Hemin-mediated construction of iridium oxide with superior stability for the oxygen evolution reaction

Abstract

The biological molecule chloroprotoporphyrin IX iron(III) (hemin) was investigated as a novel catalyst to reduce the oxygen produced in IrO_x-electroflocculation preparation and aid the catalytic stability of IrO_x for the oxygen evolution reaction (OER) in both alkaline and acidic solutions. Hemin was deposited on glassy carbon (GC) electrodes through electropolymerisation with approximately 42% coverage and this mitigated the detrimental effect of accumulated oxygen on the subsequent growth of IrO_x. When the IrO_x catalyst was fabricated through 5 min of electroflocculation (coverage of approximately 8.43 × 10⁻⁸ mol cm⁻² Ir), the stability time (the time required to reach the overpotential of 1.0 V at 10 mA cm⁻²) was approximately 7 h, which is 5-fold higher than that of the process without polymerised hemin, and 29.1-fold higher than that of the electroflocculation method reported in the literature. With the aid of hemin-catalysed oxygen depletion, the IrO_x at the interface with the polymerised hemin is apparently deposited in a smaller and well-defined structure. The in situ spectroelectrochemical results revealed a lower dissolution rate for this specific IrO_x structure, which facilitated subsequent electroflocculation towards the formation of a continuously structured deposit resistant to degradation. In 1 M NaOH, the IrO_x catalyst sustained electrolysis (10 mA cm⁻²) for more than 20 h. In 1 M H₂SO₄, the overpotential (η_{t=24 h}) remained below 0.35 V. To the best of our knowledge, this is one of the few studies that have demonstrated a hydrous IrO_x catalyst that can maintain its stability for over 24 h of the OER.