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...

No abstract

The aggregate size in fuel cell catalyst inks depends on the type of dispersion medium, particle concentration, and addition of stabilizing agents. In this work, ink stability and particle size of carbon black and carbon black/Nafion dispersions in four nonaqueous media, viz., methanol, ethanol, isopropanol and ethyl acetate are studied. Based on visual inspection, isopropanol is found to be the best medium for dispersion of carbon black inks. To rationalize this observation, a semi-empirical model based on diffusionlimited aggregation was developed to evaluate the rate of particle aggregation and predict the ink stability time for each dispersion medium. The proposed model supports the experimental observation by qualitatively predicting the same relationship between carbon stability and the dispersion media. The model also showed that the dielectric constant of the dispersion medium and the particle zeta potential are primarily responsible for the ink stability. Particle size for the different inks was determined by dynamic light scattering with and without dilution. Experimental results show that Nafion is a strong stabilizing agent, increasing the ink stability and decreasing the particle size of carbon aggregates. The beneficial effects of Nafion are independent of its concentration and are observed even at Nafion volume fractions of 10 wt%. The interaction energy is found to be a strong function of the surface potential for the dispersion medium with a higher dielectric constant. Most catalyst layer (CL) fabrication methods for polymerelectrolyte fuel cells (PEFCs) to date are based on wet deposition of a colloidal dispersion, i.e., the catalyst ink, onto either a membrane or a diffusion medium.1,2 The CL ink is usually a mixture of carbonsupported platinum particles, ionomer and a dispersion medium (DM). Ink stability, defined as the ability of particles to remain dispersed in the DM, and a reduced aggregate size are critical to deposition methods such as inkjet printing 3-6 and spray coating. 1,7,8 Inkjet printing allows for controlled spacial resolution, 6 however in order to create appropriate droplet sizes, the ink has to be ejected from micrometer size nozzles, 5 easily resulting in nozzle clogging. Inkjet printing of microporous layers using carbon ink has also been found to be extremely difficult unless ionomer solution, acting as a stabilizing agent, is added to the ink. 9 Similarly, spray coating requires inks to go through a nozzle, therefore clogging issues are likely to be a challenge when small nozzle sizes are used. Control over the aggregate size and the right selection of a DM to achieve the desired viscosity and surface tension therefore becomes critical in these deposition techniques. Further, the ink stability may also be important to larger commercial fabrication processes to increase the ink storage time.Even though the importance of appropriate ink recipes has been recognized, 1,10 very few studies have aimed at understanding the impact of DM on the ink stability. The type of DM, par...

Image analysis and numerical simulation algorithms are introduced to analyze the micro‐structure, transport, and electrochemical performance of thin, low platinum loading inkjet printed electrodes. A local thresholding algorithm is used to extract the catalyst layer pore morphology from focused ion beam scanning electron microscopy (FIB‐SEM) images. n‐point correlation functions, such as auto‐correlation, chord length, and pore‐size distribution are computed to interpret the micro‐structure variations between different images of the same catalyst layer. Pore size distributions are in agreement with experimental results. The catalyst layer exhibits anisotropy in the through‐plane direction, and artificial anisotropy in the FIB direction due to low slicing resolution. Microscale numerical mass transport simulations show that transport predictions are affected by image resolution and that a minimum domain size of 200 nm is needed to estimate transport properties. A micro‐scale electrochemical model that includes a description of the ionomer film resistance and a multi‐step electrochemical reaction model for the oxygen reduction reaction is also presented. Results show that the interfacial mass transport resistance in the ionomer film has the largest effect on the electrochemical performance.

No abstract

No abstract

No abstract

Membrane electrode assemblies were degraded by voltage cycling in hydrogen/air atmosphere. The impact of degradation on fuel cell performance was measured by various electrochemical characterization techniques. Loss of electrochemically active surface area was correlated to kinetic voltage losses at low current density as well as losses at high current density due to oxygen transport limitations. It was found that the oxygen transport resistance scales proportionally to the inverse of normalized platinum surface area. The change in the catalyst layer structural properties due to voltage cycling was visualized by electron microscopy. A new method of calculating changes in platinum loading of degraded samples by transmission electron microscopy is presented and shows redistribution of platinum in the catalyst layer due to platinum dissolution.