Carmakers are under increasing pressure to clean up their act when it comes to the environment. There's growing demand for greener cars - but the focus so far has been on making the traditional combustion engine more efficient. A more complex solution is using hydrogen fuel cells - which can power vehicles without producing any harmful emissions. One of the leading companies in the field is Stuttgart-based Daimler - it's spent more than any other company on R&D for fuel-cell-powered vehicles. And now it believes mass production could just be a few years away...