Gas Analysis of Carbon Oxidation in a Coin‐Type Direct Carbon Fuel Cell

Carbon oxidation behaviors were illuminated in terms of gas composition in a coin‐type direct carbon fuel cell. The main gas species in the anode chamber at 850 °C was mostly carbon monoxide, which was generated from the chemical reaction of carbon and molten carbonates. The concentration of CO was reduced as time passed because the reactivity of carbonates was weakened. The open circuit voltage was directly dependent on the CO concentration. The gases in the anode chamber had a vertical concentration distribution; the highest CO and the lowest CO₂ concentrations were observed near the electrode. However, the voltage in the polarization state was less dependent on the gas composition. A polarization state of 150 mA cm^–2 allowed the oxidation of CO, resulting in an increased CO₂ concentration near the electrode. The enlarged CO₂ partial pressure facilitated CO generation through the recombination of carbonate ions (CO₃^2–). Decreasing the temperature from 850 to 750 °C reduced the level of carbon monoxide at the anode. The presence of CO as a main component in the anode concludes that the oxidation of solid carbon takes place through the gasification of carbon to CO, then electrochemically to CO₂.     

Dynamic Behavior of a PEM Fuel Cell During Electrochemical CO Oxidation on a PtRu Anode

The dynamic behaviour of a single PEM fuel cell (PEMFC) with a PtRu/C anode catalyst using CO containing H₂ as anode feed was investigated at ambient temperature. The autonomous oscillations of the cell potential were observed during the galvanostatic operation with hydrogen anode feed containing CO up to 1000 ppm. The oscillations were ascribed to the coupling of the adsorption of CO (the poisoning step) and the subsequent electrochemical oxidation of CO (the regeneration step) on the anode catalyst. The oscillations were dependent on the CO concentration of the feed gas and the applied current density. Furthermore, it was found that with CO containing feed gas, the time average power output was remarkably higher under potential oscillatory conditions in the galvanostatic mode than during potentiostatic operation. Accompanying these self-sustained potential oscillations, oscillation patterns of the anode outlet CO concentration were also detected at low current density (<100 mA/cm²). The online measurements of the anode outlet CO concentrations revealed that CO in the anode CO/H₂ feed was partially electrochemically removed during galvanostatic operation. More than 90% CO conversion was obtained at the current densities above 125 mA/cm² with low feed flow rates (100–200 mL/min).     

The Effect of H2S on the Performance of SOFCs using Methane Containing Fuel

In recent years, the interest for using biogas derived from biomass as fuel in solid oxide fuel cells (SOFCs) has increased. To maximise the biogas to electrical energy output, it is important to study the effects of the main biogas components (CH₄ and CO₂), minor ones and traces (e.g. H₂S) on performance and durability of the SOFC. Single anode‐supported SOFCs with Ni–Yttria‐Stabilised‐Zirconia (YSZ) anodes, YSZ electrolytes and lanthanum‐strontium‐manganite (LSM)–YSZ cathodes have been tested with a CH₄–H₂O–H₂ fuel mixture at open circuit voltage (OCV) and 1 A cm^–2 current load (850 °C). The cell performance was monitored with electric measurements and impedance spectroscopy. At OCV 2–24 ppm H₂S were added to the fuel in 24 h intervals. The reforming activity of the Ni‐containing anode decreased rapidly when H₂S was added to the fuel. This ultimately resulted in a lower production of fuel (H₂ and CO) from CH₄. Applying 1 A cm^–2 current load, a maximum concentration of 7 ppm H₂S was acceptable for a 24 h period.     

Ru-catalyzed anode materials for direct hydrocarbon SOFCs

A solid oxide fuel cell using a thin ceria-based electrolyte film with a Ru-catalyzed anode was directly operated on hydrocarbons, including methane, ethane, and propane, at 600 °C. The role of the Ru catalyst in the anode reaction was to promote the reforming reaction of the unreacted hydrocarbons by the produced steam and CO₂, which avoided interference from steam and CO₂ in the gas-phase diffusion of the fuels. The resulting peak power density reached 750 mW cm⁻² with dry methane, which was comparable to the peak power density of 769 mW cm⁻² with wet (2.9 vol.% H₂O) hydrogen. More important was the fact that the cell performance was maintained at a high level regardless of the change in the methane utilization from 12 to 46% but was significantly reduced by increasing the hydrogen utilization from 13 to 42%. While the anodic reaction of hydrogen was controlled by the slow gas diffusion, the anodic reaction of methane was not subject to the onset of such a gas-diffusion process.     

Effect of methane concentration on reaction behavior in direct-methane solid oxide fuel cells with (Ce,Gd)O2−x-impregnated FeCr layer for fuel processing

A solid oxide fuel cell (SOFC) with a Ni-yttria-stabilized zirconia anode of 1 cm² area was set up with a porous disk of gadolinia-doped ceria-impregnated FeCr as a gas diffusion layer (GDL) under direct-methane feeding. In this setup of SOFC plus GDL, the tests at 800 °C and ambient pressure show that the current density, the methane conversion rate, the product formation rates, and the CO₂ selectivity increased with increasing methane concentration. The major reaction in the GDL is CO₂ reforming of methane to produce the syngas (CO plus H₂). The anodic electrochemical oxidation of CO from GDL results in an overall rate of CO₂ formation being much larger than that of CO formation. There is a synergy between the rate of reaction in the GDL and that over the anode.     

The influence of oxygen additions to hydrogen in their electrode reactions at Pt/Nafion interface

The influence of oxygen gas added to hydrogen in their electrode reactions at the Pt/Nafion interface was investigated using ac impedance method. The electrochemical cell was arranged in either electrolytic (hydrogen enrichment) or galvanic (fuel cell) mode. The impedance spectra of the electrode reaction of a H₂/O₂ gas mixture were taken in each mode as a function of the gas composition, electrode surface roughness and the cell potential. The spectrum taken for the anodic reaction of electrolytic arrangement confirmed the anodic oxygen reduction reaction (AOR, the local consumption of hydrogen by the added oxygen) by showing an independent arc distinguishable from that for hydrogen oxidation. But the independent arc was not revealed in the spectrum taken on a smooth (low surface area) electrode or on a Pt/C anode of the galvanic cell. At any cell current density, the electrolytic mode showed its anodic overpotential much higher (nearly three times higher at the current density of 100 mA cm⁻²) than the potential registered in galvanic mode implying that the oxygen gas in the mixture engages more active and independent AOR at the anode of the electrolytic cell.     

Electrochemical reforming of CH4−CO2 mixed gas using porous Gd-doped ceria electrolyte with Cu electrode

This paper reports on the composition and flow rate of outlet gas and current density during the reforming of CH₄ with CO₂ using three different electrochemical cells: cell A, with Ni−GDC (Gd-doped ceria: Ce_0.8Gd_0.2O_1.9) cathode/porous GDC electrolyte/Cu−GDC anode, cell B, with Cu−GDC cathode/ porous GDC electrolyte/Cu−GDC anode and cell C, with Ru−GDC cathode/ porous GDC electrolyte/ Cu−GDC anode. In the cathode, CO₂ reacts with supplied electrons to form CO fuel and O²⁻ ions (CO₂+2e⁻→CO+O²⁻). Too low affinity of Cu cathode to CO₂ in cell B reduced the reactivity of the CO₂ with electrons. The CO fuel, O²⁻ ions and CH₄ gas were transported to the anode through the porous GDC mixed conductor of O²⁻ ions and electrons. In the anode, CH₄ reacts with O²⁻ ions to produce CO and H₂ fuels (CH₄+O²⁻→2 H₂+CO+2e⁻). The reforming efficiency at 700−800 °C was lowest in cell B and highest in cell A. The Cu anode in cells A and C worked well to oxidize CH₄ with O²⁻ ions (2Cu+O²⁻→Cu₂O+2e⁻, Cu₂O+CH₄→2Cu+CO+2H₂). However, a blockage of the outlet gas occurred in all the cells at 700−800 °C. The gas flow is inhibited due to a reduction in pore size in the cermet cathode, as well as sintering and grain growth of Cu metal in the anode during the reforming.     

The electrochemistry of nitrate—amide melts: The effects of melt acidity on the discharge characteristics of lithium anodes in room temperature nitrat

Lithium anodes immersed in “neutral” nitrate—amide melts rapidly form a relatively stable passivating film. Lithium anodes polarize rapidly on the application of current densities greater than approximately 0.5 mA cm⁻². The addition of small amounts of Brønsted-Lowry acids causes rapid depassivation. Once the added acid is consumed, the anode once again passivates. In highly acidic melts, the lithium anode corrodes at a rapid rate with the evolution of NH₃, along with smaller amounts of N₂O, CO₂ and H₂O. Current densities of 1–10 mA cm⁻² can be applied with minimal anodic polarization in the acidic melts. Test cells have shown that discharge lifetime and cell voltage/current density profiles are also regulated by melt acidity. Simple test cells using lithium anodes and cerium(IV) cathodes that have open circuit voltages greater than 4 V and that can be discharged at current densities of 0.1 mA cm⁻² with minimal polarization have been demonstrated.     

Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system

This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H₂ fuel and circulate CO₂ gas in the production process of electric power. A large electric power was measured in the H₂–O₂ gas system and the CO–O₂ gas system at 1073 K. The formation process of O²⁻ ions in the endothermic cathodic reaction (1/2O₂+2e⁻→O²⁻) controlled the cell performance in both the gas systems. The electric power of the H₂–CO₂ gas system, which allowed to change CO₂ gas into a CO fuel (H₂+CO₂→H₂O+CO) in the cathode, was 1/31–1/11 of the maximum electric power for the H₂–O₂ gas system. This result is related to the larger endothermic energy for the formation of O²⁻ ions from CO₂ molecules at the cathode (CO₂+2e⁻→CO+O²⁻) than from O₂ molecules. The CO–H₂O gas system and the H₂–H₂O gas system was expected to produce a H₂ fuel in the cathode (CO+H₂O→H₂+CO₂, H₂+H₂O→H₂+H₂O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H₂O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.     

Modeling of CO2 gasification of carbon for integration with solid oxide fuel cells

This modeling study focuses on gasification of carbon by CO₂ in a minimally fluidized bed containing a solid oxide fuel cell (SOFC). Kinetic parameters for a five‐step reaction mechanism characterizing the Boudouard reaction (C + CO₂ → 2CO) were determined thermogravimetrically at 1 atm from 973 to 1273 K. Experimentally determined kinetic parameters are employed in a transport model that predicts velocities and gas concentration profiles established in the carbon bed as a consequence of convection, diffusion, and heterogeneous reaction. The model is used to simulate the effect of an imbedded SOFC, in contact with the carbon bed. Although the model does not assume particular I‐V characteristics for the fuel cell, it indicates that current densities in the practical range of 100–1000 mA/cm² can be supported. Results show that temperature strongly affects the current density, whereas CO₂ flow rate has only a weak effect. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Preparation of BaZr0.1Ce0.7Y0.2O3–δ Based Solid Oxide Fuel Cells with Anode Functional Layers by Tape Casting

Solid oxide fuel cells (SOFCs) based on the proton conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte were prepared and tested in 500–700 °C using humidified H₂ as fuel (100 cm³ min^–1 with 3% H₂O) and dry O₂ (50 cm³ min^–1) as oxidant. Thin NiO‐BZCY anode functional layers (AFL) with 0, 5, 10 and 15 wt.% carbon pore former were inserted between the NiO‐BZCY anode and BZCY electrolyte to enhance the cell performance. The anode/AFL/BZCY half cells were prepared by tape casting and co‐sintering (1,300 °C/8 h), while the Sm_0.5Sr_0.5CoO_3–δ (SSC) cathodes were prepared by thermal spray deposition. Well adhered planar SOFCs were obtained and the test results indicated that the SOFC with an AFL containing 10 wt.% pore former content showed the best performance: area specific resistance as low as 0.39 Ω cm² and peak power density as high as 0.863 W cm^–2 were obtained at 700 °C. High open circuit voltages ranging from 1.00 to 1.12 V in 700–500 °C also indicated negligible leakage of fuel gas through the electrolyte.     

Effect of Ethane and Propane in Simulated Natural Gas on the Operation of Ni–YSZ Anode Supported Solid Oxide Fuel Cells

Solid oxide fuel cells with Ni–yittria‐stabilised zirconia (YSZ) anode supports were tested on surrogate natural gas fuels (methane containing 2.5–10% ethane and 1.25–5% propane) and compared with results for pure methane. Inert anode‐side diffusion barriers were found to help suppress coking on the Ni–YSZ anodes. However, carbonaceous deposits were observed on anode compartment surfaces and the barrier layers for all of the natural gas compositions tested. The addition of air to the natural gas was shown to suppress this coking. For natural gas with 5% ethane and 2.5% propane, the addition of 33% air yielded stable, coke‐free operation at 750 °C and 800 mA cm^–2. Cell performance on this fuel was only slightly worse than for the same cell operated with dry hydrogen.     

Rates of methane formation from carbon dioxide and hydrogen over a ruthenium catalyst

The Sabatier Reaction, 4H₂ + CO₂ 2H₂O + CH₄ + 43 kcal was examined experimentally at 1 atm using a 0.5% ruthenium on alumina catalyst. Data are given from 62 experimental runs made in a 4.15 cm³ isothermal reactor with from 0 to 85% conversion in the inlet feed gases and feed flow ratios (H₂ to CO₂) of 1.9 to 3.9, and temperatures from 400 to 680 °F. Empirical correlations, based on modified gas phase kinetics, were made to describe the reaction rate over the experimental range.     

A Study on Process Parameters of Direct Ethanol Fuel Cell

Ethanol is one of the promising future fuels of Direct Alcohol Fuel Cells (DAFC). The electro‐oxidation of ethanol fuel on anode made of carbon‐supported Pt‐Ru electrode catalysts was carried out in a lab scale direct ethanol fuel cell (DEFC). Cathode used was Pt‐black high surface area. The membrane electrode assembly (MEA) was prepared by sandwiching the solid polymer electrolyte membrane, prepared from Nafion^® (SE‐5112, DuPont USA) dispersion, between the anode and cathode. The DEFC was fabricated using the MEA and tested at different catalyst loadings at the electrodes, temperatures and ethanol concentrations. The maximum power density of DEFC for optimized value of ethanol concentration, catalyst loading and temperature were determined. The maximum open circuit voltage (OCV) of 0.815 V, short circuit current density (SCCD) of 27.90 mA/cm² and power density of 10.30 mW/cm² were obtained for anode (Pt‐Ru/C) and cathode (Pt‐black) loading of 1 mg/cm² at a temperature of 90°C anode and 60°C cathode for 2M ethanol.     

Bilayer Anodes for Improved Reformate Tolerance of PEM Fuel Cells

The concept of bilayer anodes for improved reformate tolerance was analysed using model calculations. It was found that for a bilayer anode to give good results the catalyst in the layer adjacent to the backing should enable CO oxidation at low potentials and have a relatively low rate of H₂ oxidation. The catalyst in the second layer should enable fast H₂ oxidation, and have limited CO adsorption. Experimental results are presented showing that the bilayer anode works especially well with reformate gas. It was found that the water gas shift equilibrium plays an important role in the mechanism for reformate tolerance. The implementation of the bilayer concept seems to be very useful in optimising both CO and CO₂ tolerance of PEMFCs, including at high fuel utilisation.     

Thermodynamics on Landfill Gas Reforming

Landfill gas is a type of methane‐rich biogas which supplies renewable resources for clean fuels production. In this paper, the characteristics and optimum conditions of simulated landfill gas and biogas reforming reactions for H₂ production are investigated. The temperature, varied from 373.15 K to 1273.15 K, and pressure, varied from 1.013 bar to 40.013 bar, applied for the reforming system are evaluated. In addition, the effect of steam concentration, traces of hydrocarbons, and the ratio of C/H/O are analyzed using thermodynamic theories. Both the calculation and analyzed results demonstrate that the reforming system is primarily comprised of endothermic reactions. It favors lower pressure and higher temperature. Traces of hydrocarbons would result in a slight increase to CO for this system. A high ratio of CO₂ would result in more production of CO in the reforming process. Preliminary experiments on fuel cells indicate this gas‐reforming simulation is an elementary theory for fuel supply.     

Mathematical modeling of solid oxide fuel cells at high fuel utilization based on diffusion equivalent circuit model

Mass transfer and electrochemical phenomena in the membrane electrode assembly (MEA) are the core components for modeling of solid‐oxide fuel cell (SOFC). The general MEA model is simply governed with the Stefan‐Maxwell equation for multicomponent gas diffusion, Ohm's law for the charge transfer and the current‐overpotential equation for the polarization calculation. However, it has obvious discrepancy at high‐fuel utilization or high‐current density. An advanced MEA model is introduced based on the diffusion equivalent circuit model. The main purpose is to correct the real‐gas concentrations at the triple‐phase boundary by assuming that the resistance of surface diffusion is in series with that of the gaseous bulk diffusion. Thus, it can obtain good prediction of cell performance in a wide range by avoiding the decrement of effective gas diffusivity via unreasonable increment of the electrode tortuosity in the general MEA model. The mathematical model has been validated in the cases of H₂? H₂O, CO? CO₂ and H₂? CO fuel system. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

CO tolerance on PtMo/C electrocatalysts prepared by the formic acid method

In this work the activity of PtMo/C based materials prepared by the formic acid method was evaluated as electrocatalysts for the hydrogen oxidation reaction in the presence of CO, in polymer electrolyte fuel cells. A very high electrocatalytic activity was observed for an anode formed by PtMo/C (60:40) and supplied with H₂ containing 100 ppm of CO, which presented an overpotential loss of 100 mV at 1 A cm⁻², compared with pure hydrogen. Several electrode configurations based on Mo/C and PtMo/C CO filtering layers, having a Pt-based catalyst layer, were evaluated. In all cases an enhancement of electrocatalytic performance was observed, as compared with the standard Pt/C electrode. It is concluded that the CO tolerance is achieved through an electrochemical surface reaction of adsorbed CO with surface oxides, as proposed by the bifunctional mechanism, acting together with a heterogeneous chemical reaction of CO with water molecules catalyzed by Mo species and resulting in a lowering of the CO concentration in the gas channels of the electrode.     

不同燃料SOFC的理论电池电动势及其性能

以NH₃以及3% H₂O增湿的H₂、CH₄、C₃H₈和煤炭地下气化(underground coal gasification,UCG)气为燃料,用最小Gibbs自由能法计算平衡气体组分和理论电池电动势,并测试在NiO-GDC‖GDC‖Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3-δ(B_0.9CFN)阳极支撑固体氧化物燃料电池(SOFC)中的电池开路电压、电池性能和长期稳定性。结果表明,以上述气体作燃料的SOFC热力学计算理论电动势均高于1.05 V,而由于GDC电解质在还原气氛下存在电子电导,导致碳氢燃料在NiO-GDC‖GDC‖B_0.9CFN阳极支撑电池中的开路电压略小。中低温下,碳氢燃料相对缓慢的动力学过程和GDC电解质快速的氧离子传输速率,使得以UCG气、CH₄和C₃H₈为燃料的电池实际积炭比理论预测少。以UCG气为燃料的SOFC在500、550、600和650℃的最高功率密度分别高达0.151、0.299、0.537和0.729 W·cm^-2,在0.6 V恒压放电120 h后性能没有明显衰减,且阳极表面无积炭产生,表明直接UCG气SOFC具有广阔的应用前景。     

Theoretical investigation of a water‐gas‐shift catalytic membrane for diesel reformate purification

The novel application of a catalytic water‐gas‐shift membrane reactor for selective removal of CO from H₂‐rich reformate mixtures for achieving gas purification solely via manipulation of reaction and diffusion phenomena, assuming Knudsen diffusion regime and the absence of hydrogen permselective materials, is described. An isothermal, two‐dimensional model is developed to describe a tube‐and‐shell membrane reactor supplied with a typical reformate mixture (9% CO, 3% CO₂, 28% H₂, and 15% H₂O) to the retentate volume and steam supplied to the permeate volume such that the overall H₂O:CO ratio within the system is 9:1. Simulations indicate that apparent CO:H₂ selectivities of 90:1 to >200:1 at H₂ recoveries of 20% to upwards of 40% may be achieved through appropriate design of the catalytic membrane and selection of operating conditions. Under these conditions, simulations predict an apparent hydrogen permeability of 2.3 × 10^?10 mol m^?1 Pa, which compares favorably against that of competing hydrogen‐permselective membranes. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4334–4345, 2013