The mechanism of the water decomposition via the PZEC effect relies on the piezoelectric properties of the materials. Although the piezoelectric effect has been known for over 100 years and has been demonstrated in many fields, little work has been done to address its application in wet conditions (such as in solution) and particularly in the direct conversion of mechanical energy to chemical energy.

...conditions. In this study, we use microfibers of ZnO and dendritic BaTiO₃ to initiate a phenomenon and drive a nonspontaneous redox reaction, the formation of H₂ and O₂ gases from water, by using mechanical energy. Here, we show the capabilities of these materials for scavenging energy waste from the environment, such as noise and vibration, to generate hydrogen and oxygen gases.

—Hong et al.

The researchers, led by UW-Madison geologist and crystal specialist Huifang Xu, grew nanocrystals of two common crystals, zinc oxide and barium titanate, and placed them in water. When pulsed with ultrasonic vibrations, the nanofibers flexed and catalyzed a chemical reaction to split the water molecules into hydrogen and oxygen.

When the fibers bend, asymmetries in their crystal structures generate positive and negative charges and create an electrical potential. Smaller fibers bend more easily than larger crystals and therefore also produce electric charges easily. So far, the researchers have achieved an 18% efficiency with the nanocrystals, higher than most experimental energy sources.

The physics and chemistry of generating hydrogen and oxygen gases from pure water arise from the combination of piezoelectric properties of ZnO fibers and BaTiO₃ dendrites and the redox reaction of water. Both ZnO and BaTiO₃ are well-characterized piezoelectric materials...Specific morphological aspects of ZnO and BaTiO₃ such as fibers and dendrites will acquire electric potentials on their surfaces if an external mechanical energy is applied that results in a bending (deformation) of the fibers or dendrites. The strain induced electric potential formed on the fiber or dendritic surface in wet conditions (i.e., in pure water) is available for the reduction and oxidation reaction via charge transfer to species such as water molecules adsorbed on the surface.

Note that the developed potential must be greater than the standard redox potential of water (1.23 eV) to make electrons available to initiate the redox reaction in this experiment. Residual charges or potentials lower than 1.23 eV will not participate in reactions to form H₂ and O₂ from water.

—Hong et al.

In addition, they noted, in the PZEC effect, the catalyst—i.e., the zinc oxide and barium titanate—participated in the direct water splitting reaction by donating strain-induced electrons and holes without being oxidized, reduced, or decomposed. TEM and XRD observations showed that no metal species or other extra phases appeared in our experiment samples before and after the reactions.

Because the fiber and plate sizes can be tuned, said Xu, even small amounts of mechanical noise—such as a vibration or water flowing—could bend the fibers and plates. With the right technology, Xu envisions this method being useful for generating small amounts of power from a multitude of small sources—for example, walking could charge a cell phone or music player and breezes could power streetlights.

Using fibrous ZnO and dendritic BaTiO₃ catalysts with piezoelectric properties, we have demonstrated the PZEC effect for generating H₂ and O₂ from water. We have successfully verified a direct conversion of mechanical energy to chemical energy. Finding an optimum fiber length and introducing the resonance frequency of ZnO and BaTiO₃ for the direct water splitting process, it may be possible to obtain a much greater H₂ and O2 production rate.

Utilizing the piezoelectric fibrous samples, the phenomena demonstrated could usher in a new era in the field of recycling environmental energy wastes. Vibrational energy waste generated in the environment from noise, wind power, running water, or water wave action can be scavenged or harvested as a driving force for direct water splitting, thereby forming H₂ and O₂ by means of PZEC fiber arrays implanted on a substrate. The fiber arrays can also be used to harvest artificial energy wastes such as traffic noise and vibrations and convert them into hydrogen and other chemical energies.

The principle of the PZEC effect using these fibers could be a very important step forward in nanotechnology that recycles the energy wastes from the environment into precious alternative chemical energy. This work will open a new field of study on hydrogen generation, redox reactions, and energy recycling.

—Hong et al.

The new paper is co-authored by graduate student Kuang-Sheng Hong, research scientist Hiromi Konishi and mechanical engineering professor Xiaochun Li, all at UW-Madison. Xu’s research is supported by grants from the UW-Madison Graduate School, National Science Foundation, NASA Astrobiology Institute and the US Department of Energy.

Resources

• Kuang-Sheng Hong, Huifang Xu, Hiromi Konishi and Xiaochun Li (2010) Direct Water Splitting Through Vibrating Piezoelectric Microfibers in Water. J. Phys. Chem. Lett., Article ASAP doi: 10.1021/jz100027t