Nitrogen-Doped Graphene Nanocomposites as Bifunctional

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Functional Nanostructured Materials (including low-D carbon)

Ir/g-C3N4/NG nanocomposites as bifunctional electrocatalysts for overall water splitting in acidic electrolyte Binbin Jiang, Tao Wang, Yafei Cheng, Fan Liao, Konglin Wu, and Mingwang Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11970 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

Ir/g-C3N4/NG nanocomposites as bifunctional electrocatalysts for overall water splitting in acidic electrolyte Binbin Jiang,1,2 Tao Wang,1* Yafei Cheng,2 Fan Liao,2 Konglin Wu,3 Mingwang Shao2* 1 Provincial Key Laboratory of Functional Coordination Compounds and Nanomaterials, School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing, 246001, P. R. China 2 Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, P. R. China 3 Center of Single-atom, Clusters and Nanomaterials (CAN), Key Laboratory of Functional Molecular Solids, the Ministry of Education, Anhui Laboratory of Molecule-based Materials (State Key Laboratory Cultivation Base), Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241002, P. R. China. KEYWORDS: Ir; G-C3N4; NG; Water splitting; Acidic electrolyte

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ABSTRACT Nitrogen doped graphene (NG) chemically coupled with graphitic carbon nitride (g-C3N4) may facilitate the kinetics of overall electrochemical water splitting. Herein, a facile strategy is adopted to synthesize monodispersed Ir nanoparticles on g-C3N4/NG layers. Benefiting from the synergistic effect between different components of the catalyst, the optimal Ir/g-C3N4/NG catalyst with low content of Ir, (5.9 wt%) exhibits highly active for electrochemical water splitting in acidic electrolyte. Specifically, as a HER catalyst, the optimal Ir/g-C3N4/NG exhibits Tafel slope of 22 mV·dec-1. The optimal catalyst requires an overpotential of 22 mV to reach the current density of 10 mA·cm-2, the value of which is superior to Ir/NG (32 mV) and 20 wt% Pt/C (28 mV) catalysts; as an OER catalyst, it also achieve the Tafel slope of only 72.8 mV·dec-1. At the overpotential of 300 mV, the mass activity of the optimal Ir/gC3N4/NG catalyst is 2.8 times as large as that of 5.7 wt% Ir/NG catalyst. More significantly, as a bifunctional catalyst, the optimal Ir/g-C3N4/NG achieves current density of 10 mA·cm−2 with potential of only 1.56 V and displays good stability for overall water splitting. This work provides a new strategy to design highly efficient acidic catalysts for electrochemical overall water splitting.


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INTRODUCTION Electrochemical water splitting is one of the most promising technologies to the development of renewable energy conversion.1-4 Currently, despite the superior efforts in HER or OER have been achieved, few electrocatalysts can operate well toward both HER and OER in acidic electrolyte due to their unstable electrocatalytic ability in harsh environment. More importantly, the design of bifunctional catalyst for electrochemical water splitting is of paramount significance due to simple the production process and reducing the cost, which has attracted much interest in recent years.5-8 The proton exchange membrane water electrolyzer (PEMWE) is considered as the most potential technology for the electrochemical water splitting.9-11 In general, the Ir-based nanocomposites are considered as the optimal bifunctional catalyst for electrochemical water splitting in acidic electrolyte.12-14 Nevertheless, it is limited by low reserves and high cost. In the OER process, it still operates at large overpotential due to the quite sluggish kinetics in acidic electrolyte.5,

9

Despite great efforts devoted to overcome these

obstacles, the development of highly efficient electrocatalysts with low dosage of noble metals is still a major challenge.15-18 Among various strategies, the interesting choice is to load low content of noble metals on high specific-surface substrates to moderate the electrocatalytic activity.6, 17, 19, 20

Recently, the unusual physicochemical properties of graphitic carbon nitride (g-C3N4) make it an attractive component for constructing outstanding catalysts.21-24 Although the bare g-C3N4 has poor conductivity, incorporating it onto electron-conductive matrixes can address this obstacle.25 It is worth noting that due to the pyridinic N species in g-C3N4, the nearby sp2-hybridized C atoms have highly positive charge density, which is in favor of the adsorption of reactants and the charge transportation between the reaction species and catalyst surface, resulting in improved



OER performance.24,

25

Additionally, by chemically coupling g-C3N4 with nitrogen doped

graphene (NG), the hybrid manifested the high HER activity due to the moderate adsorption−desorption behavior,25 and thereby facilitated the kinetics of overall electrochemical water splitting. Besides, the g-C3N4 can help achieve good dispersion without aggregation for active materials, exposing the active sites.24, 26, 27 Furthermore, the g-C3N4 was regarded as a robust supporter due to the chemical stability.28-31 Based on the above representation, we predict that the electrocatalytic activity would be greatly improved by constructing Ir/g-C3N4/NG hybrid. Herein, the monodispersed Ir nanoparticles firmly anchored on g-C3N4/NG were obtained via a facile method. As expected, compared with 5.7 wt% Ir/NG, the Ir/g-C3N4/NG catalyst displays significantly enhanced electrocatalytic ability for water splitting. Significantly, as a bifunctional catalyst, the optimal Ir/g-C3N4/NG exhibits highly activity and stability for electrochemical overall water splitting in acidic electrolyte. The electrochemical performance suggests that the gC3N4/NG hybrid is vital to enhanced catalytic kinetics, as well as improving the intrinsic electrocatalytic ability and exposing active sites.

RESULTS AND DISCUSSION Characterization. The 5.7 wt% Ir/NG, Ir/g-C3N4/NG and 6.2 wt% Ir/g-C3N4 nanocomposites were produced by a solvothermal approach. The components and structures of the obtained nanocomposites are characterized by X-ray powder diffraction (XRD), thermogravimetric analysis (TGA) and inductively coupled plasma mass spectrometry (ICP-MS) (Figures 1, S1 and Table S1). Ir/g-C3N4/NG nanocomposites are labeled based on Ir content (Table S1).


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Figure 1. (A) XRD patterns of the 4.8 wt% Ir/g-C3N4/NG, 5.9 wt% Ir/g-C3N4/NG and 8.1 wt% Ir/g-C3N4/NG; (B) TGA curve of 5.9 wt% Ir/g-C3N4/NG in the air condition; (C) the full XPS spectra of 5.9 wt% Ir/g-C3N4/NG; (D) high-resolution C 1s; (E) N 1s; and (F) Ir 4f.

The Fourier Transform infrared spectroscopy (FTIR) spectra of the pure g-C3N4, NG and gC3N4/NG are shown in Figure S2. The peak at 806 cm-1 is indexed to tri-s-triazine units of gC3N4,32 which confirm that the g-C3N4/NG is successfully obtained.23 The structure of obtained catalysts is investigated by XRD (Figure 1 A). The broad peak at 2θ values of ~24 and 43° may be assigned to the (002) and (100) planes of NG, which is in accordance with the previous



report.33 The diffraction peaks indexed to g-C3N4 are not observed because the peaks are submerged in the peaks of NG. For the 8.1 wt% Ir/g-C3N4/NG, the peaks of (111) plane of Ir (JCPDS No. 06-0598), (100) plane of NG and (200) plane of g-C3N4 are overlapped at around 43°. The 4.8 wt% Ir/g-C3N4/NG and 5.9 wt% Ir/g-C3N4/NG show no obvious diffraction peaks for Ir due to low content of Ir. Figure 1B shows TGA for 5.9 wt% Ir/g-C3N4/NG catalyst in the air conduction. After 800 oC, the sample shows a 5 wt% residue weight, which is attributed to the decomposition of g-C3N4/NG hybrid into CO2 and N2. The weight of residue is consistent with the results of ICP-MS, which indicate the 5.9 wt% Ir/g-C3N4/NG with low content of Ir was successfully synthesized. The chemical composition and electronic structure of the 5.9 wt% Ir/g-C3N4/NG nanocomposite is further investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 1C, the Ir 4f, C 1s, N 1s, and O 1s can be clearly observed from the XPS full scan spectra of 5.9 wt% Ir/g-C3N4/NG nanocomposite. To be specific, the high resolution C 1s spectrum is analyzed (Figure 1D). The peak at 284.8 eV corresponds to C-C, and the peak at 288.3 eV is assigned to C-N=C, which is derived from the g-C3N4/NG hybrid.34 The N 1s peak (Figure 1E) is deconvoluted into four peaks, which belongs to pyridinic N (398.6 eV), pyrrolic N (400.1 eV), quaternary N (401.7 eV) and oxidized N (402.5 eV), respectively.35, 36 The high resolution Ir 4f shows two components: the peaks located at 61.5 eV and 64.5 eV are corresponding to Ir(0), while the peaks located at 62.9 eV and 66.0 eV are indexed to Ir(Ⅳ) species, which indicate the Ir nanoparticles are decorated on the g-C3N4/NG through the Ir-O bonds.37, 38 The percentage of Ir(0) species is determined to be 67.6% based on peak areas. From the XPS spectrum of highresolution O 1s for 5.9 wt% Ir/g-C3N4/NG (Figure S3), the peaks at 531.4 eV, 532.6 eV and 533.6 eV are assigned to O=C-OH, C-O-C and C-OH, respectively. The peak at 530.2 eV is


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assigned to Ir-O,37 confirming the Ir nanoparticles are attached to the g-C3N4/NG through Ir-O bonds.

Figure 2. (A) SEM and (B) TEM images of g-C3N4/NG; and 5.9 wt% Ir/g-C3N4/NG (C) TEM image, (D) HRTEM image, (E) HAADF-STEM image, and (F) the EDS mapping of C, N, O and Ir. The inset in (C) is the size distribution of the Ir nanoparticles.



The morphology and structure of as-prepared catalysts were also investigated by SEM and TEM. The g-C3N4/NG hybrid was obtained by hydrothermal method with the help of hydrazine and NH3·H2O. From Figures 2A and 2B, the thin and wrinkled nanosheets of g-C3N4/NG hybrid are clearly observed, suggesting that they are held together and form the nanosheets. However, g-C3N4 shows sheet-like structure with aggregation and overgrowth (Figure S4). For the 5.9 wt% Ir/g-C3N4/NG, TEM images (Figures 2C and 2D) clearly reveal that ultrasmall Ir nanoparticles with the diameter of 1.93 nm are uniformly dispersed on the surface of g-C3N4/NG without aggregation or overgrowth. The average spacing and density of particles also confirm that the Ir nanoparticles are well distributed (Figures S5 and S6). The high-resolution TEM (HRTEM) image (Figure 2D) shows the crystallinity of the as-prepared composite and the lattice spacing of 0.22 nm corresponds to (111) plane of Ir. The High angle annular dark field scanning TEM (HAADF-STEM mapping) also indicates that the Ir nanoparticles are homogeneously decorated on the surface of g-C3N4/NG (Figure 2E). The corresponding energy dispersive spectroscopy (EDS) mapping (Figure 2F) show element distribution of carbon (blue), nitrogen (red), oxygen (orange) and iridium (green). Furthermore, the morphology of 4.8 wt% Ir/g-C3N4/NG, 8.1 wt% Ir/g-C3N4/NG, 5.7 wt% Ir/NG, 6.2 wt% Ir/g-C3N4, 6.1 wt% Ir/g-C3N4/NG and 5.6 wt% Ir/gC3N4/NG are also investigated (Figure S7). Compared with 5.7 wt% Ir/NG, the distribution of Ir nanoparticles is close for 6.2 wt% Ir/g-C3N4 (Figures S6 and S8). Hydrogen evolution reaction. Pt is regarded as the most effective catalyst for HER in acid electrolyte. Usually, the Ir exhibits the low electrocatalytic ability for HER even though it has a value of H adsorption free energy (∆GH*) more close to zero than Pt.39 Interestingly, by combining NG with g-C3N4 effect, Ir/g-C3N4/NG is very promising for improving HER performances. To gain insight into the HER performance of Ir-based catalysts, a electrochemical


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testing of pure Ir, g-C3N4/NG, Ir/g-C3N4/NG, 6.2 wt% Ir/g-C3N4, 5.7 wt% Ir/NG and 20 wt% Pt/C are carried out in detail as follows (Figures 3 and S9). In Figure 3A, the g-C3N4/NG shows negligible performance. Compared with pure Ir, the 5.9 wt% Ir/g-C3N4/NG and 5.7 wt% Ir/NG catalysts exhibit improved activity. The most exciting point is that the 5.9 wt% Ir/g-C3N4/NG display better catalytic ability than 20 wt% Pt/C and 5.7 wt% Ir/NG at the applied potential. To deeply realize the effect of C3N4 and Ir on the HER behaviors, various Ir/g-C3N4/NG with different contents of g-C3N4 or Ir are synthesized and their HER performance is measured. Figures 3B and 3C show that 5.9 wt% Ir/g-C3N4/NG exhibits best HER performances. To reach the current density of 10 mA·cm−2, 5.9 wt% Ir/g-C3N4/NG catalyst only requires the overpotential of 22 mV, which is smaller than that of 20 wt% Pt/C catalyst (the corresponding overpotential is 28 mV). To gain the insights into intrinsic catalytic activity of 5.9 wt% Ir/g-C3N4/NG for HER, we investigated electrocatalytic kinetics from the Tafel slope (Figure 3D), which is determined by the rate-limiting step.40 The Tafel slope of 5.9 wt% Ir/g-C3N4/NG is 22.0 mV·dec−1, suggesting that the Tafel recombination step is the dominant process, indicating that it has the rapid charge transfer process.17, 41-43 Surprisingly, Tafel slope is also significantly lower than those of the Ir or Pt based catalyst such as Ir/CC (30 mV·dec-1),44 IrCo PHNCs (26.6 mV·dec-1),16 IrCo@NC-500 (23 mV·dec-1),17 PtNiCu (28 mV·dec-1),39 Pt-Ni Ass (27.7 mV·dec-1)40 and ALD50Pt/NGNs (29 mV·dec-1).45 The turnover frequency (TOF) also reveals the intrinsic catalytic activity and is calculated based on reported method.46 At overpotential of 30 mV, the TOF of 5.9 wt% Ir/gC3N4/NG reaches to 5.50 s-1, which is 18.3, 4.3 and 8.4 folds than those of pure Ir, 5.7 wt% Ir/NG and 20 wt% Pt/C catalysts (Table S2), respectively. The mass activity is an important criteria and the various catalysts is also calculated, as shown in Figures 3E and S10. The 5.9 wt%



Ir/g-C3N4/NG catalyst offers the mass acitivity (5.52 A·mgmetal-1) at the overpotantial of 30 mV. The value is about 18.3, 4.3 and 8.6 larger than those of pure Ir, 5.7 wt% Ir/NG and 20 wt% Pt/C catalysts, respectively. The electrochemcial activity of 5.9 wt% Ir/g-C3N4/NG is attributed to the fast kinetics of HER process. Additionally, the HER activity of 5.9 wt% Ir/g-C3N4/NG is also better than other reported catalysts (Table S3), indicating it is highly efficient catalysts for HER.

Figure 3. (A) LSV curves of g-C3N4/NG, 6.2 wt% Ir/g-C3N4, pure Ir, 5.7 wt% Ir/NG, 5.9 wt% Ir/g-C3N4/NG and 20 wt% Pt/C catalysts for HER with 95% iR compensation; (B) LSV curves of different content Ir for Ir/g-C3N4/NG catalysts for HER with 95% iR compensation; (C) LSV curves of different content g-C3N4 for Ir/g-C3N4/NG catalysts for HER with 95% iR compensation; (D) The Tafel slopes of 5.9 wt% Ir/g-C3N4/NG, 5.7 wt% Ir/NG and 20 wt% Pt/C catalysts; (E) the mass activity of as-prepared catalysts at overpotential of 20 mV, 30 mV, 40 mV and 50 mV; (F) The stability of 5.9 wt% Ir/g-C3N4/NG catalyst.


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Stability is another important factor of HER catalysts. The durability of the catalysts is conducted by the cyclic voltammetry (CV) and chronopotentiometry methods. Figure 3F shows a slight activity loss (6 mV) after 2000 CV cycles. After the 36,000 s chronopotentiometry test, the potential shift is less than 0.03 V (Figure S11). The morphology and composition of the 5.9 wt% Ir/g-C3N4/NG are also investigated. After a long time test, morphology (Figure S12) and composition (Figure S13) of the 5.9 wt% Ir/g-C3N4/NG are the almost unchanged. Oxygen evolution reaction. In general, Ir based composites are considered as state-of-the-art catalysts for OER in acidic media. Before the OER test, the surface of Ir nanoparticles is transformed into oxidized state (Figure S14). Figures 4, S15 and S16 show the anodic polarization curves of as-prepared catalysts for OER in O2 saturated 0.5 M H2SO4. From Figure S15, the 6.2 wt% Ir/g-C3N4 and g-C3N4/NG exhibit insignificant current for OER. Compared with the pure Ir, all Ir/g-C3N4/NG and 5.7 wt% Ir/NG catalysts exhibit obviously enhancement on the OER performance (Figures 4A and 4B). Further electrochemical characterization suggested that 5.9 wt% Ir/g-C3N4/NG catalyst exhibits the largest OER enhancement, it affords the current density of 10 mA·cm-2 at only of 287 mV, while the 5.7 wt% Ir/NG shows an overpotential at 330 mV, which implied g-C3N4 play an important role in OER process (Figure 4C). Additionally, the 5.9 wt% Ir/g-C3N4/NG also delivers greatly electrocatalytic ability for OER as compared with IrO2/g-C3N4/NG with similar content of Ir (Figure S17). To evaluate the improved OER activity, the mass activity of obtained catalyst is calculated and as shown in Figure 4D. With incorporating of g-C3N4, the 5.9 wt% Ir/g-C3N4/NG catalyst exhibits the highest mass activity (2.31 A·mgIr-1), which is 2.8 and 4.4 folds higher than those of 5.7 wt% Ir/NG (0.81 A·mgIr-1) and pure Ir (0.52 A·mgIr-1), respectively.



Figure 4. (A) LSV curves of 4.8 wt% Ir/g-C3N4/NG, 5.9 wt% Ir/g-C3N4/NG, 8.1 wt% Ir/gC3N4/NG, 5.7 wt% Ir/NG and pure Ir for OER with 95% iR compensation; (B) LSV curves of 6.1 wt% Ir/g-C3N4/NG, 5.9 wt% Ir/g-C3N4/NG, 5.6 wt% Ir/g-C3N4/NG and 5.7 wt% Ir/NG for OER with 95% iR compensation; (C) The overpotential of OER at current densities of 10 mA·cm−2 of as-prepared catalysts; (D) The mass activity of as-prepared catalysts at the overpotential of 300 mV; (E) The Tafel slopes of 5.9 wt% Ir/g-C3N4/NG and 5.7 wt% Ir/NG catalysts; (F) The stability of 5.9 wt% Ir/g-C3N4/NG catalyst. The Tafel slope is also an inherent property activity of catalysts, which reflects the rate controlling step of OER process. The Tafel slope of 5.9 wt% Ir/g-C3N4/NG catalyst is 72.8 mV·dec-1 (Figure 4E), indicating that the 5.9 wt% Ir/g-C3N4/NG has a faster charge transfer kinetic. According to the previous report,47 the Tafel slop of 72.8 mV·dec-1 reflects a chemical rate is the determining step, in which an intermediate OH species is rearranged via a surface


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reaction. Table S4 has summarized the OER activity of this catalyst and some of the previously reported catalysts for comparison. It is seen that the 5.9 wt% Ir/g-C3N4/NG catalyst exhibits an outstanding OER activity. To deeply understand the intrinsic property activity of catalysts, the turnover frequency (TOF) of as-prepared catalyst is shown in Table S2. Compared with pure Ir (0.26 s-1) and 5.7 wt% Ir/NG (0.41 s-1), the 5.9 wt% Ir/g-C3N4/NG exhibits high TOF (1.15 s-1), indicating an excellent intrinsic activity, which suggest that the g-C3N4/NG can facilitate the kinetics for OER.

Figure 5. (A) Polarization curve of the overall water splitting of 5.9 wt% Ir/g-C3N4/NG catalyst; (B) The LSV curves of 5.9 wt% Ir/g-C3N4/NG catalyst for HER and OER.

The stability is also an important criterion for OER electrocatalysts. The long-term stability of the 5.9 wt% Ir/g-C3N4/NG is evaluated by the CV (Figure 4F) and chronopotentiometry technique (Figure S18). From the Figure 4F, there is only a slight activity loss (15 mV) after 2000 CV cycles. Furthermore, the morphology and composition of 5.9 wt% Ir/g-C3N4/NG are also investigated after OER test. Form Figure S19, there is no obviously change in morphology, indicating the good stability in the acidic conduction for 5.9 wt% Ir/g-C3N4/NG. From the XPS



spectrum of Ir 4f (Figure S20), the peak shifts to higher value, indicating that the surface of Ir nanoparticles is oxidized. According to previous reports, the process is irreversible oxidation.6, 16 The design of highly efficient bifunctional catalysts for HER and OER has drawn widespread attention in recent years. Nevertheless, a few works focus on developing catalysts in acidic electrolyte for overall water splitting. Herein, the 5.9 wt% Ir/g-C3N4/NG nanocomposite are employed as bifunctional catalysts for overall water splitting (Figure 5A), with the corresponding polarization curves (the red one). The voltage difference (∆V) between the HER and OER reactions are calculated from the data in Figure 5B. The ∆V (the black one) fits with the curve of 5.9 wt% Ir/g-C3N4/NG for overall water splitting (Figure 5A). From the Figure 5A, the 5.9 wt% Ir/g-C3N4/NG catalyst affords a current density of 10 mA·cm-2 at a cell voltage of 1.56 V in acidic electrolyte. The value is lower than the recently reported bifunctional catalysts in acidic electrolyte, such as IrNi NCs (1.58 V at 10 mA·cm-2)6 and IrCoNi/CFP (1.52 V at 2 mA·cm-2).16 Furthermore, the stability of 5.9 wt% Ir/g-C3N4/NG catalysts for overall water splitting is evaluated by chronopotentiometry technique at the current density of 10 mA·cm-2 (Figure S21) in the 0.5 M H2SO4. Indeed, the impressive electrocatalytic activity of 5.9 wt% Ir/g-C3N4/NG is due to the fast charge transport process and large electrochemical active surface area. From Figure S22, the electrochemical impedance spectroscopy (EIS) is employed to investigate the electrochemical kinetics for HER and OER. According to pervious reports, the low value of resistance of the charge-transfer (Rct) indicates a fast reaction rate.40, 48 The 5.9 wt% Ir/g-C3N4/NG exhibits rapid charge transfer process due to the low value of Rct.


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Figure 6. Cyclic voltammetry (CV) graphs of (A) 5.9 wt% Ir/g-C3N4/NG, (B) 6.2 wt% Ir/gC3N4 and (C) 5.7 wt% Ir/NG at different scan rates at 20, 60, 100, 140 and 180 mV·s−1; and (D) plots of the current density versus the scan rate for 5.9 wt% Ir/g-C3N4/NG, 5.7 wt% Ir/NG and 6.2 wt% Ir/g-C3N4. The Brunauer-Emmett-Teller test shows the 5.9 wt% Ir/g-C3N4/NG has highest specific surface area (Figure S23), which is beneficial to electrochemical water splitting. To evaluate the electrochemically active surface areas of as-prepared catalysts, the double layer capacitance (Cdl) are measured by CV method. The CV curves are obtained at different scan rates (20, 60, 100, 140 and 180 mV·s−1) in the potential range of 0.7-0.9 V vs. RHE (Figure 6). The halves of the anodic and cathodic current density at 0.8 V vs. RHE are plotted versus the scan rates and shown in Figure 6D. The Cdl of 5.9 wt% Ir/g-C3N4/NG catalyst is 26.7 mF·cm-2, which is 1.62 and 127 times than that of 5.7 wt% Ir/NG and 6.2 wt% Ir/g-C3N4 catalysts, respectively. This increase in electrochemically active surface area results in the enhanced electrocatalytic performance. According to previous reports, the g-C3N4/NG hybrid can also efficiently facilitated the kinetics



of overall electrochemical water splitting.25 Thereby, the 5.9 wt% Ir/g-C3N4/NG catalyst exhibits high activity toward overall electrochemical water splitting.

CONCLUSIONS In summary, we presented a simple method for the direct synthesis of g-C3N4/NG supported ultra small Ir nanoparticle with low content. The g-C3N4/NG can effectively enhance the catalytic kinetic and prevent Ir nanoparticles from aggregation to help expose active sites. The catalysts exhibit high activities for both HER and OER in 0.5 M H2SO4. For HER, the 5.9 wt% Ir/g-C3N4/NG displays the best activity, and exceeds commercial Pt/C. For OER, 5.9 wt% Ir/gC3N4/NG can achieve current density of 10 mA·cm−2 at the low overpotential of 287 mV. As a bifunctional electrocatalyst for overall water splitting, it reaches the current density of 10 mA·cm−2 at potential of 1.56 V. Considering the electrocatlalytic activity of water splitting and the simple synthesized approach, we believe that the Ir/g-C3N4/NG catalyst holds greatly potential in fields of electrocatalysis and energy conversion.

Methods Synthesis of Ir/g-C3N4/NG nanocomposite. Ir/g-C3N4/NG nanocomposite was synthesized via the simple chemical reduction approach. Firstly, g-C3N4/NG (10 mg) was added in ethylene glycol (20 mL) by ultrasound to form the homogeneous dispersion. Then, the different volume of hexachloroiridium acid hydrate (H2IrCl6·xH2O, Ir 0.035% in HCl) was added. The mixed solution was transferred into a Teflon-lined autoclave and heated at 200 °C for 12 h. Finally, the product was washed with double distilled water and absolute ethanol, respectively. And it was


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dried in a vacuum drying oven at 30 °C. The Ir, Ir/NG and Ir/g-C3N4 nanocomposites were obtained by a similar procedure. The preparation of catalysts for electrochemical characterizations. To prepared working electrode, 2.00 mg catalysts were dispersed the 1 mL mixed solution (the volume ratio of water, isopropanol and Nafion solution (0.5 wt%): 5 : 3 : 2) by ultrasonically. The 4.0 µL of the catalyst ink (loading amount is 6.7 µgIr·cm-2 for 5.9 wt% Ir/g-C3N4/NG, determined by ICP-MS) was loaded onto a glassy carbon electrode (GCE) (area: 0.0707 cm2).

ASSOCIATED CONTENT Supporting Information. The experimental details, the structural characterization, the electrocatalytic data, the stability data, the TEM image and XPS after stability test, the electrochemical impedance spectroscopy test for HER and OER, the TOF for HER and OER, the comparison of recently reported HER and OER catalysts.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (T Wang), and [email protected] (MW Shao)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes



The authors declare no competing financial interests. ACKNOWLEDGMENT The project was supported by the Ministry of Science and Technology of China (Grant 2017YFA0204800), 111 project, Natural Science Research Project of Anhui Province Education Department (KJ2017A350, AQKJ2015B001), the National Natural Science Foundation of China (No. 21501004). REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. (3) Zou, X. X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (4) Wang, J. H.; Cui, W.; Liu, Q.; Xing, Z. C.; Asiri, A. M.; Sun, P. X. Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv. Mater. 2016, 28, 215-230. (5) Li, J.; Zheng, G. F. One-Dimensional Earth-Abundant Nanomaterials for Water-Splitting Electrocatalysts. Adv. Sci. 2017, 4, 1600380. (6) Pi, Y. C.; Shao, Q.; Wang, P. T.; Guo, J.; Huang, X. Q. General Formation of Monodisperse IrM (M = Ni, Co, Fe) Bimetallic Nanoclusters as Bifunctional Electrocatalysts for Acidic Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1700886.


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Graphical Abstract


Nitrogen-Doped Graphene Nanocomposites as Bifunctional

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