Transformation Characterization of Ni(OH)2/NiOOH in Ni-Pt Films Using an Electrochemical Quartz Crystal Microbalance for Ethanol Sensors

The transformation characterization of Ni(OH)2/NiOOH in Ni-Pt films was investigated using an electrochemical quartz crystal microbalance (EQCM) in an alkaline electrolyte under various experiment conditions. The Ni-Pt films were prepared by electro-deposition on gold-coated quartz crystal chips. When cyclic voltammetry (CV) was performed continually, the OH ions diffused into the Ni-Pt film to form Ni(OH)2. The transformation from mixed type Ni(OH)2/NiOOH to -Ni(OH)2/ -NiOOH occurred gradually. The phase transformation of Ni(OH)2/NiOOH was strongly affected by the composition, deposited weight, and aging conditions of the Ni-Pt films. The results show that high Pt content or high deposited weight of Ni-Pt films postponed the transformation to the -Ni(OH)2/ -NiOOH phase. -Ni(OH)2 was directly formed by dipping fresh Ni-Pt films in KOH solution while mixed type Ni(OH)2 was obtained after aging fresh Ni-Pt films in humid O2 atmosphere. These two pre-aging conditions accelerate the phase transformation of Ni(OH)2/NiOOH into -Ni(OH)2/ -NiOOH. After 1050 and 2100 cycles of the electro-oxidation process of ethanol, the Ni(OH)2 phase in Ni-Pt films still remained in a -Ni(OH)2-like phase with some -Ni(OH)2. The well-defined -Ni(OH)2 phase that formed in the Ni-Pt film after 1050 cycles of CV was more stable in a KOH solution than in a humid O2 atmosphere at room temperature. Ni-Pt films were also evaluated as a sensing element for an ethanol sensor. [doi:10.2320/matertrans.M2010079]