It wouldn't hurt to have acid free paper in your 
portfolios, but the best thing you can do for 
your cels is to store them in the archival-type 
bags just like you would for comic books. The 
paint won't come off onto the bags, and you can 
also maneuver the cels without dirtying or 
accidentally scratching them.

I see nothing wrong with using backing boards, 
except it would make your book thicker and harder 
to carry. I would cut the board larger than the 
entire cel so that the cel is supported fully by 
the board.

---Trebuchet