Effect of nitrogen doping on the catalytic activity of carbon nano-onions for the oxygen reduction reaction in microbial fuel cells

Fe, N, S co-doped cellulose paper carbon fibers as an air-cathode catalyst for microbial fuel cells

The heteroatoms and transition metal co-doped carbon-based catalysts are an important way to improve the catalytic activity of oxygen reduction reaction (ORR). Herein, we reported a facile method to obtain iron, nitrogen, and sulfur co-doped cellulose paper carbon fibers as catalysts (Fe-N-S/CFs) for ORR in microbial fuel cells (MFCs) with the adsorption recovery of Congo red molecules from dye wastewater. The thermal treatment promoted the etching of carbon surface by ferric ions, resulting in increased surface roughness for forming the defective carbon structure. The rich active species and defective carbon formed on the etched surface to enhance the electroactive surface area and effective sites. Fe-N-S/CFs catalysts achieved high half-wave potential due to the synergy effect between chemical components and defect structures. The assembled single-chamber air cathode MFC gained a high maximum power density of 1773 ± 40 mW m^-2 versus Pt/C MFC of 1325 ± 94 mW m^-2. This work provides a strategy for recovering dye molecules from wastewater to prepare non-precious metal catalysts for enhancing ORR activity.

Screening of Carbon-Supported Platinum Electrocatalysts Using Frumkin Adsorption Isotherms

An important stage in the development of platinum electrocatalysts on carbon support is the analysis of their basic parameters. Cyclic voltammetry is an effective tool for analyzing the structural and electrochemical properties of such electrocatalysts. Using Frumkin adsorption isotherms, the contribution of the platinum surface to the hydrogen adsorption region was well described by three peaks corresponding to different crystal structures. The screening was carried out for platinum black and platinum electrocatalysts supported by carbon black, reduced graphene oxide (RGO), carbon nanotubes (CNTs), and nanofibers (CNFs). For most samples, the peak contribution to the electrochemical surface area (ESA) and corresponding hydrogen adsorption energies had close values, but the parameters deviated for Pt black and RGO-based samples was observed. The dependence of the calculated peak parameters on the number of accelerated stress test cycles was used to evaluate the effect of the type of carbon support on the stability of the electrocatalyst and the structure of platinum nanoparticles. The experimental results indicate a high degree of stability and differences in the degradation mechanisms of electrocatalysts based on nanostructured carbon compared to carbon black, which are explained by differences in the metal-support interaction and corrosion resistance of nanostructured carbon supports.

Magnetized manganese-doped watermelon rind biochar as a novel low-cost catalyst for improving oxygen reduction reaction in microbial fuel cells

Microbial fuel cells (MFCs) are promising equipment for water treatment and power generation. The catalyst used in the oxygen reduction reaction (ORR) at the cathode is a critical factor for efficacy of MFCs. Therefore, it is important to develop cost-effective cathode catalysts to enhance application of MFCs. In the current study, a novel cathode catalyst was developed, which was annealed with watermelon rind as raw material and transition metals including iron, and manganese were introduced. The 700Mn/Fe@WRC catalyst, which was annealed at 700 °C, exhibited excellent electrochemical performance. The high relative content of pyridine nitrogen caused by the inherent nitrogen element of the watermelon rind and the high content of iron and manganese elements introduced resulted in increase in electrochemical surface area to 657.6 m²/g. The number of electrons transferred ORR was 3.96, indicating that ORR occurs through a four-electron pathway. The maximum power density of MFCs was 399.3 ± 7.4 mW/m² with a fitting total internal resistance of 15.242 Ω, and the removal efficiency of COD was 97.1 ± 1.2%. The cost of the 700Mn/Fe@WRC catalyst was approximately 0.15 $/g, which is significantly lower compared with Pt/C (33.0 $/g). Experimental verification showed that the 700Mn/Fe@WRC prepared using the economical watermelon rind biochar (WRC) is an excellent substitute for non-precious metal catalysts used in MFCs.

All carbon hybrid N-doped carbon dots/carbon nanotube structures as an efficient catalyst for the oxygen reduction reaction

This paper reports the facile and scalable synthesis of hybrid N-doped carbon quantum dots/multi-walled carbon nanotube (CD/CNT) composites, which are efficient alternative catalysts for the oxygen reduction reaction (ORR) in fuel cells. The N-doped CDs for large-scale production were obtained within 5 minutes via a one-step polyol process using ethylenediamine (ED) in the presence of hydrogen peroxide as an oxidizing agent. For comparison, different CDs were also prepared using ethylene glycol (EG) and ethanolamine (EA) in the same manner. Physicochemical characterization suggested the successful formation of a CD(ED)/CNT hybrid without individual CD(ED)s and CNTs. The N-doped CD(ED)/CNT catalyst exhibited excellent electrocatalytic activity in an alkaline solution compared to other composites (CD(EG)/CNT and CD(EA)/CNT). The Tafel slope (−60.9 mV dec⁻¹) and durability (∼9.1% decay over 10 h) for CD(ED)/CNT were superior to high-performance Pt/C catalysts. The electrochemical double-layer capacitance on the CD(ED)/CNT hybrid showed apparent improvement of the active surface area because of N-doping and highly decorated CDs on the CNT wall. These results provide an innovative approach for the potential application of all carbon hybrid structures in electrocatalysis.

The ORR measurements showed that the CD(ED)/CNT catalyst was superior to CD(EG)/CNT and CD(EA)/CNT. They even surpassed the activity of commercial Pt/C in terms of durability, Tafel slope, and MeOH tolerance.

Carbon nano-onion-powered optically transparent and economical dye-sensitized solar cells

The integration of dye-sensitized solar cells (DSSCs) with building roof panels, windows, and various decorative outdoor installations is presently an important research topic for their immediate commercialization potential because of their power generation capability, sustainability, and aesthetic appearance. For industrial applications, Pt counter electrodes (CEs) need to be replaced with Pt-free CEs because of their limited sources and cost. An ideal CE should be economical, abundant, and have excellent electrochemical stability and activity, with easy processing in bulk. As an alternative practical CE, we introduce for the first time a carbon nano-onion (CNO)-based CE for transparent DSSCs. We developed a simple, energy-efficient one-step synthesis technique for fabricating high-quality CNOs in bulk quantities without any sophisticated instrumentation and expensive nanodiamond precursors. CNO CEs proved to be promising in terms of optical transparency, reasonable electrochemical redox activity for the I-/I3- redox couple, and exchange current density comparable to Pt CEs. CNO-powered DSSCs demonstrated an optical transparency of >55% with a significant solar energy conversion efficiency of 5.17%. The intrinsic hydrophilicity of the as-synthesized CNO eliminates the use of a binder or an additive, unlike in the case of other carbon allotropes. Our results demonstrate the possibility of using CNO-based CEs as promising substitutes for scarce and expensive Pt-based CEs for low-cost DSSCs.

Electrochemical synthesis of carbon nano onions

Electrochemical synthesis of hollow and solid carbon nano onions from simple aromatic compounds in acetonitrile on a platinum plate electrode by a multi-potential steps method.