Shawn Litster

Active Water Management for PEM Fuel Cells

Proton exchange membrane (PEM) fuel cells require humidified gases to maintain proper membrane humidification, but this often results in a problematic accumulation of liquid water. Typically, excessive air flow rates and serpentine channel designs are used to mitigate flooding at the cost of system efficiency. In this paper, we present an active water management system that decouples water removal from oxidant delivery. The system uses a porous carbon flow field plate as an integrated wick that can passively redistribute water within the fuel cell. The system also employs an external electro-osmotic (EO) pump that actively removes excess water from the channels and gas diffusion layer. For a 25 cm 2 fuel cell with 23 parallel air channels, we demonstrate a 60% increase in maximum power density over a standard graphite plate with a low air stoichiometry of 1.3. EO pumping represents a negligible parasitic load, consuming typically less than 0.5% of the fuel cell power. Experimental and modeling results show that simple passive water transport through the porous carbon alone can prevent flooding at certain operating conditions and flow field dimensions. However, active water management with EO pumping facilitates robust operation with a high volumetric power density across all operating conditions.

Two-phase transport in porous gas diffusion electrodes

The accumulation of liquid water in electrodes can severely hinder the performance of PEMFCs. The accumulated water reduces the ability of reactant gas to reach the reaction zone. Current understanding of the phenomena involved is limited by the inaccessibility of PEMFC electrodes to in situ experimental measurements, and numerical models continue to gain acceptance as an essential tool to overcome this limitation. This Chapter provides a review of the transport phenomena in the electrodes of PEM fuel cells and of the physical characteristics of such electrodes. The review draws from the polymer electrolyte membrane fuel cell literature as well as relevant literature in a variety of fields. The focus is placed on two-phase flow regimes in porous media, with a discussion of the driving forces and the various flow regimes. Mathematical models ranging in complexity from multi-fluid, to mixture formulation, to porosity correction are summarized. The key parameters of each model are identified and, where possible, quantified, and an assessment of the capabilities, applicability to fuel cell simulations and limitations is provided for each approach. The needs for experimental characterization of porous electrode materials employed in PEMFCs are also highlighted.

Gas Transport Resistance in Polymer Electrolyte Thin Films on Oxygen Reduction Reaction Catalysts.

Significant reductions in expensive platinum catalyst loading for the oxygen reduction reaction are needed for commercially viable fuel cell electric vehicles as well as other important applications. In reducing loading, a resistance at the Pt surface in the presence of thin perfluorosulfonic acid (PFSA) electrolyte film, on the order of 10 nm thick, becomes a significant barrier to adequate performance. However, the resistance mechanism is unresolved and could be due to gas dissolution kinetics, increased diffusion resistance in thin films, or electrolyte anion interactions. A common hypothesis for the origin of the resistance is a highly reduced oxygen permeability in the thin polymer electrolyte films that coat the catalyst relative to bulk permeability that is caused by nanoscale confinement effects. Unfortunately, the prior work has not separated the thin-film gas transport resistance from that associated with PFSA interactions with a polarized catalyst surface. Here, we present the first characterization of the thin-film O2 transport resistance in the absence of a polarized catalyst, using a nanoporous substrate that geometrically mimics the active catalyst particles. Through a parametric study of varying PFSA film thickness, as thin as 50 nm, we observe no enhanced gas transport resistance in thin films as a result of either interfacial effects or structural changes in the PFSA. Our results suggest that other effects, such as anion poisoning at the Pt catalyst, could be the source of the additional resistance observed at low Pt loading.

Spatially resolved, in situ potential measurements through porous electrodes as applied to fuel cells.

We report the development and use of a microstructured electrode scaffold (MES) to make spatially resolved, in situ, electrolyte potential measurements through the thickness of a polymer electrolyte fuel cell (PEFC) electrode. This new approach uses a microfabricated apparatus to analyze the coupled transport and electrochemical phenomena in porous electrodes at the microscale. In this study, the MES allows the fuel cell to run under near-standard operating conditions, while providing electrolyte potential measurements at discrete distances through the electrodes thickness. Here we use spatial distributions of electrolyte potential to evaluate the effects of Ohmic and mass transport resistances on the through-plane reaction distribution for various operating conditions. Additionally, we use the potential distributions to estimate the ionic conductivity of the electrode. Our results indicate the in situ conductivity is higher than typically estimated for PEFC electrodes based on bulk polymer electrolyte membrane (PEM) conductivity.

An Electro-osmotic Fuel Pump for Direct Methanol Fuel Cells

This work reports on the design and performance evaluation of a miniature direct methanol fuel cell(DMFC)integrated with an electro_osmotic(EO)pump for methanol delivery.Electro-osmotic pumps require minimal parasitic power while boasting no moving parts and simple fuel cell integration.Here ,aneletro-osmotic pump is realized from a commercially available porous glass frit.We characterize a custom-fabricated DMFC with a free convection cathode and coupled to an extennal electro-osmotic pump operated at applied potentials of 4.0,7.0,and 10V.Maximum gross power density of our free convection DMFC(operated at 50°)is 55 mW/cm2 using 4.0 mol/L concentration methanol solution supplied by the EO pump.Experimental results show that electro-osmotic pumps can deliver 2.0,4.0 and 8.0mol/L methanol/water mixtures to DMFCs while utilizing ~5.0% of the fuel cell power.Furthermore ,we discuss pertinent design considerations when using electro-osmotic pumps with DMFCs and areas of further study.

Molten catalytic coal gasification with in situ carbon and sulphur capture

A molten catalytic process has been demonstrated for converting coal into a synthesis gas consisting of roughly 20% methane and 80% hydrogen using alkali hydroxides as both catalysts and in situ CO2 capture agents. Baselines studies were also conducted using no catalyst, weak capture agents (CaSiO3) and strong in situ capture agent for acid gases (CaO). While a similar gas composition can be achieved using CaO rather than alkali hydroxides, the rate of syngas production is greater when using molten alkali hydroxides than when using CaO as the in situ capture agent for acid gases, such as HCl, H2S and CO2. Parametric studies were conducted to understand the effects of temperature, pressure, catalyst composition, steam flow rate and the ratio of coal to alkali hydroxide on the performance of the molten catalytic gasifier in terms of kinetics and syngas composition. To measure the amount and the rate of coal conversion, we have developed a method for quantifying the coal conversion as the reduction charge remaining, which is related to the chemical oxygen demand remaining in the coal. At temperatures between 800 °C and 900 °C, we measured first-order steam-coal gasification rates using sub-bituminous coal of 2 h−1 in a fixed bed reactor while capturing significant quantities of both H2S and CO2, and while also generating 20% methane plus ethane in the syngas on a dry volume basis.

Image segmentation of nanoscale Zernike phase contrast X-ray computed tomography images

Zernike phase contrast is a useful technique for nanoscale X-ray computed tomography (CT) imaging of materials with a low X-ray absorption coefficient. It enhances the image contrast by phase shifting X-ray waves to create changes in amplitude. However, it creates artifacts that hinder the use of traditional image segmentation techniques. We propose an image restoration method that models the X-ray phase contrast optics and the three-dimensional image reconstruction method. We generate artifact-free images through an optimization problem that inverts this model. Though similar approaches have been used for Zernike phase contrast in visible light microscopy, this optimization employs an effective edge detection method tailored to handle Zernike phase contrast artifacts. We characterize this optics-based restoration method by removing the artifacts in and thresholding multiple Zernike phase contrast X-ray CT images to produce segmented results that are consistent with the physical specimens. We quantitatively evaluate and compare our method to other segmentation techniques to demonstrate its high accuracy.

Multifunctional Hydrogels with Reversible 3D Ordered Macroporous Structures

Three‐dimensionally ordered macroporous (3DOM) hydrogels prepared by colloidal crystals templating display highly reversible shape memory properties, as confirmed by indirect electron microscopy imaging of their inverse replicas and direct nanoscale resolution X‐ray microscopy imaging of the hydrated hydrogels. Modifications of functional groups in the 3DOM hydrogels result in various materials with programmed properties for a wide range of applications.

A Two-Liquid Electroosmotic Pump for Portable Drug Delivery Systems

Portable drug delivery systems present an opportunity to improve patient mobility and reduce drug dosage. Infusion pumps for drug delivery are heavily used in hospital and home care settings to administer a variety of therapies such as chemotherapy, antimicrobials, analgesia, anesthesia, and post-operative and chronic pain management. We are developing electroosmotic (EO) pumps for drug delivery applications. EO pumps offer active dosage control, are compact, use low power, and have no moving parts. We here explore a two-liquid EO pump that decouples the drug from the working electrolyte with a series of collapsible membranes and enables EO pumping of a wide variety of medications.Copyright

Water Management at the Cathode of a Planar Air-Breathing Fuel Cell with an Electroosmotic Pump

Water management is a significant challenge in portable fuel cells and particularly in fuel cells with air-breathing cathodes. Liquid water condensation and accumulation at the cathode surface is unavoidable in a passive design operated over a wide range of ambient and load conditions. Excessive flooding of the open cathode can lead to a dramatic reduction of fuel cell power. We report a novel water management design based on a hydrophilic and electrically conductive wick in conjunction with an electroosmotic (EO) pump. A prototype air- breathing fuel cell with the proposed water management design successfully functioned under severe flooding conditions, including ambient temperature 10oC and relative humidity 80 %, for up to 6 h without any observable cathode flooding or loss of performance.