ExxonMobil is "probably further" than 25 years away from bringing algae-based fuels to the market due to the technical challenges in scaling up the technology, said Rex Tillerson, chairman and CEO, whose company has committed $600 million into its algal fuel venture. "What we've come to understand is the hurdle is pretty high and the hurdle seems to exist at the basic science level, which means it's even more difficult to solve," Tillerson said.