Tw o-step solar-driven thermochemical fuel productionh as received significantr ecent attention for solar energy storage because of its potential for high efficiency and, when deployed using non-stoichiometric oxides, its chemical simplicity. [1][2][3][4][5][6] Thea pproachr elies on the large changes in oxygen content that suitable reactive oxides [7][8][9][10][11] undergo in response to variations in oxygenp artial pressure and temperature. Ty pically,t he cycle is performed under temperature-swing operation, in which the oxide is reduced at high temperature, and oxidation by steam or carbon dioxide at al ower temperature generates H 2 ,C O, syngas, [12,13] or even methane. [14] Isothermal pressure-swingc ycling [15][16][17][18][19] alleviates the thermomechanicalc hallenges associated with rapid thermal swings, [20][21][22] but penalizes fuel production per cycle.I nb otha pproaches, the expectation that material kinetics limit fuel production rates has driven the investigation of high-temperature diffusivity and surface reactivity.[23] Here we show that at high temperatures and operationalc onditions of interest, the fuel productivityf rom typicalc andidate materials is rate-limited, not by these kinetic properties but rather by thermodynamic considerations.In this work we investigate the specific case of isothermal pressure-swing cycling,recognizing that the analysis is readily generalized to the case of arbitrary combinations of thermal and chemical potential driving forces.I nt he isothermal process,ablock of porous reactive oxide is placed within ar eactor held at af ixed temperature, T 0 (set here at 1500 8C) and is exposed sequentially to reducinga nd oxidizing atmospheres. These conditions are produced, respectively, by sweepinga ni nert gas and then steam or carbond ioxide.T he variationinmaterial stoichiometry through this process is describedb yt he function d P O 2 ; T 0 ÀÁ ,i nw hich d is the oxygen non-stoichiometry in, for example,t he chemical formula CeO 2-d or LaAlO 3-d .When the material kinetics are infinitely rapid, the oxygen content in the oxide will be fixed by the rate at which the gas can be supplied or removed from the region surroundingt he material. Under the assumptions that the samplei ss mall enough that no stoichiometry gradiente xists across it and that the gas flow through the reactor is characterized by plug flow,t he oxygenp artial pressurea tagiven time t of the gas exiting the reactor, P O 2 ;out t ðÞ ,w ill be equal to the oxygen partial pressureo fagas in equilibrium with the material at its given nonstoichiometry d t ðÞ .T hat is,w hen reactor conditions are ideal, an oxide with sufficiently high diffusivitya nd surface reaction ratew illr eleaseo xygen instantaneously up to the thermodynamic capacity of the gas flowing across the sample.R ecognition of this implication of infinitely fast reaction kinetics permits numerical treatmento ft he rate of material stoichiometry change,a nd hence of gas evolution. The treatment of this problem yields important guidance for rea...
Production of chemical fuels by isothermal pressure-swing cycles has recently 12 generated significant interest. In this process a reactive oxide is cyclically exposed to an inert gas, 13 which induces partial reduction of the oxide, and to an oxidizing gas of either H2O or CO2, which 14 reoxidizes the oxide, releasing H2 or CO. At sufficiently high temperatures and sufficiently low gas flow rates, both the reduction and oxidation steps become limited only by the flow of gas across the material and not by material kinetic factors. In this contribution, we develop a numerical model describing fuel production rates in this gas-phase limited regime. The implications of this behavior are explored under all possible isothermal pressure-swing cycling conditions, and the outcome is optimized in terms of fuel production rate as well as fuel conversion and utilization of input gas of all types. Fuel production rate is maximized at infinitesimally small cycle times and attains a value that is independent of material thermodynamics. Gas utilization is maximized at infinitesimally small gas inputs, but the values can be made independent of cycle time, depending on manipulation of flow conditions. Gas-phase conditions (temperature, oxidant and reductant gas partial pressures, and CO2 vs H2O as oxidant) have a strong impact on fuel production metrics. Under realistic, finite cycle times, material thermodynamics play a measurable role in establishing fuel production rates. KEYWORDS. Solar fuels, thermochemical cycle, kinetics, thermodynamics, ceria INTRODUCTION. The storage of solar energy in chemical fuels has been a major research goal of the past decade.[1] Solar-driven two-step thermochemical fuel production has emerged as an attractive route towards this goal by harnessing the high efficiencies, and potentially low costs,
Solid oxide fuel cells are a promising clean energy technology due to their high efficiency and fuel flexibility, but current commercialization efforts are limited by expensive components needed to withstand high operating temperatures [1]. Attempts to decrease operating temperature have been stymied in large part by cathode materials with high polarization resistance [2], and efforts to improve performance through composite electrode morphologies [3] lead to complex geometries that prevent rigorous comparisons between different materials. We present a case study employing a novel methodology [4] for electrode characterization that enables fundamental property determination of multiple electrode compositions and allows for rigorous performance comparisons. In this work, the entire composition phase space of a state-of-the-art ion- and electron-conducting solid oxide fuel cell cathode material (La0.6Sr0.4Co1-xFexO3-δ) is examined with unprecedented compositional resolution (x=0 to x=1 with Δx=0.05). Gradient pulsed laser deposition was employed to obtain a compositionally graded thin film on a (100)-oriented 8 mol% Y2O3-ZrO2 electrolyte substrate. The film was patterned using photolithography and ion milling to obtain electronically isolated circular microdot electrodes ranging from 80-500 µm in diameter. Microelectrode impedance spectroscopy was performed with a robotic scanning probe in an environmental chamber to obtain relevant electrochemical parameters. The measured impedance spectra are consistent with a two-phase boundary electrochemical pathway including bulk ionic conduction through the oxide. A monotonic increase in electrochemical resistance is observed from La0.6Sr0.4CoO3–δ (LSC) to La0.6Sr0.4FeO3–δ (LSF) along with a decrease in chemical capacitance corresponding to a decrease in reducibility and oxygen vacancy concentration. This case study demonstrates the rich insights that can be gleaned from this high-throughput approach and its promising application toward searching for new high-performance solid oxide fuel cell electrode materials. 1. Yang, Z.G., Recent advances in metallic interconnects for solid oxide fuel cells. International Materials Reviews, 2008. 53(1): p. 39-54. 2. Kuklja, M.M., et al., Combined theoretical and experimental analysis of processes determining cathode performance in solid oxide fuel cells. Physical Chemistry Chemical Physics, 2013. 15(15): p. 5443-5471. 3. Sun, C.W., R. Hui, and J. Roller, Cathode materials for solid oxide fuel cells: a review. Journal of Solid State Electrochemistry, 2010. 14(7): p. 1125-1144. 4. Usiskin, R.E., et al., Probing the reaction pathway in (La0.8Sr0.2)(0.95)MnO3+delta using libraries of thin film microelectrodes. Journal of Materials Chemistry A, 2015. 3(38): p. 19330-19345.
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