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Morphological and Electronic Tuning of NiP Through Iron Doping towards Highly Efficient Water Splitting Hao Sun, Yuxiang Min, Wenjuan Yang, Yuebin Lian, Ling Lin, Kun Feng, Zhao Deng, Muzi Chen, Jun Zhong, Lai Xu, and Yang Peng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02264 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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ACS Catalysis

Morphological and Electronic Tuning of Ni2P Through Iron Doping towards Highly Efficient Water Splitting Hao Sun,†,§,‖ Yuxiang Min,‡,‖ Wenjuan Yang,⊥,‖ Yuebin Lian,†,§ Ling Lin,†,§ Kun Feng,‡ Zhao Deng,*,†,§ Muzi Chen,# Jun Zhong,‡ Lai Xu,*,‡ and Yang Peng*,†,§ †

Soochow Institute of Energy and Material Innovations, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China. [email protected]; [email protected] § Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Soochow University, Suzhou 215006, China. ‡ Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China. E-mail: [email protected] ⊥

#

Department of Chemistry, Tsinghua University, Beijing 100084, China. Analysis and Testing Center, Soochow University, Suzhou 215123, China.

ABSTRACT: Efficient water electrolysis for hydrogen production constitutes a key segment for the upcoming hydrogen economy, but has been impeded by the lack of high-performance and low-cost electrocatalysts for, ideally, simultaneously expediting the kinetics of both hydrogen and oxygen evolution reactions (HER and OER). In this study, the favored binding energetics of OER and HER reaction intermediates on iron-doped nickel phosphides are first predicted by DFT simulations, and then experimentally verified through the fabrication of Fe-doped Ni2P nanoparticles embedded in carbon nanotubes using MOF arrays on nickel foam as the structural template. Systematic investigations on the effect of phosphorization and Fe-doping reveal while the former endows a larger benefit on OER than on HER, the later enables not only modulating the electronic structure, but also tuning the micro-morphology of the catalyst, synergistically leading to both enhanced HER and OER. As a result, extraordinary performances of constant water electrolysis are demonstrated requiring only a cell voltage of 1.66 V to afford a current density of 500 mA cm-2, far outperforming the benchmark electrode couple composed of Pt/C and RuO 2. Post-electrolysis characterizations combined with DFT inspection further reveal that while the Fe-doped Ni2P species are mostly retained after prolonged HER, they are in-situ converted to Fe/P-doped γ-NiOOH during OER, serving as the actual OER active sites with high activity. KEYWORDS: electrocatalysis, overall water splitting, transition-metal phosphides, metal-organic frameworks, iron-doping

1. Introduction Water electrolysis for hydrogen production offers a promising route to convert and store the abundant but intermittent renewable energy resources such as solar, wind, and tide etc. on a large scale without leaving any carbon footprints.1-2 At the core of this technology is the development of high-efficiency but low-cost catalysts to expedite the kinetics of both the cathode and anode reactions, namely, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).3-4 Until now, Pt-based nanomaterials5 and Ru/Ir-based metal oxides6 are still regarded as the benchmark materials for catalyzing HER and OER, respectively, but their low natural abundance and high cost greatly hinder prevailing implementation, especially in industrial mass production. 7-8 Besides, apart from the added material and processing costs, using different catalyst for each half reaction of water splitting is somewhat inefficient.9 For instance, while Pt/C is most efficient in acids, RuO2 has a better stability and activity in alkaline eletrolytes.10-12 Recently, a variety of bifunctional materials have shown great promise in catalyzing overall water splitting, including metal,13-14 metal derivatives,15-18 porous carbon,19-20 and metal-organic frameworks (MOFs)21-22 etc., but most of them can only operate steadily with low current densities. In order to meet the stringent requirements from realworld applications requiring high current densities at low power input,23-24 the exploitation of high-performance bifunctional catalysts based on earth-abundant elements is on demand.

Transition-metal phosphides (TMPs), such as FeP,25-26 CoP,27-28 Ni2P29 etc., have arisen as a class of highly potent electrocatalysts for water splitting due to their remarkable activities towards both HER and OER, as well as their low fabrication costs. Recent studies have shown incorporating foreign metal atoms into the lattice of these metal phosphides enables to further adjust the atomic coordination and electronic structure, resulting in improved catalytic activities.10, 30-33 For example, Yu and co-workers synthesized a series of 2D Cobased bimetallic phosphide nanosheets and confirmed with computational investigations that the doping of secondary metals into the primitive CoP crystals can effectively modulate the electronic structure to optimize the binding energies of OER intermediates.34 Through the fabrication of core-shell Ni@Ni2P-Ru heterostructures, Dai and co-workers showed the introduction of Ru into Ni2P enables to effectively optimize the hydrogen adsorption energy (ΔGH) and thus promote the HER kinetics.35 Despite of these remarkable advances, a systematic investigation and fundamental understanding on the dopantmodulated catalytic process is still lacking, especially regarding the transition of active species along the reaction courses. 36 In addition, aiming to maximizing the H2 production and minimizing the energy input simultaneously, it is highly desired to further extend the kinetics and stability of the TMP catalysts through proper material and electrode design, especially in large scale heavy-duty operations.

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In this work, we first conducted DFT simulations to interrogate the effect of iron doping on the binding of OER and HER intermediates to the modeled Ni2P surfaces. Then, based on the simulated results predicting favored intermediate binding on the iron-doped nickel phosphides, we devised a synthetic strategy to fabricate Fe-doped Ni2P nanoparticles embedded in carbon nanotubes, using MOF arrays directly grown on nickel foam (NF) as the structural template. The Ni(BDC) MOF we chose as the precursor in this work has a tunable 2D lamellar structure, and after carbonization and phosphorization generates a large amount of carbon nanotubes encapsulating nanoparticles of metal phosphides. This hierarchical catalyst structure greatly promotes the material conductivity, the electrochemical surface area, the exposure of active sites, as well as the charge and mass transfer. More importantly, without other compositional interference such as nitrogen-doped carbon and multiple phases of phosphides, the electronic structure of the Ni2P can be effectively modulated by iron-doping, resulting in much enhanced active sites and clearer understanding of the activity origin. As a result, extraordinary HER and OER activities are achieved, leading to an impressive performance of overall water splitting requiring only a cell voltage of 1.66 V to afford a current density of 500 mA cm-2, significantly outperforming the benchmark electrode couple composed of Pt/C and RuO2. 2. Results and discussion

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2.1 DFT Simulation. Under alkaline conditions OER typically takes four elementary steps, including sequentially: (i) OH- adsorbed onto the catalyst surface, losing one e- to form *OH; (ii) *OH reacts with a second hydroxide ion, releasing one H2O molecule and one e- to form *O; (iii) *O further associates with another OH-, resulting in the formation of a peroxide group (*OOH) by giving out a third electron; (iv) *OOH reacts with the fourth OH- and release one H2O, one O2 and one e-.37-39 Accordingly, the OER kinetics can be assessed by calculating and comparing the free energy of each of these elementary steps.40-42 In the current work, due to the fact transition metals and their phosphides are easily oxidized in air, partial surface oxidation was introduced to the Ni (111) and Ni2P (111) lattice configurations (Figure 1a).34 Figure 1b shows the free energy diagrams of the OER intermediates adsorbed onto the oxidized surfaces of Ni, Ni2P, and Fe-doped Ni2P, revealing an up-hill endothermic nature for all elementary steps. In particular, the third elementary step (O*→OOH*) displays the highest free energy change (∆G3), which is rate-limiting for all catalysts. Obviously, ∆G3 of the Fe-doped Ni2P (2.60 eV) is lower than that of Ni2P (2.74 eV), and both are significantly smaller than that of Ni (111) (3.10 eV), predicting that doping of Fe into Ni2P might help boost its OER performance. In addition, the projected density of states (DOS) for both the un-doped and Fe-doped Ni2P with and without O* or OOH* absorption were calculated as shown in Figure S1. After the Fe

Figure 1. Theoretical prediction and experimental design. (a) Structure models of Ni (111), Ni 2P (111) and Fe-doped Ni2P (111) used for DFT calculation. (b) The free energy diagrams of OER intermediates on the above modelled surfaces. (c) The Gibbs free energies of absorbed hydrogen atom (ΔGH*) for HER. Inset: Volcano plot depicting the HER overpotentials as a functional of ΔGH*. (d) Schematic illustration of the catalyst preparation.

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doping, two peaks emerge near the Fermi level of Ni2P, implying that there is a greater possibility that Ni receives partial electrons from the dopant. The d-band center (εd) of the Ni atoms in Ni2P and Fe-doped Ni2P was calculated to be -1.36 eV and -1.58 eV, respectively (Table S1). From the perspective of the d-band theory, the Ni atoms with a higher d-band center should lead to a stronger binding between the metal center and the coordinated ligands.43-44 Therefore, In the case of Fe-doped Ni2P, the doping might decrease the overall absorption of reaction intermediates towards the sweet spot of optimal binding.45 The εd values of Ni upon binding with O* reveal an upshift of the d band center (+0.2 eV) for Fe-doped Ni2P, facilitating the absorption of redox species for the next-stage elemenatry reaction. On the other hand, a slight downshift of the d band center (-0.04 eV) was observed for the Fe-doped Ni2P with OOH* binding, well explaining the reduced free energy change of this rate-determining step. For HER, the Gibbs energy of H* (ΔGH*) severs as an important predictor for activity assessment. As stated by the Sabatier principle, a catalyst should bind the intermediates neither too strongly (ΔGH* < 0 eV) nor too loosely (ΔGH* > 0 eV), i.e., the optimum situation to realize an active HER electrocatalyst is ascribed to reaction intermediates that are thermoneutrally bound (ΔGH* = 0 eV) at zero overpotential.46-48 By correlating the ΔGH* with the catalytic performance, it can be generally summarized as a volcano relationship and used as a guide for the catalyst development.33 Similar to OER, we also performed DFT simulation to compare the values of ΔG H* for Ni, Ni2P, and Fe-Ni2P. The results are shown in Figure 1c, where the partially oxidized Ni (111) surface expresses a negative exothermic ΔGH* of -0.14 eV, located at the left side of the volcano (Figure 1c, inset) and indicating a relatively strong binding of H*. On the other side, the adsorption of H* on Ni2P is too weak with a positive ΔGH* (0.26 eV) located at

the right end of the volcano, which is even worse than that of Ni (111) regarding the absolute values. Fortunately, the ΔG H* of Ni2P can be effectively tuned by Fe doping, and even the addition of a small amount of Fe can dramatically reduce the ΔGH* of Ni2P to -0.01 eV. This ΔGH*, which is very close to the top of the volcano, should greatly benefit the kinetics of hydrogen conversion. Similar phenomenon was also seen by Rappe et al. when doping into Ni2P with a proper amount of 2p nonmetals and heavier chalcogens that provided nearly thermoneutral H adsorption.33 To validate the computed results experimentally, we then fabricate Fe-doped Ni2P nanoparticles embedded in carbon nanotubes and inspect their catalytic OER and HER activities as detailed in the following sections. 2.2 Catalyst Fabrication and Characterization. Arrays of Ni(BDC) directly grown on NF were adopted as the structural template for fabricating the Ni, Ni2P and Fe-doped Ni2P nanoparticles embedded in carbon nanotubes after carbonization and phosphorization (Figure 1d). The iron content was adjusted by varying the proportion of metal salts in synthesizing the MOF precursors, where the Fe/Ni feeding ratios of 0/1, 1/2, 1/1, 2/1 were applied. Accordingly, the asprepared MOF samples are denoted as NF@Ni(BDC), [email protected](BDC), NF@Fe1-Ni(BDC), and NF@Fe2Ni(BDC). We note that when the amount of iron salts was further increased, the resulted MOFs failed to fully cover the surface of nickel foam, leading to non-uniform surface growth. Cross-section scanning electron microscopy (SEM) images show the coating thickness of MOFs on nickel foam is ~20 m for all samples (Figure S2). X-ray diffraction (XRD) analysis reveals characteristic Bragg peaks associated with both the Ni(BDC) and NF underneath (Figure S3), confirming a good crystallinity of the surface-immobilized MOFs.49 Notably, some minor diffraction peaks of Ni(BDC), such as the (010) and (020) planes, are weakened or even disappeared with increasing

Figure 2. Spectroscopic characterizations of various catalyst samples stripped off from the nickel foam. (a) XRD patterns of Ni/C, Ni2P/C, Fe2-Ni/C and Fe2-Ni2P/C. (b) Shift of XRD (111) peaks for Ni2P/C samples with different amount of Fe-doping. (c) XPS P 2p, (d) Fe 2p3/2 and (e) Ni 2p3/2 spectra of the Fe2-Ni2P/C sample. (f) Ni K-edge XANES spectra of Fe2-Ni2P/C, Ni2P/C, Ni foil and NiO.

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Fe-doping, indicating subtle change of crystallinity. This change is also reflected in the top-view SEM images (Figure S4a-d), showing a more disordered and slender lamellar structure with increasing iron content. The lamellar sheet structure of the Fe-doped Ni-MOFs is further confirmed by transmission electron microscopy (TEM, Figure S5) and atomic force microscopy (AFM) (Figure S6) images, showing the linear decrease of sheet thickness with increasing Fe-doping. Thermogravimetric analysis (TGA) revealed similar thermal stability among all Fe-doped Ni-MOFs, starting to decompose at ~450 oC in argon after the volatilization of absorbents in the MOF channels (Figure S7). Collectively, these observations suggest a tunable packing morphology and sheet structure by varying the amount of Fe-doping. All samples of Fe-doped Ni-MOFs grown on NF were then annealed at 600 °C in Ar to grow carbon nanotubes encapsulating metal nanoparticles from the MOF precursors. Based on our prior investigations, we found there exists an optimal balance between the carbonization degree and the metal particle aggregation for the MOF-converted catalysts,50 and in the current case carbonization at 600 °C is the turning point where both HER and OER performances reach the best. The thus obtained samples are designated as NF@Ni/C, [email protected]/C, NF@Fe1-Ni/C and NF@Fe2-Ni/C, respectively. By stripping off the carbonized surface layer from the nickel foam through extensive high-power sonication, the actual Ni/Fe ratios can be quantified by inductively coupled plasma atomic emission spectrometer (ICP-AES), giving values of 19.3, 16.0 and 13.7 for Fe0.5-Ni/C, Fe1-Ni/C and Fe2-Ni/C, respectively (Table S2). Apparently, the actual Fe/Ni ratios are drastically different from the stoichiometric values based on reactant feedings, indicating nickel is more readily to coordinate with the BDC ligands than iron does. XRD patterns taken on the stripped Fex-Ni/C powders (Figure S8a) reveal the emergence of two new peaks with increasing Fe-doping at 43.6°and 50.9°, respectively corresponding to the (111) and (200) planes of nickel-iron alloy (JCPDS 47-1405). Meanwhile, both the Ni

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(111) and (200) planes at 44.4°and 51.7°(JCPDS 04-0850) shift to lower angles, confirming the doping of Fe into nickel lattice (Figure S8b). The weak and broad peaks at around 26° correspond to the (002) plane of graphitized carbon, which is further supported by the D and G bands observed in Raman spectra (Figure S9a and Table S3). SEM images show that the carbonization process does not change the overall morphology of the surface layers, retaining the original microstructure of MOFs (Figure S10). TEM analysis, however, reveals a large amount of carbon nanotubes extending out from the carbonized 2D MOF sheets, and at the tip end of each nanotube resides a metal nanoparticle, confirmed by the lattice fringes characterized by HR-TEM (Figure S11, S12) and chemical states identified by XPS (Figure S13). Inside the nanosheets, some conglomerated larger particles embedded in the carbon matrix can be visualized. The as-obtained NF@Fex-Ni/C samples were further subjected to phosphorization at 350 °C using sodium hypophosphite as the phosphor source, and the resulted products are denoted as NF@Ni2P/C, [email protected]/C, NF@Fe1-Ni2P/C and NF@Fe2-Ni2P/C, respectively. The phosphorizing temperature was chosen so to ensure a complete phosphorization of surface metal nanoparticles but meanwhile to avoid phosphorizing the NF substrate. XRD spectra of all the phosphorized samples display characteristic patterns of Ni2P with two minor variations. First, as the iron content increases, an additional peak located at 43.5°, corresponding to the (103) plane of nickel-iron bimetal phosphide (JCPDS 54-1126), starts to emerge (Figure 2a, S13). Second, the characteristic N2P peaks located at 40.5°, 44.3°, 47.1° and 54.0°, respectively corresponding to the (111), (201), (210) and (300) planes (JCPDS 04-0850), shift to lower angles with the increase of Fedoping due to the substitution of Ni by Fe with a larger atomic radii (Figure 2b),30 which is coincident with former observations on the carbonized samples (Figure S8b). XPS survey spectrum taken on the Fe2-Ni2P/C sample clearly presents the co-existence of Ni, Fe, C, P and O elements (Figure

Figure 3. Morphological and microstructural characterizations of NF@Fe2-Ni2P/C. (a-b) SEM images of NF@Fe2-Ni2P/C with different magnifications. (c) Elemental mapping images of Fe2-Ni2P/C. (d) TEM image of Fe2-Ni2P/C. (e) HR-TEM images of Fe2Ni2P/C. Inset: selected-area diffraction pattern (top) and high-resolution lattice fringes (bottom). (f) AFM image of Fe2-Ni2P/C and the corresponding height profile along the marked red line.

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S15a). The P species with binding energy peaks located at 129.4 and 130.3 eV are assigned to the P 2p 3/2 and P 2p1/2 states of metal phosphides, and the peaks at 133.3 and 134.5 eV represent the -POx species owing to air exposure (Figure 2c). The C 1s peak of Fe2-Ni2P/C is almost identical to that of Fe2Ni/C (Figure S15b vs. Figure S13d), indicating that the phosphorization process had little effect on the carbon matrix, which is also reflected by the unchanged D and G bands in Raman spectra (Figure S9b and Table S3). The Fe 2P3/2 spectrum (Figure 2d) reveals two major species including the Fe-P at 707.1 eV and its oxidized counterpart (Fe-POx) at 711.7 eV. The high-resolution Ni 2p3/2 peak in Figure 2e consists of three sub-peaks at 852.0, 852.7, and 853.5 eV, corresponding to a small fraction of Ni0, the majority of Ni-P and some partially oxidized Ni-POx, respectively. Notably, when comparing the Ni 2p3/2 peaks of Fe2-Ni2P/C with the undoped Ni2P/C sample (Figure S15c), a downshift of ~0.2 eV can be observed for the former, indicating partial electron transfer from Fe to Ni, in response to the difference in work functions of the two metals (5.2 eV for Ni vs. 4.8 eV for Fe). This trend of electron transfer is consistent with the DOS change on Fe-doped Ni2P illustrated in the theoretical calculation section. As the above binding-energy shift of Ni 2p3/2 is subtle due to the small amount of Fe-doping, synchrotron X-ray absorption near edge structure (XANES) analysis was carried out to further probe the change of local atomic and electronic structure in the Fe2-Ni2P/C sample caused by Fe-doping. As shown in Figure 2f, both the Fe2-Ni2P/C and undoped Ni2P/C samples exhibit similar Ni K-edge XANES spectra, revealing a much smaller oxidation state than the Ni2+ in NiO, but slightly higher than the Ni0 in Ni foil. A closer look at the whit-line peaks of both Fe2Ni2P/C and Ni2P/C (Figure 2f, inset) is able to distinguish a slightly reduced Ni oxidation state in Fe2-Ni2P/C,12, 51 echoing the above XPS analysis. On the other hand, the iron atoms in Fe2-Ni2P/C shows a slight oxidation when compared to the pristine Fe foil, as a result of the loss of partial electrons (Figure

S16). Collectively, both the XPS and XANES analysis suggest even a small amount of Fe-doping enables to modulate the electronic structure of Ni2P, albeit in a subtle way, for mediating surface absorbates and reactions.30-31 No apparent change of morphology and microstructure was observed for all phosphorized products when compared to their carbonized precursors (Figure S17 vs. Figure S11). As shown in Figure 3a, b, the sample of NF@Fe2-Ni2P/C manifests as an array of 2D lamellar sheets closely packed on the surface of nickel foam with a thickness of ~20 μm (Figure S18), which should greatly facilitate the infiltration of electrolytes and exposure of active sites during electrolysis. The weight of the catalyst loading can be estimated by measuring the mass difference after stripping it off from the nickel foam, resulting in an average catalyst loading of 3.9 ±0.3 mg cm-2 for NF@Fe2Ni2P/C (Table S4). Elemental mapping by Energy Dispersive X-ray spectroscopy (EDX) reveals the weight percentages of Ni, Fe, P, C and O are 71.6, 5.1, 17.8, 3.3 and 2.1 wt %, respectively (Figure 3c, S18), matching well with previous ICP-AES results. TEM images show the nanoscopic feature of carbon nanotubes encapsulating nanoparticles was well inherited after the phosphorization process (Figure 3d, S20), and the average size of the nanoparticles is 5.8 ± 0.1 nm (Figure S21). HR-TEM reveals that the nanoparticles are mainly Ni2P, with an interplanar spacing of 0.22 nm resolved for its (111) planes (Figure 3e). In addition, bright diffraction spots corresponding to the (111), (201), (210) and (300) planes of Ni 2P are highlighted in the selected-area diffraction pattern (SADP), affirming the metal nanoparticles are mostly converted to phosphides (Figure 3e, inset), which is in good agreement with the XPS results. AFM height measurements on Fe2-Ni2P/C reveal that the thicknesses of the lamellar nanosheets after carbonization and phosphorization increases from 5.1 ± 0.2 to 5.5 ± 0.2 nm (Figure 3f vs. Figure S4d), suggesting a slight expansion of the lamellar sheet structure in comparison to the original MOF precursor, which, again, should be beneficial for

Figure 4. Electrocatalytic oxygen evolution and hydrogen evolution performances. (a) OER polarization curves of various catalyst samples. (b) OER Tafel plots obtained by chronopotentiometry. (c) EIS spectra of various catalyst samples recorded at a constant potential of 1.53 V. (d) HER polarization curves and (e) the corresponding Tafel plots of various catalyst samples. (f) EIS spectra recorded at an HER potential of -0.1 V.

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electrolyte infiltration. The above characterizations on NF@Fe2-Ni2P/C unambiguously demonstrate that metal particles, despite being embedded in thin carbon layers, can be successfully converted into phosphides. 2.3 OER, HER and Overall Water Splitting. The catalytic OER activities of all samples were examined in 1 M KOH electrolyte and all potentials are referenced to the reversible hydrogen electrode (RHE). In order to examine the effect of Fedoping and phosphorization on the OER performance, linear scanning voltammetry (LSV) were performed on both the carbonized precursors and phosphorized catalysts at a scan rate of 1 mV s-1. In line with our theoretical prediction, both the phosphorization and iron-doping help improve the OER performance. Specifically, NF@Fe2-Ni2P/C present the highest current density across the tested voltage range, suggesting a high reaction kinetics (Figure 4a), whereas the NF substrate without catalyst loading exhibits negligible OER activity. We note that the large anodic peak centered around 1.37 V caused by the oxidation of metal and metal phosphides prior to water oxidation severely masks the early OER signatures, making the estimation of 10 (the overpotential to reach a current density of 10 mA cm-2) by LSV very difficult. To address this issue, chronopotential curves were acquired to extract the 10 values for all catalysts. As shown in Figure S22, NF@Fe2-Ni2P/C exhibits the lowest 10 of 205 mV, better than those of NF@Fe2Ni/C (228 mV), NF@Ni2P/C (246 mV) and NF@Ni/C (271 mV). Notably, the performance of NF@Fe2-Ni2P/C is even better than the commercial RuO2 catalyst, loaded onto the nickel foam with an equivalent amount (NF@RuO2, 4 mg cm-2, 10 = 291 mV). Figure 4b presents the Tafel plots extracted also from the chronopotential curves, with NF@Fe 2-Ni2P/C displaying the smallest Tafel slope of 52 mV dec-1, lower than those of NF@Fe2-Ni/C (69 mV dec-1), NF@Ni2P/C (122 mV dec-1), NF@Ni/C (124 mV dec-1) and NF@RuO2 (66 mV dec-

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). More strikingly, NF@Fe2-Ni2P/C is able to deliver a high current density of 1000 mA cm-2 at 1.53 V, with an overpotential of only 300 mV. To further correlate the amount of Fe-doping with the OER activity of the catalysts, LSV and chronopotential plots were compared for the phosphorized samples with various iron contents (Figure S22, S23). Both the 10 values and Tafel slopes decrease with increasing Fe-doping, endorsing the benefit of a second metal doping in promoting the electrocatalytic activity. We surmise that this activity enhancement is attributed to not only the slender and disordered lamellar structure resulted from Fe-doping, but also the change of electronic structure due to bimetallic alloying. To further help understand the observed difference in OER performance, the electrochemically surface area (ECSA) of all samples were compared by calculating the double-layer capacitance (Cdl) via cyclic voltammetry (CV) scanned at different rates in the 0.23-0.43 V potential range without any redox processes (Figure S24a). The higher the Cdl, the higher the ECSA.52-53 Figure S24b shows NF@Fe2-Ni2P/C has the highest Cdl (297 mF cm-2), indicating that both Fedoping and phosphorization can synergistically increase the ECSA, likely due to the resulted morphology and crystalline changes. Furthermore, Nyquist plots (Figure 4c) obtained at 1.53 V (vs. RHE) under OER conditions reveal a small series ohmic resistance (Rs) of ~2.5 Ω for all samples, benefited from the excellent current-collecting capability of the carbon-coated NF, and a dramatically decreased charge transfer resistance (Rct) for NF@Fe2-Ni2P/C (1.8 Ω), much smaller than those of other samples.54-55 To necessitate high-efficiency overall water splitting, HER is the other equally important half reaction as OER. Among all tested catalysts, NF@Fe2-Ni/C demonstrated the best HER activity at low current densities with an 10 of 33 mV and Tafel slope of 26 mV dec-1, close to that of NF@Pt/C (4.0 mg cm-2)

Figure 5. Comparison of overall water splitting for the bifunctional NF@Fe2-Ni2P/C couple vs. the NF@Pt/C||NF@RuO2 couple. (a) Polarization curves of overall water splitting in a two-electrode configuration. (b) Overall water splitting powered by a single AA battery with a nominal voltage of 1.50 V. (c) Chronopotentiometric plots of water electrolysis with constant current densities of 10, 100 and 300 mA cm-2.

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exhibiting an 10 of 34 mV and Tafel slope of 24 mV dec-1 (Figure 4d, e). By comparison, the 10 and Tafel slope of NF@Fe2-Ni2P/C are 39 mV and 30 mV dec-1, respectively. However, at higher current densities larger than ca. 150 mA cm2 , NF@Fe2-Ni2P/C exhibits lower overpotentials to reach the same current density of NF@Fe2-Ni/C, suggesting the advantage of NF@Fe2-Ni2P/C in heavy-duty applications. This trend might be attributed to the lower charge transfer resistance at higher current densities (Figure S25) as a result of the nearly doubled ECSA of the phosphorized samples (Figure S24), which helps effectively reduce local current density and dissipate the electric energy into redox species. In fact, the NF@Fe2-Ni2P/C catalyst is capable of reaching 1000 mA cm-2 at an overpotential of only 183 mV. We note that unlike OER, for HER the benefit gained from phosphorization is less than that from Fe-doping, which is further evidenced by the lower activity of NF@Ni2P/C in comparison to NF@Ni/C. The HER activities of all phosphorized catalysts exhibit a linear increase with increasing Fe-doping (Figure S26), signifying the role of Fe-doping in promoting the HER activity of Ni2P. The low Tafel slope of NF@Fe2-Ni2P/C suggests a Tafel mechanism of HER with the combination of hydrogen atoms as the ratelimiting step.56 Nyquist plots obtained at -0.1 V (vs. RHE) under HER conditions indicate a small Rs value (2.8 Ω) for all samples, and a low Rct of 4 Ω for NF@Fe2-Ni2P/C, slightly higher than those of NF@Fe2-Ni/C and NF@Pt/C but much lower than those of NF@Ni/C and NF@Ni2P/C (Figure 4f). Importantly, the relative sluggish HER kinetics of NF@Ni2P/C and the great improvement by Fe-doping is in excellent agreement with previous results of DFT simulation conforming to the thermoneutral binding and Sabatier principle. Given the outstanding OER and HER kinetics of NF@Fe2Ni2P/C in 1M KOH electrolyte, we further constructed a twoelectrode cell for testing its competence in overall water splitting. For comparison, an electrolyzer with NF@RuO2 as the anode and NF@Pt/C as the cathode was also inspected. Remarkably, the cell voltage of the NF@Fe2-Ni2P/C couple to afford a current density of 100 mA cm-2 is as low as 1.57 V (Figure 5a), substantially lower than that of the benchmark Pt/C||RuO2 couple, which requires 1.69 V. Noteworthy, even electrolyzers composed of the bifunctional NF@Fe 2-Ni/C and NF@Ni/C couples demonstrated better performance of overall water splitting than the Pt/C||RuO2 couple, requiring 1.60 and 1.66 V, respectively, to reach 100 mA cm-2 (Figure S27). What’s more, since the NF@Fe2-Ni2P/C couple is capable of delivering more than 10 mA cm-2 at 1.50 V, water electrolysis can be powered by just using a single AA battery (Figure 5b and Video S1). Electrochemical stability is an important metric to assess the long-term catalytic performance. Figure 5c compares the chronopotentiometric plots of water electrolysis for the bifunctional NF@Fe2-Ni2P/C catalyst and the NF@Pt/C||NF@RuO2 couple in a two-electrode configuration with constant current densities of 10, 100 and 300 mA cm-2, respectively. During the 22-hour test, the benchmark Pt/C||RuO2 couple was able to deliver 10 mA cm-2 at a cell voltage of 1.53 V, but is still higher than NF@Fe 2-Ni2P/C requiring only 1.48 V. At this low current density, both catalysts can maintain a stable current output without notable voltage decay. However, once the testing current density was increased to 300 mA cm-2, the voltage of the Pt/C||RuO2 couple increased from 1.82 to 1.98 V with a total increment of 160 mV in 22 hours, whereas the voltage of the NF@Fe2-Ni2P/C couple only

Figure 6. Post-electrolysis characterizations and DFT inspection of post-OER oxyhydroxides. (a) XRD patterns of post-electrolysis Fe2-Ni2P/C. (b) XPS Ni 2p3/2 spectra of post-OER Fe2-Ni2P/C. (c) SEM image of post-OER NF@Fe2-Ni2P/C. (d) HR-TEM images of post-OER Fe2-Ni2P/C. Inset: selected-area diffraction pattern and high-resolution lattice fringes. (e) Schematic diagram of the phase transition between nickel hydroxides and oxyhydroxides at the anode during OER. (f) The free energy diagram of OER intermediates on the surfaces of undoped γ-NiOOH (101) and Fe/Pdoped γ-NiOOH (101). increased by 38 mV, affirming NF@Fe2-Ni2P/C has better stability than Pt/C-RuO2 at higher current density. In order to further challenge the long-term stability of the catalyst, we also tested the NF@Fe2-Ni2P/C electrolyser at the applied potential of 1.60 V for a prolonged duration of 110 h (Figure S28), and the current density only decreased by 2.7% from 156 mA cm -2 to 152 mA cm-2 through the whole testing period. To examine the Faraday efficiency of the electrolyzer, an upward-delivery cylinder method was employed to quantify the H2 and O2 production (Figure S29). With an applied current of ~25 mA at 1.53 V, ~20 mL H2 and ~10 mL O2 were collected in 90 min, accounting for an HER rate of 14.8 L s-1 cm-2 and a Faraday efficiency of 99.79%, demonstrating unprecedented kinetics and great potential of electrolytic H2 production in large scale. 2.4 Post-catalysis Analysis. In order to check if there is any change in composition and morphology of the NF@Fe2-Ni2P/C catalyst after prolonged water electrolysis, we further characterized both the cathode (post-HER) and anode (postOER) catalysts after continuous electrolysis for 20 h. XRD analysis (Figure 6a) reveals no change in material composition for NF@Fe2-Ni2P/C on the cathode side, and all diffraction peaks belonged to the Fe-doped Ni2P are retained. XPS of Ni 2p3/2 and Fe 2p3/2 taken on the post-HER catalyst (Figure S30) identifies the same species as the fresh sample (Figure 2),

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confirming the majority of Ni and Fe in the post-HER Fe2Ni2P/C sample maintain their phosphide state, which is in agreement with the above XRD results. Both SEM and TEM images show no morphological and microstructural changes for NF@Fe2-Ni2P/C on the cathode side after the prolonged electrolytic reaction (Figure S31a-b, S32a-c), with particleembedded carbon nanotubes and lattice fringes of Ni2P (111) clearly visible from the HR-TEM images (Figure S33). Instead, the situation at the anode side is quite different. Despite of the lamellar sheet structure that remained unchanged after OER (Figure 6c, S31c-d), TEM images show the original feature of nanotubes is lost, resulting in numerous floc sheet structures dispersed in the amorphous carbon matrix (Figure 6d, S30d-f). The SADP image in the upper inset of Figure 6d indicates a polycrystalline nature with four major diffraction planes, corresponding to the (006), (101), (012) and (110) planes of α-Ni(OH)2, which is further confirmed by the (101) lattice fringes with an interplanar distance of 0.27 nm in the HRTEM image (Figure 6d, bottom inset). XRD analysis also confirms the formation of α-Ni(OH)2 (Figure 6a), in addition to a small amount of Ni2P remnant that were not oxidized inside larger Ni2P nanoparticles. Similar observations were also made in previous studies on other metal and metal phosphide catalysts, which can be readily oxidized to form hydroxides or oxides at the high anodic potentials of OER.36, 57 Further comparing the XRD patterns of post-OER Ni2P/C and Ni/C samples, we note that the latter still contains a large amount of unoxidized Ni whereas the former is mostly converted to α-Ni(OH)2 (Figure S34), suggesting metal phosphides, even encapsulated in carbon nanotubes, are more susceptible to oxidation under OER conditions. This argument is further corroborated by XPS analysis, revealing the co-existence of Ni2+ and Ni3+, as well as the Fe2+ and Fe3+ states in the post-OER NF@Fe2-Ni2P/C catalyst (Figure 6b, S33). Furthermore, no metallic Ni and Fe states were detected on the surface of the catalyst, and the intensity of P 2p signals is drastically reduced, likely due to the oxidation to form phosphates dissolved into the electrolyte. Based on the above investigation, it is now clear that in the process of OER metal hydroxides are formed in-situ and replace the original phosphides as the new catalytically active sites, which can be further witnessed by the attenuation of the metal/metal phosphide oxidation peaks when comparing the LSV curves before and after continuous OER (Figure S36). Since the conversion of metal phosphides to metal hydroxides is a continuous process, and the catalytic stability shows almost no attenuation after prolonged electrolysis (Figure S28, S36), it strongly suggests that the in-situ converted Fe/P-doped αNi(OH)2 nanosheets share similar OER activity, if not better than, as their phosphide precursors. There have been a few studies pointing out the α-Ni(OH)2 can be further oxidized to the γ-NiOOH phase at high potential in alkaline media, acting as the actual catalytic sites during OER (Figure 6e).58-62 To further investigate the origin of the OER activity for Fe/P-doped α-Ni(OH)2, DFT investigation was conducted on its oxidized γNiOOH phase by comparing the free energy of reaction intermediates. Figure 6f indicates while the elementary step of (O* → OOH*) is rate-limiting for Fe/P-doped γ-NiOOH, with ∆G3 = 1.84 eV, (OH* → O*) is the rate-limiting step for the undoped γ-NiOOH (∆G2 = 1.94 eV). Calculations on DOS show that after Fe doping, the εd value increases from -2.77 eV for γNiOOH to -2.59 eV for Fe-doped γ-NiOOH (Figure S37 and Table S5), effectively pushing the energitics of intermediate binding towards the optimum from the direction opposite to the

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aforementioned Ni2P with iron doping as a result of the much weaker intermediate adsorption here. Moreover, ∆G3 of the Fe/P-doped γ-NiOOH is even lower than that of Fe-doped Ni2P (∆G3 = 2.60 eV, Figure 1b), corroborating the high electrochemical activity and stability of the catalyst despite of a phase conversion, and this further rationalizes the ultrahigh stability of water splitting observed in the current study. 3. Conclusions To summarize, in the current study we first predicted through theoretical calculations that doping of Fe into Ni2P can simultaneously promote both OER and HER activities, which was then verified experimentally by fabricating Fe-doped Ni2P nanoparticles embedded in carbon nanotubes using MOF arrays directly grown on nickel foam as the structural template. The Fe-doping enables not only modulating the electronic structure, but also tuning the micro-morphology of the catalyst, both synergistically leading to enhanced HER and OER. In comparison, the phosphorization step endows a larger benefit on OER than on HER. Benefiting from the hierarchical structure incorporating the macroporous nickel foam, the MOFderived 2D thin sheets, and the well-dispersed active sites comprising Fe-doped Ni2P nanoparticles encapsulated in carbon nanotubes, remarkable catalytic kinetics were achieved, affording a current density of 10 mA cm-2 at an overpotential of 205 mV for OER, and 39 mV for HER. When used for overall water splitting, the NF@Fe2-Ni2P/C electrode couple only required cell voltages of 1.57 and 1.66 V to achieve the current densities of 100 and 500 mA cm-2, respectively, significantly outperforming the benchmark electrode couple composed of Pt/C and RuO2. Moreover, the NF@Fe2-Ni2P/C catalyst demonstrated excellent high-current stability, showing no attenuation for more than 100 hours at a current density of 150 mA cm-2. Post-electrolysis characterization revealed that while the Fe-doped Ni2P species are HER active, during OER the Fe/P-doped α-Ni(OH)2 are formed in situ and after converted to the γ-NiOOH phase serve as the actual OER active sites with high electrochemical activity and stability. By developing a highly potent bifunctional catalyst composed of Fe-doped Ni2P nanoparticles embedded in carbon nanotubes, and providing insights into its catalytic activity origin, this study paves the way for realizing high-efficiency but low-cost electrocatalysts for stable overall water splitting on mass production scale.

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]; *[email protected]. ORCID Zhao Deng: 0000-0002-0008-5759 Lai Xu: 0000-0003-2473-3359 Yang Peng: 0000-0002-6780-2468 Author Contributions ‖H.S.,

Y.M. and W.Y. made a significant contribution throughout the work and contributed equally to this work. Z.D. and Y.P. conceived the original idea and supervised the project; Y.M. and L.X. performed the DFT simulation of OER; W.Y. performed the DFT simulation of HER; Y.L. and L.L. supervised the electrocatalytic performance of the samples; K.F.

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and J.Z. carried out the XANES characterization; M.C. carried out the TEM characterization. All authors have given approval to the manuscript. Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXXXX Detailed experimental procedures, DFT calculations methods, and additional figures and tables, video showing the demonstration of water-splitting device powered by an AA battery.

ACKNOWLEDGMENT This work is supported by Natural Science Foundation of China (No. 21701118), Natural Science Foundation of Jiangsu Province (No. BK20161209, No. BK20160323), Natural Science Research Project of Jiangsu Higher Education Institutions (18KJA480004), Six Talent Summit Project in Jiangsu Province (No. XCL-057, XCL-062, XNY-042), the Key Technology Initiative of Suzhou Municipal Science and Technology Bureau (SYG201748) and Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. Dr. Lai Xu also acknowledges the support from “Talents in demand in the city of Suzhou”, the Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project, and Joint International Research Laboratory of Carbon-Based Functional Materials and Devices.

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50. Sun, H.; Lian, Y.; Yang, C.; Xiong, L.; Qi, P.; Mu, Q.; Zhao, X.; Guo, J.; Deng, Z.; Peng, Y., A hierarchical nickel– carbon structure templated by metal–organic frameworks for efficient overall water splitting. Energy Environ. Sci. 2018, 11 (9), 2363-2371. 51. Xiao, Z.; Wang, Y.; Huang, Y.-C.; Wei, Z.; Dong, C.L.; Ma, J.; Shen, S.; Li, Y.; Wang, S., Filling the oxygen vacancies in Co3O4 with phosphorus: an ultra-efficient electrocatalyst for overall water splitting. Energy Environ. Sci. 2017, 10 (12), 2563-2569. 52. McCrory, C. C.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F., Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137 (13), 4347-57. 53. Zhang, H.; An, P.; Zhou, W.; Guan, B. Y.; Zhang, P.; Dong, J.; Lou, X. W. D., Dynamic traction of lattice-confined platinum atoms into mesoporous carbon matrix for hydrogen evolution reaction. Sci. Adv. 2018, 4 (1), eaao6657. 54. Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S., Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. J. Am. Chem. Soc. 2013, 135 (28), 10274-7. 55. Li, Y.; Li, F.-M.; Meng, X.-Y.; Wu, X.-R.; Li, S.-N.; Chen, Y., Direct chemical synthesis of ultrathin holey iron doped cobalt oxide nanosheets on nickel foam for oxygen evolution reaction. Nano Energy 2018, 54, 238-250. 56. Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H., MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. J. Am. Chem. Soc. 2011, 133 (19), 7296-9.

57. Stern, L.-A.; Feng, L.; Song, F.; Hu, X., Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles. Energy Environ. Sci. 2015, 8 (8), 23472351. 58. Klaus, S.; Cai, Y.; Louie, M. W.; Trotochaud, L.; Bell, A. T., Effects of Fe Electrolyte Impurities on Ni(OH)2/NiOOH Structure and Oxygen Evolution Activity. J. Phys. Chem. C 2015, 119 (13), 7243-7254. 59. Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y., Efficient water oxidation using nanostructured alpha-nickel-hydroxide as an electrocatalyst. J. Am. Chem. Soc. 2014, 136 (19), 7077-84. 60. Louie, M. W.; Bell, A. T., An investigation of thinfilm Ni-Fe oxide catalysts for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2013, 135 (33), 12329-37. 61. Su, X.; Wang, Y.; Zhou, J.; Gu, S.; Li, J.; Zhang, S., Operando Spectroscopic Identification of Active Sites in NiFe Prussian Blue Analogues as Electrocatalysts: Activation of Oxygen Atoms for Oxygen Evolution Reaction. J. Am. Chem. Soc. 2018, 140 (36), 11286-11292. 62. Gorlin, M.; Chernev, P.; Ferreira de Araujo, J.; Reier, T.; Dresp, S.; Paul, B.; Krahnert, R.; Dau, H.; Strasser, P., Oxygen Evolution Reaction Dynamics, Faradaic Charge Efficiency, and the Active Metal Redox States of Ni-Fe Oxide Water Splitting Electrocatalysts. J. Am. Chem. Soc. 2016, 138 (17), 5603-14.

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