Eunjik Lee

Rational syntheses of core–shell Fe@(PtRu) nanoparticle electrocatalysts for the methanol oxidation reaction with complete suppression of CO-poisoning and highly enhanced activity

We report the syntheses of ternary nanoparticles (NPs) of Fex@(PtRu)(1−x)/2 (x = 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)3 in 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.

One-step sonochemical syntheses of Ni@Pt core-shell nanoparticles with controlled shape and shell thickness for fuel cell electrocatalyst.

We demonstrate a facile one-step method to synthesize Ni@Pt core-shell nanoparticles (NPs) with a control over the shape and the Pt-shell thickness of the NPs. By adjusting the relative reactivity of the Pt and Ni reagents in ultrasound-assisted polyol reactions, two Ni@Pt NP samples of the same composition (Ni/Pt=1) and size (3-4 nm) but with different particle shape (octahedral vs. truncated octahedral) and different Pt-shell thicknesses (1-2 vs. 2-3 monolayer) are obtained. The control is achieved by using different Ni reagents, Ni(acac)2 (acac=acetylacetonate) and Ni(hfac)2 (hfac=hexafluoroacetylacetonate). A reaction mechanism that can explain all of the observations is proposed. The Ni@Pt NPs show up to threefold higher mass activity than pure Pt NPs in oxygen reduction reaction. Between the two Ni@Pt NP samples, the one composed of octahedral NPs with the thicker Pt-shell has higher activity than the other.

Epitaxial growth of Pd nanoparticles on molybdenum disulfide by sonochemistry and its effects on electrocatalysis

In this work, we have synthesized Pd/MoS2 samples in which about 7 nm sized Pd NPs are formed on a MoS2 surface with the Pd content varied from 14.4 wt% to 33.6 wt% and studied their electrocatalytic activity for oxygen reduction reaction (ORR) in an alkaline medium. The syntheses were achieved by single-step sonochemical reactions between palladium acetylacetonate and 2H-MoS2 in ethylene glycol. Based on the structural characterization data, the Pd NPs are firstly grown epitaxially on the MoS2 surface in the [1−10]Pd||[100]MoS2 and (111)Pd||(001)MoS2 relationship. As the Pd-content increases, Pd NPs are formed on top of the first layer of Pd NPs as well as on the available MoS2 surface. The epitaxially grown Pd NPs experience a tensile strain and charge-transfer to MoS2, which raises the d-band center of Pd, lowering the on-set potential in ORR. On the other hand, the enhanced adsorption of O2 on MoS2 can facilitate the ORR kinetics of Pd NPs. The observed ORR data on Pd/MoS2 can be explained as the net result of these two opposing effects of the MoS2 support.

Multi-component electrocatalyst for low-temperature fuel cells synthesized via sonochemical reactions.

This review presents recent advances in multi-component electrocatalysts for low-temperature fuel cells (FCs) synthesized via sonochemical reactions. As a feasible approach to develop novel electrocatalysts that can overcome the many problems of the prevailing Pt electrocatalysts, Pt- or Pd-based alloy and core-shell M@Pt nanoparticles (NPs) have been pursued. Synthesizing NPs with desirable properties often turn out to be challenging. Sonochemistry generates extreme conditions via acoustic cavitation, which have been utilized in the syntheses of various Pt and Pd NPs and Pt- and Pd-based alloy NPs. Especially, it has been reported that several M@Pt core-shell NPs can be synthesized by sonochemistry, which is hard to achieve by other methods. The principles of sonochemistry are presented with examples. Also alloy NPs and core-shell NPs synthesized by sonochemistry and those by other methods are compared.

Facile sonochemical synthesis of amorphous NiFe-(oxy)hydroxide nanoparticles as superior electrocatalysts for oxygen evolution reaction

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 (Ni100-xFex, 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. Ni70Fe30(H) sample showed the lowest overpotential of 292mV at 10mAcm-2geo and the lowest Tafel slope of 30.4mVdec-1, outperforming IrOx/C (326mV, 41.7mVdec-1). Our samples are highly durable based on the chronopotentiometry data at the current density of 10mAcm-2geo for 2h which show that Ni70Fe30 sample maintains the steady-state potential, contrary to the time-varying IrOx/C.