[Show abstract][Hide abstract] ABSTRACT: An in situ, local potential measurement techniquewas further developed and applied to all-vanadium redox flowbatteries to determine the potential distribution within multilayer electrodes of the battery. Micro-scale potential probes enabled in situ measurement of local potential in electrode layers between the cell flow field and membrane. The local redox potentials were recorded for different operating conditions and states of charges. To further analyze the behavior of potential distribution in the through-plane direction, a mathematical model was developed and the species distribution as well as the flux density of any individual component was modeled in terms of contributions from convective, diffusive and electrophoretic fluxes at each operating condition. Good agreement was achieved between the mathematical model prediction and experimental data with maximum error of 8%. Both mathematical simulation and experimental data confirmed the distribution of potential in the through plane direction as a function of discharge current density, predicting the lowest potential in a region close to the flow plate.
Preview · Article · Jan 2016 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: In this work, we report on the performance of Diels Alder poly(phenylene) membranes in vanadium redox flow batteries. The membranes were functionalized with quaternary ammonium groups to form an anion exchange membrane (QDAPP) and with sulfonic acid groups to form a cation exchange membrane (SDAPP). Both membrane classes showed similar conductivities in the battery environment, suggesting that the ion conduction mechanism in the material is not strongly affected by the moieties along the polymer backbone. The resistance to vanadium permeation in QDAPP was not improved relative to SDAPP, further suggesting that the polarity of the functional groups do not play a significant role in the membrane materials tested. Both QDAPP and SDAPP outperformed Nafion membranes in cycling tests, with both achieving voltage efficiencies above 85% while maintaining 95% coulombic efficiency while at a current density of 200 mA/cm2.
Preview · Article · Nov 2015 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: A model of transport across the ion-exchange membrane in allvanadium redox flow batteries has been proposed based on concentrated solution theory for species with high concentration. The model is based upon the Stefan-Maxwell multicomponent diffusion equation where the fluxes of the species including protons (H+), bisulfate (HSO-4), water (H2O) and the sulfonate functional groups (-SO-3) are fully coupled. The driving force for species transport has been modeled in terms of concentration and electrostatic potential gradients. The ionic transference numbers as well as water electro-osmosis drag coefficient has been calculated for different acid concentrations.
No preview · Article · Sep 2014 · ECS Transactions
[Show abstract][Hide abstract] ABSTRACT: The net water drag in a polymer electrolyte fuel cell is a result of a variety of transport processes. There is often a need to tailor the water balance to achieve higher performance under dry operating conditions. The motivation of this collected work is to understand the methods by which we can control the water transport in a PEFC via engineering of the components and architecture. Experimental and numerical models have been developed to measure and predict this, and various pathways for control of water storage and transport will be discussed. In particular, control of the micro and macroporous layer thermal and mass transport resistance, cell architecture, and microporous layer|catalyst layer interface will be shown to be capable of manipulating the net water storage and drag coefficient.
No preview · Article · Aug 2014 · ECS Transactions
[Show abstract][Hide abstract] ABSTRACT: The present work demonstrates an innovative concept of obtaining enhanced performance via laser treatment of the cathode-side diffusion medium (DM) while mitigating identified degradation modes. A diffusion medium was modified such that hydrophilic heat affected zones (HAZ) were introduced, which led to localized water redistribution. However, no perforation was created, thus mitigating accelerated degradation of the catalyst layer and diffusion medium. This material was compared to a diffusion medium with 100-mu m diameter perforations that contained heat affected zones surrounding the perforations. In-situ net water drag experiments indicate that at low humidity and low-to-moderate current densities, a non-perforated microporous layer (MPL) forces more water to back diffuse from the cathode to the anode. However, when more water is produced at higher currents or the inlet streams are close to saturation, the non-perforated MPL acts as a barrier to prevent liquid water in the cathode DM from moving toward the anode. Furthermore, a computational model showed that the thermal gradients introduced as a result of the perforations can significantly change the water transport, particularly due to phase-change induced flow. This work adds understanding to the role of the MPL and the laser-induced heat affected zones in polymer electrolyte fuel cell performance. (c) 2014 The Electrochemical Society. All rights reserved.
No preview · Article · Jul 2014 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: An experimental study to investigate the through-plane thermal conductivity of three different diffusion media (DM) used in polymer electrolyte fuel cells (PEFCs) as a function of compression (from 0.1 MPa to 2 MPa) and saturation (from 0 to 25%) was performed. Additionally, measurements to determine the stress–strain relationship for the materials were made using an optical microscope. Both compression and water content had a significant impact on the through-plane thermal conductivity, which should be accounted for in multiphase modeling efforts. An analytical expression for the theoretical maximum of the through-plane thermal conductivity, as a function of both compression and saturation, was developed to help understand the nature of liquid connectivity in saturated pores. Additionally, a relationship was developed to predict actual thermal conductivity of the tested materials as a function of both compression and saturation based on experimentally measured data.
No preview · Article · Jun 2014 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: In high-power polymer electrolyte fuel cell systems, prevention of anode dry-out through enhanced back flux of water and restriction of evaporative losses is needed. One potential method to engineer the back flux of water to the anode is to utilize an asymmetric anode and cathode micro-porous layer configuration to independently tailor anode and cathode thermal and mass transport resistances. Extensive experimental tests have been performed to study the impact of asymmetric MPL configuration on the net water drag coefficient using sets of MPL with different thermal and mass transport resistances. It was observed that with asymmetric MPL alignment between the anode and cathode, the net water drag coefficient could be significantly altered, opening the door to enhanced high power performance at anode-dryout conditions.
[Show abstract][Hide abstract] ABSTRACT: Limiting current measurements were used to evaluate oxygen transport resistance in the catalyst layer of a polymer electrolyte fuel cell (PEFC). The pressure independent oxygen transport resistance in the electrode was evaluated with two different cell architectures, and two different cathode Pt loadings (0.4 and 0.07 mgPt.cm-2). The total oxygen transport resistance is divided into intermolecular gas diffusion and a pressure independent component, which could be attributed to Knudsen diffusion or dissolution film resistance. The pressure-independent oxygen transport resistance in the catalyst layer was measured to vary between 13.3 and 34.4 s/m. It is shown that the pressure independent oxygen transport resistance increases with reduced Pt loading, but that effect is exacerbated by using conventional channel/lands. The results indicate that the open metallic element architecture improves the oxygen transport resistance in ultra-low Pt loading electrodes due to enhanced water management at the catalyst layer.
[Show abstract][Hide abstract] ABSTRACT: A printed circuit board with imbedded shunt resistors was developed and implemented to obtain distributed data in real time in an all-vanadium redox flow battery. This work discusses the development and design considerations for implementing this diagnostic approach. The in-situ data from this approach can be used to assess build quality, material performance, operating conditions, flow field design and degradation. In this work, the lateral current spread and influence of electrode material properties via a split electrode configuration was observed. It was shown that current shifts away from a hydrophobic electrode.
[Show abstract][Hide abstract] ABSTRACT: Anode dry-out is the main mechanism identified to limit operation in an open metallic element (OME) PEFC. The fundamental water transport mechanisms in the OME PEFC were examined in order to engineer further improved performance and higher temperature operation required for efficient heat rejection. Specifically, the net water drag (NWD) was measured over a range of conditions and analyzed with respect to electrochemical impedance spectroscopy and performance. As the cell operating temperature was increased, the effect of back diffusion was reduced due to the diminishing liquid water content in the cathode catalyst layer, and at critical liquid water content, anode dry-out was triggered primarily through electro-osmotic drag. Addition of cathode humidity was shown to promote high temperature operation mostly due to improved water back diffusion. The same mechanism can be achieved by creating a pressure differential across the membrane, with higher pressure on the cathode side. Stable operation was demonstrated at 90 °C using a polymer electrolyte membrane. Real time NWD measurements during transient anodic dry-out conditions were consistent with gradual membrane dehydration. The trade-off between liquid water overshadowing cathode catalyst sites and its contribution in promoting back diffusion is a key factor in systems with anode dry-out limited operation.
No preview · Article · Oct 2013 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: In this study, the performance and reaction location in a VRFB was investigated with a carbon paper electrode that was modified to include a thin layer of multi-walled carbon nanotubes. The nanotube layer was introduced into the electrode at locations on the negative and positive electrode nearest the membrane and nearest the flow field. Results with the nanotube layer on the positive electrode yielded small changes in the performance, despite the location. However, when the nanotube layer was introduced on the negative electrode, the performance was greatly improved when it was closest to the current collector. In this configuration, an 8% increase in power density and a 65 mV increase in cell voltage were observed, compared to a cell with raw carbon paper. This is attributed to the increase in active area of the nanoporous structure in a location where the reaction is favored to occur.
No preview · Article · Jan 2013 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: A comprehensive 2D + 1 multi-phase computational model has been applied to polymer electrolyte fuel cells with a porous metallic flow field to investigate operating strategies which enable high power density operation in dry, elevated temperature environment. Extensive experimental model validation has been completed under a wide range of temperature, pressure, stoichiometry and humidity conditions. Both qualitative and quantitative agreement has been achieved in regard to voltage, area-specific resistance, and net water drag coefficient. Internal water distribution predictions show that modest changes in operating parameters on the cathode side help maintain a hydrated anode stream and thus effectively push the envelope of stable operating temperature 20 degrees C higher, enabling more efficient heat dissipation in coolant system. Results also show that thermo-osmotic water flux across the membrane, as observed and measured experimentally, can be significant compared to electro-osmosis under high current (> 2 A/cm(2)) hot and dry conditions, even with thin electrolyte membranes.
No preview · Article · Nov 2012 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: Performance and mass transport of a polymer electrolyte fuel cell (PEFC) with an open metallic element (OME) flow field architecture were analyzed in comparison to a conventional parallel channel/land (C/L) fuel cell, using low humidity at the anode and dry oxidant at the cathode. Under identical conditions the OME cell was able to operate at a current density of 3 A cm−2, recording a peak power of 1.2 W cm−2, compared to 0.9 W cm−2 using a parallel cell. Area specific resistance (ASR) was lower for the OME cell as a result of more uniform compression and reduced contact resistance. Electrochemical impedance spectroscopy (EIS) revealed great improvement in mass transport compared to a parallel C/L cell. A heliox mixture at the cathode of both cells revealed improved mass transport for the parallel cell, but revealed no oxygen gas phase transport limitation at high current densities for the OME architecture. In fact, it was shown that with an OME architecture, limitation at ultra-high current density results from dehydration at the anode and not reactant mass transport. This also indicates that ionomer film resistances at the electrode do not preclude operation at extremely high currents.
No preview · Article · Nov 2012 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: The performance of a vanadium flow battery with no-gap architecture was significantly improved via several techniques. Specifically, gains arising from variation of the overall electrode thickness, membrane thickness, and electrode thermal treatment were studied. There is a trade-off between apparent kinetic losses, mass transfer losses, and ionic resistance as the electrode thickness is varied at the anode and cathode. Oxidative thermal pretreatment of the carbon paper electrode increased the peak power density by 16%. Results of the pretreatment in air showed greater improvement in peak power density compared to that obtained with pretreatment in an argon environment. The highest peak power density in a VRB yet published to the author's knowledge was achieved at a value of 767 mW cm(-2) with optimized membrane and electrode engineering.
Preview · Article · Jul 2012 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a vanadium redox flow battery with a peak power density of 557 mW cm−2 at a state of charge of 60%. This power density, the highest reported to date, was obtained with a zero-gap flow field cell architecture and non-wetproofed carbon paper electrodes. The electrodes were comprised of stacked sheets of carbon paper and optimized through systematic variation of the total electrode thickness. We anticipate significant reductions in the ultimate system cost of redox flow battery systems based on this design.
No preview · Article · May 2012 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: A comprehensive 2D+1 computational model has been developed to explore the operation of a polymer electrolyte fuel cell (PEFC) with a porous flow field, also called open metallic element (OME), in the ultra-high current density regime (>2A/cm(2)). The computational model has been validated to a greater extent than previously published multi-phase models, including in-situ experimental measurement of performance, high frequency resistance (HFR) and net water drag coefficient under a wide range of input relative humidity (RH) conditions. The combined experimental and modeling investigation found that, with the OME used as the flow field, gas phase transport of oxygen is not the limiting factor, even in the ultra-high current regime. The use of OME results in significant performance improvement compared to the conventional land-channel architecture at high current. Instead of oxygen transport limitation, however, anode dry-out limits performance, as confirmed by net water drag data from both experiment and model. With the OME architecture, diffusion flow is the dominant transport mechanism of water from the catalyst layer to the flow field, compared to capillary action and convection. Results also highlight the utility of experimentally-determined anode dry-out limits for validating multi-phase models.
Preview · Article · Jan 2012 · Journal of The Electrochemical Society
[Show abstract][Hide abstract] ABSTRACT: The purpose of this work is to explore engineered interfacial and structural architecture in fuel cell diffusionmedia (DM). Perforations were introduced via lasers on samples of virgin DM that contained hydrophobic content. Depending on laser choice, some laser-cut samples displayed a "heat affected zone" (HAZ) at the catalyst layer | microporous layer interface, characterized by a region surrounding each perforation where hydrophobic content was removed. At 50% inlet relative humidity, DM with homogeneously dispersed 100-mu m perforations and a HAZ displayed a 25% power density increase compared to virgin DM. Analyzing the oxygen concentration dependence in the double-Tafel region showed transport resistances were dominated by oxygen at moderate current values. Electrochemical impedance spectroscopy (EIS) and neutron radiography results indicated charge-and mass-transport impedances and liquid water redistribution play an important role, depending on the operating current density. Results suggested two mechanisms for the increased performance of the 100-mu m DM with HAZ: i) liquid water storage and through-plane water redistribution led to rehydration of the catalyst layer and membrane, and ii) in-plane water redistribution led to improved oxygen transport through the DM. The results of this study shed light on the importance of interfacial and structural architecture of fuel cell DM.
Preview · Article · Jan 2012 · Journal of The Electrochemical Society