1 3
Topics in Current Chemistry (2018) 376:42
4.3.2 Nickel‑Based Composite Oxides
Due to the deactivation of some pure nickel-based catalysts, the studies on the
mixed oxides have been investigated extensively [100–102]. Ding et al. [103] successfully synthesized spinel-type NiCo 2 O 4 as urea electro-catalyst by chemical
deposition and simple annealing treatment. The formed NiCo 2 O 4 nanoparticles have
a size of about 10 nm, and these particles are agglomerated to form a rough surface with large coarse particles during the calcination stage. This loosely packed
structure forms a lot of mesopores, while the pore size mostly distributed around
12 nm, providing a large surface area (190.1 m
2
g
−1
). Electrochemical tests show
that the current density of NiCo 2 O 4 catalyst increased by twice the monovalent NiO,
the onset potential decreased by 80 mV, and both the durability and stability were
greatly enhanced. Thus, the composition and structure of composite oxides deserve
further study [44]. More recently, spinel nickel manganese oxides (MnNi 2 O 4 ,
NiMn 2 O 4 , and Ni 1.5 Mn 1.5 O 4 ) were applied as the urea electro-catalyst by Sivakumar
et al. [104]. Cyclic voltammetric measurements showed that the Ni 1.5 Mn 1.5 O 4 (loading as 50 µg cm
−2
) modified the electrochemical performance at 0.29 V with the
highest current of 6.9 mA cm
−2
(seven times better than the NiO and four times better than the MnNi 2 O 4 and NiMn 2 O 4 ), which was possibly attributed to the appearance of different phases. Liang et al. [88] fabricated the NiMoO 4 ·xH 2 O nanosheets
on nickel foam (NiMoO 4 ·xH 2 O NS/NF) by hydrothermal synthesis at 160 °C. The
EASA of NiMoO 4 ·xH 2 O NS/NF calculated by Randles–Sevcik equation (Eq. 9) was
valued at 0.47 cm
2
, which was 1.35 times that of Ni(OH) 2 NAs/NF. Electrochemical
impedance spectroscopy (EIS) analysis also exhibited much lower impedance for the
NiMoO 4 ·xH 2 O NAs than that of the Ni(OH) 2 NAs and resulted in a markedly faster
kinetics towards urea electro-oxidation. As a consequence, it was meaningful that
the incorporation of Mo elements could be beneficial to the elimination of surface
deposition on the normal nickel hydroxide catalysts, thus providing a higher catalytic activity with good stability.
Herein, I p , A, n, C, v, and D represent the peak current, the EASA, the number
of electrons involved in the reaction, the concentration of the reactant, the scan rate,
and the diffusion coefficient of the reactant species, respectively.
More recently, highly porous rod-like Ni–Mo–O catalysts were designed for
efficient water-to-hydrogen conversion via alkaline urea electrolysis [105]. Typically, the pre-synthesized NiMoO 4 ·xH 2 O nanorods (via one-step hydrothermal)
were treated with Ar-protected annealing process to obtain the porous rod-like
NiMoO 4 (named NiMoO-Ar). When switching the shielding gas from Ar to H 2 /
Ar, the annealing process generated the derived Ni/NiO/MoO x composite (named
NiMoO-H 2 ). From the XPS results of NiMoO-Ar, it was noted that the incorporation of Mo with high oxidation state of +6 enabled an easier transition of Ni
2+
to
Ni
3+
. Consequently, in an alkaline electrolyte, the porous rod-like NiMoO 4 performed exceptional electro-catalytic property for urea oxidation reaction (UOR)
while derived Ni/NiO/MoO x exhibited platinum-like activity for hydrogen evolution
(9)
I P = 2.99 × 10
5 nAC urea
(1 − )n 0
0.5 D
0.5 v
0.5
Reprinted from the journal
63
Topics in Current Chemistry (2018) 376:42
4.3.2 Nickel‑Based Composite Oxides
Due to the deactivation of some pure nickel-based catalysts, the studies on the
mixed oxides have been investigated extensively [100–102]. Ding et al. [103] successfully synthesized spinel-type NiCo 2 O 4 as urea electro-catalyst by chemical
deposition and simple annealing treatment. The formed NiCo 2 O 4 nanoparticles have
a size of about 10 nm, and these particles are agglomerated to form a rough surface with large coarse particles during the calcination stage. This loosely packed
structure forms a lot of mesopores, while the pore size mostly distributed around
12 nm, providing a large surface area (190.1 m
2
g
−1
). Electrochemical tests show
that the current density of NiCo 2 O 4 catalyst increased by twice the monovalent NiO,
the onset potential decreased by 80 mV, and both the durability and stability were
greatly enhanced. Thus, the composition and structure of composite oxides deserve
further study [44]. More recently, spinel nickel manganese oxides (MnNi 2 O 4 ,
NiMn 2 O 4 , and Ni 1.5 Mn 1.5 O 4 ) were applied as the urea electro-catalyst by Sivakumar
et al. [104]. Cyclic voltammetric measurements showed that the Ni 1.5 Mn 1.5 O 4 (loading as 50 µg cm
−2
) modified the electrochemical performance at 0.29 V with the
highest current of 6.9 mA cm
−2
(seven times better than the NiO and four times better than the MnNi 2 O 4 and NiMn 2 O 4 ), which was possibly attributed to the appearance of different phases. Liang et al. [88] fabricated the NiMoO 4 ·xH 2 O nanosheets
on nickel foam (NiMoO 4 ·xH 2 O NS/NF) by hydrothermal synthesis at 160 °C. The
EASA of NiMoO 4 ·xH 2 O NS/NF calculated by Randles–Sevcik equation (Eq. 9) was
valued at 0.47 cm
2
, which was 1.35 times that of Ni(OH) 2 NAs/NF. Electrochemical
impedance spectroscopy (EIS) analysis also exhibited much lower impedance for the
NiMoO 4 ·xH 2 O NAs than that of the Ni(OH) 2 NAs and resulted in a markedly faster
kinetics towards urea electro-oxidation. As a consequence, it was meaningful that
the incorporation of Mo elements could be beneficial to the elimination of surface
deposition on the normal nickel hydroxide catalysts, thus providing a higher catalytic activity with good stability.
Herein, I p , A, n, C, v, and D represent the peak current, the EASA, the number
of electrons involved in the reaction, the concentration of the reactant, the scan rate,
and the diffusion coefficient of the reactant species, respectively.
More recently, highly porous rod-like Ni–Mo–O catalysts were designed for
efficient water-to-hydrogen conversion via alkaline urea electrolysis [105]. Typically, the pre-synthesized NiMoO 4 ·xH 2 O nanorods (via one-step hydrothermal)
were treated with Ar-protected annealing process to obtain the porous rod-like
NiMoO 4 (named NiMoO-Ar). When switching the shielding gas from Ar to H 2 /
Ar, the annealing process generated the derived Ni/NiO/MoO x composite (named
NiMoO-H 2 ). From the XPS results of NiMoO-Ar, it was noted that the incorporation of Mo with high oxidation state of +6 enabled an easier transition of Ni
2+
to
Ni
3+
. Consequently, in an alkaline electrolyte, the porous rod-like NiMoO 4 performed exceptional electro-catalytic property for urea oxidation reaction (UOR)
while derived Ni/NiO/MoO x exhibited platinum-like activity for hydrogen evolution
(9)
I P = 2.99 × 10
5 nAC urea
(1 − )n 0
0.5 D
0.5 v
0.5
Reprinted from the journal
63