A group of German and Australian scientists have developed a new way of producing hydrogen from sunlight using genetically engineered algae. The scheme produces 13 times as much of the gas than a previous method of getting tiny plants to make it, and according to the team leader, it might be commercialised within five years, with a prototype appearing this year.
One lot of algae last two weeks, but after that the soup of dead algae could itself be used as fuel (or perhaps as chemical feedstock). That would release CO2, but growing the replacement algae would by definition take up as much of the gas as was released.