters. Under constraint of low manufacturing costs, it must ensure a homogeneous reactant distribution and a good water management, in terms of efficient water removal and sufficient membrane hydration. Numerous studies were realized in the last decade to investigate the effect of the flow field pattern on performance (see review papers 1,2 ). The local distribution of parameters over the active area was modeled 1 or measured in segmented cells. 3,4 The distribution and dynamics of water droplets in the channels were visualized for instance by optical or neutron imaging, 5-12 to test different fuel cell designs 7-11 or to search correlations with the local current density on segmented cells. 5,6,12 The majority of these studies were realized on full-size cells supplied by gases at technical stoichiometry (<2). Since local operation parameters are not controlled, the local impact of different flow field patterns on the performance cannot be directly tested. In addition, the local influence of the water content on the performance is difficult to understand in such cells. Indeed, liquid water droplets in the channels can impact the gas flow distribution at the full-size...

Impact of Hydrophobic Coating on Mass Transport Losses in PEFCs

“…A slight decrease with increasing humidification is observed, which can be explained by a better ionic conductivity of the ionomeric phase inside the CL. 41 This is contradictory to some claims in literature 55 where increased losses with high coating loads of the GDL are attributed to flooding the CL. However, we are currently not aware of any publication demonstrating this claim.…”

“…Besides the changes in the pore morphology, thermal conductivity measurements on GDLs with different coating loads were reported to reduce conductivity of SGL GDLs by a factor of two by applying a coating of 20 wt% PTFE compared to the material with 5 wt% PTFE. 40 Pulsed gas analysis (PGA).-In previous publications of our group, 30,37,41 the pulsed gas analysis (PGA) method has shown its usefulness for detailed studies of "bulk" and "non-bulk" mass transport losses. 30 "Bulk losses" originate from diffusive transport limitations in the GDL while "non-bulk" losses (related to Knudsen diffusion and thin film diffusion 42 ) can be primarily related to diffusive transport limitations in the CL.…”

“…The cells were imaged with the membrane parallel to the beam axis (in-plane) with a tilted imaging setup. 33,41,44,45 A pixel size of 5 × 66 μm/pixel with an effective resolution (FWHM) of approx. 30 μm across and 198 μm along the membrane was obtained.…”

Operando Properties of Gas Diffusion Layers: Saturation and Liquid Permeability

“…High gas saturation leads to the accumulation of liquid water in the porous gas diffusion layers, especially under the flow field ribs, which results in performance drops due to increased gas transport resistance in particular at the cathode. [7][8][9][10] This effect occurs also at under-saturated conditions when water cannot be transported at sufficient rates anymore in the vapor phase through the GDL and can happen even below * Electrochemical Society Member.…”

Influence of Hydrophobic Coating of Gas Diffusion Layers on the Performance and Water Transport inside PEFC

“…We performed in-plane measurements with a tilted imaging setup (18,22,29,30). A pixel size of 5 x 66 µm/pixel with an effective resolution (FWHM) of approx.…”