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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.

In this study, highly graphitic nitrogen-doped carbon nano-onions (N-CNOs) were prepared by a one-step, direct, in situ flame synthesis technique and their potential applications as catalysts for oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were evaluated for the first time. The ORR activity of the CNO, N-CNO, and the commercial Pt/C were measured using a rotating ring-disk electrode (RRDE). The reaction mechanism for the N-CNO was found to follow a four-electron transfer pathway and possess a higher onset potential in RRDE measurement than CNOs. The ORR activity of N-CNO was 5.4 times better than that of CNO, which was attributed to the introduction of nitrogen to the carbon faramework. The MFC fabricated with the N-CNO cathode produced a maximum power density of 49.6 mW m–2, which was approximately double the performance of the CNO-based MFC. The performance of N-CNO was low compared to Pt/C but the cost per power was only 1/310th. These results confirmed that N-CNOs could be used as a low-cost alternative and an energy-efficient metal-free ORR catalyst for practical MFC applications.

Flower-shaped Pt-Pd nanostructures supported on carbon (Pt-Pd/C) were successfully synthesized by simple wet-chemical processing by controlling the rate of reduction of a dissolved metal precursor. L-ascorbic acid (AA) was employed as a green and clean reductive agent for the coreduction of [PtCl6]2− and [PdCl4]2− in an aqueous solution. The electrocatalytic activity of nanoporous Pt-Pd/C on the oxygen reduction reaction (ORR) was investigated by rotating disk electrode voltammetry in a 0.1 M HClO4 solution. In particular, the Pt77-Pd23/C catalyst showed surpassing ORR activity and acceptable stability under acidic conditions compared to the commercial Pt electrocatalyst. The outstanding electrocatalytic activity was achieved by a combination of the high surface area of flower-like nanostructures and the bimetallic synergetic effect. The morphology, composition and structure of the Pt-Pd/C nanoflowers were confirmed by scanning transmission electron microscopy (STEM), TEM, X-ray photoelectron spectroscopy and X-ray diffraction. Overall, the Pt-Pd/C bimetallic nanostructures are expected to become promising cathode material in fuel cells to replace monometallic Pt ones.

This paper introduces a facile one-step process to synthesize highly interconnected nanoporous Ir-Pd alloys supported on carbon that exhibit excellent bifunctional electrocatalytic activities for both the oxygen reduction and oxygen evolution reactions with reasonable stability in alkaline electrolytes. Nanoporous Pd networks with crystalline {111} faces were shown experimentally to serve mainly as active sites for the oxygen reduction reaction, whereas the Ir nanoparticles incorporated in the Pd nanoframe networks, where the optimized Ir:Pd ratio was 0.23:0.77 (n = 10), were responsible for the oxygen evolution reaction. Such three-dimensional architectures provide a high density of active sites for the oxygen electrochemical reaction and facilitate electron transport. More importantly, the nanoporous Ir-Pd alloy nanocomposites exhibited similar stability for the oxygen reduction reaction but superior catalytic activity to the commercial Pd catalyst in alkaline solutions. In addition, the materials were also highly active for the oxygen evolution reaction, e.g., a small overpotential at 10 mA cm−2 (1.628 V vs. reversible hydrogen electrode), making it a high-performance bifunctional catalyst for both the oxygen electrochemical reaction. Rotating ring-disk electrode measurements showed that the oxygen reduction and oxygen evolution reactions on the Ir-Pd catalysts proceeded predominantly through the desired 4-electron pathway.

Microbial fuel cells (MFCs) are a potential sustainable energy resource by converting organic pollutants in wastewater to clean energy. The performance of MFCs is influenced directly by the electrode material. In this study, a ternary PANI-TiO2-GN nanocomposite was used successfully to improve the performance of both the cathode and anode MFC. The PANI-TiO2-GN catalyst exhibited better oxygen reduction reaction activity in the cathode, particularly as a superior catalyst for improved extracellular electron transfer to the anode. This behavior was attributed to the good electronic conductivity, long-term stability, and durability of the composite. The immobilization of bacteria and catalyst matrix in the anode facilitated more extracellular electron transfer (EET) to the anode, which further improved the performance of the MFCs. The application of PANI-TiO2-GN as a bifunctional catalyst in both the cathode and anode helped decrease the cost of MFCs, making it more practical.

This paper reports the synthesis of N-doped carbon foams (NCFs) using simple, environment-friendly, and self-developed freezing method. A robust structure of NCFs along with high surface area, hierarchical pore structure, and nitrogen/oxygen functionalities allows their application as free-standing electrode. Microbial fuel cell equipped with differently prepared NCFs as free-standing anode and metal-free cathode generates significantly higher power density, 35.74 W m−3 than conventional electrodes. Furthermore, NCF with an optimized resorcinol-formaldehyde content shows significantly high-charge storage capacity, 799 F g−1, at 0.25 A g−1 and also delivers a high energy density, 111 Wh kg−1, equivalent to that of a Li-ion battery.

Platinum nanoparticles (PtNPs) have long been regarded as the benchmark catalyst for the oxygen reduction reaction (ORR) in the cathode of microbial fuel cells (MFCs). On the other hand, the practical applications of these catalysts are limited by the high cost and scarcity of Pt. Therefore, developing an alternative catalyst to PtNPs for efficient ORR activity is essential for meeting the future demands for practical applications in MFCs. In this study, sulfur-doped graphene (S-GN) was synthesized via the environmental friendly, economical and facile one pot electrochemical exfoliation of graphene in a unique combination of electrolytes, which both catalyzed the exfoliation reaction and acted a sulfur source. The initial activity of S-GN as an ORR active catalyst was examined by cyclic voltammetry (CV), which showed that the as-synthesized S-GN exhibited better ORR activity than the plain material. Furthermore, the application of S-GN as a cathode material was also studied in MFCs. The results showed that the MFC equipped with the S-GN cathode produced a maximum power density of 51.22 ± 6.01 mW m−2, which is 1.92 ± 0.34 times higher than that of Pt/C. The excellent performance of S-GN as a cathode catalyst in MFCs could be due to the doping of graphene with heteroatoms, which increased the surface area and improved the conductivity of graphene through a range of interactions. Based on the above MFC performance, the as-synthesized S-GN catalyst could help reduce the cost and scale up the design of MFCs for practical applications in the near future.