Recently, the team of Academician Guo Wanlin/Professor Yin Jun of Nanjing University of Aeronautics and Astronautics published a research paper titled "Sparking potential over 1200 V by a falling water droplet" in Science Advances.
In this paper, a comprehensive model of drip-generated water-volt device is proposed, which considers the droplet spreading dynamics on the macro scale, the dynamic process of EDL formation on the micro scale and the circuit capacitance. The dominant factor of the output voltage of the two-electrode configuration device is revealed, and the theoretical limit of the output of the device is given. This is used as a guide to optimize the device to achieve drip generating spark potential of more than 1200 V. The gas ionization at normal temperature and pressure is driven by the kilovolt pulse output, and the hydrogen production by water electrolysis is realized by combining the impedance matching design of the electrolytic water system.
Research background
In liquid-solid mixed systems, selective migration of ions through restricted channels changes local potentials, classic examples include action potentials in the nervous system and flow potentials in electrokinetic effects. However, the limitation of the rate of ion migration means that the rise in potential usually takes several milliseconds, and the voltage amplitude is limited to a few hundred millivolts.
In recent years, the water-volt effect of direct electricity generation by the interaction between functional materials and water provides a new way to utilize water energy. Among them, rainfall is widely distributed and contains huge energy, and obtaining electricity from falling raindrops has attracted wide attention. Through the introduction of polarized surfaces, transistor-inspired volume effects, patterned electrodes, surface charge injection and other methods, the output voltage of drip generating water voltages has been increased from the initial 10 mV, 1 V to 100 V.
In the face of the above challenges, the team of Academician Guo Wanlin/Professor Yin Jun of Nanjing University of Aeronautics and Astronautics proposed in this paper a comprehensive model considering the dynamic process of droplet diffusion at the macro scale, EDL formation at the micro scale, and circuit capacitance, systematically revealing the factors affecting the output performance of the dual-electrode drip generation water voltage device. The kilovolt output of the water-volt device is realized by optimizing the device, which is called spark potential.
First, the research team analyzed the operating principle of the device and its equivalent circuit. The drop and spread process converts the mechanical energy at the solid-liquid interface into electrical energy, which is equivalent to charging the capacitor C1. The moving boundary of the droplet then contacts the upper electrode and introduces C2, causing the electrical energy stored in C1 and CP to be released into C2 instantaneously. In addition, in the actual electrical test, there is also a circuit capacitor Ccir.
According to theoretical analysis, the dynamics of water/upper electrode contact is the key to determine the output voltage, and the influence of circuit capacitance can not be ignored. The research team first optimized the microsecond contact dynamics of the device, so that the droplet has a larger diffusion area and a higher impact sliding speed at the macro level, and a shorter EDL relaxation time and a thinner EDL thickness at the micro level at the moment of contact with the upper electrode. Vpeak was then increased to 1200 V by placing the upper electrode near the edge of the lower electrode, optimizing the size of the upper electrode, and optimizing the oscilloscope probe to minimize Ccir.
Further, the dependence between the output performance of the dual-electrode configuration device and the load impedance is clarified. The gas ionization at normal temperature and pressure is driven by the kilovolt pulse output under high impedance load, and the system impedance matching design is combined with the optimization of the electrolytic water system to achieve the electrolytic hydrogen production of water under low impedance load.
Significance and prospect
In this work, for the first time, a single drop induced output of a water-volt device over kilovolts is achieved, and a multi-scale dynamic model is established to reveal the factors affecting the output performance of the device and explain its internal mechanism. Based on this, targeted optimization has achieved an unprecedented high voltage output of over 1 kV, marking the water volt technology into the kilovolt era. This expands the device application scenario to gas ionization, water electrolysis and other application fields, provides a new vision for the development and utilization of water volt technology, and promotes the application of water volt in the field of high pressure and green energy.
The first author of the paper is Li Luxian, a doctoral student at Nanjing University of Aeronautics and Astronautics, with professors Guo Wanlin and Yin Jun as co-corresponding authors.
Links: http://www.science.org/doi/10.1126/sciadv.adi2993