Engineering the electronic structure of high performance FeCo bimetallic cathode catalysts for microbial fuel cell application in treating wastewater.

The development of high-performance, strong-durability and low-cost cathode catalysts toward oxygen reduction reaction (ORR) is of great significance for microbial fuel cells (MFCs). In this study, a series of bimetallic catalysts were synthesized by pyrolyzing a mixture of g-C3N4 and Fe, Co-tannic complex with various Fe/Co atomic ratios. The initial Fe/Co atomic ratio (3.5:0.5, 3:1, 2:2, 1:3) could regulate the electronic state, which effectively promoted the intrinsic electrocatalytic ORR activity. The alloy metal particles and metal-Nx sites presented on the catalyst surface. In addition, N-doped carbon interconnected network consisting of graphene-like and bamboo-like carbon nanotube structure derived from g-C3N4 provided more accessible active sites. The resultant Fe3Co1 catalyst calcined at 700 °C (Fe3Co1-700) exhibited high catalytic performance in neutral electrolyte with a half-wave potential of 0.661 V, exceeding that of the commercial Pt/C (0.6 V). As expected, the single chamber microbial fuel cell (SCMFC) with 1 mg/cm2 loading of Fe3Co1-700 catalyst as the cathode catalyst afforded a maximum power density of 1425 mW/m2, which was 10.5% higher than commercial Pt/C catalyst with the same loading (1290 mW/m2) and comparable to the Pt/C catalyst with 2.5 times higher loading ( 1430 mW/m2). Additionally, the Fe3Co1-700 also displayed better long-term stability over 1100 h than the Pt/C. This work provides an effective strategy for regulating the surface electronic state in the bimetallic electro-catalyst.

Enhancement of Mass Transport for Oxygen Reduction Reaction Using Petal-Like Porous Fe-NC Nanosheet.

Nitrogen-coordinated single-atom catalysts (SACs) have emerged as a new frontier for accelerating oxygen reduction reaction (ORR) owing to the optimal atom efficiency and fascinating properties. However, augmenting the full exposure of active sites is a crucial challenge in terms of simultaneously pursuing high metal loading of SACs. Here, petal-like porous carbon nanosheets with densely accessible Fe-N4 moieties (FeNC-D) are constructed by combining the space-confinement of silica and the coordination of diethylenetriaminepentaacetic acid. The resulted FeNC-D catalyst possesses an enhanced mesoporosity and a balanced hydrophobicity/hydrophilicity, which can facilitate mass transport and advance the exposure of inaccessible Fe-N4 sites, resulting in efficient utilization of active sites. By virtue of the petal-like porous architecture with maximized active site density, FeNC-D demonstrates superior ORR performance in a broad pH range. Remarkably, when utilized as the air cathode in Zn-air battery (ZAB) and microbial fuel cell (MFC), the FeNC-D-based device displays a large power density (356 mW cm-2 for ZAB and 1041.3 mW m-2 for MFC) and possesses remarkable stability, substantially outperforming the commercial Pt/C catalyst.

Enhancing oxygen reduction reaction in air-cathode microbial fuel cells treating wastewater with cobalt and nitrogen co-doped ordered mesoporous carbon as cathode catalysts.

The sluggish oxygen reduction reaction (ORR) on the cathode severely limits the energy conversion efficiency of microbial fuel cells (MFCs). In this study, cobalt and nitrogen co-doped ordered mesoporous carbon (Cox-N-OMC) was prepared by heat-treating a mixture of cobalt nitrate, melamine and ordered mesoporous carbon (OMC). The addition of cobalt nitrate remarkably improved the ORR reactivity, compared to the nitrogen-doped OMC catalyst. By optimizing the dosage of cobalt nitrate (x = 0.6, 0.8 and 1.0 g), the Co0.8-N-OMC catalyst displayed excellent ORR catalytic performances in neutral media with the onset potential of 0.79 V (vs. RHE), half-wave potential of 0.59 V and limiting current density of 5.43 mA/cm2, which was comparable to the commercial Pt/C catalyst (0.86 V, 0.60 V and 4.76 mA/cm2). The high activity of Co0.8-N-OMC catalyst was attributed to the high active surface area, higher total nitrogen amount, and higher relative distribution of graphitic nitrogen and pyrrolic nitrogen species. Furthermore, single chamber microbial fuel cell (SCMFC) with Co0.8-N-OMC cathode exhibited the highest power density of 389 ± 24 mW/m2, chemical oxygen demand (COD) removal of 81.1 ± 2.2% and coulombic efficiency (CE) of 17.2 ± 2.5%. On the other hand, in the Co1.0-N-OMC catalyst, increasing the cobalt dosage from 0.8 to 1.0 g resulted in more oxidized-N species, and the reduced power generation in SCMFC (360 ± 8 mW/m2). The power generated by these catalysts and results of electrochemical evaluation were strongly correlated with the total nitrogen contents on the catalyst surface. This study demonstrated the feasibility of optimizing the dosage of metal to enhance wastewater treatment capacity.

Enhancing oxygen reduction reaction by using metal-free nitrogen-doped carbon black as cathode catalysts in microbial fuel cells treating wastewater.

Microbial fuel cells (MFCs) is promising to combat environmental pollution by converting organic waste to electricity. One critical problem for practical application of MFCs treating wastewater is sluggish oxygen reduction reaction (ORR) on cathode. This study focused on developing novel metal-free cost-effective cathodic catalysts to enhance power generation of MFCs. Specifically, carbon powder (Vulcan XC-72R) was modified with acid treatment and pyrazinamide (as nitrogen precursor), and subsequently pyrolyzed at different temperatures. For CN-X (X = 700-1000 °C) materials, chemical compositions (the doping contents of nitrogen species, oxygen-containing groups, and sulfur-containing groups) were altered with pyrolysis temperature. Linear sweep voltammetry showed that CN-800 exhibited the highest ORR activity, with an onset potential of 0.215 V and a half-wave potential of -0.096 V (vs. Ag/AgCl). Electrochemical measurements clearly presented an enhancement of ORR activity by treating carbon powder with sulfuric acid and nitrogen doping, which was well correlated with voltage output in single chamber MFCs (SCMFCs). On the other hand, for the nitrogen-doped cathode catalysts, the best performance in SCMFCs was directly related with the amount of pyridinic nitrogen species and total nitrogen amount. The MFC operated with CN-800 exhibited a maximum power density of 371 ± 3 mW/m2 with the chemical oxygen demand (COD) removal of 77.2 ± 1.5% and coulombic efficiency (CE) of 8.6 ± 0.3%. Furthermore, the MFC with CN-800 exhibited an excellent stability over longer than 580 h of operation with 1.5% voltage reduction. CN-800 possessed comparable COD removal efficiency to conventional costly Pt/C, and exhibited distinct cost-effectiveness for MFC practical applications in wastewater treatment.