While it might not seem immediately obvious for a quietly purring car that drips water out its tailpipe, even hydrogen-powered vehicles have a carbon problem.

Given all the electricity needed to get hydrogen compressed and ready for the tank, that juice will need to come from solar, wind or (whisper it) nuclear power to get the fuel cells under the hood nearer to zero carbon output over the fuel’s ”life-cycle.” According to a Volkswagen manager developing the technology, the current mix of power generated on the U.S. grid is still too dirty.

The efficiency of the fuel cell is what makes these prototypes greener than the average gas gazzler, but then there’s also the “chicken and egg” problem of fuel distribution to consider. John Tillman, VW’s program manager for U.S. advanced powertrain research, said hydrogen refueling stations can cost two to three times as much as a gasoline equivalent to build.

“Government’s going to have to get involved in some ways,” he said ahead of a test drive of a VW Passat Lingyu, which was showcased at last year’s Beijing Olympics. “There won’t be enough fuel (at first) to make it profitable.”

Speaking at the California Fuel Cell Partnership headquarters, just outside Sacramento, Tillman also showed reporters what he called the world’s busiest hydrogen pump — owing to the many car companies involved: Ford, Toyota, Hyundai and Nissan all display signage outside.

And while these erstwhile rivals are not working on specific fuel-cell technology together, the partnership does offer them a chance to ask — and try to answer — tough questions about poultry, and which comes first.

As part of my job covering the world of alternative fuels, General Motors last week gave me the keys to a hydrogen-powered SUV, the Chevrolet Equinox.  You won’t find the Equinox in any showrooms, and in fact, the car I drove for four days is one of just 100 such vehicles in the United States.

Despite their small numbers, GM and others hope hydrogen-powered fuel cell vehicles will be critical to reducing greenhouse gases and our dependence on ever-costlier gasoline. Hydrogen can be produced by breaking apart water molecules, and it is also made by stripping hydrogen from fossil fuel natural gas. To see an animation of how a fuel cell works, click here.

I had fun driving emissions-free for a few days, but refueling once I had run low on hydrogen wasn’t so fun. That’s because there are just four places in the L.A. area where you can refuel a hydrogen car, and the lack of that infrastructure is one of the biggest impediments to getting them in the mass market.

Below is a brief video of my few days driving on hydrogen.

Video produced and edited by Syantani Chatterjee