Oxygen reduction reaction activity and the microbial community in response to magnetite coordinating nitrogen-doped carbon catalysts in bioelectrochemical systems

Bioproduction of cerium-bearing magnetite and application to improve carbon-black supported platinum catalysts

BACKGROUND

Biogeochemical processing of metals including the fabrication of novel nanomaterials from metal contaminated waste streams by microbial cells is an area of intense interest in the environmental sciences.

RESULTS

Here we focus on the fate of Ce during the microbial reduction of a suite of Ce-bearing ferrihydrites with between 0.2 and 4.2 mol% Ce. Cerium K-edge X-ray absorption near edge structure (XANES) analyses showed that trivalent and tetravalent cerium co-existed, with a higher proportion of tetravalent cerium observed with increasing Ce-bearing of the ferrihydrite. The subsurface metal-reducing bacterium Geobacter sulfurreducens was used to bioreduce Ce-bearing ferrihydrite, and with 0.2 mol% and 0.5 mol% Ce, an Fe(II)-bearing mineral, magnetite (Fe(II)(III)2O4), formed alongside a small amount of goethite (FeOOH). At higher Ce-doping (1.4 mol% and 4.2 mol%) Fe(III) bioreduction was inhibited and goethite dominated the final products. During microbial Fe(III) reduction Ce was not released to solution, suggesting Ce remained associated with the Fe minerals during redox cycling, even at high Ce loadings. In addition, Fe L2,3 X-ray magnetic circular dichroism (XMCD) analyses suggested that Ce partially incorporated into the Fe(III) crystallographic sites in the magnetite. The use of Ce-bearing biomagnetite prepared in this study was tested for hydrogen fuel cell catalyst applications. Platinum/carbon black electrodes were fabricated, containing 10% biomagnetite with 0.2 mol% Ce in the catalyst. The addition of bioreduced Ce-magnetite improved the electrode durability when compared to a normal Pt/CB catalyst.

CONCLUSION

Different concentrations of Ce can inhibit the bioreduction of Fe(III) minerals, resulting in the formation of different bioreduction products. Bioprocessing of Fe-minerals to form Ce-containing magnetite (potentially from waste sources) offers a sustainable route to the production of fuel cell catalysts with improved performance.

Mechanisms of magnetic sensing and regulating extracellular electron transfer of electroactive bacteria under magnetic fields

Electroactive bacteria can display notable plasticity in their response to magnetic field (MF), which prompted bioelectrochemical system as promising candidates for magnetic sensor applications. In this study, we explored the sensing and stimulatory effect of MF on current generation by Geobacter sulfurreducens, and elucidated the related molecular mechanism at the transcriptomic level. MF treatment significantly enhanced electricity generation and overall energy efficiency of G. sulfurreducens by 50 % and 22 %, respectively. The response of current to MFs was instantaneous and reversible. Cyclic voltammetry analysis of the anode biofilm revealed that the redox couples changed from -0.31 to -0.39 V (vs. Ag/AgCl), suggesting that MFs could alter electron transfer related components. Differential gene expression analysis further verified this hypothesis, genes associated with electron transfer were upregulated in G. sulfurreducens under MF treatment relative to the control group, specifically, genes encoding periplasmic c-type cytochromes (ppcA and ppcD), outer membrane cytochrome (omcF, omcZ, omcB), pili (pilA-C, pilM, and pilV2), and ribosome. The enhanced bacterial extracellular electron transfer process was also linked to the overexpression of the NADH dehydrogenase I subunit, the ABC transporter, transcriptional regulation, and ATP synthase. Overall, our findings shed light on the molecular mechanism underlying the effects of magnetic field stimuli on EAB and provide a theoretical basis for its further application in magnetic sensors and other biological system.

Effect of gas atmosphere on hydrogen production in microbial electrolysis cells

Inert gas is often used in the deoxygenation of microbial electrolysis cells (MECs) to maintain growth and viability of anaerobes. However, the effects of the gas atmosphere on hydrogen production and microbial community of MECs are often neglected. Here, the performances and biofilm microbiomes of MECs pre-sparged with different gases were compared. MECs pre-sparged with argon gas (Ar) yielded more hydrogen (3.73 ± 0.13 mol-H₂/mol-acetate) and a higher hydrogen production rate (2.99 ± 0.17 L-H₂/L-reactor-day) than MECs pre-sparged with N₂ (3.41 ± 0.13 mol-H₂/mol-acetate and 2.27 ± 0.28 L-H₂/L-reactor-day, respectively). Microbiome analysis indicated that the relative abundance of Geobacter increased from 59.25% to 77.79% when the gas atmosphere in MECs shifted from N₂ to Ar. Hydrogen production may have been catalyzed by nitrogenase from Geobacter and photosynthetic bacteria in MECs pre-sparged with Ar. These findings suggested that the gas atmosphere substantially influences the microbiome of anode biofilms and Ar sparging is most effective for enhancing hydrogen production in MECs.

Effect of waterproof breathable membrane based cathodes on performance and biofilm microbiomes in bioelectrochemical systems

A novel method for fabricating air-cathodes was developed by assembling an activated carbon (AC) catalyst together with a waterproof breathable membrane (WBM) and stainless steel mesh (SSM) to reduce manufacturing costs of bioelectrochemical systems (BESs). WBMs made of different materials were tested in the assembly, including a hybrid of polypropylene and polyolefin (PPPO), polyethylene (PE), and polyurethane (PU), and compared against poly tetrafluoroethylene (PTFE)-based cathodes. Results showed that the maximum power density of the activated carbon-stainless steel mesh-polyurethane (AC@SSM/PU) assembly was 2.03 W/m² while that of conventional carbon cloth cathode assembly (Pt@CC/PTFE) was 1.51 W/m². Compared to conventional cathode fabrication, AC@SSM/PU had a much lower cost and simpler manufacturing process. Illumina Miseq sequencing of 16S rRNA gene amplicons indicated that microbiomes were substantially different between anode and cathode biofilms. There was also a difference in the community composition between different cathode biofilms. The predominant population in the anode biofilms was Geobacter (38-75% relative abundance), while Thauera and Pseudomonas dominated the cathode biofilms. The results demonstrated that different types of air-cathodes influenced the microbial community assembly on the electrodes.

An excellent alternative composite modifier for cathode catalysts prepared from bacterial cellulose doped with Cu and P and its utilization in microbial fuel cell

In this study, bacterial cellulose doped with phosphorus and copper via freeze-drying and high-temperature pyrolysis was used to prepare MFC cathode catalysts. After a series of characterization, the synthesized catalyst showed a three-dimensional network with a specific surface area of 580.09 m²/g. Due to the doping of Cu and P, more active sites were induced in the pores of bacterial cellulose and subsequently improved catalytic activity. The prepared catalyst was coated on the air cathode surface of the MFC to obtain the maximum output power and current density of 1177.31 mW/m² and 6.73 A/m², respectively, which were higher than those of Pt (1044.93 mW/m² and 6.02 A/m²). This work aimed to improve bioelectrical generation in MFC and find alternative commercial Pt catalysts.