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John O | April 2019

Researchers transform wood into flexible, thermoelectric membrane that generates ionic voltage

By Josh Perry, Editor


Researchers from the University of Maryland (College Park, Md.) transformed a piece of wood into a flexible membrane that utilizes the natural nanostructures in the material and small temperature differentials to generate ionic voltage, which is the same electric current that the human body runs on.


Researchers used the nanochannels in wood to move ions and create electricity out of small temperature differentials. (University of Maryland)


According to a report from the university, wood contains nanoscale channels that typically move water from the roots to the leaves. It is these channels that researchers used to regulate the movement of ions.


“The researchers used basswood, which is a fast-growing tree with low environmental impact,” the report explained. “They treated the wood and removed two components – lignin, that makes the wood brown and adds strength, and hemicellulose, which winds around the layers of cells binding them together. This gives the remaining cellulose its signature flexibility.  This process also converts the structure of the cellulose from type I to type II which is a key to enhancing ion conductivity.”


A thin membrane sliced from the flexible material was bordered with platinum electrodes and sodium-based electrolyte was put into the cellulose. The channel walls become charged with ions, which creates an electrical field. The sodium ions in the electrolyte move into the aligned channels.


The research was recently published in Nature Materials. The abstract read:


“Converting low-grade heat into useful electricity requires a technology that is efficient and cost effective. Here, we demonstrate a cellulosic membrane that relies on sub-nanoscale confinement of ions in oxidized and aligned cellulose molecular chains to enhance selective diffusion under a thermal gradient.


“After infiltrating electrolyte into the cellulosic membrane and applying an axial temperature gradient, the ionic conductor exhibits a thermal gradient ratio (analogous to the Seebeck coefficient in thermoelectrics) of 24 mV K–1—more than twice the highest value reported until now. We attribute the enhanced thermally generated voltage to effective sodium ion insertion into the charged molecular chains of the cellulosic membrane, which consists of type II cellulose, while this process does not occur in natural wood or type I cellulose.


“With this material, we demonstrate a flexible and biocompatible heat-to-electricity conversion device via nanoscale engineering based on sustainable materials that can enable large-scale manufacture.”

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