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

Here is Why You Need An Adapter

The outlet and plug type used in Pitcairn Islands is I 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

Pitcairn Islands

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

Type I