PEMFC Catalyst Layers: The Role of Micropores and Mesopores on Water Sorption and Fuel Cell Activity

484

394

Polymer-electrolyte fuel cells are a promising energy-conversion technology. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, especially in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several critical areas for future research. Fuel cells may become the energy-delivery devices of the 21 st century. Although there are many types of fuel cells, polymer-electrolyte fuel cells (PEFCs) are receiving the most attention for automotive and small stationary applications. In a PEFC, fuel and oxygen are combined electrochemically. If hydrogen is used as the fuel, it oxidizes at the anode releasing proton and electrons according toThe generated protons are transported across the membrane and the electrons across the external circuit. At the cathode catalyst layer, protons and electrons recombine with oxygen to generate waterAlthough the above electrode reactions are written in single step, multiple elementary reaction pathways are possible at each electrode. During the operation of a PEFC, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. * Electrochemical Society Active Member. z E-mail: azweber@lbl.govOver the last several decades significant progress has been made in increasing PEFC performance and durability. Such progress has been enabled by experiments and computation at multiple scales, with the bulk of the focus being on optimizing and discovering new materials for the membrane-electrode-assembly (MEA), composed of the proton-exchange membrane (PEM), catalyst layers, and diffusionmedia (DM) backing layers. In particular, continuum modeling has been invaluable in providing understanding and insight into processes and phenomena that cannot be resolved or uncoupled through experiments. While modeling of the transport and related phenomena has progressed greatly, there are still some critical areas that need attention. These areas include modeling the catalyst layer and multiphase phenomena in the PEFC porous media.While there have been various reviews over the years of PEFC modeling 1-7 and issues, [8][9][10][11][12][13][14] as well as numerous books and book chapters, there is a need to examine critically the field in terms of what has been done and what needs to be done. This review serves that purpose with a focus on transport modeling of PEFCs. This is not meant to be an exhaustive review...

Section: Catalyst-layer Modelingmentioning

confidence: 99%

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Weber

Borup

Darling³

et al. 2014

484

394

“…• by considering the contact angle of Ketjen Black carbons reported by Soboleva et al 45 All electrochemical parameters for CLs are obtained from the experiments discussed in the MEA fabrication and testing section in a follow up article. 46 The thermal related parameters, i.e., GDLs, CLs and PEM thermal conductivity as well as all PEM transport properties used in the model are from the previous OpenFCST non-isothermal MEA model validation study reported by Bhaiya et al 20 The double trap and dual path kinetic parameters used in the model are given in Moore et al 22 and Wang et al, 47 respectively.…”

Section: F532mentioning

confidence: 99%

A Mixed Wettability Pore Size Distribution Based Mathematical Model for Analyzing Two-Phase Flow in Porous Electrodes

Zhou

Pütz

Secanell

2017

A multi-dimensional, non-isothermal, two-phase membrane electrode assembly (MEA) numerical model is developed where the micro-structure of the porous layers is characterized by a mixed wettability pore size distribution (PSD). The PSD model is used to predict local water saturation based on gas and liquid pressures, and can be used to study the effect of varying pore size and wettability. The MEA model accounts for gas transport via molecular and Knudsen diffusion, liquid water transport, sorbed water transport by back-diffusion, electro-and thermo-osmosis, and heat generation and transport. Multi-step kinetic models are used to predict anode and cathode electrochemical reactions. Local transport losses are accounted for using a local transport resistance. The PSD model is used to predict capillary pressure vs. saturation and saturation vs. relative liquid permeability curves based on PSDs from several GDLs obtained using mercury intrusion porosimetry. The PSD-based MEA model electrochemical performance predictions are also compared to experimental data from the literature. Results show that the numerical model is able to capture the performance changes associated with varying temperate and the introduction of a micro-porous layer. Improving the performance of polymer electrolyte fuel cells (PEFCs) at high current density is critical for reducing stack size, cost and weight in automobile applications. Water accumulation in the electrode however, limits fuel cell performance at high current densities. [1][2][3] In order to mitigate the excessive water buildup in the cell, which hinders mass transport, gas diffusion layer (GDL) and catalyst layer (CL) microstructures and wettabilities must be advanced to achieve sufficient water retention in the electrolyte, reject excess water in vapor form and alleviate complete flooding of the electrode. For example, by modifying the weight percentage (wt%) of the hydrophobic content in the GDL, Wang et al. 4 reported a 10%-20% increase in performance. To understand fuel cell behavior in the two-phase region and optimize the design of GDL and CL, a two-phase model which includes microstructural information of the porous layers is needed.Two approaches have commonly been used to study liquid water transport in fuel cells. The first approach involves the solution of a saturation equation obtained by replacing liquid pressure by saturation in Darcy's law. [5][6][7] In this approach the Levertt J-function is commonly used to relate saturation with capillary pressure. The Leverett J-function is however, an empirically measured curve for a given material and it is obtained from porous media structure analysis. Thus, it is difficult to optimize the structure of the porous layers using Leverett J-functions. The second approach, known as the multi-phase approach, involves solving mass and momentum equations for gas and liquid phases independently and then relating the capillary pressure to saturation using a closure equation based on micro-structural information such as a pore size dist...

“…To achieve this, we adapted the ultrasonic spray-coating technique to AEMFCs in order to probe the effect of high-boiling solvents in catalyst layer formation, characterizing the resultant CCMs in situ, in AEMFCs, and ex situ using porosimetry and scanning electron microscopy, contextualizing these results with relevant electrochemical data, as in previous PEMFC studies by our group. 34,35 No true benchmark for AEMFC function has yet been established, but FAA-3 represents one of the few high-performance anion-exchange ionomers and unreinforced membranes commercially available, and is the subject to the most extensive parametric studies to date. 36 Consequently, we chose these materials to undertake studies of structure-property relationships of catalyst layers.…”

Section: F354mentioning

confidence: 99%

The Control and Effect of Pore Size Distribution in AEMFC Catalyst Layers

Britton

Holdcroft

2016

Self Cite

Catalyst ink for anion-exchange catalyst coated membranes based on FuMA-Tech FAA-3 membranes and ionomer typically requires high-boiling solvents. Here, we investigate the disproportionate effect of even small quantities of high-boiling solvent in the catalyst ink on the catalyst layer microstructure. High porosity in the mesoporous regime, 20-100 nm, is found to be an essential characteristic of effective anion-exchange catalyst layers for increasing membrane hydroxide ion conductivity and reducing mass-transport losses. High porosity in the nanoporous regime (<20 nm pore diameter) facilitates improvements in the kinetic region of polarization curves at the expense of mass-transport losses. New strategies are introduced to improve the control of distribution of pore sizes in the catalyst layer and to increase the mesoporosity. Beginning-of-life power densities for O 2 /H 2 anion exchange membrane fuel cells (AEMFCs), under zero backpressure, were accordingly increased from 276 to 428 mW · cm −2 , placing it among the highest AEMFC power densities reported in the literature under the conditions studied, representing a significant improvement over previously reported performances for FAA-3. The study highlights a need to develop anion-exchange solid polymer ionomers soluble in low-boiling solvents for preparing catalyst inks, and a more rigorous evaluation of porosimetry data and catalyst layer preparation methods for O 2 /H 2 polymer electrolyte fuel cells rely on the electrochemical oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) (Fig. S1). In proton-exchange membrane fuel cells (PEMFC), the reactions occur a highly acidic environment and are facile on Pt electrocatalayst.1 An alkaline environment opens the possibility of using stable, high activity, non-noble catalysts, 1-3 but analogous hydroxide-exchange membrane fuel cells (AEMFC) are less welldeveloped and the hydroxide ion diffuses more slowly than protons. 4 Nonetheless, comparable membrane conductivities have been shown. 5 Moreover, compared to liquid-based alkaline fuel cells, AEMFCs may confer a higher power density, may have the capacity to function with impurities in the fuel, and have the potential to operate in CO 2 -containing air, 2 especially under high current load. 6 Additionally, as most AEMFCs are hydrocarbon in nature, they may display a lower fuel crossover compared to perfluorosulfonate-based ionomers.A barrier to the development of AEMFC technology concerns the hydroxide-conducting polymer; namely, its poor chemical stability and low ion conductivity under typical fuel cell conditions. The same issue applies even more to the ionomer in the catalyst layer, which experiences rapid variations in hydration-state. In a typical anionexchange ionomer, both the polymer backbone and functional groups are subject to hydroxide attack, e.g., via β-Hoffman elimination or direct nucleophilic displacement of the functional groups.7,8 Cleavage of the polymer backbone primarily affects the mechanical properties of the membrane,...