Oxygen Reduction Reaction Electrocatalysts Derived from Iron Salt and Benzimidazole and Aminobenzimidazole Precursors and Their Application in Microbial Fuel Cell Cathodes

Co-Doped Zeolite-GO Nanocomposite as a High-Performance ORR Catalyst for Sustainable Bioelectricity Generation in Air-Cathode Single-Chambered Microbial Fuel Cells

High-performance cobalt (Co) nanoparticles supported on a zeolite-graphene oxide (1:2) matrix (catalyst Z2) are synthesized through a facile reduction method. In multipoint Brunauer-Emmett-Teller (MBET) surface area analysis, catalyst Z2 demonstrates a higher surface area compared with other synthesized catalysts, indicating the presence of a larger number of catalytic active sites, and supports outstanding ORR performance due to an improved electron-transfer rate and a higher number of redox-active sites. Furthermore, it is observed that catalyst Z2 is an excellent electrocatalytic material due to its low charge-transfer resistance and higher oxygen reduction reaction (ORR) activity. Herein, the electrocatalytic investigation suggests that catalyst Z2 at a potential of 483 mV and a reduction current of -0.382 mA displays a higher electrocatalytic performance and higher stability toward ORR compared with other synthesized catalysts and even the standard Pt/C catalyst. Also, when catalyst Z2 is applied as an air-cathode ORR electrocatalyst for a single-chambered microbial fuel cell (SC-MFC), the SC-MFC coated with catalyst Z2 generates the maximum power density of 416.78 mW/m2, which is 306% higher than that of SC-MFC coated with Pt/C (102.67 mW/m2). In fact, the longer stability and electronic conductivity have contributed to an outstanding ORR activity of the nanocomposite due to its porous surface morphology and the presence of the functional groups in the zeolite-GO support matrix. In brief, Co (cobalt) nanoparticles doped on a zeolite-GO (1:2) support matrix are promising cathode electrocatalysts in the practical application of MFCs and other related devices.

Contribution of configurations, electrode and membrane materials, electron transfer mechanisms, and cost of components on the current and future development of microbial fuel cells

Microbial fuel cells (MFCs) are a technology that can be applied to both the wastewater treatment and bioenergy generation. This work discusses the contribution of improvements regarding the configurations, electrode materials, membrane materials, electron transfer mechanisms, and materials cost on the current and future development of MFCs. Analysis of the most recent scientific publications on the field denotes that dual-chamber MFCs configuration offers the greatest potential due to the excellent ability to be adapted to different operating environments. Carbon-based materials show the best performance, biocompatibility of carbon-brush anode favors the formation of the biofilm in a mixed consortium and in wastewater as a substrate resembles the conditions of real scenarios. Carbon-cloth cathode modified with nanotechnology favors the conductive properties of the electrode. Ceramic clay membranes emerge as an interesting low-cost membrane with a proton conductivity of 0.0817 S cm^-1, close to that obtained with the Nafion membrane. The use of nanotechnology in the electrodes also enhances electron transfer in MFCs. It increases the active sites at the anode and improves the interface with microorganisms. At the cathode, it favors its catalytic properties and the oxygen reduction reaction. These features together favor MFCs performance through energy production and substrate degradation with values above 2.0 W m^-2 and 90% respectively. All the recent advances in MFCs are gradually contributing to enable technological alternatives that, in addition to wastewater treatment, generate energy in a sustainable manner. It is important to continue the research efforts worldwide to make MFCs an available and affordable technology for industry and society.

Waste Face Surgical Mask Transformation into Crude Oil and Nanostructured Electrocatalysts for Fuel Cells and Electrolyzers

A novel route for the valorization of waste into valuable products was developed. Surgical masks commonly used for COVID 19 protection by stopping aerosol and droplets have been widely used, and their disposal is critical and often not properly pursued. This work intended to transform surgical masks into platinum group metal-free electrocatalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) as well as into crude oil. Surgical masks were subjected to controlled-temperature and -atmosphere pyrolysis, and the produced char was then converted into electrocatalysts by functionalizing it with metal phthalocyanine of interest. The electrocatalytic performance characterization towards ORR and HER was carried out highlighting promising activity. At different temperatures, condensable oil fractions were acquired and thoroughly analyzed. Transformation of waste surgical masks into electrocatalysts and crude oil can open new routes for the conversion of waste into valuable products within the core of the circular economy.

Nanostructured Fe-N-C as Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution

The development of electrocatalysts for energy conversion and storage devices is of paramount importance to promote sustainable development. Among the different families of materials, catalysts based on transition metals supported on a nitrogen-containing carbon matrix have been found to be effective catalysts toward oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) with high potential to replace conventional precious metal-based catalysts. In this work, we developed a facile synthesis strategy to obtain a Fe-N-C bifunctional ORR/HER catalysts, involving wet impregnation and pyrolysis steps. Iron (II) acetate and imidazole were used as iron and nitrogen sources, respectively, and functionalized carbon black pearls were used as conductive support. The bifunctional performance of the Fe-N-C catalyst toward ORR and HER was investigated by cyclic voltammetry, rotating ring disk electrode experiments, and electrochemical impedance spectroscopy in alkaline environment. ORR onset potential and half-wave potential were 0.95 V and 0.86 V, respectively, indicating a competitive performance in comparison with the commercial platinum-based catalyst. In addition, Fe-N-C had also a good HER activity, with an overpotential of 478 mV @10 mAcm−2 and Tafel slope of 133 mVdec−1, demonstrating its activity as bifunctional catalyst in energy conversion and storage devices, such as alkaline microbial fuel cell and microbial electrolysis cells.

Iron-Based Electrocatalysts for Energy Conversion: Effect of Ball Milling on Oxygen Reduction Activity

In this work, we synthesized new materials based on Fe(II) phthalocyanine (FePc), urea and carbon black pearls (BP), called Fe-N-C, as electrocatalysts for the oxygen reduction reaction (ORR) in neutral solution. The electrocatalysts were prepared by combining ball-milling and pyrolysis treatments, which affected the electrochemical surface area (ECSA) and electrocatalytic activity toward ORR, and stability was evaluated by cyclic voltammetry and chronoamperometry. Ball-milling allowed us to increase the ECSA, and the ORR activity as compared to the Fe-N-C sample obtained without any ball-milling. The effect of a subsequent pyrolysis treatment after ball-milling further improved the electrocatalytic stability of the materials. The set of results indicated that combining ball-milling time and pyrolysis treatments allowed us to obtain Fe-N-C catalysts with high catalytic activity toward ORR and stability which makes them suitable for microbial fuel cell applications.

Platinum Group Metal-Free Catalysts for Oxygen Reduction Reaction: Applications in Microbial Fuel Cells

Scientific and technological innovation is increasingly playing a role for promoting the transition towards a circular economy and sustainable development. Thanks to its dual function of harvesting energy from waste and cleaning up waste from organic pollutants, microbial fuel cells (MFCs) provide a revolutionary answer to the global environmental challenges. Yet, one key factor that limits the implementation of larger scale MFCs is the high cost and low durability of current electrode materials, owing to the use of platinum at the cathode side. To address this issue, the scientific community has devoted its research efforts for identifying innovative and low cost materials and components to assemble lab-scale MFC prototypes, fed with wastewaters of different nature. This review work summarizes the state-of the-art of developing platinum group metal-free (PGM-free) catalysts for applications at the cathode side of MFCs. We address how different catalyst families boost oxygen reduction reaction (ORR) in neutral pH, as result of an interplay between surface chemistry and morphology on the efficiency of ORR active sites. We particularly review the properties, performance, and applicability of metal-free carbon-based materials, molecular catalysts based on metal macrocycles supported on carbon nanostructures, M-N-C catalysts activated via pyrolysis, metal oxide-based catalysts, and enzyme catalysts. We finally discuss recent progress on MFC cathode design, providing a guidance for improving cathode activity and stability under MFC operating conditions.

Superhydrophobic Air-Breathing Cathode for Efficient Hydrogen Peroxide Generation through Two-Electron Pathway Oxygen Reduction Reaction

Electrochemical catalysis of carbon-based material via two-electron pathway oxygen reduction reaction (ORR) offers great potential for in situ hydrogen peroxide (H₂O₂) production. In this work, we tuned catalyst mesostructure and hydrophilicity/hydrophobicity by adjusting polytetrafluoroethylene (PTFE) content in graphite/carbon black/PTFE hybrid catalyst layer (CL), aimed to improving the two-electron ORR activity for efficient H₂O₂ generation. As the only superhydrophobic CL with initiating contact angles of 141.11°, PTFE_0.57 obtained the highest H₂O₂ yield of 3005 ± 58 mg L^-1 h^-1 (at 25 mA cm^-2) and highest current efficiency (CE) of 84% (at 20 mA cm^-2). Rotating ring disk electrode (RRDE) results demonstrated that less PTFE content in CLs results in less electrons transferred and better selectivity toward two-electron ORR. Though the highest H₂ concentration (2 μmol L^-1 at 25 mA cm^-2) was monitored from PTFE_0.57 which contained the lowest PTFE, the CE decreased inversely with increasing content of PTFE, which proved that the H₂O₂ decomposition reaction was the major side reaction. Higher PTFE content increased the hydrophilicity of CL for excessive H⁺ and insufficient O₂ diffusion, which induced H₂O₂ decomposition into H₂O. Simultaneously, the electroactive surface area of CLs decreased with higher PTFE content, from 0.0041 m² g^-1 of PTFE_0.57 to 0.0019 m² g^-1 of PTFE_4.56. Besides, higher PTFE content in CL leads to the increase of total impedance (from 14.5 Ω of PTFE_0.57 to 18.3 Ω of PTFE_4.56), which further hinders the electron transfer and ORR activity.

Iron-streptomycin derived catalyst for efficient oxygen reduction reaction in ceramic microbial fuel cells operating with urine

In recent years, the microbial fuel cell (MFC) technology has drawn the attention of the scientific community due to its ability to produce clean energy and treat different types of waste at the same time. Often, expensive catalysts are required to facilitate the oxygen reduction reaction (ORR) and this hinders their large-scale commercialisation. In this work, a novel iron-based catalyst (Fe-STR) synthesised from iron salt and streptomycin as a nitrogen-rich organic precursor was chemically, morphologically and electrochemically studied. The kinetics of Fe-STR with and without being doped with carbon nanotubes (CNT) was initially screened through rotating disk electrode (RDE) analysis. Then, the catalysts were integrated into air-breathing cathodes and placed into ceramic-type MFCs continuously fed with human urine. The half-wave potential showed the following trend Fe-STR > Fe-STR-CNT ≫ AC, indicating better kinetics towards ORR in the case of Fe-STR. In terms of MFC performance, the results showed that cathodes containing Fe-based catalyst outperformed AC-based cathodes after 3 months of operation. The long-term test reported that Fe-STR-based cathodes allow MFCs to reach a stable power output of 104.5 ± 0.0 μW cm-2, 74% higher than AC-based cathodes (60.4 ± 3.9 μW cm-2). To the best of the Authors' knowledge, this power performance is the highest recorded from ceramic-type MFCs fed with human urine.