Enhanced CO2 electroreduction via interaction of dangling S bonds and Co sites in cobalt phthalocyanine/ZnIn2S4 hybrids

The strong Co–S interaction between CoPc and the dangling S bonds in CoPc/ZIS hybrids can enhance CO2 electroreduction to CO.

Metal phthalocyanines/porphyrins constitute an interesting class of materials with some obvious advantages, such as easy accessibility, chemical stability and structural tunability at the molecular level. [20][21][22] It has been known that they can be used as electrocatalysts for CO 2 reduction. [23][24][25] Incorporation of cobalt porphyrin into covalent organic frameworks (COFs) can significantly improve their catalytic activity for reducing CO 2 to CO, and they exhibited a faradaic efficiency (FE) of 90% together with an optimized initial turnover frequency as high as 3 s À1 . 26 In other cases, iron porphyrin, cobalt phthalocyanine (CoPc) and cobalt porphyrin were immobilized onto carbon nanotubes (CNTs), [27][28][29] which can catalyze the electroreduction of CO 2 to CO with remarkable activity, selectivity and durability in aqueous solution. However, the immobilization of metal phthalocyanines/porphyrins is achieved through p-p interaction between the macrocyclic complexes and the support, which is not a direct interaction between the catalytic sites and the supports. 26,28 Because the metal centers in metal phthalocyanines/porphyrins are viewed as the catalytic sites in CO 2 electroreduction, 27,30 we believe that direct electronic interaction between the metal centers in phthalocyanines/ porphyrins and the support would affect the catalytic performance for CO 2 electroreduction more signicantly.
As an important semiconductor material of ternary chalcogenides, ZnIn 2 S 4 (ZIS) has attracted considerable attention because of its special electrical and optical properties. 31,32 The defects of ZIS are easily formed, which could manipulate the energy band structure, carrier concentration, spin nature, and phonon vibration as well as migration. 33 More importantly, the surface composition and electronic structure can be controlled by defects. 34,35 It is noted that the catalytic activity of metal phthalocyanines/porphyrins is closely related to the surface composition and electronic structure of the supports. 24 Therefore, ZIS can act as an excellent support to immobilize metal phthalocyanines/porphyrins and the interaction between the catalytic sites and supports can be tuned.
Herein, we developed an efficient strategy to facilitate CO 2 electroreduction by immobilization of CoPc onto ZIS nanosheets. It was discovered that the FE of CO could reach 93% with a current density of 8 mA cm À2 and a mass activity of 266 mA mg (CoPc) À1 under optimal conditions. The high catalytic activity of CoPc/ZIS hybrids is attributed to the strong Co-S interaction between Co active sites and the dangling S bonds in the ZIS support. The strong Co-S interaction facilitated CO 2 activation, leading to superior kinetics for CO production. As far as we know, this is the rst study on the enhancement of CO 2 electroreduction by Co-S interaction between the metal center in the macrocyclic complexes and the support. Besides, the Co-S interaction was studied in detail based on a series of control experiments.

Results and discussion
ZIS nanosheets were fabricated via a hydrothermal method (see the ESI † for details). and 0.00022 S cm À1 , respectively. They are very similar, so the inuence of the electrical conductivities on the CoPc activity for CO 2 reduction can be neglected.
To further reveal the ne structure of the synthetic ZIS nanosheets, their HAADF-STEM image was directly utilized to provide the atom arrangements in the ZIS nanosheets (Fig. 1B). It reveals that ZIS-200 has an interplanar spacing of 0.331 nm and a dihedral angle of 60 . This corresponds to the (100) and (010) planes of hexagonal ZIS, respectively. In addition, abundant Zn defects were observed in the magnied image, as shown in Fig. 1C. 31 ZIS-180 was also characterized by HAADF-STEM, and the Zn defects were also observed ( Fig. S6 †). However, the amount of Zn defects in ZIS-180 was less than that in ZIS-200.
CoPc/ZIS hybrids were prepared by blending CoPc and ZIS in N,N-dimethylformamide (DMF) assisted by ultrasound. For clarity, x-CoPc/ZIS-180 and x-CoPc/ZIS-200 refer to the hybrids, and x represents the content (wt%) of CoPc. The CoPc content in CoPc/ZIS hybrids under different conditions can be calculated from the Co content determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The SEM and TEM images (Fig. S7 †) conrm that 6.2-CoPc/ZIS-200 had a sheet-like morphology with an average thickness of 10 nm. No aggregated CoPc particles can be observed. The composition of the hybrids was determined by energy dispersive X-ray spectroscopy, indicating the homogeneous distribution of Co, N, Zn, In and S in the obtained nanosheets (Fig. 1d).
Furthermore, X-ray photoelectron spectroscopy (XPS) was employed to study the surface chemical states of the Co species and the interaction between Co species and the ZIS support. As shown in Fig. S8, † compared to the Co 2p peak of pure CoPc (780.87 eV), 37 the Co 2p peaks of 6.2-CoPc/ZIS-200 and 6.2-CoPc/ ZIS-180 shied to lower binding energies by 1.03 eV and 0.67 eV, respectively. The S 2p peaks of 6.2-CoPc/ZIS-200 and 6.2-CoPc/ ZIS-180 shied to higher binding energies by 0.2 eV and 0.11 eV, compared with the pure ZIS (Fig. S9 †). These results were consistent with previous literature, 38 indicating a strong interaction between the Co and S in CoPc/ZIS hybrids. The results also suggest that the interaction between CoPc and ZIS-200 is stronger than that between CoPc and ZIS-180. Besides, XRD and Raman spectra in Fig. 1E and S10 † provide further evidence for the formation of CoPc/ZIS hybrids.
To prepare the working electrode, we dispersed the hybrids in acetone with Naon D-521 to form a homogeneous ink, and then spread them on carbon paper (CP). The performance of electroreduction of CO 2 for the as-prepared electrodes was initially investigated using linear sweep voltammetry (LSV) in N 2 or CO 2 saturated 0.5 M KHCO 3 aqueous solution with a standard three-electrode conguration. As illustrated in Fig. 2A, 6.2-CoPc/ZIS-200 showed better CO 2 catalytic activity than CoPc/ ZIS-180 and CoPc. A well-dened peak appears at around À0.84 V versus the reversible hydrogen electrode (RHE), suggesting the maximum CO 2 reduction on the CoPc/ZIS catalyst. In the meantime, the onset potential with a signicant current increase can be found in CO 2 saturated 0.5 M KHCO 3 aqueous solution as shown in Fig. S11, † indicating the reduction of CO 2 .
To further verify the occurrence of predominant CO 2 reduction other than the H 2 evolution reaction (HER), controlled potential electrolysis of CO 2 was performed in a typical H-type cell to quantify the gas product by gas chromatography (GC) and the liquid product by nuclear magnetic resonance (NMR) spectroscopy, and the results are given in Fig. 2B and C and S12. † Only two products, CO and H 2 , were detected with a combined faradaic efficiency of around 100%. We found that with the increase of CoPc content, both the faradaic efficiency (FE) of CO and the current density over the CoPc/ZIS-200 and CoPc/ZIS-180 increased gradually at each given potential. When the CoPc content increased to 6.2 wt%, the FE(CO) and current density increased very slowly with further increase of the CoPc content. Therefore, we focus on 6.2-CoPc/ZIS hybrids in the following studies.
For 6.2-CoPc/ZIS-200, FE(CO) increased with increasing applied potential, and reached over 90% at À0.73 to À0.83 V vs. RHE. Aer that, FE(CO) decreased dramatically, indicating that the HER becomes the main reaction at a more negative applied potential. The maximum FE(CO) occurred at À0.83 V vs. RHE, and reached 93% with a 2-fold current density compared to that of CoPc. At this potential, the mass activity of 6.  Fig. S13, † CoPc was directly loaded on CP, which showed a much lower FE(CO) and current density. Only H 2 could be detected when ZIS was coated, suggesting that ZIS was not active in CO 2 reduction. In order to eliminate the photocatalytic effect of ZIS, the electroreduction of CO 2 over 6.2-CoPc/ZIS-200 was also conducted in the dark. The FE and current density of CO 2 electroreduction in the dark were the same as that in light, indicating that the ZIS cannot be affected by light. Thus, the high catalyst efficiency originated from a synergistic effect of the catalyst and the support. Furthermore, a long-term operation was conducted at À0.83 V vs. RHE for 6.2-CoPc/ZIS-200 with a time of 15 h. The current density and the FE(CO) did not change with time during the entire period, as shown in Fig. 2D. This indicated that 6.2-CoPc/ ZIS-200 was stable in the process of CO 2 electroreduction. In order to clarify the difference in catalytic activity for electrochemical CO 2 reduction, we studied the Tafel plots of 6.2-CoPc/ZIS-200 and 6.2-CoPc/ZIS-180. As shown in Fig. 3A, the Tafel slopes for 6.2-CoPc/ZIS-200 and 6.2-CoPc/ZIS-180 were 141 mV dec À1 and 169 mV dec À1 , respectively. They were much lower than those of analogous CoPc (270 mV dec À1 ) 39 and cobalt porphyrin (278 mV dec À1 ). 26 Moreover, the electrochemical activities of the different CoPc/ZIS electrodes were characterized by single-sweep polarography. According to the Randles-Sevcik equation, the reduction current density at À0.83 V vs. RHE plotted against the square root of the scan rate is shown in Fig. 3B. 6.2-CoPc/ZIS-200 had a larger slope than 6.2-CoPc/ZIS-180, leading to increased electrochemically active surface area and more catalytically active sites for CO 2 electrochemical reduction. The results provide more evidence that 6.2-CoPc/ZIS-200 had a higher activity for reducing CO 2 than 6.2-CoPc/ZIS-180, and the reasons will be discussed in the following sections.
The actual surface chemical states of Co atoms are strongly dependent on the surface states of ZIS, which would affect the catalytic activity. From Fig. 4A, it can be observed that ZIS-200 and ZIS-180 possessed a similar electron paramagnetic resonance (EPR) signal (g ¼ 2.003), corresponding to the electrons captured by Zn-defects. 40 The difference in the EPR signal intensity indicates that ZIS-200 had an obviously higher concentration of Zn-defects than ZIS-180. The formation of Zndefects resulted in the dangling S bonds, which could accommodate electrons. The dangling S bonds can affect the Co centers of CoPc molecules directly via the strong Co-S interaction (Fig. 4B). In addition, ZIS-220 (obtained at 220 C) was also studied to further compare the effect of Zn-defects. As shown in the EPR results in Fig. S14, † the concentration of Zn-defects in ZIS-220 was similar to that in ZIS-200. We also studied the CO 2  electroreduction activity over CoPc/ZIS-220. The faradaic efficiency of CO and the total current density were 92.6% and 8.1 mA cm À2 at À0.83 V vs. RHE, respectively, which was similar to the results over 6.2-CoPc/ZIS-200. Combined with the results of 6.2-CoPc/ZIS-180, we can conclude that the CO 2 electroreduction activity over CoPc/ZIS correlated with the concentration of Zn-defects.
In order to further clarify the Co-S interaction in 6.2-CoPc/ ZIS-200, the X-ray absorption near-edge structure (XANES) spectra and Fourier-transformed Co K-edge extended X-ray absorption ne structure (EXAFS) spectra were recorded. As shown in Fig. 4C, the absorption edge position of Co for 6.2-CoPc/ZIS-200 was shied to a lower energy compared with the pure CoPc, suggesting that the Co atoms exist in a lower oxidation state affected by the Co-S interaction. This transformation was also veried by the shi of the Co 2p XPS peak to lower binding energy (Fig. S8 †). As shown in the Fourier transform of the EXAFS spectra of 6.2-CoPc/ZIS-200 (Fig. 4D), the peaks at around 1.6Å and 1.9Å can be attributed to the Co-N bond and the Co-S bond, respectively. The quantitative coordination conguration of the Co atom can be obtained by EXAFS tting (Fig. S15 and Table S2 †). These results indicate that Co-S interaction exists in 6.2-CoPc/ZIS-200. In situ XAFS was used to study the Co-S interaction during the electroreduction (Fig. S16 †). The Co-S interaction can be observed at À0.83 V vs. RHE, as shown in Fig. S17. † This indicates that the Co-S interaction exists during the electroreduction.
Furthermore, we also used the zeta-potentials to verify the surface states of ZIS. From Fig. S18, † the zeta potentials for ZIS-200 and ZIS-180 were À35 mV and À20 mV, respectively. This demonstrates that negative charge exists on the surface of ZIS, and the negative charge increases with increasing Zn-defect concentrations. As the content of CoPc increased, the zetapotential value gradually shied to the positive side. It is worth noting that the zeta-potential values remained stable when a certain amount of CoPc was loaded on ZIS, indicating that the dangling S bonds have been fully covered by CoPc. As shown in Fig. 2B and C, a similar trend can be found between FE(CO) and the zeta-potential values when the content of CoPc changed. We can also nd that ZIS-200 needs more CoPc to neutralize the negative charge, which proved the presence of more dangling S bonds in ZIS-200. This further provided evidence for enhancing CO 2 electroreduction on CoPc/ZIS hybrids via Co-S interaction.
From the above results, we can draw a conclusion that the enhanced CO 2 electroreduction on CoPc/ZIS hybrids was closely related to the Co-S interaction. According to previous literature on Co-based catalysts, Co + is considered as the active site for CO 2 reduction. Thus Co 2+ /Co + redox transition is crucial for the activation of reduction of CO 2 over CoPc/ZIS. As shown in the XPS Co 2p spectra and XANES spectra, the Co signal of 6.2-CoPc/ ZIS-200 shied to lower oxidation states (Fig. S8 † and 4C). This indicated that the transition from Co 2+ to Co + over 6.2-CoPc/ZIS-200 became easier, which can facilitate CO 2 reduction. This conclusion was also veried by LSV (Fig. S19 †). Signicant Co 2+ / Co + redox transition can be observed at a more positive potential for 6.2-CoPc/ZIS-200 than CoPc. The positive shi of the Co 2+ /Co + redox potential suggests more Co(I) sites in the 6.2-CoPc/ZIS-200 catalyst than CoPc at low overpotentials. Thus, we can conclude that the enhancement of CO 2 reduction over CoPc/ZIS hybrids results from the easier Co 2+ /Co + redox transition, which was achieved by the Co-S interaction.

Conclusions
In summary, we synthesized CoPc/ZIS hybrids as efficient catalysts for CO 2 electrochemical reduction to CO in aqueous electrolyte. The amount of Zn-defects could be controlled by varying the hydrothermal temperature for the synthesis of ZIS nanosheets, which provided dangling S bonds to interact with CoPc molecules. For the CO 2 electrochemical reduction, 6.2-CoPc/ZIS-200 hybrids exhibited a FE of 93% for CO production with a current density of up to 8 mA cm À2 and a mass activity of up to 266 mA mg (CoPc) À1 . The detailed study indicated that the excellent catalytic performance could be mainly attributed to the strong Co-S interaction between CoPc molecules and the dangling S bonds in the ZIS support. This work provides a protocol for enhancing the efficiency of CO 2 electrochemical reduction by regulating the electronic interaction via dangling bonds.

Conflicts of interest
There are no conicts to declare.