Consecutive Redox Cycles Research Articles

Investigation on the activity of Ni doped Ce0.8Zr0.2O2 for solar thermochemical water splitting combined with partial oxidation of methane

The incredible potential of metal oxide-based two-step solar-driven thermochemical water splitting technology lies in its ability to harness limitless solar energy to produce clean and renewable hydrogen fuel. However, achieving high redox kinetics and lowering the reaction temperature remain the primary challenges in hydrogen production. The CeO2–ZrO2 system is a well-known oxygen carrier for thermochemical water splitting on account of its high temperature stability, quick kinetics, and redox cyclability. While recent studies have explored the combination of methane partial oxidation with metal oxide reduction, the reduction effect of cerium-zirconium oxide is relatively low, and reactivity still needs improvement. The nickel-doped system exhibits good redox performance and is regenerative. In this study, different mass percentages nickel-loaded oxygen carriers mNi/Ce0.8Zr0.2O2 (m = 0, 1, 3, 5, 7, 9) were prepared using the impregnation method. Redox properties of oxygen carriers were investigated in a fixed bed reactor and various characterizations were performed. To assess the durability of mNi/Ce0.8Zr0.2O2, 60 long-cycle experiments were also conducted. The results indicate that the lattice oxygen in mNi/Ce0.8Zr0.2O2 exhibits remarkable selectivity for the partial oxidation of CH4 at temperatures ranging from 880 to 925 °C. The best reactivity performance was achieved with a nickel loading ratio of 5 wt%, and after 60 consecutive redox cycles, 5Ni/Ce0.8Zr0.2O2 demonstrates remarkable robustness, maintaining its redox activity.

Mid-temperature chemical looping methane reforming for hydrogen production via iron-based oxygen carrier particles

Chemical looping steam methane reforming (CL-SMR) via iron-based oxygen carriers is a promising method for efficient hydrogen production. To overcome challenges such as high reaction temperatures (>850 °C) and scarcity of low-cost, durable oxygen carriers (OCs), we have developed iron-based particles mixed with various ratios of nickel-based particles to achieve remarkable performance in CL-SMR at 600 °C. The mixed particles showed 85.23% methane conversion and 3.47 and 1.01 mL/min/gOC hydrogen production rates in the reduction and steam oxidation steps, respectively, in the two-step CL-SMR reaction. In the three-step CL-SMR reaction, air oxidation led to full recovery of oxygen carriers, enhancing methane conversion to 93.30% and elevating hydrogen production rate to 1.41 mL/min/gOC during steam oxidation. Precise control over methane conversion and hydrogen production in the three-step CL-SMR system is achievable by manipulating the mixing ratios of iron-based to nickel-based OC particles. Comprehensive experimental tests were conducted, covering practical aspects like support materials, gas velocity, and steam-to-carbon ratios. The outstanding cyclic stability of OC particles was confirmed over 200 consecutive redox cycles at 600 °C. The mid-temperature iron-based oxygen carrier particles, integrated with chemical looping demonstration project, might provide a powerful approach toward more efficient and scalable hydrogen production.

Solar‐Driven Redox Splitting of CO2 Using 3D‐Printed Hierarchically Channeled Ceria Structures

AbstractFuel produced from CO2 and H2O using solar energy can contribute to making aviation more sustainable. Particularly attractive is the thermochemical production pathway via a ceria‐based redox cycle, which uses the entire solar spectrum as the source of high‐temperature process heat to directly produce a syngas mixture suitable for synthetizing kerosene. However, its solar‐to‐fuel energy efficiency is hindered by the inadequate isotropic topology of the ceria porous structure, which fails to absorb the incident concentrated solar radiation within its entire volume. This study presents the design and 3D‐print of hierarchically channeled structures of pure ceria by direct ink writing (DIW) to enable volumetric radiative absorption while maintaining high effective densities required for maximizing the fuel yield. The complex interplay between radiative heat transfer and thermochemical reaction is investigated in a solar thermogravimetric analyzer with samples exposed to high‐flux irradiation, mimicking realistic operation of solar reactors. Channeled structures with a stepwise optical thickness achieve a higher and more uniform temperature profile compared to that of state‐of‐art isotropic structures, doubling the volume‐specific fuel yield for the same solar flux input. Thermomechanical stability of the ceria graded structures, DIW‐printed using a novel ink formulation with optimal rheological behavior, is validated by performing 100 consecutive redox cycles.

Sintering inhibition enables hierarchical porosity with extreme resistance to degradation during redox cycling of Fe-Mo foams

High-temperature (800 ºC) steam-hydrogen redox cycling, relevant to grid-scale energy storage, is studied for iron-based freeze-cast lamellar foams. In contrast to previously studied Fe, Fe-Ni, and Fe-Co foams that rapidly degrade, Fe-25Mo foams feature a much-enhanced structural damage resistance. Utilizing in-situ x-ray diffraction, microscopy, and x-ray tomography, strong sintering inhibition is observed in Fe-Mo foams, creating a hierarchically porous lamellar structure. This leads to (i) wide channels between lamellae, enabling high macroscopic porosity (∼78%) which can accommodate gas flow as well as volumetric expansion without lamellar contact, and (ii) microporosity within lamellae, providing additional free volume to accommodate expansion during oxidation, limiting both swelling of the lamellae and the formation of Kirkendall pores. These combined effects enable a near-complete reversibility of the microstructure during cycling, preventing damage produced via internal lamellar buckling, cracking, contacting and sintering, with a remarkably high porosity (65%) remaining after 50 consecutive redox cycles.

Rational design and reduction kinetics of efficient Ce-Co oxygen carriers for chemical looping reforming of methane

One of the most significant topics in chemical looping reforming technology is to design and prepare appropriate oxygen carriers with high reactivity and excellent stability. Starting from preparation strategy and doping engineering, this work focuses on the study for chemical looping reforming of methane over the cobalt-doped Ce-based oxygen carriers synthesized via solution combustion method with the assistance of coconut shell. The reactivity of as-prepared oxygen carriers was evaluated by isothermal redox experiments, and the apparent reduction kinetics of CH4 over representative oxygen carriers was investigated on the fixed bed. A series of characterization analyses (including XRD, XPS, H2-TPR, BET, SEM, TEM and Raman) indicated that the introduction of cobalt decreases the crystallite size of composite oxygen carrier, as well increases their oxygen vacancy concentration and lattice oxygen mobility compared with the pure CeO2. Moreover, benefitting from the interaction effect between carbon material and microwave field, a moderate amount of coconut shell additive could not only further facilitate above positive changes, but enhance the interaction between Ce and Co. As consequence, for the Ce9Co1Oδ-10CS oxygen carrier, syngas with a H2/CO ratio of about 2.2 can be stably obtained, and both CH4 conversion and CO selectivity exceed 90%, and it maintains high stability in consecutive redox cycles. From the kinetic study, it has been demonstrated that the reduction of CeO2, Ce9Co1Oδ and Ce9Co1Oδ-10CS oxygen carriers are well represented by the phase-boundary controlled (contracting cylinder) mechanism, and the activation energies are calculated to be 128.73, 91.18 and 46.82 kJ/mol in turn, which also corresponds well to the reaction performance of oxygen carriers.

Novel ferrocene-containing amphiphilic block copolymer nanoparticles as high-capacity charge carriers in aqueous redox flow systems

Size-exclusion polymer electrolytes are promising charge carriers to diminish the crossover and allowing commercially available low-cost porous membranes in redox flow batteries. Boosting the solubility in water and maximizing the number of redox sites to enhance the capacity of these polymeric systems is challenging. New highly water dispersed amphiphilic diblock copolymers are reported here, with an average concentration value of 1.7 10−3 mmol of Ferrocene (Fc)-linked moieties per mg of polymer, determined by total X-ray reflection fluorescence. These redox amphiphilic block copolymers are stabilized in water as spherical nanoparticles (20 nm) by using a simple phase solvent inversion procedure. We evidence a maximum polymer dispersibility value of 6 g/L in water, for long-term stable polymer nanoparticle suspensions, yielding a theoretical capacity value of 4.78 mAh at 10.5 mM Fc. Further adjustment of the ionic conductivity and pH of these stable redox block copolymer suspensions has rendered a conductivity value of 44.5 mS/cm at pH values close to a neutral one, by adding a variety of salt supports. Studies using a 3-electrode configuration cell reveal an efficient charge transport between each of the Fc motifs in the polymer nanoparticle. A capacity value of 3.1 mAh with no transient of the polymer nanoparticles crosswise the cheap porous membrane is evidenced when cycled as polycatholyte material in a Zn hybrid aqueous redox flow battery. The particle size and electronic changes of these novel amphiphilic redox block copolymer electrolytes during consecutive redox cycles have also been monitored by dynamic light scattering and ultraviolet-visible spectroscopy, respectively. The analysis of the results enables the understanding of the main mechanisms behind their non-fully reversible capacity. Among them, aggregation and sedimentation, along with retention inside the graphite felt electrode acting the latter as a filter. These insights will aid the design of future polymer electrolyte materials and redox flow battery components with better performance and cost.

Microbial Fe cycling in a simulated Precambrian ocean environment: Implications for secondary mineral (trans)formation and deposition during BIF genesis

Banded Iron Formations (BIFs) are ancient marine chemical sediments that contain various Fe-bearing minerals such as hematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3) and a variety of FeII-/FeIII-silicates. The prevailing opinion is that primary Fe(III) (oxyhydr)oxides, such as ferrihydrite (simplified formula of Fe(OH)3), were precipitated from the ocean’s photic zone by marine plankton, and a fraction of these minerals was subsequently transformed into secondary magnetite and siderite by dissimilatory Fe(III)-reducing bacteria (DIRB). However, aside from broad estimates, it is currently unknown what fraction of the primary Fe(III) minerals was sedimented to the seafloor where it was eventually lithified, and what fraction was reduced by DIRB in the water column, thus forming a microbial Fe cycle in the water column. To test this, we conducted Fe cycling experiments with marine phototrophic Fe(II)-oxidizing bacteria and DIRB under conditions mimicking the Precambrian ocean water column with elevated Fe(II) and Si concentrations. We followed secondary mineral formation over three consecutive redox cycles (oxidation followed by reduction) over a time interval of up to 58 days to determine which mineral phases would ultimately have settled as BIF forming sediments. We used wet geochemical methods to follow Fe speciation, measured dissolved silica and volatile fatty acid (VFA) concentrations, determined cell-mineral associations using fluorescence and electron microscopy, and characterized the mineralogy of the precipitates using 57Fe-Moessbauer spectroscopy and X-ray diffraction (XRD). Our results showed that both the absence of silica and an increasing number of Fe cycles favored the formation of more crystalline minerals, such as goethite (α-FeOOH). However, in the presence of high concentrations of monomeric silica, as suggested for ancient oceans (2.2 mM), only short-range ordered (SRO) Fe(III) minerals such as ferrihydrite were observed. These did not transform into the more thermodynamically stable goethite during repeated Fe cycling. Interestingly, no magnetite formed in any of the setups. Instead, increasing Si concentrations favored the formation of increasing quantities of Fe(II) minerals. Microscopy revealed a tight association between microbial biomass and minerals formed. Dissolved silica analysis showed the removal of Si from solution congruent with Fe(II) oxidation and a release of Si during Fe(III) reduction. Together, these results suggest an important role of co-precipitated biomass as well as silica for secondary mineral formation by either constraining crystal growth and/or inhibiting Fe(II)-induced mineral transformation. Overall, our results imply that microbial Fe cycling during settling of primary ferrihydrite through the photic zone in a Precambrian ocean would have resulted in the partial transformation of ferrihydrite into secondary Fe(II) mineral phases in the water column. This would have resulted in the accumulation of mixtures of a ferrihydrite-silica composite and Fe(II) minerals in the initial BIF forming sediments.

Effect of Natural Ilmenite on the Solid Biomass Conversion of Inhomogeneous Fuels in Small-Scale Bubbling Fluidized Beds

The application of oxygen carriers as alternative bed material in fluidized bed combustion originates from chemical lopping processes. They serve as oxygen transport agents undergoing consecutive redox cycles. Thereby, oxygen carriers can provide surplus oxygen in oxygen-lean areas of fluidized bed combustion processes. In turn, re-oxidation takes place in oxygen-rich reactor parts. A more homogeneous combustion and reduced CO emissions follow during steady-state operation. However, especially regarding solid biomass conversion, inhomogeneous fuel qualities result in transient combustion conditions. Therefore, this research deals with the influence of the oxygen carrier ilmenite on solid biomass conversion. Separated batch experiments with methane (volatile), char and wood pellets took place in a laboratory bubbling fluidized bed reactor. They reveal that ilmenite enhances the in-bed CO2 yield by up to 63% during methane combustion. Batch char experiments confirm that solid–solid reactions with ilmenite are negligible. However, heterogeneous gas–solid reactions reduce the O2 partial pressure and limit the char conversion rate. The batch wood pellet experiments show that the ilmenite oxygen buffering effect is mitigated due to high local oxygen demand around the pellets and limited pellet distribution in the bed. Finally, the continuous operation in a 100 kWth BFB with inhomogeneous fuel input indicates a higher in-bed fuel conversion and confirms lower CO emissions and less fluctuation in the flue gas during inhomogeneous fuel supply.

Ca-based sorbents as precursors of oxygen carriers in chemical looping combustion of sulfurous fuels

Limestone Chemical Looping Combustion process (LCL-C™) patented by Alstom Power Inc. proposes the formation of a CaSO4 oxygen carrier via sulfur retention process when supplying a Ca-based sorbent together with sulfurous fuels in the fuel reactor. In the present research work, four Ca-based sorbents, three limestones and one dolomite with different physico-chemical characteristics, have been evaluated as oxygen carrier precursors during consecutive redox cycles by thermogravimetric analysis. 5% H2 and 3000 vppm H2S (N2 to balance) and 10% O2 (N2 to balance) were selected as reducing and oxidizing gases, respectively. Additionally, an anhydrite ore has also been studied for comparison purpose. Results from SEM-EDX characterization of the sorbent particles revealed the existence of three different sulfation patterns: (1) uniform distribution, (2) core–shell structure and (3) high sulfur content with high limitation to be converted into CaSO4. All sorbents exhibited high oxygen transport capacity (Roc = 13.3–20.8%), except for the material showing pattern (3) (Roc = 2.6%). Likewise, the materials presented high reactivity with rate index values ranged between 11.0 and 18.8%/min. Therefore, the Ca-based sorbents herein assessed can be considered suitable precursors for the formation of CaSO4 based oxygen carriers to be used in CLC systems burning sulfurous fuels.

The role of Al2O3, MgO and CeO2 addition on steam iron process stability to produce pure and renewable hydrogen

Steam iron process represents a technology for H2 production based on iron redox cycles. FexOy are reduced by syngas/carbon to iron, which is subsequently oxidized by steam to produce pure H2. However, the system shows low stability.In this work, the effect of promoters (Al2O3, MgO and CeO2) on FexOy stability is investigated (10 consecutive redox cycles). Bioethanol is used as a reducing agent. The particles are synthesized by coprecipitation method, analysed by BET, XRD, SEM and tested in a fixed bed reactor (675 °C, 1 bar). Pure H2 is obtained controlling the FexOy reduction degree feeding different amounts of ethanol (4.56–1.14 mmol) until no CO is detected in oxidation. The results show that the promoters not only improve the thermal stability of FexOy but also affect its redox activity and react with iron forming spinel structures. MgO led to the highest activity and cyclability (H2 = 0.15 NL; E = 35%).

Consecutive Fe redox cycles decrease bioreducible Fe(III) and Fe isotope fractionations by eliminating small clay particles

Previous work has shown that about 10% of total clay-bound Fe(III) in unaltered nontronite NAu-1 is bioreducible, although it remains unclear how much of the bioreducible Fe pool persists after repeated oscillations between anoxic and oxic conditions. Here, we report on results from an experiment where we monitored the abundance of bioreducible Fe(III) in NAu-1 over three consecutive redox cycles using chemical extractions and Fe isotope analysis to document the changes in the nature and extent of Fe atom exchange. During each cycle, NAu-1 was reduced biotically by Shewanella oneidensis MR-1 and then re-oxidized abiotically by O2 via aeration. By the third reduction period (RP3), the bacteria were only able to reduce 5.7% of the total clay Fe, that is, 40% less than during the first reduction period (RP1). The decrease in bioreducible Fe(III) is attributed to preferential reductive dissolution of Fe(III) from the finest clay particles. Extrapolation of the observed trend implies that, once the reducible Fe of the finest clay particles is removed, around 4% of the total Fe of the clay remains permanently redox-active, presumably as Fe atoms within the octahedral mineral structure that are accessible to the bacteria. The proposed particle size-dependent evolution of bioreducible Fe(III) from RP1 to RP3 is supported by the observed increasing crystalline domain size, preferential Fe dissolution from the smallest aggregates, and decreasing Fe isotope fractionation factors between aqueous Fe(II) and structural Fe(III) and between solid-bound Fe(II) and structural Fe(III). Our results imply that, in redox dynamic environments, the fraction of insoluble clay-bound Fe that is potentially renewable for use by Fe-reducing bacteria is a function of the evolving size distribution of the clay particles.

Understanding of the oxygen uncoupling characteristics of Cu[sbnd]Fe composite oxygen carriers for chemical-looping gasification

A CuFe oxygen carrier (OC) with oxygen uncoupling capability is proposed to be applied in chemical-looping gasification with oxygen uncoupling (CLGOU) technology to increase the gasification rate. Density functional theory analysis shows that a chemical reaction occurs after Cu4O4 cluster adsorption onto α - Fe2O3 (001) surface. Extension of bond length and transfer of charge indicate the composite oxide has been activated. An optimal molar ratio of 2:1 of CuO to Fe2O3 is determined for the preparation the OC and the oxygen transport capacity is determined at 3.65 wt% with a starting oxygen uncoupling temperature is 749.5 °C. The composite OC exhibits good low-temperature oxygen uncoupling characteristics. This OC exhibits stable behaviour and satisfactory oxygen recyclability over 15 consecutive redox cycles at 900 °C / 700 °C oxygen uncoupling and absorbing temperatures. The high stability is corresponding to the calculation result (high adsorption energy of CuO onto Fe2O3). And oxygen uncoupling rates are much higher than the lower limit (0.4 wt%/min) for cracking. The chemical phases and surface micromorphology all retain stable after cycles. Using combined Coats-Redfern and Malek method, the oxygen uncoupling kinetic model of CuFe OC is determined as a shrinking core model (m = 3).

Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO2

AbstractThermochemical splitting of CO2 and H2O via two‐step metal oxide redox cycles offers a promising approach to produce solar fuels. Perovskite‐type oxides with the general formula ABO3 have recently gained attention as an attractive redox material alternative to the state‐of‐the‐art ceria, due to their high structural and thermodynamic tunability. A novel Ce‐substituted lanthanum strontium manganite perovskite‐oxide composite, La3+0.48Sr2+0.52(Ce4+0.06Mn3+0.79)O2.55 (LSC25M75) is introduced, aiming to bridge the gap between ceria and perovskite oxide‐based materials by overcoming their individual thermodynamic constraints. Thermochemical CO2 splitting redox cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an infrared furnace reactor over 100 consecutive redox cycles demonstrates a twofold higher conversion extent to CO than one of the best Mn‐based perovskite oxides, La0.60Sr0.40MnO3. Based on complementary in situ high temperature neutron, synchrotron X‐ray, and electron diffraction experiments, unprecedented structural and mechanistic insight is obtained into thermochemical perovskite oxide materials. A novel CO2 splitting reaction mechanism is presented, involving reversible temperature induced phase transitions from the n = 1 Ruddlesden–Popper phase (Sr1.10La0.64Ce0.26)MnO3.88 (I4/mmm, K2NiF4‐type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden–Popper phase (Sr2.60La0.22Ce0.18)Mn2O6.6 (I4/mmm, Sr3Ti2O7‐type) at re‐oxidation temperature (1000 °C) after the CO2 splitting step.

An in-depth investigation of BaO2/BaO redox oxides for reversible solar thermochemical energy storage

An in-depth investigation of the BaO2/BaO redox couple for reversible solar thermochemical energy storage has been performed. Beyond improving the cyclability and energy storage density of the material, an additional objective of this work is to better understand inconsistencies in the literature surrounding this chemistry. Sintering of BaO2/BaO is a hindrance for practical high temperature energy storage. MgO-stabilized BaO-based materials were synthesized by co-precipitation methods and investigated for improving the longevity and reactivity of the redox materials. The effect of the calcination environment during synthesis on sample purity was evaluated. Energy storage capacity and stability of commercial BaO2 and BaO2 made by co-precipitation methods were compared over 25 consecutive redox cycles. The effect of different amounts of MgO was studied, and it was discovered that with increasing amounts of MgO, the stability and reactivity of BaO2 increases while the energy storage density decreases. 15 wt% MgO/BaO2 exhibits the greatest stable weight gain/loss with a value of 6.62% (estimated 338 kJ/kg). A BaO2/MgO sample with 33 wt% MgO shows good storage capacity with weight gain/loss at 5.65% (corresponds to approximately 290 kJ/kg) over 200 redox cycles.

Evaluation of the redox capability of manganese‑titanium mixed oxides for thermochemical energy storage and chemical looping processes

Manganese oxides are capable of releasing molecular oxygen and regenerate in air under determined conditions. This fact makes these materials interesting for applications in different areas, such as thermochemical energy storage processes, oxygen production by chemical looping air separation (CLAS) or CO2 capture-oriented processes, namely chemical looping combustion (CLC) or chemical looping with oxygen uncoupling (CLOU).In this work, the potential to release molecular oxygen and regenerate in air of different manganese‑titanium mixed oxides with mass fractions of titanium from 0 to 50%, were evaluated in a TGA apparatus. Besides, the reactivity of the mixed oxide samples to the main fuel gases (CO, H2 and CH4) has been also evaluated. Consecutive redox cycles have been performed at different temperatures (850 and 940 °C) using nitrogen during reduction and air in oxidation. The oxygen transport capacity as well as the rate index for oxygen uncoupling and gas-solid reactions have been determined and compared to that found for other materials reported in literature. Moreover, solid phase identification in the different mixed oxide samples by XRD has been addressed in order to determine the oxygen release/regeneration mechanism. Results show a great potential of these materials as low-cost and environmentally friendly oxygen carriers.

Solar Reactor Demonstration of Efficient and Selective Syngas Production via Chemical‐Looping Dry Reforming of Methane over Ceria

Technologies that facilitate the conversion of CH4 and/or CO2 with concentrated sunlight provide a viable strategy for storing solar energy in the form of liquid fuels and reducing anthropogenic greenhouse gas emissions. Herein, a scalable prototype receiver‐reactor is developed to experimentally demonstrate the chemical‐looping, dry reforming of methane over ceria with simulated concentrated solar radiation. Optimal operating conditions are identified by investigating wide ranges of parameters like temperature, gas flowrate, inlet CH4 concentration, initial oxygen nonstoichiometry, and particle size. Ultimately, a selectivity to H2 and CO of greater than 0.93 is observed at reactant conversions of 0.69 and 0.88 for CH4 and CO2, respectively. As a result, the calorific value of the products relative to the reactants is upgraded, and a solar‐to‐fuel conversion efficiency of 10.06% is attained, higher than the previously reported record of 7%. Near‐perfect selectivity to syngas is achieved by operating with low reactant residence times, and if reactions were initiated over oxygen‐deficient ceria. Reactant conversion is enhanced through a reduction in particle size, which enables more rapid kinetics via an increase in surface oxygen availability. Stable performance is demonstrated over 10 consecutive redox cycles under conditions that maximized efficiency for the system presented herein.

Effect of calcination condition on the performance of iron ore in chemical-looping combustion

The calcination of oxygen carrier is the essential step prior to its application in chemical looping combustion, which is significantly influenced by calcination conditions. In this study, a natural iron ore was treated under the combined effect of three key calcination parameters (heating rate, temperature, and duration time). The reactivity performance of calcined iron ores was evaluated with CO as fuel, and the iron ore with the best performance and the parameter with the greatest impact were screened. The results showed that the iron ores treated under different calcination conditions showed evident difference in their reactivity, in which calcination temperature showed the biggest impact on the physicochemical properties of fresh iron ore. The best calcined iron ore was obtained under the heating rate of 15 °C/min, the temperature of 1050 °C, and the duration time of 60 min within the selection limits of three calcination parameters, and its cyclic test showed that the maintained good physical structure of best calcined iron ore guaranteed it against the reactivity decay, a stable recyclability performance was achieved during the consecutive redox cycles. The findings in this study could provide a reference for the further optimization of calcination parameters suitable for commercial oxygen carriers.

Attrition of ilmenite ore during consecutive redox cycles in chemical looping combustion

Chemical looping combustion divides combustion into separate reduction and oxidation (redox) reactions that are linked via a particulate oxygen carrier. The resistance of the oxygen carrier to attrition is an important factor that affects its lifetime. Here, we examined the effects of consecutive redox cycles and degrees of reduction on the attrition of ilmenite ore. First, samples were prepared by subjecting ilmenite to 0 to 150 redox cycles in a fluidized-bed reactor. Next, the resistance of the samples to attrition was examined in a laboratory-scale fluidized bed at room temperature. The attrition rate increased with the number of redox cycles, particularly between 50 and 100 redox cycles, and degree of reduction. Observations of the samples via scanning electron microscopy and measurements of their tapped density and reduction rate revealed that the rapid increase was the result of surface morphological changes, including migration of iron toward the particle surfaces.

Microwave-assisted synthesis of cobalt-manganese oxide for supercapacitor electrodes

Multi-electron surface facilitating swift movement of electrons on the surface of the electrode during the faradaic charge storage reaction establish transition metal oxides as desired electrode materials for supercapacitor application. Cobalt oxide and manganese oxide are electrochemically active materials whose theoretical capacitance is not yet achieved due to certain parameters which are expected to be resolved when they are put together as a single material. In the present work, cobalt manganese oxide, a mixed transition‐metal oxide is prepared via a facile microwave-assisted method. The crystalline phases, vibrational properties and surface morphology of the synthesized oxide are examined. The electrochemical properties of the synthesized oxide are explored in 1 M KOH aqueous electrolyte within a potential window of −0.2 V to 0.7 V. The prepared pseudocapacitive mixed transition metal oxide has a specific capacitance of 323 F g−1 at 0.5 A g−1 current density with 103.9% capacitance retention after 2000 consecutive redox cycles. The enhanced electrochemical properties of the material suggest that the prepared cobalt manganese oxide can be a note-worthy supercapacitor electrode material.

Particle design and oxidation kinetics of iron-manganese oxide redox materials for thermochemical energy storage

High-temperature thermochemical energy storage shows promise in aiding concentrating solar power plants to meet variable, grid-scale electricity demand. In this work, manganese oxide-based mixed metal oxide particles have been designed and tested for thermochemical energy storage. Particles are designed for high energy storage capacity, flowability, and physical and chemical stability. We evaluate the effects of Al2O3, Fe2O3, and ZrO2 in Mn2O3-based spray-dried particles in a TGA between 650 °C and 1200 °C over six consecutive redox cycles. Results are compared with thermodynamic predictions from 400 to 1400 °C under oxidizing and reducing atmospheres. A mixture of 2:1 Fe2O3:Mn2O3 formed iron manganese oxide spinel (MnFe2O4) on calcination, demonstrated the highest thermochemical activity. Intensive mixing was also investigated as a method for preparing MnFe2O4 spinel. Sodium contained in binders/precursors used for spray drying results in the formation of slag phases which negatively impact spray dried particle redox. Spherical particles formed by intensive mixing provided for superior redox reaction. Differential scanning calorimetry (DSC) was performed on MnFe2O4 particles prepared by intensive mixing. The oxidation reaction kinetics of MnFe2O4 was investigated using solid-state kinetics theory and XRD analysis. The reaction proceeds by two different reaction mechanisms. The reaction first proceeds by a diffusion-controlled reaction mechanism (192 ± 2 kJ mol−1 activation energy) with no phase change, followed by a nucleation-growth reaction mechanism (181.4 ± 0.3 kJ mol−1 activation energy). The work reported here supports using low cost MnFe2O4 spinel, formed by intensive mixing of a 2:1 Fe2O3:Mn2O3 composition, as a desirable thermochemical storage active material. Additionally, these results demonstrate the benefit of operating the redox cycle through a cation-vacancy mechanism where the spinel phase maintains its crystal structure and where the reaction rate is stable/robust regardless of particle size.