Nano Ce2O2S with Highly Enriched Oxygen-Deficient Ce3+ Sites

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Nano Ce2O2S with highly enriched oxygen-deficient Ce3+ sites supported by N and S dual-doped carbon as an active oxygen-supply catalyst for oxygen reduction reaction Liu Yang, Zhuang Cai, Liang Hao, Zipeng Xing, Ying Dai, Xin Xu, Siyu Pan, Yaqiang Duan, and Jinlong Zou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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ACS Applied Materials & Interfaces

Nano Ce2O2S with highly enriched oxygen-deficient Ce3+ sites supported by N and S dual-doped carbon as an active oxygen-supply catalyst for oxygen reduction reaction

Liu Yanga,b, Zhuang Caia,b, Liang Haoa,b, Zipeng Xinga,b, Ying Daia,c*, Xin Xua,b, Siyu Pana,b, Yaqiang Duana,b, and Jinlong Zoua,b,*

a

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's

Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China.

b

Key Laboratory of Chemical Engineering Process and Technology for High-Efficiency Conversion,

College of Heilongjiang Province, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China.

c

School of Civil Engineering, Heilongjiang Institute of Technology, Harbin 150050, China.

Corresponding author (s): *Ying Dai, Jinlong Zou. a

Xuefu Road 74#, Nangang District, Harbin, 150080, China.

Tel.: +86-451-86608549; Fax: +86-451-86608549. E-mail: [email protected] (Y. Dai); [email protected] (J. L. Zou).

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ABSTRACT Design of rare-earth-metal oxide/oxysulfide catalysts with high activity and durability for oxygen reduction reaction (ORR) is still a grand challenge at present. In this study, Ce-species (Ce2O2S/CeO2)/N, S dual-doped carbon (Ce-species/NSC) catalysts with promising oxygen storage/release capacity are prepared at different temperatures (800–1000 oC) to enhance the ORR efficiency. Mechanisms for the effects of temperature on crystalline phase transition between CeO2 and Ce2O2S and structure evolution of Ce-species/NSCs are inferred to better understand their catalytic activity. Porous Ce2O2S/NSC (950 oC) catalyst as the air-breathing cathode exhibits the maximum power density of 1087.2 mW m–2, which is higher than those of other Ce-species/NSCs and commercial Pt/C (989.13 mW m–2) in microbial fuel cells. The decline of power density of Ce2O2S/NSC (950 oC) cathode is 8.7 % after 80 d operation, which is far lower than that of Pt/C (36.7 %). Ce2O2S/NSC (950 oC) has a four-electron selectivity towards ORR and a low charge transfer resistance (5.49 Ω), contributing to the high ORR activity and durability. The promising ORR catalytic activity of Ce2O2S/NSC (950 oC) is attributed to its high specific surface area (338.9 m2 g–1), varied active sites, high electrical conductivity and sufficient oxygen vacancies in Ce2O2S skeleton. The high content of Ce3+ in Ce2O2S/NSC (950 oC) facilitates the formation of more oxygen-deficient Ce3+ sites that generate more oxygen vacancies to release/store more oxygen to stabilize available oxygen for ORR. Thus, this study provides a new perspective for preparation and application of this new type of ORR catalysts.

KEYWORDS: Ce2O2S; N, S dual-doping; Oxygen supplier; Oxygen vacancy; Porous structure

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INTRODUCTION In recent years, microbial fuel cells (MFCs) as one of the important sustainable technologies, which directly converts organic substances into electricity via microorganism catalytic activity, has been attracted extensive attention due to the satisfactory interest in energy storage and conversion.1,2 In a MFCs, electrons can transfer from the anode to the cathode through an external circuit to generate electricity.3 Oxygen is generally considered as the most optimal electron acceptor in MFCs cathode.4 Therefore, the oxygen supply and transfer is a pivotal step for cathodic oxygen reduction reaction (ORR) performance. So far, for most of the air-breathing MFCs, noble metal Pt and its alloys are still recognized as the ideal cathode catalysts towards ORR.5 However, it has many intractable issues in terms of subjecting to high cost6, scarcity in supply, low CO/sulfur tolerance and poor long-term operation durability, which seriously hinder the widespread application of MFCs for wastewater treatment.7 It is an urgent task with great challenges to replace the expensive Pt-based catalysts with a low-cost and highly active and durable catalyst in MFCs.8

Currently, the rare-earth-metals based catalysts as the most developing non-precious metal catalysts have attracted more and more attentions for their unique properties.9 Among them, Sm, Ce, and Gd oxides have been successfully used for ORR, because their super semiconductor property can contribute to the varied composition and structure and the particularity of 4f electrons.10,11 Among the many rare-earth-metal oxides, CeO2 has been attracted much more attentions as the important component in catalysts and/or co-catalytic supports, for its unique redox property and high oxygen storage capacity.12 Since the possession of these properties, numerous reactions relying on the high oxidation ability of CeO2-based materials have been come into use.13 Inderjeet Singh et al. successfully use CeO2 nanoparticles (NPs) as the cathode catalyst to enhance the oxygen transport and reduction because of their capability for exchanging oxygen from the ambient environment by the transfer between Ce4+ and Ce3+.14,15 These reactions can lead to a rapid diffusion of oxygen

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through the lattice, which thus reduces the electrode overpotential and mass transport losses.12,16

Some studies have reported that the ternary oxysulfides evidently exhibit superior electrochemical property to binary metal sulfides.17,18 For the first time, Cheng et al. have applied flower-like rare-earth oxysulfide (Ce2O2S) as a new developing anode for Li-ion batteries.19 The electrochemical performance of flower-like Ce2O2S is superior to the reported bare CeO2 due to the Ce2O2S with a high specific area and a porous structure can provide more active sites and accelerate the lithium ions diffusion.19 To the best of our knowledge, there is no study investigating the possible ORR activity of Ce2O2S in fuel cells, especially in MFCs. However, the poor electrical conductivity of Ce2O2S (including CeO2) should inhibit their ORR activity to be explored and exhibited.20 This hence leaves room for further investigations, especially when the Ce2O2S is used as the pollutants-tolerant (Wastewater and cathodic biofilm can be considered as the relatively harsh environment for ORR) catalytic component for enhancing the ORR activity in MFCs.

It is worth noting that a series of researches have proposed that metal composites supported by carbon materials are effective to improve the electrical conductivity and available surface area that leads to an efficient oxygen adsorption.21 In particular, nitrogen (N)-doped carbon has received more attentions, because the pyridinic-N can decrease the energy of oxygen adsorption by changing the energy band structure between the adjacent carbon atoms, which can energetically facilitate the oxygen adsorption.22,23 Recent researches show that doping binary or ternary heteroatoms into carbon skeleton can effectively enhance the ORR activity24, which is better than that of single atom doping, due to the synergetic effects contributed by the multiple heteroatoms co-doping.25 Yan and his co-workers also indicate that N and S dual-doped graphene provides a large amount of defects and ensures an intimate contact between metal oxide and carbon matrix.26 Its ORR performance is almost comparable to that of state-of-the-art Pt/C catalyst. Therefore, the binary or ternary doping

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of heteroatoms into the carbon framework, leading to the formation of more ORR active centers, is a highly promising method for preparing the ORR catalysts/supports, which still remains a big challenge.27

Herein, we report a facile method to synthesize an N, S dual-doped carbon supported Ce2O2S cathode catalyst (Ce2O2S/NSC) by using pomelo skins and thiourea as the carbon and N, S sources, respectively, together with the cerium (III) nitrate hexahydrate as the Ce precursor. Pomelo skins, as a typical wasted biomass with low-cost and abundant function groups, can immobilize various ions in their sponge-like structures, which can be considered as a promising carbon source.28 The hybrid structure between carbon supports and Ce2O2S NPs is expected to overcome the harsh ORR environments and possess superior stability and durability. The inferred mechanisms for the effects of temperature on the crystalline phase transition (CeO2→Ce2O2S→CeO2) and structure evolution of Ce-species/NSC catalysts are investigated to better understand their catalytic activity for ORR. The promising oxygen storage capacity of Ce2O2S lattice is expected to release oxygen in a low oxygen environment, which can correspondingly enhance the ORR activity. The synergistic interactions between heteroatoms-doped carbon and Ce2O2S not only enhance the exposure of more active sites, but also change the surface property of Ce2O2S/NSC to facilitate the O2 adsorption and reduction, which can significantly improve the ORR activity.27 Moreover, the ORR pathways of these type catalysts still remain elusive to date, which eagerly needs to be clarified. These results will show that the Ce-species/NSC catalysts can compete with Pt-based catalysts by the advantages of low cost and excellent ORR activity and durability, especially for MFCs.

EXPERIMENTAL SECTION Synthesis of Ce-species/NSC catalysts The Ce-species/NSC catalysts were fabricated by using a one-step in-situ reduction method. Firstly,

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the fresh pomelo skins were washed and cut up into small pieces and then dried in an oven under 60 o

C. After that, 2.1712 g of Cerium (III) nitrate hexahydrate and 0.7612 g of thiourea (molar ratio 1:

2) were dissolved in 50 mL deionized water with magnetic stirring for 3 h under the room temperature. The treated pomelo skins (3.0 g) were soaked into the above-mentioned mixture solution and then a flavescent polymer gel was obtained after standing for 24 h. In this procedure, Ce3+ should be sufficiently anchored into the sponge-like structure of pomelo skins. The obtained mixture was dried at 80 oC in an oven overnight. The catalysts were obtained by directly pyrolyzing the dried mixtures in a tubular furnace at the target temperature of 800–1000 oC under the N2 atmosphere (60 mL min–1) for 2 h and the heating rate was fixed at 5 oC min–1. The final products were labeled as Ce-species/NSC-x (x=800, 850, 900, 950 and 1000).

Air-cathode fabrication The cathode was consisted of a gas diffusion layer (GDL), a catalyst layer (CL) and a stainless steel mesh.29 The stainless steel mesh was treated with acetone to remove impurities. The mixture with carbon black and PTFE (mass ratio of 3: 7) was rolled on one side of stainless steel mesh and then sintered at 340 oC for 20 min to obtain the GDL.30 After that, the mixture with catalyst (0.25g) and PTFE (mass ratio of 1: 1) was rolled on the other side of stainless steel mesh to obtain the CL. As a comparison, the cathode with commercial Pt/C (10 wt. %) was fabricated using the same method.

MFCs setup and operation Three replicates were conducted to examine the reproducibility of the data in the same condition. Single-chamber MFCs was made with a cylindrical plastic tube with a length of 4 cm and an inner diameter of 3 cm.28 The total volume of MFCs was 28 mL and the cathode had the projected area of 7 cm2. The anode was made of a graphite fiber brush that both length and diameter were 2.5 cm. A high conductive and corrosion resistive titanium wire was used to connect the anode and cathode to

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assemble an external circuit.31 The cell was inoculated with the mixture of the well-run MFCs effluent, phosphate buffer saline solution (PBS), and 1.0 g L–1 of glucose. In each cycle (approximately 2–3 d), the electrolyte was refreshed when the voltage decreased under 100 mV.29 After the fourth or fifth cycle, the electrolyte was changed to the mixture of PBS and glucose (1.0 g L–1). The external loaded-resistance was 1000 Ω. The operation temperature of all of the MFCs was kept at 30 oC and the output voltage was recorded at each minute by a data acquisition card. The operation time of MFCs was lasted for approximately 80 d.

Materials characterization Powder X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer using Cu Kα1 radiation to analyze the crystalline phases. Morphology and nanoscale structure of all synthesized catalysts were determined by transmission electron microscopy (TEM) and scanning electron microscope (SEM), respectively. The chemical states of the catalysts were analyzed by using X-ray photoelectron spectroscopy (XPS). The surface area of the catalysts was obtained by using the Brunauer-Emmett-Teller (BET) technique at –195.8 oC and the pore size distribution was obtained from the N2 adsorption/desorption isotherms using the Barrett-Joyner-Halenda (BJH) method.

Electrochemical measurements Electrochemical measurements were performed on a CHI760E electrochemical working station, and all of the measurements were conducted at room temperature. 50 mM PBS was used as the electrolyte. The cyclic voltammetry (CV) test was performed in a three-electrode system where an Ag/AgCl electrode was used as the reference, a polished platinum sheet (1 cm2) electrode was used as the counter electrode and a glassy carbon disk electrode with a diameter of 0.4 cm was employed as the working electrode. To prepare modified-GC electrode ink, the prepared catalyst power (5 mg) was well-dispersed in a mixture of 100 mL of ethanol and 50 mL of Nafion by ultrasonic dispersion

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for 30 min. Then 5.0 µL of suspension was dropped onto the GC disk surface and dried in the air. The potential range of CV test was from –0.8 to +0.3 V (vs Ag/AgCl) with a scan rate of 50 mV s–1.32 The linear sweep voltammetry (LSV) was performed over the potential range from +0.9 to –0.3 V (vs Ag/AgCl) at a scan rate of 1.0 mV s–1. The cathode of MFCs, a platinum plate with an area of 1.0 cm2 and an Ag/AgCl electrode were employed as the working, counter and reference electrodes, respectively. To further evaluate the ORR performance of catalysts, LSV was carried out on a rotating disk electrode with different rotating speeds from 225 to 2025 rpm in an oxygen saturated electrolyte. The applied potential range was from +0.6 to –1.0 V (vs Ag/AgCl) with a scan rate of 5.0 mV s–1. The electron transfer number (n) of per oxygen molecule can be calculated from the Koutecky-Levich (K-L) equation.5 Tafel plots of the cathodes were measured at the open circuit voltage (OCP) in a three-electrode system with a scan rate of 1mV s–1 to obtain the exchange current density (j0, A cm –2).33 Electrochemical impedance spectroscopy (EIS) was carried out at the original OCP with a wide frequency range from 100 kHz to 10 mHz after 80 d operation.29 The values of charge transfer resistance (Rct), ohm resistance (Ro) and Warburg resistance (W) were obtained by fitting the equivalent circuit (EC). The polarization and power density curves were acquired by varying the external resistance from 5000 to 50 Ω. The potassium dichromate oxidation method was used to test the chemical oxygen demand (COD) of effluents.28 The coulombic efficiency (CE) was calculated according to the previously reported equation.32

RESULTS AND DISCUSSION The structure and chemical character of Ce-species/NSCs The crystalline phases of the prepared samples carbonized at different temperatures are investigated. As shown in Fig. 1a, no diffraction peaks can be detected in the XRD pattern of Ce-species/NSC-800. The broad peaks at around 2θ= 28.5˚, 33.0˚, 47.4˚, 56.3˚, and 59.0˚ are assigned to the (111), (200), (220), (311), and (222) lattice planes of CeO2 NPs in 8


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Ce-species/NSC-850, respectively. The six diffraction peaks centered at 2θ= 25.8˚, 29.0˚, 37.0˚, 45.5˚, 47.8˚, and 53.3˚ are assigned to the (100), (101), (102), (110), (103), and (112) lattice planes of Ce2O2S NPs in Ce-species/NSCs-x (x=900 and 950), respectively. Stronger diffraction intensity of (101) lattice plane of Ce2O2S is observed for Ce2O2S/NSC-950, suggesting that it has a higher crystallinity degree than that of Ce2O2S/NSC-900. There are no other detectable impurities peaks in XRD patterns, suggesting the high purity of Ce2O2S and CeO2 NPs. Furthermore, the CeO2 and Ce2O2S feature a cubic structure of Fm-3m space group (JCPDS, No.33-1002) and a trigonal structure of P-3m1 space group (JCPDS, No.26-1085), respectively. According to the full-width at half-maximum of XRD patterns and the Debye-Scherrer equation34, the average grain sizes of Ce2O2S and CeO2 NPs are approximately 13.4 and 5.3 nm, respectively. Note that the crystalline phase transition between CeO2 and Ce2O2S may be attributed to the reduction of Ce4+ to Ce3+ induced by carbon monoxide and/or sulfion during the pyrolysis process.

XPS is employed to further probe the different binding environment of N, S, C and O atoms and the valence information of Ce in Ce-species/NSCs. The survey spectra of Ce-species/NSCs show a series of peaks corresponding to the S 2p, C 1s, N 1s, O 1s and Ce 3d (Fig. S1a, Supporting Information), indicating that N and S atoms have successfully doped into the carbon skeleton. The percentage contents (wt. %) of these atoms (Ce, O, C, S and N) in Ce-species/NSCs are shown in Table S1. As temperature increases, the content of C increases gradually, which may be ascribed to the release of some small molecules including some N/S-species and O-groups from the carbon frameworks.35 Note that as carbonization temperature increases, the content of Ce decreases obviously. It is attributed to that a part of Ce-species on the surface are gradually wrapped (embedded) in the deeper carbon skeleton as the temperature increases.

As shown in Fig. 1b and Fig. S1(b-e), the high-resolution of Ce 3d spectra can be resolved into five

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pairs of spin-orbit peaks, which are related to the different states of Ce (Ce3+ and Ce4+) as previously reported.14 The transition between Ce4+ and Ce3+ in Ce-species/NSCs is related to the hybridization orbit between Ce 4f and O 2p.36 The reduction of Ce4+ to Ce3+ is usually accompanied by the simultaneous release of oxygen, which is conductive to increase the oxygen concentration on the catalyst surface.12 The spin-orbit split doublet of v and u represents a main peak (Ce 3d5/2) and a weak satellite peak (Ce 3d3/2), respectively. The peaks of v (882.7±0.2 eV), v'' (887.7±0.1 eV), v''' (898.05±0.3 eV), u (900.8±0.4 eV), u'' (906.3±0.3 eV), and u''' (916.3±0.3 eV) are attributed to the different states of Ce4+, while the peaks of v0 (880.8±0.3 eV), v' (885.2±0.1 eV), u0 (899.5±0.4 eV ), and u' (903.7±0.3 eV) are attributed to the two possible states of Ce3+.34 The v'' of Ce2O2S/NSC-950 at lower binding energy (887.05 eV) may result from the change of chemical binding environment around Ce. Note that the Ce3+ concentration is closely correlated to the oxygen vacancies. It is reported that the high content of Ce3+ may contribute to the formation of more oxygen-deficient Ce3+ sites that generate more oxygen vacancies to release/store more oxygen for oxygen reduction on the catalyst.14 The concentration of Ce3+ can be determined by the following equations (1)–(3).34 As shown in Table 1, the calculated percentage contents of Ce3+ in Ce2O2S/NSC-x (x=900 and 950) are 32.77 and 33.42 %, respectively, which are higher than those of Ce-species/NSC-x (x=800, 850 and 1000). Ce3+= v0+ v'+ u0+ u'

(1)

Ce4+= v +v'' +v''' +u +u''+ u'''

(2)

Ce 3+ [Ce ]= Ce 3+ + Ce 4 +

(3)

3+

The oxygen peaks can be used as another way to evaluate the Ce valence states. The O 1s spectra can be fitted into two main peaks (Fig. 1c and Fig. S2). The peak centered at low binding energy (529.3 ± 0.2 eV) is assigned to the lattice oxygen (Oα), while the peak at high binding energy (531.4 ± 0.2 eV) is attributed to the C−O (oxygen vacancy (Oβ)).34,37 As shown in Table 1, the NSC-800 10


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displays the lowest content of oxygen vacancy (69.13 %). The maximum percentage content of oxygen vacancy is obtained by CeO2/NSC-850 (78.38 %), which is higher than those of Ce-species/NSC-x (x=800, 900, 950 and 1000). These results are not consistent with the Ce3+ contents in Ce 3d spectra, which can be attributed to that the incorporation of S2– (i.e. the substitution of O2–) in Ce2O2S should decrease the oxygen vacancy content.

As shown in Fig. 1d and Fig. S3, the two peaks at around 161.2 and 162.1 eV correspond to the S 2p3/2 and S 2p1/2, respectively, which are attributed to the presence of Ce3+−Sx (Ce−S) bonds in Ce2O2S/NSC-x (x=900 and 950). The formation of Ce−S bonds indicates that a part of Ce3+ is bound to S atoms rather than O atoms, which should decrease the oxygen vacancy content. The peaks centered at 161.3 and 162.6 ± 0.2 eV are assigned to the S2– in CeO2/NSC-x (x=850 and 1000), which may originate from the residues in precursor.38 The two peaks at around 163.7 and 164.7 ± 0.1 eV are assigned to the thiophene-like S (C−S−C).24,39 The formation of thiophene-like S (C−S−C) conforms that the S atoms have successfully doped into the carbon edges.24 It has reported that S-doped carbon skeletons can induce the structural defects due to its larger radius than either N or C, which can act as the highly active sites for ORR.25 The two peaks at 167.3 ± 0.2 and 168.7 ± 0.2 eV correspond to the oxidized S.40 The high content of oxidized S (C−SOx−C) may originate from the adsorption of oxygen at the active sites.41 No peaks of S 2p3/2 and/or S 2p1/2 are found in NSC-800, demonstrating that the S atoms primarily bind with carbon and oxygen atoms to form the C−S−C and C−SOx−C at lower temperature (Fig. S3a). These results suggest that the carbonization temperature is a key factor for controlling the phase transformation in Ce-species/NSC.

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As shown in Fig. 1e and Fig. S4, the N1s spectra can be identified to four types of N-species including pyridinic N (397.9 ± 0.2 eV), pyrrolic N (400.3 ± 0.2 eV), graphitic N (401.6 ± 0.3 eV) and oxidized N (403.7 ± 0.2 eV), all of which play an important role in the ORR process.26,39,42 As previously reported, the presence of pyridinic N and pyrrolic N demonstrates that the N atoms have been successfully doped in the carbon skeleton.43 The pyridinic N can polarize the carbon matrix due to its different electronegativity from the adjacent carbon atoms, which facilitates the chemisorption of oxygen molecules.23 The oxidized N at higher binding energy can weak the O−O bond, which is favorable to supply more oxygen intermediates. Moreover, the graphitic N can endow the catalyst with improved conductivity to induce a four-electron (4e−) transfer mechanism.22, 43

It can be predicted that the different N-species contents in Ce-species/NSCs will lead to various

ORR performance. The percentage contents of these N functionalities in Ce-species/NSCs are displayed in Table S2. Pyridinic N and pyrrolic N account for a larger proportion in all of the samples and the oxidized N is absent in Ce-species/NSC-x (x=800, 850 and 1000). The proportions of pyrrolic N and graphitic N decrease obviously in Ce2O2S/NSC-x (x=900 and 950), while the proportions of pyridinic N increase dramatically, indicating that a part of pyrrolic N and graphitic N may convert into the oxidized N. Note that the proportion of pyridinic N in Ce2O2S/NSC-950 is the highest among all of the samples, which may contribute to the better catalytic activity toward ORR.

As shown in Fig. 1f and Fig. S5, the spectra of C 1s can be decomposed into four components including C−C (284.6 eV), C−S (285.1 ± 0.2 eV), C−N (286.5 eV) and C−O (288.8 eV), which once again confirms that both N and S atoms are simultaneously doped into the carbon skeleton.24, 44

Furthermore, the bond located at the low binding energy (around 282.5 eV) is attributed to the C

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atoms bound in carbide-like environments in Ce2O2S/NSC-950.27

The pore size and morphology of Ce-species/NSCs To explore the porosity properties of the samples, BET specific surface area (SSA) is measured by the N2 adsorption-desorption technique. As shown in Fig. S6, all of the isotherms exhibit a type Ⅳ pattern with an obvious H3 hysteresis loop, indicating that the pore volume is mainly originated from mesopores in Ce-species/NSCs. The peak positions and peak sharps in pore size distribution curves (insets of Fig. S6) for all of the samples also suggest the mesoporous structure nature of Ce-species/NSCs. The more detailed information of SSA, average pore width and total pore volume are listed in Table S3. Ce2O2S/NSC-950 shows the highest SSA of 338.9 m2 g–1, which is higher than those of other samples. Note that the SSA of Ce2O2S/NSC-900 (333.8 m2 g–1) is nearly equal to that of Ce2O2S/NSC-950, which should be attributed to the similar average pore size and total pore volume. As the carbonization temperature increases, all of the SSA displays a rising trend while the SSA of CeO2/NSC-1000 decreases sharply. At 1000 oC, the collapse/closing of mesopores structure may be happened to generate the larger pores, which correspondingly decreases the SSA. The decrease of SSA may result in the loss of some surface groups with promising functionality in the pore surface. Note that the larger SSA and more porous structure of Ce2O2S/NSC-950 and Ce2O2S/NSC-900 are of great importance for improving the ORR activity by affording more active sites for the effective adsorption and dissociation of oxygen molecules on the catalyst surface, and favoring more efficient mass transport of ORR-relevant species including H2O2, OH– and •OH radicals.25 Furthermore, more C atoms on the porous frameworks can be effectively substituted by the particular types of N and/or S atoms that are responsible for ORR activity. Therefore, the

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Ce2O2S/NSC-950 and Ce2O2S/NSC-900 may have superior catalytic activity to others.

The morphologies of Ce-species/NSCs are investigated by SEM measurements. As shown in Fig. 2, all of the catalysts exhibit the rough surface and porous structure. The particles varied in sizes and shapes are stacked on the surface of Ce-species/NSCs-x (x=800, 850, 900 and 950). The overlap and aggregation of particles on the surface of CeO2/NSC-1000 may block off the pore channels. It should be noted that Ce2O2S/NSC-950 with some randomly-distributed Ce2O2S NPs exhibits many well-formed mesopores with a relatively uniform size of approximately 15 nm. The porous structure of Ce2O2S/NSC-950 can facilitate the fast electron transfer and the more oxygen transport to the active sites, which are essential for ensuring a good diffusion of the ORR-relevant reactants. This structure may also contribute to the low mass transport resistance and the exposure of more active sites, both of which are conducive to improve the catalytic ORR activity.35

Electrocatalytic activity of Ce-species/NSCs in neutral solution To investigate the electrocatalytic ORR activity, the well-dispersed catalysts (commercial Pt/C and Ce-species/NSC) are dropped onto a glassy-carbon electrode for CV tests. As shown in Fig. 3a, the maximum current density is obtained by Ce2O2S/NSC-950 (–9.03 mA cm–2) and the second is Ce2O2S/NSC-900 (–8.52 mA cm–2), both of which are higher than that of commercial Pt/C (–7.74 mA cm–2). The NSC-800 has the lowest current density of –2.43 mA cm–2. The higher catalytic activity of Ce2O2S/NSC-x (x=900 and 950) than those of CeO2/NSC-x (x=850 and 1000) is partly attributed to the presence of Ce2O2S, indicating that Ce2O2S should have better electrochemically catalytic activity for ORR than that of CeO2. Note that the inferior performance of CeO2/NSC-x (x=850, 1000) may be also caused by the lower SSA and pore volume, which cannot afford sufficient pathways in the pore structure to facilitate the oxygen transport to the active sites.3 14


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LSV is also measured to evaluate the natures of these catalysts towards ORR. As shown in Fig. 3b, the positive order of current density for Ce-species/NSCs is exhibited as follows: Ce2O2S/NSC-950 (–5.72 mA cm–2) > Ce2O2S/NSC-900 (–5.36 mA cm–2) > commercial Pt/C (–4.94 mA cm–2) > CeO2/NSC-850 (–3.71mA cm–2) > CeO2/NSC-1000 (–3.49 mA cm–2) > NSC-800 (–2.06 mA cm–2), consistent with the CV results. Both CV and LSV results confirm that Ce2O2S/NSC-x (x=900 and 950) show superior ORR activity to other catalysts in terms of their high current density. The high ORR activity of Ce2O2S/NSC-x (x=900 and 950) can be partly ascribed to their good conductivity (the existence of graphitic N) of N, S dual-doped carbon supports, which can offer the electrons an easy access to the active sites.45 The functional Ce2O2S NPs should also play an important role in improving their ORR performance. Furthermore, the synergistic effects between N, S dual-doped carbon skeleton and Ce2O2S NPs are also conductive to supply more adherences for oxygen to couple with H+ and electrons, which can reduce the kinetics limitation for ORR.

Performance of MFCs with Ce-species/NSCs and Pt/C cathodes The single chamber air-cathode MFCs equipped with Ce-species/NSCs and commercial Pt/C cathodes are operated with the same anode for more than 1900 h. The output voltages of MFCs with different cathodes are shown in Fig. S7. The CeO2/NSC-850 cathode has a shorter start-up time, while its stability and durability is inferior to those of Ce-species/NSC-x (x=800, 900, 950 and 1000) cathodes. The highest output voltage (average) is achieved by Ce2O2S/NSC-950 cathode with the maximum voltage of approximately 0.57 V, followed by the Ce2O2S/NSC-900 cathode with the maximum voltage of approximately 0.556 V, both of which are higher than that of commercial Pt/C

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(approximately 0.55 V). The output voltages of Ce2O2S/NSC-x (x=900 and 950) almost have no decline after 1900 h, which are far more durable than those of Ce-species/NSC-x (x=800, 850 and 1000). It should be noticed that the output voltage of commercial Pt/C is stable in the 500 h and then deteriorates severely, which may be caused by the poison of sulfides and the attachment of biofilm on its surface.32

Power density and polarization curves of MFCs are measured when the cell voltage is stable (marked as the initial cycle). The initial power densities of all of the MFCs are illustrated in Fig. 4a. The Ce2O2S/NSC-950 cathode obtains the highest power density of 1087.20 mW m–2, which is higher than that of commercial Pt/C (989.31 mW m–2). Summary of the performances of MFCs with Ce2O2S/NSC-950 and other closely-related cathodes are shown in Table S4.46–48 To be more specific, the maximum power densities obtained by Ce2O2S/NSC-900 (1054.18 mW m–2), CeO2/NSC-850 (891 mW m–2), CeO2/NSC-1000 (817.61 mW m–2) and NSC-800 (712.4 mW m–2) cathodes present the same trend with the output voltage in Fig. S7. It can be inferred that the different performance should be related to the different activities of cathode catalysts. As shown in Fig. 4b, the potentials of all of the anodes are varied slightly, while the potentials of all of the cathodes are different from each other, suggesting that the anode has little influence on the overall performance.49 These results fully prove that the output performance of MFCs is greatly influenced by the ORR activity of cathode catalysts.29

To get a better insight into the durability of MFCs for long-term operation, the power density and polarization curves are measured after approximately 80 d (marked as the final cycle). As shown in

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Fig. 4c, the declines of the maximum power densities of the cathodes are different. It is noticeable that the power density of Ce2O2S/NSC-950 cathode (997.30 mW m–2) is still higher than those of other cathodes, which only declines 8.3 % after 80 d operation. The maximum power densities of Ce-species/NSC-x (x=800, 850, 900 and 1000) cathodes are 584.34, 736.77, 944.03 and 703.06 mW m–2, which correspondingly decline 18.0, 17.31, 10.45 and 14.0 %, respectively. However, the maximum power density of commercial Pt/C declines 36.7 %, indicating that the Ce-species/NSCs exhibit the superior stability and durability to commercial Pt/C. The power density should be negatively influenced by the coverage of the gradually-formed biofilm on the Ce-species/NSCs cathode surface during the operation.32 The biofilm on cathode surface can block the mass/electron transfer pathways and inactivate the surface active sites to suppress the catalytic activity.7 The average COD removal rate (93.9 %) and CEs (20.96 %) of MFCs with Ce2O2S/NSC-950 cathode are higher than those of commercial Pt/C (88.5 and 20.08 %) and other Ce-species/NSC-x (x=800, 850, 900 and 1000) cathodes (Fig. 4e and Table S5). This reflects that the Ce2O2S/NSC-950 possesses the highest ability for recycling electrons (high CEs) because of its high electrical conductivity and catalytic ORR activity.32

As shown in Fig. 4f (Table S5), the superior stability and durability of Ce-species/NSCs is inseparable with the active constituents in their structures. The high activity and durability of Ce-species/NSCs may be ascribed to the synergistic effects between pyridinic N and the oxygen vacancy that guarantees the sufficient oxygen supply on active sites. The pyridinic N embedded in the carbon skeleton can provide abundant active sites to support the chemisorption of O2, which ensures the sufficient oxygen concentration on the catalyst surface during ORR.35 Furthermore, the

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large proportion oxygen vacancy in the active components (Ce2O2S/CeO2) can store the oxygen in the suitable environment and release/supply the oxygen in the harsh environment. The biofilm (considered as “harsh environment”) formed on the catalyst surface will hinder the oxygen transport to the active sites and the bacteria in the biofilm may compete the oxygen with the catalysts. As mentioned above, the sufficient oxygen will be efficiently used by Ce-species/NSC catalysts to suppress the overgrowth of bacteria and reduce the ORR overpotential.

EIS analysis is carried out to obtain the various resistances of the cathode reaction.50 An equivalent circuit for the electrode is used to explain the EIS results. Rct, Ro and W are obtained by fitting the data in Nyquist plots (Fig. S8 and Table S6). Previous studies have shown that the low Rct is conductive to accelerate the electron transfer, which contributes to a higher output voltage.33 The Rct of Ce2O2S/NSC-x (x=900 and 950) is lower than that of commercial Pt/C, which can be attributed to their superior ORR activity and electrical conductivity to Pt/C. The low Rct of Ce2O2S/NSCs should be partly ascribed to that the pyridinic N and graphitic N in the carbon skeleton can facilitate the electrons delivery. The large surface area and porous structure of catalyst can provide more electrocatalytic active sites and mass transfer channels for enhancing the diffusion of H+ and O intermediates.42,43 In addition, the differences of Ro among different cathodes are minor due to the slight change of cathodic composition (GDL, CL and stainless steel mesh).33 The W (concentration loss) is originated from the changeable concentration of the reactants on the surface of electrode, which is much less than the R0 and Rct, indicating that the concentration loss does not exert an important influence on the performance of MFCs.45

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The detailed structure and ORR pathway of Ce2O2S/NSC-950 TEM is used to further explore the detailed structure of Ce2O2S/NSC-950 for its excellent performance in MFCs. As shown in Fig. S9a, the diameters of Ce2O2S NPs are calculated to be 10–15 nm (the inset of Fig. S 9a), in agreement with the XRD results. Abundant Ce2O2S NPs are well distributed on carbon matrix, which can be served as oxygen suppliers to boost the oxygen transfer to accelerate the oxygen reduction rate. Fig. S 9b shows the high-resolution TEM images of Ce2O2S/NSC-950, which clearly confirms that some Ce2O2S NPs are successfully embedded in the carbon matrix. The lattice fringe distance is 0.307 nm, which corresponds to the (101) plane of Ce2O2S NPs. As shown in the inset of Fig. S9b, the (101) lattice plane of Ce2O2S is further confirmed by the diffraction rings in the image of the selected area electron diffraction (SAED). These results together verify the homogeneous distribution of closely interconnected Ce2O2S (as active sites) on N, S dual-doped carbon skeleton.

As shown in Fig. 5 (a and b), LSV tests are conducted on a rotating disk electrode with different rotating speeds from 225 to 2050 rpm to obtain the kinetics parameters of Ce2O2S/NSC-950 and commercial Pt/C. The electron transfer number (n) is calculated by the K-L equation to assess the catalyst selectivity (4e− or 2e−) towards ORR.51 As shown in Fig. 5c, at the potentials of −0.3, −0.4 and −0.5 V, the K-L plots display good linearities, which demonstrate the first-order reaction kinetics with respect to the concentration of dissolved O2.5 The n values at −0.3, −0.4 and −0.5 V (vs Ag/AgCl) are 3.16, 3.40 and 3.72, respectively, which are comparable to those of commercial Pt/C. This result implies that the Ce2O2S/NSC-950 follows a 4e− process toward ORR where O2 is directly reduced to H2O or OH− with little intermediates.52 The well-dispersed Ce2O2S NPs

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embedded in the carbon matrix can not only generate different active centers to contribute to an efficient 4e− ORR process, but also bind to the ORR intermediates moderately to obtain a low overpotential.16 Its well-ordered mesoporous structure can give the electrons and H+ more access to the active sites where the reduction of oxygen is significantly promoted. Furthermore, the co-doping of N and S into the carbon skeleton can induce more useful structural-defects (active sites) to facilitate a 4e− reduction pathway.39 The existence of pyridinic N and thiophene-like S can polarize the carbon matrix to facilitate the charge dislocation, which correspondingly endows the carbon matrix with high activity for adsorption and activation of oxygen.25

The information of exchange current density (j0) and the rate of the redox reaction are obtained by the Tafel curves. The j0 is mainly employed to evaluate the catalytic activity of the Ce2O2S/NSC-950 and commercial Pt/C that can disclose the intrinsic transfer rate of electron between electrolyte and electrode.33 As shown in Fig. 5d, the Tafel lines display a subdued and linear trend after a steep increase. The good linearities of Tafel regression are exhibited in the overpotential range from 80 to 100 mV. As presented in Table S7, the j0 of Ce2O2S/NSC-950 is 0.54 × 10−5 A cm−2, which is higher than that of commercial Pt/C (0.47 × 10−5 A cm−2), illustrating that the fast reaction rate and the high efficiency of the electron utilization are obtained by Ce2O2S/NSC-950 during the ORR process.53 The uniform distribution of N and S species in the carbon matrix effectively hampers the aggregation and overlap of Ce2O2S NPs to contribute to the resistance-less pathways, which are readily applicable for the fast electron transfer.

The ORR mechanisms of Ce2O2S/NSC catalysts

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High efficiency of the adsorption, dissociation and reduction of oxygen is a decisive factor to determine the catalytic activity of catalysts. As shown in Scheme1, the oxygen molecules diffuse to the catalyst surface to form the absorbed oxygen molecules; meanwhile the O intermediates are generated from the O–O bond breaking. Then, the formed O intermediates can be successfully reduced to H2O.16 For the Ce2O2S/NSC-950, the abundant structural defects (active sites) in NSCs caused by dual-doped N, S atoms and the sufficient oxygen vacancy in the structure of Ce2O2S are conductive to absorb the oxygen molecules.25,26 The adsorption of O intermediates on the catalyst surface can be considered as the origin of overpotential, meanwhile the high binging energy between the O intermediates and surface active sites can also hinder the ORR process. Thus, it is reasonable to consider that a favorable catalyst should have the moderate binding energy to couple with the O intermediates, which can compromise the reaction obstacle during the adsorption, dissociation and reduction processes.8,51 According to the ORR measurements, the Ce2O2S/NSCs have the superior ORR activity to those of CeO2/NSCs, proving that the Ce2O2S/NSCs may possess the moderate binding energy to adsorb O-species, which indirectly highlights the importance of appropriate Ce3+-deficient active sites. Based on the XPS results, the binding energy of the main peak (V0) of Ce3+ in Ce2O2S/NSC-x (x=900 and 950) is lower than that of Ce3+ in CeO2/NSC-x (x=850 and 1000), which may be ascribed to the higher electronegativity of the O atom compared to the S atom. The high binding energy usually results in a strong adsorption of O intermediates and a difficult desorption of H2O, which in turn corrode the active sites (oxygen vacancy) for further adsorbing oxygen. Thus, the CeO2/NSC-x (x=850 and 1000) catalysts have a lower ORR activity.16

CONCLUSION In summary, we have synthesized the novel Ce-species/NSC catalysts by using a one-step in-situ 21



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reduction approach and tuning the carbonization temperatures to explore the mechanism of the crystalline phase transition. The increase of carbonization temperature leads to the transition of crystalline phase from CeO2 to Ce2O2S. Ce2O2S/NSC-950 displays the superior ORR activity to that of commercial Pt/C in terms of its higher current densities in CV (–9.03 mA cm–2) and LSV (–5.72 mA cm–2) tests and lower Rct (5.49 Ω), ensuring the viability of the as-prepared catalyst. The origin of the promising ORR activity of Ce2O2S/NSC-950 is ascribed to its moderate binding energy that leads to the adsorption of O-species in an opportune way to reduce the overpotential. The well-ordered mesoporous structure, abundant active sites including the structural defects originated from the N, S dual-doping and sufficient oxygen vacancy also contribute to the higher ORR activity of Ce2O2S/NSC-950. Moreover, Ce2O2S/NSC-950 as the MFCs cathode catalyst shows higher power density (1087.20 mW m–2) and durability (decline of 8.7 % after 80 d) than those of commercial Pt/C (989.31 mW m–2 and decline of 36.7 %). More importantly, the Ce-species/NSCs with high ORR catalytic activity and remarkable operation durability may pave a new avenue to further develop the application of rare-earth metal oxysulfides, especially for the cathode of MFCs.

ASSOCIATED CONTENT Supporting Information Percentage contents (wt.%) of the atoms (Ce, O, C, S and N) in Ce-species/NSC-x (x= 800, 850,900, 950 and 1000) (Table S1), percentage contents (wt.%) of N-species in Ce-species/NSC-x (x=800, 850, 900, 950 and 1000) (Table S2), pore characteristics of Ce-species/NSC composites (Table S3), the comparison of the performances of MFCs with Ce2O2S/NSC-950 and other closely-related catalysts (Table S4), the maximum power density in initial and final cycles, decline rate of power density, average CEs and COD removal rates, pyridinic N and oxygen vacancy contents of Ce-species/NSC-x (x=800, 850, 900 ,950 and 1000) (Table S5), the electrochemical impedance 22


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fitting results of different cathodes (Table S6), exchange current densities from the linear region of the Tafel plots (Table S7), XPS survey of the Ce-species/NSCs (a) and high resolution XPS of Ce 3d spectra for Ce-species/NSC-x (x=800 (b), 850 (c), 900 (d), and 1000 (e)) (Fig. S1), high resolution XPS of O 1s spectra for Ce-species/NSC-x (x=800 (a), 850(b), 900 (c), and 1000 (d)) (Fig. S2), high resolution XPS of S 2p spectra for Ce-species/NSC-x (x=800 (a), 850(b), 900 (c), and 1000 (d)) (Fig. S3), high resolution XPS of N 1s spectra for Ce-species/NSC-x (x=800 (a), 850(b), 900 (c), and 1000 (d)) (Fig. S4), high resolution XPS of C 1s spectra for Ce-species/NSC-x (x=800 (a), 850(b), 900 (c), and 1000 (d)) (Fig. S5), N2 adsorption/desorption isotherms and pore size distributions (inset) for the Ce-species/NSC-x (x= 800 (a), 850 (b), 900 (c), 950 (d) and 1000(e)) (Fig. S6), the output voltage of MFCs for Ce-species/NSC-x (x = 800 (a), 850 (b), 900 (c), 950 (d) and 1000(f)) and Pt/C (e) cathodes (Fig. S7), Nyquist plots of Ce-species/NSCs and Pt/C cathodes in MFCs (Fig. S8), and TEM (a) and high resolution TEM images of Ce2O2S/NSC-950 (b) (Fig. S9). This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *Phone/Fax: (+86) 451 8660 8549; E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the support by National Natural Science Foundation of China (51578218, 51108162, 21473051 and 51508342), Natural Science Foundation of Heilongjiang Province (QC2015009), Postdoctoral Science Foundation of Heilongjiang Province (LBH-Q14137), 23



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Scientific and technological innovation talents of Harbin (2016RQQXJ119), and Excellent Young Teachers Fund of Heilongjiang University and Hundred Young Talents in Heilongjiang University.

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Table 1 Percentage contents (wt. %) of Ce3+and O-species in Ce-species/NSC-x (x= 800, 850, 900, 950 and 1000). Samples

Ce3+

Lattice oxygen

Oxygen vacancy

NPGC-800

26.72

30.87

69.13

CeO2/NPGC-850

27.65

21.62

78.38

Ce2O2S/NPGC-900

32.77

21.74

78.26

Ce2O2S /NPGC-950

33.42

28.88

71.12

CeO2/NPGC-1000

27.05

22.16

77.84

31



★

b

▲

★

CeO2

▲Ce2O2S

★

★

▲

★

▲

▲

Ce-species/NSC-950

▲ ▲ ▲

▲

▲

▲

Ce-species/NSC-900

★ ★

★

Ce3d

u'''

★

▲

▲

Intensity(a.u.)

Ce-species/NSC-1000

★ Ce-species/NSC-850 ★

Intensity( a.u.)

a

u

V''' V

u' u''

V' V''

u0

V0

Ce-species/NSC-800 10

20

30

40

50

60

70

870

80

880

d

oxygen vacancy lattice oxygen

O 1s

Intensity(a.u.)

Intensity(a.u.)

C

524

526

528

530

532

534

536

538

540

S2p

910

920

C-S-C S2p3/2 S2p1/2

158

160

162

164

166

168

170

172

174

Binding Energy(eV)

f

pyridinic N

N1s

900

C-SOx-C

Binding Energy(eV)

e

890

Binding Energy(eV)

2Theta degree

C1s

Intensity( a.u.)

pyrrolic N

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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graphitic N oxidized N

392

394

396

398

400

402

404

406

408

C-C C-S C-N

280

282

284

286

C-O

288

290

Binding Energy(eV)

Binding Energy(eV)

Fig. 1 XRD patterns of Ce-species/NSC-x(x=800, 850, 900, 950, 1000) (a); High resolution XPS spectra of Ce 3d (b), O 1s (c), S 2p (d), N 1s (e) and C 1s (f) in Ce-species/NSC-950.

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Fig. 2 SEM images of the Ce-species/NSCs-x (x=800 (a), 850 (b), 900 (c), 950(d) and 1000 (e)).

33



Current Density(mA cm-2)

a9 6 3 0 -3

CeO2/NSC-1000 Ce2O2S/NSC-950 Ce2O2S/NSC-900 CeO2/NSC-850 NSC-800 Pt/C

-6 -9 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential (V vs Ag/AgCl)

b0 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

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-1

CeO2/NSC-850 NSC-800 Pt/C

CeO2/NSC-1000 Ce2O2S/NSC-950 Ce2O2S/NSC-900

-2 -3 -4 -5 -6 -0.2

0.0

0.2

0.4

0.6

0.8

Potential (V vs Ag/AgCl) Fig. 3 CV curves (a, under saturated oxygen in 50 mM PBS solution with a scan rate of 1mV s–1 ) and LSV curves (b, under nitrogen-saturated in 50 mM PBS solution at a scan rate of 50 mV s–1) of Ce-species/NSC-x (x=800, 850, 900, 950 and 1000) and Pt/C.

34


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1200

1.0

CeO2/NSC-850 NSC-800

b 0.6

0.9

0.5

Pt/C

0.8

1000

0.7 0.6

800

0.5 600

0.4 0.3

400

0.2 200 0.1 0 0.0

Electrode Potential (V)

Power Density (mW m-2)

a

Voltage(V)

0.5

1.0

1.5

2.0

2.5

3.0

3.5 -2

4.0

CeO2/NSC1000 Ce2O2S/NSC950 Ce2O2S/NSC900

0.3 0.2

cathode

0.1 0.0 -0.1

anode

-0.2 -0.3 -0.5

4.5

0.0

-2

Power Density (mW m )

1200

1.0

CeO2/NSC-850 NSC-800

d 0.6

0.9

0.5

Pt/C

0.8

1000

0.7 0.6

800

0.5 600

0.4 0.3

400

0.2 200 0.1

Electrode Potential (V)


0.5

1.0

1.5

2.0

2.5

3.0

Current density (A

Current Density (A m )

c

CeO2/NSC850 NSC800 Pt/C

0.4

0.0

Voltage(V)


0.4

3.5

4.0

4.5

m-2)


0.3 0.2

cathode

0.1 0.0 -0.1

anode

-0.2 -0.3

0.0

0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-0.5

0.0

-2

Current Density (A m )

f

90

80

80

70

70

60

40


50 40 30

30 20

20

10

10 0

5

10

15

2.0

droop rate(%) pyridinicN(%) average COD remove rate(%) average Coulumbic efficiency(%)

2.5

3.0

3.5

800

14.01

600

33.8

8.27

10.45

17.32

32.7

33.7

42.26

17.98

39.1

400 200 0

20

oxygen vacancy(%) the initial cycle the finial cycle

1000

77.84

0

0

1200

Power Density (mW m-2)

90

Coulumbic efficiency (%)

100

50

1.5

4.0

4.5

Current density (A m )

100


1.0

-2

e 110

60

0.5

1000

Cycle number (n)

71.12

950

78.26

78.38

69.13

900

850

800

100 80 60

40 20

Coulumbic efficiency (%)

0.5

COD removal rate (%)

0.0

COD removal rate (%)

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

X

Fig. 4 Power density and polarization curves of MFCs with Ce-species/NSCs and Pt/C cathodes at the initial (a and b) and final cycles (c and d); CEs and COD removal rates of MFCs with different cathodes (e); the relationship among the initial/final maximum power density, decline rate of power density, average CEs and COD removal rates, pyridinic N and oxygen vacancy content in Ce-species/NSCs (f).

35



b0

Ce2O2S/NSC-950

Current Density (mA cm-2)

Current Density (mA cm-2)

a0 -1

225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm

-2

-3

-4 -0.6

-0.4

-0.2

0.0

-1

Pt/C

-2 -3 -4

225 rpm 400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm

-5 -6 -7 -8 -9 -10

0.2

-0.6

-0.4

0.35

i-1 (cm2 mA-1)

0.30

-0.3 V : n=3.53 -0.4 V : n=3.76 -0.5 V : n=4.02

0.25 0.20

Pt/C

0.15 0.65

0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25

-0.3 V : n=3.16 -0.4 V : n=3.40 -0.5 V : n=3.72

Ce2O2S-950

0.02

0.03 0.04 ω-1/2 (rpm-1/2)

d

-5.0

0.0

0.2

-5.5 -6.0 -6.5 -7.0

-5.05

-5.10

-5.15

-5.20 0.080

-7.5

0.085

0.090

0.095

0.100

overpotential (V vs. OCP)

0.00

0.05

Ce2O2S/NSC-950 Pt/C log|current density| (A cm-2)

0.40

log|current density|(A cm-2)

c

-0.2



A

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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0.02

0.04

0.06

0.08

Overpotential (V vs. OCP)

Fig. 5 RDE tests of the cathodic catalysts with Ce2O2S/NSC-950 (a) and commercial Pt/C (b); Koutecky-Levich plots of Ce2O2S/NSC-950 and Pt/C at the potentials of –0.3, –0.4 and –0.5 V (c); Tafel plots of Ce2O2S/NSC-950 and Pt/C (d) and the linear fit for Tafel plots in the overpotential range from 80 to 100 mV (inset).

36


0.10

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Scheme 1. The mechanism of Ce2O2S/NSC catalysts towards ORR process in MFCs

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Graphic Abstract


Nano Ce2O2S with Highly Enriched Oxygen-Deficient Ce3+ Sites

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