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

Abstract

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), [Mn 2 (DOT)(H 2 O) 2 ] n (Mn-SKU-1) and [Mn 2 (DOT) 2 (BPY) 2 (THF)] n (Mn-SKU-2) (DOT = 2,5dihydroxyterephthalate; BPY = 4,4′-bipyridine). Mn-SKU-1 contains dimeric Mn(II) centers where the two corner-shared MnO 6 octahedra fuse to give rise to an infinite Mn 2 O 10 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 N 2 produced exclusively porous α-Mn 2 O 3 composites (PSKU-1 and PSKU-2), with the BET surface area of 90.8 (for PSKU-1) and 179.3 m 2 g −1 (for PSKU-2). These mesoporous MOF-derived α-Mn 2 O 3 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 Mn 2 O 3 .

Milestones of Electrocatalyst Development for Direct Alcohol Fuel Cells

“…94.7% (after 40k s CA) [202] FeCoNi-CNTs 1.0 m KOH 0.82 V RHE 7.1 mA cm −2 (limited current density) 98.6% (after 40 h CA) [199] P-NS-MnO 2 @Mn 0.1 m KOH 0.87 V RHE 2.32 mA cm −2 (current density@0.88 V RHE ) 97.1% (after 10k s CA) [204] CoNi@NC 1.0 m KOH 0.83 V RHE 6.89 A g −1 (mass activity@0.83 V RHE ) − [205] NiCo/hCN 0.1 m KOH 0.84 V RHE 45.9 mA cm −2 (kinetic current density@0.85 V RHE ) 861 mW cm −2 (AEMFC power density) 88% (after 70 h) [206] FeCo-HNC/CNT 0.1 m KOH 0.902 V RHE 42.33 A g -1 (mass activity@0.85 V RHE ) No change after 10k CVs [207] [Mn 2 (DOT) 2 (BPY) 2 (THF)]n 0.1 m KOH 0.81 V RHE 6.0 mA cm −2 (current density@0.9 V RHE ) 90% (after 10 h) [208] CuMn 2 O 4 nano-octahedra/C 1.0 m KOH 0.881 V RHE 37.6 A g −1 (mass activity@ 0.85 V RHE ) 65.2% (after 10k CVs) [209] FeCo/NC-Mo 2 TiC 2 0.1 m KOH 0.887 V RHE 4.0 mA cm −2 (kinetic current density@0.9 V RHE ) 501 mW cm 2 (AEMFC power density)…”

Stereospecific Single-Pot Route to Chiral Imidazolidines from Aziridines Using a 2D Cu Metal–Organic Framework

“…Therefore, designing a robust heterogeneous catalytic system addressing these entailing issues becomes crucial. Metal–organic frameworks (MOFs) are an important class of coordination polymers composed of organic linkers and metal ions forming an extended network of well-arranged metal nodes with permanent porosity. − The robust crystalline framework, presence of a microenvironment, and cocatalytic sites alongside an array of arranged metal centers make MOFs a steadfast cost-effective heterogeneous catalyst for various important catalytic transformations. − In this context, the design of an MOF-based catalytic system could pave an alternative heterogeneous approach toward the accessibility of biologically important imidazolidine motifs. Our previous studies include one-pot synthesis of β-arylsulfonamide and the C–N coupling reaction catalyzed using copper MOFs HKUST-1 and Cu-1D, respectively, wherein the clarity in the role of unsaturated Lewis acid sites was established. , We herein report the synthesis of a novel two-dimensional (2D) MOF with pre-existing coordinatively unsaturated sites (CUS) as an efficient ready-to-use heterogeneous catalyst for the synthesis of imidazolidine through aziridine bond cleavage in a single-pot using a nonchlorinated solvent system.…”