Estimation of the electrochemical active site density of a metal-free carbon-based catalyst using phosphomolybdate (PMo12) as an adsorbate.

A method to estimate the electrochemical active site density (SD) of carbon (C) and nitrogen-doped carbon (N/C-900) using phosphomolybdate (PMo12) as a probe molecule is proposed. The complete coverage of the active sites by the probe molecules is established irrespective of the adsorbate concentration (1, 5, or 10 mM), potential cycling (1 or 10 cycles) and cleaning time (2, 5, or 10 min). A conversion factor derived from a smooth and polished glassy carbon disk of known geometrical area is used to estimate the electrochemical active surface area (ECSA) of the carbon catalyst from the SD. The relatively higher SD values estimated from DC voltammetry than from large-amplitude Fourier-transform alternating-current voltammetry (FTacV) is indicative of the contribution of capacitive charge in the former. Adsorbed probe molecules (PMo12) can readily be desorbed from the catalyst surface by cycling the electrode to lower potentials. The active site density of N/C-900 (∼0.36 × 1019 sites g-1) is higher than that of C (∼0.17 × 1019 sites g-1).

Kinetics of the Oxygen Evolution Reaction (OER) on Amorphous and Crystalline Iridium Oxide Surfaces in Acidic Medium.

Amorphous and crystalline IrO2 catalysts are synthesized by the Adams method and characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The oxygen evolution reaction (OER) is investigated on both the catalyst surfaces in 0.5 M H2SO4 electrolyte. The Tafel slope estimated in the temperature range of 293-333 K on the two surfaces indicates a change in the rate-limiting steps. The data are also analyzed in terms of the Eyring equation to estimate the activation enthalpy (ΔH#) and pre-exponential factor (Af) as a function of overpotential and therefore the charge-transfer coefficient (α). The estimated α values suggest strong electrocatalysis on both the surfaces. While the ΔH# plays a decisive role in the electrocatalysis on the amorphous sample, the trend of Af indicates that an increase in the entropy on the crystalline surface is pivotal in reducing the reaction barrier.

Low-frequency inductive features in the electrochemical impedance spectra of mass-transport limited redox reactions.

Low-frequency (lf) inductive features in the impedance spectra of electron-transfer reactions at the electrode-electrolyte interface in acidic and alkaline media are investigated. The trend in lf equivalent circuit (EC) parameters (inductor (L) and series resistance (R0)) is analysed as a function of overpotential for the two widely investigated electrochemical reactions, viz., methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). The decreasing trend in the lf EC parameter values of the reactions not limited by mass-transport (such as the MOR) and that in the mixed kinetic-diffusion controlled region for mass-transport limited reactions (ORR) are attributed to the faster surface relaxation with applied overpotential, whereas the increase of the L and R0 values in the mass-transport limiting region in an acidic medium is shown to originate from the extra hindrance caused by the limited supply of reactant species. Such analysis of lf EC parameters with overpotential is important to understand the underlying physical processes and to establish equivalent circuit models.

Influence of Nitrogen Doping into Carbon on the Activation Barrier of ORR in Alkaline Medium: An Investigation Based on Eyring Analysis.

The oxygen reduction reaction (ORR) is investigated on metal-free carbon (Vulcan XC-72) and nitrogen-doped (∼≤1%) carbon (N/C-900) in 0.1 M KOH. The product distribution (O2 to OH- and HO2-) as a function of overpotential (η) in the temperature range of 293-323 K is analyzed using a rotating ring-disk electrode (RRDE) assembly. The kinetic current due to reduction of O2 to HO2- is estimated and used in the Eyring analysis to determine the change in enthalpy of activation (ΔH#). It is shown that doping of carbon with nitrogen (even with ≤1 wt %) causes substantial increase in the number of active sites (almost 2-fold) and reduction in ΔH# at any η. Moreover, ΔH# is a stronger function of η on N/C-900 as compared to that on the carbon surface.

Relationship between the electron-transfer coefficients of the oxygen reduction reaction estimated from the Gibbs free energy of activation and the Butler-Volmer equation.

The rate of electron-transfer reactions, irrespective of whether electrochemical or electrocatalytic, is universally explained on the basis of Butler-Volmer (B-V) theory. The charge-transfer coefficient (α) obtained is typically in the range of 0.0-1.0, and is 0.6 ± 0.1 for the oxygen reduction reaction (ORR) on Pt, which is the subject of the present investigation. Alternatively, α can be estimated from the derivative of the change in Gibbs free energy of activation (ΔG#) with respect to the overpotential (η) and has the unreasonably high value of 1.1 ± 0.2. The origin of the difference in the α values obtained from these two methods is investigated. The value of α greater than 1.0 stems from the alternative potential-dependent lower energy barrier path for the formation of the activated complex, offered by the electrified catalyst surface. For the electrocatalytic reaction, the α value derived from the ΔG# is the true kinetic parameter. The theoretical background of such processes is presented to justify our claims.

The impact of overpotential on the enthalpy of activation and pre-exponential factor of electrochemical redox reactions.

The kinetics of the V5+/V4+ redox reaction is investigated in a three-electrode configuration on a Vulcan XC-72 modified glassy carbon rotating disk electrode at four different temperatures (25 to 40 °C, with 5 °C interval). The values of enthalpy of activation (ΔH#) and pre-exponential factor (Af) estimated using the Eyring equation are in the range of 0.25-0.53 eV (24-51 kJ mol-1) and -1.3 to 5, respectively. The Eyring plots tend to diverge with overpotential, causing an increase in the values of the estimated ΔH# and Af. This is perhaps due to the retarding effect of the precipitates/adsorbates on the electrode surface. The investigation of the kinetics suggests that the V5+/V4+ redox reaction is electrocatalysed through an increase in the entropy of activation (ΔS#).

Kinetics of Hydrogen Evolution Reactions in Acidic Media on Pt, Pd, and MoS2.

Hydrogen evolution reaction (HER) are investigated on Pt, Pd, and MoS2 in a 0.5 M H2SO4 electrolyte in a rotating disk electrode (RDE) configuration in the temperature range of 285-335 K. The reaction is temperature-sensitive on all of the three catalyst surfaces at their respective overpotential ranges. The kinetic parameters (activation enthalpy (ΔH#), free energy of activation (ΔG#), and pre-exponential factor (Af)) toward HER are obtained from the Arrhenius and Eyring relations, and the overall kinetics on the catalyst surfaces is analyzed. ΔH# for HER is a strong function of the overpotential in the case of both Pt and Pd. On the other hand, the trend in Af suggests that the electrocatalysis of HER on MoS2 originates from an increase in entropy factor, perhaps due to the solvent-dipole interaction at the interface. Such analysis is pivotal to the investigation of electrocatalysis of HER, especially on surfaces for which determination of active-site density is not established.

What contributes to the internal mass-transport resistance of redox species through porous thin-film electrodes?

Transport of redox species through porous thin-film electrodes is investigated using electrochemical impedance spectroscopy (EIS). Redox species of small size and fast electron-transfer kinetics show two arcs in the EIS pattern: a high frequency arc corresponding to the charge-transfer process (electron-transfer) and a low frequency arc corresponding to the mass-transport process (transport of the redox species from the bulk of the solution to the electrode interface). Often, the features of the Nyquist plot corresponding to the transport of the redox species through the porous electrode and that through the bulk of the electrolyte are not resolved. It is shown that the resolution of such features depends on the (1) composition of the porous thin-film, (2) electron-transfer kinetics, (3) interaction of the redox species with the electrode components, and (4) bulkiness of the redox species and (5) its concentration.

Resolving charge-transfer and mass-transfer processes of VO2+/VO2 + redox species across the electrode/electrolyte interface using electrochemical impedance spectroscopy for vanadium redox flow battery.

Electrochemical impedance spectroscopy is used to investigate the charge-transfer and mass-transfer processes of VO2+/VO2 + (V4+/V5+) redox species across the carbon-modified glassy carbon disk electrode/electrolyte interface. The features of the EIS patterns depend on the potential, concentrations of the redox species and mass-transport conditions at the electrode/electrolyte interface. With the starting electrolyte containing either only V4+ or V5+ redox species, EIS shows a straight line capacitor feature, as no oxidation or reduction reaction take place at the measured open circuit potential (OCP). With the electrolyte containing equimolar concentration of V4+ and V5+, EIS pattern has both charge-transfer and mass-transfer features at the equilibrium potential. The features of the charge-transfer process are observed to be influenced by the mass-transfer process. Optimum concentrations of the V4+/V5+ redox species and supporting H2SO4 electrolyte are required to resolve the EIS features corresponding to the underlying physical processes. The semi-infinite linear diffusion characteristics of the V4+/V5+ redox species observed with a static condition of the electrode converges to that of a finite diffusion under hydrodynamic condition.

Electrocatalysis of Oxygen Reduction Reaction on Shape-Controlled Pt and Pd Nanoparticles-Importance of Surface Cleanliness and Reconstruction.

Shape-controlled precious metal nanoparticles have attracted significant research interest in the recent past due to their fundamental and scientific importance. Because of their crystallographic-orientation-dependent properties, these metal nanoparticles have tremendous implications in electrocatalysis. This review aims to discuss the strategies for synthesis of shape-controlled platinum (Pt) and palladium (Pd) nanoparticles and procedures for the surfactant removal, without compromising their surface structural integrity. In particular, the electrocatalysis of oxygen reduction reaction (ORR) on shape-controlled nanoparticles (Pt and Pd) is discussed and the results are analyzed in the context of that reported with single crystal electrodes. Accepted theories on the stability of precious metal nanoparticle surfaces under electrochemical conditions are revisited. Dissolution, reconstruction, and comprehensive views on the factors that contribute to the loss of electrochemically active surface area (ESA) of nanoparticles leading to an inevitable decrease in ORR activity are presented. The contribution of adsorbed electrolyte anions, in-situ generated adsorbates and contaminants toward the ESA reduction are also discussed. Methods for the revival of activity of surfaces contaminated with adsorbed impurities without perturbing the surface structure and its implications to electrocatalysis are reviewed.

Electrochemical estimation of active site density on a metal-free carbon-based catalyst.

Nitrogen-doped carbon is synthesized by the heat-treatment of carbon in an ammoniacal atmosphere at different temperatures. The active site density and electrochemically active surface area (ESA) of carbon and nitrogen-doped carbon catalysts are estimated from the charge due to oxidation of the adsorbed anthraquinone-2-sulfonate (AQS) probe molecule. In the potential window of interest and over a range of concentrations, there is no unwanted side reaction or polymerization of the probe molecule that interferes with the electrochemical estimation of active site density. Most importantly, the adsorbed AQS can easily be removed from the electrode surface by potential cycling. The ORR activity and active site density of the catalysts derived from AQS-adsorption have similar trends. The active site density and turnover frequency towards ORR estimated using the AQS-adsorption method are in line with those reported in the literature by other methods. On the other hand, the results show that the wetted surface area estimated from the double layer capacitance does not always correlate with catalytic activity.

Electrochemical estimation of the active site density on metal-free nitrogen-doped carbon using catechol as an adsorbate.

Carbon is heat-treated with a nitrogen-containing precursor (ammonia) to obtain nitrogen-doped carbon and the composition is estimated using CHN and XPS analysis. The active site density of the carbon and nitrogen-doped carbon is quantified using 1,2-dihydroxybenzene (catechol) molecules as an adsorbate in phosphate buffer (pH 7) solution. The features of the voltammograms of the catechol-adsorbed high surface area carbon and nitrogen-doped carbon are similar to that of the polished nitrogen-grafted glassy carbon electrode (GCE) reported in the literature. At the same time, the polished GCE does not show any well-defined catechol adsorption features. It is found that the adsorption charge (obtained by integrating the peak area, after subtracting the background) is in the order of N/C 900 > N/C 1000 > N/C 800 > N/C 700 > C. A similar trend is observed in their oxygen reduction reaction (ORR) activity in 0.1 M KOH. Moreover, the turnover frequency (ToF) of the catalysts is calculated and it is comparable to that reported in the literature using other methods for non-precious catalysts. Therefore, the adsorption charge can be correlated with the active site density of the carbon and nitrogen-doped carbon samples.

Reconstruction and dissolution of shape-controlled Pt nanoparticles in acidic electrolytes.

Shape-controlled nanoparticles are of utmost scientific and technological importance because of their facet-dependent physical and chemical properties. Under long-term electrochemical conditions, little is known about the stability and fate of these nanoparticles with selected exposed crystallographic orientations (facets) of high surface energy, while it is generally accepted that the surface area decreases. Therefore, the reconstruction and dissolution of platinum nanocubes (Pt-NCs), platinum cuboctahedral (Pt-CO) and platinum polycrystalline (Pt-PC) nanoparticles are investigated using voltammetry and in situ irreversible adsorption of Bi and Ge; the cleanliness of the Pt nanoparticles and the purity of the electrolyte solution are established with systematic voltammetric analysis in a H2SO4 electrolyte of different concentrations (0.01, 0.05, 0.5 and 1 M). The voltammetric results suggest that the {100} terrace sites undergo reconstruction/dissolution at a much higher rate relative to that of the {111} ordered bi-dimensional terrace sites and the reconstruction leads to the formation of {110}/{100} step sites. Therefore, the stability of the Pt-NCs is lower than that of the Pt-CO nanoparticles. The gradual decrease in the Hupd area on prolonged cycling in the lower potential range (0.06-0.6 and 0.06-0.8 V) is attributed to the accumulation of oxy-anions from the electrolyte on the Pt surface. Moreover, dissolution of highly energetic Pt sites also contributes to the reduction in the Hupd area, unlike that observed with low index Pt single crystal surfaces. On cycling to higher potential limits (1.0 and 1.2 V), the adsorbed anions are replaced with the oxygenated species or oxide; the protective oxide layer helps to stabilize the electrochemical surface area (ESA) of the Pt nanoparticles. With cycling, both Pt-NCs and Pt-CO eventually get converted to Pt-PC. These results are supported with cyclic voltammograms, irreversible adsorption of Bi and Ge, and HR-TEM.

Chloride (Cl(-)) ion-mediated shape control of palladium nanoparticles.

The shape control of Pd nanoparticles is investigated using chloride (Cl(-)) ions as capping agents in an aqueous medium in the temperature range of 60-100 °C. With weakly adsorbing and strongly etching Cl(-) ions, oxygen plays a crucial role in shape control. The experimental factors considered are the concentration of the capping agents, reaction time and reaction atmosphere. Thus, Pd nanoparticles of various shapes with high selectivity can be synthesized. Moreover, the removal of Cl(-) ions from the nanoparticle surface is easier than that of Br(-) ions (moderately adsorbing and etching) and I(-) ions (strongly adsorbing and weakly etching). The cleaned Cl(-) ion-mediated shape-controlled Pd nanoparticles are electrochemically characterized and the order of the half-wave potential of the oxygen reduction reaction in oxygen-saturated 0.1 M HClO4 solution is of the same order as that observed with single-crystal Pd surfaces.

Sodium borohydride treatment: a simple and effective process for the removal of stabilizer and capping agents from shape-controlled palladium nanoparticles.

The inherent property of palladium to form hydride is effectively exploited for the removal of adsorbed stabilizer and capping agents. Formation of hydride on exposure of Pd nanoparticles to sodium-borohydride weakens the metal's interaction with the adsorbed-impurities and thus enables their easy removal without compromising the shape, size and dispersion.

Oxygen reduction reaction and peroxide generation on shape-controlled and polycrystalline platinum nanoparticles in acidic and alkaline electrolytes.

Shape-controlled Pt nanoparticles (cubic, tetrahedral, and cuboctahedral) are synthesized using stabilizers and capping agents. The nanoparticles are cleaned thoroughly and electrochemically characterized in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes, and their features are compared to that of polycrystalline Pt. Even with less than 100% shape-selectivity and with the truncation at the edges and corners as shown by the ex-situ TEM analysis, the voltammetric features of the shape-controlled nanoparticles correlate very well with that of the respective single-crystal surfaces, particularly the voltammograms of shape-controlled nanoparticles of relatively larger size. Shape-controlled nanoparticles of smaller size show somewhat higher contributions from the other orientations as well because of the unavoidable contribution from the truncation at the edges and corners. The Cu stripping voltammograms qualitatively correlate with the TEM analysis and the voltammograms. The fractions of low-index crystallographic orientations are estimated through the irreversible adsorption of Ge and Bi. Pt-nanocubes with dominant {100} facets are the most active toward oxygen reduction reaction (ORR) in strongly adsorbing H2SO4 electrolytes, while Pt-tetrahedral with dominant {111} facets is the most active in 0.1 M HClO4 and 0.1 M NaOH electrolytes. The difference in ORR activity is attributed to both the structure-sensitivity of the catalyst and the inhibiting effect of the anions present in the electrolytes. Moreover, the percentage of peroxide generation is 1.5-5% in weakly adsorbing (0.1 M HClO4) electrolytes and 5-12% in strongly adsorbing (0.5 M H2SO4 and 0.1 M NaOH) electrolytes.