Nanocarbon Electrocatalysts for Oxygen ... - Wiley Online Library

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Qing Li, Ruiguo Cao, Jaephil Cho,* and Gang Wu* replacement for liquid electrolytes, the alkaline fuel cells (AFC) technology has been revitalized and new momentum has been imparted to it.[1] The alkaline anion exchange membrane fuel cell (AAEMFC) presents several potential advantages compared to its acidic Nafion-based counterparts, including much improved kinetics of electrochemical reactions, materials stability, and easy water management. Thus, AAEMFCs may hold great potential to become the most efficient low-temperature fuel cell in the near future. Metal-air batteries, representing the most promising energy storage systems for portable (electronics), mobile (electrical vehicles), and stationary (microgrids) applications, have attracted much attention because of their very high energy density compared to that of other rechargeable batteries.[2–4] Unlike the traditional intercalation electrodes in Li-ion batteries, the porous cathode in the metalair cell is capable of taking reactant O2 from the atmosphere, instead of storing them in the electrodes. The Zn-air batteries have a high theoretical energy density (1086 Wh kg−1), which is about three times higher than the Li-ion batteries.[4] Due to the low cost and high safety, Zn-air batteries have been suggested to be the most promising metalair system for practical applications. Meanwhile, aqueous Li-air batteries could deliver higher energy density than Zn-air batteries.[4] Compared to non-aqueous systems, by using a waterstable lithium electrode (WSLE) and an aqueous electrolyte, alkaline based Li-air batteries avoid the problems resulting from non-aqueous electrolytes, such as the precipitation of Li oxides on the porous cathode (low capacity) and high polarization resistance of the air electrode. According to the battery reaction 4Li + 6H2O +O2 = 4(LiOH·H2O), the energy density of this alkaline aqueous system (including oxygen) is 2450 Wh kg−1, which is only about 30% lower than that of the non-aqueous system, but comparable with another high energy density Li battery system of Li/sulphur (about 2500 Wh kg−1).[5] However, research on Li-air batteries is still at an early stage and they are far from practical applications.[6] The spectacular progress in these advanced energy technologies notwithstanding, the developments of high-efficient

Alkaline oxygen electrocatalysis, targeting anion exchange membrane fuel cells, Zn-air batteries, and alkaline-based Li-air batteries, has become a subject of intensive investigation because of its advantages compared to its acidic counterparts in reaction kinetics and materials stability. However, significant breakthroughs in the design and synthesis of efficient oxygen reduction catalysts from earth-abundant elements instead of precious metals in alkaline media remain in high demand. Carbon composite materials have been recognized as the most promising because of their reasonable balance between catalytic activity, durability, and cost. In particular, heteroatom (e.g., N, S, B, or P) doping can tune the electronic and geometric properties of carbon, providing more active sites and enhancing the interaction between carbon structure and active sites. Importantly, involvement of transition metals appears to be necessary for achieving high catalytic activity and improved durability by catalyzing carbonization of nitrogen/carbon precursors to form highly graphitized carbon nanostructures with more favorable nitrogen doping. Recently, a synergetic effect was found between the active species in nanocarbon and the loaded oxides/sulfides, resulting in much improved activity. This report focuses on these carbon composite catalysts. Guidance for rational design and synthesis of advanced alkaline ORR catalysts with improved activity and performance durability is also presented.

1. Introduction Polymer electrolyte fuel cells (PEFCs), one of the most promising energy conversion technologies available today, have major advantages over gasoline combustion, including better overall fuel efficiency and reduction in CO2 and other emissions. Due to recent progress in anion-exchange membranes (AEM) capable of conducting hydroxide ions (OH−) as a

Dr. Q. Li, Dr. G. Wu Materials Physics and Applications Division Los Alamos National Laboratory Los Alamos, NM 87545, USA E-mail: [email protected] Dr. R. Cao, Prof. J. Cho Interdisciplinary School of Green Energy Ulsan National Institute of Science and Technology Ulsan, 689–798, Korea E-mail: [email protected]

DOI: 10.1002/aenm.201301415

Adv. Energy Mater. 2014, 4, 1301415

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Nanocarbon Electrocatalysts for Oxygen Reduction in Alkaline Media for Advanced Energy Conversion and Storage

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AAEMFC and electrically rechargeable metal-air batteries continue to face challenges, including the lack of efficient and robust oxygen cathodes. The oxygen cathodes in fuel cells and metal-air cells perform the similar function of catalyzing oxygen reduction (ORR) and evolution reaction (OER) (especially for rechargeable metal-air batteries). However, high ORR and OER overpotential is still the main obstacle to making these technologies viable. Significant efforts have been put to study the mechanism of ORR/OER and the search for highly efficient catalysts. However, owing to the complexity of the oxygen reduction process, which involves several steps and many intermediates, the mechanism is still poorly understood. The mechanism and kinetics of oxygen reduction greatly depend on the catalyst and the solution environments, which make oxygen reduction proceed through different reaction pathways. In acidic solution, oxygen can be reduced to water with a standard thermodynamic potential at 1.229 V in the overall reaction O2 + 4H+ + 4e− → H2O. On the other hand, in basic solution, hydroxide is produced with a standard thermodynamic potential at 0.401 V in the overall reaction O2 + 2H2O + 4e− → 4OH−. While ORR in acidic solutions has been intensively investigated, its mechanism in basic solutions is less well understood. Recently, developing efficient ORR catalysts in alkaline media has become more important than ever before, due to a particular interest for alkaline fuel cells (AFCs) and metal-air batteries that use basic solutions as the electrolytes.[7] State-of-the-art catalysts for these oxygen reactions based on such precious metals as Pt, Pd, Ir or Au have been studied from the point of view of activity and durability. For example, in a fuel cell, it has been estimated that approximately 80% of the Pt loading in electrodes is used for catalyzing the ORR at the cathode.[8] However, the prohibitive cost and scarcity of precious metals have limited their widespread implementation. The development of cost-effective ORR catalysts with high activity and durability is desperately needed. During the last decade, several breakthroughs in the development of high-performance non-precious metal catalysts (NPMCs) at Los Alamos National Laboratory (LANL),[9–18] Ulsan National Institute of Science and Technology (UNIST),[2,19–23] and other research centers[24–31] indicate that the catalysts derived from earth-abundant elements (e.g., C, N, S, Fe, Co, and Mn) have the potential to efficiently catalyze these reactions and generate clean energy via direct electrochemical conversion.[8,32] Among various catalyst formulations, carbon composite materials with or without transition metals (e.g., Co, Fe, and Mn) have been recognized as the most promising ORR NPMC for alkaline fuel cells and metal-air batteries. Carbon materials have been most widely used for air cathodes because of their reasonable balance among catalytic activity, electron conductivity, surface area, and cost. However, traditional carbon materials (e.g., carbon blacks, nanotubes, and fibers) generally exhibit insufficient catalytic activity for the ORR for high-performance alkaline fuel cells or metal-air batteries.[33] Several approaches have been developed to modify these carbon materials in an effort to improve their catalytic performance. Chemical modification of carbon was found effective to improve intrinsically catalytic activity and stability of carbon materials.[31,34–37] In particular, heteroatom (e.g., N, S, B, or P) doping can tune the electronic and geometric properties of carbon, providing more active sites

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Qing Li is a postdoctoral research associate at Los Alamos National Laboratory. He received his B.Sc. from Wuhan University in 2005 and his Ph.D. from Peking University in 2010, both in chemistry. His research interests include functional nanomaterials and their applications in PEM fuel cells, metal-air batteries, and biosensors. Jaephil Cho is a professor and dean in Interdisciplinary School of Green Energy at UNIST (Korea). He is a director of the Converging Research Center for Innovative Battery and of the IT Research Center. His current research is focused mainly on Li-ion, redox-flow, and Zn-Air batteries, as well as nanomaterials for energy storage. Gang Wu is a scientist at Los Alamos National Laboratory (LANL). He completed his Ph.D. in 2004 at the Harbin Institute of Technology, concentrating on electrodeposition and electrocatalysis. After postdoctoral training at Tsinghua University (2004–2006), the University of South Carolina (2006–2008), and LANL (2008–2010), he became a research scientist at LANL in 2010. Wu is an electrochemist and materials scientist focusing on the development of nanostructured catalysts and electrode materials for energy storage and conversion technologies (e.g., polymer electrolyte fuel cells, metal-air batteries, and lithium ion batteries).

and enhancing the interaction between carbon structure and active catalytic sites.[31,34,35,38] Compared to other heteroatoms, nitrogen doping plays a more critical role in modifying the carbon structure, due to the comparable atomic size of nitrogen and carbon, as well as the presence of five valence electrons in the nitrogen atoms available to form strong covalent bonds with carbon atoms.[26] It was experimentally proven that the nitrogen-doped carbon materials turn out to be more compartmentalized and disordered than their undoped analogues.[37] Those defects may serve as active sites and result in much




2. Nanocarbon Composite ORR Catalysts for Alkaline Fuel Cells Due to the low cost, environmental acceptability, good corrosion resistance, high electrical conductivity, and fair ORR activity in alkaline media, carbon materials are viewed as ideal ORR catalysts for cathodes in AFCs and Zn-air batteries. Generally, among different allotropes of carbon, carbon materials with


ordered graphitic structure (e.g., nanotube, fiber, onion-like carbon, graphene) are expected to facilitate the electron transfer rate and exhibit good electrochemical stability. Independent of the participation of transition metals, importantly, heteroatom doping into carbon materials has been commonly recognized as indispensable for significantly improving ORR activity relative to pristine carbon materials.[24,47] In this section, we mainly discuss the role of heteroatomic doping (e.g., N, S, B, or P) and the effect of transition metals (e.g., Fe, Co, Mn) on the enhancement of the ORR in alkaline media for fuel cells. In addition, transition-metal compound/nanocarbon hybrid catalysts, which promise to be a new type of fuel cell cathode, are also reviewed. 2.1. Metal-Free Nitrogen-Doped Nanocarbon Catalysts 2.1.1. Nitrogen-Doped Carbon Nanotube Catalysts As a representative nanocarbon material that has been studied for more than two decades, carbon nanotubes (CNTs) offer several notable advantages over carbon blacks as supports for fuel cell electrocatalysts.[48] Those advantages include improved mass transfer of reagents/products, enhanced electronic conductivity, and higher resistance to corrosion.[49–53] Recently, vertically aligned nitrogen-doped CNTs were synthesized to efficiently activate ORR in alkaline media, yielding comparable activity to the state-of-the-art Pt/C catalysts.[47] It was found that the nitrogen-induced charge delocalization could facilitate the Yeager model (bridge model) chemisorption of O2 on the N-doped CNT catalysts rather than Pauling model (end-on adsorption), which could effectively weaken the O-O bonding. Later, a density functional theory study was carried out to investigate the electronic structure properties of pristine and nitrogen-doped carbon nanotubes.[36] The usefulness of local reactivity descriptors was examined to predict the reactivity of carbon/nitrogen atom sites on the external surface of the tubes. The properties determined included the electrostatic potential and average local ionization energy on the surfaces of the investigated tubes. Two types of N-doped structures were considered: pyridinic and graphitic nitrogen. The potential distribution and electronic environment of the pristine tube are significantly altered upon N doping and strongly dependent on the doped sites. In particular, pyridinic nitrogen was found to exhibit a lower formation energy when compared to graphitic nitrogen and was the most stable configuration compared to others. The results indicated that the N-doping tends to decrease the band gap of CNT and the substitution of nitrogen atom does not disturb the sp2-hybridization of the surrounding carbon atoms to form an additional impurity state. Thus, the extra valence electrons from the N dopant occupied the CNT conduction band and shifted the Fermi level toward the conduction band, which would improve the conductivity of the CNT. Furthermore, the nitrogen atom tends to activate the surface toward electrophilic/radical attack. On the other hand, there is a good correlation between the minima of the local ionization energies and chemical shielding isotropy values at the sites of nitrogen atoms, providing an effective way to rapidly assess the chemical environments of the nitrogen sites of N-doped CNTs. The understanding of the specific role of nitrogen functionalities in nitrogen-doped CNT for the ORR



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stronger adsorption of oxygen molecules and the higher activity for heterogeneous peroxide decomposition. These experimental observations are in good agreement with theoretical studies that nitrogen can be viewed as an n-type carbon dopant, thereby donating electrons to carbon, and facilitating the ORR.[39] In addition, there is increasing evidence suggesting that S or B doping into carbon is also critical to enhancing catalytic activity for the ORR.[40–43] While the heteroatom- (e.g., N or S) doped carbon catalysts with respectable ORR activity in alkaline media can be synthesized without involving metal content, the presence of transition metals (e.g., Fe, Co, Mn) during the synthesis can yield the carbon catalysts with greatly improved activity and durability for the ORR. Apart from the possibility of a direct participation in the active site, these transition metals are indispensable for in situ forming highly graphitized carbon nanostructures, due to the possible carbonization of nitrogen/carbon precursors used during the catalyst synthesis.[13,32] The formation of nanostructures during the synthetic process can be controlled by varying the employed nitrogen/carbon precursors, transitional metals, and thermal treatment conditions.[11,13] These carbon nanostructures may serve as a matrix for the ORR-active moieties containing nitrogen or metal atoms with improved catalytic activity and durability.[9] In turn, precise control of the interactions between precursors of the metal and carbon/ nitrogen during the synthesis can optimize nitrogen doping and resulting morphologies in terms of maximizing catalytic performance for the ORR. In addition to modifying the carbon by nitrogen doping in the absence or presence of transition metals, directly coupling inorganic metal compounds (e.g., oxides, sulfides, carbides, nitrides) with nanocarbon (e.g., nanotubes, graphene, fibers) is likely an effective way to enhance the ORR activity of carbon materials. The promotional mechanism is due to possible chemical bonding between the transition-metal compound and nanocarbon materials,[44] yielding a synergistic effect on enhancing charge transfer and on increasing catalyst stability on these hybrid materials for the ORR in alkaline media.[15,45,46] Here, we primarily focus on the recent development of nanocarbon electrocatalysts for ORR in alkaline media and their applications in anion exchange membrane fuel cells and metal-air batteries. Various carbon composite materials including metalfree heteroatom (N and S)-doped nanocarbon, transition metalderived nitrogen-doped carbon (M-N-C) catalysts, and transitionmetal compound/nanocarbon hybrids are discussed in terms of their correlations among synthesis, structures/morphology, ORR activity, and the corresponding alkaline fuel cells or metalair batteries performance under real working conditions.

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been developed to develop graphene-based catalysts. A N-doped graphene catalyst was synthesized using a physical method via exposing graphene to nitrogen plasma,[57] which also exhibited high ORR activity. In addition, large-area graphene can be developed by solution casting of 4-aminobenzoyl edge-functionalized graphite (EFG), which was produced by edge-selectively functionalizing pristine graphite with 4-aminobenzoic acid by a “direct” Friedel-Crafts acylation reaction in a polyphosphoric acid/phosphorus pentoxide medium.[58] Upon the heat treatment, the EFG film becomes an N-doped graphene film and displays outstanding electrocatalytic activity for the ORR in alkaline media. In the graphene family, graphene oxide (GO) is a major precursor for preparing graphene and has recently attracted much attention. GO is usually made through oxidation of graphite powder under harsh chemical Figure 1. Different nitrogen functionalities in a graphitic plane, N1: pyridinic; N2: pyrrolic; conditions. The resulting material is functionalized by various oxygen-containing N3: graphitic-center (N-Qcenter); N4: graphitic-valley (N-Qvalley). Reproduced with permission.[54] groups, such as hydroxyl, epoxy, and carCopyright 2012, American Chemical Society. boxyl, onto both sides of the sheets. Due to the Coulombic repulsion among the oxygen functional groups, process was further studied.[54] As shown in Figure 1, certain the GO flakes can be exfoliated into single or multilayered types of graphitic nitrogen (N-Qvalley) located at the edge of graphene sheets, which can be well-dissolved into water. A graphene planes exhibited the highest catalytic ORR activity, complete reduction process for GO can effectively restore the compared to other doping sites.[54] The edge plane nitrogen π-conjugated structure and lead to highly electron conductive functionalities N-Qvalley and pyridinic sites were capable of graphene materials.[59–61] Thus, GO might be one of the most catalyzing four-electron pathways for the ORR process; the promising graphene materials to provide controllable defects bulk N-Qcenter sites could catalyze a two-electron pathway via or vacancy via tuning its chemical reduction process. Recently, a peroxide intermediate. These results should be useful for using GO as precursors, a facile and catalyst-free thermal designing and developing efficient metal-free catalysts based annealing approach was developed for large-scale synthesis on N-doped CNTs. of N-doped graphene using cyanamide,[26] leading to high and controllable nitrogen contents up to 12.0 at%. The synthesis 2.1.2. Nitrogen-Doped Graphene Catalysts procedure is summarized in Figure 2a. Firstly, the sodium dodecylbenzenesulfonic acid functionized GO and cyanaGraphene, a single atomic layer of graphite with sp2 bonded mide were annealed at 550 °C to generate polymeric carbon carbon atoms arranged in the honeycomb structure, has nitride-graphene composites (CN-G). Further heat treatment attracted great interest since its discovery by Novoselov et al. of the CN-G at 800, 900, and 1000 °C gave rise to CN decomin 2004.[55] A particular interest in graphene for developing position and thereby generated N-doped graphene. Figure 2b advanced energy material is to use it as an efficient catalyst for displays a schematic representation of a resulting graphene ORR and OER, due to its unique physical and chemical properplane doped with different nitrogen atoms, including pyridinic ties, such as high surface area (theoretical value ≈2630 m2 g−1), and graphitic nitrogen. Experimentally, well-defined graphene high chemical stability, excellent conductivity, unique grasheets obtained at 900 °C without the presence of any residual phitic basal plane structure, and the easiness of functionalicarbon nitride were demonstrated using transmission eleczation.[56,57] Learning from the knowledge of preparation of tron microscopy (TEM; Figure 2c). The resulting N-doped gravertically aligned nitrogen-doped CNTs, a new type of metalphene catalyst shows better methanol tolerance, higher kinetic free catalyst of nitrogen-doped graphene was further develcurrent density (6.67 mA cm−2, at −0.4 V vs. Ag/AgCl), and oped by the same researchers for ORR in alkaline media.[24] improved durability (≈87% after 10 000 cycles) while catalyzing The N-doped graphene film was prepared via a chemical vapor the ORR in an alkaline solution, compared to commercial Pt/C deposition (CVD) method using nitrogen-containing gas and a catalysts. Regarding the possible active sites in the nitrogenNi-coated SiO2/Si substrate with Ni as a catalyst. The resulting doped graphene catalysts, the electrochemical performance N-doped graphene film possesses remarkable electrocatawas more dependent on the content of graphitic-nitrogen, lytic properties for the ORR, attesting the important role of relative to pyridinic nitrogen. A higher ratio of graphitic- to N-doping in enhancing catalytic activity for carbon materials. pyridinic-nitrogen from 900 °C treated samples than those Following this pioneering work, a number of methods have 1301415 (4 of 19)





PROGRESS REPORT Figure 2. a) Synthesis route for nitrogen-doped graphene (NG) from graphene oxide and cyanamide; b) schematic representation of NG with different nitrogen functionalities; and c) TEM image of NG after pyrolyzing at 900 °C. Reproduced with permission.[26] Copyright 2012, American Chemical Society.

from 800 °C and 1000 °C might be responsible for the higher catalytic activity. These results demonstrated that nitrogendoped graphene materials can be prepared using GO and wellselected nitrogen precursors and their activity is found to be very dependent on the type of nitrogen doping. Furthermore, nitrogen precursors may play important role for the preparation of nitrogen-doped graphene. In order to clearly elucidate the correlation of nitrogen precursors, the resulting nitrogen doping, and the corresponding catalyst activity, various nitrogen containing species including ammonia, polyaniline (PANI), and polypyrrole (PPy) were studied for annealing with GO.[62] Graphitic- and pyridinicnitrogen doping was dominant in the ammonia/GO, while annealing of PANI/GO and PPy/GO tended to generate pyridinic and pyrrolic nitrogen moieties, respectively. Importantly, the content of pyridinic nitrogen could influence the onset potential for the ORR. Meanwhile, the four-electron selectivity was found to be dependent on the content of graphitic nitrogen.[62]


2.1.3. Nitrogen-Doped Graphene/CNT Composite Catalysts As both CNT and graphene can be doped with nitrogen in a controlled way, exhibiting promising activity for the ORR, integrating their benefits simultaneously may provide a new opportunity to develop advanced metal-free carbon catalysts. Most recently, a new kind of N-doped graphene-CNT nanocomposite was synthesized by a facile hydrothermal process at a relatively low temperature (180 °C) using GO, oxidized CNT (OCNT), and ammonia as precursors (Figure 3a).[60] Typical morphology for the nanocomposites is shown in Figure 3b,c. The nanotubes have a uniform diameter of 9–15 nm. The atomic percentages of nitrogen content on the graphene and on the CNT measured by the elemental analysis (square regions marked with 1 and 2 in Figure 3c) are 3.2 at% and 1.3 at%, respectively. Remarkably, the N-doped graphene-CNT nanocomposite (NG-NCNT) electrode exhibited a more positive onset potential and a much larger limiting current for the ORR in 0.1 M KOH solution, compared to the standalone CNT, NCNT, NG, and a physical mixture of GO and OCNT. Thus, a



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Figure 3. a) Schematic illustration of the preparation of the NG-NCNT nanocomposites and b,c) STEM images of the nanocomposites. Repoduced with permission.[60] Copyright 2013, Wiley.

synergistic effect between graphene and CNT to enhance electrochemical activity is very likely. 2.1.4. Sulfur-Doped Nanocarbon Catalysts In addition to nitrogen doping, other heteroatoms, such as boron,[41,43] phosphorus,[63] sulfur,[64] and iodine,[65] have also been reported as dopants in carbon nanotubes/graphene to activate ORR in alkaline media. Noteworthy, Bandosz and coworkers found that reduced GO (rGO) in confined space of silica gel nanopores doped with sulfur shows high catalytic activity for the ORR in alkaline medium and exhibits a superior tolerance to the presence of methanol.[40] In particular, the treatment with hydrogen sulfide at 800 °C of rGO resulted in an incorporation of 2.8 at% sulfur in the GO layers. X-ray photoelectron spectroscopy (XPS) analysis indicated that this sulfur is mainly in the form of elemental sulfur/polysulfides (0.18 at%), R–SH groups (2.28 at%), C–S–C/R–S2–OR configuration (0.21 at%), and R2–S = O (0.13 at%) functionalities. The good performance of the S-doped material is linked to the coexistence of sulfur and oxygen on the surface in equal atomic quantities and a unique porosity being the replica of the silica pores. The former leads to the positive charge on the carbon atoms, which are the reaction sites. Importantly, enhanced hydrophobicity of the surface and the resulting meso/micropores could further enhance the adsorption of O2. In addition, N and S dual-doped mesoporous graphene has also been reported as a metal-free catalyst for the ORR in alkaline medium.[42] The density functional theory calculations revealed that the introduction of sulfur to the carbon matrix can be associated with the 1301415 (6 of 19)


mismatch of the outermost orbitals of sulfur and carbon due to their similar electronegativity. Thus the S atom is positively charged and can be viewed as the catalytic center for ORR. The synergistic performance enhancement is calculated to be a result of the redistribution of spin and charge densities brought about by the dual doping of S and N atoms, which leads to a large number of carbon atom active sites. 2.1.5. Boron- or Phosphorus-Doped Nanocarbon Catalysts Recently, ternary doping of nitrogen, boron, and phosphorus into carbon was further optimized for ORR activity improvement.[66] The B-doping is found to reinforce the sp2 structure of graphite and increases the portion of pyridinic-N sites in the carbon lattice, whereas P-doping enhances the charge delocalization of the carbon atoms and produces carbon structures with increased edge sites. The ORR activity of the N-doped carbon catalysts is increased by 11–15% due to the additional B-doping, but there was an increase of 100–108% in the case of additional P-doping (based on mass activities of the catalysts at 0.6 V vs. RHE). Therefore, the charge delocalization of the carbon atoms or the number of open edge sites is a significant factor in determining the ORR activity of N-doped carbon. Thus, further understanding the crucial role of the doping microstructure in ORR performance enhancement is of significance in designing and optimizing advanced metal-free carbon composite electrocatalysts. It is noteworthy that although the heteroatom-doped carbonbased catalysts are capable of efficiently catalyzing the ORR in alkaline media,[47,67] they still suffer from the low activity and high peroxide yield during the ORR in acidic electrolyte.[13,21]




2.2. Nanostructured M-N-C Composite Catalysts It was widely accepted that transition metals such as Fe, Co or Mn are able to induce even higher activity and better durability for such-prepared heteroatom-doped carbon catalysts.[8,32] The most likely mechanism for the promotional role of transition metals is to catalyze the graphitization process of nitrogen/carbon precursors to form nanocarbon with more favorable nitrogen doping toward the ORR. Additionally, there is increasing evidence showing transition metals, in particular Fe, could directly participate into MNx sites with much improved intrinsic activity. Initial efforts toward developing such type of M-N-C catalysts were conducted by Jasinski[68] and Yeager,[69] by pyrolyzing transition-metal macrocycles at temperatures exceeding 700 °C. The resulting materials demonstrated significantly enhanced ORR activity and stability of the resulting catalysts, relative to unheated macrocycles. However, the heat treatment resulted in a loss of the original macrocyclic structure and formation of materials with highly heterogeneous morphology, which typically depends on the heating temperature,[10,11] gas atmosphere,[70] nitrogen precursors,[14] and the type of supporting template used.[71] Recent advances in the development of high-performance M-N-C cathode catalysts indicated better-performing catalysts required a careful and creative choice of precursors and supports as well as synthesis conditions (control over precursor reactions in solution, heat-treatment temperature and atmosphere, post-treatment conditions, etc.).[10,13,21] 2.2.1. General Synthetic Procedures Generally, the synthesis of M-N-C catalysts starts from the solution reaction, followed by the first heat-treatment step, the acid leaching, and the second heat-treatment.[10] It was found that performance improvement was observed with M-N-C catalyst after applying the acid leaching and the second heat-treatment steps.[72] The acid leaching aims to remove the unstable and unreactive phases from the porous catalysts, which likely leads to an exposure of additional active sites.[14] Meanwhile, the acid leaching, especially using oxidative acid (e.g., H2SO4, HNO3) may introduce oxygen-containing functional groups, which increases the hydrophilicity of the catalyst and may even block active sites. Thus, the subsequent second heat treatment in inert atmosphere can be envisioned to ‘‘repair’’ the partially oxidized surface and increase the active site density at the expense of some surface area.[10] As the onset ORR potentials are nearly identical, the second heat treatment does not change the nature of active sites present. 2.2.2. Effect of the Type of Transition Metal The type of transition metals in the precursors has been proven to play a critical role in the M-N-C catalysts and can be tied to


activity enhancement after the heat treatment. In one type of catalyst derived from polyacrylonitrile, metal, and carbon, Fe and Co lead to the formation of the active centers with higher activity towards the ORR in alkaline solution, when compared to other transition metals (e.g., Zn, Ni, Mn, Cu, Cr).[73] Furthermore, the two metals likely play different roles in contributing to the active-site formation during the synthesis. For instance, in Co-N-C catalysts derived from ethylene diamine (EDA) or polyaniline (PANI),[10] Co species appear to generate the active sites that have the electrochemical properties (e.g., onset potential, Tafel slope) similar to those exhibited by metal-free N-C catalysts.[10] That means Co species may just assist in the nitrogen doping into carbon lattice, but do not directly participate in the active sites. Unlike for Co, there is increasing evidence to support an assumption that Fe species are able to be directly involved in the active sites stabilized by coordinating nitrogen and carbon, similar to a Fe-Nx moiety.[25,74] By simultaneously taking advantage of the catalytic properties of Co and Fe, it was found that bimetallic CoFe-based catalysts exhibit improved activity and performance durability for the ORR in acid media.[9,10,30] Likewise, in alkaline solution, a bimetallic catalyst composed of FeCo-Nx moieties embedded into reduced GO (rGO) integrates the benefits from Fe and Co, exhibiting better ORR activity than either Fe-N-rGO or Co-N-rGO catalyst alone.[75] 2.2.3. Effect of Nitrogen/Carbon Precursors In addition to the metal center, nitrogen precursors used in the synthesis were also found to be of great significance to the ORR activity and durability. Generally, three different groups of nitrogen precursors have been employed to develop M-N-C catalysts in alkaline media: 1) C≡N-based non-aromatic precursors, such as cyanamide and dicyanamide; 2) C-N-based nonaromatic amine precursors, such as ethylenediamine (EDA); and 3) aromatic precursors, such as aniline and melamine. Recently, a Fe-N-carbon nanotube (CNT)/nanoparticle (CNP) composite ORR catalyst for alkaline media was developed from heat-treating cyanamide, iron acetate, and carbon black (Black Pearl 2000) at 950 °C in N2 atmosphere.[18] As shown in Figure 4, the Fe-N-CNT/CNP catalyst contains dominant CNTs, ca. 20–30 nm in outer diameter and ca. 10 µm in length, and homogeneously distributes in the CNP phase. The bamboolike defects on CNT represent a typical morphological feature of nitrogen doping and believed to serve as active sites for O2 adsorption. Contrary to the commonly used approaches such as CVD and plasma etching, the synthesis of the Fe-N-CNT/CNP catalyst allows to achieve the desired CNT dispersion in a single step, without any additional treatments. The onset ORR potential and half-wave potential (E1/2 ≈ 0.93 V vs. RHE) measured with the Fe-N-CNT/CNP composite catalyst in 0.1 M NaOH is by ca. 20 mV higher than that for Pt/C. More importantly, the developed catalysts reveal improved activity even after 10 000 potential cycles. In the meantime, a GO-based Fe catalyst was also developed by using cyanamide for the ORR in alkaline media.[26] However, the activity of the resulting catalysts is not promising (E1/2 ≈ 0.75 V vs. RHE) and no such N-CNT structures are observed. Thus, for such type of M-N-C catalysts, even though similar nitrogen precursors are used, the differences in



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The different behavior for the ORR activity suggests that, among different possible active sites (CNx, MNx or MCNx, M = Co or Fe), the most active structures for the ORR in both electrolytes are likely not the same due to their different reaction mechanisms. In particular, the abundant metal-free CNx structures in alkaline media may be active enough to catalyze ORR, but their intrinsic activity is still insufficient in acidic media, compared to transition-metal derived carbon catalysts.

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Figure 4. N-Fe-CNT/CNP catalyst: a–c) morphology and d) ORR polarization plots before cycling durability test. e) ORR polarization plots measured during cycling durability in O2-saturated 0.1 M NaOH (cycling in the potential range 0.6–1.0 V at 50 mV s−1; N-Fe-CNT/CNP loading 1.0 mg cm−2). Reproduced with permission.[18] Copyright 2013, Macmillan Publishers Ltd.

synthetic strategies have a significant impact on the structures, properties, and performance of the final catalysts. Recently, PANI, a cheap and non-toxic aromatic polymer, was selected as a precursor of nitrogen and carbon in the M-N-C catalyst synthesis (Figure 5a).[9,14,76] The heat treatment of PANI was thought to facilitate the incorporation of nitrogencontaining active sites into the graphitized carbon matrix in the presence of iron and/or cobalt. The use of such a polymer as

a nitrogen precursor promised a more uniform distribution of nitrogen sites on the catalyst surface and possible increase in the active-site density.[9] A Co-N-C catalyst derived from PANI for oxygen reduction in alkaline media was found dominant by nitrogen-doped graphene sheets (Figure 5b). The catalyst shows improved ORR activity and stability relative to a Pt/C reference catalyst and a Fe catalyst derived from the same nitrogen precursor (Figure 5c).

Figure 5. Graphene-rich PANI-Co-C catalysts: a) synthesis procedure. Reproduced with permission.[9] Copyright 2011, American Association for the Advancement of Science. b) Graphene morphology dominant in the catalyst and c) ORR polarization plots recorded with all three catalysts after 1000 cycles. Catalyst loading: 0.6 mg cm−2 for non-precious metal catalysts, 60 µgPt cm−2 for Pt/C; electrolyte 0.1 M NaOH; rotation rate 900 rpm; temperature 25 °C. Potential cycling: range −0.6 to 0.2 V vs. 3.0 M Ag/AgCl scan rate 50 mV s−1; N2-saturated 0.1 M NaOH solution). Reproduced with permission.[76] Copyright 2011, The Electrochemical Society.

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PROGRESS REPORT Figure 6. Diversified carbon nanostructures observed in heat-treated M-N-C catalysts. Reproduced with permission.[11] Copyright 2012, American Chemical Society.

2.2.4. In Situ Formed Carbon Nanostructures The in situ formed graphitized carbon nanostructures in catalysts can likely link to the oxygen reduction activity and may be critical to active sites. Importantly, different carbon nanostructures are derived from different nitrogen and transition metals precursors. No such graphene-sheet structures were found in such catalysts synthesized using different nitrogencarbon precursors (ethylenediamine, dicyanamide), as shown in Figure 6. For example, when treating PANI without adding any metal, no special carbon nanostructure was observed, indicating that the transitional metal is indispensable for catalyzing the graphitization of nitrogen-carbon precursors and for forming highly graphitized nanocarbon. When ethylene diamine and Co were used, carbon nanotubes and onion-like carbons were formed. When another type of nitrogen carbon precursor dicyanamide and Fe was heat-treated together, the graphene-like tubes appear.[16] We also studied other precursors including PPy and melamine, and the formation of graphene is so far only observed with the PANI-derived catalysts. Thus, the aromatic structure in the PANI may be a key to forming graphene, probably due to their structural similarities. Such graphene-rich morphology in the catalysts may benefit the ORR electrocatalysis by improving electron conductivity and corrosion resistance.[16]

2.2.5. ORR Mechanism in Alkaline for NPMCs From the mechanism point of view, oxygen reduction in aqueous alkaline media is a complicated electrocatalytic


reaction. Many species have been proposed as intermediates in this multistep reaction, including O, OH, O2−, and HO2−, leading to a great number of possible mechanisms.[77] In general, the products of oxygen reduction in alkaline media are OH− (complete reduction, with the transfer of four electrons) and peroxide (partial reduction, with transfer of two electrons). The complete reduction can be further divided into two categories: direct four-electron pathway and two + two route with hydrogen peroxide (existing as HO2− in alkaline media) as an intermediate. In particular, ORR on carbon-supported Fephthalocyanine (FePc) and Co-phthalocyanine (CoPc) catalysts in alkaline media was studied using density functional theory calculations.[78] It was inferred that oxygen adsorption on the transition metal sites was the key step to control the ORR onset potential. For the molecular catalysts, the lower the O2 adsorption energy, the higher the kinetics of the ORR that can be expected. Moreover, the density functional theory calculations indicate that the ORR pathways, 2e− or 4e−, are mostly determined by the structures of H2O2 adsorption on FePc and CoPc catalysts. The breaking of the O-O bond during H2O2 adsorption accounts for the 4e− ORR on FePc catalyst molecule, while the lack of the O-O bond breaking process leads to the 2e− ORR on the CoPc catalyst molecule. Importantly, OH adsorption on the catalysts is believed to be an important factor in determining the stability of the catalysts in alkaline media. It was predicted using density functional theory calculations that OH adsorption is more energetically favorable than O2 adsorption on both FePc and CoPc catalyst molecules. This will cause the produced OH molecule to occupy the active sites of catalysts and hamper further progress (O2 adsorption) of ORR.



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2.3. Nanocarbon/Transition-Metal Compound Hybrids Metal oxides, in particular manganese and cobalt oxides, are found to be active for oxygen reaction in alkaline solution.[46,79] However, inherent low conductivity is one of the important drawbacks that limit their application as ORR cathodes in alkaline fuel cell. In order to overcome this challenge, these simple metal oxides need to be combined with other highly electrically conductive materials for preparing composite ORR electrocatalysts. Recently, a class of nanocarbon/transition metal oxides or sulfides hybrid catalysts for the ORR in alkaline media were extensively studied, including Co3O4/N-rmGO,[45] MnCo2O4/NrmGO,[46] CoO/N-CNT,[80] and Co1−xS/rGO.[27] When depositing these nanocrystals onto the nanocarbon, a synergetic effect may raise between the active species in carbon and these loaded transition metal nanocrystals, resulting in much improved activity.

2.3.1. MnOx/Nanocarbon Catalysts

alkaline media when the composition of the NiMnOx particles is relevantly chosen (i.e., without Ni segregated phase) and this catalyst may be an alternative to be used at the cathode of a direct borohydride fuel cell.[94] However, the obstacle that needs to be addressed is the deactivation of MnOx/C catalysts in concentrated LiOH or KOH electrolytes (e.g., concentration > 5 M) because of an insufficient activity of water, thereby limiting the necessary proton insertion into the MnOx lattice, a prerequisite for ORR activity.[87] In addition, when LiOH electrolyte is used, Li+ ions may insert into the MnOx lattice and stabilize MnIII and the oxygen groups at the carbon surface, which prevents their role of being a redox-mediating species and further blocks the catalytic process, eventually yielding an increased ORR overpotential.[96] 2.3.2. Other Metal Oxide/Nanocarbon Catalysts In addition to manganese-based catalysts, a large variety of metal oxides have also been studied as ORR catalysts in alkaline solution, including CoOx,[15,45] Cu2O,[99,100] and perovskitebased oxides.[101] More recently, 3D N-doped graphene aerogel (N-GA)-supported Fe3O4 nanoparticles (Fe3O4/N-GAs) as efficient cathode catalysts for the ORR in alkaline solution were developed.[102] As illustrated in Figure 7a for the synthesis procedure of the 3D Fe3O4/N-GAs catalyst, graphene oxide, iron acetate, and PPy were hydrothermally assembled at 180 °C for 12 h to form a graphene-based 3D hydrogel. As a result,

To date, carbon-supported MnOx is among the most extensively studied catalyst for oxygen reduction in alkaline media and the electrocatalytic properties of MnOx have been demonstrated to be highly dependent on its chemical composition, texture, morphology, oxidation state, and crystalline structure.[81–98] Manganese oxides can also serve in catalytic oxygen evolution reactions (OERs), thus making them attractive as bifunctional catalysts for oxygen electrochemistry. Chatenet and co-workers proposed a four-electron mechanism of ORR on carbon-supported MnOx in alkaline solution[87] and has been discussed previously.[19] However, compared with noblemetal catalysts, manganese oxides are still less active, particularly in terms of overpotentials and ORR selectivity. As mentioned above, integrating conductive carbon materials is a common strategy to enhance the activity of manganese oxides.[94] Examples of MnOx/ nanocarbon composites for metal-air battery applications will be addressed later in Section 4. In addition, doping with cations (e.g., Ni, Mg and Ca) and coating with metals have also been reported as efficient methods toward activity enhancement of manganese oxides. Previous studies have shown that nanostructured Ni(II)-doped MnOx nanoparticles supported on high area carbon exhibit remarkable catalytic activity for the oxygen reduction.[87,93] The four-electron ORR pathway is favored on such materials, probably because the doping transition metal could stabilize an intermediate MnIII/MnIV phase, which enhances the oxygen bond splitting.[84] Such a feature renders them very attractive electrocatalysts for the air cathode of an alkaline fuel cell AFC. Furthermore, MnOx-based catalysts exhibit high tolerance to fuels including methanol, Figure 7. a) Fabrication process and b) typical SEM image of Fe3O4/N-GAs. c) Rotating ring ethanol, and borohydrides. For instance, disk electrode (RRDE) test of the ORR on Fe O /N-GAs, Fe O /N-GSs, Fe O /N-CB in an 3 4 3 4 3 4 NiMnOx dispersed onto M1000 carbon turn O2-saturated 0.1 M KOH electrolyte at a rotation rate of 1600 rpm. Reproduced with permisout to be tolerant to the presence of NaBH4 in sion.[102] Copyright 2012, American Chemical Society. 1301415 (10 of 19)





able product used in acidic proton exchange membrane fuel cells, the resources for a good anion exchange membrane with high ionic conductivity and good mechanical properties are still very limited. Thus, evaluating ORR catalyst in an anion exchange membrane environment is challenging, but it is valuable to study the newly developed cathode catalysts in alkaline fuel cells compared to liquid electrolyte-based rotating disk electrode (RDE) tests. The ORR activity of NPMCs measured in the RDE and in a fuel cell are found to be very different. For example, a reverse discrepancy of activity was observed with a FePc/C catalyst in AAEMFCs and in RDE, with fuel cell performance significantly inferior to that in RDE.[99] Based on the density functional theory calculations, it is deduced that OH adsorption on the catalysts is an important factor that mitigates the ORR activity and stability in alkaline media. Generally, OH adsorption has two negative effects on the kinetics of oxygen reduction as these species block a part of real surface area and change Gibbs energy of adsorption of oxygen reduction intermediates.[103] Despite of the challenge to study the ORR catalysts in real AAEMFCs, some research groups have studied such nanocarbon composite catalysts using robust anion exchange membranes,

3. NPMC Cathodes in Alkaline Anion Exchange Membrane Fuel Cells Although a large number of NPMCs were developed and demonstrated superior ORR activity and stability in aqueous alkaline media, to date few of these materials have been applied and studied in a real alkaline anion exchange membrane fuel cell. The possible reasons for the absence of real fuel cell evaluation are the lack of qualified anion exchange membranes and ionomers. In contrast to Nafion membranes that are a standard commercially avail-


Figure 8. a,b) Catalyst morphology of the EDA-CoFe-C catalyst and its AEMFC performance. Reproduced with permission.[30] Copyright 2008, Elsevier. c) Fuel cell testing conditions: cell temperature: 50 °C; cathode catalyst loadings: 4 mg cm−2 for EDA-CoFe-C and 0.4 mgPt cm−2 for Pt/C; flow rates: 200 mL min−1 (H2) and 400 mL min−1 (O2); membrane: A201 membrane with a thickness of 28 µm (ion-exchange capacity 1.8 mmol g−1; conductivity 42 mS cm−1). Reproduced with permission.[72] Copyright 2011, Elsevier.



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Fe3O4 nanoparticles nucleate and grow on the graphene surface with simultaneous incorporation of nitrogen species into the graphene lattice. The as-prepared hydrogel was directly dehydrated via a freeze-drying process to maintain the 3D monolithic architecture and then heated at 600 °C for 3 h under N2 to obtain the final catalyst. The Fe3O4/graphene hybrids exhibit an interconnected macroporous framework of graphene sheets with uniform dispersion of Fe3O4 nanoparticles. The scanning electron microscopy (SEM) image (Figure 7b) indicates that a significant portion of the nanoparticles are encapsulated within the graphene layers, suggesting efficient assembly between the Fe3O4 and the graphene sheets. In studying the effects of the carbon support on the Fe3O4 for the ORR, Fe3O4/N-GAs show a more positive onset potential, higher oxygen reduction density, lower H2O2 yield, and higher electron transfer number in alkaline media, when compared to Fe3O4 nanoparticles supported on N-doped carbon black or N-doped graphene sheets (Figure 7c). This attests to the importance of the 3D macropores and high specific surface area of the GA support for improving the ORR performance. Better durability was observed with the Fe3O4/NGas, relative to the commercial Pt/C catalyst. As a new class of ORR catalyst, transition metal-compound/nanocarbon hybrids have attracted more and more attention. However, to date most of these catalysts offer inferior performance to that of the M-N-C catalysts obtained via a high-temperature approach, as discussed in Section 2.2. This is partially due to the insufficient inherent activity of oxides or sulfide and low electrical conductivity. Thus, further optimization of the interaction between metal compounds and nanocarbon as well as their morphologies can significantly impact the catalyst activity. Further physical characterization and simulation studies will refine our understanding of the active sites of ORR in the hybrid catalysts and guide to develop such-prepared catalysts with enhanced activity and stability.[15] Meanwhile, it is worth noting that such nanocarbon/metal oxides (sulfides) hold great promise to be efficient ORR/OER bifunctional cathode catalysts, due to the wellknown high OER activity and stability of oxides (sulfides) with spinel and perovskite structures.

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showing promising fuel cell performance. So far, although metal-free heteroatom-doped catalysts show excellent activity for the ORR in alkaline media, there is no further fuel cell research reported. Here, two selected examples are discussed using an M-N-C catalyst and a transition-metal compound/nanocarbon catalyst. As shown in Figure 8a,b, an EDAderived binary CoFe catalyst (EDA-CoFe-C) was prepared via a heat treatment at 900 °C with dominant nitrogen-doped onion-like carbon structures.[30] The EDA-CoFe-C cathode was successfully demonstrated in an anion exchange membrane fuel cell (Figure 8c), using an A201 membrane (Tokuyama Corporation, Japan), composed of a hydrocarbon main chain and quaternary ammonium groups as ion-exchange sites.[72] In a cell test, the OCVs were found to be 0.97 and 1.04 V for the EDACoFe-C and Pt/C catalysts, respectively. The corresponding maximum power densities were measured at 177 and 196 mW cm−2. At high potentials, the performance of EDACoFe-C was slightly lower than that of Pt/C. At Figure 9. CoO/rGO(N) catalysts: a) catalyst morphology; b) optimized rGO(N)-Co(II)-Ointermediate potentials, both catalysts showed Co(II)-rGO(N) using highest occupied molecular orbital (HOMO) at the B3LYP/6–31G level very similar activity. However, the lower perfor- of theory. Carbon (gray), nitrogen (blue), cobalt (green), and oxygen (red); c) steady-state RDE mance of EDA-CoFe-C observed at low poten- polarization plots for ORR on CoO/rGO(N) catalyst and other controls (CoO, rGO, rGO(N), tials may be attributed to high mass-transfer Pt/C) in 0.1 M O2-saturated KOH at 25 °C and 900 rpm; and d) anion exchange membrane H2 and/or ionic resistance in the cathode caused (at 1 atm, 57% RH) /O2 (at 1 atm, 100% RH) fuel cell tests using Pt/C (square symbol) and CoO/rGO(N) cathodes (circle symbol) at 60 °C. The cell voltage–current polarization curves by a high catalyst loading and the considerable (filled symbols) on the left axis and the power current curves (open symbols) on the right axis. thickness of the electrode. Reproduced with permission.[15] Copyright 2013, American Chemical Society. Recently, we developed a novel graphene nanocomposite catalyst composed of high loading CoO (24.7 wt%, Co) coupled with nitrogen-doped cells, mitigating the large ohmic resistance and significant reduced grapheme oxide (rGO(N)), as shown in (Figure 9a).[15] mass transport limitations in cathodes. As a result, the thickDuring the catalyst synthesis, prior to the incorporation of ness of the cathode layer in the MEAs is less than 20 nm, which CoO, nitrogen was doped into reduced GO using hydrazine as is close to the MEAs prepared with Pt/C catalysts. Compared to a reducing agent. This method may enhance the efficiency of the-state-of-the-art Pt/C cathode, the cell with the CoO/rGO(N) nitrogen doping compared to other methods of preparing the cathode has only a slightly decreased OCV around 38 mV nitrogen-doped graphene catalysts by mixing metal precursor, (Figure 9d). A similar downward voltage shift in the low curnitrogen source, and GO together.[45] In addition, compared to rent density range was observed for the CoO/rGO(N) cathode. other nitrogen-doped GO generated from a heat-treatment up At typical operating voltage of ≈0.6 V, the power outcome of the to 1000 °C,[26] the newly developed method avoids high temCoO/rGO(N) catalyst approaches that of the Pt catalysts. During perature treatments and is more facile with higher catalyst a life test up to 240 h for the cell with CoO/rGO(N) cathode at a yielding. In the CoO/ rGO(N) catalysts, the Co(II) was identiconstant cell voltage of 0.6 V, approximately 50% performance fied as a dominant cobalt species and most likely coordinately loss was observed. However, a comparable loss has been also coupled with pyridinic N doped into graphene planes, as eviobserved for the Pt/C cathode. In addition, the degradation was dent from X-ray absorption spectra and density functional accompanied by an increase in cell ionic resistance, indicating theory calculations (Figure 9b). In particular, density functional that the gradual loss of hydroxide ions in the membrane is likely theory calculations suggest a stable structure represented as the main reason for the performance degradation. CO2 permearGO(N)-Co(II)-O-Co(II)-rGO(N). With this unique structure, tion from ambient air into the cell test chamber was found to a synergistic effect between rGO(N) and CoO may facilitate be the primary cause for the rapid decrease in the membrane the ORR in alkaline media, yielding a much improved activity hydroxide conductivity observed during the life test. Another (E1/2 ≈ 0.83 V vs. RHE) and four electron selectivity, when compossible cause of the cell performance loss could be the poor pared to either rGO(N) or CoO along (Figure 9c). Furthermore, stability of the anion exchanging ionomer used because of its the CoO/rGO(N) catalyst was tested in an anion-exchangeexcessive degree of water uptake at elevated temperature. With membrane alkaline fuel cell using the AS4 anion exchange the currently available ionomer materials, the decrease in the ionomer (Tokuyama).[15] The high Co loading in the catalyst anion exchange membrane fuel cell performance could not yet can significantly reduce the thickness of cathode layer in fuel be unambiguously assigned to the catalyst stability. 1301415 (12 of 19)





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4. Nanocarbon Composite Catalysts for Oxygen Reduction in Metal-Air Batteries Recently, cost-effective metal-air batteries with high energy density have attracted significant interest because of the demand for long-range electric vehicles propulsion.[4,104,105] Similar to the AFC’s configuration, metal-air batteries also contain air electrodes that need ORR catalysts to reduce oxygen molecules coming from the ambient air for use as the cathode reactant.[19] On the anode side, the metals act as fuels to react with oxygen. Among many kinds of metal-air batteries with different metals as anodes, Li-air batteries and Zn-air batteries are considered to be the most promising candidates to electrify future electric vehicles (EVs) for long-distance range.[2] Because the overpotentials of the oxidation of metals in metal-air batteries are relatively low, the performance of metal-air batteries, to a great extent, depends on the efficiency of catalysts at oxygen cathodes. 4.1. NPMC Cathodes in Aqueous Li-Air Batteries Li-air batteries, which can theoretically deliver superhigh energy density, hold greater promise for the transportation applications than lithium ion batteries and other metal-air battery systems. Li-air batteries include four configurations: aprotic, aqueous, solid, and mixed aqueous/aprotic.[104] While all four different architectures assume the lithium metal as anode materials, they include different electrolytes and cathode components. Apart from these differences, all four configurations need a high-efficiency air-breathing cathode with ORR catalysts to suppress the oxygen polarization and improve the round-trip efficiency.[106–108] Among them, two configurations (aqueous and mixed aqueous/aprotic) utilize aqueous electrolytes at the cathode sides. Advantages of aqueous and mixed aqueous Li-air batteries include that the discharge reaction products are soluble in electrolyte, eliminating the cathode passivation, volume expansion, and electrical conductivity issues caused by the solid discharge reaction products formed in aprotic Li-air batteries. Alkaline solutions are commonly applied to aqueous Li-air batteries because both the anode and cathode are more stable than in acidic solutions.[109] Thus, in principle, the ORR catalysts discussed above can be transplanted to Li-air batteries with alkaline electrolytes. The research on aqueous Li-air batteries still has not become a hot topic, compared to its counterpart of nonaqueous Li-air batteries because of the need for a water-stable protection layer for the lithium anode increases the complexity of battery assembly. Here, we only selected two examples to discuss the role of ORR catalysts in the performance of aqueous Li-air batteries. Graphene and its composite have been intensively investigated as ORR catalysts for both fuel cells and metal-air batteries. Zhou and co-workers employed metal-free graphene nanosheets (GNSs) as an air electrode in a Li-air battery with a hybrid electrolyte (Figure 10).[110] The GNSs demonstrated very high efficiency, close to that of a commercial Pt/C catalyst. At 0.5 mA cm−2, the Li-air battery with GNS cathode delivered a constant discharge voltage of 3.00 V vs. Li/Li+, only 50 mV lower than that with state-of-the-art commercial Pt/C catalyst and much higher than that with the acetylene black (AB) cathode.


Figure 10. Structure of the rechargeable Li-air battery based on GNSs as an air electrode. Reproduced with permission.[110] Copyright 2011, American Chemical Society.

Furthermore, the same researchers used iron phthalocyanine (FePc) combined with different carbon materials, including GNSs, CNTs, and AB, as ORR catalysts and evaluated their performance in a hybrid Li-air battery.[111] In half-cell tests, the FePc supported on different carbon materials demonstrated very good performance with a low overpotential close to commercial Pt/C catalyst and performed a four-electron ORR mechanism. Importantly, Li-air battery tests demonstrated that the FePc supported on CNTs outperformed that on GNSs and AB by showing a higher discharge potential and much more stable cycling performance, indicating a stronger synergistic effect between FePc and CNTs. 4.2. NPMC Cathodes in Zn-Air Batteries The Zn-air battery is a very important energy storage device as an alternative to electrify EVs in the future. Highly concentrated potassium hydroxide solution (e.g., 6.0 M) is commonly used as the electrolyte in Zn-air batteries. However, the low efficiency of currently used ORR catalysts still limits the development of Zn-air batteries with high energy density and power density. Catalysts are needed for the cathode, aiming to minimize the high ORR overpotential in cathodes. As we discussed above, metal oxides, especially manganese oxides, are important ORR catalysts dominantly used in Li-air and Zn-air batteries.[112–124] Combining manganese oxides with conductive carbon materials, such as graphite, carbon nanotubes, and graphene, has proven to be effective at improving electrical conductivity and chemical stability. Recently, one example has been demonstrated via a facile solution-based route to anchor Mn3O4 on the surface of ionic liquid (IL) modified rGO.[22] The introduction of the IL moiety in the composite exhibited a promotional role in facilitating the ORR activity of



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Figure 11. Schematic synthesis procedure of Fe/Fe3C functionalized melamine foam infiltrated with N-doped KB and the resutling catalyst morphology. Reproduced with permission.[21] Copyright 2013, Wiley-VCH.

rGO and Mn3O4. The measured catalytic activity was found to be greatly dependent on the loading of Mn3O4 on graphene nanosheets. Comparing to a higher Mn3O4 (52.5%) content, the composite with a lower Mn3O4 (19.2%) content exhibited better activity and higher four-electron selectivity during the ORR, significantly mitigating the parasitic effect of peroxide generation. This hybrid catalyst was used in a primary Zn-air cell using 6.0 M KOH as an electrolyte, generating a maximum peak power density of 120 mW cm−2. Later, another high-efficient MnOx-based air electrode for Zn-air batteries was developed by depositing amorphous manganese oxides (MnOx) nanowires onto Ketjetblack carbon via a polyol method.[20] The air electrode based on this composite demonstrated a peak power density as high as 190 mW cm−2 in a Zn-air battery, showing comparable performance obtained using commercial Pt catalysts. The large surface area of the amorphous MnOx nanowires, coupling with high density of surface defects on carbon supports, potentially offers more active sites for oxygen adsorption, thus significantly enhancing the ORR activity. Even though MnOx is commonly used in both Li-air and Zn-air batteries and has demonstrated considerably high activity, its activity and stability are still major concerns for practical applications. In order to further address these activity and stability issues, heat-treated M-N-C composite catalysts, which are popular ORR catalysts thoroughly investigated for fuel cell applications,[32] have been studied in Zn-air batteries. For example, a novel ORR catalyst of Fe/Fe3C-functionalized melamine foam incorporated with Ketjenblack (KB) has been developed as a highly efficient electrocatalyst for the ORR in Zn-air batteries.[21] The unique feature in the composite is that the porous KB carbon with unique tetrapod as a bone support greatly facilitates mass transport (Figure 11).[21] Introduction of KB carbon improves

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the surface area of this composite and facilitates the adsorption of oxygen molecules from air. Melamine, a trimer of cyanamide with 67% nitrogen by mass, was used as a nitrogen/ carbon precursor. After heat-treatment at 900 °C followed by an acid leaching, the composite exhibited very high ORR activity in alkaline solutions. However, the possible active sites in the composite catalyst still remain unknown. Apart from heat-treated M-N-C catalysts, rational design of non-heat-treated catalysts with well-defined active sites have attracted significant interest because of their clear molecular structures. Recently, a bio-inspired method was developed to synthesize a Fe-N-C composite by anchoring iron phthalocyanine on a carbon nanotube using a pyridyl group as a bridge linker (Figure 12).[23] The novel design for this composite was inspired by the unique structure feature of the active site for ORR in cytochrome c oxidase (CcO) in respiratory chain, in which the iron center contains five-coordinated structure with an axial ligand from the back side. Interestingly, the developed catalyst demonstrated higher electrocatalytic activity for ORR than the state-of-the-art Pt/C catalyst in 0.1 M KOH solution as well as in concentrated 1.0 M and 6.0 M KOH solutions special for Zn-air batteries. Comparing to previous low-temperature synthesized M-N-C catalyst (typically less than 100 cycles in alkaline media), the durability of this bio-inspired catalyst is excellent, reaching over 1000 cycles. Theoretical calculations suggest that the rehybridization of Fe 3d orbitals with the ligand orbitals coordinated from the axial direction results in a significant change in electronic and geometric structure, leading to enhanced performance and stability. Unlike ambiguous chemical structures and unclear active sites in the heat-treated Fe-N-C catalysts, this bio-inspired structure was rationally designed with clearly defined active site proven by




PROGRESS REPORT Figure 12. a) Schematic diagram of the structure of FePc-Py-CNTs composite. b) High-resolution TEM (HR-TEM) image of FePc-Py-CNTs composite. c) Raman spectra of pristine carbon nanotubes and FePc-Py-CNTs composite. D band and G band peaks are marked with arrows. d) Fourier-transformed module of extended X-ray absorption fine structure (EXAFS) signals for FePc, FePc-CNTs, and FePc-Py-CNTs. e) Linear scanning voltammograms of FePc-CNTs, FePc-Py-CNTs, and commercial Pt/C catalyst. f) Half-wave potential as a function of cycle number of FePc-Py-CNTs and commercial Pt/C catalyst during durability test. Electrolyte: 0.1 M KOH, scan rate: 10 mV s−1. Reproduced with permission.[23] Copyright 2013, Macmillan Publishers.

thorough characterizations and thus creates the opportunity for further optimization of structures and morphology of active sites. Despite the significant efforts to develop ORR NPMCs for primary Zn-air batteries, considerable work has been done to search for bifunctional catalysts for electrically rechargeable Zn-air batteries.[125–130] Significant progress in the development of ORR/OER bifuntional catalyst has been made by Chen and co-workers, as shown in Figure 13.[131] A new class of corecorona structured catalyst (CCBC) with high activity and good durability was successfully demonstrated for rechargeable

Zn-air batteries. The CCBC is composed of nitrogen-doped carbon nanotubes (NCNTs) as a high ORR active component and lanthanum nickelate (LaNiO3) as a high OER active component. The LaNiO3 also acts as a support material for the synthesis of NCNTs via a CVD process. A synergistic effect could occur between the core material of LaNO3 and the corona material of NCNTs, which result in the excellent performance in rechargeable Zn-air batteries. More recently, Dai and co-workers demonstrated an advanced electrically rechargeable Zn-air battery that provides higher efficiency and durability than similar batteries made with costly metal catalysts, such as platinum and iridium.[132] The high-performance Zn-air batteries employ a novel cathode composed of a CoO/carbon nanotube (CoO/NCNT) hybrid as an ORR catalyst and Ni-Felayered double hydroxide as an OER catalyst (NiFe layered double hydroxide-LDH/CNT) (Figure 14).[132] Due to the strong coupling and a possible synergistic effect between the nanocrystals and carbon nanotubes, the hybrid materials exhibited superior ORR and OER activity compared to precious metal catalysts in 6.0 M KOH electrolyte. In particular, the CoO/N-CNT hybrid ORR catalyst showed higher activity relative to state-of-the-art commercial Pt/C catalyst. As an OER catalyst, the NiFe LDH/CNT outperformed the wellknown best benchmark catalyst Ir/C. As a result, the fabricated primary Zn-air battery Figure 13. a) Scheme of a Zn–air battery and the reactions taking places on the electrodes. using the new bifunctional cathode exhibThe CCBC catalyst is applied onto the positive electrode which catalyzes the ORR and OER ited a high discharge peak power density reactions. b) SEM and TEM images of the CCBC illustrating the NCNT on the surface of the core particle. Reproduced with permission.[131] Copyright 2012, American Chemical Society. of 265 mW cm−2 and a respectable current




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Figure 14. a–c) Schematic structure, SEM, and TEM images of a CoO/N-CNT hybrid. d–f) Schematic structure, SEM, and TEM images of a NiFe LDH/CNT hybrid. Scale bars are 200 nm, 5 nm, 200 nm, and 20 nm, in panels (b,c,e,f), respectively. Reproduced with permission.[132] Copyright 2013, Macmillan Publishers Ltd.

density of 200 mA cm−2 at 1.0 V, generating high energy density more than 700 Wh kg−1. That is two times higher than conventional lithium ion batteries. In a rechargeable Zn-air battery using a tri-electrode configuration, the bifunctional catalysts exhibited an unprecedented small charge-discharge voltage polarization of ≈0.70 V at 20 mA cm−2, demonstrating high reversibility and stability during deeply charge-discharge cycling tests up to 200 h (Figure 15).[132] This approach indicated that an optimal combination of effective ORR and OER catalysts in the cathode can yield high energy efficiency for rechargeable metal-air batteries. It is also suggested that there

is still the possibility to further improve the energy density of Zn-air batteries by developing advanced bifunctional catalysts for both the ORR and the OER.

5. Summary and Perspective Oxygen reduction and evolution reactions in alkaline media are one pair of the most important electrochemical processes and are crucial for advanced energy conversion and storage technologies, such as alkaline fuel cells, Li-air batteries, and

Figure 15. a) A schematic of the tri-electrode configuration with a CoO/N-CNT ORR catalyst loaded on a CFP electrode (1.0 mg cm−2) and carbonfree NiFe LDH OER catalyst loaded on Ni foam (≈5 mg cm−2) electrode for discharge and charge, respectively. b) Charge and discharge polarization (V–I) curves of the tri-electrode Zn-air battery using CoO/N-CNT and NiFe LDH (red) compared with the one using Pt/C and Ir/C (black). c) Cycling performance of the tri-electrode Zn-air battery using CoO/N-CNT and NiFe LDH at 20 mA cm−2 and a 20-h cycle period compared with the tri-electrode battery using Pt/C and Ir/C. d) The same CoO/N-CNT and NiFe LDH electrodes from (c) were used for a subsequent cycling experiment with a fresh Zn anode at 50 mA cm−2 and a 4-h cycle period. Reproduced with permission.[132] Copyright 2013, Macmillan Publishers Ltd.

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for electrically rechargeable metal-air batteries because transition-metal-based oxides, sulfides, and nitrides are well-known active catalysts for the OER. The underlying hypothesis is to assign ORR and OER catalysis to two different components in a bifunctional material. Compared to other possible configuration, the core-shell structure may provide the optimal structural effect. The ORR active nitrogen-doped nanocarbon (e.g., graphene, CNT) acting as shells are porous and highly graphitic, so they will provide facile electron transfer pathways and mass transport channel for the OER active cores (e.g., oxides, sulfide, carbide). In turn, the oxide cores can serve as supports or templates for the synthesis of nitrogen-doped nanocarbon, as we have demonstrated previously using TiO2.[12] The chemically coupling between nanocarbon and transition-metal-compounds likely results in a synergistic effect on the enhancement of catalytic activity toward the oxygen reactions, which is evident from density functional theory calculations combined with physical spectroscopy experiments (e.g., EXAFS, XPS). Thus, optimized interactions between two components, particularly at the interface of nanocarbon and oxide particles, will lead to maximum activity and durability required by the dual mode of catalyst operation. This approach will allow for the ORR active N-C/M-N-C catalysts to be a part of bifunctional catalysts for the rechargeable electrochemical processes. In order to fully implement the advanced non-precious metal ORR or OER catalysts into the alkaline fuel cells and metalair batteries, electrode layer design is the key to enhance the activity viable for practical systems. Fabrication of cathodes with optimized microstructure and morphology can maximize ionic/electronic conductivity and oxygen/product mass transport within the catalyst layers.

Acknowledgements Q.L. and R.C. contributed equally to this work. Financial support from the Los Alamos National Laboratory Early Career Laboratory-Directed Research and Development (LDRD) Program (20110483ER) for this work is gratefully acknowledged. This research was also supported by the MSIP (Ministry of Science, ICT & Future Planning), Korea, under the C-ITRC (Convergence Information Technology Research Center) support program (NIPA-2013-H0301–13–1009) supervised by the NIPA (National IT Industry Promotion Agency). Received: September 16, 2013 Revised: November 15, 2013 Published online: January 30, 2014

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Zn-air batteries. Advanced ORR/OER catalysts made from earth-abundant elements with sufficient activity and stability, as replacements of precious metals in alkaline media, are highly desired. Nanocarbon composite catalysts, especially nitrogen-doped nanocarbon catalysts without N-C and with transition metal involvement (M-N-C) have received considerable attention as the most promising non-precious metal catalysts for the ORR in alkaline media. Although a vigorous debate is ongoing regarding whether metal atoms participate directly in active sites, or merely catalyze their formation, the presence of nitrogen-dopants in carbon structures is indispensable for yielding much improved ORR activity, relative to pristine carbon materials. While metal-free carbons with nitrogen doping are capable of yielding high activity for the ORR, the presence of transition metal is essential to in situ generate highly graphitized carbon nanostructures, resulting in more active and durable catalysts. It is noteworthy that although the metal-free catalysts are capable of efficiently catalyzing the ORR in alkaline media, they still suffer from the low activity and poor durability in acid. This observation suggests that, among different possible active sites (CNx, MNx, or MCNx, M = Co or Fe), the most active structures for the ORR in both media are likely not the same due to their different reaction mechanisms. The nitrogen-doped carbon moieties such as CNx structures may be active enough in alkaline media to catalyze ORR, but still far more sufficient in acidic media. The morphology of carbon nanostructures in such synthesized catalysts are controllable during the catalyst synthesis by tuning the nitrogen/ carbon precursors, type of transition metals, and synthesis conditions (heating temperatures, metal contents, post-treatments), which can be closely correlated with both catalyst activity and durability. The resulting graphitized carbons are of importance ad a host matrix for these active nitrogen moieties, regardless of if they are bound to metal centers or not. An understanding of how the detailed synthetic chemistry during the solution reactions and subsequent high-temperature treatments influences the formation of active sites and density of active sites is still required. Future efforts in the synthesis of M-N-C catalysts are likely to focus primarily on the precise control of the interactions between nitrogen/carbon and metal precursors in order to produce catalysts with optimum chemical composition and morphology, as well as to maximize the population of ORR active sites. Continuously exploring new nitrogen/carbon precursors, using multi-metals, tuning precursor ratios, modifying the support surface/structure, and optimizing heat-treatment and post-treatment conditions are possible strategies in synthesis to further enhance catalyst activity and performance durability. As the morphologies in the nitrogen-doped nanocarbon via a high temperature approach is highly heterogeneous, it remains a challenge to distinguish the active sites from each individual component. Elucidating the exact nature of the active site structures will pave the way for controllable design and the synthesis of a NPMC with higher ORR activity and improved stability. Meanwhile, nanocarbon/transition-metal-compound hybrids are new type of non-precious metal catalysts for the ORR and OER. Although their ORR activity is inferior to the heat-treated M-N-C catalysts in alkaline media, they have the potential to be developed as effective ORR/OER bifunctional cathode catalysts

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