How to discipline a 15 month old?

Hi ladies,
Not sure if any of your toddlers are beginning to be naughtier but mine is. He understands the word no and is sometimes continuing a behaviour I tell him no to many times. Two times I have become very frustrated and angerily stuck him in his playpen for a time out for me to cool off. I don't want to act hastily out of frustration, I want to teach him. Any advice? He's my first so still new.