A directional piezoelectric sensor based on anisotropic PVDF/MXene hybrid foam enabled by unidirectional freezing

Directional sensors endowed by inherent anisotropic structure have drawn great attention in intelligent sensing systems. Unidirectional lyophilization is the mainstream technique to architect anisotropic structure particularly for water dispersible systems as the ice template can be readily removed at the melting point of water (0 degrees C). However, it is impractical for water insoluble polymers such as piezoelectric polymer systems as it features extremely low lyophilization temperature (for instance, -61 degrees C). So far, no piezoelectric polymer derived anisotropic devices has been realized. In this work, for the first time, we succeeded the constructing of anisotropic PVDF/Mxene device with directionally oriented micropores through delicate formulation design which enables the maintaining of oriented template even at temperature above 0 degrees C. As a result, the as-fabricated PVDF/Mxene composite foam exhibits distinct anisotropic behavior, featuring compressive modulus along the direction of penetrating microchannels being approximately 1.7 times of that in the orthogonal direction. The anisotropic foam is applied as directional sensor with piezo-current as the output signal, which displays a 3.93fold higher sensitivity difference in the direction perpendicular to the microchannel than in its orthogonal direction. Moreover, the foam exhibits the highest sensitivity of 41.3nA/kPa over pressure range of 2.5-20 kPa, which outperforms state-of-the-art polymer piezoelectric sensors. This novel strategy provides a practical avenue for architecting water-insoluble polymer-derived devices with anisotropic structure in a green and energy-saving manner, which could spur innovations from the fields of electromagnetic interference shielding, energy storage, piezoresistive, thermal insulation and so on wherever anisotropic characteristic is desired.