3D Pt nanoflowers, which are composed of numerous single‐crystal nanowires, are successfully synthesized by a facile chemical procedure, at room temperature, without surfactant or template. The Pt nanoflowers adhere to carbon paper, exhibiting an enlarged electroactive surface area comparable to that of a commercial Pt/C electrode.

In this study, the performance of a polyaniline (PANI) based supercapacitor where electroactive PANI films were prepared on carbon paper electrodes from a nonaqueous solution with an organic acid (CF 3 COOH) as the proton source was investigated. The use of nonaqueous media as electrolyte led to an increase of the electroactivity window from 0.75 V in aqueous media up to 1.0 V. Low frequency capacitance, evaluated by electrochemical impedance spectroscopy, of about 150 F/g is reported. Scanning electron microscopy indicated a highly porous material for deposited charges greater than 1 C/cm 2 . Constant current charge/discharge cycling of a symmetric supercapacitor based on PANI in nonaqueous medium was performed in a two-electrode cell configuration and a loss of about 60% of the discharge capacity was demonstrated after 1000 cycles. Tetramethylammonium methanesulfonate (Me 4 NCF 3 SO 3 ) was also used instead of tetraethylammonium tetrafluoroborate (Et 4 NBF 4 ) as supporting electrolyte in acetonitrile for the charge/discharge testing of the PANI-PANI capacitor. Energy and power densities of approximately 3.5 Wh/kg and 1300 W/kg, respectively, were developed by this supercapacitor for a cell voltage of 1 V and a discharge time of 20 s. On the other hand, an asymmetrical supercapacitor with polypyrrole and polyaniline used as positive and negative electrodes, respectively, displayed slightly improved performance. Indeed, an energy density of 5 Wh/kg and a power density of 1200 W/kg were reported for discharge time of about 20 s with 1 M Me 4 NCF 3 SO 3 /acetonitrile as electrolyte.

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Fe-based catalysts for O 2 reduction have been prepared on four carbon supports ͑Vulcan, Black Pearl, Norit, and a developmental carbon, RC2, from Sid Richardson Carbon Corp.͒. Four preparation procedures have been used with each carbon support and the catalytic activities for the reduction of oxygen in H 2 SO 4 , pH 1, and in polymer electrolyte membrane fuel cell tests are compared for all the catalysts, which are nominally loaded with 0.2 wt % Fe, using iron II acetate as Fe precursor. The catalytic activity of these Fe-based catalysts greatly depends upon the chosen carbon support and also upon the preparation procedure used. The results are rationalized in terms of N content at the surface of the catalysts; the larger the N content, the better the catalytic activity. The best catalysts are obtained after refluxing either RC2 or Norit in HNO 3 before adsorbing iron acetate on the oxidized carbon supports and heat-treating the resulting materials at 900°C in an atmosphere containing NH 3 . The surface nitrogen content of these catalysts, measured by XPS, is 2.5 and 4.1 atom %, respectively. For the Fe-based catalyst prepared on Norit and tested in fuel cell, the mass activity at low current regime, expressed in A/mg Fe, is only slightly lower than the A/mg Pt recorded for a state-ofthe-art, Pt-based membrane electrode assembly.Polymer electrolyte membrane ͑PEM͒ fuel cells are efficient and nonpolluting electrical power generators based on two electrochemical reactions: the oxidation of hydrogen and the reduction of oxygen. 1 As PEM fuel cells operate at low temperature ͑80°C͒, they require catalysts to generate useful currents. However, only Pt and its alloys have been used to date in full-size applications, because non-noble metals corrode in the acidic environment (pH ϳ 1) of the fuel cell membrane. Pt remains, however, an expensive metal of low abundance and its cost is a strong driver toward reducing precious metal loading or even replacing Pt-based catalysts in PEM stack systems with catalysts based on non-noble metals. The latter alternative might be possible at the cathode, using Fe or Co-based catalysts, provided that the non-noble metal-based catalysts display activity and stability approaching that of Pt. To the present date, only N 4 -metal macrocycles have been of interest in this respect.

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Hybrid electrochemical supercapacitors based on carbon and conducting polymers as negative and positive electrodes, respectively, have been investigated. Poly-͑3-fluorinatedphenyl͒thiophene and poly͑ethylenedioxythiophene͒ derivatives showing various ranges of electrochemical activity and capacitance values were evaluated as positive electrodes. It was shown that the mass and capacitance of the polymers have a significant effect on the charge/discharge characteristics and performance of such hybrid electrochemical supercapacitors. The experimental conditions that should be used to obtain specific charge/discharge curves are presented. A linear charge/discharge curve can be obtained between 0 and 3 V when the weight of conducting polymer is larger than that of the negative carbon electrode. In contrast, a battery-like charge/discharge curve is recorded when a smaller conducting polymer weight, relative to that of the carbon electrode, is used.

Multiwalled carbon nanotubes (MWCNTs) were grown on the fibers of a commercial porous carbon paper used as carbon-collecting electrodes in fuel cells. The tubes were then covered with Pt nanoparticles in order to test these gas diffusion electrodes (GDEs) for oxygen reduction in H2SO4 solution and in H2/O2 fuel cells. The Pt nanoparticles were characterized by cyclic voltammetry, transmission electron microscopy, and X-ray photoelectron spectroscopy. The majority of the Pt particles are 3 nm in size with a mean size of 4.1 nm. They have an electrochemically active surface area of 60 m2/g Pt for Pt loadings of 0.1-0.45 mg Pt/cm2. Although the electroactive Pt surface area is larger for commercial electrodes of similar loadings, Pt/MWCNT electrodes largely outperform the commercial electrode for the oxygen reduction reaction in GDE experiments using H2SO4 at pH 1. On the other hand, when the same electrodes are used as the cathode in a H2/O2 fuel cell, they perform only slightly better than the commercial electrodes in the potential range going from approximately 0.9 to approximately 0.7 V and have a lower performance at lower voltages.

Cobalt tetramethoxyphenyl porphyrin (CoTMPP) adsorbed on a high area carbon support (Vulcan XC72-R) and heat-treated at 900°C under inert atmosphere was studied as electrocatalyst for the reduction of O 2 to H 2 O 2 in acid medium. Experiments performed on rotating ring-disc electrode (RRDE) and gas diffusion electrode (GDE) show that the catalyst performance depends on the cobalt loading, going through a maximum at 0.2 wt. % Co. For higher cobalt loadings, a growing part of oxygen is reduced into water, decreasing therefore the selectivity of the catalyst. These results are interpreted in terms of a further reduction of H 2 O 2 on Co-based catalytic sites before leaving the catalytic layer. For a GDE polarized at )150 mV vs. saturated calomel electrode (SCE) and loaded with 0.9 lg cm )2 of 0.2 wt. % Co-based catalyst, a H 2 O 2 production rate of 300 lmol h )1 cm )2 was obtained which is five times higher than the H 2 O 2 production rate measured with Vulcan. In these conditions, the selectivity of the Co-based catalyst for H 2 O 2 production is 92%. The good agreement observed between RRDE and GDE results confirms the relevance of using RRDE experiment for screening these non-precious metal catalysts for further GDE applications.