(Nanowerk Spotlight) When water evaporates and condenses in the atmosphere, it carries energy – the same process that drives weather patterns and can be harnessed for power generation. While power plants use temperature differences between water vapor and cooler surfaces to generate electricity on an industrial scale, creating smaller devices that can efficiently capture and store this type of energy has proven challenging. The ability to transform ambient humidity into usable electrical power could provide a constant, renewable energy source, particularly in humid regions.
Previous attempts to create devices that both harvest moisture energy and store electricity have faced significant technical hurdles. Conventional materials perform well at either capturing moisture or storing energy, but not both. The interfaces between moisture-capturing and energy-storing components tend to degrade rapidly, preventing long-term use. Additionally, many prototype devices show promising performance in laboratory conditions but fail in real-world environments with varying humidity and temperature.
Metal-organic frameworks (MOFs) – materials with microscopic pores that can trap and hold water molecules – emerged as promising candidates for moisture capture. Meanwhile, advances in electrolyte materials improved energy storage capabilities. The challenge lay in combining these functions effectively in a single stable device.
A research team has now developed a supercapacitor that successfully merges these capabilities. Their work, published in Advanced Functional Materials (“Bismuth-Based Metal-Organic Frameworks for Water Vapor Capture and Energy Storage”), uses new bismuth-based metal-organic frameworks combined with specialized electrolytes that respond to moisture. The device can capture water vapor from air and convert it directly into stored electrical energy.
Figure shows the structure of two novel metal-organic frameworks (MOFs) developed in this research. MOF 1 is composed of bismuth ions coordinated with H4ABTC molecules (an organic linker compound) and water molecules, while MOF 2 has a similar structure but incorporates DMF solvent molecules instead of some water molecules. The coordination environment – how atoms connect to each other – is shown for a) MOF 1 and c) MOF 2, with bismuth atoms in yellow, hydrogen in pink, and other components in blue. Panels b) and d) illustrate the different pore sizes in these structures, with MOF 2 showing larger pores that improve its moisture-capturing ability. The dotted lines indicate distances between hydrogen atoms, which are important for understanding how water molecules interact with the structure. Panels e) and f) demonstrate how MOF 1 transforms into MOF 2 when DMF molecules replace some of the original components, creating a more open structure with enhanced performance. The structural differences between these MOFs explain why MOF 2 performs better at capturing water vapor and storing energy. (Image: Reprinted with permission by Wiley-VCH Verlag)
The researchers created two types of metal-organic frameworks using bismuth and organic compounds. Rather than simply coating these materials onto electrodes, they grew them directly on carbon paper using a technique called in situ growth. This method creates stronger bonds between the materials, improving durability. They then added polyaniline, a conducting polymer that enhances both electrical conductivity and moisture absorption.
The key innovation lies in the interface between the electrode and electrolyte. The team used polyoxometalates – compounds that respond to moisture – as the electrolyte. These materials work synergistically with the metal-organic frameworks, improving both moisture capture and energy storage.
In practical terms, the device performs remarkably well. Under humid conditions (90% relative humidity at 70 °C), it achieves an energy density of 40.40 watt-hours per kilogram – enough to power small electronic devices. For comparison, typical supercapacitors store around 5-10 watt-hours per kilogram. The device maintains over 92% of its capacity after 1,000 charge-discharge cycles, suggesting good durability.
Testing under various conditions revealed consistent performance across temperatures from 0 to 70°C and humidity levels between 40% and 90%. The researchers demonstrated that three devices connected in series could power an LED light, showing practical potential.
The superior performance stems from careful materials engineering. The metal-organic framework structure provides abundant sites for water molecule capture, while the polyaniline creates a network that enhances both electrical conductivity and moisture absorption. The electrolyte serves dual purposes – conducting electrical charge and participating in energy storage reactions.
This technology opens new possibilities for sustainable power generation, particularly in humid environments. Potential applications range from powering sensors and small electronic devices to supplementing larger-scale energy storage systems. The device’s ability to function across varying environmental conditions suggests practical use in different climates.
Future research will focus on scaling the technology for specific applications and improving performance under extreme conditions. The work demonstrates that properly engineered materials can effectively harvest energy from atmospheric moisture, potentially contributing to renewable energy solutions.
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