76
step in both the direct formate pathway (6.22) and the indirect pathway (6.23) are
favorable at all potentials due to negligible activation barrier. However, the second
step involving the removal of HCOO(ad) and CO(ad) in the two pathways are unfavorable at E < 0.5 V because of the large activation barrier. The oxidation of CO(ad)
in the indirect pathway (6.23) becomes progressively more significant at higher
electrode potentials as the activation barrier of the second step reduces producing
the second anodic peak [89]. In the cathodic scan, the strong peak on Pt(100) and
Pt(110) is more intense than either anodic peak because of the removal of the poisoning intermediates at high potentials. Cathodic and anodic scans retrace themselves in the case of Pt(111), suggesting considerable lower amount of strongly
bound intermediates [91].
Because the oxidation of FA at Pt is significant only at positive potentials beyond
that of technological interest (0.6 V vs. RHE), the research in the direct formic acid
fuel cells (DFAFC) has been focused onto addition of a co- catalyst that can lower
the oxidation onset. Adzic group has shown that underpotentially deposited foreign
metal (e.g. Pb, Bi) can be a significant promoter of the FA oxidation on both polyand single-crystal electrode surfaces [90, 91]. Further work has shown that cubic
Mn-Pt alloy nanocrystals and Pt-Ru alloy surface can effectively reduce the poisoning effect, although the peak current still occurs at high oxidation potentials [92,
93]. It was shown that alloying Pt with Pb can significantly promote the FA oxidation due to the effect of Pb atoms, which weaken the adsorption strength of CO, and
consequently its easier removal, so that the Pt 3 Pb alloy catalyst at a typical working
voltage for fuel cells shows 33 times greater activity than that of pure Pt [94].
Interestingly, the Pt monolayer shell over Pt 3 Pb core further improves the activity,
which was primarily ascribed to the suppression of formate pathway and thus avoidance of stable formate intermediates [95]. This example shows that less expensive
Pt-based catalysts could be generated by covering a nonnoble metal core with Pt
monolayer shell, where Pt shell works as the active catalyst that simultaneously
protects the nonnoble metal in the core. It appears that catalysts of nonnoble metals
only would be the ultimate direction for fuel cells in general, including DFAFCs
(Fig. 6.5).
6.7 Oxygen Evolution Reaction
The oxygen evolution reaction (OER) is a fundamental process in many electrochemical technologies, including fuel cells, electrolyzers, and water-splitting
devices, as well as in natural photosynthesis in plants and green algae. Despite
numerous investigations, the exact mechanism of the OER is still lacking. It is generally accepted that the reaction has four elementary steps in acidic solutions:
H O e H OH ads
2
→ +
+
−
+
(6.24)
6 Important Electrocatalytic Reactions
step in both the direct formate pathway (6.22) and the indirect pathway (6.23) are
favorable at all potentials due to negligible activation barrier. However, the second
step involving the removal of HCOO(ad) and CO(ad) in the two pathways are unfavorable at E < 0.5 V because of the large activation barrier. The oxidation of CO(ad)
in the indirect pathway (6.23) becomes progressively more significant at higher
electrode potentials as the activation barrier of the second step reduces producing
the second anodic peak [89]. In the cathodic scan, the strong peak on Pt(100) and
Pt(110) is more intense than either anodic peak because of the removal of the poisoning intermediates at high potentials. Cathodic and anodic scans retrace themselves in the case of Pt(111), suggesting considerable lower amount of strongly
bound intermediates [91].
Because the oxidation of FA at Pt is significant only at positive potentials beyond
that of technological interest (0.6 V vs. RHE), the research in the direct formic acid
fuel cells (DFAFC) has been focused onto addition of a co- catalyst that can lower
the oxidation onset. Adzic group has shown that underpotentially deposited foreign
metal (e.g. Pb, Bi) can be a significant promoter of the FA oxidation on both polyand single-crystal electrode surfaces [90, 91]. Further work has shown that cubic
Mn-Pt alloy nanocrystals and Pt-Ru alloy surface can effectively reduce the poisoning effect, although the peak current still occurs at high oxidation potentials [92,
93]. It was shown that alloying Pt with Pb can significantly promote the FA oxidation due to the effect of Pb atoms, which weaken the adsorption strength of CO, and
consequently its easier removal, so that the Pt 3 Pb alloy catalyst at a typical working
voltage for fuel cells shows 33 times greater activity than that of pure Pt [94].
Interestingly, the Pt monolayer shell over Pt 3 Pb core further improves the activity,
which was primarily ascribed to the suppression of formate pathway and thus avoidance of stable formate intermediates [95]. This example shows that less expensive
Pt-based catalysts could be generated by covering a nonnoble metal core with Pt
monolayer shell, where Pt shell works as the active catalyst that simultaneously
protects the nonnoble metal in the core. It appears that catalysts of nonnoble metals
only would be the ultimate direction for fuel cells in general, including DFAFCs
(Fig. 6.5).
6.7 Oxygen Evolution Reaction
The oxygen evolution reaction (OER) is a fundamental process in many electrochemical technologies, including fuel cells, electrolyzers, and water-splitting
devices, as well as in natural photosynthesis in plants and green algae. Despite
numerous investigations, the exact mechanism of the OER is still lacking. It is generally accepted that the reaction has four elementary steps in acidic solutions:
H O e H OH ads
2
→ +
+
−
+
(6.24)
6 Important Electrocatalytic Reactions