Flywheel energy storage systems (FESS) are an excellent short duration grid energy storage solution; however, their cost and energy storage capacity are typical barriers to their widespread commercialization. FESS can be designed by optimizing the shape of the flywheel rotor, choice of rotor material, operating speed and rotor radius. This study optimizes the flywheel rotor shape at various operating speeds and outer radii. It is found that the energy capacity of the rotor can be improved by choosing an ideal combination of operating speed and rotor radius. Our earlier work showed that including the cost of the FESS as an optimization objective could significantly alter the FESS design [1]. Therefore, the cost effectiveness of the FESS is also studied by comparing rotors made from different materials on an energy-per-cost basis, while the cost ratio of the materials is varied.

A new approach to analyze two-phase water transport in microporous layer used on polymer electrolyte fuel cells is presented. The effect of mixed wettability in MPLs is studied by means of a novel pore-size distribution (PSD) model, which is used to estimate saturation, liquid relative permeability, gas relative permeability, liquid-vapour interfacial surface area, and total pore surface area. With the proposed model, micro-structural information of the MPL often neglected by older models is taken into consideration, without adding the burden of large-scale computations usually associated with pore network models and Lattice-Boltzmann methods. The model is applied to study the effects of mixed wettability on ultra-thin catalyst layer fuel cells, where two-phase flow plays a key role.

Polymer electrolyte fuel cells (PEFC) provide the option of a remote power source with high efficiency and minimal green-house gases, NO x , SO x and particulate matter. To protect the PEFC stack from the environment in which remote power sources are required, an actively controlled enclosure to provide optimal temperature and relative humidity to the open-air cathode PEFC stack is studied. A mathematical model of a transient, non-isothermal, lumped parameter, open-cathode fuel cell stack is developed and coupled with an enclosure model. The open-cathode fuel cell stack mathematical model includes characterization of the cathode channel, the anode channel and the membrane electrode assembly (MEA). The transient mass and energy transport equations for the coupled system are solved to determine the optimal operating conditions for the PEFC stack within the enclosure.

The effect of ionomer loading on ultra-thin polymer electrolyte membrane fuel cell (PEMFC) electrodes fabricated using an inkjet printer is studied under a variety of temperatures and relative humidity conditions. Catalyst layer inks with varying amounts of Nafion were prepared and loaded in inkjet cartridges. The inks were then jetted on a Nafion membrane using an inkjet printer in order to produce fuel cell electrodes with low loadings. The Pt loading for the fabricated electrodes was estimated to be in the range of 0.009–0.01 mg/cm 2 per printer pass. SEM imaging shows that the thickness of a catalyst layer fabricated using 10 passes is approximately 3 microns and the catalyst layer agglomerates are about 200nm in radius. In order to determine the optimal Nafion content for this fabrication method, the electrochemical active area and steady state performance for the fabricated catalyst coated membranes (CCMs) was measured and also compared to a conventional CCM fabricated using spray deposition. Results show that optimal performance is achieved with a 40wt% Nafion loading under the operating conditions tested. The improved performance can be attributed to decrease in cell resistance and increase in proton conductivity.

Porous media is an essential part of polymer electrolyte membrane fuel cells (PEMFC). In order to optimize fuel cell performance and reduce catalyst consumption, mass transport in fuel cells needs to be improved. Understanding and modelling of mass transport in porous media of fuel cell (e.g. gas diffusion layer (GDL), micro porous layer (MPL) etc.) requires a knowledge of transport properties like diffusivity, permeability and Knudsen diffusivity. Current research is focused on experimental measurement of transport properties of porous media. A counter-diffusion bridge (Wicke-Kallenbach setup) has been used to estimate permeability, Knudsen diffusivity and effective diffusivity of GDLs and MPLs. The obtained transport properties are used with the recent theoretical models of multicomponent mass transport to estimate transport in fuel cells. The experimental studies show that conventional effective approximations like Bruggeman correlations are highly overpredicting and do not fully account for all the frictional forces in porous media.

A major challenge facing PEM fuel cell researchers is the development of a cathode electrode model that accurately accounts for the oxygen reduction reaction (ORR) kinetics. Rotating disk electrode (RDE) and micro-electrode experiments consistently show a doubling of the Tafel slope at a cell potential of approx. 0.75V. However, current mathematical models repeatedly use a Tafel kinetic model with constant Tafel slope, which may lead to inaccurate results. Recently, Wang et al. proposed an intrinsic, kinetic model that assumes the ORR is comprised of four intermediate steps and two intermediate adsorbed species, with the free energies of adsorption and activation used as the kinetic parameters. This model is capable of predicting the doubling of the Tafel slope and the coverage of the assumed intermediate species. The model is extended to include changes in the oxygen concentration with changing overpotential and is then implemented in an in-house two dimensional, cathode model. Results show the doubling of the Tafel slope, resulting in significant losses in the low potential region compared to Tafel kinetics. Also shown is the site blocking effect of the intermediate species, in particular Pt oxide, which completely covers the platinum surface on the catalyst layer in the presence of high oxygen concentration, i.e. at low overpotentials. Increasing the cell potential favors the reductive transition intermediate step, which consumes the Pt oxide and subsequently increases the available platinum surface. The change in Tafel slope is caused by this transition from a completely oxide covered surface to an oxide free surface.

Porous transport layers are an integral part of polymer electrolyte fuel cells (PEMFC). In order to optimize the catalyst layer performance and reduce catalyst consumption, a thorough understanding of mass transport through porous media is necessary. Currently, there is a lack of experimental measurements of effective mass transport properties of porous transport layers. Further, mass transport theories in the literature, such as the binary friction model by Kerkhof [1], have not been extensively validated for porous media. In the present study, mass transport measurements have been performed on the porous media of a PEMFC, namely a GDL and an MPL. The experimental setup described by Pant et al. [2] has been used. The setup uses the diffusion bridge/counter-diffusion technique for the mass transport measurements. The experimental setup has the advantage that it can be used to perform studies for pure diffusion and convection-diffusion mass transport. The setup also facilitates measurement of permeability of porous media, which can then be used in convection-diffusion studies. Preliminary permeability measurements of GDL and MPL from the setup show good agreement with values available in literature. In preliminary experimentation, the conventional diffusivity correlations like Bruggeman equation have been found to overpredict the diffusivities.

The catalyst layer of a polymer electrolyte fuel cell is commonly represented in mathematical models as an agglomerate structure of carbon catalyst-support particles. There are two prevailing assumptions for the structure of the agglomerates. The first is that the pores are filled with perfluorosulfonated-ionomer (PFSI). The second is that the pores are hydrophilic and are flooded only with liquid water during operation. The objective of this work is to develop numerical models for single water-filled and ionomer-filled agglomerates in a cathode catalyst layer of a polymer electrolyte membrane fuel cell (PEMFC), and investigate the properties of oxygen transport, proton transport, and reaction kinetics. The two models provide different solutions for the distribution of oxygen and protons, and produce a different reaction profile within the agglomerate. Previous numerical water-filled ionomer models in the literature have neglected the effect of the ionomer thin film. Therefore, the results obtained for both ionomer and water-filled models could not be easily compared. In this article, the equations developed relate the assumed structure of the agglomerates to the structure of the catalyst layer (CL). Results compare the effect of the thin film thickness in the two different types of agglomerates and relate the phenomena occurring within the agglomerates to overall catalyst layer performance.

Study of gas diffusion is critical in understanding the process of mass transfer in porous media, which is an integral part of polymer electrolyte membrane fuel cells (PEMFCs). An experimental method is presented to study the mass transfer processes in micro-nano capillaries, which is further extended to study the transport in the porous media of fuel cells. A diffusion bridge setup, similar to the one presented by Remick and Geankoplis [1] has been used. The experimental setup facilitates the study of binary and multicomponent mixture transport through micro-nano capillaries and porous media. The setup can perform studies for two cases viz., pure diffusion and convection-diffusion. Using pressure controls in both channels, the pressure gradient across the capillaries is varied to study the convection diffusion process in detail. The results obtained from the study will be used to review various models of mass transport available in literature.

Multicomponent mass transport in the micro/nano scale is one of the most important phenomena in many engineering processes such as fuel cells. Therefore, a through understanding of the underlying physics and modeling techniques for mass transport is of crucial importance. According to Kerkhof et al. [1], due to the approximations made in the derivation of the classic transport equations for diffusion-convective dominated problems, the commonly used approach of modeling these problems by the Maxwell-Stefan and Navier-Stokes equations results in trends that are contrary to experiments even for elementary problems like gaseous counter diffusion in capillaries. A new formulation to model molecular fluid transport in multi component systems is then presented. The main aim of this study is to develop a combined experimental-numerical program to study the validity of the classical and the proposed formulations. We discuss the development of an in-house written C/C++ Object Oriented program that solves the classical and recently presented formulation numerically.

A framework is presented to estimate the micro-structural parameters of cathode fuel cell electrodes by means of a nonlinear least-squares method. This work represents the first attempt in the literature to characterize the structure of the catalyst layer by numerical parameter estimation using a two-dimensional membrane electrode assembly model with an ionomer-filled agglomerate catalyst layer approximation. The framework is developed by coupling a two-dimensional model to an optimization based least-squares algorithm in DAKOTA. The algorithm, NL2SOL, minimizes the sum-of-squares of the residuals for any number of data points and parameters. Employing the proposed methodology allows for accurate characterization of the electrode structure and quantification the quality of the curve fit. Extension of this methodology allows for parameter estimation as novel materials are incorporated into fuel cell construction. Results indicate that curves can be fit using micro-structural and electrochemical parameters consistent with values published in the literature. However, the quality of the fit deteriorates for large data sets over the entire range of operating conditions.