Cobalt oxyhydroxide with highly porous structures as active and stable phase for efficient water oxidation

Accepted Manuscript Cobalt oxyhydroxide with highly porous structures as active and stable phase for efficient water oxidation Jian Du, Chao Li, Xilong Wang, Timothy G.J. Jones, Han-Pu Liang PII:

S0013-4686(19)30340-8

DOI:

https://doi.org/10.1016/j.electacta.2019.02.083

Reference:

EA 33674

To appear in:

Electrochimica Acta

Received Date: 17 December 2018 Revised Date:

1 February 2019

Accepted Date: 20 February 2019

Please cite this article as: J. Du, C. Li, X. Wang, T.G.J. Jones, H.-P. Liang, Cobalt oxyhydroxide with highly porous structures as active and stable phase for efficient water oxidation, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.02.083. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

ACCEPTED MANUSCRIPT

Cobalt Oxyhydroxide with Highly Porous Structures as Active and Stable Phase for Efficient Water Oxidation

†

RI PT

Jian Du,†, § Chao Li,† Xilong Wang,† Timothy G. J. Jones,‡ Han-Pu Liang†, ǁ, *

Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China.

Center of Materials Science and Optoelectronics Engineering，University of Chinese

SC

ǁ

‡

M AN U

Academy of Sciences, Beijing 100049, P. R. China

Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge, CB3 0EL, UK.

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

TE D

§

* Corresponding author. Email: [email protected] Abstract

EP

Providing the direct experimental evidence to confirm active and stable phase in

AC C

cobalt-based electrocatalysts for oxygen evolution reaction (OER) in alkaline medium is crucial to design and fabricate high-performance electrocatalysts. Herein, we report the in situ fabrication of pure cobalt oxyhydroxide (CoOOH) phase with highly porous structures during OER testing process, which exhibits outstanding performance for OER with a low overpotential of 245 mV at 10 mA cm-2 and retain stable catalytic activity for 250 h with no appreciable decay. Experimental characterizations revealed that the superior catalytic activity of as-formed CoOOH

ACCEPTED MANUSCRIPT phase mainly arises from the highly porous structures, which endows the electrocatalyst with high specific surface area (~ 200 m2 g-1), numerous accessible channels and abundant active sites. Evidence has been produced showing that the in

RI PT

situ formed pure CoOOH is the active and stable phase in cobalt-based electrocatalysts for OER in alkaline medium. In addition, the synthetic strategy

conditions using electrochemical oxidation method.

SC

applied here could provide a novel way to fabricate nanoporous structures under mild

M AN U

Keywords: Pure CoOOH phase, Highly porous structures, Water oxidation, XAFS, Active phase

1. Introduction

TE D

The electrochemical water splitting has attracted considerable interest because it provides an effective way for the generation of clean and sustainable energy sources to alleviate the energy crisis and environmental issues [1-8]. Nevertheless, the

EP

oxidative half reaction of oxygen evolution reaction (OER) on anodic electrode is

AC C

kinetically sluggish due to the multiple proton-coupled electron transfer process, thus leading to a large overpotential and hindering the large-scale application of electrochemical water splitting [9, 10]. Hence, it is necessary to deploy an effective electrocatalyst to expedite this inherently sluggish process and reduce the overpotential. To date, the most active electrocatalysts for OER are RuO2 or IrO2, but their scarcity and high costs severely limit their application [11, 12]. Thus, it is of significant importance to develop highly active and stable OER electrocatalyst using earth-abundant elements.

ACCEPTED MANUSCRIPT In the past few years, there has been a dramatic growth of interest in the development of cobalt-based electrocatalysts for OER in alkaline environment, such as sulfide [13, 14], selenide [15], telluride [16], nitride [17], phosphide [18], oxide

RI PT

[19], oxide perovskites [6], hydroxide [20] and oxyhydroxide [21]. It has been found that these cobalt-based electrocatalysts could be able to form a metal oxyhydroxide layer at the surface of electrocatalysts when being operated at potentials significantly

SC

above the thermodynamic requirements with advanced in situ X-ray techniques [22,

M AN U

23]. Consequently, these electrocatalysts would be in situ transformed into the electrocatalysts/CoOOH layer hybrids during OER testing in alkaline medium. Furthermore, it has been confirmed by Bell et al.[24] that the formed CoOOH is the major active phase for OER in alkaline medium through the density functional theory

TE D

(DFT) calculations. However, it is worth noting that these efficient electrocatalysts in reported literatures are hybrids and no pure CoOOH phase formed during OER testing was reported to provide the direct evidence to consolidate the fact that CoOOH is the

EP

active and stable phase for OER in Co-based electrocatalysts.

AC C

Herein, we demonstrate the first experimental verification that pure CoOOH phase can be formed and works as highly active and stable phase for OER during electrochemical testing in alkaline medium. In particular, the generated pure CoOOH phase features highly porous structures with a high specific surface area of 200 m2 g-1 and exhibit excellent OER performance with a low overpotential of 245 mV at the current density of 10 mA cm-2, which are among the best of the reported Co-based electrocatalysts. X-ray diffraction (XRD) reveals that the pure CoOOH maintains

ACCEPTED MANUSCRIPT stable phase during OER testing. Furthermore, the X-ray absorption fine structure spectroscopy (XAFS) demonstrates that the electronic structure of formed pure CoOOH phase is distorted and provides abundant active sites to accelerate the

RI PT

electron transfer rate, thereby leading to the enhancement of the electrocatalytic activity. 2. Experimental section

SC

2.1 Chemicals

M AN U

Cobalt (II) acetate tetrahydrate (C4H6CoO4•4H2O), Poly(N-vinyl-2-pyrrolidone) (PVP, Mw=55000), sodium hypochlorite (NaClO) solution, commercial IrO2 and RuO2 were bought from Aladdin Industrial Corporation. Isopropanol, ethylene glycol and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent

TE D

Beijing Co., Ltd. Nafion solution (5 wt. %) was supplied by DuPont. Ultra-pure deionized water (18.2 MΩ/cm) was used in all experiments. All the chemicals were

EP

used as received.

AC C

2.2 Synthesis of cobalt alkoxyacetate (CoEGAc) nanodisks 0.8 g Cobalt (II) acetate tetrahydrate (C4H6CoO4•4H2O) and 0.6 g PVP was added

into a solution of 100 mL ethylene glycol and 2.0 mL ultra-pure deionized water. After vigorous stirring for 30 min, the mixture was transferred into a 250 mL Teflon lined autoclave, sealed and heated at 180 oC for 3 h. The autoclave was cooled down to room temperature naturally. The final pink product was collected by centrifuging

ACCEPTED MANUSCRIPT the mixture, washed with isopropanol for three times, and then dried at 40 oC for further characterization.

RI PT

2.3 Synthesis of standard CoOOH

A solution of 10 mL (1.0 M) KOH was dropwise added into 10 mL (0.5 M) solution of C4H6CoO4•4H2O under stirring and pink precipitations of Co(OH)2 was

SC

obtained. Then, 10 mL (0.5 M) NaClO solution was dropped into the above suspensions to oxide pink Co(OH)2 into CoOOH. Later, the suspension was

overnight.

2.4 Materials characterization

M AN U

centrifuged, washed with deionized water for several times and then dried at 40 °C for

TE D

Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advanced diffractometer using Cu Kα radiation. Scanning electron microscopy (SEM)

EP

and transmission electron microscopy (TEM) images were taken using a Hitachi S-4800 microscope and a FEI Tecnai T20, respectively. High resolution transmission

AC C

electron microscopy (HRTEM) images and elemental mapping was recorded using FEI Tecnai F20 microscope with an accelerating voltage of 200 kV. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained on Varian 800 FTIR spectrometer using the potassium bromide (KBr) pellet technique. Raman spectra were recorded on DXR Raman microscope with a laser excitation of 532 nm. N2 adsorption-desorption measurements were conducted on Autosorb-iQ adsorption analyzer at 77 K. Prior to the measurements, the samples were degassed at 50 °C

ACCEPTED MANUSCRIPT under vacuum for 5 h. The specific surface area of the products was estimated by method of Brunauer-Emmett-Teller (BET). Pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method from desorption isotherm. X-ray

RI PT

photoelectron spectroscopy (XPS) was collected on a K-Alpha X-ray photoelectron spectrometer (Thermo Scientific) with Al Kα micro focused monochromated X-ray source. Atomic force microscopy (Multimode 8, AFM) with PFT-QNM mode was

SC

carried out to characterize the surface morphology. X-ray absorption spectroscopy

M AN U

(XAFS) measurements of Co K-edge were conducted on the 06ID superconducting wiggler sourced hard X-ray microanalysis beamline at the Beijing Synchrotron Radiation Facility. To obtain the quantitative structural parameters around central atoms, least-squares curve parameter fitting was performed using the ARTEMIS

TE D

module of IFEFFIT software packages. The corresponding EXAFS fitting curves, CN (coordination number), R (Å) (bond distance) and the Debye-Waller factor (σ2) are

EP

given in Fig. S15 and Table S2, respectively.

AC C

2.5 Electrochemical measurement

Electrochemical measurements were performed in a three-electrode system at an

electrochemical station (CHI 660E). Hg/HgO electrode and Pt wire was used as reference and counter electrode, respectively. All measured potential vs. Hg/HgO were converted to the reversible hydrogen electrode (RHE) via the Nernst equation (1). ERHE=EHg/HgO + 0.0591pH + E0Hg/HgO (1)

ACCEPTED MANUSCRIPT The overpotential (η) for OER was calculated as the follow equation (2).

η=ERHE - 1.23 (2)

RI PT

The electrolyte was 1.0 M KOH aqueous solution and it was deaerated with a high-purity N2 flow for 30 minutes prior to oxidation. The working electrode was a thin layer of Nafion-impregnated catalyst cast on a glassy carbon rotating disk

SC

electrode (RDE) as detailed in the following: 4.5 mg pink CoEGAc precursor and 10 µL of Nafion solution (5 wt. %) were dispersed in 990 µL of absolute ethanol solution

M AN U

by sonicating for 3 min. Then, 5 µL of this solution was loaded onto the polished glassy carbon electrode with a diameter of 4 mm (0.1256 cm2). To load the catalyst on carbon cloth, 12 mg of pink precursor and 10 µL of Nafion solution (5 wt. %) were firstly added in 990 µL of absolute ethanol solution and sonicated for 3 min. Next,

TE D

100 µL of suspension was dropped on carbon cloth with a geometric area of 1 cm2. The electrochemical oxidation synthesis was conducted by applying a galvanostatic

EP

current density of 10 mA cm-2 to the precursor of the CoEGAc nanodisks for 3 h and

AC C

20 h, respectively. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 0.1 Hz at 1.55 V vs. RHE with a perturbation of 5 mV. Linear sweep voltammetry (LSV) was recorded with a scan rate of 10 mV s-1 at 1600 rpm after iR-correction. For comparison, commercial RuO2 and IrO2 were directly tested under the same conditions without the electrochemical oxidation process and the loading amount was 0.16 mg cm-2 in each case. Tafel slopes were calculated from the corresponding LSV curves by plotting overpotential (η) against log (J). The stability of the catalyst was assessed by chronopotentiometry at a constant

ACCEPTED MANUSCRIPT current density of 10 mA cm-2. The electrochemical surface area (ECSA) was estimated by measuring the capacitive current associated with double-layer charging from the scan-rate dependence of CVs. For this reason, the non-faradaic potential

RI PT

window of CVs was 0.28-0.36 V vs. Hg/HgO. The double layer capacitance (Cdl) was estimated by plotting the △J=(Ja-Jc) at 0.32 V vs. Hg/HgO against the scan rate. Where, Ja and Jc are the anodic and cathodic current densities, respectively. The scan

AC C

EP

TE D

3. Results and discussion

M AN U

linear slope was twice the value of Cdl.

SC

rates were 6 mV s-1, 8 mV s-1, 10 mV s-1, 12 mV s-1, 14 mV s-1 and 16 mV s-1. The

Fig. 1 Typical (a-c) SEM images and (d-f) TEM images of pristine CoEGAc nanodisks. Firstly, CoEGAc precursor was fabricated by a facile hydrothermal synthesis method (experimental section in supporting information) and a plausible mechanism for the formation of the CoEGAc precursor had ever been proposed by Chakroune et

ACCEPTED MANUSCRIPT al.[25] and Cao et al.[26] The scanning electron microscopy (SEM) image of the as-synthesized CoEGAc precursor in Fig. 1a shows that they are relatively uniform nanodisk structures with a diameter varying from 3 µm to 4 µm. Fig. 1b gives the

RI PT

high-magnification SEM image of one individual nanodisk, in which the smooth surface could be observed. The average thickness of the nanodisk is measured to be 40-50 nm from the cross-section SEM image in Fig. 1c. Moreover, the transmission

SC

electron microscopy (TEM) images in Fig. 1d-f further confirm that these nanodisks

M AN U

are nonporous structures. The crystal structure of CoEGAc nanodisk is characterized by X-ray diffraction (XRD) and shown in Fig. S1. It is evident that one sharp peak is present at 10.4 o, which is a typical feature from the coordination of organic molecules with Co (II) cations and matches well with the reported crystal structure of CoEGAc

EP

CoOOH-3 CoOOH-20

AC C

Intensity (a. u.)

of CoEGAc precursor.

TE D

[26]. No signals from other crystal impurities were detected, indicating the high purity

JCPDS No. 07-0169, CoOOH

10

20

30

40

50

60

70

2 Theta (degree) Fig. 2 XRD patterns of formed nanoporous CoOOH-3 and CoOOH-20 nanodisks.

ACCEPTED MANUSCRIPT Then, the as-synthesized CoEGAc precursor was directly used as electrocatalyst for OER testing by applying a galvanostatic current density of 10 mA cm-2 in 1.0 M KOH solution for 20 h. It could be found from the chronopotentiometric curve in Fig. S2

RI PT

that the potential continues to decrease sharply in the first 3 h and then slowly decline to reach a stable status after 16 h. Two intermediate samples after electrochemical testing for 3 h and 20 h were collected and analyzed. The corresponding XRD

SC

patterns of these two intermediate samples are shown in Fig. 1g, where two broad

M AN U

diffraction peaks located at 20.2 º and 38.8 º and two weak peaks at 50.6 º and 65.3 º are recorded. These diffraction peaks could be indexed to the diffraction of (003), (012), (015) and (110) planes of β-CoOOH phase according to the JCPDS No. 07-0169. Moreover, it should be mentioned that both the patterns are similar,

TE D

suggesting that the formed CoOOH phase are stable during OER testing. In addition, it is evident that all the diffraction peaks associated with the CoEGAc is absent upon careful comparison with the XRD pattern of pristine CoEGAc (Fig. S1). No other

EP

crystal impurities were detected. This result suggests that CoEGAc has already been

AC C

completely transformed into pure CoOOH phase after electrochemical testing for 3 h and 20 h. Thus, the corresponding samples are named CoOOH-3 and CoOOH-20, respectively.

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 3 (a-b) TEM and (c) HRTEM images of formed nanoporous CoOOH-3 nanodisks. (d-e) TEM and (f) HRTEM images of formed nanoporous CoOOH-20

AC C

nanodisk.

EP

nanodisks. (g) Elemental mapping images of one individual nanoporous CoOOH-20

The typical low-magnification TEM (Fig. 1a and 1d) and SEM (Fig. S3a-d) images

of CoOOH-3 and CoOOH-20 show that the formed pure CoOOH phase still keep similar morphology and thickness to CoEGAc nanodisks. The high-magnification TEM images of CoOOH-3 in Fig. 1b and CoOOH-20 in Fig. 1e confirm that both CoOOH nanodisks are nanoporous structures and the diameter of pores are estimated to be 3~4 nm. Moreover, the high-magnification SEM images of CoOOH-3 in Fig.

ACCEPTED MANUSCRIPT S3a and CoOOH-20 in Fig. S3b show the presence of interconnected small nanoparticles on the surface of nanodisks and the diameter of these nanoparticles are approximately 15 nm and 17 nm, respectively. It appears that longer testing time

RI PT

causes the formation of larger nanoparticles and rougher surface. Fig. 3c and 3f show the high-resolution TEM (HRTEM) images of nanoporous CoOOH-3 and CoOOH-20 nanodisks. The visible lattice fringes in both images, with an interplanar spacing of

SC

0.231 nm, can be assigned to the (012) plane of β-CoOOH. The high-angle annular

M AN U

dark field (HAADF)/scanning transmission electron microscopy (STEM) image and the corresponding elemental mapping images of one nanoporous CoOOH-20 nanodisk in Fig. 3g suggest that the nanodisk is consisted of Co and O elements, which are uniformly distributed throughout the whole nanodisk. In addition, the

TE D

atomic force microscopy (AFM) images in Fig. S4 further confirm the presence of small nanoparticles and the formation of rough surface on the CoOOH-3 and CoOOH-20 nanodisks, compared to the smooth surface of CoEGAc nanodisk. The

EP

thickness of CoEGAc, CoOOH-3 and CoOOH-20 are around 50 nm. Based on the

AC C

analyses of SEM, TEM and AFM, it could be envisaged that the nanopores are likely originated from the internanoparticle spaces in the CoOOH nanodisks due to the formation of interconnected nanoparticles. The specific surface area and highly porous nature of formed CoOOH phase is

further

investigated

by

Brunauer-Emmett-Teller

(BET)

gas

sorptometry

measurements. Fig. 4a compares the N2 adsorption-desorption isotherms of pristine CoEGAc precursors, nanoporous CoOOH-3 and CoOOH-20 nanodisks. Both of the

ACCEPTED MANUSCRIPT CoOOH-3 and CoOOH-20 nanodisks are found to exhibit typical type IV hysteresis loop, which is characteristic of mesoporous materials [27, 28]. According to BET analysis, the total specific area of nanoporous CoOOH-3 and CoOOH-20 nanodisks

RI PT

are 201.8 m2 g-1 and 193.2 m2 g-1, respectively, whereas the precursor of CoEGAc has a low specific surface area of 35.5 m2 g-1. Fig. 4b gives the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution calculated from the desorption

SC

curves in Fig. 4a, indicating that the average pore size distribution of CoOOH-3 and

M AN U

CoOOH-20 nanodisks are 3.4 nm and 3.8 nm, respectively. Therefore, it could be perceived that the remarkably enlarged specific surface area of CoOOH-3 and CoOOH-20 nanodisks could be attributed to the formation of numerous nanopores when compared with that of CoEGAc nanodisks.

(b) 0.010

200

50 0 0.0

0.2

EP

100

0.4

0.6

0.8

-1

0.004 0.002 0.000

1.0

Relative pressure (P/P0)

AC C

(c)

0.006

10

2

2+

Co 2p

CoOOH-20

4000 3500 3000 2500 2000 1500 1000 500 -1

Wavelength (cm )

Intensity (a. u.)

CoOOH-3

100

Pore width (nm)

(d)

CoEGAc

Intensity (a. u.)

CoEGAc CoOOH-3 CoOOH-20

3

150

0.008

-1

TE D

dV/dW (cm g nm )

CoEGAc CoOOH-3 CoOOH-20

3

-1

Quantity absorbed (cm g )

(a)

Co sat.

CoEGAc

2+

Co

sat. 3+

CoOOH-3

3+

Co

Co sat.

3+

CoOOH-20

Co

sat.

3+

Co

815 810 805 800 795 790 785 780 775 770

Binding energy (eV)

Fig. 4 (a) Nitrogen adsorption/desorption isotherms, (b) pore size distribution plot

ACCEPTED MANUSCRIPT calculated by the BJH formula in desorption branch isotherm, (c) FT-IR spectra and (d) high-resolution of Co 2p spectra of pristine CoEGAc nanodisks, formed nanoporous CoOOH-3 and CoOOH-20 nanodisks.

RI PT

Fig. 4c details the Fourier transform infrared (FT-IR) spectroscopy of pristine CoEGAc, CoOOH-3 and CoOOH-20 nanodisks. It can be clearly seen that both the spectra of nanoporous CoOOH-3 and CoOOH-20 nanodisks show a pronounced

SC

characteristic peak of CoOOH at 582 cm-1, which is in agreement with that reported

M AN U

by Robert G. D. et al.[29] In the case of pristine CoEGAc, two sharply peaks are observed in 2852 cm-1 and 1095 cm-1, respectively, which could be attributed to the stretching vibration of –CH2 and –C-OH groups of C2H5O2- anions. Meanwhile, the broad bands in 1636 cm-1 and 1351 cm-1 also confirm the stretching vibrations of

TE D

COO- groups of acetate ions. Thus, it seems that the precursor appears to be cobalt alkoxyacetate, which is consistent with that reported by Chakroune et al.[25] and Cao et al.[26] By comparing these FT-IR spectra, it can be seen that the bands associated

EP

with the organic functional groups are absent from those of nanoporous CoOOH-3

AC C

and CoOOH-20 nanodisks, indicating the complete removal of organic components during OER testing, which is also responsible for the formation of nanopores. Fig. 4d compares the high-resolution X-ray photoelectron spectroscopy (XPS)

spectra of Co 2p of CoEGAc, CoOOH-3 and CoOOH-20. It is evident that both the spectra of nanoporous CoOOH-3 and CoOOH-20 nanodisks show two similar major peaks at binding energies of 779.7 eV and 794.7 eV, which could be assigned to the Co3+ oxidation state of CoOOH and are consistent with previous report of CoOOH

ACCEPTED MANUSCRIPT [30, 31]. In contrast, in the case of CoEGAc, two major peaks were located at binding energies of ~ 780.9 eV and ~ 796.9 eV, which can be assigned to the Co 2p3/2 and Co 2p1/2 of Co2+ oxidation state, along with two shake-up satellite (abbreviated as “sat.” )

RI PT

peaks at ~ 787.1 and ~ 803.0 eV. All these features indicated that the Co2+ in the CoEGAc has been completely oxidized into Co3+ oxidation states after OER testing for 3 h and 20 h. Raman spectroscopy has been demonstrated to be a very sensitive

SC

and useful technique to characterize carbon material, such as carbon nanotubes and

M AN U

graphene [10]. Raman spectra in Fig. S5 confirm that well-known D and G bands from typical carbon material, normally in the range of 1700-1200 cm-1, are absent from these samples, indicating the absence of residual carbon. Therefore, all the above results clearly demonstrate that CoEGAc nanodisks have been completely in

TE D

situ transformed into pure nanoporous CoOOH nanodisks. Based on the above analyses, a plausible mechanism for the formation of CoOOH phase could be concluded. During the OER process, the operating potential seems to cause the

EP

decomposition of CoEGAc. The oxidation of Co2+ into Co3+ in the presence of OH- in

AC C

electrolyte results in the rupture of the chemical bonds between Co2+ and organic molecules to form CoOOH phase. Therefore, numerous nanopores are formed in the nanodisks because of the removal of fractured organic molecules.

ACCEPTED MANUSCRIPT (a)

IrO2 CoOOH-20 on CC

40 20 0 1.2

1.3

1.5

1.6

Current density (mA cm )

0.25 0.20 0.15

(d) 0.55 0.50

Overpotential (V)

η=300 mV 80 60

20 0

0.45 0.40 0.35 0.30

CoOOH-3 CoOOH-20 on GC RuO2

C GC on GC on CC on GC on G 0 3 on uO 2 OH- OOH-20oOOH-2 rO 2 R I O o C C Co

-1 mV .2 ec d 98 V CoOOH-20 on CC 2m 87-1. ec mV d-1 59.9 V dec -1 53.7 m V dec m .9 3 5

IrO2

0.25

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

0.4

-2

3.9 mF cm

6

8

10

12

14

EP

0.0

-1

Scan rates (mV s )

16

Potential (V)

0.8

TE D

-2

1.2

-2

m -2 Fc 6m . cm 4 5 F 1m 49.

-2

Log (J/mA cm )

(f) 2.0

CoEGAc CoOOH-3 CoOOH-20

-1

dec

M AN U

40

1.6

Ja-Jc (mA cm )

0.30

CC GC GC GC n GC -3 on H-20 o H-20 on RuO 2 on IrO 2 on H O O O CoO CoO CoO

100

(e)

J=10 mA cm

0.10

1.7

Potential vs. RHE

(c) -2

1.4

-2

0.35

RI PT

60

on GC

SC

80

Overpotential (V)

-2

Current density (mA cm )

(b) 0.40 CoOOH-3 CoOOH-20 RuO2

1.8

1.6

1.4 40

80

120

160

200

240

Time (h)

AC C

Fig. 5 (a) LSV polarization curves of commercial RuO2, IrO2, nanoporous CoOOH-3 and CoOOH-20 nanodisks at a scan rate of 10 mV s-1 for OER in 1.0 M KOH after the iR correction. Loading based on CoEGAc mass: 0.18 mg cm-2 on GC electrode and 1.2 mg cm-2 on CC. (b) Corresponding overpotentials of electrocatalysts at current density of 10 mA cm-2. (c) The current densities of electrocatalysts at η=300 mV. (d) Tafel slopes derived from the LSV curves. (e) The capacitive current at 1.22 V as a function of the scan rates for nanoporous CoOOH-20 nanodisks and pristine

ACCEPTED MANUSCRIPT CoEGAc nanodisks on GC. Inset: CVs of nanoporous CoOOH-20 nanodisks with different scan rates from 6 to 16 mV s-1. (f) Galvanostatic curve of nanoporous CoOOH-20 nanodisks at a current density of 10 mA cm-2 on CC.

RI PT

The electrocatalytic activities of in situ formed pure nanoporous CoOOH nanodisks were investigated on glass carbon (GC) electrode and carbon cloth (CC) for OER. For comparison, commercial RuO2 and IrO2 were also tested under the same conditions

SC

and their SEM images are given in Fig. S6 and Fig. S7, respectively. It can be clearly

M AN U

seen from the linear sweep voltammetry (LSV) polarization curves in Fig. 5a that nanoporous CoOOH-20 nanodisks deliver the most negative onset potential than CoOOH-3, commercial RuO2 and IrO2 on GC electrode. Moreover, the onset potential of the nanoporous CoOOH-20 on CC electrode is more negative than that on GC

TE D

electrode, which could be tentatively attributed to the higher geometric surface and catalysts loadings. To quantify these differences, the corresponding overpotential (η) of catalysts at the current density of 10 mA cm-2 is tabulated in Fig. 5b. On GC

EP

electrode, to reach a current density of 10 mA cm-2, the nanoporous CoOOH-20

AC C

nanodisks require the smallest overpotential of only 277 mV, which is 23, 53 and 53 mV less than that for nanoporous CoOOH-3 nanodisks, commercial RuO2 and IrO2, respectively. Furthermore, the corresponding overpotential (η) is merely 245 mV when nanoporous CoOOH-20 nanodisks are evaluated on CC electrode. Moreover, Fig. 5c shows that the current density of nanoporous CoOOH-20 nanodisks at η=300 mV on GC is 32.7 mA cm-2, which is about 1.85, 5.32 and 6.78 times higher than that of nanoporous CoOOH-3 nanodisks, commercial RuO2 and IrO2, respectively. The

ACCEPTED MANUSCRIPT nanoporous CoOOH-20 on CC can reach the highest current density of 103.8 mA cm-2 at η=300 mV, based on its full scale LSV curve in Fig. S8. Furthermore, the OER kinetics performance of nanoporous CoOOH-3 and CoOOH-20 nanodisks were

RI PT

evaluated by Tafel slopes according to the Tafel equation. It is well known that the low Tafel slope represents rapid reaction kinetics and high OER activity [10]. The Tafel slopes are derived from LSV curves and a linear dependency of overpotential vs.

SC

log (J) is drawn in Fig. 5d. The Tafel slopes of nanoporous CoOOH-3 nanodisks,

M AN U

commercial RuO2 and IrO2 on GC are 59.9, 98.2 and 87.2 mV dec-1, respectively. However, the Tafel slope of nanoporous CoOOH-20 nanodisks is only 53.7 mV dec-1 on GC and 53.9 mV dec-1 on CC, respectively. Hence, it can be seen that the nanoporous CoOOH-20 nanodisks possess rapid reaction kinetics for OER.

TE D

To better understand the intrinsic reason for the difference of OER activity, the electrochemical impedance spectroscopy (EIS) plots of CoEGAc, CoOOH-3 and CoOOH-20 were measured and the corresponding results were shown in Fig. S9. The

EP

EIS plots in Fig. S9 show that CoEGAc displayed the largest solution resistance (Rs),

AC C

indicating the worst conductivity [32]. With the electrochemical oxidation time increased from 3 h to 20 h, the conductivity of CoOOH was improved and the charge transfer resistance (Rct) was decreased, indicating the enhanced catalytic activity. This phenomenon might be caused by the appearance of numerous porous structures and the enlargement of average pore size, which is beneficial to fast penetration of electrolyte and facilitates the access of reactants (OH-) in the electrolyte to the active

ACCEPTED MANUSCRIPT sites within the nanodisks. Thus, CoOOH-20 exhibits better electrocatalytic activity than CoOOH-3. Based on the analysis of overpotentials and Tafel slopes, it can be found that the

RI PT

formed nanoporous CoOOH-20 nanodisks exhibit the best OER performance among these four samples. In order to better compare the catalytic activity of nanoporous CoOOH-20 nanodisks with reported literature, a series of Co-based electrocatalysts

SC

are summarized and listed in Table S1. The overpotential of 277 mV on GC and 245

M AN U

mV on CC at 10 mA cm-2 in the present work indicate the outstanding catalytic performance for OER among those reported Co-based electrocatalysts, as well as commercial RuO2 or IrO2. It also should be mentioned that the electrocatalytic activities of commercial RuO2 and IrO2 obtained in this work are comparable to those

TE D

of reported RuO2 and IrO2 [33].

In addition, the electrochemical surface area (ECSA) of samples was estimated using a typical cyclic voltammetry (CV) method at different scan rates (Fig. S10) [34].

EP

As is shown in Fig. 5e, the double-layer capacitance (Cdl) of the nanoporous

AC C

CoOOH-20 nanodisks is 54.6 mF cm-2, which is 13 times higher than 3.9 mF cm-2 of pristine CoEGAc nanodisks on GC. It seems that the ECSA of CoEGAc after OER testing is remarkably enhanced, which could be attributed to the formation of numerous nanopores in the nanodisks. Moreover, the Cdl of CoOOH-20 is slightly higher than that of CoOOH-3, indicating that CoOOH-20 could provide more active sites during OER process and thereby leading to better catalytic activity.

ACCEPTED MANUSCRIPT The long-term stability is one of the significant issues for an electrocatalyst especially for large-scale applications. In this case, the catalytic stability of nanoporous

CoOOH-20

nanodisks

on

CC

electrode

was

evaluated

by

RI PT

chronopotentiometric measurements at a constant current density of 10 mA cm-2 and the result is shown in Fig. 5f. It is clearly found that the nanoporous CoOOH-20 nanodisks maintains constant catalytic activity for 250 h without visible attenuation,

SC

indicating the ultra-stable property of electrocatalyst over long-term testing. After the

M AN U

stability testing, the electrocatalysts were further characterized. The SEM and TEM images in Fig. S11 and Fig. S12 demonstrate that CoOOH still remains nanoporous structures with coarse surface. XRD pattern in Fig. S13 indicates that CoOOH still maintains well crystal structures after stability testing. Therefore, all these

TE D

electrochemical testing results demonstrate that the formed nanoporous CoOOH-20 nanodisks manifest highly active and stable properties for OER in alkaline medium. (a) 15 CoOOH-20 Standard CoOOH

0

CoOOH-20 Standard CoOOH

Co-Co

16

Co-O

12

3

FT (k χ(k))

EP

5

AC C

3

k χ(k)

10

(b)

-5

8 4

-10

0

2

4

6

8

10

12

14

0

0

1

2

3

4

5

6

-1

k (Å )

R (Å)

Fig. 6 (a) Co K-edge EXAFS k3χ(k) oscillation functions and (b) the corresponding FT curves of formed CoOOH-20 and standard CoOOH. To have a further insight into the excellent electrochemical performance of formed CoOOH-20, X-ray absorption fine structure spectroscopy (XAFS) was conducted. For

ACCEPTED MANUSCRIPT comparison, standard CoOOH was synthesized according to the reported method by Ding et al.[35] The corresponding SEM images and XRD pattern are given in Fig. S14a-c. Fig. 6a gives the Co K-edge EXAFS k3χ(k) oscillation curves of CoOOH-20

RI PT

and standard CoOOH, in which the noticeable difference in amplitude could be observed, indicating the different Co local atomic arrangements. Moreover, the Fourier transforms curves of CoOOH in Fig. 6b are characterized by two main peaks

SC

at 1.90 Å and 2.83 Å, corresponding to the nearest Co-O and next nearest Co-Co

M AN U

coordination, respectively. It is evident that the intensities of CoOOH-20 are lower than that of standard CoOOH. To quantify these differences, the fitting curves and the obtained fitting parameters are shown in Fig. S10 and Table S2, respectively. As shown in Table S2, the coordination number (CN) of Co-O decreases from 6.0 for

TE D

standard CoOOH to 5.3 for nanoporous CoOOH-20. The low CN could be attributed to the nanoporous structures of CoOOH-20 nanodisks, in which a significant fraction of the total number of cobalt atoms are at the surface and are coordinatively

EP

undersaturated with respect to standard CoOOH, resulted in the presence of a large

AC C

amount of dangling bonds in the surface. It should be noted that the morphology of standard CoOOH (Fig. S14a-b) is nanoflakes with the diameter of 100-150 nm, which is much smaller than CoOOH-20 nanodisks. Hence, the lower CN of CoOOH-20 than that of standard CoOOH further confirms the advantages of highly nanoporous structures. As evident from Fig. S16, the absorption edge of the CoOOH-20 shows that peak shifts towards lower energy in comparison with that of standard CoOOH, indicating the lower valence state of Co, which is in consistent with that reported by

ACCEPTED MANUSCRIPT Huang et al.[21] Therefore, based on the above analyses, these results consolidate that the excellent OER activity is mainly attributed to the fact that nanoporous structures endow the nanodisks with more active sites and accessible channels. These

RI PT

nanoporous features are in favour of the access and diffusion of OH- in electrocatalysts, the evolution of oxygen molecules, as well as the coordinatively undersaturated Co atoms at the surface serving as active sites to accelerate the

SC

electron transfer rate, thereby ultimately leading to the effective OER performance

M AN U

[36]. 4. Conclusions

In summary, pure CoOOH phase with highly porous structures is formed during OER testing when cobalt alkoxyacetate was initially deployed as electrocatalysts. It

TE D

seems that the electrochemical oxidation of Co2+ into Co3+ in alkaline medium could cause the rupture of cobalt alkoxyacetate and the releasing of the degraded molecules,

EP

which results in the formation of both CoOOH and nanopores. The formed nanoporous CoOOH-20 nanodisks exhibit highly enhanced performance for OER in

AC C

alkaline solution with a low overpotential of 245 mV at the current density of 10 mA cm-2 and the catalytic activities are very stable for more than 250 h with no appreciable decrease. In addition, XRD pattern demonstrates that the formed pure CoOOH maintains stable phase during OER testing. More importantly, this finding provides a direct experimental evidence to consolidate the fact that the CoOOH is the active and stable phase in Co-based electrocatalysts for OER, which will have

ACCEPTED MANUSCRIPT significant implications for the design and fabrication of series highly effective OER electrocatalysts. Acknowledgements

RI PT

H.-P. Liang is thankful for support from the “Hundred Talent Program” of Chinese Academy of Sciences (RENZI[2015] 70HAO, Y5100619AM), DICP&QIBEBT

SC

UN201804, Dalian National Laboratory for Clean Energy (DNL), CAS, and director innovation fund (QIBEBT SZ201801). The authors also thank Dr. Andrew W.

M AN U

Meredith of Schlumberger Cambridge Research for helpful discussions. References

[1] N. Suen, S. Hung, Q. Quan, N. Zhang, Y. Xu, H. Chen, Electrocatalysis for the

TE D

oxygen evolution reaction: recent development and future perspectives. Chem. Soc. Rev. (46) 2017 337-365.

[2] K. Wu, D. Shen, Q. Meng, J. Wang, Octahedral Co3O4 particles with high

EP

electrochemical surface area as electrocatalyst for water splitting. Electrochim.

AC C

Acta (288) 2018 82-90.

[3] L. Seitz, C. Dickens, K. Nishio, Y. Hikita, J. Montoya, A. Doyle, C. Kirk, A. Vojvodic, H. Y. Hwang, J. K. Norskov, T. F. Jaramillo, A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction. Science (353) 2016 1011-1014.

ACCEPTED MANUSCRIPT [4] M. Shakeel, M. Arif, G. Yasin, B. Li, A. Khan, F. Khan, M. Baloch, Hollow mesoporous architecture: A high performance bi-functional photoelectrocatalyst for overall water splitting. Electrochim. Acta (292) 2018 992-1000.

RI PT

[5] M. Kanan, D. Nocera, In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+. Science (321) 2008 1072-1075.

SC

[6] J. Suntivich, K. May, H. Gasteiger, J. Goodenough, Y. Shao-Horn, A perovskite

Science (334) 2011 1383-1385.

M AN U

oxide optimized for oxygen evolution catalysis from molecular orbital principles.

[7] Y. Chen, J. Prange, S. Duhnen, Y. Park, M. Gunji, C. Chidsey, P. McIntyre, Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water

TE D

oxidation. Nat. Mater. (10) 2011 539-544.

[8] S. Cobo, J. Heidkamp, P. Jacques, J. Fize, V. Fourmond, L. Guetaz, B. Jousselme,

EP

V. Ivanova, H. Dau, S. Palacin, M. Fontecave, V. Artero, A Janus cobalt-based catalytic material for electro-splitting of water. Nat. Mater. (11) 2012 802-807.

AC C

[9] Y. Jiao, Y. Zheng, M. Jaroniec, S. Qiao, Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. (44) 2015 2060-2086.

[10] L. Han, S. Dong, E. Wang, Transition-metal (Co, Ni, and Fe)-based electrocatalysts for the water oxidation reaction. Adv. Mater. (28) 2016 9266-9291.

ACCEPTED MANUSCRIPT [11] W. Luc, F. Jiao, Synthesis of nanoporous metals, oxides, carbides, and sulfides: beyond nanocasting. Acc. Chem. Res. (49) 2016 1351-1358.

[12] M. S. Balogun, W. Qiu, Y. Huang, H. Yang, R. Xu, W. Zhao, G. R. Li, H. Ji, Y.

RI PT

Tong, Cost-effective alkaline water electrolysis based on nitrogen-and phosphorus-doped self-supportive electrocatalysts. Adv. Mater. (34) 2017

SC

1702095.

[13] Y. Liu, C. Xiao, M. Lyu, Y. Lin, W. Cai, P. Huang, W. Tong, Y. Zou, Y. Xie,

M AN U

Ultrathin Co3S4 nanosheets that synergistically engineer spin states and exposed polyhedra that promote water oxidation under neutral conditions. Angew. Chem. Int. Edit. (54) 2015 11231-11235.

TE D

[14] S. Dou, L. Tao, J. Huo, S. Wang, L. Dai, Etched and doped Co9S8/graphene hybrid for oxygen electrocatalysis. Energy Environ. Sci. (9) 2016 1320-1326.

EP

[15] Y. Liu, H. Cheng, M. Lyu, S. Fan, Q. Liu, W. Zhang, Y. Zhi, C. Wang, C. Xiao, S. Wei, B. Ye, Y. Xie, Low overpotential in vacancy-rich ultrathin CoSe2

AC C

nanosheets for water oxidation. J. Am. Chem. Soc. (136) 2014 15670-15675.

[16] I. McKendry, A. Thenuwara, J. Sun, H. Peng, J. Perdew, D. Strongin, M. J. Zdilla, Water oxidation catalyzed by cobalt oxide supported on the mattagamite phase of CoTe2. ACS Catal. (6) 2016 7393-7397. [17] P. Chen, K. Xu, Z. Fang, Y. Tong, J. Wu, X. Lu, X. Peng, H. Ding, C. Wu, Y. Xie, Metallic Co4N porous nanowire arrays activated by surface oxidation as

ACCEPTED MANUSCRIPT electrocatalysts for the oxygen evolution reaction. Angew. Chem. Int. Edit. (54) 2015 14710-14714.

[18] Y. Pan, K. Sun, S. Liu, X. Cao, K. Wu, W. Cheong, Z. Chen, Y. Wang, Y. Li, Y.

RI PT

Liu, D. Wang, Q. Peng, C. Chen, Y. Li, Core-shell ZIF-8@ZIF-67-derived CoP nanoparticle-embedded N-doped carbon nanotube hollow polyhedron for efficient

SC

overall water splitting. J. Am. Chem. Soc. (140) 2018 2610-2618.

[19] Y. Wang, T. Zhou, K. Jiang, P. Da, Z. Peng, J. Tang, B. Kong, W.-B. Cai, Z.

M AN U

Yang, G. Zheng, Reduced mesoporous Co3O4 nanowires as efficient water oxidation electrocatalysts and supercapacitor electrodes. Adv. Energy Mater. (4) 2014 1400696.

TE D

[20] Y. Jiang, X. Li, T. Wang, C. Wang, Enhanced electrocatalytic oxygen evolution of alpha-Co(OH)2 nanosheets on carbon nanotube/polyimide films. Nanoscale (8)

EP

2016 9667-9675.

[21] J. Huang, J. Chen, T. Yao, J. He, S. Jiang, Z. Sun, Q. Liu, W. Cheng, F. Hu, Y.

AC C

Jiang, Z. Pan, S. Wei, CoOOH Nanosheets with high mass activity for water oxidation. Angew. Chem. Int. Edit. (54) 2015 8722-8727.

[22] A. Bergmann, E. Martinez-Moreno, D. Teschner, P. Chernev, M. Gliech, J. de Araujo, T. Reier, H. Dau, P. Strasser, Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution. Nat. Commun. (6) 2015 8625.

ACCEPTED MANUSCRIPT [23] C. Tung, Y. Hsu, Y. Shen, Y. Zheng, T. Chan, H. Sheu, Y. Cheng, H. Chen, Reversible adapting layer produces robust single-crystal electrocatalyst for oxygen evolution. Nat. Commun. (6) 2015 8106.

RI PT

[24] M. Bajdich, M. Garcia-Mota, A. Vojvodic, J. Norskov, A. Bell, Theoretical investigation of the activity of cobalt oxides for the electrochemical oxidation of

SC

water. J. Am. Chem. Soc. (135) 2013 13521-13530.

[25] N. Chakroune, G. Viau, S. Ammar, N. Jouini, P. Gredin, M. Vaulay, F. Fiévet,

M AN U

Synthesis, characterization and magnetic properties of disk-shaped particles of a cobalt alkoxide: CoII(C2H4O2). New J. Chem. (29) 2005 355-361. [26] A. Cao, J. Hu, H. Liang, W. Song, L. Wan, X. He, X. Gao, S. Xia, Hierarchically

TE D

structured cobalt oxide (Co3O4): the morphology control and its potential in sensors. J. Phys. Chem. B (110) 2006 15858-15863.

EP

[27] L. Peng, Y. Ouyang, W. Li, H-J. Qiu, Y. Wang, Facile synthesis of mesoporous Co3O4–carbon nanowires array nanocomposite for the enhanced lithium storage.

AC C

Electrochim. Acta (190) 2016 126-133.

[28] Y. Sa, K. Kwon, J. Cheon, F. Kleitz, S. Joo, Ordered mesoporous Co3O4 spinels as stable, bifunctional, noble metal-free oxygen electrocatalysts. J. Mater. Chem. A (1) 2013 9992-10001.

[29] R. Delaplane, J. Ibers, J. Ferraro, J. Rush, Diffraction and spectroscopic studies of the cobaltic acid system HCoC2–DCoO2. J. Chem. Phys. (50) 1969 1920-1927.

ACCEPTED MANUSCRIPT [30] D. Dhawale, S. Kim, D-H. Park, J-H. Choy, S. Al-deyab, K. Ariga, E. Kim, A. Vinu,

Hierarchically ordered

porous

CoOOH thin-film

electrodes

for

high-performance supercapacitors. ChemElectroChem (2) 2015 497-502.

RI PT

[31] X. Han, C. Yu, S. Zhou, C. Zhao, H. Huang, J. Yang, Z. Liu, J. Zhao, J. Qiu, Ultrasensitive iron-triggered nanosized Fe-CoOOH integrated with graphene for

SC

highly efficient oxygen evolution. Adv. Energy Mater. (7) 2017 1602148.

[32] M.-S. Balogun, W. Qiu, H. Yang, W. Fan, Y. Huang, P. Fang, G. Li, H. Ji, Y.

M AN U

Tong, A monolithic metal-free electrocatalyst for oxygen evolution reaction and overall water splitting. Energ. Environ. Sci. (11) 2016 3411-3416.

[33] C. McCrory, S. Jung, J. Peters, T. Jaramillo, Benchmarking heterogeneous

16977-16987.

TE D

electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. (135) 2013

EP

[34] B. Long, H. Yang, M. Li, M.-S. Balogun, W. Mai, G. Ouyang, Y. Tong, P. Tsiakaras, S. Song, Interface charges redistribution enhanced monolithic etched

AC C

copper foam-based Cu2O layer/TiO2 nanodots heterojunction with high hydrogen evolution electrocatalytic activity. Appl. Catal. B-Environ. (243) 2019 365-372.

[35] Y. Ding, J. Zhao, B. Li, X. Zhao, C. Wang, M. Guo, Y. Lin, The CoOOH-TMB oxidative system for use in colorimetric and test strip based determination of ascorbic acid. Mikrochim. Acta (185) 2018 131.

ACCEPTED MANUSCRIPT [36] M. Ledendecker, G. Clavel, M. Antonietti, M. Shalom, Highly porous materials as tunable electrocatalysts for the hydrogen and oxygen evolution reaction. Adv.

AC C

EP

TE D

M AN U

SC

RI PT

Funct. Mater. (25) 2015 393-399.