Development and mechanism investigation of TiO2/Co hydrogel microgenerator utilizing humidity gradient

Utilizing ambient humidity to drive electron motion in nanomaterials and generate usable electricity is promising for self-powered electronics and sensors. This paper developed a humidity gradient-driven microgenerator (HGDM) by coating a cobalt hydrogel on part of the TiO2 nanoparticle film and employing multi-walled carbon nanotubes as electrodes. The humidity gradient was established between the bottom hygroscopic hydrogel and the top electrode in contact with ambient air. Results demonstrated that the TiO2/Co hydrogel HGDMs can achieve an open-circuit voltage of 0.95 V at 20 °C and 50–80% relative humidity (RH). The single generator (size: 5 × 4 cm2) exhibited a current output of approximately 0.1 mA, which remained stable for over 100 h with less than 1% decay. The increase in hydrogel films’ height first increased then decrease the performance, and the optimal value was provided. By alternately introducing 10% and 80% RH air humidity cycles, the generator responded rapidly with a wet/dry output ratio over 400, and the delay time maintained within 1.0 s after 1000 cycles. The output performance dropped severely when the air temperature was higher than 40 °C. In practical applications, a series of 5–7 micro-generators successfully delivered consistent power supply to LED lights and calculators for a duration exceeding one week, and the voltage output can be recovered to its initial value after less than 5 h of recharging. Mechanistic studies have revealed that the primary driving force for power generation is the humidity gradient rather than vapor evaporation. Because the diameter of the nanopores (around 200 nm) are much smaller than the Debye length of the double electric layer (960 nm), ion-selective permeability in TiO2 nanochannels is also important which can prevent nearly 90% OH– from passing through the channels. This hydrogel microgenerator exhibits wide-ranging application prospects, and our study contributes to enhancing its performance and elucidating the mechanism.