Iron-Based Composites as

Article Cite This: Chem. Mater. 2018, 30, 4762−4769

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Cost-Effective Vertical Carbon Nanosheets/Iron-Based Composites as Efficient Electrocatalysts for Water Splitting Reaction Yao Zhang,† Kun Rui,†,‡ Zhongyuan Ma,† Wenping Sun,*,‡ Qingqing Wang,† Peng Wu,† Qiao Zhang,† Desheng Li,† Min Du,† Weina Zhang,† Huijuan Lin,† and Jixin Zhu*,† †

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Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China ‡ Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Wollongong, NSW 2522, Australia S Supporting Information *

ABSTRACT: Developing low-cost and highly active catalysts is vital to achieve efficient electrochemical water splitting for hydrogen production, which is considered as a very promising approach for renewable energy storage. Herein, an efficient and cost-effective electrode architecture constructed by vertically aligned carbon nanosheets (VCNs) and iron oxyhydroxide/nitride (VCNs@FeOOH//VCNs@Fe4N) is designed and synthesized for water splitting in alkaline medium. Benefiting from the highly exposed active sites, accelerated mass and electron transport, and synergistic effect of multiple components, the composite electrodes deliver unprecedented catalytic performance with high activity and excellent durability. The VCNs@FeOOH composite electrode exhibits an overpotential of as low as 179 mV at 10 mA cm−2 for oxygen evolution reaction (OER), while VCNs@Fe4N shows a low overpotential of 172 mV for hydrogen evolution reaction (HER) at 10 mA cm−2. More significantly, a full electrolyzer cell with VCNs@Fe4N as the cathode and VCNs@FeOOH as the anode exhibits an appealing operation voltage of 1.6 V at 10 mA cm−2 with superior durability. The present results provide new insight into designing robust catalysts toward practical water splitting devices and metal−air batteries.

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and environmental friendliness.30,31 However, applications of Fe-based catalysts with poor intrinsic electrical conductivity and mass-transfer ability deserve more exploration, which were considered to be less electrocatalytically active previously.32,33 Recently, considerable efforts have been focused on the direct growth of nanostructured catalysts on current collector as self-supported electroactive electrodes without using Nafion or other polymeric binders.34−37 Despite distinct advantages of enhanced electrical contact, enlarged active sites exposure, favored mass transport, etc., rational design of three-dimensional (3D) composite electrodes with desirable building blocks and controlled composition remains a critical challenge. As is well-known, two-dimensional (2D) nanosheet materials with atomic thickness have shown great potential for electrocatalysis benefiting from their dramatically increased surface/volume ratio and abundant active sites, making them ideal as subunits in integrated 3D electrodes.38−42 In particular, vertically aligned 2D carbon architectures manifest superior chemical stability and good electrical conductivity, which is

INTRODUCTION Because of the rapid growth of fossil fuel consumption and corresponding environmental degradation, developing green and renewable energy technologies such as fuel cells, rechargeable metal−air batteries, and water splitting devices is of paramount importance.1−10 Among them, electrochemical water splitting coupled with efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is widely regarded as a promising technology.11−17 Therefore, enormous efforts have been devoted to searching for highly efficient and robust electrocatalysts for the practical operation of overall water splitting.18−22 To date, noble-metal-based catalysts remain the most efficient catalysts reported, e.g., Pt for HER and IrO2 and RuO2 for OER, whose scarcity and high cost considerably render water splitting of low economic appeal.23,24 In this context, it is urgent to develop sustainable, non-noble-metal materials to effectively catalyze OER and HER in alkaline solutions. Over the past few years, transition metal compounds including oxides, sulfides, phosphides, etc., have emerged as alternative electrocatalysts.25−29 Specifically, iron (Fe)-based materials have been receiving increasing interest as promising electrocatalysts for both OER and HER owing to advantageous features of natural abundance, low cost, © 2018 American Chemical Society

Received: April 24, 2018 Revised: June 26, 2018 Published: June 27, 2018 4762

DOI: 10.1021/acs.chemmater.8b01699 Chem. Mater. 2018, 30, 4762−4769

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The morphology and microstructure information on all samples were investigated by field emission scanning electron microscopy (FE-SEM). As seen from Figure 1a and Figure S1,

highly desirable to act as a backbone by integrating Fe-based catalysts for the construction of a 2D heterojunction.18,42−44 Previously, OER performance of FeOOH was improved by constructing composite structures with Co, CeO2, and CNTs with one-dimensional (1D) configurations on conductive substrates.45−47 Moreover, Fe-based nitrides derived from FeOOH also hold great potential for HER electrocatalysis with higher electrical conductivity.31,48 Nevertheless, facile incorporation of vertically aligned carbon nanosheet arrays with Febased oxyhydroxides/nitrides as self-supported catalysts has not been reported, to the best of our knowledge. Herein, vertically aligned carbon nanosheet iron-based composite architectures were synthesized by a facile approach suitable for large-scale production and were demonstrated to be efficient electrocatalysts for water splitting in alkaline medium. The vertically aligned carbon nanosheets (VCNs) with nitrogen doping serve as highly conductive backbones to provide reliable electron transport for Fe-based catalysts. The strong bonding between the active materials (FeOOH/Fe4N) and VCNs also endows the composite electrodes with substantial stability. The VCNs@FeOOH and VCNs@Fe4N exhibited superior OER and HER catalytic activity in alkaline medium, respectively. Notably, the VCNs@FeOOH//VCNs@ Fe4N couple also delivered exceptional performance in an alkaline electrolyzer for overall water splitting.

Figure 1. Typical field emission scanning electron microscopy (FESEM) images. (a, b) Nitrogen-doped vertically aligned carbon nanosheets (VCNs) on nickel foam. (c, d) Carbon nanosheets with FeOOH composites (VCNs@FeOOH) and (e, f) carbon nanosheets with Fe4N composites (VCNs@Fe4N).

interconnected carbon nanosheets were aligned vertically on the surface of nickel foam. A high-magnification SEM image (Figure 1b) further revealed that these well-defined thin nanosheets with a thickness of 3−5 nm have a lateral dimension of up to hundreds of nanometers. These porous 2D arrays supported on the 3D macroporous nickel foam substrate possessed superior advantages due to high porosity and surface area, forming a large electrode−electrolyte interface and facilitating efficient charge transfer as well as rapid mass diffusion without any block of binder. According to the X-ray diffraction (XRD) result (Figure S2), one peak located at around 25.6° can be ascribed to the (002) peak of graphitic carbon.49,50 Three well-resolved peaks at 1369, 1583, and 2721 cm−1 in the Raman spectrum corresponding to the D-band, G-band, and 2D-band, respectively, further certified the formation of graphitic carbon nanosheets composed of layer graphene (Figure S3).51 Through a facile and mild interfacial reaction, VCNs@ FeOOH composite nanosheets arrays were vertically grown on the 3D nickel foam substrate. No significant differences were observed between the FESEM images of VCNs and VCNs@ FeOOH as evidenced in Figure 1c and Figure S4, indicating the uniform distribution of FeOOH nanosheets over the entire VCNs. At a higher magnification (Figure 1d), the thickness of these thin nanosheets barely increased as compared to VCNs, keeping a distance of around 200 nm with neighboring sheets. Meanwhile, 3D nickel foam substrates were less densely covered by these laterally extended composite nanosheets featured with more wrinkles and edge bending, beneficial to exposure of enriched active sites for catalytic reaction. After further heat treatment of VCNs@FeOOH under a NH3 atmosphere, a 3D porous hierarchical composite structure constituted by highly branched assembly of interconnected fine nanoparticles (ca. 30 nm) is illustrated in Figure 1e,f. The asobtained coral-like VCNs@Fe4N heterostructure on nickel foam substrate inherited structural merits from 2D VCNs@ FeOOH template, exhibiting abundant void space within oriented self-assembly without severe aggregation. In addition, it is worth noting that VCNs acted as a structure-directing agent during the formation of desirable VCNs@FeOOH and VCNs@Fe4N heterostructures, as indicated by the morphol-

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RESULTS AND DISCUSSION The synthesis process of VCNs@FeOOH and VCNs@Fe4N mainly involves a room-temperature interfacial reaction followed by proper thermal treatment as illustrated in Scheme 1. First, VCNs were synthesized using our previously reported Scheme 1. Schematic Illustration for the Formation of Vertically Aligned Carbon Nanosheets (VCNs) and Composite Electrodes for Overall Water Splitting

salt-templating method.8,49 Specifically, ionic liquid (IL) carbon precursor 1-ethyl-3-methylimidazolium dicyanamide (Emim-dca) and salt mixture (ZnCl2/KCl) were thoroughly ground and uniformly casted to a pretreated commercial nickel foam. After carbonization under inert gas atmosphere at 900 °C for 3 h, VCNs were obtained strongly adhereing on nickel foam. Herein, VCNs were further employed as an ideal 3D conductive substrate with delicate 2D building blocks to support Fe-based catalytically active species. The in situ growth of FeOOH nanosheets on VCNs (VCNs@FeOOH) was then achieved by immersing VCNs in ferric nitrate solution at room temperature. Furthermore, VCNs@FeOOH was mildly transformed to VCNs@Fe4N via thermal treatment in NH3 flow at 500 °C for 2 h. 4763

DOI: 10.1021/acs.chemmater.8b01699 Chem. Mater. 2018, 30, 4762−4769

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importantly, the diffraction rings in selected area electron diffraction (SAED) pattern (Figure 2g) showed the polycrystalline structure of Fe4N nanoparticles. Meanwhile, the good crystallinity of Fe4N can be also confirmed by XRD with three strong diffraction peaks centered at around 42.1°, 48.8°, and 71.5° (Figure S2), which belong to (111), (200), and (220) lattice planes of cubic Fe4N (JCPDS No. 86-0231) according to the literature.56 Furthermore, the lattice fringes with a d-spacing of 0.22 nm in the high-resolution TEM (HRTEM) image can be well indexed to the (111) plane of Fe4N (Figure S8). X-ray photoelectron spectroscopy (XPS) was further carried out to examine the chemical composition and oxidation states of the as-prepared VCNs@FeOOH and VCNs@Fe4N. Figure 3a shows the high-resolution C 1s core level of VCNs@

ogies of controlled samples in the absence of VCNs, e.g., Ni@ FeOOH and Ni@Fe4N (Figure S5). Further insight into the microstructure and composition of composite electrocatalysts was provided by conducting transmission electron microscopy (TEM). As shown in Figure 2a,

Figure 2. Typical transmission electron microscopy (TEM) images of hybrid electrocatalysts. (a) TEM image of VCNs@FeOOH. (b, c) Dark-field TEM image and corresponding elemental mapping images of VCNs@FeOOH with orange for C, red for N, purple for Fe, and green for O. (d) TEM image of VCNs@Fe4N. (e, f) Dark-field TEM image and corresponding elemental mapping images of VCNs@Fe4N with purple for C, red for N, and green for Fe. (g) Corresponding SAED pattern for VCNs@Fe4N. Figure 3. X-ray photoelectron spectroscopy (XPS) of (a−d) VCNs@ FeOOH and (e, f) VCNs@Fe4N, indicating the presence of C, N, Fe, and O in the composite electrocatalysts.

these folding sheet-like composites displayed high transparency under the electron beam, indicating their ultrathin nature.52 At a higher magnification, as-observed lattice fringes (highresolution TEM image in Figure S6) can be well indexed to (101) planes of hexagonal FeOOH (JCPDS No. 76-0123). Energy dispersive spectroscopy (EDS) elemental mapping images corresponding to the dark-field image of VCNs@ FeOOH (Figure 2b) further revealed the uniform distribution of C, N, Fe, and O (Figure 2c). Moreover, characteristic peaks located at around 467, 540, and 667 cm−1 in the Raman spectra of VCNs@FeOOH and Ni@FeOOH (Figure S7) can be well assigned to FeOOH in accordance with previous reports,53−55 while typical D-band, G-band, and 2D-band features are ascribed to the presence of VCNs. In the case of VCNs@Fe4N, ultrathin carbon nanosheets were evenly loaded with nanoparticles as observed by TEM in Figure 2d. Additionally, the homogeneous distribution of C, N, and Fe was exhibited by EDS elemental mapping (Figure 2f) based on the corresponding dark-field image in Figure 2e. More

FeOOH, which can be deconvoluted into one dominant peak of graphitic (sp2 hybridized) carbon (C−C) at 284.7 eV as well as C−N bond (285.5 eV) and C−O bond (286.2 eV).57−59 A similar C 1s spectrum was also obtained for VCNs@Fe4N and is presented in Figure S9. Besides, the detection of N in VCNs@FeOOH further revealed the N-doping feature of graphitic carbon nanosheets. According to the deconvolution of the N 1s spectrum (Figure 3b), three resolved peaks at 398.6, 400.0, and 400.9 eV can be ascribed to pyridinic N, pyrrolic N, and graphitic N, respectively.60,61 As shown in the Fe 2p spectrum (Figure 3c), two resolved peaks at 2p3/2 (713.5 eV) and 2p1/2 (725.7 eV) and the satellite peak of 2p3/2 at 719.9 eV indicated the presence of Fe as Fe3+ in VCNs@ FeOOH, which is consistent with previous reports.52,62 The O 1s region spectrum of VCNs@FeOOH was also analyzed (Figure 3d), suggesting the existence of three distinct species, 4764

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species. In addition, double-layer capacitance (Cdl) at the solid−liquid interface was measured to assess the electrochemically active surface area (ECSA) of different electrocatalysts. Cyclic voltammetry (CV) curves were obtained at different scan rates over a potential range of 1.05−1.15 V vs RHE without apparent faradaic processes (Figure S11). As expected, a much higher linear slope can be obtained for VCNs@FeOOH from current density difference (Δj) at 1.10 V vs RHE plotted against scan rate (Figure S12), e.g., a higher Cdl of 22.3 mF cm−2 as compared to VCNs (13.8 mF cm−2), Ni@FeOOH (2.4 mF cm−2), and bare Ni foam substrate (1.8 mF cm−2). The larger electrochemical active surface area of VCNs@FeOOH (ca. 10 times of pristine Ni@FeOOH) can be attributed to the porous architecture of self-supported electrode constructured by 2D ultrathin nanosheet building blocks, resulting in the enhanced OER activity. A comparison of VCNs@FeOOH with recently reported non-noble-metalbased OER electrocatalysts is summarized in Table S2, demonstrating a comparably high catalytic performance under alkaline conditions. Additionally, the long-term electrochemical stability of VCNs@FeOOH was evaluated with chronopotentiometric measurements at a current density of 25 mA cm−2. As presented in Figure 4d, VCNs@FeOOH retained high catalytic activity over 12 h without evident voltage degradation. Specifically, only 37 mV overpotential increase was encountered over 12 h, corresponding to an activity loss of less than 13.8%. This result further demonstrated the desirable stability of VCNs@FeOOH for OER. The excellent stability of the VCNs@FeOOH electrode was further confirmed by exsitu XRD, Raman, and SEM (Figure S13). Both the XRD pattern and Raman spectrum scarcely changed after the stability test, while the SEM images revealed the well-preserved morphology of VCNs@FeOOH after OER, consistent with the stable performance of the VCNs@FeOOH electrode in alkaline media. In view of the cathode reaction of water splitting, HER performance of VCNs@Fe4N was further examined by carrying out corresponding electrochemical measurements under alkaline conditions (0.1 M KOH) at room temperature. As shown in Figure 5a, a negligible catalytic activity was revealed for the bare Ni foam substrate according to the polarization curves, whereas current densities significantly increased for as-obtained self-supported 3D electrodes. A low overpotential of 172 mV was achieved for VCNs@Fe4N at a current density of 10 mA cm−2 (Figure 5b). In contrast, higher overpotentials of 331 and 319 mV were demanded for Ni@ Fe4N and VCNs, respectively. Furthermore, the corresponding Tafel plots were derived to evaluate the HER kinetic process of 3D electrodes. VCNs@Fe4N delivered a Tafel slope of 80 mV dec−1, which is smaller than Ni@Fe4N (217 mV dec−1) and VCNs (220 mV dec−1) (Figure 5c). As expected, VCNs@ Fe4N catalyst derived from the VCNs@FeOOH with a deposition time of 24 h has the lowest overpotential and smallest Tafel slope, achieving the best electrochemical activity upon HER (Figure S14). This superior electrocatalytic performance is comparable to state-of-the-art non-noblemetal-based HER catalysts (Table S3). Similarly, the effective ECSAs of VCNs@Fe4N, Ni@Fe4N, VCNs, and Ni foam toward HER were obtained according to CV scans from 0.08 to 0.18 V vs RHE (Figure S15), demonstrating a decrease in the order of VCNs@Fe4N > Ni@Fe4N > VCNs > Ni foam (Figure S16). A 2 times larger ECSA was achieved for VCNs@ Fe4N than Ni@Fe4N, indicating a more advantageous 3D

e.g., Fe−O−H bond (513.3 eV), C−O bond (532.0 eV), and CO (533.2 eV).52,55,63 On the other hand, the valence state of Fe species in VCNs@Fe4N was carefully examined by the high-resolution Fe 2p spectrum (Figure 3e). The coexistence of mixed oxidation states (+3 and +2) was revealed by the fitting, while the presence of Fe(0) can be attributed to the formation of trace metallic Fe by NH3 reduction. Moreover, a distinct peak corresponding to the Fe−N bond (397.8 eV) originating from Fe4N emerged in the N 1s spectrum of VCNs@Fe4N (Figure 3f), in addition to three typical Ncontaining groups in N-doped VCNs.60,64 To explore the potential application of these as-prepared 3D electrodes in electrochemical water splitting, the electrocatalytic activity of VCNs@FeOOH toward OER was first evaluated in a three-electrode electrochemical system (see details in the Experimental Section). Figure 4a displays typical

Figure 4. Electrocatalytic activities for oxygen evolution reaction (OER). (a) Comparative polarization curves of Ni foam, Ni@ FeOOH, VCNs, and VCNs@FeOOH (electrolyte: 0.1 M KOH; scan rate: 1 mV s−1). (b) Corresponding overpotentials at a current density of 10 mA cm−2. (c) Tafel slopes of Ni foam, Ni@FeOOH, VCNs, and VCNs@FeOOH. (d) Chronopotentiometric curve for VCNs@ FeOOH with a constant current density of 25 mA cm−2.

polarization curves obtained at 1 mV s−1 in 0.1 M KOH with VCNs@FeOOH, Ni@FeOOH, VCNs, and nickel foam directly used as working electrodes. Significant oxygen evolution electrocatalytic activities were obtained for these samples. Among them, VCNs@FeOOH delivered the best electrocatalytic activity. To be more precise, overpotentials to achieve a current density of 10 mA cm−2 were summarized for these electrocatalysts (Figure 4b). As seen, VCNs@FeOOH requires the lowest overpotential of 179 mV, which is 110, 141, and 301 mV lower than those of Ni@FeOOH, VCNs, and Ni foam, respectively. Superior OER performance of VCNs@ FeOOH was also confirmed by its smaller Tafel slope (57 mV dec−1) derived from the linear sweep voltammetry (LSV) curve (Figure 4c) than other controlled samples, e.g., Ni@ FeOOH (87 mV dec−1), VCNs (138 mV dec−1), and Ni foam substrate (357 mV dec−1), indicating a more favorable reaction kinetics of VCNs@FeOOH for OER. Here, the catalytic activity of VCNs@FeOOH can be further tuned by varying the content of FeOOH with altered deposition time (Table S1 and Figure S10). The optimal catalytic performance can be achieved with a deposition time of 24 h, which can be ascribed to the wise incorporation of moderate FeOOH active 4765

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lyzer can be driven by a single-cell AA battery with a voltage of 1.5 V in 1.0 M KOH (see corresponding movie in the Supporting Information). The evolution of numerous small bubbles near both electrodes can be clearly observed, indicating that produced O2 and H2 are readily released from the surface of catalysts. As discussed above, superior electrochemical performance of VCNs@FeOOH//VCNs@Fe4N can be attributed to the following aspects: (1) N-doped VCNs have the advantages of large electrode electrolyte interface, strong interaction with the current collector, and low contact resistance. Simultaneously, the interconnected VCNs serve as growth templates for Fe-based catalytic components, ensuring the successful establishment and stability of 3D structure. (2) Both the VCNs@FeOOH nanosheets featured with wrinkles and curled edges and the coral-like VCNs@Fe4N nanoparticles endowed with large surface area have provided abundant active sites during electrocatalytic reactions, contributing to favorable catalytic reactions. (3) Successful construction of 3D VCNs@ FeOOH//VCNs@Fe4N architectures on Ni foam substrates further enables the synergistic effect from multiple components, resulting in the acceleration of both mass and electron transport as well as promoted electrical conductivity, contributing to substantially boosted electrochemical activity. Therefore, N-doped vertically aligned carbon nanosheets coupled with iron oxyhydroxide/nitride (VCNs@FeOOH// VCNs@Fe4N) are promising electrocatalysts with high activity and excellent durability for electrochemical water splitting.

Figure 5. Electrocatalytic activities for hydrogen evolution reaction (HER). (a) Comparative polarization curves of Ni foam, VCNs, Ni@ Fe4N, and VCNs@Fe4N (electrolyte: 0.1 M KOH; scan rate: 1 mV s−1). (b) Corresponding overpotentials at a current density of 10 mA cm−2. (c) Tafel slopes of Ni foam, VCNs, Ni@Fe4N, and VCNs@ Fe4N. (d) Chronopotentiometric curve of VCNs@ Fe4N with a constant current density of −10 mA cm−2.

nanostructure provided by the presence of VCNs with enhanced electron transport along with increased active sites. Furthermore, prolonged chronopotentiometric experiment at a fixed current density of −10 mA cm−2 for 12 h showed a stable potential response, indicating remarkable durability of VCNs@ Fe4N for HER (Figure 5d). Ex-situ XRD and SEM measurements were also performed on VCNs@Fe4N after the stability test (Figure S17). As seen, the crystal phase was maintained after the cycling reaction. Meanwhile, well-preserved morphologies of porous 3D self-supported catalysts demonstrated the robust stability of as-prepared composite electrodes. In order to approach the real application, the catalytic performance toward water splitting was measured in a twoelectrode device by employing VCNs@FeOOH as anode and VCNs@Fe4N as cathode (inset of Figure 6a). As shown in

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CONCLUSIONS In summary, vertically aligned carbon nanosheets/iron-based composite architectures were synthesized by a facile approach suitable for large-scale production and were demonstrated to be efficient electrocatalysts for water splitting in alkaline medium. The composite architectures are enriched in active sites and are also very beneficial for facilitating mass diffusion and charge transfer, all of which are of great significance to enhanced electrochemical water splitting. The VCNs@ FeOOH and VCNs@Fe4N composite electrodes exhibited excellent electrocatalytic activity for OER (179 mV at 10 mA cm−2) and HER (172 mV at 10 mA cm−2), respectively. Notably, exceptional overall water splitting performance was achieved in an alkaline electrolyzer employing the two composite electrodes. The present results open a new avenue to design efficient and cost-effective electrocatalysts for water splitting and also shed light on the development of catalysts for metal−air batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. (a) Water splitting performance of VCNs@FeOOH// VCNs@Fe4N couple in 0.1 M KOH solution in a two-electrode configuration. Inset: optical photograph showing the generation of H2 and O2 bubbles. (b) Catalytic stability of VCNs@FeOOH//VCNs@ Fe4N couple for overall water splitting at 10 mA cm−2.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01699. Experimental details, additional figures, e.g., lowmagnification SEM images and EDS mapping result for VCNs and VCNs@FeOOH, XRD patterns of all samples, Raman spectra of VCNs, VCNs@FeOOH, and Ni@FeOOH, SEM images of Ni@FeOOH and Ni@ Fe4N, HRTEM image of VCNs@FeOOH and VCNs@ Fe4N, C 1s XPS spectrum of VCNs@Fe4N, electrocatalytic activities of VCNs@FeOOH with different mass loading, cyclic voltammograms of all samples,

Figure 6a, the polarization curve of water electrolysis presented a current density of 10 mA cm−2 achieved with an external bias of 1.6 V, which was superior to most of the previously reported results for water splitting in alkaline solution (Table S4). Moreover, the long-term stability test of this two-electrode electrolyzer was also performed (Figure 6b), delivering a maintained voltage around 1.61 V at 10 mA cm−2 for 10 h. In addition, such VCNs@FeOOH//VCNs@Fe4N water electro4766

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structure and morphology analysis after OER/HER (XRD pattern, Raman spectrum, and SEM images), comparison tables of the OER, HER, and overall water splitting performance (PDF) Movie S1 (AVI)

AUTHOR INFORMATION

Corresponding Authors

*(W.S.) E-mail: [email protected]. *(J.Z.) E-mail: [email protected]. ORCID

Wenping Sun: 0000-0003-3021-6382 Jixin Zhu: 0000-0001-8749-8937 Author Contributions

Y.Z. and K.R. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (2017YFA0207201), National Natural Science Foundation of China (21501091), the NSF of Jiangsu Province (BK20170045), the Recruitment Program of Global Experts (1211019), the Six Talent Peak Project of Jiangsu Province (XCL-043), and the National Key Basic Research Program of China (973) (2015CB932200). W. Sun acknowledges financial support from the Australian Research Council (ARC) DECRA Grant (DE160100596) and AIIM FOR GOLD Grant (2017, 2018). K. Rui acknowledges the fellowship from the China Scholarship Council (CSC) and the financial support from the China Postdoctoral Science Foundation (2016M600404 and 2017T100360) and Jiangsu Postdoctoral Science Foundation (1701094C). Q. Zhang is grateful for the support from the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX170955).

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