Energy harvesting system driven by the interaction of conductive nanomaterials and water

Abstract

Emerging nano-hydroelectric technology utilizes hydraulic flow on nanomaterials to generate electricity, which gains increasing attention due to its simplicity, sustainability, renewability, eco-friendliness, and ubiquity. Up to date, several designs of nano-hydroelectric devices were devised for both liquid and vapor water as resources. However, the nano-hydroelectric devices are yet to be a practical energy source for the following reasons. First, the driving force for hydraulic flow on nanomaterials was too weak to produce high electrical power. Second, autonomous energy harvesting from the ambient environment was unavailable since the devices require periodical water supplement for continuous electricity generation. Lastly, the energy conversion efficiency of previous nanomaterials was not sufficient that even the state-of-art nano-hydroelectric generator requires more than tens of thousands of units to operate a low-power electronic. In this thesis, transpiration-driven electrokinetic power generator (TEPG), a novel water-powered energy harvester, was investigated with a series of developments

self-operating TEPG, MXene-based TEPG, MXene/polyaniline-based TEPG. Mimicking the natural transpiration process of plants, TEPG exploits the capillary water flow in asymmetrically-wetted carbon-coated cotton fabric to generate electricity. Multi-dimensional analysis of TEPG reveals clear energy generation mechanisms. Accumulation of protons induced by the electrical double layer formed at the solid (carbon)/liquid (water) interface gives rise to the potential difference between the wet and dry sides. The interaction of conductive carbon channels and water generates electrical current driven by the pseudo-streaming mechanism. Followed by the first work, an artificial hydrological cycle is applied to TEPG to continuously and autonomously generate electric power. The artificial hydrological cycle was formed by using deliquescent calcium chloride ($CaCl_2$), collecting water vapor from the surrounding environment to supply water to the TEPG in a closed system continuously. The self-operating TEPG generates stable electrical energy for more than two weeks by itself in the range of 15 ~ 60% relative humidity. Finally, MXene (titanium carbide, $Ti_3C_2$) nanosheets were utilized as electrokinetic converting materials to facilitate electrokinetic–conversion of TEPG to improve energy generation efficiency. Owing to the two-dimensional feature, strong cationic affinity, and the metallic conductivity, MXene-based TEPG could achieve 170-folds of improved electrical power than carbon-based TEPGs. Moreover, the combination of MXene with polyaniline reinforced ionic diffusivity and electrical network of MXene. The optimized MXene/polyaniline-based TEPG generates a maximum voltage of $0.54$ $V$, a current of $8.2$ $mA$, and an energy density of $30.9$ $mW/cm^3$ by using $30$ $\mu L$ of NaCl solution, which provides sufficient electric power to operate a low-power electronics and charge a commercialized battery.