With the rapid development of informatization and the Internet of Things, the demand for self-powered electronic devices is increasing, and nanogenerators based on green energy are particularly promising for self-powered electronic devices. Hydrovoltaic power generation is an emerging hydropower generation technology that converts low-level energy contained in water into electricity through electrodynamic effects through the interaction of functional nanomaterials with raindrops, water waves, water evaporation, and ambient moisture. Among them, natural water evaporation is a spontaneous and ubiquitous process of absorbing environmental heat energy. As water flows through narrow channels in surface-charged materials, the interaction at the solid-liquid (water) interface is governed by an electrical double layer (EDL), through which ions in the water selectively migrate through the diffusion layer of the EDL due to electrostatic action, resulting in a flow potential and current. When power generation is induced by water evaporation, such nanogenerators are called water evaporation-induced electricity generators (WEIGs).
WEIGs usually use a variety of nanomaterials with high surface area and high surface charge density (such as carbon nanomaterials, semiconductor nanomaterials, metal oxide nanomaterials, etc.) to construct solid-liquid interface interactions. However, most WEIGs typically take a "bottom-up" approach to construct microchannels between nanomaterials, which are complex, expensive, non-biodegradable, and non-renewable. Therefore, considering cost and sustainability, there are higher requirements for the simple manufacture of biodegradable WEIGs.
Plants exhibit amazing water, nutrient and ion transport capacity under transpiration. Wood has a natural layered porous structure and multi-scale pores, with micron-sized tracheids with large diameters and thin cell walls providing water transport channels, while small-diameter, thick-walled nano-scale tracheids provide both excellent mechanical strength support and ion transport channels. The wood cell wall is composed of lignin, hemicellulose and cellulose, and the cellulose nanofibers (CNFs) embedded in the lignin and hemicellulose of the cell wall are vertically arranged with abundant charged groups (hydroxyl and carboxyl groups), and their surface charge density can be enhanced by surface chemical modification, with higher ion selectivity. The unique structure and modifiable properties of wood reveal its potential to harvest electricity from hydrothermal evaporative energy.
Professor Zhou Guofu's team and associate researcher Zhang Zhen and Lin Junyi of South China Normal University's research group studied a high-performance and biodegradable all-wood WEIG. Balsa wood (BW) was progressively delignified and hemicellulose treated by a "top-down" approach to expose more vertically aligned and surface-negatively charged CNFs that could be used as ionic nanofluidic channels for WEIG. Evaporation is used to drive the capillary flow of the aqueous solution in the CNFs nanofluid channel, so that the counterions in the solution are selectively transported upwards and accumulate at the top of the WIG to form a potential difference, which can achieve continuous power output. The performance of one-step delignified balsa wood (DBW) and two-step delignification and hemicellulose balsa wood (CBW) were compared, and the carbon electrodes were assembled into BW, DBW and CBW-based WEIGs (BWG, DBWG and CBWG), respectively. Due to the increased specific surface area, hydrophilicity, and surface charge density, the open-circuit voltage (Voc) and short-circuit current (Isc) of DBWG and CBWG are significantly increased at 0.2 V and 0.37 V, respectively. Compared with DBW, CBW's mechanical properties deteriorate and the structure collapses, especially in water, so CBWG's power generation performance is unstable. However, DBWG retains the inherent hierarchical porous structure of BW, has better mechanical properties, and the corresponding power output is more stable, so this paper mainly focuses on DBWG in the performance optimization of WEIGs. In addition, the DBWG itself can be used as a self-powered environmental sensor due to its fast response to environmental factors such as humidity, temperature, light, and wind speed that affect water evaporation. The authors further investigated the effects of different electrolyte solutions on the performance of DBWG. DBWG (40 mm × 40 mm × 2 mm) showed a Voc of 0.77 V and an Isc of 148 μA in a 1.2 M CaCl2 solution with a maximum load power of 8.35 μW.
In this paper, we propose a high-performance, sustainable and easy-to-prepare all-wood WEIG, which provides a new idea for the research of self-powered electronic devices. The research results were published in Advanced Functional Materials (https://doi.org/10.1002/adfm.202314231) under the title of All Wood-Based Water Evaporation-Induced Electricity Generator. The first unit of the paper is the South China Institute of Advanced Optoelectronics, South China Normal University, the first author of the paper is Lin Junyi, a 2021 master's student, and the corresponding authors of the paper are Zhang Zhen, associate researcher of Zhou Guofu's team of South China Normal University, Professor Fu Shiyu of South China University of Technology, and Professor Wang Xiuli of Sichuan University. This paper was supported by the National Key R&D Program of China, the Natural Science Foundation of Guangdong Province and the Youth Enhancement Project, the National Natural Science Foundation of China, and Synsys Technology.
Figure 1.Schematic diagram of the preparation and mechanism of an all-wood water evaporation induction generator (DBWG). The left panel shows the transpiration of water, nutrients, and ions along the tree cells, DBW is prepared by one-step delignification of BW, which retains the layered and porous structure of the wood and shows more exposed CNFs, shows the hierarchical structure of microscale wood cells, cellulose fibers, CNFs, and molecular-scale cellulose chains, the middle panel shows the sandwich structure of the DBWG (DBW is located between the two electrodes), and the right panel shows the multiphase transport in the DBWG, including water absorption, Ion separation and migration; the electrical energy generated by DBWG is attributed to the current potential and EDL.
- Preparation and characterization of 1.BW, DBW, and CBW
The pore structure, composition, mechanical properties, specific surface area, surface charge density and hydrophilicity of BW, DBW and CBW were studied by one-step method of NaClO2 solution and two-step method of NaOH and NaClO2 solution. The results showed that DBW and CBW had higher porosity, with specific surface areas of about 1.85 and 4.03 m2 g-1, respectively, which were 10 and 21 times that of BW (0.19 m2 g-1), respectively. The zeta potentials for BW, DBW, and CBW are approximately -21.4, -29.2, and -22.7 mV, respectively. In addition, delignification and hemicellulose treatments changed the hydrophobicity of BW, DBW became more hydrophilic, while CBW became superhydrophilic. Compared with DBW, CBW is mainly composed of cellulose, which becomes very soft, has poor mechanical properties, and is very prone to structural collapse in water. Compared with most wood treatment methods using strong acids or alkalis, the preparation method in this paper is simple, and the prepared DBW not only retains the graded porous structure of wood, but also has excellent mechanical properties, higher surface charge density (-29.2 mV), specific surface area (1.85 m2 g-1) and hydrophilicity (water contact angles of 83.9° and 88.6°).
Figure 2. a) Schematic diagram of the structure and composition of BW, DBW and CBW, b), c) and d) SEM images of the appearance and top-down and side-view of BW DBW and CBW, respectively.
Figure 3. a, b) Fourier transform infrared spectra of BW, DBW and CBW, c) X-ray diffraction pattern and d) XPS spectra, e) relative content of elements C and O calculated from XPS spectra and f) C═O content in C1s.
Figure 4. a) Specific surface area and physical adsorption pore size distribution of BW, DBW and CBW, c) d) Comparison of pore cross-sections and water contact angles of capillaries for BW, DBW and CBW, e) water absorption of BW, DBW and CBW membranes (40 mm × 40 mm × 10 mm), and f) Zeta potential of BW, DBW and CBW.
- 2. Power generation performance of BWG, DBWG and CBWG
Due to the increase in specific surface area, hydrophilicity, and surface charge density, the Voc and Wasc of DBWG and CBWG were significantly increased. Compared with DBW, CBWG's power generation performance is not stable due to the deterioration of CBW's mechanical properties and structural collapse, especially in water. The DBWG retains the inherent hierarchical porous structure of BW while providing a stable power output. DBWG has a Voc of 0.20 V, an Isc of 1.5 μA, and a maximum load power of 0.92 μW at 40% RH and 26°C.
Figure 5. a) Schematic diagram of WEIG power generation. b) c) Voc and Isc of the three wood WEIGs;f) Comparison of the Voc, Isc and resistance of the three wood WEIGs;d) Voc and Isc of the DBWG at different external load resistances;e) Power of the three wood WEIGs with different external load resistances;f) Comparison of the Voc and Isc and resistance of the three wood WEIGs;g) SEM of the DBW and CBW showing intact pore structure of the DBW and collapse of the internal pore structure of the CBW;h) The process of DBWG being discharged in deionized water for more than 14 hours over a long period of time (see Wasc measurement circuit diagram). Note: DBWG, cross-section, 40 mm × 40 mm; thickness 10 mm).
- 3. The influence of environmental factors on the performance of DBWG
Since environmental factors affect the evaporation of water, the power generation performance of DBWG is largely influenced by environmental factors. Under conditions of low relative humidity, high winds, strong light exposure and high temperatures, DBWG has a higher Voc value. Notably, DBWG has demonstrated a fast response to temperature. As a result, the DBWG itself can be used as an ambient self-powered sensor.
Figure 6. DBWG responds to electrical signals from its surroundings. a) Schematic diagram of water permeation and water evaporation in DBWG, b) Response of DBWG Voc to changes in relative humidity (25 °C) under atmospheric conditions, c) Response of DBWG Voc to changes in airflow velocity, d) Real-time voltage of DBWG immersed in DI water under xenon lamp irradiation or heating, e) DBWG Voc value under xenon lamp irradiation, and f) Voc of DBWG at different temperatures.
- 4. Effect of electrolytes on DBWG performance
The effects of electrolyte types and concentrations on DBWG were further studied. The results show that the performance of DBWG can be significantly improved by using salt electrolytes. By analyzing the ionic conductivity and electric double layer of DBW under different CaCl2 concentrations, the mechanism of electrolyte effect on DBWG performance was further discussed and revealed. At low concentrations (<10-2 M), the Debye length is similar to the channel size, and the EDL overlaps, resulting in a constant ionic conductivity of DBW around 1.46 mS/cm, which is determined by the surface charge of the CNFs channel in the DBW, independent of the ion concentration of the electrolyte solution. At high concentrations (>10-2 M), the ionic conductivity is positively correlated with the solution concentration. DBWG with a thickness of 2 mm stably produces 0.77 V Voc and 148 μA Wasc in 1.2 M CaCl2 solution, with a maximum load power of 8.35 μW.
Figure 7. Effect of electrolytes on DBWG. a) b) Voc and Wasc of DBWG in different electrolytes; c) Power of DBWG in different electrolytes under different external loads;d) Ionic conductivity of DBW in different salt electrolytes;e) Voc and Isc of DBWGs at different CaCl2 concentrations;f) Ionic conductivity of DBW-X and DBW-Y as a function of CaCl2 volume concentration, showing two distinct regions: concentration-dominant and surface charge-dominant;g) Schematic diagram of multiphase transport in the DBW cell wall and ion transport behavior in the CaCl2 electrolyte; h) Voc and Isc at different thicknesses with DBW.
5. Serial, parallel and application of DBWG
By connecting multiple DBWGs in series or parallel, the Voc and Isc of DBWGs can be significantly increased. Using the DBWG as a power source to charge the capacitor, the LED light was successfully lit.
Figure 8.Demonstration of amplified and light-emitting LEDs of three 100 μF capacitors charged in series by DBWGs. a) DBWGs and their Vocs in series, b) Voltage-time curves of commercial capacitors with different capacitors (100, 470 μF) when charging in CaCl2, c) Output voltage when charging 100 μF capacitors in series, and d) Three charging capacitors in series can light up an LED. Source: Frontiers of Polymer Science