All
electrochemical experiments were performed on an Autolab PGSTAT302N
potentiostat from Metrohm equipped with the FRA32 module and operated
with Nova 1.11.2 software. Each measurement was conducted on a freshly
cleaved HOPG surface. Before the experiment, the CE was flame-cleaned
with a blue butane flame and the RE was thoroughly washed with DI
water. To avoid the contamination of the HOPG by the adsorption of
air-bound hydrocarbons, a phenomenon well established in the literature
for the basal plane of graphite,27 (link) the
solution was deposited on the WE within 1 min of cleaving the surface.
Unless specified otherwise, the applied potential, E, throughout the main text is referred vs Ag/AgCl(sat. KCl) (seeSection S1.2 in the SI). The experimental
protocol used for the static measurements was composed of consecutive
potential pulses from 0 to −2 V vs Ag/AgCl wire with a step
of 50 mV. For the positive side, the same approach was followed in
the potential range between 0 and +1.5 V vs Ag/AgCl wire. The duration
of the pulses was adjusted accordingly (varying from 5 to 15 s) for
each electrolyte concentration in the range of 0.1–16 m for
KF and 0.1–20 m for CsF based on preliminary dynamic experiments
for the determination of the time required to attain equilibrium;
the latter was indicated by the plateau in the calculated CA values.
A similar strategy was adopted for the dynamic measurements, in which
the potential was directly stepped from 0 to either −2 or +1.5
V vs Ag/AgCl wire three consecutive times. Each repetition represents
one cycle. Once again, the duration of the potential pulse was chosen
based on the required time needed (0.5 s) to ensure a steady state
response for the electrolyte concentration used. For the investigation
of the surface processes occurring during cathodic and anodic polarization
in different electrolyte concentrations, cyclic voltammetry (CV) experiments
were carried out over a potential range from 0 to −2 and 0
to +1.5 V vs Ag/AgCl(sat. KCl), respectively, at a
scan rate of 1 V s–1. Electrochemical impedance
spectroscopy (EIS) measurements were performed in the frequency range
between 20 kHz and 10 Hz, using an imposed AC rms amplitude of 7 mV
peak-to-peak. The EIS experimental data was evaluated for its compliance
with Kramers–Kronig (KK) criteria by fitting the AC response
of the system to the admittance representation of a theoretical circuit
containing a ladder of n RC elements in series, with
an additional capacitance and/or inductance in parallel to the ladder
structure, using the software developed by Boukamp.28 (link) The choice of the aforementioned equivalent circuit relies
on the blocking nature of the electrodes under study (the impedance
increases to infinity as frequency approaches zero), which renders
the Voigt-type approximation inappropriate.28 (link) The compliance with KK criteria was assured for all data by the
values of the relative residuals, calculated to be less than 0.5%
for both the real and imaginary parts of the impedance and the chi-square
parameter which was found to be on the order of 10–7 for the complete data series. All of the experiments were conducted
inside a faraday cage.
electrochemical experiments were performed on an Autolab PGSTAT302N
potentiostat from Metrohm equipped with the FRA32 module and operated
with Nova 1.11.2 software. Each measurement was conducted on a freshly
cleaved HOPG surface. Before the experiment, the CE was flame-cleaned
with a blue butane flame and the RE was thoroughly washed with DI
water. To avoid the contamination of the HOPG by the adsorption of
air-bound hydrocarbons, a phenomenon well established in the literature
for the basal plane of graphite,27 (link) the
solution was deposited on the WE within 1 min of cleaving the surface.
Unless specified otherwise, the applied potential, E, throughout the main text is referred vs Ag/AgCl(sat. KCl) (see
protocol used for the static measurements was composed of consecutive
potential pulses from 0 to −2 V vs Ag/AgCl wire with a step
of 50 mV. For the positive side, the same approach was followed in
the potential range between 0 and +1.5 V vs Ag/AgCl wire. The duration
of the pulses was adjusted accordingly (varying from 5 to 15 s) for
each electrolyte concentration in the range of 0.1–16 m for
KF and 0.1–20 m for CsF based on preliminary dynamic experiments
for the determination of the time required to attain equilibrium;
the latter was indicated by the plateau in the calculated CA values.
A similar strategy was adopted for the dynamic measurements, in which
the potential was directly stepped from 0 to either −2 or +1.5
V vs Ag/AgCl wire three consecutive times. Each repetition represents
one cycle. Once again, the duration of the potential pulse was chosen
based on the required time needed (0.5 s) to ensure a steady state
response for the electrolyte concentration used. For the investigation
of the surface processes occurring during cathodic and anodic polarization
in different electrolyte concentrations, cyclic voltammetry (CV) experiments
were carried out over a potential range from 0 to −2 and 0
to +1.5 V vs Ag/AgCl(sat. KCl), respectively, at a
scan rate of 1 V s–1. Electrochemical impedance
spectroscopy (EIS) measurements were performed in the frequency range
between 20 kHz and 10 Hz, using an imposed AC rms amplitude of 7 mV
peak-to-peak. The EIS experimental data was evaluated for its compliance
with Kramers–Kronig (KK) criteria by fitting the AC response
of the system to the admittance representation of a theoretical circuit
containing a ladder of n RC elements in series, with
an additional capacitance and/or inductance in parallel to the ladder
structure, using the software developed by Boukamp.28 (link) The choice of the aforementioned equivalent circuit relies
on the blocking nature of the electrodes under study (the impedance
increases to infinity as frequency approaches zero), which renders
the Voigt-type approximation inappropriate.28 (link) The compliance with KK criteria was assured for all data by the
values of the relative residuals, calculated to be less than 0.5%
for both the real and imaginary parts of the impedance and the chi-square
parameter which was found to be on the order of 10–7 for the complete data series. All of the experiments were conducted
inside a faraday cage.
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