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9 figures, 5 tables.-- Supplementary information available Active and durable electrocatalysts for the oxygen evolution reaction (OER), capable of replacing noble metal catalysts, are required to develop efficient and competitive devices within the frame of the water electrolysis technology for hydrogen production. In this work, we have investigated tantalum based catalysts supported on carbon nanofibers (CNF) for the first time. The effect of CNF characteristics and the catalyst annealing temperature on the electrochemical response for the OER have been analyzed in alkaline environment using a rotating ring disc electrode (RRDE). The best OER activity and oxygen efficiency were found with a highly graphitic CNF, despite its lower surface area, synthesized at 700 °C, and upon a catalyst annealing temperature of 800 °C. The ordering degree of carbon nanofibers favors the production of oxygen in combination with a low oxygen content in tantalum oxides. The most active catalyst exhibited also an excellent durability. The authors want to thank the Ministerio de Economía, Industria y Competitividad (MICINN) and FEDER for the received funding in the project of reference ENE2017-83976-C2-1-R, and to the Gobierno de Aragón (DGA) for the funding to Grupo de Investigación Conversión de Combustibles (T06_17R). J.C. Ruiz-Cornejo acknowledges DGA for his PhD grant. D. Sebastián acknowledges the MICINN for the Ramón y Cajal research contract (RyC-2016-20944). Peer reviewed

Carbon xerogels represent nowadays an outstanding alternative to carbon blacks for the preparation of efficient fuel cell electrocatalysts, due to their easily tunable and well developed mesoporous structure. To further improve both activity and durability of Pt/C catalysts, the introduction of heteroatoms (such as O, N, S, P, B, etc.) in the structure of carbon materials has been proposed. In the present work, highly mesoporous carbon xerogels (CXGs) have been subjected to a sulfurization process with elemental sulfur. The insertion of S into the carbon matrix does not compromise their mesoporous structure. Pt catalysts supported on sulfurized carbon xerogels show enhanced catalytic activity towards both the methanol electro-oxidation and the oxygen electro-reduction reactions, exceeding not only the performance of the catalyst supported on the bare xerogel, but also of the catalyst supported on a commercial carbon black. The sulfurization treatment is also effective in improving the resistance of Pt/CXG catalysts towards corrosion phenomena occurring at the fuel cell cathode. The authors wish to thank the Spanish Ministry of Economy and Competitiveness (Secretaría de Estado de I+D+I) and FEDER for financial support under the project CTQ2011-28913-C02-01. Peer reviewed

9 páginas.- 1 tabla.- 18 figuras. Several graphite-based electrodes are investigated for vanadium flow battery applications. These materials are characterized both as-received and after chemical or electrochemical treatments in order to vary the content of oxygen functional groups on the electrode surface. The surface properties of the samples are investigated by X-ray photoelectron spectroscopy. Electrochemical performance is evaluated by cyclic voltammetry and electrochemical impedance spectroscopy measurements in a three electrode half-cell. The chemical treatment with HNO3 causes a cleaning of the electrode surface from adsorbed oxygen species or labile bonded functional groups in highly graphitic samples. Whereas, carbonaceous materials characterized by smaller graphitic domains or a higher degree of amorphous carbon show an increase of oxygen functional groups upon chemical and electrochemical pre-treatments. In both cases, an increase of oxygen species content on the surface above 5% causes a decrease of electrochemical performance for the redox battery determined by an increase of ohmic and charge transfer resistance Authors from CNR-ITAE acknowledge the financial support from “Ministero dello Sviluppo Economico – Accordo di Programma MSE-CNR per la Ricerca del Sistema elettrico Nazionale”. Peer reviewed

The low oxidation kinetics of alcohols and the need for expensive platinum group metals are still some of the main drawbacks for the commercialization of energy efficient direct alcohol fuel cells. In this work, we investigate the influence of nitrogen doping of ordered mesoporous carbon (CMK) as support on the electrochemical activity of PtRu nanoparticles. Nitrogen doping procedures involve the utilization of pyrrole as both nitrogen and carbon precursor by means of a templating method using mesoporous silica. This method allows obtaining carbon supports with up to 14 wt. % nitrogen, with an effective introduction of pyridinic, pyrrolic and quaternary nitrogen. PtRu nanoparticles were deposited by sodium formate reduction method. The presence of nitrogen mainly influences the Pt:Ru atomic ratio at the near surface, passing from 50:50 on the bare (un-doped) CMK to 70:30 for the N-doped CMK catalyst. The electroactivity towards the methanol oxidation reaction (MOR) was evaluated in acid and alkaline electrolytes. The presence of nitrogen in the support favors a faster oxidation of methanol due to the enrichment of Pt at the near surface together with an increase of the intrinsic activity of PtRu nanoparticles.

Developing efficient, durable, and low cost catalysts based on earth-abundant elements for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for renewable energy conversion and energy storage devices. We report herein a simple one-pot procedure for the synthesis of non-precious metals N-doped graphene composites employing urea as nitrogen source, and their application as bifunctional catalysts for both the ORR and OER in alkaline environment. In this study, the effects of the addition of Ti and Co on the structure and performance of the N-doped graphene composites are investigated. The incorporation of Ti leads to a composite with both anatase and rutile TiO2 crystalline phases as well as Ti3+ species stabilized upon hybridization with N-doped reduced graphene oxide. The ORR onset potential for the Ti-based composite is 0.85 V (vs. RHE) and the number of electrons transferred is 3.5, showing superior stability than Pt/C after accelerated potential cycling in alkaline solution. However, this composite shows low activity and stability for the OER. On the other hand, the composite consisting of metallic Co and Co3O4 nanocrystals grown on N-doped reduced graphene oxide exhibits the best performance as bifunctional catalyst, with ORR and OER onset potentials of 0.95 V and 1.51 V (vs. RHE), respectively, and a number of electrons transferred of 3.6 (ORR). The results reveal the presence of important structural features such as metallic Co as the predominant crystalline component, amorphous Co3O4 phase and the coordination of Co-N-doped graphene. All of them seem to be fundamental for the high activity and stability towards ORR and OER. Authors acknowledge financial support given by Spanish Ministry of Economy and Competitiveness (MINECO) through projects ENE2014-52158-C2-1-R and 2-R (co-founded by FEDER). J. M. Luque and G. Lemes also thank MINECO and Aragon Government, respectively, for their Ph.D. grants. Peer reviewed

In recent decades, significant efforts have been focused on the direct electrochemical oxidation of alcohol and hydrocarbon fuels. Organic liquid fuels are characterized by high energy density and the electromotive force associated with their electrochemical combustion to CO2 is comparable to that of hydrogen combustion to water. Among the liquid organic fuels, methanol and ethanol have promising characteristics in terms of reactivity at low temperatures, storage and handling. Compared with ethanol, methanol has the significant advantage of faster reaction kinetics and higher selectivity to CO2 formation for the electrochemical oxidation process. Highly dispersed carbon-supported bimetallic PtRu and PtSn alloy catalysts are widely recognized among the most performing anode formulations for these processes. Alloying Pt with Ru and Sn promotes oxidation of methanol and ethanol by the adsorption of OH species at considerably lower overpotentials and, thus, favoring the occurrence of a bifunctional mechanism. Furthermore, the electronic effect caused by a second metal on the neighboring Pt atoms affects the strength of CO adsorption on the catalyst surface. This causes a decrease of the coverage of poisoning CO intermediate species. Catalysts characterized by a high degree of alloying and metallic behavior on the surface appear to be very active towards methanol oxidation. However, beside the alloyed catalysts, noble metal oxides (IrOx, RuOx) and valve metal oxides (SnOx, TiOx and VOx) can be suitable promoters for methanol and ethanol oxidation in acidic environment. An effective use of such oxide promoters in combination with the alloy catalysts can provide a multifunctional catalytic system. Recent studies carried out in our laboratory have shown that IrOx can give rise to a significant promoting effect, even larger than that of RuOx, both in the case of methanol and ethanol oxidation. Whereas, the electrocatalytic enhancement produced by the valve metal oxides is generally lower than IrOx and RuOx but well evident. These results are interpreted in terms of the different water displacement mechanisms for the various oxides and the related effects on adsorbed CO removal. The effect of temperature is also discussed with reference to the coverage of adsorbed methanolic residues or change in selectivity in the case of ethanol electro-oxidation. For the ethanol oxidation process, anode catalyst selectivity towards CO2, acetic acid and acetaldehyde reaction products is discussed in relation to the alloy and oxide content in the catalyst. In particular, SnOx species on the surface of Sn-rich Pt-Sn-based electrocatalyst appear to assist the further oxidation of ethanolic adsorbates, leading to larger yields of acetic acid and CO2. In addition to the enhancement of reaction rates, there is an effect of the promoter on the stability of the bimetallic alloy as evidenced by accelerated stress tests. All these evidences seem to indicate that a multifunctional catalyst may represent a valid route to enhance performance and reliability of methanol and ethanol electro-oxidation processes at low temperature.

Xerogel–nanofiber carbon composites (XNCCs) have been easily synthesized by using a Ni catalyst supported on carbon xerogel (CXG), growing randomly oriented carbon nanofibers (CNFs) within the coralline-like structure of the xerogel (CXG). This novel composite combines the advantages of xerogel and fiber nanostructures. The interactions between these phases as well as their effect as a support on Pt electrocatalysts for the oxygen reduction reaction (ORR) have been investigated. Platinum catalysts supported on different XNCCs (varying in terms of CXG and CNF contents) as well as on bare CXG and CNFs have been synthesized using a microemulsion route. They have been characterized in terms of structure, morphology and porosity and investigated for the ORR in a half-cell configuration. The catalyst supported on the XNCC with a 44% CNF content shows the best electrochemical behavior. This catalyst formulation leads to a catalytic activity 5 times higher than that obtained on a Vulcan-based catalyst at low overpotential and 2.5 times higher at large overpotential. Accelerated degradation tests also show better stability for the composite support-based catalyst. Compared to bare CNF and CXG supports, a stabilization effect is envisaged by the presence of highly graphitic CNFs within the composite structure. CNR-ITAE authors acknowledge the financial support through the PRIN 2010-11 project “Advanced nanocomposite membranes and innovative electrocatalysts for durable polymer electrolyte membrane fuel cells (NAMED-PEM)”. CSIC-ICB authors gratefully acknowledge financial support given by the Ministry of Economy and Competitiveness through the Project CTQ2011-28913-CO2-01. Peer reviewed