In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on

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In-situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co3N for Hydrogen Production in Alkaline Solution Zhe Xu, Wenchao Li, Yadong Yan, Hongxu Wang, Heng Zhu, Meiming Zhao, Shicheng Yan, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04596 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Materials & Interfaces

InIn-situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co3N for Hydrogen Production in Alkaline Solution Zhe Xu, †

† §

Wenchao Li,

† §

†

†

†

†

†,*

†,‡

Yadong Yan, HongXu Wang, Heng Zhu, Meiming Zhao, Shicheng Yan, and Zhigang Zou

National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Eco-Materials and

Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu 210093, P. R. China ‡

Jiangsu Province Key Laboratory for Nanotechnology, School of Physics, Nanjing University, Nanjing, Jiangsu 210093, P. R. China

KEYWORDS: electrocatalysis, water splitting, hydrogen evolution reaction, transition metal nitrides, interface kinetics ABSTRACT: Sluggish water dissociation kinetics on non-precious metal electrocatalysts limit the development of economical hydrogen production from water-alkali electrolysers. Here, using Co3N electrocatalyst as a prototype, we find that during water splitting in alkaline electrolyte a cobalt-containing hydroxide formed on the surface of Co3N, which greatly decrease the activation energy of water dissociation (Volmer step, a main rate-determining step for water splitting in alkaline electrolyte). Combining the cobalt ion poisoning test and theoretical calculations, the efficient hydrogen production on Co3N electrocatalyst would benefit from the favorable water dissociation on in-situ formed cobalt-containing hydroxide and low hydrogen production barrier on the nitrogen sites of Co3N. As a result, the Co3N catalyst exhibits a low water-splitting activation energy (26.57 kJ mol-1) that approaches to the value of platinum electrodes (11.69 kJ mol-1). Our findings offer a new insight to understanding the catalytic mechanism of nitride electrocatalysts, thus contributing to the development of economical hydrogen production in alkaline electrolyte.

■ INTRODUCTION Water splitting electrocatalysis is a cost-effective 1 and convenient route to producing hydrogen fuels. The efficient hydrogen evolution reaction (HER) was achieved by using noble metals such as Pt, Ir, and Rh, which generate high cathodic current density at very low 2 overpotential either in acid solution or alkaline solution. However, the widespread application of noble metals is limited by their high cost and low earth-abundance. Inexpensive electrocatalysts such as transition metal 3-4 5-7 8 9-11 oxides, nitrides, sulfides, and phosphides, have been explored as potential alternatives to noble metals. In 5, 12-13 particular, transition metal nitrides, such as the 14-15 cobalt-containing nitrides, arouse great research interest. Firstly, metal characteristics of the nitrides dramatically facilitate the electron transportation and avoid the formation of Schottky barriers at both catalyst-electrolyte and catalyst-support electrode interfaces, thus reducing the additional overpotential 5, 12 requirement for water splitting. Secondly, a good 8 catalyst must be an excellent ad-/desorption material. Non-metal elements (such as N, S, P, and Se) in compounds can be used to adjust the energy of adsorption and desorption of the reaction intermediates by adjusting the electronic structure of catalysts and the interactions 7-8, 11, 16-17 between metal atoms. Therefore, non-metal

elements in compounds play a very important role in water splitting reaction and give those materials excellent catalytic performance. When the HER takes place in alkaline solution, the first reaction step of water splitting is water dissociation, named as the Volmer step: H2O + e + * → H* + OH ( * 18 denotes a site on the electrode surface), meaning that water molecule will adsorb on or bond with catalysts. Due to the low thermodynamic stability of nitrides compared to oxides, the OH from water dissociation or dissolved oxygen in electrolyte may oxidize the surface 19 of nitride electrocatalysts to form oxides or hydroxides, resulting from the strong interactions between those oxygen-containing species and metal sites of nitrides. Indeed, the replacement of nitrogen by oxygen was observed on the surface of Ta3N5 via a self-limiting 20 surface oxidation process. In addition, the oxygen in nitrides probably originates from two routes: the residual oxygen from oxide precursors due to incompletely nitriding reaction or oxidation of nitrides during the 21 cooling process. These results confirmed that oxygen impurities are common and thermodynamically stable on surface of nitrides. However, the effect of oxides or hydroxides on the catalytic reaction occurred on the nitride electrolysts is not fully understood. Here, we use the Co3N as a prototypical catalyst to explore the catalytic mechanism for hydrogen production 1 / 12

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in alkaline electrolyte. We found that surface of Co3N will undergo a partial self-oxidation reaction to form 2+ Co-containing hydroxides. A poisoning test of Co ions by KSCN is performed to confirm the catalytic effect of the in-situ formed Co-containing hydroxides on water dissociation. Theoretical calculations indicated that nitrogen atoms in Co3N play an important role in optimally adsorbing H*. Therefore, we concluded that the efficient hydrogen production on Co3N electrocatalyst will benefit from the favorable water dissociation on the in-situ formed cobalt-containing hydroxides and low hydrogen production barrier on the nitrogen sites of Co3N. Our results can provide valuable insights to developing efficient nitride electrocatalysts for economical hydrogen production in alkaline electrolysers.

EXPERIMENTAL SECTION Electrocatalyst preparation. Cobalt plate with a purity of 99.95% was washed with HCl solution (pH = 1) under sonication for 15 min, and then rinsed with deionized water and ethanol repeatedly for thoroughly removing oxide film, impurities and residual acid on the surface. To prepare a cobalt oxide electrode, a piece of cleaned cobalt sheet was placed in the middle location of o a Muffle furnace, and heated to 600 C with a rate of 10 o -1 C min in air and maintained for 3 h with a naturally cooling to room temperature (denoted as CoOx plate electrode). To obtain a cobalt nitride electrode, the CoOx plate electrode was placed in the tube furnace (inner o diameter, 50 mm) and heated to 500 C with a rate of 10 o -1 C min under a flowing NH3 atmosphere (1 bar, 500 mL -1 min ) and maintained for 3 h, then the system was naturally cooled down to room temperature under a flowing NH3 atmosphere. Materials Characterization. The crystal structures of these as-prepared products were determined by powder X-ray diffraction (XRD, Rigaku Ultima III, Cu Kα radiation). X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 Versa Probe (ULVAC-PHI, Japan) with monochromatized Al Kα X-ray radiation (1486.6 eV). The energy resolution of the electrons analyzed by the hemispherical mirror analyzer was about 0.2 eV. The binding energy was determined in reference to the C 1s line at 284.8 eV. The morphology for the samples was observed with a transmission electron microscope (TEM, FEI Tecnai G2 F30 S-Twin, Hillsboro, OR). Scanning electron microscopy (SEM) images were collected with a field-emission scanning electron microscope (FE-SEM, Nova NanoSEM 230, FEI, USA) operating at 15 kV. Transmission electron microscopy (TEM) was performed using a JEOL JEM 2100F at an acceleration voltage of 300 kV.

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Electrochemical characterization. To evaluate the electrocatalytic HER activity of catalysts, the electrochemical measurements were carried out in a standard three-electrode system connected to a CHI660E electrochemical workstation, a saturated Ag/AgCl electrode and a graphite rod were used as the reference and counter electrodes, respectively. HER polarization curves were measured in 1M KOH solution with a scan -1 rate of 5 mV s . The electrolyte was purged with high purity N2 gas before carried out the electrochemical test. The iR compensation was carried out by iR compensation procedure on the CHI660E electrochemical workstation. All chronoamperometric and chonopotentiometric curves were shown without iR compensation. Unless otherwise stated, all experiments were performed at ambient temperature and electrode potentials were converted to the reversible hydrogen electrode (RHE) according to the Nernst equation of ERHE = EAg/AgCl + 0.197 V + 0.059pH, where EAg/AgCl is the applied potential and 0.197 V is the reference potential of Ag/AgCl with respect to the RHE scale. Theoretical calculations. The (002) surfaces of both Co3N and pure Co were used in our study. A six-layer slab and a (2×2) supercell were constructed. During the structural optimization calculations, the atoms in the two bottom layers were fixed in their bulk positions, and those in the other four layers were allowed to relax. A vacuum layer as large as 15 Å was used along the c-axial direction normal to the surface to avoid periodic interactions. The chemisorption energies of H on surfaces were defined as ∆Eads = EH/slab - (Eslab + 1/2EH ), where the EH/slab was the total energy of H atom on the surface of Co3N or Co. Eslab was the total energy of the surface of Co3N or Co, and EH was the energy of hydrogen molecule in gas phase. The first two terms were calculated with the same parameters. The third term was calculated by setting the isolated H2 in a box of 12 Å × 12 Å × 12 Å. The Gibbs free-energy change (∆Gads) of H on these surfaces are defined as ∆ Gads = ∆ Eads+ ∆ EZPE - T ∆ S, where ∆Eads is the adsorption energy of the atomic H on the Co3N or Co surfaces, ∆ EZPE is the difference in zero-point energy between the adsorbed hydrogen and hydrogen in the gas phase. ∆S is the entropy change of one H atom from the absorbed state to the gas phase. Since the H atom is binding on the surface, the entropy of the adsorbed hydrogen can be negligible. Therefore, the ∆ S can be estimated by -1/2×S0, in which S0 is the standard entropy of H2 with gas phase at pressure of 1 bar and pH = 0 at 300 K. In summary, the Gibbs free-energy change (∆Gads) of H can be described as ∆Gads = ∆Eads+ ∆ EZPE - 0.0615 eV. The calculations were carried out using density

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2

2

high-resolution transmission electron microscopy (TEM) image (Figure Figure 1b) 1b exhibited clear lattice fringers of 0.231 nm and 0.214 nm, corresponding to (100) and (002) plane of Co3N, respectively. These evidences demonstrated that the Co3N plate electrode is successfully prepared. In addition, we used X-ray photoelectron spectroscopy (XPS) to explore the surface oxidation states and binding energy information. The XPS survey spectrum of Co3N plate electrode (Figure Figure S3 S3) indicated that only O, N and Co elements are observed. Cobalt nitrides such as Co2N, Co3N, and Co4N, are regarded as a metallic interstitial compound. The small nitrogen atoms incorporating into the interstices of the cobalt-based framework makes the nitride exhibit metal-like properties. The formation of nitrides narrows the filled states of d-band of parent metals, resulting in that the similar 28 electronic structure of nitrides to noble metals. In the Co 2p XPS spectrum (Figure Figure 1c), 1c the two peaks at 777.9 eV 0 (2p3/2) and 793.3 eV (2p1/2) can be assigned to the Co in 5, 29, 30 the Co3N. In addition, another Co 2p3/2 binding energy, Co 2p1/2-Co 2p3/2 splitting, and energy separation between the main peak (2p3/2) and its satellite peak (2p3/2 sat.) are 780.9 eV, 15.7 eV, and 5.7 eV, respectively, which 2+ 31, are well ascribed to the binding energy of Co in CoO. 32 O 1s core-level XPS spectrum (Figure Figure 1d) 1d can be deconvoluted into three peaks: 529 eV for lattice oxygen, 531.1 eV for chemically adsorbed hydroxyl on Co sites, and 532.5 eV for physically adsorbed water molecule. These facts mean that the partial oxidation occurred on the surface of Co3N, probably resulting from the oxygen incorporating into the surface lattice.

functional theory method with the Perdew-Burke-Ernzerbof (PBE) form of generalized 22 gradient approximation functional (GGA). The Vienna 23-26 ab-initio simulation package (VASP) was employed. The plane wave energy cutoff was set as 400 eV. The Fermi scheme was employed for electron occupancy with an energy smearing of 0.1 eV. The first Brillouin zone 27 was sampled in the Monkhorst-Pack grid. The 2×2×1 k-point mesh for the surface calculation. The energy -6 -1 (converged to 1.0 × 10 eV atom ) and force (converged -1 to 0.01eV Å ) were set as the convergence criterion for geometry optimization. The spin polarization was considered in all calculation.

RESULTS AND DISCUSSIONS Briefly, to prepare the Co3N film electrode, the nitridation of pre-oxidized Co plate was performed at 500 o -1 C for 3 h under a NH3 flow of 500 mL min . As shown in Figure S1, S1 scanning electron microscopy (SEM) observations present that the thickness of nitride overlayer on Co substrates is about 6 µm. The particle size of Co3N (about 100 nm) on the surface layer of film is obviously larger than that of raw Co particles (about 20 nm), indicating that the particle aggregation and growth occurred during the phase transformation into Co3N. And this nitride overlayer exhibits a close contact with the Co substrate, indicating the good charge transfer between the overlayer and substrate. The X-ray diffraction (XRD) pattern of the cobalt nitride plate is shown in Figure 1a. 1a All the XRD peaks are well indexed to hexagonal Co3N (JCPDS, No. 06-0691) with no substrate or impurity phases signal (Figure Figure S2 S2a). XRD analysis confirmed that o the pre-oxidation at 600 C for 3 h in air induced the formation of CoOx (a mixture of Co3O4 and CoO) layer on the surface of Co plate (Figure Figures Figures S2b). Meanwhile, the

0

(b)

Current density / mAcm

-20 -40

Co

-60

CoOx

-80

Co3N Pt -0.4

No. 06-0691

20

30

40

50

60

70

80

2-Theta / Degree

(c)

Co-N

Co 2p

777.9 eV

(d)

780.9 eV Sat.

Co-N

793.3 eV

Intensity / a.u.

796.6 eV

OH

O 1s

Co-O Co-O

Sat.

802.3 eV

786.6 eV

780

790

800

Binding energy / eV

810

525

528

-

300

531

-10

-0.34V

-40

-0.39V -50

-0.44V 5

10

15

Time / h

534

537

Binding energy / eV

Figure 1. Characterization of Co3N plate electrode. (a) XRD pattern, (b) High-resolution TEM lattice image, (c) Co 2p and (d) O 1s XPS spectra.

20

64.3 mV dec-1

101.6 mV dec-1

50.47 mV dec-1

0.4

0.8 1.2 1.6

Log ( j )

-0.29V

-60

Co Co3N Pt

-1.2 -0.8 -0.4 0.0

(d)

-30

58.0 mV dec

100 0

-0.19V -0.24V

-20

-1

CoOx

200

-0.1

0

0

H 2O

2-

O

770

-0.2

Potential / V vs. RHE

Current density / mA cm

-2

(c)

Co3N JCPDS Card

10

-0.3

400


(103)

(110)

(102)

(100) (002)

Intensity / a.u.

-100

Co3N

500

-2

(a)

(101)

(a)

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


Overpotential / mV

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-0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40 -0.45 -0.50

25

0 10 20 30 40 50 60 70 80 90 100

Time / h

Figure 2. (a) HER polarization curves of Co3N, pure cobalt, cobalt oxide (CoOx), and Pt plate electrodes in 1M KOH solution (scan -1 rate = 5 mVs , with iR compensation). (b) Tafel plots obtained from polarization curves shown in Fig. 2a. (c) Multistep chronoamperometric curve of Co3N at different overpotentials, starting at 190 mV and ending at 440 mV with an increment of 50 mV (without iR compensation). (d) Long-term stability -2 chonopotentiometric curve of Co3N at j = 10 mA cm (without iR compensation).

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The HER polarization curves of Co3N plate electrode in 1M KOH solution are shown in Figure 2a. 2a -2 To achieve the 10 mA cm for HER, both the pure Co and CoOx plate electrodes require the overpotential to be larger than 400 mV. In contrast, the corresponding HER overpotential of Co3N plate electrode was only 230 mV, which is close to the performance of Pt plate (144 mV). It indicated that Co3N exhibited a pronounced HER activity comparing with cobalt-based metal or oxides, which shows good performance for water splitting (Table Table S1). S1 To gain further insight into the high activity for HER on Co3N plate electrode, the Tafel slopes were calculated, as shown in Figure 2b. 2b The Co3N electrode exhibited a Tafel -1 slope of 101.6 mV dec , larger than those of the Pt -1 33 polycrystalline plate (50.47 mV dec ) , (Pt/C 39 mV -1 34 -1 35 dec ) , pure cobalt (64.3 mV dec ) and CoOx (58.1 mV -1 -1 7, 36 dec ).. A theoretical Tafel slope of 118 mV dec is predicted when the initial water adsorption and subsequent water dissociation (Volmer step, H O + e + * → H* + OH) are the rate-determining step (RDS) for HER in alkaline solution. Therefore, the Tafel slope of -1 101.6 mV dec of Co3N electrode, significantly close to -1 the theoretical Tafel slope of 118 mV dec , means that the RDS of Co3N catalyst for HER in 1M KOH is Volmer 7 step. Furthermore, the Tafel slopes of the CoOx and pure cobalt plate electrodes mean that their RDS are the Volmer-Heyrovsky (H* + H2O + e → * + OH + H2↑) 36 or the Volmer-Tafel step (2H* →H2↑ + *). The exchange current density (j 0) is an important reaction kinetic descriptor. The j 0 was calculated by formula η = blog(j /j 0), where η and b are overpotential and Tafel slope, respectively). As shown in Table S2 S2, the -2 Co3N plate electrode exhibits a j 0 value of 0.05 mA cm , 3 which is about 6.4 x 10 times higher than the pure cobalt -6 -2 (8 x 10 mA cm ) and greatly outperforms the CoOx (1.33 -7 -2 x 10 mA cm ). As a non-precious metal catalyst, the Co3N plate electrode has exceeded many catalysts in the aspect of exchange current density (Table Table S3 S3), demonstrating the inherently high activity of Co3N catalyst. We used the electrochemical impedance spectroscopy (EIS) technique to further verify the difference in electrocatalytic kinetics between Co and Co3N. The Nyquist plots of Co and Co3N electrodes were shown in (Figure Figure S4 S4) and fitted to a typical Randle’s equivalent circuit. A semicircle in the high-frequency range of the Nyquist plot is assigned to the polarization resistance (Rct) at the interface between catalyst and electrolyte. which is related to the electron transfer 16, 37-38 kinetics of the overall rate of the HER. The value of 2 Rct at the Co3N-electrolyte interface is about 2.6 Ω cm , which is about 98.9% lower than that at Co-electrolyte 2 interface (225.4 Ω cm ). Thus, the lower Rct suggests a faster electron transfer at the catalyst-electrolyte interface. 2

The electronic structure of Co3N at the liquid-solid interface probably contributes to the different charge 28 transfer resistance. Moreover, the non-metal element would promote the adsorption and desorption of 11, 39, 40 intermediates, and the Co3N facilitates the in-situ formation of hydroxide on its surface. As the consequence, the HER kinetics of Co3N is improved, so the intermediates are prone to accept electron, 41 contributing to lower Rct for Co3N electrode. After 20-cycling CV scanning, the HER activity of Co3N reaches to a steady state (Figure Figure S5 S5). To assess the catalytic stability of Co3N catalyst for HER in alkaline solution, a multistep chronoamperometric curve is recorded for the Co3N catalyst in a wide overpotential range from 190 to 440 mV with a potential increment of 50 mV (Figure Figure 2c 2c). The result shows that the current density can remain stable in 1M KOH solution at each overpotential range, indicating the highly electrochemical stability of Co3N catalyst, in good agreement with the -2 stability test for 100 h HER at 10 mA cm (Figure Figure 2d). 2d After a long-term HER for 100 h, O 1s core-level XPS analysis indicates that the lattice oxygen content significantly increase (Figure Figure 3a), 3a meaning that the partial surface oxidation occurred during the cathodic electrochemical process. In addition, compared with fresh-prepared Co3N catalyst, the peak of OH for Co3N shifted to 530.8 eV after 100 h HER, stronger binding 2+ energy between Co and OH , which suggests the phase change from CoO to Co(OH)2 on the catalyst surface. The surface reconstruction of Co3N after long-term HER test also revealed that its surficial composition was changed (Figure Figure S6a S6a). 6a As shown in Figure S6 S6b, the consequent HR-TEM analysis shows that the clear lattice distance of 0.230 nm is assigned to the plane (002) of Co(OH)2 (JCPDS No. 30-0443). This result further confirms the existence of hydroxide. Since the Co-based hydroxide would prevent the excessive permeability of electrolyte, the thickness of our in-situ formed Co-containing hydroxide is about 5 nm. The Co 2p3/2 peak at 780.0 eV and Co 2p1/2-Co 2p3/2 splitting of 16 eV confirm the formation of Co(OH)2 on the surface (Figure Figure 3b), 3b in agreement with the previously observed Co-based species 42-43 in alkaline electrolyte. Apart from those, a small peak 0 at 777.6 eV of Co is observed in Co3N after 100 h HER, indicating the activated state can maintain a long-term stability in HER. The Tafel slope of Co3N electrode after -1 100 h test significantly decreases to 79.5 mV dec (Figure Figure S7 S7a), which indicates the Co-containing hydroxides promote dissociation of water. However, because of the content of Co3N decrease, activity of the Co3N electrode after HER 100 h test slightly decreased (Figure Figure S7 S7b), which confirms that the hydrogen production is catalyzed on Co3N. Therefore, as the HER 4 / 12


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44-45

Intensity / a.u.

OH-

531.1 eV

Co3N Co3N after 100h HER

O2-

525

528

531

534

537

540

Binding energy / eV

(b)

Co 2p

2p3/2 780 eV

2p1/2 796 eV 16 eV

after 100h HER

2p3/2 780.9 eV

2p1/2 796.6 eV

15.7 eV

Co3N

770

780

790

800

810

Binding energy / eV Figure 3. 3 (a) O 1s and (b) Co 2p core-level XPS spectra of Co3N plate electrode before and after 100 h long term HER test in 1M KOH.

proceeds, the hydroxides generated by catalytic reaction promote the dissociation of water, and Co3N still plays an important role for hydrogen generation. To full understand the effect of surface Co-containing hydroxide on hydrogen production over Co3N catalyst, we performed a poisoning test by adding 10 mM potassium thiocyanate (KSCN) into the 1M KOH electrolyte. The thiocyanate ion (SCN ), which is a strong complexant, can coordinate with partial transition metal 6, ion center at the axial position, such as Fe, Ni, and Co. 44-46 As shown in Figure 4a, 4a the existence of thiocyanate ions can obviously suppress the HER activity, and current density of polarization curve decreased significantly from -2 21.5 to 9.5 mA cm at η = 350 mV, with an obvious decrease of the onset potential. It is due to stronger nucleophilic ability of SCN on Co ions than other 6, adsorption groups such as OH and H2O in the solution. 44-45 Therefore, the adsorption of SCN on Co ions competes with the adsorption of H2O, inhibiting the progress of water dissociation step, which indicates a direct contribution of Co ions to the HER in alkaline solution. Moreover, after rinsing the thiocyanate-poisoned electrode with water, the HER activity can almost completely recover in fresh KOH electrolyte without KSCN (Figure Figure 4a). 4a This phenomenon suggests that the SCN ions replacing other adsorption groups coordinate with the Co sites on the Co3N catalyst, thus suppressing

In order to further prove the the HER activity. interactions between Co ions and SCN , cyclic voltammetry of pure cobalt plate is given in Figure 4b, 4b which is tested between 0-0.8V vs.RHE in 1M KOH with and without SCN in alkaline solution. A wide oxidation wave at 0.35-0.7 VRHE resulted from the overlying 0/+1 oxidation currents for Co (oxidation peak ≈ 0.4 VRHE) +1/+2 47 and Co (oxidation peak ≈ 0.5 VRHE) . Adding the SCN ions into the electrolyte, the oxidation wave decreased evidently, suggesting that the SCN in alkaline solution suppressed cobalt oxidation reaction due to their 44, 48 strong interactions. Therefore, due to the additional electrons from SCN ligand leading to strong combination 44, 48 with Co ions, we believe that the SCN ions adsorbed on cobalt ions occupied the catalytic sites and stabilized the oxidation state of cobalt during HER, thus changing redox characteristics of Co ions and significantly inhibiting the water reduction kinetics. Indeed, when decreased the pH value of electrolyte from 14 to 12, the HER performance of Co3N electrode significantly decreased due to the low stability of Co(OH)2 layer in electrolyte with low pH value (Figure Figure S8 S8). This further demonstrated that the Co(OH)2 contributed to the water dissociation kinetics. 0

(a) Current density / mAcm-2

O 1s 530.8 eV

-10 -20

Co3N in KOH -30

Co3N in KOH +KSCN

-40

Poisoned Co3N after rinsing tested in KOH

-50 -0.4

-0.3

-0.2

-0.1

0.0

Potential / V vs. RHE 0.6

(b) Current density / mAcm-2

(a)

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


Co+1/+2

Co

0.4

0/+1

0.2

0.0

Co

-0.2

Co with KSCN 0.0

0.2

0.4

0.6


0.8

Figure 4. 4 HER polarization curves of (a) Co3N catalyst without (orange curve) and with (cyan curve) 10 mM KSCN in 1M KOH, and the poisoned Co3N after rinsing with water (olive curve). Scan rate is 5 mV s-1, without iR compensation. (b) Cyclic voltammetry of pure cobalt plate in 1 M KOH with (red curve) and without (black curve) 10 mM KSCN between 0 ~ 0.8 V. Scan rate is 100 -1 mV s .

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ACS Applied Materials & Interfaces To further assess the alkaline electrolyte, the activation energy (Ea) for electrodes is determined relationship,

kinetic barriers of HER in apparent electrochemical Co plate and Co3N plate by using the Arrhenius

. , where j0 is the

exchange current density, T is the temperature, and R is 38, 49-50 the gas constant. The plots of current density versus applied potential as a function of temperature for both Co and Co3N plate electrode are shown in Figure 5. 5 As the o temperature is increased from 30 to 60 C in 1M KOH, there are different degrees of increase in the current density at lower applied potentials. An Arrhenius plot, logarithm of the exchange current density versus reciprocal of temperature, revealed a good linear function. The apparent Ea obtained from the slope of Arrhenius plot -1 -1 is 64.3 kJ mol for Co and 26.57 kJ mol for Co3N, respectively. The activation energy of Co plate was in 49 good agreement with the previously reported values. The activation energy of the Co3N plate electrode is significantly close to that of platinum electrode (11.69 kJ -1 2, 51 mol ), which would originate from the inherently catalytic activity of Co3N and the great reduction of water dissociation barrier by Co-containing oxides or hydroxides.

(c)

0.1

e- + H +

30 oC

60 oC

∆GH* / eV

Co3N-HN1

-0.2

-0.4

H*

Co3N-HN2 Co3N-HCo1 Co3N-HCo2 Co3N-HCo3 Co-H

-10

-0.5 -1.6

Ea = 64.3 kJ mol-1

-1.8

Log (j0)

Current density / mA cm

-2

Co

Co3N-TN

-0.1

0

(a)

1/2 H2

0.0

-0.3

-20

-2.0 -2.2 -2.4

-30

-2.6 0.0030

0.0034

0.0032

1/T / K-1

-40 -0.4

-0.3

-0.2

-0.1

0.0


(b)

0

Co3N -20

30 oC

-40

-60

-1

Ea = 26.6 kJ mol

-0.6 -0.7 -0.8

-80

-100 -0.4

-0.9 0.0030

0.0032

0.0034

1/T / K-1

-0.3

Reaction coordinate Figure 6. 6 Structures of H* adsorption on (a) (002) Co3N-N terminated and (b) (002) Co3N-Co terminated surface, including the top site (T) over an atom, the bridge site (B) over a bond, and the hollow site (H) over a ring on these surfaces. Note: adsorptions on the top and bridge sites are unstable and the H atom moves to hollow sites. Royal blue sphere: Co; Yellow orange sphere: N. (c) HER free energy diagram calculated at the equilibrium potential for various sites. TN and HN represent the top site and hollow site with N acting as the terminal, HCo represents the hollow site with Co acting as the terminal.

60 oC -0.5

Log (j0)

Current density / mA cm-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

Page 6 of 12

-0.2

-0.1

0.0


Figure 5. 5 The HER linear sweep voltammetry of (a) Co plate electrode and (b) Co3N plate electrode as a function of temperature o -1 from 30-60 C in 1M KOH with 5mV s scan rate. Inset: Arrhenius plot of the exchange current density.

In order to explore the role of Co3N catalyst in HER, the catalytic active sites for hydrogen production on Co3N can be elucidated by using density functional theory (DFT) calculations. The adsorbed free energy (∆GH*) of hydrogen free radical (H*), an important intermediate for hydrogen generation, is considered as a direct thermodynamic descriptor to identify the catalytic active 8 sites. The (002) surface of Co3N with some possible adsorption sites of H*, including the top site (T), the bridge site (B) and hollow site (H), was constructed on Co terminated and N terminated surface (Figure Figure 6a, 6a, 6b, 6b, S99), respectively. For comparison, the H* Figure S adsorption is also carried out on the (002) Co hollow site of Co electrode (the top and bridge sites are unstable), as 6 / 12


Page 7 of 12

shown in Figure S1 S100. We find that the calculated∣∆GH*∣ for most of the adsorption sites on (002) surface of Co3N are smaller than those for H* adsorbed on (002) surface of Co electrode (Figure Figure 6c and Table S4), S4 proving the intrinsic activity of Co3N catalyst. The Co3N-HCo1 site, where the H atom adsorbs over the hollow site of three Co atoms with one N atom at the bottom, is the location of the optimized adsorbed H atom, which proved the important role of N atom in HER. Indeed, it was well demonstrated that non-metal elements, such as N, P, and S, have strong electronegativity to adsorb positively 52 charged protons in HER. The nitriding treatment has also been proved to promote the oxygen evolution 12, 29, 53 reaction (OER) and CO2 reduction. Introducing nitrogen atoms into the lattice can adjust the interaction force between metal atoms and increase the conductivity 12, 17 of the material, which increases the electron transfer rate and reduces the additional energy requirement. In addition, the stronger electronegativity of nitrogen element in Co3N catalyst can optimize adsorption of the 5-6, 8, 11, 39, 54-55 intermediates to improve the activity of HER. In the case of Co3N, the in-situ formed Co-containing hydroxide would promote the H* near the N sites of Co3N to form H2. According to the theoretical results, we believe that the high activity of Co3N in hydrogen generation from the alkaline electrolyte would originate from the low water dissociation barrier on Co-containing hydroxide and low hydrogen generation barrier on near N sites of Co3N. Co3N 40

Co 450

Overpotential / mV

Current density / mA cm-2

(a)50

30

20

10

400

60 mV/dec

350 300

70 mV/dec

250 200 -0.5

0.0

0.5

1.0

1.5

2.0

Log (j)

0 1.30 1.35 1.40 1.45 1.50 1.55 1.60 1.65 1.70

Potential / V vs. RHE 0.6

16

Co3N

2

18

Co

-Z`` / ohm cm

2

(b) 20

-Z`` / ohm 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


14

It is well demonstrated that oxygen evolution reaction (OER) occurred on Co-containing nitride catalysts follows a similar catalytic process with the Co-containing oxide or hydroxide, that is, the OER occurs on the in-situ formed Co-containing oxide or hydroxide layer and the Co-containing nitride under the oxide layer seems to be a highly efficient conductor for 12 electron transfer. To verify whether the Co3N catalyst could perform a highly active OER, we examined OER polarization curve in 1 M KOH (Figure Figure 7a). 7a Co3N catalyst exhibits a more negative onset potential and -2 lower overpotential (330 mV) at 10 mAcm than pure Co -2 plate (overpotential 426 mV at 10 mAcm ). Here, application of high positive voltage (>1.5 V vs.RHE) in strong alkaline solution can induce the formation of Co-containing hydroxides on the surface of Co3N, which contribute to the efficient OER. The Tafel slope of Co3N -1 for OER is about 70 mV dec , significantly close to that -1 of pure cobalt electrode (60 mV dec ), indicating the nearly same OER rate-determining step for Co3N catalyst and Co. This result further suggests that same Co-containing active species acting as catalytic sites form on the surface of both Co3N and Co electrodes and are 29 mainly responsible for efficient OER . EIS measurements could reveal the charge transfer properties between the electrode system and electrolyte. According to the Nyquist plots shown in Figure 7b, 7b the high-frequency semicircle appears for Co3N plate electrode, which is attributed to the interface charge transfer between Co3N and Co substrate. The small equivalent radius of this high-frequency semicircle suggests the low charge transfer resistance between Co3N and the Co substrate. The low-frequency semicircle of Co3N plate electrode is remarkably smaller than that of Co plate electrode at the same bias (1.6 VRHE), which reflects the charge transfer at the interface between catalytic layer and electrolyte. This low electrode-electrolyte resistance means that in situ formed Co-containing species on Co3N are beneficial to OER kinetics.

0.4

CONCLUSIONS

0.2

12 0.0

10

1.4

1.5

8

1.6

1.7

1.8

Z` / ohm cm2

1.9

2.0

6 4 2 0 0

2

4

6

8

10

12

14

16

18

20

Z` / ohm cm2

Figure Figure 7. 7 (a) Polarization curves of Co3N plate and pure cobalt plate for OER in 1M KOH solution (Scan rate 5mV s-1, with iR correction). Inset shows Tafel plots of polarization curves. (b) Nyquist plots of the Co3N plate and pure cobalt plate at 1.6 V vs. RHE in 1M KOH solution. Inset shows the Nyquist plots for the

high-frequency region.

In summary, an oxidation-nitridation method was developed for directly synthesizing Co3N catalyst on cobalt plate. Co3N catalyst preforms excellent activity in both HER and OER. Using the kinetic methods, including Tafel slope, EIS analysis, exchange current density, and apparent activation energy, we have demonstrated that the Co3N is an excellent HER catalyst. The poisoning test of Co ions by SCN ions and theoretical calculations are performed to identify the HER mechanism on Co3N. We find that efficient HER on Co3N electrocatalyst in 7 / 12



alkaline electrolyte benefits from the favorable water dissociation (Volmer step) on in-situ formed cobalt-containing hydroxide, lowing the hydrogen production barrier on the nitrogen sites of Co3N. Our findings shed light on the important role of the in-situ formed cobalt hydroxides on surface of Co-containing nitride catalysts for HER.

Page 8 of 12

Single Crystals in Alkaline Electrolytes. J. Electroanal.

Chem. 2002, 524-525, 252–260. (3) Gong, M.; Zhou, W.; Tsai, M. C.; Zhou, J.; Guan, M.; Lin, M. C.; Zhang, B.; Hu, Y.; Wang, D. Y.; Yang, J.; Pennycook, S. J.; Hwang, B. J.; Dai, H. Nanoscale Nickel Oxide/Nickel Heterostructures for Active Hydrogen Evolution Electrocatalysis. Nat. Commun. 2014, 5, 4695. (4) Chou, N. H.; Ross, P. N.; Bell, A. T.; Tilley, T. D.

ASSOCIATED CONTENT

Comparison

of

Cobalt-Based

Nanoparticles

as

Electrocatalysts for Water Oxidation. ChemSusChem

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern, SEM image, XPS spectrum, CV plots, J-V curves.

2011, 4 , 1566-1569. (5) Gao, D.; Zhang, J.; Wang, T.; Xiao, W.; Tao, K.; Xue, D.; Ding, J. Metallic Ni3N Nanosheets with Exposed Active Surface Sites for Efficient Hydrogen Evolution. J.

Mater. Chem. A 2016, 4 , 17363-17369.

AUTHOR INFORMATION

(6) Yin, J.; Fan, Q.; Li, Y.; Cheng, F.; Zhou, P.; Xi, P.;

Corresponding Author * E-mail: [email protected]

Sun, S. Ni-C-N Nanosheets as Catalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2016, 138, 14546-14549.

ORCID Shicheng Yan: 0000-0002-3432-9117

(7) You, B.; Liu, X.; Hu, G.; Gul, S.; Yano, J.; Jiang, D. E.; Sun, Y. Universal Surface Engineering of Transition

Author Contributions § These authors contributed equally to this work.

Metals for Superior Electrocatalytic Hydrogen Evolution in Neutral Water. J. Am. Chem. Soc. 2017, 139, 12283-12290.

Notes The authors declare no competing financial interest.

(8) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J.

ACKNOWLEDGMENT

K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles

This work was supported primarily by the National Basic Research Program of China (2013CB632404), the National Natural Science Foundation of China (51572121, 21603098 and 21633004), the Natural Science Foundation of Jiangsu Province (BK20151265, BK20151383 and BK20150580), the Fundamental Research Funds for the Central Universities (021314380133 and 021314380084), the Postdoctoral Science Foundation of China (2017M611784), Six talent peaks project in Jiangsu Province (YY-013) and the program B for outstanding PhD candidate of Nanjing University (201702B084).

as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127 , 5308-5309. (9) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135 , 9267-9270. (10) Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP.

Chem. Mater. 2015, 27 , 3769-3774.

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Nanomaterials:

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The Table of Contents (TOC)

Accelerating Volmer step by surface hydroxide: hydroxide: H2O + e + * → H* + OH The in-situ formed hydroxide layer on Co3N catalyst was demonstrated to be having important role on accelerating water dissociation kinetics

12 / 12


Page 12 of 12

In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on

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