A water coordinated Ni complex and a 2D Ni-MOF: topology dependent highly enhanced electrocatalytic OER activity.

Metal-organic frameworks (MOFs) with their tunable topology, functionality and coordination environment have been considered as potential materials for various applications including electrocatalysis. Herein, we have synthesised a water coordinated nickel based 2D metal-organic framework (Ni-MOF) and a coordination complex (Ni-C) and investigated their electrocatalytic OER activity. The Ni-MOF showed a 2D sheet structure with one water coordination whereas a four water molecule coordinated charged complex was formed in the Ni-C. Thermogravimetric analysis (TGA) confirmed their water coordination and good thermal stability. Interestingly, electrocatalytic OER studies showed strongly enhanced activity for the Ni-MOF and that it required a low overpotential (194 mV) to produce a geometric current density of 10 mA cm-2. The Ni-C required 225 mV to produce 10 mA cm-2. The post-catalytic analysis suggested that the Ni-MOF and Ni-C are converted to nickel hydroxides/oxyhydroxides during electrocatalysis and acted as the catalytic centre. The low Tafel slope and charge transfer resistance further supported the higher activity of Ni-MOF based nickel hydroxides/oxyhydroxides. Chronoamperometric studies revealed the excellent stability of the Ni-MOF based catalyst over 72 h. The present study revealed the potential of developing highly active electrocatalysts based on Ni-MOFs by optimizing the topology and coordination environment.

Highly enhanced electrocatalytic OER activity of water-coordinated copper complexes: effect of lattice water and bridging ligand.

The use of metal-organic compounds as electrocatalysts for water splitting reactions has gained increased attention; however, a fundamental understanding of the structural requirement for effective catalytic activity is still limited. Herein, we synthesized water-coordinated mono and bimetallic copper complexes (CuPz-H2O·H2O, CuPz-H2O, CuBipy-H2O·H2O, and CuMorph-H2O) with varied intermetallic spacing (pyrazine/4,4'-bipyridine) and explored the structure-dependent oxygen evolution reaction (OER) activity in alkaline medium. Single crystal structural studies revealed water-coordinated monometallic complexes (CuMorph-H2O) and bimetallic complexes (CuPz-H2O·H2O, CuPz-H2O, CuBipy-H2O·H2O). Further, CuPz-H2O·H2O and CuBipy-H2O·H2O contained lattice water along with coordinated water. Interestingly, the bimetallic copper complex with lattice water and shorter interspacing between the metal centres (CuPz-H2O·H2O) showed strong OER activity and required an overpotential of 228 mV to produce a benchmark current density of 10 mA cm-2. Bimetallic copper complex (CuPz-H2O) without lattice water but the same intermetallic spacing and bimetallic complex with increased interspacing but with lattice water (CuBipy-H2O·H2O) exhibited relatively lower OER activity. CuPz-H2O and CuBipy-H2O·H2O required an overpotential of 236 and 256 mA cm-2, respectively. Monometallic CuMorph-H2O showed the lowest OER activity (overpotential 271 mV) compared to bimetallic complexes. The low Tafel slope and charge transfer resistance of CuPz-H2O·H2O facilitated faster charge transfer kinetics at the electrode surface and supported the enhanced OER activity. The chronoamperometric studies indicated good stability of the catalyst. Overall, the present structure-electrocatalytic activity studies of copper complexes might provide structural insight for designing new efficient electrocatalysts based on metal coordination compounds.

Highly enhanced dye adsorption of MoO3 nanoplates fabricated by hydrothermal-calcination approach in presence of chitosan and thiourea.

Water pollution by organic dyes poses great challenge to the environment and living organism. Hence effective removal of organic dyes by cost effective methods have received significant attention in recent years. Herein, we report the complete removal of organic dyes (rhodamine B), methylene blue) and eosin yellow) from water via effective adsorption by MoO3 catalyst. Hydrothermally synthesised MoO2 (1) and amorphous MoSx (2) using ammonium molybdate without and with thiourea exhibited low dye adsorption. In contrast, crystalline micro/nanoplates of MoO3 (3 and 4) obtained from calcination of 1 and 2 showed highly enhanced dye adsorption. Particularly 4 showed higher dye adsorption compared to 3. UV-Visible absorption studies confirmed complete removal of organic dyes upon stirring with MoO3 catalyst. Dye removal studies further revealed that cationic dyes are adsorbed faster than anionic dye that could be attributed to the surface charge of MoO3. Interestingly, the adsorbed dyes were not released from MoO3 for more than 50 days. The exhausted MoO3 catalyst can be recovered by annealing at 400 °C. MoO3 catalyst has also been used as packing materials in dropper column and demonstrated effective removal of dyes by passing through dyes separately as well as mixture.