Redox Cycling Experiments Research Articles

Robust Superconductivity in Infinite-Layer Nickelates.

The recent discovery of nickelate superconductivity represents an important step toward understanding the four-decade mastery of unconventional high-temperature superconductivity. However, the synthesis of the infinite-layer nickelate superconductors shows great challenges. Particularly, surface capping layers are usually unitized to facilitate the sample synthesis. This leads to an important question whether nickelate superconductors with d9 configuration and ultralow valence of Ni1+ are in metastable state and whether nickelate superconductivity can be robust? In this work, a series of redox cycling experiments are performed across the phase transition between perovskite Nd0.8Sr0.2NiO3 and infinite-layer Nd0.8Sr0.2NiO2. The infinite-layer Nd0.8Sr0.2NiO2 is quite robust in the redox environment and can survive the cycling experiments with unchanged crystallographic quality. However, as the cycling number goes on, the perovskite Nd0.8Sr0.2NiO3 shows structural degradation, suggesting stability of nickelate superconductivity is not restricted by the ultralow valence of Ni1+, but by the quality of its perovskite precursor. The observed robustness of infinite-layer Nd0.8Sr0.2NiO2 up to ten redox cycles further indicates that if an ideal high-quality perovskite precursor can be obtained, infinite-layer nickelate superconductivity can be very stable and sustainable under environmental conditions. This work provides important implications for potential device applications for nickelate superconductors.

Evidence of redox cycling as a sub-mechanism in hydrogen production during ethanol steam reforming over La0.7Sr0.3MnO3-x perovskite oxide catalysts

Ethanol steam reforming (ESR) is of societal interest. In this work, experiments were conducted to ascertain if some of the H2 is produced by a redox cycle involving H2O filling oxygen vacancies over reducible oxide catalysts. Redox cycling experiments were performed over La0.7Sr0.3MnO3-x(100) in ultra-high vacuum. It was found that H2 was produced from redox cycling with alternating ethanol and water exposures over La0.7Sr0.3MnO3-x(100), with both half-cycles occurring at temperatures ≤800 K. In the first half-cycle, ethanol ‘directly’ reduced the surface to create oxygen vacancies (not by a CO intermediate), and in the second half-cycle water filled oxygen vacancies to make H2. The H2 production during the water exposure has a half-cycle turnover frequency of >3.2 × 10-2 molecules site-1 s−1 in the temperature range of 700–800 K, which is fast enough to be part of the ESR full catalytic cycle. Flowing both reactant gases together, ethanol and water, over La0.7Sr0.3MnO3-x(100) and La0.7Sr0.3MnO3-x powders significantly increases hydrogen production compared to pure ethanol. The results suggest that steady state ESR includes a sub-mechanism of ethanol ‘directly’ reducing the surface to create oxygen vacancy, and water filling oxygen vacancy to make some of the H2 by a Mars van Krevelen type mechanism.

Multifunctional nanopore electrode array method for characterizing and manipulating single entities in attoliter-volume enclosures

Structurally regular nanopore arrays fabricated to contain independently controllable annular electrodes represent a new kind of architecture capable of electrochemically addressing small collections of matter—down to the single entity (molecule, particle, and biological cell) level. Furthermore, these nanopore electrode arrays (NEAs) can also be interrogated optically to achieve single entity spectroelectrochemistry. Larger entities such as nanoparticles and single bacterial cells are investigated by dark-field scattering and potential-controlled single-cell luminescence experiments, respectively, while NEA-confined molecules are probed by single molecule luminescence. By carrying out these experiments in arrays of identically constructed nanopores, massively parallel collections of single entities can be investigated simultaneously. The multilayer metal–insulator design of the NEAs enables highly efficient redox cycling experiments with large increases in analytical sensitivity for chemical sensing applications. NEAs may also be augmented with an additional orthogonally designed nanopore layer, such as a structured block copolymer, to achieve hierarchically organized multilayer structures with multiple stimulus-responsive transport control mechanisms. Finally, NEAs constructed with a transparent bottom layer permit optical access to the interior of the nanopore, which can result in the cutoff of far-field mode propagation, effectively trapping radiation in an ultrasmall volume inside the nanopore. The bottom metal layer may be used as both a working electrode and an optical cladding layer, thus, producing bifunctional electrochemical zero-mode waveguide architectures capable of carrying out spectroelectrochemical investigations down to the single molecule level.

Characteristic Study of Briquette Cyanobacteria as Fuel in Chemical Looping Combustion with Hematite as Oxygen Carrier

Due to the more and more serious cyanobacteria bloom problem, it is particularly urgent to find a technology suitable for large-scale disposal and the efficient recovery of abundant nitrogen and phosphorus resources in cyanobacteria. The combination of chemical looping combustion (CLC) and biomass densification technology is thought to be a promising utilization selection. Based on the experimental results, the mechanical strength and energy density of briquette cyanobacteria are evidently increased with the compressive load; whereas, 10% is the optimal moisture content in the densification process. A higher heating rate in TGA would result in the damage of the internal structure of the briquette cyanobacteria, which are conducive to the carbon conversion efficiency. The presence of a hematite oxygen carrier would enhance the carbon conversion and catalyzed crack liquid products. CO2 yield is increased 25 percent and CH4 yield is decreased 50 percent at 900 °C in the CLC process. In addition, the lower temperature and reduction atmosphere in CLC would result in a lower NO emission concentration. The reactivity and porous property of hematite OC in CLC also increased during 10 redox cycle experiments. The CLC process accelerates the generation of CaH2P2O7 and CaHPO4 in cyanobacteria ash, which is more conducive to phosphorus recovery.

Solar thermochemical fuel production from H2O and CO2 splitting via two-step redox cycling of reticulated porous ceria structures integrated in a monolithic cavity-type reactor

Solar thermochemical H2O and CO2 splitting cycles represent an efficient route for converting high temperature concentrated solar heat into valuable chemical energy carriers (solar fuels). A new monolithic solar reactor compatible with ceria redox reactions was designed, constructed and tested under concentrated solar radiation. The ceria redox material was shaped and integrated as reticulated porous structures with controlled cell sizes and gradient (10–60 ppi, pores per inch) enabling efficient volumetric solar radiation absorption and micro-scale interconnected porosity favouring the solid-gas reactions. Temperature-swing redox cycling experiments were performed to demonstrate solar reactor reliability during continuous operation. The foams were first thermally activated by increasing the reactor temperature (1400–1450 °C) for O2 release and then exposed to H2O or CO2 stream to produce pure H2 or CO (700–1100 °C), allowing cyclic operation in the same reactor. The influence of operating conditions (including reduction and oxidation temperatures, pressure and type of oxidizing gas) on reactor performance was investigated. An increase of the reduction temperature or a decrease of the operating pressure improved both the ceria reduction extent and fuel production yields (up to 341 μmol/g), while a decrease of the CO:CO2 ratio (by increasing total inlet gas flow-rate) or an increase of the inlet CO2 concentration enhanced oxidation rates (up to 9.3 mL/g/min). The obtained fuel production rates outperformed the maximum previously reported values by up to 8 times using the highly-reactive manufactured ceria porous foams cycled between 1400 °C and 900 °C with oxidation performed in 100% CO2 upon dynamic cooling. An average H2/CO production of ∼280 Ncm3/cycle (64 cycles performed) was achieved with solar-to-fuel efficiency up to ∼7.5% and remarkable material performance stability.

Redox stability of metal-supported fuel cells with nickel/gadolinium-doped ceria anode

Metal-supported fuel cells (MSCs) are promising candidates for not only stationary but also mobile applications. Their appeal is in their potential to withstand reoxidation of the anode, which might occur by an interruption of the fuel supply or an emergency shutdown of the fuel cell system. A novel nickel/gadolinium-doped ceria anode (Ni/GDC) was recently introduced in a MSC concept of Plansee, almost doubling power density compared to cells with a nickel/yttria-doped zirconia (Ni/YSZ) anode. In this study, both cell concepts are compared concerning their ability to tolerate harsh redox cycles. Therefore, controlled redox cycles of the anodes were conducted at different temperatures. The response of the cell's power output to the redox cycling experiments was continuously recorded. In the case of MSCs with a Ni/YSZ anode, strong degradation occurs after redox cycling. In contrast, cells with a Ni/GDC anode exhibit significantly improved redox tolerance and cell performance improves with the number of redox cycles. For understanding this behavior, microstructural investigations of the Ni/GDC anode and the adjacent electrolyte were performed by FIB-SEM. The long-term redox behavior of MSCs with a Ni/GDC anode was also investigated by conducting more comprehensive redox cycles at 400 °C, 500 °C, and 600 °C.

Ni Substitution Effect on the Fe Position of Spinel Fe2MnO4 Particles for Chemical Looping Combustion.

The purpose of this study was to use a spinel structure to improve the performance and stability of chemical looping combustion processes. The oxygen carrier employed was Fe2MnO4, in which Ni was substituted at the Fe sites. Fe2-xNixMnO4 spinel particles were successfully synthesized by a sol-gel method. The obtained particles were characterized by X-ray diffraction (XRD), scanning electron microscopy, and CH4-/CO-temperature programmed desorption experiments. The XRD analysis confirmed that all the synthesized particles presented spinel structure. The performance of the particles was evaluated in redox cycle experiments under H2/air and CH4/air at 850 °C using a thermogravimetric analyzer. The Ni-substituted particles exhibited a higher performance than Fe2Mn1, being Fe1Ni1Mn1 the sample with the highest oxygen transfer capacity (19.68 wt% in H2/air and 15.90 wt% in CH4/air).

Redox cycles with doped calcium manganites for thermochemical energy storage to 1000 °C

Redox cycles of doped calcium manganite perovskites (CaMnO3−δ) are studied for cost-effective thermochemical energy storage at temperatures up to 1000 °C for concentrating solar power and other applications. If the thermodynamics and kinetics for heat-driven reduction can be tailored for high temperatures and industrially accessible low O2 partial pressures (PO2⩾10-4 bar), perovskite redox cycles can offer high specific energy storage at temperatures much higher than state-of-the-art molten-salt subsystems. To this end, a range of A-site and B-site doped CaMnO3−δ were screened for their reducibility at 900 °C and PO2≈10-4 bar via thermogravimetric analysis. For compositions with high reducibility, notably A-site doped Ca1−xSrxMnO3−δ (x=0.05 and 0.10) and B-site doped CaCryMn1−yO3−δ (y=0.05 and 0.10), oxygen non-stoichiometry δ with respect to temperature and PO2 were measured and used to fit thermodynamic parameters of a two-reaction, point-defect model of the redox process for the two prominent crystalline phases (orthorhombic and cubic) that the perovskites occupy during the cycle. The fits compare favorably to differential scanning calorimetry measurements with the magnitude of the overall reduction enthalpies decreasing as the degree of reduction increases and the perovskites shift from orthorhombic to cubic crystalline phases. Based on thermodynamic limits, redox cycles of both Ca1−xSrxMnO3−δ compositions between air at 500 °C and PO2≈10-4 bar at 900 °C can store and release up to ≈700 kJ kg−1 with over 50% of the total energy stored as chemical energy. This is approximately 140 kJ kg−1 more chemical energy than the thermodynamic limits for CaCryMn1−yO3−δ compositions under the same cycle conditions. Approaching these thermodynamic limits for the specific energy storage of these redox cycles in a concentrating solar plant requires fast kinetics for perovskite reduction in the solar receiver and for reoxidation in the heat recovery reactor. Isothermal packed-bed redox cycling experiments of Ca1−xSrxMnO3−δ and CaCryMn1−yO3−δ compositions at temperatures up to 1000 °C show that reoxidation is fast compared to reduction. Thus, specific thermochemical energy storage is limited by residence times available for high-temperature reduction. The Sr-doped compositions approach higher fractions (≈90% or more) of the specific energy storage equilibrium limit after 300 s of reduction in the packed bed configuration above 800 °C and completely reoxidize in ⩽20 s in air. Non-isothermal cycling with heating from 500 °C to 900 °C in low PO2≈10-4 bar and subsequent reoxidation during cooling in air back to 500 °C demonstrate excellent chemical stability over 1000 cycles for all doped CaMnO3−δ compositions tested. The results suggest that these redox cycles may offer a viable energy storage subsystem with long-term stability for future concentrating solar plants and other high-temperature energy storage applications.

Fe2O3–Al2O3 oxygen carrier materials for chemical looping combustion, a redox thermodynamic and thermogravimetric evaluation in the presence of H2S

Alumina-supported Fe2O3 oxygen carrier material (OCM) system is among the most promising OCM systems for solid and gaseous fuel CLC. This work utilizes a comprehensive thermogravimetric and thermodynamic equilibrium approach to redox and CLOU performance, oxygen transfer capacity, reduction rate and sulfur tolerance of the Fe2O3 impregnated on Al2O3 OCM. Thermodynamic evaluations reveal that the beneficial composition range lies in a wide range of 7.5–34% molar Fe2O3 ratios. This is the range at which aluminum-rich corundum phase, i.e., (Al, Fe)2O3, remains stable throughout the oxidizing to very reducing oxygen partial pressures in fuel reactor. The experimental system in this study contains 20 mass% Fe2O3, i.e., XFe = 13.8% molar which lies well within this interval. Deep redox cycle experiments confirm the thermodynamic modeling and during the long residence time of this experiment, the sample is almost fully reduced and exhibits its thermodynamic redox oxygen capacity of close to 1.5 mass%. Extension of the deep redox cycles to 15 cycles induces no performance deterioration in terms of capacity, rate of reduction or morphological failure. The redox experiment under sour reducing gas indicates no H2S poisoning for the 20 mass% Fe2O3 supported on Al2O3 OCM. The findings that this system is not affected with the H2S content of the gas, and the prediction of the SO2 release from the fuel reactor is in good agreement with our recent reactor testing findings available in the literature.

Effect of Reduction Treatment on CO Oxidation with CeO2 Nanorod‐Supported CuOx Catalysts

AbstractUnderstanding the correlation of thermal treatment with catalyst activity provides mechanistic information about how the catalysts can be activated or deactivated. In this paper, we report that a comparative study was conducted on CeO2 nanorod‐supported CuOx catalysts before and after reduction treatment to gain insight on the effects of the copper oxidation state and catalyst–support interfacial interactions on CO oxidation. X‐ray diffraction, Raman spectroscopy, hydrogen temperature‐programmed reduction (H2‐TPR), and transmission electron microscopy (TEM) were used in a comprehensive way to study the effects of CuOx (0≤x≤1) composition as well as their spatial distribution on CeO2 nanorods on the H2 consumption and CO oxidation of the catalysts. These techniques were then used to gain a better understanding of the correlation between the different copper species (α, β, and γ‐type CuOx) and the multiple reduction (H2 consumption) peaks, found in the H2‐TPR data during the first run and during in situ reduction and redox cycling experiments. Also concluded from this study is that an abundance of surface defects are found on CeO2 nanorods from high‐resolution TEM, which consequently may be critical to strong CuOx(0≤x≤1)–CeO2−x(0≤x≤0.5) interactions, therefore resulting in the improved low‐temperature catalytic activity.

Reduction and oxidation performance evaluation of manganese-based iron, cobalt, nickel, and copper bimetallic oxide oxygen carriers for chemical-looping combustion

Transition metal oxides like Fe2O3, Co3O4, NiO, and CuO have mainly been used in the chemical-looping combustion (CLC) reaction as oxygen carrier particles because of their outstanding performances due to high reduction potential. However, monometal oxides have a serious problem of rapid deactivation due to agglomeration between each particle after several redox cycles. In order to solve this problem, bimetal oxides based on Mn, which undergoes no deactivation, were synthesized. Mn-based bimetal oxygen carrier particles Fea-Mnb-Ox, Coa-Mnb-Ox, Nia-Mnb-Ox, and Cua-Mnb-Ox were synthesized by using a sol–gel method. The range of a and b in the particle label obtained in this study is 0.5–2.5, depending on the metal content added. The chemical and physical properties of the prepared particles were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), CH4-/CO-temperature programmed desorption, and thermogravimetric analysis (TGA). The XRD results showed that the prepared particles comprised two- or three-phase mixed metal oxides. The Fe2.0-Mn1.0-Ox particle was in the phases of Fe2Mn1O4 and Fe20.16Mn11.84O48, respectively, and the Co2.0-Mn1.0-Ox particle had the phase of Co2Mn1O4. In addition, Ni2.0-Mn1.0-Ox particle has NiO and Ni1Mn2O4 phases, and Cu2.0-Mn1.0-Ox particle exist in the phases of CuO, Cu2O and Cu1Mn1O2, respectively. The distributions of added elements on the particles were observed from the SEM mapping results. The oxygen carrier capacities of the particles were tested by isothermal H2/air and CH4/air redox cycle experiments using TGA at 850°C. The improved results obtained in this study revealed that the Cu2.0-Mn1.0-Ox particle was an enhanced bimetal oxide oxygen carrier for CLC under CH4/air conditions. In particular, among the synthesized particles, the Cu2.0-Mn1.0-Ox particle stably showed 16.7wt% oxygen transfer capacity over 10 redox cycle under CH4/air at 850°C.

Thermochemical energy storage in strontium-doped calcium manganites for concentrating solar power applications

Thermochemical energy storage (TCES) using redox cycles of reducible perovskite oxides can potentially provide higher specific energy capacities and storage temperatures than molten-salt systems for large-scale energy storage in concentrating solar power (CSP) and other applications. Perovskites from earth abundant cations, such as CaMnO3, are needed for cost-effective solutions, but such materials must demonstrate favorable thermodynamics for high specific TCES (chemical+sensible) and favorable kinetics for heat-driven reduction and exothermic re-oxidation in the redox cycle. This paper explores the thermodynamics and kinetics of Ca1-xSrxMnO3-δ(x=0.05and0.1) particles for TCES redox cycles where the particles are heated and reduced in N2 (PO2≈10-4bar) to high temperatures TH up to 1000°C in a solid-particle solar receiver. Chemical and sensible energy stored in the reduced perovskite particles is released as needed to a power cycle via re-oxidation and cooling of the material. Variation of oxygen non-stoichiometry (δ) of Ca1-xSrxMnO3-δ with temperature and PO2 is measured via thermogravimetric analysis. Reaction enthalpy for reduction and re-oxidation is determined by fitting the variation of δ with temperature and PO2 to a two-reaction point defect model. The fits compare favorably to differential scanning calorimetry with overall reaction enthalpies varying significantly with δ. For CSP applications, limited time in the solar receiver for perovskite reduction requires fast kinetics to achieve high specific TCES. Once the material is characterized thermodynamically, kinetic measurements for particle reduction and re-oxidation are performed in a packed-bed reactor to assess rates of O2 release and uptake at various temperatures and PO2 with particles of diameter between250and425μm. Packed-bed experiments indicate that the low values of A-site Sr-doping stabilize the CaMnO3-δ structure and allow large δ at high temperatures and PO2≈10-4bar. In these time-limited redox cycling experiments between 500and900°C, mass-transfer limited kinetics allows the Ca0.95Sr0.05MnO3-δ to reach a specific TCES of 620kJkg−1, which is 78% of the equilibrium limit of 791kJkg−1. X-ray diffraction after 1000 redox cycles showed that the Ca1-xSrxMnO3-δ particles have excellent phase stability for the desired redox conditions.

Improvement of reduction and oxidation performance of MMgOx (M = Fe, Co, Ni, and Cu) particles for chemical looping combustion

The objective of this study was to use Mg to improve the performance in chemical looping combustion. Oxygen carrier particles were synthesized in which Mg was doped into the M-sites with Fe, Co, Ni, and Cu to produce FeaMgbOx, CoaMgbOx, NiaMgbOx, and CuaMgbOx, respectively, with M/Mg ratios ranging from 1.5 to 3.0. The structure and properties of the prepared samples were characterized using XRD, SEM, CH4-/CO-TPD, XPS, and TGA. The XRD results explore that the Fe2Mg1Ox sample has a pure cubic structure of Fe2MgO4, whereas the Co2Mg1Ox and Ni2Mg1Ox samples exist as a solid solution. Nevertheless, the mixed compound formation of orthorhombic structure of Cu2MgO3 and cubic structure of MgO has been observed in Cu2Mg1Ox. The CH4 absorption is the highest on the Cu2Mg1Ox sample according to the CH4-TPD results. The CO-TPD and H2–TPR results showed that the adsorption of the Cu and Mg complex metal oxide is higher than that of the pure metal oxide. The oxygen carrier capacity of the samples was studied by isothermal H2 (FR)/air (AR) and CH4 (FR)/air (AR) redox cycle experiments at 850°C using TGA. The promising results obtained from chemical looping combustion indicate that the CuaMgbOx samples are potentially useful for CLC in H2 and CH4 atmospheres.

Redox Cycling Behavior of Catecholamines and Their Mixtures at Different Diffusion Distances: Steps Toward Quantitative Speciation

Investigation of catecholamines (dopamine: DA, norepinephrine: NE and epinephrine EP) has been of importance, because of the crucial roles that these molecules play in neurological function and related diseases. Due to the similar chemical structures and oxidation potentials of these compounds, it has been a challenge to use simple electrochemical techniques to differentiate them in mixtures.1 Redox cycling is known to have the capability to differentiate between species based on the propensity of the compounds to diffuse to and produce an electrochemical response at the neighboring electrode.2 The oxidized species generated at an electrode (generator) can diffuse to neighboring electrodes held at a reducing potential (collector) that will yield the original species. This process can occur multiple times and can produce an amplified signal. If the oxidized form of the species is electrochemically inactive, selectivity can be achieved for the electroactive molecules at the collector. Chemical reactions that can happen during the transfer between generator and collector can also convert the oxidized species to an electroinactive one and prevent a signal at the collector electrode, as well. This can be the basis of separating and detecting species using redox cycling. In their ECC’ mechanism, catecholamines undergo a two-electron oxidation (E) to yield the o-quinone.3 The o-quinone form can then go through an internal cyclization reaction (C) to yield the leucoaminochrome. The leucoaminochrome and o-quinone forms can react with each other (C’) to yield the original catecholamine and the aminochrome form. The internal cyclization reaction that is involved in the C part of the mechanism occurs at different rates for the different catecholamines. The apparent rate constants at pH=7.4 in phosphate buffer have been reported to be 0.13 ± 0.05 s-1, 0.98 ± 0.52 s-1 and 87 ± 10 s-1 for DA, NE and EP, respectively.4 Due to the difference in these rate constants, the species can be distinguished by spatial distribution based on the survival of the o-quinone from the following chemistry. Step C in the mechanism can also affect the number of times the species can go back and forth between the electrodes and therefore the amplification of the signal, depending on the species. The purpose of our research is to study the fundamental electrochemical redox cycling behavior of DA, NE and EP. By understanding this behavior, it can then be determined whether their simultaneous detection in more complex samples will be feasible. Recent developments from our laboratory will be discussed. We are using microfabricated chips with individually-addressable microelectrode arrays to activate various combinations of electrodes in the redox cycling experiments. By activating different electrodes on the array, greater distances between the collector and generator electrodes can be achieved leading to longer transit times for the molecules to move between electrodes. This study will lead to the generation of a calibration map for different gaps, ratios of catecholamines, and concentrations. There is a greater decrease in collector signal with increasing gap width (beyond the loss due to diffusion) for species with greater rate constants for step C. Epinephrine does not exhibit a reduction signal when the gap is set at 4-µm.2 Norepinephrine’s signal decreases as a function of increasing gap more sharply than dopamine’s. There is no observed collector signal for NE at the collector electrode, which is activated 28-µm away from the collector. The oxidation signal of the mixtures of catecholamines decreases even further due to the following chemistry producing the leucoaminochrome and increased concentrations of o-quinone and leucoaminochrome molecules. References Hawley, M. D.; Tatawawadi, S. V.; Piekarski, S.; Adams, R. N. Journal of the American Chemical Society 1967, 89, 447-450. Hu, Mengjia; Fritsch, Ingrid. Analytical Chemistry 2015, 87, 2029−2032.Ciolkowski, E. L.; Maness, K. M.; Cahill, P. S.; Wightman, R. M.; Evans, D. H.; Fosset, B.; Amatore, C. Analytical Chemistry 1994, 66, 3611-3617.Ciolkowski, E. L.; Cooper, B. R.; Jankowski, J. A.; Jorgenson, J. W.; Wightman, R. M. Journal of the American Chemical Society 1992, 114, 2815-2821.

Investigation of K2CO3‐Catalyzed Pyrolysis and Steam Gasification of Coal Char

AbstractPyrolysis and steam gasification experiments of coal char with and without K2CO3 were performed using a thermogravimetric analyzer to investigate the effect of catalyst loading method on gasification reactivity at different temperatures (650–800 °C), and an additional redox cycle experiment of catalyzed char‐steam gasification was conducted in a fixed‐bed reactor. The results showed that, at low temperature, the solution impregnation method had a larger effect on the gasification reactivity than the physical blending method; at temperatures above 700 °C, the differences of two methods diminished due to the high mobility of potassium. In catalytic pyrolysis, the release of CO and CO2 indicated that K2CO3 reacted with carbon. The redox cycle experiments suggest that the potassium reacted with carbon to produce intermediates of the form K2O2C, K2OC, and K2C. The K2C intermediate may help to explain the release of more H2 during the initial period of catalytic coal char–steam gasification.

Fe–Mn based minerals with remarkable redox characteristics for chemical looping combustion

Searching for naturally occurring minerals/ores which can only be crushed down and sieved before they are used as oxygen carrier materials (OCM) is one of the main quests of the chemical looping combustion (CLC) community. Three cheap iron and manganese based minerals directly taken from the mining industry are investigated in this study. These minerals are Sinai manganese mineral (Egypt), Guizhou iron mineral (China) and Kryvbas iron mineral (Ukraine). Samples are studied with respect to oxygen capacity and redox kinetics by isothermal hydrogen and methane redox cycling experiments (800–1000°C) and are compared to ilmenite as a widely used state-of-the-art reference. As one of the goals of this benchmarking is to spot minerals which are needless of pre-treatment and activation, no thermal treatment on the samples was performed prior to testing. Degradation issues have also been studied and investigated by SEM. Both Sinai manganese mineral and Kryvbas iron mineral perform well in respect to oxygen capacity and redox kinetics. However the post characterisation demonstrates that cation diffusion is much higher for Fe-based systems compared to the Fe–Mn system which shows much less swelling after 200 cycles.

Redox cycling in nanoporous electrochemical devices

Nanoscale redox cycling is a powerful technique for detecting electrochemically active molecules, based on fast repetitive oxidation and reduction reactions. An ideal implementation of redox cycling sensors can be realized by nanoporous dual-electrode systems in easily accessible and scalable geometries. Here, we introduce a multi-electrode array device with highly efficient nanoporous redox cycling sensors. Each of the sensors holds up to 209,000 well defined nanopores with minimal pore radii of less than 40 nm and an electrode separation of ~100 nm. We demonstrate the efficiency of the nanopore array by screening a large concentration range over three orders of magnitude with area-specific sensitivities of up to 81.0 mA (cm(-2) mM(-1)) for the redox-active probe ferrocene dimethanol. Furthermore, due to the specific geometry of the material, reaction kinetics has a unique potential-dependent impact on the signal characteristics. As a result, redox cycling experiments in the nanoporous structure allow studies on heterogeneous electron transfer reactions revealing a surprisingly asymmetric transfer coefficient.

Redox-cycling studies of anode-supported microtubular solid oxide fuel cells

Accidental reoxidation is one of the most critical issues of SOFC anodes, leading to performance degradation or even producing catastrophic failure of the cell. In this work several redox cycling experiments over microtubular SOFC cells are reported. Oxidation kinetic parameters have been determined for this geometry by Thermogravimetric Analysis (TGA). Oxidation kinetics is known to be diffusion limited at typical operation conditions and, for this reason, strictly dependant on geometry. A linear relationship was found between expansion kinetics and damage on SOFC anodes in a series of dilatometric experiments for the same temperature. A microtubular Ni-YSZ/YSZ/LSM-YSZ cell was also redox cycled several times at partial oxidation conditions (22% oxidation) and performance degradation was measured by j-V characterization and the loss was found to be approximately 2% in performance per cycle.

Electrochemical oxidation of berberine and mass spectrometric identification of its oxidation products

Electrochemical oxidation of the isoquinoline alkaloid berberine in aqueous medium was studied by cyclic and differential pulse voltammetry at a glassy carbon electrode (GCE). Two anodic peaks of the quaternary form of berberine were observed at +1.2V and +1.4V (vs. SCE) in acidic and neutral solutions. When the anodic polarization exceeded the value of +1.1V, the redox active film is formed on the GCE surface. The formation of adsorbed film was well-documented by quasireversible redox couple at +0.25V which was studied in redox cycling experiments. In alkaline medium, a new anodic peak at +0.5V appeared due to oxidation of berberine pseudobase to 8-oxoberberine. Solutions of berberine at different pH were subjected to controlled potential electrolysis on platinum gauze electrode and analyzed using liquid chromatography (HPLC) equipped with electrospray ionization/quadrupole time-of-flight mass spectrometry. The main water soluble monomeric product of berberine oxidation under physiological-near experimental conditions, OP1, was identified as demethyleneberberine cation (2,3-dihydroxy-9,10-dimethoxy-5,6-dihydroisoquinolino[3,2-a]isoquinolin-7-ium).

Fabrication of a nanoporous dual‐electrode system for electrochemical redox cycling

AbstractNanoporous electrodes are promising tools for highly sensitive electrochemical detection of molecules by redox cycling. In here, we report on a novel approach for the on‐chip fabrication of nanoporous dual‐electrode systems. The fabrication is based on template assisted dry etching into an electrode–insulator–electrode structure using a nanoporous alumina film as a mask. The mask itself is patterned by directly anodizing an aluminum layer on top of the chip. Homogeneous growth of the alumina nanopores on a wafer scale is achieved by adding a titanium intermediate film between the top electrode and the aluminum substrate as a stopping layer. The resulting electrodes are separated by approximately 100 nm and have pore diameters in the range of 30–50 nm. Thus they are ideally suited for redox cycling experiments based on fast diffusion of molecules between the electrodes. Integrating this process into microfabricated electrode structures promises to yield a stable and sensitive electrochemical sensor platform.magnified image Redox cycling scheme for a nanoporous dual‐electrode system. The close distance of anode and cathode provides a short diffusion path of oxidized and reduced molecules, as depicted by the arrows. The inlet shows a close‐up of the molecules within a pore. Different colours indicate different oxidation states.