As global energy consumption accelerates at an alarming rate, the develop- ment of clean and renewable energy conversion and storage systems has become more important than ever. Although the ...efficiency of energy conversion and storage devices depends on a variety of factors, their overall performance strongly relies on the structure and properties of the component materials. 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. As a building block for carbon materials of all other dimensionalities (such as 0D buckyball, 1D nanotube, 3D graphite), the two-dimensional (2D) single atomic carbon sheet of graphene has emerged as an attractive candidate for energy applications due to its unique structure and properties. Like other materials, however, a graphene-based material that possesses desirable bulk properties rarely features the surface characteristics required for certain specific applications. Therefore, surface functionalization is essential, and researchers have devised various covalent and noncovalent chemistries for making graphene materials with the bulk and surface properties needed for efficient energy conversion and storage. In this Account, I summarize some of our new ideas and strategies for the controlled functionalization of graphene for the development of efficient energy conversion and storage devices, such as solar cells, fuel cells, supercapacitors, and batteries. The dangling bonds at the edge of graphene can be used for the covalent attachment of various chemical moieties while the graphene basal plane can be modified via either covalent or noncovalent functionalization. The asymmetric functionalization of the two opposite surfaces of individual graphene sheets with different moieties can lead to the self-assembly of graphene sheets into hierarchically structured materials. Judicious application of these site-selective reactions to graphene sheets has opened up a rich field of graphene-based energy materials with enhanced performance in energy conversion and storage. These results reveal the versatility of surface functionalization for making sophisticated graphene materials for energy applications. Even though many covalent and noncovalent functionalization methods have already been reported, vast opportunities remain for developing novel graphene materials for highly efficient energy conversion and storage systems.
Carbon atoms in the graphitic carbon skeleton can be replaced by heteroatoms with different electronegative from that of the carbon atom (i.e., heteroatom doping) to modulate the charge distribution ...over the carbon network. The charge modulation can be achieved via direct charge transfer with an electron acceptor/donor (i.e., charge transfer doping) or through introduction of defects (i.e., defective doping). Various doping strategies, including heteroatom doping, charge‐transfer doping, and defective doping, have now been devised for modulating the charge distribution of numerous graphite carbon materials to impart new properties to carbon materials. Consequently, carbon nanomaterials with defined doping have recently become prominent members in the carbon family, promising for a variety of applications, including catalysis, energy conversion and storage, environmental remediation, and important chemical production and industrial processes. The purpose of this review is to present an overview on the doping of carbon materials for metal‐free electrocatalysis, especially the development of doping strategies and doping‐induced structure and property changes for potential catalytic applications. Current challenges and future perspectives in the doped carbon‐based metal‐free catalyst field are also discussed.
Various doping strategies, including heteroatom‐doping, charge‐transfer doping, and defective‐doping, have been devised for modulating the charge distribution of carbon materials to impart new properties. Doped nanocarbons are promising materials for various applications, including catalysis, energy conversion and storage, environmental remediation, and chemical production and industrial processes. A concise, but comprehensive and critical, overview of this field is presented.
Rationally designed N, S co‐doped graphitic sheets with stereoscopic holes (SHG) act as effective tri‐functional catalysts for the oxygen reduction reaction, hydrogen evolution reaction, and oxygen ...evolution reaction, simultaneously. The multifunctional electrocatalytic activities originate from a synergistic effect of the N, S heteroatom doping and unique SHG architecture, which provide a large surface area and efficient pathways for electron and electrolyte/reactant transports.
Besides their use in fuel cells for energy conversion through the oxygen reduction reaction (ORR), carbon‐based metal‐free catalysts have also been demonstrated to be promising alternatives to ...noble‐metal/metal oxide catalysts for the oxygen evolution reaction (OER) in metal–air batteries for energy storage and for the splitting of water to produce hydrogen fuels through the hydrogen evolution reaction (HER). This Review focuses on recent progress in the development of carbon‐based metal‐free catalysts for the OER and HER, along with challenges and perspectives in the emerging field of metal‐free electrocatalysis.
Free agents: Carbon‐based metal‐free catalysts have been intensively researched as alternatives to noble‐metal/metal oxide catalysts for the oxygen evolution reaction (OER) in metal–air batteries and for the splitting of water through the hydrogen evolution reaction (HER). This Review gives an overview of recent developments in this area, with a focus on the OER and HER.
Electrocatalysts are required for clean energy technologies (for example, water‐splitting and metal‐air batteries). The development of a multifunctional electrocatalyst composed of nitrogen, ...phosphorus, and fluorine tri‐doped graphene is reported, which was obtained by thermal activation of a mixture of polyaniline‐coated graphene oxide and ammonium hexafluorophosphate (AHF). It was found that thermal decomposition of AHF provides nitrogen, phosphorus, and fluorine sources for tri‐doping with N, P, and F, and simultaneously facilitates template‐free formation of porous structures as a result of thermal gas evolution. The resultant N, P, and F tri‐doped graphene exhibited excellent electrocatalytic activities for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The trifunctional metal‐free catalyst was further used as an OER–HER bifunctional catalyst for oxygen and hydrogen gas production in an electrochemical water‐splitting unit, which was powered by an integrated Zn–air battery based on an air electrode made from the same electrocatalyst for ORR. The integrated unit, fabricated from the newly developed N, P, and F tri‐doped graphene multifunctional metal‐free catalyst, can operate in ambient air with a high gas production rate of 0.496 and 0.254 μL s−1 for hydrogen and oxygen gas, respectively, showing great potential for practical applications.
A hat‐trick: The combination of a self‐powered water‐splitting unit with a Zn–air battery, based on a N, P, and F tri‐doped metal‐free multifunctional graphene electrocatalyst, exhibits high hydrogen and oxygen gas production rates from water. ORR=oxygen reduction reaction, OER=oxygen evolution reaction, HER=hydrogen evolution reaction.
The oxygen reduction reaction (ORR) plays an important role in renewable energy technologies, such as fuel cells and metal–air batteries. Along with the extensive research and development of ...nonprecious metal catalysts (NPMCs) to reduce/replace Pt for electrocatalytic reduction of oxygen, a new class of heteroatom-doped metal-free carbon catalysts has been recently developed, which, as alternative ORR catalysts, could dramatically reduce the cost and increase the efficiency of fuel cells and metal–air batteries. The improved catalytic performance of heteroatom-doped carbon ORR catalysts has been attributed to the doping-induced charge redistribution around the heteroatom dopants, which lowered the ORR potential and changed the O2 chemisorption mode to effectively weaken the O–O bonding, facilitating ORR at the heteroatom-doped carbon electrodes. Subsequently, this new metal-free ORR mechanism was confirmed by numerous studies, and the same principle has been applied to the development of various other efficient catalysts for not only ORR in fuel cells but also oxygen evolution reaction (OER) in metal–air batteries and hydrogen evolution reaction (HER) in water-splitting systems. However, there are still some concerns about possible contributions of metal impurities to the ORR activities of these carbon catalysts. To avoid unnecessary confusion, therefore, it is important to clarify the situation. In this Perspective, we provide important aspects of the metal-free ORR catalysts with a critical analysis of the literature, along with perspectives and challenges of this rapidly growing field of practical significance.
Owing to their high energy density and power density, supercapacitors exhibit great potential as high-performance energy sources for advanced technologies. Recently, carbon nanomaterials (especially, ...carbon nanotubes and graphene) have been widely investigated as effective electrodes in supercapacitors due to their high specific surface area, excellent electrical and mechanical properties. This article summarizes the recent progresses on the development of high-performance supercapacitors based on carbon nanomaterials and provides various rational concepts for materials engineering to improve the device performance for a large variety of potential applications, ranging from consumer electronics through wearable optoelectronics to hybrid electric vehicles.
Single Fe atoms dispersed on hierarchically structured porous carbon (SA‐Fe‐HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within ...micropores of the porous carbon support. The single‐atom Fe catalysts have a well‐defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA‐Fe‐HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E1/2=0.81 V), but have better long‐term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA‐Fe‐HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E1/2=0.89 V), fuel selectivity, and long‐term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA‐Fe‐HPCs are promising non‐platinum‐group metal ORR catalysts for fuel‐cell technologies.
Stay single: Single‐atom catalysts with Fe atoms dispersed on hierarchical porous carbons were prepared by pyrolysis of iron phthalocyanine and unsubstituted phthalocyanine complexes confined within micropores of the porous carbon supports. The resulting catalysts outperformed the commercial Pt/C electrode in alkaline electrolytes and showed an electrocatalytic activity comparable to the commercial Pt/C electrode in acidic media with a better long‐term stability.
Metal–organic frameworks (MOFs) and MOF‐derived materials have recently attracted considerable interest as alternatives to noble‐metal electrocatalysts. Herein, the rational design and synthesis of a ...new class of Co@N‐C materials (C‐MOF‐C2‐T) from a pair of enantiotopic chiral 3D MOFs by pyrolysis at temperature T is reported. The newly developed C‐MOF‐C2‐900 with a unique 3D hierarchical rodlike structure, consisting of homogeneously distributed cobalt nanoparticles encapsulated by partially graphitized N‐doped carbon rings along the rod length, exhibits higher electrocatalytic activities for oxygen reduction and oxygen evolution reactions (ORR and OER) than that of commercial Pt/C and RuO2, respectively. Primary Zn–air batteries based on C‐MOF‐900 for the oxygen reduction reaction (ORR) operated at a discharge potential of 1.30 V with a specific capacity of 741 mA h gZn–1 under 10 mA cm–2. Rechargeable Zn–air batteries based on C‐MOF‐C2‐900 as an ORR and OER bifunctional catalyst exhibit initial charge and discharge potentials at 1.81 and 1.28 V (2 mA cm–2), along with an excellent cycling stability with no increase in polarization even after 120 h – outperform their counterparts based on noble‐metal‐based air electrodes. The resultant rechargeable Zn–air batteries are used to efficiently power electrochemical water‐splitting systems, demonstrating promising potential as integrated green energy systems for practical applications.
A Co@N‐C material, derived from new chiral metal–organic frameworks, exhibits excellent bifunctional electrocatalytic activities toward both oxygen reduction and oxygen evolution reactions, along with a remarkable performance for primary/rechargeable Zn–air batteries (powering water‐splitting systems), outperforming their counterparts based on Pt/C and Pt/C + RuO2, respectively.
The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out with noble metals (such as Pt) and metal oxides (such as RuO₂ and MnO₂) as catalysts, ...respectively. However, these metal-based catalysts often suffer from multiple disadvantages, including high cost, low selectivity, poor stability and detrimental environmental effects. Here, we describe a mesoporous carbon foam co-doped with nitrogen and phosphorus that has a large surface area of ∼1,663 m(2) g(-1) and good electrocatalytic properties for both ORR and OER. This material was fabricated using a scalable, one-step process involving the pyrolysis of a polyaniline aerogel synthesized in the presence of phytic acid. We then tested the suitability of this N,P-doped carbon foam as an air electrode for primary and rechargeable Zn-air batteries. Primary batteries demonstrated an open-circuit potential of 1.48 V, a specific capacity of 735 mAh gZn(-1) (corresponding to an energy density of 835 Wh kgZn(-1)), a peak power density of 55 mW cm(-2), and stable operation for 240 h after mechanical recharging. Two-electrode rechargeable batteries could be cycled stably for 180 cycles at 2 mA cm(-2). We also examine the activity of our carbon foam for both OER and ORR independently, in a three-electrode configuration, and discuss ways in which the Zn-air battery can be further improved. Finally, our density functional theory calculations reveal that the N,P co-doping and graphene edge effects are essential for the bifunctional electrocatalytic activity of our material.
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