Controllable Preparation Of Heteroatom-doped Sludge-derived Carbons And Investigation Of Their Capacitive And Oxygen Reduction Reaction Performance

The activated sludge represents the main by-product of the wastewater treatment process;it is prone to generate bad odor and pollute the air during stacking.The treatment of the sludge remains difficult and the commonly used methods such as compositing,landfill,and incineration can easily lead to secondary pollution that causes adverse effects on human health;therefore,it is desirable to develop more environment-friendly and sustainable technologies for appropriate sludge treatment and disposal.Taking into consideration that the sludge has abundant carbon elements and contains large levels of other heteroatoms including Fe,N,P,S elements,we attempt to use it as a precursor for synthesizing heteroatom-doped carbon-based material that can find application as electrode materials in many electrochemical devices.A critical concern regarding the sludge-derived carbon preparation is whether the complex structural properties of the resulting carbon materials can be reproduced in a well-controlled manner,considering that the properties and compositions of sludge vary considerably depending on the characteristics of the wastewater,the processes and techniques adopted for wastewater treatment,and the chemicals added during treatment.To address this issue,we run a prerequisite step to identifying the key factors that determine the physicochemical properties?i.e.,surface area,porosity,conductivity,doping atoms and levels,and chemical stability?of sludge-derived carbon that are consequently responsible for its capacitive performance.It was demonstrated that the physicochemical and electrochemical properties are remarkably different among four different sludge-derived carbons:those obtained from municipal wastewater sludge,coking wastewater sludge,phenol-acclimated municipal wastewater sludge,and phenol-acclimated coking wastewater sludge?denoted as MSC,CSC,P-MSC,and P-CSC,respectively?.Hierarchical mesoporous graphene-like carbon was obtained from the acclimated sludge?particularly for P-CSC?with a high surface area,sufficient degree of graphitization,good electrical conductivity,and rich N and P doping.Accordingly,the P-CSC electrode delivered a large specific capacitance?416.1F g^-11 detected with the three-electrode mode at 1 Ag^-11 in 1M H₂SO₄ solution?,good rate capability(retention ratio of 80.7%from 1 to 15 Ag^-1),and long-term cycling stability?retention ratio of 96.9%after 10,000 cycles?,outperforming the MSC,CSC,and P-MSC electrodes.After conducting a series of control experiments,we determined that the contents of Fe,N,and P in the sludge,and the affinity of the microorganisms and particles/flocs in the sludge are crucial to the final characteristics of the sludge-derived carbon.To prepare sludge-derived carbon with improved electrochemical performance and explore its application in many electrochemical fields,we further regulated and defined the chemical compositions and morphology of the coking wastewater sludge flocs by acclimation of the coking wastewater sludge with successive feedings of ammonium.The nitrified sludge flocs are attractive precursors for producing N,S dual-doped carbon with a hierarchical mesoporous graphene-like structure via the simple one-step pyrolysis method without the addition of external N-and/or S-containing organic compounds,chemical activation agents,or graphitization catalyst precursors.Owing to its unique features,the resulting nitrified sludge floc derived carbon exhibits outstanding electrochemical performance.The specific capacitance of the obtained material determined in 1 M H₂SO₄ at a current density of 1 A g^-11 is889 Fg^-1;it also shows an excellent cycling stability with only 1.2%loss in capacitance after10000 cycles at a current density of 20 Ag^-1.This carbon material also achieves superior activity towards the four-electron oxygen reduction reaction?ORR?and proves to be a promising metal-free ORR electrocatalyst showing comparable electrocatalytic performance,higher selectivity,and longer durability as compared to the commercial Pt/C benchmark.These findings might provide insights into the controllable synthesis of sludge-derived carbon that has multifunctional applications as electrode materials,and suggest a promising method for resource recovery from the sludge treatment.