The reduction of selenate mediated by underpotential deposited Cu on Au(poly), in aqueous 0.1 M HClO4 was carried out wirelessly using a vanadium oxyanion-based redox couple to “pin” the potential of Au(poly) negative to the underpotential deposition of Cu, but positive to its bulk deposition, to yield an adsorbed layer of one or more forms of Cu selenide, CuxSe. Evidence in support of this mechanism was obtained by comparing the features found in linear scan voltammograms of such CuxSe modified Au(poly) surfaces, that were removed from the aforementioned solution, rinsed with neat 0.1 M HClO4 and then immersed in the latter electrolyte, with those obtained by assembling the same interface by conventional electrochemical techniques reported earlier in our laboratories. A similar tactic was implemented to wirelessly reduce selenite in 0.1 M HClO4, which yielded in the scan toward positive potentials a single peak attributed to adsorbed Se oxidation.

The study of electrochemical systems relies on external electronic instrumentation, such as a potentiostat, to control the electrode potential and, thus, the rates of redox processes. We introduce herein a novel paradigm that employs solution-phase redox couples to “pin” the potential of an electrode and thus promote electron transfer reactions without the use of additional instrumentation, an approach we have dubbed Wireless Heterogeneous Electrocatalysis, WHE. In this study, we illustrate, as a model system, the redox-mediated reduction of selenious acid (H₂SeO₃) on Au electrodes via WHE, a process that yields adsorbed elemental Se as the sole product, using the V⁴⁺|V³⁺ redox couple to pin the potential of the Au electrode. Shown in Panel A , Fig. 1 is a cyclic voltammogram of a flame-annealed Au wire in deaerated 0.1 mM Na₂SeO₃ in 0.5 M H₂SO₄, recorded at a scan rate of ν = 100 mV/s. The current for E < ca. 0.68 V vs RHE, is due to the reduction of H₂SeO₃, and the peak centered at ca. 1.12 V vs RHE is ascribed to the oxidation of adsorbed elemental Se to yield back H₂SeO₃. Shown in Panel B, Fig. 1 (blue trace) is a linear scan voltammogram, LSV, in the range 0.685 to 1.2 V vs. RHE obtained in the same solution as in Panel A, for an Au surface devoid of elemental Se, which, not surprisingly, displays no evidence of Se oxidation. Also shown in this figure is an LSV recorded in the same potential range recorded in the same solution following the addition of a mixture of VOSO₄ and VCl₃, until the open-circuit potential (OCP) reached a value of ca. 0.55 V vs. RHE, which is negative to the onset of H₂SeO₃ reduction. Clear evidence for this process is provided by the LSV (see red trace), which displays a peak at about 1.12 V vs. RHE characteristic of Se oxidation. 1 Figure 1

In this study, ohmic microscopy 1 and electrostatic stimulation of electrochemical interfaces 2 were implemented to achieve spatially and temporally resolved images of CO monolayers adsorbed on Pt(poly), CO|Pt(poly), surfaces. Whereas electrostatic stimulation makes it possible to induce localized changes in the interfacial composition, ohmic microscopy allows, in certain cases, local images of such interfaces to be created by using microreference electrodes 1 to monitor the flow of current in the solution in the immediate vicinity of the electrode. In our experiments, we successfully triggered the oxidation of CO|Pt(poly) via electrostatic stimulation at the center of a Pt(poly) disk electrode creating a circular region virtually devoid of adsorbed CO, and halted its propagation along the electrode surface by stepping the potential down to the double-layer (DL) region. This is schematically shown in the insert, Fig. 1, where the center circle represents an area devoid of CO, B, and the grey annulus (B), one with adsorbed CO. As is well known, the double layer capacity of bare Pt surface is markedly larger than that of a CO covered Pt surface; hence, the local current flowing toward the two areas, would be proportional to the local potential measure between the two microreference electrodes, Δφ so l , This is illustrated in Fig. 1, which shows Δφ so l vs time profiles of a few hundred cyclic voltammetric cycles performed over an area of Pt covered with CO, shaded in light blue, i.e. A in the Insert, and another devoid of CO, shaded in yellow (B in the Insert). Efforts are underway to create images of the electrode surface by using an XY computer control table to raster the entire electrode surface. This tactic will allow monitoring the ingress of CO molecules from the annulus to the central bare disk as time progress, or, equivalently, determine their surface diffusion coefficient. References: (1) Feng, Z.; Georgescu, N. S.; Scherson, D. A. New Advances in Ohmic Microscopy. Russ. J. Electrochem. 2017 , 53 (9), 1003–1010. https://doi.org/10.1134/S1023193517090051. (2) Han, Q.; Georgescu, N. S.; Gibbons, J.; Scherson, D. Electrode Stimulation: Redox Reactions Induced by Modulating the Electrostatic Potential in Solution. Electrochim. Acta 2019 , 325 , 134957. https://doi.org/10.1016/j.electacta.2019.134957. Figure 1

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.

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 oxidation of adsorbed carbon monoxide on Pt electrodes, CO(ads)|Pt, in aqueous electrolytes ranks among the most studied reactions in electrocatalysis both from theoretical[1]and experimental viewpoints.[2][3] Much of the interest in this electron transfer process stems from the role CO(ads) plays as an impurity that affects adversely the operation of H 2 |O 2 fuel cells. Consensus appears to have been reached regarding the role of adsorbed hydroxyl, OH . (ads) in controlling the activation and ultimate oxidation of CO(ads). The present contribution illustrates the use of electrochemical stimulation techniques developed in our laboratory[4] to explore new aspects of this process not as yet unveiled by the use of more conventional methods. As described in our earlier publications,[4,5] electrode stimulation refers to changes in the rates of heterogeneous electron transfer processes involving adsorbed species induced by modifications in the electrostatic potential in the solution immediately adjacent to the electrode generated by the passage of current between an electrode placed very close to the working electrode, WE, polarized at constant potential, and a second electrode far away from it. The electrochemical cell used for these experiments consisted of a glass tube covered at each end with custom-made Teflon caps and, thus, similar in design to that described by Qi et al.[4] Experiments were performed in Ar-purged 0.1 M H 2 SO 4 involving CO adsorbed at saturation coverages on polycrystalline Pt, polarized at E WE = 0.8 V, a potential very close to the onset of CO(ads) oxidation with a Au disk electrode placed directly above it working as a stimulating electrode, SE, i.e. face to face configuration. Shown in Fig. 1 are plots of the current flowing through the SE (red), i SE and i WE (black) (see Left Panel) and their expanded views during stimulation (Middle Panel) and CO full oxidation (Right Panel), respectively. As shown in the middle panel in this figure. the passage of a negative i SE , elicits a positive i WE ascribed to CO oxidation, which persists even when i SE turns positive, until the end of the stimulation process (see Right Panel), which takes several seconds. In brief, a single stimulation lasting 140 ms can trigger the full oxidation of the entire adsorbed CO layer in CO|Pt over a subsequent period of ca. 4 s. The most likely explanation for this unique effect may be found in the formation of adsorbed OH . on sites previously covered with CO, as the CV of bare Pt would strongly suggest, which will promote the subsequent oxidation of CO(ads), a process that will propagate through the entire Pt surface. Further support for this mechanism was provided by an experiment in which the WE potential was stepped down to 0.4 V immediately after the stimulation was applied, which stopped virtually instantaneously the further oxidation of CO(ads). Experiments are now in progress to image the surface of the CO|Pt electrode during and after the stimulation to monitor in real time the rates of propagation and to provide a more details theoretical model to account for this phenomenon. References 1 Gao, W.; Mueller, J.E.; Jiang, Q.; Jacob, T. Angew. Chem. Int. Ed. 2012 , 51 , 9448 –9452 2 Scott, S.B.; Kibsgaarda, J.; Vesborga, P.C.K.; Chorkendorff, I. Electrochim. Acta 2021 , 374 , 137842 3 Scott, S.B.; Kibsgaarda, J.; Vesborga, P.C.K.; Chorkendorff, I . Electrochim. Acta 2021 , 374 , 137844 4 Han, Q.; Georgescu, N. S.; Gibbons, J.; Scherson, D. Electrochim. Acta 2019 , 325 , 134957 5 Heer, A.S.; Mantelli, H; Han, Q.; Georgescu, N. S.; Scherson, D. in preparation. Acknowledgement Support for this work provided by NSF, CHE-1808592. Fig. 1. Plots of i WE and i SE recorded for E WE = 0.8 V prior, during and following WE stimulation (see text for details) (Left Panel). The middle and right panels show expanded views of the data in the left panel during stimulation and CO full oxidation, respectively. Figure 1

In a recent communication (1), we described a novel method of controlling the electrostatic potential in the solution neighboring a working electrode, we have dubbed the monopolar electrode , ME, polarized at a constant potential, EME, using a potentiostat. This tactic involved passing current between a second or stimulating electrode , SE, placed in front, and at a relatively small distance from the ME, and a distant auxiliary electrode, AE. Depending on the polarity of the current flowing through the SE, the electrostatic potential just outside the ME could be modified so as to increase or decrease the surface overpotential, and, thus modulate the rates of redox reactions at the ME|electrolyte interface. This presentation will review critical aspects of ohmic microscopy, and illustrate with two examples the electrode stimulation effect, using a ring disk electrode, RRDE, in which current flowing through the disk acting as the SE, is employed to induce reactions on the ring, acting as the ME, namely: i. the oxidation of adsorbed carbon monoxide on Pt using a Pt|glassy carbon RRDE and ii. the reduction of selenite to elemental Se on Au in aqueous 0.1 M HClO4 using a Au|Au RRDE. In both these cases, the stimulation current was generated by scanning the potential of SE within its double layer region, so as to minimize any changes in the actual composition of the solution adjacent to the ME, while monitoring the current flowing through the ME, IME . Under these conditions, the electrostatic potential in solution follows the primary current density. The efficiency of the stimulation process was determined by coulometric analysis of the CO oxidation current and in the case of selenite via a coulometric analysis of the current due to the oxidation of elemental Se produced by the reduction of selenite. The stimulation efficiency, in this latter case, could be accurately determined from a coulometric analysis of the peak for Se oxidation observed by subsequently scanning the ME linearly toward positive potentials to yield selenite. Excellent quantitative agreement was obtained between IME and EME as a function of Idisk and theoretical simulations employing COMSOL using parameters extracted from independent measurements performed under otherwise identical experimental conditions. This overall approach will open new prospects for gaining insight into the rates of surface diffusion and other interfacial dynamics phenomena. Acknowledgements This work was supported by a grant from NSF, CHE-1412060 References Han, Q.; Georgescu, N. S.; Gibbons, J.; Scherson, D. Acta 2019, 325, 134957.

Changes in the electrostatic potential within an aqueous acidic solution induced by the passage of current between the Au disk of a stationary Au|Au ring disk electrode and a distant counter electrode, Idisk made it possible to stimulate the reduction of selenite to elemental selenium at the concentric Au ring electrode polarized at Eringo just positive to its otherwise expected onset. This effect was ascribed to variations in the surface overpotential, and, thus, in the rates of selenite reduction along the ring, we define, hereafter, as a monopolar electrode. The stimulation efficiency could be accurately determined from a coulometric analysis of the peak for Se oxidation observed by subsequently scanning the ring linearly toward positive potentials to yield selenite. Shown in the figure below is a series of linear scan \voltammograms, LSV, recorded at a scan rate of 50 mV/s for the ring polarized at Eringo in the range 0.39 - 0.45 V (see legend) following Nstim = 40 between Edisk = 0.75 and 1.00 V, where the black curve corresponds to the LSV recorded without any prior stimulation under otherwise identical conditions. As clearly indicated the stripping charge associated with the oxidation of elemental Se increases as the potential at which the ring is polarized is decreased. Excellent quantitative agreement was obtained between the current flowing through the ring, Iring , and Eringo as a fucntion of Idisk and theoretical simulations employing COMSOL using parameters extracted from independent measurements performed under otherwise identical experimental conditions. This novel tactic is expected to open new prospects for gaining insight into surface diffusion and other interfacial dynamics phenomena. Acknowledgements This work was supported by the US NSF Award 1808592 Literature Cited 1) Han, Q.; Georgescu, N. S.; Gibbons, J.; Scherson, D. Electrochim. Acta 2019, 325, 134957 Figure 1

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.

According to Newman[1], the rates of heterogeneous redox reactions can be written in terms of the surface overpotential, η s , i = io{exp(aanFηs/RT)- exp(-acnFηs/RT)}, where η s is defined as “the potential of the working electrode, relative to a reference electrode of the same kind placed in the solution adjacent to the surface of the working electrode.” On this basis, η s can significantly alter the current flowing across the electrode-electrolyte interface. Due to its intrinsic resistive character, the flow of electricity through the electrolyte will elicit changes in the electrostatic potential within that phase, which can be conveniently measured using reference electrodes, as has been implemented in this[2-4] and other laboratories[5]. To be described in this presentation is a novel means of inducing local changes in η s by passing current between a stimulating electrode, SE, and a distant counter electrode in the same solution. The overall tactic was demonstrated using a redox active self-assembled monolayer of N-ethyl-N’-octadecyl-4,4’-bipyridinium dibromide (EOB) adsorbed on a Au(poly) disk electrode as the WE and a Au(poly) disk of larger diameter placed parallel to the WE with its normal axis aligned along the corresponding axis of the WE, as the SE. The potential of the WE, EWE, was set at EWE= -0.35 V, i.e. the layer was polarized in its fully oxidized state (See the cyclic voltammogram in the Insert, Fig 1), while the SE placed at a distance d = 0.5 mm from the WE was scanned from 0.65 V up to 0.8 V, and back at ν = 1 V/s, yielding a iSE vs time curve shown in red dots in Fig. 1. The response of the WE is shown in blue in Fig. 1. As clearly indicated, iSE > 0 elicits a corresponding iWE < 0 due to the change in the electrostatic potential in the vicinity of the WE. Once iSE is reversed, the previously reduced fraction of EOB is oxidized returning the redox layer to its original fully oxidized state. This explanation is also consistent with the increase in iWE induced by a decrease in d (See black, green and magenta curves in Fig. 1). Implementation of this tactic will enable application of a local potential in a region next to a working electrode and thus promote electrochemical processes in that region without the need of a mask. Acknowledgements This work was supported by NSF, CHE-1808592. References [1] J. Newman, K.E. Thomas-Alyea. Electrochemical Systems, 3rd Edition, Wiley-Interscience, 2004. [2] Y. Chen, A. Belianinov, D.A. Scherson. J. Phys. Chem. C, 112 (2008), 8754-8758. [3] C. A. Cartier, D. Kumsa, Z. Feng, H. Zhu, D. A. Scherson, Anal. Chem., 84 (2012), 7080-7084. [4] Z. Feng, N. S. Georgescu, D. A. Scherson, Russian Journal of Electrochemistry, 53 (2017),1003-1010. [5] I. Plettenberg, G. Wittstock, Energy Technology, 4 (2016), 1-8. Figure 1

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.

This paper describes a GaNFET-based high-speed charge injection circuit to study fast redox processes at electrode-electrolyte interfaces. The circuit allows the rates of electrode processes, which are much faster than those accessible with a conventional potentiostat, to be measured. It is able to inject charge across the interface within a few nanoseconds, and also to hold the potential generated across the cell following injection for up to 1 s without appreciable (less than 1%) decay. In addition, the circuit can still monitor the current flowing through the cell, as in a conventional potentiostat. Preliminary test results with both a dummy load and a custom two-electrode electrochemical cell confirm the functionality of the proposed circuit.

The pK(a) of bicarbonate ion adsorbed on the surface of Pt(111), HCO3-(ads), in CO2-saturated KClO4/HClO4 aqueous solutions has been determined by judicious application of a theoretical model originally proposed by Smith and White (Langmuir 1993, 9 (1), 1-3) to explain voltammetric features found for non-redox active alkyl chain monolayers bearing carboxylic moieties irreversibly bound to gold surfaces. The analysis herein presented relied on HCO3-(ads) and CO32- (ads) coverages derived from in situ Fourier transform infrared spectroscopy measurements as a function of potential and pH reported by Martinez-Hincapie et al. (J. Phys. Chem. C 2016, 120 (29), 16191-16199), as input parameters, yielding a pK(a) for HCO3-(ads) of ca. 2.6 +/- 0.2. This value is significantly smaller than that of HCO3-(aq) in bulk solutions, a phenomenon associated with the bonding of the species to the electrode surface, a factor that markedly modifies its acid base characteristics. (C) 2019 Published by Elsevier Ltd.

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.

Au surfaces modified by Pd electrodeposits have been found to enhance the rates of nitrate electroreduction in acidic aqueous solutions containing Cd2+ over those of the bare Au substrate in the same media. Rather surprisingly, similar electrocatalytic effects were also observed for ensembles of Au and Pd nanoparticles supported on otherwise inert glassy carbon (GC) surfaces under the same conditions, which suggest that this phenomenon does not originate solely from specific interactions between the two metals. In fact, the overall electrocatalytic activity of GC surfaces decorated with either a fixed number of Au in one case, or a fixed number of Pd nanoparticles in the other, displayed much lower activity than GC surfaces covered with a mixture of half of each of them. This unique behavior may be explained by the ability of Pd to reduce NO2-, which is known to be the main reaction product of NO3- reduction on underpotential deposited Cd on Au. (C) 2019 The Electrochemical Society.

Introduction The rates of electrode reactions are frequently expressed in terms of the surface overpotential, ηs, defined as “the potential of the working electrode relative to a reference electrode of the same kind placed in the solution adjacent to the surface of the working electrode.”1 Changes in ηs are most commonly effected by means of a potentiostat, a device that allows accurate control of the potential of the working electrode with respect to a reference electrode in solution.2 This communication describes a method that makes it possible to promote heterogeneous electron transfer reactions at a working electrode, WE, polarized at a fixed potential, EWE, by changing the electrostatic potential in the electrolyte “just outside its double layer”, ϕdl, and thus ηs. Experimental The body of the electrochemical cell used in our experiments was a polypropylene centrifuge tube with its bottom end cut off to expose a circular hole ca. 5 mm in diameter. A commercial Au disk microelectrode (12.5 µm diameter), Au µ-el, inserted into the hole of the cell facing upwards, was used as WE, and a carbon rod and a Ag/AgCl as counter and reference electrodes, respectively. The stimulating electrode, SE, was a Pt disk (1.6 mm diameter) placed directly above the WE. The potentials of the WE and SE were controlled independently using a Metrohm Autolab potentiostat. The cell was filled with 0.1 M HClO4 and then purged with Ar prior to the measurements. EWE was fixed at a prescribed value, while the potential of the SE, ESE, was scanned at a rate ν = 50 V/s, starting at 1.2 V, down to -0.25 V and back to 1.2 V for all experiments. Results The cyclic voltammogram of the Au µ-el, WE, recorded at a scan rate ν = 50 V/s in deaerated 0.1 M HClO4 (see insert, Fig. 1), displayed characteristic features ascribed to the formation of Au oxide and its subsequent reduction. ESE was set at 1.2 V for ca. 1 s, and then scanned at ν = 50 V/s down to -0.25 V, and back again to 1.2 V, yielding the “unfolded” voltammogram in the upper panel, Fig. 1. The current response of the Au working µ-el, iWE, during this scan, when fixing EWE= 1.15 V, is shown in green in the lower panel, Fig. 1. As clearly indicated, for ESE < 0.5 V during the scan in the negative direction, both |iSE| and |iWE| markedly increased, albeit in opposite directions, reaching a maximum at ca. 22 ms. Whereas |iSE| decreased by ca. 30% toward the end of the scan in negative direction, |iWE| dropped to a very small value similar to that found prior to the stimulation. These results are consistent with the formation of Au oxide stimulated by a shift in ηs toward positive values induced by iSE. In fact, the sudden drop in iWE signals the end of Au oxide formation at the maximum applied ηs. During the entire scan toward positive potentials, iWE was very small, indicating that the value of ηs achieved was insufficient to effect the reduction of the Au oxide formed. This is in agreement with the cyclic voltammogram of Au, for which the onset of Au oxide reduction is ca. 0.9 V (see insert, Fig. 1). In striking contrast, very small iWE was observed in similar experiments in which EWE = 0.4 V (see black, Fig. 1) regardless of the direction of the scan. We ascribe this behavior to changes in the structure of the interface, which may involve a redistribution of charge in the diffuse double layer, as well as a possible reorientation of water dipoles at the WE surface. Strong evidence that the response of the WE is indeed derived from changes in ϕdl was provided by experiments in which the distance between WE and SE, δ, was varied. As shown in Fig. 2, increasing δ led to a decrease in ηs, thus, in a corresponding decrease in the magnitude of the peak current associated with Au oxide formation. This behavior, as well as the lack of proportionality between the peak current and δ, are both in agreement with theoretical predictions. Acknowledgments This work was supported by a grant from NSF, CHE-1412060. References 1. Newman, J. S.; Thomas-Alyea, K. E., Electrochemical systems. Third ed.; J. Wiley: Hoboken, N.J, 2004. 2. Bard, A. J.; Faulkner, L. R., Electrochemical methods: fundamentals and applications. 2nd ed.; Wiley: New York, 2001. Figure 1

Herein, a method is presented that allows quantitative determination of faradaic efficiencies for dinitrogen (N-2) generation during the electrochemical oxidation of hydroxylamine (NH2OH), fN2NH2OH, on a polycrystalline gold Au(poly) disk electrode in aqueous electrolytes over a wide pH range. This tactic involves the use of an impinging jet electrolyte configuration incorporating a gas porous ring connected in turn to a mass spectrometer. The actual amount of N-2 generated at the Au(poly) disk was assayed using the oxidation of hydrazine (N2H4) in aqueous phosphate buffer (pH 7). This redox process yields N-2 as the only product, allowing a direct correlation to be established between the changes in the partial pressures of N-2 and the current flowing through the disk electrode. An analysis of the data collected revealed a strong dependence of fN2NH2OHboth on pH and the applied potential. Although values of fN2NH2OH as high as 20 to 30 % were found in acid and neutral media over a narrow potential region, those in alkaline solution were far smaller in the entire potential range examined.