Template-Free Synthesis of Hollow Iron Phosphide- Phosphate Composite Nanotubes for Use as. Efficient and Durable Oxygen Evolution Catalysts

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1 Template-Free Synthesis of Hollow Iron Phosphide- Phosphate Composite Nanotubes for Use as Efficient and Durable Oxygen Evolution Catalysts Junyuan Xu, Dehua Xiong,,, Isilda Amorim and Lifeng Liu,* International Iberian Nanotechnology Laboratory (INL), Avenida Mestre José Veiga, Braga, Portugal State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan , P. R. China KEYWORDS: water splitting, oxygen evolution reaction, iron phosphide, phosphate, hollow nanostructures Corresponding Author Abstract The oxygen evolution reaction (OER) is a half-cell reaction that is important for a number of electrochemical devices, especially for water electrolyzers. Developing efficient, durable and lowcost OER electrocatalysts comprising earth-abundant elements has been a focus of electrocatalysis 1

2 research. Herein, we report a cost-effective, scalable and template-free approach to preparing hollow iron phosphide-phosphate (FeP-FePxOy) composite nanotubes (NTs), which is realized by hydrothermal synthesis of iron oxy-hydroxide nanorod precursors, followed by a post phosphorization treatment. The hollow interior of NTs results from the Kirkendall effect occurring upon phosphorization. When used as OER electrocatalysts in alkaline medium, the as-synthesized FeP-FePxOy composite NTs exhibit excellent catalytic activity, only requiring a low overpotential of 280 mv to deliver the benchmark current density of 10 ma cm -2, showing a small Tafel slope of 48 mv dec -1 and a high turnover frequency of 0.10 s -1 at the overpotential of 350 mv. Moreover, the composite NTs demonstrate outstanding long-term catalytic stability, capable of sustaining constant electrolysis at 10 ma cm -2 for at least 42 h without any degradation, showing substantial promise for use as efficient and low-cost anode catalysts in water electrolyzers. Introduction Since the seminal experimental work on the hydrogen (H2) evolution performance of nickel phosphide nanoparticles (NPs) 1, transition metal phosphides (TMPs) have in the past few years attracted considerable research interest for use as a new class of electrocatalysts to expedite the hydrogen evolution reaction (HER), with an aim to substitute the expensive and scarce platinumbased catalysts Early studies about TMP catalysts were primarily focused on their use in the HER. It was discovered later on that TMPs are also able to catalyze, in alkaline solutions, the oxygen evolution reaction (OER) the half-cell reaction that is more difficult to accomplish than the HER 11,12. Hence, TMPs can be employed as bifunctional catalysts for the overall alkaline water splitting

3 Among various TMP electrocatalysts investigated by far, iron phosphide (FeP) is of particular interest because both Fe and P are Earth-forming elements, and therefore have high natural abundance and a great potential for lowering production cost of water electrolyzers. FeP nanorods (NRs) were firstly reported in 2014, which exhibited reasonably good HER performance 20,21. Subsequently, FeP was also synthesized and used as OER catalysts To improve the catalytic activity of iron phosphide catalysts, nanostructure engineering has been done and a variety of iron phosphide nanostructures including nanotubes (NTs) 18, NRs 23-25, sea urchin 26 and nanowire (NW) arrays 27,28 have been extensively reported. Hollow nanostructures can offer comparatively large surface area and potentially more catalytically active sites, and therefore have recently drawn much attention. Yan and coworkers synthesized FeP NTs using ZnO NRs as the sacrificial template and studied the NTs electrocatalytic performance for OER 18, and they observed a low overpotential (η) of 288 mv for achieving the anodic current density of 10 ma cm -2 as well as a Tafel slope as small as 43 mv dec -1. Very recently, hollow FeP microcubes were also successfully obtained using metal-organic frame (MOF) as scaffold and/or precursors, 29 and improved electrocatalytic activity based on these FeP hollow structures has been observed 18,29. Notwithstanding some progress, developing a template-free method allowing large-scale synthesis of hollow FeP-based nanocatalysts remains challenging. In particular, the catalytic performance of hollow FeP-based catalysts towards the OER has been less explored by far. 18 In this article, we report a simple, cost-effective and scalable template-free approach to the preparation of hollow iron phosphide-phosphate (FeP-FePxOy) composite NTs, which is realized by hydrothermal synthesis of iron oxy-hydroxide (FeOOH) NR precursors, followed by a post phosphorization treatment in red phosphorus vapor at an elevated temperature. The as-obtained composite NTs exhibit outstanding electrocatalytic performance for OER in alkaline solution, 3

4 requiring only a low η of 280 mv to deliver a current density of 10 ma cm -2, a small Tafel slope of 48 mv dec -1 and particularly a high turnover frequency of 0.10 s -1 at η = 350 mv, outperforming both the state-of-the-art commercial RuO2 NPs and many other Fe-based OER catalysts reported so far in the literature. Moreover, the composite NTs also demonstrate excellent long-term catalytic stability toward the OER. Experimental Procedures Hydrothermal synthesis of FeOOH NRs. All chemicals used in this work were of analytical grade and purchased from Sigma-Aldrich. The FeOOH precursor NRs were synthesized through a simple hydrothermal approach modified according to a previous report 30. Typically, 0.15 M FeCl3 6H2O and 1 M NaNO3 were dissolved into 35 ml deionized (DI) water under vigorous magnetic stirring. After 20 min, the mixed solution was transferred into a Teflon-lined steel autoclave reactor. The reactor was sealed and heated up to 100 C, and kept at this temperature for 24 h. Subsequently, the reactor was cooled down naturally to room temperature. The obtained yellow precipitates (i.e., FeOOH NRs) were washed several times using DI water and ethanol, respectively, and then collected by centrifugation. The collected powders were dried in a vacuum oven at 60 C for further use. Fabrication of FeP-FePxOy composite NTs. Hollow FeP-FePxOy NTs were obtained by phosphorizing the FeOOH NR powders in phosphorus vapor at an elevated temperature. In a typical experiment, phosphorus red (1.0 g) and the FeOOH NRs (0.3 g) were loaded in a porcelain boat and separated with a distance of ca. 2 cm, with the red phosphorus at the upstream side and FeOOH NRs at the downstream side in a tube furnace. A piece of porcelain sheet was placed on the top of the boat to increase the local concentration of phosphorus vapor. The ramping rate of 4

5 the furnace was set to 5 C min -1. After phosphorization treatment, the furnace was naturally cooled down to room temperature. The phosphorization treatments were performed in the temperature range of C for a certain period of time (30 60 min) in high purity N2 (99.999%) atmosphere. To obtain optimal phosphorization conditions, the loading of red phosphorous was tuned as well. Moreover, NaH2PO2 was also used as an alternative source of phosphorous in the experiments. The optimized conditions under which the obtained catalysts show best OER performance are as follows: 1.0 g red phosphorous and 0.3 g FeOOH NRs phosphorized at 500 C for 30 min. Structural characterization. The morphology, microstructure and chemical composition of samples were examined by filed-emission scanning electron microscopy (FE-SEM, FEI Quanta 650) and transmission electron microscopy (TEM, FEI Titan ChemiSTEM operating at 200 kv). The crystallographic structure of samples was studied by X-ray diffractometry (XRD, PANalytical X Pert PRO) using Cu Kα radiation (λ = Å) and a PIXcel detector. The surface chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). Electrode preparation and electrocatalytic tests. The catalyst ink was prepared by ultrasonically dispersing 5 mg of catalysts into 1 ml of ethanol containing 50 μl of Nafion solution (5 wt %). To prepare an electrode for catalytic tests, 50 μl of the catalyst ink was loaded on a polished glassy carbon (GC) electrode with an exposed area of 0.78 cm 2, leading to a loading mass of ca. 0.3 mg cm -2. The electrode was then dried in air at room temperature. All electrocatalytic tests were carried out in a three-electrode cell at room temperature with a Biologic VMP-3 potentiostat/galvanostat. 1.0 M KOH was used as the electrolyte. The catalyst-loaded GC, a Pt wire and a saturated calomel electrode (SCE, with saturated KCl solution) were utilized as working, counter, and reference electrodes, respectively. The SCE reference was calibrated prior to each measurement in H2/Ar- 5

6 saturated 0.5 M H2SO4 solution using a clean Pt wire as the working electrode. Unless otherwise stated, all potentials are reported versus the reversible hydrogen electrode (RHE) by converting the measured potentials according to the following equation: URHE = USCE ph (1) Cyclic voltammetry (CV) was performed at a scan rate of 5 mv s -1 in the potential range of 1.0 to 1.8 V vs RHE. An ir-correction of 85 % was applied to compensate the potential drop between the reference and working electrodes, which was measured by a single-point high-frequency impedance measurement. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a d.c. voltage of 1.50 V vs RHE in the frequency range of Hz with a 10 mv sinusoidal perturbation. The catalytic stability of FeOOH NRs and FeP-FePxOy composite NTs catalysts was assessed using chronopotentiometry (CP) at a constant current density of 10 ma cm -2 at room temperature with the electrolyte stirred vigorously. Calculation of turnover frequency (TOF). The TOF values were calculated through the following equation 31 : TOF (s -1 ) = (j A) / (4 F n) (2) where j (A cm -2 ) is the current density at a given overpotential, A = 0.78 cm 2 is the geometric surface area of the electrode, F = C mol -1 stands for the Faraday constant, and n (mol) is mole number of iron loaded on the GC electrode. All metal cations in the catalysts were assumed to be catalytically active, so the calculated value represents the lower limit of TOF. For FeP- FePxOy composite NTs, the Fe content in the catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Shimadzu ICPE-9000). Specifically, 20 mg of catalysts were dispersed in 12 g of concentrated nitric acid in an autoclave, which was then kept in an electric oven at 180 C for 12 h. Subsequently, the acidic solution was diluted in a 50 ml 6

7 volumetric flask. The analyses were done three times using ca. 10 ml solution each time to obtain an average value (59 wt% Fe). Results and Discussion Figure 1. Morphology, microstructure and composition of the FeP-FePxOy composite NTs. (a) SEM image. (b) low-magnification TEM image. (c) high-magnification TEM image. Inset: HRTEM image and FFT-ED pattern. (d) TEM-EDX spectrum. (e) STEM-HAADF image and elemental maps of Fe, P, O and their overlay. Scale bars: 80 nm. 7

8 FeP-FePxOy composite NTs were obtained upon the phosphorization treatment of hydrothermally synthesized FeOOH NR powders. The as-prepared FeOOH precursor NRs are spindle-like and have a typical diameter of nm and a length of ca. 800 nm (Figure S1). XRD examination confirmed that the NRs consist exclusively of tetragonal β-feooh (JCPDS No , Figure S2), and no crystalline phases other than β-feooh were observed. After phosphorization treatment under optimized conditions (i.e. in red phosphorous vapor, 500 C, 30 min), the yellow FeOOH NR powders turned to black. Nevertheless, the spindle-like morphology of NRs did not change except that almost all NRs were found to become hollow, upon a close inspection using SEM (Figure 1a). Figure 1b shows a representative TEM image of the asobtained FeP-FePxOy NTs, where the hollow interiors can be distinguished more clearly, in agreement with the SEM observation. We hypothesize that the formation of hollow NTs stems from the Kirkendall effect taking place at the surface of each individual NR. The phosphorization of FeOOH firstly occurred at the outermost surface of NRs, forming a layer of iron phosphide. Since the O and P species have different diffusion rates at the high temperature, the inter-diffusion of O and P will be balanced by a flux of vacancies. When these vacancies accumulate to a certain level, they tend to coalescence into voids inside the NRs, eventually leading to the formation of hollow interiors. Similar Kirkendall processes were also used to create other hollow nanostructures previously 32,33. On the other hand, the dehydrogenation of FeOOH at high temperatures (i.e., loss of OH groups) might contribute to the formation of hollow NTs as well. To gain further information about the microstructure, high-resolution TEM (HRTEM) investigation was carried out. As illustrated in Figure 1c, the tube walls comprise crystallized domains embedded in an amorphous matrix, and the measured inter-planar spacing of the crystallite is about nm, corresponding to the lattice distance of (202) crystal planes of orthorhombic FeP (JCPDS No. 65-8

9 2595). The high crystallinity of the embedded nanocrystals is verified by the fast Fourier transformation electron diffraction (FFT-ED) analysis, where a well-defined spotted ED pattern is observed (Figure 1c, inset). Extensive SEM and TEM energy-dispersive X-ray spectroscopy (EDX) analyses revealed that the resulting hollow NTs consist of Fe, P and O elements (Figures 1d and S3), and the Cu signal comes from the TEM grid used. Elemental mapping over a single NT was performed in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode. As displayed in Figure 1e, Fe, P and O all distribute evenly over the NT. It is noted that the O does not simply result from the surface oxidation upon exposing the sample to air, and it instead can be observed across the tube walls together with the elements of Fe and P. Therefore, FePxOy may have formed during the phosphorization treatment and been present in the form of amorphous matrix in NT walls. Figure 2. (a) XRD pattern, (b) Fe 2p, (c) P 2p, and (d) O 1s XPS spectra of the FeP-FePxOy composite NTs. 9

10 To examine the crystal structure of the obtained NTs on a macroscopic scale, XRD measurements were carried out. The sample prepared under optimized conditions illustrates a pattern that can be indexed to the orthorhombic FeP (JCPDS No ), as shown in Figure 2a by the characteristic diffraction peaks located at 32.7, 37.2, 46.3, 47.1, 48.4, and 50.3, respectively. No diffraction peaks from either FeOOH or iron phosphate could be detected, indicating that on the one hand, FeOOH was completely converted to FeP or FePxOy; on the other hand, FePxOy should exist in amorphous phase. To further investigate the surface chemical states of the resulting hollow NTs, XPS analysis was performed. Figure 2b shows the high-resolution Fe 2p3/2 XPS spectrum of FeP-FePxOy prepared under optimized conditions. The characteristic binding energy (BE) peaks at ev generally relates to the Fe 2p contribution of metal phosphides 18,20-28,34, which is a good indication of Fe P bond formation; while the BE peaks at and ev may be associated with the Fe III in FePxOy, as pointed out before in the literature 18,35. As far as the P 2p spectrum is concerned (Figure 2c), two BE peaks appear at and ev, which can be assigned to the low-valence P and the 2p3/2 and 2p1/2 core levels of central P atoms in phosphide 20-28, respectively, indicating that Fe P bonds have formed. It is noted that a strong peak arising from P O bonding is observed at ev, which should result from FePxOy that forms due to the outward diffusion of O and inward diffusion of P during the phosphorization treatment. Figure 2d shows the O 1s XPS spectrum where two components can be de-convoluted: the BE peak at ev can be attributed to O P bonding 18,34, and the peak at ev arises from the adsorbed OH groups. In conjunction with the above STEM elemental mapping and XRD results, it is concluded that the walls of the as-obtained NTs are composed of crystalline FeP domains and amorphous iron phosphate (FePxOy) matrices. 10

11 Figure 3. OER performance of the FeP-FePxOy composite NTs and control catalysts measured in O2-saturated 1.0 M KOH electrolyte with a catalyst loading of 0.3 mg cm -2. (a) ir-corrected polarization curves (i.e. the reduction branches of the CV curves shown in Figure S4) recorded at 5 mv s -1. The inset shows the potentials needed for the catalysts to deliver current densities of 10, 20, and 50 ma cm -2. (b) Tafel plots. (c) Nyquist plots measured at 1.50 V vs RHE. (d) The TOF values calculated at η = 300, 350 and 400 mv. The electrocatalytic activity of FeP-FePxOy NTs towards the OER was investigated in O2- saturated 1.0 M KOH using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Prior to the catalytic test, pre-activation was carried out by repetitive CV scans at 50 mv s -1 in the potential range of V vs RHE until a steady state CV curve was obtained. 11

12 The electrocatalytic activity of FeOOH NRs, commercially available RuO2 NPs and other control catalysts prepared under different phosphorization conditions was also measured and compared to that of FeP-FePxOy NTs. Figure 3a shows the reduction branches of ir-corrected CV curves of the FeOOH NRs, RuO2 NPs and FeP-FePxOy catalysts (see CV curves in Figure S4). The GC electrode shows no anodic current up to 1.8 V vs RHE, indicating that it is not catalytically active at all towards the OER. After loading with FeOOH NRs, the anodic current density is markedly improved, and the phosphorization treatment further significantly enhances the OER activity. The η needed at a specific anodic current density has been broadly used as a performance indicator of OER catalysts 36,37. η needed for the catalysts to deliver 10, 20, and 50 ma cm -2 (i.e., η10, η20, and η50) are compared in the inset of Figure 3a. To reach current densities of 10, 20, and 50 ma cm -2, FeOOH NRs need η of 350, 370, and 397 mv, respectively, while FeP-FePxOy NTs merely demand η of 280, 295, and 318 mv, showing a significant cathodic shift in η. The OER activity of FeP- FePxOy NTs is not only better than that of commercial RuO2 catalysts (η10 = 308 mv, η20 = 351 mv, and η50 = 436 mv), but also outperforms that of many other mono-metallic phosphide catalysts reported recently in the literature, such as Ni2P nanowires (η10 = 290 mv) 11, FeP NRs and FeP NTs (η10 = 288 mv 18 or 300 Mv 23 or 350 mv 25 ), and CoP mesoporous NR array (η10 = 290 mv 38 ), albeit it is inferior to that of recently reported Fe(PO3)2/Ni2P composites (η10 = 177 mv) due to the very low loading mass (0.3 mg cm -2 vs 8 mg cm -2 for Fe(PO3)2/Ni2P). 34 It is worth mentioning that the OER activity of FeP-FePxOy NTs is also found to be superior to that of some bi-metallic phosphides which are supposed to have better performance than mono-metallic phosphides due to the synergistic effect 24,31, Detailed comparison between the FeP-FePxOy NTs and other non-precious OER catalysts is summarized in Table S1. It is worth mentioning that all other iron phosphide control catalysts prepared under different phosphorization conditions 12

13 (Figures S5-S7) show OER performance substantially inferior to that of the FeP-FePxOy composite NTs obtained under optimized phosphorization conditions (i.e. in red phosphorous vapor, 500 C, 30 min), regardless of their morphologies and crystal phase compositions (Figure S8). The OER kinetics of FeOOH NRs, FeP-FePxOy NTs and commercial RuO2 NPs was evaluated by Tafel analysis (Figure 3b). The Tafel slope of FeP-FePxOy NTs is 48 mv dec -1, indicating that the first discharge (M + OH MOH + e ) and subsequent chemical adsorption (MOH + OH MO + H2O) processes are not the rate-limiting steps (RDS) 42. In contrast, the FeOOH NRs presents a Tafel slope of 59 mv dec -1, which implies that although the first discharge step is not RDS for FeOOH NRs, the chemical adsorption of OH that follows the first discharge step limits the overall reaction rate. The small Tafel slope observed for FeP-FePxOy NTs suggests that the OER occurs faster at the surface of the hollow composite NTs catalysts. This is further corroborated by EIS analysis which characterizes the charge transfer kinetics at the catalyst/electrolyte interface during the OER. Figure 3c displays the EIS Nyquist plots of FeOOH NRs and FeP-FePxOy NTs catalysts, which can be fitted using an equivalent circuit model consisting of an equivalent series resistance (Rs), and two parallel combinations of a resistance and a constant phase element (R1 CPE1 and Rct CPE2). The fitting results show that the charge transfer resistance (Rct) of FeP-FePxOy NTs is only 15 Ω (Table S2), substantially smaller than that of FeOOH NRs (55 Ω), indicating that the charge transfer taking place at the FeP-FePxOy NTs is much faster. This agrees well with the above Tafel analysis. The electrocatalytic activity of FeP-FePxOy composite NTs was further assessed on the basis of turnover frequency that represents an intrinsic metric independent of the catalyst s size, shape and surface area. TOF was calculated assuming all metal species in the catalysts were active toward 13

14 the OER (i.e., the lower limit). Figure 3d shows the TOF values of FeP-FePxOy NTs as well as FeOOH NRs calculated at η = 300, 350, and 400 mv, respectively. Similar to that observed for the apparent OER activity (Figure 3a), the FeP-FePxOy NTs show substantially higher intrinsic activity than the FeOOH precursor NRs, indicating that the phosphorization treatment indeed greatly promotes the OER performance of the catalysts. Remarkably, the TOF value of FeP- FePxOy NTs can be as high as 0.1 s -1 at η = 350 mv, significantly higher than that of some nonprecious OER catalysts including FeP NRs 23,25, O-doped CoP on reduced graphene oxide, 31 Ni0.75V0.25 LDH nanosheets 43, and Ni3Se2 thin films 44 (Table S1). Figure 4. Chronopotentiometric curves of the FeP-FePxOy NTs and FeOOH NRs recorded at a constant current density of 10 ma cm -2 at room temperature. The long-term catalytic stability of FeP-FePxOy composite NTs was tested at a fixed current density of 10 ma cm -2 and compared to that of FeOOH NRs. Figure 4 shows the chronopotentiometric (CP) curves of both catalysts. The potential required to maintain 10 ma cm - 2 is ca V vs RHE for FeP-FePxOy NTs, lower than that needed for FeOOH NRs (i.e., 1.60 V), 14

15 and both catalysts can sustain constant OER electrolysis up to ca. 40 h without any degradation. In fact, η10 was found to decrease by ca. 10 mv after the 40-h testing for both FeP-FePxOy NTs and FeOOH NRs, implying that the catalysts were likely further activated during the CP test. It is known that non-oxide electrocatalysts will undergo structural and compositional changes upon the long-term testing under OER conditions 31, We have examined the FeP-FePxOy catalysts after the extended OER electrolysis for ca. 42 h using SEM and TEM. It was very challenging to resolve individual nanostructures like what was done for the as-prepared ones (Figure 1), given that the Nafion film wrapped the catalysts and only the agglomerates of catalysts were visible under SEM and TEM investigation. While virtually no hollow tubular structures could be distinguished, many thin sheet-like structures, most probably composed of FeOOH, appeared after the long-term stability test (Figure S9), in agreement with the previous report on Fe(PO3)2 catalysts for OER electrolysis 34. Conclusions In summary, we report a simple and scalable template-free method for synthesizing low-cost FeP-FePxOy composite NTs. Comprehensive structural and composition characterization demonstrates that the NTs comprise crystalline FeP nanoparticles embedded in an amorphous FePxOy matrix. The as-prepared FeP-FePxOy composite NTs exhibit outstanding apparent and intrinsic electrocatalytic activities toward the oxygen evolution reaction and excellent long-term operational stability in alkaline solution. Given the simplicity of the synthetic method, low-cost of catalytic materials and high catalytic performance, the FeP-FePxOy composite NTs hold substantial promise for use as efficient and cheap anodic catalysts in water electrolyzers. 15

16 ASSOCIATED CONTENT Supporting Information. Morphology and structure FeOOH precursor NRs; SEM-EDX spectrum of FeP-FePxOy composite NTs; Cyclic voltammograms of FeOOH NRs and FeP-FePxOy NTs; XRD, SEM, EDX, and OER performance of control catalysts prepared under different phosphorization conditions; SEM and TEM characterization of FeP-FePxOy NTs after the extended stability test; Performance comparison with other non-precious OER electrocatalysts; Summary of EIS fitting parameters. AUTHOR INFORMATION (L.F. Liu) X.J.Y. and X.D.H. contributed equally Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the European Horizon 2020 project Critcat under the grant agreement number L.F.L. acknowledges the financial support from the Portuguese Foundation of Science and Technology (FCT) under the projects IF/2014/01595 and PTDC/CTM-ENE/2349/2014" (grant agreement No ). 16

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