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

Estimation and prediction of summer evapotranspiration from a northern wetland

Master of ScienceNatural Resources and EnvironmentUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/113964/1/39015003271098.pd

Beyond Ceria: Theoretical Investigation of Isothermal and Near-Isothermal Redox Cycling of Perovskites for Solar Thermochemical Fuel Production

Thermodynamic data for several LaMnO3-based perovskites indicates that in the high oxygen partial pressure (pO2) range (e.g., 10–7 to 10–3 atm), where isothermal thermochemical redox cycling is viable, they can undergo larger changes in oxidation state than ceria for a given change in pO2. This suggests that these materials may be more optimal for isothermal operation than ceria and offers the potential for more efficient H2/CO production via thermochemical splitting of H2O/CO2. To investigate this hypothesis, we developed a thermodynamic process model to predict the solar-to-fuel efficiencies of La1–x(Sr,Ca)xMn1–yAlyO3 perovskites and compared results to ceria and Zr-doped ceria. The calculations were performed for isothermal or near-isothermal cycling from 1473 to 1773 K. Four methods of lowering the reduction pO2 were considered: inert gas sweeping, mechanical vacuum pumping, electrochemical oxygen pumping, and thermochemical oxygen pumping. Considering a reduction pO2 of 10–6 atm and a gas-phase heat recovery effectiveness of 95%, the calculations showed that the perovskites outperformed ceria and Zr-doped ceria during isothermal operation in terms of fuel production and efficiency regardless of the pO2 reduction method. For example, at 1773 K, the calculated efficiencies were 35.17% for La0.6Sr0.4Mn0.6Al0.4O3 and 28.26% for ceria when implementing thermochemical oxygen pumping. Other methods of lowering the reduction pO2 resulted in lower efficiencies, where electrochemical oxygen pumping > inert gas sweeping > vacuum pumping. Small temperature swings using inert gases to lower the pO2 resulted in the highest efficiencies overall. For example, with a reduction temperature of 1773 K and a temperature swing of 100 K, the efficiency of the ceria-based cycle was 35.18% and with a temperature swing of 300 K, the efficiency of the La0.6Ca0.4MnO3 cycle was 40.75%. Importantly, in the case of inert gas sweeping, the efficiency of the ceria-based cycle exceeds that of the candidate materials when the temperature swing is low. The theoretical calculations within this work show that perovskites have the potential for improved solar-to-fuel efficiencies during isothermal or near-isothermal redox cycling beyond those achievable by ceria