Covalent attachment of [Ni(alkynyl-cyclam)]2+ catalysts to glassy carbon electrodes

Article abstract of DOI:10.1039/c8cc00718g

Surface modification of glassy carbon electrodes (GCEs) with molecular electrocatalysts is an important step towards developing more efficient heterogeneous CO₂ reduction materials. Here, we report direct anodic electrografting of [Ni(alkynyl-cyclam)]²⁺ catalysts to the surface of GCEs in one simple step using inexpensive earth-abundant chemicals. When modified, these electrodes show reversible electrochemistry in organic solvents with zero peak-to-peak separations (ΔE = 0) and non-diffusive I (V) profiles that are typical for heterogeneous redox materials. CPE of these electrodes showed enhanced formation of H₂ gas relative to CO compared to homogeneous catalysts.

Full text of DOI:10.1039/c8cc00718g

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Covalent attachment of [Ni(alkynyl-cyclam)]2+ catalysts to glassy
carbon electrodes.
Received 9th January 2018,
Accepted 00th January 20xx
Almagul Zhanaidarova, Curtis E. Moore, Milan Gembicky, and Clifford P. Kubiak *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Surface modification of glassy carbon electrodes (GCEs) with
molecular electrocatalysts is an important step towards developing
more efficient heterogeneous CO2 reduction materials. Here, we
report direct anodic electrografting of [Ni(alkynylꢀcyclam)]2+ catalysts
to the surface of GCEs in one simple step using inexpensive earthꢀ
abundant chemicals. When modified, these electrodes show
reversible electrochemistry in organic solvents with zero peakꢀtoꢀpeak
separations (∆E = 0) and nonꢀdiffusive I(V) profiles that are typical for
heterogeneous redox materials. CPE of these electrodes showed
enhanced formation of H2 gas relative to CO compared to
homogeneous catalysts.
represent heterogeneous systems and are capable of
increased efficiency due to the absence of catalyst diffusion
limitations.
There are only a few reports in the literature of surface
modification of GCEs with molecular electrocatalysts, despite
the fact that glassy carbon is one of the most common
materials used in electrochemistry. Here we report the
successful covalent attachment of macrocyclic [Ni(alkynyl-
cyclam)]2+ electrocatalysts to the surface of GCE. Despite high
interest in its catalytic properties there are no previous reports
in the literature of a covalent attachment of [Ni(cyclam)]2+ to
the surface of a GCE.
[Ni(cyclam)]2+ is a low cost homogeneous electrocatalyst
widely known for its high reactivity towards electrochemical
reduction of CO2 to CO at low potential (-1.2 V vs. NHE) and its
ability to operate in aqueous media with high turnover rates and
selectivity towards CO production at metal2,3 and glassy carbon
Surface modification of glassy carbon electrodes (GCEs)
with molecular catalysts has been one of the most active and
promising areas of electrocatalysis in the past decade. Surface
modification of electrode materials through the covalent
attachment of molecular catalysts is an essential step towards
creation of new, scalable, robust electrocatalyst materials with
unique structures and properties.1 These materials can be
used in basic electrochemical studies, energy conversion and
storage, aerospace, biomedical and semiconductor industries.
Development of surface confined catalytic systems allows
combining the most advantageous properties of homogeneous
catalysis such as high reactivity and selectivity with those of
heterogeneous catalysis - simple product separation, low
catalyst loadings and simple catalyst regeneration. In
homogeneous electrocatalysis, the diffusion of reactive species
to the electrode surface is an important factor that affects the
rate of catalysis. Electrodes with surface-attached catalysts
electrodes4. Incorporation of
a
[Ni(cyclam)]2+ catalyst to the
electrode surface can significantly increase electrocatalytic rates
and provide all of the benefits of heterogeneous catalysts. The
recent report by Leem et al. describes a self-assembled Nicyclam-
BTC on ITO glass and its enhanced electrocatalysis for water
oxidation5. The incorporation of [Ni(cyclam)]2+ into an artificial
metalloenzyme described by Schneider et al. improved the
selectivity of CO2 reduction over hydrogen production by these
enzymes6. In the paper by Neri and co-workers [Ni(cyclam-COOH)]2+
was immobilized onto FTO surfaces for photoelectrochemical CO2
reduction7, Kashiwagi et al. described polymer encapsulated
[Ni(cyclam)]2+ at a graphite felt electrode and showed that these
modified electrodes can efficiently reduce various organic
substrates8. Even though glassy carbon surface modification is
challenging there are several techniques that allow efficient
modification of electrode surfaces, including modification of
a.A. Zhanaidarova, Prof. C. P. Kubiak
Materials Science and Engineering Department, Department of Chemistry and
Biochemistry
University of California San Diego
electrodes through Click chemistry9, hydrogen plasma treatment10
,
9500 Gilman Drive, Mail code 0358, La Jolla, California
E-mail: ckubiak@ucsd.edu
electrochemical assisted attachment11, etc. Anodic electrografting
of ferrocenyl derivatives and porphyrins through ethynyl linkages to
glassy carbon and other electrode surfaces was previously
described by the Geiger group.12 According to this method, the key
step in the mechanism of anodic electrografting through terminal
b.Dr. C. E. Moore, Dr. M. Gembicky,
Crystallography Facility, Urey Hall 5128
University of California San Diego
9500 Gilman Drive, Mail code 0358, La Jolla, California
Electronic Supplementary Information (ESI) available: experimental data including
cyclic voltammetry experiments, synthesis and characterization. See
DOI: 10.1039/x0xx00000x
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1
2
3
alkynes is the formation of alkynyl radicals. One-electron oxidation
Homogeneous
electrochemical
CO2
reduction
increases the acidity of the alkyne causing the loss of a proton and experiments using the [Ni(alkynyl-cyclam)]2+ complexes
formation of alkynyl radicals which react with the surface of a GCE. showed reactivity towards CO2 reduction in MeCN/water
Following this method, we successfully modified GCEs through the mixtures and are good candidates for further development of
alkynyl radical electrografting procedure with [Ni(alkynyl-cyclam)]2+ a heterogeneous system with attached Ni(alkynyl-cyclam)
molecules. Newly modified electrodes were studied using complexes. Homogeneous CO2 reduction by [Ni(hexynyl-
electrochemical methods such as cyclic voltammetry (CV), scan rate cyclam)]2+ in acetonitrile/water mixture showed catalytic
dependence, and differential pulse voltammetry (DPV). The linear current enhancement at -1.65 V vs. Fc+/0 at a glassy carbon
dependence of peak current versus scan rate confirmed the electrode under CO2 atmosphere (Figure S2). Notably, the
heterogeneous nature of surface bound redox species.
The synthesis of new alkynyl-cyclam ligands as well as new in comparison to the parent catalyst [Ni-cyclam]2+ at the same
nickel complexes [Ni-propargyl-cyclam]2+ ), [Ni-pentynyl- conditions16. Similar current increases were detected for
cyclam]2+ ) and [Ni-hexynyl-cyclam]2+
) are described here complexes and in the presence of 20% water under a CO2
for the first time. The synthetic route for alkynyl-cyclams atmosphere.
includes the following steps: selective tri-Boc-protection of Cyclic voltammetry of complex
reduction potential is almost 0.2 V less negative for complex 3
(
1
(
2
(
3
1
2
1
(0.5 mM) in
cyclam13, N-functionalization using propargyl bromide14, 5- DCM/TBAPF6 solution showed the NiIII/II couple at +0.35 V vs.
iodo-1-pentyne or 6-iodo-1-hexyne, deprotection in TFA/DCM Fc+/0 as well as propargyl oxidation at +1.65 V vs. Fc+/0 (Figure
solution, which resulted in the formation of a trifluoroacetate 2A). Multiple scans (up to 10 anodic scans) through the alkynyl
salt. The salt was basified with 2 M NaOH and the ligand was group resulted in the attachment of [Ni(propargyl-cyclam)]2+ at
extracted with chloroform15 (Scheme 1). The corresponding the GCE surface, as determined by CV after thorough rinsing of
nickel complexes were obtained using the newly synthesized the modified electrode in DCM and transferring the electrode
ligands and nickel chloride hexahydrate (NiCl
2⋅6H2O). A color to a pure electrolyte solution.
change from green to purple was observed upon ligand
Electrodes modified with complex
1
(mod-GCE-1) were
coordination in anhydrous ethanol. Purple crystals suitable for studied using cyclic voltammetry and differential pulse
the X-ray analysis were obtained through vapor diffusion in voltammetry experiments. The reversible NiIII/II redox couple
DCM/pentane. Crystal structures for [Ni(propargyl-cyclam)]2+ was observed at -0.1 V vs. Fc+/0 and the peak to peak
(complex
1
), [Ni(pentynyl-cyclam)]2+ (complex
2
)
and separation ΔE1/2 was nearly 0 V which indicates the
Ni(hexynyl-cyclam)]2+ (complex 3) are shown in Figure 1.
heterogeneous nature of the surface attached species and the
absence of diffusion limitations (Figure 2B). Scan rate
dependence studies were performed on newly prepared
electrodes and were taken within the scan rate range of 10 –
70 mV/s (Figure 2C). The peak current of NiIII/II redox couple
increased linearly with increasing scan rate. The linear fit is
another confirmation of the heterogeneous nature of the
surface bound [Ni(propargyl-cyclam)]2+ (Figure S7). Similar
behavior was observed for a GCE modified with complex
(mod-GCE-2) or complex (mod-GCE-3). he CV of complex
2
3
T
2
showed a reversible NiIII/II couple at +0.38 V vs. Fc+/0 and the
pentynyl oxidation peak at +1.93 V vs. Fc+/0 (Figure S5). After
10 scans mod-GCE-2 showed a reversible NiIII/II peak at -0.04 V
vs. Fc+/0 (Figure S1A) and the scan rate studies showed a linear
dependence of peak current vs. scan rate (Figure S8). Cyclic
voltammetry of complex
3 in 0.1M TBAPF6 /DCM solution
Scheme 1. Synthesis of [Ni(alkynyl-cyclam)]2+ complexes.
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showed the NiIII/II redox couple at +0.35 V vs. Fc+/0 and the
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Experimental surface coverages were obtained from the
hexynyl oxidation at +1.8 V vs. Fc+/0 (Figure S3). A GCE differential pulse voltammetry (DPV) and are based on the
modified with
3
showed reversible NiIII/II redox couple at -0.03 electrochemical charge (Q) passed through the surface of the
V vs. Fc+/0 with minimal ΔE1/2 in pure TBAPF6/DCM solution electrode. Determination of the surface coverage of modified
(Figure S1B). The peak current increased linearly with electrodes was calculated using Equation 117:
increasing scan rate similar to the scan rate dependence of
mod-GCE-1 (Figure S9). The NiIII/II redox couple of complexes
and for homogeneous solution and surface attached
species are summarized in Table S1. The redox potential where n – number of electrons, F – Faradaic constant, A – the
1,
(1) Γ = Q/nFA
2
3
difference
homogeneous species in solution decreased with the adsorbed molecules per cm2.
increasing length of a carbon chain linker.
(ꢀE) between surface attached species and electrode surface area, Γ – the surface coverage in moles of
A
Figure 3. DPV experiment of mod-GCE-3 in 0.1 M TBAPF6/DCM.
DPV experiments for the electrode modified with
[Ni(hexynyl-cyclam)]2+ showed a charge of 0.88 uC that
corresponds to the surface coverage of 1.28x10-10 mol/cm2 and
is in exact agreement with the theoretical value of 1.28x10-10
mol/cm2 (Figure 3). Monolayer coverage was determined in all
three cases. Detailed surface coverage calculations for GCE
modified with complex
1, 2 and 3 are provided in SI.
Theoretical and experimental surface coverages for all
modified GC electrodes are summarized in Table 1. Theoretical
surface coverages of the modified electrodes were calculated
according to previously described methods for Fc molecules18
and are provided in SI.
B
Heterogeneous CO2 reduction experiments using newly
modified GCEs were carried out in 0.1 M TBAPF6/MeCN
solution with 20% water as a proton source. The CV shows a
current increase at -1.45 V vs. Fc+/0 under CO2 and reached a
plateau at -2.3 V vs. Fc+/0 (Figure 4).
In order to carry out controlled potential electrolysis (CPE)
experiments, modified Tokai glassy carbon electrodes with larger
surface area (2 cm2) were prepared by anodic electrografting. CPE
experiments were run in CO2 saturated MeCN/water solution
and the headspace of the cell was sampled for gas
chromatography analysis. When held at -2.4 V for 1 h, the
Table 1. Theoretical and experimental surface coverage of glassy carbon
electrodes modified with [Ni(alkynyl-cyclams)]2+.
C
mod-GCE-1 mod-GCE-2
mod-GCE-3
11.23
0.88
Diameter (d, Å)
Charge (uC)
8.13
1.54
2.53
2.25
10.36
1.02
1.62
1.46
Figure 2. CV of [Ni(propagyl-cyclam)]2+ in 0.1 M TBAPF6 /DCM solution.
Scan rate 100 mV/s (A). CV of mod-GCE-1 in 0.1 M TBAPF6 in DCM. Scan
rate 100 mV/s (red), 5 mV/s (black) (B). CV of mod-GCE-1 in 0.1 M
TBAPF6 in DCM at various scan rates (C).
Theor. (10-10 mol/cm2)
Exp. (10-10 mol/cm2)
1.28
1.28
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average Faradaic efficiency was determined to be 89% for H2 Research through grants FA9550-13-1-0020 and FA9550-17-
and 7% for CO (Table S2 and Fig. S13). CPE for homogenous Ni- 0198. This research was funded in part through the Internal
alkynyl cyclams were carried out at the same conditions with Research and Technology Development program of the Jet
Propulsion Laboratory, California Institute of Technology,
under a contract with the National Aeronautics and Space
Administration.
Conflicts of interest
There are no conflicts to declare.
Notes and references
‡ Experimental data including cyclic voltammetry experiments,
synthesis and characterization are provided in ESI.
§
CCDC 1580804, 1580806, and 1580807 contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by The Cambridge Crystallographic Data
Centre.
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Figure 4. CV of mof-GCE-3 in 0.1 M TBAPF6/MeCN with 20% water
under CO2 (red) and under N2 (blue) and blank GCE under CO2 (green)
and under N2 (black).
average FE 54% for CO and 39% for H2. These results are
consistent with previous findings that functionalized
16
Ni(cyclam)2+ will catalyze CO2 reduction to CO and H2
.
Functionalized -NH groups on cyclam ligand appear to increase
H2 production and lower selectivity for CO. It was also
previously shown that the interaction between the cyclam
ligand and the electrode surface has a significant influence on
the efficiency of Ni-cyclam catalysts, where the flatness of the
ligand to the electrode surface facilitates production of CO.3
Ni-alkynyl cyclam molecules may undergo structural hindrance
when attached to the GCE surfaces which resulting in
increased evolution of H2 gas.
We have modified the surface of the GCE with
[Ni(alkynyl-cyclam)]2+
catalysts
using
direct
anodic
electrografting. To the best of our knoweledge, this is the first
report of the direct attachment of a molecular catalyst to the
surface of GCE. Electrochemical studies performed on these
electrodes showed that [Ni(alkynyl-cyclam)]2+ molecules were
attached to the surface in heterogeneous fashion, eliminating
diffusion limitations that are typical for homogeneous
catalysis. CPE studies of the modified electrodes increased the
hydrogen evolution reaction (HER) relative to CO2 reduction to
CO during 1 h electrolysis. Structural configuration and
orientation of cyclam ligand plays an important role in product
distribution. Functionalized -NH groups on cyclam ligand and
orientation of the attached Ni-cyclam catalysts towards the
surface of the electrode may also play a crucial role for the
selective CO2 reduction by these systems.
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Acknowledgements
This work was supported by the National Science
Foundation (CHE-1461632) and Air Force Office of Scientific
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