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Researchers improve aluminum-air battery performance by using flax straw extract as corrosion inhibitor

In a paper in ChemSusChem, a team from Israel and Russia reports substantial improvements in the performance of aluminum-air batteries by adding flax straw extract (3 vol %) to the alkaline solution to act as a mixed-type aluminum corrosion inhibitor.

The Aluminum-air battery, a metal-air battery system which uses a catalytic air cathode in combination with an electrolyte and an aluminum anode, has been of great interest for a number of years due to its high theoretical specific energy. (Earlier post, earlier post, earlier post.) However, parasitic hydrogen evolution caused by the corrosion of the aluminum anode during the discharge process is a well-known obstacle to commercialization of the system, as it not only causes additional consumption of the anode material but also increases the ohmic loss in the cell.

One of the possible approaches to overcome this challenge is to mitigate Al corrosion by the addition of inhibitors to the alkaline media. However, unlike the inhibition protection of technological equipment (pipeline, heat exchanger, and tubes) there are additional and unique requirements for anode corrosion inhibitors in batteries. In the case of the Al anode, it is beneficial to achieve corrosion protection by the suppression of only the cathodic process (hydrogen evolution reaction). The anodic process should remain intact, similar to that that occurs in the inhibitor-free solution as the suppression of the anodic process by the inhibitors may affect the performance of the entire battery negatively.

… In this work, we present the results obtained from our study of the inhibition efficiency of a flax straw extract added to an alkaline electrolyte to suppress Al anode corrosion. The extract was prepared from a flax straw, stored for an extended period in a flax plant, and gradually, under the influence of light, temperature, and microorganisms, was subjected to a humification process. Humified flax straw may contain a high concentration of humic substances (HS), which includes a mixture of humic and fulvic acids.

The researchers studied the inhibition effect of flax straw extract on Al corrosion in 5m KOH by using different electrochemical techniques, weight-loss tests, electrochemical impedance spectroscopy, FTIR spectroscopy, SEM, and other test methods.

A protective film covered the suface of the Al electrodes during exposure to the alkaline solution containing flax straw extract.

They found that flax straw extract suppressed Al corrosion markedly (with 50% efficiency) at a relatively low concentration (3 vol%) in the alkaline solution. An increase of the flax straw extract concentration in the alkaline solution from 1 to 3 vol% reduces the Al corrosion rate; how- ever, the reduction in the inhibition efficiency is reduced as the concentration of the flax straw increases.

For practical battery applications, only low concentrations of an organic inhibitor should be added to the electrolyte. This is a prerequisite to avoid drastic damage to the electrochemical performances (working potential, capacity, and rate capabilities). In lieu of this, Al–air cells that utilize alkaline solutions that contain the flax straw extract show a markedly higher discharge capacity (longer discharge time) than other electrolytes. We suggest that the use and application of such a natural inhibitor is the next step, with the aim to develop hybrid organic–inorganic systems tailored form Al in strong alkaline media to enable sustainable Al alkaline batteries.

Resources

  • Grishina, E., Gelman, D., Belopukhov, S., Starosvetsky, D., Groysman, A. and Ein-Eli, Y. (2016) “Improvement of Aluminum–Air Battery Performances by the Application of Flax Straw Extract,” ChemSusChem doi: 10.1002/cssc.201600298

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