Jiaxin Lia,
Baogang Zhang*a,
Qinan Songa and
Alistair G. L. Borthwickb
aSchool of Water Resources and Environment, China University of Geosciences Beijing, Key Laboratory of Groundwater Circulation and Evolution, Ministry of Education, Beijing 100083, China. E-mail: zbgcugb@gmail.com; baogangzhang@cugb.edu.cn; Fax: +86 10 8232 1081; Tel: +86 10 8232 2281
bSchool of Engineering, The University of Edinburgh, The King's Buildings, Edinburgh EH9 3JL, UK
First published on 24th March 2016
Improvement of microbial fuel cells (MFCs) via bioelectricity recovery is urgently needed in micro-energy devices nowadays. Herein, we demonstrate that the power outputs of double-chamber air-cathode catalyst free MFCs can be increased by approximately 40% through the cyclical redox of vanadium (between V and IV) in the catholyte. The enhanced performance can be explained by the regenerated vanadium(V) resulting from aeration of residual vanadium(IV) on the cathode surface as well as an accelerated electron transfer rate in the aqueous solution. This study provides a simple and cost-efficient strategy to improve the performance of MFCs.
Severe water pollution events result from the presence of high concentrations of vanadium that arise from vanadium mining and vanadium pentoxide production.14 Such events can lead to severe environmental damage.15 As a kind of regenerative fuel cell, vanadium redox flow batteries (VRFB) stores relatively large scale amounts of energy by using V(V)/V(IV) and V(III)/V(II) redox couples in positive and negative cells,16 separated by a proton-conducting membrane.17 The equation of the vanadium redox couple reactions of V(V)/V(IV) is given below, with redox potential of 0.991 V.18,19
VO2+ + 2H+ + e− ↔ VO2+ + H2O EΘ = 0.991 V | (1) |
Considering the excellent redox characteristics and abundant existence of vanadium(V) in metallurgical wastewater, it can be introduced in the cathode of MFCs for performance boost.
The present study demonstrated that the power outputs could indeed be enhanced by adding V(V) to the double-chamber air-cathode MFCs. Changes to the valence states of vanadium and its electrochemical behaviors were investigated. The results indicated that the addition of non-consumptive V(V) could be an effective means to enhance the power outputs of MFCs.
Chemical oxygen demand (COD) was measured by fast digestion spectrophotometric method (DR5000, HACH, USA). Concentrations of remaining V(V) and generated V(IV) were measured by spectrophotometric method.22,23 V(III) and V(II) were not monitored, given that they were rarely generated under this condition.24 Total vanadium was detected by inductively coupled plasma mass spectroscopy. Voltages were recorded by a data acquisition system (PMD1208LS, Measurement Computing Corp., Norton, MA, USA) at 5 min intervals. Polarization curves were drawn with external resistances ranging from 5000 to 10 Ω using a resistor box to evaluate the performance of the MFCs and the maximum power density. Coulombic efficiency (CE) was calculated as reported previously.20 Cyclic voltammetry (CV) was performed at a scan rate of 0.01 V s−1 in the range of −0.6 V to +0.8 V using an electrochemical workstation (VMP3, Bio-Logic Science Instruments, France) with cathode electrode and Ag/AgCl as working and reference electrodes, respectively.25 Electrochemical impedance spectroscopy (EIS) measurements were carried out for the cathode of the MFCs over a frequency range of 100 kHz to 1 mHz with an AC signal of 10 mV amplitude.26
O2 + 2H+ + 2e− → H2O2 | (2) |
H2O2 + 2H+ + 2e− → 2H2O | (3) |
O2 + 4H+ + 4e− → 2H2O | (4) |
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Fig. 1 The voltage outputs for MFC-VO, MFC-V, and MFC-O, with 100 Ω external resistance during the three cycles operation. |
V(V) had the capacity to act as an alternative electron acceptor given its relatively higher electrode potential (eqn (1)), as indicated by MFC-V (Fig. 1) and our previous research.24 Although the standard electrode potential of V(V) is lower than that of oxygen, it has larger solubility, and so the cathode potential could be improved (from 425.3 ± 7.4 mV to 125.9 ± 11.2 mV in a typical cycle) and the voltage outputs enhanced. By comparison, ORR is a gas phase reaction with large mass transfer resistance and much lower oxygen solubility;30 hence, the voltage outputs of MFC-O were lower than those of MFC-V (Fig. 1).
Fig. 1 showed that the voltage outputs of MFC-VO were enhanced substantially with the increase of cathode potential in the presence of both V(V) and oxygen (from 498.4 ± 10.1 mV to 155.1 ± 18.5 mV during a typical cycle). Redox potentials of V(V) and oxygen based on Nernst equation were presented in eqn (5) and (6). Both V(V) and oxygen had relatively higher redox potentials, and were present together, leading to the higher voltage outputs of MFC-VO.
![]() | (5) |
![]() | (6) |
The power outputs of the three types of MFCs also exhibited similar behavior to those of voltage outputs, inferred from polarization curves (Fig. 2). MFC-VO achieved the highest maximum power density of 553.9 mW m−2 with current density of 1315.6 mA m−2, and the maximum power densities of MFC-V and MFC-O were representative.24,27,28 This was achieved due to the improvement in cathode potential with addition of V(V) in MFC-VO, confirming that addition of higher redox potential V(V) is a priming strategy to improve the power outputs of MFCs. Moreover, CEs of MFC-O and MFC-VO were 10.4% and 11.2%, respectively, both lower than that of MFC-V (16.9%), as oxygen could penetrate the proton exchange membrane and the generated electrons from organics oxidation could be consumed by oxygen instead of being transferred to anode.31
CV was used to evaluate the redox activities of cathode. The results are shown in Fig. 3 and it could be seen that MFC-VO had the largest closed-curve area, which was to say the electric quantity integrated by the closed curves was the biggest, implying the highest electricity production.32,33 MFC-VO also exhibited an apparent reduction peak corresponding to positive current due to V(V) reduction, while the oxidation peak was slightly weak due to low concentration of generated V(IV) as well as the fast oxidation process by oxygen. Furthermore, the voltammogram for the catholyte containing V(V) indicated that the current flow was significantly higher in most potential regions than that for MFC-O. These results demonstrated that the addition of V(V) to the aqueous solution played an important role with respect to electron transfer in MFCs.
Fig. 4a showed the results obtained using EIS performed to evaluate the circuit resistance and kinetics for ORR in MFCs.34,35 It could be seen from the Fig. 4a that the modular curves show that maximum resistance was obtained for MFC-O, followed by MFV-VO and MFC-V. This sequence occurred because the vanadium-containing solution had better conductivity than that with oxygen in the gas phase and the addition of vanadium reduced the cathode resistance. The EIS data were also analyzed using the one-time constant model36 and the phase angles in the low-frequency region of the spectra indicated that the polarization resistance of the MFC-O was the highest due to the lack of catalyst for ORR.37 Moreover, the Nyquist plots also confirmed these results and the calculated ohmic resistances for MFC-VO, MFC-V and MFC-O were 23.3 Ω, 20.2 Ω and 23.9 Ω, respectively (Fig. 4b). Additionally, monolayer reaction was observed in the MFC-VO as could be seen in the phase angle curves in Fig. 4a, implying that no solid vanadium chemicals were attached to the cathode surface which might hinder the oxygen reduction reaction. This phenomenon differed from that of similar metal ions acting as electron acceptors in the cathode. For example, it was previously reported that Cr(VI) could also work as an alternative electron acceptor.38,39 However, Cr(VI), being an electron consumer, experienced great difficulty in undergoing in situ regeneration. Moreover, Cr(VI) reduced to Cr(III) with the latter attached on the cathode surface, which could affect the long-time operation of MFCs. In short, the addition of non-consumptive V(V) was more superior to enhance the performance of MFCs with long durability.
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Fig. 5 Time histories of V(V) and V(IV) concentrations in the catholyte of MFC-VO and MFC-V during the 4 d operating period. |
Vanadium ion with higher redox potential acted as electron acceptor in the cathode chamber, as indicated in MFCs employing two different electron acceptors.40 In the present study, V(V) took precedence of oxygen as it possessed higher redox potential, and so was reduced. Although it could improved the power outputs of MFCs, the electron-consuming V(V) would also increase costs and adversely affect the environment. When oxygen was simultaneously introduced, the generated V(IV) irons were further oxidized to V(V) quickly, realizing the regeneration of V(V) and avoiding the need for extra addition of V(V) during the operation. Oxygen was the terminal acceptor for electrons from anode. Fig. 6 showed the possible pathways for the entire process schematically.
Additionally, the effect of the change of conductivity with the addition of V(V) was also examined. When V(V) of 150 mg L−1 was added to the phosphate buffer solution, a slight increase of conductivity was observed, from 4.98 mS cm−1 to 5.44 mS cm−1. For a test conducted with the same conductivity (5.44 mS cm−1) adjusted by NaCl, hardly any increase in power output was measured. This provided further evidence that the enhanced power outputs were due to reaction of V(V) rather than increase in conductivity. Meanwhile, these two species of vanadium were also measured in the anode chamber of MFC-VO with the same frequency as performed in the cathode chamber. Almost no V(V) or V(IV) ions were detected in the anode chamber and the total vanadium in the cathode chamber kept unchanged, indicating that vanadium ions were almost unable to penetrate the proton exchange membrane, thus preventing poisoning of microbes by toxic vanadium in the anode chamber and vanadium loss in the cathode chamber.
Moreover, other materials with higher redox potential could also be considered as candidates to improve MFC performance. Rhoads et al. used biomineralized manganese oxides as cathodic reactants, whereby MnO2 was reduced to soluble Mn2+, and Mn2+ was then reoxidized with oxygen to manganese dioxide.41 However, this strengthening effect might be limited due to the reducing reaction of MnO2 in the solid phase. The characteristics of their reduction products were also worth careful consideration; for example, chromium(VI) had previously been examined intensively in MFCs, but was found unsuitable to be employed in the aerated cathode because its reduction was insoluble Cr(III), which deposited on the cathode surface to block electrode reactions and was hardly oxidized again to Cr(VI) by oxygen.42 Moreover, the present research demonstrated similar benefits to a study of the use of Fe(III) to stimulate bioelectricity generation with regeneration of Fe(III) catalyzed by microbes, which required carbon sources as well as proper growth conditions.43 However, the system proposed herein could function well with substantially less added chemicals and wider applicability to the environment. Wastewaters from vanadium mining and vanadium pentoxide production containing high concentration of V(V) could be directly employed as catholyte with oxygen being introduced in to improve the power outputs, while stopping aerating in the same system could realize the remediation of these wastewaters as indicated by MFC-V as well as our previous study.20 In summary, the system proposed in the present research could generate higher bioelectricity without expensive catalysts, and without consumption of high redox potential materials, which were major advantages in the practical applications of MFCs.
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