Publications:

 

 

  1. The Reversible Behavior of K2Fe(VI)O4 in Aqueous Media: In Situ 57Fe Mössbauer and Synchrotron X-Ray Spectroscopy Studies

http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=ESLEF600000600001200A260000001&idtype=cvips&prog=normal
S. Ghosh, W. Wen, R. C. Urian, C. Heath, V. Srinivasamurthi, W. M. Reiff, V. Naschitz, S. Licht, and S. Mukerjee*
Electrochemical and Solid-State letters. 6(12), A260 (2003)

 

Abstract
The chemistry of the Fe(VI) compound, K2FeO4, was investigated in aqueous potassium hydroxide electrolyte for its potential use in secondary storage systems. High charge storage K2FeO4 material was synthesized using alkaline hypochlorite oxidation of ferric nitrate and characterized by in situ X-ray diffraction and Mössbauer spectroscopy. The discharge-charge profile of the cathode, measured vs. a zinc anode, was semiquantitatively correlated with changes in Fe6+/Fe3+ ratio and the nature of structural transformation at each stage by in situ Mössbauer and in situ synchrotron X-ray diffraction spectroscopy. The data in conjunction with the charge-discharge profile clearly indicates the rechargeability of K2FeO4 with a charge efficiency of about 36% of the total current passed under the prevailing experimental conditions. These results are potentially important to the design of Fe6+ compound analogs for use in secondary energy storage systems.

 

  1. Oxygen Permeation Studies on Alternative Proton Exchange Membranes Designed for Elevated Temperature Operation

L. Zhang, C. Ma and S. Mukerjee*
Electrochimica Acta., 48, 1845 (2003)

 

Abstract
Kinetic and mass transport properties were investigated for the oxygen reduction reaction in Nafion 117 and a sulfonated poly (arylene ether sulfone) membrane (SPES-40, 40% sulfonated groups/repeat unit) under 1 atm oxygen pressure, 100% relative humidity in a temperature range of 303–343 K using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)/proton exchange membrane (PEM) interface. Oxygen reduction kinetics was found to be similar for both Nafion® 117 and SPES-40 membrane at the Pt microelectrode interface. The temperature dependence of O2 permeation parameters showed same trends for both the membranes studied, there was an increase in D and a concomitant decrease in C. Despite lower equivalent weight and hence higher water content SPES-40 exhibited relatively close values of D with Nafion® 117. The results are discussed in the context of their different microstructures. Values of C showed a closer relationship to water content and the percent volume of aqueous phase in the respective membranes. The values of overall oxygen permeability were significantly higher in Nafion® 117, with a higher positive slope in its variation with temperature.

 

  1. An Investigation of Proton Conduction in Select PEM’s and Reaction Layer Interfaces Designed for Elevated Temperature Operation

http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6TGK-48V81K6-4-11&_cdi=5257&_user=2403224&_orig=browse&_coverDate=07%2F15%2F2003&_sk=997809998&view=c&wchp=dGLzVlz-zSkWb&md5=078285aa9d4ddbe6c4173193f0adc7a8&ie=/sdarticle.pdf
C. Ma, L. Zhang, S. Mukerjee*, D. Ofer and B. Nair
J. Membrane Science, 219, 123 (2003)

 

Abstract
The proton conductivity of several alternative proton exchange membranes, i.e. SPES-40 (a sulfonated polyarylene ether sulfone), SPSS-40 (sulfonated polysulfide sulfone) and SPES-PS (a polyether sulfone post-sulfonated) were studied using a four-probe ac-impedance method as a function of temperature. Further, proton conductivity was also investigated for the same ionomers in the form of micro-aggregates such as those typically encountered in the reaction layer (the interfacial layer of the electrode containing the catalyst). For this a new configuration of the conventional reaction layer in a membrane electrode assembly (MEA) was used, which enabled the isolation of proton conductivity to be the principle contributor to the ac-impedance. The results under 100% relative humidity, showed that SPES-40 has similar proton conductivity as Nafion® in the membrane within our experimental conditions. The values for the other membranes investigated were lower.  Attempts to correlate these observed differences with parameters such as equivalent weight (EW), water uptake (λ), acidity (pKa), etc. showed that the prime contributor was the difference in microstructure of the membranes. Conductivity of these polymeric ionomers when present as micro-aggregates in the reaction layer showed very different values as compared to the bulk membranes. There was a great divergence in conduction as a function of increase in temperature with Nafion® showed
a far greater rate of increase of conductivity than SPES-50 and SPES-PS. Blends of these ionomers with Nafion® showed intermediate values, albeit lower with characteristics closer to Nafion®. Single cell PEM polarization curves were measured for both Nafion® 117 and SPES-40 membrane keeping the ionomer in the reaction layer same as the membrane. Comparison of the performance showed similar ohmic polarization characteristics. However, their performance in the low current density activation polarization region indicated poorer oxygen reduction reaction kinetics with SPES-40 material as compared to Nafion®.

 

  1. Electrocatalysis of Reformate Tolerance in Proton Exchange Membranes Fuel Cells

http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6TGB-49J8KRP-4-1K&_cdi=5250&_user=2403224&_orig=browse&_coverDate=09%2F15%2F2003&_sk=994459999&view=c&wchp=dGLbVlW-zSkWz&md5=57e95739fbb61ff48f2f6d580b608267&ie=/sdarticle.pdf
Richard C. Urian, Andrea F. Gullá and S. Mukerjee
J. Electroanalytical Chemistry, 554-555, 307 (2003)

 

Abstract
Electrocatalysis of anode electrode tolerance resulting from the presence of both CO and CO2 in the reformer feed was investigated for Pt, Pt_/Ru (1:1) and various atomic ratios of supported Pt:Mo electrocatalysts in proton exchange membrane fuel cells (PEMFCs). In order to elucidate the effects of CO and CO2 in the reformer feed, separate systematic studies were conducted with varying levels of CO and CO2 in H2. The results were used to explain those obtained with a fixed reformate composition: 45% H2, 10 ppm CO, 15% CO2, 1%CH4 balanced with N2. Results with CO in H2 showed that PtMo/C exhibits at least a threefold better CO tolerance as compared to PtRu/C and fourfold with respect to Pt/C. The variation of PtMo atomic composition has a negligible effect on CO tolerance. Additional surface poisoning was detected for all the electrocatalysts studied in the molar ratio (H2:CO2, 40:60 to 60:40). The presence of a reduced CO2 species was confirmed using cyclic voltammetry. An ensemble effect was proposed to explain the variation of tolerance to CO2 as a function of Pt: Mo atomic ratio, this is in contrast to the effect in the presence of adsorbed CO. Interestingly, the overpotential losses in the presence of H2:CO2 for PtMo/C (1:1) and PtRu/C (1:1) were very close.  As the Pt content of the PtMo/C alloys was increased, the overpotential losses followed those observed for pure Pt, clearly demonstrating a relationship between overpotential loss and Pt site availability. Despite similar overpotential losses between Pt/C and PtMo/C (5:1), both of which were greater than PtRu/C (1;1) the overpotential loss observed for PtMo in a CO2_/CO reformate mix was far better than for both PtRu/C and Pt/C.