Formation of Mercury Adatom Layer by Simple Dipping Method for the Oxygen Reduction and Its Application to an Al-air Fuel Cell

Abstract

Intensity (CPS)Figure 1. Au4f and Hg4f peaks by X-ray photoelectron spectroscopy for Hg adatom layers prepared by dipping (solid) and underpotential deposition (dashed) methods.The electrocatalytic reduction of oxygen is of prime importance in view of its practical applications for developing cathodes in fuel cells and metal-air batteries. [1][2][3][4] Since the cathode performance is directly affected by the oxygen reduction overpotential, a lot of effort has been poured on the development of better electrocatalysts having less potential loss. Reaction mechanisms and rates vary with electrode materials, 5,6 crystallographic orientations, 7,8 and reaction environments. 9 In aqueous solution, oxygen reduction usually proceeds via consecutive two-electron processes, being reduced to hydrogen peroxide and then to water. There is no doubt that the best way is to reduce oxygen directly to water. This four-electron process, however, is not easy to achieve. Despite the fact that many ingenious methods and new materials have been devised, the search for the four-electron reduction electrocatalysts is still under way. They include various materials such as carbon-supported platinum, 10 metal macrocycles, 11,12 and 1,4,5,8-naphthalenetetronemodified gold.13 Biological systems such as laccase 14 and cytochrome c oxidase 15 have also been investigated. Recently Matsumoto and coworkers 16,17 reported that mercury adatom-modified gold electrodes prepared by underpotential deposition (upd) showed catalytic activities toward both oxygen and hydrogen peroxide reduction. Oxygen reduction, notably enough, proceed by four-electron reduction to water on the Au/Hg electrodes, while two-electron process has been observed both on Au and Hg electrodes. 18In this work, we report a simple and reproducible way to prepare mercury-deposited gold electrodes by simply dipping Au in a mercury ion-containing solution. Thus prepared electrodes were used as a cathode in an alkaline-air fuel cell. Fig. 1 shows XPS spectra for Hg-adlayers formed on the Au surface by dipping and UPD. Two peaks at 84.6 eV (or 84.5 eV) and 88.3 eV (or 88.2 eV) correspond to Au4f 7/2 and Au4f 5/2 , respectively, and two peaks at 100.5 eV (or 100.4 eV) and 104.5 eV (or 104.4 eV) to Hg4f7/2 and Hg4f5/2, respectively. 19The essentially same Hg peak positions within experimental error indicate that the oxidation state of Hg in both electrodes is identical. Metallic mercury gives a binding energy of 4f7/2 at 99.9 eV.19 Upon oxidation to Hg 2+ it increases to 100.5 eV with 104.6 eV for 4f 5/2 . 19,20 Our XPS results indicate that mercury in both electrodes exists in a divalent state. Hg 2+ ions are adsorbed on the Au surface by dipping. The possible reason why † This paper is dedicated to Professor Hasuck Kim for his outstanding contribution to electrochemistry and analytical chemistry.Hg is still in +2 state for Au-Hg(upd) is that Hg is oxidized by oxygen in air during sample transfer.Panel A of Fig. 2 is the linear sweep voltammetry (LSV) of oxygen reduction at a Au-Hg(di...