Rate Of Reoxidation Research Articles

Reoxidation dynamics of the methylene blue reduced by laser-synthesized silver nanoparticle

The catalytic reduction of methylene blue (MB) by metal nanoparticles and subsequent reoxidation is widely recognized and often referred to as a "redox reaction." The reduction mechanism of MB has been extensively studied and is well understood but its reoxidation has received little attention. Here we report an experimental study on the reoxidation dynamics of MB reduced by silver nanoparticles (Ag NPs). Ag NPs synthesized using the Pulsed Laser Ablation (PLAL) technique are used to reduce MB in the presence of the reducing agent NaBH4. The reduction/reoxidation dynamics are monitored using UV−Vis spectroscopy through the decrease/increase of characteristic MB absorbance peak and color switching between methylene blue and leucomethylene (LMB). The influence of the various volumetric concentrations of Ag NPs and NaBH4 on reoxidization dynamics of reduced MB is investigated. The reoxidation of reduced MB exhibited almost linear growth over time for different concentrations of both Ag NPs and NABH4 used in the reduction process. Higher NaBH4 concentrations appeared to decelerate the reoxidation process, while the concentration of Ag NPs had no discernible impact on the reoxidation rate. The reduction of reoxidized MB after adding additional amounts of Ag NPs and NaBH4 revealed that Ag NPs remained stable in the reaction medium but NaBH4 deteriorated. Therefore, the deterioration of the NaBH4 is identified as the major cause of the reoxidation of the reduced MB. A series of reduction-reoxidation experiments using chemically synthesized Ag NPs indicated that MB reoxidation is not limited to laser-synthesized Ag NPs.

'Seeing' the electromagnetic spectrum: spotlight on the cryptochrome photocycle.

Cryptochromes are widely dispersed flavoprotein photoreceptors that regulate numerous developmental responses to light in plants, as well as to stress and entrainment of the circadian clock in animals and humans. All cryptochromes are closely related to an ancient family of light-absorbing flavoenzymes known as photolyases, which use light as an energy source for DNA repair but themselves have no light sensing role. Here we review the means by which plant cryptochromes acquired a light sensing function. This transition involved subtle changes within the flavin binding pocket which gave rise to a visual photocycle consisting of light-inducible and dark-reversible flavin redox state transitions. In this photocycle, light first triggers flavin reduction from an initial dark-adapted resting state (FADox). The reduced state is the biologically active or 'lit' state, correlating with biological activity. Subsequently, the photoreduced flavin reoxidises back to the dark adapted or 'resting' state. Because the rate of reoxidation determines the lifetime of the signaling state, it significantly modulates biological activity. As a consequence of this redox photocycle Crys respond to both the wavelength and the intensity of light, but are in addition regulated by factors such as temperature, oxygen concentration, and cellular metabolites that alter rates of flavin reoxidation even independently of light. Mechanistically, flavin reduction is correlated with conformational change in the protein, which is thought to mediate biological activity through interaction with biological signaling partners. In addition, a second, entirely independent signaling mechanism arises from the cryptochrome photocycle in the form of reactive oxygen species (ROS). These are synthesized during flavin reoxidation, are known mediators of biotic and abiotic stress responses, and have been linked to Cry biological activity in plants and animals. Additional special properties arising from the cryptochrome photocycle include responsivity to electromagnetic fields and their applications in optogenetics. Finally, innovations in methodology such as the use of Nitrogen Vacancy (NV) diamond centers to follow cryptochrome magnetic field sensitivity in vivo are discussed, as well as the potential for a whole new technology of 'magneto-genetics' for future applications in synthetic biology and medicine.

From copper concentrate to Cu-based metal oxides for thermochemical heat storage: Impurity impact analysis and redox performance optimization

Copper oxide is a promising material for thermochemical heat storage, but its high cost remains a challenge for large-scale applications. To address this issue, we explore using relatively cheap copper concentrate as a starting material. We heat treat the concentrate and find that it mainly consists of CuO and CuFe2O4. Our thermogravimetric analysis shows that the re-oxidation performance of the concentrate is poor, with only 12.5 % reaction, which is about 32.5 % lower than that of commercial copper oxide (≥99.5 %). Our investigation reveals that impurities in the concentrate inhibit the re-oxidation of copper oxide, and their removal is essential to improve performance. Based on these findings, we optimize the existing purification process that removes impurities to less than 0.1–0.5 wt%. The resulting copper oxide has a content above 99 % and an extraction rate of over 97 %, which can minimize the waste of raw materials and reduce the influence of impurities. The redox properties of the purified copper oxide are comparable to those of commercially available copper oxide. On this basis, the self-made CuO was modified by doping with cheap MgO and Al2O3, whose reduction conversion rate and reoxidation rate could still reach 97.0 % and 96.3 % after 250 redox cycles.

The dissolution of gold in chloride media under ambient conditions-a re-assessment of mechanisms

This study has shown that the use of methods that measure the initial rates of dissolution of gold using iron(III) as the oxidant under ambient conditions can be deceptive in that, in the absence of efficient methods to re-oxidize iron(II) to iron(III), the rates rapidly decrease with time resulting in predictable poor recoveries. Nevertheless, the results obtained have confirmed that gold(I) is the main product of dissolution with iron(III) as the oxidant and that the initial rate increases parabolically with increasing chloride concentration, is proportional to the square root of the iron(III) concentration and is independent of the acidity. These observations are consistent with the effects of chloride and acid concentrations on the anodic dissolution of gold. The mixed potential model of oxidative dissolution was shown to be operative in this system.The rate of homogeneous re-oxidation of iron(II) to iron(III) by dissolved oxygen under ambient conditions is too slow to be of value in sustaining the high potentials required. On the other hand, the use of low concentrations of chlorate have been shown to be effective in sustaining the reaction as a result of the generation of small quantities of chlorine by oxidation of chloride.

A Conserved Second Sphere Residue Tunes Copper Site Reactivity in Lytic Polysaccharide Monooxygenases.

Lytic polysaccharide monooxygenases (LPMOs) are powerful monocopper enzymes that can activate strong C-H bonds through a mechanism that remains largely unknown. Herein, we investigated the role of a conserved glutamine/glutamate in the second coordination sphere. Mutation of the Gln in NcAA9C to Glu, Asp, or Asn showed that the nature and distance of the headgroup to the copper fine-tune LPMO functionality and copper reactivity. The presence of Glu or Asp close to the copper lowered the reduction potential and decreased the ratio between the reduction and reoxidation rates by up to 500-fold. All mutants showed increased enzyme inactivation, likely due to changes in the confinement of radical intermediates, and displayed changes in a protective hole-hopping pathway. Electron paramagnetic resonance (EPR) and X-ray absorption spectroscopic (XAS) studies gave virtually identical results for all NcAA9C variants, showing that the mutations do not directly perturb the Cu(II) ligand field. DFT calculations indicated that the higher experimental reoxidation rate observed for the Glu mutant could be reconciled if this residue is protonated. Further, for the glutamic acid form, we identified a Cu(III)-hydroxide species formed in a single step on the H2O2 splitting path. This is in contrast to the Cu(II)-hydroxide and hydroxyl intermediates, which are predicted for the WT and the unprotonated glutamate variant. These results show that this second sphere residue is a crucial determinant of the catalytic functioning of the copper-binding histidine brace and provide insights that may help in understanding LPMOs and LPMO-inspired synthetic catalysts.

Mechanistic modeling of redox balance effects on the fermentation of eucalyptus wood-derived xylose to acetone-butanol-ethanol

A Michaelis-Menten-like kinetic model is proposed to simulate the effects of redox balance on Clostridium beijerinckii NCIMB 8052 metabolism during fermentation of biomass-derived xylose to acetone-butanol-ethanol (ABE). The model builds on previous metabolic kinetic models by introducing six mass balance differential equations for the redox pairs NADH/NAD+, NADPH/NADP+, and oxidized/reduced ferredoxin. Batch cultures were conducted using laboratory-grade or eucalyptus-derived xylose, and kinetic parameters were estimated by minimizing the difference between experimental (substrate, metabolites, and cell) and model results using a genetic optimization algorithm. The simulations agreed well with experimental data, and a sensitivity analysis showed that the rate of reoxidation of reduced ferredoxin used for NADPH regeneration is the most relevant reaction for the synthesis of acids and ABE. Furthermore, an increase in NADPH consumption due to the reduction of eucalyptus-derived furfural to furfuryl alcohol was only significant for the production of butanol. Differences in the values of the Michaelis-Menten constants, Km’s, depending on the xylose source, suggest that a decrease in the conversion rates of lactate to pyruvate and acetate to acetyl-CoA considerably affected the biosynthesis of butanol from eucalyptus-derived xylose. The proposed model was effective in assessing the effects of redox balance on ABE fermentation, suggesting its use for the design of metabolic engineering strategies to improve butanol synthesis from furfural-containing biomass hydrolysates.

Exploration of LiCoO2-doped cobalt oxide composite for thermochemical energy storage at high temperature

Due to the high energy storage density, long-term energy storage and long-distance transport possibility, as well as operation possibility under open system, metal oxide-based thermochemical energy storage (TCS) was considered as a promising candidate for efficient energy utilization. Herein, lithium cobalt oxide (LiCoO2)-doped cobalt oxide composite was proposed and investigated for the metal oxide-based TCS application. Doping with LiCoO2 diminished the onset temperature of reduction and re-oxidation by 50 °C and 65 °C, respectively. Also, doping with appropriate amount of LiCoO2 contributed to improve the re-oxidation rate of cobalt oxide-based system under a high cooling rate, resulting the discharging heat (423.8 kJ/kg) increased significantly in comparison with pure cobalt oxide (289.8 kJ/kg) under temperature cooling rate of 20 °C/min. It was found that doping with LiCoO2 decreased the apparent activation energy of reduction and oxidation. Meanwhile, the results obtained from in-situ X-ray diffraction indicate that LiCoO2 was involved in the reduction/re-oxidation process. Moreover, excellent repeatability of the composite was attested in 30 cycles. Hence, doping with LiCoO2 was verified as an effective method to expanding the possibility of cobalt oxide for TCS application.

Screening of optimal dopants on cobalt-based ceramics for high-temperature thermochemical energy storage

To date, numerous dopants have been investigated to promote the initial heat storage performance or sintering resistance of Co3O4 for thermochemical energy storage. Owing to the different synthesis methods and heat storage test conditions, the performances of these materials are not comparable. In this work, nine dopants, including four unreported dopants, were systematically compared and screened under the same conditions. Results demonstrated that for five redox cycles, the Si- and Mg- doped sample had adverse effects on the conversion rate, and the others maintained the conversion rate above 0.95. Cu and Zr doping exhibited the highest re-oxidation rate values, and those of the others were lower than that of pure cobalt oxide. Moreover, except for Cu and Zr, all dopants adversely affected energy density. According to aforementioned heat storage properties, Cu-, Zr-doped samples, and pure cobalt oxide were subjected to 100 cycles for further comparison. The Cu-doped sample exhibited a higher re-oxidation rate and energy density, and the Zr-doped sample exhibited more stable cyclability. Finally, by doping Cu and Zr, the micromorphology evolution was more stable than that of pure oxide for multiple cycles, and X-ray photoelectron spectroscopy results revealed that the higher surface oxygen was the primary reason for the superior performance facilitated by Cu and Zr. The Cu- and Zr-modified cobalt oxides were the potential candidates for thermochemical energy storage.

Towards Ge-based electronic devices: Increased longevity of alkanethiol-passivated Ge(100) in low humidity environments

Germanium is a critically important material for future complementary metal-oxide-semiconductor devices, however, to maximise its potential it is necessary to develop a robust passivation process that prevents Ge re-oxidation for a queue time of 24 h. Self-assembled monolayers (SAMs) of alkanethiols on Ge have previously been shown to inhibit oxidation; however, re-oxidation eventually occurs when exposed to ambient conditions. Herein, it is shown that humidity plays a key role in the degradation of the SAM, ultimately resulting in re-oxidation. To demonstrate this, thiol-passivated Ge(100) surfaces are exposed to controlled humidity environments with different levels of relative humidity (RH). The rate of re-oxidation of the Ge surfaces are tracked using X-ray photoelectron spectroscopy and water contact angle analysis to discern what role RH plays in the re-oxidation of the Ge and the degradation of the SAM passivation. Atomic force microscopy data is presented to show that humidity-mediated re-oxidation of the Ge has little or no impact on the route mean square roughness of those surfaces. Finally, atomistic modelling of thiol-SAM passivated Ge in the presence of water molecules has been studied using first principles density functional theory in order to simulate experimental conditions and to understand the atomic level processes that determine stability in hydrophilic and hydrophobic configurations.

Monitoring by in situ NAP-XPS of active sites for CO2 methanation on a Ni/CeO2 catalyst

Ni/CeO2 catalysts are very active and selective for total hydrogenation of CO2 to methane, but the nature of the active sites is still unclear. The surface of a Ni/CeO2 catalyst has been monitored under CO2 methanation conditions by Near Ambient Pressure-XPS (NAP-XPS) using synchrotron radiation, and has been concluded that the species involved in the redox processes taking place during the CO2 methanation mechanism are the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ pairs. In addition, a small fraction of nickel is present on the catalyst surface forming NiO and Ni2+-carbonates/hydroxyls (around 20% of the total surface nickel), but these species do not participate in the redox processes of the methanation mechanism.Under CO2 methanation conditions the H2 reduction rate of the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ couples is much faster than their CO2 reoxidation rate (2 times faster, at least, at 300ºC), but a certain proportion of nickel always remains oxidized under reaction conditions. The high activity of Ni/CeO2 catalysts for CO2 methanation is tentatively attributed to the simultaneous presence of Ni2+-CeO2 and Ni0 active sites where CO2 and H2 are expected to be efficiently dissociated, respectively.

Effect of Alkaline Additives over V-Based Catalysts Supported on y- Al2O3 for Propane Oxidative Dehydrogenation

The olefins product are more reactive than the corresponding alkanes, and may easily be super-oxidised into CO and CO2 in selective oxidation of alkanes. The proper balance of acid and base sites on the V-based catalyst surface plays a decisive role in limiting the complete oxidation of alkanes. Therefore, our goal was to evaluate the promoting effect of alkali metals and the support effect using boehmite as precursor for the propane oxidative dehydrogenation (ODH) reaction. Catalysts were prepared via co-impregnation of V and Na or K on a synthesized alumina support. The following characterization techniques were used: N2 adsorption-desorption, X-ray diffraction (XRD), temperature programmed reduction (TPR) and isopropanol decomposition testes to evaluate the acid-base character. The catalyst performance in the propane ODH reaction was evaluated at the O2:C3H8:He molar ratios of 5:2:4, 6:1:4, and 4:3:4. The K-doped catalysts exhibited higher propene selectivity owing to the modification of acid-base character that rendered the weaker interaction between olefinic intermediate and more basic catalyst surface. A high molar fraction of O2 of the reaction mixture minimized coke formation and a high reoxidation rate possibly increased catalytic activity and propene selectivity.

Oxidative Dehydrogenation of Propane over Vanadium Catalyst Supported on Alkali-Modifiedx-Al2O3

The propane oxidative dehydrogenation (ODH) reaction has been considered as an alternative method for propene production owing to its exothermic nature, which renders it environmentally friendly. The use of alkaline promoters for supported V catalysts can increase propene selectivity and partially inhibit the formation of CO and CO2. Our goal was to evaluate the promoting effect of K and Na and the support effect using gibbsite as precursor for the propane ODH reaction. Catalysts were prepared via co-impregnation of V and alkali metals on a previously prepared alumina support and were characterized using N2 adsorption-desorption, X-ray diffraction, temperature-programmed reduction, and isopropanol decomposition tests to evaluate their acid-base properties. The activity of the synthesized catalysts for the propane ODH reaction was evaluated at the O2:C3H8:He molar ratios of 5:2:4, 6:1:4, and 4:3:4. The addition of alkali metals to the V catalysts increased propane conversion and propene selectivity; moreover, both parameters increased with increasing molar fraction of O2 in the reactants. K doping increased the propene selectivity of the doped catalysts, because it inhibited a large fraction of catalytic surface acidic sites. A high molar fraction of O2 in the reactants facilitated the regeneration of the catalyst, whereas a high reoxidation rate improved catalytic activity and propene selectivity.

Determining the Role of Fe-Doping on Promoting the Thermochemical Energy Storage Performance of (Mn1- x Fex )3 O4 Spinels.

Mn oxides are promising materials for thermochemical heat store, but slow reoxidation of Mn3 O4 to Mn2 O3 limits efficiency. In contrast, (Mn1- x Fex )3 O4 oxides show an enhanced transformation rate, but fundamental understanding of the role played by Fe cations is lacking. Here, nanoscale characterization of Fe-doped Mn oxides is performed to elucidate how Fe incorporation influences solid-state transformations. X-ray diffraction reveals the presence of two distinct spinel phases, cubic jacobsite and tetragonal hausmannite for samples with more than 10% of Fe. Chemical mapping exposes wide variation of Fe content between grains, but an even distribution within crystallites. Due to the similarities of spinels structures, high-resolution scanning transmission electron microscopy cannot discriminate unambiguously between them, but Fe-enriched crystallites likely correspond to jacobsite. In situ X-ray absorption spectroscopy confirms that increasing Fe content up to 20% boosts the reoxidation rate, leading to the transformation of Mn2+ in the spinel phase to Mn3+ in bixbyite. Extended X-ray absorption fine structure shows that FeO length is larger than MnO,but both electron energy loss spectroscopy and X-ray absorption near edge structure indicate that iron is always present as Fe3+ in octahedral sites. These structural modifications mayfacilitate ionic diffusion during bixbyite formation.

Superior thermochemical energy storage performance of the Co3O4/CoO redox couple with a cubic micro-nanostructure

Co3O4 has been regarded as one of the most promising materials for redox energy storage due to its high theoretical conversion and complete redox reversibility. However, when undergoing charge-discharge cycles at high temperature, the material becomes sintered. The reversibility of the cobaltous oxides highly deteriorates, and the reoxidation reaction rate notably decreases. In addition, the obvious thermal hysteresis in the redox reaction process also limits the energy storage performance of the material. In this work, Co3O4 with different micro-nanostructured morphologies is synthesized to evaluate its thermochemical energy storage performance. The capacity of the synthesized and commercial Co3O4 is tested by thermogravimetric analysis (TGA). The fresh and recycled oxides are well characterized. Results suggest that all samples show complete reversibility, and the conversion decreases slightly after 30 cycles. The synthesized samples show better performance, especially cubic Co3O4. The reoxidation rate of the commercial sample is approximately 150 μmol/min/g, but that of the cubic sample is approximately 300-200 μmol/min/g in the first five cycles. The oxidation rate of the cubic sample decreases and finally remains at about 180 μmol/min/g from the 5th cycle to the 30th cycle. After 5 charge-discharge cycles, the commercial Co3O4 is severely sintered, while the cubic oxides are only slightly sintered. After 30 redox cycles, the morphology of commercial oxides completely changed, while that of the cubic oxide retains the approximate shape of a cube. Moreover, the thermal hysteresis values of the commercial and cubic Co3O4 are 24.7°C and 10.1°C, respectively, suggesting that the energy loss of cubic oxides is much smaller. The higher exothermic temperature of the cubic Co3O4 redox process also indicates that it can supply high-grade thermal energy.

Combined Iodine- and Sulfur-Based Treatments for an Effective Passivation of GeSn Surface

GeSn alloys are metastable semiconductors that have been proposed as building blocks for silicon-integrated short-wave and mid-wave infrared photonic and sensing platforms. Exploiting these semiconductors requires, however, the control of their epitaxy and their surface chemistry to reduce non-radiative recombination that hinders the efficiency of optoelectronic devices. Herein, we demonstrate that a combined sulfur- and iodine-based treatments yields effective passivation of Ge and Ge0.9Sn0.1 surfaces. X-ray photoemission spectroscopy and in situ spectroscopic ellipsometry measurements were used to investigate the dynamics of surface stability and track the reoxidation mechanisms. Our analysis shows that the largest reduction in oxide after HI treatment, while HF+(NH4)2S results in a lower re-oxidation rate. A combined HI+(NH4)2S treatment preserves the lowest oxide ratio <10 % up to 1 hour of air exposure, while less than half of the initial oxide coverage is reached after 4 hours. These results highlight the potential of S- and I-based treatments in stabilizing the GeSn surface chemistry thus enabling a passivation method that is compatible with materials and device processing.

LaNi1-xCoxO3 perovskites for methane combustion by chemical looping

Perovskite oxides with composition LaNi1-xCoxO3 (x = 0.0; 0.2; 0.5; 1.0) were studied in the chemical looping combustion (CLC) of methane. In situ XRDs under methane flow at different temperatures of LaNi0.5Co0.5O3 and LaNi0.8Co0.2O3 samples, suggest that a different reduction mechanism is activated depending on the composition of the starting material: LaNi0.5Co0.5O3 reduction involved the formation of LaNi0.5Co0.5O2.5 and Co3O4 as intermediate phases, whereas LaNi0.8Co0.2O3 showed an oxygen-deficient perovskite structure as well as spinel and NiO phases. At temperatures higher than 700 °C, in both samples, the final phases evidenced under methane flow were Ni0, Co0 and La2O3. The substitution of nickel for cobalt contributed to a decrease in the quantity of active oxygen for the methane combustion reaction, making the catalysts less active. However, the insertion of cobalt improved the re-oxidation rate of the catalysts and lead to materials that were less contaminated by coke deposits. During methane CLC, the formation of coke tended to decrease for all samples with the progress of the reaction. After CLC process, the catalysts external morphology was appeared much more uniform. Among the LaNi1-xCoxO3 samples resistant to coke formation (x = 0.5 and 1.0), LaNi0.5Co0.5O3 lead to a complete reduction in less time and is therefore considered the best CLC sample of the series.

Photochemical degradation of iron(III) citrate/citric acid aerosol quantified with the combination of three complementary experimental techniques and a kinetic process model

Abstract. Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially in the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing the reduction of iron(III) and the oxidation of carboxylate ligands. In the presence of O2, the ensuing radical chemistry leads to further decarboxylation, and the production of .OH, HO2., peroxides, and oxygenated volatile organic compounds, contributing to particle mass loss. The .OH, HO2., and peroxides in turn reoxidize iron(II) back to iron(III), closing a photocatalytic cycle. This cycle is repeated, resulting in continual mass loss due to the release of CO2 and other volatile compounds. In a cold and/or dry atmosphere, organic aerosol particles tend to attain highly viscous states. While the impact of reduced mobility of aerosol constituents on dark chemical reactions has received substantial attention, studies on the effect of high viscosity on photochemical processes are scarce. Here, we choose iron(III) citrate (FeIII(Cit)) as a model light-absorbing iron carboxylate complex that induces citric acid (CA) degradation to investigate how transport limitations influence photochemical processes. Three complementary experimental approaches were used to investigate kinetic transport limitations. The mass loss of single, levitated particles was measured with an electrodynamic balance, the oxidation state of deposited particles was measured with X-ray spectromicroscopy, and HO2. radical production and release into the gas phase was observed in coated-wall flow-tube experiments. We observed significant photochemical degradation with up to 80 % mass loss within 24 h of light exposure. Interestingly, we also observed that mass loss always accelerated during irradiation, resulting in an increase of the mass loss rate by about a factor of 10. When we increased relative humidity (RH), the observed particle mass loss rate also increased. This is consistent with strong kinetic transport limitations for highly viscous particles. To quantitatively compare these experiments and determine important physical and chemical parameters, a numerical multilayered photochemical reaction and diffusion (PRAD) model was developed that treats chemical reactions and the transport of various species. The PRAD model was tuned to simultaneously reproduce all experimental results as closely as possible and captured the essential chemistry and transport during irradiation. In particular, the photolysis rate of FeIII, the reoxidation rate of FeII, HO2. production, and the diffusivity of O2 in aqueous FeIII(Cit) ∕ CA system as function of RH and FeIII(Cit) ∕ CA molar ratio could be constrained. This led to satisfactory agreement within model uncertainty for most but not all experiments performed. Photochemical degradation under atmospheric conditions predicted by the PRAD model shows that release of CO2 and repartitioning of organic compounds to the gas phase may be very important when attempting to accurately predict organic aerosol aging processes.

Compositional and operational impacts on the thermochemical reduction of CO2 to CO by iron oxide/yttria-stabilized zirconia.

Ferrites have potential for use as active materials in solar-thermochemical cycles because of their versatile redox chemistry. Such cycles utilize solar-thermal energy for the production of hydrogen from water and carbon monoxide from carbon dioxide. Although ferrites offer the potential for deep levels of reduction (e.g., stoichiometric conversion of magnetite to wüstite) and correspondingly large per-cycle product yields, in practice reactions are limited to surface regions made smaller by rapid sintering and agglomeration. Combining ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves the cyclability of the ferrites and enables a move away from powder to monolithic systems. We have studied the behavior of iron oxides composited with YSZ using thermogravimetric analysis under operando conditions. Samples in which the iron was fully dissolved within the YSZ matrix showed greater overall extent of thermochemical redox and higher rate of reaction than samples with equal iron loading but in which the iron was only partially dissolved, with the rest existing as agglomerates of iron oxide within the ceramic matrix. Varying the yttria content of the YSZ revealed a maximum thermochemical capacity (yield per cycle) for 6 mol% Y2O3 in YSZ. The first thermochemical redox cycle performed for each sample resulted in a net mass loss that was proportional to the iron oxide loading in the material and was stoichiometrically consistent with complete reduction of Fe2O3 to Fe3O4 and further partial reduction of the Fe3O4 to FeO. Mass gains upon reaction with CO2 were consistent with re-oxidation of the FeO fraction back to Fe3O4. The Fe dissolved in the YSZ matrix, however, is capable of cycling stoichiometrically between Fe3+ and Fe2+. Varying the re-oxidation temperature between 1000 and 1200 °C highlighted the trade-off between re-oxidation rate and equilibrium limitations.

Additive Effect of Iron and Copper Sulfates in Heteropolyacid Solution As a Cathode Reaction Mediator on the Performance of Redox Flow PEFCs

Since the Industrial Revolution, we have relied heavily on fossil resources. Even today, they are indispensable for our cultural activities, but environmental problems such as global warming and soil pollution caused by their combustion have been concerned. These issues have grown an interest in alternative fuels. In this situation, hydrogen has been regarded as promising next-generation energy sources in terms of energy content and environmental load. Moreover, fuel cells, which directly convert the chemical energy of hydrogen into electrical energy, are cleaner and more efficient than conventional power generation devices.Polymer electrolyte fuel cells (PEFCs) have been already commercialized as power sources for automobiles due to low operating temperature and high current density. However, the carbon-supported platinum or platinum-based alloy is widely applied as the cathode. Therefore, these fuel cells are very costly and required to reduce the platinum usage for more widespread application of PEFCs.We proposed a redox flow PEFC as one of the solutions to this problem [1]. The redox flow PEFC employing the heteropolyacid solution of H6PV3Mo9O40 as a redox mediator for the cathode reaction was constructed. In this system, the cathode is free from noble metals for electrocatalyst. The electrochemical reduction of heteropolyanion over the carbon cathode and the subsequent re-oxidation of the partially reduced heteropolyanion by exposure to the dissolved oxygen in the regenerator are important processes for the continuous power generation. Especially, the improvement of re-oxidation rate contributes to a stable operation with higher current density. Therefore, the development of catalyst to promote this process will lead to the improved performance.For the heteropolytungstates of H3PW12O40 and H5SiVW11O40, it has been reported that the re-oxidation of their reduced anion species are efficiently catalyzed by a combination of copper ion and iron ion [2].The catalytic pathway consists of a reversible reaction between Cu(II) and Fe(II), followed by fast oxidation of reduced heteropolyanion and Cu(I) by Fe(III) and O2, respectively, to regenerate Fe(II) and Cu(II). In this study, therefore, the influence of the addition of iron and copper sulfates to the H6PV3Mo9O40 solution on the cell performance was evaluated. As a result, the obtained current density and the re-oxidation rate were improved by the addition. Also, in the discharge operation at a constant current the terminal voltage remained higher in using the solution with these sulfates than without additives. Accordingly, the addition of iron and copper sulfates to H6PV3Mo9O40 solution contributed to the improvement of the performance of the redox flow PEFCs.-------------------------------------------------------------------------------------[1] T. Matsui, E. Morikawa, S. Nakada, T. Okanishi, H. Muroyama, Y. Hirao, T. Takahashi, K. Eguchi. ACS Appl. Mater. Inter.18119-18125 (2016).[2] M. Kim, M. Chamack, Y. V. Geletii, C. L. Hill, Inorg. Chem. 311-318 (2018)

Improving the redox performance of Mn2O3/Mn3O4 pair by Si doping to be used as thermochemical energy storage for concentrated solar power plants

Thermal energy storage plays a significant role in concentrated solar power plants. Particularly, thermochemical energy storage has been proposed as a promising future candidate due to its high energy density. One of the most studied thermochemical reactions for this application are redox reactions using metallic oxides. The simplicity of using air as heat transfer fluid and reactant comprises important advantages. Among the studied materials, Mn2O3 has been deeply investigated due to its low cost and toxicity, although its reversibility has been shown to be highly deteriorated when exposed to high temperatures. The main negative effects found in the material are grain growth and densification, known as sintering, causing cyclability loss. Doping of ceramic materials has been proven to be an effective technique to control the sintering effects caused by high temperatures, especially in the production of metallurgical products. In this work, the effect of doping Mn2O3 with Si4+ as a sintering inhibitor for thermochemical energy storage has been studied deeply for the first time. The capacity to withstand 40 redox cycles with different quantities of dopant using two different synthesis routes was tested by TGA analysis. As a main result, dopant segregation on the grain boundaries caused a noticeable reduction of the grain growth and particle shrinkage. Additionally, all the doped samples presented a re-oxidation rate improvement. These two effects worked in a synergetic way providing a noticeable improvement on the cycling behavior of the material, which was particularly improved for low Si content (x = 0.02) using sol-gel synthesis method.