Biogenic Precursor To Controlled Synthesis Of Heteroatom-doped Sludge-derived Carbons And Their Performance Towards Electrocatalytic Reduction Of Oxygen

Multi-heteroatom self-doped porous carbon derived from sludge can be obtained via high-temperature carbonization,owing to the fact that the sludge inherently contains abundant elemental C and large amounts of elemental Fe,N,and P.The potential use of sludge-derived carbon as the oxygen reduction reaction?ORR?electrocatalyst offers the advantages of resource recovery from the waste sludge and replacing high-cost precious metal catalysts for fuel cell applications.Considering that the composition of sludge can be varied depending on many factorssuch as wastewater quality and treatment process,the controllable synthesis of carbon with desirable properties and performance remains a challenge.Herein,we demonstrate a strategy to tune the sludge composition by microbial enrichment,which is the key to attain functional carbon materials that show promises for application as the ORR electrocatalysts.Firstly,N,P-doped porous graphene-like carbon with outstanding ORR performance was synthesized by pyrolysis of surplus sludge from the secondary sedimentation tank;this sludge functioned as a self-doped,self-activated,and self-templated precursor by acclimation with continuous feedings of phenol.The results show that more amounts of microorganisms are enriched after acclimation,with increasing contents of N,P,Fe,as well as C atoms.The phenol-acclimated sludge pyrolyzed at 800??PSC-800?features a BET surface area as high as 402.4 m²·g^-1,which is much higher than that of the raw sludge carbon?SC-800?(262.4 m²·g^-1).X-ray photoelectron spectroscopic data suggests that the PSC-800 has higher levels of heteroatom doping,which is conducive to the formation of oxygen reduction active sites,such as pyridinic nitrogen and graphitic nitrogen.The obtained PSC-800 exhibits excellent ORR activity in alkaline media with the onset potential of 0.93 V.In comparison to the commercial Pt/C catalyst,the PSC-800 shows comparable catalytic activity in terms of the close onset potential,kinetic-limiting current and electron transfer number?approximate to 4?,higher tolerance to methanol toxicity,and superior stability.The increasing pyrolysis temperature results in the formation of an ordered graphitic structure;however,the excessively high temperature induces the drop in the amounts of the heteroatoms that are doped in the carbon framework.Secondlly,an attempt was made to obtain carbon from the primary sludge of the wastewater treatment plants,which often contains coagulants such as polyferric flocs?PFS?.To achive this,this sludge represented by PFS was inoculated by dissimilatory iron-reducing bacteria,Shewanella oneidensis MR-1,resulting in a biogenic precursor?MR-1@vivianite?.Accordingly,well-dispersed Fe₂P nanoparticles encapsulated within heteroatom-doped graphene-like carbon were obtained via the simple pyrolysis process.It is found that the strong interaction between the proteins of bacteria and the biogenic vivianite plays a significant role in preventing agglomeration of Fe₂P nanoparticles and facilitates formation of mesoporous hierarchical carbons with large surface area and high conductivity.The crystalline size of Fe₂P nanoparticles is tunable in a range from 3 nm to 80 nm by modulating the properties of the bacteria@vivianite precursor,which varies significantly depending on the molar ratio?D:A?of electron donors?i.e.,lactate?to electron acceptors?i.e.,PFS?for acclimation.The resulting Fe₂P?3nm?@BC catalyst derived from the bacteria@vivianite precursor at a D:A ratio of 8:1 exhibits substantially improved ORR activity in alkaline media with an onset potential of 1.01 V and a half-wave potential of 0.85 V,which is close to the Pt/C catalyst.The microbial fuel cell?MFC?equipped with the developed Fe₂P?3nm?@BC cathode achieves a maximum power density of 1859.3 mW m^-2,an increase of 759.4 mW m^-2 relative to that obtained from the Pt/C cathode.These findings might provide insights into the controllable synthesis of sludge-derived carbon that has an application as oxygen reduction electrocatalyst,and suggest a promising new method for resource recovery from the sludge treatment.