Metal–Organic Framework-Derived Rare Earth Metal (Ce-N-C)-Based Catalyst for Oxygen Reduction Reactions in Dual-Chamber Microbial Fuel Cells

Abstract

Pt supported on carbon (Pt/C) is deemed as the state-of-the-art catalyst towards oxygen reduction reactions (ORRs) in chemical and biological fuel cells. However, due to the high cost and scarcity of Pt, researchers have focused on the development of Earth-abundant non-precious metal catalysts, hoping to replace the traditional Pt/C catalyst and successfully commercialize the chemical and biological fuel cells. In this regard, electrocatalysts made of transition metals emerged as excellent candidates for ORRs, especially the electrocatalysts made of Fe and Co in combination with N-doped carbons, which produce potentially active M-N4-C (M=Co, Fe) ORR sites. At present, however, the transition metal-based catalysts are popular; recently, electrocatalysts made of rare earth metals are emerging as efficient catalysts, due to the fact that rare earth metals also have the potential to form rare earth metal-N4-C active sites, just like transition metal Fe-N4-C/Co-N4-C. In addition, mixed valance states and uniqueness of f-orbitals of the rare earth metals are believed to improve the redox properties of the catalyst that helps in enhancing ORR activity. Among the rare earth metals, Ce is the most interesting element that can be explored as an ORR electrocatalyst in combination with the N-doped carbon. Unique f-orbitals of Ce can induce distinctive electronic behavior to the catalyst that helps to form stable coordination structures with N-doped carbons, in addition to its excellent ability to scavenge the OH● produced during ORRs, therefore helping in catalyst stability. In this study, we have synthesized Ce/N-C catalysts by a metal–organic framework and pyrolysis strategy. The ORR activity of Ce/N-C catalysts has been optimized by systematically increasing the Ce content and performing RDE studies in 0.1 M HClO4 electrolyte. The Ce/N-C catalyst has been characterized systematically by both physicochemical and electrochemical characterizations. The optimized Ce/N-C-3 catalyst exhibited a half-wave potential of 0.68 V vs. RHE. In addition, the Ce/N-C-3 catalyst also delivered acceptable stability with a loss of 70 mV in its half-wave potential when compared to 110 mV loss for Pt/C (10 wt.%) catalyst, after 5000 potential cycles. When Ce/N-C-3 is used as a cathode catalyst in dual-chamber microbial fuel cells, it delivered a volumetric power density of ⁓ 300 mW m−3, along with an organic matter degradation of 74% after continuous operation of DCMFCs for 30 days.