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Design of basal plane active MoS2 through one-step nitrogen and phosphorus co-doping as an efficient pH-universal electrocatalyst for hydrogen evolution
Author’s Accepted Manuscript Design of Basal Plane Active MoS 2 through Onestep Nitrogen and Phosphorus Co-doping as an Efficient pH-universal electrocatalyst for Hydrogen Evolution Kaian Sun, Lingyou Zeng, Sihui Liu, Lei Zhao, Houyu Zhu, Jinchong Zhao, Zhi Liu, Dongwei Cao, Yongchun Hou, Yunqi Liu, Yuan Pan, Chenguang Liu
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S2211-2855(19)30112-0 https://doi.org/10.1016/j.nanoen.2019.02.006 NANOEN3450
To appear in: Nano Energy Received date: 13 September 2018 Revised date: 5 January 2019 Accepted date: 2 February 2019 Cite this article as: Kaian Sun, Lingyou Zeng, Sihui Liu, Lei Zhao, Houyu Zhu, Jinchong Zhao, Zhi Liu, Dongwei Cao, Yongchun Hou, Yunqi Liu, Yuan Pan and Chenguang Liu, Design of Basal Plane Active MoS 2 through One-step Nitrogen and Phosphorus Co-doping as an Efficient pH-universal electrocatalyst for Hydrogen Evolution, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design of Basal Plane Active MoS2 through One-step Nitrogen and Phosphorus Co-doping as an Efficient pH-universal electrocatalyst for Hydrogen Evolution
Kaian Suna, Lingyou Zenga, Sihui Liua, Lei Zhaoa, Houyu Zhub, Jinchong Zhaoa, Zhi Liua, Dongwei Caob, Yongchun Houb, Yunqi Liua*, Yuan Panc*, Chenguang Liua
a
State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China
University of Petroleum (East China), Qingdao, 266580, China. b
College of Science, China University of Petroleum (East China), 66 West Changjiang
Road, Qingdao 266580, China c
Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China
[email protected] [email protected]
*
Corresponding authors.
Abstract The exploration of low-cost, stable, and highly active noble-metal-free electrocatalyst for hydrogen evolution reaction (HER) in a wide pH range is crucial but still challenging task for renewable energy techniques. MoS2-based materials have been considered as a promising electrocatalyst for HER. However, corresponding studies have been hampered by the lack of effective routes to fully utilize the large number of inert basal plane for catalyzing HER, especially under alkaline media. Herein, a novel ammonia
1
ions-guided-nitrogenization-phosphorization strategy is developed to prepare N and P co-doped MoS2 with active basal plane for efficient catalyzing HER with a quite low overpotential of 116 and 78 mV in 0.5 M H2SO4 and 1.0 M KOH to achieve a current density of 10 mA•cm−2, respectively. Experimental studies and theoretical calculations confirm Mo-N-P sites in the basal plane of MoS2 can not only accelerate HER kinetics, but also result in energetic favorability and structure stability. Furthermore, outstanding performances are also obtained under both sea and river water, vastly broadening the application prospects.
Graphical Abstract TOC
A novel ammonia ions-guided-nitrogenization-phosphorization strategy was developed to prepare N and P co-doped MoS2 with active basal plane for HER under a wide pH value, especially sea and river water. The active basal plane is originated from the new formation Mo-N-P site.
Keywords: Molybdenum disulfide; Hydrogen evolution reaction; Basal plane; Density functional theory.
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1. Introduction Ever-increasing global concerns about environmental pollutions and energy crisis have kindled a great desire for alternative clean energy sources and renewable carriers. Molecular hydrogen (H2), is not only considered as one of the most promising clean fuel sources, but also regarded as an ideal carrier to store energy from renewable energy sources (e.g., solar and wind) into the chemical bond via electrochemical hydrogen evolution reaction (HER) [1]. To improve the HER energy conversion efficiency, a leapfrog development is needed to design inexpensive commercially available electrocatalyst to the replacement of high-cost and scarce precious-metal catalysts (i.e., Pt, Pd, and Rh) [2]. Up to date, transition metal sulfides [3], selenides [4], phosphides [5], carbides [6], and nitride [7] have been widely investigated. Among these, MoS2 has been emerged as a promising candidate in transition metal sulfides, due to its cost-effective, earth-abundant, and high performance under acidic condition. Unfortunately, the HER activity and stability of MoS2-based materials under alkaline medium is still far less than that of benchmark Pt/C, limiting its potential of being a pH-universal electrocatalyst to address large-scale industrial requirements. Toward the goal of making MoS2 as an efficient HER catalyst under alkaline media, it has recently been proven that interface engineering, including MoS2/Ni3S2 [8], NiS2/MoS2 [9], Co-Ex-MoS2 [10], Ni(OH)2/MoS2 [11], etc. can tune the electronic structure at the edge sites of MoS2 for reducing the kinetic energy barrier during the hydrogen generation pathway. Although the significant progresses have been achieved, the activation of
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the dominant basal plane, which constitutes the majority of bulk MoS2, still remains a challenge under alkaline condition. As an easily-scalable and cost-effective method, doping with non-metal heteroatoms into MoS2 can not only tailor the electronic structure effectively, but also trigger basal plane active sites to fully utilize MoS2 for HER under acidic condition [12]. However, the application of non-metal heteroatoms doped MoS2 as a pH-universal electrocatalyst has not been implemented yet. In specific, inspired by the enhanced electro-catalysis activity of carbon-based materials via co-doping with two elements, one with higher electro-negativity and one with lower than that of C (2.55), for example, N (3.04) and B (2.04) or P (2.19) [13], we anticipate that substituting S (2.58) of MoS2 with N and P may create a unique electronic structure to achieve desired applications. Herein, we develop a novel ammonia ions-guided-nitrogenization-phosphorization (AGNP) strategy toward the preparation of N and P co-doped MoS2 array anchored on carbon cloth (NP-MoS2/CC) as a self-supported electrode for efficient catalyzing HER over a broad pH value. Distinct from previous studies [14], the present AGNP strategy relies on the intercalated NH4+ in MoS2. The intercalated NH4+ guaranteed in situ homogeneously doping of N without production of unstable crystal structure and the introduction of N further favor the co-doping of P at low temperature [15]. In addition, this strategy can be extended to fabricate single P or N doped MoS2 grown on CC (P-MoS2/CC and N-MoS2/CC, respectively) with fixed structure to NP-MoS2/CC, providing a scalable pathway to systematically investigate the effect of co-doping.
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Electrochemical measurements demonstrated that the activity and stability of NP-MoS2/CC were much higher than that of P-MoS2/CC or N-MoS2/CC under both acidic and alkaline condition. Impressively, the activitiy of NP-MoS2/CC under both sea and river water was superior to those of all previously reported electrocatalysts and outperform commercial Pt/C catalyst. Experimental and theoretical results revealed that the excellent activity and durability originate from the new formation Mo-N-P site. Our study and understanding may shed some new lights for designing highly efficient pH-universal HER electrocatalyst.
2. Results and Discussions The AGNP strategy for NP-MoS2/CC involved two steps (Fig. 1a. The detail experiment processes are shown in Supporting Information). Firstly, NH4+ intercalated MoS2 nanosheets arrays (NH4+-MoS2/CC) was prepared by a hydrothermal process according to our previous report [16]. Urea was added into the reaction system to release NH4+, which could be absorbed between the interlayer of MoS2. CC with rough surface and abundant surface oxygen/hydrogen-containing functional groups offered an ideal scaffold to anchor NH4+-MoS2 for meeting the requirements of innovative energy conversion technologies, due to its feature of excellent electrical conductivity, stability, and flexibility. A polyethylene glycol/water mixed solvent was used to slow down the dipole-dipole interactions between primary crystal nuclei, facilitating the orientation growth of MoS2 nanosheets on CC. Scanning electron micros-copy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis and mass spectrometry (TG-MS) characterizations were revealed the product structure
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as NH4+-MoS2/CC (Fig. S1 and S2). Then, a low-temperature one-step nitrogenization and phosphorization process was conducted at 400 °C for 1 h in Ar atmosphere. The intercalated NH4+ and NaH2PO2 acted as precursors to generate free NH4+ and PH3, respectively. The free NH4+ and PH3 then simultaneously reacted with MoS2 to form NP-MoS2/CC (Fig. S3). In particular, this strategy can be easily extended to fabricate other single non-metal heteroatoms doped MoS2, including N- and P-MoS2/CC. The optimized N- (ca. 3.2 atom%) and P-MoS2/CC (ca. 1.6 atom%) have a similar structure to NP-MoS2/CC, as revealed by SEM, TEM, XRD, Brunauer-Emmett-Teller (BET), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) (Fig. S4-11 and Table S1). The structure of NP-MoS2/CC was firstly investigated by SEM, showing that the array structure is well preserved after the second step reaction (Fig. 1b). The whole surface of CC is homogeneously covered by cross-linked nanosheets, with an average size of 200-400 nm, resulting in a rough and highly porous structure. N2 sorption isotherm of NP-MoS2/CC exhibits a typical IV isotherm, presenting a characteristic of mesoporous. The BET specific surface area is 40 m2•g−1 and the pore width distributes in a wide range of 2-20 nm (Fig. S12). Such an edge-terminal nanostructure with abundant mesoporous facilitate the electrode kinetics and accelerate the transport of the evolved H2. A closer observation on the edge of NP-MoS2/CC by the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and the corresponding EDS element mapping illustrate that N, P, S, and Mo elements are uniformly distributed through the as-grown NP-MoS2 nanosheets and the bulk Mo/S/P/N atom ratio is 33.2:62.6:1.2:2.9, in agreement with SEM-EDS results (Fig. 1c-h and Fig. S13). The phase purity was confirmed by XRD analysis (Fig. 1i and 1j).
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Eliminating the peaks arising from the CC (JCPDS card No. 65-6212), all the peaks are well-matched with the hexagonal 2H phase MoS2 (2H-MoS2) with space group P63/mmc (JCPDS card No. 37-1492). This suggests that N and P co-doping does not cause phase structure changes of 2H-MoS2, whithout the formation of unstable 1T phase MoS2, MoPx or MoNx. A magnification at (100) diffraction peak shows an obvious shift to higher angle compared with a physical mixture of single N- and P-MoS2, indicating that there possibly exists strong interaction between N and P rather than a simple phase separation condition. According to Bragg's equation, the d-spacing of the (100) plane can be estimated to be about 0.25 nm, which is in accordance with the results of TEM and smaller than that of bulk MoS2 (Fig. 1k). To further evaluate the local atomic structure around Mo quantitatively, extended X-ray absorption fine structure (EXAFS) was carried out. The Fourier transform (FT) curves of EXAFS present two main peaks at around 1.9 and 2.9 Å in all samples, corresponding to Mo−S and Mo−Mo bond, respectively (Fig. 2a) [17]. However, a new shoulder peak at around 1.5 Å corresponding to Mo−N bond can be observed N-MoS2 and NP-MoS2 [18]. Notably, the Mo−N distance in NP-MoS2 is slightly longer than that of N-MoS2 and the intensity ratios of the Mo-S/Mo-N peaks for NP-MoS2 are lower than that of N-MoS2, which possibly resulted from the missing atoms in the Mo coordination sphere after P incorporation. Moreover, because the Mo-P peak have an overlapping position in the Mo-S peak [19], the nature of doping was further identified by soft X-ray absorption near-edge spectroscopy (XANES). Except for the peaks of phosphate species, the lowest spin-orbit doublet at around 129.8 and 130.8 eV can be observed in P-MoS2 and NP-MoS2, corresponding to the 2p 3/2 → * and 2p 1/2 → * transition, respectively [20]. These two features can be attributed to the Mo atoms attached to the
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doped P atoms [21]. In comparison to P-MoS2 samples, the * transition of NP-MoS2 exhibits a slight energy shift to higher energies and a new N-P peak at around 133.1 eV, consistent with the depth profile XPS results (Fig. 2b and S14) [22]. These changes in the XANES features suggest a strong chemical interaction occurs between Mo-N bond and Mo-P bond, corresponding to Mo-N-P site in the NP-MoS2. Moreover, the strong interaction between N and P is also confirmed by Raman and Fourier transform infrared (FTIR) spectroscopy. Raman spectra reveal a new Raman band at around 986.4 cm−1, corresponding to the N–P bond, alongside the major vibration bands for the in-plane (E1 2g) and out-of-plane (A1g) modes of 2H-MoS2 at around 371.2 and 401.4 cm−1, respectively (Fig. 2c) [23]. For the FTIR results, the peak at around 1438.3 cm−1 correspond to N−P stretching modes (Fig. 2d) [24]. Additionally, it should be pointed that N can encourage the co-doping of P at low temperature (400 °C), whereas single P doping can only be achieved at high temperature (600 °C) (Fig. S15). The electrocatalytic HER performance of NP-MoS2/CC was investigated in 0.5 M H2SO4 and 1.0 M KOH solution using a standard three-electrode setup. For comparison, bare CC, commercial 20% Pt/C, P-, N-, and 2H-MoS2/CC were also tested in parallel. The bare CC shows negligible activity, whereas the Pt/C shows superior activity in both acid and alkaline media. As expected, single N and P doping can boost the activity of MoS2 under acid media. Interestingly, N or P are also an effective promoter to modulate HER performance of MoS2 under alkaline media. More interestingly, the performance of single element doped MoS2 can be further enhanced by the incorporation of a second heteroatom. The NP-MoS2/CC shows quite low overpotentials of 116 and 78 mV at the current densities of 10 mA•cm−2 in 0.5 M H2SO4 and 1.0 M KOH, respectively (Fig. 3a and 3b). Impressively, the overpotential under alkaline media is superior to those of all
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previously reported MoS2-based electrocatalyst and very close to commercial Pt/C catalyst (Table S2 and S3). The extracted Tafel slopes of the NP-MoS2/CC are 58.4 and 51.6 mV•dec−1 at low overpotential ranges in 0.5 M H2SO4 and 1.0 M KOH, respectively, suggesting the HER occurs on NP-MoS2/CC via a Volmer-Heyrovsky mechanism (Fig. S16 and S17) [25]. Moreover, the NP-MoS2/CC shows considerably smaller charge transfer resistances and obviously higher double-layer capacitances (Cdl) in comparison with P- or N- MoS2/CC, indicating that N and P co-doping could either accelerate HER kinetics or facilitate active site proliferation (Fig. S18-20 and Table S4). The turnover frequencies (TOF) value of NP-MoS2/CC was determined to be 0.29 and 0.28 H2 s−1 at an overpotential of 200 mV, which is larger than that of and P-MoS2/CC (0.14 and 0.12 H2 s−1) and N-MoS2/CC (0.12 and 0.04 H2 s−1), in 0.5 M H2SO4 and 1.0 M KOH, respectively (Fig. S21). Since NP-MoS2/CC has a lower N and P contents than the corresponding N- or P-MoS2 and the similar morphology among all samples, the increased TOF value indicates that the superior activity could be attributed to the synergistic effect between N and P. To verify the real structure in charge of the HER, we further compared HER activity of the fresh NP-MoS2/CC sample with the aged sample, which prepared by partially oxidation in O2-saturated aqueous solution over five days. The polarization curve of aged sample shows no performance degradation compared with the fresh sample (Fig. S22). XPS results indicate that the aged sample has been partially oxidized with abvious Mo-O, P-O and especially N-P peaks, but without Mo-P peaks (Fig. S23). Previously study indicates that the oxidation of nanostructure initiates at the edge, thus, the synergistic effect is mainly originated from the new formed Mo-N-P site on basal plane, which can also enhance the durability for HER catalysis in a significant manner. After 2000 scanning cycles ranging from +0.2 to −0.2 V, the
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overpotential of NP-MoS2/CC shows an ignorable increment of 3.4 mV and 5.1 at 50 mA•cm−2 in acidic and alkaline media, respectively. In contrast, N- or P-MoS2/CC suffer from severe performance degradation. After the cycling tests, the NP-MoS2/CC retains its primary morphology, while the structure of P or N-MoS2/CC are slightly collapsed as shown in SEM images (Fig. S24). XPS and TEM tests demonstrate no obvious change in chemical compositions, especially N-P species for used NP-MoS2 nanosheets (Fig. S25 and S26). Furthermore, considering the demands of practical application, chronopotentiometric curves were recorded at large current densities of 500 and 1000 mA•cm−2. The overpotentials can be maintain at around 316 and 295 mV at the current density of 500 mA•cm−2 and only 15 and 19% decay at the current density of 1000 mA•cm−2 during 20 h of continuous operation in 0.5 M H2SO4 and 1.0 M KOH, respectively (Fig. 3c and 3d). Density functional theory calculations were performed to gain molecular-level understanding of the excellent HER activity. Firstly, the possibility of Mo-N-P site incorporation into the MoS2 lattice on basal plane was evaluated. Different types of N and P co-doped atomic models were adopted (Fig. S27-29). The formation energy is obviously decreased with Mo-N-P site formation. Since the electro-negativity of the S is located between the P and N, the N atom is first polarized by Mo, and then can draw electrons from P directly, verifying by Bader charge (Table S5) and XPS analysis (Fig. S30). Therefore, Mo-N-P site is favoured by co-doping with N, which leading to energetic favourability and structure stability, corresponding to the experimental results. Then, the kinetic energy barrier of water dissociation (∆GH O) was calculated. The ∆GH O is con2
2
sidered as a key rate-limiting Volmer step under alkaline media [27]. The value of ∆GH O 2
of N-MoS2 is 3.17 eV, still making the extremely difficult to dissociate water on inert
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basal plane (Fig. 3e and S31). However, after P doping into the MoS2 lattice, the value of ∆GH O on basal plane is significantly reduced, owing to that the presence of P and 2
neighbour S help to stabilize the hydroxide (OH*) and hydrogen (H*) intermediates, respectively, and hence favour the initial water dissociation process. Strikingly, with introduction of N co-dopant to form Mo-N-P site, the ∆GH O value on NP-MoS2 further 2
reduces to 0.72 eV, approaching that of Pt (0.44 eV) [28]. Meanwhile, the basal plane of NP-MoS2 can provide not only effectively sites for water dissociation, but also an appropriate OH* and H* acceptor sites. The OH* adsorption energy (∆GOH*) on P atom and H* adsorption energy (∆GH*) on neighbour S atom in NP-MoS2 are −0.74 and 1.89 eV, respectively, which are much weaker than those of P-MoS2 (−1.03 and 0.39 eV, respectively), indicating the desorption hydroxyl and proton are facilitated remarkably and preventing the blockage of active sites to increase the rate of proton formation (Fig. S32). Subsequently, the generated proton is favorably adsorbed on P atom, owning to the lower electro-negativity of P than S. It is worth noting that the ∆GH* value on P atom in NP-MoS2 is −0.21 eV, which is closer to thermo-neutral state (i.e., ∆GH* ≈ 0 eV) than P atom in P-MoS2 (−0.96 eV), denoting a balance between the transfer of proton and the removal of H2 on P atom in NP-MoS2 for further accelerating HER kinetics under alkaline media [29]. The significant suppression of strongly bonding proton on P atom originates from the downshift of P p orbits and the decreased electronic states around the Fermi level [30], attributed to the formation of N-P bond (Fig. S33). Particularly, N-P site yields the smallest |∆GH*| in sharp contrast with those of optimized sites in all counterparts, whether doping on basal plane or S edge, confirming the superior HER performance under acidic media (Fig. 3f and S34-S37). However, the activity on edge doping site is easy to be inhibited by oxygen, and thus, the greatly reduced ∆GH O, ap2
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propriate proton and hydride acceptor ability, and oxidation resistance property on basal plane of NP-MoS2 suggest the intrinsic HER activity can be effectively improved under both acidic and alkaline condition [31], in agreement with the results of HER measurement. In addition, considering of the part of single N dopants surrounding to N-P site in our as-grown NP-MoS2 nanosheets, the function of surrounding single N dopants was investigated to approach more realistic conditions. The N-MoS2 exhibits a narrow band gap of 1.04 eV in comparison with pristine 2H-MoS2 (1.65 eV), indicating the improvement of the intrinsic electron transfer capacity, in agreement with EIS measurements (Fig. S38). Combining the evidences between theoretical calculations and experimental results, the remarkable enhancement of HER activity in a wide pH value is originated from: (i) The formation of the Mo-N-P sites increases the energetic favorability and structure stability of co-doping. (ii) The electronic interactions between N and P trigger a unique electronic structure to active basal plane of MoS2 for accelerating HER kinetics. (iii) The uniform distribution of surrounding single N dopants effectively facilitates electron transfer to Mo-N-P sites over the basal plane, further bringing the enhanced HER catalytic activities. On the basis of the promising activity of the as-prepared NP-MoS2/CC in the wide pH value, this material was further tested in weak acidic river (Dingjia river, pH = 6.8) and alkaline sea (Yellow sea, pH = 7.9) water. NP-MoS2/CC yields a overpotential of 345.4 mV to achieve a current density of 10•mA cm−2 under sea water, showing a higher activity than the reported those-of-art electrocatalyst (Table S6). Remarkably, its activity outperforms Pt/C at a large current density under both sea and river water (>21.8 and 1.9 mA cm−2, respectively) and remains stable for 8 h (Fig. 4a, 4b and S39). Meanwhile, the Faradic efficiencies of H2 end with 90.9 and 96.7% after 8 h reaction, in
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stark contrast to that of 79.6 and 91.6% for Pt/C under sea and river water, respectively, indicating that the side reactions can be effectively suppressed within the NP-MoS2/CC (Fig. 4c and 4d). These results reveal that the as-obtained NP-MoS2/CC has tremendous application potential in industrial water electrolysis.
Conclusions In summary, a novel AGNP strategy was developed toward the preparation of NP-MoS2/CC, which enables high catalytic activity and long-term stability under a wide pH value, especially real water system. Experimental and theoretical studies demonstrated that the new formation Mo-N-P site could trigger the appropriate basal plane sites for dissociation water, proton and hydride acceptor, thus dramatically enhancing the catalytic performance. This work may shed some new lights toward the development of highly efficient, pH-universal, and inexpensive HER electrocatalyst for industrial application especially.
Acknowledgements This work has been financially support by the National Natural Science Foundation of China (No. 21676300 and 21776315), the Beijing Natural Science Foundation (No. 2184104), the Fundamental Research Funds for the Central Universities (No.
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16CX06007A), the PetroChina Innovation Foundation (2017D-5007-0402), and the China Postdoctoral Science Foundation (No. 2017M610076).
Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at http:// dx.doi.org/10.1016/j.nanoen.XXXXX.
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Author Biography
Kaian Sun received his B.S. degree in 2015 from the Shandong University of Technology. Now his is a Ph.D. candidate in Professor Liu’s group at China University of Petroleum. His research interest focuses on the electrochemical energy conversion and storage using transitional metal-based catalyst.
Lingyou Zeng received his master degree (2016) and B.S. degree (2013) in College of Chemical Engineering at China University of Petroleum (UPC). He is now pursuing his Ph.D. under the supervision of Prof. Yunqi Liu and Prof. Chenguang Liu. His present research interests mainly focus on the design and synthesis of noble metal-free, 3D nanostructured materials for electrocatalytic water splitting.
Sihui Liu received her bachelor degree in Chemical Engineering and Technology at Liaoning Shihua University in 2016. She is now pursuing her postgraduate degree at China University of Petroleum in the group of Prof. Yunqi Liu. Her current research interests focus on the design and synthesis of advanced materials for hydrogen evolution reaction.
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Lei Zhao received her mater degree in 2018 from the China University of Petroleum (East China). Now her is a Ph.D. candidate in Professor Mintova group at China University of Petroleum. Her research interest focuses on the synthesis and advance application of nanoscale zeolite.
Houyu Zhu received his Ph.D. degree in 2012 from the China University of Petroleum (UPC). He worked as a Post.Doc. Associate at the University of Notre Dame from 2012 to 2014. Now he is a lecturer of the School of Materials Science and Engineering at the UPC. His research mainly focuses on the design and screening of petroleum-related catalyst materials.
Jinchong Zhao is a Ph.D. candidate in Professor Liu’s group at China University of Petroleum (East China) now. His research interest focuses on the fuel refining and catalysis on the energy field.
Zhi Liu received his mater degree (2017) and B.S. degree (2014) in College of Chemical Engineering at China University of Petroleum (UPC). He is now pursuing his Ph.D. under the supervision of Prof. Yunqi Liu and Prof. Chenguang Liu. His present research interests mainly focus on the reaction mechanisms and materials design over selective catalytic reduction
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(SCR) NOx by NH3.
Dongwei Cao is currently a PhD student at the college of science at China University of Petroleum under the supervision of Dr. Wenpei Kang and Prof. Daofeng Sun. He received his B.S. (2014) and master (2017) degree in college of chemical engineering at China University of Petroleum. His research interests focus on the design and synthesis of novel nanomaterials for energy storage and conversion.
Yongchun Hou received his B.S. degree in 2016 from the Qufu Normal University. Now his is a master candidate in Professor Guo’s group at China University of Petroleum. His research interest focuses on the hydrogen oxidation reaction on the ceria-based solid oxide fuel cells anodes.
Yunqi Liu is a professor in State Key Laboratory of Heavy Oil Processing and College of Chemical Engineering at China University of Petroleum. He was awarded a Ph.D. in Applied Chemistry from China University of Petroleum in 2000. Dr. Liu then worked two years as a postdoctoral follow in Dalian Institute of Chemical Physics at Chinese Academy of Sciences. His research interests include petroleum refining and chemical engineering, catalytic materials and catalysts for oil production and advanced materials for electrochemical water splitting and renewable energy applications. Dr. Yuan Pan received his Ph.D. in College of Chemical Engineering at China University of Petroleum (East China) in 2016. After postdoctoral work at Tsinghua University with Prof. Yadong Li,
21
he joined the College of Chemical Engineering at China University of Petroleum (East China) as an associate professor in 2019. His research interests focus on the design and synthesis of novel nanomaterials, clusters and single-atom materials for energy-related catalytic application.
Chenguang Liu is a professor in State Key Laboratory of Heavy Oil Processing and College of Chemical Engineering at China University of Petroleum. Dr. Liu received his Ph.D. in 1991 in Applied Chemistry at China University of Petroleum. His current research interests include petrochemistry, petroleum refining and chemical engineering, green fine chemical technology and renewable energy and oxygen-containing fuels applications.
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Fig. 1. Structural characterizations of NP-MoS2/CC. (a) Schematic illustration of the synthesis process. (b) SEM image. (c-h) HAADF-STEM and EDS mapping images. (i, j) XRD patterns. (k) TEM images.
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Fig. 2. The chemical composition and state of NP-MoS2. (a) Mo K-edge FT-EXAFS. (b) P L3-edge XANES. (c) Raman and (d) FTIR spectra.
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Fig. 3. Electrochemical activity. (a, b) Polarization curves in 0.5 M H2SO4 and 1.0 M KOH, respectively. (c, d) Chronopotentiometric curves at large current densities under 0.5 M H2SO4 and 1.0 M KOH, respectively. The insets in (c) and (d) show the overpotentials at 50 mA·cm−2 before and after 2000 cycles over 0.5 M H2SO4 and 1.0 M KOH, respectively. (e) The calculated ∆GH O value. (f) The calculated ∆GH* value. 2
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Fig. 4. Application in real water system. (a, b) Polarization curves. The insets are time-dependent current density curves. (c, d) The Faradic efficiency.
Video S1 is “The video recording displays the process of hydrogen evolution under sea water using NP-MoS2/CC at a current density of 50 mA·cm−2”. Video S2 is “The video recording displays the process of hydrogen evolution under river water using NP-MoS2/CC at a current density of 50 mA·cm−2”.
26
HIGHLIGHTS
A
new
ammonia
ions-guided-nitrogenization-phos phorization strategy was developed
to
prepare
a
novel
self-supported electrode with N and P co-doped MoS2 array anchored on carbon cloth.
The HER activity of this material is close to Pt/C under alkaline or acidic condition and outperforms Pt/C at a large current density under real water system.
DFT calculation and advanced spectroscopic techniques were performed to confirm a new Mo-N-P site on basal plane is the real structure in charge of the HER.
27
PII: DOI: Reference:
www.elsevier.com/locate/nanoenergy
S2211-2855(19)30112-0 https://doi.org/10.1016/j.nanoen.2019.02.006 NANOEN3450
To appear in: Nano Energy Received date: 13 September 2018 Revised date: 5 January 2019 Accepted date: 2 February 2019 Cite this article as: Kaian Sun, Lingyou Zeng, Sihui Liu, Lei Zhao, Houyu Zhu, Jinchong Zhao, Zhi Liu, Dongwei Cao, Yongchun Hou, Yunqi Liu, Yuan Pan and Chenguang Liu, Design of Basal Plane Active MoS 2 through One-step Nitrogen and Phosphorus Co-doping as an Efficient pH-universal electrocatalyst for Hydrogen Evolution, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design of Basal Plane Active MoS2 through One-step Nitrogen and Phosphorus Co-doping as an Efficient pH-universal electrocatalyst for Hydrogen Evolution
Kaian Suna, Lingyou Zenga, Sihui Liua, Lei Zhaoa, Houyu Zhub, Jinchong Zhaoa, Zhi Liua, Dongwei Caob, Yongchun Houb, Yunqi Liua*, Yuan Panc*, Chenguang Liua
a
State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China
University of Petroleum (East China), Qingdao, 266580, China. b
College of Science, China University of Petroleum (East China), 66 West Changjiang
Road, Qingdao 266580, China c
Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China
[email protected] [email protected]
*
Corresponding authors.
Abstract The exploration of low-cost, stable, and highly active noble-metal-free electrocatalyst for hydrogen evolution reaction (HER) in a wide pH range is crucial but still challenging task for renewable energy techniques. MoS2-based materials have been considered as a promising electrocatalyst for HER. However, corresponding studies have been hampered by the lack of effective routes to fully utilize the large number of inert basal plane for catalyzing HER, especially under alkaline media. Herein, a novel ammonia
1
ions-guided-nitrogenization-phosphorization strategy is developed to prepare N and P co-doped MoS2 with active basal plane for efficient catalyzing HER with a quite low overpotential of 116 and 78 mV in 0.5 M H2SO4 and 1.0 M KOH to achieve a current density of 10 mA•cm−2, respectively. Experimental studies and theoretical calculations confirm Mo-N-P sites in the basal plane of MoS2 can not only accelerate HER kinetics, but also result in energetic favorability and structure stability. Furthermore, outstanding performances are also obtained under both sea and river water, vastly broadening the application prospects.
Graphical Abstract TOC
A novel ammonia ions-guided-nitrogenization-phosphorization strategy was developed to prepare N and P co-doped MoS2 with active basal plane for HER under a wide pH value, especially sea and river water. The active basal plane is originated from the new formation Mo-N-P site.
Keywords: Molybdenum disulfide; Hydrogen evolution reaction; Basal plane; Density functional theory.
2
1. Introduction Ever-increasing global concerns about environmental pollutions and energy crisis have kindled a great desire for alternative clean energy sources and renewable carriers. Molecular hydrogen (H2), is not only considered as one of the most promising clean fuel sources, but also regarded as an ideal carrier to store energy from renewable energy sources (e.g., solar and wind) into the chemical bond via electrochemical hydrogen evolution reaction (HER) [1]. To improve the HER energy conversion efficiency, a leapfrog development is needed to design inexpensive commercially available electrocatalyst to the replacement of high-cost and scarce precious-metal catalysts (i.e., Pt, Pd, and Rh) [2]. Up to date, transition metal sulfides [3], selenides [4], phosphides [5], carbides [6], and nitride [7] have been widely investigated. Among these, MoS2 has been emerged as a promising candidate in transition metal sulfides, due to its cost-effective, earth-abundant, and high performance under acidic condition. Unfortunately, the HER activity and stability of MoS2-based materials under alkaline medium is still far less than that of benchmark Pt/C, limiting its potential of being a pH-universal electrocatalyst to address large-scale industrial requirements. Toward the goal of making MoS2 as an efficient HER catalyst under alkaline media, it has recently been proven that interface engineering, including MoS2/Ni3S2 [8], NiS2/MoS2 [9], Co-Ex-MoS2 [10], Ni(OH)2/MoS2 [11], etc. can tune the electronic structure at the edge sites of MoS2 for reducing the kinetic energy barrier during the hydrogen generation pathway. Although the significant progresses have been achieved, the activation of
3
the dominant basal plane, which constitutes the majority of bulk MoS2, still remains a challenge under alkaline condition. As an easily-scalable and cost-effective method, doping with non-metal heteroatoms into MoS2 can not only tailor the electronic structure effectively, but also trigger basal plane active sites to fully utilize MoS2 for HER under acidic condition [12]. However, the application of non-metal heteroatoms doped MoS2 as a pH-universal electrocatalyst has not been implemented yet. In specific, inspired by the enhanced electro-catalysis activity of carbon-based materials via co-doping with two elements, one with higher electro-negativity and one with lower than that of C (2.55), for example, N (3.04) and B (2.04) or P (2.19) [13], we anticipate that substituting S (2.58) of MoS2 with N and P may create a unique electronic structure to achieve desired applications. Herein, we develop a novel ammonia ions-guided-nitrogenization-phosphorization (AGNP) strategy toward the preparation of N and P co-doped MoS2 array anchored on carbon cloth (NP-MoS2/CC) as a self-supported electrode for efficient catalyzing HER over a broad pH value. Distinct from previous studies [14], the present AGNP strategy relies on the intercalated NH4+ in MoS2. The intercalated NH4+ guaranteed in situ homogeneously doping of N without production of unstable crystal structure and the introduction of N further favor the co-doping of P at low temperature [15]. In addition, this strategy can be extended to fabricate single P or N doped MoS2 grown on CC (P-MoS2/CC and N-MoS2/CC, respectively) with fixed structure to NP-MoS2/CC, providing a scalable pathway to systematically investigate the effect of co-doping.
4
Electrochemical measurements demonstrated that the activity and stability of NP-MoS2/CC were much higher than that of P-MoS2/CC or N-MoS2/CC under both acidic and alkaline condition. Impressively, the activitiy of NP-MoS2/CC under both sea and river water was superior to those of all previously reported electrocatalysts and outperform commercial Pt/C catalyst. Experimental and theoretical results revealed that the excellent activity and durability originate from the new formation Mo-N-P site. Our study and understanding may shed some new lights for designing highly efficient pH-universal HER electrocatalyst.
2. Results and Discussions The AGNP strategy for NP-MoS2/CC involved two steps (Fig. 1a. The detail experiment processes are shown in Supporting Information). Firstly, NH4+ intercalated MoS2 nanosheets arrays (NH4+-MoS2/CC) was prepared by a hydrothermal process according to our previous report [16]. Urea was added into the reaction system to release NH4+, which could be absorbed between the interlayer of MoS2. CC with rough surface and abundant surface oxygen/hydrogen-containing functional groups offered an ideal scaffold to anchor NH4+-MoS2 for meeting the requirements of innovative energy conversion technologies, due to its feature of excellent electrical conductivity, stability, and flexibility. A polyethylene glycol/water mixed solvent was used to slow down the dipole-dipole interactions between primary crystal nuclei, facilitating the orientation growth of MoS2 nanosheets on CC. Scanning electron micros-copy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analysis and mass spectrometry (TG-MS) characterizations were revealed the product structure
5
as NH4+-MoS2/CC (Fig. S1 and S2). Then, a low-temperature one-step nitrogenization and phosphorization process was conducted at 400 °C for 1 h in Ar atmosphere. The intercalated NH4+ and NaH2PO2 acted as precursors to generate free NH4+ and PH3, respectively. The free NH4+ and PH3 then simultaneously reacted with MoS2 to form NP-MoS2/CC (Fig. S3). In particular, this strategy can be easily extended to fabricate other single non-metal heteroatoms doped MoS2, including N- and P-MoS2/CC. The optimized N- (ca. 3.2 atom%) and P-MoS2/CC (ca. 1.6 atom%) have a similar structure to NP-MoS2/CC, as revealed by SEM, TEM, XRD, Brunauer-Emmett-Teller (BET), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) (Fig. S4-11 and Table S1). The structure of NP-MoS2/CC was firstly investigated by SEM, showing that the array structure is well preserved after the second step reaction (Fig. 1b). The whole surface of CC is homogeneously covered by cross-linked nanosheets, with an average size of 200-400 nm, resulting in a rough and highly porous structure. N2 sorption isotherm of NP-MoS2/CC exhibits a typical IV isotherm, presenting a characteristic of mesoporous. The BET specific surface area is 40 m2•g−1 and the pore width distributes in a wide range of 2-20 nm (Fig. S12). Such an edge-terminal nanostructure with abundant mesoporous facilitate the electrode kinetics and accelerate the transport of the evolved H2. A closer observation on the edge of NP-MoS2/CC by the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) and the corresponding EDS element mapping illustrate that N, P, S, and Mo elements are uniformly distributed through the as-grown NP-MoS2 nanosheets and the bulk Mo/S/P/N atom ratio is 33.2:62.6:1.2:2.9, in agreement with SEM-EDS results (Fig. 1c-h and Fig. S13). The phase purity was confirmed by XRD analysis (Fig. 1i and 1j).
6
Eliminating the peaks arising from the CC (JCPDS card No. 65-6212), all the peaks are well-matched with the hexagonal 2H phase MoS2 (2H-MoS2) with space group P63/mmc (JCPDS card No. 37-1492). This suggests that N and P co-doping does not cause phase structure changes of 2H-MoS2, whithout the formation of unstable 1T phase MoS2, MoPx or MoNx. A magnification at (100) diffraction peak shows an obvious shift to higher angle compared with a physical mixture of single N- and P-MoS2, indicating that there possibly exists strong interaction between N and P rather than a simple phase separation condition. According to Bragg's equation, the d-spacing of the (100) plane can be estimated to be about 0.25 nm, which is in accordance with the results of TEM and smaller than that of bulk MoS2 (Fig. 1k). To further evaluate the local atomic structure around Mo quantitatively, extended X-ray absorption fine structure (EXAFS) was carried out. The Fourier transform (FT) curves of EXAFS present two main peaks at around 1.9 and 2.9 Å in all samples, corresponding to Mo−S and Mo−Mo bond, respectively (Fig. 2a) [17]. However, a new shoulder peak at around 1.5 Å corresponding to Mo−N bond can be observed N-MoS2 and NP-MoS2 [18]. Notably, the Mo−N distance in NP-MoS2 is slightly longer than that of N-MoS2 and the intensity ratios of the Mo-S/Mo-N peaks for NP-MoS2 are lower than that of N-MoS2, which possibly resulted from the missing atoms in the Mo coordination sphere after P incorporation. Moreover, because the Mo-P peak have an overlapping position in the Mo-S peak [19], the nature of doping was further identified by soft X-ray absorption near-edge spectroscopy (XANES). Except for the peaks of phosphate species, the lowest spin-orbit doublet at around 129.8 and 130.8 eV can be observed in P-MoS2 and NP-MoS2, corresponding to the 2p 3/2 → * and 2p 1/2 → * transition, respectively [20]. These two features can be attributed to the Mo atoms attached to the
7
doped P atoms [21]. In comparison to P-MoS2 samples, the * transition of NP-MoS2 exhibits a slight energy shift to higher energies and a new N-P peak at around 133.1 eV, consistent with the depth profile XPS results (Fig. 2b and S14) [22]. These changes in the XANES features suggest a strong chemical interaction occurs between Mo-N bond and Mo-P bond, corresponding to Mo-N-P site in the NP-MoS2. Moreover, the strong interaction between N and P is also confirmed by Raman and Fourier transform infrared (FTIR) spectroscopy. Raman spectra reveal a new Raman band at around 986.4 cm−1, corresponding to the N–P bond, alongside the major vibration bands for the in-plane (E1 2g) and out-of-plane (A1g) modes of 2H-MoS2 at around 371.2 and 401.4 cm−1, respectively (Fig. 2c) [23]. For the FTIR results, the peak at around 1438.3 cm−1 correspond to N−P stretching modes (Fig. 2d) [24]. Additionally, it should be pointed that N can encourage the co-doping of P at low temperature (400 °C), whereas single P doping can only be achieved at high temperature (600 °C) (Fig. S15). The electrocatalytic HER performance of NP-MoS2/CC was investigated in 0.5 M H2SO4 and 1.0 M KOH solution using a standard three-electrode setup. For comparison, bare CC, commercial 20% Pt/C, P-, N-, and 2H-MoS2/CC were also tested in parallel. The bare CC shows negligible activity, whereas the Pt/C shows superior activity in both acid and alkaline media. As expected, single N and P doping can boost the activity of MoS2 under acid media. Interestingly, N or P are also an effective promoter to modulate HER performance of MoS2 under alkaline media. More interestingly, the performance of single element doped MoS2 can be further enhanced by the incorporation of a second heteroatom. The NP-MoS2/CC shows quite low overpotentials of 116 and 78 mV at the current densities of 10 mA•cm−2 in 0.5 M H2SO4 and 1.0 M KOH, respectively (Fig. 3a and 3b). Impressively, the overpotential under alkaline media is superior to those of all
8
previously reported MoS2-based electrocatalyst and very close to commercial Pt/C catalyst (Table S2 and S3). The extracted Tafel slopes of the NP-MoS2/CC are 58.4 and 51.6 mV•dec−1 at low overpotential ranges in 0.5 M H2SO4 and 1.0 M KOH, respectively, suggesting the HER occurs on NP-MoS2/CC via a Volmer-Heyrovsky mechanism (Fig. S16 and S17) [25]. Moreover, the NP-MoS2/CC shows considerably smaller charge transfer resistances and obviously higher double-layer capacitances (Cdl) in comparison with P- or N- MoS2/CC, indicating that N and P co-doping could either accelerate HER kinetics or facilitate active site proliferation (Fig. S18-20 and Table S4). The turnover frequencies (TOF) value of NP-MoS2/CC was determined to be 0.29 and 0.28 H2 s−1 at an overpotential of 200 mV, which is larger than that of and P-MoS2/CC (0.14 and 0.12 H2 s−1) and N-MoS2/CC (0.12 and 0.04 H2 s−1), in 0.5 M H2SO4 and 1.0 M KOH, respectively (Fig. S21). Since NP-MoS2/CC has a lower N and P contents than the corresponding N- or P-MoS2 and the similar morphology among all samples, the increased TOF value indicates that the superior activity could be attributed to the synergistic effect between N and P. To verify the real structure in charge of the HER, we further compared HER activity of the fresh NP-MoS2/CC sample with the aged sample, which prepared by partially oxidation in O2-saturated aqueous solution over five days. The polarization curve of aged sample shows no performance degradation compared with the fresh sample (Fig. S22). XPS results indicate that the aged sample has been partially oxidized with abvious Mo-O, P-O and especially N-P peaks, but without Mo-P peaks (Fig. S23). Previously study indicates that the oxidation of nanostructure initiates at the edge, thus, the synergistic effect is mainly originated from the new formed Mo-N-P site on basal plane, which can also enhance the durability for HER catalysis in a significant manner. After 2000 scanning cycles ranging from +0.2 to −0.2 V, the
9
overpotential of NP-MoS2/CC shows an ignorable increment of 3.4 mV and 5.1 at 50 mA•cm−2 in acidic and alkaline media, respectively. In contrast, N- or P-MoS2/CC suffer from severe performance degradation. After the cycling tests, the NP-MoS2/CC retains its primary morphology, while the structure of P or N-MoS2/CC are slightly collapsed as shown in SEM images (Fig. S24). XPS and TEM tests demonstrate no obvious change in chemical compositions, especially N-P species for used NP-MoS2 nanosheets (Fig. S25 and S26). Furthermore, considering the demands of practical application, chronopotentiometric curves were recorded at large current densities of 500 and 1000 mA•cm−2. The overpotentials can be maintain at around 316 and 295 mV at the current density of 500 mA•cm−2 and only 15 and 19% decay at the current density of 1000 mA•cm−2 during 20 h of continuous operation in 0.5 M H2SO4 and 1.0 M KOH, respectively (Fig. 3c and 3d). Density functional theory calculations were performed to gain molecular-level understanding of the excellent HER activity. Firstly, the possibility of Mo-N-P site incorporation into the MoS2 lattice on basal plane was evaluated. Different types of N and P co-doped atomic models were adopted (Fig. S27-29). The formation energy is obviously decreased with Mo-N-P site formation. Since the electro-negativity of the S is located between the P and N, the N atom is first polarized by Mo, and then can draw electrons from P directly, verifying by Bader charge (Table S5) and XPS analysis (Fig. S30). Therefore, Mo-N-P site is favoured by co-doping with N, which leading to energetic favourability and structure stability, corresponding to the experimental results. Then, the kinetic energy barrier of water dissociation (∆GH O) was calculated. The ∆GH O is con2
2
sidered as a key rate-limiting Volmer step under alkaline media [27]. The value of ∆GH O 2
of N-MoS2 is 3.17 eV, still making the extremely difficult to dissociate water on inert
10
basal plane (Fig. 3e and S31). However, after P doping into the MoS2 lattice, the value of ∆GH O on basal plane is significantly reduced, owing to that the presence of P and 2
neighbour S help to stabilize the hydroxide (OH*) and hydrogen (H*) intermediates, respectively, and hence favour the initial water dissociation process. Strikingly, with introduction of N co-dopant to form Mo-N-P site, the ∆GH O value on NP-MoS2 further 2
reduces to 0.72 eV, approaching that of Pt (0.44 eV) [28]. Meanwhile, the basal plane of NP-MoS2 can provide not only effectively sites for water dissociation, but also an appropriate OH* and H* acceptor sites. The OH* adsorption energy (∆GOH*) on P atom and H* adsorption energy (∆GH*) on neighbour S atom in NP-MoS2 are −0.74 and 1.89 eV, respectively, which are much weaker than those of P-MoS2 (−1.03 and 0.39 eV, respectively), indicating the desorption hydroxyl and proton are facilitated remarkably and preventing the blockage of active sites to increase the rate of proton formation (Fig. S32). Subsequently, the generated proton is favorably adsorbed on P atom, owning to the lower electro-negativity of P than S. It is worth noting that the ∆GH* value on P atom in NP-MoS2 is −0.21 eV, which is closer to thermo-neutral state (i.e., ∆GH* ≈ 0 eV) than P atom in P-MoS2 (−0.96 eV), denoting a balance between the transfer of proton and the removal of H2 on P atom in NP-MoS2 for further accelerating HER kinetics under alkaline media [29]. The significant suppression of strongly bonding proton on P atom originates from the downshift of P p orbits and the decreased electronic states around the Fermi level [30], attributed to the formation of N-P bond (Fig. S33). Particularly, N-P site yields the smallest |∆GH*| in sharp contrast with those of optimized sites in all counterparts, whether doping on basal plane or S edge, confirming the superior HER performance under acidic media (Fig. 3f and S34-S37). However, the activity on edge doping site is easy to be inhibited by oxygen, and thus, the greatly reduced ∆GH O, ap2
11
propriate proton and hydride acceptor ability, and oxidation resistance property on basal plane of NP-MoS2 suggest the intrinsic HER activity can be effectively improved under both acidic and alkaline condition [31], in agreement with the results of HER measurement. In addition, considering of the part of single N dopants surrounding to N-P site in our as-grown NP-MoS2 nanosheets, the function of surrounding single N dopants was investigated to approach more realistic conditions. The N-MoS2 exhibits a narrow band gap of 1.04 eV in comparison with pristine 2H-MoS2 (1.65 eV), indicating the improvement of the intrinsic electron transfer capacity, in agreement with EIS measurements (Fig. S38). Combining the evidences between theoretical calculations and experimental results, the remarkable enhancement of HER activity in a wide pH value is originated from: (i) The formation of the Mo-N-P sites increases the energetic favorability and structure stability of co-doping. (ii) The electronic interactions between N and P trigger a unique electronic structure to active basal plane of MoS2 for accelerating HER kinetics. (iii) The uniform distribution of surrounding single N dopants effectively facilitates electron transfer to Mo-N-P sites over the basal plane, further bringing the enhanced HER catalytic activities. On the basis of the promising activity of the as-prepared NP-MoS2/CC in the wide pH value, this material was further tested in weak acidic river (Dingjia river, pH = 6.8) and alkaline sea (Yellow sea, pH = 7.9) water. NP-MoS2/CC yields a overpotential of 345.4 mV to achieve a current density of 10•mA cm−2 under sea water, showing a higher activity than the reported those-of-art electrocatalyst (Table S6). Remarkably, its activity outperforms Pt/C at a large current density under both sea and river water (>21.8 and 1.9 mA cm−2, respectively) and remains stable for 8 h (Fig. 4a, 4b and S39). Meanwhile, the Faradic efficiencies of H2 end with 90.9 and 96.7% after 8 h reaction, in
12
stark contrast to that of 79.6 and 91.6% for Pt/C under sea and river water, respectively, indicating that the side reactions can be effectively suppressed within the NP-MoS2/CC (Fig. 4c and 4d). These results reveal that the as-obtained NP-MoS2/CC has tremendous application potential in industrial water electrolysis.
Conclusions In summary, a novel AGNP strategy was developed toward the preparation of NP-MoS2/CC, which enables high catalytic activity and long-term stability under a wide pH value, especially real water system. Experimental and theoretical studies demonstrated that the new formation Mo-N-P site could trigger the appropriate basal plane sites for dissociation water, proton and hydride acceptor, thus dramatically enhancing the catalytic performance. This work may shed some new lights toward the development of highly efficient, pH-universal, and inexpensive HER electrocatalyst for industrial application especially.
Acknowledgements This work has been financially support by the National Natural Science Foundation of China (No. 21676300 and 21776315), the Beijing Natural Science Foundation (No. 2184104), the Fundamental Research Funds for the Central Universities (No.
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16CX06007A), the PetroChina Innovation Foundation (2017D-5007-0402), and the China Postdoctoral Science Foundation (No. 2017M610076).
Appendix A. Supporting Information Supplementary data associated with this article can be found in the online version at http:// dx.doi.org/10.1016/j.nanoen.XXXXX.
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Author Biography
Kaian Sun received his B.S. degree in 2015 from the Shandong University of Technology. Now his is a Ph.D. candidate in Professor Liu’s group at China University of Petroleum. His research interest focuses on the electrochemical energy conversion and storage using transitional metal-based catalyst.
Lingyou Zeng received his master degree (2016) and B.S. degree (2013) in College of Chemical Engineering at China University of Petroleum (UPC). He is now pursuing his Ph.D. under the supervision of Prof. Yunqi Liu and Prof. Chenguang Liu. His present research interests mainly focus on the design and synthesis of noble metal-free, 3D nanostructured materials for electrocatalytic water splitting.
Sihui Liu received her bachelor degree in Chemical Engineering and Technology at Liaoning Shihua University in 2016. She is now pursuing her postgraduate degree at China University of Petroleum in the group of Prof. Yunqi Liu. Her current research interests focus on the design and synthesis of advanced materials for hydrogen evolution reaction.
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Lei Zhao received her mater degree in 2018 from the China University of Petroleum (East China). Now her is a Ph.D. candidate in Professor Mintova group at China University of Petroleum. Her research interest focuses on the synthesis and advance application of nanoscale zeolite.
Houyu Zhu received his Ph.D. degree in 2012 from the China University of Petroleum (UPC). He worked as a Post.Doc. Associate at the University of Notre Dame from 2012 to 2014. Now he is a lecturer of the School of Materials Science and Engineering at the UPC. His research mainly focuses on the design and screening of petroleum-related catalyst materials.
Jinchong Zhao is a Ph.D. candidate in Professor Liu’s group at China University of Petroleum (East China) now. His research interest focuses on the fuel refining and catalysis on the energy field.
Zhi Liu received his mater degree (2017) and B.S. degree (2014) in College of Chemical Engineering at China University of Petroleum (UPC). He is now pursuing his Ph.D. under the supervision of Prof. Yunqi Liu and Prof. Chenguang Liu. His present research interests mainly focus on the reaction mechanisms and materials design over selective catalytic reduction
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(SCR) NOx by NH3.
Dongwei Cao is currently a PhD student at the college of science at China University of Petroleum under the supervision of Dr. Wenpei Kang and Prof. Daofeng Sun. He received his B.S. (2014) and master (2017) degree in college of chemical engineering at China University of Petroleum. His research interests focus on the design and synthesis of novel nanomaterials for energy storage and conversion.
Yongchun Hou received his B.S. degree in 2016 from the Qufu Normal University. Now his is a master candidate in Professor Guo’s group at China University of Petroleum. His research interest focuses on the hydrogen oxidation reaction on the ceria-based solid oxide fuel cells anodes.
Yunqi Liu is a professor in State Key Laboratory of Heavy Oil Processing and College of Chemical Engineering at China University of Petroleum. He was awarded a Ph.D. in Applied Chemistry from China University of Petroleum in 2000. Dr. Liu then worked two years as a postdoctoral follow in Dalian Institute of Chemical Physics at Chinese Academy of Sciences. His research interests include petroleum refining and chemical engineering, catalytic materials and catalysts for oil production and advanced materials for electrochemical water splitting and renewable energy applications. Dr. Yuan Pan received his Ph.D. in College of Chemical Engineering at China University of Petroleum (East China) in 2016. After postdoctoral work at Tsinghua University with Prof. Yadong Li,
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he joined the College of Chemical Engineering at China University of Petroleum (East China) as an associate professor in 2019. His research interests focus on the design and synthesis of novel nanomaterials, clusters and single-atom materials for energy-related catalytic application.
Chenguang Liu is a professor in State Key Laboratory of Heavy Oil Processing and College of Chemical Engineering at China University of Petroleum. Dr. Liu received his Ph.D. in 1991 in Applied Chemistry at China University of Petroleum. His current research interests include petrochemistry, petroleum refining and chemical engineering, green fine chemical technology and renewable energy and oxygen-containing fuels applications.
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Fig. 1. Structural characterizations of NP-MoS2/CC. (a) Schematic illustration of the synthesis process. (b) SEM image. (c-h) HAADF-STEM and EDS mapping images. (i, j) XRD patterns. (k) TEM images.
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Fig. 2. The chemical composition and state of NP-MoS2. (a) Mo K-edge FT-EXAFS. (b) P L3-edge XANES. (c) Raman and (d) FTIR spectra.
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Fig. 3. Electrochemical activity. (a, b) Polarization curves in 0.5 M H2SO4 and 1.0 M KOH, respectively. (c, d) Chronopotentiometric curves at large current densities under 0.5 M H2SO4 and 1.0 M KOH, respectively. The insets in (c) and (d) show the overpotentials at 50 mA·cm−2 before and after 2000 cycles over 0.5 M H2SO4 and 1.0 M KOH, respectively. (e) The calculated ∆GH O value. (f) The calculated ∆GH* value. 2
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Fig. 4. Application in real water system. (a, b) Polarization curves. The insets are time-dependent current density curves. (c, d) The Faradic efficiency.
Video S1 is “The video recording displays the process of hydrogen evolution under sea water using NP-MoS2/CC at a current density of 50 mA·cm−2”. Video S2 is “The video recording displays the process of hydrogen evolution under river water using NP-MoS2/CC at a current density of 50 mA·cm−2”.
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HIGHLIGHTS
A
new
ammonia
ions-guided-nitrogenization-phos phorization strategy was developed
to
prepare
a
novel
self-supported electrode with N and P co-doped MoS2 array anchored on carbon cloth.
The HER activity of this material is close to Pt/C under alkaline or acidic condition and outperforms Pt/C at a large current density under real water system.
DFT calculation and advanced spectroscopic techniques were performed to confirm a new Mo-N-P site on basal plane is the real structure in charge of the HER.
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