DC generator circuit
DC Motor can generate negative voltage which destroy most of circuit especially electrolytic capacitor. So you need to rectify it using full bridge rectifier unless you can make sure that motor always turn in 1 direction you can use single diode.
anyway diode have significant power loos in low voltage application due to vorward voltage. Try to use low forward voltage diode to minimize power loss.
Or you can use mosfet as full bridge rectifier for higher efficiency.
see this link