Bioinspired superwetting surface based high-performance hydrovoltaic device

Abstract

The rise in climate crisis, population explosion, and water and energy shortage sparks global attention towards the pursuit of renewable energy sources. Our society calls for the next-generation renewable energy sources that facilitates the transition from carbon-intensive based fossil fuels to fully decarbonized energy sources. Water occupies 70% of earth’s surface and absorbs nearly half of the solar energy that reaches the earth’s surface, which is the largest energy reservoir on earth. Traditional hydraulic power generation relies exclusively on water dam to convert mechanical energy of high-flux water flow into electricity generation. In spite of this centralized water energy, tremendous water energies featured with low-frequency, wide distribution and easy accessibility that manifests in a wide spectrum of forms such as rain, dew, fog, ocean wave and even water evaporation remain largely untapped. Recently, with an aim to exploit these underutilized energies, a myriad of water-enable electricity generators, such as hydrovoltaic technology, piezoelectric nanogenerator (PENG), pressure-retarded osmosis, reverse electrodialysis and triboelectric nanogenerator (TENG), emerge to largely expand the scope of current energy scavenging schemes. Despite notable progress, these energy harvesting techniques dedicated by interfacial charge generation and transfer is still constrained by low efficacy, unreliable output in long-term operation, and difficulty for commercial scalability.

Nature is an expert to sustainably and efficiently manipulate water and energy. Particularly, biological organisms leverage ingenious surface topography design to dynamically interact with ubiquitous hydrological environment, and achieve efficient water and energy exchange yet with minimum materials, which provides continuous inspirations for engineering surfaces for water energy harvesting. In this thesis, by fundamentally understanding key principles underpinning biological surface for controlling liquid-solid interactions, we developed bio-inspired superwetting surface for efficient mass and momentum transfer as well as rapid interfacial charge generation and transfer, thereby rendering a high-performance and robust hydrovoltaic device for sustainable energy production.

First, we present an overview of development milestones of hydrovoltaic technology, starting from the fundamentals underpinning hydroelectric generation to recent breakthrough in emerging hydrovoltaic devices for energy scavenging from entire hydrological cycle. Particularly, we elaborately discuss recent progress on the design of efficient hydrovoltaic devices that takes inspiration from biological surface with fascinating topological properties. Then, we also highlight the remaining challenges faced with hydrovoltaic devices on real-life application scenarios.

Second, we develop a droplet-based electricity generator incorporating Kelvin water dropper (K-DEG) to overcome the inherent limitations of low charge density and long charging time in previous design of DEG. Leveraging the rational integration of Kelvin water dropper and DEG, our K-DEG can achieve a remarkable high surface charge density of 358 μC/m² within a short charging time less than one second, thereby generating an output voltage up 2000V and instantaneous power density of 10^5 W, which is respectively 14 times and 2000 times than that of DEG. The key of such a remarkable improvement lies in the synergistic coupling of two droplet energy scavengers, in which Kelvin water dropper injects a large amount of ionized charges on the surface of DEG, while DEG with a transistor-like architecture can fully release these surface charges into electricity generation during one single droplet impingement. Alternatively, owing to the continuous charge replenishment from the Kelvin water dropper, the DEG can maintain a long-term stability of a high-density surface charge, thereby mitigating the inevitable deterioration of surface charge density resulted from the unwanted counter-ions adsorption in aqueous environment in previously reported DEG devices.

Third, in addition to intermittent and discrete droplet energy, we also developed evaporation-driven hydrovoltaic devices to convert ambient low-grade thermal energy into electricity through ubiquitous yet often neglected water evaporation process. As compared to conventional pressure-driven electrokinetic potential, the proposed hydrovoltaic device partially submerged in water leverages natural evaporation process to establish a potential difference towards the water diffusing path without any external energy input such as pressure, standing out as a promising self-sustaining hydroelectric energy sources. This evaporation-induced energy harvesting device is composed of a layer of aluminum oxide nanoparticles deposited on the ceramic substrate through blade coating, a top and bottom L-shape carbon paste electrode that are respectively patterned on the coating layer. The core components of this hydrovoltaic devices is loosely packed aluminum oxide nanoparticles that form positively charged nano/micro channels characterized with high hydrophilicity, large surface area and high ion permselectivity, imparting a micro-scale re-distribution of ions on water-solid interface, thereby synergically building up a macro-scale potential difference towards the direction of water capillary flow from bottom to top during evaporation. Moreover, leveraging the “cooling effect” brought by hydrovoltaic device during water evaporation, we integrate this hydrovoltaic device with a photovoltaic cell with an aim to dissipate excess heat generated on solar cell, rendering a more appreciate working temperature and superior photoelectric conversion efficiency for a solar panel. We envision that this hydrovoltaic device not only hold promise to efficiently harvest previously underutilized yet ubiquitous water energy that manifests in the form of evaporation but also shows wide applicability to integrate with other renewable energy scavengers such as solar cell.

In summary, this thesis reports two high-performance hydrovoltaic devices with distinctly contrast surface wettability, such as superhydrophobic surface for instantaneous shedding of droplet in the case of droplet energy harvesting and superhydrophilic surface for rapid liquid wicking in the case of evaporation-driven electricity generator. Inspired by the biological surface with elegant control of surface physico-chemical properties such as wettability, materials, composition and structure, we develop hydrovoltaic devices with superwetting surfaces to ultimately achieve massive transfer of mass and momentum as well as efficient charge generation and transportation, which provides a promise guidance for the design of high-performance sustainable water energy harvesters.

Date of Award	29 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pingan ZHU (Supervisor) & Zuankai Wang (External Co-Supervisor)