Oxygen-Containing Nanoclusters on the Surface of Pt-Electrodes and Oxygen Reduction Reaction in Alkaline Medium

Aleksey M Trunov

doi:10.1155/2018/7164578

Abstract

Analysis of the role of oxygen-containing nanoclusters in oxygen reduction reaction (ORR) on Pt-electrodes in alkaline media is provided on the basis of the concept of electrochemical processes with slowed stage of consecutive heterogeneous chemical reaction (ConHCR). Under the ConHCR concept, the main factor determining the ORR characteristics is energetic inhomogeneity of electrode surface (EIES) according to Temkin. A new concept, according to which EIES is determined by the Gibbs energy of formation of oxygen-containing surface structures with inclusions of surface defects of the platinum crystal structure, Pts,d, is formulated. A correlation between the level of EIES of Pt-electrodes and packing density of Pts,d atoms on the surface of Pt(hkl) monocrystals is determined. The concept, according to which the stationary potential of ORR process is considered as a “mixed potential” of two reactions (electrochemical reduction of surface atom PtIIs,d and consecutive oxidation of PtIs,d by molecular oxygen), is substantiated. It is proposed that the formation of surface nanocluster transition state [⁎(OO)PtIIs,d(OH)] defines the rate of the entire ORR process on Pt-electrodes in alkaline media.

Highlights

Oxygen reduction reaction (ORR) is used in many electrochemical energy transformation devices
ORR process is experimentally studied in detail and, as reflected in a number of classical reviews [1,2,3,4], there is an experimental determination that platinum and platinum based alloys are characterized by the highest electrochemical activity
A change of reaction paths correlates with change of state of monocrystal surface: introduction of Br− ions result in disappearance from cyclic voltammograms of peaks corresponding to formation of oxygen-containing structures

Summary

Introduction

Oxygen reduction reaction (ORR) is used in many electrochemical energy transformation devices (e.g., in lowtemperature fuel cells). (3) Nearly identical values of parameters s,dΔGa∗ and s,dΔGk∗ for cases of Pt(111) and Pt(100) monocrystals reflect reversibility of reaction (7) and, correspondingly, the equality of potentials of peaks I on anodic and cathodic CVA branches (see Table 2). Note that values of s,dΔGa∗ parameter correspond to potentials of anodic peaks on smooth Pt-electrodes at 0.7 and 1.1 V (see Table 2). The author considered formation of surface oxygen-containing structure s,dPtI(OH) according to reaction (7) as the most probable pathway explaining electrochemical processes on Pt-electrodes in absence of molecular oxygen. A change of reaction paths correlates with change of state of monocrystal surface: introduction of Br− ions result in disappearance from cyclic voltammograms of peaks corresponding to formation of oxygen-containing structures (see Table 1). The study [19] reports that there is no influence of any type of anion on state of Pt-electrode surface in alkaline media

Discussion of ORR Redox Cycle on Pt-Electrodes

Conclusion