Electrocatalytic activity of nanoparticulate TiO2 coated onto Ta-doped IrO2/Ti substrates: Effects of the TiO2 overlayer thickness

Abstract

The trade-off between short-term efficiency and long-term durability in electrocatalysis has always been a tough choice. This study examines the electrocatalytic activity of double-layered oxide electrodes comprising a ~1 μm-thick Ta-IrO2 underlayer (coated on a Ti plate) and ~3.5 μm- and 5.5 μm-thick nanoparticulate TiO2 overlayers (TEC-1 and TEC-2, respectively) in NaClO4 and NaCl solutions as non-active and active supporting electrolytes, respectively. The physicochemical features of both TEC samples, including the crystalline pattern, elemental states, morphology, and electrochemically active surface area, are very similar, except for the TiO2 overlayer thickness. The TEC-1 electrode shows ~54% lower electrode/electrolyte interfacial charge-transfer resistances than those of TEC-2. In the bulk electrolysis for the decomposition of ammonia, the current density with the TEC-1 electrode is two-fold larger than that with TEC-2 under the same potential, due to the relatively low interfacial charge transfer with TEC-1. Electron spin resonance spectroscopic analysis reveals that OH and O2−/HOO are the primary reactive oxygen species responsible for the ammonia decomposition in NaClO4. In the NaCl electrolyte, reactive chlorine species (i.e., free chlorines) decompose ammonia with significantly faster kinetics compared to the case with the reactive oxygen species. Regardless of the electrolyte type, TEC-1 is always more active than TEC-2. However, prolonged bulk electrolysis tests over 300 h show that TEC-2 is more durable than TEC-1, maintaining its electrocatalytic activity and faradaic efficiency. The effect of the TiO2 overlayer thickness on the efficiency and durability of the electrodes is discussed in detail.