Skutterudite-Type Ternary Co1âxNixP3 Nanoneedle Array

Subscriber access provided by - Access paid by the | UCSB Libraries

Letter

Skutterudite-type Ternary Co1-xNixP3 Nanoneedle Arrays Electrocatalysts for Enhanced Hydrogen and Oxygen Evolution Qiang Fu, Tao Wu, Gang Fu, Tangling Gao, Jiecai Han, Tai Yao, Yumin Zhang, Wenwu Zhong, Xianjie Wang, and Bo Song ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00908 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 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 Energy Letters

Skutterudite-type Ternary Co1−xNixP3 Nanoneedle Arrays Electrocatalysts for Enhanced Hydrogen and Oxygen Evolution Qiang Fu,† Tao Wu, ‡ Gang Fu, ∫ Tangling Gao,∫ Jiecai Han, § Tai Yao,* §, ¶ Yumin Zhang, § Wenwu Zhong, ∆ Xianjie Wang, *† and Bo Song *† , §, ¶, ∆ †

Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China

‡

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing

210094, P. R. China ∫

Institute of Petrochemistry, Heilongjiang Academy of Sciences, Harbin 150040, P. R. China

§

Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150001,

P. R. China ¶

Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology,

Harbin 150001, P. R. China ∆

School of Advanced Study, Taizhou University, Taizhou, 317000, China

ABSTRACT: Developing earth-abundant and low cost electrocatalysts for water splitting is important for the conversion systems of renewable and clean energy. Herein, under the guidance of theoretical calculations, a new type of skutterudite-type ternary cobalt nickel phosphide (Co1−xNixP3) nanoneedle arrays (NAs) are fabricated on carbon cloth for the splitting of water. The electronic structure was tuned by doping appropriate amount of Ni, and the resultant 1 ACS Paragon Plus Environment

ACS Energy Letters 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

Co0.93Ni0.07P3 displayed good catalytic activity toward hydrogen evolution reaction (HER) with an overpotential (η10) of 87 mV vs. reversible hydrogen electrode (RHE) when the current density reaches −10 mA cm−2 and the Tafel slope of the catalyst is 60.7 mV dec−1 in alkaline electrolyte. Besides, the Co0.93Ni0.07P3 also exhibited an OER activity (η20 of 221 mV vs. RHE and the Tafel slope of 83.7 mV dec−1). These skutterudite-based Co1−xNixP3 electrocatalysts show promising potential in the applications of overall water splitting in alkaline environment.

For Table of Contents Only

2 ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24 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 Energy Letters

With the rapid expansion of human society, the need for energy and its related services is progressively increasing. In order to meet the growing energy demands of future generations, significant research efforts have been devoted to searching clean and sustainable energy sources.1, 2 Among these new-type energy sources, hydrogen (H2) is one of the best energy carrier with ideal energy density that can well address the increasingly serious environmental and energy crisis issues which is caused by the overexploitation and use of fossil fuels, such as coal, oil, and natural gas etc.3, 4 By far, steam reforming or partial oxidation of coal gasification and methane are still the main methods to produce H2, whose proportion could reach as high as ~ 90%.5 However, only a small amount of H2 is achieved by the electrochemical splitting of water, which is considered as one of the most promising pathways for sustainable and clean H2 production.6 In general, water could be split in to oxygen and hydrogen, and the splitting process involves two separated reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and the two processes are both kinetically sluggish in nature. Therefore, in order to produce H2 in large scale, highly efficient catalysts for HER are of great importance and highly desirable.7, 8 In contrast with the well-investigated HER process,9-11 the current bottleneck required to be overcome to further improve the water-splitting technique is the OER process, which involves for sequential proton-coupled electron transfers and is of thermodynamically and kinetically demanding.12,

13

To improve the sluggish kinetics of these two half reactions,

development of high-efficient and cost effective electrocatalysts for HER and OER would be a promising strategy; however, it still remains a major challenge for the researchers. Presently, 3 ACS Paragon Plus Environment


platinum (Pt) and oxides of some noble metal (such as IrO2 and RuO2) are still the most efficient HER and OER electrocatalysts, respectively.14, 15 However, their high cost and scarcity severely restrict the large-scale applications of precious metal-based electrocatalysts in energy conversion.16-18 Therefore, tremendous research efforts have been devoted to searching or designing first-row (3d) transition-metal compounds-based electrocatalysts as low-cost alternative for the noble metal-based catalysts because of their earth abundance, high activities, and gram-scale synthesis pathways.19, 20 Until now, a series of non-precious metal-based electrocatalysts has been synthesized to enhance the catalytic activity toward HER and OER.21-28 Among them, transition metal compounds that contain phosphorus have been proved to be promising catalysts for HER.29-35 As a representative, cobalt (Co)-based phosphides have been well investigated owing to their high catalytic activity.36, 37 For instance, Tian et al. reported that nanoporous cobalt phosphide (CoP) nanowire arrays grown on carbon cloth (CC) could be an efficient HER electrocatalyst over the wide range of pH (0–14).38 Callejas et al. found that CoP nanoparticles (NPs) showed an enhanced HER performance than Co2P NPs, and predicted that a high P:Co ratio would provide more active sites for catalytic activity toward HER.39 To verify this, Wu et al. reported the synthesis of self-supported 3D-structured nanoporous CoP3 nanoneedle arrays (NAs) on conductive carbon fiber paper, which shows good catalytic activity for both HER and OER. However, there are still some areas that need to be improved for the overall electrocatalytic activity, and the real active species for OER performance has not yet been elucidated in CoP3-based electrocatalysts.40 Moreover, the doping engineering (tuning the electronic 4 ACS Paragon Plus Environment

Page 4 of 24

Page 5 of 24 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 Energy Letters

structures) is considered as an effective strategy to reduce the hydrogen adsorption free energy in catalysts, and then, enhance the catalytic activity.23, 41 Therefore, tuning the electronic structures and controlling the morphologies of the skutterudite-type electrocatalysts are two important aspects to boost their catalytic activity for practical application, which could be a promising but challenging task. Herein, a series of skutterudite-type Co1−xNixP3 NAs grown on CC were synthesized under the density functional theory (DFT) calculations guidance and then the samples were used for both HER and OER measurement in alkaline electrolyte. It was found that appropriate Ni doping could effectively tune the electronic properties and optimize the hydrogen absorption free energy of the parent CoP3, thus leading to the enhancement in the catalytic activity. Among them, Co0.93Ni0.07P3

exhibited the superior HER performance with η10 of 87 mV vs. RHE, and a Tafel

slope of 60.7 mV dec−1 in 1 M KOH. Interestingly, Co0.93Ni0.07P3 also exhibited η20 of 221 mV for OER in 1 M KOH, although the Co0.93Ni0.07P3 served only as the precursor, and the real catalysts were the Co(Ni) oxide/oxyhydroxide transformed from the surface oxidation of Co0.93Ni0.07P3

during the OER process. This study not only demonstrates a new approach to enhance

the electrocatalytic activity toward HER and OER of skutterudite-type electrocatalysts, which enables us to achieve excellent HER and OER performance via simple hydrothermal reaction with a following phosphidation treatment, but also paves the way for further design and fabrication of skutterudite-type electrocatalysts for both HER and OER, and other applications via the engineering of electronic structures.



Figure 1. Hydrogen adsorption free energy ∆ on different sites for CoP3(200) surfaces: (a) CoP3(200) surface without Ni doping, (b) CoP3(200) surface with one Ni atom doped on the terminated atoms, and (c) CoP3(200) surface with four Ni atoms doped on the terminated atoms. (The top P atom in the middle of the lattice is considered as P-1 site and the top P atom located at the lattice is considered as P-2 site). 6 ACS Paragon Plus Environment

Page 6 of 24

Page 7 of 24 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 Energy Letters

Bulk CoP3 is a skutterudite-type cubic crystal structure (space group Im3) in which six phosphorous atoms constitute a distorted octahedral configuration with one Co atom in the middle of the octahedron, while each phosphorous atom is surrounded by other two phosphorous atoms and two Co atoms (Figure S1). As HER usually happens on the surface, the model of most stabel surface structure is required to theoretically evaluate the HER electrocatalytic activity of CoP3. In order to determine the most stable surface of CoP3, the ordered and clean (200), (211), (220), and (013) surfaces were constructed, starting from the bulk structure of CoP3. Then, the surface energies of different surfaces were examined and the hydrogen adsorption energies on the most stable surface were compared. The surfaces with low Miller-index are usually considered to be much more stable than the surfaces with higher index; therefore, herein the first four surfaces with low Miller-index corresponding to (200), (211), (220), and (013) were considered (Figure S2). To determine the most stable surface structure(s), the surface energies of these four surfaces of CoP3 according to following equation (1) were calculated and compared: =

(1)

where Eslab and Ebulk represent the total energy of the surface slab and the bulk atoms (eV per atom), respectively. N and A represent the number of atoms in the slab and the surface area, respectively. According to the calculation results, the energies of each surface are listed in Table S1. As shown in the list, the surface energy of the (200) surface is the lowest and the stabilities of the clean surfaces follow the trend of (200) > (220) > (013) > (211). Then, we focused on the calculations of the hydrogen adsorption energy on the (200) surface in the following discussions. In general, the HER process in alkline usually contains two main steps: (1) water dissociation 7 ACS Paragon Plus Environment


step; and (2) the H* on the active sites convert into H2. Usually, the HER activity chould be well described by hydrogen adsorption free energy ∆ . To disscuss the effect of Ni-doping on catalytic activity toward HER in CoP3 structure, (200) surfaces with three different terminated structures were constructed including : 1) four Co atoms + four P atoms; 2) one Ni atom + three Co atoms + four P atoms; and 3) four Ni atoms + four P atoms. These three surfaces with different terminated atoms can qualitatively represent pristine CoP3, CoP3 with small amounts of doped Ni, and CoP3 with a little more doped Ni, respectively. Figure 1 exhibits the three different (200) surfaces and hydrogen adsorption free energy ∆ on different surfaces. Distinctly, the results provide the conclusion that the hydrogen adsorption free energy is very large (> 0.30 eV) irrespective of the Co site or the Ni site, indicating that both the Co and Ni sites are not crucial for the catalytic activity of CoP3 toward HER. It most likely originates from the large energy difference between 3d orbital of Co or Ni and 1s orbital of H atom. In contrast with the Co and Ni, adsorption of the H atom on the P sites of CoP3(200) is much more favorable. Noteworthy, two different P sites exist according to the different symmetries: P-1 site and P-2 site, as denoted in Figure 1. The hydrogen adsorption free energy in pristine CoP3 is 0.21 eV and −0.04 eV for P-1 site and P-2 site, respectively (Figure 1a). With increasing Ni content (Figure 1b), the hydrogen adsorption free energy on P-2 site increases from −0.04 to −0.01 eV, which is comparable with the optimal hydrogen adsorption free energy of 0 eV. Then, Co(Ni)P3 with the small amount of doped Ni will exhibit an enhanced catalytic activity for HER. However, if the Ni content on the CoP3(200) is further increased, the hydrogen adsorption free energy decreases to −0.10 eV (P-2 site) and −0.02 eV (P-1 site), respectively (Figure 1c). Thus, the HER activity 8 ACS Paragon Plus Environment

Page 8 of 24

Page 9 of 24 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 Energy Letters

of Co(Ni)P3 deteriorates, that is, the HER performance of Co(Ni)P3 first becomes better with an increase of Ni content and then gradually gets worse.

Figure 2. (a) Schematic illustration of the synthesis of Co1−xNixP3 NAs, (b) XRD patterns of as-prepared Co1−xNixP3 (x = 0, 0.05, 0.07, and 0.08) samples, vertical bars at the bottom are the expected Bragg positions for CoP3 (PDF #73-1239), (c) The magnified region of 2θ = 36−37.5°, (d) SEM image of Co0.93Ni0.07(OH)F NAs, and (e) Co0.93Ni0.07P3 NAs after phosphidation treatment of (d). Scale bars: 5 µm. To verify the theoretical calculations, a two-step procedure, as shown in Figure 2a, was used to synthesize a series of Co1−xNixP3 (x = 0, 0.05, 0.07, and 0.08) samples (experiment details could be found in the Supporting Information). X-ray diffraction (XRD) was used to characterize the crystal structure of the as-synthesized samples (Figures 2b and c). Clearly, all the 9 ACS Paragon Plus Environment


Page 10 of 24

samples agree well with the cubic structure of the skutterudite-type CoP3 (PDF #73-1239, space group Im3, a = b = c = 7.707 Å). Noteworthy, little doping of Ni atoms (0 < x < 0.08) does not change the crystal structure, and all the as-prepared specimens still maintain the cubic structure. A closer look at the (0 1 3) diffraction peak in the 2θ range of 36–37.5° shows a slight shift to the lower angle with the increase of Ni content due to the lattice expansion when the bigger Ni ions partially supersede the Co ions. Raman features of the as-synthesized Co1−xNixP3 samples (Figure S3) clearly demonstrate eight Raman active modes (4Tg + 2 Ag + 2Tg) in the range of 200–600 cm−1 as observed in pristine CoP3,40, 42 further verifying the formation of skutterudite-type crystal structure. Scanning electron microscopy (SEM) images (Figure 2d, Figure S4) reveal the homogeneous growth of Co0.93Ni0.07(OH)F with NAs (~3–5 µm in length and ~70–100 nm in width) morphological features on the CC, and similar morphological features with other Ni contents are displayed in Figures S5a, c, and e. After the phosphidation treatment, SEM image (Figures 2e and S5) reveals that the as-prepared Co1−xNixP3 NAs mainly retain the original morphologies of the corresponding precursor of Co1−xNix(OH)F NAs. Table 1 Electrochemical data of various Co1−xNixP3 NAs for HER and OER catalysis in 1 M KOH Catalysts

ηHER a

TSHER b

Cdl

Rct

ηOER c

TSOER

CoP3

136 mV

75.1

36

128.5

330 mV

138.3

Co0.95Ni0.05P3

91 mV

108.7

67

41.7

282 mV

92.8

Co0.93Ni0.07P3

87 mV

60.7

80

12.3

221 mV

83.7

Co0.92Ni0.08P3

107 mV

72

49

85.1

281 mV

177.9


Page 11 of 24 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 Energy Letters

ηHER(mV) refers to the overpotential when the current density reaches −10 mA cm−2 during the HER process. b TSHER represents Tafel slope and the unit is mV dec−1. c ηOER refers to the

a

overpotential when the current density reaches 20 mA cm−2 during the OER process. Cdl is given in the unit of mF cm−2 and Rct in Ω. For the better comprehensive understanding of the role of electrical structures tuned by Ni doping, the electrocatalytic activities of Co1−xNixP3 NAs toward HER were investigated in 1 M KOH using the as-grown samples directly as the working electrodes, and the commercial Pt/C (20 wt.%) was also systematically examined for comparison. The key electrochemical results for as-synthesized samples with different x values are also summarized in Table 1.

Figure 3. Electrochemical performance of as-prepared Co1−xNixP3 NAs compared to that of the commercial Pt/C catalyst in 1 M KOH: (a) J–V curves after iR correction of the performance of 11 ACS Paragon Plus Environment


Page 12 of 24

Co1−xNixP3 NAs toward HER and (b) Tafel plots for the corresponding sample, (c) Electrochemical double-layer capacitance measurements of the corresponding samples, and (d) EIS Nyquist plots of various Co1−xNixP3 NAs, and the inset is the fitted equivalent circuit. The iR-corrected polarization curves of the samples (Figure 3a) revealed the limited performance of the pristine CoP3 NAs toward HER, acquiring an overpotential (η10) of 136 mV when the electrocatalytic current density reaches j = −10 mA cm−2. In contrast, Co1−xNixP3 with x = 0.05, 0.07, and 0.08 could achieve a geometric catalytic current density of j = −10 mA cm−2 at η of 91, 87, and 107 mV vs. RHE, respectively. Herein, the as-obtained η10 of 87 mV for Co0.93Ni0.07P3 NAs was at least comparable or even better than the most transition-metal phosphide, phosphosulfide, oxide, and selenide in alkaline electrolyte (Table S2). Furthermore, the Tafel slopes acquired by extrapolating the linear region of η vs. log j (Figure 3b) are calculated to be 75.1, 108.7, 60.7, and 72 mV dec−1 (after iR correction) for CoP3, Co0.95Ni0.05P3, Co0.93Ni0.07P3, and Co0.92Ni0.08P3, respectively. All the values of the Tafel slopes fell within the range of 60–110 mV dec−1, indicating that the HER process taking place on the surface of Co1−xNixP3 follows

a

mechanism.43 To

Volmer–Heyrovsky

evaluate

the

effective

electrochemically active surface area (ECSA) of the as-synthesized catalysts, the electrochemical double-layer capacitances (Cdl) were estimated via the cyclic voltammetry (CV) method on the assumption that all the as-synthesized Co1−xNixP3 NAs samples roughly possess the same surface areas and physical size. Derived from the CV curves with various scan rates (Figure S6), the Cdl values are 36, 67, 80, and 49 mF cm−2 for CoP3 NAs, Co0.95Ni0.05P3 NAs, Co0.93Ni0.07P3 NAs, and 12 ACS Paragon Plus Environment

Page 13 of 24 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 Energy Letters

Co0.92Ni0.08P3 NAs, respectively (Figure 3c). This indicates that doping Ni atoms leads to the generation of more HER-active sites in Co1−xNixP3, and also verifies the theoretical prediction that doping Ni could effectively tune the electronic structures of skutterudite-type electrocatalysts. For instance, with the increase of x to 0.07, at least two times catalytically active sites than pristine CoP3 NAs with almost the same catalyst loading were achieved. Further increasing x to 0.08 leads to the reduction of the ECSA, thus clearly indicating that the appropriate amount of doped-Ni may promote the high activity toward HER due to the generation of more active sites. To further investigate the electrode kinetics of the HER process, we measured the electrochemical impedance spectroscopy (EIS) of each sample. As shown in Figure 3d, the Nyquist plots with semi-circle features were fitted using a Randles equivalent circuit (inset in Figure 3d). Then the charge transfer resistance (Rct) was extracted to be 128.5, 41.7, 12.3, and 85.1 Ω for CoP3, Co0.95Ni0.05P3, Co0.93Ni0.07P3, and Co0.92Ni0.08P3, respectively. This indicates that tuning the electronic structures of skutterudite-based electrocatalysts can significantly facilitate the charge transfer process at the interface of Co1−xNixP3 NAs. Among them, Co0.93Ni0.07P3 shows the best HER catalytic activity, including the lowest overpotential, Tafel slope, and Rct, while more or less Ni doping leads to the deterioration of the HER activity (Figure S7), which is in well agreement with the aforementioned DFT calculations results.



Figure 4. Electrochemical catalytic performance of the Co1−xNixP3 NAs in 1 M KOH: (a) J–V curves after iR correction of the performance of Co1−xNixP3 NAs toward OER and (b) Tafel plots for the corresponding sample, (c) OER polarization curves for the Co0.93Ni0.07P3 NAs sample before and after 1000 cycles of the accelerated stability test, and (d) Chronoamperometric profile of Co0.93Ni0.07P3 NAs at 1.5 V in 1 M KOH. Recent studies have revealed that transition-metal phosphides may be excellent candidates as precursors for OER catalysts.44-46 In this study, the OER performance of as-synthesized samples in 1 M KOH was measured in order to evaluate the potential of Co1−xNixP3 NAs. Figure 4a illustrates that the Co0.93Ni0.07P3 NAs can achieve a geometric current density of 20 mA cm−2 at the overpotential (η20) of 221 mV after iR correction. In contrast, CoP3 NAs, Co0.95Ni0.05P3 14 ACS Paragon Plus Environment

Page 14 of 24

Page 15 of 24 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 Energy Letters

NAs, and Co0.92Ni0.08P3 NAs require significantly larger overpotentials (η) of 330, 282, and 281 mV to derive the same current density (20 mA cm−2), respectively, indicating the as-synthesized Co0.93Ni0.07P3 NAs as promising OER precursor. Notably, the catalytic activity toward OER exhibited similar trend as a function of x in contrast with the catalytic performance toward HER. The Tafel slope were obtained by extrapolating the liner region of overpotential (η) vs. log j (Figure 4b), and the value for CoP3 NAs, Co0.95Ni0.05P3 NAs, Co0.93Ni0.07P3 NAs, and Co0.92Ni0.08P3 NAs were 138.3, 92.8, 83.7, and 177.9 mV dec−1 (after iR correction), respectively. The results of catalytic performance for both HER and OER indicate a prominent electrocatalytic behavior of Co1−xNixP3 NAs. Similarly, to better understand the electrode kinetics during OER, EIS measurement was performed. Noticeably, all the Nyquist plots (Figure S8) with semi-circle features could be well fitted using the Randles equivalent circuit (inset in Figure S8) to extract the Rct of 20.5, 41.7, 12.3, and 85.1 Ω for CoP3, Co0.95Ni0.05P3, Co0.93Ni0.07P3, and Co0.92Ni0.08P3, respectively. This further demonstrates that tuning the electronic structures could greatly facilitate the charge transfer process at the interface of Co1−xNixP3 NAs. For the further practical application, the electrocatalysts must be of good stability and durability. The polarization curves for Co0.93Ni0.07P3 NAs after 1000 cycles exhibit only a slight degradation for both the OER (Figure 4c) and HER (Figure S9a) processes. Further, the amperometric i–t curves showed that, even after 10h continuous measurement, the catalytic current density could maintain at around 20 mA cm−2 for OER (Figure 4d) and −20 mA cm−2 for HER (Figure S9b), respectively. The negligible degradation of the catalytic performance for both HER and OER springs from the mechanical loss, and thus loss of catalytic active sites during drastic gas evolution process. 15 ACS Paragon Plus Environment


Figure 5. Structural characterizations of the Co0.93Ni0.07P3 NAs before and after OER measurement: XPS spectra of (a) Co 2p, (b) P 2p, and (c) O 1s; and (d) Raman spectra of the sample before and after OER test. Recent investigations demonstrated that the intrinsic stability of the metal phosphides was poor during OER test. Nonetheless, the surfaces of the metal phosphides get converted into metal oxides/hydroxides, after initial several cycles, which are considered as the real active species for OER.47-49 To examine the stability of the Co1−xNixP3 NAs toward OER, XPS spectra before and after OER were obtained (Figure 5). Before the OER test, besides the two satellite peaks (denoted as “Sat.”) located at 787.8 and 804.8 eV, the Co 2p spectrum (Figure 5a) shows two main deconvoluted peaks located at 779.9 and 794.7 eV, which can be ascribed to Co 2p3/2 and 16 ACS Paragon Plus Environment

Page 16 of 24

Page 17 of 24 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 Energy Letters

Co 2p1/2 for the Co–P bond,50, 51 respectively. Furthermore, the deconvoluted peaks located at 781.5 and 796.5 eV are assigned to the oxidation peaks of Co2+,18 indicating that before the OER test, the surfaces of Co1−xNixP3 NAs were partially oxidized. After the OER measurement, it was found that the relative intensity of the oxidation peak of Co2+ increased, indicating the transformation of more Co0.93Ni0.07P3 NAs into oxidized cobalt or cobalt hydroxide.52, 53 The P 2p XPS spectrum (Figure 5b), before OER test, exhibits three characteristic peaks of P 2p1/2, P 2p3/2, and P–O bond, located at ~129.9, 130.8,54 and 134.8 eV,55 respectively. It is easy to understand the formation of P–O bond which was derived from the phosphidation process of Co1−xNix(OH)F to Co1−xNixP3 NAs. After the OER test, the XPS peak for P–O bond still dominated the sample, while the P 2p1/2 and P 2p3/2 peaks almost disappeared, clearly indicating that at least the Co–P bonds on the surface were converted into oxides or hydroxide.47, 56 The peak located at ~132.1eV could be attributed to P-C bond, which may be formed during the high temperature phosphidation process between the P and the carbon cloth.57 Moreover, before the OER measurement, the O 1s XPS spectrum (Figure 5c) could be deconvolved into two peaks, which were located at 531.7 and 533.2 eV, respectively, assigned to hydroxyl groups (Co–OOH) and absorbed H2O molecule.58-60 After the OER test, the intensity of these two peaks significantly decreased and the peak located at 530.7 eV corresponding to the Co–O bond, became dominant, thus providing a convincing evidence for the formation of oxides on the surface of Co0.93Ni0.07P3 NAs during the OER process.47, 58 This result was also reflected in HER measurement after the OER test (Figure S10). Moreover, Raman and XRD characterizations of Co0.93Ni0.07P3 NAs exhibited a distinct structure transformation during the OER test. Obviously, the Raman 17 ACS Paragon Plus Environment


characteristic mode of only Co3O4 (located at 609 cm−1)61 could be observed (Figure 5d) and the XRD pattern could be also indexed into Co3O4 (Figure S11a) after OER test. The above mentioned results lead to the conclusion that the as-synthesized Co0.93Ni0.07P3 NAs get transformed into Co (Ni) oxide or hydroxide during the OER process, which actually serve as the excellent OER-precursors. A comparative XPS, Raman and XRD study of Co0.93Ni0.07P3 NAs indicates no obvious change on the catalyst surface before and after the HER process (Figure S11b and Figure S12), which further confirms that the as-synthesized Co1−xNixP3 NAs are promising candidates for the HER process in alkaline media. Moreover, the SEM images of the Co0.93Ni0.07P3 after HER and OER test are shown in Figure S13, which proved the as prepared sample are of good morphology stability. In summary, under the guidance of density functional theory calculations, skutterudite-type Co1−xNixP3 NAs as an efficient electrocatalyst for both HER and OER-precursor were achieved on CC through a simple hydrothermal synthesis followed by the phosphidation treatment. Among all samples, Co0.93Ni0.07P3 NAs showed the most remarkable catalytic activity for HER with the η10 of 87 mV and a Tafel slope of 60.7 mV dec−1. However, the structural analyses in case of OER indicates that the Co1−xNixP3 NAs actually acted as the OER-precursor, and the derived oxide or hydroxide was proved to be the real active species for the catalytic process. The Co0.93Ni0.07P3 NAs as OER-precursor exhibited the best catalytic activity for OER with η20 of 221 mV and a Tafel slope of 83.7 mV dec−1. The excellent performance of the as-prepared catalysts in alkaline media for both HER and OER, combined with the facile and scalable synthetic strategy, make the as-synthesized Co1−xNixP3 NAs very promising earth-abundant alternatives for 18

ACS Paragon Plus Environment

Page 18 of 24

Page 19 of 24 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 Energy Letters

noble metal-based electrocatalysts. This study also offers some new insights into the designing and constructing of novel skutterudite-type electrocatalysts for hydrogen and oxygen evolution or other catalytic applications via tuning the electronic structure, which would be a promising strategy for the practical application. ASSOCIATED CONTENT Supporting Information. Figures S1–S13, and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail for B.S.: [email protected]. *E-mail for X.W.: [email protected]. *E-mail for T. Y.: [email protected]. ORCID Bo Song: 0000-0003-2000-5071

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the Major State Basic Search Program (No. 2014CB46505), National Natural Science Foundation of China (Grant Nos. 51372056, 51472064, 51672057, 51722205), Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 11421091), International Science & Technology Cooperation Program of China (2012DFR50020), the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRETIV.201801), the Natural Science Foundation of Heilongjiang Province (Grant No. E2018032), and the Program for New Century Excellent Talents in University (NCET-13-0174).



REFERENCES (1) Chu, S.; Majumdar, A. Opportunities and Challenges for a Sustainable Energy Future. Nature 2012, 488, 294-303. (2) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (3) Dresselhaus, M. S.; Thomas, I. L. Alternative Energy Technologies. Nature 2001, 414, 332-337. (4) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (5) Ding, Q.; Song, B.; Xu, P.; Jin, S. Efficient Electrocatalytic and Photoelectrochemical Hydrogen Generation Using MoS2 and Related Compounds. Chem 2016, 1, 699-726. (6) Turner, J. A. A Realizable Renewable Energy Future. Science 1999, 285, 687-689. (7) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem., Int. Ed. 2014, 126, 5531-5534. (8) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: an Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900. (9) Yin, Y.; Zhang, Y.; Gao, T.; Yao, T.; Zhang, X.; Han, J.; Wang, X.; Zhang, Z.; Xu, P.; Zhang, P.; et al. Synergistic Phase and Disorder Engineering in 1T-MoSe2 Nanosheets for Enhanced Hydrogen-Evolution Reaction. Adv. Mater. 2017, 29, 1700311. (10) Cabán, A. M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H. C.; Tsai, M. L.; He, J. H.; Jin, S. Efficient Hydrogen Evolution Catalysis Using Ternary Pyrite-Type Cobalt Phosphosulphide. Nat. Mater. 2015, 14, 1245-1251. (11) Voiry, D.; Yang, J.; Chhowalla, M. Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction. Adv. Mater. 2016, 28, 6197-6206. (12) Kim, J. H.; Youn, D. H.; Kawashima, K.; Lin, J.; Lim, H.; Mullins, C. B. An Active Nanoporous Ni(Fe) OER Electrocatalyst via Selective Dissolution of Cd in Alkaline Media. Appl. Catal., B-Environ. 2018, 225, 1-7. (13) Song, S.; Zhang, H.; Ma, X.; Shao, Z.; Baker, R. T.; Yi, B. Electrochemical Investigation of Electrocatalysts for the Oxygen Evolution Reaction in PEM Water Electrolyzers. Int. J. Hydrogen Energy 2008, 33, 4955-4961. (14) Xing, Z.; Han, C.; Wang, D.; Li, Q.; Yang, X. Ultrafine Pt Nanoparticle-Decorated Co(OH)2 Nanosheet Arrays with Enhanced Catalytic Activity toward Hydrogen Evolution. ACS Catal. 2017, 7, 7131-7135. (15) Reier, T.; Oezaslan, M.; Strasser, P. Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials. ACS Catal. 2012, 2, 1765-1772. (16) Merki, D.; Hu, X. Recent Developments of Molybdenum and Tungsten Sulfides as 20 ACS Paragon Plus Environment

Page 20 of 24

Page 21 of 24 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 Energy Letters

Hydrogen Evolution Catalysts. Energy Environ. Sci. 2011, 4, 3878-3888. (17) Liu, Q.; Liu, Q.; Kong, X. Anion Engineering on Free-Standing Two-Dimensional MoS2 Nanosheets toward Hydrogen Evolution. Inorg. Chem. 2017, 56, 11462-11465. (18) Chen, X.; Cheng, M.; Chen, D.; Wang, R. Shape-Controlled Synthesis of Co2P Nanostructures and Their Application in Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 3892-3900. (19) Zhou, H.; Yu, F.; Sun, J.; Zhu, H.; Mishra, I. K.; Chen, S.; Ren, Z. Highly Efficient Hydrogen Evolution from Edge-Oriented WS2(1-x)Se2x Particles on Three-Dimensional Porous NiSe2 Foam. Nano Lett. 2016, 16, 7604-7609. (20) Xu, X.; Song, F.; Hu, X. A Nickel Iron Diselenide-derived Efficient Oxygen-evolution Catalyst. Nat. Commun. 2016, 7, 12324. (21) Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; et al. Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets. J. Am. Chem. Soc. 2016, 138, 7965-7972. (22) Ding, Q.; Song, B.; Xu, P.; Jin, S. Efficient Electrocatalytic and Photoelectrochemical Hydrogen Generation Using MoS2 and Related Compounds. Chem 2016, 1, 699-726. (23) Staszak-Jirkovsky, J.; Malliakas, C. D.; Lopes, P. P.; Danilovic, N.; Kota, S. S.; Chang, K. C.; Genorio, B.; Strmcnik, D.; Stamenkovic, V. R.; Kanatzidis, M. G.; et al. Design of Active and Stable Co-Mo-Sx Chalcogels as pH-universal Catalysts for the Hydrogen Evolution Reaction. Nat. Mater. 2016, 15, 197-203. (24) Caban-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H. C.; Tsai, M. L.; He, J. H.; Jin, S. Efficient Hydrogen Evolution Catalysis Using Ternary Pyrite-type Cobalt Phosphosulphide. Nat. Mater. 2015, 14, 1245-1251. (25) Lukowski, M. A.; Daniel, A. S.; English, C. R.; Meng, F.; Forticaux, A.; Hamers, R. J.; Jin, S. Highly Active Hydrogen Evolution Catalysis from Metallic WS2 Nanosheets. Energy Environ. Sci. 2014, 7, 2608-2613. (26) Stern, L. A.; Feng, L.; Song, F.; Hu, X. Ni2P as a Janus Catalyst for Water Splitting: the Oxygen EvolutionActivity of Ni2P Nanoparticles. Energy Environ. Sci. 2015, 8, 2347-2351. (27) Liang, H.; Meng, F.; Caban-Acevedo, M.; Li, L.; Forticaux, A.; Xiu, L.; Wang, Z.; Jin, S. Hydrothermal Continuous Flow Synthesis and Exfoliation of NiCo Layered Double Hydroxide Nanosheets for Enhanced Oxygen Evolution Catalysis. Nano Lett. 2015, 15, 1421-1427. (28) Song, F.; Hu, X. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477. (29) Xiao, P.; Sk, M. A.; Thia, L.; Ge, X.; Lim, R. J.; Wang, J.-Y.; Lim, K. H.; Wang, X. Molybdenum Phosphide as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction. Energy Environ. Sci. 2014, 7, 2624-2629. (30) Ledendecker, M.; Krick Calderon, S.; Papp, C.; Steinruck, H. P.; Antonietti, M.; Shalom, M. The Synthesis of Nanostructured Ni5P4 Films and Their Use as a Non-noble Bifunctional Electrocatalyst for Full Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 12538-12542. (31) You, B.; Jiang, N.; Sheng, M.; Bhushan, M. W.; Sun, Y. Hierarchically Porous 21 ACS Paragon Plus Environment


Urchin-Like Ni2P Superstructures Supported on Nickel Foam as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2015, 6, 714-721. (32) Wu, L.; Pu, Z.; Tu, Z.; Amiinu, I. S.; Liu, S.; Wang, P.; Mu, S. Integrated Design and Construction of WP/W Nanorod Array Electrodes Toward Efficient Hydrogen Evolution Reaction. Chem. Eng. J. 2017, 327, 705-712. (33) Ahn, S. H.; Manthiram, A. Direct Growth of Ternary Ni–Fe–P Porous Nanorods onto Nickel Foam as a Highly Active, Robust Bi-functional Electrocatalyst for Overall Water Splitting. J. Mater. Chem. A 2017, 5, 2496-2503. (34) Li, D.; Liao, Q.; Ren, B.; Jin, Q.; Cui, H.; Wang, C. A 3D-composite Structure of FeP Nanorods Supported by Vertically Aligned Graphene for the High-performance Hydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5, 11301-11308. (35) Wu, T.; Pi, M.; Wang, X.; Zhang, D.; Chen, S. Three-dimensional Metal-organic Framework Derived Porous CoP3 Concave Polyhedrons as Superior Bifunctional Electrocatalysts for the Evolution of Hydrogen and Oxygen. Phys. Chem. Chem. Phys. 2017, 19, 2104-2110. (36) Wang, X.; Tong, R.; Wang, Y.; Tao, H.; Zhang, Z.; Wang, H. Surface Roughening of Nickel Cobalt Phosphide Nanowire Arrays/Ni Foam for Enhanced Hydrogen Evolution Activity. ACS Appl. Mater. Interfaces 2016, 8, 34270-34279. (37) Han, A.; Chen, H.; Zhang, H.; Sun, Z.; Du, P. Ternary Metal Phosphide Nanosheets as a Highly Efficient Electrocatalyst for Water Reduction to Hydrogen over a Wide pH Range from 0 to 14. J. Mater. Chem. A 2016, 4, 10195-10202. (38) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-supported Nanoporous Cobalt Phosphide Nanowire Arrays: an Efficient 3D Hydrogen-evolving Cathode over the Wide Range of pH 0-14. J. Am. Chem. Soc. 2014, 136, 7587-7590. (39) Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP. Chem. Mater. 2015, 27, 3769-3774. (40) Wu, T.; Pi, M.; Zhang, D.; Chen, S. 3D Structured Porous CoP3 Nanoneedle Arrays as an Efficient Bifunctional Electrocatalyst for the Evolution Reaction of Hydrogen and Oxygen. J. Mater. Chem. A 2016, 4, 14539-14544. (41) Wang, H.; Lu, Z.; Kong, D.; Sun, J.; Hymel, T. M.; Cui, Y. Electrochemical Tuning of MoS2 Nanoparticles on Three-dimensional Substrate for Efficient Hydrogen Evolution. ACS nano 2014, 8, 4940-4947. (42) Ogita, N.; Kondo, T.; Hasegawa, T.; Takasu, Y.; Udagawa, M.; Takeda, N.; Ishikawa, K.; Sugawara, H.; Kikuchi, D.; Sato, H.; et al. Raman Scattering Investigation of Skutterudite Compounds. Physica B: Condensed Matter 2006, 383, 128-129. (43) Yu, X. Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon Coated Porous Nickel Phosphides Nanoplates for Highly Efficient Oxygen Evolution Reaction. Energy Environ. Sci. 2016, 9, 1246-1250. (44) Yu, J.; Cheng, G.; Luo, W. Hierarchical NiFeP Microflowers Directly Grown on Ni Foam for Efficient Electrocatalytic Oxygen Evolution. J. Mater. Chem. A 2017, 5, 11229-11235. 22 ACS Paragon Plus Environment

Page 22 of 24

Page 23 of 24 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 Energy Letters

(45) Han, A.; Chen, H.; Sun, Z.; Xu, J.; Du, P. High Catalytic Activity for Water Oxidation Based on Nanostructured Nickel Phosphide Precursors. Chem. Commun. 2015, 51, 11626-11629. (46) 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, 6874-6878. (47) Liu, P. F.; Li, X.; Yang, S.; Zu, M. Y.; Liu, P.; Zhang, B.; Zheng, L. R.; Zhao, H.; Yang, H. G. Ni2P(O)/Fe2P(O) Interface Can Boost Oxygen Evolution Electrocatalysis. ACS Energy Lett. 2017, 2, 2257-2263. (48) Dutta, A.; Samantara, A. K.; Dutta, S. K.; Jena, B. K.; Pradhan, N. Surface-Oxidized Dicobalt Phosphide Nanoneedles as a Nonprecious, Durable, and Efficient OER Catalyst. ACS Energy Lett. 2016, 1, 169-174. (49) Jin, S. Are Metal Chalcogenides, Nitrides, and Phosphides Oxygen Evolution Catalysts or Bifunctional Catalysts? ACS Energy Lett. 2017, 2, 1937-1938. (50) Li, J.; Liu, G.; Liu, B.; Min, Z.; Qian, D.; Jiang, J.; Li, J. An Extremely Facile Route to Co2P Encased in N,P-codoped Carbon Layers: Highly Efficient Bifunctional Electrocatalysts for ORR and OER. Int. J. Hydrogen Energy 2018, 43, 1365-1374. (51) Pan, Y.; Liu, Y.; Lin, Y.; Liu, C. Metal Doping Effect of the M-Co2P/Nitrogen-Doped Carbon Nanotubes (M = Fe, Ni, Cu) Hydrogen Evolution Hybrid Catalysts. ACS Appl. Mater. Interfaces 2016, 8, 13890-901. (52) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Graphitic Carbon Nitride Nanosheet-carbon Nanotube Three-dimensional Porous Composites as High-performance Oxygen Evolution Electrocatalysts. Angew. Chem., Int. Ed. 2014, 53, 7281-7285. (53) Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C. Resolving Surface Chemical States in XPS Analysis of First Row Transition Metals, Oxides and Hydroxides: Cr, Mn, Fe, Co and Ni. Appl. Surf. Sci. 2011, 257, 2717-2730. (54) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X. Carbon Nanotubes Decorated with CoP Nanocrystals: a Highly Active Non-noble-metal Nanohybrid Electrocatalyst for Hydrogen Evolution. Angew. Chem., Int. Ed. 2014, 53, 6828-6832. (55) Li, D.; Baydoun, H.; Verani, C. N.; Brock, S. L. Efficient Water Oxidation Using CoMnP Nanoparticles. J. Am. Chem. Soc. 2016, 138, 4006-4009. (56) Liang, H.; Gandi, A. N.; Anjum, D. H.; Wang, X.; Schwingenschlogl, U.; Alshareef, H. N. Plasma-Assisted Synthesis of NiCoP for Efficient Overall Water Splitting. Nano Lett. 2016, 16, 7718-7725. (57) Latorre-Sanchez, M.; Primo, A.; Garcia, H. P-doped Graphene Obtained by Pyrolysis of Modified Alginate as a Photocatalyst for Hydrogen Generation From Water-Methanol Mixtures. Angew. Chem., Int. Ed. 2013, 52, 11813-11816. (58) Hu, F.; Zhu, S.; Chen, S.; Li, Y.; Ma, L.; Wu, T.; Zhang, Y.; Wang, C.; Liu, C.; Yang, X.; et al. Amorphous Metallic NiFeP: A Conductive Bulk Material Achieving High Activity for Oxygen Evolution Reaction in Both Alkaline and Acidic Media. Adv. Mater. 2017, 29, 1606570. (59) Yuan, C.; Li, J.; Hou, L.; Zhang, X.; Shen, L.; Lou, X. W. D. Ultrathin Mesoporous NiCo2O4Nanosheets Supported on Ni Foam as Advanced Electrodes for Supercapacitors. Adv. 23 ACS Paragon Plus Environment


Funct. Mater. 2012, 22, 4592-4597. (60) Yan, K. L.; Shang, X.; Li, Z.; Dong, B.; Li, X.; Gao, W. K.; Chi, J. Q.; Chai, Y. M.; Liu, C. G. Ternary mixed metal Fe-doped NiCo2O4 nanowires as efficient electrocatalysts for oxygen evolution reaction. Appl. Surf. Sci. 2017, 416, 371-378. (61) Yeo, B. S.; Bell, A. T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2011, 133, 5587-5593.


Page 24 of 24

Skutterudite-Type Ternary Co1âxNixP3 Nanoneedle Array

Recommend Documents

Skutterudite-Type Ternary Co1âxNixP3 Nanoneedle Array