P-Doped Ag Nanoparticles Embedded in N-Doped Carbon Nanoflake

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P-Doped Ag Nanoparticles Embedded in N-Doped Carbon Nanoflake: An Efficient Electrocatalyst for the Hydrogen Evolution Reaction Xuqiang Ji, Bingping Liu, Xiang Ren, Xifeng Shi, Abdullah M. Asiri, and Xuping Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04732 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Sustainable Chemistry & Engineering

P-Doped Ag Nanoparticles Embedded in N-Doped Carbon Nanoflake: An Efficient Electrocatalyst for the Hydrogen Evolution Reaction Xuqiang Ji,†,‡,║ Bingping Liu,‡,║ Xiang Ren,† Xifeng Shi,§ Abdullah M. Asiri,ʃ and Xuping Sun*,† †

Institute of Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, China, ‡College of Chemical and Environmental Engineering, Qingdao University, Qingdao 266071, China, §College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China, and ʃ Chemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia * E-mail: [email protected] ABSTRACT: Electrolytic hydrogen generation needs efficient and durable electrocatalysts for the hydrogen evolution reaction. In this Communication, for the first time, we report on the development of P-doped Ag nanoparticles embedded in N-doped carbon nanoflake (denoted as “P-Ag@NC”) for effective hydrogen evolution electrocatalysis. When tested in 0.5 M H2SO4, such PAg@NC demands overpotential of only 78 mV to drive a catalytic current density of 10 mA cm-2, which is 198 mV less than that of Ag@NC counterpart. Remarkably, this catalyst also shows strong long-term electrochemical durability with the preservation of its catalytic activity for at least 85 h. Density functional theory calculations suggest that P doping brings the optimization of hydrogen adsorption free energy to a more thermo-neutral value. KEYWORDS: Ag, P doping, Hydrogen evolution, Electrocatalyst, Density functional theory

The depletion of fossil fuels and increased environmental concerns have triggered an urgent need to search for clean and sustainable alternatives.1,2 Hydrogen is regarded as an ideal such candidate with outstanding gravimetric energy density.3,4 Several techniques including steam reforming, partial oxidation and coal gasification emit carbon dioxide and thus accelerate greenhouse effect. Electrochemical water splitting offers an environmentally-friendly route to make hydrogen from electricity produced from intermittent renewable energy resources like solar energy, wind, and wave power, etc. Electrolytic hydrogen production however needs effective electrocatalysts for the hydrogen evolution reaction (HER) to attain high current density at low overpotential.5-15 Acid-stable HER catalysts must be used for proton exchange membrane-based electrolysis units because of the strongly acidic conditions.16 Pt is the most active HER catalyst in acids.17 For Pt, however, the crust abundance is only 0.005 ppm18 and the market price is $995 per ounce (Kitco Platinum Index, Jan 31, 2018), hindering its wide use. Thus, it is highly needed to explore non-Pt alternatives satisfying both good catalytic performance and reasonable cost. Compared to Pt, noble metals of Pd, Rh, Ru and Ir have higher abundance and much recent effort has been put to develop efficient HER catalysts based on such elements to alleviate the Pt dependency.19-29 As a less expensive noble metal, Ag has also been explored to construct hybrid catalysts, including Ag-Ag2S/MoS2,30 Ag2S/CuS,31 MoS2/Ag2S32 and Ag2S/Ag.33 Among them, Ag2S/CuS shows the best activity with the demand of overpotential of 193 mV to deliver 10 mA

cm-2 in 0.5 M H2SO4.31 On the other hand, efficient electrocatalysis requires the good dispersion of nanocatalysts. Particularly, encapsulating nanocatalysts in conductive carbon matrix not only avoids effectively particles aggregation but facilitates electron transfer, which is of great benefit to the electrochemical performance.34,35 To the best of knowledge, however, Ag-based such catalyst has not been reported before.

Figure 1. (a) XRD patterns for NC, Ag@NC and P-Ag@NC. (b) XPS survey spectrum for P-Ag@NC. XPS spectra for P-Ag@NC in the (c) Ag 3d, (d) P 2p, (e) N 1s, and (f) C 1s regions.

Here, we report the first development of P-doped Ag nanoparticles embedded in N-doped cabon nanoflake (PAg@NC) for hydrogen evolution electrocatalysis in acids. When tested in 0.5 M H2SO4, such P-Ag@NC shows high HER activity with the need of overpotential of only 78 mV to attain a catalytic current density of 10 mA cm-2, 198 mV less

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than that of Ag@NC. Remarkably, this catalyst is also superior in long-term electrochemical durability with the retention of its activity for at least 85 h. Density functional theory (DFT) calculations reveal that the hydrogen adsorption free energy (∆GH*) on Ag@NC decreases dramatically from 2.015 to 0.709 eV after P doping, and such thermo-neutral ∆GH* is favourable for optimal HER activity. P-Ag@NC was fabricated by thermal annealing of AgNO3, urea and glucose with the presence of NaH2PO2, and another two control samples of NC and Ag@NC were also synthesized (see ESI for preparative details). Figure 1a shows the X-ray diffraction (XRD) patterns of NC, Ag@NC and P-Ag@NC. NC shows diffraction peak of carbon at 25.96°.36 Compared to NC, Ag@NC shows extra diffraction peaks at 38.1°, 44.3°, 64.4°, and 77.38° indexed to the (111), (200), (220), and (311) crystal planes of metallic Ag, respectively (JCPDS No. 870597).37 The diffraction peaks of Ag are slightly shifted toward higher Bragg angle for P-Ag@NC, implying successful P doping of [email protected] Figure 1b shows the X-ray photoelectron spectroscopy (XPS) survey spectrum for PAg@NC, also confirming the exsitence of P in P-Ag@NC. However, NC (Figure S1) and Ag@NC (Figure S2) show no peak of P element. Figure 1c-1f present the XPS spectra of PAg@NC in the Ag 3d, P 2p, N 1s and C 1s regions. In the Ag 3d region (Figure 1c), the peaks at 368.6 and 374.6 eV are assigned to the binding energies (BEs) of Ag 3d5/2 and Ag 3d7/2, respectively.39 The BE at 133.35 eV in the P 2p region (Figure 1d) is assigned to oxidized P species, resulting from air contact of [email protected] And peaks at 130.2 and 129.3 eV are attributed to P-Ag bond. N 1s region shows two peaks at 399.7 and 401.5 eV corresponding to N-C=C and quaternary N, respectively (Figure 1e).41 In Figure 1f, the peaks at 284.8 and 286.5 eV can be ascribed to C-C/C=C and C=C-N bonds, respectively.41 High-resolution XPS spectra for NC and Ag@NC are shown for comparison (Figure S3 and S4).

Figure 2. (a, b) SEM images of P-Ag@NC. (c) TEM images of PAg@NC. (d, e) HRTEM image taken from P-Ag@NC. (f) SEM image and corresponding EDX elemental mapping of P, Ag and N for P-Ag@NC.

Figure 2a and 2b show the scanning electron microscopy (SEM) images of P-Ag@NC, suggesting the formation of flake-like nanostructures. NC and Ag@NC are also nanoflakes in nature (Figure S5). The low-magnification transmission electron microscopy (TEM) image for P-Ag@NC shows the co-existence of a large amount of nanoparticles with nanoflake (Figure 2c). A closer view of the hybrid nanoflake reveals that the nanoparticle was embedded in the NC matrix (Figure 2d).

Such nanoparticle shows well-resolved lattice fringes with an interplanar distance of 0.25 nm (Figure 2e), slightly larger than the value of 0.235 nm for the (111) plane of metallic Ag, providing another piece of evidence to support P doping. Energy dispersive X-ray spectrum (EDX) analysis for P-Ag@NC (Figure S6) concludes that the atomic ratio of P/Ag/N/C is roughly 1.02:5.53:25.57:67.88. The SEM image and corresponding EDX elemental mapping images further indicate that all elements are uniformly distributed in the hybrid (Figure 2f). For comparison, corresponding results for NC and Ag@NC are also presented in Figure S7 and S8.

Figure 3. (a) LSV curves for P-Ag@NC, Ag@NC, Pt/C, NC, and bare GCE with a scan rate of 2 mV s−1 for HER. (b) Tafel plots for P-Ag@NC, Ag@NC, and Pt/C. (c) LSV curves recorded for P-Ag@NC before and after 1000 CV cycles. (d) Time-dependent current density curve at a fixed overpotential of 200 mV for PAg@NC. All experiments were performed in 0.5 M H2SO4.

We examined the electrocatalytic HER activity of PAg@NC (loading: ~0.8 mg cm-2) deposited on glassy carbon electrode (GCE) in 0.5 M H2SO4 using a typical threeelectrode configuration. For comparison, Ag@NC, NC, and commercial Pt/C (20 wt%) were deposited on GCEs and examined under the same test conditions. Bare GCE was also tested. Since as-measured reaction currents cannot directly reflect the intrinsic behavior of catalysts due to the effect of solution resistance, all electrochemical data were corrected with ohmic potential drop (iR) losses for further analysis.42-44 Overpotentials were reported on a reversible hydrogen electrode (RHE) scale. Figure 3a shows the linear sweep voltammetry (LSV) curves with a scan rate of 2 mV s-1. Bare GCE has no catalytic activity for the HER and NC also shows very poor HER activity. Pt/C is an excellent HER catalyst with the need of overpotential of 26 mV to approach a catalytical current density of 10 mA cm-2. Notably, P-Ag@NC is also highly active for the HER and demands overpotential of 78 mV to drive 10 mA cm-2, 198 mV less than that of Ag@NC. This overpotential compares favorably to the behaviors of all reported Ag-based HER electrocatalysts in acids (Table S1). Figure 3b shows the Tafel plots. Pt/C gives a Tafel slope of 58 mV dec-1. The Tafel slope for P-Ag@NC (107 mV dec−1) is much lower than that for Ag@NC (434 mV dec−1), implying more favorable catalytic kinetics on [email protected] We also probed the stability of P-Ag@NC by continuous cyclic volt-


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ammetry (CV) scanning with a scan rate of 100 mV s-1. After 1000 CV cycles in 0.5 M H2SO4, the polarization curve shows negligible difference compared with the initial one (Figure 3c). The long-term durability of P-Ag@NC was further evaluated by HER electrolysis at a fixed overpotential of 200 mV, suggesting this catalyst retains its catalytic activity for at least 85 h (Figure 3d). The morphology and composition of PAg@NC are unchanged after long-term stability test and further confirmed by corresponding SEM (Figure S9), TEM (Figure S10) and XPS (Figure S11). The generated gas was confirmed by gas chromatography analysis and measured quantitatively using a calibrated pressure sensor to monitor the pressure change in a cathode compartment of H-type electrolytic cell.46 The Faradaic efficiency was determined as 100% by comparing the quantity of practically evolved hydrogen with theoretically calculated one (Figure S12). Compared with the performance in acidic solution, P-Ag@NC demonstrates poor catalytic activity in both alkaline and neutral media, with large overpotentials at 10 mA cm-2 (252 mV in 1.0 M KOH and 210 mV in 1.0 M phosphate buffer saline), while the good stability is presented (Figure S13 and S14).

thermal-neutral value (0.709 eV), which is of great benefit for optimal HER activity. In conclusion, P-Ag@NC has been developed by thermal annealing of AgNO3, urea, and glucose using NaH2PO2 as P source. Such P-Ag@NC shows much superior catalytic activity to undoped Ag@NC and needs overpotential of 78 mV to drive 10 mA cm-2 in 0.5 M H2SO4, 198 mV lower than that of Ag@NC. Moreover, this catalyst also shows strong long-term electrochemical durability. Such superior activity for PAg@NC is attributed to its more thermo-neutral hydrogen adsorption free energy. This study not only provides us an attractive non-Pt catalyst material in water-splitting devices toward electrochemical production of hydrogen fuel, but would open an exciting new avenue to explore using P as an effective dopant to enhance the hydrogen-evolving activity of transition metal catalysts for applications.

ASSOCIATED CONTENT Supporting Information Experimental section; XPS spectra; SEM images; EDX spectrum; EDX elemental mapping images; TEM images; FE data; LSV curves; Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Author Contributions ║

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Figure 4. Structural models for hydrogen adsorption on (a) NC, (b) Ag@NC and (c) P-Ag@NC. (d) The calculated free-energy diagram of HER at equilibrium potential for NC, Ag@NC, PAg@NC and Pt.

It is established that the HER activity is correlated with ∆GH* on the catalyst surface.47 ∆GH* is a natural parameter used to quantify the bond strength between hydrogen and active sites, and a ∆GH* value closed to 0 eV indicates optimal HER activity with intermediate binding energies. We applied DFT to caculate the ∆GH* on NC, Ag@NC, P-Ag@NC and Pt. Figure 4a-4c show the theoretical models of NC, Ag@NC and P-Ag@NC used in DFT calculations, and the C atom next to N atom is the active adsorption site for H*. As shown in Figure 4d, Pt as the most active HER catalyst has a ∆GH* value of approximately -0.09 eV, which is in good agreement with the recent study.48 NC has less thermal-neutral ∆GH* of 2.625 eV. The combination of metallic Ag with NC can slightly reduce the hydrogen adsorption strength on the catalyst surface, with a ∆GH* value of 2.015 eV. Moreover, the doping of P atoms into Ag can further decrease the ∆GH* of Ag@NC to a more

This work was supported by the National Natural Science Foundation of China (No. 21575137). We also appreciate Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

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P-doped Ag nanoparticles embedded in N-doped carbon nanoflake (P-Ag@NC) behaves an efficient and durable HER electrocatalyst with overpotential of only 78 mV to drive 10 mA cm-2, 198 mV less than that for undoped Ag@NC counterpart.

6 ACS Paragon Plus Environment

P-Doped Ag Nanoparticles Embedded in N-Doped Carbon Nanoflake

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