Boosting hydrogen evolution by using covalent frameworks of fluorinated cobalt porphyrins supported on carbon nanotubes

Article abstract of DOI:10.1039/c9cc06916j

A cobalt(ii) tetrakis(2,3,5,6-tetrafluoro-4-ethynylphenyl)porphyrin (FCoP-H) was designed and synthesized. With carbon-nanotube-templated polymerization, covalent porphyrin frameworks using FCoP-H can be synthesized via the Hay-coupling. The resulting FCo

Full text of DOI:10.1039/c9cc06916j

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Boosting hydrogen evolution by using covalent
frameworks of fluorinated cobalt porphyrins
supported on carbon nanotubes†
Cite this: Chem. Commun., 2019,
55, 12647
Received 5th September 2019,
Accepted 26th September 2019
Gelun Xu,‡ Haitao Lei,‡ Guojun Zhou, Chaochao Zhang, Lisi Xie, Wei Zhang
and
Rui Cao
*
DOI: 10.1039/c9cc06916j
rsc.li/chemcomm
A cobalt(II) tetrakis(2,3,5,6-tetrafluoro-4-ethynylphenyl)porphyrin Although this method is easy and straightforward, the weak inter-
(FCoP-H) was designed and synthesized. With carbon-nanotube- actions between molecular catalysts and CNTs usually lead to poor
templated polymerization, covalent porphyrin frameworks using electron transfer ability and stability, and thus poor catalytic
FCoP-H can be synthesized via the Hay-coupling. The resulting performance. Two immobilization strategies have been used to
FCoP@CNT is efficient to catalyze the hydrogen evolution reaction improve the molecule–CNT interactions. The first one is to intro-
(HER) in aqueous media, and its performance is superior, in terms of duce large conjugated groups onto catalyst molecules, which will
both lower overpotentials and larger catalytic currents, to covalent result in the increased p–p interactions with CNTs.37,38 The other
porphyrin frameworks using non-fluorinated cobalt porphyrin one is to covalently graft catalyst molecules on CNTs.17,32 In both
analogues and also monomeric fluorinated cobalt porphyrins cases, improved catalytic performance is realized. In addition to
simply adsorbed on CNTs. This work combines the merits of CNT- these methods, CNT-templated polymerization is also a valuable
templated covalent frameworks and fluorinated cobalt porphyrins method for the immobilization of catalyst molecules. For example,
to significantly improve the HER performance. This strategy is through the oxidative coupling of meso-ethynyl groups, covalent
valuable to be explored in other electrocatalytic systems using porphyrin frameworks can be synthesized on CNTs to show high
molecule-engineered carbon materials.
catalytic performance for oxygen electrocatalysis.39,40
Metal porphyrins have been shown to be highly active for the
Hydrogen is considered to be a clean and renewable energy HER.41–49 Strong electron withdrawing groups at the meso- and/or
carrier.1–3 The electrocatalytic HER provides an appealing strategy b-positions can cause large shifts of the reduction waves to the
to generate H2 and meanwhile to convert electrical energy to anodic direction and thus can boost catalytic HER performance by
chemical energy.4,5 Tremendous efforts have been made in the decreasing overpotentials.18 Herein we report the design and
past decade to develop cheap, efficient and robust HER electro- synthesis of CoII tetrakis(2,3,5,6-tetrafluoro-4-ethynylphenyl)porphyrin
catalysts.6–12 This leads to the identification of a variety of (FCoP-H, Fig. 1a) and its CNT-templated covalent framework
molecular complexes of Fe,13–15 Co,16–21 Ni22–27 and Cu28,29 as FCoP@CNT, which combines the merits of covalent porphyrin
active HER catalysts. Despite these achievements, however, frameworks and the strong electron withdrawing feature of
immobilization of molecular catalysts on appropriate electrode tetrafluorophenyl substituents. We show that FCoP@CNT is
materials is critical to make them practically useful.30 It is much more efficient for the HER in aqueous solutions than
demonstrated that immobilization of molecular catalysts can simply adsorbed monomeric Co porphyrins on CNTs and also
improve the performance, stability and recycling of catalysts.
the CNT-templated covalent framework using non-fluorinated
Owing to large surface area, high chemical stability, and CoII tetrakis(4-ethynylphenyl)porphyrin (CoP-H, Fig. 1a). This
good electrical conductivity, carbon nanotubes (CNTs) have been work presents a CNT-templated polymerization strategy of fluori-
used as supporting electrode materials.17,31–36 Typically, molecular nated porphyrins, which will be valuable for other electrocatalytic
catalysts can be simply loaded on CNTs via physical adsorption. systems using molecule-engineered carbon materials.
Monomeric FCoP-SiMe3 was first synthesized (Fig. 1a and
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University,
Xi’an 710119, China. E-mail: ruicao@ruc.edu.cn
Scheme S1, ESI†). Details of synthesis and characterization are
given in the ESI.† Crystals of FCoP-SiMe3, suitable for single
crystal X-ray diffraction studies, were obtained. Crystallographic
studies revealed that FCoP-SiMe3 crystallized in the monoclinic
P21/n space group (Fig. 1b). The Co ion is coordinated by the
porphyrin ligand through four N atoms, giving a square-planar
† Electronic supplementary information (ESI) available: Experimental details;
Fig. S1–S21 and Tables S1 and S2. CCDC 1950759. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9cc06916j
‡ These authors contributed equally to this work.
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Fig. 2 CVs of 1 mM FCoP-SiMe3 (a) and CoP-SiMe3 (b) in DMF with
increasing acetic acid. (c and d) Plots of icat measured at À2.25 V vs.
ferrocene against the concentration of acetic acid with 1 mM FCoP-SiMe3
and CoP-SiMe3, respectively. (e and f) Plots of icat measured at À2.20 V vs.
ferrocene against the concentration of FCoP-SiMe3 and CoP-SiMe3,
respectively, with 40 mM acetic acid.
Fig. 1 (a) Molecular structures of Co porphyrins. (b) Thermal ellipsoid plot
(50% probability) of the X-ray structure of FCoP-SiMe3. Hydrogen atoms
are omitted. (c) CVs of FCoP-SiMe3 and CoP-SiMe3 in DMF with 50 mV sÀ1
scan rate at 20 1C. (d) Synthetic route to FCoP@CNT.
FCoP-SiMe3 and CoP-SiMe3, their first reduction waves do not
show changes after the addition of acetic acid, but the second
geometry. The average Co–N distance is 1.975(3) Å, which is reduction waves change to catalytic ones. This is consistent
consistent with a CoII electronic structure. The four ortho- and with previous studies, showing that Co0 is the catalytically
meta-positions of the phenyl groups are all occupied by a fluoride active species for the HER.50 In both cases, the catalytic
atom, and the para-position bears a trimethylsilylethynyl group. currents increased linearly with the concentrations of acid
This confirms the structure we designed. In addition, the (Fig. 2c and d) and catalyst (Fig. 2e and f), implying the
identity and the purity of the bulk sample were confirmed by molecular nature of the catalysis. It is worth noting that
high-resolution mass spectrometry and elemental analysis, the onset overpotential of FCoP-SiMe3 is smaller than that of
showing an ion at a mass-to-charge ratio of 1343.1683, which CoP-SiMe3 by 4250 mV. This result is consistent with the
matches the calculated value of 1343.1720 with expected cathodic shifts of the redox waves of CoP-SiMe3 as compared
isotopic distribution (Fig. S8, ESI†). As a control, the non- to FCoP-SiMe3, further highlighting the positive role of tetra-
fluorinated analogue CoP-SiMe3 was also synthesized and fluorophenyl groups in improving the HER performance.
characterized (Fig. 1a and Scheme S2, ESI†).
After removing the protecting trimethylsilyl groups, the result-
The cyclic voltammogram (CV) of FCoP-SiMe3, measured in ing FCoP-H and CoP-H were adopted for subsequent CNT-
dimethylformamide (DMF) under N2 (Fig. 1c), displays two reversible templated polymerization. The products were carefully washed
redox waves at À1.07 and À2.01 V vs. ferrocene, which correspond to to remove unreacted monomeric porphyrins. Details of synthesis
the formal CoII/CoI and CoI/Co0 redox couples, respectively.50 For are given in the ESI.† The successful formation of FCoP@CNT
CoP-SiMe3, it shows a reversible CoII/CoI wave and an irreversible and CoP@CNT was evidenced by UV-vis spectroscopy (Fig. 3a and
CoI/Co0 wave at À1.27 and À2.36 V vs. ferrocene, respectively. b), transmission electron microscopy (TEM, Fig. 3c and d) and
These waves are shifted to the cathodic direction by 4200 mV as energy-dispersive X-ray spectroscopy (EDX, Fig. S17 and S18, ESI†).
compared to those of FCoP-SiMe3, which is consistent with the The loaded amount of catalysts was determined by inductively
strong electron withdrawing feature of the tetrafluorophenyl coupled plasma mass spectrometry (ICP-MS), giving 0.23 wt% and
substituents of FCoP-SiMe3.
0.18 wt% of Co in FCoP@CNT and CoP@CNT, respectively.
In UV-vis spectroscopy, the strong Soret band of FCoP-SiMe3
An electrocatalytic HER was first studied in DMF with acetic
acid as the proton source. As shown in Fig. 2a and b, for both at 410 nm is red-shifted to 426 nm in FCoP@CNT (Fig. 3a).
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Fig. 4 (a) LSVs of FCoP@CNT and FCoP/CNT. (b) LSVs of CoP@CNT and
CoP/CNT. (c) LSV comparison of FCoP@CNT and CoP@CNT. (d) Tafel
plots. Conditions: 0.5 M H2SO4, N2 atmosphere, 50 mV sÀ1 scan rate,
and 20 1C.
Fig. 3 UV-vis spectra of (a) FCoP@CNT, FCoP/CNT, FCoP-SiMe3 and (b)
CoP@CNT, CoP/CNT, CoP-SiMe3. TEM images of (c) untreated CNTs and
(d) FCoP@CNT. The thin layers of covalent Co porphyrin frameworks on
CNTs are labelled.
overpotentials and larger catalytic currents, than simply adsorbed
monomeric Co porphyrins on CNTs. This improvement is likely
In addition to this 16 nm shift, the Soret band of FCoP@CNT due to the stronger p–p interactions between the molecules of
becomes much broader. In sharp contrast, simply adsorbed covalent porphyrin frameworks and CNTs, which will facilitate the
monomeric FCoP-SiMe3 on CNTs, denoted as FCoP/CNT, does interfacial electron transfer.
not show such a red shift for the Soret band. This result not
It is interesting to compare the catalytic performance of
only showed the loading of intact Co porphyrin molecules on FCoP@CNT and CoP@CNT. As shown in Fig. 4c, the onset
CNTs, but also suggested the presence of strong p–p interactions overpotential of FCoP@CNT (576 mV) is smaller than that of
between the catalyst molecules and CNTs.37,39 For CoP@CNT, a CoP@CNT (648 mV). At 800 mV overpotential, the catalytic
similar red shift of the Soret band by 420 nm is also observed current of FCoP@CNT (25.1 mA cmÀ2) is two times larger than
as compared to monomeric CoP-SiMe3 (Fig. 3b). In TEM, an that of CoP@CNT (11.5 mA cmÀ2). In addition, the Tafel plot
untreated CNT has a very smooth surface with few flaws of FCoP@CNT (126 mV decÀ1) is also smaller than that of
(Fig. 3c), while the FCoP@CNT shows the formation of thin CoP@CNT (167 mV decÀ1, Fig. 4d). These results demonstrate
layers coated on the surface of CNTs (Fig. 3d), further confirm- that FCoP@CNT outperforms CoP@CNT to catalyze the HER.
ing the successful polymerization.
This improvement is due to the strong electron withdrawing
In order to examine electrocatalytic HER features, the linear tetrafluorophenyl substituents in the structure of FCoP@CNT.
sweep voltammogram (LSV) of FCoP@CNT on a glassy carbon The durability of FCoP@CNT and CoP@CNT for the HER was
(GC) electrode was recorded in 0.5 M H2SO4 aqueous solution also studied. Controlled potential electrolysis at 750 mV over-
under N2, displaying a catalytic current with an onset overpoten- potential showed stable currents for 47 h (Fig. S19 and S20,
tial of 576 mV (measured at current density j = 1.0 mA cmÀ2
Fig. 4a). This performance is comparable to immobilized mole- graphy, giving a faradaic efficiency of 94% (Fig. S21, ESI†).
cular catalysts loaded on carbon materials operating under In summary, we report the synthesis and electrocatalytic
,
ESI†). The produced H2 gas was determined by gas chromato-
similar conditions (Table S1, ESI†). Note that only a few HER property of a covalent framework using fluorinated CoII
molecular complexes can stably catalyze the HER in such strongly porphyrins. This FCoP@CNT is much more efficient to catalyze
acidic media. The rigid and stable coordination provided by the HER in aqueous media than a covalent framework using
porphyrin macrocycles is crucial to prevent demetallization under non-fluorinated Co porphyrin analogue CoP@CNT and mono-
highly acidic conditions.32,37 The formation of gas bubbles on the meric fluorinated Co porphyrins simply adsorbed on CNTs. The
surface of the GC electrode is indicative of the evolution of H2. significantly improved efficiency, in terms of both low over-
Significantly, the LSV of FCoP/CNT displays a much smaller potentials and large catalytic currents, is a result of combining
catalytic current with a larger onset overpotential of 705 mV under the merits of CNT-templated covalent frameworks and the
the same conditions. Similarly, LSVs of CoP@CNT and CoP/CNT strong electron withdrawing features of the tetrafluorophenyl
also show the catalytic HER with onset overpotentials of 648 and substituents. This work highlights the role of molecular design
743 mV, respectively (Fig. 4b). These results suggest that the in boosting the catalytic efficiency. The CNT-templated poly-
covalent Co porphyrin frameworks grown on CNTs are much merization strategy using fluorinated porphyrins will be applic-
more efficient to catalyze the HER, in terms of both lower able for other electrocatalytic and electrochemical systems.
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for the Central Universities, and the Research Funds of Shaanxi
Normal University.
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