At ARPA-E's recent Energy Innovation Summit in Washington, DC, Program Director Grigorii Soloveichik presented his vision for the future of transportation: hybrid electric vehicles that combine the advantages of both plug-in battery and fuel cell technologies.
This "optimal solution" will require a new generation of fuel cell that is "fast, furious, and flexible." Fast, in terms of start-up / shut-down time. Furious, in terms of energy density. And flexible, in terms of fuel choice - specifically sustainable liquid fuels, like ammonia.