1 3
Topics in Current Chemistry (2018) 376:42
urea electro-oxidation on nickel nanowire arrays electrode reaches 0.25 V (vs. Ag/
AgCl), which corresponds to the generation potential of NiOOH from Ni(OH) 2 , thus
corresponding to indirect E-C mechanism. Compared to the nickel plate electrode,
the OOP of the nickel nanowire electrode shows a certain degree of negative shift
(– 21 mV), which can attribute to the superior nanostructure of nickel catalyst. The
reversed CV curve that suffers a sharp peak at 0.55 V is interpreted as secondary
oxidation caused by the reaction intermediate. According to the DFT calculations,
the surface of nickel catalyst might be covered by the intermediate product CO or
CO 2 species and then the catalyst was poisoned. Ojani et al. [77] have studied the
AC impedance curves of methanol electro-oxidation on the surface of Pt–Pd binary
catalysts. They found that the Nyquist semi-arc showed clockwise and counterclockwise changes with the polarization potential. According to their conclusions, the
reverse half arc at potentials 0.2–0.5 V (vs. SCE) represents that the electrode surface is adsorbed and covered by CO intermediate species. Afterwards, in alkaline
urea oxidation process, the intermediate product CO was further electro-oxidized to
form CO 2 at the high potential with the reversed scanning. The generated CO 2 follows on reacting with the alkali, resulting in a large number of surface active sites
instantaneously exposed. Thus, a sudden rise of the current density appeared in the
reversed CV curves (Fig. 6a). This means that the coverage and poisoning by CO 2
species on the catalyst surface was slowed down. On the other hand, because CO
appeared as the intermediate product or final product, an odd reverse semi-circle
was observed at 0.47 V in the Nyquist plots (Fig. 6b). Daramola et al. [74] thought
that this phenomenon was due to the fact that CO and OH
–
was easy to be adsorbed
on Ni electrode surface during the electrochemical oxidation course. As a result,
the catalyst gets poisoned on its surface, limiting the activity and causing electrode
deactivation. Therefore, in order to further promote the development of alkaline
urea electrolysis cells, it is imperative to develop anode catalytic materials with high
activity and excellent stability.
4 Development of Anodic Catalysts for Urea Electro‑Oxidation
For the purpose of improving the electrical output performance of DUFCs and the
effectiveness of hydrogen production from urea electrolysis, the most significant
pathway is to develop anode catalysts that can reduce onset oxidation potential
(OOP) while perform their high activity. Initially, precious metal catalysts such as
Ti–Pt–Ir, Ti–Pt, and Ru–TiO 2 were utilized to enhance the urea electro-catalytic performance, which obtained no obvious effect but high cost. Instead, since NiOOH has
been proved as the intermediate transition catalyst towards urea electro-oxidation,
recent research has paid more attention to increase the NiOOH formation efficiency
of nickel–based catalyst [76]. In the literatures, nickel–based catalysts with various
micromorphology and elemental composition have been optimized in order to perform low OOP while high oxidation current density. Specifically, present research
work on the anodic catalyst mainly focuses on nickel-based metals, nickel-based
hydroxides, nickel-based oxides, novel nickel-based compounds, and supported
Reprinted from the journal
53
Topics in Current Chemistry (2018) 376:42
urea electro-oxidation on nickel nanowire arrays electrode reaches 0.25 V (vs. Ag/
AgCl), which corresponds to the generation potential of NiOOH from Ni(OH) 2 , thus
corresponding to indirect E-C mechanism. Compared to the nickel plate electrode,
the OOP of the nickel nanowire electrode shows a certain degree of negative shift
(– 21 mV), which can attribute to the superior nanostructure of nickel catalyst. The
reversed CV curve that suffers a sharp peak at 0.55 V is interpreted as secondary
oxidation caused by the reaction intermediate. According to the DFT calculations,
the surface of nickel catalyst might be covered by the intermediate product CO or
CO 2 species and then the catalyst was poisoned. Ojani et al. [77] have studied the
AC impedance curves of methanol electro-oxidation on the surface of Pt–Pd binary
catalysts. They found that the Nyquist semi-arc showed clockwise and counterclockwise changes with the polarization potential. According to their conclusions, the
reverse half arc at potentials 0.2–0.5 V (vs. SCE) represents that the electrode surface is adsorbed and covered by CO intermediate species. Afterwards, in alkaline
urea oxidation process, the intermediate product CO was further electro-oxidized to
form CO 2 at the high potential with the reversed scanning. The generated CO 2 follows on reacting with the alkali, resulting in a large number of surface active sites
instantaneously exposed. Thus, a sudden rise of the current density appeared in the
reversed CV curves (Fig. 6a). This means that the coverage and poisoning by CO 2
species on the catalyst surface was slowed down. On the other hand, because CO
appeared as the intermediate product or final product, an odd reverse semi-circle
was observed at 0.47 V in the Nyquist plots (Fig. 6b). Daramola et al. [74] thought
that this phenomenon was due to the fact that CO and OH
–
was easy to be adsorbed
on Ni electrode surface during the electrochemical oxidation course. As a result,
the catalyst gets poisoned on its surface, limiting the activity and causing electrode
deactivation. Therefore, in order to further promote the development of alkaline
urea electrolysis cells, it is imperative to develop anode catalytic materials with high
activity and excellent stability.
4 Development of Anodic Catalysts for Urea Electro‑Oxidation
For the purpose of improving the electrical output performance of DUFCs and the
effectiveness of hydrogen production from urea electrolysis, the most significant
pathway is to develop anode catalysts that can reduce onset oxidation potential
(OOP) while perform their high activity. Initially, precious metal catalysts such as
Ti–Pt–Ir, Ti–Pt, and Ru–TiO 2 were utilized to enhance the urea electro-catalytic performance, which obtained no obvious effect but high cost. Instead, since NiOOH has
been proved as the intermediate transition catalyst towards urea electro-oxidation,
recent research has paid more attention to increase the NiOOH formation efficiency
of nickel–based catalyst [76]. In the literatures, nickel–based catalysts with various
micromorphology and elemental composition have been optimized in order to perform low OOP while high oxidation current density. Specifically, present research
work on the anodic catalyst mainly focuses on nickel-based metals, nickel-based
hydroxides, nickel-based oxides, novel nickel-based compounds, and supported
Reprinted from the journal
53