This paper presents a 2-D transient, isothermal model of a vanadium redox flow battery that can predict the species crossover and related capacity loss during operation. The model incorporates the species transport across the membrane due to convection, diffusion, and migration, and accounts for the transfer of water between the half-cells to capture the change in electrolyte volume. The model also accounts for the side reactions and associated changes in species concentration in each half-cell due to vanadium crossover. A set of boundary conditions based on the conservations of flux and current are incorporated at the electrolyte|membrane interfaces to account for the steep gradients in concentration and potential at these interfaces. In addition, the present model further improves upon the accuracy of existing models by incorporating a more complete version of the Nernst equation, which enables accurate prediction of the cell potential without the use of a fitting voltage. A direct comparison of the model predictions with experimental data shows that the model accurately predicts the measured voltage of a single charge/discharge cycle with an average error of 1.83%, and estimates the capacity loss of a 45 cycle experiment with an average error of 4.2%.

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Alloying nanocrystalline materials to stabilize them against grain growth is proving a critical enabling strategy for the processing and usage of bulk nanocrystalline parts. Alloying elements that segregate strongly to grain boundaries can lead to a preference for nanocrystalline structure, and to be most stable the grain boundary segregated state would need to be preferred to forming any other phase or solute configuration, including a solid solution, ordered compounds, or solute precipitates. In this paper, a stability criterion is developed by comparing the enthalpy of the grain boundary segregated state against such stable bulk phases. This enthalpic criterion is also translated into a lattice model framework to enable the use of Monte Carlo simulations to incorporate entropic and geometric effects in assessing nanocrystalline stability. Monte Carlo simulations show that entropy can play a role in stabilizing nanocrystalline states, leading to duplex structures, and also in forming a grain boundary network preferentially over a disordered or amorphous-like bulk phase.

This study investigated the influence of micro-porous layer (MPL) surface topology on the polymer electrolyte fuel cell (PEFC) performance through both experimental characterization and computational modeling. Studies were performed considering MPLs with varying degrees of surface roughness and crack size/density. In two instances, we systematically varied the topology by introducing water transport channels into the MPL. We experimentally observed that transport channels consistently increased the maximum current density. To investigate the reasons for the performance improvement, a two-dimensional, multi-phase, multi-fluid model was formulated with discrete domains for morphological features, including liquid water retention within interfacial voids and cracks. The model results provide evidence that the performance improvement with the modified MPLs is due to improved liquid water removal from the catalyst layer (CL). Liquid water moves laterally through the CL|MPL interface until it reaches the milled lines and from there it is removed in through-plane direction through the fibrous diffusion media (DM) and into the gas channel. A key outcome of this work is that we demonstrate a CL|MPL interface model that can accurately capture the effects of varying MPL surface morphology on PEFC performance and can be used in the design of new, optimized CL|MPL interface morphologies.

The instability of nanocrystalline materials against both grain growth and bulk phase separation is a principal challenge in their production and usage. This article reviews the thermodynamic stabilization of nanocrystalline structures by alloying, where a nanocrystalline state is considered to be stable if the nanostructure has the lowest free energy available to the alloy system, such that it is stable both against grain growth and the formation of bulk second phases. The thermodynamic accessibility of nanocrystalline structures in the alloy phase space introduces configurational degrees of freedom both at the atomic scale of the grain boundary structure and at the mesoscale level of the grains and grain boundary topology, which should be considered when identifying the equilibrium state. This article presents a survey of the kinds of thermodynamic models and simulations that have been developed to search for equilibrium nanocrystalline states. The review emphasizes the utility of Monte Carlo simulations to assess the thermodynamic stability of nanocrystalline states, including methods that have been proposed to account for degrees of freedom at both the atomic and grain scales. Although atomic scale simulations provide detailed segregation energetic information, the topological degrees of freedom in nanoscale polycrystals seem to be more critical considerations in the free energy description for identifying whether a nanocrystalline state is stable, and these are better addressed with mesoscale lattice-based simulation methods. A variety of interesting new nanostructural alloy states awaits further exploration by computational methods.