Topics in Current Chemistry (2018) 376:42
1 3
the high cost of Ni-Rh or other Ni-noble catalysts remains an objective problem
that restricts their large-scale application for the alkaline urea electrolytic cells in
the future. By contrast, cheap transition metals provide an effective solution due to
the fact that doping the transition metal (such as Co, Fe, Zn, Mn, Cr, etc.) into Ni
catalyst can induce more defects of nickel crystal structure to form more electrochemical active sites, thereby reducing the onset oxidation potential. Furthermore,
the addition of these transition metals can also hinder the electrolysis of water (side
reactions), so as to improve the electro-catalytic performance. Taking this into consideration, the researches on binary or multivariate nickel-based alloy catalysts have
gradually achieved significant success in recent years. Yan et al. [82] successfully
electrodeposited Ni-Co alloy on the Ti substrate. From the experimental data, it was
observed that the Ni-Co alloy catalyst can greatly reduce the over-potential of urea
oxidation at a maximum of 150 mV, so that the interference of side reactions such as
oxygen evolution can be reduced or avoided for less energy consumption. Cao et al.
[54] utilized a dynamic hydrogen template method to prepare three-dimensional
porous Ni-Co electrode on nickel foam substrates. By changing the composition
of the electrodeposition solution (nickel salt: cobalt salt = 10:0, 8:2, 5:5, 2:8, 0:10),
they obtained Ni–Co@Ni foam electrode with different structures and morphologies
(noted as Ni 10, Ni 8, Ni 5, Ni 2, Ni 0). As the corresponding SEM images shown
in Fig. 8, all the deposits were porous and loose, consisting of abundant micronsized particles. Benefiting from the unique micromorphology, Ni 2 electrode had
a maximum real surface area (461.50 cm
2
) as the geometric area fixed at 1 cm
2
.
When applied as the anodic catalyst of DUPFC, the Ni 2 electrode also performed
the best power density of 13.8 mW cm
−2
at a low OCV (0.8 V), contributing to a
proper doping of Co element, which was beneficial to the expose of more nickel
active sites and the generation of NiOOH. However, the oxidation current density
produced by Ni–Co catalyst is accidentally lower than the Ni catalyst in the case of
equivalent amounts of Ni–Co and pure Ni catalyst. That is mainly ascribed to: (1)
during the co-deposition process, the deposition rate of Co atoms is faster that might
lead to partially covering the surface of Ni atoms, reducing the catalytic sites of the
active ingredient Ni; (2) Co has no activity towards the electro-oxidation of urea. On
Fig. 8 SEM images of Ni foam (a) and Ni 2 (b) electrode prepared by hydrogen template method Reproduced with permission from Ref. [54]
Reprinted from the journal
56
1 3
the high cost of Ni-Rh or other Ni-noble catalysts remains an objective problem
that restricts their large-scale application for the alkaline urea electrolytic cells in
the future. By contrast, cheap transition metals provide an effective solution due to
the fact that doping the transition metal (such as Co, Fe, Zn, Mn, Cr, etc.) into Ni
catalyst can induce more defects of nickel crystal structure to form more electrochemical active sites, thereby reducing the onset oxidation potential. Furthermore,
the addition of these transition metals can also hinder the electrolysis of water (side
reactions), so as to improve the electro-catalytic performance. Taking this into consideration, the researches on binary or multivariate nickel-based alloy catalysts have
gradually achieved significant success in recent years. Yan et al. [82] successfully
electrodeposited Ni-Co alloy on the Ti substrate. From the experimental data, it was
observed that the Ni-Co alloy catalyst can greatly reduce the over-potential of urea
oxidation at a maximum of 150 mV, so that the interference of side reactions such as
oxygen evolution can be reduced or avoided for less energy consumption. Cao et al.
[54] utilized a dynamic hydrogen template method to prepare three-dimensional
porous Ni-Co electrode on nickel foam substrates. By changing the composition
of the electrodeposition solution (nickel salt: cobalt salt = 10:0, 8:2, 5:5, 2:8, 0:10),
they obtained Ni–Co@Ni foam electrode with different structures and morphologies
(noted as Ni 10, Ni 8, Ni 5, Ni 2, Ni 0). As the corresponding SEM images shown
in Fig. 8, all the deposits were porous and loose, consisting of abundant micronsized particles. Benefiting from the unique micromorphology, Ni 2 electrode had
a maximum real surface area (461.50 cm
2
) as the geometric area fixed at 1 cm
2
.
When applied as the anodic catalyst of DUPFC, the Ni 2 electrode also performed
the best power density of 13.8 mW cm
−2
at a low OCV (0.8 V), contributing to a
proper doping of Co element, which was beneficial to the expose of more nickel
active sites and the generation of NiOOH. However, the oxidation current density
produced by Ni–Co catalyst is accidentally lower than the Ni catalyst in the case of
equivalent amounts of Ni–Co and pure Ni catalyst. That is mainly ascribed to: (1)
during the co-deposition process, the deposition rate of Co atoms is faster that might
lead to partially covering the surface of Ni atoms, reducing the catalytic sites of the
active ingredient Ni; (2) Co has no activity towards the electro-oxidation of urea. On
Fig. 8 SEM images of Ni foam (a) and Ni 2 (b) electrode prepared by hydrogen template method Reproduced with permission from Ref. [54]
Reprinted from the journal
56