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International audience; Nanostructured electrocatalysts for microbial fuel cell air-cathodes were obtained via use of conductive carbon blacks for the synthesis of high performing 3D conductive networks. We used two commercially available nanocarbons, Black Pearls 2000 and multiwalled carbon nanotubes, as conductive scaffolds for the synthesis of nanocomposite electrodes by combining: a hydrothermally carbonized resin, a sacrificial polymeric template, a nitrogenated organic precursor and iron centers. The resulting materials are micro-mesoporous, possess high specific surface area and display N-sites (N/C of 3-5 at%) and Fe-centers (Fe/C

[EN] In this work, we report the synthesis of Fe/N/C electrocatalysts using triazine based porous organic polymers as precursors. Iron-doped triazine porous organic polymers were obtained by in situ polymerization of iron pre- cursor and 1,2- or 1,4- dicyanobenzene (DCB). In order to obtain the actual catalyst, the polymer obtained was subjected to thermal treatment under NH3. The catalysts obtained exhibit activity and durability for the oxygen reduction reaction in acid electrolyte. Thorough characterization of the catalysts reveal the formation of several types of iron species, including metallic iron, iron carbides and Fe-Nx moieties. The latter species is the main responsible for the high activity measured for the oxygen reduction reaction in acid electrolyte. 1,2-DCB results in more active catalysts than 1,4-DCB due to the higher fraction of FeNx ensembles in the former, probably because vicinal positions of N-bearing groups are more prone to coordinate Fe atoms., This work was supported by PEGASUS project, funded by the European Union’s Horizon 2020 Programme (call JTI-FCH-2017-1) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 779550, and by project ENE2016-77055-C3-3-R from the Spanish Ministry of Economy and Competitiveness (MINECO). We acknowledge the Spanish CRG BM25-SpLine, MINECO, The Spanish National Research Council (CSIC) and The European Synchrotron (The ESRF) for provision of synchrotron radiation facilities.

Summary Metal oxide electrocatalysts with minimal or no reliance on precious metals can display outstanding activity in oxygen catalysis and thus hold promise as materials for sustainable energy storage/conversion technologies. We review recent progress in the preparation of oxides for bifunctional oxygen reduction/evolution applications (ORR/OER) and include a summary of benchmark performances achieved over the past 3 years. Parallel and complementary progress on development of activity descriptors for the OER in oxides is also covered; special attention is dedicated to synergistic effects and the importance of support choice and oxide/support interactions to maximize activity and ensure stability. Finally, exciting mechanistic insights on OER and bifunctional ORR/OER oxide catalysis gained from recent in situ and operando X-ray experiments from the recent literature are discussed.

The development and commercialization of direct methanol fuel cells (DMFCs) as energy conversion devices remains a challenge despite their advantages in terms of energy density and energy-conversion efficiency. The bottleneck for the development of DMFCs is mainly caused by the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode of fuel cells, and the effect of the so-called methanol crossover in state-of-the-art Pt/C electrocatalysts. Herein, we report for the first time an easily scalable continuous flow method based on ultraspray pyrolysis (USP) for the preparation of Pt nanoparticles directly embedded on highly porous carbon spheres. A study on the effect that post-synthesis treatment procedures have on the level of graphitization and catalytic properties is described. Use of USP results in a substantial reduction of the final Pt content with respect to typical Pt/C electrocatalysts, while yielding also excellent durability and tolerance to methanol crossover under acidic conditions. These results demonstrate that the USP method reported herein is a good candidate for its use in the preparation of ORR catalysts in commercial applications.

Porous manganese carbonate was obtained via solvothermal synthesis using ethanol and urea. The manganese carbonate was subsequently used as a precursor to synthesise mesoporous manganese oxides via thermal treatments at three various temperatures. X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) results shows that γ-MnO2 is synthesised at 380 and 450 °C while Mn2O3 is produced at the annealing temperature of 575 °C. X-ray absorption spectra show that γ-MnO2 converts completely to Mn2O3 after annealing over the 450–575 °C range. The oxides obtained at 380 °C and 450 °C possess extremely high specific surface area, which is of interest for catalytic applications. The oxides were investigated as electrocatalysts for the oxygen evolution reaction; the oxide prepared at the lowest annealing temperature was found to be the optimum catalyst with an overpotential of 427 ± 10 mV at a current density of 10 mA cm−2, normalised by the geometric area. The improved catalytic activity was related to the presence of defect-rich and highly porous manganese dioxide at the lowest annealing temperature.

Metal-free nitrogenated amorphous carbon electrodes were synthesised via dc plasma magnetron sputtering and post-deposition annealing at different temperatures. The electrocatalytic activity of the electrodes towards the oxygen reduction reaction (ORR) was studied as a function of pH using cyclic voltammetry with a rotating disk electrode. The trends in onset potential were correlated to the carbon nanostructure and chemical composition of the electrodes as determined via Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Results suggest that: 1) the ORR activity in acidic conditions is strongly correlated to the concentration of pyridinic nitrogen sites. 2) At high pH, the presence of graphitic nitrogen sites and a graphitized carbon scaffold are the strongest predictors of high ORR onsets, while pyridinic nitrogen site density does not correlate to ORR activity. An inversion region where pyridine-mediated activity competes with graphitic-N mediated activity is identified in the pH region close to the value of pKa of the pyridinium cation. The onset of the ORR is therefore determined by the activity of different sites as a function of pH and evidence for distinct reduction reaction pathways emerges from these results.

Metal-free carbon electrodes with well-defined composition and smooth topography are prepared via sputter deposition followed by thermal treatment with inert and reactive gases. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy show that three carbons of similar N/C content that differ in N-site composition are thus prepared: an electrode consisting of almost exclusively graphitic-N (NG ), an electrode with predominantly pyridinic-N (NP ), and one with ≈1:1 NG :NP composition. These materials are used as model systems to investigate the activity of N-doped carbons in the oxygen reduction reaction (ORR) using voltammetry. Results show that selectivity toward 4e-reduction of O2 is strongly influenced by the NG /NP site composition, with the material possessing nearly uniform NG /NP composition being the only one yielding a 4e-reduction. Computational studies on model graphene clusters are carried out to elucidate the effect of N-site homogeneity on the reaction pathway. Calculations show that for pure NG -doping or NP -doping of model graphene clusters, adsorption of hydroperoxide and hydroperoxyl radical intermediates, respectively, is weak, thus favoring desorption prior to complete 4e-reduction to hydroxide. Clusters with mixed NG /NP sites display synergistic effects, suggesting that co-presence of these sites improves activity and selectivity by achieving high theoretical reduction potentials while facilitating retention of intermediates.

The optimization of the synthesis of Fe/N/G catalysts for the oxygen reduction reaction (ORR) has been investigated. The synthesis of Fe/N/G comprises a solid state reaction of the precursors in a planetary ballmilling followed by a thermal treatment in ammonia/nitrogen atmosphere. First, the influence of the concentration of NH 3 during the pyrolysis step was studied. The results obtained show that the influence of the NH 3 concentration during the pyrolysis step on ORR activity is more critical in acid medium. Next, the effect of the nature of the iron precursor in the structure and catalytic performance of the catalysts obtained was evaluated. Representative iron precursors containing Fe atoms in different oxidation states and containing or not heteroatoms (N or S) into their structure have been studied. The catalyst displaying the highest activity for the ORR in alkaline and acid electrolyte is the one obtained when iron phthalocyanine was used as iron precursor.

Fe/N/C catalysts are very active for the oxygen reduction reaction (ORR); however, the nature of the active site(s) is not fully understood. In this work, we study the performance of different types of N/C and Fe/N/C catalysts for the ORR, and the effect of the addition of NaCN. Phthalocyanine and graphene have been studied as model metal-free catalysts for the ORR. Fe-phthalocyanine (FePhcy), Fe-phthalocyanine dispersed in graphene (FePhcy/G) and Fe/N/G, have been used as model Fe-containing catalysts. FePhcy and FePhcy/G only contain Fe atoms coordinated to 4 nitrogen atoms. On the other hand, different species such as Fe-Nx and Fe3C coexist in Fe/N/G. In addition, C C ensembles are present in the graphene present in FePhcy/G and Fe/N/G. The ORR activity is characteristic of each catalyst, being the highest for the catalysts containing FeN4 ensembles. The addition of CN− results in the selective poisoning of the Fe-containing sites but it does not suppress the ORR activity of the Graphene containing samples. In-situ infrared spectroscopy studies during the ORR reveal that CN− poisoning of the Fe sites is reversible, desorbing at potentials less positive than ca. 600 mV. As a consequence, the ORR activity of the Fe-containing sites is recovered gradually.

Graphene-like (G), multiwalled carbon nanotubes (CNTs) and active carbon (AC) have been used as carbon matrix for the synthesis of Fe/N/C catalysts for the oxygen reduction reaction. A thorough physicochemical characterization of the electrocatalysts, including X-ray photoelectronic and X-ray absorption spectroscopies, reveal that the formation of Fe/Nx ensembles is favored when the graphene-like or the CNTs are used as the carbon matrix. As a result, the catalyst prepared with the graphene matrix (Fe/N/G) records the highest activity for the ORR in the series of 3.1 A g −1 at 0.9 V. This very high ORR activity positions these catalysts as a realistic alternative to replace Pt/C cathodes in alkaline fuel cells. Moreover, using graphene as the carbon matrix endows the catalyst with very high stability during the ORR showing stable catalytic performance for the ORR even after being subjected to severe treatments of 3000 cycles up to 1.4 V. In situ IRRA spectra demonstrate that such a high stability for the ORR relates to the excellent resistance against corrosion of the graphene-based catalyst.

This work demonstrates a stand-alone power source that integrates a paper-based hydrogen fuel cell with a customized chemical heater that produces hydrogen in-situ upon the addition of a liquid. The presented approach operates by capillary action and takes advantage of the hydrogen released as a by-product of an exothermic reaction used in point-of-care diagnostics. The paper-based fuel cell produces a maximum power of 25.8 mW (103.2 mW cm−2), which is suitable for powering a diversity of electrical devices such as commercially available digital pregnancy tests and glucometers. While device shape and dimensions can be customized, here it is shown that the fuel cell can be designed in a compact form factor and footprint comparable to a lateral flow test while providing a remarkable power output. This approach holds great promise for powering portable diagnostics, as the generated electric power could enable device functionalities required for advanced assays, such as device timing, actuation, and signal quantification. Part of the same liquid sample that is to be analyzed (urine, saliva, water, etc) could be used to trigger the hydrogen generation and start the fuel cell operation., J.P. Esquivel acknowledges support from Marie Curie International Outgoing Fellowship (APPOCS - GA.328144) within the 7th European Community Framework Programme. J. Buser and P. Yager were supported by a grant from DARPA DSO/BTO--HR0011-11-2-0007, awarded to Yager at the University of Washington. P. Yager was supported in part by funds from the Department of Bioengineering at the University of Washington. S. Rojas acknowledges financial support from project ENE2013-42322R. N. Sabaté would like to thank financial support received from ERC Consolidator Grant (SUPERCELL - GA.648518).

The co-incorporation of S and N into multiwalled carbon nanotubes (CNTs) and the effects for the catalytic performance for the oxygen reduction reaction (ORR) in acidic and alkaline electrolytes has been studied. Subjecting CNTs to different ballmilling periods results in the formation of defects in their graphitic structure. Heteroatoms such as N and S can be actually incorporated into such defects leading to active catalysts for the ORR. In fact, the ORR activity in acid and alkaline media increases with the increasing amount of heteroatoms, especially N, actually incorporated in the catalysts. The use of precursors containing both N and S into their structure such as thiourea, results in a higher incorporation of surface N atoms than with similar N-containing precursors. As a consequence the ORR activity of the S/N/CNTs based catalysts is higher than that of N/CNT ones. This promotional effect of the presence of S is more significant when the ORR is measured in alkaline media suggesting that S-incorporation into the carbon matrix could actually play a direct role for the ORR., This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D-006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from project ENE2013-42322-R from the Spanish Ministry of Economy and Competitiveness is also acknowledged.

Non-precious metal catalysts for the oxygen reduction reaction (ORR) based upon the incorporation of different amounts of N into the CC network of multiwalled carbon nanotubes (CNTs) have been prepared by using CNTs and urea as the carbon and nitrogen sources, respectively. First, and with the aim of generating different levels of defects in the carbon network, the CNTs have been subjected to ballmilling during different periods of time between 0 and 150 h. Then, urea was mixed with the treated CNTs, subjected to further ballmilling and pyrolized at 800 °C. The number of defects, and as a consequence, the amount of N incorporated into the CNTs, increases with the duration of the ballmilling time. Moreover, the structure of the CNTs obtained after longer ballmilling times collapses leading to a carbon material with a high degree of microporisity. The performance of the N/CNT for the ORR, in terms of both the onset potential and mass current activity, increases with the amount of N actually incorporated into the CNT. Moreover, the H2O2 formation during ORR varies with the morphology of the catalyst. Thus, the formation of H2O2 is favored with the electrocatalysts in which the CNT structure is preserved, whereas the total reduction of O2 to H2O is favored for the electrocatalysts in which micropores are formed., This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D-006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from Project 201080E116 from the CSIC is also acknowledged.

In this work a series of N–modified graphene composites with different N/C ratios have been synthesised. The incorporation of Fe atoms into the N–modified graphene composites leads to the formation of Fe/N/C ensembles on the outer graphene layers along with Fe3C and metallic Fe phases in the bulk of the graphite nanoplates as revealed by X-ray absorption and XPS analyses. The adequate choice of the N/C atomic ratio of precursors to prepare Fe/N/graphene based materials is crucial to obtain electrocatalysts with an optimal performance for the ORR. The activity for the oxygen reduction reaction (ORR) of the Fe/N–graphene based electrocatalysts increases with increasing amount of accessible nitrogen, that is, with the amount of nitrogen by surface area.

The behavior of Fe-based non-precious metal catalysts (NPMCs) in different electrolytes and the repercussion for the oxygen reduction reaction (ORR) has been studied. For this matter, a series of Fe-based NPMC electrocatalysts have been prepared from different carbon sources, carbon black and multiwalled carbon nanotubes. The catalysts have been subjected to chemical treatments in 0.5 M H2SO4 and thoroughly characterized. Their performance for the ORR in different electrolytes e.g. HClO4, H2SO4, CF3SO3H, KOH and NaOH has been studied. Higher ORR rates have been recorded in the alkaline electrolytes as compared to the acid ones. Remarkably, the effect of the electrolyte is almost negligible when measured at a given pH value; i.e., the ORR performance is not affected by the nature of the anion when measured in acid electrolytes or the cation when measured in alkaline electrolytes. On the other hand, the activity of NPMCs for the ORR decreases remarkably after treatment of the catalysts in 0.5 M H2SO4. This effect accounts to both the removal of active sites for the ORR during acid treatment and to the blockage of active sites due to the presence of adsorbed sulfates., This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant number (D- 006-432). The authors, therefore, acknowledge with thanks DSR technical and financial support. Economic support from projects ENE2010-15381 from the Spanish Ministry of Science and Innovation and Project 201080E116 from the CSIC is also acknowledged.