Molybdenum-doped Porous Cobalt Phosphide Nanosheets for

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Molybdenum-doped Porous Cobalt Phosphide Nanosheets for Efficient Alkaline Hydrogen Evolution Yechuang Han, Pengfei Li, Zhenfei Tian, Chao Zhang, Yixing Ye, Xiaoguang Zhu, and Changhao Liang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00924 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Energy Materials

Molybdenum-doped Porous Cobalt Phosphide Nanosheets for Efficient Alkaline Hydrogen Evolution Yechuang Han a ,b,c ‡, Pengfei Li a ‡, Zhenfei Tian a, Chao Zhang a, b, Yixing Ye a, Xiaoguang Zhua and Changhao Liang a, b *. a

Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and

Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China. b Department

of Materials Science and Engineering, University of Science and Technology of

China, Hefei 230026, China. c State

Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Centre

of Chemistry for Energy Materials (iChEM), and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.

‡

These authors contributed equally to this work.

*

Corresponding author.

E-mail: [email protected] Tel: +86 55165591129; Fax: Tel: +86 55165591434;

ACS Paragon Plus Environment

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ABSTRACT The electrocatalytic water splitting allow the production of high-purity and industrially affordable hydrogen fuel. However, the lack of highly active and robust non-noble-metal electrocatalysts has hindered the at-scale production of hydrogen for years. Here, a nickel foam supported porous Modoped CoP nanosheets (pCoMo-P/NF), was employed as an alkaline electrocatalyst for hydrogen evolution reaction (HER), which was obtained from CoMoAl layered triple hydroxides (CoMoAlLTH) derived phosphides with Al-species-dissolved. The unique advantage in such synthesis process is that the introduced Mo and Al elements in Co(OH)2 matrix efficiently prevent the aggregation of doped CoP nanocrystals during high temperature phosphorization. Then, due to the optimized electronic structure for accelerating H* adsorption after Mo doping, as well as the increased electrochemically active surface area and decreased electrode/electrolyte interfacial resistance after dissolution of Al-species, such pCoMo-P/NF electrocatalyst requires only 49 mV overpotential to drive 10 mA cm−2 current density in 1.0 M KOH, together with prolonged stability with negligible activity decay after 3000 cycles cyclic voltammetry test or chronopotentiometric measurement under a static overpotential of 100 mV for 10 h. This work not only provides a synergistic strategy to design both efficient and durable HER electrocatalyst, but also investigates the crucial elemental doping effects in phosphide electrocatalysts as well as in the transition of hydroxides to phosphides.

KEYWORDS: hydroxides derived phosphides, transition, molybdenum doping, porous nanosheets, alkaline hydrogen evolution.


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1. INTRODUCTION. Hydrogen as a promising alternative to carbon-based fossil fuel is clean, regenerable and has the highest gravimetric energy density.1,2 In many of the innovative approaches to provide carbonfree and affordable hydrogen, electrochemical water splitting stands out because which can produce high-purity hydrogen in combination with abundant solar or wind power systems.3-5 The practical electrolytically produced hydrogen is mainly from water electrolysis and chloralkaline industry under alkaline environment, however, there are two fundamental catalytic limitations that hindered the large-scale electrochemical hydrogen production.6,7 First, the kinetics of hydrogen generation are about 2–3 orders slower in alkaline solutions than those in acidic conditions and therefore a larger activation overpotential are required to achieve the desired net current.8,9 The second limitation is the lack of cost-effective and robust electrocatalysts. Hence, to obtain both high-performance and low-cost hydrogen evolution reaction (HER) electrocatalysts, two general strategies were applied to develop earth-abundant electrocatalyst with high performance: (i) enhance the intrinsic activity of each active site or (ii) increase the number of active sites on a given electrode.10 Under the guidance of above-mentioned strategies, various well-designed materials have been selected for HER in alkaline solutions, such as carbon-based materials,11 metal alloys,12 chalcogenides,13 carbides,14 nitrides,15 borides,16 oxides,17 and phosphides.18 Among these catalysts, nanoscale transition-metal phosphides, with engineered structure or doped with foreign elements, have shown outstanding electrocatalytic HER performance due to their suitable charge transfer capabilities from metal to P as well as the facilitated proton coupled electron transfer process during HER.19-22 Note that Co centers are widely accepted as excellent water dissociation active centers, while Mo centers own superior adsorption properties towards hydrogen.23,24 Thus,


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a well-designed Co-Mo-based phosphide may achieve a Pt-resembling Gibbs free energy for hydrogen adsorption and effectively accelerate the HER kinetics in alkaline solutions.19,25 In addition, recent researchers also found that the morphology engineering of nanocatalysts, such as creating a porous structure, may expose more active sites and further enhance the activity of nanocatalysts26,27 Thus the combination of foreign element doping and morphology-engineering is supposed to increase the intrinsic activity of each active site as well as the number of active sites on a given electrode. Moreover, elevated temperature is usually applied during the synthesis of metal phosphide electrocatalysts and the high-temperature-induced agglomeration of phosphides will block the mass and charge diffusion channels and reduce the number of active sites.28 In this regard, a rational synthetic route that can prevent the agglomeration of phosphides during preparation is thus also desirable, but which remains a big challenge.29 Herein, a porous Mo-doped CoP electrocatalyst supported on nickel foam (pCoMo-P/NF) was obtained from CoMoAl-LTH/NF derived phosphides with Al-species-dissolved. The synthesized electrocatalyst nanocrystalline is confirmed to be Co0.76Mo0.24P with an average particle size of 4.11 nm. There are three main favorable factors that endow such pCoMo-P/NF with high HER performance. First, the size of doped CoP nanocrystals is decreased because the introduced Mo6+ and Al3+ increased the dispersity of Co2+ in its hydroxide precursor and can protect the doped CoP nanocrystals from agglomeration during high-temperature phosphorization. Second, the electronic structures of CoP is optimized and resemble to that of Pt after Mo doping. Third, ECSA is remarkably increased and the electrode/electrolyte interfacial resistance is lowered after Al species dissolution. As a result, the pCoMo-P/NF electrocatalyst only need 49 mV overpotential to drive 10 mA cm−2 current density in 1.0 M KOH solution, and pCoMo-P/NF electrocatalyst also shows larger current density at high overpotential in compared with that of commercial Pt/C


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electrocatalyst. Importantly, the pCoMo-P/NF electrocatalyst exhibits long-time stability with negligible activity decay after 3000 cycles CV cycling test or chronopotentiometric measurement under a static overpotential of 100 mV for 10 h. Therefore, our work not only provides a synergistic strategy to design both highly active and robust HER electrocatalysts, but also investigates the crucial elemental doping effects in phosphide electrocatalysts as well as in the transition of hydroxides to phosphides. 2. RESULTS AND DISCUSSION 2.1 Synthesis and Characterization of CoMo(Al)-P Catalysts

Figure 1. (a) Schematic of the synthesis of CoMo(Al)-P/NF through phosphorization of CoMoAL-LTH/NF. (b) SEM image of nickel foam supported CoMo(Al)-P. Inset is the magnified SEM and TEM image of vertical


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CoMo(Al)-P nanosheets. (c) TEM image and its corresponding (d) HR-TEM images of CoMo(Al)-P. (e) STEM image and (f) EDS elemental mapping of a representative CoMo(Al)-P nanosheet.

NF-supported phosphides were prepared through phosphorization of their hydroxide precursors under phosphine (PH3) atmosphere (Figure 1a), and the PH3 was generated via thermal decomposition of sodium hypophosphite (NaH2PO2) as showing in below reaction.30 ∆, 𝐴𝑟

2𝑁𝑎𝐻2𝑃𝑂2

𝑁𝑎2𝐻𝑃𝑂4 + 𝑃𝐻3↑

The obtained product, where Co and Mo species existed in the form of phosphides while Al species mainly existed in the form of oxides, was named as CoMo(Al)-P (Figure S1). More details about the microstructure of CoMo(Al)-P nanosheets were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The as-prepared CoMo(Al)-P, with a similar honeycomb arrangement as its CoMoAl-LTH precursor, consists of nanosheets with thickness of 5.3 nm (Figure 1b and Figure S2). As shown in Figure 1c, d, the CoMo(Al)-P nanosheets were mainly composed of densely adjacent nanocrystals (marked with dashed lines). In Figure 1e and Figure 1f, scanning transition electron microscopy (STEM) image of CoMo(Al)P nanosheets and the corresponding elemental mapping images demonstrates that Mo and Al elements were uniformly distributed in CoMo(Al)-P nanosheet with constitutes of Co0.76Mo0.24(Al)P (Figure S3). CoMo-P electrocatalyst, with the similar layered structure and a closely Co/Mo atomic ratio as that of Co0.76Mo0.24(Al)P, was also synthesized and characterized for comparison (Figure S4).


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Figure 2. XRD patterns of CoMo(Al)-P, CoMo-P and CoP.

The powder X-ray diffraction (XRD) pattern (Figure 2) demonstrates that, the as-synthesized CoP, CoMo-P, and CoMo(Al)-P, with mainly exposed (011) and (211) crystal surfaces, can be well indexed to the standard diffraction patterns of orthorhombic CoP phase (JCPDS No. 29-0479). Notably, under the same preparation and characterization conditions for the above samples (see Experimental Section), CoMo(Al)-P shows the lowest diffraction intensity, followed by CoMo-P and CoP. Similar experimental phenomenon has also been recorded by previous literature, but less attention was paid on the elemental doping effects during the transition of hydroxides to phosphides.31,32 This trend should be ascribed to the introduction of Mo6+ and Al3+ in their hydroxide matrix precursors. Specifically, in the host structure of cobalt hydroxides, the introduced Mo6+ and Al3+ would not only isomorphous substitution of partial Co2+ to form stable brucite-like layers with edge-sharing MO6 (M = Co, Mo, Al) octahedra (Figure S5), but also increase the dispersity of Co2+.33-36 During conversion of hydroxides to phosphides, the aforementioned two factors will slow down the growth rate of CoP nanocrystals and protect these nanocrystals from aggregation. As shown in Figure S6, the average size of CoP nanocrystals, prepared through phosphorization of Co(OH)2 precursor, was 10.59 nm. By contrast, the average size of doped CoP nanocrystals in CoMoAl-P and CoMo-P samples were dramatically decreased to 4.11 nm and 5.89


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nm, respectively. Thus, we observed that the smaller size and higher dispersity of CoP nanocrystal results in a decreasing sequence in diffraction peak intensity and crystallinity for CoP, CoMo-P, CoMoAl-P.37 2.2 Electrocatalytic HER performance

Figure 3. Alkaline HER activity of nickel foam (NF), CoP/NF, CoMo-P/NF, CoMo(Al)-P/NF, Pt/C-20/NF and Pt/C-40/NF electrocatalysts. (a) Polarization curves. The inset is optical photography showing the generated H2 bubbles on CoMo(Al)-P/NF. (b) Tafel slope. (c) Overpotential (η) of CoMo(Al)-P/NF and Pt/C-40/NF at a current density of 10 mA cm−2. (d) Time-dependent current density curves of CoMo(Al)-P/NF and Pt/C-40/NF under a static overpotential of 100 mV for 10 h.

The electrocatalytic HER activity of as-prepared electrocatalysts were examined in 1.0 M KOH solution. The overpotential needed for NF, CoP/NF, CoMo-P/NF, CoMo(Al)-P/NF, Pt/C-20/NF, Pt/C-40/NF to reach a current density of 10 mA cm−2 were 248 mV, 138 mV, 78 mV, 64 mV, 59 mV and 26 mV (Figure 3a). Noted that although Pt/C-20/NF and Pt/C-40/NF required less overpotential than that of CoMo(Al)-P/NF at the current density of 10 mA cm−2, but the current density of CoMo(Al)-P/NF increased faster at higher overpotential and surpassed Pt/C-20/NF and Pt/C-40/NF at the potential of −0.103 V and −0.262 V (vs. RHE), respectively. That means in industrially water electrolysis for hydrogen production, usually operate at high current density, the CoMo(Al)-P/NF electrocatalyst is more applicable than expensive Pt/C electrocatalyst.38,39


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Evidently, large amount of H2 bubbles were generated and then separate from the CoMo(Al)-P/NF during HER test (inset of Figure 3a). As shown in Figure 3b and Figure S7, CoMo(Al)-P/NF possess smaller Tafel slope of ~55.02 mV dec−1 than CoMo-P/NF (~59.06 mV dec−1), CoP/NF (~98.13 mV dec−1) and Pt/C-20/NF (~75.36 mV dec−1), only larger than that of Pt/C-40/NF (~36.28 mV). The low Tafel slope indicates that the HER proceeded on CoMo(Al)-P/NF followed a Volmer–Heyrovsky mechanism, while Heyrovsky step was the rate determining step. This result further suggests that the water dissociation step (Volmer step) is efficiently promoted with the participation of CoMo(Al)-P/NF electrocatalyst.40,41

Figure 4. Alkaline HER stability of CoMo(Al)-P/NFand Pt/C-40/NF electrocatalysts. (a) and (b) Polarization curves at different CV cycling numbers. (c) Overpotential (η) of CoMo(Al)-P/NF and Pt/C-40/NF at a current density of 10 mA cm−2. (d) Time-dependent current density curves of CoMo(Al)-P/NF and Pt/C-40/NF under a static overpotential of 100 mV for 10 h.

The durability of CoMo(Al)-P/NF electrocatalyst toward HER was evaluated via prolonged CV cycling test and chronopotentiometric measurement. As shown in Figure 4a, the electrocatalytic


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HER activity of CoMo(Al)-P/NF displays an obviously increase for the first 1500 cycles CV test, and the polarization curves of CoMo(Al)-P/NF after 1500 and 3000 cycles CV were almost overlapped, suggesting its good durability. By contrast, Pt/C-40/NF electrocatalyst presents continuous activity decay during prolonged CV test (Figure 4b). Specifically, as observed in Figure 4c, the overpotential required for CoMo(Al)-P/NF to drive 10 mA cm−2 current density was prominently reduced from 64 mV to 49 mV after 1500 cycles CV tests, and the Tafel slope decreased from 55.02 mV dec−1 to 43.31 mV dec−1 (Figure S8). Such excellent HER performance of CoMo(Al)-P/NF is comparable to reported advanced phosphide electrocatalysts (Table S1). Nevertheless, in sharp contrast, the overpotential needed for Pt/C-40/NF to reach 10 mA cm−2 current density increased from the initial value of 26 mV to 45 mV and 68 mV after 1500 and 3000 cycles, respectively. Furthermore, under 100 mV overpotential, the current density of CoMo(Al)P/NF increased initially and then kept stable, while Pt/C-40/NF shows continuous current density decay during reaction (Figure 4d and Figure S9). Thus, CoMo(Al)-P/NF not only possess comparable activity to that of commercial Pt/C, but also excellent stability during long-time HER. 2.3 The effects of Mo, Al species in CoMo(Al)P during HER To further investigate the effects of doped Mo and co-existed Al species in CoMo(Al)P electrocatalyst during HER, additional theoretical analysis were also carried out. Density functional theory (DFT) calculations were firstly performed to uncover the intrinsic influence of Mo dopants on the HER activity of CoP. Here, we only consider two of the mainly exposed crystal surfaces, (211) and (011), which were determined from XRD patterns of CoP nanocrystals. On the clean (211) and (011) surfaces, hydrogen preferentially adsorbs at the Co-Co bridge site (Figure 5a). With the introduction of Mo, the initial Co-Co bridge is broken and the new Co-Mo bridge is formed, leading to the hydrogen atom transferring from bridge site to the near Co-top site. The


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calculated free energy of the adsorbed H (ΔGH*) on Mo doped CoP (211) (0.003 eV) and Mo doped CoP (011) (0.25 eV) are much closer to the thermoneutral than that on CoP (211) (−0.33 eV) and (011) (0.51 eV), suggesting that Mo doping could facilitate the hydrogen adsorption/desorption process (Figure 5b).

Figure 5. (a) Optimized atomic structure of H atoms adsorbed on CoP (011) (left), (211) (right) and Mo doped CoP (011) and (211) surfaces. Blue, gray, yellow and green balls represent Co, P, Mo, H atoms, respectively. (b) Free-energy diagram for HER on CoP and Mo doped CoP surfaces. (c) The density of states (DOS) of transition metal d orbitals before and after Mo doped CoP (211) surface. (d) The partial density of states (PDOS) of H s orbitals and transition metal d orbitals on CoP (211) surface. The Fermi level is set to be 0 eV.

To gain insights into the mechanism at the level of electronic structure, the density of states (DOS) around the Fermi level which is mainly originated from the d states is investigated before


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and after Mo-doping (Figure 5c). Previous study has demonstrated that the d band center model would be a good descriptor of the adsorbate-metal interaction, since it lie between the bonding and antibonding states which derive from the coupling between the adsorbate valence states and transition metal d states.42 Impressively, the d-band center (Ed) of CoP (211) and Mo doped CoP (211) relative to the Fermi level (EF) are calculated to be −2.31 eV and −2.39 eV, respectively (inset of Figure 5c), clearly indicating the d band center is shifted away from the Fermi level after Mo doping. It also suggests the antibonding states are lowered and the interaction between adsorbate and surface is weakened. On the other hand, the coupling between H s states and transition metal d states is directly observed based on the partial density of states (PDOS), as plotted in Figure 5d. It could be found that the interactions between H s states and Co&Mo d states is weaker than that between H s states and Co d states through the integration of states at the energy of −6 eV to 0 eV, suggesting the weaker interactions between adsorbate and surface after Mo doping. These studies provide the intrinsic elucidation that the downshift of d band center with more electron filling of the antibonding states after Mo doping decreases the adsorption energy of H and meanwhile facilitates the desorption of H from the catalyst surface for HER catalysis.


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Figure 6. (a) Typical TEM image, (b) HR-TEM image, (c) STEM image and corresponding elemental mapping images of (d) CoMoAl-P after 3000 cycles CV tests.

Subsequently, TEM and energy-dispersive spectroscopy (EDS) analysis were employed to determine the structure—activity relationship of CoMo(Al)-P electrocatalyst during HER. As shown in Figure 6a, the post-catalysis CoMo(Al)-P featured high porosity and was renamed as porous Mo-doped CoP electrocatalyst (pCoMo-P). The HR-TEM image shows that Mo doped CoP nanocrystals maintained in the pCoMo-P electrocatalyst, the space distance between two adjacent lattice fringes was 0.249 nm, which corresponds to the (111) plane of CoP (Figure 6b). In addition, the STEM image once again displays the representative porous structure of pCoMo-P which consisted of closely adjacent Mo doped CoP nanocrystals (Figure 6c). Notably, due to the generated porous structure, the double layer capacitance (Cdl) of pCoMo-P/NF (post-catalysis CoMo(Al)-P/NF) increased from 65.22 mF cm−2 to 118.74 mF cm−2 (Figure S10), which indicates that the ECSA of post-catalysis pCoMo-P electracatalyst was increased after prolonged CV cycling tests. Besides that, the pCoMo-P electrocatalyst also shows a lower electrode/electrolyte interfacial resistance than its freshly prepared CoMo(Al)-P electrocatalyst (Figure S12). Thus, the porous structure in phosphide electrocatalysts is highly desirable for it can provides more accessible catalytically active sites and facilitate electron/mass transportation during HER.26,27 The elemental mapping images in Figure 6d, combined with EDS shown in Figure S12, demonstrates that the Mo and Al elements existed in pCoMo-P electrocatalyst. But the atomic ratio of Al to M (M= Co, Mo) in electrocatalyst decreased from the initial value of 14.9% (CoMo(Al)-P) to 1.4% (pCoMo-P). Accordingly, the dissolution of Al species play a critical role in modifying the morphology and further boosting the electrocatalytic HER activity of CoMo(Al)-P/NF during dynamical reaction.43-45 By the way, the CoMo-P nanosheets, with no Al species added in its


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hydroxide precursors, did not exhibit porous structure or activity increase during CV cycling test (Figure S13).

Figure 7. Detailed high-resolution XPS survey of Co 2p, P 2p, Mo 3d, Al 2p in CoMoAl-P catalyst before (a1, b1, c1, d1) and after (a2, b2, c2, d2) 3000 cycles CV tests.

Based on the above analysis, detailed high-resolution X-ray photoelectron spectroscopy (XPS) surveys, toward the surface chemistry of freshly prepared CoMo(Al)-P and pCoMo-P (postcatalysis CoMo(Al)-P), were also investigated to obtain more information regarding the origin of much improved electrocatalytic HER performance. As shown in Figure 7a, the peaks located at 778.8 eV and 793.9 eV can be attributed to the 2p3/2 and 2p1/2 of cobalt phosphides, respectively, and those peaks at 781.1 eV (2p3/2) and 797.7 eV (2p1/2) can be assigned to the cobalt oxides caused by exposure to air.46-48 The peaks at 785.5 eV and 803.4 eV corresponded to two satellite peaks of cobalt. Notably, more Co-P sites were exposed and accessible to reactant after prolonged CV tests (Figure 7a2) than its initial state (Figure 7a1). This result can also be verified by the XPS


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analysis of P. The peaks at 129.3 eV (P 2p3/2) and 130.2 eV (P 2p1/2), belong to the phosphides in pCoMo-P (Figure 7b2), were stronger than that of the freshly obtained CoMo(Al)-P (Figure 7b1).49 This phenomenon reconfirmed that the oxidized P species (133.3 eV, P-O bond) had been decreased and allowed more catalytically active sites to be exposed in HER. It is worth noting that the phosphorous sites are not simple spectators during HER, for phosphorous and metal sites in transition-metal phosphides respectively act as proton-acceptor sites and hydride-acceptor sites during catalytic hydrogen evolution, which endow phosphides with high activity toward HER.50 As shown in Figure 7c, Mo element displays two kinds of valence states in freshly synthesized and post-catalysis CoMo(Al)-P (after 3000 cycles CV tests), include Mo5+ at 231.2 eV and 234.3 eV, and Mo6+ at 232.7 eV and 236.0 eV.45 But only Mo6+ existed in its hydroxide precursor (Figure S14).51 This result indicates that Mo6+ were partially reduced to Mo5+ during phosphorization and the Mo5+ can be maintained in reaction. Notably, coordinatively unsaturated Mo5+ sites are highly efficient in transferring electrons to adsorbed reactants, therefore, enhanced the HER activity of CoP electrocatalyst after doping with the pentavalent Mo.52 This conclusion was in consistent with the aforementioned theoretical analysis. Figure 7d1 shows two peaks of Al, the weak peak at 74.4 eV can be attribute to the Al(OH)3 which can also be observed in its hydroxide precursors (Figure S15), and the newly emerged peak at 75.4 eV belongs to the aluminum native oxide.53 After 3000 cycles CV tests (Figure 7d2), the peaks of Al species disappeared. This result, together with the EDS analysis of pCoMo-P in Figure S12, indicate that almost all Al species in CoMo(Al)-P electrocatalyst decomposed and dissolved in strong alkaline solution during HER. Considering that the dissolution of Al species would not only create a great number of defects in the Mo-doped CoP lattices, but also expose more catalytically active Co and


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Mo sites on the surface of catalyst.43,45 Therefore, as a result, the electrocatalytic HER activity of pCoMo-P electrocatalyst was further enhanced. 3. CONCLUSIONS In summary, a Mo-doped CoP porous nanosheets supported on nickel foam (pCoMo-P/NF), mainly consisted Co0.76Mo0.24P nanocrystals with an average size of 4.11 nm, was obtained from CoMoAl-LTH/NF derived phosphides with Al-species-dissolved. Primarily, the introduced Mo and Al elements in Co(OH)2 matrix can efficiently prevent the aggregation of doped CoP nanocrystals during high temperature phosphorization. Then, both experimental and theoretical analysis confirmed that the Mo-doped CoP, in which (211) and (011) were the mainly exposed crystal surfaces, can achieve a Pt-resembling Gibbs free energy for H* adsorption and efficiently facilitate hydrogen evolution from water. Also, in alkaline solutions, the dynamic dissolution of Al species in CoMoAl-P would expose more catalytically active Co, Mo and P sites and lower the electrode/electrolyte interfacial resistance, which further endow the as-prepared electrocatalyst with higher activity toward HER. As a result, pCoMo-P/NF electrocatalyst shows a very high HER activity with only 49 mV overpotential to drive 10 mA cm−2 current density in 1.0 M KOH, and this value is comparable to that of commercial 20 wt% Pt/C. Also, the current density of pCoMoP/NF increased faster and superior to that of commercial 40 wt% Pt/C at higher overpotential (η > 248.5 mV, 1.0 M KOH). Importantly, the pCoMo-P/NF electrocatalyst also exhibits prolonged stability with negligible activity decay after 3000 cycles of CV test or chronopotentiometric measurement under a static overpotential of 100 mV for 10 h. This work not only provides an efficient strategy to design both highly active and robust HER electrocatalyst, but also investigates the crucial doping effects in both phosphide electrocatalyst as well as in the transition of hydroxides to phosphides.


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4. EXPERIMENTAL SECTION Materials and chemicals: NF (1 mm thickness, 300 g m−2 areal density, 99.8% purity) was purchased from Liyuan Lithium Technology Center. Co(NO3)2·6H2O, Al(NO3)3·9H2O, (NH4)6Mo7O24·4H2O, urea (CO(NH2)2), NH4F, and NaH2PO2·H2O were purchased from Sigma Aldrich. Commercial 20% Pt/C and 40% Pt/C was purchased from Alfa Aesar. All chemicals used in this work were of analytical grade and without further purification. Deionized water (resistance, 18 MΩ cm−1) was used throughout all experiments. Synthesis of CoMoAl-LTH/NF, CoMo-LDH/NF, and Co(OH)2/NF: CoMoAl-LTH, CoMoLDH, and Co(OH)2 were synthesized through a urea hydrolysis route.54 Initially, NF (1 cm × 3 cm) was carefully washed with the assistance of sonication to remove surface oxide layer in 1 M HCl solution, ethanol, and deionized water consecutively for 20 min. To synthesize CoMoAl layered triple hydroxides on NF (CoMoAl-LTH/NF), 150 mg of Co(NO3)2·6H2O, 30.3 mg of (NH4)6Mo7O24·4H2O, 32.2 mg of Al(NO3)3·9H2O, 300 mg of urea, and 100 mg of NH4F were firstly dissolved in 40 mL of deionized water. Then, the cleaned NF was immersed in the hybrid solutions and then transferred into a 50-mL Teflon-lined stainless-steel autoclave. Subsequently, autoclave was sealed and kept at 120 °C for 12 h to grow hydroxides on NF. Finally, the CoMoAlLTH/NF samples were washed with deionized water and dried at 60 °C for 12 h. CoMo-LDH/NF and Co(OH)2/NF samples were prepared and pre-treated with the same procedure except adding Al(NO3)3·9H2O or (NH4)6Mo7O24·4H2O during hydrothermal process. Synthesis of CoMo(Al)-P/NF, CoMo-P/NF and CoP/NF: CoMoAl-LTH/NF, CoMo-LDH/NF, and Co(OH)2/NF were directly transformed into phosphide with the assistance of in situ generated PH3 via thermal decomposition of hypophosphites. Briefly, the obtained NF-supported hydroxide samples and 500 mg of NaH2PO2·H2O were placed on the separate positions of ceramic boat inside


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a tube furnace, and NaH2PO2·H2O powder was at the upstream of gas flow. After flushing with Ar to squeeze out air for 30 min, both samples and NaH2PO2·H2O powder were heated to 350 °C and kept for 2 h (heating rate, 2 °C min−1). After naturally cooling to ambient temperature in Ar atmosphere, the atomic ratio in the obtained phosphide samples were detected by EDS and named as CoMo(Al)-P/NF, CoMo-P/NF, and NF@CoP. In Particular, the Al species dissolved, porous post-catalysis CoMo(Al)-P/NF was renamed as pCoMo-P/NF. It should be noticed that the unreacted and poisonous PH3 during phosphating process was absorbed with concentrated CuSO4 solution. Materials characterization: Before characterization of as-synthesized samples, the obtained materials were stripped off from NF under ultrasound and then collected by vacuum freeze-drying method. Before XRD analysis test, 15 mg of powder sample was extruded into a wafer shape with 2 cm in diameter. Phase structure of the prepared catalysts was analyzed through Philips X’Pert system with Cu Kα radiation (λ = 0.15419 nm, scan step size = 0.0334, and time per step = 120 s). The surface chemical constituents of the prepared catalysts were analyzed by XPS (Thermo ESCACLB 250). The morphologies of the prepared samples were observed by FESEM (Hitachi SU 8020) at an accelerating voltage of 10 kV. TEM images, high-angle annular dark-field (HAADF) scanning TEM images, and EDS elemental mapping images were captured by a FEI Tecnai TF 20 operated at 200 kV. Electrochemical measurements: All electrochemical experiments were conducted at room temperature (25 °C) with a standard three-electrode electrochemical workstation (Zahner IM6ex). The as-synthesized electrodes were directly used as working electrodes. A carbon paper and an Ag/AgCl electrode were adopted as the counter and reference electrodes, respectively. All measured potentials in this work were calibrated to reversible hydrogen electrode (RHE) via the


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following equations: ERHE = EAg/AgCl + 0.197 + 0.059 pH. Prior to electrochemical data acquisition, the working electrode was pre-activated with 20 cycles of CV test in 1.0 M KOH at a scan rate of 50 mV s−1 (−0.2~0.2 V vs. RHE). Then, polarization curves were obtained using linear sweep voltammetry (LSV) with a scan rate of 5 mV s−1. The Tafel slopes were determined by fitting the linear regions of the Tafel plots to the Tafel equation (η = b log(j) + a) by replotting the polarization curves. Electrochemical impedance spectroscopy (EIS) was performed with frequency from 10−1 to 105 Hz and an amplitude of 10 mV. The long-term stability of as-obtained electrocatalysts was evaluated by using chronopotentiometric and CV measurements. All the electrochemical measurements were recorded without iR correction. For comparison, 2 mg of commercial 20 wt% or 40 wt% Pt/C (Alfa Aesar) was dispersed in a 1-mL solution containing 0.5 mL of ethanol, 0.4 mL of deionized water, and 0.1 mL of 5 wt% Nafion solution with the assistance of ultrasonication. Then, 50 µL of the homogeneous ink was evenly dropped on the surface of 1 cm2 NF and then dried in air at 60 °C. The obtained electrodes were named as Pt/C-20/NF and Pt/C-40/NF. Computational methods：The structure relaxation and total-energy calculation are carried out using the density functional theory within the generalized gradient approximation (GGA), as implemented in the VASP 5.4.1 package.55 Electronic exchange and correlation are described by Perdew−Burke−Ernzerhof (PBE) functional.56 All-electron plane-wave basis sets with the projector augmented wave (PAW) potentials are adopted with 1s1, 3s23p3, 3d74s2 and 4d55s1 treated as valence electron configuration for H, P, Co, and Mo respectively, and the cutoff energy is set to be 400 eV. A dense enough k points sampling is checked with energy tolerance in 1 meV/atom. The surfaces are represented by periodic slab models. A vacuum larger than 10 Å thick is inserted in each model to avoid interaction with imaging free surfaces. Similarly, the lattice


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parameters of each slab supercell chosen according to the corresponding optimized bulk parameters, is also larger than 10 Å to avoid interaction between the adsorbed hydrogen atom and its image. The optB88-vdW functional was used to calculate the adsorption energy, which is an efficient method to approximately account for the long-range vdW interaction.57 The hydrogen adsorption free energy ΔGH* = E(surf+H) − E(surf) − 1/2E(H2) + ΔEZPE − TΔS, where ΔEZPE and ΔS are the difference in the zero-point energy and entropy between the adsorbed H atom and the gaseous phase H2. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.xxxxxx. Figures S1-S15 showing XPS, SEM, TEM, XRD characterization and electrochemical measurements of as-prepared phosphide catalysts. Table S1 showing the activity comparison of phosphide electrocatalysts in alkaline HER. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. L.) Author Contributions ‡ Han, Y. C. and Li, P. F. have contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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We gratefully acknowledge the financial support from the National Basic Research Program of China (2014CB931704), the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201627), the National Natural Science Foundation of China (NSFC, No. 11504375, 11604320, 11674321, 51571186) and the Anhui Provincial Natural Science Foundation (1508085QA21). We also appreciate Dr. Hailong Wang and Dr. Yanfen Wen for their assistance in paper polishing. REFERENCES (1) Dunn, S. Hydrogen Futures: Toward a Sustainable Energy System. Int. J. Hydrogen Energy 2002, 27 (3), 235-264. (2) Staffell, I.; Scamman, D.; Abad, A. V.; Balcombe, P.; Dodds, P. E.; Ekins, P.; Shah, N.; Ward, K. R. The Role of Hydrogen and Fuel Cells in the Global Energy System. Energy Environ. Sci. 2019, 12 (2), 463-491. (3) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I. B.; Norskov, J. K. Computational High-throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5 (11), 909-913. (4) Holladay, J. D.; Hu, J.; King, D. L.; Wang, Y. An Overview of Hydrogen Production Technologies. Catal. Today 2009, 139 (4), 244-260. (5) Rausch, B.; Symes, M. D.; Chisholm, G.; Cronin, L. Decoupled Catalytic Hydrogen Evolution from a Molecular Metal Oxide Redox Mediator in Water Splitting. Science 2014, 345 (6202), 1326-1330. (6) Chi, J.; Yu, H. M. Water Electrolysis based on Renewable Energy for Hydrogen Production. Chinese J. Catal. 2018, 39 (3), 390-394.


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Table of Contents

Molybdenum-doped Porous Cobalt Phosphide Nanosheets for Efficient Alkaline Hydrogen Evolution

A nickel foam supported porous Mo-doped CoP nanosheets (pCoMo-P/NF) was employed as an alkaline electrocatalyst for hydrogen evolution reaction. The introduced Mo in CoP can modulate its electronic structures and achieve a Pt-resembling Gibbs free energy for H* adsorption, while the dynamic dissolution of Al species would induce more active sites and decreased interfacial resistance.


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Molybdenum-doped Porous Cobalt Phosphide Nanosheets for

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