Analysis of representative elementary volume and through-plane regional characteristics of carbon-fiber papers: diffusivity, permeability and electrical/thermal conductivity

Understanding the transport processes that occur in carbon-fiber papers (CFPs) used in fuel cells, electrolyzers, and metal-air/redox flow batteries is necessary to help predict cell performance and durability, optimize materials and diagnose problems. The most common technique used to model these thin, heterogeneous, anisotropic porous media is the volume-averaged approximation based on the existence of a representative elementary volume (REV). However, the applicability of the continuum hypothesis to these materials has been questioned many times, and the error incurred in the predictions is yet to be quantified. In this work, the existence of a REV in CFPs is assessed in terms of dry effective transport properties: mass diffusivity, permeability and electrical/thermal conductivity. Multiple sub-samples with different widths and thicknesses are examined by combining the lattice Boltzmann method with X-ray tomography images of four uncompressed CFPs. The results show that a meaningful length scale can be defined in the material plane in the order of 1–2 mm, which is comparable to the rib/channel width used in the aforementioned devices. As for the through-plane direction, no distinctive length scale smaller than the thickness can be identified due to the lack of a well-defined separation between pore and volume-averaged scales in these inherently thin heterogeneous materials. The results also show that the highly porous surface region (amounting up to 20% of the thickness) significantly reduces the through-plane electrical/thermal conductivity. Overall, good agreement is found with previous experimental data of virtually uncompressed CFPs when approximately the full thickness is considered.The authors thank the support team of Calcul Quebec and Compute Canada for their help during the simulation campaign, as well as Dr. Dula Parkinson and Dr. Alastair MacDowell at the Advanced Light Source (ALS) for help in obtaining the tomographic images. This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-AC02-05CH11231, Project ENE2015-68703-C2-1-R (MINECO/FEDER, UE) and the research grant 'Ayudas a la Investigation en Energia y Medio Ambiente' awarded to the first author by the Spanish lberdrola Foundation. I.V. Zenyuk and A.D. Shum would like to acknowledge support from the National Science Foundation under CBET Award 1605159. X-ray tomography experiments were performed on beamline 8.3.2 at the ALS (Lawrence Berkeley National Laboratory), which is a national user facility funded by the Department of Energy, Office of Basic Energy Sciences under contract DE-ACO2-05CH11231. Numerical calculations were performed on the supercomputing clusters Briaree, Colosse, Guillimin and Mp2, managed by Calcul Quebec and Compute Canada. The operation of these supercomputers is funded by the Canada Foundation for Innovation (CFI), Ministere de l'Economie, de l'Innovation et des Exportations du Quebec (MEIE), RMGA and the Fonds de recherche du Quebec -Nature et technologies (FRQ-NT)

Implications of inherent inhomogeneities in thin carbon fiber-based gas diffusion layers: A comparative modeling study

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.electacta.2018.09.089. © 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Thin porous media are present in multiple electrochemical energy devices, where they provide key transport and structural functions. The prototypical example is gas diffusion layers (GDLs) in polymer-electrolyte fuel cells (PEFCs). While modeling has traditionally been used to explore PEFC operation, this is often accomplished using volume-averaged (VA) formulations, where the intrinsic inhomogeneities of the GDL are smoothed out and the lack of defining a representative elementary volume is an ever-present issue. In this work, the predictions of a single-phase VA PEFC model are compared to those of a pore-scale PEFC model using GDL tomograms as a part of the meshed domain to delineate important aspects that VA models cannot address. The results demonstrate that while VA models equipped with suitable effective properties can provide a good average estimate for overall performance, the lack of accounting for real structures limits their predictive power, especially for durability and degradation behavior where large deviations are found in the spatial distributions. Furthermore, interfacial effects between the GDL and the microporous layer are explored with the pore-scale model to understand the implications of the layered geometry. It is shown that the actual microstructure of the GDL/MPL transition region can significantly affect the fluxes across the sandwich, something that VA models cannot easily consider. Interfacial design is recognized as a key quality control parameter for large-scale MEA manufacturing and assembly.Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy [contract DE-AC02-05CH11231]MINECO/FEDER [Project ENE2015-68703-C2-1-R]Spanish Iberdrola Foundation [grant ‘Ayudas a la Investigación en Energía y Medio Ambiente’]National Science Foundation [CBET Award 1605159

Mechanisms of electrocatalysis of oxygen reduction by metal porphyrins in trifluorom ethane sulfonic acid solution

This study examines the oxygen reduction reaction (ORR) in a homogeneous catalyst system, comparing between the outer-sphere and inner-sphere electron-transfer mechanisms. The rate constants are measured using aqueous trifluoromethane sulfonic acid (TFMSA) and water-soluble M*meso-tetra(pyridyl)porphine chloride complexes [M*TMPyP, M* = Fe(III), Co(III), Mn(III) and Cu(II)] at given pH and molar ratio of metal complexes to oxygen. An outer-sphere model consistent with Marcus theory explains that an outer-sphere electron transfer mechanism occurs in the activation-control region. However, higher rate constants than predicted suggests that a possible reaction pathway is a quasi-redox mechanism associated with the formation of an intermediate bond between M’TMPyP [M’ = Fe(II), Co(II), Mn(II) and Cu(I)]with O2followed by proton-activated decomposition. An increase in the catalyst turnover frequency was also observed upon addition of imidazole base, indicating the role of protonation is crucial to the ORR mechanism. The results are encouraging for replacement of platinum with non-noble metal-polymer complex systems for oxygen reduction in that the reorganization barrier for reaction pathway significantly decreases. The positive effect of proton activation on the catalytic activity of the homogeneous redox catalysts is of considerable interest for future studies.In a three-dimensional, molecular catalysis model, the predicted results using the measured reaction rate suggest that the non-noble metal catalysts can be used for practical electrochemical cell designs

Probing water distribution in compressed fuel-cell gas-diffusion layers using X-ray computed tomography

X-ray computed tomography was used to investigate geometrical land and channel effects on spatial liquid-water distribution in gas-diffusion layers (GDLs) of polymer-electrolyte fuel cells under different levels of compression. At low compression, a uniform liquid-water front was observed due to water redistribution and uniform porosity; however, at high compression, the water predominantly advanced at locations under the channel for higher liquid pressures. At low compression, no apparent correlation between the spatial liquid water and porosity distributions was observed, whereas at high compression, a strong correlation was shown, indicating a potential for smart GDL architecture design with modulated porosity. Keywords: X-ray computed tomography, Gas-diffusion layers, Water saturation, Land-channel effects, Compression, Polymer-electrolyte fuel cell