Active Non-precious Metal Catalysts Research Articles

One-Pot construction of highly active hetero-interface over porous non-stoichiometric perovskite with gel combustion strategy to enhance toluene purification activity

Constructing active perovskite nanostructures by a simple method is of great value and significance to the practical application of non-precious metal oxide-based catalysts. Herein, a surfactant-assisted gel combustion approach was adopted to fabricate a state-of-the-art LaMnO3 perovskite catalyst with a rich porous structure and hetero-interface, which can be active for low-temperature purification of toluene. Due to the large amount of gas produced during the decomposition of surfactant, the obtained catalysts own a rich porous structure, which results in a higher specific surface area and exposes more active sites. Significantly, the highly active Mn2O3-LaMnO3 hetero-interface can be generated over the non-stoichiometric perovskite successfully through adjusting the proportion of B-site element. The introduction of hetero-interfaces alters the surface structure of the material with more oxygen vacancies, leading to a significant promoting effect. It is found that the T90 of toluene (1000 ppm in air) conversion over the sample with 30 % excess manganese (La: Mn = 1: 1.3) is only 289 °C at a relatively high weight hourly space velocity (WHSV = 120,000 mL•g−1•h−1), which is 87 °C lower than that of stoichiometric LaMnO3 perovskite. This work presents a versatile strategy for preparing highly active non-precious metal catalysts for industrial applications.

Catalytic activity of Ni–Cr–Fe–Mo porous materials for hydrogen evolution reaction in alkaline media

The hydrogen evolution reaction (HER) based on water electrolysis represents an ideal approach for hydrogen production. The advancement of highly active non-precious metal catalysts holds paramount importance. This research employed powder metallurgy sintering to prepare Ni–Cr–Fe and Ni–Cr–Fe–Mo porous materials. It investigated the impact of Mo doping and sintering temperature on the catalytic activity and microstructural influence of hydrogen evolution materials. Furthermore, the materials' electrochemical performance and long-term stability in 6 mol/L KOH were examined. In complicated systems, the current work provides a feasible method for producing hydrogen from Ni–Cr–Fe–Mo porous materials in a strong alkali environment. The results exhibited that when Mo content is 5% and the sintering temperature is 800 °C, the porosity of Ni–Cr–Fe–Mo porous materials is up to 39.9% and the catalytic activity is up to 97%. At the same time, the current density decays to 7.1% after 1000 cycles of electrolysis and shows good HER stability after 48 h. The enhanced catalytic activity for hydrogen evolution emanated from the positive synergy among the transition metal elements.

Catalytic oxidation of formaldehyde by MnOx-decorated anatase TiO2{001}: Investigation of the effect of calcination temperature on catalytic activity and reaction mechanism

The removal of indoor formaldehyde (HCHO) is crucial for human health. The design of highly catalytically active nonprecious metal catalysts has major challenges, and the use of transition metal element modification to improve the catalytic performance is an effective strategy. In this study, the active component Mn was loaded on highly active crystalline anatase TiO2{001}, and the effect of the calcination temperature on the catalytic activity was investigated. The important influence of temperature on the catalyst valence distribution, surface active oxygen species, and content was thoroughly determined by characterization techniques, such as XPS, H2-TPR, and O2-TPD. The catalytic reaction process was further analyzed by in situ infrared technology to determine the reaction intermediates and to clarify the catalyst reaction poisoning mechanism; additionally, a reasonable HCHO catalytic reaction pathway was proposed with the combination of density function theory calculations. In addition, oxygen defects on the catalyst surface were determined by EPR, and based on theoretical calculations, the oxygen defects could further reduce the reaction energy barrier through the Langmuir-Hinshelwood mechanism. By exposing specific crystal surfaces and increasing the number of oxygen defects, the catalytic activity could be effectively improved; based on these results, our study provides a concept for the subsequent development of nonprecious metal catalysts.

Anionic phosphorous and sulfur regulate self-supported Ni-Fe-based electrocatalyst for water-splitting under large current density

Nickel-iron based (Ni-Fe) catalysts are widely used in the electrolysis of water due to their high activity and low cost. However, the development of highly active non-precious metal catalysts is still a great challenge. Herein, self-supported nickel–iron phosphide is fabricated through a sulfurization/phosphorization approach (Ni-Fe-P-S-1/Ni-Fe-P-S-2). The prepared catalysts show excellent catalytic performance and stability. The HER performance of Ni-Fe-P-S-1 reaches 500 mA cm−2 with small overpotentials of 289 mV in alkaline freshwater. Meanwhile, the Ni-Fe-P-S-2 requires 312 and 375 mV to reach 500 mA cm−2 in alkaline freshwater and alkaline seawater for OER. Remarkably, at the current density of 500 mA cm−2, the Ni-Fe-P-S-1||Ni-Fe-P-S-2 electrolyzer requires only 1.85 V in alkaline. Interestingly, sustainable energies can power the electrolyzer effectively. The work provides methods for potentially promising and cost-effective electrocatalysts.

Carbon-coated Fe3O4@cotton fiber biomass carbon for the oxygen reduction reaction in air cathode microbial fuel cells

Iron-based oxide catalysts on biomass carbon fibers can jointly use their respective advantages of chemical composition and structure to develop catalysts for the oxygen reduction reaction (ORR). In this study, carbon-coated Fe3O4@cotton fiber biomass carbon (C@Fe3O4@CFBC) was prepared as an ORR catalyst through hydrothermal growth and calcination treatment for air cathode microbial fuel cells (MFCs). The carbon coating layer enhanced the O2 adsorption energy, decreased the overpotential, and promoted surface activation of the Fe3O4 catalyst. The carbon coating layer can modulate the surface electronic structure of the Fe3O4 catalyst with C–O–Fe bonding and improve the electrical conductivity to accumulate electrons on the Fe3O4 surface to enhance the ORR performance. The CFBC supported Fe3O4 nanoparticles to prevent severe aggregation and expose more effective surface-active sites. The C@Fe3O4@CFBC catalyst achieved a high positive half-wave potential (0.801 V) and low Tafel slope (73.7 mV·dec−1) compared with the Pt/C catalyst (0.877 V and 94.3 mV·dec−1). The assembled air cathode MFCs achieved a desirable maximum power density (1059 ± 16 mW·m−2) based on 1266 ± 94 mW·m−2 of Pt/C MFC devices. This work provides a strategy for designing highly active non-precious metal catalysts for air cathode MFCs.

Self-adaptively electronic transfer of ternary Ni3Ga0.8In0.2/SiO2 alloy catalysts toward enhancing selectivity hydrogenation

Non-noble metal-based alloy catalysts are extensively utilized in diverse applications. Nevertheless, finding a suitable preparation strategy to precisely moderate the catalytic active site remains a substantial challenge and subsequently improves the atomic utilization and reaction efficiency. Herein, the non-noble trimetallic alloy catalysts (Ni3Ga0.8In0.2/SiO2) were fabricated by using the strategy of incorporating doping (Indium as doping metal) and modulating the Ni3Ga bimetallic alloy in which to precisely tune the electronic structure of catalysts. The dispersion of alloy nanoparticles on the support was investigated by HRTEM, alongside the valence and elemental distribution of the components by EDS, XRD, and XPS, demonstrating the successful alloying of the composite. Moreover, other metal-doped Ni-Ga-M/SiO2 (M=Cu, Pt) catalysts and Ni-Ga alloy were prepared and evaluated catalytic performance by the model reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The experimental results revealed that suitable metal doping enhanced the reaction rate significantly, with the optimal effect of Indium among the different doping metals. Particular synergies arise from the homogeneous metals' similar atomic size and electronic structure. Theoretical calculations by Density Function Theory (DFT) have revealed that the doping of Indium resulted in electron transfer at the active interface of the catalyst, the gain of additional electrons by Ni, and the creation of electron vacancies at the active interface as a result of the electron deficiency of Ga and In, which enables the optimization of the catalytic reaction. In addition, the strategy can be extended to synthesize other trimetallic nanoparticle catalysts, which is expected to provide a new avenue for precise optimization and preparation of highly active non-precious metal catalysts.

Dual-template synthesis of defect-rich mesoporous Co3O4 for low temperature CO oxidation

CO oxidation is a benchmark in heterogeneous catalysis for evaluation of redox catalysts due to its practical relevance in many applications and the fundamental problems associated with its very high activity at low temperatures. Among which, Co3O4 is one of the most active non-precious metal catalysts. Exposed crystal planes and cobalt sites are considered to be important for its high catalytic activity. Herein, we demonstrate an enhanced CO oxidation activity by a defect-rich mesoporous Co3O4 that prepared by a designed dual-template method. Two different kinds of silicas are used as hard-templates at the same time, resulting in a defect-rich mesoporous Co3O4 with a surface area as high as 169 m2/g. This catalyst exhibited a very high catalytic activity for low temperature CO oxidation with a light-off temperature at −73 oC under the space velocity of 80,000 mL h-1 gcat-1. Further studies reveal that the high surface area promotes the lattice oxygen mobility, surface rich of Co2+ species and active oxygen species are crucial for the high catalytic activity. Moreover, the dual-template approach paves a way towards the design and construction of high-surface-area mesoporous metal oxides for various applications.

(Carl Wagner Memorial Award Address) Towards Platinum-free Fuel Cells for Affordable Zero-emission Vehicles

Green hydrogen from wind and solar electricity is necessary to decarbonize certain sectors of our economy that are inaccessible by renewable electricity and it has the potential to reduce more than 30% of the global carbon emission. For the transportation sector, green hydrogen can help decarbonization through its use in fuel cells. Green hydrogen refers to hydrogen that is produced by water electrolyzers powered by renewable electricity. For low temperature fuel cells and electrolyzers, polymer electrolytes play a critical role in controlling their cost, performance, and durability, and consequently their economic viability. In this presentation, I will focus on our recent work on hydroxide exchange membrane fuel cells (HEMFCs). More specifically I will show the roadmap we have developed for this technology, the progress we have made in developing one of the most stable membranes and some of the most active nonprecious metal catalysts. I will also try to answer the fundamental question: why are hydrogen oxidation reactions are slower in base than in acid for precious metal catalysts? Figure 1

Non-precious metal cathodes for anion exchange membrane fuel cells from ball-milled iron and nitrogen doped carbide-derived carbons

Iron and nitrogen doping of carbon materials is one of the promising pathways towards replacing Pt/C in polymer electrolyte fuel cell cathodes. Here, we show a synthesis method to produce highly active non-precious metal catalysts and study the effect of synthesis parameters on the oxygen reduction reaction (ORR) activity in high-pH conditions. The electrocatalysts are prepared by functionalizing silicon carbide-derived carbon (SiCDC) with 1,10-phenanthroline, iron(II)acetate and, optionally polyvinylpyrrolidone, by ball-milling with ZrO2 in dry or wet conditions, followed by pyrolysis at 800 °C. The catalysts are characterized by scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, N2 physisorption and inductively coupled plasma mass spectrometry. By optimizing the ball-milling conditions, we achieved a reduction in the size of SiCDC grains from >1 μm to 200 nm without negatively affecting the high BET area of catalysts derived from SiCDC. This resulted in increased ORR activity in both rotating disk electrode and anion exchange membrane fuel cell (AEMFC) environments, and improved mass-transport properties of the cathode layer in fuel cell. The ORR activity at 0.9 V in AEMFC of the optimized iron and nitrogen-doped SiCDC reaches 52 mA cm−2, exceeding that of a Pt/C cathode at 36.5 mA cm−2.

Ultrathin cobalt pyrophosphate nanosheets with different thicknesses for Zn-air batteries

Two dimensional (2D) ultrathin nonprecious metal based catalysts show excellent electrocatalytic activities, due to the larger surface areas, more catalytic sites and more interconnected electron-transfer access than their bulk counterparts. Here, we synthesized cobalt pyrophosphate (Co2P2O7) nanosheets with different thickness by a simple and efficient one-step hydrothermal process. The catalytic performance of the obtained Co2P2O7 was investigated via diverse electrochemical measurement. Due to the unique 2D structure and the flexible coordination of pyrophosphate group, the as-prepared Co2P2O7 catalyst had excellent electrocatalytic performance and good stability, which could rank among the most active nonprecious metal catalysts for oxygen evolution reaction and oxygen reduction reaction. In addition, the ultrathin Co2P2O7 nanosheets exhibited good performance as the air cathode catalyst for zinc air batteries.

Oxygen Reduction Reaction Catalytic Sites on Carbon-Coated Fe3C Catalyst

Proton-exchange membrane fuel cells (PEMFC) are promising chemical-to-electrical energy conversion devices which can potentially replace combustion engine in automobiles. Their commercialization is currently suppressed primarily due to the price of the platinum catalyst for the cathodic oxygen reduction reaction (ORR). Required platinum loading per fuel-cell car is about 10 times higher than for the conventional one [1], which results in a significant contribution to the car price. Therefore, either the platinum loading needs to be drastically decreased, or a stable and active non-precious metal catalyst has to be developed. Recently, a new type of ORR catalyst composed of carbon-coated iron carbide nanoparticles has been synthesized [2]. It exhibits only slightly lower activity than, and comparable durability to platinum in acidic solutions. It has been suggested that a new ORR catalytic site may have been discovered [3]. In this work we study the catalyst by means of the Density Functional Theory to identify its active sites. Spin-polarized DFT calculations are performed using the Vienna Ab Initio Simulation Package (VASP) with PAW potentials, BEEF-vdW exchange-correlation functional [4], plane wave cutoff of 400 eV and vacuum layer of 14 Å. The model system consists of Fe3C/carbon interface and a few explicit water molecules to model the liquid environment. The following factors influencing the activity are taken into account: (i) thickness of the carbon layer (single- or multilayer graphene), (ii) defects in the carbon structure, (iii) nitrogen doping, (iv) adsorbate coverage, (v) presence of spectator adsorbates. A few other catalyst structures, which are independent of the presence of iron carbide, but still may exist in the real system, are also studied. These include catalysts with porphyrin-iron moiety embedded in the graphene structure and defective graphene with or without nitrogen doping. As a result, a few possible structures of catalytic sites in the carbon-coated Fe3C catalyst are proposed. [1] H.A. Gasteiger, D.R. Baker, R.N. Carter, W. Gu, Y. Liu, F.T. Wagner, P.T. Yu, in Hydrogen and Fuel Cells: Fundamentals, Technologies and Applications (D. Stolten, Ed.) Ch. 1 p. 3, Wiley-VCH, Weinheim, 2010. [2] Y. Hu, J. O. Jensen, W. Zhang, L. N. Cleemann, W. Xing, N. J. Bjerrum, Q. Li, Angew. Chem. Int. Ed., 2014, 53, 3675. [3] J.-P. Dodelet, R. Chenitz, L. Yang, M. Lefèvre, ChemCatChem, 2014, 6, 1866. [4] J. Wellendorff, K.T. Lundgaard, A. Møgelhøj, V. Petzold, D.D. Landis, J.K. Nørskov, T. Bligaard, K.W. Jacobsen, Phys. Rev. B, 2012, 85, 235149.

(Invited Plenary) Platinum-Free Fuel Cells for Affordable Zero-Emission Cars

One of the grand challenges facing humanity today is the development of an alternative energy system that is safe, clean, and sustainable and where combustion of fossil fuels no longer dominates. A distributed renewable electrochemical energy and mobility system (DREEMS) could meet this challenge. At the foundation of this new energy system, we have chosen to study a number of electrochemical devices including fuel cells, electrolyzers, and flow batteries. For all these devices electrocatalysis and polymer electrolytes play a critical role in controlling their performance, cost, and durability, and thus their economic viability. In this presentation, I will focus on our recent work on hydroxide exchange membrane fuel cells (HEMFCs) which can work with nonprecious metal catalysts and inexpensive hydrocarbon polymer membranes. More specifically I will show the roadmap we have developed for this technology, the progress we have made in developing the most stable membranes and the most active nonprecious metal catalysts. I will also discuss why hydrogen oxidation reactions are slower in base than in acid for precious metal catalysts.

Electro-Polymerized Metallocorroles As Electrocatalysts for Oxygen Reduction

The most active non-precious metal catalysts (NPMCs) for oxygen reduction (ORR) to date are the pyrolyzed catalysts, inspired from Heme-like complexes. These are usually composed of iron and/or cobalt coordinated by nitrogens, claimed to resemble the structure of porphyrins and phthalocynines, on the surface of a carbon support. Unfortunately, the exact structure of the catalytic sites remains a mystery and the solution for this conundrum seems be almost impossible. The advantages of using such catalysts are: (1) their high activity and (2) low price, which is derived from the cost of their precursors. The disadvantages are: (1) low durability compared to precious metal catalysts, and (2) their unknown structure, which limits their further improvement in order to obtain both the activity and durability benchmarks needed to become good alternatives for precious metals. One of the most promising options to resolve these issues is to find non-pyrolyzed molecular non-precious metal catalysts for ORR that could be tuned to match the necessary requirements. Recently, Metallo-corroles, a relatively new family of molecular catalysts was reported to have very good potential as non-precious metal catalysts for ORR. Inspired from the extensive work on the electropolymerization of metalloporphyrins, and its merits, we electropolymerized of metallocorroles on carbon electrodes to (1) increase their site density in order to overcome their relatively low electrocatalytic turnover frequency (when compared to platinum) and (2) use the proximity of active sites to move from 2- to 4-electroreduction of oxygen. Metallocorroles substituted with anilines were studied and electropolymerized. They show a significant enhancement in the overpotential required for the execution of ORR and tendency for 4-electron reduction to water. The electropolymerization, its characterization and the effect on the electrocatalytic activity of the electropolymerized metallo-corroles vs. the monomers will be presented.

Identification of Iron Nanoparticles As Active Species for Oxygen Reduction in Non-Precious Metal Catalysts

The widespread use of fuel cells is currently limited by the lack of efficient and cost-effective catalysts for the oxygen reduction reaction (ORR). Fe-based non-precious metal (NPM) catalysts exhibit promising activity and stability as an alternative to state-of-the-art Pt catalysts. However, the identity of the active species in NPM catalysts remains elusive, impeding the development of new catalysts. For the first time, we report the identification the catalytic species in an active NPM catalyst using Cl2 and H2 treatments to decrease heterogeneity. Additionally, we demonstrate the reversible deactivation and reactivation of the catalyst using the Cl2 and H2 treatments. Mössbauer and X-ray absorption spectra reveal that carbon-encapsulated Fe nanoparticles present in the as-prepared and H2-treated catalyst, but absent in the Cl2-treated catalyst are responsible for the observed ORR activity and stability of the catalyst. These findings suggest a new direction for the design and synthesis of enhanced NPM ORR catalysts.

(Invited Plenary) Improving the Catalytic Properties of Non-Precious Metal Catalysts - Milestones in the Development of Me-N-C Catalysts

Me-N-C catalysts are today’s most active non-precious metal catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). The application of these catalysts dates back to 1964 when Jasinski was the first demonstrating ORR activity of different phthalocyanines in alkaline electrolyte. The most important steps in improvement of ORR activity and stability were made by the heat-treatment of MeN4-macrocycles in 1976 and the pyrolysis of independent metal, nitrogen and carbon sources in 1989 as shown by Jahnke et al. respectively Gupta et al.. However, especially during the last decade important progress in the development and fundamental understanding of these catalysts was gained. Based on these findings the ORR activity was boosted significantly. This motivated researchers worldwide to contribute to the development of Me-N-C catalysts and to the controversy of the origin of ORR activity. In this contribution the most important milestones in the development of Me-N-C catalysts will be summarized. Based on this rational criteria can be defined for the further optimization of these catalysts with respect to a final implementation in the car.

Is Ammonium Peroxydisulate Indispensable for Preparation of Aniline-Derived Iron-Nitrogen-Carbon Electrocatalysts?

Iron and nitrogen co-doped carbon (Fe-N-C) materials are among the most active non-precious metal catalysts that could replace Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. The synthesis of the Fe-N-C catalysts often involves the use of aniline as the precursor for both N and C and ammonium peroxydisulfate (APS) as an indispensable oxidative initiator for aniline polymerization. Herein, a detailed structure and catalytic ORR performance comparison of aniline-derived Fe-N-C catalysts synthesized with and without the use of APS is reported. The APS-free preparation, which uses Fe(III) ions as the Fe source as well as the aniline polymerization initiator, results in a simple Fe-N-C catalyst with a high activity for the ORR. We show that APS is not necessary for the preparation and even detrimental to the performance of the catalyst.

(Invited) Efficient Electrocatalysis of Oxygen Reduction with Metallo-Corroles

The most active non-precious metal catalysts (NPMCs) for oxygen reduction (ORR) to date are the pyrolyzed catalysts, inspired from Heme-like complexes. These are usually composed of iron and/or cobalt coordinated by nitrogens, claimed to resemble the structure of porphyrins and phthalocynines, on the surface of a carbon support. Unfortunately, the exact structure of the catalytic sites remains a mystery and the solution for this conundrum seems be almost impossible. The advantages of using such catalysts are: (1) their high activity and (2) low price, which is derived from the cost of their precursors. The disadvantages are: (1) low durability compared to precious metal catalysts, and (2) their unknown structure, which limits their further improvement in order to obtain both the activity and durability benchmarks needed to become good alternatives for precious metals. One of the most promising options to resolve these issues is to find non-pyrolyzed molecular non-precious metal catalysts for ORR that could be tuned to match the necessary requirements. Recently, Metallo-corroles, a relatively new family of molecular catalysts was reported to have very good potential as non-precious metal catalysts for ORR. The electro-reduction of oxygen was investigated, using a series of first row transition metal β-pyrrole-brominated 5,10,15-tris(pentafluorophenyl)-corroles [M(tpfcBr8), M=Mn, Fe, Co, Ni and Cu] as catalysts in alkaline solutions. The kinetics of the oxygen reduction reaction (ORR) on these catalysts was evaluated using the rotating disk electrode method in a solution of 0.1M KOH. These corroles were adsorbed on a high surface area carbon powder (BP2000) prior to electrochemical measurements, to create a unique composite material. The comparison between the corroles with different metal centers showed a favorable catalytic performance of the ORR in the case of Fe and Co Corroles. The mechanism by which these corroles catalyze the ORR was studied by the rotating ring disk electrode (RRDE) technique, showing that in the case of alkaline solutions, there is a favorable 4 electron transfer mechanism. When comparing the performance of these catalysts in alkaline solutions, to their performance in acidic media, it was clear that the former was substantially higher, both in the onset potentials of the ORR and in the overall kinetic parameters.

A highly active non-precious metal catalyst based on Fe–N–C@CNTs for nitroarene reduction

An efficient Fe–N–C@CNTs for the hydrogenation of nitroarenes was prepared. ε-Fe3N is the active site and nitrogen/carbon atoms serve as bridges to transport the dissociated hydrogen atoms via spillover effect.

Synthesis–structure–performance correlation for poly-aniline–Me–C non-precious metal cathode based on mesoporous carbon catalysts for oxygen reduction reaction in low temperature fuel cells

In this work, attempt is made to development of active non-precious metal catalysts (NPMCs) for the oxygen reduction reaction in polymer electrolyte fuel cells (PEFCs) based on the heat treatment of poly-aniline/transition metal/carbon precursors. All the materials have been characterized by X-ray diffraction (XRD) small and wide angle, N2 adsorption–desorption isotherms, high-resolution transmission electron microscopy (TEM), Scanning electron microscope (SEM) and X-ray photo-electron spectroscopy (XPS). Moreover for electrochemical evaluation of samples, Rotating Disk electrode (RDE) technique and Fuel Cell test were employed. The results showed that onset potential for the optimized sample is about 0.1 v less than the commercial catalyst whereas exchange current density of the optimized sample (at 0.2 V vs. Reversible Hydrogen Electrode (RHE)) is about 15 mA cm−2 more than platinum electro-catalyst. Finally, the polarization curves for the fabricated membrane electrode assembly (MEAs) with overall catalyst loading of 2 mg cm−2 demonstrated that the optimized catalyst shows suitable performance compared with E-tek commercial platinum sample.

The key role of metal dopants in nitrogen-doped carbon xerogel for oxygen reduction reaction

Highly active non-precious metal catalysts based on nitrogen-doped carbon xerogel (NCX) for the oxygen reduction reaction (ORR) is prepared with resorcinol(R)-formaldehyde (F) resin as carbon precursor and NH3 as nitrogen source. NCX samples doped with various transition metal species are investigated to elucidate the effect of transition metals on the structure and ORR activity of the products. As-prepared NCX catalysts with different metals are characterized using nitrogen-adsorption analysis, X-ray diffractometry, X-ray photoelectron spectroscopy, and Raman spectroscopy. The structural properties and ORR activities of the catalysts are altered by addition of different metals, and NCX doped with iron exhibits the best ORR activity. Metal doping evidently promotes the formation of more micropores and mesopores. Raman and XPS studies reveal that iron, cobalt, and nickel can increase pyridinic-N contents and that iron can catalyse the formation of graphene structures and enhance quaternary-N contents. Whereas the total N-content does not determine ORR activity, Metal-N4/C-like species generated from the interaction of the metals with nitrogen and carbon atoms play important roles in achieving high ORR activity.