Enhancement Effect of Borate Doping on the Oxygen Evolution Activity

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The Enhancement Effect of Borate Doping on the Oxygen Evolution Activity of #-Nickel Hydroxide Zhao Zhang, Tianran Zhang, and Jim Yang LEE ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00210 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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ACS Applied Nano Materials

The Enhancement Effect of Borate Doping on the Oxygen Evolution Activity of α-Nickel Hydroxide Zhao Zhang, Tianran Zhang, and Jim Yang Lee* Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore. Email: [email protected]

KEYWORDS. Water splitting, oxygen evolution reaction, nickel hydroxide, borate, electrocatalysis

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ABSTRACT

The availability of low-cost oxygen evolution catalysts is critical to the successful commercialization of renewable energy conversion and storage systems based on solar energy and water splitting reactions. We demonstrate here that a moderate amount (~12 wt%) of borate doping of crystalline α-Ni(OH)2, a common low-cost catalyst, can lower the Tafel slope of the oxygen evolution reaction (OER) in 0.1 M KOH aqueous solution from the typical value of 60 mV decade-1 to 43 mV decade-1; and the overpotential for 10 mA cm-2 of current density from 390 mV to 340 mV. Borate doping also improves the stability of α-Ni(OH)2 in OER. The borate-doped α-Ni(OH)2 can even surpass the benchmark Ir/C precious metal catalyst in terms of activity and stability. These results demonstrate the potential of borate doping as a strategy to increase the catalytic activity of base metal water oxidation catalysts.


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INTRODUCTION The oxygen evolution reaction (OER) is central to the operation of electrochemical devices such as rechargeable metal-air batteries and water electrolyzers. Through this electrochemical reaction of water, molecular oxygen can be generated via four protoncoupled electron transfer (PCET) processes.1-2 The implementation of OER in practice is, however, fraught with challenges. The greatest one being its sluggish kinetics, which has to be ameliorated by effective catalysis.3-4 The platinum group metals (PGMs, mostly Ir, Ru and their oxides) are the most active OER catalysts known to date.5 The high cost of PGMs is their greatest disadvantage. The availability of low-cost alternatives to the PGM catalysts is essential for the development of sustainable renewable energy systems based on solar energy and water splitting reactions. Several earth-abundant first-row transition metal (e.g. Mn, Fe, Co, and Ni) compounds, most notably the oxides, phosphates, borides, perovskites, and layered double hydroxide;6-16 have shown acceptable catalytic activity and stability. Among the transition-metal-based catalysts which have been investigated to date, nickel hydroxide has drawn the most interest and investigations due to its accessibility and catalytic properties. The OER performance of nickel hydroxide is the best among the transition metal-hydroxides.17 The hydroxide structure is also detected during the oxygen electrochemical reactions on phosphide and perovskite catalysts. These in-situ formed hydroxides have been used to rationalize the increase in catalytic activity after the pristine catalysts have undergone “activation” or “conditioning” treatments.8,

18

Recently, Yan et al. reported that nanostructured α-Ni(OH)2 hollow

spheres are more catalytically active and stable than RuO2 and β-Ni(OH)2 in OER.19



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Nonetheless the overpotential for OER on Ni(OH)2 is still significant. α-Ni(OH)2 nanosheets, for example,20 require an overpotential of 420 mV to sustain a current density of 10 mA cm-2. The performance of Ni(OH)2 can be improved through morphological21 or compositional22 modifications. Other approaches that have been considered include increasing the number of active sites,23 improving the external electron transport efficiency,24 and invoking an electron-withdrawing inductive effect.25-27 Many of these modifications are implemented via cationic substitution (the substitution of nickel with another transition metal), whilst very little is known about the effect of anionic substitution. Anionic substitution has the following envisaged advantages: since anions are directly coordinated to the metal centres, they can affect the electronic properties of the metal centres more directly than a cationic substitution which relies on the metalmetal interaction. In addition, cationic modification necessitates the partial substitution of the catalytically active metal, and as such could lead to the loss of the active metal centres. Anionic substitution, on the other hand, does not impose a similar trade-off. Boron has been used in transition-metal catalysts before but not as a dopant.28-30 A recent study from the Nocera group reported an interesting enhancement effect of borate anions in the electrolyte on OER.31 It is therefore of interest to evaluate the effectiveness of an alternative use of borate in OER, i.e. by using borate as a dopant for nickel hydroxide. Herein, we report that the OER activity of α-nickel hydroxide can be improved via borate anionic doping. Higher catalytic activity and improved catalytic stability for OER in alkaline solution, which surpass even a benchmark Ir/C catalyst, were experimentally demonstrated. We theorize that borate is incorporated into the α-Ni(OH)2 crystal lattice, and serves as a proton acceptor to facilitate the PCET pathway in oxygen evolution. In 4 ACS Paragon Plus Environment

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addition, by comparing the durability of borate-doped vs borate-free α-Ni(OH)2 in prolonged operation, we also observed the stabilization of the catalytic Ni(II)/Ni(III) centers by the incorporated borate anions. EXPERIMENTAL SECTION Chemicals. Nickel (II) sulfate hexahydrate (NiSO4·6H2O, > 99.0 %), potassium tetraborate tetrahydrate (K2B4O7·4H2O, ≥ 99.5%), nickel(II) chloride hexahydrate (NiCl2·6H2O, 99.9%), ammonium sulfate ((NH4)2SO4, ≥ 99.5%), zinc sulfate heptahydrate (ZnSO4·7H2O, 99.999%), Nafion solution (5 wt%), sodium hydroxide (NaOH, ≥ 98%), and urea (NH2CONH2, 98.0 %) from Sigma Aldrich; carbon black (Ketjen black EC 600JD) from Shanghai Tengmin Corp.; and 20 wt% Ir on Vulcan XC72 (Ir/C) catalyst from Premetek; were used as received. Deionized water was used as the universal solvent throughout. Synthesis of Borate-Doped α-Ni(OH)2.

15 mL aqueous potassium tetraborate

tetrahydrate (K2B4O7·4H2O) solution (300 mM) was added slowly to 15 mL aqueous NiSO4·6H2O solution (250 mM). A solid product was formed immediately. After stirring for an hour, the mixture was transferred to a Teflon-lined autoclave and heated at 150 °C for 24 h. The autoclave was then cooled to room temperature naturally, and its content was centrifuged. The solid residue after washing with water and ethanol thrice, was vacuum-dried at 60 °C to yield the borate-doped α-Ni(OH)2 final product (denoted henceforth as B-NOH). B-NOH with different B contents were synthesized by using the same quantity of the K2B4O7 solution in different concentrations (50–500 mM). We have also tested the generality of the borate anionic substitution method by preparing boratedoped Co(OH)2. The preparation simply involved replacing the 15 mL aqueous 5 ACS Paragon Plus Environment


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NiSO4·6H2O solution (250 mM) in the above procedure with 15 mL CoSO4·7H2O (250 mM) aqueous solution. Synthesis of Borate-Free α-Ni(OH)2 Pristine α-Ni(OH)2 (denoted henceforth as NOH) nanosheets with the same rhombohedral structure as the B-NOH nanosheets had to be prepared by a different method in the literature.32 In brief, 50 mL aqueous solution of dissolved NiCl2·6H2O (96 mM) and urea (1.92 M) were homogenized for 10 min. The mixture was kept at 90 °C for 48 h, and then centrifuged to recover the solid residue. The solid after washing thoroughly with water and ethanol, was dried at 80 °C for 6 h to yield the final product. Borate-free Co(OH)2 for comparison with borate-doped Co(OH)2 was also similarly prepared by using CoSO4·7H2O (96 mM) as the metal precursor. Materials Characterization. Crystal structure determination by X-ray diffraction (XRD) was conducted on a Bruker D8 advance X-ray diffractometer using a Cu Kα radiation (1.5405 Å) source. Morphological examinations were based on field-emission transmission electron microscopy (FETEM, on a JEOL 2100F), field-emission scanning electron microscopy (FESEM, on a JEOL JSM-6700F); and atomic force microscopy (AFM, on a Bruker Dimension ICON microscope operating in the tapping mode). Surface characterization was based on X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra DLD spectrometer with dual anode X-ray gun (Al/Mg) and focused monochromatic Al x-ray gun, using the binding energy of the C1s peak of adventitious carbon as the reference (284.50 eV). Elemental analysis was based on inductively coupled plasma atomic emission spectroscopy (ICP-OES) on a Thermo Scientific iCAP 6200 duo inductively coupled Plasma optical emission spectrometer. NMR spectra of solution samples were recorded by a Bruker AVANCE 500 Ultrashield NMR 6 ACS Paragon Plus Environment

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Spectrometer. An aqueous solution of Et2O‧BF3 was used as the reference (0 ppm) for the 500 MHz

11

B solution spectra. The borate solutions were prepared by dissolving

potassium tetraborate (0.3 M) in deionized water and pH adjusted to 9.0 and 13.0 by a 0.1M KOH aqueous solution. Electrochemical Measurements. A glassy carbon (GC) disk electrode after polishing to a mirror finish, was rinsed, sonicated in water, and dried. For the preparation of the working electrode, 4 mg catalyst and 1 mg carbon (Ketjen Black) were dispersed in 1 mL of ultrapure water (18.18 MΩ·cm)/ethanol (1:1 v/v) mixture, and added with 20 µL Nafion solution (mixed with 1M NaOH aqueous solution to pH = 9.0). The suspension was sonically homogenized for 30 min. 12 µL of the suspension was drop-cast on the GC electrode to a mass loading of 0.306 mg cm-2. Electrochemical measurements commenced at least an hour later after the complete solvent evaporation at room temperature. A commercial Ir/C (20 wt%) catalyst was similarly used to prepare a benchmarking GC electrode with the same catalyst loading. Cyclic voltammetry (CV) and electrochemical capacitance measurements were carried out in a standard three electrode system using a computer-controlled Autolab type III potentiostat and 0.1 M KOH electrolyte. The catalyst-loaded GC electrode, a Pt foil electrode, and a 3M KCl Ag/AgCl electrode formed the working electrode, counter electrode, and reference electrode respectively. Electrolyte was purged with flowing O2 for at least 30 min prior to the measurements and the flow of O2 continued during the measurements. The electrochemically active surface area (ECSA) of the catalysts were measured by the CV measurements of electrochemical capacitance. All potentials are reported with reference to the reversible hydrogen electrode (RHE) at pH = 13 (E = + 0.9783 V vs. 3M KCl 7 ACS Paragon Plus Environment


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Ag/AgCl). Current densities (mA cm-2, based on the electrode geometric area) were measured in the 1.1-1.8V (vs RHE) potential range at the scan rate of 5 mV s-1. The stability of the catalysts on the GC electrode was evaluated by CV for 50 cycles. The following equation (1) was used to calculate the catalyst’s turnover frequency (TOF):24

TOF =

×

(1)

× ×

where j is the current density (A cm-2) at 300 mV of overpotential, A is the area of the GC electrode, n is the number of electrons exchanged in the overall reaction, F is the Faraday constant (96485 C mol-1), and m is the number of moles of catalytic metal deposited on the GC electrode.


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RESULTS AND DISCUSSION The crystal structure, morphology, and surface composition of all synthesized compounds in this study were characterized by XRD, FETEM, FESEM, and XPS respectively.

Figure 1. TEM images of (a) NOH and (b) B-NOH, (c) XRD patterns of NOH and BNOH, (d) B 1s XPS spectrum of B-NOH, (e) O 1s XPS spectra of NOH and B-NOH, and (f) Ni 2p XPS spectrum of B-NOH The TEM images in Figure (1a, b) reveal NOH and B-NOH (represented by the BNOH with ~12 wt% of boron - the most OER active catalyst in this study, vide infra) as nanosheets with a lateral size of 40 nm and 50 nm respectively. The prevalence of the nanosheet structure was also confirmed by SEM (Figure S1). The thickness of the nanosheets as measured by atomic force microscopy was ~6 nm (Figure S2). All XRD diffraction peaks in Figure 1c can be indexed to a series of reflections from rhombohedral α-Ni(OH)2 (JCPDS card no. #38-0715, a = b = 3.08 Å and c = 23.41 Å).32 The lack of 9 ACS Paragon Plus Environment


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significant difference between the XRD patterns of B-NOH and NOH indicates that they are isostructural; except for some slight shifts of the (003) and (006) peaks to higher Bragg angles in the case of B-NOH. These shifts are common, and are normally caused by the size and shape of intercalated species in the interlayer galley.33-34 The decrease in interplanar distance that resulted in higher Bragg angles was verified by high-resolution TEM (Figure S3). Hence the tetraborate anions, with a radius larger than K+ and water molecules, are situated within the crystal lattice proper and are not present in the interlayer as intercalated species. The nanosheet thickness as estimated from the widths of the (003) and (006) peaks was about 6 nm, consistent with the AFM measurements. The peak at 191.9 eV in the B 1s XPS spectrum of B-NOH (Figure 1d) is typical of threecoordinated borate species.35 The borate presence can also be inferred from the O 1s XPS spectra (Figure 1e) as follows. The broad O1s feature in both NOH and B-NOH can be deconvoluted into three components – as Ni-O (530.8 eV), Ni-OH (531.9 eV), and H2O (533.4 eV) respectively. The presence of B-O species in B-NOH, the binding energy of which overlaps with that of Ni-OH36, could cause an apparent increase of the Ni-OH component in the B-NOH spectrum.37 The peak at 855.5 eV in the Ni 2p spectrum of NOH (Figure 1f) can be assigned to Ni(II) of Ni(OH)2. The peak shifted slightly to 855.7 eV after borate doping (Figure S4a), an indication of the inductive effect of the borate anion in action .38


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30

70

Tafel Slope Value (mV/dec)

Current Density (mA/cm2)

a B-NOH NOH 20 wt% Ir/C

20

10

0

-10 1.1

1.2

c 30

20

1.3

1.4

1.5

1.6

b 61

60

54 50

43 40

30

20

1.7

d

Voltage (V vs RHE)

B-NOH

10

NOH

Ir/C

B-NOH NOH

0.015

B-NOH NOH 20 wt% Ir/C i (mA)


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


0.010

0.005

0

-10 1.1

1.2

1.3

1.4

1.5

1.6

1.7

0.000

20

Voltage (V vs RHE)

30

40

50

v (mV s-1)

Figure 2. (a) iR-corrected cyclic voltammograms of OER on B-NOH, NOH, and Ir/C, (b) comparison of Tafel slopes, (c) cyclic voltammograms of OER after 50 cycles, and (d) Electrochemical double-layer capacitance, CDL of NOH and B-NOH The OER performance of B-NOH in 0.1 M KOH was measured by cyclic voltammetry in a three-electrode cell, and compared with NOH and a benchmark Ir/C catalyst at the same catalyst loading. Cyclic voltammetry showed the same OER onset potential (the potential where OER current density is 2 mA cm-2, a value low enough to omit the contributions from the oxidation of Ni(II) to Ni (III) in Ni-based catalysts) for NOH and Ir/C, at 1.55V (Figure 2a, red and blue lines). The OER onset potential was lower in the case of B-NOH, at 1.53V (the black line in Figure 2a). Beyond the onset potential, B11 ACS Paragon Plus Environment


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NOH also showed a generally higher current density due to its significantly small Tafel slope (Figure 2b). Specifically, the Tafel slopes were 43, 54, 61 mV decade-1 for B-NOH, NOH, and Ir/C respectively. The TOFs of oxygen evolution, calculated at 1.53 V (overpotential = 0.3 V) and based on the catalyst total Ni contents, were 0.024 s−1 for BNOH and 0.012 s−1 for NOH, indicating the B-NOH is able to make more effective use of the catalytic metal. B-NOH also emerged top in the OER stability test (50 CV cycles; Figure 2c). The chemical states of Ni and B after the stability test were determined by XPS (Figure S4). The lack of observable changes between the Ni 2p spectra before and after the test (Figure S4a, b) indicates good Ni stability in the B-NOH and NOH catalysts. The B1s XPS spectrum likewise confirmed the presence of B after extended cycling (Figure S4c). On the other hand, the Ir 4f spectrum of Ir/C after the stability test (Figure S4d) showed a positive shift of 0.5 eV. Although this observation has not been reported in the literature before, the likelihood is there that the change is associated with catalyst deactivation. In an extended stability test carried out at the constant current density of j = 10 mA cm−2, BNOH was also able to maintain a fixed potential for 10 h with only a slight increase from 1.56 to 1.64 V (Figure S5). It has been shown previously that exfoliation of layered double hydroxide-based catalysts could increase their OER activity by virtue of the increase in surface area.23, 39 The surface area effect on the activity of B-NOH was therefore evaluated by comparing the electrochemically active surface areas (ECSAs) of B-NOH and NOH from doublelayer capacitance (CDL) measurements.40 The ECSAs calculated as such were 15.01 m2/gm for NOH and 15.00 m2/gm for B-NOH (Figure 2d and Figure S6). The sameness 12 ACS Paragon Plus Environment

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of these numbers indicates that the higher catalytic activity of B-NOH cannot be attributed to a surface area effect. OER in general occurs by a 4-step PCET mechanism, with each step involving the transfer of one proton and one electron (Figure 3a)41 on the catalyst surface (Figure 3b). A Tafel slope of 40 – 60 mV decade-1 is often used to suggest the formation of the adsorbed peroxide intermediates as the rate-determining step (RDS) (step 3 in Figure 3a)42-45. Hence the similarity of Tafel slopes for B-NOH and NOH suggests the same RDS on both catalysts. An illustration of the PCET mechanism with Ni as the active centers is shown in Figure 3c. Borate has previously been used in the electrolyte primarily to buffer the local pH.46 Since borate is now coordinated directly to the catalytic metal, the function of immobilized borate in B-NOH is expected to be different from the pH regulation role of dissolved borate. Some deductions may however be inferred from the general Lewis acid properties of borates. We posit that the reversible transformation from the three-coordinated BO33- to the four-coordinated BO33-–OHenhances the proton-accepting property (PAP) of integrated borates, and consequently the OER performance of B-NOH. Such a transformation is known to be kinetically facile in alkaline solution.47 While the determination of reaction intermediates on solid catalysts is not possible at this time due to lack of suitable in-situ/operando characterization techniques, the transformability of borate under the alkaline OER environment can be verified by the 11B NMR spectrometry of potassium tetraborate aqueous solution (0.3 M) (Figure S7). The NMR spectrum of the borate solution at pH 9.0 (the pH of the asprepared borate solution) showed a chemical shift of 10 ppm which agrees well with three-coordinated borates (sp2 hybridized boron),45 while the solution at pH 13 (the 13 ACS Paragon Plus Environment


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prevalent pH in alkaline OER) showed a chemical shift of ~1 ppm consistent with the four-coordinated borates (sp3 hybridized boron).46 The four-coordinated borate is capable of accepting a foreign proton and releases a water molecule subsequently to revert back to the three-coordinated borate, thereby completing a cycle of facilitated proton transfer in PCET. This mechanism is distinctively different from the buffering action of dissolved borates which is based on the equilibrium between H3BO3 and its conjugate base in the electrolyte. The mechanism is also different from other heteroatom (e.g. nitrogen or sulfur) doping which works by altering the electronic properties of the catalytic centers rather than participating directly in the OER process.


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Figure 3. (a) The PCET mechanism of OER in the literature, "S" represents a surface site (b) the surface structure of B-NOH, and (c) the proposed RDS on a borate-coordinated Ni active center. The Nocera group46 was the first to report the use of dissolved borates for OER in pH neutral solution. It is therefore of interest to compare the performance of latticeimmobilized borates with dissolved borates. The electrocatalytic activity of B-NOH for OER in a pH neutral environment was measured in an aqueous solution with 2M 15 ACS Paragon Plus Environment


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(NH4)2SO4 and 0.1M ZnSO4 (pH = 7.0).1 The resulting Tafel plots (Figure S8a) show a substantial increases in the Tafel slope (from 40-60 mV dec-1 to >100 mV dec-1) when the electrolyte was switched from an alkaline to a pH neutral solution. The Tafel slope of NOH was also ~42% higher than that of B-NOH in neutral solution. The addition of dissolved borates to the electrolyte (as 0.1 M K2B4O7) increased the current density of BNOH slightly without significant changes in the Tafel slope, indicating that the protonaccepting ability of B-NOH was derived mainly from the lattice-immobilized borates (Figure S8b). Table 1. A literature survey of some of the best-performing OER catalysts in 0.1 M KOH Catalyst B-NOH NOH Ir/C Ir/C Delithiated LiCo0.33Ni0.33Mn0.33O2 Pr0.5Ba0.5-xCaxCoO3-δ Delithiated LiCoO2

Tafel slope Overpotential (mV) at j (mV decade-1) = 10 mA cm−2 43 340 54 360 61 380 60 360

Reference This work This work This work Ref21

38

310

Ref6

73 57

440 390

Ref48 Ref49

4NiHPO4·Ni3(PO4)2

48

360

Ref50

NiFe-LDH Nanostructured α-Ni(OH)2 β-Ni(OH)2 nanoparticles

50 42 65

230 330 390

Ref21 Ref19 Ref21

The overpotential required to sustain 10 mA cm-2 of current density is often used as one of the performance indicators for OER electrocatalysts.23, 51 The overpotentials and Tafel

1

The use of a Zn-based electrolyte is to prepare for potential deployment in Zn-air batteries. 16 ACS Paragon Plus Environment

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slopes of some of the best-performing OER catalysts in 0.1M KOH solution are shown in Table 1. The Tafel slope of B-NOH at 43 mV decade-1 and its overpotential of 340 mV are on par with several highly effective OER catalysts in the literature.

a

0.32

b

40

c

70

65

55

0.38

50

0


B-NOH NOH

NiⅡ

10

NiⅢ

0

NiⅡ 1.2

4

10

30

11

12

13

45

50

55

60

65

14

0

B / wt %

e

20

-10 1.1

0.36

Binding Energy (eV)

d 30

60

0.34

4

10

f Current Density (mA/cm2)

Fe 3p

Tafel slopes (mV/dec)

Overpotential (V)

Intensity (a.u.)

Ni 3p


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


B-NOH NOH

20

10

0

20

11

12

13

14

B / wt %

B-NOH NOH

10

0

NiⅢ 1.3

1.4

1.5

Voltage (V vs RHE)

1.6

1.7

-10 1.1

1.2

1.3

1.4

1.5

1.6

1.7

Voltage (V vs RHE)

1.1

1.2

1.3

1.4

1.5

1.6

1.7

Voltage (V vs RHE)

Figure 4. (a) XPS spectrum covering the Ni 3p and Fe 3p regions, (b) the volcano curve of OER overpotential as a function of the boron content of B-NOH, (c) the volcano curve of OER Tafel slope as a function of the boron content of B-NOH, and (d-f) OER voltammograms of B-NOH and NOH, (d) initially, (e) after 50 cycles, and (f) after 100 cycles. It is known that iron impurities in a catalyst originated from the precursors can lead to the observation of an increased OER performance.52-53 In order to confirm that the BNOH results were free from the effect of Fe impurities, XPS measurements were carried out in the Fe 2p and 3p spectral regions. The unary peak in the 700 to 740 eV region (Figure S9) contravenes the binary peaks of Fe 2p but agrees well with the Ni Auger peak. Furthermore, no signal was detected in the 50 to 75 eV Fe 3p region (Figure 4a). 17 ACS Paragon Plus Environment


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Therefore, the presence of Fe impurities in B-NOH can be ruled out as a contributing factor to the observed enhancement of catalytic effects. We also investigated the catalytic performance of B-NOH with different B contents. The results showed that the OER activity as measured by either the overpotential required for the current density of 10 mA cm-2 (Figure 4b), or the Tafel slope (Figure 4c), varied with the catalyst boron content as a volcano curve. Such volcano trend can be rationalized as follows: at the low end of B concentrations, there were not sufficient borates to help with the proton transfer and make notable contributions to the turnover activity at the Ni sites; and hence the OER overpotential stayed about the same as that of NOH. The OER overpotential began to decrease when the B content was higher than 4 wt%. A sample with B content of 11.9 wt% (Ni/B ratio of 56.5/43.5) provided the best overall performance in terms of OER overpotential and Tafel slope. Beyond that the OER overpotential started to rise with the B content. This can be understood in terms of the reduction of the Ni sites, compounded by the steric hindrance from the crowding of borate groups on the catalyst surface. We also observed a borate stabilization effect of the Ni catalytic centers. In the OER voltammograms of B-NOH and NOH catalysts (Figure 4d), the oxidation and reduction peaks around 1.42 V can be attributed to the following transformation reaction between α-Ni(OH)2 and γ-NiOOH, with the latter being generally considered as a catalytically more active phase19, 54-56 Ni(OH)2 + OH– ↔ NiOOH + H2O + e–. It was found that the Ni(II)/Ni(III) redox features on NOH diminished after extended cycling (Figure 4e, 50th cycle), while they remained relatively unchanged in the case of 18 ACS Paragon Plus Environment

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B-NOH. The Ni(II)/Ni(III) redox features were completely obliterated after 100 cycles (Figure 4f) in NOH while they were still detectable in B-NOH (albeit diminished). These measurements manifested the stabilization effect of the borate group on the Ni centers. It is also known that a high valence state of the catalytic metal center is preferred for the OER.6, 56 This is because the high valence state active sites are more able to promote the interaction between the water molecule and an adsorbed O atom on the active sites. The diminished Ni(II)/Ni(III) redox features with cycling indicates the degradation of high valence Ni sites in NOH. The same was not found for B-NOH, an indication that the high valence Ni sites are rendered more stable by the robustness of the borate structure due to the strength of the B-O bonds (relative to the O-H bonds) and a bridge-bonded structure (Figure S10). A similar performance enhancement effect was also observed in Co(OH)2. Co(OH)2 (denoted as COH, Figure S11a) and B-doped Co(OH)2 with boron content of 9.2 wt% (denoted as B-COH, Figure S11a, b) were synthesized and their OER performance was compared in 0.1 M KOH. Overpotential and Tafel slope were both lower in the B-COH sample (390 mV at 10 mA cm−2 and 62 mV dec−1 respectively relative to 413 mV at 10 mA cm−2 and 68 mV dec−1 for COH) (Figure S12a, b), demonstrating that the enhancement effect of boron doping in water oxidation catalysts

(via the PCET

mechanism) also applies to cobalt OER catalysts. CONCLUSION In summary, we have developed a facile method to produce borate-doped α-Ni(OH)2 nanosheets by hydrothermal synthesis. The as-synthesized material surpasses both boratefree α-Ni(OH)2 and the standard Ir/C catalyst in OER in terms of a smaller Tafel slope 19 ACS Paragon Plus Environment


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(43 mV decade-1), a smaller overpotential to reach a current density of 10 mA cm-2 (340 mV), and higher catalytic stability. The performance enhancements can be understood in terms of the lattice-integrated borate groups participating in the PCET mechanism of OER to facilitate proton transfers. The work here highlights a new strategy to design OER catalysts by promoting the proton-accepting property of the solid catalyst in the PCET mechanism, and the alternative of using anionic substitution as a catalyst modification method. ASSOCIATED CONTENT The following files are available free of charge. Supplementary characterization and chemical structure (PDF) AUTHOR INFORMATION * E-mail: [email protected] ACKNOWLEDGMENT Z.Z. acknowledges the National University of Singapore for his research scholarship. T.R. Zhang acknowledges the grants from the Advanced Energy Storage Program of the Science and Engineering Research Council (SERC), Singapore (R-265-000-436-305) and from the Cambridge Centre for Advanced Research in Energy Efficiency in Singapore under the Campus for Research Excellence and Technological Enterprise (CREATE) program (R-279-000-424-592). REFERENCES


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Enhancement Effect of Borate Doping on the Oxygen Evolution Activity

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