In this work, we presented a novel, facile, and template-free strategy for fabricating graphene-like N-doped carbon as oxygen reduction catalyst in sustainable microbial fuel cells (MFCs) by using an ion-inducing and spontaneous gas-flow tailoring effect from a unique nitrogen-rich polymer gel precursor which has not been reported in materials science. Remarkably, by introduction of trace platinum- and cobalt- precursor in polymer gel, highly dispersed sub-10 nm PtCo nanoalloys can be in situ grown and anchored on graphene-like carbon. The as-prepared catalysts were investigated by a series of physical characterizations, electrochemical measurements, and microbial fuel cell tests. Interestingly, even with a low Pt content (5.13 wt %), the most active Co/N codoped carbon supported PtCo nanoalloys (Co–N–C/Pt) exhibited dramatically improved catalytic activity toward oxygen reduction reaction coupled with superior output power density (1008 ± 43 mW m–2) in MFCs, which was 29.40% higher than the state of the art Pt/C (20 wt %). Notability, the distinct catalytic activity of Co–N–C/Pt was attributed to the highly efficient synergistic catalytic effect of Co–Nx–C and PtCo nanoalloys. Therefore, Co–N–C/Pt should be a promising oxygen reduction catalyst for application in MFCs. Further, the novel strategy for graphene-like carbon also can be widely used in many other energy conversion and storage devices.Keywords: graphene-like carbon; microbial fuel cells; N/Co-dual doping; PtCo nanoalloys; synergistic catalyst;

Porous C/Ag nanohybrids were one type of emerging and promising alternative oxygen reduction catalysts for Pt, which was crucial to energy conversion and storage devices. Herein, a template-free, sustainable and in-situ method was employed for preparing N-doped hollow carbon tube @ hierarchically porous carbon supporting homogeneous Ag nanoparticles (HCT@HPC@AgNPs) by one-pot carbonization with the assist of “chelating effect”. It was very interesting that the “chelating effect” can inhibit the self-aggregation of AgNPs during its growing process, since Ag+ can be well immobilized on precursor. Moreover, during the carbonization process, an amazing N-doped hollow carbon tube @ porous carbon was also achieved by the synergistic effect of thermal degradation, spontaneous bubble-template and self-doping. The resultant HCT@HPC@AgNPs was investigated by physical characterizations and electrochemical tests. The results indicated that HCT@HPC@AgNPs delivered distinct electrocatalytic activity towards oxygen reduction reaction (ORR), long-term durability and anti-toxic capacity in alkaline electrolyte, which was comparable or even better than Pt/C. Thus, HCT@HPC@AgNPs was a promising ORR catalyst for application in the field of energy and catalysis.

A highly active and cost-effective Pt-free catalyst for oxygen reduction reaction (ORR) is significantly important for air-cathode microbial fuel cells (MFCs). In this study, a novel low-cost iron–nitrogen–carbon nanorod network-anchored graphene (Fe–N–C/G) nanohybrid was prepared for use as an efficient ORR catalyst. The morphology, chemical composition, and ORR catalytic activity of the as-prepared Fe–N–C/G were investigated by a series physical measurements and electrochemical tests. Finally, it the nanohybrid was employed as an ORR electrocatalyst in the practical air-cathode MFCs. Remarkably, Fe–N–C/G exhibited a comparable catalytic performance and stability in a neutral medium along with even better power generation performance (1601 ± 59 mW m−2) in MFCs as compared to the-state-of-the-art Pt/C catalyst (1468 ± 58 mW m−2). The superior ORR activity of Fe–N–C/G should be attributed to its N/Fe co-doping, the introduction of graphene, as well as the unique micro-nano structure, which can dramatically favor the oxygen reduction kinetics. Therefore, the cost-effective Fe–N–C/G can be one of the most promising ORR catalysts for application in a neutral medium and practical air-cathode MFCs.

•1 Low-cost N/B co-doped carbon was prepared from renewable biomass.•2 NB-HCT exhibited comparable ORR activity and better stability than Pt/C.•3 Superior ORR activity of NB-HCT was ascribed to synergistic catalysis effect.Low-cost and highly active catalysts for oxygen reduction reaction (ORR) were of great importance to the development of fuel cell. Herein, we presented a novel N/B co-doped carbon as cost-effective and stable ORR catalyst from renewable biomass. The as-prepared carbon showed an interesting hollow tube morphology that derived from its precursor. The N/B co-doped sample (NB-HCT) possessed bigger electrochemical active area (296.4 m2 g−1), and also contained much more ORR active-N/B species than those of single N or B doped carbon. Notability, the on-set potential and half-wave potential of NB-HCT were 0.929 V (vs. RHE) and 0.810 V (vs. RHE), respectively, which were much more positive than others and even comparable to Pt/C, revealing it had distinct ORR activity. Remarkably, the superior ORR activity of NB-HCT could be mainly ascribed to the synergistic effect of ORR active-N and B-C, which might result in the possible formation of high-active B-C-N nanostructure. Therefore, NB-HCT should be one of promising ORR catalysts to alternative precious Pt/C and promote the development of fuel cell.Download high-res image (208KB)Download full-size image

The exploration of highly active and cost-effective catalysts for the oxygen reduction reaction is vitally important to facilitate the improvement of metal–air batteries and fuel cells. Herein, super-active catalysts made from an interesting metal–polymer network (MPN) that consist of Fe–Nx–C, B–N and Fe3O4/Fe3C alloys were prepared via facile one-pot carbonization. The achieved catalysts possessed an amazing porous structure that was derived from the MPN with the assistance of a “bubble-template”. Remarkably, the content of highly active Fe–Nx–C can be regulated by introducing graphene, and the ORR activity of the catalyst was enhanced dramatically with an increase in the Fe3O4/Fe3C alloy content. The most active BNFe–C–G2 catalyst exhibited superior ORR activity/stability, and was then employed as an air cathode electrocatalyst in a microbial fuel cell. The results showed that the output voltage and power density of BNFe–C–G2 were significantly improved to 575 ± 11 mV and 1046.2 ± 35 mW m−2, respectively. These values are 4.5% and 44.44% higher than those of commercial Pt/C. Thus, due to the synergistic electrocatalysis of the Fe–Nx–C, B–N and Fe3O4/Fe3C alloys, the super-active and low-cost BNFe–C–G2 material should be a promising ORR catalyst for application in biofuel cells, and in many other energy conversion and storage devices.

•A novel method is used to study the extracellular electron transfer (EET) mechanism.•The mediated EET process can be studied alone by “resolution-contrast” method.•The dominant bacteria and the evolution of microbial community have been analyzed.•The main potential redox active compounds have been discussed.Due to the synergy existing in mixed bacteria, electricity-generation performances of the mixed bacteria microbial fuel cell (m-MFC) are better than that of the pure bacteria microbial fuel cell (p-MFC). It is necessary to explore the electron transfer mechanisms of the m-MFC to further improve electricity generation. In this study, a facile “resolution-contrast” method was employed in the two-chamber m-MFC to investigate the electron transfer mechanism using mixed bacteria as the anodic inoculums. The anode was wrapped with a mixed cellulose esters membrane to eliminate the effects of direct extracellular electron transport, while the mediator electron transport was investigated by electrochemical methods. The results showed that both transfer methods existed simultaneously in the m-MFC. The main exoelectrogens were Escherichia coli, Pseudomonas aeruginosa, and Brevundimonas diminuta, and the main redox mediators included 1-hydroxyanthracene-9,10-dione, phenazine-1-carboxylic acid and 2,4-diacetyl phloroglucinol.

A “Spontaneous bubble-template” method is fascinating in that bubbles are formed in situ during material processing and employed as a template for fabricating unique structures, which has not been reported in material science. It is sustainable, green and efficient in that no extra additives or post-treatment are used. Herein, novel metal–polymeric framework derived hierarchically porous carbon/Fe3O4 nanohybrids are prepared using a “spontaneous bubble-template” method by one-step carbonization. During the carbonization process, N and Co are self-doped on porous carbon in which in situ grown nano Fe3O4 is embedded (Fe3O4@N/Co–C). The as-prepared Fe3O4@N/Co–C displays a three-dimensional interpenetrating morphology (electrochemical active area: 729.89 m2 g−1) with well-distributed Fe3O4 nanoparticles (20–50 nm) which are coated with a carbon layer (3–5 nm). Fe3O4@N/Co–C exhibits remarkable oxygen reduction activity in biofuel cells with a distinct output voltage (576 mV) and power density (918 mW m−2), which are 3.6% and 17.8% higher than those of Pt (0.5 mg cm−2), respectively. Besides biofuel cells, Fe3O4@N/Co–C may also have the potential for application in chemical fuel cells, since it demonstrates better oxygen reduction activity in electrochemical measurements. Thus, with the virtues of its low-cost, facile synthesis and large-scale preparation, Fe3O4@N/Co–C is a promising electrocatalyst for the oxygen reduction reaction and application in biofuel cells.

•Stainless steel fiber felt (SSFF) is used as a novel cathode for hydrogen production.•SSFF cathode significantly enhance the hydrogen production performance in MEC.•SSFF is a low-cost and high efficient cathode to replace Pt catalytic cathode.Microbial electrolysis cell (MEC) is a promising technology for sustainable production of hydrogen from biodegradable carbon sources. Employing a low-cost and high efficient cathode to replace platinum catalyzed cathode (Pt/C) for hydrogen generation is a challenge for commercialization of MEC. Here we show that a 3D macroporous stainless steel fiber felt (SSFF) with high electrochemical active surface area has an excellent catalytic activity for hydrogen generation, which is comparable to Pt/C cathode and superior to stainless steel mesh (SSM) cathode in the single-chamber MEC. The SSFF cathode (mean filter rating 100 μm) produces hydrogen at a rate of 3.66 ± 0.43 m3 H2 m−3d−1 (current density of 17.29 ± 1.68 A m−2), with a hydrogen recovery of 76.37 ± 15.04% and overall energy efficiency of 79.61 ± 13.07% at an applied voltage of 0.9 V. The performance of SSFF cathode improves over time due to a decrease in overpotential which caused by corrosion. These results demonstrate that SSFF can be a promising alternative for Pt catalytic cathode in MEC for hydrogen production.

So far, the effect of the carbon matrix on ORR catalytic efficiency over carbon/cobalt oxide nanohybrids in biofuel cells has not been investigated, which is vital to guiding the scientific research on ORR catalysts. Moreover, although cobalt oxide crystals have been reported with electrocatalytic activity, studies on square-like nano cobalt oxide are very few, and it has not been reported as an oxygen reduction reaction (ORR) catalyst, let alone used in biofuel cells. Thus, herein, square-like nano cobalt oxide anchored on nitrogen-doped graphene (NG/Co-NS), carbon nanotube (CNT/Co-NS) and carbon black (CB/Co-NS) were prepared by a one-pot hydrothermal method for the application as an ORR catalyst in microbial fuel cells (MFC). The results indicated that NG/Co-NS exhibited outstanding ORR activity with a more positive on-set potential (−0.05 V vs. Ag/AgCl) and higher limiting diffusion current (5.8 mA cm−2 at −0.8 V) than CB/Co-NS and CNT/Co-NS, attributed to the synergistic catalytic effect of NG and Co-NS. Besides, in MFC tests, the maximum power density of NG/Co-NS was improved significantly to 713.6 mW m−2, which was 24.9% higher than Pt/C (571.3 mW m−2, 0.2 mg Pt cm−2). In addition, the internal resistance of MFCs with NG/Co-NS was lower than CB/Co-NS and CNT/Co-NS, which favored the electricity generation performance. Thus, NG/Co-NS was promising material for an alternative oxygen reduction reaction electrocatalyst of Pt/C in MFCs.

Biocathodes have shown great promise for developing low-cost cathodes for hydrogen production in microbial electrolysis cells (MEC). To promote the performance of hydrogen production with a biocathode, we constructed a graphene modified biocathode and assessed the performance of the modified biocathode by setting different cathode potentials. The results indicated that it was feasible to promote the current density, electron recovery efficiency (ERE) and hydrogen production rate by a modified biocathode using graphene. At −1.1 V (vs. Ag/AgCl), the hydrogen production rate of the graphene modified biocathode even achieved 2.49 ± 0.23 m3 per m3 per day with 89.12 ± 6.03% of ERE at a current density of 14.07 ± 0.06 A m−2, which were about 2.83 times, 1.38 times and 2.06 times that of the unmodified biocathode, respectively. The hydrogen production performance of the graphene modified biocathode was close to that of the platinum catalyzed cathode and superior to that of the stainless steel mesh cathode at −1.1 V.

Novel three-dimensional (3D) macroporous cathodes for microbial fuel cells (MFCs) are constructed by using stainless steel felt (SSF) as the diffusion backing and the current collector, instead of two-dimensional (2D) materials such as a carbon cloth (CC) or stainless steel mesh (SSM), thereby resulting in an enlarged surface area for the oxygen reduction reaction (ORR). Different amounts of carbon black (CB) are applied in the base layers to optimize the performance of those SSF cathodes. The MFCs using the SSF cathodes with CB loading of 1.56 mg cm−2 (SSF-1.56) achieve a maximum power density of 1315 ± 6 mW m−2, which is 60% and 42% higher than those using the CC and SSM cathodes, respectively. The results show that the cathode of SSF-1.56 exhibits an excellent catalytic activity for ORR as well as a reduced total internal resistance, thanks to the improved three-phase interface (TPI) that not only facilitates the electron transfer, the proton transfer and the oxygen diffusion, but also offers a large surface for the ORR at the cathode. Our research also demonstrates that the SSF cathodes with an optimal CB loading will benefit the advancement of MFCs in practical application.

•Effect of different anodic pH microenvironments on MFC performance was discussed.•The main influence factors to electricity generation were analyzed.•Optimum conditions for the maximum output voltage of MFC were available.•The major metabolites produced by the mixed culture were observed.The two-chamber microbial fuel cell (MFC) was operated in batch mode, using acclimated hydrogen-producing mixed bacteria as the anodic inoculum, artificial sucrose wastewater as the substrate (sucrose concentration 10.0 g/L). The performance of the MFC was analyzed at different anodic pH microenvironments, such as the initial pH of the anolyte of 8.57, 7.3, 7.0 and 6.0, respectively, while anodic pH-controlled of 7.3 and 7.0. It showed that the best performance was obtained when the MFC was carried out at anodic pH-controlled of 7.3. Taking the interaction of factors into consideration, we adopted response surface methodology (RSM) to investigate the effects of sucrose concentration, operating temperature and ferrous sulfate concentration on the performance of MFC. The optimum condition for maximum output voltage of the two-chamber MFC (external resistance 1000 Ω) was thus obtained.

•NiCo2O4 nanoplatelets were in-situ growing on carbon cloth as ORR catalyst in biofuel cells.•Binder-free cathode with the lower internal resistance.•Binder-free cathode was low-cost.•NiCo2O4-CFC shows better power generation performance than Pt/C.Air-cathode microbial fuel cells (MFCs) was one of most promising sustainable new energy device as well as an advanced sewage treatment technology, and thoroughly studies have been devoted to lower its cost and enhance its power generation. Herein, a binder-free and low-cost catalyst air-cathode was fabricated by in-situ electro-deposition of NiCo2O4 nanoplatelets on carbon cloth, followed by feasible calcinations. The catalytic activity of catalyst air-cathode was optimized by varying the deposition time. And the optimal air-cathode was installed in real MFCs and exhibited distinct maximum out-put power density (645 ± 6 mW m−2), which was 12.96% higher than commercial Pt/C (571 ± 11 mW m−2). Noted that its remarkable electricity generation performance in MFCs should absolutely attributed to the well catalytic activity for oxygen reduction reaction, and more likely ascribed to its low internal resistance since binder-free catalyst air-cathode can facilitate the electron/charge transfer process. Therefore, it was an efficient strategy to improve the electricity generation performance of MFCs by using this binder-free catalyst air-cathode, which was also potential for application in many other electrochemical devices.

A “Spontaneous bubble-template” method is fascinating in that bubbles are formed in situ during material processing and employed as a template for fabricating unique structures, which has not been reported in material science. It is sustainable, green and efficient in that no extra additives or post-treatment are used. Herein, novel metal–polymeric framework derived hierarchically porous carbon/Fe3O4 nanohybrids are prepared using a “spontaneous bubble-template” method by one-step carbonization. During the carbonization process, N and Co are self-doped on porous carbon in which in situ grown nano Fe3O4 is embedded (Fe3O4@N/Co–C). The as-prepared Fe3O4@N/Co–C displays a three-dimensional interpenetrating morphology (electrochemical active area: 729.89 m2 g−1) with well-distributed Fe3O4 nanoparticles (20–50 nm) which are coated with a carbon layer (3–5 nm). Fe3O4@N/Co–C exhibits remarkable oxygen reduction activity in biofuel cells with a distinct output voltage (576 mV) and power density (918 mW m−2), which are 3.6% and 17.8% higher than those of Pt (0.5 mg cm−2), respectively. Besides biofuel cells, Fe3O4@N/Co–C may also have the potential for application in chemical fuel cells, since it demonstrates better oxygen reduction activity in electrochemical measurements. Thus, with the virtues of its low-cost, facile synthesis and large-scale preparation, Fe3O4@N/Co–C is a promising electrocatalyst for the oxygen reduction reaction and application in biofuel cells.