Partha P. Mukherjee

Modeling of Two-Phase Behavior in the Gas Diffusion Medium of PEFCs via Full Morphology Approach

A full morphology (FM) model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell (PEFC). The three-dimensional (3D) fibrous microstructure for the nonwoven gas diffusion layer (GDL) microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.

Impact of GDL structure and wettability on water management in polymer electrolyte fuel cells

A key performance limitation in polymer electrolyte fuel cells (PEFC), called the mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. The cathode gas diffusion layer (GDL) is a primary contributor to mass transport loss owing to the blockage of available pore space by liquid water thereby rendering hindered oxygen transport to the active reaction sites in the electrode. The GDL, typically a fibrous non-woven carbon paper or a woven carbon cloth, thus plays an important role in the water management of a PEFC. This Feature Article provides a systematic description of the development of pore-scale models coupled with realistic microstructural delineation as well as micron-resolution imaging techniques to study the profound influence of the underlying structure and surface wettability on liquid water transport and interfacial dynamics in the fuel cell GDL. A pore-network model and a two-phase lattice Boltzmann model coupled with stochastic generation of GDL microstructures are elaborated. Concurrently, optical diagnostics of water dynamics at GDL interfaces and X-ray micro-tomographic imaging of liquid water distribution inside the GDL of an operating fuel cell are discussed.

Pore-scale modeling of two-phase transport in polymer electrolyte fuel cells—progress and perspective

examined to model the underlying transport mechanisms in the PEFC with different levels of complexities. Due to the macroscopic nature, these two-phase models fail to resolve the underlying structural influence on the transport and performance. Mesoscopic modeling at the pore-scale offers great promise in elucidating the underlying structure-transport-performance interlinks in the PEFC porous components. In this article, a systematic review of the recent progress and prospects of pore-scale modeling in the context of two-phase transport in the PEFC is presented. Specifically, the efficacy of lattice Boltzmann (LB), pore morphology (PM) and pore network (PN) models coupled with realistic delineation of microstructures in fostering enhanced insight into the underlying liquid water transport in the PEFC GDL and CL is highlighted.

Stochastic Microstructure Reconstruction and Direct Numerical Simulation of the PEFC Catalyst Layer

A direct numerical simulation DNS model of species and charge transport in the cathode catalyst layer of a polymer electrolyte fuel cell has been developed. The 3D porous microstructure of the catalyst layer has been reconstructed based on a stochastic technique using the low-order statistical information porosity, two-point correlation function as obtained from 2D transmission electron microscopy TEM micrographs of a real catalyst layer. In this microscopically complex structure, the DNS model solves point-wise accurate conservation equations, thereby obtaining a pore-scale description of concentration and potential fields. DNS predictions are further compared with the one-dimensional macrohomogeneous results to establish appropriate correlations for effective transport properties as input into macroscopic computational fuel cell models. Finally, the utility of the stochastic reconstruction technique coupled with the DNS model is demonstrated through addressing the influence of microstructural inhomogeneity on the fuel cell performance.

Anisotropic Heat and Water Transport in a PEFC Cathode Gas Diffusion Layer

A nonisothermal, two-phase model was developed to investigate simultaneous heat and mass transfer in the cathode gas diffusion layer GDL of a polymer electrolyte fuel cell PEFC. The model was applied in two-dimensions with the in-plane i.e., channel-to-land and through-plane i.e., catalyst layer-to-channel directions to investigate the effects of anisotropy of GDL. For the first time, the anisotropy in the GDL properties was taken into account and found to be an important factor controlling the temperature distribution in the GDL. The maximum temperature difference in the GDL was found to be a strong function of GDL anisotropy. A temperature difference of up to 5°C at a cell voltage of 0.4 V was predicted for an isotropic GDL while it reduced to 3°C for an anisotropic GDL. Significant effect of temperature distribution on liquid water transport and distribution was also observed. In addition, the latent heat effects due to condensation/evaporation of water on the temperature and water distributions were analyzed and found to strongly affect the two-phase transport.

Direct Numerical Simulation Modeling of Bilayer Cathode Catalyst Layers in Polymer Electrolyte Fuel Cells

A pore-scale description of species and charge transport through a bilayer cathode catalyst layer CL of a polymer electrolyte fuel cell using a direct numerical simulation DNS model is presented. Two realizations of the bilayer catalyst layer structure are generated using a stochastic reconstruction technique with varied electrolyte and void phase volume fractions. The DNS calculations predict that a higher electrolyte phase volume fraction near the membrane–CL interface provides an extended active reaction zone and exhibits enhanced performance. A higher void phase fraction near the gas diffusion layer aids in better oxygen transport. The effects of cell operating conditions in terms of low inlet relative humidity and elevated cell operating temperature on the bilayer CL performance are also investigated. Low humidity and elevated temperature operations exhibit overall poorer performance compared to the high humidity and the low-temperature operations, respectively.

Evaluating Pristine and Modified SnS2 as a Lithium-Ion Battery Anode: A First-Principles Study

Li intercalation and diffusion in pristine and modified SnS2 interlayer are studied by a first-principles approach. The results predict that the octahedral interstitial site is energetically favored for Li intercalation. The minimum energy path of Li diffusion in SnS2 interlayer is investigated by climbing image nudged elastic band method. It is found that Li atom diffuses from one energetically favored octahedral interstitial site to the neighbor one via tetrahedral interstitial site. The expansion of interlayer spacing is beneficial for decreasing the diffusion barrier. Ce dopant negatively impacts the Li diffusivity although it can optimize the interlayer spacing. Geometric structures of LixSnS2 (0 < x ≤ 3) are investigated to understand the lithiation-induced volume expansion and atomic structure change. The lithiation process can be divided into two stages. When Li content (x in LixSnS2) is less than 1, the volume expansion is not dramatic and only S atoms capture electrons from Li atoms. When Li content is larger than 1, Sn(4+) cations are significantly reduced, S-Sn-S trilayer gradually decomposes, and LixS2 (1 ≤ x ≤ 3) layer forms between two Sn monolayers. The mechanism of volume expansion is elucidated in this study.

Columnar order in jammed LiFePO4 cathodes: ion transport catastrophe and its mitigation

The high-rate, high-capacity potential of LiFePO4-based lithium-ion battery cathodes has motivated numerous experimental and theoretical studies aiming to realize such performance through nano-sizing, tailoring of particle shape through synthesis conditions, and doping. Here, a granular mechanics study of microstructures formed by dense jammed packings of experimentally and theoretically inspired LiFePO4 particle shapes is presented. A strong dependence of the resultant packing structures on particle shapes is observed, in which columnar structures aligned with the [010] direction inhibit diffusion along [010] in anisotropic LiFePO4. Transport limitations are induced by [010] columnar order and lead to catastrophic performance degradation in anisotropic LiFePO4 electrodes. Further, judicious mixing of nanoplatelets with additive nanoparticles can frustrate columnar ordering and thereby enhance the rate capability of LiFePO4 electrodes by nearly an order of magnitude.

Li2S Film Formation on Lithium Anode Surface of Li–S batteries

The precipitation of lithium sulfide (Li2S) on the Li metal anode surface adversely impacts the performance of lithium-sulfur (Li-S) batteries. In this study, a first-principles approach including density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations is employed to theoretically elucidate the Li2S/Li metal surface interactions and the nucleation and growth of a Li2S film on the anode surface due to long-chain polysulfide decomposition during battery operation. DFT analyses of the energetic properties and electronic structures demonstrate that a single molecule adsorption on Li surface releases energy forming chemical bonds between the S atoms and Li atoms from the anode surface. Reaction pathways of the Li2S film formation on Li metal surfaces are investigated based on DFT calculations. It is found that a distorted Li2S (111) plane forms on a Li(110) surface and a perfect Li2S (111) plane forms on a Li(111) surface. The total energy of the system decreases along the reaction pathway; hence Li2S film formation on the Li anode surface is thermodynamically favorable. The calculated difference charge density of the Li2S film/Li surface suggests that the precipitated film would interact with the Li anode via strong chemical bonds. AIMD simulations reveal the role of the anode surface structure and the origin of the Li2S formation via decomposition of Li2S8 polysulfide species formed at the cathode side and dissolved in the electrolyte medium in which they travel to the anode side during battery cycling.

Mesoscale elucidation of the influence of mixing sequence in electrode processing.

Mixing sequence during electrode processing affects the internal microstructure and resultant performance of a lithium-ion battery. In order to fundamentally understand the microstructure evolution during electrode processing, a mesoscale model is presented, which investigates the influence of mixing sequence for different evaporation conditions. Our results demonstrate that a stepwise mixing sequence can produce larger conductive interfacial area ratios than that via a one-step mixing sequence. Small-sized cubical nanoparticles are beneficial for achieving a high conductive interfacial area ratio when a stepwise mixing sequence is employed. Two variants of multistep mixing have been investigated with constant temperature and linearly increasing temperature conditions. It is found that the temperature condition does not significantly affect the conductive interfacial area ratio. The homogeneity of binder distribution in the electrode is also studied, which plays an important role along with the solvent evaporation condition. This study suggests that an appropriate combination of mixing sequence and active particle size and morphology plays a critical role in the formation of electrode microstructures with improved performance.