Electrochemical, spectroscopic and theoretical studies of a simple bifunctional cobalt corrole catalyst for oxygen evolution and hydrogen production

Abstract

Six cobalt and manganese corrole complexes were synthesized and examined as single-site catalysts for water splitting. The simple cobalt corrole [Co(tpfc)(py)2] (1, tpfc = 5,10,15-tris(pentafluorophenyl)corrole, py = pyridine) catalyzed both water oxidation and proton reduction efficiently. By coating complex 1 onto indium tin oxide (ITO) electrodes, the turnover frequency for electrocatalytic water oxidation was 0.20 s(−1) at 1.4 V (vs. Ag/AgCl, pH = 7), and it was 1010 s(−1) for proton reduction at −1.0 V (vs. Ag/AgCl, pH = 0.5). The stability of 1 for catalytic oxygen evolution and hydrogen production was evaluated by electrochemical, UV-vis and mass measurements, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), which confirmed that 1 was the real molecular catalyst. Titration and UV-vis experiments showed that the pyridine group on Co dissociated at the beginning of catalysis, which was critical to subsequent activation of water. A proton-coupled electron transfer process was involved based on the pH dependence of the water oxidation reaction catalyzed by 1. As for manganese corroles 2–6, although their oxidizing powers were comparable to that of 1, they were not as stable as 1 and underwent decomposition at the electrode. Density functional theory (DFT) calculations indicated that water oxidation by 1 was feasible through a proposed catalytic cycle. The formation of an O–O bond was suggested to be the rate-determining step, and the calculated activation barrier of 18.1 kcal mol(−1) was in good agreement with that obtained from experiments.

Reactivity and Mechanism Studies of Hydrogen Evolution Catalyzed by Copper Corroles

“…The catalytic HER activity observed for 1 considerably exceeds that reported for the cobalt corrole analogue [(Cor)Co(py) 2 ] (py = pyridine). 63 Although [(Cor)Co(py) 2 ] functions well as a heterogeneous electrocatalyst when it was deposited on working electrodes, it degrades by oxidative dimerization after losing its pyridine ligands in acidic solution (see above for discussions), which makes the mechanistic studies challenging.…”

Artificial Photosynthesis: Molecular Systems for Catalytic Water Oxidation

“…In this regard, cobalt was shown to be superior to the other studied metals, by enabling highly favorable thermodynamics for the overall H 2 O oxidation event. 426 Another interesting and bifunctional cobalt corrole complex [Co(tpfc)(pyr) 2 ] (332, Figure 90; tpfc = 5,10,15-tris-(pentafluorophenyl)corrole; pyr = pyridine) for both electrochemical O 2 evolution and H 2 production was very recently reported by Lei et al 427 Electrochemical measurements of cobalt corrole complex 332 in an acetontrile solution at a glassy carbon electrode revealed two reversible waves at E 1/2 = 1.01 and 1.57 V vs NHE. From theoretical studies, the authors could ascribe the first oxidation to a corrole-centered event.…”

Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions

“…On the other hand molecular complexes based on porphyrin and corrole architectures, such as nickel porphyrins, copper corroles, etc. have also shown their possible use 38, 39, 40, 41, 42. It is worth mentioning that recently designed Co corrole complex 5,15‐bis‐(pentafluorophenyl)‐10‐(4)‐(1‐pyrenyl)phenyl containing a triphenylphosphine axial ligand by Cao and co‐workers is the first example of a molecular HER catalyst that is active and stable in a wide pH range (0–14) 43…”