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Li Y, Chen H, Lim K, Deng HD, Lim J, Fraggedakis D, Attia PM, Lee SC, Jin N, Moškon J, Guan Z, Gent WE, Hong J, Yu YS, Gaberšček M, Islam MS, Bazant MZ, Chueh WC. Fluid-enhanced surface diffusion controls intraparticle phase transformations. NATURE MATERIALS 2018; 17:915-922. [PMID: 30224783 DOI: 10.1038/s41563-018-0168-4] [Citation(s) in RCA: 33] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/03/2017] [Accepted: 08/14/2018] [Indexed: 06/08/2023]
Abstract
Phase transformations driven by compositional change require mass flux across a phase boundary. In some anisotropic solids, however, the phase boundary moves along a non-conductive crystallographic direction. One such material is LiXFePO4, an electrode for lithium-ion batteries. With poor bulk ionic transport along the direction of phase separation, it is unclear how lithium migrates during phase transformations. Here, we show that lithium migrates along the solid/liquid interface without leaving the particle, whereby charge carriers do not cross the double layer. X-ray diffraction and microscopy experiments as well as ab initio molecular dynamics simulations show that organic solvent and water molecules promote this surface ion diffusion, effectively rendering LiXFePO4 a three-dimensional lithium-ion conductor. Phase-field simulations capture the effects of surface diffusion on phase transformation. Lowering surface diffusivity is crucial towards supressing phase separation. This work establishes fluid-enhanced surface diffusion as a key dial for tuning phase transformation in anisotropic solids.
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Gent WE, Lim K, Liang Y, Li Q, Barnes T, Ahn SJ, Stone KH, McIntire M, Hong J, Song JH, Li Y, Mehta A, Ermon S, Tyliszczak T, Kilcoyne D, Vine D, Park JH, Doo SK, Toney MF, Yang W, Prendergast D, Chueh WC. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nat Commun 2017; 8:2091. [PMID: 29233965 PMCID: PMC5727078 DOI: 10.1038/s41467-017-02041-x] [Citation(s) in RCA: 164] [Impact Index Per Article: 23.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2017] [Accepted: 11/02/2017] [Indexed: 01/05/2023] Open
Abstract
Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17-x Ni0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.
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Zhang L, Sun L, Guan Z, Lee S, Li Y, Deng HD, Li Y, Ahlborg NL, Boloor M, Melosh NA, Chueh WC. Quantifying and Elucidating Thermally Enhanced Minority Carrier Diffusion Length Using Radius-Controlled Rutile Nanowires. NANO LETTERS 2017; 17:5264-5272. [PMID: 28817772 DOI: 10.1021/acs.nanolett.7b01504] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The minority carrier diffusion length (LD) is a crucial property that determines the performance of light absorbers in photoelectrochemical (PEC) cells. Many transition-metal oxides are stable photoanodes for solar water splitting but exhibit a small to moderate LD, ranging from a few nanometers (such as α-Fe2O3 and TiO2) to a few tens of nanometers (such as BiVO4). Under operating conditions, the temperature of PEC cells can deviate substantially from ambient, yet the temperature dependence of LD has not been quantified. In this work, we show that measuring the photocurrent as a function of both temperature and absorber dimensions provides a quantitative method for evaluating the temperature-dependent minority carrier transport. By measuring photocurrents of nonstoichiometric rutile TiO2-x nanowires as a function of wire radius (19-75 nm) and temperature (10-70 °C), we extract the minority carrier diffusion length along with its activation energy. The minority carrier diffusion length in TiO2-x increases from 5 nm at 25 °C to 10 nm at 70 °C, implying that enhanced carrier mobility outweighs the increase in the recombination rate with temperature. Additionally, by comparing the temperature-dependent photocurrent in BiVO4, TiO2, and α-Fe2O3, we conclude that the ratio of the minority carrier diffusion length to the depletion layer width determines the extent of temperature enhancement, and reconcile the widespread temperature coefficients, which ranged from 0.6 to 1.7% K-1. This insight provides a general design rule to select light absorbers for large thermally activated photocurrents and to predict PEC cell characteristics at a range of temperatures encountered during realistic device operation.
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Sinclair R, Lee SC, Shi Y, Chueh WC. Structure and chemistry of epitaxial ceria thin films on yttria-stabilized zirconia substrates, studied by high resolution electron microscopy. Ultramicroscopy 2017; 176:200-211. [DOI: 10.1016/j.ultramic.2017.03.015] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2016] [Revised: 12/15/2016] [Accepted: 12/28/2016] [Indexed: 11/26/2022]
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Guan Z, Chen D, Chueh WC. Analyzing the dependence of oxygen incorporation current density on overpotential and oxygen partial pressure in mixed conducting oxide electrodes. Phys Chem Chem Phys 2017; 19:23414-23424. [DOI: 10.1039/c7cp03654j] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The oxygen incorporation reaction involves the transformation of an oxygen gas molecule to two lattice oxygen ions in a mixed ionic and electronic conducting solid. The current density is modeled as functions of both oxygen partial pressure and overpotential.
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31
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Shi Y, Lee SC, Monti M, Wang C, Feng ZA, Nix WD, Toney MF, Sinclair R, Chueh WC. Growth of Highly Strained CeO 2 Ultrathin Films. ACS NANO 2016; 10:9938-9947. [PMID: 27934073 DOI: 10.1021/acsnano.6b04081] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Large biaxial strain is a promising route to tune the functionalities of oxide thin films. However, large strain is often not fully realized due to the formation of misfit dislocations at the film/substrate interface. In this work, we examine the growth of strained ceria (CeO2) thin films on (001)-oriented single crystal yttria-stabilized zirconia (YSZ) via pulsed-laser deposition. By varying the film thickness systematically between 1 and 430 nm, we demonstrate that ultrathin ceria films are coherently strained to the YSZ substrate for thicknesses up to 2.7 nm, despite the large lattice mismatch (∼5%). The coherency is confirmed by both X-ray diffraction and high-resolution transmission electron microscopy. This thickness is several times greater than the predicted equilibrium critical thickness. Partial strain relaxation is achieved by forming semirelaxed surface islands rather than by directly nucleating dislocations. In situ reflective high-energy electron diffraction during growth confirms the transition from 2-D (layer-by-layer) to 3-D (island) at a film thickness of ∼1 nm, which is further supported by atomic force microscopy. We propose that dislocations likely nucleate near the surface islands and glide to the film/substrate interface, as evidenced by the presence of 60° dislocations. An improved understanding of growing oxide thin films with a large misfit lays the foundation to systematically explore the impact of strain and dislocations on properties such as ionic transport and redox chemistry.
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Lim J, Li Y, Alsem DH, So H, Lee SC, Bai P, Cogswell DA, Liu X, Jin N, Yu YS, Salmon NJ, Shapiro DA, Bazant MZ, Tyliszczak T, Chueh WC. Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles. Science 2016; 353:566-71. [DOI: 10.1126/science.aaf4914] [Citation(s) in RCA: 295] [Impact Index Per Article: 36.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2016] [Accepted: 07/08/2016] [Indexed: 01/29/2023]
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33
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Gent WE, Li Y, Ahn S, Lim J, Liu Y, Wise AM, Gopal CB, Mueller DN, Davis R, Weker JN, Park JH, Doo SK, Chueh WC. Persistent State-of-Charge Heterogeneity in Relaxed, Partially Charged Li1- x Ni1/3 Co1/3 Mn1/3 O2 Secondary Particles. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2016; 28:6631-6638. [PMID: 27187238 DOI: 10.1002/adma.201601273] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/05/2016] [Revised: 04/14/2016] [Indexed: 06/05/2023]
Abstract
Ex situ transmission X-ray microscopy reveals micrometer-scale state-of-charge heterogeneity in solid-solution Li1- x Ni1/3 Co1/3 Mn1/3 O2 secondary particles even after extensive relaxation. The heterogeneity generates overcharged domains at the cutoff voltage, which may accelerate capacity fading and increase impedance with extended cycling. It is proposed that optimized secondary structures can minimize the state-of-charge heterogeneity by mitigating the buildup of nonuniform internal stresses associated with volume changes during charge.
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Gopal CB, Gabaly FE, McDaniel AH, Chueh WC. Origin and Tunability of Unusually Large Surface Capacitance in Doped Cerium Oxide Studied by Ambient-Pressure X-Ray Photoelectron Spectroscopy. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2016; 28:4692-4697. [PMID: 27031580 DOI: 10.1002/adma.201506333] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/21/2015] [Revised: 02/13/2016] [Indexed: 06/05/2023]
Abstract
The volumetric redox (chemical) capacitance of the surface of CeO2-δ films is quantified in situ to be 100-fold larger than the bulk values under catalytically relevant conditions. Sm addition slightly lowers the surface oxygen nonstoichiometry, but effects a 10-fold enhancement in surface chemical capacitance by mitigating defect interactions, highlighting the importance of differential nonstoichiometry for catalysis.
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Li Y, Meyer S, Lim J, Lee SC, Gent WE, Marchesini S, Krishnan H, Tyliszczak T, Shapiro D, Kilcoyne ALD, Chueh WC. Effects of Particle Size, Electronic Connectivity, and Incoherent Nanoscale Domains on the Sequence of Lithiation in LiFePO₄ Porous Electrodes. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2015; 27:6591-6597. [PMID: 26423560 DOI: 10.1002/adma.201502276] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/12/2015] [Revised: 08/12/2015] [Indexed: 06/05/2023]
Abstract
High-resolution X-ray microscopy is used to investigate the sequence of lithiation in LiFePO4 porous electrodes. For electrodes with homogeneous interparticle electronic connectivity via the carbon black network, the smaller particles lithiate first. For electrodes with heterogeneous connectivity, the better-connected particles preferentially lithiate. Correlative electron and X-ray microscopy also reveal the presence of incoherent nanodomains that lithiate as if they are separate particles.
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Nelson Weker J, Li Y, Shanmugam R, Lai W, Chueh WC. Tracking Non-Uniform Mesoscale Transport in LiFePO4Agglomerates During Electrochemical Cycling. ChemElectroChem 2015. [DOI: 10.1002/celc.201500119] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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37
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Mueller DN, Machala ML, Bluhm H, Chueh WC. Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions. Nat Commun 2015; 6:6097. [DOI: 10.1038/ncomms7097] [Citation(s) in RCA: 258] [Impact Index Per Article: 28.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2014] [Accepted: 12/12/2014] [Indexed: 12/22/2022] Open
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38
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Feng ZA, Machala ML, Chueh WC. Surface electrochemistry of CO2reduction and CO oxidation on Sm-doped CeO2−x: coupling between Ce3+and carbonate adsorbates. Phys Chem Chem Phys 2015; 17:12273-81. [DOI: 10.1039/c5cp00114e] [Citation(s) in RCA: 71] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Strong coupling between carbonate coverage, surface Ce3+concentration and overpotential reveals rate-limiting step in CO oxidation and CO2reduction.
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39
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Li Y, El Gabaly F, Ferguson TR, Smith RB, Bartelt NC, Sugar JD, Fenton KR, Cogswell DA, Kilcoyne ALD, Tyliszczak T, Bazant MZ, Chueh WC. Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes. NATURE MATERIALS 2014; 13:1149-1156. [PMID: 25218062 DOI: 10.1038/nmat4084] [Citation(s) in RCA: 104] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/19/2014] [Accepted: 08/06/2014] [Indexed: 06/03/2023]
Abstract
Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode lithium iron phosphate (LiFePO4; LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes.
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40
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Feng ZA, El Gabaly F, Ye X, Shen ZX, Chueh WC. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nat Commun 2014; 5:4374. [PMID: 25007038 DOI: 10.1038/ncomms5374] [Citation(s) in RCA: 71] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2014] [Accepted: 06/11/2014] [Indexed: 12/24/2022] Open
Abstract
Electrochemical incorporation reactions are ubiquitous in energy storage and conversion devices based on mixed ionic and electronic conductors, such as lithium-ion batteries, solid-oxide fuel cells and water-splitting membranes. The two-way traffic of ions and electrons across the electrochemical interface, coupled with the bulk transport of mass and charge, has been challenging to understand. Here we report an investigation of the oxygen-ion incorporation pathway in CeO2-δ (ceria), one of the most recognized oxygen-deficient compounds, during hydrogen oxidation and water splitting. We probe the response of surface oxygen vacancies, electrons and adsorbates to the electrochemical polarization at the ceria-gas interface. We show that surface oxygen-ion transfer, mediated by oxygen vacancies, is fast. Furthermore, we infer that the electron transfer between cerium cations and hydroxyl ions is the rate-determining step. Our in operando observations reveal the precise roles of surface oxygen vacancy and electron defects in determining the rate of surface incorporation reactions.
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Chen C, Chen D, Chueh WC, Ciucci F. Modeling the impedance response of mixed-conducting thin film electrodes. Phys Chem Chem Phys 2014; 16:11573-83. [DOI: 10.1039/c4cp01285b] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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42
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Siegel DA, Chueh WC, El Gabaly F, McCarty KF, de la Figuera J, Blanco-Rey M. Determination of the surface structure of CeO2(111) by low-energy electron diffraction. J Chem Phys 2013; 139:114703. [DOI: 10.1063/1.4820826] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
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43
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Chueh WC, El Gabaly F, Sugar JD, Bartelt NC, McDaniel AH, Fenton KR, Zavadil KR, Tyliszczak T, Lai W, McCarty KF. Intercalation pathway in many-particle LiFePO4 electrode revealed by nanoscale state-of-charge mapping. NANO LETTERS 2013; 13:866-872. [PMID: 23362838 DOI: 10.1021/nl3031899] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.
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Ye X, Melas-Kyriazi J, Feng ZA, Melosh NA, Chueh WC. A semiconductor/mixed ion and electron conductor heterojunction for elevated-temperature water splitting. Phys Chem Chem Phys 2013; 15:15459-69. [DOI: 10.1039/c3cp52536h] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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45
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Chueh WC, Haile SM. Electrochemistry of Mixed Oxygen Ion and Electron Conducting Electrodes in Solid Electrolyte Cells. Annu Rev Chem Biomol Eng 2012; 3:313-41. [DOI: 10.1146/annurev-chembioeng-073009-101000] [Citation(s) in RCA: 75] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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46
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El Gabaly F, McDaniel AH, Grass M, Chueh WC, Bluhm H, Liu Z, McCarty KF. Electrochemical intermediate species and reaction pathway in H2 oxidation on solid electrolytes. Chem Commun (Camb) 2012; 48:8338-40. [PMID: 22781193 DOI: 10.1039/c2cc33229a] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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47
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Chueh WC, Hao Y, Jung W, Haile SM. High electrochemical activity of the oxide phase in model ceria-Pt and ceria-Ni composite anodes. NATURE MATERIALS 2011; 11:155-161. [PMID: 22138788 DOI: 10.1038/nmat3184] [Citation(s) in RCA: 96] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/01/2011] [Accepted: 10/26/2011] [Indexed: 05/31/2023]
Abstract
Fuel cells, and in particular solid-oxide fuel cells (SOFCs), enable high-efficiency conversion of chemical fuels into useful electrical energy and, as such, are expected to play a major role in a sustainable-energy future. A key step in the fuel-cell energy-conversion process is the electro-oxidation of the fuel at the anode. There has been increasing evidence in recent years that the presence of CeO(2)-based oxides (ceria) in the anodes of SOFCs with oxygen-ion-conducting electrolytes significantly lowers the activation overpotential for hydrogen oxidation. Most of these studies, however, employ porous, composite electrode structures with ill-defined geometry and uncontrolled interfacial properties. Accordingly, the means by which electrocatalysis is enhanced has remained unclear. Here we demonstrate unambiguously, through the use of ceria-metal structures with well-defined geometries and interfaces, that the near-equilibrium H(2) oxidation reaction pathway is dominated by electrocatalysis at the oxide/gas interface with minimal contributions from the oxide/metal/gas triple-phase boundaries, even for structures with reaction-site densities approaching those of commercial SOFCs. This insight points towards ceria nanostructuring as a route to enhanced activity, rather than the traditional paradigm of metal-catalyst nanostructuring.
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Ciucci F, Chueh WC, Goodwin DG, Haile SM. Surface reaction and transport in mixed conductors with electrochemically-active surfaces: a 2-D numerical study of ceria. Phys Chem Chem Phys 2011; 13:2121-35. [DOI: 10.1039/c0cp01219j] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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49
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Chueh WC, Yang CK, Garland CM, Lai W, Haile SM. Unusual decrease in conductivity upon hydration in acceptor doped, microcrystalline ceria. Phys Chem Chem Phys 2011; 13:6442-51. [DOI: 10.1039/c0cp02198a] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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50
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Chueh WC, Haile SM. A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2010; 368:3269-3294. [PMID: 20566511 DOI: 10.1098/rsta.2010.0114] [Citation(s) in RCA: 80] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
We present a comprehensive thermodynamic and kinetic analysis of the suitability of cerium oxide (ceria) for thermochemical fuel production. Both portions of the two-step cycle, (i) oxygen release from the oxide at 1773 and 1873 K under inert atmosphere, and (ii) hydrogen release upon hydrolysis at 1073 K, are examined theoretically as well as experimentally. We observe gravimetric fuel productivity that is in quantitative agreement with equilibrium, thermogravimetric studies of ceria. Despite the non-stoichiometric nature of the redox cycle, in which only a portion of the cerium atoms change their oxidation state, the fuel productivity of 8.5-11.8 ml of H(2) per gram of ceria is competitive with that of other solid-state thermochemical cycles currently under investigation. The fuel production rate, which is also highly attractive, at a rate of 4.6-6.2 ml of H(2) per minute per gram of ceria, is found to be limited by a surface-reaction step rather than by ambipolar bulk diffusion of oxygen through the solid ceria. An evaluation of the thermodynamic efficiency of the ceria-based thermochemical cycle suggests that, even in the absence of heat recovery, solar-to-fuel conversion efficiencies of 16 to 19 per cent can be achieved, assuming a suitable method for obtaining an inert atmosphere for the oxygen release step.
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