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Single-atom Pt supported on holey ultrathin g-C3N4 nanosheets as efficient catalyst for Li-O2 batteries
Journal Pre-proofs Single-atom Pt supported on holey ultrathin g-C3N4 nanosheets as efficient catalyst for Li-O2 batteries Wen Zhao, Jun Wang, Rui Yin, Boya Li, Xiaoshuai Huang, Lanling Zhao, Lei Qian PII: DOI: Reference:
S0021-9797(19)31568-1 https://doi.org/10.1016/j.jcis.2019.12.102 YJCIS 25842
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
4 October 2019 10 December 2019 23 December 2019
Please cite this article as: W. Zhao, J. Wang, R. Yin, B. Li, X. Huang, L. Zhao, L. Qian, Single-atom Pt supported on holey ultrathin g-C3N4 nanosheets as efficient catalyst for Li-O2 batteries, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.102
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Single-atom Pt supported on holey ultrathin g-C3N4 nanosheets as efficient catalyst for Li-O2 batteries
Wen Zhaoa, Jun Wang*a, Rui Yina, Boya Lib, Xiaoshuai Huanga, Lanling Zhao*c, Lei Qian*a aKey
Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry
of Education), Shandong University, 17923 Jingshi Road, Jinan 250061, China. E-mail: [email protected], [email protected]. bSchool
of Mechanical Electronic & Information Engineering, China University of Mining &
Technology-Beijing, Beijing 100083, P. R. China. cSchool
of Physics, Shandong University, Jinan 250100, P. R. China. E-mail:
[email protected].
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Abstract As for electrocatalysis, single-atom metal catalysts have been proved to lower the cost and utilize precious metals more efficiently. Herein, single-atom Pt catalyst supported on holey ultrathin g-C3N4 nanosheets (Pt-CNHS) was synthesized via a facile liquid-phase reaction of g-C3N4 and H2PtCl6. The single-atom Pt can achieve high dispersibility and stability, which can promote the utilization efficiency as well as enhance the electrochemical activity. When employed as Li-O2 batteries’ cathode catalyst, Pt-CNHS exhibits excellent electrocatalytic activity. Li-O2 batteries utilizing Pt-CNHS show much higher discharge specific capacities than those with pure CNHS. Li-O2 batteries with Pt-CNHS cathode can be cycled stably for 100 times under the discharge capacity of 600 mAh g-1. Based on experimental results and density functional theory calculations, the superior electrocatalytic activity of Pt-CNHS can be ascribed to the large surface area, the enhanced electrical conductivity and the efficient interfacial mass transfer through Pt atoms and porous structure of CNHS. Keywords: Single-atom Pt; g-C3N4; Density functional theory calculations; Electrocatalysis; Li-O2 batteries
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1. Introduction Li-O2 battery is a promising energy storage system for future electric vehicles since it possesses a high specific energy (up to 3560 Wh Kg-1) [1-4]. However, in terms of practical applications, there still are several unsolved problems, such as poor cycling efficiency, low rate capability and limited short life span[5, 6]. These unsatisfactory electrochemical characteristics are mainly attributed to the polarization during discharging and charging[7, 8], which are caused by the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)[9]. Developing efficient electrocatalysts is an ideal way to solve this problem. Various catalysts have been explored to achieve high electrocatalytic activity, including noble metals[10, 11], bimetallic nanoparticles[12, 13], transition metal oxides[14, 15], perovskites[16, 17], metal nitride[18, 19], metal carbide[20, 21] and metal sulfide[22, 23]. Among them, platinum (Pt) is a frequently used catalytic material[24, 25]. However, restricted by its high cost and rarity, many researches focus on increasing Pt utilization efficiency as well as enhancing its catalytic activity[26-28]. Previous studies show that the catalytic properties of Pt are largely related to its structure. Therefore, numerous efforts have been conducted to control the size of Pt nanoparticles, design core-shell structures and construct metal-support interactions[29-31]. By employing these methods, favorable catalytic property of Pt-based materials has been achieved. Therefore, constructing a novel Pt-containing catalyst with unique structure and morphology is of great importance to optimize the performance of Li-O2 batteries. Recently, single-atom Pt based catalysts have been prepared and achieved excellent electrocatalytic activity. For instance, Single-atom Pt doped on TiN nanoparticles exhibits excellent electrocatalytic performance in terms of oxygen reduction, formic acid oxidation as well as methanol oxidation[26]. Atomically dispersed Pt with high dispersibility was supported on mesoporous Al2O3 to exhibit enhanced stability[32]. Similarly, single-atom Pt supported on FeOx showed high activity and stability for CO oxidation[33]. Also, Pt atoms 3
were doped on MoS2 surface to achieve high electrocatalytic activity towards hydrogen-evolution reaction[34]. The single-atom form attracts great interests since it could maximize the Pt atom utilization as well as achieve high catalytic activity. The synthesis of graphitic-carbon nitride (g-C3N4) can be realized by a series of polycondensation reactions of nitrogen-rich materials[35, 36]. g-C3N4 shows a planar phase similar with graphite, but unlike graphite, g-C3N4 contains both graphite-like and pyridine-like nitrogen atoms. Also, in g-C3N4, every carbon atom is bonded to both graphitic and pyridinic nitrogen atoms[37]. Recently, g-C3N4 has attracted tremendous attention due to its unusual properties, including high chemical stability, low-cost preparation and special foam-like structure[38]. Due to its nitrogen-rich property and facile synthesis, g-C3N4 is suitable for serving as the catalyst for photocatalysis, fuel cells and CO2 capture[39, 40]. It was reported by Yi et al.[7] that graphitic-C3N4@carbon paper was used as the cathode in Li-O2 batteries, and the batteries obtain high rate capability as well as good cycling stability. Zheng et al.[41] prepared nanoporous g-C3N4@carbon metal-free electrocatalysts and used them as ORR catalyst. In g-C3N4, there are many removable electrons between the layers which can complex with abundant metal ions and adjust the conjugation structure[42]. Many researches have tried to combine g-C3N4 with non-precious metals and perovskite oxides, such as Co-g-C3N4[40], Fe-g-C3N4[43], Ni-Co3O4-C3N4[44] and g-C3N4-LaNiO3[10]. Pt possesses superior electrical conductivity and excellent electrocatalytic activity. Combining Pt with g-C3N4 is assumed to promote both the mass transfer and catalytic activity effectively. Some researchers have successfully prepared g-C3N4-Pt composites. For instance, Hu et al.[45] prepared g-C3N4-Pt nanohybrids which exhibited great catalytic activity and high stability towards methanol oxidation. Li et al.[46] employed single-atom Pt to catalyze H2 evolution. However, as far as we know, few researches are focused on loading single-atom Pt on g-C3N4 to catalyze the reactions in Li-O2 batteries.
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Based on these previous studies, we prepared single-atom Pt decorated holey ultrathin g-C3N4 nanosheets (Pt-CNHS) as a novel cathode catalyst for Li-O2 batteries. Various in-plane holes on the g-C3N4 layers can offer more active sites for the reactions in Li-O2 batteries[47]. Furthermore, the cross-layer diffusion paths resulted from the holes may promote the mass transfer and electron distribution efficiency of CNHS[48], which are in favor of the electrocatalytic performance. Besides improving the catalytic process and mass transportation, high surface area of CNHS can also promote the wetting of electrode by electrolyte and the accommodation of Li2O2. By loading isolated Pt atoms uniformly and stably on the CNHS matrix, the maximum utilization of Pt metal as well as good electrocatalytic performance can be achieved. The capacity, cycling stability and rate performance of Pt-CNHS were investigated. As demonstrated by the density functional theory calculations, the improved electrocatalytic performance of Pt-CNHS is attributed to the synergistic effects between single-atom Pt with modified charge distribution and CNHS, which provide local build-in electric field to stable Pt atoms and enhance their battery performance. 2. Experimental methods 2.1 Synthesis of bulk g-C3N4 Bulk g-C3N4 (CNB) was synthesized by thermal polycondensation of melamine. Specifically, 20 g melamine powder was calcinated under 500 °C for 2 h (2 °C min-1) in an alumina crucible and then calcinated under 520 °C for another 2 h (under the same heating rate). The resulted yellow product was grounded into powder for next-step treatment. 2.2 Synthesis of g-C3N4 nanosheets and holey ultrathin g-C3N4 nanosheets g-C3N4 nanosheets (CNS) were synthesized by further calcination of CNB. 5 g CNB was uniformly spread in an alumina ark and then calcinated under 520 °C for 4.5 h (2 °C min-1). Afterwards, the light-yellow product was obtained. The holey ultrathin g-C3N4 nanosheets (CNHS) were synthesized by increasing the calcination time to 6 h. 5
2.3 Synthesis of Pt-CNHS The Pt-CNHS was prepared by liquid-phase reaction of CNHS and H2PtCl6·6H2O followed by a heat treatment. Firstly, 50 mg CNHS was dispersed in 15 ml deionized water to form an aqueous dispersion and 30 mg H2PtCl6·6H2O was added into it. Next, the mixture was continuously stirring at 70 °C for 6, 8 and 10 h (marked as Pt-CNHS-6, Pt-CNHS-8 and Pt-CNHS-10, respectively). Pt-CNB and Pt-KB were also synthesized using the same method by replacing CNHS with CNB and Ketjen Black with a stirring duration for 8 h. The resulted products were washed by centrifuging using deionized water and ethanol. Finally, they were dried at 60 °C for 12 h under vacuum, then heated to 125 °C for 1 h under vacuum. 2.4 Physical characterization X-ray diffraction (XRD) measurements were conducted using a D8 Advance X-ray generator and diffractometer with Cu Kα radiation (λ=1.5418 Å). Fourier transform infrared spectroscopy (FTIR) were collected by using a Nicolet Nexus 410 spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ThermoFisher K-Alpha instrument. Inductively coupled plasma (ICP) was carried out on an HK-2000 emission spectrophotometer. The Extended X-ray absorption fine structure spectroscopy (EXAFS) was conducted on beam line BL14W1[49] of the Shanghai Synchrotron Radiation Facility (SSRF), in the fluorescent mode with silicon drift fluorescence detector. The station was operated with a Si (311) double crystal monochromator. The synchrotron was operated at the energy of 3.5 GeV and the current between 150-210 mA. The photon energy was calibrated with the first inflection point of Pt L3-edge in Pt metal foil. Brunauer-Emmett-Teller (BET) analysis were performed using N2 adsorption/desorption on a Micromeritics Instrument Corp ASAP2460 instrument. Field emission scanning electron microscopy (FESEM) and Energy disperse Spectroscopy (EDS) images were obtained on a SU-70 FESEM. High Resolution transmission electron microscope (HRTEM) observations were employed on a Tecnai G2 F20 microscope. The high-angle annular dark-field scanning transmission electron microscopy 6
(HAADF-STEM) characterization was performed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. 2.5 Electrochemical investigation To prepare the cathodes tested in Li-O2 batteries, as-prepared catalysts were mixed with Super P and poly(tetrafluoroethylene) (PTFE) at the mass ratio of 4:4:2 in isopropanol. Afterwards, the mixture was uniformly pasted on a carbon paper and dried at 120 °C for 12 h under vacuum. The Li-O2 batteries were assembled with a Li foil anode, a glass fiber separator and a catalyst coated carbon paper as cathode. Galvanostatic discharge/charge tests were conducted on a LAND CT 2001A battery tester using constant current mode. All the measurements were carried out in O2 from 2.35 V to 4.35 V. Rate capability was tested by applying different current densities. Cyclic voltammetry (CV) experiments and Electrochemical impedance spectroscopy (EIS) were conducted on a RST5002F electrochemical workstation to investigate the kinetics of the ORR and the OER. CV was tested with a scan rate of 0.1 mV s-1 from 2.35 to 4.35 V and EIS was performed at an amplitude sine wave of 0.01 V and the frequency range from 100 kHz to 0.01 Hz. 2.6 Theoretical calculations All calculations were performed using the Vienna abinitio simulation package (VASP)[50, 51]. The projector augmented wave (PAW) potentials were employed to describe the nulcei-electron interactions, and the exchange-correlation between electrons was described using the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) form[52]. A C48N64 unit cell with the volume of 28.45Å 14.23Å 20.00 Å was used during the calculations. The cutoff energy of plane wave basis was set as 550 eV. The total energy convergence criterion and the residual forces were set to be 110-5 eV and 110-2 eV/Å, respectively. A 1 2 1 K-point grid centered at the gamma () point using Monkhorst Pack Scheme was applied. 7
3. Results and Discussion Scheme 1 shows the schematic illustration for synthetic process of Pt-CNHS. In this work,
Scheme 1 Schematic illustration of the process for preparing CNHS and Pt-CNHS.
foam-like CNHS with abundant holes was prepared by thermal polycondensation of melamine in air. Afterwards, single-atom Pt was loaded on CNHS via a facile liquid phase reaction. The digital photos of samples in different stages are shown in Supporting Information. Figure S1a, b, c, d and e are in correspondence with melamine, CNB, CNS, CNHS and Pt-CNHS, respectively. The crystal structure of different samples was analyzed by XRD. Figure S2a shows the XRD pattern of melamine. The XRD patterns of the CNB, CNS, CNHS and Pt-CNHS in Figure 1a demonstrate that the diffraction peaks of melamine disappeared and two major diffraction peaks corresponding to g-C3N4 appeared. The two dominant diffraction peaks at 13.1° and 27.3° are associated with the in-plane trigonal N linkage of tri-s-triazine motifs and the periodic stacking of layers for conjugated aromatic systems, respectively[39]. The decreased intensity of peak at 13.1° is mainly due to the decrease of in-plane structural units, which is caused by the oxidation etching parts of melems during the long-time calcination[53]. The peak of CNB has a little right shift, indicating a decreased gallery distance between the basic sheets in the nanosheets[47]. In this case, the heating treatment during thermal oxidation process should lead to a denser packing and thus shorten the gallery distance observed[47]. The peak intensity at 27.3° dramatically decreases, indicating that g-C3N4 bulk was successfully exfoliated to nanosheets[39, 54]. Meanwhile, the loading of Pt atoms may lead to a slight decrease of layer space of the nanosheets, causing the increase of 8
the peak intensity of Pt-CNHS at 27.3°. Compared with bare CNHS, the diffraction pattern of Pt-CNHS shows no obvious difference, implying that Pt decoration did not bring obvious influence to CNHS’s crystal structure. The diffraction peaks of Pt element do not appear in the XRD pattern of Pt-CNHS, possibly due to its low content and single atom size. The FT-IR spectra of CNB, CNS, CNHS and Pt-CNHS in Figure 1b show that nanosheets exhibit similar peaks as the bulk. The broad peaks which are from 3000 to 3500 cm-1 and from 900 to 1700 cm-1 can be attributed to N–H stretching vibrations and s-triazine derivatives, respectively. The band at 2150 cm-1 is associated with cyano terminal groups with C≡N[1, 41, 55, 56]. It clearly demonstrates that the structures of CNS and CNHS did not change. These results also confirm that Pt loading do not have influences on the CNHS structure.
Figure 1 (a) XRD patterns, (b) FT-IR spectra and (c) XPS survey spectra of CNB, CNS, CNHS and Pt-CNHS; (d) C 1s, (e) N 1s and (f) Pt 4f XPS spectra of CNHS and Pt-CNHS.
Surface chemical compositions of the composites were analyzed by XPS measurements. The XPS survey spectra in Figure 1c indicates the existence of C and N elements in both CNHS and Pt-CNHS. Furthermore, it shows that Pt atoms were successfully loaded on CNHS in the Pt-CNHS sample. In C 1s spectra of CNHS and Pt-CNHS, the two peaks with binding energy of 284.72 and 288.23 eV are assigned to graphitic carbon and sp2-bonded carbon (N=C–N), respectively (Figure 1d)[36, 39, 57]. The electronic structure variation of CNHS 9
after the introduction of Pt can be observed from the N 1s spectra in Figure 1e. The four peaks in the N 1s spectrum of pure CNHS with binding energy at 398.69, 399.25, 401.12 and 404.15 eV are assigned to sp2-bonded nitrogen in N-containing aromatic rings (C–N=C), sp3 tertiary nitrogen (N–C3), amino functional groups (C–NHx) and charging effects or π excitations, respectively[46]. After Pt was introduced to CNHS, the above peaks move to higher binding energy. The binding energy shift suggests that a hybrid heterojunction was formed due to the electronic interaction between Pt and g-C3N4. Ong et al.[58] reported a similar finding of the binding energy shift in the Pt/g-C3N4 photocatalysts. Deconvolution of the Pt 4f spectrum in Figure 1f shows the presence of two peaks with binding energy of 71.4 and 74.5 eV, which are assigned to Pt 4f7/2 and Pt 4f5/2 signals, respectively[59, 60]. According to the ICP results, the weight ratio of Pt element in Pt-CNHS is approximately 0.77 wt%. The dispersion of Pt species on CNHS network was analyzed by EXAFS spectroscopy. Figure 2a shows the Fourier transform (FT) of the Pt L3-edge EXAFS oscillations of Pt-CNHS, in comparation with that of standard Pt-foil. The peak of Pt-CNHS at 1.5 Å can be assigned to Pt-C/N contribution[46]. Meanwhile, the absence of Pt-Pt contribution at 2.6 Å suggests that Pt exists mainly in the form of isolated atoms in Pt-CNHS[32]. X-ray absorption near edge structure (XANES) was used to investigate the electronic structure of Pt species (Figure 2b). The white line peak in the XANES indicates an electronic transfer from 2p2/3 to unoccupied 5d states[26]. Therefore, the increase intensity of the white line represents fewer electrons in the d orbital and stronger interaction between CNHS and Pt[61-64]. In Figure 2b, the white line intensity of Pt-CNHS is much higher than that of Pt-foil, confirming that Pt species are charged by electron transfer between Pt and CNHS, which is in consistent with the FT-EXAFS results[65].
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Figure 2 (a) The K3-weighted FT-EXAFS of Pt-CNHS and Pt-Foil; (b) The normalized XANES spectra at Pt L3 edge of Pt-CNHS and Pt-Foil.
BET analysis and pore size distributions of CNB, CNHS and Pt-CNHS were measured by N2 adsorption-desorption measurements to investigate the evolution of porous structure. As shown in Figure S3a, b and c, CNB, CNHS and Pt-CNHS features BET surface areas of 7.1854, 207.8474 and 61.6146 m2 g-1, respectively. High specific surface area of CNHS can provide abundant active sites and enough space for discharge product storage. As illustrated, all three samples show high adsorption capacities in high relative pressure (P/P0 > 0.8), indicating the presence of meso- and macropores[36, 39]. The pore size distribution curves in Figure S2d, e and f can further confirm the above results. The pore size distribution of CNHS becomes broader and shows a sharp peak at ~4 nm, suggesting its porous structure[39]. The large number of small pores on the CNHS provide ample tri-phase active sites for ORR and OER as well as enough channels for oxygen diffusion. Moreover, they can promote electrolyte immersion on their surfaces, enabling excellent electrochemical performance. As a result, these advantages will aid realizing higher specific capacity and better rate performance of Li-O2 batteries. FESEM was employed to find out the changes among different samples. Figure S2b shows that melamine exhibits a coarse granule-like structure with irregular shapes. Figure S4a and b
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demonstrate that CNB exists in the shape of particles and is composed of multiple nanosheets. Therefore, CNB can be exfoliated into nanosheets through
Figure 3 FESEM images of (a) CNHS and (b) Pt-CNHS; (c) element mapping of Pt-CNHS; (d) TEM image of Pt-CNHS; (e) HAADF-STEM image and (f) magnified HAADF-STEM image.
further thermal treatment. The FESEM images of CNS in Figure S4c and d clearly show that it presents an obvious sheet-like structure and the lateral extension of the nanosheets can reach to several hundred nanometers. FESEM image of CNHS nanosheets in Figure 3a indicates that the basic structure of CNHS is similar with that of CNS, but there are some pores dispersing on the surface of the nanosheets. The in-plane nanopores distribute in the entire CNHS layer, which can create new exposed active edges[39]. The holey ultrathin nanosheets structure of CNHS is in favor of loading Pt atoms on them. After Pt were loaded on CNHS, its structure exhibits no obvious change (Figure 3b). EDS element mapping images of C, N and Pt elements in Figurec3c show the uniform dispersion of those elements in Pt-CNHS. The porous structure was further observed by TEM analysis. As shown in Figure 3d, Pt-CNHS exhibits a 2D sheet-like structure with enormous mesopores of several tens of nanometers present in the layers. The interaction between g-C3N4 and Pt resulted in the excellent activity for electron migration process[59]. As a powerful tool for discerning 12
individual heavy atoms, HAADF-STEM was used to confirm the distribution and configuration of Pt species on CNHS. Figure 3e and f clearly show that the bright spots corresponding to Pt atoms are uniformly dispersed on CNHS, demonstrating that Pt exists exclusively as isolated single atoms. The electrocatalytic activities of Pt-CNHS and CNHS towards ORR and OER were investigated using CV curves from 2.35 to 4.35 V (Figure S5a and b). For the pure CNHS cathode, the low anodic and cathodic currents indicate there are almost no ORR or OER activities. With the addition of Pt, both anodic and cathodic currents of the composite cathode increase. During cathodic scan of Pt-CNHS, the onset voltage is about 3.7 V with a larger current density, indicating that the cathode has superior ORR activity. From the EIS spectra of Pt-CNHS and CNHS fresh electrodes in Figure S5c and d, the obvious squashed semicircles in high-frequency region prove that the charge transfer resistance of CNHS and Pt-CNHS are both faradaic processes. Compared with Pt-CNHS, pure CNHS shows a much higher charge transfer resistance, which suggests that the single-atom Pt can reduce the resistance effectively. The easier electron transfer and ion diffusion process inside Pt-CNHS cathode are in favor of realizing good rate capability[66, 67]. The Li-O2 battery performances of the as-made materials were evaluated by assembling 2032-type coin cells. Figure 4a reveals the galvanostatic discharge performances of Pt-CNHS samples with different reaction time, CNHS, Pt-CNB and Pt-KB in the first discharge/charge cycle from 2.35 to 4.35 V under the current density of 100 mA g-1. Pt-CNHS exhibits the highest initial discharge capacity among the cathodes, which can reach to 17059.5 mAh g-1. The initial discharge capacity of pure CNHS cathode is only 5890.1 mAh g-1, highlighting the advantages of nanosheets structure of CNHS and the introduction of Pt. Also, the discharge voltage plateau of the Pt-CNHS-8 catalyst (2.8 V) is higher than Pt-CNHS-6 (~2.75 V), Pt-CNHS-10 (~2.75 V) and pure CNHS (~2.65 V). The smaller overpotential of Pt-CNHS suggests that it exhibits better catalytic activity than pure CNHS, mainly because of the 13
introduction of single-atom Pt[10]. A Li-O2 battery using only carbon paper as cathode was assembled to eliminate the capacity of itself. The result shows that the discharge/charge capacities of the carbon paper are limited (80 mAh g-1, as shown in Figure S6), indicating that its capacity can be negligible.
Figure 4 (a) The initial discharge profiles of pure CNHS, Pt-CNHS-6, Pt-CNHS-8, Pt-CNHS-10, Pt-CNB-8 and Pt-KB-8 at 100mA g-1; typical discharge/charge profiles of (c) CNHS and (d) Pt-CNHS under a capacity limit of 600 mAh g-1 at 100mA g-1; (d) the initial discharge profiles of Pt-CNHS at different current densities; (e) discharge/charge terminal voltages of CNHS and Pt-CNHS.
Furthermore, the capacity-limited cycling method was used to test cycling performance of CNHS and Pt-CNHS. Figure 4b and c present the selected discharge/charge profiles of Pt-CNHS cathode and pure CNHS cathode at the current of 100 mA g-1 under the specific capacity of 600 mAh g-1. Compared with CNHS cathode, Pt-CNHS cathode can be cycled more stably. The discharge/charge terminal voltages of pure CNHS cathode reach 2.0 V/4.6 V just after 70 cycles, while those of Pt-CNHS cathode can still maintain at 2.4 V/4.6 V after 100 cycles. During the initial discharge/charge cycle, the charge voltage plateau of Pt-CNHS cathode (3.84 V) is about 100 mV lower than pure CNHS cathode, demonstrating that the introduction of Pt lowers the charge overpotential. The Pt-CNHS cathode exhibits a 69% round-trip efficiency, which is higher than that of pure CNHS cathode (65%). The relatively 14
high round-trip efficiency is of great importance for electrochemical energy storage systems[1]. The Pt-CNHS cathode also shows good rate capabilities. When the current density varied from 200, 400, 600 to 800 mA g-1, the Pt-CNHS cathode exhibits the discharge capacities of about 13843.9, 10283.7, 9562.7 and 5964.7 mAh g-1, respectively (Figure 4d). Figure 4e displays the terminal discharge/charge voltages for galvanostatic discharge/recharge curves of the pure CNHS and Pt-CNHS cathodes at the current density of 100 mA g-1 under the capacity of 600 mAh g-1. It is clearly that the charge terminal voltage of Pt-CNHS cathode is lower than pure CNHS cathode during the whole cycling process. The discharge terminal voltage of CNHS cathode drops quickly after the 50th cycle, while that of Pt-CNHS cathode can remain stable over 100 cycles. This indicate that Pt-CNHS cathode could deliver a more stable cycling performance, due to the loading of single-atom Pt. The Pt-CNHS cathode exhibits excellent electrocatalytic performance in both discharge and charge process, which may be on account of the sufficient reaction sites on the cathode. This could lead to the formation and decomposition of discharge products, which are stored in the porous structure of CNHS. Besides, the efficient synergistic effect of the single-atom Pt and CNHS matrix also promotes the reaction process[1]. The large specific surface area and uniform pore structures are in favor of loading Pt atoms, while Pt atoms are likely to affect the nucleation and growth of Li2O2 by serving as the catalytic active sites[68]. Ex-situ XRD and FESEM were carried out to investigate composition and morphology changes of Pt-CNHS and the reversibility of electrochemical reactions. Figure 5a shows the XRD patterns at different discharge/charge stages. The characteristic Li2O2 peaks appeared after the 1st full discharging to 2.35 V, indicating the formation of discharge product (Li2O2). After recharged and the 100th fixed-capacity cycle, the peaks of Li2O2 disappear, demonstrating that the Pt-CNHS cathode can efficiently catalyze Li2O2 to form and decompose during cycling.
This implies that the Pt-CNHS cathode exhibits an excellent
cycling stability. EIS curves of Pt-CNHS cathode at different discharge/recharge stages are 15
shown in Figure 5b. After firstly discharged to 2.35 V, the charge-transfer resistance of the cathode becomes larger compared with the fresh cathode. The formation of discharge product-Li2O2 causes higher electrical resistance and might prevent the transfer of electrons in the electrochemical reaction[69, 70]. After recharged to 4.35 V, the charge-transfer resistance of Pt-CNHS cathode becomes almost the same as the fresh cathode, suggesting Li2O2 has disappeared and the formation and decomposition of discharge product is reversible. After continuously cycled for 100 times, the charge-transfer resistance increases slightly, confirming its remarkable catalytic property[71, 72].
Figure 5 (a) XRD patterns and (b) EIS plots of Pt-CNHS cathode at different discharge/charge stages; FESEM images of Pt-CNHS cathode at different stages: (c) fresh, (d) fully discharged to 2.35 V, (e) recharged to 4.35 V and (f) after the 100th cycle under fixed-capacity. 16
FESEM images in different stages can further prove the above results. As shown in Figure 5d, the surface of Pt-CNHS is covered with the close-packed discharge products as a well dispersed layered form after the battery was discharged to 2.35 V. The wrinkle of original nanosheets vanish and the nanosheets have become thicker compared the fresh electrode (Figure 5c). The solubility of LiO2 will affect the morphology of Li2O2 (particles or films). Other factors such as donor ability of solvent, effective current density and overpotential also have influence[73-75]. Film-like Li2O2 grows via a surface growth model of low soluble LiO2, while the particle-shaped or toroid-shaped Li2O2 grows through a solution mechanism due to the high soluble LiO2[76-78]. In this case, Li2O2 film is formed via a surface-adsorption growth model on Pt-CNHS, benefitting the contact between discharge product and cathode and the charge transfer between them[79]. The solid products disappeared and the layered structure reappeared after the battery was first charged to 4.35 V, as shown in Figure 5e. Furthermore, the image in Figure 5f shows that the morphology of the cathode remained the same after 100th cycle, under the capacity of 600 mAh g-1. The FESEM results are in accordance with the XRD and EIS results. Table S1 lists the Li-O2 battery performance of the Pt-CNHS cathode in comparison with various representative g-C3N4-based and Pt-based catalysts reported in the literature. It clearly demonstrated that the Pt-CNHS cathode exhibits better electrocatalytic performance than those of other representative electrocatalysts under similar testing conditions. The excellent capacity and cycling stability, which are attributed to the enhanced electrical conductivity and the efficient interfacial mass transfer, make Pt-CNHS a promising material to serve as the cathode catalyst in Li-O2 batteries. The above results demonstrate the versatile advantage of Pt-CNHS cathode in Li-O2 batteries regarding the high capacity, low overpotential and improved cycling stability. As indicated in Figure 6a, there are some possible reasons to explain this: (1) the holey ultrathin nanosheets structure with large surface area can facilitate the fast transportation of O2 and 17
electrolyte as well as offer more space to accommodate the discharge products; (2) the interfacial interaction between CNHS and single-atom Pt are stable enough to anchor the single-atom Pt, making them firmly and uniformly distributed on CNHS ; (3) the Pt atoms with high reaction activity embedded on CNHS may act as effective reaction sites, accelerating reaction kinetics and improving the catalytic properties of the Pt-CNHS cathode. The density functional theory (DFT)calculations can further confirm the result. The partial and total density-of-states (DOS) for CNHS and Pt-CNHS are displayed in Figure 6b and 6c. DOS peaks located at around -1eV for Pt-CNHS is associated with Pt 5d electrons and the absolute DOS value is related to the percentage of Pt atoms in the system. Compared to CNHS, the Pt-CNHS shows narrower band gap, which should be attribute to the existence of Pt 5d electrons and less dependent on the Pt percentage. Furthermore, the narrower band gap of the Pt-CNHS also demonstrates that the delocalization occurs with the decoration of Pt atom on CNHS. This unique structure could definitely lead to the increase of the electrical conductivity of Pt-CNHS, favorable for enhancing its electrochemical activity and battery performance. From Figure S7 and Figure 6e, it can be seen that the charge distribution of a single Pt atom and the CNHS are both rich and homogeneous. However, the charge distribution of single Pt atoms on the CNHS changes obviously. The charge distribution of the neighboring CNHS also slightly varies, which demonstrates that the single Pt atom interacts strongly with the CNHS to form a stable and high active structure. More clearly evidence can be seen in Figure S8, and it is found that Pt atom possesses an imbalanced charge distribution. The local electric field could cause a local in-built driving force and intensely enables the efficient electrons and ions transport in the electrode[80-82], resulting in superior specific capacity and cycling stability of the Pt-CNHS cathode.
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Figure 6(a) Schematic illustration for the reaction process during cycling; DOS of (b) CNHS and (c) Pt-CNHS; differential charge density distributions of (d) CNHS and (e) Pt-CNHS. Yellow and green color indicate the charge depletion and accumulation, respectively.
4. Conclusions In conclusion, single-atom Pt supported on CNHS was successfully prepared through a facile liquid phase method. By introducing single-atom Pt as co-catalyst, the utilization efficiency of Pt metal has been maximized and the composite shows favorable electrocatalytic activity towards Li-O2 batteries. The assembled batteries using Pt-CNHS as catalyst could deliver a 17059.5 mAh g-1 discharge capacity under the current density of 100 mA g−1, while that of pure CNHS is only 5890.1 mAh g-1. Pt-CNHS can still achieve a discharge capacity of 5964.7 mAh g-1 when current density increased to 800 mA g-1, showing its good rate capability. When discharge/charge capacities were limited to 600 mAh g−1 under the current density of 100 mA g-1, Pt-CNHS cathode can be continuously cycled for 100 times without obvious terminal voltage variation. The excellent catalytic performance of Pt-CNHS is on 19
accounted of the synergistic effect between Pt atoms and holey nanosheet graphitic-C3N4. The porous structure of CNHS with large surface area facilitates the fast mass transportation and electrolyte wetting, as well as provides more space to accommodate Li2O2. Single-atom Pt can also help with the efficient interfacial mass transfer due to the high electrical conductivity. Meanwhile, the single-atom Pt can serve as excellent reaction sites to remain stable and accelerate reaction kinetics during cycling, due to its imbalanced charge distribution on CNHS. Therefore, this co-catalyst strategy is promising for designing ideal cathode for Li-O2 batteries, which can reduce the cost of precious metals and provide favorable rechargeability as well as high specific capacity. Acknowledgements This work is supported by the National Nature Science Foundation of China (No. 51672162), China Postdoctoral Science Foundation (2017M622198), Natural Science Foundation of Shandong Province (ZR2017BEM018), the Open Project Program of Key Laboratory for Analytical Science of Food Safety and Biology, Ministry of Education (FS18010), and the Fundamental Research Funds of Shandong University (No.2017JC035). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.
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Graphical Abstract
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Wen Zhao: Investigation, Methodology, Data analysis, Writing - original draft. Jun Wang: Supervision, Review & editing draft. Rui Yin: Investigation. Boya Li: Investigation. Xiaoshuai Huang: Investigation. Lanlin Zhao: Theory calculations. Lei Qian: Supervision, Review & editing draft.
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Declaration of interests ☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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S0021-9797(19)31568-1 https://doi.org/10.1016/j.jcis.2019.12.102 YJCIS 25842
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
4 October 2019 10 December 2019 23 December 2019
Please cite this article as: W. Zhao, J. Wang, R. Yin, B. Li, X. Huang, L. Zhao, L. Qian, Single-atom Pt supported on holey ultrathin g-C3N4 nanosheets as efficient catalyst for Li-O2 batteries, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.102
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Single-atom Pt supported on holey ultrathin g-C3N4 nanosheets as efficient catalyst for Li-O2 batteries
Wen Zhaoa, Jun Wang*a, Rui Yina, Boya Lib, Xiaoshuai Huanga, Lanling Zhao*c, Lei Qian*a aKey
Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry
of Education), Shandong University, 17923 Jingshi Road, Jinan 250061, China. E-mail: [email protected], [email protected]. bSchool
of Mechanical Electronic & Information Engineering, China University of Mining &
Technology-Beijing, Beijing 100083, P. R. China. cSchool
of Physics, Shandong University, Jinan 250100, P. R. China. E-mail:
[email protected].
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Abstract As for electrocatalysis, single-atom metal catalysts have been proved to lower the cost and utilize precious metals more efficiently. Herein, single-atom Pt catalyst supported on holey ultrathin g-C3N4 nanosheets (Pt-CNHS) was synthesized via a facile liquid-phase reaction of g-C3N4 and H2PtCl6. The single-atom Pt can achieve high dispersibility and stability, which can promote the utilization efficiency as well as enhance the electrochemical activity. When employed as Li-O2 batteries’ cathode catalyst, Pt-CNHS exhibits excellent electrocatalytic activity. Li-O2 batteries utilizing Pt-CNHS show much higher discharge specific capacities than those with pure CNHS. Li-O2 batteries with Pt-CNHS cathode can be cycled stably for 100 times under the discharge capacity of 600 mAh g-1. Based on experimental results and density functional theory calculations, the superior electrocatalytic activity of Pt-CNHS can be ascribed to the large surface area, the enhanced electrical conductivity and the efficient interfacial mass transfer through Pt atoms and porous structure of CNHS. Keywords: Single-atom Pt; g-C3N4; Density functional theory calculations; Electrocatalysis; Li-O2 batteries
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1. Introduction Li-O2 battery is a promising energy storage system for future electric vehicles since it possesses a high specific energy (up to 3560 Wh Kg-1) [1-4]. However, in terms of practical applications, there still are several unsolved problems, such as poor cycling efficiency, low rate capability and limited short life span[5, 6]. These unsatisfactory electrochemical characteristics are mainly attributed to the polarization during discharging and charging[7, 8], which are caused by the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)[9]. Developing efficient electrocatalysts is an ideal way to solve this problem. Various catalysts have been explored to achieve high electrocatalytic activity, including noble metals[10, 11], bimetallic nanoparticles[12, 13], transition metal oxides[14, 15], perovskites[16, 17], metal nitride[18, 19], metal carbide[20, 21] and metal sulfide[22, 23]. Among them, platinum (Pt) is a frequently used catalytic material[24, 25]. However, restricted by its high cost and rarity, many researches focus on increasing Pt utilization efficiency as well as enhancing its catalytic activity[26-28]. Previous studies show that the catalytic properties of Pt are largely related to its structure. Therefore, numerous efforts have been conducted to control the size of Pt nanoparticles, design core-shell structures and construct metal-support interactions[29-31]. By employing these methods, favorable catalytic property of Pt-based materials has been achieved. Therefore, constructing a novel Pt-containing catalyst with unique structure and morphology is of great importance to optimize the performance of Li-O2 batteries. Recently, single-atom Pt based catalysts have been prepared and achieved excellent electrocatalytic activity. For instance, Single-atom Pt doped on TiN nanoparticles exhibits excellent electrocatalytic performance in terms of oxygen reduction, formic acid oxidation as well as methanol oxidation[26]. Atomically dispersed Pt with high dispersibility was supported on mesoporous Al2O3 to exhibit enhanced stability[32]. Similarly, single-atom Pt supported on FeOx showed high activity and stability for CO oxidation[33]. Also, Pt atoms 3
were doped on MoS2 surface to achieve high electrocatalytic activity towards hydrogen-evolution reaction[34]. The single-atom form attracts great interests since it could maximize the Pt atom utilization as well as achieve high catalytic activity. The synthesis of graphitic-carbon nitride (g-C3N4) can be realized by a series of polycondensation reactions of nitrogen-rich materials[35, 36]. g-C3N4 shows a planar phase similar with graphite, but unlike graphite, g-C3N4 contains both graphite-like and pyridine-like nitrogen atoms. Also, in g-C3N4, every carbon atom is bonded to both graphitic and pyridinic nitrogen atoms[37]. Recently, g-C3N4 has attracted tremendous attention due to its unusual properties, including high chemical stability, low-cost preparation and special foam-like structure[38]. Due to its nitrogen-rich property and facile synthesis, g-C3N4 is suitable for serving as the catalyst for photocatalysis, fuel cells and CO2 capture[39, 40]. It was reported by Yi et al.[7] that graphitic-C3N4@carbon paper was used as the cathode in Li-O2 batteries, and the batteries obtain high rate capability as well as good cycling stability. Zheng et al.[41] prepared nanoporous g-C3N4@carbon metal-free electrocatalysts and used them as ORR catalyst. In g-C3N4, there are many removable electrons between the layers which can complex with abundant metal ions and adjust the conjugation structure[42]. Many researches have tried to combine g-C3N4 with non-precious metals and perovskite oxides, such as Co-g-C3N4[40], Fe-g-C3N4[43], Ni-Co3O4-C3N4[44] and g-C3N4-LaNiO3[10]. Pt possesses superior electrical conductivity and excellent electrocatalytic activity. Combining Pt with g-C3N4 is assumed to promote both the mass transfer and catalytic activity effectively. Some researchers have successfully prepared g-C3N4-Pt composites. For instance, Hu et al.[45] prepared g-C3N4-Pt nanohybrids which exhibited great catalytic activity and high stability towards methanol oxidation. Li et al.[46] employed single-atom Pt to catalyze H2 evolution. However, as far as we know, few researches are focused on loading single-atom Pt on g-C3N4 to catalyze the reactions in Li-O2 batteries.
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Based on these previous studies, we prepared single-atom Pt decorated holey ultrathin g-C3N4 nanosheets (Pt-CNHS) as a novel cathode catalyst for Li-O2 batteries. Various in-plane holes on the g-C3N4 layers can offer more active sites for the reactions in Li-O2 batteries[47]. Furthermore, the cross-layer diffusion paths resulted from the holes may promote the mass transfer and electron distribution efficiency of CNHS[48], which are in favor of the electrocatalytic performance. Besides improving the catalytic process and mass transportation, high surface area of CNHS can also promote the wetting of electrode by electrolyte and the accommodation of Li2O2. By loading isolated Pt atoms uniformly and stably on the CNHS matrix, the maximum utilization of Pt metal as well as good electrocatalytic performance can be achieved. The capacity, cycling stability and rate performance of Pt-CNHS were investigated. As demonstrated by the density functional theory calculations, the improved electrocatalytic performance of Pt-CNHS is attributed to the synergistic effects between single-atom Pt with modified charge distribution and CNHS, which provide local build-in electric field to stable Pt atoms and enhance their battery performance. 2. Experimental methods 2.1 Synthesis of bulk g-C3N4 Bulk g-C3N4 (CNB) was synthesized by thermal polycondensation of melamine. Specifically, 20 g melamine powder was calcinated under 500 °C for 2 h (2 °C min-1) in an alumina crucible and then calcinated under 520 °C for another 2 h (under the same heating rate). The resulted yellow product was grounded into powder for next-step treatment. 2.2 Synthesis of g-C3N4 nanosheets and holey ultrathin g-C3N4 nanosheets g-C3N4 nanosheets (CNS) were synthesized by further calcination of CNB. 5 g CNB was uniformly spread in an alumina ark and then calcinated under 520 °C for 4.5 h (2 °C min-1). Afterwards, the light-yellow product was obtained. The holey ultrathin g-C3N4 nanosheets (CNHS) were synthesized by increasing the calcination time to 6 h. 5
2.3 Synthesis of Pt-CNHS The Pt-CNHS was prepared by liquid-phase reaction of CNHS and H2PtCl6·6H2O followed by a heat treatment. Firstly, 50 mg CNHS was dispersed in 15 ml deionized water to form an aqueous dispersion and 30 mg H2PtCl6·6H2O was added into it. Next, the mixture was continuously stirring at 70 °C for 6, 8 and 10 h (marked as Pt-CNHS-6, Pt-CNHS-8 and Pt-CNHS-10, respectively). Pt-CNB and Pt-KB were also synthesized using the same method by replacing CNHS with CNB and Ketjen Black with a stirring duration for 8 h. The resulted products were washed by centrifuging using deionized water and ethanol. Finally, they were dried at 60 °C for 12 h under vacuum, then heated to 125 °C for 1 h under vacuum. 2.4 Physical characterization X-ray diffraction (XRD) measurements were conducted using a D8 Advance X-ray generator and diffractometer with Cu Kα radiation (λ=1.5418 Å). Fourier transform infrared spectroscopy (FTIR) were collected by using a Nicolet Nexus 410 spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a ThermoFisher K-Alpha instrument. Inductively coupled plasma (ICP) was carried out on an HK-2000 emission spectrophotometer. The Extended X-ray absorption fine structure spectroscopy (EXAFS) was conducted on beam line BL14W1[49] of the Shanghai Synchrotron Radiation Facility (SSRF), in the fluorescent mode with silicon drift fluorescence detector. The station was operated with a Si (311) double crystal monochromator. The synchrotron was operated at the energy of 3.5 GeV and the current between 150-210 mA. The photon energy was calibrated with the first inflection point of Pt L3-edge in Pt metal foil. Brunauer-Emmett-Teller (BET) analysis were performed using N2 adsorption/desorption on a Micromeritics Instrument Corp ASAP2460 instrument. Field emission scanning electron microscopy (FESEM) and Energy disperse Spectroscopy (EDS) images were obtained on a SU-70 FESEM. High Resolution transmission electron microscope (HRTEM) observations were employed on a Tecnai G2 F20 microscope. The high-angle annular dark-field scanning transmission electron microscopy 6
(HAADF-STEM) characterization was performed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. 2.5 Electrochemical investigation To prepare the cathodes tested in Li-O2 batteries, as-prepared catalysts were mixed with Super P and poly(tetrafluoroethylene) (PTFE) at the mass ratio of 4:4:2 in isopropanol. Afterwards, the mixture was uniformly pasted on a carbon paper and dried at 120 °C for 12 h under vacuum. The Li-O2 batteries were assembled with a Li foil anode, a glass fiber separator and a catalyst coated carbon paper as cathode. Galvanostatic discharge/charge tests were conducted on a LAND CT 2001A battery tester using constant current mode. All the measurements were carried out in O2 from 2.35 V to 4.35 V. Rate capability was tested by applying different current densities. Cyclic voltammetry (CV) experiments and Electrochemical impedance spectroscopy (EIS) were conducted on a RST5002F electrochemical workstation to investigate the kinetics of the ORR and the OER. CV was tested with a scan rate of 0.1 mV s-1 from 2.35 to 4.35 V and EIS was performed at an amplitude sine wave of 0.01 V and the frequency range from 100 kHz to 0.01 Hz. 2.6 Theoretical calculations All calculations were performed using the Vienna abinitio simulation package (VASP)[50, 51]. The projector augmented wave (PAW) potentials were employed to describe the nulcei-electron interactions, and the exchange-correlation between electrons was described using the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) form[52]. A C48N64 unit cell with the volume of 28.45Å 14.23Å 20.00 Å was used during the calculations. The cutoff energy of plane wave basis was set as 550 eV. The total energy convergence criterion and the residual forces were set to be 110-5 eV and 110-2 eV/Å, respectively. A 1 2 1 K-point grid centered at the gamma () point using Monkhorst Pack Scheme was applied. 7
3. Results and Discussion Scheme 1 shows the schematic illustration for synthetic process of Pt-CNHS. In this work,
Scheme 1 Schematic illustration of the process for preparing CNHS and Pt-CNHS.
foam-like CNHS with abundant holes was prepared by thermal polycondensation of melamine in air. Afterwards, single-atom Pt was loaded on CNHS via a facile liquid phase reaction. The digital photos of samples in different stages are shown in Supporting Information. Figure S1a, b, c, d and e are in correspondence with melamine, CNB, CNS, CNHS and Pt-CNHS, respectively. The crystal structure of different samples was analyzed by XRD. Figure S2a shows the XRD pattern of melamine. The XRD patterns of the CNB, CNS, CNHS and Pt-CNHS in Figure 1a demonstrate that the diffraction peaks of melamine disappeared and two major diffraction peaks corresponding to g-C3N4 appeared. The two dominant diffraction peaks at 13.1° and 27.3° are associated with the in-plane trigonal N linkage of tri-s-triazine motifs and the periodic stacking of layers for conjugated aromatic systems, respectively[39]. The decreased intensity of peak at 13.1° is mainly due to the decrease of in-plane structural units, which is caused by the oxidation etching parts of melems during the long-time calcination[53]. The peak of CNB has a little right shift, indicating a decreased gallery distance between the basic sheets in the nanosheets[47]. In this case, the heating treatment during thermal oxidation process should lead to a denser packing and thus shorten the gallery distance observed[47]. The peak intensity at 27.3° dramatically decreases, indicating that g-C3N4 bulk was successfully exfoliated to nanosheets[39, 54]. Meanwhile, the loading of Pt atoms may lead to a slight decrease of layer space of the nanosheets, causing the increase of 8
the peak intensity of Pt-CNHS at 27.3°. Compared with bare CNHS, the diffraction pattern of Pt-CNHS shows no obvious difference, implying that Pt decoration did not bring obvious influence to CNHS’s crystal structure. The diffraction peaks of Pt element do not appear in the XRD pattern of Pt-CNHS, possibly due to its low content and single atom size. The FT-IR spectra of CNB, CNS, CNHS and Pt-CNHS in Figure 1b show that nanosheets exhibit similar peaks as the bulk. The broad peaks which are from 3000 to 3500 cm-1 and from 900 to 1700 cm-1 can be attributed to N–H stretching vibrations and s-triazine derivatives, respectively. The band at 2150 cm-1 is associated with cyano terminal groups with C≡N[1, 41, 55, 56]. It clearly demonstrates that the structures of CNS and CNHS did not change. These results also confirm that Pt loading do not have influences on the CNHS structure.
Figure 1 (a) XRD patterns, (b) FT-IR spectra and (c) XPS survey spectra of CNB, CNS, CNHS and Pt-CNHS; (d) C 1s, (e) N 1s and (f) Pt 4f XPS spectra of CNHS and Pt-CNHS.
Surface chemical compositions of the composites were analyzed by XPS measurements. The XPS survey spectra in Figure 1c indicates the existence of C and N elements in both CNHS and Pt-CNHS. Furthermore, it shows that Pt atoms were successfully loaded on CNHS in the Pt-CNHS sample. In C 1s spectra of CNHS and Pt-CNHS, the two peaks with binding energy of 284.72 and 288.23 eV are assigned to graphitic carbon and sp2-bonded carbon (N=C–N), respectively (Figure 1d)[36, 39, 57]. The electronic structure variation of CNHS 9
after the introduction of Pt can be observed from the N 1s spectra in Figure 1e. The four peaks in the N 1s spectrum of pure CNHS with binding energy at 398.69, 399.25, 401.12 and 404.15 eV are assigned to sp2-bonded nitrogen in N-containing aromatic rings (C–N=C), sp3 tertiary nitrogen (N–C3), amino functional groups (C–NHx) and charging effects or π excitations, respectively[46]. After Pt was introduced to CNHS, the above peaks move to higher binding energy. The binding energy shift suggests that a hybrid heterojunction was formed due to the electronic interaction between Pt and g-C3N4. Ong et al.[58] reported a similar finding of the binding energy shift in the Pt/g-C3N4 photocatalysts. Deconvolution of the Pt 4f spectrum in Figure 1f shows the presence of two peaks with binding energy of 71.4 and 74.5 eV, which are assigned to Pt 4f7/2 and Pt 4f5/2 signals, respectively[59, 60]. According to the ICP results, the weight ratio of Pt element in Pt-CNHS is approximately 0.77 wt%. The dispersion of Pt species on CNHS network was analyzed by EXAFS spectroscopy. Figure 2a shows the Fourier transform (FT) of the Pt L3-edge EXAFS oscillations of Pt-CNHS, in comparation with that of standard Pt-foil. The peak of Pt-CNHS at 1.5 Å can be assigned to Pt-C/N contribution[46]. Meanwhile, the absence of Pt-Pt contribution at 2.6 Å suggests that Pt exists mainly in the form of isolated atoms in Pt-CNHS[32]. X-ray absorption near edge structure (XANES) was used to investigate the electronic structure of Pt species (Figure 2b). The white line peak in the XANES indicates an electronic transfer from 2p2/3 to unoccupied 5d states[26]. Therefore, the increase intensity of the white line represents fewer electrons in the d orbital and stronger interaction between CNHS and Pt[61-64]. In Figure 2b, the white line intensity of Pt-CNHS is much higher than that of Pt-foil, confirming that Pt species are charged by electron transfer between Pt and CNHS, which is in consistent with the FT-EXAFS results[65].
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Figure 2 (a) The K3-weighted FT-EXAFS of Pt-CNHS and Pt-Foil; (b) The normalized XANES spectra at Pt L3 edge of Pt-CNHS and Pt-Foil.
BET analysis and pore size distributions of CNB, CNHS and Pt-CNHS were measured by N2 adsorption-desorption measurements to investigate the evolution of porous structure. As shown in Figure S3a, b and c, CNB, CNHS and Pt-CNHS features BET surface areas of 7.1854, 207.8474 and 61.6146 m2 g-1, respectively. High specific surface area of CNHS can provide abundant active sites and enough space for discharge product storage. As illustrated, all three samples show high adsorption capacities in high relative pressure (P/P0 > 0.8), indicating the presence of meso- and macropores[36, 39]. The pore size distribution curves in Figure S2d, e and f can further confirm the above results. The pore size distribution of CNHS becomes broader and shows a sharp peak at ~4 nm, suggesting its porous structure[39]. The large number of small pores on the CNHS provide ample tri-phase active sites for ORR and OER as well as enough channels for oxygen diffusion. Moreover, they can promote electrolyte immersion on their surfaces, enabling excellent electrochemical performance. As a result, these advantages will aid realizing higher specific capacity and better rate performance of Li-O2 batteries. FESEM was employed to find out the changes among different samples. Figure S2b shows that melamine exhibits a coarse granule-like structure with irregular shapes. Figure S4a and b
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demonstrate that CNB exists in the shape of particles and is composed of multiple nanosheets. Therefore, CNB can be exfoliated into nanosheets through
Figure 3 FESEM images of (a) CNHS and (b) Pt-CNHS; (c) element mapping of Pt-CNHS; (d) TEM image of Pt-CNHS; (e) HAADF-STEM image and (f) magnified HAADF-STEM image.
further thermal treatment. The FESEM images of CNS in Figure S4c and d clearly show that it presents an obvious sheet-like structure and the lateral extension of the nanosheets can reach to several hundred nanometers. FESEM image of CNHS nanosheets in Figure 3a indicates that the basic structure of CNHS is similar with that of CNS, but there are some pores dispersing on the surface of the nanosheets. The in-plane nanopores distribute in the entire CNHS layer, which can create new exposed active edges[39]. The holey ultrathin nanosheets structure of CNHS is in favor of loading Pt atoms on them. After Pt were loaded on CNHS, its structure exhibits no obvious change (Figure 3b). EDS element mapping images of C, N and Pt elements in Figurec3c show the uniform dispersion of those elements in Pt-CNHS. The porous structure was further observed by TEM analysis. As shown in Figure 3d, Pt-CNHS exhibits a 2D sheet-like structure with enormous mesopores of several tens of nanometers present in the layers. The interaction between g-C3N4 and Pt resulted in the excellent activity for electron migration process[59]. As a powerful tool for discerning 12
individual heavy atoms, HAADF-STEM was used to confirm the distribution and configuration of Pt species on CNHS. Figure 3e and f clearly show that the bright spots corresponding to Pt atoms are uniformly dispersed on CNHS, demonstrating that Pt exists exclusively as isolated single atoms. The electrocatalytic activities of Pt-CNHS and CNHS towards ORR and OER were investigated using CV curves from 2.35 to 4.35 V (Figure S5a and b). For the pure CNHS cathode, the low anodic and cathodic currents indicate there are almost no ORR or OER activities. With the addition of Pt, both anodic and cathodic currents of the composite cathode increase. During cathodic scan of Pt-CNHS, the onset voltage is about 3.7 V with a larger current density, indicating that the cathode has superior ORR activity. From the EIS spectra of Pt-CNHS and CNHS fresh electrodes in Figure S5c and d, the obvious squashed semicircles in high-frequency region prove that the charge transfer resistance of CNHS and Pt-CNHS are both faradaic processes. Compared with Pt-CNHS, pure CNHS shows a much higher charge transfer resistance, which suggests that the single-atom Pt can reduce the resistance effectively. The easier electron transfer and ion diffusion process inside Pt-CNHS cathode are in favor of realizing good rate capability[66, 67]. The Li-O2 battery performances of the as-made materials were evaluated by assembling 2032-type coin cells. Figure 4a reveals the galvanostatic discharge performances of Pt-CNHS samples with different reaction time, CNHS, Pt-CNB and Pt-KB in the first discharge/charge cycle from 2.35 to 4.35 V under the current density of 100 mA g-1. Pt-CNHS exhibits the highest initial discharge capacity among the cathodes, which can reach to 17059.5 mAh g-1. The initial discharge capacity of pure CNHS cathode is only 5890.1 mAh g-1, highlighting the advantages of nanosheets structure of CNHS and the introduction of Pt. Also, the discharge voltage plateau of the Pt-CNHS-8 catalyst (2.8 V) is higher than Pt-CNHS-6 (~2.75 V), Pt-CNHS-10 (~2.75 V) and pure CNHS (~2.65 V). The smaller overpotential of Pt-CNHS suggests that it exhibits better catalytic activity than pure CNHS, mainly because of the 13
introduction of single-atom Pt[10]. A Li-O2 battery using only carbon paper as cathode was assembled to eliminate the capacity of itself. The result shows that the discharge/charge capacities of the carbon paper are limited (80 mAh g-1, as shown in Figure S6), indicating that its capacity can be negligible.
Figure 4 (a) The initial discharge profiles of pure CNHS, Pt-CNHS-6, Pt-CNHS-8, Pt-CNHS-10, Pt-CNB-8 and Pt-KB-8 at 100mA g-1; typical discharge/charge profiles of (c) CNHS and (d) Pt-CNHS under a capacity limit of 600 mAh g-1 at 100mA g-1; (d) the initial discharge profiles of Pt-CNHS at different current densities; (e) discharge/charge terminal voltages of CNHS and Pt-CNHS.
Furthermore, the capacity-limited cycling method was used to test cycling performance of CNHS and Pt-CNHS. Figure 4b and c present the selected discharge/charge profiles of Pt-CNHS cathode and pure CNHS cathode at the current of 100 mA g-1 under the specific capacity of 600 mAh g-1. Compared with CNHS cathode, Pt-CNHS cathode can be cycled more stably. The discharge/charge terminal voltages of pure CNHS cathode reach 2.0 V/4.6 V just after 70 cycles, while those of Pt-CNHS cathode can still maintain at 2.4 V/4.6 V after 100 cycles. During the initial discharge/charge cycle, the charge voltage plateau of Pt-CNHS cathode (3.84 V) is about 100 mV lower than pure CNHS cathode, demonstrating that the introduction of Pt lowers the charge overpotential. The Pt-CNHS cathode exhibits a 69% round-trip efficiency, which is higher than that of pure CNHS cathode (65%). The relatively 14
high round-trip efficiency is of great importance for electrochemical energy storage systems[1]. The Pt-CNHS cathode also shows good rate capabilities. When the current density varied from 200, 400, 600 to 800 mA g-1, the Pt-CNHS cathode exhibits the discharge capacities of about 13843.9, 10283.7, 9562.7 and 5964.7 mAh g-1, respectively (Figure 4d). Figure 4e displays the terminal discharge/charge voltages for galvanostatic discharge/recharge curves of the pure CNHS and Pt-CNHS cathodes at the current density of 100 mA g-1 under the capacity of 600 mAh g-1. It is clearly that the charge terminal voltage of Pt-CNHS cathode is lower than pure CNHS cathode during the whole cycling process. The discharge terminal voltage of CNHS cathode drops quickly after the 50th cycle, while that of Pt-CNHS cathode can remain stable over 100 cycles. This indicate that Pt-CNHS cathode could deliver a more stable cycling performance, due to the loading of single-atom Pt. The Pt-CNHS cathode exhibits excellent electrocatalytic performance in both discharge and charge process, which may be on account of the sufficient reaction sites on the cathode. This could lead to the formation and decomposition of discharge products, which are stored in the porous structure of CNHS. Besides, the efficient synergistic effect of the single-atom Pt and CNHS matrix also promotes the reaction process[1]. The large specific surface area and uniform pore structures are in favor of loading Pt atoms, while Pt atoms are likely to affect the nucleation and growth of Li2O2 by serving as the catalytic active sites[68]. Ex-situ XRD and FESEM were carried out to investigate composition and morphology changes of Pt-CNHS and the reversibility of electrochemical reactions. Figure 5a shows the XRD patterns at different discharge/charge stages. The characteristic Li2O2 peaks appeared after the 1st full discharging to 2.35 V, indicating the formation of discharge product (Li2O2). After recharged and the 100th fixed-capacity cycle, the peaks of Li2O2 disappear, demonstrating that the Pt-CNHS cathode can efficiently catalyze Li2O2 to form and decompose during cycling.
This implies that the Pt-CNHS cathode exhibits an excellent
cycling stability. EIS curves of Pt-CNHS cathode at different discharge/recharge stages are 15
shown in Figure 5b. After firstly discharged to 2.35 V, the charge-transfer resistance of the cathode becomes larger compared with the fresh cathode. The formation of discharge product-Li2O2 causes higher electrical resistance and might prevent the transfer of electrons in the electrochemical reaction[69, 70]. After recharged to 4.35 V, the charge-transfer resistance of Pt-CNHS cathode becomes almost the same as the fresh cathode, suggesting Li2O2 has disappeared and the formation and decomposition of discharge product is reversible. After continuously cycled for 100 times, the charge-transfer resistance increases slightly, confirming its remarkable catalytic property[71, 72].
Figure 5 (a) XRD patterns and (b) EIS plots of Pt-CNHS cathode at different discharge/charge stages; FESEM images of Pt-CNHS cathode at different stages: (c) fresh, (d) fully discharged to 2.35 V, (e) recharged to 4.35 V and (f) after the 100th cycle under fixed-capacity. 16
FESEM images in different stages can further prove the above results. As shown in Figure 5d, the surface of Pt-CNHS is covered with the close-packed discharge products as a well dispersed layered form after the battery was discharged to 2.35 V. The wrinkle of original nanosheets vanish and the nanosheets have become thicker compared the fresh electrode (Figure 5c). The solubility of LiO2 will affect the morphology of Li2O2 (particles or films). Other factors such as donor ability of solvent, effective current density and overpotential also have influence[73-75]. Film-like Li2O2 grows via a surface growth model of low soluble LiO2, while the particle-shaped or toroid-shaped Li2O2 grows through a solution mechanism due to the high soluble LiO2[76-78]. In this case, Li2O2 film is formed via a surface-adsorption growth model on Pt-CNHS, benefitting the contact between discharge product and cathode and the charge transfer between them[79]. The solid products disappeared and the layered structure reappeared after the battery was first charged to 4.35 V, as shown in Figure 5e. Furthermore, the image in Figure 5f shows that the morphology of the cathode remained the same after 100th cycle, under the capacity of 600 mAh g-1. The FESEM results are in accordance with the XRD and EIS results. Table S1 lists the Li-O2 battery performance of the Pt-CNHS cathode in comparison with various representative g-C3N4-based and Pt-based catalysts reported in the literature. It clearly demonstrated that the Pt-CNHS cathode exhibits better electrocatalytic performance than those of other representative electrocatalysts under similar testing conditions. The excellent capacity and cycling stability, which are attributed to the enhanced electrical conductivity and the efficient interfacial mass transfer, make Pt-CNHS a promising material to serve as the cathode catalyst in Li-O2 batteries. The above results demonstrate the versatile advantage of Pt-CNHS cathode in Li-O2 batteries regarding the high capacity, low overpotential and improved cycling stability. As indicated in Figure 6a, there are some possible reasons to explain this: (1) the holey ultrathin nanosheets structure with large surface area can facilitate the fast transportation of O2 and 17
electrolyte as well as offer more space to accommodate the discharge products; (2) the interfacial interaction between CNHS and single-atom Pt are stable enough to anchor the single-atom Pt, making them firmly and uniformly distributed on CNHS ; (3) the Pt atoms with high reaction activity embedded on CNHS may act as effective reaction sites, accelerating reaction kinetics and improving the catalytic properties of the Pt-CNHS cathode. The density functional theory (DFT)calculations can further confirm the result. The partial and total density-of-states (DOS) for CNHS and Pt-CNHS are displayed in Figure 6b and 6c. DOS peaks located at around -1eV for Pt-CNHS is associated with Pt 5d electrons and the absolute DOS value is related to the percentage of Pt atoms in the system. Compared to CNHS, the Pt-CNHS shows narrower band gap, which should be attribute to the existence of Pt 5d electrons and less dependent on the Pt percentage. Furthermore, the narrower band gap of the Pt-CNHS also demonstrates that the delocalization occurs with the decoration of Pt atom on CNHS. This unique structure could definitely lead to the increase of the electrical conductivity of Pt-CNHS, favorable for enhancing its electrochemical activity and battery performance. From Figure S7 and Figure 6e, it can be seen that the charge distribution of a single Pt atom and the CNHS are both rich and homogeneous. However, the charge distribution of single Pt atoms on the CNHS changes obviously. The charge distribution of the neighboring CNHS also slightly varies, which demonstrates that the single Pt atom interacts strongly with the CNHS to form a stable and high active structure. More clearly evidence can be seen in Figure S8, and it is found that Pt atom possesses an imbalanced charge distribution. The local electric field could cause a local in-built driving force and intensely enables the efficient electrons and ions transport in the electrode[80-82], resulting in superior specific capacity and cycling stability of the Pt-CNHS cathode.
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Figure 6(a) Schematic illustration for the reaction process during cycling; DOS of (b) CNHS and (c) Pt-CNHS; differential charge density distributions of (d) CNHS and (e) Pt-CNHS. Yellow and green color indicate the charge depletion and accumulation, respectively.
4. Conclusions In conclusion, single-atom Pt supported on CNHS was successfully prepared through a facile liquid phase method. By introducing single-atom Pt as co-catalyst, the utilization efficiency of Pt metal has been maximized and the composite shows favorable electrocatalytic activity towards Li-O2 batteries. The assembled batteries using Pt-CNHS as catalyst could deliver a 17059.5 mAh g-1 discharge capacity under the current density of 100 mA g−1, while that of pure CNHS is only 5890.1 mAh g-1. Pt-CNHS can still achieve a discharge capacity of 5964.7 mAh g-1 when current density increased to 800 mA g-1, showing its good rate capability. When discharge/charge capacities were limited to 600 mAh g−1 under the current density of 100 mA g-1, Pt-CNHS cathode can be continuously cycled for 100 times without obvious terminal voltage variation. The excellent catalytic performance of Pt-CNHS is on 19
accounted of the synergistic effect between Pt atoms and holey nanosheet graphitic-C3N4. The porous structure of CNHS with large surface area facilitates the fast mass transportation and electrolyte wetting, as well as provides more space to accommodate Li2O2. Single-atom Pt can also help with the efficient interfacial mass transfer due to the high electrical conductivity. Meanwhile, the single-atom Pt can serve as excellent reaction sites to remain stable and accelerate reaction kinetics during cycling, due to its imbalanced charge distribution on CNHS. Therefore, this co-catalyst strategy is promising for designing ideal cathode for Li-O2 batteries, which can reduce the cost of precious metals and provide favorable rechargeability as well as high specific capacity. Acknowledgements This work is supported by the National Nature Science Foundation of China (No. 51672162), China Postdoctoral Science Foundation (2017M622198), Natural Science Foundation of Shandong Province (ZR2017BEM018), the Open Project Program of Key Laboratory for Analytical Science of Food Safety and Biology, Ministry of Education (FS18010), and the Fundamental Research Funds of Shandong University (No.2017JC035). The authors thank beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.
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Graphical Abstract
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Wen Zhao: Investigation, Methodology, Data analysis, Writing - original draft. Jun Wang: Supervision, Review & editing draft. Rui Yin: Investigation. Boya Li: Investigation. Xiaoshuai Huang: Investigation. Lanlin Zhao: Theory calculations. Lei Qian: Supervision, Review & editing draft.
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Declaration of interests ☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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