Publications by authors named "Namgee Jung"

20 Publications

  • Page 1 of 1

Emerging carbon shell-encapsulated metal nanocatalysts for fuel cells and water electrolysis.

Nanoscale 2021 Sep 23;13(36):15116-15141. Epub 2021 Sep 23.

Center for Hydrogen·Fuel Cell Research, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea.

The development of low-cost, high-efficiency electrocatalysts is of primary importance for hydrogen energy technology. Noble metal-based catalysts have been extensively studied for decades; however, activity and durability issues still remain a challenge. In recent years, carbon shell-encapsulated metal ([email protected]) catalysts have drawn great attention as novel materials for water electrolysis and fuel cell applications. These electrochemical reactions are governed mainly by interfacial charge transfer between the core metal and the outer carbon shell, which alters the electronic structure of the catalyst surface. Furthermore, the rationally designed and fine-tuned carbon shell plays a very interesting role as a protective layer or molecular sieve layer to improve the performance and durability of energy conversion systems. Herein, we review recent advances in the use of [email protected] type nanocatalysts for extensive applications in fuel cells and water electrolysis with a focus on the structural design and electronic structure modulation of carbon shell-encapsulated metal/alloys. Finally, we highlight the current challenges and future perspectives of these catalytic materials and related technologies in this field.
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http://dx.doi.org/10.1039/d1nr01328aDOI Listing
September 2021

Enhancement of Catalytic Activity and Durability of Pt Nanoparticle through Strong Chemical Interaction with Electrically Conductive Support of Magnéli Phase Titanium Oxide.

Nanomaterials (Basel) 2021 Mar 24;11(4). Epub 2021 Mar 24.

Fuel Cell Laboratory, Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Korea.

In this study, we address the catalytic performance of variously sized Pt nanoparticles (NPs) (from 1.7 to 2.9 nm) supported on magnéli phase titanium oxide (MPTO, TiO) along with commercial solid type carbon (VXC-72R) for oxygen reduction reaction (ORR). Key idea is to utilize a robust and electrically conductive MPTO as a support material so that we employed it to improve the catalytic activity and durability through the strong metal-support interaction (SMSI). Furthermore, we increase the specific surface area of MPTO up to 61.6 m g to enhance the SMSI effect between Pt NP and MPTO. After the deposition of a range of Pt NPs on the support materials, we investigate the ORR activity and durability using a rotating disk electrode (RDE) technique in acid media. As a result of accelerated stress test (AST) for 30k cycles, regardless of the Pt particle size, we confirmed that Pt/MPTO samples show a lower electrochemical surface area (ECSA) loss (<20%) than that of Pt/C (~40%). That is explained by the increased dissolution potential and binding energy of Pt on MPTO against to carbon, which is supported by the density functional theory (DFT) calculations. Based on these results, we found that conductive metal oxides could be an alternative as a support material for the long-term fuel cell operation.
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http://dx.doi.org/10.3390/nano11040829DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8063942PMC
March 2021

Facile Strategy for Mass Production of Pt Catalysts for Polymer Electrolyte Membrane Fuel Cells Using Low-Energy Electron Beam.

Nanomaterials (Basel) 2020 Nov 6;10(11). Epub 2020 Nov 6.

Graduate School of Energy Science and Technology (GEST), Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Korea.

There are so many variables affecting the large-scale chemical synthesis of nanoparticles that mass production remains a challenge. Here, using a high-efficiency compact electron beam generator irradiating a low-energy electron beam, we fabricate carbon-supported Pt nanoparticles (Pt/C) in an open chamber to present the applicability of an electron beam to the mass production of metal nanocatalysts for polymer electrolyte membrane fuel cells (PEMFCs). The amount of dispersants (glycerol) and radical scavengers (isopropyl alcohol, IPA), the most important factors in the electron beam-induced fabrication process, is systematically controlled to find the conditions for the synthesis of the particle structure suitable for PEMFC applications. Furthermore, the effects of the structural changes on the electrochemical properties of the catalysts are thoroughly investigated. Through in-depth studies, it is clearly revealed that while dispersants control the nucleation step of monomers affecting the degree of dispersion of nanoparticles, radical scavengers with strong oxidizing power have an effect on the particle growth rate. Therefore, this study is expected to present the applicability of low-energy electron beam to the mass production of metal nanocatalysts for PEMFCs, and to provide insights into the fabrication of nanoparticles using low-energy electron beams.
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http://dx.doi.org/10.3390/nano10112216DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7694981PMC
November 2020

Quaternary Ammonium-Bearing Perfluorinated Polymers for Anion Exchange Membrane Applications.

Membranes (Basel) 2020 Oct 26;10(11). Epub 2020 Oct 26.

Fuel Cell Laboratory, Korea Institute of Energy Research, Daejoen 34129, Korea.

Perfluorinated polymers are widely used in polymer electrolyte membranes because of their excellent ion conductivity, which are attributed to the well-defined morphologies resulting from their extremely hydrophobic main-chains and flexible hydrophilic side-chains. Perfluorinated polymers containing quaternary ammonium groups were prepared from Nafion- and Aquivion-based sulfonyl fluoride precursors by the Menshutkin reaction to give anion exchange membranes. Perfluorinated polymers tend to exhibit poor solubility in organic solvents; however, clear polymer dispersions and transparent membranes were successfully prepared using -methyl-2-pyrrolidone at high temperatures and pressures. Both perfluorinated polymer-based membranes exhibited distinct hydrophilic-hydrophobic phase-separated morphologies, resulting in high ion conductivity despite their low ion exchange capacities and limited water uptake properties. Moreover, it was found that the capacitive deionization performances and stabilities of the perfluorinated polymer membranes were superior to those of the commercial Fumatech membrane.
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http://dx.doi.org/10.3390/membranes10110306DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7693359PMC
October 2020

Modulating Catalytic Activity and Durability of PtFe Alloy Catalysts for Oxygen Reduction Reaction Through Controlled Carbon Shell Formation.

Nanomaterials (Basel) 2019 Oct 19;9(10). Epub 2019 Oct 19.

Graduate School of Energy Science and Technology (GEST), Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Korea.

Demand on synthetic approaches to high performance electrocatalyst with enhanced durability is increasing for fuel cell applications. In this work, we present a facile synthesis of carbon shell-coated PtFe nanoparticles by using acetylacetonates in metal precursors as carbon sources without an additional polymer coating process for the carbon shell formation. The carbon shell structure is systematically controlled by changing the annealing conditions such as the temperature and gas atmosphere. PtFe catalysts annealed at 700 °C under H-mixed N gas show much higher oxygen reduction reaction (ORR) activity and superior durability compared to a Pt catalyst due to the ultrathin and porous carbon shells. In addition, when increasing the annealing temperature, the carbon shells encapsulating the PtFe nanoparticles improves the durability of the catalysts due to the enhanced crystallinity of the carbon shells. Therefore, it is demonstrated that the developed hybrid catalyst structure with the carbon shells not only allows the access of reactant molecules to the active sites for oxygen reduction reaction but also prevents the agglomeration of metal nanoparticles on carbon supports, even under harsh operating conditions. The proposed synthetic approach and catalyst structure are expected to provide more insights into the development of highly active and durable catalysts for practical fuel cell applications.
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http://dx.doi.org/10.3390/nano9101491DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6835385PMC
October 2019

Electrochemical Analysis for Demonstrating CO Tolerance of Catalysts in Polymer Electrolyte Membrane Fuel Cells.

Nanomaterials (Basel) 2019 Oct 8;9(10). Epub 2019 Oct 8.

Graduate School of Energy Science and Technology (GEST), Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 34134, Korea.

Since trace amounts of CO in H gas produced by steam reforming of methane causes severe poisoning of Pt-based catalysts in polymer electrolyte membrane fuel cells (PEMFCs), research has been mainly devoted to exploring CO-tolerant catalysts. To test the electrochemical property of CO-tolerant catalysts, chronoamperometry is widely used under a CO/H mixture gas atmosphere as an essential method. However, in most cases of catalysts with high CO tolerance, the conventional chronoamperometry has difficulty in showing the apparent performance difference. In this study, we propose a facile and precise test protocol to evaluate the CO tolerance via a combination of short-term chronoamperometry and a hydrogen oxidation reaction (HOR) test. The degree of CO poisoning is systematically controlled by changing the CO adsorption time. The HOR polarization curve is then measured and compared with that measured without CO adsorption. When the electrochemical properties of PtRu alloy catalysts with different atomic ratios of Pt to Ru are investigated, contrary to conventional chronoamperometry, these catalysts exhibit significant differences in their CO tolerance at certain CO adsorption times. The present work will facilitate the development of catalysts with extremely high CO tolerance and provide insights into the improvement of electrochemical methods.
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http://dx.doi.org/10.3390/nano9101425DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6835550PMC
October 2019

Rational Design of Ultrathin Gas Barrier Layer via Reconstruction of Hexagonal Boron Nitride Nanoflakes to Enhance the Chemical Stability of Proton Exchange Membrane Fuel Cells.

Small 2019 10 16;15(44):e1903705. Epub 2019 Sep 16.

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, 16419, Republic of Korea.

Hexagonal boron nitride (hBN) has great potential as a promising gas barrier layer in proton exchange membrane fuel cells (PEMFCs) as it shows high proton conductivity as well as excellent gas-blocking capability. However, structural defects and mechanical damage during the transfer of the hBN layer and membrane swelling have limited the application of hBN sheets to PEMFCs. Here, an ultrathin gas barrier layer is successfully fabricated on a proton exchange membrane via reconstruction of mechanically exfoliated hBN nanoflakes using a direct spin-coating process. The hBN-coated layer effectively suppresses the gas crossover and inhibits the formation of reactive oxygen radicals in the electrodes without reducing the proton conductivity of the membrane. It is also demonstrated that the structural advantages of hBN-coated gas barrier layers promise high performance of a unit cell even after a open-circuit voltage (OCV) hold test for 100 h. Furthermore, through in-depth postmortem analyses, a time-dependent degradation mechanism of membrane electrode assembly under the OCV condition is rationally proposed.
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http://dx.doi.org/10.1002/smll.201903705DOI Listing
October 2019

Boosting Fuel Cell Durability under Shut-Down/Start-Up Conditions Using a Hydrogen Oxidation-Selective Metal-Carbon Hybrid Core-Shell Catalyst.

ACS Appl Mater Interfaces 2019 Aug 26;11(31):27735-27742. Epub 2019 Jul 26.

Center for Fuel Cell Research , Korea Institute of Science and Technology (KIST) , Seoul 136-791 , Republic of Korea.

Performance degradation generated by reverse current flow during fuel cell shut-down/start-up is a big challenge for commercialization of polymer electrolyte membrane fuel cells in automobile applications. Under transient operating conditions, the formation of H/O boundaries on Pt surfaces and the occurrence of undesired oxygen reduction reaction (ORR) in an anode cause severe degradation of carbon supports and Pt catalysts in a cathode because of an increase of the cathode potential up to ∼1.5 V. Herein, to directly prevent the formation of H/O boundaries in the anode, we propose a unique metal-carbon hybrid core-shell anode catalyst having Pt nanoparticles encapsulated in nanoporous carbon shells for selective H permeation. This hybrid catalyst exhibits high hydrogen oxidation reaction (HOR) selectivity along with fully subdued ORR activity during long-term operation because of the excellent stability of the carbon molecular sieves. Furthermore, the HOR-selective catalyst effectively suppresses the reverse current flow in a single cell under shut-down/start-up conditions.
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http://dx.doi.org/10.1021/acsami.9b06309DOI Listing
August 2019

Boosting the oxygen reduction activity of a nano-graphene catalyst by charge redistribution at the graphene-metal interface.

Nanoscale 2019 Mar;11(11):5038-5047

Graduate School of Energy Science and Technology (GEST), Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea.

N-Doped carbon materials have been intensively studied to replace Pt catalysts for the oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFCs). However, the low doping level in these catalysts results in a limited number of ORR active sites, so high catalyst loading is still required. Hence, the electrode thickness becomes extra thick, causing large mass transfer resistance in AEMFCs. In this study, we propose a unique hybrid catalyst concept utilizing charge redistribution at the graphene-transition metal interface to modify the electronic structure of graphene and simultaneously create multiple carbon active sites. The hybrid catalyst consists of n-type nano-graphene shells (NGS) three-dimensionally coated on the surface of transition metal nanoparticles highly dispersed on carbon supports. The n-type NGS catalysts efficiently facilitate oxygen adsorption owing to facile charge transfer from the metal nanoparticles underneath and provide abundant active carbon sites owing to their structural benefits. As a result, despite the same catalyst loading, the NGS catalyst shows high ORR activity and greater durability than a carbon-supported Pt (Pt/C) catalyst.
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http://dx.doi.org/10.1039/c8nr10327eDOI Listing
March 2019

Unraveling the Factors Affecting the Electrochemical Performance of MoS-Carbon Composite Catalysts for Hydrogen Evolution Reaction: Surface Defect and Electrical Resistance of Carbon Supports.

ACS Appl Mater Interfaces 2019 Feb 24;11(5):5037-5045. Epub 2019 Jan 24.

Graduate School of Energy Science and Technology (GEST) , Chungnam National University , Daejeon 34148 , Republic of Korea.

In MoS-carbon composite catalysts for hydrogen evolution reaction (HER), the carbon materials generally act as supports to enhance the catalytic activity of MoS nanosheets. The carbon support provides a large surface area for increasing the MoS edge site density, and its physical structure can affect the electron transport rate in the composite catalysts. However, despite the importance of the carbon materials, direct observation of the effects of the physical properties of the carbon supports on the HER activity of MoS-carbon composite catalysts has been hardly reported. In this work, we conduct an experimental model study to find the fundamental and important understanding of the correlation between the structural characteristics of carbon supports and the HER performance of MoS-carbon composite catalysts using surface-modified graphitic carbon shell (GCS)-encapsulated SiO nanowires ([email protected] NWs) as support materials for MoS nanosheets. The surface defect density and the electrical resistance of [email protected] NWs are systematically modulated by control of H gas flow rates during the carbon shell growth on the SiO NWs. From in-depth characterization of the model catalysts, it is confirmed that the intrinsic catalytic activity of MoS-carbon composites for the HER is improved linearly with the conductance of the carbon supports regardless of the MoS edge site density. However, in the HER polarization curve, the apparent current density increases in proportion to the product of the number of MoS edge sites and the conductance of [email protected] NWs.
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http://dx.doi.org/10.1021/acsami.8b19072DOI Listing
February 2019

Self-Assembled Dendritic Pt Nanostructure with High-Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction.

ChemSusChem 2017 08 28;10(15):3063-3068. Epub 2017 Jun 28.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, 08826, Republic of Korea.

The durability issues of Pt catalyst should be resolved for the commercialization of proton exchange membrane fuel cells. Nanocrystal structures with high-index facets have been recently explored to solve the critical durability problem of fuel cell catalysts as Pt catalysts with high-index facets can preserve the ordered surfaces without change of the original structures. However, it is very difficult to develop effective and practical synthetic methods for Pt-based nanostructures with high-index facets. The current study describes a simple one-pot synthesis of self-assembled dendritic Pt nanostructures with electrochemically active and stable high-index facets. Pt nanodendrites exhibited 2 times higher ORR activity and superior durability (only 3.0 % activity loss after 10 000 potential cycles) than a commercial Pt/C. The enhanced catalytic performance was elucidated by the formation of well-organized dendritic structures with plenty of reactive interfaces among 5 nm-sized Pt particles and the coexistence of low- and high-index facets on the particles.
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http://dx.doi.org/10.1002/cssc.201700852DOI Listing
August 2017

A simple synthesis of urchin-like Pt-Ni bimetallic nanostructures as enhanced electrocatalysts for the oxygen reduction reaction.

Chem Commun (Camb) 2016 Jan 10;52(3):597-600. Epub 2015 Nov 10.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul, 151-742, Korea.

The synthesis of urchin-like Pt-Ni bimetallic nanostructures is achieved by a controlled one-pot synthesis. Pt-Ni nanostructures have superior oxygen reduction reaction activities in both with and without specific anion adsorption electrolytes due to the geometric and alloying effects.
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http://dx.doi.org/10.1039/c5cc08088fDOI Listing
January 2016

Green synthesis of carbon-supported nanoparticle catalysts by physical vapor deposition on soluble powder substrates.

Sci Rep 2015 Sep 18;5:14245. Epub 2015 Sep 18.

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea.

Metal and metal oxide nanoparticles (NPs) supported on high surface area carbon (NP/Cs) were prepared by the physical vapor deposition of bulk materials on an α-D-glucose (Glu) substrate, followed by the deposition of the NPs on carbon supports. Using Glu as a carrier for the transport of NPs from the bulk materials to the carbon support surfaces, ultrafine NPs were obtained, exhibiting a stabilizing effect through OH moieties on the Glu surfaces. This stabilizing effect was strong enough to stabilize the NPs, but weak enough to not significantly block the metal surfaces. As only the target materials and Glu are required in our procedure, it can be considered environmentally friendly, with the NPs being devoid of hazardous chemicals. Furthermore, the resulting NP/Cs exhibited an improvement in activity for various electrochemical reactions, mainly attributed to their high surface area.
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http://dx.doi.org/10.1038/srep14245DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4585564PMC
September 2015

P-modified and carbon shell coated Co nanoparticles for efficient alkaline oxygen reduction catalysis.

Chem Commun (Camb) 2014 Dec;50(100):15940-3

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea.

Described herein is the development of a novel Co-based oxygen electrode catalyst coupled with unique carbon structures. The present carbon shell coated Co nanoparticles of which the surface composites are modified by phosphorus incorporation, exhibit efficient oxygen reduction activities as well as oxygen evolving properties.
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http://dx.doi.org/10.1039/c4cc07143cDOI Listing
December 2014

Edge-exposed MoS2 nano-assembled structures as efficient electrocatalysts for hydrogen evolution reaction.

Nanoscale 2014 Feb 6;6(4):2131-6. Epub 2013 Dec 6.

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Republic of Korea.

Edge-exposed MoS2 nano-assembled structures are designed for high hydrogen evolution reaction activity and long term stability. The number of sulfur edge sites of nano-assembled spheres and sheets is confirmed by Raman spectroscopy and EXAFS analysis. By controlling the MoS2 morphology with the formation of nano-assembled spheres with the assembly of small-size fragments of MoS2, the resulting assembled spheres have high electrocatalytic HER activity and high thermodynamic stability.
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http://dx.doi.org/10.1039/c3nr05228aDOI Listing
February 2014

Chemical tuning of electrochemical properties of Pt-skin surfaces for highly active oxygen reduction reactions.

Phys Chem Chem Phys 2013 Oct;15(40):17079-83

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Korea.

Pt-skin surfaces were successfully fabricated by the chemical deposition of additional Pt on corrugated Pt-Ni nanoparticles with Pt-skeleton surfaces. Compared to the Pt-skin formed by heat annealing, the chemically-tuned Pt-skin had a higher Pt coordination number and surface crystallinity, which resulted in superior ORR activity and durability.
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http://dx.doi.org/10.1039/c3cp52807cDOI Listing
October 2013

Correlation between platinum nanoparticle surface rearrangement induced by heat treatment and activity for an oxygen reduction reaction.

Phys Chem Chem Phys 2013 Aug;15(32):13658-63

Center for Nanoparticle Research, Institute for Basic Science, and School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea.

Heat treatment of nanoparticles could induce the surface rearrangement for more stable facet exposure induced by thermodynamics. By changing the heat treatment environment, we confirmed the correlation between the oxygen reduction activity and the effect of surface oxide and the degree of surface rearrangement of Pt nanoparticles. Native surface oxide was not a critical factor for oxygen reduction activity. However, the degree of surface rearrangement could affect the activity, which was confirmed by the surface sensitive techniques such as CO(ad) oxidation and potential of zero total charge. Analysis indicated that the driving force for nanoparticle surface rearrangement was affected by the heat treatment environment such as gas, in our case.
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http://dx.doi.org/10.1039/c3cp51520fDOI Listing
August 2013

Tailoring the Electronic Structure of Nanoelectrocatalysts Induced by a Surface-Capping Organic Molecule for the Oxygen Reduction Reaction.

J Phys Chem Lett 2013 Apr 4;4(8):1304-9. Epub 2013 Apr 4.

†Center for Nanoparticle Research, Institute for Basic Science (IBS), and School of Chemical and Biological Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea.

Capping organic molecules, including oleylamine, strongly adsorbed onto Pt nanoparticles during preparation steps are considered undesirable species for the oxygen reduction reaction due to decreasing electrochemical active sites. However, we found that a small amount of oleylamine modified platinum nanoparticles showed significant enhancement of the electrochemical activity of the oxygen reduction reaction, even with the loss of the electrochemically active surface area. The enhancement was correlated with the downshift of the frontier d-band structure of platinum and the retardation of competitively adsorbed species. These results suggest that a capping organic molecule modified electrode can be a strategy to design an advanced electrocatalyst by modification of electronic structures.
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http://dx.doi.org/10.1021/jz400574fDOI Listing
April 2013

A compact BrFAFC (bio-reformed formic acid fuel cell) converting formate to power.

Chem Commun (Camb) 2011 Apr 24;47(13):3972-4. Epub 2011 Feb 24.

School of Chemical and Biological Engineering, Bio-MAX Institute, Seoul National University, Sillimdong 56-1, Seoul 151-744, Korea.

A compact BrFAFC can directly convert formate to power without hydrogen storage and poisoning effect by CO at mild temperature. We are the first to establish the performance of the BrFAFC with high power density. Furthermore, this BrFAFC can be manufactured in a simple design for use in portable fuel cells.
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http://dx.doi.org/10.1039/c0cc05225fDOI Listing
April 2011
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