CeO₂ doped Pt/C as an efficient cathode catalyst for an air-cathode single-chamber microbial fuel cell

Abstract

Incorporation of nanophase ceria into the cathode catalyst Pt/C was used as alternative cathode catalysts for the oxygen reduction reaction in an air-cathode single-chamber microbial fuel cell (SCMFC) for the first time. Electrochemical results reveal that the ceria doped Pt/C catalysts exhibited a higher catalytic performance than the pure Pt/C catalyst. The highest voltage of 580 mV and the maximum power density of 840 ± 24 mW m⁻² were achieved from the 3 wt% CeO₂ doped Pt/C cathode, which was larger than that of the pure Pt/C cathode.

---

Introduction

The microbial fuel cell (MFC) is a recently developed promising technology for energy recovery, which can generate electricity from wastewater or biomass through a microbial metabolic process.^1–3 Several types of MFCs have been developed, including two-chamber, single-chamber, up-flow, flat, and tubular designs.^4–8 Electricity generation has been demonstrated from glucose, organic acids, domestic, animal wastewaters, and sewage sludges, etc.^9–13 Among the various MFCs, the membraneless air-cathode single-chamber microbial fuel cell (SCMFC) is the most likely configuration to be scaled up for sludge treatment due to its high power output, simple structure, and relatively low cost.^14–17

It is proved that factors such as the catalyst type, the binder materials as well as the cathode structures can affect the performance of the air cathode. Traditional catalyst platinum (Pt) was often used as the oxygen reduction reaction (ORR) catalyst on the cathodes of air-cathode SCMFCs. However, in addition to the expensive cost, Pt catalyst was easily poisoned by some common anions like sulfide in wastewater and electrode reactions were seriously interfered in this case. Therefore, developing effective and low-cost catalysts for ORR, such as NiO, MnO₂, WO₃ and V₂O₅ have been aroused extensive research interest.^18–22

Cerium dioxide (CeO₂) was routinely used as oxygen storage cathode material for catalytic support in automotive exhaust, solid oxide fuel cell (SOFC),²³ direct methanol fuel cell (DMFC),²⁴ and proton exchange membrane (PEM) fuel cell,²⁵ due to its prominent ability of oxygen storage, transportation and release. CeO₂ belongs to fluorite oxide whose cation is able to reversibly shift between two steady oxidation states (Ce⁴⁺ and Ce³⁺), as a result, it can be used to control the concentration of oxygen on the surface of catalysts. This causes higher reaction kinetics or catalytic activity.^26–29

In view of these facts mentioned above, this work deals with the use of optimized loading by nano-CeO₂ doped commercial Pt/C cathode to improve the performance of membraneless air-cathode SCMFCs. The material and electrochemical performance of this MFC is compared with that observed using commercial Pt/C cathode by X-ray Diffraction (XRD), transmission electron microscope (TEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammogram (CV), respectively. Moreover, the output voltages, power density and polarization curves of air-cathode SCMFCs were also investigated.

Experimental

Materials

Cerium oxide (CeO₂), sodium acetate, Na₂HPO₄, NaH₂PO₄, NH₄Cl, KCl, and isopropyl alcohol were purchased from Aladdin Chemical Reagent Co. Ltd., China. Poly-tetrafluoroethylene (PTFE), carbon black, non wet-proofed carbon cloth, carbon brush, Nafion dispersion solution and modules of all air-cathode SCMFCs were all purchased from Hesen Co. Ltd, China.

Preparation of catalysts

In a typical preparation of CeO₂–Pt/C catalyst, various amounts of nano-CeO₂, 60 mg commercial 20 wt% Pt/C catalyst, 50 μL deionized water, 400 μL Nafion solution and 200 μL isopropyl alcohol were mixed into a vial, which magnetic stirred for 30 minutes. The CeO₂–Pt/C catalysts with various CeO₂ content of 1, 3, 5, and 10 wt% were branded as CPC1, CPC2, CPC3, CPC4, respectively. As a reference, the pure Pt/C was branded as PC.

Preparation of electrode

Non wet-proofed carbon cloth (effective area of 7 cm²) were used as cathode of air-cathode SCMFC. Carbon base was prepared by mixed solution of 25 mg commercial carbon black and 300 μL poly-tetrafluoroethylene (PTFE), which painted on the air facing side of carbon cloth. After calcined at 370 °C for 25 min, four PTFE diffusion layers were painted on carbon base. After calcined at 370 °C for 12 min, the catalyst prepared above were painted on the other side of carbon cloth. Natural air dry for 24 h at room temperature.

MFC construction and operation

All air-cathode SCMFCs tests were implemented using a air-cathode SCMFC with an resultful volume of 28 mL and inoculated with anaerobic activated sludge. The anode was made of preprocessed carbon brush, which was placed on opposite side with the air-cathode. The solution in air-cathode SCMFCs were comprised of 12.5 mL anaerobic activated sludge and 12.5 mL culture media. The culture media (1 L) contained sodium acetate (1 g), Na₂HPO₄ (4.57 g), NaH₂PO₄ (2.13 g), NH₄Cl (0.31 g), KCl (0.13 g), vitamin (5 mL) and minerals (12.5 mL). The vitamin contained the following component in 1 L: biotin (2 mg), folic acid (2 mg), vitamin B₆ (10 mg), riboflavin (5 mg), thiamine (5 mg), niacin (5 mg), pantothenic acid (5 mg), vitamin B₁₂ (0.1 mg), p-aminobenzoic acid (5 mg) and lipoic acid (5 mg). The minerals contained the following component in 1 L: nitrilotriacetic acid (1.5 g), MgSO₄·7H₂O (6.15 g), MnSO₄·H₂O (0.5 g), NaCl (1 g), FeSO₄·7H₂O (0.1 g), CaCl₂·2H₂O (0.1 g), CoCl₂·6H₂O (0.1 g), ZnCl₂ (0.13 g), CuSO₄·5H₂O (0.01 g), AlK(SO₄)₂·12H₂O (0.01 g), H₃BO₄ (0.01 g) and Na₂MoO₄·2H₂O (0.054 g). MFCs were conducted in batch mode with 1000 Ω external resistance and controlled at 30 °C.

Analytical methods

Cyclic voltammogram (CV, potential in the range of −0.6 to 0.6 V at a scan rate of 0.1 V s⁻¹) was carried out in a potentiostat (CHI660E Chenhua Instrument Co., China) with a three-electrode arrangement Ag/AgCl, a Pt sheet electrode and a modified electrode were used as reference electrode, counter electrode and working electrode, respectively, which were put in 50 mM phosphate buffer (pH 7.0). The characterize structure of the catalysts were studied from X-ray Diffraction (XRD) patterns, by using a Bruker D8-A25 X-ray Diffractometer. The microstructure of the catalysts were characterized by using a transmission electron microscope (TEM, JEOL-JSM-2100). The electrochemical impedance spectroscopy (EIS) of the cells were measured by Zennium Electrochemical Workstation (Germany) and identified by an energy dispersive X-ray spectrometer (EDX).

The output voltage was collected every minute across a 1000 Ω resistor connected between the air cathode and anode by using a data acquisition system (MPS-010602). The power density and polarization curves were estimated by changing the resistance from 5000 Ω to 50 Ω.

Results and discussion

The characterize structure of samples

The XRD patterns of pure Pt/C, CeO₂ and CeO₂–Pt/C are shown in Fig. 1. The Pt showed the characteristic diffraction peak at 2θ = 40.04° (111), 46.11° (200), 67.23° (220), 81.05° (311), 85.09° (222). The C revealed the characteristic diffraction peak at 2θ = 24.93° (006) and the sharp peaks of CeO₂ exhibited good crystallinity. The special peaks of CeO₂ could be found in the patterns of the CeO₂–Pt/C, which indicated that CeO₂ was doped successfully into the Pt/C and the crystal structure of CeO₂ was well maintained. In other words, the character and structure of CeO₂ in the Pt/C catalyst was nearly no changed.

Fig. 1 X-ray diffraction patterns of Pt/C, CeO₂ and CeO₂–Pt/C.

Fig. 2(a) and (b) show the TEM images of Pt/C and CeO₂–Pt/C catalysts, respectively. It can be seen from Fig. 2(a) that the Pt particles are uniformly dispersed on the carbon surface and show the dot-like morphology. From the TEM image of CeO₂–Pt/C in Fig. 2(b), it can be clearly seen that the CeO₂ are homogeneously dispersed on the Pt/C support with the average size approximately 60 nm. The Pt/C and CeO₂–Pt/C catalysts were also investigated by EDS (Fig. S1†) and the element Ce and Pt were clearly detected.

Fig. 2 TEM images of (a) Pt/C and (b) CeO₂–Pt/C.

The electrochemical properties of samples

Cyclic voltammogram (CV) is probably the most widely used technique for the electrochemical characterization of electrocatalysts, which is conducted to assess the ORR activity of different catalysts. The important parameters for evaluating catalytic activities of different cathodes are peak current density and peak-to-peak separation (E_pp). Typically, the higher redox current densities indicate the larger active area for ORR. The E_pp is associated with not only the rate of electron transfer, but also the porosity of electrode.^30–32 Fig. 3 shows the typical cyclic voltammogram cures of Pt/C catalysts with varying CeO₂. As shown in Fig. 3, the peak current densities with an order of CPC2 > CPC3 > CPC4 > CPC1 > PC demonstrate that the catalysts could display the corresponding catalytic activity in the similar order. The E_pp decreases in the order of PC (521 mV) > CPC4 (485 mV) > CPC1 (483 mV) > CPC3 (458 mV) > CPC2 (440 mV), indicating that the CPC2 possesses lowest overpotential loss. Taken together, the order of the five catalysts on the ORR performance is CPC2 > CPC3 > CPC4 > CPC1 > PC, and the CPC2 has the highest ORR activity. This may be due to the electrical conductivity of CeO₂ was poor, the electrical conductivity changed little and the ORR of catalyst increased greatly when a small amount of CeO₂ doped Pt/C catalyst (<3 wt%). However, the electrical properties and ORR were all restricted when excess CeO₂ doped Pt/C catalyst (>3 wt%).

Fig. 3 Cyclic voltammogram curves of the different cathodes in oxygen saturated buffer solution.

Electrochemical impedance spectroscopy (EIS) can be efficiently applied to analyze the impedance of catalyst (or mediators) by applying a low amplitude alternating current (AC) signal in mid-frequency range. The EIS analysis of air-cathode SCMFCs with different cathodic catalysts (PC and CPC2) were showed in Fig. 4. The polarization resistance of 10.51 and 27.3 Ω was exhibited for PC-SCMFC and CPC2-SCMFC, respectively. The cathode performance of air-cathode SCMFC should depend on mutual cooperation of catalytic activity and electrical conductivity of catalysts. Although the polarization resistance was improved by CeO₂ doped in Pt/C catalyst, the catalytic activity of Pt/C catalyst was enhanced by CeO₂ doped in Pt/C catalyst. Therefore, the air-cathode SCMFC should have the best efficiency when CPC2 as cathode catalyst.

Fig. 4 EIS of PC-SCMFC and CPC2-SCMFC.

Comparison of performance of air-cathode SCMFCs

The voltage–time curves of air-cathode SCMFCs with different cathodic catalysts were showed in Fig. 5. After the boot process, the output voltages of the five air-cathode SCMFCs stabilized for a period of time, and reduced slowly on account of consumption of the culture media, then quickly increased after the replacement of the fresh culture media. In addition, it was also found that the output voltages of CPC1–4 SCMFCs were enhanced when compared with the PC-SCMFC. Specifically, CPC2-MFCs showed the highest voltage, approximately 580 mV, followed by that of CPC1-SCMFC and CPC3-SCMFC (550 mV and 540 mV), however, CPC4-SCMFC and PC-SCMFC were only 530 mV and 505 mV.

Fig. 5 Voltage–time curves with different cathodic catalysts.

The maximum power density (P_max) curves and polarization curves of air-cathode SCMFCs are presented in Fig. 6(a) and (b), respectively. As showed in Fig. 6(a), the obvious changes of power density curves were ascribed to various CeO₂–Pt/C catalyst as cathode of air-cathode SCMFCs. The power density of PC, CPC1, CPC2, CPC3 and CPC4 were 617 ± 12, 800 ± 21, 840 ± 24, 702 ± 17, and 645 ± 15 mW m⁻², respectively. The air-cathode SCMFC with CPC2 cathode has the highest power density of 840 ± 24 mW m⁻², which further proved the electrochemical properties of CPC2 cathode catalyst. From Fig. 6(b), the stabilized performance of air-cathode SCMFC was conducted by changing the external circuit resistance, the ohmic internal resistance (R_in) of air-cathode SCMFCs were also calculated from the slope of voltage versus current density (Table 1). The lowest R_in was 205 ± 16 Ω in air-cathode SCMFC with PC cathode, and the highest R_in was 268 ± 14 Ω in air-cathode SCMFC with CPC4 cathode, respectively. The ohmic internal resistance (R_in) was improved with increasing the amount of CeO₂ from 0 to 10 wt%, which was consistent with the above conclusion that the polarization resistance was improved when the amount of CeO₂ increased gradually.

Fig. 6 (a) Power densities versus current densities curves, (b) voltage versus current densities curves, (c) electrode potentials (vs. Ag/AgCl) as a function with different cathodes.

Table 1 Comparison of different air-cathode SCMFCs parameters with various cathode materials

Cathode	Rin (Ω)	OCV (mV)	Pmax (mW m^−2)	CE (%)
PC	205 ± 16	709 ± 2	617 ± 12	30 ± 1
CPC1	210 ± 12	736 ± 5	800 ± 21	31 ± 1
CPC2	218 ± 10	750 ± 8	840 ± 24	31 ± 2
CPC3	234 ± 13	732 ± 5	702 ± 17	29 ± 2
CPC4	268 ± 14	719 ± 3	645 ± 15	28 ± 3

---

In order to verify the impact of cathode on the performance of air-cathode SCMFCs, the anode potentials and cathode potentials were surveyed respectively and showed in Fig. 6(c). In each polarization test, the anode potentials of all the air-cathode SCMFCs were essentially the same, while the cathode potential of the air-cathode SCMFCs with different cathode were much distinctive, representing that the changing of the performance of air-cathode SCMFCs was only due to the changing of the air-cathode. Other performance parameter of the air-cathode SCMFCs with different cathodes were also summarized in Table 1, the open circuit voltage (OCV) of the air-cathode SCMFC with CPC2 cathode was 750 ± 8 mV, followed by air-cathode SCMFC with CPC1 cathode (736 ± 5 mV) and air-cathode SCMFC with CPC3 cathode (732 ± 5 mV), which were higher than that of CPC4 cathode (719 ± 3 mV) and PC cathode (709 ± 2 mV). The air-cathode SCMFCs with various cathodes exhibited the similar coulombic efficiency (CE) value, which may be owing to the analogous structure of the electrodes and reactor.

Conclusion

In summary, the CeO₂–Pt/C material was firstly used as a cathode catalyst in air-cathode SCMFCs. The ORR of the cathode catalysts and the performance of the air-cathode SCMFCs were enhanced when CeO₂ doped Pt/C catalyst as cathode. The highest output voltage (580 mV), the maximum power density (840 ± 24 mW m⁻²), and the best ORR of catalyst were obtained by the use of 3 wt% CeO₂–Pt/C cathodes (CPC2) in air-cathode SCMFCs. Moreover, there was no change in the character and structure of Pt/C when CeO₂ doped in Pt/C catalyst.

Notes and references

1. V. B. Oliveira, M. Simões, L. F. Melo and A. M. F. R. Pinto, Biochem. Eng. J., 2013, 73(8), 53.
2. B. Logan, Nat. Rev. Microbiol., 2009, 7(5), 375.
3. B. E. Logan and K. Rabaey, Science, 2012, 337(6095), 686.
4. B. Y. Xiao, Y. P. Han, X. Liu and J. X. Liu, Int. J. Hydrogen Energy, 2014, 39(29), 16419.
5. J. M. Moon, S. Kondaveeti and B. Min, Fuel Cells, 2015, 15(1), 230.
6. Y. L. Oon, S. A. Ong and L. N. Ho, Bioresour. Technol., 2015, 186, 270.
7. Z. H. Xu, B. C. Liu, Q. C. Dong and Y. Lei, Bioresour. Technol., 2015, 197, 244.
8. R. A. Timmers, D. P. B. T. B. Strik and H. V. M. Hamelers, Biomass Bioenergy, 2013, 51(4), 60.
9. X. Li, N. Zhu and W. Yun, Bioresour. Technol., 2013, 128(1), 454.
10. M. G. Hosseini and I. Ahadzadeh, J. Taiwan Inst. Chem. Eng., 2013, 44(4), 617.
11. H. Wang, L. Lu and D. Liu, Bioresour. Technol., 2015, 195, 25.
12. R. Karthikeyan, A. Selvam and K. Y. Cheng, Bioresour. Technol., 2016, 200, 845.
13. J. M. Sonawane, E. Marsili and P. C. Ghosh, Int. J. Hydrogen Energy, 2014, 39(36), 21819.
14. S. Cheng, W. Liu and G. Jian, Biosens. Bioelectron., 2014, 56(9), 264.
15. A. Tremouli, A. Intzes and P. Intzes, J. Appl. Electrochem., 2015, 45, 1.
16. Z. Liang, Z. Xun and H. Kashima, Bioresour. Technol., 2015, 179, 26.
17. Z. Nengwu, C. Xi and Z. Ting, Bioresour. Technol., 2011, 102(1), 422.
18. D. B. Kim, H. J. Chun and Y. K. Lee, Int. J. Hydrogen Energy, 2010, 35, 313.
19. C. M. Zhao, H. J. Wang, F. Peng, J. H. Liang, H. Yu and J. Yang, Langmuir, 2009, 25(13), 7711.
20. Q. Wen, S. Wang and J. Yan, J. Power Sources, 2012, 216(11), 187.
21. J. Zhao, W. Chen and Y. Zheng, Mater. Chem. Phys., 2009, 113(2), 591.
22. T. Maiyalagan and F. N. Khan, Catal. Commun., 2009, 10(10), 433.
23. Y. Li, Y. Zhang and X. Zhu, J. Power Sources, 2015, 285, 354.
24. H. Kunitomo, H. Ishitobi and N. Nakagawa, J. Power Sources, 2015, 297, 400.
25. F. Xu, R. Xu and S. C. Mu, Electrochim. Acta, 2013, 112(12), 304.
26. J. H. Xu, J. Harmer, G. Q. Li, T. Chapman, P. Collier, S. Longworthb and S. C. Tsang, Chem. Commun., 2010, 46(11), 1887.
27. H. P. Zhou, H. S. Wu, J. Shen, A. X. Yin, L. D. Sun and C. H. Yan, J. Am. Chem. Soc., 2010, 132(11), 4998.
28. C. Yeung and S. C. Tsang, J. Phys. Chem. C, 2009, 113(15), 6074.
29. G. Balducci, M. S. Islam, J. Kašpar, P. Fornasiero and M. Graziani, Chem. Mater., 2000, 12(3), 677.
30. M. K. Wang, A. M. Anghel and B. Marsan, J. Am. Chem. Soc., 2009, 131(44), 15976.
31. Z. Huang, X. Liu, K. Li, Y. Luo and H. Li, Electrochem. Commun., 2007, 9(4), 596.
32. J. D. Roy-Mayhew, D. Bozym, C. Punckt and I. Aksay, ACS Nano, 2010, 4(10), 6203.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04012h

---