https://www.researchgate.net/publication/292138426_A_Review_of_Ammonia_Fuel_Cells
A Review of Ammonia Fuel Cells
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DOI: 10.13140/RG.2.1.1700.7129
Arizona State University, In Progress, DOI:10.13140/RG.2.1.1700.7129
Arizona State University, In Progress, DOI:10.13140/RG.2.1.1700.7129
1st Melvin Mathew4.51 · Arizona State University
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DOI: 10.13140/RG.2.1.1700.7129 ·Available from: Melvin Mathew, Jan 28, 2016
A Review of Ammonia Fuel Cells
Amar Thaker a, Melvin Mathew a, Naimee Hasib a, Nick Herringer a
Abstract: Ammonia is an important chemical which is widely available. Besides having high energy density per dollars as a
fuel for fuel cells, production techniques for ammonia are well established. In this review, different production techniques for
ammonia, mechanisms behind the operation of different ammonia fuel cells, different catalysts used for various types of fuel
cells and their design limitations are reviewed.
1 Introduction
Fuel cells have been a topic of discussion with respect to their
application in energy systems. Hydrogen is the primary fuel
used in such cells. Hydrogen has been shown to achieve effi-
ciencies above 80 % but, its low volumetric energy density has
been considered a disadvantage especially when transporta-
tion of the fuel from source to application is needed. Fur-
thermore, along with being explosive and leaking easily, H2
is a very dangerous fuel because it is odorless. To improve
the energy density, various hydrogen carriers like NaBH4are
employed.
Ammonia is a promising hydrogen carrier due to its high
hydrogen density, ease of liquefaction at ambient temperature
and low production cost1,2 . Technologically, use of ammonia
both as a fuel (direct operation) and as a hydrogen carrier (in-
direct operation) is possible. It is a better choice when com-
pared to hydrogen gas and carriers like sodium borohydride
due to the storage and distribution systems already in place.
Further, it is a non-carbon fuel thereby, causing no pollution.
This review aims to study more into the present technologies
to use ammonia both as a fuel and a source of hydrogen that
can be used as a fuel. A detailed elucidation of the production
process, catalysts used and limitations have been discussed.
2 Production of Ammonia
Ammonia can be used in fuel cells as an ideal carbon free fuel.
NH3is one of the most highly produced inorganic chemicals.
It is produced about 150 million tons per year3. Half of all
hydrogen that is produced in different chemical industries is
used to produce NH3. A typical modern NH3producing plant
first converts natural gas like methane or liquefied petroleum
gases like propane or butane into gaseous H2. These H2are
combined with N2to produce NH3via Haber-Bosch process4.
Renewable energy like solar power, wind energy, biomass can
also be used to produce NH3.5
2.1 Haber-Bosch Ammonia Production Process:
The conventional Haber-Bosch (HB) process for producing
Ammonia is considered one of the greatest inventions of the
20th century. It requires medium temperature (about 500°C),
very high pressure ( 250 atm) and reaction accelerating iron
catalyst to produce Ammonia and generally the yield of this
process is 10-20%. In 2012 production of 140m tons of NH3
consumed about 2% of the worlds energy6. Apart from large
energy requirements to achieve reaction conditions, the cur-
rent production method is inefficient because of its need of
hydrogen gas obtained from natural gas. The byproduct of the
process is carbon dioxide. Traditional HB process produces
roughly about 2 ton of CO2/ton of C free NH37.
So two of the major problems of this process are:(1) it
has a large carbon footprint associated with it, which is one
the largest contributors to global warming and environmental
degradation (2) it is not energy efficient (it needs lots of energy
but yield is lower and achieving high pressure and temperature
is expensive).
2.2 Lichts Ammonia Production Process:
Recently researcher Stuart Licht has developed a unique
method of a low-energy process to synthesize ammonia using
just air, water, and sunlight, by electrifying water and bubbling
it through a matrix of iron oxide, and sodium and potassium
hydroxide.7In this case the associate problem of a larger CO2
footprint and less energy efficiency have been improved a lot.
Stuart Licht and his colleagues at George Washington Uni-
versity came up with an innovative approach the scientists
were long awaiting. The method of using water as a source
of hydrogen is not new but electrolysis always involves higher
cost and hence became less appealing. In this method, air and
water instead of NG is used here as a source of H2. We all
know air is made up of 78% N2. In this process the bubble of
wet air is passed through a mixture of tiny particles of iron ox-
ide and molten NaOH & KOH. When electricity is applied,
FeO captures electron to extract H2from water and allows
H2O and air to interact directly to form NH3.
This method claims to use two-third of the energy used
in the HB process. Also the environmental emission here is
1–6 |1
much less than the HB process as there is no CO2produced
as a byproduct. High temperature and pressure is also not re-
quired here like in the HB process. Solar thermal electrochem-
ical production (STEP) technique is used here where hydrogen
can be produced as a by product which in turn is a great source
for fuel in a Hydrogen Fuel Cell.
2.3 Ammonia Production Process by Biogas:
Production from biomass is another way of utilizing renew-
able technology to get rid of excessive carbon release. The
technique is to gasify biomass to produce hydrogen enriched
syngas for ammonia production8.. In the conventional HB
process the major environmental concern comes from the pro-
duction of hydrogen from fossil fuels where natural gas is used
as a feedstock and heavy fuel oil used as the fuel of the re-
former. This environmental emission could be greatly reduced
by using biodiesel or additional syngas production via biomass
gasification.
The major benefit of producing Ammonia from biomass
gasification is: it involves a higher number of hydrogen which
in turns lower the power requirement since the process con-
version efficiency is higher. The other benefit of biogas is: it
generally has a very low sulfur content which eliminates the
need of desulphurization of NG, which is a major preliminary
step of a traditional HB process.
2.4 Solar Thermochemical Ammonia Production Pro-
cess:
NH3can be synthesized by visible light irradiation through
Nitrogen Photo-fixation. Nitrogen from the air is introduced
in the first reaction chamber and concentrated solar radiation
is introduced into the reaction chamber near 100-1250 °C. N2
then passes to the second reaction chamber. H2comes from
steam that is introduced in second reaction chamber at 700
°Cwith manganese nitride and manganese Oxide. NH3is pro-
duced from the second chamber.9
3 Mechanism behind various Ammonia Fuel
Cells
As mentioned earlier, Ammonia Fuel Cells can be direct or
indirect fuel cell depending upon where the decomposition of
ammonia molecules takes place. Mechanisms for both of them
are reviewed and included below.
3.1 Indirect Ammonia Fuel Cell:
Most of the current literature in this mode of Ammonia Fuel
Cell is divided into two major modes: thermal 10 and electrol-
ysis/electrooxidation.11 There are also papers that talk about
hydrolysis from NH3products.12
3.1.1 Thermal Decomposition/Catalytic cracking:
2 NH3+ heat N2+ 3 H2{1}
Thermodynamically speaking this equation is very favorable
at high temperatures of around 200°C and leads to 99% con-
version of ammonia to H2.10 The problem according to litera-
ture lies in its kinetics. Therefore, most of the literature is ded-
icated to finding the best and also the cheapest catalyst to bring
down the kinetic barrier for the reaction. Keeping the brevity
of this review paper, the best results obtained were 13 who used
nanosized Ni particles on SBA-15 support and achieved a rate
of hydrogen generation of 33.3 mmol of H2per gram of cata-
lyst per min.
Mechanism: According to literature14 it is a three step pro-
cess. 1) adsorption of ammonia onto catalyst sites, 2) cleavage
of N-H bond on adsorbed ammonia, 3) recombination desorp-
tion of N2atoms
NH3NH3surface {2}
NH3surface + NH2surface + Hsurface {3}
2 Nsurface N2{4}
There was a consensus among the researchers that combina-
tive desorption of nitrogen atoms is the rate determining step
in the decomposition of the reaction. Therefore the active re-
search in the field is for finding the catalyst which reduces the
barrier potential for this step the most at the least cost.
3.1.2 Electrolysis/electro oxidation:This method is used
where the scalability might be an issue. The amount of H2
produced depends upon the current supplied to the external
reactor. It was first proposed by Vitse et. al15.
Mechanism:
2 NH3+ 6 OH–N2+ 6 H2O + 6 e–{5}
6 H2O + 6 e–3 H2+ 6 OH–{6}
The thermodynamic potential for above two equations are -
0.77eV and -0.82eV. The most widely accepted mechanism
of ammonia oxidation is 1) the adsorption of ammonia on to
Pt surfaces, 2) dehydrogenation of ammonia into various ad-
sorbed intermediates (N, NH, NH2), 3) reaction of the inter-
mediates to form N2H2surface, N2H3surface & N2H4surface which
then react with OH to produce nitrogen16. Again, major work
here is dedicated to finding good catalyst for its kinetics.
2|1–6
3.2 Direct Ammonia Fuel Cell:
For these fuel cells there is no need for an external reactor
to supply the H2for cell. To generate electricity, ammonia
is directly used in cell. Literature talks about using Polymer
Electrolyte Membrane Fuel Cell (PEMFC), Solid Oxide Fuel
Cell (SOFC) and Alkaline Fuel Cell (AFC)for this mode of
operation
3.2.1 Polymer Electrolyte Membrane Fuel Cell:The lit-
erature has consensus about the difficulty in operating direct
ammonia fuel cell with PEM. The Nafion©membrane used is
acidic and it reacts with basic products of ammonia to create
salts, which increases the resistance of the cell rapidly. This is
an irreversible process making the cell inefficient.
3.2.2 Alkaline Membrane and Solid Oxide Fuel Cell:
To solve the above mentioned problem of acidic Nafion mem-
brane, researcher17 have tried working with alkaline mem-
brane transporting hydroxide ion across. These Alkaline
Membrane Fuel Cells (AMFC) works under the same prin-
ciples as AFC.
O2+ 2 H2O + 4 e–4 OH–{7}
and anode
2 NH3+ 6 OH–N2+ 6 H2O + 6 e–{8}
Thus overall the reaction adds up to be
4 NH3+ 3 O22 N2+ 6 H2O{9}
AMFC works well but have very low power density at room
temperature even after using expensive platinum alloy cata-
lyst.
As for Direct Ammonia Fuel Cells, up to date the best
choice is to operate at high temperature using Solid Oxide Fuel
Cell (SOFC) technology.Vayenas and Farr 18 made the first at-
tempt to fuel an yttrium-stabled zirconia (YSZ) based SOFC
with ammonia, where the electrolyte was an oxygen carrier.
The reaction carried at the cathode was following:
2 NH3+ 5 O –
22 NO + 3 H2O + 10 e–{10}
2 NH3+ 3 NO 5
2N2+ 3 H2O{11}
The above reaction of NO production is rate limiting
because of the slow diffusion of O2through the elec-
trolyte,therefore, NO is produced at the SOFC anode.
As we can see in Figure 1, especially in the case of Ammo-
nia Fuel Cells, the presence of a good catalyst is even more
important than for H2fuel cell. We will discuss the types of
catalyst for different Ammonia Fuel Cells.
Fig. 1 IV plot for an SOFC having an input fuel of ammonia and a
further plot showing the activity of the same cell having an input
fuel of hydrogen. No catalyst was in use. The results confirm that
ammonia is relatively inactive without the catalyst.21
4 Design of catalyst for Ammonia Fuel cell
Catalysts for ammonia production typically utilize potas-
sium promoted iron,19however some processes use ruthenium
based catalysts. Catalysis of ammonia in fuel cells is much
different however and mostly centers around nickel catalysts.
Nickel as a catalyst in ammonia SOFC poses a big advantage
because it reduces the cost of the fuel cell by avoiding the use
of more expensive metals, such as platinum.20 Ammonia is
most viable in Alkaline Membrane Fuel Cells (AMFCs) and
Solid Oxide Fuel Cells (SOFC) and can be used in (PEMFC)
but is unadvisable because of unwanted chemical reaction be-
tween the basic ammonia fuel and the acidic Nafion membrane
as well as poisoning of the platinum catalyst3.
4.1 Catalyst for SOFC:
The use of ammonia in SOFC requires some slight deviation
from typical material selection, specifically in the electrolyte.
The standard yttria stabilized zirconia (YSZ) electrolyte can
be used with ammonia fuel, however the transport of oxygen
ions causes the production of NO so a proton conducting elec-
trolyte is used instead, usually a doped Ba structure such as
BaCeO3or BaZrO33. In a SOFC, the catalyst is incorporated
in the electrode material so if YSZ is used as the electrolyte
then the anode will likely be a Ni/YSZ cermet while if a Ba
based structure such as BaCeY is used, the anode will be a
composite of the Ba structure and Ni such as Ni/BaCeY3,22.
The anode will likely be the support of the SOFC and opti-
mally contains about 50% Ni20. It is important to note that Ru
has been shown to perform better than Ni as a catalyst for the
decomposition of ammonia, however Ni is much cheaper than
Ru and performs similarly under high temperature conditions
and is thus much more economically viable for a SOFC20.
The cathode of an ammonia fed SOFC is rather typical as a La
1–6 |3
structure doped with Sr, such as LaSrCoO23 .
4.2 Catalyst for AMFC:
Ammonia can also be used as fuel in an AMFC, using a typi-
cal anion exchange membrane such as Tokuyama A201 24. A
PtRu/C composition can be used for the anode, however the
alkaline conditions of the AMFC allow for the use of a Ni/C
composition, such as CDN/C, which have been shown to out-
perform PtRu/C anodes when a higher loading is used and still
be a cheaper catalytic alternative17 . The cathode also presents
the opportunity to use non-noble metal catalysts as a MnO2/C
can be used effectively to reduce oxygen and water. Addi-
tional advantages to using ammonia in an AMFC are that the
presence of CO2at the cathode has been shown to improve the
performance of the AMFC so air is a readily viable option in-
stead of pure oxygen17. A disadvantage to using ammonia in
an AMFC however is that NO may be produced from reaction
of ammonia at the cathode if cross over occurs3.
5 Limitations
Ammonia has been discussed both as a direct source of fuel
and as a source of hydrogen gas that may be then consumed
for the operation of the fuel cell. These ideas have been exten-
sively studied for the past decade and the results are promising
but, there is a wide range of limitations. Primarily, ammonia
is a toxic gas and when mixed in air can form a combustible
mixture. But, if there is a leak, detection above 1 ppm is easy
due to the gas pungent odor.
5.1 Limitations when using directly as a fuel
5.1.1 Inability to use popular PEMFC design: Systems
based on polymer electrolyte membranes have been a technol-
ogy of interest in the recent years due to the low operation tem-
peratures required. Ammonia cannot be applied directly as a
fuel in these acidic mediums. This hampers the use of ammo-
nia as a fuel in PEMFC with Nafion electrolytes. The acidic
H+replaces the NH4ions both in the catalyst layer ionomer
and the bulk membrane causing serious decrease in conduc-
tivity. The reduced conductivity leads to an overall reduction
of efficiency and power output.25 .
5.1.2 Higher Ohmic resistance: As an option, we can
use ammonia as a fuel source in alkaline systems with potas-
sium hydroxide electrolytes. These Alkaline Fuel Cell sys-
tems are more bulky and have a lower output voltage. The use
of a liquid electrolyte instead of the thin Nafion membrane
increases the size of the cell. The increased size, causes an
increase in the distance travelled by the ion and thereby, the
ohmic resistance is comparatively greater. New research ven-
tures have investigated the possibility of an alkaline electrolyte
membrane system but, most of the reported works do not reach
an efficiency at par with the PEMFCs.26,27
5.1.3 Higher activation over-potential and poisoning of
catalyst: Now, if ammonia is consumed at the anode towards
production of energy from the cell, greater loss may be ex-
pected due to the higher activation potential (0.43V) associ-
ated with reaction of ammonia at the anode.28,29 Also, the
combustion of ammonia produces nitrogen gas that has been
reported to adsorb onto the platinum used in the catalyst layer.
This causes poisoning of the catalyst layer.25,29,30 Poisoning
calls for the replacement of the catalyst layer which can prove
to be a tedious and costly process. This poisoning of the
catalysts can lead to effectively having to operate the cell in
batches. Operating fuel cells in batch mode makes its opera-
tion similar to that of a simple galvanic cell.
5.1.4 Incomplete combustion of Ammonia:When used
as a fuel directly, the combustion process is based on an equi-
librium. Hence, the combustion is never complete and this
causes reduced efficiency and production of waste ammonia
gas. Ammonia Fuel Cells have been operated at room tem-
perature but the power density is rather low, which could be
related to the low catalytic activity of the electrode materials
at low operating temperatures17 hence, SOFC is definitely a
good option for the production of energy. The consumption of
ammonia in Solid Oxide Fuel Cells have come to be a topic of
interest in the recent years and the combustion characteristics
are found to be good but, the waste oxides of nitrogen pro-
duced during combustion may cause suffocation and health
problems. It is known that the performance of the ammonia
fed SOFC is highly dependent on the extent of ammonia de-
composition in the SOFC31,32.
5.2 Limitations when used as a source of hydrogen
The production of hydrogen from cracking of ammonia is an-
other way to utilize the gas in fuel cells. Firstly, it is a two-
step process, hence the investment in a large project would
be greater. Also, ammonia cracking is an equilibrium reac-
tion. No equilibrium reaction reaches completion and there
is always some reactants in the product stream. That means,
there are always traces of ammonia in the H2N2mixture ex-
iting the cracking reactor. The presence of ammonia in the
product stream may cause pollution or, may affect the com-
bustion within the hydrogen fuel cell. To purify the product
stream, a stripper is required that removes the ammonia and
nitrogen gases from the product stream. The removal of ni-
trogen from the stream is proven to be hard and the process
has some residual nitrogen in the outlet. This fact might be
a drawback in case the produced hydrogen stream is fed into
Proton Exchange Membrane Fuel Cell (PEMFC).33
4|1–6
6 Conclusion
Using suitable catalysts, ammonia fuel cell works at par, and
in some designs, better than hydrogen fuel cells. Ammonia of-
fers a clean and reliable source of energy, without many of the
problems associated with the traditional hydrogen economy.
Using ammonia as direct fuel has its advantage but besides
using SOFC, there are little competitive designs to make Am-
monia Fuel Cells a popular alternative. Challenges currently
facing AFCs are similar to those faced by other fuel cells; most
of the current research is dedicated to finding better as well as
cheaper catalyst materials and innovative designs around the
three phase region in the cell. In brief, by not being challenged
by the storage problems of H2, once a compact design of AFC,
working at low temperature, is available at competitive prices,
ammonia may very well be the fuel that makes the technology
commercially viable.
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1–6 |5
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