A rotating ring-disk electrode, RRDE, method is herein described, whereby a Se-modified Pt ring electrode is used to measure the rates of oxygen generation, derived from the heterogeneous hydrogen peroxide decomposition, het-HPD, on the surface of selected disk materials, under open circuit. OC, conditions. This technique was inspired by the earlier work of Shigehara and Anson ( J. Phys. Chem. 1982, 86 (14), 2776-2783), who employed a high area carbon paste ring electrode modified by an adsorbed layer of a rather unique Co macrocycle to accomplish the same goal. As first discovered by Mo et al. in our laboratory ( J. Electrochem. Soc. 2003, 150 (1), E39), a Pt surface bearing 0.83 ML of elemental Se, Se(0.83)|Pt, catalyzes the oxygen reduction reaction, ORR, in 0.5 M H 2 SO 4 , yielding Levich plots, i lim vs ω 1/ 2 , where i lim is the limiting current, and ω is the rotation rate of the electrode, consistent with a strict 2e - process. Such plots, however, displayed a non-zero intercept, signaling complications in the ORR mechanism. As will be shown in this work, this behavior could be accounted for by introducing a first order het-HPD step to the otherwise strict 2e - ORR. Application of this formalism to data collected for a Se(0.83)|Pt disk polarized at a potential of 0.175 V vs RHE, a value well within the range where the ORR proceeds under diffusion limited conditions in the aforementioned electrolyte, yielded an estimated average first order rate constant in peroxide for the het-HPD step, = 1 ´ 10 -4 ± 0.2 cm s -1 . Measurements were then performed using a Se(0.83)|Pt ring of a RRDE polarized at that same potential to determine by fitting the data collected for Teflon, glassy carbon, Se(0.83)|Pt and Pt disks using COMSOL simulations. As evidenced from the results obtained for [H 2 O 2 ] = 0.3 and 1.0 mM solutions in the same base electrolyte, the intrinsic activity of Teflon and GC for the het-HPD was found to be negligible compared to that of Pt, which yielded values of as high as ca. 1.7 ´ 10 -9 mol cm -2 s -1 . Overall, this method may be regarded as far simpler and more versatile compared to that proposed by Shigehara and Anson, as it avoids the use of chemicals not available from commercial sources. ACKNOWLEDGMENT This work was supported by NSF, CHE1808592

Recent findings in our laboratories have shown that copper 1 and cadmium 2 underpotential deposited on polycrystalline Au electrodes, can mediate the reduction of in 0.1 M HClO 4 aqueous solutions, to yield irreversibly adsorbed elemental Se allowing its detection down to the nM range. Current efforts are focused on gaining a better understanding of the the factors that govern this unique electrocatalytic phenomenon by employing a combination of electrochemical, microgravimetric, and optical techniques, using Cu as the UPD species. Correlations have been sought between the rates of this process and the coverage of Cu(UPD), the concentration of and, to a more limited extent, the applied potential in 0.1 M HClO 4 solutions. Experimental conditions were selected to unveil the functional dependence of the kinetics on each of the aforementioned factors, while keeping the others constant, in solutions both quiescent and in the presence of convective flow. A number of interesting observations were made based on the data collected. In particular, shown in Panel C, Fig. 1, are plots of the charge associated with the oxidation of adsorbed Cu and Se, Q Cu and Q Se , respectively (see Panel C), obtained from the area under the peaks shaded in light blue and yellow in Panels A and B, Fig, 1, respectively, vs (see below). These data were recorded during linear potential scans following polarization of the electrode at E hold = 0.325 V for various times, t hold (see Panels A-C, Fig. 1), where the linear character of the data in solutions is consistent with Cu(UPD) proceeding under strict diffusion control. As indicated by the Q Se data, the electrode displayed electrocatalytic activity for reduction only for Q Cu > ca. 80 μC cm 2- . Additional insight was obtained from measurements of the electrode weight as a function of using a quartz crystal microbalance (see Panel D, Fig.1), which yielded evidence for adsorption only for Q Cu > ca. 80 μC cm 2- , pointing to the formation of a surface-bound adduct, as a necessary step for reduction to ensue. On this basis, we propose that for Q Cu > ca. 80 μC cm 2 the mechanism for reduction involves, as a first step, the reversible formation of the adduct, Eq. (1) followed by its irreversible reduction, generating adsorbed elemental Se, Cu|Se(ads), Eq. (2). Atomically resolved images of Cu(UPD) on Au(111) obtained with a scanning tunneling microscope for the closely related sulfate ion, strongly suggest that the active site involved adsorption on the empty Au sites of the so-called √3´x√3 superstructure (see Insert in Panel A, Fig. 1). A similar conclusion was drawn based on measurements performed with rotating Cu ring-polycrystalline Au disk electrodes, RRDE, at constant Cu(UPD) coverage and applied potential for various solution compositions. Quantitative analyses of all the data, which included numerical simulations, yielded k f /k r , and k ET values in the range (2.4 – 3.23) ´ 10 6 cm 3 mol -1 and (2.5 – 9) ´ 10 -3 s -1 , respectively. Acknowledgments Support for this work was provided by NSF CHEM 1808592 References 1. Strobl, J. R.; Scherson, D. A., The Reduction of Selenate Mediated by Underpotential Deposited Copper on Gold Electrodes in Acidic Solutions: Analytical Applications. Journal of The Electrochemical Society 2016, 163 (13), H1066. 2. Han, Q.; Strobl, J.; Scherson, D., Communication—Selenate Reduction Induced by Cadmium Underpotential Deposition on Gold in an Aqueous Acidic Electrolyte. Journal of The Electrochemical Society 2019, 166 (8), H283. Figure Caption Figure 1

Underpotential deposited Cu on the low index faces of single crystal Au electrodes has been found to promote the reduction of selenate, SeO2-4 (aq), in 0.1 M HClO4 solutions, a behavior analogous to that reported earlier in our laboratory for polycrystalline Au (Strobl et al. J. Electrochem. Soc. 2016, 163 (13), H1066). Sequential potential step-linear scan voltammetry data collected for Au(111) film electrodes in solutions containing Cu2+(aq) in the mu M range afforded evidence that the onset for the electrocatalytic activity occurs for Cu(UPD) coverages, theta Cu approximate to 0.18, the same value at which complementary microgravimetric data displayed a clear increase in mass. On this basis, a reaction mechanism has been proposed involving the initial reversible formation of an adsorbed adduct, we denote as Cu|SeO24 (ads), followed by a first order irreversible reduction to yield a yet to be identified species, which we denote as Cu|Se(ads), as the rate determining step. Support for this reaction scheme was obtained from numerical solutions of the coupled differential equations that govern the time evolution of Cu|SeO- 4 (ads) and Cu|Se(ads), yielding best agreement with the experimental data for values of the equilibrium constant for adduct formation and the adduct reduction rate constant of 2.5 x 106 cm3 mol-1, and 9.1 x 10-3 s-1, respectively. This unique electrocatalytic effect has been attributed to a shift in the potential of zero charge of the bare Au substrates induced by Cu(UPD), which promotes the adsorption of HSeO-4 (aq) at potentials more negative than those found for the bare substrates, allowing access to overpotentials high enough for its activation and further reduction to ensue.

The reduction of selenate, SeO2-4 (aq), in 0.1 M HClO4 solutions, induced by underpotential deposition, UPD, of Cu on polycrystalline Au electrodes was investigated using the rotating ring-disk electrode, RRDE, technique. Design and implementation of electrode potential-rotation rate protocols made it possible to determine the rates of SeO24 (aq)reduction as a function of Cu coverage, 0Cu, as determined by the Bruckenstein method (Swathirajan et al. J. Phys. Chem. 1982, 86, 2480-2485). In agreement with the results reported recently for Au(111) film electrodes (Strobl et al. Electrochimica Acta 2024, 493, 144,298), the reaction was found to proceed only for 0Cu above a critical value, i.e. ca. 0.39, in this case, and the mechanism is consistent with an initial reversible formation of adsorbed Cu|SeO2-4 (ads), followed by its subsequent irreversible reduction, to yield a yet to be identified species denoted as Cu|Se(ads), as the rate determining step. Best fits of the kinetic model yielded values of the equilibrium constant for adduct formation, K, and first order rate constant for adduct reduction, kET, in the range (2.4 - 45) x 106 cm3 mol- 1 and (0.55 - 30) x 10-3 s- 1, respectively, which are close to those found for Au (111). This unique electrocatalytic effect has been attributed to a shift in the potential of zero charge of the bare substrate toward more negative values, induced by the metal UPD, which promotes the adsorption of the oxyanion at potentials more negative than those found for the bare substrate, making it possible to access overpotentials large enough for its further reduction to ensue.

Normal incidence differential reflectance, or NIDR, is an in-situ technique suitable for high time resolution monitoring of the interfacial structure and composition of electrode-solution interfaces. Recent efforts have been focused in our laboratory on formulating quantitative models for interpreting the potential-, and coverage-dependence of NIDR signals for use in monitoring interfacial dynamics at Au electrodes in various electrolytes. A simple model capturing the aforementioned dependences will be presented. Within the framework of this model, different limiting behaviors are anticipated depending on the optical properties of the adsorbate and the nature of its interactions with the Au surface. In addition, some exemplary data sets on Au electrodes will be discussed, namely cyclic voltammetry and potential steps on single crystals and polycrystalline Au surfaces in aqueous perchloric acid solutions. The NIDR responses to these potential perturbations can be rationalized on the basis of the optical properties of perchlorate and its interaction with the surface. All NIDR data are consistent with adsorbed perchlorate exhibiting optical properties that deviate minimally from that of water, as well as perchlorate physisorbing on Au rather than chemisorbing. In essence, the NIDR signal is proportional to the charge stored on the Au surface, with no change in said proportionality with perchlorate adsorption, rendering this adsorbate essentially ‘invisible’ to NIDR. This has obvious implications when using perchlorate salts as a supporting electrolyte in NIDR experiments, as perchlorate will produce no NIDR response and therefore will not convolute with the NIDR response arising from other adsorbates. To demonstrate this in practice, NIDR experiments for Se covered Au electrodes in aqueous perchloric acid electrolytes will be discussed and the clear dependence of the NIDR response on Se coverage will be demonstrated.

The oxygen reduction reaction, ORR, in aqueous electrolytes, may rank among the most studied heterogeneous electron transfer processes. Despite the extraordinary efforts of numerous research groups worldwide, critical questions still remain unanswered regarding critical aspects of this technologically important reaction and the dependence of its mechanism and rates on the nature of the electrode material. In their pioneering studies, Zurilla et al.1 examined the ORR on polycrystalline gold, Au(poly), in alkaline solutions, using rotating ring-disk electrode, RRDE, techniques. According to these authors, the experimental evidence collected was consistent with a mechanism involving an initial one-electron transfer to yield adsorbed superoxide ion, O2 -(ads), which, subsequently, underwent a second order heterogeneous dismutation generating solution phase peroxide and oxygen, O2(aq) and HO2 -(aq). The same mechanism was later invoked by Adzic et al.2 for the ORR on Au(100), a surface that displayed extraordinary activity compared to other low index faces of Au. More recently, Ignaczak et al.3 put forward theoretical arguments that support the view that the initial RDS is actually of the outer sphere type, yielding a solvated species, i.e. O2 -(aq), which is then followed by a subsequent, fast, outer sphere one electron-transfer, generating solution phase peroxide, HO2 -(aq), and not by a second order dismutation of as was postulated earlier in the literature. This is shown as the ‘outer sphere pathway’ in Scheme 1. Scheme 1: Proposed inner and outer sphere pathways for ORR on Au in basic solution. This presentation will describe the use of RRDE methods to examine the kinetics of the ORR on Au(poly) in 0.1 M NaOH + 0.9 M NaClO4 aqueous electrolytes containing oxygen in one case and peroxide in the other. The results obtained were found to be consistent with Ignaczak et al.’s mechanism, or equivalently an inner sphere pathway (Scheme 1) which includes a fast equilibrium between O2 -(aq), HO2 -(aq) and their corresponding adsorbed counterparts, O2 -(ads) i.e. and HO2 -(ads) however, a critical assessment of in situ vibrational spectroscopy data published in the literature has raised questions regarding the assignment of the spectral features reported for these adsorbed species. Also considered in this model is the direct reduction of HO2 -(aq) and O2(aq) to generate OH-(aq). Quantitative analyses of data collected in O2-saturated and Ar-purged containing solutions made it possible to determine a unique set of kinetic rate constants for the various steps in the proposed mechanism over a wide potential range. References: Zurilla, R. W.; Sen, R. K.; Yeager, E. The Kinetics of the Oxygen Reduction Reaction on Gold in Alkaline Solution. Electrochem. Soc. 1978, 125 (1103-1109). Adzic, R. R.; Markovic, N. M.; Vesovic, V. B., Structural Effects in Electrocatalysis Oxygen Reduction on the Au(100) Single Crystal Electrode. Electroanal. Chem. 1984, 165, 105-120. Ignaczak, A.; Nazmutdinov, R.; Goduljan, A.; Moreiro de Campos Pinto, L.; Juarez, F.; Quaino, P.; Santos, E.; Schmickler, W. A scenario for oxygen reduction in alkaline media. Nano Energy 2016, 26, 558-564. Figure 1

The adsorption of perchlorate, ClO4- (aq), sulfate, SO42- (aq) and selenate, SeO42- (aq) on Au(111)-textured single crystals from aqueous acidic solutions was examined as a function of the applied potential, E , by simultaneous voltammetry, normal incidence differential reflectance spectroscopy, Delta R/R, and electrochemical quartz crystal microbalance, EQCM, techniques. In particular , Delta R/R vs E data collected in pure 0.1 M HClO4 or the same electrolyte containing either 1 mM Na2SO4 or 1 mM Na2SeO4 could be quantitatively accounted for by using a simple model introduced in similar studies reported earlier in this laboratory. This model regards the Delta R/R response as arising from a sum of contributions from bare and adsorbate covered areas of the surface, which are, in each case, linear functions of the applied potential. Adsorption isotherms for ClO4- (aq), and SO42- (aq) on Au(111) from the literature allowed fitting of the model to data obtained in 1 mM Na2SO4 in 0.1 M HClO4 solutions. The two oxyanions were found to co-adsorb over a wide potential range, with SO42- (aq) fully displacing ClO4- (aq) for E > 1.25 V vs RHE. Analogous Delta R/R measurements involving 1 mM Na2SeO4 in 0.1 M HClO4 solutions combined with the model allowed calculation of the coverage of the two oxyanions as a function of E. The experimental isotherms enabled calculation of the mass densities for the adsorbed oxyanions, rho(th), yielding values virtually identical to those derived from EQCM experiments, rho(exp), for data recorded in the range ca. E < 0.73 V vs RHE in all three solutions. At higher potentials, however, rho(exp) were significantly larger than those predicted by the optical measurements, a behavior consistent with water co-adsorption. (C) 2021 Elsevier Ltd. All rights reserved.

A 3-mercapto-1-propanol (3M1P)-modified Au ring electrode has been employed to monitor solution phase superoxide, O-2(-)(aq), generated at the surface of a glassy carbon (GC) disk of a ring-disk rotating electrode (RRDE) in 0.1 M NaOH aqueous solutions. Measurements performed at various rotation rates afforded unambiguous evidence that maximum yields for O-2(-)(aq) is attained at potentials associated with a current minimum in the polarization curve at E-disk = 0.3 V vs RHE, with a ca. 18% faradaic efficiency. In stark contrast, no O-2(-)(aq) could be detected in virtually identical experiments involving Au and Pt disk electrodes. This finding indicates that, if superoxide is involved in the mechanism of O-2 (aq) reduction on these metals, it remains adsorbed on the surface and is subsequently reduced, or, if desorbed, the rate of its reduction would be high enough for its concentration to decrease to undetectable levels at the functionalized Au-ring under the experimental conditions employed. (C) 2019 Elsevier Ltd. All rights reserved.

Underpotential deposition (UPD) of Cd on polycrystalline Au, Au(poly), has been found to catalyze the reduction of selenate in 0.1 M HClO4 to yield, following stripping of the Cd-UPD layer, elemental Se. Under certain conditions, the rates of this process, as measured by the charge under the peak associated with Se oxidation, were found to be nearly constant, with values which increased with the applied overpotential. (C) The Author(s) 2019. Published by ECS.

In analogy with many other oxyanions, the electrochemical reduction of selenate, SeO4 2-(aq) in aqueous solutions on most electrode surfaces is typically characterized by slow kinetics. Typical reduction strategies employ excessive amounts of cationic reducing agents like Cu(I) and Fe(II) compounds [1, 2]. Recently, Cu UPD on Au(poly) was found not only to promote reduction of SeO4 2-(aq) in acidic electrolytes, but advantage was taken of the resulting Cu/Se stripping peaks to develop an exceedingly sensitive analytical method for SeO4 2-(aq) detection down to the nM range [3]. An example of some typical qSe vs. [Na2SeO4] calibration curves, where qSe represents the charge under the Se stripping peak, (see insert in Fig. 1), are shown in Fig. 1. The rates of SeO4 2-(aq) reduction as catalyzed by Cu UPD are very time dependent, indicative of a complex electrocatalytic mechanism. Insight into some aspects of this process has been gained from studies involving the low index faces of single crystal Au via hold/strip voltammetry coupled with Normal Incidence Differential Reflectance (NIDR) and in the case of Au(111), by Electrochemical Quartz Crystal Microgravimetry (EQCM) studies. It has been found that a minimum surface coverage of Cu UPD is required before any reduction of SeO4 2-(aq) occurs, and that reduction slows over time as the surface saturates with Se atoms. However, excessively high Cu UPD coverages were found to hinder SeO4 2-(aq) reduction on the Au(111) plane. Some simple kinetic models have been formulated to attempt to capture the observed behavior. Additionally, at fixed coverages of Cu UPD, rates of SeO4 2-(aq) reduction increase with decreasing applied potential. To this end, the possible electrocatalytic properties of other metals capable of undergoing UPD at potentials more negative than those of Cu were considered. The first candidate to be examined was Cd, a metal for which the work function in bulk form is much lower than that of Cu [4] and, thus, its UPD should occur at more negative potentials. In addition, Cd UPD on Au has shown electrocatalytic activity for the reduction of NO3 -(aq), yet another example of a difficult to reduce oxyanion, in aqueous electrolytes [5]. It was found that SeO4 2-(aq)reduction can indeed be catalyzed by Cd UPD on Au, as shown in Fig. 2. In contrast to Cu UPD catalyzed reduction, it was found that under certain conditions the reduction rates of SeO4 2-(aq) were time independent when mediated by Cd UPD, as shown in Fig 3. References: Murphy, A. P. Eng. Chem. Res. 1988, 27, 187-191. Baeshov, A.; Khovakov, B. E.; Buketov, E. A. Doklady Akademii SSSR. 1985, 278, 818-820. Strobl, J. R.; Scherson, D. A. Electrochem. Soc. 2016, 163, H1066-H1068. R. Rumble, D. R. Lide and T. J. Bruno, CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data, Boca Raton : CRC Press, [2018], 99th edition. K. Xing and D. A. Scherson, J. Electroanal. Chem. Interfacial Electrochem., 199, 485 (1986). ACKNOWLEDGEMENTS: This work was supported by a grant from NSF, CHE-1412060. Figure 1

Channel electrodes, in the wall of a microfluidic channel, allow generator-collector schemes with solution species generated at an upstream electrode and collected at a downstream electrode, possibly with homogenous reactions occurring during the mass transport between the electrodes. These are useful devices to study electrocatalytic reaction mechanisms, especially for oxidation of small organic molecules, where there are multiple pathways and multiple soluble intermediates and products. We briefly review our recent experimental and theoretical advances toward this goal, which include the implementation of a PdH reference in a side channel [1], a new semianalytical method for convective diffusion in rectangular channels [2], and a numerical investigation of the validity of the Lévêque approximation and neglect of axial diffusion for impedance at a single channel electrode [3]. The experimental and theoretical study of the double channel generator-sensor impedance are then described for a reversible solution redox couple (Ru(II/III) hexammine complex) [4]. By using galvanostatic generation of the ac signal at the upstream working electrode and the working sense connection to measure the ac potential at the downstream sensor electrode, we were able to implement a generator-sensor electrode scheme with a single potentiostat. We define the "downstream impedance" as the ratio of the ac potential measured downstream to the ac current measured upstream. The downstream impedance shows beautiful spirals. The phase may be interpreted simply in terms of the propagation time between the two electrodes relative to the period of the a.c. signal. This simple picture predicts that the phase is linear with the frequency. If there were no diffusive spread as the concentration wave moved downstream, the impedance would be a circle in the complex plane. However, diffusion across the channel during downstream propagation leads to a decreasing amplitude with frequency and therefore spirals rather than circles. The full solution of the convective-diffusion problem was solved numerically using COMSOL and showed spirals in good agreement with the experimental results. A dimensional analysis for the simplifying assumptions of the Lévêque approximation and neglect of axial diffusion shows that the impedance depends only on a single parameter, a dimensionless frequency Ω. An analytical solution of the downstream impedance under these assumptions was possible for the case of zero frequency, which was found to agree with the numerical solution within 10% except at the lowest flow rates. As predicted by this model, when normalized by the zero-frequency impedance, the complex plane plots for all flow rates fall on a common curve. Bode plots of the normalized impedance also fall on common curves. The slope of log(phase) vs log(Ω) plots is 1 for Ω<1 as predicted by the simplistic model. We thank the Research Council of Norway, the Natural Science and Engineering Research Council of Norway, and our respective institutions for financial support. [1] E.V. Fanavoll, D.A. Harrington, S. Sunde, G. Singh, F. Seland, Electrochim. Acta., 225, 69 (2017). [2] T. Holm, S. Sunde, F. Seland, D.A. Harrington, J. Electroanal. Chem., 745, 72 (2015). [3] T. Holm, M. Ingdal, E.V. Fanavoll, S. Sunde, F. Seland, D.A. Harrington, Electrochim. Acta., 202, 84 (2016). [4] T. Holm, M. Ingdal, J.R. Strobl, E.V. Fanavoll, S. Sunde, F. Seland, D.A. Harrington, Electrochim. Acta., 229, 452 (2017).

Despite decades of research, no consensus has been reached regarding basic aspects of the mechanism of the oxygen reduction reaction (ORR) on Au electrodes in aqueous neutral and alkaline solutions1, 2., Most of the problems from an experimental viewpoint stem from the lack of techniques capable of identifying species believed to be involved as intermediates in the reaction, including both adsorbed and solution-phase superoxide and peroxide. Recently, we developed a rotating ring-disk technique incorporating a judiciously functionalized Au ring displaying remarkably high specificity toward solution-phase superoxide in neutral solutions which made it possible to detect such species generated by the reduction of dioxygen at a bare Au disk electrode as a function of the applied potential3. Our contribution illustrates the use of in situ differential reflectance spectroscopy to monitor the adsorption of hydroxyl ion on a Au rotating disk electrode during dioxygen reduction, as well as that of on line mass spectrometry under forced convection to measure the rates of heterogeneous peroxide disproportionation on the same electrode surface using a jet impinging electrolyte arrangement4. In basic solution, reduction of oxygen on Au electrodes is thought to proceed at least in part through a hydrogen peroxide intermediate. Many authors have speculated that subsequent disproportionation of the peroxide intermediate to oxygen and water plays an important mechanistic role, but measurements of disproportionation rates have not been attempted1, 2. This is most likely due to the difficulty of deconvoluting this reaction rate from the rates of oxygen / peroxide reduction. To this end, we measured both the consumption rate of peroxide using a rotating ring disk electrode (RRDE) and the production rate of oxygen by on-line mass spectrometry on a Au electrode at open circuit in 0.1 M NaOH. Shown in Figure 1 are plots of the partial pressure of dioxygen monitored with the mass spectrometer at two fixed disk potentials, i.e. 0.2 and -1.2 V, at which peroxide is oxidized and reduced respectively under diffusion limited conditions, yielding maximum and minimum O2 signal. The cell was then disconnected and the open circuit potential, after which the O2 signal settled at an intermediate value 1/3 of the way between that seen at 0.2 and -1.2 V. This spontaneous production of O2 from H2O2 without net current through the disk can only be explained by invoking H2O2 disproportionation. Furthermore, since only 1 O2 is produced per 2 H2O2 via disproportionation, the O2 signal at OCP represents consumption of 2/3 of the total H2O2 reaching the Au electrode. Fig. 1: A Au disk electrode surrounded by a porous, gas-permeable Teflon ring was mounted in the wall-jet configuration (flow rate 0.45 mL/s), and the O2 pressure passing through the Teflon ring was monitored by mass spec while the following electrochemical experiment was conducted: the Au electrode potential was held at either 0.2 (black) or -1.2 V vs Ag/AgCl (red) for 40 s, and then the circuit was opened. The solution was 0.1 M NaOH with 1 mM H2O2 (solid lines) or without H2O2 (dotted lines). The selected potentials of 0.2 and -1.2 V induce mass transport limited oxidation and reduction of H2O2, respectively, and so will yield the maximum and minimum O2 signal. Other authors have invoked superoxide as yet another intermediate in the reduction of oxygen on Au in alkaline solutions. Also to be discussed in this presentation is a theoretical analysis of the mechanism shown in Scheme I, which accounts for the contributions associated with all three species to the measured currents. References: Zurilla, R. W.; Sen, R. K.; Yeager, E. Electrochem Soc. 1978, 125, 1103-1109. Kim, J.; Gewirth, A. A. Phys. Chem. B. 2006, 110, 2565-2571. Feng, Z.; Georgescu, N. S.; Scherson, D. A. Chem. 2016, 88, 1088-1091. Treufeld, I.; Jebaraj, A. J. J.; Xu, J.; Martins de Godoi, D.; Scherson, D. A. Chem. 2012, 84, 5175-5179. Figure 1

A method for measuring downstream concentration effects through electrochemical impedance spectroscopy at double channel electrodes is demonstrated. An ac current perturbation is applied at an upstream working electrode and the resulting ac potential response at a downstream sensing electrode is measured. This generator-detector scheme is implemented with a single potentiostat. Experimental data for a reversible redox couple are presented and good agreement is found with numerical simulations. A qualitative explanation of the features is given which lays the foundation for a more rigorous theoretical treatment. Relationships with flow rate and frequency are found that can scale the data to lie on a universal curve.(C) 2017 Elsevier Ltd. All rights reserved.

The heterogeneous dismutation of hydrogen peroxide (HDHP) in aqueous electrolytes is an important but rarely addressed process that can play a critical role in the analysis of the data for the oxygen reduction reaction (ORR). On this basis, it is essential to develop reliable experimental methods for determining its rates and their dependence on among other factors the composition of the electrolyte. A potentially powerful method to evaluate the activity of materials for the HDHP reaction introduced decades ago by Anson and coworkers [1] relies of the use of a ring disk electrode whereby the disk is made of the material to be examined and the ring is used to detect oxygen generated by the disk via the HDHP in peroxide containing solutions. As discussed in the aforementioned publication, the ring must be active for the ORR, but inactive for the reduction of hydrogen peroxide, i.e. a strict two-electron reducer. Such conditions were achieved in that work by employing a ring modified with a transition metal macrocycle found to display these rather stringent conditions. The strategy employed in our group, involves the use of a Se-modified Pt ring electrode and takes advantage of the ability of this surface to reduce rather exclusively dioxygen to peroxide demonstrated earlier in our group in efforts aimed at the development of an oxygen concentrator [2]. As shown in Figure 1, a Pt disk electrode covered by 0.34 ML of Se polarized at 0.175 V vs RHE in 0.5 M H2SO4 is capable of yielding mass transport limited currents for the 2-electron reduction of oxygen to hydrogen peroxide [2], as evidenced by the slope of the Levich plot for ORR. However, statistical analysis of these data indicated a small positive intercept. Additional experiments in which peroxide was intentionally added to the solution gave rise to a rather linear increase in the magnitude of the intercept with peroxide concentration, without changing the slope of the Levich plot. This interesting behavior suggests that although Se modified Pt is largely selective towards the 2 electron reduction of oxygen, hydrogen peroxide also yields a small amount of current via some unidentified reaction. Mechanistic investigations are currently underway, and the possibility of using this electrode for oxygen and peroxide detection is also under investigation. References: Shigehara, K.; Anson, F. C. J. Phys. Chem. 1982, 86, 2776 – 2783. Mo, Y.; Scherson, D. A. J. Electrochem. Soc. 2003, 150, E39 – E46. Figure 1

Reduction of aqueous selenate (SeO4 2-) to lower oxidation states of selenium is regarded as a very difficult reaction and thus far, only very few methods have yielded positive results [1-6]. One such previous study asserts that gold electrodes can catalyze reduction of commercial selenate at potentials below 0.6 V vs SHE [1], forming an elemental selenium deposit that can be anodically stripped above 0.9 V vs SHE, (see peaks A and B, black curve in Fig. 1 recorded in 1 mM Na2SeO4 in 0.1 M HClO4). However, commercially supplied selenate usually contains some selenite (SeO3 2-), a far more electroactive selenium oxyanion believed to yield elemental selenium and/or selenides as the product(s) of reduction. In fact, as shown by experiments performed in our laboratories, such voltammetric features vanished upon removal of selenite from the selenate solutions by adsorption on TiO2(see red curve in Fig. 1). This contribution will present strong evidence that the reduction of selenate under the same conditions can proceed on a layer of underpotentially deposited copper on Au, yielding a corresponding layer of CuxSe. To be discussed are various aspects of the formation mechanism of this CuxSe using potentiostatic deposition and linear sweep stripping voltammetry (LSSV) experiments. During LSSV experiments, Cu can be anodically stripped, leaving behind a layer of adsorbed elemental Se (peak C, Fig. 2). At higher potentials, this layer is oxidized to selenite (peak B, Fig. 2) [7-9]. Repetitive formation and oxidation of the CuxSe product on gold electrodes via an AC deposition / stripping waveform may allow for continuous turnover of selenate into selenite. References: Ivandini, T. A.; Einaga, Y. Electrocatalysis. 2013, 4, 367-374. Koshikumo, F.; Murata, W.; Ooya, A.; Imabayashi, S.-I. Electrochem. Soc. Japan. 2013, 81, 350-352. Kozhakov, B. E.; Baeshov, A.; Buketov, E. A.; Zhurinov, M. Zh. Elektrokhimiia. 1985, 21, 550-551. Kikuchi, E.; Tanaka, M.; Liang, R.; Sakamoto, H. Trans. Mat. Res. Soc. Japan. 2004, 29, 2337-2340. Hayashi, H.; Kanie, K.; Shinoda, K.; Muramatsu, A.; Suzuki, S.; Sasaki, H. Chemosphere. 2009, 76, 638-643. Ladriere, J. Bulletin des Societes Chimiques Belges. 1973, 82, 99-122. Lister, T. E.; Stickney, J. L. J. Phys. Chem. 1996, 100, 19568-19576. Huang, B. M.; Lister, T. E.; Stickney, J. L. Surface Science. 1997, 392, 27-43. Alanyalioglu, M.; Demir, U.; Shannon, C. J. Electroanal. Chem. 2004, 561, 21-27. Acknowledgements: This work was supported by a grant from the NSF, # CHE-1412060. Figure 1. Cyclic voltammograms recorded with a Au wire in a solution of 1 mM Na2SeO4 before (black curve) and after purification (red curve, see text) in 0.1 M HClO4at a scan rate ν = 0.2 V/s. Figure 2. Linear scans recorded at ν = 0.1 V/s obtained for a Au RDE after holding the potential at Ehold = 0.375 with the electrode rotating at ω = 1500 rpm in 0.1 M HClO4, 1 mM Na2SeO4 (purified) and 1.2 mM Cu(ClO4)2 solutions for thold = 10 (red), 100 (blue) and 1000 s (green). Figure 1

Voltammetric techniques have been employed to show that underpotential deposited Cu on polycrystalline Au electrodes in aqueous 0.1 M HClO4 catalyzes the reduction of purified selenate, SeO42-, to yield a layer of adsorbed copper selenide, CuxSe. Subsequent oxidation of this layer led to the loss of Cu, leaving behind adsorbed, elemental Se, which could be oxidized to selenite, SeO32-, at higher potentials. Application of this method made it possible to detect SeO42- down to nM levels. Voltammetric features observed on Au(poly) in Cu2+-free 0.1 M HClO4 containing SeO42- reported earlier in the literature could be attributed to the reduction of SeO32- impurities present in the commercial chemical. (C) The Author(s) 2016. Published by ECS. All rights reserved.

Reduction of aqueous selenate (SeO4 2-) to lower oxidation states of selenium is typically a very slow reaction, but important to the detection and removal of soluble selenium from wastewater. Electrocatalytic reduction of selenate has not been exhaustively explored; only a few electrocatalytic methods for reducing selenate have been reported to date [1-6]. Commercially available selenate is often contaminated with selenite (SeO3 2-), a species that can be readily reduced to yield elemental selenium or selenides. No selenate reduction studies to date acknowledge the presence of this impurity. Furthermore, not all these studies unambiguously show that selenate itself is reduced, as opposed to these traces of selenite. Previous literature claims that gold electrodes are capable of reducing selenate at electrode potentials below 0.6 V vs SHE [1], forming a selenium deposit that strips anodically above 0.9 V vs SHE. Cyclic voltammetry experiments in this work reproduced the reported redox features, shown in Fig. 1 as peak A and B, respectively. However, when selenite was removed from selenate solutions with a titania adsorbent, no redox activity is seen on gold electrodes. It can be concluded that the previous report [1] was in fact measuring currents associated with trace selenite reduction. Selenate undergoes specific adsorption on gold and no other reaction. Peak C in Fig. 1 can be attributed to specific adsorption of selenate. Figure 1: Cyclic voltammetry at 0.2 V/s on Au wire electrode with 1mM of selenate in 0.1 M HClO4. Two purities of selenate were used. The first was selenate decahydrate purchased from Alfa Aesar at a purity of 99.9%, used as received. The other sample of selenate was the same, but was exposed to a titania adsorbent in order to remove traces of selenite. A cyclic voltammogram is also shown after selenite addition. We have found that the chemical inertness of selenate can be overcome by addition of copper (II) salts. Copper (II) can be reduced to an adlayer of underpotential deposited copper on gold electrodes between 0.64 and 0.24 V SHE. Once formed, we have found that this adlayer induces slow reduction of selenate, yielding a thin copper selenide film. The mechanism for CuxSe formation was investigated using potentiostatic deposition, followed by linear sweep stripping voltammetry. This technique allows determination of Cu and Se stripping charges separately, observed in Fig. 2 as peak D and E, respectively. At lower electrode potentials (< 0.24 V vs SHE), where copper (II) is reduced to bulk copper metal, selenate both accelerates Cu deposition and is reduced. Selenium co-deposits with bulk copper formed in selenate / copper (II) solutions. In this regime, selenate reduction is much faster than reduction by UPD copper. Figure 2: Linear sweep stripping voltammograms at 0.1 V/s after potential holds at 0.325 V on a Au electrode. Solution was 0.1 M HClO4 with 5 mM Cu(ClO4)2 and 1 mM Na2SeO4(titania purified). Potential hold duration for each curve is indicated in the legend. References: Ivandini, T. A.; Einaga, Y. Electrocatalysis. 2013, 4, 367-374. Koshikumo, F.; Murata, W.; Ooya, A.; Imabayashi, S.-I. Electrochem. Soc. Japan. 2013, 81, 350-352. Kozhakov, B. E.; Baeshov, A.; Buketov, E. A.; Zhurinov, M. Zh. Elektrokhimiia. 1985, 21, 550-551. Kikuchi, E.; Tanaka, M.; Liang, R.; Sakamoto, H. Trans. Mat. Res. Soc. Japan. 2004, 29, 2337-2340. Hayashi, H.; Kanie, K.; Shinoda, K.; Muramatsu, A.; Suzuki, S.; Sasaki, H. Chemosphere. 2009, 76, 638-643. Ladriere, J. Bulletin des Societes Chimiques Belges. 1973, 82, 99-122. Acknowledgements: This work was supported by a grant from the NSF. Figure 1

The activity in lattice-gas systems with geometric constraints is shown to be the ratio of the number of particles to the number of available sites. The key role of sites available for occupation is emphasized. Available sites may be different for different species and are not necessarily just unoccupied sites. Location-specific or non-local constraints are allowed. An analytical expression for the number of available sites is given for the hard-hexagon model. The utility of an expression for available sites is illustrated for the non-trivial case of a mixed Langmuir/hard-hexagon adsorption system, where the influence of the Langmuir adsorbates on the hard-hexagon phase transition is investigated. The dependence on available sites indicates how to extend these results to the kinetic regime and simulations of kinetic voltammograms for the hard-hexagon model are given as an example. (C) 2013 AIP Publishing LLC.

A key step in the electro-oxidation of formic acid to CO2 on Au electrodes is the reversible formation of an adsorbed intermediate, proposed to be formate. A central feature of this reaction is the marked dependence of anodic current onset and magnitude on supporting electrolyte identity and concentration. Past studies show that increasing affinity of the supporting anion for Au drastically dampens reaction rates and formate vibrational intensities at a given potential. A thus far untapped method for studying this adsorbed intermediate is collection of capacitance data. Using dynamic Electrochemical Impedance Spectroscopy (dEIS), capacitance data in solutions from 1 to 0.005 molal sulfuric or perchloric acid with 1 to 0.005 molal formic acid were acquired. Capacitance curves in sulfuric/formic acid and perchloric/formic acid solutions on Au reveal adsorption peaks for formate at higher potentials than supporting anions. This data suggests formate must adsorb onto previously anion-covered surfaces to undergo oxidation. The suppression of current onset and magnitude is discussed in terms of competition of formate with the supporting anions for sites on the Au electrode. This process is drastically different for each supporting electrolyte. Sulfate undergoes a disorder-order phase transition to a 2D solid-like adlayer at high coverages along with co-adsorbed water. This phenomenon is not known for perchlorate. The sulfuric/formic acid system displays strongly inhibited formate adsorption, with formate adsorption peaks shifting to much higher potentials and decreasing in size as sulfuric acid activity increases. The perchloric/formic acid system shows near perfect co-adsorption, with nearly constant peak size as a function of perchloric acid activity. Experimental results are compared with modelling results for competitive adsorption based on hard-hexagon and Frumkin isotherms. The authors thank the Natural Sciences and Engineering Research Council of Canada for financial support of this work, including the award of a Alexander Graham Bell Scholarship to J.R.S.