Degradation of Ni-Zr0.9Sc0.1O1.95 anode in H2 + H2O at low temperature: Influence of nickel surface charge

The present paper reports on the data of the Ni-Zr_0.9Sc_0.1O_1.95 anode polarization resistance long-term test (1500 h) at 600 °С in the wet hydrogen and 30% H₂ + 70% H₂O. The results are presented for two type of anodes: initial and impregnated with ceria. The fast degradation of both types of anodes in 30% H₂ + 70% H₂O was observed. After the long-term tests in 30% H₂ + 70% H₂O at 600 °C, heating and exposure at 900 °C in wet hydrogen leads to the restore of anode performance. At the termination of the 1500 h test, the area polarization specific resistance of Ni-Zr_0.9Sc_0.1O_1.95 remained almost unchanged as compared to the initial value, whereas for impregnated anodes the polarization resistance increased three times. The observed phenomena were explained by the OH^? ions adsorption at the positively charged nickel surface. During the long-term tests at 600 °С the electrode microstructure did not change and the significant sintering of highly disperse ceria was not observed.     

Reactivity of Ni,Cr and Zr doped ceria in CO2 splitting for CO production via two-step thermochemical cycle

This work focuses on modification and screening of ceria-based oxides for solar H₂O/CO₂ splitting via two-step thermochemical cycle. Ce_1-xM_xO_2-δ (M = Zr, Ni, Cr; x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized via sol-gel method and tested for CO₂-splitting via two-step thermochemical cycles. Reduction was conducted at 1500 °C through a ramp rate of 10 °C/min and oxidation was performed at 1000 °C isothermally. Both Ni and Cr showed low solubility in ceria and no or very limited promoting effect on CO productivity. Cr could be reduced in the first reduction step but cannot be oxidized by CO₂ in the following oxidation step. Zr doped sample showed advantages in both CO productivity and lattice stability. 15% Zr doped exhibited the best performance with the CO productivity of 315.40 μmol/g. However, the oxidation rate of Zr doped samples was much lower than that of pure ceria. Compromise between fuel productivity and fast kinetics should be made in practical application.     

Electricity production from lignocellulosic biomass by direct coupling of a gasifier and a Nickel/Yttria-stabilized Zirconia-based solid oxide fuel cell. Part 1: From gas production to direct electricity production

This work presents the direct coupling of a gasification pilot according to the patented concept by S3D Company and of a Ni-YSZ-based SOFC. The composition of gas issued from the gasifier is rather stable, with H₂ ≈ 15%, CO ≈ 15%, CH₄ ≈ 1%, CO₂ ≈ 20% and N₂ ≈ 49%. Before injecting directly the gas on the cell, a preliminary test of the home-made cell consisting of an industrial Fiaxell Nickel/Yttria-stabilized Zirconia-based anode-electrolyte assembly covered by praseodymium nickelate oxide was performed with H₂N₂ mixtures. The cell is tested at 750 °C and 850 °C, with a maximum power density of 1.4 W cm^?2 at 850 °C when fueled with a 76%–24% H₂Ar mixture. The effect of the dilution of the gas is also studied, and validates the use of a bag containing the gas issued from the gasifier. In these conditions, using exclusively the fuel issued from biomass, without any additional purification steps, maximum power densities of 340 mW cm^?2 and 113 mW cm^?2 can be obtained at 850 °C and 750 °C respectively.     

High activity oxide Pr0.3Sr0.7Ti0.3Fe0.7O3−δ as cathode of SOEC for direct high-temperature steam electrolysis

A high activity ferrite Pr_0.3Sr_0.7Ti_0.3Fe_0.7O_3?δ (PSTF) has been synthesized and examined as a cathode of solid oxide electrolysis cell (SOEC) for direct high-temperature steam electrolysis. The SOEC with a configuration of PSTF|YSZ|LSM-YSZ was operated under H₂O concentrations ranging from 20%H₂O/Ar to 60%H₂O/Ar and exhibited excellent electrochemical performances. Polarization resistance of the electrolyzer was as small as 0.43 Ω cm² in 60%H₂O/Ar at 1.85 V at 800 °C. According to AC impendence spectra analyzing, gas diffusion process was the rate-determine-step (RDS) under smaller current density, while under larger current density, transport properties in the electrodes and the interfaces of electrode/electrolyte was RDS. The electrochemical properties of PSTF cathodes were systematically investigated and compared when they were exposed to gas atmosphere with and without safe gas (H₂). The obtained results demonstrated that PSTF electrode could conceivably avoid any hydrogen feeding for steam electrolysis.     

23,000 h steam electrolysis with an electrolyte supported solid oxide cell

An electrolyte supported solid oxide cell of 45 cm² area was operated in the steam-electrolysis mode during more than 23,000 h before scheduled shutdown, of which 20,000 h with a current density of j = ?0.9 A cm^?2. The cell consisted of a scandia/ceria doped zirconia electrolyte (6Sc1CeSZ), CGO diffusion-barrier/adhesion layers between electrolyte and electrodes, a lanthanum strontium cobaltite ferrite (LSCF) oxygen electrode, and a nickel/gadolinia-doped ceria (Ni/GDC) steam/hydrogen electrode. Voltage degradation in the operation period with j = ?0.9 A cm^?2 was 7.4 mV/1000 h (0.57%/1000 h) and the increase in the area specific resistance 8 mΩ cm²/1000 h. The final cell voltage was 1.33 V (at 851 °C cell temperature). After dismantling, the cell showed no mechanical damage at electrolyte and H₂/H₂O electrode; a small fraction of the oxygen electrode was delaminated. Impedance spectroscopy applied at the steady state DC current density confirmed a degradation dominated by an increasing ohmic term, mainly due to ionic conductivity decay in the electrolyte. In addition, a small non-ohmic and at least partly reversible O₂ electrode contribution to degradation was identified, affected by a pollution from the (compressor) purge air.     

Electrochemical investigations of cobalt-free perovskite cathode material for intermediate temperature solid oxide fuel cell

A novel cobalt-free perovskite zinc-doped lanthanum strontium iron oxide (La_0.8Sr_0.2Zn_xFe_1?xO_3?δ, LSZF, x = 0.1–0.3) is synthesized and evaluated as cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC) with samarium doped ceria (SDC) electrolyte. LSZF cathode at x = 0.2 composition demonstrates the remarkable electrochemical activity at intermediate temperature (550 °C): such as, high electrical conductivity (13.63 S cm^?1), excellent thermal stability with SDC electrolyte (12.10 μK^?1), high surface area (4.52 m² g^?1), extremely reduced area specific resistance (0.69 Ω cm^?2) and low activation energy (0.117 eV). Furthermore, single fuel cells are fabricated using LSZF as a cathode, which exhibits the excellent performance by achieving the high power density of 409 mW cm^?2 under natural gas as a fuel and ambient air as an oxidant at 550 °C with good stability over 10 h. All experimental results indicate that the LSZF is a promising cathode material for natural gas based intermediate temperature fuel cell applications.     

Hydrogen-rich gas production from pyrolysis of wet sludge in situ steam agent

A series of wet sludge samples with different moisture contents were pyrolyzed in situ steam in a bench-scale fixed bed reactor in order to examine the influence of moisture and temperature on product distribution and gas composition. The results demonstrated that inherent moisture in wet sludge had a great effect on the product yield. The pyrolysis of wet sludge (43.38% moisture content) at 800 °C exhibited maximum H₂ yield (7.76 mol kg^?1 dry basis wet sludge) and dry gas yield (0.61 Nm³ kg^?1) and H₂ content of 42.13 vol%. When the moisture exceeded 43.38%, H₂ yield and gas yield both tended to decline. It was also shown that the elevated temperature exhibited a significant influence on gas content increase and tar reduction; at the same time, H₂ yield and H₂ content were increased from 1.83 mol kg^?1 dry basis wet sludge and 16.67 vol% to 9.15 mol kg^?1 dry basis wet sludge and 45.67 vol%, respectively, as temperature increased from 600 °C to 850 °C. LHV of fuel gas varies from 15.49 MJ Nm^?3 to 11.65 MJ Nm^?3 because of decrease in CH₄ and C₂H₄ content as temperature increasing. In conclusion, hydrogen rich gas production by pyrolysis of wet sludge which avoided pre-drying process and utilized in situ steam agent from wet sludge is an economic method.     

Effect of anode thickness and Cu content on consolidation and performance of planar copper-based anode-supported SOFC

Planar, Cu-containing Gadolinia-doped ceria anode-supported solid oxide fuel cells to be used at intermediate temperature (500–750 °C) were produced in the present work. The Intermediate temperature solid oxide fuel cells were fabricated using Li₂O as sintering aid for Gadolinia-doped ceria, varying the anode-to-electrolyte thickness ratio (r) from 2 to 10 and the CuO content in the anode from 45 vol% to 55 vol%. Co-sintering of the thermo-pressed green cells was carried out at 900 °C for 3 h. The electrolyte densification was favoured by increasing the r value, this being accounted for the enhanced compressive stresses induced by the supporting anode on the electrolyte upon sintering. Larger CuO content positively influences the overall cell performance, due to the improved electronic conductivity of the anode. Nevertheless, CuO concentration cannot exceed 50 vol% because of the tensile stresses (and corresponding flaws) generated in the electrolyte for larger amount. IT-SOFC containing 50 vol% CuO was characterized by an Open Circuit Voltage ≈0.82 V and a maximum power density of 200 mW cm^?2 at 700 °C.     

Physical characterization and electrochemical performance of copper–iron–ceria–YSZ anode‐based SOFCs in H2 and methane fuels

Thermal instability and poor electrochemical activity of copper‐ceria‐YSZ anodes at the solid oxide fuel cells (SOFCs) operation temperature (>700 °C) necessitates the use of new strategy to improve the performance of respective anodes for direct hydrocarbon SOFCs. In the present study, iron is incorporated into copper–ceria–YSZ anodes in order to investigate the structural, morphological, and electrochemical properties by using various techniques such as X‐ray diffraction, elemental mapping, current–voltage testing, and electrochemical impedance spectroscopy. X‐ray diffraction shows that copper promotes the reduction of iron oxide, and formation of cubic phase of copper–iron metals is observed after reduction in H₂ at 800 °C. Elemental mapping shows better distribution of metal catalyst inside the pores of copper–ceria–YSZ anodes at 800 °C in the presence of iron. The maximum power densities of copper–ceria–YSZ anodes and copper–iron–ceria–YSZ anodes are observed to be 140 and 195 mW cm^?2 in H₂ fuel and 70 and 90 mW cm^?2 in CH₄ fuel at 800 °C. The maximum power density increases with the increase in Cu–Fe metal loading, temperature and with the addition of 1‐wt% Pd in copper–iron–ceria–YSZ anodes. The decrease in performance from 125 to 100 mW cm^?2 is observed during the exposure of CH₄ fuel for 46 h. Electrochemical impedance spectra show an increase in ohmic and total resistance of cell because of sintering and carbon formation, which affects the catalytic activity of anode lowering the performance of SOFC as suggested by post SEM analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

Ni-samaria-doped ceria (Ni-SDC) anode-supported solid oxide fuel cell (SOFC) operating with CO

The performance of nickel-samaria-doped ceria (Ni-SDC) anode-supported cell with CO-CO₂ feed was evaluated. The aim of this work is to examine carbon formation on the Ni-SDC anode when feeding with CO under conditions when carbon deposition is thermodynamically favoured. Electrochemical tests were conducted at intermediate temperatures (550–700 °C) using 20 and 40% CO concentrations. Cell operating with 40% CO at 600–700 °C provided maximum power densities of 239–270 mW cm^?2, 1.5 times smaller than that achieved with humidified H₂. Much lower maximum power densities were attained with 20% CO (50–88 mW cm^?2). Some degradation was observed during the 6 h galvanostatic operation at 0.1 A cm^?2 with 40% CO fuel at 550 °C which is believed due to the accumulation of carbon at the anode. The degradation in cell potential occurred at a rate of 4.5 mV h^?1, but it did not lead to cell collapse. EDX mapping at the cross-section of the anode revealed that carbon formed in the Ni-SDC cell was primarily deposited in the anode section close to the fuel entry point. Carbon was not detected at the electrolyte-anode interface and the middle of the anode, allowing the cell to continue operation with CO fuel without a catastrophic failure.     

Effect of Sr2+ doping on sintering behavior,microstructural development and electrical properties of LaPO4·nH2O nanorods prepared by dry mechanical milling

The effect of Sr²⁺ doping on the presence of second phases, sintering behavior, microstructural development, and electrical properties of LaPO₄·nH₂O nanorods (La_1?xSr_xPO_4?x/2 where x = 0.025 and 0.05) obtained by a dry mechanochemical milling process was investigated. When Sr²⁺ is present monazite-type La_1?xSr_xPO₄ nanopowders were obtained instead of rhabdophane-type LaPO₄·nH₂O. In addition, Sr²⁺ doping implies a larger P/La ratio and it enhances the formation of lanthanum tryoxophosphate (La(PO₃)₃), a thermodinamically stable phase, in doped samples. Dilatometric studies reveal a shift of the maximum shrinkage rate at lower temperatures for doped samples, with larger shifts with higher Sr²⁺ contents. This shift is related to the presence of oxygen vacancies but also to a higher content of La(PO₃)₃. Furthermore, the derivative of the linear shrinkage curves for all the samples showed peaks at temperatures higher than 1300 °C that are associated to the volatilization of P₄O₁₀ gas and to the recrystallization of monazite from the incongruent melting of La(PO₃)₃. After the dilatometric tests at 1500 °C the samples showed polygonal grains with a bimodal size distribution. For the doped samples the smaller grains do not present Sr²⁺ in their composition and it is related to those grains form from the recrystallization of monazite-LaPO₄ formed in turn from La(PO₃)₃. The total conductivity of the studied samples (x = 0.05) is higher for the samples sintered at 1000 °C for 1 h than for those sintered at 1500 °C without any dwell time. It can be due in part to the fact that the smaller grains of the samples sintered at 1500 °C do not contain Sr²⁺ and it can hinder the charge transport.     

Reversible H2/H2O electrochemical conversion mechanisms on the patterned nickel electrodes

The patterned nickel (Ni) electrode enables to quantify the triple-phase boundary (TPB) length and Ni surface area as well as exclude the interference of bulk gas diffusion. In this study, the patterned Ni electrodes are investigated in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H₂O/H₂. The experimental test shows the patterned Ni electrode keeps stable and intact only at the specific operating condition due to instability of Ni at the H₂O-containing atmosphere. The effects of the temperature, partial pressure of H₂O and H₂ on the electrochemical performance are measured. The electrochemical performance has a positive correlation with the temperature, partial pressure of H₂ and H₂O. Further, the experimental results are compared with the mechanism containing two-step charge-transfer reaction used in the existing literature. An analytical calculation is performed to indicate the rate-limiting steps may be different for SOFC and SOEC modes. In SOFC mode, H₂ electrochemical oxidation could be dominated by both charge transfer reaction at low polarization voltage and by the charge-transfer reaction H(Ni) + O^2?(YSZ) → OH^?(YSZ) + (Ni) + e^? at high polarization voltage, however in SOEC mode, H₂O electrochemical reduction is considered to be dominated by H₂O(YSZ) + (Ni) + e^? → OH^?(YSZ) + H(Ni).     

Dual ionic conductive membrane for molten carbonate fuel cell

Within this study, the electrochemically inert, molten carbonate fuel cell (MCFC) $\text{γ}$-LiAlO₂ matrix is replaced by oxygen ion conducting ceramics, typical for solid oxide fuel cell (SOFC) application. Such solution leads to synergistic ion transport both by molten carbonate mix (CO₃^2-) and yttria-stabilized zirconia (YSZ) or samaria-doped ceria (SDC) matrix (O^2-).Single unit cell tests confirm that application of hybrid ionic membrane increases the performance (power density) of the MCFC over pure $\text{γ}$-LiAlO₂ for a wide range of operating temperatures (600 °C–750 °C). Cell power density with SDC and YSZ matrices is 2% and 13% higher, respectively, compared to the $\text{γ}$-LiAlO₂ at typical 650 °C operating temperature of MCFC.     

Study of the decomposition of a 0.62LiBH4–0.38NaBH4 mixture

This work highlights the dehydrogenation mechanisms of a 0.62LiBH₄–0.38NaBH₄ mixture in the range of 25–650 °C in flowing Ar. The dehydrogenation starts from 287 °C followed by two decomposition steps at 488 °C and 540 °C. These peak temperatures are in the range of 470 °C (for pure LiBH₄)–580 °C (for pure NaBH₄) due to different Pauling electronegativity values for Li⁺ (0.98) and Na⁺ (0.93) that affects the stability and decomposition temperatures. The 1^st step of dehydrogenation is accompanied with precipitation of LiH, Li₂B₁₂H₁₂ and B in between 287 and 520 °C; whilst the 2^nd step of dehydrogenation is mainly accompanied by the precipitation of Na and B when temperature is higher than 520 °C. The total amount of H₂ released is 10.8 wt.% that exceeds the estimated amount (8.9 wt.%), indicating less metal dodecaborate (than that for pure LiBH₄) is formed during the decomposition.     

Dehydrogenation and rehydrogenation of a 0.62LiBH4-0.38NaBH4 mixture with nano-sized Ni

The dehydrogenation reaction pathway of a 0.91 (0.62LiBH₄-0.38NaBH₄)-0.09Ni mixture in the temperature range of 25–650 °C in flowing Ar and the cycling stability in H₂ are presented. No H₂ is released immediately after melting at 225 °C. The major dehydrogenation occurs above 350 °C. Adding nano-sized Ni reduces the dehydrogenation peak temperatures by 20–25 °C, leading to three decomposition steps where Ni₄B₃ and Li_1.2Ni_2.5B₂ are found in the major dehydrogenation products for the 1^st and the 3^rd step; whilst the Ni-free mixture decomposes through a two-step decomposition pathway. A total of 8.1 wt% of hydrogen release from the 0.91 (0.62LiBH₄-0.38NaBH₄)-0.09Ni mixture is achieved at 650 °C in Ar. This mixture has a poor hydrogen cycling stability as its reversible hydrogen content decreases from 5.1 wt% to 1.1 wt% and 0.6 wt% during three complete desorption-absorption-cycles. However, the addition of nano-sized Ni facilitates the reformation of LiBH₄.     

Solution combustion synthesized lithium cobalt oxide as a catalytic precursor for the hydrolysis of sodium borohydride

Solution combustion synthesis (SCS) has recently been explored as one method to synthesize metal oxides (e.g. Co₃O₄) that can serve as catalytic precursors for the hydrolysis of sodium borohydride (NaBH₄). In this work, SCS is used to produce the mixed metal oxide lithium cobalt oxide (LiCoO₂) from a solution of cobalt nitrate, lithium acetate, and glycine. Its subsequent use as an effective catalyst precursor for NaBH₄ hydrolysis is characterized and compared to commercially available LiCoO_2. To remove residual impurities from the SCS material the materials were heated at a rate of 10 °C min^?1 and held for 2 h at temperatures ranging from 500 to 800 °C and subsequently characterized. It was found that the layered phase of LiCoO₂ results at heat treat temperatures above 700 °C. Using a 0.6 wt.% aqueous solution of NaBH₄ at 25 °C and a 1 wt.% catalyst precursor loading, an optimized HGR of 2.09 L min^?1 g_cat^?1 was achieved for the solution combustion synthesized LiCoO₂. In contrast, at the same conditions, a HGR of 0.29 L min^?1 g_cat^?1 was obtained for commercial materials even though the specific surface area was much higher.     

Long-term stability of Ni–Sn porous metals for cathode current collector in solid oxide fuel cells

Ni–Sn porous metals with different concentrations of Sn were prepared as potential current collectors for solid oxide fuel cells (SOFCs). The weight increase of these species was evaluated after heat-treatment under elevated temperatures in air for thousands of hours to evaluate the long-term oxidation resistance. Ni–Sn porous metals with 5–14 wt% of Sn exhibited excellent oxidation resistance at 600 °C, although oxidation became significant above 700 °C. Intermetallic Ni₃Sn was formed at 600 °C due to phase transformation of the initially solid solutions of Sn in Ni in the porous metals. For the porous metal with 10 wt% of Sn, the oxidation rate constant at 600 °C in air was estimated to be 8.5 × 10^?14 g² cm^?4 s^?1 and the electrical resistivity at 600 °C was almost constant at approximately 0.02 Ω cm² up to an elapsed time of 1000 h. In addition, the gas diffusibility and the power-collecting ability of the porous metal were equivalent to those of a platinum mesh when applied in the cathode current collector of a SOFC operated at 600 °C. Ni–Sn porous metals with adequate contents of Sn are believed to be promising cathode current collector materials for SOFCs for operation at temperatures below 600 °C.     

A decade of ceria based solar thermochemical H2O/CO2 splitting cycle

Since 2006, ceria is used as a redox reactive material for production of H₂, CO, and syngas via a two-step solar driven thermochemical H₂O/CO₂ splitting cycle. Different forms of phase pure ceria were studied over a wide range of temperatures and oxygen partial pressures. To increase the redox reactivity and long-term stability, the effects of incorporation of different dopants in to the ceria fluorite structure (in varying proportions) were studied in detail. A variety of solar reactors, loaded with ceria based ceramics, were designed and developed to investigate the performance of these materials towards thermal reduction and H₂O/CO₂ splitting reactions. The thermodynamics and reaction kinetics of ceria based solar thermochemical H₂O/CO₂ splitting cycles were also explored heavily. This paper presents a detailed chronological insight into the development of ceria-based oxides as reactive materials for solar fuel production via thermochemical redox H₂O/CO₂ splitting cycles.     

Optimization of renewable hydrogen-rich syngas production from catalytic reforming of greenhouse gases (CH4 and CO2) over calcium iron oxide supported nickel catalyst

Multi-response optimization of hydrogen-rich syngas from catalytic reforming of greenhouses (methane and carbon dioxide over Calcium iron oxide supported Nickel (15 wt%Ni/CaFe₂O₄) catalyst was performed by varying reaction temperature (700–800 °C), feed ratio (0.4–1.0) and gas hourly space velocity (10,000–60,000 h^?1)) using response surface methodology. Four response surface methodology (RSM) models were obtained for the prediction of reactant conversion and the product yield. The analysis of variance (ANOVA) conducted on the model showed that the parameters have significant effect on the responses. Optimum conditions for the methane dry reforming over the 15 wt%Ni/CaFe₂O₄ catalyst were obtained at reaction temperature, feed ratio and gas hourly space velocity (GHSV) of 832.45 °C, 0.96 and 35,000 mL g^?1 h^?1 respectively with overall desirability value of 0.999 resulting in the highest methane (CH₄) and carbon dioxide (CO₂) conversions of 85.00%, 88.00% and hydrogen (H₂) and carbon monoxide (CO) yields of 77.82% and 75.76%, respectively.     

Development of methanol‐fueled low‐temperature solid oxide fuel cells

Low‐temperature solid oxide fuel cell (SOFC, 300–600°C) technology fueled by methanol possessing significant importance and application in polygenerations has been developed. Thermodynamic analysis of methanol gas‐phase compositions and carbon formation indicates that direct operation on methanol between 450 and 600°C may result in significant carbon deposition. A water steam/methanol ratio of 1/1 can completely suppress carbon formation in the same time enrich H₂ production composition. Fuel cells were fabricated using ceria–carbonate composite electrolytes and examined at 450–600°C. The maximum power density of 603 and 431 mW cm^?2 was achieved at 600 and 500°C, respectively, using water steam/methanol with the ratio of 1/1 and ambient air as fuel and oxidant. These results provide great potential for development of the direct methanol low–temperature SOFC for polygenerations. Copyright © 2010 John Wiley & Sons, Ltd.