Fe2P(O) Interface Can Boost Oxygen Evolution

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Letter

Ni2P(O)/Fe2P(O) Interface Can Boost Oxygen Evolution Electrocatalysis Peng Fei Liu, Xu Li, Shuang Yang, Meng Yang Zu, Porun Liu, Bo Zhang, Lirong Zheng, Huijun Zhao, and Hua Gui Yang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00638 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Ni2P(O)/Fe2P(O) Interface Can Boost Oxygen Evolution Electrocatalysis Peng Fei Liu,† Xu Li,† Shuang Yang,† Meng Yang Zu,† Porun Liu,‡ Bo Zhang,§ Li Rong Zheng,# Huijun Zhao,‡ and Hua Gui Yang*,† †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials

Science and Engineering, East China University of Science and Technology, Shanghai 200237, China. ‡

Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University,

Queensland 4222, Australia. §

State Key Laboratory of Molecular Engineering of Polymers, Department of

Macromolecular Science, Fudan University, Shanghai 200438, China. #

Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049,

China.

ABSTRACT: Oxygen evolution reaction (OER) plays a paramount role in renewable energy technologies. However, the slow kinetics of OER seriously limit their overall performance and commercialization. Here, we rationally design metallic Ni2P/Fe2P interface, which can be in-situ oxidized to Ni2P(O)/Fe2P(O) interface to enhance OER efficiency, with active doped oxyhydroxides and phosphates on the surface and conductive phosphide in the bulk. The resulting catalysts require a low overpotential

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of 179 mV to achieve a current density of 10 mA/cm2 (without iR compensation), and can continuously drive OER for 120 hours without any obvious degradation, which rivals most reported OER catalysts. These results suggest that we are able to design multi-component metallic precatalysts to construct most active surface layers and conductive bulks, further boosting OER performance for real-world electrolysis utilization.

Table of Contents graphic.

Electrochemical water splitting and CO2 reduction play a paramount role in the conversion of sustainable electricity to fuels and chemicals.1-5 Unfortunately, the bottleneck of these processes is the oxygen evolution reaction (OER), in which large overpotentials are required to drive the reaction.6-10 Therefore, great efforts have been devoted to explore efficient and earth-abundant electrocatalysts, such as fist-row (3d) transition metal based oxides,7,

11

(oxy)hydroxides,9,

12, 13

phosphate10 and borate

composites,14 perovskite oxides,6, 15 and metal-organic frameworks.16, 17 Afterwards, known strategy like nanostructuring has been proposed to expose more active sites with increment of surface areas.18-22 Meanwhile, another alternative focuses on nanocompositing with conductive supports to regulate the electron transfer ability, 2 ACS Paragon Plus Environment

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and simultaneously to achieve better dispersion and reduced aggregation of electrocatalysts.11, 23-25 However, these approaches to advance catalytic activity would eventually result in limitations from mass and/or charge transport, thus restricting further improvements of OER performances.22

Recently, alloying has been investigated as an effective means to improve intrinsic activity, with NiFe based oxyhydroxides being the state-of-the-art alloys.7, 9, 11, 26, 27

Furthermore, several operando measurements and density-functional theory

calculations have demonstrated surface doped metal species as OER active sites, rather than bulk oxyhydroxides.28-30 In response, we anticipate incorporating Fe sites into NiOOH surface (NiOOH:Fe) or Ni sites into FeOOH surface (FeOOH:Ni) would generate active metal coordination environments, thus significantly boosting the OER process for alloyed catalysts. Meanwhile, phosphorus has been demonstrated for accommodating electrons on surface and increasing local charge density, because of lone-pair electrons in 3p orbitals and vacant 3d orbitals. Thus, surface phosphates have also been reported to mediate water oxidation via proton-coupled electron transfer and accelerate OER.31,

32

Moreover, the metallic property has also been

considered to improve OER performances.33-35 Thus, Ni2P,36 Ni3Se2,37 Ni3N,38 NixFe1-xSe39 and Ni3FeN40 have been discovered as precatalysts for OER with metallic bulk and active oxyhydroxides surface. Nonetheless, these in-situ oxidized surface species are crudely formed and the conductive bulk might be destroyed for durability test, which is not active enough to motivate OER. Therefore, subtle design


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strategies for fabricating metallic electrocatalysts to induce most active surface species and keep conductive bulk are urgently needed.

Herein, we rationally construct Ni2P(O)/Fe2P(O) interfaces, with active surface oxidized reorganizations (doped oxyhydroxides and phosphates) and durable conductive phosphide bulk (Figures 1 and S1). Benefitting from this unique structure, the oxidized multiple Ni2P/Fe2P (M-Ni2P/Fe2P-O) catalyst exhibits a low onset overpotential (η) of 163 mV and requires η of 179 and 251 mV to drive the current densities (j) of 10 and 100 mA/cm2 (not iR compensated), respectively, with a small Tafel slope of 42.7 mV/dec. M-Ni2P/Fe2P-O can consistently motivate OER at j of 40 mA/cm2 for more than 120 hours’ (h) durability test. Motivated by these results, we believe that multi-component metallic compound interfaces, especially for the traditional 3d transition metal phosphides, would be artistic precatalysts to generate efficient and durable catalysts for real-world OER related energy technologies.

To fabricate multiple Ni2P/Fe2P precatalyst, a hierarchical hydroxide precursor was firstly prepared. After traditional low-temperature phosphidation treatment, the multiple Ni2P/Fe2P heterostructures were obtained, with small Ni2P nanoparticles on Fe2P nanoplates, vertically grown on surface phosphidized Ni foam substrate (M-Ni2P/Fe2P). Figure S2 show a typical field-emission scanning electron microscopy (FESEM) images of as-prepared hydroxide precursors. As precursor, the Ni(OH)x nanosheets decorated Fe(OH)x nanoplates were uniformly distributed on the Ni foam substrate, which was also surface-oxidized during the hydrolysis process (named as 4 ACS Paragon Plus Environment

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M-Ni(OH)x/Fe(OH)x).

The

overall

size

and

layered

morphology

of

M-Ni(OH)x/Fe(OH)x nearly retained well after the thermal treatment, besides that the surface became much rougher and Ni(OH)x nanosheets turned into irregular grainy shapes to form Ni2P particles (Figure 2a-c). For comparison, single Ni2P/Fe2P (S-Ni2P/Fe2P) precatalysts with relatively less Ni2P/Fe2P interfaces and other controlled samples were also synthesized and characterized by FESEM (Figures S3, S4, S5 and S6).

In Figure 2d, TEM image of M-Ni2P/Fe2P-O exhibits a plate-like morphology. The corresponding selected area electron diffraction (SAED) pattern confirms that Ni2P and Fe2P own high crystallinity and belong to the same crystal structure (Figure 2e). The high-resolution TEM (HRTEM) image further illustrates that the crystalline Ni2P/Fe2P generates an amorphous layer (Figure 2f). The spacing of the fringes in the bulk is about 5.22 Å, which is characteristic to (010) facet of Ni2P or Fe2P.36, 41 According to previous reports, we anticipate that the NiOOH formed on the Ni2P surfaces and FeOOH formed on the Fe2P surfaces when being oxidized during OER test (Figures S1 and S3). Simultaneously, active NiOOH:Fe and FeOOH:Ni would be achieved near the Ni2P/Fe2P interface. In the Ni2P/Fe2P interface areas, we also applied a mask in the fast Fourier transform algorithm (FFT) image and further performed inverse FFT algorithm. In Figure 2g, notable lattice distortions and edge dislocations can be recognized (circled by white colour). Overall, we visually observed the existence of Ni2P/Fe2P interface and generation of amorphous oxyhydroxide layers after OER test, from the microscopy results. To further 5 ACS Paragon Plus Environment

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demonstrate the M-Ni2P/Fe2P interface components, the focused ion beam technique (FIB) combined with transmission electron microscopy (TEM) was carried out. Figure 2h exhibits the TEM image of a FIB cut foil from M-Ni2P/Fe2P, which displays the cross-section morphology with layered Fe2P and grainy Ni2P. Moreover, the elemental mapping images of Ni, Fe and P were also conducted, indicating the existence of Ni2P and Fe2P interfaces (Figure S7). In the magnified region (Figure 2i-k), Ni and Fe atoms mutually appeal on the Fe2P and Ni2P surface, respectively, which might generate NiOOH:Fe and FeOOH:Ni structures during the OER process.

To determine the crystalline structure of Ni2P/Fe2P before and after OER test, X-ray diffraction (XRD) was conducted. In Figure 3a, XRD pattern of M-Ni2P/Fe2P before OER test matches well with hexagonal Fe2P structure. After OER durability test of 2 h and 70 h, samples of M-Ni2P/Fe2P-O still keep the crystalline nature of phosphide. Besides diffraction signals of phosphides and Ni foam, no detectable peaks after OER test were recognized, demonstrating the generated layers might stay amorphous phase, coinciding with HRTEM results. Other compared samples were also characterized (Figures S8, S9, S10 and S11).

The chemical composition and surface valence states were investigated by X-ray photoelectron spectroscopy (XPS). Firstly, we synthesized NiFeP sample through directly phosphidation of NiFe hydroxide precursors, which was insufficient of Ni2P/Fe2P interfaces. Furthermore, M-Ni2P/Fe2P, NiFeP, their precursors and samples obtained after OER test were studied (Figures S12, S13 and S14), indicating different 6 ACS Paragon Plus Environment

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electronic structures between these samples. In the high-resolution XPS spectra of P 2p region, pristine sample of M-Ni2P/Fe2P shows notable features of phosphides, with characteristic peaks at 129.46 and 130.29 eV (Figure 3b).42 Undergoing stability test for 70 h, trace amount of P element for M-Ni2P/Fe2P-O was identified on the surface (Figures 3b, S15 and S16). However, negligible signals of phosphides were detected on the M-Ni2P/Fe2P-O surface. Both samples exhibit signals of P-O on the surface.31 In addition, we also collected M-Ni2P/Fe2P-O after different time of stability test, and observed that the signals of phosphates became weaker during the electrolysis process but existed even after 100 hours’ test (Figure S16). It is mentionable that surface phosphates could can act as a proton transport mediator and accelerate OER process.31-33 While for the O 1s spectra, the O 1s spectra can be deconvolved into three bands, corresponding to O-P at 529.75 eV, O-metal (O-Ni or O-Fe) in metal phosphates at 531.56 eV and O-H in H2O at 533.09 eV (Figure 3c).43 For the Fe 2p region, the signals of Fe-P centred around at 706.00 eV become weaker after OER test, suggesting Fe2P on the surface were reconstructed (Figure 3d).44 Meanwhile, the peak of Fe-O bonds shifts to a more positive value, indicating Fe was oxidized with a higher valence state. Before OER test, the Ni 2p spectra exhibits four prominent peaks, with 852.78 and 870.00 eV associated to Ni-P bonds, and 856.70 and 874.50 eV assigned to Ni-O bonds (Figure 3e).42 In comparison, the M-Ni2P/Fe2P-O displays no response of Ni-P bonds on the surface. Likewise, the high-valence Ni species on the oxidized surface occupy the dominant content after OER test. From the XPS results, we conclude that metal phosphides (Ni2P or Fe2P) on the surface have been 7 ACS Paragon Plus Environment

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rearranged and oxidized to high-valence states, accompanying with surface generated phosphates, which contribute to high OER activity of these precatalysts.

To further analyse the changes of oxidation states, X-ray absorption near-edge structure (XANES) characterization was performed. In the Fe K-edge XANES spectra, the valences of S-Ni2P/Fe2P and M-Ni2P/Fe2P were notably lower than those of

S-Ni(OH)x/Fe(OH)x

and

M-Ni(OH)x/Fe(OH)x, illustrating the

successful

phosphidation treatment (Figure 4a). After OER test, both Ni2P/Fe2P samples were oxidized with increased Fe valences. The oxidized NiFeP (NiFeP-O) exhibits the highest valence of Fe among all these samples, revealing that the NiFeP sample could be easily oxidized during the OER process. Comparing Ni2P/Fe2P-O and NiFeP-O samples, we reason that metallic iron phosphide species still exist in the bulk. Analogously, the Ni K-edge XANES spectra were also recorded. In Figure 4b, S-Ni2P/Fe2P and M-Ni2P/Fe2P samples show deep phosphidation feature with lower valence than that of NiO (Ni2+). After OER test, the valences of Ni2P/Fe2P-O are still lower than that of NiO, implying the bulk components keep the metallic phosphide phase. In sharp comparison, the Ni species of NiFeP sample were significantly oxidized. These XANES results combined with XPS characterization demonstrate that Ni2P/Fe2P samples with abundant interfaces can be in-situ oxidized to generate highly active oxyhydroxide layers with surface phosphates, retaining the phosphide structure in the bulk.


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The extended X-ray absorption fine structure (EXAFS) spectra were conducted to detect the changes of coordination structures. In the local structure around Fe sites, both initial Ni2P/Fe2P samples feature with characteristic Fe-P bonds, which was different with the Fe-O bonds in controlled samples (Figure 4c). Likewise, the local structure of Ni sites for Ni2P/Fe2P samples characterize with Ni-P bonds (Figure 4e). After OER test, both Ni2P/Fe2P-O samples show a significant decrease in Fe-X (X represents non-metallic element, O or P) and Ni-X bond distances (Figure 4c-f). The bond distances of Fe-X and Ni-X in NiFeP-O are similar of those in controlled S-Ni(OH)x/Fe(OH)x sample, suggesting that most metal-P bonds were destroyed and oxidized with metal-oxygen bonds in the bulk (Figure 4d,f). Nevertheless, metal-P bonds in Ni2P/Fe2P-O samples could still be observed, featured with longer Fe-X or Ni-X bonds than those in controlled samples. Overall, combing with XRD results, we conclude that metallic phosphides still exist in the bulk for the Ni2P/Fe2P samples when being oxidized, which offers good electron transfer ability for OER process.

The OER performances were measured in a three-electrode cell containing 1 M KOH solution. The lineal sweep voltammetry (LSV) was employed to obtain polarization curves of Ni2P/Fe2P-O and controlled samples. The M-Ni2P/Fe2P-O sample only required η of 179 mV to reach a benchmark j of 10 mA/cm2 (η10 = 179 mV, based on projected geometric area). For comparison, Ni2P-O consumed η10 of 291 mV, S-Ni2P/Fe2P-O needed η10 of 213 mV, and NiFeP-O required η10 of 225 mV. The OER performance of M-Ni2P/Fe2P-O also surpassed that of IrO2 benchmark, other hydroxide controls, and corresponding annealed samples (Figures 5a,c and S17). 9 ACS Paragon Plus Environment

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Notably, in the region of large j, M-Ni2P/Fe2P-O also performed better than controlled samples, with η20 of 193 mV, η100 of 251 mV, and η400 of 379 mV. These performances obtained from M-Ni2P/Fe2P-O evidence higher activity than recent reported state-of-art 3d transition metal based oxyhydroxide catalysts and compound precatalysts (Table S1).

The catalytic kinetics of Ni2P/Fe2P catalysts were assessed by Tafel plots derived from LSV curves. As shown in Figure 5b, the resultant Tafel slope of M-Ni2P/Fe2P-O (42.7 mV/dec) approximates that of S-Ni2P/Fe2P-O (43.2 mV/dec), suggesting the similar OER mechanism. Moreover, the Tafel slopes of Ni2P/Fe2P-O are much smaller than that of NiFeP-O, IrO2, Ni2P-O and other hydroxide compared samples (Figure 5b,c). Previous reports have demonstrated that Tafel slopes can be influenced by mass transport and electron transfer ability.13,

45

Therefore, it is reasonable to

conclude that the lower Tafel slope of Ni2P/Fe2P precatalysts are likely attributed to the facile electron transfer ability of the metallic phosphide core. Additionally, the electrochemical impedance spectroscopy (EIS) was conducted to test the conductivity of these catalysts (Figure S18). The Nyquist diagrams of all M-Ni(OH)x/Fe(OH)x, S-Ni2P/Fe2P-O, and M-Ni2P/Fe2P-O catalysts show an apparent semicircle in the high frequency range, which should be associated with charge transfer resistance (Rct). M-Ni2P/Fe2P-O featured the smallest diameter of semicircle, revealing lowest charge transfer resistance among these samples, which is also in a good agreement with the Tafel analysis.


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We further measured the double layer capacitance using a simple cyclic voltammetry (CV) method (Figure S19) to estimate the electrochemically surface area (ECSA). In Figure 5d, we could clearly found that M-Ni2P/Fe2P-O possesses the highest 2Cdl of 119.0 mF/cm2, followed by S-Ni2P/Fe2P-O (43.2 mF/cm2), NiFeP-O (35.97 mF/cm2) and Ni2P-O (23.7 mF/cm2). Next, we further rebuilt the polarization curves normalized to the ECSA, shown in Figure 5e. M-Ni2P/Fe2P-O and S-Ni2P/Fe2P-O catalysts show slight difference, which we ascribe to that they own the same active structures. In comparison, the Ni2P/Fe2P-O catalysts behaved better than NiFeP-O and Ni2P-O normalized to ECSA.

To evaluate the OER stability of M-Ni2P/Fe2P-O catalysts, we performed water oxidation at the constant j of 40 mA/cm2 continuously for about 120 h (Figure 5f). We observed slight degradation for stability test, which might result from the leaching phosphates on the surface (Figures S16 and S20). Meanwhile, the multi-step chronopotentiometric test was also conducted, with j started from 40 to 240 mA/cm2 (Figure S21). The potentials at each step remain nearly unchanged, implying good conductivity, mass transportation, and mechanical robustness of this electrode. The good stability combined excellent OER activity demonstrate M-Ni2P/Fe2P-O can serve as efficient OER electrocatalyst for large-scale real-word energy-related electrolyzers.

Based on these results, we conclude that the high intrinsic OER activity of Ni2P/Fe2P-O might result from synergistic effects of Ni2P(O)/Fe2P(O) interfaces, that 11 ACS Paragon Plus Environment

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is active surface layers and metallic phosphide bulk. On the surface, phosphates would act as a proton-coupled electron transfer mediator, further expediting the kinetics of oxygen evolution.32, 33 Meanwhile, doped oxyhydroxides could accelerate the formation of O-O bonds and lower the activation barrier for OER.28-30 In the bulk, Ni2P/Fe2P-O could remain conductive phosphide phase, and promote the electron transfer process.

In summary, we have demonstrated Ni2P(O)/Fe2P(O) interface can serve as effective catalysts for efficient OER process, consisting of highly active layers on the surface and metallic phosphide core in the bulk. Significantly, the oxidized hierarchically multiple Ni2P/Fe2P-O catalysts exhibit high electrocatalytic activity with η10 of 179 mV, and excellent catalytic stability with long-term durability test for 120 h. We believe Ni2P(O)/Fe2P(O) catalysts are promising for alkaline OER catalysis in operational devices, and also provide guidelines to subtly design multi-component metallic compounds to induce highly efficient electrocatalytic structures based on other 3d transition metal interfaces.


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Figure 1. The metallic Ni2P/Fe2P interfaces would be in-situ oxidized with the anion permeation in the alkaline environment, to form highly active surface reorganizations (Fe doped NiOOH and Ni doped FeOOH along with phosphates) on the conductive phosphide surface. Notes: in the parts of surface accelerated OER, step 1 means that proton-coupled electron transfer would be mediated by phosphates, and step 2 illustrates that doped metal oxyhydroxides would efficiently boost the formation of O-O bonds and oxygen generation.


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Figure 2. Top-down FESEM images of (a-c) M-Ni2P/Fe2P. (d) TEM image of M-Ni2P/Fe2P-O scraped off from the Ni foam substrate. (e) The corresponding SAED pattern of M-Ni2P/Fe2P-O. (f) HRTEM image of M-Ni2P/Fe2P-O. Fe2P, Ni2P and generated amorphous oxyhydroxide layers were masked by yellow lines. (g) TEM image reconstructed by inverse FFT algorithm. Lattice distortions and edge dislocations in the Ni2P/Fe2P interface areas were circled by white color. Cross-section TEM images of (h) M-Ni2P/Fe2P and corresponding elemental mapping images of (i) Ni, (j) Fe and (k) P.


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Figure 3. (a) XRD patterns of pristine M-Ni2P/Fe2P and samples after stability test for 2 h and 70 h, respectively. High-resolution XPS spectra of M-Ni2P/Fe2P before and after OER test for (b) P 2p, (c) O 1s, (d) Fe 2p, and (e) Ni 2p, respectively.


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Figure 4. Comparison of (a) Fe K-edge and (b) Ni K-edge XANES spectra collected on different samples. Comparison of (c) Fe and (e) Ni EXAFS data in R-space collected on pristine Ni2P/Fe2P and selected controlled samples before OER test. Comparison of (d) Fe and (f) Ni EXAFS data in R-space collected on Ni2P/Fe2P-O samples obtained after OER test and controlled samples.


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Figure 5. (a) LSV curves and (b) corresponding Tafel plots of different catalysts. The polarization curves were obtained with a backward scan rate of 1 mV/s, which were not iR-corrected. (c) Overpotentials needed to reach the current densities of 10 and 100 mA/cm2 and Tafel slopes for different catalysts. (d) Charge current density 17 ACS Paragon Plus Environment

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differences (∆j = ja - jc) plotted against scan rates. (e) OER current densities corrected by ECSA against the potentials of Ni2P-O, S-Ni2P/Fe2P-O and M-Ni2P/Fe2P-O samples,

respectively.

(f)

Chronopotentiometric

curves

obtained

with

the

M-Ni2P/Fe2P-O catalyst with constant current density of 40 mA/cm2 for about 120 h.


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ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Experimental section, additional SEM and TEM figures, XRD and XPS analysis, electrochemical data, and Table S1 for comparison of OER performance (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions P.F.L. and X.L. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (21573068, 21503079), SRF for ROCS, SEM, SRFDP, Program of Shanghai Subject Chief Scientist (15XD1501300), Shanghai Municipal Natural Science Foundation (14ZR1410200), Fundamental Research Funds for the Central Universities (WD1313009) and 111 Project (B14018). This work has also benefited from ID-20B beamline of the Advanced Photon Source (APS) at Argonne National 19 ACS Paragon Plus Environment

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Laboratory, the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) and the 1W1B beamline of Beijing Synchrotron Radiation Facility (BSRF).

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