Eugene S. Smotkin

Methanol Oxidation on Single‐Phase Pt‐Ru‐Os Ternary Alloys

Methanol oxidation was studied on arc-melted Pt-Ru-Os alloys and on fuel cell catalysts prepared by the NaBH{sub 4} reduction of metal chloride salts. Both the arc-melted alloys and the high surface area catalysts have x-ray diffraction patterns indicative of single-phase face-centered cubic lattices. Hydrogen adsorption/desorption measurements on the polished alloy electrodes, in the presence of adsorbed CO (25 C), show that selected ternary alloys have significant hydrogen adsorption/desorption integrals at adsorption potentials where Pt:Ru (1:1) was fully blocked and higher integrals at all adsorption potentials studied up to 400 mV vs. the reference hydrogen electrode. In situ diffuse reflection Fourier transform infrared spectroscopy of the fuel cell anodes showed that the alloy catalysts had reduced CO coverage relative to Pt, with the ternary catalyst showing the least coverage. Steady-state voltammetry of the arc-melted alloys at 25 C confirmed that Pt-Ru-Os (65:25:10) is more active than Pt-Ru (1:1), particularly above 0.6 V. Pt-Ru-Os (65:25:10) methanol fuel cell performance curves were consistently superior to those of Pt-Ru (1:1) (e.g., typically at 90 C, 0.4 V; 340 mA/cm{sup 2} with Pt-Ru-Os vs. 260 mA/cm{sup 2} with Pt-Ru).

Carbon supported and unsupported Pt–Ru anodes for liquid feed direct methanol fuel cells

A comparative study of the use of supported and unsupported catalysts for direct methanol fuel cells has been performed. The effect of catalyst loading, fuel concentration and temperature dependence on the anode, cathode and full fuel cell performance was determined in a fuel cell equipped with a reversible hydrogen reference electrode. Although the measured specific activities of supported catalysts were as much as 3-fold greater than the unsupported catalysts, membrane electrode assemblies prepared with supported catalysts showed no improvement with loadings above 0.5 mg/cm 2 . Fuel cells utilizing 0.46 mg/cm supported catalyst electrodes performed as well as unsupported catalyst electrodes with 2 mg/cm 2 . The temperature dependence and methanol concentration dependence studies both suggest increased methanol permeation through the thinner supported catalyst layers relative to the unsupported catalyst layers.

A methanol impermeable proton conducting composite electrolyte system

The concept of a proton conducting, methanol impermeable composite electrolyte system is demonstrated. A three-layered laminar electrolyte consisting of a dense, methanol impermeable protonic conductor sandwiched between proton permeable electronically insulating layers permits the selective transport of protons while eliminating methanol crossover to the cathode. We demonstrate the selectivity of the composite electrolyte using palladium foil sandwiched between two Nafion Tm polymer membranes. The open-circuit voltage for an H 2 /O 2 fuel cell utilizing this composite electrolyte is unaffected by introduction of methanol to the H 2 fuel stream, whereas conventional polymer electrolyte cells suffer severe degradation of performance due to methanol crossover.

Resistance and polarization losses in aqueous buffer–membrane electrolytes for water-splitting photoelectrochemical cells

The recent development of inexpensive catalysts for the oxygen evolution reaction has suggested that efficient photoelectrochemical cells (PECs) might be constructed from terrestrially abundant materials. Because these catalysts operate in aqueous buffer solutions at neutral to slightly basic pH, it is important to consider whether electrolytic cells can have low series loss under these conditions. Water-splitting or fuel-forming PECs will likely require porous separators or electrolyte membranes to separate the cathode products from oxygen produced at the anode. For this reason we analyze the individual potential losses in electrolytic systems of buffer solutions and commercially available anion- and cation-exchange membranes. Potentiometric analysis and pH measurements were employed to measure the potential losses associated with solution resistance, membrane resistance, and pH gradient formation at the current density (25 mA cm−2) expected for efficient PECs. The membrane pH gradient is the most problematic source of loss in these systems, but monoprotic buffers can minimize the pH gradient by diffusion of the neutral acidic or basic form of the buffer across the membrane. These results suggest that water-splitting PECs can be viable with properly chosen membrane–buffer combinations.

The effect of plasticizers on transport and electrochemical properties of PEO-based electrolytes for lithium rechargeable batteries

Abstract The effect of plasticizers (or solvent additives) on the transport and electrochemical properties of polymer electrolytes has been examined. First, the mobility of cations and anions of LiN(CF3SO2)2 (LiTFSI) was increased by doping the PEO-based electrolytes with ethylene carbonate and poly(ethylene glycol dimethyl) ether. However, the cation mobility in PEO-based electrolytes doped with propylene carbonate was suppressed resulting in a lower cation transference number. Nevertheless, all the plasticized electrolytes have higher bulk conductivity than the non-plasticized, even in the case the cation transference number decreased. This is due to increased ion mobilities and/or higher degree of ion dissociation upon the introduction of plasticizers. A study of the electrochemical stability of the electrolytes, by linear sweep voltammetry, shows that the plasticizers have a negligible effect on the electrolyte stability windows. Finally, the electrolyte/lithium metal electrode interfacial impedance was monitored by A.C. impedance as a function of storage time. The formation and growth rate of the passivation layer was diminished in the presence of the plasticizers.

Array membrane electrode assemblies for high throughput screening of direct methanol fuel cell anode catalysts

A fuel cell using an array membrane electrode assembly has been developed for the high throughput screening of fuel cell electrocatalysts. Standard membrane and electrode assembly methods are used. The use of modified fuel cell hardware permits the use of realistic catalyst exposure histories and steady state reaction conditions. The array fuel cell requires no supplemental electrolytes. The performance of the array fuel cell is demonstrated by the testing of one prepared in-house and three commercially available fuel cell catalysts. Within the potential range of a DMFC anode (i.e. 0.3–0.4 V), the catalyst rankings were PtRu (Johnson Matthey)>PtRu oxide (E-Tek)>PtRu (reduced by NaBH4)>Pt.

Elucidating the Ionomer-Electrified Metal Interface

The competitive adsorption of Nafion functional groups induce complex potential dependencies (Stark tuning) of vibrational modes of CO adsorbed (CO(ads)) on the Pt of operating fuel cell electrodes. Operando infrared (IR) spectroscopy, polarization modulated IR spectroscopy (PM-IRRAS) of Pt-Nafion interfaces, and attenuated total reflectance IR spectroscopy of bulk Nafion were correlated by density functional theory (DFT) calculated spectra to elucidate Nafion functional group coadsorption responsible for the Stark tuning of CO(ads) on high surface area fuel cell electrodes. The DFT calculations and observed spectra suggest that the side-chain CF3, CF2 groups (i.e., of the backbone and side chain) and the SO3(-) are ordered by the platinum surface. A model of the Nafion-Pt interface with appropriate dihedral and native bond angles, consistent with experimental and calculated spectra, suggest direct adsorption of the CF3 and SO3(-) functional groups on Pt. Such adsorption partially orders the Nafion backbone and/or side-chain CF2 groups relative to the Pt surface. The coadsorption of CF3 is further supported by Mulliken partial charge calculations: The CF3 fluorine atoms have the highest average charge among all types of Nafion fluorine atoms and are second only to the sulfonate oxygen atoms.

In Situ Fourier Transform Infrared‐Diffuse Reflection Spectroscopy of Direct Methanol Fuel Cell Anodes and Cathodes

In situ Fourier transform infrared-diffuse reflection spectroscopy (FTIR-DRS) was used to study both the adsorbed and desorbed species produced on high surface area anodes and cathodes of direct methanol/oxygen fuel cells. The authors investigated platinum-ruthenium and platinum black as anodes. The cathodes studied were platinum black. The primary product detected on both Pt-black and Pt-Ru anodes at low methanol/water vapor ratios (P{sub methanol}: 15.2 kPa) was CO{sub 2}. Consistent with previous work, CO adsorption is more prevalent on Pt-black than on Pt-RU. In addition to CO and CO{sub 2}, vibrational modes due to formic acid, methylformate, and formaldehyde are detected by FTIR-DRS under potentiostatic control. At higher methanol/water vapor ratios (P{sub methanol}: 38.0 kPa) and low potentials (0.10 to 0.50 V), formaldehyde is the only product at the Pt-Ru anode. Methylformate and formic acid vibrational modes appear at potentials from 0.60 to 0.80 V. CO{sub 2} and methanol are observed at open circuit on the cathode side as a result of methanol permeation from the anode to the cathode region. CO{sub 2} increases in the cathode region with increasing anode potential.

Carbon riveted Pt/C catalyst with high stability prepared by in situ carbonized glucose

Carbon riveted Pt/C catalyst was designed and prepared by in situ carbonized glucose in this paper. Characterization results show that carbon riveted Pt/C prevents Pt nanoparticles from coalescence. Its stability is markedly enhanced due to the anchoring effect of the carbon layers formed during the carbon riveting process.

Comparison of High-Throughput Electrochemical Methods for Testing Direct Methanol Fuel Cell Anode Electrocatalysts

The screening and testing of fuel cell electrocatalysts often involves comparisons under conditions that do not closely match their use in membrane electrode assemblies. We compared the activities of several commercial and homemade Pt and PtRu catalysts for electrochemical methanol oxidation by four different techniques; disk electrode linear sweep voltammetry in aqueous methanol/ sulfuric acid solutions, optical fluorescence detection in aqueous methanol solutions containing a fluorescent acid-base indicator, steady-state voltammetry in a 25 electrode array fuel cell with a large common counter electrode, and steadystate voltammetry in a conventional direct methanol fuel cell. The fluorescence detection method, which is a high-throughput technique developed for large arrays of electrocatalysts, can distinguish active from inactive catalysts, but it does not accurately rank active catalysts. Both the disk electrode and array fuel cell methods gave a reliable ranking of the catalysts studied. The best agreement occurred between the array fuel cell and single electrode fuel cell catalyst rankings. A wide range of catalytic activities was found for PtRu catalysts of the same nominal composition that were prepared by different methods.