High-performance piezoelectric nanogenerators for self-powered nanosystems: quantitative standards and figures of merit.

Harvesting energies from the atmosphere cost-effectively is critical for both addressing worldwide long-term energy needs at the macro-scale, and achieving the sustainable maintenance-free operation of nanodevices at the micro-scale (Wang and Wu 2012 Angew. Chem. Int. Ed. 51 11700-21). Piezoelectric nanogenerator (NG) technology has demonstrated its great application potential in harvesting the ubiquitous and abundant mechanical energy. Despite of the progress made in this rapidly-advancing field, a fundamental understanding and common standard for consistently quantifying and evaluating the performance of the various types of piezoelectric NGs is still lacking. In their recent study Crossley and Kar-Narayan (2015 Nanotechnology 26 344001), systematically investigated dynamical properties of piezoelectric NGs by taking into account the effect of driving mechanism and load frequency on NG performance. They further defined the NGs' figures of merit as energy harvested normalized by applied strain or stress for NGs under strain-driven or stress-driven conditions, which are commonly seen in the vibrational energy harvesting. This work provides new insight and a feasible approach for consistently evaluating piezoelectric nanomaterials and NG devices, which is important for designing and optimizing nanoscale piezoelectric energy harvesters, as well as promoting their applications in emerging areas e.g. the internet of things, wearable devices, and self-powered nanosystems.