Electrochemical Surface Treatment of Discontinuous Carbon Fibers

Abstract

We developed an operationally simple electrolytic design for the surface treatment of short carbon fibers. Using X-ray photoelectron spectroscopy (XPS), we demonstrated that the electrochemical surface treatment of discontinuous fibers is highly reproducible, uniform, and tunable. Specifically, total amounts of surface oxygen and nitrogen contents (0 to 17 atomic %) as well as surface oxygen-to-nitrogen ratio (1:0 to 1:2) vary significantly over the ranges of each processing parameter: applied voltage (1.5–21 V), location of carbon fiber (i.e., anode, cathode, or mixed mode), initial temperature (3–70.5 °C), and ammonium bicarbonate concentration (0.005–0.75 M). Optimized processing conditions afforded carbon fibers that have similar surface compositions (86.3 ± 1.1 at. % C, 8.9 ± 0.8 at. % O, 4.7 ± 0.6 at. % N) as those of commercially available continuous fibers. In addition, these fibers retain their mechanical properties (tensile strength and tensile modulus) and exhibit no detectable surface damage based on single fiber tensile tests and scanning electron microscopy (SEM). We also performed a number of control experiments to develop a proposed mechanism for the surface functionalization of the carbon fiber. These mechanistic studies demonstrated that water splitting contributes significantly to the oxidation of carbon fibers and that other species in the chemical equilibria of ammonium bicarbonate (and not just its individual ions) play a significant role in functionalizing carbon fiber surfaces.

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“…As we know, during water electrolysis, large amounts of radicals such as hydroxyl ( • OH) and hydroperoxyl ( • OOH) radicals are generated. 32 These radicals will attack the carbon atoms, leading to the opening of graphitic rings. Such an attack not only results in the oxidation of carbon atoms but sometimes also induces the cleavage of C-C bonds.…”

“…As can be seen, when CC is electro-oxidized under 35 V, the potential of CC is around 23.40 V, more than 13.25 V higher than the potential of CC when CC is oxidized in a 0.5 M NH 4 Cl solution under a DC voltage of 16 V, which might explain why N and Cl elements could not be observed in Tran et al's work. 32 The G band is due to the stretching vibration in the plane of sp 2 -hybridized carbon atoms, whereas the D band signifies the carbon structure defects. 27 The I D /I G ratio of pristine CC is 0.84, which increases to 0.91 for O 15 -CC/ NH 4 Cl because the incorporation of oxygen functionalities and doping of heteroatoms created a lot of defects on the carbon networks.…”

“…The amino radicals can react with the carbon atoms, leading to an increase in the N content. 32 Moreover, the deconvolution of the N 1s core-level peak shows that after the electroreduction, the contributions from both pyrrolic-N and pyridinic-N increase substantially (Figure S8C), which will certainly promote the catalytic activity of CC toward the oxygen reaction. 37,38 However, different from N, the atomic concentration of Cl decreases after the electroreduction.…”

“…30 nm thick, which is caused by the etching of the exfoliated carbon layer. As we know, during water electrolysis, large amounts of radicals such as hydroxyl ( • OH) and hydroperoxyl ( • OOH) radicals are generated 32 . These radicals will attack the carbon atoms, leading to the opening of graphitic rings.…”

“…It is worth mentioning that in addition to C and O, two more elements, N (N 1s ~402.23 eV) and Cl (Cl 2p ~200.61 eV), are also observed in the XPS spectrum of O 15 ‐CC/NH 4 Cl, demonstrating that the 50 V DC voltage not only induces the incorporation of oxygen functionalities but also causes the co‐doping of N and Cl atoms. Tran et al 32 performed the electro‐oxidation of CC in a 0.5 M NH 4 Cl solution under a DC voltage of 16 V, and N was barely detectable and Cl was not detectable on XPS measurement, 32 implying that the relatively low DC voltage and the high NH 4 Cl concentration could not induce the doping of N and Cl heteroatoms. Figure shows the N 1s and Cl 2p core‐level XPS peaks of O 15 ‐CC/NH 4 Cl, wherein the N 1s peak is deconvoluted into pyridinic‐N (400.22 eV), pyrrolic‐N (401.05 eV), graphitic‐N (402.44 eV), and oxidized‐N (403.23 eV), 33 while the Cl 2p peak is fitted into two sets of 2p 1/2 –2p 3/2 doublets.…”