You Do Need power plug adapter while traveling from Sierra Leone 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 Sierra Leone are D and G. This is how they look like.

Sierra Leone

In Sierra Leone, outlets of type D and G are in use.

Type D

Type G

Niger

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

Type C

Type D

Type E

Type F