Significance of the apparent limit of anodic oxide film formation at Pt: saturation coverage by the quasi two-dimensional state

Structural Evolution of Pt, Au and Cu Anodes by Electrolysis up to Contact Glow Discharge Electrolysis in Alkaline Electrolytes*

Applying a voltage to metal electrodes in contact with aqueous electrolytes results in the electrolysis of water at voltages above the decomposition voltage and plasma formation in the electrolyte at much higher voltages referred to as contact glow discharge electrolysis (CGDE). While several studies explore parameters that lead to changes in the I-U characteristics in this voltage range, little is known about the evolution of the structural properties of the electrodes. Here we study this aspect on materials essential to electrocatalysis, namely Pt, Au, and Cu. The stationary I-U characteristics are almost identical for all electrodes. Detailed structural characterization by optical microscopy, scanning electron microscopy, and electrochemical approaches reveal that Pt is stable during electrolysis and CGDE, while Au and Cu exhibit a voltage-dependent oxide formation. More importantly, oxides are reduced when the Au and Cu electrodes are kept in the electrolysis solution after electrolysis. We suspect that H2 O2 (formed during electrolysis) is responsible for the oxide reduction. The reduced oxides (which are also accessible via electrochemical reduction) form a porous film, representing a possible new class of materials in energy storage and conversion studies.

Study of the formation and quick growth of thick oxide films using platinum nanoelectrodes as a model electrocatalyst

We report a study of the formation and quick growth of thick films of platinum oxide on platinum nanoelectrodes at low anodic potentials. Here, structurally well-defined platinum nanoelectrodes are used as a model platform for nanoscale platinum electrocatalysts. Platinum films are formed on the surface of the nanoelectrode upon application of a constant anodic potential in an acidic environment for an extended time period. A current spike is initially observed, which is attributed to capacitance charging, the oxidation of water, and the initial oxidation of the platinum surface. A finite residual current follows the initial current spike, which is composed of both water oxidation and the oxidation of platinum metal concealed beneath the growing oxide layer. These films are observed to be structurally irreversible, grow to be relatively thick, and protrude out of the glass insulating material encasing the nanoelectrode due to the added volume of the oxygen incorporated into the growing platinum oxide film. Once reduced, the platinum metal remains protruding out of the glass, and its presence is confirmed by both SEM imaging and cyclic voltammetry. Steady-state voltammetric data shows a finite increase in the diffusion-limited faradaic current of the nanoelectrode, relative to the initial steady-state current, after the oxidation/reduction of the platinum which is due to an increased area of the protruding platinum metal. A minimum apparent rate of ∼1.2 nm/min can be calculated for the growth of the platinum oxide film. The use of platinum nanoelectrodes has shown several distinct advantages in this study, including better control of the size and morphology of the individual electrocatalysts, the ability to image using electron microscopy, and the ability to use voltammetry to evaluate the geometry of the electrode quickly.

In-situ X-ray absorption spectroscopy study of Pt and Ru chemistry during methanol electrooxidation

Methanol electrooxidation in a 0.5 M sulfuric acid electrolyte containing 1.0 M CH3OH was studied on 30% Pt/carbon and 30% PtRu/carbon (Pt/Ru = 1:1) catalysts using X-ray absorption spectroscopy (XAS). Absorption by Pt and Ru was measured at constant photon energy in the near edge region during linear potential sweeps of 10-50 mV/s between 0.01 and 1.36 V vs rhe. The absorption results were used to follow Pt and Ru oxidation and reduction under transient conditions as well as to monitor Ru dissolution. Both catalysts exhibited higher activity for methanol oxidation at high potential following multiple potential cycles. Correlation of XAS data with the potential sweeps indicates that Pt catalysts lose activity at high potentials due to Pt oxidation. The addition of Ru to Pt accelerates the rate of methanol oxidation at all potentials. Ru is more readily oxidized than Pt, but unlike Pt, its oxidation does not result in a decrease in catalytic activity. PtRu/carbon catalysts underwent significant changes during potential cycling due to Ru loss. Similar current density vs potential results were obtained using the same PtRu/carbon catalyst at the same loading in a membrane electrode assembly half cell with only a Nafion (DuPont) solid electrolyte. The results are interpreted in terms of a bifunctional catalyst mechanism in which Pt surface sites serve to chemisorb and dissociate methanol to protons and carbon monoxide, while Ru surface sites activate water and accelerate the oxidation of the chemisorbed CO intermediate. PtRu/carbon catalysts maintain their activity at very high potentials, which is attributed to the ability of the added Ru to keep Pt present in a reduced state, a necessary requirement for methanol chemisorption and dissociation.