1 3
Topics in Current Chemistry (2018) 376:42
research work related to urea electro-oxidation started early in the 1970s and
was applied in the field of bioengineering, such as portable bio-electrochemical
sensors and artificial kidney rejuvenation. Actually, as long as supplying certain
electrical energy to an aqueous solution that contains urea, urea will be oxidized
to other inorganic materials. More recently, the technology of urea electro-oxidation is mainly developed and applied in direct urea fuel cells (DUFCs) as anodic
reaction or the field of urea electrolysis for H 2 production.
This review summarizes the current advances in urea electro-oxidation. Adequate
literature is available in the whole review and some main issues not mentioned in the
existing literature are also addressed in this review. Specifically, a detailed survey
of the applications for urea electro-oxidation, which mainly include direct urea fuel
cells and urea electrolysis, are organized in Sect. 2. In-depth mechanisms of urea
electro-oxidation in neutral or alkaline media are discussed in Sect. 3. The development of anodic catalysts is of great significance towards DUFCs and urea electrolysis, thus are mainly introduced and classified in Sect. 4, where the major challenges
and remedies are also mentioned as a matter of interest. Finally, future challenges
and prospects are proposed in Sect. 5.
2 Application of Urea Electro‑Oxidation
2.1 Direct Urea Fuel Cells (DUFCs)
The fuel cell is a new type of high-efficiency, low-pollution power generation device,
which has been highly favored since its invention [26–37]. Simultaneously, DUFCs
are recently considered as promising advanced fuel cells, of which the fuel sources
mainly come from industrial urea effluent or animal urine. This type of device,
which uses the above urea as anodic fuels while O 2 or H 2 O 2 as cathodic oxidants,
can generate electricity as well as degrade waste water [38–46].
Tao et al. [10] firstly designed the working urea fuel cell and successfully applied
this technology to generate electricity. The urea electro-oxidation takes place in
the anodic compartment of this cell, and oxygen reduction reaction occurs in the
cathodic compartment. They also found that when Pt/C catalyst was used for both
cathode and anode, the open circuit voltage (OCV) reaches ~ 0.5 V, while the peak
power density was about 0.55 mW cm
−2
. Later in 2011, Tao et al. [47] made further
efforts to optimize the anode catalyst and continued to increase the open circuit voltage and power density of DUFCs. Nickel nanoparticles with sizes between 2 and
3 nm were designed and firstly reported as anodic catalyst in the urine fuel cell.
The power density was greatly enhanced to 1.5 mW cm
−2
while the OCV was also
broadened to 0.65 V. Recently, Basumatary et al. developed an alkaline air cathode DUFC using Cu/ZnO@MWCNT, which demonstrated power outputs of 12 mW
cm
−2
in 0.35 M urea at 20 °C (the highest power output reported at room temperature) with an OCV of 0.9 V [41]. Compared with other alkaline fuel cells (AFC),
DUFCs’ electrical output performance is relatively low [48–51]. Nevertheless, it
is still considered that urea can be promising anodic fuels mainly due to their special merits as follows: (1) they have higher volumetric energy density (16.9 MJ l
−1
)
Reprinted from the journal
43
Topics in Current Chemistry (2018) 376:42
research work related to urea electro-oxidation started early in the 1970s and
was applied in the field of bioengineering, such as portable bio-electrochemical
sensors and artificial kidney rejuvenation. Actually, as long as supplying certain
electrical energy to an aqueous solution that contains urea, urea will be oxidized
to other inorganic materials. More recently, the technology of urea electro-oxidation is mainly developed and applied in direct urea fuel cells (DUFCs) as anodic
reaction or the field of urea electrolysis for H 2 production.
This review summarizes the current advances in urea electro-oxidation. Adequate
literature is available in the whole review and some main issues not mentioned in the
existing literature are also addressed in this review. Specifically, a detailed survey
of the applications for urea electro-oxidation, which mainly include direct urea fuel
cells and urea electrolysis, are organized in Sect. 2. In-depth mechanisms of urea
electro-oxidation in neutral or alkaline media are discussed in Sect. 3. The development of anodic catalysts is of great significance towards DUFCs and urea electrolysis, thus are mainly introduced and classified in Sect. 4, where the major challenges
and remedies are also mentioned as a matter of interest. Finally, future challenges
and prospects are proposed in Sect. 5.
2 Application of Urea Electro‑Oxidation
2.1 Direct Urea Fuel Cells (DUFCs)
The fuel cell is a new type of high-efficiency, low-pollution power generation device,
which has been highly favored since its invention [26–37]. Simultaneously, DUFCs
are recently considered as promising advanced fuel cells, of which the fuel sources
mainly come from industrial urea effluent or animal urine. This type of device,
which uses the above urea as anodic fuels while O 2 or H 2 O 2 as cathodic oxidants,
can generate electricity as well as degrade waste water [38–46].
Tao et al. [10] firstly designed the working urea fuel cell and successfully applied
this technology to generate electricity. The urea electro-oxidation takes place in
the anodic compartment of this cell, and oxygen reduction reaction occurs in the
cathodic compartment. They also found that when Pt/C catalyst was used for both
cathode and anode, the open circuit voltage (OCV) reaches ~ 0.5 V, while the peak
power density was about 0.55 mW cm
−2
. Later in 2011, Tao et al. [47] made further
efforts to optimize the anode catalyst and continued to increase the open circuit voltage and power density of DUFCs. Nickel nanoparticles with sizes between 2 and
3 nm were designed and firstly reported as anodic catalyst in the urine fuel cell.
The power density was greatly enhanced to 1.5 mW cm
−2
while the OCV was also
broadened to 0.65 V. Recently, Basumatary et al. developed an alkaline air cathode DUFC using Cu/ZnO@MWCNT, which demonstrated power outputs of 12 mW
cm
−2
in 0.35 M urea at 20 °C (the highest power output reported at room temperature) with an OCV of 0.9 V [41]. Compared with other alkaline fuel cells (AFC),
DUFCs’ electrical output performance is relatively low [48–51]. Nevertheless, it
is still considered that urea can be promising anodic fuels mainly due to their special merits as follows: (1) they have higher volumetric energy density (16.9 MJ l
−1
)
Reprinted from the journal
43