High Activity Hydrogen Evolution Catalysis by Uniquely Designed Amorphous/Metal Interface of Core shell Phosphosulfide/N-Doped CNTs

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1 Communication Hydrogen Evolution High Activity Hydrogen Evolution Catalysis by Uniquely Designed Amorphous/Metal Interface of Core shell Phosphosulfide/N-Doped CNTs Dong Jun Li, Joonhee Kang, Ho Jin Lee, Dong Sung Choi, Sung Hwan Koo, Byungchan Han,* and Sang Ouk Kim* A cost effective hydrogen evolution reaction (HER) catalyst that does not use precious metallic elements is a crucial demand for environment-benign energy production. The family of earth-abundant transition metal compounds of nitrides, carbides, chalcogenides, and phosphides is one of the promising candidates for such a purpose, particularly in acidic conditions. However, its catalytic performance is still needed to be enhanced through novel material designs and crystalline engineering. Herein, a chemically and electronically coupled transition metal phosphosulfide/n-doped carbon nanotubes (NCNT) hybrid electrocatalyst is fabricated via a two-step synthesis. The uniquely designed synthesis leads to the material morphology featuring a core shell structure, in which the crystalline metal phosphide core is surrounded by an amorphous phosphosulfide nanoshell. Notably, due to the favorable modification of chemical composition and surface properties, core shell CoP@PS/NCNT exhibits the noticeable HER activity of approximately ma cm 2 with excellent durability, which is one of the highest active nonnoble metal electrocatalysts ever reported thus far. Hydrogen is a promising energy carrier for clean and sustainable human society. Direct electrolysis and photoelectrolysis have attracted a great deal of attention for the effective and eco-friendly production of hydrogen fuels. [1 5] Unfortunately, without high cost noble metal catalysts such as Pt, initiation of the hydrogen turnover requires a significant activation energy barrier. As such, the efficiency of overall energy conversion is deteriorated for real-world application. It is an urgent task to surpass the expensive rare-earth materials with active Dr. D. J. Li, H. J. Lee, Dr. D. S. Choi, S. H. Koo, Prof. S. O. Kim National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly Department of Materials Science & Engineering KAIST Daejeon 34141, Republic of Korea sangouk.kim@kaist.ac.kr Dr. J. Kang, Prof. B. Han Department of Chemical and Biomolecular Engineering Yonsei University Seoul 03722, Republic of Korea bchan@yonsei.ac.kr The ORCID identification number(s) for the author(s) of this article can be found under DOI: /aenm nonprecious catalysts toward the hydrogen evolution reaction (HER) for a widespread of sustainable energy storage and conversion. [6] To date, a variety of nonprecious-metalbased catalysts, especially, the family of earth-abundant transition metal compounds of nitrides, [7,8] carbides, [9,10] chalcogenides, [11 14] and phosphides, [15 19] have been explored for HER catalysts. It has been reported that novel material designs relying on chemical synthesis or crystalline engineering (facets, polymorph, interface, defect, doping, surface modification) can enhance HER activity. Nonetheless, in order to avoid the undesired charge transfer loss and gas bubble trap, a welldesigned, atomically and electronically coupled interface between fully tailored catalyst morphology and high surface area of conductive support turned out to be essential. [20] In this regard, unlike widely used oxygen functionalized carbon supports with severely damaged electrical properties, electron-rich N-doped carbon nanotubes (CNTs) and graphene not only provide additional electrons to graphitic carbons to maintain high electroconductivity but also afford novel synthesis platform for the synergistic hybrid structures of ultimate practical utilization. [12] We present a unique nanostructured core shell hybrid catalysts composed of transition metal phosphosulfide (CoP@PS, MoP@PS, CoMoP@PS) decorated at N-doped carbon nanotubes (NCNTs). Nanoscale thick amorphous MS x (M = Co, Mo, CoMo) layers were directly deposited on the large area NCNT surfaces by low-temperature wet chemical process and subsequent phosphidation. Quasi-amorphous phosphosulfide (PS) nanoshells are formed around the crystalline core particles, whose S-doping effect greatly enhances the HER activity of nanoparticles to attain the noticeable HER activity of ma cm 2 with excellent durability. It is noteworthy that this is the first report for core shell transition metal phosphosulfide nanostructure formation with a clear evolution of amorphous PS surface. The underlying catalytic mechanisms were investigated by density functional theory (DFT), which revealed the principal role of hybrid interfaces for the catalytic activity. Taken together, this hybrid offers one of the highest active nonnoble metal electrocatalysts ever reported thus far (1 of 7)

2 Figure 1. Characterization of core shell hybrid. a) High-resolution TEM image of core shell nanocrystal grown over NCNT strand, where amorphous nanoshell wrapping the crystalline core particle was clearly observed. Scale bar: 5 nm. b) HAADF-STEM image and corresponding EELS line scanning analysis of core shell CoP@PS nanoparticle. c) XRD pattern of the synthesized hybrid that was compared with the standard pattern of CoP. d) XPS analysis of Co 2p, P 2p, and S 2p spectra of the core shell CoP@PS/NCNT. Core shell transition metal phosphosulfide/ncnt hybrids could be synthesized from various precursors (Figure S1, Supporting Information) by typical low temperature wet chemical reaction. Owing to the spontaneous electrostatic coordination among cobalt metal linker with polysulfide ions (S 5 2 ), [21,22] 2 nm thick amorphous CoS x layers were nucleated at the N-dopant sites (weakly posited charged) of CNT surfaces [12c] (Figure S2, Supporting Information) (see the Supporting Information for synthetic details). Afterward, thoroughly washed CoS x /NCNT hybrids were phosphidated at 700 C (see the Supporting Information for synthetic details). Upon this step, the as-deposited amorphous metal sulfide was entirely transformed into nm size metal phosphosulfide nanoparticles to reduce surface tension, where amorphous PS nanoshells encapsulate the crystalline cores (Figure 1a,b). [21] During the high-temperature phosphidation conversion, quasi-amorphous CoS x film not only underwent the local atomic diffusion as well as motion via thermal stimulation but also varied its stress governed by the intrinsic surface tension, which eventually drives the morphological regulation at high temperature. More importantly, because of the simultaneous flow of the phosphorus precursor and H 2 gas, it induced the reduction of CoP x S y accompanying H 2 S release in parallel, which finally forms an amorphous phosphosulfide phase nanoshell at the surface and the crystalline nanoparticle in the core rather than forming crystalline cobalt sulfide compound. [21,25] Fringe lattice spacing of crystalline core shows an interlayer distance of nm for the (211) plane of CoP, which is also consistent with the X-ray diffraction (XRD) in Figure 1c. To unveil the chemical composition of amorphous shell, electron energy loss spectroscopy (EELS) line scanning was employed, which suggests that the outside edges are clearly protruded with P and S signals, whereas Co and P elements are mainly distributed in the center cores (Figure 1b). Besides, energy-dispersive X-ray (EDX) elemental mappings of the core shell CoP@PS/NCNT shown in Figure S3 (Supporting Information) confirm the distribution of Co, P, S, and N over C. This again verifies that the core shell nanoparticles are intimately formed at NCNT surfaces. As shown in Figure 1d, the surface composition and oxidation states of hybrids were investigated before and after phosphidation with X-ray photoelectron spectroscopy (XPS). Before phosphidation, the Co 2p 3/2 and 2p 1/2 core level peaks are located at and ev, respectively, coincidental with the Co 2p core level of CoS x. [22] The Co 2p region after phosphidation exhibits dominant Co 2p doublet at and ev, indicative of a cobalt phosphide species formation. [23] In the case (2 of 7)

3 Figure 2. Characterization of core shell Co incorporated hybrid. a) High-resolution TEM image of core shell attached on NCNT, where amorphous nanoshell wrapping the crystalline core particle was shown clearly. Scale bar: 5 nm. b) XRD pattern of the synthesized hybrid compared with the standard pattern of MoP. c) HAADF-STEM image demonstrates the densely attached core shell nanoparticles over NCNT. Scale bar: 40 nm. d) Corresponding EDS mapping analysis within the green line rectangular region in (c) shows rather distribution of each element. e) EDS line scanning spectra of core shell Co-MoP@PS nanocrystal. of P 2p region, appearance of doublet peaks (P 2p 3/2 at ev and P 2p 1/2 at ev) indicates the bonding between P and Co. [24] By contrast, the phosphate peak at the high binding energy of ev is very weak, which means that the surface of core shell CoP@PS/NCNT is less prone to be oxidized. The direct comparison of S 2p core level before and after the phosphidation illustrates the decrease or even complete disappear of varied sulfur species, such as terminal S 2 2 ligands ( ev), the bridging ligands, the apical ligand ( ev), and the residual sulfur at higher binding energies ( ev) in the hybrid. [25] This signifies the removal of polysulfide species during the phosphidation procedure. Our synthetic protocol is compatible with various available precursors for relevant core shell transition metal phosphosulfide/ncnt hybrids. Figure 2a and Figure S4a (Supporting Information) show the high-resolution transmission electron microscopy images of core shell CoMoP@PS/NCNT and MoP@ PS/NCNT, synthesized from (NH 4 ) 2 MoS 4, and CoCl 2 as the precursors for Co, Mo, and S, respectively, (see the Supporting Information for synthetic details). The XRD results illustrate both hybrids have the primary peaks originated from MoP (JCPDS no ) (Figure 2b; Figure S4b, Supporting Information). The difference, however, just lies in the Co element that serves as dopant in the core shell CoMoP@PS/NCNT. Furthermore, the lattice spacing of nm at the core matches with MoP (101) plane, which is also in accordance with XRD analysis above. Scanning transmission electron microscopy and energy dispersive spectrometer (EDS) mapping in Figure 2c,d demonstrate that core shell nanoparticles were uniformly attached on the NCNT surfaces. Noticeably, the chemical composition of amorphous shell was detected to be composed of P and S, based on the EDX line scanning measurement (Figure 2e). In addition, XPS analysis shows the influence of Co doping with the shift of Mo 3d 5/2 and Mo 3d 3/2 binding energies from and ev (CoMoP@PS/NCNT) to and ev (MoP@PS/NCNT) (Figure S5, Supporting Information). However, the variation trend of P, S 2p spectra before and after the phosphidation is the same with that observed from the core shell CoP@PS/NCNT synthesis (Figure S5, Supporting Information). Therefore, it clarifies the coherent formation of amorphous nanoshell throughout the uniquely designed two-step synthesis protocol. Recently, it has been demonstrated that the transition metal phosphosulfides show promising hydrogen turnover activity. Significantly, the proton reduction kinetics was highly associated with the chemistry and structure of catalyst surface. [11 14] Along with the core shell morphology formation of our hybrid (3 of 7)

4 Figure 3. HER activity of hybrid electrocatalyst. a) Polarization curves, b) onset potential (left) and overpotential (right) analysis of different HER catalyts. c) Tafel plots of core shell and CoS P/NCNT in acid solution. d) Cycling stability of core shell before and after 1000 cycles and e) time dependence of overpotential variation under 20 ma cm 2 current density. catalysts with unique chemical structure, the modification of proton adsorption/desorption energy and active sites may greatly influence on the HER activity. The electrochemical HER tests are performed using three-electrode setup in the acidic condition of 0.5 m H 2 SO 4 solution (see the Supporting Information for details). Noteworthy that all polarization curves are corrected for ir loss. The typical polarization curve (I V plot) demonstrates that the core shell CoP@PS/NCNT presents a low overpotential of 80 mv versus RHE (@ 10 ma cm 2 ) (Figure 3a,b), which is inferior to the catalytic activity of commercial Pt/C ( ma cm 2 ) but better than CoS P/NCNT without nanoshell ( ma cm 2 ) in the same HER test setup (see the Supporting Information for synthesis of CoS P/NCNT). More significantly, the catalytic overpotential of core shell CoP@PS/NCNT is comparable to those of the best transition metal phosphosulfide HER catalysts ever reported thus far, such as CoPS ( 65 to ma cm 2 ) and MoPS ( ma cm 2 ) [17,18] and other high-performance HER catalysts (Table S1, Supporting Information). Apart from that, HER activity of the other catalysts such as, CoMoP@PS/NCNT and MoP@PS/NCNT synthesized through the same approach are also evaluated as shown in Figure S8 (Supporting Information), in which both of CoMoP@PS/NCNT and MoP@PS/NCNT exhibit inferior HER performance compared to CoP@PS/NCNT. To evaluate the charge transfer process during the electrocatalysis, electrochemical impedance spectroscopy of various hybrid catalysts were characterized. As shown in Figure S9 and summarized Table S3 (Supporting Information), CoP@PS/NCNT exhibits the lowest charge transfer resistance, which further supports the higher HER activity of CoP@PS/NCNT compared to other catalysts from the polarization analysis (4 of 7)

5 Figure 4. a) The top view of the most stable configurations of hydrogen adsorption on the CoP (101), (101), and CoS P (101) surfaces. Purple, light-blue, green, and white atoms correspond to Co, P, S, and H, respectively. b) Free energy diagram is calculated on the corresponding surfaces. To understand the intrinsic HER activity of core shell CoP@PS/NCNT, Tafel plots based on polarization curves were acquired. Tafel slopes of 53, 59 mv per decade were obtained for core shell CoP@PS/NCNT, CoS P/NCNT, respectively, as shown in Figure 3c. Compared to CoS P/NCNT, the core shell CoP@PS/NCNT hybrids possess a relatively lower Tafel slope, indicating that it follows favorable HER mechanism of Volmer Heyrovsky pathway. Based on the Tafel slope analysis, the exchange current density of different catalysts was also evaluated (Table S2, Supporting Information). The intrinsic per-site hydrogen molecule evolution is another important metric to evaluate the HER activity of a catalyst. We used electrochemical capacitance surface area measurements to determine the active surface area. [18] This was further used to calculate the average activity of each site, namely, a per-site turnover frequency (TOF) (see the detail in Figure S6 and the equation in the Supporting Information). The electrochemical active surface area (ECSA) of core shell CoP@PS/NCNT hybrid is estimated to be 306. The corresponding hydrogen TOF from ECSA is then calculated to be as high as 0.06 s 1 at η = 100 mv versus RHE and ph = 0. Stability is another significant criterion for HER catalyst. The catalytic durability of core shell CoP@PS/NCNT is characterized by continuous cyclic voltammetry between 0.2 and 0.2 V versus RHE at 50 m s 1 scan rate (Figure 3d). Only a minor deterioration of overpotential was observed after 1000 cycling. Also, continuous hydrogen production from chronopotentiometry measurement further implies the remarkable stability of core shell CoP@PS/NCNT hybrids with only 5 mv increment of overpotential at 20 ma cm 2 after 250 min operation (Figure 4e). According to Sabatier principle, [26] HER catalytic activity over a wide range of materials has been proposed to be well described by the adsorption free energy of H at catalyst surface. This underlines that there is an optimal value of H adsorption free energy maximizing the HER rate. Using DFT calculations, herein, we mapped thermodynamic free energy diagrams for HER over the catalyst surfaces of CoP(101), CoP@PS(101), and CoS P(001). To simulate the hybrid interfaces between core metal phosphide and amorphous nanoshells we setup model systems as shown in Figure 4a, where S from amorphous nanoshell functions as dopants at the interface. Figure 4b denotes thermodynamic free energy diagrams for our model catalysts toward HER as a function of reaction intermediates (see the Supporting Information for the detailed computational description). Our calculations indicated that H adsorbs at the bridge site of Co Co in both CoP and CoP@PS with similar adsorption energies (H slightly prefers CoP to CoP@PS only by ev). The next H absorption is more favorable for nearby P-sites in both catalysts, but with a considerably low energy barrier for CoP@PS. This can be ascribed to the different electronegativity between S and P (S has a higher electron affinity). Thus, electrons are partially transferred from P to S. It leads to, then, that the P in CoP@PS would be more deficient of lone pair electrons than that in bare CoP and thus, has a higher basicity (proton-acceptor). This is the underlying mechanism for the higher HER activity of CoP@PS. In pyritetype CoS P, the successive hydrogen adsorption mechanism is the same as the CoP@PS. CoS P shows a lower energy barrier for H adsorption in the reaction step H ads + H ads. Due to the stronger binding energy with H, which makes H 2 desorption difficult, the CoS P, in fact, shows a lower HER activity than CoP@PS. Taken together, CoP@PS is the most promising catalyst among the three model nanoparticles shown in Figure 4b. We have demonstrated the unique synthesis approach for various core shell metal phosphosulfide/ncnt hybrids. Notably, core shell CoP@PS/NCNT exhibits remarkable HER performance, also outperforming CoS P/NCNT counterpart. This is attributed to the significant enhancing effect from nanometer thick amorphous PS layer on the HER turnover (5 of 7)

6 process and additional synergy effect arising from the innovative hybridization between conductive NCNT forest electrodes and active materials. Importantly, the remarkable HER activity of core shell hybrids could match with that of the best metal phosphosulfide HER catalysts ever reported. Furthermore, these typical features allow our nonprecious-metalbased hybrid catalyst structures to be a prospective candidate for other hydrogenation reactions. Experimental Section Experimental details are shown in the Supporting Information. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements D.J.L. and J.K. contributed equally to this work. This work was supported by the Global Frontier Hybrid Interface Materials (GFHIM) (Grant No. 2013M3A6B ), Nano Material Technology Development Program (Grant No. 2016M3A7B ), and the Multi-Dimensional Directed Nanoscale Assembly Creative Research Initiative (CRI) Center (Grant No. 2015R1A3A ) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning. Conflict of Interest The authors declare no conflict of interest. Keywords carbon nanotubes, catalysts, doping, hydrogen evolution, interfaces, metal phosphosulfide Received: October 8, 2017 Revised: November 24, 2017 Published online: [1] M. S. Dresselhaus, I. L. Thomas, Nature 2001, 414, 332. [2] J. A. Turner, Science 2004, 305, 972. [3] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, [4] a) M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 2010, 110, 6446; b) Y. Shi, B. Zhang, Chem. Soc. 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