A conformal van der Waals graphene coating enabled high-performance piezo-ionic sensor for spatial, gesture, and object recognition

Abstract

Transforming mechanical forces into electrical signals is vital in self-powered strain sensors. Compared to piezoelectric materials, piezo-ionic materials show great potential in self-powered strain sensors because of the passive and direction identification characteristics arising from the ion gradient under pressure. However, the low signal output (less than 5 mV) of piezo-ionic sensors might be influenced by the bioelectricity when used in practical applications. In addition, the normally used metal electrodes face fracture under high and long-term strain. Here, we used a dry coating method to prepare highly conformal, highly electrically conductive (minimum sheet resistance of 16 ohm sq⁻¹), and highly deformed graphene electrodes with a van der Waals (vdW) structure. Such conformal vdW graphene (vdWGR) electrodes could enhance the signal output to 90.3 mV and resist a large strain of 50% bending variation with 10 000 cycles, avoiding bioelectricity interference and electrode fracture during use. Interestingly, the vdWGR-assisted piezo-ionic sensor exhibited a special “bouncing back” peak when touched by soft objects, which could be utilized for object recognition. We demonstrated its applications in spatial, gesture, and object recognition by utilizing the piezo-ionic strain sensor's differences in the peak value, directionality, and shapes. The preparation method for high-performance vdW graphene electrodes can be extended to other flexible electrodes or electronics. Besides, the piezo-ionic effect identifying the softness or hardness of objects might assist piezo-ionic sensors in more applications.