IrO2-Decorated Titania Nanotubes as Oxygen Evolution Anodes

Abstract

In this work, we have used both plain titania nanotubes, TNTs, and their reduced black analogues, bTNTs, that bear metallic conductivity (prepared by solid state reaction of TNTs with CaH₂ at 500 °C for 2 h), as catalyst supports for the oxygen evolution reaction (OER). Ir was subsequently been deposited on them by the galvanic replacement of electrodeposited Ni by Ir(IV) chloro-complexes; this was followed by Ir electrochemical anodization to IrO₂. By carrying out the preparation of the TNTs in either two or one anodization steps, we were able to produce close-packed or open-structure nanotubes, respectively. In the former case, larger than 100 nm Ir aggregates were finally formed on the top face of the nanotubes (leading to partial or full surface coverage); in the latter case, Ir nanoparticles smaller than 100 nm were obtained, with some of them located inside the pores of the nanotubes, which retained a porous surface structure. The electrocatalytic activity of IrO₂ supported on open-structure bTNTs towards OER is superior to that supported on close-packed bTNTs and TNTs, and its performance is comparable or better than that of similar electrodes reported in the literature (overpotential of η = 240 mV at 10 mA cm^-2; current density of 70 mA cm^-2 and mass specific current density of 258 mA mg_Ir^-1 at η = 300 mV). Furthermore, these electrodes demonstrated good medium-term stability, maintaining stable performance for 72 h at 10 mA cm^-2 in acid.

Keywords: acid water electrolysis; black titanium dioxide; dimensionally stable anodes; galvanic replacement; iridium oxide; titanium dioxide nanotubes.

Figure 1

SEM micrographs of: (a) two-step/close-packed TNTs; (b) one-step/open-structure TNTs; (c) close-packed IrO_x(Ni)/bTNTs and (d) open-structure IrO_x(Ni)/bTNTs.

Figure 2

(a) SEM micrographs of close-packed IrO_x(Ni)/TNTs; (b) the corresponding EDS mapping analysis.

Figure 3

XPS spectra for the as-prepared (red dotted line) and used (green dotted line) for OER after 72 h IrO_x/bTNTs electrode: (a) Ir 4f; (b) Ti 2p; (c) O 1s; and (d) C 1s spectra.

Figure 4

Stabilized cyclic voltammograms (obtained at 50 mV s⁻¹, in deaerated 0.1 M HClO₄) of the open-structure IrO_x/bTNTs recorded in three potential windows, with an upper potential limit of +0.6 V_RHE (solid line), +1.2 V_RHE (dotted line), and +1.5 V_RHE (dashed line).

Figure 5

Stabilized cyclic voltammograms (obtained at 50 mV s⁻¹, in deaerated 0.1 M HClO₄) recorded in the potential window between hydrogen and oxygen evolution for the (a) close-packed IrO_x(Ni)/bTNTs and TNTs and (b) open-structure IrO_x/bTNTs. Current density, j_geom, is per electrode substrate projected geometric area.

Figure 6

(a) Current density (per electrode substrate geometric area) vs. applied potential curves corrected for the uncompensated solution resistance (obtained at 5 mVs⁻¹, in deaerated 0.1 M HClO₄) and their corresponding Tafel plots (Inset). Current density, j_geom, is per electrode substrate projected geometric area; (b) same as above but with current density, j_charge, normalized per electroactive IrO_x charge.

Figure 7

Nyquist plots of electrochemical impedance spectroscopy at the open-structure IrO_x/bTNTs electrode: (a) at potentials of +1.50 and +1.55 V_RHE and (b) at the potential of E = +1.50 V_RHE, as adapted to the equivalent electrical circuit R_sol(R_tQ_t)(R_pQ_p)(R_ctQ_dl).

Figure 8

Potential vs. time curve (chronopotentiometry) at a constant current density of 10 mA cm⁻² for 72 h in 0.1 M HClO₄ electrolyte.

Figure 9

SEM micrographs of an open-structure IrO_x(Ni)/bTNT after 72 h of continuous operation at 10 mA cm⁻² in 0.1 M HClO₄ electrolyte, at two different magnifications (a) and (b) as indicated by the scale bars (1 μm and 500 nm, respectively).