You Do Need power plug adapter while traveling from Equatorial Guinea 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 Equatorial Guinea are C and E. This is how they look like.

Equatorial Guinea

In Equatorial Guinea, outlets of type C and E are in use.

Type C

Type E

Sierra Leone

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

Type D

Type G