•We proposed an easy-to-fabricate cathode derived from carbonized kraft paper.•Preparation process of cathode had a significant effect on the Pmax of MFCs.•Separate KP pyrolysis and FePc heat treatment improved cathode performance.•MFCs with the KP-HT-FePc-HT process had the highest Pmax of 830 ± 31 mW m−2.•An optimal multiple-layer cathode assembly improved the power output.In this paper, we proposed a simple method for preparing a binder-free air-cathode using carbonized kraft paper as the support and iron (II) phthalocyanine (FePc) as the catalyst. The results indicated that the oxygen reduction reaction (ORR) performance of the air-cathodes was dependent on the fabrication steps. A cathode (KP-HT-FePc-HT) fabricated by pyrolyzing kraft paper at 1000 °C followed by FePc heat treatment at 700 °C showed the highest Pmax of 830 ± 31 mW m−2 compared to FePc/KP-HT (363 ± 48 mW m−2) prepared by direct pyrolysis. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy and electrochemical tests showed that the superior electrocatalytic activity of KP-HT-FePc-HT was attributable to its higher content of pyridinic-N. This study demonstrated that the FePc/KP-based binder-free air-cathode had the advantages of low cost, easy fabrication, environmental benefits, and good scalability and therefore could serve as a good alternative for the air-cathode of MFCs.Download high-res image (209KB)Download full-size image

•Pyrolyzed Chlorella pyrenoidosa were used as the ORR catalyst for MFCs.•The obtained catalyst (CP900) exhibited a higher activity than Pt/C.•The MFC with CP900 showed a higher Pmax than that with Pt/C.•CP900 has superior stability than Pt/C in MFC relevant conditions.•The effect of pyrolysis temperature on the CP catalysts was investigated.Lack of low-cost, stable, and effective catalysts for the oxygen reduction reaction (ORR) is one of the key factors limiting the practical application of microbial fuel cells (MFCs). In this paper, a non-metal high-performance ORR catalyst, prepared by directly pyrolyzing Chlorella pyrenoidosa (CP) in N2 atmosphere, was proposed. It was found that the ORR activity of the CP catalysts was highly dependent on the carbonization temperatures. The MFC with the catalyst carbonized at 900 °C (CP900) delivered the highest Pmax (maximum power density) value of 2068 ± 30 mW m−2, which was 13% higher than that with commercial 20 wt% Pt/C (1826 ± 37 mW m−2) at the same catalyst loading. CP900 also showed good structural stability, maintaining 57.4% of the activity after 10,000 s operation at −0.3 V (vs. Ag/AgCl), significantly higher than 48.5% for Pt/C. The Brunauer–Emmet–Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electrochemical analyses indicated that the superior performance of CP900 was due to the high graphitization, the appropriate N and P doping, and the improved catalyst utilization due to the presence of abundant mesopores and macropores. These results demonstrated that CP900 could be a cost-efficient, stable and high performance alternative to the commercial Pt/C for MFC applications.

•The response of a stacked microbial fuel cell to variable operating conditions is studied.•Connection mode significantly influences the stack performance.•An optimal number of cells should be considered for the future design of MFC stack.•Voltage reversal occurs when switching to a series connection or decreasing the load.•The stack performance could be improved by increasing flow rate to an extent.The stacked microbial fuel cell (MFC) is a potential pathway for future applications, and its response to variable operating conditions is important for practical operation. In this study, a stacked MFC with serpentine flow field was constructed to investigate the stack performance and response to cell number, connection type, variable loads and electrolyte flow rates. The results showed that the highest maximal power (22.2 mW) was observed in a series connection, which was 12.1% and 29.1% higher than the maximal power in the parallel and hybrid connection, respectively. With the increasing number of cells, a gradually decreasing increase in the voltage output was found in the parallel stack and series stack at a high load, while the series stack showed first an increase and later a decrease in the voltage output at low load. Voltage reversal was observed when switching to a series connection or decreasing the load, resulting in a decreased stack voltage. The performance of the stack could be improved to an extent by increasing the electrolyte flow rates.

To simplify the cathode fabrication, we herein report a bamboo charcoal tube-derived (BCT) air cathode for microbial fuel cells (MFCs). This monolithic cathode was prepared by carbonizing a bamboo tube in a N2 atmosphere at 900 °C. The scanning electron microscope indicates that the porous structure of the BCT cathode can provide possible channels for oxygen supply and proton transfer, and therefore can offer the formation of the triple phase interfaces required for oxygen reduction reaction (ORR) in the bulk of BCT. It is also shown that compared to the cathode with bamboo charcoal powder (BC) and Pt/C, the BCT cathode exhibited the lowest oxygen mass transfer coefficient of 4.5 × 10−5 cm s−1. Additionally, the rotating ring-disk electrode tests demonstrate that the BCT cathode exhibited a considerable activity toward ORR. Consequently, the MFC with BCT delivered a comparable maximum power density (40.4 ± 1.5 W m−3) to that with Pt/C (37.7 ± 2.5 W m−3) and the highest coulombic efficiency of 55.87 ± 1.0% as compared with the MFCs with BC (29.9 ± 0.4%) and Pt/C (17.2 ± 0.4%). These results indicate that BCT could serve as a low cost, simple fabrication and high-performance alternative cathode for MFCs.

•We proposed a hybrid microbial fuel cell stack for self-sustaining pH control.•The hybrid stack was based on single and double chamber MFCs.•The hybrid stack demonstrated a high operational stability.•An improved stack performance was obtained in the hybrid stack.•The hybrid stack showed efficient conversion of acetate to electricity.Proton accumulation in the anode chamber is the major problem that affects the operational stability and electricity generation performance of double chamber microbial fuel cells (MFCs). In this study, a hybrid microbial fuel cell stack (DS–DS stack) based on single (SCMFCs) and double chamber MFCs (DCMFCs) is proposed for self-sustaining pH control in the MFC stack. It is found that the aerobic microbial oxidation of acetate by the biofilm that is attached to the air cathode of SCMFCs is responsible for the self-sustaining removal of accumulated H+ in the effluent of DCMFCs. Compared with the stack that solely consists of SCMFCs (SS–SS stack) or DCMFCs (DD–DD stack), the hybrid stack exhibits the highest electricity output performance and the most effective conversion of acetate into electricity at high power levels. Furthermore, the hybrid stack demonstrates the operation time of 15.7 ± 1.1 h when the operating voltage is above 0.8 V. This value is much higher than that of the DD–DD (8.5 ± 2.4 h) and SS–SS (8.1 ± 1.4 h) stacks, which suggests that the hybrid stack had a good operational stability.

•Effect of proton transfer on unbuffered microbial fuel cells (MFCs) was investigated.•MFCs with low (MFC-LPP) or high H+ permeability (MFC-HPP) of separator were developed.•MFC-HPP had a much higher power than MFC-LPP mainly due to better proton transfer.•The increasing flow rate enhanced proton transfer and improved MFC-HPP performance.•Unbuffered MFCs could be operated in sequential anode–cathode flow for application.Microbial fuel cells (MFCs) require a better understanding of proton transfer especially under buffer-less condition for practical application. In this study, two continuous-flow tubular MFCs using separators with either low (MFC-LPP) or high proton permeability (MFC-HPP) were operated under buffer-less condition. The results showed that MFC-HPP had a better cathode performance and 125% higher maximum power density than MFC-LPP. The higher performance of MFC-HPP was mainly attributed to the enhanced proton transfer and increased catholyte conductivity, suggesting that the proton transfer from the anode to the cathode was crucial to the performance of unbuffered MFCs. It was also found that promoting the proton transfer by increasing the electrolyte flow rate led to a further performance improvement of MFC-HPP. Based on energy efficiency analysis, additional energy gain can be obtained from the unbuffered MFCs when it was operated below a critical flow rate.

•A novel method to increase the MFC performance by rotating the anode was proposed.•Anode rotation can enhance the mass transport inside the carbon-brush anode.•The rotating anode can improve the electrochemical activity of the biofilm.A novel method was proposed to improve the power output of microbial fuel cells (MFCs) by rotating the carbon-brush anode. The MFC with a rotating anode produced a peak power density of 210±3 W/m3 and a maximum current density of 945±43 A/m3, 1.4 and 2.7 times higher than those of the non-rotating case, respectively. The difference of the electrochemical impedance spectroscopy and cyclic voltammetry before and after anode rotation clearly suggested that the mass transfer to the spiral space was enhanced by the rotating anode. Furthermore, Tafel plots analysis also revealed that the rotating anode can improve the electrochemical activity of the biofilm.

•The long-term effect of anode inner diameter on MFC performance was studied.•Anode structure impacts the electrochemical activity of the biofilm.•Bamboo charcoal tubes with inner diameter of 2 mm are the optimal anode structure.In the present study, the bamboo charcoal tube derived from the carbonization of the bamboo tube was employed as the anode. The effect of inner diameter of bamboo charcoal tube anodes was experimentally investigated on the microbial fuel cells (MFCs) performance to obtain an optimal structure. After successful star-up, bamboo charcoal tube anodes with different inner diameters (1 mm, 1.5 mm, 2 mm and 3 mm in inner diameter were named as MFC-D1, MFC-D1.5, MFC-D2 and MFC-D3) resulted in various voltage output. However, MFC-D2 and MFC-D3 still kept stable output while the MFC-D1 and MFC-D1.5 performances had a significant drop for a long-term operation (after operation for 30 days). Scanning electron microscope and electrochemical impedance spectroscopy results indicated that the reduction in the powder density for MFC-D1 and MFC-D1.5 attributed to a compact and thicker biofilm on the anode surface leading to the increased the internal resistance of MFCs. Furthermore, compared with other anodes, the highest power density (3303 W/m3) for MFC-D2 suggested that the tubular bamboo charcoal with 2 mm in diameter was more suitable for electricity generation.

•A new method for anode modification (CC-NA) was proposed.•Quinoid groups were generated on the CC-NA anode.•Quinoid groups reduced the charge transfer resistance of the CC-NA anode.•CC-NA anode showed the highest performance and smallest internal resistance.In this study, performances of two-chamber microbial fuel cells (MFCs) with surface-modified carbon cloth anodes by four methods are compared including soaking in aqueous ammonia (CC-A), electrolysis in phosphate buffer (CC-P), electrolysis in nitric acid (CC-N) and electrolysis in nitric acid followed by soaking in aqueous ammonia (CC-NA). It is found that performances of all these modified anodes are better than that of the bare one. The MFC with the anode modified by CC-NA yield the maximum power density of 3.20 ± 0.05 W m−2, which is 58%, 36%, 35% and 28% higher than those of the MFC with untreated anode (CC-C, 2.01 ± 0.02 W m−2), and treated by the CC-A (2.35 ± 0.15), CC-N (2.38 ± 0.02 W m−2) and CC-P (2.50 ± 0.08 W m−2) methods, respectively. The active biomass and EIS analysis for the anodes indicated that the highest power output of the CC-NA MFC may primarily attributed to the enhanced electron transfer resulting from the existence of quinoid groups on the CC-NA surface, rather than the biomass effect. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopic analyses of the CC-NA anode further demonstrate that the formation of quinoid groups contributes to the improvement in the MFC performance by accelerating the electron transfer between electrochemically active bacteria (EAB) and the anode surface.

•Effect of inert particle concentration on the operation of MFCs was studied.•The maximum power density decreased with increasing the particle concentration.•Active biomass dropped significantly at high particle concentrations.•SiO2 particles led to an increase in the charge transfer resistance at anode.Considering the promising application of microbial fuel cells (MFCs) in the wastewater treatment, the inherent solid particles in the wastewater may affect the MFC performance. In this paper, the effect of inert particle concentration on the operation of MFCs is investigated by adding silicon dioxide (SiO2) particles into the anolyte. The results show that the existing SiO2 particles in the anolyte result in a decreased active biomass and a reduced electrochemical activity of the biofilm. The anode ohmic resistance is almost the same for MFCs with various SiO2 particle concentrations in the anolyte, while an increase in the charge transfer resistance is observed. A small amount of inert particles have little influence on the MFC. However, when the MFC is operated with the anolyte containing more than 500 mg L−1 SiO2 particles, the performance decreases significantly due to the low electrochemical activity and high internal resistance of the anode.

•A combined system of PBRs and MFCs was proposed for additional biohydrogen recovery.•Inhibitory VFAs and H+ generated in PBR1 were removed by the MFCs.•Byproduct removal in MFCs resulted in a notably improved hydrogen production in PBR2.•A glucose utilization efficiency of 97.6% was achieved in the combined system.To obtain additional hydrogen recovery from the downstream photosynthetic biohydrogen reactor (PBR), a system (PBR1–MFCs–PBR2) that combined PBRs with three single chamber microbial fuel cells (MFCs) was proposed in this study. The results revealed that the PBR2 in PBR1–MFCs–PBR2 showed a hydrogen production rate of 0.44 ± 0.22 mmol L h−1, which was 15 and 4 times higher than those obtained by direct connecting the two PBRs (PBR1–PBR2) and pH regulated system (PBR1–pH regulation–PBR2), respectively. In addition, the PBR1–MFCs–PBR2 exhibited the highest glucose utilization (ηg) of 97.6 ± 2.1 %, while lower ηg values of 75.6 ± 2.2% and 70.1 ± 1.2% was obtained for PBR1–PBR2 and PBR1–pH regulation–PBR2, respectively. These improvements were due to the removal of inhibitory byproduct and H+ from the PBR1 effluent by the MFCs.

•We proposed an alternative method for fabricating GFB electrodes.•Two series of the anodes with various brush lengths and fiber loadings were made.•Increasing L at a constant m induced an improved MFC performance until L = 30 mm.•No obvious trend was found for the electrodes with different m and similar L.An alternative method for fabricating graphite fiber brush (GFB) electrodes was proposed. Two series of GFB electrodes with different lengths (L) and loaded fiber masses (m) were fabricated. The effects of m/L on the biomass distribution, active biomass content, electrochemical behavior and MFC performance were investigated. For the electrodes with a similar m but different L, substrate supply within the interior of GFB electrodes improved with L, leading to higher biomass content and consequently the improved performance. However, a complex trend was found for the electrodes with different m and similar L, due to the opposing trends of substrate supply and actual functional area for electrochemically active bacteria with m. Furthermore, m-normalized biomass content and power density of the GFB electrodes increased with decreasing of m/L ratio due to the improved graphite fiber utilization until 0.014 g mm−1, below which they remained constant since the utilization of graphite fibers plateaued.

A composite carbon prepared by blending Vulcan XC-72 carbon with multi-walled carbon nanotubes (MWCNTs) had been used as a support of PtRu alloy catalyst (PtRu/C + MWCNTs) for methanol oxidation reaction. The obtained electrocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of the composite carbon support on the alloy content and PtRu surface state was discussed. The electrocatalytic property of the PtRu/C + MWCNTs catalyst was investigated by cyclic voltammetry (CV), chromoamperometry, CO stripping voltammetry and DMFC single cell test. The results demonstrated that, compared to the PtRu/C and PtRu/MWCNTs catalysts, the PtRu/C + MWCNTs catalyst exhibited superior electrocatalytic activity for methanol oxidation.