Phosphorus and Aluminum Codoped Porous NiO Nanosheets as

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Phosphorus and Aluminum Co-Doped Porous NiO Nanosheets as Highly Efficient Electrocatalysts for Overall Water Splitting Zhao Li, Wenhan Niu, Le Zhou, and Yang Yang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00174 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Energy Letters

Phosphorus and Aluminum Co-Doped Porous NiO Nanosheets as Highly Efficient Electrocatalysts for Overall Water Splitting Zhao Li1#, Wenhan Niu2#, Le Zhou1, and Yang Yang1,2* 1

Department of Materials Science and Engineering, University of Central Florida, Orlando, FL

32826, United States 2

NanoScience Technology Center, University of Central Florida, Orlando, FL 32826, United

States *Email: [email protected] #

These authors contributed equally to this work.

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Abstract Herein, we presented a facile way to fabricate phosphorus and aluminum co-doped nickel oxidebased nanosheets by using layered double hydroxide (AlNi-LDH) as precursors, which showed an overall water splitting performance in alkaline solution. The co-doping of phosphorus and aluminum into nickel oxide nanosheets leads to an optimum balance among surface chemical state, electrochemically active surface area and density of active site. As a result, it can afford a current density of 100 mA cm-2 at the overpotential of 310 mV for oxygen evolution reaction (OER) and a current density of 10 mA cm-2 at the overpotential of 138 mV for hydrogen evolution reaction (HER) in 1 M KOH, respectively. When it was used as a bifunctional catalyst in a two-electrode water splitting device, a potential of 1.56 V was achieved at the current density of 10 mA cm-2.

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The growing energy demand has recently become a great issue all over the world due to the increasing depletion of fossil fuel. Thus, it is imperative to explore renewable energy resources to alleviate current energy crisis.1-5 Hydrogen, as a clean source, is regarded as one of the most promising and sustainable alternatives to fossil energy. Electrochemical water splitting is an efficient way for hydrogen generation, which consists of two main reactions including hydrogen evolution reaction (HER) on cathode and oxygen evolution reaction (OER) on the anode.6, 7 Extensive studies show that platinum group metals (PGMs) such as Ir, Ru, and Pt-based materials are the state-of-the-art catalysts for HER and OER owning to their excellent electronic conductivity and highly efficient active sites.8-10 However, their high cost, scarcity and poor stability greatly hinder the wide-commercialization of water splitting device.11, 12 As a result, the development of high-efficiency and cost-effective PGM-free catalysts is a primary work for wide application of hydrogen energy at present.13, 14 Recently, transition metal-based catalysts have been reported as newly alternative earthabundant catalysts for water splitting.8, 11, 15-19 Among them, nickel-based materials such as NiX (X=O, P, S, and Se) have attracted extensive attention due to their high electrocatalytic performance in water splitting.12, 18, 20-22 The high activity of nickel-based catalysts is primarily attributed to the optimal active sites on nickel surface for the absorption of proton and hydride, resulting in a favorable condition for O, P or S sites in the structure bonding with reaction intermediates in water splitting.23-26 Despite many strategies have been developed for improving the catalytic performance of Ni-based catalysts, the relatively low electrochemical surface area (ECSA) remains a great issue that leads to poor mass transfer and limited active sites.8, 27 Thus, it is worth to implant the additional active sites in Ni-based catalysts which can facilitate the electrochemical process and simultaneously increase the ECSA for water splitting. The Al

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element could be a good candidate, due to the dissolvable and nontoxic nature of acid and an alkaline electrolyte. In addition, there are studies reporting that the transition metal atoms such as Ni and Co could obtain additional electrons from adjacent Al atoms when the Al element doping into transition metals, leading to the optimal interaction between transition metal sites and reactant molecules.28, 29 Moreover, conventional transition metal catalyst electrodes are generally fabricated by loading powdered catalysts with polymer binders on the current collector, which will result in the decay of catalytic activity by the separation between catalysts and current collector during electrolysis.30,

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Therefore, it is important to develop a facile and scalable method for the

fabrication of a binder-free catalytic electrode with abundant active sites for water splitting application. In this work, we presented a novel strategy for the fabrication of nickel foam (NF) supported phosphorus (P) and aluminum (Al) co-doped porous nickel oxide nanosheets (referred as PANiO). Benefitting from good electron transfer properties, sufficiently exposed active sites and large ECSA, PA-NiO was applied as a bifunctional catalyst in overall water splitting, which showed a small potential of 1.56 V at a current density of 10 mA cm-2 in alkaline solution. The experimental observation and characterization demonstrated that the superior performance of PA-NiO was derived from the P and Al co-doping, which can improve its intrinsic electron transfer and durability, and porous structure leading to sufficient exposure of active site with facilitated mass transfer property. The synthetic procedure for PA-NiO is illustrated in Figure 1a. Firstly, NF supported AlNi-LDH as a precursor was synthesized by hydrothermal method. Then, the NF supported AlNi-LDH was phosphorized in the nitrogen atmosphere to obtain NF supported PA-NiO (Figure S1).

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ACS Energy Letters

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to investigate the morphology and composition of the samples, as illustrated in Figure 1 (b-f). Figure 1b shows that the layered structure of PA-NiO was vertically grown on the NF. The size and thickness of the PA-NiO nanosheet are calculated to be 200 nm and 6 nm, respectively. In contrast, PA-NiO shows a similar morphology to that of AlNi-LDH precursor (Figure S2), confirming the structure stability of PA-NiO during pyrolysis. Energy-dispersive Xray spectroscopy (EDS) data in Figure S3 confirms the co-existence of Ni, Al, P and O elements in PA-NiO. Interestingly, TEM images in Figure 1c show that AlNi-LDH as the precursor has a nanosheet-like structure with a smooth surface. However, after the phosphorization, the sample (PA-NiO nanosheets) derived from the AlNi-LDH shows a nanosheet morphology with a porous surface, ascribing to the dehydration of AlNi-LDH at a high temperature (Figure 1 d).32-34 Highresolution TEM (HRTEM) image in Figure 1e further confirms that the mesopores with around 5 nm diameter are highly distributed within PA-NiO surface, which likely plays a favorable role in the facilitation of mass transfer during electrolysis.34 The lattice fringe spacing of 0.20 nm in PANiO (insert in Figure 1e), which belongs to the (200) planes of NiO, is slightly smaller than that in standard NiO (0.21 nm), due to the P and Al co-doping into NiO.34-36 Meanwhile, the disordered lattices are observed in PA-NiO, which verifies the existence of defect sites in the surface. Whereas, the increased defect sites could lead to a decrease of total surface energy and further improve the electronic conductivity of NiO, which are good for electrocatalytic activity.37 Transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) element mapping images (Figure 1f) show the uniform distribution of Ni, Al, O and P elements in whole PA-NiO nanosheets, indicating that the P and Al elements were uniformly doped into the PANiO nanosheet by the phosphorization process.

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The crystal structures of AlNi-LDH and PA-NiO were identified by X-ray diffraction measurements (XRD), as shown in Figure 2a. The XRD pattern of AlNi-LDH shows a series of characteristic peaks at 11.35°, 22.80°, 32.48°, 34.93°, 39.27°, 46.5°, 61.07° and 62.29° which agree well with (003), (006), (101), (012), (015), (018), (110) and (113) planes of AlNi-LDH (JCPDS No. 15-0087).18,

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On the other hand, the XRD pattern of PA-NiO shows three

characteristic peaks of (110), (200) and (220) crystalline planes, which belongs to the NiO phase (JCPDS No. 47-1049). It suggests that a phase transformation process took place in the phosphorization process of AlNi-LDH to PA-NiO. Moreover, these peak intensities of PA-NiO are much lower than those of standard NiO (JCPDS No. 47-1049), revealing that the P doping into NiO resulting in the lattice disorder and low crystallization of PA-NiO.41 In addition to the P doping into NiO, Al was also identified as co-dopant in PA-NiO as evidenced by no crystalline Al composite phase found in XRD pattern.34 It is worth mentioning that Al as the dopant has been reported as a decoration component in improving the thermal stability of PA-NiO under high temperature.42 Raman spectroscopies further confirms the vibrational modes of AlNi-LDH and PA-NiO, as depicted in Figure 2b. The peaks located at 485 cm-1, 556 cm-1 and 1060 cm-1 correspond to the typical vibrational modes of AlNi-LDH.43-46 Besides, there are four peaks at 354 cm-1, 545 cm-1 and 722 cm-1 and 1080 cm-1 in PA-NiO, corresponding to the Ni-O bonding in PA-NiO and the first-order transverse optical (TO), longitudinal optical (LO), 2TO and 2LO phonon modes of NiO, respectively.47, 48 Note that the Raman spectra of AlNi-LDH are similar to that of PA-NiO, indicating a low doping concentration of P in NiO. X-ray photoelectron spectroscopy (XPS) measurement was employed to investigate the chemical state of PA-NiO. The XPS survey spectrum in Figure 2c illustrates the existence of Ni,

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Al, O and P elements in PA-NiO. High-resolution XPS of Ni 2p in Figure 2d shows that two fitted peaks at 855.27 eV and 873.09 eV correspond to Ni 2p3/2 and Ni 2p1/2. The two peaks at 861.45 eV and 879.40 eV belong to the satellites Ni 2p. These values are consistent with those of Ni2+.39, 49 Interestingly, the Ni 2p spectra for PA-NiO is shifted to a higher binding energy by 0.3 eV when compared with that of AlNi-LDH (Figure S4), which confirms that the doping of P and Al into NiO leads to a lower electron density of Ni.50 As shown in Figure 2e, two peaks at about 73.05 eV and 75.31 eV in Al 2p region coincide with that of Al dopant in NiO.51, 52 Figure 2f shows that the binding energy of P 2p is at 133.09 eV, corresponding to the P dopant in NiO.41 To better understand the effects of chemical composition and structure on determining the catalytic performance of PA-NiO, the control samples including NF supported AlNi-LDH, phosphatized NF (Ni-P) (Figure S5-S6) and NF supported commercial RuO2 (or Pt/C) were also prepared and subsequently compared by electrochemical analysis under the same condition. The OER activities of all samples were tested by linear sweep voltammogram (LSV) measurement in a standard three-electrode system with 1 M KOH aqueous electrolyte. As shown in Figure 3a, the PA-NiO electrode displays a good OER activity with more negative onset potential at 1.46 V when compared with 1.49 V for ALNI-LDH, 1.52 V for Ni-P，1.56 V for NF and 1.48 V for RuO2, respectively. Note that, the peak around 1.36 V is assigned to the oxidation peak of Ni (II) to Ni (III or IV).53 The remarkable OER activity of PA-NiO was further demonstrated by comparing the overpotentials at different anodic current densities (60 and 100 mA cm-2) with other electrodes. As shown in Figure 3b, the PA-NiO affords lower overpotentials of 290 and 310 mV at 60 and 100 mA cm-2, respectively, when compared with 320 and 330 mV of the pure AlNi-LDH, 330 and 340 mV of Ni-P, and 380 and 430 mV of the state-of-the-art RuO2 electrodes. Furthermore, as shown in Table S1, the OER activity of PA-NiO even outperforms

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those of recently reported non-precious electrocatalysts in 1 M KOH. In the meanwhile, the excellent OER performance in 0.1 M KOH was also identified for the PA-NiO electrode. For example, in Figure S7, PA-NiO shows the superior catalytic activity with a lower onset potential of 1.28 V compared with 1.36 V of AlNi-LDH, 1.39 V of Ni-P and 1.41 V of RuO2. Additionally, the small potentials of 1.37 and 1.38 V are needed to achieve the current densities of 60 mA cm-2 and 100 mA cm-2, respectively, on PA-NiO electrode. These values are much smaller than 1.49 and 1.51 V of the AlNi-LDH, 1.56 and 1.60 V of Ni-P, and 1.54 and 1.57 V of the state-of-theart RuO2 electrodes. The kinetics of OER on all electrodes can be confirmed by the Tafel plots (Figure 3c). The PA-NiO exhibits the lowest Tafel slope with only 36 mV dec-1 among the series, indicating that the kinetics of OER is much more enhanced on the PA-NiO electrode. The high stability of PANiO electrode was also identified by the chronoamperometry, as shown in Figure 3d. Almost no decay of current density was observed on PA-NiO electrode after 7 h operation, revealing the robustly active site in PA-NiO catalyst during OER. It is likely reasonable that Al element acting as a dopant in PA-NiO prevents the structural aggregation of NiO during the catalytic reaction.34 This view can be also supported by SEM images of PA-NiO electrode after the stability test (Figure S8). It is interesting that the Al content of PA-NiO was decreased sharply after the OER stability test (Figure S3 and S9), but the chronoamperometry curve maintained the current ratio over 100%, owing to the sufficiently exposed active sites. The ECSA of these materials was evaluated for their electrochemical double layer capacitance (Cdl) elucidated in Figure 3e. Cdl is linearly proportional to ECSA and the linear slope of plots of △j = ja-jc at 0.1 V against scan rate is equivalent to twice of the Cdl.53, 54 It can be sure that the slope of PA-NiO is much steeper than those of other two catalysts, AlNi-LDH and Ni-P, suggesting that PA-NiO possesses a higher

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ACS Energy Letters

ECSA. Note that the ECSA of PA-NiO increases after the OER stability test when compared to the fresh one, which is in accordance with the decreased Al content of PA-NiO after stability test by the partial/slight dissolution of Al element (Figure S10). Furthermore, electrochemical impedance spectroscopy (EIS) was employed to examine the electron transfer of all electrodes. As shown in Nyquist plot (Figure 3f), the PA-NiO possesses a much smaller semicircle when compared with those of AlNi-LDH and Ni-P electrodes, meaning that PA-NiO has a better electron conductivity.55 Interestingly, the PA-NiO still maintains a high conductivity for OER (Figure S11). This good performance of PA-NiO can be primarily attributed to two factors: Firstly, the P doping could increase the intrinsic activity by providing more active site and current carrier;56 Secondly, the Al doping could further provide a synergetic effect by mediating the electronic state of adjacent Ni atom, and on enlarging the ECSA to expose more active sites by the partial/slight dissolution of Al element.15, 57, 58 To further evaluate the potential application of NF supported porous PA-NiO acting as a bifunctional electrocatalyst in water splitting, the HER performance of PA-NiO was tested in an alkaline electrolyte. As illustrated in Figure 4a-b, the PA-NiO catalyst shows a good HER performance in 1 M KOH with more positive onset potential at -67 mV, which outperforms those of AlNi-LDH, Ni-P, and NF with onset potentials at -92, -109 mV and -158 mV, separately. Additionally, the PA-NiO only requires overpotentials of 138 and 162 mV to achieve current densities of 10 mA cm-2 and 20 mA cm-2, respectively, which are close to those of commercial Pt/C, and better than 179 and 211 mV of AlNi-LDH and 201 and 235 mV of Ni-P. In addition, the HER activity of PA-NiO is comparable or even much better than those of reported catalysts, as shown in Table S2. For a fair comparison, the OER activities for all electrodes were normalized with respect to the ECSA. The excellent OER activity of PA-NiO was also

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demonstrated by affording the highest current density at 1.54 V (Figure S12). We further examined the HER performance of PA-NiO in 0.1 M KOH (Figure S13), which was consistent with the results tested in 1 M KOH. Besides. the Tafel slope of PA-NiO electrode is calculated to be 81 mV dec-1 (Figure 4c), which is much smaller than 130 mV dec-1 of AlNi-LDH and 134 mV dec-1 of Ni-P, respectively. It means that the PA-NiO electrode is more favorable for the absorption of hydrogen and hydride with faster kinetics. The electrocatalytic stability of PA-NiO was tested by the chronoamperometric method at a potential of -0.25 V for 7 h (Figure 4d). The result displays a small loss of current density as 5 % when compared to the initial one, which is comparable to those of recently reported results.6, 59-63 The remarkable OER and HER performances of PA-NiO in the alkaline electrolyte indicates an outstanding bifunctional property for water splitting. The practicability of PA-NiO electrode for water splitting was further examined by itself acting as both anode and cathode in a twoelectrode system in 1 M KOH. As shown in Figure 4e, the LSV curves show that the PA-NiO electrode could achieve a current density of 10 mA cm-2 at 1.56 with iR compensation and 1.59 V without iR compensation, respectively, which are comparable to 1.54 V of RuO2//Pt/C, and lower than 1.66 V of AlNi-LDH and 1.69 V of Ni-P and the leading results reported in other studies, respectively (inset in Figure 4e and Table S3). In addition, the faradaic efficiency of water splitting on the PA-NiO electrode was calculated to be 86.9% (Figure S14). Finally, we conducted the stability test for the PA-NiO electrode at varied current densities from 10 to 200 mA cm-2 via chronopotentiometry (CP) (Figure 4f). At the initial stage of CP test, PA-NiO electrode achieves a stable potential of 1.59 V at 10 mA cm-2, consistent with the overpotential observed in LSV curve (Figure 4e). With the current density increasing from 100 to 200 mA cm2

, the potentials were increased to 2.03 and 2.27 V, respectively. Nevertheless, when the current

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ACS Energy Letters

density of CP returned back to 100 and 10 mA cm-2, sequentially, the potential at 100 and 10 mA cm-2 almost kept the same values with those of original one. Thus, the result further verifies the outstanding stability of PA-NiO electrocatalyst in water splitting application. For identifying the effect of Al doping, the PA-NiO with varied Al concentrations were synthesized and subsequently compared by LSV measurements. The result from Figure S15 indicates that the PA-NiO with the elemental ratio of Ni: Al= 2:1 shows the best performance for OER and HER in series samples, confirming the favorable effect of the Al doping on improving the catalytic activity. In addition, the P doping also acts a critical role in improving the activity of the Nibased catalyst, as evidenced by the enhanced OER activity of A-NiO sample without the P doping (Figure S16). In summary, we developed a facile and scalable approach for the fabrication of NF supported porous PA-NiO nanosheets catalyst for overall water splitting application. The PA-NiO catalyst exhibited both good OER and HER activity in alkaline electrolyte. Specifically, a small potential of 1.54 V was required to achieve a current density of 10 mA cm-2 on PA-NiO electrode. The remarkable performance of PA-NiO electrocatalyst can be attributed to the further doping of Al into P doped NiO resulting in the enhancement of the electron conductivity, the increased ECSA, and the enhanced stability. Besides, the nanoporous surface of PA-NiO also provides sufficiently exposed active sites and facilitated mass transfer for water splitting reactions. Overall, this work will open a paradigm for the development of transition metal electrocatalysts for energy conversion reactions. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials and electrochemical characterizations.

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situ X-ray diffraction, absorption and electrochemical impedance spectroscopies. Phys. Chem. Chem. Phys. 2017, 19, 8681-8693. (30) Li, C.; Wang, L.; Wei, M.; Evans, D. G.; Duan, X., Large oriented mesoporous selfsupporting Ni–Al oxide films derived from layered double hydroxide precursors. J. Mater. Chem. 2008, 18, 2666. (31) Lin, H. Y.; Lee, T. H.; Sie, C. Y. Photocatalytic hydrogen production with nickel oxide intercalated K4Nb6O17 under visible light irradiation. Interfaces J. Hydrogen Energy 2008, 33, 4055-4063. (32) Chen, M. T.; Lu, M. P.; Wu, Y. J.; Song, J.; Lee, C. Y.; Lu, M. Y.; Chang, Y. C.; Chou, L. J.; Wang, Z. L.; Chen, L. J. Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays. Nano Lett. 2010, 10, 4387-4393. (33) Zhao, Y.; Zhao, Y.; Waterhouse, G. I. N.; Zheng, L.; Cao, X.; Teng, F.; Wu, L. Z.; Tung, C. H.; O'Hare, D.; Zhang, T. Layered-Double-Hydroxide Nanosheets as Efficient Visible-LightDriven Photocatalysts for Dinitrogen Fixation. Adv. Mater. 2017, 29, 1703828. (34) Wang, J.; Song, Y.; Li, Z.; Liu, Q.; Zhou, J.; Jing, X.; Zhang, M.; Jiang, Z. In Situ Ni/Al Layered Double Hydroxide and Its Electrochemical Capacitance Performance. Energy & Fuels 2010, 24, 6463-6467. (35) George, G.; Anandhan, S. Synthesis and characterisation of nickel oxide nanofibre webs with alcohol sensing characteristics. RSC Adv. 2014, 4, 62009-62020. (36) Wang, B.; Liu, Q.; Qian, Z.; Zhang, X.; Wang, J.; Li, Z.; Yan, H.; Gao, Z.; Zhao, F.; Liu, L. Two steps in situ structure fabrication of Ni–Al layered double hydroxide on Ni foam and its electrochemical performance for supercapacitors. J. Power Sources 2014, 246, 747-753. (37) Das, N.; Saha, B.; Thapa, R.; Das, G.; Chattopadhyay, K., Band gap widening of nanocrystalline nickel oxide thin films via phosphorus doping. Physic. E: Low-dimensional Systems and Nanostructures 2010, 42, 1377-1382. (38) Rebours, B.; d'Espinose de la Caillerie, J. B.; Clause, O. Decoration of nickel and magnesium oxide crystallites with spinel-type phases. J. Am.Chem. Soc.1994, 116, 1707-1717. (39) Memon, J.; Sun, J.; Meng, D.; Ouyang, W.; Memon, M. A.; Huang, Y.; Yan, S.; Geng, J. Synthesis of graphene/Ni–Al layered double hydroxide nanowires and their application as an electrode material for supercapacitors. J. Mater. Chem. A 2014, 2, 5060.

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(40) Yang, W.; Gao, Z.; Wang, J.; Ma, J.; Zhang, M.; Liu, L. Solvothermal one-step synthesis of Ni-Al layered double hydroxide/carbon nanotube/reduced graphene oxide sheet ternary nanocomposite with ultrahigh capacitance for supercapacitors. ACS Appl. Mater. Interfaces 2013, 5, 5443-5454. (41) Gao, Z.; Wang, J.; Li, Z.; Yang, W.; Wang, B.; Hou, M.; He, Y.; Liu, Q.; Mann, T.; Yang, P.;et al. Graphene Nanosheet/Ni2+/Al3+Layered Double-Hydroxide Composite as a Novel Electrode for a Supercapacitor. Chem. Mater. 2011, 23, 3509-3516. (42) Momodu, D.; Bello, A.; Dangbegnon, J.; Barzeger, F.; Fabiane, M.; Manyala, N. P3HT:PCBM/nickel-aluminum layered double hydroxide-graphene foam composites for supercapacitor electrodes. J. Solid State Elect. 2014, 19, 445-452. (43) Zhou, G.; Wang, D. W.; Yin, L. C.; Li, N.; Li, F.; Cheng, H. M. Oxygen Bridges between NiO Nanosheets and Graphene for Improvement of Lithium Storage. ACS Nano 2012, 6, 32143223. (44) Wang, W.; Liu, Y.; Xu, C.; Zheng, C.; Wang, G. Synthesis of NiO nanorods by a novel simple precursor thermal decomposition approach. Chem. Phys. Lett. 2002, 362, 119-122. (45) Peck, M. A.; Langell, M. A. Comparison of Nanoscaled and Bulk NiO Structural and Environmental Characteristics by XRD, XAFS, and XPS. Chem. Mater. 2012, 24, 4483-4490. (46) Wang, Q.; Shang, L.; Shi, R.; Zhang, X.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C.-H.; Zhang, T. NiFe Layered Double Hydroxide Nanoparticles on Co,N-Codoped Carbon Nanoframes as Efficient Bifunctional Catalysts for Rechargeable Zinc-Air Batteries. Adv. Energy Mater. 2017, 7, 1700467. (47) Hess, A.; Kemnitz, E.; Lippitz, A.; Unger, W.; Menz, D. ESCA, XRD, and IR characterization of aluminum oxide, hydroxyfluoride, and fluoride surfaces in correlation with their catalytic activity in heterogeneous halogen exchange reactions. J. Cata. 1994, 148, 270-280. (48) Ohtsu, N.; Oku, M.; Obara, K.; Ito, S.; Shisido, T.; Wagatsuma, K. Oxidation behavior of NiAl alloy at low temperatures. Surf. Interfaces Anal.2007, 39, 528-532. (49) Liang, H.; Meng, F.; Caban-Acevedo, M.; Li, L.; Forticaux, A.; Xiu, L.; Wang, Z.; Jin, S. Hydrothermal continuous flow synthesis and exfoliation of NiCo layered double hydroxide nanosheets for enhanced oxygen evolution catalysis. Nano Lett. 2015, 15, 1421-7. (50) Song, F.; Hu, X. Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. Nat. Commun. 2014, 5, 4477.

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(51) Dong, C.; Kou, T.; Gao, H.; Peng, Z.; Zhang, Z. Eutectic-Derived Mesoporous Ni-Fe-O Nanowire Network Catalyzing Oxygen Evolution and Overall Water Splitting. Adv. Energy Mater. 2017, 8, 1701347. (52) Wang, Z.; Li, P.; Chen, Y.; He, J.; Liu, J.; Zhang, W.; Li, Y. Phosphorus-doped reduced graphene oxide as an electrocatalyst counter electrode in dye-sensitized solar cells. J. Power Sources 2014, 263, 246-251. (53) Wang, A. L.; Xu, H.; Li, G. R., NiCoFe layered triple hydroxides with porous structures as high-performance electrocatalysts for overall water splitting. ACS Energy Lett. 2016, 1, 445−453 (54) Liu, W.; Bao, J.; Guan, M.; Zhao, Y.; Lian, J.; Qiu, J.; Xu, L.; Huang, Y.; Qian, J.; Li, H. Nickel-cobalt-layered double hydroxide nanosheet arrays on Ni foam as a bifunctional electrocatalyst for overall water splitting. Dalton Trans. 2017, 46, 8372-8376. (55) Anantharaj, S.; Karthick, K.; Venkatesh, M.; Simha, T. V. S. V.; Salunke, A. S.; Ma, L.; Liang, H.; Kundu, S. Enhancing electrocatalytic total water splitting at few layer Pt-NiFe layered double hydroxide interfaces. Nano Energy 2017, 39, 30-43. (56) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically oriented cobalt selenide/NiFe layered-double-hydroxide nanosheets supported on exfoliated graphene foil: an efficient 3D electrode for overall water splitting. Energy Environ. Sci. 2016, 9, 478-483. (57) Li, S.; Wang, Y.; Peng, S.; Zhang, L.; Al‐Enizi, A. M.; Zhang, H.; Sun, X.; Zheng, G. CoNi‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts. Adv. Energy Mater.2016, 6, 1501661. (58) Duan, J.; Chen, S.; Vasileff, A.; Qiao, S. Z. Anion and cation modulation in metal compounds for bifunctional overall water splitting. ACS Nano 2016, 10, 8738-8745. (59) Liu, W.; Bao, J.; Guan, M.; Zhao, Y.; Lian, J.; Qiu, J.; Xu, L.; Huang, Y.; Qian, J.; Li, H. Nickel-cobalt-layered double hydroxide nanosheet arrays on Ni foam as a bifunctional electrocatalyst for overall water splitting. Dalton Trans 2017, 46, 8372-8376. (60) Anantharaj, S.; Karthick, K.; Venkatesh, M.; Simha, T. V. S. V.; Salunke, A. S.; Ma, L.; Liang, H.; Kundu, S. Enhancing electrocatalytic total water splitting at few layer Pt-NiFe layered double hydroxide interfaces. Nano Energy 2017, 39, 30-43. (61) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically oriented cobalt selenide/NiFe layered-double-hydroxide nanosheets supported on exfoliated graphene foil: an efficient 3D electrode for overall water splitting. Energy Environ. Sci. 2016, 9, 478-483.

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(62) Li, S.; Wang, Y.; Peng, S.; Zhang, L.; Al‐Enizi, A. M.; Zhang, H.; Sun, X.; Zheng, G. CoNi‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts. Adv. Energy Mater. 2016, 6, 1501661. (63) Duan, J.; Chen, S.; Vasileff, A.; Qiao, S. Z. Anion and cation modulation in metal compounds for bifunctional overall water splitting. ACS Nano 2016, 10, 8738-8745.

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ACS Energy Letters

Figure 1. (a) Schematic illustration of the fabrication process of the NF supported porous PANiO electrocatalyst. (b) SEM images of NF supported porous PA-NiO at low magnification and corresponding high magnification of PA-NiO (insert). (c, d) TEM images of AlNi-LDH and PANiO, respectively. (e) HRTEM image of PA-NiO showing the local structure from (d) and the lattice fringe (insert). (f) EDS elemental mapping of PA-NiO.

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Figure 2. (a) XRD patterns of AlNi-LDH and PA-NiO supported on nickel foam. (b) Raman spectra of AlNi-LDH and PA-NiO scraped from nickel foam. (c) XPS full spectrum and highresolution XPS spectrum of (d) Ni 2p, (e) Al 2p and (f) P 2p for PA-NiO scraped from nickel foam.

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ACS Energy Letters

Figure 3. OER performance of PA-NiO, AlNi-LDH, Ni-P, and RuO2 supported on nickel foam in 1 M KOH. (a) LSV curves. (b) corresponding onset potentials, overpotentials at the current densities of 60 and 100 mA cm-2. (c) and Tafel plots. (d) The chronoamperometry curve of PANiO catalyst at 1.6 V. (e) Comparison studies of capacitive current density as a function of scan rate. (f) EIS of PA-NiO, AlNi-LDH, and Ni-P measured at 1.55 V.

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Figure 4. HER and overall water splitting performance of PA-NiO, AlNi-LDH, Ni-P and RuO2//Pt/C (or Pt-C) supported on nickel foam in 1 M KOH. (a) LSV curves. (b) corresponding onset potentials and required overpotentials at the current densities of 10 and 20 mA cm-2. (c) and Tafel plots. (d) The chronoamperometry curve of PA-NiO catalyst under a constant overpotential of 252 mV. (e) The LSV curves with PA-NiO electrode as both anode and cathode for overall water splitting with and without iR compensation at the scan rate of 5 mV s-1. The insert shown in (e) is the comparison of the potentials required to afford a current density of 10 mA

cm-2 with

different

electrodes

tested

in

a

two-electrode

configuration.

Chronopotentiometric curves of the PA-NiO carried out at different current densities.

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(f)

Phosphorus and Aluminum Codoped Porous NiO Nanosheets as

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