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Title: Toward High-Precision Control of Transformation Characteristics in VO 2 through Dopant Modulation of Hysteresis
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Author(s) / Creator(s):
; ; ; ; ; ;
Date Published:
Journal Name:
The Journal of Physical Chemistry C
Page Range / eLocation ID:
21223 to 21231
Medium: X
Sponsoring Org:
National Science Foundation
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  1. This paper addresses the use of Ce 0.8 Gd 0.2 O 2−δ (GDC) infiltration into the Ni–(Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 (YSZ) fuel electrode of solid oxide cells (SOCs) for improving their electrochemical performance in fuel cell and electrolysis operation. Although doped ceria infiltration into Ni–YSZ has recently been shown to improve the electrode performance and stability, the mechanisms defining how GDC impacts electrochemical characteristics are not fully delineated. Furthermore, the electrochemical characteristics have not yet been determined over the full range of conditions normally encountered in fuel cell and electrolysis operation. Here we present a study of both symmetric and full cells aimed at understanding the electrochemical mechanisms of GDC-modified Ni–YSZ over a wide range of fuel compositions and temperatures. Single-step GDC infiltration at an appropriate loading substantially reduced the polarization resistance of Ni–YSZ electrodes in electrolyte-supported cells, as measured using electrochemical impedance spectroscopy (EIS) at various temperatures (600–800 °C) in a range of H 2 O–H 2 mixtures (3–90 vol% H 2 O). Fuel-electrode-supported cells had significant concentration polarization due to the thick Ni–YSZ supports. A distribution of relaxation times approach is used to develop a physically-based electrochemical model; the results show that GDC reduces the reaction resistance associated with three-phase boundaries, but also appears to improve oxygen transport in the electrode. Increasing the H 2 O fraction in the H 2 –H 2 O fuel mixture reduced both the three-phase boundary resistance and the gas diffusion resistance for Ni–YSZ; with GDC infiltration, the electrode resistance showed less variation with fuel composition. GDC infiltration improved the performance of fuel-electrode-supported full cells, which yielded a maximum power density of 2.28 W cm −2 in fuel cell mode and an electrolysis current density at 1.3 V of 2.22 A cm −2 , both at 800 °C. 
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  2. Abstract

    Unveiling the underlying mechanisms of properties of functional materials, including the luminescence differences among similar pyrochlores A2B2O7, opens new gateways to select proper hosts for various optoelectronic applications by scientists and engineers. For example, although La2Zr2O7(LZO) and La2Hf2O7(LHO) pyrochlores have similar chemical compositional and crystallographic structural features, they demonstrate different luminescence properties both before and after doped with Eu3+ions. Based on our earlier work, LHO‐based nanophosphors display higher photo‐ and radioluminescence intensity, higher quantum efficiency, and longer excited state lifetime compared to LZO‐based nanophosphors. Moreover, under electronic O2−→Zr4+/Hf4+transition excitation at 306 nm, undoped LHO nanoparticles (NPs) have only violet blue emission, whereas LZO NPs show violet blue and red emissions. In this study, we have combined experimental and density functional theory (DFT) based theoretical calculation to explain the observed results. First, we calculated the density of state (DOS) based on DFT and studied the energetics of ionized oxygen vacancies in the band gaps of LZO and LHO theoretically, which explain their underlying luminescence difference. For Eu3+‐doped NPs, we performed emission intensity and lifetime calculations and found that the LHOE NPs have higher host to dopant energy transfer efficiency than the LZOE NPs (59.3% vs 24.6%), which accounts for the optical performance superiority of the former over the latter. Moreover, by corroborating our experimental data with the DFT calculations, we suggest that the Eu3+doping states in LHO present at exact energy position (both in majority and minority spin components) where oxygen defect states are located unlike those in LZO. Lastly, both the NPs show negligible photobleaching highlighting their potential for bioimaging applications. This current report provides a deeper understanding of the advantages of LHO over LZO as an advanced host for phosphors, scintillators, and fluoroimmunoassays.

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  3. [RuCp*(1,3,5-R 3 C 6 H 3 )] 2 {Cp* = η 5 -pentamethylcyclopentadienyl, R = Me, Et} have previously been found to be moderately air stable, yet highly reducing, with estimated D + /0.5D 2 (where D 2 and D + represent the dimer and the corresponding monomeric cation, respectively) redox potentials of ca. −2.0 V vs. FeCp 2 +/0 . These properties have led to their use as n-dopants for organic semiconductors. Use of arenes substituted with π-electron donors is anticipated to lead to even more strongly reducing dimers. [RuCp*(1-(Me 2 N)-3,5-Me 2 C 6 H 3 )] + PF 6 − and [RuCp*(1,4-(Me 2 N) 2 C 6 H 4 )] + PF 6 − have been synthesized and electrochemically and crystallographically characterized; both exhibit D + /D potentials slightly more cathodic than [RuCp*(1,3,5-R 3 C 6 H 3 )] + . Reduction of [RuCp*(1,4-(Me 2 N) 2 C 6 H 4 )] + PF 6 − using silica-supported sodium–potassium alloy leads to a mixture of isomers of [RuCp*(1,4-(Me 2 N) 2 C 6 H 4 )] 2 , two of which have been crystallographically characterized. One of these isomers has a similar molecular structure to [RuCp*(1,3,5-Et 3 C 6 H 3 )] 2 ; the central C–C bond is exo , exo , i.e. , on the opposite face of both six-membered rings from the metals. A D + /0.5D 2 potential of −2.4 V is estimated for this exo , exo dimer, more reducing than that of [RuCp*(1,3,5-R 3 C 6 H 3 )] 2 (−2.0 V). This isomer reacts much more rapidly with both air and electron acceptors than [RuCp*(1,3,5-R 3 C 6 H 3 )] 2 due to a much more cathodic D 2 ˙ + /D 2 potential. The other isomer to be crystallographically characterized, along with a third isomer, are both dimerized in an exo , endo fashion, representing the first examples of such dimers. Density functional theory calculations and reactivity studies indicate that the central bonds of these two isomers are weaker than those of the exo , exo isomer, or of [RuCp*(1,3,5-R 3 C 6 H 3 )] 2 , leading to estimated D + /0.5D 2 potentials of −2.5 and −2.6 V vs. FeCp 2 +/0 . At the same time the D 2 ˙ + /D 2 potentials for the exo , endo dimers are anodically shifted relative to those of [RuCp*(1,3,5-R 3 C 6 H 3 )] 2 , resulting in much greater air stability than for the exo , exo isomer. 
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