Solar-Thermochemical Hydrogen Production Using Thin Film Ald Ferrites and Other Metal Oxides

Production of renewable hydrogen is achievable via two-step redox cycles using metal oxide-based intermediates. Concentrated solar energy is capable of decomposing the metal oxide in the first high temperature step, and in the second step water is reacted with the reduced metal oxide to produce H2 and regenerate the starting material. The thermodynamics of relevant ferrite-based water splitting cycles has been investigated using the thermodynamics software package FactSage. The effect of different metal substitutions in MxFe3-xO4, has been explored, and indicates that Co and Ni based ferrites are both superior to Fe3O4. Additionally, it is shown that increasing the inert gas concentrations has a direct effect on the reduction temperature. Increasing the amount of cobalt results in lowering the thermal reduction requirements, but does not necessarily translate to more H2&nbsp;production. For values of x &gt; 1, the amount of reducible iron decreases, and results in less H2&nbsp;production at elevated reduction temperatures. Oxidation of reduced species is shown to be achievable at temperatures greater than when &Delta;Grxn &gt; 0 if large excesses of water are introduced. More H2&nbsp;is expected to be present at equilibrium for ferrite based reactions compared to ceria based water splitting cycles, because the degree of reduction is approximately three times greater. Atomic layer deposition (ALD) has been used as a means to synthesize thin films of iron oxide, which can be used as reactive intermediates in solar redox cycles. Conformal films of amorphous iron (III) oxide and &alpha;-Fe2O3 have been coated on zirconia nanoparticles (26 nm) in a fluidized bed reactor by atomic layer deposition. Ferrocene and oxygen were alternately dosed into the reactor at temperatures between 367 ᵒC and 534 ᵒC. Self-limiting chemistry was observed via in situ mass spectrometry, and by means of induced coupled plasma &ndash; atomic emission spectroscopy analysis. Film conformality and uniformity were verified by high resolution transmission electron microscopy, and the growth rate was determined to be 0.15 &Aring; per cycle. Iron oxide (&gamma;-Fe2O3) and cobalt ferrite (CoxFe3-xO4) thin films have also been synthesized via ALD on high surface area (50 m2/g) m-ZrO2 supports. The oxide films were grown by sequentially depositing iron oxide and cobalt oxide, and adjusting the number of iron oxide cycles relative to cobalt oxide to achieve desired stoichiometry. Samples were chemically reduced in a flow reactor equipped with in situ x-ray diffraction. They were also subjected to chemical reduction and oxidation in a stagnation flow reactor to test activity for use in chemical looping cycles to produce H2&nbsp;via water splitting. &gamma;-Fe2O3&nbsp;films chemically reduced in mixtures of H2, CO, and CO2 at 600 &deg;C formed Fe3O4&nbsp;and FeO phases, and exhibited a trend-wise decrease in H2&nbsp;production rates upon cycling. Co0.85Fe2.15O4 films were successfully cycled without deactivation and produced four times more H2&nbsp;than &gamma;-Fe2O3, principally due to the formation of a CoFe alloy upon reduction. For comparison, a mechanically milled mixture of &alpha;-Fe2O3&nbsp;and ZrO2&nbsp;powders with similar iron loading to the thin films did not maintain high activity to water splitting due to sintering and grain growth. Cobalt ferrites are deposited on Al2O3 substrates via ALD, and the efficacy of using these in a ferrite water splitting redox cycle to produce H2&nbsp;is studied. Experimental results are coupled with thermodynamic modeling, and results indicate that CoFe2O4 deposited on Al2O3&nbsp;is capable of being reduced at lower temperatures than CoFe2O4&nbsp;(200oC-300oC) due to a reaction between the ferrite and substrate to form FeAl2O4. Significant quantities of H2 are produced at reduction temperatures of only 1200oC, whereas, CoFe2O4&nbsp;produced little or no H2&nbsp;until reduction temperatures of 1400oC. CoFe2O4/Al2O3&nbsp;was capable of being cycled at 1200oC reduction/ 1000oC oxidation with no obvious deactivation. Cobalt ferrite (Co0.9Fe2.1O4) and iron oxide (Fe3O4) thin films deposited via ALD on m-ZrO2&nbsp;supports are utilized in a high temperature water splitting redox cycle to produce H2. Both materials were thermally reduced at 1450oC and oxidized with H2O (20-40%) at temperatures between 900oC and 1400oC in a stagnation flow reactor. Oxidation of iron oxide was more rapid than the cobalt ferrite, and the rates of both materials increased with temperature, even up to 1400oC. At elevated oxidation temperatures (T &gt; 1250oC) we observed simultaneous production of H2&nbsp;and O2, due to both thermal reduction and water oxidation operating in equilibrium. A kinetic model was developed for the oxidation of cobalt ferrite from 900oC to 1100oC, in which there was an initial reaction order limited regime, followed by a slower diffusion limited regime characterized well by the parabolic rate law. The activation energy and H2O reaction order during the reaction order regime were 119.76 &plusmn; 8.81 kJ/mole and 0.70 &plusmn; 0.32, respectively, and the activation energy during the diffusion limited regime was 191 &plusmn; 19.8 kJ/mol. The feasibility of using commercially available, un-doped, ceria (CeO2) felts in a thermochemical redox cycle to produce H2&nbsp;has been explored, and a detailed kinetic analysis of the oxidation reaction is discussed. Reduction is achieved at 1450 ᵒC, and the subsequent H2 producing step is studied from 700 to 1200oC and H2O mole fractions of 0.04 to 0.32. The O2&nbsp;and H2&nbsp;equilibrium compositions remain constant for up to 30 redox cycles, and sintering appears to be abated by microscopy analysis. The average amount of H2&nbsp;produced is 280.9 &plusmn; 45.8 &mu;moles/g CeO2. The re-oxidation rates are faster on a per mass basis than similar ferrite based-cycles because the surface area is largely unaffected by thermal cycling. The oxidation reaction is governed by a first order reaction mechanism (1-&alpha;) at low temperatures and conversions, but at higher temperatures the mechanism transitions to a second order reaction (1-&alpha;)2. This is attributed to the onset of the thermodynamically favored reverse reaction at elevated temperatures. The activation energy is calculated between 700 and 900oC from 0.2&lt;&alpha;&lt;0.5, and determined to be 35.5 &plusmn; 13.3 kJ/mol. An Arrhenius expression, coupled with a first order reaction mechanism is used to model the experimentally observed reaction rates where the forward reaction was predominant.</p

Driving the solar thermal reforming of methane via a nonstoichiometric ceria redox cycle

This talk will be focused on a prospective solar driven methane reforming process using a nonstoichiometric ceria-based redox cycle. Compared to the traditional temperature swing process that accompanies solar-thermal redox cycles, the introduction of methane during the reduction step provides the ability to operate the cycle isothermally, or with smaller temperature swings, because the required reduction temperature decreases. As a result, the valuable solar energy that is utilized in the process is used more efficiently because sensible heating requirements are reduced, and the overall solar conversion efficiency is enhanced. Furthermore, compared to typical iron oxide based materials that are often used in similar chemical looping cycles, ceria has inherent kinetic and thermodynamic benefits that render it more suitable for isothermal operation where efficiencies are greater. Please click Additional Files below to see the full abstract

Reactive Absorption of CO2 Using Ethylaminoethanol Promoted Aqueous Potassium Carbonate Solvent

Atmospheric concentration of CO2, which is considered as one of the major greenhouse gases (GHGs), has increased up to 398 ppmv as of 2015. CO2 concentration in atmosphere was 280 ppmv in pre-industrial era, and due to the continuous discharge, it is expected to increase up to 550 ppmv by 2050. Many of the major industrial sources of CO2 emissions are natural gas fired power plants, synthesis gas used in integrated gasification combined cycle (IGCC) and power generation, gas streams produced after combustion of fossil fuels or other carbonaceous materials, and oxyfuels. Reactive absorption of CO2 from the industrial off gases by using chemical solvents is considered as one of the most common, efficient, and cost effective technologies utilized by the industry for CO2 capture. The captured CO2 can be stored by using the geological or oceanic sequestration approaches. As an alternative to geological or oceanic sequestration, the captured CO2 can be re-energized into CO by using solar energy and combined with H2, which can be generated from different methods, to produce syngas. The syngas produced can be further processed to liquid fuels such as methanol, gasoline, jet fuel, etc. via the catalytic Fischer-Tropsch process. In past, a variety of chemical solvents (mostly aqueous amines and there derivatives) have been used for CO2 capture from different gaseous streams via reactive absorption. Though the amines are attractive for the CO2 capture application, there are several disadvantages such as very strong corrosion to equipment and piping, high energy requirement during the stripping of CO2 and they are prone to oxidative and thermal degradation. Recently, use of aqueous potassium carbonate (K2CO3) as a solvent for the absorption of CO2 has gained widespread attention. The usage of K2CO3 has been employed in a number on industries for the removal of CO2 and H2S. Due to its high chemical solubility of CO2, low toxicity and solvent loss, no thermal and oxidative degradation, low heat of absorption, and absence of formation of heat stable salts, K2CO3 seems to be more attractive compared to the conventional amines towards CO2 capture. However, K2CO3 solvent shows slow rate of reaction with CO2 and, consequently, low mass transfer in the liquid phase as compared to the amine solvents. Hence, several investigators are focused towards improving the rate of reaction of CO2 in K2CO3 solvent with the help of different types of promoters. In this paper, the kinetics of absorption of CO2 into an aqueous K2CO3 (20 wt %) promoted by ethylaminoethanol (EAE) solution (hereafter termed as APCE solvent) was studied in a glass stirred cell reactor using a fall in pressure method. Reactive absorption of CO2 in EAE promoted aqueous K2CO3 solution (APCE solvent) was studied at different initial EAE concentrations (0.6 to 2 kmol/m3) and reaction temperatures (303 to 318 K). The reaction between the CO2 and APCE solvent was very well represented by the zwitterion mechanism. The N2O analogy was employed for the determination of H_(CO2) in the APCE solvent. The H_(CO2) was observed to be decreased by 5 and 31% due to the increase in the EAE concentration from 0.6 to 2 kmol/m3 and reaction temperature from 303 to 318 K, respectively. The D_(CO2) in the APCE solvent was also decreased by 21% due to the similar increase in the initial EAE concentration. In contrast, the D_(CO2) increased with the rise in the reaction temperature from 303 to 318 K by a factor of 1.678. The rate of absorption of CO2 in the APCE solvent was observed to increase by 35.10% and 47.59% due to the increase in EAE concentration (0.6 to 2 kmol/m3) and reaction temperature (303 to 318 K). The absorption kinetics was observed to be of overall second order i.e. first order with respect to both CO2 and EAE concentrations, respectively. The rate constant (k_2) for the absorption of CO2 in the APCE solvent was observed to be equal to 45540 m3/kmol√s at 318 K. The temperature dependency of k_2 for the CO2 – APCE solvent system was experimentally determined as: k_2 = (1.214 × [10]^18)√exp(( − 9822.7)/T). Findings of this study indicate EAE as a promising promoter for the aqueous K2CO3 solution.qscienc

Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: A review

This review summarizes state of the art metal oxide materials used in two-step thermochemical redox cycles for the production of H2 and CO from H2O and CO2 using concentrated solar energy. Advantages and disadvantages of both stoichiometric (e.g. iron oxide based cycles) and nonstoichiometric (e.g. ceria based cycles) materials are discussed in the context of thermodynamics, chemical kinetics, and material stability. Finally, a perspective aimed at future materials development and requirements necessary for advances of process efficiencies is discussed.ISSN:1369-7021ISSN:1873-410

Diffusion of oxygen in ceria at elevated temperatures and its application to H2O/CO2 splitting thermochemical redox cycles

Determination of reaction and oxygen diffusion rates at elevated temperatures is essential for modeling, design, and optimization of high-temperature solar thermochemical fuel production processes, but such data for state-of-the-art redox materials, such as ceria, is sparse. Here, we investigate the solid-state reduction and oxidation of sintered nonstoichiometric ceria at elevated temperatures relevant to solar thermochemical redox cycles for splitting H2O and CO2 (1673 K ≤ T ≤ 1823 K, 3 × 10–4 atm ≤ pO2 ≤ 2.5 × 10–3 atm). Relaxation experiments are performed by subjecting the sintered ceria to rapid oxygen partial pressure changes and measuring the time required to achieve thermodynamic equilibrium state. From such data, we elucidate information regarding ambipolar oxygen diffusion coefficients through comparison of experimental data to a numerical approximation of Fick’s second law based on finite difference methods. In contrast to typically applied analytical approaches, where diffusion coefficients are necessarily concentration independent, such a numerical approach is capable of accounting for more realistic concentration dependent diffusion coefficients and also accounts for transient gas phase boundary conditions pertinent to dispersion and oxygen consumption/evolution. Ambipolar diffusion coefficients are obtained in the range 1.5·10–5 cm2 s–1 ≤ D̃ ≤ 4·10–4 cm2 s–1 between 1673 and 1823 K. These results highlight the rapid nature of ceria reduction to help guide the design of redox materials for solar reactors, the importance of accounting for transient boundary conditions during relaxation experiments (either mass based or conductivity based), and point to the flexibility of using a numerical analysis in contrast to typical analytical approaches.ISSN:1932-7455ISSN:1932-744

Oxygen nonstoichiometry and thermodynamic characterization of Zr doped ceria in the 1573-1773 K temperature range

This work encompasses the thermodynamic characterization and critical evaluation of Zr4+ doped ceria, a promising redox material for the two-step solar thermochemical splitting of H2O and CO2 to H2 and CO. As a case study, we experimentally examine 5 mol% Zr4+ doped ceria and present oxygen nonstoichiometry measurements at elevated temperatures ranging from 1573 K to 1773 K and oxygen partial pressures ranging from 4.50 × 10−3 atm to 2.3 × 10−4 atm, yielding higher reduction extents compared to those of pure ceria under all conditions investigated, especially at the lower temperature range and at higher pO2. In contrast to pure ceria, a simple ideal solution model accounting for the formation of isolated oxygen vacancies and localized electrons accurately describes the defect chemistry. Thermodynamic properties are determined, namely: partial molar enthalpy, entropy, and Gibbs free energy. In general, partial molar enthalpy and entropy values of Zr4+ doped ceria are lower. The equilibrium hydrogen yields are subsequently extracted as a function of the redox conditions for dopant concentrations as high as 20%. Although reduction extents increase greatly with dopant concentration, the oxidation of Zr4+ doped ceria is thermodynamically less favorable compared to pure ceria. This leads to substantially larger temperature swings between reduction and oxidation steps, ultimately resulting in lower theoretical solar energy conversion efficiencies compared to ceria under most conditions. In effect, these results point to the importance of considering oxidation thermodynamics in addition to reduction when screening potential redox materials.ISSN:1463-9084ISSN:1463-907

Reticulated porous ceria undergoing thermochemical reduction with high-flux irradiation

A numerical and experimental analysis is performed on the solar-driven thermochemical reduction of ceria as part of a H2O/CO2-splitting redox cycle. A transient heat and mass transfer model is developed to simulate reticulated porous ceramic (RPC) foam-type structures, made of ceria, exposed to concentrated solar radiation. The RPC features dual-scale porosity in the mm-range and μm-range within its struts for enhanced transport. The numerical model solves the volume-averaged conservation equations for the porous fluid and solid domains using the effective transport properties for conductive, convective and radiative heat transfer. These in turn are determined by direct pore-level simulations and Monte-Carlo ray tracing on the exact 3D digital geometry of the RPC obtained from tomography scans. Experimental validation is accomplished in terms of temporal temperature and oxygen concentration measurements for RPC samples directly irradiated in a high-flux solar simulator with a peak flux of 1200 suns and heated to up to 1940 K. Effective volumetric absorption of solar radiation was obtained for moderate optically thick structures, leading to a more uniform temperature distribution and a higher specific oxygen yield. The effect of changing structural parameters such as mean pore diameter and porosity is investigated.ISSN:0017-9310ISSN:1879-218

Morphological Characterization and Effective Thermal Conductivity of Dual-Scale Reticulated Porous Structures

Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO2 and H2O. They feature dual-scale porosity with mm-size pores for effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography. Total and open porosity, pore size distribution, mean pore diameter, and specific surface area are extracted from the computer tomography (CT) scans. The 3D digital geometry is then applied in direct pore level simulations (DPLS) of Fourier’s law within the solid and the fluid phases for the accurate determination of the effective thermal conductivity at each porosity scale and combined, and for fluid-to-solid thermal conductivity from 10−5 to 1. Results are compared to predictions by analytical models for structures with a wide range of porosities 0.09–0.9 in both the strut’s µm-scale and bulk’s mm-scale. The morphological properties and effective thermal conductivity determined in this work serve as an input to volume-averaged models for the design and optimization of solar chemical reactors.ISSN:1996-194