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Ion-conducting wood membranes

22.07.2021

US scientists have coated ordinary wood with a hydrogel and turned it into an ion-selective material. Membranes made of such material let only positively charged ions through, which can be used to generate electricity at the border of fresh and salt water. The results of the study are published in the journal ACS Nano.

Scientists from the University of Maryland, led by Lian Bing Hu (Liangbing Hu) managed to make ion-selective membranes from a more affordable material - wood. Wood is a composite material that consists of cellulose fibers in a matrix of the tougher lignin polymer. Wood has a porous structure, and due to negative charges on the surface, it even has some ion-selective properties - it passes predominantly positively charged ions. However, the pores in wood are too large, and the ion selectivity of wood is lower than that of artificial materials. In addition, wood is an anisotropic material, that is, its properties differ greatly depending on the direction. The conductivity is higher for porous cross-sections, and the strength is higher for longitudinal cuts (and also the area of \uXNUMXb\uXNUMXbtransverse cuts is limited by the diameter of the tree, while longitudinal cuts can have a much larger area). And finally, wood is not stable enough: with prolonged contact with water, its fibers gradually swell and the structure of the material is broken. Scientists have tried to improve the properties of wood with a polymer hydrogel coating.

The hydrogel contains carboxyl groups, which donate a proton in an aqueous medium and turn into a negatively charged COO- fragment. By covering the tree with hydrogel, the scientists intended to increase the density of negative charges on the surface of the material. And so it happened - the measurement of the zeta potential showed that the concentration of negative charges on the surface of the material almost doubled - from minus 1,49 to minus 2,53 millicoulombs per square meter. As a result, the ionic conductivity of the membranes increased by two orders of magnitude compared to untreated wood. In transverse sections, the conductivity was still higher than in longitudinal ones, but not by much - 1,29 millisiemens per centimeter compared to 0,97 millisiemens per centimeter. And hydrogel additives made the membranes stronger - the tensile strength of the longitudinal sections increased from 16,9 to 52,7 megapascals, and the transverse ones - from 1,8 to 10,7 megapascals. The authors believe that the reason is the formation of additional hydrogen bonds between cellulose fibers.

In terms of strength-conductivity ratio, the new longitudinal membranes turned out to be better than most of the known analogs. But their main advantage is low price and scalability. Wood is an inexpensive and renewable material, and the use of longitudinal cuts will make membranes of several square meters that can be used to generate energy on a large scale.

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