Necklace-like Multishelled Hollow Spinel Oxides with Oxygen

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Necklace-Like Multi-Shelled Hollow Spinel Oxides with Oxygen Vacancies for Efficient Water Electrolysis Shengjie Peng, Feng Gong, Linlin Li, Deshuang Yu, Dongxiao Ji, Zhe Hu, Zhiqiang Zhang, Shulei Chou, Tianran Zhang, Yonghua Du, and Seeram Ramakrishna J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05134 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Journal of the American Chemical Society

Necklace-Like Multi-Shelled Hollow Spinel Oxides with Oxygen Vacancies for Efficient Water Electrolysis Shengjie Peng,†,‡ Feng Gong,＃ Linlin Li,*, ‡ Deshuang Yu,‡ Dongxiao Ji,† Tianran Zhang,§ Zhe Hu,⊥ Zhiqiang Zhang,*,‖ Shulei Chou,*,⊥ Yonghua Du,£ and Seeram Ramakrishna‡ †

Department of Mechanical Engineering, National University of Singapore, 117574, Singapore

‡

Jiangsu Key Laboratory of Electrochemical Energy Storage Technologies, College of Materials Science and

Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ＃

School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 611731,

China §

‖

Department of Chemical and Biomolecular Engineering, National University of Singapore, 119260, Singapore

Key Laboratory for Functional Material, Educational Department of Liaoning Province, University of Science and

Technology Liaoning, Anshan, 114051, China ⊥

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong,

Innovation Campus, Squires Way, North Wollongong, NSW 2522, Australia £Institute

of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833, Singapore

Supporting Information ABSTRACT: ABSTRACT: The durability and reactivity of catalysts can be effectively and precisely controlled through the careful design and engineering of their surface structures and morphologies. Herein, we develop a novel “adsorptioncalcination-reduction” strategy to synthesize spinel transitional metal oxides with a unique necklace-like multi-shelled hollow structure exploiting sacrificial templates of carbonaceous microspheres, including NiCo2O4 (NCO), CoMn2O4,

dual-electrode, alkaline water electrolyzer. Calculations based on Density functional theory (DFT) reveal a mechanism for the promotion of the catalytic reactions based on a decrease in the energy barrier for the formation of intermediates resulting from the introduction of oxygen vacancies through the reduction process. This method could prove to be an effective general strategy for the preparation of complex, hollow structures and functionalities.

and NiMn2O4. Importantly, benefiting from the unique structures and reduction treatment to offer rich oxygen vacancies, the unique reduced NCO (R-NCO) as a bifunctional

INTRODUCTION

electrocatalyst exhibits the dual characteristics of good sta-

Hydrogen as a promising clean energy source for sus-

bility as well as high electrocatalytic activity for both the

tainable energy applications has received much research

hydrogen evolution reaction (HER) and oxygen evolution

interest propelled by the rapid exhaustion of fossil fuels

reaction (OER). It is observed at 1.61 V cell voltage, a 10

coupled with increasing global environmental concerns over

mA cm-2 water splitting current density is obtained from the

the use of such fuels. The electrocatalytic splitting of water i.e. hydrogen evolution reaction (HER), presents a simple

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Page 2 of 17

yet efficient method to produce H2, a cleaner, more sustain-

significant efforts to develop it as a straightforward and

able alternative to conventional fuels. Advanced catalysts

general strategy for hollow spheres with multiple shells.3, 10,

for the HER are therefore in high demand to obtain high

13-21, 23, 25

current densities at reduced

overpotentials.1

By using the carbonaceous microspheres as tem-

To date, alt-

plates, Wang group has developed a variety of multi-shelled

hough the best HER electrocatalysts are Pt-group metals,

metal oxide hollow microspheres (e.g., SnO214, Fe2O319,

their high cost and low abundance have become key barri-

ZnO25, Co3O426, TiO227, MnO228). Therefore, based on the

ers to large-scale application. On the other hand, it has

research of this template-assisted synthesis, quasi-one-

been demonstrated that the efficiency of the water-splitting

dimensional (1D) metal oxides with complex hollow struc-

reaction can be enhanced by using bifunctional HER and

tures is expected to achieve using non-spherical templates.

OER catalysts in alkaline media. With alkaline water elec-

Furthermore, quasi 1D complex hollow structure in water

trolysis already being widely deployed commercially, the

splitting are rarely reported, although metal oxides can

use of these catalysts offers an interesting prospect of fur-

show high catalytic activity.29-30 On the other hand, defect

ther simplifying the system and lowering the cost of produc-

engineering, especially in the case of surface oxygen va-

ing hydrogen.2 Therefore, the engineering of these high-

cancies, has been regarded as a good approach to control

activity, non-precious metal based, bifunctional catalysts

the electronic structure, number of active sites for catalysis,

capable of both OER and HER catalysis in alkaline media is

and the reactive species’ adsorption energies.31-33 The in-

of tremendous significance, and is fast becoming a topic of

corporation of oxygen vacancies is found to catalytically

focus for water splitting research.

activate neighboring atoms to enhance the active site’s re-

It has been widely acknowledged that the size, shape,

activity along with increasing the density of states ap-

composition, and surface structure of catalysts are im-

proaching the Fermi level. These effects combine to yield

portant parameters to determine the HER and OER catalyt-

improved electron transfer efficiency and the destabilization

ic activity.3-9 Being a novel and distinctive category of func-

of the metal-oxygen bonds, thereby allowing faster interme-

tional material, hollow nanostructures have drawn much

diate exchange effects and the superior electrocatalytic

attention to themselves because of their potential applica-

performance.34-35 As such, the design and construction of a

tions in catalysis, energy, and sensors.10-12 Over the past

quasi 1D complex hollow structure containing oxygen va-

decades, there have been great strides in the development

cancies would be highly sought after for water splitting ap-

of effective methods for the synthesis of diverse hollow

plications. In recent years, spinel transition metal oxides

structures.13-17 However, many of the reported hollow struc-

(TMOs), in particular the ternary and mixed oxide varieties,

tures have relatively straightforward, single-shell configura-

possessing attributes such as relative abundance, low-cost,

tions, which limit the opportunities to modulate their poten-

minimal toxicity, and rich redox chemistry, have been thor-

tial properties. Recently, there has been much activity sur-

oughly studied and show great promise as electrode mate-

rounding the rational design and preparation of hollow

rials for the storage and conversion of energy.36-37 As a key

structures with multilevel interior structures which not only

member of the TMO family, NiCo2O4 (NCO), an abundant

imbue them with high specific areas, multiphase heteroge-

multiple oxidation state ternary metal oxide, has shown

neous interfaces, and an abundance of internal voids, but

promise when applied in supercapacitors and batteries (in-

additionally result in tunable chemical and physical local

cluding metal-air batteries), among other energy storage

microenvironments.18-24 These structures may provide novel

solutions. The high conductivity and larger quantity of elec-

approaches to modify the catalyst’s characteristics for both

trochemically active sites and controlled structures, result in

fundamental studies and in different applications. Among

NCO demonstrating superior electrocatalytic activities to-

the various approaches for preparing multi-shelled hollow

ward oxygen reduction and evolution reactions.38-41 Howev-

structures, template-assisted synthesis is recognized as the

er, the application of NiCo2O4 as a bifunctional electrocata-

gold standard in the synthesis of highly complex, hollow

lyst in water splitting has rarely been reported.42-43 Recently,

particles. To our knowledge, Wang group first introduced

reduction treatment has proven effective to introduce oxy-

this template-assisted synthesis method and has devoted

gen vacancies into metal oxides and modify the intrinsic electronic structure of metal oxides, leading to improved

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electrocatalytic performance over the pristine oxides.44-45

set to 2 ºC min-1 in air to develop the necklace-like triple

Creating a hybrid of the aforementioned design paradigms,

shelled, concentric hollow structures. To investigate the

we expect that reduced NCO (R-NCO) with multiple hollow

effect of temperature, the temperature was varied from 310,

structures can serve as a superior bifunctional catalyst for

340, to 370 ºC with other conditions remaining constant,

the splitting of water.

and the obtained samples are denoted as NCO-310, NCO-

Herein, we report the rational synthesis of novel reduced necklace-like complex spinel transitional metal oxides (RTMOs)

hollow

structure,

including

NiCo2O4

340, and NCO-370, respectively. Synthesis of R-TMO. The reduced TMO were synthe-

(NCO),

sized by soaking the pristine powder in 1 M NaBH4 aqueous

CoMn2O4, and NiMn2O4. This strategy involves the facile

solution for 2 h, then collected by centrifugation, rinsed with

carbon template-assisted synthesis of complex hollow

DI water then subsequently kept for 12 h in a vacuum oven

structure, followed by reduction. This delicate manner can

maintained at 80 ºC to dry.

bring additional possibilities of engineering the necklace-like

Structure characterizations. Field emission scanning elec-

structure with a tunable hollow interior. Importantly, reduc-

tron microscope (FESEM), model JEOL JSM-7600F along

tion treatment of the multi-shelled hollow NCO successfully

with transmission electron microscope (TEM), model JEOL

introduces oxygen vacancies, which results in an increase

2100F, were used to analyze the morphology and micro-

in the number of active sites, enhanced capacity for elec-

structure of the as-prepared samples. The phase and con-

tron transport, and optimization of the adsorption energy,

stitution were characterized with a combination of X-ray

thereby significantly improving both the durability as well as

diffraction (XRD), model Bruker D8 ADVANCE, using a

catalytic activity for OER and HER simultaneously in a

wavelength of 1.5418 Å, Cu Kα, and X-ray Photoelectron

strongly alkaline medium. Furthermore, the basic R-NCO

Spectroscopy (XPS), model PHI, PHI5300 system with

water electrolyzer configured in a dual-electrode system

coupled with a Kratos Axis Ultra DLD electron spectrome-

needs only a 1.61 V cell voltage to achieve current density

ter. Fourier transform infrared (FTIR) spectroscopy was

of 10mA cm-2. The superior catalytic performance can be

conducted using a Perkin Elmer Spectrum GX instrument

ascribed to the reduction treatment and the novel, complex

between 400 to 4000 cm-1. Thermogravimetric analysis

hollow structures.

(TGA), model Q500 was performed with temperature ramp

EXPERIMENTAL SECTION

rate set at 10 ºC min-1 in air. The elemental analysis was

Synthesis of necklace-like carbon. Typically, sucrose (10 g) was dissolved in distilled water (80 mL) by stirring the mixture charged to a Teflon-lined autoclave (100 mL). The autoclave was then sealed, and transferred to an electric oven maintained at 190 ºC for 2 h. Afterwards, the black products obtained were rinsed with distilled (DI) water and ethanol repeatedly, then dried overnight at 80 ºC. Synthesis of TMO. Taking the preparation of NCO as an example. Necklace-like carbon (0.5 g) were dispersed in Ni(NO3)2 · 5H2O (10 M, 60 mL) and Co(NO3)2 · 5H2O aqueous solution (10 M, 60 mL) by agitation with a magnetic stirrer for 2 h followed by ultrasonication for 0.5 h. The obtained mixture was then charged to a Teflon-lined autoclave, sealed and heated to 180 ºC for 10 h to ensure Ni and Co ions adsorbed on the surface of necklace-like carbon sufficiently. Then, the Ni-Co/C precursor was obtained by centrifuge. The as-prepared precursor was subsequently

carried out using an inductively coupled plasma atomic emission spectroscope (ICP-AES), model Thermo Elemental IRIS 1000. Raman spectra measurements were recorded with a Renishaw Raman system, with excitation set at 514 nm. The photoluminescence (PL) spectra of the photocatalyst were detected with a Cary Eclipe florescence spectrometer (Varian, USA) with use of a Xe lamp as the excitation source. Co and Ni K edge XAFS (X-Ray Absorption Fine Structure spectra) were measured under room temperature using the transmission mode of the XAFCA beamline [beamline paper] in the Singapore Synchrotron Light Source. Extended X-ray absorption fine structure (EXAFS) data was interpreted utilizing WINXAS 3.1 code, where it was normalized, then transformed to momentum space (k) from the initial energy space. Background absorption was also eliminated and the x(k) function was extracted in the range of 1.5~10.6 Å-1. The Fourier transform

subject to calcination at 450 ºC for 1 h, with the ramp rate


3


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of x(k) weighted by k3, (i.e., k3x(k) in R space) was per-

sampling was employed for the Brillouin zone integrations.

formed using the Bessel function.

The bottoms layers of NiCo2O4 were fixed to model the bulk

Electrochemical

characterization.

All

electrochemical

measurements were conducted at room temperature by

properties of the catalyst. The hydrogen absorption energies were calculated relative to the energy of H2 (g) as:

∆E = E(slab+ H) − E(slab) − 12 E(H2 ) (1)

using Autolab electrochemical work station (Autolab Instrument) bipotentiostat. The Ag/AgCl electrode, Platinum foil, and nickel foam were respectively applied as the reference, counter, and the working electrodes with a 1.0 M KOH electrolyte. The catalyst slurry was prepared by mixing the catalyst (5mg), 5% Nafion (0.04 mL), ethanol (0.2 mL), along with DI water (0.8 mL). Drop casting was employed to fabricate the electrodes wherein the slurry was deposited onto 1 ×2 cm2 nickel foam (loading mass is approximately ∼2.5 mg

The associated free energy of H is determined from the equation:

∆G = ∆E + ∆ZPE − T∆S (2) where ∆ZPE is the change in zero-point energy, ∆S being the entropy change between the adsorbed state and gas phase, and T being room temperature (298 K). The free energy profile of the OER process in an alkaline

cm-2). The Pt/C (20 wt%) or RuO2 electrodes were also fab-

medium was calculated based on the approach that has

ricated by using the same procedure. Linear sweep volt-

been widely applied to investigate various catalysts. In an

ammetry was conducted at a 5 mV·s-1 scan rate to measure

alkaline medium, the overall OER process is assumed to be

the OER and HER catalyst performance of the various

4OH− → O2 + 2H2O + 4e− , which consists of four elemen-

working electrodes, with the polarization measurements

tary reaction steps as bellows:

being iR-corrected. Reversible hydrogen electrodes (RHE)

HO * + OH − → O * + H 2 O + e −

were used to calibrate the reference electrode. Cyclic voltammetry (CV) method was then employed to evaluate the double-layer capacitance (Cdl). The voltage window of cyclic voltammograms was 1.15−1.25 V vs RHE. Computational methodology. The absorption energies and free energies both in HER and OER were computed using DFT with implementation in the Vienna ab initio Simulation Package (VASP). In order to simulate the various

(1)

∆G1 = ∆G(HO* ) − ∆G(O* ) − ∆G(H 2O) − (e− − OH− ) − eU O* + OH− → HOO* + e−

(2)

∆G2 = ∆G (O * ) − ∆G ( HOO * ) − (e − − OH − ) − eU HOO* + OH − → OO* + H 2O + e −

(3)

∆G3 = ∆G(HOO* ) − ∆G(OO* ) − ∆G(H2O) − (e− − OH− ) − eU

OO* + OH− → HO* + O2 + e−

(4)

∆G4 = ∆G ( OO * ) − ∆G ( HO * ) − ∆G (O 2 ) − (e − − OH − ) − eU

electron-ion interactions, projector augmented wave (PAW)

where ∆G1-4 represent the free energies of the above four

potentials were utilized. Meanwhile, in order to compute the

reaction steps in the OER process. The free energies of

electronic exchange correlation energy, a generalized gra-

each

dient approximation (GGA) of the Perdew - Burke - Ern-

∆ G = ∆ E + ∆ ZPE − T ∆ S − eU . ∆E is the calculated DFT ener-

zerhof (PBE) was selected. The long-range interactions

gy. ∆ZPE and ∆S are the changes in zero point energy and

(van der Waals’ (vdWs) forces) between the catalyst and

entropy, respectively. These corrections are adopted from

the adsorbate was described by means of the PBE density

standard tables for gas molecules. U is the applied poten-

function along with vdWs correction (opt88-vdw-DF). The

tial. The free energies of H2O, O2 and OH- were calculated

plane wave cut-off energy was fixed at 400 eV. For the

by using DFT energy of H2O and H2 in the gas phase.

structural relaxation, the convergence criteria were set to be 10-6 eV for the electronic energy and to be 0.02 eV∙Å-1 for the residual forces, respectively. The unit cells of bulk NiCo2O4 have lattice parameters a = b = 5.81 Å and c = 8.15 Å. The vacuum layer thickness fixed at 18 Å for subsequent calculations in order to eliminate the influence of adjacent systems. A 3×3×1 Γ-centered Monkhorst–Pack k point

intermediate

are

obtained

by

RESULTS AND DISCUSSION The multistep synthesis process involves adsorption, calcination, and reduction. As illustrated in Scheme 1, firstly, templates of necklace-like carbonaceous spheres, obtained by a facile hydrothermal method, having average diameters approximately 1 μm are deployed to prepare the complex hollow structures. Secondly, necklace-like carbon can absorb metal precursors with the presence of abundant func-


4

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tional groups on the surface of the carbon (Figure S1 and

form with an interplanar spacing (~0.281 nm) of the lattice

S2 in Supporting Information). The surface functional

attributable to the NCO’s (220) plane. Nonetheless, the

groups on the carbon microspheres, such as -OH hydroxyl

presence of a disordered edge layer detected clearly on the

and -COOH carboxylic acid, allow for easy adsorption of

R-NCO but not on the pristine NCO surface (Figure S3 in

metal ions without any

modifications.13

In order to realize

Supporting Information). This indicates the reduction treat-

the penetration of metal ions deep into the carbon template,

ment employed would impact the surface structures of

the reaction solution undergoes hydrothermal reaction at

NCO, which might have a significant effect on the electro-

180 ºC continuously for 10 h. After calcination in air for the

chemical properties. Furthermore, the elemental mapping

removal of the carbon template, the absorbed carbon is

analysis in Figure 1e certifies the successful preparation of

transformed into necklace-like TMOs with complex hollow

the triple-shelled hollow product, while also offering compel-

structures. Additionally, the internal hollow shell could be

ling evidence of the homogenous presence and dispersion

tuned by modifying the calcination temperature. Thirdly, the

of the elements Co, Ni, and O within this unique structure.

necklace-like TMOs complex hollow structure is well pre-

To confirm the versatility of the method, necklace-like multi-

served after reduction by soaking in NaBH4 solution. The

shelled NiMn2O4, CoMn2O4, R-NiMn2O4, and R-CoMn2O4,

compared structures of R-NCO and NCO show the oxygen

are synthesized (Figure S5 and S6 in Supporting Infor-

vacancies can be produced during this step. Finally, the

mation). TEM and HRTEM images verify that the similar

necklace-like R-TMO with the complex hollow structure

features in morphology and uniformity were prepared suc-

could function as bifunctional electrocatalysts for HER and

cessfully using the template technique described with pre-

OER, demonstrating superior electrocatalytic performance

cise control of the thermal treatment. The chemical compo-

in water splitting. Furthermore, based on the first-principle

sitions of the samples are confirmed by ICP measurement

DFT calculations, the oxygen vacancies optimize the H at-

(Table S1 in Supporting Information). These examples illus-

om adsorption in HER and oxygen-intermediates adsorption

trate the versatility of the synthesis route described.

in OER, thereby resulting in the electrochemical performance improvements for the OER and HER.

that the peaks correspond to the NiCo2O4 cubic phase

Three different types of TMOs with a necklace-like multishelled

hollow

structure,

such

as

XRD patterns of both the pristine NCO and R-NCO show

NiCo2O4

(JCPDS: 73-1702), indicating the preservation of the NCO

(NCO),

crystal structure after reduction (Figure S7 in Supporting

CoMn2O4, and NiMn2O4, have been fabricated successfully

Information). The creation of superficial oxygen vacancies

as reported herein. NCO is used as the archetype to illus-

on the reduced NCO was confirmed by XPS, as shown in

trate the novel synthesis process. As shown in the FESEM

Figure 2a and 2b. A deeper analysis unveiled that the Co

images in Figure 1a, R-NCO presents a quasi-one-

2p3/2 and Ni 2p3/2 binding energies were shifted higher com-

dimensional structure with a length up to several microme-

pared with those of pristine NCO. Significantly, deconvolu-

ters. R-NCO nanoparticles form chains of inter-connected

tion of the O 1s band in the R-NCO yielded three peaks

spheres approximately 500 nm in diameter. TEM results

located at 529.3, 530.7, and 532.7 eV, that are attributed to

shown in Figure 1b and c clearly show that the R-NCO pre-

the metal-oxygen bonds, defect and surface adsorption.

sents a necklace-like structure, which is built up triple-

Relatively higher proportion of the peak at 530.7 eV is ob-

shelled hollow spheres. It is apparent that the overall neck-

served in R-NCO, again indicating the large number of su-

lace-like structure is well maintained after reduction, sug-

perficial defects like oxygen vacancies after reduction.46

gesting the excellent robustness of the nanostructures (Fig-

The XPS results indicate that the NCO after reduction pro-

ure S3 and Figure S4 in Supporting Information). Thus, it is

duce oxygen vacancies or defects near the surface of NCO.

concluded that the necklace-like precursors in fact act as

XAFS analysis was conducted to further disclose the dis-

the self-sacrificing templates for preparing the final neck-

tinct oxygen vacancies in the R-NCO. Figure 2c shows X-

lace-like NCO samples. High magnified TEM (HRTEM) is

ray absorption near edge structure (XANES) spectra for Co

used to further study the effects of reduction on the struc-

K-edge presenting higher intensity compared to the that of

ture of NCO samples. Figure 1d reveals that R-NCO is uni-

pristine NCO, indicating that the R-NCO is structurally more


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disordered than pristine NCO and the formation of oxygen

leads to a triple-shelled structure with an inner solid core.

vacancies in the NaBH4-treated NCO.13 In addition, the

By the time the temperature approaches 450 ºC, rapid

slight shift of the peak to low coordination is observed for

combustion of the composite spheres is likely to have oc-

the R-NCO, which is caused by the oxygen vacancies com-

curred due to the quick heating rates employed to achieve

pared to the pristine NCO. Fourier-transformed data in Fig-

the target temperature, with the eventual result of the heat

ure 2d obviously show the substantial lowering of the peak

treatments yielding multi-shelled hollow structures.25 The

intensities representing the Co coordination bonding in R-

complete removal of carbon template and formation of NCO

NCO. This parallels the reduction in coordination number

can be proved by TGA (Figure S13 in Supporting Infor-

for Co in R-NCO versus pristine NCO, thereby suggesting

mation). Therefore, it naturally follows that when the heat

the presence of a lower concentration of oxygen vacancy

treatment temperature is relatively low, the degradative

defects in the pristine NCO sample. Additionally, this de-

oxidation of the proceeds slowly, hence leading to a slow

crease in Co coordination number suggests a higher degree

shrinking of the carbonaceous microspheres which allow

of disorder in the system, signifying the existence of dan-

the metal ions to accumulate in the matrix of the carbona-

gling bonds and surface unsaturation. These distortions are

ceous template and crosslink, forming the shell of oxides.

likely to result in an increase of surface energy, which

Further increasing the temperature culminates in separation

should facilitate improved stability. Also, the result of Ni K-

between the shell of oxide and the eroding carbonaceous

edge XAFS spectra and corresponding Fourier-transformed

template due to the incompatible rates of formation and

data bears many similarities to the that of Co (Figure S8 in

disintegration respectively, which can form different shells

Supporting Information). Furthermore, XRD Rietveld re-

of NCO.47 Finally, the reduction of NCO by NaBH4 can pro-

finement, Raman and PL spectra were obtained for NCO

duce R-NCO.

and R-NCO (Figure S9, S10, and S11 and Table S2 in

The unique necklace-like multi-shelled hollow nanostruc-

Supporting Information). All these observations evidence

tures endows R-NCO with superior performance in electro-

the presence of oxygen vacancies in R-NCO.

catalytic reactions. The water splitting activity of the R-NCO

To understand the formation process of such unique mul-

is first examined in electrochemical OER employing a

ti-shelled NCO necklace-like hollow structures, we per-

standard three-electrode system in a 1 M KOH electrolyte.

formed temperature-dependent experiments to demonstrate

To investigate the effect of different shells on the catalytic

the continual evolution of morphology from a spherical core-

performance, polarization curves obtained using NCO with

single shell morphology to a spherical core-double-shelled

varying heat treatment temperatures are compared (Figure

morphology, and eventually to a multi-shelled, hollow

S14 in Supporting Information). The polarization curves of

sphere morphology. (Figure S12 in Supporting Information).

NCO and R-NCO in Figure 3a show the presence of an

Compared to the pristine necklace-like carbon at room tem-

oxidation peak around 1.32 V (prior to onset of OER), which

perature, there is no visible change after hydrothermal

can be attributed to the oxidation of Co2+ to Co3+ or Co4+

treatment to absorb metal ions (Figure S12a and b in Sup-

and Ni2+ to Ni3+ 30, 48-49. The R-NCO displays high OER per-

porting Information). Increasing the heat treatment tempera-

formance with a low onset potential at 1.44 V, whereas the

ture (310 ºC) resulted in the sample becoming rougher and

anodic currents measured for NCO and RuO2 renders high

the formation of a concentric shell surrounding a solid

onset potential of ∼1.48 V and 1.49 V respectively. In addi-

sphere - likely the result of carbonization accompanied by a

tion, R-NCO was shown to give a 10.0 mA cm-2 current

sharp volume contraction (Figure 8c in Supporting Infor-

density at 1.47 V, under that of NCO at 1.51 V and NCO

mation). When the temperature reaches 340 ºC, a certain

with other different shells. Noticeably, the OER activity of R-

depth of the outward facing surface of the core undergoes

NCO exceeds that of RuO2. Interestingly, the R-NCO’s out-

burning, which is accompanied by a contraction in volume

standing OER catalytic activity even surpasses many of the

of the inner portion of the core, leaving a spherical core-

state-of-the-art alkaline media electrocatalysts based on

double shelled structure (Figure 8d in Supporting Infor-

oxides (Table S3 in Supporting Information). Tafel plots

mation). Further increases in the temperature to 370 ºC

(Figure 3b) were also calculated to analyze the catalytic


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kinetics for the evolution of oxygen. The Tafel slope for R-

only allow the evolved bubbles of hydrogen and oxygen gas

NCO (50 mV decade-1) is significantly less than both NCO

to escape, but also promote the efficient interaction with the

(69 mV decade-1) and RuO2 (63 mV decade-1), but compa-

electrolyte, and aids in the diffusion and transport of the

rable with the best reported OER catalysts in published

various reactive species (such as -OH ions and water mole-

literature (Table S1 in Supporting Information).

cules) .8, 50 This unique structure also offers sufficient void

We next evaluated the catalytic activities of R-NCO and

space for hydrogen, oxygen, and electrolyte fast transport in

the comparative samples undergoing HER in an electrolyte

addition to the formation of local triple phase (solid-liquid-

of 1 M KOH. At the same for OER, R-NCO demonstrated

gas) regions - necessary for the transport of both products

exceptional electrocatalytic activity, displaying a low onset

and reactants during HER and OER.51 Second, reduction

potential of 90 mV while achieving excellent current densi-

treatment can provide a favorable electron transportation

ties of 10 and 50 mA cm-2 with overpotentials of 135 and

route, lowering the distance and barrier for electron

240 mV respectively (Figure 3c), whereas NCO displayed a

transport, thereby expediating the reaction kinetics as con-

less impressive performance having a 180 mV onset over-

firmed by the EIS measurements (Figure S21 in Supporting

potential while also requiring a 236 mV overpotential to at-

Information). The lower resistance of R-NCO with an inter-

tain 10 mA cm-2 current density. The OER performance of

connected multi-shelled architecture leads to the formation

R-NCO surpasses NCO with different heat treatment tem-

of a percolated network of conductive pathways which facili-

peratures (Figure S15 in Supporting Information). It should

tates charge separation.52 Third, the rich oxygen vacancies

be noted that R-NCO can produce 334 mV at 300 mA cm-2,

induced by the reduction treatment can afford numerous

notably exceeding Pt/C (305 mV at 300 mA cm-2). Pro-

active electrocatalytic sites, which was evaluated by elec-

cessing the Tafel plot data with linear fitting results in a

trochemical double-layer capacitance (Cdl). By calculating

Tafel slope of 52 mV dec-1 for R-NCO (Figure 3d). Further-

the gradient from the linear relationship of the current densi-

more, R-NCO possesses good cycling stability for OER and

ty against the scan, Cdl of R-NCO is confirmed at 45.7 mF

HER (Figure S16 in Supporting Information). The above

cm-2, which is superior to that of NCO (25.3 mF cm-2) and

results provide quite a good match with numerous other

NCO with different heat treatment, as observed in Figure

previously reported OER elecrocatalyst based on non-

S22, S23 and S24 in Supporting Information. The observed

noble-metals, although they are a little lower than Pt/C (Ta-

trend is nearly identical to the specific surface area trend

ble S4 in Supporting Information). Furthermore, the HER

(Figure S25 in Supporting Information).Since Cdl is directly

and OER performance of necklace-like multi-shelled hollow

correlated to the catalyst’s active surface area, these find-

NCO is better than that of hollow NCO with other controlled

ings show that multi-shelled hollow necklace NCO after

number of shells and bulk NCO obtained by a sol-gel meth-

reduction treatment are more effective in increasing active

od (Figure S17, S18 and S19 in Supporting Information).

surface area compared to NCO; thus, higher exposure to

With regard to the CoMn2O4 and NiMn2O4, R-CoMn2O4 and

and improved utilization efficiency of the electroactive sites

R-NiMn2O4 can much improve the HER and OER electro-

on the greater active surface area of NCO significantly in-

catalytic activity (Figure S20 in Supporting Information).

fluence its ultrahigh electrocatalytic activity.53 Furthermore,

This further demonstrates the benefits of the reduction pro-

the accessible Co and Ni cations surrounding the oxygen

cess and the multi-shelled architecture which increases the

deficient sites potentially could preferentially adsorb and

number of active sites and facilitates electron-transfer to

have interactions with OH- to promote the transfer of elec-

improve electrocatalysis kinetics.

trons between the surface of the catalyst and the intermedi-

The superior performance of R-NCO, in comparison to that of NCO and sol-gel derived NCO, can be traced to a number of beneficial characteristics, which form the basis for the development of advanced multi-shelled hollow structures and reduction treatment. First, the multi-shelled hollow structure possesses numerous accessible channels to not

ate reaction species, which is advantageous for subsequent OER and HER process.54-55 The conclusion can thus be drawn that the prevalence of defect-rich surfaces of R-NCO and the unique necklace-like multi-shelled hollow structure synergistically render R-NCO with excellent catalytic activity in both HER and OER.


7


Page 8 of 17

Encouraged by the exceptional bifunctional property of R-

the PDOS of Co was significantly modulated and the spin

NCO, the overall water splitting is further performed in an

polarization of Co was greatly improved, which may enable

electrolyzer of a two-electrode conﬁguration by using the R-

Co to be more active catalytic sites, promoting the water

NCO as both the positive and negative electrode. As shown

splitting reactions.

in Figure 3e, R-NCO electrodes afford a current density of

As concerns HER, ΔGH* is usually used as a crucial de-

10 mA cm-2 at the operating potential of 1.61 V in 1.0 M

scriptor in predictions of the theoretical activity for hydrogen

KOH, which is lower than NCO and comparable to the

evolution reaction in a basic electrolyte. A good catalyst

benchmarking RuO2//Pt/C electrode. Furthermore, the cata-

must have a moderate H adsorption energy approximately 0

lytic activity of the R-NCO based electrolyzer is among the

eV, resulting in optimal HER activity having reduced reac-

best recently reported for water-splitting catalyst materials

tion barriers owing to the equilibrium between the adsorp-

(Table S3 in Supporting Information). Parallelly, R-NCO

tion/desorption steps. The optimized structures of H adsorp-

electrodes demonstrate good long-term stability. At applied

tion at NCO and R-NCO are illustrated in Figure S28 in

potentials of 1.61 V, maintenance of a 10 mA cm-2 current

Supporting Information, respectively. Figure 4b shows the

density for 50 h presented no difficulty, with bubbles contin-

computed HER free energy plots. Although the ΔGH* of Pt is

uously evolved and released from the surfaces of both elec-

the smallest, R-NCO exhibits lower ΔGH* than pristine NCO,

trodes (inset in Figure 3f). The XRD spectra of the catalyst

indicating that reduction will undoubtedly enhance the HER

material after 50 h in operation indicated no deviation from

performance in a basic solution.

the original phase composition of R-NCO (Figure S26 in Supporting Information). In addition, the surface morphology of R-NCO after the durability test is also stable (Figure S27 in Supporting Information). These results illuminate several aspects of rational design for low-cost and energyefficient water splitting bifunctional electrocatalysts.

Generally, OER typically undergoes a four-electron step process in alkali media.59 Since the elementary steps may be correlated with each other, we analyzed the adsorption energies of OOH*, OH*, O* and O2 species adsorbed on RNCO during the OER process. The molecular structures after the adsorptions of OOH*, OH*, and O* on the active

To probe deeper into the HER and OER catalytic mecha-

site of Co in R-NCO and NCO in an alkaline environment

nisms of R-NCO, first-principle calculations using DFT

are given in Figure 4c and Figure S29 in Supporting Infor-

methods were performed to uncover the electronic struc-

mation. Based on these molecular structures, reaction free

tures and catalytic reaction schemes for R-NCO (Figure 4).

energy on all possible sites of R-NCO can be calculated by

A recent study demonstrated Co cores having octahedral

the DFT method. Figure 4d and e show the energy dia-

geometries showed greater activity toward the adsorption of

grams of OER, where free energies of various intermedi-

water molecules near oxygen vacancies on the surface to

ates adsorbed by NCO and R-NCO at different constant

obtain a coordination number of 6.56-57 We therefore based

potentials were calculated. At zero potential, all the steps

the active site for HER on the Co atoms for the purpose of

are endothermal reaction for NCO and R-NCO. At the equi-

the DFT computation. Figure 4a displays the projected den-

librium potential (U = 1.23 V), HO* and HOO* formation is

sity of states (PDOS) of Co d orbital，Ni d orbital and O p

exothermic, while the other steps are endothermic on R-

orbital. Compared with the pristine NCO, the PDOS of Co d

NCO. Therefore, for pristine NCO, the conversion of OOH*

orbital in R-NCO shifts to the low-energy direction and pre-

to OO* is the rate-determining step (RDS) since it has the

sents broader peaks, which is induced by the oxygen va-

largest reaction free energy of 1.76 eV at zero potential.

cancy with the reduction treatment. A leftward shift of the

Contrary to that of pristine NCO, the RDS for R-NCO is the

DOS is indicative of the shift in the distribution of electrons

formation of O2 molecular, which has a lowered free binding

in the d-band away from the Fermi level. This leads to the

energy of 1.30 eV at zero potential. Therefore, R-NCO has

catalyst being less active in chemical bonding with other

a better OER activity than pristine NCO, largely agree with

species, thus lowering adsorption energy.58 The weak ad-

the above mentioned experiment results. Overall, our com-

sorption of intermediates may promote the HER and OER.

putations show that the reduction treatment reduces the

In other words, with the introduction of oxygen vacancies,

energy barriers for every step, thereby lowering the free


8

Page 9 of 17 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


energies of each elementary reaction step, thus fulfilling an important function in influencing the catalytic activity.

In summary, uniform necklace-like multi-shelled R-TMO with rich oxygen vacancies was successfully synthesized by versatile

sacrificial

This work was supported by the Singapore National Research Foundation (NRF-CRP10-2012-06), China Jiangsu

CONCLUSIONS

a

ACKNOWLEDGMENT

template

assisted

“adsorption-

calcination-reduction” strategy capable of controlling the interior architecture and number of shells. The multi-shelled hollow necklace-like R-TMO demonstrated better OER and HER activity, improved kinetics, and durability when compared to pristine TMO. The water splitting performance was found to be superior when contrasted with other reported oxide-based electrodes to the best of our knowledge. The outstanding performance can be attributed to the unique necklace-like hollow structure and reduction treatment, resulting in greater active surface areas, improved mechanical stability, and enhanced mass and charge transport. The theoretical calculations further confirm that the optimized intermediate bindings by reduction contribute greatly toward improving the NCO’s activity in HER and OER. Therefore, our proposed synthesis strategy may enrich our knowledge and material design to provide intriguing possibilities to develop many other oxides, sulfide and selenides with similar unique structures toward affordable energy and catalysis.

Specially Appointed Professor, the Fundamental Research Funds

for

the

Central

Universities

(NE2017004,

NS2018040), National Natural Science Foundation of China (No. 21542017), Jiangsu Provincial Founds for Natural Science Foundation (BK20170793), Nankai 111 project, No. B12015.

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Figure Captions


11


Page 12 of 17

Scheme 1. 1 a) Schematic illustration of the formation process of R-TMO with a necklace-like multi-shelled hollow structure for water splitting. I) The absorption of metal ions on the carbon, II) calcination of the absorbed carbon and III) reduction of the TMO to obtain R-TMO with a necklace-like multi-shelled hollow structure. b) Schematic illustration of creating oxygen vacancy defects on the surface of NCO after reduction, which is applied as a bifunctional electrocatalyst for water splitting to produce H2 and O2 in 1.0 M KOH aqueous solution.


12

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Figure 1. a) SEM image, b, c) TEM, and d) HRTEM images of R-NCO; e) elemental mapping of Ni, Co, and O element in R-NCO, respectively. The inset in a) and b) show the corresponding magnified image of a single cracked nanosphere and a photograph of necklace.


13


Page 14 of 17

Figure 2. a) Co 2p and b) O 1s XPS spectra of pristine NCO and R-NCO; c) Co K-edge EXAFS data and d) the corresponding k3-weighted Fourier-transformed data of pristine NCO and R-NCO.


14

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Figure 3. OER and HER electrocatalytic properties of R-NCO and NCO. a) Polarization curves of carbon, NCO, R-NCO, and RuO2 for OER; and b) their Tafel plots; c) Polarization curves of carbon, NCO, R-NCO, and Pt for HER, and d) their Tafel plots; e) Polarization curves of RNCO served as both cathode and anode electrocatalysts in a two-electrode configuration at a scan rate of 5 mV s-1 in 1.0 M KOH; f) Current density vs time curves of HER and OER with RNCO recorded for over 50 h.


15


Page 16 of 17

Figure 4. a) Calculated DOS curves for pristine NCO and R-NCO; b) Calculated free energy diagram of the HER on pristine NCO and R-NCO; c) Optimized geometry of adsorption structure of OH*, O*, OOH*, and O2 on R-NCO model, respectively; d and e) Schematic illustration of reaction paths for OER on pristine NCO and R-NCO at d) zero potential and e) equilibrium potential at 1.23 V.


16

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SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).


17

Necklace-like Multishelled Hollow Spinel Oxides with Oxygen

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