Rationally Designed Manganese-Based Metal-Organic Frameworks as Altruistic Metal Oxide Precursors for Noble Metal-Free Oxygen Reduction Reaction

The sluggish oxygen reduction reaction (ORR) at the cathode is challenging and hinders the growth of hydrogen fuel cells. Concerning kinetic values, platinum is the best known catalyst for ORR; however, its less abundance, high cost, and corrosive nature warrant the development of low-cost catalysts. We report the hydrothermal synthesis of two novel Mn-based metal-organic frameworks (MOFs), [Mn₂(DOT)(H₂O)₂]_n (Mn-SKU-1) and [Mn₂(DOT)₂(BPY)₂(THF)]_n (Mn-SKU-2) (DOT = 2,5-dihydroxyterephthalate; BPY = 4,4'-bipyridine). Mn-SKU-1 contains dimeric Mn(II) centers where the two corner-shared MnO₆ octahedra fuse to give rise to an infinite Mn₂O₁₀ cluster, whereas the two Mn(II) ions coordinate to DOT and BPY moieties to give rise to a pillared structure in Mn-SKU-2 and form a 3D → 3D homo-interpenetration MOF with a twofold interpenetrated net. The pyrolysis of as-synthesized Mn-MOFs at 600 °C under N₂ produced exclusively porous α-Mn₂O₃ composites (PSKU-1 and PSKU-2), with the BET surface area of 90.8 (for PSKU-1) and 179.3 m² g^-1 (for PSKU-2). These mesoporous MOF-derived α-Mn₂O₃ composites were modified as cathode materials for the electrocatalytic reduction of oxygen. The onset potential for the oxygen reduction reaction was found to be 0.90 V for PSKU-1 and 0.93 V for PSKU-2 versus RHE in 0.1 M KOH solution, with the current density of 4.8 and 6.0 mA cm^-2, respectively, at 1600 rpm. Based on the RDE/RRDE results, the electrocatalytic oxygen reduction occurs majorly via the four-electron process. The electrocatalyst PSKU-2 is cheap, easy to use, retains 90% of its activity after 10 h of continuous use, and offers higher recyclability than Pt/C. The onset potential maximum current density and kinetic values (J_k = 11.68 mA cm^-2 and Tafel slope = 85.0 mV dec^-1) obtained in this work are higher than the values reported for pure Mn₂O₃.

ZIF-67 Derived MnO2 Doped Electrocatalyst for Oxygen Reduction Reaction

In this study, zeolitic imidazolate framework (ZIF-67) derived nano-porous carbon structures that were further hybridized with MnO2 were tested for oxygen reduction reaction (ORR) as cathode material for fuel cells. The prepared electrocatalyst was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Energy Dispersive X-ray Analysis (EDX). Cyclic voltammetry was performed on these materials at different scan rates under dissolved oxygen in basic media (0.1 M KOH), inert and oxygen rich conditions to obtain their I–V curves. Electrochemical impedance spectroscopy (EIS) and Chronoamperometry was also performed to observe the materials’ impedance and stability. We report improved performance of hybridized catalyst for ORR based on cyclic voltammetry and EIS results, which show that it can be a potential candidate for fuel cell applications.

Facile synthesis of N-doped graphene supported porous cobalt molybdenum oxynitride nanodendrites for the oxygen reduction reaction

Exploring an inexpensive, active and stable electrocatalyst as an alternative to expensive Pt for the oxygen reduction reaction (ORR), porous Co-Mo-ON alloy nanodendrites supported on nitrogen-doped graphene (Co-Mo-ON/NG) have been synthesized by a two-step solid state heating method. The Co-Mo-ON/NG nanodendrites offer high ORR activity and superior electrochemical stability both in acidic and alkaline media. This superiority is due to the synergistic effect of NG, enhanced catalytic efficiency by Mo, highly active intrinsic surface area and exposure of catalytic facets of nanodendritic morphology towards the ORR. The Co-Mo-ON/NG nanodendrites show a 4e- ORR process with 0.710 V and 0.915 V onset potentials in 0.5 M H2SO4 and 0.1 M KOH, respectively. The Co-Mo-ON/NG nanodendrites show extreme electrochemical stability in terms of 96% and 97% current retention for 40 000 s in both acidic and alkaline media, respectively, long term durability for continuous 2000 cycles and greater resistance to methanol than the commercial Pt/C catalyst. Furthermore, Mo-ON/NG, Co-ON/NG, and Co-Mo-ON are also tested to evaluate the effect of Mo-doping and NG on the electrocatalytic activity of Co-Mo-ON/NG nanodendrites. Owing to their low cost, easy synthesis, outstanding ORR performance, and extreme durability, Co-Mo-ON/NG nanodendrites emerged as a promising non-precious and highly stable ORR electrocatalyst in fuel cell applications.