Non-precious Oxygen Reduction Reaction Research Articles

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.

Graphene wrapped Fe7C3 nanoparticles supported on N-doped graphene nanosheets for efficient and highly methanol-tolerant oxygen reduction reaction

Green and efficient non-precious metal electrocatalysts for oxygen reduction reaction (ORR) are prepared to meet the increasing demand for clean, secure and sustainable energy. Herein, we report a novel and environmentally friendly strategy for synthesis of graphene-wrapped iron carbide (Fe7C3) nanoparticles supported on hierarchical fibrous N-doped graphene with open-mesoporous structures (Fe7C3/NG) by simply annealing the mixture of melamine, iron (II) phthalocyanine (FePc) and Fe2O3. The effects of the pyrolysis temperature and the molar ratio of FePc to melamine were critically examined in the controls. Remarkably, the Fe7C3/NG obtained at 800 °C (i.e. Fe7C3/NG-800) manifested the forward shifts in the onset potential (0.98 V) and half-wave potential (0.85 V) with respective to commercial Pt/C (50 wt%) in 0.1 M KOH, coupled with the great enhancement in the durability (still remained 92.11% of its initial current density even after 40,000 s) and strong methanol tolerance. This research presents a promising strategy for developing Pt-free non-precious efficient ORR electrocatalysts in fuel cells.

Ultrafine Co@nitrogen-doped carbon core-shell nanostructures anchored on carbon nanotubes for highly efficient oxygen reduction

The rational design of non-noble-metal electrocatalysts for oxygen reduction reaction (ORR) with both excellent activity and robust stability still remains a key challenge nowadays. Herein, N-doped carbon wrapped Co nanoparticles core-shell nanostructures grafted on carbon nanotubes (Co@NC@CNTs) were achieved by a simple pyrolysis process using ZIF-67 and CNTs as precursors. Most importantly, this unique structure of Co@NC@CNTs is beneficial to increase the contact area of N-doped carbon and Co, inhibit the aggregation of Co@NC core-shell nanoparticles and protect the Co from dissolution, thus improving the electrocatalytic performance and stability for ORR. As a result, the well-defined Co@NC@CNTs electrocatalyst exhibits excellent ORR activity with a high onset potential, half-wave potential and limited current density, comparable to the commercial Pt/C in alkaline electrolyte. Furthermore, the Co@NC@CNTs electrocatalyst presents outstanding electrochemical durability and methanol tolerance in comparison with Pt/C. This strategy will open a new avenue toward the development of nonprecious high-performance ORR catalysts.

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.

Sandwich-like electrode with tungsten nitride nanosheets decorated with carbon dots as efficient electrocatalyst for oxygen reduction

Developing efficient and cost-effective oxygen reduction reaction (ORR) catalysts for fuel cells is quite necessary but still full of challenges. In this study, tungsten nitride nanosheets (WN NSs) decorated with nitrogen-containing carbon dots hybrid materials are developed using a facile two-step route. Specifically, a sandwich-like porous electrode, which contains a N-doped carbon layer encapsulated with uniform carbon dots (N-CDs) and a sheet-like WN for both-side formation of N-CDs, is obtained and designed as a novel type of ORR catalysts (denoted as N-CDs@WN). The N-CDs@WN hybrid shows superior catalytic activity via a four-electron pathway and demonstrates excellent stability over long periods of time and resistance to methanol. The excellent electrocatalytic properties of N-CDs@WN hybrids benefited from a hierarchical structure with large specific area, numerous active sites, and synergistic effect between CDs and WN that can facilitate the conductivity and mass transfer during the ORR process. Compared to other carbon dots-based nanocomposites, our facile synthetic route for N-CDs@WN not only presents the advantages of being relatively green, simple, and cost-effective but also is an attractive alternative for the electrocatalyst in the non-precious ORR.

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.

(Invited) Active Carbon Supports for Pt Cathode Catalysts in PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely considered one of environmentally friendly and highly efficient electric power sources for a wide variety of power applications ranging from portable and stationary power supplies to transportation. However, some of the most critical barriers to their commercialization are the intrinsic high cost and insufficient long-term performance durability. Among others, the spherical carbon black particle support used for Pt catalyst in MEAs have a number of intrinsic structural disadvantage. It is partially responsible for current insufficient performance and durability of MEAs. They include (1) inefficient O2 transport due to significant tortuosity in carbon black-based cathode, (2) non-uniform distribution of ionomer due to significant microporosity of carbon blacks, (3) catastrophic corrosion during start-up/shut-down events due to low corrosion resistance, (4) low electrical conductivity, and (5) weak binding sites with Pt nanoparticles. We has developed a new type of high-surface areas (up to 850 m2/g) and large size nitrogen-diped graphene tubes (NGT) in a highly controlled manner and at high yields. The diameters of these graphene tubes can be tuned from 50 to 500 nm, which is much larger than conventiona carbon nanotubes (20 nm). It should be noted that the graphene tube is low cost using inexpensive dicyandiamide as precursors ($1-2/kg) and the synthesis method is easily scaled up. Apart from the obvious advantage of high electronic conductivity and enhanced corrosion resistance, the large tube size provided better Pt nanoparticle dispersion and more facil mass transfer including O2 and H2O. Importantly, the NGT exhibited oxygen reduction reaction (ORR) activity in acidic media comparable with state of the art PGM-free catalsyts. Thus, the novel ORR active nitrogen-doped graphene tubes are able to not only offer remarkable support effects by geometrically and electronically modifying the deposited Pt nanoparticles, but also provide a large amount of complementary nonprecious ORR active sites, thereby significantly improving the overall activity and durability of the Pt catalysts. Thus, relative to conventional carbon blacks, the highly graphitized and active N-GT is an ideal support for low-PGM catalysts for high-performance MEAs with enhanced mass transport and durability along with ideal interfaces with ionomer.

Rational Design of Metal-Organic Frameworks/Gels As Efficient Catalysts for Oxygen Reduction Reaction

The growing pollution and energy crisis emphasize the need for the development of clean energy, such as fuel cells and metal-air batteries, in order to reduce the dependence on fossil fuels. In such clean energy devices, oxygen reduction reaction (ORR) is a key process in determining practical performance. Unfortunately it suffers from sluggish kinetics, resulting in large overpotentials and high efficiency penalties. To address this issue, rational design of non-precious metal catalysts with ORR performance comparable or even superior to precious metal is much needed. Metal-organic frameworks (MOFs) are porous crystalline materials assembled from metal nodes and organic linkers. MOFs possess high surface areas, open metal sites, and tunable pore structures, and are therefore promising for ORR electrocatalysis, in pristine phases or as self-sacrificial templates (Fig. 1A). Besides MOFs, another class of metal-organic coordinated materials that are of great interest are metal-organic gels (MOGs). MOGs are viscoelastic solid-like materials assembled by metal-organic coordination in conjunction with other interactions. MOGs possess great inclusion properties, design flexibility and ease in synthesis, highlighting themselves as precursors for producing ORR electrocatalysts (Fig. 1A). Herein, a series of MOFs/MOGs has been synthesized for ORR electrocatalysis. The catalytic ORR performance of the catalysts was evaluated through a rotating disk electrode technique, and the corresponding voltammograms are summarized in Fig. 1B. Several catalysts exhibit ORR activity comparable or greater than that of a commercial Pt/C benchmark, suggesting rational design approach of MOFs/MOGs offers a general solution for production of highly effective non-precious ORR electrocatalysts. Figure 1

Enhancement of Fe-N-C carbon catalyst activity for the oxygen reduction reaction: effective increment of active sites by a short and repeated heating process.

Controlling the formation of Fe–N–C catalytic sites is crucial to activate the oxygen reduction reaction (ORR) for realization of non-precious electrocatalysts in proton exchange membrane fuel cells (PEMFCs). We present a quantitative study on the effect of a newly obtained thermal history on the formation of Fe–N–C catalytic sites. A short and repeated heating process is employed as the new thermal history, where short heating (1 min) followed by quenching is applied to a sample with arbitrary repetition. Through electrochemical quantitative analysis, it is found that the new process effectively increases the Fe–N–C mass-based site density (MSD) to almost twice that achieved using a conventional continuous heating process, while the turn-over frequency (TOF) is independent of the process. Elemental analysis shows that the new process effectively suppresses the thermal desorption of Fe and N atoms during the initial formation stage and consequently contributes to an increase in the Fe–N–C site density. The resultant catalytic activity (gravimetric kinetic current density (0.8 V vs. RHE)) is 1.8 times higher than that achieved with the continuous heating process. The results indicate that fine control of the thermal history can effectively increase the catalytic activity and provide guidelines for further activation of non-precious ORR electrocatalysts for PEMFCs.

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.

Chemical-state evolution of Ni in Mn[sbnd]Ni/polypyrrole nanocomposites under bifunctional air electrode conditions, investigated by quasi-in situ multi-scale soft X-ray absorption spectroscopy

Rechargeable metal-air batteries are highly prospective energy conversion and storage devices with enormous potential impact on broad deployment of renewable energies as well as on transportation. Nevertheless, efficient and durable systems are not yet available, mainly owing to limited chemical stability and durability of bifunctional air electrodes, that undergo poorly understood changes in chemical state and space distribution of the electrocatalytic materials. Manganese oxides (MnOx) are popular non-precious oxygen-reduction reaction (ORR) catalysts due to their low cost and relatively high activity. Unfortunately, the stability of MnOx under ORR conditions and its performance in the oxygen-evolution reaction (OER) are not completely satisfactory. Previous work in our group has shown that mixed MnNi oxides exhibit a notably improved ORR activity and durability, strongly enhanced by dispersing the oxides in a polypyrrole (PPy) matrix. Moreover, PPy exhibits high stability as well as catalytic activity in the OER. This study thus focuses on the ORR/OER behaviour of MnNi/PPy nanocomposite and is aimed at following the changes in chemical state distribution of dopant Ni at the sub-microscale. These changes result from the cathodic and anodic operation of this material under conditions relevant to the oxygen electrode of Zn-air batteries. Our study is based on quasi-in situ identical-location soft-X ray absorption microspectroscopy at the Ni L-edge, complemented by spectroscopy performed through ptychography for the case of ORR. Electrochemical operation results in a space- and potential-dependent rearrangement of the distribution of elemental and oxidized Nickel and in a change of their ratios. Operation under ORR conditions is accompanied by an increase of the oxidized Nickel fraction owing to the presence of an oxidizer in the electrolyte. As expected, oxidizing OER conditions cause an increase of the oxidized Nickel fraction.

Oxygen-Reduction Active Carbon Supports for Pt Cathode Catalysts in Polymer Electrolyte Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely considered one of environmentally friendly and highly efficient electric power sources for a wide variety of power applications ranging from portable and stationary power supplies to transportation. However, some of the most critical barriers to their commercialization are the intrinsic high cost and insufficient long-term performance durability. Among others, the spherical carbon black particle support used for Pt catalyst in MEAs have a number of intrinsic structural disadvantage. It is partially responsible for current insufficient performance and durability of MEAs. They include (1) inefficient O2 transport due to significant tortuosity in carbon black-based cathode, (2) non-uniform distribution of ionomer due to significant microporosity of carbon blacks, (3) catastrophic corrosion during start-up/shut-down events due to low corrosion resistance, (4) low electrical conductivity, and (5) weak binding sites with Pt nanoparticles. University at Buffalo, SUNY has developed a new type of high-surface areas (up to 850 m2/g) and large size nitrogen-diped graphene tubes (NGT) in a highly controlled manner and at high yields. The diameters of these graphene tubes can be tuned from 50 to 500 nm, which is much larger than conventiona carbon nanotubes (20 nm). It should be noted that the graphene tube is low cost using inexpensive dicyandiamide as precursors ($1-2/kg) and the synthesis method is easily scaled up. Apart from the obvious advantage of high electronic conductivity and enhanced corrosion resistance, the large tube size provided better Pt nanoparticle dispersion and more facil mass transfer including O2 and H2O. Importantly, the NGT exhibited oxygen reduction reaction (ORR) activity in acidic media comparable with state of the art PGM-free catalsyts. Thus, the novel ORR active nitrogen-doped graphene tubes are able to not only offer remarkable support effects by geometrically and electronically modifying the deposited Pt nanoparticles, but also provide a large amount of complementary nonprecious ORR active sites, thereby significantly improving the overall activity and durability of the Pt catalysts. Thus, relative to conventional carbon blacks, the highly graphitized and active N-GT is an ideal support for low-PGM catalysts for high-performance MEAs with enhanced mass transport and durability along with ideal interfaces with ionomer.

Copper-Based Electrocatalysts with Multinuclear Catalytic Sites for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) to water is catalyzed at cathodes in polymer electrolyte fuel cells (PEFCs). For widespread applications of PEFCs, low-cost cathode materials with high catalytic activity and durability are highly desirable. Since natural ORR catalysts such as multi-copper oxidases catalyze the ORR with almost no overpotential (ca. 20 mV)[1]and the enzymatic ORR occurs at a multinuclear copper complex, mimicking enzymatic active sites would allow us to develop non-precious metal-based ORR electrocatalysts for practical applications. Herein we report our research progress in synthetic and mechanistic studies on copper-based electrocatalysts for the ORR.[3,4]Our findings suggest that the design and synthesis of multinuclear copper cores might be a key to develop highly efficient and durable copper-based electrocatalysts for the ORR. References. [1] N. Mano, V. Soukharev, A. Heller, J. Phys. Chem. B, 110, 11180 (2006). [2] M.S. Thorum, J. Yadav, A.A. Gewirth, Ang. Chem. Int. Ed., 48, 165 (2008). [3] M. Kato et al., Phys. Chem. Chem. Phys., 17, 8638-8641 (2015). [4] M. Kato, N. Oyaizu, K. Shimazu, I. Yagi, J. Phys. Chem. C, in press. DOI: 10.1021/acs.jpcc.5b11663

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

2D Nanoporous Fe-N/C Nanosheets as Highly Efficient Non-Platinum Electrocatalysts for Oxygen Reduction Reaction in Zn-Air Battery.

It is an ongoing challenge to fabricate nonprecious oxygen reduction reaction (ORR) catalysts that can be comparable to or exceed the efficiency of platinum. A highly active non-platinum self-supporting Fe-N/C catalyst has been developed through the pyrolysis of a new type of precursor of iron coordination complex, in which 1,4-bis(1H-1,3,7,8-tetraazacyclopenta(1)phenanthren-2-yl)benzene (btcpb) functions as a ligand complexing Fe(II) ions. The optimal catalyst pyrolyzed at 700 °C (Fe-N/C-700) shows the best ORR activity with a half-wave potential (E1/2 ) of 840 mV versus reversible hydrogen electrode (RHE) in 0.1 m KOH, which is more positive than that of commercial Pt/C (E1/2 : 835 mV vs RHE). Additionally, the Fe-N/C-700 catalyst also exhibits high ORR activity in 0.1 m HClO4 with the onset potential and E1/2 comparable to those of the Pt/C catalyst. Notably, the Fe-N/C-700 catalyst displays superior durability (9.8 mV loss in 0.1 m KOH and 23.6 mV loss in 0.1 m HClO4 for E1/2 after 8000 cycles) and better tolerance to methanol than Pt/C. Furthermore, the Fe-N/C-700 catalyst can be used for fabricating the air electrode in Zn-air battery with a specific capacity of 727 mA hg-1 at 5 mA cm-2 and a negligible voltage loss after continuous operation for 110 h.

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

Porous VO(x)N(y) nanoribbons supported on CNTs as efficient and stable non-noble electrocatalysts for the oxygen reduction reaction.

Novel nanocomposites of carbon nanotubes supported porous VOxNy nonoribbons (VOxNy-CNTs) have been synthesized by the annealing of the sol-gel mixture of CNTs and V2O5 under NH3 atmosphere as well as the ageing process in air. Besides the morphological and structural characterizations revealed by TEM, SEAD, EDS, XRD and XPS measurements, typical electrochemical tests including cyclic voltammetry (CV), rotating disk electrode (RDE) and chronoamperometry have been employed to determine the oxygen reduction reaction (ORR) performance of VOxNy-CNTs. Inspiringly, the results indicate that VOxNy-CNTs catalyst exhibits a 0.4 mA/cm2 larger diffusion-limited current density, a 0.10 V smaller onset potential value, a 10.73% less of ORR current decay and an excellent methanol-tolerance compared with commercial Pt/C catalyst. Therefore, we have reasonable grounds to believe that this new VOxNy-CNTs nanocomposites can be regarded as a promising non-precious methanol-tolerant ORR catalyst candidate for alkaline fuel cells.

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.