Liming Dai,*,†,‡ Yuhua Xue,†,‡ Liangti Qu,* Hyun-Jung Choi, and Jong-Beom Baek* †Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, School of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China School of Energy and Chemical Engineering/Center for Dimension-Controllable Covalent Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon, Ulsan, 689-798, South Korea

Metal-free catalysts for oxygen reduction reaction.

Replacing the rare and precious platinum (Pt) electrocatalysts with earth-abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high-performance sustainable energy devices. However, the challenging issues associated with non-Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth-abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth-abundant materials for boosting ORR.

Earth-Abundant Nanomaterials for Oxygen Reduction.

Heterogeneous nanocarbon materials are being increasingly investigated and deployed in numerous new technologies and devices for sustainable energy conversion and storage. Nanocarbons often consist of fullerene, graphene and carbon nanotubes. Their derivatives include quantum dots, nanofibres, nanoribbons, nanospheres/capsules and other nanostructured morphologies. The heterogeneous forms of these nanocarbons stem from the implantation of alien atoms into the aromatic carbon lattice or the covalent grafting of functional groups onto the carbon basal plane or edge sites. Heterogeneous nanocarbons have shown remarkable advantages in solar cells, water splitting, supercapacitors, lithium ion batteries and catalysis. This review focuses on recent progress in the experimental and computational studies of the roles of heteroatoms in heterogeneous nanocarbons for electrocatalytic oxygen reduction reaction (ORR). Critical perspectives are devoted to the ambiguous phenomena in this emerging research area. The long standing debate about the active sites is discussed with an emphasis on more rational development of advanced nanocarbon-based electrocatalysts for ORR.

Heterogeneous nanocarbon materials for oxygen reduction reaction

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play curial roles in electrochemical energy conversion and storage, including fuel cells and metal-air batteries. Having rich multidimensional nanoarchitectures [for example, zero-dimensional (0D) fullerenes, 1D carbon nanotubes, 2D graphene, and 3D graphite] with tunable electronic and surface characteristics, various carbon nanomaterials have been demonstrated to act as efficient metal-free electrocatalysts for ORR and OER in fuel cells and batteries. We present a critical review on the recent advances in carbon-based metal-free catalysts for fuel cells and metal-air batteries, and discuss the perspectives and challenges in this rapidly developing field of practical significance.

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Carbon-based electrocatalysts for advanced energy conversion and storage

Active sites and the catalytic mechanism of nitrogen-doped graphene in an oxygen reduction reaction (ORR) have been extensively studied but are still inconclusive, partly due to the lack of an experimental method that can detect the active sites. It is proposed in this report that the active sites on nitrogen-doped graphene can be determined via the examination of its chemical composition change before and after ORR. Synchrotron-based X-ray photoelectron spectroscopy analyses of three nitrogen-doped multilayer graphene samples reveal that oxygen reduction intermediate OH(ads), which should chemically attach to the active sites, remains on the carbon atoms neighboring pyridinic nitrogen after ORR. In addition, a high amount of the OH(ads) attachment after ORR corresponds to a high catalytic efficiency and vice versa. These pinpoint that the carbon atoms close to pyridinic nitrogen are the main active sites among the different nitrogen doping configurations.

Observation of active sites for oxygen reduction reaction on nitrogen-doped multilayer graphene

A novel monolithic carbon sponge is prepared by a ZnCl2-assisted carbonization of commercial melamine foam (MF) at 800 °C in a N2 atmosphere. The as-prepared nitrogen/oxygen co-doped carbon sponges (NOCSs) can be directly applied as monolithic electrodes for supercapacitors without using any additives. It is found that ZnCl2 activation plays an important role in improving the supercapacitive behavior of NOCSs. Compared with the NOCS without ZnCl2 activation treatment, the optimal NOCS-1/10, obtained with a ZnCl2/MF weight ratio of 1 : 10, has the best capacitive performance due to its unique porous architecture, rich pseudocapacitance-active species, and good electron/ion transfer feature. It exhibits a high specific capacitance (242 F g−1 at a current density of 0.5 A g−1 and 325 F g−1 at a scan rate of 10 mV s−1) and good electrochemical cyclability (97% after 10 000 cycles at 5.0 A g−1). When being further assembled into an all-solid-state supercapacitor (PVA/KOH electrolyte), it can deliver an energy density of 4.33 W h kg−1 at a power density of 250 W kg−1 and 3.13 W h kg−1 at a power density of 3 kW kg−1.

Nitrogen/oxygen co-doped monolithic carbon electrodes derived from melamine foam for high-performance supercapacitors

Synthesis of three-dimensional flowerlike nitrogen-doped carbons by a copyrolysis route and the effect of nitrogen species on the electrocatalytic activity in oxygen reduction reaction

Key roles of bisulfite (HSO ) in food quality assurance and human health necessitate a reliable analytical method for rapid, sensitive, and selective detection of HSO . Herein, a new red-emitting ratiometric fluorescence probe, BIQ, is reported for sensitive and selective detection of HSO in food samples and live animals. Probe BIQ recognizes HSO via a 1,4-nucleophilic addition reaction. As a result of this specific reaction, emission intensities at 625 and 475 nm are dramatically changed, allowing the detection of HSO in a ratiometric fluorescence model in an aqueous solution. The obvious changes of solution color from pink to transparent and fluorescence color from rose-red to cyan allow the detection of HSO by naked eyes. Furthermore, probe BIQ has fast response in color and fluorescence (<2 min), excellent selectivity, and a low detection limit (0.29 μM), which enables its application in HSO detection in food samples and live organisms. The practical applications of probe BIQ are then demonstrated by the visualization of HSO in live animals (zebrafish and nude mouse) as well as the determination of HSO in white wine and sugar.

Wenjun Kang

Papers

Nitrogen/oxygen co-doped monolithic carbon electrodes derived from melamine foam for high-performance supercapacitors

Synthesis of three-dimensional flowerlike nitrogen-doped carbons by a copyrolysis route and the effect of nitrogen species on the electrocatalytic activity in oxygen reduction reaction

Red-Emission Probe for Ratiometric Fluorescent Detection of Bisulfite and Its Application in Live Animals and Food Samples.

KOH Chemical-Activated Porous Carbon Sponges for Monolithic Supercapacitor Electrodes

Boosting the potassium-ion storage performance of a carbon anode by chemically regulating oxygen-containing species.