Catalytic Activity For Oxygen Reduction Research Articles

An 2D Polymer Used As Ingredient of Fe/N/C Composite Towards Oxygen Reduction Catalyst In Acidic Medium.

AbstractCarbon‐based transition metal/nitrogen material is one of the most promising non‐precious metal catalysts for the oxygen reduction reaction. And N‐containing polymers are considered as a kind of excellent N precursor because they are more likely to form ordered and stable carbon‐based active layer during heat treatment. However, further research is still needed due to the inferior oxygen reduction catalytic activity compared with Pt in acidic condition. Herein, we design the Fe/N/C catalyst through synthesize a special nitrogen‐rich polymer poly‐hexaazatrinaphthylene as N source and subsequently calcine with the mixture of ferric chloride and Vulcan XC‐72. The physical results indicate that the oxygen reduction catalytic activity is mainly related to the amorphous carbon, pyridinic‐N, and Fe‐N4 coordination geometry. Electrochemical tests indicated that the sample pyrolyzed at 800°C presents relative high oxygen reduction activity in 0.5 mol L−1 H2SO4 via an efficient four‐electron‐dominant pathway.

Novel and highly efficient cathodes for Li-O2 batteries: 3D self-standing NiFe@NC-functionalized N-doped carbon nanonet derived from Prussian blue analogues/biomass composites

Traditional 3D self-standing electrodes for lithium-oxygen (Li-O2) batteries require a high cost and complex preparation process that restrict their development. Herein, discarded biomass (pomelo peel, PP) is used as raw material and in-situ grew NiFe-Prussian blue analogues on its surface. Then a novel 3D binder-free and self-standing NiFe@NC/PPC electrode is obtained through the carbonization process and directly used as a cathode in Li-O2 batteries.Compared with PPC (carbonized pomelo peel) and NiFe@NC/CP (NiFe@NC catalysts coated on carbon paper), NiFe@NC/PPC gathered the advantageous characteristics of i) efficient diffusion rates for O2/electrolyte and ii) good catalytic activity for oxygen reduction reaction/oxygen evolution reaction (ORR/OER). The as fabricated Li-O2 batteries, with NiFe@NC/PPC as cathode, are performed a relatively high specific capacity (13.79 mAh cm−2), a long cycle life (290 cycles) at a current density of 0.1 mA cm−2. Therefore, considering the convenience of synthesis method and the inexpensive raw materials, this is a feasible way to prepare high performance and low cost cathodes for Li-O2 batteries.

Synthesis of octahedral Pt-Ni-Ir yolk-shell nanoparticles and their catalysis in oxygen reduction and methanol oxidization under both acidic and alkaline conditions.

Fuel cells are expected to be one of the most promising alternatives to the increasingly scarce fossil fuels, and Pt is the most commonly used catalyst for anodic and cathodic electrochemical reactions. To realize large-scale commercialization, it is most urgent to improve the efficiency of Pt and reduce the cost. Here, we synthesized an octahedral Pt-Ni-Ir yolk-shell catalyst through stepwise co-deposition (SCD), surface-limited Pt deposition (SLPD) and Ni-coordinating etching (NCE) processes. Experimental studies showed that the catalytic activities of the as-prepared trimetal yolk-shell catalyst were several times higher than that of the commercial Pt/C towards oxygen reduction and methanol oxidization under both acidic and alkaline conditions. This work may be extended to designing other multimetallic functional materials with complex hierarchical nanostructures, which is conducive to greatly enhancing the performance.

Versatile synthesis of CuPt nanocatalysts for oxygen reduction reaction

The synthesis of catalysts in the lab is typically limited to low mass quantities, and it makes the large quantity production of catalysts a challenge. The High Energy Ball Milling technique allows for the processing of more massive amounts. In this work, we prepared CuPt nanoparticles, and their oxygen reduction catalytic activity was evaluated. The nanoparticles were prepared by galvanic displacement of Cu coming from a High Energy Ball Milling process. Alumina from the wear of the milling vials was present in the catalyst. Hydrodynamic curves from rotating-disk electrode measurements showed a catalytic activity seven times higher on the alloy than that observed on platinum-loaded carbon black. The catalytic activity was not negatively affected by the presence of the Al2O3 impurities.

Co3O4 nanoparticles on porous bio-carbon substrate as catalyst for oxygen reduction reaction

Abstract The synthesis of low-cost, environmentally-friendly, and stable non-noble metallic catalyst for oxygen reduction reaction (ORR) is quite important in electrochemical fields, for ORR is the essential cathodic process in fuel cells. Here, we report a simple and general method to fabricate Co3O4 nanoparticles supported on porous carbon using butterfly wings as template and carbon source. The as-synthesized catalyst shows excellent catalytic activity for oxygen reduction with its performance close to commercial 20 wt% Pt/C. Compared to free Co3O4 particles, the enhanced performance mainly comes from the synergistic effect of Co3O4 and bio-carbon substrate. The carbonized butterfly wing shows unique porous structure with both high surface area and good connectivity, providing a firm and conductive support for active oxide nanoparticles. This work is expected to provide new ideas for the development of other porous carbon-based ORR catalysts.

Systematic Control of Redox Properties and Oxygen Reduction Reactivity through Colloidal Ligand-Exchange Deposition of Pd on Au.

Core-shell nanoparticles of Au@Pd with precise submonolayer, monolayer, or multilayer structure were synthesized using ligand-exchange reactions of palladate ions onto colloidal Au nanocrystals. Decoupling the palladate adsorption step from the subsequent reduction enables excellent precision, uniformity, and tunability in the Pd shell thickness. The redox properties of the surface Pd are directly correlated to the thickness of the Pd shell with a >+200 mV shift in the PdO reduction potential for submonolayer Au@Pd nanoparticles compared to pure Pd. Using these precisely controlled core-shell materials, the oxygen reduction catalytic activity can be directly correlated to PdO reduction potential and Pd surface coverage on Au. When the Pd oxide reduction peak is shifted by +240 mV compared to pure Pd, a 50 mV reduction in overpotential and a 4-fold increase in kinetic current density for oxygen reduction are observed. Colloidal ligand-exchange synthesis may be particularly useful for noble metal core-shell catalysts as a strategy to subtly tune the electronic properties of surface atoms in order to lower overpotential and increase catalytic turnover.

(Invited) Influencing the Catalytic Activity for Oxygen Reduction and Evolution in Aqueous and Non-Aqueous Electrolytes: Support and Cations

The idea of a battery employing a lightweight metal anode and an oxygen cathode has inspired many electrochemists during the last decades. This is mainly due to the theoretically high specific energies, which would allow for surpassing some of the shortcomings of conventional lithium–ion batteries. However, one of the main issues to address is the stability of the metal anode in contact with the electrolyte and possibly oxygen. This is why the oxygen reduction in organic electrolytes has recently attracted much attention [1]. Organic electrolytes, unlike aqueous electrolytes, generally exhibit an increased compatibility with metal anodes like lithium or sodium, which are unstable in aqueous electrolytes due to their low redox potential. Although lithium is the most promising metal energy-wise, other alkaline and earth alkaline metals would still yield superior specific energies in comparison with lithium-ion batteries. This is why this work aims at elucidating the role of the metal cation on the ORR and OER in organic solvents. Previous DEMS (differential electrochemical mass spectrometry)-studies imply that there might be a charge-density related effect within the alkaline metals: Monovalent Cations with high charge densities seem to foster the formation of the corresponding peroxide [2]. However, recent observations using Ca2+ as a cation show that the picture is obviously more complicated when using the earth alkaline metals. Despite its low charge density, Ca2+ fosters the peroxide at gold electrodes [3]. Within the earth alkaline series, the (average) number of electrons transferred per oxygen molecule increased in the order: z(Ca2+)<z(Sr2+)<z(Ba2+)~z(Mg2+). Furthermore, the solubility and stability of the products is also significantly influenced by the cation. In aqueous electrolytes, oxides are the best bifunctional electrode materials. We have recently shown that the activity of silver for oxygen reduction is increased when Ag particles are in contact with Co3O4 particles. Also, the activity of Co3O4 for oxygen evolution is increased in such a mixed catalyst. [4] We will demonstrate here that to some extent such effects can also be observed when other support materials instead of silver are used, therefore, part of the effect may be due to conductivity effects. However, model experiments using smooth electrodes demonstrate that there is also a true synergistic effect between Ag and the Co-oxide. We will further demonstrate that this effect is also observed for various perovskites mixed with or deposited onto Ag. XPS analysis has shown that the presence of Ag+ in contact with Co3O4 facilitates the redox switching of Co3O4, which might be the reason for the enhanced catalytic activity of the mixture.[5] Using differential electrochemical mass spectrometry (DEMS) and 18O- isotope labeling it has been shown that the lattice oxygen is participating in the OER at oxides.[6-9] This procedure is applied to Co3O4 and the mixed catalyst as well. The active area thus determined will be compared to that estimated from BET data and a simple ball model. [1] K.M. Abraham, Z. Jiang, J. Electrochem. Soc. 143 (1996) 1-5. [2] C. Bondue, P. Reinsberg, A.A. Abd-El-Latif, H. Baltruschat, Phys. Chem. Chem. Phys. 17 (2015) 25593–25606. [3] P. Reinsberg, C.J. Bondue, H. Baltruschat, J. Phys. Chem. C 120 (2016) 22179-22185. [4] H.M.A. Amin, H. Baltruschat, D. Wittmaier, K.A. Friedrich, Electrochimica Acta 151 (2015) 332-339. [5] H.M. Amin, C.J. Bondue, S. Eswara, U. Kaiser, H. Baltruschat, Electrocatalysis 8 (2017) 540-553. [6] M. Wohlfahrt-Mehrens, J. Heitbaum, Journal of Electroanalytical Chemistry 237 (1987) 251-260. [7] S. Fierro, T. Nagel, H. Baltruschat, C. Comninellis, Electrochemistry Communications 9 (2007) 1969-1974. [8] K. Macounova, M. Makarova, P. Krtil, Electrochemistry Communications 11 (2009) 1865-1868. [9] H.M.A. Amin, H. Baltruschat, Phys. Chem. Chem. Phys. 19 (2017) 25527-25536.

Electronic coupling induced high performance of N, S-codoped graphene supported CoS2 nanoparticles for catalytic reduction and evolution of oxygen

A simple synthetic method is developed for the synthesis of CoS2/N, S-codoped graphene. The result shows the existence of a strong electronic coupling between CoS2 and N, S-codoped graphene. The pyrrolic and pyridinic type nitrogen and S in the form of C-S-C in N, S-codoped graphene are found to be the anchoring sites of the CoS2 nanoparticles. As a bifunctional catalyst, the CoS2/N, S-codoped graphene exhibits an oxygen reduction onset potential of 0.963 V vs. RHE and delivers an oxygen evolution overpotential of 393 mV at the current density of 10 mA cm−2. Its oxygen reduction and evolution catalytic activities are comparable to those of the Pt/C and the state-of-art RuO2/C, respectively. Most impressively, the CoS2/N, S-codoped graphene exhibits a potential gap of 771 mV. This value is lower than those of most bifuntional catalysts reported, clearly indicating its potential use as the bifunctional catalyst to replace the noble-metal based catalysts for practical applications. Additionally, our results also suggest a great importance to prepare a single pure phase CoS2 in improving the catalytic bifunctionality of the CoS2/N, S-codoped graphene. The primary Zn-air battery with CoS2/N, S-codoped graphene shows a higher discharge peak power density than that with Pt/C.

Influence of oxygen ion enrichment on optical, mechanical, and electrical properties of LSCF perovskite nanocomposite

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is a mixed ionic electronic conductor with excellent surface catalytic activity for oxygen reduction. This work demonstrated that introduction of pure oxygen ion conductor to LSCF particles can significantly influence in-plane electronic conduction at the surface of LSCF-samarium-doped ceria (SDC) composite. The composite functional layer was prepared by mixing 50 wt% SDC particles with LSCF particles obtained from glycine–nitrate process. Homogeneous LSCF-SDC composite layer deposited by screen printing on an SDC substrate has been studied with and without LSCF current-collecting layer (CCL). The microstructural, optical, Raman, mechanical and electrical properties, and interfacial polarization resistance (Rp) of the prepared powders were evaluated. Results revealed that addition of oxygen ion conductor SDC exerted negligible effect on the phase structure and specific surface area but significantly influenced the band gap, oxygen vacancies, and electrical conductivity of LSCF. SDC addition significantly increased area specific resistance (ASR) of LSCF from 0.138 Ω cm2 to 0.481 Ω cm2 at 800 °C, thereby blocking the conduction path among LSCF particles. Rp value of LSCF-SDC composite can be improved by more than six times by enlarging the in-plane electronic conduction with thin CCL. Electrochemical measurement revealed that LSCF CCL reduced the Rp value, resulting in the lowest ASR of 0.087 Ω cm2 at 800 °C for the LSCF–SDC composite.

Alginate derived Co3O4/Co nanoparticles decorated in N-doped porous carbon as an efficient bifunctional catalyst for oxygen evolution and reduction reactions

Fabricating high-performance and low-cost bifunctional electrocatalysts for both oxygen evolution and reduction reactions is vital for the practical application of renewable energy conversion. Herein, a hybrid of Co3O4/Co active species incorporated in N-doped porous carbon matrix, denoted as Co3O4/Co-NPC, has been prepared using a scalable and facile method involving the carbonization of alginate composite after a Co ion exchange with sustainable seaweed biomass. The 3D Co3O4/Co-NPC exhibits good bifunctional oxygen electrocatalytic performance in alkaline medium. The proposed catalyst gives the smaller onset potential (1.450 V vs. RHE) and overpotential (277 mV) at 10 mA cm−2 current density, indicating a superior oxygen evolution reaction activity to commercial IrO2 catalyst. It also performs the excellent oxygen reduction catalytic activity with larger onset potential (0.91 V vs. RHE), more positive half wave potential (0.806 V vs. RHE) and bigger diffusion-limiting current density (−5.86 mA cm−2 at 0.4 V). The Co3O4/Co-NPC hybrid experiences an almost ideal 4e process with better long-term stability and anti-methanol ability than commercial Pt/C catalyst in 0.1 M KOH. The prominent bifunctional activity of Co3O4/Co-NPC can be due to the synergistic effects of the well dispersed Co3O4/Co active species and N-doped porous carbon nanostructure.

Dicationic ionic liquid mediated fabrication of Au@Pt nanoparticles supported on reduced graphene oxide with highly catalytic activity for oxygen reduction and hydrogen evolution

Ionic liquids as templates or directing agents have attracted great attention for shaping-modulated synthesis of advanced nanomaterials. In this work, reduced graphene oxide supported uniform core-shell Au@Pt nanoparticles (Au@Pt NPs/rGO) were fabricated by a simple one-pot aqueous approach, using N-methylimidazolium-based dicationic ionic liquid (1,1-bis(3-methylimadazoilum-1-yl)butylene bromide, [C4(Mim)2]2Br) as the shape-directing agent. The morphology evolution, structural information and formation mechanism of Au@Pt NPs anchored on rGO were investigated by a series of characterization techniques. The obtained nanocomposites displayed superior electrocatalytic features toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) compared with commercial Pt/C catalyst. This approach provides a novel route for facile synthesis of nanocatalysts in fuel cells.

The plasma-induced formation of silver nanocrystals in aqueous solution and their catalytic activity for oxygen reduction

Ag nanocrystals with different architectures are synthesized using a submerged plasma discharge without the involvement of any chemicals. The Ag architecture relies on the electron density in the plasma that could enable the Ag ions to be reduced instantaneously to generate a large number of small Ag nanoparticles. With a low electron density of 7.1 × 10−22 m−3, the Ag nanowires with a corrugated structure induced by twinning and stacking faults are formed along the entire longitudinal 〈111〉 direction. However, with a high electron density 13.7 × 10−22 m−3, the Ag nanodendrites are constructed with a defect-free structure. Due to the unique structure composed of twins and stacking faults, the Ag nanowires show a specific current density that is 2.7 times higher than the Ag nanodendrites towards the oxygen reduction reaction. This work not only suggests a synthetic route to the formation of nanowires with structural defects but also offers a rational design of electrocatalysts with enhanced catalytic activity.

In Situ Self-Template Synthesis of Fe-N-Doped Double-Shelled Hollow Carbon Microspheres for Oxygen Reduction Reaction.

Herein, we reported a special Fe-N-doped double-shelled hollow carbon microsphere (Fe-N-DSC) which was prepared by a facile, in situ polymerization followed by pyrolysis. With porous ferroferric oxide (Fe3O4) hollow microspheres as the templates, where pyrrole monomers were dispersed around the outer surface and prefilled the interior space. By adding hydrochloric acid, Fe3+ ions were released to initiate polymerization of pyrrole on both the outer and inner surfaces of Fe3O4 microspheres until they were completely dissolved, resulting in the Fe-containing polypyrrole double-shelled hollow carbon microspheres (Fe-PPY-DSC). The Fe-PPY-DSC was then pyrolyzed to generate the Fe-N-DSC. The Fe3O4 hollow microspheres played trifunctional roles, i.e., the template to prepare a double-shelled hollow spherical structure, the initiator (i.e., Fe3+ ions) for the polymerization of pyrrole, and the Fe source for doping. The Fe-N-DSC exhibited a superior catalytic activity for oxygen reduction as comparable to commercial Pt/C catalysts in both alkaline and acidic media. The high catalytic performance was ascribed to the special porous double-shelled hollow spherical structure, which provided more active sites and was beneficial to a high-flux mass transportation.

DFT Prediction of the Catalytic Oxygen Reduction Activity and Poisoning-Tolerance Ability on a Class of Fe/S/C Catalysts

The detailed density functional theory calculations were performed to screen the oxygen reduction reaction (ORR) catalytic activity of several kinds of Fe−Sx/C catalysts such as Fe−S2/C, Fe−S2×2/C, Fe−S3/C, Fe−S4/C, and Fe−S6/C. The results suggest that only the Fe−S2/C structure could maintain its initial planar structure after the geometry optimization, indicating that it is most likely to have ORR catalytic ability. Further investigations find that the O2 molecule is firstly chemisorbed on the Fe−S2 active site with a stable side-on mode, and then is gradually reduced to H2O following a 4e− OOH dissociation pathway. Both thermodynamic adsorption energies of ORR intermediates and kinetic activation energies of proton transfers indicate that the current Fe−S2/C catalytic site possesses the comparable catalytic activity to that of precious Pt catalysts. Furthermore, compared with Pt(111) surface, it also has very excellent tolerance to some impurities such as sulfur compounds (SO2, H2S), carbon compounds (CO), and nitrogen compounds (NO, NH3).

Hierarchically Porous Co3C/Co-N-C/G Modified Graphitic Carbon: A Trifunctional Corrosion-Resistant Electrode for Oxygen Reduction, Hydrogen Evolution and Oxygen Evolution Reactions

Developing highly active and robust electrode catalysts for oxygen reduction reaction, hydrogen evolution reaction, and oxygen evolution reaction is critical for realizing efficient energy conversion and storage devices. In this work, we synthesize a hierarchically porous electrocatalyst (Co3C/Co-N-C/G) via a two-step process including hydrothermal and pyrolysis procedures. During the hydrothermal process, an acid-base reaction is induced to create abundant defects in the materials. Physicochemical characterizations of the catalyst suggest that the starting material of pyridine-3,4-dicarbonitrile leads to graphitic carbon structure, and stable Co-N-C and Co3C active centers are generated. Due to the synergistic effects among the components, the catalyst exhibits excellent catalytic activity for oxygen reduction and hydrogen evolution reactions, with performance comparable to those of the benchmark 20wt% Pt/C, as well as good catalytic activity for oxygen evolution reaction, with a performance rivaling that of RuO2. The high catalytic performance of Co3C/Co-N-C/G makes it a promising candidate for energy conversion and storage devices.

Nitrogen-doped carbon nanotubes with encapsulated Fe nanoparticles as efficient oxygen reduction catalyst for alkaline membrane direct ethanol fuel cells

Exploring low-cost and highly efficient non-precious metal electrocatalysts toward oxygen reduction reaction is crucial for the development of fuel cells. Herein, we report the synthesis of bamboo-like N-doped carbon nanotubes with encapsulated Fe-nanoparticles through high-temperature pyrolysis of multiple nitrogen complex consisting of benzoguanamine, cyanuric acid, and melamine. As-prepared catalyst exhibits high catalytic activity for oxygen reduction with onset potential of 1.10 V and half-wave potential of 0.93 V, as well as low H2O2 yield (<1%) in alkaline medium. The mass activity of the catalyst at 1.0 V (0.58 A g−1) can reach 43% of state-of-the-art commercial Pt/C. This catalyst also exhibits high durability and ethanol tolerance. When it was applied in alkaline membrane direct ethanol fuel cell, the peak power density could reach to 64 mW cm−2, indicating its attractive application prospect in fuel cells.

Electrochemical Study of (La0.6Sr0.4)0.99CoO3-δ Thin Film Microelectrodes

Solid Oxide Fuel/Electrolysis Cells (SOFC/SOEC, collectively termed SOC) are one of the most promising reversible energy conversion/storage technologies. Long term durability is required for such devices to become economically feasible. One approach to make SOCs more durable and at a lower cost is to decrease the operation temperature. However, lowering the operation temperature of SOCs has shown to be challenging due to the difficulty in finding suitable oxygen electrodes which have high catalytic activity for oxygen reduction and fast ionic transport. (La,Sr)CoO3-δ (LSC) is one of the promising cathode materials due to its high electronic and ionic conductivity as well as good catalytic activity for oxygen reduction at intermediate temperatures (500-700 °C)1. However, LSC is characterized by low chemical stability. Multiple degradation mechanisms are reported for LSC such as zirconate formation due to reactivity with YSZ (electrolyte) at operating temperature, decomposition at low pO2, SrO enrichment of and precipitation at the surface2 and Cr poisoning3 , 4. Several studies on the electrochemical properties of oxygen electrode materials are reported in the literature. Most of the studies are performed on porous electrodes with the purpose of having a realistic scenario; however we still lack fundamental understanding of the underlying degradation mechanism. Even though porous electrode studies have provided invaluable information about the degradation of the oxygen electrode, the deconvolution of geometrical effects from the intrinsic properties of the material is very difficult. Therefore, the presented work aims to study the oxygen reduction mechanism using geometrically well-defined dense model electrodes. Gd doped CeO2 (CGO) was deposited on polished single crystals YSZ (100) and on top of the CGO layer was deposited a (La0.6Sr0.4)0.99CoO3-δ(LSC40) using pulsed laser deposition (PLD). The thin CGO film (~100 nm) was deposited to avoid any reaction between the YSZ and LSC40 (250 nm). Subsequently, using photolithography and ion beam etching the microelectrode arrays with varying diameters (from 100 µm to 5 µm) were produced. Each sample has 4 macro-electrodes which were used as counter-electrode while performing electrochemical measurements. To observe the effect of temperature on the film microstructure and chemistry one sample was heat treated for 16 hours. SEM images, AFM and ToF-SIMS reveal similar behavior for both heat treated and as-deposited films. ToF-SIMS depth profiling reveals a Sr and Co rich surface compared to the bulk of the LSC40 for both samples. The difference between the two samples are in the distribution of common impurities, such as silica. After the heat treatment, the Si signal is higher in the LSC/CGO and CGO/YSZ interfaces (See Figure 1 a-b)). The electrochemical measurements were recorded in a Controlled Atmosphere High Temperature Scanning Probe Microscope (CAHT-SPM)5which can reach temperatures up to 850 °C. The impedance spectra reveal a high frequency intercept which is related to the conductivity of the electrolyte, a small arc at high/middle frequencies originating from the electrode electrolyte interface, and finally a low frequency arc which is related to the air electrode interface. The evolution of the impedance spectra with changing temperature, atmosphere, polarization and microelectrode size combined with surface chemical analysis and imaging techniques reveals information about the underlying degradation mechanism of LSC electrodes. 1. Adler, S. B. Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes. Chem. Rev. 104,4791–4843 (2004). 2. Lee, W., Han, J. W., Chen, Y., Cai, Z. & Yildiz, B. Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites. J. Am. Chem. Soc. 135,7909–7925 (2013). 3. Zhao, L., Drennan, J., Kong, C., Amarasinghe, S. & Jiang, S. P. Insight into Surface Segregation and Chromium Deposition on La0.6Sr0.4Co0.2Fe0.8O3-δ Cathodes of Solid Oxide Fuel Cells. J. Mater. Chem. A 2,11114–11123 (2014). 4. Fergus, J. W. Effect of Cathode and Electrolyte Transport Properties on Chromium Poisoning in Solid Oxide Fuel Cells. Int. J. Hydrogen Energy 32,3664–3671 (2007). 5. Hansen, K. V., Wu, Y., Jacobsen, T., Mogensen, M. B. & Theil Kuhn, L. Improved Controlled Atmosphere High Temperature Scanning Probe Microscope. Rev. Sci. Instrum. 84, 73701 (2013). Figure 1

Imidazoles-Derived PGM-Free Cathode Catalysts for Oxygen Reduction Reaction

Conventionally platinum based catalysts are most commonly used cathodic catalysts in fuel cells. The high cost of platinum is a serious limiting factor for the widespread deployment of fuel cell technology. Fe-N-C materials have been explored to be some of the best performing substitutes. However the cost of synthesis, activity and durability are issues that still need to be addressed. In this work we explore the use of novel synthesis techniques with small organic molecules to synthesize Fe-N-C catalysts. Hydrothermal and pyrolysis synthesis strategies are used on templated and template free precursors to obtain the desired catalytic materials. Benzimidazole carboxylic acid is treated with a dual templating protocol that involves sodium salt formation and sacrificial silica hard template. The process then involves high temperature pyrolysis followed by template removal to yield the final product. A highly active and stable Fe-N-C electrocatalyst possessing a unique 3D porous graphene “nano-stack” morphology is thus synthesized. Control experiments with single template were performed to prove the advantage of the dual template. The oxygen reduction catalytic activity of these benzimidazole derived catalysts in alkaline electrolyte is studied rotating disk electrode methods. The dual templated catalyst shows an improvement over the commercial Pt/C catalyst along with excellent stability (as performed under the Department of Energy- Durability Working Group protocols). Furthermore, template-free synthesis of an Fe-N-C ORR catalyst is achieved by the direct pyrolysis of ethylenediaminetetraacetic acid ferric sodium salt. Detailed physical characterization of the catalyst is performed by surface area measurement, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. Rotating Ring Disk Electrode measurements. A detailed analysis of the effects of synthesis temperature on graphitization, surface area and their concurrent effects on the catalytic performance is brought forth in this work. Finally, we also establish a novel hydrothermal-pyrolysis approach to generate an active Fe-N-C catalyst from molecules like glucose and methyl-imidazole. The process simultaneously involves formation of metal-nitrogen bonds and partial carbonization resulting in in-situ active site formation. The glucose-imidazole catalysts termed as GLU-IMID-C exhibit good ORR activity in acidic and alkaline electrolyte. We perform detailed structural analysis of these catalysts using such techniques as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET surface area analysis and Transmission Electron Microscopy (TEM). The electrocatalytic behavior of the catalysts towards oxygen reduction is studied separately in acidic and alkaline electrolytes by using rotating ring disk electrode measurements. A general precursor-structure-performance relationship of these catalysts and their individual performance trends in both electrolytes has been established in this work.

Role of Local Carbon Structure Surrounding FeN4Sites in Boosting the Catalytic Activity for Oxygen Reduction

Development of effective nonprecious metal and nitrogen codoped carbon catalysts for the oxygen reduction reaction (ORR) requires a fundamental understanding of the mechanisms underlying their catalytic activity. In this study, we employed the first-principles density functional theory calculations to predict some key parameters (such as activation energy for O–O bond breaking and free-energy evolution as a function of electrode potential) of ORR on three FeN4-type active sites with different local carbon structures. We find that the FeN4 site surrounded by eight carbon atoms and at the edge of micropores has the lowest activation energy (about 0.20 eV) for O–O bond breaking among the three FeN4-type active sites for promoting a direct four-electron ORR. Consequently, our computational results suggest that introduction of micropores in the nonprecious metal catalysts could enhance their catalytic activity for ORR through facilitating the formation of FeN4–C8 active sites with high specific activity.

Solvothermal co-gelation synthesis of N-doped three-dimensional open macro/mesoporous carbon as efficient electrocatalyst for oxygen reduction reaction

A facile solvothermal method is developed to synthesize N-doped 3D bicontinuous macro/mesoporous carbon through co-gelation of in-situ polymerized N-enriched resin together with in-situ formed colloidal silica packing domains. The resultant material possesses abundant pyridinic/graphitic N, respectable surface area, and smooth pore channels (interconnected macropores with open mesopores ca. 16nm). Due to the combined contributions of above features, it shows high oxygen reduction catalytic activity with excellent durability and tolerance to methanol crossover effect.