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

Here is Why You Need An Adapter

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

Mauritania

In Mauritania, outlets of type C are in use.

Type C

Niger

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

Type C

Type D

Type E

Type F