Graphdiyne@NiOx(OH)y heterostructure for efficient overall water splitting

Chao Zhang ac, Yurui Xue *ab, Lan Hui ac, Yan Fang ac, Yuxin Liu ac and Yuliang Li *abc
aInstitute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. E-mail: xueyurui@iccas.ac.cn; ylli@iccas.ac.cn
bScience Center for Material Creation and Energy Conversion, School of Chemistry and Chemical Engineering, Institute of Frontier and Interdisciplinary Science, Shandong University, Jinan 250100, P. R. China
cUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China

Received 25th March 2021 , Accepted 13th May 2021

First published on 13th May 2021


Abstract

Graphdiyne (GDY), a rising star of two-dimensional (2D) carbon materials consisting of unique sp-/sp2-cohybridized carbon atoms, has been demonstrated to be an ideal platform for developing efficient catalysts with high activity and long-term durability. In this work, a new GDY-based heterostructure of GDY@NiOx(OH)y was successfully synthesized and used as an efficient electrocatalyst for water splitting. The creative incorporation of GDY with NiOx(OH)y endows the heterostructure with unique advantages; for example, the mixed valent Ni species, the strong interactions between GDY and NiOx(OH)y, and the greatly enhanced charge transfer ability, which are significantly beneficial for enhancing the catalytic activity and long-term durability. When applied in alkaline OWS electrolysis, GDY@NiOx(OH)y||GDY@NiOx(OH)y exhibited a voltage of only 1.54 V to achieve 10 mA cm−2, with robust stability over 100 h at 20 mA cm−2, much better than most of the reported benchmark catalysts, making them outstanding among water-splitting electrocatalysts.


Introduction

Electrochemically splitting water into hydrogen and oxygen through hydrogen and oxygen evolution reactions (HER/OER; H2O → H2 + O2) provides a promising route for H2 production to conquer the serious energy crisis and environmental problems.1–5 However, the sluggish kinetics of these electrode reactions still largely limit the overall efficiency and lead to large overpotentials. The ideal catalysts for catalysing water splitting are noble-metal-based materials, such as Pt-based materials for the HER and Ru/Ir-based materials for the OER. But the high cost, scarcity, and low stability under harsh conditions have hindered their practical applications. In the past few years, scientists have made great efforts to synthesize transition metal-based catalysts, such as sulphides,6,7 oxides,8–10 phosphides,11–13,54,55 and hydroxides,14–16 for efficient water splitting. However, the practical application was limited by their low conductivity, unsatisfactory activity and stability. Nickel (Ni) is one of the most abundant elements present on the earth and has many advantages that are beneficial for synthesizing high-performance catalysts, such as low-cost and multiple valence states. Among Ni-based materials, nickel hydroxide is the simplest nickel-based material and has a good behaviour for the decomposition of H–OH, which is beneficial for improving the OER performances.17 But it was still constrained by low activity for converting the hydrogen intermediates into H2,18–20 thus restricting its use as a bifunctional catalyst for OWS.

GDY, a novel carbon allotrope containing sp and sp2 carbon atoms, has shown many unique properties including π-conjugated networks, unique electronic and chemical structures, unevenly distributed surface charge, natural porous structures, high conductivity, and excellent chemical stability,21–24 and exhibited transformative performances in various fields from catalysis to energy conversion and storage, etc.25–57 Our recent studies have demonstrated that the superior advantages of GDY enable the facile and controllable fabrication of the ideal interface structure with high activity. Besides, the valence states of the active sites could be effectively tuned by GDY for accelerated catalytic reaction kinetics. The porous structures of GDY could also facilitate the adsorption and desorption of intermediates, resulting in an improvement in the catalytic performances. Benefiting from these fascinating properties, great amounts of GDY-based electrocatalysts have been synthesized and applied in catalysis, such as HER, OER, OWS, ORR, NRR, and CO2RR.

In this work, we report a hierarchical three-dimensional (3D) heterostructure of GDY@NiOx(OH)y synthesized through a facile and controllable method (Fig. 1). The 3D porous structure of GDY@NiOx(OH)y provides the highest electrical conductivity and the largest electrochemical catalytically active surface, offering more active sites towards catalytic processes. Experimental results showed that the heterostructure had excellent catalytic activity and stability for the HER, the OER,and OWS, in alkaline conditions. When being used as both anodic and cathodic electrodes, the obtained alkaline electrolyzer requires a small cell voltage of 1.54 V (vs. RHE) to achieve 10 mA cm−2, and excellent long-term stability with almost no OWS activity loss over 100 h at 20 mA cm−2.


image file: d1qm00466b-f1.tif
Fig. 1 Schematic representation of the synthetic route for GDY@NiOx(OH)y.

Results and discussion

The GDY@NiOx(OH)y catalyst was prepared through a facile three-step strategy, including the first growth of graphdiyne film on carbon cloth, followed by the in situ growth of Ni(OH)2 nanosheets on graphdiyne surfaces forming GDY@Ni(OH)2. The as-prepared GDY@Ni(OH)2 sample was then annealed at 220 °C to form GDY@NiOx(OH)y (Fig. 1). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the morphology of the samples. Fig. S1 (ESI) shows that the carbon cloth possesses a 3D interweaved structure with smooth surfaces. Fig. 2a–c show the uniform growth of graphdiyne (GDY) nanosheet arrays on the smooth surface of carbon cloth without any aggregates after the cross-coupling reaction, forming a three-dimensional (3D) porous electrode, which could increase the electrochemical area. After a simple chemical deposition process, nickel hydroxide (Ni(OH)2) nanosheets with smooth surfaces were grown on the surface of GDY nanosheets. The GDY@Ni(OH)2 heterostructure was obtained (Fig. 2d–f). Subsequently, the samples were annealed at the temperature below the decomposition of Ni(OH)2 to remove the intramolecular adsorbed water which seriously limits the conductivity of the electrode. As shown in Fig. 2j–i, the resulting NiOx(OH)y showed more rough surface and possessed a larger pore structure, as compared to the pristine Ni(OH)2 nanosheets, beneficial for increasing the electrochemically active surface area and improving the mass transport efficiency and finally enhancing the catalytic performances.
image file: d1qm00466b-f2.tif
Fig. 2 Low and high magnification SEM images of (a–c) GDY nanowalls, (d–f) GDY@Ni(OH)2 nanosheets, and (g–i) GDY@NiOx(OH)y heterostructures.

Transition electron microscopy (TEM) images (Fig. 3a, b and Fig. S2a, ESI) of GDY@NiOx(OH)y nanosheets confirmed the porous characteristics of the NiOx(OH)y nanosheets. High-resolution TEM (HRTEM) images of GDY@NiOx(OH)y (Fig. 3c, d–h and Fig. S2b–d, ESI) show the layered arrangement of the heterostructures. The NiOx(OH)y nanosheets exhibited interplanar distances (0.116 nm and 0.175 nm) corresponding to (202) and (102) of Ni(OH)2 (Table S1, ESI), which was consistent with the results of the X-ray diffraction patterns (XRD). The interplanar distance of GDY was 0.361 nm, consistent with other reports.24–26 Besides, the HRTEM images clearly showed the intimate interaction between GDY and NiOx(OH)y nanosheets, which is beneficial for improving the electronic transport behaviour. The energy-dispersive X-ray spectroscopy (EDX) mapping images (Fig. 3e–h) revealed the uniform distribution of C, Ni, and O in the nanosheets.


image file: d1qm00466b-f3.tif
Fig. 3 (a and b) TEM images and (c and d) high-resolution TEM (HRTEM) images of GDY@NiOx(OH)y. Scanning TEM image (e) and corresponding elemental mapping images (f–h) of C, Ni and O in the GDY@NiOx(OH)y nanosheet.

The structural properties of the catalysts were first studied using X-ray diffraction (XRD). As shown in Fig. S3 (ESI), the nickel hydroxide hydrate obtained by a chemical precipitation method shows typical peaks at 11.2°, 22.5°, 33.6°, 34.5° 38.6° and 45.3° (Table S1, ESI), which correspond to (003), (006), (101), (012), (015), (113) and (018), respectively. For GDY@Ni(OH)2, the peaks at 22.5° and 38.6° disappeared, and other peaks shifted to larger angles, which indicates the presence of strong interactions between GDY and Ni(OH)2. After the annealing treatment, the hydrated water in nickel hydroxide was removed completely. GDY@NiOx(OH)y exhibits an obvious shift in the XRD peaks to smaller angles (Fig. 4a and Fig. S3, ESI). The peaks at 33.3°, 38.2°, and 59.6° could be attributed to the crystal plane of nickel hydroxide (JCPDS No. 14-0117). Fig. 4b shows the Raman spectra of the samples. The successful synthesis of GDY@NiOx(OH)y was demonstrated by the presence of typical peaks at 488.1, 1096.3, 1364.7, 1597.1, 1968.3 and 2174.1 cm−1, corresponding to the lattice vibration of NiOx species (488.1 cm−1), Ni(OH)2 species (1096.3 cm−1),58 and the D band (1364.7 cm−1), G band (1597.1 cm−1) and vibrations of the diacetylene bond (1968.3 and 2174.1 cm−1) of GDY,59 indicating the successful incorporation of GDY and NiOx(OH)y. Compared with GDY (ID/IG = 0.75), the ID/IG of GDY@NiOx(OH)y increased to 0.8, indicating the increased defects of the sample which implies the generation of more active sites.60Fig. 4c shows the X-ray photoelectron spectroscopy (XPS) survey spectra of the samples. GDY@NiOx(OH)y contain carbon, nickel and oxygen elements. As shown in Fig. 4d, the C 1s XPS spectra of GDY@NiOx(OH)y showed five sub-peaks at 284.5 (sp2 C–C), 285.1 (sp C–C), 286.5 (C–O), 288.6 (C[double bond, length as m-dash]O) and 290 eV (π–π* transition), respectively. The ratio of sp- to sp2-carbon is 2, confirming the integrity of GDY after chemical deposition and annealing at 220 °C. The additional π–π* satellite peak at 290 eV reflects the interactions between GDY and NiOx(OH)y. Compared with pure GDY, the peaks shifted to lower binding energies, indicating the charge transfer from NiOx(OH)y to GDY. For Ni 2p (Fig. 4e), Ni(OH)2 showed two sub-peaks at 855.7 and 861.4 eV, respectively, corresponding to the Ni2+ species;61–63 while two additional peaks at 856.7 and 864.7 eV corresponding to Ni3+ were observed for GDY@NiOx(OH)y (Fig. S5a, ESI).64 These results confirmed the mixed valent states of Ni species in GDY@NiOx(OH)y, which might be beneficial for enhancing the catalytic activities. Moreover, compared to pristine GDY@Ni(OH)2, the O 1s XPS spectrum of GDY@NiOx(OH)y shows an additional peak at 529.2 eV (Ni1–O, Fig. 4f), which indicates the formation of new nickel–oxygen species in the catalyst.65 Our additional experiments showed that there was no change in the valence states for pure Ni(OH)2 annealed under the same conditions (Fig. S5b, ESI). These results reveal the important role of GDY in fabricating new catalysts with superior advantages for efficient catalysis.


image file: d1qm00466b-f4.tif
Fig. 4 (a) XRD patterns of GDY@NiOx(OH)y, GDY, and GDY@Ni(OH)2. (b) Raman and (c) XPS survey spectra of GDY@NiOx(OH)y, GDY, GDY@Ni(OH)2, and Ni(OH2). (d) C 1s XPS spectra of GDY@NiOx(OH)y and GDY. (e) Ni 2p3/2 XPS spectra of GDY@NiOx(OH)y and Ni(OH)2. (f) O 1s XPS spectra of GDY@Ni(OH)2 and GDY@NiOx(OH)y.

OER performances of GDY@NiOx(OH)y were first tested in a typical three-electrode system. The GDY, Ni(OH)2, GDY@Ni(OH)2 and Ni(OH)x were also tested for reference. As expected, GDY@NiOx(OH)y (Fig. 5a) exhibited the best OER activity with the smallest overpotentials (η) of 292 mV and 393.4 mV to drive 10 and 100 mA cm−2, respectively, much better than GDY (497.4 mV at 10 mA cm−2), Ni(OH)2 (436.8 mV at 10 mA cm−2, 554.3 mV at 100 mA cm−2), GDY@Ni(OH)2 (354.4 mV at 10 mA cm−2, 470.7 mV at 100 mA cm−2), Ni(OH)x (398.6 mV at 10 mA cm−2, 573.3 mV at 100 mA cm−2), and most of the reported Ni(OH)2-based catalysts (Table S3, ESI). GDY@NiOx(OH)y showed the smallest Tafel slope of 97.8 mV dec−1 among all samples, revealing the fastest OER reactive kinetics. Long-term stability is also an important factor for a catalyst toward practical applications. The stability of GDY@NiOx(OH)y was evaluated via cyclic voltammetry (CV) and current–time curve at constant voltage (it). As shown in Fig. 5c, the catalytic activity of GDY@NiOx(OH)y was almost the same before and after 8000 continuous CV tests. Besides, the OER activity of GDY@NiOx(OH)y could also be maintained after continuous electrolysis for 190 h at 20 mA cm−2 (η = 331.7 mV) (Fig. 5d). There were no changes in the structure as evidenced by SEM images (Fig. S6a–c, ESI), TEM images (Fig. S6d–f, ESI) and XPS results (Fig. S8, ESI). These results revealed the excellent OER activity and robust long-term stability of GDY-based heterostructures.


image file: d1qm00466b-f5.tif
Fig. 5 (a) CV curves and (b) Tafel slopes of the samples for the OER in 1.0 M KOH. (c) CV curves of GDY@NiOx(OH)y before and after the CV cycling test (inset: time-dependent current density curve of GDY@NiOx(OH)y for the OER). (d) Polarization curves and (e) Tafel slopes of the samples for the HER in 1.0 M KOH. (f) Polarization curves of GDY@NiOx(OH)y before and after CV cycling tests (inset: time-dependent current density curve of GDY@NiOx(OH)y for the HER). (g) CV curves of GDY@NiOx(OH)y for OWS in a two-electrode system in 1.0 M KOH. (h) Time-dependent current density curve of GDY@NiOx(OH)y||GDY@NiOx(OH)y for OWS. (i) Nyquist plots of GDY@NiOx(OH)y, GDY@Ni(OH)2, and Ni(OH)2.

The electrocatalytic performance of GDY@NiOx(OH)y for the HER was further tested under 1.0 M KOH conditions. As shown in Fig. 5e, GDY@NiOx(OH)y shows an overpotential of 154.3 mV at 10 mA cm−2, which is much smaller than that of GDY (466.8 mV, Ni(OH)2) (464.8 mV), GDY@Ni(OH)2 (290.9 mV), Ni(OH)x (326.3 mV), and most of the reported electrocatalysts (Table S4, ESI). GDY@NiOx(OH)y showed a smaller Tafel slope of 183.8 mV dec−1 (Fig. 4f) than GDY (316.6 mV dec−1), Ni(OH)2 (363.5 mV dec−1), GDY@Ni(OH)2 (423.7 mV dec−1) and Ni(OH)x (454.4 mV dec−1), indicating the most kinetically favourable process of GDY@NiOx(OH)y. GDY@NiOx(OH)y also shows high long-term stability for the HER (Fig. 5g and h), which was further confirmed by the SEM (Fig. S7a–c, ESI) and TEM images (Fig. S7d–f, ESI) after 180 h of continuous electrolysis at 35 mA cm−2 (η = 210.5 mV). In light of the outstanding OER and HER activities, an alkaline electrolyzer using GDY@NiOx(OH)y as both the anodic and cathodic electrodes (GDY@NiOx(OH)y||GDY@NiOx(OH)y) was produced. Remarkably, it exhibited a voltage of only 1.54 V to achieve 10 mA cm−2, with robust stability for over 100 h at 20 mA cm−2 (Fig. 5i). These are much better than those of most of the reported benchmark catalysts (Table S4, ESI).

To better understand the origin of the excellent electrocatalytic activity of the catalysts, the electrical impedance spectra (EIS) and electrochemical surface area (ECSA) of the catalysts were measured. The EIS plots were fitted by an equivalent circuit model (Fig. 5i and Fig. S9, ESI). GDY@NiOx(OH)y exhibited the smallest charge transfer resistance (Rct) of 4.90 Ω as compared to that of GDY@Ni(OH)2 (Rct = 5.76 Ω), GDY (Rct = 2246 Ω) and Ni(OH)2 (Rct = 434.5 Ω) (Table S5, ESI), indicating the most facilitated charge transfer kinetics of GDY@NiOx(OH)y. ECSAs of the samples were estimated through capacitance measurements (Fig. S10, ESI). GDY@NiOx(OH)y exhibits the highest double-layer capacitance (Cdl) value of 4.6 mF cm−2 compared to CC (2.0 mF cm−2), GDY (4.5 mF cm−2) and GDY@Ni(OH)2 (3.8 mF cm−2). This result reveals the largest ECSA of GDY@NiOx(OH)y, benefitting from the enhancement of the catalytic activity.

The XPS measurements were further performed to gain more insights into the structural evolution of GDY@NiOx(OH)y during electrocatalysis. Fig. 6a–d show a detailed XPS analysis of the heterostructures during continuous working. High-resolution Ni 2p and O 1s XPS spectra of the GDY@NiOx(OH)y are shown in Fig. 6a and b. It was observed that the percentage of Ni2+ increased from 32.9% to 82.4%, 87.3% and 88.1% as the reaction time increased from 1 h to 2 h and 3 h (Fig. 6c). O 1s XPS spectra revealed the increase of the Ni1–O species. These findings revealed that the relative proportion of Ni2+ and Ni3+ could effectively tune the catalytic activity of the catalysts.


image file: d1qm00466b-f6.tif
Fig. 6 High-resolution (a) Ni 2p and (b) O 1s XPS spectra of the GDY@NiOx(OH)y at different HER electrocatalysis times. (c) The percentage of Ni2+ and Ni3+ species in GDY@NiOx(OH)y after different HER times according to (a). (d) The percentage of –OH and M–O–M species in GDY@NiOx(OH)y after different HER times according to (b).

Conclusions

In summary, we reported the successful synthesis of 2D GDY@NiOx(OH)y heterostructures on flexible CC, forming a 3D porous electrode, by using a simple three-step strategy. Benefitting from the unique properties of GDY, GDY@NiOx(OH)y heterostructures show mixed valence states, greatly optimized charge transfer behaviour and accelerated reaction kinetics, as well as the largest electrochemical active surface area, therefore endowing it with excellent catalytic activity and long-term stability. For example, in an alkaline electrolyzer, GDY@NiOx(OH)y||GDY@NiOx(OH)y exhibits a low voltage of 1.54 V to reach 10 mA cm−2, which could be maintained for 100 h. Our study marks an important step forward in the rational design and synthesis of non-noble metal-based electrocatalysts for highly efficient and robust overall water splitting devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the support from the National Nature Science Foundation of China (21790050 and 21790051), the National Key Research and Development Project of China (2016YFA0200104 and 2018YFA0703501), and the Key Program of the Chinese Academy of Sciences (QYZDY-SSW-SLH015).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00466b

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