Urea electrolysis: Direct hydrogen production from urine

Article abstract of DOI:10.1039/b905974a

A new technology has been developed that accomplishes the direct conversion of urine and urea to pure hydrogen via electrochemical oxidation with an inexpensive nickel catalyst.

Full text of DOI:10.1039/b905974a

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Urea electrolysis: direct hydrogen production from urinew
Bryan K. Boggs, Rebecca L. King and Gerardine G. Botte*
Received (in Cambridge, UK) 25th March 2009, Accepted 11th June 2009
First published as an Advance Article on the web 1st July 2009
DOI: 10.1039/b905974a
A new technology has been developed that accomplishes the
direct conversion of urine and urea to pure hydrogen via
electrochemical oxidation with an inexpensive nickel catalyst.
Our results demonstrate that human urine, with an average
6
concentration of 0.33 M urea, can be electrochemically
oxidized with an inexpensive transition metal, nickel,
according to eqn (1–4).
The utilization of wastewater for useful fuel has been
gathering recent attention due to society’s need for alternative
energy sources. The electrooxidation of urea found at high
concentrations in wastewater simultaneously accomplishes
fuel production and remediation of harmful nitrogen
compounds that currently make their way into the atmosphere
and groundwater. Pure hydrogen was collected in the cathode
compartment at 1.4 V cell potential, where water electrolysis
does not occur appreciably. It was determined that an
inexpensive nickel catalyst is the most active and stable for
the process.
À
2
CO(NH )2(aq) + 6OH
À
-
N2(g) + 5H
2
O
(l) + CO2(g) + 6e
(1)
(2)
(3)
(4)
À
À
Ni(OH)2(s) + OH - NiOOH(s) + H O + e
2
(l)
À
À
6H
2
O
(l) + 6e - 3H2(g) + 6OH
CO(NH
2
)
2(aq) + H (l) - N2(g) + 3H2(g) + CO2(g)
2
O
Urea is oxidized at the anode (eqn (1)) at a standard electrode
potential of À0.46 V vs. SHE. The Gibb’s energy of urea was
calculated as the crystalline Gibb’s energy plus the energy for
dissolution of urea from its crystalline state. The oxidation of
Urine is the most abundant waste on Earth. The largest
constituent of urine is urea, which is a significant organic
source of H, C, O, and N. Despite the numerous benefits of
Ni(OH) to NiOOH at the anode (eqn (2)) is a competing
2
reaction that attributes to current during electrolysis and
occurs at 0.49 V vs. SHE. Alkaline reduction of water
(eqn (3)) occurs on the cathode at À0.83 V vs. SHE. Overall
in eqn (4), an electrolytic cell potential of only 0.37 V is
thermodynamically required to electrolyze urea at standard
conditions. This is significantly less than the 1.23 V required
to electrolyze water theoretically generating 70% cheaper
hydrogen.
1
using urea/urine for hydrogen production, there is not a
1
single technology that directly converts urea to hydrogen.
,2
In addition to sustaining hydrogen resources, such a process
could denitrificate urea-rich water that is commonly purged
into rivers, creeks, and tributaries from municipal wastewater
treatment plants. Currently, nitrate concentration in these
À1
waters is regulated at 10 mg L , but available denitrification
3
technologies are expensive and inefficient. Converting urea to
Nitrogen is generated from the anode demonstrating nitrate
remediation of wastewater while water is reduced at the
cathode producing valuable hydrogen for the impending
hydrogen economy.
valuable products before it naturally hydrolyzes to ammonia,
which generates gas-phase ammonia emissions and contributes
to ammonium sulfate and nitrate formation in the atmosphere,
4
will save billions of dollars spent each year on health costs.
Fig. 2a shows the cyclic voltammogram (CV) comparison of
different electrocatalysts (Pt, Pt–Ir, Rh, and Ni) for the
electrooxidation of urea in alkaline media. Polarization curves
between the various metals in the presence and absence of
Here we demonstrate a technology for improving hydrogen
resources for energy sustainability by recycling waste materials
such as human excreta. We have developed an electrochemical
5
process that produces H from urine/urea as shown in Fig. 1.
2
À1
0.33 M urea and 5 M KOH at a scan rate of 10 mV s from
À0.1 to 0.8 V versus Hg/HgO reference supported by a Luggin
capillary at 25 1C shows that Ni is the most active catalyst in
terms of current density. Scanning the potential more negative
than À0.1 V revealed no oxidation current. The electrodes
2
were 4 cm based on geometric area of Ti foil (inert) deposited
with an average 10.0 Æ 0.1 mg of respective metal. The counter
2
electrode was a 25 cm Pt foil. All electrochemical experiments
were performed in a conventional three-electrode cell powered
by a Solartron 1281 Multiplexer potentiostat. Fig. 2b,
constant voltage analysis at 1.4 V in 5 M KOH–0.33 M urea
at 25 1C, further shows Ni is the most stable and active
electrocatalyst for the electrooxidation of urea in alkaline
media. This potential was chosen from the fact that water
Fig. 1 Schematic representation of the direct urea-to-hydrogen
process.
Department of Chemical and Biomolecular Engineering,
Ohio University, Athens, OH, USA. E-mail: botte@ohio.edu;
Fax: +1 740 593 0873; Tel: +1 740 593 9670
w Electronic supplementary information (ESI) available: Experimental
details, cyclic voltammograms, constant voltage analyses, and gas
chromatographs. See DOI: 10.1039/b905974a
7–9
reduction is kinetically friendly at À0.83 V vs. SHE
(
standard hydrogen electrode), and the electrooxidation of
urea is occurring at 0.55 V vs. Hg/HgO according to Fig. 2c.
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Chem. Commun., 2009, 4859–4861 | 4859

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Fig. 3 (a) Cyclic voltammograms obtained in 5 M KOH + 0.33 M
urea for the NOMN electrode with various scan rates (n) from
À1 À1
5
mV s to 95 mV s . (b) The plot of cathodic current density
1/2
variation with n
.
metallic substrates (Ni foil, Ni gauze, Ti foil, and Ti gauze)
that have been electroplated with 10.0 Æ 0.1 mg of Ni using a
Watts bath then activated following the procedure developed
1
0
by Vaze et al. yield higher current densities than those
of M/Ni, where M represents the metallic substrate.
NOMN electrodes were used for the remaining electro-
chemical behavior analyses. Fig. 3 demonstrates that there is
an influence of scan rate on the cyclic voltammetry behavior of
NOMN electrodes. The electrooxidation of urea in this system
was characterized with CVs from 0.0 to 0.6 V versus Hg/HgO
Fig. 2 Anode catalyst analysis at 25 1C: (a) cyclic voltammograms
obtained in 5 M KOH with and without the presence of urea on Ti-foil
À1
À1
at scan rates of 5 to 95 mV s . Fig. 3a shows that the cathodic
supported electrodes with a 10 mV s scan; (b) constant voltage test
peak does not shift in potential as the scan rate increases in the
presence of urea. The curves are shown from 0.0 to 0.5 V for
scaling purposes. Fig. 3b indicates that cyclic voltammetry
peak cathodic currents (Ipc) followed a linear correlation with
with 1.4 V potential step with 5 M KOH–0.33 M urea; (c) cyclic
voltammogram of Ni/Ti electrode in the absence (grey) and presence
(black) of 0.33 M KOH in 5 M KOH solution.
2
+
2
Nickel in basic media is rapidly converted to Ni(OH)
2
(Ni
)
the square root of the scan rate (R = 0.976). Together, these
3
which is further oxidized to NiOOH (Ni ). This transition
+
criteria confirm that the production of NiOOH from Ni(OH)2
is a reversible diffusion-controlled process. The increase in
cathodic currents as a function of scan rate indicates that the
electrooxidation of urea is slower than the electrooxidation of
nickel species to a higher valence state. Therefore, we hypo-
thesize that the rate-limiting step is the reaction between
NiOOH and urea absorbed on the surface.
2
+
3+
from Ni
behavior of small organic compounds.
Ni(OH) to NiOOH is represented by anodic peak a
shows that urea electrolysis begins at the same potential where
to Ni
enhances catalytic electrooxidation
9
–11
The oxidation of
. Fig. 2c
2
1
3
+
NiOOH is formed, suggesting that Ni is the active form for
urea oxidation. This is seen as an increase of current density
at a
1
in the presence of urea. Furthermore, a change in slope
The electrocatalytic behavior of the NOMN electrode
towards urea oxidation in basic media was further studied
with cyclic voltammetry and constant voltage analyses at
varying operating conditions. It was found that the
current density increases with temperature. Also, higher
due to the onset of water electrolysis can be seen at more
positive potentials.
We found that nickel oxyhydroxide modified nickel
electrodes (NOMN) for urea electrooxidation on different
4
860 | Chem. Commun., 2009, 4859–4861
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concentrations of KOH favor the reaction rate. As the
concentration of KOH exceeded 5 M, the NOMN electrode
lost activity as seen by a decrease in current density during
constant voltage analyses. This could be due to faster
disappearance of the oxide layer, which was visibly evident.
Energy dispersive X-ray (EDX) microanalyses of a Ti foil
Table 1 Energy and hydrogen cost comparison between urea and
water electrolysis based on an energy cost of $ 0.07 kWh
À1
À1
À1
2
H cost/$ kg
Electrolysis
Energy/Wh g
Urea
Water
37.5
53.6
2.63
4.13
(
99.99% pure) electrode (deposited with 10.0 Æ 0.1 mg of Ni
analysis mentioned in the ESIw, voltages for both urea
1.4 V) and water electrolysis (2.0 V) were found that resulted
and then activated into a NOMN electrode) before and after
urea electrolysis at 1.4 V for 30 minutes in 5 M KOH–0.33 M
urea shows that the amount of atomic carbon and oxygen on
the electrode surface increases during electrolysis. This may
be contributed to adsorption of products onto the surface.
As a result, the surface atomic composition of Ni decreases
leading to decay in the current density during the constant
voltage study.
(
in cell currents of 20 mA. Using these voltages for comparison,
we found that 30% less energy is required for urea electrolysis,
which generated 36% cheaper hydrogen compared to water
electrolysis.
In the past, research pertaining to urea electrolysis
exclusively involved the possibility of developing artificial
kidneys for portable dialysis devices utilizing platinum
12–15
Anode and cathode gases were collected separately in a
Hoffman apparatus filled with a solution of 5 M KOH in the
presence and absence of 0.33 M urea and analyzed via gas
chromatography. The electrolyses were performed at a
constant voltage of 1.5 V and 25 1C for 22 hours. Currents
observed were 20 mA and less than 1 mA in the presence and
absence of urea, respectively. This verifies that water electro-
electrodes in acidic buffers.
There is great interest in the
scientific community for finding non-platinized catalyst
alternatives such as Ni for hydrogen production. We have
demonstrated that the technology is effective for both urea
and urine.
Notes and references
lysis is not occurring to an appreciable extent. Pure H
observed at the cathode while N (96.1%) with trace amounts
of O (1.9%) and H (2.0%) were detected at the anode for
2
was
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3
2
Ni(OH) - NiOOH. Carbon dioxide was not detected as part
5
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2
this new technology utilizing inexpensive Ni. The electrolysis
2
003.
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9
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Chem., 2007, 610, 163–170.
(
see ESIw, Fig. S4). Theoretically, hydrogen can be produced
À1
À1
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at $0.69 kg based on an electricity cost of $0.07 kWh and
the proposed electrochemical reactions (eqn (1–4)) that have
been developed from electrochemical data and gas analyses.
Table 1 shows energy consumption (Wh per gram of
hydrogen) and cost of hydrogen comparison between urea
and water electrolysis at experimental conditions with Ni
anodes. Utilizing the same cell configuration as the GC
1
1
1
This journal is ꢀc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4859–4861 | 4861