Dusan Spernjak

In situ characterization of the catalyst layer in a polymer electrolyte membrane fuel cell

The catalyst layer CL in the polymer electrolyte membrane fuel cell PEMFC has not been explored in detail due to its complexities and difficulties associated with accessing it. It is important to understand water ingress and egress at the CL as it offers potential for water management by manipulating the evaporation rate. A technique for in situ visual characterization of the CL is presented. This is accomplished by designing and fabricating a catalyst-visible operational fuel cell and developing a microvisualization system. The dynamics of microdroplets on the CL surface including formation, growth, coalescence, and removal were visualized in an operating PEMFC. The liquid water behavior at the interface of the CL and the gas diffusion layer GDL were shown to promote the periodic droplet reemergence on the GDL surface in the flow channels. Mechanisms of water condensation and transport within the CL pores are discussed with respect to pore architecture and wetting properties. It has been shown that reduction of pore size and CL thickness alleviates flooding therein and promotes better catalyst utilization. Evaporation was identified as one of the distinguishing mechanisms of the CL, and one of the future challenges will be to control this mechanism. The experimental results should prove useful in clarifying the role of the CL in water management, and in refining models used to optimize PEMFC performance.

Accurate measurement of the through-plane water content of proton-exchange membranes using neutron radiography

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeders paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. ...

Measurement of Water Content in Polymer Electrolyte Membranes Using High Resolution Neutron Imaging

Sufficient water content within a polymer electrolyte membrane (PEM) is necessary for adequate ionic conductivity. Membrane hydration is therefore a fundamental requirement for fuel cell operation. The hydration state of the membrane affects the water transport within, as both the diffusion coefficient and electro-osmotic drag depend on the water content. Membranes water uptake is conventionally measured ex situ by weighing free-swelling samples equilibrated at controlled water activity. In the present study, water profiles in Nafion{reg_sign} membranes were measured using the high-resolution neutron imaging. The state-of-the-art, 10 {micro}m resolution neutron detector is capable of resolving water distributions across N1120, N1110 and N117 membranes. It provides a means to measure the water uptake and transport properties of fuel cell membranes in situ.

Effect of Hydrophilic Treatment of Microporous Layer on Fuel Cell Performance

The gas diffusion layer in a polymer electrolyte fuel cell is the component primarily responsible for effective water management under a wide variety of conditions. The incorporation of hydrophilic alumosilicate fibers in the microporous layer leads to an improvement in the fuel cell performance associated with a decrease in the mass transport resistance especially under high RH operation. This improvement in performance is obtained without sacrificing performance under low RH conditions. The alumosilicate fibers create domains that wick liquid water away from the catalyst layer. The improved mass transport performance is corroborated by AC impedance and neutron radiography analysis and is consistent with an increase in the average pore diameter inside the microporous layer.

Vortex-Induced Motion of Floating Structures: CFD Sensitivity Considerations of Turbulence Model and Mesh Refinement

Vortex-Induced-Motion (VIM) is an important issue in offshore engineering as it impacts the integrity of the mooring system for floating structures such as oil platforms and wind turbine platforms. Understanding and predicting VIM is a challenging task because of the inherent complexity of vortex structure shedding and fluid-structure interaction (FSI) in high Reynolds number flows. Computational Fluid Dynamics (CFD) is one of the key tools in VIM studies and optimization of the offshore systems design. We report a CFD sensitivity study with focus on turbulence model, mesh refinement, and time-step selection. Experimental measurements in a tow-tank facility are used to validate the CFD results. Three types of tank tests are modeled numerically: current drag, oscillating free decay, and VIM. The effect of turbulence model is evaluated by comparing Delayed Detached Eddy Simulation (DDES) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) models. The influence of mesh refinement and time step is investigated by using the grid convergence index (GCI). For present geometry and flow conditions (Re∼105), the DDES turbulence model demonstrates better agreement with experimental measurement in model scale VIM compared to the URANS model. In addition, DDES simulation captures the vortex structure more realistically, as evidenced by Q-criteria and turbulent eddy viscosity distribution. Finally, we show how the mesh refinement and time step selection affect simulation accuracy. Two viscous-flow commercial solvers are tested: the finite-volume solver ANSYS-Fluent™, and the finite-element solver Altair AcuSolve™. The results of this CFD Sensitivity study provide useful guidelines for CFD simulation of FSI and VIM problems for offshore engineering applications.Copyright

Durability Improvements Through Degradation Mechanism Studies

The durability of polymer electrolyte membrane (PEM) fuel cells is a major barrier to the commercialization of these systems for stationary and transportation power applications. By investigating cell component degradation modes and defining the fundamental degradation mechanisms of components and component interactions, new materials can be designed to improve durability. To achieve a deeper understanding of PEM fuel cell durability and component degradation mechanisms, we utilize a multi-institutional and multi-disciplinary team with significant experience investigating these phenomena.

Vortex-Induced Motion of Deep-Draft Semisubmersibles: A CFD-Based Parametric Study

Deep Draft Column Stabilized Floaters (DDCSFs) provide a viable dry-tree platform for ultra-deep water applications. A particular area of concern for this concept is its susceptibility to Vortex-Induced-Motions (VIM) due to its deeper draft that results in higher column slenderness ratios than conventional semisubmersibles. The VIM characteristics of platforms have traditionally been assessed through experimental measurements but in recent years Computational Fluid Dynamics (CFD) has been used alongside experiments to predict VIM. Besides solving problems that cannot be addressed by other analysis methods, CFD can be seamlessly integrated into the overall concept design cycle where different geometric parameters can be varied to optimize the performance of the platform. While tank experiments give little fluid flow information without extensive instrumentation, CFD solutions provide flow details that can be used to improve design. CFD also provides a cost-effective solution given the high cost of experiments and facility scheduling constraints. One such DDCSF concept is the Paired-Column semi-submersible (PC-Semi) and the design has a pair of columns instead of one at each of the four sides of the platform. Several design parameters of the columns, like the inter-column spacing, the cross-sectional areas, and the shape of the respective columns can be tuned to minimize the heave response. This study takes a detailed look at the effect of the change in the column-dependent parameters on the overall VIM characteristics of the PC semisubmersible. A two-step approach is presented in which the first step is to identify aspects of the physics that are unique to VIM and then formulate a methodology to predict it within the framework of a set of commercially available CFD tools. The methodology is verified further through a comparison with available experimental data. The second step is to apply the verified methodology and CFD tools to predict the VIM performance of a range of PC-Semi concepts obtained through a systematic variation of the column-dependent design parameters in model scale. Through this exercise the critical design parameters that can either improve or prove detrimental to the VIM performance of the PC-Semi design are identified.Copyright

Zr-doped ceria additives for enhanced PEM fuel cell durability and radical scavenger stability

Doped ceria compounds demonstrate excellent radical scavenging abilities and are promising additives to improve the chemical durability of polymer electrolyte membrane (PEM) fuel cells. In this work, Ce0.85Zr0.15O2 (CZO) nanoparticles were incorporated into the cathode catalyst layers (CLs) of PEM fuel cells (based on Nafion XL membranes containing 6.0 μg cm−2 ion-exchanged Ce) at loadings of 10 and 55 μg cm−2. When compared to a CZO-free baseline, CZO-containing membrane electrode assemblies (MEAs) demonstrated extended lifetimes during PEM chemical stability accelerated stress tests (ASTs), exhibiting reduced electrochemical gas crossover, open circuit voltage decay, and fluoride emission rates. The MEA with high CZO loading (55 μg cm−2) demonstrated performance losses, which are attributed to Ce poisoning of the PEM and CL ionomer regions, which is supported by X-ray fluorescence (XRF) analysis. In the MEA with the low CZO loading (10 μg cm−2), both the beginning of life (BOL) performance and the performance after 500 hours of ASTs were nearly identical to the BOL performance of the CZO-free baseline MEA. XRF analysis of the MEA with low CZO loading reveals that the BOL PEM Ce concentrations are preserved after 1408 hours of ASTs and that Ce contents in the cathode CL are not significant enough to reduce performance. Therefore, employing a highly effective radical scavenger such as CZO, at a loading of 10 μg cm−2 in the cathode CL, dramatically mitigates degradation effects, which improves MEA chemical durability and minimizes performance losses.

Experimental Investigation of Liquid Water Formation and Transport in a Transparent Single-Serpentine PEM Fuel Cell

Liquid water formation and transport was investigated by direct experimental visualization in an operational transparent single-serpentine PEM fuel cell. We examined the effectiveness of various gas diffusion layer (GDL) materials in removing water away from the cathode and through the flow field over a range of operating conditions. Complete polarization curves as well as time evolution studies after step changes in current draw were obtained with simultaneous liquid water visualization within the transparent cell. At similar current density (i.e. water production rate), lower level of cathode flow field flooding indicated that liquid water had been trapped inside the GDL pores and catalyst layer, resulting in lower output voltage. No liquid water was observed in the anode flow field unless cathode GDLs had a microporous layer (MPL). MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in anode flow field flooding close to the H2 exit.Copyright