High Catalytic Activity For Oxygen Reduction Reaction Research Articles

Highly Durable PtNi Alloy on Sb-Doped SnO2 for Oxygen Reduction Reaction with Strong Metal-Support Interaction.

Carbon-supported Pt is currently used as catalyst for oxygen reduction reaction (ORR) in fuel cells but is handicapped by carbon corrosion at high potential. Herein, a stable PtNi/SnO2 interface structure has been designed and achieved by a two-step solvothermal method. The robust and conductive Sb-doped SnO2 supports provide sufficient surfaces to confine PtNi alloy. Moreover, PtNi/Sb0.11 SnO2 presents a more strongly coupled Pt-SnO2 interface with lattice overlap of Pt (111) and SnO2 (110), together with enhanced electron transfer from SnO2 to Pt. Therefore, PtNi/Sb0.11 SnO2 exhibits a high catalytic activity for ORR with a half-wave potential of 0.860 V versus reversible hydrogen electrode (RHE) and a mass activity of 166.2 mA mgPt -1 @0.9 V in 0.1 M HClO4 electrolyte. Importantly, accelerated degradation testing (ADT) further identify the inhibition of support corrosion and agglomeration of Pt-based active nanoparticles in PtNi/Sb0.11 SnO2 . This work highlights the significant importance of modulating metal-support interactions for improving the catalytic activity and durability of electrocatalysts.

Bamboo-like carbonitride nanotubes with multi-type active sites for oxygen reduction reaction in both alkaline and acid mediums

To omni-directionally utilize carbon-based material in nanoscale and improve its catalytic activity for oxygen reduction reaction (ORR), bamboo-like carbonitride nanotubes (bCNTs) with high-density multi-type active sites (CoO@Co–N-bCNT) are facilely synthesized via a multi-step method referring to thermal pyrolysis of Co2+ complexed melamine, acid leaching and second annealing. Multi-type active sites including encapsulated Co nanoparticles, intercalated Co/CoO species, Co-Nx coordinated sites and defect-rich surface are present in the as-prepared CoO@Co–N-bCNT electrocatalyst. The types and densities of these active sites are easily tuned via the ratio of melamine to Co2+ in precursors and subsequent treatment. Due to the high-density multi-type active sites and bamboo-like tubular structure, CoO@Co–N-bCNT electrocatalyst exhibits high catalytic activity for ORR with high stability in both alkaline and acid electrolytes. Quasi-solid-state zinc air battery (ZAB) assembled with the CoO@Co–N-bCNT as the cathode exhibits open circle voltage of 1.39 V and peak power density of 14.9 mW cm−2. Two-cell series of quasi-solid-state ZABs can light LED indicator for 7 h, suggesting its promising practical application. The studies provide facile strategy to design the carbon-based electrocatalysts with high performance and tune their active sites.

Design of High-Performance Co-Based Alloy Nanocatalysts for the Oxygen Reduction Reaction.

Co-based nanoalloys show potential applications as nanocatalysts for the oxygen reduction reaction (ORR), but improving their activity is still a great challenge. In this paper, a strategy is proposed to design efficient Co-M (M=Au, Ag, Pd, Pt, Ir, and Rh) nanoalloys as ORR catalysts by using density functional theory (DFT) calculations. Through the Sabatier analysis, the overpotential as a function of ΔGOH * is identified as a quantitative descriptor for analyzing the effect of dopants and atomic structures on the activity of the Co-based nanoalloys. By adopting the suitable dopants and atomic structures, ΔGOH * accompanied by overpotential could be adjusted to the optimal range to enhance the activity of the Co-based nanoalloys. With this strategy, the core-shell structured Ag42 Co13 nanoalloy is predicted to have the highest catalytic activity for ORR among these Co-based nanoalloys. To give a deeper insight into the properties of Ag-Co nanoalloys, the structure, thermal stability, and reaction mechanism of Ag-Co nanoalloys with different compositions are also studied by using molecular simulations and DFT calculations. It is found that core-shell Ag42 Co13 exhibits the highest structural and thermal stability among these Ag-Co nanoalloys. In addition, the core-shell Ag42 Co13 shows the lowest ORR reaction energy barriers among these Ag-Co nanoalloys. It is expected that this kind of strategy could provide a viable way to design highly efficient heterogeneous catalysts in extensive applications.

Electrospun single iron atoms dispersed carbon nanofibers as high performance electrocatalysts toward oxygen reduction reaction in acid and alkaline media

The metal (Fe/Co), nitrogen co-doped carbon represent an important class of oxygen reduction reaction (ORR), which can be obtained via the thermal treatment of transition-metal macrocycles (TMMs). However, the N4-chelate complex with metal atom (M-N4) moieties as major activity site for ORR are easily destroyed to form inorganic metal species during simple pyrolysis of TMMs. In this report, polyacrylonitrile (PAN) nanofibers were prepared by electrospinning containing a small amount of hemin (chloroprotoporphyrin IX iron(III), TMMs). The electrospun nanofibers were converted into Fe, N co-doped carbon nanofibers (Fe-N-CNFs) through preoxidized and thermal treatment. The PAN macromolecules can prevent hemin from aggregation during the process of pyrolysis. The Fe elemental mapping demonstrated that Fe species probably existed in a single atom state. The Fe K-edge X-ray absorption fine structure spectrum of Fe-N-CNFs proved that the Fe-N4 moieties have been successfully reserved. The X-ray photoelectron spectra of Fe-N-CNFs indicated that the amount of Fe-N4 moieties increased with the increased percent of hemin. Therefore, the Fe-N-CNFs exhibited the higher catalytic activity for ORR compared with Pt electrocatalysts. Furthermore, the Fe1-N-CNFs displayed higher stability and methanol tolerance than Pt/C.

Heteroatom (N, P, B) Doped Hierarchical Porous Carbons from Coal Tar Pitch with High Specific Surface Area for Oxygen Reduction Reaction

Heteroatom (N, P, B) Doped Hierarchical Porous Carbons from Coal Tar Pitch with High Specific Surface Area for Oxygen Reduction Reaction Fang Dong, Jinli Qiao* College of Environmental Science and Engineering, Donghua University, 2999 Ren’min North Road, Shanghai 201620, China. *qiaojl@dhu.edu.cn In the past few decades, fuel cells, as a kind of clean and efficient energy sources, which is considered to be the most promising solution to the increasingly serious energy crisis of energy conversion equipment, has attracted significant attention.1 Oxygen reduction reaction (ORR) in fuel cells plays a decisive role in determination of performance, and electrocatalysts with high-performance ORR are essential for practical applications. Up to now, the platinum or its alloyed nanoparticles have been used to catalyze the sluggish ORR, however, their high cost as well as the limited natural abundance hinder the development and commercialization of the interrelated technologies.2 The development of new ORR catalysts free of noble metals is thus the next challenge for the long-term viability and sustainability of fuel cells. Due to the introduction of porous structure, porous carbon materials with high specific surface area, porous rich and aperture can be adjusted, which has the advantages of high chemical stability and good electrical conductivity, and this has been widely used in the aspects such as catalysis, adsorption and electrochemical energy storage.3 Recent years witnessed the dramatic interest in using nitrogen-doped porous carbon as one of the promising ORR catalysts. Besides N, other heteroatoms have also been introduced into carbon materials and they also show enhanced catalytic activity for ORR.4 In this work, we choose N, P and B as doped heteroatoms to fabricate the hierarchical porous carbons derived from coal tar pitch. The catalytic ORR activities of the heteroatom (N, P, B) doped hierarchical porous carbons from coal tar pitch are studied and the results show that heteroatom (N, P, B) doped with hierarchical porous carbon from coal tar pitch has high specific surface area and high catalytic activity for ORR. References 1 M.L. Xiao, J.B. Zhu, L.G. Feng, C.P. Liu, W. Xing, Advanced Materials, 27, 2521 (2015). 2 M. Sahoo, S. Ramaprabhu, Energy, 119, 1075 (2017). 3 L. Osmieri, R. Escudero-Cid, A.H.A. Monteverde Videla, P. Ocón, S. Specchia, Applied Catalysis B: Environmental, 201, 253 (2017). 4 M.J. Wu, Q.W. Tang, F. Dong, Z.Y. Bai, L. Zhang, J.L. Qiao, Journal of Catalysis, 352 , 208 (2017).

The Effect of Oxygen Partial Pressure and Composition on the Oxygen Reduction Reaction Properties of Ba0.5Sr0.5Fe1-XCuxO3-δ as a Cathode for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) face hurdles in market acceptability due to their high operating temperatures, which typically ranges from 750 oC – 1000 oC. Thus, current research is moving towards lowering the operating temperature to an intermediate temperature range from 500 oC - 700 oC by developing intermediate temperature (IT)-SOFCs. However, during IT operation, conventional cathodes show sluggish oxygen reduction reaction (ORR) kinetics. Till now, perovskite-type oxides have been at the forefront because of their mixed ionic electronic conductive (MIEC) nature. Additionally, Co-based perovskites have shown high catalytic activity for ORR in IT-SOFC cathodes. However, Co-leads to additional cost, high thermal expansion coefficient and toxicity. In this study, ORR properties of the Co-free perovskite-type Ba0.5Sr0.5Fe1-x Cu x O3-δ (x = 0.05 and 0.2) (BSFCu) cathodes were investigated by utilizing electrochemical impedance spectroscopy (EIS). BSFCu powders were synthesized through the sol-gel method with a final sintering temperature of 1050 °C. Powder X-ray diffraction was used test chemical compatibility between BSFCu and La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) powders up to 1100 °C in air. Symmetrical cells of the following configurations BSFCu-LSGM/LSGM/BSFCu-LSGM were prepared for electrochemical measurements using varying ratios of cathode to electrolyte. Using a well-controlled set up, the effect of pO2 on the area specific resistance (ASR) for ORR of symmetrical cells was studied. BSFCu0.05-LSGM/LSGM/BSFCu0.05-LSGM showed the lowest ASR of 0.03 Ω/cm2 under ambient air at 800 °C, when sintered at 1000 °C. These ASR values are competitive with typical values for Co-based cathodes as reported in the literature. Thermo-gravimetric analysis, under O2/Ar atmospheres showed reversible oxygen intake/release capability of BSFCu powders, making them potential candidates for applications in oxygen-storage technologies.

Particle size effect of Ag-nanocatalysts deposited on carbon as prepared by γ-radiation induced synthesis

The effect of silver particle sizes on the catalytic performance of Ag/C electrodes for oxygen reduction reaction (ORR) was studied. The Ag particles were precipitated from AgNO3 solutions on Vulcan XC-72 carbon as support by γ-radiation induced synthesis method. The structural and morphological characterizations of the electrode materials were done by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that particles with smaller diameters, 11 ± 6 nm, possess higher catalytic activity for ORR (50 mA/cm2 at 0.3 V) as compared to those with larger diameters, 41 ± 5 nm, ORR activity is 25 mA/cm2 at 0.3 V. The observed effect may be explained by an increased amount of low coordinated atoms in smaller particles as compared to the larger ones.

Microwave-assisted synthesis of carbon nanotubes threaded core-shell CoPx/Co-Nx-C@CNT and its performance as an efficient bifunctional oxygen catalyst for the rechargeable zinc-air battery

The development of bifunctional cathode catalysts with advantages such as high activity, low cost, and high durability is highly desirable and challenging for zinc-air batteries (ZABs) in energy applications. In this work, a novel type of bifunctional catalyst (CoPx/Co-Nx-C@CNT) simultaneously effective for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) was prepared via a simplified all-in-one phosphorization-carbonization synthesis, which is distinctively different from the previously reported carbonization and postphosphorization route. With the integration of efficient catalytic factors such as Co-Nx-C–coordinated moieties, cobalt phosphide, graphitic carbon shells, as well as a supportive and conductive matrix, the as-prepared CoPx/Co-Nx-C@CNT exhibits high catalytic activity for ORR and OER in 0.1 M KOH, with an output positive onset potential of 0.92 V vs reversible hydrogen electrode (RHE) for ORR and a low potential of 1.69 V vs RHE at 10 mA cm−2 for OER. Remarkably, the assembled liquid and all-solid-state ZAB based on the as-prepared CoPx/Co-Nx-C@CNT showed superior cycling performance than the results of its Pt/C-based counterparts and some recently reported cobalt phosphide–based ZABs. The good flexibility and reversibility of the all-solid-state ZAB also verified its feasibility in applications such as wearable electronics.

Composite Metal Oxide‐Carbon Nanotube Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions

AbstractElectrocatalysts capable of performing both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) are key components for energy technologies including fuel cells, and water electrolysis. High‐performance catalysts based on earth abundant components are therefore a prime research theme. Herein, we report the development of a series of novel composites based on nanostructured iron‐doped CoWO4 particles electrically linked to conductive carbon nanotubes (CNTs) using a facile one‐pot hydrothermal method. It is shown that tuning of the iron content of Co1–xFexWO4−CNT composites (x=0–1) allows structural and reactivity control. The Co1–xFexWO4−CNT composites are active and stable OER and ORR electrocatalysts, and follow the Sabatier principle. For Co0.5Fe0.5WO4−CNT, minimum overpotentials of η=290 mV and low Tafel slopes of 42 mV dec−1 are achieved at a current density of j=10 mA cm−2 for OER, and the Co0.5Fe0.5WO4−CNT shows high catalytic activity for ORR with half‐wave potential of 0.74 V. As such, the composites show high potential as OER and ORR electrocatalysts, so that technologically viable, noble‐metal‐free, tunable electrocatalysts with relevance for alkaline water electrolysis can be designed from the bottom up.

Phase Diversity of Nickel Phosphides in Oxygen Reduction Catalysis

AbstractMotivated by the challenge to find a low‐cost catalyst with high activity for the oxygen reduction reaction (ORR), transition metal phosphides (TMPs) appear to be one of the most burgeoning alternatives to noble metal based electrocatalysts. In addition to the low cost, TMPs have numerous interesting features such as high conductivity and chemical stability. Since, the catalytic activity of TMPs is highly dependent on the metal/phosphorous ratio, herein, we report the investigation of nickel‐phosphide/carbon composites of different stoichiometries (NixPy/C) with tractable nickel phosphide phases as promising electrocatalyst for ORR under alkaline and acidic conditions. The NixPy/C composites are obtained by carbonization of a Ni‐struvite (NiNH4PO4 ⋅ H2O) coated phenol‐formaldehyde resin at different temperatures, resulting in variations of the formed NixPy phases. As expected, it is found that the electrocatalytic ORR performance of the NixPy/C composites highly depends on the predominant phase of nickel phosphide formed. The highest catalytic activity for ORR in alkaline as well as acidic media was found for the NixPy/C composite with the highest proportion of Ni2P as a predominant phase, obtained at 800 °C. For composites with NiP2 and Ni12P5 as the predominant phase, obtained at lower and higher temperatures, respectively, a lower catalytic activity was found.

Barrier-free Interface Electron Transfer on PtFe-Fe2C Janus-like Nanoparticles Boosts Oxygen Catalysis

Summary Designing highly efficient interface catalysts with new interface-enhancing mechanisms for the oxygen reduction reaction (ORR) in acid solution still remains a significant challenge. Here, we report a class of stable PtFe-Fe 2 C Janus-like nanoparticle (NP) interface catalysts with an unrevealed barrier-free interface electron-transfer property that greatly boosts ORR catalytic activity and stability. The PtFe-Fe 2 C Janus-like NPs showed much higher catalytic activity for ORR than either PtFe or Fe 2 C NPs in both acidic and alkaline electrolytes. Density functional theory simulations revealed that a barrier-free interface electron transfer on the interface of PtFe-Fe 2 C Janus-like NPs is the main factor in enhancing ORR activity. This interface electron-transfer property makes them the most active for ORR among all reported PtFe-based nanocatalysts. We further demonstrate that this barrier-free interface electron-transfer property can be readily generalized to other systems, such as the hydrogen evolution reaction and H 2 O 2 reduction electrocatalysis, to achieve better electrocatalytic enhancement.

Green Synthesis of Three-Dimensional Hybrid N-Doped ORR Electro-Catalysts Derived from Apricot Sap.

Rapid depletion of fossil fuel and increased energy demand has initiated a need for an alternative energy source to cater for the growing energy demand. Fuel cells are an enabling technology for the conversion of sustainable energy carriers (e.g., renewable hydrogen or bio-gas) into electrical power and heat. However, the hazardous raw materials and complicated experimental procedures used to produce electro-catalysts for the oxygen reduction reaction (ORR) in fuel cells has been a concern for the effective implementation of these catalysts. Therefore, environmentally friendly and low-cost oxygen reduction electro-catalysts synthesised from natural products are considered as an attractive alternative to currently used synthetic materials involving hazardous chemicals and waste. Herein, we describe a unique integrated oxygen reduction three-dimensional composite catalyst containing both nitrogen-doped carbon fibers (N-CF) and carbon microspheres (N-CMS) synthesised from apricot sap from an apricot tree. The synthesis was carried out via three-step process, including apricot sap resin preparation, hydrothermal treatment, and pyrolysis with a nitrogen precursor. The nitrogen-doped electro-catalysts synthesised were characterised by SEM, TEM, XRD, Raman, and BET techniques followed by electro-chemical testing for ORR catalysis activity. The obtained catalyst material shows high catalytic activity for ORR in the basic medium by facilitating the reaction via a four-electron transfer mechanism.

Facile Preparation and High Activity of TiO2 Nanotube Arrays toward Oxygen Reduction in Alkaline Media

Four TiO2 electrodes have been investigated for the oxygen reduction reaction (ORR) in 2 M NaOH. Anodic oxidation of Ti-foil was used as the final or the first step in the preparation of four electrodes based on TiO2: amorphous (Ti/AM-TiO2), two anatase forms (Ti/A400-TiO2 and Ti/A550-TiO2), and, finally, lithium-anatase form (Ti/A-Li0.25TiO2), all of them obtained as TiO2 nanotube arrays. The electrodes’ structure was examined by Raman spectroscopy. Cyclic voltammetry and chronoamperometry were employed for the study of kinetics of ORR. Ti/A-Li0.25TiO2 and Ti/A400-TiO2 electrodes showed the highest catalytic activity for ORR with the highest cathodic peak current density and the lowest Tafel slope values. Number of electrons exchanged during ORR at TiO2 electrodes was in the range from 2.3 to 2.8. Additionally, TiO2 electrodes showed good stability as well as acceptable selectivity in the presence of borohydride, methanol or ethanol as typical fuels for fuel cells. Thus this study introduces a simple, low-cost preparation route of TiO2-based electrodes with high efficiency for ORR for energy conversion and storage applications.

Facile synthesis of Co(OH)2 magnetic nanoflake deposited on reduced graphene oxide nanoflake as an efficient bi-functional electrocatalyst for oxygen evolution/reduction reactions in alkaline media

Here, a new hybrid material consisting Co(OH)2 magnetic nanoflake (MNF-Co(OH)2) grown on reduced graphene oxide nanoflake (MNF-Co(OH)2@RGONF) was synthesized as an excellent bi-functional electrocatalyst for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline media, by a facile and inexpensive technique. The onset potential, current density at 2.025V (vs. RHE), and Tafel slope of OER in 1.0molL−1 KOH for MNF-Co(OH)2@RGONF modified glassy carbon electrode (MNF-Co(OH)2@RGONF/GCE) were about 1.505mV (vs. RHE), 227mAcm−2 and 63mVdec−1, respectively. So that the current density and Tafel slope of this electrocatalyst are better than commercial IrO2. Furthermore, it was concluded that MNF-Co(OH)2@RGONF/GCE due to four-electron transfer mechanism, an overvoltage about 775mV (vs. RHE), better durability and stronger resistance rather than Pt/C (20%) at 0.436V in KOH 0.10molL−1 (with the addition of methanol to electrochemical cell), has a high catalytic activity for ORR.

Electrochemical Behavior of Manganese Oxide Nanoparticles for Oxygen Reduction Reaction Environment in PEM Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are well known as high energy conversion efficient, low noise, and pollution-free devices to produce power and electricity from hydrogen. The preparation of electrocatalysts with high activity, poison tolerance and durability, however, remains a major challenge for the commercialization of PEMFCs [1, 2]. Recently, metal oxides have become a hot topic due to their high stability; additionally some of these oxide can act as co-catalysts in conjunction with platinum. As a search of new metal oxides supports or electrocatalysts, manganese oxides were studied for their high catalytic activity among non-precious metal oxides. Therefore, they might be a good candidate for oxygen reduction reaction (ORR) which occurs in the PEMFCs. It has been shown that manganese oxide can decompose the hydrogen peroxide ions, formed as an intermediate phase during ORR, by a disproportionation mechanism which conducts the ORR to follow the complete reduction reaction with a four electron transfer per O2molecule [3]. Recently, Gorlin et al. [4] developed an active manganese oxide nanostructured thin film with high catalytic activity for ORR in alkaline media. The results showed that the manganese oxide thin film was more active for the ORR than that of carbon supported iridium and ruthenium nanoparticles with an activity very close to platinum nanoparticles supported on carbon. Furthermore, Roche et al. [5] indicated that manganese oxide catalyst, chemically deposited onto high specific surface area carbon, can have promising activity for the ORR in neutral pH solution. Although manganese oxide have shown favorable ORR activity in alkaline and neutral media, an extensive literature search did not yield any studies on ORR activity of manganese oxide catalysts conducted in acidic media of PEMFCs. Moreover, there appears to be no verdict on the stability of manganese oxides in PEMFC environment which further motivates the current study [6-9]. In this study, the effect of manganese oxidation state and particle size on ORR catalytic activity and stability of manganese oxide nanoparticles in PEMFC environment were investigated. The Mn2O3 and Mn3O4 nanoparticles were characterized by field emission scanning electron microscopy (FE-SEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction analysis (XRD). Thereafter, the ORR activity and durability of the manganese oxide were studied by cyclic voltammetry (CV) and accelerated stability test (AST) in N2 and O2 saturated 0.1 M HClO4 electrolyte using rotating disk electrode (RDE) technique. The results showed that the ORR activity of manganese oxide nanoparticles strongly depends on their metal oxidation states and particle size. The catalytic activity of manganese oxide nanoparticles increased with increase in the particle size. Moreover, the Mn2O3 nanoparticles catalyst with +3 oxidation state exhibited higher ORR activity compared to the Mn3O4 nanoparticles catalyst with mixed (+2, +3) oxidation state. The ORR activities of all catalysts, however, decreased after 5000 potential cycles from 0.6-1.0 V vs. RHE.

From melamine sponge towards 3D sulfur-doping carbon nitride as metal-free electrocatalysts for oxygen reduction reaction

Development of new and efficient metal-free electrocatalysts for replacing Pt to improve the sluggish kinetics of oxygen reduction reaction (ORR) is of great importance to emerging renewable energy technologies such as metal-air batteries and polymer electrolyte fuel cells. Herein, 3D sulfur-doping carbon nitride (S-CN) as a novel metal-free ORR electrocatalyst was synthesized by exploiting commercial melamine sponge as raw material. The sulfur atoms were doping on CN networks uniformly through numerous S–C bonds which can provide additional active sites. And it was found that the S-CN exhibited high catalytic activity for ORR in term of more positive onset potential, higher electron transfer number and higher cathodic density. This work provides a novel choice of metal-free ORR electrocatalysts and highlights the importance of sulfur-doping CN in metal-free ORR electrocatalysts.

A comparison study of oxygen reduction on the supported Pt, Pd, Au monolayer on WC(0001)

Using the first principles methods, the geometric and electronic structures of the metal monolayers supported on WC(0001) surfaces (MML/WC(0001) (M = Pt, Pd, and Au)) and their catalytic activity toward the oxygen reduction reaction (ORR) were comparatively studied. Both the kinetics and the density of states results demonstrated that the direct dissociation of O2 on all three MML/WC(0001) surfaces are almost impossible. Yet the barriers of the formation and dissociation of OOH on AuML/WC(0001) are much smaller than those on the PtML/WC(0001) and the PdML/WC(0001) surfaces, implying that the AuML/WC(0001) exhibits the highest catalytic activity for ORR via a combination of 2e− hydrogenation of O2 and 4e− dissociation of OOH. The rate-limiting step barrier of 0.83 eV for the hydrogenation of OH forming H2O is also comparable to that on the traditional Pt-based catalysts. The deactivation mechanism of PtML/WC(0001) and the performance of PdML/WC(0001) for ORR were identified. The present study is conductive to designing new efficient catalyst without using of the precious Pt for efficiently promoting ORR.

Template-free synthesis of hierarchical yolk-shell Co and N codoped porous carbon microspheres with enhanced performance for oxygen reduction reaction

The structures and compositions of materials have important influences on their performance. Herein, hierarchically structured yolk-shell Co and N codoped porous carbon microspheres (YS-Co/N-PCMs) have been successfully synthesized by using low-cost melamine, formaldehyde and cobalt acetate as raw materials via a facile template-free hydrothermal method and a subsequent pyrolysis. The formation process of the yolk-shell precursor is systematically investigated, involving a morphological evolution process from solid microspheres, ultrathin and wrinkled shells wrap, to yolk-shell structure formation. More importantly, the unique structure combines the favorable features towards oxygen reduction reaction (ORR), such as high surface area, sufficient Co-Nx and graphitic N active sites and suitable pore structures. As a result, the YS-Co/N-PCMs catalyst shows high catalytic activity for ORR in alkaline media for fuel cells, which not only outperforms commercial Pt-based catalysts in terms of resistance to methanol crossover and long-time stability, but is also better than many non-precious metal doped carbon-based catalysts reported previously. In addition, the YS-Co/N-PCMs catalyst also has high catalytic activity toward oxygen evolution reaction (OER). Therefore, the YS-Co/N-PCMs catalyst may serve as a promising alternative to Pt/C catalyst for ORR and OER in alkaline media.

Nitrogen‐Doped Annealed Nanodiamonds with Varied sp2/sp3 Ratio as Metal‐Free Electrocatalyst for the Oxygen Reduction Reaction

AbstractNitrogen‐doped nanocarbon materials as metal‐free electrocatalysts for the oxygen reduction reaction (ORR) have attracted great attention, owing to their catalytic activity close to Pt/C in alkaline media. Annealed nanodiamonds (AND) possess a special structure, because we can finely control the carbon sp2/sp3 ratio. This provides a good opportunity to study the relationship between ORR performance (activity and stability) and the special structure of AND. We have used nitrogen‐doped annealed nanodiamonds (NAND) with varied carbon sp2/sp3 ratio as metal‐free electrocatalysts for ORR in alkaline media. NAND showed higher catalytic activity for ORR with higher content of sp2 carbon and larger amount of nitrogen species present as pyridinic groups. More importantly, NAND displayed stability superior to other nitrogen‐doped nanocarbon materials as a result of transformation of carbon from sp3 to sp2 during electrochemical cycling.

Density Functional Theory Study of Oxygen Reduction Reaction Mechanism on Pt Alloy Catalysts

Power generation is one of the key challenges in the current century due to decreasing natural energy resources, population growth and the goal to reduction of carbon emissions. Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention for potential power generation technology especially for mobile application. However, PEMFC currently faces several challenges: high cost of Platinum group metals catalysts, the slow kinetics of oxygen reduction reaction (ORR) at the cathode and the durability of cathode catalysts in harsh acidic environment.Alloying Pt modifies the surface electronic structures and at the same time reduces the use of expensive Pt from the catalyst design. Recently, 3d-transition metals (TMs) alloyed Pt catalysts have been reported to show superior ORR activity as compared to pure Pt catalyst.1-5 For example, the ORR activity enhancement for bimetallic Pt3Ni(111) extended single crystal surface is reported to be 10 times (10´) higher than pure Pt(111) surface and 90´ higher than the commercial Pt/C nanoparticle catalysts.1 With these encouraging studies, catalysts design by controlling composition, sizes and shapes of Pt-based alloy nanoparticle are now under extensive investigations.Within this contribution, we employed an atomistic Monte Carlo (MC) simulation method to predict the equilibrium surface structures of Pt alloy surfaces. Based on the calculated surface composition profile, we built Pt surface-segregated surface slab models for Pt and Pt alloy catalysts. The possible ORR mechanism on the (111) and (100) surfaces of Pt and Pt alloys are explained by first-principles density functional theory (DFT) calculations. Our DFT computations predict that alloying Pt with transition metal alters the adsorption energetics of the ORR intermediates and changes the mechanism of ORR compared to pure Pt catalyst. The ORR on Pt and Pt alloy catalysts can proceed via one of the following ORR mechanisms: O2 dissociation, OOH dissociation or H2O2 dissociation ORR mechanism.Our calculations show that the ORR occurs via an OOH dissociation mechanism on the pure Pt(111) surface. In contrast, we predict that the ORR on Pt(100) surface proceeds via an O2 dissociation ORR mechanism. Our calculated activation energy for the rate determining step (RDS) on the Pt(111) surface (0.79 e V)3 is very similar to that (0.80 eV)4 on the Pt(100) surface. This suggests that the (111) and (100) facets of Pt nanocrystals show similar catalytic activity for ORR.We further studied the mechanism of ORR on Pt surface segregated modified Pt/TM(111) (TM =Fe, Co, Ni) and Pt3TM(111) (TM=Ti, V, Fe, Ni) surfaces. We found that the activation energy of the RDS on modified Pt/TM(111) and Pt3TM(111) surfaces is always smaller than that on the pure Pt(111) surface. Thus modified Pt/TM(111) and Pt3TM(111) surfaces have superior catalytic activity for ORR compared to the ORR activity of pure Pt(111) surface. The DFT calculations further predict that the most favorable ORR mechanism on the modified Pt(111) surfaces is H2O2 dissociation ORR mechanism. We found that Ni modified Pt/TM(111) and Pt3TM(111) surfaces show highest catalytic activity for ORR. Contrarily, our DFT calculations demonstrates that the catalytic activity of modified Pt/Ni(100) surface is very similar to that of pure Pt(100) surface. All these predictions are in excellent agreement with previous experimental measurements. Acknowledgements This work was funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant no. DE-FG02-09ER16093). We also acknowledge the research grant from the EERE program of the U.S. Department of Energy (Grant no. DE-AC02-06CH11357). References (1) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493-497.(2) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241-247.(3) Duan, Z. Y.; Wang, G. F. Phys. Chem. Chem. Phys. 2011, 13, 20178-20187.(4) Duan, Z. Y.; Wang, G. F. J. Phys. Chem. C 2013, 117, 6284−6292.(5) Kattel, S.; Duan, Z. Y.; Wang, G. F. J. Phys. Chem. C 2013, 117, 7107−7113.