Heterocyclic-Additive-Activated Dinuclear Dysprosium Electrocatalysts for Heterogeneous Water Oxidation

Heterogeneous catalysis based on air-stable lanthanide complexes is relatively rare, especially for electrochemical water oxidation and reduction. Therefore, it is highly desired to investigate the synergy caused by cocatalysts on the lanthanide complex family for heterogeneous catalysis because of their structural diversity, air/moisture insensitivity, and easy preparation under an air atmosphere. Two mononuclear and three dinuclear dysprosium complexes containing a series of Schiff-base ligands have been demonstrated as robust electrocatalysts for triggering heterogeneous water oxidation in alkaline solution, in which the complex [Dy₂(hmb)₂(OAc)₄]·MeCN(3) was revealed to have the best activity toward heterogeneous water oxidation among all five complexes in the present study. The molecular activation of dysprosium complexes has also been investigated with a series of N-containing heterocyclic additives [i.e., 4-(dimethylamino)pyridine (DMAP), bis(triphenylphosphine)iminium chloride ([PPN]Cl), indole, and quinoline]. In particular, the corresponding overpotential was effectively enhanced by 211 mV (at a current density of 10 mA cm^-2) with the assistance of DMAP. On the basis of electrochemical and ex situ/in situ spectroscopic investigations, the best catalyst, DMAP-complex 3 on a carbon paper electrode, was confirmed with well-maintained molecular identity during heterogeneous water oxidation free of forming any dysprosium oxide and/or undesired products.