Revisiting Tractable Strategies to Determine the Activity/Inactivity of Electrocatalysts towards O2/•OH Production

The measurement of the electroactivity of anode materials toward the generation of hydroxyl radicals or oxygen evolution from water oxidation is overriding to develop novel catalysts with tuned electrocatalytic applications. Thus, this work experimentally revisits chemical and electrochemical characterization techniques with the aims of establishing a comparison of the electrochemical performance of typical IrO 2 , RuO 2 , SnO 2 -Sb, and Boron doped diamond (BDD) electrodes toward H 2 O oxidation under feasible experimental conditions, and highlighting those conditions where variability cannot be removed. Tafel slopes corrected for ohmic resistance and active areas are 127, 175, 42, 48 mV dec − 1 for BDD, SnO 2 -Sb, IrO 2 and RuO 2 , respectively; while in the same order, the onset potential for water oxidation is: 2.49, 2.29, 1.47, 1.38 V vs SHE, measured at a current density of 0.5 mA cm − 2 . Additionally, a new interpretation is presented for the analysis of electrochemical impedance spectroscopy (EIS), and its connection with the OER mechanism: in the potential zone where Tafel is applicable is possible to appreciate the dielectric properties of the substrate for non-active anodes and elucidating the variation in the rate-determining step of water oxidation. EIS results suggest the possibility to use the time-constant associated with this stage as a parameter of the active or non-active behavior of the above electrodes. The advantages and drawbacks of different trapping molecules used to detect the • OH generation in electrocatalysis is discussed. The photoluminescence of coumarin could be used as a fast and reliable method for the evaluation of the electrocatalyst activity toward the • OH production. Electrochemical analysis.— A three-electrode cell was used to carry out these electrochemical experiments. Saturated calomel electrode and a pure graphite rod (0.5 cm diameter × 16 cm long, Alfa Aesar, 99.999%) were used as reference electrode and counter elec- trode, respectively. Meanwhile RuO 2 , IrO 2 , SnO 2 -Sb, BDD were used as working electrode (1.6 cm 2 of geometric area). Initially, cyclic voltammetry (CV) was conducted in a VMP3 (Bio-Logic Science In- strument) potentiostat/galvanostat in 0.1 M H 2 SO 4 electrolyte using a scan rate of 20 mV s − 1 at room temperature, to stabilize the catalyst and remove any impurity remaining from the synthesis. Linear sweep voltammetry (LSV) was used to determine Tafel slopes at 1 mV s − 1 in 0.1 M HClO 4 (Sigma-Aldrich). This electrolyte was selected because no specific adsorption is generated on electrode surface in its presence. Air was bubbled for 15 minutes to get the redox couple O 2 /H 2 O, and the polarization curves were corrected from ohmic drop using the methodology proposed by Shub and Reznik. 10 The electrochemical response of Tafel slopes collected under air conditions were compared to those obtained using O 2 and N 2 bubbling (Supplementary Material 1). The electroactive area was determined in 3 mM Ru(NH 3 ) 6 Cl 3 (Sigma-Aldrich) and 0.5 M KCl (J. T. as supporting electrolyte, using the chronoamperometry method in the diffusion control regime described by Cottrell equation. The sample time was from 5 to 10 ms, in order to ensure the length of the diffusion layer was similar to the electrode roughness. In this case, N 2 was bubbled before each (EIS) potentiostatic a VMP3 potentiostat/galvanostat frequency response ana-lyzer A wave 10 was used it was linearization of the response to the but enough a response that was detectable from the measurement The varied from 10 5 to 10 − 2 Hz at 10 points per decade. EIS spectra were obtained in 0.1 M HClO 4 at potentials indicated in Supplementary Material 2 including the zone in which Tafel slope was determined. At least two replicate EIS experiments at each potential. Virtually for each set of confidence reproducibility of the measurements. emission slit was set to be both 2.5 nm. In the same direction, ad- ditional tests were conducted with N, N-Dimethyl-p-4-nitrosoaniline (Sigma-Aldrich, RNO) as trapping molecule of • OH. The degrada-tion studies were carried out in 6 ppm RNO in 0.1 M phosphate buffer (pH 6.9), imposing a current density of 3.13 mA cm − 2 for 120 min for all the electrocatalysts analyzed. The RNO solution was recircu- lated by peristaltic pump trough an USB4000 fiber optic spectrometer (Ocean Optics) coupled to a UV-VIS-NIR light source (Ocean Optic, DH-2000), the tracking peak is approximately 440 nm.