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Topological phononic crystals (PCs) offer a novel approach to control the behavior of acoustic or elastic waves. In this paper, we design the gradient PC structures with coupled interfaces to realize topological rainbow trapping and broadband acoustic energy harvesting. Based on the geometric symmetry of PC unit cells, we construct the coupled topological interfaces by amalgamating two PCs with distinct topological phases. Also, the gradient modulation of structural parameters along the coupled interfaces is employed for the generation of rainbow trapping. Subsequently, a polyvinylidene fluoride (PVDF) film is mounted along the coupled interfaces of the gradient PC structures, facilitating the conversion of acoustic energy into electrical energy. The numerical and experimental results show that the acoustic waves at different frequencies stop and are amplified at different positions of the coupled interfaces. Compared with the harvester in a bare structure, the topological PC energy harvester significantly enhances the output power at different excited frequencies. In the experiments, the maximum amplification ratio of the output power reaches 91. Besides, it is demonstrated that the topological rainbow trapping of acoustic waves exhibits robustness against random structural disorders. Notably, the coupled interfaces have broadband and multi-mode features, which are promising for other applications such as selective filtering and sensing enhancement.

In this study, we design the 3D-printed phononic crystal (PnC) beam with the topological interface state for harvesting the mechanical energy of flexural waves. The PnC beam is formed by arranging periodic grooves on its surface. The PnC beam with either topologically trivial or non-trivial phase can be achieved via changing the distance between the grooves. The topological interface state is then generated by combining two PnCs with distinct topological phases. The existence of the interface state of the PnC beam is verified both numerically and experimentally. To convert the mechanical energy into the electricity, a piezoelectric disc is attached at the interface of the proposed PnC beam. Compared to the reference beam harvester, the measured output power is significantly amplified by the PnC harvester at the frequency corresponding to the interface state. Furthermore, the PnC beam energy harvester based on the topological state exhibits robustness against geometrical disorders.