Ultrafast Metal‐Free Microsupercapacitor Arrays Directly Store Instantaneous High‐Voltage Electricity from Mechanical Energy Harvesters

Abstract Harvesting renewable mechanical energy is envisioned as a promising and sustainable way for power generation. Many recent mechanical energy harvesters are able to produce instantaneous (pulsed) electricity with a high peak voltage of over 100 V. However, directly storing such irregular high‐voltage pulse electricity remains a great challenge. The use of extra power management components can boost storage efficiency but increase system complexity. Here utilizing the conducting polymer PEDOT:PSS, high‐rate metal‐free micro‐supercapacitor (MSC) arrays are successfully fabricated for direct high‐efficiency storage of high‐voltage pulse electricity. Within an area of 2.4 × 3.4 cm2 on various paper substrates, large‐scale MSC arrays (comprising up to 100 cells) can be printed to deliver a working voltage window of 160 V at an ultrahigh scan rate up to 30 V s−1. The ultrahigh rate capability enables the MSC arrays to quickly capture and efficiently store the high‐voltage (≈150 V) pulse electricity produced by a droplet‐based electricity generator at a high efficiency of 62%, significantly higher than that (<2%) of the batteries or capacitors demonstrated in the literature. Moreover, the compact and metal‐free features make these MSC arrays excellent candidates for sustainable high‐performance energy storage in self‐charging power systems.


2D two-phase model for PEDOT: PSS
To gain more insight into dependence of the CV curves and working voltage window (WVW) of the MSCs on the material morphology in the PEDOT:PSS electrodes (especially the width of the PEDOT-rich region), we employ the 2D two-phase model [1] developed by Volkov et al.   to conduct computer simulations.As illustrated in Fig. 1b, the two-phase model represents a nano geometry of the PEDOT:PSS electrode (of thickness lp) which consists of the electronically-conductive PEDOT regions (of width wPEDOT) and the ionically-conductive PSSrich regions (of width wPSS).The metal current collector and electrolyte are located on the left and right sides, respectively.The electron (hole) and ion transport inside the two-phase electrode is modeled with the modified Nernst-Plank-Poisson approach [1] .The boundary conditions are constrained based on the assumption that only holes transport in the PEDOT region, while only ions transport in the PSS and the electrolyte regions, as detailed below.
There are three types of chemical species in the systems, hole (h), cations (+) and anions (−).
Their flux densities are described by the continuity equation (S1) where   and   are respectively the concentration and flux of a chemical specie with i being h, + or − t is time,  is the Faraday constant,  is the electric potential,  0 is the permittivity of the vacuum,   = 81 is the dielectric permittivity of the system, and  fixed = 1 mM is the concentration of residual negative ions on the PSS chains and only exists in the PSS regions.
In the PEDOT-rich region, only holes can transport, and the hole flux  ℎ ⃗⃗⃗ obeys the modified Nernst-Planck equation as where  ℎ is hole concentration, the additional factor (1 being the concentration of accessible sites of holes,  ℎ is the hole diffusivity in PEDOT,  =   ( is the molar constant, and  = 300 K is the temperature).The boundary conditions for Eq.(S1) and (S3) in the PEDOT region are specified in Figure S2a.
In the PSS-rich region and electrolyte region, only ions can transport so that modified Nernst-Planck equation reads as where  ± is the diffusivity of the cations (+) and anions (−),  max is the maximum ion concentration in the regions.The last term in Eq. (S4) is a correction because of the finite size of ions [1] .The boundary conditions for Eq.(S1) and (S4) in the PSS and electrolyte regions are specified in Figure S2b.Note that the continuity boundary condition for ion concentrations holds at the interface between PSS regions and electrolyte.
Finally, throughout all the regions, the hole/ion concentrations are correlated to the potential distribution according to the Poisson's equation (S2).The boundary conditions for Eq.(S2) specified in Figure S2c.Note that the continuity boundary condition for potential V holds at all the internal interfaces (PEDOT-PSS, PEDOT-electrolyte, and PSS-electrolyte).The potential applied at the metal current collector is , where  min and  max are the minimum and maximum applied potential, respectively,  is the scan rate, and  0 = ( max −  min )  ⁄ is the half cycle period.With the applied  in (), Eq. ( 1)-( 4) can be self-consistently solved to get the distribution of  ℎ ,  ± , and V.The current density is obtained by integration over the PEDOT region as The plot of () against  in () gives the simulated CV curves.

Figure S1 :
Figure S1: Electrochemical performance of MSCs fabricated through mask-based process with PEDOT:PSS dispersions doped with EG at different ϕEG.a-c, CV curves (at the scan rate of 25 mV s -1 ).d, Nyquist plot, (inset) Photograph of the MSC with ϕEG = 5 vol%.e, Capacitance retention versus scan rates.

Figure S4 :
Figure S4: Characterization of PEDOT:PSS DIW inks.a, Viscosity as a function of shear rate for different mass concentration, (inset) Photographs of the 1.4% and 3% inks placed upside down.b, Photographs of printed patterns with the inks of different concentration (scale bar 1 mm).c, Demo of printed conducting polymer circuits on paper substrates that can be used to light up an LED.

Figure S5 :
Figure S5: Cross-sectional SEM images of the MSCs with 25 printing passes.

Figure S6 :
Figure S6: Transmission line method (TLM) used to measure the conductivity of the DIW printed PEDOT:PSS patterns.a, Photograph of the DIW printed PEDOT:PSS lines and inkjet printed silver contacts on photopaper.The PEDOT:PSS lines have length ranging from 1 to 5 mm, and width ranging from 1.2 to 1.5 mm.b, Plot of resistance against line length for various thickness (ranging from 16 to 86 μ ).c, Sheet resistance and conductivity of printed PEDOT:PSS lines.

Figure S7 :Figure S8 :
Figure S7: Schematic of the fabrication process of the fully-printed MSC arrays.

Figure S9 :
Figure S9: Additional electrochemical characterization of the MSCs with different thickenss.a, GCD curves at current density of 10 mA cm -2 .b, Cycling performance for 10000 CV tests for the MSC of 107-µm-thick electrode, (inset) CV curves after different number of cycles.c, CV curves at the scan rate of 1000 mV s -1 .d, Bode plot and Nyquist plot (inset) of MSCs.

Figure S11 :
Figure S11: Additional electrochemical performance of the large-scale MSC arrays.a,b, GCD curves at 50 µA (a) and Nyquist plot (b) of the MSC arrays with different cell number.

Figure S12 :
Figure S12: Dependence of WVW on the cell number of the MSC arrays in the literature.

Figure S13 :
Figure S13: Characterization of flexibility of the large-scale MSC array.Photographs (ae) and CV curves at 25 V s -1 (f) of the MSC array under the different bending angles.

1 Figure S14 :
Figure S14: The self-charging power system.a, Photograph of the self-charging power system consisting of the DEG, bridge rectifier and printed 100-cell MSC array.b, Output current from DEG with a load resistance of 10 kΩ.The bridge rectifier used in this work is BAS4002A-RPP (Infineon Technologies).

Figure S15 :
Figure S15: Comparison of rate capability between 6-cell MSC arrays of doped and undoped PEDOT: PSS.a,b, CV curves at the scan rate of (a) 300 mV s -1 and (b) 6000 mV s -1 .c, The overall capacitance versus scan rate.

Figure S16 :Figure S17 :
Figure S16: Electrochemical performance of the MSC array of different cell numbers.a, Photograph of the MSC array on carton paper substrate (used in Fig. 3d-g, the electrodes are printed with 2 DIW passes of PEDOT:PSS ink).b, GCD curves at the current of 10 μA.c, Capacitance calculated from the GCD curves.

Figure S18 :
Figure S18: Equivalent circuit diagram for testing average output power of the DEG.

Table S1 .
The meaning and Initial Value of the parameters in the simulation model.

Table S2 .
Summary of the electrochemical performance of the state-of-the-art printed SCs.

Table S4 .
Summary of the energy storage performance of the state-of-the art self-charging power systems.