Engineering Cobalt Defects in Cobalt Oxide for Highly Efficient

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Engineering Cobalt Defects in Cobalt Oxide for Highly Efficient Electrocatalytic Oxygen Evolution Rongrong Zhang, Yong-Chao Zhang, Lun Pan, Guo-Qiang Shen, Nasir Mahmood, YuHang Ma, Yang Shi, Wenyan Jia, Li Wang, Xiangwen Zhang, Wei Xu, and Ji-Jun Zou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01046 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Engineering Cobalt Defects in Cobalt Oxide for Highly Efficient Electrocatalytic Oxygen Evolution Rongrong Zhang,†,‡ Yong-Chao Zhang,†,‡ Lun Pan,*,†,‡ Guo-Qiang Shen,†,‡ Nasir Mahmood,†,‡ YuHang Ma,† Yang Shi,† Wenyan Jia,† Li Wang,†,‡ Xiangwen Zhang,†,‡ Wei Xu,δ Ji-Jun Zou*,†,‡ †

Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, China. ‡

Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,

China. δ

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100049, P. R. China. KEYWORDS: cobalt oxide, metal defects, electrocatalytic oxygen evolution, electron delocalization

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ABSTRACT: Defect engineering is an effective way to modulate the electric states and provide active sites for electrocatalytic reactions. However, most studied oxygen vacancies are unstable and susceptible under the oxygen circumstance. Here, we in-situ fabricated cobalt-defected Co3-xO4 for efficient oxygen evolution reaction (OER). XAFS and PALS characterizations show the crystals are abundant with Co vacancies and distortion structure. DFT calculations indicate the metal defects lead to obvious electronic delocalization, which enhances the carrier transport to participate the water

splitting

reactions

along

the

defective

conducting

channels,

and

the

water

adsorption/activation on catalyst surface. Therefore, cobalt-defected Co3-xO4 shows remarkably high OER activity by delivering much lower overpotential of 268 mV@10 mA·cm−2 (with a small Tafel slope of 38.2 mV/dec) for OER in KOH electrolyte, compared with normal Co3O4 (376 mV), IrO2 (340 mV) and RuO2 (276 mV). This work opens up a promising approach to construct electronic delocalization structure in metal oxides for high-performance electrochemical catalysts.


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1. Introduction Extensive research on electrochemical and photochemical water splitting have been conducted to solve the severe energy crisis and environmental contamination, because they are promising to produce clean and efficient fuel by renewable energy sources.1-2 However, the oxygen evolution reaction (OER) is a bottleneck in the whole process of water splitting and requirs a high overpotential (η), due to the thermodynamic uphill reaction involving a stepwise complex fourelectron redox process, and thus an efficient electrocatalyst is needed to lower the energy barrier. Currently Ir and Ru-based catalysts are commonly used for OER,3 nevertheless, their high cost and scarcity limit their large-scale application for water splitting. Recently, earth-abundant transition metal oxides and perovskite oxides/hydroxides have been considered to be promising in OER, owing to their high abundance, low cost, and environment-friendly natures.4-10 However, to fabricate highly active OER electrocatalysts, a rational design of simple and effective strategies is still needed. Electrical conductivity and water adsorption capability are two key points that influence the OER performance, both of which are strongly correlated with the electronic configuration, especially the electron delocalization of a catalyst,11 and defects engineering has been regarded as one important approach to realize the delocalization of electrons.12-14 Currently, oxygen vacancies are the most studied defects in metal oxides for heterogeneous catalysis, which are able to influence the surface electronic structure and thus the activity significantly.14 The approaches applied to producing oxygen vacancies including plasmaengraving,15 NaBH4 treatment,16 Ar/air-assisted thermal annealing,12 and so on. Generally, a new state will be formed within the band gap of the O-defected oxides, in which the electrons associated with the Metal-O bonds tend to be delocalized, resulting in the much higher electrical conductivity and catalytic activity. However, the durability of oxygen defects is susceptible under oxygen circumstance.14 And it is a complex process to introduce vacancies after complete catalyst synthesis, which may has an unsatisfying stability compared with the in-situ vacancies production during synthesis of catalysts. Alternatively, another kind of defect, i.e. metal defect, is very stable and can 3 ACS Paragon Plus Environment

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also cause electrons delocalization and provide adsorption/reaction active sites.17 Actually in previous work we have developed a universal method to in-situ fabricate metal-defected oxides like ZnO and TiO2,17-18 which shows significantly enhanced photocatalytic performance. Co3O4 has been regarded as one of most active OER non-noble electrocatalysts, and many strategies have been used to optimize its electron structure like coupling with other metals and to enhance the conductivity like coated with carbon.19-21 However, one can achieve more efficient electrocatalyst for practical application if the intrinsic activity of Co3O4 crystals can be enhanced significantly. With these considerations, here we modulated the electron states and surface electronic structure of Co3O4 by generating metal defects in the crystals, and acheived a much lower overpotential of 268 mV@10 mA·cm−2 (with a small Tafel slope of 38.2 mV/dec) for OER in KOH electrolyte, compared with normal Co3O4 (376 mV), IrO2 (340 mV) and RuO2 (276 mV). This insitu metal-defect fabrication method can be utilized for the preparation of other oxides, and can also be used in many other OER-related applications, such as copper electrowinning, electrocatalytic water splitting, fuel cells, and metal-air batteries.1-2, 22-25 2. Experimental Section 2.1. Materials Cobalt acetate, glycerol, isopropanol, ethanol, and potassium hydroxide were all purchased from Tianjin Guangfu Fine Chemical Research Institute. Nafion solution was obtained from DuPont Company. Milli-Q ultrapure water (>18 MΩ·cm) was used in all experiments. RuO2 powder (99.9% metals basis) and IrO2 powder (99.9% metals basis) were purchased from Alfa Aesar. 2.2. Synthesis of Co-defected Co3-xO4 1.0 g cobalt acetate and 37.9 g glycerol were mixed into a 100 mL Teflon-lined autoclave under 1-hour magnetic stirring for the well mixture. Then the Teflon-lined autoclave was heated in an oven at 180 °C and maintained for 2 hours. After that the obtained purple powders were collected, washed with absolute ethanol for several times, dried at 60 °C for 12 h, and finally calcined in air at 4 ACS Paragon Plus Environment

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300, 500 and 700 °C for 2 hours with a heating rate of 5 °C min−1, namely as Co-300, Co-500 and Co-700, respectively. 2.3. Structure characterization The X-ray absorption fine structure spectroscopy (XAFS) was performed at the 1W2B beam line, Beijing Synchrotron Radiation Facility. The storage ring runs 2.5 GeV electrons at 250 mA constantly during the experiments. The incident beam is monochromatized by Si (111) double crystal monochromators. The raw XAFS data were preprocessed following the conventional procedure: background removal, normalization, Fourier transformation to k-space, and k3 weighted EXAFS oscillations. X-ray photoelectron spectrum (XPS) analysis was conducted with a PHI-1600 X-ray photoelectron spectroscope equipped with Al Kα radiation, and the binding energy was calibrated by the C 1s peak (284.8 eV) of contamination carbon. Positron annihilation lifetime spectra (PALS) were measured with a fast/slow coincidence ORTEC system with a time resolution of ~201 ps (full width at half maximum). X-ray diffraction (XRD) patterns were recorded using D/MAX-2500 X-ray diffractometer equipped with Cu Kα radiation at 40 kV and 140 mA at a scanning rate of 5°·min−1. Thermogravimetric analysis (TG) was conducted on a TGA Q500 thermogravimeter at a rate of 5 °C·min−1 under air flow. Raman measurements were carried out at room temperature, and the signals were recorded by a FT-Raman Spectrophotometer RFS 100/S (Bruker). An Nd: YAG laser of the 633-nm line was used as the excitation source. The obtained Raman spectra were recorded with a resolution of approximately 1.5 cm−1. Transmission electron microscope (TEM) analysis was carried out using a Tecnai G2 F-20 transmission electron microscope with a field-emission gun operating at 200 kV. 2.4. Measurements of electrochemical properties Electrochemical properties, such as linear sweep voltammetry (LSV) and cyclic voltammetry (CV), were measured using IVIUMSTAT (Ivium Technologies BV) in a three-electrode cell, with a graphite rod as the counter electrode, an Hg/HgO as the reference electrode and 1 M KOH solution 5 ACS Paragon Plus Environment

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as electrolyte. The scan rate of LSV is 5 mV/s, while it is 10 mV/s for CV. The working electrode was prepared by dip-coating Co3O4 dispersion solution on a glassy carbon electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sinusoidal ac perturbation of 10 mV applied over the frequency range of 0.01~105 Hz. 2.5. Computation The first-principles calculations were based on density functional theory (DFT) and the projector augmented wave method as implemented in the Vienna Ab initio Simulation Package (VASP).26-28 The Perdew-Burke-Ernzerh of spin-polarized generalized gradient approximation was used for the exchange-correlation potential and the plane-wave cutoff was set to 400 eV. The unit cell with Fd3m group space includes 24 Co atoms and 32 O atoms, and in each cell one atom is removed or inserted. The calculation was performed using generalized gradient approximation with the DFT+U with U-J = 3.3 eV for Co 3d. A G-centered 3 × 3 × 3 k mesh together with an energy cutoff of 400 eV was used in geometry optimization process and static calculations. The convergence criteria for total energy were 10−5 eV. All the atoms were relaxed until HellmannFeynmann forces on each atom were reduced to less than 0.02 eV·Å−1. 3. Results and discussion 3.1. Crystal structure of Co3-xO4 Initially the precursor of Co3O4 was fabricated by solvothermal treatment of cobalt acetate in glycerol and its structure was recorded by XRD pattern (Figure 1a). The precursor shows a sharp peak at ca. 10° and three weak peaks at ca. 21°, 34° and 60°, which are ascribed to glycerolatocobalt (II) (CoGly).29 CoGly has a layered crystal structure consisting of parallel chains of Co−O−Co−O skeleton with terminated −O−C−C(C−OH)−O− groups

30-31

(inset of Figure 1a,

and Figure S1, Supporting Information, SI). Importantly, such layered structure benefits the formation of metal defects in oxide crystals during thermal calcination.17-18 Thermal gravity analysis of CoGly shows a single weight loss at 283.6 °C (Figure S2, SI) with no further weight loss observed with increasing temperature, indicating the completed removal of 6 ACS Paragon Plus Environment

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inter-layer organic groups by means of releasing CO2 and H2O (as shown in derivative TG curve). As revealed in Figure 1b, the XRD patterns of the samples calcined at 300, 500 and 700 ºC have typical cubic Fd3m Co3O4 structure (JCPDS No. 42-1467), without any other phases or impurities. The diffraction peaks become stronger and sharper with the increase of calcination temperature, suggesting the improved crystallinity and larger crystal size. TEM and SEM characterizations (Figure 1d-f, Figure S3, SI) were further conducted, and a small increase in crystal size is observed, e.g. 12.2 nm for Co-300 and 32.3 nm for Co-700 (Table S1, SI). In HR-TEM images (insets in Figure 1d-f), the lattice fringe spacing of ca. 0.46 nm refers to Co3O4 (111) facets.32-33 (c)

(3

11 )

(b)

(a)

A1g

(440)

Counts

(511) (422)

(400)

(222)

(220)

(111)

Intensity (a.u.)

CoGly

F2g 616

Co-700

194

476

F2g 518

Co-700

670 467

604

Co-500

192

510

Co-500 465

667

Co-300

F2g

Eg

672

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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603

190

507

Co-300

Co3O4

0

20

40

60

2θ (degree)

80

100

20

30

40

50

60

2θ (degree)

70

80

800

700

600

500

400

Raman shift (cm-1)

300

200

Figure 1. (a) XRD pattern and crystal structure (inset) of CoGly. (b) XRD patterns, (c) Raman spectra, and (d-f) TEM/ HR-TEM (insets) images of Co-300, Co-500 and Co-700, respectively. Raman spectroscopy was also applied to test the structural properties of the samples, as shown in Figure 1c. They exhibit similar spectrum with five Raman-active modes at about 194 cm−1 (F2g), 476 cm−1 (Eg), 518 cm−1 (F2g), 616 cm−1 (F2g) and 672 cm−1 (A1g), confirming the formation of pure cubic phase of Co3O4.34 Moreover, the decrease in calcined temperature leads to the right shift of 7 ACS Paragon Plus Environment

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Raman peak, which means a smaller short-range order in Co-300. Besides, the reduction in peak intensity of F2g (194 cm−1) for Co-300 compared with Co-500 and Co-700 indicates its smaller long-range order.35 The smaller long-range and short-range orders in Co3O4 crystals suggest the existence of equally distributed defected sites, expecially in Co-300. 3.2. Cobalt defects in Co3-xO4 Although Co-300, Co-500 and Co-700 are all cubic Co3O4 phase, the elemental analysis shows they have different compositions (Table 1). The Co/O atom ratio of Co-700 determined by TEMEDS and XPS are very close to the normal Co3O4 (Co/O = 0.75). However, both Co-300 and Co500 show relatively lower Co/O ratio, especially in case of Co-300 (Co/O ratio is ca. 0.6). The results suggest the presence of Co defects (VCo) or interstitial oxygen (Oint) in Co3O4 crystals, and their concentration is in the order of Co-300 > Co-500 > Co-700≈Co3O4. Table 1. Co/O atom ratio and positron lifetime parameters of Co-300, Co-500 and Co-700. Co/O atom ratio

Defect concentration

τ1 (ps) b

samples

Co-300

Co-500

Co-700

EDS

XPS

(%) a

0.605

0.591

8.7

0.677

0.747

0.671

0.748

I1 (%) c

τ2 (ps) b

I2 (%) c

280.2

67.5

427.0

32.5

(0.0074)

(4.5)

(0.0150)

(4.5)

235.0

43.3

378.6

56.7

(0.0081)

(3.0)

(0.0057)

(3.0)

230.5

42.2

394.4

57.8

(0.0047)

(1.2)

(0.0026)

(1.2)

4.3

0.1

a

The ratio of defect Co atoms and all atoms. b Lifetime of positrons annihilated at different vacancies. c Relative intensities of different vacancies. Values in parentheses are uncertainties. Then, the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies were carried out to finely investigate the structure. As shown in Figure 2a, the Co K-edges for Co-300 and Co-500 are approximately 0.18 eV and 0.13 eV higher in energy than that of standard Co3O4, respectively, confirming they possess a relatively higher oxidation state 8 ACS Paragon Plus Environment

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owing to the lower Co/O ratio.36 Then, R space was obtained by Fourier transforming from wavevector k (Figure 2b). All samples show four prominent peaks mainly caused by the single scattering of Co-O (Reff 1.923 Å), Co-Co1 (Reff 2.858 Å), Co-Co2 (Reff 3.351 Å), Co-Co3 (Reff 4.950 Å), respectively. The amplitude of R space depends on the coordination number and mean-square disorder, with the positive correlation to the high coordination number and negative correlation to the low mean-square disorder, and vice versa.37 From the peaks of Co-Coi (i= 1, 2, 3), it is obvious that Co-300 and Co-500, especially the former, have obvious lower Co coordination number and higher mean-square disorder, indicating the presence of large amount Co defects in the crystals (8.7 % for Co-300, Table 1). 20

1.6

(b)

(a) Co-300 Co-500 model Co3O4

0.8

Co-Co1

16 -4 |χ χ(R)| (Å )

1.2

normalized xµ µ(E)

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

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1.5

1.4

model Co3O4 Co-300 Co-500

Co-O Co-Co2

12 Co-Co3

8

0.4 1.3

1.2 7728

0.0 7700

4

7710

7720

7730

7730

7740

7732

7750

0 7760

0

1

2

3

4

5

6

R (Å)

Energy(eV)

Figure 2. (a) XANES, and (b) Fourier transforms of k-space oscillations for Co3O4, Co-500 and Co300. Furthermore, the first three main peaks were fitted using three paths (powerful single scattering) up to 3.4 Å. As shown in Table 2 and Figure S4 (SI), the coordination numbers of bonding Co atom for Coi in different distance shell of Co-500 and Co-300 are both less than the model Co3O4, which leads to the increase of mean-square disorder and the shortage of distance to neighboring atoms, consistent with the analysis of Raman spectra. Importantly, Co-300 possesses the lowest coordination number of Co atoms and highest mean-square disorder, further confirming the existence of abundant Co vacancies in the crystal. 9 ACS Paragon Plus Environment

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Table 2. Fitted parameters of Co K-edge EXAFS curves for model Co3O4, Co-500 and Co-300. path R (Å) a S0 2 b Nc s2 (Å-2) d Reff (Å) e Co-O 1.915(0.004) 5.33 0.0036(0.0002) 1.923 model 0.786 Co-Co1 2.854(0.003) 4.00 0.0033(0.0002) 2.858 Co-Co2 3.357(0.004) 8.00 0.0062(0.0002) 3.351 Co-O 1.914(0.005) 5.33 0.0035(0.0003) 1.923 Co-500 0.786 Co-Co1 2.854(0.005) 3.94 0.0035(0.0003) 2.858 Co-Co2 3.355(0.009) 7.75 0.0063(0.0007) 3.351 Co-O 1.911(0.004) 5.33 0.0036(0.0003) 1.923 Co-300 0.786 Co-Co1 2.854(0.004) 3.80 0.0036(0.0005) 2.858 Co-Co2 3.354(0.007) 7.50 0.0066(0.0006) 3.351 a b c Actual distance to neighboring atom; Amplitude attenuation factor; Coordination number of neighboring atom; d Mean-square disorder of neighbor distance; e Theoretical distance to neighboring atom. Values in parentheses are uncertainties. samples

It is also reported that metal vacancies in metal oxides exist in form of point defect.13 Positron annihilation lifetime spectra (PALS) can give direct information about the dimension and contents of free-volume holes in solids through measuring positrons annihilation in low density regions around dislocations or vacancies.38 The PALS measurements were then conducted, and as shown in Table 1, all samples exhibit two distinct lifetime values (τ1, τ2), corresponding to two kinds of vacancies with different size. The shorter lifetime (230.5~280.2 ps) is assigned to the annihilation of positrons in the single metal vacancy, while the longer one (378.6~427.0 ps) is the annihilation of vacancy clusters.18 Both of the shorter and longer lifetime for Co-300 are higher than the normal Co3O4 (Co-500 and Co-700), and Co-300 has the highest τ1 lifetime and I1 intensity, indicative of the most abundant single metal vacancies existing in Co-300.


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Figure 3. Optimized cell structures of (a) perfect crystal (Co24O32), (b) Co-defected (Co23O32), and (c) Oint (Co24O33) Co3O4. (d) Schematic formation of Co vacancies in Co3-xO4 via the thermal calcination of CoGly precursor. To figure out the defected sites of nonstoichiometric Co3-xO4, the density functional theory (DFT) calculations were conducted. It was found the formation of Co2+ defects is more preferential than Co3+ defects, owing to the lower formation energy of the former (Figure S5, SI), suggesting the majority of cobalt vacancies are Co2+ defects. Then defected and normal Co3O4 crystals were constructed (Figure 3a-3c), and the calculations predict the presence of VCo makes the a axis shrunken obviously compared with perfect crystals, which is in agreement with the XRD data where Co-300 shows obvious a axis shrunken as compared with normal Co3O4 (Table S2, SI). In addition, the presence of Oint results in the expansion of a axis, which is contrary with XRD data, again confirming the nonstoichiometry of Co3-xO4 is caused by the formation of cobalt vacancies. Actually in the synthetic process of defected Co3O4, the lamellated structure of CoGly precursor plays a very crucial role to introduce metal defects. As shown in Figure 3d, during thermal calcination, inter-chains organic groups of CoGly is gradually removed with surface terminal O atoms retained, then the remaining Co−O−Co parallel lattice chains simultaneously conjunct with each other to form Co3O4 crystals. Importantly, the bonding of terminal retained O 11 ACS Paragon Plus Environment

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atoms with the surface Co atoms causes many metal voids in the crystal lattice, resulting into abundant Co vacancies in the tortuous lattice of Co-300. However, the higher calcined temperature of 500 °C or 700 °C will lead to re-crystallization of crystal lattice and decrease of VCo concentration.17, 39 Importantly, the electronic structures of Co3O4 and Co-defected Co3-xO4 were studied by DFT calculation. In the total and projected charge density of states (Figure 4a), the normal Co3O4 shows a direct band gap of 1.2 eV as reported previously.40 Differently, Co3-xO4 clearly shows an increased density of state for the occupied states from ca. 0.50 eV above the Fermi level (the shadowed part in Figure 4a) and then a very small energy gap with respect to Co3O4. Meanwhile, from the crystal structure and partial charge density in Figure 4b,4c, compared with normal Co3O4, the presence of Co vacancies bring distortion to neighboring atoms and leads to obvious electronic delocalization (see the charge density mapping). Specifically, the neighboring Co atoms show appreciable charge depletion but more overlap areas of electron wave function with neighboring O atoms. Therefore, the charge density around the conduction band edge in Co-defected Co3-xO4 is increased considerably and more dispersive than in bulk Co3O4. These changes should be beneficial for faster carrier transport to participate the water splitting reactions along the defective conducting channels.41 Furthermore, the formation of VCo is accompanied by structure distortion, which helps to decrease surface energy and hence ensure better structure stability.41 In addition, the rapid electron transport along the defective conducting channels allows for low corrosion rates and hence guarantees long-term durability in the aqueous electrolyte.42 Accordingly, it is expected that Codefected Co3-xO4 can achieve efficient and robust electrochemical performances.


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40

(a)

Total Cobalt Oxygen

Co3O4

30 20

DOS (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10 0 40

Co-defected Co3-xO4

30 20 10 0

-6

-4

-2

0

2

4

6

Energy (eV)

Figure 4. (a) Total and projected charge density of states for Co3O4 and Co-defected Co3-xO4. Optimized cell structures and the corresponding charge density mapping of normal Co3O4 (Co24O32, b) and Co-defected Co3-xO4 (Co23O32, c). 3.3. OER performance of Co3-xO4 To investigate the electrocatalytic OER performance, linear sweep voltammetry (LSV) was employed to obtain polarization curves of Co-300, Co-500 and Co-700 in 1 M KOH electrolyte, as shown in Figure 5a. Notably, a sharp increased anodic current response starting at an estimated onset potential (Eonset) of 1.42 V (defined as the potential required to reach an OER current density of 0.1 mA·cm−2) is observed for Co-300 electrode, showing a remarkably improved catalytic activity compared to other electrodes (Eonset for Co-500 and Co-700 is 1.46 V and 1.48 V, respectively). Besides Eonset, the operational potential at 10 mA·cm−2 (Ej=10) is another key parameter for OER performance evaluation.1 When the thermodynamic OER potential (E (H2O/O2) = 1.23 V) is used as the reference, the Co-300 electrode shows a very low overpotential of 268 mV at 10 mA·cm−2, which is considerably lower than that of Co-500 (367 mV), Co-700 (376 mV), IrO2 (340 mV) and RuO2 (276 mV) (commercial catalysts, see the electrodes preparation in SI), and Odefected Co3O4-x (Table S3, SI). Importantly, when the current density reaching 50 mA/cm2, Co300 performs much better activity (with overpotential of 309 mV) than Co-500 (422 mV), Co-700 (428 mV), IrO2 (416 mV) and RuO2 (415 mV). The selectivity of OER is usually expressed by 13 ACS Paragon Plus Environment

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faradaic efficiency as measured by rotating ring-disk electrode (RRDE).43-45 The average electron transfer number (N) of the five catalysts obtained by using a RRDE are all about 4.0 (Figure S6, SI). And all the faradic efficiency of O2 generation reaction were almost 100% (Table S4, SI), indicating OER follows a four-electron pathway to generate oxygen molecules. In addition, Co-300 performs a good stability with no obvious de-activity in 10000 seconds, with an efficiency of ca. 98.0% and only 4.2% increased overpotential at 10 mA/cm2 (after 2000 cycles) (Figure 5b). After OER stability test, there are no obvious variations of Co-defect concentration (Table S5, SI), indicating the defected structure is stable. The results indicate that the presence of Co vacancies in Co3O4 can significantly improve the OER performance.

(b)

(a) Co-300 Co-500 Co-700 IrO2

30

30

0.8

RuO2

20 10

Before After 2000-cycles test

0.6

j (mA/cm2)

40

98.0%

1.0

j/j0(%)

Current density (mA/cm2)

50

0.4 0.2

0

20

10

0 1.0

1.4

1.5

1.6

0

1.7

0.6

1.6

1.8

2000

4000

6000

8000

10000

Time/s

(d)

Co-300 Co-500 Co-700 IrO2

57.2 mV/dec

1.6

Potential(V vs. RHE)

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RuO2 0.4

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Figure 5. OER performance of Co-300, Co-500, Co-700, IrO2 and RuO2. (a) The polarization curves of OER. (b) Stability: current-time chronoamperometric response of Co-300 at 1.50 V vs.


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ACS Catalysis

RHE, and LSV before and after 2000 cycles (inset). (c) TOF for OER. (d) Tafel plots. The current density is based on glassy carbon electrode (0.07068 cm2) in this work. The turnover frequency (TOF) is an important index (defined as the number of moles of O2 evolved per unit time) to evaluate the intrinsic activty.46 From Figure 5c, the TOF of Co-300 is 0.39 s−1 at the overpotential of 350 mV, which is 4.1 times higher than that of RuO2 (0.095 s−1), and ≥10 times higher than IrO2 (0.039 s−1), Co-500 (0.016 s−1) and Co-700 (0.011 s−1). Tafel slopes are derived from potential-current logarithmic plots and shown in Figure 5d. The calculated Tafel slope of Co-300 is 38.2 mV/dec, much lower than IrO2 (53.8 mV/dec), RuO2 (53.4 mV/dec), Co500 (57.2 mV/dec) and Co-700 (58.9 mV/dec), indicating Co-300 has the faster reaction rate constant and better electrocatalytic kinetics (with current density increasing faster with smaller overpotential change). It is worth noting that the anodic peaks in the potential region from 1.35 to 1.45 V vs. RHE are attributed to oxidation of Co3+ to Co4+, which is positively related to the OER performance.47 The electronic delocalization structure and reapportion of charge due to the abundant Co vacancies in Co-300 make Co3+ easier to oxidize to Co4+ species (see the obvious Co3+→Co4+ oxidation peak for Co-300, while very trace oxidation peaks observed for other samples, Figure 5a), which favors the formation of more active oxyhydroxide phase under electrocatlaysis conditions.48 Moreover, the electrochemical surface area (ECSA, calculated by the cyclic voltammograms (CV) technique according to double layer theory, SI) of Co-300 (266.0 cm2) is much higher than Co-500 (53.7 cm2) and Co-700 (26.2 cm2) (Figure S7, SI), which means much more active sites are produced over Co300. As the above mentioned, the electrical conductivity and water adsorption capability of electrocatalyst are two vital factors of OER.11 The electrochemical impedance spectroscopy (EIS) was tested to determine the electrical conductivity of the electrodes. As shown in Figure 6a, the charge transfer resistance (Rct) of Co-300 with abundant Co vacancies is 100.5 Ω, which is comparable to RuO2 (127.8 Ω) and much smaller than Co-500 (716.6 Ω), Co-700 (933.5 Ω), IrO2 15 ACS Paragon Plus Environment

ACS Catalysis

(337.2 Ω), which confirm the important role of electronic delocalization structure (caused by Co vacancies) in promoting the electrical conductivity atributed to the faster carrier transport along the defective conducting channels (Figure 4).11 In addition, it is extremely important to enhance the affinity between electrodes and electrolytes for electrocatalysis applications.49 The surface wettability of defected Co3-xO4 was evaluated through water contact angle measurements. Figure S8 (SI) shows Co-300 has a better wettability with smaller contact angle of 12.5°, in comparison with normal Co3O4 reported in literatures (36° or 45°).49-50 Meanwhile, as the initial and important step of OER process, H2O adsorption on catalyst surface was investigated by DFT calculations. As shown in Figure 6b, the H2O adsorption energies on 3-fold Co sites for both pristine Co3O4 and Co-defected Co3-xO4 are negative, indicating H2O molecules are adsorbed and splitted on the surface.51 Importantly, one hydrogen atom of H2O was drawn to surface oxygen (with a binding length of 1.746 ffi) nearby adsorbed Co site on Co3-xO4, and the adsorbed energy (Eads) of water on Co3-xO4 (−1.07 eV) is lower than the normal Co3O4 (−0.98 eV), suggesting the Co vacancies benefit H2O adsorption for OER process. 1000

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ACS Catalysis

Figure 6. (a) Electrochemical impedance spectroscopy (EIS) (inset: EIS fitting model, Rs, electrolyte resistance; Rct, charge-transfer resistance; Cd, capacitive reactance). (b) The optimized structures of water adsorbed at Co (3 fold) on (111) surface (top and site views) of Co3O4 (left) and Co-defected Co3-xO4 (right). Deformation charge density of H2O adsorbed structure of Co3O4 (c) and Co-defected Co3-xO4 (d). In order to investigate the changes of electronic structure by the adsorbed water, the deformation charge density ∆ρ(r) was calculated via ∆ρ(r)= ∆ρtotal(r) - ∆ρwater(r) - ∆ρsurface(r) (where ∆ρtotal(r), ∆ρwater(r) and ∆ρsurface(r) are electron density of the water-adsorption system, isolated water molecule and Co3O4/Co3-xO4 surface).52 As shown in Figure 6c,6d, the deformation charge density of adsorbed H2O on Co3O4 and Co3-xO4 show an apparent difference. The density of two Hw atoms (of water) in the Co3O4 system keep a symmetrical shape but one Hw atom in adsorbed water of Co3-xO4 shows obvious interaction with Os (surface O of Co3-xO4), indicating Co-defected Co3-xO4 has stronger interaction with adsorbate H2O than normal Co3O4.53 This strengthened force elongates the H-OH bond length of adsorbed water to 1.014 Å, whereas it is only 0.986 Å for normal Co3O4 surface. The above results indicate the Co-defected surface is advantageous for the adsorption of water and further cleavage of H-OH for water splitting. Accordingly, the Co-defected structure can not only improve the electronic conductivity but also favor the evolution of oxygen reaction, thus resulting in excellent OER activity. 4. Conclusions In this work, abundant Co vacancies have been introduced into Co3-xO4 to modulate the electronic structure for high electrocatalytic OER performance. The existence of Co vacancies in Co3O4 leads to obvious delocalized electrons structure, providing more active catalytic sites with high electrical conductivity and initial water adsorption ability, exhibiting a remarkably high electrocatalytic OER performance (with a low overpotential of 268 mV@10 mA·cm−2 and a small Tafel slope of 38.2 mV/dec). Importantly, Co-defected Co3-xO4 (Co-300) shows ≥10 times higher 17 ACS Paragon Plus Environment

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turnover frequency than normal Co3O4. Therefore, this result presents metal-defected oxides as a new kind of efficient electrocatalyst for OER. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L. Pan); [email protected] (J.-J. Zou) ACKNOWLEDGMENT The authors appreciate the support from the National Natural Science Foundation of China (21506156, 21676193, 51661145026) and the Tianjin Municipal Natural Science Foundation (16JCQNJC05200, 15JCZDJC37300). The authors also appreciate the help of XAS analysis from Beijing Synchrotron Radiation, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China, and the help of contact angle measurement from Professor Xiang-Gao Li and Dr. Jia Yuan. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images, element analysis, TG and DTG profile, EXAFS curve, contact angle, rotate ring disk electrode voltammogram, cyclic voltammogram, electron transfer number and Faradaic efficiency of Co-defected and normal Co3O4; Optimized cell structure and lattice structure of CoGly chain, Co23O32, and normal Co3O4; Electrochemical measurement methods; Detailed DFT calculations. REFERENCES (1) Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J.-J.; Wang, Z. L. Electrocatalytic Oxygen Evolution Reaction for Energy Conversion and Storage: A Comprehensive Review. Nano Energy 2017, 37, 136-157. (2) Huang, Z.-F.; Wang, J.; Peng, Y.; Jung, C.-Y.; Fisher, A.; Wang, X. Design of Efficient 18 ACS Paragon Plus Environment

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Engineering Cobalt Defects in Cobalt Oxide for Highly Efficient

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