In the framework of hybrid energy harvesting for scavenging ambient motion, this paper proposes a cooperative piezoelectric electromagnetic energy harvesting system to harvest rotational energy. In particular, while the actual process of harvesting energy is accomplished by the electromagnetic device, the piezoelectric element is used for improving the AC/DC conversion efficiency of the former. To do so, a half wave voltage doubler using MOSFETs driven by a piezo element is employed. The low voltage output (order of magnitude of mV) of the electromagnetic system and the low conversion abilities of the piezoelectric transducer in the proposed mechanical structure justifies the motivation behind this work. Simulations followed by the experimental validations are exposed and discussed, highlighting the improvement of energy conversion efficiency of an electromagnetic transducer, giving a power gain of 27 with respect to the DC power obtained with standard silicon diodes.