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Nano Letters Letter are homogeneous throughout the cathode product, charac- terized by uniform diameters of 200 to 300 nm, and with length of 20 to 200 μm. The fibers are prepared by electrolysis at a 10 cm2 coiled galvanized steel wire cathode (shown) and an oxygen generating nickel anode in 730 °C molten Li2CO3, initiated by a low current of 5 mA/cm2 cathode, followed by constant current electrolysis at high current (100 mA cm2) for 2 to 4 Ah. The cooled product consists of fibers mixed with solidified electrolyte. Product readily falls of the cooled cathode when it is uncoiled. The Coulombic efficiency is over 80% (and approaches 100% with carefully recovery of all product after washing); the product (after washing off the electrolyte) consists of >80% pure carbon nanofibers. SEM are shown after washing electrolyte from the product. The washed cathode product is further characterized by X-ray powder diffraction, XRD, and Raman spectroscopy in Figure 2. Figure 2. Top: Raman of the carbon products. Bottom: X-ray powder diffraction of the cathode carbon product. The XRD diffraction peaks in Figure 2 at 26° and 43° are assigned to the hexagonal graphite (002) and diffraction planes (JCPDS card files no. 41-1487) within the CNF (specifically, the stacking of parallel graphene layers and the size of graphene layer, respectively).19 The resolved XRD peaks at 43° (100 plane) and 44° (101 plane) is evidence of homogeneity of the synthesized CNFs. Raman spectrum was recorded to study the degree of graphitization of the carbon nanofibers. The Raman spectrum exhibits two sharp peaks observed at 1350 and 1580 cm−1, which correspond to the disorder-induced mode (D band) and the high frequency E2g first order mode (G band), respectively. The intensity ratio between D band and G band (ID/IG) is an important parameter to evaluate the graphitization and is 0.70 in our case, which is consistent with commercial hollow carbon nanofiber samples.30 All of the above information indicates the formation of good CNFs. In the absence of a nucleating metal, such as Ni, CNFs are not evident during molten electrolysis. This is shown in the Supporting Information in which a Pt or Ir anode is used instead of the Ni electrode. Subsequent to electrolysis in a Ni- free environment (Li2CO3 at 730 °C with 6 molal Li2O utilizing either a Pt or Ir, rather than Ni, anode), amorphous and platelet 6145 structures are seen instead of fibers, indicative of partially formed multilayered graphene/graphite structures. Electron dispersive spectroscopy elemental analysis indicates that the amorphous and platelet structures are composed of >99% carbon. We had not previously anticipated the oxygen-generating anode effects on the structure of the carbon formed at the cathode during carbonate electrolysis. As demonstrated here, these anode effects promote significant carbon nanofiber formation. We have investigated Pt, Ir, and Ni, and each can be effective as molten carbonate oxygen generating ano- des.7,8,23−29 Whereas Ir exhibits no corrosion following hundreds of hours use in molten carbonates, the extent of Ni corrosion is determined by the cation composition of the carbonate electrolyte. A nickel anode undergoes continuous corrosion in a sodium and potassium carbonate electrolyte,25 it is stable after initial minor corrosion in lithium carbonate electrolytes,8 and no corrosion of the nickel anode is evident in barium/lithium carbonate electrolytes.22,25 In lithium carbonate electrolytes, we have quantified the low rate of nickel corrosion at the anode as a function of anode current density, electrolysis time, temperature, and lithium oxide concentration.17 The Ni lossata100mAcm−2 NianodeinLi2CO3 at750°Cwith0or 5 molal added Li2O is respectively 0.5 or 4.1 mg cm−2 of anode subsequent to 600 s of electrolysis and increases to 4.6 or 5.0 mg cm−2 subsequent to 1200 or 5400 s of electrolysis. The Ni loss increases to 7.0 mg or 13.8 mg cm−2, respectively, subject to higher current (1000 mA cm−2) or higher temperature (950 °C). Each of these nickel losses tends to be negligible compared to the mass of Ni used in the various Ni wire or Ni shim configured anodes. Nickel oxide has a low solubility of 10−5 mol NiO per mol of molten Li2CO3,31 equivalent to 10 mg Ni per kg Li2CO3. This low, limiting solubility constrains some of the corroded nickel to the anode surface as a thin oxide overlayer, with the remainder as soluble oxidized nickel available for reduction and redeposition at the cathode. The characteristic CNF structure is observed when the electrolysis is initiated at a gradually increasing current density, or with an initial low current (1 h of 5 mA cm−2 at the cathode) followed by an extended high current electrolysis such as at 100 mA cm−2 (for several hours). However, when the electrolysis starts directly at only a high (100 mA cm−2) current density. The cathode product is principally amorphous (and only ∼25% CNF). The linear EDS map on the middle, right lower side of Figure 1 shows elemental variation along the 6 μm path of the EDS scan from pure Ni at the start of the fiber to pure carbon along the remainder of the fiber. We interpret this mechanistically as follows: due to its low solubility and lower reduction potential, nickel (in this case originating from the anode) is preferentially deposited at low applied electrolysis currents (5 or 10 mA cm−2). This is evidenced by the low observed electrolysis voltage (<0.7 V) and sustains the formation of nickel metal cathode deposits, which appear to be necessary to nucleate CNF formation. The high concentration of electrolytic [CO32−] ≫ [Ni2+] and mass diffusion dictates that higher currents will be dominated by carbonate reduction. The subsequent higher electrolysis voltage thermodynamically required to deposit carbon24 is only observed at higher applied currents (>20 mA cm−2). Hence, without the initial application of low current, amorphous carbon will tend to form, while the CNF structures are readily formed following the low current nickel nucleation activation. The lithium carbonate electrolyte has an abundant Li ion 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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