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The oxygen reduction reaction is one ofthe limiting stepsin microbial fuel cell performance. M-N-C catalysts (M as transition metal) represent the best compromise of optimal cost, electrocatalytic activity and durability. The Fe-based catalysts were shown to be the best compared to Co-, Mn-, Ni-based catalysts. The addition of the second transition metal such as Mn was shown to increase the selectivity of the reaction and reduce peroxide production. The use of different N-C precursors resulted in diverse surface chemistry that directly affects the performance. Generally, surface chemistry plays a critical role in the electrocatalytic activity. Integration of the catalyst in the air-breathing cathode is also discussed with a performance that is enhanced by: i) increased catalyst loading; ii) the addition of graphene to structure.

The effects of the synthesis steps of a platinum group metal-free (PGM-free) catalyst on the surface chemistry, morphology, and electrochemical activity in acidic and alkaline media toward the oxygen reduction reaction (ORR) were studied. Each step exhibits a positive impact on catalyst activity. In acid media, etching of the silica template is the major contributor to the enhancement of the half-wave potential, as the ORR active sites formed during the first pyrolysis become more accessible. Further processing steps result even in higher accessibility and utilization of the 4e– transfer sites. In alkaline media, the second pyrolysis is a critical step that favors the complete reduction of oxygen to water, as the peroxide production is significantly diminished. The large heterogeneity in the porosity at each synthesis step indicates that this parameter needs to be further studied to attain better control of the morphology of the PGM-free catalyst, as it is an important factor that contributes to the active site utilization. The acid etching and second pyrolysis increase the meso- and macroporosity. Understanding the effects of each of the synthesis steps on the chemical composition, morphology and ORR activity of the PGM-free catalyst provides the necessary feedback for the design of synthetic schemes that increase the catalysts’ performance.

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The effect of tris(hydroxymethyl)aminomethane (Tris) on the activity of metal−nitrogen−carbon (M−N−C) platinum metal group-free (PGM-free) catalysts for oxygen reduction reaction (ORR) is studied. Electrochemical tests performed in the acidic and alkaline electrolyte and comparison between electrochemical observations for metal-free and metal-containing catalysts along with density functional calculations showed the multiplex mechanism of oxygen reduction in which numerous species perform partial and full reduction of oxygen to peroxide or water, respectively. The important role of protons in the mechanism of oxygen reduction is highlighted.

The impact of the electrolyte’s pH on the catalytic activity of platinum group metal-free (PGM-free) catalysts toward the oxygen reduction reaction (ORR) was studied. The results indicate that the ORR mechanism is determined by the affinity of protons and hydroxyls toward multiple functional groups present on the surface of the PGM-free catalyst. It was shown that the ORR is limited by the proton-coupled electron transfer at pH values below 10.5. At higher pH values (>10.5), the reaction occurs in the outer Helmholtz plane (OHP), favoring hydrogen peroxide production. Using a novel approach, the changes in the surface chemistry of PGM-free catalyst in a full pH range were studied by X-ray photoelectron spectroscopy (XPS). The variations in the surface concentration of nitrogen and carbon species are correlated with the electron transfer process and overall kinetics. This study establishes the critical role of the multitude of surface functional groups, presented as moieties or defects in the carbonaceous “backbone” of the catalyst, in mechanism of oxygen reduction reaction. Understanding the pH-dependent mechanism of ORR provides the basis for rational design of PGM-free catalysts for operation across pH ranges or at a specific pH of interest. This investigation also provides the guidelines for developing and selecting ionomers used as “locally-confined electrolytes”, by taking into account affinities and possible interactions of specific functional groups of the PGM-free catalysts with protons or hydroxyls facilitating the overall ORR kinetics.

The oxygen reduction reaction (ORR) is one of the major factors that is limiting the overall performance output of microbial fuel cells (MFC). In this study, Platinum Group Metal-free (PGM-free) ORR catalysts based on Fe, Co, Ni, Mn and the same precursor (Aminoantipyrine, AAPyr) were synthesized using identical sacrificial support method (SSM). The catalysts were investigated for their electrochemical performance, and then integrated into an air-breathing cathode to be tested in “clean” environment and in a working microbial fuel cell (MFC). Their performances were also compared to activated carbon (AC) based cathode under similar conditions. Results showed that the addition of Mn, Fe, Co and Ni to AAPyr increased the performances compared to AC. Fe-AAPyr showed the highest open circuit potential (OCP) that was 0.307 ` 0.001 V (vs. Ag/AgCl) and the highest electrocatalytic activity at pH 7.5. On the contrary, AC had an OCP of 0.203 ` 0.002 V (vs. Ag/AgCl) and had the lowest electrochemical activity. In MFC, Fe- AAPyr also had the highest output of 251 ` 2.3 mWcm"2, followed by Co-AAPyr with 196 ` 1.5 mWcm"2, Ni-AAPyr with 171 ` 3.6 mWcm"2, Mn-AAPyr with 160 ` 2.8 mWcm"2 and AC 129 ` 4.2 mWcm"2. The best performing catalyst (Fe-AAPyr) was then tested in MFC with increasing solution conductivity from 12.4 mScm"1 to 63.1 mScm"1. A maximum power density of 482 ` 5 mWcm"2 was obtained with increasing solution conductivity, which is one of the highest values reported in the field.

Catalytic activity towards the oxygen reduction reaction (ORR) of PGM-free electrocatalysts integrated with an enzyme (bilirubin oxidase, BOx) in neutral media was studied. The effects of chemical and morphological characteristics of PGM-free materials on the enzyme enhancement of overall kinetics was investigated. The surface chemistry of the PGM-free catalyst was studied using X-ray Photoelectron Spectroscopy. Catalyst surface morphology was characterized using two independent methods: length-scale specific image analysis and nitrogen adsorption. Good agreement of macroscopic and microscopic morphological properties was found. Enhancement of ORR activity by the enzyme is influenced by chemistry and surface morphology of the catalyst itself. Catalysts with a higher nitrogen content, specifically pyridinic moieties, showed the greatest enhancement. Further, catalysts with a higher fraction of surface roughness in the range of 3-5 nm exhibited greater performance enhancement than catalysts lacking features of this size.

Iron(II) phthalocyanine (FePc) deposited onto two different car- bonaceous supports was synthesized through an unconven- tional pyrolysis-free method. The obtained materials were stud- ied in the oxygen reduction reaction (ORR) in neutral media through incorporation in an air-breathing cathode structure and tested in an operating microbial fuel cell (MFC) configura- tion. Rotating ring disk electrode (RRDE) analysis revealed high performances of the Fe-based catalysts compared with that of activated carbon (AC). The FePc supported on Black-Pearl carbon black [Fe-BP(N)] exhibits the highest performance in terms of its more positive onset potential, positive shift of the half-wave potential, and higher limiting current as well as the highest power density in the operating MFC of (243 7) mW cm2, which was 33 % higher than that of FePc support- ed on nitrogen-doped carbon nanotubes (Fe-CNT(N) ; 182 5 mW cm2). The power density generated by Fe-BP(N) was 92 % higher than that of the MFC utilizing AC ; therefore, the utilization of platinum group metal-free catalysts can boost the performances of MFCs significantly.