Substituting parchment for foil in slow cooker A slow cooker gooey brownie recipe calls for using aluminum foil as a liner. 

Can I substitute parchment paper for the foil?

Since the OP seems to be long gone, I'm going to edit in this recipe, since it's the first Google hit if you search for "slow cooker gooey brownie recipe", and it does call for lining the slow cooker with foil. The recipe also calls for the foil to be generously buttered.
My experience would suggest that unless the aluminum foil is used to form some kind of shape such as a seal around the food, parchment paper would be superior.  Aluminum foil is better for forming shapes.  Parchment paper is better for non-stick qualities.  Parchment paper has a temperature limit of around 400 degrees F, whereas aluminum foil will tolerate much higher temperatures (technically it will melt around 1200 degrees F).  Most slow cookers wouldn't be too hot for either one, but they vary, so you may want to check yours to be sure.