Variation Of Electrode Potential Research Articles

Electro-Chemo-Mechanical Coupling in Hierarchical Nanoporous Gold-Polypyrrole-Water Hybrids

Nanoporous (np) metals made by dealloying is a novel class of nanostructured materials that offers numerous functionalities, ranging from sensing, energy storage, to (electro-)catalysis and beyond. A substantial amount of research in np materials has been focused on np gold due to its excellent structural and chemical stability, adjustable pore size, high conductivity, biocompatibility, and easy fabrication process via free corrosion or electrochemical dealloying. A considerable number of studies have been conducted to investigate np gold infiltrated with an aqueous electrolyte [1]. In that way, the material is considered as a hybrid so that one can reversibly tune the gold surface state by manipulating the superficial charge densities or adsorbate coverages via the applied electric potential [1]. The above-mentioned approach results in a number of intriguing properties of np Au revealed upon electrode potential variation in aqueous electrolytes such as actuation [2, 3], sensing [4], tunable strength [5] and elastic modulus [6]. The behavior is well-studied in case of weak ion adsorption and surface oxidation.One of the promising methods towards further improvement of functional and mechanical properties of np metals was implemented by Roschning and Weissmueller [7]. It was shown that the actuation strain of np Au can be enhanced via electrodeposition of a conductive polymer polypyrrole (PPy) on the gold surfaces. The actuation strain of the np Au-PPy hybrids strongly scales with the PPy phase fraction. Moreover, Li et al. [8] observed variations of effective elastic modulus in np Au-PPy-water hybrids. Thus, introducing the conductive polymer into a pore space of the np metal yields a hybrid actuator with a significantly improved actuation amplitude and yield strength.Here, we employ PPy for surface functionalization of hierarchical (hc) np Au (Figure 1) to explore the impact of the conductive polymer coating on the functional behavior of the hierarchically structured np metals. Np metals with structural hierarchy made by dealloying [9] open up new opportunities for multifunctional hard-soft hybrids. The larger pores at the higher hierarchical level (characterized by diameters around 120 nm) can promote fast mass exchange, while nanopores at the lower hierarchy level (below 30 nm) provide a large surface area and high chemical activity. Actuation behavior of the hybrid materials was analyzed in situ in a dilatometer upon potential cycling in a 1 M HClO4 aqueous electrolyte. The elastic response was examined in situ in a dynamic mechanical analyzer in the same electrolyte. We revealed pronounced changes of the macroscopic length and Young’s modulus in response to the voltage-induced redox reactions of the polymer films at the np electrodes. In the contribution, we discuss the origin of the electro-chemo-mechanical coupling phenomena.[1] Shao et al., in Nanoporous Gold: From an ancient technology to a high-tech material. RSC (2012).[2] Kramer et al. Nano Lett., 4(5) 793 (2004).[3] Jin et al. Nano Lett., 10(1) 187 (2010).[4] Stenner et al., Adv. Funct. Mater. 26 (28), 5174 (2016).[5] Jin et al., Science 332, 1179 (2011.)[6] Mameka et al., Acta Mater. 76, 272 (2014).[7] Roschning et al., Adv. Mater. Interfaces, 2001415 (2020).[8] Li et al., Acta Mater., 212 (1), 116852 (2021).[9] Shi et al., Science 371, 1026 (2021).Figure 1. Scanning electron micrograph of hierarchical nanoporous Au-PPy hybrid microstructure. Figure 1

Changes in the negative logarithm of end-tidal hydrogen partial pressure indicate the variation of electrode potential in healthy Japanese subjects

Molecular hydrogen (H2) is produced by human colon microbiomes and exhaled. End-tidal H2 sampling is a simple method of measuring alveolar H2. The logarithm of the hydrogen ion (H+)/H2 ratio suggests the electrode potential in the solution according to the Nernst equation. As pH is defined as the negative logarithm of the H+ concentration, pH2 is defined as the negative logarithm of the H2 effective pressure in this study. We investigated whether changes in pH2 indicated the variation of electrode potential in the solution and whether changes in end-tidal pH2 could be measured using a portable breath H2 sensor. Changes in the electrode potential were proportional to ({mathrm{pH}}_{2}-2times mathrm{pH}) in phosphate-buffered solution (pH = 7.1). End-tidal H2 was measured in the morning (baseline) and at noon (after daily activities) in 149 healthy Japanese subjects using a handheld H2 sensor. The median pH2 at the baseline was 4.89, and it increased by 0.15 after daily activities. The variation of electrode potential was obtained by multiplying the pH2 difference, which suggested approximately + 4.6 mV oxidation after daily activities. These data suggested that changes in end-tidal pH2 indicate the variation of electrode potential during daily activities in healthy human subjects.

Coinage metals doped 1T′ WS2 as efficient bifunctional electrocatalyst towards ORR and HER: A first principles study

The electrocatalytic activities of Cu, Ag and Au doped 1T′ WS2 as electrocatalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in acidic medium are explored by means of first principles calculations. The stability of these doped systems are inspected in terms of formation energy and charge transfer analysis upon doping. Highest amount of charge transfer towards the dopant is observed for systems with lowest formation energy. For all the doped systems, charge is transferred to the dopant that substitutes the anionic S atom validating the doping scheme and stability of the doped systems. ORR proceeds only via the energetically efficient four electron pathway on Cu doped 1T′ WS2 with a monotonically exothermic free energy profile. In the case of Ag and Au doped structures, ORR occurs competitively via both two and four electron pathways due to the marginal free energy difference in the 2nd protonation step between these two pathways and produces both H2O or H2O2 competitively as the end product. Study of the effect of electrode potential variation on the free energy profiles reveals the overpotential for ORR on Cu, Ag and Au doped 1T′ WS2 to be 0.85 V, 0.80 V and 0.66 V respectively via the four electron pathway. Proneness of Cu doped 1T′ WS2 towards only the four electron pathway and low overpotential for ORR makes it the promising cathode electrocatalyst for ORR among the considered systems. Further study of the HER activity of Cu, Ag and Au doped 1T′ WS2 reveals the Gibbs free energy of H+ ion adsorption on Cu doped 1T′ WS2, Ag doped 1T′ WS2 and Au doped 1T′ WS2 to be −0.14 eV, −0.17 eV and 0.18 eV respectively indicating high efficiency of those system towards HER in acidic medium.

In-situ detection of LiMn2O4 dissolution during electrochemical cycling by

Herein we describe a modified Swagelok electrochemical cell for characterizing individual electrode behavior as part of full cell operation, including individual electrode potentials and dissolution phenomena. The case study presented is that of the non-aqueous Li-LiMn2O4 system. Outcomes from the work include characterization of lithium negative electrode potential variation with applied current, lithium corrosion and passivation in the electrolyte, and LiMn2O4 dissolution with cycling. It is also demonstrated that Mn(III) is the dissolution product. Application of the technique to other systems, including electrode materials and electrolytes, is discussed.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy.

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal |liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversiblycontrolled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. Thisis experimentallydemonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Electrochemical Impedance Spectroscopy on the Performance Degradation of LiFePO4/Graphite Lithium-Ion Battery Due to Charge-Discharge Cycling under Different C-Rates

Lithium-ion batteries (LIBs) using a LiFePO4 cathode and graphite anode were assembled in coin cell form and subjected to 1000 charge-discharge cycles at 1, 2, and 5 C at 25 °C. The performance degradation of the LIB cells under different C-rates was analyzed by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy. The most severe degradation occurred at 2 C while degradation was mitigated at the highest C-rate of 5 C. EIS data of the equivalent circuit model provided information on the changes in the internal resistance. The charge-transfer resistance within all the cells increased after the cycle test, with the cell cycled at 2 C presenting the greatest increment in the charge-transfer resistance. Agglomerates were observed on the graphite anodes of the cells cycled at 2 and 5 C; these were more abundantly produced in the former cell. The lower degradation of the cell cycled at 5 C was attributed to the lowered capacity utilization of the anode. The larger cell voltage drop caused by the increased C-rate reduced the electrode potential variation allocated to the net electrochemical reactions, contributing to the charge-discharge specific capacity of the cells.

In operando measurement of localised cathode potential to mitigate DMFC temporary degradation

An innovative external reference electrode technique has been applied to the cathode of an operating DMFC in order to identify variations in electrode potential across the active area of the cell. The evolution of cathode potential at two different locations in the cell was monitored during operation, with the primary focus on studying the potential dynamics during the temporary degradation recovery procedure, the so-called refresh cycle. The results highlight for the first time a non-uniform local recovery of temporary degradation at the cathode during refresh cycles, associated with varying rates of platinum oxide reduction across the cell, which could lead to current density redistribution and contribute to an uneven degradation of the components. The technique shows great promise for the improvement of long term DMFC performance via optimisation of refresh cycle protocols.

Direct Electrodeposition of Aluminum Nanowires from a Room Temperature Ionic Liquid: An Electrochemical 2D-3D-1D Process

Single-crystalline Al nanowires with diameters under 100 nm are successfully prepared by direct electrodeposition from a room temperature ionic liquid without using templates, additives or catalysts for the first time. We propose the three stages of nanowires growth mechanism including electrodeposition of the initial two dimensional nanofilm, subsequent electrochemical formation of three dimensional nano-nuclei and final one dimensional growth of nanowires. The electrodeposited Al nanowires can grow to lengths of up to 50 μm in 120 s with an average growth rate more than 0.4 μm/s, reaching a high aspect ratio over 1000. This study shows that the dimensions of nanowires are precisely controlled through the variation of electrode potential and are influenced by the electrodeposition temperature and time. The morphology of Al nanowires is very uniform and the nanowire diameters electrodeposited at −1.6 V/25°C within 60 s are in the range of 17 nm to 50 nm. It is found that diffusion strongly influences the growth process.

Statistical Mechanics of ‘Unwanted Electroactuation’ in Nanoporous Supercapacitors

Nanoporous electrodes have the potential to increase the surface electrode interfacial area and the stored energy density of a supercapacitor. However, structural deformation of the electrode can become apparent when the size of the pore is comparable to the size of a charging ion. After many cycles this could cause wear and degradation. We present a theoretical study of this ‘Unwanted Electroactuation’ in a carbon electrode wetted with an ionic liquid. We incorporate changes of the carbon-carbon bond length due to electrochemical doping of the pore walls and steric effects related to counterion insertion into the pore via a modified Ising model of charge storage. When considering the total electrode deformation these effects either complement or compete with each other, depending on the polarisation of the electrode. Our model shows qualitative agreement with the features of the experimentally observed expansion caused by variation of electrode potential.

Electrochemical Gating of Charge Transport in Single Macrocycle Molecular Switches

Molecular switches, i.e., molecular scale devices that can be turned on and off, are of long standing interest. In the quest for miniaturization of electronic devices, the development of electrical switches of molecular dimensions has been identified as an important target of practical importance. While much progress has been made in designing functional molecular scale electrical switches, i.e., molecular cores whose intrinsic conductivity can be externally modulated, the question of contact resistance has not been sufficiently addressed. Our novel strategy for the regulation of charge transport through single molecule junctions combines external stimuli using a variation of electrode potential in an electrochemical environment, internal modulation of molecular structures and optimization of anchoring groups. Our synthetic collaborators have designed redox-active benzodifuran (BDF) compounds as functional electronic units which we have used to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ).[1] The conductance of BDF terminated thiol, the standard anchoring group for molecular scale electronic components, can be tuned by changing the electrode potentials in an ionic liquid electrolyte to show a clear single molecule field effect transistor (FET) behavior. We hypothesized that we could optimize the response by appropriate choice of molecular anchoring group.[2] Guided by our earlier investigations, a BDF molecule tailored with carbodithioate (-CS2-) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only increases the junction conductance, but also substantially improves the FET response by enhancing the on/off ratio from 2.5 to 8. [1] Regulating a Benzodifuran Single Molecule Redox Switch via Electrochemical Gating and Optimization of Molecule/Electrode Coupling, Zhihai Li, Hui Li, Songjie Chen, Toni Froehlich, Christian Schönenberger, Michel Calame, Silvio Decurtins, Shi-Xia Liu and Eric Borguet, Journal of the American Chemical Society 136, 25 8867-8870 (2014). [2] Optimizing Single-Molecule Conductivity of Conjugated Organic Oligomers with Conjugated Carbodithioate Linkers, Yangjun Xing, Tae-Hong Park, Ravindra Venkatramani, Shahar Keinan, David N. Beratan, Michael J. Therien, and Eric Borguet, Journal of the American Chemical Society 132 (23), 7946-7956 (2010). Figure 1

Particle concentration distribution measurements in stirred tanks using a new experimental technique: time and frequency domain analyses

A novel experimental technique has been used to measure particle concentration distributions in agitated tanks. This approach consisted in using pyrite (FeS2) particles and a Pt electrode connected to a potentiostat to measure variations in the electrode potential caused by galvanic interactions between FeS2 particles and the Pt electrode. In order to discern collisions between the particles and the Pt electrode from random variations in electrode potential, a time and a frequency domain method of analysis were used. The latter showed no periodicity in the collision rate indicating that the process was stochastic. The presented method is convenient because of its simplicity and it can provide important information regarding the hydrodynamic behaviour of solids in agitated tanks, particularly when non-intrusive techniques are not an option. Finally, the advantages and drawbacks of the technique, and its relevance to hydrometallurgical and other chemical processes are discussed.

Variation of electrode potential and charge transfer bands in series of tetravalent ruthenium complexes [RuCl6−n(PrCN)n]n−2

A truncated series of ruthenium(IV) complexes [RuCl(6−n)(RCN)n]n−2 (n=0–3; R=n-propyl) has been electrogenerated in situ and characterized spectroscopically. This substitution series allowed the examination of the variation of electrode potential of Ru+4/Ru+3 couple and the accompanying trends in ligand-to-metal charge-transfer (LMCT) spectra. The Cl→Ru4+ charge transfer manifold red-shifted from n=0–2, and then moved back to the blue edge upon the third substitution of chloride with nitrile. Moreover, the anodic shift in Ru+4/Ru+3 redox couple upon the third chloride replacement was barely half the expected change. These irregular trends contradict the results of the DFT studies and the tenet of ligand additivity principle.

A metal ion release study of CoCrMo exposed to corrosion and tribocorrosion conditions in simulated body fluids

Several studies have investigated the presence of metal ions in different human fluids (i.e. blood, urine, serum and plasma) of patients using CoCrMo bearing implants. The general conclusion is that the degradation mechanisms of the implants (the joint action of biocorrosion and wear) lead to an increase of dissolved metals and particles leading to implants failure and tissue inflammation. However, the levels of metallic ions in those fluids can vary depending on the involved degradation mechanism of the implant (corrosion or tribocorrosion). Having a tool able to identify the origin of the metal ion concentration in patients using metal bearing implants could allow faster and easier clinical treatments and failure detection.This work has investigated the metal ion release of a low carbon CoCrMo subjected to different electrochemical and tribocorrosion conditions in NaCl, phosphate buffer solution (PBS) and PBS with protein (albumin) at the human body conditions of 37°C. Cobalt is the most concentrated ion and has the largest concentration ratio in all testing conditions, with the exception of the corrosion tests in PBS with albumin. Albumin drastically increases the molybdenum concentration in the electrolyte after corrosion tests at applied potentials below the transpassive region of the alloy. In the tribocorrosion tests, an increase of metal ion concentration with potential was found, while this effect was not observed in the corrosion tests, thus electrode potential variations are critical for metal ion release of CoCrMo alloys under tribocorrosion conditions. Combining ICP-MS analysis with electrochemical testing gives mechanistic information on the involved redox reactions, oxide formation and amount of wear debris in corrosion and tribocorrosion situations of biomedical alloys. This is needed for the interpretation of the in vivo data.

Highly disparate activity regions due to non-uniform potential distribution in microfluidic devices: Simulations and experiments

In a microfluidic flow cell activity pattern can occur along a thin band electrode due to the potential distribution in the cell. For quantitative characterization of the pattern formation exclusively due to electric effect and for elimination of interactions of reaction sites from concentration distribution along the flow channel, a partial differential equation model is formulated for the spatiotemporal variation of electrode potential with Butler–Volmer kinetics limited by mass transfer. At constant applied circuit voltage, with increase of the electrode size a limiting current is achieved because of the spatial pattern formation. The limiting current arises due to the formation of high activity at the downstream edge, and low (nearly open circuit potential) activity at the upstream edge. The spatial pattern (e.g., ratio of active vs. inactive region) depends on the electrode size, the applied voltage, the conductivity of the electrolyte, and the distance from the downstream electrode edge to the reservoir. It is also shown that by placing equally spaced insulating stripes on the electrode much of the activity can be retained and the current does not decrease significantly due to the lessened surface area (as long as the surface area of the insulating stripes is less than about 50% of the entire electrode area). The model simulations are interpreted with a coupled ordinary differential equation model of segmented electrodes and the occurrence of the strong edge effects is confirmed with experiments of a four-electrode electrode array in a microfluidic flow cell with ferrocyanide oxidation.

Determination of diffusion coefficient of lithium in substituted LiMn1.95Cr0.05O4 spinel using impedance technique

Electrodiffusion properties of chromium-substituted lithium-manganese spinel Li x Mn1.95Cr0.05O4 intended for application as a cathodic material for lithium-ion batteries is studied. The studies are carried out at 25°C using the electrochemical impedance spectroscopy technique in alkyl-carbonate electrolyte. In the analysis of impedance spectra, the apparatus of electric equivalent circuits was employed to determine surface layer resistances, double electric layer capacitance, differential intercalation capacity, chemical diffusion coefficient D of lithium, and other electrode characteristics. The issues of substantiating the choice of electric equivalent circuits and correct interpretation of their elements are discussed; dependences of the calculated model parameters on the electrode potential (lithium concentration in the electrode) are analyzed. The chemical diffusion coefficient of Li+ in Li x Mn1.95Cr0.05O4 found on the basis of the impedance spectra is in the range of 10−9 to 10−12 cm2/s under electrode potential variation in the range of 3.5–4.5 V (vs. Li/Li+) with a pronounced minimum of D in the middle of this range. Repeated cycling of the electrode is accompanied by a gradual increase in resistance of the solid-electrolyte interphase (SEI).

Properties of screen printed electrocardiography smartware electrodes investigated in an electro-chemical cell

BackgroundECG (Electrocardiogram) measurements in home health care demands new sensor solutions. In this study, six different configurations of screen printed conductive ink electrodes have been evaluated with respect to electrode potential variations and electrode impedance.MethodsThe electrode surfaces consisted of a Ag/AgCl-based ink with a conduction line of carbon or Ag-based ink underneath. On top, a lacquer layer was used to define the electrode area and to cover the conduction lines. Measurements were performed under well-defined electro-chemical conditions in a physiologic saline solution.ResultsThe results showed that all printed electrodes were stable and have a very small potential drift (less than 3 mV/30 min). The contribution to the total impedance was 2% of the set maximal allowed impedance (maximally 1 kΩ at 50 Hz), assuming common values of input impedance and common mode rejection ratio of a regular amplifier.ConclusionOur conclusions are that the tested electrodes show satisfying properties to be used as elements in a skin electrode design that could be suitable for further investigations by applying the electrodes on the skin.

Combined effects of Tafel kinetics and Ohmic potential drop on the nonlinear responses of electrochemical systems to low-frequency sinusoidal perturbation of electrode potential – New approach using the Lambert W-function

Nonlinearity effects of electrochemical systems kinetics on impedance measurements are re-examined in the low-frequency (LF) domain through symbolic manipulation and numerical computation performed with Mathematica. It is a new approach to some problems not yet completely resolved in electrochemical impedance spectroscopy despite a number of publications on this subject in the specialized literature. This article is focused on electrochemical systems governed by Tafel kinetics under steady-state conditions, so-called Tafelian systems below, and perturbed by a sinusoidal variation of electrode potential vs. time, at low frequency. First, harmonic analysis of the system response to sinusoidal perturbation of potential with negligible Ohmic drop effects is dealt with. Next, the combined effects of Tafel kinetics and Ohmic drop are thoroughly examined. The current–potential response at low frequency is modelled using the Lambert W-function. New formulations are derived for the nonlinear polarization resistance of Tafelian systems in the presence of Ohmic drop. Closed-form or infinite-series formulations are derived for the amplitudes of fundamental and harmonic components of periodic current. Finally, the validity condition for impedance measurements carried out in the LF domain is derived for Tafelian systems.

Proton exchange membrane fuel cell contamination model: Competitive adsorption followed by a surface segregated electrochemical reaction leading to an irreversibly adsorbed product

A generic, transient fuel cell kinetic loss mathematical model was developed for the case of contaminants that partially cover the catalyst surface with irreversibly adsorbed products. The model was derived using step changes in contaminant concentration, constant operating conditions and disregarding liquid water scavenging effects. The closed form solutions were validated using H 2S, SO 2 and COS data from a single source. The model needs to be validated against other data sets and transient operating conditions more representative of automotive applications. A method is proposed to determine kinetic rate constants and relies on tests with a reactant, a contaminant and, a reactant and a contaminant mixture. The method is useful to evaluate the presence of interactions between reactant and contaminant related adsorbates, and, to minimize electrode potential variations during controlled cell voltage measurements. Model parameters were similar for all contaminants suggesting a common adsorbate configuration. The model also expands the number of previously derived cases. All models in this inventory, derived with the assumption that the reactant is absent, lead to different dimensionless current vs. time behaviors similar to a fingerprint. These model characteristics facilitate contaminant mechanism identification. Model predictions include a limit of 0.7 ppb contaminant concentration in the reactant stream to minimize cell performance losses during the 5000 h automotive application life. This tolerance limit represents a worse case scenario because it does not take into account performance recovery resulting from drive cycle operation or the addition of mitigation strategies. A cell performance loss increase of 40% is also predicted for a catalyst loading decrease from 0.4 to 0.1 mg Pt cm −2.

Raman Spectroelectrochemistry of Molecules within Individual Electromagnetic Hot Spots

The role of chemical enhancement in surface-enhanced Raman scattering (SERS) remains a contested subject. We study SERS spectra of 4-mercaptopyridine molecules excited far from the molecular resonance, which are collected from individual electromagnetic hot spots at concentrations close to the single-molecule limit. The hot spots are created by depositing Tollen's silver island films on a transparent electrode incorporated within an electrochemical cell. Analysis of the intensity of the spectra relative to those obtained from individual rhodamine 6G molecules on the same surface provides a lower limit of approximately 3 orders of magnitude for the chemical enhancement. This large enhancement is likely to be due to a charge transfer resonance involving the transfer of an electron from the metal to an adsorbed molecule. Excitation at three different wavelengths, as well as variation of electrode potential from 0 to -1.2 V, lead to significant changes in the relative intensities of bands in the spectrum. It is suggested that while the bulk of the enhancement is due to an Albrecht A-term resonance Raman effect (involving the charge transfer transition), vibronic coupling provides additional enhancement which is sensitive to electrode potential. The measurement of potential-dependent SERS spectra from individual hot spots opens the way to a thorough characterization of chemical enhancement, as well to studies of redox phenomena at the single-molecule level.

Electronic structure models of oxygen adsorption at the solvated, electrified Pt(111) interface

The adsorption of molecular oxygen is the first step in the oxygen reduction reaction. Influences of interfacial water structure and electrode potential on oxygen adsorption to the Pt(111) surface were evaluated using density functional theory. Two approaches were used to model the electrification of the interface, an applied homogeneous electric field and the double-reference method of Filhol, Taylor, and Neurock. The free energy change for molecular oxygen replacement of water at the surface shows qualitatively different trends between the two models. The inclusion of solvation effects and direct control of the electrode potential offered by the double-reference method lead to the conclusion that O(2) replacement of water is favorable at all potentials studied, and O(2) binding becomes more favorable with increasing potential. This trend is contrary to that observed using an external electric field model to represent the electrochemical double layer, and arises due to the compounded effect of potential on water-surface, oxygen-surface, and water-molecular oxygen interactions. These results indicate that oxygen replacement of adsorbed water does not limit the overall oxygen reduction reaction rate at a proton-exchange membrane fuel cell cathode. The impacts of aspects of model construction (number of water layers, water density) and variation of electrode potential on the O(2)-Pt(111) interaction are described.