Ball milling liquid metal with natural plant compounds produces a black powder that purifies seawater and generates electricity using only sunlight.
(Nanowerk Spotlight) A silver droplet of gallium-indium alloy reflects most incoming light. This reflectivity poses a fundamental problem for researchers hoping to use liquid metal in solar energy devices: the material bounces away the very sunlight it needs to capture. Liquid metal offers valuable properties for solar applications, including electrical conductivity of approximately 3.4 × 10⁶ S m⁻¹ and surface electrons that can resonate with incoming light to generate heat. But that reflective surface, combined with a stubborn resistance to mixing evenly into other materials, has kept these advantages out of reach.
A research team at Sichuan University in China found a fix in the chemistry of oak bark and gallnuts. By coating liquid metal droplets with polyphenols, the plant compounds responsible for the astringency in red wine and the bitter taste of unripe fruit, they transformed the reflective silver material into a light-absorbing black powder.
Their work, published in the journal Advanced Functional Materials (“Polyphenolic Mechanochemistry‐Mediated Liquid Metal Hydrogels for Efficient Solar‐Powered Desalination and Electricity Generation”), demonstrates a device that purifies seawater and generates electricity simultaneously using only sunlight. The resulting device achieved an evaporation rate of 3.60 kg m⁻² h⁻¹ under one-sun illumination (the equivalent of normal sunlight intensity) while generating stable electrical outputs of 250 mV and 0.257 mA, surpassing all previously reported liquid metal-based evaporators.
Such a device could help address two overlapping crises. Billions of people worldwide lack access to safely managed drinking water, and hundreds of millions more have no electricity. Solar-powered desalination could tackle both problems at once by evaporating seawater to collect purified vapor while capturing electrical potential generated during the process.
Fabrication and characterization of liquid metal gallic acid (LMGA). (Image: Reproduced with permission by Wiley-VCH Verlag) (click on image to enlarge)
Traditional approaches using carbon materials, semiconductors, or precious metal nanoparticles have each fallen short due to cost, narrow light absorption, or manufacturing complexity.
The team’s approach centers on ball milling, a common industrial technique that uses grinding balls inside a rotating container to pulverize and mix materials. They placed liquid metal gallium-indium alloy, gallic acid (a polyphenol found naturally in gallnuts, sumac, and oak bark), and zirconia grinding beads into a planetary ball mill. The intense shearing forces broke the liquid metal into nanoscale droplets while gallic acid spontaneously coated each droplet’s surface.
This polyphenol coating transforms the material’s optical properties. Unmodified liquid metal registers a lightness value of 45.28 on a standardized scale. After treatment, the coated material drops to just 9.09, appearing jet black rather than silver. Spectroscopy measurements confirmed that the treated material absorbs light effectively across the entire solar spectrum from 200 to 2500 nm, spanning ultraviolet, visible, and near-infrared wavelengths.
The darkening occurs through several mechanisms. Gallic acid molecules form coordination bonds with gallium atoms on the metal surface, creating a stable shell approximately 10 nm thick around each droplet. During ball milling, the polyphenols undergo oxidation and polymerization reactions, generating structures that extend the material’s conjugated electron system and shift absorption toward longer wavelengths. Breaking the liquid metal into particles smaller than optical wavelengths also suppresses its metallic luster through light diffraction effects.
The polyphenol shell simultaneously solved the dispersion problem. Uncoated liquid metal droplets settled to the bottom of a container within 15 minutes, but coated particles remained suspended even after 60 minutes. Surface charge measurements shifted from +5.53 mV for uncoated metal to −17.53 mV for the coated version, reflecting the abundant phenolic groups now present. This negative charge creates electrostatic repulsion between particles, preventing aggregation.
The researchers incorporated their polyphenol-coated liquid metal into hydrogels made from polyvinyl alcohol and chitosan. Molecular dynamics simulations showed that binding energy between coated liquid metal and the hydrogel matrix reached 31,769.31 kJ mol⁻¹, far exceeding the 20,294.34 kJ mol⁻¹ measured for uncoated metal. Compression tests confirmed this difference: hydrogels containing coated liquid metal withstood 51.3 kPa of pressure, a 242% improvement over pure polymer hydrogels.
Beyond simple heating, the hydrogel activates water molecules to make evaporation more efficient. The abundant hydroxyl and carboxyl groups on the polyphenol coating interact with water molecules to disrupt their hydrogen bonding networks. This creates more “intermediate water,” loosely bound molecules requiring less energy to vaporize than bulk water. Calorimetry measurements showed the hydrogel’s evaporation enthalpy dropped to 1,623.6 J g⁻¹, well below pure water’s 2,265.5 J g⁻¹.
The electricity generation mechanism exploits a hydrovoltaic effect. As water evaporates from the hydrogel surface, capillary action pulls replacement water upward through microscale channels. The negatively charged liquid metal surfaces attract positive ions in the water, creating an electric double layer. This directional ion migration establishes a potential difference across the material, producing measurable current.
When the team tested the system in simulated seawater with 3.5% salinity, concentrations of sodium, magnesium, calcium, and potassium ions fell by several orders of magnitude, meeting World Health Organization drinking water standards. Over seven days of continuous operation, the evaporator maintained an average rate of 3.56 kg m⁻² h⁻¹ without salt crystallization.
Outdoor tests in Chengdu, China using actual Bohai Sea water produced 16.06 kg m⁻² of freshwater during eight hours of operation, enough for a modestly sized evaporator to meet an adult’s daily drinking water needs. The electrical output powered small electronic devices including a mini-fan, digital watch, and calculator when energy from multiple capacitors was combined.
The use of inexpensive, naturally derived polyphenols rather than specialized synthetic coatings suggests potential for economical manufacturing at scale. Demonstrating simultaneous freshwater production and electricity generation from a single device, this work opens pathways toward integrated systems capable of serving communities that currently lack both clean water and reliable power.
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