Interfacial charge polarization in Co2P2O7@N, P co-doped carbon nanocages as Mott-Schottky electrocatalysts for accelerating oxygen evolution reaction

Journal Pre-proof Interfacial Charge Polarization in Co2 P2 O7 @N, P Co-doped Carbon Nanocages as Mott-Schottky Electrocatalysts for Accelerating Oxygen Evolution Reaction Da Liang, Cheng Lian, Qiucheng Xu, Miaomiao Liu, Honglai Liu, Hao Jiang, Chunzhong Li

PII:

S0926-3373(19)31163-4

DOI:

https://doi.org/10.1016/j.apcatb.2019.118417

Reference:

APCATB 118417

To appear in:

Applied Catalysis B: Environmental

Received Date:

15 September 2019

Revised Date:

5 November 2019

Accepted Date:

11 November 2019

Please cite this article as: Liang D, Lian C, Xu Q, Liu M, Liu H, Jiang H, Li C, Interfacial Charge Polarization in Co2 P2 O7 @N, P Co-doped Carbon Nanocages as Mott-Schottky Electrocatalysts for Accelerating Oxygen Evolution Reaction, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118417

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Interfacial Charge Polarization in Co2P2O7@N, P Co-doped Carbon Nanocages as Mott-Schottky Electrocatalysts for Accelerating Oxygen Evolution Reaction

Da Liang,a Cheng Lian,b Qiucheng Xu,a Miaomiao Liu,a Honglai Liu,b Hao Jiang a,* and

a

ro of

Chunzhong Li a,b,*

Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center

-p

of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University

re

b

lP

of Science and Technology, Shanghai 200237, China

na

*Corresponding author, Tel.: +86-21-64250949, Fax: +86-21-64250624 E-mail: [email protected] (Prof. H. Jiang), [email protected] (Prof. C. Li)

Jo

ur

Graphical abstract

Highlights 

The Co2P2O7@N,P-C nanocages are prepared via facile ligands exchange reaction.



The Mott-Schottky junction effectively modulated the electronic structure of Co2P2O7.



The Co2P2O7@N,P-C nanocages exhibit superior OER activity and durability.



The DFT calculations demonstrate the rate-determined step of OER has been accelerated.

Abstract heterogeneous

non-noble

metal

electrocatalysts

to

modulate

the

ro of

Developing

valence-electron state near the Fermi level of metal centers is the pivotal for efficient oxygen evolution reaction (OER). Herein, we report a Mott-Schottky heterojunction electrocatalyst of

-p

the Co2P2O7@N, P co-doped carbon nanocages, in which the metallic N, P co-doping carbon

re

layer as a co-catalyst can effectively modulate the overfilled Co center e g orbital occupation of the Co2P2O7 nanoparticles and stabilize the microstructure. The density functional theory

lP

(DFT) calculations also reveal the build-in electric field promoted local charge polarization in the heterojunction interface greatly boosts the targeted intermediate (OOH*) adsorption with

na

a higher intrinsic activity. The as-obtained electrocatalysts manifest superior OER catalytic performance with an overpotential of only 310 mV at a current density of 50 mA cm-2 and

ur

negligible current loss for 100 h in 1.0 M KOH, much lower than the benchmark RuO2 (370 mV). This work demonstrates a heterointerface charge polarization concept to accelerate OER

Jo

in efficient electrocatalysts for water splitting.

Keywords: cobalt pyrophosphates; N, P co-doped carbon; co-catalysts; Mott-Schottky heterojunction; oxygen evolution.

1. Introduction The oxygen evolution reaction (OER) plays a highly determinant role in water-splitting devices, metal-air batteries and other renewable energy technologies [1-5]. A high overpotential is generally required because the breaking of O-H bond and the formation of O=O bond involve a four-electron transfer process with a sluggish reaction kinetics for OER [6]. Currently, considerable efforts have been devoted to exploiting effective non-precious electrocatalysts to decrease the overpotential, such as transition metal hydroxides, sulfides

ro of

and phosphides [7-9]. However, these catalysts usually suffer from the obvious microstructure collapse in the process of surface hydroxylation [10-11]. The cobalt pyrophosphate (Co2P2O7) has recently become a new star candidate because of its intrinsic high activity and favorable

-p

kinetics [12-14]. More charmingly, the diverse coordination environments of the pyrophosphate groups benefit the structural stability [15-17]. However, the activity of the

re

reported Co2P2O7-based catalysts is still far from the benchmarks RuO2 and IrO2 largely due

lP

to its semiconductor nature and the overfilled Co center eg orbital occupation [17-22], where the latter is detrimental to the rate-determining step (CoIV → CoIII) of OER.

na

The typical protocol using co-catalysts to modulate the electron density near the Fermi level of metal atom centers has been extensively applied to tailor the d-band orbital filling

ur

[23-24]. Constructing heterogeneous catalysts should be the ideal approach to change the charges flow across the heterointerface between catalyst and co-catalyst [25-26]. In order to

Jo

precisely regulate the positive and negative charge distribution, it is important to build a Mott-Schottky heterojunction, in which the difference of Fermi level will drive the flow of valence-electrons through the heterojunction interface [27-29]. It is well-accepted that carbonaceous materials as co-catalysts have prominent advantages in terms of their tunable electron structure and chemical stability [30]. More impressively, the heteroatoms co-doping (e.g. boron, nitrogen, phosphorus, sulfur, etc.) can greatly increase electrical conductivity and

work function, and meantime can break the initial conjugate electron coordination environment even enabling the metallicity [31-34]. The metallic carbon as a co-catalyst would effectively reduce the valence electron density of the Co center of Co2P2O7, then optimizing its

d-band

orbital

occupation.

However,

designing

Co2P2O7-based

Mott-Schottky

electrocatalysts with optimized microstructure that can realize the efficient and stable water splitting still have been seldom reported to date. It is documented that phytic acid (PA) with rich phosphate groups could easily crosslink

ro of

on the surface of metal-organic frameworks (ZIF-67) [35-36], and then reacts with Co ions to form a uniform coating layer (Co-phytate). Inspired by this feature, we herein report the Co2P2O7@N, P

co-doped

carbon

(Co2P2O7@N,P-C)

nanocages

as

Mott-Schottky

-p

electrocatalysts prepared via the ligands exchange reaction of 2-methylimidazole (2MIM) and phytate (phy) followed by the subsequent pyrolysis process. The in-situ N, P co-doping

re

carbon matrix exhibits a metallic characteristic mainly verified by the temperature-dependent

lP

resistivity. The resultant heterojunction interface effectively modulates the overfilled Co center eg orbital occupation. The turnover frequency (TOF) of the Co2P2O7@N,P-C nanocages at the overpotential of 320 mV was almost triple that of the corresponding Co2P2O7

na

alone. An overpotential of only 270 mV was applied to achieve a current density of 10 mA cm-2 for over 100 hours, almost the best report for cobalt-based electrocatalysts to our

ur

knowledge. The density functional theory (DFT) calculations and the XPS measurements

Jo

further demonstrate the local charge polarization in the heterojunction interface caused by build-in electric field remarkably promotes the targeted intermediate (OOH*) adsorption, leading to the enhancement of intrinsic activity.

2. Experimental 2.1. Synthesis of the Co2P2O7@N,P-C nanocages

200 mg of ZIF-67 powders were dissolved in a mixture solution of ethanol and deionized water (Vwater: Vethanol =1:1), stirred for 30 min. Then 8 mL of phytic acid solution was injected into the ZIF-67 solution and stirred for 32 min. The solution was centrifuged and washed with deionized water and ethanol for three times, respectively. The Co-phy/2MIM precipitates were obtained after dried at 80 °C for 12 h. When carbonized the Co-phy/2MIM under Ar/H2 flow at 550 °C for 3 h, the Co2P2O7@N,P-C nanocages were obtained. As controls, we also synthesized the corresponding the Co2P2O7 and N,P-C nanocages just through annealing the

ro of

Co2P2O7@N,P-C at air and etching the Co2P2O7 in 3 M H2SO4 solution, respectively. 2.2. Structural characterization

The powder X-ray diffraction was used to measure the crystal structure of samples on

-p

Rigaku D/Max 2550. Field emission scanning electron microscopy (FESEM) was used to examine the morphologies on Hitachi S-4800 (20 kV). Transmission electron microscopy

re

(TEM) images and corresponding high-angle annular dark field (HAADF) images were

lP

obtained with JEOL JEM-2100F. The X-ray photoelectron spectroscopy (XPS) and the ultraviolet photoemission spectroscopy (UPS) were performed to investigate the chemical state of samples, recorded on a Thermo Scientific ESCALAB 250Xi and a VG Scienta R4000

na

analyzer (monochromatic He I light source of 21.2 eV), respectively. A Thermo Electron NEXUS 670 FT-IR spectrometer was used to perform the Fourier Transform-Infrared

ur

Spectroscopy (FT-IR) spectra. The UV-Vis diffuse reflectance spectra (DRS) of the samples

Jo

were measured on a Varian Cary 500 Scan UV-Vis spectrophotometer with barium sulfate as the reference. Photoluminescence (PL) spectra were measured on a LS-55 Lumine fluorescence spectrophotometer. 2.3. Electrochemical measurements The electrochemical tests were performed in O2-saturated 1.0 M KOH solution by a standard three electrode system, with saturated Ag/AgCl (3 M KCl) and graphite electrode as

reference electrode and counter electrode, respectively. For the preparation of working electrode, 5 mg of synthesized sample was mixed with 50 μL of Nafion solution (5 wt%) and 950 μL of ethanol by sonication of at least 1 h to prepare the catalyst ink, then 35 μL of the ink was drop-coated on carbon fiber paper (0.5 × 0.7 cm2) with a loading density of 0.5 mg cm-2. In 1.0 M KOH electrolyte, potentials were referenced to a reversible hydrogen electrode (RHE) by adding 1.0236 V (0.1976 + 0.0591 × pH). The Linear sweep voltammetry (LSV) was tested to measure the OER electrochemical activities on CHI 660E electrochemical

ro of

analyzer. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS, 0.01 Hz - 100 KHz) were performed on Autolab PGSTAT302N electrochemical workstation. Double-layer capacitance measurements were made by taking CV at a scan rate that ranged

-p

from 10 to 800 mV s-1. The TOF values of the catalysts were calculated from the equation: TOF = (J × A)/(2 × F × n), where J is the current density at a given overpotential, A is the

re

surface area of the CC, F is the Faraday constant (96485 C mol -1), and n is the number of

2.4. Computational details

lP

moles of the active sites of the catalyst deposited on the electrode.

First, the heterojunction of Co2P2O7 and N,P-C was optimized by the DFT. Second, the

na

Co2P2O7 crystal was cleaved on the (100) plane, and this (100) plane is also packed with N,P-C surface to form the heterojunction of Co2P2O7@N,P-C. The free energy profile for

ur

oxygen evolution is calculated by the density functional theory (DFT) and the transition state

Jo

theory (TST). The TST representing the binding free energy is calculated by: ∆G = ∆E + ∆EZPE − T∆S, where ∆E is the binding energy, ∆EZPE is the zero-point energy correction, T = 298 K is the temperature and ∆S is the change of entropy. The ∆EZPE and ∆S for different absorption species are taken from previous studies by assuming that they are surface independent [34,37-38]. The binding energies ∆E for OH*, OOH*, and O* were calculated by the DFT, respectively. The DFT calculation for binding energy was performed in the Dmol 3

module of the Material Studio software. The PW91 method was used to approximate the exchange correlation functional [39]. The convergence of force for optimization is 0.01 eV Å−1, and that of total energy of structure is set to 10−5 eV for all calculations. The reaction free energy change at an applied potential U is expressed as: ΔG(U) = ΔG – eU.

3. Results and discussion

na

lP

re

-p

ro of

3.1. Synthesis and characterizations of Co2P2O7@N,P-C nanocages electrocatalyst

Fig. 1. (a) Schematic illustration of the preparation process of the Co2P2O7@N,P-C nanocages; (b) in-situ

ur

UV-Vis spectra of the ligands exchange reaction of 2MIM and phytate; (c) FT-IR spectra of ZIF-67 before

Jo

and after ligands exchange; (d) XRD pattern of the Co2P2O7@N,P-C nanocages.

The Co2P2O7@N,P-C nanocages have been prepared by a simple ligands exchange

reaction of 2MIM and phytate followed with the subsequent pyrolysis process, schematically illustrated in Fig. 1a. Specifically, the uniform metal-organic framework ZIF-67 rhombic dodecahedrons with sizes of ~200 nm were firstly synthesized by self-assembly of Co2+ and 2MIM (Fig. S1a-b) with positively charged surface. With the help of electrostatic attraction,

the negatively charged PA with high P content is easily coated on the surface of ZIF-67, stabilizing the morphology. Subsequently, the protons of PA will diffuse into ZIF-67 to break the coordination bonds between cobalt and 2MIM (superhigh N content), hence releasing Co2+ and dissociative protonated 2MIM. With increasing the reaction time, the concentration gradient drives the Co2+ to coordinate with the surface phytate (Co-phy). The Co-phy/2MIM nanocages are then obtained, as shown in Fig. S1c-d. Fig. S2 also shows the Zeta potential change before and after exchange. To clarify the ligands exchange process, in-situ UV-Vis

ro of

spectra are performed (Fig. 1b), where the absorption peaks at 535, 564, and 585 nm are assigned to the Co-2MIM bonds characteristic peaks of ZIF-67 [40]. It can be observed that the Co-2MIM bonds were gradually broken with the reaction time until reaching a balance at

-p

32 min. The FT-IR (Fig. 1c) spectra further confirm the characteristic peaks of Co-phy and Co-2MIM in Co-phy/2MIM nanocages [40-41]. Such a formation process is also verified by

re

the corresponding TEM images for each step of Fig. 1a. After calcination, the final

lP

Co2P2O7@N,P-C nanocages are obtained, in which 2MIM and phytate respectively act as the nitrogen and phosphorus sources. The XRD pattern in Fig. 1d is indexed to the Co2P2O7 diffraction peaks (JCPDS No. 49-1091). No other impurity can be detected. The XPS spectra

na

also indicate the N and P elements have been successfully doped into carbon skeleton (Fig. S3). The content of N,P-C in the Co2P2O7@N,P-C nanocages is 18 wt% according to TG

ur

measurement in Fig. S4. As controls, the corresponding Co2P2O7 and N,P-C nanocages (Fig.

Jo

S5) were prepared after burning carbon and etching Co2P2O7 nanoparticles, respectively.

ro of -p

re

Fig. 2. (a) SEM and (b,c) TEM images of the Co2P2O7@N,P-C nanocages; (d) TEM image of the N,P-C

lP

nanocages; (e) HRTEM and (f) HAADF-mapping images of the Co2P2O7@N,P-C nanocages.

The morphology and detailed microstructure have been characterized by scanning

na

electron microscopy (SEM) and transmission electron microscopy (TEM), as shown in Fig. 2. The polyhedrons have been well-maintained with sizes of ~200 nm (Fig. 2a). The

ur

low-magnification TEM image (Fig. 2b) confirms the hollow feature. Further magnifying the wall (Fig. 2c), we can find that the Co2P2O7 nanoparticles are incorporated into carbon

Jo

skeleton. To verify this point, the Co2P2O7 nanoparticles have been etched by the diluted sulfuric acid. The N, P co-doped carbon nanocages are obtained in Fig. 2d with abundant pores in the wall. The interface between N,P-C and Co2P2O7 is exhibited in high-resolution TEM image (Fig. 2e), in which one side corresponds to the (012) plane of Co2P2O7 with lattice fringe of 0.29 nm and another side is assigned to defective carbon. The inset of Fig. 2e gives the fast Fourier transformation (FFT) pattern in the white dotted box, which can also be

attributed to the (012) plane. The SAED pattern of the Co2P2O7@N, P-C nanocages in Fig. S6 further proves the monoclinic cobalt pyrophosphate crystalline structure. The high-angle annular dark field (HAADF) image and corresponding EDS mappings of the Co2P2O7@N,P-C in Fig. 2f demonstrate the uniform distribution of elements. These results confirm the successful construction of the heterogeneous electrocatalyst composed of

lP

re

-p

ro of

Co2P2O7 nanoparticles incorporated into N, P co-doped carbon nanocages.

Fig. 3. (a) Co 2p XPS and (b) PL spectra of the Co2P2O7@N,P-C and the Co2P2O7; (c) work function

na

values of the Co2P2O7, the N,P-C, and the Co2P2O7@N,P-C (inset showing the corresponding UPS spectra); (d) schematic energy band diagrams of the heterojunction between Co2P2O7 and N,P-C (Evac = vacuum energy, Ec = conduction band, Ev = valence band, Ef = Fermi level, Eg = energy gap, W = work

Jo

ur

function, Φ = depletion region).

To investigate the effects of the heterojunction on electronic interaction, Fig. 3a gives the

comparison of Co 2p XPS spectra of the Co2P2O7@N,P-C and the pure Co2P2O7 nanocages. An obvious positive shift of 0.4 eV for Co 2p3/2 and Co 2p1/2 peaks could be observed for the Co2P2O7@N,P-C, which implies electrons transfer from the Co2P2O7 to the N,P-C. This electrons transfer can also be proved by the FT-IR analysis (Fig. S7). In their photoluminescence (PL) spectra (Fig. 3b), the decreased emission intensity of

Co2P2O7@N,P-C indicates a higher charge transfer efficiency [42]. In addition, the conduction edge of Co2P2O7 (semiconductor) may show an upshift trend according to the blue-shift of emission for Co2P2O7@N,P-C [43]. To further characterize the interfacial charge polarization and band structure alteration, the ultraviolet photoelectron spectra (UPS) of the Co2P2O7@N,P-C, the pure Co2P2O7 and the pure N,P-C are analyzed (Fig. 3c). The cutoff energy (Ecutoff) values of these three samples are given by normalized secondary electron cutoff (SECO) spectra (inset in Fig. 3c). The work function (W) can be calculated according

ro of

to the following equation: W = hv - |Ecutoff - Ef|, with the hv and Ef are 21.2 and 0 V in our case. The values of the Co2P2O7@N,P-C, the Co2P2O7 and the N,P-C are 4.7, 3.8 and 5.2 eV, respectively. Notably, the N,P-C also shows a metallic feature. A direct evidence is the

-p

observation of the temperature-dependent resistivity (ρ-T) test in Fig. S8a, which showing a gradually increased resistivity with increasing the temperature [43]. The XPS valance band

re

spectrum (Fig. S8b) further confirms the metallicity with the appearance of Fermi edge [44].

lP

According to the theory of semiconductor physics, a Mott-Schottky heterojunction of metallic N,P-C and N-type semiconductor Co2P2O7 is achieved. To study the contact interface between N,P-C and Co2P2O7, the energy band diagrams before and after contact are plotted in Fig. 3d.

na

The bandgap (Eg) and the position of Fermi level (Ef - Ev) could be conceived based on the Tauc plot and XPS valance band spectrum of the Co2P2O7 (Fig. S9). It is reported that the

ur

reducing valence electrons for the cobalt with octahedral geometry can greatly decrease the

Jo

occupancy of eg orbital with an enhanced adsorption ability for targeted intermediate (OOH*) [23-24,31]. In this case, the built-in electric field caused by Mott-Schottky heterojunction can effectively drive valence electrons from the Co2P2O7 to the N,P-C, until the same Fermi levels on both sides of the interface, leading to the remarkable reduction of valence electrons, which therefore accelerate the kinetics of the rate-determining step (CoIV → CoIII). 3.2. Electrochemical activity of Co2P2O7@N,P-C nanocages electrocatalyst

ro of

Fig. 4. (a) The OER polarization curves and (b) Tafel plots of the RuO2, the Co2P2O7, the N,P-C, and the Co2P2O7@N,P-C; (c) the overpotential value and Tafel slope of the Co2P2O7@N,P-C compared with the

-p

reported advanced Co-based electrocatalysts; (d) Nyquist plots and (e) TOF values of the Co2P2O7, the

re

N,P-C, and the Co2P2O7@N,P-C; (f) the stability test of Co2P2O7@N,P-C.

The OER tests of all the samples are carried out in 1.0 M KOH solution. Fig. 4a shows

lP

the linear sweep voltammetry (LSV) curves at a scan rate of 2 mV s-1. It is obvious that the Co2P2O7@N,P-C nanocages exhibit a RuO2-like overpotential of 270 mV to achieve a current

na

density of 10 mA cm-2, which is much lower than the Co2P2O7 nanocages (η10 = 320 mV). To drive a current density of 50 mA cm-2, the Co2P2O7@N,P-C delivers an extremely low

ur

overpotential of 310 mV, far less than the commercial RuO2 (η50 = 370 mV) and the Co2P2O7 (η50 = 400 mV). This result indicates the Co2P2O7@N,P-C has a faster reaction kinetics in

Jo

OER. The corresponding Tafel slope of the Co2P2O7@N,P-C also gives a lowest value of 49.1 mV dec-1 in Fig. 4b. Comparative study on the effect of Co concentration in the Co2P2O7@N,P-C to the OER performance are provided in Fig. S10. Comprehensive comparisons with Co-based OER electrocatalysts have been summarized in Fig. 4c and Table S2. The catalytic activity of the Co2P2O7@N,P-C is at least among the best report for all Co-based electrocatalysts, e.g. the metaphosphate NaCo(PO3)3 nanoparticles [45] and the

colloidal Co2P nanocrystals [46]. Such exceptional high catalytic performance is dominantly attributed to the design of Mott-Schottky heterogeneous nanocages electrocatalysts, affording rapid charge transfer in the process of OER. Fig. 4d gives the Nyquist plots of the three samples. The Co2P2O7@N,P-C exhibits a very small charge transfer resistance (Rct) of ~5.0 Ω compared with the Co2P2O7 (~21.0 Ω) and the N,P-C (~6.0 Ω), further verifying the accelerated reaction kinetics and faradaic process. To clarify the intrinsic activity, the turnover frequency (TOF) of the three samples are provided in Fig. 4e, in which the

ro of

Co2P2O7@N,P-C displays a highest intrinsic catalytic activity with a TOF value of 0.103 s-1 at an overpotential of 320 mV, almost 3.5 times and 6 times of the Co2P2O7 and the N,P-C. The electrochemical active surface area (ECSA) normalized LSV curves are also provided in Fig.

-p

S11. The normalized current density of the Co2P2O7@N,P-C at an overpotential of 300 mV is 0.035 mA cm-2, which is much higher than the Co2P2O7 (0.012 mA cm-2), further indicating a

re

distinctly improved intrinsic activity. In addition, the Co2P2O7@N,P-C can work continuously

lP

for 100 hours of chronopotentiometry (CP) test with no distinct overpotential change, as shown in Fig. 4f. The inset of Fig. 4f and Fig. S12 give the SEM and TEM images of the Co2P2O7@N,P-C after CP test, in which the nanocages are well-maintained, therefore

na

exhibiting outstanding catalytic stability.

To deeply investigate the effect of Mott-Schottky heterojunction on the intrinsic catalytic

ur

activity, the XPS spectra are applied to investigate the chemical state variation of the

Jo

Co2P2O7@N,P-C and the Co2P2O7 electrocatalysts. After OER, the Co2P2O7@N,P-C shows a relatively higher Co3+/Co2+ ratio of 2.14 compared to the Co2P2O7 (1.27) in Fig. S13a, implying that the interfacial charge polarization of Mott-Schottky heterojunctions is beneficial for the generation of targeted OOH* intermediate. The kinetics of the rate-determining step is then accelerated, which is in good agreement with the corresponding O 1s spectra in Fig. S13b that the Co2P2O7@N,P-C shows more OOH* intermediate adsorption. It is noted that

the N,P-C mainly acts as co-catalyst to engineer the activity of the Co2P2O7 catalysts. The N 1s and P 2p spectra after OER (Fig. S14) indicate that there is almost no variation for N and P contents of carbon layer, directly revealing the highly stable N, P co-doped carbon skeleton combined with its negligible OER catalytic activity. This advantage is also the reason that the Co2P2O7@N,P-C nanocages can be well-maintained after OER.

re

-p

ro of

3.3. Theoretical calculations of OER for Co2P2O7@N,P-C nanocages electrocatalyst

lP

Fig. 5. (a) The side view and the top view of charge density of the heterojunction of Co2P2O7 and N,P-C (the yellow and green represent charge depletion and accumulation in the space, respectively); (b) and (c)

na

the models and binding energy of OER intermediates on the active site of the Co2P2O7@N,P-C and the Co2P2O7; (d) and (e) the calculated free-energy diagram of OER on the active site of the Co2P2O7@N,P-C

ur

and the Co2P2O7.

Jo

The DFT calculations are further implemented to gain theoretical insight onto the excellent intrinsic activity of the Co2P2O7@N,P-C nanocages electrocatalysts. Fig. 5a gives the optimized model of the Co2P2O7@N,P-C with spatial charge distribution. It can be observed the depletion and accumulation of charge density in the Co2P2O7 and the N,P-C, respectively. This result indicates the electrons injection from the Co2P2O7 to the N,P-C, which fits well with the aforementioned Mott-Schottky junction model in Fig. 3d. The

resultant decrement of average valence charge of Co near the interface will result in the enhancement of catalytic activity. Subsequently, the (100) facets of the pristine and carbon-engineered Co2P2O7 are chosen as the models to investigate the adsorption of intermediates. Obviously, the adsorption energies of OH*, O*, and OOH* intermediates at Co site for the Co2P2O7@N,P-C (Fig. 5b) are much lower than the Co2P2O7 (Fig. 5c). This result indicates the interfacial charge polarization energetically contributes to the adsorption of OER intermediates, especially the key OOH* intermediates, which is consistent with the above

ro of

XPS observations. The Gibbs free energies of various electron transfer steps are calculated at different applied potentials under pH = 14. At the equilibrium potential (U = 0.402 V), the most elevated endothermic step is directly associated with the rate-determined step and the

-p

corresponding theoretical onset overpotentials. As exhibited in Fig. 5d and Fig. 5e, it can be observed that the Co2P2O7@N,P-C shows a much lower onset overpotential of 0.218 V than

re

that of the Co2P2O7 (0.298 V). Therefore, the interfacial charge polarization caused by the

lP

Mott-Schottky heterojunction enables a higher intrinsic activity and the accelerated reaction

4. Conclusions

na

kinetics.

ur

In summary, we have designed and synthesized a Mott-Schottky heterojunction electrocatalyst of the Co2P2O7@N, P co-doped carbon nanocages by the ligands exchange

Jo

reaction of 2MIM and phytate as well as the subsequent pyrolysis process for oxygen evolution. The Co2P2O7@N,P-C electrocatalyst shows a very high TOF value at the overpotential of 320 mV, about three times of the corresponding Co2P2O7 nanocages. An overpotential of only 270 mV is required to achieve 10 mA cm-2 for over 100 hours, almost the best report for cobalt-based electrocatalysts. Such exceptional high OER performance is mainly ascribed to the enhancement of intrinsic catalytic activity driven by the interfacial

charge polarization of the Mott-Schottky heterojunction. The robust N,P-C layer with metallicity can not only effectively modulate the overfilled Co center eg orbital occupation of the Co2P2O7 nanoparticles as a co-catalyst, but also ensure a continuous and long-term work by stabilizing the microstructure of the electrocatalysts. The DFT calculations and the XPS measurements further reveal the Mott-Schottky electrocatalysts have a lower the targeted intermediate (OOH*) adsorption energy and accelerated reaction kinetics for rate-determined

ro of

step of OER.

Notes

-p

The authors declare no competing financial interests.

re

Acknowledgements

This work was supported by the National Natural Science Foundation of China

lP

(21838003 and 51621002), the Social Development Program of Shanghai (17DZ1200900), the Shanghai Scientific and Technological Innovation Project (18JC1410600), the National

na

Program for Support of Top-Notch Young Professionals, and the Fundamental Research

ur

Funds for the Central Universities (222201718002).

References

Jo

[1] Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, Combining theory and experiment in electrocatalysis: Insights into materials design. Science 355 (2017) eaad4998.

[2] Q. Xu, H. Jiang, H. Zhang, Y. Hu, C. Li, Heterogeneous interface engineered atomic configuration on ultrathin Ni(OH)2/Ni3S2 nanoforests for efficient water splitting. Appl. Catal. B: Environ. 242 (2019) 60-66.

[3] H. Zhang, H. Jiang, Q. Xu, Y. Hu, C. Li, Rapid low-temperature synthesis of hollow CuS0. 55

nanoparticles for efficient electrocatalytic water oxidation. Chem. Eng. Sci. 195 (2019)

665-670. [4] Y. Zhang, J. Fu, H. Zhao, R. Jiang, F. Tian, R. Zhang, Tremella-like Ni3S2/MnS with ultrathin nanosheets and abundant oxygen vacancies directly used for high speed overall water splitting. Appl. Catal. B: Environ. 257 (2019) 117899. [5] X. Yuan, L. Zhang, L. Li, H. Dong, S. Chen, W. Zhu, C. Hu, W. Deng, Z. Zhao, J. Gong,

dioxide reduction. J. Am. Chem. Soc. 141 (2019) 12.

ro of

Ultrathin Pd-Au shells with controllable alloying degree on Pd nanocubes toward carbon

[6] L. Yu, J. F. Yang, B. Y. Guan, X. W. Lou, Hierarchical hollow nanoprisms based on

-p

ultrathin Ni‐ Fe layered double hydroxide nanosheets with enhanced electrocatalytic activity towards oxygen evolution. Angew. Chem. Int. Ed. 57 (2018) 172-176.

re

[7] H. Jiang, Q. He, X. Li, X. Su, Y. Zhang, S. Chen, S. Zhang, G, Zhang, J. Jiang, P. M.

lP

Ajayan, L. Song, Tracking structural self‐ reconstruction and identifying true active sites toward cobalt oxychloride precatalyst of oxygen evolution reaction. Adv. Mater. 31 (2019) 1805127.

na

[8] P. Cai, J. Huang, J. Chen, Z. Wen, Oxygen‐ containing amorphous cobalt sulfide porous nanocubes as high‐ activity electrocatalysts for the oxygen evolution reaction in an

ur

alkaline/neutral medium. Angew. Chem. Int. Ed. 56 (2017) 4858-4861.

Jo

[9] L. Yan, L. Cao, P. Dai, X. Gu, D. Liu, L. Li, Y. Wang, X. Zhao, Metal‐ organic frameworks derived nanotube of nickel-cobalt bimetal phosphides as highly efficient electrocatalysts for overall water splitting. Adv. Funct. Mater. 27 (2017) 1703455.

[10] H. Jiang, Q. He, Y. Zhang, L. Song, Structural self-reconstruction of catalysts in electrocatalysis. Acc. Chem. Res. 51 (2018) 2968-2977. [11] S. Jin, Are metal chalcogenides, nitrides, and phosphides oxygen evolution catalysts or

bifunctional catalysts? ACS Energy Lett. 2 (2017) 1937-1938. [12] H. Kim, J. Park, I. Park, K. Jin, S. E. Jerng, S. H. Kim, K. T. Nam, K. Kang, Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst. Nat. Commun. 6 (2015) 8253. [13] H. Du, W. Ai, Z. L. Zhao, Y. Chen, X. Xu, C. Zou, L. Wu, L. Su, K. Nan, T. Yu, C. M. Li, Engineering Morphologies of Cobalt Pyrophosphates nanostructures toward greatly enhanced electrocatalytic performance of oxygen evolution reaction. Small 14 (2018)

ro of

1801068. [14] D. Zhao, Q. Shao, Y. Zhang, X. Huang, N-Doped carbon shelled bimetallic phosphates for efficient electrochemical overall water splitting. Nanoscale 10 (2018) 22787-22791.

-p

[15] R. Gao, X. Lai, J. Huang, X. Du, Y. Yan, Y. Sun, G. Zou, J. Xiong, Phosphate‐ based electrocatalysts for water splitting: recent progress. ChemElectroChem 5 (2018)

re

3822-3834.

lP

[16] M. W. Kanan, J. Yano, Y. Surendranath, M. Dinca, V. K. Yachandra, D. G. Nocera, Structure and valency of a cobalt-phosphate water oxidation catalyst determined by in situ X-ray spectroscopy. J. Am. Chem. Soc. 132 (2010) 39.

na

[17] A. Gajuta, P. George, N. Devanna, Synthesis, characterization and antibacterial studies of Co(II), Ni(II) and Zn(II) complexes of schiff base derived from 4-methoxy aniline. Int. J.

ur

Pharma. Bio. Sci. 8 (2017) 148-159.

Jo

[18] W. P. Wang, H. Pang, M. L. Jin, X. Shen, Y. Yao, Y. G. Wang, Y. C. Li, X. D. Li, C. Q. Jin, R. C. Yu, Studies on the structural stability of Co2P2O7 under pressure. J. Phys. Chem. Solids 116 (2018) 113-117.

[19] T. Zhou, Y. Du, D. Wang, S. Yin, W. Tu, Z. Chen, A. Borgna, R. Xu, Phosphonate-based metal-organic framework derived Co-P-C hybrid as an efficient electrocatalyst for oxygen evolution reaction. ACS Catal. 7 (2017) 6000-6007.

[20] H. Wan, R. Ma, X. Liu, J. Pan, H. Wang, S. Liang, G. Qiu, T. Sasaki, Rare cobalt-based phosphate nanoribbons with unique 5-coordination for electrocatalytic water oxidation. ACS Energy Lett. 3 (2018) 1254-1260. [21] Y. Z. Jin, Z. Li, J. Q. Wang, R. Li, Z. Q. Li, H. Liu, J. Mao, C. K. Dong, J. Yang, S. Z. Qiao, X. W. Du, Tuning spin state of rock‐ salt‐ based oxides by manipulation of crystallinity for efficient oxygen electrocatalysis. Adv. Energy. Mater. 8 (2018) 1703469. [22] J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. Shao-Horn, A perovskite

ro of

oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 334 (2011) 1383-1385.

[23] M. R. Gao, X. Cao, Q. Gao, Y. F. Xu, Y. R. Zheng, J. Jiang, S. H. Yu, Nitrogen-doped

-p

graphene supported CoSe2 nanobelt composite catalyst for efficient water oxidation. ACS Nano 8 (2014) 3970-3978.

re

[24] N. K. Oh, C. Kim, J. Lee, O. Kwon, Y. Choi, G. Y. Jung, H. Y. Lim, S. K. Kwak, G. Kim,

lP

H. Park, In-situ local phase-transitioned MoSe2 in La0.5Sr0.5CoO3-δ heterostructure and stable overall water electrolysis over 1000 hours. Nat. Commun. 10 (2019) 1723. [25] M. R. Gao, Y. F. Xu, J. Jiang, Y. R. Zhang, S. H. Yu, Water oxidation electrocatalyzed by

na

an efficient Mn3O4/CoSe2 nanocomposite. J. Am. Chem. Soc. 134 (2012) 2930-2933. [26] X. H. Li, M. Antonietti, Metal nanoparticles at mesoporous N-doped carbons and carbon

ur

nitrides: functional Mott-Schottky heterojunctions for catalysis. Chem. Soc. Rev. 42 (2013)

Jo

6593-6604.

[27] C. Li, Y. Liu, Z. Zhuo, H. Ju, D. Li, Y. Guo, X. Wu, H. Li, T. Zhai, Local charge distribution engineered by Schottky heterojunctions toward urea electrolysis. Adv. Energy Mater. 8 (2018) 1801775. [28] Z. H. Xue, J. T. Han, W. J. Feng, Q. Y. Yu, X. H. Li, M. Antonietti, J. S. Chen, Tuning the adsorption energy of methanol molecules along Ni‐ N‐ doped carbon phase boundaries

by the Mott-Schottky effect for gas‐ phase methanol dehydrogenation. Angew. Chem. Int. Ed. 57 (2018) 2697. [29] Y.X. Liu, H. H. Wang, T. J. Zhao, B. Zhang, H. Su, Z. H. Xue, X. H. Li, J. S. Chen, Schottky barrier induced coupled interface of electron-rich N-doped carbon and electron-deficient Cu: in-built Lewis Acid-base pairs for highly efficient CO2 fixation. J. Am. Chem. Soc. 141 (2019) 38. [30] Y. Lin, L. Yang, Y. Zhang, H. Jiang, Z. Xiao, C. Wu, G. Zhang, J. Jiang, L. Song,

ro of

Defective carbon-CoP nanoparticles hybrids with interfacial charges polarization for efficient bifunctional oxygen electrocatalysis. Adv. Energy Mater. 8 (2018) 1703623.

[31] X. Wang, L. Zhuang, T. He, Y. Jia, L. Zhang, X. Yan, A. Du, Z. Zhu, X. Yao, S. H. Yu,

-p

Grafting cobalt diselenide on defective graphene for enhanced oxygen evolution reaction. iScience 7 (2018) 145-153.

re

[32] J. Zhang, L. Qu, G. Shi, J. Liu, J. Chen, L. Dai, N, P‐ codoped carbon networks as

lP

efficient metal‐ free bifunctional catalysts for oxygen reduction and hydrogen evolution reactions. Angew. Chem. Int. Ed. 55 (2016) 2230-2234. [33] K. Gao, B. Wang, L. Tao, B. V. Cunning, Z. Zhang, S. Wang, R. S. Ruoff, L. Qu, Efficient

na

metalfree electrocatalysts from N‐ doped carbon nanomaterials: mono‐ doping and co‐ doping. Adv. Mater. 31 (2019) 1805121.

ur

[34] Y. P. Zhu, Y. Jing, A. Vasileff, T. Heine, S. Z. Qiao, 3D synergistically active carbon

Jo

nanofibers for improved oxygen evolution. Adv. Energy Mater. 7 (2017) 1602928. [35] R. Ravichandran, V. Seitz, J. R. Venugopal, R. Sridhar, S. Sundarrajan, S. Mukherjee, E. Wintermantel, S. Ramakrishna, Mimicking native extracellular matrix with phytic acid‐ crosslinked protein nanofibers for cardiac tissue engineering. Macromol. Biosci. 13 (2013) 366-375. [36] Q. Xu, H. Jiang, Y. Li, D. Liang, Y. Hu, C. Li, In-situ enriching active sites on co-doped

Fe-Co4N@NC nanosheet array as air cathode for flexible rechargeable Zn-air batteries. Appl. Catal. B: Environ. 256 (2019) 117893. [37] A. Valdes, Z. W. Qu, G. J. Kroes, J. Rossmeisl, J. K. Norskov, Oxidation and photo-oxidation of water on TiO2 surface. J. Phys. Chem. C 2 (2008) 9872-9879. [38] R. V. Mom, J. Cheng, M. T. M. Koper, M. Sprik, Modeling the oxygen evolution reaction on metal oxides: the influence of unrestricted DFT calculations. J. Phys. Chem. C 118 (2014) 4095-4102.

correlation energy. Phys. Rev. B 45 (1992) 13244-13249.

ro of

[39] J. P. Perdew, Y. Wang, Accurate and simple analytic representation of the electron-gas

[40] J. Qin, S. Wang, X. Wang, Visible-light reduction CO2 with dodecahedral zeolitic

-p

imidazolate framework ZIF-67 as an efficient co-catalyst. Appl. Catal. B: Environ. 209 (2017) 476-482.

re

[41] P. Li, Z. Jin, J. Yang, Y. Jin, D. Xiao, Highly active 3D-nanoarray-supported

(2016) 153-161.

lP

oxygen-evolving electrode generated from cobalt-phytate nanoplates. Chem. Mater. 28

[42] Q. Xu, Y. Liu, H. Jiang, Y. Hu, H. Liu, C. Li, Unsaturated sulfur edge engineering of

na

strongly coupled MoS2 nanosheet-carbon macroporous hybrid catalyst for enhanced hydrogen generation. Adv. Energy Mater. 9 (2019) 1802553.

ur

[43] Z. Zhuang, Y. Li, Z. Li, F. Lv, Z. Lang, K. Zhao, L. Zhou, L. Moskaleva, S. Guo, L. Mai,

Jo

MoB/g‐ C3N4 interface materials as a schottky catalyst to boost hydrogen evolution. Angew. Chem. Int. Ed. 57 (2018) 496-500.

[44] X. Huang, H. Li, Z. Tu, L. Liu, X. Wu, J. Chen, Y. Liang, Y. Zou, Y. Yi, J. Sun, Wei, Xu, D. Zhu, Highly conducting neutral coordination polymer with infinite two-dimensional silver–sulfur networks. J. Am. Chem. Soc. 140 (2018) 15153-15156. [45] R. Gond, D. K. Singh, M. Eswaramoorthy, P. Barpanda, Water oxidation electrocatalyzed

by an efficient Mn3O4/CoSe2 nanocomposite. Angew. Chem. Int. Ed. 131 (2019) 1-6. [46] H. Li, Q. Li, P. Wen, T. B. Williams, S. Adhikari, C. Dun, C. Lu, D. Itanze, L. Jiang, D. L. Carroll, G. L. Donati, P. M. Lundin, Y. Qiu, S. M. Geyer, Colloidal cobalt phosphide nanocrystals as trifunctional electrocatalysts for overall water splitting powered by a

Jo

ur

na

lP

re

-p

ro of

zinc-air battery. Adv. Mater. 30 (2018) 1705796.