Stéphane Ruggeri

Influence of carbon on the electrode characteristics of MgNi prepared by mechanical alloying

Abstract The influence of carbon addition on the characteristics of MgNi alloy prepared by mechanical alloying and used as metal hydride electrode has been studied. Our results indicate that, despite its low proportion, carbon addition has a major deleterious effect on the electrode performance, i.e. the initial discharge capacity decreases exponentially from 522 to 332 mA h g −1 with carbon content increasing from 0 to 3.5 wt.%. This exponential decay cannot be only explained by a decrease in the H solubility due to the dissolution of C atoms in the hydrogen interstitial sites. The cycle life of the electrode is not influenced by the carbon content. The particle morphology and the amorphous structure of the material are unmodified by the carbon addition. In addition, our results indicate that the hydrogen diffusivity in MgNi is unchanged by carbon addition ( D H / a 2 =3.2×10 −5 s −1 ). In contrast, the exchange current density I 0 decreases from 92 to 62 mA g −1 with increasing carbon content in the alloy. This results indicate that carbon limits the charge-transfer reaction at the surface of the alloy, which must have an influence on the initial discharge capacity of the alloy. In addition, the electrochemical PCT curves reveal that the slopes of the isotherms become steep with C addition indicating a wider distribution of energy levels for hydrogen. This is considered as the major reason for the observed decrease in the initial discharge capacity of the MgNiC x alloys with increasing x .

Fe-Based Electrocatalysts for Oxygen Reduction in PEMFCs Using Ballmilled Graphite Powder as a Carbon Support

Active Fe-based electrocatalysts were prepared using ballmilled graphite powder as a carbon support. The best performing catalysts were achieved by acid-washing, iron-loading, and pyrolyzing the ballmilled graphite powders. Only 1 h of ballmilling was required to produce optimal catalytic activity. High-energy ballmilling of pristine graphite powder under nitrogen was shown to reduce crystallite size, increase nitrogen content, increase surface area, increase degree of disorder, and inevitably introduce metallic impurities. Acid-washing treatment of ballmilled graphite powders reduced, but did not completely eliminate, metallic impurities. Iron enrichment and pyrolysis of acid-washed, ballmilled graphite powder was shown to increase catalytic activity, have little effect on crystallite size, increase surface area, and decrease degree of disorder. It was found that catalytic activity increases as crystallite size decreases, degree of disorder and nitrogen content increase, and micropore specific surface area increases. Fuel cell test results have shown that the order of increasing maximum power density follows the order of increasing catalytic activity. Interestingly, the optimal crystallite size parameter L a (20-30 A) and maximum activity for catalysts made with either ballmilled graphite powder or carbon black is almost the same.

Influence of Structural Properties of Pristine Carbon Blacks on Activity of Fe ∕ N ∕ C Cathode Catalysts for PEFCs

Four series of carbon blacks, with various disordered carbon content, have been fabricated at the Sid Richardson Carbon Corporation and were used as supports to obtain heat-treated Fe/N/C electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). All catalysts were very active and differed by about one order of magnitude in their electrocatalytic activity. Two pristine carbon blacks were then selected to determine, by Raman spectroscopy and X-ray diffraction, which structural parameters of the pristine carbons are important in electrocatalysis. It was found that W D (the full width at half-maximum of the D band in the Raman spectrum) is indicative of the disordered phase content in the pristine carbon black. The pristine carbon black having the largest W D also yielded the best ORR electrocatalytic activity. L a and L c are structural parameters representing the lateral (L a ) and vertical (L c ) extensions of the graphitic crystallites. They do not change with the catalytic activity, at least not with a change of one order of magnitude of the catalytic site density. Only the distance between the graphene layers in the graphitic crystallites (d 002 ) changes with the electrocatalytic activity, but only marginally. This is, however, believed to be an indirect effect related to the gasification of disordered carbon found between the graphitic crystallites in pristine carbon black.

Optimization of the Ball-Milling Parameters for the Synthesis of Amorphous MgNi Alloy Used as Negative Electrode in Ni-MH Batteries

The optimization of the ball milling parameters resulted in the synthesis with a milling duration equal to 10 hours of amorphous MgNi having an initial discharge capacity over 520 mAh/g. Further milling results in a partial crystallization of amorphous MgNi into nanocrystalline MgNi 2 and Mg2Ni, which decreases significantly the electrode performance. This study also shows that it is possible to obtain an amorphous and electroactive material in large scale (1 kg of alloy per batch) using an industrial high-energy attritor. In addition, it was demonstrated that the carbon added at the beginning of the milling to avoid powder welding, in spite of its small proportion (1 wt.%), has a notable influence on the performance of the material. This study also confirms the major loss of activity during cycles. However, the partial substitution of Ti for Mg leads to a remarkable improvement of the cycle lifetime of the alloy.

Synthesis of nanocrystalline CaNi5-Based alloys and use for metal hydride electrode

The CaNi 5 -based alloys were synthesized by mechanical alloying followed by a low temperature annealing. Study shows that both substitution and annealing treatment affect the hydrogen storage capacity. Mm substitution for Ca results in an increased plateau pressure. The Zn and Al substitution for Ni leads to a bigger unit cell volume, a lower plateau pressure and a reduced capacity. Annealing treatment of the mechanically alloyed CaNi 5 -based alloys is a necessary process for obtaining good hydrogen storage capacity. The synthesized Ca-Mm-Ni-Zn-Al multicomponent nanocrystalline alloys exhibit fast kinetics on gas phase and electrochemical reaction and significantly improved cycle stability on electrochemical reaction.