MnCo2 O4 Anchored on Nitrogen-Doped Carbon Nanomaterials as an Efficient Electrocatalyst for Oxygen Reduction

Space-Confined Anchoring of Fe-Nx on Concave N-Doped Carbon Cubes for Catalyzing Oxygen Reduction

Carbon-based electrocatalysts with atomically dispersed Fe-N-C display promising performance for oxygen reduction reaction (ORR) amongst non-precious electrocatalysts. Nonetheless, increasing the number and utilization of Fe-N-C active sites is challenging. Designing morphologies and adjusting the pore structure of carbon-based electrocatalysts would boost the mass transfer, enhance the utilization of the active sites, and increase the overall ORR performance. In this work, a concave N-doped carbon cubes structure adorned with highly external Fe-N_x was designed and produced by the space-confined induced strategy. The optimal electrocatalyst revealed excellent ORR activity in both alkaline and acidic electrolytes, with half-wave potentials of 0.86 and 0.75 V, respectively. The superior performance arose from its unique concave structure, possessing more accessible active sites with improved intrinsic activity, which holds promising potential for preparing advanced ORR electrocatalysts.

Three-Dimensional-Order Macroporous AB2O4 Spinels (A, B =Co and Mn) as Electrodes in Zn-Air Batteries

In this work, atomically substituted three-dimensionally ordered macroporous (3DOM) spinels based on Co and Mn (MnCo₂O₄ and CoMn₂O₄) were synthetized and used as cathodic electrocatalysts in a primary Zn-air battery. Scanning/transmission electron microscopy images show a 3DOM structure for both materials. Skeleton sizes of 114.4 and 140.8 nm and surface areas of 65.3 and 74.6 m² g^-1 were found for MnCo₂O₄ and CoMn₂O₄, respectively. The increase in surface area and higher presence of Mn³⁺ and Mn⁴⁺ species in the CoMn₂O₄ 3DOM material improved battery performance with a maximum power density of 101.6 mW cm^-2 and a specific capacity of 1440 mA h g^-1, which shows the highest battery performance reported to date using similar spinel materials. The stability performance of the electrocatalyst was evaluated in half-cell and battery cell systems, showing the higher durability of CoMn₂O₄, which was related to its better capability to perform the electrocatalytic process as adsorption, electron transfer, and desorption. It was found through density functional theory calculations that the CoMn₂O₄ spinel has a higher density of states in the Fermi level vicinity and better conductivity. Finally, the unique shape of 3DOM spinels promoted a high interaction between electroactive species and catalytic sites, making them suitable for oxygen reduction reaction applications.

Spinel MnCo2 O4 Nanoparticles Supported on Three-Dimensional Graphene with Enhanced Mass Transfer as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The rational design of highly efficient and durable oxygen reduction reaction (ORR) catalysts is critical for the commercial application of fuel cells. Herein, three-dimensional graphene (3D-G) is synthesized by the template method, which used coal tar pitch as the carbon source and nano MgO as the template. Then, spinel MnCo₂ O₄ is in situ supported on the 3D-G by a facile hydrothermal method, giving MnCo₂ O₄ /3D-G. The resultant MnCo₂ O₄ /3D-G retains the multilayered mesoporous graphene structure where MnCo₂ O₄ nanoparticles are deposited on the inner walls of pores in the 3D-G. The catalyst MnCo₂ O₄ /3D-G shows high electrocatalytic activity with a half-wave potential of 0.81 V versus reversible hydrogen electrode, which is clearly superior to those of MnCo₂ O₄ /reduced graphene oxide (0.78 V), MnCo₂ O₄ /carbon nanotubes (0.74 V), MnCo₂ O₄ /C (0.72 V), and 20 wt % Pt/C (0.80 V). The electron transfer number of MnCo₂ O₄ /3D-G indicates a four-electron process of ORR. The durability test demonstrates that the MnCo₂ O₄ /3D-G catalyst has a much better durability than 20 wt % Pt/C. Our work makes an inspiring strategy to prepare high-performance electrocatalysts for the development of fuel cells.