A new anaerobic treatment system that combined a microbial electrolysis cell (MEC) with membrane filtration using electrically conductive, porous, nickel-based hollow-fiber membranes (Ni-HFMs) was developed to treat low organic strength solution and recover energy in the form of biogas. This new system is called an anaerobic electrochemical membrane bioreactor (AnEMBR). The Ni-HFM served the dual function as the cathode for hydrogen evolution reaction (HER) and the membrane for filtration of the effluent. The AnEMBR system was operated for 70 days with synthetic acetate solution having a chemical oxygen demand (COD) of 320 mg/L. Removal of COD was >95% at all applied voltages tested. Up to 71% of the substrate energy was recovered at an applied voltage of 0.7 V as methane rich biogas (83% CH4; <1% H2) due to biological conversion of the hydrogen evolved at the cathode to methane. A combination of factors (hydrogen bubble formation, low cathode potential and localized high pH at the cathode surface) contributed to reduced membrane fouling in the AnEMBR compared to the control reactor (open circuit voltage). The net energy required to operate the AnEMBR system at an applied voltage of 0.7 V was significantly less (0.27 kWh/m3) than that typically needed for wastewater treatment using aerobic membrane bioreactors (1-2 kWh/m3).

Single-atom metal (SA-M) catalysts with high dispersion of active metal sites allow maximum atomic utilization. Conventional synthesis of SA-M catalysts involves high-temperature treatments, leading to low yield with a random distribution of atoms. Herein, a nature-based facile method to synthesize SA-M catalysts (M = Fe, Ir, Pt, Ru, Cu, or Pd) in a single step at ambient temperature, using the extracellular electron transfer capability of Geobacter sulfurreducens (GS), is presented. Interestingly, the SA-M is coordinated to three nitrogen atoms adopting an MN 3 on the surface of GS. Dry samples of SA-Ir@GS without further heat treatment show exceptionally high activity for oxygen evolution reaction when compared to benchmark IrO 2 catalyst and comparable hydrogen evolution reaction activity to commercial 10 wt% Pt/C. The SA-Ir@GS exhibits the best water-splitting performance compared to other SA-M@GS, showing a low applied potential of 1.65 V to achieve 10 mA cm −2 in 1.0 M KOH with cycling over 5 h. The density functional calculations reveal that the large adsorption energy of H 2 O and moderate adsorption energies of reactants and reaction intermediates for SA-Ir@GS favorably improve its activity. This synthesis method at room temperature provides a versatile platform for the preparation of SA-M catalysts for various applications by merely altering the metal precursors.

Metal nanoclusters can be synthesized following the top-down or bottom-up approaches involving multiple chemical and/or physical steps where strong ligands, toxic chemicals, and high temperature and pressure are normally applied. In contrast, biological methods eliminate the use and generation of hazardous substances and do not require the application of high temperature and pressure during the synthesis process. Biological methods based on the extracellular electron transfer (EET) capability of electroactive bacteria (EAB) are considered a promising sustainable route for the synthesis of metal nanoparticles; however, a fine control of the size of the nanoparticles has not been achieved yet. Herein, we report a facile biological-based synthesis method of size-controlled palladium (Pd) nanoclusters using the EET capability of Geobacter sulfurreducens. By controlling the metal precursor concentrations and dosing and incubation times, we synthesized size-controlled Pd nanoclusters (1.0 ± 0.6 to 4.8 ± 1.4 nm) anchored on the surface of G. sulfurreducens. The as-synthesized Pd nanoclusters anchored on the surface of G. sulfurreducens cells (Pd/GS) were tested for their performance as bifunctional electrocatalysts for the overall alkaline water splitting reaction. Despite a very low mass loading of 0.002 mg Pd cm −2 , the hybrid material (i.e., Pd/GS) showed exceptionally higher activity for hydrogen evolution reaction (HER) when compared to benchmark 10 wt % Pt/C and Pd/C. Similarly, Pd/GS showed higher activity for oxygen evolution reaction (OER) when compared to Pd/C and comparable OER activity when compared to benchmark IrO 2 . The findings in this study provide a promising sustainable route for designing size-controlled metal nanoclusters anchored on the surface of EAB as efficient and low-cost electrocatalysts for various applications.

In article number 2010916, Srikanth Pedireddy, Pascal E. Saikaly, and co‐workers report a facile synthesis strategy of single‐atom catalysts (SACs) at room temperature by harnessing the extracellular electron transfer capability of Geobacter sulfurreducens. This strategy can be successfully extended for the synthesis of various transition metal SACs by merely altering the metal precursors. Without further heat treatment, the dried catalysts exhibit excellent electrocatalytic activity for oxygen and hydrogen evolution reactions.

A new hybrid system that combines a microbial electrolysis cell and membrane bioreactor using electrically conductive, porous, hollow-fiber membrane was developed to treat low organic strength solutions (300 mg COD/L) and recover energy in the form of biogas. This new system is called an anaerobic electrochemical membrane bioreactor (AnEMBR). The nickel-based hollow-fiber serves the dual function of cathode electrode for hydrogen evolution and membrane to filter the treated effluent. The system was operated for 70 days and recovered up to 71% of the substrate energy as methane rich biogas due to biological conversion of the hydrogen evolved at the cathode surface to methane by hydrogenotrophic methanogens. At an applied voltage of 0.5 V the energy density of the produced biogas exceeded the electrical energy required to operate the system by 53%. Findings also suggest that hydrogen bubble formation, cathode potential and localized alkaline pH at the cathode membrane surface may help mitigate membrane fouling.