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

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)=3.2x10(-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 MgNiCx alloys with increasing x. (C) 2002 Elsevier Science B.V. All rights reserved.