Highly Efficient Electrocatalysis and Mechanistic Investigation of Intermediate IrO_x(OH)_y Nanoparticle Films for Water Oxidation

Abstract

A new transparent iridium oxide (IrO x ) film on fluorine-doped tin oxide (FTO) electrodes were achieved from a homogeneous precursor complex solution by employing a facile spin-coating technique. The composition of the nanostructure and crystallinity of the IrO x film is tunable by a simple annealing treatment of a compact complex layer, which is responsible for their significantly different electrocatalytic performances for water oxidation. Transmission electron microscopy (TEM) observations showed uniformly dispersed small IrO x nanoparticles of dimensions ca. 2–5 nm for the film annealed at 300 °C, and the nanoparticles gradually agglomerated to form relatively large particles at higher temperatures (400 and 500 °C). The IrO x films prepared at different annealing temperatures are characterized by Raman spectroscopic data to reveal intermediate IrO x (OH) y nanoparticles with two oxygen binding motifs: terminal hydroxo and bridging oxo at 300 and 350 °C annealing, via amorphous IrO x at 400 °C, transforming ultimately to crystalline IrO2 nanoparticles at 500 °C. Cyclic voltammetry suggests that the intrinsic activity of catalytic Ir sites in intermediate IrO x (OH) y nanoparticles formed at 300 °C annealing is higher in comparison with amorphous and crystalline IrO x nanoparticles. Electrochemical impedance data showed that the charge transfer resistance (R ct = 232 Ω) for the IrO x (OH) y film annealed at 300 °C is lower relative to that of films annealed at higher temperatures. This is ascribable to the facilitated electron transfer in grain boundaries between smaller IrO x particles to lead the efficient electron transport in the film. The high intrinsic activity of catalytic Ir sites and efficient electron transport are responsible for the high electrocatalytic performance observed for the intermediate IrO x (OH) y film annealed at 300 °C; it provides the lowest overpotential (η) of 0.24 V and Tafel slope of 42 mV dec–1 for water oxidation at neutral pH, which are comparable with values for amorphous IrO x ·nH2O nanoparticle films (40–50 mV dec–1) reported as some of the most efficient electrocatalysts so far.

Remarkable Improvement in Water Oxidation Catalysis by Moderate Heat Treatment of a Crystalline Silver‐Based Thin Film Developed In‐Situ From Silver‐ions in Acetate Solution

“…The enhancement in the OER activity with increase of catalyst film thickness is attributed to an increase in the density of surface active sites which facilitate adsorption of OH species from the solution . It has been previously reported that the heat treatment can highly improve the catalytic performance of metal‐oxide based catalysts for OER as well as enhancing its mechanical strength,,,,, thus, after the initial electrodeposition, the resultant Ag x O films on FTO were annealed at three different temperatures (50, 75 and 100 °C) for 20 min. Heating of the Ag x O film above 100 °C causes its thermal decomposition ,,.…”

Characterization of Interfacial Charge‐Transfer Photoexcitation of Polychromium‐Oxo‐Electrodeposited TiO2 as an Earth‐Abundant Photoanode for Water Oxidation Driven by Visible Light

“…Interest in finding sustainable energy sources has become increasingly important because of the imminent depletion of global fossils fuels and increasing concern about climate change . Widespread attention has been paid to artificial photosynthesis and related catalysts as one of the promising energy‐providing systems for the future . The development of an efficient photoanode for visible‐light‐driven water oxidation to evolve O 2 is a key task because such a photoanode is considered to be the energy demanding bottleneck in artificial photosynthesis.…”

Solar‐Driven Production of Hydrogen Peroxide from Water and Dioxygen

“…[72] Various metal oxides reported for H 2 Oo xidation are included in Figure 1. [73][74][75][76] The selectivity diagram shows that IrO 2 ,R hO 2 ,a nd PtO 2 are thermodynamically located in thes electivity region for O 2 evolution and indeed they catalyze the electrochemical four-electron/four-proton oxidation of H 2 Ot o O 2 , [77][78][79][80][81][82][83][84] whereas TiO 2 is close to the border of weak OH adsorption( red region), where both OHC andH 2 O 2 are formed. [85][86][87][88][89][90]…”

“…Manganese oxides (MnO x )h ave attracted special attention as water oxidation catalysts due to their naturala bundance, low toxicity,a nd compositional similarity to nature's water splitting inorganic cluster,t he oxygen-evolving complex (OEC) of photosystem II (PSII). [73][74][75][76][77][78][79][80] Although MnO x usually acts as a water oxidationc atalyst for four-electron/four-proton oxidation of H 2 Ot oe volve O 2 , [1,[91][92][93][94][95][96][97][98][99] electrocatalytic H 2 Oo xidation with MnO x in the presenceo fabutylammonium sulfate (BAS)/base mixture (1.0 m)a tp H10a taconstant appliedp otential resulted in production of H 2 O 2 rather than O 2 . [100] Faradaic efficiency for the H 2 O 2 production during the controlled potential electrolysis for 10 min over an MnO x electrocatalyst in BAS electrolyte (1.0 m)a ta na ppliedp otentialo f0 .8 Vv ersusA g/AgCl reached1 00 %i nt he pH region between1 0.0 and 10.4 ( Figure 2).…”