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Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting
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Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting Chi Zhang, Yunchao Xie, Heng Deng, Cheng Zhang, Jheng-Wun Su, Yuan Dong, Jian Lin* Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO 65211, United States
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abstract
Article history:
Electrochemical water splitting is a promising technology for mass hydrogen production.
Received 2 November 2017
Efficient, stable, and cheap electrocatalysts are keys to realizing this strategy. However,
Received in revised form
high price and preciousness of commonly used noble metal based catalysts severely hinder
20 February 2018
this realization. Herein, we report nickel iron phosphide (Ni-FexP) bifunctional electro-
Accepted 22 February 2018
catalyst via the in-situ growth of NieFe(OH)x on nickel foam (NieFe(OH)x/NF) followed by
Available online xxx
low-temperature phosphidation. As a hydrogen evolution reaction (HER) catalyst, the NiFexP/NF only needs an overpotential of 119 mV to drive a current density of 10 mA/cm2 in
Keywords:
a base media. It also shows excellent activity toward oxygen evolution reaction (OER) with
Nickel iron phosphide
low overpotentials of 254 mV, 267 mV, and 282 mV at 50, 100 and 200 mA/cm2, respectively.
HER
Moreover, when this bifunctional catalyst is used for overall water splitting, a low cell
OER
voltage of 1.62 V is needed to deliver a current density of 10 mA/cm2, which is superior to
Overall-water splitting
commercial electrolyzer and it also shows remarkable stability.
Electrocatalysis
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to continuous growth of global energy demand and environmental concerns, in the past decades, great efforts have been devoted to explore clean and renewable energy sources to replace fossil fuel [1]. Hydrogen has been widely regarded as a suitable candidate because of its high density, high energy conversion efficiency, and environmental friendliness [2]. Electrochemical water splitting (2H2O / 2H2 þ O2) is a promising way to realize a mass production of pure molecular hydrogen without carbon emission [3,4]. At present, RuO2 and Pt/C are benchmark catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. However, the high price, poor stability,
and scarcity severely restrict their potentials [5,6]. Therefore, it is highly desirable and imperative to develop earthabundant and efficient OER and HER catalysts. In recent years, transitional metal sulfides, selenides, carbides, nitrides, phosphides, and phosphates have been widely surveyed as water splitting electrocatalysts [7e16]. Among them, transitional metal phosphides have attracted great attention because of their high catalytic activity and earth abundance [17e19]. Since Sun's group reported the topotactic fabrication of free-standing CoP nanowire arrays on carbon paper with excellent HER activity in acid, neutral, and basic electrolytes [20], much exciting progress toward various metal phosphides such as Ni2P [21,22], MoP [23], and WP [24] have been achieved. Very recently, experimental and theoretical studies have confirmed that the existence of P atom can
* Corresponding author. E-mail address: [email protected] (J. Lin). https://doi.org/10.1016/j.ijhydene.2018.02.157 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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significantly reduce the absolute value of Gibbs free energy of hydrogen adsorption (DG H), which is thought to be beneficial to enhance the catalytic performance toward HER [25]. Moreover, easily formed P-defects can balance binding intensity between intermediates (*OOH and *O) and catalytic sites to decrease the free-energy barrier [26], thus enhancing the OER activity. On the other hand, many studies have proved that bimetallic based compounds with certain metal ratios usually exhibit better catalytic performance than their corresponding monometallic counterparts [27e30]. For instance, Husam's group reported NiCoP as a superior bifunctional catalyst towards HER and OER in alkaline media [10]. The experimental and theoretical results suggest that the electronic structure of the monometallic phosphide was altered by introducing extrinsic metals to dramatically enhance the catalytic activity. Among different metal configurations, the synergistic effect between Ni and Fe has drawn extensive attention because researchers find out that even a trace amount of Fe incorporation can greatly improve the turn over frequency of nickel based compounds and then dramatically enhance their OER performance [31]. Although the exact mechanism of this synergistic effect is still under debate, the strategy has been widely deployed in different material systems [32e35]. For instance, Yang’ group demonstrated that by the introduction of Fe, the HER catalytic activity and stability of iron-nickel sulfide are greatly improved in acidic solution [27]. Recent work shows that Fe doped Ni2P as an OER catalyst is superior than pure Ni2P. It results in an overpotential of 215 mV when current density is 50 mA/cm2 [36]. Very recently, Ahn and Mathiram adopted a post-MOF conversion strategy to synthesize amorphous carbon incorporated porous NieFe phosphide nanorod on nickel foam (porous NieFeeP@C/NF) through several synthesis steps [37]. However, the MOF treatment is necessary to obtain porous structure and boost the catalytic performance. Despite these progress, to the best of our knowledge, there is little report on Ni incorporated Fe2P dominated bimetallic compounds as a single-functional electrocatalyst let alone demonstrating their bifunctional HER and OER activities. In this paper, we reported bimetallic NieFe phosphide (Ni-FexP) directly synthesized on a nickel foam via in situ growth of NieFe(OH)x on nickel foam by coprecipitation and followed by a low temperature phosphidation. The synthesized Ni-FexP acts as a highly efficient bifunctional catalyst towards HER and OER in alkaline solution. It affords an overpotential of 176 mV at 50 mA/cm2 for HER and an overpotential of 254 mV at 50 mA/cm2 for OER, respectively. Finally, a typical two-electrode full-cell alkaline water electrolyzer based on this bifunctional electrocatalyst was demonstrated. It shows an overpotential of 1.62 V at a current density of 10 mA/cm2 and an extraordinary stability of more than 140 h at a constant current density of 10 mA/cm2.
reported wet chemistry method with a minor modification [12]. In a typical synthesis, 0.022 g trisodium citrate and 0.3336 g FeSO4$7H2O were added into a 250 mL flask containing 120 mL deionized water. After vigorously stirring, a clear light green solution was formed. A piece of nickel foam (2 cm 1 cm) was washed with 3 M HCl, ethanol, and deionized water several times. Then the pre-cleaned nickel foam was immersed into the prepared solution and heated at 90 C for 2 h in an oil bath with a slow stirring. After cooling down to the room temperature naturally, the precursor was taken out and washed with deionized water and ethanol. To prepare Ni-FexP/NF, a well known low-temperature phosphidation method was adopted [20,28,38]. To be specific, the obtained NieFe(OH)x/NF and 0.4 g NaH2PO2 were placed at two positions of a porcelain boat with a distance of ~5 cm in between and then heated at 300 C for 2 h under a static argon atmosphere. After cooling down to room temperature naturally, the Ni-FexP/NF was taken out and ready for test. The mass loading is around 8 mg/cm2 based on the mass change and the elements ratio. As a comparison, iron foam supported iron phosphide (FexP/IF) was prepared through the same procedure. It affords an overpotential of 213 mV at 50 mA/cm2 for HER and an overpotential of 360 mV at 50 mA/ cm2 for OER, respectively (Figs. 2e3). The results further demonstrated that the increase of catalytic activity was caused by the introduction of nickel.
Characterization X-ray diffraction (XRD) patterns were carried out on a Rigaku X-ray diffractometer with a Cu Ka radiation (l ¼ 0.15406 nm) to explore crystalline property. The surface morphology was observed with Hitachi S-4800 field emission scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) and EDS elemental mapping were performed with an EDS detector equipped in the Hitachi S-4800 FESEM. High-
Experimental Fabrication of Ni-FexP/NF All chemicals were used as received without any further purification. The NieFe(OH)x/NF was prepared via a previously
Fig. 1 e (a) Schematic illustration of Ni-FexP/NF. (b) SEM images of (b, c) NieFe(OH)x/NF and (d, e) Ni-FexP/NF (f) EDS elemental mappings of Fe, Ni, and P in Ni-FexP/NF. (g) High-resolution TEM image of Ni-FexP/NF.
Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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Fig. 2 e HER in 1 M KOH. (a) iR-corrected polarization curves of NF, NieFe(OH)x/NF, Ni-FexP/NF, Pt/C and FexP/IF recorded at a scan rate of 2 mV/s. (b) Polarization curves-derived Tafel slopes for the corresponding electrocatalysts. (c) Nyquist plots of NF, NieFe(OH)x/NF, Ni-FexP/NF and FexP/IF. (d) Polarization curves of the Ni-FexP/NF before and after 1000 cycles of CV scans. Inset: time dependence of the potential at a constant current density of 10 mA/cm2 for Ni-FexP/NF.
Fig. 3 e OER in 1 M KOH. (a) iR-corrected polarization curves of NF, NieFe(OH)x/NF, Ni-FexP/NF, RuO2 and FexP/IF recorded at a scan rate of 2 mV/s. (b) Polarization curves-derived Tafel slopes for the corresponding electrocatalysts. (c) Nyquist plots of NF, NieFe(OH)x/NF, Ni-FexP/NF and FexP/IF. (d) Polarization curves of the Ni-FexP/NF before and after 1000 cycles of CV scans. Inset: time dependence of the potential at a constant current density of 10 mA/cm2 for Ni-FexP/NF.
Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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resolution transmission electron microscopy (HRTEM) were recorded on a JEOL-2100F microscope. HRTEM was analyzed using a digital micrograph software (Gatan Inc.). X-ray photoelectron spectroscopy (XPS) measurement was performed on a PHI Quantera equipped with an Al Ka (1253.6 eV) radiation.
Electrochemical measurements All electrochemical measurements were conducted by a CHI 708E electrochemical workstation (CH instruments, Inc.,). A standard three electrode setup was used, including a piece of Ni-FexP/NF as the working electrode, an alkaline/mercurous oxide electrode (Hg/HgO) as the reference electrode, and graphite rod as the counter electrode. The Hg/HgO was calibrated with respect to reversible hydrogen electrode (RHE), as shown in Fig. S6. Linear sweep voltammetry (LSV) measurements were conducted in 1 M KOH with a scan rate of 2 mV/s to obtain the polarization curves of HER and OER, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed at overpotentials of 200 mV and 250 mV in a frequency range from 0.01 Hz to 100000 Hz at an AC amplitude of 5 mV for HER and OER, respectively, Chronopotentiometric measurements were performed at a current density of 10 mA/ cm2 to evaluate the long-term stability. An accelerated degradation test was conducted at a scan rate of 100 mV/s. Pt/ C and RuO2 inks were prepared by dispersing 4 mg commercialized Pt/C (20 wt% Pt) or RuO2 purchased from Alfa Aesar in a solution which contains 750 mL deionized water, 250 mL ethanol, and 30 mL 5 wt% Nafion (Alfa Aesar). Then the sonicated solution was casted onto a nickel foam (1 cm 1 cm) with the same mass loading of Ni-FexP/NF and air-dried at the room temperature.
Results and discussion As shown in Fig. 1a, synthesis of Ni-FexP on a nickel foam (NiFexP/NF) starts from in situ growth of the NieFe(OH)x on NF (NieFe(OH)x/NF) [12], followed by conversion of the Nie Fe(OH)x/NF via a low-temperature topotactic phosphidation [20,28,38]. In the final free-standing active material, the nickel foam was used as a conductive substrate. During the synthesis it also serves as the nickel source. First, Fe2þ is oxidized to Fe3þ with the assistance of O2 and H2O and then Fe3þ oxidizes the NF to obtain Ni2þ. Then both of them are coprecipitated on the surface of NF to achieve NieFe(OH)x/NF. The phosphidation of the as-synthesized NieFe(OH)x was achieved by the PH3, a thermal decomposition product of NaH2PO2. In the process, after the in situ growth of the Nie Fe(OH)x on NF, the color of NF is changed from silver-white to brown-yellow; then further changed to black after phosphidation (Fig. S1). The morphology and structures of the synthesized materials were first characterized by scanning electron microscopy (SEM). Fig. 1b shows that the NF is uniformly covered by the NieFe(OH)x which is composed of nanosheets (Fig. 1c). Micro cracks caused by the redox-etching are well observed, a common phenomenon when using NF as substrates for preparing self-standing electrocatalysts [6,10,12]. After phosphidation, the whole structure becomes
rougher as shown in Fig. 1d and Fig. S3. The nanostructures are transformed into interconnected porous structure (Fig. 1e). Similar morphology and structure transitions have been previously reported [37,39]. This high porous feature facilitates the mass transport of reactants, intermediates, and the final products, promoting the catalytic performance. The EDS elemental mapping results reveal that Ni, Fe, and P atoms with an atomic ratio of 4.51%, 38.87%, and 56.63% are uniformly distributed on the NF (Fig. 1f). This atomic percentage indicates that Fe is the dominant metal element and Ni is an extrinsic metal element in the final bimetallic NieFe phosphide. To get insight into the structures of the material, highresolution transmission electron microscopy (HRTEM) was performed (Fig. 1g). Through thorough investigation, the obtained HRTEM image of synthesized Ni-FexP/NF shows dominant lattice fringes with an interplane spacing of 0.221 nm, which can be assigned to the (111) plane of Fe2P. Its slightly smaller spacing compared with pure Fe2P can be attributed to the smaller atomic radius of substituted nickel atoms than that of iron atoms in the Ni-FexP/NF. The existence of dominant Fe2P is further proved by the XRD patterns. As shown in Fig. S4, the diffraction peaks of as-synthesized Ni-FexP/NF appear at 40.27 , 47.28 and 52.91 which can be indexed to (111), (210), and (002) planes of Fe2P (PDF#85-1725), respectively. To demonstrate the high-efficiency bifunctional electrocatalysts, the HER performance of Ni-FexP/NF was first assessed in 1 M KOH using a standard three-electrode setup. As control samples, bare NF, NieFe(OH)x/NF, commercial Pt/C and FexP/IF were also examined. The potentials were measured versus Hg/HgO reference electrode for all experimental tests and they are reported versus reversible hydrogen electrode (RHE). As expected, Pt/C shows an outstanding electrochemical activity towards HER with nearly zero onset potential while bare NF shows a poor HER activity as shown in the polarization curves (Fig. 2a). In most cases, the overpotential (h10), the potential required to deliver a current density of 10 mA/cm2, is viewed as a key parameter to characterize catalytic performance. Fig. 2a shows that h10 of Pt/C and Ni-FexP/NF is 20 mV and 119 mV, respectively. In contrast, 127 mV, 170 mV and 250 mV are resulted for FexP/IF, NieFe(OH)x/NF and NF to yield the same current density, suggesting that Ni-FexP/NF has much superior electrochemical activity. It is worth noting that as the current rate increases the overpotential of the Ni-FexP/NF (e.g. 190 mV at 80 mA/cm2) is getting closer to that of Pt/C (e.g. 168 mV at 80 mA/cm2), suggesting that the Ni-FexP/NF has less ratelimited production than Pt/C. This trend is the same to the case of Ni-FexP/NF and FexP/IF. It is either better than or comparable to recently demonstrated metal phosphides HER catalysts (Table S1) [17,20,37,40e45]. The Tafel slopes derived from the respective polarization curves are shown in Fig. 2b to explore the reaction mechanism of Ni-FexP/NF. The Tafel slope (bf) of Pt/C is 30 mV/dec while Ni-FexP/NF exhibits a Tafel slope of 80 m/dec which is smaller than those of FexP/IF, Nie Fe(OH)x/NF and NF. The theoretical bf values of Volmer, Heyrovsky and Tafel reactions are supposed to be 120 mV/dec, 40 mV/dec and 30 mV/dec, respectively [7]. Since the Tafel slope of Ni-FexP/NF lies in between of 40 mV/dec and 120 mV/ dec, it indicates that the Ni-FexP/NF proceeds a VolmerHeyrovsky mechanism.
Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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Fig. 4 e (a) XPS survey, (bed) High-resolution XPS spectra of Fe 2p, Ni 2p and P 2p peaks for the Ni-FexP/NF before and after the OER tests.
In order to reveal the effect of the surface area in the catalytic performance, the electrochemical active surface area (ECSA) was estimated by evaluating the electrochemical double layer capacitance (Cdl), as Cdl is linearly proportional to the ECSA. As shown in Fig. S5, Ni-FexP/NF has much larger Cdl (339 mF/cm2) than NieFe(OH)x/NF (138 mF/cm2), indicating that more active sites are created after phosphidation. Nyquist plots obtained from the electrochemical impedance spectroscopy show that compared with FexP/IF, NieFe(OH)x and NF, Ni-FexP/NF has much smaller charge transfer resistance, indicating that after phosphidation and the nickel incorporation, the charge transport kinetics of Ni-FexP/NF has been greatly enhanced. This is also in good agreement with the results shown in Fig. 2aeb. The HER stability of Ni-FexP was explored by continuous cyclic voltammetry (CV) scanning between 0.05 and 0.25 V at a scan rate of 100 mV/s. There is no obvious change between initial polarization curve and final one after 1000 cycles, indicating its good stability. The longterm stability of the Ni-FexP/NF under a constant current density of 10 mA/cm2 was tested (inset of Fig. 2d). The increase of the overpotential after test is only 37 mV, suggesting it can maintain high catalytic activity for at least 30 h. We further measured the OER activities of the Ni-FexP/NF in 1 M KOH. As control samples, NF, NieFe(OH)x/NF, benchmark RuO2 and FexP/IF were tested for comparison. As presented in Fig. 3a, RuO2 exhibited great OER activity, showing an overpotential of 286 mV at 100 mA/cm2, whereas NF shows a negligible catalytic performance toward OER. But the Ni-
FexP/NF only requires an overpotential of 267 mV to achieve the same current density, and the potential only increases to 282 mV at 200 mA/cm2, indicating it is superior to RuO2 in the OER activity. Moreover, it is very clear that the onset overpotential of Ni-FexP is much smaller than that of NieFe(OH)x/ NF and FexP/IF. The overpotentials required to achieve 50, 100 and 200 mA/cm2 for NieFe(OH)x/NF are 297 mV, 314 mV and 336 mV, respectively. The Tafel slopes are 100, 85, 47, 44, and 37 mV/dec for NF, FexP/IF, NieFe(OH)x/NF, RuO2 and Ni-FexP/ NF, respectively. Obviously, the Ni-FexP/NF exhibits the smallest Tafel slope, implying favorable reaction kinetics for the OER. In addition, its long-term stability was also assessed. As shown in Fig. 4d, after 1000 cycles the polarization curves show negligible loss in the current density. An electrolysis experiment at a fixed current density of 10 mA/cm2 further confirmed the superior durability of Ni-FexP/NF (inset of Fig. 4d). The good OER performance of Ni-FexP/NF can be attributed to the synergistic interaction between Ni and Fe in the phosphide and porous structure after thermal phosphidation by providing more active sites as revealed by the ECSA (Fig. S5) [46]. It should be noted that for bifunctional catalysts, the ECSAs for HER and OER are different and the value of Cdl also highly depends on the scan rates [10]. Recently, researchers showed that some non-oxidebased OER catalysts such as metal selenides, sulfides and phosphides were transformed into corresponding metal oxide/hydroxide during catalysis test [9,10,47,48]. Thus, Xray photoelectron spectroscopy (XPS) measurements of Ni-
Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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test, the peaks of 128.1 eV and 132.0 eV are ascribed to the binding energies of phosphides and phosphates (PeO species), respectively. After OER test, the only peak of P 2p at 133.4 eV appears, which can be assigned to phosphates. These observations indicate that after OER test the surface of the Ni-FexP was oxidized to form a (hydro)oxides and phosphates overlayer [10,51]. Inspired by the superior HER and OER performance of NiFexP/NF, Ni-FexP/NF was used as both a cathode and an anode to build a full-cell setup for water splitting in 1.0 M KOH. Fig. 5a shows polarization curves of Ni-FexP/NFjjNi-FexP/NF electrolyzer recorded at 2 mV/s. Remarkably, this system requires only an overpotential of 1.62 V to reach a current density of 10 mA/cm2, which is very close to that of a full cell built with precious metal based electrocatalysts (Pt/CjjRuO2,~1.57 V) [52] and is better than or comparable to the previously reported transition-metal-based bifunctional catalysts (Table S2) [10,12,29,33,37,45,52e55]. Furthermore, a long-term stability has been tested at a constant current density of 10 mA/cm2. As shown in Fig. 5b even after 140 h, it still can retain high catalytic activity with a slight increase in the potential, indicating its superior stability.
Conclusions
Fig. 5 e (a) Polarization curve of water electrolysis for NiFexP/NFjjNi-FexP/NF full cell at a scan rate of 2 mV/s in 1.0 M KOH. (b) Chronopotentiometric curve of water electrolysis for Ni-FexP/NFjjNi-FexP/NF full at a constant current density of 10 mA/cm2 at room temperature.
FexP/NF before and after OER catalysis were performed to probe the surface composition and the element chemical states. As shown in the XPS survey spectra, Ni-FexP/NF consists of Fe, Ni, P, O, and C before and after OER test (Fig. 4a). The C element could result from contaminants of carbon containing compounds. In the high-resolution XPS spectra of Fe 2p3/2 peaks before OER test (Fig. 4b), the peak centering at 707.3 eV can be ascribed to Fedþ (0 < d < 3) in NiFexP. Meanwhile, another two peaks centering at 710.0 eV and 712.2 eV can be attributed to Fe2þ and Fe3þ, respectively [49]. However, after OER test (Fig. 4b), the Fe 2p3/2 only displays one peak at 710.5 eV, indicating full surface oxidation of iron species. Before OER test, the Ni 2p3/2 spectrum of Ni-FexP/NF (Fig. 4c) displays two peaks centering at 855.6 eV and 852.5 eV, respectively. The former peak corresponds to the reduced Nidþ (0 < d < 2) and the latter one is assigned to NieP in Ni-FexP [50]. After OER test (Fig. 4c), apart from the disappearance of the peak of 852.5 eV, the peak of Ni 2p3/2 shifts to 855.6 eV compared with 857.0 eV, which may be caused by electron density transfer from Ni to P. In the P 2p spectra (Fig. 4d), before OER
In this work, self-supported Ni-FexP were successfully synthesized via in situ growth of NieFe(OH)x on Ni foam and followed by the topotactic phosphidation. We demonstrate that Ni-FexP/NF shows superior catalytic activity and excellent stability as both a HER and an OER electrocatalyst in an alkaline electrolyte. It exhibits an overpotential of 191 mV at a current density of 80 mA/cm2 for HER and 267 mV at a current density of 100 mA/cm2. It affords an overpotential of 254 mV at 50 mA/cm2 for OER. The built full cell based on this bifunctional catalyst exhibits an overpotential of 1.62 V and stability of over 140 h. The high catalytic performance of the Ni-FexP/ NF can be attributed to following reasons: (1) In situ growth on Ni foam circumvents the utilization of a binder and conductive agent which has been demonstrated to increase the “dead volume” so that the flow of electrons between the Ni foam and Ni-FexP/NF is improved; (2) The synergistic interaction between Ni and Fe in the phosphide greatly promotes the catalytic activity; (3) Ni-FexP/NF has good electronic conductivity and high density of active sites.
Acknowledgement The work was supported by NASA Missouri Space Consortium (Project: 00049784), NSF IGERT program (Award number: 1069091), United States Department of Agriculture (Award: 2018-67017-27880), and NSF SBIR program (Award number: 1648003).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.02.157.
Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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[46] Xu B, Chen Z, Zhang H, Sun Y, Li C, Novel Ni. (S0. 49Se0. 51) 2 porous flakes array on carbon fiber cloth for efficient hydrogen evolution reaction. Int J Hydrogen Energy 2017;42:30119e25. [47] Xu X, Song F, Hu X. A nickel iron diselenide-derived efficient oxygen-evolution catalyst. Nat Commun 2016;7. [48] Wang Z, Li J, Tian X, Wang X, Yu Y, Owusu KA, et al. Porous nickeleiron selenide nanosheets as highly efficient electrocatalysts for oxygen evolution reaction. ACS Appl Mater Interfaces 2016;8:19386e92. [49] Fujii T, de Groot FMF, Sawatzky GA, Voogt FC, Hibma T, Okada K. In situ XPS analysis of various iron oxide films grown by NO2-assisted molecular-beam epitaxy. Phys Rev B 1999;59:3195e202. [50] Gong M, Li Y, Wang H, Liang Y, Wu JZ, Zhou J, et al. An advanced NieFe layered double hydroxide electrocatalyst for water oxidation. J Am Chem Soc 2013;135:8452e5. [51] Chang J, Xiao Y, Xiao M, Ge J, Liu C, Xing W. Surface oxidized cobalt-phosphide nanorods as an advanced oxygen evolution catalyst in alkaline solution. ACS Catal 2015;5:6874e8. [52] Zhu W, Yue X, Zhang W, Yu S, Zhang Y, Wang J, et al. Nickel sulfide microsphere film on Ni foam as an efficient bifunctional electrocatalyst for overall water splitting. Chem Commun 2016;52:1486e9. [53] Jiang N, You B, Sheng M, Sun Y. Electrodeposited cobaltphosphorous-derived films as competent bifunctional catalysts for overall water splitting. Angew Chem 2015;127:6349e52. [54] Luo J, Im J-H, Mayer MT, Schreier M, Nazeeruddin MK, Park N-G, et al. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 2014;345:1593e6. [55] Chen P, Xu K, Tao S, Zhou T, Tong Y, Ding H, et al. Phasetransformation engineering in cobalt diselenide realizing enhanced catalytic activity for hydrogen evolution in an alkaline medium. Adv Mater 2016;28:7527e32.
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Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting Chi Zhang, Yunchao Xie, Heng Deng, Cheng Zhang, Jheng-Wun Su, Yuan Dong, Jian Lin* Department of Mechanical & Aerospace Engineering, University of Missouri, Columbia, MO 65211, United States
article info
abstract
Article history:
Electrochemical water splitting is a promising technology for mass hydrogen production.
Received 2 November 2017
Efficient, stable, and cheap electrocatalysts are keys to realizing this strategy. However,
Received in revised form
high price and preciousness of commonly used noble metal based catalysts severely hinder
20 February 2018
this realization. Herein, we report nickel iron phosphide (Ni-FexP) bifunctional electro-
Accepted 22 February 2018
catalyst via the in-situ growth of NieFe(OH)x on nickel foam (NieFe(OH)x/NF) followed by
Available online xxx
low-temperature phosphidation. As a hydrogen evolution reaction (HER) catalyst, the NiFexP/NF only needs an overpotential of 119 mV to drive a current density of 10 mA/cm2 in
Keywords:
a base media. It also shows excellent activity toward oxygen evolution reaction (OER) with
Nickel iron phosphide
low overpotentials of 254 mV, 267 mV, and 282 mV at 50, 100 and 200 mA/cm2, respectively.
HER
Moreover, when this bifunctional catalyst is used for overall water splitting, a low cell
OER
voltage of 1.62 V is needed to deliver a current density of 10 mA/cm2, which is superior to
Overall-water splitting
commercial electrolyzer and it also shows remarkable stability.
Electrocatalysis
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to continuous growth of global energy demand and environmental concerns, in the past decades, great efforts have been devoted to explore clean and renewable energy sources to replace fossil fuel [1]. Hydrogen has been widely regarded as a suitable candidate because of its high density, high energy conversion efficiency, and environmental friendliness [2]. Electrochemical water splitting (2H2O / 2H2 þ O2) is a promising way to realize a mass production of pure molecular hydrogen without carbon emission [3,4]. At present, RuO2 and Pt/C are benchmark catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. However, the high price, poor stability,
and scarcity severely restrict their potentials [5,6]. Therefore, it is highly desirable and imperative to develop earthabundant and efficient OER and HER catalysts. In recent years, transitional metal sulfides, selenides, carbides, nitrides, phosphides, and phosphates have been widely surveyed as water splitting electrocatalysts [7e16]. Among them, transitional metal phosphides have attracted great attention because of their high catalytic activity and earth abundance [17e19]. Since Sun's group reported the topotactic fabrication of free-standing CoP nanowire arrays on carbon paper with excellent HER activity in acid, neutral, and basic electrolytes [20], much exciting progress toward various metal phosphides such as Ni2P [21,22], MoP [23], and WP [24] have been achieved. Very recently, experimental and theoretical studies have confirmed that the existence of P atom can
* Corresponding author. E-mail address: [email protected] (J. Lin). https://doi.org/10.1016/j.ijhydene.2018.02.157 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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significantly reduce the absolute value of Gibbs free energy of hydrogen adsorption (DG H), which is thought to be beneficial to enhance the catalytic performance toward HER [25]. Moreover, easily formed P-defects can balance binding intensity between intermediates (*OOH and *O) and catalytic sites to decrease the free-energy barrier [26], thus enhancing the OER activity. On the other hand, many studies have proved that bimetallic based compounds with certain metal ratios usually exhibit better catalytic performance than their corresponding monometallic counterparts [27e30]. For instance, Husam's group reported NiCoP as a superior bifunctional catalyst towards HER and OER in alkaline media [10]. The experimental and theoretical results suggest that the electronic structure of the monometallic phosphide was altered by introducing extrinsic metals to dramatically enhance the catalytic activity. Among different metal configurations, the synergistic effect between Ni and Fe has drawn extensive attention because researchers find out that even a trace amount of Fe incorporation can greatly improve the turn over frequency of nickel based compounds and then dramatically enhance their OER performance [31]. Although the exact mechanism of this synergistic effect is still under debate, the strategy has been widely deployed in different material systems [32e35]. For instance, Yang’ group demonstrated that by the introduction of Fe, the HER catalytic activity and stability of iron-nickel sulfide are greatly improved in acidic solution [27]. Recent work shows that Fe doped Ni2P as an OER catalyst is superior than pure Ni2P. It results in an overpotential of 215 mV when current density is 50 mA/cm2 [36]. Very recently, Ahn and Mathiram adopted a post-MOF conversion strategy to synthesize amorphous carbon incorporated porous NieFe phosphide nanorod on nickel foam (porous NieFeeP@C/NF) through several synthesis steps [37]. However, the MOF treatment is necessary to obtain porous structure and boost the catalytic performance. Despite these progress, to the best of our knowledge, there is little report on Ni incorporated Fe2P dominated bimetallic compounds as a single-functional electrocatalyst let alone demonstrating their bifunctional HER and OER activities. In this paper, we reported bimetallic NieFe phosphide (Ni-FexP) directly synthesized on a nickel foam via in situ growth of NieFe(OH)x on nickel foam by coprecipitation and followed by a low temperature phosphidation. The synthesized Ni-FexP acts as a highly efficient bifunctional catalyst towards HER and OER in alkaline solution. It affords an overpotential of 176 mV at 50 mA/cm2 for HER and an overpotential of 254 mV at 50 mA/cm2 for OER, respectively. Finally, a typical two-electrode full-cell alkaline water electrolyzer based on this bifunctional electrocatalyst was demonstrated. It shows an overpotential of 1.62 V at a current density of 10 mA/cm2 and an extraordinary stability of more than 140 h at a constant current density of 10 mA/cm2.
reported wet chemistry method with a minor modification [12]. In a typical synthesis, 0.022 g trisodium citrate and 0.3336 g FeSO4$7H2O were added into a 250 mL flask containing 120 mL deionized water. After vigorously stirring, a clear light green solution was formed. A piece of nickel foam (2 cm 1 cm) was washed with 3 M HCl, ethanol, and deionized water several times. Then the pre-cleaned nickel foam was immersed into the prepared solution and heated at 90 C for 2 h in an oil bath with a slow stirring. After cooling down to the room temperature naturally, the precursor was taken out and washed with deionized water and ethanol. To prepare Ni-FexP/NF, a well known low-temperature phosphidation method was adopted [20,28,38]. To be specific, the obtained NieFe(OH)x/NF and 0.4 g NaH2PO2 were placed at two positions of a porcelain boat with a distance of ~5 cm in between and then heated at 300 C for 2 h under a static argon atmosphere. After cooling down to room temperature naturally, the Ni-FexP/NF was taken out and ready for test. The mass loading is around 8 mg/cm2 based on the mass change and the elements ratio. As a comparison, iron foam supported iron phosphide (FexP/IF) was prepared through the same procedure. It affords an overpotential of 213 mV at 50 mA/cm2 for HER and an overpotential of 360 mV at 50 mA/ cm2 for OER, respectively (Figs. 2e3). The results further demonstrated that the increase of catalytic activity was caused by the introduction of nickel.
Characterization X-ray diffraction (XRD) patterns were carried out on a Rigaku X-ray diffractometer with a Cu Ka radiation (l ¼ 0.15406 nm) to explore crystalline property. The surface morphology was observed with Hitachi S-4800 field emission scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) and EDS elemental mapping were performed with an EDS detector equipped in the Hitachi S-4800 FESEM. High-
Experimental Fabrication of Ni-FexP/NF All chemicals were used as received without any further purification. The NieFe(OH)x/NF was prepared via a previously
Fig. 1 e (a) Schematic illustration of Ni-FexP/NF. (b) SEM images of (b, c) NieFe(OH)x/NF and (d, e) Ni-FexP/NF (f) EDS elemental mappings of Fe, Ni, and P in Ni-FexP/NF. (g) High-resolution TEM image of Ni-FexP/NF.
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Fig. 2 e HER in 1 M KOH. (a) iR-corrected polarization curves of NF, NieFe(OH)x/NF, Ni-FexP/NF, Pt/C and FexP/IF recorded at a scan rate of 2 mV/s. (b) Polarization curves-derived Tafel slopes for the corresponding electrocatalysts. (c) Nyquist plots of NF, NieFe(OH)x/NF, Ni-FexP/NF and FexP/IF. (d) Polarization curves of the Ni-FexP/NF before and after 1000 cycles of CV scans. Inset: time dependence of the potential at a constant current density of 10 mA/cm2 for Ni-FexP/NF.
Fig. 3 e OER in 1 M KOH. (a) iR-corrected polarization curves of NF, NieFe(OH)x/NF, Ni-FexP/NF, RuO2 and FexP/IF recorded at a scan rate of 2 mV/s. (b) Polarization curves-derived Tafel slopes for the corresponding electrocatalysts. (c) Nyquist plots of NF, NieFe(OH)x/NF, Ni-FexP/NF and FexP/IF. (d) Polarization curves of the Ni-FexP/NF before and after 1000 cycles of CV scans. Inset: time dependence of the potential at a constant current density of 10 mA/cm2 for Ni-FexP/NF.
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resolution transmission electron microscopy (HRTEM) were recorded on a JEOL-2100F microscope. HRTEM was analyzed using a digital micrograph software (Gatan Inc.). X-ray photoelectron spectroscopy (XPS) measurement was performed on a PHI Quantera equipped with an Al Ka (1253.6 eV) radiation.
Electrochemical measurements All electrochemical measurements were conducted by a CHI 708E electrochemical workstation (CH instruments, Inc.,). A standard three electrode setup was used, including a piece of Ni-FexP/NF as the working electrode, an alkaline/mercurous oxide electrode (Hg/HgO) as the reference electrode, and graphite rod as the counter electrode. The Hg/HgO was calibrated with respect to reversible hydrogen electrode (RHE), as shown in Fig. S6. Linear sweep voltammetry (LSV) measurements were conducted in 1 M KOH with a scan rate of 2 mV/s to obtain the polarization curves of HER and OER, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed at overpotentials of 200 mV and 250 mV in a frequency range from 0.01 Hz to 100000 Hz at an AC amplitude of 5 mV for HER and OER, respectively, Chronopotentiometric measurements were performed at a current density of 10 mA/ cm2 to evaluate the long-term stability. An accelerated degradation test was conducted at a scan rate of 100 mV/s. Pt/ C and RuO2 inks were prepared by dispersing 4 mg commercialized Pt/C (20 wt% Pt) or RuO2 purchased from Alfa Aesar in a solution which contains 750 mL deionized water, 250 mL ethanol, and 30 mL 5 wt% Nafion (Alfa Aesar). Then the sonicated solution was casted onto a nickel foam (1 cm 1 cm) with the same mass loading of Ni-FexP/NF and air-dried at the room temperature.
Results and discussion As shown in Fig. 1a, synthesis of Ni-FexP on a nickel foam (NiFexP/NF) starts from in situ growth of the NieFe(OH)x on NF (NieFe(OH)x/NF) [12], followed by conversion of the Nie Fe(OH)x/NF via a low-temperature topotactic phosphidation [20,28,38]. In the final free-standing active material, the nickel foam was used as a conductive substrate. During the synthesis it also serves as the nickel source. First, Fe2þ is oxidized to Fe3þ with the assistance of O2 and H2O and then Fe3þ oxidizes the NF to obtain Ni2þ. Then both of them are coprecipitated on the surface of NF to achieve NieFe(OH)x/NF. The phosphidation of the as-synthesized NieFe(OH)x was achieved by the PH3, a thermal decomposition product of NaH2PO2. In the process, after the in situ growth of the Nie Fe(OH)x on NF, the color of NF is changed from silver-white to brown-yellow; then further changed to black after phosphidation (Fig. S1). The morphology and structures of the synthesized materials were first characterized by scanning electron microscopy (SEM). Fig. 1b shows that the NF is uniformly covered by the NieFe(OH)x which is composed of nanosheets (Fig. 1c). Micro cracks caused by the redox-etching are well observed, a common phenomenon when using NF as substrates for preparing self-standing electrocatalysts [6,10,12]. After phosphidation, the whole structure becomes
rougher as shown in Fig. 1d and Fig. S3. The nanostructures are transformed into interconnected porous structure (Fig. 1e). Similar morphology and structure transitions have been previously reported [37,39]. This high porous feature facilitates the mass transport of reactants, intermediates, and the final products, promoting the catalytic performance. The EDS elemental mapping results reveal that Ni, Fe, and P atoms with an atomic ratio of 4.51%, 38.87%, and 56.63% are uniformly distributed on the NF (Fig. 1f). This atomic percentage indicates that Fe is the dominant metal element and Ni is an extrinsic metal element in the final bimetallic NieFe phosphide. To get insight into the structures of the material, highresolution transmission electron microscopy (HRTEM) was performed (Fig. 1g). Through thorough investigation, the obtained HRTEM image of synthesized Ni-FexP/NF shows dominant lattice fringes with an interplane spacing of 0.221 nm, which can be assigned to the (111) plane of Fe2P. Its slightly smaller spacing compared with pure Fe2P can be attributed to the smaller atomic radius of substituted nickel atoms than that of iron atoms in the Ni-FexP/NF. The existence of dominant Fe2P is further proved by the XRD patterns. As shown in Fig. S4, the diffraction peaks of as-synthesized Ni-FexP/NF appear at 40.27 , 47.28 and 52.91 which can be indexed to (111), (210), and (002) planes of Fe2P (PDF#85-1725), respectively. To demonstrate the high-efficiency bifunctional electrocatalysts, the HER performance of Ni-FexP/NF was first assessed in 1 M KOH using a standard three-electrode setup. As control samples, bare NF, NieFe(OH)x/NF, commercial Pt/C and FexP/IF were also examined. The potentials were measured versus Hg/HgO reference electrode for all experimental tests and they are reported versus reversible hydrogen electrode (RHE). As expected, Pt/C shows an outstanding electrochemical activity towards HER with nearly zero onset potential while bare NF shows a poor HER activity as shown in the polarization curves (Fig. 2a). In most cases, the overpotential (h10), the potential required to deliver a current density of 10 mA/cm2, is viewed as a key parameter to characterize catalytic performance. Fig. 2a shows that h10 of Pt/C and Ni-FexP/NF is 20 mV and 119 mV, respectively. In contrast, 127 mV, 170 mV and 250 mV are resulted for FexP/IF, NieFe(OH)x/NF and NF to yield the same current density, suggesting that Ni-FexP/NF has much superior electrochemical activity. It is worth noting that as the current rate increases the overpotential of the Ni-FexP/NF (e.g. 190 mV at 80 mA/cm2) is getting closer to that of Pt/C (e.g. 168 mV at 80 mA/cm2), suggesting that the Ni-FexP/NF has less ratelimited production than Pt/C. This trend is the same to the case of Ni-FexP/NF and FexP/IF. It is either better than or comparable to recently demonstrated metal phosphides HER catalysts (Table S1) [17,20,37,40e45]. The Tafel slopes derived from the respective polarization curves are shown in Fig. 2b to explore the reaction mechanism of Ni-FexP/NF. The Tafel slope (bf) of Pt/C is 30 mV/dec while Ni-FexP/NF exhibits a Tafel slope of 80 m/dec which is smaller than those of FexP/IF, Nie Fe(OH)x/NF and NF. The theoretical bf values of Volmer, Heyrovsky and Tafel reactions are supposed to be 120 mV/dec, 40 mV/dec and 30 mV/dec, respectively [7]. Since the Tafel slope of Ni-FexP/NF lies in between of 40 mV/dec and 120 mV/ dec, it indicates that the Ni-FexP/NF proceeds a VolmerHeyrovsky mechanism.
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Fig. 4 e (a) XPS survey, (bed) High-resolution XPS spectra of Fe 2p, Ni 2p and P 2p peaks for the Ni-FexP/NF before and after the OER tests.
In order to reveal the effect of the surface area in the catalytic performance, the electrochemical active surface area (ECSA) was estimated by evaluating the electrochemical double layer capacitance (Cdl), as Cdl is linearly proportional to the ECSA. As shown in Fig. S5, Ni-FexP/NF has much larger Cdl (339 mF/cm2) than NieFe(OH)x/NF (138 mF/cm2), indicating that more active sites are created after phosphidation. Nyquist plots obtained from the electrochemical impedance spectroscopy show that compared with FexP/IF, NieFe(OH)x and NF, Ni-FexP/NF has much smaller charge transfer resistance, indicating that after phosphidation and the nickel incorporation, the charge transport kinetics of Ni-FexP/NF has been greatly enhanced. This is also in good agreement with the results shown in Fig. 2aeb. The HER stability of Ni-FexP was explored by continuous cyclic voltammetry (CV) scanning between 0.05 and 0.25 V at a scan rate of 100 mV/s. There is no obvious change between initial polarization curve and final one after 1000 cycles, indicating its good stability. The longterm stability of the Ni-FexP/NF under a constant current density of 10 mA/cm2 was tested (inset of Fig. 2d). The increase of the overpotential after test is only 37 mV, suggesting it can maintain high catalytic activity for at least 30 h. We further measured the OER activities of the Ni-FexP/NF in 1 M KOH. As control samples, NF, NieFe(OH)x/NF, benchmark RuO2 and FexP/IF were tested for comparison. As presented in Fig. 3a, RuO2 exhibited great OER activity, showing an overpotential of 286 mV at 100 mA/cm2, whereas NF shows a negligible catalytic performance toward OER. But the Ni-
FexP/NF only requires an overpotential of 267 mV to achieve the same current density, and the potential only increases to 282 mV at 200 mA/cm2, indicating it is superior to RuO2 in the OER activity. Moreover, it is very clear that the onset overpotential of Ni-FexP is much smaller than that of NieFe(OH)x/ NF and FexP/IF. The overpotentials required to achieve 50, 100 and 200 mA/cm2 for NieFe(OH)x/NF are 297 mV, 314 mV and 336 mV, respectively. The Tafel slopes are 100, 85, 47, 44, and 37 mV/dec for NF, FexP/IF, NieFe(OH)x/NF, RuO2 and Ni-FexP/ NF, respectively. Obviously, the Ni-FexP/NF exhibits the smallest Tafel slope, implying favorable reaction kinetics for the OER. In addition, its long-term stability was also assessed. As shown in Fig. 4d, after 1000 cycles the polarization curves show negligible loss in the current density. An electrolysis experiment at a fixed current density of 10 mA/cm2 further confirmed the superior durability of Ni-FexP/NF (inset of Fig. 4d). The good OER performance of Ni-FexP/NF can be attributed to the synergistic interaction between Ni and Fe in the phosphide and porous structure after thermal phosphidation by providing more active sites as revealed by the ECSA (Fig. S5) [46]. It should be noted that for bifunctional catalysts, the ECSAs for HER and OER are different and the value of Cdl also highly depends on the scan rates [10]. Recently, researchers showed that some non-oxidebased OER catalysts such as metal selenides, sulfides and phosphides were transformed into corresponding metal oxide/hydroxide during catalysis test [9,10,47,48]. Thus, Xray photoelectron spectroscopy (XPS) measurements of Ni-
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test, the peaks of 128.1 eV and 132.0 eV are ascribed to the binding energies of phosphides and phosphates (PeO species), respectively. After OER test, the only peak of P 2p at 133.4 eV appears, which can be assigned to phosphates. These observations indicate that after OER test the surface of the Ni-FexP was oxidized to form a (hydro)oxides and phosphates overlayer [10,51]. Inspired by the superior HER and OER performance of NiFexP/NF, Ni-FexP/NF was used as both a cathode and an anode to build a full-cell setup for water splitting in 1.0 M KOH. Fig. 5a shows polarization curves of Ni-FexP/NFjjNi-FexP/NF electrolyzer recorded at 2 mV/s. Remarkably, this system requires only an overpotential of 1.62 V to reach a current density of 10 mA/cm2, which is very close to that of a full cell built with precious metal based electrocatalysts (Pt/CjjRuO2,~1.57 V) [52] and is better than or comparable to the previously reported transition-metal-based bifunctional catalysts (Table S2) [10,12,29,33,37,45,52e55]. Furthermore, a long-term stability has been tested at a constant current density of 10 mA/cm2. As shown in Fig. 5b even after 140 h, it still can retain high catalytic activity with a slight increase in the potential, indicating its superior stability.
Conclusions
Fig. 5 e (a) Polarization curve of water electrolysis for NiFexP/NFjjNi-FexP/NF full cell at a scan rate of 2 mV/s in 1.0 M KOH. (b) Chronopotentiometric curve of water electrolysis for Ni-FexP/NFjjNi-FexP/NF full at a constant current density of 10 mA/cm2 at room temperature.
FexP/NF before and after OER catalysis were performed to probe the surface composition and the element chemical states. As shown in the XPS survey spectra, Ni-FexP/NF consists of Fe, Ni, P, O, and C before and after OER test (Fig. 4a). The C element could result from contaminants of carbon containing compounds. In the high-resolution XPS spectra of Fe 2p3/2 peaks before OER test (Fig. 4b), the peak centering at 707.3 eV can be ascribed to Fedþ (0 < d < 3) in NiFexP. Meanwhile, another two peaks centering at 710.0 eV and 712.2 eV can be attributed to Fe2þ and Fe3þ, respectively [49]. However, after OER test (Fig. 4b), the Fe 2p3/2 only displays one peak at 710.5 eV, indicating full surface oxidation of iron species. Before OER test, the Ni 2p3/2 spectrum of Ni-FexP/NF (Fig. 4c) displays two peaks centering at 855.6 eV and 852.5 eV, respectively. The former peak corresponds to the reduced Nidþ (0 < d < 2) and the latter one is assigned to NieP in Ni-FexP [50]. After OER test (Fig. 4c), apart from the disappearance of the peak of 852.5 eV, the peak of Ni 2p3/2 shifts to 855.6 eV compared with 857.0 eV, which may be caused by electron density transfer from Ni to P. In the P 2p spectra (Fig. 4d), before OER
In this work, self-supported Ni-FexP were successfully synthesized via in situ growth of NieFe(OH)x on Ni foam and followed by the topotactic phosphidation. We demonstrate that Ni-FexP/NF shows superior catalytic activity and excellent stability as both a HER and an OER electrocatalyst in an alkaline electrolyte. It exhibits an overpotential of 191 mV at a current density of 80 mA/cm2 for HER and 267 mV at a current density of 100 mA/cm2. It affords an overpotential of 254 mV at 50 mA/cm2 for OER. The built full cell based on this bifunctional catalyst exhibits an overpotential of 1.62 V and stability of over 140 h. The high catalytic performance of the Ni-FexP/ NF can be attributed to following reasons: (1) In situ growth on Ni foam circumvents the utilization of a binder and conductive agent which has been demonstrated to increase the “dead volume” so that the flow of electrons between the Ni foam and Ni-FexP/NF is improved; (2) The synergistic interaction between Ni and Fe in the phosphide greatly promotes the catalytic activity; (3) Ni-FexP/NF has good electronic conductivity and high density of active sites.
Acknowledgement The work was supported by NASA Missouri Space Consortium (Project: 00049784), NSF IGERT program (Award number: 1069091), United States Department of Agriculture (Award: 2018-67017-27880), and NSF SBIR program (Award number: 1648003).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.02.157.
Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157
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Please cite this article in press as: Zhang C, et al., Ternary nickel iron phosphide supported on nickel foam as a high-efficiency electrocatalyst for overall water splitting, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.157