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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

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

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

A PtCu electrocatalyst was synthesized using a galvanic displacement route obtaining nanoparticles with a semi-spherical morphology and an average size of 4 nm, supported on carbon black (Vulcan). The crystallographic characterization by X-ray diffraction showed a certain degree of Pt-Cu alloying. The electrocatalytic activity of the prepared electrocatalyst for the electro-oxidation of ethanol in alkaline media was investigated. A 2-fold increase of the peak current density and a negative shift of the potential were recorded in half-cell experiments for the bimetallic catalyst compared to a commercial Pt/C. The presence of Cu promotes ethanol oxidation in alkaline electrolyte by hindering the Pt-H adsorption at low overpotentials. Additionally, the PtCu electrocatalyst was used as anode in an anion-exchange-membrane direct ethanol fuel cell (AEM-DEFC) exhibiting about 2-fold higher power density than the benchmark Pt/C.

This review paper presents the most recent research progress on carbon-based composite electrocatalysts for the oxygen evolution reaction (OER), which are of interest for application in low temperature water electrolyzers for hydrogen production. The reviewed materials are primarily investigated as active and stable replacements aimed at lowering the cost of the metal electrocatalysts in liquid alkaline electrolyzers as well as potential electrocatalysts for an emerging technology like alkaline exchange membrane (AEM) electrolyzers. Low temperature electrolyzer technologies are first briefly introduced and the challenges thereof are presented. The non-carbon electrocatalysts are briefly overviewed, with an emphasis on the modes of action of different active phases. The main part of the review focuses on the role of carbon–metal compound active phase interfaces with an emphasis on the synergistic and additive effects. The procedures of carbon oxidative pretreatment and an overview of metal-free carbon catalysts for OER are presented. Then, the successful synthesis protocols of composite materials are presented with a discussion on the specific catalytic activity of carbon composites with metal hydroxides/oxyhydroxides/oxides, chalcogenides, nitrides and phosphides. Finally, a summary and outlook on carbon-based composites for low temperature water electrolysis are presented.

9 páginas, 1 tabla, 8 figuras. Carbon nanofibers (CNFs) characterized by different mean diameter, BET surface area, pore volume and crystallinity were prepared and studied as supports for PtRu nanoparticles to investigate the influence of the support characteristics on the performance for the electrooxidation of alcohols. A modified microemulsion procedure was used to deposit the metal nanoparticles minimizing the effect of the support on the catalyst particle size. PtRu nanoparticles of ca. 2 nm size were obtained despite the relatively low surface area of CNFs (95-185 m2 g-1) with a good distribution on the surface as confirmed by TEM micrographs and the high values of the electrochemically active surface areas (110-140 m2 g-1) determined by electrochemical CO stripping. The most appropriate PtRu dispersion was achieved for those carbon nanofibers showing the best compromise in terms of BET surface area and graphicity. A cross-analysis of the supports physico-chemical properties, ECSA and mass activity for the methanol and ethanol oxidation reactions suggests that both PtRu dispersion and electronic properties as determined by the effect of CNF crystallinity play a significant role in determining the electrocatalytic activity. Different electrocatalytic activity behavior with CNF properties were found for methanol and ethanol. Methanol oxidation is favored using highly crystalline CNFs as PtRu support, despite their low surface area, whereas ethanol oxidation is hindered by diffusional problems when using highly graphitic CNFs due to their low pore volume, so the activity is maximized supporting PtRu on CNFs with a more accessible porosity. The authors wish to thank FEDER and the Spanish Ministry of Economy and Competitiveness for the financial support to projects CTQ2011-28913-C02-01 and -02. Peer reviewed