You Do Need power plug adapter while traveling from Congo to Sierra Leone.

Here is Why You Need An Adapter

The outlet and plug type used in Sierra Leone are D and G and in Congo are C, D and E. This is how they look like.

Congo

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

Type C

Type D

Type E

Sierra Leone

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

Type D

Type G