Electrochemical Removal of Carbon Monoxide in Reformate Hydrogen for Fueling Proton Exchange Membrane Fuel Cells

A twin-cell electrochemical filter is demonstrated to reduce the CO concentration in reformate hydrogen. In this design, the potential and gas flow are switched between the two filter cells so that alternative CO adsorption and oxidation occur in each cell while providing a continuous flow of H2 to a fuel cell. The effects of filter switching time and applied potential on the CO concentration of gas exiting the filter are presented here for a CO concentration of 1000 ppm in nitrogen flowing at 100 cm3/min. The parasitic loss of hydrogen from a corresponding reformate stream was estimated to be 1.5%

A Boundary-Layer Model of a Parallel-Plate Electrochemical Reactor for the Destruction of Nitrates and Nitrites in Alkaline Waste Solutions

Electrochemical processes appear to be attractive for treating low level nuclear wastes. The development of a simple divided electrochemical-cell model operating in a batch mode, used for the reduction of nitrates and nitrites from nuclear wastes, is presented. This model, based on a boundary-layer approach, is simple and yet encompasses the key features of a previously developed distributed-parameter model that includes diffusion, migration, and convection as the flux components. Because it dramatically reduces computation time, this boundary-layer model is well suited for use in a complex interactive flowsheet model and for optimization studies. The boundary-layer model is used to predict partial current densities, reservoir concentrations, and off-gas compositions as a function of time. Good agreement between simulated and experimental data (i.e., nitrate and nitrite concentrations and off-gas compositions) is observed over the course of a batch run. In addition, a comparison with a rigorous distributed-parameter model is made to illustrate the accuracy and robustness of this model. The results of selected case studies are shown, and a preliminary batch optimization is carried out to show how the model can be used to maximize the destruction of nitrates and nitrites

Multimetallic Electrocatalysts of Pt, Ru, and Ir Supported on Anatase and Rutile TiO2 for Oxygen Evolution in an Acid Environment

Anatase and rutile TiO2 were investigated as stable supports for different multimetallic nanoparticles (i.e., Pt:Ru, Pt:Ir, Pt:Ru:Ir, and Ir:Ru) and tested for activity toward the oxygen evolution reaction (OER). Overall, Ir:Ru had the highest activity toward OER (i.e., current per gram of metal) compared to the other multimetallic combinations studied. This bimetallic supported on anatase TiO2 had a 53% higher current per gram of metal than an unsupported electrocatalyst of the same composition. The higher catalyst utilization of the supported electrocatalysts for OER is consistent with small, well-dispersed nanoparticles, which were observed in high resolution transmission electron microscopy images

Effect of Titanium Dioxide Supports on the Activity of Pt-Ru toward Electrochemical Oxidation of Methanol

TiO2and Nb-TiO2 were investigated as stable supports for Pt-Ru electrocatalysts towards methanol oxidation. X-ray photo-electron spectroscopy (XPS) data for all these TiO2-based supports show oxidation states of Ti4+, with no Ti3+, suggesting low electronic conductivity. However, the deposition of metal nanoparticles onto the supports at loadings of 60 wt% metal dramatically increased conductivity, making these electrodes (metal particles + support) suitable for electrochemistry even though the supports have low conductivity. For some of these TiO2-based supports, the activity of Pt-Ru towards methanol oxidation was excellent, even surpassing the activity of the same electrocatalysts supported on carbon. The activity of the electrocatalyst depended on TiO2 crystalline structure, the addition of Nb into the support and the weight loading of metal. For example, using anatase Nb-TiO2 as a support increased the electrochemical activity of Pt-Ru by 83% compared to the same electrocatalysts supported on either carbon Vulcan XC-72R or rutile Nb-TiO2. This electrode was also 64% more active than the one that had anatase TiO2 as the support with no Nb. Finally, increasing the weight loading of metal from 5 to 60% increased the conductivity by 5 orders of magnitude and the activity by a factor of 20

Development of a Novel CO Tolerant Proton Exchange Membrane Fuel Cell Anode

Typically Pt is alloyed with metals such as Ru, Sn, or Mo to provide a more CO-tolerant, high-performance proton exchange membrane fuel cell (PEMFC) anode. In this work, a layer of carbon-supported Ru is placed between the Pt catalyst and the anode flow field to form a filter. When oxygen is added to the fuel stream, it was predicted that the slow H2 kinetics of Ru in this filter would become an advantage compared to Pt and Pt:Ru alloy anodes, allowing a greater percentage of O2 to oxidize adsorbed CO to CO2. With an anode feed of H2, 2% O2, and up to 100 ppm CO, the Pt + Ru filter anode performed better at 70°C than the Pt:Ru alloy. The oxygen in the anode feed stream was found to form a hydroxyl species within the filter. The reaction of these hydroxyl groups with adsorbed CO was the primary means of CO oxidation within the filter. Because of the resulting proton formation, the Ru filter must be placed in front of and adjacent to the Pt anode and must contain Nafion in order to provide the ionic pathways for proton conduction, and hence achieve the maximum benefit of the filter

Estimation of Diffusion Coefficient of Lithium in Carbon Using AC Impedance Technique

The validity of estimating the solid phase diffusion coefficient, Ds, of a lithium intercalation electrode from impedance measurement by a modified electrochemical impedance spectroscopy (EIS) method is studied. A macroscopic porous electrode model and concentrated electrolyte theory are used to simulate the synthetic impedance data. The modified EIS method is applied for estimating Ds. The influence of parameters such as the exchange current density, radius of active material particle, solid phase conductivity, porosity, volume fraction of inert material, and thickness of the porous carbon intercalation electrode, the solution phase diffusion coefficient, and transference number, on the validity of Ds estimation, is evaluated. A simple dimensionless group is developed to correlate all the results. It shows that the accurate estimation of Ds requires large particle size, small electrode thickness, large solution diffusion coefficient, and low active material loading. Finally, a full model method is developed for the cases where the modified EIS method does not work well

Electrowinning of Nonnoble Metals with Simultaneous Hydrogen Evolution at Flow-Through Porous Electrodes III. Time Effects

The electrowinning of zinc from a recirculating alkaline zincate solution at a flow-through porous electrode was investigated. Experimental results were obtained to test the predictions of a mathematical model. The effects of electrolyte flow rate, cell current, and electrode thickness on the concentration-time relations and coulombic efficiency-time relations were studied. The experimental results show that the highest recovery rate was obtained at both high flow rates and cell currents. There is, however, a practical limit for increasing both the cell current and the electrolyte flow rate. Reasonable agreement between the model predictions and experimental results was obtained. A case study is presented to show how the model equations are used to predict the relationship between time, cell current, flow rate, and coulombs passed which are required to recover 95% of the zinc content of the electrolyte