Xi
Chen
ac,
Danyan
Zhang
ac,
Xuchen
Zheng
ac,
Chao
Zhang
ac,
Yang
Gao
a,
Chengyu
Xing
a,
Siao
Chen
ac,
Han
Wu
ac,
Yurui
Xue
*ab and
Yuliang
Li
*ac
aCAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: ylli@iccas.ac.cn
bShandong Provincial Key Laboratory for Science of Material Creation and Energy Conversion, Science Center for Material Creation and Energy Conversion, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. E-mail: yrxue@sdu.edu.cn
cUniversity of Chinese Academy of Sciences, Beijing 100049, China
First published on 13th April 2023
Water electrolysis provides an effective method for energy efficient hydrogen production. The challenge in this field is how to synthesize electrocatalysts with high activity and stability in water electrolysis reactions. Herein, a new heterostructure of graphdiyne-iron oxyhydroxide was synthesized by controlled growth of GDY films on the surface of FeOOH nanowires (FeOOH@GDY). An interface structure with an electron-donor (GDY) and electron-acceptor (Fe) was formed, which showed the obvious incomplete charge transfer between GDY and Fe atoms at the interfaces. This produces infinite active sites which guarantee excellent catalytic activity. The GDY grown on the surfaces of oxyhydroxides endows the electrocatalyst with high stability. These unique and fascinating properties make FeOOH@GDY show excellent catalytic activities toward overall water splitting (OWS) with a small cell voltage of 1.43 V at 10 mA cm−2 under ambient conditions and excellent long-term stability.
Among various methods for enhancing catalytic performances, constructing efficient interface structures by incorporating catalysts with carbon materials can effectively improve the electrocatalytic selectivity, activity, and stability by enhancing the conductivity, stability, and the number of surface-active sites and the adsorption/desorption ability of reactants/intermediates.8–10 However, traditional carbon-based materials are usually prepared under extremely harsh conditions, which could easily destroy the composition and structure of active sites and result in low catalytic activity and stability.
Compared with traditional carbon-based materials, GDY shows many fascinating characteristics for electrocatalysis, including sp-/sp2-cohybridized and large π-conjugated all-carbon two-dimensional networks, natural pores, large specific surface area, high conductivity, high intrinsic activity, and excellent stabilities.11–14 Specifically, the uneven charge distribution on the surface of GDY endows it with infinite active sites and high intrinsic activity. And the unique property of GDY that it can be controllably grown on arbitrary material surfaces allows the selective construction of active interfaces with determined composition and structures. Moreover, the unique incomplete charge transfer between GDY and metal atoms could increase the number and the stability of the active site.15–18 Benefitting from these superiorities, GDY has shown excellent performances and attracted greatly increasing interest from the fields of electrocatalysis,19–28 photocatalysis,29–31 energy conversion,32–37etc.38–40
In this study, the highly active and stable OER, HER, as well as OWS were achieved on the heterostructure of FeOOH@GDY with electronic-donating and withdrawing interface structures, resulting in infinite active sites which guarantee excellent catalytic activity. The GDY grown on surfaces of oxyhydroxides endows the electrocatalyst with high stability. For example, FeOOH@GDY shows small overpotentials of 205 and 38 mV at 10 mA cm−2 for the OER and HER in 1.0 M KOH. When assembled into an electrolyzer, the catalyst can drive 10 mA cm−2 at a low cell voltage of 1.43 V for more than 115 hours.
Fig. 1 Schematic representation of the controlled synthesis route to FeOOH@GDY through (a) the growth of FeOOH nanowires and (b) the in situ growth of GDY. |
TEM images confirm the nanowire morphology of FeOOH NWs with an average diameter of ∼30 nm (Fig. 3a) and a polycrystalline structure (Fig. 3b–d). As characterized by HRTEM (Fig. 3c and d), FeOOH NWs have two types of lattice spacings of 0.269 and 0.245 nm, which can be ascribed to the (130) and (111) planes of FeOOH, respectively. After in situ growth of GDY on FeOOH, the nanowire width increases to ∼40 nm (Fig. 3e). The intimate interface between the FeOOH and GDY (Fig. 3f) confirms the successful construction of the heterostructure of FeOOH@GDY. Fringe spacing of GDY increased from 0.365 nm (pure GDY, Fig. S2†) to 0.385 nm (FeOOH@GDY, Fig. 3g), and the lattice distance of the (130) plane of FeOOH decreased to 0.256 nm. Interestingly, after the growth of GDY on the surface of FeOOH, the crystal phase of (111) disappeared. Elemental mapping images (Fig. 3i) show homogeneous distribution of Fe, O, and C elements over the FeOOH@GDY nanowire. The change in the crystal structure of FeOOH was further confirmed by the XRD results. As shown in Fig. 3j, FeOOH shows two obvious peaks at 34.7° and 36.6° which correspond to (130) and (111), respectively (ICDD PDF # 081-0436). Only the (130) phase could be observed in FeOOH@GDY. These results reveal that the in situ growth of GDY effectively induces the transition of the (111) phase to the catalytically active (130) phase,41,42 which helps improve the catalytic activity. Raman spectra (Fig. 3k) show that FeOOH@GDY contains two dominant regions of FeOOH species (295 and 406 cm−1) and GDY (D band: 1396 cm−1; G band: 1590 cm−1; diyne links: 1933 and 2194 cm−1), further confirming the successful incorporation of FeOOH and GDY. This was also confirmed by the infrared spectrometry (IR; Fig. S3†). Compared with pure GDY and FeOOH, the peaks exhibit negative shifts, which indicates the strong interactions between FeOOH and GDY.25 In addition, FeOOH@GDY has a larger intensity ratio of D and G bands (ID/IG) of 0.97 than pure GDY (0.89), indicating the formation of more defects after the growth of GDY, which helps improve the electrochemical activity.15
The chemical states of the samples were further studied by X-ray photoelectron spectroscopy (XPS). After the in situ growth of GDY on FeOOH, the intensity of the C 1s peak is greatly enhanced. Fig. 4a shows the C 1s XPS spectrum of GDY with four characteristic peaks corresponding to C–C (sp2), C–C (sp), C–O, and CO, and a C–C (sp2) to C–C (sp) ratio of 0.5. For FeOOH@GDY, two additional peaks at 283.2 and 289.55 eV corresponding to C–Fe and π–π* transitions, respectively, were observed. The formation of a C–Fe bond would effectively enhance the stability of the formed interface structures. Besides, the XPS survey spectra (Fig. S4†) show the co-existence of C, O, and Fe in FeOOH@GDY. These further confirm the successful growth of GDY on FeOOH NWs. The high-resolution Fe 2p XPS spectra of FeOOH and FeOOH@GDY shown in Fig. 4b are well fitted into two spin–orbit doublets of Fe2+ (at 710.35 and 723.87 eV) and Fe3+ (at 712.37 and 726.70 eV).43 Compared with those of GDY, C 1s XPS peaks of FeOOH@GDY shift to higher binding energies (BEs) by 0.21 eV and the Fe 2p XPS peaks of FeOOH@GDY shift to lower BEs, which indicates the efficient charge transfer from GDY to FeOOH. The O 1s XPS spectra (Fig. 4c) could be divided into O–Fe (529.68 eV), –OH (531.35 eV), and H2O (531.5 eV) species, respectively. The valence spectra (Fig. 4d) show that the top of the valence band of FeOOH@GDY (ΔE = 1.33 eV) is closer to the Fermi level than those of GDY (ΔE = 1.54 eV) and FeOOH (ΔE = 1.67 eV), indicating the improved conductivity of the catalyst. In order to examine the components of FeOOH@GDY from the surface to the inner part, XPS depth profiling (C 1s, Fe 2p and O 1s) was conducted (Fig. 4e).25 As the etching depth increases, the C 1 s (Fig. 4f), Fe 2p (Fig. 4g) and O 1s (Fig. 4h) XPS spectra of FeOOH@GDY show that there are no changes in the compositions and chemical states of FeOOH@GDY, indicating the robust stability of the catalyst. As the etching depth increases, the content of Fe increases while the content of C decreases (Fig. 4i), confirming the successful coating of GDY on FeOOH.
The HER catalytic performance was further studied in 1.0 M KOH. FeOOH@GDY has the smallest overpotential of 38 mV at 10 mA cm−2, better than FeOOH (90 mV@10 mA cm−2), GDY (158 mV@10 mA cm−2), CC (304 mV@10 mA cm−2), 20 wt% Pt/C (43 mV@10 mA cm−2) and some reported FeOOH-based electrocatalysts (Fig. 5e and Table S2†). Moreover, FeOOH@GDY shows good HER performance with low overpotentials of 87, 117, 135, and 161 mV at high current density (100, 300, 500 and 1000 mA cm−2). The smallest Tafel slope of 46 mV dec−1 is shown for FeOOH@GDY in contrast with FeOOH (167 mV dec−1), GDY (65 mV dec−1), CC (230 mV dec−1), and Pt/C (59 mV dec−1) in Fig. 5f, revealing the faster reaction kinetics of FeOOH@GDY. The HER on FeOOH@GDY proceeds most likely via a Volmer–Heyrovsky mechanism.44 The applied overpotential shows no significant increases at 10 mA cm−2 after 28000 continuous CV cycling cycles and decrease in the current densities after 441 h electrolysis (Fig. 5g), which demonstrate the excellent long-term stability of FeOOH@GDY. In contrast, FeOOH presented poor stability during the electrolysis process and its HER activity decreased sharply (Fig. S6b†). These results indicate that the heterostructures of GDY and FeOOH could significantly improve the stability of the catalyst.
Nyquist plots were obtained by EIS measurements (Fig. 5h) and fitted with an R(QR)(QR) equivalent circuit model (Fig. S7†) containing the solution resistance (Rs), the charge transfer resistance (Rct) and the gas adsorption resistance (Rad). As presented in Table S3 and Fig. S8,† FeOOH@GDY shows the smallest Rs (4.93 Ω) and Rct (0.31 Ω) compared to FeOOH (Rs = 7.68 Ω, Rct = 1170 Ω), GDY (Rs = 5.98 Ω, Rct = 37.56 Ω) and CC (Rs = 5.49 Ω, Rct = 1080 Ω), indicating the enhanced charge transfer ability and good electrical conductivity of FeOOH@GDY, which could effectively accelerate the catalytic kinetics of the heterostructure. Higher calculated double-layer capacitance (Cdl) of FeOOH@GDY (5.3 mF cm−2) than GDY (4.5 mF cm−2), FeOOH (1.8 mF cm−2) and CC (1.5 mF cm−2) indicates its higher density of catalytically active sites (Fig. 5i and S9†). Accordingly, the calculated electrochemical active surface area (ECSA) of FeOOH@GDY is 132.5 cm2, larger than that of GDY (112.5 cm2), FeOOH (45.0 cm2) and CC (37.5 cm2),45 demonstrating the largest exposed catalytic sites of FeOOH@GDY to the reactants and higher surface roughness assigned to the introduction of GDY, which is beneficial for the electrolysis. Remarkably, FeOOH@GDY has better catalytic activities for both the OER and HER processes, higher conductivity and larger ECSA than pure FeOOH and GDY. These results indicate that catalytic activity comes from the newly formed interface structures with unique incomplete charge transfer between GDY and Fe atoms.
The water electrolysis measurements were performed in 1.0 M KOH, in which FeOOH@GDY was used as both the cathode and anode (Fig. 6a). FeOOH@GDY‖FeOOH@GDY shows the best OWS activity with the lowest cell voltage of 1.43 V (an overpotential of 198 mV) at 10 mA cm−2 (Fig. 6b), lower than that of 20 wt% Pt/C‖RuO2 (1.63 V), FeOOH‖FeOOH (1.64 V), GDY‖GDY (1.83 V) and CC‖CC (1.85 V). After a 115 h chronopotentiometry test (Fig. 6c), there is negligible variation in both catalytic activity and morphology (Fig. S10†) of FeOOH@GDY, indicating the reliable long-term performance of the FeOOH@GDY‖FeOOH@GDY electrode. These results demonstrate the excellent activity and long-term stability of FeOOH@GDY for water electrolysis under alkaline conditions, outperforming other reported Fe-based catalysts (Fig. 6d and Table S4†).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta01176c |
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