Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation

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A Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation Zhe Xu, Wenchao Li, Xiaohui Wang, Bing Wang, Zhan Shi, Cheng Dong, Shicheng Yan, and Zhigang Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09247 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Materials & Interfaces

A Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation †

†

†

†

§

‡

†*

†‡

Zhe Xu, Wenchao Li, Xiaohui Wang, Bing Wang, Zhan Shi, Cheng Dong , Shicheng Yan, and Zhigang Zou

† National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Eco-Materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu 210093, P. R. China ‡ School of Physics, Nanjing University, Nanjing, Jiangsu 210093, P. R. China § Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China * E-mail: [email protected]

KEYWORDS: electrocatalysis, water oxidation, oxygen evolution catalyst, photoelectrochemical water splitting, photoanode ABSTRACT: Developing efficient and stable oxygen evolution catalyst (OEC) is a critical step to overcome the sluggish kinetics of water oxidation. Here, we hydrothermally synthesized a novel OEC, cobalt germanium hydroxide, CoGeO2(OH)2. The inherent Cobonded hydroxyl groups facilitate the formation of active OER intermediates. Meanwhile, the facile leaching of Ge at the OECelectrolyte interface contributes to surface reconstruction generating Co-based (oxy)hydroxides, which would weaken its lattice constraint and suppress the excessive corrosion in the OEC bulk. As a result, CoGeO2(OH)2 reveals good catalytic activity and sta-2

bility. This CoGe-based OEC achieves the overpotential at 10 mA cm (η@10mA) of ~340 mV and the turnover frequency (TOF) of -1

-2

~0.08 s . And the electrolysis kept at ~ 10 mA cm could be sustained for over 350 h. In addition, this p-type CoGeO2(OH)2 is demonstrated to be an effective electrocatalytic overlayer on n-type Ta3N5 photoanode, remarkably decreasing the onset for nearly 400 mV and increasing the photocurrent density at 1.23 VRHE about 3.8 times.

/ 18Environment ACS Paragon 1Plus

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■ INTRODUCTION To meet the global demand of clean renewable energy, the water splitting and CO2 reduction are investigated to obtain ecofriendly fuels and industrial chemicals, such as hydrogen, hydrogen peroxide, and hydrocarbons etc.

1-3

However, these energy con4

versions are generally hindered by the poor oxygen evolution reaction (OER) at the electrode-electrolyte interface. Developing effective and stable oxygen evolution catalysts (OECs) is a key criterion for improving the electrochemical energy conversion. 5

More and more precious/transition metal and their compounds are being exploited as OECs to overcome the sluggish OER kinetics. +

-

The precious metal-based OECs, such as Ir, Ru, IrO2, and RuO2, exhibit great OER activity in both acidic (2H2O → O2 + 4H + 4e ) -

-

6

and alkaline (4OH →O2 + H2O + 4e ) aqueous solution environments. The scarcity and high cost of precious metals restrict their 7

widespread commercial applications, thus non-noble-metal based OECs are urgently required. Currently, the alternative OECs mainly include 3d transition metals such as Mn, Fe, Co, and Ni and their compounds. emerged as a favorable candidate for OER according to the“volcano” trend of OER activity, stability and low cost.

13-14

8-9

10-12

Among them, Co-based OECs have besides its abundant reserve, thermal

Various Co-based oxides, nitrides, phosphides, sulfides, and selenides, such as Co3O4, CoxN, CoxP, CoSx,

and CoSex, were developed and applied as bifunctional electrocatalysts, though they might suffer from unsatisfactory stability during anodic OER process.

15-19

In addition, the two-dimension ultrathin films, ultrathin metal-organic frameworks (MOFs), or conduc-

tive support with high surface area were developed to improve the OER performances.

20-23

The density functional theory (DFT) results revealed that the large overpotential of water oxidation would be dependent on 24

energy difference of different adsorbed OER intermediates. This means that the huge OER overpotential (>~400 mV) is attributed 25

to the four-electron transfer process with complex proton-coupled intermediates (OH*, O*, OOH*). Generally, the OER activity is sensitive to the surface properties of OECs, following the Sabatier principle: the excellent OEC binds intermediates to active sites 26

neither too tightly nor too weakly. Adding heteroatom (e.g. Mn, Fe, Ni, Ce etc.) is an effective method to adjust adsorption/desorption of intermediates.

27-28

Recently, the interaction between intermediates and the Co-based catalyst gel was tuned with -2

29

the Fe and W elements, which realizes an excellent OER performance (overpotential of 223 mV at 10 mA cm ). Furthermore, the different configurations of Co ions with bridging O would lead to the slow single-Co-site catalysis or the fast dual-Co-site cataly30

sis. In addition, the formation of active intermediates on OECs would affect water oxidation. The typical spinal Co3O4 consists of 2+

2+ Td

one Co in the tetrahedral site (Co

3+

3+ Oh

) and two Co in the octahedral site (Co

2+ Td

). Substituting Co

3+ Oh

and Co

2+

3+

by Zn and Al , 31

respectively, demonstrated that the formation of CoOOH associated with Co ions is responsible for OER activity. Hence, the ex-


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ploitation of the OEC with novel element coordination environment is still necessary to understand the relationship between crystal structure and mechanism of OER activity for designing efficient OECs. In this work, we report a facile hydrothermal route to fabricate the cobalt-germanium hydroxide CoGeO2(OH)2 as the novel electrocatalyst (donated as CoGe-OEC) and investigate its OER activity. Though owning small Brunauer-Emmett-Teller (BET) area 2

-1

-2

of ~16 m g and limited electrochemical surface area (ECSA) of ~13.6 mF cm , the CoGeO2(OH)2 still exhibits good activity and stability for water oxidation. Similar to the OH groups in layered double hydroxide, the inherent Co-bonded hydroxyl groups of CoGeO2(OH)2 probably facilitate the formation of active intermediates for water oxidation. Moreover, the facile leaching of inactive Ge ions at OEC-electrolyte interface contributes to the amorphization on the OEC surface during the forming Co-based (oxy)hydroxides, which would release lattice constraint and prevent the further corrosion of OEC in alkaline solution. Therefore, -2

this bulk CoGeO2(OH)2 presents good OER activity and stability, which achieved the overpotential at 10 mA cm (η@10mA) of ~340 -2

mV and the excellently stable operation at 10 mA cm for 350 h. In addition, this p-type electrocatalyst also expressed favorable function to improve the photoelectrochemical (PEC) performance of n-type photoanodes. The modification of CoGeO2(OH)2 on the -2

Ta3N5 photoanode not only enhanced its photocurrent density by 2.8 times to 3.5 mA cm at 1.23 VRHE, but effectively lowered the onset potential about 400 mV.

■ EXPERIMENTAL SECTION Fabrication Fabrication of CoGeO2(OH)2 and corresponding (photo)electrode

Synthesis of CoGeO2(OH)2. The precursor Na2GeO3 was first synthesized by sintering a stoichiometric mixture of Na2CO3 (99%, o

Sinopharm Chemical Reagent Co., Ltd) and GeO2 (99%, Aladdin) at 850 C for 10 h. The as-prepared powder was subsequently dissolved into deionized water (DI water) for 10 mM Na2GeO3 solution. The Na2GeO3 solution (10 mM, 40 mL) was dropped into the CoCl2 (99%, Sinopharm Chemical Reagent Co., Ltd) aqueous solution (10 mM, 40 mL). The mixed colloidal solution was o

stirred at room temperature for 20 min, then transferred into the 100 mL Teflon-lined hydrothermal autoclave at 160 C for 24 h. o

The precipitation was ultrasonically cleaned in DI water and collected by centrifugation, followed by treatment at 60 C in a vacuum drying oven to obtain the crystalline CoGeO2(OH)2, donated as CoGe-OEC. For comparison, the solution containing 0.1 M CoCl2 and NH4OH (25%, Sinopharm Chemical Reagent Co., Ltd) with the pH of 10 was made to prepare Co(OH)2 by a similar o

hydrothermal method (150 C for 24h) according to a previous report.

32

Modification of CoGeO2(OH)2 on different substrates. The CoGe-OEC (10 mg) was suspended in the ethanol solution (500 -2

µL), which was loaded on a glassy carbon rotating disk electrode (GC-RDE) with the loading amount about 0.2 mg cm , observing



Page 4 of 18

its intrinsic electrocatalytic activity. The Nafion solution (5v/v%) was added to strengthen the adherence of OEC to the GC-RDE. To inspect the practical performance, the OEC was also electrophoresed onto fluorine-doped tin oxide (FTO) glass and Ta3N5 pho33

toanodes for electrochemical/photoelectrochemical measurements. Ta3N5 photoanode was synthesized as previous report. The Ta o

o

plate was oxidized at 610 C for 30 min, followed by nitriding treatment with the NH3 gas at 850 C for 500 min. Typically, the electrophoretic bath consists of OEC powders (15 mg), iodine (5 mg) and acetone (30 mL) with the assistance of sonication. During electrophoresis, the FTO or Ta3N5 photoanode acting as work electrode was placed parallel to a Ti foil (1 cm*2 cm) counter elec2

trode. The area of the electrode exposed in the electrophoretic bath is about 1 cm . The constant voltage of 20 V was applied for electrophoresis. The mean loading amount of OEC on FTO was obtained on twenty work electrodes. And the electrophoretic period was changed from 10 s to 5 min adjusting the amount of OEC on work electrode. Characterization The crystallinity of these as-prepared products was determined by powder X-ray diffraction (XRD, Rigaku Ultima III) with a o

D8/Advance diffractometer using Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA (the 2θ range, 20-80 ). X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 Versa Probe (ULVAC-PHI, Japan) with monochromatized Al Kα X-ray radiation (1486.6 eV). The energy resolution of the electrons analyzed by the hemispherical mirror analyzer is about 0.2 eV. The binding energy was determined by reference to the C 1s line at 284.8 eV. The morphology for the samples was observed with a transmission electron microscope (TEM, FEI Tecnai G2 F30 S-Twin, Hillsboro, OR), a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6700F) and a atomic force microscope (AFM, Asylum Research, MFD-3D-SA). The UV-Vis absorption spectra were obtained by Ultraviolet-visible spectrophotometer (UV, Shimadzu UV-2550). The Fourier transfer-infrared spec-1

-1

troscope (FT-IR, Bomen DA-8 spectrometer) was obtained with a resolution of 4 cm in the range of 400-4000 cm to verify the existence of hydroxyl. The tablet-like sample was prepared by compressing the mixture of vacuum-dried CoGe-OEC (1 mg) and IR-grade KBr (200 mg). And the amount of hydroxyl was approximately estimated by thermogravimetry (TG, Netzsch Co., STA o

o

-1

449 F3 Jupiter) experiment operated from room temperature to 800 C at a heating rate of 10 C min in the air. The specific surface area was analysed with a physical method of Brunauer-Emmett-Teller gas adsorption (BET, Micrometrics Co. Tri-Star 3000). N2 o

o

adsorption isotherms at -196 C were determined volumetrically. The samples were pre-outgassed at 100 C for 3 h. And the surface area was determined from the N2 adsorption isotherm. Electrochemical/Photoelectrochemical Characterization The electrochemical/photoelectrochemical measurements were conducted in a three-electrode system using an electrochemical analyzer (CHI-660D, Shanghai Chenhua, China) and rotating disk electrode (RRDE-3A, ALS Co., Japan). The CoGe-OEC coated / 18Environment ACS Paragon 4Plus

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GC-RDE, FTO, or Ta3N5 photoanodes were used as a working electrode. A Pt foil and saturated calomel electrode (SCE) were applied for the counter and reference electrode, respectively. The electrolyte was aqueous 1 M NaOH solution (pH = 13.6). The electrochemical impedance spectra (EIS) were measured using an electrochemical analyzer (Solartron 1260 + 1287, AMETEK, Berwyn, PA) with a 10 mV amplitude perturbation and frequencies between 100 kHz and 0.01 Hz. The Mott−Schottky curves were obtained using an electrochemical analyzer (2273, Princeton Applied Research, AMETEK). All the potentials described in the context were converted to the reversible hydrogen electrode (RHE) potential, which was calculated following the formula: VRHE = VSCE + 0.241 + 0.059pH (where pH is the pH value of electrolyte). To investigate the intrinsic OER performance, the linear sweep voltammetry -1

(LSV) was operated for OECs loaded on GC-RDE at the rate of 5 mV s with 85% iR compensation. For electrochemical meas2

urements of the planar OEC/FTO electrodes, the area exposed in electrolyte was about 1 cm geometric surface area (GSA). Cyclic -1

voltammetry (CV) was performed at a scan rate of 10 mV s without iR compensation. For the photoelectrochemical measurement, the light source was AM 1.5 G simulated sunlight (100 mW cm− ). 2

■ RESULTS AND DISCUSSION The XPS spectra were obtained to characterize the chemical components and valence states on CoGe-OEC. As revealed in Figure 1a, the Co 2p XPS spectrum was mainly deconvoluted into four peaks with the binding energy (BE) of 796.7 eV for Co 2p1/2 and 780.5 eV for Co 2p3/2, and their respective satellite peaks at 803.7 and 787.5 eV. These BEs are very similar to those of Cobased oxides and hydroxides (i.e. Co3O4, CoO, Co(OH)2).

34-35

And, the main peaks of Co 2p1/2 and Co 2p3/2 present the spin-orbit

2+ 36

splitting value of 16.2 eV, which can be assigned to Co . The Ge 3d XPS spectrum is displayed in Figure 1b. The BE at 32.2 eV 4+ 37

suggests that the valence state of germanium is Ge . Moreover, the surface of CoGe-OEC contains not only metal cations but a certain amount of hydroxyl groups. Figure 1c illustrates that the O 1s XPS spectrum can be fitted into four peaks. The peaks at 533.2 and 531.7 eV are assigned to adsorbed water molecule and Co-bonded hydroxyl groups (Co-OH), respectively. BEs of 531.2 and 530.3 eV are associated with Ge-O and Co-O bonds, respectively.

39-40

34, 38

And the

4+

Since electronegativity of Ge (2.01) is

2+

larger than Co (1.88), the Ge-O bond has larger BE than the Co-O bond. In addition to the XPS analysis, the energy dispersive spectrometer (EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were applied to analyse the composition of CoGe-OEC. As shown in Figure S1 S1 and Table S1, S1 the stoichiometric ratio of Co:Ge remains to about 1:1 in the bulk, which is consistent with initial ratio of precursor. The similar atomic ratios of Co:Ge obtained from the surface (XPS) and bulk (EDS,



ICP-AES) measurements indicate that no phase segregation occurs. It means that both Co and Ge are uniformly distributed in the bulk and surface.

(b)

Co 2p

Ge 3d

803.7

787.5

800 795 790 785 Binding energy (eV)

780

775

36

35

O 1s

32.2

Intensity (a.u.)

796.7

805

(c)

780.5

Intensity (a.u.)

(a)

Intensity (a.u.)

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

Page 6 of 18

34 33 32 31 Binding energy (eV)

30

29

531.2 531.7 530.3

533.2

534

532

530 528 Binding energy (eV)

526

524

Figure 11.. XPS spectra for as-prepared CoGe-OEC, (a) Co 2p, (b) Ge 3d and (c) O 1s

The FT-IR spectra (Figure Figure 2a) further confirm the formation of Co-bonded hydroxyl groups for as-prepared CoGe-OEC. The -1

sharp peaks at 3625.5 and 3573.5 cm are similar to stretching of free non-hydrogen bonded OH groups in cobalt hydroxide. o

41-42

As -1

the sample was heated at 80 C to prepare the dry OEC/KBr tablet for FT-IR test, no broad band at the region of 3500~3000 cm is 43

-1

observed, ruling out the interference of adsorbed water molecule. The absence of the FT-IR peak for Ge-OH (3676 cm ) also sug2+

44

-1

gests the OH groups would be connected with Co ions. The peaks at 819.6 and 698.1 cm are associated with Ge-O vibration. -1

45

-1

In the low frequency region (600~400 cm ), the peaks at 570.8, 539.9 and 457.0 cm are ascribed to Co-O stretching and Co-OH bending vibrations.

46-48

The wavenumbers of Co-O and Ge-O are slightly different with the values reported for Co(OH)2 and GeO2 in

this fingerprint region, because the chemical environments of Co and Ge ions in CoGe-OEC are different from those in cobalt hydroxides or germanium oxides. Basis of these evidences, the possible chemical formula of the CoGe-OEC is CoGeOx(OH)y. The amount of OH groups in CoGeOx(OH)y was approximately estimated by thermosgravimetric (TG) method. The TG curve was displayed in Figure 2b, and the differential scanning calorimetry (DSC) curve was shown in Figure S2. S2 The first-step weight loss o

(~1.63 Wt%) occurred below 100 C, which is due to the desorption of adsorbed water molecule. The major mass loss step taken o

place when the temperature was higher than 400 C with weigh loss of 7.35 Wt%. This loss would be associated with the decompoo

49

sition of OH groups in the crystalline catalyst, a typical removal of lattice OH groups at above 300 C. According to the weight loss, we can obtain the mole ratio of OH groups to Co is close to 2:1, thus we determined that the chemical formula of the CoGeOEC is CoGeO2(OH)2.


Page 7 of 18

ACS Applied Materials & Interfaces (a)

(b) 100

100 % ∆ = - 1.63 %

Step 1 98.37 %

1000

800

700

600

500

Wavelength ( nm )

TG (%)

457.0

570.8 539.9

698.1 819.6 900

96 ∆ = - 7.35 %

Step 2

94 400

819.6

3573.5

3625.5

Intensity (a.u.)

98

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

4000 3500 3000 2500 2000 1500 1000 Wavelength ( nm )

92

500

90

91.02 %

0

100 200 300 400 500 600 700 800 900 Temperature (oC)

-1

Figure 2. 2. (a) FT-IR spectra. The inset highlights the range of 1000 ~ 400 cm . (b) TG measurement of CoGe-OEC. Step 1 means the desorption of adsorbed water molecule, and Step 2 means the decomposition of OH groups.

As shown in Figure 3a, 3a the sharp peaks of the XRD pattern for as-prepared CoGeO2(OH)2 suggest the formation of a novel electrocatalyst with well crystallinity. Its pattern is different from that of the Na2GeO3 precursor or the known Co or Ge-containing compounds (Figure Figure S3 S3). In addition, the XRD patterns in Figure S4 S4 are attributed to the samples synthesized by Na2GeO3 and CoX 2-

-

-

(X=SO4 , NO3 and CH3COO ), instead of CoCl2. All the similar patterns illustrate that the identical CoGeO2(OH)2 samples were formed, even replacing the anion legend of precursor. And the precursor anions are not inserted into the lattice space causing peaks -3

shift. The true density of this novel compound is tested to be ~4.0844 g cm . According to the structure simulation (Figure Figure S5) S5 and the above-mentioned spectroscopic analysis, the CoGeO2(OH)2 would be considered to belong to hexagonal system with space group of P6122(178). The calculated lattice parameter is a = b = 5.467 Å and c = 44.54 Å. The unit cell consists of 18 Co3

o

o

o

o

GeO2(OH)2 molecule with the volume of 1150 Å . And, the typical peaks at 11.9 , 18.7 , 23.9 , and 24.6 are assigned to (006), (100), (0012), and (108) facets, respectively. The SEM image (Figure Figure 3b) 3b reveals that as-prepared CoGeO2(OH)2 synthesized by hydrothermal reaction of CoCl2 and Na2GeO3 is almost the hexagonal nanoplate with size of 50~100 nm in wideness and 30-60 nm in thickness. The average thickness of single nanoplate is 45.396 nm, verifying by atomic force microscope (AFM) measurement. (Figure Figure S6). Figure 3c, d) S6 And the TEM images (Figure d further indicate that CoGeO2(OH)2 is the single crystalline nanoplate with hexagonal structure, especially proved by the selected area electron diffraction (SAED). The CoGeO2(OH)2 nanoplate shows clear lattice fringes with the lattice space of 0.432 nm, which is assigned to the (0012) facet according to the structure simulation. These morphological features were well consistent with the XRD results. Meanwhile, the typical Co-based oxides (i.e. the commercial Co3O4 and CoO, Aladdin) as well as the hydroxide Co(OH)2 synthesized by similar hydrothermal method were taken as reference samples. As shown in Figure S7, S7 the XRD patterns confirmed the single-phase Co(OH)2 (JCDPS No. 30-0443), Co3O4 (JCDPS No.42-1467) and CoO (JCDPS No.65-2902). The SEM (Figure Figure S8) Figure S9) S8 and TEM (Figure S9 images reveal the microstructures of these Co-based OECs. Similar to CoGeO2(OH)2, the Co(OH)2 particles also show hexagonal nanoplate. And their clear lattice spaces of 0.237, 0.234



Page 8 of 18

and 0.246 nm are assigned to the facets of Co(OH)2 (101), Co3O4 (311) and CoO (111), respectively. These observations are consistent with XRD results.

O

O

Figure 3. (a) XRD pattern of CoGe-OEC powders. Inset displays the details of peaks at the region from 23 to 25 . (b) SEM, (c) TEM and (d) high resolution TEM (HR-TEM) images of pristine CoGe-OEC. Inset is the selected area electron diffraction (SAED) image.

The CoGeO2(OH)2 and other OECs were deposited on GC-RDE to probe the intrinsic OER activity. These samples were sus-2

pended in ethanol solution and the loading amount was regulated to be 0.2 mg cm . The linear sweeping voltammetry (LSV) curves shown in Figure 4a were measured on GC-RDE with 85% iR compensation, which would eliminate the mass-transport limit on 50

OER at high bias. The inset of Figure 4a illustrates that the onset of CoGeO2(OH)2 arises before 1.50 VRHE. And the 10 mA cm

-2

current density occurs at overpotential of ~340 mV (η@10mA). Although the OER activity of CoGeO2(OH)2 is not as good as commercial RuO2 (Figure Figure S10), Table S2), S10 its η@10mA is lower than 400 mV for most typical bulk cobalt oxides (Table S2 suggesting the good OER activity of CoGeO2(OH)2.

51-52

Moreover, we measured the OER performance of commercial GeO2 in basic solution containing 0.5 M 53

Na2SO4 (pH = 12), since Ge species would be dissolved in strong basic solution (pH>13, i.e. 1 M NaOH). As shown in Figure S11, S11 GeO2 does not reveal effective OER activity. Comparing with Co(OH)2, Co3O4, and CoO, the CoGeO2(OH)2 exhibits lower onset potential and higher current density at the overpotential over 300 mV. The mass activation of CoGeO2(OH)2 at 350 mV overpoten-1

-1

-1

-1

tial is about 49.5 A g , which is larger than those of Co(OH)2 (27.1 A g ) , Co3O4 (1.8 A g ) and CoO (1.6 A g ). The double-layer capacitance (Cdl) is widely used to estimate the electrochemically active surface area (ECSA) for electrocatalysts operated in elec54

trolyte. As shown in Figure 4b, 4b the Cdl was approximately evaluated from scan rate dependent charging current density. The corresponding CV curves of each OEC (Figure Figure S12) S12 were obtained after cyclic scans (50 cycles) for electrochemical activation. The / 18Environment ACS Paragon 8Plus

Page 9 of 18

current density-potential (J-V) curves depending on the scan rate are nearly rectangular, which indicates that they are non-Faradaic capacitance rather than pseudocapacitance. The non-Faradaic currents were then fitted for Cdl, following the equation Cdl=ic/υ, where 4

ic is the charging current density and υ is the scan rate. The ic is the average value of anodic current density (i+) and cathodic current density (i-) at the central potential within the scan region, following the equation ic=(|i+|+|i-|)/2. The Cdl of CoGeO2(OH)2 (13.6 -2

-2

-2

-2

mF cm ) is obviously larger than those of Co(OH)2 (7.7 mF cm ), Co3O4 (11.2 mF cm ) and CoO (8.2 mF cm ), suggesting that the CoGeO2(OH)2 is able to provide more active sites for water oxidation. This result means that increasing the specific surface area may be an efficient route to optimize the OER activity of CoGeO2(OH)2, as demonstrated in the nanostructured Co-based cata55-57

CoGeO2(OH)2

60

(b) 0.6

Co(OH)2

Commercial Co3O4 Commercial CoO

40

20

Current density (mA cm-2)


(a)


lysts.

60

CoGe-OEC

40 1.57 V

20 onset

0 1.2

1.4

1.6

13.6 mF cm-2 0.5

11.1 mF cm-2

0.4

8.2 mF cm-2

0.3

7.7 mF cm-2 CoGeO2(OH)2

0.2

Co(OH)2 0.1

Commercial Co3O4

Potential (V, vs. RHE)

0

0.0

1.2

1.4 Potential (V, vs. RHE)

1.6

(c) 50

Commercial CoO 0

10

20 30 40 Scan rate (mV s-1)

50

60

(d) 0.6

18

30 20 6 10 0 CoGeO2(OH)2

Co(OH)2

Commercial Commercial Co3O4 CoO

0

Overpotential (mV)

12

Cdl for ECSA (mF cm-2)

CoGeO2(OH)2

40 SBET (cm2 g-1)

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


Co(OH)2 Commercial Co3O4 Commercial CoO

108 mV dec-1 74 mV dec-1

0.4

58 mV dec-1 60 mV dec-1 0.2

0.0

0.5 Log (J/(mA cm-2))

Figure 4. Electrochemical characterization on GC-RDE: (a) LSV curves of CoGeO2(OH)2, Co(OH)2, commercial Co3O4, and commercial CoO. The -1 measurement was carried out in 1 M NaOH (pH = 13.6) with 85 % iR compensation. The scan rate was 5 mV s . (b) Non-Faradaic charging current densities of CoGeO2(OH)2, Co(OH)2, commercial Co3O4, and commercial CoO at different scan rates. The slope reflects the double-layer capacitance (Cdl) on OECs. (c) BET surface (SBET) and Cdl associated with electrochemically active surface area (ECSA) for CoGeO2(OH)2, Co(OH)2, commercial Co3O4, and commercial CoO. (d) Tafel slopes of CoGeO2(OH)2, Co(OH)2, commercial Co3O4, and commercial CoO.

The specific surface area CoGeO2(OH)2, Co(OH)2, commercial Co3O4, and commercial CoO was determined by N2 adsorption58

desorption BET tests. As displayed in Figure 4b and S13, S13 the N2 adsorption-desorption isotherms show negligible adsorption at -1

59

ppo < 0.8, and no obvious hysteresis is observed. These plots illustrate that all the OECs comply with the type-II isotherm, which means the CoGeO2(OH)2 is just the compact OEC bulk without porous structure. It is worth noting that the SBET of CoGeO2(OH)2 is 2

-1

2 -1

2 -1

2 -1

~16.6 m g , which is lower than those of Co(OH)2 (16.8 m g ), Co3O4 (~25.8 m g ) and CoO (~19.1 m g ). Nevertheless, the η@10mA of CoGeO2(OH)2 is lower than those Co-based OECs, indicating that CoGeO2(OH)2 possesses the better OER activity. According to / 18Environment ACS Paragon 9Plus

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-2

previous report, the specific capacitance of ~ 0.04 mF cm is generally chosen to evaluate the ECSA property. The OER performances were normalized by BET surface area or ECSA (Figure Figure 4c). 4c As shown in Figure S14, S14 the CoGeO2(OH)2 still exhibits better surface area or ECSA normalized OER performance than other OECs. These results further indicate that CoGeO2(OH)2 owns relatively high OER activity. To gain more insight into OER kinetics, Tafel slope derived from anodic polarization was fitted following the equation = 60

tion = blogJ + a, where b is Tafel slope, is current density and a is the exchange current density. As shown in Figure 4d, 4d Co-1

-1

GeO2(OH)2 exhibits the Tafel slope of ~60 mV dec , similar to Co(OH)2 (58 mV dec ) synthesized by hydrothermal method. The Tafel slope has been typically used to infer the rate-determining step among the four-electron/proton transfer process. And a Tafel -1

61

slope of 60 mV dec indicates the water oxidation on CoGeO2(OH)2 is affected by the reaction step after the first electron transfer.

And similar slopes of CoGeO2(OH)2 and Co(OH)2 might suggest that they have similar OER rate-determining step. Obviously, the -1

-1

Co3O4 (74 mV dec ) and CoO (108 mV dec ), similar to the previously reported CoOx,

62-67

show the larger Tafel slope than those

hydroxyl-containing compounds, probably indicative of the rich hydroxyl groups contributing to high activity of CoGeO2(OH)2. The catalytic activity of CoGeO2(OH)2 was further checked on planar electrode (FTO) with the loading amount of 0.2 mg -2

st

th

cm . As shown in Figure 5a, 5a the initial (1 ) and 50 cyclic CV curves for CoGeO2(OH)2/FTO were recorded at a scan rate of 10 mV -1

s without iR compensation. In the first cycle of CV test, CoGeO2(OH)2 exhibits an anodic peak at ~1.4 VRHE during the initial oxi2+

3+

4+

68

dation. This broad peak prior to OER onset is attributed to the valence transition of Co ions (Co →Co →Co ). The large split69

ting of the anodic and cathodic peaks indicates the irreversible redox, that is, this catalyst is activated initially. After electrochemical activation, the anodic peak diminished and shifted to ~1.05 VRHE, which is similar to the electrochemical behavior of CoOOH.

51

70

And the surface amorphization accompanied with leaching of non-catalytic ions is commonly found for crystalline OECs, which would decrease the lattice constraint. As a result, the catalytic activity and electric conductivity of catalytic layer improved evidently.

71-72

It is demonstrated by the decrease of the η@10mA from 550 mV to 480 mV for CoGeO2(OH)2 during the formation of catalytic

layer (Figure Figure 5a). 5a According to the integrated anodic peak (4.85 mC), the mole amount of Co species participating in OER is approximately -8

5.03×10 mol, assuming that the valence transition at 1.05 VRHE originated from one-electron process. The turnover frequency -1

73

(TOF) was calculated to be ~ 0.08 s at η = 300 mV, by the equation of TOF = J/(4Fn), where J is current density, F is Faradaic constant and n is the mole amount of active sites. The relatively high TOF for the bulk CoGeO2(OH)2 confirmed the excellent cata25

lytic ability of this novel Co-based catalyst. Then, the Co(OH)2, commercial Co3O4 and commercial CoO deposited on FTO were / 18Environment ACS Paragon10 Plus

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also applied for CV tests (Figure Figure S15). S15 The η@10mA of these three OECs are larger than 500 mV, which further indicates the good OER performance on CoGeO2(OH)2. (b)

1st cycle 50th cycle

25

1.40 V 1.45 V 1.47 V 1.50 V

40

20

2

-Z`` (Ω cm )


(a) 30

15 10

20

5 0

0

0.6

0.8

1.0 1.2 1.4 1.6 Potential (V, vs. RHE)

1.8

2.0

(c) 15

0

20

40

60

80

Z` (Ω Ω cm2) (d) 1.5

1/C2 (×109 F-2 cm4)


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


10

5

1.0

0.5

0.0

0

0

50

100

150 200 Time (h)

250

300

350

0.8

1.0

1.2

1.4

1.6

Potential (V, vs. RHE)

Figure 5. Electrochemical measurements on planar FTO substrate: (a) Cyclic voltammogram (CV) curves for CoGeO2(OH)2 before and after 50 cycling scans for electrochemical activation. The CV measurements were performed without iR compensation. (b) Nyquist plots of CoGeO2(OH)2 -2 at different bias around onset potential after electrochemical activation. (c) Stability measurement at overpotential of 480 mV for ~ 10 mA cm . (d) -2 Mott-Schottky test of CoGeO2(OH)2. All these electrochemical measurements were carried out for the planar CoGeO2(OH)2/FTO electrode (1 cm ) in 1 M NaOH. 74

The EIS measurements for CoGeO2(OH)2 were then conducted to further understand its electrochemical property, and the corresponding equivalent circuit is drawn in Figure S16. S16 The Nyquist plots displayed in Figure 5b illustrate that no OER occurs until the bias is applied to 1.45 VRHE, because the EIS spectrum still shows the typical capacitor behavior at low frequency region. When the bias was increased from 1.45 to 1.50 VRHE, the electrode-electrolyte interface transfer resistance (Rct,EC|E) continuously decreased. The radius of low-frequency semicircle gradually reduced with increasing the applied potentials, indicative of the reduction of Rct,EC|E. As shown in Figure S17, S17 the Nyquist plots collected at 1.58 VRHE could reflect the charge transfer properties at OEC-electrolyte interface. The low-frequency semicircle of CoGeO2(OH)2 indicates remarkably smaller Rct,EC|E, demonstrating its faster charge transfer. The OER performances revealed by CV curves (Figure Figure 5a) 5a are consistent with these EIS behaviors. The potentiostatic technique at the overpotential of 480 mV was performed for stability test. As displayed in Figure 5c, 5c the -2

OER operation could be sustained at the current density of ~10 mA cm for 350 h. The test was paused every 50 h to refresh the electrolyte. After the stability test, the TEM observation (Figure Figure S18a) S18a reveals that the hexagonal structure was remained during -2

potentiostatic test for 350 h (at about 10 mA cm ), and the edges of nanoplates become rough. The HR-TEM image (Figure Figure S18b) S18b / 18Environment ACS Paragon11 Plus


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clearly shows that 5-7 nm Co(OH)2 and CoOOH layer formed on the surface of CoGeO2(OH)2, identifying active sites of Co ions. 3+

The orbital splitting value of 15.1 eV for Co 2p3/2 and Co 2p1/2 in Co XPS spectrum (Figure Figure S19) S19 confirmed the formation of Co 75

during OER. The EDS, ICP and XPS analyses for the finial sample indicate that the loss amount of Ge was ~20 at%, mainly occurred on CoGeO2(OH)2 surface (Table Table S1 S1). We measured the atomic ratio of Co:Ge in the first and seventh batches of electrolyte after OER test, which are about 1:1.82 and 1:0.01, respectively. It means that the leaching of Ge species on accompanying with the formation of Co-based (oxy)hydroxide catalytic layer, and the Ge leaching might be suppressed after the surface transformation. The similar suppression of surface corrosion has been demonstrated on Co4N nanowires due to the formation of Co-based hydrox56

ides. In addition, the O 1s XPS spectrum reveals that the surface species, Co-O and OH groups, increased obviously. These evidences also indicated that the (oxy)hydroxide was formed on CoGeO2(OH)2. The OER was catalyzed by redox of Co ions to facili76

tate OER to proceed. Different to the other Co-based catalysts, the existence of inherent Co-bonded hydroxyl groups and the 4+

leaching of Ge ions probably promote the surface reconstruction for high-valence Co ions acting as active sites. The Mott-Schottky plot (Figure Figure 5d) 5d suggests the CoGeO2(OH)2 is a p-type semiconductor. This feature would be useful for 77

constructing heterojunction with n-type photoanodes to accelerate the charge separation and OER kinetics. We modified the as-2

prepared Ta3N5 photoanode with CoGeO2(OH)2 (~0.02 mg cm ) by electrophoresis. As shown in Figure 6a, 6a the onset potential of Ta3N5 was remarkably reduced from ~1.10 to ~0.70 VRHE after the modification of CoGeO2(OH)2. And the photocurrent density at -2

1.23 VRHE was improved from 0.9 to 3.5 mA cm . The cathodically shifted onset potential and improved current density demonstrated that the p-type CoGeO2(OH)2 is a potential OEC useful to photoelectrochemical water splitting. Indeed, as shown in Figure 6b, 6b the CoGeO2(OH)2 overlayer does not obviously hinder the light absorption of Ta3N5 photoanode, suggesting that the CoGeO2(OH)2 can achieve high transparent OEC modification on surface of photoanodes due to that low loading amount can afford the high OER activity.

Figure 6. 6 (a) The chopped current density-potential (J-V) curves for Ta3N5 (red) and CoGeO2(OH)2 modified Ta3N5 (blue) photoanodes under AM -2 -1 1.5G simulated sunlight (100 mW cm ). The scan rate is 30 mV s and the electrolyte is 1 M NaOH. (b) Transmittance spectra for FTO, CoGeO2(OH)2/FTO. Inset is the light absorption of Ta3N5 and CoGeO2(OH)2/Ta3N5 photoanodes. The loading amounts of CoGeO2(OH)2 on FTO and

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Ta3N5 photoanode were adjusted to be ~0.02 mg cm . And the CoGeO2(OH)2/FTO and CoGeO2(OH)2/Ta3N5 were activated at 1.8 VRHE for 30 min without AM 1.5 G illumination before spectroscopic tests.

■ CONCLUSIONS As described in this work, we synthesized a novel hexagonal cobalt-germanium hydroxide, CoGeO2(OH)2, acting as electro4+

catalyst for water oxidation. The inherent Co-bonded hydroxyl groups and the facile leaching of Ge at OEC-electrolyte interface promote the formation of superficial active species. And the stable catalytic layer prevents the corrosion of CoGeO2(OH)2, thus sta-2

bilizing the current density at 10 mA cm for nearly 350 h. This p-type CoGeO2(OH)2 can be applied for the surface modification of photoanodes to accelerate kinetics of water oxidation.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD pattern, SEM image, TEM image, DSC curve, LSV curves, AFM image, N2 adsorption-desorption isotherms, XPS spectrum, CV plots, and EIS simulations.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Shicheng Yan: 0000-0002-3432-9117 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported primarily by the National Natural Science Foundation of China (51872135, 51572121, 21603098 and 21633004), the Natural Science Foundation of Jiangsu Province (BK20151265, BK20151383 and BK20150580), the Fundamental Research Funds for the Central Universities (021314380133 and 021314380084), the Postdoctoral Science Foundation of China (2017M611784), Six talent peaks project in Jiangsu Province (YY-013) and the program B for outstanding PhD candidate of Nanjing University (201702B084).

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Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation

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