Effects of Structures on the Sensing Properties of Long-Term Stable IrOx pH Electrodes

Abstract

Iridium oxide electrodes made by melted Li2CO3 + Na2O2 in a semi-closed/closed reactor exhibited fast response and high stability. A slight periodic fluctuation of potential change with standard deviation < 0.65 mV in 48 h and < 2.30 mV (0.04 pH) over 139 h were observed. The composition, structure, and sensing mechanism of the pH sensitive substance was examined. The XRD pattern and XPS results suggested the newly produced material Hy-xLix[LiIr2]O6 was a mixture of H5-xLix[LiIr2]O6 and H3-xLix[LiIr2]O6 with a HCrO2 structure. Approximately 95% of Li+ was replaced by H+ after acid treatment. The pH sensing process was found to be an irreversible redox reaction between the tetravalent and trivalent iridium oxides according to the CV and XPS results. The presence of lithium ion in the lattice combined with well-crystallized small crystals corresponded to the fast response during the pH sensing process. The crystal structure may help to protect iridium from the interference of weak oxides, and thus may maintain stability with a relatively acceptable potential drift in the natural redox condition. The large portion of OH− existing in the oxide lattice detected by XPS confirmed the well-hydrated structure, which may also help the electrode to maintain high stability.