Comments Regarding the Non-Miscible Solvent Microcapillary Method for Superoxide Detection in Aqueous Electrolytes

Certain aspects of the nanoelectrochemical method reported by Zhou et al. ( J. Am. Chem. Soc., 137 , 6517 (2015)) for the detection of solution phase superoxide, O . − 2 ( aq ) , generated by the oxygen reduction reaction, ORR, on polycrystalline Pt in aqueous electrolytes, have been critically assessed. Experiments performed under conditions similar to those employed by these authors have shown that upon formation of the liquid-liquid interface, as required by this technique, benzotriﬂuoride, BTF, undergoes partial dissolution into the aqueous phase and subsequent adsorption on the Pt electrode. As evidenced by data collected with a Pt | Pt rotating ring disk electrode, this effect induces signiﬁcant changes in the kinetics and mechanism associated with the ORR on the otherwise bare electrode, a factor that might limit the overall utility of this tactic as a reliable tool for elucidating detailed pathways involved in this important redox process. Also included in this communication is a mathematical model that allows for the concentration of O . − 2 ( aq ) next to the disk of a RRDE to be determined during the ORR based on the magnitude of the current collected with a Au ring speciﬁcally functionalized to detect exclusively O . − 2 ( aq ) , using data reported earlier in our laboratories for a glassy carbon disk electrode as a model system.

with a bipotentiostat (Pine Instruments, Model AFCBP1), and the rotation rate of the RRDE, ω, adjusted with a commercial rotator (Pine Instruments, Model AFMSRX). All the data were recorded using a National Instruments acquisition card (USB-6009) programmed in Labview.
Unless otherwise specified, all experiments were performed in Ar (Airgas, PP300, 99.998%)-purged 0.1 M Na 2 SO 4 prepared from Na 2 SO 4 (J. T. Baker, 99.9%) and ultrapure water (UPW, 18.3 M cm, EASYpure UV system, Barnstead) yielding a pH of ca. 5.5. Oxygen (Airgas, 99.999% research grade), and hydrogen peroxide (Fisher Scientific, certified ACS, 31.5%) were used as received. A gold wire (ca. 5 cm 2 area) and a Ag/AgCl (3.5 M KCl) were used as counter and reference electrodes, respectively. In order to mimick the conditions of the original experiments reported by Zhou et al., 1 an amount of BTF (Sigma Aldrich, anhydrous 99.9%) just sufficient to yield, upon full dissolution, a concentration of 100 ppm (0.68 mM), and thus below the solubility limit of BTF in pure water, i.e. 451 ppm (3.1 mM) at 25 • C, 6 was added to the 0.1 M Na 2 SO 4 aqueous solution in which the electrochemical measurements were performed. As one might have expected based on the lack of miscibility with water 7 and its higher density, ca. 1.19 g/cm 3 at 20 • C 8 , most of the BTF fell to the bottom of the cell forming a droplet. The actual concentration of BTF in the aqueous phase was then determined from standardized UV visible spectra recorded with a Cary 50 spectrophotometer using BTF solutions in 0.1 M Na 2 SO 4 of concentrations in the range 0.04 to 0.33 mM, yielding a value of 0.23 mM (see Fig. S1 in the Supplementary Material). For these spectroscopic measurements, a homogeneous 0.33 mM BTF in 0.1M Na 2 SO 4 stock solution was prepared by brief ultrasonic agitation. Additional experiments were also carried using Na 2 SO 4 (Fisher, 99% + purity) to examine the role of a lower quality chemical on the electrochemical results.

Results and Discussion
Voltammetric measurements in Ar-purged solutions.-Shown in Panel A, Fig. 1, are cyclic voltammetric curves collected with the disk of the Pt|Pt RRDE in Ar-purged 0.1 M Na 2 SO 4 (J. T. Baker) at a scan rate ν = 10 mV/s under stagnant conditions over the range −0.2 < E < 1.0 V (black) and −0.2 < E < +0,2 V vs Ag/AgCl (blue). The uncharacteristic lack of full (kinetic) reversibility of the hydrogen adsorption features is due to the fact that the electrolyte lacks any significant buffer capacity. Under such conditions, the solution in the immediate neighborhood of the electrode turns alkaline during hydrogen adsorption leading to a shift in its subsequent desorption toward higher potentials (vs the pH independent Ag/AgCl reference  Figure  S2 in the Supplementary Material), where the extent of the blockage under quiescent conditions was found to be ca. 44%. In addition to displaying affinity for Pt surfaces BTF was also found to undergo oxidation at potentials positive to the onset of oxide formation on Pt in the neat electrolyte, as evidenced by the much higher charge observed in voltammetric scans where the positive limit was extended to 1.0 V vs Ag/AgCl (see Figure 2).   no major changes in the diffusion limited currents nor in the onset potential for the ORR could be discerned between the two sets of data, the presence of the surfactant gave rise to two significant effects: i. A shift in the half-wave potential, E 1/2 , toward negative potentials as large as 35 mV (see Table I, for 900 rpm) indicative of losses in activity due to the presence of the organic adsorbate. These shifts were even larger when a lower quality Na 2 SO 4 (Fisher, 99%+) was used (see values in parenthesis in Table I) and perhaps more pronounced in the even lower quality Na 2 SO 4 (Sigma-Aldrich, ≥98% purity) employed in Ref. 1. ii. An increase in the amount of H 2 O 2 (aq) and/or O − 2 detected by the Pt ring by a factor of two over almost the entire potential range Although the reasons for the large discrepancy between the values reported by Zhou et al. 1 and those widely accepted in the literature still remain unexplained, the fact that they assumed the disproportionation of O − 2 to be a pseudo first order, as opposed to a second order reaction as experiments have shown, 2 may have been a contributing factor. As is well known, the half-life for a second order reaction depends on the initial concentration of the reactant, whereas for a first order analogue it does not. Table I • The values in parenthesis were found with Na 2 SO 4 (Fisher). An alternate method for detecting O − 2 (aq) generated by the ORR in aqueous electroytes was recently reported by Feng et al. 3 This tactic relies on the use of a RRDE, whereby solution phase O 2 is being reduced at the disk of the RRDE to yield O − 2 (aq) (and probably other products as well), which then escapes into the bulk electrolyte where it can undergo homogeneous disproportionation according to and/or be oxidized at a judiciously functionalized Au ring of the RRDE virtually impervious to the presence of peroxide in solution. As will shown in the next section, this strategy makes it possible to determine the concentration of O − 2 (aq) in the neighborhood of the disk electrode based on the values of the ring current for the oxidation of O − 2 (aq), assuming the latter proceeds under diffusion limited conditions.

Theoretical Aspects
The primary objective of this section is to determine quantitative correlations between the concentration of O − 2 in the immediate vicinity of the disk and the magnitude of the ring current measured at steady state with a RRDE as a function of pH. The problem so stated involves finding solutions to the time-independent convective diffusion equation for O − 2 (aq) in Eq. 2 below, where [O − 2 ] and D O − 2 represent the concentration and diffusion coefficient of O − 2 , respectively, u r and u z are the fluid velocities along the radial, r, and axial, z, coordinates, and R is the pH-dependent rate of the second-order dismutation of O − 2 given by 2 where the rate constant k dis , is given in units of cm 3 mol −1 s −1 . Numerical solutions of Eq. 2, subject to the boundary conditions specified in Table II were obtained using COMSOL, assuming the system could be accurately represented by the finite 2-dimensional axisymmetric domain shown schematically in Figure 4, and explicit expressions for the fluid velocities as prescribed by the Levich formalism, 9 i.e.: u r = 0.51023 · (ω 3 /ν) 1/2 r z [4] u z = −0.51023 · (ω 3 /ν) 1/2 z 2 [5] where ω is the rotation rate of the electrode in rad/s, and ν is the kinematic viscosity of the solution, i.e. ca. 0.01 cm 2 /s. Such calculations were performed for a series of rotation rates, in the range of 400 to 1600 rpm and for pH 7.4 and 10.0. As indicated in Figure 4, the height of the domain was assumed to be three times longer the thickness of the diffusion boundary layer, δ, given by where D O − 2 = 6.7 × 10 −6 cm 2 /s 1, . 10  ] bulk , and independent of the radial distance along the disk, r 1 . The simulated current for a single-electron process flowing across the ring (boundary 5) is given by: where F is Faraday's constant, and boundary 5 represents the thickness of the ring electrode. Figure 5 Figure 6, which provide valuable information to elucidate the overall mechanism of the ORR in the disk. Unfortunately, the actual mechanism of the ORR on many surfaces including carbon is not known with certainty and the rates of the elementary processes involved have not as yet been determined with sufficient accuracy. This scarcity of key information makes it very difficult to gain additional insights regarding the ORR based strictly on the data analyzed in this work.

Conclusions
The information presented in this brief communication raises serious doubts regarding the reliability of the nanoelectrochemical method reported by Zhou et al. 1 for the determination of mechanistic aspects of a surface sensitive reaction, such as the ORR on Pt, owed to the possible adsorption of the organic solvent on the electrode surface. In fact, as shown by Yang and McCreery, 11 the presence of an organic on an electrode surface can lead to enhancements in the amount of O − 2 produced during the ORR. On this basis it is quite possible that a fraction of O − 2 detected by Zhou et al. may have originated from the presence of adsorbed BTF on Pt. Yet an additional concern regarding this strategy is the magnitude of the half-life of O − 2 dismutation in the unbuffered aqueous solution reported by Zhou et al., which is orders of magnitude shorter than accepted values in the literature. In contrast, the RRDE method developed by Feng et al. not only avoids many of these complications, but offers rather simple means for the quantitative electrochemical detection of solution phase O − 2 , provided the rates of O − 2 dismutation are not too fast for its concentration at the ring to drop to negligible values. In closing, and, from a general perspective, the solubility of the organic solvents most commonly employed in the implementation of this SECM capillary method in pure water is relatively high, ranging from 3.1 mM for BTF up to 88 mM for dichloroethane (DCE). It is, therefore, quite likely that their presence in the aqueous phase can alter the properties and behavior of the systems, including interfacial processes, being examined.