Boosting ORR electrocatalytic performance of metal-free mesoporous

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Boosting ORR electrocatalytic performance of metalfree mesoporous biomass carbon by synergism of huge specific surface area and ultra-high pyridinic nitrogen doping Baohua Zhang, Chunping Wang, Di Liu, Yijian Liu, Xili Yu, and Liang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01876 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Sustainable Chemistry & Engineering

Boosting ORR electrocatalytic performance of metal-free mesoporous biomass carbon by synergism of huge specific surface area and ultra-high pyridinic nitrogen doping Baohua Zhang,†,§,¶ Chunping Wang,†,¶ Di Liu,† Yijian Liu,† Xili Yu,*,† Liang Wang*,‡ †

Department of Chemical Engineering, School of Environmental and Chemical

Engineering, Shanghai University, No.333 Nanchen Road, BaoShan District, Shanghai 200444, P.R. China §

Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education,

Donghua University, No. 2999 People's North Road, Songjiang District, Shanghai 201620, P. R. China ‡

Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical

Engineering, Shanghai University, No.99 Shangda Road, BaoShan District, Shanghai 200444, P.R. China To whom correspondence should be addressed: Tel: +86-21-66135276. *E-mail: [email protected] (L. Wang); [email protected] (X. L. Yu). ABSTRACT: Oxygen reduction reaction (ORR) plays a critical position in direct methanol fuel cells. However, electrocatalytic materials currently utilized in ORR is a rare and expensive metal Pt, so it is vital to develop cathode catalysts with cheap and high ORR activity. Herein, chitosan, a natural material made from chitin, were employed as a complex precursor of carbon source and nitrogen (N) source to synthesize N-doped mesoporous biomass carbon ORR catalysts. Adding different pore agents regulated specific surface area and N type of catalysts. The relationship between the properties of the catalysts and their ORR electrocatalytic performance was investigated. It was lucky found that the addition of ferric nitrate as a pore forming agent created huge specific surface area of the N-doped mesoporous biomass carbon (1190 m2/g) significantly. More importantly, the synthesized catalyst was doped by whole pyridinic-N at high content (11.58 at%), and inhibited the 1

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ACS Sustainable Chemistry & Engineering 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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two-electron reaction efficiently, promoted the four-electron reaction and accelerated the ORR reaction rate. Furthermore, it provided significant catalytic activity with robust methanol tolerance, and notable cycle-stability, indicating the practical applicability of the huge-surface-area, ultra-high pyridinic-N-doped mesoporous biomass carbon catalyst. Keywords: Oxygen reduction reaction, Chitosan, Biomass carbon, Pyridinic nitrogen, Specific surface area INTRODUCTION Energy is the foundation of national economic and social development. For a long time in the past, fossil energy has made a main contribution to the development of human society. Unfortunately, fossil energy is non-renewable resource, excessive reliance on fossil energy has drastically made their prices rising, which leads to an energy crisis. Meanwhile, the extensive use of fossil fuels has brought a series of problems for the sustainable development of natural environment, such as exhaust gas and dust such as CO2, NOx, and SOx. The greenhouse effect and environmental pollution caused by these problems will seriously threaten human survival. Therefore, it urgently needs to develop clean, efficient and sustainable green energy sources to replace traditional fossil energy sources in order to realize the long-term sustainable development of human society. A fuel cell is a kind of energy conversion device that can directly convert chemical energy into electrical energy1. It gets the advantages of diversified fuel sources, high energy conversion efficiency, environmental friendliness, rapid start-up at room temperature, and wide application range. It is of great significance for solving the two major problems facing the world today: the energy crisis and environmental pollution. So, it has received extensive attention from worldwide governments and researchers. Compared with other fuel cells, direct methanol fuel cells (DMFCs) have taken in more and more consideration due to their unique strength. In addition to the general advantages of fuel cells, DMFCs also has its own advantages: use a liquid fuel rich in methanol, low cost, easy to store and replenish fuel, simple and reliable battery structure, and great flexibility. However, up to now, Pt is the most widely employed 2


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electrocatalytic oxygen reduction reaction (ORR) cathode catalyst in DMFCs, but the low Pt reserves, high price, and vulnerability to intermediate product poisoning have seriously hampered the commercialization of DMFCs.2-5 Therefore, in order to reduce the amount of platinum even wholly replace the use of platinum, the development of low cost, high ORR activity and anti-methanol non-Pt catalyst has been the subject of research in the DMFCs application field.6-7 Carbon materials act a pivotal part in ORR research. For instance, nanomaterial catalysts such as Pt and its alloys require carbon of high specific surface area as the carrier material to reduce the amount of catalyst used, enlarge the dispersion and use efficiency of the catalyst, and inhibit agglomeration of catalyst particles during reaction process. Although such materials are low-cost and excellent stability8-9, their catalytic effect is far from that of Pt metal owing to their low catalytic activity, and therefore there is a need to find ways to increase its catalytic activity. In recent, ORR electrocatalytic performance of carbon materials was illustrated to enhance by doping and pretreatment of heteroatoms, such as nitrogen (N) atoms8-16, especially high pyridinic-N content12-16. At the same time, the pretreatment of carbon materials can bring more surface-active functional groups, expose fresh carbon edges, micro particles and defects, or enhance the specific surface area through the formation of porous membranes of the surface17-18. The surface state of the carbon material renders them activated, thereby enhancing the ORR catalytic performance19. Additionally, it can broaden its ORR activity and stability in alkaline and acidic electrolytes.20-22 Chitosan, a sustainable biomass-derived inexpensive N precursor can be utilized to introduce N-active sites within the porous carbon material. Wu et al12 reported a two-step route employing chitosan to synthesize S, N co-doped graphitic carbon nanoparticles. The catalyst contained 3.4% N, and its specific surface area was owned by 579 m2/g-1. It exhibited that ORR electrocatalytic activity of catalyst overcomes traditional Pt/C catalysts. Zhang et al23 investigated an approach to produce cobalt/nitrogen doped carbon nanotubes for high ORR performance by cobalt catalyzed carbonization of chitosan. The catalyst displayed the highest BET surface areas up to 337 m2/g-1. Furthermore, the pyridinic-N percentage of catalyst was 49 at% 3



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in aggregate doped-N (9.1 at%). Despite these chitosan-based catalysts exhibited relatively good electrocatalysis activity, it seems too difficult to achieve large specific surface area and high pyridinic-N content of catalysts simultaneously. Herein, we synthesized a porous biomass carbon material with chitosan as a precursor, improving the specific surface area and regulating N type of catalysts by adding varies pore forming agents. The relationship between the pore diameter and specific surface area of the N-doped mesoporous biomass carbon and their ORR electrocatalytic properties was investigated. Furthermore, N contents and types of synthesized catalysts for enhancing ORR activities were analyzed. RESULTS AND DISCUSSION

Figure 1. The SEM image of NC-1 (a, b), NC-2 (c, d) and NC-3 (e, f). And the EDS image of NC-3 (g, h, i, j).

4


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Figure 2. The TEM image of NC-1 (a) NC-2 (b) and NC-3 (c). The HRTEM image of NC-3 (d) The structure of synthesized materials was characterized by scanning transmission electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images of NCs product can be seen in the Figure 1. According to SEM observation, NC-1 has an obvious porous structure (Figure 1b), while the NC-1 consists of irregular shapes (Figure 1a). However, with the addition of TiO2, the number of pores and the pore size of the NC-2 increases (Figure 1c and d). Importantly, the number of holes in the NC-3 material increases more by adding iron nitrate and densely-distributed of layered hole structure can be clearly seen from the cross section (Figure 1e and f). During the heating process, these metal compounds play a role to format abundant pore, and porous construction would be revealed after the metal is completely removed by acid washing after pyrolysis (Figure S1), which is benefit for the surface/interface contact and reaction between electrode and electrolyte19. As shown by the corresponding energy dispersive spectroscopy (EDS) results, only C, N, and O atoms was existed in the NC-3 material (Figures 1h-j), demonstrating that the remaining iron particles are withdrawn by washing with HCl. TEM observation reveals the pore structure (blue rectangles) of NC-1 (Figure 2a) and macropore structure of NC-2 (Figure 2b). From the TEM image (Figure 2c); NC-3 displays approximate well-arranged porous structure, because iron nitrate is transferred to FeO 5



species in high temperature (Figure. S1), which to form the good porous structure of carbonized product. The resulting products also possess a great deal of graphitic-phase structures at the edge of NC-3 (Figure 2d), which act a pivotal part

(a) 600 400 200

0.0

NC-1 NC-2 NC-3

0.2

0.4

0.6

0.8

1.0

dV/dD Pore Volume (cm3/g⋅nm)

in catalytic activity22. Quantity Adsorbed (cm3/g STP)

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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(b)

NC-1 NC-2 NC-3

0.2

0.1

0.0 0

Relative Pressure (P/Po)

5

10

15

20

Pore Diameter (nm)

Figure 3. Nitrogen adsorption desorption isotherms and corresponding pore size distributions of NC-1, NC-2 and NC-3. The porous structures of synthesized materials were further illustrated by N2 adsorption-desorption tests. As shown in Figure 3a, the similar shape of three samples are given in the isothermal curves. They might be allocated to I-type adsorption-desorption isotherm24,25. The phenomenon of a sharp rise of the N2 isotherm is appeared in the NC-1 and NC-3 material under the low relative pressure, further claiming the existence of great deal of micropores. Gas adsorption capacity of NC-1 is virtually not increased when the pressure increases, indicating that the micropore's surface area and pore volume contribute more at this point. This is the behavior of gas adsorption and desorption when the micropore occupy the major part. However, gas adsorption capacity of NC-3 is extended when the pressure increases, which is indicating that the NC-3 sample has more and larger holes than NC-1 (Figure 3a). The pore-size distribution curves were showed in Figure 3b. For the NC-1 sample, the pore size ranged from 1.8 nm to 4.1 nm, respectively. However, the NC-3 sample display more and larger mesopores compared with the NC-1 sample, assuming that smaller pores could convert bigger and more after adding ferric nitrate. More information

about

the

brunauer–emmett–teller

(BET)

surface

area

and

barret–joyner–halenda (BJH) pore volume of the NC samples is summarized in Table 1. Although the pore size distribution of the NC-3 is closely resembled that of NC-2, 6


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the BET surface area of the NC-3 has remarkably raised from 743 m2/g to 1190 m2/g with a significantly increased pore volume from 0.76 cm3/g to 1.10 cm3/g. The allude to above results illustrate that these samples contained unlimited micro- and meso-pores, especial well-organized porous structure in the NC-3 sample. This orderly arrangement porous architecture of the samples would take such advantage to adsorption and transport of electrolyte inside samples. In the summary, these superiorities improve the ORR rate capability of the porous carbon materials26. Table 1. BET surface area and BJH pore volume of the NC samples Samples

Sbet (m2/g)

Smeso/macro (m2/g)

Smicro (m2/g)

VPoretotald (cm3/g)

Vmicro (cm3/g)

Vmeso/macro (cm3/g)

Pore Size (nm)

NC-3 NC-2 NC-1

1190 743 602

1085 720.5 17

105 585

1.10 0.76 0.44

0.047 0.308

1.053 0.20 0.128

4.2 4.3 2.3

Figure 4. (a) XRD patterns, (b) Raman spectra and (c) The survey XPS spectrums of NCs. High-resolution XPS N1s spectra of NC-1 (d), NC-2 (e) and NC-3 (f). The Figure 4a shows the X-ray diffraction (XRD) patterns for the CNs. There are appeared two obvious diffraction peaks correspond to the (002) diffraction of the graphitic layer-by-layer structure and the (100) diffraction of graphite, respectively. In contrast, for the NC-3 sample, the peak at 24° turned gently, displaying that the crystalline structure of NC-3 is no change in addition of ferric nitrate. Furthermore, the graphite degree of samples was characterized by Raman spectra in Figure 4b, two 7



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notable peaks located at 1360 cm−1 and 1589 cm−1 are assigned to the D and G bands of samples, respectively. It is important to note that the integrated intensity ratio (ID/IG) of the NC-3 (1.24) sample was below that of the NC-1 (1.36) and NC-2 (1.39), exhibiting that the graphitization degree of NC-3 is highest in the samples. It stems from the fact that during the bonfire process, the iron element easily interacts with the heteroatoms of the organic precursor, thereby promoting graphitization and reinforcing the attaching of heteroatoms in the biomass carbon material. The Raman results display the excellent conductivity capacity of NC-3, which is benefit for its electrocatalysis ORR performance. The relative elemental analysis of the produced samples was included in X-ray photoelectron spectroscopy (XPS) spectra, and it indicated that all of NCs consists of three elements, that are C, N and O (Figure 4c). And there is not any iron in the NC-3 sample or Ti in the NC-2 sample indicates that acid wash removed all metal ions (Figure S1). Generally, the N1s spectrum can be separated into three peaks at 401.0 eV, 399.8 eV, and 398.3 eV, which can be allocated to graphitic N, pyrrolic N, and pyridinic N, respectively.27 With the aim of revealing the chemical valence states of N in detail, the N1s spectra were disintegrated into distinct peaks in light of varied mentioned above valence states of N (Figure 4d-f). The binding energy and the relative atom ratio of N about the samples are outlined in Table S1. NC-3 has the highest N atomic ratio in the three samples. In the NC-1, there is abundant graphitic N (main content), pyridinic N, and pyrrolic N whereas only graphitic N in NC-2 and pyridinic N in NC-3. This phenomenon indicates that this may be the addition of iron promotes the formation of pyridinic N under high temperature annealing. It is worth observing that pyridinic N could reduce the adsorption energy of O227, with a plenty of pyridinic-N species nurse to operate ORR through a 4e- mechanism28-29.

8


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Figure 5. (a) The CV curves of NC-3 on a GC electrode in N2 or O2-saturated 0.1 M KOH solution. (b) The LSV of samples in O2-saturated 0.1 M KOH solution. (c) The LSV of NC-3 with different rotation rates from 400 to 1600 rpm (Inset, K-L plots of NC-3 derived from -0.7 to -0.85 V). (d) The number of electrons transferred per O2 molecule at different potentials based on the K–L equation for NC-3, NC-2, and NC-1. (e) RRDE linear sweep voltammograms of NC-1, NC-2, and NC-3, at a rotation rate of 1600 rpm. (f) Plots of H2O2 yield and the number of transferred electrons n for NCs calculated from the ring and disk currents in O2 saturated 0.1 M KOH solution at a scan rate of 5 mV/s and a rotational speed of 1600 rpm for comparison. ORR is a critical electrochemical operation in fuel cells and is normally appraised by several factors such as the initial potential, oxygen reduction peak and electron transfer number30. To evaluate the ORR electrocatalytic performance of the NC-3, cyclic voltammetry (CV) measurements were estimated in N2- and O2-saturated 0.1 M KOH solution at a scan rate of 50 mV/s. Figure 5a showed that any obvious peaks were observed within the ORR potential range in N2-saturated solution voltammograms. On the contrary, manifest ORR activities were inspected with separate peak potentials via adding the O2 into the reaction solution. Particularly, the 9



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more positive ORR peak potential of NC-3 at -0.2 V was beheld in O2-saturated solution, implying a more comfortable ORR process for NC-3. Linear sweep voltammetry (LSV) of NCs were collected in O2-saturated 0.1 M KOH on a rotating disk electrode (RDE), and the onset potential of catalysts was obtained from RDE linear sweep, with the rotating rate of 1600 rpm (Figure 5b)31. The values of the onset potentials for NC-3, NC-2, and NC-1 samples were -0.01 V, -0.22 V, and -0.04 V, respectively. It is note that the ORR onset potential of NC-3 catalyst positively alters to -0.01 V, compared with -0.04 V of NC-1 due to the specific surface area and pyridinic-N content enlarged in NC-3, pointing out a more facile reactivity for ORR on the latter. Despite NC-2 has an increase in specific surface area compared to NC-1 (Table 1), its pyridinic-N content is lower than that of NC-1 (Table S1), and so the electrocatalytic performance is not satisfactory. As discussed above, it is clear that the ORR activity was not simply dependent on the total amount of the doping N32,33, specific surface area and N species’ type of catalyst should be synergism to the ORR catalytic performance. The LSVs of NC-3 with different rotation rates needed to be measured in O2-saturated 0.1 M KOH solution. As shown in Figure 5c, the current density increased with rotation rate because of the shortened diffusion distance15,34. The LSVs of NC-1 and NC-2 show in the Figure S2. We all knew that we can calculate the number of electrons transferred per oxygen molecule (n) involved in the ORR process from the slope of the Kouteckye-Levich (K-L) equation35,36, which is implied in the inset of Figure 5c. K-L plots of NC-3 catalyst exhibited excellent linearity over the potential range of -0.7 V to -0.85 V, implying similar electron transfer numbers for ORR at different electrode potentials. The electron transfer number of NC-3 reached 3.6-3.9 within the potential change from -0.7 V to -0.85 V. In term of K–L plots for NC-1 and NC-2 (Figure S2c), the electron transfer number values of NC-2 totaled 1.32-1.4 and the NC-1 only demonstrated an n of 1.5-1.6, implying a limited 2ereduction route (Figure 5d). The results were clarified that NC-3 exhibited a better four-electron reaction and shows the excellent ORR performance of the material, which is outstanding consistent with the CV data. The rotating ring disk electrode 10


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(RRDE) measurements were further achieved for NC-1, NC-2 and NC-3 (Figure 5e), and the production yields of H2O2 are shown in Figure 5f. By comparison, the NC-3 catalyst shows lower H2O2 yield than NC-1 and NC-2 at all potentials within the range from -0.3 V to -0.8 V, clarifying a higher ORR efficiency of NC-3. In addition, the corresponding electron transfer number is 3.98 for NC-3, which is also in agreement with the K−L results. The boost of electrochemical activity of NC-3 could be reasonably assigned to synergism of higher surface area (Table 1) and higher pyridinic-N content likewise the well-organized porous structure, which served as an electronic channel to authorize the adsorption and transport of electrolyte inside samples.37 For the reason that small organic fuel molecules such as methanol, may pass through the polymer electrolyte membrane from anode to cathode, and severely threaten the activity of the whole cell. The crossover influence should be deliberated for practical application in methanol fuel cells. The methanol crossover effect was appraised on both NC-3 and commercial Pt/C catalysts. In the present study, NC-3 samples showed basic catalytic properties. Cathodic oxygen reduction peaks of Pt/C (20% by weight) faded out in O2-saturated electrolytes containing 3 M methanol (Figure S3b). In opposite, the current density of NC-3 was only slightly altered (Figure S3a), which illustrated that NC-3 sample displayed better methanol tolerance than the commercial Pt/C catalyst. The cyclic curve is essentially the same for the initial one in the O2-saturated KOH electrolyte after 300 CV cycles. This result demonstrates excellent stable catalytic activities of NC-3 (Figure S4a) and Pt/C (20 wt%) (Figure S4b) are similar as described in other papers. CONCLUSIONS In summary, mesoporous structural biomass carbon material with whole pyridinic-N-doping with high content and ultra-high specific surface area of 1190 m2/g has been prepared via a simple way by using renewable biomass derived chitosan as precursor. An elevated electrocatalytic performance of NC-3 was exhibited due to synergism of high pyridinic-N content and large specific surface area, 11



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while the well-organized porous structure served as an electronic channel to authorize the adsorption and transport of electrolyte inside samples. The resultant product NC-3 demonstrates fantabulous activity toward ORR in alkaline solution in regard to excellent performance and substantial stability. Convenient and low-cost synthesis, and superior ORR activity surely makes NC-3 a hopeful and eco-friendly electrocatalyst to replace Pt. Additionally, NC-3 is highly scheduled to be used as porous carbon-based electrode material for further applications and open up a new area for green synthesis from naturally sustainable biomass. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Authors *Tel.: +(86)-21-66135276. Fax: +(86)-21-66138025. E-mail: [email protected] (L. Wang). * Tel.: +(86)-21-66135276. Fax: +(86)-21-66138025. E-mail: [email protected] (X. L. Yu). ORCID Liang Wang: 0000-0002-3771-4627 Author Contributions ¶ Baohua Zhang and Chunping Wang contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 21671129, 21571124, and 21671131), the Shanghai Sailing Program (No. 16YF1404400),

and

Natural

Science

Foundation

of

Guangxi

(No.

2017GXNSFBA198216). We thank the Laboratory for Microstructures of Shanghai University. 12


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C16mimBF4

assisted

synthesis

of

poly

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Synergistic effects between huge specific surface area and ultra-high pyridinic nitrogen doping in metal-free biomass carbon for boosting ORR performance.

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Boosting ORR electrocatalytic performance of metal-free mesoporous

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