Surface Interrogation Mode Research Articles

Inducing and real-time monitoring of lead (de)sulfation processes using scanning electrochemical microscopy for applications in the refurbishment of lead-acid batteries

Lead acid batteries (LABs) remain as a mature, cost-effective energy storage technology for a wide variety of applications. Hard sulfation is one of the primary reasons for the short lifetime of LABs. This phenomenon is primarily characterized by the formation of large, non-redox active crystals of PbSO4 which reduce the negative active material utilization by slowing down the electron transfer (ET) kinetics on the surface. Here, we report a microelectrochemical technique based on the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) to locally probe the phenomenon of sulfation and the loss of ET therein. Thereafter, we validate our technique through an existing in-situ refurbishing strategy for sulfated anodes using EDTA-based chelation treatment and the recovery of ET. Energy dispersive X-ray spectroscopy (EDX) was used to verify the formation of PbSO4 and its subsequent removal post treatment. We propose that our reported microelectrochemical technique promises to be useful in evaluating possible chelator candidates for the refurbishment of sulfated LABs. Finally, we also introduce ammonium acetate as a promising chelator candidate for the removal of PbSO4 and use our proposed technique to evaluate the superior recovery of ET during ammonium acetate refurbishment. We hope this technique will be useful in evaluating the surface treatment of LABs and other energy storage electrodes which undergo redox-induced passivation processes.

Estimating Time since Deposition of Bloodstains by Scanning Electrochemical Microscopy

AbstractA novel strategy has been developed for determining the time since deposition (TSD) of bloodstains on poly (vinylidene difluoride) (PVDF) membrane using scanning electrochemical microscopy (SECM) in the surface interrogation mode. In this approach, the estimation of elapsed time was based on the changes of tip current depending on the reaction between the tip‐generated reductant [Ru(NH3)6]2+ and hemichrome (HC) formed during bloodstain aging. Lateral scanning results of bloodstains with two timelines (4.5‐61 h and 2–144 h) were analyzed to establish a current‐TSD aging fitting curve for providing quantitative predictions. The results indicate a high correlation between tip current and TSD of a bloodstain. Finally, this method was effectively expanded to differentiate blood fingermarks (BFMs) between 3.5+1.5 h and 75.5+1.5 h, which was very helpful to establish the relevant association of multiple bloodstains. It shows the great promise for practical use in the dating of BFMs or bloodstains.

Quantification of Surface Reactive Oxygen Species at Co-Modified BiVO4 with Surface Interrogation Mode of Scanning Electrochemical Microscopy

Ultrasensitive electrochemical and optical imaging approaches are critical for realizing local chemical information particularly for systems such as sustainable energy harvesting and conversion. Scanning electrochemical microscopy (SECM)-based electrochemical and spectroelectrochemical techniques will be described in this presentation to illustrate ultrasensitive quantitative analysis of catalytic surfaces and localized redox activities with improved spatial and spectral and temporal resolutions. Examples of SECM based on ultramicroelectrodes, nanoelectrodes, and optical fiber electrodes and optical imaging techniques will be discussed. These techniques help understanding fundamental aspects of an electrochemical process, such as local heterogeneities in catalytic reaction and stability issues of functional low-dimension metallic and semiconductor electrode materials with synergistic functions for sunlight harvesting, conversion, and storage into chemical fuels. These studies will provide insights into developing unique catalytic and photonic properties of nanostructured electrode materials for the efficient and selectivity of the solar-to-fuel conversion.

Quantification of Surface Reactive Oxygen Species at Co-Modified BiVO4 with Surface Interrogation Mode of Scanning Electrochemical Microscopy

Our recent study suggests that cobalt modification can greatly improve the photoelectrochemical (PEC) water oxidation activities of n-BiVO4 (ACS Appl. Energy Mater., 1(5), 2283 (2018)). This enhancement is attributed to (1) the formation of a top layer of Co3O4 at BiVO4 surface as a cocatalyst and (2) the enrichment in electronic conductivity of BiVO4 upon Co doping. Here, surface interrogation mode of scanning electrochemical microscopy (SI-SECM) technique equipped with an ultramicroelectrode (UME) tip was employed to in-situ interrogate and quantify the surface adsorbed reactive oxygen species at Co-BiVO4 generated during PEC water oxidation. Experimental conditions of PEC water oxidation including applied potential and illumination time were found to play an important role in determining the surface coverage of the oxygen species. Their spontaneous decay under the open circuit condition was also characterized by increasing the time delay before interrogating the Co-BiVO4 surface.

Numerical Deconvolution of Surface Interrogation Scanning Electrochemical Microscopy Experiments on Platinum During Hydrogen Evolution

AbstractIt is experimentally challenging to deconvolute the potential‐dependent adsorption of the different intermediates that occur during the hydrogen evolution reaction (HER) in alkaline media. This difficulty has limited our understanding regarding why the HER kinetics are more sluggish in alkaline media compared to acidic media. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI‐SECM) to investigate the surface adsorbed species that form during the HER in alkaline media on polycrystalline platinum, Pt(poly). To deconvolute the different adsorbed intermediates, we developed a detailed COMSOL‐based kinetic model to rapidly simulate the SI‐SECM titration reactions under our experimental conditions. Utilization of this rapid‐kinetic model overcomes the limitation of SI‐SECM not having the ability to simultaneously resolve multiple surface adsorbed intermediates. We demonstrate that these numerical simulations can separate the potential dependant formation of the underpotential deposition of hydrogen (H(UPD)) from the overpotential deposition (H(OPD)) of hydrogen. In addition, our simulations show that a spectator species may also exist on the surface during HER potentials. Our simulations also show that at full H2‐producing potentials, the surface of Pt(poly) is fully saturated with intermediates. Comparison between the potential‐dependent adsorption of H(OPD) and Tafel analysis reveal that the Heyrovsky step is likely rate‐determining in alkaline media. However, in alkaline media the Heyrovsky step transitions from first‐order in H(OPD) at low H(OPD) coverage to zero‐order at high H(OPD) coverage, due to surface saturation of adsorbed intermediates. Tafel analysis in acidic media shows that the Heyrovsky step is likely rate‐determining, but remains first‐order in Had over a larger potential range. These fundamental insights reveal that a sluggish Heyrovsky step is a major contributor to the attenuated kinetics of the HER in alkaline media.

Coulometric Titration of Active Sites at Mesostructured Cobalt Oxide Spinel by Surface Interrogation Mode of Scanning Electrochemical Microscopy

Cobalt-based transition-metal oxides are promising candidates for the oxygen evolution reaction (OER). However, a complex interplay between the catalyst crystal structures and material morphologies...

Catalytic Water Oxidation Reaction at Nanostructured Titania Modified with Carbonized Metal Organic Framework

Solar energy can be used for direct water splitting to produce hydrogen through a photoelectrochemical (PEC) reaction at semiconducting electrodes. Thus, the overall efficiency of the PEC system is dependent on electrocatalytic or photoelectrocatalytic electrode materials for efficient light absorption, charge separation and transport, in proton reduction and water oxidation half reactions. Herein, a new hybrid catalytic electrode comprised of NH2-MIL-125, a titanium-based metal organic framework (MOF), derived nanostructured Titania electrode material. XRD, SEM and EDX characterizations suggest that nanostructured Titania-MOF electrode is comprised of an underlying layer of TiO2nanorods and the upper layer of distributed clusters of MOF derived disk-shaped nanoparticles (NPs) with an island morphology. The NH2-MIL-125 NPs are formed through a colloid seed-assisted solvothermal reaction and afterwards carbonized while clustering during a pyrolysis process. The resulting nanostructure proved to be composed of C, O, N, and Ti elements. The Titania was employed as the anode substrate of the MOF NPs because it exhibits significantly improved activity for water oxidation and this reaction is greatly enhanced upon coating with carbonized MOF structure. The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is applied in this study to investigate reactive surface states (RSSs) on the nanostructured electrodes, owing to its selectivity to surface species, capability to in situ quantify for both surface coverage and reactivity of adsorbed RSSs, and usefulness to obtain spatially and temporally-resolved information.

Interrogating the Surface Intermediates and Water Oxidation Products of Boron‐Doped Diamond Electrodes with Scanning Electrochemical Microscopy

AbstractBoron‐doped diamond (BDD) electrodes are widely used in electrochemical sensing and water purification owing to their chemical and structural stability under harsh reaction conditions. Water oxidation at BDD electrodes is known to produce reactive oxygen species, but the discharged and surface chemistries involved in these processes have not been studied in depth. Here, we present scanning electrochemical microscopy (SECM) studies of electrogenerated intermediates and products formed on sp2 carbon‐containing BDD electrodes that display stark differences in their reactivity as a function of electrolyte type and pH during water oxidation. The most reactive and abundant species discharged from the electrode were observed at pH 11 in sulfate electrolyte. With the surface interrogation mode of SECM, two kinetically distinct surface intermediates were clearly distinguished, with one forming two orders of magnitude faster than the other but displaying a slower desorption rate. The surface coverage of these species was estimated in the range of 4–7×10−5 mol/cm2 for the first, and 3–4.4×10−5 mol/cm2 for the second one. SECM imaging suggested that regions of increased product evolution have decreased electron transfer kinetics and limited surface sites for intermediate binding. This work establishes methods for studying highly reactive intermediates found in BDD and paves the way for the inspection of other interfaces where solvolysis impacts their reactivity and evolution.

Insights into the Active Electrocatalytic Areas of Layered Double Hydroxide and Amorphous Nickel–Iron Oxide Oxygen Evolution Electrocatalysts

Layered double hydroxide (LDH) and amorphous nickel–iron (oxy)hydroxides (Ni1–xFexOOH) are emerging catalysts for the electrochemical oxygen evolution reaction (OER). It is still unresolved if the layered two-dimensional (2D) structure allows for active catalytic sites to exist below the traditional electrode/electrolyte interface. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) to directly measure active site densities in situ. We determined that Ni0.8Fe0.2OOH LDH showed a 10-fold increase in the active site density compared to rock salt Ni0.8:Fe0.2 oxide, giving direct evidence that water and hydroxide in the interlayer are able to create stable NiIV/FeIV active species at layers below the electrode/electrolyte interface. This result suggests that electrolyte permeability of the 2D LDH structure is a major contributor for its increased catalytic activity. Amorphous Ni0.8:Fe0.2 oxide also exhibits an anomalously high active site density and higher activ...

In Situ Quantification of Surface Intermediates and Correlation to Discharge Products on Hematite Photoanodes Using a Combined Scanning Electrochemical Microscopy Approach.

Hematite is a promising photoanode for solar driven water splitting. Elucidating its surface chemical pathways is key to improving its performance. Here, we use redox titrations in the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM) to quantitatively probe in situ the reactivity and time evolution of surface species formed on hematite during photo assisted water oxidation. Using SI-SECM, two distinct populations of oxidizing surface species were resolved with measured ksi of 316 m3/(mol·s) and 2 m3/(mol·s) for the more and less reactive species, respectively. While the surface coverage of both species was found to increase as a function of applied bias, the rate constants did not change appreciably, suggesting that the mechanism of water oxidation is independent of bias potential. In the absence of applied potential, both populations exhibit decay that is well described by second order kinetics, with kd values of 1.2 × 105 ± 0.2 × 105 and 6.3 × 103 ± 0.9 × 103 m2/(mol·s) for the fast and slow reacting adsorbates, respectively. Using transient substrate generation/tip collection mode, we detected the evolution of as much as 1.0 μmol/m2 of H2O2 during this decay process, which correlates with the coverage observed by one of the titrated species. By deconvoluting the reactivity of multiple adsorbed reactants, these experiments demonstrate how SI-SECM enables direct observation of multiple adsorbates and reaction pathways on operating photoelectrodes.

Probing the hydrogen adsorption affinity of Pt and Ir by surface interrogation scanning electrochemical microscopy (SI-SECM)

Scanning electrochemical microscopy in the surface interrogation mode (SI-SECM) has been used to probe the strength of metal-adsorbed H bonds at Pt and Ir. The generally accepted view is that this technique can only give meaningful results for similar probe and substrate electrode dimensions. However, it is shown that SI-SECM can also provide valuable information for H adsorption even when the substrate is much larger than the microelectrode probe (of 500 and 25μm diameter respectively), for properly chosen substrate and mediator combinations that minimize the tip currents under substrate open circuit potential conditions. Linear sweep voltammetry at a microelectrode positioned at a very small probe–substrate distance (2.5μm) showed remarkable positive feedback in the presence of an oxidizable mediator (TMPD) when a full or partial H monolayer was electrochemically pre-formed on the Pt or Ir substrate. The magnitude and shapes of chronoamperometric responses at the tip were interpreted in terms of variation of H coverage with time and changes in the open circuit potential of the substrate. The higher affinity of Ir than Pt for adsorbed H has been used to validate the approach.

Quantification of Active Sites at Spinel Type Metal Oxide Catalysts by Surface Interrogation Scanning Electrochemical Microscopy

Nanostructured spinel type metal oxides are promising catalysts for the oxygen reduction reaction (ORR) in fuel cells as well as for the oxygen evolution reaction (OER) in electrolysers under alkaline conditions [1]. For a more comprehensive understanding of the oxygen electrochemistry the enlightenment of the interaction between catalyst structure and occurring surface reaction will be a major step [2]. Therefore, new experimental approaches are needed to identify material patterns that determine the electrocatalytic activity. Especially, following the oxidation states of metal atoms as function of potential and time will provide new insights into the ORR and OER mechanisms. A versatile and powerful method in the assessment of electrocatalysts is scanning electrochemical microscopy (SECM). The spatial activity and selectivity of a catalytic material is immediately visualized by analyzing the current response of a movable probe electrode [3]. Recently, Rodríguez-López et al. [4] described a surface interrogation mode of SECM that allows the detection of adsorbates depending on time and substrate potential. This method is based on a transient feedback of a redox mediator with which a coulometric titration is performed. By integrating the current response at the tip electrode the quantification of active catalytic sites is accessible. In this study, we apply the surface interrogation mode to detect the amount of active sites at spinel type metal oxide catalyst under alkaline fuel cell conditions. Therefore, the catalyst powder is supplied in cavity-microelectrodes which were prepared by electrochemical etching of a metal wire [5]. Through the quantification of active sites and the examination of their reactivity new insights to the ORR of alkaline fuel cells are achieved. The results will be compared to a commercial carbon-supported platinum fuel cell catalyst. 1. X. Deng and H. Tüysüz, ACS Catal., 4, 3701 (2014). 2. I. Katsounaros, S. Cherevko, A. R. Zeradjanin and K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 53, 102 (2014). 3. G. Wittstock, M. Burchardt, S. E. Pust, Y. Shen and C. Zhao, Angew. Chem. Int. Ed., 46, 1584 (2007). 4. J. Rodríguez-López, M. A. Alpuche-Avilés and A. J. Bard, J. Am. Chem. Soc., 130, 16985 (2008). 5. A. Minguzzi, C. Locatelli, O. Lugaresi, A. Vertova and S. Rondinini, Electrochim. Acta, 114, 637 (2013).

Redox titration of gold and platinum surface oxides at porous microelectrodes.

The extension of the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is demonstrated for porous electrode materials. These materials are often high surface area powders which are very important electrocatalysts for instance in fuel cells or water electrolyzers. The powdered electrocatalyst material is filled into a cavity-microelectrode which is then operated as the sample electrode in SECM. After a surface oxide generation step, the oxides on the porous sample are reduced by [Ru(NH3)6]2+ formed at the microelectrode probe of the SECM while the sample is at open circuit potential. Such porous electrodes pose the difficulty to cope with unavoidable variations in the filling of the cavity and to access the entire surface by the mediator. The electrochemically active surface area is used to compensate the variation in filling. It can also be used for calculating coverages of surface oxides for a better comparison between different electrodes. We found a complete and fast accessibility for all investigated porous electrodes which is based on electron transfer. Therefore, we propose a "vertical feedback" mechanism analogous to SECM feedback experiments on extended flat samples at open circuit potential. Moreover, the current transients indicate that distinctive oxide species with different kinetics are present. Taken together, these measures ensure consistent determination of oxide coverages for nanoporous gold and carbon-supported platinum nanoparticles.

In Situ Reactive Imaging of Photoassisted Water Splitting Reaction Intermediates on n-Doped Strontium Titanate Using Surface Interrogation Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) gives spatially resolved information about electrochemical reactions occurring on surfaces, and easily probes reaction mechanisms. The in situ surface study of photoelectrochemical processes, such as water splitting at semiconductor electrodes, is challenging due to its fast kinetics and the multiple reactive intermediates formed. By combining the surface interrogation mode (SI-) of SECM [1] with transport simulations, the reactivity of multiple adsorbed intermediates resulting from water splitting on gently n-doped strontium titanate (STO) was investigated. Using this technique, we addressed the in situ detection of long lived holes and photogenerated hydroxyl radical on STO. Combined with the measurements on the local production of oxygen using the collection mode of SECM, our methodology provides an integrated view of reactivity at photocatalytic surfaces. In our SI-SECM measurements, a micro- or nano-electrode used as SECM tip was positioned close to the STO surface and activated to generate a redox titrant that provided information about the surface coverage and reaction kinetics of adsorbed intermediates. By addressing different parts on the electrode surface, it is possible to obtain information about different surface condition son the STO electrode. This combined methodology brings spatial resolution to SI-SECM, and measured different adsorbate coverage on pristine and defective STO (100) surfaces. For the first time, SI-SECM was adapted to the reactive imaging of adsorbates to show how these affect photocatalytic performance on a heterogeneous surface. [1] Rodríguez-López, J. The Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): an approach to the study of adsorption and (electro)catalysis at electrodes. In Electroanalytical Chemistry, a series of advances. Vol. 24. Bard, A.J. and Zoski, C.G., Eds. 2012, CRC Press, pp. 287-352.

(Invted) The Use of Redox Nano-Titrations for Elucidating Reactive Heterogeneity on Electrodes for Energy Conversion and Charge Storage

We present on the use of the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM)[1] for the quantification of reactive intermediates at heterogeneous reactive electrode structures. In this mode, an SECM probe is approached to the electrode under study and a pulse of reactive redox mediator is directed to the substrate while it rests at open circuit. Since the tip operates in the feedback mode, if this pulse induces a chemical reaction on the surface species, an increased flux of the redox mediator sensed by the tip can be used to obtain the local titrated charge and the kinetics of reaction between the mediator and the surface species. Here, we will present how SI-SECM can be used to quantify by titration the reactive species present on an extended surface of a relatively inert electrode. This technique was used for interfaces of interest for energy conversion and storage: 1) for the detection of photogenerated hydroxyl radical on a semiconducting surface, and 2) for the detection of redox active polymer particles [2] on single layer graphene (SLG). Both semiconducting and SLG interfaces permit only the reactivity of reactive species on their surface to be detected. Studies using finite element simulations indicate that the efficiency and extent of titration of a surface are strongly dependent on the SECM tip electrode size and on the tip-substrate distance. As the SECM tip size is made smaller, the apparent titrated charge increases. Likewise, the distance between the SECM tip and substrate determines the titrated charge – very small tip-substrate gaps lead to a constrained area, while large tip-substrate gaps increase the area titrated at the expense of a decreased signal-to-background. An optimum distance can be obtained. We will present on a system that validates these findings, as well as examples of interest to our laboratory, which include the titration of photogenerated intermediates on semiconductors for water splitting, and the titration of redox active polymer particles for charge storage in redox flow batteries. In the first case, we were able to decouple the reactivity of hydroxyl radicals and obtained their coverage and reaction kinetics. In the second case, the relaxed tip-substrate geometry suggested by our simulated findings allowed to evaluate the charge stored in polydisperse polymer nanoparticles for flow batteries. SI-SECM redox nano-titrations will allow to evaluate the role and importance of reactive intermediates in charge storage and energy conversion. [1] Rodríguez-López, J. The Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): an approach to the study of adsorption and (electro)catalysis at electrodes. In Electroanalytical Chemistry, a series of advances. Vol. 24. Bard, A.J. and Zoski, C.G., Eds. 2012, CRC Press, pp. 287-352[2] Gavvalapalli, N.; Hui, J.; Cheng, K.; Lichtenstein, T.; Shen, M.; Moore, J.S.; Rodríguez-López, J. J. Am. Chem. Soc. 2014, 136, 16309-16316

Iridium Oxidation as Observed by Surface Interrogation Scanning Electrochemical Microscopy

The formation of surface oxides on most metal, including noble metal, electrodes occurs before the onset of the oxygen evolution reaction (OER). An understanding of changes in surface structure and composition caused by the oxidation process is important to the field of electrocatalysis of the OER. In this work, the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of −OH(ads) and −H(ads) species generated at the surface of polycrystalline iridium ultramicroelectrodes (UMEs) in 2 M NaOH. This system was selected because the iridium oxides are among the most effective and stable electrocatalysts for the OER. We introduce the redox pair Fe(III/II)–TEA as a mediator for stable surface interrogation at pH ≥ 12. This is the first time that SI-SECM experiments have been carried out at such an extreme pH. Monolayer coverage of −OH(ads) and −H(ads) was Qθ=1,OH = 456 ± 2.0 μC cm–2 and Qθ=1,H = 224.2 ± 0.2 μC cm–2, respectively. At potentials m...

Quantification of photoelectrogenerated hydroxyl radical on TiO2 by surface interrogation scanning electrochemical microscopy

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM) was used for the detection and quantification of adsorbed hydroxyl radical ˙OH((ads)) generated photoelectrochemically at the surface of a nanostructured TiO(2) substrate electrode. In this transient technique, a SECM tip is used to generate in situ a titrant from a reversible redox pair that reacts with the adsorbed species at the substrate. This reaction produces an SECM feedback response from which the amount of adsorbate and its decay kinetics can be obtained. The redox pair IrCl(6)(2-/3-) offered a reactive, selective and stable surface interrogation agent under the strongly oxidizing conditions of the photoelectrochemical cell. A typical ˙OH((ads)) saturation coverage of 338 μC cm(-2) was found in our nanostructured samples by its reduction with the electrogenerated IrCl(6)(3-). The decay kinetics of ˙OH((ads)) by dimerization to produce H(2)O(2) were studied through the time dependence of the SI-SECM signal and the surface dimerization rate constant was found to be ~k(OH) = 2.2 × 10(3) mol(-1) m(2) s(-1). A radical scavenger, such as methanol, competitively consumes ˙OH((ads)) and yields a shorter SI-SECM transient, where a pseudo-first order rate analysis at 2 M methanol yields a decay constant of k'(MeOH) ~ 1 s(-1).

Surface interrogation mode of scanning electrochemical microscopy for oxidation study of adsorbed CO generated from serine on platinum

Abstract We report a new method for detection and oxidation of adsorbed carbon monoxide (CO(ads)) generated from serine on a polycrystalline platinum ultramicroelectrode (UME) by bromine (Br2) using in situ surface interrogation (SI) mode of scanning electrochemical microscopy (SECM). In the SI mode, tip and substrate are both Pt UMEs, and CO(ads) on Pt substrate, generated from serine, can be oxidized by the tip-generated Br2 giving a positive response. Dosing CO(ads) from serine instead of purging CO gas expands the newly introduced reaction of Br2 with CO(ads) and further enhances the hope to get rid of CO(ads) on Pt for fuel cells.

Reaction of Various Reductants with Oxide Films on Pt Electrodes As Studied by the Surface Interrogation Mode of Scanning Electrochemical Microscopy (SI-SECM): Possible Validity of a Marcus Relationship

The surface interrogation mode of scanning electrochemical microscopy (SI-SECM), an in situ method for the quantification of adsorbed intermediates at electrodes at open circuit, was used to evaluate the rate of reaction of four redox mediators of different reducing power (methyl viologen+ > Ru(NH3)62+ > Fe(II)[EDTA]2− > Fe(CN)64−) with electrogenerated platinum oxides (PtOx) at different pHs. The rate constant determined by SI-SECM was sensitive to the nature of the Pt oxide formed at different potentials. A simplified model with finite element method simulation was used to fit the SI-SECM experimental response. The extraction of kinetic constants and their comparison to the potential (reducing power) of the mediator yielded a Marcus-theory type relationship. Although these are clearly not outer sphere reactions, if subjected to the Marcus theory treatment, one can extract an estimated value of λ = 1.3 ± 0.3 eV for the reduction of “place exchanged” oxide, which forms with oxide coverage values larger than 0.5 (as PtO). Steady state feedback experiments were also conducted in which the reduced form of the mediators was oxidized at Pt with different oxide coverage, i.e., over a wide range of potentials. A comparison of SI-SECM kinetic analysis with the Pt substrate at open circuit to the case where an electrode bias is applied indicates that for Fe(II)[EDTA]2−, the feedback current follows the oxide coverage at the Pt substrate. We suggest that thin layers of oxide provide a chemical route in which the oxide mediates oxidation of the reduced electrogenerated species; this route is governed by the reactivity of the different species of oxide formed at the electrode and not necessarily by other effects, such as direct electron tunneling through the oxide layer.

Scanning Electrochemical Microscopy: Surface Interrogation of Adsorbed Hydrogen and the Open Circuit Catalytic Decomposition of Formic Acid at Platinum

The surface interrogation mode of scanning electrochemical microscopy (SECM) is extended to the in situ quantification of adsorbed hydrogen, H(ads), at polycrystalline platinum. The methodology consists of the production, at an interrogator electrode, of an oxidized species that is able to react with H(ads) on the Pt surface and report the amounts of this adsorbate through the SECM feedback response. The technique is validated by comparison to the electrochemical underpotential deposition (UPD) of hydrogen on Pt. We include an evaluation of electrochemical mediators for their use as oxidizing reporters for adsorbed species at platinum; a notable finding is the ability of tetramethyl-p-phenylenediamine (TMPD) to oxidize (interrogate) H(ads) on Pt at low pH (0.5 M H(2)SO(4) or 1 M HClO(4)) and with minimal background effects. As a case study, the decomposition of formic acid (HCOOH) in acidic media at open circuit on Pt was investigated. Our results suggest that formic acid decomposes at the surface of unbiased Pt through a dehydrogenation route to yield H(ads) at the Pt surface. The amount of H(ads) depended on the open circuit potential (OCP) of the Pt electrode at the time of interrogation; at a fixed concentration of HCOOH, a more negative OCP yielded larger amounts of H(ads) until reaching a coulomb limiting coverage close to 1 UPD monolayer of H(ads). The introduction of oxygen into the cell shifted the OCP to more positive potentials and reduced the quantified H(ads); furthermore, the system was shown to be chemically reversible, as several interrogations could be run consecutively and reproducibly regardless of the path taken to reach a given OCP.