Varying Electrode Potential Research Articles

(Invited) Electroenzymatic Hydrogen Production: Mediated/Direct Electron Transfer Strategies and Continuous Product Analysis

Metalloenzymes such as hydrogenases and nitrogenases are of interest to electrochemists for mechanistic study as well as applied biotechnological hydrogen and ammonia production. In order to interface these enzymes with electrodes mediated and direct electron transfer methods have been reported, each having their strengths and weaknesses. In addition to establishing efficient electron transfer with enzymes, dynamic product analysis is also desirable since rates of product formation (and the distribution of products) can be determined during an experiment with varying electrode potential (and thermodynamic driving force for heterogeneous electron transfer).Our recent efforts concerning the design of electrode:enzyme interfaces are presented here, including (i) mediated vs. direct electron transfer strategies, (ii) rotating ring disk electrochemistry, and (iii) continuous mass spectrometry and gas chromatography [1-2]. In this work, an [FeFe]-hydrogenase is employed as a model enzyme for molecular hydrogen formation, with a view to expand these approaches towards enzymatic electrocatalytic studies of nitrogenases. Limitations and difficulties of the presented approaches will be discussed.

Relaxation Times of Ionic Liquids under Electrochemical Conditions Probed by Friction Force Microscopy.

Ionic liquids (ILs) represent an important class of liquids considered for a broad range of applications such as lubrication, catalysis, or as electrolytes in batteries. It is well-known that in the case of charged surfaces, ILs form a pronounced layer structure that can be easily triggered by an externally applied electrode potential. Information about the time required to form a stable interface under varying electrode potentials is of utmost importance in many applications. For the first time, probing of relaxation times of ILs by friction force microscopy is demonstrated. The friction force is extremely sensitive to even subtle changes in the interfacial configuration of ILs. Various relaxation processes with different time scales are observed. A significant difference dependent on the direction of switching the applied potential, i.e., from a more cation-rich to a more anion-rich interface or vice versa, is found. Furthermore, variations in height immediately after the potential step and the presence of trace amounts of water are discussed as well.

Inductive Effect Alone Cannot Explain Lewis Adduct Formation and Dissociation at Electrode Interfaces.

Understanding breaking and formation of Lewis bonds at an electrified interface is relevant to a large range of phenomena, including electrocatalysis and electroadsorption. The complexities of interfacial environments and associated reactions often impede a systematic understanding of this type of bond at interfaces. To address this challenge, we report the creation of a main group classic Lewis acid-base adduct on an electrode surface and its behavior under varying electrode potentials. The Lewis base is a self-assembled monolayer of mercaptopyridine and the Lewis acid is BF3, forming a Lewis bond between nitrogen and boron. The bond is stable at positive potentials but cleaves at potentials more negative of approximately -0.3 V vs Ag/AgCl without an associated current. We also show that if the Lewis acid BF3 is supplied from a reservoir of Li+BF4- electrolyte, the cleavage is completely reversible. We propose that the N-B Lewis bond is affected both by the field-induced intramolecular polarization (electroinduction) and by the ionic structures and ionic equilibria near the electrode. Our results indicate that the second effect is responsible for the Lewis bond cleavage at negative potentials. This work is relevant to understanding the fundamentals of electrocatalytic and electroadsorption processes.

Temperature dependence of differential capacitance in the electric double layer.Symmetric valency 1:1 electrolytes.

The differential capacitance of an electric double layer formed by an aqueous solution of KNO3 on a glassy carbon electrode is measured by impedance analysis at constant frequency. Results are obtained at electrolyte concentrations of 0.1 mol/dm3, 0.5 mol/dm3, and 1.0 mol/dm3, and at a series of temperatures, viz., 288 K, 298 K, 308 K, 318 K, and 328 K. The differential capacitance envelopes reveal a rich, complex pattern of maxima, minima, and local minima, whose magnitude and position change with a change in solution concentration. At the two lower concentrations, the temperature dependence of the capacitance, for example, at zero electrode potential, shows an alternating positive-negative behavior, while at the highest concentration of 1.0 mol/dm3, the slope of the differential capacitance-electrode potential curve is always positive. The experimental results are supplemented by a numerical grand canonical Monte Carlo simulation study of a restricted primitive model double layer but with an off-center cationic charge achieved by displacing the charge center from the ion sphere center toward its surface. The simulations, performed at the electrolyte concentration of 1.0 mol/dm3 and constant cation charge center displacement, and at varying electrode potentials and temperatures, show, in general, a negative temperature dependence of the differential capacitance. However, this temperature dependence can also be positive for a negative electrode charge and for a sufficiently large gradient of the cation charge center displacement with temperature. This feature is seen to be associated with an increase in the entropy of formation of the double layer.

Modelling the Electrode/Electrolyte Interfaces

Electrochemical reactions are mainly governed by the interface properties and changing electrode potential. Understanding the nature of electrochemical electrode/electrolyte interfaces is thus a crucial step in the optimization of systems for energy conversion and storage. However, a generally accepted method to consider varying electrode potentials in first-principles calculations does not exist, and modelling interfaces remains one of the biggest challenges for the density functional theory (DFT) community. Implicit solvent model implemented in some DFT codes [1, 2] together with the inclusion of explicit solvent molecules was used on lithium system and the obtained results were in agreement with experimental values [3]. We use the same methodological approach to study Mg2+ solvated in monoglyme (DME) and in ethylene carbonate (EC). Using molecular DFT (Gaussian) we first determine the explicit first solvation shell of Mg2+ in DME and EC. The solvated Mg2+ complex is then placed in front of Mg electrode in the plane wave DFT calculations (VASP). Potential dependent grand canonical energies, equilibrium (redox) potentials and differential capacitance are obtained for both systems and are in excellent agreement with experimental results. We further develop the method to probe stability regions of the solvents at the interface. We find that the stability of the solvent molecules is drastically modified in the presence of Mg2+ . Furthermore, thepresented methodological approach can be used to study decomposition pathways. The obtained results give valuable insights into why some solvents are not appropriate for Mg battery systems, and how electrolyte performance can be improved. [1] K. Mathew et al., The Journal of chemical physics 140, 084106 (2014).[2] K. Letchworth-Weaver et al., Physical Review B 86, 075140 (2012).[3] N. Lespes et al., Journal of chemical theory and computation 11, 33753382 (2015).[4] C. D. Taylor et al., Physical Review B 73, 165402 (2006).

Visualizing charges accumulated in an electric double layer by three-dimensional open-loop electric potential microscopy.

Charges accumulated in an electric double layer (EDL) play key roles in various interfacial phenomena and electronic devices. However, direct imaging of their spatial distribution has been a great challenge, which has hindered our nano-level understanding of the mechanisms of such interfacial phenomena and functions. In this study, we present direct imaging of charges accumulated at an electrode-electrolyte interface using three-dimensional open-loop electric potential microscopy (3D-OL-EPM). Conventional OL-EPM allows us to visualize two-dimensional potential distributions in liquid yet the zero of the measured potential is not well defined due to the influence of the long-range (LR) interaction between the cantilever and the sample. Here, we present practical ways to reduce such an influence by improving the equation for the potential calculation and subtracting the LR contribution estimated from a Z potential profile. These improvements enabled the calibration of the measured potential values with respect to the bulk solution potential. With these improvements, we visualized opposite charge accumulation behaviors on a polarizable and non-polarizable electrode with a varying electrode potential. Combining OL-EPM with a 3D tip scanning method, we also performed a 3D-OL-EPM measurement on a Cu fine wire and visualized the nanoscale distribution of the charges accumulated at the interface. Such real-space information on the charge distributions in an EDL should provide valuable insights into the mechanisms of interfacial phenomena and functions that are important in various academic and industrial research on electronic devices, electrochemistry, tribology and life sciences.

(Invited) Electrochemically Tuned Metamaterials: New Scenarios Navigated By Theory

The notion of metamaterials has emerged in this century, primarily in the context of new structures with unusual optical properties, such as specially designed plasmonic materials. But metamaterials can be considered much more broadly, in the context of what is alternatively called smart materials, or even more modetly functional materials which can perform different kind of novel functions at our will. N. Zheludev, opening a META-2014 congress in Singapore, made a provocative statement: “The time of metamaterials is over. It is the time of …tuneable metamaterials”. Electrochemistry, known to physicists as a “surface science with a joy stick — the potentiostat” (B.Ocko) is actually in a best position to deliver innovations on this front. It can be used to tune the properties of nano and micro- structures designed to be electrochemically controlled. In such structures, varying electrode potentials rearranges the ionic distributions, remarkably affecting system properties. In this keynote I will discuss few most recent examples of electroactive/electrotuneable materials, the study of many of which are still in progress, and the design of which has been navigated or strongly influenced by the theory. These are Electrovariable nanoplasmonic mirrors and trace analyte sensors [1-2].Electrotuneable friction [3]Ultrananoporous supercapacitors [4,5]Electroactuators [6]Reverse electroactuators — harvesters of mechanical energy, such as AC-current generating shoes [7]Electrochemical single molecule rectifiers [8] Looking pretty much applied, all this systems, however, rest on interesting physics, contain challenges for theory, themselves emerged as derivatives of knowledge that electrochemistry accumulated by now. [1] J. Edel, A.A. Kornyshev, A. Kucernak, M. Urbakh, “Fundamentals and applications of self-assembled plasmonic nanoparticles at interfaces”, Chemical Soc. Reviews (2016) - DOI: 10.1039/c5cs00576k. [2] J.B. Edel, A.A.Kornyshev, M.Urbakh. “Self-assembly of nanoparticle arrays for use as mirrors, sensors, and antennas”, ACS NANO, 7, 9526-9632 (2013). [3] O. Fajardo, F. Bresme, A.A. Kornyshev, M. Urbakh, Electrotunable lubricity with ionic liquid nanoscale films, Scientific Reports (NPG) 5 # 7698 (2015); “Electrotunable friction with ionic liquid lubricants: how important is the molecular structure of ions?” J.Phys.Chem.Lett. 6, 3998-4004 (2015). [4] M.V.Fedorov, A.A.Kornyshev, Ionic liquids at electrified interfaces, Chem.Rev. 114, 2978−3036 (2014). [5] S. Kondrat, A.A.Kornyshev, Pressing a spring: what does it take to maximize the energy storage in nanoporous supercapacitors? Nanoscael Horizons, B1, 45-52 (2016). [6] A. Lee, R. Colby, A.A. Kornyshev, “Electroactuation with single charge carrier ionomers: the roles of electrostatic pressure and steric strain”, Soft Matter 9,3767-3776 (2013). [7] A.B. Kolomeisky, A.A. Kornyshev, 2016, Current-generating 'double layer shoe' with a porous sole, J. Phys.Cond.Matter 28, ISSN: 0953-8984 (2016). [8] K.C.M. Cheung, X. Chen, T.Albrecht, A. A. Kornyshev, “Principles of a single-molecule rectifier in electrolytic environment”,J.Phys.Chem.C (2016) - DOI: 10.1021/acs.jpcc.5b10320.

Implications of boron doping on electrocatalytic activities of graphyne and graphdiyne families: a first principles study.

Dispersive force corrected density functional theory is used to map the oxygen reduction reaction (ORR) kinetics of six kinds of graphyne (Gy) and graphdiyne (Gdy) systems (namely αGy, βGy, γGy, δGy, 6,6,12Gy, RGy and Gdy) with substitutional boron (B) atom doping. To this end, the most favorable sites for B doping of each structures are determined by comparing their formation energies and then the best configuration for di-oxygen (O2) adsorption is computed by analyzing the corresponding adsorption energies. Even though oxygen adsorption is found to be energetically favorable on all of these and all Gys and Gdy are found to distinctly favor the four electron pathways for ORR, a reaction scheme with monotonically exothermic ΔG is observed only for B doped RGy. Further computations performed by varying electrode potential indicated this monotonically exothermic nature of the ΔG of B doped RGy to persist in the range 0-0.22 V and also indicated the first (H(+) + e) transfer step to be the rate limiting step.

Theory and Simulation of Space Charge Layers in All Solid State Batteries

Due to their good expected safety properties, Li batteries based on solid electrolytes provide an attractive alternative to conventional liquid systems. However, a number of obstacles have yet to be overcome for all-solid-state batteries to be competitive on the market. One of the most severe problems in Lithium all solid state batteries is the as yet rather limited power density, which is mainly ascribed to a strongly increased interfacial resistance. The cause of this resistance is the formation of space-charge layers near the electrodes, which go along with very high electric fields. Understanding in detail the mechanisms and parameters involved in the space-charge-layer formation will be an important step towards improved solid-state systems with more favorable properties. However, although experimental studies on this subject are readily available, a mathematically (and thermodynamically) rigorous description of the space-charge regions in a solid electrolyte seems to be still lacking. In this work, we propose a new mathematical model for ionic transport in a solid electrolyte, which is based on first principles only. To this end, the conservation equations for mass, momentum and energy are augmented by constitutive relations derived from a universal entropy principle and a free-energy functional which describes the properties of the system on the macroscopic scale. It is important to note that boundary-layer like structures are intrinsically present in our model and do not have to be augmented a posteriori as, e.g., in Gouy-Chapman theory for fluids. We provide analytical and numerical solutions for a solid electrolyte in contact with metal electrodes, which we evaluate to study the parametric dependencies of the cation and potential distributions under the influence of varying electrode potentials and varying interface properties. We compare our results both with the predictions of classical Poisson-Nernst-Planck theory and with experimental results [1]. Estimates on the width of the space charge regions are given as well as predictions on the behaviour of this region under different conditions. We observe that there exist some fundamental differences between space-charge layers in fluid and in solid electrolytes. While the space-charge regions are usually symmetric in a fluid, this is not the case in the solid electrolyte, due to an asymmetric distribution of the charge carriers. Furthermore, the space-charge regions tend to be much wider in the solid electrolyte than in a fluid under comparable conditions. Consistent with experiments [2] we find that the space charge regions and thus the distribution of electric fields can be systematically modified by carefully choosing layers of materials with varying dielectric properties. These modifications can be used to decrease considerably interfacial resistances and thus strongly improve the power density of All Solid State Batteries. [1] K. Yamamoto et al., Angew. Chem. Int. Ed. 49 (2010), pp 4414-4417. [2] C. Yada, A. Ohmori, K. Ide, H. Yamasaki, T. Kato, T. Saito, et al., Dielectric modification of 5V-class cathodes for high-voltage all-solid-state lithium batteries, Adv. Energy Mater. 4 (2014) 1–5. doi:10.1002/aenm.201301416.

SERS and in situ SERS spectroscopy of riboflavin adsorbed on silver, gold and copper substrates. Elucidation of variability of surface orientation based on both experimental and theoretical approach

Surface-enhanced Raman scattering and in situ surface-enhanced Raman scattering spectra have been collected to study influences of (i) used metal and (ii) applied electrode potential on orientation of adsorbed riboflavin molecules. Special in situ SERS spectroelectrochemical cell was used to obtain in situ SERS spectra of riboflavin adsorbed on silver, gold and copper nanostructured surfaces. Varying electrode potential was applied in discrete steps forming a cycle from positive values to negative and backward. Observed spectral features in in situ SERS spectra, measured at alternate potentials, have been changing very significantly and the spectra have been compared with SERS spectra of riboflavin measured ex situ. Raman spectra of single riboflavin molecule in the vicinity to metal (Ag, Au and Cu) clusters have been calculated for different mutual positions. The results demonstrate significant changes of bands intensities which can be correlated with experimental spectra measured at different potentials. Thus, the orientation of riboflavin molecules adsorbed on metal surfaces can be elucidated. It is influenced definitely by the value of applied potential. Furthermore, the riboflavin adsorption orientation on the surface depends on the used metal. Adsorption geometries on the copper substrates are more diverse in comparison with the orientations on silver and gold substrates.

Challenges in the first-principles description of reactions in electrocatalysis

Abstract In spite of the strong relevance of reactions in electrocatalysis, in particular for the electrochemical energy conversion and storage, the number of theoretical studies addressing electrocatalytic reactions from first principles is still limited. This is due to the fact that there are two factors adding considerable complexity to the theoretical treatment: the presence of the electrolyte at the electrode surface and varying electrode potentials. Still, there are promising approaches to cope with these problems allowing a realistic theoretical description of reactions in electrocatalysis. It will be demonstrated that ab initio molecular dynamics simulations based on periodic density functional theory calculations can contribute to an understanding of the structures and reactions at water/metal interfaces. In order to model varying electrode potentials, an explicit counter electrode has been implemented in a periodic density functional theory code, and first preliminary results using this implementation will be presented.

Kinetic Model of Hydrogen Evolution at an Array of Au-Supported Catalyst Nanoparticles

A model is presented for studying the electrocatalytic activity at an array of catalyst nanoparticles that is deposited on a catalytically inactive support material. The objective is to rationalize effects on apparent reactivity of sizes and packing densities of nanoparticles. In the current version, the focus of the model is on the contribution of the spillover effect of adsorbed hydrogen to the overall rate of the hydrogen evolution reaction. For nanoparticles of fixed size, the current density per catalyst surface area exhibits a peculiar maximum as a function of the surface particle density; the optimum particle density varies with the rate of spillover of adsorbed hydrogen from particle to support and with adsorbate surface diffusion. For fixed particle density, the current density per catalyst surface area increases strongly with decreasing particle size. The model was used to fit experimental current densities for different catalyst particle coverage at varying electrode potentials. A good agreement between experimental data and calculated results was found. The fits provide key parameters of surface processes on the catalyst/support system.

Potential-Dependent Studies on the Interaction between Phenylalanine-Substituted Bombesin Fragments and Roughened Ag, Au, and Cu Electrode Surfaces

In this work, we report systematic surface-enhanced Raman spectroscopy (SERS) and generalized two-dimensional correlation analysis (G2DCA) studies of the structures of five specifically modified phenylalanine-substituted C-terminal bombesin 6-14 fragments (BN(6-14)). The fragments studied have all been tested as chemotherapeutic agents in cancer therapy, and they form amino acid sequences in bombesin: cyclo[d-Phe(6),His(7),Leu(14)]BN(6-14), [D-Phe(6),Leu-NHEt(13),des-Met(14)]BN(6-14), [D-Phe(6),Leu(13)-((R))-p-Cl-Phe(14)]BN(6-14), [D-Phe(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), and [D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14). We adsorbed these fragments onto roughened Ag, Au, and Cu electrode surfaces, using a potential range from -1.200 to 0.400 V, at physiological pH. We compared the adsorption mechanism of each fragment on these substrates, as well any changes observed with varying electrode potential, to determine the relationship between adsorption strength and geometry of each of the peptides wherever it was possible. For example, we showed that none of these fragments directly interact with the Ag, Au, and Cu surfaces via residues of Phe (phenylalanine) and Trp(8) (L-tryptophane at position 8 of the BN amino acid sequence) or by an amide bond, due to a very small shift in wavenumber of their characteristic vibrations. Specific interactions were recognized from the broadening, wavenumber shift, and increase in intensity of the W18 Trp(8) mode near 759 cm(-1) and decrease in nu(12) vibration frequency of the Phe residue. In general, more intense SERS bands were observed due to the Phe ring, compared with the Trp(8) ring, which suggested a preferential adsorption of phenylalanine over tryptophane. For [D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), the data also suggest some interaction of a D-Tyr(6) residue (D-tyrosine at position 6). Finally, only slight rearrangements of these moieties on the substrates are observed with changes in electrode potential.

Electrogenerative Processes for Environmental Applications

AbstractThe applications of electrogenerative processes which could be an alternative to current environmental treatment technologies are briefly reviewed. This process does not require an external supply of energy due to the spontaneous chemical reaction that takes place in the reactor and generates an external flow of current as a by‐product. The operation is attractive in reducing operational costs, especially when dealing with solutions of low ion concentrations. Electrogenerative reactors which can be operated in various configurations and their designs are described. The rate and selectivity for particular reactions can be controlled by varying electrode potentials and with suitable choices of electrodes. This review paper aims to invoke the interest of industries to consider the merits of electrogenerative processes as a replacement for the current processes and practices. The applications of electrogenerative processes to the removal and recovery of metal ions, electrosynthesis of organic chemicals and leaching of sulfide minerals are discussed.

Raman spectroelectrochemical study of Toluidine Blue, adsorbed and electropolymerized at a gold electrode

The redox dye Toluidine Blue (TB), adsorbed and electropolymerized at a roughened gold electrode, has been studied by Raman spectroscopy at λ ex of 676.4 nm in an electrochemical cell. Most of the spectral bands have been assigned based on density functional theory (DFT) calculations. The number and position of the bands, as well as their intensity depend on electrode potential and solution pH, allowing one to discern different redox forms of TB or its polymer. Raman spectra, obtained in an acidic solution (pH 1.0) at a low electrode potential (−0.2 to 0.0 V vs. Ag/AgCl) are of low intensity, and correspond to the reduced (leucoform) of TB. At a higher potential (0.2–1.0 V), the intensity increases significantly, presumably due to resonance enhancement, and new bands appear, corresponding to oxidized form of TB. In a pH-neutral solution, the changes in spectra with varying electrode potential are less expressed. The observed changes in band positions and intensities have been analyzed.

Raman spectroelectrochemical study of polyaniline and sulfonated polyaniline in solutions of different pH

Raman spectroelectrochemical study has been done with electrochemically prepared films of polyaniline and a copolymer of polyaniline and metanilic acid using a green laser excitation (532 nm). The experimental variables included solutions of different pH between 0.5 and 9.0, and varying electrode potential between 0.0 and 0.8 V versus Ag/AgCl. Raman bands within the wavenumber limits of 500–1700 cm −1 have been analysed, and their changes, proceeding with varying of electrode potential and solution pH, have been interpreted. It has been stressed that the spectral changes of polymer films proceed continuously rather than stepwise by changing the electrode potential. Considering leucoemeraldine and pernigraniline forms of polyaniline as fully reduced and oxidised structures, respectively, it could be concluded that many different redox forms can exist between these two limiting forms, rather than the only possible emeraldine form.

Characterization of the first layer and second layer adsorbates on Au electrodes using ATR-IR spectroscopy

The first and second layer adsorbates on Au thin film electrodes were elucidated with respect to deproptonation of organic monolayers or Cu-upd (under potential deposition) on Au(1 1 1) using attenuated total reflection-infrared spectroscopy. The v(ClO) band from ClO 4 − ions at the second layer, located outside the uracil monolayer, shows invariant frequency, while the ClO 4 − band at the bare Au surface increases with the potential. The Cl − ions coadsorb with sulfate species during the Cu-upd process. In contrast to the XPS data, the surface coverage of the halide ions does not change significantly at the second upd peak. In addition, we found a robust method to improve the adhesion properties of the Au thin films onto a Si prism base, which enables in situ monitoring of 11-mercaptoundecanoic acid-self-assembled monolayer formation and the following deprotonation process with varying electrode potential or pH in electrolyte solutions. Simultaneously, the electrolyte ions and water species in the vicinity of the first layer adsorbates were feasibly characterized without disturbance by bulk solution species.

The electrochemical modulation reflectance spectrum characteristic of MnO 2 film electrodes

The electrochemical modulation reflectance spectra (EMRS) of MnO 2 film electrodes in 1 mol l −1 NH 4Cl solutions were obtained. An electrolytic cell was specially designed to meet both the requirements of the reflectance spectroscopy and the electrochemical measurements. The typical spectra of γ/(β, R*), γ and γ/α-MnO 2 exhibited typical France–Keldysh oscillations. Hence, the energy gap ( E g) value of these samples can be determined by means of the three-point method of Aspnes. The spectra of γ-MnO 2 films with varying electrode potential have also been obtained, by which the flat band potential ( ϕ fb) of γ-MnO 2 can be located. This information plays an important role in studies on the semiconductor behavior of γ-MnO 2 and on the structure of the γ-MnO 2 ∣ electrolyte interface.

An extrapolation method for the fast evaluation of the fluoride ion-selective electrode equilibrium potential in continuous flow analysis

The fluoride ion-selective electrode has been used in continuous flow analysis to determine the fluoride concentration in aqueous solution. The varying electrode potentials, E(t), were collected in the first 60 s by an interfacing computer, and the equilibrium potential of the electrode was predicted using an extrapolation method. This value agrees to within ±0.5 mV of the comparative value obtained by allowing sufficient time for the electrode to reach a steady state. Like flow-injection (FI), continuous flow (CF) analysis with this extrapolation method significantly shortens the time required for routine fluoride concentration measurements, but it also achieves a higher sensitivity than FI. The method has been applied to the determination of fluoride in city tap water.

The participation of surface decomposition intermediates in charge transfer processes at dark p‐type GaP/electrolyte interfaces

AbstractThe competition between the anodic decomposition of the dark p‐GaP electrode in aqueous electrolytes and the stabilization of this electrode by ferrous ions was studied by rotating ring‐disk experiments. The same competition mechanism was found to operate as that, formerly deduced for the illuminated n‐GaP/Fe2+ electrode. This mechanism also explains the observed decrease of the anodic current density with increasing Fe2+ concentration at given electrode potential. Quantitative differences in reactivity between dark p‐type and illuminated n‐type GaP are interpreted on the basis of the difference in band edge position under circumstances of anodic current flow, as deduced from capacitance results. The change in surface charge with varying electrode potential, observed by capacitance measurements, is discussed.