Ultrathin Graphdiyne-Wrapped Iron Carbonate Hydroxide Nanosheets

8 hours ago - FeCH@GDY/NF (Figure 4a) exhibits the best OER activity giving the smallest overpotentials (η) of 260 mV and 328 mV to drive 10 and 100 ...
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Ultrathin Graphdiyne-Wrapped Iron Carbonate Hydroxide Nanosheets toward Efficient Water Splitting Lan Hui,†,‡ Dianzeng Jia,*,† Huidi Yu,‡ Yurui Xue,*,‡,§ and Yuliang Li*,‡,§ †

Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, P. R. China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P.R. China ‡

S Supporting Information *

ABSTRACT: We employed a two-step strategy for preparing ultrathin graphdiyne-wrapped iron carbonate hydroxide nanosheets on nickel foam (FeCH@GDY/NF) as the efficient catalysts toward the electrical splitting water. The introduction of naturally porous GDY nanolayers on FeCH surface endows the pristine catalyst with structural advantages for boosting catalytic performances. Benefited from the protection of robust GDY nanolayers with intimate contact between GDY and FeCH, the combined material exhibits high long-term durability of 10 000 cycles for oxygen-evolution reaction (OER) and 9000 cycles for hydrogen evolution reaction (HER) in 1.0 M KOH. Such excellent bifunctional OER/HER performance makes FeCH@ GDY/NF quite qualified for alkaline two-electrode electrolyzer. Remarkably, such electrocatalyst can drive 10 and 100 mA cm−2 at 1.49 and 1.53 V, respectively. These results demonstrate the decisive role of GDY in the improvement of electrocatalytic performances, and open up new opportunities for designing cost-effective, efficient, and stable electrocatalysts for sustainable oxygen/hydrogen generation. KEYWORDS: carbon material, graphdiyne, iron-layered double hydroxide, heterostructure, bifunctional electrocatalysts, overall water splitting

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INTRODUCTION Electrical splitting water into oxygen and hydrogen provides an efficient and sustainable strategy for renewable energy technologies.1−10 But the sluggish OER kinetics seriously limited the overall water splitting efficiency, which causes a large overpotential at operating condition. Precious electrocatalysts (e.g., RuO2 for OER; Pt-based materials for HER) are currently viewed as benchmarking electrocatalysts, however, the high-cost and scarcity greatly hampered their widespread applications.3,11 Besides, electrocatalysts efficient in acidic media might be inactive in basic one and vice versa. Obviously, integrating these different OER-HER electrocatalysts together for overall water splitting (OWS) would cause larger operation complexity, higher cost and lower catalytic efficiency. Developing low-cost and efficient electrocatalysts, which can work in the same electrolyte (especially in basic one), is therefore of great interest and significance.9,12−14 Numerous of electrocatalysts (e.g., transition metal (TM)-based hydroxides,14,15 oxides,14,16−18 sulfides,19 nitrides20 and phosphides21−23) have been recently developed to meet the practical demand. However, their low conductivity, catalytic activity, and stability have seriously restricted their practical uses. Recently, incorporating catalysts with conductive substrates (e.g., graphene,19 carbon nanotubes,22,24 Ni foam,23 and C3N418,25) has been considered as an alternative way for © XXXX American Chemical Society

fabricating new electrocatalysts. Although this strategy showed improvements in water splitting performance, the high overpotentials of the obtained electrocatalysts are still far from the commercialization for global energy demand. The design of new high-performance and low-price electrocatalysts is still the critical challenge. Graphdiyne (GDY),26−32 a novel 2D monolayer of sp/sp2hybridized carbons, has been first synthesized by our group33 and has been successfully applied in numerous fields, for example, electrocatalysis34−40 (e.g., HER, OER, oxygen reduction reaction (ORR) and OWS), energy storage,41−44 lithium-ion batteries,45−47 solar cells,48−51 etc. Remarkably, each benzene ring is bridged by butadiyne linkages (−CC− CC−), endowing it with uniformly distributed triangular pores, high electronic conductivity, structural flexibility and chemical stability.31−33,52 The porous structure is very beneficial for the electron and mass transport and the gas evolution. Our recent experiments demonstrated the interaction between TMs and GDY could cause an evident electrons Special Issue: Graphdiyne Materials: Preparation, Structure, and Function Received: January 31, 2018 Accepted: March 15, 2018

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DOI: 10.1021/acsami.8b01887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Optical photographs of (i) pure Ni foam, (ii) FeCH/NF, and (iii) FeCH@GDY/NF. Low- and high-magnification SEM images of (b,c) pure Ni foam, (d−f) FeCH/NF, (g−i) FeCH@GDY/NF, respectively. Electrocatalyst Characterizations. FESEM (Hitachi Model S− 4800) and TEM (JEM−2100F, 200 kV) were used for morphological characterizations. X−ray powder diffraction (XRD) and X−ray photoelectron spectroscopy (XPS) were studied by a Rigaku D/ max−2500 rotation anode x−ray diffractometer with monochromatized Cu Kα radiation (λ = 1.54178 Å) and Thermo Scientific ESCALab 250Xi with 200 W monochromated Al Kα radiation, respectively. Electrochemical Measurements. CHI−760E electrochemical workstation was used for electrochemical studies. The as-synthesized samples, graphite plate, saturated calomel electrode (SCE) were employed as the working electrode, counter electrode and reference electrode, respectively. The OER and HER were measured in O2saturated/H2-saturated 1.0 M KOH at 1 mV s−1. .

transport from metal to surrounding carbon atoms in GD, which is very beneficial for improving catalytic activity.34 Besides, the presence of triple bonds results in many charged carbon atoms in GDY, which makes it rich in active sites for catalysis. The coating of GDY on the surface of electrocatalysts could therefore result in a new class of thermally and electrically conducting, catalytically active and chemically inert surfaces. Inspiringly, GDY would be an ideal material for fabricating high-performance electrocatalysts. In this study, we report the successful growth of ultrathin graphdiyne-wrapped iron carbonate hydroxide nanosheets array on Ni foam (FeCH@GDY/NF). The underlying concepts to this work is to design an electrocatalyst that can function as efficient bifunctional OER/HER electrodes in the same alkaline electrolyte. We demonstrate the excellent OER and HER performances of this FeCH@GDY/NF with high catalytic current densities, small Tafel slopes and high durability outperforming all recently reported TMCH catalysts. When used as bifunctional electrocatalysts in an alkaline water electrolyzer, it can drive 10 and 100 mA cm−2 at 1.49 and 1.53 V, respectively.

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RESULTS AND DISCUSSION A two-step strategy was used for the design and synthesis of graphdiyne-wrapped iron carbonate hydroxide nanosheets array on a nickel foam substrate (FeCH@GDY/NF), including the first synthesis of iron carbonate hydroxide nanosheets array on nickel foam (FeCH/NF) followed by the in situ growth of a ultrathin graphdiyne coatings on its surface. Figure 1a shows optical photographs of samples, revealing the 3D porous characteristics of them, which could provide accessible channels for ion/mass transport and diffusion during the water splitting process. The originally silver colored Ni foam changed to reddish brown, and finally to black with the large pores in the foam still visible. SEM images (Figure 1b, c) indicated that the NF has a 3D porous structure and smooth surface. Figure 1d−f shows the SEM images of FeCH/NF recorded at different areas. As can be seen, a layer of interconnected FeCH

EXPERIMENTAL SECTION

Preparation of FeCH@GDY/NF. A polytetrafluoroethylene (Teflon)−lined stainless steel autoclave containing Fe(NO3)3 (10 mmol), CO(NH2)2 (45 mmol) and a piece of freshly cleaned Ni foam was kept at a temperature of 120 °C for 6 h. After completion of the reaction, the brown FeCH/NF were obtained. FeCH/NF was then used as the substrate for growing graphdiyne layer according to ref 33. The synthesized FeCH@GDY/NF sample was then used for experiments. B

DOI: 10.1021/acsami.8b01887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a, b) TEM images, (c) electron diffraction pattern and (d) high-resolution TEM image of FeCH nanosheets. (e) Scanning TEM with elemental mapping images of (f) Fe and (g) O of FeCH. (h, (i) TEM images, (j) electron diffraction pattern, and (k, l) high-resolution TEM images of FeCH@GDY nanosheets. (m) STEM and elemental mapping images of C, Fe, and O of FeCH@GDY.

images (Figure 2e−g) confirmed the uniform distribution of Fe and O elements in FeCH. The same to SEM images, slightly winkled folds can be seen from the TEM images of FeCH@ GDY nanosheets (Figure 2h, i). The single crystallinity of the sample is also maintained (Figure 2j). The measured lattice fringe spacing of FeCH@GDY (0.242 nm, Figure 2k, l) is smaller than that of FeCH (0.245 nm, Figure 2d), which might be due to the strong associate interactions between FeCH and GDY. STEM image and EDX mapping images (Figure 2m) show the homogeneous distribution of C, Fe, and O elements within the whole nanosheets. Typical X-ray powder diffraction (XRD) patterns of asprepared samples were shown in Figure 3a. Excluding the typical peaks (44.4 and 51.8°) originating from the NF, the diffraction peaks of our samples were found to be in good agreement with iron carbonate hydroxide hydrate (Fe2(OH)2CO3; JCPDS No. 33−0650). After in situ growth of the GDY layer, there are obvious shifts of the diffraction peaks to lower angles, which could be due to the compressive strain resulting from the existence of the GDY thin layer and

nanosheets are vertically aligned and uniformly distributed on the Ni foam, forming an ordered array, which can act as the substrate for the growth of GDY layer. After the cross-coupling reaction, the FeCH@GDY nanosheets (Figure 1g−i) shows ultrathin nanosheet morphology similar to that of FeCH. FeCH@GDY nanosheets are grown upright on top of the Ni foam, maintaining the ordered array morphology, but with more wrinkled folds, which might originate from the strong interactions between GDY and FeCH. This would result in larger contact area and more available adsorption sites, thus generating more catalytic sites during water splitting. Energydispersive X-ray spectroscopy (EDS) analysis confirmed the only existence of Fe, C, and O elements FeCH@GDY nanosheet (Figure S1). TEM images further confirm the successful synthesis of 2D and ultrathin FeCH nanosheets (Figure 2a, b). The selectedarea electron diffraction (SAED) pattern collected from the nanosheet indicated the single crystallinity of the sample (Figure 2c). The lattice fringe spacing of FeCH (Figure 2d) is 0.245 nm. Scanning TEM (STEM) and elemental mapping C

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Figure 3. (a) XRD patterns of (i) NF, (ii) FeCH@GDY/NF, and (iii) FeCH/NF. (b) Raman spectra and (c) C 1s core-level XPS spectra of (i) FeCH@GDY/NF and (ii) GDY, respectively. (d) Fe 2p core-level XPS spectra of (i) FeCH@GDY/NF and (ii) FeCH/NF.

Figure 4. (a) CV curves and (b) Tafel slopes of samples toward OER in 1.0 M KOH. (c) LSV curves of FeCH@GDY/NF and FeCH/NF recorded before and after respective OER cycling tests. FeCH/NF shows a 33% decrease in j at 1.6 V. (d) Polarization curves and (e) Tafel slopes of samples toward HER in 1.0 M KOH. (f) LSV curves of FeCH@GDY/NF and FeCH/NF recorded before and after respective HER cycling tests. FeCH/NF shows a 42% decrease in j at −0.4 V.

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DOI: 10.1021/acsami.8b01887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) CV curves for as-synthesized samples in a two-electrode system; (b) time-dependent current density curves of FeCH@GDY/NF at 10 mA cm−2 in an alkaline electrolyzer.

Figure 6. (a) Nyquist plots for as-synthesized samples. (b) Magnified plots of a. (c) CV curves of FeCH@GDY/NF in potential range of −0.19− 0.09 V at 10 mV s−1 (red), 40 mV s−1 (purple), 80 mV s−1 (orange), 120 mV s−1 (pinkish purple), 180 mV s−1 (blue), and 220 mV s−1 (black). (d) Dependence of capacitive current density on scan-rates.

interactions between the FeCH and GDY, making FeCH@ GDY/NF more favorable for electrochemical water splitting. OER performances of the as-prepared samples was first studied. FeCH/NF, GDY/NF, RuO2 and NF were used as references. FeCH@GDY/NF (Figure 4a) exhibits the best OER activity giving the smallest overpotentials (η) of 260 mV and 328 mV to drive 10 and 100 mA cm−2, which outperform most of the recently reported nonprecious OER electrocatalysts (please see comparisons in the Table S1). FeCH@GDY/NF shows the smallest Tafel slope of 54.5 mV dec−1 among all samples (Figure 4b). The fact that FeCH@GDY/NF exhibits larger current density and smaller Tafel slopes than FeCH/NF and GDY/NF indicate the introduction of GDY could effectively improve the catalyst’s activity. Long-term durability is also an important merit for practical applications. For pure FeCH/NF (Figure 4c), the current density decreased from 108 mA cm−2 to 73 mA cm−2 (−33%) after only 4000 cycles, although there is almost no loss in current density for FeCH@ GDY/NF even after 10 000 cycles. Besides, we examined the morphology of FeCH@GDY/NF after cycling tests. As shown in Figure S2, the morphologies were well preserved, revealing its excellent structural stability. These results demonstrated the decisive role of GDY in improving electrocatalytic perform-

lead to a shrinkage of the unit cell, in accordance with the above HRTEM results (Figure 2). In the Raman spectra (Figure 3b), the successful preparation of FeCH@GDY/NF was evidenced by the presence of typical peaks at 478, 578, 680, 1382.2, 1589.8, 1928.7, and 2182.7 cm−1. The peaks at 478 cm−1 correspond to Fe2+−OH and Fe3+−OH stretching vibration modes.50 The vibration modes appeared in the range of 500− 800 cm−1 can be assigned to the M−O, O−M−O, and M−O− M (M = Fe) vibrations.53,54 The band centered at 1382.2 and 1589.8 cm−1 are assignable to D band and G band of GDY.32 Moreover, the typical peaks corresponding to the vibration of the triple bond were observed at 1928.7 and 2182.7 cm−1.32 The increase in the intensity ratio of the D and G-bands (ID/IG; for GDY, ID/IG = 0.88; for FeCH@GDY/NF, ID/IG = 0.95) indicates the increased defects in FeCH@GDY/NF, which means more active sites are formed. C 1s XPS spectra for pure GDY and FeCH@GDY/NF was shown in Figure 3c. Compared with pure GDY, we see clearly an additional π → π* transition peak at 290.8 eV for FeCH@GDY/NF. The peak area ratio of sp/sp2− is 2, indicating each benzene ring are linked by diine in GDY layer. As shown in Figure 3d, an obvious larger energy shift in Fe 2p (from 711.7 to 713.1 eV) was observed for FeCH@GDY/NF compared to FeCH/NF sample. These results indicated the presence of the electron E

DOI: 10.1021/acsami.8b01887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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enables efficient ion/mass transfer to the catalyst’s active sites. The obvious associate interactions between GDY and FeCH as evidenced by XRD, XPS and Raman spectra would be helpful to accelerate the sluggish kinetics for water splitting, thus leading to greatly improved overall water splitting efficiency. Furthermore, the incorporation of robust and chemical inert GDY on FeCH not only provides an excellent coating to effectively prevent the corrosion, but also reduces the solution and charge transfer resistance during the catalysis. Finally, the 3D morphology of FeCH@GDY/NF can effectively promote the gas evolution and release from the architecture because of its open framework, resulting in extraordinary durability.58,59

ances. The Faradaic efficiency of OER on FeCH@GDY/NF was found to be about 92% (Figure S3a). The HER performance of FeCH@GDY/NF was further studied. FeCH@GDY/NF shows a small ηof 148 mV at 10 mA cm−2 (Figure 4d), compared favorably with other reported electrocatalysts (Table S2). As shown in Figure 4e, FeCH@ GDY/NF showed a small Tafel slope of 84.7 mV dec−1, very close to that of Pt (82.1 mV dec−1) (Figure 4e). This indicated the Volmer−Heyrovsky mechanism of FeCH@GDY/NF in HER process,55 in which Heyrovsky is the rate-limiting step. As shown in Figure 4f, the polarization curves for FeCH@GDY/ NF showed a negligible variation in current density after 9000 cycles. Whereas for pure FeCH/NF (inset of the Figure 4f), the current density decreased from 195.4 mA cm−2 to 113.7 mA cm−2 (−42%) after only 2600 cycles. The Faradaic efficiency of the HER on FeCH@GDY/NF was about 94% (Figure S3b). SEM images (Figure S4) measured after the continuous cycling tests show almost no changes in morphology, indicating the excellent structural stability of FeCH@GDY/NF electrodes toward HER in alkaline conditions. These results demonstrate that the GDY does in fact improve the HER performance of FeCH. Inspired by the superior bifunctional OER/HER performances of FeCH@GDY/NF, the FeCH@GDY/NF was used for overall water splitting in 1.0 M KOH. Figure 5a shows the CV curves of samples. FeCH@GDY/NF||FeCH@GDY/NF shows much higher performance than others; it requires only 1.49 and 1.53 V to reach 10 and 100 mA cm−2, respectively. This is much better than other reported electrodes (Table S3) such as NiFe LDH/Ni foam (1.70 V@10 mA cm−2),56 Co1Mn1CH/NF (1.67 V@10 mA cm−2),15 and Fe−CoP (1.60 V@10 mA cm−2).57 Long-term stability were also determined conducted, which showed that the FeCH@GDY/NF||FeCH@GDY/NF electrolyzer was exceptionally stable with a negligible overpotential loss (Figure 5b). To gain deeper insights into the electron and charge transfer ability of the electrocatalysts, electrical impedance spectra (EIS) was measured (Figure 6a,b) and fitted to an equivalent circuit model (Figure S5, Table S4). FeCH@GDY/NF exhibits the solution resistance (Rs) of 2.46 Ω and the charge transfer resistance (Rct) of 15.98 Ω, much smaller than that of FeCH/ NF (Rs = 3.0 Ω, Rct = 1833 Ω), and GDY/NF (Rs = 3.57 Ω, Rct = 620.7 Ω). We attribute the greatly improved charge transfer behavior and water splitting kinetics to the introduction of GDY, an intrinsic active and highly electrical conductive element, in the electrocatalyst.33 The electrochemical surface area (ECSA) of electrocatalysts was further determined through a cyclic voltammograms (CVs) method (Figure 6c, d; Figure S6).3,11 FeCH@GDY/NF shows the Cdl value of 1.0 mF cm−2 which is about 2.6 times and 7.7 times than FeCH/NF (0.39 mF cm−2) and GDY/NF (0.13 mF cm−2), respectively, suggesting a significantly increased active surface area after the coating of thin GDY layer. According to above-discussed, FeCH@GDY/NF has significant advantages in water splitting. First, the 2D nanosheet morphology can result in larger contact area and more catalytic sites, which are beneficial to the catalytic activity. Second, the introduction of intrinsic active and highly electrical conductive GDY in the electrocatalyst can greatly improve catalyst’s charge transfer behavior and water splitting kinetics. It is possible that the graphdiyne changed the structure of iron carbonate hydroxide nanosheets and thus induced the increase of active sites. Besides, the naturally porous structure of GDY coating

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CONCLUSIONS In summary, we reported the ultrathin graphdiyne-wrapped iron carbonate hydroxide nanosheets on nickel foam (FeCH@ GDY/NF) could as the efficient bifunctional OER/HER catalyst for overall water splitting in 1.0 M KOH. Our experiments indicated that the introduction of GDY endows the catalysts with high ECSA, improved charge transport behavior, facilitated kinetics, and extraordinary long-term stability. FeCH@GDY/NF can deliver 10 mA cm−2 at η of 260 mV for OER and 148 mV for HER and shows high durability (at least 10 000 cycles for OER, and 9000 for HER). When used as a two-electrode electrolyzer, FeCH@GDY/NF drives 10 and 100 mA cm−2 at 1.49 and 1.53 V, respectively. All these results solidly demonstrated the GDY has intrinsically enhanced the catalytic performances of pristine catalysts. Our results marks an important step forward in designing efficient electrocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01887. EDS image of freshly prepared FeCH@GDY/NF, SEM, and XPS results of FeCH@GDY/NF after stability tests, CV curves of as-prepared catalysts, comparison of the OER and overall water splitting performances (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.J.). *E-mail: [email protected] (Y.X.). *E-mail: [email protected] (Y.L.). ORCID

Yuliang Li: 0000-0001-5279-0399 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Key Research and Development Project of China (2016YFA0200104), the National Nature Science Foundation of China (21790050 and 21790051), and the Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-SLH015).

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