You Do Need power plug adapter while traveling from Guinea-Bissau 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 Guinea-Bissau is C. This is how they look like.

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C

Sierra Leone

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

Type D

Type G