Detection of OH radicals as the effect of Pt particles in the membrane of polymer electrolyte fuel cells

The formation of OH radicals (OH) for polymer electrolyte fuel cell (PEFC) was investigated with a homemade test cell by means of a fluorescence probe method for several kinds of membrane electrode assembly (MEA) consisting of Nafion. After 12-h operation under open circuit condition, OH was detected at both the anode and the cathode sides but the amount was much larger for the anode side. The formation of OH considerably depended on the amount of Pt/C catalyst loaded in the MEA. Dispersion of Pt particles in Nafion membranes increased the OH formation, but Pt ions did not. Based on the results, the formation mechanism was discussed.     

Investigation of capacitance transients in CuInS2 due to ionic migration

Transport of mobile ions in n-TiO₂/n-CuInS₂/p-CuInS₂ thin-film devices is studied with the transient ion-drift (TID) method. In contrast to the normal TID method, a mobile ion profile is created in the CuInS₂ layer, which can be described by the Gouy-Chapman theory. Activation energies for diffusion of 0.5 and 1.0 eV are found. We postulate that these activation energies are related to the associated defect, ( In_Cu)^x, which introduces a deep electronic state inside the bandgap of CuInS₂. This defect can accept or release an electron and drift out of the depletion region. This will lower the concentration of recombination centers in the depletion region, which explains the self-healing property of CuInS₂.     

Laminar burning velocities and flame instabilities of butanol isomers-air mixtures

Laminar burning velocities and flame instabilities of the butanol-air premixed flames and its isomers are investigated using the spherically expanding flame with central ignition at initial temperature of 428 K and initial pressures of 0.10 MPa, 0.25 MPa, 0.50 MPa and 0.75 MPa. Laminar burning velocities and sensitivity factor of n-butanol-air mixtures are computed using a newly developed kinetic mechanism. Unstretched laminar burning velocity, adiabatic temperature, Lewis number, Markstein length, critical flame radius and Peclet number are obtained over a wide range of equivalence ratios. Effect of molecular structure on laminar burning velocity of the isomers of butanol is analyzed from the aspect of CH bond dissociation energy. Study indicates that although adiabatic flame temperatures of the isomers of butanol are the same, laminar burning velocities give an obvious difference among the isomers of butanol. This indicates that molecular structure has a large influence on laminar burning velocities of the isomers of butanol. Branching (CH3) will decrease laminar burning velocity. Hydroxyl functional group (OH) attaching to the terminal carbon atoms gives higher laminar burning velocity compared to that attaching to the inner carbon atoms. Calculated dissociation bond energies show that terminal CH bonds have larger bond energies than that of inner CH bonds. n-Butanol, no branching and with hydroxyl functional group (OH) attaching to the terminal carbon atom, gives the largest laminar burning velocity. tert-Butanol, with highly branching and hydroxyl functional group (OH) attaching to the inner carbon atom, gives the lowest laminar burning velocity. Laminar burning velocities of iso-butanol and sec-butanol are between those of n-butanol and tert-butanol. The instant of transition to cellularity is experimentally determined for the isomers of butanol and subsequently interpreted on the basis of hydrodynamic and diffusion-thermal instabilities. Little effect on flame instability is observed for the isomers of butanol. Critical flame radii are the same for the isomers of butanol. Peclet number decreases with the increase in equivalence ratio.     

Uniqueness in the low temperature oxidation of cycloalkanes

This work examines the inherent features in the low temperature oxidation of cycloalkanes which distinguish cyclic alkanes from open-chain alkanes. The first part of the discussion is based on the recent motored-engine studies of cyclic hydrocarbons, [Yang and Boehman, Proc. Combust. Inst. 32, p. 419; Yang and Boehman, Combust. Flame, 157, p. 495], and focuses on the formation of conjugate olefins in low temperature oxidation. While less reactive than linear alkanes of similar size, cyclic hydrocarbons produced significant amounts of conjugate olefins during low temperature oxidation, which is uncharacteristic of linear alkanes. Conformational analyses in this paper and in a companion paper reveal that the inhibited low temperature chain branching and the promoted olefin formation are due to the steric structures of the cyclic compounds limiting the number of hydrogens available to the (1,5) H-shift but alternatively enhancing the opportunity for the (1,4) H-shift during the isomerization of the fuel peroxy radicals, ROO → QOOH.The second part of this work focuses on the role of methyl substitution in low temperature oxidation of cycloalkanes, which is drastically different from that of linear alkanes. Ab initio calculations are conducted on cyclohexane and methylcyclohexane to compute the activation energy of the (1,5) and (1,4) H-shift with full consideration of species conformation. The presence of the methyl group is found to enable low activation-energy channels in the (1,5) H-shift. Next, the impact of methyl substitution on the formation of conjugate olefins is discussed for methylcyclohexane and methylcyclopentane. Based on the experimentally determined yields of conjugate olefin isomers, estimations are made of the fraction of each fuel radical that is converted to conjugate olefins. For both compounds, more tertiary radicals are converted to conjugate olefins than secondary radicals, and primary radicals have the least fraction being converted.     

Decomposition and hydrocarbon growth processes for hexenes in nonpremixed flames

Hexene and other large alkenes are present in practical fuels, are decomposition products of alkanes, and play an important role in aromatics formation due to their degree of unsaturation. The experiments in this paper examined the decomposition and hydrocarbon growth mechanisms of all 13 isomers of hexene in soot-forming nonpremixed flames. Specifically, C3 to C12 hydrocarbon concentrations were measured on the centerline of atmospheric-pressure methane/air coflowing nonpremixed flames doped with 4000 ppm of each isomer. The hexene decomposition rates and hydrocarbon product concentrations showed that the primary decomposition mechanism was unimolecular allylic CC fission for 12 of the isomers, and allylic CH fission for 2,3-dimethyl-2-butene, which does not contain any allylic CC bonds. Other decomposition mechanisms such as H-atom abstraction and six-center elimination were much less important. The allylic radicals produced by the initial hexene decomposition subsequently dissociated to propadiene, butadiene, pentadiene, and hexadiene. Allyl also formed butene by methyl addition and propene by H-atom abstraction from H₂ and CH₄. The different alkadienes appeared to promote different benzene formation pathways: propadiene decomposed to propargyl and therefore favored C3 + C3 routes, whereas the larger alkadienes decomposed to butenyne and therefore favored C4 + C2 routes. The decomposition mechanisms and primary products identified here can be generalized to other alkenes.     

A study of the catalysis of cobalt hydroxide towards the oxygen reduction in alkaline media

A cobalt hydroxide modified glassy carbon (Co(OH)₂/GC) electrode has been fabricated by a galvanostatic electrodeposition method. The catalytic activity for the oxygen (O₂) reduction reaction (ORR) of this electrode in alkaline media is studied by cyclic voltammetry, rotating disk electrode voltammetry, and rotating ring-disk electrode voltammetry. The O₂ reduction at the Co(OH)₂/GC disk electrode has been found to undergo an electrochemical process followed by sequential disproportionation of the electrochemical reduction intermediates, i.e., superoxide anion (O₂⁻) and hydrogen peroxide anion (HO₂⁻) in 0.1 M KOH solution. The Co(OH)₂ is first found to possess an excellent catalytic activity not only for the disproportionation of the O₂⁻ produced into O₂ and HO₂⁻ but also for that of the HO₂⁻ produced, combined with electrochemical reduction of O₂ mediated by surface functional groups at the carbon electrode surface. The Co(OH)₂ is a potential electrode material for the ORR in alkaline fuel cells and metal-air batteries.     

The effect of platinum in a Nafion membrane on the durability of the membrane under fuel cell conditions

The effect of platinum on free radical generation and membrane degradation in proton exchange membrane (PEM) fuel cells is investigated using three typical cell configurations. Examinations of the fluoride emission rates (FERs) under different testing conditions indicate that platinum deposited in the membrane plays an important role as a catalytic center for the formation of H₂O₂ and HO free radicals, leading to PEM degradation. The chemical durability of the membranes is tested in accelerated Fenton tests. It confirms the formation of free radicals in the presence of platinum in the decomposition of H₂O₂ by colorimetric method with dimethyl sulfoxide (DMSO) as the trapping agent. In addition, structural and morphological changes of the membranes are characterized using FT-IR spectroscopy and scanning electron microscopy (SEM).