Circadian humidity fluctuation induced capillary flow for sustainable mobile energy

The search for sustainable and readily available energy sources is a critical global challenge. While solar and wind energy are significant contributors, their intermittent nature and geographical limitations drive the need for alternative solutions. This research explores the untapped potential of ambient humidity fluctuations, a ubiquitous and continuous natural phenomenon, as a source of energy. Previous studies have demonstrated various drop-based generators that convert liquid motion into electricity, such as those using droplet spreading, falling, bouncing, or dragging. However, these often rely on significant liquid movement and don't harness the subtle, yet continuous, changes in ambient humidity. This work presents a novel approach: a device that continuously harvests energy from the daily cyclical variations in air humidity using static ionic liquid droplets. The core hypothesis is that moisture absorption/desorption within the ionic liquid droplet causes directional flow along the solid/liquid interfaces, which in turn induces a potential difference due to ion redistribution. This approach offers a distinct advantage, leveraging the ubiquitous and consistent nature of daily humidity fluctuations for continuous, low-power energy generation, eliminating the need for large-scale motion and offering a potentially widespread and readily accessible energy source.

Existing literature highlights several drop-based generators that leverage the movement of liquid droplets for electricity generation. These methods typically rely on dynamic processes like droplet spreading, falling, bouncing, or dragging to create a voltage. One approach uses the motion of organic or water droplets on surfaces to generate millivolts due to the drawing potential effect. Another involves impinging water droplets onto a PTFE surface to yield higher power output. Additionally, liquid-liquid triboelectric nanogenerators utilizing falling droplets across a liquid membrane have been developed with high charge collection efficiency. These studies focus on dynamic droplet movement to harvest instantaneous electrical energy. While some research explores humidity-related energy generation, these often involve short-duration power generation due to rapid saturation of the system. This study differentiates itself by utilizing the continuous, subtle fluctuations in ambient humidity to generate sustained electrical power using static droplets, representing a novel mechanism for energy harvesting from a ubiquitous and under-utilized environmental resource.

The study employs an innovative design using spherical cap-shaped ionic liquid (1-Octyl-3-methylimidazolium chloride) droplets pinned on a poly(dimethylsiloxane) (PDMS) nanowire array. The PDMS nanowire array is chemically modified to enhance power generation and provides a pinning effect for the ionic liquid droplet, preventing the triple-phase contact line from freely advancing or receding. The ionic liquid was chosen for its negligible evaporation losses and high moisture absorption/desorption capabilities. The device's open-circuit voltage (V_oc) was measured using a digital multimeter under various humidity conditions, both outdoors and in controlled indoor environments. In-situ microscopic imaging, using microspheres embedded in the ionic liquid, was employed to directly observe the directional capillary flow induced by moisture absorption and desorption. The experiments investigated the influence of relative humidity (RH) on the flow rate and power generation performance. Furthermore, molecular dynamics (MD) simulations were conducted to elucidate the mechanism behind the moisture-mediated surface gradient and the moisture-induced cation-anion interactions within the ionic liquid. The MD simulations used LAMMPS to investigate the interactions between the modified PDMS nanowires and the ionic liquid, focusing on radial distribution functions (RDF) to quantify interactions at varying water content. The simulations investigated the effects of pressure gradients on ion velocity and charge distribution near the PDMS surface to understand the generation of potential differences. The fabrication of the PDMS nanowire array involved mixing PDMS prepolymer and curing agent, followed by depositing conductive adhesive and Ag wires onto an AAO template. After removing the template, the PDMS nanowire array was embedded within Ag/AgCl electrodes. Characterization techniques included SEM, EDS, and XPS. Power generation measurements were performed using a digital multimeter under various conditions: outdoors, in controlled indoor environments, and under varying humidity levels.

The experimental results demonstrated a strong correlation between moisture absorption/desorption and power generation. Continuous voltage output ranging from ~0.14 to ~0.15 V was observed over three days under fluctuating relative humidity (RH). In-situ microscopic imaging confirmed the hypothesis of directional capillary flow induced by moisture absorption/desorption. Inward flow of microspheres was observed during moisture absorption, and outward flow during desorption. Analysis showed that flow velocity increases with increasing RH, ranging from 0.2–0.8 μm/s at 5% RH to 65–100 μm/s at 90% RH. MD simulations revealed that absorbed water molecules enhance the interactions between the PDMS nanowires and the ionic liquid's anions, leading to an asymmetric distribution of cations and anions within the flow, causing a potential difference. The simulations further showed that the confined space within the nanowire array restricts the movement of cations, promoting charge separation. Under wind conditions, the streaming potential was further improved, likely due to accelerated moisture absorption/desorption. Finally, the device demonstrated its utility by successfully powering an LED and an LCD screen, showcasing the potential for utilizing this ubiquitous environmental resource for low-power applications.

The findings confirm the feasibility of harvesting energy from subtle daily humidity fluctuations using a simple yet effective device. The continuous voltage output achieved over extended periods highlights the potential of this approach for providing a constant, low-power energy source. The mechanism of directional capillary flow driven by moisture absorption/desorption, confirmed by both experimental observations and MD simulations, provides a fundamental understanding of this novel energy harvesting process. The successful powering of electronic devices underscores the practical implications of this technology. The study's results expand the scope of sustainable energy resources, adding a readily accessible and consistently available energy source to the existing options like solar and wind energy. The ubiquitous nature of humidity fluctuations makes this approach potentially deployable in various locations and climates, offering a promising solution for low-power applications in diverse settings.

This research successfully demonstrates a novel method for harvesting energy from ubiquitous humidity fluctuations. The device uses the directional capillary flow of ionic liquid droplets on a nanowire array to generate continuous electrical power. This approach offers a sustainable and readily accessible energy source, potentially applicable to various low-power electronic devices. Future research could focus on optimizing device design for increased power output, exploring different ionic liquids and nanowire materials, and investigating applications in remote or off-grid environments.

While this study demonstrates the feasibility of energy harvesting from humidity fluctuations, several limitations exist. The power output is currently relatively low, suitable primarily for small-scale applications. The long-term stability and durability of the device under various environmental conditions require further investigation. The influence of other environmental factors, beyond humidity and wind, should also be explored to assess the robustness of the energy harvesting process.