You Do Need power plug adapter while traveling from Niger to Nauru.

Here is Why You Need An Adapter

The outlet and plug type used in Nauru is I and in Niger are C, D, E and F. This is how they look like.

Niger

In Niger, outlets of type C, D, E and F are in use.

Type C

Type D

Type E

Type F

Nauru

In Nauru, outlets of type I are in use.

Type I