Gas-Flow Tailoring Fabrication of Graphene-like CoâNxâC Nanosheet

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Gas-flow tailoring fabrication of graphene-like Co-NxC nanosheet supported sub-10 nm PtCo nanoalloys as synergistic catalyst for air-cathode microbial fuel cells Chun Cao, Liling Wei, Qiran Zhai, Jiliang Ci, Weiwei Li, Gang Wang, and Jianquan Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04564 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 25, 2017

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ACS Applied Materials & Interfaces

Gas-flow tailoring fabrication of graphene-like Co-Nx-C nanosheet supported sub-10 nm PtCo nanoalloys as synergistic catalyst for air-cathode microbial fuel cells

Chun Cao ab, Liling Wei a*, Qiran Zhai c, Jiliang Ci bd, Weiwei Li ab, Gang Wang ab and Jianquan Shen a* a

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Green

Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P R China b

c

University of Chinese Academy of Sciences, Beijing, 100049, P R China College of Chemistry and Molecular Engineering, Peking University, Beijing

100871, P. R. China d

State Engineering Research Center of Engineering Plastics, Technical Institute of

Physics and Chemistry, Chinese Academy of sciences, Beijing 100190, China

*Corresponding Author: E-mail: [email protected] (Liling Wei); [email protected] (Jianquan Shen)

ACS Paragon Plus Environment


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Abstract In this work, we presented a novel, facile and template-free strategy for fabricating graphene-like N-doped carbon as oxygen reduction catalyst in sustainable microbial fuel cells (MFCs) by using amazing ion-inducing and spontaneous gas-flow tailoring effect from special nitrogen-rich polymer-gel precursor, which has not been reported in the material science. Remarkably, by introducing trace platinum- and cobalt- precursor in polymer-gel, highly-dispersed sub-10 nm PtCo nanoalloys can be in-situ grown and anchored on graphene-like carbon. The as-prepared catalysts were investigated by a series of physical characterizations, electrochemical measurements and microbial fuel cells tests. Interestingly, even with a low Pt content (5.13 wt%), the most active Co/N co-doped carbon supported PtCo nanoalloys (Co-N-C/Pt) exhibited dramatically improved catalytic activity towards oxygen reduction reaction coupled with superior out-put power density (1008 ± 43 mW m-2) in MFCs, which was 29.40% higher than the-state-of-the-art Pt/C (20 wt%). Notability, the distinct catalytic activity of Co-N-C/Pt was attributed to the high-efficient synergistic catalytic effect of Co-Nx-C and PtCo nanoalloys. Therefore, Co-N-C/Pt should be one of promising oxygen reduction catalysts for application in MFCs. Besides, the novel strategy for graphene-like carbon also can be widely used in many other energy conversion and storage devices. Key words: Microbial fuel cells; Graphene-like carbon; Synergistic catalyst; N/Co-dual doping; PtCo nanoalloys

1. Introduction


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Recently, air-cathode microbial fuel cells (MFCs) have been attracting increasing attention in the field of renewable energy1-4, since it can convert the chemical energy within organic compound to electrical energy efficiently, which was inexhaustible, eco-friendly

and

no

pollution5-6.

However,

the

potential

widespread

commercialization of MFCs was hindered by the heavy usage of noble metal-based catalysts for oxygen reduction reaction (ORR)7-11. Therefore, significant efforts have been devoted to exploring cost-effective catalysts with promising ORR activity, including (1) transition metals based catalysts12-14 , (2) heteroatom doped carbon materials15-19 and (3) low content Pt catalysts20-25. Although lots of catalysts based on transition metals and doped carbon materials had been prepared in the past decades, their electrocatalytic activity nearly cannot compare favorably with Pt-based catalysts26-28, due to their sluggish oxygen reduction reaction kinetics. Moreover, once utilized in real MFCs, most of Pt-free catalysts would soon lose ORR activity owing to the catalyst deterioration and the inhibition of microorganism12, 15. Noted that the ORR active components in Pt-free catalysts might be gradually damaged by complicate chemicals within electrolyte, such as metabolites from microorganism. Therefore, it seem that low-content Pt catalysts should be promising ones for reducing the cost of MFCs and retaining the catalytic activity as well. Nevertheless, as the active ingredient for oxygen reduction, the catalytic activity of catalysts with low content Pt might be weakened. Therefore, to solve this problem, one of the most effective emerging strategies was to fabricate Pt/N-doped carbon nanohybrids, which can deliver a high-active activity even with a low content Pt,



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ascribed to the well-known synergistic catalytic effect27. Furthermore, the stability and durability also can be enhanced in Pt/N-doped carbon nanohybrids29. Naturally, graphene and carbon nanotube were firstly selected as doped carbon matrix for compositing with Pt nanocrystal, due to their well electroconductivity as well as unique micro-nano morphology30-32. However, it was a pity that the cost of them was relatively high that cannot meet the requirements of large-scale usage for MFCs. Besides, there was still a very fatal flaw for graphene that was impermeable and airtight, which can significantly obstacle the oxygen diffusion and proton transfer in the air-cathode, and thus leading to low oxygen reduction kinetics and awful power generation performances in MFCs33-34. Subsequently, extensive researches have been focusing on constructing low-cost graphene-like or carbon-nanotube-like carbon materials for Pt/N-doped carbon nanohybrids35-37. However, almost all of them were prepared by template method, which was nearly involved with complicated operation and time-consuming38-39. In addition, lots of Pt nanocrystal was achieved by chemical reduction40, solvothermal synthesis41-42 and electrodeposition43-44, which all included meticulous chemical steps, excessive use of organic chemicals and surfactants. In the present work, a facile template-free one-pot carbonization strategy was provided for fabricating graphene-like carbon by using amazing ion-inducing and spontaneous gas-flow tailoring effect from special polymer-gel precursor, which have not been reported in the material science. Moreover, by introducing trace platinum chloride acid in polymer-gel, highly-dispersed and low-content Pt nanocrystal can be in-situ formed and anchored on carbon nanosheet. Likewise, heteroatom cobalt and


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nitrogen were also meanwhile introduced to form final low-content Pt/cobalt and nitrogen binary doped carbon nanosheet (L-Pt/Co-N-CNS). It should be noted that platinum ion and cobalt ion can be well immobilized in polymer gel by the coordination effect and electrostatic interaction, which would inhibit their spontaneous aggregation during carbonization process and therefore resulted in highly-dispersed Pt nanoscrystal as well as excellent doping efficiency of Co. More importantly, the L-Pt/Co-N-CNS electrocatalyst exhibited dramatically improved catalytic activity towards ORR coupled with superior power generation performance in MFCs, as benchmarked with commercial high-content Pt/C catalyst. 2. Experimental and method 2.1 Materials Polyacrylonitrile (PAN, average Mw 150 000) and potassium chloroplatinate (K2PtCl6) was bought from Sigma Aldrich. Carbon felt, Pt/C (JM 20wt%) catalyst and whole fluorine sulfonic acid polyvinyl fluoride (Nafion, 5wt%) were purchased from Hesheng Co., Ltd (Shanghai). CoCl26H2O, NaOH, HCl and hydroxylamine hydrochloride were purchased from Beijing Chemical Works and used directly without any further purification. 2.2 Synthesis of catalysts Prior to prepare self-induced polymer hydrogel, polyacrylamidoxime (PAO) should be firstly synthesized from PAN by well known Michael addition reaction. Briefly, 0.5 g of PAN was dispersed into 25 mL of distilled water under continuous stirring, followed by adding NaOH (0.6 g) and hydroxylamine hydrochloride (1.04 g)



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when the temperature was gradually rising to 65 oC. Then, sealed the reactor and reacted for 4 h until white PAN was completely converted to yellow, indicting the Michael addition reaction was finished. Finally, the obtained PAO was collected by filtration, washing and drying at 50 oC in oven over night. For preparation of self-induced polymer hydrogel, 0.5 g of PAO powder was put into 10 mL serum bottle, then 5 mL of the ternary solution that containing K2PtCl6 (2 mgPt mL-1), HCl (1.2 M) and CoCl2 (0.2 mgCo mL-1) was subsequently added into serum bottle. The polymer hydrogel would be gradually formed under continuous stirring for about 1 h. In addition, in order to clearly observe the formation process of polymer hydrogel, the above serum bottle only should be placed statically, and the PAO would slowly swell from bottom until reach the top of liquid surface at about 1 h. Besides, the polymer hydrogel that not containing K2PtCl6 or CoCl2 also can be obtained by the similar preparation method, and only remove the corresponding raw from water solution. For synthesis of catalysts, the above achieved polymer hydrogel was freeze-dried, and carbonized at 900 oC for 2 h in the tubular furnace with the protection of nitrogen. The finally synthesized carbon materials were crushed into powder using mortar for 30 min, by which it was easier for catalysts to perform various electrochemical tests and physical characterizations. 2.3 Physical and electrochemical characterizations Transmission electron microscope (TEM, JEM-2200FS) and scanning electron microscope (SEM, JSM-6700F) was conducted to observe the morphology and


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structure of obtained catalysts. X ray diffraction (XRD, Rigaku) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific) were used to analyze the chemical composition of carbon catalysts. The thickness of carbon nanosheet was measured by atomic force microscope (AFM, Multimode-8, Bruker). The

electrochemical

experiments

were

conducted

on

electrochemical

workstation (CHI660e, Shanghai Chenhua Co., Ltd) using three electrodes system in phosphate buffer solution (PBS, pH=7.0) with catalysts modified glass carbon electrode (3 mm in diameter), Pt wire and Ag/AgCl (Saturated KCl, 197 mV vs. SHE) as working electrode, counter electrode and reference electrode, respectively. To prepare catalysts modified work electrode, 5 mg catalyst and 50 µL nafion (5 wt%) were dispersed in 0.5 mL water with the assistance of ultrasonic for 30 min to achieve uniformly distributed ink. And then exactly 5 µL of ink was dropped on the surface of glass carbon electrode (GC) and dried in air for 12 h. The GC was polished by polishing powder for 15 min to form fresh surface before modification. Therefore, for LSV tests, the loading of prepared catalysts and Pt/C both was 0.64 mg cm-2. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were carried out to investigate the ORR activity at the potential ranged from 0.2 to -0.8 V, and the scan rate was 10 mV s-1. Prior to the CV and LSV tests, the electrolyte was bubbled with O2 or N2 for 30 min to achieve O2-saturated and O2-free condition. The AC impedance was conducted to measure the internal resistance and charge transfer resistance of different catalysts modified MFCs. The amplitude potential was 10 mV and the frequency ranged from 0.1 Hz to 1×106 Hz. By LSV with different rotation



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speed, the exact electron transfer number (n) can be fitted and calculated using Koutecky-Levich equations (Eq. 1 and Eq. 2) below. 1⁄ = 1⁄ + 1⁄ ⁄

Eq. 1

= 0.62 ⁄ ⁄

Eq. 2

Where, j and jL are the measured current density and the diffusion limiting current densities, respectively. ω represents the angular velocity (ω=2πN, N is the linear rotation speed); B could be calculated from the slope of Levich equation (Eq. 2); n, F and C0 are the overall number of electrons transferred in oxygen reduction reaction, the Faraday constant (F=96,485 C mol-1) and the bulk concentration of O2 (C0=1.2×10-3 mol L-1), respectively. D0 (1.9×10-5 cm2 s-1) is the diffusion coefficient of O2 in 100 mM PBS and ν (0.01 cm2 s-1) is the kinetic viscosity of the electrolyte (100 mM PBS). The constant 0.62 is adopted when the rotating speed is expressed in rad s-1. 2.4 MFC configuration and performance tests To check the practical catalytic activity of catalysts, various catalysts modified air-cathode was installed in standard cubic air-cathode MFC (volume=27 cm2, the area of air-cathode was 7.065 cm-2) with the 50 mM PBS (1 g L-1 sucrose) as culture medium. The loading content was 4 mg cm-1 for as-prepared catalysts, and 2.5 mg cm-1 for Pt/C. That is to say, the loading Pt content was 0.5 mg cm-1, which was the standard content for MFC tests12, 15. Carbon felt (Projected area: 4 cm2) was used as anode, and vaccinated with the bacteria that have been used for more three years in our lab45-46. The MFCs was operated in constant temperature incubator (35 oC) under


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continuous stirring. A resistance (1000 Ω) was loaded between the anode and air-cathode during operation, and the out-put voltage was recorded by electrochemical collector. The culture medium would be refreshed once the out-put voltage fell down to about 100 mV. For investigating the out-put power density of MFCs, the external resistance was varied from 5 K Ω to 50 Ω, and recorded the out-put voltages accordingly. The out-put power density can be calculated by the follow equation (Eq. 3):

/R "# A%# P = E

Eq. (3)

Where, Ecell, Rext and Acat are the external voltage, external resistance and the projected area of air-cathode, respectively. 3. Results and Discussion 3.1 Synthesis of catalysts

Fig. 1 (a) the formation process of PAO hydrogel within 60 min; (b) the obtained PAO hydrogel; (c) graphene-like carbon from PAO hydrogel; the possible formation mechanism of graphene-like carbon nanosheet from PAO hydrogel: the diagram of



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PAO hydrogel (d), ion-inducing process (e) and gas-flow tailoring process (f). The catalysts were prepared by a feasible three steps method including the preparation of polymer hydrogel, freeze drying and carbonization process. As shown in Fig. 1(a), it was amazing that with the presence of HCl or H2PtCl6, the tight PAO powder would be spontaneously swell up over time at room temperature, and finally convert to jelly-like polymer hydrogel within 60 min. Noted that this special phenomenon have not been reported anywhere, which was discovered accidentally in our experiments. Furthermore, the formed hydrogel can well retain the solid-state morphology and not flow down even turned the bottle upside down (Fig. 1(b)), suggesting the self-induced PAO hydrogel was steady and the interaction between water and PAO polymer chain was strong enough. Besides, the possible formation mechanism of the polymer hydrogel was provided in Fig. 1. From Fig. 1(d), the abundant hydrogen bonds would be appeared in the interlayer of PAO, since there were plenty of -OH and -NH2 groups within PAO, which resulted in a tight structure. Nevertheless, once the H+ or/and M+ (Pt4+, Co2+) were added in PAO, the hydrogen bonds would be destroyed by the protonation and coordination effect of H+ or/and M+, and therefore the polymer layer would swell up gradually till all of H+ or/and M+ were exhausted (Fig. 1(e)). Noted that the H+ or/and M+ was just like the scissors and cut open the interaction between PAO layers. In addition, some residual hydrogen bonds and introduced M+ would be served as the cross-linking point between PAO layers, and finally leading to the stable hydrogel. The achieved hydrogel was then freezing-dried, followed by carbonization at 900 oC


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with the protection of nitrogen, and the carbon-based catalysts were obtained (Fig. 1(c)). The different catalysts can be prepared by varying the feeding raw materials, and the catalysts that only containing Co or Pt were abbreviated as Co-N-C and N-C/Pt, respectively. The catalyst that containing Co and Pt was abbreviated as Co-N-C/Pt, and the catalysts without Co and Pt was abbreviated as N-C. The morphology of catalysts was observed by scanning electron microscope (SEM) and transmission electron microscope (TEM). It can be seen clearly from Fig. 2(a)-(h) that all of N-C, Co-N-C, N-C/Pt and Co-N-C/Pt delivered an interesting structure of graphene-like carbon nanosheet with the size about 2-10 µm. Moreover, the thickness of achieved carbon nanosheet was investigated by atomic force microscope (AFM), and that was ~30 nm in average, as shown in Fig. 2(k-i). The formation of amazing carbon nanosheet should be attributed to the effect of “ion-inducing” as well as “gas-flow tailoring”. “Ion-inducing” represented the protonation and coordination effect of H+ or/and M+ in precursor (hydrogel) as we mentioned above, by which the interaction between interlayer of PAO had been weakened and even been totally broken (Fig. 1(e)). This process was just like the intercalation effect and swelling action of oxidant for preparation of graphene oxide by typical Hummers method. Subsequently, the effect of “gas-flow tailoring” would be in charge during carbonization process, which meant that gas-flow would tailor the ion-induced PAO and exfoliate it layer by layer to form PAO nanosheet (Fig. 1(f)). It should be stressed that abundant gas including ammonia, water vapour, nitrogen, carbon monoxide and carbon dioxide would be released from precursor at the



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temperature of 200-300 oC11, since PAO contained lots of -NH2 and -OH. The released gas would gradually joint into gas-flow within the interlayer of ion-induced PAO, and tailor the PAO until flowing out. Remarkably, the “gas-flow tailoring” effect was also very similar to the function of ultrasonic-exfoliating process within Hummers method for preparing graphene oxide suspension. Then, the exfoliated PAO layer would be further carbonized at higher temperature and finally converted to the interesting carbon nanosheet. Therefore, the “ion-inducing” and “gas-flow tailoring” effect should be of great importance for fabricating the unique carbon nanosheet. Besides, from the insert graphs in Fig. 2(f-h), the obvious nanoparticles were uniformly anchored on the surface of N-C/Pt and Co-N-C/Pt, but not Co-N-C, suggesting the nanoparticles only can be formed with the presence of Pt. Additionally, the size distribution of nanoparticles (Fig. 2(i-j)) were calculated as 5 ± 3 nm and 6 ± 4 nm for N-C/Pt and Co-N-C/Pt, respectively.


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Fig. 2 The SEM images of N-C (a), Co-N-C (b), N-C/Pt (c) and Co-N-C/Pt (d); The TEM images of N-C (e), Co-N-C (f), N-C/Pt (g) and Co-N-C/Pt (h), the inset graphs in (f)-(h) were the corresponding relatively HRTEM images; the HRTEM images of N-C/Pt (i) and Co-N-C/Pt (j), the insert graphs were the corresponding lattice space images; the AFM image (k) and the thickness plots (l) of N-C; the TEM-EDS images (m) and corresponding C (n), N (o), Pt (p) and Co (q) elemental signals of Co-N-C/Pt. To identify the composition of nanopartciles, the space lattice parameter (Fig. 2(i-j), insert graph) and the EDS images (Fig. 2(m-q)) were also obtained. The space lattice of nanopartciles in N-C/Pt and Co-N-C/Pt was calculated as 0.226 nm and 0.215 nm which was consistent with Pt and PtCo nanocrystal, respectively, revealing the sub-10 nm Pt or PtCo nanoparticles were successfully prepared in N-C/Pt and



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Co-N-C/Pt47. Noted that the lattice of Co-N-C/Pt was a little smaller than that of N-C/Pt, ascribed to the lattice contraction of PtCo nanocrystal48. Furthermore, the strong signal of Pt in EDS image (Fig. 2(p)) also confirmed the formation of Pt nanoparticles. During the carbonization process, the introduced Pt4+ would be in-situ reduced by amidoxime group to form Pt nanoparticles, and meanwhile uniformly anchored on the surface of obtained carbon nanosheet. Interestingly, due to the immobilization effect of amidoxime group for Pt4+, the spontaneous aggregation process of Pt nanoparticles would be inhibited and therefore resulted in the small size sub-10 nm Pt nanoparticles. Noted that the clear signal of C, N and Co in the corresponding carbon nanosheet indicated that both of N and Co were doped into carbon nanosheet. However, it was also confusing that the Co signal was also very strong in the corresponding region of Pt nanoparticle, which might suggesting the introduction of Co elemental in Pt nanocrystal and the formation of PtCo nanoalloys. Therefore, to further clarify the composition of catalysts, XRD and XPS were performed and analyzed. (a)

Co-N-C/Pt Co-N-C N-C/Pt N-C JM 20% Pt/C

Pt CoN Graphtic C 111 111

002

200

220

(b)

N-C/Pt Co-N-C/Pt (111) (200)

311

111 200 100 111 220

200

10

20

30

40

50

60

311

70

2θ

80

90

40

42

44

46

48

50

2θ

Fig. 3 The XRD patterns of various catalysts (a) and (b) the high resolution XRD patterns of N-C/Pt and Co-N-C/Pt.


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As depicted in Fig. 3(a), all of catalysts showed the two obvious broad peaks at about 26° and 46° that indexed to the crystal planes of (002) and (100) for graphitic carbon (JCPDS 75-1621), revealing some graphitic carbon crystallite was formed within all catalysts during carbonization process. Furthermore, the characteristic peak located at about 36° in Co-N-C/Pt and Co-N-C was ascribed to the (111) plane of CoN phase, demonstrating the CoN moiety was achieved, which can significantly facilitate the oxygen reduction reaction. However, its XRD signal was very weak and cannot be found in SEM or TEM observation, therefore CoN phase might just be some very small cluster with very few N and Co atoms, in which the N and Co atoms should have some interaction with carbon to form Co-Nx-C species. Besides, the typical XRD patterns of N-C/Pt and commercial Pt/C showed several characteristic peaks at around 41.3°, 47.2°, 68.9° and 83.1°, which can be accordingly assigned to the (111), (200), (220) and (311) planes of the face-centered-cubic (fcc) Pt (JCPDS, 04-0802), indicating the formation of Pt crystal in N-C/Pt as confirmed by EDS results. It was interesting to note that the diffraction peaks of Co-N-C/Pt shift slightly toward higher angles compared to those of the N-C/Pt (Fig. 3(b). Meanwhile, the two diffraction peaks appearing at about 41.8° (111) and 47.9° (200) were agreement with those of PtCo nanoalloys with a face-centered-cubic structure, implying a lattice contraction that arose from the partial substitution of Pt atoms by Co atoms to form an alloy phase20, 49.



(a)

C 1s

(b)

C 1s

C-C 284.4 eV C-N/C-O 285.5 eV C=O 288.8 eV

(c)

Pt 4f

Pt(0) 4f 7/2 71.9 eV Pt(0) 4f 5/2 75.2 eV

N-C

O 1s

N-C/Pt

Intensity (a.u.)

Intensity (a.u.)

N 1s

Co 2p Pt 4f Co-N-C/Pt N-C/Pt

Pt(II) 76.4 eV

Intensity (a.u.)

Pt(II) 72.8 eV

Co-N-C

N-C/Pt

Co-N-C Co-N-C/Pt Co-N-C/Pt

N-C

200

400

600

280

800

282

284

(d)

Co 2p

Co(III) 2p 3/2

288

290

292

N1 397.9 eV

75

80

Binding energy (eV)

(f)

ORR-active N moiety N1 pyridinic N

N3 400.4 eV N4 401.5 eV

6.6

N2 Co-N N5 403.5 eV

Co(II) 2p 1/2

Satellite 1

70

8 N 1s

(e)

Co(III) 2p 1/2

Co(II) 2p 3/2

286

Binding energy (eV)

Binding energy (eV)

N4 graphitic N

N-C

6

5.8

N3 pyrrolic N

Satellite 2

Intensity (a.u.)

N5 N-oxidized

Co-N-C/Pt

N-C/Pt N2 399.1 eV

Co-N-C

Molar fraction (%)

0

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4.5

4 3.98

3.2 3.42

2

2.16

Co-N-C 1.35 Co-N-C/Pt

0 780

790

Binding energy (eV)

800

810

395

400

405

Binding energy (eV)

N-C

N-C/Pt

Co-N-C

Co-N-C/Pt

Fig. 4 (a) the whole XPS survey of various catalysts; the high resolution XPS spectra of O 1s (b), Pt 4f (c), Co 2p (d) and N 1s (e); (f) the content of N species of various catalysts. The chemical composition and valance states of various catalysts were analyzed by XPS technique (Fig. 4). From the whole XPS spectrum in Fig. 4(a), three obvious peaks for C 1s, N 1s and O 1s can be clearly found in all catalysts, suggesting nitrogen was successfully in-situ doped into them. Moreover, the elemental signal for P4f and Co 2p only appeared in the corresponding Pt/Co-containing samples, pointing out they were also introduced into catalysts by extra addition. Based on the peak intensity, the atomic ratio of different element was calculated and listed in Table. S1. The C content was the dominant species for all samples and nearly remained unchanged. Interestingly, the N content was gradually increasing from N-C, Co-N-C, N-C/Pt to Co-N-C/Pt, attributed to the strong immobilization of introduced metal ion (Co2+ and Pt4+) which had heavy interaction between N-precursor (amidoxime group)


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and therefore reduced the N-loss during the carbonization process. It was worth to note that the high N content could boost the oxygen reduction reaction as reported12, 50. Compared to N content, the O species nearly showed a reverse trend from N-C, Co-N-C, N-C/Pt to Co-N-C/Pt, which can absolutely facilitate the electro-catalytic activity

since

the

lower

O

content

represented

the

relatively

higher

electro-conductivity. The high resolution XPS spectrum of C 1s (Fig. 4(b)) showed three peaks at 284.4, 285.5 and 288.8 eV, corresponding to C-C, C-N/C-O and C=O, respectively. Likewise, the high resolution XPS spectrum of Pt (Fig. 4(c)) showed two strong peaks at about 71.9 and 76.2 eV, which can be assigned to Pt 4f7/2 and Pt 4f5/2, respectively. Remarkably, these two peaks of Co-N-C/Pt slightly shifted to high binding energy compared with the N-C/Pt, indicating that the electronic structure of Pt was changed within Co-N-C/Pt, attributed to the combined effects of electron transfer and lattice expansion/compression due to the formation of PtCo alloys as confirmed by XRD results. They were further split into four peaks at ~ 71.9, 72.8, 76.2, and 78.4 eV, revealing the presence of Pt (0) and Pt (II) on the surface of PtCo alloys. Noted that the formation of PtCo alloys within Co-N-C/Pt was favorable to weaken the binding energy between the absorbed oxygenated species and the active sites, and thus improved the electro-catalytic activity for ORR. In the case of Co, the high-resolution XPS spectrum (Fig. 4(d)) can be deconvoluted into six peaks presented at binding energy of 780.4 eV, 782.1 eV, 795.7 eV, 798.8 eV and 788.4/803.7 eV, which were indexed to Co(III) 2p3/2, Co(II) 2p3/2, Co(III) 2p1/2, Co(II) 2p1/2 and a pair of satellite peaks, respectively, proving a mixed valence state



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of Co was doped into Co-N-C/Pt and Co-N-C. The mixed valence state of Co can easily accept and donate electron to form “electron-flow” from catalyst to oxygen, and therefore boosted the oxygen reduction reaction. Furthermore, the doped Co also could be served as ORR active sites and significantly enhanced the ORR catalytic activity of Co-N-C/Pt and Co-N-C. Besides, it was well known that the N species were vitally important to the ORR activity, and only some special species such as pyridinic-N, graphitic-N and metal-N were active for ORR. Therefore the high-resolution of the N1s signal for different catalysts were also studied by means of XPS-peak-differentating analysis. From Fig. 4(e), nitrogen signals of Co-N-C/Pt and Co-N-C was deconvoluted into five distinguishable peaks at 397.9, 399.1, 400.4, 401.6 and 403.6 eV, which can be accurately attributed to the pyrrolic-N (N1), Co-N (N2), pyridinic-N (N3), graphitic-N (N4) and oxidized-N (N5), respectively. Therefore, based on the XRD and XPS results, it was believed that the component of Co-Nx-C was formed in Co-N-C/Pt and Co-N-C, which was super active for ORR51-52. Noted that Co-Nx-C component contained CoN phase, Co-N moiety and doped nitrogen. However, for N-C/Pt and N-C, only N1, N3, N4 and N5 can be found in their nitrogen signals since Co was not introduced in their precursors. Based on the peak intensity, the content of various N species were calculated and depicted in Fig. 4(f). By introducing Co and Pt, it can be seen clearly that the N content especially the ORR active N species (Co-N, pyridinic-N, graphitic-N)12,

53

were dramatically improved to 3.98 for Co-N-C/Pt,

which was 298 %, 84.3 % and 16.4 % higher than N-C (1.35), Co-N-C (2.16) and


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N-C/Pt (3.42), respectively, suggesting the ORR activity can be significantly enhanced by incorporating Co and Pt to form more and more ORR active N species. Additionally, the Pt content were 5.62 wt% and 5.13 wt% for N-C/Pt and Co-N-C/Pt (Table S1), which was only about a quarter of commercial Pt/C (20 wt%) catalysts. Therefore, the cost of N-C/Pt and Co-N-C/Pt was greatly reduced compared to Pt/C (20 wt%) catalysts. Overall, on the basis of above results, N-C/Pt and Co-N-C/Pt might possess distinct ORR activity. 3.2 Electrochemical evaluation for ORR

0 0.0

O2 N2

-0.5

-1.0

-2

N-C Co-N-C N-C/Pt JM 20% Pt/C Co-N-C/Pt

-4

-1.5 -0.6

-0.4

-0.2

0.0

0.2

-0.8

-0.6

0

0.30

0.0

-1 2 -1 j (mA/cm )

0.25

900 1200 1600 2000 2500

-4

-0.6

-0.4

-0.2

0.0

Potential (V vs Ag/AgCl)

3

2

1

N-C

0.2

4

0.20

0.4

0.15

0.07

0.08

Co-N-C

N-C/Pt

Pt/C

Co-N-C/Pt

-0.7

-0.6

-0.5

-0.4

(f)

3

2

1

0 0.06

at -0.8 V

0

0.2

-0.4 V -0.5 V -0.6 V -0.7 V -0.8 V

Co-N-C/Pt

-0.8

-0.2

(e)

(d)

-2

-6

-0.4

(c)

4



Electron transfer number (n)

-0.8

5

-2

(b) Current density (mA cm-2)

Current density (mA cm-2)

(a)

Limiting current density (mA cm )

0.5

Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.09

-1/2

ω (rad/s)

-0.8


Fig. 5 (a) the CV plots of Co-N-C/Pt in O2-saturated (red line) and N2-saturated (blue dash line) PBS electrolyte; (b) the LSV plots of various catalysts in O2-saturated electrolyte at 1600 rpm; (c) the current density of various catalysts at -0.8 V; (d) the LSV plots of Co-N-C/Pt under different rotation speed; (e) the Koutecky-Levich plots of Co-N-C/Pt at different potentials; (f) the electron transfer number plots of various catalysts.



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To evaluate the ORR activity of as-prepared catalysts, CV combined with LSV were performed on RDE in PBS solution (pH = 7.0), since they were desired to be used as electro-catalysts in real MFCs. From Fig. 5(a) and Fig. S1, obvious responsive current peaks for oxygen reduction were appeared in O2-saturated electrolyte for Co-N-C/Pt and other catalysts, and were nearly totally disappeared in N2-saturated solution, proving all of catalysts were exactly possessed the ORR activity. Furthermore, the ORR peak of Co-N-C/Pt was about 0.025 V, which was more positive than N-C/Pt (-0.025 V), Co-N-C (-0.175 V), N-C (-0.241 V) and even comparable to Pt/C, indicating it had better electrocatalytic activity. Fig. 5(c) showed the LSV plots of Co-N-C/Pt under various rotation speed. The current density of Co-N-C/Pt keep increasing along with the rotation speed from 500 to 2500 rpm, and other catalysts also exhibited similar results in Fig. S2(a)-(c). The LSV plots of all catalysts that measured in O2-saturated electrolyte at 1600 rpm were shown in Fig. 5(b), it was clearly that Co-N-C/Pt exhibited more positive on-set potential, half-wave potential, and bigger limiting current density (at -0.8 V) than the rests. In details, the on-set potential of Co-N-C/Pt was enhanced to 0.181 V compared to N-C/Pt (0.162 V), Co-N-C (-0.019 V), N-C (-0.081 V) and even a little superior to Pt/C (0.176 V). Moreover, the half-wave potential of Co-N-C/Pt was 0.025 V which was only 10 mV negative than Pt/C (0.015 V), but 155 mV, 270 mV and 356 mV positive than N-C/Pt (-0.130 V), Co-N-C (-0.245 V) and N-C (-0.331 V), respectively, implying the electro-catalytic activity can be exactly improved by the synergistic catalysis of PtCo alloys and N/Co co-doping. Besides, the limiting current density of various catalysts


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was illustrated in Fig. 5(c), and the Co-N-C/Pt also exhibited the biggest limiting current density than the others, which should ascribed to the abundant effective active sites within Co-N-C/Pt. From Fig. 5(e)-(d) and Fig. S2, the kinetic process of oxygen reduction reaction with different as-synthesized catalysts can be fitted well by Koutecky-Levich equation, and the electron transfer number (n) of oxygen reduction reaction at different potentials were calculated (Fig. 5(f)). The electron transfer number of Co-N-C/Pt nearly made no great difference and about 3.65-3.95 on average from -0.4 V to -0.8 V, which was higher than others. Thus, it was a very efficient catalytic process of oxygen reduction reaction with a four electron pathway for Co-N-C/Pt in a neutral PBS electrolyte. 3.3 Synergistic catalysis of Co-Nx-C and PtCo alloys Undoubtedly, the superior oxygen reduction activity of Co-N-C/Pt was attributed to the high-efficient synergistic catalytic effect of Co/N co-doped carbon and PtCo nanoalloys. In details, it was well known that PtCo nanoalloys possessed the outstanding catalytic activity for ORR, due to its low energy barrier for cracking oxygen molecule. However, it also should be stressed that the ORR catalytic kinetic process on PtCo alloys was mainly limited by its poor adsorption capacity for oxygen molecule. Interestingly, by introducing and doping Co, the high-active Co-Nx-C component (including Co-N, pyridinic-N and graphitic-N) was greatly improved within graphene-like carbon sheet, which should absolutely facilitate the oxygen reduction reaction of Co-N-C/Pt, since Co-Nx-C had great adsorption capacity for oxygen molecule as reported51,

54

. Notability, the graphene-like carbon nanosheet



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would absolutely provided sufficient active site exposure for Co-Nx-C and PtCo nanoalloys and further promoted the ORR.

Fig. 6 The possible synergistic catalytic mechanism of Co-Nx-C and PtCo nanoalloys. The possible synergistic catalytic effect of Co-Nx-C and PtCo nanoalloys was proposed according to literatures55-57 and illustrated in Fig. 6. Firstly, oxygen molecule would be adsorbed on the active sites of Co-Nx-C by sharing some electron with one oxygen atom as shown in step 1. Then, another oxygen atom would be anchored on PtCo nanoalloys (step 2) and meanwhile the inter-atomic force of O2 can be weakened for accepting proton and electron (step 3). After accepting one more proton and electron for single oxygen atom (step 4), the final was formed and would be divorced from active sites of Co-Nx-C as well as the PtCo nanoalloys (step 5), and the four electron pathway ORR process was completed. 3.4 MFC performance and analysis To check the practical ORR activity and stability, Co-N-C/Pt modified air-cathode microbial fuel cells (Co-N-C/Pt-MFC) was inoculated by bio-anode and operated for more than one week. For comparision, a commercial Pt/C (0.5 mgPt cm-2), N-C/Pt and Co-N-C modified air-cathode microbial fuel cells were also running at the


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same condition. Accordingly, they were abbreviated as Pt-MFC, N-C/Pt-MFC and Co-N-C-MFC, hereinafter. The image of air-cathode microbial fuel cells and the working principles were illustrated in Fig. 7. From Fig. 7(a), it was obvious that MFCs was consisted of anode, air-cathode and an interflow chamber that filled with PBS electrolyte. The front image of air-cathode was shown in Fig. 7(b), it contained current collector (carbon cloth), catalytic layer (facing the electrolyte) and the gas diffusion layer (facing the air). For the working principles of MFCs, it can be seen clearly from Fig. 7(c) that the microorganism was closely anchored on the surface of anode, and consumed the fuel (sucrose) to generate carbon dioxide, proton (H+) as well as electrons. Next, the electrons and proton would transfer to air-cathode via external circuit or electrolyte. At the side of air-cathode, oxygen would go through the gas diffusion layer and reach the catalytic layer. And with activation of catalysts, it would finally accept the electrons and protons to complete the oxygen reduction reaction. Therefore, the electron-flow was generated by MFCs.

Fig. 7 The image of air-cathode microbial fuel cells (a), air-cathode (b) and the working principles (c).



700

Co-N-C/Pt JM Pt/C

(a) 600

N-C/Pt Co-N-C

(c) 0

Current density (A m-2)

Voltage (mV)

500 400 300 200 100

-10

-20 Co-N-C N-C/Pt JM Pt/C Co-N-C/Pt

-30

-40 0 0

50

100

150

200

-0.8

Time (h)

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential (V vs Ag/AgCl) 0.015

(d)

(b) 1000 Co-N-C/Pt JM Pt/C N-C/Pt Co-N-C

N-C/Pt Co-N-C JM Pt/C Co-N-C/Pt

0.010

2

800

-Z''(ohm m )

-2

Power density (mW m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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600

400

0.005

200

0

0.000 0

1

2

3

4

5

0.01

0.02

Current density (A m-2)

0.03

0.04

0.05

2

Z' (ohm m )

Fig. 8 (a) the V-t plots, (b) out-put power density plots, (c) LSV curves and (d) AC impedance spectrums of various catalysts modified MFCs. Fig. 8(a) showed the V-t plots of various catalysts modified MFCs, one can see that the out-put voltages were increased sharply until reached a stable voltage platform once the culture medium was refreshed, and then fell down rapidly when the substrate was consumed totally after about 20-50 h. In details, the out-put voltage platform of Co-N-C/Pt-MFC was calculated as 581 ± 9 mV, which was 5.25%, 19.55% and 34.18% higher than Pt-MFC (552 ± 5 mV), N-C/Pt-MFC (486 ± 7 mV) and Co-N-C-MFC (433 ± 8 mV), respectively, indicating Co-N-C/Pt possessed a distinct ORR catalytic activity in practical application and even superior to the state of the art commercial Pt/C catalyst (20 wt%). Moreover, from Fig. 8(b), it was remarkable that the maximum power density of Co-N-C/Pt-MFC was significantly enhanced to 1008 ± 43 mW m-2, which was 29.40%, 64.98% and 175.41% higher than that of Pt-CFC (779 ± 28 mW m-2), N-C/Pt-MFC (611 ± 37 mW m-2) and Co-N-C-MFC (366 ± 28


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mW m-2), respectively. Herein, it should be noted that the practical catalytic activity of various catalysts in real MFCs may changed compared with that testing on RDE, since the microorganism can absolutely influence the ORR catalytic activity. Besides, the resistance of MFCs also had a great impact on the out-put power density. And the lower internal resistance can result into a higher power density for MFCs with the same ORR catalysts. Therefore, LSV and AC impedance was directly performed on real MFCs after long-term MFCs operation using catalysts modified air-cathode, bio-anode and Ag/AgCl as working electrode, counter electrode and reference electrode, respectively. The LSV plots of different MFCs were depicted in Fig. 8(c), the responsive current of Co-N-C/Pt-MFC bigger than all of rests from 0.4 V to -0.8 V, indicating Co-N-C/Pt had the distinct practical catalytic activity for ORR in real MFCs. Furthermore, the Nyquist graphs of different MFCs were shown in Fig. 8(d), through the analysis and fitting by equivalent circuit (Fig. S3), the resistance including Rs (ohmic resistance), Rct (charge transfer resistance) and Rd (diffusion resistance) of various MFCs were calculated and listed in Table S2. In details, Co-N-C/Pt-MFC showed a significantly smaller Rs (22.4 Ω), Rct (30.1 Ω), Rd (55.4 Ω) and total resistance (107.9 Ω) than Pt-MFC (29.6 Ω, 31.5 Ω, 63.6 Ω and 124.7 Ω for Rs, Rct, Rd and Rtotal respectively) and others, ascribed to its well electro-conductivity and abundant big holes resulted from stacking of carbon nanosheet, which would therefore favor electron/charge/mass transfer process in ORR and promote the power generation in real MFCs. 4. Conclusion



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Herein, a facile template-free strategy had been provided to fabricate graphene-like carbon nanosheet by using amazing ion-inducing and spontaneous gas-flow tailoring effect from special polymer-gel precursor. Since the polymer-gel contained sufficient nitrogen source, the nitrogen can be in-situ doped in final carbon nanosheet. Meanwhile, by introducing trace platinum- and cobalt- precursor in polymer-gel, the highly-dispersed and low-content PtCo nanoalloys would be grown and anchored on N/Co dual-doped graphene-like carbon during carbonization. Interestingly, even with a low Pt content (5.13 wt%), the most active Co-N-C/Pt exhibited dramatically improved catalytic activity towards oxygen reduction reaction as well as superior out-put power density (1008 ± 43 mW m-2) in microbial fuel cells (MFCs), which was 29.40% higher than the state of the art Pt/C (20 wt%) catalyst. Remarkably, the distinct catalytic activity of Co-N-C/Pt should be ascribed to the synergistic catalytic effect of Co-Nx-C and PtCo nanoalloys. Therefore, Co-N-C/Pt should be one of promising oxygen reduction catalysts for application in MFCs. More importantly, the facile preparation method of graphene-like carbon might be widely used in the field of energy conversion and storage devices. Supporting Information CV plots of catalysts, LSV plots of catalysts, koutecky-levich plots and the fitting results, nyquist graphs, elemental content from XPS, resistance distribution results. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21403251).


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A.;

Bhauriyal,

P.;

Rawat,

K.

S.;

Pathak,

B.,

Pt3Ti

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TOC Graphene-like carbon was fabricated using amazing ion-inducing and spontaneous gas-flow tailoring effect, and the achieved Co-N-C/Pt exhibited remarkable catalytic activity towards ORR due to the synergistic catalytic effect of Co-Nx-C and PtCo nanoalloys.


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Gas-Flow Tailoring Fabrication of Graphene-like CoâNxâC Nanosheet

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Gas-Flow Tailoring Fabrication of Graphene-like CoâNxâC Nanosheet