Solvent-Dependent Adsorption of Perfluorosulfonated Ionomers on a Pt(111) Surface Using Atomic Force Microscopy

The adsorption behavior of perfluorosulfonated ionomers (PFSIs) on a Pt(111) surface in various solvents is investigated by in situ atomic force microscopy (AFM) and discussed on the basis of aggregation of PFSIs in the liquid phase. The AFM images show that, in an aqueous solution of PFSI (0.1 wt % Nafion + 99.9 wt % water), PFSI aggregates with a lateral size of 20-200 nm adsorb on the Pt(111) surface. In a PFSI solution containing a small amount of 1-propanol (0.1 wt % Nafion + 99.5 wt % water + 0.4 wt % 1-propanol), however, slightly smaller aggregates adsorb on the Pt(111) surface. Such solvent-dependent sizes of adsorbed aggregates are in reasonable agreement with apparent hydrodynamic radii of PFSIs in the corresponding solutions determined by dynamic light scattering (DLS) while assuming the formation of spherical aggregation. Interestingly, a step-terrace structure characteristic to a clean Pt(111) surface is observed in a propanol-rich PFSI solution (0.1 wt % Nafion + 44.45 wt % water + 55.45 wt % 1-propanol) but X-ray photoelectron spectroscopy clearly indicates the existence of fluorocarbon species at the Pt(111) surface, suggesting the formation of a smooth adsorbed layer of PFSIs in a lying down configuration. Absence of any features assignable to aggregates in DLS data suggests well-dispersion of PFSIs in such propanol-rich solution without aggregations. Thus, the adsorbed structure of PFSIs at Pt surfaces can be controlled by tuning the composition of mixed solvent, which affects the aggregation of PFSI in the liquid phase.

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

An electrochemical method to estimate the active site density of metal-free electrocatalysts using catechol adsorption.

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

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

The role of surface oxygenated-species and adsorbed hydrogen in the oxygen reduction reaction (ORR) mechanism and product selectivity on Pd-based catalysts in acid media

Surface adsorbed species can significantly alter the catalytic activity and product selectivity.

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.

Reduction of graphene oxide – a comprehensive electrochemical investigation in alkaline and acidic electrolytes

Electrochemical characterization to investigate the extent of reduction and the nature of reminiscent oxygen moieties in GO-based materials.