Dried leaves convert small temperature differences into electricity through ion movement, offering a natural, biodegradable platform for efficient thermal energy harvesting that rivals engineered thermoelectric materials.
(Nanowerk Spotlight) Ions flow constantly through plant tissues, enabling critical processes like nutrient uptake, water balance, and stress signaling. These ion movements are tightly regulated, yet their response to gentle temperature differences—such as those caused by sunlight warming a leaf surface—has rarely been studied in terms of electricity generation.
This gap matters. Across the planet, vast amounts of low-level thermal energy go unused: from ambient heat in natural environments to small temperature gradients created by waste heat in buildings or electronic devices. Finding ways to convert these mild temperature differences into electricity could open new pathways for sustainable energy technologies.
Today’s thermoelectric systems are based mostly on electronic materials—rigid semiconductors that are often expensive, toxic, or poorly suited to flexible applications. Ionic thermoelectrics, which rely on the movement of ions rather than electrons, have emerged as a softer, potentially biodegradable alternative. But their performance has been limited. Engineered gels and polymers produce only small voltages, and efforts to improve them face steep trade-offs between conductivity, stability, and environmental impact.
Plant tissues, by contrast, already possess many features these synthetic systems aim to replicate. Leaf cell walls contain hydrated polymers—like pectin and cellulose—that guide ion transport through natural, nanoscale networks. These materials are abundant, self-assembled, and compatible with living environments.
Yet until now, no one had measured whether they could convert heat into electricity efficiently. A new study (Advanced Materials, “High Ionic Seebeck Effect in Natural Leaves”) changes that, demonstrating that leaves themselves can act as high-performance ionic thermoelectric generators—revealing a functional property of plant matter that had gone unnoticed in both biology and energy research.
A team of researchers from Korea University and Seoul National University tested this idea using leaves from the Ficus elastica, or Indian rubber plant. In controlled experiments, they exposed leaf samples to mild temperature gradients and measured the resulting thermovoltage. Fresh leaves exhibited ionic Seebeck coefficients of about -0.64 millivolts per degree Kelvin, indicating that negatively charged ions (anions) were the primary carriers.
When dried, however, the same leaves produced voltages nearly 1000 millivolts per Kelvin—an order of magnitude beyond typical ionic thermoelectric materials. A single 2 cm section of desiccated leaf generated nearly 7 volts under a 10 K temperature difference.
Experimental setup and ionic thermovoltage analysis. a) Photographs of a Ficus elastica leaf used the experiments. b) Left: Schematic of the leaf-based thermodiffusion measurement setup. Right: Photograph of the assembled electrode-leaf-electrode thermodiffusion cell (leaf dimensions: width (W) = 1 cm and length (L) = 2 cm). c) Thermographic image showing the temperature gradient (ΔT) applied to the leaf using a Peltier device. d) Schematic illustrating ion thermodiffusion in cellulose and pectin matrices under a temperature gradient. Cellulose facilitates cation diffusion, while its hydroxyl groups interact with anions (left); pectin promotes anion diffusion via carboxyl (COO−) group interactions with cations (right). e) Ionic thermovoltage (ΔVi) curves recorded over five heating/cooling cycles for a fresh leaf-based cell. ΔVipeak denotes the peak voltage at each ΔT. f) Plot of ΔVi versus ΔT, showing a linear relationship. Error bars indicate the standard deviation measured from five heating/cooling cycles. g) Histogram of ionic Seebeck coefficients (Si, mV K−1) obtained from 50 separate leaf-based thermodiffusion cells. h) Schematic of the sample preparation for ion chromatography analysis, involving leaf segmentation after applying a temperature gradient. i) Ion concentration differences (ΔCcold-hot) between the cold and hot regions of the leaf, determined by ion chromatography. Error bars represent the standard deviation from three independent ion chromatography measurements of ΔCcold-hot. (Image: Reprinted from DOI:10.1002/adma.202510413, CC-BY) (click on image to enlarge)
To explain this result, the researchers analyzed how different parts of the leaf contribute to ion transport. Plant cell walls are built from two major charged polymers: pectin, which contains carboxyl groups that bind positively charged ions and enhance anion diffusion, and cellulose, which binds anions and supports cation movement. In hydrated leaves, pectin remains expanded and flexible, making it the dominant channel for ion flow. Using ion chromatography, the team showed that phosphate and nitrate ions accumulated asymmetrically across the temperature gradient—consistent with anion thermodiffusion.
As the leaves dried, the ion transport behavior changed. Pectin collapses when dehydrated, losing its ability to conduct ions. Cellulose, in contrast, retains its structural integrity and some bound water. This shift causes a transition from anion to cation dominance. Around 12 hours into the drying process, the sign of the Seebeck coefficient began to reverse. After 24 hours, potassium ions became the dominant carriers, and the coefficient turned positive.
The nature of the electrode material also shaped the measurements. When carbon tape was used instead of silver paste, the voltage signal strengthened and changed polarity. The researchers attributed this to differences in surface ion adsorption and screening effects. Carbon tape, which interacts less strongly with ions at the surface, allows more of the internal thermovoltage to appear across the device. With carbon tape and 4-day dried leaves, the team measured Seebeck coefficients of up to 971 millivolts per Kelvin.
To account for this enhancement, the researchers proposed a dielectric capacitive model. As the leaf dries, a thin insulating layer of cellulose forms at the surface, separating the inner ion-rich matrix from the electrode. Instead of forming a typical double layer at the interface, the system develops a capacitive voltage across this dielectric barrier. This adds to the thermodiffusion voltage generated inside the tissue. Simulations incorporating both the dielectric layer and bulk ion movement closely matched the measured voltages. The model showed that thinner dielectrics with lower permittivity produced stronger capacitive amplification.
To test this mechanism independently, the team built artificial thermodiffusion cells using salt-soaked cellulose paper with and without added dielectric layers. A low-permittivity polymer layer—such as PDMS—enhanced the voltage output significantly. As predicted, dielectric layers with lower relative permittivity produced larger voltage gains, confirming the model’s key predictions.
These structural changes also impacted the energy conversion performance. The power factor—an efficiency metric that combines Seebeck coefficient and conductivity—initially dropped during early drying stages due to reduced conductivity. But after 24 hours, as the Seebeck coefficient rose sharply, the power factor increased by more than 100-fold compared to fresh leaves. The dimensionless figure of merit (ZT), which includes thermal conductivity, reached a value of 5.6 at room temperature—among the highest ever reported for ionic thermoelectric systems.
The researchers also demonstrated that this effect persists in living tissue. They attached carbon tape electrodes to a still-living leaf and applied heat to one side using a Peltier device. The resulting voltage followed a predictable temperature dependence and was comparable to that of excised samples. Using a focused light source to generate heat gave the same result. Even after multiple heating and cooling cycles, the leaf remained physiologically active, as confirmed by chlorophyll fluorescence imaging.
These findings show that leaves can serve as functional ionic thermoelectric materials, capable of generating measurable voltages from small temperature differences. The system requires no synthetic processing, no toxic components, and no structural alteration to the tissue. Because it is based on ordinary plant material, it offers a fully biodegradable platform for passive energy harvesting.
This approach could find applications in self-powered sensors, distributed environmental monitors, or plant-integrated electronic systems. It also suggests new research directions in biomimetic thermoelectrics, where engineered materials replicate the structure and transport properties of natural tissues. By showing that living plant matter can convert thermal energy into electricity under ambient conditions, the study expands what’s known about both biological energy transport and sustainable materials engineering.
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