Preparation Of Three-dimensional Porous Metal-nitrogen-carbon Cathode Catalysts And Their Application In Microbial Fuel Cells

It is of great significance to develop clean and sustainable energy storage and conversion devices with the consumption of fossil energy and environmental degradation.Microbial fuel cell(MFC)is a renewable and pollution-free device,which converts organic matters in sewage into electricity by the oxidation of electrogenic bacteria.On the one hand,the sluggish oxygen reduction reaction(ORR)gave rise to excessive overpotential at the cathode,which seriously limited the practical application of MFC.To substitute the expensive and naturally scarce precious metal catalysts,the transition metal-nitrogen-carbon catalysts with low cost,durability,and high electrocatalytic activity had been developed,which promoted the large-scale application of MFC.On the other hand,the microbial community in the anode was related to the efficiency of organic matter oxidation and extracellular electron transfer.As the difference in external conditions,the microbial community of anode biofilms was affected by the cathode coated with different ORR performance catalysts.In this paper,two 3D hierarchical porous transition metal-nitrogen-carbon catalysts were developed with the template method.The ORR performance and stability of catalysts were improved by the structural design and the dopant of heteroatom.The microbial community was detected when the excellent ORR catalysts were equipped in MFC,which achieved the overall improvement of MFC power production performance.(1)the 3D hierarchical porous Co-N-C(3DHP Co-N-C)catalysts were designed and synthesized by a metal-organic framework ZIF-67 as the precursor and SiO₂ sphere as the hard template.The 3DHP Co-N-C-2 with an approximate 129 nm macropore structure exhibited excellent ORR performance in 0.1 M KOH solution with a half-wave potential of 0.80 V vs.RHE and 50 m M solution(E_1/2=0.72 V vs.RHE).The more content of the Co-N_x active site was exposed in the specific macropore-mesopore-micropore hierarchal structure,which accelerated the electrolyte transport and oxygen diffusion.The MFC with 3DHP Co-N-C-2 as the cathode catalysts possessed excellent performance with a maximum power density of426.9±7.87 m W m^-2 and favorable durability after 50 d of operation.In addition,16S r DNA results revealed the different abundance of dominant electrogenic bacterias and some important non-electrogenic bacterias in the anode biofilm in MFCs when equipped with cathode catalysts with different ORR activity.And 3DHP Co-N-C-2 was found to be beneficial to the synergistic effect of electrogenic and non-electrogenic bacteria,which enhanced power generation and wastewater treatment performance of the whole MFC.(2)The 3D hierarchical porous copper,nitrogen,and boron co-doped carbon(3DHP Cu-N/B-C)catalyst was synthesized by the dual template of SiO₂ and Zn Cl₂.The higher content of the Cu-Nx active site was obtained with a dopant of B by H3BO3.The introduced B with low electronegativity increased the electronic density around the abundant Cu-Nx active sites,which further enhanced the bonding energy of the reaction intermediate and facilitated the efficiency of the ORR process as well as achieved the stabilization of the active sites.3DHP Cu-N/B-C catalyst exhibited excellent ORR performance in 0.1 M KOH electrolyte and 50 m M PBS electrolyte.Furthermore,the MFC with 3DHP Cu-N/B-C catalyst showed a prominent maximum power density of 760.14±19.03 m W m^-2 and long-term durability.Moreover,the high-throughput sequencing of 16S r DNA presented that the MFC with 3DHP Cu-N/B-C catalyst revealed a high relative abundance of anaerobic electroactive bacteria in the anode biofilms and the fewer content of aerobic bacteria in cathode biofilms,which boosted the power output and long-term stability.