Topics in Current Chemistry (2018) 376:42
1 3
Remarkably, in the urea electrolysis cell employed 2 selfsame electrodes for the sake
of sustaining urea electrolysis and hydrogen production, the C@Ni–Fe/NF reached
a greater current efficiency value ca. 62% at 0.6 V than NiFe/NF (52%) or Ni/NF
(44%). Barakat et al. [85] decorated nickel and manganese nanoparticles on carbon
nanofibers (noted as NiMn-CNFs) by high-temperature calcination of electrospun
mats method. The normal TEM image (Fig. 9d) displayed that the prepared nanofibers composed of amorphous matrix decorated by uniformly distributed nanoparticles. CVs as shown in Fig. 9c, the NiMn-CNFs electro-catalyst performed the catalytic activity of 300 mA cm
−2
g
−1
at 0.5 V with an OOP of 290 mV. Compared with
pure Ni-CNFs, the incorporation of Mn elements distinctly enhanced the electrocatalytic activity as well as lowered the OOP toward urea electro-oxidation. Considering that the Cr element was rarely noticed in the nickel–based multi-metal field,
Singh et al. [86] used direct borohydride reduction to synthesize NiCr/C electrode
as a stable catalyst for urea oxidation. The Cr content in the electro-catalyst layers was optimal by dominating the atomic ratio of nickel salt and chromium salt
in the reaction solution. The results showed that the electro-catalyst with 40%
amount of Cr (noted as Ni 60 Cr 40 /C) exhibited the best catalytic performance of
90 mA cm
−2
at a potential of 0.55 V vs. Ag/AgCl, which confirmed the increase
in the Ni
2+
/Ni
3+
active sites on the electrode surface. Tafel slope of the Ni 60 Cr 40 /C
catalyst was observed as 16 mV decade
−1
, which was worse than 30 mV decade
−1
of Ni/C, indicating improved charge-transfer kinetics. On the other hand, Shi et al.
[87] noticed that the doping of Mo species was also selectable for enhancing the
catalytic performance of bare Ni catalyst since the Mo-doping catalyst exhibited a
strong anti-poisoning characteristic for the carbon monoxide intermediate. Therefore, they fabricated Ni–Mo composite nanocatalysts on graphene support (Ni–Mo/
graphene) by simple reduction methods. The effect of doping amount of Mo species on the catalyst structure as well as properties was investigated in detail. Consequently, the optimal Ni 2 Mo 1 /graphene catalyst with selected meso-porous construction (particle size: 20–40 nm, pore size: 3.75 nm) exhibited higher catalytic current
(128 mA cm
−2
at 0.53 V) and decent kinetics (Tafel slope: 120 mV dec
−1
, OOP:
0.39 V) by comparison with the Ni/graphene catalyst in 0.33 M urea and 1 M KOH
electrolytes. It was the premium structural/electronic effects between Mo species
and Ni
3+
that contributed to the outstanding activity for the electro-oxidation of urea
at the Ni-Mo/graphene catalyst.
4.2 Nickel‑Based Hydroxide
4.2.1 Monovalent Nickel Hydroxide
Nickel hydroxide has a distinctive layer structure that results in two types of
crystals: α-Ni(OH) 2 and β-Ni(OH) 2 . The randomly stacked non-stoichiometric
Ni(OH) 2–x formed the c-axis of α-Ni(OH) 2 (i.e., layered stacking direction), therefore, extraneous anions are able to intercalate between layers to counteract the positive charge of based layer. According to the size of the inserted anion, the spacing
of α-Ni(OH) 2 on the bottom surface is distributed as 7.5–31.7 Å, which is conducive
Reprinted from the journal
58
1 3
Remarkably, in the urea electrolysis cell employed 2 selfsame electrodes for the sake
of sustaining urea electrolysis and hydrogen production, the C@Ni–Fe/NF reached
a greater current efficiency value ca. 62% at 0.6 V than NiFe/NF (52%) or Ni/NF
(44%). Barakat et al. [85] decorated nickel and manganese nanoparticles on carbon
nanofibers (noted as NiMn-CNFs) by high-temperature calcination of electrospun
mats method. The normal TEM image (Fig. 9d) displayed that the prepared nanofibers composed of amorphous matrix decorated by uniformly distributed nanoparticles. CVs as shown in Fig. 9c, the NiMn-CNFs electro-catalyst performed the catalytic activity of 300 mA cm
−2
g
−1
at 0.5 V with an OOP of 290 mV. Compared with
pure Ni-CNFs, the incorporation of Mn elements distinctly enhanced the electrocatalytic activity as well as lowered the OOP toward urea electro-oxidation. Considering that the Cr element was rarely noticed in the nickel–based multi-metal field,
Singh et al. [86] used direct borohydride reduction to synthesize NiCr/C electrode
as a stable catalyst for urea oxidation. The Cr content in the electro-catalyst layers was optimal by dominating the atomic ratio of nickel salt and chromium salt
in the reaction solution. The results showed that the electro-catalyst with 40%
amount of Cr (noted as Ni 60 Cr 40 /C) exhibited the best catalytic performance of
90 mA cm
−2
at a potential of 0.55 V vs. Ag/AgCl, which confirmed the increase
in the Ni
2+
/Ni
3+
active sites on the electrode surface. Tafel slope of the Ni 60 Cr 40 /C
catalyst was observed as 16 mV decade
−1
, which was worse than 30 mV decade
−1
of Ni/C, indicating improved charge-transfer kinetics. On the other hand, Shi et al.
[87] noticed that the doping of Mo species was also selectable for enhancing the
catalytic performance of bare Ni catalyst since the Mo-doping catalyst exhibited a
strong anti-poisoning characteristic for the carbon monoxide intermediate. Therefore, they fabricated Ni–Mo composite nanocatalysts on graphene support (Ni–Mo/
graphene) by simple reduction methods. The effect of doping amount of Mo species on the catalyst structure as well as properties was investigated in detail. Consequently, the optimal Ni 2 Mo 1 /graphene catalyst with selected meso-porous construction (particle size: 20–40 nm, pore size: 3.75 nm) exhibited higher catalytic current
(128 mA cm
−2
at 0.53 V) and decent kinetics (Tafel slope: 120 mV dec
−1
, OOP:
0.39 V) by comparison with the Ni/graphene catalyst in 0.33 M urea and 1 M KOH
electrolytes. It was the premium structural/electronic effects between Mo species
and Ni
3+
that contributed to the outstanding activity for the electro-oxidation of urea
at the Ni-Mo/graphene catalyst.
4.2 Nickel‑Based Hydroxide
4.2.1 Monovalent Nickel Hydroxide
Nickel hydroxide has a distinctive layer structure that results in two types of
crystals: α-Ni(OH) 2 and β-Ni(OH) 2 . The randomly stacked non-stoichiometric
Ni(OH) 2–x formed the c-axis of α-Ni(OH) 2 (i.e., layered stacking direction), therefore, extraneous anions are able to intercalate between layers to counteract the positive charge of based layer. According to the size of the inserted anion, the spacing
of α-Ni(OH) 2 on the bottom surface is distributed as 7.5–31.7 Å, which is conducive
Reprinted from the journal
58