Will pepper spray work on rats?

Sure, but you might want to consider the consequences of using pepper spray in your home for the purposes of driving out vermin. There are specialty sprays out there for wildlife, mostly for use against dogs and bears. Those use a fogger style nozzle, which acts much like a fire extinguisher. That has the added effect of lingering in a room, which is great if you want to keep someone away from you in your home, but it will linger for about an hour or so, you may not want to use it in your home. See ledefensetech.Wordpress.com for more information on pepper sprays.