110th Anniversary: A Total Water Splitting Electrocatalyst Based on

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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110th Anniversary: A Total Water Splitting Electrocatalyst Based on Borate/Fe Co-Doping of Nickel Sulfide Zhao Zhang, Tianran Zhang, and Jim Yang Lee*

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Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore ABSTRACT: The design of tunable low-cost electrocatalysts efficient in both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is central to the clean production of hydrogen without the reliance on platinum group metals (PGMs), which are the best known electrocatalysts for these reactions today. In this study, a catalyst-integrated electrode with a tunable composition (borate/Fe dual-doped Ni3S2 hollow microspheres on nickel foam) was fabricated by a self-templating method where the dopant contents can be easily varied by the precursor compounds. The catalyst-integrated electrode showed impressive OER and HER activities and good catalyst stability in an alkaline electrolyte. The performance of this electrode for total electrochemical water splitting was quite exceptional, requiring only a small overpotential of 364 mV to yield 50 mA cm−2 of current density. The strong total water splitting performance can be attributed to the use of complementary anionic (borate) and cationic (Fe) doping of the nickel sulfide structure to improve various elementary processes in the OER and HER mechanisms.

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INTRODUCTION Hydrogen, with a high energy density of ∼282 kJ mol−1, is a clean fuel at the point of use but can only be a sustainable alternative to conventional fossil fuels if it is produced from renewable resources with renewable energy.1,2 Electrochemical water splitting (2H2O → 2H2 + O2) is one such technology when the electrons are generated by solar and wind energy. The best known electrocatalysts that are currently available commercially are IrO2 for the anodic oxygen evolution reaction (OER) and Pt/C for the cathodic hydrogen evolution reaction (HER). Since these catalysts are based on low earth-abundant and expensive platinum group metals (PGMs),3,4 they are not scalable for industrial application of water electrolysis technologies. Low-cost catalysts which can substitute for IrO2 in OER and Pt/C in HER have been an active area of research for years. Many of these catalysts are based on oxides,5 hydroxides,6 sulfides,7,8 and phosphides9 of earthabundant transition metals and have shown comparable performance as PGMs. Among these electrocatalysts examined to date, nickel sulfide has drawn significant recent interest because of its generally all-rounded electrochemical performance (except for its HER activity which is an area for improvement), low material cost, and simple synthesis.7,10 It is known that the overpotentials in water electrolysis are mainly caused by the energy barriers in elementary reaction steps involving water molecules (e.g., dissociation of the water molecule and adsorption of the water molecule and H atom on the catalyst surface).11,12 In our previous work,13 borate doping was found to be an effective anionic substitution for nickel sulfide to improve its performance in both OER and HER. The enhancement effect of borate is believed to originate from the coordination transformation of borate in alkaline solution, © XXXX American Chemical Society

which gives rise to a localized negative charge that facilitates dissociation of the water molecule. Inspired by a recent report that Fe doping of nickel sulfide could improve water adsorption and optimize hydrogen adsorption on the catalyst surface,14 we wanted to explore whether Fe substitution can be administered concurrently with borate doping to further enhance the water splitting reaction outcome. Since an electrocatalyst performance is strongly dependent on its microstructure, morphology, and integration with the electrode,15 we selected a synthesis which could render nickel sulfide as Ni3S2 hollow microspheres on nickel foam as a stable catalyst-integrated three-dimensional electrode for use in total water electrolysis.16 The amounts of Fe and borate precursors in the synthesis were varied to tune the extents of cation and anion doping in borate/Fe dual-doped Ni3S2 hollow microspheres on Ni foam (Fe-NiS-(BO)/NF) to deliver the best balance of OER and HER properties. The experimental measurements borne out our expectation. The best catalyst-integrated electrode of this study showed good water splitting performance including longterm stability (95.7% retention of initial current density after 10 h of electrolysis), low OER (250 mV) and HER (200 mV) overpotentials, and consequently a total overpotential of 364 mV for complete water splitting at the current density of 50 mA cm−2. The measured performance surpasses the current total water splitting catalysts in the literature and also the Pt/ C/NF||IrO2/NF (412 mV at 50 mA cm−2) dual-PGM catalyst Received: Revised: Accepted: Published: A

April 11, 2019 June 28, 2019 June 28, 2019 June 28, 2019 DOI: 10.1021/acs.iecr.9b01976 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Morphology of Fe-NiS-(BO)/NF: (a) SEM image, (b) FESEM image and TEM image (inset in (b). TEM images of reaction products sampled at different reaction times: (c) 1 h, (d) 4 h, and (e) 8 h. Corresponding SEM images are (f) 1 h, (g) 4 h, and (h) 8 h.

foam was removed, washed with water several times, and dried at 60 °C in a vacuum. For comparative studies, Fe-free boratedoped Ni3S2 was also prepared by the same procedure sans the use of iron sulfate. Likewise, borate/Fe-free undoped Ni3S2 was also prepared without Fe and borate precursors. The catalyst loading on NF was in the range of 4−6 mg cm−2. Materials Characterization. The crystal structures of the synthesized materials were determined by X-ray diffraction (XRD) on a Bruker D8 advance X-ray diffractometer using a Cu Kα (1.5405 Å) source. Morphology examination, on the other hand, was based on field-emission transmission electron microscopy (FETEM, on a JEOL 2100F microscope) and field-emission scanning electron microscopy (FESEM, on a JEOL JSM-6700F microscope). The catalyst elemental compositions were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Thermo Scientific iCAP 6200 duo spectrometer based on samples removed from NF by ultrasonication. The catalyst surface composition and constituent oxidation states were analyzed by X-ray photoelectron spectroscopy (XPS) on a Kratos Axis Ultra DLD spectrometer fitted with a dual anode (Al/Mg) Xray gun and a focused monochromatic Al X-ray gun. The measured binding energies were corrected by referencing the C 1s peak of adventitious carbon to 284.50 eV. Electrochemical Measurements. The electrochemical performance of the catalysts for OER and HER in 1 M KOH was evaluated by linear sweep voltammetry (LSV) and galvanostatic measurements on an Autolab type III potentiostat/galvanostat in the standard three electrode configuration. Following the published best practice of excluding any presence of PGM in the evaluation of non-PGM catalysts,17 the three-electrode cell contained a catalyst-loaded NF working electrode, a graphite rod counter electrode, and a 3 M KCl Ag/AgCl reference electrode with a 3 M KCl salt

system. More importantly the study establishes the general utility of borate doping to different nickel sulfide phases (i.e., Ni3S4 and Ni3S2) and the feasibility of using concurrent anion (borate) and cation (Fe) doping to further improve the total water splitting performance of nickel sulfide catalysts.

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EXPERIMENTAL SECTION Chemicals. Potassium hydroxide (KOH, ≥85%), nickel(II) sulfate hexahydrate (NiSO4·6H2O, >99.0%), iron(II) sulfate heptahydrate (FeSO4·7H2O, ≥99%), potassium tetraborate tetrahydrate (K 2 B 4O 7 ·4H2 O, ≥99.5%), ethylene glycol (HOCH2CH2OH, 99.8%), polytetrafluoroethylene solution ((CF2CF2)n, 60 wt % dispersion in ethanol), iridium(IV) oxide (IrO2, 99.9%), and L-cysteine (HSCH2CH(NH2)COOH, 97%) from Sigma-Aldrich and 20 wt % Pt on Vulcan XC-72 (Pt/C) catalyst from Premetek were used as received. Nickel foam (NF, 99.8%) was supplied by Taiyuan Lizhiyuan Co. Ltd. (China) and pretreated before use (vide infra). Deionized water (DI water) was the universal solvent unless indicated otherwise. Synthesis of Borate/Fe Dual-Doped Nickel Sulfide on Nickel Foam. The nickel foam was ultrasonicated in acetone, 10% HCl aqueous solution, and DI water successively and then vacuum-dried at 60 °C. A previous method of preparation of Fe-doped nickel sulfide (FeNi2S4)16 was suitably modified to support dual borate/Fe doping and catalyst deposition on a nickel foam. Briefly, 1.5 mmol FeSO4 7H2O, 2 mmol NiSO4· 6H2O, 1.5 mmol K2B4O7·4H2O, and 8 mmol L-cysteine were dissolved in 40 mL of a glycol−DI water mixture (1:1 v/v) and rigorously stirred for 30 min. This mixture and a pretreated nickel foam (3 cm × 2 cm) were transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 8 h. After naturally cooling to room temperature, the nickel B

DOI: 10.1021/acs.iecr.9b01976 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. (a) XRD patterns of as-synthesized materials and (b) expanded XRD patterns in the region of interest. The red line shows the standard diffraction peak of Ni3S2 (JCPDS No. 44-1418). XPS spectra of as-synthesized NiS/NF, NiS-(BO)/NF, and Fe-NiS-(BO)/NF in (c) Ni 2p, (d) S 2p, (e) B 1s, and (f) Fe 2p regions (inset is the Fe 3p spectrum).

hydrogen gases released were collected and measured to calculate the Faradaic efficiency.

bridge. All measured potentials were converted to the reversible hydrogen electrode (RHE) scale at pH 14 (E = +1.0374 V with respect to the 3 M KCl Ag/AgCl reference electrode). The catalyst activity and stability in OER and HER were measured in a deaerated 1 M KOH solution (pH 14) at 25 °C (under flowing N2 after 30 min of N2 purging). All linear sweep voltammograms were based on the same catalyst loading and scan rate (5 mV s−1) and iR-corrected. The electrochemically active surface areas (ECSAs) of the catalysts were estimated by their double layer capacitance in the non-Faradaic region using scan rates from 5 to 50 mV s−1. The OER and HER activities of 20% Pt/C and IrO2 on NF were also measured for performance benchmarking. The working electrodes for the latter were prepared by ultrasonicating 8 mg Pt/C (or IrO2 powder) and 1 mL polytetrafluoroethylene dispersion in ethanol for 30 min, followed by drop-casting the catalyst ink onto a NF (1 cm × 1 cm) foam and air-drying at room temperature. The catalyst loading was kept at ∼5.0 mg cm−2. Electrolytic cells for water splitting were assembled by using two identical catalyst-loaded nickel foams as the anode and the cathode in 1 M KOH solution. The oxygen and

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RESULTS AND DISCUSSION

Borate/Fe dual-doped Ni3S2 on nickel foam (denoted henceforth as Fe-NiS-(BO)/NF) was prepared by a facile one-step method. The nickel foam (NF) provided a connected 3D macrostructure and conductive substrate for the deposition of the dual-doped electrocatalyst (Figure 1a). The scanning electron microscopy (SEM) image in Figure 1b shows the catalyst as uniform microspheres of Fe-NiS-(BO) attached to the NF substrate. The partially broken shells of some microspheres revealed their hollow interior, which is also apparent in the TEM images as material density changes (inset of Figure 1b). The mechanism of hollow microsphere formation was investigated by examining the morphologies of the products formed at different reaction times by TEM (Figure 1c−e) and SEM (Figure 1f−h). Figure 1c and f shows that solid microspheres were formed after a reaction time of 1 h. Some decomposition of the solid interior occurred after 4 h of C


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decrease in the Ni content from 64.5 to 55.9 atom % with 10.6 atom % of Fe doping is an indication of cation substitutional doping. The surface composition and elemental chemical states before and after anion and cation codoping were analyzed by XPS. The analysis of the surface Ni, S, B, and Fe contents was respectively, based on the high-resolution Ni 2p, S 2p, B 1s, and Fe 3p XPS spectra (Figure 2c−f). The three sets of peaks in the Ni 2p spectrum of NiS/NF (Figure 2c) suggest the presence of Ni in multiple oxidation states. The two peaks centered at 852.17 and 868.72 eV correspond well with the 2p3/2 and 2p1/2 states of Ni (0) and the two peaks at 855.45 and 873.05 eV with the 2p3/2 and 2p1/2 states of Ni2+. The two broad peaks around 860.96 and 878.91 eV are the satellite features of Ni 2p3/2 and Ni 2p1/2, respectively. The SEM image in Figure 1a suggests a Ni3S2 layer thicker than 10 nm on the NF. Since the detection depth from the surface was ∼10 nm based on the Al X-ray source, the detected Ni(0) XPS signal was most likely generated by Ni3S2 and not from the underlying Ni foam. The copresence of Ni (0) and Ni2+ is expected from the Ni3S2 stoichiometry.19 No peak shift in the Ni 2p spectrum was found after borate doping (NiS-(BO)/ NF) due to the similar electronegativity of the borate group (average electronegativity of 2.59)20 with a S anion (2.58). On the other hand, Fe doping shifted the Ni 2p3/2 peak negatively from 855.45 eV (NiS/NF) to 855.30 eV (Fe-NiS-(BO)/NF) due to the lower electronegativity of Fe (1.83) relative to Ni (1.91). The electronegativity argument also suggests easier electron transfer from Fe to neighboring S atoms than from Ni to S atoms. This could also be a factor contributing to the relative ease of Fe doping of the nickel sulfide structure.14 Two peaks featured prominently in the S 2p XPS spectrum of NiS/ NF (Figure 2d). The peak at ∼162.5 eV could be deconvoluted into two overlapping peaks centered at 162.40 and 163.90 eV, which may be assigned to Sn2− and metal-S bonds of S.21 The isolated peak at 168.76 eV is characteristic of S species in high oxidation states found among sulfurcontaining compounds after exposure to air.22 The S 2p XPS spectra of NiS-(BO)/NF and NiS/NF are largely similar due to the similar electronegativity of borate and S anions as mentioned before. On the other hand, the peaks due to Sn2− and metal-S bonds were shifted negatively after Fe doping (from 162.4 to 161.82 eV and from 163.9 to 163.41 eV, respectively). The suggestion of a higher electron density around the S atoms in Fe-NiS-(BO)/NF is consistent with the findings from the Ni 2p spectra. Lastly, the 191.8 eV peak in the B 1s spectra in Figure 2e confirms the presence of threecoordinated borate species in both NiS-(BO)/NF and Fe-NiS(BO)/N.23 It was however difficult to analyze Fe in Fe-NiS(BO)/NF based on the Fe 2p spectrum (the most intense XPS peak of Fe) due to the overlap of the binding energy of Fe 2p with the Ni LMM state (Figure 2f). We resorted to the analysis of the Fe 3p state instead and confirmed the presence of Fe in Fe-NiS-(BO)/NF with two peaks (55.9 and 57.9 eV) attributable to Fe2+ and Fe3+, respectively. The OER and HER performances of all catalysts in this study were evaluated separately in standard three-electrode cells with a 1 M KOH electrolyte. iR correction was applied to all measurements. The OER overpotential, a key performance indicator for OER electrocatalysis, was defined as the potential difference from 1.23 V (vs RHE) which is required to support a current density of 50 mA cm−2 (η50). At this low current density, the contribution from the Ni(II)/Ni(III) redox

reaction, initiating the development of a core−shell structure (Figure 1d and g). After 8 h of reaction, the core was emptied, and shells with hollow interiors were formed (Figure 1e and h). There was limited morphology change thereafter. The formation of the Ni3S2 hollow spheres could be understood in terms of a previously known mechanism.16 According to a study from the Wang group,16 the L-cysteine molecules are predisposed to form large spherical particles of polypeptides. Due to the coordination affinity of the L-cysteine thiol group (−SH) for metal cations to form metal-L-cysteine complexes,18 the L-cysteine residues on the polypeptide particle exterior could easily bind the Fe2+ and Ni2+ cations in the precursor solution.16 During hydrothermal processing, the polypeptide decomposed and released H2S as the sulfur source (reaction 1) to convert the immobilized Ni2+-L-cysteine complex into a Ni3S2 deposit on the walls of the spherical core. The deposit gradually thickened while the core shrunk with the progress of the reaction. Finally, the L-cysteine core was completely depleted leaving Ni3S2 as the residual shell over a hollow interior. HSCH 2CHNH 2COOH + H 2O → CH3COCOOH + NH3 + H 2S

(1)

The crystal structure of Fe-NiS-(BO)/NF was analyzed by X-ray powder diffraction (XRD) (Figure 2a). For comparison, Fe-free borate-doped Ni3S2/NF (denoted henceforth as NiS(BO)/NF) and undoped Ni3S2/NF (denoted henceforth as NiS/NF) were also similarly analyzed. All three diffraction patterns confirm the presence of Ni3S2 (JCPDS No. 44-1418) on NF. No peaks attributable to a borate structure (as BO3 according to the XPS spectrum in Figure 2e, vide infra) or Fe sulfides were detected, and hence, the dual doping did not result in the formation of new crystal phases. A closer look at the dominant (110) peak indicated shifts to lower Bragg angles (2θ = 31.07° and d value of 2.87 Å for NiS-(BO)/NF and 2θ = 31.03° and d value of 2.88 Å for Fe-NiS-(BO)/NF in comparison with 2θ = 31.14° and d value of 2.86 Å for NiS/ NF). These shifts are indications of an expanded lattice due to the incorporation of borate and Fe into the Ni3S2 crystal lattice (borate ion (191 pm) is larger than the sulfide ion (184 pm), and the Fe cation (78 pm) is larger than the Ni cation (69 pm)) (Figure 2a). The comparable ionic radii of the host and dopant ions eased substitutional doping without affecting the structural integrity of the nickel sulfide host. The ICP analyses of the Ni, S, B, and Fe contents in NF-free NiS, NiS-(BO), and Fe-NiS-(BO) (Table 1) confirmed the occurrence of substitutional doping. The Ni contents before and after borate doping were about the same (62.9 atom % for NiS and 64.5 atom % for NiS-(BO)), but the S content decreased notably from 37.1 to 25.8 atom % with borate doping. On the other hand, the Table 1. Elemental Compositions of Catalysts Ultrasonically Removed from NF element

NiS (atom %)

NiS-(BO) (atom %)

Fe-NiS-(BO) (atom %)

Ni S B

62.9 37.1 − − − −

64.5 25.8 9.7 (B/S: 37.6) − −

55.9 24.0 9.5 (B/S: 39.6) 10.6 (Fe/Ni: 18.9)

Fe

D


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Figure 3. (a) iR-corrected polarization curves of OER on NF, NiS/NF, NiS-(BO)/NF, Fe-NiS-(BO)/NF, and IrO2/NF. (b) Comparison of OER Tafel slopes. (c) iR-corrected polarization curves of HER on NF, NiS/NF, NiS-(BO)/NF, Fe-NiS-(BO)/NF, and Pt/C. (d) Comparison of HER Tafel slopes. Chronoamperometry of (e) OER and (f) HER on Fe-NiS-(BO)/NF for a period of 10 h.

Table 2. Literature Survey of Selected High Performance OER Catalysts in 1 M KOHa catalyst

η at 10 mA cm−2 (mV)


Tafel slope (mV decade−1)

Fe-NiS-(BO)/NF NiS-(BO)/NF NiS/NF IrO2/NF IrO2/NF Fe-Ni3S2/FeNi MoS2/Ni3S2 heterostructures High-index faceted Ni3S2/NF MoS2-Ni3S2 HNRs/NF NiCo2S4 NW/NF

− − − − − 282 218 260 249 260

250 290 368 317 362 382 276 − 314 343

54.5 65.9 88.0 63.2 90 54 88 − 57 40

ref this this this this 27 28 29 30 8 21

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a

HNRs, heteronanorods; NW, nanowire.

E


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Industrial & Engineering Chemistry Research Table 3. Literature Survey of Selected High Performance HER Catalysts in 1 M KOH catalyst



Tafel slope (mV decade−1)

Fe-NiS-(BO)/NF NiS-(BO)/NF NiS/NF Pt/C/NF Pt/C/NF NixCo3−xS4/Ni3S2/NF MoS2/Ni3S2 heterostructures High-index faceted Ni3S2/NF MoS2-Ni3S2 HNRs/NF NiCo2S4 NW/NF

140 177 237 40 48 136 110 223 98 210

200 272 339 126 189 − 158 − 172 −

57.9 77.8 101.2 40.1 170 107 83.1 − 61 58.9


work work work work

a

HNRs: heteronanorods; NW: nanowire.

Figure 4. Determination of electrode electrochemically active surface areas (ECSAs) via electrochemical capacitance measurements. Cyclic voltammetry at different scan rates for (a) NiS/NF, (b) NiS-(BO)/NF, and (c) Fe-NiS-(BO)/NF. (d) Plot of measured capacitive current as a function of scan rate. ECSA-normalized current densities (j/ECSA) of different electrodes for (e) OER and (f) HER. F


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Figure 5. (a) OER and (b) HER polarization curves of Fe-NiS-(BO)/NF prepared with different borate contents in the feed (0,5, 1, 1.5, and 2 mmol). (c) Overpotentials at a current density of 50 mA cm−2.

reaction at ∼1.4 V is relatively minor.24−26 Figure 3a shows the outstanding OER activity of Fe-NiS-(BO)/NF, with η50 as low as 250 mV which compares well with the rest of the catalysts (η50 = 290 mV for Fe-free NiS-(BO)/NF, η50 of 368 mV for undoped NiS/NF, and η50 = 417 mV for the NF substrate). The activity of Fe-NiS-(BO)/NF also surpasses that of the reference IrO2/NF catalyst (η50 = 317 mV). The other important performance indicator, the OER Tafel slope (A), was calculated from the following equation ij j yz η = A·logjjjj zzzz jj z k 0{

Figure 3d), which is much smaller than those of NiS-(BO)/NF (77.8 mV dec−1), NiS/NF (101.2 mV dec−1), and NF (134.6 mV dec−1). Fe-NiS-(BO)/NF is also one of the best of transition-metal-based HER catalysts based on our literature survey conducted to date (Table 3). In summary, the concurrent cation and anion doping of the Fe-NiS-(BO)/NF catalyst has integrated remarkable OER and good HER activities in a common catalyst, which ranks among the best of the non-PGM research catalysts developed to date. Catalyst stability is the natural next test for new catalysts which have shown good activity and favorable catalysis kinetics. Fe-NiS-(BO)/NF also performed well in the OER and HER durability tests (Figure 3e and f) where 96.8% and 92% of the initial activities (current density) could be retained after 10 h of OER and HER, respectively. Fe-NiS-(BO)/NF is therefore not only an active OER and HER catalyst but also a durable one. Since high surface area contributes to high catalytic activity,32,33 the electrochemically active surface areas (ECSAs) of NiS/NF, NiS-(BO)/NF, and Fe-NiS-(BO)/NF were also estimated from electrochemical capacitance measurements and compared. The measurements were taken in the −0.5−0.5 V voltage window where no Faradaic process occurred.34 Based on the cyclic voltammograms recorded at different scan rates, CDL was calculated from the slope of a linear plot of double-layer capacitive current (taken as the midpoint current from the voltammogram) against scan rate (Figure 4a−d). ECSA was then calculated from CDL by the following equation

(2)

where η is the measured overpotential, j the current density, and j0 is the exchange current density. A small Tafel slope is indicative of favorable OER kinetics. The OER Tafel slope was the lowest from Fe-NiS-(BO)/NF (54.5 mV dec−1, Figure 3b), followed by IrO2/NF (63.2 mV dec−1), NiS-(BO)/NF (65.9 mV dec−1), NiS/NF (88.0 mV dec−1), and NF (117.9 mV dec−1). A comparison of OER η50 and the Tafel slope with current OER catalysts in 1 M KOH (Table 2) clearly places the Fe-NiS-(BO)/NF catalyst as among the top of the PGM alternatives. The HER activity of the catalysts was assessed likewise. FeNiS-(BO)/NF again emerged as having the highest HER activity (Figure 3c), where its overpotential of 200 mV to support 50 mA cm−2 of current density (defined as η50-HER) is closer to that of the reference Pt/C/NF catalyst (126 mV) than NiS-(BO)/NF (272 mV), NiS/NF (339 mV), and NF (440 mV). In addition, Fe-NiS-(BO)/NF also showed favorable HER kinetics by having a Tafel slope comparable to that of Pt/C/NF (57.9 and 40.1 mV dec−1, respectively, in

ECSA = G

C DL CS

(3) DOI: 10.1021/acs.iecr.9b01976 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. (a) OER and (b) HER polarization curves of Fe-NiS-(BO)/NF prepared with different Fe precursor contents in the feed (0, 0,5, 1, 1.5, and 2 mmol). (c) Overpotentials at a current density of 50 mA cm−2.

50 mA cm−2 (η50-OER, η50-HER, and η50 (calculated as the sum of η50-OER and η50-HER)) were calculated and compared (Figure 5c). The η50 of Fe-NiS-(BO)/NF-B0.5 was 555 mV, which decreased to 484 mV for Fe-NiS-(BO)/NF-B1 and 450 mV for Fe-NiS-(BO)/NF-B1.5. The trend was reversed with further increases of the borate feed content, resulting in a η50 of 524 mV in the case of Fe-NiS-(BO)/NF-B2. The “near optimal” anion (borate) doping in this study was therefore a B/S atomic ratio of 39.6 from the NiS-(BO)/NF-B1.5 catalyst. An excess of borate content is believed to reduce the number of Ni active sites and increase the steric hindrance of the borate groups crowding on the catalyst surface. The “optimal” cation composition was also determined from similar OER and HER measurements. The results showed that a Fe/Ni atomic ratio of 18.9 provided the “near optimal” Fe-doping content for total water splitting (Figure 6). OER in general involves four sequential electron transfer steps (eqs I−IV).

where CS is the capacitance of an ideal planar surface with few irregularities. A value of 25 μF cm−2 is typical for the 1 M KOH electrolyte.34 The ECSAs calculated as such were 149.6, 154.4, and 158 cm2 for NiS/NF, NiS-(BO)/NF, and Fe-NiS(BO)/NF, respectively. These values are high among the transition metal catalysts.35 The surface area contribution to the measured activities was discounted by normalizing the measured activities of the catalysts by their ECSAs (Figure 4e and f). It was found that the catalysts still displayed the same trend of activity differences. The good OER and HER performance of Fe-NiS-(BO)/NF was therefore chemical in origin and primarily due to the enhancement of the intrinsic activity of Ni3S2 by anion and cation doping. The above understanding highlights the possibility of catalyst activity tuning via catalyst composition optimization. The cation and anion dopant contents of NiS were therefore varied by adjusting the amounts of borate and Fe precursors in the synthesis, and the performance of the resulting catalysts in OER and HER was measured and compared. Two series of catalysts were preparedone by fixing the amount of the Fe precursor salt (iron(II) sulfate, 1.5 mmol) in the synthesis and varying only the borate content (Fe-Fe-NiS-(BO)/NF-B0.5, Fe-NiS-(BO)/NF-B1, Fe-NiS-(BO)/NF-B1.5, and Fe-NiS(BO)/NF-B2 where the number indicates the borate feed content in mmol) and the other by fixing the borate feed content (1.5 mmol) and varying the Fe precursor content in the synthesis (Fe-NiS-(BO)/NF-Fe0, Fe-NiS-(BO)/NF-Fe0.5, Fe-NiS-(BO)/NF-Fe1, Fe-NiS-(BO)/NF-Fe1.5, and Fe-NiS(BO)/NF-Fe2, where the number indicates the Fe feed content in mmol). The OER and HER polarization curves of these catalysts are shown in Figure 5a and b, based on which the overpotentials for OER, HER, and total water splitting at

H 2O(l) + * ↔ OHads + H+ + e− OHads ↔ Oads + H+ + e−

(I) (II)

Oads + H 2O(l) ↔ OOHads + H+ + e−

(III)

OOHads ↔ * + O2 (g) + H+ + e−

(IV)

Tafel analysis is often used to deduce the rate-determining step (RDS) in OER. A Tafel slope of ∼40 mV dec−1, for example, suggests the formation of OOHads (step III) as the RDS.36−38Similarly, a Tafel slope of ∼60 mV dec−1 suggests the formation of Oads (step II) as the RDS,37−40 and a Tafel slope greater than 120 mV dec−1 suggests the formation of H


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Figure 7. (a) iR-corrected polarization curves measured in experimental water electrolyzers. (b) % retention of initial current over 10 h of operation in Fe-NiS-(BO)/NF||Fe-NiS-(BO)/NF; inset shows the polarization curves before and after the 10 h test. (c) Evolution of H2 and O2 from Fe-NiS(BO)/NF for the determination of Faradaic efficiency. The circles and squares represent the amounts of H2 and O2 generated, respectively. The red line and black lines represent the theoretical amounts of H2 and O2 calculated based on 100% Faradaic efficiency. All measurements were taken in 1 M KOH electrolyte.

Table 4. Literature Survey of Selected High-Performance Bifunctional Catalysts for Total Water Splitting in 1 M KOHa catalysta

potential at 10 mA cm−2 (V)

potential at 50 mA cm−2 (V)

Fe-NiS-(BO)/NF NiS-(BO)/NF NiS/NF Pt/C/NF||IrO2/NF CoSx/Ni3S2/NF 3D hierarchical porous Ni3S4/NF Fe17.5%-Ni3S2/NF CoP/NF||Co2P/NF Fe- and Ni-MOFs/NF FePx/Fe-N-C/NPC

1.38 1.48 1.57 1.55 1.57 1.61 1.54 1.56 1.50 1.58

1.59 1.61 1.70 1.64 1.86 1.79 1.66 1.68 1.80 −


work work work work

a

The catalyst was applied to both anode and cathode unless stated otherwise; MOF, metal−organic framework; NPC, N- and P-codoped carbon.

adsorption, ΔG(Hads), on a catalyst surface can therefore be used as a HER activity descriptora HER catalyst should adsorb Hads neither too strongly nor too weakly with ΔG(Hads) close to zero as the optimal value. As for the effect of Fe doping, it has been shown that Fe doping of nickel sulfide would move ΔG(Hads) to this optimum as in the case of Pt.14 This could rationalize the high HER activity of Fe-NiS-(BO)/ NF relative to native and borate-doped NiS/NF. In addition, it has been reported that Fe doping could also give rise to the formation of a bimetallic Fe-Ni (oxy)hydroxide (Fe-NiOOH) phase during OER, which is often regarded as catalytically more active than NiOOH.47 While there have been some previous attempts at anion and cation codoping (e.g., Fe/O codoped Co2P,48), the dopants were however not selected to simultaneously benefit both OER and HER. Since Fe-NiS-(BO)/NF has shown good activity in both OER and HER, its suitability as a total water splitting catalyst was tested in a two-electrode experimental electrolyzer using Fe-NiS-(BO)/NF as the anode and cathode (Figure 7a). The iR-corrected cell voltage required for total water splitting to deliver 50 mA cm−2 of current density was as low as 1.59 V, which outperformed the NiS-(BO)/NF||NiS-(BO)/NF (1.61 V) and NiS/NF||NiS/NF (1.70 V) electrolyzers easily. The overwhelmingly high OER activity of this catalyst also made it possible to eclipse the performance of an electrolyzer with dual PGM catalysts Pt/C/NF||IrO2/NF (1.64 V) despite the lower HER activity of Fe-NiS-(BO)/NF relative to that of PGM. The performance represents a notable improvement over the recent non-PGM total water splitting systems (Table 4) most notably

OHads (step I) as the RDS.38,40 Therefore, OER Tafel slopes of NiS/NF, NiS-(BO)/NF, and Fe-NiS-(BO)/NF of 88.0, 65.9, and 54.5 mV dec−1, respectively, are indicative of Oads formation (step II) as the RDS, and borate and Fe doping has helped promote Oads formation (step II). In our last study, we proposed that three-coordinated borate could transform into four-coordinated borate via reactions with hydroxyl ions (the Lewis base) in an alkaline electrolyte. This would facilitate the proton transfer from adsorbed OH to form Oads on the catalytic metal center.41 Since Fe-NiS-(BO)/NF contained both Fe(II) and Fe(III) which could withdraw electrons from Ni, OHads on Ni was rendered less stable, thus lowering the activation energy of Oads formation (step II). HER, on the other hand, occurs by the Volmer−Heyrovsky or Volmer−Tafel mechanism involving three elementary reaction steps:42 the Volmer reaction (H2O + e− → Hads + OH−), the Heyrovsky reaction (Hads + H2O + e− → H2 + OH−), or the Tafel reaction (Hads + Hads → H2), with Tafel slopes of 120, 40, and 30 mV decade−1, respectively.43 The HER Tafel slopes of 57.9, 77.8, and 101.2 mV decade−1 for FeNiS-(BO)/NF, NiS-(BO)/NF, and NiS/NF are all within the range of 40−120 mV decade−1, which can be used to suggest the prevalence of the Volmer reaction, i.e., the adsorption of H2O and the dissociation of H2Oads into Hads atom and OH− as the RDS.44−46The partial negative charge on fourcoordinated borate (under alkaline condition) can also alleviate the bond breaking in adsorbed H2O on the catalyst surface. A catalyst’s HER activity is also dependent on the Hads adsorption strength on a catalyst surface. The free energy of I


Article

Industrial & Engineering Chemistry Research CoSx/Ni3S2/NF||CoSx/Ni3S2/NF (1.86 V),7 3D hierarchical porous Ni3S4/NF||Ni3S4/NF (1.79 V),49 CoP/NF||Co2P/NF (1.68 V),50 and Fe17.5%-Ni3S2/NF||Fe17.5%-Ni3S2/NF (1.66 V).14 The η50 (364 mV) measured from the two-electrode electrolyzer was close to that of the three-electrode cell (450 mV, as the sum of η50-OER and η50-HER), despite the different cell configurations. The experimental electrolyzer could also retained 95.7% of its initial current density after 10 h of operation (Figure 7b). Hence, the total water splitting performance of Fe-NiS-(BO)/NF ranks at the top of research catalysts in recent reports. Furthermore, the evolution of O2 and H2 as a function of reaction time is shown in Figure 7C, showing almost 100% Faradaic efficiency for both HER and OER.

M.; Artero, V. A Janus cobalt-based catalytic material for electrosplitting of water. Nat. Mater. 2012, 11 (9), 802−807. (2) Turner, J. A. Sustainable hydrogen production. Science 2004, 305 (5686), 972−974. (3) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 2013, 135 (45), 16977−16987. (4) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Nørskov, J. K. Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. In Materials for Sustainable Energy: A Collection of Peer-Reviewed Research and Review Articles from Nature Publishing Group; World Scientific, 2011; pp 280−284. (5) Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. Efficient electrocatalytic oxygen evolution on amorphous nickel−cobalt binary oxide nanoporous layers. ACS Nano 2014, 8 (9), 9518−9523. (6) Lu, X.; Zhao, C. Electrodeposition of hierarchically structured three-dimensional nickel−iron electrodes for efficient oxygen evolution at high current densities. Nat. Commun. 2015, 6, 6616− 6623. (7) Shit, S.; Chhetri, S.; Jang, W.; Murmu, N. C.; Koo, H.; Samanta, P.; Kuila, T. Cobalt Sulfide/Nickel Sulfide Heterostructure Directly Grown on Nickel Foam: An Efficient and Durable Electrocatalyst for Overall Water Splitting Application. ACS Appl. Mater. Interfaces 2018, 10 (33), 27712−27722. (8) Yang, Y.; Zhang, K.; Lin, H.; Li, X.; Chan, H. C.; Yang, L.; Gao, Q. MoS2−Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2017, 7 (4), 2357−2366. (9) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Carbon Nanotubes Decorated with CoP Nanocrystals: A Highly Active Non-Noble-Metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 53 (26), 6710−6714. (10) Wu, Y.; Liu, Y.; Li, G.-D.; Zou, X.; Lian, X.; Wang, D.; Sun, L.; Asefa, T.; Zou, X. Efficient electrocatalysis of overall water splitting by ultrasmall NixCo3−xS4 coupled Ni3S2 nanosheet arrays. Nano Energy 2017, 35, 161−170. (11) Huang, y.; liu, q.; Lv, J.; Babu, D. D.; Wang, W.; Wu, M.; Yuan, D.; Wang, Y. Co-intercalation of Multiple Active Units in Graphene by Pyrolysis of Hydrogen-bonded Precursors for Zinc−air Batteries and Water Splitting. J. Mater. Chem. A 2017, 5, 20882−20891. (12) Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon coated porous nickel phosphides nanoplates for highly efficient oxygen evolution reaction. Energy Environ. Sci. 2016, 9 (4), 1246− 1250. (13) Zhang, Z.; Zhang, T.; Lee, J. Y. Electrochemical Performance of Borate-Doped Nickel Sulfide: Enhancement of the Bifunctional Activity for Total Water Splitting. ChemElectroChem. 2019, 6 (5), 1443−1449. (14) Zhang, G.; Feng, Y.-S.; Lu, W.-T.; He, D.; Wang, C.-Y.; Li, Y.K.; Wang, X.-Y.; Cao, F.-F. Enhanced Catalysis of Electrochemical Overall Water Splitting in Alkaline Media by Fe Doping in Ni3S2 Nanosheet Arrays. ACS Catal. 2018, 8 (6), 5431−5441. (15) Yu, X.-Y.; Yu, L.; Lou, X. W. D. Metal Sulfide Hollow Nanostructures for Electrochemical Energy Storage. Adv. Energy Mater. 2016, 6 (3), 1501333. (16) Wang, H.; Tang, J.; Li, Y.; Chu, H.; Ge, Y.; Baines, R.; Dong, P.; Ajayan, P. M.; shen, j. f.; Ye, M. Template-free solvothermal preparation of ternary hollow balloons as RuO2-like efficient electrocatalysts for oxygen evolution reaction with superior stability. J. Mater. Chem. A 2018, 6, 19417−19424. (17) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Highperformance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011, 332 (6028), 443−447. (18) Chang, K.; Chen, W. L-cysteine-assisted synthesis of layered MoS(2)/graphene composites with excellent electrochemical performances for lithium ion batteries. ACS Nano 2011, 5 (6), 4720− 4728. (19) Liu, X.; Li, Y. X.; Chen, N.; Deng, D. Y.; Xing, X. X.; Wang, Y. D. Ni3S2@Ni foam 3D electrode prepared via chemical corrosion by

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CONCLUSION In summary, a simple hydrothermal synthesis was used to prepare hollow borate/Fe dual-doped Ni3S2 microspheres on nickel foam NF (Fe-NiS-(BO)/NF). The 3D catalystintegrated electrode prepared as such showed outstanding catalytic performance in both OER and HER with smaller overpotentials and lower Tafel slopes than borate-doped (NiS(BO)/NF) and undoped NiS (NiS/NF). The Fe-NiS-(BO)/ NF with the optimized Fe and borate contents was evaluated for total water splitting in both two- and three-electrode configurations. The low η50 of Fe-NiS-(BO)/NF of 1.59 V easily surpassed the performance of an alkaline water electrolyzer with state-of-the-art PGM catalysts (1.64 V). The good performance of Fe-NiS-(BO)/NF in OER was therefore more than enough to compensate for its lesser HER performance relative to PGM (a common feature among all current non-PGM alternatives). This study demonstrates the potential in using complementary cation and anion doping to improve the OER and HER properties of existing non-PGM catalysts, which can be a new strategy to broaden the search for new catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhao Zhang: 0000-0002-0417-3182 Tianran Zhang: 0000-0003-2837-4971 Jim Yang Lee: 0000-0003-1569-9718 Author Contributions

This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.Z. acknowledges the National University of Singapore for his research scholarship. T.Z. acknowledges the grants from the National Research Foundation (NRF), Prime Minister’s office, Singapore, under the Campus for Research Excellence and Technological Enterprise (CREATE) program.

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REFERENCES

(1) Cobo, S.; Heidkamp, J.; Jacques, P.-A.; Fize, J.; Fourmond, V.; Guetaz, L.; Jousselme, B.; Ivanova, V.; Dau, H.; Palacin, S.; Fontecave, J


Article

Industrial & Engineering Chemistry Research sodium sulfide and using in hydrazine electro-oxidation. Electrochim. Acta 2016, 213, 730−739. (20) Mohammad. Electronegativity, Interaction Parameter and Electronicpolarizability for Bisumth Borate Metal Glasses. Phys. Int. 2011, 2 (1), 21−24. (21) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26 (26), 4661−4672. (22) Liang, Y.; Liu, Q.; Luo, Y.; Sun, X.; He, Y.; Asiri, A. M. Zn 0.76 Co 0.24 S/CoS 2 nanowires array for efficient electrochemical splitting of water. Electrochim. Acta 2016, 190, 360−364. (23) Choi, N.-S.; Yew, K. H.; Kim, H.; Kim, S.-S.; Choi, W.-U. Surface layer formed on silicon thin-film electrode in lithium bis(oxalato) borate-based electrolyte. J. Power Sources 2007, 172 (1), 404−409. (24) Batchellor, A. S.; Boettcher, S. W. Pulse-Electrodeposited Ni− Fe (Oxy)hydroxide Oxygen Evolution Electrocatalysts with High Geometric and Intrinsic Activities at Large Mass Loadings. ACS Catal. 2015, 5 (11), 6680−6689. (25) Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.; Yachandra, V. K.; Nocera, D. G. Structure-activity correlations in a nickel-borate oxygen evolution catalyst. J. Am. Chem. Soc. 2012, 134 (15), 6801−6809. (26) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient water oxidation using nanostructured alpha-nickelhydroxide as an electrocatalyst. J. Am. Chem. Soc. 2014, 136 (19), 7077−7084. (27) Cao, L. M.; Hu, Y. W.; Tang, S. F.; Iljin, A.; Wang, J. W.; Zhang, Z. M.; Lu, T. B. Fe-CoP Electrocatalyst Derived from a Bimetallic Prussian Blue Analogue for Large-Current-Density Oxygen Evolution and Overall Water Splitting. Adv. Sci. 2018, 5 (10), 1800949. (28) Yuan, C. Z.; Sun, Z. T.; Jiang, Y. F.; Yang, Z. K.; Jiang, N.; Zhao, Z. W.; Qazi, U. Y.; Zhang, W. H.; Xu, A. W. One-Step In Situ Growth of Iron-Nickel Sulfide Nanosheets on FeNi Alloy Foils: HighPerformance and Self-Supported Electrodes for Water Oxidation. Small 2017, 13 (18), 1604161. (29) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X.; Feng, X. Interface Engineering of MoS2 /Ni3 S2 Heterostructures for Highly Enhanced Electrochemical OverallWater-Splitting Activity. Angew. Chem., Int. Ed. 2016, 55 (23), 6702−6707. (30) Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-index faceted Ni3S2 nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting. J. Am. Chem. Soc. 2015, 137 (44), 14023−14026. (31) Li, X.; Liu, P. F.; Zhang, L.; Zu, M. Y.; Yang, Y. X.; Yang, H. G. Enhancing alkaline hydrogen evolution reaction activity through Ni− Mn 3 O 4 nanocomposites. Chem. Commun. (Cambridge, U. K.) 2016, 52 (69), 10566−10569. (32) Song, F.; Hu, X. Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. Nat. Commun. 2014, 5, 4477− 4482. (33) Jiang, W. J.; Niu, S.; Tang, T.; Zhang, Q. H.; Liu, X. Z.; Zhang, Y.; Chen, Y. Y.; Li, J. H.; Gu, L.; Wan, L. J.; Hu, J. S. CrystallinityModulated Electrocatalytic Activity of a Nickel(II) Borate Thin Layer on Ni3 B for Efficient Water Oxidation. Angew. Chem., Int. Ed. 2017, 56, 6572−6577. (34) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 2013, 135 (45), 16977−16987. (35) Liang, X.; Dong, R.; Li, D.; Bu, X.; Li, F.; Shu, L.; Wei, R.; Ho, J. C. Coupling of Nickel Boride and Ni(OH)2 Nanosheets with Hierarchical Interconnected Conductive Porous Structure Synergizes the Oxygen Evolution Reaction. ChemCatChem 2018, 10, 4555− 4561.

(36) Louie, M. W.; Bell, A. T. An investigation of thin-film Ni−Fe oxide catalysts for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2013, 135 (33), 12329−12337. (37) Lyons, M. E.; Doyle, R. L. Oxygen evolution at oxidised iron electrodes: a tale of two slopes. Int. J. Electrochem. Sci. 2012, 7 (10), 9488−9501. (38) Doyle, R. L.; Godwin, I. J.; Brandon, M. P.; Lyons, M. E. Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes. Phys. Chem. Chem. Phys. 2013, 15 (33), 13737−13783. (39) Birss, V. I.; Damjanovic, A.; Hudson, P. Oxygen evolution at platinum electrodes in alkaline solutions II. Mechanism of the reaction. J. Electrochem. Soc. 1986, 133 (8), 1621−1625. (40) Fang, Y.-H.; Liu, Z.-P. Mechanism and tafel lines of electrooxidation of water to oxygen on RuO2 (110). J. Am. Chem. Soc. 2010, 132 (51), 18214−18222. (41) Zhang, Z.; Zhang, T.; Lee, J. Y. Enhancement Effect of Borate Doping on the Oxygen Evolution Activity of α-Nickel Hydroxide. ACS Applied Nano Materials 2018, 1 (2), 751−758. (42) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In situ cobalt-cobalt oxide/N-doped carbon hybrids as superior bifunctional electrocatalysts for hydrogen and oxygen evolution. J. Am. Chem. Soc. 2015, 137 (7), 2688−2694. (43) Mahmood, N.; Yao, Y.; Zhang, J. W.; Pan, L.; Zhang, X.; Zou, J. J. Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions. Adv. Sci. (Weinh) 2018, 5 (2), 1700464. (44) Gao, W.-K.; Qin, J.-F.; Wang, K.; Yan, K.-L.; Liu, Z.-Z.; Lin, J.H.; Chai, Y.-M.; Liu, C.-G.; Dong, B. Facile synthesis of Fe-doped Co 9 S 8 nano-microspheres grown on nickel foam for efficient oxygen evolution reaction. Appl. Surf. Sci. 2018, 454, 46−53. (45) Laursen, A. B.; Kegnæs, S.; Dahl, S.; Chorkendorff, I. Molybdenum sulfidesefficient and viable materials for electro and photoelectrocatalytic hydrogen evolution. Energy Environ. Sci. 2012, 5 (2), 5577−5591. (46) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically oriented cobalt selenide/NiFe layered-double-hydroxide nanosheets supported on exfoliated graphene foil: an efficient 3D electrode for overall water splitting. Energy Environ. Sci. 2016, 9 (2), 478−483. (47) Dionigi, F.; Strasser, P. NiFe-Based (Oxy)hydroxide Catalysts for Oxygen Evolution Reaction in Non-Acidic Electrolytes. Adv. Energy Mater. 2016, 6 (23), 1600621. (48) Duan, J.; Chen, S.; Vasileff, A.; Qiao, S. Z. Anion and Cation Modulation in Metal Compounds for Bifunctional Overall Water Splitting. ACS Nano 2016, 10 (9), 8738−8745. (49) Ren, J.-T.; Yuan, Z.-Y. Hierarchical Nickel Sulfide Nanosheets Directly Grown on Ni Foam: A Stable and Efficient Electrocatalyst for Water Reduction and Oxidation in Alkaline Medium. ACS Sustainable Chem. Eng. 2017, 5 (8), 7203−7210. (50) Li, H.; Li, Q.; Wen, P.; Williams, T. B.; Adhikari, S.; Dun, C.; Lu, C.; Itanze, D.; Jiang, L.; Carroll, D. L.; Donati, G. L.; Lundin, P. M.; Qiu, Y.; Geyer, S. M. Colloidal Cobalt Phosphide Nanocrystals as Trifunctional Electrocatalysts for Overall Water Splitting Powered by a Zinc-Air Battery. Adv. Mater. 2018, 30 (9), 1705796. (51) Raja, D. S.; Lin, H.-W.; Lu, S.-Y. Synergistically well-mixed MOFs grown on nickel foam as highly efficient durable bifunctional electrocatalysts for overall water splitting at high current densities. Nano Energy 2019, 57, 1−13. (52) Qin, Q.; Jang, H.; Li, P.; Yuan, B.; Liu, X.; Cho, J. A Tannic Acid−Derived N-, P-Codoped Carbon-Supported Iron-Based Nanocomposite as an Advanced Trifunctional Electrocatalyst for the Overall Water Splitting Cells and Zinc−Air Batteries. Adv. Energy Mater. 2019, 9, 1803312.

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