Electrochemical Response of Bound Electrolyte Ions at Oriented Hematite Surfaces: A Local Electrochemical Impedance Spectroscopy Study

The electrochemical response of millimeter-sized hematite (α-Fe 2 O 3 ) electrode surfaces to bound ions of NaCl, NH 4 Cl, and NaHCO 3 salts was monitored by alternating current scanning electrochemical microscopy (AC-SECM). Local electrochemical impedance spectroscopy (LEIS) measurements along 100 μm lines on the (001) and (012) faces of hematite were used to extract capacitance and resistance parameters affected by bound inorganic ions. Equivalent circuit modeling was used to suggest that (1) double layer capacitances are affected by the spatial distribution of ions, and that (2) compact plane capacitance and resistance are affected by the closeness of association of ions to surface hydroxo groups. This study confirms the sensitivity of the technique to electrolyte ion binding, and provides new and key insight into the micrometer-scale electrochemical properties of iron oxides exposed to environmentally relevant conditions.