S-Doped Porous Graphene-Like

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Facile and scale synthesis of Co/N/S-doped porous graphene-like carbon architectures as electrocatalysts for sustainable zinc-air battery cells Zhao-yang Chen, Heng Liu, Long-cheng Zhang, Qiu-lin Li, Maowen Xu, and Shu-Juan Bao ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Sustainable Chemistry & Engineering

Facile and scale synthesis of Co/N/S-doped porous graphene-like carbon architectures as electrocatalystsfor sustainable zinc-air battery cells Zhao-yang Chen, Heng Liu, Long-cheng Zhang, Qiu-lin Li, Mao-wen Xu*, Shu-Juan Bao* Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, Institute for Clean Energy and Advanced Materials, School of Materials and Energy, Southwest University, Chongqing 400715, PR China E-mail: [email protected] ABSTRACT Developing low cost and efficient electrocatalysts for oxygen reduction reaction (ORR) largely beneficial for preparing regenerative zinc-air batteries. Herein, ultra-small cobalt nanoparticles were uniformly embedded in N,S co-doped hierarchically graphene-like carbon nanostructures(Co@NSC) by a scalable facile pyrolysis method. Due to the hierarchical pore structures, high surface area, and synergistic effect between the dual-doped heteroatoms and transition metal, the resultant Co@NSC displayed high electrochemical activity. After H2SO4 treatment, the electrochemical performance of Co@NSC-acid enhanced with a positive half-wave potential (0.82Vvs.RHE), high electron number(4e-) and a low HO2- yield. As a air-cathode catalyst in the rechargeable zinc-air battery, the Co@NSC-acid showed a comparable open circuit voltage (1.42 V), remarkable peak power density (73.5 mWcm-2), and high charge-discharging cycling stability. Keywords: Oxygen reduction reaction, Electrocatalysis, hierarchical pore structures , Co@NSC-acid, Zinc-air batteries Introduction Zinc-air batteries(ZABs) are considered to be a promising and effective electrochemical energy storage and conversion device to minimize the consumption of fossil fuel,1-4 in which the cathode utilizes oxygen as a reactant rather than storing heavy active materials. Compared with other devices, ZABs have the advantage of

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high power density, rechargeability, lower operation risks and environmental friendly.5-7 However, the sluggish kinetics of the ORR process of air electrode hinder further application of these devices without outstanding catalysts.8-11 Currently, the high cost and limited reserves of platinum-based catalysts restrict the large-scale application of zinc-air batteries.12-16 Hence, developing economically available, high poison resistance, excellent catalytic performance catalysts is an urgent demand for the production of zinc-air batteries sooner rather than later. With this focus, Dai’s group found that nitrogen-doped carbon nanotubes can deliver surprising ORR electrocatalytic activity due to heteroatom-disturbed electroneutrality in the sp2 lattice, subsequently creating favorable sites for molecular oxygen adsorption or splitting.17 Following Dai’s work, more researchers have discovered that the catalytic activity of carbon materials can be enhanced by dual-heteroatoms(N, P,S, B etc.) doping.18-19 A density functional theory study unveiled that a synergistic effect occurred by dual-doping carbon framework, in which the charge and spin densities at different sites changed, resulting in more active sites towards single-doped samples.20 In addition, the dually-doped carbons can provide more flexibility to regulate the surface electronic configuration by optimizing doping sites, dopant ration and densities.21 Although these dual-heteroatoms doping enhanced the performances of carbon materials, their onset potentials is still far away from Pt/C. Recently, transition metals embedded carbon materials attracted more attention, and the synergistic effects of heteroatoms doping and transition metal were considered on improving the charge transfer and ORR activity of carbon based electrocatalysts.22-24 For examples, Da-Hee Kwak et al. fabricated Fe-N-S doped catalysts, which displayed outstanding activity.25 Sun et al. prepared a S-doping Fe/N/C catalyst delivered high power density.26 These works among numerous others confirm that M/N/S tri-doping could effectively enhanced the catalytic activity of carbon materials. Yet, the low production yields, expensive toxic precursors and complicated preparation equipment of such catalysts limit their further large-scale development. In this work, an effective and simple method was applied to embed cobalt into


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N,S co-doped graphene-like carbon using melamine and cysteine as the starting carbon sources. During a simple stirring process, melamine polymerized and covalently bonded with the highly active thiol groups of L-cysteine through an amide reaction, while Co2+ was absorbed uniformly by L-cysteine via electrostatic attraction or coordination interaction. After annealing at a high temperature, Co/N/S co-doped hierarchically graphene-like carbon nanostructures(Co@NSC) was formed. By removing most cobalt nanoparticles after H2SO4 treatment, the electrochemical activity of the as-prepared catalysts(Co@NSC-acid) was further improved. The primary Zn-air battery composed with the Co@NSC-acid catalyst acted as cathode electrocatalyst exhibiting high power density and charge-discharge stability. The reaction process and microstructure of the as-designed catalyst are illustrated in Fig. S1. Experimental Synthesis of samples Synthesis of Co@NSC, Co@NSC-acid and NSC: 2.0 g melamine and 500 mg L-cysteine ground and dissolve in 30 mL of ethanol and stirred for 30 min. Then, 500 mg cobalt nitrate was dissolved in 10 mL anhydrous ethanol and added to the above solution dropwise understirring. The product was collected by filtration, washed with ethanol, and further dried in a vacuum at 80°C. Co@NSC sample were obtained by annealing the precursor in N2 at 600ºC for 2 h and then heated to 800ºC for 2 h at a rate of 2ºC/min. Then were naturally cooled to room temperature. Co@NSC-acid was prepared by keeping Co@NSC in 0.5 M H2SO4 solution overnight, then were collected by centrifugation and washed with water and ethanol. NSC was fabricated following the same experiment procedure but without adding cobalt nitrate. Liquid Zn-air battery testing The Zn-air battery, an air cathode was prepared by loading Co@NSC-acid or 20%Pt/C mixed with 5 wt% Nafion solution on carbon fiber paper (1×1 cm3). The



catalyst loading mass was 1 mg/cm2, which was dried at 60°C for 4h in an oven. The anode was paired with a polished zinc plate(2×5×0.5cm3) and assembled with home-made electrochemical equipment. Before testing, the electrolyte(6M KOH aqueous solution containing 0.2 M Zn(Ac)2) was bubbled with oxygen for 15 min. Electrochemical measurements were carried out using an electrochemical workstation. Results and Discussion

Fig. 1 (a,b)FESEM images of the Co@NSC; (c) AFM image and the corresponding line of Co@NSC; (d)Low-magnification TEM image of Co@NSC; (e-f)Different magnification TEM images of the Co@NSC-acid. The typical morphology of the catalysts is characterized in detail by FESEM and TEM measurements. A well-defined, large lateral size and 3D interconnected network nanoarchitecture can be seen in Fig.1a and b. Atomic force microscopy(AFM) analyses shows that the thickness of NSC flake is about 1~1.5 nm (Fig. 1c), corresponding to about four or less graphene sheets. The large lateral size, while still maintaining atomic thickness, can provide large ultrahigh specific surface area and exposure plenty of active sites for ORR. The corresponding length profile of NSC displayed in Fig. S3. More detailed microstructures of the catalyst were observed by TEM, which reveal homogeneous distribution of ultra-small cobalt nanoparticles on


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the surface of the porous NSC nanosheets (Fig.1d). A schematic illustration of the preparation process, high-magnification image of Co@NSC and the corresponding elemental mapping showed in Fig. S1. The size of cobalt particles is about 14~17nm (Fig. S2). Furthermore, we used concentrated sulphuric acid to remove the metallic cobalt core and obtain hollow graphene spheres. Afterwards, HRTEM analysis was applied to characterize the microstructure of the catalyst. As shown in Fig. 1e and 1f, the mesopores are around ~3.8 nm, and the carbon layers of ~0.8 nm thickness wrapped on the metallic cobalt. Clear HRTEM pictures of Co@NSC-acid are displayed in Fig. S4. This three-dimensional flexible nanosheets with interconnected, entangled, and porous morphology can offer a continuous electron transmission channel to promote mass transfer.27-28 In addition, abundant pores edges and defects can be seen. This specific structure is beneficial to enhance ORR activity by offering rich active sites and more mass transfer pathways.

Fig.2 (a)XRD patterns of Co@NSC with Co@NSC-acid; (b)Co 2p XPS spectra; (c)N 1s XPS spectra; (d)S 2p XPS spectra of Co@NSC with Co@NSC-acid; (e)Raman Spectrum;(f)TGA curves annealed in O2 atmosphere. The chemical composition and crystal structure of the catalysts were examined by XRD. In Fig. 2a, two samples reveal a relatively wide diffraction peak at 25.5°, corresponding to the (002) facets of graphene. The diffraction peaks located at 44.2°,



51.5°, and 75.9°, are well indexed to the (111), (200), and (220) planes of Co(JCPDS. 89-4307). As shown in Fig.2a, the peak intensities of cobalt decreased greatly after acid leaching, indicating that most of cobalt was removed. XPS was utilized to analyze the surface electronic state and doping type of nitrogen, sulphur, and cobalt species in Co@NSC and Co@NSC-acid. Because XPS is a surface analysis technology, it is difficult to detect signals inside the carbon shells, and therefore, the Co2p spectrum is mainly exhibits Co(II) and Co(III) species (Fig. 2b).29After acid leaching, the cobalt signal was suppressed in the XPS spectra. The detailed N1s spectra (Fig. 2c) of Co@NSC and Co@NSC-acid composites can be devided into three sub-peaks, including graphitic-N (400.8 eV), pyridinic-N (398.2 eV) and pyrrolic-N (400.1 eV), further confirming the doping of N in the carbon framework.30-31 According to previous theoretical and experimental study, both graphitic and pyridinic nitrogen play a crucial role in improving the electrocatalysis activity through increasing spin density and disturb the charge distribution in the π conjugation.20,

32-33

They has the lowest barrier for the rate-limiting first electron

transfer as well as high selectivity for the four-electron reduction pathway.34-35 In the high-resolution S2p spectrum(Fig. 2d), the peak located at 163.8 eV and 165.2 eV could be assigned to the C-S-C specious.36 Combining XRD results and XPS data, we have reason to acknowledge that Co, N, and S are doped to the carbon structure rather than physically adsorbed on the carbon sheets. The carbon structure of Co@NSC and Co@NSC-acid were further investigated by Raman spectroscopy (Fig. 2e), which revealed two pronounced D-band and G-band peaks centered at 1332 and 1573 cm-1, respectively. The peak intensity ratio, ID/IG, was determined to be about 1.02, suggesting that more defects and disordered structures exists in the as-prepared 3D carbon network.37 Based on thermogravimetric analysis (TGA)(Fig. 2f) indicates that the loading capacity of Co is about 5.59 wt% after acid etching (after a simple caculation). The corresponding TGA curves of metal Co@NSC displayed in Fig. S5. The specific surface area and pore size distribution of Co@NSC and Co@NSC-acid were measured by Brunauer-Emmett-Teller (BET). It is obvious that Co@NSC-acid in possession of more abundant pore size (1.5-10.5 nm) towards Co@NSC(Fig. S6a),


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which is attributed to plenty of hollow graphene spheres due to the removal of the metal cores by acid leaching and the decomposition of g-C3N4 under high temperatures. These small pores could potentially act as a stoichiometric electron acceptor and host for various electron-donating guest species, indicating their influence on the reaction kinetics and selectivity of catalysts. Type IV nitrogen-adsorption isotherm with a H3-type hysteresis loop can be seen in Fig. S6b, revealing the mesoporous character of our prepared materials,38-40 and the higher specific surface area(397.5 m2g−1) of Co@NSC-acid. Combining the morphology images and BET data, we know that the synthesized Co@NSC-acid has a cross-linked network, developed porous nanostructure, and large surface area. These structural advantages offer multidimensional electron transport pathways, which could enhance the electrochemical performance of the as-synthesized products.41-42

Fig. 4. Electrochemical properties evaluation of NSC, Co@NSC, Co@NSC-acid, and Pt/C in 0.1M KOH electrolyte: (a)CV curves; (b)Linear sweep voltammetry (LSV) curves (1600 rpm, 5 mVs-1); (c)Polarization curves at various rotation rates; (d)RRDE voltammograms of Co@NSC-acid at a rotation rate of 1600 rpm; (e)Electron transfer number and H2O2 yield of Co@NSC-acid. (f)A comparison with transition metal@NC materials reported in literature about loading(left) and onset potential vs. RHE(right). The electrocatalytic activity of the catalysts toward ORR were first measured by cyclic voltammetry(CV) (Fig. 4a) at 0.1M KOH solutions. For N2-saturated solutions,



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no redox features were observed for the three samples (NSC, Co@NSC, Co@NSC-acid), while a clear oxygen reduction peak can be observed in O2-saturated solutions. Impressively, Co@NSC-acid displayed a more positive reduction peak (-0.08 V) than Co@NSC(-0.12 V) and NSC(-0.18 V). The as-prepared samples were further measured by linear sweep voltammetry (LSV) at 1600 rpm(Fig. 4b). According to the LSV curves, the Co@NSC catalyst exhibits a positive onset potential(0.92Vvs.RHE), which is superior to that of NSC(0.85Vvs.RHE) and close to 20 wt% Pt/C(0.93Vvs.RHE). Additionally, after H2SO4 treatment, the onset potential of the catalyst (Co@NSC-acid) showed no change, while the half-wave potential and the

cathodic

current

density

of

Co@NSC-acid

catalysts

was

enhanced

to(0.82Vvs.RHE) and 4.5 mA/cm2 towards Co@NSC(0.79 Vvs.RHE; 3.6 mA/cm2). A tafel plots of the four samples shown in Fig. S7. The Co@NSC-acid catalyst displays the smallest tafel slope among these materials and close to Pt/C, further demonstrating its excellent electrocatalytic performance. In addition, the electrochemical double-layer capacitance(Cdl) is used to assess the electrochemically active surface area (ECSA), which is determined by measuring CV curves of NSC, Co@NSC and Co@NSC-acid with various scan rates from 10 mV/s to 50mV/s. As shown in Fig. S8, the Co@NSC-acid exhibits the highest Cdl probably because of its large surface area. The better catalytic activity of Co@NSC-acid may be accountable to several reasons. First, the slack and hierarchical 3D porous nanostructures could provide abundant active sites on the Co@NSC-acid surface to facilitate the quick electrolyte/reactant diffusion and reduce the diffusion path length of electrolyte ions. Second, after acid etching, the catalytic activity increases obviously, suggesting that metal cobalt is not engaging directly in catalytic reaction. After the acid etching, more defects and disordered structures will be formed in its 3D carbon network, especially the sites surrounding the metal core, while produced more small pores could offer more channels to oxygen diffusion. Co-N-C or Co-S-C moieties formed in the preparation process remained well and still keep good electrochemical activity after acid etching. Third, the large lateral size, while still maintaining atomic thickness can possess strong chemical resilience and exposure plenty of active sites for ORR.


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Fourth, both Co@NSC and Co@NSC-acid possess good conductivity. Co@NSC-acid exhibited faster mass transport process than Co@NSC(Fig. S9), a good conductive framework structure in favor for quick charge transfer, thus resulting in outstanding activity.29, 43-47 In order to further understand the internal kinetics of Co@NSC-acid for ORR, LSV measurements at different rotating speeds were carried out (Fig. 4c). It is clear that the current density improved as the rotation rate increased. This can be attributed to the fact that the rotating ring-disk electrode could effectively eliminate the concentration polarization. The corresponding polarization curves of Co@NSC and NSC at various rotation rates are shown in Fig. S10~S11. Furthermore, the kinetic parameters were assessed using the Koutecky-Levich(K-L) equation (see SI). The K-L plots of Co@NSC-acid show good linearity and a consistent slope, indicating first-order reaction kinetics and similar electron transfer number in the potential range of 0.2 to 0.7 V (Fig S12).The RRDE tests performed at 1600rpm were employed

to

explore

the

average

electron

transfer

number

(N)

and

hydrogenperoxide(HO2-) yields, as shown in Fig. 4d. The value of N and HO2-% yield of Co@NSC-acidwere 3.81 and 3.94, respectively. The hydrogenperoxide(HO2-) yields below 10% (Fig. 4e) based on the equation (see SI), which further confirms that ORR occurs via a direct four electron pathway. The optimal Co@NSC-acid catalyst displayed a outstanding performance compared with M/N/C catalysts reported in literature in regard to catalyst loading, onsetpotential (EORRonset) (Fig. 4f) and half-wave potential (EORRhalf) (Fig.S13). The catalyst mass loading was 0.2 mg/cm2, which is relatively lower compared with other reported values. Under this condition, the Co@NSC-acid electrode still showed a clear positive EORRonset(0.92 V vs. RHE ) and EORRhalf(E1/2 = 0.82V vs. RHE). The results prove that the as-designed Co@NSC-acid catalyst display an excellent ORR catalytic activity. The ORR catalytic activity comparison of the as-prepared Co@NSC-acid catalyst with other Co/carbon-based materials was listed in Table S1. It is found that Co@NSC-acid catalyst outperform most Co/carbon-based catalysts in the aspect of onset potential and half-wave potential. In addition, oxygen evolution(OER) activity of as-prepared samples in KOH solution was also evaluated by LSV(Fig S14).



Fig. 5. (a)Methanol resistance test of Co@NSC-acid and Pt/C at -0.2V; (b)Chronoamperometric responses of Co@NSC-acid and Pt/C catalyst; (c)With addition of 20% methanol (v/v) into the electrolyte of Co@NSC-acid; (d)ORR polarization curves(1600rpm) of the Co@NSC-acid after 2500 and 5000 potential cycles; (e)Open circuit voltage measurement; (f)Current-voltage and current-power density curves of Zinc-air batteries using Co@NSC-acid and Pt/C catalysts; (g)Schematic of a Zinc-air batteries; (h)The discharge and charge voltage profiles of Zn-air batteries with Co@NSC-acid. The methanol tolerant sensitivity of Co@NSC-acid and Pt/C were measured by adding methanol into the electrolyte. As shown in Fig. 5a, the current of Pt/C quickly shifted after the rapid injection of methanol into the alkaline medium, while no significant decrease in the current of Co@NSC-acid is observed. The stability of the catalyst was evaluated using chronoamperometric. In Fig. 5b, the Co@NSC-acid catalysts exhibited excellent durability, where the initial current only decay by 16% after the same condition. While the Pt/C catalyst showed a greater current loss of 38%


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occurred for Pt/C after operation for the same time. In general, the Co@NSC-acid catalyst displayed better stability and methanol resistance than commercial Pt/C catalyst. The corresponding polarization curves before and after adding methanol solutions(Fig.5c) indicate the better methanol tolerance of Co@NSC-acid. ORR polarization curves after 2500 and 5000 cycles were applied to investigate the durability of Co@NSC-acid(Fig. 5d). It showed 10 and 23 mV negative shifts after manifold cycles for E1/2. To measure the practical application potential of Co@NSC-acid, a primary Zn-air battery was composed by loading Co@NSC-acid onto carbon paper for using as the air cathode. The battery made of Co@NSC-acid displayed an open circuit voltage of (1.42 V), which is slightly lower than that of Pt/C (1.44 V) (Fig. 5e). This result is consistent with the positive onset potential of the linear sweep voltammetry(LSV) curves observed in Fig. 4b. The power density curves of the Co@NSC-acid and Pt/C showed in Fig. 5f. Along with the increase in current density, the voltage decreased, resulting in a higher peak power density (73.5 mW cm-2 ) close to Pt/C (81.2 mW cm-2). More importantly, the stability of the zinc-air battery was evaluated by a cycle discharging-charging test at 2 mA/cm2(Fig. 5h). After a long cycle test, the final charge-discharge value has no obvious change compared to the initial value. This result further reveals that the Co@NSC-acid cathodic electrode have a good stability. Conclusion In summary, the Co/N/S-doped porous graphene-like carbon electrocatalyst by a simple one-pot soft-template method, demonstrates great potential as a viable alternative

to

Pt/C

in

electrochemical

applications.

The

dual-doped

heteroatoms(nitrogen and sulfur) and transition metal units displayed different functionalities that synergistically improved ORR activity. After a careful acid etching, the electrochemical performance of Co@NSC-acid was further enhanced, as indicated by the half-wave potential and limiting current density. Compared with a commercial 20% Pt/C, the catalyst Co@NSC-acid catalyst displayed a nearly onset potential, lower H2O2 yields, and sufficient stability towards Pt/C. When



Co@NSC-acid is used for zinc-air battery, it also exhibited nearly open circuit voltage and power density. More importantly, the battery assembled from the Co@NSC-acid air electrode showed an outstanding stability. Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Schematic illustration of the preparation process, FESEM and TEM images and elemental mapping(Fig. S1); Size distribution(Fig. S2); the corresponding length profile of NSC(Fig. S3); TEM images of Co@NSC-acid(Fig. S4); TGA curves(Fig. S5); Pore size distribution curves and nitrogen adsorption−desorption isotherms(Fig. S6); Tafel plots(Fig. S7); CV curves with various scan rates(Fig. S8); Nyquist plots(Fig. S9); Polarization curves of Co@NSC and Co@NSC-acid at various rotation rates(Fig. S10~S11); K-L plots of Co@NSC-acid(Fig. S12); A comparison about half-wave potential vs. RHE(Fig. S13); OER Polarization curves(Fig. S14); ORR performance comparison(Table S1). Material characterizations and electrochemical measurements; Koutecky-Levich (K-L) equations. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21773188), Natural Science Foundation of Chongqing (cstc2018jcyjAX0714),Chongqing Engineering Research Center for Micro-Nano Biomedical Materials and Devices, Chongqing Key Laboratory for Advanced Materials and Technologies. Notes and References 1. Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J.-E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H., Advanced zinc-air batteries based on high-performance hybrid electrocatalysts. Nat. Commun 2013, 4, 1805, DOI 10.1038/ncomms2812. 2. Cheng, F.; Chen, J., Metal–air batteries: from oxygen reduction electrochemistry to cathode catalysts. Chem. Soc. Rev. 2012, 41 (6), 2172-2192, DOI 10.1039/c1cs15228a.


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Table of Contents Graphic

Facile and scale synthesis of Co/N/S-doped porous graphene-like carbon architectures as electrocatalysts for sustainable zinc-air battery cells.


S-Doped Porous Graphene-Like

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