Consecutive Redox Cycles Research Articles

Solar-Driven Thermochemical Water-Splitting by Cerium Oxide: Determination of Operational Conditions in a Directly Irradiated Fixed Bed Reactor

Concentrated solar energy can be transformed into electricity, heat or even solar fuels, such as hydrogen, via thermochemical routes with high exergetic efficiency. In this work, a specific methodology and experimental setup are described, developed to assess the production of hydrogen by water splitting making use of commercial cerium oxide, ceria (CeO2), in a solarized reactor. A fixed bed reactor, directly irradiated by a 7 kWe high flux solar simulator (HFSS) was used. Released H2 and sample temperature levels were continuously monitored. Three tests were carried out consisting of three consecutive redox cycles each, with irradiances in the range of 1017–2034 kWm−2. It was necessary to achieve a compromise between sample temperatures (higher temperatures lead to higher reduction rates) and sample stability, since absorbed radiation can degrade a sample at lower temperature (1280–1480 °C) than in a conventional infrared oven (T > 2000 °C). Irradiating the surface of the sample with an irradiance of 2034 kWm−2 (270 W of total radiation power) during 9.5 min eventually degraded the sample, resulting in a conversion into stoichiometrically reduced oxide (Ce2O3) of 11%. A similar conversion was achieved (9.7%) after 2 min of irradiation at 270 W (100% of radiation), but without irreversibly damaging the sample.

High Iron and Calcium Coal Ash As the Oxygen Carrier for Chemical Looping Combustion

The potential of Zhundong’s coal ash as a novel oxygen carrier (OC) in chemical-looping combustion (CLC) was evaluated in a thermogravimetric analysis (TGA) under both the CO/Ar and air atmospheres. A single reduction–oxidation cycle was carried out in the TGA reactor to determine the optimum reaction temperature, which was 1123 K. The cyclic stability and agglomeration tendency of ash also was evaluated by consecutive redox cycles. On the basis of the kinetic analysis, the apparent activation energy of ash was about 26–29 kJ/mol and the reduction process of ash was divided into three stages. The reaction control steps for these stages were gas diffusion, diffusion and kinetics, and kinetics, respectively. Besides, the reactions in the first and second stages corresponded to the reduction of Fe2O3 to Fe and CaSO4 to CaS. In the third stage, the reaction occurred between the CaS and CaSO4.The products of the equilibrium reaction between CO and ash were obtained using Factsage (software), while the transiti...

Exploring the thermochemical heat storage capacity of AMn2O4 (A = Li or Cu) spinels

At present, there is a clear interest in developing redox materials with improved properties for high temperature thermochemical energy storage. Chemical modification of manganese oxides with cations such Li and Cu can produce, among other phases, LiMn2O4 and CuMn2O4 spinels, which are feasible candidates for heat storage due to their redox capacity. In this work, these materials were synthesized by Pechini method, and the characterization results confirmed the formation of the targeted phases with some minor contribution of Mn3O4. Thermogravimetrical redox tests in air established that both materials experience fully reversible redox transformations when the temperature is varied between 900 and 1000 °C. These assays showed the stability of both Cu and Li mixed oxides after five consecutive redox cycles and, in accordance, the XRD confirmed that the two samples retaining their spinel crystal structure after the treatment. However, in these conditions reduction temperatures are higher than 940 °C for both oxides and the enthalpies of these transformations are modest, with a maximum value of 36 kJ/kg for LiMn2O4. Alternatively, if the reduction is performed in argon and the oxidation in air, it is possible to increase the amount of oxygen exchanged in the gas-solids reactions and, accordingly, the heat storage capacity. Therefore, the heat recovered in the re-oxidation of CuMn2O4 at 700 °C was 144 kJ/kg (34 kJ/mol), while LiMn2O4 showed an enthalpy of 209 kJ/kg (37 kJ/mol). These changes in the composition of the atmosphere do not affect to the stability of the system and the conversion is maintained after five consecutive cycles. In all cases, the initial spinel phase is recovered after reoxidation, which takes place at remarkably fast rates. Analysis of the intermediate reduced materials reveals a significant complexity of the redox transformations, which imply the formation of LiMnO2 and CuMnO2, among other phases. Accordingly, considering the stability of these systems, as well as the relatively high enthalpies CuMn2O4 and LiMn2O4 appear to be promising materials for thermochemical energy storage.

Reactive stability of promising scalable doped ceria materials for thermochemical two-step CO2 dissociation

This work reports an improved and stable oxygen exchange capacity (OEC) of optimized doped ceria Ce1−xMxO2−δ (M = Zr, Hf, Nb) materials for two-step thermochemical CO2 splitting over 50 consecutive redox cycles (7 days).

A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore cycle for enhanced solar thermochemical water splitting

A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore stoichiometric redox cycle with superior H2 production capacities is identified and corroborated for two‐step solar thermochemical water splitting (STWS). During the first thermal reduction step (1400°C), a reaction between CeO2 and SnO2 occurred for all the CeO2–xSnO2 (x = 0.05–0.20) solid compounds, forming thermodynamically stable Ce2Sn2O7 pyrochlore rather than metastable CeO2‐δ. Consequently, substantially higher reduction extents were achieved owing to the reduction of CeIV to CeIII. Moreover, in the subsequent reoxidation with H2O (800°C), H2 production capacities increased by a factor of 3.8 as compared to the current benchmark material ceria when x = 0.15, with the regeneration of CeO2 and SnO2 and the concomitant reoxidation of CeIII to CeIV. The H2O‐splitting performance for CeO2–0.15SnO2 was reproducible over seven consecutive redox cycles, indicating the material was also robust. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3450–3462, 2017

Polypyrrole linear actuation tuned by phosphotungstic acid

Co-doping polypyrrole (PPy) with dodecylbenzenesulfonate and multicharged phosphotungstate anions (PT) from the phosphotungstic acid (PTA) led to free-standing PPy/DBS-PT films, which were studied for their linear actuation properties. FTIR and Raman spectra revealed that PT was successfully embedded in PPy/DBS during electropolymerisation. Isometric stress and isotonic strain measurements in aqueous electrolyte under various electrochemical experiments showed an increase in the obtainable strain and stress, which was attributed to the electrocatalytic role PTA plays during the electropolymerisation. This results in lower synthesis potential and the formation of more compact films in comparison to PPy/DBS films under equal conditions. With the improved structure as well as the higher-charged immobilized PT dopant, 17 times higher conductivities, 1.7 times higher redox charge, and 1.8 times higher specific capacitance were obtained (at equal frequency). Energy dispersive X-ray (EDX) spectra indicated that contrary to some other published works, the majority of PT stayed stably in the film during consecutive redox cycles.

Activity study of NiO-based oxygen carriers in chemical looping steam methane reforming

This study evaluates the performance of NiO-based oxygen carriers supported on ZrO2, TiO2, SiO2, Al2O3 and NiAl2O4 as conventional reforming catalysts at low temperature (650°C) as well as oxygen transfer materials (OTMs) for chemical looping steam methane reforming. Conventional reforming experiments with pre-reduced OTMs demonstrated satisfactory performance of NiO/Al2O3, NiO/ZrO2 and NiO/NiAl2O4 at 650°C, with less than 8% relative decrease in conversion after 10h time-on-stream. On the other hand, NiO/SiO2 and NiO/TiO2 were found to have low activity (<50% initial CH4 conversion) and deactivated rapidly, eventually dropping to less than 10% CH4 conversion after 2h on stream. The three most promising materials were tested under chemical looping steam methane reforming conditions for twenty consecutive redox cycles in a fixed bed flow unit. NiO/ZrO2 exhibited good activity with initial CH4 conversion higher than 80% and had very good stability. Deactivation was minimal after 20 consecutive redox cycles, corresponding to 20h exposure to CH4/steam. NiO/Al2O3 and NiO/NiAl2O4 demonstrated similar high initial activity, but also high deactivation, leading to methane conversion of 59 and 63% conversion respectively at the end of the test.

Synergistic effects of mixtures of iron ores and copper ores as oxygen carriers in chemical-looping combustion

Iron ore is a cheap and nontoxic oxygen carrier in chemical-looping combustion (CLC). However, it performs good recyclability while low reactivity in the CLC of solid fuels. Copper ore exhibits very high reactivity and oxygen uncoupling behavior, while suffers from tendency towards sintering and agglomeration during consecutive redox cycles at a higher temperature (e.g., 900–1000°C). In this work, mixtures of hematite and copper ore were used as oxygen carriers for CLC of syngas and coal. Through the isothermal redox experiments at 950°C in a thermogravimetric analyzer, it is found that there are synergistic effects between iron ore and copper ore, and copper ore could be more efficiently utilized when the mixing ratio of copper ore is maintained 10–20wt%. As the mixing ratio of copper ore is 20wt%, the reduction reaction of the mixer OC is no longer endothermic, which is beneficial to the controllability of the fuel-reactor temperature. The fluidized-bed experiments were carried out to verify the reactivity of the mixing ore OCs at 950°C. It is observed that the mixing OC with 20wt% copper ore has a better reactivity with the gasification products (especially H2) of low-volatile anthracite than the pure hematite, and leads to a higher fuel conversion rate and CO2 yield. The mixtures of iron ore and copper ore are expected to address simultaneously reactivity, recyclability, cost and environmental concerns of oxygen carriers.

Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dual-scale porosities.

Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO2. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with dmean = 2.5 mm and porosity = 0.76-0.82, enables volumetric absorption of concentrated solar radiation for efficient heat transfer to the reaction site during endothermic reduction, while the smaller void size range within the struts, with dmean = 10 μm and strut porosity = 0-0.44, increases the specific surface area for enhanced reaction kinetics during exothermic oxidation with CO2. Characterization is performed via mercury intrusion porosimetry, scanning electron microscopy, and thermogravimetric analysis (TGA). Samples are thermally reduced at 1773 K and subsequently oxidized with CO2 at temperatures in the range 873-1273 K. On average, CO production rates are ten times higher for samples with 0.44 strut porosity than for samples with non-porous struts. The oxidation rate scales with specific surface area and the apparent activation energy ranges from 90 to 135.7 kJ mol(-1). Twenty consecutive redox cycles exhibited stable CO production yield per cycle. Testing of the dual-scale RPC in a solar cavity-receiver exposed to high-flux thermal radiation (3.8 kW radiative power at 3015 suns) corroborated the superior performance observed in the TGA, yielding a shorter cycle time and a mean solar-to-fuel energy conversion efficiency of 1.72%.

Global kinetic evaluation during the reduction of CoWO4 with methane for the production of hydrogen

CoWO4 (CW) promoted with 10% Ni (CW–Ni) and 10% La2O3 (CW–La) were used as oxygen carriers in redox cycles of CH4/Ar and H2O/Ar to evaluate their global kinetics (reaction rate, order, constant and activation energy) with methane for the production of hydrogen. CoWO4 was synthesized by co-precipitation from equimolar solutions of Na2WO4 and Co(NO3)2 and calcined at 950 °C. Ni and La nitrate solutions were incorporated to CoWO4 by impregnation. Kinetic data was collected by thermogravimetric analysis (TGA) at 2, 5 and 8% CH4/Ar and at 850, 900 and 950 °C for two consecutive redox cycles. Oxidation was performed using 5% H2O/Ar at 900 °C. Results indicate a global first order reaction for the three materials. Activation energies during the first cycle for CW, CW–Ni and CW–La were 52.8, 32 and 33.4 kcal/mol, respectively, thus reflecting the influence of Ni and La2O3 in a greater reaction rate with respect to CW.

Behavior of ilmenite as oxygen carrier in chemical-looping combustion

For a future scenery where will exist limitation for CO 2 emissions, chemical-looping combustion (CLC) has been identified as a promising technology to reduce the cost related to CO 2 capture from power plants. In CLC a solid oxygen-carrier transfers oxygen from the air to the fuel in a cyclic manner, avoiding direct contact between them. CO 2 is inherently obtained in a separate stream. For this process the oxygen-carrier circulates between two interconnected fluidized-bed reactors. To adapt CLC for solid fuels the oxygen-carrier reacts with the gas proceeding from the solid fuel gasification, which is carried out right in the fuel-reactor. Ilmenite, a natural mineral composed of FeTiO 3, is a low cost and promising material for its use on a large scale in CLC. The aim of this study is to analyze the behavior of ilmenite as oxygen-carrier in CLC. Particular attention was put on the variation of chemical and physical characteristics of ilmenite particles during consecutive redox cycles in a batch fluidized-bed reactor using CH 4, H 2 and CO as reducing gases. Reaction with H 2 was faster than with CO, and near full H 2 conversion was obtained in the fluidized-bed. Lower reactivity was found for CH 4. Ilmenite increased its reactivity with the number of cycles, especially for CH 4. The structural changes of ilmenite, as well as the variations in its behavior with a high number of cycles were also evaluated with a 100 cycle test using a CO + H 2 syngas mixture. Tests with different H 2:CO ratios were also made in order to see the reciprocal influence of both reducing gases and it turned out that the reaction rate is the sum of the individual reaction rates of H 2 and CO. The oxidation reaction of ilmenite was also investigated. An activation process for the oxidation reaction was observed and two steps for the reaction development were differenced. The oxidation reaction was fast and complete oxidation could be reached after every cycle. Low attrition values were found and no defluidization was observed during fluidized-bed operation. During activation process, the porosity of particles increased from low porosity values up to values of 27.5%. The appearance of an external shell in the particle was observed, which is Fe enriched. The segregation of Fe from TiO 2 causes that the oxygen transport capacity, R OC, decreases from the initial R OC = 4.0% to 2.1% after 100 redox cycles.

Curvature and Strength of Ni-YSZ Solid Oxide Half-Cells After Redox Treatments

One of the main drawbacks of anode-supported solid oxide fuel cell technology is the limited capability to withstand reduction and oxidation (“RedOx”) of the Ni phase. This study compares the effect of RedOx cycles on curvature and strength of half-cells, composed of a nickel-yttria-stabilized-zirconia (Ni-YSZ) support, a Ni-YSZ anode, and an 8YSZ electrolyte. Five different treatments are studied: (i) reduction at 600°C, (ii) reduction at 1000°C, (iii) 1RedOx cycle at 750°C, (iv) 5RedOx cycles at 750°C, and (v) 5RedOx cycles at 600°C. The strength is measured by the ball-on-ring method, where it is calculated analytically from the force. In this calculation the thermal stresses are estimated from the curvature of the half-cell. For each treatment, more than 30 samples are tested. About 20 ball-on-ring samples are laser cut from one original 12×12 cm2 half-cell. Curvature and porosity are measured for each sample before and after RedOx treatments. The first observations show that increasing the reduction temperature enhance strength but does not influence the curvature, whereas 1RedOx cycle at 750°C increases the curvature without changing the strength. Consecutive RedOx cycles seem to decrease anode-supported cell strength but this is coupled to lower porosity of the tested samples.

Fabrication of Redox Tolerant Anode with an Electronic Conductive Oxide of Y-Doped SrTiO3

Composite anodes consisting of Ni, Y-doped (YST), and -stabilized (YSZ) were fabricated; the Ni species was impregnated to YST–YSZ powder and then the resulting powder was fired onto the electrolyte at for 5 h in air. The conductive oxide of YST sintered in air showed a conductivity of after reduction in humidified hydrogen (1.2% ) for 100 h and behaved as a metallic conductor with respect to the temperature dependence of conductivity. The influence of the nickel content in the anode of Ni/YST–YSZ on the electrochemical property was studied under 0.6% atmosphere. The cermet of 10 wt % Ni/YST–YSZ showed the minimal polarization resistance among the catalysts with the nickel content in the range of 5–50 wt %. An electrolyte-supported cell employing this anode exhibited a power density of at 0.5 V at despite the extremely low content of Ni. Stable power generation was attained at a constant terminal voltage of 0.7 V in humidified hydrogen (0.6% ) for 21 h. Even after five consecutive redox cycles, the cell performance remained unchanged mainly due to the stable porous framework of YST–YSZ with a low Ni content.

Ilmenite Activation during Consecutive Redox Cycles in Chemical-Looping Combustion

Ilmenite, a natural mineral composed of FeTiO3, is a low-cost and promising oxygen carrier (OC) for solid fuels combustion in a chemical-looping combustion (CLC) system. The aim of this study is to analyze the behavior of ilmenite as an OC in CLC and the changes in its properties through redox cycles. Experiments consisting of reduction−oxidation cycles in a thermogravimetric analyzer were carried out using the main products of coal pyrolysis and gasification, that is, CH4, H2, and CO, as reducing gases. Characterizations of ilmenite through scanning electron microscopy−energy-dispersive X-ray (SEM−EDX), X-ray diffraction (XRD), Hg porosimetry, N2 fisisorption, He pycnometry, and hardness measures have been performed. Both fresh and previously calcined at 1223 K ilmenite have been used as initial OCs. Fresh ilmenite reacts slowly; nevertheless, there is a gain in reactivity in reduction as well as in oxidation with the number of cycles. This activation occurs for all tested fuel gases and is faster if ilm...

Stability of Ni/Y-doped SrTiO3-YSZ Anode in Reduction-oxidation Cycles

A composite anode consisting of 3 vol% Ni, Y-doped SrTiO3 (YST) and Y2O3 stabilized ZrO2 (YSZ) was fabricated: the Ni species was impregnated to YST-YSZ powder and then the resulting powder was fired onto the electrolyte at 1200ºC for 5 h in air. A single cell employing this anode exhibited the power density of 0.28 W cm-2 at 0.7 V at 1000ºC despite its extremely low content of Ni. Stable power generation was attained at the constant terminal voltage of 0.7 V in humidified hydrogen (0.6% H2O-H2) for 22 h. Even after five consecutive redox cycles, the cell performance remained unchanged mainly due to the stable porous framework of YST-YSZ with low Ni content.

RedOx study of anode-supported solid oxide fuel cell

The common technology for solid oxide fuel cells (SOFC) is based on a cermet anode of nickel with yttrium stabilized zirconia (YSZ), often used as the supporting structure. One of the main limitations of this anode technology is the redox tolerance. In this study, two techniques are used to quantify the anode expansion after one redox cycle at different temperatures. First, the increase of porosity is measured using scanning electron microscopy (SEM) image quantification. The second method considers the anode expansion above the electrolyte fracture limit by measuring the crack width in the electrolyte layer. An influence of redox cycling temperature on anode volume expansion was already observed by Ettler et al. but not fully understood. Here a model based on nickel oxidation mechanisms is proposed to explain the relation between redox temperature and volume expansion. The same methods are used to quantify anode expansion after consecutive redox cycles at constant temperature. In particular, small cracks caused at low oxidation temperature (600°C) were closed again after a subsequent reduction. The quantifying technique is then applied to cells tested in real stack conditions. The cell corners can undergo redox cycles depending on stack design and fuel utilization. The study of this location can give information on the number of cycles experienced by the anode support. Moreover, cracks in the electrolyte show infiltration of NiO that close the openings. Important infiltration can produce a short circuit between anode and cathode. The presence of strontium chromate on the infiltrated NiO was observed.

Redox properties of doped and supported copper–ceria catalysts

Copper-doped ceria catalysts feature in a variety of catalytic reactions. One important application is selective hydrogen combustion via oxygen exchange, which forms the basis of cyclic oxidative dehydrogenation. This paper describes the synthesis of monophasic (doped) and biphasic (supported) Cu-ceria catalysts, that are then characterized using a combination of temperature programmed reduction (TPR) and X-ray diffraction (XRD) methods. The catalysts are analyzed both as fresh samples and after redox cycling at 550-800 degrees C. TPR and XRD characterization clarify the role of the active sites on the catalyst surface and the copper-ceria interactions. Depending on the catalyst type, reduction occurs at approximately 110 degrees C, approximately 150 degrees C, or approximately 190 degrees C. The reduction at 110 degrees C is ascribed to highly dispersed copper species doped in the ceria lattice, and that at 190 degrees C to CuO crystallites supported on ceria. Remarkably, both types converge to the 150 degrees C feature after redox cycling. The reduction temperature of the doped catalyst increases after redox cycling, indicating that stable Cu clusters form at the surface. Conversely, the reduction temperature of the "supported" catalyst decreases after redox cycling, and the CuO crystallites disappear. With this knowledge, a copper-doped ceria catalyst is analyzed after application in selective hydrogen combustion (16 consecutive redox cycles at 550 degrees C). No CuO crystallites are observed, and the sample reduces at approximately 110 degrees C. This suggests that copper-doped ceria is the active oxygen exchange phase in selective hydrogen combustion.

TPR, TPO, and TPD examinations of Cu 0.15Ce 0.85O 2− y mixed oxides prepared by co-precipitation, by the sol–gel peroxide route, and by citric acid-assisted synthesis

Temperature-programmed reduction (TPR), oxidation (TPO), and desorption (TPD) studies were performed on three copper–ceria mixed oxide samples having the same nominal composition, Cu 0.15Ce 0.85O 2− y , but prepared in three different ways: by co-precipitation, the sol–gel peroxide route, and the sol–gel citric acid route. The obtained results reveal that despite a drastic initial drop in specific surface area after consecutive redox cycles, the hydrogen consumption remains constant. This is because CuO is highly dispersed over the surface of CeO 2 nanocrystallites and remains highly dispersed even after the agglomeration of CeO 2 nanocrystallites in a denser secondary structure. The dispersed CuO is reduced to Cu 0 during the TPR, forming agglomerated metal particles on the surface of partially reduced CeO 2. However, after subsequent temperature-programmed oxidation all the Cu 0 is oxidized back into CuO and redispersed over the CeO 2 crystallites.

Mechanism of redox transformation of titanocene dichloride centers immobilized inside a polypyrrole matrix—EQCM and XPS evidences

We report electrochemical quartz crystal microbalance (EQCM) results for electrodeposition of titanocene derivatized polypyrrole p(Tc3Py) films and redox transformation of polypyrrole matrix and titanocene centers immobilized in the film. Films of p(Tc3Py), Tc3Py = Tc(CH 2) 3NC 4H 4 (Tc = Cl 2TiCpCp′, Cp = C 5H 5, Cp′ = C 5H 4) were obtained from acetonitrile solutions of monomer on a Pt disc or thin Au layer evaporated on 10 MHz quartz crystals. Polymerization efficiency, derived from the slope of the change of resonant frequency as a function of the deposition charge ranged from 54% to 75%. A gradual loss of redox activity of Tc centers during consecutive redox cycles of p(Tc3Py) film in TBAFP 6/THF solutions is discussed in terms of elimination of Cl − ions from the Tc complex and accommodation of solvent molecule. The EQCM data are supported by XPS results. The preliminary studies performed in TEACl in AN solution have shown that the presence of Cl − ions in the solution markedly inhibits the loss of redox activity of Tc centers immobilized in the polymer matrix.