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.

The cathodic oxygen reduction reaction (ORR) is an important process in fuel cells and metal-air batteries. [1][2][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][5][6][7][8][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] Quantum mechanics calculations [4] indicate that the enhanced catalytic activity of VA-NCNTs toward ORR can be attributed to the electron-accepting ability of the nitrogen species, which creates net positive charges on the CNT surface to enhance oxygen adsorption and to readily attract electrons from the anode for facilitating the ORR. Uncovering this new ORR mechanism in nitrogen-doped carbon nanotube electrodes is significant as the same principle could be applied to the development of various other metal-free efficient ORR catalysts for fuel-cell applications and even beyond fuel cells. Indeed, recent intensive research efforts in developing metal-free ORR electrocatalysts have led to a large variety of carbon-based metal-free ORR electrocatalysts, including heteroatom (N, B, or P)-doped carbon nanotubes, graphene, and graphite. [4][5][6][7][8][9][10][11][12][13][14] More recently, we have successfully synthesized vertically aligned carbon nanotubes co-doped with N and B (VA-BCN) and demonstrated a significantly improved electrocatalytic activity toward the ORR, with respect to CNTs doped with either N or B, only due to a synergetic effect arising from the N and B co-doping. [15] However, most of the reported carbon-based ORR electrocatalysts (particularly, heteroatom-doped nanotubes and graphene) were produced by chemical vapor-deposition (CVD) processes involving vacuum-based elaborate and careful fabrication, which are often too tedious and too expensive for mass production. As demonstrated in this study, therefore, it is of great importance to develop a facile approach to BCN graphene as low-cost and efficient ORR electrocatalysts. The recent availability of solution-exfoliated graphite oxide (GO) [16] allows the mass production of graphene and derivatives by conventional physicochemical treatments of GO.Herein, we have developed a facile approach to metalfree BCN graphene of tunable B/N co-doping levels as efficient...

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.

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.

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 (CO 2 -capturing) approaches to low-cost mass production of high-quality GNs for applications where GOs have been exploited and beyond.carbon dioxide | eco-friendly | edge-functionalization | graphite A s a building block for carbon nanomaterials of all other dimensionalities, such as 0D buckyball, 1D nanotubes, and 3D graphite, graphene nanosheets (GNs) with carbon atoms densely packed in a 2D honeycomb crystal lattice have recently attracted tremendous interest for various potential applications (1). Several techniques, including the peel-off by Scotch tape (2), epitaxial growth on SiC (3), chemical vapor deposition (CVD) (4, 5), and solution exfoliation of graphite oxide (GO) (6), have been reported for producing GNs. Although the Scotch tape method led to the Nobel-Prize-winning discovery of high quality GNs (2), it is unsuitable for large-area preparation of GN films due to technique difficulties. On the other hand, large-area thin GN films up to 30 in. have been prepared by CVD (7). However, the CVD process involves extremely careful fabrication processes, which appears to be too tedious and too expensive for mass production. The widely reported solution exfoliation of graphite into GO, followed by solution reduction (8-10), allows the mass production of GNs via an all-solution process. Due to strong interactions between the hexagonally sp 2 -bonded carbon layers in graphite, however, the solution exfoliation requires the involvement of hazardous strong oxidizing reagents (e.g., HNO 3 , KMnO 4 , and/or H 2 SO 4 ) and a tedious multistep process (8,9,11,12). Such a corrosive chemical oxidation often causes severe damage to the carbon basal plane to introduce a large number of chemical and topological defects (13). As a result, postexfoliation reduction of GO into reduced graphene oxide (rGO) is essential in order to restore the graphitic basal plane for the resultant GNs (6,[14][15][16][17][18][19]. To make the matter worse, the reduction reaction also involves hazardous reducing reagents (e.g., hydrazine, NaBH 4 ) with a limited reduction conversion (approximately 70%) (20). The reduced GO (rGO) still contains considerable oxygenated groups and structural defects, and thus additional...