Stability of Mn-Doped TiO₂ Thin-Film Anodes during Water Oxidation Reactions

Incorporating manganese atoms into TiO₂ has been shown to improve its activity for water oxidation. However, its activity has been observed to steadily degrade under constant applied potential, especially at more oxidizing applied potentials. The compositional profile and electrochemical stability of Mn-doped TiO₂ (Mn:TiO₂) thin-film electrodes precisely fabricated by atomic layer deposition (ALD) were evaluated to elucidate the mechanisms of electrode degradation. Although ALD enables the deposition of Mn dopants throughout the film thickness, only Mn within 1.5 nm of the surface was observed to be redox active. Annealing Mn:TiO₂ films in air leads to the diffusion of Mn toward the electrode surface and oxidation of the Mn and alters the redox behavior. Annealing endows greater stability to Mn redox behavior but does not fully stabilize the water oxidation current, suggesting that the two electrochemical processes are not precisely correlated. In situ inductively coupled plasma mass spectrometry reveals that Mn is lost from Mn:TiO₂ films at potentials greater than 1.8 V vs RHE. However, the initial Mn loss is equivalent to less than a single ALD layer of MnO_<i>x</i>. Longer-term constant potential experiments suggest that Mn is lost from up to 1 nm of the electrode surface but that this loss may account for only a portion of the diminished water oxidation activity.