Development of plasma-sprayed molybdenum carbide-based anode layers with various metal oxides for SOFC.

Air plasma-sprayed (APS) coatings provide an ability to deposit a range of novel fuel cell materials at competitive costs. This work develops three separate types of composite anodes (Mo-Mo2C/Al2O3, Mo-Mo2C/ZrO2, Mo-Mo2C/TiO2) using a combination of APS process parameters on Hastelloy®X for application in intermediate temperature proton-conducting solid oxide fuel cells. Commercially available carbide of molybdenum powder catalyst (Mo-Mo2C) and three metal oxides (Al2O3, ZrO2, TiO2) was used to prepare three separate composite feedstock powders to fabricate three different anodes. Each of the modified composition anode feedstock powders included a stoichiometric weight ratio of 0.8:0.2. The coatings were characterized by scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, nanoindentation, and conductivity. We report herein that three optimized anode layers of thicknesses between 200 and 300µm and porosity as high as 20% for Mo-Mo2C/Al2O3 (250-µm thick) and Mo-Mo2C/TiO2 (300µm thick) and 17% for Mo-Mo2C/ZrO2 (220-µm thick), controllable by a selection of the APS process parameters with no addition of sacrificial pore-forming material. The nanohardness results indicate the upper layers of the coatings have higher values than the subsurface layers in coatings with some effect of the deposition on the substrate. Mo-Mo2C/ZrO2 shows high electrical conductivity

Electrochemical Investigation of the O-2(g),Ni/YSZ System Using Cyclic Voltammetry

Cyclic voltammetry was used for the electrochemical study of a Ni thin film (similar to 880 nm) deposited on an yttria-stabilized zirconia (YSZ) solid electrolyte pellet at temperatures between 350 and 450 degrees C at 20 kPa O-2 and atmospheric pressure. NiO formed electrochemically by the O2- species supplied from the electrolyte upon anodic polarization, while it grew according to the parabolic growth law, in agreement with Wagner's oxidation theory of metals at high temperatures. A model for the NiO formation was proposed, where NiO is formed at the Ni/YSZ interface and grows by the outward diffusion of Ni2+ species through NiO, which is determined as the rate-limiting step. This implies the autoinhibition of NiO formation by its continuous growth. The above result agrees with Wagner's theory; however, a difference between the latter and the current model is the means of supplying oxygen for NiO formation. Also, O-2 evolution reaction is not firmly affected by NiO formation because it is controlled by the electronic current. An apparent activation energy of the limiting Ni2+ diffusion step was calculated to be 35 kJ/mol under 400 mV and decreased with potential increase

Investigation of the CO oxidation rate oscillations using electrochemical promotion of catalysis over sputtered-Pt films interfaced with YSZ

The oscillatory behaviour of CO oxidation was studied at 250 °C and atmospheric pressure using an electrochemical catalyst composed of a thin (60 nm) sputtered-Pt film interfaced with an yttria-stabilized zirconia membrane. Oscillations of CO oxidation rate showed a perfect correlation with those of the electrochemical potential values. Electrochemical promotion of catalysis was used to initiate and stop the oscillatory behaviour. Small current application induced a permanent effect on the oscillatory behaviours. An extremely small negative current (−17 μA) led to a 4-fold increase of the catalytic activity and created oscillations that were stable even after current interruption. This permanent effect in the oscillatory behaviour of CO oxidation rate is observed for the first time using EPOC. This has been interpreted by the higher tendency of the nanometric-Pt particles to form PtOx in thin sputtered films. Keywords: CO oscillatory behaviour, Sputtered-Pt films, EPOC, Oxide mode

Permanent electrochemical promotion of C3H8 oxidation over thin sputtered Pt films

The effect of “permanent electrochemical promotion of catalysis” (P-EPOC) was studied for the first time in the catalytic deep oxidation of C3H8 over a thin (∼150 nm) sputtered Pt film on YSZ, under excess of oxygen at 350 °C. Short positive potential application (+1 V) resulted in a 5.6-fold increase of the catalytic rate, where C3H8 conversion reached 33%, while the apparent Faradaic efficiency was ∼330. After positive current interruption the catalytic rate remained in a highly active steady-state, determined by the total charge of the anodic polarization step. Restoration of the catalytic activity to the initial value occurred only by a similar negative potential imposition. This new stable steady-state after current interruption can be interpreted by storage of a non-reactive oxygen species upon anodic polarization at the proximity of the Pt/YSZ interface and subsequent enhanced migration of spillover Oδ− species from the electrolyte support to the Pt/gas interface under open-circuit conditions. Keywords: Permanent EPOC, NEMCA, Pt electrodes, C3H8 oxidatio

Investigation of the "permanent" electrochemical promotion of catalysis (P-EPOC) by electrochemical mass spectrometry (EMS) measurements

The phenomenon of electrochemical promotion of catalysis (EPOC) is most often fully reversible. Subsequent to long-lasting polarization, however, the new steady-state open-circuit catalytic activity after current interruption may remain significantly higher than that before polarization. This phenomenon has been reported as "permanent electrochemical promotion of catalysis" (P-EPOC). The catalytic oxidation Of C2H4 was studied over a Pt/YSZ/Au electrochemical cell under excess O-2 at 375 degrees C, combining cyclic voltammetry and mass spectrometry. it has been found that after positive current interruption, the catalytic rate remains in a highly active P-EPOC steady-state, where it is almost double than the initial open-circuit rate. During this highly active steady-state, the application of a similar negative current for a similar time period has been found to result in the return of the catalytic rate to the initial open-circuit state. Similar reversibility of the rate has been observed after cyclic voltammetry experiments where after a complete potential oscillation the open-circuit rate is almost the same to that before polarization. These establish the reported mechanism for the origin of P-EPOC, on promoting species storage and concomitant migration to the metal/gas interface after positive current interruption, through the three phase boundaries. (C) 2009 Elsevier B.V. All rights reserved

Effect of Diesel Oxidation Catalysts on the Diesel Particulate Filter Regeneration Process

AIR+LLI:STS:ABO:CGO:PVEInternational audienceA Diesel Particulate Filter (DPF) regeneration process was investigated during aftertreatment exhaust of a simulated diesel engine under the influence of a Diesel Oxidation Catalyst (DOC). Aerosol mass spectrometry analysis showed that the presence of the DOC decreases the Organic Carbon (OC) fraction adsorbed to soot particles. The activation energy values determined for soot nanoparticles oxidation were 97 +/- 5 and 101 +/- 8 kJ mol(-1) with and without the DOC, respectively; suggesting that the DOC does not facilitate elementary carbon oxidation. The minimum temperature necessary for DPF regeneration was strongly affected by the presence of the DOC in the aftertreatment. The conversion of NO to NO(2) inside the DOC induced the DPF regeneration process at a lower temperature than O(2) (Delta T = 30 K). Also, it was verified that the OC fraction, which decreases in the presence of the DOC, plays an important role to ignite soot combustion