| Microbial electrosynthesis(MES)is expected to achieve the conversion of CO2 to high value-added compounds,coupling inorganic hydrogen evolution reaction(HER)with biocatalytic conversion.The cathodes in MES system have problems in the performance of hydrogen evolution,metal leaching and by-production of reactive oxygen species(ROS),all of which may limit the coupling effect with microorganism and affect operation efficiency.In this work,focusing on the biocompatibility of cathode,the interaction between cathode and biosynthesis system was systematically studied to reveal the main limitations in MES system.Relying on the rational selection and preparation of cathode,the MES system was operated efficiently and stably.In order to make insight into the operating mechanism and main limitations of MES system.the MES system,coupling HER electrocatalysts with H2-autotrophic bacterium,Cupriavidus necator H16,was taken as the model.Meanwhile,poly-β-hydroxybutyrate(PHB)can be produced via the MES system from CO2.The MES system equipped with stainless steel(SS)had been built and the operation parameters and biocompatibility indexes were investigated.It had been confirmed that the hydrogen evolution rate and the dissolution of Fe,Cr,Ni were not the limitations.The production of ROS was monitored and the concentration of H2O2 was quantified to be 611 μmol L-1,which was higher than the half-maximal inhibitory concentration(IC50)of C.necator H16.Therefore,the excessive production of ROS is the main limitation for MES system operation.To address the problems above,Mn3O4 was selected and decorated on the surface of SS by electrodeposition,which is a kind of nanozymes capable of scavenging ROS.The particle size and loading of Mn3O4 during electrodeposition were optimized,and the scan rate of 5 mV s-1 and the electrolyte concentration of 1 mmol L-1 were determined to be the optimal condition.In order to evaluate the biocompatibility of Mn3O4-decorated SS cathode(SS+Mn3O4).the dissolution concentration of Mn was monitored during MES operation,low Mn leaching was detected meaning low biotoxicity.On the other,the ROS yields of SS+Mn3O4 was determined.Beneficial from the physical separation and immediate decomposition of SS+Mn3O4,the ROS yields of SS+Mn3O4,including ·O2-,·OH and H2O2.can be reduced by 14.0%,47.9%and 25.1%compared with the unmodified SS cathode,respectively.Better biocompatibility of SS+Mn3O4 can be concluded concerning spot assay experiment.261.7 mg L-1of PHB and 7.70%of energy conversion efficiency(ECE)were attained during MES system operation equipped with SS+Mn3O4.Furtherly,in order to reduce the intrinsic ROS yields of cathode materials,porphyrinic triazine-based frameworks anchored with metals(Fe,Co,Ni and Cu)were rationally prepared and porphyrinic triazine-based frameworks anchored with Co(Co@PTF)was selected and as the electrocatalyst for its best catalytic performance of HER.Co@PTF showed excellent biocompatibility.On one hand,the dissolution concentration of Co was far lower than the IC50 of C.necator H16,which can be attributed to low Co loading and the stabilizing effect of metalloporphyrins.On the other,beneficial from the synergistic effect of Co-Nx,the electron transfer number for oxygen reduction reaction(ORR)of Co@PTF was determined to be 3.75,thus the by-production of ROS,including ·O2-,·OH and H2O2,was effectively reduced owing to its high selectivity for the 4e--ORR.Excellent biocompatibility of Co@PTF can be concluded concerning spot assay experiment.Furtherly,the MES system equipped with Co@PTF successfully converted CO2 to PHB with excellent stability,whose maximum concentration of PHB reached 252.2 mg L-1 and ECE got to 4.66%.This work provides theoretical support and novel strategies for the preparation and modification of biocompatible cathodes in MES system and shows significance in the study of coulping mechanism. |
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