Research On Cathodic Structure And Catalyst Of Air-cachode Microbial Fuel Cells

Taking advantage of bacterium consuming the organics in the wastewater,microbial fuel cell （MFC） directly converts the bioenergy to electricity and it isbecoming a new research direction in the field of environmenetal engineering.Among many different types of MFC, air-cathode microbial fuel cell （ACMFC）provides relatively high theoretical cell voltage because it uses the oxygen in the airas the cathodic catalyst for oxygen reduction reaction （ORR）. However, theexpensive platinum （Pt）, which serves as the indispensable ORR catalyst incathodes, dramatically increases the cost of ACMFCs. The relative research onACMFCs is still limited to the laboratory scale. Many low-cost substitutes for Pthave been developed and the cost of ACMFCs is indeed reduced, however, at thecost of degrading the electricity generation capacity. Reducing the cost of ACMFCwhile keeping its high electricity generation capacity is becoming an urgentproblem to be solved in the field of research on ACMFC.Focusing the goal of reducing cost and improving cathodic performance ofACMFCs, cathodic materials and structure was studied, with the detailed contentsas below:Catalytic activities of Pd/C and Ru/C for ORR were investigated by testingpower densities of a single-chamber hexahedral ACMFC and comparison with Pt/Cwas made as well. Results showed that Pd/C and Ru/C were both equivalent to Pt/Cin terms of columbic efficiencies and COD removal rates of ACMFCs and theirmaximum power densities （MPDs） were respectively90.3%and85.5%of that ofPt/C. The amount of electricity generation per cost for Pd/C and Ru/C wererespectively1.8times and6times as that for Pt/C.Low-cost nitrogen-doped carbon powders （NDCP） with high catalytic activityfor ORR and good stability were prepared and used as the cathodic catalyst ofACMFCs. In order to further improve the effect of the nitrogen-doped treatment, ahybrid pretreatment was introduced consisting of hydrochloric acid immersion andheat treatment. The nitrogen content and the types of key nitrogen functional groupwere compared in NDCPs with and without the hybrid pretreatment. Results showedthat the catalytic activity of NDCP （without the pretreatment） was1.1times as that of untreated carbon powders. The pretreatment can obviously enhance the N-dopedeffect, with the corresponding maximum power density of ACMFC increased by12.4%. Three key N-functional groups were found to be pyridinic-type nitrogen,pyrrolic-like nitrogen and chemisorbed nitrogen oxide. The grain sizes of carbonpowders and the dosage of catalysts were optimized in order to improve thecathodic performance. Reuslts showed that the grain size of1600nm was preferredand increasing the dose of NDCP could considerably improve the MPD of ACMFC.The final conclusion achieved was that the prepared NDCP （with the pretreatment）was competent enough to replace the expensive Pt/C. The MPD of the ACMFCbased on NDCP was almost equal to that of ACMFC based on Pt/C, i.e.926.0mW·m^-2for NDCP vs.980.6mW·m^-2for Pt/C, while the cost of ACMFCs wasreduced by78%. Moreover, the NDCP cathode processed much smaller polarizationloss, which was only half of that of Pt/C cathode. More importantly, NDCP showedmuch higher long-term stability than Pt/C. The MPD of ACMFC based on NDCPnearly remained unchanged after running30periods. By contrast, the MPD ofACMFC based on Pt/C decreased by as much as13%and tended to decreasefurther.Optimizations of cathodic structure was conducted in terms of the gradient ofcatalytic layer, the cooling rate when preparing diffusion layer, ash content andgrain sizes of carbon powders of the carbon base layer, and so on. Results showedthat the MPD of ACMFCs was increased by keeping the total dose of catalysts andbinder both constant while changing the single catalyst laer to the gradient multiplecatalytic layers in which the amount of catalysts gradually decreased along thedirection from the cathodic electrolyte to the outside air. The cathodic performancealso benefited from the rapid cooling when preparing the diffusion layer, becausethe developed pore passage was easier for oxygen to pass and then reach thecatalytic layer completing ORR reaction. There existed an optimum grain size,around1000mesh, for carbon powders in the carbon base layer. The lower the ashcontent was, the better the performance of ACMFCs.Based on the previous prepared low-cost NDCP catalyst and the optimizedcathodic structure, the effect of sulfide on the catalytic activity of NDCP wasstudied and compared with Pt/C. Results showed that existence of sulfide greatlydecreased the activity of Pt/C. When concentration of Na2S solution was0.2g/L, theMPD of ACMFC with Pt/C cathode was reduced by21.3%as compared to the neutral electrolyte. By contrast, NDCP cathodes nearly disregard the existence ofsulfide and the corresponding MPD of ACMFC larger than that of Pt/C by11.4%under the same concentration of Na2S. With such NDCP cathodes, not only the costof ACMFCs can be substantially reduced, but larger electricity output can be alsoachieved when ACMFCs operated in the presence of sulfide.