Electrocatalysis at Platinum and Bimetallic Alloys - Fuel Cell Catalysis: A Surface Science Approach - page 314

10.2 ANODE CATALYSTS FOR CO-TOLERANT HOR

When pure hydrogen is used as the fuel, the overpotential for the hydrogen oxidation
reaction (HOR) at the Pt anode is negligibly small.

H 2 ! 2H þ þ 2e (10 : 1)

However, the Pt anode is seriously poisoned by trace amounts of carbon monoxide in
reformates (fuel gas reformed from hydrocarbon), because CO molecules strongly
adsorb on the active sites and block the HOR [Lemons, 1990; Igarashi et al., 1993].
Therefore, extensive efforts have been made to develop CO-tolerant anode catalysts
and cell operating strategies to suppress CO poisoning, such as anode air-bleeding
or pulsed discharging.
To evaluate the catalytic activity or to investigate the reaction mechanism, planar
electrodes with well-defined characteristics such as surface area, surface and bulk
compositions, and crystalline structure have often been examined in acidic electrolyte
solutions. An appreciable improvement in CO tolerance has been found at Pt with
adatoms such as Ru, Sn, and As [Watanabe and Motoo, 1975a, 1976; Motoo and
Watanabe, 1980; Motoo et al., 1980; Watanabe et al., 1985], Pt-based alloys Pt-M
(M ¼ Ru, Rh, Os, Sn, etc.) [Ross et al., 1975a, b; Gasteiger et al., 1994, 1995;
Grgur et al., 1997; Ley et al., 1997; Mukerjee et al., 2004], and Pt with oxides
(RuO x H y ) [Gonzalez and Ticianelli, 2005; Sugimoto et al., 2006].
We have found new CO-tolerant catalysts by alloying Pt with a second, nonpre-
cious, metal (Pt-Fe, Pt-Co, Pt-Ni, etc.) [Fujino, 1996; Watanabe et al., 1999;
Igarashi et al., 2001]. In this section, we demonstrate the properties of these new
alloy catalysts together with Pt-Ru alloy, based on voltammetric measurements, elec-
trochemical quartz crystal microbalance (EQCM), electrochemical scanning tunneling
microscopy (EC-STM), in situ Fourier transform infrared (FTIR) spectroscopy, and
X-ray photoelectron spectroscopy (XPS).

10.2.1 Screening Tests of Sputtered Pt Alloys
by Rotating Disk Electrode

We prepared thin film Pt alloy electrodes by Ar-sputtering Pt and the second metal
targets simultaneously onto a disk substrate at room temperature (thickness
approximately 200 nm). The resulting alloy composition was determined by gravime-
try and X-ray fluorescent analysis (EDX). Grazing incidence (q¼ 18) X-ray
diffraction patterns of these alloys indicated the formation of a solid solution with a
face-centered cubic (fcc) crystal structure.
A standard rotating disk electrode (RDE) setup with a gas-tight Pyrex cell was used
for the experiment on CO adsorption and the HOR. A Pt wire was used as counter-
electrode. A reversible hydrogen electrode, RHE(t), kept at the same temperature as
that of the cell (t,in8C), was used as the reference. All the electrode potentials in
this chapter will be referenced to RHE(t). The electrolyte solution of 0.1 M HClO 4