Iron Nanoparticles Embedded in Carbon Nanotubes for Reversible Li-CO₂ Batteries

As a main reason of greenhouse effect, CO₂ emission should be reduced to alleviate further environmental pollution such as global warming and climate changes.^[1] Considering wide and substantial utilization of fossil fuels in various industries, it is hard to diminish the emission in a short time. To handle the problems, carbon capture and storage (CCS) technologies have emerged as new breakthrough of relieving the amount of CO₂ in atmosphere, fixing and storing the gaseous CO₂ to solid phases.^[2] Li-CO₂ batteries as electrochemical CCS technology have been spotlighted for their bi-functional applications enabling both energy storage system and environment-friendly CO₂ fixation.^[3] The Li-CO₂ batteries operate with CO₂ (Mw. CO₂: 44) as a cathode material, which is two-fold lighter than transition metal (e.g. Mw. LiCoO₂: 98) in Li-ion batteries, indicating distinguished energy potentials. However, insulating discharge products and inactive reactions of the Li-CO₂ cells during cycling lead to cell degradation, showing poor cyclability and low efficiency. Therefore, to utilize the Li-CO₂ batteries as next-generation energy storage system, various catalysts should be introduced, facilitating the discharge/charge reaction.

In both economic and eco-friendly respects, using bio-waste as a precursor has attracted the attention in the field of catalyst development.^[4] Hemoglobin obtained from blood-wastes (such as slaughter and medical wastes) has been introduced as a cathode catalyst.^[5-6] It is composed of four globular protein chains each with a Fe-centered porphyrin called heme, which is capable of changing their oxidation numbers. With redox reactions of centered iron, hemoglobin can bind with CO₂, interacting directly with the cell reaction in Li-CO₂ batteries. Furthermore, various cation (e.g. iron) and anion (e.g. oxygen, nitrogen, and carbon) resources in hemoglobin can be employed as catalytic components. Diverse treatments and procedures can be performed to maximize catalytic effects of hemoglobin-derived catalyst.

In this presentation, we introduce a new method, applying capillary action of hemoglobin precursor into CNT, to fabricate Fe nanoparticles embedded in N-doped CNT (Fe NPs@N-CNT) catalysts. By simple treatments, we successfully synthesized Fe NPs@N-CNT and examined the morphological and structural characteristics of the catalyst. The pore size of CNT provides strong force of capillary motion to draw the precursor solution into the tubes and subsequent segregation of Fe components are achieved, showing crystalline structure in TEM analysis. We performed the electrochemical cell tests, employing Fe NPs@N-CNT catalysts, and confirmed the catalytic activities of reduced charge overpotential and enhanced capacity in both discharge and charge process. The utilization of bio-waste as the catalysts provides environmental merits for developing eco-friendly Li-CO₂ batteries and moreover, the novel application of capillary motion to fabricate Fe NPs@N-CNT catalysts offers new pathway to develop various catalytic materials.

[1] A. M. Appel, J. E. Bercaw, A. B. Bocarsly, H. Dobbek, D. L. DuBois, M. Dupuis, J. G. Ferry, E. Fujita, R. Hille and P. J. Kenis, Chemical reviews, 2013, 113, 6621-6658.

[2] N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C. S. Adjiman, C. K. Williams, N. Shah and P. Fennell, Energy & Environmental Science, 2010, 3, 1645-1669.

[3] Y. Qiao, J. Yi, S. C. Wu, Y. Liu, S. X. Yang, P. He and H. S. Zhou, Joule, 2017, 1, 359-370.

[4] Z. Ma, K. Wang, Y. Qiu, X. Liu, C. Cao, Y. Feng and P. Hu, Energy, 2018, 143, 43-55.

[5] W.-H. Ryu, F. S. Gittleson, J. M. Thomsen, J. Li, M. J. Schwab, G. W. Brudvig and A. D. Taylor, Nature communications, 2016, 7, 1-10.

[6] J.-Y. Lee, H.-S. Kim, J.-S. Lee, C.-J. Park and W.-H. Ryu, ACS Sustainable Chemistry & Engineering, 2019, 7, 16151-16159.