Porous Multishelled Ni2P Hollow Microspheres as an Active

Sep 18, 2017 - The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the d...
1 downloads 14 Views 4MB Size
Download PDF
Article pubs.acs.org/cm

Porous Multishelled Ni2P Hollow Microspheres as an Active Electrocatalyst for Hydrogen and Oxygen Evolution Hongming Sun,† Xiaobin Xu,‡ Zhenhua Yan,† Xiang Chen,† Fangyi Cheng,*,†,‡ Paul S. Weiss,‡ and Jun Chen†,§ †

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China ‡ Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States § State Key Laboratory of Elemento-Organic Chemistry, Innovative Collaboration Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Tailoring the morphology and microstructure of electrocatalysts is important in improving catalytic performance. Herein, porous multishelled Ni 2 P hollow microspheres assembled by nanoparticles were prepared through a simple and economical self-templating approach followed by phosphorization. Compared with nanoparticles and hierarchical solidinterior microspheres, the synthesized multishelled, hollow microstructures of Ni2P exhibit significantly higher electrocatalytic activity for the hydrogen evolution reaction in a 1 M KOH electrolyte. Additionally, a NiOOH layer is formed on the surface of Ni2P during anodic polarization, as revealed by electron microscopy, X-ray photoelectron spectroscopy, and in situ Raman analysis. The Ni2P/NiOOH derivative outperforms the benchmark RuO2 in catalyzing the oxygen evolution reaction. Furthermore, pairing the carbon fiber paper-supported multishelled Ni2P as both the anode and cathode results in superior overall alkaline water splitting performance, generating 10 and 20 mA cm−2 current densities at applied cell voltages of only 1.57 and 1.64 V, respectively, together with outstanding durability. These results suggest that further elaboration of the design of multishelled and hollow structured metal phosphides is desirable for application in hydrogen and oxygen evolution electrocatalysis.

■

INTRODUCTION Splitting of water into hydrogen and oxygen through electrocatalytic processes is now widely regarded as an ideal alternative for producing sustainable, renewable, clean hydrogen fuel energy.1,2 High-activity electrocatalysts are required to decrease the high overpotential (usually >1.8 V vs the theoretical limit of 1.23 V) of practical water electrolysis involving the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER).3 Pt-based and Ir/ Ru-based compounds are the state-of-the-art electrocatalysts for the HER and OER, respectively.4,5 However, the prohibitive cost and scarcity preclude their large-scale applications. Therefore, considerable efforts have been dedicated to exploiting alternative electrocatalytic materials based on abundant, available, and lower-cost elements. For example, transition metal alloys,6 carbides,7 phosphides,8 chalcogenides,9,10 nitrides,11 non-metal materials,12 and their hybrid structures13 have been employed as HER catalysts and usually show their best catalytic activity in strong acidic electrolytes, and transition metal oxides14,15 and oxy/hydroxides16,17 have © 2017 American Chemical Society

been investigated for electrocatalytic OER in alkaline electrolytes. It is desirable to exploit a catalyst that possesses high activities for both HER and OER in the same electrolyte (an alkaline or acidic solution), which could simplify setups, avoid the production of different electrocatalysts, and decrease the cost.18 Exhilaratingly, some transition metal phosphides,19,20 chalcogenides,21 borides,22 nitrides,23 oxides,24 and their composites25 are effective as overall water splitting catalysts. Among them, Ni2P, which features good electrical conductivity, high stability over a wide pH range, and high elemental abundance of both Ni and P, has been experimentally and theoretically demonstrated as an attractive HER catalyst mimicking biological hydrogen-evolving catalyst [NiFe] hydrogenases.3,26 This set of properties has triggered the investigation of a variety of Ni2P nanostructures such as nanocrystals,27 nanoparReceived: August 27, 2017 Revised: September 18, 2017 Published: September 18, 2017 8539

DOI: 10.1021/acs.chemmater.7b03627 Chem. Mater. 2017, 29, 8539−8547

Article

Chemistry of Materials ticles,28,29 and nanowires.30 Moreover, Ni2P was reported to serve as an interesting precatalyst toward the OER.31,32 However, the real active species of Ni2P-based materials remains elusive, being ambiguously identified as nickel oxides,31,32 oxyhydroxide,33 a hydroxide/oxyhydroxide hybrid,34 or even a mixture of oxides, hydroxides, and oxyhydroxides.35 In addition, the HER and OER performance of the reported Ni2P is often not comparable with that of the Pt/ RuO2 couple, limiting its practical application in water electrolysis. Designing the microstructure and morphology to maximize the accessible active sites is effective for improving the performance of energy storage and converion devices.10,36 Three-dimensional (3D) hollow structures with porous multilayer shells have attracted a tremendous amount of attention as they feature abundant interior space and large surface areas for fast diffusion and enhanced reaction kinetics. Porous 3D multishelled hollow structures have found versatile applications, being used in catalysis, solar cells, sensors, and biomedicine.37 Hitherto, different types of transition metal oxides (e.g., Co 3 O 4 , 38 CeO 2 , 39 Fe 3 O 4 , 40 etc.), metal sulfides (e.g., NiCo2S441), organosilica,42 and carbonaceous materials43 with multishelled hollow structures have been synthesized by different methods. However, to the best of our knowledge, multishelled and hollow-structured pure metal phosphides have not been exploited to date. Herein, we report the electrocatalytic application of porous multishelled Ni2P hollow microspheres (hereafter abbreviated as multishelled Ni2P), which were synthesized by a selftemplate method followed by phosphorization. For the HER and OER, the prepared multishelled Ni2P show electrocatalytic activity remarkably higher than that of Ni2P nanoparticles and solid-interior microspheres. By combining ex situ transmission electron microscopy, X-ray photoelectron spectroscopy, and in situ Raman analysis, we reveal the formation of NiOOH on the surface of multishelled Ni2P during anodic polarization. The asformed Ni2P/NiOOH core−shell nanostructure results in a low overpotential of 270 mV to reach a current density of 10 mA cm−2 and a low Tafel slope of 40.4 mV decade−1, outperforming the benchmark RuO2. Furthermore, an alkaline electrolyzer was assembled by employing carbon fiber papersupported multishelled Ni2P (abbreviated as Ni2P/CP) as both the cathode and anode electrodes. This electrolyzer generates H2/O2 at ∼1.57 V and 10 mA cm−2 and can operate continuously for at least 20 h with negligible performance degradation. The results indicate that Ni2P with multishelled and hollow structure is a promising economical yet highefficiency electrocatalyst for overall water splitting.

■

with NaH2PO2 at the upstream side of the furnace; the Ni:P ratio was 1:7.5. Subsequently, the sample was heated at 300 °C for 60 min in an Ar atmosphere and then naturally cooled to ambient temperature under Ar. Hierarchical solid Ni2P microspheres were synthesized by using similar procedures but without C6H12O6·H2O in the preparation of NiO. For comparison, monodisperse Ni2P nanoparticles were prepared by a previously reported method.26 Typically, Ni(acac)2 (250 mg), 1octadecene (4.5 mL), oleylamine (6.4 mL), and tri-n-octylphosphine (2 mL) were added to a three-neck round-bottom flask. The solvent mixture was stirred and heated to 120 °C under vacuum for 1 h. Then, the solution was heated to 320 °C under an Ar atmosphere and maintained at 320 °C for 2 h. After the solution had cooled to room temperature, the resulting nanoparticles were collected by centrifugation and washed several times using a solvent mixture of hexanes and ethanol (1:3 by volume). The resulting nanoparticles were resuspended and stored in hexanes. Characterization of Material. X-ray diffraction (XRD) was recorded on a Rigaku Mini Flex 600 powder diffractometer (Rigaku Mini Flex 600 X-ray generator; Cu Kα radiation; λ = 1.5406 Å). Scanning electron microscopy (SEM) measurements were taken on a JEOL JSM-7500F microscope at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) imaging was performed on a Philips Tecnai F20 system operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were taken with a PerkinElmer PHI 1600 ESCA system. The specific surface area of the sample was measured by nitrogen adsorption−desorption isotherms at 77 K with a BELSORP-Mini instrument. Elemental analysis was tested by inductively coupled plasma atomic emission spectroscopy (ICPAES) (PerkinElmer Optima 83000). In situ Raman spectra were recorded using a confocal Raman microscope (DXR, Thermo-Fisher Scientific) with 633 nm excitation from an argon ion laser. Each spectrum was recorded (40 s duration and 10 s interval) on the Ni2P/ CP electrode, which was anodically scanned from 1.0 to 1.7 V at a rate of 1 mV s−1. Electrochemical Tests. HER and OER catalytic activity measurements were taken with computer-controlled workstation bipotentiostats (AFCBP1, Pine Instrument) in a three-electrode cell configuration at room temperature. The saturated calomel electrode (SCE) and Pt were used as the reference and counter electrode, respectively. All measured potentials were converted to the reversible hydrogen electrode (RHE), according to the equation E (RHE) = E (SCE) + 1.053 V in 1.0 M KOH (pH ≈13.75). The working electrode was a glassy carbon electrode coated with a thin layer of catalyst, which was prepared by ultrasonically mixing 9.0 mg of as-prepared catalyst and 1.0 mg of Vulcan-72 carbon in a solution containing 50 μL of Nafion and 950 μL of isopropanol solvent. A 7.0 μL portion of the prepared ink was transferred to the surface of a rotation disk electrode (RDE) using a microsyringe. The catalyst-afforded electrode was allowed to dry at room temperature. Catalyst loading was approximately 0.283 mg cm−2 on the RDE. To study the hydrogen evolution reaction (HER) on multishelled Ni2P, the electrode was first subjected to continuous potential cycling between 0.2 and −0.5 V at a rate of 50 mV s−1 in argon-saturated 1 M KOH until stabilized voltammograms were obtained. Linear sweep voltammograms were recorded at 5 mV s−1 to test the HER activity. The OER activity was examined in oxygensaturated KOH (1 M) by linear sweep voltammograms from 1 to 1.8 V at a rate of 5 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were taken in the frequency range of 100 kHz to 0.1 Hz. Water electrolysis was performed in a single-compartment cell, in which carbon fiber paper electrodes (0.6 cm × 0.6 cm) modified with catalysts (loading of 2.0 mg cm−2) serve as both the anode and the cathode. To prepare the electrodes, 72 μL of the catalyst ink described above was dripped onto the carbon fiber paper. The electrodes were dried and further heated at 50 °C in vacuum for 2 h. Linear sweep voltammetry of water electrolysis was recorded from 1.0 to 2.0 V at a scan rate of 1 mV s−1 in 1 M KOH. All polarization curves were iRcorrected following the equation E = Em − iRs (where Rs is the resistance of the solution, E is the corrected potential, and Em is the measured potential).

MATERIALS AND METHODS

Preparation of Materials. The preparation of multishelled Ni2P hollow microspheres followed a two-step route: synthesis of a multishelled NiO hollow microsphere precursor and phosphidation of the NiO precursor to Ni2P using NaH2PO2 as a phosphorus source. In a typical synthesis, 0.56 g of CO(NH2)2, 0.18 g of NiCl2·6H2O, and 5.4 g of C6H12O6.H2O were dissolved in 70 mL of deionized water. Then, the aqueous solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 20 h. After the solution had naturally cooled to room temperature, black or brown product was collected by centrifugation, washed with ethanol and distilled water, and dried at 60 °C for 12 h. After that, the sample was calcined at 450 °C for 4 h in air to form multishelled NiO hollow microspheres. For preparing multishelled Ni2P, the NiO precursor and NaH2PO2 were loaded at two separate positions in a porcelain boat 8540

DOI: 10.1021/acs.chemmater.7b03627 Chem. Mater. 2017, 29, 8539−8547

Article

Chemistry of Materials

Figure 1. Synthetic schematic illustration and material characterization of the multishelled Ni2P. (a) Schematic illustration. (b) X-ray diffraction pattern and the reference (JCPDS Card No. 65-1989). (c) Scanning electron micrograph of the particle. (d) Transmission electron microscopy (TEM) image of the particles. (e) Elemental mapping from TEM imaging. (f) High-magnification TEM image of the shell of a sphere. The inset shows the size distribution histogram of the nanoparticles in the shell. (g) High-resolution TEM image and the corresponding selective area electron diffraction pattern (inset).

■

RESULTS AND DISCUSSION Multishelled Ni2P was synthesized by a two-step strategy. As illustrated schematically in Figure 1a, multishelled NiO hollow microspheres were prepared first using a previously reported self-templating method with modified procedures.39 Then, the multishelled NiO precursor was converted to multishelled Ni2P via phosphidation in Ar. Figure 1b shows the XRD pattern of the as-synthesized sample. The diffraction data are readily indexed to the hexagonal structure of Ni2P (JCPDS Card No. 65-1989, space group P6̅2m), without discernible diffraction peaks of other phases. The morphology and microstructure of multishelled Ni2P were characterized by SEM and TEM. As seen in the SEM image (Figure 1c), the as-prepared samples present a spherical morphology with diameters of approximately 1.5−2.5 μm. Small pores can be seen on the surfaces of the microspheres. From the TEM image (Figure 1d), the Ni2P microspheres are hollow with four-layer spherical shells, as revealed by the obvious contrast between the dark periphery and the pale intervals of the microspheres. Elemental mapping with energy-dispersive X-ray spectroscopy (EDX) (Figure 1e) of multishelled Ni2P hollow microspheres reveals that Ni and P elements are uniformly distributed in the whole microsphere from inside to out. The detailed microstructure of the outer shell of a typical hollow microsphere is shown in Figure 1f. We conclude that the shells are composed of closely interconnected Ni2P nanoparticles with sizes of 12−17 nm, which is in agreement with the crystallite size (15.2 nm) determined from the (111) peak width of the XRD pattern using the Debye−

Scherrer equation. High-resolution TEM (HRTEM) images of the nanocrystals (Figure 1g) show well-resolved lattice fringes with interplanar distances of 0.221 and 0.292 nm, which correspond to the (111) and (110) planes of Ni2P, respectively. The observed bright rings in the corresponding selected area electron diffraction (SAED) pattern (Figure 1g, inset) could be indexed to the (300), (120), (201), and (111) planes of hexagonal Ni2P and indicate the polycrystalline nature of the sample. The porous, multishelled, and hollow structure of Ni2P is entirely inherited from the NiO precursor, which is assembled by nanocrystals with smaller sizes (8−12 nm) (Figure S1). The texture of the multishelled NiO and Ni2P samples was characterized by nitrogen adsorption/desorption isotherms (Figure S2). The distribution of pore sizes deduced from desorption branches using the Barrett−Joyner−Halenda (BJH) method is within 2−16 and 2−25 nm for the NiO precursor and Ni2P product, respectively. The Brunauer−Emmett−Teller (BET) specific surface area of Ni2P is determined to be 31.6 m2 g−1, which is smaller than that of NiO (69.1 m2 g−1) because of the large sizes of Ni2P nanoparticles that build up the multishelled hollow microspheres. For comparison, we also prepared hierarchical solid-interior microspheres of Ni2P (abbreviated as hierarchical Ni2P) and Ni2P nanoparticles (denoted as nanostructured Ni2P). Hierarchical Ni2P is composed of aggregated nanoparticles and characterized by high phase purity and porosity (Figures S3 and S4). The nanostructured Ni2P is monodisperse with a size of 13−19 nm (Figures S5 and S6), which is close to that of nanoparticles 8541

DOI: 10.1021/acs.chemmater.7b03627 Chem. Mater. 2017, 29, 8539−8547

Article

Chemistry of Materials

NAs/CC (218 mV),48 Co-NRCNTs (370 mV),49 Ni2P film (183 mV),50 and Co/CoP (135 mV)51 (Table S2). Figure 2b shows the Tafel plots derived from the polarization curves of multishelled Ni2P, hierarchical Ni2P, and Pt/C. The determined Tafel slope of the Pt/C benchmark is 32.9 mV decade−1, close to reported values.46,52 At lower overpotentials, the multishelled Ni2P exhibits a Tafel slope of 86.4 mV decade−1, which is much smaller than those of hierarchical Ni2P (108.4 mV decade−1) and nanostructured Ni2P (125.4 mV decade−1) suggesting more rapid HER kinetics. This Tafel slope falls within the range of 40−120 mV decade−1, revealing that the HER occurs through a Volmer−Heyrovsky mechanism.22 A slightly upward deviation is observed in Tafel plots of Pt/C and hierarchical Ni2P in the high-overpotential regime, which is possibly due to transport limitation at a high current density. EIS was also tested to understand the electrochemical properties of multishelled Ni2P. As shown in Figure 2c, the charge-transfer resistance (Rct), corresponding to the semicircular region, is indicative of charge-transfer kinetics.53 At an overpotential of 150 mV, the multishelled Ni2P electrode exhibits an Rct significantly lower than those of hierarchical Ni2P, nanostructured Ni2P, and bare carbon, indicating a lower charge-transfer resistance. We attribute the superior HER electrocatalytic activity of multishelled Ni2P to the intrinsically favorable conductivity of Ni2P and the multishelled porous structure that allows facile electrolyte accessibility and electrical contact. The turnover frequency (TOF) for each active site is a good figure of merit for comparison of intrinsic activities among different catalytic materials.54 To calculate the TOF, the upper limit of available active sites of Ni2P is determined to be ∼2.00 × 1015 cm−2, by evaluating the BET specific surface area and tentatively assuming all surface atoms (Ni and P) are active as the active sites of Ni2P are not known explicitly.26 The calculated TOFs of multishelled Ni2P are 0.18 and 1.41 s−1 at overpotentials of 100 and 200 mV, respectively. These values outperform that of hierarchical Ni2P (0.38 s−1 at η = 200 mV) and nanostructured Ni2P (0.19 s−1 at η = 200 mV), suggesting excellent HER catalytic activity of multishelled Ni2P. In addition, the TOF values based on Cdl of multishelled Ni2P at overpotentials of 100 and 200 mV are 0.14 and 1.10 s−1, respectively, which are close to the TOFs based on BET. To evaluate the durability of the as-prepared catalysts in the HER, continuous cyclic voltammogram (CV) sweeps were performed from −0.20 to 0.20 V at a scanning rate of 100 mV s−1 in a 1 M KOH solution. After 1000 cycles, the multishelled Ni2P electrode retained performance almost similar to that in the initial cycle (Figure 2d). The durability of multishelled Ni2P was further tested by polarization at a constant overpotential of 100 mV, with >90% current retention after 20000 s (Figure 2d, inset). Additionally, ICP-AES analysis indicated undetectable nickel in the KOH solution after HER measurement (Table S1). These results further demonstrated the HER durability of multishelled Ni2P. The electrochemically active surface areas (ECSAs) of multishelled Ni2P, hierarchical Ni2P, and nanostructured Ni2P were evaluated from the electrochemical double-layer capacitance (Cdl) by collecting CVs at different scan rates within the potential window of 0.04−0.14 V (Figure S8). As shown in Figure 2e, the capacitance of the multishelled Ni2P (6.95 mF cm−2) is nearly 5 times of that of hierarchical Ni2P (1.38 mF cm−2) and >2 times that of nanostructured Ni2P (2.95 mF cm−2), indicating that the multishelled architecture and

constituting multishelled Ni2P. After being loaded on the RDE electrode, the nanostructured Ni2P is agglomerated severely, as shown in Figure S7. To assess the electrocatalytic HER activity, the prepared multishelled Ni2P was mixed with carbon (Vulcan XC-72) and loaded onto rotating disk electrodes (RDEs) with a mass loading of 0.283 mg cm−2. Electrochemical measurements in a 1 M KOH solution employed a standard three-electrode setup. For comparison, bare carbon, nanostructured Ni2P, hierarchical Ni2P, and the benchmark Pt/C catalyst (20 wt % Pt) were also tested. Figure 2a shows the linear sweep curves for the different

Figure 2. (a) Linear sweep voltammetry (LSV) polarization curves of bare carbon, nanostructured Ni2P, hierarchical Ni2P, multishelled Ni2P, and benchmark Pt/C in 1 M KOH at a scan rate of 5 mV s−1. (b) Corresponding Tafel plots with linear fittings. (c) Nyquist plots recorded at an overpotential of 150 mV. (d) Polarization curves of multishelled Ni2P in 1 M KOH before and after 1000 cycles at a scan rate of 100 mV s−1 between 0.1 and −0.25 V. The inset in panel d illustrates the chronoamperometry (I−t) curves of multishelled Ni2P obtained at an overpotential of 100 mV. (e) Scan rate dependence of the current densities of multishelled Ni2P, hierarchical Ni2P, and nanostructured Ni2P at 0.09 V. (f) Electrochemical active surface area (ECSA)-normalized initial LSV curves from panel a.

GCEs modified with bare carbon, nanostructured Ni2P, hierarchical Ni2P, multishelled Ni2P, and Pt/C. The Pt/C electrode exhibits excellent HER activity with almost no overpotential, while the bare carbon shows negligible catalytic activity. Remarkably, the multishelled Ni2P enabled a small onset overpotential of 10 mV (j = 1.0 mA cm−2) and a rapid cathodic current increase as more negative potentials were applied. The overpotential driving a cathodic current density of 10 mA cm−2 is 98 mV, much lower than that observed on hierarchical Ni2P (298 mV) and nanostructured Ni2P (214 mV). Such a low overpotential requirement of multishelled Ni2P at pH 14 compares favorably to those of most reported nonprecious HER catalysts, such as CoP/CC (209 mV),44 Ni− Co−P nanocubes (150 mV),45 Ni2P/graphene/nickel foam (150 mV),46 Mo2C/carbon microflowers (100 mV),47 FeP 8542

DOI: 10.1021/acs.chemmater.7b03627 Chem. Mater. 2017, 29, 8539−8547

Article

Chemistry of Materials

proposed as the real active material for OER as discussed below. The Tafel plots derived from polarization curves were constructed to investigate the OER kinetics. As shown in Figure 3b, the Tafel slope for the multishelled Ni2P electrode is 40.4 mV decade−1, which is lower than those of multishelled NiO (57.9 mV decade−1), nanostructured Ni2P (57.2 mV decade−1), hierarchical Ni2P (71.7 mV decade−1), and RuO2 (64.3 mV decade−1). This value is also lower than those of most reported OER catalysts (Table S3), indicating favorable OER kinetics for the multishelled Ni2P electrode. The EIS results (Figure 3c) also indicate that the multishelled Ni2P gives rise to chargetransfer resistance that is much lower than that of multishelled NiO, nanostructured Ni2P, hierarchical Ni2P, and RuO2. Moreover, polarization curves (Figure 3d) show insignificant changes between the first and 1000th cycle, and negligible decay of activity is observed in a continuous chronoamperometric test of 20000 s (Figure 3d inset), indicating superior catalytic stability of multishelled Ni2P toward the OER. To gain further insight into OER on Ni2P, the electrode was structurally and compositionally analyzed after polarization at 270 mV for 20000 s. The spherical morphology of Ni2P is essentially maintained after extended polarization, as evidenced by SEM imaging (Figure S9a). The retention of the multishelled and porous structure can be also viewed by TEM (Figure 4a). Notably, EDX elemental mapping analysis

nanosized subunits could provide a much larger accessible surface area for electrochemical reactions. Meanwhile, the porous shells, which provide a rapid transport channel for the electrolyte and generated hydrogen, also contribute to the high catalytic activity. To test this idea, the electrocatalytic HER current densities of Ni2P samples were normalized to ECSA. The ECSA-normalized initial curves (Figure 2f) indicate that multishelled Ni2P shows an onset overpotential lower than and a current density larger than those of hierarchical Ni2P and nanostructured Ni2P. The improvement of the normalized activity highlights the significant contribution of the porous multishelled architecture for optimizing HER performance. The OER, involving four sequential proton-coupled electrontransfer steps and oxygen−oxygen bond formation, is a thermodynamically and kinetically demanding process in water electrolysis.16,55 The high HER activity of multishelled Ni2P motivated us to assess its OER activity in O2-saturated 1.0 M KOH. The benchmark RuO2, hierarchical Ni2P, nanostructured Ni2P, multishelled NiO, and bare carbon were also tested for comparison. As shown in Figure 3a, multishelled

Figure 3. (a) Linear sweep voltammetry (LSV) polarization curves of bare carbon, multishelled NiO, nanostructured Ni2P, hierarchical Ni2P, multishelled Ni2P, and the benchmark RuO2 in 1 M KOH at a scan rate of 5 mV s−1. (b) Corresponding Tafel plots with linear fits. (c) Nyquist plots recorded at an overpotential of 150 mV. (d) Polarization curves of multishelled Ni2P before and after 1000 cycles at a scan rate of 100 mV s−1 between 1.0 and 1.6 V. The inset shows the current density vs time (I−t) curve of multishelled Ni2P obtained at an overpotential of 270 mV over 20000 s.

Figure 4. (a) Transmission electron microscopy (TEM) image of multishelled Ni2P after a 20000 s OER stability test. The inset is the related schematic diagram. (b) Corresponding elemental mapping and (c) high-resolution TEM image of multishelled Ni2P. (d) Ni 2p and (e) P 2p XPS spectra of the multishelled Ni2P before and after OER electrocatalysis in 1.0 M KOH. (f) In situ Raman spectra of the Ni2P/ CP electrode during a linear sweep voltammetry scan.

Ni2P displays the smallest overpotential requirement of 270 mV to reach a current density of 10 mA cm−2, while hierarchical Ni2P, nanostructured Ni2P, and multishelled NiO need larger overpotentials of 359, 320, and 478 mV, respectively. Thus, the catalytic activity is significantly improved by chemical transformation from NiO to Ni2P in spite of their similar hollow multishelled microarchitecture. Furthermore, the multishelled Ni2P manifests an overpotential 50 mV lower than that of benchmark RuO2, demonstrating its superior OER catalytic activity. The oxidation peak (1.35−1.45 V) associated with the transition from Ni2+ to Ni3+ shows the differences among the LSV curves of multishelled Ni2P, hierarchical Ni2P, and multishelled NiO, which correlate with OER electrocatalysis. The higher intensity of the oxidation peak of multishelled Ni2P indicates more active species (e.g., NiOOH),20 which is

reveals a homogeneous distribution of Ni, P, and O (Figure 4b). The presence of O suggests oxidation of Ni2P during the OER electrocatalysis process. In HRTEM (Figure 4c), the interplanar spacings of the fringes of 0.208 and 0.214 nm in the marginal area are well indexed to the (210) and (111) planes of NiOOH, respectively (JCPDS Card No. 27-956). Meanwhile, the fringes assigned to the (200) plane of Ni2P are detected in the core region of the microspheres. Thus, a thin layer of NiOOH is formed during the OER process. The surface chemical changes of the synthesized Ni2P before and after OER measurement are studied by X-ray photo8543

DOI: 10.1021/acs.chemmater.7b03627 Chem. Mater. 2017, 29, 8539−8547

Article

Chemistry of Materials electron spectroscopy (XPS), a surface-sensitive analytical technique. For fresh Ni2P, two peaks (Figure 4d) at 853.1 eV (2p3/2) and 870.0 eV (2p1/2) in the Ni 2p region correspond well to the Niδ+ species in Ni2P, while the peak at 855.8 eV is ascribed to Ni−O species from surface oxidation.56 With respect to the P 2p core level spectrum (Figure 4e), two peaks are observed at 128.9−129.6 and 132.4−133.4 eV. The former is ascribed to reduced P (Pδ−), while the latter reflects metal phosphate species, which is due to the surface passivation and/ or residue phosphate precursor.52 From the XPS analysis, Ni 2p3/2 (853.1 eV) and P 2p3/2 (129.3 eV) in Ni2P are positively and negatively shifted, respectively, relative to the binding energies of metallic Ni (852.5−852.9 eV) and element P (130.2 eV), implying a transfer of electron density from Ni to P.57 After the OER test, the sample presents two main peaks at 856.1 eV (2p3/2) and 873.8 eV (2p1/2) with a spin energy separation of 17.7 eV and two shakeup satellite peaks at 880.0 and 861.5 eV in the Ni 2p region, suggesting the presence of Ni3+ on the surface.58 The higher-resolution spectrum in the O 1s region (Figure S9b) exhibits three peaks at 529.0, 521.5, and 534.3 eV. The peaks of 529.0 and 521.5 eV are consistent with the typical Ni−O bonds in NiOOH and protonated oxygen from OH− groups, while the peak of 534.3 eV correlates to the adsorbed H2O or O2.59 As shown in Figure 4e, the P signal is not detected in the high-resolution P 2p XPS spectra on the post-OER multishelled Ni2P electrode. These results suggest the electrochemical oxidation from Ni2P to NiOOH on the surface of multishelled Ni2P during the OER. To identify the composition of the actual catalytic species under OER conditions, a potential-dependent in situ Raman investigation was performed on a Ni2P/CP electrode. At potentials of