Asymmetric Push–Pull Type Co(II) Porphyrin for Enhanced Electrocatalytic CO2 Reduction Activity

Molecular electrocatalysts for electrochemical carbon dioxide (CO2) reduction has received more attention both by scientists and engineers, owing to their well-defined structure and tunable electronic property. Metal complexes via coordination with many π-conjugated ligands exhibit the unique electrocatalytic CO2 reduction performance. The symmetric electronic structure of this metal complex may play an important role in the CO2 reduction. In this work, two novel dimethoxy substituted asymmetric and cross-symmetric Co(II) porphyrin (PorCo) have been prepared as the model electrocatalyst for CO2 reduction. Owing to the electron donor effect of methoxy group, the intramolecular charge transfer of these push–pull type molecules facilitates the electron mobility. As electrocatalysts at −0.7 V vs. reversible hydrogen electrode (RHE), asymmetric methoxy-substituted Co(II) porphyrin shows the higher CO2-to-CO Faradaic efficiency (FECO) of ~95 % and turnover frequency (TOF) of 2880 h−1 than those of control materials, due to its push–pull type electronic structure. The density functional theory (DFT) calculation further confirms that methoxy group could ready to decrease to energy level for formation *COOH, leading to high CO2 reduction performance. This work opens a novel path to the design of molecular catalysts for boosting electrocatalytic CO2 reduction.


Introduction
Recently, metal complexes consisting of transition metal ions with heteroatom-embedded organic molecules as ligands are emerging as good catalyst materials in wide electrochemical application, due to the existence of occupied d z orbitals for the favorable catalytic CO 2 reduction activity [1][2][3][4]. As the candidates of a molecular electrocatalyst, the metal complex with well-defined molecular structure could be ready to control by changing of various metal ions and organic ligands (e.g., dipyridine, terpyridine, dipyrromethane, porphyrin) with different π-conjugated systems, achieving tunable electrocatalytic performance [5][6][7]. Moreover, these metal complexes could be used as key building blocks for the preparation of organic porous polymers and single-atom carbon materials [8,9]. Different from other metal complexes, porphyrin possesses planar macrocyclic aromatics with extended π-electron conjugation, endowing them with some optical/electronic characteristics, like a broad photoabsorption wavelength, a narrow bandgap, and fast electron acceptors, among other properties [8,[10][11][12][13][14]. Therefore, metal porphyrins have been proven as good candidates to be the electrocatalysts for CO 2 reduction. So far, the research of porphyrin-based molecular electrocatalysts mostly focuses on the development of new porphyrin-based ligands for the improvement of CO 2 reduction reactions (CO 2 RR).

Synthesis Description
The synthetic route to two methoxy-functionalized CoPor (asymmetric and crosssymmetric CoPor named as as-PorCo-OMe and cs-PorCo-OMe, respectively) are given in Figure 1a. The key intermediate of imine-containing dipyrromethane derivate (DMP-imine) was prepared from 2,6-dimethylbenzaldehyde using a three-step reaction in total yield of 57%, according to the reported work [29]. The 2,2 -((2,5-dimethoxyphenyl)methylene)bis(1Hpyrrole) (DmpMP) was synthesized by condensation reaction of 2,5-dimethoxybenzaldehyde with pyrrole in good yield of 82%. Then, as-PorCo-OMe was prepared by a one-pot reflux reaction of DmpMP, DMP-imine and cobalt acetate [Co(OAc) 2 ] in ethanol for 18 hrs. The pure as-PorCo-OMe was purified by alumina column chromatography with PE and DCM (v/v = 8:2) as a crimson solid in the yield of 17%. For cs-PorCo-OMe, the 5,10,15,20-tetrakis(2,5-dimethoxyphenyl) porphyrin was firstly synthesized from 2,5-dimethoxybenzaldehyde and pyrrole in propionic acid for 24 h, and the crude purple solid was filtered and used without purification. After reaction with Co(OAc) 2 in DMF, the cs-PorCo-OMe was obtained and purified by alumina column chromatography with PE and DCM (v/v = 6:4) as a crimson solid in yield of 15%. The detailed synthesis information and NMR spectra on these compounds is provided in Section 4 and Supplementary Materials ( Figures S1-S4). The target molecular weight of as-PorCo-OMe and cs-PorCo-OMe are confirmed by mass spectrometry (MS). Figure 1b shows that the MS results are consistent with the predicted values of targeted compounds, suggesting the successful preparation of as-PorCo-OMe and cs-PorCo-OMe.
PorCo-OMe are confirmed by mass spectrometry (MS). Figure 1b shows that the MS results are consistent with the predicted values of targeted compounds, suggesting the successful preparation of as-PorCo-OMe and cs-PorCo-OMe.

Structural Characterization
The structures of the as-prepared complexes were characterized by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, the absorption bands between 1596 and 1445 cm −1 are attributed to the C = C vibration peaks of aromatic (like phenyl and pyrrole) groups, and absorption band at 1352 cm −1 is the C=N bond in the backbone of porphyrin [30], while the peak at 997 cm −1 is associated with the vibration of the Co-N bond [31]. These results demonstrate the successful preparation of Co(II) porphyrin derivates. The stretching vibration peaks of C-H is at 2965 and 2922 cm −1 for methyl and aromatic groups, respectively [18]. Moreover, the intensity of peak at 2965 cm −1 in cs-PorCo-OMe is stronger than that of as-PorCo-OMe, due to existence of eight methoxy group in cs-PorCo-OMe. The C-O bands in as-PorCo-OMe and cs-PorCo-OMe are found at 1211 and 1258 cm −1 , respectively, suggesting the stronger conjugation effect of the methoxy bond in cs-PorCo-OMe [32,33]. The chemical states of elementals of these complexes have also been investigated. Figures S5 and S6 show that elements of cobalt (Co), carbon (C), oxygen (O) and nitrogen (N) are displayed and both two complex show the similar high-resolution XPS results. In Figure 2b, the Co 2p high-resolution XPS spectra of as-PorCo-OMe exhibits two main peak at 779.1 and 794.8 eV, resulting from Co(II) atom with Co 2p 3/2 and Co 2p 1/2 binding energies, respectively [18]. For N 1s XPS spectra, these complexes exhibit two peaks at 397.8 and 401.1 eV, attributing to the pyrrolic N and Co-N structures, respectively (Figure 2c) [34]. In addition, the C 1s XPS spectra can be separated into three peaks at 284.1, 286.1 and 287.4 eV, indicating the bend energy of C-C/C=C, C-O and C=N, respectively (Figure 2d) [35]. These results demonstrate the accurate structure of Co(II) porphyrins with various methoxy substituents.

Structural Characterization
The structures of the as-prepared complexes were characterized by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). As shown in Figure 2a, the absorption bands between 1596 and 1445 cm −1 are attributed to the C = C vibration peaks of aromatic (like phenyl and pyrrole) groups, and absorption band at 1352 cm −1 is the C=N bond in the backbone of porphyrin [30], while the peak at 997 cm −1 is associated with the vibration of the Co-N bond [31]. These results demonstrate the successful preparation of Co(II) porphyrin derivates. The stretching vibration peaks of C-H is at 2965 and 2922 cm −1 for methyl and aromatic groups, respectively [18]. Moreover, the intensity of peak at 2965 cm −1 in cs-PorCo-OMe is stronger than that of as-PorCo-OMe, due to existence of eight methoxy group in cs-PorCo-OMe. The C-O bands in as-PorCo-OMe and cs-PorCo-OMe are found at 1211 and 1258 cm −1 , respectively, suggesting the stronger conjugation effect of the methoxy bond in cs-PorCo-OMe [32,33]. The chemical states of elementals of these complexes have also been investigated. Figures S5 and S6 show that elements of cobalt (Co), carbon (C), oxygen (O) and nitrogen (N) are displayed and both two complex show the similar high-resolution XPS results. In Figure 2b, the Co 2p high-resolution XPS spectra of as-PorCo-OMe exhibits two main peak at 779.1 and 794.8 eV, resulting from Co(II) atom with Co 2p 3/2 and Co 2p 1/2 binding energies, respectively [18]. For N 1s XPS spectra, these complexes exhibit two peaks at 397.8 and 401.1 eV, attributing to the pyrrolic N and Co-N structures, respectively (Figure 2c) [34]. In addition, the C 1s XPS spectra can be separated into three peaks at 284.1, 286.1 and 287.4 eV, indicating the bend energy of C-C/C=C, C-O and C=N, respectively (Figure 2d) [35]. These results demonstrate the accurate structure of Co(II) porphyrins with various methoxy substituents.

Electronic Structures
The photophysical properties of as-synthesized materials was investigated by ultraviolet and visible adsorption (UV-Vis) spectroscopy in dichloromethane (DCM) (Figure 3a). The Soret band of as-PorCo-OMe shows a strong absorbance at 398 nm, indicating the π-π* transition of porphyrin backbones, while its Q band is located between 498 and 564 nm indicating the n-π* transition from donor-acceptor structure [36]. Compared with that of as-PorCo-OMe, cs-PorCo-OMe has the enhanced push-pull effect, due to the increasing number of 2,5-dimethoxyphenyl groups, leading to the obvious red-shift phenomenon in the UV-Vis spectrum [37]. Furthermore, the board single peak of Q band suggests the symmetric structure of cs-PorCo-OMe [29]. On the basis of their UV-Vis results, the optical bandgap (E g ) of as-PorCo-OMe and cs-PorCo-OMe can be calculated to be 2.17 and 2.18 eV, respectively, by using Tauc measurement (Figure 3b). The decrease of bandgap in these complexes manifests the donor effect of the methoxy group, but, the slight change is caused by steric effect of α-functionalized methoxy substituent.

Electronic Structures
The photophysical properties of as-synthesized materials was investigated by ultraviolet and visible adsorption (UV-Vis) spectroscopy in dichloromethane (DCM) ( Figure  3a). The Soret band of as-PorCo-OMe shows a strong absorbance at 398 nm, indicating the π-π* transition of porphyrin backbones, while its Q band is located between 498 and 564 nm indicating the n-π* transition from donor-acceptor structure [36]. Compared with that of as-PorCo-OMe, cs-PorCo-OMe has the enhanced push-pull effect, due to the increasing number of 2,5-dimethoxyphenyl groups, leading to the obvious red-shift phenomenon in the UV-Vis spectrum [37]. Furthermore, the board single peak of Q band suggests the symmetric structure of cs-PorCo-OMe [29]. On the basis of their UV-Vis results, the optical bandgap (Eg) of as-PorCo-OMe and cs-PorCo-OMe can be calculated to be 2.17 and 2.18 eV, respectively, by using Tauc measurement ( Figure 3b). The decrease of bandgap in these complexes manifests the donor effect of the methoxy group, but, the slight change is caused by steric effect of α-functionalized methoxy substituent.
The cyclic voltammetry (CV) measurement was exploited to characterize to the electronic structures of methoxy-substituted Co(II) porphrins. The CV curves, performed in argon (Ar)-saturated 0.1 M TBAPF6 DCM solution, are given in Figure 3c. The as-PorCo-OMe exhibits an irreversible one-electron reduction, while cs-PorCo-OMe has two successive reduction processes, indicating that the electron could be delocalized effectively over the molecular backbone, due to the symmetric structure of cs-PorCo-OMe [38]. The peak around −0.7~−0.9 V is the reduction reaction of Co(II)-to-Co(I) [39]. Based on the onset of first reduction potential, the LUMO energy level of as-PorCo-OMe and cs-PorCo-OMe is −3.39 and −3.63 eV, respectively. Following the equation:

DFT Calculation
To gain deep insight into the electronic and geometric structures, frontier orbitals of these complexes were performed by DFT calculations (Figure 4a). The LUMOs of as-PorCo-OMe and cs-PorCo-OMe mainly reside on the backbone of porphyrin, indicative The cyclic voltammetry (CV) measurement was exploited to characterize to the electronic structures of methoxy-substituted Co(II) porphrins. The CV curves, performed in argon (Ar)-saturated 0.1 M TBAPF 6 DCM solution, are given in Figure 3c. The as-PorCo-OMe exhibits an irreversible one-electron reduction, while cs-PorCo-OMe has two successive reduction processes, indicating that the electron could be delocalized effectively over the molecular backbone, due to the symmetric structure of cs-PorCo-OMe [38]. The peak around −0.7~−0.9 V is the reduction reaction of Co(II)-to-Co(I) [39]. Based on the onset of first reduction potential, the LUMO energy level of as-PorCo-OMe and cs-PorCo-OMe is −3.39 and −3.63 eV, respectively. Following the equation:

DFT Calculation
To gain deep insight into the electronic and geometric structures, frontier orbitals of these complexes were performed by DFT calculations (Figure 4a). The LUMOs of as-PorCo-OMe and cs-PorCo-OMe mainly reside on the backbone of porphyrin, indicative of their similar LUMO energy level at −2.05, and −2.08 eV, respectively. For HOMO, the porphyrin core and partial 2,5-dimethoxybenzene are covered, demonstrating the donor effect of methoxy group in the substituents [40]. With the increasing number of substituents, the push-pull effect becomes stronger. The calculated energy levels of methoxy-substituted porphyrins are well agreement with the tested results from CVs, and the detailed information is provided in Table 1.

Electrocatalytic CO2RR
The electrocatalytic CO2RR performance of methoxy-substituted Co(II) porphyrins were evaluated in a the 0.5 M KHCO3 electrolyte using an H-type three-electrode cell with Nafion-117 as separator. All potentials are applied to the reversible hydrogen electrode (RHE) [43]. The electrocatalytic activity of methoxy-substituted Co(II) porphyrins in the Ar-and CO2-saturated electrolyte was studied by the linear sweep voltammetry measure-  Based on pervious works, the Co(II) porphyrin has been approved as a good candidate to be the electrocatalyst for the CO 2 -to-CO reduction via a four-step reaction [41,42]. Thus, the DFT was carried out to investigate the reaction kinetics of the electrochemical CO 2 reduction process with as-synthesized molecular catalysts ( Figure S7). As shown in Figure 4b, the formation of *COOH is the rate-limiting step in CO 2 RR in this reaction energetics evolution. The free energy path of the conversion of CO 2 to *COOH (∆ *GCOOH ) requires 0.65 and 0.64 eV, respectively, for as-PorCo-OMe, and cs-PorCo-OMe, which is similar to that of 5,15-bis(2,6-dimethylphenyl) Co(II) porphyrin (DMP-CoPor). The methoxy substitution could provide the electron donor effect on a bit of enhancement of electrocatalytic activity.

Electrocatalytic CO 2 RR
The electrocatalytic CO 2 RR performance of methoxy-substituted Co(II) porphyrins were evaluated in a the 0.5 M KHCO 3 electrolyte using an H-type three-electrode cell with Nafion-117 as separator. All potentials are applied to the reversible hydrogen electrode (RHE) [43]. The electrocatalytic activity of methoxy-substituted Co(II) porphyrins in the Arand CO 2 -saturated electrolyte was studied by the linear sweep voltammetry measurement. Figure S9 illustrates that the current densities of these molecules is higher in CO 2 atmosphere than that in the Ar-saturated condition, suggesting the presence of electrocatalytic activity of the Co(II) porphyrin core [44]. As shown in Figure 5a, both of as-PorCo-OMe and cs-PorCo-OMe generate the increasing current densities with the increase of potential from −0.4 to −1.0 V versus RHE. Compared with cs-PorCo-OMe, the as-PorCo-OMe shows higher electrocatalytic activity. This result may result from the lower steric hindance effect of as-PorCo-OMe than that of cs-PorCo-OMe, leading to the fast electron transfer from carbon nanotubes to catalysts for enhanced electrochemical CO 2 RR [29]. PorCo-OMe. The catalytic activities of these methoxy-substituted Co(II) porphyrins wa comprehensively evaluated by the index of turnover frequency (TOF). In Figure 5d, the TOF values of two molecules gradually increased from the potential from −0.4 and −1.0 V vs. RHE, owing to the increase of current density at high potential. Compared with cs PorCo-OMe, as-PorCo-OMe exhibits better TOF performance in the whole potentials, in dicative its good electrochemical activity for CO2RR application. Furthermore, the TOF o as-PorCo-OMe (2880 h −1 at −0.7 V vs. RHE) also is superior to many reported state-of-the art porphyrin-based electrocatalysts [10,18,29,[45][46][47][48][49][50]. These results demonstrate the effi cient electron and proton transfer kinetics for the push-pull type as-PorCo-OMe with weak steric hindrance.  The Tafel slope represents a reaction kinetic of rate determining steps involved in electrocatalysis, which can be calculated from the polarization curves [51]. In Figure 6a the as-PorCo-OMe shows the Tafel value of 145 mV dec −1 , which is smaller than that of cs PorCo-OMe (200 mV dec −1 ), indicating that as-PorCo-OMe has the higher catalytic activity of *COOH formation in CO2 reduction reaction via electron/proton transfer [52,53]. To evaluate the electrochemical behavior of as-prepared complexes in electrocatalytic  The CO 2 RR products were tested by the online gas chromatography (GC) and offline NMR techniques ( Figure S10), which confirms that only CO and H 2 were found during the reduction reaction, suggesting the high selectivity of methoxy-substituted Co(II) porphyrins. The CO Faraday efficiencies (FE CO ) of two complexes are given in Figure 5b. As expected, the FE CO of as-PorCo-OMe reaches as high as 94.7%, which is much larger than those of DMP-CoPor (85.5%) [18], suggesting that the electron donor of methoxy substitution has the positive influence for electrocatalytic CO 2 reduction by push-pull effect. Moreover, FE CO of as-PorCo-OMe also is better than that of cs-PorCo-OMe (84.5%), as well as reported PorCo-TPP (91%) [39], PorCo-MOF (76%) [45] and Co protoporphyrin (40%) [46]. Correspondingly, the partial current densities of methoxy-substituted CoPors for CO production increase with the increase of potentials. As the example of at −0.7 V, the specific current density of as-PorCo-OMe is over two times higher than that of cs-PorCo-OMe. The catalytic activities of these methoxy-substituted Co(II) porphyrins was comprehensively evaluated by the index of turnover frequency (TOF). In Figure 5d, the TOF values of two molecules gradually increased from the potential from −0.4 and −1.0 V vs. RHE, owing to the increase of current density at high potential. Compared with cs-PorCo-OMe, as-PorCo-OMe exhibits better TOF performance in the whole potentials, indicative its good electrochemical activity for CO 2 RR application. Furthermore, the TOF of as-PorCo-OMe (2880 h −1 at −0.7 V vs. RHE) also is superior to many reported state-ofthe-art porphyrin-based electrocatalysts [10,18,29,[45][46][47][48][49][50]. These results demonstrate the efficient electron and proton transfer kinetics for the push-pull type as-PorCo-OMe with weak steric hindrance.
The Tafel slope represents a reaction kinetic of rate determining steps involved in electrocatalysis, which can be calculated from the polarization curves [51]. In Figure 6a, the as-PorCo-OMe shows the Tafel value of 145 mV dec −1 , which is smaller than that of cs-PorCo-OMe (200 mV dec −1 ), indicating that as-PorCo-OMe has the higher catalytic activity of *COOH formation in CO 2 reduction reaction via electron/proton transfer [52,53]. To evaluate the electrochemical behavior of as-prepared complexes in electrocatalytic CO 2 RR, electrochemical impedance spectroscopy (EIS) was carried out [54]. The charge transfer resistance (R ct ) derived from the Nyquist plot exhibits that the resistance of 33.25 Ω for as-PorCo-OMe is lower than that of cs-PorCo-OMe (38.69 Ω) (Figure 6b), demonstrating the superior electron transfer ability of as-PorCo-OMe. Furthermore, the electrochemical capacitances from CV between −0.26 and −0.16 eV vs. RHE show that as-PorCo-OMe provides the higher electrochemical active surface area (Figure 6c and Figure S11), benefiting from its asymmetric push-pull structure and low steric hindrance effect. Thus, as-PorCo-OMe has been approved as the good catalyst for electrocatalytic CO 2 RR application. The durability performance of as-PorCo-OMe was investigated at −0.7 V vs. RHE (potential for best FE CO ) (Figure 4d). After testing for 12 hr, the FE CO of as-PorCo-OMe remains over 93% and its current density has a low loss, and Figure S12 shows that Co 2p and N1s XPS spectra have a neglect binding energy change after the cycle experiment, demonstrating that such asymmetric Co(II) porphyrin exhibits a good electrochemical stability during long-term working.
PorCo-OMe has been approved as the good catalyst for electrocatalytic CO2RR app tion. The durability performance of as-PorCo-OMe was investigated at −0.7 V vs. R (potential for best FECO) (Figure 4d). After testing for 12 hr, the FECO of as-PorCo-O remains over 93% and its current density has a low loss, and Figure S12 shows that C and N1s XPS spectra have a neglect binding energy change after the cycle experim demonstrating that such asymmetric Co(II) porphyrin exhibits a good electrochemical bility during long-term working.

Conclusions
In summary, a novel kind of push-pull type Co(II) porphyrins with methoxy su tutions have been prepared efficiently by using 2,5-dimethoxybenzaldehyde as star material. The structures of these methoxy-substituted molecules have been confirmed various measurement like MALDI-TOF MS, FTIR and XPS spectroscopy. Compared w that of as-PorCo-OMe, cs-PorCo-OMe shows the slight red-shift absorption properties low bandgap, due to the limited donor effect of methoxy substitution in these structu Such as-prepared Co(II) porphyrins bearing electrocatalytic active site of cobalt ion wo be applied as electrocatalysts for CO2RR. In a CO2-saturated KHCO3 aqueous solution PorCo-OMe exhibits the better electrochemical CO2-to-CO performance including FEC 94.7% and TOF of 2880 h −1 at −0.7 V vs. RHE than those of cs-PorCo-OMe and repo DMP-CoPor, which is almost in agreement with that of DFT calculation. Therefore, work provides a new molecular engineering strategy for boosting electrocatalytic CO via methoxy functionalization.

Conclusions
In summary, a novel kind of push-pull type Co(II) porphyrins with methoxy substitutions have been prepared efficiently by using 2,5-dimethoxybenzaldehyde as starting material. The structures of these methoxy-substituted molecules have been confirmed by various measurement like MALDI-TOF MS, FTIR and XPS spectroscopy. Compared with that of as-PorCo-OMe, cs-PorCo-OMe shows the slight red-shift absorption properties and low bandgap, due to the limited donor effect of methoxy substitution in these structures. Such as-prepared Co(II) porphyrins bearing electrocatalytic active site of cobalt ion would be applied as electrocatalysts for CO 2 RR. In a CO 2 -saturated KHCO 3 aqueous solution, as-PorCo-OMe exhibits the better electrochemical CO 2 -to-CO performance including FE CO of 94.7% and TOF of 2880 h −1 at −0.7 V vs. RHE than those of cs-PorCo-OMe and reported DMP-CoPor, which is almost in agreement with that of DFT calculation. Therefore, this work provides a new molecular engineering strategy for boosting electrocatalytic CO 2 RR via methoxy functionalization.
Synthesis of cs-Por-OMe. Pyrrole (400 mg, 5.96 mmol) and 2,5-dimethoxybenzaldehyde (1002 mg, 6.04 mmol) were dissolved in propionic acid (200 mL). The solution was heated to 110 • C under an N 2 atmosphere. After stirring for 24 h, the crude product was obtained via precipitation in the methanol. The pure purple solid was obtianed by washing with methanol until it was a transparent color (612 mg, 12%). 1  Synthesis of cs-PorCo-OMe. The obtained cs-Por-OMe (300 mg, 0.35 mmol) and Co(OAc) 2 (800 mg, 4.52 mmol) was dissolved in DMF (30 mL). The solution was heated to 100 • C for 8 h under an N 2 atmosephere. After reaction, the solvent was removed and the solid was precipitated in the methanol and purified by a silica gel column chromatography (PE/DCM = 6:4) to collect cs-PorCo-OMe (304 mg, 95%).

Characterizations
NMR spectra were obtained from a Bruker Avance III 500 MHz spectrometer using CDCl 3 as solvents. MALDI-TOF mass spectrometry was recorded on autoflex speed TM TOF Mass Spectrometer. FTIR spectra were performed on Perkin Elmer Spectrum 100 spectrometer with KBr. XPS spectra were measured with a PHI 5000C ESCA System using C 1s (284.8 eV) as reference. UV-Vis spectra were recorded on a Lambda 950 spectrophotometer. CV tests were performed using 0.1 M TBAPF 6 DCM solution as an electrolyte with the CH CHI 660E instrument.

Electrode Preparation
Firstly, catalysts (1 mg) were dispersed well in the commercial CNTs (9 mg) ( Figure S8), then Nafion solution (2 mL, 0.5 wt. %) was added and stirred for 12 h. A quantity of 100 µL of mixed ink was dropped on carbon paper (surface: 1 cm 2 ) until dry to achieve the working electrode with catalyst loading of 0.05 mg cm −2 .

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.