75
cells like methanol [81]. Although the intrinsic energy content of formic acid is
lower than that of methanol (two electrons per molecule of formic acid versus six
electrons per molecule of methanol), formic acid can be used in high concentration
because of its low crossover rate [82]. This is due to the repulsion between HCOO
−
in formic acid and the sulfuric group in the surface of Nafion membrane [83].
Formic acid is a liquid fuel at room temperature and a strong electrolyte. It can
maintain the performance from a broad range of fuel concentration unlike ethanol
[83]. Theoretically, formic acid–oxygen fuel cell will produce electromotive force
of 1.45 V. This is higher compared to DMFC (1.18 V) and H 2 –O 2 PEMFC
(1.23 V) [84].
The proposed oxidation of formic acid can take the following possible pathways [85]:
HCOOH ad
COOH ad H e
CO
H
e
( )→
( )+ + →
+
+
+
−
+
−
2
2
2
(6.21)
HCOOH ad
HCOO ad H e
CO
H
e
( )→
( )+ + →
+
+
+
−
+
−
2
2
2
(6.22)
HCOOH ad
CO ad H O CO
H
e
( )→ ( )+
→
+
+
+
−
2
2
2
2
(6.23)
Following the adsorption of FA on the surface, the HCOOH(ad) can undergo one
of the two direct pathways, (6.21) and (6.22), that differ only on the position of the
first hydrogen that detaches. In the indirect pathway (6.23), the first non-Faradaic
step produces adsorbed CO, which is further oxidized to CO 2 through the reaction
with water. However, recent interest in FA electrooxidation have shown that the
reaction pathway may lead through a series of reactions involving CH(OH) 2 (ad)
species produced by interaction with adsorbed H
+
, that leads to CHO(ad) and further to CO(ad) [86].
Thus far, platinum and palladium were found to be the most active for the FA
oxidation reaction. However, while the initial activity of Pd and Pd-based catalysts
is high, palladium suffers from dissolution in acidic solutions during anodic oxidation of formic acid and its vulnerability to the reaction intermediates. The high
activity is ascribed to the high tolerance of Pd to CO contamination, as inferred
from IR spectroscopy [86]. Additionally, a Pd thin film deposited on Au shows a
high CO-poisoning tolerance and an improved activity in the low oxidation potential region.
On the other hand, Pt and Pt-based catalysts are less susceptible to corrosion but
suffer from the high cost of Pt [87, 88]. FA oxidation on three low-index Pt surfaces
is shown in Fig. 6.5. Two peaks are observed in the anodic scan on Pt(100) and
Pt(110), around 0.5 and 1.0 V, while the pronounced peak between 0.4 and 0.6 V is
observed in the subsequent cathodic scan. The first peak in anodic scan is usually
ascribed to the direct pathway. It is small because of the accumulation of “poisoning” species on Pt surface, that is species that undergo a chemical reaction but cannot be removed from the surface, thus blocking the Pt sites for further oxidation of
FA. DFT calculations carried at various positive potentials have shown that the first
6.6 Formic Acid Oxidation Reaction
cells like methanol [81]. Although the intrinsic energy content of formic acid is
lower than that of methanol (two electrons per molecule of formic acid versus six
electrons per molecule of methanol), formic acid can be used in high concentration
because of its low crossover rate [82]. This is due to the repulsion between HCOO
−
in formic acid and the sulfuric group in the surface of Nafion membrane [83].
Formic acid is a liquid fuel at room temperature and a strong electrolyte. It can
maintain the performance from a broad range of fuel concentration unlike ethanol
[83]. Theoretically, formic acid–oxygen fuel cell will produce electromotive force
of 1.45 V. This is higher compared to DMFC (1.18 V) and H 2 –O 2 PEMFC
(1.23 V) [84].
The proposed oxidation of formic acid can take the following possible pathways [85]:
HCOOH ad
COOH ad H e
CO
H
e
( )→
( )+ + →
+
+
+
−
+
−
2
2
2
(6.21)
HCOOH ad
HCOO ad H e
CO
H
e
( )→
( )+ + →
+
+
+
−
+
−
2
2
2
(6.22)
HCOOH ad
CO ad H O CO
H
e
( )→ ( )+
→
+
+
+
−
2
2
2
2
(6.23)
Following the adsorption of FA on the surface, the HCOOH(ad) can undergo one
of the two direct pathways, (6.21) and (6.22), that differ only on the position of the
first hydrogen that detaches. In the indirect pathway (6.23), the first non-Faradaic
step produces adsorbed CO, which is further oxidized to CO 2 through the reaction
with water. However, recent interest in FA electrooxidation have shown that the
reaction pathway may lead through a series of reactions involving CH(OH) 2 (ad)
species produced by interaction with adsorbed H
+
, that leads to CHO(ad) and further to CO(ad) [86].
Thus far, platinum and palladium were found to be the most active for the FA
oxidation reaction. However, while the initial activity of Pd and Pd-based catalysts
is high, palladium suffers from dissolution in acidic solutions during anodic oxidation of formic acid and its vulnerability to the reaction intermediates. The high
activity is ascribed to the high tolerance of Pd to CO contamination, as inferred
from IR spectroscopy [86]. Additionally, a Pd thin film deposited on Au shows a
high CO-poisoning tolerance and an improved activity in the low oxidation potential region.
On the other hand, Pt and Pt-based catalysts are less susceptible to corrosion but
suffer from the high cost of Pt [87, 88]. FA oxidation on three low-index Pt surfaces
is shown in Fig. 6.5. Two peaks are observed in the anodic scan on Pt(100) and
Pt(110), around 0.5 and 1.0 V, while the pronounced peak between 0.4 and 0.6 V is
observed in the subsequent cathodic scan. The first peak in anodic scan is usually
ascribed to the direct pathway. It is small because of the accumulation of “poisoning” species on Pt surface, that is species that undergo a chemical reaction but cannot be removed from the surface, thus blocking the Pt sites for further oxidation of
FA. DFT calculations carried at various positive potentials have shown that the first
6.6 Formic Acid Oxidation Reaction