Hollow Mn 3 O 4 nanoparticles (diameter = 31 nm, cavity diameter = 16 nm, and shell thickness = 7 nm) were attached to the surface of multiwall carbon nanotubes (MWCNT). A suspension of hollow Mn 3 O 4 /MWCNT with Nafion™ was dropcast onto a glassy carbon electrode, and the electrochemical reduction of oxygen in aqueous solution was investigated with this electrode. We assess the role of MWCNT, hollow Mn 3 O 4 , and Nafion™ in the performance of the electrode, and investigate the kinetics of the oxygen reduction reaction. The electrode exhibits outstanding performance in measures of cathodic current density and onset potential, and performed similarly well in acidic, neutral, and alkaline conditions.

Operating microbial fuel cells (MFCs) under extreme pH conditions offers a substantial benefit. Acidic conditions suppress the growth of undesirable methanogens and increase redox potential for oxygen reduction reactions (ORRs), and alkaline conditions increase the electrocatalytic activity. However, operating any fuel cells, including MFCs, is difficult under such extreme pH conditions. Here, we demonstrate a pH-universal ORR ink based on hollow nanospheres of manganese oxide (h-Mn 3 O 4 ) anchored with multiwalled carbon nanotubes (MWCNTs) on planar and porous forms of carbon electrodes in MFCs (pH = 3–11). Nanospheres of h-Mn 3 O 4 (diameter ∼ 31 nm, shell thickness ∼ 7 nm) on a glassy carbon electrode yielded a highly reproducible ORR activity at pH 3 and 10, based on rotating disk electrode (RDE) tests. A phenomenal ORR performance and long-term stability (∼106 days) of the ink were also observed with four different porous cathodes (carbon cloth, carbon nanofoam paper, reticulated vitreous carbon, and graphite felt) in MFCs. The ink reduced the charge transfer resistance ( R ct ) to the ORR by 100-fold and 45-fold under the alkaline and acidic conditions, respectively. The current study promotes ORR activity and subsequently the MFC operations under a wide range of pH conditions, including acidic and basic conditions.

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