When Toyota introduced the Mirai fuel-cell vehicle three years ago the automaker tossed in an appealing incentive, $15,000 worth of the hydrogen needed to power it up. At the time, it cost just over $70 to fill Mira’s 5.6-kilogram tank. Today, you’ll pay over $200 – if you can find the lightweight gas, as many of the stations providing it have run dry. And that’s threatening to short-circuit the “hydrogen economy” that fuel-cell proponents have predicted is just around the corner.