1

Electrochemical transformations of polymers formed from nickel (II) complexes with salen-type

ligands in aqueous alkaline electrolytes

Nikita Kuznetsov1, Peixia Yang2, Georgy Gorislov3, Yuri Zhukov1, Vladimir Bocharov1, Valery

Malev1, 4, Oleg Levin1*

1Saint Petersburg University, St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg,

199034 Russian Federation

2School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001 China

3 NPC Shtandart Ltd., Malyy pr. P.s., 4,St. Petersburg, 197110 Russian Federation

4Institute of Cytology, Russian Academy of Sciences, Tikhoretsky pr. 6, St. Petersburg, 194064 Russian

Federation

* Corresponding author

E-mail: o.levin@spbu.ru, tel. +7(812)428-69-00

Electronic Supplementary Information

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3.2. Cyclic voltammetry of salen-type polymer films on GC substrates

0.0 0.2 0.4 0.6 0.8

-2

0

2

4

6

8

10 (a)

j / m

A cm

-2

E vs (Ag / AgCl / KClsat.) / V

Cycle 1 Cycle 2 Cycle 10 Cycle 20 Cycle 30 Cycle 40 Cycle 50 Cycle 60

0.0 0.2 0.4 0.6 0.8

-2

0

2

4

6

8

10 (b)

j / m

A cm

-2

E vs (Ag / AgCl / KClsat.) / V

Cycle 1 Cycle 2 Cycle 10 Cycle 20 Cycle 30 Cycle 40 Cycle 50 Cycle 60

Fig. SI1. Cyclic voltammograms of polymeric films hydrolysis (Гpolymer = 5·10–8 mol cm−2) in a 0.2 mol

dm−3 NaOH solution, with a scan rate of 50 mV s−1: (a) poly[Ni(Salen)]; (b) poly[Ni(3-MeOSalen)].

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-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3 (a)

j / m

A cm

-2

E (vs Ag / Ag+ / CH3CN) / V

glassy carbon Qsyn = 1.1 mC cm-2

Qsyn = 2.1 mC cm-2

Qsyn = 4.3 mC cm-2

Qsyn = 8.6 mC cm-2

Qsyn = 17.2 mC cm-2

Qsyn = 34.3 mC cm-2

0 5 10 15 20 25 30 350

2

4

6

8

10

12 (b)

QR

ed (C

H3C

N) /

mC

cm

-2

Qsyn / mC cm-2

Y = 0.6157 X + 0.3337

Fig. SI2. (а) Cyclic voltammograms of poly[Ni(3-MeOSalen)] films with different polymerization

charges Qsyn in a 0.1 mol dm−3 Et4NBF4 / CH3CN solution, scan rate is 10 mV s−1; (b) Dependence of the

poly[Ni(3-MeOSalen)] reduction charge, QRed (CH3CN), calculated from CV on the polymerization

charge Qsyn.

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During prolonged cycling in an alkaline solution, the change in the shape of peaks is observed,

corresponding to a phase transformation of nickel hydroxide. The loss of charge under the cathodic

reduction curve after 1000 cycles is no more than 20% of its maximum.

0 100 200 300 400 500 600 700 800 900 10000.00.51.01.52.02.53.03.54.04.5 (a)

QNi

(OH)

2 mC

cm-2

Number of cycles

0.0 0.2 0.4 0.6 0.8-4

-2

0

2

4

6

8

10

12

14 (b)

Cycle 100 Cycle 500 Cycle 1000 Cycle 2000

j / m

A cm

-2

E vs (Ag / AgCl / KClsat.) / V

Fig. SI3. (a) Dependence of charge under the cathodic curve on the cycle number during prolonged

polymer hydrolysis; (b) characteristic CVA curves.

Cyclic voltammetry, various scan rates

As was mentioned in the text, the electrochemical behavior of Ni-CME during hydrolysis is almost

independent of the presence of substituents in the initial complex, the difference can only be observed in

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the first cycle. This indicates that the structure of oxidized ligands is not fully preserved, and

electrochemically identical products are formed. Thus, from this point on, without loss of generality, the

results are presented for hydrolysis products of poly[Ni(3-MeOSalen)], which we studied more

extensively. Voltammograms of the resulting products in both their shape and the half-sum of peak

potentials (Fig. SI1) are close to voltammograms of one-electron nickel hydroxide recharge process:

Ni(OH)2 + OH– → NiOOH + H2O + e

The electrode after hydrolysis with the resulting product was rinsed with deionized water, put in a fresh

alkaline solution, and then characterized by cyclic voltammetry with various scan rates.

0.0 0.2 0.4 0.6 0.8

-0.05

0.00

0.05

0.10

0.15

0.20 (a) 2 mV/s 5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s 500 mV/s 1000 mV/s 2000 mV/s 5000 mV/s

j v-1 /

A s

V-1 c

m-2

E vs (Ag / AgCl / KClsat.) / V

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0 (b)

lg jp, anodic

lg jp, cathodic

Y = -3,67 + 0,59 X

Y = -4,15 + 0,85 X

Y = -3,23 + 0,501 X

Y = - 3,69 + 0,78 X

lg (

j p / A

cm

-2)

lg (v / mV s-1)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 (c)

E p / V

lg (v / mV s-1)

Ep, anodic

Ep, cathodic

Fig. SI4. (a) Cyclic voltammograms of Ni-CME formed from poly[Ni(3-MeOSalen)] (Гpolymer = 5×10–8

mol cm−2) in a 0.2 mol dm−3 NaOH solution under various scan rates; (b) dependence of current

densities on the scan rate; (c) dependence of peak potentials on the scan rate.

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The shift of cathodic and anodic peaks relative to each other suggests the complex nature of the recharge

process, which can be limited by one or several of the following stages: (a) electron injection at the

substrate-modifier interface, (b) electron and counter-ion diffusion in the modifier and (c) counter-ion

injection at the modifier-solution interface.

As shown in Fig. SI5b, c, a significant increase in anodic currents can be observed in solutions with added

aliphatic alcohols, which, according to the literature, can be attributed to the catalytic oxidation of

alcohols and hydroxide anions by Ni(III) centers. Therefore, we can assume that the process (a) will not

be limiting.

Dependences in Fig. SI4b can be presented as several linear regions. Evidently, at moderate scan rates (up

to 200 mV s−1) Ni-CME behaves like thin layers with no significant diffusion restrictions on the transfer

of charge carriers. Because of that, a scan rate of 50 mV s−1 was chosen for conducting experiments,

under which Ni-CME acts as an effective electron transfer mediator. The diffusion contribution in the

modifier volume increases alongside the increase in scan rate and only produces significant effect in the

last region.

Based on that, we can assume that the initial slope is mostly defined by the slow intercalation of counter-

ions (OH–) into the modifier layer from the solution under fast Ni(II)/Ni(III) recharge. A special case was

described by a model in the work of Vorotyntsev [1], which qualitatively explains the shift of anodic and

cathodic peaks relative to each other. However, in this model, for the purposes of describing the counter-

ion injection, the Butler-Volmer equation was used, which did not account for double-layer effects on the

interfaces, and so the current of the peak does not depend on the scan rate. A model that takes the double-

layer structure into account will be published in later works.

As the observed dependence in Fig. SI4b is close to linear in the chosen range of scan rates used in the

experiments, it is possible to estimate, using equation ГNi-CME = QNi-CME/1·FS, a molar load of the

obtained modifier from the charges under the cathodic voltammogram curve, since only one electron is

used in the recharge process.

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3.3. Determining the hydrolysis products

XRD of the reference samples of nickel hydroxide (II)

Fig. SI6. X-ray powder diffractograms of α-Ni(OH)2 (a) and β-Ni(OH)2 (b) samples used as reference

compounds in this work.

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3.3.1. X-ray photoelectron spectroscopy

Table SI1. XPS elemental composition (atomic %) of poly[Ni(Salen)], poly[Ni(3-MeOSalen)] and their

hydrolized forms.

Element poly[Ni(salen)] poly[Ni(3-MeOSalen)]

initial after hydrolysis initial after hydrolysis

C(1s) 68.56 56.76 64.76 63.66

N(1s) 5.25 0.00 5.67 0.00

O(1s) 24.77 37.43 27.21 33.78

Ni(2p) 1.41 5.82 2.36 2.56

3.3.2. Raman spectroscopy and scanning electron microscopy

Fig. SI7. EDX spectra of nanoparticles of Ni-CME coating presented in Fig. 9

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3.4. Electrocatalytic oxidation of water and aliphatic alcohols at Ni-CME electrodes

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

60

70

80 (a)j /

mA

cm-2

E (vs Ag / AgCl / KClsat.) / V

0 M EtOH 0.05 M EtOH 0.20 M EtOH 1.00 M EtOH

0.2 M NaOH + X M EtOH

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80 (b)

j / m

A cm

-2

E (vs Ag / AgCl / KClsat.) / V

0 M EtOH 0.05 M EtOH 0.10 M EtOH 0.20 M EtOH 0.50 M EtOH 1.00 M EtOH

1 M NaOH + X M EtOH

Fig. SI8. Cyclic voltammograms of Ni-CME (Гpolymer = 5·10–8 mol cm−2) with a scan rate of 50 mV s−1, in

solutions containing various ethanol concentrations and (a) 0.2 mol dm−3 NaOH; (b) 1.0 mol dm−3

NaOH.

10

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

60

j / m

A cm

-2

E (vs Ag / AgCl / KClsat.) / V

0.1 M NaOH 0.2 M NaOH 0.5 M NaOH 1.0 M NaOH

Fig. SI9. Cyclic voltammograms of Ni-CME (Гpolymer = 5·10–8 mol cm−2) in solutions containing various

concentrations of NaOH, with a scan rate of 50 mV s−1

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50 (a)

j / m

A cm

-2

E (vs Ag / AgCl / KClsat.) / V

0.1 M NaOH 0.1 M NaOH + 0.35 M MeOH 0.1 M NaOH + 1.00 M MeOH

0.1 M NaOH + X M MeOH

0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50 (b)

j / A

cm

-2

E (vs Ag / AgCl / KClsat.) / V

0.2 M NaOH 0.2 M NaOH + 0.35 M MeOH 0.2 M NaOH + 1.00 M MeOH

0.2 M NaOH + X M MeOH

Fig. SI10. Cyclic voltammograms of Ni-CME (Гpolymer = 5·10–8 mol cm−2) with a scan rate of 50 mV s−1

in (a) 0.1 mol dm−3 NaOH and (b) 0.2 mol dm−3 NaOH solutions containing various methanol

concentrations.

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0.0 0.2 0.4 0.6 0.8 1.0

0

10

20

30

40

50

60

70 (a)

j / m

A cm

-2

E (vs Ag / AgCl / KClsat.) / V

0.1 M NaOH + 1 M EtOH 0.2 M NaOH + 1 M EtOH 0.5 M NaOH + 1 M EtOH 1.0 M NaOH + 1 M EtOH

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50 (b)

j / m

A cm

-2

CNaOH / mol dm-3

1 M EtOH, E = 0.55 V 1 M EtOH, E = 0.60 V

X M NaOH + 1 M EtOH

Fig. SI11. (a) Cyclic voltammograms of Ni-CME (Гpolymer = 5·10–8 mol cm−2) with a scan rate of 50 mV

s−1 in solutions containing various NaOH concentrations and 1 M EtOH; (b) dependence of the ethanol

oxidation currents (at potentials of 0.55 V and 0.60 V) on the NaOH concentration.

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Bibliography

[1] M.A. Vorotyntsev, L.I. Daikhin, M.D. Levi, Isotherms of electrochemical doping and cyclic

voltammograms of electroactive polymer films, Journal of Electroanalytical Chemistry, 332 (1992) 213-

235.