| The gradually exhausting traditional energy sources and serious environmental pollution are two major problems of the world today.Vigorously developing renewable clean energy has become the most urgent task and topics for humanity.Bio-electrochemical system?BES?,including microbial fuel cell?MFC?and microbial electrolysis cell?MEC?,is a completely new biological technology.During organic wastewater treatment process,it can recover electricity or hydrogen.Thus,this technology has attracted increasing research attention.However,the technology is still in its infancy.Many problems have to be resolved and improved.The cathodic electron acceptor and hydrogen evolution catalyst have significant effects on electricity generation and hydrogen production for BES.The cathodic electron acceptors of MFC and the hydrogen evolution catalysts of MEC have been systematically studied in the present dissertation.NaBrO3 and Ag2O were tested as possible cathodic electron acceptors in MFC,andtheirperformanceswereinvestigated.Mg?OH?2/grapheneand MoS2/graphene were synthesized,and the hydrogen evolution reaction activity and stability were studied.Meanwhile,the biocathode of MEC was cultivated.In addition,the biodiversity was analyzed using acer sequencing techniques,and the excellent bacteria were selected for hydrogen production.The main contents are as follows:1.A new idea of bromate with high oxidation-reduction potential as a cathodic electron acceptor was proposed.Sodium bromate was selected as cathodic electron acceptor for dual chamber MFC.The effects of sodium bromate concentration and initial catholyte pH on the electricity production of MFC were investigated.The mechanism of BrO3-reduction was also discussed.Experimental results showed that the MFC performance improved with increasing sodium bromate concentration and decreasing catholyte pH.The optimal open circuit potential?1.635 V?and maximum voltage output?0.538±0.013 V?were obtained with sodium bromate concentration of 100 mM at pH3.0.The specific reaction steps of BrO3-reduction to Br-under acidic conditions were given,which provided the basis for further study of the reaction mechanism.2.Ag2O/Ag electrode was prepared through the oxidation of sterling silver by electrochemical method.The prepared electrode was used as a solid state electron acceptor in a single chamber MFC.The surface morphology and phase of the Ag2O/Ag electrode were characterized by scanning electron microscopy?SEM?and X-ray powder diffraction?XRD?.SEM image showed that several cube crystal particles appeared on the surface of sterling silver after electrochemical oxidation.The particle size of these crystals was approximately0.61.5?m.The characteristic peak corresponding to Ag2O?111?plane appears in the XRD pattern,indicating that the coordinated tetrahedrons of[Ag4-O5]6-were priority overlapped on the?111?plane to form a stable Ag2O.MFC testes showed that the performance of Ag2O electron acceptor was stable in electricity production and regeneration.The maximum voltage output in 100 cycles was maintained at 0.470.5 V,and the overpotential loss for Ag2O was only 0.0210.006 V.The energy required for electrochemical reoxidation accounts for4050%of the energy produced by the MFC.The above results indicated that Ag2O was a good MFC cathodic electron acceptor.3.The Mg?OH?2/graphene composite was synthesized as hydrogen evolution catalyst by simple hydrothermal method with MgSO4 and graphene oxide as precursors.The composite was formed by electrostatic self-assembly with positive charge Mg2+and negative charge graphene.Due to the spatial limitation of graphene,the aggregation of Mg?OH?2 was prevented,and the Mg?OH?2 morphology evolved from a 2D compact structure to a 3D flower-like superstructure,which increased the specific surface area of the Mg?OH?2.Linear sweep voltammetry tests showed that the composite synthesized from 50 wt.%MgSO4·7H2O and 50 wt.%GO with a surface density of 1.5 mg·cm-22 exhibited the best catalytic activity for hydrogen evolution reaction.The tafel slope of Mg?OH?2/graphene carbon paper electrode was 45 mV·dec-11 due to the synergetic effect of Mg?OH?2 and graphene.In the MEC tests,the hydrogen production rate,hydrogen recovery and cathodic hydrogen recovery with Mg?OH?2/graphene cathode were respectively 0.42±0.07 m3H2·m-3d-1,71.4±12.0%and 82.8±9.2%,which were all higher than those obtained with Pt/C cathode.The experiments were carried out for 2 months,the performance of Mg?OH?2/graphene cathode remained stable.Most importantly,the cost of Mg?OH?2/graphene cathode was low,and only one in sixty of Pt/C cathode.4.A series of MoS2/graphene composites were synthesized via simple hydrothermal method.Then,they were loaded on the carbon-based electrode.SEM and TEM images showed that Mo S2 nanosheets and graphene were tightly bonded to facilitate rapid transport of electron,which improved their electrochemical performance.The layer thickness of Mo S2/graphene materials with a three-dimensional layered structure was 515 nm.The molybdenum sulfide loaded on graphene was amorphous Mo S2.Compared with the pure MoS2,the specific surface area of the MoS2/graphene was 9 times than that of pure MoS2,and the ultrathin layer structure was beneficial to the exposure of more catalytically active sites of MoS2.Linear sweep voltammetry tests showed that the composite synthesized from 50 wt.%?NH4?2MoS4 and 50 wt.%graphene oxide with a surface density of 1.5 mg·cm-22 exhibited the best catalytic activity for hydrogen evolution reaction.The tafel slope of the best electrode was 38.3 mV·dec-11 due to the synergetic effect of Mo S2 and graphene.In the MEC tests,the hydrogen production rate,hydrogen recovery,cathodic hydrogen recovery and coulomb efficiency obtained with MoS2/graphene cathode were respectively 0.424±0.041 m3H2·m-3d-1,70.47±6.78%,78.86±2.49%and 89.11±5.87%,which were higher than those obtained with Pt/C cathode,indicating that the MoS2/graphene had good catalytic activity for hydrogen evolution.The experiments were carried out for 3 months,the hydrogen-producing current and gas production of MoS2/graphene cathode remained stable,suggesting that the catalytic stability of MoS2/graphene was better.In addition,MoS2/graphene cathode was inexpensive.These advantages made it suitable for practical application.5.The biocathode of MEC was cultivated,and a comprehensive analysis was performed on the starting-up process,hydrogen production efficiency and biological diversity of the biocathode.The results showed that only 163 h was required for successful start-up,and a current density of 14.75 A·m-22 was obtained at an applied voltage of 0.7 V.In the formal hydrogen-production stage,the hydrogen recovery and hydrogen production rate of biocathode were respectively 71.22±8.98%and 0.428±0.054 m3H2·m-3d-1,respectively.These values were higher than those obtained with Pt/C cathode.The results of acer sequencing showed that environment had great influence on the enrichment and elimination of microorganisms.The microbial populations in the MEC were less diverse than those of the original aerobic activated sludge?AAS?and the anodophilic biofilm of MFC.At the phylum level,Proteobacteria was evidently dominant in the anodophilic biofilm of MEC?MECan?and the cathodophilic biofilm of MEC?MECca?,accounting for 59.73%and 53.14%,which was far higher than that in AAS and MFC.At the class level,Alphaproteobacteria and Gammaproteobacteria were the dominant bacteria in MEC,accounting for35.92%and 39.05%of the total bacteria,respectively.While the two classes accounted for 21.8%in MFC and 15.98%in AAS.It was proved that the applied electric field was beneficial to the microbial enrichment of these two classes.Compared to AAS,the proportion of Clostridia in MFC and MEC was greatly increased,indicating that the bacteria could not only produce electricity,but also could produce hydrogen.Pseudomonas,Petrimonas and Azospirillum were the dominant hydrogenogens in MEC at the genus level,the applied electric field could be beneficial to their enrichment. |
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