Two-dimensional mathematical model of an air-cathode microbial fuel cell with graphite fiber brush anode
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A two-dimensional mathematical model has been developed for characterizing and predicting the dynamic performance of an air-cathode MFC with graphite fiber brush used as anode. The charge transfer kinetics are coupled to the mass balance at both electrodes considering the brush anode as a porous matrix. The model has been used to study the effect of design (electrode spacing and anode size) as well as operational (substrate concentration) parameters on the MFC performance. Two-dimensional dynamic simulation allows visual representation of the local overpotential, current density and reaction rates in the brush anode and helps in understanding how these factors impact the overall MFC performance. The numerical results show that while decreasing electrode spacing and increasing initial substrate concentration both have a positive influence on power density of the MFC, reducing anode size does not affect MFC performance till almost 60 % brush material has been removed. The proposed mathematical model can help guide experimental/pilot/industrial scale protocols for optimal performance.Keywords:
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The structure of cobalt formed by electrodeposition and the influence of the pH of the plating solution and the cathode potential was studied by potentiodynamic measurements and X-ray diffraction. It was found that the level of overpotential significantly affects the structure of the formed cobalt. When electrodeposition is performed far from equilibrium conditions, i.e., at a high overpotential, face-centered cubic (fcc) cobalt is deposited while at low overpotential hexagonal close packed Co is formed with a lower rate of hydrogen evolution. A higher overpotential is needed in a neutral compared to acidic solution in order to enhance the evolution of hydrogen that is required for the formation of fcc cobalt. © 2002 The Electrochemical Society. All rights reserved.
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Oxygen evolution reaction (OER) involves multiple electron-transfer processes, resulting in a high activation barrier. Developing catalysts with low overpotential and high intrinsic activity toward OER is critical but challenging. Here we report a major advancement in decreasing the overpotential for oxygen evolution reaction. Ni foam-supported Fe-doped β-Ni(OH)2 nanosheets achieve an overpotential of 219 mV at the geometric current density of 10 mA cm–2. To our knowledge, this is the best value reported for Ni- or Fe hydroxide-based OER catalysts. In addition, the catalyst yields a current density of 6.25 mA cm–2 at the overpotential of 300 mV when it is normalized to the electrochemical surface area of the catalyst. This intrinsic catalytic activity is also better than the values reported for most state-of-the-art OER catalysts at the same overpotential.
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In the search for nonprecious metal catalysts for the hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) have been proposed as promising candidates. Here, we present a facile method for significantly decreasing the overpotential required for catalyzing the HER with colloidally synthesized WSe2. Solution phase deposition of 2H WSe2 nanoflowers (NFs) onto carbon fiber electrodes results in low catalytic activity in 0.5 M H2SO4 with an overpotential at −10 mA/cm2 of greater than 600 mV. However, two postdeposition electrode processing steps significantly reduce the overpotential. First, a room-temperature treatment of the prepared electrodes with a dilute solution of the alkylating agent Meerwein's salt ([Et3O][BF4]) results in a reduction in overpotential by approximately 130 mV at −10 mA/cm2. Second, we observe a decrease in overpotential of approximately 200–300 mV when the TMDC electrode is exposed to H+, Li+, Na+, or K+ ions under a reducing potential. The combined effect of ligand removal and electrochemical activation results in an improvement in overpotential by as much as 400 mV. Notably, the Li+ activated WSe2 NF deposited carbon fiber electrode requires an overpotential of only 243 mV to generate a current density of −10 mA/cm2. Measurement of changes in the material work function and charge transfer resistance ultimately provide rationale for the catalytic improvement.
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To meet the requirements for industrial water splitting to generate hydrogen, it is urgent, but still quite challenging to develop highly active and stable electrocatalysts for large-current-density hydrogen evolution reaction (HER). Herein, Ru-incorporated NiSe2 (Ru-NiSe2 ) was designed and synthesized. The introduction of Ru results in the formation of hierarchically structured Ru-NiSe2 with large electrochemical active surface area, and well-modified electronic structure. As expected, the as-fabricated Ru-NiSe2 displays impressive HER activity in 1.0 M KOH, with a low overpotential of 180.8 mV to reach the current density of 1000 mA cm-2 . Ru-NiSe2 also presents outstanding long-term stability at high current densities, owing to its high intrinsic chemical stability, and strong catalyst-support interface. Notably, when performed at a certain current density of 1000 mA cm-2 , the overpotential increase after 90 h is only 13 mV. Such excellent HER performance of Ru-NiSe2 demonstrates its great potential for practical use in industrial water splitting.
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The promise and challenge of electrochemical mitigation of CO2 calls for innovations on both catalyst and reactor levels. In this work, enabled by our high-performance and earth-abundant CO2 electroreduction catalyst materials, we developed alkaline microflow electrolytic cells for energy-efficient, selective, fast, and durable CO2 conversion to CO and HCOO–. With a cobalt phthalocyanine-based cathode catalyst, the CO-selective cell starts to operate at a 0.26 V overpotential and reaches a Faradaic efficiency of 94% and a partial current density of 31 mA/cm2 at a 0.56 V overpotential. With a SnO2-based cathode catalyst, the HCOO–-selective cell starts to operate at a 0.76 V overpotential and reaches a Faradaic efficiency of 82% and a partial current density of 113 mA/cm2 at a 1.36 V overpotential. In contrast to previous studies, we found that the overpotential reduction from using the alkaline electrolyte is mostly contributed by a pH gradient near the cathode surface.
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Noble metal
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