Topics in Current Chemistry (2018) 376:42
1 3
1 Introduction
Human society consumes a large amount of fossil energy, which triggers serious
environmental concerns and excessive emission of greenhouse gases. In order to
maintain the normal operation of the global economy and preserve the ecological balance, new and clean power alternatives of petrochemical energy must be
searched for and developed [1, 2]. Hydrogen is a clean and novel energy resource,
but the production, storage, and transportation of hydrogen still suffer from great
challenges that severely restrict the large-scale application of hydrogen energy.
Therefore, exploring a qualified hydrogen energy carrier is one of the main ways
to overcome these technical obstacles. In general, normal hydrogen energy carriers need to be considered in several ways, such as environmental compatibility,
safety and cost, etc. [3–7].
Urea, as an organic material that contains carbon, nitrogen, oxygen, and hydrogen elements, is considered a hydrogen-rich chemical fuel (hydrogen content
reaches 6.67 wt%) [8, 9]. Therefore urea is attractively focused on as an alternative anodic fuel for application in fuel cells [10]. In fact, urea sources are abundant from human urine, urea-containing waste water, and industrial urea. On the
one hand, human urine contains 2–2.5 wt% of urea, which is equal to the average molar concentration of 0.33 mol l
−1
. On the other hand, industrial production discharges amounts of urea-containing waste water with different urea contents. However, the urea-rich waste water or urine can be naturally hydrolyzed to
ammonia (NH 3 ) if without any treatment and released to the atmosphere, accelerating the formation of acid rain as well as causing damage to the environment
[11]. In addition, ammonia is unstable in the air, which means it is easily oxidized
to form pollutants such as nitrates, nitrites, and nitrogen oxides. These harmful nitrogenous substances will directly infiltrate the soil and drinking water as
rainwater, posing a great threat to the ground and human health. Furthermore,
because of the high energy density of urea, direct emissions may cause a great
deal of energy waste. Therefore, the utilization of rational methods to remove
nitrogen-containing components from effluent and make better treatment for
urea is becoming an increasingly important energetic and environmental issue
[12]. Traditional urea treatments mainly include hydrolysis [12, 13], adsorption
[14–17], biodegradation [18, 19], and chemical oxidation [20–23], but the high
cost of required equipment of these methods as well as excessive energy consummation greatly limit their boarder application. For instance, in order to obtain a
better degradation effect, these methods are usually operated under high temperature conditions, which may result in inactivation and denaturation of the biological active enzymes (such as urease) [14, 24, 25].
Fortunately, the electro-oxidation method of urea possesses the advantages of
simple operation, large quantity of processing, long operation cycle, and stable
production of non-toxic CO 2 , N 2 with available H 2 (in basic solution). More significantly, urea electro-oxidation under alkaline condition can be effectively realized without the utilization of precious metals, which greatly reduces the catalyst
cost and increases the possibility of practical application. As a matter of fact,
Reprinted from the journal
42
1 3
1 Introduction
Human society consumes a large amount of fossil energy, which triggers serious
environmental concerns and excessive emission of greenhouse gases. In order to
maintain the normal operation of the global economy and preserve the ecological balance, new and clean power alternatives of petrochemical energy must be
searched for and developed [1, 2]. Hydrogen is a clean and novel energy resource,
but the production, storage, and transportation of hydrogen still suffer from great
challenges that severely restrict the large-scale application of hydrogen energy.
Therefore, exploring a qualified hydrogen energy carrier is one of the main ways
to overcome these technical obstacles. In general, normal hydrogen energy carriers need to be considered in several ways, such as environmental compatibility,
safety and cost, etc. [3–7].
Urea, as an organic material that contains carbon, nitrogen, oxygen, and hydrogen elements, is considered a hydrogen-rich chemical fuel (hydrogen content
reaches 6.67 wt%) [8, 9]. Therefore urea is attractively focused on as an alternative anodic fuel for application in fuel cells [10]. In fact, urea sources are abundant from human urine, urea-containing waste water, and industrial urea. On the
one hand, human urine contains 2–2.5 wt% of urea, which is equal to the average molar concentration of 0.33 mol l
−1
. On the other hand, industrial production discharges amounts of urea-containing waste water with different urea contents. However, the urea-rich waste water or urine can be naturally hydrolyzed to
ammonia (NH 3 ) if without any treatment and released to the atmosphere, accelerating the formation of acid rain as well as causing damage to the environment
[11]. In addition, ammonia is unstable in the air, which means it is easily oxidized
to form pollutants such as nitrates, nitrites, and nitrogen oxides. These harmful nitrogenous substances will directly infiltrate the soil and drinking water as
rainwater, posing a great threat to the ground and human health. Furthermore,
because of the high energy density of urea, direct emissions may cause a great
deal of energy waste. Therefore, the utilization of rational methods to remove
nitrogen-containing components from effluent and make better treatment for
urea is becoming an increasingly important energetic and environmental issue
[12]. Traditional urea treatments mainly include hydrolysis [12, 13], adsorption
[14–17], biodegradation [18, 19], and chemical oxidation [20–23], but the high
cost of required equipment of these methods as well as excessive energy consummation greatly limit their boarder application. For instance, in order to obtain a
better degradation effect, these methods are usually operated under high temperature conditions, which may result in inactivation and denaturation of the biological active enzymes (such as urease) [14, 24, 25].
Fortunately, the electro-oxidation method of urea possesses the advantages of
simple operation, large quantity of processing, long operation cycle, and stable
production of non-toxic CO 2 , N 2 with available H 2 (in basic solution). More significantly, urea electro-oxidation under alkaline condition can be effectively realized without the utilization of precious metals, which greatly reduces the catalyst
cost and increases the possibility of practical application. As a matter of fact,
Reprinted from the journal
42