The oxygen evolution reaction of anode catalyst is an important reaction in polymer electrolyte membrane water electrolysis. Recent research has focused on understanding the factors governing the catalytic properties of nanostructured IrO
2 in order to decrease the use of expensive and precious IrO
2. We demonstrate here the equivalent diameter (
De) effect (lateral size effect) of IrO
2 nanosheets for the oxygen evolution reaction. IrO
2 nanosheets with four different
De from 110 to 260 nm and same thickness (∼1.5 nm) were prepared. The electrical double layer capacitance, which should be proportional to the electrochemically active surface area, of IrO
2 nanosheets was independent of
De. On the other hand, the pseudo-capacitance, which is related to surface redox processes, and the oxygen evolution reaction activity increased with decreasing
De. The enhanced oxygen evolution reaction activity is discussed based on the electrochemically active surface area and edge effect.
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