What exactly are sad piano chords?
Typically people have always regarded the minor chords as being the sad sounding chords. They are however made all the more sad sounding when a single minor chord is suddenly played after a major chord.
Other types of chords have their own properties too. Suspended chords for example have a certain kind of ambiguity about them, augmented chord are sometimes thought of as being spooky or dramatic sounding.
Experiment with the different types of chords and identify emotions with them. When you are improvising or trying to compose then you will be able to use these emotions when choosing your chords.