The Detection of Solution PHASE Superoxide As a Reaction Intermediate of Oxygen Reduction in Neutral Aqueous Solutions
Authors: Feng Zhange , Scherson Daniel 
Published: February 2020

Despite decades of research, certain aspects of the mechanism of dioxygen, O2, reduction on solid electrodes in aqueous electrolytes still remain poorly understood.1 Particularly elusive has been the role of solution phase superoxide, a relatively short lived species, as a reaction intermediate. A method is herein described that allows for solution phase superoxide generated via the reduction of dioxygen in neutral aqueous solutions at a rotating disk electrode to be oxidized at a concentric Au ring electrode bearing a covalently linked monolayer of 3-mercapto-1-propanol, a modified surface that blocks the oxidation of solution phase of hydrogen peroxide 2.
Experimental
All measurements were performed at room temperature in a conventional three electrode glass cell using either a gold rotating disk electrode (RDE, Pine Instruments, area: 0.164 cm2), Au ring- glassy carbon (GC) disk RRDE (Pine Instruments: AFE7R9GGCAU) as working electrodes. A Au wire and a Ag/AgCl in 3.5M KCl contained within a Pt cracked seal capillary served as counter and reference electrodes, respectively. Experiments were carried out in 0.1M PB of pH 7.4 prepared, respectively, by mixing Na2HPO4 (J.T. Baker Ultrapure Bioreagent 99.7%), and NaH2PO4 (J.T. Baker Ultrapure Bioreagent 99.9%), using ultrapure water (UPW, 18.3 MΩ cm, EASYpure UV system, Barnstead). Shown as inserts in Fig. 1 are typical cyclic voltammetric curves recorded with the Au electrodes employed in this study in deaerated 0.1 PB solutions of pH 7.4 displaying characteristic features associated with the clean surfaces. Such Au electrodes were chemically modified by immersing the RDE or RRDE in a solution 10 mM 3-mercapto-1-propanol (MP, Sigma Aldrich, 95%) in ethyl alcohol (AAPER, 200 proof) for 20 h, as described by Gobi et al.3

Shown in thick lines in Fig. 1 are dynamic polarization curves recorded with the Au RDE at a scan rate, n = 10 mV/s and a rotation rate, w = 900 rpm, in O2-saturated 0.1 M PB aqueous solutions, pH 7.4, before (blue) and after (black) adding 1mM H2O2. These results clearly indicates that the MP-modified Au electrode blocks the reduction of O2 and both the oxidation and reduction of H2O2 over the specified potential range.
Shown in Fig 2 top panel is the dynamic polarization curves were then recorded at ν = 10 mV/s, for the glassy carbon (GC) disk electrode of the Au|GC RRDE in O2–saturated PB aqueous solutions of pH 7.4 and pH 10 at ω = 900 rpm. The results obtained in the experiments in which the Au ring was modified by a covalently linked MP monolayer also yielded an increase in iring at the onset potential for O2 reduction on the disk. iring in this case, displayed a bell shaped curve with a maximum at Edisk ca. -0.4 V In agreement with the data shown in Fig. 1, the increase in iring for the MP-modified Au induced by adding H2O2 to the buffered solution to a concentration of 1 mM amounted to ca. 3 nA for pH 7.4 at 0.4 V, i.e. less than an order of magnitude smaller than those found in lower panels, Fig. 2. It may concluded on this basis that the species being oxidized by the MP-modified Au ring is indeed with a very minor contribution due to solution phase peroxide, which appears to reach a maximum at Edisk = -0.4 and thus consistent with its further reduction as Edisk becomes from negative.
ACKNOWLEDGMENT
This work was supported by a grant from NSF, CHE-1412060
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Figure 1