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Lee, Eunjik,Park, Ah-Hyeon,Park, Hyun-Uk,Kwon, Young-Uk Butterworth Heinemann 2018 Ultrasonics sonochemistry Vol.40 No.1
<P><B>Abstract</B></P> <P>In this work, we present facile synthesis of amorphous Ni/Fe mixed (oxy)hydroxide (NiFe(H)) nanoparticles (NPs) and their electrocatalytic performance for oxygen evolution reaction (OER) in alkaline media. a-NiFe(H) NPs have received lots of attention as OER electrocatalysts with many desirable properties. By using a simple sonochemical route, we prepared amorphous Ni and Fe-alkoxide (NiFe(A)) NPs whose composition can be controlled in the entire composition range (Ni<SUB>100−x</SUB>Fe<SUB>x</SUB>, 0≤x≤1). These samples are composed of extremely small NiFe(A) NPs with Ni and Fe atoms homogeneously distributed. NiFe(A) NPs are readily converted into corresponding electrocatalytically active NiFe(H) NP by a simple electrochemical treatment. Electrochemical analysis data show that the OER activity of amorphous NiFe(H) samples follows the volcano-type trend when plotted against the Fe content. Ni<SUB>70</SUB>Fe<SUB>30</SUB>(H) sample showed the lowest overpotential of 292mV at 10mAcm<SUP>−2</SUP> <SUB>geo</SUB> and the lowest Tafel slope of 30.4mVdec<SUP>−1</SUP>, outperforming IrO<I> <SUB>x</SUB> </I>/C (326mV, 41.7mVdec<SUP>−1</SUP>). Our samples are highly durable based on the chronopotentiometry data at the current density of 10mAcm<SUP>−2</SUP> <SUB>geo</SUB> for 2h which show that Ni<SUB>70</SUB>Fe<SUB>30</SUB> sample maintains the steady-state potential, contrary to the time-varying IrO<I> <SUB>x</SUB> </I>/C.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Small and amorphous NiFe(H) nanoparticles are synthesized by a sonochemical method. </LI> <LI> OER activity of NiFe(H) show the volcano-type trend following the Fe content. </LI> <LI> Ni<SUB>70</SUB>Fe<SUB>30</SUB>(H) showed the superior activity and durability for OER, outdoing the IrO<I> <SUB>x</SUB> </I>. </LI> <LI> Our facile method can be useful in the discovery of more improved OER catalysts. </LI> </UL> </P>
Lee, Eunjik,Jang, Ji-Hoon,Kwon, Young-Uk Elsevier 2015 Journal of Power Sources Vol.273 No.-
<P><B>Abstract</B></P> <P>In this work, we synthesized a series of manganese cobalt spinel oxide (Mn<SUB> <I>x</I> </SUB>Co<SUB>3−<I>x</I> </SUB>O<SUB>4</SUB>) nanoparticles (NPs) covering the whole composition range (<I>x</I> = 0.0. 0.4, 0.8, 1.4, 1.9, and 3.0) and investigated their electrocatalytic properties in relation with the oxygen reduction reaction (ORR) in alkaline media. The NP samples were synthesized by sonochemical reactions of Mn(OAc)<SUB>2</SUB> and Co(OAc)<SUB>2</SUB> (Ac = acetyl) in a water-dimethylformamide mixed solvent. The four samples in the Co-rich side have the cubic structure whereas the other two samples in the Mn-rich side have the tetragonal structure. The X-ray photoelectron spectroscopy and electrochemical analyses data indicate that the distribution of manganese and cobalt ions between the two metal ion sites of the spinel structure in our NP samples conforms to that of the bulk counterparts in the literature. The electrocatalytic data show that the ORR mechanism is changed when the structure is changed from cubic to tetragonal. The highest ORR activity was observed with the <I>x</I> = 0.4 sample. The electrochemical stability of this sample is higher than that of commercial Pt/C.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We synthesized Mn<SUB> <I>x</I> </SUB>Co<SUB>3−<I>x</I> </SUB>O<SUB>4</SUB> NP samples with <I>x</I> = 0.0, 0.4, 0.8, 1.4, 1.9, and 3.0. </LI> <LI> The substituted Mn in sample is mainly on the Mn<SUP>3+</SUP> ions in the octahedral site. </LI> <LI> The ORR mechanism is changed when the structure is changed from cubic to tetragonal. </LI> <LI> The highest ORR activity was observed with the <I>x</I> = 0.4 sample. </LI> <LI> The electrochemical stability of <I>x</I> = 0.4 sample is higher than that of Pt/C. </LI> </UL> </P>
Ultra-high capacitance hematite thin films with controlled nanoscopic morphologies.
Liu, Jingling,Lee, Eunjik,Kim, Yong-Tae,Kwon, Young-Uk RSC Pub 2014 Nanoscale Vol.6 No.18
<P>We synthesized α-Fe2O3 (hematite) thin films with two different nanoscopic morphologies through self-assembly between a Fe-precursor and a Pluronic tri-block copolymer (F127) followed by aging and calcination. Relative humidity (RH) during the aging step of the spin-coated films was found to be critical in determining the morphologies. A network structure of nanowires 6 nm in diameter formed when the RH was 75%. The resulting nanowire hematite thin film (NW) had 150-250 nm-sized macropores. When the RH was 0%, a mesoporous hematite thin film (MP) with a wormlike pore structure and a pore size of 9 nm formed. Investigation of the electrochemical properties of these films revealed that they had very high specific capacitances of 365.7 and 283.2 F g(-1) for NWs and MPs, respectively, at a current density of 3 A g(-1) in a 0.5 M Na2SO3 electrolyte. Both of these capacitance values are considerably higher than those previously reported for hematite-based electrodes. We attributed this to the high porosity of the thin films, which enables ready access of electrolyte ions to the electrode surfaces, and their ultra-thin size, comparable to that of the depletion layer, allowing the low conductivity of hematite to be overcome. The higher capacitance of NWs than MPs is likely due to the accelerated electron transport through the crystalline nanowires in NWs.</P>
Matin, Md. Abdul,Lee, Eunjik,Kim, Hyunchul,Yoon, Won-Sub,Kwon, Young-Uk The Royal Society of Chemistry 2015 Journal of Materials Chemistry A Vol.3 No.33
<P>We report the syntheses of ternary nanoparticles (NPs) of Fex@(PtRu)(1−x)/2(<I>x</I>= 0.0, 0.30, 0.34, 0.38, and 0.44) with Fe cores and PtRu alloy shells, which exhibit greatly improved electrocatalytic properties for the methanol oxidation reaction (MOR). The syntheses were achieved by one-step sonochemical treatments of Pt(acac)2, Ru(acac)3, and Fe(acac)3in ethylene glycol. The NPs are characterized by X-ray diffractometry, transmission electron microscopy (TEM), and inductively coupled plasma-adsorption emission spectroscopy for the particle size, morphology, and composition, respectively. The formation of core-shell NPs has been proven by scanning TEM-energy dispersive X-ray spectroscopy, and the electronic structures of the elements have been investigated by X-ray photoelectron spectroscopy and X-ray adsorption near edge spectroscopy. The ternary NPs show enhanced MOR electrocatalytic activity compared to a commercial PtRu-alloy (PtRu) by a factor of up to 2.5 based on the forward current density data. More importantly, the ternary NPs show complete suppression of CO-poisoning. Chronoamperometry data for MOR on the ternary NPs show improved stability over Pt/C and PtRu references.</P>
Hyun-Uk Park,Eunjik Lee,Jeongyun Jang,Min Ho Seo,Gu-Gon Park 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
In this work, we propose a method to easily control the size of the core-shell nanoparticles (NPs) by controlling the pressure of the inert gas during the heat treatment process. As synthesized Fe@Pt/C catalysts were conducted heat treatment with varied pressure control (1, 40, 80 bar) under inert atmosphere. The average size of the metal NPs was distributed in the range of about 3 to 5 nm, and the particle size was clearly reduced in inverse proportion to the pressure. By scanning transmission electron microscopy (STEM) analysis, we also confirmed that the catalysts show clear M core-Pt shell structure. We also confirmed that the NPs retained the core-shell structure even after heat treatment at all pressure conditions and were successfully applied to the oxygen reduction reaction (ORR) electrocatalysts, confirming that the mass activity was higher than commercial Pt/C (HiSPEC4000) catalyst. Theoretical calculations show the correlation between the size of metal NPs and external pressure. Herein, we introduce a novel particle size control method and our unique synthetic approach can be a promising way to obtain intermetallic structure NPs with controlled particle size.
Park, Hyun-Uk,Lee, Eunjik,Kwon, Young-Uk Elsevier 2017 International journal of hydrogen energy Vol.42 No.31
<P><B>Abstract</B></P> <P>In this study, we synthesized Pt nanoparticles (NPs) with small amounts of Mn (≤11.4 at%) included, hence Pt(Mn) NPs, on titanium carbide (TiC) support (denoted as Pt(Mn)/TiC) in three different Pt loadings (16.0–33.2 wt%) and investigated their electrocatalytic performance for methanol oxidation reaction (MOR) in acidic media. The syntheses were achieved via one-pot sonochemical reactions of Pt(acac)<SUB>2</SUB> and Mn(acac)<SUB>2</SUB> (acac = acetylacetonate) in ethylene glycol in the presence of TiC particles and without any other additives. The Pt(Mn) NPs were uniform in size (4–6 nm) and were evenly deposited on the TiC surface. The electronic structure of Pt in Pt(Mn)/TiC samples, probed by X-ray photoelectron spectroscopy (XPS) and other techniques, is systematically changed with the Pt loading, by which enhanced electrocatalytic properties from pure Pt are expected. In addition, the TiC support contributes to enhancing the electrocatalytic properties of Pt(Mn) NPs through its high conductivity, chemical resistance to corrosion, and the TiO<SUB>2</SUB> formed on the surface which exerts the bifunctional mechanism to reduce the CO poisoning on Pt. The electrochemical performance of Pt(Mn)/TiC was investigated by the rotating disk electrode (RDE) technique. The specific and mass MOR currents are, respectively, 1.6–2.2 and 0.9–1.4 times higher in Pt(Mn)/TiC samples than in commercial Pt/C. All Pt(Mn)/TiC samples show 93–98% of the initial electrochemical surface areas after 3000 potential cycles, superior electrochemical stability to commercial Pt/C (86%).</P> <P><B>Highlights</B></P> <P> <UL> <LI> Well-dispersed Pt(Mn) NPs supported on TiC are synthesized by a facile sonochemical method. </LI> <LI> The NPs are 4–6 nm size and uniformed distributed on TiC surfaces. </LI> <LI> Pt(Mn)/TiC samples showed improved specific MOR activity than that of Pt/C. </LI> <LI> TiC support can lead to the enhancement of activity and durability for MOR of Pt(Mn)/TiC samples. </LI> </UL> </P>