Investigating the Effect of Microstructure and Surface Functionalization of Mesoporous N-Doped Carbons on V⁴⁺/V⁵⁺ Kinetics

Abstract

State-of-the-art electrode materials for all-vanadium redox flow batteries are based on carbon. Unfortunately, the impact of the carbon structure, i.e., microstructure/crystallinity, surface functional groups, and porosity/morphology/surface area, on the electrochemical performance is still unclear. This is due to the fact that usually several structural characteristics are varied due to synthesis or post-treatment procedures at the same time. Therefore, this paper shows systematically how microstructure, porosity, and surface functional groups vary with carbonization and graphitization temperature (ranging from 700 to 1500 °C) for a mesoporous N-doped carbon (MPNC). Changes in the material’s structure (e.g., morphology, porosity, crystal structure, surface functionalization), determined by scanning and transmission electron microscopy, X-ray diffraction (pair distribution function analysis), X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and N2 sorption measurements, are correlated to changes in wettability, conductivity, and electrochemical kinetics, investigated by H2O sorption measurements, cyclic voltammetry, and electrochemical impedance spectroscopy in the VO2+ electrolyte, respectively. We found that the kinetics of the VO2+/VO2 + reaction increases with an increase in sp2-C content and therefore an increase in crystallite size and conductivity of the mesoporous N-doped carbon. Nevertheless, the largest current for the VO2+/VO2 + reaction for the same amount of carbon during cyclic voltammetry is observed for the MPNC carbonized at an intermediate temperature, 1000 °C, as a result of its larger wettability and thus available surface area compared to the MPNC carbonized at 1500 °C.

High‐Efficiency Monolithic Photosupercapacitors: Smart Integration of a Perovskite Solar Cell with a Mesoporous Carbon Double‐Layer Capacitor

“…High-resolution transmission electron microscopy (HRTEM) images (Figure 2d) reveal weakly defined lattice fringes in the carbon nanospheres, which suggest a moderate degree of graphitization. [27] This is in accordance with Raman spectra analysis of the samples (Figure S3a, Supporting Information) that shows domination of two broad absorption bands at 1604 and 1354 cm À1 corresponding to the G band and defect band D, respectively. Fitting of the spectra (Figure S3b, Supporting Information) enabled us to distinguish the presence of additional bands at 1350, 1500, and 1620 cm À1 that reflects contribution of defects and amorphous carbon.…”

“…The elemental analysis determined the N content as 3.3 wt%, which is in good agreement with our previously reported MPNC materials carbonized at 1000 C but with larger pore sizes. [27] Nitrogen physisorption was applied to characterize the porosity of the material. From the fitting of the data to a Brunauer-Emmett-Teller (BET) model, the specific surface area was calculated to be as high as 825 m 2 g À1 with 82% related to mesopores and macropores and 18% related to micropores (as estimated from the cumulative surface area) (Figure S5a, Supporting Information).…”

“…The MPNC nanosphere synthesis is based on a hard-templating approach previously reported by us for fuel cell catalyst supports and vanadium redox-flow battery electrodes, involving the oxidative polymerization of aniline (as carbon and nitrogen precursor) in the presence of spherical silica nanoparticles (NPs) as template, and was adapted to yield MPNC nanospheres with 7 nm pores. [26,27] During the synthesis, silica/polyaniline self-assembly yielded homogenously sized composite nanospheres, which, after pyrolysis at elevated temperature (1000 C under inert atmosphere) and etching for template removal, resulted in spherical MPNC particles with a homogenous mesoporous structure imprinted by the silica nanotemplates.…”

Rapid wet-chemical oxidative activation of graphite felt electrodes for vanadium redox flow batteries

“…37,38 Typically for VRFB electrode analysis, most focus is given to the two peaks located between 1200-1600 cm À1 which correspond to the G-band (1580 cm À1 ) and D-band (1350 cm À1 ). 14,[39][40][41] The D-band originates in breathing modes of sp 2 -atoms in rings and its occurrence signals imperfect graphitic order. The G-band reects bond stretching or pairs of sp 2 hybridized atoms in rings and chains and therefore indicates conjugated sp 2 hybridized systems, including graphitic domains.…”

Molybdenum-assisted reduction of VO2+ for low cost electrolytes of vanadium redox flow batteries

“…Fig. 4c and d show two parameters, Rs and Rct, evaluated by fitting experimental results to the equivalent circuit consisting of ohmic resistance of bulk electrolyte (Rs), the charge transfer resistance across electrode/electrolyte (Rct), the constantphase-element (CPE) describing the electrochemical double layer, and the Warburg coefficient (Z) resulting from vanadium ion diffusion 54 . For both the positive and negative electrolytes, Rs values were independent of molybdenum concentration.…”