Cobalt based catalysts are promising bifunctional electrocatalysts for both oxygen reduction and oxygen evolution reactions (ORR and OER) in unitized regenerative fuel cells (URFCs) operating with alkaline electrolytes. Here we report a hybrid composite of cobalt nanoparticles embedded in nitrogen-doped carbon (Co/N-C) via a solvothermal carbonization strategy. With the synergistic effect arising from the N-doped carbon and cobalt nanoparticles in the composite, the Co/N-C hybrid catalyst exhibits highly efficient bifunctional catalytic activity and excellent stability toward both ORR and OER. The ΔE (oxygen electrode activity parameter for judging the overall electrocatalytic activity of a bifunctional electrocatalyst) value for Co/N-C is 0.859 V, which is smaller than those of Pt/C and most of the non-precious metal catalysts in previous studies. Furthermore, the Co/N-C composite also shows better bifunctional catalytic activity than its oxidative counterparts, which could be attributed to the high specific surface area and the efficient charge transfer ability of the composite, as well as the good synergistic effect between N-doped carbon and the Co nanoparticles in the Co/N-C composite.

It is highly crucial and challenging to develop bifunctional oxygen electrocatalysts for oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) in rechargeable metal-air batteries and unitized regenerative fuel cells (URFCs). Herein, a facile and cost-effective strategy is developed to prepare mesoporous Fe-N-doped graphene-like carbon architectures with uniform Fe3C nanoparticles encapsulated in graphitic layers (Fe3C@NG) via a one-step solid-state thermal reaction. The optimized Fe3C@NG800-0.2 catalyst shows comparable ORR activity with the state-of-the-art Pt/C catalyst and OER activity with the benchmarking RuO2 catalyst. The oxygen electrode activity parameter ΔE (the criteria for judging the overall catalytic activity of bifunctional electrocatalysts) value for Fe3C@NG800-0.2 is 0.780 V, which surpasses those of Pt/C and RuO2 catalysts as well as those of most nonprecious metal catalysts. Significantly, excellent long-term catalytic durability holds great promise in fields of rechargeable metal-air batteries and URFCs.

The design and fabrication of efficient and inexpensive electrodes for oxygen evolution reaction (OER) is essential for energy-conversion technologies. Herein, high OER activity is achieved using hollow mesoporous NiCo2O4 nanocages synthesized via a Cu2O-templated strategy combined with coordination reaction. The NiCo2O4 nanostructures with a hollow cavity, large roughness and high porosity show only a small overpotential of ∼0.34 V at the current density of 10 mA cm(-2) and a Tafel slope of 75 mV per decade, which is comparable with the performance of the best reported transition metal oxide based OER catalysts in the literature. Meanwhile, the positive impacts of the nanocage structure and the Ni incorporation on the electrocatalytic performance are also demonstrated by comparing the OER activities of NiCo2O4 nanocages with Co3O4 nanocages, NiCo2O4 nanoparticles and 20 wt% Pt/C. Moreover, the NiCo2O4 nanocages also manifest superior stability to other materials. All these merits indicate that the hollow mesoporous NiCo2O4 nanocages are promising electrocatalysts for water oxidation.

Despite tremendous progress in developing doped carbocatalysts for the oxygen reduction reaction (ORR), the ORR activity of current metal-free carbocatalysts is still inferior to that of conventional Pt/C catalysts, especially in acidic media and neutral solution. Moreover, it also remains a challenge to develop an effective and scalable method for the synthesis of metal-free carbocatalysts. Herein, we have developed nitrogen and phosphorus dual-doped hierarchical porous carbon foams (HP-NPCs) as efficient metal-free electrocatalysts for ORR. The HP-NPCs were prepared for the first time by copyrolyzing nitrogen- and phosphorus-containing precursors and poly(vinyl alcohol)/polystyrene (PVA/PS) hydrogel composites as in situ templates. Remarkably, the resulting HP-NPCs possess controllable nitrogen and phosphorus content, high surface area, and a hierarchical interconnected macro-/mesoporous structure. In studying the effects of the HP-NPCs on the ORR, we found that the as-prepared HP-NPC materials exhibited not only excellent catalytic activity for ORR in basic, neutral, and acidic media, but also much better tolerance for methanol oxidation and much higher stability than the commercial, state-of-the-art Pt/C catalysts. Because of all these outstanding features, it is expected that the HP-NPC material will be a very suitable catalyst for next-generation fuel cells and lithium-air batteries. In addition, the novel synthetic method described here might be extended to the preparation of many other kinds of hierarchical porous carbon materials or porous carbon that contains metal oxide for wide applications including energy storage, catalysis, and electrocatalysis.

Via a solvothermal carbonization process, an enriched graphitic N-doped carbon-supported Fe3O4 nanoparticles composite was prepared which exhibits similar high catalytic activity but superior stability to Pt/C for the oxygen reduction reaction.

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A bottom-up approach was introduced to prepare nitrogen and phosphorus dual-doped multilayer graphene with high dopant content and well-developed porosity, which leads to high catalytic activity in hydrogen evolution reaction with comparable onset overpotential (0.12 V) and Tafel slope (79

A nanocomposite of cobaltosic oxide and nitrogen-doped graphene (Co3O4/N-G) was prepared by the facile hydrothermal method. Morphology characterizations show that the Co3O4 nanoparticles with crystalline spinel structure are uniformly dispersed on the nitrogen-doped graphene nanosheets, and the graphene weight fraction in Co3O4/N-G composite is estimated to be ∼20%. Meanwhile, electrochemical measurements reveal that the as-prepared Co3O4/N-G nanocomposite exhibits a high catalytic activity and long-term stability in neutral electrolyte. Moreover, the use of Co3O4/N-G as cathode catalyst for oxygen reduction in microbial fuel cells (MFCs) to produce electricity was also investigated. The obtained maximum power density was 1340 ± 10 mW m–2, which was as high as almost four times that of the plain cathode (340 ± 10 mW m–2), and only slightly lower than that of a commercial Pt/C catalyst (1470 ± 10 mW m–2). All the results prove that a Co3O4/N-G hybrid can be a good alternative to platinum catalysts for practical MFC applications.