G.E. Marnellos

Methane activation on a La0.6Sr0.4Co0.8Fe0.2O3 perovskite catalytic and electrocatalytic results

The catalytic and electrocatalytic behavior of the La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) perovskite deposited on yttria-stabilized zirconia (YSZ) was studied during the reaction of methane oxidation. Experiments were carried out in a well-mixed (CSTR) reactor at atmospheric pressure, in the 600–900°C range. When, instead of co-feeding with methane in the gas phase, oxygen was electrochemically supplied as O2−, considerable changes in the methane conversion and product selectivity were observed. Catalytic and electrocatalytic results were compared to those obtained when the LSCF served as a dense mixed-conducting membrane supplying oxygen to the methane feed stream because of the oxygen partial-pressure gradient across the membrane.

Effect of carbon type on the performance of a direct or hybrid carbon solid oxide fuel cell

The impact of carbon type on the performance of the direct carbon fuel cell (DCFC) or hybrid carbon fuel cell (HCFC) is investigated by utilizing bare carbon or carbon/carbonate mixtures as feedstock, respectively. In this regard, four different types of carbons, i.e. bituminous coal (BC), demineralised bituminous coal (DBC), anthracite coal (AC) and pine charcoal (PCC), are employed as fuels in a SOFC of the type: carbon (carbonate)|Cu–CeO2/YSZ/Ag|Air. The results reveal that in the absence of carbonates (DCFC configuration) the optimum performance, in terms of maximum power density (Pmax), is obtained for the charcoal sample, which demonstrated a power output of ∼12 mW cm−2 at 800 °C, compared to 3.4 and 4.6 mW cm−2 with the anthracite and bituminous samples, respectively. Demineralization treatment of bituminous coal is found to improve the DCFC performance resulting in a maximum power density of 5.5 mW cm−2. A similar trend in terms of maximum power density, i.e., PCC > DBC > BC > AC, is obtained in the hybrid carbon fuel cell (HCFC) employing a eutectic mixture of lithium and potassium carbonates (62 mol% Li2CO3 + 38 mol% K2CO3) in the anode compartment at a carbon/carbonate weight ratio of 4:1. An enhancement of up to 185% in the maximum power density is achieved by admixing molten carbonates with carbon feedstock, with its extent being dependent on carbon type and temperature. The obtained results are interpreted on the basis of carbon physicochemical characteristics and their impact on DCFC performance. It is found that the observed trend in volatile matter, porosity and structure disorder is perfectly correlated with the achieved power output. In contrast, high ash and sulfur contents notably inhibit the electrochemical performance. The superior performance demonstrated by pine charcoal in conjunction with its availability and renewable nature, reveals the potential of biomass as feedstock in both DCFCs and HCFCs.

Ethyl Acetate Abatement on Copper Catalysts Supported on Ceria Doped with Rare Earth Oxides

Different lanthanide (Ln)-doped cerium oxides (Ce0.5Ln0.5O1.75, where Ln: Gd, La, Pr, Nd, Sm) were loaded with Cu (20 wt. %) and used as catalysts for the oxidation of ethyl acetate (EtOAc), a common volatile organic compound (VOC). For comparison, both Cu-free (Ce-Ln) and supported Cu (Cu/Ce-Ln) samples were characterized by N2 adsorption at −196 °C, scanning/transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and temperature programmed reduction in H2. The following activity sequence, in terms of EtOAc conversion, was found for bare supports: CeO2 ≈ Ce0.5Pr0.5O1.75 > Ce0.5Sm0.5O1.75 > Ce0.5Gd0.5O1.75 > Ce0.5Nd0.5O1.75 > Ce0.5La0.5O1.75. Cu addition improved the catalytic performance, without affecting the activity order. The best catalytic performance was obtained for Cu/CeO2 and Cu/Ce0.5Pr0.5O1.75 samples, both achieving complete EtOAc conversion below ca. 290 °C. A strong correlation was revealed between the catalytic performance and the redox properties of the samples, in terms of reducibility and lattice oxygen availability. Νo particular correlation between the VOC oxidation performance and textural characteristics was found. The obtained results can be explained in terms of a Mars-van Krevelen type redox mechanism involving the participation of weakly bound (easily reduced) lattice oxygen and its consequent replenishment by gas phase oxygen.

Polarization studies in the Pd|SrCe0.95Yb0.05O2.975|Pd proton conducting solid electrolyte cell

Abstract The steady-state current–overpotential characteristics of the H 2 –Pd–SCY interphase (SCY=SrCe 0.95 Yb 0.05 O 2.975 ) were studied as a function of hydrogen partial pressure (10–100 kPa) and temperature (400–550°C) in the single chamber reactor cell: Pd∣SCY∣Pd. Based on the experimental results, a reaction model for the charge transfer reaction at the three-phase-boundary, is proposed. Results are compared to those obtained in the single chamber reactor cell: Ag∣SCY∣Ag.

Effect of cobalt loading on the solid state properties and ethyl acetate oxidation performance of cobalt-cerium mixed oxides

Cobalt-cerium mixed oxides were prepared by the wet impregnation method and evaluated for volatile organic compounds (VOCs) abatement, using ethyl acetate (EtAc) as model molecule. The impact of Co content on the physicochemical characteristics of catalysts and EtAc conversion was investigated. The materials were characterized by various techniques, including N2 adsorption at -196°C, scanning electron microscopy (SEM), X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) to reveal the structure-activity relationship. The obtained results showed the superiority of mixed oxides compared to bare CeO2 and Co3O4, demonstrating a synergistic effect. The optimum oxidation performance was achieved with the sample containing 20wt.% Co (Co/Ce atomic ratio of ca. 0.75), in which complete conversion of EtAc was attained at 260°C. In contrast, temperatures above 300°C were required to achieve 100% conversion over the single oxides. Notably, a strong relationship between both the: (i) relative population, and (ii) facile reduction of lattice oxygen with the ethyl acetate oxidation activity was found, highlighting the key role of loosely bound oxygen species on VOCs oxidation. A synergistic Co-Ce interaction can be accounted for the enhanced reducibility of mixed oxides, linked with the increased mobility of lattice oxygen.

Catalytic and electrocatalytic oxidation of methane on palladium electrodes in a solid electrolyte cell

The catalytic oxidation of methane on polycrystalline palladium films was studied at 550-750°C and atmosheric total pressure. The reaction was studied under both open and closed-circuit. Under open circuit, and when yttria-stabilized zirconia (YSZ) was used as solid electrolyte, the technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the Pd electrode during reaction. The main products were those of complete oxidation, i.e. CO2 and H2O. Under closed-circuit, the effect of electrochemical oxygen “pumping” to or from the catalyst was studied. Non-faradaic (NEMCA) phenomena were observed but the reaction rate enhancement factors (A) were not as large as with previously studied catalytic systems.

Assessment of biochar as feedstock in a direct carbon solid oxide fuel cell

The feasibility of employing biochar as a fuel in a direct carbon fuel cell (DCFC) or a hybrid carbon fuel cell (HCFC) is investigated in the present study, by utilizing bare biochar or biochar/carbonate mixture as feedstock, respectively. Three different types of biochars, i.e., pistachio shells (PI), pecan shells (PE) and sawdust (SD) are used as feedstock in a solid oxide fuel cell (SOFC) of a type: Biochar|Co–CeO2/YSZ/Ag|Air. All samples were characterized by means of chemical composition (ultimate/proximate analysis), mercury porosimetry, N2 adsorption–desorption isotherms (BET method), thermogravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), to obtain a close correlation between cell performance and biochar characteristics. The electrochemical measurements reveal that the optimum performance, in terms of maximum power density (Pmax), is obtained for the PI biochar, which demonstrated a power output of 15.5 mW cm−2 at 800 °C, compared to 14 and 10 mW cm−2 for PE and SD biochars, respectively. The obtained cell performance results are interpreted on the basis of biochar physicochemical characteristics and AC impedance spectroscopy studies. The superior performance of PI biochar is attributed to a synergistic effect of several physicochemical characteristics, involving the porosity, the acidity, the volatile matter, the carbon and hydrogen content as well as the population of oxygenated surface functionalities.

Electrode polarization and electrical properties of the La0.6Sr0.4Co0.8Fe0.2O3 − δ, O2/yttria stabilized zirconia interface: Effect of gas phase composition and temperature

The steady-state current-overpotential characteristics of the O2,La0.6Sr0.4Co0.8Fe0.2O3 − δ/YSZ interface have been studied as a function of oxygen partial pressure and temperature. Ideal Nernst behaviour is observed in the temperature range between 400–900 °C and oxygen pressure range between 0.5–100 kPa. The results of I − η measurements indicate that in the potential range 0.05–0.25 V, the apparent anodic and cathodic charge transfer coefficients are close to unity: a = c = 1. The logarithm of the equilibrium exchange current density (I0) shows a positive dependence on the logarithm of the oxygen partial pressure with a slope m = 0.25 ± 0.05. These observations are in agreement with a proposed reaction model in which the diffusion of singly ionized oxygen adatoms (Oads−) on the oxide surface is assumed to be the rate determining step of the electrode reaction.

Carbon to electricity in a solid oxide fuel cell combined with an internal catalytic gasification process

Abstract This study explores strategies to develop highly efficient direct carbon fuel cells (DCFCs) by combining a solid-oxide fuel cell (SOFC) with a catalyst-aided carbon-gasification process. This system employs Cu/CeO 2 composites as both anodic electrodes and carbon additives in a cell of the type: carbon|Cu-CeO 2 /YSZ/Ag|air. The study investigates the impact on in situ carbon-gasification and DCFC performance characteristics of catalyst addition and variation in the carrier gas used (inert He versus reactive CO 2 ). The results indicate that cell performance is significantly improved by infusing the catalyst into the carbon feedstock and by employing CO 2 as the carrier gas. At 800 °C, the maximum power output is enhanced by approximately 40% and 230% for carbon/CO 2 and carbon/catalyst/CO 2 systems, respectively, compared with that of the carbon/He configuration. The increase observed when employing the catalyst and CO 2 as the carrier gas can be primarily attributed to the pronounced effect of the catalyst on carbon-gasification through the reverse-Boudouard reaction, and the subsequent in situ electro-oxidation of CO at the anode three-phase boundary.

Electrocatalytic synthesis of ammonia at atmospheric pressure

Ammonia was successfully synthesized from nitrogen and hydrogen at atmospheric pressure in a solid-state proton-conducting cell-reactor. Two types of reactors were used, one double-chamber and one single-chamber cell. In the form of protons (H + ), hydrogen was electrochemically transported through the solid electrolyte, from anode to cathode (palladium) over which ammonia was synthesized. This novel process eliminates the thermodynamic requirement for high-pressure operation.