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Zheng, Yongping,Song, Kyeongse,Jung, Jaepyeong,Li, Chenzhe,Heo, Yoon-Uk,Park, Min-Sik,Cho, Maenghyo,Kang, Yong-Mook,Cho, Kyeongjae American Chemical Society 2015 Chemistry of materials Vol.27 No.9
<P>Li-O-2 batteries provide high-capacity energy storage, but for aprotic Li-O-2 batteries, it is reported that the charge-discharge efficiency is ultimately limited by the crystal growth of insoluble Li2O2 on the porous cathode. Catalysts have been reported to improve the nucleation and morphology of Li2O2, which helps achieve high energy densities. We provide a new insight into the catalytic mechanism of the oxygen reduction reaction (ORR) in aprotic Li-O-2 batteries the oxygen sites on the surface play a more important role than the exposed metal sites via a study based on the density functional theory (DFT) examining alpha-MnO2 surfaces. Lithium ions from electrolytes are found to interact with the surface oxygen sites and form surface lithium sites, facilitating further growth of Li2O2. A larger number of initial growth points with uniform distribution makes Li2O2 well dispersed, forming small particles, which benefit both the ORR and oxygen evolution reactions (OER). This design concept for oxygen sites has been successfully validated by the real Li-O-2 cell experiments with alpha-MnO2 nanowire cathodes.</P>
Li, Chenzhe,Moon, Junghwan,Yun, Jung-Hoon,Kim, Hyunsu,Cho, Maenghyo Elsevier 2017 Polymer Vol.129 No.-
<P><B>Abstract</B></P> <P>The strain generated by photoresponsive polymer actuators under external applied loads is a key performance parameter. Herein, we investigated the photogenerated strain under uniaxial loads in densely crosslinked azobenzene-incorporated liquid crystalline polymer (azo-LCP) actuators. Spectral analysis indicated that the liquid crystalline (LC) moieties and incorporated azobenzene moieties behave independently under load. The degree of LC mesogen distortions decreased as the applied load increased, whereas azobenzene isomerization showed high tolerance towards the external load. Such stress-response differences led to universal “two-stage” photogenerated strain behavior, with rapid decreases under low applied loads and steady linear decreases under relatively high loads. More importantly, we found that light intensity has little influence on important parameters such as the loading stage and actuator failure stress. However, these properties could be optimized by changing the crosslinking density (either azobenzene or side-chain LC concentration) in the polymeric network.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Applied loads can significantly alternate the photogenerated strain in densely crosslinked azo-LCP actuators. </LI> <LI> The LC mesogen distortions and azobenzene isomerization behave independently resulting “two-stage” photogenerated strain. </LI> <LI> The photogenerated strain under loads and effective actuation range can be well tuned by changing the cross-linked density. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Kweun, Joshua Minwoo,Li, Chenzhe,Zheng, Yongping,Cho, Maenghyo,Kim, Yoon Young,Cho, Kyeongjae Elsevier 2016 APPLIED SURFACE SCIENCE - Vol.370 No.-
<P><B>Abstract</B></P> <P>Designing metal-oxides consisting of earth-abundant elements has been a crucial issue to replace precious metal catalysts. To achieve efficient screening of metal-oxide catalysts via bulk descriptors rather than surface descriptors, we investigated the relationship between the electronic structure of bulk and that of the surface for lanthanum-based perovskite oxides, LaMO<SUB>3</SUB> (M=Ti, V, Cr, Mn, Fe, Co, Ni, Cu). Through density functional theory calculations, we examined the d-band occupancy of the bulk and surface transition-metal atoms (<I>n</I> <SUB>Bulk</SUB> and <I>n</I> <SUB>Surf</SUB>) and the adsorption energy of an oxygen atom (<I>E</I> <SUB>ads</SUB>) on (001), (110), and (111) surfaces. For the (001) surface, we observed strong correlation between the <I>n</I> <SUB>Bulk</SUB> and <I>n</I> <SUB>Surf</SUB> with an <I>R</I>-squared value over 94%, and the result was interpreted in terms of ligand field splitting and antibonding/bonding level splitting. Moreover, the <I>E</I> <SUB>ads</SUB> on the surfaces was highly correlated with the <I>n</I> <SUB>Bulk</SUB> with an <I>R</I>-squared value of more than 94%, and different surface relaxations could be explained by the bulk electronic structure (e.g., LaMnO<SUB>3</SUB> vs. LaTiO<SUB>3</SUB>). These results suggest that a bulk-derived descriptor such as <I>n</I> <SUB>Bulk</SUB> can be used to screen metal-oxide catalysts.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Bulk-surface relationship was predicted by the ligand field nature of metal oxides. </LI> <LI> Antibonding and bonding d-bands occupancy clarified the bulk-surface relationship. </LI> <LI> Different surface relaxations were explained by the bulk electronic structures. </LI> <LI> Transition from the bulk to the surface state was simulated by oxygen adsorption. </LI> </UL> </P>
Zheng, Yongping,Yang, Dae-Soo,Kweun, Joshua M.,Li, Chenzhe,Tan, Kui,Kong, Fantai,Liang, Chaoping,Chabal, Yves J.,Kim, Yoon Young,Cho, Maenghyo,Yu, Jong-Sung,Cho, Kyeongjae Elsevier 2016 Nano energy Vol.30 No.-
<P><B>Abstract</B></P> <P>Bio-inspired non-precious-metal catalysts based on iron and cobalt porphyrins are promising alternatives to replace costly platinum-based catalysts for oxygen reduction reaction (ORR) in fuel cells. However, the exact nature of the active sites is still not clearly understood, and further optimization design is needed for practical applications. Here, we report a rational catalyst design process by combining density functional theory (DFT) calculations and experimental validations. Two sets of square-planar (MN<SUB>x</SUB>C<SUB>4−x</SUB>) and square-pyramid (MN<SUB>x</SUB>C<SUB>5−x</SUB>) active centers (M=Mn, Fe, Co, Ni) incorporated in graphene were examined using DFT. Fe-N<SUB>5</SUB> and Co-N<SUB>4</SUB> sites were identified theoretically to have the best performance in fuel cells, while Ni-N<SUB>x</SUB>C<SUB>4−x</SUB> sites catalyze the most H<SUB>2</SUB>O<SUB>2</SUB> byproduct. Graphene samples with well-dispersed incorporations of metals were synthesized, and the following electrochemical measurements show an excellent agreement with the theoretical predictions, indicating that a successful design framework and systematic understanding toward the catalytic nature of these materials are established.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Graphene based catalysts design for ORR is demonstrated by combining experiments and modellings. </LI> <LI> Iron porphyrin like active site is unraveled to be five nitrogen coordinated as FeN<SUB>5</SUB>. </LI> <LI> Cobalt porphyrin like active site is shown to be four nitrogen coordinated as CoN<SUB>4</SUB>. </LI> <LI> Nickel porphyrin like catalyst is potentially used for catalytic synthesis of H<SUB>2</SUB>O<SUB>2</SUB>. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Li, Chenzhe,Yun, Jung-Hoon,Kim, Hyunsu,Cho, Maenghyo American Chemical Society 2016 Macromolecules Vol.49 No.16
<P>The photomechanical behavior of azobenzene-functionalized liquid-crystalline polymers (azo-LCPs) is closely related to UV light propagation. Here, we report the effect of light absorption by the LC host and the concerted isomerism of azobenzene on this property. In situ measurements of light absorptivity and computer simulations revealed that the light propagation of a liquid-crystalline polymer is affected by the UV absorption of the LC host under the same concerted isomerism of azobenzene. The lower UV light absorption of the LC host results in deeper UV light propagation and a sharper isomerization profile, which eventually induces faster bending when compared to the higher UV light absorption of the LC host. On the other hand, increasing the azobenzene concentration in the polymer greatly decreases the speed of light propagation, which is regarded as one of the factors inhibiting further increases in the bending speed of azo-LCP films.</P>
Chenzhe Li,Maenghyo Cho,Kyeongjae Cho 대한기계학회 2014 대한기계학회 춘추학술대회 Vol.2014 No.11
We present theoretical evidence for the phase evolution and stability of Fe substituted RMn₂O<SUB>5-δ</SUB> (R=Bi, Sm or Y) mullite structures. Our density-functional simulations show evidence for the R site dependence of the mullite phase evolution with the Fe content in the solid solution. For Fe in R=Y or Sm compounds, the orthorhombic mullite phases are predicted to possess substitution limits. Compounds with x more than 0.5 or 0.75 in R[Fe<SUB>x</SUB>Mn<SUB>1-x</SUB>]₂O<SUB>5-δ</SUB> stoichiometry formula are found to be unstable under the high temperature calcination condition. For Fe in R=Bi compounds, a sub-phase transition behavior is found to preserve the orthorhombic structure with oxygen content variation in the tested compounds.
Li, Chenzhe,Thampy, Sampreetha,Zheng, Yongping,Kweun, Joshua M,Ren, Yixin,Chan, Julia Y,Kim, Hanchul,Cho, Maenghyo,Kim, Yoon Young,Hsu, Julia W P,Cho, Kyeongjae IOP 2016 Journal of Physics, Condensed Matter Vol.28 No.12
<P>Understanding and effectively predicting the thermal stability of ternary transition metal oxides with heavy elements using first principle simulations are vital for understanding performance of advanced materials. In this work, we have investigated the thermal stability of mullite <I>R</I>Mn<SUB>2</SUB>O<SUB>5</SUB> (<I>R</I> = Bi, Pr, Sm, or Gd) structures by constructing temperature phase diagrams using an efficient mixed generalized gradient approximation (GGA) and the GGA + <I>U</I> method. Simulation predicted stability regions without corrections on heavy elements show a 4–200 K underestimation compared to our experimental results. We have found the number of <I>d</I>/<I>f</I> electrons in the heavy elements shows a linear relationship with the prediction deviation. Further correction on the strongly correlated electrons in heavy elements could significantly reduce the prediction deviations. Our corrected simulation results demonstrate that further correction of <I>R</I>-site elements in <I>R</I>Mn<SUB>2</SUB>O<SUB>5</SUB> could effectively reduce the underestimation of the density functional theory-predicted decomposition temperature to within 30 K. Therefore, it could produce an accurate thermal stability prediction for complex ternary transition metal oxide compounds with heavy elements.</P>
윤정훈,Chenzhe Li,정하영,최준명,조맹효 한국전산구조공학회 2016 한국전산구조공학회논문집 Vol.29 No.6
Photo responsive polymer(PRP) is well known for its photo deformation under UV light, and goes back to its original shape in visible light due to the photoisomerization of the azobenzene inside the PRP. In this paper, dynamic study of the vibration in PRP is discussed. In order to predict photo-deformation of the PRP a multiscale modeling is introduced which covers quantum level photo excitation, microscopic morphology, and macroscopic deformation of the PRP. A simple 1D beam model is introduced to model dynamic bending behavior of the PRP. Through fast Fourious transformation analysis, we identify that vibration frequency of the PRP can be controlled by light polarization angle.