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Kinetics and mechanism of solar-thermochemical H2 production by oxidation of a cobalt ferrite-zirconia composite

Energy and Environmental Science

Scheffe, Jonathan R.; McDaniel, Anthony H.; Allendorf, Mark; Weimer, Alan W.

Accurate knowledge of water splitting kinetics is essential for the design and optimization of high-temperature thermochemical cycles for solar-driven fuel production, but such crucial data are unavailable for virtually all redox materials of potential practical value. We describe an investigation of the redox activity and oxidation kinetics of cobalt ferrite, a promising material for this application that is representative of a broader class of metal-substituted ferrites. To enable repetitive cycling, ferrites must be supported on another oxide to avoid sintering and deactivation. Consequently, we synthesized a composite material using atomic layer deposition of cobalt and iron oxides on zirconia, a commonly used ferrite "support", to create a well-controlled, uniformly distributed composition. Our results show that the support is not an innocent bystander and that dissolved iron within it reacts by a different mechanism than embedded iron oxide particles in the matrix. Samples were thermally reduced at 1450 °C under helium and oxidized with steam at realistic process temperatures ranging from 900 °C to 1400 °C. Experiments within a fluid-dynamically well-behaved stagnation-flow reactor, coupled with detailed numerical modelling of the transient H2 production rates, allow us to effectively deconvolve experimental artefacts from intrinsic material behaviour over the entire time domain of the oxidation reaction. We find that second-order reaction and diffusion-limited mechanisms occur simultaneously at different oxidation rates and involve iron in two separate phases: (1) reduced Fe dissolved in the ZrO2 support and (2) iron oxide located at the interface between embedded ferrite particles and the zirconia matrix. Surprisingly, we also identified a catalytic mechanism occurring at the highest temperatures by which steady-state production of H 2 and O2 occurs. The results reported here, which include Arrhenius rate constants for both oxidation mechanisms, will enable high-fidelity computational simulation of this complex, but promising approach to renewable fuel production. © 2013 The Royal Society of Chemistry.

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TYPE Journal Article YEAR 2013
ScopusOSTI

Nano-ordering of donor-acceptor interactions using Metal-Organic Frameworks as scaffolds

ECS Transactions

Leong, Kirsty; Foster, Michael E.; Wong, Bryan M.; Spoerke, Erik D.; Gough, Dara; Deaton, Joseph C.; Allendorf, Mark

Metal-Organic Frameworks (MOFs) are nanoporous materials with tunable pore sizes that can accommodate and stabilize small molecules. Because of their long-range order and wellunderstood pore environment, the nano-confinement of donoracceptor materials within MOFs offers a new methodology for creating uniform phase-segregated donor-acceptor interfaces. Phase segregation and the photo-physical effects of confining α,ω-Dihexylsexithiophene (DH-6T) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in several MOFs and the potential role of the MOF in creating a nano-heterojunction for organic photovoltaics are discussed. We demonstrate infiltration of both molecules into MOF pores and use luminescence and absorption spectroscopies to characterize the MOF-guest energy transfer processes. Comparison with density functional theory allows us to determine the energetics and band alignment within the MOF. The results demonstrate the utility of MOFs as scaffolds for sub-nanoscale ordering of donor and acceptor species within a highly uniform environment, allowing both the interaction and separation distance to be much more controlled than in the classical bulk heterojunction. © The Electrochemical Society.

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TYPE Conference YEAR 2013
ScopusOSTIDOI
Results 176–200 of 256
Results 176–200 of 256