Why is a high use of renewable energy necessary to make hydrogen fuel an environmentally sound alternative?

Hydrogen is similar to electricity. Although an electric vehicle, for an example, doesn't create any tail pipe emissions from the vehicle, that may not be the case where the electricity was made. If the electricity was made from an old, smoking, worn out gasoline powered generator in order to charge the electric vehicle, the total pollution created would be much more than that made by a regular gasoline car. However, if the electricity is made by an environmentally friendly renewable source, such as solar or wind, then the total pollution created in powering the electric vehicle would be much less, perhaps even zero. 

Hydrogen can be made from electricity or petroleum. If made from petroleum it wouldn't be much different than gasoline because the rest of the petroleum would have to be used somewhere. If the hydrogen is made from electricity then the question again is where is the electricity being made. 

Your question is also a good one because it highlights the "high use" of renewable energy. The production and use of hydrogen in less efficient than running just off of electricity. So you'd have to produce a whole lot more electricity to make hydrogen to drive a car a certain distance than to charge a battery-electric car and drive the same distanc

Electrons are transferred to ferredoxin.