Y-doped SrTiO3 based sulfur tolerant anode for solid oxide fuel cells

A solid oxide fuel cell (SOFC) anode with high sulfur tolerance was developed starting from a Y-doped SrTiO₃ (SYTO)-yttria stabilized zirconia (YSZ) porous electrode backbone, and infiltrated with nano-sized catalytic ceria and Ru. The size of the infiltrated particles on the SYTO-YSZ pore walls was 30–200 nm, and both infiltrated materials improved the performance of the SYTO-YSZ anode significantly. The infiltrated ceria covered most of the surface of the SYTO-YSZ pore walls, while Ru was dispersed as individual nano-particles. The performance and sulfur tolerance of a cathode supported cell with ceria- and Ru-infiltrated SYTO-YSZ anode was examined in humidified H₂ mixed with H₂S. The anode showed high sulfur tolerance in 10–40 ppm H₂S, and the cell exhibited a constant maximum power density 470 mW cm⁻² at 10 ppm H₂S, at 1073 K. At an applied current density 0.5 A cm⁻², the addition of 10 ppm H₂S to the H₂ fuel dropped the cell voltage slightly, from 0.79 to 0.78 V, but completely recovered quickly after the H₂S was stopped. The ceria- and Ru-infiltrated SYTO-YSZ anode showed much higher sulfur tolerance than conventional Ni-YSZ anodes.     

Durability of solid oxide fuel cells using sulfur containing fuels

The usability of hydrogen and also carbon containing fuels is one of the important advantages of solid oxide fuel cells (SOFCs), which opens the possibility to use fuels derived from conventional sources such as natural gas and from renewable sources such as biogas. Impurities like sulfur compounds are critical in this respect. State-of-the-art Ni/YSZ SOFC anodes suffer from being rather sensitive towards sulfur impurities. In the current study, anode supported SOFCs with Ni/YSZ or Ni/ScYSZ anodes were exposed to H₂S in the ppm range both for short periods of 24 h and for a few hundred hours. In a fuel containing significant shares of methane, the reforming activities of the Ni/YSZ and Ni/ScYSZ anodes were severely poisoned already at low H₂S concentrations of ∼2 ppm H₂S. The poisoning effect on the cell voltage was reversible only to a certain degree after exposure of 500 h in the state-of-the-art cell, due to a loss of percolation of Ni particles in the Ni/YSZ anode layers closest to the electrolyte. Using SOFCs with Ni/ScYSZ anodes improved the H₂S tolerance considerably, even at larger H₂S concentrations of 10 and 20 ppm over a few hundred hours.     

Glass fiber entrapped sorbent for reformates desulfurization for logistic PEM fuel cell power systems

Glass fiber entrapped ZnO/SiO₂ sorbent (GFES) was developed to remove sulfur species (mainly hydrogen sulfide, H₂S) from reformates for logistic PEM fuel cell power systems. Due to the use of microfibrous media and nanosized ZnO grains on highly porous SiO₂ support, GFES demonstrated excellent desulfurization performance and potential to miniaturize the desulfurization reactors. In the thin bed test, GFES (2.5 mm bed thickness) attained a breakthrough time of 540 min with up to 75% ZnO utilization at 1 ppm breakthrough. At equivalent ZnO loading, GFES yielded a breakthrough time twice as long as the ZnO/SiO₂ sorbent; at equivalent bed volume, GFES provided a three times longer breakthrough time (with 67% reduction in ZnO loading) than packed beds of 1–2 mm commercial extrudates. GFES is highly regenerable compared with the commercial extrudates, and can easily be regenerated in situ in air at 500 °C. During 50 regeneration/desulfurization cycles, GFES maintained its desulfurization performance and structural integrity. A composite bed consisting of a packed bed of large extrudates followed by a polishing layer of GFES demonstrated a great extension in gas life and overall bed utilization. This approach synergistically combines the high volume loading of packed beds with the overall contacting efficiency of small particulates.     

Self-sustained operation of a kWe-class kerosene-reforming processor for solid oxide fuel cells

In this paper, fuel-processing technologies are developed for application in residential power generation (RPG) in solid oxide fuel cells (SOFCs). Kerosene is selected as the fuel because of its high hydrogen density and because of the established infrastructure that already exists in South Korea. A kerosene fuel processor with two different reaction stages, autothermal reforming (ATR) and adsorptive desulfurization reactions, is developed for SOFC operations. ATR is suited to the reforming of liquid hydrocarbon fuels because oxygen-aided reactions can break the aromatics in the fuel and steam can suppress carbon deposition during the reforming reaction. ATR can also be implemented as a self-sustaining reactor due to the exothermicity of the reaction. The kW_e self-sustained kerosene fuel processor, including the desulfurizer, operates for about 250 h in this study. This fuel processor does not require a heat exchanger between the ATR reactor and the desulfurizer or electric equipment for heat supply and fuel or water vaporization because a suitable temperature of the ATR reformate is reached for H₂S adsorption on the ZnO catalyst beds in desulfurizer. Although the CH₄ concentration in the reformate gas of the fuel processor is higher due to the lower temperature of ATR tail gas, SOFCs can directly use CH₄ as a fuel with the addition of sufficient steam feeds (H₂O/CH₄ ≥ 1.5), in contrast to low-temperature fuel cells. The reforming efficiency of the fuel processor is about 60%, and the desulfurizer removed H₂S to a sufficient level to allow for the operation of SOFCs.     

Synthesis and characterization of new ternary transition metal sulfide anodes for H2S-powered solid oxide fuel cell

A number of ternary transition metal sulfides with general composition AB₂S₄ (where A and B are different transition metal atoms) have been prepared and investigated as potential anode catalysts for use in H₂S-powered solid oxide fuel cells (SOFCs). For the initial screening, polarization resistance of the materials was measured in a two electrode symmetrical cell at 700–850 °C. Vanadium-based materials showed the lowest polarization resistance, and so were chosen for subsequent full cell tests using the configuration [H₂S, AV₂S₄/YSZ/Pt, air] (where A = Ni, Cr, Mo). MoV₂S₄ anode had superior activity and performance in the full cell setup, consistent with results from symmetrical cell tests. Polarization curves showed MoV₂S₄ had the lowest potential drop, with up to a 200 mA cm⁻² current density at 800 °C. The highest power density of ca. 275 mW cm⁻² at 800 °C was obtained with a pure H₂S stream. Polarization resistance of materials was a strong function of current density, and showed a sharp change of slope attributable to a change in the rate-limiting step of the anode reaction mechanism. MoV₂S₄ was chemically stable during prolonged (10 days) exposure to H₂S at 850 °C, and fuel cell performance was stable during continuous 3-day operation at 370 mA cm⁻² current density.     

Autothermal reforming of JP8 on a Pt/Rh catalyst: Catalyst durability studies and effects of sulfur

Autothermal reforming (ATR) of commercial grade JP8 was performed on a Pt/Rh catalyst deposited on a monolith. This study investigated catalyst performance under three test conditions: (i) 120 startup and shutdown cycles, (ii) 80 h of continuous operation with sulfur-free fuel, and (iii) 370 h of testing with JP8 containing 125 ppm of sulfur. Axial reactor temperature profiles and gas composition data showed that startup and shutdown cycling had no impact on catalyst performance. When durability testing was done with fuel containing 125 ppm of sulfur, the catalyst deactivated initially, which was reflected by a decrease in H₂ concentration and decrease in fuel conversion. However, after 250 h of operation the activity stabilized at 66% fuel conversion and product concentrations were constant for the remaining 120 h of testing. The presence of sulfur resulted in higher CO selectivity, lower H₂ concentrations, and lower fuel conversions compared to data with sulfur-free fuel. The data suggests that the presence of sulfur primarily affects steam reforming reactions, and CO oxidation. Regeneration was attempted with air and with fuel-lean combustion but initial H₂ yields and carbon selectivity were not achieved.     

The effect of H2S on the performance of Ni-YSZ anodes in solid oxide fuel cells

Biomass-derived fuel, e.g. biogas, is a potential fuel for solid oxide fuel cells (SOFCs). At operating temperature (∼850 °C) reforming of the carbon-containing biogas takes place over the Ni-containing anode. However, impurities in the biogas, e.g. H₂S, can poison both the reforming and the electrochemical activity of the anode.Tests of single anode-supported planar SOFCs were carried out in the presence of H₂S under current load at 850 °C. The cell voltage dropped as we periodically added 2-100 ppm H₂S to an H₂-containing fuel in 24 h intervals, but it regenerated to the initial value after we turned off the H₂S. Evaluation of the changes of the cell voltage suggests that saturation coverage was reached at approximately 40 ppm H₂S. A front-like movement of S-poisoning over the anode was seen by monitoring the in-plane voltage in the anode. Furthermore, impedance spectra showed that mainly the polarization resistance increased when adding H₂S. These changes in resistance were found to happen at 1212 Hz, which is related to reactions at the anode-electrolyte interface. These findings can be used to identify S-related effects on the performance, when an SOFC is fuelled with biogas or other fuels with H₂S impurities and thus help in the development of more sulfur tolerant SOFCs.     

Deactivation of Pt/VC proton exchange membrane fuel cell cathodes by SO2, H2S and COS

Sulfur contaminants in air pose a threat to the successful operation of proton exchange membrane fuel cells (PEMFCs) via poisoning of the Pt-based cathodes. The deactivation behavior of commercial Pt on Vulcan carbon (Pt/VC) membrane electrode assemblies (MEAs) is determined when exposed to 1 ppm (dry) of SO₂, H₂S, or COS in air for 3, 12, and 24 h while held at a constant potential of 0.6 V. All the three sulfur compounds cause the same deactivation behavior in the fuel cell cathodes, and the polarization curves of the poisoned MEAs have the same decrease in performance. Sulfur coverages after multiple exposure times (3, 12, and 24 h) are determined by cyclic voltammetry (CV). As the exposure time to sulfur contaminants increases from 12 to 24 h, the sulfur coverage of the platinum saturates at 0.45. The sulfur is removed from the cathodes and their activity is partially restored both by cyclic voltammetry, as shown by others, and by successive polarization curves. Complete recovery of fuel cell performance is not achieved with either technique, suggesting that sulfur species permanently affect the surface of the catalyst.     

Tolerance tests of H2S-laden biogas fuel on solid oxide fuel cells

Biogas is a variable mixture of methane, carbon dioxide and other gases. It is a renewable resource which comes from numerous sources of plant and animal matter. Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can directly use clean synthesized biogas as fuel. However, trace impurities, such as H₂S, Cl₂ and F₂ in real biogas can cause degradation in cell performance. In this research, both uncoated and coated Ni-YSZ anode-supported cells were exposed to three different compositions of synthesized biogases (syn-biogas) with 20 ppm H₂S under a constant current load at 750-850 °C and their performance was evaluated periodically using standard electrochemical methods. Postmortem analysis of the SOFC anode was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that H₂S causes severe electrochemical degradation of the cell when operating on biogas, leading to both complete electrochemical and mechanical failure. The Ni-CeO₂ coated cell showed excellent stability during CH₄ reforming and some tolerance to H₂S contamination.     

The effects of hydrogen sulfide on the polymer electrolyte membrane fuel cell anode catalyst: H2S-Pt/C interaction products

The performance of a polymer electrolyte membrane fuel cell (PEMFC) operating on a simulated hydrocarbon reformate is described. The anode feed stream consisted of 80% H₂, ∼20% N₂, and 8 ppm hydrogen sulfide (H₂S). Cell performance losses are calculated by evaluating cell potential reduction due to H₂S contamination through lifetime tests. It is found that potential, or power, loss under this condition is a result of platinum surface contamination with elemental sulfur. Electrochemical mass spectroscopy (EMS) and electrochemical techniques are employed, in order to show that elemental sulfur is adsorbed onto platinum, and that sulfur dioxide is one of the oxidation products. Moreover, it is demonstrated that a possible approach for mitigating H₂S poisoning on the PEMFC anode catalyst is to inject low levels of air into the H₂S-contaminated anode feeding stream.     

Sr2Fe4/3Mo2/3O6 as anodes for solid oxide fuel cells

Sr₂Fe_4/3Mo_2/3O₆ has been synthesized by a combustion method in air. It shows a single cubic perovskite structure after being reduced in wet H₂ at 800 °C and demonstrates a metallic conducting behavior in reducing atmospheres at mediate temperatures. Its conductivity value at 800 °C in wet H₂ (3% H₂O) is about 16 S cm⁻¹. This material exhibits remarkable electrochemical activity and stability in H₂. Without a ceria interlayer, maximum power density (P_max) of 547 mW cm⁻² is achieved at 800 °C with wet H₂ (3% H₂O) as fuel and ambient air as oxidant in the single cell with the configuration of Sr₂Fe_4/3Mo_2/3O₆|La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ (LSGM)| La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF). The P_max even increases to 595 mW cm⁻² when the cell is operated at a constant current load at 800 °C for additional 15 h. This anode material also shows carbon resistance and sulfur tolerance. The P_max is about 130 mW cm⁻² in wet CH₄ (3% H₂O) and 472 mW cm⁻² in H₂ with 100 ppm H₂S. The cell performance can be effectively recovered after changing the fuel gas back to H₂.     

Fuel-flexible operation of a solid oxide fuel cell with Sr0.8La0.2TiO3 support

Solid oxide fuel cells with Sr_0.8La_0.2TiO₃ anode-side supports, Ni- Sm-doped ceria adhesion layer, Ni- Y₂O₃-stabilized ZrO₂ (YSZ) anode active layer, YSZ electrolyte, and La_0.8Sr_0.2MnO₃(LSM)–YSZ cathode are described. These cells are stable in simulated natural gas at current densities as low as 0.2 A cm⁻². This represents much-improved stability against coking in natural gas, compared with conventional Ni–YSZ anode-supported SOFCs which rapidly coke, even at higher current densities. Cell operation in H₂ fuel with 50–100 ppm, H₂S results in an initial decrease in cell power density, but no long-term degradation occurs and full recovery to the initial performance level is observed after dry H₂ fuel flow is restored. Degradation is not observed during or after seven redox cycles between H₂ and air.     

Sulfur behavior in chemical looping combustion with NiO/Al2O3 oxygen carrier

Chemical looping combustion (CLC) is a novel technology where CO₂ is inherently separated during combustion. Due to the existence of sulfur contaminants in the fossil fuels, the gaseous products of sulfur species and the interaction of sulfur contaminants with oxygen carrier are a big concern in the CLC practice. The reactivity of NiO/Al₂O₃ oxygen carrier reduction with a gas mixture of CO/H₂ and H₂S is investigated by means of a thermogravimetric analyzer (TGA) and Fourier Transform Infrared spectrum analyzer in this study. An X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to evaluate the phase characterization of reacted oxygen carrier, and the formation mechanisms of the gaseous products of sulfur species are elucidated in the process of chemical looping combustion with a gaseous fuel containing hydrogen sulfide. The results show that the rate of NiO reduction with H₂S is higher than the one with CO. There are only Ni and Ni₃S₂ phases of nickel species in the fully reduced oxygen carrier, and no evidence for the existence of NiS or NiS₂. The formation of Ni₃S₂ is completely reversible during the process of oxygen carrier redox. A liquid phase sintering on the external surface of reduced oxygen carriers is mainly attributed to the production of the low melting of Ni₃S₂ in the nickel-based oxygen carrier reduction with a gaseous fuel containing H₂S. Due to the sintering of metallic nickel grains on the external surface of the reduced oxygen carrier, further reaction of the oxygen carrier with H₂S is constrained, and there is no increase of the sulfidation index of the reduced oxygen carrier with the cyclical reduction number. Also, a continuous operation with a syngas of carbon monoxide and hydrogen containing H₂S is carried out in a 1 kW_th CLC prototype based on the nickel-based oxygen carrier, and the effect of the fuel reactor temperature on the release of gaseous products of sulfur species is investigated.     

The study of Au/MoS2 anode catalyst for solid oxide fuel cell (SOFC) using H2S-containing syngas fuel

Au/MoS₂ is a promising anode catalyst for conversion of all components of H₂S-containing syngas in solid oxide fuel cell (SOFC). MoS₂-supported nano-Au particles have catalytic activity for conversion of CO when syngas is used as fuel in SOFC systems, thus preventing poisoning of MoS₂ active sites by CO. In contrast to use of MoS₂ as anode catalyst, performance of Au/MoS₂ anode catalyst improves when CO is present in the feed. Current density over 600 mA cm⁻² and maximum power density over 70 mW cm⁻² were obtained at 900 °C, showing that Au/MoS₂ could be potentially used as sulfur-tolerant catalyst in fuel cell applications.     

Exploring the effects of symmetrical and asymmetrical relative humidity on the performance of H2/air PEM fuel cell at different temperatures

This article is dedicated to study the interlinked effects of symmetric relative humidity (RH), and asymmetric RH on the performance of H₂/air PEM fuel cell at different temperatures. The symmetric and asymmetric RH were achieved by setting the cathode relative humidity (RHC) and anode relative humidity (RHA) as equal and unequal values, respectively. The cell performance was evaluated by collecting polarization curves of the cell at different RH, RHC and RHA and at different cell temperatures (T_cell). The polarization curves along with the measured internal cell resistance (membrane resistance) were discussed in the light of the present fuel cell theory. The results showed that symmetric relative humidity has different impacts depending on the cell temperature. While at RH of 35% the cell can show considerable performance at T_cell = 70 °C, it is not so at T_cell = 90 °C. At T_cell = 70 °C, the cell potential increases with RH at lower and medium current densities but decreases with RH at higher currents. This was attributed to the different controlling processes at higher and lower current densities. This trend at 70 °C is completely destroyed at 90 °C. Operating our PEM fuel cell at dry H₂ gas conditions (RHA = 0%) is not detrimental as operating the cell at dry Air (O₂) conditions (RHC = 0%). At RHA = 0% and humidified air, water transport by back diffusion from the cathode to the anode at the employed experimental conditions can support reasonable rehydration of the membrane and catalysts. At RHA = 0, a possible minimum RHC for considerable cell operation is temperature dependent. At RHC = 0 conditions, the cell can operate only at RHA = 100% with a loss that depends on T_cell. It was found that the internal cell resistance depends on RH, RHA, RHC and T_cell and it is consistent with the observed cell performance.     

The effect of H2S and CO mixtures on PEMFC performance

Proton exchange membrane fuel cells (PEMFCs) most likely will use reformed fuel as the primary source for the anode feed which always contains carbon dioxide (CO) and hydrogen sulfide (H₂S). Trace amount of CO and H₂S can cause considerable cell performance losses. A comparison between the effect of CO and that of H₂S on PEMFC performance was made in this paper. Under the same conditions, the H₂S poisoning rate is much higher than CO because of different adsorption intensity. When the fuel stream contains the gas mixture (25 ppm CO and 25 ppm H₂S), the fuel cell performance deteriorates more quickly than 50 ppm CO but slowly than 50 ppm H₂S and can be only partially recovered by reintroducing neat H₂. The resulting effects of the mixtures can be divided into two parts roughly: during the inception phase, the cell voltage drops quickly and the actual values of anode overvoltage are bigger than the corresponding calculated values; then the deterioration rate of the cell performance decreases gradually.     

Chemical reactor network modeling of a microwave plasma thermal decomposition of H2S into hydrogen and sulfur

The conventional treatment method for H₂S is the Claus process, which produces sulfur and water. This results in a loss of the valuable potential product hydrogen. H₂S treatment would be more economically valuable if both hydrogen and sulfur products could be recovered. Based on standard heats of formation analysis, the theoretical energy required to produce hydrogen from H₂S dissociation is only 20.6 kJ/mol of H₂ as compared to 63.2 kJ/mol of H₂ from steam methane reforming and 285.8 kJ/mol of H₂ from water electrolysis. Among the many thermal decomposition methods that have been explored in the literature, Micro-wave plasma dissociation of H₂S prevails as the method of choice to attain the best conversion and energy efficiency. Chemical kinetics simulations using an ideal flow reactor network have been carried out on the CHEMKIN-PRO software package and they support these findings. The reactor network simulates the thermal plasma behavior in the plasma torch, the plasma reactor, and the sulfur condenser. Two chemical kinetics mechanisms have been used and the results show an almost complete conversion of H₂S into hydrogen and sulfur in the plasma reactor at an optimum temperature of about 2400 K at atmospheric pressure. While the most challenging task of the process is found to be the plasma cooling rate at the sulfur condenser where very fast quenching is required to conserve the hydrogen product from converting back to H₂S.     

Effect of Al2O3 content on the adsorptive properties of Cu/ZnO/Al2O3 for removal of odorant sulfur compounds

Cu/ZnO/Al₂O₃ adsorbents for removal of odorant sulfur compounds were prepared with various Al/Cu molar ratios by co-precipitation method. The sulfur removing ability as a function of Al/Cu molar ratio of the adsorbents for t-butyl mercaptan (TBM), tetrahydro thiophene (THT), dimethyl disulfide (DMS) and H₂S were investigated at 250 °C and 6000 h⁻¹ space velocity. Based on the results of adsorption capacity and characterization by various techniques, the optimum Al/Cu ratio for maximum sulfur removal capacity is found to be at Al/Cu molar ratio of 0.15 which possesses the well-dispersed Cu species with high reducibility. The adsorption capacity is highest for H₂S followed by TBM, DMS and THT. The main role of Al₂O₃ component is to provide the dispersion of CuO species homogeneously with small particle formation and high reducibility.     

Effect of H2O2 on Nafion properties and conductivity at fuel cell conditions

During PEM fuel cell operation, formation of H₂O₂ and material corrosion occurs, generating trace amounts of metal cations (i.e., Fe²⁺, Pt²⁺) and subsequently initiating the deterioration of cell components and, in particular, PFSA membranes (e.g., Nafion). However, most previous studies of this have been performed using conditions not relevant to fuel cell environments, and very few investigations have studied the effect of Nafion decomposition on conductivity, one of the most crucial factors governing PEMFC performance. In this study, a quantitative examination of properties and conductivities of degraded Nafion membranes at conditions relevant to fuel cell environments (30-100%RH and 80 °C) was performed. Nafion membranes were pre-ion-exchanged with small amounts of Fe²⁺ ions prior to H₂O₂ exposure. The degradation degree (defined as loss of ion-exchange capacity, weight, and fluoride content), water uptake, and conductivity of H₂O₂-exposed membranes were found to strongly depend on Fe content and H₂O₂ treatment time. SEM cross-sections showed that the degradation initially took place in the center of the membrane, while FTIR analysis revealed that Nafion degradation preferentially proceeds at the sulfonic end group and at the ether linkage located in the pendant side chain and that the H-bond of water is weakened after prolonged H₂O₂ exposure.     

Structural modifications to nickel cermet anodes in fuel cell environments

Restructuring of Ni in cermet anodes of solid oxide fuel cells (SOFCs) has been studied using both bulk fuel cells and thin foil anodes. The bulk cells were button cells (23 mm in diameter) with cermet anodes (30-70 μm thick) made up of nickel and gadolinium-doped ceria (Ni/CGO). The cells were operated (under current load) at 700 °C in moist H₂ or moist H₂ with low levels of H₂S. Scanning electron microscopy (SEM) was used to characterize the microstructure before and after testing. The thin foil samples (100-150 nm thick) were cermets of nickel and yttria doped zirconia (Ni/YSZ) and these were exposed (without current load) at 700 °C to dry H₂, moist H₂ or moist H₂ with H₂S (1 ppm). Transmission electron microscopy (TEM) and SEM were used to analyze the microstructural changes in these samples. The anodes from the bulk cells exhibited terracing of Ni grains in all instances, with the extent of terracing increasing with exposure to H₂S, and with increasing H₂S levels and exposure time. The thin foil anodes showed much more extensive Ni restructuring leading to agglomeration and faceting of Ni grains. This was accompanied by debonding from YSZ, commencing at triple points, where some combination of three Ni/YSZ grains meet. The amount of restructuring increased with increasing H₂ concentration in the gas, and was accelerated by the presence of H₂S and/or H₂O. Evidence is presented that indicates that terracing may represent the early stages of Ni agglomeration.