Nickel aluminate spinel was studied as a potential candidate electrode material. When NiAl_2O_4 powders were reduced in a hydrogen atmosphere, nickel metal phase was observed as well as the spinel phase which had undergone a change of cation distribution. The former observation was confirmed by TEM/EDS analysis. The Ni-Al-O stability diagram was calculated from thermodynamic data and shows that a transformation, stoichiometric spinel → Ni + nonstoichiometric spinel, occurs in a critical Po_2 range of 1.9 × 10~(-18) ~ 1.0 × 10~(-20) atm. at 650°C. In addition, it was found that the nonstoichiometric vacancies introduced with decreasing Po_2 occupied mainly the tetrahedral interstitial cation sites, rather than occupying octahedral sites for charge neutralization. These results suggest that NiAI_2O_4 may be a candidate anode material.
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