Facile Synthesis of Amorphous Ternary Metal BoridesâReduced

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Energy, Environmental, and Catalysis Applications

Facile Synthesis of Amorphous Ternary Metal Borides - Reduced Graphene Oxide Hybrid with Superior Oxygen Evolution Activity Jean Marie Vianney Nsanzimana, Raksha Dangol, Vikas Reddu, Shuo Dou, Yuecheng Peng, Khang Ngoc Dinh, Zhenfeng Huang, Qingyu Yan, and Xin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17836 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Materials & Interfaces

Facile Synthesis of Amorphous Ternary Metal Borides - Reduced Graphene Oxide Hybrid with Superior Oxygen Evolution Activity Jean Marie Vianney Nsanzimana,† Raksha Dangol,‡ Vikas Reddu,† Shou Duo,† Yeucheng Peng,† Khang Ngoc Dinh,‡ Zhenfeng Huang,† Qingyu Yan,‡* and Xin Wang†* †School

of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore). E-mail: [email protected] ‡School

of Material Science and Engineering, Nanyang Technological University, Singapore 639798 (Singapore). E-mail: [email protected] ABSTRACT: Metal borides represent an emerging family of advanced electrocatalyst for oxygen evolution reaction (OER). Herein, we present a fast and simple method of synthesizing iron-doped amorphous nickel boride on reduced graphene oxide (rGO) sheets. The hybrid exhibits outstanding OER performance and stability in prolonged OER operation. In 1.0 M KOH, only 230 mV is required to afford a current density of 15 mA cm-2 with a small Tafel slope of 50 mV dec-1. DFT calculations lead to a suggestion that the in-situ formation of MOxHy during electrochemical activation acts as active sites for water oxidation. The superior OER activity of the as-prepared catalyst is attributed to (i) unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents and, (iii) strong coupling of active material and highly conductive rGO. This work not only provides new perspectives to design highly effective material for OER, but also opens a promising avenue to tailor the electrochemical properties of metal borides which could be extended to other materials for energy storage and conversion technologies. KEYWORDS: amorphous • metal borides • 2D hybrid • graphene oxide • electrocatalyst • oxygen evolution reaction • synergistic

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1. INTRODUCTION Hydrogen, as a potential energy carrier, permits the storage of renewable electricity in the form of chemical bonds. Its use can mitigate carbon dioxide emission from the combustion of non-renewable fossil fuel.1-2 Though electrochemical water splitting technology has been wellestablished among other techniques as a clean and efficient technology for hydrogen production due to the possibility of coupling to other renewable sources such as solar and wind energy, it is still limited by the sluggish anodic oxygen evolution reaction (OER).3-5 As this sluggish electrochemical reaction is also involved in many energy storage and conversion technologies, it has become a hot topic over last decades.6 The state-of-the-art for OER is precious metalbased catalysts such as Ir- and Ru-based oxides and it’s composites. However, the high cost and scarcity of precious metals make their large-scale applications and commercial utilization both uneconomical and impractical.7-8 In light of these limitations, it is crucial to develop a low‐cost fabrication route of an efficient and earth-abundant material with abundant active sites, and good stability.9 Up to date, earth-abundant 3d transition metals-based material have been proposed as an alternative to precious metal-based catalyst for OER due to its electrocatalytic activity, lowcost and stability.10-11 These include but not limited to the monometal catalyst such as nickel, cobalt, and iron based oxides and (oxo)hydroxides.12-14 It was further found that doping of other metals to form multimetal catalyst offers a great advantage for modulating the electronic structure which results into the synergistic effect of active centers, and hence advanced OER activity.15-18 Especially, the Ni-based catalyst has been explored due to its stability against corrosion during the OER in alkaline medium and the doping of other metals had found to tailor the electrocatalytic activities.19-21 Though the OER active center in transition metal-based catalyst is oxidized metal, metal non-oxides material such as phosphides, nitrides, sulfides, selenides, and borides show promising activity compared to metal alloy and metal oxides.15, 18 2 ACS Paragon Plus Environment

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This has been attributed to the unusual effect of this metal-metalloid which acts as a precursor and thus, undergoes both morphological and chemical change in highly oxidative potentials.22-23 As a result, an in-situ formation of oxo/hydroxo metal species on the surface is noticed during the electrochemical oxidation process which functions as catalytic sites for OER with superior performance.15, 24 Recently, metal borides have received a new interest as efficient material which even outperform precious metal-based catalyst in some cases.15, 25 Various shapes from spherical particles to ultrathin nanosheets and different structures such as crystalline and amorphous structure had been reported.26-27 The amorphous nature of metal borides produced in an aqueous medium is important as it allows the tunable incorporation of other metals by controlling the metal ratio in metal salt solution, and this does not affect the nature of the final product. It also shows advanced activity mostly due to abundant defect sites.28 Hence, from OER catalyst perceptive, nanosized, amorphous metal boride material which allow exposure of more active sites is preferred. Among the techniques to boost the electrocatalytic activity is to prepare catalyst hybrid on highly conductive materials such as Ni foam and reduced graphene oxide (rGO).29-31 Thus, it is of a great interest to develop facile method to synthesize nanosized, amorphous metal borides on a conductive material with high activity. Herein, we synthetized an amorphous iron-doped nickel boride coupled to co-reduced GO sheets via one-step reduction reaction at room temperature for OER. The choice of rGO as substrate is because of its high electric conductivity, good electrochemical stability and ultralarge specific surface area.32 By incorporation of iron in nickel boride and controlling the ratio of GO to active material, the as-prepared electrocatalyst showed excellent OER catalytic performance and stability in alkaline medium. A very low overpotential of 230 mV is required for achieving the current density of 15 mA cm-2 with a small Tafel slope of 50 mV dec-1, which was better than Ni-B/G and commercial RuO2 catalyst at the same mass loading. To our 3 ACS Paragon Plus Environment


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knowledge, the as-prepared material exhibits outstanding activities compared to metal borides catalyst reported to date. This simple, facile and one-step hybridization of ternary metal boride on highly conductive co-reduced GO in a short period of time can be extended to other metal borides as well. 2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Graphene oxide (GO) was synthesized by mixing 2 g of graphite powder with 100 mL of sulfuric acid (H2SO4) in ice bath at 4 oC for 30 minutes. Afterwards, 12 g of potassium permanganate (KMnO4) was added slowly and maintained at 4 oC for one hour. Before adding 200 mL of deionized water (DIW), the solution was removed from the ice bath and transferred in the water bath. After two hours at 40 oC, the DIW was added slowly and when the temperature was below 60 oC, hydrogen peroxide (H2O2) was added drop by drop. The product was filtered and rinsed with 400 mL 10 % hydrochloric acid (HCl) for three times. Before using, GO was dispersed in water and the suspension were dialyzed for two weeks. The amorphous metal boride - graphene oxide hybrids were prepared by reducing aqueous mixture containing metals precursor (iron (II) sulfate heptahydrate (FeSO4.7H2O) and nickel (II) nitrate hexahydrate (Ni(NO3)2.6H2O)) and graphene oxide with a strong reducing agent, sodium borohydride (NaBH4), which also acts as the boron source. Typically, the total moles of the metal salts source were 1.25 mmoles in 45 mL and the reducing solution was 5 mL. The reduction of metal (M2+) ions in an aqueous solution results into a precipitate of metal boride via the following reaction:25, 33 2M2+ + 4BH4- + 9H2O → M2B + 12.5H2 + 3B(OH)3

(1)

Herein an adaptation had been used, instead of drop by drop addition of reducing solution to metal sources, rapid addition was used in the presence of saturated Argon (Ar) environment. The BH4-/M2+ ion ratio was maintained at three times to ensure the formation of metal borides.34 4 ACS Paragon Plus Environment

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To optimize the as-prepared material, we investigated the effect of (1) iron to nickel ratio, (2) different GO amounts, and (3) reduction rate. The addition of reducing solution caused rapid transformation of solution to black precipitate well dispersed in the solution. The black precipitates were collected and were washed subsequently with ultra-purified water to reach neutral pH and with ethanol three times to remove excess ions and other undesired dissolved species such as Na+, and Cl- ions. Afterward, the samples were dried for 12 hours in Ar saturated environment at 50 oC. The products were collected for structure and electrochemical characterization. To analyze the behavior of GO in reduction environment of metal ions in aqueous solution, the same amount of GO suspension used for optimized catalyst (30%) was treated by NaBH4. The obtained black product was also cleaned in DIW and ethanol and dried in the same environment as metal boride - reduced GO hybrid catalyst. 2.2. Catalyst Characterization. The surface morphologies of the as-prepared powders were studied by field emission scanning electron microscopy (FESEM, JEOL JSM 6701F) coupled with energy dispersive X-ray spectroscopy (EDX). Furthermore, the transmission electron microscopy (TEM) images and selected area diffraction (SAED) pattern were taken from JEOL 2010 HR while the TEM mapping and TEM-EDX were from JEOL 2100F at an accelerating voltage of 200 kV. Structural characterization was determined by recording the X-ray diffraction (XRD) patterns using the Cu Kα radiation (λ = 1.5414 Å) in Bragg–Brentano configuration. The N2 absorption and desorption isotherms were acquired by using the Brunauer–Emmett–Teller (BET) method on Quantachrome Autosorb-6 to investigates the specific surface areas and pore size distribution. The surface electronic states and composition of the as-prepared catalyst and after OER test (activation by cyclic voltammetry (CV) till a reproducible voltammogram were obtained and treated by chronopotentiometry at a fixed current density of 20 mA cm-2 for one hour) were carried out by X-ray photoelectron spectroscopy (XPS). XPS spectra were collected by a Kratos AXIS UltraDLD instrument 5 ACS Paragon Plus Environment


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equipped with a monochromatic Al Kα (1486.71 eV) X-ray source and a hemispherical analyser. 2.3. Electrochemical Measurement. Electrochemical measurements of the as-prepared material and commercial RuO2 catalyst for comparison were conducted in a three-electrode cell using a rotating disk electrode (RDE, PINE Research Instrumentation) with an Autolab PGSTAT302N

(Metrohm,

Switzerland)

potentiostat

workstation

equipped

with

electrochemical impedance spectroscopy (EIS). The graphite rod and Hg/HgO (1.0 M KOH) were used as the counter and reference electrodes, respectively. All the electrochemical measurements were conducted in O2-saturated alkaline 1.0 M KOH electrolyte at room temperature. The electrolyte was prepared by using potassium hydroxide (KOH) pellets and ultrapure H2O (18.2 MΩ, Millipore). Before loading the catalyst aliquot on GC electrodes, they were polished using aqueous alumina suspension on felt polishing pads, washed and sonicated in ethanol for five minutes, and finally, rinsed with DIW. The catalyst and the commercial Ruthunium (IV) oxide (RuO2, 99.9% metals basis, aladdin) catalyst suspensions were prepared by dispersing 5 mg of catalyst in 1.0 mL of solution containing 725 µL of ethanol, 235 µL of water and 40 µL of 5 wt% Nafion solution, followed by sonication for 20 minutes. Then appropriate aliquot was pipetted into pretreated GC surface (0.196 cm2) to give a loading of 0.3 mg/cm2, and then dried under ambient environment. The physical mixture of the unsupported metal boride and GO treated by NaBH4 to mimic the co-reduced GO during reduction process was prepared using 30% of treated-GO. The linear sweep voltammetry (LSV) polarization curves were recorded at a sweep rate of 5 mV s-1 under continuous stirring of 1600 rpm to avoid accumulation of gas bubbles over the GC electrode. Prior to LSV polarization curve recording, the materials were exposed to cyclic voltammetry (CV) polarization curve along the potential window of 0.2 V to 0.8 V vs. Hg/HgO at 100 mV s-1. In this work, after recording polarization curves, the iR drop was compensated 6 ACS Paragon Plus Environment

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at 97% which was measured by the EIS technique at 0.6 V vs. Hg/HgO. The stability test was carried out by recording CV profiles of 1000 cycles at 100 mV s-1 from 0.2 V to 0.6 V vs. Hg/HgO. Furthermore, the chronopotentiometric responses were measured at a fixed current density of 10 mA cm-2. All measured potentials reported in this paper were converted from vs. Hg/HgO to vs. RHE by using the following equation:21, 35 Evs. RHE = Evs. Hg/HgO + 0.095 + 0.059 pH

(2)

In addition, all the reported current densities were normalized to the area of the electrode. The overpotential was calculated as follows: η = E (vs. RHE)-1.23

(3),

considering O2/H2O equilibrium at 1.23 V vs. RHE. 2.4. Turnover of Frequency (TOF) and Electrochemically Active Surface Area (ECSA). The TOF was calculated as the following: TOF = J × A / (4F × m)

(4),

where J is the OER current density calculated from LSV collected at 5 mV s-1, F is the Faraday constant, A is the geometrical surface area of the electrode (GC = 0.196 cm-2), and m is the amount of electroactive Ni, Fe, and/or Ni-Fe sites, which was obtained from integrating the charge amount of metal redox peaks area. The ECSA was measured by electrochemical double layer capacitance method. The cyclic voltammetry (CV) used for electrochemical double layer capacitance (Cdl) calculation was acquired in a potential window where no Faradaic process occurred. To derive the Cdl, the following equation was used: Cdl = Ic/ν, (equation 5), where Cdl was the double-layer capacitance (mF cm-2) of the electroactive materials, Ic was charging current (mA cm-2) and ν was scan rate (mV s-1). The ECSA of a catalyst sample was calculated from the double layer



capacitance according to the following equation: ESCA = Cdl/Cs (equation 6), where Cs was the specific capacitance (0.040 mF cm−2). 2.5. Computational methods. The electronic structures and adsorption free energy were computed by Vienna ab initio simulation package (VASP) using spin-polarized density functional with the Hubbard model (DFT+U).36-37 The projector augmented wave (PAW) model with Perdew–Burke–Ernzerhof (PBE) function was employed to describe the interactions between core and electrons.38-39 An energy cutoff of 500 eV was used for the planewave expansion of the electronic wave function. The Brillouin zones of all systems were sampled with gamma-point centered Monkhorst–Pack grids. A 3 × 3 × 3 and 3 × 3 × 1 Monkhorst Pack k-point setup were used for bulk and slab geometry optimization. The force and energy convergence criterion were set to 0.02 eV Å-1 and 10-5 eV, respectively. 3. RESULTS AND DISCUSSION Due to the presence of a large amount of hydroxyl and carboxyl groups on GO, metal ions were adsorbed on the surface of GO, and sodium borohydride (NaBH4) was used both as reducing reagent and boron source. The chemical reduction process was carried out by rapid addition of reducing solution under Ar at room temperature. It has been reported that regardless of the other parameters, rapid mixing in the presence of Ar results into metal borides.34 The obtained metal boride was strongly coupled to simultaneously co-reduced GO sheets in a short duration compared to conventional method whereby the reducing agent is added dropwise and considerable time is required for the complete reaction. Scheme 1 shows a schematic representation of the catalyst synthesis (denoted as FexNiy-B/G). The morphology of a typical catalyst (FeNi3-B/G) was first studied by electron microscopy. In high-resolution transmission electron microscopy (HRTEM) image (Figure 1a,b) and at higher magnification FESEM (Inset Figure 1a), the rGO sheets were found to have been covered by nanostructures. The energy dispersed X-ray spectroscopy (EDX) spectrum (Figure 8 ACS Paragon Plus Environment

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S1b) contains Fe, Ni, B, C, O elements and gives 2 : 1 (the Inset Table in Figure S1b) for metal to boron ratio which indicates the formation of metal borides with a general formula of M2B (M2B/G) and agrees with the literature on transition metal borides produced by chemical reduction in aqueous medium.25, 33 The absence of dots and appearance of halo rings which are from the substrate in the selected area angle diffraction (SAED) pattern (the inset of Figure 1b) suggest the amorphous nature. On the contrary, GO is reported to show the presence of dots in SAED pattern, and hence, this shows that GO was simultaneously reduced to rGO, which got covered by metal boride.40 This is also observable from HRTEM image in Figure S2 whereby the bare GO shows a smooth surface while the surface of FexNiy-B/G is rougher and covered by nanostructures. The amorphous nature and highly porous morphologies of the ultrathin nanostructure is supported by the absence of lattice fringe in the HRTEM image (Figure 1c). Figure 1d and Figure S3 show high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images for the freshly prepared and after OER test material, as well as corresponding TEM elemental mapping of Ni, Fe, C, B, and O which reveal homogeneous distribution of the constituents on the rGO. From TEM-EDX result, the iron to nickel ratio was 0.27:0.73 which is close to 1:3 in the metal precursor solution for FeNi3-B/G. The structure of the as-prepared hybrid catalyst was investigated by X-ray diffraction (XRD) (Figure 2a). FeNi3-B/G catalyst showed two weak peaks at 2θ = 10o and at 2θ = 25o. The former matched with the strong peak of (001) plane observed in the XRD pattern of GO (Figure S4a), while the latter is (002) plane from reduced GO which implies partially reduction of GO, hence denoted G.41-42 The absence of any other strong peak which is also observed for unsupported ternary metal boride (FeNi3-B) shows the amorphous nanostructure of the product which agrees with reported metal borides produced by the chemical reduction of metal salt in an aqueous solution (Figure 2a).27 The shoulder peaks at 2θ = 35o and at 2θ = 61o may arises from the metal and/or boron oxides formed inevitably in the procedure of storage and 9 ACS Paragon Plus Environment


measurements under atmospheric exposure.43 XRD pattern of NaBH4-treated GO (Figure S4b) shows a peak at 25o for (002) plane and very weak peak for (001) plane reveals partially coreduction of GO in the presence of reducing agent and agrees with partially reduction of GO observed for the as-prepared FeNi3-B/G catalyst. The addition of reducing solution to GO suspension showed color change, validating the co-reduction of GO (Figure S5a-c). The weak peaks in FeNi3-B/G suggest that the metal boride nanostructure covered the co-reduced GO as observed in TEM mapping (Figure 1d). In Figure S5d, the FeNi3-B/G prepared by dropwise approach is about three times in powder volume compared to the as-prepared FeNi3-B/G material by fast method which reveals poor packing of active material per volume. The exothermic reaction of metal ions in aqueous medium of the latter in contact with NaBH4 may have contributed to the co-reduction of GO compared to strong peaks for the dropwise sample (Figure S6), and led to strong hybridization of metal boride nanoparticles to GO.44 The special surface areas of unsupported FeNi3-B and FeNi3-B/G hybrid were analysed by the BET method. The N2 absorption and desorption isotherms show a typical H3 hysteresis loop of IV type curve (Figure 2b), indicating the presence of mesopores structures.45 The pore size distribution curves in the inset Figure 2b show a single well-defined peak at ca. 3.8 nm, further suggesting mesopores material. The Brunauer–Emmett–Teller (BET) specific surface areas and pore volumes of FeNi3-B/G hybrid were calculated to be 56 m² g-1 and 0.44 cm3 g-1, respectively. On the other hand, the unsupported FeNi3-B showed smaller values 38 m² g-1 and 0.17 cm3 g-1). The elemental composition and the chemical states were studied by X-ray photoelectron spectroscopy (XPS). As shown in the XPS survey of FeNi3-B/G (30% GO) (Figure S7a), the presence of Ni, Fe, B, C, and O was found with 5.12%, 1.63%, 3.46%, 69.36%, and 20.44% atomic percentages, respectively. The metal to boron ratio was found to be around two, which shows that metal boride material with the general formula of transition metal borides (M2B) 10 ACS Paragon Plus Environment

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was produced.25, 33 The ratio of iron to nickel (0.24:0.76) followed the metal ions’ ratio in metal salt solution (FeNi3-B/G), showing a fast, easy and efficient method for doping iron. As revealed by the B 1s XPS spectrum (Figure 2c), boron (B) in a prolonged stored catalyst showed a peak at 188.3 eV for boron bonding metallic elements.46-48 The positive shift in binding energy (BE) compared to pure B (187.1 eV),49 and to nickel boride (187.9 eV) shows enhanced electron transfer from B to the vacant d-orbital of the metallic elements.46 Additionally, peaks of oxidized B species such as borates were present at 192.4 eV and 193.4 eV. Both nickel and iron XPS 2p spectra reveal existence of both elemental and oxidized states (Figure 2d and Figure S8a) which was consistent with the literature that metal borides exposed and stored for longtime in air are covered by a thin layer of hydroxides or oxides, hence identified as metal boride/metal boron-oxide complex nanostructure.25, 50-52 It is clear that no obvious change during OER test in Fe 2p XPS spectrum (Figure 2d) for iron bonding to boron at 706.2 eV emphasizing the presence of core@oxide layer nanostructure.47, 51 Both metals experience severe oxidation during water oxidation due to the formation of MOxHy which acts as active surface species. The O 1s and C 1s spectra in Figure S8b-c show not only the peak for carbonaceous bonds at 284.6 eV in pristine GO and graphite,53-54 but also other peaks from graphene oxide and oxidized species due to exposure to air. After OER test, the Boron content showed a significant decrease (Figure 2c) while there was an increase in metal oxidized species and oxygen (Figure S7, S8b and Table S1), showing the formation of stable metal (hydroxy-)oxide which acts as the catalytic center for OER. It is a common behavior of oxygenevolving catalysts to undergo structural transformation and progressive oxidation during the process.47, 55 We investigated the electrocatalytic activity of the as-obtained metal boride material in an O2-saturated 1.0 M KOH aqueous electrolyte. A typical three-electrode electrochemical cell was used where the rotating disk (GC), mercury/mercury oxide (Hg/HgO) electrode, and 11 ACS Paragon Plus Environment


graphite rod were the working, reference, and counter electrodes, respectively. For comparison, the OER performance of GO and RuO2 catalyst was measured at the same conditions. The currents measured in this study were normalized to the surface area of GC (surface area = 0.196 cm2) while the potentials were all calibrated to the reversible hydrogen electrode (RHE) reference for comparison purpose. In the cyclic voltammogram (CV) plot (Figure 3a), there was a negative shift to lower potential with respect to the initial cycles of CV testing. As can be seen, steady state was attained in about 30 cycles and the cycles became reproducible as the 30th and 50th cycles overlapped. The oxidative and reductive redox peaks in Figure 3a also showed an increase and become reproducible which shows permanent morphological transformation during the electrochemical activation. Most of the metal-metalloid oxygen-evolving electrocatalysts, especially metal borides, undergo in-situ activation due to continuous growth of a metal hydroxide/oxyhydroxide layer which acts as the active site for water oxidation.25, 47, 50 This is in agreement with the increased content of oxidized species and oxygen content increased while B content decreased as observed in Table S1 from the XPS analysis after OER test. This is similar to the phenomenon observed in metal phosphides, whereby, they transform to oxo(hydroxy-) metal species and act as the catalytic center for water oxidation.56 The water oxidation activity of FeNi3-B/G catalyst was compared with binary metal borides (Ni-B/G and Fe-B/G), commercial RuO2 catalyst, and bare GO in 1.0 M KOH. Linear sweep voltammetry (LSV) polarization curves were collected at a sweep rate of 5 mV s-1 and compensated for iR-drop as shown in Figure 3b. A positive shift in oxidation peak appeared from 1.39 V to 1.43 V vs. RHE for Ni-B/G and FeNi3-B/G, respectively. The former is assigned to the Ni2+/Ni3+/4+ redox process, while the latter one is associated with both Fe and Ni redox processes which reveals synergistic effect for water oxidation. It is clear that the anodic current increased tremendously for FeNi3-B/G in comparison to other samples, including RuO2 12 ACS Paragon Plus Environment

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catalyst. As a consequence, it required an overpotential of only 230 mV to reach a current density of 15 mA cm-2, which is comparable to or even outperforms many recently developed nickel-based borides, oxides, hydroxides, borates, phosphides, and chalcogenides, such as NixB (η10 = 380 mV),50 Ni–Bi/G (η10 = 338 mV),57 Ni2B (η10 = 350 mV),58 Ni2P (η10 = 360 mV),58 CoFe2O4/PANI-MWCNT (η10 = 314 mV),59 Fe–Ni–B (η10 = 294,15 NiFeB (η10 = 251 mV),51 Fedoped NiOx (η10 = 310 mV),60 borate-doped α-Ni(OH)2 (η10 = 340 mV),61 and CoFe ‐ N ‐ CN/CNTs (η10 = 285 mV)62 (see Table S2 for more). Tafel slope is another crucial parameter to characterize OER catalyst kinetics.63 The Tafel slope which reveals the correlation between overpotential and steady-state current density was evaluated by analyzing Tafel plots (Figure S9). The linear portions at low overpotentials, derived from anodic polarization curve, were fitted to the Tafel equation: η = b log j + a (equation 7), where η defines the overpotential, j is the current density, and b is the Tafel slope.64-65 The Tafel slope of FeNi3-B/G is 50 mV dec-1, much smaller than the rest control samples, demonstrating the favorable OER reaction kinetics. It is clear in Figure 3c that the asprepared ternary metal boride hybrid outperforms the binary metal boride hybrid due to synergistic effect from material constituents.48 Consistent results are obtained from electrochemical impedance spectroscopic measurement (Figure 3d). The advanced OER performance of the as-presented material has first been attributed to the amorphous nature of metal boride nanostructure. Secondly, the strong coupling of metal boride with rGO enhanced the electrical conductivity and allowed for better charge transfer. Thirdly, once the catalyst is activated under OER process, it undergoes in-situ transformation on the surface and the presence of conducting metal-metalloid at the core ensures high conductivity of the catalyst. Lastly, the synergistic effect of iron and nickel in the active material is crucial for the high efficiency of the catalyst.



The optimization of the catalytic activity was performed by first controlling the percentage of GO in metal salt solution. First, all the supported ternary metal boride hybrid catalyst outperformed the unsupported ternary metal boride, FeNi3-B (0% G), (Figure 3e), showing the crucial importance of highly conductive 2D rGO to boost OER performance.66-67 30% of rGO was then identified to be optimal. E.g. the overpotential required to afford a 15 mA cm-2 was higher for FeNi3-B, FeNi3-B/G with 15% G, and 45% G when compared to FeNi3-B/G with 30% G by 58 mV, 45 mV, and 36 mV, respectively (Figure 3f). Additionally, these catalysts have a similar Tafel slope and differ only by 5 mA dec-1 (Figure S10). This may imply that these catalyst hybrids and unsupported ternary metal boride with similar composition share a similar OER catalytic mechanism while the rGO support assists in electric conductivity. To understand the high performance of FeNi3-B/G, electrochemical surface area (ECSA) was calculated from the electrochemical double-layer capacitance (Cdl) derived from cyclic voltammograms (Figure S11-12). The Cdl of FeNi3-B/G in Figure S11e is determined to be 0.25 mF cm-2 which is much higher than 0.17, 0.04 and 0.017 mF cm-1 for FeNi3-B, Ni-B/G, and Fe-B/G, respectively. The higher ECSA value for FeNi3-B/G compared to binary and ternary catalysts studied in this work represents a larger electrochemical surface area, suggesting larger exposure of active sites. To investigate if this hybrid material exhibits strong and direct contact with co-reduced GO, FeNi3-B catalyst mixed with NaBH4-treated GO was tested (Figure S13). The results showed inferior performance (η15 = 270 mV compared to 230 mV for hybrid material, Figure S13a) and poor charge transfer (Figure S13b) due to poor contact between the unsupported catalyst and the highly conductive substrate. Additionally, the optimized catalyst prepared by rapid mixing rate shows better performance compared to the catalyst prepared by slow rate (dropwise addition) (Figure S14). The latter required 298 mV to deliver 15 mA cm-2 compared to 230 mV for the former (Figure S14a). It also outperforms the catalyst prepared by slower reduction rate 14 ACS Paragon Plus Environment

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of 1.0 mL dropwise followed by faster rapid reduction whereby 266 mV@15 mA cm-2 was required. Additionally, the larger Tafel slope for the samples prepared by slow rate in Figure S14b reveals poor OER kinetics. This correlates with larger semicircles observed in Figure S14c which are even larger compared to unsupported FeNi3-B (Figure S10b) which indicates slow charge transfer due to poor conductivity. This may be associated with poor reduction of GO as shown above in the XRD pattern analysis (Figure S6). The first semicircle represents the charge transfer resistance, while the second represents the resistance from the mass transfer during electrocatalytic reactions.68 Hence, the appearance of the second semicircle (Figure S14c), may suggest higher mass transfer resistance. To get insight into the mechanism of metal borides toward water oxidation, the electronic structures and adsorption free energy were investigated by DFT calculations. The best catalyst (at the top of activity volcano curve) can be found at the O* binding energy around 3.0 eV.69 The pristine MB phase is not active and suitable for OER (Figure S15a-c). On the other hand, if O adsorption is too strong, the material would be spontaneously oxidized leading to the formation of MOxHy active species. This agrees with the treatment of the catalyst by CV technique (till a reproducible curve) before every OER measurements (Figure 3a). In Figure S15d-f, the O adsorption energy are -3.02 eV, -1.97 eV, and -1.85 eV for Fe-B/G, Ni-B, and FeNi3-B/G, respectively, which follows the order of electrochemical performance, Fe-B/G < Ni-B/G < FeNi3-B/G. Based on many literature reports, it has been observed that the in situ changes of metal phosphides to their corresponding oxide/oxyhydroxide are very OER active and this is similar to metal borides is composed of MOOH, while the core is M2B.24, 58 As shown in Figure 3b, nickel boride hybrid (Ni-B/G) is more active than iron boride (FeB/G). By incorporation of iron into nickel boride, much improved activity is observed (Figure 4). This iron doping caused the electronic modulation of nickel boride and hence provided advanced electrochemical performance.70 The effect of iron doping is then further investigated 15 ACS Paragon Plus Environment


by varying the iron to nickel ratio (Figure 4a,b). The LSV polarization curves in Figure 4a shows that 1:3 (FeNi3-B/G) offers the best activity. Similar Tafel slopes observed could suggest similar OER kinetics despites iron to nickel ratio variation (Figure 4b). The difference in oxygen-evolving activity due to varying iron to nickel ratios shows the synergy between iron and nickel for optimum OER performance.47, 71 The intrinsic activity of the catalyst was studied by the Turnover frequency (TOF). The TOF values calculated at 300 mV vs. RHE (Table S3) shows that the as-prepared FeNi3-B/G hybrid showed eight folds TOF value compared to FeNi3-B (0.14 s-1) which further suggest advanced catalytic activity of hybrid material. Doping of iron boosted the intrinsic activity whereby the TOF FeNi3-B/G hybrid (1.3 s-1) is around 14 times compared to Ni-B/G. Another critical criterion to evaluate a competent catalyst for practical electrochemical application is its stability.72 For this reason, the stability was probed by continuous CV sweeps for 1000 cycles in an alkaline solution at 100 mV s−1 (Figure 4c). The initial iR-corrected LSV polarization curves overlapped with the one measured after 1000 continuous cycles. The catalyst stability was further probed by chronopotentiometry technique at a fixed current density (10 mA cm-2) for 12 hours (Figure 4d). It shows a promising stability with negligible decay over the time. The stability of the as-prepared material has been attributed to the strongcoupling between the active catalyst and co-reduced GO. Furthermore, the presence of oxidized metal, boron-metal covalent bond, and in-situ transformation of metal boride catalyst which occur in OER activation process have also contributed to the stability.25, 47, 50 4. CONCLUSION Amorphous metal boride - reduced GO hybrid was successfully synthesized by a simple and one-step fast reduction process. The iron incorporation in nickel boride and the ratio of active material to GO were found to be the determining factors for boosting the water oxidation performance. The optimized and ternary metal boride hybrid possesses remarkable stability and 16 ACS Paragon Plus Environment

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catalytic activity in alkaline electrolyte compared to the binary metal borides hybrid and commercial RuO2 catalyst. It only needs an overpotential of 15 mA cm-2 at 230 mV with a small Tafel slope of 50 mV dec-1 which shows advanced kinetics. This highly efficient OER performance is attributed to the strong coupling of ternary iron nickel boride nanostructure with co-reduced GO, iron-doping boost the synergistic effect which resulted into robust metal (hydroxy-)oxides as catalytic center, and finally, from unique amorphous structure. This work provides a facile approach for preparing amorphous transition metal borides coupled to coreduced GO yet outstanding activity and hence, it could be extended to other energy storage and conversion technologies. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxx. Digital images, and additional SEM images, XRD patterns, EDX, XPS spectra, electrocatalytic measurements, and a table for performance comparison (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail, [email protected] (W.X.). *E-mail: [email protected] (Q.Y.Y). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS



This project is funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. We also acknowledge financial support from the academic research fund AcRF tier 1 (M4011784, RG6/17) and tier 2 (M4020246, ARC10/15), Ministry of Education, Singapore. The authors also acknowledge the Facility for Analysis, Characterization, Testing and simulation (FACTS), Nanyang Technological University, Singapore, for use of the TEM JEOL 2010 UHR, and FESEM JEM-7600F facilities. Conflict of Interest The authors declare no conflict of interest.

REFERENCES (1) Ghoniem, A. F., Needs, Resources and Climate Change: Clean and Efficient Conversion Technologies. Prog. Energy Combust. Sci. 2011, 37 (1), 15-51. (2) Schultz, M. G.; Diehl, T.; Brasseur, G. P.; Zittel, W., Air Pollution and Climate-Forcing Impacts of a Global Hydrogen Economy. Science 2003, 302 (5645), 624-627. (3) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G., Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110 (11), 6474-6502. (4) Busch, M.; Halck, N. B.; Kramm, U. I.; Siahrostami, S.; Krtil, P.; Rossmeisl, J., Beyond the Top of the Volcano? – a Unified Approach to Electrocatalytic Oxygen Reduction and Oxygen Evolution. Nano Energy 2016, 29, 126-135. (5) Koper, M. T. M., Thermodynamic Theory of Multi-Electron Transfer Reactions: Implications for Electrocatalysis. J. Electroanal. Chem. 2011, 660 (2), 254-260. (6) Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J.-J.; Wang, Z. L., Electrocatalytic Oxygen Evolution Reaction for Energy Conversion and Storage: A Comprehensive Review. Nano Energy 2017, 37, 136-157. (7) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z., Design of Electrocatalysts for Oxygenand Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44 (8), 20602086. (8) Park, S.; Shao, Y.; Liu, J.; Wang, Y., Oxygen Electrocatalysts for Water Electrolyzers and Reversible Fuel Cells: Status and Perspective. Energy Environ. Sci. 2012, 5 (11), 93319344. (9) Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M., Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46 (2), 337-365. (10) Wang, D.; Astruc, D., The Recent Development of Efficient Earth-Abundant TransitionMetal Nanocatalysts. Chem. Soc. Rev. 2017, 46 (3), 816-854. 18 ACS Paragon Plus Environment

Page 18 of 28

Page 19 of 28 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


(11) Song, F.; Bai, L.; Moysiadou, A.; Lee, S.; Hu, C.; Liardet, L.; Hu, X., Transition Metal Oxides as Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solutions: An Application-Inspired Renaissance. J. Am. Chem. Soc 2018, 140 (25), 7748-7759. (12) McAteer, D.; Godwin Ian, J.; Ling, Z.; Harvey, A.; He, L.; Boland Conor, S.; Vega‐Mayoral, V.; Szydłowska, B.; Rovetta Aurélie, A.; Backes, C.; Boland John, B.; Chen, X.; Lyons Michael, E. G.; Coleman Jonathan, N., Liquid Exfoliated Co(OH)2 Nanosheets as Low‐Cost, yet High‐Performance, Catalysts for the Oxygen Evolution Reaction. Adv. Energy Mater. 2018, 8 (15), 1702965. (13) Nardi Katie, L.; Yang, N.; Dickens Colin, F.; Strickler Alaina, L.; Bent Stacey, F., Creating Highly Active Atomic Layer Deposited NiO Electrocatalysts for the Oxygen Evolution Reaction. Adv. Energy Mater. 2015, 5 (17), 1500412. (14) Zhao, B.; Zhang, L.; Zhang, Q.; Chen, D.; Cheng, Y.; Deng, X.; Chen, Y.; Murphy, R.; Xiong, X.; Song, B.; Wong, C. P.; Wang, M. S.; Liu, M., Rational Design of Nickel Hydroxide‐Based Nanocrystals on Graphene for Ultrafast Energy Storage. Adv. Energy Mater. 2017, 8 (9), 1702247. (15) Nsanzimana, J. M. V.; Peng, Y.; Xu, Y. Y.; Thia, L.; Wang, C.; Xia, B. Y.; Wang, X., An Efficient and Earth-Abundant Oxygen-Evolving Electrocatalyst Based on Amorphous Metal Borides. Adv. Energy Mater. 2018, 8 (1), 1701475. (16) Kim Ju, S.; Kim, B.; Kim, H.; Kang, K., Recent Progress on Multimetal Oxide Catalysts for the Oxygen Evolution Reaction. Adv. Energy Mater. 2018, 8 (11), 1702774. (17) Roger, I.; Shipman, M. A.; Symes, M. D., Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting. Nat. Rev. Chem. 2017, 1, 0003. (18) Li, Y.; Zhang, H.; Jiang, M.; Zhang, Q.; He, P.; Sun, X., 3d Self-Supported Fe-Doped Ni2P Nanosheet Arrays as Bifunctional Catalysts for Overall Water Splitting. Adv. Funct. Mater. 2017, 27 (37), 1702513. (19) Zhou, T.; Cao, Z.; Zhang, P.; Ma, H.; Gao, Z.; Wang, H.; Lu, Y.; He, J.; Zhao, Y., Transition Metal Ions Regulated Oxygen Evolution Reaction Performance of Ni-Based Hydroxides Hierarchical Nanoarrays. Sci. Rep. 2017, 7, 46154. (20) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W., Revised Oxygen Evolution Reaction Activity Trends for First-Row Transition-Metal (Oxy)Hydroxides in Alkaline Media. J. Phys. Chem. Lett. 2015, 6 (18), 3737-3742. (21) Nsanzimana, J. M. V.; Peng, Y.; Miao, M.; Reddu, V.; Zhang, W.; Wang, H.; Xia, B. Y.; Wang, X., An Earth-Abundant Tungsten–Nickel Alloy Electrocatalyst for Superior Hydrogen Evolution. ACS Appl. Nano Mater. 2018, 1 (3), 1228-1235. (22) Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B., The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-Catalysts”. ACS Energy Lett. 2016, 1 (1), 195-201. (23) Xu, X.; Song, F.; Hu, X., A Nickel Iron Diselenide-Derived Efficient Oxygen-Evolution Catalyst. Nat. Commun. 2016, 7, 12324. (24) Dutta, A.; Pradhan, N., Developments of Metal Phosphides as Efficient OER Precatalysts. J. Phys. Chem. Lett. 2017, 8 (1), 144-152. (25) Masa, J.; Weide, P.; Peeters, D.; Sinev, I.; Xia, W.; Sun, Z.; Somsen, C.; Muhler, M.; Schuhmann, W., Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution. Adv. Energy Mater. 2016, 6 (6), 1502313. (26) Schaefer, Z. L.; Ke, X.; Schiffer, P.; Schaak, R. E., Direct Solution Synthesis, Reaction Pathway Studies, and Structural Characterization of Crystalline Ni3B Nanoparticles. J. Phys. Chem. C 2008, 112 (50), 19846-19851. (27) Carenco, S.; Portehault, D.; Boissière, C.; Mézailles, N.; Sanchez, C., Nanoscaled Metal Borides and Phosphides: Recent Developments and Perspectives. Chem. Rev. 2013, 113 (10), 7981-8065. 19 ACS Paragon Plus Environment


(28) Lian, K.; Thorpe, S. J.; Kirk, D. W., The Electrocatalytic Activity of Amorphous and Crystalline Ni-Co Alloys on the Oxygen Evolution Reaction. Electrochimica Acta 1992, 37 (1), 169-175. (29) Ji, L.; Meduri, P.; Agubra, V.; Xiao, X.; Alcoutlabi, M., Graphene‐Based Nanocomposites for Energy Storage. Adv. Energy Mater. 2016, 6 (16), 1502159. (30) Chaudhari, N. K.; Jin, H.; Kim, B.; Lee, K., Nanostructured Materials on 3d Nickel Foam as Electrocatalysts for Water Splitting. Nanoscale 2017, 9 (34), 12231-12247. (31) Zhou, D.; Jia, Y.; Yang, H.; Xu, W.; Sun, K.; Zhang, J.; Wang, S.; Kuang, Y.; Liu, B.; Sun, X., Boosting Oxygen Reaction Activity by Coupling Sulfides for High-Performance Rechargeable Metal–Air Battery. J. Mater. Chem. A 2018, 6 (42), 21162-21166. (32) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk Ji, W.; Potts Jeffrey, R.; Ruoff Rodney, S., Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22 (35), 3906-3924. (33) Ganem, B.; Osby, J. O., Synthetically Useful Reactions with Metal Boride and Aluminide Catalysts. Chem. Rev. 1986, 86 (5), 763-780. (34) Glavee, G. N.; Klabunde, K. J.; Sorensen, C. M.; Hadjapanayis, G. C., Borohydride Reductions of Metal Ions. A New Understanding of the Chemistry Leading to Nanoscale Particles of Metals, Borides, and Metal Borates. Langmuir 1992, 8 (3), 771-773. (35) Zhou, L.-J.; Huang, X.; Chen, H.; Jin, P.; Li, G.-D.; Zou, X., A High Surface Area Flower-Like Ni-Fe Layered Double Hydroxide for Electrocatalytic Water Oxidation Reaction. Dalton Trans. 2015, 44 (25), 11592-11600. (36) Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47 (1), 558-561. (37) Kresse, G.; Hafner, J., Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49 (20), 1425114269. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865-3868. (39) Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59 (3), 1758-1775. (40) Anoop, G.; Rani, J. R.; Lim, J.; Jang, M. S.; Suh, D. W.; Kang, S.; Jun, S. C.; Yoo, J. S., Reduced Graphene Oxide Enwrapped Phosphors for Long-Term Thermally Stable Phosphor Converted White Light Emitting Diodes. Sci. Rep. 2016, 6, 33993. (41) Song, J.; Wang, X.; Chang, C.-T., Preparation and Characterization of Graphene Oxide. J Nanomater. 2014, 2014, 6. (42) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M., Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4 (8), 4806-4814. (43) Chen, H.; Ouyang, S.; Zhao, M.; Li, Y.; Ye, J., Synergistic Activity of Co and Fe in Amorphous Cox–Fe–B Catalyst for Efficient Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2017, 9 (46), 40333-40343. (44) Khurana, J. M.; Gogia, A., Synthetically Useful Reactions with Nickel Boride. A Review. Org. Prep. Proced. Int. 1997, 29 (1), 1-32. (45) Sing, K. S.; Everett, D. H.; Haul, R. A.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T., Reporting Physisorption Data for Gas/Solid Systems. In Handbook of Heterogeneous Catalysis. In Handbook of Heterogeneous Catalysis, G. Ertl; H. Knözinger; F. Schüth; Weitkamp, J., Eds. 2008, DOI: 10.1002/9783527610044.hetcat0065. (46) Legrand, J.; Gota, S.; Guittet, M. J.; Petit, C., Synthesis and XPS Characterization of Nickel Boride Nanoparticles. Langmuir 2002, 18 (10), 4131-4137.


Page 20 of 28

Page 21 of 28 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


(47) Nsanzimana, J. M. V.; Reddu, V.; Peng, Y.; Huang, Z.; Wang, C.; Wang, X., Ultrathin Amorphous Iron–Nickel Boride Nanosheets for Highly Efficient Electrocatalytic Oxygen Production. Chem. Eur. J. 2018, 24, doi:10.1002/chem.201802092. (48) Klemenz, S.; Schuch, J.; Hawel, S.; Zieschang, A.-M.; Kaiser, B.; Jaegermann, W.; Albert, B., Synthesis of a Highly Efficient Oxygen-Evolution Electrocatalyst by Incorporation of Iron into Nanoscale Cobalt Borides. ChemSusChem 2018, 11 (18), 3150-3156. (49) Moulder, J. F.; Chastain, J.; King, R. C., Handbook of X-Ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of Xps Data. Eden Prairie, Minn.: Physical Electronics: 1995; pp 172. (50) Masa, J.; Sinev, I.; Mistry, H.; Ventosa, E.; de la Mata, M.; Arbiol, J.; Muhler, M.; Roldan Cuenya, B.; Schuhmann, W., Ultrathin High Surface Area Nickel Boride (NixB) Nanosheets as Highly Efficient Electrocatalyst for Oxygen Evolution. Adv. Energy Mater. 2017, 7 (17), 1700381. (51) Liu, G.; He, D.; Yao, R.; Zhao, Y.; Li, J., Amorphous NiFeB Nanoparticles Realizing Highly Active and Stable Oxygen Evolving Reaction for Water Splitting. Nano Research 2018, 11 (3), 1664-1675. (52) 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., Crystallinity‐Modulated Electrocatalytic Activity of a Nickel(II) Borate Thin Layer on Ni3B for Efficient Water Oxidation. Angew. Chem. Int. Ed. 2017, 56 (23), 6572-6577. (53) Ganguly, A.; Sharma, S.; Papakonstantinou, P.; Hamilton, J., Probing the Thermal Deoxygenation of Graphene Oxide Using High-Resolution in Situ X-Ray-Based Spectroscopies. J. Phys. Chem. C 2011, 115 (34), 17009-17019. (54) Cao, X.; Cui, L.; Wang, X.; Yang, W.; Liu, J., Nickel‐Borate/Reduced Graphene Oxide Nanohybrid: A Robust and Efficient Electrocatalyst for Oxygen Evolution Reaction in Alkaline and near Neutral Media. ChemCatChem 2018, 10 (10), 2826. (55) Yang, W.; Zhu, K.; Zhu, X., Application of in Situ Techniques for the Characterization of NiFe Based Oxygen Evolution Reaction (OER) Electrocatalysts. Angew. Chem. Int. Ed. 2018, doi:10.1002/anie.201802923. (56) Liu, K.; Wang, F.; He, P.; Shifa Tofik, A.; Wang, Z.; Cheng, Z.; Zhan, X.; He, J., The Role of Active Oxide Species for Electrochemical Water Oxidation on the Surface of 3d‐Metal Phosphides. Adv. Energy Mater. 2018, 8 (15), 1703290. (57) Yang, M.-Q.; Dan, J.; Pennycook, S. J.; Lu, X.; Zhu, H.; Xu, Q.-H.; Fan, H. J.; Ho, G. W., Ultrathin Nickel Boron Oxide Nanosheets Assembled Vertically on Graphene: A New Hybrid 2d Material for Enhanced Photo/Electro-Catalysis. Mater. Horiz. 2017, 4 (5), 885-894. (58) Masa, J.; Andronescu, C.; Antoni, H.; Sinev, I.; Seisel, S.; Elumeeva, K.; Barwe, S.; Marti-Sanchez, S.; Arbiol, J.; Roldan Cuenya, B.; Muhler, M.; Schuhmann, W., Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides. ChemElectroChem. 2018, doi:10.1002/celc.201800669. (59) Liu, Y.; Li, J.; Li, F.; Li, W.; Yang, H.; Zhang, X.; Liu, Y.; Ma, J., A Facile Preparation of CoFe2O4 Nanoparticles on Polyaniline-Functionalised Carbon Nanotubes as Enhanced Catalysts for the Oxygen Evolution Reaction. J. Mater. Chem. A 2016, 4 (12), 4472-4478. (60) Wu, G.; Chen, W.; Zheng, X.; He, D.; Luo, Y.; Wang, X.; Yang, J.; Wu, Y.; Yan, W.; Zhuang, Z.; Hong, X.; Li, Y., Hierarchical Fe-Doped NiOx Nanotubes Assembled from Ultrathin Nanosheets Containing Trivalent Nickel for Oxygen Evolution Reaction. Nano Energy 2017, 38, 167-174. (61) Zhang, Z.; Zhang, T.; Lee, J. Y., Enhancement Effect of Borate Doping on the Oxygen Evolution Activity of Α-Nickel Hydroxide. ACS Appl. Nano Mater. 2018, 1 (2), 751-758. (62) Liu, Y.; Li, F.; Yang, H.; Li, J.; Ma, P.; Zhu, Y.; Ma, J., Two-Step Synthesis of Cobalt Iron Alloy Nanoparticles Embedded in Nitrogen-Doped Carbon Nanosheets/Carbon Nanotubes for the Oxygen Evolution Reaction. ChemSusChem 2018, 11 (14), 2358-2366. 21 ACS Paragon Plus Environment


(63) Fang, Y.-H.; Liu, Z.-P., Tafel Kinetics of Electrocatalytic Reactions: From Experiment to First-Principles. ACS Catal. 2014, 4 (12), 4364-4376. (64) Shinagawa, T.; Garcia-Esparza, A. T.; Takanabe, K., Insight on Tafel Slopes from a Microkinetic Analysis of Aqueous Electrocatalysis for Energy Conversion. Sci. Rep. 2015, 5, 13801. (65) García-Osorio, D. A.; Jaimes, R.; Vazquez-Arenas, J.; Lara, R. H.; Alvarez-Ramirez, J., The Kinetic Parameters of the Oxygen Evolution Reaction (OER) Calculated on Inactive Anodes via EIS Transfer Functions: •OH Formation. J. Electrochem. Soc. 2017, 164 (11), E3321-E3328. (66) Jixin, Z.; Dan, Y.; Zongyou, Y.; Qingyu, Y.; Hua, Z., Graphene and Graphene‐Based Materials for Energy Storage Applications. Small 2014, 10 (17), 3480-3498. (67) Chen, D.; Feng, H.; Li, J., Graphene Oxide: Preparation, Functionalization, and Electrochemical Applications. Chem. Rev. 2012, 112 (11), 6027-6053. (68) Qi, J.; Zhang, W.; Xiang, R.; Liu, K.; Wang, H.-Y.; Chen, M.; Han, Y.; Cao, R., Porous Nickel-Iron Oxide as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction. Adv. Sci. 2015, 2 (10), 1500199-1500199. (69) Huang, Z.-F.; Wang, J.; Peng, Y.; Jung, C.-Y.; Fisher, A.; Wang, X., Design of Efficient Bifunctional Oxygen Reduction/Evolution Electrocatalyst: Recent Advances and Perspectives. Adv. Energy Mater. 2017, 7 (23), 1700544. (70) Acosta, D.; Ramírez, N.; Erdmann, E.; Destéfanis, H.; Gonzo, E., Transition Metals as Dopants on Nickel Borides: Their Catalytic Activity Effect on Hydrogenation Reactions. Catal. Today 2008, 133-135, 49-55. (71) Xiao, H.; Shin, H.; Goddard, W. A., Synergy between Fe and Ni in the Optimal Performance of (Ni,Fe)OOH Catalysts for the Oxygen Evolution Reaction. Proc. Natl. Acad. Sci. U. S. A. 2018. (72) McCrory, C. C. L.; 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.


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Figures and Caption

Scheme 1. Schematic representation for the transition metal borides - reduced graphene oxide (M2B/G).



Figure 1. (a) HRTEM image of the fresh-prepared FeNi3-B/G hybrid catalyst. The inset Figure 1(a) is the high magnification FESEM image. (b,c) HRTEM images. The inset in Figure 1(b) is SAED pattern. (d) HAADF-STEM image and elemental mapping images of the as-prepared FeNi3-B/G catalyst (30% GO).


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Figure 2. (a) XRD pattern of graphene oxide (GO), unsupported catalyst (FeNi3-B), and FeNi3B/G catalyst. The inset is the magnified picture of the boxed area from Figure 2(a). (b) N2 adsorption–desorption isotherm curves and pore size distribution curves (inset) of the unsupported FeNi3-B and FeNi3-B/G. High resolution XPS spectra of the B 1s (c), and (d) Fe 2p core level of FeNi3-B/G catalyst and after CV activation followed by chronopotentiometry at a fixed current density of 20 mA cm-2 for one hour.



Figure 3. (a) Activation of amorphous FeNi3-B/G by continuous cyclic voltammetry between 0.2 and 0.8 V vs. Hg/HgO at a scanning rate of 100 mV s-1 in 1.0 M KOH at room temperature, the arrows indicate the direction of current increases. (b) iR-corrected LSV polarization curves of FeNi3-B/G, Ni-B/G, Fe-B/G, RuO2, and bare GO at 5 mV s-1. (c) Overpotential at j = 15 mA cm-2 and Tafel slope. (d) Nyquist plots, the inset is enlarged Figure 3d. (e,f) iR-corrected LSV polarization curves and overpotential required to deliver 15 mA cm-2 for FeNi3-B/G (0 %, 15%, 30%, and 45% of G), respectively.


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Figure 4. (a) iR-corrected polarization curves for FexNiy-B/G (1:1, 1:2, 1:3, 1:6, and 1:9). (b) corresponding Tafel plot. (c,d) Electrochemical stability tests. (c) iR-corrected LSV polarization curves of FeNi3-B/G, initial and after continuous 1000 cycles. (d) Chronopotentiometric curve at a potential required to afford 10 mA cm-2 in 1.0 M KOH at room temperature.



TOC figure


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Facile Synthesis of Amorphous Ternary Metal BoridesâReduced

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