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Origin of unusual spinel-to-layered phase transformation by crystal water
Yang, Eunjeong,Kim, Heejin,Kim, Sangryun,Kim, In,Kim, Jaehoon,Ji, Hyunjun,Choi, Jang Wook,Jung, Yousung Royal Society of Chemistry 2018 Chemical science Vol.9 No.2
<▼1><P>Crystal water mediated phase transition: the underlying thermodynamic and kinetic role of crystal water is investigated using <I>ab initio</I> calculations.</P></▼1><▼2><P>It is well known that many layered transition metal oxides can transform into a spinel structure upon repeated battery cycling, but a phase transition in the opposite direction is rare. Recently, the transformation from spinel Mn<SUB>3</SUB>O<SUB>4</SUB> to layered MnO<SUB>2</SUB> was observed during the operation of a Mg battery in aqueous conditions, resulting in high performance Mg batteries. We hereby use <I>ab initio</I> calculations to unveil the mechanism by which crystal water plays a critical role in this unique transformation. Once inserted into the spinel form, a water molecule donates an electron, offering a key structural and thermodynamic driving force to initiate the transformation process. These crystal water molecules then get favorably clustered into a planar form in the layered structure and act as a stabilizing agent for birnessite. Kinetically, the inserted crystal water dramatically promotes the necessary rearrangement of Mn during the transition by lowering the activation barrier by >2 eV. The present structural, thermodynamic and kinetic understanding of the crystal water-driven phase transition provides novel insights to further the design of related low dimensional hydrated materials for multi-valent cathodes.</P></▼2>
Yang, Eunjeong,Ji, Hyunjun,Kim, Jaehoon,Kim, Heejin,Jung, Yousung The Royal Society of Chemistry 2015 Physical chemistry chemical physics Vol.17 No.7
<P>Recently a group of two-dimensional materials called MXenes have been discovered and they have demonstrated their potential in Li rechargeable batteries. Herein, the Na storage and ion migration properties of M<SUB>2</SUB>C-type MXenes (M = Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo) were investigated using density functional theory (DFT) calculations, and were compared to the Li case. Based on the average voltage and migration barrier of surface ions, we suggest that M = Ti, V, Cr, Mn, and Mo are suitable for sodium ion battery (SIB) anodes. These screened M<SUB>2</SUB>C materials can provide a theoretical capacity of 190–288 mA h g<SUP>−1</SUP> by accommodating two alkali ions per formula unit. They also exhibit an activation barrier of 0.1–0.2 eV for ionic motion, suggesting that the M<SUB>2</SUB>C materials are promising for high-power applications. The underlying aspects of the voltage differences between M<SUB>2</SUB>C materials are also discussed using electrostatic considerations.</P> <P>Graphic Abstract</P><P>MXenes are predicted to be a family of promising Na anode materials with desirable electrochemical properties using density functional theory. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp05140h'> </P>
Affordable method of video recording for ecologists and citizen-science participants
Yang, Eunjeong,Lee, Keesan,Ha, Jung-moon,Kim, Woojoo,Song, Ho-Kyung,Hwang, Injae,Lee, Sang-im,Jablonski, Piotr G. The Ecological Society of Korea 2018 Journal of Ecology and Environment Vol.42 No.6
Observations and video documentation of interactions between animals living in dens, cavities, and other enclosed spaces are difficult, but they play an important role in field biology, ecology, and conservation. For example, bird parents visiting nests and feeding their nestlings may provide crucial information for testing of ecological hypotheses and may easily attract attention of participants of citizen-science ecological and conservation projects. Because of the nest concealment of cavity-nesting birds, their behaviors in the nest can only be studied by using video surveillance. Professional wildlife surveillance systems are extremely expensive. Here, we describe an inexpensive video setup that can be constructed with relatively little effort and is more affordable than any previously described system. We anticipate that the relatively low cost of about 250 USD for a battery-operated system is an important feature for citizen-science type of projects and for applications in heavily populated areas where the potential for theft and vandalism may be high. Based on our experiences, we provide methodological advice on practical aspects of using this system in the field for ecological research on birds. We highlight the low cost, easiness of construction, and potential availability to a large number of observers taking part in wildlife monitoring projects, and we offer technical help to participants of such research projects.