The Preparation And Performance Study Of Carbon-based Manganese Cobaltites As Cathodic Electrocatalysts

Microbial fuel cell (MFC), an emerging renewable energy technology, utilizes microorganisms as catalysts to degrade organic matter or inorganic matter and harvests electricity. Because of environmental compatibility, mild operation conditions, low cost and so on, MFC provides a new way to develop renewable energy and to solve the problem such as high operating cost and secondary pollution in wastewater treatment. Thus, MFC has recently attracted attention in the field of energy and environment.MFC has good development prospect because of its potential advantages, but commercialization of MFC still remains impracticable. This is due to the low power density output of MFC. Especially, both the high overpotential and sluggish kinetics of cathodic oxygen reduction reaction (ORR) obstruct improvement in the performance of MFC. In this study, the carbon-based manganese cobaltites were prepared, characterized and used as cathodic catalysts in MFC power output. Furthermore, the possibility of using as-prepared products as the alternative ORR catalysts of Pt/C was evaluated.Firstly, a series composites of manganese cobaltite and carbon black named’in situ-MnCo2O4/C’ were prepared via hydrothermal method. The effect of MnCo2O4 loading ratio on the in situ-MnCo2O4/C ORR catalytic activity was investigated. The experiment results show when the MnCo2O4:C molar ratio is 1:60, in situ-MnCo2O4/C-60 possesses superior ORR catalytic activity. The electrochemical measurements indicate the in situ-MnCo2O4/C-60 favors a quasi-4-electron oxygen reduction reaction path, and its cathodic peak potential(Ep=-0.111V) shows a 11 mV negative shift compared with Pt/C (Ep=-0.100 V). The MnCo2O4 nanoparticles (average size of 10 nm) with a crystalline spinel structure are well dispersed on carbon black based on XRD, TEM, SEM, XPS analysis.Then, carbon black supported Cu-doped Mn-Co spinel-type oxides (MnxCu1-xCo2O4/C, x=0,0.2,0.4,0.6,1) were synthesized via hydrothermal method. The effects of doping ratio on morphology, crystal phase and ORR catalytic activity of MnxCu1-xCo2O4/C were discussed. TEM and XRD results show the Cu doping has no significant influence on the MnxCu1-xCo2O4/C crystal structure and apparent morphology, and all the MnxCu1-xCo2O4 nanoparticles with spinel structure and uniform size are well dispersed on carbon black. BET results demonstrate the texture property is effected by Cu doping significantly, the Mno.6Cuo.4Co204/C owns optimum special surface area (159 m2/g). The CV measurements show Mno.6Cuo.4Co204/C has superior ORR catalytic activity (Ep=-0.160 V), but slightly lower than Pt/C (Ep=-0.100 V).Furthermore, the MnCo2O4 specie was directly anchored on the carbon felt surface (MnCo2O4-CF) via hydrothermal-annealing method. The ORR catalytic activity was compared between binder-free electrode and catalyst coating electrode with Nafion binder. XRD and SEM analysis show the as-prepared MnCo2O4 specie with spinel structure is well coated on the surface of carbon fiber, forming a uniform layer floc. The CV measurements show the ORR peak potential of MnCo2O4-CF (Ep=-0.360 V) is positive shift than coating electrode(Ep=-0.410 V).Finally, the in situ-MnCo2O4/C-60 and MnxCu1-xCo2O4/C were used parallelly as cathodic catalysts for MFCs power output respectively. The polarization curves demonstrate that maximum power density of in situ-MnCo2O4/C-60 is 545 mW/m2, which is far higher than that of the plain cathode (Pmax=214 mW/m2) and slightly lower than that of commercial Pt/C (Pmax=689 mW/m2); Mn0.6Cu0.4Co2O4/C with largest special surface area owns the best power (Pmax=469 mW/m2, OCV=0.64 V) output, which is close to that of 20 wt% Pt/C (Pmax=550 mW/m2, OCV=0.72 V).In this paper, the carbon-based Mn-Co spinel oxides as cathodic electrochemical catalysts were systematically investigated. The main effects on ORR catalytic activity are confirmed. It is found that the coupling effect, the loading of active component, metal ion doping, binder addition have significant effects on carbon-based spinel oxides ORR catalytic activity, which provide ideas for the research of non-noble metal catalysts. Also, this study is helpful for electrode materials research in other fuels system.