Figure 3.

(a) Triboelectric generation mechanism. (initial state) There is no charge on both surfaces. (i) contacted state: charge separation between the two surfaces by contact electrification; (ii) releasing state: differently charged contact surface induces charge flows between the top and bottom electrodes; (iii) separated state: charge neutral state is reached; (iv) pressing state: electrical equilibrium state breaks again and induced charges flow in opposite direction; (b) Triboelectric output voltages from non Pd-functionalized ITO and PET contact pair. There is no output voltage change since Pd does not react with H₂ gas; (c) Triboelectric output voltage of the H₂ gas sensor with a Pd-functionalized ITO and PET contact pair in varying H₂ concentrations. The output voltage sublinearly increases until H₂ concentration reaches 1%. The output voltages are saturated above 3% H₂ concentration; (d) Energy band diagrams of contact surfaces. Work function (WF) of Pd moves close to donor surface states (E₀) of PET as PdH_x is formed, increasing surface charge exchange.