Energy storage in in vivo synthesizable biominerals

Abstract

With the move toward the use of greener materials for powered vehicles, environmentally-benign synthesis of energy materials is becoming important. Here, the energy storage capability of biominerals from the jaws of a marine bloodworm, Glycera dibranchiate, is demonstrated, implying the possibility of a bio-factory (or in vivo synthesis) for energy storage.

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Over the past decade, biomaterials have been extensively studied for medical applications, such as new pharmaceuticals, tissue engineering, and artificial organs, due to their excellent biocompatibility.^1–4 Nowadays, biomaterials further expand their boundaries to various functionalities in semiconductors, sensors, and display devices.^5–10 Mimicking the structure and synthetic route of biomaterials in living organisms or direct use of the biomaterials often provides insights in enhancing the performance of devices due to similarities in the material requirements for functional devices and those of living organisms.

The feasibility of biological materials as a template for the synthesis of energy storage materials has recently been investigated.^11–13 Pioneering works done by Nam et al. showed that organic biomolecules can be used as a structural template to provide growth sites for electrode materials in batteries.¹¹ In this study, Co₃O₄ nanoparticles were grown onto genetically modified M13 viruses as templates, resulting in good electrochemical performance. In vitro synthesized, self-assembled peptides were also used as templates for the fabrication of battery electrodes.^14–16 Facile control of the peptide nanostructures by self-assembly could result in ideal electrode nanostructures for Li rechargeable batteries. Further evidence of this approach was reported by Ryu et al. who fabricated Fe phosphate electrode materials on peptide nanostructured templates by mimicking the natural biomineralization process of bone formation in which Ca phosphates form on collagen.¹⁶

Recently, Chen et al. reported that a biomaterial itself can function as an active electrode for Li batteries.¹⁷ Organic Li₂C₆O₆ extracted from biomass was capable of storing four Li ions, delivering a specific capacity of 580 mAh g⁻¹, which is substantially higher than that of conventional cathode materials (∼170 mAh g⁻¹). Such organic electrodes can potentially offer sustainable life cycles from production to consumption without generating additional CO₂. Thus, the use of such chemistry is believed to be able to result in greener and more sustainable batteries.

We envision that electrode materials can be produced from ‘bio-factories,’ once electrochemically active biomaterials are identified from the living organisms. Fig. 1 briefly illustrates the concept of the bio-factory. The living organisms that possess electrochemically active biominerals are farm-bred on a large scale. The biominerals are extracted for use in battery fabrication and the productivity of biominerals from the living organisms can be optimized by genetic engineering. The production of battery materials from bio-factories has several advantages over production from a ‘real’ factory, including environmental friendliness, non-toxicity to humans, non-necessity of large building sites, and potentially lower production cost. The greener production of Li batteries also reduces the environmental burdens of large-volume Li battery production.

Fig. 1 Schematic illustration of a ‘bio-factory’. Electrochemcially active biominerals can be extracted from the living organisms and used as electrodes for Li rechargeable batteries.

In this study, we discovered that Cu₂Cl(OH)₃, a Cu-based natural biomineral found in the jaws of a marine bloodworm Glycera dibranchiate, can efficiently convert electric energy into chemical energy and vice versa through a reversible electrochemical reaction with Li ions. Cu₂Cl(OH)₃ is the only Cu-based biomineral known to exist in nature, while Ca-, Si-, and Fe-based biominerals are commonly observed in other living organisms.^18–20 Although previous studies have shown that some organic biomaterials can be used as active electrode materials, the intrinsic inferiority of organic materials with regard to long-term stability is a major drawback in their application to long-life energy devices.^21–24 This study reveals that an inorganic biomineral, Cu₂Cl(OH)₃, can be a promising electrode material for Li rechargeable batteries. This material reversibly stores and releases Li ions via a conversion reaction that is widely observed in transition metal oxide, nitride, and fluoride electrodes.^25–29

We chemically synthesized Cu₂Cl(OH)₃ to examine the energy storage capability of the Cu₂Cl(OH)₃ biomineral-based electrode. A detailed description of the synthesis is provided in the Supplementary Information.† The X-ray diffraction (XRD) pattern shown in Fig. 2a indicates that the synthesized Cu₂Cl(OH)₃ is composed of two phases with similar structures, atacamite and clinoatacamite. Although atacamite is the most common crystalline form of Cu₂Cl(OH)₃, clinoatacamite can be regarded as a distorted structure of atacamite with similar atomic arrangements (see Supplementary Fig. S1a and b†). The crystal information for both phases is summarized in Fig. S1.†Fig. 2b shows that the synthesized Cu₂Cl(OH)₃ is a polyhedral shape of a few hundred nanometers in dimension. Magnified transmission electron microscope (TEM) images reveal that individual particles are composed of a number of well-formed crystalline primary nanoparticles. (Fig. S2†).

Fig. 2 (a) XRD pattern and (b) SEM image of the synthesized Cu₂Cl(OH)₃.

The electrochemical properties of Cu₂Cl(OH)₃ were examined using a CR2016-type coin cell with elemental Li as a counter electrode. Fig. 3a shows charge–discharge profiles of the Cu₂Cl(OH)₃ electrode operated in the 0.01–3 V range at a current rate of 100 mA g⁻¹. An abnormally high specific capacity was obtained at the first discharge (∼1100 mAh g⁻¹), indicating that a significant amount of Li was used to irreversibly form surface passivation layers. After several subsequent cycles, stable cycling was observed with a specific capacity of about 500 mAh g⁻¹, which is comparable to the theoretical capacity of Cu₂Cl(OH)₃ based on the 2Cu²⁺/Cu⁰ redox reaction.

Fig. 3 Electrochemistry of Cu₂Cl(OH)₃: (a) charge–discharge profiles for the initial 5 cycles and (inset) capacity retention, in situ XRD patterns during (b) discharge and (c) charge, and ex situ TEM images of (d) as-prepared, (e) discharged, and (f) charged electrodes.

To verify the detailed Li storage mechanism in Cu₂Cl(OH)₃, XRD analysis was performed on the electrode during both the discharge and charge processes using an in situ electrochemical cell. The evolution of the XRD patterns during the first discharge process in Fig. 3b reveals that the original Cu₂Cl(OH)₃ structure completely disappears, as indicated in the dotted box. Furthermore, the crystal structure is not recovered during subsequent charging, as shown in Fig. 3c. This behavior is similar to the phase behavior generally observed for an electrode that undergoes a conversion reaction.^27–29 When the conversion reaction occurs, the pristine crystalline phase deforms into a nanocomposite of the amorphous or nanosized Li_nX (X = anion) and metal, which is hardly observable with XRD. Ex situ TEM analysis (Fig. 3d–f) of the electrode shows that a corresponding microscopic structural change occurred during the electrochemical reaction. Single Cu₂Cl(OH)₃ particles, a few hundred nanometers in size, were destroyed to a size of tens of nanometers after discharge. The morphology of the particles was not recovered after subsequent charge, consistent with the XRD results. A magnified images of the discharged state (Supplementary Fig. S3†) shows that the nanoparticles are embedded in the matrix, as is commonly observed in conversion reaction compounds in which metal nanoparticles are embedded in the Li_nX matrix.^27–29 From the XRD and TEM analysis, it is reasonable to assume that Cu₂Cl(OH)₃ underwent a conversion reaction: however, the precise identification of reaction products was not successful. Further study of the reaction mechanism is necessary and currently under way.

In summary, the possibility of energy storage in the Cu-based biomineral, Cu₂Cl(OH)₃, was examined to assess its use as an electrode material in Li rechargeable batteries. Cu₂Cl(OH)₃ delivered approximately 500 mAh g⁻¹ through the conversion reaction. It is noteworthy that Cu₂Cl(OH)₃ is a novel-class of electrode material, containing Cl⁻ and (OH)⁻ as anions, which are not common for electrode materials. Chemically synthesized Cu₂Cl(OH)₃ was investigated in this study, hence further study on ‘real’ Cu₂Cl(OH)₃ biomineral from the marine bloodworm should be followed to demonstrate the feasiliby of the bio-factory. In addition, purification and scale-up of the biomineral electrode still remains a challenge. There are numerous natural biominerals that contain other transition metal ions, such as Fe and Mn, which can serve as excellent redox elements. Thus, significant unexplored opportunities for energy device applications exist in natural biominerals with different electrochemical properties.

This research was supported by the Energy Efficiency and Resources R&D program (20112020100070), Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (20114010203120) funded by the Ministry of Knowledge Economy, and the National Research Laboratory (ROA-2008-000-20041-0) program of the National Research Foundation (NRF), Republic of Korea. The work was also supported by the Northeastern Center for Chemical Energy Storage, and Energy Frontier Research Center funded by the U.S. DOE, BES under award No. DE-SC0001294.

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Footnote

† Electronic Supplementary Information (ESI) available: detailed description on experiments, crystal structures of atacamite and clinoatacamite, and TEM images. See DOI: 10.1039/c2ra20671d/

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