Electrode Corrosion and Storage Capacity

Deterioration of electrode performance due to corrosion of electrode components is a critical problem. The susceptibility of MH electrodes to corrosion is essentially determined by two factors surface passivation due to the presence of surface oxides or hydroxides, and the molar volume of hydrogen, V, in the hydride phase. As pointed out by Willems and Buschow [40], V is important since it governs alloy expansion and contraction during the charge-discharge cycle. Large volume changes 

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Cobalt is invariably present in commercial MHt battery electrodes. It tends to increase hydride thermodynamic stability and inhibit corrosion. However, it is also expensive and substantially increases battery costs thus, the substitution of Co by a lower/cost metal is desirable. Willems and Buschow [40] attributed reduced corrosion in LaNi 5 vCoi (x= 1 -5) to low Vn. Sakai et al. [47 J noted that LaNi25Co25 was the most durable of a number of substituted LaNi5 iCoi alloys but it also had the lowest storage capacity. 

Two types of fundamental metal hydride electrodes comprising the AB, and ABj classes of alloys are currently of interest. The AB, alloys with A = rare earth or mischmetal, B = Ni and/or other transition metal are investigated. LaNi, has been well-investigated because of its utility in conventional hydrogen storage applications, but it is very expensive and corrodes rapidly. The commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The partial substitution of Ni by Co, Ce, Mn and Al increases the thermodynamic stability of the hydride phase, the corrosion resistance and hence the cycle life. However, the substitution reduces the hydrogen storage capacity. 

The rare earth composition of commercial electrodes is also related to electrode corrosion. This was noted by Sakai et al. [44], who found that the presence of Nd and Ce inhibited corrosion when substituted in part for La in Lai xZ x(NiCoAl)5 (Z = Ce or Nd) electrodes. However no explanation for the effect was noted. Willems [23] prepared an electrode having the composition of La sNd 2Ni2,5Co2 4Si i, which retained 88% of its storage capacity after 400 cycles. He attributed its long cycle life to a low Vh of 2.6 A. ... 

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