Catalytic coatings comprising a mixture of IrO 2 and Ta 2 O 5 were prepared at 50-80 mol% Ir by thermal decomposition at 470°C, and the crystallographic structures and surface morphologies were investigated. XRD analysis and high-resolution SEM observation using low accelerated incident beam revealed that decreasing Ir ratio made a phase transition of IrO 2 from crystalline to partly amorphous, and nano IrO 2 particles of 10 nm or less and nano chains made of them, which were uniformly dispersed on the flat area of the coating, were generated at 50-60 mol% Ir. The relationship between nano-scale surface morphology of the coating and electrocatalysis for oxygen evolution in H 2 SO 4 solutions was also investigated. The active surface area and oxygen evolution current increased with reducing Ir ratio. The electrocatalysis of the IrO 2 -Ta 2 O 5 coatings for oxygen evolution is possible to be attributed to nano IrO 2 particles.
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