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Heterogeneous Bimetallic Phosphide/Sulfide Nanocomposite for Efficient Solar-EnergyDriven Overall Water Splitting Yanmei Xin,† Xiang Kan,‡ Li-Yong Gan,*,‡,§ and Zhonghai Zhang*,† †
School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai 200241, China Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu 610031, China § School of Material Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, China ‡
S Supporting Information *
ABSTRACT: Solar-driven overall water splitting is highly desirable for hydrogen generation with sustainable energy sources, which need efficient, earth-abundant, robust, and bifunctional electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, we propose a heterogeneous bimetallic phosphide/sulfide nanocomposite electrocatalyst of NiFeSP on nickel foam (NiFeSP/NF), which shows superior electrocatalytic activity of low overpotentials of 91 mV at −10 mA cm−2 for HER and of 240 mV at 50 mA cm−2 for OER in 1 M KOH solution. In addition, the NiFeSP/NF presents excellent overall water splitting performance with a cell voltage as low as 1.58 V at a current density of 10 mA cm−2. Combining with a photovoltaic device of a Si solar cell or integrating into photoelectrochemical (PEC) systems, the bifunctional NiFeSP/NF electrocatalyst implements unassisted solar-driven water splitting with a solar-to-hydrogen conversion efficiency of ∼9.2% and significantly enhanced PEC performance, respectively. KEYWORDS: electrochemistry, phosphosulfide, nanosheet, solar energy, water splitting
H
necessary and vital for circumambulating the thermodynamic uphill barrier and for expediting sluggish kinetics of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In addition, it is also essential for practical applications to simplify the setup with the same electrocatalysts (used as anode for OER and cathode for HER) with small overpotentials in the same electrolytes. At present, the noble metal Pt is still the state-of-the-art electrocatalyst for HER, and the noble metal oxide IrO2 is regarded as one of the most efficient electrocatalysts for OER in acidic electrolytes, but is not a perfect candidate in alkaline solution due to its slow anodic dissolution,8,9 while, very recently, the electrocatalyst NiFeOx has been proposed to be a state-of-the-art catalyst for OER in alkaline solution.10,11 However, the high cost and scarcity of noble-metal-based electrocatalysts and the relatively complicated setup with separated HER and OER cells (connected through an ion exchange membrane) seriously limit their implementation into large-scale applications. Motivated by addressing these
ydrogen (H2) is one of the most promising global molecules as fuels for fulfilling the clean and renewable energy demand of human society and as indispensable chemicals for petroleum refining and ammonia synthesizing.1,2 Given that more than 95% of hydrogen is produced through thermochemical catalytic steam methane reforming,3 which involves severe fossil energy carrier loss and vast greenhouse gas release, exploration of alternative pathways for H2 generation in a cost-effective manner is significantly desirable for reducing fossil fuel consumption and alleviating CO2 emissions. Currently, the most attractive strategy for hydrogen generation is solar-driven water splitting with solar energy as sustainable energy input and water as renewable and abundant hydrogen-containing resource.4 There are two styles for solar water splitting: (1) photovoltaic (PV)-assisted electrolytic water splitting and (2) photoelectrochemical (PEC) water splitting.5 According to a very recent outlook,6,7 the PV-assisted electrochemical water splitting is a promising strategy for efficient hydrogen generation, and it is easier to integrate with current industrial systems, such as electroplating and electrolysis, for practical and scalable applications. For efficient solar-energy-driven photovoltaic-assisted electrolytic water splitting, effective and robust electrocatalysts are © 2017 American Chemical Society
Received: July 17, 2017 Accepted: September 12, 2017 Published: September 12, 2017 10303
DOI: 10.1021/acsnano.7b05020 ACS Nano 2017, 11, 10303−10312
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Cite This: ACS Nano 2017, 11, 10303-10312
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Figure 1. DFT calculations of crystal structures of (a1) side view and (a2) top view of NiS, top views of (a3) NiFeS, (a4) NiSP, (a5) NiFeSP (I), and (a6) NiFeSP (II) with Ni-terminated (0001) surfaces (gray, yellow, red, and blue balls represent Ni, S, Fe, and P atoms, respectively); (b) band structure of P63/mmc NiS; (c) calculated Gibbs free energy diagrams of NiS, NiFeS, NiSP, Ni2P NiFeSP, and Pt for HER; (d) H2O adsorption energies on the NiS, NiFeS, NiSP, Ni2P, NiFeSP, and Pt surfaces.
mA cm−2) for HER and 240 mV (50 mA cm−2) for OER in the same alkaline media; in addition, the NiFeSP/NF also implements overall water splitting with a cell voltage as low as 1.58 V at a current density of 10 mA cm−2 with long-term stability, superior to most of the bifunctional electrocatalysts reported so far. Furthermore, the potential utilizations of this NiFeSP/NF electrocatalyst for solar-energy-driven overall water splitting have also been investigated with coupling of a commercially available Si solar cell and with integration into PEC systems, which contribute a stable photocurrent density of 7.5 mA cm−2 with a solar-to-hydrogen conversion efficiency of ∼9.2% and significantly enhanced PEC performance.
concerns, enormous efforts have been devoted to search for or design bifunctional and low-cost alternatives with earthabundant elements, including transition-metal phosphides, chalcogenides, oxides, carbides, borides, and nitrides.12−22 Particularly, phosphides and sulfides are receiving incredible attention, and a series of binary monometallic (Ni, Fe, Co, Cu, and Mo) phosphides and sulfides have emerged as interesting electrocatalysts with high catalytic activity.23−34 Furthermore, enlightened from the natural highly active [NiFe] hydrogenase for biological HER with bimetallic active centers,35,36 recently, ternary bimetallic phosphides and sulfides, such as NiCoP, NiCo2S4, and NiFeS,37−40 have been proposed to further enhance electrocatalytic performance due to the synergistic effect of the two components. On the contrary, we have noticed that another type of ternary electrocatalysts of monometallic phosphosulfides, such as MoP|S, CoS|P, FePS3, and CoPS,41−46 have also been proposed and demonstrate excellent electrocatalytic activity through crossing substitutions between phosphorus and sulfur. Inspired by both ternary electrocatalysts of bimetallic phosphides (sulfides) and monometallic phosphosulfides, we propose a nanocomposite electrocatalyst involving metal phosphides and sulfides, and the mutual substitution will further modify their electronic structure, tune the hydrogen adsorption free energy, and consequently dramatically enhanced electrocatalytic performance can be expected. Herein, a heterogeneous bimetallic phosphide/sulfide electrocatalyst of NiFeSP is rationally designed, which predicts that simultaneously formation of metal−P and metal−S bonds will optimize the electronic structure, balance surface free energy for reactant adsorption or desorption, and thus enhance overall water splitting activity. The NiFeSP on nickel foam (NiFeSP/NF) is fabricated through facile procedures and, as expected, presents effective bifunctional electrocatalytic activity with overpotentials of 91 mV (−10 mA cm−2, 150 mV at −50
RESULTS AND DISCUSSION DFT calculations have been first performed to determine if the formation of Ni−P and Fe−S bonds in pristine NiS would alter its electronic structure, surface adsorption energy of atomic hydrogen and molecular H2O, and thus the potential electrocatalytic water splitting activity. NiS in space group P63/mmc has a hexagonal structure, as shown in Figure 1(a1, a2), and presents a zero band gap, that is, a metallic nature (Figure 1b),47 which is favorable for fast electron transfer in electrocatalytic processes. The Gibbs free energy change of the adsorbed H* (|ΔGH*|), with * being the adsorption site, can be commonly used to evaluate the HER activity of catalysts.48 Thus, the Gibbs free energies for H adsorption (ΔGH*) on pristine and composite (0001) surfaces of NiS are calculated. We found that H adsorption on a S-terminated surface is endothermic. Thus, in the following only Ni-terminated surfaces are considered. As shown in Figure 1a, one topmost surface Ni/S atom is substituted by Fe and P atoms, named NiFeS (Figure 1a3) and NiSP (Figure 1a4), respectively. It is found that P dopant prefers to stay at inner layer sites. For the (Fe, P)-co-doped surface (named NiFeSP), two configurations are taken into 10304
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Figure 2. (a) XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image, and (e) HAADF-STEM and EDS elemental mapping images of NiFeSP.
doped surface, thus showing weaker adsorption. Therefore, the theory simulation results suggest that NiFeSP has potential to be a highly active electrocatalyst for water splitting. Encouraged by these promising theoretical results, we synthesized NiFeSP/NF samples through a three-step process (Figure S2), and the detailed experimental procedures can be found in Methods. Briefly, first, a cleaned NF was partially substituted by Fe in FeCl3 aqueous solution through an ionexchange process (Figure S3) with the sample color changing from silver to yellow (Figure S4); second, bimetallic sulfide NiFeS/NF (Figures S5 and S6) was synthesized through a hydrothermal method with thiourea as sulfur resource; third, the sulfur in NiFeS/NF was incompletely substituted with phosphorus in a high-temperature phosphodization process to obtain the final product, NiFeSP/NF. X-ray diffraction (XRD) technique was first employed to characterize the crystal structures of NiFeSP/NF, and the XRD patterns (Figure 2a) presented dominant crystal phases of NiS (JCPDS No. 862280), Ni2P (JCPDS No. 74-1385), and FeS (JCPDS No. 760965); in addition, some weak peaks were also detected and can be ascribed to Fe2P4O12 (JCPDS No. 76-0223) due to the inevitable oxidation of iron phosphides when exposed to air. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to depict the morphologies of NiFeSP/NF. As shown in Figure 2b, the SEM image of NiFeSP presented a graphene-like nanosheet structure with a rough and curved surface, the structure of which was inherited and topotactically converted from NiFeS (Figure S7). The TEM image in Figure 2c provides a clearer image of a graphene-like nanosheet structure with ultrathin thickness, and the high-resolution TEM (HRTEM) in Figure 2d shows wellresolved lattice fringes with an interplanar distance of 0.475 nm in the bulk phase, which is close to that in the (110) plane of pure NiS (0.480 nm) (Figure S8). The slight shrinkage can be ascribed to the substitution of iron and phosphorus into NiS. In
account, as displayed in Figure 1a5 and a6, and the former is energetically more favorable, with an attraction between Fe and P. That is, the surface Fe substitution is able to induce segregation of the inner P to the surface; accordingly, the former configuration is used to model H and H2O adsorption. Since HER includes the reversible adsorption and desorption of H, an ideal catalyst should neither bind H too strongly nor too weakly. This means |ΔGH*| should be close to thermoneutral, i.e., |ΔGH*| = 0.49 According to Figure 1c, ΔGH* of H adsorption on pristine NiS (0001) is −1.02 eV. Single doping of Fe or P increases and decreases the Gibbs free energy, respectively, but co-doping of Fe and P insignificantly lowers ΔGH* to −0.13 eV, which is close to that on Pt.50 In addition, the ΔGH* of H adsorption on Ni2P, a well-investigated HER electrocatalyst,36,51,52 is also calculated and presents a value of −0.29 eV, which further reveals the potential advantages of NiFeSP composites for good HER performance. Meanwhile, molecular H2O adsorption is also important for water dissociation on an electrocatalyst surface in alkaline media for HER and OER;53 thus H2O adsorption energy (ΔE) is further calculated on these surfaces, and the results are presented in Figure 1d. The overall trend is the same as that of ΔGH*, indicating higher and lower H2O binding activity on NiFeS and NiSP surfaces, respectively, compared with that on a NiS surface. The Ni2P shows a H2O adsorption energy of −0.22 eV, and the NiFeSP presents a medium H2O adsorption energy of −0.49 eV, more than twice that on Pt(111), which guarantees effective water adsorption. To further explore the binding strength on those surfaces, projected d-orbital density of states of metal atoms for H and H2O adsorption are calculated (Figure S1), and distinct peaks can be seen around the Fermi level in the Fe-doped case, rationalizing the strongest adsorption of H and H2O on this surface according to the dband theory.54 In contrast, the d-orbital of Fe in NiFeSP is relatively delocalized with respect to those of pristine and P10305
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Figure 3. Core-level XPS of (a) Ni 2p, (b) S 2p, (c) Fe 2p, and (d) P 2p of NiFeSP.
and Ni3+. These results are different from those in Ni2P, NiS, and NiFeS samples (Figure S11), where only Ni0, Ni2+, and Ni3+ can be detected, respectively, implying that the doping of P altered the Ni surrounding chemical environment. In addition, compared to NiFeS/NF samples, the Ni(III) 2p binding energy in NiFeSP/NF presented obvious positive shifts (857.1 eV in NiFeSP vs 855.8 eV in NiFeS for Ni(III) 2p3/2 and 874.8 eV in NiFeSP vs 873.3 eV in NiFeS for Ni(III) 2p1/2), which further indicated that the doping of P contributed more electrons due to its weaker electronegativity than S. The corelevel S 2p XPS of NiFeSP, as shown in Figure 3b, was deconvoluted into five peaks, and the strong peaks at 162.0 and 163.1 eV correspond to S 2p1/2 and S 2p3/2 of S2− with a satellite peak at 169.0 eV.59 In addition, another strong peak at 163.5 eV can be assigned to a characteristic bimetallic metal−S bond,40 and a very weak peak with an energy of 161.1 eV can be ascribed to the formation of an Fe−S bond.60 The Fe 2p profile (Figure 3c) was well fitted using photoelectron peaks at 706.0, 711.4, and 724.9 eV, corresponding to Fe(0), Fe(III) 2p3/2, and Fe(III) 2p1/2, respectively.60,61 In addition, two extra peaks with higher energy at 713.4 and 729.6 eV were part of the splitting peaks of Fe 2p3/2 and Fe 2p1/2, which implied the existence of iron with higher chemical states than Fe(III), which is in accordance with the XRD data with formation of Fe2P4O12. The core-level P 2p XPS spectrum is also displayed in Figure 3d, and two peaks at 130.3 and 132.4 eV can be ascribed to P 2p3/2 and P 2p1/2, respectively,62,63 which characterized the formation of metal−phosphide. Another strong peak at 134.7 eV could be ascribed to oxidized phosphorus arising from superficial oxidation of metal phosphides as a result of air contact.64 According to the above-mentioned XPS analysis, the NiFeSP sample at the nearsurface mainly comprised Ni2+, Ni3+, Fe3+, Fe6+, S2−, and P3−. The doping of P increased the electron density of the active center of Ni, which would be helpful for fast electron transfer, and the existence of Fe would contribute to retain the P on the surface rather than in the bulk.
addition, lattice fringes on the nanosheet edge with interplanar distances of 0.221 and 0.215 nm were also observed and correspond to the (111) plane of Ni2P and the (202) plane of FeS, respectively, which is in accord with XRD analysis. Furthermore, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and corresponding energy dispersive spectroscopy (EDS) elemental mapping images were further employed to investigate the elemental distribution in the NiFeSP nanosheet. As shown in Figure 2e, the elements Ni, Fe, S, and P are uniformly distributed on an individual nanosheet, and only small amounts of Fe and P with atomic ratios of 1.21% and 3.78%, respectively, were detected (Figure S9), which further suggested the successful implantation of Fe and P to form a heterogeneous phosphide/sulfide nanocomposite. X-ray photoelectron spectroscopy (XPS) technique was employed to determine the surface composition and chemical states of NiFeSP/NF. The XPS survey can be found in Figure S10, which indicates the existence of Ni, Fe, S, P, and O elements in the NiFeSP sample, and the atomic ratios of these elements are also presented. The P element presented a relatively higher atomic ratio in XPS analysis (16.31%) than that in EDS analysis (3.78%); on the contrary, the S element presented a relatively lower atomic ratio in XPS analysis (2.54%) than that in EDS analysis (39.85%), which suggested that most of the P element was distributed on the top surface of the catalyst in Ni2P form, while most of the S element was stored in the bulk phase of Ni2S; these results have also been revealed in the HRTEM image. The core-level XPS of Ni 2p in NiFeSP is presented in Figure 3a, where two peaks with binding energies located at 853.6 and 870.8 eV can be assigned to Ni(II) 2p3/2 and Ni(II) 2p1/2, respectively.55,56 Another two peaks at 857.1 and 874.8 eV can be assigned to Ni(III) 2p3/2 and Ni(III) 2p1/2, respectively,57,58 and two extra shakeup satellite peaks were also detected with higher banding energy of 861.5 and 879.7 eV for Ni 2p3/2 and Ni 2p1/2, respectively,56 suggesting that the Ni in NiFeSP was in divalent states of Ni2+ 10306
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Figure 4. (a) iR-corrected HER polarization curves of NiFeSP/NF along with NF, NiS/NF, NiFeS/NF, NiSP/NF, and Pt/C for comparison; (b) HER Tafel plots for the corresponding electrocatalysts; (c) iR-corrected OER polarization curves of NF, NiS/NF, NiFeS/NF, NiSP/NF, NiFeSP/NF, and NiFeOx; (d) OER Tafel plots for the corresponding electrocatalysts; (e) electrochemical impedance spectroscopy (EIS) Nyquist plots of NF, NiS/NF, NiFeS/NF, NiSP/NF, and NiFeSP/NF, bottom inset is the enlarged Nyquist plots of NiSP/NF and NiFeSP/NF, and top inset is the fitted equivalent circuit; (d) long-term stability tests of NiFeSP/NF carried out for HER at a constant current density of −10 mA cm−2 and for OER at a constant current density of 50 mA cm−2.
with the increase of the overpotential, and that value fell within the range of 38−116 mV dec−1, suggesting that the HER taking place on the NiFeSP/NF surface would follow a Volmer− Heyrovsky mechanism.66 The OER performance on NiFeSP/NF was also investigated in the same alkaline solution of 1.0 M KOH along with NF, NiS/NF, NiFeS/NF, NiSP/NF, and NiFeOx/NF for comparison. Prior to water oxidation, the oxidation peaks at around 1.40 V on the NF-based electrodes can be attributed to the transformation of Ni2+ → Ni3+.67 As shown in Figure 4c, the iRcorrected OER polarization curves revealed excellent OER activity on NiFeS/NF and NiSP/NF, with low overpoentials of 260 and 280 mV, respectively, at an OER current density of 50 mA cm−2, which was superior to that on NiFeOx/NF, with an overpotential of 290 mV at 50 mA cm−2. Very interestingly, the NiFeSP/NF presented the best OER catalytic activity (with the same synthesis parameters as that for HER, Figure S13) among these electrocatalysts, with an overpotential of 240 mV at 50 mA cm−2. Figure 4d shows OER Tafel plots, and the Tafel slope for NiFeSP/NF was calculated to be 76.3 mV dec−1,
The HER performances of NF, NiS/NF, NiFeS/NF, NiSP/ NF, NiFeSP/NF, and Pt/C were evaluated and compared using a three-electrode system in 1.0 M KOH. As shown in Figure 4a, both NiFeS/NF and NiSP/NF samples presented improved HER activity to that on pristine NiS/NF. The chemical compositions of NiFeSP/NF samples were well adjusted through tuning the synthesis parameters (Figure S12), and as expected, the optimized NiFeSP/NF sample achieved an excellent HER activity with a low onset potential near 0 mV (onset potential, defined as the potential at −1 mA cm−2), and an overpotential as low as 94 mV at a current density of −10 mA cm−2 and 150 mV at −50 mA cm−2. Even though this value is higher than that on Pt/C (19 mV at −10 mA cm−2), it is still comparable to the state-of-the-art Pt-free HER catalysts in alkaline solution (Table S1). The Tafel plots of these electrocatalysts were calculated from polarization data and are presented in Figure 4b. Pt/C showed a Tafel slope of 37.2 mV dec−1, consistent with the reported values in the literature.36,65 The NiFeSP/NF presented a relatively small Tafel slope of 82.6 mV dec−1, which suggested a sudden rise of the HER velocity 10307
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Figure 5. (a) Overall electrocatalytic water splitting characteristics of NiFeSP/NF with two NiFeSP/NF electrodes. Inset: Digital photograph of the two-electrode configuration; (b) chronopotentiometric measurement of the overall water splitting at 10 mA cm−2; (c) schematic illustration of the solar-driven overall water splitting cell with a commercial planar Si solar cell; (d) chronoamperometric measurement of the solar-driven overall water splitting cell under chopped simulated sunlight (AM 1.5G); photoelectrochemial performance of NiFeSP deposited on (e) nanoporous BiVO4 photoanode for water oxidation and (f) Cu2O nanowires for water reduction under illumination of simulated sunlight.
which is lower than that on NiS/NF (89.6 mV dec−1), NiFeS/ NF (86.7 mV dec−1), and NiSP/NF (182.8 mV dec−1) and higher than the value of 45.8 mV dec−1 on NiFeOx/NF, further indicating excellent OER performance of NiFeSP/NF in alkaline solution (Table S2). For a more accurate comparison of the electrocatalytic activity, the surface areas of electroctalysts (NF, NiS/NF, NiSP/NF, NiFeS/NF, and NiFeSP/NF) have been measured with a nitrogen absorption method with the Brunauer− Emmett−Teller (BET) model and are presented in Figure S14 and Table S3. In addition, the mass loading of our electrocatalysts has also been measured after decomposing the nickel foam support in an acidic solution, and the mass of NiS, NiFeS, NiSP, and NiFeSP can be determined to be 4.0, 3.7, 4.5, and 4.2 mg (NF with a geometric area of 1.0 cm−2), respectively. From these results, the NiFeSP/NF did not
present a much higher BET surface area (even lower than NiS and NiFeS samples) and was also not overloaded, which indicated that the superior electrocatalytic performance of NiFeSP cannot be ascribed to the increasing surface area or overdose loading effects; it originated from its excellent electrocatalytic activity. The formation of metal−S and metal−P bonds resulted in an efficient bifunctional electrocatalyst of NiFeSP for both HER and OER, and the altering of the electronic structure from elemental substitution was further revealed through an electrochemical impedance spectroscopy (EIS) technique. As presented in Figure 4e, NiFeSP presented the smallest semicircular arc (bottom inset in Figure 4e), implying its best electron transfer activity. The EIS data of NiFeSP were well fitted, and the equivalent circuit (top inset in Figure 4e) was comprised of a series resistor (Rs), charge transfer resistance 10308
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efficiency of ∼9.2%. Our result suggested that simultaneous formation of metal−S and metal−P bonds could effectively alter the electronic structure and tune the reactant adsorption energy, thus dramatically enhancing the catalytic activity. Due to its facile and scalable synthetic procedures, this strategy might be versatile for enhancing electrocatalytic and photoelectrocatalytic activities for many other applications.
(RCT), constant phase element (CPE1, CPE2), and resistance (R). The NiFeSP showed a small CPE value of 0.28 Ssn and rapid charge transfer kinetics of 0.145 Ω between the electrode and the electrolyte, which can be ascribed to the formation of metal−P and metal−S bonds with metallic properties. In addition, a small Rs value of 0.995 Ω was also fitted and corresponded to the seamless contact between the electrocatalyst NiFeSP and the current collector NF. The long-term electrochemical stability of NiFeSP/NF was evaluated with chronopotentiometric measurements at a current density of −10 mA cm−2 for HER and 50 mA cm−2 for OER, respectively. As presented in Figure 4f, NiFeSP/NF exhibited an outstanding long-term stability for 25 h with insignificant voltage decay. The morphology, crystal structure, and elemental composition of NiFeSP/NF after long-term HER and OER measurements were characterized with SEM (Figure S15), XRD (Figure S16), and XPS (survey and corelevel XPS of Ni and Fe in Figure S17). The graphene-like nanosheet structure was well maintained, and multicrystal structures from NiS, Ni2P, FeS, and trace Fe2P4O12 were also detected; in addition, the elements Ni, Fe, O, S, and P can also all be detected from XPS analysis, and the chemical valence states of Ni and Fe presented insignificant changes. From the above data, we can identify that the NiFeSP/NF was an excellent bifunctional electrocatalyst for HER and OER, and two identical NiFeSP/NF electrodes were employed to set up an electrolysis cell for overall water splitting. The polarization curve of water electrolysis with a two-electrode configuration (inset in Figure 5a) presented a current density of 10 mA cm−2 with an external bias of 1.58 V (Figure 5a), which was superior to most previously reported results and reached the stat-of-the-art performance for bifunctional overall water splitting in alkaline solution (Table S4). The long-term durability of the two-electrode configuration for overall water splitting was also evaluated (Figure 5b) and maintained a voltage of 1.58 V at 10 mA cm−2 for 20 h. Finally, the capability of solar-energy-driven water splitting on bifunctional NiFeSP/NF was estimated after coupling with a commercial planar Si solar cell (Figure S18) and integrating into a PEC system.68 As shown in Figure 5c, with solar energy as the only energy input (AM 1.5G simulated sunlight), the solar cells provided 1.6 V bias on the two-electrode electrolysis cell and generated a photocurrent density of 7.5 mA cm−2 with the repeated on−off cycles of simulated solar light illumination for 60 min (Figure 5d), which yielded an excellent Faradaic efficiency of 98.7% (96.5% with anion exchange membrane) (Figure S19) and a high solar-to-hydrogen conversion efficiency of ∼9.2%. Furthermore, after deposition of NiFeSP on nanoporous BiVO4 as photoanode (Figure S20a) for PEC water oxidation (Figure 5e) and on Cu2O nanowires (Figure S20b) as photocathode for PEC water reduction (Figure 5f), both significantly enhance the PEC water splitting performance, which indicated the excellent bifunctional electroataytic activity of NiFeSP and enlarged its application range.
METHODS Chemicals and Materials. Nickel foam (99.6%, 40 × 10 × 1 mm3) and copper foam were purchased from Jinjia Metal, China. Hydrochloric acid (HCl), ethanol, acetone, iron(III) chloride (FeCl3), thiourea (CH4N2S), sodium hypophosphite (NaH2PO2), potassium hydroxide (KOH), sodium hydroxide (NaOH), potassium iodide (KI), nitric acid (HNO3), bismuth nitrate pentahydrate (Bi(NO3)3· 5H2O), p-benzoquinone, vanadyl acetylacetonate (VO(acac)2), and dimethyl sulfoxide (DMSO) were purchased from Macklin Chemical and used as received. Pt/C (10% Pt) was purchased from SigmaAldrich. All aqueous solutions were prepared using deionized water (DI) with a resistivity of 18.2 MΩ cm. Preparation of NiFeSP on NF. First, a piece of NF was sonicated in 3.0 M HCl for 10 min to remove the NiO layer on the top surface, then washed with ethanol and deionized water, respectively, and dried in air. The cleaned NF was placed in FeCl3 solution for 30 min, and the color of NF turned from silver to yellow because of the partial replacement of Ni with Fe. Then, the NiFe/NF with CH4N2S was transferred into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h to get NiFeS/NF, which was finally converted to NiFeSP/NF in a phosphidation process with NaH2PO2 as phosphorus resource: NiFeS/NF and NaH2PO2 powder were put in two porcelain boats with NaH2PO2 powder at the upstream side of the furnace; subsequently, the furnace was heated to 350 °C with a heating rate of 2 °C min−1 under an Ar atmosphere and then naturally cooled to ambient temperature under an Ar atmosphere. In addition, the concentrations of FeCl3, CH4N2S, and NaH2PO2 have been optimized for preparation of NiFeSP. For comparison, NiS/NF, NiFeS/NF, and NiSP/NF have also been prepared in this work. Preparation of NiFeOx on NF. The state-of-the-art electrocatalyst of NiFeOx for OER was synthesized from a reported electrodoposition method.69,70 The solution with 0.4 mM (total metals) Ni(NO3)2/ FeCl2 was first buffered to pH = 9.0 and used to deposit NiFeOx on the NF electrode at 0.909 V vs SCE in a three-electrode configuration without stirring. The loading mass can be controlled through deposition time. Deposition of NiFeSP on BiVO4 and Cu2O. The BiVO4 photoelectrode was synthesized as previously reported.71 The BiOI film was first electrodeposited on FTO glass with a potential of −0.1 V vs Ag/AgCl in an electrolyte composed of 0.04 M Bi(NO3)3, 0.4 M KI, and 0.23 M p-benzoquinone solutions at room temperature for 5 min. Then, the obtained BiOI/FTO electrode was rinsed with 0.2 mL of DMSO containing 0.2 M VO(acac)2, followed by annealing in a muffle furnace at 450 °C for 2 h. After cooling to room temperature, the electrodes were soaked in 1 M NaOH solution for 30 min with gentle stirring to remove the excess V2O5 on the BiVO4. Finally, the pure BiVO4 electrode was rinsed with deionized water and dried in ambient air. The Cu2O nanowire photocathode was fabricated through simple electrochemical anodization/annealing processes, as previously reported.72 First, copper foam (CF) was anodized in an alkali solution (3 M NaOH) for 30 min under 45 mA cm−2 to form a Cu(OH)2 nanowire. The as-anodized nanowire was annealed in the tube furnace at 550 °C for 4 h under nitrogen to convert the Cu(OH)2 nanowire to a Cu2O nanowire. The NiFeSP was detached from NF in an ultrasonic cleaning bath and was spin coated on the BiVO4 and Cu2O. Structural Characterization. The morphologies of electrodes were characterized by SEM (Hitachi S4800) and TEM (JEOL JEM 2100). The crystalline structures of electrodes were analyzed by XRD on a Bruker D8 Discover diffractometer using Cu Kα radiation (1.540 598 Å). Chemical compositions and status were analyzed by
CONCLUSION In summary, under guidance of DFT calculations, we proposed and synthesized an effective bimetallic phosphide/sulfide nanosheet electrocatalyst of NiFeSP/NF, which achieved superior performance for both HER and OER in alkaline media than most reported earth-abundant electrocatalysts so far and also implemented efficient solar-driven overall water splitting with an outstanding solar-to-hydrogen conversion 10309
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ACS Nano XPS on an Axis Ultra instrument (Kratos Analytical) under ultrahigh vacuum (
