Hybridized Mechanical and Solar Energy-Driven Self-Powered Hydrogen Production

Highlights A hybridized mechanical and solar energy-driven hydrogen production system was developed. A rotatory disc-shaped triboelectric nanogenerator (RD-TENG) enables to harvest mechanical energy from water flow and functions as a sufficient external power source. WO3/BiVO4 heterojunction is fabricated as photoanodes in the self-powered photoelectrochemical (PEC) cell, and the hydrogen production rate reaches to 7.27 μL min−1 under sunlight illumination with the energy conversion efficiency of 2.59%. Electronic supplementary material The online version of this article (10.1007/s40820-020-00422-4) contains supplementary material, which is available to authorized users.


Supplementary Figures
UPS spectra of (a) WO3 and (b) BiVO4. The top of valence band is calculated to be about -7.2 eV and -6.9 eV (compared to the vacuum level) by subtracting the width of the He I UPS spectrum from the excitation energy (21.2 eV)  Rs is the series resistances. Cbulk is the capacitance of the space charge depletion region at the electrode surface. Rtrap is the resistance for trapping holes by surface states. Ctrap shows the amount of active sites in surface states. Rct is the resistance for charge transfer across the interface.   Supporting Note S1 Figure S1 shows the UPS spectra of WO3 and BiVO4. Ecutoff is determined by linear extrapolation to zero of the yield of secondary electrons. From Fig. S1, Ecutoff = 16.8 eV for WO3 and Ecutoff = 17.3 eV for BiVO4. The HOMO energy is determined using the incident photon energy, hv = 21.2 eV, Ecutoff, and Eonset. Also, Eonset = 2.8 eV for WO3 and Eonset = 3.0 eV for BiVO4. Thus, the HOMO energies are obtained directly from the UPS measurements, For WO3, EHOMO = -7.2 eV, and EHOMO = -6.9 eV for BiVO4. The LUMO energies were calculated using the HOMO levels and the optical gaps (Eg) obtained from the onset of absorption (Fig. 1f). Eg = 2.58 eV for WO3 and Eg = 2.41 eV for BiVO4. Thus for WO3, ELUMO = -4.62 eV; and ELUMO = -4.49 eV for BiVO4.

Supporting Note S2
The slopes from the Mott-Schottky plots are used to estimate the carrier densities using Eq. S2: where e0 is the electron charge (1.602 × 10 -19 C), ε is the dielectric constant of WO3 (2.3), BiVO4 (60), ε0 is the permittivity of vacuum (8.854 × 10 -12 F m -1 ), Nd is the donor density and V is the potential applied at the electrode. Capacitances were derived from the electrochemical impedance obtained at each potential with 10,000 Hz frequency in the dark. The Mott-Schottky slopes of both samples are positive which indicates that they are n-type semiconductors with electrons as majority carriers. The slopes are used to estimate carrier densities. The charge carrier densities after calculation for WO3 and BiVO4 are 6.81 × 10 23 and 7.30 × 10 19 cm -3 , respectively. Moreover, BiVO4 can only be considered as a modification in our experiment and very small quantity, so the result of the WO3/BiVO4 heterojunction can be supposed to be as the WO3, while the value is 6.81 × 10 23 cm -3 . The higher carrier density of the WO3/BiVO4 heterojunction photoanode compared with the individual sample BiVO4 could partly provide a benefit to the performance.

Supporting Note S3
Overall energy conversion efficiency (η) = Output Energy Total Input Energy = E 1 E 2 + E 3 where E1 is the energy of produced hydrogen involved, E2 is the input mechanical energy to drive a RD-TENG, and E3 is the input sunlight light energy.
When the rotation speed is 160 rpm, the calculation process of different energies is as follows: (1) Under illumination, the H2 production rates reached 7.27 μL/min, thus, where n1 and n'1 is moles of hydrogen under illumination and dark, , ΔG is standard Gibbs free energy.
(2) The rated torque of the rotary motor to drive the RD-TENG is 8.5 N m, thus, E 2 = T · n 9550 · t = 8.5 × 160 9550 × 1000 × 1 3600 × 60s = 2.37 J where T is the torque of the rotary motor, n is the rotation speed, t is the time.
In conclusion,