You Do Need power plug adapter while traveling from Pitcairn Islands 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 Pitcairn Islands is I. This is how they look like.

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I

Sierra Leone

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

Type D

Type G