(Invited) Modeling and Characterization of Porous Media with Application to Electrochemical Energy Conversion and Storage Devices

Abstract

Porous media are an integral part of electrochemical energy conversion and storage devices. Among them, we have gas diffusion layers (GDLs) and catalyst layers (CLs) used in polymer electrolyte membrane fuel cells (PEMFCs), as well as fibrous electrodes used in redox flow batteries (RFBs). These porous media must fulfill several critical functions, such as providing a transport pathway for reactants/products through its pore volume and ensuring charge and heat conduction through its solid structure. CLs and active electrodes have the added functionality of providing a reactive surface area. In addition, percolation transport in ion exchange membranes (IEMs) plays a critical role in PEMFCs, RFBs, and related electrochemical devices. On the way to broad commercialization, a thorough understanding of the mass, charge and heat transport properties of these components is crucial to achieving improved performance and durability.Mathematical modeling is one of the key tools used to design new components and optimize the operation of the afore-mentioned devices. However, there are several approaches to achieve this modeling. The most common technique is to use the macroscopic continuum hypothesis, which treats porous media as homogeneous domains with effective transport properties [1,2]. Examples of effective properties are the permeability used in Darcy’s law, the tortuosity factor used to correct Fick’s law of diffusion, or the effective electrical and thermal conductivities used in Ohm’s and Fourier’s laws. Nevertheless, the scientific community has long questioned the applicability of the macro-homogeneous approach to model the thin (thickness~10-500 μm) and heterogeneous porous media used in these technologies. The vast body of work presented in the literature has shown that macro-homogeneous models can be conveniently configured to describe global behaviors and trends. However, the need of guiding the development of improved components, predicting localized effects such as degradation, and diagnosing operation and performance problems demands the development of more advanced multiphysics, multiphase and multiscale models. For this reason, it has become increasingly common to model transport in porous media at the pore scale, either using pore-network modeling (PNM) or direct numerical simulation (DNS) [3-8]. The incorporation of key information extracted from pore-scale models into macroscopic models is necessary to understand electrochemical systems at multiple scales, while keeping computational cost moderate for engineering applications.In this talk, research activities performed in terms of modeling and characterization of porous media in PEMFCs, RFBs and Li-ion batteries will be presented. An overview of pore-scale and macroscopic modeling techniques will be provided, along with the development of composite models aiming to combine the ease of implementation of macroscopic continuum modeling and the computational power of PNM [9-10]. Several topics will be addressed, including two-phase transport in GDLs, mass-transport in RFBs, proton transport in copolymer membranes, and the effect of interfaces and microstructural heterogeneities in GDLs and Li-ion electrodes [1-11].[1] P.A. García-Salaberri et al., J. Power Sources 359 (2017) 634–655.[2] P.A. García-Salaberri et al., Processes 8 (2020) 775.[3] P.A. García-Salaberri et al., Int. J. Heat Mass Trans. 86 (2015) 319–333.[4] P.A. García-Salaberri et al., J. Power Sources 296 (2015) 440–453.[5] P.A. García-Salaberri et al., Electrochim. Acta 295 (2019) 861–874.[6] P.A. García-Salaberri et al., Int. J. Heat Mass Transf. 127 (2018) 687–703.[7] Z.A. Khan, P.A. García-Salaberri, T. Heenan, R. Jervis, P. Shearing, D. Brett, A. Elkamel, J.T. Gostick, J. Electrochem. Soc. 167 (2020) 040528.[8] M. Muñoz-Lorente, J.M. Rubio-Hammo, A. Forner-Cuenca, F.R. Brushett, P.A. García-Salaberri, Analysis of species mass transport in fibrous electrodes of redox flow batteries, Proc. InterPore 2019.[9] P.A. García-Salaberri et al., ECS Trans. 97 (7) 615.[10] P.A. García-Salaberri, Int. J. Heat Mass Trans. (2020), accepted.[11] N. Ureña, M.T. Pérez-Prior, P.A. García-Salaberri, Polymers (2020), submitted.