Surface‐Immobilized Single‐Site Iridium Complexes for Electrocatalytic Water Splitting

Abstract

Catalytic water splitting using solar energy represents an attractive potential solution for affordable and renewable energy. [1,2] To construct a (photo)electrochemical H 2 /O 2 evolution system, oxygen evolving catalysts (OECs) need to be immobilized on a conducting surface.[1] Many metal complexes containing single or more catalytic sites have been tested for water oxidation; [2][3][4][5][6][7] however, the design and implementation of a stable and efficient molecular water oxidation system that operates at high catalytic turnover number (TON) and frequency (TOF) for extended periods of controlled-potential electrolysis (CPE), with moderate overpotential and high current density, are challenging. [8][9][10] Herein we disclose robust immobilized [(L 2 bpy)Ir2+ (L is -PO 3 H 2 or -COOH, bpy is 2,2'-bipyridine, Cp* is pentamethylcyclopentadiene) complexes on ITO (indium tin oxide) surface (ITO/Cat) for electrocatalytic water oxidation ( Figure 1). The aqua complexes were obtained by Cl to H 2 O ligand exchange before immobilization on the ITO surfaces. The mono-iridium catalysts are modified with carboxylate and phosphonate linkers that are known to anchor covalently on ITO.[1, 9] At 1.75 V (vs. NHE; NHE = normal hydrogen electrode) the system operates with a high TOF of 6.7 s À1 and has TONs of more than 210 000, well in excess of the maximum TON of 28 000 reported before.[9] The oxygen generation current densities are higher than 1.70 mA cm À2 , which is one to two orders of magnitude higher than the reported densities of approximately 50 mA cm À2 .[ 2+ molecular complex is a highly competent catalytic system for electrochemical oxygen evolution.The synthesis and characterization of the complexes are detailed in the Supporting Information ( Figure S1). [12,13] In situ ligand exchange from Cl to H 2 O and deprotonation are monitored by UV/Vis spectroscopy ( Figure S2 in the Supporting Information). CV of an ITO/Cat.Ir-COOH in aqueous acid (pH 1) shows a catalytic current wave at approximately 1.22 V that sharply grows until approximately 1.31 V, and leads to an O 2 evolution current with tiny oxygen 2+ (Cat.Ir-PO 3 H 2 and Cat.Ir-COOH for L = PO 3 H 2 and COOH, respectively) on ITO for electrochemical water oxidation. The metalstabilizing Cp* ligand is highlighted in brown-red.

Efficient electrochemical water oxidation in neutral and near-neutral systems with a nanoscale silver-oxide catalyst

“…[5][6] Recently, many molecular complexes of precious metals like Ru and Ir and their oxides along with other inorganic materials of transition metals have been studied for electrocatalytic water oxidation. [7][8][9][10] But, due to instability and poor performance, catalytic water oxidation systems are not yet capable of being employed in large scale applications. The overall efficiency of the water oxidation process is largely limited by the slow kinetics of the oxygen evolution reaction (OER) that requires a high overpotential to drive it.…”

Electrochemical and Spectroscopic Study of Mononuclear Ruthenium Water Oxidation Catalysts: A Combined Experimental and Theoretical Investigation

“…2,8,11,[16][17][18][19] Recently, a shift of attention has occurred, directing more effort towards developing mononuclear WOCs. 8,20 A large number of ruthenium [4][5][6][7][8][9]13,[21][22][23] and iridium 8,11,14,[24][25][26][27][28][29] based catalysts have been reported. Density Functional Theory (DFT) has been used extensively to predict the reaction mechanisms and the electronic properties of several multi-and mono-nuclear molecular catalysts.…”

Ni‐Based Electrocatalyst for Water Oxidation Developed In‐Situ in a HCO3−/CO2 System at Near‐Neutral pH

“…[ 7 ] In particular, a water electrolysis system working in a near-neutral pH solution and at a moderate overpotential would be ideal to make a light-driven fuel generation device. [ 8,9 ] Precious metal oxides such as ruthenium oxide and iridium oxide are ideal candidates for electrochemical water oxidation, but their use in large scale terrestrial applications is impeded by their high prices and limited availability. [10][11][12] Recently, non-noble transition metal-oxides derived electrocatalytic systems have attracted widespread scientifi c interest because of their good catalytic activity for anodic oxygen evolution and abundant availability.…”