Microwave assisted flow synthesis of Ir-IrOx electrocatalysts for acidic oxygen evolution reaction: synthesis conditions and electrocatalytic performance correlation

Scalable synthesis of the nanoparticulate iridium-based electrocatalysts (Ir-IrO_x) for the oxygen evolution reaction (OER) in acidic media is demonstrated using single phase microwave- (MW-) assisted milli-fluidic continuous flow synthesis (MF-CFS) technique. The flow reactions were initiated in the MW cavity reactor, where Ir⁴⁺ was reduced by NaBH₄ to form Ir-IrO_x nanoparticles (NPs) of particle size ranging from 2.0 to 3.5 nm. Effects of (i) the Ir precursor concentration, (ii) the pre-mixing time interval between precursor/reducing agent mixing and injection of the mixture in the MW cavity (t_pre−mix) and (iii) the residence time in the MW cavity (t_r) were investigated to optimize the reaction conversion efficiency (𝜂_c) and the electrochemical performance of the synthesized electrocatalysts. The value of 𝜂_c was impressively increased from 52% to ~ 98% by changing the value of t_r from 2.4 min to 4.7 min while keeping all other parameters constant. The Ir-IrO_x electrocatalysts synthesized under optimal synthesis conditions demonstrate an excellent OER mass activity of 351 A g⁻¹ compared to that of 305 A g⁻¹ for the commercial IrO₂. During a potentiodynamic accelerated stress test (AST), the synthesized electrocatalysts show an electrochemical stability comparable to or higher than that of the commercial IrO₂. The Ir-IrO_x sample having the optimal OER performance exhibited an OER activity retention of 40% compared to 31% for the commercial IrO₂. Longer t_r and additional MW exposure of the reaction bath was found to decrease the oxidation number of the Ir-IrO_x surface efficiently, enhancing both the OER activity and durability of the synthesized Ir-IrO_x sample.