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Nano Letters Letter beginning of the carbon nanofiber exhibits pure copper and carbon, providing evidence that the deposited copper acts as a nucleation point to initiate CNF formation. Each of the nucleation metals which we have observed that promote CNF growth have in common that they require a low reduction/ deposition potential (which is less than that required for carbon growth) and lead to higher CNF yield when deposited from low concentrations at low electrochemical current density. This range of metals including nickel, copper, iron, and cobalt enhancing the high yield production of CNFs by molten electrolysis correlates with the metals that catalyze CVD growth of CNFs.10,19 As observed in Figures 1 and 4, and as further delineated in the Supporting Information, Ni, Cu, Co, or Fe nucleates the high-yield, high-rate production of carbon nanofiber formation in the presence of zinc metal. However, little or uncontrolled CNF formation is observed in the absence of zinc. In each of the prior experiments, zinc metal (which melts at 420 °C) is present in the form of the coating over the steel cathode formed from conventional, galvanized steel wire. A high fraction of the carbon product is consistently CNFs when galvanized (with zinc) steel is used as the cathode and is not when either iron wire or 316 stainless steel shim was employed as the cathode. As noted, we observe that electrolyses initiated at a high current generated a profusion of amorphous graphites, and a variety of carbon nanostructures rather than a high yield of nanofibers, while electrolyses initiated with a low current step prior to the high current can generate a high yield of uniform CNFs. Deposition of Ni, Co, Cu (or Fe) at low current density, from electrolytes which have a low concentration of the metal, dissolved as the oxide, lead to small, isolated nucleation sites on the cathode, as evidenced by EDS, which promote CNF growth. A solid metal cathode does not have these character- istics and does not lead to homogeneous CNF growth. As seen in the SI, an overabundance of Fe leads to uncontrolled growth, which in the extreme would tends toward an undefined randomly distributed (amorphous) carbon. Combined with an absence of zinc metal, the absence of this low current step produced amorphous graphites (Supporting Information) or a profusion of nanofiber structures (Supporting Information) with diameters ranging from 0.2 to 4 μm and either circular or rectangular duct-like cross sections. It will be of interest in future studies to isolate conditions that refine the distribution of these different structures. As previously noted, the low current step occurs at a potential of <0.7 V, which is sufficient to form metal nucleation sites at the cathode, but is thermodynamically energetically insufficient to reduce carbo- nate to solid carbon. A proposed mechanism of the observed high yield CNF that is consistent with the observed zinc activation and metal nucleation effects is presented and the observation that the STEP CNF electrolysis chamber is readily scalable are described in the Supporting Information. The demonstrated CNF synthesis can be driven by any electric source. As an alternative to conventionally generated electrical, we have also driven the CNF synthesis using electric current as generated by an illuminated efficient concentrator photovoltaic operating at maximum power point (SI). Conclusions. Here, we show a new high yield pathway to produce carbon nanofibers directly from atmospheric or exhaust CO2 in an inexpensive molten electrolysis. Formation of a highly valued, compact, readily stored form of carbon 6147 directly from carbon dioxide may provide a new pathway to mitigate this greenhouse gas. Today, carbon nanofibers require 30- to 100-fold higher production energy compared to aluminum production. We present the first high yield, inexpensive synthesis of carbon nanofibers from the direct electrolytic conversion of CO2, dissolved in molten carbonates to CNFs at high rates using scalable, inexpensive nickel and steel electrodes. The structure is tuned by controlling the electrolysis conditions, such as the addition of trace nickel to act as CNT nucleation sites, limits to the electrolytic oxide concentration, inclusion of zinc, and control of current density. New infrastructure and merchandise built from CNFs would provide a repository to store atmospheric CO2. The Raman, XED, and SEM characterization provides fundamental evidence of the high yield and purity of the CNF synthesis. Future papers will explore application level testing of the strength, thermal, and electrical conductivities and lithium ion intercalation properties of the CNFs synthesized from CO2 in molten salts tuned for applications under various electro- chemical conditions. It is evident in the Supporting Information that a range of carbon nanostructures is attainable and future studies will probe conditions to characterize and optimize g■rowth of these structures. ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nano- lett.5b02427. (i) Conventional CO2 to carbon formation in carbonates, (ii) Ni catalyzed CO2 to CNF formation, (iii) other metals’ action on molten electrolytic carbon formation, (iv) a mechanism of high yield electrolytic CNF formation, (v) scalability of the CNT formation, and (vi) high rate CO2 splitting in carbonates at high solar efficiency (PDF) AUTHOR INFORMATION ■ Corresponding Author *E-mail: slicht@gwu.edu. Notes T■he authors declare no competing financial interest. ACKNOWLEDGMENTS This project was supported in part by a grant from the United States National Science Foundation 1230732. We are grateful to the Director of the George Washington University Institute for Nanotechnology Prof. Michael Keidar and his research group members Dr. Alexey Shashurin and Xiuqi Fang for their h■elpinattainingtheRamanspectroscopy. REFERENCES (1) pluscomposites, Composites: Materials of the Future: Part 4: Carbon fibre reinforced composites, at: http://www.pluscomposites. eu/publications, directly accessed July 16, 2015 at: http://www. pluscomposites.eu/sites/default/files/Technical%20series%20- %20Part%204%20- %20Carbon%20fibre%20reinforced%20composites_0.pdf. (2) Kim, H. C.; Fthenakis, V. J. Ind. Ecol. 2013, 17, 528−541. (3) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670−5703. (4) Shen, Y.; Yan, L.; Song, H.; Yang, J.; Yang, G.; Chen, X.; Zhou, J.; Yu, Z.-Z.; Yang, S. Angew. Chem., Int. Ed. 2012, 51, 12202−12205. DOI: 10.1021/acs.nanolett.5b02427 Nano Lett. 2015, 15, 6142−6148PDF Image | One-Pot Synthesis of Carbon Nanofibers from CO2
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