There are certain motors/generators that are designed to work/output a DC voltage. They can only do this because they have brushes in contact with a commutator ring which will connect a specific set of windings based on position of the shaft. They are more expensive,have a much higher friction (considerable loss when you are trying to make it turn with the least amount of effort), and they are not known for being efficient. Automobiles used to use generators back in the 30's but all of them now use alternators (AC output) and rectify it to DC with diodes. They are cheaper and more efficient. Almost all the axial flux generator designs you will see for wind generators are 3 phase, which cogg less and rectify into a cleaner DC voltage with diodes. Diodes are cheap, btw.
