2.5. Electrochemical Measurements

Electrochemical measurements were run with a standard three-electrodes setup in a lab-made glass cell with an Ag/AgCl (3 M KCl) electrode as reference electrode and a Pt wire as counter electrode. The measurements were recorded by using a multichannel VSP potentiostat/galvanostat (BioLogic), equipped with EC-Lab v. 10.1x software for data acquisition. The reported potentials are referred to the reversible hydrogen electrode (RHE). The measured potentials vs. the Ag/AgCl reference electrode were converted to the RHE scale by using the Nernst Equation: ERHE=EAg/AgCl+EAg/AgCl0+0.059 pH. E_RHE is the converted potential vs. RHE, E_Ag/AgCl is the experimental potential measured against the Ag/AgCl reference electrode, and EAg/AgCl0 is the standard potential of Ag/AgCl (3 M KCl) at 25 °C (i.e., 0.210 V). The catalytic activity of MnO_x films was tested at pH = 7.0 in 0.1 M sodium phosphate buffer. Cyclic voltammetries (CVs) were performed in the range between 0.6 and 2.0 V vs. RHE with a sweep rate of 20 mV·s⁻¹. Current density was calculated by considering the geometrical area of the FTO substrate covered by the MnO_x film (~4 cm²).

For Tafel plot analysis, current density was recorded stepwise between 1.4 V and 2.0 V vs. RHE until reaching quasi-stationary currents. Therefore, the electrode potential was increased by 40 mV steps and held for 5 min. The potential was corrected for the ohmic drop, hence, impedance spectra were recorded at every potential/current step at 10 kHz with a modulation amplitude of 20 mV in order to determine the ohmic resistance of the solution (R_s ~ 40 Ω). The overpotential (η) values reported in both Tafel plot and Table 1 were calculated as follows: η=ERHE−ERHE0−i R, where ERHE0 is the standard potential of the water splitting reaction (~1.23 V) and i is the current. EIS curves were recorded using the same potentiostat described above from 0.1 Hz to 0.5 MHz, with an alternating current (AC) amplitude of 20 mV at different applied potentials (i.e., 1.0 V, 1.2 V and 1.4 V vs. Ag/AgCl).

Catalytic properties of the MnO_x films prepared by different methods.

^a Potential measured from the last CV cycle; ^b Current density measured from the last CV cycle; ^c Total O₂ produced in 1 h under an applied potential of 2.0 V vs. RHE.

Oxygen and hydrogen evolutions were measured by a Varian 490 micro-GC equipped with a Molsieve column during chrono-amperometry test at a fixed potential of 2.0 V vs. RHE, after purging of the cell with a continuous Ar flow for 1 h. An Argon flow rate of 25 Nml·min⁻¹ was used in order to carry the evolved O₂ and H₂ gases from the cell to the micro-GC, while the solution was maintained under continuous stirring to increase the mass transfer of the dissolved gases to the gaseous media.