Fabrication of Cobalt Porphyrin. Electrochemically Reduced Graphene

May 23, 2014 - Here we report on an experimental study of an electrocatalyst for the hydrogen evolution reaction (HER) based on cobalt porphyrin and e...
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Fabrication of Cobalt Porphyrin. Electrochemically Reduced Graphene Oxide Hybrid Films for Electrocatalytic Hydrogen Evolution in Aqueous Solution Dekang Huang, Jianfeng Lu, Shaohui Li, Yanping Luo, Chen Zhao, Bin Hu, Mingkui Wang,* and Yan Shen* Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, HuaZhong University of Science and Technology, Luoyu Road 1037, Wuhan 430074, PR China S Supporting Information *

ABSTRACT: Here we report on an experimental study of an electrocatalyst for the hydrogen evolution reaction (HER) based on cobalt porphyrin and electrochemically reduced graphene oxide (ERGO) functional multilayer films, which are prepared by the alternating layer-by-layer (LBL) assembly of negatively charged graphene oxide (GO) and positively charged [tetrakis (N-methylpyridyl) porphyrinato] cobalt (CoTMPyP) in combination with an electrochemical reduction procedure. The resulting [ERGO@CoTMPyP]n multilayer films display relatively high electrocatalytic activity and superior stability toward HER in alkaline media. Electrochemical studies indicate that CoTMPyP in the multilayer films is the active catalyst for the reduction of protons to dihydrogen.

1. INTRODUCTION Increasing energy demands and pressing environmental concerns have stimulated intensive research on the development of renewable and clean energy alternatives.1,2 Hydrogen is considered to be such a promising energy carrier with zero carbon emission and a high energy density (140 MJ kg1−) which far exceeds those of gasoline and coal.3,4 Today’s hydrogen is mainly produced from fossil sources, but the production chain still leaves a carbon footprint. At present, the most appealing way to produce hydrogen in high purity and large quantities is the water-splitting process, either through the electrochemical or photochemical route.5,6 However, the existence of a large overpotential in an electrocatalysis process makes it urgent to design highly active electrocatalysts for the hydrogen evolution reaction (HER).7 As we know, platinum is the unbeatable catalyst for HER which has an exchange current density (J0) of 4.5 × 10−4 A cm−2 and a Tafel slope (b) as small as 30 mV per decade, but its high cost and low abundance present problems for making catalysts based on precious metals economically competitive on a global scale.8−10 Thus, an enormous amount of attention has been devoted to developing non-noble metal complexes based on earth-abundant materials with high activity and superior stability.11 Over the past few decades, a series of cobalt complexes, such as cobalt glyoxime complexes,12−14 polypyridine cobalt complexes,15 N4 macrocyclic complexes,16 and cobalt complexes of base-containing multiphosphines,17,18 have been © XXXX American Chemical Society

reported to display remarkable activity toward HER with high Faradaic yields at low overpotentials. However, most of these molecular catalysts can operate smoothly only in nonaqueous solvents, which greatly limits their application in water-splitting devices. Therefore, a great deal of research has been carried out to develop highly active cobalt complexes which could work efficiently in aqueous solutions. For instance, Dey et al. developed an electronically tuned Co(III) corrole that could catalyze HER from aqueous sulfuric acid at as low as −0.3 V (vs NHE) with a turnover frequency of 600 s−1 and ≫107 catalytic turnovers.19 Recently, Artero et al. have shown that a diiminedioxime cobalt catalyst grafted to the carbon nanotube surface mediated H2 generation (55 000 turnovers in 7 h) from fully aqueous solutions at low-to-medium overpotentials and exhibited superior stability even after extensive cycling.20 This encouraging result motivates us to search other cobalt complexes with similar molecular structure for HER. Cobalt porphyrins have been found to be effective catalysts for the oxygen reduction reaction21 and oxygen evolution reaction22 but have been rarely explored as electrocatalysts for hydrogen production. Due to the similar structure between diiminedioxime cobalt and cobalt porphyrins in which one cobalt atom is coordinated with four nitrogen atoms, we have high Received: March 23, 2014 Revised: May 19, 2014

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Scheme 1. Fabrication Process of an [ERGO@CoTMPyP]n Thin Film with the Layer-by-Layer Assemble Method, Including Step 1 Deposition of GO and Step 2 Deposition of CoTMPyP Followed by an Electrochemical Reduction Procedure

applications including sensors,37 nonlinear optics,38 electronic devices,39 selective-area patterning,40 and electrocatalysis.41 Herein we communicate an experimental study of an electrocatalyst for HER based on cobalt porphyrin and electrochemically reduced graphene oxide (ERGO) functional multilayer films, which were prepared by the alternating layerby-layer (LBL) assembly of negatively charged graphene oxide (GO) and positively charged [tetrakis (N-methylpyridyl) porphyrinato] cobalt (CoTMPyP) in combination with an electrochemical reduction procedure (Scheme 1). The prepared multilayer films were characterized by UV−vis absorption spectra, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy.

expectations for the application of cobalt porphyrins as high HER electrocatalysts in aqueous solvents. Graphene, a 2D network of hexagonally structured sp2hybridized carbon atoms, has been an ideal construction component for fabricating ultrathin film materials due to its favorable electronic structure, impressive surface area and outstanding mechanical strength.23−26 However, due to the poor solubility of graphene, it is difficult to allow for the formation of large-scale thin films on different substrates through various solution processing methods, such as layer-bylayer assembly27 and Langmuir−Blodgett assembly.28 The frequently used method of synthesizing scalable chemically modified graphene is via graphene oxide (GO), which is obtained from the exfoliation of graphite oxide.29,30 The oxygen-containing functional groups in GO, such as carboxyl and phenolic groups at the edges and hydroxyl and epoxide on the basal planes, make it easy to functionalize with other materials and highly soluble in various solvents, including water for forming stable colloidal suspensions. The aqueous suspensions could be readily used to fabricate GO-based thin films, which can convert to graphene thin films readily through electrochemical reduction,31 chemical reduction,32 and thermal reduction.33 Layer-by-layer (LBL) assembly, which is based on the consecutive absorption of monolayers of oppositely charged species from dilute solutions by electrostatic interactions, has been introduced by Decher and co-workers.34 The properties of thin films can be easily tailored by the numbers of layers, the ionic strength, the pH of deposition solutions, and any postpreparative treatments (rinsing protocols, drying, etc.) .35,36 Due to its simplicity and robustness, this method has been successfully applied to the fabrication of multilayer nanocomposite thin films with finely controlled nanometer thickness and conformal structures and resulted in a wide range of

2. EXPERIMENTAL SECTION 2.1. Preparation of Graphene Oxide (GO) and [Tetrakis (NMethylpyridyl) Porphyrinato] Cobalt (CoTMPyP). Graphite oxide was synthesized according to the procedure reported in a previous publication.29 Note that a modified purification procedure was utilized for the synthesis of GO, in which the product was isolated by repeated mixture and centrifugation steps. Water-soluble CoTMPyP was prepared according to our previous report.42 The presence of the target macrocyclic structure of CoTMPyP was verified by 1H NMR (DMSO-d6) δ 9.45 (d, J = 5.1 Hz, 8H), 9.25 (s, 8H), 8.88 (d, J = 6.6 Hz, 8H), 4.73 (s, 12H) (Figure S1). In order to prepare 0.2 mg mL−1 GO and 0.2 mg mL−1 CoTMPyP for thin film assembly, as-synthesized GO and CoTMPyP were dispersed in deionized water (18.2 MΩ cm−1, Millipore) under ultrasonication (40 kHz, 120 W) for 2 h and 30 min, respectively. 2.2. Preparation of [ERGO@CoTMPyP]n Thin Films. The multilayer films were fabricated with a layer-by-layer (LBL) assembly technique using negatively charged GO and positively charged CoTMPyP in combination with an electrochemical reduction procedure on different substrates (Scheme 1). Prior to the LBL assembly, both ITO glasses and quartz slides were cleaned in a freshly prepared piranha solution (concentrated H2SO4 (98 wt %)/H2O2 (30 wt %) = 3:1 vol/vol) for 15 min and then rinsed extensively with B

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Figure 1. (a) TEM image of graphene oxide, (b) molecular structure of CoTMPyP, and (c) SEM image of an [ERGO@CoTMPyP]11 thin film (top view on ITO glass). impedance spectroscopy (EIS) measurements were obtained at −0.5 V (vs RHE) using a CHI 750D potentiostat in 0.1 M KOH. An ac signal amplitude of 5 mV was applied from 105 to 10−1 Hz. The potential related to the hydrogen evolution reaction was corrected to a reversible hydrogen electrode (RHE) by adding a value of (0.241 + 0.059 pH) V at room temperature.

deionized water. (Caution! Piranha solution is dangerous when placed in contact with organic material.) The surface of cleaned ITO glass and the quartz slide was negatively charged due to the existence of a native thin oxide layer.43 Subsequently, a piece of clean ITO (or quartz slide) was immersed in 0.3 wt % solution of poly(diallyldimethylammonium chloride) (PDDA, MW < 100 000) for 10 min to make the surface positively charged. Then, the substrate was immersed in a 0.2 mg mL−1 solution of GO for 15 min, rinsed with deionized water, dried with N2, and then immersed in a 0.2 mg mL−1 solution of CoTMPyP for 15 min. This procedure gave a single deposition cycle. In this article, n denotes the depostion cycle number for the repeated deposition procedure. Once thin films with a desired cycle number n were prepared, the corresponding ([GO@CoTMPyP]n)-film-modified electrodes were electrochemically reduced in N2-saturated 0.1 M phosphate-buffered solution (PBS, pH 7.4) at a scan rate of 50 mV s−1 between 0.0 and −1.2 V (vs SCE) to get a [ERGO@CoTMPyP]n thin film. In the case of glassy carbon (GC) electrodes, they were first polished to a mirrorlike surface with 1.0 and 0.3 μm α-Al2O3 suspensions on a polishing microcloth successively and then were cleaned ultrasonically in deionized water for 3 min. Finally, the GC electrodes were treated with 4-aminobenzoic acid (4-ABA)41 and immersed in PDDA for 10 min. Then the subsequent [ERGO@ CoTMPyP]n-film-modified GC electrode was fabricated according to the above-mentioned procedure. 2.3. Instruments and Measurements. 1H NMR was recorded on a Bruker AV400 400 MHz with tetramethylsilane as an internal standard. UV−vis absorption spectra were collected on a SHIMADZU UV-3600 UV−vis spectrometer. Raman spectra were collected using a LabRAM HR800 Raman spectrometer (Horiba JobinYvon Co.) at an excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD600W XPS instrument with Al Kα X-ray radiation as the X-ray source for excitation. The binding energy was referenced to the C 1s spectrum of the carbon support at 284.6 eV. A Sirion 200 (FEI Company, Holland) was utilized to obtain the field-emission scanning electron microscopy (FESEM) micrographs. Transmission electron microscopy (TEM) images were taken on a Tecnai G2 F30 microscope (FEI Company, Holland). Atomic force microscopy (AFM) images were recorded by using a Nanoscope 3D (Veeco) instrument. The samples for Raman, XPS, and FESEM measurements used ITO as the substrate. For Raman, XPS, and FESEM measurements, samples were assembled onto ITO substrates. For UV−vis spectroscopy characterization, the samples were assembled on quartz slides. The samples for AFM study were assembled on silicon wafers. Electrochemical tests were carried out with a CHI 750D potentiostat (Chenghua Co. Shanghai) in a conventional threeelectrode electrochemical cell using the prepared [ERGO@CoTMPyP]n/PDDA/4-ABA/GC electrode (3 mm, Chenghua Co. Shanghai) as the working electrode, a twisted platinum wire as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. Linear sweep voltammetry with a scan rate of 2 mV s−1 was conducted in N2-saturated 0.1 M KOH for the HER. Electrochemical

3. RESULTS AND DISCUSSION In this work, uniform [ERGO@CoTMPyP]n multilayer films on various substrates were fabricated by means of the LBL method followed by electrochemical reduction. Scheme 1 shows the process for the fabrication of an [ERGO@ CoTMPyP]n film on ITO glass, serving as an example. GO was synthesized by the liquid-phase exfoliation method. As illustrated by the TEM image in Figure 1a, the obtained GO sample showed a single-layer platelet structure with a wrinkle characteristic. The cleaned ITO glass was first immersed in PDDA solution to change the substrate surface to positively charged. Subsequently, [GO@CoTMPyP]n multilayer films were fabricated by the alternating LBL assembly of negatively charged GO and positively charged CoTMPyP. The molecular structure of CoTMPyP is shown in Figure 1(b). Due to a large number of oxygen-containing groups on the surface or edge parts, GO presents a negative charge property when it is dispersed in the solution at pH 7. However, CoTMPyP is positively charged in this environment. Therefore, the electrostatic interaction acts as the driving force for the LBL assembly of GO and CoTMPyP. Finally, [ERGO@CoTMPyP]n multilayer films were obtained after an electrochemical reduction procedure. Detailed information on multilayer fabrication can be found in the Experimental Section. The as-obtained [ERGO@CoTMPyP]11 multilayer films exhibited occasional folds, crinkles, and rolled edges as indicated by the FESEM image (Figure 1(c)), which is consistent with our previous report on [PDDA@ERGO]n multilayer films.41 It was observed that the oxygen content was significantly decreased after electrochemical reduction (as discussed later). The remaining oxygen-containing groups offer the possibility of electrostatic interactions between CoTMPyP and ERGO. Additional interactions, such as π−π stacking and the van der Waals force between the cobalt porphyrin and ERGO through the planar benzene rings, also existed.21 These interactions can improve the stability of the obtained thin films. It is significant that, after assembly with CoTMPyP, the G band of ERGO was shifted from 1601 to 1607 cm−1 as shown in Figure S5. This is assigned to the charge transfer from ERGO to CoTMPyP. This C

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Figure 2. UV−vis absorption spectra of (a) a GO solution, (b) a CoTMPyP solution, and (c) [GO@CoTMPyP]n (n = 1, 3, 5, 7, 9, 11) on quartz slides. The inset of panel c shows the linear relationship between the absorbance at 446 nm and the number of depostion cycles. (d) Image of the bare quartz slide (left) and the [GO@CoTMPyP]11-modified quartz slide (right).

Figure 3. (a) CV curves of [GO@CoTMPyP]n/PDDA/4-ABA/GC (n = 1, 3, 5, 7, 9, 11) electrodes in N2-saturated 0.1 M PBS (pH 7.4) solution at a scan rate of 50 mV s−1 within the potential range from 0 to −1.2 V (vs SCE). The inset shows the CV curve of the [GO@CoTMPyP]7/PDDA/4ABA/GC electrode in N2-saturated 0.1 M PBS (pH 7.4) solution. (b) Peak current and peak potential against the number of depostion cycles.

polyoxometalate in previous reports.42,46 This result suggests that CoTMPyP molecules are uniformly dispersed over the platelet structure of GO. The surface density (mol cm−2) of CoTMPyP in the film could be calculated with eq 1, which was derived from the Beer−Lambert law modified for 2D concentration47

feature significantly suggests the interaction between CoTMPyP and ERGO.44 The successful synthesis of [GO@CoTMPyP]n thin films with the LBL assembly process was confirmed by UV−vis spectroscopy characterization (Figure 2). The spectra of GO in water showed an absorption peak at 226 nm, which could be attributable to the π−π* transition of aromatic C−C bonds. The shoulder at around 300 nm was attributed to the n−π* transition of CO bonds (Figure 2a).45 As shown in Figure 2b, CoTMPyP in solution displayed a characteristic Soret band (due to a strong transition to the second excited state, S0 → S2) at 447 nm and a Q band at 554 nm (due to a weak transition to the first excited state, S0 → S1).43 The absorbance of GO was not clearly observed upon the buildup of [GO@CoTMPyP]n thin films (Figure 2c) due to the perturbation by the response of porphyrin at about 214 nm. The absorbance at 447 nm was found to increase linearly with cycle number n from 1 to 11 (inset of Figure 2c), indicating a homogeneous increase of CoTMPyP during the assembly cycles. This result also suggests that after adding CoTMPyP, the structure built in a previous deposition cycle was not damaged. Note that the Soret absorption peaks of porphyrin remained constant during the whole assembly process, which was different from the layer-bylayer assembly process between CoTMPyP and Au or

Γ = 10−3D/ε

(1) −1

−1

where ε is the molar absorption coefficient (M cm ) of CoTMPyP (∼99021 M−1 cm−1 at 447 nm) and D is the absorbance of CoTMPyP (∼0.0218 for the [GO@CoTMPyP]1 film). The Γ value of CoTMPyP in the thin films was calculated to be about 1.2 molecule nm−2, indicating the formation of a very densely packed film. Figure 2d presents an image of quartz slides before and after the self-assembly procedure, indicating the successful depostion of [GO@CoTMPyP]n thin films. Atomic force microscopy (AFM) was used to check the thickness of the [GO@CoTMPyP]7 film. As shown in Figure S2, a section analysis revealed an average thickness of 7.3 nm for the [GO@CoTMPyP]7 film, which means that the thickness of the [GO@CoTMPyP]1 thin film is about 1.04 nm. Cyclic voltammetry (CV) was also performed to monitor the formation of the [GO@CoTMPyP]n film by using a molecular redox probe of [Fe(CN)6]3−/4−. It is known that the substantial D

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sp3 fraction in GO renders this material insulating, which largely suppresses the oxidation−reduction current of the [Fe(CN)6]3−/4− probe.48 Figure S3 shows the CVs of various [GO@CoTMPyP]n/PDDA/4-ABA/GC electrodes in the presence of [Fe(CN)6]3−/4−. The peak currents gradually decreased as cycle number n increased from 1 to 11, indicating the homogeneous increase of GO in [GO@CoTMPyP]n films during the assembly process. The electrochemical reduction procedure has been proven to be an effective route for converting GO to graphene.41 Figure 3a shows the first segment of CVs for the [GO@CoTMPyP]n/ PDDA/4-ABA/GC electrodes in N2-saturated 0.1 M PBS (pH 7.4) within the potential range from 0.0 to −1.2 V (vs SCE). The CVs presented well-defined cathodic current peaks for various films. The large cathodic current observed in Figure 3a can be attributed to the electrochemical reduction of the oxygen-containing functional groups of GO.49 All of the samples presented a starting reduction potential at about −0.65 V. The characteristic peak from the first segment was no longer observed in successive scans (inset of Figure 3a) due to the irreversible conversion process of oxygen-containing functional groups of GO. As presented in Figure 3b, the peak current and potential varied linearly with deposition cyclic number n, indicating that ERGO was homogeneously deposited onto the GC electrode surface. The peak cathodic current reflected the increased mass loading of GO in the films as deposition cycle number n increased. The transformation of GO to ERGO could be also monitored by the electrochemical current for the [Fe(CN)6]3−/4− probe. Figure S4 shows the CVs of GC, [GO@ CoTMPyP]7/PDDA/4-ABA/GC, and [ERGO@CoTMPyP]7/ PDDA/4-ABA/GC electrodes in the presence of [Fe(CN)6]3−/4−. A reversible electrochemical response for the [Fe(CN)6]3−/4− probe was observed on the bare GC electrode. While the electrode was modified with the [GO@CoTMPyP]7 film, this current was seriously suppressed due to the insulating nature of GO as discussed above. However, it was found that the [ERGO@CoTMPyP]7-film-modified electrode exhibited an increased redox peak current and a decreased peak separation compared to those of the bare GC electrode due to an augmenting of conductivity and the reactive edge defects on ERGO. The reduction of GO was further confirmed with Raman spectroscopy. The Raman spectra of [GO@CoTMPyP]7 and [ERGO@CoTMPyP]7 are shown in Figure S5. The samples displayed a strong G band at 1607 cm−1 and a weak D band at 1358 cm−1. The D/G intensity ratio increased from 0.79 for [GO@CoTMPyP]7 to 0.93 for [ERGO@ CoTMPyP]7 thin films, suggesting a decrease in the sp2 domain in graphene induced by the electrochemical reduction. Details of the chemical changes during the electrochemical reduction of GO were further elucidated by XPS measurements. Figure 4a shows the high-resolution C 1s XPS spectrum of the [GO@CoTMPyP]7 film, which can be fitted with three different components of oxygen-containing functional groups at binding energies of 284.6 eV (C−C), 286.5 eV (C−O−C and C−OH), and 288.2 eV (O−CO), respectively.50,51 The C 1s XPS spectrum of corresponding films after electrochemical reduction exhibited a significant decrease in the spectrum signal of oxygen-containing moieties (Figure 4b), particularly the (C− O−C and C−OH) groups at 286.5 eV. This result indicates an efficient reduction of oxygen-containing functional groups of GO in the sample. Figure 4c shows the high-resolution N 1s XPS spectra of the [ERGO@CoTMPyP]7 film. Two distinct peaks were observed that could be attributed to pyridine-type

Figure 4. (a) High-resolution spectrum of C 1s of [GO@ CoTMPyP]7. High-resolution spectra of (b) C 1s, (c) N 1s, and (d) Co 2p of [ERGO@CoTMPyP]7.

nitrogen (398.6 eV) and Co-coordinated nitrogen (401.5 eV). Figure 4d shows the high-resolution Co 2p XPS spectra of the [ERGO@CoTMPyP]7 film. Co 2p characteristic photoelectron peaks were found to appear at 781.3 and 796.1 eV.21 The difference of 14.8 eV between Co 2p3/2 and Co 2p1/2 peaks and the absence of a shakeup satellite on the Co 2p core-level signals are clear evidence of the presence of the Co(II) ion.20 The N/Co atom ratio in the [ERGO@CoTMPyP]7 film was revealed to be 8.4:1, which was slightly higher than the atom ratio of 8:1 as expected from the chemical composition of CoTMPyP (Figure 1). This difference can be explained by the presence of PDDA in the [ERGO@CoTMPyP]7 film. So far, hydrogen production catalyzed by cobalt complexes occurred rarely in aqueous solutions.52 For example, cobalt microperoxidase-11 was reported to reduce a proton to hydrogen in neutral water under aerobic conditions. However, a high overpotential (852 mV) was needed in this system.53 In the following study, the [ERGO@CoTMPyP]n thin film was demonstrated to be an effective electrocatalyst for HER in alkaline solutions. The electrocatalytic activity of the [ERGO@ CoTMPyP]n thin film for HER was first evaluated with linear sweep voltammetry in an N2-saturated 0.1 M KOH solution at room temperature (Figure 5a). The catalytic activity of the GC electrode was first examined, which showed negligible HER activity. In contrast, [ERGO@CoTMPyP]n thin films displayed pronounced activity toward HER with a more positive onset potential and a larger current density. When a potential (more negative than −0.6 V vs RHE) was applied to the working electrodes, anodic current flow (Figure 5a) and hydrogen bubbles on the [ERGO@CoTMPyP]n electrode surface (Figure S6) were observed in an electrolyte 0.1 M KOH solution. The catalytic activity of [ERGO@CoTMPyP]n thin films was saturated when the bilayer number was increased to 7. As the bilayer number increased from 1 to 7, the current density increased from 0.222 to 1.729 mA cm−2 at a potential of −0.5 V (vs RHE). Meanwhile, the overpotential at a current density of 1 mA cm−2 decreased from 572 to 474 mV due to an enhancement in the active site density generated in the thin film. Minor changes were observed with further increases in the deposition cyclic number (Table 1) probably due to a E

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saturation of active sites. Noted that the overpotential (474 mV) at a current density of 1 mA cm−2 for [ERGO@ CoTMPyP]7 thin films is lower than for the diimine-dioxime cobalt catalyst (590 mV),20 suggesting the high activity of these thin films toward HER in alkaline media. The kinetic parameters were estimated from the polarization curves according to the Tafel equation η = a + b log( −j) ⎛ 2.3RT ⎞ ⎟log j , a = −⎜ 0 ⎝ αF ⎠

Table 1. Kinetic Parameters of HER and Rct in N2-Saturated 0.1 M KOH on [ERGO@CoTMPyP]n/PDDA/4-ABA/GC (n = 1, 3, 5, 7, 9, 11) Electrodes

a

b (mV dec−1)

αa

1 3 5 7 9 11

112 112 109 116 116 115

0.53 0.53 0.54 0.51 0.50 0.51

j0 (mA cm−2) 7.62 1.85 2.49 8.46 7.02 6.64

× × × × × ×

10−6 10−5 10−5 10−5 10−5 10−5

η (mV)b

j (mA cm−2)c

Rct (Ω)

572 529 502 474 483 482

0.222 0.538 0.963 1.729 1.435 1.481

2416 1084 496 283 373 313

b=

2.3RT αF

(3)

where j is the current density, j0 is the exchange current density, α is the charge-transfer coefficient, η is the overpotential, F is the Faraday constant, R is the gas constant, T is the absolute temperature, b is the Tafel slope, and a is the Tafel intercept. The numerical values of kinetic parameters are listed in Table 1. The Tafel slopes (∼114 mV dec−1) obtained for various films are close to the theoretical one for the Volmer−Heyrovsky mechanism (∼118 mV dec−1) (Figure 5b).54 The highest exchange current density (8.46 × 10−5 mA cm−2) was determined on an [ERGO@CoTMPyP]7 film. The electrocatalytic activity of the [ERGO@CoTMPyP]7 thin film for HER was also investigated in an N2-saturated 0.5 M H2SO4 solution. As shown in Figure S7, at a current density of 1 mA cm−2 the overpotential was estimated to be about 460 mV in the pH 0 solution, which was comparable to the value (474 mV) obtained in 0.1 M KOH (pH 13). The current densities at the potential of −0.6 V (vs RHE) in both solutions were almost the same. These results indicate that solution pH has a negligible influence on the activity of the [ERGO@ CoTMPyP]7 thin film toward HER. Electrochemical impedance spectroscopy (EIS) is a useful technique for studying the electrode kinetics in HER. Figure 5c presents Nyquist plots of [ERGO@CoTMPyP]n thin films in the frequency range from 100 mHz to 100 kHz. The EIS measurements were recorded at −0.5 V (vs RHE) in 0.1 M KOH. As shown in Figure 5c, only one semicircle was observed for various films. Its diameter decreased as the cyclic number n increased from 1 to 7. Afterward, the semicircle increased as the cyclic number increased further. The absence of Warburg impedance indicates that ionic transportation in electrolytes is rapid enough that the electrochemical reaction on the electrode surface is kinetically controlled.55 Therefore, the catalytic system that can be described by a simple equivalent electrical circuit is shown in the inset of Figure 5c. The resistance element R1 is attributed to the uncompensated for solution

Figure 5. (a) Polarization curves for the hydrogen evolution reaction on GC and [ERGO@CoTMPyP]n/PDDA/4-ABA/GC (n = 1, 3, 5, 7, 9, 11) electrodes in an N2-saturated 0.1 M KOH solution at a scan rate of 2 mV s−1. (b) Corresponding Tafel plot of [ERGO@CoTMPyP]n/ PDDA/4-ABA/GC (n = 1, 3, 5, 7, 9, 11) electrodes. (c) Impedance spectra of [ERGO@CoTMPyP]n/PDDA/4-ABA/GC (n = 1, 3, 5, 7, 9, 11) electrodes in 0.1 M KOH solution. Symbols, experimental data; solid lines, fitted data. The inset shows the equivalent circuit. (d) Stability test for the [ERGO@CoTMPyP]7/PDDA/4-ABA/GC electrode.

n

(2)

α is the Tafel intercept. bAt j = 1 mA cm−2. cAt η = 500 mV.

Figure 6. (a) CVs of [ERGO@CoTMPyP]1/PDDA/4-ABA/GC and [ERGO]1/PDDA/4-ABA/GC electrodes in N2-saturated 0.1 M KOH at a scan rate of 50 mV s−1. (b) CVs of [ERGO@CoTMPyP]7/PDDA/4-ABA/GC and ERGO/GC electrodes in N2-saturated 0.1 M KOH at a scan rate of 50 mV s−1. F

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4. CONCLUSIONS Active catalysts for HER based on cobalt porphyrin and ERGO were prepared by alternating layer-by-layer assembly in combination with an electrochemical reduction procedure. It was found that the resulting [ERGO@CoTMPyP]n films increase their electrocatalytic activity in alkaline solutions with respect to HER as the depostion cycle number increases to 7. A detailed investigation demonstrates that the active site in [ERGO@CoTMPyP]n catalysts origins from the Co−N4 moiety.

resistance. The resistance element Rct is attributed to the charge-transfer resistance. A low value of Rct corresponds to a fast reaction. A constant phase element (CPE1) represents the double-layer capacitance under HER conditions. As listed in Table 1, Rct for [ERGO@ CoTMPyP]n thin films decreased from 2416 to 283.3 Ω with increasing cyclic number n up to 7 and then increased with further increasing cyclic number. The [ERGO@CoTMPyP]7 sample presented the smallest value of Rct at 283.3 Ω. This result is consistent with the exchange current analysis. Stability is another concern for HER catalysts. To evaluate the HER stability of catalysts, long-term potential cycling of the [ERGO@CoTMPyP]7 film was conducted by taking a potential scan at a scan rate of 2 mV s−1 and at an accelerated scanning rate of 200 mV s−1 after 1000 cycles from −0.1 to −0.6 V. As shown in Figure 5d, only a slight activity loss was observed after 1000 cycles for the [ERGO@CoTMPyP]7 film, indicating the good durability of this catalyst in alkaline solutions. A set of controlled experiments was carried out to find out the active sites in the [ERGO@CoTMPyP]n film toward HER. Figure 6a compares the CVs of [ERGO@CoTMPyP]1/PDDA/ 4-ABA/GC and [ERGO]1/PDDA/4-ABA/GC electrodes in N2-saturated 0.1 M KOH at a scan rate of 50 mV s−1. The [ERGO]1/PDDA/4-ABA/GC electrode presented poor performance toward HER as reflected by a low reduction current density (red cuve). This result indicates the principle contribution of CoTMPyP to HER. When the deposition cyclic number n increased to 7, an augmented catalytic current was observed for the sample in Figure 6b (black curve). For comparsion purposes, an ERGO/GC electrode fabricated via dip-coating a 1 mg GO solution on a GC electrode following an electrochemical reduction procedure was tested (red curve in Figure 6b). Herein, the amount of ERGO in the ERGO/GC electrode was larger than that of the [ERGO@CoTMPyP]7/ PDDA/4-ABA/GC electrode. However, the [ERGO@CoTMPyP]7/PDDA/4-ABA/GC electrode exhibited a much larger reduction current than the ERGO/GC electrode, indicating a higher catalytic activity for the former. These results clearly demonstrate that CoTMPyP plays a critical role in the [ERGO@CoTMPyP]n thin film activity toward HER. Therefore, the main role of ERGO in the functional thin film would be a high conductive support for CoTMPyP. This is beneficial to the HER process on the thin film catalyst. It has been reported that in most cobalt-based HER catalysts the active site is the cobalt atom, which can be coordinated with four20 or more nitrogen atoms (coded as Co−Nx).52 In this study, an analogous porphyrin without a Co core, coded as TMPyP, was synthesized to clarify whether Co−N4 is the active site in the [ERGO@CoTMPyP]n film. Figure S8 presents the 1 H NMR characterization of as-synthesized TMPyP, which is similar to that of CoTMPyP. Figure S9a shows a further characterization of UV−vis spectroscopy of TMPyP in solution. Compared to CoTMPyP, the characteristic Soret band of TMPyP was negatively shifted to 420 nm due to the demetallization of cobalt. Figure S9b shows the polarization curves for [ERGO@CoTMPyP]7/PDDA/4-ABA/GC and [ERGO@TMPyP]7/PDDA/4-ABA/GC electrodes in N2-saturated 0.1 M KOH solution. The [ERGO@TMPyP]7/PDDA/4ABA/GC electrode presented limited activity with respect to HER compared to the [ERGO@CoTMPyP]7/PDDA/4-ABA/ GC electrode, demonstrating Co−N4 as the active center in the [ERGO@CoTMPyP]n catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of CoTMPyP and TMPyP. Raman spectra of ERGO, [GO@CoTMPyP]7, and [ERGO@CoTMPyP]7 films. AFM images of the [ERGO@CoTMPyP]7 film. CV characterization of the [ERGO@CoTMPyP]n film. UV−vis absorption spectrum of TMPyP. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86 27 87793313. Tel: +86 27 87793419. *E-mail: [email protected]. Fax: +86 27 87793313. Tel: +86 27 87793419. Author Contributions

D.H. and J.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the 973 Program of China (2013CB922104, 2014CB643506, and 2011CBA00703), the NSFC (21103057, 201173091, and 21161160445), and the CME through the Program of New Century Excellent Talents in University (NCET-10-0416) is gratefully acknowledged. We thank the Analytical and Testing Centre at the HUST for characterizing various samples. Y.S. and M.W. thank the Carl von Ossietzky University of Oldenburg for support as visiting scientists.

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REFERENCES

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