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Developing high-performance cathodic catalysts and exploiting new techniques to investigate the nanoscale process of the oxygen reduction reaction (ORR) are of significant importance for the wide application of microbial fuel cells (MFCs). Here, novel Fe-based clusters embedded in nitrogen doped activated carbon (Fe-clusters/NAC) catalysts were developed via a simple ball milling method. Abiotic electrochemical tests demonstrated that these Fe-clusters/NAC catalysts exhibited an enhanced ORR performance due to the formation of Fe-based clusters, and the exposure of abundant active sites including quaternary N, Fe–N and Fe-based clusters. More importantly, the Fe-clusters/NAC catalysts exhibited an even better ORR performance than Pt due to the existence of more active sites than Pt, which was confirmed by using a scanning electrochemical microscope. Acid etching treatment and density functional theory calculations further verified the superior ORR activity of Fe-based clusters. The power density of the Fe-clusters/NAC catalyst-based MFCs reached 2387 mW m−2, which is the highest value among the reported Fe–N–C catalysts under the same/similar experimental conditions over the last five years, at least 65% higher than that of AC and 50% higher than that of commercial Pt.

Activated carbon (AC) is an inexpensive and sustainable catalyst for oxygen reduction in air-cathodes of microbial fuel cells (MFCs), but its electrical conductivity is relatively poor. To improve cathode performance, five different more conductive materials were added to AC: three carbon materials (carbon black, mesoporous carbon, and carbon nanotubes), and two metal powders (inexpensive copper and inert gold). Carbon-based particles improved maximum power densities by 6–14% compared to plain AC due to reduced charge transfer resistance. Copper powder had reduced performance, likely due to toxicity effects on the anode bacteria, while gold particles were similar to plain AC. Heat treated AC mixed with carbon black produced the highest power density of 1900 ± 76 mW m−2, 41% higher than the widely used Pt air-cathode (1350 ± 55 mW m−2). The use of inexpensive carbon black with heat treatment was therefore the most effective and economical approach for improving cathode performance in MFCs.

Correction for ‘Iron-based clusters embedded in nitrogen doped activated carbon catalysts with superior cathodic activity in microbial fuel cells’ by Xiaoyuan Zhang et al., J. Mater. Chem. A, 2020, DOI: 10.1039/C9TA10797E.

Air-cathodes are a critical component for microbial fuel cells (MFCs) and need to have high catalytic performance for the oxygen reduction reaction (ORR). As an important two-dimensional material, graphene has been explored in various applications including ORR catalysts for MFCs. However, the reported graphene for MFC cathodes was usually small flakes/powders, which cannot be directly coated onto metal meshes without binders. Here, we report a binder-free nitrogen-doped graphene (NG) sheet in situ grown on nickel mesh as an efficient catalyst layer for MFC air-cathodes. By optimizing the growth parameters of NG, the maximum power density of MFCs based on NG can be boosted up to 1470 ± 80 mW m−2, which is 32% higher than that of the conventional Pt/C air-cathode. The optimized NG air-cathode has a low internal resistance (21 ± 3 Ω), only 20% of that of the Pt/C air-cathode. These results provide a proof-of-concept for the binder-free NG air-cathode as an alternative to the costly Pt cathode for MFCs.

The characteristics of several different types of diffusion layers were systematically examined to improve the performance of activated carbon air cathodes used in microbial fuel cells (MFCs). A diffusion layer of carbon black and polytetrafluoroethylene (CB + PTFE) that was pressed onto a stainless steel mesh current collector achieved the highest cathode performance. This cathode also had a high oxygen mass transfer coefficient and high water pressure tolerance (>2 m), and it had the highest current densities in abiotic chronoamperometry tests compared to cathodes with other diffusion layers. In MFC tests, this cathode also produced maximum power densities (1610 ± 90 mW m−2) that were greater than those of cathodes with other diffusion layers, by 19% compared to Gore-Tex (1350 ± 20 mW m−2), 22% for a cloth wipe with PDMS (1320 ± 70 mW m−2), 45% with plain PTFE (1110 ± 20 mW m−2), and 19% higher than those of cathodes made with a Pt catalyst and a PTFE diffusion layer (1350 ± 50 mW m−2). The highly porous diffusion layer structure of the CB + PTFE had a relatively high oxygen mass transfer coefficient (1.07 × 10−3 cm s−1) which enhanced oxygen transport to the catalyst. The addition of CB enhanced cathode performance by increasing the conductivity of the diffusion layer. Oxygen mass transfer coefficient, water pressure tolerance, and the addition of conductive particles were therefore critical features for achieving higher performance AC air cathodes.