Romesh Kumar

Effect of Voltage on Platinum Dissolution Relevance to Polymer Electrolyte Fuel Cells

One of the processes responsible for performance degradation of a polymer electrolyte fuel cell (PEFC) is the loss of the electrochemically active surface area of the platinum-based electrocatalysts, due in part to platinum dissolution. The long-term dissolution behavior of polycrystalline platinum and high-surface-area carbon-supported platinum particles was studied under potentiostatic conditions relevant to PEFC cathode conditions. The equilibrium concentration of dissolved Pt was found to increase monotonically from 0.65 to 1.1 V (vs SHE) and decrease at potentials >1.1 V. Dissolution rates measured at 0.9 V were comparable for the two types of electrodes (1.4 and 1.7 × 10 -14 g/cm 2 s).

Cathode Materials for Reduced-Temperature SOFCs

The progress toward a commercial solid oxide fuel cell (SOFC) continues to be a slow struggle due to materials, stacking, and system challenges. One long-term challenge has been the search for suitable cathode materials for use on intermediate (650-800°C) yttria-stabilized zirconia (YSZ) electrolytes and low-temperature (500-650°C) gadolinia-doped ceria (CGO) electrolytes. The present study has identified La(Sr)FeO 3 and possibly Pr(Sr)FeO 3 as potential cathode materials for YSZ. A La-deficient La(Sr)FeO 3 cathode has achieved an area-specific resistance of 0.1 Ω cm 2 at 800°C, with stable long-term performance. On CGO, Pr(Sr)CoO 3 , Gd(Sr)CoO 3 , and Sr(Co/Fe) 1 . 5 O 3 . 2 5 have all achieved an area-specific resistance close to 0.1 Ω cm 2 at 650°C, with relatively stable long-term performance, but further reduction in temperature to 600°C is necessary for efficient CGO operation.

Characterization of CuO/ZnO under oxidizing conditions for the oxidative methanol reforming reaction

Abstract Catalytic generation of hydrogen by the reaction of methanol with oxygen in the presence of steam over an industrial copper–zinc oxide catalyst was studied. Under differential oxygen conversion conditions, the catalyst remained in an oxidized state, and the main reaction was oxidation of methanol to carbon dioxide and water. The activity was proportional to the copper oxide surface area. The methanol consumption rate had a small positive order in methanol and oxygen (0.18th order) and was suppressed by water. The catalyst deactivated with time on stream due to agglomeration of copper oxide. As the reactor temperature increased, the rate of methanol oxidation increased, the oxygen conversion became very high, and the catalyst away from the reactor entrance became reduced. Then, a significant rate of hydrogen production was observed.

Bimetallic Pd-Cu oxygen reduction electrocatalysts

A series of Vulcan carbon-supported Pd-Cu catalysts with various molar ratios of Pd to Cu was prepared by co-impregnation followed by a reduction in a hydrogen atmosphere at three different temperatures. The degree of alloying between the two metals, alloy composition, and particle size and size distribution were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The electrocatalytic activity for the oxygen reduction reaction (ORR) for these various compositions was determined using the thin-film rotating disk electrode technique. Our study reveals that the Pd-Cu bimetallic electrocatalysts, with a suitable degree of alloying, offer a greatly enhanced ORR activity compared to the Pd monometallic electrocatalyst. The best electrocatalytic activities were observed for the bimetallic catalysts that showed alloy nanoparticles with a Pd-Cu molar ratio of approximately 1:1.

Thermal-hydraulic model of a monolithic solid oxide fuel cell

A mathematical model has been developed to simulate the electrochemistry and thermal hydraulics in a crossflow monolithic solid oxide fuel cell. The fuel cell stack consists of a honeycomb structure of cells with alternating layers of anode, electrolyte, cathode, and interconnect. In each cell of the stack, the porous anode (fuel) layer consists of numerous horizontal channels separated by thin walls. The porous cathode (air) layer lies below the anode and is separated from it by the dense electrolyte. The horizontal flow channels in the cathode run perpendicular to those in the anode. A dense layer of interconnect is sandwiched between successive cells of the stack. Dividing a single‐cell layer into a number of nodes, the model sets up the steady‐state heat and mass‐transfer equations for each node in a cell layer. Based on the average thermal and compositional conditions at each node and a specified cell voltage, the model calculates the Nernst potential and the resultant current, heat generation, and heat removal rates at each node. These calculations yield the temperature and the fuel and oxidant compositions and partial pressure matrices for the entire cell. The simulation also provides related performance data for the fuel cell stack, such as energy efficiency, fuel utilization, and power density. The model can be used to simulate operation with fuel gases other than hydrogen, such as coal gas or synthesis gas. A mathematical model such as this can be used to examine the effects of changing one or more of the various design variables and to evaluate the effectiveness of fabrication improvements in technology development. In the design phase, the model can be used to determine the size of the stack that will be required for a given power rating and to make design decisions regarding structure‐specific parameters, such as the thicknesses of the anode, electrolyte, cathode, and interconnect layers and dimensions of the flow channels in the anode and cathode. The model can also be helpful to the fuel cell system operator. Given a particular stack, the most favorable operating conditions can be determined by calculating a priori the effects of altering process variables, such as flow rates and feed conditions.

Oxygen-18 study of nonaqueous-phase oxidation of sulfur dioxide☆

Abstract In a study of the mechanisms of atmospheric sulfate formation, oxygen isotope ratios were measured in sulfates and in the SO 2 and water vapors from which they were formed, in the absence of liquid water. In a 3-l glass chamber, SO 2 and water vapor of various 18 O contents were isotopically equilibrated, and then air oxidation of the SO 2 to sulfate was performed by four different methods: high-voltage discharges, NO 2 addition, gamma irradiation and adsorption on activated charcoal. Isotopic equilibration between SO 2 and water vapor proceeded rapidly, resulting in a strong dependence of the δ 18 O of the sulfate on that of the water vapor. Oxidation of SO 2 on dry charcoal occurred through the apparent formation of 9-oxygen, 2-sulfur, chemisorbed molecules which decomposed to sulfate in leach water. The δ 18 O SO 2− 4 vs δ 18 O H 2 O relationships observed for these four nonaqueous-phase oxidations of SO 2 to sulfate, together with those in three previously reported aqueous-phase oxidations ( Fe 3+ -catalyzed air oxidation, charcoal-catalyzed air oxidation and H 2 O 2 oxidation), were compared to sulfate in rain and snow collected at Argonne, IL. The δ 18 O of sulfate in precipitation water was significantly higher than could be accounted for by any of the several oxidation reactions that were investigated as possible pathways in the formation of secondary sulfates in the atmosphere, either singly or in combination.

Nitrogen fixation by lightning activity in a thunderstorm

Abstract NO and NO2 produced in the atmosphere by lightning have been observed and measured. Detailed observations of NOx concentrations and storm activity have been recorded. In the presence of ozone at ≈ 30–50 ppb, the NO produced is rapidly oxidized to NO2. The NOx resulting from an individual flash of lightning, as well as from numerous area-wide lightning flashes have been measured. It is estimated that, at least in the observed storm, a flash of lightning produced 4 × 1026 molecules of NOx, with an uncertainty of from one-fourth to twice that amount.

Sulfur removal from diesel fuel-contaminated methanol

Methanol is considered to be a potential on-board fuel for fuel cell-powered vehicles. In current distribution systems for liquid fuels used in the transportation sector, commodity methanol can occasionally become contaminated with the sulfur in diesel fuel or gasoline. This sulfur would poison the catalytic materials used in fuel reformers for fuel cells. We tested the removal of this sulfur by means of ten activated carbons (AC) that are commercially available. Tests were conducted with methanol doped with 1 vol.% grade D-2 diesel fuel containing 0.29% sulfur, which was present essentially as 33-35 wt.% benzothiophenes (BTs) and 65-67 wt.% dibenzothiophenes (DBT). In general, coconut shell-based carbons activated by high-temperature steam were more effective at sulfur removal than coal-based carbons. Equilibrium sorption data showed linear increase in sulfur capture with the increase of sulfur concentration in methanol. Both types of carbons had similar breakthrough characteristics, with the dynamic sorption capacity of each being about one-third of its equilibrium sorption capacity. Results of this study suggest that a fixed-bed sorber of granular AC can be used, such as in refueling stations, for the removal of sulfur in diesel fuel-contaminated methanol.

Water balance in a polymer electrolyte fuel cell system

Polymer electrolyte fuel cell (PEFC) systems operating on carbonaceous fuels require water for fuel processing. Such systems can find wider applications if they do not require a supply of water in addition to the supply of fuel, that is, if they can be self-sustaining based on the water produced at the fuel cell stack. This paper considers a generic PEFC system and identifies the parameters that affect, and the extent of their contribution to, the net water balance in the system. These parameters include the steam-to-carbon and the oxygen-to-carbon ratios in the fuel processor, the electrochemical fuel and oxygen utilizations in the fuel cell stack, the ambient pressure and temperature, and the composition of the fuel used. The analysis shows that the amount of water lost from the system as water vapor in the exhaust is very sensitive to the system pressure and ambient temperature, while the amount of water produced in the system is a function of the composition of the fuel. Fuels with a high H/C (hydrogen to carbon atomic ratio) allow the system to be operated as a net water producer under a wider range of operating conditions.

Fuel processing for fuel cell power systems

Fuel processors to produce hydrogen from conventional and alternative fuels are being developed for use in fuel cell power generators. The design of these fuel processors hinges on many factors that include the temperature and pressure required for the conversion, the type and level of by-product that the fuel cell can tolerate, and the duty cycle of the fuel cell power system. This article reviews the types of fuels being considered for fuel cell systems, the reformer technologies being pursued, and the suitability of the reformers for specific applications. The various components needed in the fuel processor have been identified, and some results obtained from the fuel processor development work being conducted at Argonne National Laboratory have been reported.