Exceptional capacitive deionization rate and capacity by block copolymer–based porous carbon fibers

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Science Advances  17 Apr 2020:
Vol. 6, no. 16, eaaz0906
DOI: 10.1126/sciadv.aaz0906
  • Fig. 1 Schematic illustrations of PCF preparation and capacitive deionization.

    (A) Synthesis of PCF from PMMA-b-PAN via electrospinning of PMMA-b-PAN into fibers, self-assembly of PMMA-b-PAN into disordered, bicontinuous PMMA and PAN domains, and pyrolysis of PMMA-b-PAN into PCF with uniform and interconnected pores in a continuous carbon matrix. PMMA generates mesopores and PAN yields carbon. Micropores are also generated in the carbon matrix during the pyrolysis of PAN and are interconnected with the mesopores. (B) Scheme of a CDI cell during charging. The CDI electrodes include (i) block copolymer–based PCF, (ii) conventional nonmesoporous CFs, and (iii) AC. (i versus ii) Compared to PAN-derived conventional CFs that are devoid of uniform mesopores, PCF has abundant interconnected mesopores that provide large ion-accessible surface areas and fast ion diffusion. Thus, PCF has a high desalination capacity and high desalination rate. (i versus iii) Compared to AC composed of discrete carbon particles with irregular shapes and sizes, PCF offers continuous electron and ion conduction pathways both in the vertical and in-plane directions that facilitate high-rate deionization.

  • Fig. 2 Structures and morphologies.

    (A to C) Photographs of (A) PCF, (B) CF, and (C) AC adhered to Sn tapes. The area of each electrode is ~3.8 cm by 2.5 cm. Photo Credits: Tianyu Liu, Virginia Tech. (D to F) Low-magnification top-view SEM images of (D) PCF, (E) CF, and (F) AC. PCF and CF are continuous fibers while AC is made of discrete particles. (G to I) Magnified views of (G) PCF, (H) CF, and (I) AC. (Insets) Cross-sectional images. Scale bars, 100 nm.

  • Fig. 3 Characteristics of porous structures.

    (A) N2 (at 77 K) and (B) CO2 (at 273 K) physisorption isotherms of PCF, CF, and AC. (C) Surface areas of PCF, CF, and AC. Solid bars represent the BET surface areas determined from the CO2 adsorption isotherms, and dashed bars denote the effective desalination surface areas evaluated from the desalination capacity and the radius of hydrated Na+ (see Supplementary Methods). The error bars are instrument errors. (D) PSDs of PCF, CF, and AC determined by nonlinear density functional theory. The dashed vertical line marks the boundary between different pores (i.e., micropores, mesopores, and macropores). The PSDs of the micropores and mesopores are derived from CO2- and N2-physisorption isotherms, respectively.

  • Fig. 4 Chemical and electrical properties.

    (A) XPS survey spectra of PCF, CF, and AC. The light-yellow region highlights the N 1s peak. (B) The N 1s spectra of PCF and CF. The black circles are experimental data. The red, green, and blue dashed peaks represent pyridinic-N, pyrrolic-N, and graphitic-N, respectively. The solid burgundy curves are the best fittings. (C) Static contact angles of NaCl solution (500 mg liter−1) on the surfaces of PCF, CF, and AC. (D) Electrical conductivities of PCF, CF, and AC measured by a four-point probe. Inset: Scheme of a four-point probe setup. The error bars are standard deviations (SDs) based on at least five independent measurements. Because of the interparticle contact resistance, the electrical conductivity of AC is appreciably lower than those of PCF and CF. (E) Na+ diffusion resistances of PCF, CF, and AC probed by electrochemical impedance spectroscopy (EIS) in NaCl solutions (500 mg liter−1).

  • Fig. 5 Desalination performances of PCF, CF, and AC.

    (A) NaCl concentrations in brackish water and tap water before and after deionization by PCF. The NaCl concentrations were determined by ion chromatography. (B) Time-resolved NaCl desalination profiles of PCF, CF, and AC in CDI cells with an excess amount of NaCl solution. (C) NaCl desalination mass capacities of PCF, CF, and AC. (D) Gravimetric and molar desalination capacities of PCF for NaCl, KCl, MgCl2, and CaCl2 deionization. (E) CDI Ragone plots of PCF, CF, and AC, in comparison with state-of-the-art carbon electrodes. Solid and open symbols are performances of carbon electrodes with and without N-dopants, respectively. The values are summarized in table S2. The lines are a guide to the eye. (F) NaCl deionization capacity stability of PCF. The error bars represent 1 SD.

  • Table 1 Summary of the structural, elemental, physical, and CDI properties of PCF, CF, and AC.

    Physical propertiesPeak pore size (nm)
    CO2 BET surface area
    (m2 g−1)
    132.9 ± 2.457.9 ± 1.193.8 ± 2.3
    N2 BET surface area
    (m2 g−1)
    639.9 ± 2.7144.6 ± 0.21317.0 ± 6.0
    Effective desalination surface
    area (m2 g−1)
    Micropore volume (cm3 g−1)
    Meso- and macro-pore
    volume (cm3 g−1)
    Contact angle(Completely wet)(Completely wet)95°
    Elemental CompositionC (atom %)81.683.191.6
    O (atom %)
    Graphitic-N (atom %)4.94.50
    Pyridinic-N (atom %)3.93.10
    Pyrrolic-N (atom %)4.42.60
    Electrical propertiesFour-probe electrical
    conductivity (S cm−1)
    0.97 ± 0.061.02 ± 0.060.057 ± 0.004
    Equivalent series resistance
    Na+ diffusion coefficient
    (×10−11 cm2 s−1)
    CDI performanceDesalination capacity
    (mgNaCl gelectrode−1)
    Desalination rate
    (mgNaCl gelectrode−1 min−1)
    Coulombic efficiency72%85%47%

Supplementary Materials

  • Supplementary Materials

    Exceptional capacitive deionization rate and capacity by block copolymer–based porous carbon fibers

    Tianyu Liu, Joel Serrano, John Elliott, Xiaozhou Yang, William Cathcart, Zixuan Wang, Zhen He, Guoliang Liu

    Download Supplement

    This PDF file includes:

    • Supplementary Methods
    • Figs. S1 to S8
    • Tables S1 and S2
    • References

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