*2.1. Electrospinning of Nanofiber Membranes (NFMs)*

The P(VDF-TrFE) (75/25 mol%, Piezotech Inc., Pierre-Bénite, France) nanofibers were prepared as described previously [38]. Briefly, P(VDF-TrFE) powders were dissolved in the N,N-dimethylformamide (DMF) and acetone mixture solution (6:4 v/v) at 20% (w/v). The P(VDF-TrFE) spinning solution was injected into a 5-ml syringe fitted with a 22 G needle. A syringe pump (KDS101, KD Scientific, Holliston, MA, USA) was used to supply a constant flow rate of 1 mL/h. A high voltage of 15 kV was applied between the tip of the syringe needle and the grounded roller collector at a distance of 10 cm. The thickness of the electrospun NFM was regulated at ~30 μm by controlling the electrospinning time. The electrospun NFMs were dried at 65 ◦C for 10 h to volatilize the residual solvent. The annealed NFMs were kept in a vacuum oven at 135 ◦C for 4 h. The poled samples were pressed via a powder compression machine (BJ-15, BOJUNKEJI Inc., Tianjin, China) after annealing, in order to ensure no conduction between subsequent sputtering electrodes. Next, the samples were prepared by sputtering gold electrodes on both the top and bottom surfaces, then placed in a fixture and immersed completely in silicon oil. Afterwards, the silicon bath was heated to 115 ◦C by a hot plate (HCT basic, IKA Inc., Staufen, Germany) and then a polarization electric field of 80 to 100 MV/m was applied. After 30 min of thermal poling treatment, the polarization voltage was kept constant, cooled down to room temperature and finally the voltage was removed. Next, pristine P(VDF-TrFE) NFMs without any postprocessing (annealing and poling) were labeled as U-NFM, and the samples treated by annealing were coded as A-NFM. The poled samples were denoted as P-NFM. The NFMs poled with the electric field of 80 MV/m and 100 MV/m were labeled as P80-NG and P100-NG, respectively.
