Facile Dispersion of Nanosized NiFeP for Highly Effective Catalysis of

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Facile Dispersion of Nanosized NiFeP for Highly Effective Catalysis of Oxygen Evolution Reaction Zichen Liu, Gong Zhang, Kai Zhang, Huijuan Liu, and Jiuhui Qu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00471 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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ACS Sustainable Chemistry & Engineering

Facile Dispersion of Nanosized NiFeP for Highly Effective Catalysis of Oxygen Evolution Reaction ∥

∥

Zichen Liu,†, Gong Zhang,‡ Kai Zhang,†, Huijuan Liu†,‡* and Jiuhui Qu‡,§ †

State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Haidian District, Beijing 100085, China ‡

School of Environment, Tsinghua University, Beijing 100084, China

§

Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-

Environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, Haidian District, Beijing 100085, China ∥

University of Chinese Academy of Sciences, Beijing 100049, China

* Corresponding author e-mail: [email protected] (H. Liu)

ABSTRACT: Transition metal phosphides (TMPs) are commonly attributed to efficient oxygenevolving catalysts but are limited by insufficient exposure of active sites. Homogeneous dispersion and reducing size to nanoscale by modulating the combinations of metal and nonmetal P would benefit to explore their intrinsic activities. Based on size-confined synthesis strategy, nanostructured bimetallic phosphide NiFeP with dimensions of several nanometres has

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been synthesized via controlled pyrolysis of green organophosphorus source. The highly dispersed Ni0.65Fe0.35P electrocatalyst exhibited relatively low oxygen-evolution-reaction overpotential of 270 mV while supporting the current density of 10 mA/cm2 (without iR-drop corrections). Additionally, current density of 200 mA/cm2 at overpotential of 500 mV was achieved in alkaline solutions, which was 4-fold and 5-fold OER activity higher (η=500 mV) than their single-metal counterparts Ni2P and FeP, respectively, and even better than conventional method compound and noble metal benchmark.

KEYWORDS: electrocatalysis, transition metal phosphide, oxygen evolution reaction, nanosize, high dispersion, water splitting Introduction： The eco-friendly and energy producing electrocatalytic water splitting system provides an attractive approach to relieve environmental pollution and the energy crisis.

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However, the

sluggish kinetics of the four electron transfer reaction at the anode dramatically restricts the improvement of energy conversion efficiency.

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Noble metal catalysts, including IrO2 and

RuO2, are considered the state-of-art materials to catalyze the oxygen evolution reaction (OER), whereas the high cost impedes their extensive use in water splitting systems.

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Earth-abundant

transition metal phosphides (TMPs), especially Ni-Fe-P, exhibit an electronic structure similar to the noble metal-based benchmarks, and thereby can be seen as potentially ideal substitutions in terms of high OER activity in alkaline conditions.

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Various methods have been applied for

NiFeP-based electrode design. Electrodeposition has its benefits for the controllable synthesis of different nanosized structures, 11, 12 while the multi-metallic phosphide with various atomic ratios can be easily tuned via the pyrolysis process. However, conventional phosphorus sources,


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including trioctyphosphine (TOP)

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and NaH2PO214,

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, are limited by their toxicity and

uncontrollable phosphorization rate (namely inadequacy exposure of active sites) during the synthesis process. This motivates researchers to explore an alternative environmentally friendly process for the preparation of NiFeP nanostructures with high exposure of OER active sites. Phytic acid (PA) is mainly extracted from plant seeds, molasses, etc., which could be considered as a green organophosphorus source during the process of TMP fabrication.16 Benefiting from its molecular structure, namely six phosphate groups and twelve hydroxyl groups, PA exhibits a negative charge over a wide range of pH and tends to irreversibly bind with metal atoms.

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This excellent chelation property aroused our attention to consider PA as a

suitable P source. The strong interaction between the lone pair electrons on PA and unoccupied d-orbitals of the transition metals Ni and Fe ensures the irreversible capture and homogeneous dispersion of metal atoms, providing a cage-confining effect to avoid aggregation in the pyrolysis process.

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Unlike the release of toxic PH3 during the temperature-programmed

process under H2 atmosphere by conventional phosphorus source NaH2PO2, the strongly reducing H atoms from the dissociation of H2 can readily break the P-O bond of PA to form NiFeP nanostructures at the atomic level,

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with the carbon skeleton of PA simultaneously

transformed into amorphous carbon to enhance electron transfer process. Previous reports have proven that the incorporation of Fe into Ni-based (oxy)hydroxides would greatly enhance their OER activity.

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In view of the PA chelation-confinement

synthesis strategy and the synergistic effect of Ni-Fe, an alternative facile method for the fabrication of high dispersion and nanosized NiFeP catalyst was herein designed for highefficiency electrocatalytic OER (Scheme 1). In brief, a metal-containing solution was dropwise added into a PA-ethanol solution with vigorous stirring. The mixture was then maintained at 60


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℃ to form a sol-gel precursor. After pyrolysis at high temperature and thorough grinding, the black powder-like OER catalyst was obtained (see the Supporting Information for details). The as-prepared NiFeP catalyst showed a relatively low OER overpotential (η) of 270 mV when supporting a current density of 10 mA cm-2 in 1.0 M KOH solution, which was significantly lower than the values of 315, 360 and 390 mV for NiFeP-C (synthesized with a conventional inorganic phosphorus compound, NaH2PO2, as the P source), Ni2P and FeP, respectively. Furthermore, only the η of 500 mV was required to drive the current density of 200 mA cm-2. Meanwhile, combining the results of MAS NMR spectra analysis and synchrotron radiationbased X-ray absorption fine structure (XAFS) spectroscopy, we confirmed that the substitution of high oxidation state iron into the Ni2P matrix and the subsequent change of coordinate surroundings would not only increase the intrinsic conductivity, but promote the removal of P atoms for the conversion of NiFeP/NiFeOOH core-shell structure, thereby enhancing the electrocatalytic OER ultimately.

Scheme 1. Diagram of the synthesis of TMPs through a sol-gel method.


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Results and discussion: As-prepared samples were characterized by X-ray diffraction (XRD) to gain structural information. As shown in Figure 1a, the diffraction peaks of the monometallic phosphide Ni2P can be indexed to the hexagonal crystal system (ICCD card no.03-065-1989), while FeP shows typical peaks for an orthorhombic structure (a=5.1930 Å, b=3.0990 Å, and c=5.7920 Å). After the introduction of Fe3+ ions, the main diffraction peaks at 40.6°, 44.5°, and 47.2°, indexed to the (111), (201), and (210) reflections respectively, were shifted towards higher diffraction angles in contrast to those in pristine Ni2P, which was primarily due to the successful incorporation of Fe into the Ni2P lattice. The broad hump in the 2θ range of 20-30°can be attributed to the scattering of amorphous carbon. The existence of carbon materials have been further investigated by Raman spectra (Figure S1). Furthermore, 31P Solid state NMR was performed to determine the P coordination environment (Figure 1b). Compared to Ni2P, the isotropic chemical shift at 4076 ppm (referenced to ADP (adenosine diphosphate)) disappeared in NiFeP. Since the introduction of Fe would change the intrinsic electronic structure of the P atoms, the missing peak further demonstrated the substitution by Fe. Additional peaks were detected around and below 0 ppm, indicating the existence of typical phosphate species (Figure S2). 23, 24


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Figure 1. (a) XRD patterns of NiFeP, Ni2P, and FeP. (b)

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P MAS NMR spectra of Ni2P and

NiFeP referenced to ADP. (c) Typical SEM image of as-prepared NiFeP. (d) STEM image of NiFeP, where bright spots correspond to higher atomic number elements (Ni, Fe) and (e) EDS mapping with corresponding elements of Ni, Fe, and O. To obtain information on the morphology, detailed microscopic characterization was subsequently carried out. According to the scanning electron microscope (SEM) image in Figure 1c, nanostructured NiFeP particles were homogenously embedded in a porous carbon matrix. The BET surface area was calculated to be around ~800 m2/g using N2 adsorption-desorption isotherms; such a high surface area should be beneficial for the catalytic process (Figure S3). The STEM image clearly reveals the high dispersal of active sites in the amorphous carbon (Figure 1d). The relatively bright spots correspond to the higher atomic number elements (Ni and Fe), while the less bright regions represent lighter elements, including C (Z=6). Benefiting from the excellent chelation-confinement property of PA, NiFeP particles with average dimension ~3 nm were evenly distributed on the surface of the carbon matrix (Figure S4). In contrast, the


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morphology of NiFeP-C shows evidence of uncontrollable sintering occurring during the phosphorization process (Figure S5 and S6), with the dimension of tens of nanometres for active sites. Energy dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 1e, Figure S7) further indicates that the Ni and Fe atoms are homogeneously distributed in the NiFeP structure. Notably, in good agreement with previous reports on the preparation of TMP using PA as P resource, a certain amount of O was also preserved during the NiFeP crystal formation process. 20

Information on the local structure around photoabsorbers for the Fe-incorporated binary phosphide and single-metal phosphides was obtained using X-ray absorption fine structure (XAFS) spectroscopy. The Fe k-edge in the extended X-ray adsorption fine structure spectrum (EXAFS) reveals that the coordination number of P and d (Fe-P) in NiFeP are dramatically smaller and shorter than in FeP, indicating that the coexistence of Ni and Fe can trigger a decrease of the cell volume (Figure 2a, Table S1). X-ray absorption near-edge structure (XANES) data were normalized with the edge height. As depicted in Figure 2b, the Ni-K XANES shows that the Ni in NiFeP had the highest average valence state after reaction, in good agreement with the formation of larger numbers of active sites evidenced by electrochemical characterization. A Fourier-transform (FT) of the Ni K-edge EXAFS data shows remarkably different surroundings around the photoabsorber Ni between Ni2P and NiFeP (Figure 2c). Both Ni2P and NiFeP exhibit peaks corresponding to Ni-O, Ni-P, and Ni-Ni from 1 Å to 3 Å. The asymmetric outline from 2 Å to 3 Å in NiFeP reveals the presence of Ni-Fe bonds. Local bonding situations were fitted in real space (Figure S8 and Table S1). Notably, the coordination number of Ni-O in NiFeP is higher than that in Ni2P. A suitable increase in the coordination of M (metal)-O would promote the intrinsic conductivity in TMPs, as confirmed by theoretical calculations in previous research.


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M(metal)-P bonds offer a weak ” ligand effect” which can provide moderate bonding to trap

catalytic intermediates,

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while the M-M bonds provide high conductivity to decrease the

overpotential. The higher coordination number of Ni photoabsorbers in NiFeP means more active sites and high conductivity. Since Fe is more conductive than Ni, the incorporation of Fe can further improve the intrinsic conductivity.

Figure 2. (a) Fourier transformed k2-weighted EXAFS oscillations of catalysts measured at Fe Kedge. (b) X-ray absorption near edge spectra (XANES) of Ni K-edge for Ni2P and NiFeP. The electrocatalysts were oxidized in 1 M KOH by cycling the potential between 0 V to 0.65 V vs. Ag/AgCl at a scan rate of 20 mV/s for 20 min to obtain post-reaction samples. (c) Fourier transformed k2-weighted EXAFS oscillations of catalysts measured at Ni K-edge. (d) Selected cyclic voltammograms between 1.2 V and 1.53 V vs. RHE. During electrochemical reaction, TMP serves as a conductive support and the formation of a metallic (oxy)hydroxide shell provides the true active sites.

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As depicted in Figure 2d, a

redox peak at ~1.39 V vs. RHE was observed for Ni2P, which was attributed to the


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transformation between Ni(ΙΙ)(OH)2 and Ni(ΙΙΙ)OOH.

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Consistent with previous works, the

Ni(OH)2 /NiOOH redox peak shifts to a more positive potential in this well-dispersed NiFeP catalyst. This was due to the fact that Fe ions with high valence adjacent to Ni inhibit the conversion of Ni(ΙΙ) to Ni(ΙΙΙ).

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As shown in Figure S9, the high valence state of Fe ion was

confirmed by the X-ray photoelectron spectroscopy (XPS) measurement. In contrast, the NiFePC aggregates from the conventional preparation process shows a sluggish Ni2+/Ni3+ redox potential, indicating the difficulty in removing the P atoms in the NiFeP/NiFeOOH conversion process. The difference between nanosized NiFeP and bulk NiFeP-C could be attributed to the size effect, since small nanoparticles offer a higher percentage of surface atoms, which favours sufficient exposure of active sites, and have better contact with the conductive carbon matrix to accelerate the charge transfer process. The abundant periphery of the well-dispersed NiFeP particles provide the possibility to form Fe4+ as Stahl depicted, and advanced the Fe3+/Fe4+ redox potential compared to Fe-only catalysts. 29 In principle, β-NiOOH7 and FeOOH30 are considered as effective species for OER and lead to higher kinetic activity. The nanoparticles and the synergistic effect of Ni and Fe promote the efficient conversion of the active sites, consistent with the larger redox peak area of NiFeP, and lead to higher current density ultimately. The electrocatalytic performance of the investigated materials, coated on a glassy carbon (GC) electrode rotated at 1600 rpm to remove oxygen bubbles, was evaluated in a typical three electrode system. By virtue of high exposure of active sites and Ni-Fe synergy, NiFeP shows a low overpotential of 270 mV (without iR correction) when driving a current density of 10 mA cm-2 for OER in basic conditions (Figure 3a), which was significantly lower than the overpotential values of 315 mV, 360 mV, and 390 mV for NiFeP-C, pure Ni2P, and pure FeP respectively. Moreover, to achieve the large current density of 200 mA cm-2, only an


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overpotential of 500 mV was required, which is better than the other NiFe-based electrocatalysts previously studied.

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The incorporation of Fe ions and nanosized particles benefit the removal

of surface P atoms and thereby can form the actual OER sites with low overpotential and accelerate the oxidation kinetics, as depicted by the Tafel plots (Figure 3b). NiFeP shows much smaller Tafel slope (60 mV dec-1) than the values of ~99 mV dec-1 and ~123 mV dec-1 measured for Ni2P and FeP, respectively. In view of faradaic process of interconversion of Ni(II)/Ni(III) between 0.2 and 0.4 V, electrochemical impedance spectroscopy (EIS) for the OER performance was then performed at the overpotential of 0.5 V to investigate electrode kinetics (Figure 3c).32, 33 Since the size of the semicircle in the Nyquist plot is related to the charge transfer resistance (Rct), it is clear that the bimetallic phosphide exhibits lower charge transfer impedance, benefiting from Fe-doping to form a NiFe alloy with enhanced intrinsic conductivity.

Figure 3. Electrocatalytic performance of Ni2P, NiFeP, FeP, and NiFeP-C measured in 1 M KOH at room temperature. (a) Polarization curves. (b) Tafel plots. (c) Nyquist plots at


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overpotential of 0.5 V. (d) Estimation of Cdl via plotting current density variation at 1.097 V vs. RHE. The data were derived from electrochemical surface area (ECSA) measurements. Because reducing the particle size to the several nanometre range would enable high exposure of active sites (compared with NiFeP-C), the electrochemical surface area (ECSA) was then evaluated to measure the amount of catalytically active sites among the four catalysts. ECSA values were calculated from the double-layer capacitance (Cdl) in a non-Faradaic region using cyclic voltammograms (CV) with different scan rates (Figure S10). Figure 3d shows that the Cdl value of Ni2P, FeP, and NiFeP-C are 4.04 mF cm-2, 3.91 mF cm-2, and 0.51 mF cm-2 respectively, less than the value of 9.00 mF cm-2 exhibited by NiFeP. The higher ECSA value for NiFeP reveals that more active sites are exposed on the surface. In addition, chronopotentiometric measurement (V-t) of NiFeP was performed in 1 M KOH at 20 mA cm-2 to investigate the longterm stability (Figure S11). Apart from the periodic fluctuation of the potential caused by the growth and release of O2 bubbles, no obvious activity decline was observed after more than 25000 s test, demonstrating the good durability of NiFeP during the OER process. In summary, the chelation-confinement strategy has been successfully applied to synthesize TMP NiFeP as a highly active electrocatalyst for OER. Benefiting from the outstanding dispersion effect of PA and substitution by Fe ions, NiFeP exhibits the characteristics of a highperformance catalyst, such as: (1) high dispersion of particles, (2) high exposure of active sites, and (3) high electrical conductivity. The NiFeP powder shows a relatively low overpotential of 270 mV (η10) for OER, and current density of 200 mA cm-2 at overpotential of 500 mV in alkaline solutions, dramatically better than pure Ni2P, FeP, and NiFeP-C. This approach provides possibilities for designing well-dispersed and nanosized transition metal phosphides with high electrocatalytic efficiency by using the innocuous and readily available organic phosphorus


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source. The application field of PA-crosslinked TMPs can be also expanded to HER, ORR, HDS, and DMOR. Further, the powder-like catalyst can be flexibly integrated with other substrates, such as carbon cloth, for more energy conversion applications. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Materials preparation and electrochemical measurements; characterization of materials by 31

P MAS NMR, BET, TEM and corresponding EDS spectrum, powder XRD, XAFs, XPS

spectra; supplementary tables. (PDF) AUTHOR INFORMATION Corresponding Author *[email protected] ORCID Huijuan Liu: 0000-0003-0855-0202 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (No. 51708543, 51722811, and 51438011). REFERENCES


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X.; Song, L.; Yang, X.; Xiong, Y. Amorphous Metallic NiFeP: A Conductive Bulk Material Achieving High Activity for Oxygen Evolution Reaction in Both Alkaline and Acidic Media. Adv. Mater. 2017, 29, 1606570-n/a.


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Synopsis: We fabricated nanosized NiFeP with the perspective of modulating the exposure of active sites by green organophosphorus source, which can highly effective electrocatalysis water oxidation.


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Facile Dispersion of Nanosized NiFeP for Highly Effective Catalysis of

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