Effect Of Nano Mo₂C-modified Electrode Interface On Shewanella Oneidensis Bioelectrocatalysis

Microbial electrochemical systems with bioelectrocatalytic reaction as the core are the forefront of current research,which have shown obvious advantages in direct recovery of electricity from organic waste/pollutants or in electrically driven transformation/synthesis of high-value chemicals.But their efficiency has not yet reached the requirements of commercialization.It is recognized that the slow interfacial electron transfer process between electroactive microorganisms and an electrode is a crucial factor limiting the microbial electrocatalytic kinetics.Given that the microbial electrocatalytic reaction is a synergistic process that involves both microbial biocatalysis and electrochemical catalysis of an electrode,thus improving the electrochemical activity at the electrode interface is expected to be an effective way to achieve high-performance microbial electrocatalysis.In this study,effect of nano molybdenum carbide（Mo₂C）with high catalytic activity on the bioelectrocatalysis of Shewanella oneidensis MR-1,a typical and most used electroactive microorganism,was investigated systematically.To this end,the enhancement of nano-Mo₂C catalytic interface on the anodic production of electricity and cathodic reduction of fumarate by S.oneidensis,and the involved with extracellular electron transfer mechanism were studied by virtue of nanoscience,electrochemistry and transcriptomics.The main research contents and results are as follows.1.Mo₂C nanoparticles were uniformly anchored on conventional carbon felt（CF）and carbon cloth（CC）as substrate electrodes through a facile electrostatic assembly followed by high-temperature carburization approach.The prepared electrode materials were referred to as nano-Mo₂C-modified CF（Mo₂C/N-CF）and CC（Mo₂C/N-CC）electrode,respectively.When the Mo₂C/N-CF was used as an anode,the increased loading amount of bacterial cells and total protein on the Mo₂C/N-CF demonstrated the better biocompatibility of nano-Mo₂C interface than the bare carbon fiber surface,which greatly promoted the adhesion growth of S.oneidensis MR-1 cells.The plateau current density of the Mo₂C/N-CF anode posited at a potential of+0.2 V（vs.SCE）in a single-chamber half-cell was～1.508 A m^-2,which was about 2.0 and 9.5 times of that of the N-CF(product of carbonization of PDA-modified CF electrode,～0.767 A m^-2)and bare CF(～0.159 A m^-2),respectively.Moreover,the maximum power density delivered by the Mo₂C/N-CF anode in a dual-chamber MFC was～212.10 m W m^-2,which was 2.42 and 7.72 times of that of the N-CF and CF electrodes,respectively.The above results indicated that the nano-Mo₂C-modified interface significantly enhanced the extracellular electron transfer rate from the bacterial cells to the electrode.2.Effect of nano-Mo₂C-modified interface on S.oneidensis MR-1 anodic metabolism was analyzed by comparative transcriptomics.Compared to the N-CF anode,28 up-regulated genes while 40 down-regulated ones were identified in S.oneidensis MR-1 cells growth on the Mo₂C/N-CF anode.Among them,the expression of genes Hcr（encoding NADH oxidoreductase）,which are related to intracellular redox processes,were significantly up-regulated,consistent with the result of RT-q PCR.It indicated that the in vivo redox metabolic response of S.oneidensis MR-1was enhanced during the interaction with the highly active nano-Mo₂C interface,which significantly enhanced the outward electron export capacity.However,the expression of those genes related to lambda and Mu prophage was found to down-regulated,which needed to be further investigated.3.The Mo₂C/N-CC was used as a cathode to investigate the effect of nano-Mo₂C interface on electrically driven fumarate reduction by S.oneidensis MR-1.The current consumption density of the Mo₂C/N-CC cathode posited at a potential of-0.6 V（vs.SCE）was～0.41 A m^-2,about 2.7 and 4.1 folds of that of N-CC cathode(～0.15 A m^-2)and bare CC(～0.10 A m^-2),respectively.This indicated that the electrode interface modified by Mo₂C nanoparticles significantly enhanced the cathodic bioelectrocatalytic ability of S.oneidensis MR-1.The result of linear scanning voltammetry showed that the potential（～-0.4 V vs.SCE）needed for electrically driven fumarate reduction by S.oneidensis MR-1 cathode was much higher than that needed for the electrochemical hydrogen evolution,which indicated that the enhancement of fumarate reduction by the Mo₂C-modified interface was not dependent on the Mo₂C-induced electrochemical hydrogen evolution process.A comparative analysis of the interactions between the nano-Mo₂C-modified electrode interface and different S.oneidensis MR-1 mutant strains lack of Mtr pathway-associated cytochromes,as well as the endogenous electron shuttle riboflavin,revealed that the interfacial electron transfer process from the Mo₂C/N-CC cathode to S.oneidensis MR-1 was not only dependent on the Mtr pathway across the inner and outer membranes,but alos benefited from the rapid electrochemical reaction of riboflavin molecules at the Mo₂C-nano-modified interface,thereby accelerating the direct interfacial electron exchange between the electrode and the outer membrane cytochromes.In summary,it was confirmed that the highly catalytical-active nano-Mo₂C modified electrode interface can significantly promote the bioelectrocatalytic ability of Shewanella species with a significant modulating effect on intracellular energy metabolism and interfacial charge transfer.This work provides an experimental basis for an in-depth understanding of the synergistic bio-and electrochem-catalysis mechanism of microbial electrodes.