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

Here is Why You Need An Adapter

The outlet and plug type used in Equatorial Guinea are C 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

Equatorial Guinea

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

Type C

Type E