Bifunctional Copper-Doped Nickel Catalysts Enable Energy-Efficient

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Bifunctional Copper-Doped Nickel Catalysts Enable Energy-Efficient Hydrogen Production via Hydrazine Oxidation and Hydrogen Evolution Reduction Qiangqiang Sun, Liyuan Wang, Yuqian Shen, Meng Zhou, Yi Ma, Zenglin Wang, and Chuan Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01887 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Sustainable Chemistry & Engineering

Bifunctional Copper-Doped Nickel Catalysts Enable Energy-Efficient Hydrogen Production via Hydrazine Oxidation and Hydrogen Evolution Reduction Qiangqiang Sun,†,‡ Liyuan Wang,† Yuqian Shen,† Meng Zhou,† Yi Ma,† Zenglin Wang,*,† and Chuan Zhao,*,†,§ † Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, and School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources, Shangluo University, Shangluo 726000, China § School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia E-mail: [email protected]；[email protected] KEYWORDS：Hydrazine oxidation reaction, Hydrogen evolution reaction, Nanoporous NiCu, Galvanostatic electrodeposition, Dealloying, Normalization processing

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ABSTRACT: Hindered by sluggish kinetics and large over-voltages of direct hydrazine oxidation, energy-efficient electrolytic hydrogen generation from whole cell hydrazine electrolysis still remains great challenging. Herein, we present a 3D hierarchically nanotubular Ni-Cu alloy on nickel foam (Ni(Cu)/NF) and demonstrate its high-efficiency and strongdurability for hydrazine oxidation reaction (HzOR) with a required potential of merely 86 mV to afford a current density of 100 mA cm-2 in alkaline hydrazine aqueous solution. The normalization of HzOR polarization curves for Ni(Cu)/NF manifests that the super-large electrochemical active surface area (ECSA) with an eighteen-fold increase is main contributor to the excellent HzOR performance. The superior cell performance makes Ni(Cu)/NF a well alternative transition-metal-based electrocatalyst for the utilization in HzOR electrolyser. The remarkable performance towards hydrogen evolution reaction (HER) of Ni(Cu)/NF allows the use of a superior bifunctional electrocatalyst for electrolytic hydrogen production via HzOR and HER. In a two-electrode electrolyser cell employing Ni(Cu)/NF to function as the cathode and anode, an extremely low cell voltage of 0.41 V is required to afford 100 mA cm-2 with remarkable long-term stability.

INTRODUCTION Considered as an ideal energy carrier, hydrogen is widely exploited for storage and utilization of renewable energy sources [1-4]. Water electrolysis provides a simple and promising way to produce hydrogen from water. Nevertheless, the sluggish kinetics of oxygen evolution reaction (OER) greatly impedes the overall water splitting efficiency on account of the high activation energy barrier for oxygen interatomic bonding [5,6]. Hence, replacing water oxidation with other oxidizable species like glycerol, alcohol, methanol and ammonia, can open enormous opportunities for hydrogen production at high-energy efficiency [7-10]. Recently, hydrazine 2 ACS Paragon Plus Environment

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hydrate (N2H4·H2O), has been identified as a prospective hydrogen reservoir to be applied widely to direct hydrazine fuel cells (DHFCs) [11-13]. However, commercial applications of DHFCs have been impeded by the scarcity and high cost of noble-metal-based electrocatalysts required for hydrazine oxidation reaction [14-16]. Developing low-cost and high-efficient base metal electrocatalysts for HzOR is therefore highly desired. Nanostructured metal catalysts such as metal alloys [17], phosphides [18], sulfides [19], and borides [20] have gained tremendous attentions recently for HzOR [21-23]. Owing to high corrosion resistance and excellent stability, Cu-based and Ni-based catalysts also have emerged as candidates for catalyzing HzOR [24-28]. Sun and coworkers developed an ultrathin NiCo alloy nanoarrays on nickel foam (NF) substrate, showing a much higher performance towards HzOR in DHFCs than Ni nanoarray or commercial Pt/C [29]. Cu3P nanoarray supported on copper foam and NiS2 nanoarray supported on Ti mesh also have been reported as bifunctional electrocatalysts for HER and HzOR which require low cell voltages of 0.72 and 0.75 V, respectively, to deliver 100 mA cm-2 in the two-electrode electrolyzers [30,31]. Recent researches have indeed manifested that transition metal sulfides and phosphides are high-effective bifunctional catalysts for both HzOR and HER. Herein, we report a copper-doped metallic Ni electrocatalyst for HzOR. The nanoporous catalyst, prepared by a facile two-step electrodeposition-dealloying strategy onto macroporous NF, possesses ultralarge ESCA and is of high-activity and stability towards both HzOR and HER. Notably, the Ni(Cu)/NF couple in a two-electrode alkaline electrolyzer requires a cell voltage of merely 0.41 V to deliver 100 mA cm-2 for hydrogen evolution. To the best of our knowledge, there has been no report on Ni-Cu catalysts for two-electrode HzOR electrolyzer to date.

EXPERIMENTAL SECTION 3 ACS Paragon Plus Environment


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Materials. Nickel sulfate (NiSO4·6H2O), copper sulfate (CuSO4·5H2O), boric acid (H3BO3), potassium hydroxide (KOH) and absolute ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Preparation for all the standard solutions were conducted with double-distilled water. All chemical reagents were used as received of analytical grade without further purification.

Electrocatalyst synthesis. Prior to electrodeposition, NF (2.0×0.5×1.6 cm3) was first ultrasonicated in 5 M dilute hydrochloric acid for 15 min to remove the surface oxide layer, subsequently cleaned with distilled water and absolutely ethanol, then left dry in air. Same pretreatments apply to other substrate supports in this paper, such as copper foam (CF), Ni mesh (NM) and carbon cloth (CC). With the NF (or other substrates) as the working electrode, a nickel sheet (1.0×1.0 cm2) as counter electrode and a Hg/Hg2Cl2(saturated KCl) (SCE) utilized as reference electrode, the galvanostatic electrodeposition was performed on a CHI 760E electrochemical workstation with the optimal depositon parameters of 0.1 A for 600 s at room temperature. The electrolyte solution was composed of 0.5 M NiSO4, 0.075 M CuSO4 and 0.5 M H3BO3 with pH of 4.0. After the electrodepositions, dealloying of NiCu alloy was conducted at 1.0 V (versus SCE) with dissolution time of 400 s and stirring rate of 1000 rpm. Then washing with ethanol and water, Ni(Cu)/NF (or other substrates) was obtained along with naturally airdried. For comparison, on the basis of the aforementioned electrodeposition procedures, the NiCu/NF electrode was fabricated by direct electrodeposition of NiCu onto activated NF without Cu dissolution. Similarly, the Ni/NF and Cu/NF were also fabricated from the same electrolyte with the absence of CuSO4 or NiSO4, respectively. In addition, Pt/C/NF electrode was also prepared as the literature [31].


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Material characterization. The SEM images and EDX data were obtained on a HITACHI SU8020 field emission scanning electron microscope at the accelerating voltage of 10 kV and 20 kV, respectively. TEM measurements were conducted on a FEI Tecnai G2 F20 electron microscope at 200 kV to further analyze the morphologies and elemental distributions. XRD profiles of Ni(Cu)/NF were acquired from Bruker D8 diffractometer in the 2θ-range of 3080º with a step size of 0.02° and a step time of 2 s empolying a CuKα radiation. XPS measurements were conducted on a PHI-5000 X-ray photoelectron spectrometer with a AlKα radiation.

Electrocatalytic measurements. The electrochemical characterizations were performed on a CHI 760E Bipotentiostat in the H2, N2-saturated alkaline aqueous solution with a certain concentration hydrazine, and a standard three-electrode system was employed with the as-obtained catalyst used as the working electrode, a graphite rod and a saturated calomel as the auxiliary and the reference electrode, respectively. All the potential measurements were converted to reversible hydrogen electrode (RHE) scale (ERHE = ESCE + 0.242V + 0.059 pH). The HzOR activities were determined by linear sweep voltammetry (LSV) at a scan rate of 5 mV s-1. Tafel slopes were obtained from plots derived from LSV curves according to the equation η = b log j + a, where η is the overpotential, j is the current density, b is the Tafel slope and a is the Tafel constant. To acquire the double layer capacitance (Cdl) and ECSA, cyclic voltammograms were collected in the non-Faradaic region in potential window of -0.05 to 0.05 V (vs RHE) at scan rates from 5 to 200 mVs-1. Charging current density changes (∆j = |ja - jc|/2) plotted as a linear correlation with scan rates. The linear slope of fitted line was equal to the Cdl, and the ECSA values were calculated based on the proportional relationship: ECSA = Cdl / Cs. where Cs is a unit characteristic capacitance of 0.040 mFcm2. The normalization against ECSA of HzOR 5 ACS Paragon Plus Environment


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activity was achieved by correcting the LSV polarization curves based on the equation: jc = j/∆n, where jc was corrective current density, j was measured current density, ∆n was the ECSA multiple as for NF. Electrochemical impedance spectroscopy (EIS) was measured at the potential of -1.0 V (versus SCE) with frequency sweep from105 Hz to 0.1 Hz using an AC amplitude of 5mVs-1. To check the electrode durability, chronopotentiometric tests for continuous HER were carried out for 10 hours. Meanwhile, multi-current and multi-potential processes were tested under the identical experimental electrolytic device without iR compensation.

RESULTS AND DISCUSSIONS

Figure 1. Schematically formation process of the porous Ni(Cu)/NF composite through electrodeposition and dealloying.

Characterisation of Ni(Cu)/NF. Schematic diagram shows that porous Ni(Cu) coating was electrodeposited on the NF substrate by a facile two-step synthetic route (Figure 1). [32]

Briefly, nickel and copper were codeposited onto the NF surface from its salt solution, 6 ACS Paragon Plus Environment

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followed by the selective Cu dissolution via potentiostatic process. The successful in situ growth of nanoporous Ni(Cu) on NF are evidenced by the apparent color change of NF after electrodeposition and dissolution (see Figure S1). Meantime, the catalyst loadings of Ni(Cu)/NF are 0.49 mg·cm-2.

Figure 2. Low- and high-resolution SEM images for (a) bare NF, (b, c and d) Ni(Cu)/NF. (e) High-resolution (HRTEM) image of Ni(Cu)/NF. Inserts: magnified HRTEM image recorded from region marked by yellow square. (f) XRD profile of Ni(Cu) film scraped directly from NF and XPS spectra of Ni 2p, Cu 2p and O 1s in the as-prepared Ni(Cu)/NF. The top-view and cross-sectional SEM images of Ni(Cu)/NF (Figure 2a, b and Figure S2), show the formed Ni(Cu) layer is about 250 nm thick containing densely packed nanotubes with average diameter of 200 nm growing erectly on the NF (Figure 2c, d). Such hierarchal porous morphology offers wide open spaces and ultra-large ECSA, compared with the surface morphology of Ni/NF, Cu/NF, NiCu/NF (Figure S3). Figure 2e shows the HRTEM image of 7 ACS Paragon Plus Environment


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Ni(Cu) film stripping off from NF, which exhibits clear lattice fringes with inter-planar distances of 0.205 nm well indexed to the (111) plane of metal Ni (inset in Figure 2e). Figure 2f presents the XRD pattern for Ni(Cu) coatings scraped from as-prepared catalyst and the XPS spectrum of Ni(Cu)/NF. From the XRD pattern, three well-observed peaks at 44.3o, 51.6o and 76.2o are readily assigned to (111), (200) and (220) diffractions of pure Ni, respectively. [33, 34] This result identifies that the Ni(Cu) coating grown on NF surface are mainly metal Ni. From the high-resolution Ni2p spectra (Figure 2f), two weak spin-orbit peaks at 852.8 and 870.2 eV correspond well to Ni (0) in Ni(Cu)/NF. [35] In addition, two strong ones appearing at 855.4 and 873.4 eV following with two shakeup satellites located at 861.1 and 880.2 eV, which is assigned to Ni2+ resulting from surface oxidation of Ni(Cu)/NF due to exposure to air. [36,37]

For Cu 2p spectra, two spin-orbit peaks at 932.8 and 952.8 eV belong to the metallic Cu

which is silent in the XRD pattern. [38] The O1s spectrum exhibits a strong peak at 529.6 eV, which is indexed to lattice oxygen in nickel oxide. [39] This further confirms the formation of superficial NiO shell in Ni(Cu)/NF, which is evidenced by the EDX spectra (Figure S4) and Raman spectroscopy (Figure S5) (the prominent peak at 534 cm-1 is ascribed to the typical Ni-O stretching vibration. [40]

Hydrazine oxidation catalysis. The electrocatalytic HzOR activity of Ni(Cu)/NF was evaluated in a typical three-electrode alkaline electrolyzer. All the potentials obtained were converted into RHE scales. Figure 3a shows the HzOR performances of Ni(Cu)/NF with variable hydrazine concentrations. Markedly, there is no signficant voltammetric response in the working potential window in absence of hydrazine. In contrast, a significant anodic response current arises with the presence of 0.1 M hydrazine addition and it increases with the increase of hydrazine concentration. Figure 3b and S6 display the LSV curves and surface morphologies of 8 ACS Paragon Plus Environment

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Ni(Cu) films on different substrates, such as carbon cloth (CC), nickel mesh (NM), copper foam (CF) and nickel foam. It is found HzOR activity for Ni(Cu)/NM is significantly superior to that for Ni(Cu)/CC owing to outstanding conductivity of metal substrate, as evidenced by the EIS data (see in Figure S7). In addition, it is noted that Ni(Cu) film on 3D metal foam substrate generally shows higher activity than carbon cloth substrate attributed to the superlarge ESCAs (shown in Figure S8) and excellent gas bubble dissipation ability. [41,42] Among the substrates, Ni(Cu)/NF exhibits the highest electrocatalytic activity towards HzOR, and NF is therefore used as substrate hereafter. 500

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Figure 3. (a) Steady-state polarization curves of Ni(Cu)/NF in 1.0 M KOH with variable hydrazine concentration. (b) LSV polarization curves of Ni-Cu alloy films on various substrates, nickel foam (NF), Copper foam (CF), Nickel mesh (NM) and carbon cloth (CC)). We measured the catalytic HzOR performances of benchmark PtC/NF, Ni(Cu)/NF, NiCu/NF, Ni/NF, Cu/NF and bare NF in 1.0 M KOH with 0.5 M hydrazine. As displayed in Figure 4a and Figure S9a, bare NF exhibits very poor HzOR activity needing a potential of 204 mV to achieve 50 mA cm-2, in comparison to Cu/NF (140 mV), Ni/NF (109 mV) and NiCu/NF (69 mV). Whereas Ni(Cu)/NF exhibits significantly boosted activity for HzOR, requiring an extremely low potential of 38 mV to deliver a current density of 50 mA cm-2, which is indeed smaller than that 9 ACS Paragon Plus Environment


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of benchmark Pt/C/NF (52 mV). Notably, it also outperforms other reported state-of-the-art transition-metal-based HzOR catalysts like Cu3P nanoarray on Cu foam (η50 mAcm-2 = 98 mV), [30] NiS2 nanosheet array on Ti mesh (η50 mAcm-2 ≈ 85 mV), [31] CoS2 nanoarray on Ti mesh (η50 mAcm-2 ≈ 73 mV), [43] porous Ni-Cu alloy (η50 mAcm-2 ≈130 mV), [28] Cu nanowire arrays (η50 mAcm-2 ≈ 310 mV), [24] ultrathin nickel nanosheet array (η50 mAcm-2 ≈ 60 mV). [44] This superior performance makes the Ni(Cu)/NF prevail over the most efficient catalysts reported for HzOR under alkaline condition (Table S1). To better understand the HzOR kinetics, we compared the Tafel plots of the aforementioned electrodes. As observed in Figure 4b, Ni(Cu)/NF shows a quite low Tafel slope of 51.2 mV dec-1, while 221.1 mV dec-1 is required for NF, 144.1 mV dec-1 for Cu/NF, 104.1 mV dec-1 for Ni/NF, and 77.5 mV dec-1 for NiCu/NF. Namely, Ni(Cu)/NF electrode holds the lowest Tafel slope even smaller than Pt/C/NF (56.5 mV dec-1), demonstrating its favorable catalytic kinetics and fast electron transfer towards HzOR. In order to evaluate the electrode process kinetics and the charge transfer behavior between electrode and electrolyte, we recorded their EIS data in Figure 4c and Figure S9b. Semicircles in the Nyquist plots suggest that HzOR processes for the five electrodes followed an analogous kinetically controlled mechanism. The nanoporous Ni(Cu)/NF electrode presents a lowest charge transfer resistance of 0.9 Ω·cm-2 among 49.9 Ω cm-2 for NF, 5.0 Ω cm-2 for Cu/NF, 3.1 Ω cm-2 for Ni/NF and 1.9 Ω cm-2 for NiCu/NF, further supporting the high electrocatalytic HzOR kinetics of Ni(Cu)/NF.


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Figure 4. (a) The steady-state polarization curves for PtC/NF, Ni(Cu)/NF, NiCu/NF, Ni/NF, Cu/NF and bare NF towards HzOR, and (b) Tafel plots derived from the corresponding LSV curves. (c) Nyquist plots for counterparts with magnified Nyquist plots recorded from region marked by red square (inset). (d) HzOR performances of Ni(Cu)/NF electrode at variable scan rates of 5~200 mV s-1 (inset: plots showing the current density data with various scan rates at 0.3 11 ACS Paragon Plus Environment


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V vs RHE). (e) Multi-current testing of Ni(Cu)/NF for ten steps from the initial 50 mAcm-2, with a current increment of 50 mAcm-2 per 200 s. (f) Stability testing of Ni(Cu)/NF by chronopotentiometry at constant current density of 100 and 200 mAcm-2, respectively. Figure 4e displays the multiple-current chronopotentiometric curve for Ni(Cu)/NF towards HzOR in 1 M KOH aqueous solution containing 0.5 M hydrazine hydrate. The potential rapidly levels off at 0.04 V (vs RHE) at the initial current density and maintains constant for the rest 200 s. Similar potential responses can be detected for all steps tested herein up to the last 500 mA cm2

, reflecting the superb conductivity, fast mass transportation and strong mechanical robustness

of Ni(Cu)/NF. [46] To evaluate the durability of as-obtained electrode for HzOR, long-term galvanostatic electrolysis at 100 and 200 mA cm-2 were conducted, respectively (Figure 4f). After continuous electrolysis for 10 hours, the potential increments for Ni(Cu)/NF are merely 12 and 15 mV, respectively, revealing the superb durability of Ni(Cu)/NF. Figure S10 and S11 show the SEM image, XRD pattern and XPS spectra of Ni(Cu)/NF after long-term HzOR electrolysis, the well-preserved morphology and phase composition confirmed the structural recoverability. The robustness of Ni(Cu)/NF electrode is also verified by HzOR polarization curves after successive cyclic voltammetry for 1000 cycles at potential window of -0.2 ~ 0.2 V vs RHE scale. As can be seen in Figure S12, the LSV curves of Ni(Cu)/NF after long-term cycling test overlaps with the initial one almost completely, revealing the superior electrocatalytic stability of Ni(Cu)/NF holey nanotubes.

Whole cell hydrazine electrolysis using Ni(Cu)/NF. It has been found that Ni(Cu)/NF also exhibit remarkable catalytic activity toward HER in alkaline media. [32] Thus, employing Ni(Cu)/NF as a bifunctional catalyst for both HER and HzOR can realize whole cell hydrazine electrolysis for hydrogen generation. As displayed in Figure 5a, only very small 12 ACS Paragon Plus Environment

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potentials of 203 and 86 mV are required to deliver a current density of 100 mA cm-2 for HER and HzOR, respectively, demonstrating Ni(Cu)/NF is an efficient bifunctional catalyst for energy-saving electrolytic hydrogen generation. For this purpose, we constructed a two-electrode alkaline water electrolyzer employing Ni(Cu)/NF to function as both the anode and cathode. As shown in Figure 5b, this system requires cell voltages of 0.27, 0.41 and 0.64 V to afford 50, 100 and 200 mA cm-2, respectively, in the alkaline hydrazine solution. The Ni(Cu)/NF║Ni(Cu)/NF couple present outstanding electrocatalytic performance capable of affording 100 mA cm-2 at a voltage of 0.41V, which is superior to newly published state-of-the-art noble-metal-free bifunctional electrocatalyst couples, such as Ni2P/NF║ Ni2P/NF (0.45 V),[47] Cu3P/CF║Cu3P/CF (0.72 V), [30] NiS2/TiM║NiS2/TiM (0.75 V),[31] CoS2/TiM║CoS2/TiM (0.81 V).[43] The superior performance makes the Ni(Cu)/NF rank among the most transition-metal-based bifunctional catalysts for HzOR and HER under alkaline condition (Table S2). In comparison to hydrogen generation from overall water splitting, much higher cell voltage of 1.67 V (Figure 5b) is required using Ni(Cu)/NF as the cathode and anode in 1.0 M KOH. The results manifest that replacing OER with HzOR in alkaline water electrolyzer is conductive to high-efficiency electrolytic hydrogen generation. The cell stability of Ni(Cu)/NF couple was also evaluated in a two-electrode system for HzOR and HER. Figure 5c displays the chronopotentiometric curve of the Ni(Cu)/NF║Ni(Cu)/NF cell. As observed, the cell voltage remains at near 0.41 V to sustain a current density of 100 mA cm-2 and then keeps almost steady for 10 h with no significant fluctuations (< 15 mV), similar phenomenon can also be observed for during continuous 12 h testing (Figure S13). The durability is also confirmed by chronoamperometry test which shows insignificant performance attenuation (Figure S14). The generated gas at the anode and cathode 13 ACS Paragon Plus Environment


were identified to be N2 and H2 by gas chromatography (Figure S15), The volume ratio of N2/H2 in the gas chromatography results is 6:1. A calibrated pressure sensor was used to determine quantitatively the generated hydrogen by detecting the pressure difference of cathode compartment in the hermetic H-type electrolyser. [48] The Faraday Efficiency (FE) for HER was calculated to be close to unity on account of the experimental amount of generated gas matching well with the theoretical one (Figure 5d).

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generated H2 with theoretically calculated one versus electrolytic time for Ni(Cu)/NF. All the electrochemical measurements were performed in 1.0 M KOH containing 0.5 M hydrazine hydrate. 500

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11.14 7.31

(d) After ECSA normalization 0s 100 s 200 s 300 s 400 s 500 s

30

20

10

4

0

0 0

100

200

300

400

500

0.0

t (s)

0.1

0.2

0.3

0.4


Figure 6. (a) HzOR polarization curves and (b) plots showing the capacitive current density as a linear correlation with scan rates for Ni(Cu)/NF for variable dealloying time. (c) Histograms comparing the double-layer capacitance (Cdl) values for the corresponding electrodes. (d) The ECSA normalized HzOR activities of Ni(Cu)/NF with the dealloying for 0 ~ 500 s.

Understanding the electrocatalytic HzOR activity. ECSA is a key influential factor for HzOR catalysts. An increase of ECSA normally brings about the exposure of the more



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catalytic active sites and gives rise to the enhancement of HzOR performance. [49] To better understand the structural contribution of the as-prepared electrode to the electrocatalytic performance, we collected the Cdls of Ni(Cu)/NF, NiCu/NF, Ni/NF and NF using cyclic voltammetry (Figure S16) to evaluate the ECSA values. [50] As comparisons, the Cdl value of Ni(Cu)/NF electrode achieves 17.3 mFcm-2, which is markedly larger than 0.97 mFcm-2 of NF, 6.5 mFcm-2 of Ni/NF, and 7.3 mFcm-2 of NiCu/NF, indicating that Ni(Cu)/NF has the largest electrochemically active surface area. Namely, Ni(Cu)/NF holds a significantly higher surface roughness, produces more catalytic active centres and thus forms high-HzOR active sites than the other three electrodes. As a consequence, the ultra-large ECSA (an 18-fold increase to NF) of Ni(Cu)/NF contribute to enhanced activity by affording more active sites for hydrazine oxidation. In order to further evaluate the intrinsic activities of the as-prepared electrodes, we compared the HzOR performances and ECSAs of Ni(Cu)/NF obtained with variable dealloying time (0 ~ 500 s) which lead to different Cu dopant level. Figure 6a shows the HzOR performances of Ni(Cu)/NF against various dealloying time, while Figure 6b, 6c present the corresponding double-layer capacitances. As observed, the best catalyst for HzOR with the largest Cdl value is obtained at the dealloying time of 400 s, and an optimal catalytic performance is simultaneously attained. We attribute this behavior to the corresponding changes in nanostructure and ECSAs of Ni(Cu)/NF. Further dealloying time increment leads to a mild decrease in activity and ECSA, which may result from the collapsed nanotube structure caused by excessive dissolution. The effect of Cu dopant on the HzOR performance in Ni(Cu)/NF was further understood by normalization against ECSA to exclude the structural contribution. [51] As observed in Figure 6d, 16 ACS Paragon Plus Environment

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HzOR activity of as-synthesized electrode decreases with an increase of Cu dissolution time. From Table S3, Cu dopant level decreases with the increase of dissolution time suggesting more Cu dopant can enhance the catalytic activity of Ni(Cu)/NF, owing to the intermetallic synergy between Ni and Cu. [52-53] Nevertheless, as demonstrated in Figure 6a, the optimal performance of Ni(Cu)/NF to catalyze the HzOR is achieved at the dealloying time of 400 s with the lower Cu dopant of only 2% (Table S3). This evidences prove that the outstanding HzOR activity of Ni(Cu)/NF is primarily attributed to the superlarge ECSA of Ni(Cu)/NF induced by the 3D nanotubular structure. Another factor that contributes to the extraordinary performance of Ni(Cu)/NF is the unique 3D hierarchical porous microstructure, which enhances the electron transfer, promotes mass transport between electrolyte and electrode and facilitates the release of gas bubbles. [54]

CONCLUSION Nanotubular array of Ni(Cu)/NF enables it to be a top-ranking HzOR catalyst of low cost, high-efficiency and high robustness in alkaline media. A significant enhancement in the cell performance is associated with multiple advantages of Cu-doped nanotubular Ni, including the super-large ESCA the main contributors to the superior performance, the excellent hydrogen bubble dissipation ability, and high electrical conductivity. It allows for energy-efficient electrolytic hydrogen production by HzOR in lieu of OER. As a high active and strong durable bifunctional electrocatalyst for hydrazine electrolyzer, Ni(Cu)/NF requires an ultralow cell voltage of 0.41 V to afford 100 mA cm-2 with a Faradaic efficiency of close to 100% for HER. This work not only provide a fascinating cost-effective candidate catalyst for alternative design of direct hydrazine fuel cells fed with liquid fuels, but also describes a nano-synthesis strategy to better create alloy-based nanostructures for energy conversion technologies. 17 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information. Schematic illustration; Cross-sectional SEM image; EDX spectrum; Raman spectra; EIS data; Histograms for overpotentials and the charge transport resistances; SEM image after long-term test; LSV curves; Chronoamperometry curve; GC spectra; CV curves and Plots of the Cdls. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax Number: (086)-29-81530727 *E-mail:[email protected] ORCID Zenglin Wang: 0000-0002-4903-5621 Chuan Zhao: 0000-0001-7007-5946 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The project was fnancially supported by National Natural Science Foundation of China (Grant No. 21273144, No. 21603134), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2016JQ2023) and an Australian Research Council Discovery (Grant Nos. FT170100224 and DP160103107). REFERENCES (1) Li, Y.B.; Zhao, C. Enhancing water oxidation catalysis on a synergistic phosphorylated NiFe 18 ACS Paragon Plus Environment

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For Table of Contents Use Only

Bifunctional copper-doped nickel catalysts enable energy-efficient hydrogen production via hydrazine oxidation and hydrogen evolution reduction with remarkable longterm stability

Table of Contents graphic



500

0 M N2H4

(a)

(b)

0.1 M N2H4

400

0.2 M N2H4

400

j (mAcm-2)

500

j (mAcm-2)

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0.5 M N2H4 0.8 M N2H4

300

1.0 M N2H4 200

Ni(Cu)/NF Ni(Cu)/CF Ni(Cu)/NM Ni(Cu)/CC

300

200

100

100

0

0 0.00

0.05

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0.30

0.0


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Figure 3(a, b)

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500

0.16

j (mAcm-2)

400

Potential ( V vs.RHE )

(a) NF Cu/NF Ni/NF NiCu/NF Ni(Cu)/NF PtC/NF

300

200

100

(b) NF Cu/NF Ni/NF NiCu/NF Ni(Cu)/NF PtC/NF

0.12

0.08

0.04

-1

c

V

.1

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104

0.00

m

-1

V

4 14

c de

m .1

V .5 m

77

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V .1 m

de

dec

-1 -1 c dec V de m 5 . 56

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0 0.0

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35

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1

2

3

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15

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NF

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5 mv/s 10 mv/s 20 mv/s 50 mv/s 80 mv/s 100 mv/s 200 mv/s

300

200

10

400

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NiCu/NF

j (mAcm-2)

Z"(ohm)

30

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Ni(Cu)/NF

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100

380 360 340 320

Rct

5

300 0

50

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20

30

40

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50

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150

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0.4 0.3 0.2 0.1

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j = 200 mAcm-2

0.25 0.20 0.15

j = 100 mAcm-2

0.10 0.05

0.0

0.00 0

500

1000

1500

2000

0

Time (s)

Figure 4 (a ~ f)


2

4

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6

8

10


400

0.174 V

200

j (mAcm-2)

j (mAcm-2)

400

(a)

300

0.086 V

100 0 - 0.203 V

-100 -200

- 0.254 V

E100 = 0.289 V

300

(b) HER&HzOR

HER&OER

200

1.55 V

100

E200 = 0.428 V

-300 -400

0 -0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.0

0.3

0.6

0.9

1.5

1.8

2.1

2.4

2.7


Potential ( V vs RHE ) 0.35 0.55

Amount of gas (mmol)

(c)

0.50

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 49

0.45

j = 100 mAcm-2

0.40 0.35 0.30

0.30

(d)

0.25

Measured H2

0.20

Calculated H2

0.15 0.10 0.05 0.00

0.25 0

2

4

6

8

10

0

Time (h)

10

20

30

Time (min)

Figure 5 (a ~ d)


40

50

60

Page 31 of 49

500

4

(a) j (mAcm )

0s 100 s 200 s 300 s 400 s 500 s

300 200

-2

j (mAcm-2)

400

t t t t t t

3

2

(b)

=0 = 100 s = 200 s = 300 s = 400 s = 500 s

1

100

0

0 0.0

0.1

0.2

0.3

0.4

0.00

0.05

(c)

16.23

17.27

17.01 40

12

jc (mAcm-2)

13.68

8

–1

0.15

0.20

50

20 16

0.10

Scan rate (V s )


Cdl(mFcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11.14 7.31

20

10

0

0

100

200

300

400

0s 100 s 200 s 300 s 400 s 500 s

30

4

0

(d) After ECSA normalization

500

0.0

t (s)

0.1

0.2

0.3


Figure 6 (a ~ d)


0.4


NF Ni(Cu)/NF

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 49

200

400

600

800

1000 -1

Raman shift (cm )

Figure S5


1200

1400

Page 33 of 49

(a)

Ni(Cu)/Ni foam Ni(Cu)/Cu foam Ni(Cu)/Ni mesh Ni(Cu)/CC fitted curve

1.2

Rs Rct

0.4

3 2

1.7 0.9

1

1.5

2.0

2.5

4.8

4

0.8

0.0 1.0

(b)

-2

1.6

5

Rct (cm )

2.0

Z"(ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.0

Z'(ohm)

3.5

4.0

4.5

1.1

0

Ni(Cu)/NF Ni(Cu)/CFNi(Cu)/NMNi(Cu)/CC

Figure S7 (a, b)



4 Ni(Cu)/CC

500

-2

Cdl= 5.26 mF cm

Ni(Cu)/NM Cdl= 8.40 mF cm Ni(Cu)/NF

-2

Cdl= 15.50 mF cm

431.8 387.5

400

2

Ni(Cu)/CF

ECSA (cm )

3

(b)

(a)

-2

-2

 j0V vs.SCE(mAcm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 49

-2

Cdl= 17.27 mF cm

2

300 210.0 200

1

131.5 100

0 0.00

0.05

0.10

–1

0.15

Scan rate (mV s )

0.20

Ni(Cu)/CCNi(Cu)/NMNi(Cu)/CF Ni(Cu)/NF

Figure S8 (a, b)


Page 35 of 49

70

(a)

-2

j = 50 mAcm -2 j = 100 mAcm

300 226 200

60 -2

332

Rct (cm )

400

Overpotential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


193

182 123

130

49.9

50 40 30 20

108

100

(b)

96 69

51

10 0.9

0 0

NF

Cu/NF

Ni/NF

NiCu/NF Ni(Cu)/NF

1.9

Ni(Cu)/NF NiCu/NF

Figure S9 (a, b)


3.1

5.0

Ni/NF

Cu/NF

NF


(a)

(b) Ni2p Intensity(a.u.)

Intensity(a.u.)

Ni(111)

Ni(200)

+2

Ni 2p3/2

+2

Ni 2p1/2

sat.

0

0

Ni 2p1/2

sat.

Ni 2p3/2

Ni(220)

30

40

50

60

70

885

80

880

(c) Cu2p

(d) O1s

0

870

865

860

855

O2-

Intensity(a.u.)

Cu 2p3/2

0

Cu 2p1/2

955

875

Binding Energy (eV)

2Theta(degree)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 49

950

945

940

935

930

536

534

Binding Energy(eV)

Figure S11 (a ~ d)


532

530

528

Binding Energy (eV)

526

850

Page 37 of 49

500 400 -2

j (mAcm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intial 1000th cycle

300 200 100 0 -0.1

0.0

0.1

0.2

0.3

Potential (V vs. RHE)

Figure S12


0.4


0.60 0.55

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 49

0.50 -2

j = 100 mAcm

0.45 0.40 0.35 0.30 0.25 0

2

4

6

8

Time (h)

Figure S13


10

12

Page 39 of 49

200

-2

j (mAcm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

V 100

50

0 0

2

4

6

Time (h)

Figure S14


8

10


Page 40 of 49

H2 generated H2+N2 standard H2+N2

N2 H2

0.0

0.5

N2

1.0

1.5

2.0

Time(min)

Figure S15


2.5

Page 41 of 49

0.3

2.0

Bare NF

0.1

1.5

0.0 -0.1

5 mv/s 10 mv/s 25 mv/s 50 mv/s 100 mv/s 200 mv/s

Ni/NF

0.5 0.0 -0.5

-0.2 -0.3 -0.06

(b)

1.0

-2

(a)


j (mA cm )

-2

j (mAcm )

0.2

-1.0

-0.04

-0.02

0.00

0.02

0.04

-1.5 -0.06

0.06

-0.02

-0.04

0.00

0.02

0.04

0.06

Potential(V vs. SCE)

Potential (V vs. SCE) 5

1.0

4 3

0.5 0.0 -0.5

2 1


0 -1

-3

-1.5 -0.06

(d) Ni(Cu)/NF

-2

-1.0

-0.04

-0.02

0.00

0.02

0.04

-4 -0.06

0.06

-0.04

-0.02

6

(e) 4

-2

NF

Cdl= 0.97mF cm

Ni/NF

Cdl= 6.50 mF cm

NiCu/NF

Cdl= 7.31 mF cm

-2 -2

Ni(Cu)/NF Cdl= 17.27 mF cm

-2

2

0 0.00

0.00

0.02



 j (mAcm-2)

-2

(c) NiCu/NF

-2

1.5


j (mA cm )

2.0

j (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05

0.10

–1

0.15

Scan rate (mV s )

Figure S16 (a ~ e)


0.20

0.04

0.06


Schematically formation process of the porous Ni(Cu)/NF composite through electrodeposition and dealloying. 338x203mm (150 x 150 DPI)


Page 42 of 49

Page 43 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SEM images for (a) bare NF, (b, c and d) Ni(Cu)/NF. (e) HRTEM image of Ni(Cu)/NF. Inserts: magnified HRTEM image recorded from region marked by yellow square. (f) XRD pattern of Ni(Cu) coating by peeling off directly from nickel foam and XPS spectra of Ni(Cu)/NF in the Ni 2p, Cu 2p and O 1s regions. 355x211mm (150 x 150 DPI)



Schematic photographs showing the formation process of the porous Ni(Cu)/NF composite electrodes. 302x188mm (150 x 150 DPI)


Page 44 of 49

Page 45 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cross-sectional SEM image of Ni(Cu) coating scraped from as-prepared catalyst. 486x431mm (96 x 96 DPI)



SEM images of Ni/NF (a), Cu/NF (b) and NiCu/NF (c). 599x185mm (96 x 96 DPI)


Page 46 of 49

Page 47 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy dispersive X-ray spectra of the as-deposited NiCu film on Ni foam by dealloying. 260x186mm (96 x 96 DPI)



SEM images of Ni(Cu) films on different substrates, (a) carbon cloth, (b) nickel mesh, (c) copper foam and (d)nickel foam. 398x373mm (96 x 96 DPI)


Page 48 of 49

Page 49 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SEM image of Ni(Cu)/NF after HzOR electrolysis at a constant current density of 100 mA cm-2 for 10 h. 276x231mm (96 x 96 DPI)


Bifunctional Copper-Doped Nickel Catalysts Enable Energy-Efficient

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