Formic acid naturally occurs in stinging nettles and certain species of ants, but it's also manufactured cheaply and safely on an industrial level, and is typically used for preserving livestock feed. Team FAST uses a blend of 99% formic acid with a performance enhancing agent to create Hydrozine -- which has four times as much energy density as a battery -- to power the bus. The CO2 produced in splitting the hydrozine is used during the electricity-making process, so it's completely carbon neutral.
The system is 42,000 times stronger than the scale model the team originally presented in 2016 and is capable of producing 25kW of power. While Team FAST is still running final tests, it's hoped the bus will be fully operational by the end of the year.

Source: Eindhoven University of Technology