Most of his talks he mentions this. He was doing a PHD to study ultra-capacitors, or at least, was accepted to UPENN with that intention. He left to start zip2. He has also mentioned that a part of the reason he didn't proceed was that his research was very theoretical and it would've been able to find (or disprove) of a method after years pf study. He mentions some of the other breakthroughs on iltra-capacitors having the efficiency of a lead battery, but, while extremely efficient, the prototype used an extremely rare material that only was mined in a few tonnes a year. Thus the solution could never scale.