Synergistic effects of Ni1−xCox-YSZ and Ni1−xCux-YSZ alloyed cermet SOFC anodes for oxidation of hydrogen and methane fuels containing H2S

Preparation and performance of bimetallic Ni_(1−x)Co_x-YSZ and Ni_(1−x)Cu_x-YSZ anodes were tested to overcome common deficiencies of carbon and sulfur poisoning in SOFCs. Ni_1−xCo_xO-YSZ and Ni_(1−x)Cu_xO-YSZ precursors were synthesized via co-precipitation of their respective chlorides. Single cell solid oxide fuel cells of these bimetallic anodes were tested in H₂, CH₄, and H₂S/CH₄ fuel mixtures. Addition of Cu²⁺ into the NiO lattice resulted in large metal particle sizes and decreased SOFC performance. Addition of Co²⁺ into the NiO lattice to form Ni_0.92Co_0.08O-YSZ anode precursor produced a cermet with a large BET surface area and active metal surface area, thus increasing the rate of hydrogen oxidation for this sample. The performance of both bimetallics was found to quickly degrade in dry CH₄ due to carbon deposition and lifting of the anode from the electrolyte. However, Ni_0.69Co_0.31-YSZ showed superior activity in a 10% (v/v) H₂S/CH₄ fuel mixture, surpassing performance with H₂ fuel, thereby demonstrating the exciting prospect of using sulfidated Ni_(1−x)Co_x-YSZ as SOFC anodes in sulfur containing methane streams. The active anode becomes a sulfidated alloy (Ni-Co-S) under operating conditions. This anode showed enhanced performance, which surpassed those of sulfidated Ni and Co anodes, thereby suggesting a synergistic behaviour in the Ni-Co-S anode.     

Performance and stability of composite nickel and molybdenum sulfide-based anodes for SOFC utilizing H2S

Performances of four anode compositions with different weight ratios of Ni_3±xS₂ to MoS₂ (2:1, 1:1, 1:2 and 1:4) were compared for H₂S oxidation in SOFC at 700–850 °C. Their thermal and chemical stability were determined using DSC/TGA, XRD, SEM and NAA. Electrochemical stability was investigated in fuel cell mode under OCV and constant overpotential conditions. It was shown that MoS₂ disappearance previously attributed to its volatility at temperature above 450 °C was instead related to its preliminary oxidation to MoO₃ in fuel cell mode as MoO₃ is highly volatile at temperatures above 600 °C. Suppression of volatility of MoO₃ by addition of Ni_3±xS₂ was shown by DSC/TGA analysis. Highest power density ca. 300 mW cm² at 850 °C was achieved with 1:1 weight ratio anode composition. All four compositions had unstable electrochemical performance which was more pronounced under polarization conditions than at OCV.     

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.     

Effect of H2S in methane/air flames on sulfur chemistry and products speciation

Reaction behavior of H₂S/O₂ under different equivalence ratios in methane/air flames is examined. Three equivalence ratios extending from fuel-lean (Φ = 0.5), stoichiometric (Φ = 1.0), to fuel-rich (Claus condition, Φ = 3.0) are examined. The results revealed that the presence of H₂S prevents hydrogen oxidation in the primary reaction zone, while in the secondary reaction zone oxidation competition occurs between H₂ and H₂S. In presence of oxygen, oxidation of hydrogen sulfide forms sulfur dioxide. However, under Claus conditions, the depletion of oxidant causes the direction of hydrogen sulfide reaction to shifts towards the formation of elemental sulfur. Higher hydrocarbons are formed in trace amounts under Claus conditions wherein sulfur dioxide acts as a coupling catalyst which enhances the dimerization of CH₃ radical to form higher series of hydrocarbons. Under Claus conditions, sulfur deposits are formed in low temperature regions of the reactor including the sampling line. The deposits are analyzed using X-ray powder diffractometer and were found to be cyclo-S₈ (α-sulfur) with orthorhombic crystal structure. The formation of α-sulfur is mainly due to the agglomeration of elemental sulfur (S₂) during its condensation at low temperatures.     

Effect of H2S on carbon-catalyzed methane decomposition and CO2 reforming reactions

The effect of H₂S on catalytic processing of methane is of a great practical importance. In this work, the effect of small quantities (0.5–1.0 vol.%) of H₂S present in the feedstock on the methane decomposition and CO₂ reforming reactions over carbon and metal based catalysts was investigated. Activated carbon (FY5), an in-house prepared alumina-supported Ni catalyst (NiA) and the mixture of both (FY5 + NiA) were used as catalysts in this study. It was found that CH₄ and CO₂ conversions were noticeably increased when H₂S was added to the reacting mixture, which points to (i) the tolerance of carbon catalyst to H₂S and (ii) the catalytic effect of H₂S on carbon-catalyzed decomposition and dry reforming of methane. In contrast, NiA catalyst and the mixture FY5 + NiA were deactivated in the presence of H₂S in both reactions. The effect of the heating system (i.e., conventional electric resistance vs microwave heating) on the products yield of the dry reforming reaction in the presence of H₂S is also discussed in this paper.     

Evidence for H2S gas as an intermediate species in the reaction mechanism of trapping hydrogen by cobalt disulfide

Cobalt sulfide prepared by aqueous precipitation using Na₂S and a Co(II) salt is known to trap hydrogen at room temperature and low pressure. The importance of oxidation of the primary CoS precipitate with atmospheric oxygen with respect to its efficiency as a hydrogen absorber is demonstrated. This stage of oxidation produces a mixture of two solid phases: a partially crystallized cobalt hydroxide Co(OH)₂ and an amorphous cobalt sulfide CoS₂ with a Co(OH)₂/CoS₂ molar ratio of 1 as predicted by thermodynamics. This biphasic product is probably the basic cobalt sulfide CoSOH considered in older and even more recent work. This product traps molecular hydrogen with a H₂/Co molar ratio of 0.5 whereas unoxidized CoS precipitate traps almost no hydrogen (H₂/Co = 0.025). Moderate acidic treatment of the absorber at room temperature leads to the selective dissolution of Co(OH)₂. The remaining cobalt sulfide has CoS₂ stoichiometry and reacts with hydrogen to form H₂S gas and CoS. We showed that H₂S released is reactive toward bases: CoS or Na₂S were formed when H₂S reacted with Co(OH)₂ or NaOH, respectively. This proves that the hydrogen trapping reaction mechanism implies H₂S as an intermediate species.     

Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels

The double perovskite Sr₂CoMoO_6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H₂ atmosphere with the introduction of a limited concentration of oxide-ion vacancies. Stoichiometric Sr₂CoMoO₆ has an antiferromagnetic Néel temperature T_N ≈ 37 K, but after reduction in H₂ at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H₂, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr₂CoMoO_6−δ a good mixed oxide-ion/electronic conductor (MIEC). With a 300-μm-thick La_0.8Sr_0.12Ga_0.83Mg_0.17O_2.815 (LSGM) as oxide-ion electrolyte and SrCo_0.8Fe_0.2O_3−δ as the cathode, the Sr₂CoMoO_6−δ anode gave a maximum power density of 1017 mW cm⁻² in H₂ and 634 mW cm⁻² in wet CH₄. A degradation of power in CH₄ was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).     

Synthesis and electrochemical behaviour of ceria-substitution LSCM as a possible symmetric solid oxide fuel cell electrode material exposed to H2 fuel containing H2S

12.5 wt.% ceria-substituted on the A-sites of La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) for La_0.75Sr_0.125 Ce_0.125Cr_0.5Mn_0.5O_3−δ (LSCCM) has been synthesized by the sol–gel process and evaluated as the electrode materials of symmetric solid oxide cells. The orthorhombic perovskite-type structure was demonstrated using X-ray diffraction (XRD) and part of cerium has been successfully doped to LSCM and presents two valence states (+3 and +4). In addition, the surface adsorption oxygen content increases due to ceria-doping using X-ray photoelectron spectroscopy (XPS). The measured electrical conductivity shows that the addition of ceria yields increase in total conductivity in air and humidified H₂. Electrochemical performance test of yttria-stabilized zirconia (YSZ) electrolyte-supported symmetric solid oxide fuel cell with the configuration of LSCCM|YSZ|LSCCM was performed, and shows peak power density of 33.12 mW cm⁻² at 1173 K when operating in wet 3% H₂–1% H₂S, far greater than the one of LSCM in the same test conditions.     

Mechanism for SOFC anode degradation from hydrogen sulfide exposure

Recent results on solid oxide fuel cells with Ni/YSZ and Ni/GDC anodes reveal a mechanism for permanent performance degradation due to hydrogen sulfide exposure. Our results confirm the temporary performance decline observed by others but also reveal a mechanism for the long term permanent degradation. We find that hydrogen sulfide leads to nickel migration and depletion in the anode, thereby compromising electrical conductivity and cell performance.     

Catalytic properties of monometallic copper and bimetallic copper-nickel systems combined with ceria and Ce-X (X = Gd,Tb) mixed oxides applicable as SOFC anodes for direct oxidation of methane

The present contribution analyses the possibilities of both Cu and bimetallic Cu-Ni formulations combined with CeO₂-based oxides for their use as anodes of solid-oxide fuel cells (SOFC) for direct oxidation of methane. The main objective is related to examining how the metals combination and the presence of dopants like Gd and Tb into the ceria structure could affect the catalytic activity of this type of materials towards reaction with methane. For this purpose, cermets of Cu alone as well as bimetallic Cu-Ni (with 20 and 40 wt.%) have been synthesised in combination with various supports composed by oxides of Ce, Ce-Tb and Ce-Gd. The behaviour of such systems towards interaction with dry methane up to 900 °C was analysed by means of CH₄-TPR tests. Appreciable differences in the catalytic activity are revealed as a function of the presence of nickel as well as Gd or Tb dopants in the systems. The characteristics of carbonaceous deposits formed upon such interaction was analysed by means of TPO and XPS.     

Analysis of the regenerative H2S poisoning mechanism in Ce0.8Sm0.2O2-coated Ni/YSZ anodes for intermediate temperature solid oxide fuel cells

Ceria is used as a sulfur sorbent due to its high affinity for sulfide at high temperatures. In addition, the ionic conductivity of ceria can be dramatically increased by doping with rare metals, including lanthanum, samarium, and gadolinium. Therefore, to enhance sulfur tolerance and improve anode performance, we modified an Ni-based anode with a thin layer coating of Sm_0.2Ce_0.8O_2−δ (SDC) on the pore wall surface of an Ni/YSZ anode. The anode-supported cells were tested with varying H₂S concentrations (0-100 ppm) at 600 and 700 °C. The cell performance was improved in the ceria- (by 20%) and in the SDC- (by 50%) modified anode by extending the additional TPB area in the anode. Under varying H₂S exposure, the polarization resistance was reduced by ceria and the SDC coating on the anode pore wall surface, which led to improved cell performance. A porous SDC layer on the Ni/YSZ anode pore wall acted as a sulfur sorbent as well as an additional TPB area. Otherwise, ceria mainly acted as a sulfur sorbent at high concentrations of H₂S (>60 ppm).     

Fabrication and characterization of Cu–Ni–YSZ SOFC anodes for direct use of methane via Cu-electroplating

The Cu–Ni–YSZ cermet anodes for direct use of methane in solid oxide fuel cells have been fabricated by electroplating Cu into a porous Ni–YSZ cermet anode. The uniform distribution of Cu in the Ni–YSZ anode was obtained by electroplating in an aqueous solution mixture of CuSO₄·5H₂O and H₂SO₄ for 30 min with 0.1 A of applied current. When the Cu–Ni–YSZ anode was exposed to methane at 700 °C, the amount of carbon deposited on the anode decreased as the amount of Cu in the Cu–Ni solid solution increased. The power density (0.24 W/cm²) of a single cell with a Cu–Ni–YSZ anode was slightly lower in methane at 700 °C than the power density (0.28 W/cm²) of a single cell with a Ni–YSZ anode. However, the performance of the Ni–YSZ anode-supported single cell degraded steeply over 21 h because of carbon deposition, whereas the Cu–Ni–YSZ anode-supported single cell showed enhanced durability up to 200 h.     

PdAgAu alloy with high resistance to corrosion by H2S

PdAgAu alloy films were prepared on porous stainless steel supports by sequential electroless deposition. Two specific compositions, Pd₈₃Ag₂Au₁₅ and Pd₇₄Ag₁₄Au₁₂, were studied for their sulfur tolerance. The alloys and a reference Pd foil were exposed to 1000H₂S/H₂ at 623 K for periods of 3 and 30 h. The microstructure, morphology and bulk composition of both non-exposed and H₂S-exposed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). XRD and SEM analysis revealed time-dependent growth of a bulk Pd₄S phase on the Pd foil during H₂S exposure. In contrast, the PdAgAu ternary alloys displayed the same FCC structure before and after H₂S exposure. In agreement with the XRD and SEM results, sulfur was not detected in the bulk of either ternary alloy samples by EDS, even after 30 h of H₂S exposure. X-ray photoelectron spectroscopy (XPS) depth profiles were acquired for both PdAgAu alloys after 3 and 30 h of exposure to characterize sulfur contamination near their surfaces. Very low S 2p and S 2s XPS signals were observed at the top-surfaces of the PdAgAu alloys, and those signals disappeared before the etch depth reached ∼10 nm, even for samples exposed to H₂S for 30 h. The depth profile analyses also revealed silver and gold segregation to the surface of the alloys; preferential location of Au on the alloys surface may be related to their resistance to bulk sulfide formation. In preliminary tests, a PdAgAu alloy membrane displayed higher initial H₂ permeability than a similarly prepared pure Pd sample and, consistent with resistance to bulk sulfide formation, lower permeability loss in H₂S than pure Pd.     

Interactions of nickel/zirconia solid oxide fuel cell anodes with coal gas containing arsenic

The performance of anode-supported and electrolyte-supported solid oxide fuel cells was investigated in synthetic coal gas containing 0-10 ppm arsenic at 700-800 °C. Arsenic was found to interact strongly with nickel, resulting in the formation of nickel-arsenic solid solution, Ni₅As₂ and Ni₁₁As₈, depending on temperature, arsenic concentration, and reaction time. For anode-supported cells, loss of electrical connectivity in the anode support was the principal mode of degradation, as nickel was converted to nickel arsenide phases that migrated to the surface to form large grains. Cell failure occurred well before the entire anode was converted to nickel arsenide, and followed a reciprocal square root of arsenic partial pressure dependence that is consistent with a diffusion-based rate-limiting step. Failure occurred more quickly with electrolyte-supported cells, which have a substantially smaller nickel inventory. For these cells, time to failure varied linearly with the reciprocal arsenic concentration. Failure occurred when arsenic reached the anode/electrolyte interface, though agglomeration of nickel reaction products may have also contributed. Test performed with nickel/zirconia coupons showed that arsenic was essentially completely captured in a narrow band near the fuel gas inlet. Arsenic concentrations of ∼10 ppb or less are estimated to result in acceptable rates of fuel cell degradation.     

Catalytic transformation of H2S for H2 production

Hydrogen sulfide (H₂S) gas is a by-product from natural gas refining, hydrodesulfurization of various fossil fuels, and syngas cleaning from pyrolysis and gasification. Catalytic pyrolysis of H₂S provides an alternative and effective pathway to recover both H₂ and sulfur. Catalysts from hydrotalcite of ZnAl, ZnNiAl, and ZnFeAl were employed for H₂S pyrolysis and compared with TiO₂ and MoS₂ at atmospheric pressure and temperatures in the range of 923–1123 K. Kinetic analysis was carried out in a packed bed reactor which revealed the effect of H₂S partial pressures to be of the order of 0.8–1 with respect to H₂S. The developed novel catalysts showed improved performance with significantly reduced activation energy compared to TiO₂ by 30 kJ/mol as well as higher H₂S conversion during pyrolysis (17% at 1173 K) than with MoS₂ catalyst, even at high H₂S partial pressure which is necessary for viable hydrogen production. The new approach showed an alternate economical and efficient pathway of catalyst design to obtain high activity and stability for simultaneous H₂ energy and pure sulfur recovery from unwanted H₂S resources.     

Effect of substitution with Cr and addition of Ni on the physical and electrochemical properties of Ce0.9Sr0.1VO3 as a H2S-active anode for solid oxide fuel cells

Whereas Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ is an active fuel cell anode catalyst for conversion of only the H₂S content of 0.5% H₂S-CH₄ at 850 °C, inclusion of 5 wt% NiO to form a composite catalyst enabled concurrent electrochemical conversion of CH₄. A fuel cell with a 0.3 mm thick YSZ membrane and Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ as anode catalyst had a maximum power density of 85 mW cm⁻² in 0.5% H₂S-CH₄ at 850 °C, arising only from the electro-oxidation of H₂S. Using a same thick membrane, promotion of the anode with 5 wt% NiO increased the total anode electro-oxidation activity to afford maximum power density of 100 mW cm⁻² in 0.5% H₂S-CH₄. The same membrane provided 30 mW cm⁻² in pure CH₄, showing that the incremental improvement arose substantially from CH₄ conversion. Performance of each anode was stable for over 12 h at maximum power output. XPS and XRD analyses showed that an increase in conductivity of Ce_0.9Sr_0.1Cr_0.5V_0.5O₃ in H₂S-containing environments resulted from a change in composition and structure from the tetragonal oxide to monoclinic Ce_0.9Sr_0.1Cr_0.5V_0.5(O,S)₃.     

An assessment of electrolytic hydrogen production from H2S in Black Sea waters

In the deeper parts of the Black Sea basin, water is anoxic. Hydrogen sulfide (H₂S) occurs naturally, and its concentration is nearly constant, around 9.5 mg/L at 1500 m depth. Its high solubility, and the existing chemical environment facilitate its accumulation and containment in the seawater, and its extraction poses a challenge.Possibility of hydrogen and sulfur production from H₂S contained in the waters of Black Sea is investigated conceptually. A multistage process is considered which involves extraction of seawater, adsorption of H₂S, electrochemical production of hydrogen and polysulfides; fresh water production by desalination of seawater and further hydrogen production from the resulting salty solution through chlorine-alkaline electrolysis. Some consideration is included regarding the economic and environmental aspects of the process.     

A comparative study of H2S poisoning on electrode behavior of Ni/YSZ and Ni/GDC anodes of solid oxide fuel cells

The effect of sulfur poisoning on the activity and performance of Ni/Y₂O₃–ZrO₂ (Ni/YSZ) and Ni/Gd₂O₃–CeO₂ (Ni/GDC) cermet anodes of solid oxide fuel cells has been examined by polarization and electrochemical impedance spectroscopy (EIS) measurements at 800 °C. The anodes are alternately polarized in pure H₂ and H₂S-containing H₂ fuels with H₂S concentration gradually increased from 5 ppm to 700 ppm at 200 mA cm⁻² for 2 h. The results show that the anode potential of Ni/YSZ electrodes measured in pure H₂ decreases from 0.61 V to 0.34 V after exposure to H₂S-containing H₂ fuels with H₂S concentration increased from 5 to 700 ppm. On the other hand, the anode potential of Ni/GDC electrodes measured in pure H₂ decreases from 0.78 V to 0.72 V under identical test conditions. The degradation in performance for the hydrogen oxidation in H₂S-containing H₂ fuels is substantially smaller on Ni/GDC anodes, as compared to that on Ni/YSZ anodes. Similar trend is also observed for the change of the electrode polarization resistance for the hydrogen oxidation reaction on the Ni/YSZ and Ni/GDC anodes after exposure to H₂S-containing H₂ fuels. The SEM results indicate the structure modification of Ni/YSZ anodes only occurs on Ni particles, and in the case of Ni/GDC anodes, structural modification on both Ni and GDC phases occurs. The mixed ionic and electronic conductivity of GDC phase could be the primary reason for the high sulfur tolerance of the Ni/GDC cermet anodes.     

Catalytic steam reforming of methane,methanol, and ethanol over Ni/YSZ: The possible use of these fuels in internal reforming SOFC

This study investigated the possible use of methane, methanol, and ethanol with steam as a direct feed to Ni/YSZ anode of a direct internal reforming Solid Oxide Fuel Cell (DIR-SOFC). It was found that methane with appropriate steam content can be directly fed to Ni/YSZ anode without the problem of carbon formation, while methanol can also be introduced at a temperature as high as 1000 °C. In contrast, ethanol cannot be used as the direct fuel for DIR-SOFC operation even at high steam content and high operating temperature due to the easy degradation of Ni/YSZ by carbon deposition. From the steam reforming of ethanol over Ni/YSZ, significant amounts of ethane and ethylene were present in the product gas due to the incomplete reforming of ethanol. These formations are the major reason for the high rate of carbon formation as these components act as very strong promoters for carbon formation.     

Structural, catalytic/redox and electrical characterization of systems combining Cu-Fe with CeO2 or Ce1−xMxO2−δ (M = Gd or Tb) for direct methane oxidation

The present work analyses bimetallic Cu-Fe formulations combined with CeO₂ or other structurally related mixed oxides resulting from doping of the former with Gd or Tb, focusing to its possible use as anodes of solid oxide fuel cells (SOFC) for direct oxidation of methane. The main objective is the characterization of the various formulations at structural level as well as with regard to redox changes taking place in the systems upon interaction with methane, in order to evaluate the effects induced by the presence of the mentioned dopants. In the same sense, an analysis of thermal expansion and electrical properties of the systems as well as their chemical compatibilities with several electrolytic materials is performed, considering its possible implantation in SOFC single cells. For the mentioned purposes, the systems have been analysed by means of CH₄-TPR tests subsequently followed by TPO tests, as well as by XRD, Raman and XPS, with the aim of exploring structural and redox changes produced in the systems and the formation of carbon deposits during interaction with methane. The results reveal significant modifications in the structural, catalytic/redox and electrical properties of the systems as a function of the presence of Fe and/or Gd and Tb dopants in the formulation.