Reprint A Journal of © Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemcatchem.org DOI: 10.1002/cctc.201901084 Full Papers 1 2 3 Facile Synthesis of Fe/N/S-Doped Carbon Tubes as High- 4 Performance Cathode and Anode for Microbial Fuel Cells 5 6 Wei Yang,*[a] Jun Li,*[b] Linghan Lan,[b] Zhuo Li,[b] Wenli Wei,[c, d] Jia En Lu,[c] and 7 [c] 8 Shaowei Chen* 9 10 11 As arenewable energy technology, microbial fuel cell (MFC) has ahalf-wave potential of +0.81 Vvs. RHE and an electron 12 been attracting increasing attention in recent decades. How- transfer number of 3.98 at +0.6 Vvs. RHE. The Fe/N/S-doped 13 ever, practical applications of MFCs has been hampered by the carbon tubes also exhibited aremarkable performance as an 14 unsatisfactory electrode performance, in particular, at the MFC anode by facilitating bacterial growth and electron transfer 15 cathode. Herein, Fe/N/S-doped carbon hollow tubes were between the biofilm and electrode. In fact, an MFC based on 16 prepared by afacile two-stage procedure involving hydro- the carbon tubes as both cathode and anode showed a 17 thermal treatment and pyrolysis at controlled temperatures. markedly higher performance (maximum power density 18 Electrochemical studies showed that the obtained samples 479 WmÀ3)than the control MFC based on agraphene aerogel 19 exhibited an apparent electrocatalytic activity towards oxygen anode and Pt/C cathode (359 WmÀ3). These results suggest that 20 reduction reaction in both alkaline and acidic media, aperform- Fe/N/S-doped carbon composites can be used for the fabrica- 21 ance comparable to that of commercial Pt/C, and the sample tion of high-efficiency MFC electrodes. 22 prepared at 800 °Cstood out as the best among the series with 23 24 Introduction improvement in the performance of air cathode MFC.[3] 25 However, the power output of MFCs remains unsatisfactory 26 Microbial fuel cell (MFC) represents an energy conversion with regards to the need of practical applications. The perform- 27 technology that uses anode microbes to catalyze the oxidation ance of an air cathode MFC has been found to depend on a 28 of organic substrates for electricity generation, and achieves range of factors, such as the electron-transfer rate from bacteria 29 power production and wastewater treatment simultaneously.[1] to anode surface, kinetics of oxygen reduction reaction (ORR) at 30 Of these, because of the simple structure and air breathing the cathode, ion diffusion/migration between anode and 31 design, air cathode MFC has been attracting extensive attention cathode, substrate supply in biofilm, and ion diffusion in 32 in recent years.[2] In fact, significant progress in material cathode catalyst layer.[4] Among these, the construction of 33 synthesis, electrode design and operation parameter optimiza- anode and cathode stands out as the most important factor 34 tion has been achieved and contributed to anoticeable influencing the MFC performance. In particular, the sluggish 35 ORR kinetics at the cathode has been recognized as aprimary 36 bottleneck that limits the MFC performance.[5] Platinum-based 37 [a] Dr. W. Yang catalysts have been widely used to facilitate the ORR process, 38 State Key Laboratory of Hydraulics and Mountain River Engineering but their high cost, scarcity and low durability hinder their 39 College of Water Resource &Hydropower Sichuan University practical utilization.[6] Recently, carbon doped with select 40 Chengdu 610065 (P. R. China) (transition) metal and nonmetal elements has demonstrated 41 E-mail: [email protected] high ORR activity towards ORR, and can be exploited as viable 42 [b] Prof. J. Li, L. Lan, Z. Li Institute of Engineering Thermophysics alternatives to platinum.[7] Of these, ternary iron and nitrogen- 43 School of Energy and Power Engineering doped carbon (Fe/N/C) is considered as one of the most 44 Chongqing University attractive candidates for fuel cell application, due to their 45 Chongqing 400030 (P. R. China) E-mail: [email protected] remarkable ORR activity in both acid and alkaline solutions,[8] 46 [c] Dr. W. Wei, Dr. J. E. Lu, Prof. Dr. S. Chen where the catalytic active sites are generally believed to arise 47 Department of Chemistry and Biochemistry from the FeN moieties and nitrogen dopants in the carbon 48 University of California x Santa Cruz CA-95064 (USA) matrix.[9] In addition, it has been shown that sulfur doping of 49 E-mail: [email protected] the Fe/N/C catalysts can further enhance the ORR activity, 50 [d] Dr. W. Wei where the half-wave potential (E1= )can be increased by as 51 College of Chemistry and Chemical Engineering 2 Lanzhou University much as 0.3 V.[10] Experimentally, S-doping of Fe/N/C catalysts is 52 Lanzhou 730000 (P. R. China) typically achieved by asequential process which entails 53 Supporting information for this article is available on the WWW under pyrolyzing nitrogen-containing precursors to facilitate Ndop- 54 https://doi.org/10.1002/cctc.201901084 ing, followed by mechanical mixing of metal salts and sulfur 55 This manuscript is part of the Special Issue “Electrocatalysis: From Batteries precursors to incorporate metal and sulfur dopants.[11] This 56 to Clean Energy Conversion”, which is part of the wider project “Building A New Energy Economy with Catalysis”. method is rather tedious and unfavorable for homogenous 57 ChemCatChem 2019, 11,6070–6077 6070 ©2019 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Papers dispersion of the active sites.[12] Afacile approach is desired for 1 the preparation of S-doped Fe/N/C. 2 On the anode, it has been found that the dissimilatory 3 metal-reducing bacteria Geobacter species and Shewanella 4 oneidensis can use Fe3+ as electron acceptors, and utilize the Fe 5 3d electrons as along-distance electron transfer conduit.[13] 6 Interestingly, previous studies have shown that the Geobacter 7 species grows apparently, concurrently with increasing Fe3+ 8 reduction, suggesting that the introduction of Fe3+ may 9 stimulate the bacteria growth on the anode surface.[14] In 10 addition, it has been shown that elemental sulfur can mediate 11 electron-shuttling during iron reduction by dissimilatory metal- 12 reducing bacteria.[15] Therefore, one can expect that aS-doped 13 Fe/N/C carbon anode may facilitate bacteria growth and 14 electron transfer. Another factor affecting the anode perform- 15 ance is the space accommodation of bacteria growth, proton 16 transport and substrate supply.[14b] Three-dimensional graphene 17 aerogel (GA) with ahierarchical porous structure has been used 18 extensively as an anode material, and substantial progress has 19 been made in enhancing the efficiency of current generation.[16] 20 However, the performance of GA alone remains unsatisfactory, 21 due to the limited growth and cultivation of bacteria in the 22 interior of the anode. Figure 1. (a) Schematic illustration of the preparation procedure of carbon 23 tubes. (b) Bright-field and (c) dark-field TEM images, (d) high resolution TEM Herein, we prepared well-defined S-doped Fe/N/C carbon 24 images, (e) electron diffraction patterns and element mapping images of CT- tubes as electrode materials for both MFC cathode and anode 800. 25 by using aone-pot synthesis method. The obtained nano- 26 composites showed an apparent ORR activity. In addition, the 27 carbon tubes were used as astructural scaffold to fabricate a 28 three-dimensional carbon tube/graphene oxide aerogel anode, by the diffusive electron diffraction patterns (Figure 1e). In 29 where the abundant macropores were found to facilitate elemental mapping analysis, the elements of Fe, N, S, Cand O 30 bacteria growth, electrolyte transport and electron transfer. An can be seen to be distributed homogeneously within the 31 MFC using the obtained S-doped Fe/N/C as both cathode and carbon tubes (Figure 1e). In conjunction with the absence of 32 anode achieved apower density that was markedly higher than any nanoparticle in the samples, this suggests that Fe was most 33 that of acontrol based on agraphene aerogel anode and Pt/C likely atomically dispersed within the carbon matrix. 34 cathode. The structures of the carbon tube samples were further 35 probed by XRD measurements. From Figure 2a, it can be seen 36 that all samples display only two broad peaks at 2 =24° and 37 θ Results and Discussion 44° that are consistent with the (002) and (101) diffractions of 38 graphitic carbon; and the broad peaks suggest alow degree of 39 The preparation procedure of carbon tubes is schematically crystallinity of the samples. For the pre-pyrolysis CT-pre sample, 40 shown in Figure 1a. Specifically, methyl orange and FeCl two major sharp diffraction peaks can be identified at 17.45° 41 3 reacted and produced fibrillar complexes, which served as and 20.10°,most likely due to FeO formed by hydrolysis of 42 x structural templates for pyrrole polymerization, and hollow FeCl .[17] However, these diffraction features vanished after 43 3 nanotubular structures were formed due to the self-degradation pyrolysis at elevated temperature, indicating the absence of 44 of the fibrillar templates. In this process, the Fe species and metal/metal oxide nanoparticles in the samples, in good agree- 45 methyl orange were adsorbed into the polypyrrole tubes, and ment with results from TEM measurements (Figure 1). In 46 controlled pyrolysis at elevated temperatures led to the addition, it can be noticed that the carbon (002) diffraction 47 incorporation of Fe, Nand Sinto the carbon framework. SEM peaks shifted from 23.9° to 24.5° from CT-600 to CT-900, 48 measurements showed that the obtained samples indeed implying ashrinking interlayer spacing, most likely due to an 49 exhibited afibrillar structure with adiameter of 150–200 nm increasing loss of N, Sand Fe dopants with the increase of 50 and length of 5–10 m(Figure S1); and in both bright-field and pyrolysis temperature (vide infra); meanwhile, the carbon (101) 51 μ dark-field TEM studies (Figure 1b–c), the samples can be seen to diffraction peak became intensified, suggesting improved 52 exhibit ahollow tubular structure.