B.B. Beyene, et al.
Inorganica Chimica Acta 513 (2020) 119929
2.4.2. Controlled potential electrolysis and electrochemical impedance
spectroscopy
Controlled potential electrolysis (CPE) at −1.25 V vs Ag/Ag + was
conducted to prove H evolution by sampling head space gas. Hence,
2
after 105 min CPE, 16.91 coulomb of charge is passed though (Fig. 6).
Deducting the charge passed by control experiment, theoretical amount
of H
standard H
05 min, and analysing using gas chromatography (GC) practical
2
evolved is calculated to be 30 µmol by using calibration curve of
2
(Table SI-2 and Fig. SI-4). By sampling head space gas after
1
amount of hydrogen is 29.4 µmol (Fig. SI-5). Hence the Faradaic effi-
ciency of the catalyst in neutral aqueous solution is 98.3%.
In order to confirm stability of CuTPPS under electrolytic condition,
continuous controlled potential electrolysis was carried out at −1.3 V
for about 9 h (Fig. SI-6). The amount of charge produced increase lin-
early with time and no degradation of catalyst for long time implying
the stability of a molecular catalyst under experimental condition.
The role of sulphonation of phenyl group on catalytic activity was
also assessed by carrying out experiments using CuTPPS and CuTPP
Fig. 6. CPE of CuTPPS and blank in 0.1 KPi solution at −1.25 V vs Ag/Ag+.
displays a liner relationship to the scan rate, which was varied from
(
without para-sulfonic group). To clearly explain the difference, both
0.01 to 0.5 V/s. In addition, the catalytic current increases linearly with
molecules were adsorbed to glassy carbon electrode (GCE) using con-
tinuous swiping. In electrochemical modification of GCE by continuous
an increase in concentration of CuTPPS (Fig. SI-3) indicating the com-
plex functions in diffusion-controlled regime under catalytic condition.
This implies the complex serve as freely diffusing molecular catalyst in
nature. Thus hydrogen evolution reaction is first order and an ap-
sweeping, the electrode was swept for 100 cycles in the potential
window from + 1.0 to −1.0 V vs Ag/Ag+. The catalyst modified
electrode was taken out and immersed to the corresponding clean sol-
vent (in water for CuTPPS and in DCM for CuTPP) for 5 min to remove
loosely attached catalysts. Then, by taking catalyst modified electrodes
proximate model for pseudo-first order or 1st order H
tion (Eq-1) is often applied to estimate the observed rate constant (kobs
91–95].
2
evolving reac-
)
[
(
CuTPP-GCE and CuTPPS-GCE) Differential pulse voltammetry (DPV),
CPE and electrochemical impedance spectroscopy (EIS) experiments
were performed in phosphate buffer solution of pH 7. From differential
pulse voltammetry, the onset potential of CuTPPS-modified electrode is
about −0.98 V, whereas for CuTPP–modified electrode it is about
Icat
Ip
2
RTKobs
Fv
=
0.446
(1)
Where, icat is catalytic current, i
p
= plateau current of non-catalytic
−
1.38 V as shown in Fig. 7. Moreover, the catalytic current for
CuTPPS-modified electrode at −1.75 V is about 120 µA and it is about
0 µA for CuTPP–modified electrode at the same potential. This shows
(
the reversible reduction peak of Cu(II/I)), R is universal gas constant, T
the temperature in Kelvin, F = Faraday’s constant, v is the san rate and
3
kobs is the observed first order rate constant.
the catalytic activity of CuTPPS modified electrode is far better than
that of CuTPP-GCE electrode. This may be an evidence for accumula-
tion of more catalyst close to GCE as the catalytic current is directly
proportional to amount of catalyst used. Moreover, onset potential of
CuTPPS-GCE is 400 mV less negative than CuTPP-GCE. All these can be
considered as evidence for accumulation of more catalyst close to GCE
in case of CuTPPS modification. To confirm this result, CPE was con-
So, the catalytic activity is estimated by using Eq. (1) at higher scan
rate (the point at which increasing scan rate has no effect on catalytic
current enhancement, which is 0.8 V/s in this case). Hence, since
3
−1
i
p
cat = 261 and i = 3.5 (from Fig. 6), the kobs value of 8.6 × 10 s is
obtained at scan rate of 0.8 V/s. This rate is higher than some of re-
ported Cu porphyrins in organic solvents up on using proton source. For
example in Table 1 a kobs value of 6 × 10
porphyrin with ferrocene as peripheral substituent in organic solvent by
using acetic acid as a proton source [30]. Copper porphryin with penta
3
−1
s
is reported for copper
2
ducted at −1.2 V vs Ag/Ag + . The study reveals 9 mA/cm of current
2
density for CuTPPS-modified electrode and about 5.5 mA/cm for
CuTPP-modified electrodes. Therefore, DPV experimental result is in
good agreement with CV and replacing H of TPP at para position with
−1
fluoro substituent showed a TOF of 266 h , Faradaic efficiency of
8.3% with 878 mV overpotential in DMF up on addition of trifluoro
acetic acid as proton source [27,28]. In other study Cu porphyrins with
electron donating amino (–NH ) groups were reported to show kobs of
and catalytic efficiency of 75.7 in DMSO up on addition of
trifluoro acetic acid [72]. A fused copper complex showed the highest
9
SO
3
H has significant role in catalytic activity because of its nature to
anchor to electrode surface, electron withdrawing behaviour and
2
proton containing substituent which enhances H
eous solution.
2
evolution from aqu-
−1
45.11 s
The electrode modified with CuTPPS performs much better than
bare electrode and CuTPP modified electrode. It is also stable as there is
continuous charge accumulation when controlled potential electrolysis
at −1.75 V vs Ag/Ag + is conducted for about 118 min (Fig. 8).
Fig. 9 represents an electrochemical impedance spectroscopy results
for CuTPPS and CuTPP modified electrodes at −1.5 V applied potential
in the form of Nyquist plot. An electrode modified with CuTPPS show
evolving activity with kobs value of 2.1 × 106
there is no so far any report concerning Cu-based porphyrin molecular
catalysts for H evolution in water. So this work is by far important and
s
−1
[85]. Actually,
H
2
2
interesting as it involves study in aqueous solution.
Table 1
Comparison of molecular Cu-porphyrin catalysts used for H
2
evolution.
Over potential
Cu-based porphyrins
kobs (TOF)
Faradic efficiency (%)
Catalysis condition
Ref.
3
Cu(II)-para-ferocene porphyrin
Cu(II)-pentafluoro porphyrin
6 × 10 s−1
200 mV
878 mV
106 mV
570 mV
470 mV
70
91
75.7
99.9
98.3
TEAHCl/TFA in DMF
Acetic acid in DMF
TFA in DMSO
30
27,28
72
85
This work
−
1
266 h
45.1 s
−1
Cu(II)-para-NH
2
porphyrin
6
3
−1
−1
Cu(II)-fused porphyrin
Cu(II)-para-sulfonic porphyrin
2.1 × 10
8.6 × 10
s
s
TFA in DCM
In neutral H
2
O solution
4