r-GO Aerogel toward


Queensland Micro- and Nanotechnology Centre (QMNC), Griffith University, Nathan, Brisbane, QLD 4111, Australia. ACS Nano , Article ASAP. DOI: 10.1021/...
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Electronic Structure Tuning in NiFeN/r-GO Aerogel towards Bifunctional Electrocatalyst for Overall Water Splitting Yu Gu, Shuai Chen, Jun Ren, Yi Alec Jia, Cheng-Meng Chen, Sridhar Komarneni, Dongjiang Yang, and Xiangdong Yao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05971 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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A seaweed derived pathway was developed for the scalable synthesis of Ni3FeN/r-GO aerogels to trigger efficient overall water splitting. 35x25mm (600 x 600 DPI)

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Electronic Structure Tuning in Ni3FeN/r-GO Aerogel towards Bifunctional Electrocatalyst for Overall Water Splitting Yu Gu,† Shuai Chen,‡ Jun Ren,§ Yi (Alec) Jia¶, Chengmeng Chen,‡ Sridhar Komarneni,ǁ Dongjiang Yang,*,†, ¶ and Xiangdong Yao*,¶ †

Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong

Province, School of Environmental Science and Engineering, Qingdao University, Qingdao 266071, P R China. ‡

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science,

Taiyuan 030001, China. §

School of Chemical and Environmental Engineering, North University of China, Taiyuan 030051, P. R.

China. ǁ

Materials Research Institute, Department of Ecosystem Science and Management, The Pennsylvania

State University, University Park, PA16802, USA. ¶

Queensland Micro- and Nanotechnology Centre (QMNC), Griffith University, Nathan, Brisbane, QLD

4111, Australia.

ABSTRACT: Searching for the highly active, stable, and high-efficiency bifunctional electrocatalysts for overall water splitting, e.g. for both oxygen evolution (OER) and hydrogen evolution (HER), is

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paramount in terms of bringing future renewable energy systems and energy conversion processes into reality. Herein, three-dimensional (3D) Ni3FeN nanoparticles (NPs)/reduced graphene oxide (r-GO) aerogel electrocatalysts were fabricated using precursors of (Ni,Fe)/r-GO alginate hydrogels through an ion exchange process, followed by a convenient one-step nitrogenization treatment in NH3 at 700 °C. The resultant materials exhibited excellent electrocatalytic performance for OER and HER in alkaline media, with only small overpotentials of 270 mV and 94 mV at a current density of 10 mA cm-2, respectively. The good performance was attributed to abundant active sites and high electrical conductivity of the bimetallic nitrides and efficient mass transport of the 3D r-GO aerogel framework. Furthermore, an alkaline electrolyzer was set up using Ni3FeN/r-GO as both the cathode and the anode, which achieved a 10 mA cm-2 current density at 1.60 V with durability of 100 h for overall water splitting. Density functional theory (DFT) calculations support that Ni3FeN (111)/r-GO is more favorable for overall water splitting since the surface electronic structure of Ni3FeN is tuned by transferring electrons from Ni3FeN cluster to the r-GO through interaction of two metal species. Thus, the currently developed Ni3FeN/r-GO with superior water splitting performance may potentially serve as a material for use in industrial alkaline water electrolyzers.

KEYWORDS: Ni3FeN, aerogel, bifunctional electrocatalyst, overall water splitting Electrocatalytic water splitting into hydrogen and oxygen could potentially fulfill the future energy demand, since hydrogen-fuel can be compatible with the current catalytic systems in fuel cells.1-4 The oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) of electrocatalytic water splitting are crucial for the overall efficiency, which is strongly influenced by the slow kinetics and large energy barriers of both the half reactions.5 Therefore, highly active bifunctional electrocatalysts with low overpotentials for HER and OER are imperative for water

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splitting.6 Platinum (Pt)-based catalysts exhibit high-performance for HER, but only moderate activity for OER. Similarly, noble metal oxides (IrO2 and RuO2) are state-of-the-art electrocatalysts for OER, but not good for HER.7 In addition, the high cost and global reserve scarcity of these noble metals have greatly hampered their practical applications. Recently, in order to develop low-cost electrocatalysts with excellent performance for water splitting, various transition metals and their derivatives have been widely investigated8-17. In particular, metal nitrides have drew extensive attention due to the special properties of better metallic conductivity than the metal oxides. However, single-metal nitrides have not been reported to be bifunctional. For instance, Ni3N21, Co4N19 have been reported as highly active OER catalyst and MoN20 shows excellent performance in acidic media for HER. Compared with single-metal nitrides, bimetallic nitrides show bifunctional catalytic activities because the coordination of two metal species could provide both-way rich active sites and improved electronic conductivity, which result in enhanced water-splitting performance.21-23 Nevertheless, the enhanced conductivity in bimetallic nitrides

is still limited, which always results in

instability in strong alkaline electrolytes and high overpotential under long-time operation.23 To address this issue, the catalysts could be deposited on conductive carbon substrates that have interconnected porous structure to achieve accessibility of HER- and OER-relevant reactants with the catalytic centers.25,26 In addition, to reveal the catalytic mechanism on bimetallic nitrides, theoretical calculations were conducted to study adsorption behaviors of H2O on Ni3FeN. It was found that the enhancive adsorption energy was attributed to the strong energy states of Fe centers.23 However, only the calculations of H2O adsorption could not explain the bidirectional catalytic mechanism due to the different reaction mechanism of the HER and OER in water splitting process. In

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addition, these studies evaluated the effect of the electronic structures of the bimetallic nitrides, but did not consider the impact of electron transport between metal cluster and conductive carbon substrates on water splitting. This neglected the interface dynamics on the catalytic reactions. Recently, the interfacial charge transfer between semiconductor photocatalysts and carbon substrates has been studied to reveal the mechanism of enhanced photocatalytic activity of the semiconductors.27-29 This inspires us to reveal the HER and OER mechanism of bifunctional bimetallic nitrides by investigating the electron transfer between Ni3FeN and a carbon substrate. Herein, we designed robust and highly efficient bifunctional electrocatalysts of Ni3FeN nanoparticles (NPs) on reduced graphene oxide hybrid aerogels (Ni3FeN/r-GO). This synthesis was realized by using precursors of (Ni,Fe)-alginates that have a “egg-box” composed of Fe3+ and Ni2+ cations and G-blocks in alginate. Besides guiding the formation of highly porous and 3D interconncected aerogel structure, the “egg-box” structure could precisely tuning the Ni-Fe ratio and easily achieve binary cooperative complementary effect in Ni3FeN.25,26,30 In order to investigate the multiple catalytic mechanism, we have carried out comprehensive and in-depth theoretical calculations. Density functional theory (DFT) calculations on the density of states (DOS) indicate a covalent interaction of Ni, Fe, and N that can lead to enhanced electrical conductivity and tune the surface electronic distribution of bimetallic nitrides. More importantly, the calculations of charge density demonstrate that the coordination of Ni, Fe and N can promote the electron transfer from the metal cluster to the r-GO surface, which leads to the separation and redistribution of electrons and holes. The electrons and the holes accumulations are supposed to enhance HER and OER, respectively.31 Experimentally, the hybrid aerogels exhibited highly robust bifunctional catalytic activities for HER and OER. The highly efficient bifunctional

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performance has been demonstrated by an alkaline electrolyser for overall water-splitting, achieving a current density of 10 mA cm-2 at only 1.60 V with excellent stability of 100 h. RESULTS AND DISCUSSION The synthesis process of Ni3FeN/r-GO is illustrated in Figure 1. Firstly, the sodium alginate (SA) was mixed with various amounts of r-GO in solution to obtain SA/r-GO dispersion, where the SA macromolecules were combined with the r-GO nanosheets (Figure 1a). The above dispersion was poured into FeCl3 and NiCl2 solution, then the SA/r-GO dispersion was converted to hydrogel (Figure 1b), because of forming an “egg-box” structure by the coordination of the Fe3+ and Ni2+ ions and four Gblocks of SA (Figure 1c).32,33 The ratio of Ni/Fe in the gel could be precisely regulated by altering the molar ratio of Ni2+/Fe3+ in solution. Then, the hydrogels were dewatered through a lyophilization approach to obtain the 3D (Ni,Fe)-alginate/r-GO aerogels. The 3D (Ni,Fe)-alginate/r-GO aerogels were subsequently heated to 700 °C in NH3 atmosphere to obtain ultralight 3D Ni3FeN/r-GO (Figure 1d). As shown in Figure 1e and Figure S1a, the Ni3FeN NPs were supported homogeneously on the highly porous 3D r-GO aerogels, which have interconnected macropores to provide large surface areas and rapid electron transfer. These Ni3FeN NPs were encapsulated in a carbon shell that was generated by Gblocks of alginate macromolecules from the pyrolysis of the “egg-box” (Figure 1f). However, the Ni3FeN NPs aggregated markedly in the sample without addition of r-GO nanosheets (Figure S1b). Apparently, the 3D r-GO aerogel could efficiently prevent the aggregation of particles, thereby promoting particle dispersion and increasing the number of active sites. XRD patterns of Ni3FeN/r-GO-x (x = 0, 10, 20, 30, mass ratio (%) of r-GO) (Figure 2a) samples show all the characteristic peaks which could be assigned to Ni3FeN (JCPDS Card 50-1434).34 With the increase of r-GO, the intensity of the diffraction maximum of Ni3FeN was markedly reduced. The mean crystalline sizes of Ni3FeN are calculated from the diffraction peaks of Ni3FeN (111) facet in varying

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samples by the Scherrer equation, which are 26.7 nm for Ni3FeN/r-GO-30, 35.4 nm for Ni3FeN/r-GO-20, 39.3 nm for Ni3FeN/r-GO-10, and 60.7 nm for support-free Ni3FeN, respectively. Raman spectra of Ni3FeN/r-GO-20 showed the ID/IG intensity ratio of 0.94, which resulted from the high graphitic quality of the aerogel (Figure S2). The TEM image of Ni3FeN/r-GO-20 is displayed in Figure 2b. Clearly, the Ni3FeN NPs with sizes of 30–40 nm are homogeneously deposited on the r-GO layers (Figure 2b). Furthermore, the atomic force microscope (AFM) indicates the thickness of the r-GO is ~1.35 nm (Figure S3). The enlarged TEM image showed a graphitic carbon shell with a thickness of ~ 3 - 6 nm encapsulated a single Ni3FeN NP (Figure 2c and Figure S4). The delicate core/shell structure is conducive to enhancing activity and stability.35 The high-resolution TEM (HRTEM) diagram of the Ni3FeN NP provided a lattice fringe with the d-spacing of 0.215 nm, according with the (111) plane of Ni3FeN, and the corresponding FFT graph further proves the presence of (111) plane (Figure 2d). The EDS mapping spectrum indicates uniform distribution of Ni, Fe, N, C and O elements of the Ni3FeN/rGO-20 (Figure 2e). Furthermore, the weight ratio of Ni and Fe was determined to be 3:1 by using inductively coupled plasma mass spectroscopy (ICP-MS). The thermogravimetric analysis (TGA) measurement indicates that the carbon content of Ni3FeN/r-GO-20 is 49% (Figure S5). X-ray photoelectron spectroscopy (XPS) was carried out to probe the valence state of the elements. As shown in Figure 3a, it exhibits that Ni3FeN/r-GO-20 contains all expected C 1s, O 1s, N 1s, Ni 2p and Fe 2p signals. The peaks of the Ni 2p XPS spectrum at 852.7 eV (Ni 2p3/2) and 869.9 eV (Ni 2p1/2) are in accordance with a metallic state of nickel (Figure 3b). The peaks at 855.5 eV (Ni 2p3/2) and 873.1 eV (Ni 2p1/2) are related to Ni2+, suggesting the existence of trace NiO due to surface oxidation.18,22,23,36 The peaks at 711.5 eV (Fe 2p3/2) and 724.5 eV (Fe 2p1/2) reveal the existence of Fe3+, while the other two peaks at 707.1 (Fe 2p3/2) and 719.5 eV (Fe 2p1/2) are related to metallic iron nitride (Figure 3c). The N 1s XPS spectrum contains four peaks (Figure 3d). The peak at 397.5 eV is ascribed to metal nitride.

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The peaks at 398.5 eV, 400.0 eV and 401.0 eV are consistent with pyridine-like N, pyrrole-type N, and graphitic N, respectively.21,37,38 The XPS spectra of O 1s show three peaks at 529.8 eV, 531.4 eV and 533.3 eV, which are in accordance with metal−O, O−C−O, and C−OH (Figure S6a). The C 1s spectra show the graphitic C−C peak (284.6 eV), C−N (286.0 eV) and C=O (288.2 eV) (Figure S6b).39,40 These results are consistent with the XRD data. For comparison, the Ni3Fe/r-GO-20, Ni3N/r-GO-20, and Fe2N/r-GO-20 were also prepared with same amounts of r-GO (20% by mass). Ni3N/r-GO-20 and Fe2N/r-GO-20 were obtained by calcination of the Ni-alginate/r-GO-20 aerogels and the Fe-alginate/r-GO-20 aerogels at 700 °C in NH3 atmosphere. Ni3Fe/r-GO-20 was synthesized by calcining the (Ni,Fe)-alginate/r-GO-20 aerogels under Ar/H2 at 700 °C. Their FESEM and XRD images at different magnifications are shown in Figure S7 and Figure S8. Apparently, Ni3N, Fe2N and Ni3Fe NPs were uniformly distributed on r-GO layers. The BET specific surface areas, pore volume and average pore size of all the samples are in the range of 171–269 m2 g-1, 0.07–0.93 cm3 g-1 and 9.8–14.7 nm, respectively (Figure S9). The OER activity of Ni3FeN/r-GO-x was evaluated in 1 M KOH solution in a standard threeelectrode system. The LSV curves of Ni3FeN/r-GO-x are shown in Figure S10. Apparently, Ni3FeN/rGO-20 has the lowest overpotential, indicating that an appropriate amount of r-GO could lead to a better catalytic activity. For comparison, Ni3Fe/r-GO-20, Ni3N/r-GO-20, Fe2N/r-GO-20, Ni3FeN, and commercial IrO2/C were also investigated under the same conditions. As displayed in Figure 4a, Ni3FeN/r-GO-20 also shows the smallest onset potential of ∼1.48 V vs RHE and the biggest catalytic current activity for OER with the lowest overpotential of 270 mV at 10 mA cm-2. The overpotential at 10 mA cm-2 of Ni3FeN/r-GO-20 is superior to most OER catalysts reported (Table S1). When anodic current density reached 50 mA cm-2, the overpotentials of the Ni3FeN/r-GO-20, Ni3Fe/r-GO-20, Ni3N/r-

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GO-20, Fe2N/r-GO-20, Ni3FeN and IrO2/C are 298, 308, 352, 390, 421 and 400 mV, respectively. This indicates the high OER activity of Ni3FeN/r-GO-20. Furthermore, the OER kinetics was evaluated by Tafel plots derived from polarization curves. As shown in Figure 4b, the Ni3FeN/r-GO-20 exhibits the smallest Tafel slope of 54 mV dec-1, which is better than those of the Ni3Fe/r-GO-20 (57 mV dec-1), Ni3N/r-GO-20 (65 mV dec-1), Fe2N/r-GO-20 (93 mV dec-1) and Ni3FeN (116 mV dec-1). This suggests that the Ni3FeN/r-GO-20 catalyst has the fastest OER kinetics. Electrochemical impedance spectroscopy (EIS) were carried out to investigate kinetics. Compared with the Ni3Fe/r-GO-20, Ni3N/r-GO-20, Fe2N/r-GO-20 and Ni3FeN, the Ni3FeN/r-GO-20 shows a significantly decreased charge-transfer resistance (Figure 4c). The EIS results confirm that the Ni3FeN/r-GO-20 exhibits a much faster charge-transfer process during electrochemical reaction, which is in good agreement with the results of more positive onset potential and low Tafel slope. This means the bimetallic nitride has a higher electrical conductivity. Furthermore, the metallic nitride NPs supported on r-GO aerogel display lower charge-transfer resistance than that of the Ni3FeN NPs.41 This should be associated with the 3D connected hierarchical mesoporous structure of the aerogels. In addition, the high stability of catalysts toward OER is crucial to assess the overall performance in practice. For this purpose, the durability of the Ni3FeN/r-GO-20 was measured. As shown in Figure 4d, the polarization curve of Ni3FeN/r-GO-20 only slightly drifted after extended scanning of 2000 cycles. The time-dependent current density profile of the Ni3FeN/r-GO-20 was also evaluated under overpotential of 280 mV over 10 h (Figure 4d, inset). Both indicate that Ni3FeN/r-GO-20 exhibits the stability toward OER. The electrochemical HER activities of the Ni3FeN/r-GO, Ni3Fe/r-GO-20, Ni3N/r-GO-20, Fe2N/rGO-20, Ni3FeN, and commercial Pt/C on a Ni foam electrode were also evaluated in 1.0 M KOH solution by a standard three-electrode system. The LSV curves show that Ni3FeN/r-GO-20 exhibits a

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low overpotential of 94 mV to reach a 10 mA cm-2 current density, which is notably smaller than the 110 mV of Ni3Fe/r-GO-20, 156 mV of Ni3N/r-GO-20, 195 mV of Fe2N/r-GO-20 and 235 mV of Ni3FeN (Figure 5a). As expected, Pt/C revealed near-zero onset potential and a small overpotential with only 50 mV at 10 mA cm-2 current density. However, with the increase of current density, the discrepancy between Pt/C and Ni3FeN/r-GO-20 are reduced gradually. Importantly, the current density of Ni3FeN/rGO-20 becomes higher than that of the Pt/C beyond an overpotential of 213 mV, demonstrating Ni3FeN/r-GO-20 is superior to Pt/C under higher overpotentials. The overpotential of Ni3FeN/r-GO-20 at 10 mA cm-2 current density is the smallest among most HER catalyst reported recently (Table S2). The polarization curves of Ni3FeN/r-GO-x are show in Figure S11, and Ni3FeN/r-GO-20 also has the smallest onset potential, which reveals that an appropriate amount of 20% r-GO is also beneficial for HER performance. Figure 5b show that the Tafel slope of Ni3FeN/r-GO-20 is 90 mV dec-1, which is smaller than Ni3Fe/r-GO-20 (109 mV dec-1), Ni3N/r-GO-20 (111 mV dec-1), Fe2N/r-GO-20 (120 mV dec-1), and Ni3FeN (123 mV dec-1). This means that the Ni3FeN/r-GO-20 has more efficient electron transfer for HER. The Nyquist plots of Ni3FeN/r-GO-20 exhibited the smallest semicircle in the low-frequency range (Figure 5c), suggesting the lowest charge transfer resistance. The result of durability test of Ni3FeN/r-GO-20 displays in Figure 5d, showing the stable HER catalytic performance with negligible degradation after continuous CV scanning for 2000 cycles. As seen in the inset of Figure 5d, the chronoamperometric test for Ni3FeN/r-GO-20 at an overpotential of 280 mV shows that the current density is nearly constant during the continuous operation for 10 h. In addition, electrochemical doublelayer capacitance (Cdl) was measured to evaluate the effective electrode surface area. Ni3FeN/r-GO-20 exhibits the largest Cdl of 15.9 mF cm−2 which demonstrates the largest exposure of effective active sites (Figure S12).

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Given the excellent bifunctional performance, the Ni3FeN/r-GO-20 hybrid material was used as a bifunctional catalyst to further assess its performance in an electrolytic cell at ambient environment. In Figure 6a, the polarization curves of Ni3FeN/r-GO-20 show that the voltage difference (ΔV) between HER and OER are 1.59 V (vs RHE) at 10 mA cm-2. This value is very close to the result from overall water splitting test, which required external voltage of 1.60 V with a current density of 10 mA cm-2, indicating the steady state process for overall water splitting (Figure 6b). Remarkably, this water splitting potential of 1.60 V is smaller than those of transition-metal-based bifunctional catalysts such as NiFe LDH/NF (1.70 V), NiS/NF (1.64 V), NiCo2S4 NA/CC (1.68 V), Ni3S2/NF (1.76 V), NiSe/NF (1.63 V) (Table S3). Additionally, the Ni3FeN/r-GO-20 also exhibits robust stability for the overall watersplitting reaction in 1.0 M KOH. As shown in Figure 6c, when setting a voltage at 1.60 V, a steady current density of 10 mA cm-2 was retained with no obvious decay for 100 h. Phenomenally, the evolution of H2 and O2 gas bubbles could be clearly observed by applying with a voltage of 1.60 V (Figure 6d; Movie S1, Supporting Information). These results suggest that Ni3FeN/r-GO-20 is a promising candidate to replace precious metal catalysts for practical alkaline water electrolysis applications. To reveal the origin of the HER and OER activity, theoretical calculations were carried out using the density functional theory (DFT) by implementing VASP code with exchange-correlation function that is modeled by Perdew-Burke-Ernzerhof (PBE) function.42 The Ni3FeN (111) surfaces were used to construct a complex surface with r-GO because of the minimal surface energy of the (111) surface.22,23,43,44 As comparison, Ni3N (001) and Fe2N (001) surfaces were chosen to build Ni3N (001)/rGO and Fe2N (001)/r-GO interfaces. As shown in Figure 7, the two surfaces have similar structure to the Ni3FeN

(111)

surface

and

have

been

examined in the previous

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theoretical investigations.18,45,46 Computational details and model construction can be found in supporting information. The optimized structural representations of Ni3FeN (111)/r-GO, Ni3N (001)/r-GO and Fe2N (001)/r-GO are presented in Figure 7a-c. To obtain further perceptions about three interaction between the metal nitrides and r-GO, We calculated the charge density of three models. Figure 7d-f display the charge density difference between the metallic nitrides and the r-GO substrate. The pink and green regions refer to charge accumulation and charge depletion, respectively. Obviously, most of the charge transfer in Ni3N (001)/r-GO and Fe2N (001)/r-GO occurs at the interface of metallic nitride cluster/r-GO, and few charge accumulation on the r-GO surface is observed (Figure 7e and 7f). However, for Ni3FeN (111)/r-GO, abundant charge accumulation can be seen on the r-GO side due to more electrons (1.78 e-) transferred from the Ni3FeN cluster to the r-GO (Figure 7d and Table S4). Density of states (DOS) were calculated to further pinpoint the origin of fast electron transfer of Ni3FeN. As shown in Figure 7g, the band structure of Ni3FeN are consecutive near the Fermi level, which demonstrates that Ni3FeN is intrinsically metallic.18,23 The partial DOS of Ni3FeN illustrates that the d orbitals of the Ni and Fe make a major contribution to the total DOS. The partial DOS contributed by the p orbital of N is also successive, suggesting a covalent interaction among Ni, Fe, and N.23 However, Ni3N and Fe2N have only one metal track to contribute DOS value, which suggests more electron transport of Ni3FeN than those of Ni3N/Fe2N from the cluster to the r-GO. The number of electrons transferred from Ni3FeN cluster to the r-GO layer will lead to charge redistribution at the interface between Ni3FeN and r-GO. The electrons accumulation on r-GO is beneficial for enhancing C−H binding and thus improving HER activity as a whole. On the other hand, the electron transfer results in the holes accumulation on Ni3FeN, which favors to obtain the optimal value of binding energies for OER reaction intermediates and show the excellent OER performance.26,34

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The energy diagrams of OER, where free energies of various intermediates adsorbed on the r-GO by the catalysts at zero potential (U = 0 V) were calculated as shown in Figure 7h. The relative value between △GO* and △GOH* was used as an indicator to describe the OER catalytic activity. According to the universal scaling relation, the O* adsorption energy can ascertain the overpotential. All reaction steps present uphill at the zero potential (U = 0 V) on the three model surfaces. For the Ni3N (001)/r-GO and Fe2N (001)/r-GO, the theoretical overpotentials are 0.64 eV and 0.76 eV, respectively. The data are in good agreement with the experimental results that show that single-metal nitrides need high overpotential for OER. However, the Ni3FeN (111)/r-GO exhibits a theoretical overpotential as low as 0.46 eV, which indicates the best OER performance. With respect to HER, △GH* is employed as a key parameter in predicting theoretical activity for HER in an alkaline electrolyte. It is a necessary criterion for a good catalyst to have a moderate H adsorption energy of ~ 0 eV, which leads to optimal HER activity with a lower reaction barrier. The H adsorption free energy of commercial Pt is around 0.09 eV, which approaches the thermoneutrality.47 We also calculated the △G0(H*) of the three models, which are in good agreement with experimental results: bimetallic nitrides exhibit HER performance better than that of single metal nitrides. As shown in Figure 7i, Ni3FeN (111)/r-GO has the smallest ∆GH* of 0.17 eV, whereas Ni3N (001)/r-GO and Fe2N (001)/r-GO show values of 0.53 eV and 0.62 eV, respectively. CONCLUSIONS In summary, we report the synthesis of Ni3FeN/r-GO aerogels by a simple and scalable biomass conversion strategy. When evaluated as bifunctional catalyst for OER and HER, the aerogel showed an outstanding capability to achieve 10 mA cm-2 with a potential of 1.60 V and superior durability for overall water splitting. DFT calculations reveal that the synergistic effect between Ni and Fe can change the charge distribution of bimetallic nitrides and then optimize the electronic structure of material

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surface. Therefore, the electronic properties of the r-GO can be altered by the electron transfer from metal clusters, which can tune significantly the adsorption energy on the r-GO surface for OER and HER. In addition, the 3D interconnected hierarchical mesoporous structure of nanoaerogels could enhance molecular transport and catalytic activity because of the high porosity and high electrical conductivity of r-GO for a fast electron transfer. The present work provides a way in probeing a very simple, inexpensive, and environmentally-friendly biomass conversion pathway to fabricate excellent bimetallic nitride systems for overall water splitting. EXPERIMENTAL METHODS Synthesis of Ni3FeN/r-GO-x: An alginate solution (1 wt%) with different amounts of r-GO (x = 0, 10, 20, 30 mass ratio (%) of r-GO/sodium alginate) was poured into a 5 wt% aqueous solution containing Fe3+ and Ni2+ cations (with a Ni/Fe molar ratio of 10:1) (Aladdin Chemistry Co. Ltd., Shanghai, China, ⩾99.0%) under vigorous stirring to form (Ni,Fe)-alginate/r-GO hydrogel. The Ni and Fe cross-linked hydrogels were separated from the solution after 30 minutes and washed clearly several times. The hydrogels were dehydrated through lyophilization approach to form (Ni,Fe)-alginate/r-GO aerogels. The resultant NiFeA/r-GO aerogel composites were placed in a tubular furnace and treated at 700 °C for 1 h with the heating rate of 5 °C min-1 in NH3 atmosphere. Synthesis of Ni3Fe/r-GO-20: The synthesis process are the same as Ni3FeN/r-GO-20 except for the heating in Ar/H2 atmosphere. Synthesis of Ni3N/r-GO-20: Ni-alginate/r-GO-20 aerogels were treated to 700 °C for 2 h with the heating rate of 1 °C min-1 in NH3 atmosphere.

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Synthesis of Fe2N/r-GO-20: Fe-alginate/r-GO-20 aerogels were treated to 700 °C for 1 h with the heating rate of 5 °C min-1 in NH3 atmosphere. Characterization: The crystalline phases were studies by X-ray diffraction (XRD). The morphology were studied by field emission scanning electron microscopy (FESEM) and TEM, high-resolution TEM (HRTEM) and HAADF-STEM images were obtained. X-ray photoelectron spectroscopy (XPS) was carried out for chemical composition. For the test details, see our previous reports.32,33,46 Electrochemical Measurements: Electrochemical studies were carried out in a standard three electrode system controlled by a CHI 760D electrochemistry workstation. For preparing working electrode, 3.0 mg of the catalyst was dispersed in a mixture of ethanol (300 µl), water (300 µl) and Nafion (5 wt%, 30 µl). After that, 105 µl of catalyst ink was drop dried onto a 1 cm × 1 cm Ni foam (loading 0.5 mg/cm2). Catalyst loaded on Ni foam was used as the working electrode, a graphite plate as the counter electrode and Ag/AgCl electrode as the reference electrode for the HER and OER tests. The linear sweep voltammetry curves were recorded at a scan rate of 5 mV s-1 in 1 M KOH as the electrolyte. All LSV polarization curves were corrected with 95% iR-compensation before measurements. The electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range 0.01-105 Hz. The double-layer capacitances (Cdl) were estimated by CV in the 0–0.2 V versus RHE region at various scan rates (20, 40, 60, 80, 100, 120, 140 mV s−1) to evaluate the effective surface area of various catalysts. The overall water splitting test was conducted in a two-electrode device using Ni3FeN/r-GO-20 catalyst as anode and cathode. All data of the two electrode test were carried out without iR-compensation.

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Figure 1. Schematic illustration for scalable synthesis of Ni3FeN/r-GO catalysts derived from sustainable SA/r-GO hydrogels.

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Figure 2. (a) XRD patterns of the Ni3FeN/r-GO-x samples. (b) TEM images of Ni3FeN/r-GO-20. (c) The carbon shell of a Ni3FeN NP with a thickness of ~ 3 - 6 nm. (d) Fast Fourier transform (FFT) pattern and HRTEM image. (e) EDS elemental mapping images (Ni, Fe, N, C, and O).

Figure 3. XPS spectra of the Ni3FeN/r-GO-20: (a) Survey scan, (b) Ni 2p, (c) Fe 2p, and (d) N 1s.

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Figure 4. OER electrocatalytic performance. (a) Polarization curves of Ni3FeN/r-GO-20, Ni3Fe/r-GO-20, Ni3N/r-GO-20, Fe2N/r-GO-20, Ni3FeN and commercial IrO2/C in 1.0 M KOH at a scan rate of 5 mV s-1. (b) Corresponding Tafel plots. (c) Nyquist plots obtained at 200 mV overpotential. (d) Polarization curves recorded for the Ni3FeN/r-GO-20 before and after 2000 cycles of CV scan under basic conditions. (Inset: chronoamperometric curve at the overpotential of 280 mV).

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Figure 5. HER electrocatalytic performance. (a) Polarization curves of Ni3FeN/r-GO-20, Ni3Fe/r-GO-20, Ni3N/r-GO-20, Fe2N/r-GO-20, Ni3FeN and commercial Pt/C in 1.0 M KOH at a scan rate of 5 mV s-1. (b) Corresponding Tafel plots. (c) Nyquist plots obtained at 200 mV overpotential. (d) Polarization curves recorded for the Ni3FeN/r-GO-20 before and after 2000 cycles of CV scan under basic conditions. (Inset: chronoamperometric curve at the overpotential of 120 mV).

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Figure 6. (a) The polarization curves of Ni3FeN/r-GO-20 for HER and OER. (b) The overall watersplitting performance of Ni3FeN/r-GO-20, Ni3Fe/r-GO-20, Ni3N/r-GO-20, Fe2N/r-GO-20 and Ni3FeN with a scan rate of 5 mV s-1 in 1.0 M KOH without iR-correction. (c) Time-dependent current density curve for Ni3FeN/r-GO-20 in a two-electrode configuration at a potential of 1.61 V. (d) Photograph of the two-electrode electrolyzer using Ni3FeN/r-GO-20 as both anode and cathode at a potential of 1.60 V.

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Figure 7. The optimized structural representations of (a) Ni3FeN (111)/r-GO (b) Ni3N (001)/r-GO and (c) Fe2N (001)/r-GO. Calculated charge density differences of (d) Ni3FeN (111)/r-GO, (e) Ni3N (001)/rGO and (f) Fe2N (001)/r-GO. The Ni, Fe, N and C atoms are marked in wathet, tawny, blue and yellow respectively. The pink and green regions refer to increased and decreased charge distributions,

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respectively. (g) Total and partial electronic density of states (TDOS and PDOS) calculated for Ni3FeN (111), Ni3N (001), and Fe2N (001). Schematic energy profiles of different models for (h) OER pathway and (i) HER pathway.

ASSOCIATED CONTENTS Supporting Information Additional, photograph, SEM, XRD, and electrochemical characterizations and additional Tables. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We are grateful for the financial support by the National Natural Science Foundation of China (grant no. 51473081 and 51672143), Outstanding Youth of Natural Science in Shandong Province (JQ201713), and ARC Discovery Project (No. 170103317). REFERENCES

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