In the Proton Exchange Membrane (PEM) water electrolysis industry, it is important to identify an electrocatalyst which is stable in acidic environment for water oxidation reactions. The water oxidation reactions can be divided into two types: four proton and electron transfer reaction, which is the oxygen evolution reaction (OER); and two proton transfer reaction, which is the hydrogen peroxide evolution reaction (2e-WOR). The understanding of the correlation between catalysts’ structure and performance is of significance in water oxidation catalyst design. In this work, we discuss the iridium-based oxides behaviour during the OER, and explored the possibility of hydrogen peroxide generation during the water oxidation reactions using precious and non-precious metals.
For iridium-based oxides, a range of crystalline iridium-based oxide catalysts of different particle sizes ranging from 1 – 5 nm were synthesized, using a modified Adams technique. A double-layer capacitance method was developed and used to determine the in situ electrochemically active surface areas (ECSAs) of the OER catalysts in acidic media. The specific double-layer capacitance for iridium oxide was measured across a range of catalysts and determined. These ECSA measurements allow estimations of the specific activity of catalysts, an approach which has hitherto been missing from OER experiments. OER electrochemical performance was measured using the ultra-low loading, high mass transport floating electrode technique in the potential range from 1.3 – 2.0 V.
It was found that the iridium oxides showed an impressively good OER performance at high potential with ultra-low Ir loading. Representing a specific activity of 3.75 mA cm-2 (at 1.8 V), corresponding to a turnover frequency of 25 electrons site-1 s-1, and a mass activity of 9900 mA mg-1. A small specific activity increase is seen as the particle size decreases in the low potential range (<1.65 V), but this effect disappears at higher potentials with all catalysts tending towards roughly the same specific activity.
To further reduce the Ir loading, different ratios of cobalt was doped onto iridium oxides to test the OER performance in acidic media within the potential range from 1.3 - 2 V. The cobalt doped iridium oxides demonstrated much better OER activities than commercial IrO2. The geometric current density at 1.9 V of Co0.4IrOx showed 300 mA cm-2, while the in-house synthesised IrO2 was around 147 mA cm-2 and a commercial IrO2 less than 20 mA cm-2. The geometric catalyst loadings are 15 ugIr cm-2. The OER mechanism is then proposed to be potential dependent with two different regions: the low current region with surface mechanism, and the OER region with pH-performance dependency suggesting lattice oxygen evolution reaction mechanism.
The generation of hydrogen peroxide is also tested during water oxidation reactions with a range of precious and non-precious metal in both acidic and alkaline media. No H2O2 was detected in either condition during the oxidation reactions, by utilizing a modified RRDE system with oxidation reactions occur at both ring and disk electrodes.