Nickel Molybdenum Nitride Nanorods Grown on Ni Foam as Efficient

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Energy, Environmental, and Catalysis Applications

Nickel Molybdenum Nitride Nanorods Grown on Ni Foam as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting Jun-Ran Jia, Meng-Ke Zhai, Jing-Jing Lv, Bao-Xun Zhao, Hongbin Du, and Jun-Jie Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09854 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

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ACS Applied Materials & Interfaces

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Nickel Molybdenum Nitride Nanorods Grown on Ni Foam as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting

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JunRan Jia,[a] MengKe Zhai,[a] JingJing Lv,[b] BaoXun Zhao,[a] HongBin Du,*[a] JunJie Zhu*[b]

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[a]State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of

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Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing

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University, Nanjing, 210023, China. Email: [email protected]

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[b]State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and

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Chemical Engineering, Nanjing University, Nanjing, 210023, China. Email: [email protected]

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KEYWORDS: Water splitting • Electrocatalysts • Metal nitride • Hydrogen evolution • Oxygen

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evolution

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ABSTRACT: Non-noble-metal electrocatalysts for water splitting hold great promises for

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developing sustainable and clean energy sources. Herein, a highly efficient bifunctional electrode

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consisting of Ni-doped molybdenum nitride nanorods on Ni foam is prepared through topotactic

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transformation of NiMoO4 nanorods that are in situ hydrothermally grown on Ni foam. The

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electrode not only contains rich, accessible, electrochemically active sites, but also possesses

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extraordinary chemical stability. It exhibits excellent hydrogen evolution reaction (HER) and

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oxygen evolution reaction (OER) performance in 1.0 M KOH with low overpotentials of 15 mV

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and 218 mV, respectively, at a current density of 10 mA cm−2, superior to the commercial

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benchmark materials Pt/C and RuO2 under the same condition. A simple water electrolyzer using

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the obtained electrode as both the anode and cathode needs a very low cell potential of 1.49 V to

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reach a current density of 10 mA cm−2 and maintains stability for 110 h without degradation. The

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excellent performance of the electrode could be attributed to the formation of highly conductive,

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corrosion- and oxidation-resistant metal nitrides and the synergetic effect between intimately

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interconnected, electrochemically active nickel molybdenum nitride and Ni or NiO

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nanoparticles. This study shows that the use of transition metal nitrides in combination of

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nanostructured heterojunctions of multiple active components enables one to develop highly

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stable and efficient water electrolyzers without precious metals. The preparative strategy used in

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this work could be applied to devise new electrocatalysts.

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1. INTRODUCTION

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The catalytic electrolysis of water via hydrogen evolution reaction (HER) and oxygen

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evolution reaction (OER) provides a potential solution to the challenge of developing clean and

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sustainable energy sources.1,2 The major difficulty in water electrolysis is to reduce the large

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overpotentials in water-splitting electrolyzer, which highly depend on the interfaces,

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microstructures and elemental compositions of the electrocatalysts.3 RuO2 and Pt/C are the state-

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of-the-art catalysts for OER and HER, respectively, which have already been commercialized4,5.

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However, the noble metals are scare in earth crust and expensive, which have limited their

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applications. Recently, extensive research has been carried out on the use of earth-abundant

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transition metals6 and their derivatives (e.g. oxides,7-9 hydroxides,10-13carbides,14-18nitrides,19-22

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phosphides,23-27, boride28, selenides29 and sulfides30) as water-splitting electrocatalysts, some of

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which have shown promising HER or OER activities. However, most of the state-of-the-art non-

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noble metal electrocatalysts usually do not perform well over a long period of time under harsh

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operating conditions (such as pH, high electrode potentials) due to the corrosion and/or oxidation.

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The development of inexpensive and efficient non-noble metal electrocatalysts for HER and,


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particularly, OER, remains a challenge.31-33 Moreover, it is also desirable to develop highly

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efficient bifunctional noble-metal-free electrocatalysts for construction of simplified water-

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splitting electrolyzers.

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Bimetallic alloys (e.g. NiCo, FeCo, NiFe, NiMo, etc.) and derivatives have been known to

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possess better HER or OER electrocatalytic activities than the single ones owing to the

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synergistic effect of the two active components.7,10,19,21,30 38 The presence of a second metal atom

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in bimetallic alloys could offer more reaction sites and enhance the electronic conductivity.

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Moreover, bimetallic alloys allow adjustability on the elemental composition ratio, which affords

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great potential on regulating chemical properties. For instance, Ni atoms are identified as

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efficient water splitting centres, while Mo atoms are thought to have strong binding effects

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towards hydrogen. Upon alloying, the bonding strength of the Mo−H bond in NiMo catalysts is

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effectively reduced due to the presence of nickel atoms, so the dissociation and desorption of

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hydrogen proceed smoothly and the HER reactivity is greatly improved.6 However, like most

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electrocatalysts, the alloys usually suffer degradation because of corrosion and/or oxidation

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under applied electrode potentials in a strongly acidic or basic electrolyte solution.

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Transition metal nitrides have recently been showed high catalytic activities for water

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splitting due to their unique physical and chemical properties.15,34 The incorporation of nitrogen

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atoms alters the d band structure of the original metal, making it more resembling the electronic

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structure of the noble metal catalyst and thus improving the electrocatalytic activities.16 On the

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other hand , due to its small radius, the nitrogen atom tends to occupy the interstitial sites in the

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original close-packed lattice of metal atoms when forming the nitrides. The modified lattice not

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only allows transition metal nitrides to have higher electrical conductivity, but also better

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corrosion and oxidation resistance. As a result, transition metal nitrides have showed advantages


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in water splitting over pure metals or metal alloys.21 For instance, Ni3N nanosheets were found to

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be great OER catalyst in KOH solution, attributable to the formation of highly electrochemically

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active NiOOH on the surface.35 Ni3N-Ni0.2Mo0.8N nanosheets and nanotubes showed

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considerable improvement on both HER and OER.36,37.Ni-Ni0.2Mo0.8N nanoparticles were also

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demonstrated to have good HER property.38 The latter two consisted of heterostructures of two

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active ingredients, which provided synergetic effects to further improve the electrocatalytic

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performance.

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Herein we report the designed synthesis of a highly efficient bifunctional nickel molybdenum

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nitride nanorod electrocatalyst grown on Ni foam (denoted as [email protected]) for water

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splitting. The catalyst is devised by taking advantage of the synergistic effect of the multiple

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active components, highly conductive and corrosion-resistant nature of metal nitride and

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integrated 1-dimensional (1-D) nanostructures that consist of interconnecting electrochemically-

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active nanoparticles to minimize resistance potentials. The resulting [email protected]

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exhibits exceptional HER and OER performance in 1.0 M KOH, both of which surpass the

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commercial benchmark materials Pt/C and RuO2 under the same condition.

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2. EXPERIMENTAL SECTION

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Materials. Nickel nitrate hexahydrate, ammonium heptamolybdate tetrahydrate (AHM),

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potassium hydroxide and hydrochloric acid were purchased from Sinopharm Chemical Reagent

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Co. Ltd. Nickel foam (thickness 1.6 mm, 40 PPI) was purchased from Long Sheng Bao Kunshan

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Electronic Materials Co. Ltd. Commercial Pt/C (20 wt%) were purchased from Shanghai

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Macklin Biochemical Co. Ltd. RuO2 (99.9 %) was purchased from Aladdin industrial

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Corporation. All reagents were used as received without further purification.


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Synthesis of [email protected]. In order to synthesize the [email protected]

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electrocatalyst, NiMoO4 nanorods were first hydrothermally grown on Ni foam according to the

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literature.39-41 In a typical preparation, Ni foam was first cut into 4 × 4 cm2 pieces and

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successively cleaned in ethanol, 3 M HCl solution and deionized water. Then, one piece of Ni

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foam was placed into a Teflon-lined stainless steel autoclave. 80 mL of deionized water

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containing AHM (0.01 M) and Ni(NO3)2·6H2O (0.05 M) were immediately added into the

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autoclave to prevent Ni foam being oxidized. Next, the autoclave was sealed and heated at 160

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°C for 6 h. After cooled to room temperature, the substrate (denoted as Nifoam@NiMoO4) was

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washed with deionized water and then dried under an infrared light. The loading mass of the

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formed NiMoO4 nanorods on nickel foam was estimated as ∼46.3 mg cm–2. For nitridation, the

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obtained yellow nickel foam plate was put in a tube furnace and heated at 550 °C for 3 h in a

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flowing NH3 atmosphere to form the [email protected] electrocatalyst. The loading weight

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of the formed Ni0.2Mo0.8N nanorods on nickel foam was estimated as ∼42.0 mg cm–2.

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Synthesis of Nifoam@Ni4Mo/MoO2. Nifoam@Ni4Mo/MoO2 was synthesized by following the

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same procedure except for changing NH3 to H2/Ar (10 : 90 v/v) and the calcination temperature

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set at 550°C to 500°C according to the reference.6 SEM images showed nanorods grown on Ni

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foam (Figure S1), which consist of Ni, Mo and O with an atomic ratio of 0.30 : 0.46 : 1.00.

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XRPD and XPS analyses revealed the formation of Ni4Mo and MoO2 on Ni foam (Figure S1).

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The loading mass of the formed Ni4Mo/MoO2 nanorods on nickel foam was estimated as

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∼43.6 mg cm–2. It is noted that the freshly prepared Ni4Mo/MoO2 nanorods are highly reactive

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and easily oxidized in air (e.g. burst into flame even cooled down to room temperature), which

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need passivation in Ar atmosphere for several hours to prevent bursting into flame.


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Synthesis of Nifoam@Ni3N. Nifoam@Ni3N was synthesized by high temperature nitridation of

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Ni(OH)2 precursor on nickel foam, which was synthesized by following the reported method:42 A

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piece of cleaned Ni foam (4 × 4 cm2) was immersed into 80 mL of HCl solution (pH = 3.0) and

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heated at 180°C for 6 h to produce Ni(OH)2 nanosheets anchored Ni foam. The resulted nickel

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foam plate was then put in a tube furnace and heated at 550 °C for 2 h under a flowing NH3

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atmosphere. SEM images showed nanosheets grown on Ni foam (Figure S2a). EDS and XRPD

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analysis revealed the formation of Ni3N on Ni foam (Figures S2b-c). The loading mass of the

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formed Ni3N nanosheets on nickel foam was estimated as ∼25.1 mg cm–2.

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Physical Methods. X-ray powder diffraction (XRPD) patterns were recorded on a Bruker D8-

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Advance diffractometer using Cu-Kα radiation (λ = 1.54178 Å) at 40 kV and 40 mA with a scan

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rate of 0.1 s/step. Scanning electron microscopy (SEM) images and Energy-dispersive spectrum

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(EDS) were recorded on a Hitachi S-4800 field emission scanning electron microscope.

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Transmission electron microscopy (TEM) and high-resolution (HR) TEM images were collected

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on a JEM-2100 instrument (Japan). X-ray photoelectron spectroscopy (XPS) spectra were

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obtained on a PHI 5000 Versa Probe instrument. The binding energy was calibrated against the C

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1s peak energy of 284.6 eV. FT-IR spectra were collected on a Nicolet 6700 instrument (USA).

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Electrochemical measurements. The electrochemical tests were carried out on a CHI 660D

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electrochemical workstation (Chenhua, China) in a three-electrode system. The electrocatalyst-

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loaded Ni foam (1 × 1 cm2 in sizes) was directly used as a working electrode, while a graphite

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rod was used as the counter electrode and a Hg/HgO electrode as the reference electrode. Linear

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sweep voltammetry (LSV) and cyclic voltammetry (CV) were measured in a N2-saturated (for

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HER) or O2-saturated (for OER) KOH (1.0 M) solution with a scan rate of 1 mV s–1.

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Electrochemical impedance spectroscopy (EIS) were recorded with operating overpotentials set


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at +0.2 V (OER) and −0.2 V (HER).43 The frequency ranges from 100000 to 0.01 Hz. For overall

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water splitting, [email protected] was employed as both the cathode and the anode in a

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two-electrode system. All the electrochemical data were converted according to the equation:

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Potential (vs. RHE) = Emeasured,

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according to the equation: Ecompensated = Emeasured – iRs (Rs is the series resistance determined by

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EIS).44 Long-term durability tests were carried out at a current density of 10 mA cm–2 without iR

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compensation. The double-layer capacitances (Cdl) were measured, according to the reported

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method in Ref. 45, by conducting cyclic voltammograms (CVs) in 1.0 M KOH with different

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scan rates from 20 to 300 mV s−1 in a potential window of 1.025 to 1.125 V (vs. RHE). The

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capacitor charging current jc and discharging current ja at 1.075 V were plotted vs. the scan rate v

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according to the equation: |j| = v × Cdl, where Cdl was the slope of the linear fit.

vs. Hg/HgO

+ 0.059 pH + 0.098 V. The test data were corrected

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The rotating ring-disk electrode (RRDE) voltammograms were collected to determine the

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Faraday efficiency, using a RRDE configuration (Pine Research Instrumentation, USA)

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comprised of a glassy carbon disk electrode and a platinum ring electrode. The Nifoam@Ni-

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Ni0.2Mo0.8N was peeled off from nickel foam and then coated onto RRDE with Nafion as the

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binder (catalyst loading: 0.28 mg cm-2). Rotation rate was maintained at 1000 rpm. The O2/H2

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produced by OER/HER on the surface of glassy carbon disk can easily diffuse to the platinum

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ring electrode in such high rotating speed. The ring potential of the RRDE was kept constantly at

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1.2 V (HER) and 0.4 V (OER) vs. RHE to reduce the H2/O2 produced from catalyst on the disk

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electrode in 1.0 M KOH solution. The electron transfer number (N) can be calculated from the

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disk current (Id) and ring current (Ir) of the RRDE: N = n × Id/ (Id+ Ir/N), where n is the

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theoretical electron transfer number (n = 2 for HER, n = 4 for OER).46 The Faraday efficiency

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can be calculated as: Faraday efficiency = N / n ×100 %.


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For comparisons of the electrocatalytic properties, the Pt/C and RuO2 electrodes were

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prepared as follows: 10 mg of catalysts (20 wt% Pt/C and 99.9% RuO2) were dispersed in 900

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µL of 4 : 1 (v / v) water/ethanol and 100 µL of 0.5 % nafion solution by sonication for 30 min to

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form a homogeneous ink and then loaded onto a freshly cleaned 1 cm2 Ni foam.

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RESULTS AND DISCUSSION

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Material Preparation and Characterization.

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The [email protected] nanostructures were fabricated by thermal nitridation of

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Nifoam@NiMoO4, i.e. NiMoO4 nanorods grown on Ni foam. As shown in Figures 1a-b, a dense

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forest of NiMoO4 nanorods with a diameter of 0.5~1 µm and a length of ~50 µm were in situ

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grown perpendicularly onto the surface of Ni foam via a simple hydrothermal reaction. X-ray

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powder diffraction (XRPD) and element mapping analyses showed the nanorods were composed

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of crystalline NiMoO4·xH2O (JCPDS card no. 13-0128) and NiMoO4 (JCPDS card no. 33-0948)

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(Figures S3 and S4). After heated at 550°C in an ammonia atmosphere, the golden color of

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Nifoam@NiMoO4 turned black (Figure S5), implying successful nitridation.


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Figure 1. SEM images of (a, b) Nifoam@NiMoO4, and (c, d) [email protected]. (e) Element

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mapping and (f) HRTEM images of [email protected]. Insert shows the SAED pattern.

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As depicted in Figures 1c-d, the morphology of the [email protected] remained the

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same as the precursor Nifoam@NiMoO4 after nitridation at high temperature. Element mapping


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images showed that Mo, Ni, and N were uniformly distributed in the nanorods (Figure 1e).

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Energy-dispersive spectrum (EDS) suggested that the nanarods mainly consisted of Mo, Ni, N

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and O, with an atomic ratio of Ni/Mo/N/O close to 0.88 : 0.88 : 1 : 0.88 (Figure S6). XRPD

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analyses confirmed the successful nitridation (Figure S7). The four broad diffraction peaks at

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36.49°, 49.41°, 65.32° and 74.91° were related to the (200), (202), (220) and (222) planes of

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Ni0.2Mo0.8N (JCPDS card No. 29-0931). The diffraction peaks at 44.53°, 51.90°, and 76.36° were

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due to Ni (JCPDS card No. 04-0850), which were resulted from reduction of NiMoO4 by NH3

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during nitridation and from the Ni foam substrate as well. In addition, NiO (JCPDS card No. 47-

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1049) was also present in the nanorods, which was likely due to the oxidation of Ni nanoparticles

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upon exposure to air. The average crystallite sizes of Ni0.2Mo0.8N, Ni and NiO were calculated as

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11, 9 and 15 nm, respectively, based on the Scherrer equation. High-resolution transmission

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electron microscopy (HR-TEM) images revealed that the nanorods consisted of randomly

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distributed, interconnecting crystallites of ca. 5–15 nm in sizes, the lattice fringes of which could

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be attributed to those of Ni0.2Mo0.8N, Ni or NiO (Figures 1f and S8). Consistently, the selected

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area electron diffraction (SAED) patterns showed the diffraction spots belonging to these species.

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The above results suggested that after nitridation, the nanorods were converted into

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interconnecting Ni0.2Mo0.8N nano crystallites decorated by partially oxidized Ni nanoparticles.


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Figure 2. XPS spectra of [email protected]: (a) survey scan, (b) Ni 2p, (c) Mo 3d, and (d)

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N 1s regions.

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The XPS survey spectra of [email protected] revealed the presence of Mo, Ni, O and

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N (Figure 2a), in agreement with the element mapping results. The Ni 2p3/2 XPS spectrum

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showed a strong peak of the metallic Ni0 at 852.7 eV, accompanied by a slightly weaker peak of

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the divalent Ni2+ at 855.6 eV (Figure 2b). The former was due to the Ni nanoparticles reduced

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from NiMoO4 and the Ni substrate, while the latter likely came from NiO owing to partial

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oxidation of Ni0. The strong peak at 229.0 in the Mo 3d XPS spectrum (Figure 2c) could be


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deconvoluted into two peaks at 229.0 eV and 229.8 eV, respectively, which belonged to MoIII

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3d5/2 and MoIV 3d5/2. The slightly weaker peak at 232.2 eV was attributable to the overlap of

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MoVI 3d5/2, MoIII 3d3/2 and MoIV 3d3/2. There were trace amounts of MoVI species (i.e. Mo 3d5/2

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peak at 232.2 eV and 3d3/2 peak at 235.4 eV) in the sample, likely due to the oxidized surface Mo

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species.47,48 The results implied the transformation of MoVI to lower oxidation states during the

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nitridation at high temperature. The N 1s XPS spectrum gave an asymmetric peak around 396 eV

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(Figure 2d), which could be deconvoluted into three peaks at 394.8 eV, 397.1 eV and 398.8 eV,

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respectively. The first peak was due to MoIII 3p3/2, while the second peak was close to the N 1s

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value expected for the Mo/Ni–N bond (396.7 eV),38 suggesting the formation of metal nitrides.

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The third peak can be attributed to N-H groups, which implies the formation of N-H species at

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the surface of the electrocatalyst.37 The XPS results showed that [email protected] was

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composed of partially oxidized Ni nanoparticles and nickel molybdenum nitrides on Ni foam,

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consistent with the XRPD, SEM and TEM results. Quantitative calculations were made to

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determine the ratio of Ni and Ni0.2Mo0.8N, which is close to 0.6 : 1, consistent with the EDS

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results (Figure S6).

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HER Performance

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The HER and OER properties of [email protected] were performed in a 1.0 M KOH

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solution by using a standard three-electrode system, where the [email protected] foam with

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dimensions of 1 × 1 cm2 was directly used as the working electrode. Figure 3 shows the linear

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sweep voltammograms (LSV) for the HER performance of [email protected], together

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with those of several reference electrodes, including Nifoam@NiMoO4 (the precursor to

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[email protected], prepared according to Ref. 39 to 41), Nifoam@Ni4Mo/MoO2 (H2-reduced

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Nifoam@NiMoO4, a product composed of Ni4Mo/MoO2 nanorods on Ni foam, prepared according


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to Ref. 6, Figure S1), and Nifoam@Ni3N (Ni3N nanosheets on Ni foam, prepared according to Ref.

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41, Figure S2). As shown in Figure 3a, [email protected] displayed an overpotential (η10)

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of 15 mV at a current density (j) of 10 mA cm−2, which was much smaller than reference

229

materials Nifoam (263 mV), Nifoam@Ni3N (164 mV), Nifoam@NiMoO4 (262 mV), and

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Nifoam@Ni4Mo/MoO2 (97 mV). In fact, the extremely low η10 of [email protected] is one

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of the lowest amongst the state-of-the-art non-noble-metal-based HER catalysts (Table S1).

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Notably, the outstanding activity of [email protected] exceeded that of the commercial

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benchmark HER catalyst 20% Pt/C supported on Ni foam with a mass loading of 10 mg cm-2,

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which required an overpotential of 21 mV to achieve j = 10 mA cm−2. Moreover, the Tafel slope

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of [email protected] was estimated to be merely 39 mV dec−1, which was also smaller than

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those of Nifoam@Ni4Mo/MoO2 (162 mV dec−1), Nifoam@NiMoO4 (212 mV dec−1), Nifoam@Ni3N

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(183 mV dec−1), Nifoam (322 mV dec−1) and even the Pt/C on Nifoam (45 mV dec−1) (Figure 3b).

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The Faraday efficiencies of [email protected] determined by using rotating ring-disk

239

electrode (RRDE) voltammograms were 96.75±0.25% (Figure S9a). More importantly, the

240

[email protected] catalyst showed excellent HER stability for 110 h of successive testing

241

with negligible increase in the overpotentials (Figure 3d). To the best of our knowledge, such a

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low η10 value accompanied with a very small Tafel slope has scarcely been reported for non-

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noble-metal-based electrocatalysts (Table S1). The above results suggest that Nifoam@Ni-

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Ni0.2Mo0.8N is one of the most efficient HER electrocatalysts reported to date.


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Figure 3. (a) iR-corrected LSV curves, (b) Tafel and (c) Nyquist plots (at η = −200 mV) for

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HER measured in a N2-saturated 1.0 M KOH solution at a scan rate of 1 mV s–1. (d) Long-term

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durability of [email protected] at j = –10 mA cm–2 without iR compensation. The inset

249

shows polarization curves before and after the stability tests.

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To shed light on the low HER overpotentials of [email protected], electrochemical

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impedance spectroscopy (EIS) (Figure 3c) was used to study the charge-transfer resistance (Rct)

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in these materials. The observed Rct value (5.0 Ω) of [email protected] was smaller than

253

those of Nifoam@Ni4Mo/MoO2 (18.2 Ω), Nifoam@NiMoO4 (18.2 Ω), Nifoam@Ni3N (5.3 Ω), and

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Nifoam (27.1 Ω), suggesting rapid electron transport for hydrogen evolution within the electrode


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system of [email protected]. This could be attributed to the formation of highly conductive

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and electrochemically active metal nitride nanorods and the intergrowth of the nanorods on the

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Ni foam electrode that allows minimization of interface-resistance potentials.

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Figure 4. (a) iR-corrected LSV curves measured in negative polarization scanning mode, (b)

260

Tafel and (c) Nyquist plots (at η = 200 mV) for OER measured in an O2-satuarated 1.0 M KOH

261

at a scan rate of 1 mV s−1. The insert in (c) shows the enlarged plots. (d) Long-term durability of

262

[email protected] at j = 10 mA cm−2 without iR compensation. The inset in (d) shows the

263

polarization curves before and after the stability tests measured in positive polarization scanning

264

mode.


15


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Page 16 of 28

OER Performance

266

Figure 4 shows the OER performance of [email protected] measured in an O2-

267

saturated 1.0 M KOH solution. To achieve a current density of 10 mA cm−2, Nifoam@Ni-

268

Ni0.2Mo0.8N displayed the smallest overpotential of 218 mV amongst the tested materials

269

including Nifoam@Ni4Mo/MoO2 (247 mV), Nifoam@Ni3N (372 mV), Nifoam@NiMoO4 (241 mV),

270

and Nifoam (405 mV), even better than the commercial benchmark material RuO2 supported on Ni

271

foam with a mass loading of 10 mg cm-2 (245 mV). Impressively, the Tafel slope of Nifoam@Ni-

272

Ni0.2Mo0.8N was estimated to be 55 mV dec−1, which was also smaller than those of

273

Nifoam@Ni4Mo/MoO2 (181 mV dec−1), Nifoam@NiMoO4 (133 mV dec−1), Nifoam@Ni3N (81 mV

274

dec−1), Nifoam (110 mV dec−1) and even the RuO2 on Nifoam (66 mV dec−1) (Figure 4b). The OER

275

η10 and Tafel slope of [email protected] are smaller than most of the state-of-the-art non-

276

precious metal based OER catalysts (Table S2). The Faraday efficiencies of Nifoam@Ni-

277

Ni0.2Mo0.8N determined by using RRDE voltammograms were 96.25±0.75% (Figure S9b).

278

Moreover, the OER performance of [email protected] remained nearly unchanged after

279

110 h of successive tests at j = 10 mA cm−2 (Figure 4d), demonstrating excellent stability. In

280

contrast, the Nifoam@Ni4Mo/MoO2 and Nifoam@NiMoO4 electrodes underwent severe

281

degradation during the OER tests for a prolonged time (Figures S1 and S10). EIS data (Figure

282

4c) again showed that [email protected] possessed the smallest Rct in Nyquist plots (1.4 Ω),

283

suggesting that [email protected] had an excellent electron transport passway for OER.

284


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285 286

Figure 5. Polarization curves of (a) [email protected] | [email protected] and (b)

287

Pt/C | RuO2 in a two-electrode system for overall water splitting (measured in negative

288

polarization scanning mode). Inset: Durability of [email protected] for overall water

289

splitting in 1.0 M KOH at j = 10 mA cm−2.

290

Overall Water-Splitting Performance

291

Encouraged by its excellent HER and OER performance, [email protected] was used

292

as a bifunctional catalyst for both the anode and cathode in a simple two-electrode system for

293

overall water splitting tests (See video in SI). Figure 5 shows the polarization curve of the

294

bifunctional [email protected] electrolyzer in 1.0 M KOH. Only a small cell potential of

295

1.49 V (without iR compensation) was needed to achieve a current density of 10 mA cm−2. The

296

cell potential was smaller than those of the commercial RuO2−Pt/C couple (1.53 V at 10 mA cm–

297

2

298

importantly, the [email protected] bifunctional catalyst showed excellent stability for 110 h

), and most of the state-of-the-art non-noble metal bifunctional electrocatalysts (Table S3). More


17


Page 18 of 28

299

of successive testing with negligible increase in cell potentials, showing promises for practical

300

applications.

301

Structure and Activity

302

To investigate the structure-and-activity relationship of the high-performance Nifoam@Ni-

303

Ni0.2Mo0.8N electrodes, we fabricated the electrodes under varied preparative conditions and

304

reference materials Nifoam@Ni4Mo/MoO2 and Nifoam@Ni3N for comparison. Figure S11 shows

305

the XRPD patterns epared by varying nitridation time. Ni0.2Mo0.8N nanoparticles were apparently

306

formed within 10 min of heating at 550 ºC in ammonia. When the nitridation time increased from

307

10 min to 5 h, the XRPD peak intensities increased, implying better crystallinity (i.e. larger

308

crystallite sizes) of the samples prepared at prolonged nitridation time. However, even with

309

prolonged nitridation time of 5 h, no obvious XRPD peaks due to Ni3N was observed. In

310

comparison, the formation of Ni3N could be completed in 2 h at 550ºC with Nifoam@Ni(OH)2

311

(Figure S2). The HER and OER tests (Figure S12) showed that the [email protected]

312

electrode prepared in 10 min of nitridation did not behave well, likely due to incomplete

313

nitridation. Upon increasing the nitridation time to 2 h, the electrode showed the best HER and

314

OER performance, while the electrode prepared in 5 h of nitridation displayed deteriorated

315

performance, probably because of larger crystals of Ni0.2Mo0.8N. The results suggest the

316

electrode prepared by nitridation for 2 h possessed the optimized crystallinity and crystallite

317

sizes.

318

Figure S13 shows the XRPD patterns of [email protected] prepared by varying

319

nitridation temperature for 2 h. When the temperature increased from 450 °C to 800 °C, the

320

XRPD peaks due to Ni0.2Mo0.8N got stronger, suggesting the formation of larger crystallites of

321

Ni0.2Mo0.8N. Similar to the samples prepared under variable nitridation time, there were no peaks


18

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322

due to Ni3N observed in all the samples nitrided at 450 °C to 800 °C. The EDS results (Figure

323

S14) show that the nitrogen contents in the samples increased from 3.8 : 2.9 : 1 (Ni : Mo : N

324

atomic ratio) prepared at lower temperature (450 °C) to 1.7 : 1.4: 1 (500 °C), and then reached a

325

maximum of 0.8 : 0.8 : 1 at 550 °C. The compositions of the samples prepared above 550 °C

326

remained the same as those of 550 °C, suggesting the complete nitridation at 550 °C. It is noted

327

that there have been reports that shows the conversion of NiMoO4 nanosheets to Ni3N-

328

Ni0.2Mo0.8N, and NiMoO4 nanoparticles to Ni-Ni0.2Mo0.8N composites.37,38 We did not observe

329

the formation of Ni3N during the nitridation of NiMoO4 nanorods on Nifoam under various

330

conditions by means of XRPD, EDS and HRTEM studies, the reason of which is likely related to

331

the structures of the precursors. As depicted by the LSV curves in Figure S15, the electrode

332

prepared at 550 °C for 2 h presented the best HER and OER performance. The inferior

333

performance of the electrodes prepared at lower and higher temperatures could be attributed to

334

incomplete nitridation or larger crystallite sizes of the electrochemically active components. The

335

electrode prepared by nitridation for 2 h possessed the optimized composition and crystallite

336

sizes.

337

To elucidate the extraordinary HER and OER stabilities of [email protected], the

338

electrodes after the electrochemical tests were characterized by SEM, XRPD, XPS and infrared

339

spectroscopy studies. As depicted in Figure S16, SEM images showed that the morphology of

340

the [email protected] electrodes kept intact after both the HER and OER long-term

341

stability tests. XRPD analyses revealed that the two electrodes retained their crystallinity (Figure

342

S17). The Ni 2p XPS spectra of the [email protected] nanorods showed that surface Ni(0)

343

species turned into Ni(II) due to the oxidation by O2 or water during the HER and OER tests

344

(Figure S18). Detailed analysis of Ni 2p XPS spectra of the [email protected] electrode


19


Page 20 of 28

345

after the OER tests revealed that there existed NiIII species (Figure S19b), which likely served as

346

catalytic sites as shown in many Ni-based OER catalysts.49 The N 1s XPS spectra of Nifoam@Ni-

347

Ni0.2Mo0.8N displayed little change before and after long-term stability tests for HER and OER,

348

i.e. the Mo−N bond kept intact. While the Mo 3d XPS spectra of [email protected] after

349

the HER tests showed little changes compared to the pristine electrode, the intensities of the

350

MoVI peaks in [email protected] after the OER tests increased significantly, indicating the

351

oxidation of surface Mo under applied high oxidation potentials. Consistently, Fourier-

352

transformed infrared spectroscopy studies showed the appearance of two new peaks at ca. 1400

353

cm-1 and 600 cm-1 in [email protected] after the OER tests (Figure S20). These two peaks

354

could be attributed to MoOx,46 which were likely generated by surface oxidation of Ni0.2Mo0.8N

355

nanoparticles at applied high voltages. In the meanwhile, the broaden peaks at ca. 3400 cm-1

356

could also consist of the vibrations of the –OH groups in NiOOH or Ni(OH)2, which came from

357

the surface oxidation of Ni0.2Mo0.8N or Ni(0).49 The above results suggested that both the used

358

HER and OER electrodes of [email protected] were basically the same as the pristine ones,

359

although the surface species were partly oxidized into MoOx and NiOOH in strong alkali

360

environment. In contrast, Nifoam@Ni4Mo/MoO2 could not withstand the OER tests, despite the

361

fact that it exhibited excellent, stable HER activities in 1.0 M KOH. XRPD and EDS analyses

362

showed that MoO2 was dissolved in the alkali solution during the OER tests (Figure S1), likely

363

because of oxidation under the applied high OER potentials.31 Similarly, the precursor

364

Nifoam@NiMoO4 was also unstable for OER in alkali environment, confirmed by the change of

365

color and XRPD patterns before and after immersion in 1.0 M KOH for several hours (Figure

366

S10). The above results showed that the extraordinary HER and OER stabilities of Nifoam@Ni-


20

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367

Ni0.2Mo0.8N could be attributed to the formation of highly corrosion- and oxidation-resistant

368

metal nitrides.

369

The above results also shed light on the active sites in [email protected]. The nanorods

370

in [email protected] consist of interconnecting, evenly distributed Ni0.2Mo0.8N and Ni or

371

NiO nanoparticles. The surface Ni and Mo were oxidized upon exposure to air and/or in alkaline

372

media, which acted as highly-active HER and OER sites similar to many previous studies.6, 35-37,

373

50, 51

374

distributed active sites: Mo atoms acted as adsorbent toward H2O molecules and lowered the

375

energy for Volmer step, while Ni atoms served as efficient catalytic centers and lowered the

376

energy for Tafel step.5 Synergy between Mo and Ni in [email protected] resulted in great

377

HER and OER performance. Consistently, [email protected] showed better performance

378

than Nifoam@Ni3N.

Due to the N-doping, the oxidation process only occurred on the surface, producing evenly-

379

To further elucidate the high activities of [email protected], we investigated the

380

relationship between electrochemical activities and electrochemically active surface areas

381

(ECSAs) of the materials described above. The ECSA of a material is proportional to its

382

electrochemical double-layer capacitance (Cdl) in a non-Faradaic region, where the charge

383

transfer owing to the electrode reactions can be ignored and the current (j) originates only from

384

the electrical double layer charging and discharging.52 The Cdl can thus be obtained from the

385

equation |j| = v × Cdl, where v is the scan rate in CV measurements.44 As shown in Figure S21,

386

the Cdl values of the materials described above were obtained in the order of Nifoam@Ni-

387

Ni0.2Mo0.8N > Nifoam@Ni4Mo/MoO2 > Nifoam@Ni3N > Nifoam > Nifoam@NiMoO4. Nifoam@Ni-

388

Ni0.2Mo0.8N possesses the largest Cdl, i.e. the largest ECSAs amongst the tested samples,

389

consistent with the HER and OER experimental results. By comparison of the Cdl values between


21


Page 22 of 28

390

the test materials and correlation with their electrocatalytic activities, the following speculations

391

can be drawn: First, the higher Cdl values and higher electrochemical activities of the nitride

392

electrodes compared to corresponding non-nitride electrodes (i.e. [email protected] vs.

393

Nifoam@Ni4Mo/MoO2; Nifoam@Ni3N vs. Nifoam) show that the formation of metal nitrides results

394

in increase in ECSAs (i.e. Cdl), and thus improved electrocatalytic activities. The increase in

395

ECSAs could be attributed to the modification of the metals’ d bands by N in metal nitrides,

396

which rendered them more favorable for water splitting.7,8 Second, the better electrochemical

397

performance of [email protected] vs. Nifoam@Ni3N show that there is a synergetic effect

398

between Mo and Ni, as observed in many other electrocatalysts,15,16 resulting in more ECSAs

399

and thus higher activities in bimetallic catalysts. The intimate interconnections between the

400

electrochemically active Ni0.2Mo0.8N and Ni or NiO nanoparticles in the [email protected]

401

nanorods allow fast electron and mass transportation and thus facilitate the bond breakage and

402

formation during water splitting, further improving the electrocatalytic activities. Lastly, it is

403

pointed out that Nifoam@NiMoO4 possessed the lowest Cdl values, which could be attributed the

404

dissolution/oxidation of the catalyst under applied electrode potentials (1.025 to 1.125 V vs.

405

RHE) during the measurements.

406 407

3. CONCLUSIONS

408

In summary, we fabricated a bifunctional nanostructured nickel molybdenum nitride

409

electrode by topotactic transformation of NiMoO4 nanorods that were in situ grown vertically on

410

the surface of Ni foam. The nitride nanorods in the electrode (denoted as Nifoam@Ni-

411

Ni0.2Mo0.8N) consisted of interconnecting Ni0.2Mo0.8N nano crystallites decorated by partially

412

oxidized Ni nanoparticles. The electrode exhibited exceptionally low overpotentials and Tafel


22

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413

slopes for both HER and OER in 1.0 M KOH, surpassing the commercial benchmark materials

414

Pt/C and RuO2 under the same condition, respectively. A simple water electrolyzer using

415

[email protected] as both the anode and cathode required a very low cell potential of 1.49

416

V to reach a current density of 10 mA cm−2 and maintained stability for 110 h without any

417

degradation. The excellent bifunctional electrocatalytic properties of [email protected]

418

could be ascribed to the formation of highly conductive, corrosion- and oxidation-resistant metal

419

nitrides, the synergetic effect of multiple active components, and the integration of 1-D

420

nanostructures on the electrode. Because of its excellent HER and OER activities, exceptional

421

stability and simple preparation, [email protected] can be exploited as promising

422

electrodes for practical water electrolyzers. The preparative strategy developed in this work

423

could be applied to devise new highly efficient electrocatalysts.

424 425

Supporting Information.

426 427

Electronic Supplementary Information (ESI) available: Additional XRPD, EDS, SEM, XPS, STEM, XPS spectra, tables and video clips.

428

Corresponding Author

429

Hongbin Du*. Email: [email protected]

430

JunJie Zhu*. Email: [email protected]

431

Notes

432

The authors declare no competing financial interest.

433

ACKNOWLEDGEMENTS

434 435

We are grateful for financial support from the National Natural Science Foundation of China (21471075 and 21673115).

436

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437 438

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Nickel Molybdenum Nitride Nanorods Grown on Ni Foam as Efficient

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