Nitrogen-doped carbon
(N/C) and graphene (N/G) were synthesized
by the established conventional heat-treatment method, and the incorporation
of nitrogen into the carbon matrix was confirmed by CHN analysis,
X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electrochemical
impedance spectroscopy (EIS) of the prepared catalysts in argon-saturated
0.1 M KOH was performed in a three-electrode rotating disk electrode
(RDE) configuration. The capacitance derived from the low-frequency
region of the EIS patterns was used to estimate the effective density
of states [D(E
F)] of
carbon and its nitrogen-doped counterparts. Moreover, the carrier
concentrations (N
D) and flat band potentials
of the samples were obtained by Mott–Schottky analysis. The
metal-free catalyst samples were tested for possible oxygen reduction
reaction (ORR) activity in oxygen-saturated 0.1 M KOH electrolyte,
and the origin of the activity improvement with nitrogen doping of
carbon/graphene can be explained on the basis of the effective density
of states [D(E
F)], carrier
concentration (N
D), and flat band potential.
The results suggest that N/C-900 has the highest carrier concentration
and maximum flat band potential and, therefore, the highest activity
for the ORR.
Transport of redox
species (VO2+/VO2
+, V2+/V3+, Ti3+/Ti4+, and Fe2+/Fe3+) across the electrode/electrolyte
interface is investigated in a thin-film rotating disk electrode configuration
using electrochemical impedance spectroscopy (EIS). The transport
features depend on the constituents of the thin-film catalyst layer
and on the rate constant of the redox reaction. On Nafion-free porous
electrodes, semi-infinite linear and finite transport features
are observed under static and hydrodynamic conditions of
the electrode, respectively. Depending on the rate constant of the
electrochemical reaction, an equivalent circuit consisting of either
resistance (R) and constant phase element (Q) or the Warburg short (W
s)
element is proposed to explain the finite transport features. Addition
of Nafion (binder) in the electrode offers extra resistance to the
transport of redox species, which helps resolve EIS features of the
transport of redox species through the porous thin-film electrode
and that through the bulk of the electrolyte. The features of the
transport of redox species through the porous electrode media are
independent of the hydrodynamic conditions.
n-type and p-type silicon of various dopant concentrations (10 14 to 10 19 cm −3 ) are electrochemically characterized in a 1% hydrogen fluoride (HF) electrolyte with voltammetry, electrochemical impedance spectroscopy (EIS), and Mott−Schottky analysis. Voltammograms in conjunction with Mott−Schottky analysis are used to assign the EIS features to the underlying physical processes. The high-frequency (hf) semiconductor features (depletion for n-type Si and accumulation for p-type Si) are strongly influenced by the cathodic overpotentials (−0.4 to −1.5 V) and dopant concentration and are not apparent in the Nyquist plots. With an increase in dopant concentration, Si shows features similar to that of metals, with an hf reminiscence of the spacecharge capacitance. Nevertheless, the mid-frequency (mf) semicircle corresponding to the double layer dominates the EIS features. Moreover, at further lower potentials, various low-frequency (lf) features corresponding to the hydride formation and hydrogen evolution reaction are observed.
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