Aurélien Habrioux

Electronic interaction between platinum nanoparticles and nitrogen-doped reduced graphene oxide: effect on the oxygen reduction reaction

In this study, low-mass loadings (ca. 5 wt%) Pt/C catalysts were synthesized using the carbonyl chemical route allowing for the heterogeneous deposition of Pt nanoparticles on different carbon-based substrates. N-doped reduced graphene oxide, reduced graphene oxide, graphene oxide, graphite and Vulcan XC-72 were used for the heterogeneous deposition of Pt nanoparticles. The effect of the chemical nature of the carbon-based substrate on the Oxygen Reduction Reaction (ORR) kinetics at Pt nanoparticles surfaces was investigated. XPS results show that using N-doped reduced graphene oxide materials for the deposition of Pt nanoparticles leads to formation of Pt–N chemical bonds. This interaction between Pt and N allows for an electronic transfer from Pt to the carbon support. It is demonstrated that ca. 25% of the total amount of N atoms were bound to Pt ones. This chemical bond also revealed by the DFT analysis, induces changes in the oxygen adsorption energy at the platinum surface, engendering an enhancement of the catalyst activity towards ORR. In comparison with Vulcan XC-72, the mass activity at 0.9 V vs. RHE is 2.1 fold higher when N-doped reduced graphene oxide is used as substrate. In conjunction with the experimental results, DFT calculations describe the interaction between supported platinum clusters and oxygen where the support was modelled accordingly with the carbon-based materials used as substrate. It is demonstrated that the presence of N-species in the support although leading to a weaker O2 adsorption, induces elongated O–O distances suggesting facilitated dissociation. Additionally, it is revealed that the strong interaction between Pt clusters and N-containing substrates leads to very slight changes of the cluster–substrate distance even when oxygen is adsorbed at the interfacial region, thus leading to a lower resistance for electron charge transfer and enabling electrochemical reactions.

Electrochemically induced surface modifications of mesoporous spinels (Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ) as the origin of the OER activity and stability in alkaline medium

Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ materials were synthesized using a nanocasting process consisting in replicating a SBA-15 hard template. Catalysts powders obtained were characterized using different physico-chemical techniques (X-ray scattering, transmission electron microscopy, N2 physisorption and X-ray photoelectron spectroscopy) in order to deeply characterize their morphostructural properties. Electrochemical measurements performed with cyclic voltammetry and electrochemical impedance spectroscopy techniques have shown that these catalysts were liable to surface modifications induced by the applied electrode potential. These surface structural modifications as well as their effect on the electroactivity of the catalyst towards the OER in alkaline medium are discussed. The activated NiCo2O4−δ material showed particularly excellent catalytic ability towards the OER in 0.1 M KOH electrolyte. In this material Co(IV) is found to be the active species in the catalyst composition for the OER. It exhibits an overpotential as low as 390 mV at a current density of 10 mA cm−2. This catalytic activity is especially high since the oxide loading is only of 0.074 mg cm−2. Furthermore, this anode catalyst showed high stability during an accelerated durability test of 1500 voltammetric cycles.

Yttrium oxide/gadolinium oxide-modified platinum nanoparticles as cathodes for the oxygen reduction reaction.

Rare-earth-element (Y, Gd) modified Pt nanoparticles (NPs) supported on a carbon substrate (Vulcan XC-72) are synthesized via a water-in-oil chemical route. In both cases, X-ray diffraction (XRD) measurements show the non-formation of an alloyed material. Photoemission spectroscopy (XPS) results reveal that Y and Gd are oxidized. Additionally, no evidence of an electronic modification of Pt can be brought to light. Transmission electron microscopy (TEM) studies indicate that Pt-Y(2)O(3) and Pt-Gd(2)O(3) particles are well dispersed on the substrate-and that their average particle sizes are smaller than the Pt-NP sizes. The catalytic activity of the Pt-Y(2)O(3)/C and Pt-Gd(2)O(3)/C catalysts towards the oxygen reduction reaction (ORR) is studied in a 0.5 M H(2)SO(4) electrolyte. The surface and mass specific activities of the Pt-Y(2)O(3)/C catalyst towards the ORR at 0.9 V (vs. the reversible hydrogen electrode, RHE) are (54.3±1.2) μA cm(-2)(Pt) and MA=(23.1±0.5) mA mg(-1)(Pt), respectively. These values are 1.3-, and 1.6-fold higher than the values obtained with a Pt/C catalyst. Although the as-prepared Pt-Gd(2)O(3)/C catalyst has a lower catalytic activity for the ORR compared to Pt/C, the heat-treated sample shows a surface specific activity of about (53.0±0.7) μA cm(-2) Pt , and a mass specific activity (MA) of about (18.2±0.5) mA mg(-1) Pt at 0.9 V (vs. RHE). The enhancement of the ORR kinetics on the Pt-Y(2)O(3)/C and heat-treated Pt-Gd(2)O(3)/C catalysts could be associated with the formation of platinum NPs presenting modified surface properties.

Thermally Induced Strains on the Catalytic Activity and Stability of Pt-M2O3/C (M=Y or Gd) Catalysts towards Oxygen Reduction Reaction

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 °C under hydrogen/nitrogen (H2/N2) 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 °C, remaining nearly constant after heat treatment at 300 °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 6000 cycles between 0.5 and 0.95 V 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 0.9 V 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.

Enhanced HER and ORR behavior on photodeposited Pt nanoparticles onto oxide-carbon composite

The photodeposition process under ultraviolet domain for platinum nanoparticles was explored. The concomitant presence of different mechanisms during the photodeposition of Pt nanoparticles onto TiO2 in the presence of water and alcohol is evidenced. According to the process, one can devise various complex mechanisms. The presence of nanoparticulated oxide anatase phase enhances the photodeposition process of metal nanoparticles via the so-called heterogeneous photocatalysis. A description and the effect of mixing of various chemicals in the reactor reveal interesting information, which allows controlling the size of nanoparticles by the photodeposition process. This study also paves the way to decrease the amount of precious metals used in material composition used as catalysts towards hydrogen evolution reaction and oxygen reduction reaction for fuel cell technologies.

Three dimensionally ordered mesoporous hydroxylated NixCo3−xO4 spinels for the oxygen evolution reaction: on the hydroxyl-induced surface restructuring effect

Surface restructuration upon potential cycling of three dimensionally ordered NixCo3−xO4 spinels for the oxygen evolution reaction (OER) in an alkaline medium is studied using structural, spectroscopic and electrochemical techniques. It was shown that the intrinsic activity of different catalysts depends on the incorporated amount of nickel and surprisingly correlates with the CoIII/CoIV peak potential. The electrochemical activity of the OER is amazingly improved upon potential cycling. It was observed that potential cycling induces an increase of active sites up to 45% on the most effective electrocatalyst. This unexpected increase in activity is very pronounced and becomes stable after 30 voltammetric cycles. Such a phenomenon is explained by the formation of a layered mixed nickel/cobalt oxyhydroxide active site whose oxidation potential is related to the nickel amount in the catalyst. The formation of this layer is promoted by the surface hydroxylation degree of non-cycled catalysts. In these catalysts, nickel modulates the electronic properties of the active site, which modifies the adsorption energies of key oxygenated intermediates. The synthesis route proposed herein allows an efficient way for obtaining high specific surface areas as well as highly hydroxylated surfaces, the latter being the key factor in the enhancement of the electrocatalytic activity of nickel cobaltites.

Electronic modification of Pt via Ti and Se as tolerant cathodes in air-breathing methanol microfluidic fuel cells

We reported herein on the use of tolerant cathode catalysts such as carbon supported Pt(x)Ti(y) and/or Pt(x)Se(y) nanomaterials in an air-breathing methanol microfluidic fuel cell. In order to show the improvement of mixed-reactant fuel cell (MRFC) performances obtained with the developed tolerant catalysts, a classical Pt/C nanomaterial was used for comparison. Using 5 M methanol concentration in a situation where the fuel crossover is 100% (MRFC-mixed reactant fuel cell application), the maximum power density of the fuel cell with a Pt/C cathodic catalyst decreased by 80% in comparison with what is observed in the laminar flow fuel cell (LFFC) configuration. With Pt(x)Ti(y)/C and Pt(x)Se(y)/C cathode nanomaterials, the performance loss was only 55% and 20%, respectively. The evaluation of the tolerant cathode catalysts in an air-breathing microfluidic fuel cell suggests the development of a novel nanometric system that will not be size restricted. These interesting results are the consequence of the high methanol tolerance of these advanced electrocatalysts via surface electronic modification of Pt. Herein we used X-ray photoelectron and in situ FTIR spectroscopies to investigate the origin of the high methanol tolerance on modified Pt catalysts.

Decorated nanotube buckypaper as electrocatalyst for glucose fuel cells

We present novel metallic/bimetallic (Pt, Au-Pt) nanoparticle-decorated carbon nanotubes and bilirubin oxidase-decorated carbon nanotubes deposited on nano-tube buckypaper as promising supported electro-catalytic systems and as electrode material respectively for mixed-reactant biofuel cell applications at neutral pH. We found that the novel enzyme-decorated carbon nanotubes on nanotube buckypaper material is a promising cathode for glucose biofuel cells. It exhibited a high tolerance and catalytic activity resulting in higher current densities compared to carbon black based electrodes.

Tailoring nanostructured catalysts for electrochemical energy conversion systems

Abstract This review covers topics related to the synthesis of nanoparticles, the anodic and cathodic electrochemical reactions and low temperature electrochemical energy devices. The thermodynamic aspects of nucleation and growth of nanoparticles are discussed. Different methods of chemical synthesis such as w/o microemulsion, Bönnemann, polyol and carbonyl are presented. How the electrochemical reactions take place on the surface of the catalytic nanoparticles and the importance of the substrate is put in evidence. The use of nanomaterials in low temperature energy devices such as H2/O2 polymer electrolyte or proton exchange membrane fuel cell (PEMFC) and micro-direct methanol fuel cell (μDMFC), as well as recent progress and durability, is discussed. Special attention is given to the novel laminar flow fuel cell (LFFC). This review starts with the genesis of catalytic nanoparticles, continues with the surface electrochemical reactions that occur on them, and finally it discusses their application in electrochemical energy devices such as low temperature fuel cells or Li-air batteries.

Preparation and Electrochemical Properties of NiCo2O4 Nanospinels Supported on Graphene Derivatives as Earth-Abundant Oxygen Bifunctional Catalysts

This work reports on the facile synthesis and characterisation of a non-precious-metal bifunctional catalyst for oxygen reduction and evolution reactions (ORR and OER). A few-layer reduced graphene oxide-supported NiCo2 O4 catalyst is prepared using a rapid and easy two-step method of synthesis. It consists of the solvothermal poyl(vinylpyrrolidone)-assisted assembly of metal complexes onto few-layer graphene followed by a calcination step aiming at converting metal complexes into the spinel phase. Using this synthesis approach, the most active material demonstrates an outstanding activity towards the OER and ORR, making it one of the best bifunctional catalysts of these reactions ever reported. This composite catalyst exhibits improved bifunctional behaviour with a low reversibility criterion of 746 mV. The ORR process follows a four-electron pathway and the hydroxyl selectivity is higher than those with pure reduced graphene oxide or NiCo2 O4 materials, showing the synergistic effect between the two phases. Moreover, the high activity of this composite catalyst is confirmed by comparing its performance with those obtained on other cobaltite catalysts prepared using a different synthesis method, or those obtained using a different graphene-based support.