68
A potential at which metal ions are deposited onto an electrode surface is predicted by Nernst equation. However, it is well known that the deposition occurs at a
more positive potential, and the process called underpotential deposition (UPD)
continues until the first monoatomic layer fully covers the surface. The UPD process results from foreign metal–metal substrate interaction, which exceeds that
between foreign metal atoms. On Pt, underpotentially deposited hydrogen atoms
tend to adsorb on hollow sites embedded in the Pt surface’s lattice and on the bridge
sites between two adjacent Pt atoms, whereas the active HOR intermediate adsorbs
at atop sites on Pt atoms, as confirmed by IR and UV-VIS reflectance measurements
[40, 41]. Despite the two independent sites on Pt surface, DFT calculations have
shown that the HOR is significantly weakened by the lateral repulsion of hydrogen
adsorbed at UPD sites [42].
Renewed interest in the HOR/HER reactions has been observed in recent years
due to the increasing need for renewable energy technologies as the reactions offer
technical routes to cut down carbon emissions and lower fossil fuel use. Hydrogen
can be used as a fuel in proton exchange membrane fuel cells (PEMFCs), converting
chemical energy stored in molecular hydrogen into electricity through the HOR at
the anode, while the process at the cathode involves oxygen reduction from molecular oxygen in the atmosphere.
Exchange current plot as a function of the free energy for hydrogen adsorption
on different metals shows a volcano curve, explaining the significant impact of Pt
atop sites for the HOR with respect to other metals that adsorb hydrogen either too
strongly or too weakly [43]. The kinetics of the HOR on Pt in acidic media are
extremely fast as its overpotential is negligible, but in alkaline medium it is several
orders of magnitude slower [44]. Nevertheless, at other non-Pt catalyst candidates it
is often even slower [45].
6.3.1 CO Tolerance
H 2 gas for PEMFCs is commonly produced by reforming methanol or hydrocarbons
and inevitably contains small amounts of carbon monoxide. CO level as low as
5 ppm in H 2 can cause significant degradation of the HOR activity because CO
molecules adsorb strongly on the active Pt sites, thus blocking them for the
HOR. Therefore, CO tolerance is of importance for anode catalysts in H 2 -feed fuel
cells, and efforts have been made to improve it by adding a different element.
Among various anode catalysts for the HOR, Pt-Ru alloys are generally considered
as the best candidates due to high CO tolerance and acceptable durability under fuel
cell operating conditions.
A new approach for designing and synthesizing anode electrocatalysts with dramatically reduced Pt content was introduced recently, based on spontaneous deposition of a submonolayer of Pt on carbon-supported Ru nanoparticles [46, 47]. Despite
several times lower Pt loading with respect to commercial Pt 2 Ru 3 alloy, the catalyst
has enhanced activity and stability as well as CO tolerance. Pt-Sn catalysts have
6 Important Electrocatalytic Reactions
A potential at which metal ions are deposited onto an electrode surface is predicted by Nernst equation. However, it is well known that the deposition occurs at a
more positive potential, and the process called underpotential deposition (UPD)
continues until the first monoatomic layer fully covers the surface. The UPD process results from foreign metal–metal substrate interaction, which exceeds that
between foreign metal atoms. On Pt, underpotentially deposited hydrogen atoms
tend to adsorb on hollow sites embedded in the Pt surface’s lattice and on the bridge
sites between two adjacent Pt atoms, whereas the active HOR intermediate adsorbs
at atop sites on Pt atoms, as confirmed by IR and UV-VIS reflectance measurements
[40, 41]. Despite the two independent sites on Pt surface, DFT calculations have
shown that the HOR is significantly weakened by the lateral repulsion of hydrogen
adsorbed at UPD sites [42].
Renewed interest in the HOR/HER reactions has been observed in recent years
due to the increasing need for renewable energy technologies as the reactions offer
technical routes to cut down carbon emissions and lower fossil fuel use. Hydrogen
can be used as a fuel in proton exchange membrane fuel cells (PEMFCs), converting
chemical energy stored in molecular hydrogen into electricity through the HOR at
the anode, while the process at the cathode involves oxygen reduction from molecular oxygen in the atmosphere.
Exchange current plot as a function of the free energy for hydrogen adsorption
on different metals shows a volcano curve, explaining the significant impact of Pt
atop sites for the HOR with respect to other metals that adsorb hydrogen either too
strongly or too weakly [43]. The kinetics of the HOR on Pt in acidic media are
extremely fast as its overpotential is negligible, but in alkaline medium it is several
orders of magnitude slower [44]. Nevertheless, at other non-Pt catalyst candidates it
is often even slower [45].
6.3.1 CO Tolerance
H 2 gas for PEMFCs is commonly produced by reforming methanol or hydrocarbons
and inevitably contains small amounts of carbon monoxide. CO level as low as
5 ppm in H 2 can cause significant degradation of the HOR activity because CO
molecules adsorb strongly on the active Pt sites, thus blocking them for the
HOR. Therefore, CO tolerance is of importance for anode catalysts in H 2 -feed fuel
cells, and efforts have been made to improve it by adding a different element.
Among various anode catalysts for the HOR, Pt-Ru alloys are generally considered
as the best candidates due to high CO tolerance and acceptable durability under fuel
cell operating conditions.
A new approach for designing and synthesizing anode electrocatalysts with dramatically reduced Pt content was introduced recently, based on spontaneous deposition of a submonolayer of Pt on carbon-supported Ru nanoparticles [46, 47]. Despite
several times lower Pt loading with respect to commercial Pt 2 Ru 3 alloy, the catalyst
has enhanced activity and stability as well as CO tolerance. Pt-Sn catalysts have
6 Important Electrocatalytic Reactions