Efficient Hydrogen Evolution Activity and Overall Water Splitting of

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Efficient Hydrogen Evolution Activity and Overall Water Splitting of Metallic CoN Nanowires through Tunable d-orbitals with Ultrafast Incorporation of FeOOH 4

Yuwen Hu, Hao Yang, Junjie Chen, Tuzhi Xiong, M.-Sadeeq (Jie Tang) Balogun, and Yexiang Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20717 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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ACS Applied Materials & Interfaces

Efficient Hydrogen Evolution Activity and Overall Water Splitting of Metallic Co4N Nanowires through Tunable d-orbitals with Ultrafast Incorporation of FeOOH Yuwen Hu,† Hao Yang,† Junjie Chen,† Tuzhi Xiong,# M.-Sadeeq (Jie Tang) Balogun,*,# and Yexiang Tong*,†

†MOE

of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-Carbon

Chemistry & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University Guangzhou 510275, People’s Republic of China.

#College

of Materials Science and Engineering, Hunan University, Changsha 410082, Hunan, People’s

Republic of China.

ACS Paragon Plus Environment

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ABSTRACT: Cobalt nitride electrocatalysts have been investigated and proven to show excellent oxygen evolution reaction (OER) activity owing to their excellent metallic properties but their hydrogen evolution reaction (HER) properties are rarely reported due to their unsatisfactory molecular energy level especially the d-orbital. Herein, taking Co4N as a case study, we tune the d-orbital of metallic Co4N nanowires via rapid formation of iron oxyhydroxide (FeOOH). Experimental analyses show that FeOOH@Co4N/SSM exhibits excellent HER catalytic activity with considerable low onset overpotential (22 mV), small Tafel slope (34 mV dec-1) and excellent stability at current densities ranging from 20-100 mA cm-2. Additionally, theoretical assessments display that the hybridization of Co4N with FeOOH is beneficiary for optimizing and promoting the free energy of H adsorption due to the tuning of d-orbital. Overall water splitting device assembled based on bifunctional FeOOH@Co4N/SSM delivers an onset potential of 1.48 V with excellent stability up to 4 days. This shows a new strategy for designing high-performance water-splitting device based on cobalt-based electrocatalysts.

KEYWORDS: Co4N, FeOOH, hybridization, d-orbital, hydrogen evolution reaction


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INTRODUCTION One of the most promising methods of producing hydrogen is by water electrolysis.1-8 As for Pt and Pt-based electrocatalysts, the major barrier in the hydrogen production is the high-cost of Pt and Pt-based electrocatalysts. Thus, developing low-cost HER electrocatalysts such as the transition metal compounds (TMCs) attracts urgent attention. TMCs such as transition metal oxides, phosphides, sulphides, and nitrides have recently gained great attention as potential catalysts to replace the noble metals

9-12

due to their excellent earth abundant nature,

environmental benignity and low-cost compared to Pt-based catalysts. Among the commonly reported TMCs, transition metal nitrides (TMNs) have also attracted worthy awareness as matured electrocatalysts due to their unique 3d electron numbers.6, 13-17 With significant achievements in utilizing TMNs such as Ni3N, 16 Ni3FeN,

18

WN,

19

binary nitrides as

HER electrocatalysts, cobalt nitrides seem to be very inferior during hydrogen evolution. Cobalt nitrides (a 3d metals binary nitride systems) show excellent electrical conductivity, attractive resistance to corrosion and can also be termed as metallic interstitial compounds.20,

21

They

consist of cobalt–cobalt interactions with the nitrogen atoms integrating into cobalt-based framework, which gives a characteristic metallic behaviour.20, 22 Different cobalt nitrides in their binary states such as CoN, Co2N, Co3N and Co4N have theoretically and experimentally shown outstanding and impressive OER activities based on their degree of metallicity and intrinsic conductivity,20, 21 whereas their utilization as catalysts for HER is rarely reported. Recently, Wang et al. impressively boosted the HER performance of Co4N nanosheets by tailoring the d-band centers via doping of V, W and Mo doping.

23

However, few literatures have been found on

improving the HER properties of Co4N as electrocatalyst for HER. Meanwhile, cost-effective and



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efficient OER electrocatalysts also demand urgent attention24,

25

because overall water splitting

using bifunctional electrocatalyst to further reduces the cost of using Pt as HER cathode and oxides of Ru/Ir as OER anode remains a promising challenge.

26-28

Moreover, there is less report

on the use of cobalt nitrides as bifunctional catalysts for overall water splitting. Herein, we demonstrate that ultrafast formation of iron oxyhydroxide (FeOOH) on Co4N nanowires supported on three-dimensional (3D) stainless steel mesh (Co4N/SSM) could tune the d-orbitals of Co4N and enhance the hydrogen evolution properties in alkaline medium. The ultrafast formation of FeOOH on Co4N nanowires is known for its high intrinsic catalytic activity, 29

which

created

strong

interfacial

reaction

between

them.

The

optimal

hybrid

(FeOOH@Co4N/SSM) delivers onset overpotential of 22 mV and 10 mA cm-2 overpotential of 92 mV, which is better than that of Co4N/SSM. Theoretical analyses show that decoration of FeOOH on Co4N nanowires generated electronic interaction between the Co4N and FeOOH which is responsible for improving the hydrogen adsorption ability and promotes water absorption for efficient hydrogen evolution. With the excellent OER performance of FeOOH@Co4N/SSM, an alkaline electrolyzer assembled based on bifunctional FeOOH@Co4N/SSM cathode and anode displayed an onset potential of 1.48 V with excellent stability.

RESULTS Morphological Characterization We synthesized porous Co4N nanowires on 3D SSM and used it as high performance electrocatalyst for HER. Compared to other 3D current collectors such as nickel foam and carbon cloth, the SSM is cheaper and exhibits worthy catalytic performance.27 FeOOH@Co4N/SSM is obtained by preparing Co4N on SSM through hydrothermal followed by annealing and 5 s rapid


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growth of FeOOH via simple chemical bath deposition (details in experimental section and Supporting Information, Figure S1). Co4N/SSM is primarily achieved by growing Co2(OH)2CO3 nanowire precursors (Figure S2) on SSM (Figure S3) followed by annealing in ammonia (NH3) at 450 oC for 3 h. After annealing, the smooth surface of the nanowires became rough as confirmed by scanning electron microscopy (SEM) image in Figure 1a.

Figure 1. SEM images of (a) Co4N/SSM and (b) FeOOH@Co4N/SSM. TEM images of (c) Co4N and (d) FeOOH@Co4N. Inset in 2d is the HRTEM image showing the lattice fringes of the Co4N/SSM. (e) HRTEM image showing core-shell layer structure of FeOOH@Co4N/SSM from the red circle in 2d. (f-j) Elemental mapping showing the distribution of the elements present in FeOOH@Co4N/SSM sample

Transmission emission microscopy (TEM) affirmed the roughness, porosity and formation of the Co4N/SSM (Figure S4). Motivated by the electronic interaction concept that occurs when



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transition metals hybridized with another transition metal compounds, 30, 31 FeOOH was grown on Co4N/SSM in 5 s to create an electronic interaction between Co4N and FeOOH (Figure S1). Nanoflakes of FeOOH (Figure 1b) were successfully decorated on Co4N/SSM nanowires. The presence of Co4N phase in pristine Co4N/SSM and FeOOH@Co4N/SSM was confirmed by X-ray diffraction (XRD) analyses (Figure S5) with the (111), (200) and (220) phases of Co4N corresponding to PDF#41-0943. However, the peaks of FeOOH cannot be identified. Compared with the smooth nanowire surface of Co4N (Figure 1c), the TEM analysis in Figure 1d shows that the nanowires of FeOOH@Co4N hybrid are characterized with nanoflakes. The high-resolution TEM (HRTEM) image in Figure 1e (enlarge image in Figure S6) reveals that the lattice fringes obtained from nanoflakes at the edge of the nanowire in Figure 1d correspond to (100) and (101) phases of hexagonal FeOOH (PDF#76-0123), which confirmed the formation of FeOOH nanoflakes on Co4N/SSM nanowires. Additionally, lattice fringes collected from the HRTEM image in Figure 1d can also be identified with (111) phase of Co4N (Figure 1d inset). Energy dispersive X-ray spectroscopy (EDS) data in Figure 1f-j confirmed the presence and distribution of the individual elements in the nanowire, with higher concentration of Fe and O at the nanowire surface, further indicating that the coating of FeOOH on Co4N and successful hybridization of FeOOH@Co4N.

Composition Characterization Insight information on the relationship between Co4N and FeOOH was experimentally observed by X-ray photoelectron spectroscopy (XPS) in Figure 2. Figure 2a shows the high-resolution XPS spectra of Fe 2p of both Co4N and FeOOH@Co4N. According to Fe 2p high-resolution XPS spectra of the samples, Co4N sample is Fe-free, while that of FeOOH@Co4N hybrid is characterized with Fe peak as shown in Figure


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2a. Fe 2p XPS spectrum of FeOOH@Co4N is characterized with the three peaks located at 711.8, 719.6 and 725. 4 eV correspond to the Fe 2p3/2, satellite and Fe 2p1/2 of FeOOH.

32, 33

Furthermore, the O 1s

XPS spectra of the two samples are shown in Figure 2b. O 1s XPS spectra of Co4N is acknowledged with main peak of nitrides at 532.2 eV,

34-36

while that of FeOOH@Co4N consists of two distinct peaks

acknowledged with Fe-O-Fe bond (530.1 eV) and Fe-O-H bond (531.7 eV).

32, 33

Both Fe 2p and O 1s

XPS spectra of FeOOH@Co4N hybrid further affirmed the formation of FeOOH on Co4N nanowires.

Figure 2. High-resolution (a) Fe 2p, (b) O 1s, (c) N 1s and (d) Co 2p XPS spectra of Co4N/SSM and FeOOH@Co4N/SSM.



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The N peak of the N 1s XPS spectra can be identified for all the samples confirming the formation of the nitrides (Figure 2c). Elemental analyses on both Co4N and the hybrid samples displayed N content of approximately 1.54 and 1.94% (

Efficient Hydrogen Evolution Activity and Overall Water Splitting of

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