1 3
Topics in Current Chemistry (2018) 376:42
plate with the shape of “wheat ears”, leaving a gap between the “wheat” and “ear”,
and the distance between the individual nanowires is also far away from each other.
Electrochemical tests indicate the burr-like nickel nanowire catalyst can electro-catalyze urea oxidation at a lower OOP of 0.25 V vs. Ag/AgCl with a higher current
density of 160 mA cm
−2
in the 0.33 M urea and 5 M KOH solution at 0.50 V (Ag/
AgCl). This kind of open three-dimensional array structure is conducive for diffusing the fuel to the electrode surface and fully contacting with the active sites, while
the reaction product can also quickly diffuse away to the bulk solution.
4.1.2 Nickel‑Noble Metal Alloys
As we mentioned above, pure nickel catalysts are prone to be poisoned during the
electro-oxidation process, which may deactivate the active NiOOH. What’s more,
nickel catalysts always suffer shortcomings of higher starting potentials and greater
polarization losses when catalyzing the oxidation of urea. Therefore, nickel is
usually doped with other metals (such as noble metal or transition metal) for the
preparation of binary or multicomponent metal catalysts to overcome these difficulties. King et al. [80] found some precious metals showing special resistance to be
poisoned in the catalytic oxidation of urea. They introduced an electrodeposition
method to synthesis Rh–Ni, Ru–Ni, Pt–Ir–Ni, and Pt–Ni alloy electrodes on the Ni
substrate and compared their properties with the pure Ni catalyst. It was observed
that the Ni–Rh alloy catalyst exhibited the best electro-catalytic property and stability (the maximum current density reached 80 mA cm
−2
), which was probably owing
to the doping of Rh that selectively adsorbed CO or OH
−
to a certain extent, thus
inhibiting the CO poisoning phenomenon and enhancing the stability of the catalyst. From the perspective of micromorphology, there existed many cracks on the
relatively rough surface of Pt–Ir–Ni and Rh–Ni electrodes, leading to a high specific
surface area. Therefore, the introduction of Rh and Pt–Ir can not only obtain a higher
current density (about 200 times than that of the pure Ni) but also extend their catalytic life to a large extent. As a matter of fact, Rh has no catalytic activity towards
urea electro-oxidation, which demonstrates that the incorporation of Rh increases
the formation efficiency of NiOOH (generation potential reaches ~ 0.45 V vs. Hg/
HgO) and suppresses the CO poisoning phenomenon. It is the synergy between Rh
and Ni that makes the catalytic activity higher and more stable. Subsequently, Miller
et al. [81] studied the influence of deposition potential and Rh loading on the catalyst morphology and current efficiency. Additionally, they found that the more negative the deposition potential, the lower current efficiency in the electrode position
process. When Rh was deposited at a low potential, Rh-Ni alloy formed with a high
alloying degree. Nonetheless, electrochemical results showed that when the two
metals were not alloyed, which means Rh metal was deposited only on the surface of
Ni, and showed the highest activity.
4.1.3 Nickel‑Transition Metal Alloys
King et al. has come up with a promising research direction for the development
of nickel-based alloy materials as anodic electro-catalysts in DUFCs. Unfortunately,
Reprinted from the journal
55
Topics in Current Chemistry (2018) 376:42
plate with the shape of “wheat ears”, leaving a gap between the “wheat” and “ear”,
and the distance between the individual nanowires is also far away from each other.
Electrochemical tests indicate the burr-like nickel nanowire catalyst can electro-catalyze urea oxidation at a lower OOP of 0.25 V vs. Ag/AgCl with a higher current
density of 160 mA cm
−2
in the 0.33 M urea and 5 M KOH solution at 0.50 V (Ag/
AgCl). This kind of open three-dimensional array structure is conducive for diffusing the fuel to the electrode surface and fully contacting with the active sites, while
the reaction product can also quickly diffuse away to the bulk solution.
4.1.2 Nickel‑Noble Metal Alloys
As we mentioned above, pure nickel catalysts are prone to be poisoned during the
electro-oxidation process, which may deactivate the active NiOOH. What’s more,
nickel catalysts always suffer shortcomings of higher starting potentials and greater
polarization losses when catalyzing the oxidation of urea. Therefore, nickel is
usually doped with other metals (such as noble metal or transition metal) for the
preparation of binary or multicomponent metal catalysts to overcome these difficulties. King et al. [80] found some precious metals showing special resistance to be
poisoned in the catalytic oxidation of urea. They introduced an electrodeposition
method to synthesis Rh–Ni, Ru–Ni, Pt–Ir–Ni, and Pt–Ni alloy electrodes on the Ni
substrate and compared their properties with the pure Ni catalyst. It was observed
that the Ni–Rh alloy catalyst exhibited the best electro-catalytic property and stability (the maximum current density reached 80 mA cm
−2
), which was probably owing
to the doping of Rh that selectively adsorbed CO or OH
−
to a certain extent, thus
inhibiting the CO poisoning phenomenon and enhancing the stability of the catalyst. From the perspective of micromorphology, there existed many cracks on the
relatively rough surface of Pt–Ir–Ni and Rh–Ni electrodes, leading to a high specific
surface area. Therefore, the introduction of Rh and Pt–Ir can not only obtain a higher
current density (about 200 times than that of the pure Ni) but also extend their catalytic life to a large extent. As a matter of fact, Rh has no catalytic activity towards
urea electro-oxidation, which demonstrates that the incorporation of Rh increases
the formation efficiency of NiOOH (generation potential reaches ~ 0.45 V vs. Hg/
HgO) and suppresses the CO poisoning phenomenon. It is the synergy between Rh
and Ni that makes the catalytic activity higher and more stable. Subsequently, Miller
et al. [81] studied the influence of deposition potential and Rh loading on the catalyst morphology and current efficiency. Additionally, they found that the more negative the deposition potential, the lower current efficiency in the electrode position
process. When Rh was deposited at a low potential, Rh-Ni alloy formed with a high
alloying degree. Nonetheless, electrochemical results showed that when the two
metals were not alloyed, which means Rh metal was deposited only on the surface of
Ni, and showed the highest activity.
4.1.3 Nickel‑Transition Metal Alloys
King et al. has come up with a promising research direction for the development
of nickel-based alloy materials as anodic electro-catalysts in DUFCs. Unfortunately,
Reprinted from the journal
55