Preparation And Oxygen Reduction Catalytic Performance Study Of Sludge Biochar/Carbon Nanotube Composites

Heteroatom-doped non-noble metal carbon-based materials,as oxygen reduction reaction（ORR）catalysts for microbial fuel cells（MFCs）,are one of the ideal substitutes for commercial Pt/C catalysts.Sludge is rich in organic substances,inorganic substances and transition metal ions,etc,which is a suitable precursor for ORR catalysts.Paper mill sludge and electroplating sludge,which contained metal and N source,were synthesized respectively as PMS-CNT/c-b and N-EPS-CNT/C-（1:2）through N-doping and high-temperature carbonization techniques.The ORR electrocatalytic properties of them were studied by electrochemical technology and MFCs technology,and utilitzed a series of physical and chemical characterization to explore the subtle relationship between the microstructure and ORR.The main results are as follows:（1）Paper mill sludge is a semi-solid or concentrated solid residue in the process of papermaking wastewater treatment.Hollow porous carbon nanocomposites（PMS-CNT/C-b）containing Fe-Nx active sites were prepared from paper mill sludge and melamine by simple high temperature carbonization process.Scanning electron microscope（SEM）and transmission electron microscope（TEM）showed that the composition of melamine and Fe jointly promoted the formation of carbon nanotube structure.According to X-ray photoelectron spectroscopy（XPS）,PMS-CNT/C-b contained C,O,N,S and Fe elements and increased the active sites of pyridine N,Fe-Nx,pyrrole N and graphite N by adding of melamine.PMS-CNT/c-b not only has the initial potential（0.99 v（vs.RHE））consistent with that of commercial Pt/C catalyst,but also has a half wave potential（0.77 v（vs.RHE））higher than that of Pt/C catalyst by 10 m V in O₂saturated 0.1 M KOH solution.The catalyst also showed good ORR catalytic activity in O₂saturated 0.05m PBS solution.In addition,it had good resistance to methanol interference and stability in alkaline,and also had a good effect in the actual operation of MFCs,is a highly efficient ORR catalyst.（2）Electroplating sludge is a kind of waste from electroplating industry which contains heavy metals and exists in the form of complex liquid-solid mixtures.Due to the presence of refractory organic carbon,a small number of unstable organic compounds and a variety of transition metals,such as Fe,Cu and Ni,electroplating sludge combined with melamine to produce carbon nanotube catalysts（N-EPS-CNT/C-（1:2））.The nitrogen adsorption specific surface（BET）characterization of the catalyst indicateed that the adsorption isotherm belonged to type I,and the pore size distribution were between micropores and mesopores.XPS technology showed that the content of N and C of（N-EPS-CNT/C-（1:2））with the addition of melamine increased significantly,to～2.7 times and～3.6 times,respectively.Among them,active N existed in the form of pyridine N,Fe-Nx and pyrrole N/graphite N.X-ray diffraction（XRD）results showed that sharp Fe-Nx diffraction peaks appeared at 2θ=42.8°and 44.7°,indicating that active sites were generated.According to electrochemical activity tests,the catalyst showed good ORR electro-catalytic performance in both neutral and alkaline media,and also has a good effect in the actual operation of MFCs.The highest power density of N-EPS-CNT/C-（1:2）was831.75 W/m²,slightly higher than that of Pt/C catalyst（793.55 W/m²）.In summary,metal and N-doped carbon nanomaterials were obtained by reference of melamine without additional metal source,and the prepared materials showed excellent ORR performance compared with Pt based catalysts.The method has the characteristics of low cost,simple preparation and environmental friendliness,which provides a new idea for the utilization of sludge and the preparation of N-doped non-noble metal ORR catalyst.