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CeO2 decorated CoOx rod-like hybrid, supported onto holey reduced graphene (CoOx/CeO2/RGO) composite, was fabricated via a surfactant-assisted route. Its corresponding electrocatalytic performance towards oxygen reduction/evolution reactions (ORR and OER) was systematically investigated in alkaline electrolyte. Structural, morphological and compositional studies revealed changes in electronic and surface properties when CeO2 was introduced as an oxygen buffer material. The oxygen vacancies effectively enhanced the electrocatalytic activity, while the synergistic effect of co-catalyst CeO2, CoOx active-centers, and defective graphene with many voids facilitate the charge/mass transfer, making CoOx/CeO2/RGO an efficient and stable bifunctional electrocatalyst for OER/ORR with ΔE = 0.76 V (ΔE = E10 mA cm-2, OER – E1/2, ORR). This parameter is 70 mV and 270 mV lower than CoOx/RGO and the benchmark Pt/C, respectively. In addition, the OER/ORR bifunctionality of CoOx/CeO2/RGO composite outperforms that of Pt/C catalyst in a H2-O2 micro fuel cell platform.

Herein, FeCo bimetallic organic frameworks (MOFs) with different compositions were fabricated by controlling the initial molar ratio of Fe3+/Co2+ ions. Through a one-step carbonization treatment of FeCo MOFs and the melamine mixture, a series of FeCo nanoalloys encapsulated in bamboo-like N-doped carbon nanotubes (CNTs), entangled with N-doped carbon nanosheet nanocomposites (FexCoy–N–C), were prepared. The FexCoy–N–C materials presented an excellent ORR performance in alkaline medium. X-ray photoelectron spectroscopy (XPS) studies revealed the existence of ORR-active centers, such as Co–Nx, pyridinic-N and graphitic-N, which were responsible for the enhanced ORR activity. Besides, the synergistic coupling between FeCo alloy and N-doped carbon nanotubes favors the enhancement of the electrocatalytic performance. Among FexCoy–N–C electrocatalysts, the optimized Fe1Co3–N–C and Fe3Co1–N–C catalysts showed higher ORR performance. In addition, both Fe1Co3–N–C and Fe3Co1–N–C showed superior stability compared to the benchmark commercial Pt/C catalyst.

N-doped graphene decorated with cobalt-based nanoparticles (Co/CoO/NG-T) were obtained via a 1,10-phenanthroline assisted pyrolysis method. The optimized Co/CoO/NG-800 showed excellent performance with E1/2, ORR of 0.82 V vs. RHE and ηOER, 10 mA cm-2 of 407 mV. A good stability was obtained with only 20 mV negative drift in the half-wave potential after 2000 cycles, and a retention rate of 94% after 5 h. The prominent performance is attributed to the electronic interaction between Co-based species and N-doped graphene, and the presence of active pyridinic-N moieties. This work provides a revealing paradigm on the design of efficient non-precious metal-based electrocatalysts.

The performance of platinum group metal (PGM)-free bifunctional electrodes, in a regenerative H2/O2 microfluidic cell, is examined. The system is operated in an alkaline environment at 25 °C. Multiwalled carbon nanotube (CNT)-supported Ni-based nanostructures, NiO–Ni/CNT, were used to catalyze the hydrogen evolution reaction (HER) and the hydrogen oxidation reaction (HOR), whereas N-doped carbon nanohorn-supported NiFeOx–CoNy (NiFeOx–CoNy/NCNH) served as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts. The system NiO–Ni/CNT||NiFeOx–CoNy/NCNH, at 25 °C, showed an interesting performance in fuel cell mode (ca. 15 mW cm–2 at the maximum peak) and in electrolyzer mode (ca. 102 mW cm–2 at 50 mA cm–2). The round-trip efficiency, after five consecutive operating cycles, decreased from 44.76 to 39.52%.

Carbon-supported PtM nanostructured materials (M = Cr, Co, Ni, Cu, Mo, Y, Sm, Gd) were investigated for the hydrogen evolution/oxidation reactions (HER/HOR) in alkaline medium. All catalysts were synthesized by the carbonyl complex chemical route. Among PtM, Cr, Co, Ni, and Cu formed nanoalloys. M2O3 was generated with Y, Sm, and Gd, and Mo was found at a dopant concentration of Pt. The electrochemical results showed that the HER/HOR activities on PtNi/C and PtCo/C outperformed that of the Pt/C benchmark. The presence of OHads species, acting as a bifunctional mechanism, favored the HER/HOR activity on PtNi/C and PtCo/C. This process is concomitant with early DFT studies that concluded that the presence of OHads weakens the Hads and H2Oads energetics.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), namely, so-called oxygen electrode reactions, are two fundamental half-cell reactions in the energy storage and conversion devices, e.g., zinc–air batteries and fuel cells. However, the oxygen electrode reactions suffer from sluggish kinetics, large overpotential and complicated reaction paths, and thus require efficient and stable electrocatalysts. Transition-metal-based layered double hydroxides (LDHs) and their derivatives have displayed excellent catalytic performance, suggesting a major contribution to accelerate electrochemical reactions. The rational regulation of electronic structure, defects, and coordination environment of active sites via various functionalized strategies, including tuning the chemical composition, structural architecture, and topotactic transformation process of LDHs precursors, has a great influence on the resulting electrocatalytic behavior. In addition, an in-depth understanding of the structural performance and chemical-composition-performance relationships of LDHs-based electrocatalysts can promote further rational design and optimization of high-performance electrocatalysts. Finally, prospects for the design of efficient and stable LDHs-based materials, for mass-production and large-scale application in practice, are discussed.

Flower-like hierarchical three-dimensional NiFe layered double hydroxides hollow microspheres (3D NiFe-LDH HMS), as one kind of novel non-noble metal electrocatalysts, have been fabricated in a template-free route for water oxidation. Both of the concentration of ammonium fluoride and the reaction time are adjusted to obtain a series of NiFe-LDH microspheres, with different internal structures from massive to hollow generated during the hydrothermal treatment, which improve the electrocatalytic activity of the NiFe-LDH catalysts towards the evolution reaction of oxygen. The optimized NiFe-LDH-0.4M HMS show the excellent OER performance in alkaline electrolyte with η = 290 mV@10 mA cm−2, and a Tafel slope of 51 mV dec−1, which outperforms the benchmark RuO2 catalyst. The possible reason is attributed to the more exposure of active sites, and fast ion transport resulting from the hierarchical hollow structure.

Direct ethanol fuel cells are highly promising energy conversion devices, but the complete oxidation of ethanol to CO2 at low temperatures is still one of the main challenges. Titanium oxycarbide (TiO1-xCx) powder is investigated as stable and synergistic support for Pt nanoparticles during the ethanol oxidation reaction (EOR) in acidic electrolytes. EOR products are quantitatively detected online with differential electrochemical mass spectrometry. At room temperature, Pt/TiO1-xCx has a CO2 efficiency maximum of 8.9%, while only 1.7% are measured for Pt nanoparticles on Vulcan carbon (Pt/C). The reaction pathway is channeled towards the formation of C1 products, while C2 product formation is suppressed. This effect is expected to be much enhanced at elevated temperatures which makes TiO1-xCx a highly interesting catalyst support material in general.

It is increasingly useful to develop bifunctional catalysts for oxygen reduction and oxygen evolution reaction (ORR and OER) for fuel cells, metal-air rechargeable batteries, and unitized regenerative cells. Here, based on the excellent conductivity and stability of ordered mesoporous carbons, and the best ORR and OER activity of Co3O4, the composite Co3O4/N-HNMK-3 was designed and manufactured by means of a solvothermal method, using ordered N-doped mesoporous carbon (N-HNMK-3) as substrate, and then the bifunctional electrocatalytic performance corresponding to ORR, OER in alkaline media was carefully investigated. The results showed that Co3O4/N-HNMK-3 composite, a non-precious metal centered electrocatalyst, displayed excellent ORR performance (activity, selectivity, and stability) close to that of commercial 20 wt.% Pt/C and a promising OER activity near 20 wt.% RuO2/C. The outstanding bifunctional activities of Co3O4/N-HNMK-3 was assessed with the lowest △E value of 0.86 V (EOER,10 mA cm&minus, 2-EORR,&minus, 3 mA cm&minus, 2) with respect to the two commercial precious metal-based electrocatalysts.

The recent advances in electrocatalysis for the hydrogen evolution reactions in acid and alkaline media are carefully reviewed. In this sense, this short review focuses on precious and non-precious catalytic centers. The recent development of HOR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis methods are summarized. The research directions, e.g., on the further development of more active, more stable, and less expensive electrocatalysts for the anion-exchange membranes for the alkaline fuel cell technology are provided. In this context, positive perspectives to overcome the high kinetic barriers encountered in alkaline medium can be expected in the years to come.

Structural degradation in manganese oxides leads to unstable electrocatalytic activity during long-term cycles. Herein, we overcome this obstacle by using proton exchange on well-defined layered Li2MnO3 with an O3-type structure to construct protonated Li2-xHxMnO3-n with a P3-type structure. The protonated catalyst exhibits high oxygen reduction reaction activity and excellent stability compared to previously reported cost-effective Mn-based oxides. Configuration interaction and density functional theory calculations indicate that Li2-xHxMnO3-n has fewer unstable O 2p holes with a Mn3.7+ valence state and a reduced interlayer distance, originating from the replacement of Li by H. The former is responsible for the structural stability, while the latter is responsible for the high transport property favorable for boosting activity. The optimization of both charge states to reduce unstable O 2p holes and crystalline structure to reduce the reaction pathway is an effective strategy for the rational design of electrocatalysts, with a likely extension to a broad variety of layered alkali-containing metal oxides., Structural degradation in manganese oxides leads to unstable activity during long-term cycles. Herein, authors demonstrated that reduced unstable O 2p holes and the short interlayer distance of layered lithium manganese oxide are favorable for excellent electrocatalytic stability and activity.

By combining experimental and theoretical aspects, this study focuses on exploring and enhancing the activity and stability towards oxygen reduction reaction (ORR) of fluorinated carbon supported platinum nanoparticles in the acidic electrolyte. Physical characterization results revealed that fluorinated carbon support generates a C-F bonding. The interaction between Pt and fluorinated-carbon allows for a strong charge transfer from Pt to the support. This phenomenon is consistent with the density functional theory (DFT) analysis. Because of the fluorination, the electronic structure of Pt is changed, which is an advantage for improving the catalytic activity and stability of the catalyst towards ORR.

Pt deposition/anchoring technique and nature of the support influence the oxygen reduction reaction (ORR) activity in low-temperature proton exchange membrane fuel cells. Surface distribution, morphology (particle shape and size) of the deposited Pt nanoparticles (NPs) and physicochemical properties contribute to the electrocatalytic activity and durability of the catalyst. Investigations over the past two decades lead to various metal and metal oxide-based supports, along with advanced nanocarbon materials as suitable candidates since they play a vital role in defining surface morphology, particle-size distribution, crystallinity and electronic structure of the deposited Pt catalyst. Moreover, such supports improve the ORR activity and stability of the electrocatalyst due to their stronger interaction with the deposited Pt NPs. Briefly, a well-controlled and selective deposition of Pt NPs and designing of an excellent corrosion-resistant support for a promising ORR catalyst has gained more attention. Many advanced strategies are developed for the fabrication of atomically precise nanostructured Pt catalysts. This review summarises recent developments in the electrochemical, photochemical and physical deposition techniques for Pt NPs on various supports and their effects on the physicochemical properties and electrocatalytic activity towards the ORR. Financial support by the Estonian Research Council (grant PRG723) and by the EU through the European Regional Development Fund (TK141, “Advanced materials and high-technology devices for energy recuperation systems”) is gratefully acknowledged. G.M. is grateful to CAPES-PrInt (grant 88881.311799/2018e01) for the financial support.

International audience; The use of high amounts of iridium in industrial proton exchange membrane water electrolyzers (PEMWE) could hinder their widespread use for the decarbonization of society with hydrogen. Nonthermally oxidized Ir nanoparticles supported on antimony-doped tin oxide (SnO2:Sb, ATO) aerogel allow decreasing the use of the precious metal by more than 70% while enhancing the electrocatalytic activity and stability. To date, the origin of these benefits remains unknown. Here, we present clear evidence of the mechanisms that lead to the enhancement of the electrochemical properties of the catalyst. Operando near-ambient pressure X-ray photoelectron spectroscopy on membrane electrode assemblies reveals a low degree of Ir oxidation, attributed to the oxygen spill-over from Ir to SnO2:Sb. Furthermore, the formation of highly unstable Ir(III) species is mitigated, while the decrease of Ir dissolution in Ir/SnO2:Sb is confirmed by inductively coupled plasma mass spectrometry. The mechanisms that lead to the high activity and stability of Ir catalysts supported on SnO2:Sb aerogel for PEMWE are thus unveiled.

Tailoring active, durable, and cost-effective electrocatalysts is crucial for hydrogen evolution (HER) and oxidation reactions (HOR) in an alkaline medium. Herein, the synthesis, physicochemical, a...

Platinum nanoparticles were photo-deposited on acid-treated multi-walled carbon nanotubes (CNTs) and SnO2-coated CNTs. The SnO2-CNT nanocomposite was synthesised by a chemical method. Scanning and scanning transmission electron microscopy results indicated that the CNTs surface is partially covered by the SnO2 layer. The surface morphology of the Pt catalyst depends on the nature of the support material. For instance, porous Pt aggregates of various sizes are formed at specific nucleation sites on the CNT surface, while a selective deposition of Pt was observed on the metal oxide part of the SnO2-CNT. Cyclic voltammetry and CO stripping investigations in 0.1 M HClO4 solution confirmed different surface electrochemistry of the Pt catalysts. It is revealed that photo-deposition induces the formation of Pt-Sn nanoalloys due to increased interaction at the oxide interfaces. Both Pt/CNT and Pt/SnO2-CNT catalysts showed remarkable activity towards the electroreduction of oxygen in acid media. Durability measurement showed that photo-deposited Pt nanoparticles are more resistant to degradation. The corrosion-resistant nature of the SnO2-CNT support further improves the catalysts stability. Pt/CNT and Pt/SnO2-CNT catalysts retained 86% and 88% of the initial surface area respectively, after 10,000 potential cycles between 0.6 and 1.2 VRHE.

Herein, a series of three-dimensional Ni2Fe-SDS-LDH/CFP non-precious metal electrocatalysts in a simple hydrothermal route using sodium dodecyl sulfonate (SDS) as an interlayer spacer agent, and carbon fiber paper (CFP) as a conductive substrate was tailored. The electrocatalytic performance towards oxygen evolution reaction (OER) in alkaline medium was investigated. The results demonstrate that the OER activity improvement is mainly related to the increased interlayer distance from 0.76 nm to 2.49 nm depending on the intercalated amount of SDS in Ni2Fe-LDH, and resulting in the enhanced superior surface characteristics (e.g., larger specific surface area, bigger pore size and pore volume). Among all the samples, the Ni2Fe-SDS-LDH/CFP (molar ratio SDS/Fe = 1.5) showed the best OER performance (η@10 mA cm−2 = 289 mV, Tafel slope = 39 mV dec−1) comparable with the commercial IrO2 catalyst, which corresponded to the largest interlayer space of 2.49 nm.

We fabricated a NiFeO x /CoN y -C nanocomposite derived from CoPcTs-intercalated Ni 2 Fe-layered double hydroxides (Ni 2 Fe-CoPcTs-LDH), which served as high-efficiency, low-cost, and long-durability bifunctional oxygen electrocatalyst in half-cell, and a H 2 -O 2 laminar flow unitized regenerative micro-cell (LFURMC) in alkaline media. Based on the synergistic effect between Co-N y and NiFeO x centers, the non-noble hybrid catalyst NiFeO x /CoN y -C achieves a ΔE (η@jOER,10 - η@jORR,-3) = 0.84 V in alkaline solution, outperforming the commercial Pt/C, and very close to that of IrO x /C. In the fuel cell mode, the performance of NiFeO x /CoN y -C with the maximum power density of 56 mW cm −2 is similar to that of Pt/C (63 mW cm −2 ) and IrO x /C (58 mW cm −2 ); in the electrolysis mode, the calculated maximum electrical power consumed on NiFeO x /CoN y -C (237 mW cm −2 ) is more than 3 times that on Pt/C (73 mW cm −2 ), similar with that of IrO x /C. More importantly, the NiFeO x /CoN y -C shows a remarkable stability in alternating modes in a LFURMC system.

This short review analyzes the catalytic activity of nanostructured molybdenum disulfide (MoS2). The phase transition of nanostructured MoS2 semiconductor from 2H (hexagonal) to 1T (trigonal) leads to a metal-like material through so-called surface defect engineering by chemical exfoliation. The 1T phase was found to be active for the electrocatalytic hydrogen evolution reaction (HER). Various key aspects vis-à-vis the MoS2 synthesis, characterization, support interaction, and application are, herein, highlighted.

Mesoporous Co3O3.87◻0.13 (◻ represents oxygen vacancies) and Co3O3.87F0.13 electrocatalysts, integrating advantages of oxygen vacancies and fluorine anion, were deployed. Co3O3.87◻0.13 and Co3O3.87F0.13 showed enhanced oxygen evolution reaction (OER) with overpotentials of 440 mV and 430 mV at 10 mA cm−2, Tafel slopes of 56 mV dec-1 and 56 mV dec-1, turnover frequency (TOFη= 400 mV) of 0.023 s-1 and 0.042 s-1 respectively, as compared to Co3O4 (520 mV, 64 mV dec-1, 0.0005 s-1) in the alkaline medium. Physicochemical characterizations showed that oxygen vacancies and the substituted fluorine modified the electronic structure, leading to the enhanced activity for OER process, consistent with the theoretical calculation. This approach provides ideas for rational design of new defective fluorinated oxide, with the feasible extension of such an approach to a wide variety of transition metal oxides.

International audience; TiO2 Brookite, and Anatase powders were prepared by a facile and low temperature solution based method. XRD diffractometry was performed to confirm their phase composition and crystallinity. Morphology was further confirmed by TEM. N2 adsorption was used to determine surface area and pore size distribution. The difference in capacitive properties of both materials and the determination of the flat band position was obtained by Mott-Schottky analysis. Diffuse reflectance and Tauc plots allowed to obtain the band gap value. The degradation of Indigo Carmine dye with both catalysts in a photo-electrochemical microreactor was assessed by UV–vis, and Raman spectroscopy. Both materials were more active than the benchmark commercial Degussa P25 sample in the order P25 < Anatase < Brookite and explained in terms of energy band positions.

Nanostructured palladium (Pd) supported on carbon (Vulcan XC-72) substrate, prepared via carbonyl chemical route, showed Pd-mass depending morphology, reflected on the surface electrochemistry via cyclic, CO-stripping voltammograms, and performance in direct formic acid and direct methanol micro laminar flow fuel cell arrangement. Comparison was done with commercial Pd/C (30 wt.%, ETEK) catalyst. The formic acid oxidation reaction (FAOR) studied in different acid media (0.1 M HClO 4 , 0.5 M H 2 SO 4 and 0.5 M H 3 PO 4 ), indicated an enhanced FAOR activity on nanowire-like as compared to nanoparticles and nanorods as well as the commercial one in each electrolyte. Besides, the nanowire-like Pd/C catalyst also favored methanol (MOR) in 0.1 M KOH. These facts confirm favorable morphological effect of nanowires for organic fuel oxidation in both acid and alkaline media.

This review summarizes recent progress in the development of cobalt-based catalytic centers as the most potentially useful alternatives to noble metal-based electrocatalysts (Pt-, Ir-, and Ru-based) towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in acid and alkaline media. A series of cobalt-based high-performance electrocatalysts have been designed and synthesized including cobalt oxides/chalcogenides, Co⁻Nx/C, Co-layered double hydroxides (LDH), and Co⁻metal-organic frameworks (MOFs). The strategies of controllable synthesis, the structural properties, ligand effect, defects, oxygen vacancies, and support materials are thoroughly discussed as a function of the electrocatalytic performance of cobalt-based electrocatalysts. Finally, prospects for the design of novel, efficient cobalt-based materials, for large-scale application and opportunities, are encouraged.

Metal-free nitrogen-doped ordered mesoporous carbons, from home-made mesoporous carbon (CMK-3), were tailored. The different N-doped amounts was provided by melamine, as the nitrogen source, at 900°C. The corresponding electrocatalytic performance towards oxygen reduction reaction (ORR) in alkaline media was evaluated. Prior to the thermal treatment, the CMK-3 was modified by nitric acid to introduce oxygen-containing functional groups. The results showed that the method employed is efficient to prepare N-doped carbon electrocatalysts. The pyridinic-N amount reached a maximum value of 53.8 atom% for a mass ratio in the Me/HNMK-5 catalyst material. The ORR performance of this material attained an onset potential of 0.92 V vs. RHE, and a half-wave potential (E1/2) of 0.87 V vs. RHE with a four-electron pathway in alkaline medium comparable with the benchmark catalyst, 20 wt% Pt/C.

This chapter summarizes the effect of the supports on chalcogenide materials aiming at enhancing electrocatalytic and/or photocatalytic processes. The different nature of the supports from carbon, oxides, and carbon–oxide composites is discussed.

Improving the efficiency of Pt-based oxygen reduction reaction (ORR) catalysts while also reducing costs remains an important challenge in energy research. To this end, we synthesized highly stable and active carbon-supported Mo-doped PtCu (Mo-PtCu/C) nanoparticles (NPs) from readily available precursors in a facile one-pot reaction. Mo-PtCu/C displays two-to-fourfold-higher ORR half-cell kinetics than reference PtCu/C and Pt/C materials, a trend that was confirmed in proof-of-concept experiments by using a H2 /O2 microlaminar fuel cell. This Mo-induced activity increase mirrors observations for Mo-PtNi/C NPs and possibly suggests an emerging trend. Electrochemical-accelerated stability tests revealed that dealloying was greatly reduced in Mo-PtCu/C in contrast to the binary alloys PtCu/C and PtMo/C. Supporting DFT studies suggested that the exceptional stability of Mo-PtCu could be attributed to oxidative resistance of the Mo-doped atoms. Furthermore, our calculations revealed that oxygen could induce segregation of Mo to the catalytic surface, at which it effected beneficial changes to the surface oxygen adsorption energetics in the context of the Sabatier principle.

International audience; This work summarizes the advanced materials developed by various research groups for improving the stability of platinum (Pt), and Pt-based catalysts center toward the oxygen reduction reaction (ORR) in acid medium. The ORR stability enhancement of Pt catalytic center can be classified according to the different nature of the supporting materials, namely, carbon-, oxide-based-, and oxide-carbon composites. The enhancement and stability of a catalytic center can be related to either its electronic modification induced by a strong interaction with the support, another metal (alloy), or to geometric effects. In addition, other parameters come into play, the size, the morphology of the catalytic center, the temperature, the dispersion, and mass loading, along with the measuring methods. This mini-review mainly focusses on the stability improvement, depending on the substrate nature. This latter can be further modified via functionalization or by the chemical interaction nature between the substrate and catalyst.

Here, we report the effect of substrate for tuning the active reaction center of precious and non-precious electrocalysts for efficient oxygen reduction reaction (ORR) activity. We have selected surface modified carbon morphologies to study the substrate effect on Pt and CoSe2 catalytic center. Pt supported on surface modified carbon nanotube (CNT) substrates shows, defective sites is important for effective dispersion of Pt on it. CO stripping process probes the effect of Pt interaction with defective and graphitic domains of carbon support. Improved mass activity and specific activity is observed for Pt supported on pyridine functionalized CNT compared to Pt on other CNT substrate. Similarly, CoSe2 supported on nitrogen doped carbon nanohorn (NCNH) shows improved ORR activity in alkaline and acid conditions compared to other carbon based substrates. Enhanced mass activity and specific activity of CoSe2/NCNH is observed in both alkaline and acidic media compared to CoSe2 supported on other modified substrates.

International audience; Yttrium oxide and gadolinium oxide modified platinum nanoparticles supported on carbon (Vulcan XC-72), noted as Pt-M2O3/C (M=Y and Gd), were synthesized by water-in-oil nanoemulsion chemical route, followed by heat treatment at 100 and 300 degrees C under hydrogen/nitrogen (H-2/N-2) atmosphere and tested as electrocatalysts for the oxygen reduction reaction (ORR). As revealed by powder X-ray diffraction analysis, all obtained catalysts showed solely Pt face-centered-cubic structure, and absence of a secondary phase before and after heat treatments, indicating that Y2O3 and Gd2O3 are highly disordered (amorphous) and dispersed clusters. The surface Pt:M ratio (M=Y and Gd), as revealed by X-ray photoelectron spectroscopy, increased after heat treatment with respect to the value of as-prepared samples, for which the ratio was 1:1. Microstrain data extracted from Williamson-Hall plots for Pt-M2O3/C (M=Y and Gd) catalysts surprisingly increased after heat treatment at 100 degrees C, remaining nearly constant after heat treatment at 300 degrees C, whereas the value of pure Pt nanoparticles, noted as Pt/C, decreased after heat treatments. The mean particle size derived from TEM images for Pt-M2O3/C (M=Y and Gd) was almost unchanged after heat treatments, at variance with the Pt/C case where a clear increase is observed. Surface specific activity and mass activity towards ORR obtained with as-prepared Pt-M2O3/C (M=Y and Gd) catalysts were higher than those of as-prepared Pt/C catalyst. After heat treatment, the ORR activity of Pt-M2O3/C (M=Y and Gd) augmented, whereas that of Pt/C diminished. Moreover, after 6000cycles between 0.5 and 0.95V versus reversible hydrogen electrode (vs. RHE), Pt-M2O3/C (M=Y and Gd) catalysts retained a large active surface area and a high kinetic current density at 0.9V vs. RHE in comparison with Pt/C samples. These facts assess a positive effect of the interaction between M2O3 (M=Y and Gd) and Pt catalytic centers both on the catalytic activity of the material towards ORR and on its durability.

Here, we report the tuning of active reaction center of the CoSe2 nanoparticles by nitrogen doped carbon nanohorns (NCNH). CoSe2 nanoparticles supported on NCNH (CoSe2/NCNH) shows improved oxygen reduction reaction (ORR) activity compared to the same supported on carbon nanohorns and Vulcan carbon. An improved positive shift in onset potential (50 mV) and half-wave potential (150 mV) of CoSe2/NCNH compared to other catalysts indicates the intrinsic activity as well as active reaction center of CoSe2 modulated by the presence of NCNH. CoSe2/NCNH reduces molecular oxygen to hydroxide through a four-electron transfer pathway with improved electrochemical stability in alkaline condition. Air breathing direct methanol micro laminar flow fuel cells (DM-µLFFC) performance of CoSe2/NCNH as cathode catalyst shows a maximum power density and current density respectively of 10.05 mW cm-2 and 124.20 mA cm-2 with 5M methanol as fuel.

Nanocrystalline La0.5Sr0.5MnO3 (LSMO) perovskite-sized powders were successfully synthesized at heating low temperature and time using the Pechini method based on polyesterification between citric acid and ethylene glycol. The electroactivity of carbon-supported perovskite (LSMO/C) for the oxygen reduction reaction (ORR) was evaluated. The results showed that the electroactivity of LSMO/C largely depends on the calcination temperature, type of electrolyte, and mass loading. It increased remarkably with the calcination temperature in 0.1 M KOH. The ORR performance was examined in KOH, NaOH, and K2SO4. Higher electroactivity was recorded in KOH electrolyte, as compared to NaOH and K2SO4.

Hybrid Pt/TiO 2 -C catalysts were synthesized via photodeposition in this study in order to increase the kinetics and the carbonate yield of the Ethanol Oxidation Reaction. Photodeposition was chosen because it was believed that the Pt nanoparticles would deposit predominately on top of the oxide sites, which would change the reactivity of the Pt nanoparticles. First, TiO 2 -C composites with varying TiO 2 mass percentages were synthesized by sol gel using Vulcan Carbon XC-72R and titanium isopropoxide as precursors. The composites were dispersed in isopropanol and mixed with Chloroplatinic Acid such that the final Pt concentration was 20% by weight. This mixture was illuminated with UV light for 3 h and the Pt/TiO 2 -C catalysts were recovered. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analysis showed that the Pt photodeposition was almost quantitative and XRD patterns showed that the average particle size ranged from 4 to 5 nm. XPS Spectra showed that the Pt nanoparticles shifted to lower binding energies as TiO 2 concentration increased, whereas Ti binding energies shifted to higher values with higher TiO 2 concentrations. The synthesized catalysts showed increased current density in Linear Sweep Voltammetry when compared to a commercial Pt/C catalyst experiments which was attributed to increased nucleophilicity on the Pt which allowed for kinetically easier nucleophilic attacks. In-situ FTIR studies showed that the bands attributed to carbonate vibrations were more intense on the synthesized catalysts. All of these experiments allowed us to conclude that the increased electron density facilitates nucleophilic attacks and C C bond breaking.

International audience

The application of metal–organic frameworks (MOFs) in electrocatalytic energy conversion is reviewed. MOFs are promising electrocatalysts for reactions, such as oxygen reduction reaction/oxygen evolution reaction (ORR/OER), hydrogen evolution reaction/hydrogen oxidation reaction (HER/HOR), fuels oxidation and production. The applications of MOFs in these domains are diverse. They can be directly used as catalytic centres with and/or without supporting carbon materials. The frameworks can be used as supports to load metal or oxide nanoparticles. After pyrolysis, MOFs can form a composite of a metal or an oxide with carbon. Various carbon materials can be derived via a heat-treatment of MOFs, followed by an acid treatment. They can also serve as precursors to synthesize nanoalloys or composites with noble metals (e.g. Pt and Pd). Since more and more efforts have been devoted to apply MOFs to the field of energy conversion, we present a summary of the most significant results from the literature, to facilitate further development of these kinds of materials in the domain of energy conversion.

International audience; A passive silicon microfabricated direct methanol fuel cell employing a polymer anion exchange membrane has been identified as a promising integrable power supply for portable devices in the MEMS field. In this work the fabrication steps of the different components: silicon current collectors and membrane-electrode assembly (MEA), as well as the mounting approach and performance evaluation for the whole passive alkaline micro air-breathing direct methanol fuel cell (mu ADMFC) are shown. This system, with a small active area of 0.25 cm(2), was tested near of the real application conditions with totally passive fueling and at room temperature. Different MEA configurations and methanol and KOH concentrations were compared. Best performance was observed for the MEA with a previously sprayed catalytic layer on carbon cloth instead of the MEAs with the catalytic layer deposited directly onto the alkaline membrane. A maximum power density of 2.2 mW cm(-2) was achieved for 15 mu L of 1 M methanol + 4 M KOH fuel solution.

Chalcogenide electrocatalysts are very active and tolerant for the oxygen-reduction reaction (ORR) in electrolytes containing small organics. This property of tolerance allows them to be used as tolerant cathodes of direct methanol fuel cells (DMFC) that use highly concentrated methanol. Platinum chalcogenide-based cathodes of direct methanol fuel cells can overcome the problem of the formation of a mixed-potential due to the fuel crossover [Alonso-Vante et al., Patent-FR 12 55944, Appl. Filed 22 June 2012]. In this study, a comprehensive investigation on the origin of the tolerance of Pt chalcogenide electrocatalyst to small organic molecules such as methanol is provided. The XPS spectrum of Se 3d region in PtxSey/C revealed a shift of the selenium emission peak to lower binding energy as compared to a Se/C sample. This negative shift can be explained by charge transfer from the platinum surface atoms to the selenium, which is at the origin of the improved tolerance of Pt towards ORR in the presence of methanol. The physical chemical properties and electrochemical performances in half-cell and mixed-reactant microfluidic fuel cells are discussed.

One of the major issues for the development of long-term stable electrodes for fuel cells deals with the improvement of the stability of the catalyst support. Substrates with different nature can be used as support for metallic nanoparticle catalysts. Among them, carbon based materials, e.g., Vulcan XC-72, carbon nanotubes (CNTs) have been mostly considered as a choice. Highly dispersed carbon as the classical support for electrocatalytic metal nanoparticles, e.g. Pt, leads to its oxidation to CO2 occurring at E > 0.97 V vs. RHE [1-2]. This well known corrosion phenomenon leads to the reduction of the platinum active surface area [3] either by aggregation or dissolution of the metallic active centers and consequently to a decrease of the fuel cell performances. Chemical modification of carbon based materials and elaboration of carbon-oxide composites are two suitable strategies leading to an improvement of the stability of the catalytic system. Moreover, the technique for depositing platinum on the support seems also to be responsible to mastering the nature of the interaction between the catalyst and the support. In particular, the photo-deposition technique [4] is very adapted to realize strong metal substrate interaction (SMSI) figure 1 [5]. The electrocatalytic activity of a material depends on both its chemical nature and its morphology. Therefore it is evident that an interaction allows not only an increased stability of the catalyst center but also modifies the catalytic properties and therefore the fuel cell performance based on platinum electrodes.

International audience; The electroreduction of nitrogen species in alkaline medium was performed on platinum nanoparticles (1-2 nm) with metal loadings of 5 and 10 wt% Pt, synthesized by the chemical carbonyl route. The electrochemically active surface analysis evidences that the real surface area is concomitant to the Pt-loading in the support. The electrochemical analysis put in evidence that nitrite ions were reduced from -0.6 to -0.78 V and the charge transfer step is attributed to the NO2-/NO redox couple. The Tafel slope analysis confirms that from E-p to -0.78 V the reduction of NO occurs via an ECE mechanism. An analysis of Nicholson-Shain schemes and NO-saturated in solution verified these results. (C) 2012 Elsevier Ltd. All rights reserved.

Platinum, and platinum containing Yttrium (Y) species nanoparticles (NPs) supported by carbon black, Vulcan XC-72, oxidized multi-walled carbon nanotube, ox.MWCNT, pyridine functionalized multi-walled carbon nanotube, (ox.MWCNT)-Py, were prepared via carbonyl chemical route, followed by heat-treatment. X-ray diffraction (XRD) analyses showed, beside Pt fcc-phase, the presence of PtxY alloy in Pt-Y nanoobjects supported onto functionalized MWCNTs. X-ray photoelectron spectroscopy (XPS) revealed, essentially, that Pt surface was decorated by Y2O3; therefore, such nanoobjects were labeled Pt-Y. By analyzing the surface atomic ratio of Pt/N and Pt/Y in Pt-Y/(ox.MWCNT)-Py, it is suggested that Pt atoms interact with Y2O3, pyridine group, and Y nano-alloying Pt. The transmission electron microscopy (TEM) shows similar mean particle size values, in Pt-Y/Vulcan, Pt-Y/ox.MWCNT and Pt-Y/(ox.MWCNT)-Py, in opposition to Pt NPs supported onto Vulcan, ox.MWCNT and (ox.MWCNT). The surface electrochemistry demonstrated a unique behavior of Pt on Pt-Y/(ox.MWCNT)-Py with respect to other samples. The ORR kinetics of this catalyst was the highest among all the samples, revealing a synergistic effect of Y2O3, pyridine functionalization, and nano-alloyed Y alloyed on Pt toward ORR.

The Pt-M 2 O 3 /C (M = Y and Gd) catalysts were synthesized via carbonyl chemical route, and heat-treated at 300 °C. Both catalysts are nanoparticulated with a face-centered cubic ( fcc ) structure with no secondary phase present as revealed by X-ray diffraction (XRD) patterns similar to Pt/C, generated in the same way. Data analyses of Pt-M 2 O 3 /C XRD patterns via the Williamson–Hall method were performed. The stacking fault, and micro-strain values increased with respect to Pt/C catalyst. The transmission electron microscopy (TEM) further revealed that nano-particles are homogeneously dispersed in both Pt/C and Pt-M 2 O 3 /C (M = Y and Gd). The mean particle size is close, ca. 3.4 nm for Pt/C, ca. 4.1 nm for Pt-Y 2 O 3 /C and ca. 3.6 nm for Pt-Gd 2 O 3 /C sample. Although the surface electrochemical studies (cyclic and CO-stripping voltammograms) showed similar Pt surface behavior, the kinetics of oxygen reduction reaction (ORR) on Pt-M 2 O 3 /C (M = Y and Gd) catalysts was higher than the homemade Pt/C and commercial Pt/C catalysts (20 wt. %, Johnson Matthey). After 6000 potential cycles (0.6–1.0 V vs. RHE) of accelerated stability test (AST), the remaining Pt active surface (based on hydrogen underpotential deposition region) and kinetic current density (at 0.9 V vs. RHE) in Pt-M 2 O 3 /C (M = Y and Gd) catalysts were higher than the reference Pt/C, and commercial Pt/C catalysts. These findings assess the positive effect of M 2 O 3 (M = Y and Gd) in improving the activity and stability of Pt NPs towards ORR.

In this work, carbon-supported CoSe2 is used as a Pt-free cathode catalyst in a passive, air-breathing, alkaline anion exchange membrane μ-direct methanol fuel cell (AEM-μDMFC). The obtained results demonstrate the improvement in the performance, when the device is operated at high fuel concentrations, if the proposed cathode catalyst is used instead of the standard Pt/C catalyst, due to the better tolerance to methanol crossover even in alkaline medium. This result reinforces the suitability of AEM-DMFC's as a promising option for mobile devices powering.

Carbon-supported platinum nanoparticles (Pt/C) were synthesized by the carbonyl chemical route. The obtained Pt/C catalysts with different Pt mass loadings, varying from 5 to 50 wt %, showed constant morphology with particle sizes of about 2 nm and a dispersion of around 50 %. The electrochemical surface area, determined from CO stripping and hydrogen underpotential deposition, increased with the Pt mass loading, which confirmed that agglomeration was negligible. The oxygen reduction reaction (ORR) half-wave potential was positively shifted with an increase in the Pt mass loading, and the ORR kinetic current was (40±10) μA cm−2Pt. The data collected on a rotating ring-disk electrode for the ORR pathway revealed a two-electron serial mechanism (hydrogen peroxide), instead of the four-electron transfer (water). The determined turnover frequency factor indicated good selectivity of the Pt active sites for the ORR with 1 or 2 mm H2O2 in the electrolyte for samples with mass loadings ranging from 15 to 40 wt %. The ORR was markedly affected by the mass loading (5–10 wt % Pt/C) in the presence of 2 mm H2O2.

International audience

This review is devoted to discussing the state of the art in the relevant aspects of the synthesis of novel precious and non-precious electrocatalysts. It covers the production of Pt- and Pd-based electrocatalysts synthesized by the carbonyl chemical route, the synthesis description for the preparation of the most catalytically active transition metal chalcogenides, then the employment of free-surfactants synthesis routes to produce non-precious electrocatalysts. A compilation of the best precious electrocatalysts to perform the hydrogen oxidation reaction (HOR) is described; a section is devoted to the synthesis and electrocatalytic evaluation of non-precious materials which can be used to perform the HOR in alkaline medium. Apropos the oxygen reduction reaction (ORR), the synthesis and modification of the supports is also discussed as well, aiming at describing the state of the art to improve kinetics of low temperature fuel cell reactions via the hybridization process of the catalytic center with a variety of carbon-based, and ceramic-carbon supports. Last, but not least, the review covers the experimental half-cells results in a micro-fuel cell platform obtained in our laboratory, and by other workers, analyzing the history of the first micro-fuel cell systems and their tailoring throughout the time bestowing to the design and operating conditions.

In this work, platinum nanoparticles were impregnated by two different techniques, namely the carbonyl chemical route and photodeposition, onto systematically surface-modified multiwalled carbon nanotubes. The different interactions between platinum nanoparticles with sp(2)-sp(3) carbon nanodomains were investigated. The oxidation of an adsorbed monolayer of carbon monoxide, used to probe electronic catalytic modification, suggests a selective nucleation of platinum nanoparticles onto sp(2) carbon nanodomains when photodeposition synthesis is carried out. XPS attests the catalytic center electronic modification obtained by photodeposition. DFT calculations were used to determine the interaction energy of a Pt cluster with sp(2) and sp(3) carbon surfaces as well as with oxidized ones. The interaction energy and electronic structure of the platinum cluster presents dramatic changes as a function of the support surface chemistry, which also modifies its catalytic properties evaluated by the interaction with CO. The interaction energy was calculated to be 8-fold higher on sp(3) and oxidized surfaces in comparison to sp(2) domains. Accelerated Stability Test (AST) was applied only on the electronic-modified materials to evaluate the active phase degradation and their activity toward oxygen reduction reaction (ORR). The stability of photodeposited materials is correlated with the surface chemical nature of supports indicating that platinum nanoparticles supported onto multiwalled carbon nanotubes with the highest sp(2) character show the higher stability and activity toward ORR.

Oxide-Carbon composites (TiO 2 -C) and yttrium-doped TiO 2 -C (Y:TiO 2 -C), synthesized via sol-gel route, were used as supports to photo-deposit platinum nanoparticles (Pt NPs). The physical-chemical properties of these materials were investigated by Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD). The TEM images showed that Pt NPs were agglomerated and deposited onto oxide sites of the composites. Pt face-centered cubic, and TiO 2 Anatase phases were identified on both Pt/TiO 2 -C and Pt/Y:TiO 2 -C samples. The particle size, stacking fault and micro-strain of Pt NPs were estimated by Williamson-Hall method. Compared with Pt/TiO 2 -C, similar stacking faults as well as increased crystallite sizes and micro-strain values could be observed for Pt/Y:TiO 2 -C sample, indicating a minimum impact of the rare earth element on Pt. Pt/C was prepared by the same method, and used as a reference catalyst. The strong-metal/support interaction (SMSI) effect induced by the photo-deposition method, in Pt/TiO 2 -C and Pt/Y:TiO 2 -C samples, was probed by CO-stripping, and associated to the enhanced stability toward the oxygen reduction reaction (ORR) in acid medium.

Carbon supported bimetallic palladium–copper nanoparticles (PdCu/C) were prepared using a copper metal–organic framework (MOF) as sacrificial precursor. The effect of MOF was investigated by comparing the PdCu/C synthesized from a simple metal chloride precursor (CuCl 2 ). The PdCu/C nanoparticles are nanorod-like particles in both cases. Based on the analysis from Warren planar defect and size-strain isotropic models, in the MOF-derived PdCu/C sample, the crystallite size, stacking-fault and micro-strain parameters increase, whereas lattice parameter decreases, with reference to both the CuCl 2 -derived PdCu/C and Pd/C nanoparticles. This assesses a solid solution in the MOF-derived PdCu/C sample, while the alloying is less evident in the CuCl 2 -derived sample, as supported by inductively coupled plasma and X-ray photoelectron spectroscopy data. A better oxygen reduction reaction activity obtained on MOF-derived PdCu/C, as compared to other homemade and commercial samples, is correlated to Pd surface modification induced by alloying, further proved by CO-stripping voltammograms.

Name that tune: A simple and new method to produce a methanol-tolerant Pt chalcogenide catalyst for the oxygen reduction reaction (ORR) is developed. The catalyst is tuned by electrochemical stripping of the Se atoms on the surface of the catalyst. The resulting electrode nanomaterial is PtSe0.2, and it shows the highest activity ever reported for the ORR in solutions containing methanol.