Dong Wook Chang

Carbon Nanomaterials for Advanced Energy Conversion and Storage

It is estimated that the world will need to double its energy supply by 2050. Nanotechnology has opened up new frontiers in materials science and engineering to meet this challenge by creating new materials, particularly carbon nanomaterials, for efficient energy conversion and storage. Comparing to conventional energy materials, carbon nanomaterials possess unique size-/surface-dependent (e.g., morphological, electrical, optical, and mechanical) properties useful for enhancing the energy-conversion and storage performances. During the past 25 years or so, therefore, considerable efforts have been made to utilize the unique properties of carbon nanomaterials, including fullerenes, carbon nanotubes, and graphene, as energy materials, and tremendous progress has been achieved in developing high-performance energy conversion (e.g., solar cells and fuel cells) and storage (e.g., supercapacitors and batteries) devices. This article reviews progress in the research and development of carbon nanomaterials during the past twenty years or so for advanced energy conversion and storage, along with some discussions on challenges and perspectives in this exciting field.

BCN Graphene as Efficient Metal‐Free Electrocatalyst for the Oxygen Reduction Reaction

Abstract : The cathodic oxygen reduction reaction (ORR) is an important process in fuel cells and metal air batteries.[1 3] Although Pt-based electrocatalysts have been commonly used in commercial fuel cells owing to their relatively low overpotential and high current density, they still suffer from serious intermediate tolerance, anode crossover, sluggish kinetics, and poor stability in an electrochemical environment. This, together with the high cost of Pt and its limited nature reserves, has prompted the extensive search for alternative low-cost and high-performance ORR electrocatalysts. In this context, carbon-based metal-free ORR electrocatalysts have generated a great deal of interest owing to their low-cost, high electrocatalytic activity and selectivity, and excellent durability.[4 9] Of particular interest, we have previously prepared vertically aligned nitrogendoped carbon nanotubes (VA-NCNTs) as ORR electrocatalysts, which are free from anode crossover and CO poisoning and show a threefold higher catalytic activity and better durability than the commercial Pt/C catalyst.[4]

Polyaniline Grafted Reduced Graphene Oxide for Efficient Electrochemical Supercapacitors

An alternative and effective route to prepare conducting polyaniline-grafted reduced graphene oxide (PANi-g-rGO) composite with highly enhanced properties is reported. In order to prepare PANi-g-rGO, amine-protected 4-aminophenol was initially grafted to graphite oxide (GO) via acyl chemistry where a concomitant partial reduction of GO occurred due to the refluxing and exposure of GO to thionyl chloride vapors and heating. Following the deprotection of amine groups, an in situ chemical oxidative grafting of aniline in the presence of an oxidizing agent was carried out to yield highly conducting PANi-g-rGO. Electron microscopic studies demonstrated that the resultant composite has fibrillar morphology with a room-temperature electrical conductivity as high as 8.66 S/cm and capacitance of 250 F/g with good cycling stability.

Polyelectrolyte-Functionalized Graphene as Metal-Free Electrocatalysts for Oxygen Reduction

Poly(diallyldimethylammonium chloride), PDDA, was used as an electron acceptor for functionalizing graphene to impart electrocatalytic activity for the oxygen reduction reaction (ORR) in fuel cells. Raman and X-ray photoelectron spectroscopic measurements indicate the charge transfer from graphene to PDDA. The resultant graphene positively charged via intermolecular charge-transfer with PDDA was demonstrated to show remarkable electrocatalytic activity toward ORR with better fuel selectivity, tolerance to CO posing, and long-term stability than that of the commercially available Pt/C electrode. The observed ORR electrocatalytic activity induced by the intermolecular charge-transfer provides a general approach to various carbon-based metal-free ORR catalysts for oxygen reduction.

Edge-carboxylated graphene nanosheets via ball milling

Low-cost, high-yield production of graphene nanosheets (GNs) is essential for practical applications. We have achieved high yield of edge-selectively carboxylated graphite (ECG) by a simple ball milling of pristine graphite in the presence of dry ice. The resultant ECG is highly dispersable in various solvents to self-exfoliate into single- and few-layer (≤ 5 layers) GNs. These stable ECG (or GN) dispersions have been used for solution processing, coupled with thermal decarboxylation, to produce large-area GN films for many potential applications ranging from electronic materials to chemical catalysts. The electrical conductivity of a thermally decarboxylated ECG film was found to be as high as 1214 S/cm, which is superior to its GO counterparts. Ball milling can thus provide simple, but efficient and versatile, and eco-friendly (CO2-capturing) approaches to low-cost mass production of high-quality GNs for applications where GOs have been exploited and beyond.

Novel Quinoxaline-Based Organic Sensitizers for Dye-Sensitized Solar Cells

Novel quinoxaline-based organic sensitizers using vertical (RC-21) and horizontal (RC-22) conjugation between an electron-donating triphenylamine unit and electron-accepting quinoxaline unit have been synthesized and used for dye-sensitized solar cells (DSSCs), leading to the relatively high power conversion efficiencies of 3.30 and 5.56% for RC-21 and RC-22, respectively. This result indicates that the quinoxaline electron-accepting unit is quite a promising candidate in organic sensitizers.

Nitrogen-Doped Graphene Nanoplatelets from Simple Solution Edge-Functionalization for n-Type Field-Effect Transistors

The development of a versatile method for nitrogen-doping of graphitic structure is an important challenge for many applications, such as energy conversions and storages and electronic devices. Here, we report a simple but efficient method for preparing nitrogen-doped graphene nanoplatelets via wet-chemical reactions. The reaction between monoketone (C═O) in graphene oxide (GO) and monoamine-containing compound produces imine (Shiff base) functionalized GO (iGO). The reaction between α-diketone in GO and 1,2-diamine (ortho-diamine)-containing compound gives stable pyrazine ring functionalized GO (pGO). Subsequent heat-treatments of iGO and pGO result in high-quality, nitrogen-doped graphene nanoplatelets to be designated as hiGO and hpGO, respectively. Of particular interest, hpGO was found to display the n-type field-effect transistor behavior with a charge neutral point (Dirac point) located at around -16 V. Furthermore, hpGO showed hole and electron mobilities as high as 11.5 and 12.4 cm(2)V(-1)s(-1), respectively.

Graphene in photovoltaic applications: organic photovoltaic cells (OPVs) and dye-sensitized solar cells (DSSCs)

Graphene, a one-atom thick layer of graphite with a two-dimensional sp2-hybridized carbon network, has recently attracted tremendous research interest due to its peculiar properties such as good mechanical strength, high thermal conductivity, superior transparency, large specific surface area and exceptional charge transport properties. To take advantage of its unusual properties, graphene has been widely studied in various energy conversion and storage applications such as supercapacitors, fuel cells, batteries and solar cells. Amongst these applications, the development of efficient solar cells, which can convert sunlight into electricity is in high demand in order to solve up-coming energy-related and global warming issues. So far, significant effort has been devoted to using graphene for improving the overall performance of photovoltaic devices such as organic photovoltaic cells (OPVs) and dye sensitized solar cells (DSSCs). It has been reported that graphene can play diverse, but positive roles such as an electrode, an active layer, an interfacial layer and an electron acceptor in photovoltaic cells. Herein, we summarize the recent progress and general aspects of graphene in various photovoltaic cells including the synthesis, structure, properties and performance. In addition, challenges and future perspectives in this field are also discussed.

Water-dispersible, sulfonated hyperbranched poly(ether-ketone) grafted multiwalled carbon nanotubes as oxygen reduction catalysts.

To endorse sufficient water affinity to multiwalled carbon nanotubes (MWCNTs), dendritic hyperbranched poly(ether-ketone) (HPEK) was first covalently grafted to the surface of a MWCNT via a Friedel-Crafts acylation reaction. The resultant HPEK-grafted MWCNT (HPEK-g-MWCNT) was subsequently sulfonated in chlorosulfonic acid to produce sulfonated HPEK-g-MWCNT (SHPEK-g-MWCNT), which is dispersible well in water showing a zeta potential value of -57.8 mV. The SHPEK-g-MWCNT paper simply formed by filtration of aqueous dispersion has a sheet resistance as low as 63 Ω/sq. Its thin film shows a high electrocatalytic activity for oxygen reduction reaction (ORR). Thus, the newly produced water-dispersible MWCNT is a new class of high performance cathode material for ORR.

CdSe quantum dot (QD) and molecular dye hybrid sensitizers for TiO2 mesoporous solar cells: working together with a common hole carrier of cobalt complexes

Redox couples based on cobalt complexes were found to be effective in regenerating both inorganic CdSe quantum dot- and organic dye-sensitizers. The hybrid sensitizer composed of CdSe QD and ruthenium sensitizer (Z907Na) dye showed a maximum power conversion efficiency of 4.76% on using cobalt(o-phen)(3)(2+/3+) as a common redox mediator.