To overcome the limitations of piezoelectric energy harvesters such as narrow bandwidth and low power density, our group has recently demonstrated a broadband harvester, which is based on amplitude-stiffened Duffing mode resonance. This nonlinear resonance greatly increases the bandwidth by keeping the harvester resonant until jumping down to a low energy state. Furthermore, the stretching strain of the nonlinear beam produces much higher maximum extractable electrical energy than that of a linear bending-based harvester. This design has been fabricated into a compact MEMS device, which is about the size of a US quarter coin.  The test results show more than one order of magnitude improvements in both bandwidth (~20% of the peak frequency) and power density (up to 2W/cm³) in comparison to the devices previously reported. To make the energy harvester better scavenge energy from ambient vibrations, which typically have low frequency spectra and low-g excitation, we are exploring new designs based on the nonlinear resonance. We have found that it is possible to bring the working frequency down to the range of 100 Hz to several hundred Hz, and lower the excitation level to ~0.5 g, by tuning the design parameters such as the dimensions of the resonator and external proof mass. The new low frequency, low-g designs will be implemented and tested soon. We anticipate that the broadband, low frequency, low-g piezoelectric energy harvesters will be used to power a wide range of devices including portable electronic devices and self-powered wireless sensors.