Direct Ammonia Fuel Cells
Direct Ammonia Fuel Cells
A general overview, current technologies and future directions
Ammonia will be utilised as a key energy vector for storage and long-distance transport in the developing hydrogen economy. Direct ammonia fuel cells (DAFCs) have the potential to decrease the process complexity of current fuel cell technology and therefore increase overall efficiency and unit footprint where implemented. In this paper, current DAFC technologies are explored, such as solid oxide, alkaline and ammonia borane fuel cells. From this, it is shown that solid oxide fuel cells with oxygen conducting electrolyte (SOFC-O) have high experimental power outputs of 1100 mW cm–2 but have disadvantages of high nitrogen oxides (NOx) production, lower fuel utilisation and low efficiency. Alkaline and ammonia borane fuel cells are of lesser interest due to complex ammonia pretreatment, high NOx production and lower power outputs of 450 mW cm–2 and 110 mW cm–2, respectively. Solid oxide fuel cells with proton conducting electrolyte (SOFC-H) seem to have the most potential due to high theoretical power outputs, high efficiency, increased fuel utilisation and low NOx production. DAFC technology has yet to reach full commercialisation, but as the hydrogen economy develops the potential benefits of DAFCs in complexity and footprint reduction will drive further investment and development, particularly in the shipping sector.
The drive for a greener and more sustainable world has now been adopted into all industry areas. Large multinational energy and technology companies have adopted ambitious environmental, social and governance (ESG) goals and set targets for achieving net-zero emissions. Industries are moving towards cleaner processes for their commercial products and the source of their energy and chemical raw materials has come into focus. The ammonia industry is making efforts to be more sustainable and be a key enabler in a future hydrogen economy. Large ammonia producers are looking at ‘green’ and ‘blue’ avenues for ammonia generation as well as the potential downstream market and applications for its low carbon (‘blue’) or no carbon (‘green’) ammonia. Currently, ammonia is typically produced by using a natural gas or fossil fuel feedstock that is converted into hydrogen and combined with nitrogen in a 3:1 ratio. Using a colour-coding scheme similarly to hydrogen production, the common method of ammonia synthesis described above is known as ‘grey’ or ‘brown’ ammonia. This process, however, generates a lot of carbon dioxide emissions which are estimated to be between 1.6–3.8 tonnes per tonne of ammonia produced (1).
To mitigate the carbon emissions, alternatives for ammonia generation have been explored. The following are just some of the possible methods for more sustainable ammonia production. Blue ammonia is produced by the same process as grey or brown ammonia, but there are additional carbon capture and storage steps. ‘Turquoise’ ammonia uses pyrolysis to convert natural gas into carbon and hydrogen. Finally, green ammonia is a production route that uses only renewable energy, water and air to produce ammonia and is completely carbon-free.
Ammonia is already used in several industries, such as the production of chemicals, fertilisers, nitric acid, explosives and synthetic fibres. In terms of sustainability, ammonia can potentially help replace more carbon emitting technologies that use fossil fuels. For example, ammonia has the potential to be used as a fuel in vehicles and shipping (2). Furthermore, it can be used as a transport and storage vector for hydrogen that can later be reconverted for hydrogen using applications.
A technology of particular interest is DAFCs. This technology can convert ammonia directly into useable electricity without the need for complex preconditioning or reconversion back into hydrogen. This review will discuss the current and future development of DAFCs as well as competition with other ammonia-based energy production.
2. Ammonia as a Fuel
Ammonia as a fuel source is advantageous due to its very mild enthalpy of reformation, around 46 kJ mol–1 described in Equation (i) (3). This is beneficial because it allows ammonia to easily act as a hydrogen carrier. Furthermore, ammonia fuel offers many solutions to the limitations of hydrogen fuel, such as being less energy intensive to liquefy, store and transport. Figure 1 demonstrates the cost estimates for hydrogen in comparison to ammonia.
As shown in Figure 1, the cost to transport hydrogen via shipment is threefold in comparison to ammonia. This is due to the transport and storage conditions required for each of the components. As ammonia liquefies at –33°C, the energy required to keep it in phase is lower compared to hydrogen’s required temperature of –253°C (1). This allows ammonia to be more cost-competitive to store in comparison to hydrogen and even to other popular hydrocarbons. This is highlighted in Table I.
|Boiling temperature at 1 atm, °C||–33.4||–253||–161||–42.1|
|Condensation pressure at 25°C, atm||9.90||n/a||n/a||9.40|
|Lower heating value (LHV), MJ kg–1||18.6||120||50.0||46.4|
|Flammability limit (equivalence ratio)||0.63~1.40||0.10~7.1||0.50~1.7||0.51~2.5|
|Adiabatic flame temperature, °C||1800||2110||1950||2000|
|Maximum laminar burning velocity, m s–1||0.07||2.91||0.37||0.43|
|Minimum autoignition temperature, °C||650||520||630||450|
aData of boiling point and condensation point are from NIST database (4)
From Table I, it is shown that ammonia has many beneficial qualities compared to hydrogen and common fossil fuels. It can be liquid at room temperature under 10 atm pressure; its power density is comparable to other liquid fuels, as shown in Table II. Finally, it is non-flammable, vaporises easily, is not explosive and it has well established, extensive transport and storage infrastructure readily available. These favourable qualities highlight the practical potential for commercialisation as a fuel.
In operations where individuals or corporations have committed to reduce CO2 emissions, blue or green ammonia offers current operators a path to reduce direct combustion emissions on a partial and more economical basis. This is a good short-medium term option vs. introducing full scale post-combustion CO2 removal systems to industrial applications such as industrial scale furnace or boiler packages. The energy density of ammonia allows for a more efficient partial decarbonisation of existing fuel systems and could decrease the reliance on fossil fuel firing without full asset overhauls.
However, there are safety concerns with ammonia as a fuel. It is corrosive, and can potentially cause dehydration, skin burns, frostbite, eye damage, lung damage on inhalation and fatal respiratory failure if inhaled at 5000 ppm concentrations (2). These undesirable qualities necessitate strict health and environmental regulations when practically used.
2.1 Direct Combustion of Ammonia
Direct combustion of ammonia uses ammonia as a primary fuel source or additive in traditional fossil fuel combustion engines, gas turbines and furnace applications. In practical application, Figure 2, combustion is difficult to achieve because if ammonia is combusted with excess air it is partially oxidised and forms NOx and N2O as byproducts in variable concentrations, Equation (ii) (5).
N2O emissions are equivalent to approximately 300 times the greenhouse gas effect of CO2 emissions, so must be minimised in order to achieve net CO2 equivalent reductions. Additionally, there are challenges with combustion such as high ignition temperature, low flame velocity and slow chemical kinetics (5). Due to the described challenges, direct combustion of ammonia may not be favourable as a long-term solution although dosing natural gas with ammonia may be beneficial to reduce current carbon emissions through the energy transition.
2.2 Direct Ammonia Fuel Cells
DAFCs use ammonia as the primary feed to fuel cells. Instead of ammonia combusting immediately as described in the above section, the ammonia is dissociated into hydrogen and nitrogen ions, purified and the hydrogen ion instigates the necessary reactions. The reactions are the same as Section 2.1 but occur within a fuel cell to immediately produce electricity via electrochemical reaction (6). Details of this reaction are referenced elsewhere (7).
DAFCs have the potential to be a clean and efficient power generation source with around 50–65% efficiency. Fuel cells can be portable and have a lower impact on the environment due to the reduction in NOx. Furthermore, because fuel cells can directly convert a fuel into electricity using internal electrochemical reactions (6), this removes the need for turbines or other electricity generating units which can reduce industrial plant footprint and further simplifies overall process.
3. Ammonia Fuel Cells: Structure and Operation
A DAFC obtains hydrogen in a similar way to a natural gas-fed fuel cell. This type of fuel cell consists of an anode for oxidation and cathode for reduction. There is also an electrocatalyst that separates the anode and cathode from a porous and electrically conductive membrane. In the centre is an ionic membrane that is not electrically conductive (3). Figure 3 shows the schematic more in detail (8).
Hydrogen is dissociated from a natural gas or ammonia fuel by using catalyst and steam reformation. The hydrogen passes through a membrane that is electrically insulated so that the counter flow of electrons alternatively goes through an electrical circuit for power generation. The hydrogen on the other side of the membrane then recombines with atmospheric oxygen to form water (3, 9).
3.1 Direct vs. Indirect Ammonia Fuel Cells
An indirect ammonia fuel cell is in essence a normal hydrogen fuel cell, but there is the added step of dissociating the ammonia to get access to the hydrogen. The nitrogen is typically released into atmosphere and the hydrogen is used as the fuel. Indirect ammonia fuel cells require the ammonia to be cracked into nitrogen and hydrogen and then purified of any residual ammonia before entering the fuel cell. The advantage of indirect ammonia fuel cells is that the technology for hydrogen fuel cells is developed and is commercially available. The indirect fuel cell system, however, is more complex and costly than a direct ammonia-fed fuel. The extra dissociation and purification step requires additional infrastructure to handle the nitrogen byproduct and thermal cracking units. This step alone makes indirect ammonia fuel cells theoretically not cost competitive, as demonstrated by Figure 4.
Figure 4 shows the cost of transporting liquid hydrogen that includes conversion to liquid hydrogen, transportation and reconversion to gaseous hydrogen. The yellow line shows the transportation cost of the conversion of hydrogen to ammonia and reconversion back to hydrogen. Finally, the dotted line shows the cost of ammonia transportation without any conversion necessary. For hydrogen purposes, there is a critical distance between liquid ammonia and liquid hydrogen transport where short distances tend to favour liquid hydrogen, but longer distances favour liquid ammonia. However, using ammonia directly without reconversion to hydrogen is significantly more cost-effective than the other transport methods no matter the distance (10).
An example of an indirect fuel cell that can use the hydrogen from dissociated ammonia is a proton exchange membrane (PEM) fuel cell. Ammonia is decomposed and trace amounts are removed before dissociated H ions can enter the cell. Purification is required because the fuel cell’s platinum hardware is sensitive to ammonia (11). Ammonia degrades the PEM fuel cell membrane conductivity, catalyst ionomer of the anode and cathode, hydrogen oxidation reaction and oxygen reduction reaction. Based on work done previously on the effects of ammonia in PEM fuel cells, the researchers saw a loss of voltage, reduced water uptake by the catalyst ionomer and the possible formation of hydrogen peroxide which in turn reduces catalyst activation (11).
The sensitivity of current hydrogen fuel cells to ammonia, paired with the complexity and inherent efficiency losses, has led many researchers and some companies to put considerable focus and investment into DAFCs.
4. Direct Ammonia Fuel Cell Types
4.1 Solid Oxide Fuel Cells
SOFCs are unique in that they use ceramic electrodes with platinum group metal (pgm) electrolytes between the anode and cathode of the fuel cell device (12). This type of fuel cell can have high energy conversion efficiency and high degree of fuel flexibility (2). Due to the high operational temperatures of SOFCs, which are typically over 800°C, it supersedes ammonia’s cracking temperature of 500°C, allowing the ammonia to be directly used in hydrogen-based fuel cells (2). This method removes the requirement of a pretreatment stage, which was necessary for the indirect ammonia fuel cell. However, the dissociated ammonia generates both hydrogen and nitrogen which means there is potential to form NOx. There are currently two types of ammonia-fed SOFCs in development: SOFC-O and SOFC-H (2). These are defined by the electrolyte used and are expanded on in the next sections.
An SOFC-O denotes an oxygen conducting electrolyte, which transports oxygen anions across the pgm electrolyte. The ammonia is dissociated in the cell and the resulting hydrogen produced is oxidised at the anode. Oxygen is introduced at the cathode-electrolyte interface and reduced to oxygen anions. The oxygen anions are transported across the electrolyte where they combine with the dissociated hydrogen, forming water and producing an electrical current (2). Equations (iii) and (iv) represent what occurs at the anode and cathode, respectively. Figure 5 represents an SOFC-O process.
An SOFC-H is a proton conducting SOFC. The ammonia fuel breaks down to nitrogen and hydrogen at the anode and oxygen is introduced to the cathode, in the same arrangement as an SOFC-O. However, in an SOFC-H it is the resulting hydrogen ions that are transported through the electrolyte to reach the cathode-electrolyte interface. Then the hydrogen ions react with oxygen to form water vapour. The water vapour and unreacted oxygen leave on the cathode side while unreacted ammonia, nitrogen, and hydrogen leave on the anode side. The removal of water on the cathode side is the key difference between SOFC-Hs and SOFC-Os as it prevents the formation of NOx (9). Equations (v) and (vi) show what occurs at the anode and cathode, respectively. Figure 6 represents an SOFC-H process. Other potential electrode/electrolyte configurations and performance for ammonia-fed SOFC-O and SOFC-H devices, respectively, are described by Siddiqui et al. (9).
It is clear that the operating temperature has a large impact on SOFC performance (6, 9). A study by Afif et al. (6) shows that at 800°C, the equilibrium potential of a SOFC-H is greater than a SOFC-O, even with different fuel utilisation. Additionally, the efficiency of an ammonia-fed SOFC-H is roughly 10% greater than a SOFC-O and the peak power density of SOFC-H is about 20–30% higher than SOFC-O due to higher concentration of hydrogen at the anode (6). However, at temperatures above 800°C, the peak power density of an SOFC-O is about 50% higher than a SOFC-H, but this is mainly due to uncertainty in estimating the exchange current density of the SOFC-H in comparison to the rate expressions used for SOFC-O (6).
There is research into developing low temperature SOFCs, where operating temperature is typically 100°C. Unfortunately, this technology has a few application issues. Firstly, operating temperature requires the use of ionomeric membrane like hydroxide exchange membrane which allows higher conductivity and better chemical stability (13). However, the membrane is deactivated by the ammonia itself as the ammonia is not disassociated into ions before entering the fuel cell. Secondly, platinum and iridium would be the only usable metals due to their significant electrocatalytic capability under 150°C (13), but again the metals are deactivated by ‘surface poisoning’ by the ammonia feed. Further information of this research can be found elsewhere (13) but the research itself requires further maturity to overcome this known issue.
It is also noted that there is a large contrast between theoretical and experimental peak power outputs. Theory shows SOFC-H to be the optimum technology due to the high potential in peak power densities, 700 mW cm–2, and low NOx production. SOFC-O only reaches 600 mW cm–2 via the same theoretical modelling (9). This contrasts with the experimental results, where SOFC-O is shown to have the highest power output of 1100 mW cm–2 at 650°C while SOFC-H only reaches 275 mW cm–2 at 650°C. This is due to the unexpected development of hydrogen dilution at the cathode of the SOFC-H which reduces power outputs. Further developments must be made to overcome this so that SOFC-H can reach its full theoretical potential (9).
Overall, ammonia-fed SOFCs have the advantage of being an established technology, decomposing ammonia in situ and not requiring platinum or similar metals. The primary research focus is to improve stationary applications for heat and power, transportation and reduction of oxidation effects (1).
4.2 Ammonia Alkaline Fuel Cells
Ammonia alkaline fuel cells (AAFCs) use the exchange of anions. Air is fed into the cathode side and oxygen from the air reacts with water molecules to form hydroxide anions. The hydroxide anions transport through an anion exchange membrane or an alkaline electrolyte, depending on design. Ammonia is fed as a fuel on the anode side and combines with the hydroxide ions to form nitrogen and water. The reaction between oxygen and water at the cathode side consumes electrons. The reaction between ammonia and hydroxide ions at the anode sides produces electrons resulting in an electric current (9). Equations (vii) and (viii) show what occurs at the anode and cathode, respectively. Figure 7 represents an alkaline fuel cell process. Potential electrode/electrolyte configurations and performance for ammonia-fed alkaline fuel cells are demonstrated by Siddiqui et al. (9).
Another factor which makes AAFC desirable is the use of air as an oxidant rather than pure oxygen. Furthermore, if potassium hydroxide-based electrolytes are used, carbonate ions are formed which reduce the hydroxide ions and thus reduce the number of ions available to react with ammonia (9). Carbonate ions also form precipitates within the fuel cell which decrease the overall power density. Pretreatment of air via carbon dioxide scrubbing is required to overcome this issue (14, 15).
Overall, ammonia-fed alkaline fuel cells have the advantage and option of not using platinum or similar metals and are highly tolerant of ammonia. However, they have a low-moderate energy density of 450 mW cm–2 at 100°C (16), limited number of commercial suppliers and the need for carbon dioxide scrubbing. There is research into platinum-based membranes but the power output is lower than potassium or sodium hydroxide based membrane at 16 mW cm–2 at 220°C (17). The research currently focuses on increasing energy density and improving suitability for stationary applications (1). Researchers are still exploring its commercial viability in the automobile industry (2).
4.3 Ammonia Borane Fuel Cells
Another DAFC technology uses ammonia borane (NH3BH3) in the anode side to react with hydroxyl anions. Ammonia borane is desirable due to its high hydrogen density. It can be synthesised using Equation (ix), using borane-tetrahydrofuran and ammonia (17, 18).
Within the fuel cell, the hydroxyl anions are formed by the reduction of oxygen at the cathode. The electrolyte can be either a cation or anion exchange membrane. The hydroxyl anions diffuse through the membrane electrolyte and on the anode side, ammonia borane is oxidised (9). Equations (x) and (xi) show the reactions occurring at anode and overall. Potential electrode/electrolyte configurations and performance for borane fuel cells are demonstrated by Siddiqui et al. (9).
Ammonia borane is of interest as a hydrogen carrier commercially, but due to storage issues has not yet progressed beyond the research stage. Ammonia borane must be regenerated outside the hydrogen storage tank thus requiring an increase in process complexity to include regeneration infrastructure (19). Additionally, this fuel cell performs best using platinum or other precious metals (20). Zhang et al. observed that an ammonia borane fuel cell with an anion exchange membrane and platinum electrodes had power densities between 40–110 mW cm–2 (9). They also noted peak power output of 14 mW cm–2 at room temperature (21).
Even though these results are promising, ammonia borane’s complex process infrastructure partnered with its high requirements of precious metal per product can result in a low cost-efficiency product, unless correct metal recycling routes are in place or a simplified process is available.
4.4 Comparison of Fuel Cell Technologies
Key information about the DAFC technologies discussed so far is summarised in Table III. From this, it is clear that SOFCs are the most developed and efficient form of DAFC, whereas alkaline and ammonia borane fuel cells have too low energy densities or require additional process stages.
|Technology||Experimental peak power output, mW cm–2||Operating temperature, °C||Optimum peak power and temperature||Advantages||Disadvantages||References|
|SOFC-O||20–1190||500–750||1190 mW cm–2 @ 650°C||Low pgm use; no ammonia pretreatment; high experimental power output||No water; ammonia-nitrogen mixture produced; high NOx production||(2, 9)|
|SOFC-H||15–390||450–700||390 mW cm–2 @ 750°C||Reduced NOx production; high fuel utilisation; low pgm use; no pretreatment needed||Reduced power potential due to hydrogen dilution||(2, 9)|
|Ammonia alkaline||16–450||100–400||450 mW cm–2 @ 100°C||Option to use precious metals in hardware||Requires carbon scrubbing||(9, 16, 17)|
|Ammonia borane||14–315||20–45||110 mW cm–2 @ 45°C (individual cell); 315 mW cm–2 @ 45°C (using a 10 stack cell)||High hydrogen density feed||Additional regeneration stage for ammonia borane; storage issues||(9, 21)|
5. Future Directions
5.1 Direct Ammonia Fuel Cell Technology
As discussed previously, DAFC still requires maturity from its current state to ensure proper commercial production. The primary research is to improve experimental performance of DAFCs especially in open-circuit voltage, peak power density and short circuit current density. The next stages for ammonia technology, shown in Figure 8, highlight the evolution of ammonia with existing technology. The final goal encompasses integrated systems within the global power and energy infrastructure.
5.2 Commercial Viability
DAFC commercial infrastructure is currently being developed by 22 companies worldwide (22), such as Yara (23) Topsoe (24) and Mitsubishi Power (25). This is to ensure a proper foundation is in place by the time DAFCs reach full product maturity. Further commercial production of DAFC requires heavy investment to create necessary infrastructure to sustain large-scale production. This includes investments in raw material procurement, commercial process scale-up and large-scale manufacturing facilities.
Scarcity of compatible catalysts will also be an issue which can affect large-scale production of DAFC. Some catalysts can have adverse effects with ammonia and therefore there will be a limited variety of compatible metals or metal alloys which can be used in future DAFC products. Catalysts with iron-based composites or alloys are sought after as they have sufficient activity for the electrochemical oxidation of ammonia and low susceptibility to poisoning. These favourable qualities can result in the dependency on such metal or metal alloy prices and future financial fluctuations.
The pgms platinum, palladium, ruthenium and iridium are widely used materials for fuel cell hardware fabrication. Limited supply of these metals must be taken into consideration, alongside research into recycling these materials. Recycling of pgms is already at an advanced stage of technology with over 95% recovery possible (26). An attractive DAFC technology which could minimise the use of pgm would be SOFCs, since precious metal is only used for the electrolyte of the fuel cell while the electrodes are ceramic (12). The combined use of ceramics and pgm could result in a similar recycling process to that used for current automobile catalysts, which are comprised of ceramic substates and a precious metal washcoat.
As discussed earlier, the full potential of DAFC performance based on open-circuit voltage, peak power density and short-circuit current density has not yet been achieved at laboratory scale. Furthermore, life cycle studies need to be conducted to determine the life of these cells for commercial viability (22).
Overall, the primary concerns for ammonia fuel cells are the scarcity of highly compatible catalysts, growth of commercial infrastructure and precious metal management. Work is being done by several companies and universities to make ammonia fuel cells viable for commercial use. The timeline of recent and future progress is shown in Figure 9.
In closing, the initial research foundation is already in place for DAFC to be used for sustainable energy production, but further development is needed for commercial use. Direct combustion of ammonia or dosing fossil fuels with ammonia may be preferred in the short term, as it allows use of pre-existing fossil fuel infrastructure, turbines and power generation equipment without immediate phase out. Direct combustion of ammonia could therefore act as an intermediate before DAFC comes to its full realisation and maturity within the commercial market.
SOFCs, particularly SOFC-H, has the most potential due to its minimised use of precious metal, high efficiency, increased fuel utilisation, reduced NOx production and simplified process. This technology requires further investigation to overcome the hydrogen dilution at the cathode and reach its full power potential. SOFC-O, alkaline ammonia and ammonia borane fuel cells have disadvantages such as low power potentials, high process complexity and high NOx production making them unfavourable.
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Andrew Cai contributed to the early development of this article while working as a Graduate Rotation Engineer at Johnson Matthey, Oakbrook Terrace, USA from May 2021 until September 2021. During his time with Johnson Matthey, he worked on ammonia production and future uses in the energy economy.
Zoe Rozario graduated from Heriot-watt University, Edinburgh, UK, with an MEng in Chemical Engineering with Energy Engineering in 2020. She joined Johnson Matthey as a graduate within Operations directly after. She has completed five rotations in precious metals, clean air, health and catalyst technologies.