Efficient Oxygen Evolution Catalysis Triggered by Nickel Phosphide

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Efficient Oxygen Evolution Catalysis Triggered by Nickel Phosphide Nanoparticles Compositing with Reduced Graphene Oxide with Controlled Architecture Ping Li, Ran Chen, Shuanghong Tian, and Ya Xiong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01062 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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ACS Sustainable Chemistry & Engineering

Efficient Oxygen Evolution Catalysis Triggered by Nickel Phosphide Nanoparticles Compositing with Reduced Graphene Oxide with Controlled Architecture Ping Li,*a,b Ran Chen,a,b Shuanghong Tian a,b and Ya Xiong a,b a School

of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou

510275, Guangdong, P. R. China. b Guangdong

Provincial Key Laboratory of Environmental Pollution Control and Remediation

Technology, Guangzhou 510275, PR China. *E-mail: [email protected]

KEYWORDS: transition-metal phosphide, architectural control, nanostructure, anodic polarization, oxygen evolution reaction

ABSTRACT: Searching for active, durable, and economical electrocatalysts for the oxygen evolution reaction (OER) is crucial for developing future sustainable energy technologies. Herein,

ACS Paragon Plus Environment

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we present our recent endeavour in the engineer of a high-efficiency OER electrocatalyst based on nanoscale nickel phosphide particles compositing with reduced graphene oxide (rGO/Ni2P), derived from a supported metal-organic complex, through self-assembly followed with lowtemperature phosphidation. The desirable architectural and physicochemical properties, featuring sandwich-like nanosheet morphology, nanoscale Ni2P subunit building blocks and conductive substrate incorporation, endow the as-synthesized rGO/Ni2P with abundant active sites, facile mass/charge transfer, and good stability against agglomeration. As a result, rGO/Ni2P nanocomposite manifests advanced OER performance with low overpotential, fast reaction kinetics, and strong durability. Additionally, detailed structural and compositional analysis further reveals that an overlayer of NiOOH is in-situ generated on the surface of Ni2P nanoparticles upon anodic polarization, and the derived rGO/Ni2P/NiOOH performs as actual catalyst to boost OER process. This work elucidates that fine control over structural architecture and chemistry of the material promises high-performance electrocatalysis, and would inspire to develop efficient noblemetal-free OER electrocatalysts derived from transition-metal phosphides.

Introduction The increasing energy needs and environmental issues have stimulated interest in exploring renewable and clean energy sources.1 Molecular hydrogen is proposed to be a sustainable energy carrier, serving as a prospective alternative to the fossil fuels to meet future global energy demands.2 The production of molecular hydrogen by means of direct electrochemical water splitting is considered to be a particularly attractive and green route to support hydrogen economy.3,4 Unfortunately, the efficiency of hydrogen production largely depends on oxygen evolution reaction (OER) at the anode of the water electrolyzer,


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which involves a complicated multiproton-coupled electron transfer process, leading to sluggish kinetics.5,6 Precious metals, like RuO2 and IrO2, are considered the best-performed catalysts for the OER, while their high cost and low elemental abundance prohibit them from large-scale practical application.7,8 Thus, to enable hydrogen economy, developing earth-abundant and cost-effective OER electrocatalysts with high activity and stability becomes a significant theme and has attracted lots of research interest.9-12 To date, a number of alternatives, spanning from transition metal transition metal oxides,13-15 hydroxides,16-18 phosphides,19 phosphate,20 chalcogenides,21-23 nitrides,24 to metal-free materials,25,26 have been studied as promising noble-metal-free OER electrocatalysts. Among the above earth-abundant OER catalysts, transition-metal phosphides (TMP), a class of functional compounds traditionally used in magnetism, optics, energy storage and catalysis,27,28 are discovered as effective electrocatalytic materials towards OER, and even can outperform their corresponding oxides counterparts.29-31 It is believed that in situ generation of metal oxide/hydroxide/(oxy)hydroxide or their hybrid on the phosphides surface during the OER attributes to the observed high OER activity.29,32 Notwithstanding the considerable progress, most of the reported TMP at present still display limited OER performance resulting from their intrinsic properties such as poor electrical conductivity,33 and underdeveloped structural architecture with insufficient electrochemical active sites and unfavourable mass transportation. On the other hand, corrosion and aggregation of the TMP during water electrolysis deteriorate the stability of TMP-based catalysts. Therefore, the engineer of highly efficient and robust TMP-based OER catalysts remains a significant challenge.


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Herein, we establish a facile and novel route to a sandwich-like nanosheet architecture consisting of nickel phosphide nanoparticles (NPs) supported by reduced graphene oxide (rGO/Ni2P). The synthetic protocol involves a topological transformation reaction via lowtemperature phosphidation of the rGO-supported Ni-containing metal-organic complex precursor (rGO/Ni-EG) prepared by using a one-pot self-assembly route. When utilizing in catalyzing OER, core-shell structured Ni2P/NiOOH is found to generagte in situ on the rGO substrate during anodic polarization, giving rise to rGO/Ni2P/NiOOH. Owing to the well-controlled sheet-like morphology, high dispersion of the catalytic active NPs, incorporation of conductive graphene substrate, together with the occurrence of in-situ electrochemical conversion, the resulting nanocomposite exhibits remarkable electrocatalytic OER performance, ranking among the most active earth-abundant OER catalysts reported to date.

Experimental Section Materials and reagents. The following chemicals were used as received: ethylene glycol (EG, 98+%, Merck), Ni(CH3COO)2·4H2O (98%, Fluka), graphite (99.8%, Alfa Aesar), polyvinylpyrrolidone (PVP, K30, 99%, Sigma−Aldrich), ethanol (99.99%, Fisher), Sodium hypophosphite (NaH2PO2, >98%, Sigma−Aldrich). Deionized water was generated with the Elga Micromeg Purified Water system. Fabrication of the rGO/Ni2P nanocomposite. The graphene oxide (GO) was prepared via a modified Hummer's method.34 To prepare rGO/NiEG precursor, 1 mmol of Ni(CH3COO)2·4H2O and 0.32 g of PVP were firstly added in 20 mL of ethylene glycol (EG). Then ca. 2.5 mg of GO was added, and the above mixture was ultrasonicated


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for 0.5 h at room temperature. Then the above homogeneous suspension was stirred at 180 °C for 3 h, and the product was centrifuged, washed 3 times with ethanol, and dried at 80 °C for 12 h. rGO/Ni2P nanocomposite was fabricated via a low-temperature phosphidation route. In a typical synthesis, the rGO/Ni-EG precursor and NaH2PO2 were placed into a tube furnace with NaH2PO2 located at the upstream point of the tube furnace. n(metal)/n(phosphorus) is around 1/20. Then the furnace was heated to 300 °C with a heating rate of 2 °C/min and kept at 300 °C for 2 h under 20 mL/min of N2 flow, and then cooled to room temperature. Preparation of the control samples. rGO: rGO was obtained by using the similar method of rGO/Ni-EG precursor while without addition of Ni(CH3COO)2·4H2O. rGO/NiO nanocomposite: rGO/NiO nanocomposite was obtained by direct calcination of the rGO/Ni-EG precursor in the muffle furnace at 300 °C for 2 h (heating rate = 2 °C/min). Free-standing Ni2P: Firstly, the Ni-EG precursor was fabricated by using the above-mentioned procedure for rGO/Ni-EG while without the addition of GO during synthesis. Then Ni2P was fabricated by phosphidation treatment. The detailed phosphidation treatment was the same as that for the rGO/Ni2P nanocomposite while using Ni-EG as the precursor. Electrochemical measurements. The electrochemical measurements were performed by using an electrochemical workstation (Autolab, PGSTAT 302N) with a three-electrode cell with Ag/AgCl (filled with 3M KCl solution) electrode as the reference electrode, a Pt plate as the counter electrode, and a glassy carbon electrode (GCE, diameter = 3 mm) coated with the sample material as the working electrode. To prepare the working electrode, 3 mg of catalyst was firstly dispersed in a mixed solution composed of ethanol (122.2 µL), water (488.8 µL), and 5 wt% Nafion solution (25.46 µL) to prepare the


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catalyst ink. After ultrasonication for 0.5 h, the homogeneous catalyst ink (1.5 µL) was drop-casted onto the GCE (the mass loading for the catalyst: ca. 0.1 mg cm−2) and air dried overnight. For the electrochemical measurement, 1.0 M KOH was used as the electrolyte and pre-purged with O2 for 0.5 h. The working electrode was firstly scanned by cyclic voltammetry (CV, scan rate = 50 mV s-1) until the CV curves were steady, then the Linear sweep voltammetry (LSV) was recorded with a sweep rate of 5 mV s-1). All polarization curves were iR-corrected. The electrochemical double-layer capacitance (Cdl) were measured via cyclic voltammograms (CV) in a narrow potential window (1.2-1.3 V vs RHE). The plot of the current density differences (ja  jc) at 1.25 V vs RHE against the scan rate exhibits a linear relationship and the corresponding slope is equivalent to twice of the Cdl. The scan rates were 2, 5, 10, and 20 mV s-1. The stability test were carried out via a chronopotentiometric (CP) measurement with a constant current density of 10 mA cm−2 in 1.0 M KOH. In this study, the electrochemical tests were performed on at least 3 working electrodes and the average value was collected. To remove the produced gas bubbles, the working electrode was rotating at 1600 rpm. The current density was normalized to the geometrical surface area of the GCE, and the potentials were reported to a reversible hydrogen electrode (RHE) scale: E (RHE) = E (Ag/AgCl) + 0.210 + 0.0591 × pH. Materials characterization. Scanning electron microscopy (SEM) was carried out on JEOL-6700F with an energy-dispersive X-ray (EDX) analyzer (Oxford INCA). Transmission electron microscopy (TEM) was performed on JEOL JEM-2010 (200 kV). High resolution transmission electron microscopy (HRTEM) was conducted by JEOL JEM-2100F (200 kV). And the elemental mapping analysis was collected by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, model 7426). Nitrogen sorption


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measurement was taken on Quantachrome NOVA-3000 system at 77 K. The specific surface area of samples was calculated with the Brunauer–Emmet–Teller (BET) method and the pore size distribution curve was produced by using NLDFT method. The XRD patterns were performed using Bruker D8 Advance system (Cu Kα radiation). The chemical bonding information of the materials were obtained on Fourier transform infrared spectroscopy (FTIR, Bio-Rad FTS-135). The X-ray photoelectron spectroscopy (XPS) analysis was conducted on AXIS-HSi, Kratos Analytical equipped with a monochromated Al Kα X-ray source (1486.71 eV), the binding energies were corrected to the C 1s peak of the C−C bonds (284.5 eV).

Results and Discussion Preparation and characterization of the catalysts

Scheme 1. The fabrication procedure for the rGO/Ni2P nanocomposite with sandwich-like nanosheet architecture. The rGO/Ni-EG precursor was fabricated by dispersing GO sheets in ethylene glycol (EG) containing nickel acetate at elevated temperature (Scheme 1). With abundant functional groups, the GO sheets strongly adsorb Ni ions via electrostatic attraction. At increasing temperature,


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coordination reaction between GO-mounted Ni ions and EG could take place, leading to metalorganic complex (Ni-EG) selectively functionalizing on both sides of GO sheets. Meanwhile, GO is partially reduced to rGO owing to the reductibility of EG under high temperature, thus producing rGO/Ni-EG with sandwich-like architecture. From SEM and TEM images (Figure 1a, b and S2), the rGO/Ni-EG is nanosheets with rough surface, in stark contrast with the pristine GO nanosheets with smooth surface (Figure S1). The FTIR spectrum (Figure S3a) reveals the existence of the coordination interaction between EG and Ni ions, that is, the successful formation of Ni-EG complex. The strong peak at around 10° in the X-ray diffraction (XRD) pattern (Figure 2a) is typical for the stacked metal-oxygen sheets spaced by the bonded anionic alcoholate species, which is also reported in other metal alkoxides.35,36 Besides, the EDX analysis (Figure S3b) indicates the precursor is composed of Ni, C and O elements, which is well consistent with the chemistry of rGO/Ni-EG.


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Figure 1. (a) SEM and (b) TEM images of the sandwich-like rGO/Ni-EG precursor. (c) SEM image, (d, e) TEM images, (f) HRTEM image and (g) EDX elemental mapping of the sandwichlike rGO/Ni2P nanocomposite. The inset in Figure 1e is the corresponding size distribution histogram of Ni2P nanoparticles in the rGO/Ni2P nanocomposite. After low-temperature phosphidation treatment of rGO/Ni-EG precursor, the XRD pattern of the phosphided product (Figure 2a) displays new diffraction peaks ascribed to Ni2P (PDF card no. 03−0953), while the original peaks for the precursor are no longer be identified, evidencing the full conversion of rGO/Ni-EG precursor. And the particle size of Ni2P is calculated to be about 6.9 nm from (111) peak by using Debye-Scherrer equation. The SEM and TEM images (Figure 1c-e and S4) of the product show that the original nanosheet morphology is well-maintained, and numerous small nanoparticles are uniformly decorated on both sides of rGO sheets. The


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nanoparticles range from 5 to 8 nm in particle size, with a mean diameter of about 6.3 nm (inset of Figure 1e). The HRTEM image (Figure 1f) exhibits lattice fringes with a spacing of 0.22 nm assigned to the (111) planes of Ni2P, further certifying the formation of Ni2P nanocrystallites. Moreover, the EDX elemental mapping from the rGO/Ni2P nanosheet (Figures 1g and S5) demonstrates the homogeneous elemental distribution of Ni, P, and C throughout the sheet, verifying the uniform dispersion of Ni2P nanocrystallites on the rGO sheets. And the mass fraction of carbon in the composite is about 20 wt%. Additionally, nitrogen sorption measurement was performed to study the physical feature of the rGO/Ni2P nanocomposite. As displayed in Figure S6a, the nanocomposite manifests the type IV isotherm with an obvious hysteresis loop (H3 type), suggesting mesoporous structure. And the rGO/Ni2P has a BET specific surface area of 112 m2/g, a total pore volume of 0.29 cm3/g, and a broad pore size distribution from micropores to mesopores (Figure S6b). Obviously, such large surface area and hierarchical pores are particularly favorable for the full exposure of electrocatalytic active centers and mass transfer of reactants and products during catalytic applications.


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Figure 2. (a) The XRD patterns of the samples obtained at each step. The XPS spectra of rGO/Ni2P nanocomposite: (b) survey spectrum, (c) Ni 2p, and (d) P 2p. X-ray photoelectron spectroscopy (XPS) is employed to investigate the surface composition and chemical state of rGO/Ni2P. The survey spectrum in Figure 2b signifies the existence of the elements of Ni, P, C and O. From the Ni 2p spectrum in Figure 2c, the peak at 853.0 eV is corresponded to Ni in Ni2P, and the peak centered at 856.5 eV is corresponded to the oxidized Ni species due to oxidation during air exposure.37 In the P 2p spectrum in Figure 2d, the peaks centered at 129.0 and 130.0 eV are for the P−Ni species, and the peak located at 133.7 eV is for P-O species.38 The XPS result confirms the successful generation of nickel phosphide species in the final product. For comparison purpose, a series of control samples including pure rGO, freestanding Ni2P, and the calcination-derived product (e.g., rGO/NiO) with similar sandwich-


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like nanosheet morphology, were also constructed. Their detailed fabrication procedures are descripted in Experimental Section and the characterizations (including SEM, TEM, FTIR, and XRD) are displayed in Supporting Information (Figure S7-S11). Specifically, rGO was produced by the reduction of GO in EG at elevated temperature. Free-standing Ni2P was obtained by the phosphidation of the free-standing Ni-EG prepared by the similar route used in rGO/Ni-EG production while without adding GO. rGO/NiO was obtained by directly calcining rGO/Ni-EG precursor at 300 °C under air atmosphere. Here it is noteworthy that without the adding of the GO, the as-obtained Ni2P is only seriouslyaggregated particles composed of numerous secondary nanoparticles as building units (Figure S9). And the crystallite size is calculated to be 20.1 nm from XRD pattern (Figure S10) with Debye-Scherrer equation. The contrasting nanostructures indicate the critical role of rGO. During synthesis, rGO acts as template to guide the formation of sandwichlike nanosheet architecture, and meanwhile, as support to highly disperse Ni2P NPs with well-controlled particle size. Apparently, these favourable features are conducive to maximizing exposure of the active sites, making rGO/Ni2P promise high-efficient catalysis. Electrocatalytic performance of catalysts


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Figure 3. Electrochemical performance of rGO/Ni2P, rGO/Ni-EG, rGO/NiO, Ni2P, and rGO. (a) Polarization curves and (b) Tafel plots of the samples. (c) The required overpotential of the samples to reach the current density of 10 mA cm−2 obtained from the OER polarization curves. (d) Summary of the Tafel slope of the samples. (e) Electrochemical double-layer capacitance (Cdl) measurement of the rGO/Ni2P nanocomposite and Ni2P. The current density differences (ja  jc) at 1.25 V vs RHE plotted against the scan rate. The linear slope is equivalent to twice of the Cdl. In the measurement, scan rate = 5 mVs−1, catalyst loading = ca. 0.10 mgcm−2, and 1.0 M KOH as the electrolyte. The electrocatalytic performance of rGO/Ni2P for the OER was studied in a three-electrode setup with 1.0 M KOH as electrolyte. For a systematic comparison study, rGO, pure Ni2P particles, rGO/Ni-EG precursor, and sandwich-like rGO/NiO were also studied as controls. Figure 3a displays the polarization curves of all the samples. The pristine GCE shows no


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visible contribution to the OER. rGO exhibits negligible OER response, indicating without electroactive components, pure rGO is ineffective for the OER. Then we compare the overpotential required for different samples to reach a current density of 10 mA cm−2. This value is a frequently used index to appraise the catalytic activity for the OER of materials.6,39 As clearly illustrated (Figure 3a and c), rGO/Ni-EG precursor only shows modest activity with an overpotential of 321 mV at a current density of 10 mA cm−2. In intense contrast, after simple phosphidation, the resulting rGO/Ni2P gives remarkably improved activity with significantly larger current density at any certain overpotential (Figure 3a). Specifically, it demands an overpotential of as low as 283 mV to yield 10 mA cm−2, while for the air calcination-derived product, rGO/NiO, needs a larger overpotential of 318 mV, revealing that metal phosphide is more efficient than the corresponding metal oxide in the water oxidation, which agrees with the previously reported observations.30,31,40 Notably, the outstanding OER activity of rGO/Ni2P is also much better than that of the commercial RuO2, which needs 342 mV to yield a current density of 10 mA cm−2 (see the corresponding LSV curve in Figure S12a). On the other hand, as anticipated, the Ni2P particles exhibit inferior activity (320 mV at a current density of 10 mA cm−2) in comparison with that of rGO/Ni2P, revealing crucial impact of the architecture and significant contribution of rGO nanosheets. To further elucidate this point, the electrochemically active surface area (ECSA) of rGO/Ni2P and bare Ni2P were compared by electrochemical double-layer capacitance (Cdl), since Cdl is linearly proportional to the ECSA.6 The Cdl of the samples are estimated with cyclic voltammetry (CV). Figure S13a and b display CV curves of rGO/Ni2P and bare Ni2P under different scan rates in a non-Faradaic window. From the plot of differences in current density (ja - jc)


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against the scan rate (Figure 3e), rGO/Ni2P has a > 3-fold higher Cdl (2.5 mF cm−2) than that of the bare Ni2P (0.6 mF cm−2), affirming remarkably enhanced electrochemically active surface area and thus more catalytic sites available from sandwich-like construction with rGO incorporation. Moreover, the Tafel slope is also studied to study the OER kinetics of the samples. As shown in Figure 3b and d, among all the samples tested, rGO/Ni2P displays the smallest Tafel slope (43.6 mV dec−1), much smaller than those of rGO/Ni-EG (75.8 mV dec−1), rGO/NiO (51.5 mV dec−1) and Ni2P (62.6 mV dec−1), demonstrating favourable OER kinetics of rGO/Ni2P. Significantly, the rGO/Ni2P presented here, with quite low overpotential and small Tafel slope, is among the best OER electrocatalysts reported so far that include the transition metal-based and noble metal-based ones.5,6,29,39,4146

A comparison of different catalysts for the electrocatalytic OER is detailedly listed in

Table S1.


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Figure 4. (a) Stability behavior of the sandwich-like rGO/Ni2P nanocomposite under a constant current density of 10 mA cm−2. The mass loading of the catalyst was about 0.1 mg cm−2. (b,c) TEM images, (d) HRTEM image, and (e) EDX elemental mapping of the spent rGO/Ni2P after 30 h of water electrolysis. Stability is another pivotal criterion to evaluate an OER catalyst. The stability of rGO/Ni2P was studied by electrolysis at a current density of 10 mA cm−2. As displayed in Figure 4a, the activity of the catalyst is well-retained for over 30 h of continuous electrolysis, suggesting its superior stability in the prolonged electrochemical process. Furthermore, from the TEM images (Figure 4b-c and Figure S14a-b), the original nanosheet morphology is largely retained, while these nanosheets become thicker and are


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covered by overlayers. It is assumed that these overlayers are composed of the in-situ generated oxidized Ni species during electrolysis, which will be detailed below. Mechanism investigation To reveal more insight into the electrocatalytic behavior of rGO/Ni2P for the water oxidation, XPS, HRTEM and elemental mapping techniques were further performed on the spent rGO/Ni2P. In the Ni 2p spectrum in Figure S15a, the peak assigned to Ni in Ni2P is almost undetectable, while the peak corresponding to oxidized Ni species is quite intense. In detail, the two peaks centered at 855.4 eV (Ni 2p3/2) and 873.0 eV (Ni 2p1/2) with an energy separation of 17.6 eV, accompanied with two shake-up satellite peaks centered at 861.4 and 879.6 eV, are characteristic of Ni3+ species in NiOOH .14,47,48 For P 2p spectrum (Figure S15b), the total P signal weakens seriously. Only the broad peak at 133.7 eV attributed to P-O species is observed, while the peaks for P−Ni species disappear, further confirming the surface oxidation of rGO/Ni2P. In addition, from the HERTEM image (Figure 4d), the lattice fringes with spacing of 0.208 nm in the marginal region of the nanosheet can be indexed to the (210) planes of NiOOH. And the lattice fringes from the (111) planes of Ni2P still can be observed in the core, verifying the formation of Ni2P/NiOOH nanocomposite with a core-shell structure. Moreover, EDX elemental mapping (Figure 4e) illustrates that the Ni, C and O elements are uniformly distributed in the whole sample, while the P element is dominant in the inner region, further revealing the rGO/Ni2P is covered by a layer of oxidized Ni species to form a core–shell structure. The above characterizations collectively suggest that prominent electrochemical oxidation of electrocatalyst takes place during the OER, leading to the conversion of the Ni2P to NiOOH on rGO/Ni2P surface, and the in-situ generated NiOOH species coated on


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rGO/Ni2P performs as electrocatalyst for the OER (Figure 5). That is to say, herein rGO/Ni2P serves as a type of precatalyst, and the in-situ generated core-shell structured Ni2P/NiOOH derivative compositing with rGO upon anodic polarization presents excellent catalytic OER activity. Similar phenomena were also found in other TMP-based OER systems.29,49,50

Figure 5. The structural evolution of the rGO/Ni2P nanocomposite during the OER process. According to the previous studies, the general mechanism for the OER on Ni-based electrocatalysts in the alkaline media involves 4 successive elementary steps and can be expressed as follows:51,52 Ni2+ + 3OH− ↔ NiOOH + H2O + e−

(1)

NiOOH + OH− ↔ NiO(OH)2 + e−

(2)

NiO(OH)2 + 2OH− ↔ NiOO2 + 2H2O + 2e−

(3)

NiOO2 + OH− → NiOOH + O2 + e−

(4)

Overall OER: 4OH− → O2 + 2H2O + 4e−

(5)

Among these steps, the first three steps are reversible and determine the overall OER rate, whereas the step 4 is fast and irreversible. The OER on rGO/Ni2P in this study probably share a similar reaction mechanism. In the anodic OER process, surface Ni species of rGO/Ni2P is firstly oxidized into NiOOH, giving rise to the formation of rGO supported Ni2P/NiOOH as the actual electrocatalytic active sites, then the generated active centers


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promote the oxidation of OH− into the molecular oxygen. Obviously, the presence of Ni with higher valence state would be conducive to the enhanced electrostatic adsorption of OH–, and meanwhile, favorably lowers the reaction energy barrier for the nickel species conversion from lower to higher oxidation states, thus boosting the OER behavior.48,53 Furthermore, it is expected that the construction of Ni2P/NiOOH heterostructure as well as rGO incorporation would promote electron transportation and OER kinetics.

Conclusions In summary, we have described the topological preparation of sandwich-like nanostructured rGO/Ni2P composite through simple low-temperature phosphidation of the corresponding rGO-supported Ni-based metal-organic complex precursor. As revealed by TEM, EDX elemental mapping and XPS analysis, an overlayer of NiOOH species can be formed on the Ni2P surface during the anodic polarization under alkaline media. And it is found that the rGO/Ni2P/NiOOH, derived from rGO/Ni2P nanocomposite precatalyst, performs as a highly efficient OER catalyst with quite low overpotential of 283 mV at a current density of 10 mA cm−2, small Tafel slope of 43.6 mV dec−1, and excellent stability. Such outstanding electrocatalytic performance is associated with its unique architectural characteristics, including the incorporation of rGO conductive substrate to accelerate the charge transfer, high dispersion of nanoscale catalyst particles with full exposure of electrocatalytic active sites, the facile mass transportation endowed by the sandwich-like nanosheet morphology, and the on-site generation of active NiOOH species on Ni2P surface. The present work enlightens an exciting avenue to explore the engineer and


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construction of highly efficient, durable, and economical electrocatalysts based on TMP for a broad range of energy and environment related applications.

ASSOCIATED CONTENT Supporting Information. Additional SEM images, TEM images, EDX patterns, XRD patterns, FTIR spectra, N2 adsorption–desorption isotherm, polarization curve, Tafel plot, cyclic voltammetry curves, and XPS spectra of the samples. The files are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The research is supported by the Nature Science Foundations of China (21677180, 21777196); Science and Technology Research Programs of Guangzhou City (201510010083).


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TOC:

SYNOPSIS: A nanocomposite with nickel phosphide nanoparticles anchoring on reduced graphene oxide, derived from supported metal-organic complex, is finely engineered for advanced electrochemical oxygen evolution catalysis.


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Efficient Oxygen Evolution Catalysis Triggered by Nickel Phosphide

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