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Multivariate MOF-Templated Pomegranate-Like Ni/C as Efficient Bifunctional Electrocatalyst for Hydrogen Evolution and Urea Oxidation Lu Wang, Lantian Ren, Xiaorui Wang, Xiao Feng, Junwen Zhou, and Bo Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18650 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Multivariate MOF-Templated Pomegranate-Like Ni/C as Efficient Bifunctional Electrocatalyst for Hydrogen Evolution and Urea Oxidation Lu Wang, Lantian Ren, Xiaorui Wang, Xiao Feng, Junwen Zhou and Bo Wang* Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology. Beijing 100081, P. R. China. KEYWORDS: metal-organic frameworks, hydrogen evolution reaction, urea oxidation reaction, water splitting, electrocatalysis

ABSTRACT: Producing hydrogen through electrolysis is considered as a feasible strategy to quench the world’s clean-energy thirst. Compared with water electrolysis, urea electrolysis presents a more promising prospect in the way that it could carry out sewage treatment as well as energy-efficient hydrogen production at the same time. Herein, highly porous pomegranate-like Ni/C was synthesized from multivariate metal-organic frameworks and exhibits excellent hydrogen evolution activity with an unprecedentedly low overpotential of 40 mV at the current density of 10 mA cm−2 in 1 M KOH, ranking among the best earth-abundant electrocatalysts deposited on glassy carbon electrode reported to date. In addition, it also displays superb anodic urea oxidation activity with onset potential of 1.33 V vs. RHE. Furthermore, a two-electrode urea

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electrolyzer with Ni/C as both the cathode and anode electrocatalyst was fabricated and generates 52 times more hydrogen amount than the water electrolyzer under the same condition.

INTRODUCTION Hydrogen evolution (HER) via water electrolysis is pursued as a promising strategy to access clean and renewable energy.1 However, the widely studied water electrolyzer, which combines HER with oxygen evolution reaction (OER), requires prohibitive total expenditure due to the following two reasons: (1) the sluggish kinetics of OER leads to the excessive energy cost;2, 3 (2) the lack of efficient HER catalysts in alkaline condition results in the dependence on Pt.4-17 Therefore, a more energy-efficient approach for hydrogen production, which is to utilize urea oxidation to replace traditional OER,18 is explored here. Urea, which is a major component in sewage and human/animal urine, can be electrochemically oxidized to N2, denoted as UOR.18, 19 The urea electrolysis (CO(NH2)2 + H2O = N2 + 3H2 + CO2) brings three benefits: (1) the production of H2 with less energy consumption because of the lower theoretical overall voltage of urea electrolysis than water electrolysis;20 (2) the remediation of harmful nitrogen compounds in the water system; (3) applying UOR as the anodic reaction in hydrogen production could guarantee the electrolyzer safety since the generation of O2 and the mix of O2 and H2 can be avoided. Against this backdrop, it is imperative to find catalysts with bifunctionality that can both accelerate hydrogen production and reduce the energy cost of anodic half reaction by replacing OER with UOR. Metal−organic frameworks (MOFs), which are constructed by joining metal clusters with organic ligands, have attracted extensive attention in applications such as sensor,21, 22 catalysis,23, 24

clean energy,25-27 and so on. Moreover, because of their monodispersed active centers in the


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well-defined carbon-based frameworks, MOFs are also emerged as the promising platform for synthesizing highly porous electrocatalysts with controlled morphologies.28-30 Metallic Ni has been used to catalyze HER in alkaline solution since a century ago and has been also applied in UOR electrocatalysis recently.8, 18. Researchers have devoted numerous efforts to optimizing Ni-based catalysts, including applying MOFs as the precursor.8, 31 However, most of them still cannot rival against noble-metal based catalysts. One possible reason is the low surface area of the catalyst caused by direct carbonization of Ni-MOFs. For electrocatalysis, to construct porous nanostructure is one of the feasible solutions to significantly enhance the performance of catalysts. We speculate that introducing Zn into Ni-MOF can assisted the formation of high specific surface area and rich active sites in catalysts because during the calcination, the evaporative Zn in the framework, which can act as the pore-forming agent, will left pores and create open channels. Therefore, a Ni-based material derived from a Zn-doped NiMOF can be an efficient bifunctional catalyst for hydrogen production via urea electrolysis. Herein, we prepared a series of multivariate MOFs (MTV MOFs) based on Zn2+/Ni2+ and 1,3,5-trimesic acid (BTC) with varied ratios of Zn/Ni as they are lattice-matched.32 The derived Ni/C composites exhibit tunable high surface area and well-dispersed Ni active species. Remarkably, the optimized highly porous pomegranate-like Ni/C composite shows excellent HER activity with an unprecedentedly low overpotential of 40 mV at the current density of 10 mA cm−2 in 1 M KOH, which is comparable to commercial Pt/C and surpasses most nonprecious metal-based HER catalysts. In addition, it also displays superb UOR activity with an onset potential of 1.33 V vs. RHE. Furthermore, a two-electrode urea electrolyzer is successfully constructed by two identical Ni/C electrodes as the cathode and anode respectively, which gives an electrolysis current density of 10 mA cm−2 at around 1.65 V for 12 h. And this alkaline urea


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electrolyzer could produce 52 times more hydrogen amount than the alkaline water electrolyzer under the same condition (Scheme 1). Scheme 1. a) Schematic representation of the synthesis and bifunctionality of Ni/C composite. b) Comparison of HER activity in alkaline electrolyte of Ni/C-1 and other representative nonprecious metal-based catalysts deposited on glassy carbon electrode with the loading density at the range of 0.1~1 mg cm−2.

EXPERIMENTAL SECTION Materials and Characterization. All reagents and raw materials were purchased and used directly. Ni(NO3)2·6H2O, Zn(NO3)3·6H2O, 1,3,5-trimesic acid (BTC) and 2-methyl-imidazole were purchased from Sinopharm Chemical Reagent Co. Ltd. N,N-dimethylformamide (DMF) and ethanol were acquired from Beijing Chemical Works. KOH and urea were purchased from J&K Chemical Co. Pt/C catalyst (20 wt.% Pt in carbon, XRD crystallite size 3.5 nm, carbon black-XC72R) was purchased from Johnson Matthey Co. Ltd. Nafion solution was acquired from Alfa-Aesar. Milli-Q water (18 MΩ·cm) was used throughout all the experiments.


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Powder X-ray diffraction (PXRD) pattern was analyzed on a Rigaku MiniFlex 600 X-ray diffractometer with a Cu-Kα X-ray radiation source (λ = 0.154056 nm). Zn and Ni were confirmed via inductively coupled plasma−atomic emission spectroscopy (ICP−AES) analysis, which was conducted on an Axial View Inductively Coupled Plasma Spectrometer instrument (SPECTRO ARCOS EOP). Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were tested on JSM-7500F instrument at an accelerating voltage of 5.0 kV and JEM-2100 instrument at an accelerating voltage of 200 kV, respectively. Energy dispersive X-ray spectroscopy (EDS) was measured on TN5400 EDS at 20 keV. X-ray photoelectron spectroscopy (XPS) was recorded on the Thermo Scientific ESCALab 250Xi with 200 W monochromated Al Kα radiation source. Elemental analysis (C, H, N) was conducted on EuroEA Elemental Analyzer. After pretreatment of the sample (heating and degassing at 120 °C for 6 h), the nitrogen sorption isotherms (at 77 K) and the corresponding results were recorded and analyzed on a Quantachrome Instrument ASiQMVH002-5. The electrochemical characteristics were evaluated on an electrochemical workstation (CHI 760E, CH Instrumental Inc.). Solvothermal synthesis of Zn/Ni-BTC. Zn/Ni-BTC samples with Zn/Ni ratio of 1/19, 1/4, 1/1 were synthesized via a hydrothermal process reported previously with corresponding alterations. 32

To prepare NiBTC, briefly, 0.38 g Zn(NO3)2·6H2O, 0.205 g BTC, 0.055 g 2-methyl-imidazole

and 15 mL DMF were added into a 20 mL Teflon-lined autoclave and heated at 170 °C for 2 days. After allowing to room temperature, the crystals were washed with DMF and ethanol subsequently for 3 times, and finally dried under vacuum at 80 °C for 24 h. The samples with Zn/Ni ratio of 1/19 (denoted as Zn0.05Ni0.95BTC), 1/4 (denoted as Zn0.2Ni0.8BTC) and 1/1 (denoted as Zn0.5Ni0.5BTC) were prepared by the same method in the presence of


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Zn(NO3)2·6H2O with the Zn(NO3)3·6H2O/Ni(NO3)·6H2O ratio of 1/19, 1/4 and 1/1 in the starting material, respectively. The total molar amount of (Zn + Ni) was fixed to be 2.6 mmol. Zn/Ni-BTC samples with Zn/Ni ratio of 4/1 and ZnBTC were prepared via the following method.33 To prepare ZnBTC, 0.0395 g BTC, 0.1707 g Zn(NO3)2·6H2O and 10 mL DMF were put into a 20 mL glass scintillation vial and heated at 85 °C for 16 h. After allowing to room temperature, the crystals were washed with DMF and ethanol subsequently for 3 times, and finally dried under vacuum at 80 °C for 24 h. The sample with Zn/Ni ratio of 4/1 (denoted as Zn0.8Ni0.2BTC) was prepared by the same method in the presence of Zn(NO3)2·6H2O with the Zn(NO3)3·6H2O/Ni(NO3)·6H2O ratio of 4/1 in the starting material. The total molar amount of (Zn + Ni) was fixed to be 0.574 mmol. The Zn/Ni ratio in each MOF sample was calculated according to the ICP-AES analysis results and the results showed that the Zn/Ni ratios in MOF samples are consistent with the ratios in starting materials, respectively. Synthesis of Ni/C catalysts derived from Zn/NiBTC. The powder of Zn/NiBTC was placed in a tube furnace and heated from room temperature to 900 °C at a heating rate of 5 °C min−1 and then calcinated at 900 °C for two more hours under N2 flow (80 mL min−1). After naturally cooled to room temperature, the Ni/C samples were obtained. The as-prepared products were directly used without any further treatment. Preparation of working electrode. The catalyst ink was prepared by ultrasonically mixing 3.5 mg of the MOF derived Ni/C samples or 20 wt.% Pt/C with 500 µL 0.5% Nafion solution for 10 min to form homogeneous catalyst suspensions. Next, 5 µL of the asprepared ink was dropped onto the newly polished glassy carbon electrode (GCE, 5 mm in diameter) and then the modified


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electrode was dried under N2 flow. The catalyst loading density of the working electrode was 0.5 mg cm-2. Electrochemical characterizations. For HER, all electrochemical experiments were performed in a standard three-electrode system using 50 mL 1 M KOH aqueous solution as the electrolyte controlled by a CHI 760E electrochemistry workstation at room temperature. A modified GCE, a graphite rod and a Ag/AgCl (saturated KCl) electrode were used as the working electrode, counter electrode and reference electrode, respectively. For UOR, all electrochemical experiments were also carried out in a three-electrode cell using 50 mL 1 M KOH with 0.33 M urea aqueous solution as electrolyte controlled by a CHI 760E electrochemistry workstation at room temperature. A modified GCE, a Pt wire and a Ag/AgCl (saturated KCl) electrode were used as the working electrode, counter electrode and reference electrode, respectively. iR compensation was applied for all linear sweep voltammetry (LSV) curves and controlled potential electrolysis. All potentials were coverted versus reversible hydrogen electrode (RHE) based on the Nernst equation: E(RHE) = E(Ag/AgCl) + (0.197 + 0.059 × pH). η10, which is the overpotential at the current density of 10 mA cm–2, is chosen as a key parameter because 10 mA cm–2 is the current density of a realistic device with 12.3% solar-tohydrogen efficiency.34 The current density j is calculated by using the apparent surface area of the electrode. Calculation of energy efficiency and hydrogen production rate. To compare the hydrogen production of the urea electrolyzer and the water electrolyzer, the Faradic efficiency of HER is assumed to be 100% firstly. The energy efficiency (ee) is calculated from the following equation:

ee =

EΘ Ecell (1)


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E Θ (V) represents the thermodynamic potential of water splitting. Ecell (V) denotes the cell voltage. The hydrogen production rate v (L h–1) can be determined by the current density j (mA cm-2) based on the following equation:

v=

j⋅s ⋅Vm 2N A ⋅ q (2)

s (cm2) denotes the surface area of the electrode. t (h) represents the time. 2NA (mol-1) denotes the number of electrons consumed during the production of 1 mole of H2. NA is the Avogadro constant, approximately equal to 6.02×1023 mol-1. q is the elemental charge, approximately equal to 1.602×10−19 coulombs. Vm (L mol-1) is the volume occupied by 1 mole of H2, approximately equals to 22.4 L mol-1. Preparation of alkaline water/urea electrolyzer. The catalyst ink was prepared by the same method mentioned in Preparation of working electrode and carefully dropped onto the 1×2 cm2 carbon cloth leading to a catalyst loading density of 0.5 mg cm-2. Then the modified carbon cloth was dried under N2 flow. When it was used as the working electrode or counter electrode, only 1×1 cm2 carbon cloth was immersed in the electrolyte. The chronopotentiometry was conducted under stirring. RESULTS AND DISCUSSION Characterization of morphology and structure. The Ni/C catalysts were prepared through a two-step method: 1) the MTV Zn/Ni-BTC MOFs with different Zn/Ni ratios and same PXRD patterns were synthesized through solvothermal methods (Figure S1a); 2) the prepared Zn/NiBTC MOF samples were then calcinated in N2 atmosphere to form nanoporous Ni/C composites while Zn atoms evaporate during the calcination. These products were denoted as Ni/C-X, where


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the number X indicates the Zn/Ni ratio in MOF precursors. The resulting Ni/C composites were thoroughly characterized by PXRD (Figure S1b), scanning electron microscopy (SEM), energydispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), Xray photoelectron spectroscopy (XPS) and N2 sorption measurements. The morphology of quite uniform and well-dispersed porous spheres was confirmed by SEM images (Figure 1a, 1b, S2). EDS mapping images revealed that Ni was uniformly doped into the carbon shells and no diffractions were observed for Zn species in the sample obtained (Figure S3). The Ni species were confirmed as Ni metal (JCPDS 4-0850) by PXRD (Figure 1c) and HRTEM (Figure S4). The surface state of Ni/C-1 was characterized by XPS (Figure S5) and the Ni0 is the main component in the Ni/C-1.35 The NiO suggested by the Ni 2p peak at 854 eV with the O 1s peak at 529.8 eV and the Ni(OH)2 indicated by the O 1s peak at 531.5 eV could be formed during the exposure in air since metallic Ni is highly active in ambient environments, with NiO and/or Ni(OH)2 forming spontaneously on the surface of atomically clean Ni metal.35 The porosity of Ni/C samples was characterized by N2 sorption test (Figure 1d, S6). With the introduction of Zn in MOF precursors, the Brunauer−Emmett−Teller specific surface area (BET SSA) and the total pore volume of the carbonized samples show big improvement (Table S1). The BET SSA and total pore volume of Ni/C-1 are 438 m2 g–1 and 0.605 cm3 g–1, respectively. In order to further confirm that the high surface area and large pore volume are electrochemically accessible, the electrochemical surface areas (ECSA) of Ni/C samples were then evaluated by measuring the electrochemical double-layer capacitance (Cdl) in non-Faradaic region. The Ni/C-1 (2.403 F cm–2) exhibits a 3 times Cdl than Ni/C-0 (0.7205 F cm–2) (Figure S7). Generally, high surface area, including BET SSA and ECSA, could accelerate gas release, facilitate mass transport and improve the accessibility of active sites.


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Figure 1. a, b) SEM images of Ni/C-1. c) PXRD patterns of Zn0.5Ni0.5BTC, the derived Ni/C-1 and the simulated Ni-BTC. d) N2 adsorption/desorption isotherms of Ni/C-1, inset: pore width distribution of Ni/C-1. Electrocatalysis performance of HER. The electrocatalytic HER activities of Ni/C catalysts and commercial 20 wt.% Pt/C were evaluated in 1 M KOH. The polarization curves show that Ni/C-1 exhibits the highest activity among all Ni/C composites with an extremely small overpotential at 10 mA cm–2 (η10) of 40 mV, which is very close to that of Pt/C (25 mV) and is much smaller than that of Ni/C-0 (316 mV) (Figure 2a). The trend in the overpotential suggests that the introduction of pore-forming agent in MOF precursors indeed improved the electrocatalytic performance of the derived Ni/C nanoporous composites. Tafel slopes of 77, 167


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and 58 mV dec−1 were measured for Ni/C-1, Ni/C-0 and 20 wt.% Pt/C, respectively (Figure 2b). It is suggested that the smaller Tafel slope leads to a higher HER rate and indicates a faster kinetics. Generally, two possible pathways, the Volmer–Heyrovsky pathways or Volmer–Tafel pathways, has been proposed for the HER process in alkaline medium. According to previous reports, a Tafel slope fall within 40 to 120 mV dec−1 may arise from the Volmer–Heyrovsky process.36, 37 These values indicate that the Ni/C-1 is one of the top non-precious metal based HER catalysts in alkaline electrolyte (Table S2) and can be even comparable to some noblemetal based catalysts.38 The electrochemical impedance spectroscopy analysis was also performed under the overpotential of 100 mV (Figure 2c, S8). The Rct of Ni/C-0 and Ni/C-1 in 1 M KOH are calculated to be 39.48 Ω and 4.256 Ω, respectively. Generally, smaller charge transfer resistance (Rct) value corresponds to more favorable HER kinetics. In the low frequency range, Ni/C-0 shows ion diffusion behavior inside the electrode and Ni/C-1 displays semi double-layer capacitance and this is due to the porosity and the high surface area in Ni/C-1.39 To estimate the stability of Ni/C-1, controlled potential electrolysis (CPE) at overpotential of 100 mV was conducted under continuous stirring (Figure 2d). During the test, a substantial amount of bubbles of H2 gas were observed, which led to the current fluctuation in HER process. The steady current as well as the unchanged PXRD patterns and LSV curves of Ni/C-1 after CPE show the extraordinary stability of the catalyst (Figure S9).


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Figure 2. a) LSV curves of Ni/C composites derived from different MOF precursors. Wine: blank GC, black: NiBTC, red: Zn0.05Ni0.95BTC, blue: Zn0.2Ni0.8BTC, dark cyan: Zn0.5Ni0.5BTC, magenta: Zn0.8Ni0.2BTC, orange: Pt/C. b) Tafel plots of Ni/C-0, Ni/C-1 and 20 wt.% Pt/C. c) Nyquist plots of Ni/C-1 and Ni/C-0 at η of 100 mV. d) CPE of Ni/C-1 at η of 100 mV. Electrocatalysis performance of UOR. Afterwards, the electrocatalytic UOR performances of Ni/C-1 and Ni/C-0 were evaluated in the same three-electrode setup. The LSV curves of catalysts tested in the absence and presence of 0.33 M urea in 1 M KOH solution are shown in Figure 3a. Compared to the curves in 1 M KOH, an intense increase on current density with the onset potential at 1.33 V vs. RHE was clearly shown with the addition of urea. According to the


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previous reports, the onset potential of UOR is consistent with the potential of NiOOH formation and the newly generated NiOOH species are the active sites for the urea oxidation.19 Compared to Ni/C-0, Ni/C-1 exhibits much higher catalytic activity with smaller onset potential and larger current density. And this is consistent with the HER catalytic results and reveals that the high surface area can also enhance the UOR catalytic performance. Similar to the EIS result of HER, the calculated Rct of Ni/C-1 is smaller than that of Ni/C-0 (Figure 3b, S10). The stable oxidation current of Ni/C-1 recorded during 10-hour CPE under the potential of 1.65 V vs. RHE proves the outstanding durability of Ni/C-1 as a qualified UOR catalyst (Figure 3c).


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Figure 3. a) LSV curves of Ni/C-1 (red) and Ni/C-0 (black) in 1 M KOH, with (solid line) and without (dashed line) 0.33 M urea. b) Nyquist plots of Ni/C-1 and Ni/C-0 and c) CPE of Ni/C-1 at 1.65 V vs RHE. The alkaline urea electrolyzer. As encouraged by the promising dual functionalities, alkaline electrolyzers using Ni/C materials deposited on carbon cloth as catalysts for both cathode and anode were assembled. LSV curves of cells using 1 M KOH and 1 M KOH with 0.33 M urea as electrolytes are compared in Figure 4a. Obviously, in the presence of urea, the cell exhibits higher activity with a smaller cell voltage of 1.6 V at the current density of 10 mA cm–2. What’s more, when the energy efficiency of the electrolyzer is set to be 70%, the hydrogen production rate of the urea electrolyzer reaches 3.36 L h−1, which is 52 times higher than that of the water electrolyzer (0.064 L h−1). Consisted with previous results, the activity of Ni/C-1 is much higher than that of Ni/C-0 (Figure 4b). Afterwards, the stability of the urea electrolyzer was proved by chronopotentiometry under current density of 10 mA cm–2 for 12 hours (Figure 4c).


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Figure 4. a) LSV curves of an alkaline water electrolyzer using Ni/C-1 as catalyst for both HER and OER in 1 M KOH and an alkaline urea electrolyzer using Ni/C-1 as catalyst for both HER and UOR in 1 M KOH with 0.33 M urea. b) LSV curves of an alkaline urea electrolyzer using Ni/C-1 and Ni/C-0 as catalyst. c) Long-term durability tests of urea electrolyzer. Inset: A digital image presenting the evolution of H2 and N2 gas on surfaces the electrodes. CONCLUSION In conclusion, highly porous pomegranate-like Ni/C-1, which is derived from Zn/Ni-BTC with Zn/Ni ratio of 1, exhibits extremely high HER activity in alkaline electrolyte with a unprecedentedly low overpotential of 40 mV at current density of 10 mA cm–2, surpassing most documented non-precious metal-based catalysts. Additionally, Ni/C-1 shows outstanding UOR activity with the onset potential of 1.33 V vs. RHE. The bifunctionality of Ni/C-1 was further


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proved by a two-electrode alkaline urea electrolyzer applying Ni/C-1 as both anode and cathode, which could achieve 10 mA cm–2 at a cell voltage of 1.6 V and generate 52 times more hydrogen amount than the water electrolyzer under the energy efficiency of 70%. Furthermore, it is scarcely reported that producing H2 from H2O using urea oxidation as the anodic reaction. And this new method, which is to generate hydrogen via replacing traditional OER with more thermodynamically favorable UOR, could achieve sewage treatment and energy-efficient hydrogen production at the same time. This study may pave the way to the design and synthesis of efficient electrocatalysts and electrolyzers with environmental benignity and lower energy consumption. The catalytic performance of the Ni/C-1 and the total energy expenditure of hydrogen production in real sewage/urine condition are being investigated and will be reported timely.

ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

PXRD pattern, SEM images, TEM images, XPS spectra, surface area determination, resistance analysis, stablility test results and comparison table of HER performance (PDF).

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was financially supported by the 973 Program 2013CB834702; the National Natural Science Foundation of China (Grant No. 21625102, 21471018, 21404010, 21490570, 21674012); 1000 Plan (Youth).

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