The interaction of electropolymerized Co(III)TAPP (polyCoTAPP) films with adsorbed resorcinol on glassy carbon (GC) and with surface quinone functionalities on aerogel carbon (AEC) were studied using reflection UV−visible spectroscopy and X-ray photoelectron spectroscopy. A red shift of the Soret band and the appearance of new Q bands appearing after adsorption of resorcinol on a GC/polyCoTAPP film was interpreted as being due to change of the metalloporphyrin electronic structure. The photoelectron depth profiles for an AEC/polyCoTAPP film showed that the cobalt ion is mostly in the Co(III) state at the outer layers of the film, while the amount of cobalt ion in the formal +2 state gradually increases in the inner film layers. This seems to indicate the formation of charge-transfer complexes between the metalloporphyrin and reduced quinone functionalities on the AEC surface. Understanding the nature of metalloporphyrin/porous carbon structures is an important step toward the design of reliable and low-cost non-noble-metal oxygen reduction catalytic electrodes and their application in fuel cells and batteries.
Reduced p-, m-, and o-benzoquinones: hydroquinone (HQ), resorcinol (Res), and catechol (Cat), undergo irreversible monolayer adsorption in aerogel carbon (AEC) electrodes with rates of
1.7×10−4
,
7.1×10−5
, and
1.4×10−4normals−1
for HQ, Res, and Cat, respectively. The adsorbed species showed electrochemical quasi-reversible behavior in 1 M
normalH2SO4
with half-wave potentials
(E1/2)
of
+0.45
,
+0.31
, and
+0.58V
vs Ag/AgCl/KCl (saturated) for AEC/HQ, AEC/Res, and AEC/Cat, respectively. Upon adsorption of Co(III) tetra(p-sulfonatophenyl)porphyrin in these electrodes, a single reduction wave was observed and its
E1/2(∼+0.45V)
was independent of the nature of the adsorbed reduced quinone. This was attributed to a metalloporphyrin/quinone complex, which formed and stabilized at the electrode surface. This species, after being reduced, reacted with oxygen with a rate of
1.8×105normalM−1normals−1
. Mediation of oxygen reduction by these systems occurred at a relatively high potential
(∼+0.5V)
almost completely via a four-electron-transfer process.
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