Highly active sites of NiVB nanoparticles dispersed onto graphene nanosheets towards efficient and pH-universal overall water splitting

Journal Pre-proofs Highly active sites of NiVB nanoparticles dispersed onto graphene nanosheets towards efficient and pH-universal overall water splitting Muhammad Arif, Ghulam Yasin, Muhammad Shakeel, Muhammad Asim Mushtaq, Wen Ye, Xiaoyu Fang, Shengfu Ji, Dongpeng Yan PII: DOI: Reference:

S2095-4956(20)30704-X https://doi.org/10.1016/j.jechem.2020.10.014 JECHEM 1639

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Journal of Energy Chemistry

Received Date: Revised Date: Accepted Date:

29 May 2020 13 October 2020 14 October 2020

Please cite this article as: M. Arif, G. Yasin, M. Shakeel, M.A. Mushtaq, W. Ye, X. Fang, S. Ji, D. Yan, Highly active sites of NiVB nanoparticles dispersed onto graphene nanosheets towards efficient and pH-universal overall water splitting, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.10.014

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Highly active sites of NiVB nanoparticles dispersed onto graphene nanosheets towards efficient and pH-universal overall water splitting Muhammad Arifa,b, Ghulam Yasinb, Muhammad Shakeelb, Muhammad Asim Mushtaqa,b, Wen Yea, Xiaoyu Fanga, Shengfu Jib,*, Dongpeng Yana,b,c,* a

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China b State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China c College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China *

Corresponding authors. E-mail addresses: [email protected] (D. Yan), [email protected] (S. Ji). ABSTRACT Production of hydrogen (H2) and oxygen (O2) through electrocatalytic water splitting is one of the sustainable, green and pivotal ways to accomplish the ever-increasing demands for renewable energy sources, but remains a big challenge because of the uphill reaction during overall water splitting. Herein, we develop high-performance non-noble metal electrocatalysts for pH-universal water splitting, based on nickel/vanadium boride (NiVB) nanoparticles/reduced graphene oxide (rGO) hybrid (NiVB/rGO) through a facile chemical reduction approach under ambient condition. By virtue of more exposure to surface active sites, superior electron transfer capability and strong electronic coupling, the as-prepared NiVB/rGO heterostructure needs pretty low overpotentials of 267 and 151 mV to deliver a current density of 10 mA cm−2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) respectively, with the corresponding Tafel slope of 44 and 88 mV dec−1 in 1.0 M KOH. Moreover, the NiVB/rGO electrocatalysts display a promising performance in a wide-pH conditions that require low overpotential of 310, 353 and 489 mV to drive a

current density of 10 mA cm−2 for OER under 0.5 M KOH, 0. 05 M H2SO4 and 1.0 M phosphate buffer solution (PBS) respectively, confirming the excellent electrocatalytic performance among state-of-the-art Ni-based electrocatalysts for overall water splitting. Therefore, the interfacial tuning based on incorporation of active heterostructure may pave a new route to develop bifunctional, cost-effective and efficient electrocatalyst systems for water splitting and H2 production. Keywords: Electrocatalysis; Oxygen evolution reaction; Hydrogen evolution reaction; NiVB/rGO heterostructure; pH-universal 1. Introduction Owing to the rapidly run down of fossil fuels on earth, there is an extensive requirement to develop cost-effective, environmental friendly and sustainable energy conversion/storage for simultaneous production of H2 and O2 from electrochemical water splitting [1–4]. However, the commercial-scale application of overall water splitting is still very restricted, because the splitting reactions—comprising the anodic OER as well as the cathodic HER are typically uphill with high overpotentials [5–7]. Recently, Zhang and co-workers also focused on alternative reactions to OER (aqueous selective semi‐dehydrogenation of tetrahydroisoquinolines; anodic oxidation of primary amines and electrochemical synthesis of nitric acid from air) to minimize the required overpotential [8–10]. Therefore, developing highly competent electrocatalysts is obligatory for overall water splitting to fulfill the rising requirement of world energy [11]. Up to now, RuO2/IrO2 and Pt/C based precious-metals are the most useful electrocatalysts for OER and HER respectively, but they are highly limited by the relatively low durability, scarcity and high cost of precious metals. Considerable

research

enthusiasm

is

therefore

devoted

to

cost-effective,

earth-abundant, highly active and stable electrocatalysts to substitute precious

metal-based electrocatalysts towards water splitting [12–15]. Transition metal based oxides, chalcogenides, nitrides, phosphides, sulfides and their hybrids have been vastly studied as electrocatalysts [14,16–20]. Particularly, Ni based materials and their composites have received great attention owing to their high earth-abundance and acceptable redox capacity, such as NiCoP/C [21], NiCo2O4 [5], Ni−Fe−P [22], Fe2O4-NiOOH [23], NiCo2S4@NiFe-layered double hydroxide (LDH) [24], Ni2P [25], CoNi2Se4 [26], Co-Ni-B [27], NiCo-MOF-74/NF [28], NiV-LDH [11], NiFeV-LDH [29], and NiSe2 [30]. However, how to achieve the efficient overall water splitting, particularly at variable pH conditions is still a challenge. For example, the NiFe-LDH presents excellent OER activity but relatively weak HER performance in alkaline conditions, since the LDH electrocatalysts exhibit poor stability in strong acidic media [31–33]. Furthermore, the Ni-based electrocatalysts are also restricted by the poor dispersion and intrinsically low electrical conductivity during electrocatalytic process. Therefore, several approaches have been developed to explore and enhance the catalytic activity of Ni-based electrocatalysts. For example, recently, Zhang and co-workers demonstrated a very motivating, facile and in-situ mild room-temperature electrochemical approach to synthesize self-supported amorphous spinel nanosheets material (NiFe2O4-NiOOH) towards an efficient oxygen production reaction. In this sense, to enhance the electrocatalytic performance and stability, self-supported nanosheet arrays with high hydrophilic (permits quick permeation of electrolyte on the electrodes) and aerophobic (help to desorbed created bubbles easily on the electrodes) surface electrocatalysts were fabricated. Accordingly, it is concluded that the supportive effect of the in-situ formed as well as the hydrophilic and aerophobic surface made key role to the remarkable OER activity [23]. Therefore, great advancement

has

been

achieved

either

by

optimizing

morphological

micro/nanostructures or combining (doping) with metal-free elements (such as sulfur, phosphorus, nitrogen) [34–37]. Song and co-workers designed a vanadium based electrocatalyst (CoVP@CC) and demonstrated that V played a decisive role in tuning the surface electronic structure as well as facilitated to boost electrocatalytic performance. Owing to keeping variable valence states ranging from +2 to +5, V is considered as a promising candidate for overall water splitting process [38]. Furthermore, V also facilitated to prevent the nearby metal atoms from leaching out, and thus greatly improved the durability of catalyst towards water splitting process [39]. Boron is principally exciting element for water splitting process, owing to its high gravimetric H2 production potential of 277 g H2 per 1000 g of B [40]. Lately, Masa and Schumann proposed that boron is helpful to boost the ECSA by generating pores on the surface of catalyst to increase the rate OER process via the leach out of its surface species in the electrolyte [41]. Moreover, two-dimensional (2D) ultrathin carbon based materials, such as reduced graphene oxide(rGO), have been incorporated for bifunctional electrochemical overall water splitting, due to their tunable electronic structure, good conductivity, high surface area, excellent thermal/chemical stability and environmental goodwill [42–47]. Recently, the incorporation of metal-based electrocatalysts on graphene nanosheets attracted a lot of attention towards electrocatalytic energy conversion and storage applications [48–51]. Among various non-noble metal based electrocatalysts, the transition metal borides (TMB) have almost been laid aside until Vrubel and Hu et al. reported boride based compound MoB as HER active catalyst in acidic and basic conditions [52,53]. Liu et al. reported highly active amorphous NiFeB nanoparticles can be synthesized by a chemical reduction method [54]. Xu et al. prepared Ni-Co-B@NF bifunctional electrocatalyst with good catalytic activity for OER and HER [55]. It is documented

that the high electrochemical performance of TMB comes from the reverse electron transfer from boron to metals, and thus providing high electron density at the catalytic active sites of electrocatalyst [56–58]. Therefore, the tunable electronic structure and high chemical stability of TMB may supply a new platform to develop efficient electrocatalysts for pH-universal overall water splitting. In this sense, integration of TMB based electrocatalysts with rGO could not only minimize the ion transport distance, dissolution and agglomeration of electrocatalysts, but also enhance the electrical conductivity, which considerably boosted electrocatalytic activity and chemical/thermal

stability

for

bifunctional

electrochemical

water

splitting

applications. Herein, for the first time, a highly active pH-universal bifunctional water splitting electrocatalyst, NiVB (optimized Ni/V=4:1) hybridized with rGO (denoted as NiVB/rGO heterostructure) is fabricated by a facile chemical reduction method under ambient conditions (Fig. 1a). The rGO provides a required support to proceed the in situ growth and highly uniform dispersion of active sites onto the conductive ultrathin nanosheets, which help to prevent agglomeration of nanoparticles [44,59]. Benefitting from the high exposed surface area, more accessible active sites, and strong electron transfer capability between NiVB nanoparticles and rGO nanosheets, NiVB/rGO heterostructure reveals efficient bifunctional electrocatalytic activity for water splitting applications, with optimized OER overpotential of 267 mV to afford a current density value of 10 mA cm−2 as well as lower Tafel slope approximately 44 mV dec−1 in 1.0 M KOH. Particularly, the NiVB/rGO heterostructure achieves a very low HER overpotential of 151 mV with lower Tafel slope (88 mV dec−1) under basic (1.0 M KOH) condition. To the best of our knowledge, NiVB/rGO heterostructure is the superior electrocatalyst with highest HER and OER catalytic activity among TMB

electrocatalysts reported to date for overall water splitting in a universal pH range. It can be expected that such interfacial tuning based on TMB heterostructure can be extended

to

synthesize

other

multi-functional,

cost-effective

and

efficient

electrocatalyst systems towards energy storage and transfer applications. 2. Experimental 2.1. Materials Nickel hexahydrate (NiCl2∙6H2O), Vanadium (III) chloride (VCl3), sodium borohydride (NaBH4), graphite powder, sodium nitrate (NaNO3), potassium permanganate (KMnO4), potassium hydroxide (KOH), water (H2O), ethanol (C2H5OH) and sulfuric acid (H2SO4) were bought from Sinopharm Chemical Reagent (Beijing Co., Ltd.). All the aforementioned chemicals (analytical grade) were used directly without any additional purification process. Deionized (DI) H2O was used during all the experiments. 2.2. Preparation of GO Graphene oxide (GO) was prepared successfully by the oxidation of graphite powder according to the reported literature with few modifications of eliminating sodium nitrate (NaNO3) from the reaction formula [60]. In a typical procedure, 3 g of graphite powder was dispersed into 70 mL of concentrated H2SO4 under vigorous magnetic stirring in an ice bath. Under strong agitation, 9.0 g of potassium permanganate (KMnO4) was introduced slowly to maintain the temperature of the reaction mixture lower than 20 °C. Successively the reaction mixture was shifted to oil bath and robustly stirred at 40 °C for almost 30 min. Afterward, 150 mL of H2O was added in the solution and stirred at 95 °C for about 15 min. Furthermore, 500 mL of H2O was added and subsequently, 15 mL of H2O2 was dropped slowly and the color of the reaction solution turned from dark brown to yellow. The as-obtained yellow

mixture was washed to remove metal ions with 250 mL HCl (1:10) aqueous solution. The resulting solid product was placed in the open air to dry and further diluted in 600 mL of H2O to make graphite oxide dispersion. Lastly, dilute dispersion was purified for at least one week by a dialysis membrane to filter the residual metallic species. Furthermore, the resulting graphite oxide dispersion was again diluted with 1.2 L of H2O, stirred for at least 12 hours and sonicated for 0.5 hour to exfoliate above dispersion to GO. Finally, to eradicate the unexfoliated graphite, the GO dispersion was centrifuged again at 3500 rpm for 40 min. 2.3. Preparation of rGO GO was reduced by annealing at 800 °C in a tube furnace for at least 1.0 hour under continuous supply of Ar gas, subsequently cooling down naturally to room temperature. 2.4. Preparation of NiVB Amorphous NPs of NiVB were synthesized by a chemical reduction method [54]. In a typical procedure, the concentration of metal salts (NiCl2·6H2O and VCl3 was kept to 5 mmol and subsequently added in 50 mL of deionized H2O under vibrant magnetic stirring. Afterward, 15 mmol of NaBH4 was dissolved in 75 mL of deionized water and then added drop wise with the speed of ~5 mL/min in an ice-water bath using a dropping funnel. The abovementioned solution was then stirred for a further 30 min to confirm the overall reduction of Ni2+ and V3+. To obtain a final product, resulting black solution was washed and centrifuged with H2O and C2H5OH for at least three times. At the end, the purified black color products were dried at 50 °C for 12 hours under vacuum. In order to optimize the mole ratio of NiVB, different mole ratios of Ni/V solutions (2:1, 4:1, 6:1 and 8:1) were prepared by regulating the initial concentration of Ni and V salts solution. In contrast, pristine NiB and VB NPs were

also fabricated successfully by following the aforementioned experimental process. 2.5. Preparation of NiVB/rGO Typically, the specific amount of the rGO nanosheets was dispersed in 50 mL of water followed by ultrasonication for at least 2 hours. Then, the above-mentioned solution of metal salts (NiCl2·6H2O and VCl3) for NiVB were introduced into the ultrasonicated mixture of rGO dispersion under strong stirring and continuous supply of nitrogen bubbling under ambient conditions. The mixture of rGO dispersion and metal salts solution was ultrasonically treated for more than 1 hour to allow the adsorption of metal ions on the surface of rGO via the electrostatic interaction. After that, preparation of NiVB/rGO followed the same procedure as aforementioned for the synthesis of pure NiVB. 2.6. Characterizations Powder X-ray diffraction (XRD) configurations of all of the as-produced samples were performed at operating conditions of 30 mA, 40 kV and λ= 0.15418 nm (Shimadzu XRD-6000 diffractometer) by a graphite-filtered Cu Kα radiation. X-ray photoelectron spectrometry (XPS) of the as-synthesized samples was executed with Al Kα radiation (Thermo VG ESCALAB MK II). Using the C 1s line at 284.8 eV, the positions of all the BEs values were calibrated. For detailed investigations of morphological structure, scanning electron microscopy (SEM, Zeiss SUPRA 55) was performed by applying an accelerating voltage of 20 kV. Transmission electron microscope (TEM) and EDS mapping were carried out using microscopy (JEOL JEM-2010F) integrated with an EDX (Oxford X-Max N 80-TLE) spectroscopy. 2.7. Electrode preparation Typically, 0.005 g of the as-produced catalyst powder was added in 1.0 mL of deionized H2O, 0.25 mL of CH3CHOHCH3 (isopropyl alcohol) and 0.015 mL of

Nafion solution, afterwards treated ultrasonically for more than 1.0 h to acquire a homogeneous mixture. At the end, 10 μL of the above-prepared mixture was added onto the GCE (glassy carbon electrode) surface through micropipette and permitted to dry the GC at room temperature via evaporation of solvent (H2O and CH3CHOHCH3). 2.8. Electrochemical measurements In our study, all the electrochemical tests for OER and HER were accomplished on the electrochemical potentiostatic (CHI 760 E, CH Instrument Co. USA) in pH-varied electrolyte of 0.5 M H2SO4, 0.05 M H2SO4, 1.0 M KOH, 0.5 M KOH, 0.1 M KOH and PBS. All the tests were executed in a standard three-electrode system, i.e. Glassy Carbon (working) electrode having a diameter of 3 mm, Pt wire (counter electrode) and saturated KCl solution filled in Ag/AgCl electrode (reference electrode). For the activation purpose, alumina powder was used to polish GC electrode on a Nylon polishing pad. Afterwards, GC electrode was cleaned entirely with DI H2O and rinsed with acetone (solvent) for at least five (5) sec by ultrasonication. The applied potential converted to overpotential (η) from E vs. Ag/AgCl for OER as followed by equations: η = E vs. RHE ‒ 1.23 V = E vs. Ag/AgCl ‒ 0.221 V and RHE (reversible hydrogen electrode) converted by the given equation as E vs. RHE = E vs. Ag/AgCl + 1.009 V in 1.0 M KOH. Cyclic voltammograms (CVs) were performed for the potential range of 0.2–0.8 V vs. Ag/AgCl to activate the catalyst-loaded electrodes in required electrolyte with a scan rate of 0.1 mV s−1. LSV (OER and HER) curves were carried out at a scan rate value of 10 mV s−1 between potential of 0.2–0.8 V. Tafel slopes curves were attained from the LSV OER as well as HER curves. Additionally, ECSA (electrochemical active surface area) was calculated from CVs that executed at different scan rates. For this objective, CVs with potential range (0.3–0.4 V) were

recorded at different scan rates (20, 40, 60, 80, 100 and 120 mV s−1) vs. Ag/AgCl. The double layer capacitance (Cdl) was explored by plotting the ∆J = (Ja − Jc) on y-axis and the scan rate on x-axis and linear slope value is twofold of the Cdl. Tafel slopes curves were obtained from the LSV data by plotting applied voltage versus Ag/AgCl on y-axis as well as log(J) on x-axis. For the estimation of the charge transfer behaviors as well as electrical conductivity, electrochemical impedance spectroscopy (EIS) spectra of as-synthesized catalysts were recorded in 1.0 M KOH with the standard three-electrode system. The frequency range was adjusted from 100 kHz to 0.10 Hz for EIS across Ag/AgCl potential with voltage amplitude of 5 mV at voltage 0.5 V. 3. Results and discussion 3.1. Structures and morphologies The phase purity and structural characterizations of the as-prepared hybrids were scrutinized by powder X-ray diffraction (XRD) and Raman spectra as shown in Fig. 1. All XRD patterns for these as-synthesized NiVB (Ni/V=3:1, 4:1 and 6:1 as examples) reveal only one broad diffraction peak between 40° and 50°, verifying the amorphous structure of the samples (Figs. 1b and S1).

Fig. 1. (a) Schematic illustration for the synthesis procedure and electrocatalytic water splitting of NiVB/rGO heterostructure, (b) XRD patterns of rGO, NiVB and NiVB/rGO heterostructure, and (c) Raman spectrum of NiVB and NiVB/rGO heterostructure. Moreover, the XRD pattern of NiVB/rGO heterostructure exhibits only one extra peak from rGO in a resulting composite, signifying an amorphous character of NiVB/rGO ultrathin nanosheets (Fig. 1b). Additionally, the Raman spectra analysis was further used to detect the NiVB/rGO. As displayed in Fig. 1(c), there is no extra characteristic peak found in NiVB/rGO compared with the pristine rGO, in which two peaks at 1589 and 1350 cm−1 can be attributed to the sp2 C skeletal vibration and the defect-induced dispersive vibrations respectively, further enlighten the amorphous nature of the NiVB/rGO [44,61,62]. The above-mentioned two peaks at approximately 1589 cm−1 (G band) and 1350

cm−1 (D band) are also observed for NiVB/rGO heterostructure with obvious changes in the peaks intensity due to the elimination of partial oxygen moieties and the exothermic reaction with NaBH4, facilitating to the reduction of defects and disorder and signifying a consistent increase in the degree of graphitization, and thus the carbon structure is sp2 conjugated, which leads to an enhanced electrical conductivity. It is renowned that the enhancement in electrical conductivity of materials benefits high catalytic activity owing to faster charge transfer [63–65]. The morphological features and composition of the as-produced NiVB/rGO heterostructure were examined by the scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS) and the transmission electron microscopy (TEM). For the pristine NiVB (Ni/V=4:1 as an example), the morphology of NiVB illustrates small nanoparticles (NPs) with the average particle size of approximately 5–10 nm, which are in the form of clumps, indicating higher degree of agglomeration (Figs. 2b and c, S2, 3b and c). Figs. 2(a) and 3(a) display the ultrathin nanosheets morphology of rGO. Furthermore, SEM image provides a morphological insight of NiVB/rGO heterostructure (Figs. 2d and e), showing that small NiVB NPs are uniformly dispersed on and inside the surface of ultrathin rGO nanosheets without any agglomeration.

Fig. 2. SEM images of (a) rGO, (b and c) NiVB (Ni/V=4:1), (d and e) NiVB/rGO and (f) EDS mapping of the NiVB/rGO heterostructure. EDS mapping images of NiVB/rGO heterostructure also display a uniform distribution of all Ni, V, B and C elements, confirming the high dispersion of NiVB NPs onto the ultrathin rGO nanosheets (Figs. 2f, S4 and S5). Moreover, the TEM images of NiVB/rGO heterostructure reveal that the individual NiVB NPs dispersed over and/or within the entire surface of the rGO nanosheets (Figs. 3d–f and S3), which are in good agreement with the SEM result. Such a uniform localization of the NiVB NPs over the ultrathin rGO nanosheets may facilitate the increase of ECSA due to roughness and curvatures in rGO nanosheets, in which more accessible active sites and strong synergetic coupling effect could boost up electron transfer capability for overall water splitting applications [66].

Fig. 3. TEM images of (a) rGO, (b and c) NiVB (Ni/V=4:1), and (d–f) NiVB/rGO heterostructure at different resolution. To give a comprehensive understanding on the oxidation state, atomic structure and chemical bonding within NiVB/rGO heterostructure, X-ray photoelectron spectroscopy (XPS) studies were conducted. According to full-scanned XPS spectra of NiVB/rGO, the as-prepared hybrid materials include all Ni, V, B, O, and C elements from NiVB and rGO (Fig. 4a). XPS full-scanned spectra of already reported transition metal-based borides showed carbon peak intensity is similar to the XPS spectrum of NiVB [44,54,67]. The peak intensity of carbon increases obviously in NiVB@rGO spectrum by the addition of rGO to NiVB as shown in Fig. S6 (zoom on the C 1s spectrum). The Ni 2p spectrum (Fig. 4b) is fitted well with two spin orbit doublets (855.9 and 873.5 eV) over and above shakeup satellites (862.3 and 880.1 eV), the binding energy difference between the 2p3/2 and 2p1/2 components is nearly 17.6 eV, signifying the existence of the Ni2+ valence state [24,68]. Deconvoluted XPS peak of Ni 2p3/2 at 857.1 eV and the peaks for Ni 2p1/2 at 873.5 and 874.9 eV are observed for Ni2+ oxidation state [69,70]. The B 1s core-level spectrum of NiVB/rGO heterostructure confirms the existence of B species in both elemental (Bo) and

trivalent (B3+) oxidation states (Fig. 4c). Two peaks of B spectrum appear at ca. 188.1 and 192.1 eV, attributed to B 1s as an elemental and oxidized, respectively [71,72], signifying the successful synthesis of NiVB.

Fig. 4. Full-scan XPS spectra of (a) rGO, NiVB and NiVB/rGO; (b) Ni 2p, (c) B 1s, (d) O 1s and V 2p spectra, (e) zoom on the V 2p spectrum and (f) C 1s spectrum of the NiVB/rGO heterostructure. Specifically, XPS results illustrate that the binding energy (BE) of B 1s (188.2 eV) in the as-synthesized NiVB/rGO heterostructures is positively shifted relative to that of elemental B (187.1 eV), signifying that electron transfer occurred from B to vacant d-orbital of metallic Ni and V [55]. The combined XPS spectra of oxygen (O 1s) and vanadium (V 2p) are displayed in Fig. 4(d), in which the peaks of O 1s originate from NiVB/rGO with binding energy of 531 eV. The surface XPS of the V2p core level is also deconvoluted into V 2p3/2 and V 2p1/2 due to the spin–orbit splitting with the BE difference of ca.7.6 eV between the 3/2 and 1/2 components. A zoom view on the V 2p3/2 and V 2p1/2 positions are revealed in Fig. 4(e). Three different peaks with BE of

ca. 515.9 eV (light grey), 516.7 eV (light pink) and 517.5 eV (light blue) are assigned to V (III), V (IV) and V (V) valance states, respectively [11,73]. The fitting analysis and high-resolution C 1s XPS of NiVB/rGO heterostructure illustrate the presence of four different functional carbon species, including C=C bond at ~284.5 eV, C−C bond at ~285.1 eV, C=O bond at ~286.5 eV and −COO− group at ~288.5 eV (Fig. 4f). The noticeable predominant non-oxygenated (C−C and C=C) specie of C signified that the GO is successfully converted to rGO after heating treatment. 3.2. OER performances Firstly, all the polarization curves (IR corrected) were recorded from linear sweep voltammetery (LSV) tests in 1.0 M KOH with a scan rate of 10 mV s−1. In Fig. 5(a), the OER polarization curves of NiVB/rGO heterostructure exhibit a very low overpotential of 267 mV to accomplish the current density value of 10 mA cm−2, whereas higher overpotentials of 275, 335, 302, 313 and 318 mV are required to attain the same current density for the RuO2 and NiVB (Ni/V=2, 4, 6, 8) electrocatalysts, respectively. The optimized Ni/V ratio of 4 is further used for the following analysis. The low overpotential of the NiVB/rGO heterostructure specifies its electrocatalytic OER activity is superior to that of the pristine NiVB NPs and even better or comparable to the most recently reported state-of-the-art electrocatalysts (Table S1). The advanced OER performance is further verified by a lower Tafel slope value of the NiVB/rGO (44 mV dec−1) compared to that of the RuO2 (72 mV dec−1) and individual NiVB (51 mV dec−1) NPs (Fig. 5b), illustrating a favorable OER kinetics in the NiVB/rGO heterostructure. It is understood that the smaller Tafel slope value correlates to a faster electrocatalytic reaction, and thus promotes the enhanced OER activity. To further evaluate the charge transfer behaviors, electrochemical impedance

spectroscopy (EIS) was employed for both NiVB and NiVB/rGO heterostructure in 1.0 M KOH with the three-electrode system (Fig. 5c). The charge transfer resistance (Rct) as well as solution resistance (Rs) values were recorded based on the corresponding Nyquist plots. The diameter of the semicircle demonstrates the Rct, where low Rct value indicates high conductivity as well as speedy charge-transfer ability.

Fig. 5. (a) Linear scan voltammograms (LSV) OER curves of NiVB (with different Ni/V ratios), RuO2 and NiVB/rGO heterostructure; (b) corresponding Tafel slopes for OER; (c) EIS of rGO, NiVB and NiVB/rGO heterostructure; (d) the extraction of the Cdl to estimate the ECSA of RuO2, NiVB and NiVB/rGO heterostructure; (e) LSV HER curves of NiVB (with different Ni/V ratios), Pt/C and NiVB/rGO heterostructure; (f) corresponding Tafel slopes for HER. Therefore, EIS results (Fig. 5c) confirm that the lower Rct value is noticed for the NiVB/rGO heterostructure (63 Ω) compared to that of the RuO2 (90 Ω) and individual NiVB (112 Ω) NPs, which signifies its superlative electron-transfer behavior and matches well with the LSV and Tafel slope results. The double-layer capacitances (Cdl) were recorded to gain further insight into the electrochemically active surface area

(ECSA) of all the as-prepared electrocatalysts. ECSA is a decisive parameter for all those electrocatalysts used in water splitting applications, as it is believed that higher ECSA usually favors for enhancing the electrocatalytic activity of the catalyst. As shown in Figs. 5(d) and S7, the Cdl value of NiVB/rGO reaches to 41.4 mF cm−2, higher than that of the individual NiVB (34.9 mF cm−2) and RuO2 (18.4.2 mF cm−2), signifying that the heterostructure leads to enhance accessible surface area and plenteous surface-active sites, which is responsible for the advanced electrocatalytic OER performance. According to our viewpoint, an amorphous nickel/vanadium boride (NiVB) nanoparticles/reduced graphene oxide (rGO) hybrid (NiVB/rGO) electrocatalyst with high electrocatalytic activity towards overall water splitting is owing to the following motives: (1) The high intrinsic electrocatalytic activity of amorphous NiVB/rGO heterostructure materials. (2) The thin nanosheets of rGO boost the accessible surface area for water splitting reaction. (3) rGO nanosheet supports can accelerate charge transfer to advance the kinetics of water splitting processes. (4) NiVB/rGO heterostructure benefits to higher catalytic performance owing to the strong synergistic coupling effects between NiVB nanosheets and rGO support, which helps in enhancing charge transport. (5) The NiVB NPs are uniformly dispersed over and within the entire surface of ultrathin rGO nanosheets without any agglomeration, which facilitates the increase of ECSA. Consequently, these collective advantages provide

the

NiVB/rGO

electrocatalytic activity.

heterostructure

electrocatalyst

with

remarkable

Fig. 6. LSV OER curves of NiVB and NiVB/rGO heterostructure in: (a) 0.5 M KOH, (c) 0.05 M H2SO4 and (e) 1.0 M PBS, and (b, d and f) their corresponding Tafel slope profiles. The electrocatalytic activities of NiVB/rGO heterostructure for OER were further systematically investigated in pH-varied electrolytes (namely 0.5 and 0.05 M H2SO4; 1, 0.5 and 0.1 M KOH; PBS (pH = 7)) with a standard three-electrode electrochemical system (Figs. 6 and S8) for comparison analysis. The NiVB/rGO heterostructure also performs superbly in a variety of aqueous electrolyte (alkaline, acidic and neutral environment). For example, in 0.5 M KOH, the NiVB/rGO demands an overpotential of ca. 310 mV to attain a current densityof 10 mA cm−2, and the pretty low value of Tafel slope (49 mV dec−1) further verifies the advantageous kinetics for OER (Figs. 6a and b). For comparison analysis in a wide range pH, the electrocatalytic activities of the NiVB/rGO heterostructure toward OER were also systematically evaluated at acidic aqueous electrolyte. In 0.05 M H2SO4, it needs an overpotential of about 353 mV to convey a current density of 10 mA cm−2 (vs. RHE), and the small overpotential and the Tafel slope (77 mV dec−1) further confirm a rapid kinetics towards the OER

catalyzing capability (Figs. 6c and d). In neutral environment (pH = 7), the OER overpotential of NiVB/rGO is 489 mV to deliver 10 mA cm−2 with the corresponding Tafel slope of 95 mV dec−1 (Figs. 6e and f). Therefore, the obvious electrocatalytic activities of the NiVB/rGO heterostructure occur in a universal range of pH (acidic, neutral and alkaline), indicating the extremely favorable performance toward OER. The high electrocatalytic performance for the NiVB/rGO heterostructure in pH-varied electrolytes is also noticeable in comparison with the other state-of-art electrocatalysts for OER applications (Table S1). 3.3. HER performance We have also assessed the electrocatalytic HER activities of the as-synthesized NiVB and NiVB/rGO heterostructure in the same electrolyte and electrode system with that for OER. Besides the outstanding OER performance, it is found that the NiVB/rGO heterostructure is also electrocatalytically active towards HER in a wide range of pH. The HER polarization curves of NiVB/rGO heterostructure exhibit a low overpotential of 151mV to drive the current density of 10 mA cm−2. Except the Pt/C electrocatalyst (99 mV), higher overpotentials of 303, 196, 214, and 270 mV are required to attain the same current density of 10 mA cm−2 for the NiVB (Ni/V=2, 4, 6, 8) electrocatalysts, respectively (Fig. 5e).

Fig. 7. LSV HER curves of NiVB and NiVB/rGO heterostructure in (a) 0.5 M H2SO4, (c) 0.05 M H2SO4 and (e) 0.5 M KOH, and (b, d and f) their corresponding Tafel slope profiles.

Therefore, the low overpotential signifies the HER activity of the NiVB/rGO heterostructure is also enhanced relative to that of NiVB NPs and even better or at least comparable to current transition metal-based catalysts (Table S3). The superior electrocatalytic HER performance of the NiVB/rGO heterostructure is further demonstrated by a lower Tafel slope value (88 mV dec−1) compared with that of the Pt/C (151 mV dec−1) and NiVB (93 mV dec−1) NPs in Fig. 5(f), signifying a favorable HER kinetics in the NiVB/rGO heterostructure than the pristine NiVB. The enhanced electrocatalytical activity of the NiVB/rGO heterostructure for HER should be ascribed to their high electrochemical active surface area and lower charge transfer resistance as compared to NiVB NPs (Figs. 5c and d). To further examine the HER performances of the NiVB/rGO heterostructure under universal pH conditions, we measured the HER performances in alkaline (0.5

and 0.1 M KOH) and acidic (0.5 and 0.05 M H2SO4) electrolytes (Figs. 7 and S9). Exploration of the HER polarization curves in acidic (0.5 and 0.05 M H2SO4) electrolyte illustrates that the NiVB/rGO heterostructure displays a much better electrocatalytic performance comparable to that of pristine NiVB (Fig. 7a). To accomplish a current density of 10 mA cm−2, the NiVB/rGO heterostructure requires overpotentials of 146 and 278 mV in 0.5 and 0.05 M H2SO4 respectively, and similar trends are also observed for corresponding Tafel slopes values (Fig. 7a–d). For the cases in aqueous alkaline electrolytes (0.5 and 0.1 M KOH), the required overpotentials of the NiVB/rGO heterostructures are 268 and 315 mV to drive a current density of 10 mA cm−2, whereas the NiVB demonstrates higher overpotential of 307 mV. Similar behaviors are also observed for corresponding Tafel slopes values (113 and 128 mV dec−1), relative to the pristine NiVB (148 mV dec−1) (Figs. 7e and f, and S9). The detailed values of overpotentials and Tafel slopes for OER and HER at different pH conditions are shown in Table S4 and Fig. S10. It was found that both the OER and HER catalytic activities of the NiVB/rGO heterostructure in pH varied electrolytes are higher than those of most catalysts designed to date (Tables S1–S2). As described above, the high electrocatalytic performances are related to the morphological structure, oxidation state, and nature of chemical bonding in NiVB/rGO heterostructure, in which well dispersion of NiVB NPs on ultrathin rGO nanosheets plays a vital role in improving the OER and HER activity. The electrocatalytic performance can be optimized as the pH value increases towards strong basic media, which is consistent well with reported works [27,54,74].

Fig. 8. (a) Durability test of NiVB/rGO heterostructure in 1.0 M KOH electrolyte for OER, (b) polarization curves before and after 1000 CV cycles for OER, (c) durability test of NiVB/rGO in 1.0 M KOH electrolyte for HER, and (d) polarization curves for overall water splitting. 3.4. Durability and overall water splitting For the practical applications of as-prepared electrocatalysts, durability is an exceptionally cost-effective and essential parameter. The long-term durability of the NiVB/rGO heterostructure for both OER and HER is investigated by a chronoamperometry test for 12 hours (Fig. 8a–c). The durability test of the NiVB/rGO heterostructure for OER is also carried out with cyclic voltammetery (CV) measurements. The as-synthesized electrocatalyst retains an almost invariable current density after 1000 cycles of consecutive CV scanning at a scan rate of 0.1 V s−1.

Furthermore, the current density value is retained with almost negligible fluctuation in HER, signifying the excellent stability of the NiVB/rGO heterostructure for water splitting in the alkaline medium. The high stability performance demonstrates that the NiVB/rGO heterostructure can be suitable as practical electrocatalysts for water splitting. In order to further gain insights into the durability of the NiVB/rGO heterostructure electrode after water electrolysis process, we also performed the XRD, SEM and TEM characterizations. XRD patterns of NiVB/rGO heterostructure after water electrolysis process exhibits that there is no obvious change observed from XRD patterns of NiVB/rGO heterostructure before and after water electrolysis process, that confirms its high stability (Fig. S11). Moreover, SEM and TEM images indicate that there is no noticeable difference in the morphological structure of NiVB/rGO heterostructure after water electrolysis, that further approves the stability of heterostructure as it well-sustained the architecture (Figs. S12 and S13). The overall water splitting measurements were accomplished in a two-electrode system, where the NiVB/rGO heterostructure electrodes act as both anode as well as cathode for OER as well as HER, respectively in alkaline medium. The LSV polarization curves presented in Fig. 8(d) reveal that the NiVB/rGO heterostructure requires the potential of just 1.56 V to deliver a current density value of 10 mA cm−2, while the cell voltage of 1.76 V needs to deliver the current density of 100 mA cm−2 for overall water splitting reaction. Such a low potential of the NiVB/rGO heterostructure towards overall water splitting is comparable or even superior to other non-noble metal based recently reported electrocatalysts (Table S2).

4. Conclusions In summary, we have successfully synthesized nickel/vanadium boride (NiVB) NPs/reduced graphene oxide (rGO) heterostructure (denote as NiVB/rGO) by a facile chemical reduction method under ambient condition, which is further used as bifunctional electrocatalyst for overall water splitting. The NiVB NPs uniformly dispersed over and within the entire surface of the rGO nanosheets that leads to high accessible surface area, efficiently coupled with strong electronic interaction, benefits to enhance the charge transfer and boosted reaction kinetics. Compared with the pristine NiVB NPs, the NiVB/rGO heterostructure is demonstrated as a more proficient bifunctional electrocatalyst, which requires overpotentials of 267 and 151 mV to afford the current density of 10 mA cm−2 in alkaline electrolyte for OER and HER respectively, which are higher or at least comparable to the other recently reported

state-of-art

electrocatalysts.

Particularly,

the

high

bifunctional

electrocatalytic performance can be maintained in a wide range pH values, rending the NiVB/rGO as a new type of OER and HER electrocatalysts in universal conditions. Furthermore, the NiVB/rGO heterostructure is used as both the anode as well as cathode and reveals tremendous catalytic activity for overall water splitting that requires a voltage of 1.56 V to convey the current density of 10 mA cm−2. Therefore, this work not only highlights an effective low-cost electrocatalyst with promising pH-universal electrocatalytic performance, but also provides a new outlooks to explore metal boride based heterostructure towards the overall water splitting applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21771021, 21822501, 21720303, and 22061130206), the Newton Advanced Fellowship award (NAF\R1\201285), the Fok Ying-Tong Education Foundation (Grant No. 171008), the Beijing Nova Program (Grant No. xx2018115), the State Key Laboratory of Rare Earth Resources Utilization (RERU2019005), the Fundamental Research Funds for the Central Universities and the Measurements Fund of Beijing Normal University. References [1] S. Chu, A. Majumdar, Nature 488 (2012) 294-303. [2] R. Gao, Q. Dai, F. Du, D. Yan, L. Dai, J. Am. Chem. Soc. 141 (2019) 11658-11666. [3] E. Hu, Y. Feng, J. Nai, D. Zhao, Y. Hu, X.W. Lou, Energy Environ. Sci. 11 (2018) 872-880. [4] R. Gao, D. Yan, Adv. Energy Mater. 10 (2020) 1900954. [5] X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang, Z. Lin, Angew. Chem. Int. Ed. 55 (2016) 6290-6294. [6] J. Wang, W. Cui, Q. Liu, Z. Xing, A.M. Asiri, X. Sun, Adv. Mater. 28 (2016) 215-230. [7] M. Arif, G. Yasin, M. Shakeel, M.A. Mushtaq, W. Ye, X. Fang, S. Ji, D. Yan, Mater. Chem. Front. 3 (2019) 520-531. [8] C. Huang, Y. Huang, C. Liu, Y. Yu, B. Zhang, Angew. Chem. Int. Ed. 58 (2019) 12014-12017. [9] Y. Huang, X. Chong, C. Liu, Y. Liang, B. Zhang, Angew. Chem. Int. Ed. 57 (2018) 13163-13166. [10] Y. Wang, Y. Yu, R. Jia, C. Zhang, B. Zhang, Natl. Sci. Rev. 6 (2019) 730-738. [11] K. Fan, H. Chen, Y. Ji, H. Huang, P.M. Claesson, Q. Daniel, B. Philippe, H. Rensmo, F. Li, Y. Luo, L. Sun, Nat. Commun. 7 (2016) 11981. [12] F.A. Garcés-Pineda, M. Blasco-Ahicart, D. Nieto-Castro, N. López, J.R. Galán-Mascarós, Nat. Energy 4 (2019) 519-525. [13] J. Shan, T. Ling, K. Davey, Y. Zheng, S.Z. Qiao, Adv. Mater. 31 (2019) 1900510. [14] H. Sun, Z. Yan, F. Liu, W. Xu, F. Cheng, J. Chen, Adv. Mater. 32 (2020) 1806326. [15] R. Gao, H. Zhang, D. Yan, Nano Energy 31 (2017) 90-95.

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Graphical abstract

In current research work, highly efficient and pH-universal non-noble metal based electrocatalyst (NiVB/rGO heterostructure) towards bifunctional water splitting application is fabricated via a facile chemical reduction method under ambient conditions.

Highlights



Novel amorphous NiVB/rGO heterostructure is fabricated by a facile and easily scalable chemical reduction method.



The uniform dispersion of the NiVB nanoparticles over the ultrathin rGO nanosheets facilitates to prevent the agglomeration.



Unique architecture increases the electrochemical active surface area and accessibility to active sites.



The NiVB/rGO heterostructure shows tremendous electrocatalytical activity in a wide-range of pH towards overall water splitting applications.