KwangSup Eom

Cobalt-carbon nanofibers as an efficient support-free catalyst for oxygen reduction reaction with a systematic study of active site formation

Recently, major efforts have been devoted to exploring cheap and active non-precious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells for large-scale applications. Herein, we report electrospun cobalt-carbon nanofibers (Co-CNFs) as an efficient catalyst for the ORR, together with a systematic study of the active site formation. The ORR activity of the Co-CNFs increases with increasing Co content up to approximately 30 wt%, at which high ORR activity is exhibited, comparable with a commercial Pt/C catalyst in alkaline media. XPS and structural analysis reveals a Co–pyridinic Nx bond at the edge plane, and more Co nanoparticles were found in the Co-CNFs as the Co content was increased. These sites can behave as the primary and the secondary active sites for the ORR, according to a dual-site mechanism. The ORR activity of the Co-CNFs may deteriorate even if only one of these sites is limited. The high ORR activity of the Co-CNF catalysts results from the synergetic effect of dual site formation for the ORR.

Stabilization of selenium cathodes via in situ formation of protective solid electrolyte layer

The lithium/selenium (Li/Se) rechargeable battery chemistry offers a higher energy density than traditional Li ion battery cells. However, high solubility of polyselenides in suitable electrolytes causes Se loss during electrochemical cycling, and leads to poor cycle stability. This study presents a simple technique to form a protective, solid electrolyte layer on the cathode surface. This protective layer remains permeable to Li ions, but prevents transport of polyselenides, thus dramatically enhancing cell cycle stability. The greatly reduced reactivity of polyselenides with fluorinated carbonates (such as fluoroethylene carbonates [FEC]) permits using their in situ reduction for low-cost formation of protective coatings on Se cathodes.

Electrospun Nb-doped TiO 2 nanofiber support for Pt nanoparticles with high electrocatalytic activity and durability

This study explores a facile method to prepare an efficient and durable support for Pt catalyst of polymer electrolyte membrane fuel cell (PEMFC). As a candidate, Nb-doped TiO2 (Nb-TiO2) nanofibers are simply fabricated using an electrospinning technique, followed by a heat treatment. Doping Nb into the TiO2 nanofibers leads to a drastic increase in electrical conductivity with doping level of up to 25 at. % (Nb0.25Ti0.75O2). Pt nanoparticles are synthesized on the prepared 25 at. % Nb-doped TiO2-nanofibers (Pt/Nb-TiO2) as well as on a commercial powdered carbon black (Pt/C). The Pt/Nb-TiO2 nanofiber catalyst exhibits similar oxygen reaction reduction (ORR) activity to that of the Pt/C catalyst. However, during an accelerated stress test (AST), the Pt/Nb-TiO2 nanofiber catalyst retained more than 60% of the initial ORR activity while the Pt/C catalyst lost 65% of the initial activity. The excellent durability of the Pt/Nb-TiO2 nanofiber catalyst can be attributed to high corrosion resistance of TiO2 and strong interaction between Pt and TiO2.

A stable lithiated silicon–chalcogen battery via synergetic chemical coupling between silicon and selenium

Li-ion batteries dominate portable energy storage due to their exceptional power and energy characteristics. Yet, various consumer devices and electric vehicles demand higher specific energy and power with longer cycle life. Here we report a full-cell battery that contains a lithiated Si/graphene anode paired with a selenium disulfide (SeS2) cathode with high capacity and long-term stability. Selenium, which dissolves from the SeS2 cathode, was found to become a component of the anode solid electrolyte interphase (SEI), leading to a significant increase of the SEI conductivity and stability. Moreover, the replacement of lithium metal anode impedes unwanted side reactions between the dissolved intermediate products from the SeS2 cathode and lithium metal and eliminates lithium dendrite formation. As a result, the capacity retention of the lithiated silicon/graphene—SeS2 full cell is 81% after 1,500 cycles at 268 mA gSeS2−1. The achieved cathode capacity is 403 mAh gSeS2−1 (1,209 mAh cmSeS2−3).

Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell.

The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a conventional decal transfer method and an outer catalyst layer directly coated on a gas diffusion layer. The double-layered structure was used to improve the interfacial contact between the catalyst layer and membrane, to increase catalyst utilization and to modify the removal of product water from the cathode. Based on a series of MEAs with double-layered cathodes with an overall Pt loading fixed at 0.4 mg cm(-2) and different ratios of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg Pt cm(-2) and an outer layer of 0.1 mg Pt cm(-2) exhibited the best performance. This performance was better than that of the conventional single-layered electrode by 13.5% at a current density of 1.4 A cm(-2).

Bi-axial grown amorphous MoS x bridged with oxygen on r-GO as a superior stable and efficient nonprecious catalyst for hydrogen evolution

Amorphous molybdenum sulfide (MoSx) is covalently anchored to reduced graphene oxide (r-GO) via a simple one-pot reaction, thereby inducing the reduction of GO and simultaneous doping of heteroatoms on the GO. The oxygen atoms form a bridged between MoSx and GO and play a crucial role in the fine dispersion of the MoSx particles, control of planar MoSx growth, and increase of exposed active sulfur sites. This bridging leads to highly efficient (−157 mV overpotential and 41 mV/decade Tafel slope) and stable (95% versus initial activity after 1000 cycles) electrocatalyst for hydrogen evolution.

Effects of Nafion Contents on the Performance of MEAs Prepared by Decal-Transfer Method

Abstract >> Nafion ionomer located in electrode helps to increase the platinum utilization and proton conductivity.To achieve higher performance in PEMFCs, it is important an optimum Nafion content in the electrode. As theplatinum loading and fabricated method depend on the optimum Nafion content. In this study, we have examinedthe interrelationship between platinum loading and Nafion content fabricated by decal transfer method. For electrodes with 0.25 and 0.4 mg/cm 2 Pt loading, best performance was obtained at 25 wt.% Nafion ionomer loading.It is also found that MEA with 0.25 mg/cm 2 Pt, the optimum Nafion content appears differently at low and highcurrent density. Key words : PEMFC(고분자전해질연료전지), Nafion contents(나피온 함량), MEA(막-전극접합체), Pt loading(백금 사용량), Decal-transfer method(데칼전사기법) † Corresponding author : eacho@kist.re.kr [ 접수일 : 2012.4.10 수정일 : 2012.4.23 게재확정일 : 2012.4.27 ] 1. 서 론 연료전지는 수소를 연료로 전기를 생산하며 그 부산물로 물과 열만을 발생하는 친환경 전기 에너지 변환장치이다. 특히 고분자 전해질 연료전지는 저온(상온-80℃)에서 운전되고 출력밀도가 높아 자동차 및 휴대용 전원으로 활발히 연구가 진행되고 있다.