Multistage coupling water-enabled electric generator with customizable energy output

The constant circulation of water between the land, ocean, and atmosphere represents a vast, sustainable energy source. Recent advancements in water-enabled electric generators (WEGs) and hydrovoltaic technology exploit this energy using the interaction between functional materials and water in various forms (liquid, moisture). Moisture-enabled electric generators (MEGs), for instance, utilize ion diffusion in graphene oxide to convert water's chemical potential variation into electricity. Liquid flow within nanostructured materials generates streaming currents via the electric double layer at the solution-material interface, driven by evaporation or external pressure. Water droplet movement on surfaces like multi-wall carbon nanotubes also produces electricity. However, current single-stage WEGs discharge water after a single use, leading to significant energy waste. In contrast, natural processes like arid soil water absorption demonstrate multi-step water utilization, maximizing resource use. This research introduces a multistage coupling water-enabled electric generator (mc-WEG) that efficiently uses both liquid flow and moisture diffusion for synchronous electricity generation, overcoming limitations of single-stage WEGs.

Existing literature extensively covers water-enabled energy generation. Studies have explored moisture-enabled electric generation using graphene oxide frameworks, achieving efficient electricity generation from ambient humidity gradients. Other research focuses on nanofluidics for osmotic energy conversion, using nanostructured carbon materials for water-evaporation-induced electricity. Bio-inspired approaches utilize protein nanowires and microbial biofilms for energy harvesting from water evaporation and moisture flow. However, a gap remains in efficiently harnessing the multistage energy potential of water in a single device, a problem that mc-WEG directly addresses.

The mc-WEG consists of three layers: a water-flow-enabled electricity generation layer (wf-layer), a moisture-diffusion-enabled electricity generation layer (md-layer), and a polyacrylonitrile (PAN) membrane sandwiched between them. The wf-layer is a CaCl₂ asymmetrically loaded carbon fabric. The asymmetric loading creates a potential difference as water flows from the high-concentration to the low-concentration region. The PAN membrane facilitates water transfer from the wf-layer to the md-layer. The md-layer is a polyelectrolyte membrane (H2SO4-doped polystyrene sulfonic acid membrane sandwiched between PVA membranes with different LiCl contents), where moisture induces ionic concentration gradients and subsequent ion migration for electricity production. The materials were prepared using techniques such as soaking, electrospinning, and casting. The wf-layer was fabricated by soaking cotton fabric in a carbon black nanoparticle dispersion, followed by partial immersion in a CaCl2 solution. The PAN membrane was created through electrospinning, forming a porous structure on an Au mesh substrate. The md-layer was assembled layer-by-layer from PVA-LiCl membranes and an H-PSS membrane. The device was then assembled by stacking these three layers and encapsulating with polyimide tape. Characterization included SEM, EDS, water absorption and permeability tests, and electrical measurements using Keithley instruments. Various configurations of mc-WEG were tested to optimize power output and assess its performance under different conditions.

The mc-WEG successfully demonstrated multistage electricity generation. The wf-layer, utilizing liquid flow, generated -0.24 V and -31 µA under asymmetrical humidity conditions. The output was stable for extended periods. The md-layer, based on moisture-induced ion migration, showed diode characteristics in its current-voltage curve, exhibiting significant ion rectification with a maximum rectification ratio of approximately 382. When integrated, the mc-WEG generated 0.67 V and 104.89 µA from the md-layer and 0.24 V and 42.70 µA from the wf-layer, resulting in a maximum power output of -92 mW m⁻² (-11 W m⁻³). The mc-WEG design's flexibility allowed for customization through size control, space optimization, and integration design. Parallel and series configurations of the layers were tested, demonstrating adjustable output voltage and current. Outdoor testing showed stable performance over a range of temperature and humidity fluctuations. Twenty-two mc-WEG units connected in series delivered -10.32 V and -280 µA, sufficient to power a table lamp for over 30 minutes without pre-charging a capacitor. The mc-WEG's flexibility allowed it to maintain its electrical output even when folded, demonstrating excellent environmental adaptability.

The mc-WEG successfully addresses the limitations of single-stage WEGs by utilizing multistage water energy harvesting. The simultaneous use of liquid flow and moisture diffusion significantly enhances power generation efficiency. The modular and customizable design provides a high degree of freedom in tailoring the device's output for various applications, unlike previous WEGs that often required capacitor pre-charging. The stable performance in varying environmental conditions demonstrates its practical potential for widespread applications. The achieved power output surpasses many previously reported WEGs, indicating a significant advancement in the field. The simple fabrication methods, using readily available materials, also contribute to its scalability and cost-effectiveness.

This research successfully developed a multistage coupling water-enabled electric generator (mc-WEG) capable of efficiently harnessing both liquid flow and moisture diffusion for electricity generation. The customizable design, high power output, and environmental adaptability make it a promising technology for various applications. Future research could focus on optimizing the materials and structure for even higher power density and exploring integration with other energy harvesting technologies for hybrid power systems.

While the mc-WEG shows significant promise, limitations exist. The power output is still relatively low compared to traditional power sources, although it is superior to many other WEGs. The long-term stability and durability of the device under continuous operation need further investigation. The dependence on environmental humidity might limit its applications in arid regions. Further research is needed to optimize the device's performance across a wider range of environmental conditions.