Non-precious Oxygen Reduction Reaction Catalysts Research Articles

Developing First Row Transition Metal Antimonate Oxynitride and Oxysulfide Nanoparticles As Oxygen Reduction Electrocatalysts

Hydrogen fuel cells (FCs) are a promising avenue for replacing global dependence on carbon-intensive fuels especially in the transportation sector. The cathode of a FC carries out the oxygen reduction reaction (ORR), and the kinetics of the ORR is one of the primary sources of inefficiency in commercial FC devices. Commercial FC cathodes are based on expensive platinum-based catalyst architectures owing to their high activity, and therefore identifying highly active, non-precious ORR catalysts could accelerate the deployment of hydrogen fuel cells into the energy landscape. Theory calculations and experiments indicate first-row transition metal antimonates (MSbOx, M = Mn, Fe, Cr, Ni) have highly desirable ORR activity and stability characteristics. Furthermore, the material systems in this family have been extended to ternary and quaternary compositions for the ORR through the inclusion of additional transition metals to identify further opportunities for activity enhancement. In this work, we develop (oxy)sulfides and (oxy)nitrides of first-row transition metals such as manganese (MnSbOxSy and MnSbOxNz, respectively) to extend this class of materials towards enhanced activity when compared to previously reported antimonate materials. These nanoscale electrocatalysts are synthesized via colloidal means followed by high temperature surface modification under various reactive atmospheres (NH3, H2S, O2). Precise control of temperature ramps and holds enables modulation of the degree of oxidation, sulfidation, or nitridation, resulting in fully oxidized manganese antimonate nanocrystal cores with thin polycrystalline shells of (oxy)sulfide and (oxy)nitride. We tune the synthetic parameters of these materials and evaluate their enhanced electrochemical ORR activity in both alkaline and acidic conditions that are relevant to the operating conditions of anion-exchange membrane and proton-exchange membrane FCs, respectively. We perform extensive bulk and surface characterization with techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and gas adsorption BET surface area analysis. We find that converting the pure oxide into oxysulfide or oxynitride causes significant alterations to crystallinity and is accompanied by the appearance of additional crystalline phases that were not present previously as seen in XRD diffractograms. Transmission electron micrographs reveal polycrystalline nanocrystals of roughly 50 nm diameter for all compositions and show varying internal contrast patterns. The pure oxide nanocrystals form a thin shell of ~4 nm thickness after sulfidation or nitridation, and this shell may be converting into an active phase exposed to electrolyte during ORR testing and is expected to be significant for understanding the observed activity enhancement. XPS spectra indicate shifts in Sb and Mn peaks that are suggestive of changing surface-level oxidation state. We also utilize nanometer-resolution elemental mapping with TEM coupled to energy dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements both before and after catalysis to understand how degradation affected observed activity trends. Lastly, on-line degradation measurements during potential cycling on an electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) reveal potential and current density dependence of corrosion to provide insight into degradation mechanisms. Utilization of a diverse set of physical and electrochemical characterization techniques provides a thorough description of how manganese antimonate (oxy)sulfides and (oxy)nitrides carry out the ORR and could inform the future development of advanced non-precious FC cathode catalysts to drive down device cost.

Durability Assessment of Transition Metal Oxygen Electroreduction Catalysts with on-Line Flow Cell Trace Elemental Analysis

Proton exchange membrane hydrogen fuel cell (PEMFC) cathodes for commercial applications are typically based on platinum group materials that exhibit remarkable oxygen reduction reaction (ORR) activity and stability. Despite this performance, the high cost of such platinum electrodes prevents a more rapid transition to a more sustainable hydrogen economy involving PEMFCs. Platinum-free transition metal ORR catalysts have been identified and tested as high activity, low-cost alternatives with well-established protocols, though understanding the durability of these materials would require more experimental efforts. Techniques for measuring catalyst degradation typically involve activity loss measurements, surface-sensitive microscopy and spectroscopy, and liquid aliquot collection for metal loss quantification that are all performed post-catalysis for extended periods of time. Experiments using this approach provide valuable information about overall catalyst material degradation but usually cannot elucidate mechanistic or potential-dependent phenomena that may occur on shorter time scales. More rigorous experiments with synchrotron radiation have been utilized to investigate the degradation mechanisms of platinum-based fuel cell catalysts but studies on other transition metal materials are less widespread due to the high time requirements and limited accessibility of synchrotron beamlines for such experiments. In our work, we utilize an on-line electrochemical flow cell coupled with an in-house inductively coupled plasma-mass spectrometer (ICP-MS) for evaluation of fuel cell catalyst degradation in acidic electrolyte environments. Oxygen and nitrogen gas saturated electrolytes with a constant pH of 1 but differing chemical identity are flowed through the cell and across a working electrode composed of non-platinum transition metals. Dissolved metal ions are convectively transported to the ICP-MS where they are measured with up to part per trillion resolution with a time delay of ~4 seconds, enabling millisecond resolution of transient catalyst surface degradation. We evaluate the oxygen mass transport capabilities of our custom-machined flow cell against a commercial flow cell and compare to the much more established rotating disk electrode (RDE) technique for distinguishing ORR activity. Cyclic voltammetry measurements in oxygen-saturated electrolyte on various metal foils indicate potential dependent corrosion in agreement with what is expected from a Pourbaix (potential-pH) diagram of stability. In contrast, potential cycling these materials in the ORR-relevant range reveals degradation during ORR electrocatalysis that increases with ORR current to differing amounts as a function of the metal in a way that has not been previously characterized. Notably, degradation appears to be suppressed in the ORR potential range when saturating the electrolyte with nitrogen compared to oxygen, suggesting that surface-bound oxygen and ORR intermediates have a potential-dependent role in enabling degradation. Probing degradation during ORR conditions further with varied cyclic voltammetry scan rates and directions reveals more fundamental information about the timescales of catalytic corrosion processes and may inform the development of activation protocols that enhance the lifetime of ORR catalyst materials during operation. Through electrochemical flow cell testing in various pH 1 electrolytes, we make suggestions for electrolyte compositions and catalyst operation potential ranges that maintain activity and stability to advise fuel cell catalyst researchers in developing platinum-free PEMFC cathode materials. We are also developing additional flow cell configurations to allow testing of other non-precious ORR catalyst morphologies such as nanoparticle catalysts on carbon supports. The on-line ICP-MS flow cell architecture accelerates materials stability and degradation experiments for rapid assessment of platinum-free ORR electrocatalysts through in-house experiments and direct comparability to established electrochemical techniques.

Boron doping induced electronic reconfiguration of Fe-Nx sites in N-doped carbon matrix for efficient oxygen reduction reaction in both alkaline and acidic media

Developing non-precious metal-based catalysts as the substitution of precious catalysts (Pt/C) in oxygen reduction reaction (ORR) is crucial for energy devices. Herein, a template and organic solvent-free method was adopted to synthesize Fe, B, and N doped nanoflake-like carbon materials (Fe/B/N–C) by pyrolysis of monoclinic ZIF-8 coated with iron precursors and boric acid. Benefiting from introducing B into Fe–N–C, the regulated electron cloud density of Fe-Nx sites enhance the charge transfer and promotes the ORR process. The as-synthesized Fe/B/N–C electrocatalyst shows excellent ORR activity of a half-wave potential (0.90 V vs 0.87 V of Pt/C), together with superior long-term stability (95.5% current density retention after 27 h) in alkaline media and is even comparable to the commercial Pt/C catalyst (with a half-wave potential of 0.74 V vs 0.82 V of Pt/C) in an acidic electrolyte. A Zn-air battery assembled with Fe/B/N–C as ORR catalyst delivers a higher open-circuit potential (1.47 V), specific capacity (759.9 mA h g−1Zn at 10 mA cm−2), peak power density (62 mW cm−2), as well as excellent durability (5 mA cm−2 for more than 160 h) compared to those with commercial Pt/C. This work provides an effective strategy to construct B doped Fe–N–C materials as nonprecious ORR catalyst. Theoretical calculations indicate that introduction of B could induce Fe-Nx species electronic configuration and is favorable for activation of OH∗ intermediates to promote ORR process.

Rich and uncovered FeNx atom clusters anchored on nitrogen-doped graphene nanosheets for highly efficient and stable oxygen reduction reaction

It is still challenging but essential to synthesize non-noble catalysts with rich and uncovered catalytic active sites towards oxygen reduction reaction (ORR). And the limited research of the ORR properties of FeNx atom clusters also hinders their practical implementations. Here, we report a two-dimensional (2D) non-precious ORR catalyst constructed by abundant and fully exposed FeNx atom clusters anchored on nitrogen-doped graphene nanosheets (Fe-N-C/NG), which derives from three-dimensional (3D) ZIF-8, 2D graphene oxide nanosheet, and ferrocene. The optimized Fe-N-C/NG delivers outstanding ORR activities (Eonset = 0.97 V, E1/2 = 0.84 V, Jd = 5.48 mA cm−2), excellent Tafel slope (57.7 mV dec−1), remarkable long-term durability, and strong methanol tolerance in an alkaline electrolyte. Its superior catalytic activity is possibly attributed to its unique conversion of nanoarchitecture and the synergistic effects: the change of morphology effectively utilizes the advantages of 2D graphene oxide and 3D ZIF-8, which guarantees sufficient and reachable active sites; the 2D porous nanoarchitecture of Fe-N-C/NG promotes the ion diffusion and mechanical stability; both the FeNx atom clusters and N-doped graphene nanosheets facilitate the catalytic activity. This work introduces an innovative strategy to rationally design the nanoarchitecture and facilely synthesize nonprecious ORR catalysts with sufficient and uncovered active sites for high efficiency and durability.

Functionalized halloysite template-assisted polyaniline synthesis high-efficiency iron/nitrogen-doped carbon nanotubes towards nonprecious ORR catalysts

The iron-functionalized halloysite nanotudes is synthesized as a tubular template from an iron-nitrogen co-doped carbon nanocatalyst derived from the polyaniline pyrolysis of Oxygen reduction reaction (ORR). The use of iron-doped sites exposes the surface of the material to more graphite defects, effectively increasing the specific surface area of the carbon nanotubes and optimizing the density of the nitrogen-containing active sites. We have found that this iron-nitrogen co-doped carbon nanotube exhibits a superior ORR performance with a half-wave potential of 0.77 V in the 0.1 mol L−1 KOH aqueous solution. The electrochemical stability test was carried out at 1600 rpm. The half-wave potential showed only a degradation of 8 mV, which was better than the commercial 20% Pt/C (18 mV). This study developed an economical method for the direct synthesis of iron-nitrogen co-doped carbon nanotubes using a hard template.

Effect of Nitrogen Doping on Oxygen Reduction Activity of TiO2 in Acidic Media

In order to develop low-cost nonprecious metal cathodes that have both high activity of oxygen reduction reaction (ORR) and high durability as alternatives to Pt-based electrocatalysts for practical fuel cell vehicle application, we have focused on group 4 and 5 oxide-based compounds because they are insoluble in acidic media and are much less expensive than platinum. We already demonstrated that titanium and zirconium oxide-based compounds prepared from these oxy-phthalocyanines showed definite ORR activities [1,2]. However, these types of the catalysts used multi-walled carbon nano-tubes as a support and deposited carbon derived from phthalocyanine as a local electron conduction path. Carbons are fundamentally oxidized at high potential and temperature. Therefore, carbon-free nonprecious ORR catalysts are required for fuel cell vehicle application. An arc-plasma deposition method is useful to control the oxidation state of the oxides without heat-treatment. We prepared zirconium oxide-based cathode using tin-oxide based electroconductive support by arc-plasma deposition as a carbon-free nonprecious metal cathode. Fig. 1 shows the potential-ORR current density curves of the ZrOx/SnO2-based support prepared by arc-plasma deposition method, the SnO2-based support, and the TixNbyOz+Ti4O7 [3] in 0.1 M H2SO4 at 30 oC. The ZrOx/SnO2-based support had superior ORR activity. Fig. 2 shows comparison of the onset potential of the ZrOx/SnO2-based support and the TixNbyOz+Ti4O7 in 0.1 M H2SO4 at 30 oC. Both onset potentials are above 1.1 V vs. RHE, indicating that these active sites are superior in quality. Figs. 1 and 2 suggested the great potential of carbon-free nonprecious metal oxide-based cathodes. Acknowledgment The authors thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. This work was conducted under the auspices of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Program for Promoting the Reform of National Universities.

The construction of porous graphene tri-doped with B, N and Co for enhanced oxygen reduction reaction

Graphene-based materials are promising nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR). However, this kind of carbon materials are usually suffering from layer stacking and intrinsically lack of highly-efficient active sites for ORR. At present, it is still a huge challenge to develop porous multi-doped graphene. Herein, the B, N and Co tri-doped graphene with hierarchical pores is successfully synthesized via a first hydrothermal reaction of graphene oxide (GO) with Co(II) nitrate, 2-methylimidazole (2MI) and boric acid, subsequent pyrolysis and final acid leaching. The morphology, structure and components of a series of samples are systematically investigated. The as-obtained products possess abundant macro/mesopores and ORR active sites including B-bonded C (CB), pyridinic N, graphitic N, CoNC and defects owing to the formation and decomposition of the Co layered double hydroxides (Co-LDHs). After optimizing hydrothermal temperature, pyrolytic temperature and the amount of GO, the sample exhibits an excellent ORR performance in 0.1 M KOH solution. The activity, durability and methanol tolerance are comparable or even superior to those of the commercial Pt/C catalyst (20 wt%). This work provides a new strategy to construct porous multi-doped graphene as nonprecious ORR catalyst.

Controlled synthesis of graphene via electrochemical route and its use as efficient metal-free catalyst for oxygen reduction

The oxygen reduction reaction (ORR) is a critical step in the operation of fuel cells. The exploration of efficient, low cost and durable non-precious ORR catalysts, to replace the rare and expensive Pt-based catalysts, is promising towards the large-scale commercialization of fuel cells. Here, we report the fabrication of high-quality graphene (GP) with controllable layers, using a simple, versatile and cost-effective electrochemical exfoliation method. Our systematic studies revealed that the graphite precursor, electrolyte temperature, and pyrolysis conditions largely affect the number of layers and quality of the graphene, which consequently has a significant effect on its ORR performance. The optimized graphene sample was subsequently heat-treated in the NH3 atmosphere to obtain the nitrogen-doped graphene (N-GP). As a metal-free electrocatalyst, N-GP exhibit much enhanced ORR activity than GP, and comparable ORR activity to the state-of-the-art Pt/C catalyst. Moreover, it also shows better stability than Pt/C catalyst in alkaline solution.

Nitrogen and sulfur Co-doped graphene inlaid with cobalt clusters for efficient oxygen reduction reaction

Cobalt based catalysts are promising for oxygen reduction reaction (ORR) for unitized regenerative fuel cells and alkaline metal-air batteries. Herein, we report a novel non-precious ORR catalyst, in which cobalt clusters with a mean size of ∼2 nm are inlaid into porous nitrogen and sulfur co-doped graphene matrix. Benefiting from the unique structural property of Co clusters and the three-dimensional (3D) hierarchical porous architecture of nitrogen and sulfur codoped graphene, the prepared catalyst exhibits excellent ORR catalytic activity with good methanol tolerance and long-term durability in alkaline environment.

Hybrid shells of MnO2 nanosheets encapsulated by N-doped carbon towards nonprecious oxygen reduction reaction catalysts

Developing nonprecious electrocatalysts for efficient oxygen reduction reaction (ORR) have aroused increasing interest recently. Herein, α-MnO2 nanosheet-based hollow spheres encapsulated by N-doped carbon (denoted as N-Carbon/MnO2/N-Carbon) as efficient ORR electrocatalysts were reported. As started from redox reactive polystyrene/polyaniline (PS/PANI) core/shell templates, MnO2 and PANI layers were sequentially in situ loaded, leading to the formation of MnO2 nanosheet-based hollow spheres encapsulated by PANI (denoted as PANI/MnO2/PANI) after PS removal process. Finally, the calcination process ensured the crystallization of MnO2 and carbonization of PANI, resulting in the formation of N-Carbon/MnO2/N-Carbon hybrid shells. The N-Carbon/MnO2/N-Carbon hybrid shells showed excellent electrocatalytic activity towards ORR in 0.1 mol L−1 KOH aqueous solution, making the hybrids promising cathode catalysts for alkaline fuel cell applications.

Recent Progress in Oxygen Electrocatalysts for Zinc–Air Batteries

AbstractZinc–air batteries (ZABs) have attracted extensive attention due to their remarkable high theoretical energy output. They represent one of the most promising future power sources. However, many barriers restrict their application on a large scale. One of the main challenges is the sluggish rates of the oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER), which govern the discharging and charging processes of the battery, respectively. Here, recent advances related to oxygen electrocatalyst materials for ZABs are discussed. Detailed discussions will focus on unifunctional ORR electrocatalysts and bifunctional ORR and OER electrocatalysts. Pt‐based nanomaterials, as the best ORR electrocatalysts, possess the virtue of high activity, but have the disadvantages of high cost, scarcity, and poor stability. Thus, materials based on transition metals (alloys, metal oxides, metal nitrides, and spinel oxides) and metal‐free materials are widely investigated as nonprecious ORR catalysts owing to their promising catalytic activities. As for bifunctional ORR and OER electrocatalysts, the following two categories are introduced: (i) metal‐based materials, including single metal/metal‐oxides‐based materials and mixed‐metal/metal‐oxides‐based materials; and (ii) metal‐free materials. Finally, perspectives on the continuous research and limitation of the current ZAB technology are provided.

Non-Precious ORR Catalyst for PEMFC Application Using Iron, Nitrogen and Sulfur Co-Doped Few Layer Graphene

The sluggish kinetics of oxygen reduction reaction (ORR) at the cathode of proton exchange membrane fuel cells (PEMFC) demand a high loading of precious platinum (Pt) or Pt alloy electrocatalysts [1]. The extensive implementation of fuel cell technologies for stationary and vehicular applications, however, demands the replacement of high cost and less abundant Pt with low cost and natural abundant electrocatalysts. Recent studies show the use of various non-precious metals, metal alloys, metal oxides and nitrogen doped carbon as alternative catalysts for ORR [2, 3]. Even though, these non-precious metal catalysts experience severe dissolution and large agglomeration under vigorous PEMFC operational conditions and subsequent catalyst performance degradation. In this work, we developed a facile method for making non-precious ORR catalyst using nitrogen (3.7 at. %), sulfur (2.4 at. %) and iron (1.3 at. %) co-doped porous graphene (Fe-NSG). The ORR activity and durability of catalyst in acidic media was investigated by half (Fig. 1) and full cell electrochemical measurements. RDE measurements of the Fe-NSG catalyst confirm a four electron transfer ORR process with high current density. PEMFC full cell measurements of the catalyst give a maximum power density of 225 mW cm-2 at 80 °C with good stability [4]. The high ORR activity and good stability of Fe-NSG electrocatalyst can be ascribed due to, (a) the comparatively higher electronegativity of doped N and S atoms (N: 3.04, S: 2.58) with respect to carbon (C: 2.55), which induces more charged sites within the graphene lattice favorable for oxygen adsorption and reduction, (b) presence of the large amount of pyridinic nitrogen, which can coordinate with iron cations and increase the density of ORR active Fe-Nx centers, (c) an optimum amount of sulfur within the non-precious catalyst suppress the iron carbide formation, and increases the formation of Fe-N4 ORR active species, (d) the unique confined morphology of Fe nanoparticles within the graphene layers suppress the agglomeration/dissolution of metal particles and increase their interfacial contact and durability. References F. Jaouen, E. Proietti, M. Lefevre, R. Chenitz, J.-P. Dodelet, G. Wu, H. T. Chung, C. M.Johnston and P. Zelenay, Energy & Environmental Science, 2011, 4, 114-130.Z. Chen, D. Higgins, A. Yu, L. Zhang and J. Zhang, Energy & Environmental Science, 2011, 4, 3167-3192.M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science, 2009, 324, 71-74.B. P. Vinayan, T. Diemant, R. J. Behm and S. Ramaprabhu, RSC Advances, 2015, 5, 66494-66501. Figure 1

DFT Studies of the Oxygen Reduction Reaction on Pristine and N-Doped Fe3C/Graphene Catalyst

Proton exchange membrane fuel cells (PEMFCs) are considered to be a promising power generation systems for future mobile and stationary applications. One of the main factors that restrain their wide use is the sluggish kinetics of the oxygen reduction reaction (ORR) at the electrode. Platinum-based materials have been so far the most active catalysts for the ORR, but their high cost makes the technology rather exclusive. In recent years, a number of non-precious ORR catalysts have been reported [1-3]. Among them, a catalyst composed of Fe3C nanoparticles encased in graphitic layers [3] draws an increasing interest due to its high stability and activity, which may be a result of the presence of a new type of active site [4]. To explain this phenomenon, we study possible catalytic sites of Fe3C/graphene interface and its N-doped variant by means of the Density Functional Theory. Additionally we study other potentially interesting systems like Fe3C with multiple graphitic layers and its N-doped variants, as well as FeN4 moieties embedded in the graphitic layers. Free energy diagrams of the 4- and 2-electron associative pathways are calculated. The aqueous environment is modeled by including several explicit water molecules. Catalytic activity and selectivity towards H2O is considered and compared with current state-of-the-art ORR catalysts. [1] E. Proietti, F. Jaouen, M. Lefèvre, N. Larouche, J. Tian, J. Herranz, J.-P. Dodelet, Nat. Commun., 2011, 2, 416. [2] G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science, 2011, 332, 443. [3] Y. Hu, J. O. Jensen, W. Zhang, L. N. Cleemann, W. Xing, N. J. Bjerrum, Q. Li, Angew. Chem. Int. Ed., 2014, 53, 3675. [4] J.-P. Dodelet, R. Chenitz, L. Yang, M. Lefèvre, ChemCatChem, 2014, 6, 1866. Figure 1

Hydrogel-derived non-precious electrocatalysts for efficient oxygen reduction

The development of highly active, cheap and robust oxygen reduction reaction (ORR) electrocatalysts to replace precious metal platinum is extremely urgent and challenging for renewable energy devices. Herein we report a novel, green and especially facile hydrogel strategy to construct N and B co-doped nanocarbon embedded with Co-based nanoparticles as an efficient non-precious ORR catalyst. The agarose hydrogel provides a general host matrix to achieve a homogeneous distribution of key precursory components including cobalt (II) acetate and buffer salts, which, upon freeze-drying and carbonization, produces the highly active ORR catalyst. The gel buffer containing Tris base, boric acid and ethylenediaminetetraacetic acid, commonly adopted for pH and ionic strength control, plays distinctively different roles here. These include a green precursor for N- and B-doping, a salt porogen and a Co2+ chelating agent, all contributing to the excellent ORR activity. This hydrogel-based process is potentially generalizable for many other catalytic materials.

Defective Nitrogen-Doped Graphene Foam: Clarifying the Role of Nitrogen in Non-Precious ORR Catalysts

The energy industry is set to be revolutionized by polymer electrolyte membrane fuel cells (PEMFCs). Commercial PEMFC stationary units for industrial / home use are already on sale in Japan, and PEMFC vehicles will be commercialized in various countries in 2015. Platinum-decorated carbon black electrocatalyst powder is a crucial aspect of PEMFC technology. However, Pt is expensive, and limits PEMFC durability due to aggregation, dissolution, ripening, and carbon corrosion. Pt-free catalysts are therefore desirable for next-generation fuel cells. Non-precious, Pt-free catalysts for the electrochemical oxygen reduction reaction (ORR) in acid media have been the subject of intense research for the past few decades. One of the most popular materials in this field are pyrolysed mixtures of Fe/C/N-containing precursors, subjected to various heating, acid washing, and milling procedures. The mechanism for the ORR in these catalysts is not well understood, and is still debated. There are two main camps; those who see Fe as part of the catalytic center; and those who see Fe as a generator of active sites, whilst nitrogen would play the active role in the ORR. The history of this field is well-summarized in a recent review.1 Resolution of this debate is hampered by the complicated chemical structure in these materials; the possible combinations of Fe, N, and C atoms that could be ORR active are many. Therefore our approach is to simplify the system by removing Fe from the equation, and clarifying the fundamental catalytic activity of nitrogen-doped carbons. Initially we synthesized carbon nitride, and pyrolysed carbon nitrides supported on carbon black and carbon nanotubes. These materials had some inherent ORR activity, but Fe contamination was an issue,2,3,4 as with most materials approaches to this problem. We therefore developed synthesis of a truly metal-free nitrogen-doped graphene foam (GFN), which showed high inherent 4-electron oxygen reduction in acid medium.5,6 Here we synthesize GFN by combustion of nitrogen-containing sodium alkoxide, followed by washing, 1000˚C heat treatment in N2 and H2, and graphitization at 1400˚C. This is a 3D carbon with micron-scale pores encapsulated by thin defective graphene walls with a thickness of around 2 nm, with a surface area of > 700 m2/g and a nitrogen content of around 0.5 at.%. (Fig. 1). The material was confirmed to be Fe-free by ICP analysis. Linear sweep voltammograms (LSVs) of GFN (Fig. 2) show relatively high activity for an entrirely metal free catalyst (maxinum current density, -4 mA/cm2, mass activity (at 0.6 V), 2.79 A/gcat). The onset potential (at -10 µA/cm2) is around 0.85 V, similar to many Fe-containing catalysts. The electron transfer cooeficient (calculated from the ring electrode) is 3.6, incdicating majority 4-electron transfer. We conclude that Fe-free active sites may contribute significantly to the 4-electron ORR in Fe/C/N-based catalysts (although this does not mean that Fe-centers have no activity).Figure 1. (a) Schematic showing some PEMFC degradation mechanisms; (b) nitrogen-doped graphene schematic; (c) SEM image of GFN; (d) XPS N1s spectrum; linear sweep voltammograms for GFNin acid, at various rotation speed; (e) disc electrode, (f) ring electrode. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. 1. J-. P. Dodelet, Electrocatalysis in Fuel Cells, Springer (2013):271 2. Lyth, S. M., et al. J. Phys. Chem. C113.47 (2009): 20148 3. Lyth, S. M., et al. J. Electrochem. Soc. 158.2 (2011): B194 4. Lyth, S. M., et al. J. Nanosci. & Nanotech. 12.6 (2012): 4887 5. Lyth, S. M., et al. e-J. Surf. Sci. & Nanotech. 10 (2012): 296. Liu, J, et al. J. Electrochem. Soc. 161.4 (2014):F544-F550

Facile Synthesis and Evaluation of Nanofibrous Iron–Carbon Based Non-Precious Oxygen Reduction Reaction Catalysts for Li–O2 Battery Applications

Development of low cost active catalysts toward oxygen reduction reaction (ORR) is critical for the effective operation of Li–O2 battery. Porous nonprecious iron–carbon based nanofiber catalysts have been developed by electrospinning method. The catalysts demonstrated similar ORR catalytic activity for ORR as the commercial Pt-based catalysts in the aqueous half-cell voltammetry sweeps. In the Li–O2 aprotic environment, the catalyst exhibited higher on-set potentials when compared to glassy carbon and Pt disk electrodes. The results show that the nonprecious electrospun nanofiber could be an effective low cost ORR catalyst at the cathode for Li–O2 battery.

Iron phthalocyanine and nitrogen-doped graphene composite as a novel non-precious catalyst for the oxygen reduction reaction

We develop a facile method for the synthesis of an iron phthalocyanine (FePc) and nitrogen-doped graphene (NG) composite as a novel and efficient non-precious catalyst in the oxygen reduction reaction (ORR). The resulting product exhibits superior ORR catalytic activity, excellent tolerance to methanol crossover, and comparable stability to commercial Pt/C, which leads to the invention of a new non-precious catalyst for ORR in fuel cells.

Wet chemical synthesis of nitrogen-doped graphene towards oxygen reduction electrocatalysts without high-temperature pyrolysis

The oxygen reduction reaction (ORR) is one of the important reactions not only in life processes but also in artificial energy conversion systems, such as fuel cells and metal/air batteries. As one of the non-precious ORR catalysts, N-doped carbon materials show an exciting activity, but most of them were universally synthesized by high-temperature pyrolysis or annealing up to 1100 °C in the past few decades, which makes the structural manipulation of the catalysts extremely difficult. Here, we propose that ORR active N-doped carbon catalysts could, in principle, be prepared via a sophisticated wet chemical reaction between a reactive graphitic carbon template (e.g. graphene oxide) and N-containing molecules (e.g. dicyandiamide) at temperatures as low as 180 °C. Without any high-temperature treatments, for example, the as-prepared N-doped reduced graphene oxide with additional Fe-containing nanoparticles showed an impressive ORR catalytic activity that was comparable to many previous N-doped carbon from high-temperature pyrolysis. Rational utilization of the graphitic carbon template, the N-containing molecules, and the wet chemical reactions may offer a low-temperature route to create interesting ORR electrocatalysts with easier surface properties manipulation.

Development of non-precious oxygen reduction reaction catalyst for polymer electrolyte membrane fuel cells based on substituted cobalt porphyrins

Active and stable cobalt-based non-precious metal catalysts for the oxygen reduction reaction (ORR) in PEM fuel cells were developed through high-temperature pyrolysis of metal-porphyrins supported on carbon. The roles of substituted porphyrins, carbon support, and catalyst loading on ORR activity were studied using rotating disc electrode (RDE) measurements. It was observed that the carbon support plays a major role in improving the catalytic activity. The results showed that among the supported catalysts, the homemade mesocarbon-supported cobalt-porphyrin catalyst with 20 wt% loading displayed higher ORR activity; the cell performance showed maximum current density of 1.1 A cm−2 at 0.13 V in H2/O2 fuel cells.