Abstract
Kinetic and mass-transport properties were investigated for the oxygen reduction reaction for Nafion_ 117 and sulfonated poly(arylene ether sulfone) membranes, both pre- and post-sulfonated analogs (SPES-40 & SPES-PS) under 100% relative humidity and as a function of pressure (1–4 atm total pressure, 323 K) and temperature (303–353 K, 3 atm) 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)jproton exchange membrane (PEM) interface. The oxygen reduction kinetics were found to be similar for all the membranes at the Pt microelectrode interface. The temperature dependence of O2 permeation parameters showed identical trends for the membranes studied while the pressure dependence of O2 permeation parameters displayed some differences. Despite higher ion exchange capacities and hence higher water uptake, the two SPES membranes exhibited relatively lower values of D as compared to Nafion_ 117. The results are discussed in the context of their different microstructures.
Oxygen Reduction Kinetics in Low and Medium Temperature Acid Environment: Correlation of Water Activation and Surface Properties in Supported Pt and Pt Alloy Electrocatalysts
Abstract
Kinetics of oxygen reduction reaction on supported Pt and several Pt alloy electrocatalysts (PtCo/C and PtFe/C) have been investigated in terms of the effect of alloying on the initiation and extent of surface oxide formation (water activation: xH2O + Pt*(M) → (M)Pt−[OH]x + xH+ + xe-). For this, a systematic RRDE investigation has been conducted in trifluoromethane sulfonic acid (TFMSA) as a function of concentration (in the range 1 to 6 M) which corresponds to a change in mole ratio of water/acid from 50:1 in 1 M to 4:1 in 6 M TFMSA. This change in relative amount of water in the various concentrations can also be indirectly correlated to the relative humidity in an operating PEM fuel cell. The scope of this effort was (a) to confirm the shift and lowering of water activation on supported Pt alloy electrocatalysts relative to Pt at lower concentrations (1 M); (b) to compare the inherent activity for ORR on supported Pt and Pt alloy nanoparticles without the effect of oxide formation via activation of water, this was enabled at higher concentrations of TFMSA (6 M); (c) to relate the activation energy values at 1 M for Pt and Pt alloy electrocatalysts for further insight into the nature of the rate-determining step in the mechanism; and (d) to examine the relative formation of peroxides via a parallel pathway for Pt and Pt alloy electrocatalysts in 1 and 6 M TFMSA. Our results confirm that for fully hydrated systems akin to 1 M concentration the alloys shift the formation and extent of water activation on the Pt alloy surfaces; this has been correlated with in-situ XAS data (changes to Pt electronic states and short-range atomic order) as well as via direct EXAFS probe of the formation of oxygenated species above 0.75 V (typical potential for initiation of surface oxides on Pt). The lowering of oxide formation agrees well with the extent of enhancement of ORR activity. Activation energy determinations at 1 M concentration however revealed no difference between Pt and Pt alloys, indicating thereby that the rate-limiting step remains unchanged. At lower water activity (6 M) with negligible water activation (and hence surface oxides), the Pt surface was found to possess a higher activity for ORR as compared to the alloys. In addition, the determination of peroxide yield on the Pt surface showed that there was variation both in terms of alloy formation as well as the water activity at the interface. All these results have been discussed in the context of a PEM fuel cell operating in the low to medium temperature range (70−120 °C) and humidity variation (100 to 10%).
In situ Synchrotron X-ray Studies on Copper-Nickel 5 Volts Mn Oxide Spinel Cathodes for Li-ion Batteries
S. Mukerjee, X. Q. Yang, X. Sun, S. J. Lee, J. McBreen and Y. Ein-Eli Electrochimica Acta, 49, 3373 (2004)
Abstract
Partial substitution of Mn in lithium manganese oxide spinel materials by Cu and Ni greatly affects the electrochemistry and the cycle life characteristics of the cathode. Substitution with either metal or a combination of both metals in the spinel lattice structure reduces the 3.9–4.2 V potential plateaus associated with the conversion of Mn3+ to Mn4+. Higher potential plateau associated with oxidation of the substituted transition elements is also observed. These substituents also significantly alter the onset of Jahn–Teller distortions in the 3 V potential plateau. Synchrotron based in situ X-ray absorption (XAS) was used to determine the exact nature of the oxidation state changes in order to explain the overall observed capacities at different potential plateaus. The studies on LiCu0.5Mn1.5O4 show single phase behavior in the 4–5 V potential region with a good cycle life. Lower cycle life characteristic observed in cycling LiNi0.5Mn1.5O4 and LiNi0.25Cu0.25Mn1.5O4 versus Li metal are ascribed to coexistence of several phases in this potential region. However, LiCu0.5Mn1.5O4 shows onset of Jahn–Teller distortions in the 3 V potential plateau, in contrast to LiNi0.5Mn1.5O4 and LiNi0.25Cu0.25Mn1.5O4 cathode materials.
Electrocatalysis of CO Tolerance by Carbon Supported PtMo Electrocatalysts in Proton Exchange Membrane Fuel Cells
Abstract
This paper is a full version of an earlier short communication, where significantly higher (up to threefold) CO tolerance was reported for PtMo/C (atomic ratio, Pt:Mo, 3:1) relative to the current state-of-the-art PtRu/C (1:1) in a proton exchange membrane fuel cell (PEMFC) under standard operating conditions (85°C, 100% humidification, with H2 1 100 pm CO//O2). We report significantly different behavior for PtMo/C in contrast to PtRu/C, wherein there is negligible variation in CO tolerance (100 ppm CO in H2) with variations in alloying compositions (Pt:Mo, 1:1 to 5:1). Further, in contrast to Pt/C and PtRu/C, significantly lower variations in overpotential losses is observed for PtMo/C as a function of temperature (55–115°C) and CO concentrations (5–100 ppm, balance H2). In addition, excellent long-term stability is reported for PtMo/C (1:1) under steady-state conditions (constant potential conditions at 0.6 V) for a total duration of 1500 h, with anode gas composition varied between pure H2 and those with 100 ppm CO, with or without the presence of other reformate gases (primarily CO2 and N2). These are discussed in the context of detailed physicochemical characterization of the nanoparticles using a combination of X-ray diffraction, transmission electron microscopy, and in situ synchrotron X-ray absorption spectroscopy.