Back to bacteria: researchers use bacteria to produce electrode material
From Leonardo da Vinci to Velcro’s® inventor, living organism are an inspiration source for innovators since centuries. Nature often shows sustainable and efficient solutions to practical issues. The study presented here applies this approach to electrochemical energy storage. Bacteria are used as eco-efficient workers that produce iron oxide hollow shells. Those shells have been tested as electrode material.
Li-ion technology is based on a rich and varied chemistry associated with a wide array of materials that can be used as positive (LCO, LFP, NMC, LMO…) and negative electrodes (C, Sn, Si, LTO…). Currently, most of the commercial electrode materials are prepared by ceramic method (T > 400°C) with long heating periods (> 24h) and thus at high environmental cost. In reaction, the scientific community is increasingly studying alternative synthesis methods (e.g.: “chimie douce”). In France, those methods are studied at RS2E (French research network on electrochemical energy storage) under the name “eco-compatible storage”.
A team of scientists (comprising RS2E affiliates: Jennyfer Miot, Nadir Recham, Dominique Larcher and Jean-Marie Tarascon from LRCS) studied the synthesis of an iron oxyhydroxide synthetized by a bacterial pathway at room temperature, their results are published in Energy & Environmental Science.
The bacterium they used, Acidovorax sp., is 1-2µm long and 0.2µm wide. Through its metabolism it produces g-FeOOH (lepidocrocite, a crystallized solid iron oxyhydroxyde). This biomineralization process takes place within the bacterium membrane: a 40nm-thick space progressively filled with g-FeOOH by the bacterium under certain conditions. Then, this lepidocrocite-made wall can be turned into hematite through heating (< 1h) at 700°C (fig. 1). The heating process also eliminates the bacteria. The choice of hematite (α-Fe2O3) is motivated by its strong ability to store electric charges (1000 mAh/g) that made it a good experimental model.
Fig. 1: Hematite shells. Arrows are pointing at "broken" shells that give us a glimpse at their hollow shape (© RS2E/RSC).
The resulting hematite powder is composed of multiple hollow, micro-sized, rod-like shells which have retained the initial characteristics and shape of the bacteria’s periplasms (fig. 2). Researchers used the word “bacteriomorphs” to describe those shells.
Fig. 2: Hematite bacteriomorph. Hematite particles are clearly visible where the periplasm of the bacteria once was (© RS2E/RSC).
Does this unique organization lends interesting properties to hematite bacteriomorphs?
To find out, it has been compared to untextured hematite (i.e. crushed by a mortar thus destroying its bacteriomorphic structure) and abiotic hematite (i.e. produced using “standard” synthesis pathways). All samples were mixed with carbon (10% of total weight mass) to enhance electrical conductivity.
In the end, researchers obtained the following results: after 10 charge-discharge cycles at slow speed, 91% of the initial charge storage capability is retained. The retained capability drops to 18% for abiotic samples, and to 8% for untextured samples (fig. 3). Surprisingly enough, the shells retains this ability to store charges on wide speed ranges: for speeds of charge/discharge ranging from C/100 to 10C (a factor of a 1,000), 70% of the initial ability to store charges remains. These promising features are explained by the unique micrometric organization of bacteriomorphic samples that confers a better mechanical stability to the electrode upon cycling and a porosity that allows a better contact with the electrolyte.
Fig. 3: Comparison between hematite bacteriomorphs and untextured hematite electrochemical properties (© RS2E/RSC).
To this day, those results are amongst the best obtained with hematite. Researchers were able i) to demonstrate remarkable electrochemical properties that can be explained by the unique structure of the material and ii) to explore a less energy demanding synthesis pathway.
Regarding scability, it’s worth noting that large scale use of bacteria is already mastered (e.g. two million tons of glutamate are produced by bacteria every year). To go beyond the case of hematite, scientists now want to produce positive electrode materials with bacteria for Li-ion systems such as double phosphates (general formula AMPO4 with A an alkaline metal – Li, Na… – and M a 3d metal – Fe, Mn…).
Biomineralized α-Fe2O3: Texturation and electrochemical reaction with Li. J.Miot, N.Recham, D.Larcher, F.Guyot, J. Brest and J-M. Tarascon
Energy & Environmental Science, Accepted Manuscript, 11/7/2013, DOI : 10.1039/c3ee41767k.