Electrochemical Activity of Iron Phosphide Nanoparticles in Hydrogen

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Electrochemical activity of iron phosphide nanoparticles in hydrogen evolution reaction Lihong Tian, Xiaodong Yan, and Xiaobo Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01515 • Publication Date (Web): 20 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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Electrochemical activity of iron phosphide nanoparticles in hydrogen evolution reaction Lihong Tian,†,‡ Xiaodong Yan,† Xiaobo Chen†* †Department of Chemistry, University of Missouri – Kansas City, Kansas City, Missouri, 64110, USA. ‡Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Ministry of Education Key Laboratory for the Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan 430062, China

ABSTRACT Iron phosphide (FeP) has been recently demonstrated as a very attractive electrocatalyst for hydrogen evolution reaction (HER). However, understanding of its properties is far from satisfactory. Herein, we report the HER performance of FeP nanoparticles is enhanced after stability test due to reduced surface charge transfer resistance in the HER process. The synthetic temperature and reactant ratio are important to surface charge transfer resistance, the electrochemically active surface area and HER activity. Hydrogenation apparently improves the HER performance of FeP nanoparticles by reducing the surface charge transfer resistance, overpotential, and Tafel slope. Enhanced HER performance is observed after stability test for both bare and hydrogenated FeP nanoparticles in the HER due to reduced surface charge transfer

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resistance. Thus, this study may enrich our knowledge and understanding to advance HER catalysis for electrochemical hydrogen generation.

KEYWORDS: Iron phosphide, Hydrogen evolution reaction, Electrocatalyst, Water electrolysis, Hydrogenation treatment

Hydrogen is expected to be a promising alternative energy source to the traditional fossil fuel as a clean and renewable energy.1 Among several emerging clean energy technologies, electrochemical reduction of water is considered as the most applicable one.1, 2 Current protonexchange membrane technology-based water electrolysis units are limited to work under strongly acidic condition.3 Pt-based electrocatalysts show the highest performance on hydrogen evolution reaction (HER), but suffer from high cost and cannot be used widely.4,5 Nickel based alloys have been commercially used as HER catalysts, but the corrosion and instability in acidic solution inhibit their applications in proton exchange membrane (PEM) based electrolyzers.6,7 Great progresses have been made in synthesizing and exploiting novel and effective HER electrocatalysts in the past decades. Particularly, the earth-abundant 3d transition metal (such as Fe, Co and Ni) based phosphides have been prepared and demonstrated as promising catalysts for HER in acidic conditions.8-10 Among all 3d transition metal based phosphides, iron phosphides are especially attractive because Fe comprises 5% of the Earth’s crust and is the most abundant transition metal.11 For instance, Callejas et al. synthesized FeP nanoparticles by first decomposing Fe(CO)5 in a mixture of oleylamine and 1-octadecene, and then reacting with trioctylphosphine.12 The resulted FeP/Ti


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showed a overpotential of 50 mV to acquire 10 mA cm-2 in 0.5 M H2SO4.12 Zhang’s group prepared nanoporous FeP nanosheets through an anion exchange reaction of Fe18S25–TETAH (TETAH = protonated triethylenetetramine) nanosheets with phosphide ions, and the FeP nanosheets had a overpotential of 250 mV at 10 mA cm-2 in acid medium.8 However, these synthesis methods are tedious and high cost, even involving organic reagents. Therefore, it is necessary to develop simple and economic synthesis route of FeP for HER applications. In this work, we show that enhanced HER activity after stability test is achieved with FeP nanoparticles due to reduced surface charge transfer resistance in the HER process. The FeP nanoparticles are obtained by one-step phosphorization of -Fe2O3 with NaH2PO2 at low temperatures under argon atmosphere. We have investigated the influence of temperature and the reactant molar ratio (of -Fe2O3 over NaH2PO2: the starting Fe/P ratio) to understand the HER activity of FeP nanoparticles. In addition, we found hydrogenation can further improve their HER activity, different from current morphology control13,14 or direct growth on carbon materials, carbon clothes and graphene.15-17 Enhanced HER activity is achieved for both bare and hydrogenated FeP nanoparticles in the HER process. Figure 1A showed X-ray diffraction (XRD) patterns of the rhombohedral -Fe2O3 precursor (JCPDF no. 89-599) synthesized by a hydrothermal method and the FeP nanoparticles obtained at 400 °C with the starting Fe/P ratio of 1:3. The phosphorization temperature and the starting Fe/P ratio were important in forming FeP nanoparticles (Figure S1). Below 350 oC, -Fe2O3 was only partially converted into FeP; at 400 or 450 oC, pure FeP (JCPDF no. 65-2595) were obtained. The crystalline grain size was calculated by Scherrer equation:  = (kλ)/(βcosθ), where  is the mean size of the ordered (crystalline) domains, k is the shape factor with a typical value of 0.9, λ is the X-ray wavelength, β is the full width at half maximum peak height in radians, and θ is the Bragg angle. The grain size calculated


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with the most intensive signal at 2θ of 48.4° was 27.2, 31.4 and 33.3 nm for FeP-350, FeP-400 and FeP-450, respectively. As shown in Figure 1B, -Fe2O3 was completely converted into pure FeP when the molar ratio of Fe/P precursor was 1:2 or 1:3. When the molar ratio of Fe/P precursor was 1:1 or 1:4, besides the formation of FeP, some Fe2P was also observed after the reaction (Figure 1B and Figure S2-S5). In following discussions, if not specifically denoted, the FeP nanoparticles were prepared with the Fe/P molar ratio of 1:3 at 400 oC. The FeP nanoparticles obtained normally had an average size of 50 nm and were aggregated as shown by the transmission electron microscopy (TEM) image in Figure 1C. Well-resolved lattice fringes were observed in the highresolution transmission electron microscopy (HRTEM) image shown in Figure 1D, indicating the high crystallinity of FeP nanoparticles. The lattice spacing value of 0.290 nm corresponded to the (002) crystal plane of FeP. The X-ray photoelectron spectroscopy (XPS) survey spectrum displayed that Fe, P, O and C were present in the as-prepared FeP (Figure S3-S5). The main carbon signal at 284.6 eV from the tape used to fix the sample was used as reference to calibrate all the XPS spectra. The oxygen signal was likely related to some ferric hydroxide and also possibly some oxides or simply absorbed water on the surface of the FeP nanoparticles, as a thin layer of oxides are commonly observed on the surfaces of many non-oxide materials. In the Fe 2p spectrum shown in Figure 1E, two peaks at 707.1 and 720.1 eV were ascribed to the 2p 3/2 and 2p 1/2 of Fe3+ in FeP;18 the other two peaks at 711.9 eV and 725.6 eV were likely due to the 2p 3/2 and 2p1/2 of Fe3+ ions in the form of FeO(OH) on the surface.19 In the P 2p XPS spectrum shown in Figure 1F, the strong peaks located at 129.3 and 133.1 eV were assigned to the P 2p 3/2 and P 2p 1/2 of P3ions in FeP.18 These results confirmed the formation of FeP. We examined the HER performance of FeP nanoparticles in 0.5 M H2SO4 using a typical three-electrode setup. Figure 2A showed the polarization curves of FeP. The titanium foil coated


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with carbon black (Ti/C) and commercial Pt/C (20 wt% Pt) were studied for comparison. All the polarization curves were iR-corrected. All potentials in this work were reported versus reversible hydrogen electrode (RHE). Commercial Pt/C exhibited an excellent HER activity with near zero onset potential, while Ti/C electrode had a poor activity with an onset overpotential of 303 mV. The FeP nanoparticles showed an onset overpotential of 50 mV, and small overpotentials of 154 and 187 mV to reach 10 and 20 mA cm−2, respectively. These overpotentials for FeP were comparable to those of the reported FeP nanosheets,8 FeP nanoparticles on graphene,17 mixedphased Co-based catalysts,20 MoP,21 and W2P.22 Figure 2B showed the Tafel plots of FeP (1:3), Ti/C and Pt/C derived from Figure 2A. The linear portion of the Tafel plots was fitted to the Tafel equation η = b log j + a, where η was the applied overpotential, j was the current density, b was the Tafel slope and a was the intercept relative to the exchange current density. The FeP nanoparticles had a Tafel slope of 65 mV dec−1, the Ti/C electrode of 138 mV dec−1, and the Pt/C electrode of 23 mV dec−1. The HER reaction in acidic solutions can be written in two steps: the Volmer (electrochemical hydrogen adsorption: 2H+ + 2e- -> 2Had), and the Tafel reaction (chemical desorption: Had + Had -> H2) or Heyrovsky process (chemical desorption: Had + H+ + e− -> H2).23-26 A Tafel slope of 120, 40, or 30 mV dec−1 is expected if the Volmer, Heyrovsky, or Tafel step is the rate-determining step, respectively.23-26 This Tafel slope indicated that the HER occured through a Volmer–Heyrovsky mechanism and the electrochemical recombination with an additional proton to form the desorbed H2 gas was likely the rate-limiting step.25-30 The Tafel slope of FeP was close to that of Cu3P nanowires,31 FeP nanosheets,8 Ni12P5,32 Ni2P/Ni,33 MoP nanosheets,34 Co2P/CoP film,20 indicating the FeP nanoparticles had a high HER activity. The HER stability of FeP was evaluated using constant voltage technique without iR-correction (Figure 2C). Under an overpotential of 167 mV, the initial current density of FeP electrode was


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10.6 mA cm−2. The current density decreased to 8.5 mA cm−2 after 5700 s. It was noted that the current dramatically dropped when stirring was stopped, but recovered to 8.5 mA cm−2 when stirring continued. The corresponding Faradic efficiency for the HER was around 99.4% based on the calculation from the produced hydrogen over the electricity consumed during the reaction (Fig. S6). The decay of the current density was likely due to the sluggish mass transfer of the electrolyte or the gas desorption from the electrode surface in the HER, but the FeP nanoparticles may have a stable HER performance (within the timeframe being tested). To confirm that, we compared the polarization curves of FeP nanoparticles before and after stability as shown in Figure 2D. After the stability test, the FeP exhibited a better polarization curve with lightly smaller overpotentials of 151 and 177 mV at 10 and 20 mA cm−2, respectively, along with a smaller Tafel slope (Figure 2E). Meanwhile, the Nyquist plots of the electrochemical impedance spectroscopy (EIS) spectra of FeP before and after stability (Figure 2F) suggested that the charge transfer resistance Rct became smaller after the stability test (4.18 ) than before (4.75 ). This indicated that the current decay in the stability test was likely from the mass transfer diffusion during the HER. The HER activity of FeP nanoparticles were largely affected by the synthetic temperature and the ratio of Fe2O3 to NaH2PO2. The HER activity increased in the order: FeP (350 °C) < FeP (450 °C) < FeP (400 °C) as shown in Figure 3A when the Fe/P ratio was 1:3, while the overpotential at 10 mA cm−2 decreased in the order: 340 mV for FeP (350 °C) > 195 mV for FeP (450 °C) > 171 mV for FeP (400 °C), and the slope of the Tafel plots decreased in this order: 141 mV dec−1 for FeP (350 °C) > 88 mV dec−1 for FeP (450 °C) > 65 mV dec−1 for FeP (400 °C) (Figure S7). The FeP synthesized at 400 oC displayed the highest HER activity among those FeP catalysts. On the other hand, when the reaction temperature was 400 °C, the HER activity increased in the order: FeP (1:1) < FeP (1:4) < FeP (1:3)  FeP (1:2) (Figure 3B), the overpotential at 10 mA/cm2


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decreased in the order: 207 mV for FeP (1:1) > 165 mV for FeP (1:4) > 156 mV for FeP (1:3) > 154 mV for FeP (1:2), and the slope of the Tafel plots decreased in this order: 82 mV dec−1 for FeP (1:1) > 69 mV dec−1 for FeP (1:4) > 65 mV dec−1 for FeP (1:3) > 62 mV dec−1 for FeP (1:2) (Figure S8). The lower activity of FeP (1:1) and FeP (1:4) suggested that the existence of the Fe2P phase led to a lower HER activity as both samples had a mixed phase of FeP and Fe2P, while the FeP (1:2) and FeP (1:3) with pure FeP phase had higher HER activities. To understand how the HER activity of FeP nanoparticles is affected by their surface area, or more specifically, the electrochemically active surface area (ECSA), we analyzed the capacitive current as a function of scan rate as shown in Figure 3C, since the ECSA was linearly proportional to the double layer capacitance (Cdl).29,30,35,36 The linear slope represents the ECSA, which is 2fold Cdl.35,36 For FeP made from different Fe/P ratios, the slope increased in this order: 0.0010 s cm-2 S-1 for FeP (1:1) < 0.0016 s cm-2 S-1 for FeP (1:4) < 0.0025 s cm-2 S-1 for FeP (1:3) < 0.0043 s cm-2 S-1 for FeP (1:2). This suggested that a larger ECSA led to a higher HER activity, while the existence of the Fe2P phase might lead to the formation of a smaller ECSA. Figure 3D showed the relations between the HER current density at an overpotential of 160 mV, the overpotential at 10 mA cm-2 (10) and Tafel slope (kTafel) with the ESCA (2Cdl) for the FeP nanoparticles made from different Fe/P ratios. The HER current increased with the increase of the ESCA value, while the 10 and kTafel decreased with the increase of the ESCA value of the FeP nanoparticles. The HER activity of FeP nanoparticles was further correlated with the Nyquist plots as shown in Figure 3E. For FeP nanoparticles made from different Fe/ P ratios, all the Nyquist plots had one major semicircle corresponding to the charge transfer resistance (Rct) and Rct decreased in the order 29.2  for FeP (1:1) > 5.73  for FeP (1:4) > 4.52  for FeP (1:3) > 3.98  for FeP (1:2). The large Rct of FeP (1:1) and FeP (1:4) were likely due to the existence of Fe2P phase in the sample. The


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extreme large Rct value of FeP (1:1) hinted that the Fe2P might mainly existed on the surface with exposed inactive facet which limited the charge transfer process. The relationships between the HER current at 160 mV (j160), 10 and kTafel with the charge transfer resistance Rct were shown in Figure 3F. j160 decreased as the Rct increased, and 10 and Tafel slope kTafel increased. Meanwhile, as shown in Figure S9, the ECSA decreased as the Rct increased. We have shown hydrogenation as an effective approach to enhance the HER performance of metal oxide.37,38 Here, we found that hydrogenation can also improve the HER activity of the FeP catalysts. The XRD results (Figure S10 and S11) indicated that hydrogenation at 400 °C for 3 h did not apparently change the phase compositions of the FexP nanoparticles after hydrogenation. That is, hydrogenated H-FeP (1:2) and H-FeP(1:3) had a pure FeP phase, while H-FeP (1:1) and H-FeP (1:4) had a major FeP phase with a minor Fe2P phase. Based on the Scherrer estimation, hydrogenation did not alter much the crystalline size of the FexP nanoparticles as the peaks’ widths were almost the same before and after hydrogenation (Figure S11). As seen, hydrogenation improved the HER activity of FeP nanoparticles made from Fe/P ratio from 1:2 and 1:3 (Figure 4A and Figure S12). For example, after hydrogenation, the overpotential at current density of 10 mA cm−2 decreased to 189 mV from 207 mV for FeP (1:1), and to 147 mV from 154 mV for FeP (1:3). But for the FeP (1:4), hydrogenation decreased its HER activity based on the overpotential at 10 mA cm−2. Although hydrogenation reduced the overpotential at current density of 10 mA cm−2 for FeP (1:1), the activity at higher overpotential apparently decreased compared with pristine FeP (1:1). The decreased HER activity of hydrogenated FeP (1:4) and FeP (1:1) was likely related to the formation of more Fe2P phase on the surface due to the reduction of the Fe3+ in the FeP which was less active for HER. On the other hand, hydrogenation only slightly changed the Tafel slopes as shown in Figure 4B, suggesting that hydrogenation did not alter the HER mechanism.


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Figure 4C showed the capacitive current of H-FeP nanoparticles as a function of scan rate. For HFeP, the slope increased in the order: 0.0007 s cm-2 S-1 for H-FeP (1:4) < 0.0018 s cm-2 S-1 for HFeP (1:1) < 0.0027 s cm-2 S-1 for H-FeP (1:3) < 0.0036 s cm-2 S-1 for FeP (1:2). Thus, the ECSA increased in this order: H-FeP (1:4) < H-FeP (1:1) < H-FeP (1:3) < H-FeP (1:2). Figure 4D showed the HER current density at an overpotential of 150 mV (j150) plotted against ESCA (2Cdl) for the H-FeP nanoparticles. Here, the reason we used j150 for H-FeP nanoparticles instead of j160 as for FeP nanoparticles was because that the H-FeP showed a better HER performance where the overpotential needed to reach the same current density of 10 mA cm-2 was reduced after hydrogenation. Apparently, the HER current increased with the increase of the ECSA value, while the overpotential decreased with the increase of the ESCA value. However, the Tafel slope kTafel did not show an apparent relationship with the change of ECSA, likely due to the complication of the induction of the Fe2P phase in the hydrogenated FeP nanoparticles with Fe/P ratios of (1:1) and (1:4). As shown in Figure 4E, for hydrogenated FeP nanoparticles made from different Fe/ P ratios, all the Nyquist plots had one major semicircle corresponding to the charge transfer resistance (Rct) which decreased in the order 18.7  for H-FeP (1:1) > 4.93  for FeP (1:4) > 4.50  for FeP (1:3) > 3.18  for FeP (1:2). Figure 4F showed the relationships between the HER current at 150 mV (j150), 10 and kTafel with the charge transfer resistance Rct. With the increase of Rct, j150 decreased, and 10 and Tafel slope kTafel increased. These relationships in the hydrogenated FeP nanoparticles were similar to those in the pristine FeP nanoparticles. To find out why hydrogenation improved the HER performance, we compared the Nyquist plots of FeP (1:2) before and after hydrogenation (H-FeP) (Figure S13) and summarized the comparison of HER properties of the FeP nanoparticles before and after hydrogenation in Table 1. After hydrogenation, the charge transfer resistance Rct decreased from 3.98 Ω to 3.18 Ω. Hydrogenation


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also decreased the Rct values of H-FeP with other Fe/P ratios. Thus the increased HER performance may be attributed to the enhanced charge transfer properties on the surface induced by the hydrogenation. However, due to the existence of the Fe2P phase in hydrogenated FeP nanoparticles with Fe/P ratios of (1:1) and (1:4), there were no apparent relationship between the HER current at j150, 10, and kTafel against Rct. The changes of the overpotential, Tafel slope, Rct and ESCA of the FeP nanoparticles after hydrogenation were summarized in Table 1. The stability of H-FeP (1:2) was evaluated using constant voltage technique without iR-correction. Under an overpotential of 150 mV, the initial current density of FeP electrode was 10.1 mA cm −2 (Figure 5A). The current density decreased to 7.2 mA cm−2 after 6000 s. This decay of the current density over time was likely due to the sluggish mass transfer of the electrolyte in the HER. The corresponding Faradic efficiency for the HER was around 99.2% (Figure S14). As shown in Figure 5B, after the stability test, the H-FeP (1:2) exhibited a better polarization curve with lightly smaller overpotentials of 172 mV and 252 mV to reach 20 and 100 mA cm−2, respectively. The Nyquist plots of the EIS spectra of H-FeP (1:2) before and after stability (Figure S15) suggested that the charge transfer resistance Rct became smaller after stability test (3.06 ) than before stability test (3.18 ) and the contact resistance became smaller (1.14  vs. 1.54 ) as well. This indicated that the performance decay in the stability test was likely from the mass transfer diffusion during the HER. Thus, the H-FeP nanoparticles had a stable HER performance and the HER activity of HFeP nanoparticles actually increased as the HER proceeded. We measured the XRD patterns of the FeP and H-FeP nanoparticles after stability tests with the electrode materials scratched off from the titanium foil (Figure S16). The XRD analysis indicated that both nanoparticles did not change apparently after stability test.


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In conclusion, enhanced HER activity is observed after stability test for both bare and hydrogenated FeP nanoparticles due to reduced surface charge transfer resistance in HER process. The FeP nanoparticles are prepared using a low-temperature phosphorization of -Fe2O3 with NaH2PO2. Table S1 compared the activity of the current HER catalysts. Apparently, the FeP nanoparticles prepared in this work displayed attractive performance. Their HER activity is largely affected by the phosphorization conditions, i.e., the reactant ratio and phosphorization temperature. The HER current density increases with increased ECSA, but with decreased overpotential, the Tafel slope, and the surface charge transfer resistance. The catalytic performance can be further improved by hydrogenation due to the enhanced charge transfer, reduced overpotential and Tafel slope. The HER polarization curves become better during stability test due to the decrease of surface charge transfer resistance. However, the long-term HER performance decays due to limited mass transfer near the surface. Thus, overcoming surface mass transfer limitation may further provide a promising solution in future studies for developing high-performance HER catalysts.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

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Material preparation, characterization and Figure S1-S16 supplied in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT X. C. acknowledges the support from the College of Arts and Sciences, University of Missouri − Kansas City, University of Missouri Research Board, and the University of Missouri Interdisciplinary Intercampus (IDIC) Program. L. T. thanks the National Natural Science Foundation of China (no. 51302072) and the China Scholarship Council for financial support. REFERENCES (1) Wang, H.; Zhang, Q.; Yao, H.; Liang, Z.; Lee, H. W.; Hsu, P. C.; Zheng, G.; Cui, Y. Nano Lett. 2014, 14, 7138–7144. (2) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. J. Am. Chem. Soc. 2014, 136, 4897–4900. (3)Weng, S. X.; Chen, X. Nano Energy 2016, 19, 138–144. (4) Schuldiner, S. J. Electrochem. Soc. 1959, 106, 891–895. (5) Yan, X.; Li, K.; Lyu, L.; Song, F.; He, J.; Niu, D.; Liu, L.; Hu, X.; Chen, X. ACS Appl. Mater. Interfaces 2016, 8, 3208–3214. (6) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. ACS Catal. 2013, 3 166–169. (7) Morales-Guio, C. G.; Stern, L. A.; Hu, X. Chem. Soc. Rev. 2014, 43, 6555–6569. (8) Xu, Y.; Wu, R.; Zhang, J.; Shi, Y.; Zhang, B. Chem. Commun. 2013, 49, 6656–6658. (9) Wang, X.; Kolen’ko, Y. V.; Bao, X. Q.; Kovnir, K.; Liu, L. Angew. Chem. Int. Ed. 2015, 54, 8188–8192.


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(23) Conway, B. E.; Tilak, B. V. Electrochim. Acta 2002, 47, 3571–3594. (24) Marini, S.; Salvi, P.; Nelli, P.; Pesenti, R.; Villa, M.; Berrettoni, M.; Zangari, G.; Kiros, Y. Electrochim Acta 2012, 82, 384–391. (25) Pan, Y.; Liu, Y.; Zhao, J.; Yang, K.; Liang, J.; Liu, D.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. J. Mater. Chem. A, 2015, 3, 1656–1665. (26) Pan, Y.; Hu, W.; Liu, D.; Liu, Y.; Liu, C. J. Mater. Chem. A, 2015, 3, 13087–13094. (27) Pan, Y.; Liu, Y.; Liu, C. J. Power Sources, 2015, 285, 169–177. (28) Pan, Y.; Yang, N.; Chen, Y.; Lin, Y.; Li, Y.; Liu, Y.; Liu, C. J. Power Sources, 2015, 297, 45–52. (29) Pan, Y.; Lin, Y.; Chen, Y.; Liu, Y.; Liu, C. J. Mater. Chem. A, 2016, 4, 4745–4754. (30) Pan, Y.; Lin, Y.; Liu, Y. Liu, C. Catal. Sci. Technol. 2016, 6, 1611–1615. (31) Tian, J.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X. Angew. Chem. Int. Ed. 2014, 53, 9577– 9581. (32) Huang, Z.; Chen, Z.; Chen, Z.; Lv, C.; Meng, H.; Zhang, C. ACS Nano, 2014, 8, 8121–8129. (33) Shi, Y.; Xu, Y.; Zhuo, S.; Zhang, J.; Zhang, B. ACS Appl. Mater. Interfaces 2015, 7, 2376– 2384. (34) Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Alamry, K. A.; Sun, X. Appl. Catal. B: Environ. 2015, 164,144–150.


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ACS Catalysis

(35) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. J. Am. Chem. Soc. 2013, 135, 10274–10277. (36) Song, F.; Hu, X. Nat. Commun. 2014, 5, 4477/1–4477/9. (37) Yan, X.; Tian, L.; Chen, X. J. Power Sources 2015, 300, 336–343. (38) Yan, X.; Tian, L.; He, M.; Chen, X. Nano Lett. 2015, 15, 6015–6021.


15

ACS Catalysis

Figure and Figure Captions

B

# Fe2P #

#

#

#

FeP(1:1)

Intensity / a.u.

Intensity / a.u.

A Fe2O3 FeP

Fe2O3 (89-599)

FeP(1:2)

FeP(1:3) FeP(1:4)

FeP (65-2595)

30

40

50

60

30

70

40

E

725.6

711.9 720.1

730

720

707.1

710

Binding Energy / eV

60

70

F

P 2p

Intensity / a.u.

Fe 2p

50

2Theta / degree

2 Theta / degree

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 22

700

133.1

129.3

136

134

132

130

128

Binding Energy / eV

Figure 1. The XRD patterns of (A) the precursor, FeP and standard patterns of -Fe2O3 (pdf # 89599) and FeP (pdf # 65-2595); (B) with different Fe/P molar ratios at 400 °C.; (C) TEM graph of FeP; (D) HRTEM graph of FeP; The XPS spectra of Fe 2p (E) and P 2p (F) of FeP (1:3).


16

Page 17 of 22

0.5

0

j / mA cm

-40 -60 FeP Pt/C Ti/C

-80 -100 -0.4

-0.3

-0.2

-0.1

B

0.4

Overpotential / V

A

-2

-20

-1

138m

FeP Pt/C Ti/C

0.2 -1

65 mV dec 0.1

-1

23 mV dec

0.0 0.0

0.0

0.2

0

0.8

1.0

)

D

-20 -2

-10

j / mA cm

-2

0.6

Log (j/mA cm

C j / mA cm

0.4

-2

-15

stop stirring

-5

-40 -60 before stability test after stability test

-80 0

V dec

0.3

Potential vs. RHE / V

0

1000

2000

3000

4000

5000

-100

6000

-0.3

Time / s

-0.2

-0.1

0.0

Potential vs. RHE / V 2.0

E

V 65m

0.10

c De

1.5

-1

63m

V

Dec

0.2

0.4

0.6

0.8 -2

Log (j / mA cm

1.0

0.5

before stability test after stability test

0.05 0.0

F

-1

- Z" /

0.15

Overpotential / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

before stability test after stability test

1.0

0.0

1

2

)

3

4

5

6

Z' / 

Figure 2. (A) Polarization curves of the FeP, Ti/C and commercial Pt/C electrodes; (B) Tafel plots derived from (A); (C) Current−time characteristics of FeP for the stability test at an overpotential of 160 mV. (D) Polarization curves of the FeP electrode before and after stability test. (E) Tafel plots of FeP before and after stability test. (F) The Nyquist plots of FeP before and after stability test under an overpotential of 160 mV.


17

ACS Catalysis

0

A

0

-20 -2

-2

-40

j / mA cm

-60 FeP-350 FeP-400 FeP-450

-0.3

-0.2

FeP (1:1) FeP (1:2) FeP (1:3) FeP (1:4)

-80

-0.1

-100 -0.4

0.0

-0.3

0.5

0.0016 0.0010

0.1

8 6 4

190

60

80

70 64

2 0

100

0.001

0.002

-1

0.003

0.004

12

F

210

9

10

15

20

25

30

35

40

0

72 170 67

0

150 10

Z' / 

20

62

-1

5

3

FeP (1:1)

FeP(1:1) FeP(1:2) FeP(1:3) FeP(1:4)

6

190

FeP (1:4)

6

0

82 77

FeP (1:2) FeP (1:3)

-2

j160 / mA cm

- Z'' / 

9

0

58

0.005

ECSA(2Cdl)

E

3

76

10 /mV & kTafel / mV dec

40

82

170

150 20

Scan rate / mV s

12

0.0

-1

0.0

-0.1

210

FeP(1:1)

25

0.00

0.2

D

10 -2

FeP(1:1) FeP(1:2) FeP(1:3) FeP(1:4)

0.3

3

04

0. 0

j160 / mA cm

j / mA cm

-2

0.4

12

C

-0.2



FeP(1:2)

-100 -0.4

-60

FeP(1:3)

-80

-40

FeP(1:4)

j / mA cm

B

-20

10 /mV & kTafel /mV dec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

30

Rct / 

Figure 3. (A) Polarization curves of the FeP nanoparticles prepared at different temperatures; (B) Polarization curves of the FeP nanoparticles from different starting Fe/P ratios reacting at 400 °C; (C) Charging current density difference (Δj= ja−jc) plotted against scan rate for the FeP from different Fe/P ratios reacting at 400 °C; (D) HER current density at an overpotential of 160 mV (j160), the overpotential at 10 mA cm-2 (10) and Tafel slope kTafel plots vs. ESCA (2Cdl); (E) The Nyquist plots; (F) HER current at 160 mV (j160), the overpotential at 10 mA cm-2 (10) and Tafel slope kTafel plots vs. Rct for the FeP nanoparticles made from different Fe/P ratios.


18

A Overpotential / V

-2

j / mA cm

-40 -60 H-FeP (1:1) H-FeP (1:2) H-FeP (1:3) H-FeP (1:4)

-80 -100 -0.4

B

0.16

-20

-0.3

-0.2

-0.1

65mV D

0.12

-1

82mV 59mV

0.00

0.1

-2

)

200 88

9 6

80

H-FeP (1:1)

0.0018

0.45

D H-FeP (1:4)

027

0.0

0.2

j150 / mA cm

-2

j / mA cm

0.3

036

0.30

Log( j/ mA cm

C 0.0

H-FeP(1:1) H-FeP(1:2) H-FeP(1:3) H-FeP(1:4)

-1

ec

-2

12 H-FeP(1:1) H-FeP(1:2) H-FeP(1:3) H-FeP(1:4)

-1

Dec

0.15


0.4

Dec

64mV D

0.08

0.0

-1

ec

180

H-FeP (1:2)

0

72 160 64

3

40

60

80

Scan rate / mV s

100

15

20

H-FeP (1:2) H-FeP (1:3)

j150 / mA cm

- Z'' / 

-2

10

9 6

82

185 74 170 155

66

3 0

140 58 0

4

8

Z' / 

12

16

20

-1

H-FeP (1:1) H-FeP (1:2) H-FeP (1:3) H-FeP (1:4)

200


4

5

56

0.004

F

12

6

0

0.003

ECSA(2Cdl)

E

0

0.002

-1

8

2

0.001

H-FeP (1:1)

20

H-FeP (1:4)

0

140 0 0.000

-1

0.0007 0.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

H-FeP (1:3)

Page 19 of 22

24

Rct / 

Figure 4. (A) Polarization curves of the hydrogenated FeP nanoparticles with different starting Fe / P ratios; (B) Derived Tafel plots; (C) Charging current density difference (Δj= ja−jc) plotted against scan rate; (D) Current density at an overpotential of 150 mV (j150), the overpotential at 10 mA cm-2 (10) and Tafel slope (kTafel) plots vs. ESCA (2Cdl). (E) The Nyquist plots of H-FeP with different starting Fe/P ratios; (F) HER current at an overpotential of 150 mV (j150), the overpotential at 10 mA cm-2 (10) and Tafel slope (kTafel) plots against Rct.


19

ACS Catalysis

-15

0

A

B

-2

-10

j / mA cm

-2

-20

j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

-5

0

1000

2000

3000

Time / s

4000

5000

-60 H-FeP (1:2)

-80

H-FeP (1:2)

0

-40

6000

-100 -0.4

before stability test after stability test -0.3

-0.2

-0.1

0.0

Potential vs RHE / V

Figure 5. (A) Current−time characteristics of H-FeP (1:2) for the stability test at an overpotential of 150 mV. (B) Polarization curves of the H-FeP (1:2) electrode before and after stability test.


20

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 1. Comparison of HER properties for FeP nanoparticles before and after hydrogenation. FeP (1:1) FeP (1:2) FeP (1:3) FeP (1:4) 10 / mA cm-2

Tafel slope / mVdec-1

Rct / 

ESCA / s cm-2 S-1

Pristine

207

154

157

166

Hydrogenated

189

145

147

192

Pristine

82

64

65

69

Hydrogenated

82

64

59

65

Pristine

29.2

3.98

4.52

5.73

Hydrogenated

18.71

3.18

4.50

4.93

Pristine

0.0010

0.0043

0.0025

0.0016

Hydrogenated

0.0018

0.0036

0.0027

0.0007


21

ACS Catalysis

Table of Contents Graphic

0 FeP nanoparticles

-2

-20

j / mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 22

-40 -60 Improved activity after long-term HER reaction

-80 -100

-0.3

-0.2

-0.1

0.0


Enhanced activity in hydrogen evolution reaction (HER) is observed for FeP nanoparticles after the stability test due to the reduced surface charge transfer resistance in the HER process.


22

Electrochemical Activity of Iron Phosphide Nanoparticles in Hydrogen

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