Towards smart cities powered by nanogenerators: Bibliometric and machine learning–based analysis

Highlights: • Piezoelectric, triboelectric, ‎and pyroelectric nanogenerators are generally described. • Progress in nanogenerators is visualized by bibliographic and multivariate methods. • The technical/environmental performance of the nanogenerators' materials is analyzed. • Efficient, inexpensive, and green materials should be used in future nanogenerators. • The analysis results indicate that China is forefront in nanogenerators research. Nanogenerators have attracted particular attention during the last decade. A nanogenerator harness mechanical or thermal energy to produce electricity without any need for battery. In this paper, general descriptions of different types of nanogenerators, including piezoelectric, triboelectric, and pyroelectric, are presented. Next, bibliometric analysis and unsupervised machine learning–based analysis (principle component analysis) are carried out to determine the trends in this field of science. The current developments and directions for technology commercialization are briefly discussed. Additionally, the output performance and environmental consequences of active materials used in nanogenerators are analyzed by principle component analysis. The analysis reveals China's sensible investment in the nanogenerator field during the last five years. The number of articles published by Chinese scholars is over 1000 during 2015–2020 with a considerable distance from the USA. It is also deduced that European countries need to pay more attention to this field if they want to compete with the USA, China, and South Korea. Overall, due to the huge amount of waste thermal energy worldwide, more efforts and resources have to be focused and invested in developing pyroelectric nanogenerators. Moreover, there is a need to find low-cost materials having a favorable environmental profile to fabricate nanogenerators.