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

Here is Why You Need An Adapter

The outlet and plug type used in Congo are C, D and E 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

Congo

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

Type C

Type D

Type E