Ammonia Fuel Cells
Can we use conventional fuel cells with ammonia instead of hydrogen?
There are many ways to replace H2 with NH3 in conventional fuel cells. But there are some limitations. Ammonia can be used both as direct or indirect fuel for fuel cells. Indirect operation means ammonia is used to produce hydrogen, and then the hydrogen is transferred to the fuel cell. Direct operation means NH3 is used as the main fuel inside the fuel cell.
- By thermal decomposition:
2𝑁𝐻3 + heat → 𝑁2 + 3𝐻2
At a temperature of around 200 centigrades, this reaction leads to 99% conversion of ammonia to hydrogen[1]A. Thaker, M. Mathew, N. Hasib and N. Herringer, “A Review of Ammonia Fuel Cells,” 2015.. But the kinetic of the reaction is the barrier and limits the time efficiency of this process.
- By electrolysis:
2𝑁𝐻3+6𝑂𝐻¯ →𝑁2+6𝐻2𝑂+6𝑒¯
6𝐻2𝑂 + 6𝑒¯ → 3𝐻2 + 6𝑂𝐻¯
In this method, the amount of H2 produced depends on the current supplied to the external reactor. The kinetics of the reaction is again a limit and can be improved by choosing a proper catalyst.
Direct operation
With this method, no reactor is needed for a cracking process to extract H2. Fuel cells are characterized by their electrolyte material. Three types of fuel cells could use ammonia as a direct fuel. PEM fuel cells, alkaline fuel cells, and solid oxide fuel cells. We will discuss these fuel cells and their feasibility for using NH3.
1. Proton Exchange Membrane Fuel Cells (PEMFC)
A PEM (Proton Exchange Membrane) fuel cell has 4 main components. The cathode, the anode, the membrane, and the catalyst. Low-temperature proton exchange membrane fuel cells using hydrogen as the fuel have been developed for various applications including electric vehicles. But the drawback is that ammonia cannot be used in these fuel cells. Ammonia will contaminate the Pt/C anode catalyst and react with the acidic Nafion membrane [2]R. Lan and S. Tao, “Ammonia as a suitable fuel for fuel cells,” 2014.. Although ammonia can’t be directly injected into these fuel cells. There is always the option of using ammonia to produce hydrogen, then injecting the hydrogen inside the fuel cell.
- Minimize the crossover of ammonia
- Identify suitable electro-catalysts
Proton-conducting materials based on acidic properties cannot be used as electrolytes for ammonia fuel cells due to the reaction between ammonia and acids. Also, the low operation temperatures of PEMFCs are not suitable for breaking the N-H bonds, therefore the cracking of ammonia in these conditions is complicated. For temperatures below 200 degrees, with the current technology we have, it is difficult to have a direct ammonia fuel cell with good performance.
2. Alkaline Fuel Cells (AFC)
Besides proton exchange membrane, polymeric alkaline membranes are also developed for fuel cell applications. Alkaline fuel cells consume hydrogen and pure oxygen producing potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70% efficiency[3]”Platinum-free fuel cell developed in Japan,” Reuters Editorial, 2007.. The difference with PEM fuel cells is that Alkaline fuel cells (AFCs) are based on the transport of alkaline anions (usually 𝑂𝐻−) between the electrodes. By using ammonia in these fuel cells and selecting a nickel anode and a 𝑀𝑛𝑂2 cathode, the overall reaction gives:
4𝑁𝐻3 + 3𝑂2 → 2𝑁2 + 6𝐻2𝑂
NH3 Alkaline fuel cell (ESL – The Electrochemical Society)
These ammonia fuel cells have been operated at room temperature but they are not very power-dense. This is because of the low catalytic activity of the electrode materials at a low operating temperature[4]R. Lan and S. Tao, “Ammonia as a suitable fuel for fuel cells,” 2014.. The draw backs of using ammonia with AFCs are as follows:
- It is difficult to identify a good anode and cathode catalyst
- The crossover of ammonia through the polymeric membrane may decrease the open-circuit voltage and efficiency.
- The oxidation of diffused ammonia at the cathode may generate toxic NO.
- The alkaline membrane can react with 𝐶𝑂2 to form 𝐶𝑂2¯³ ions, reducing the 𝑂𝐻¯ ion conductivity.
- The current polymeric alkaline membrane cannot be operated at temperatures higher than 200 degrees.
An inorganic 𝑂𝐻− ion conductor, allowing higher working temperatures, would be the perfect electrolyte for these types of fuel cells. Also, this electrolyte would prevent ammonia crossover.
3. Solid Oxide Fuel Cells (SOFC)
The SOFC (Solid Oxide Fuel Cell) has a solid oxide or ceramic electrolyte. These fuel cells operate at high temperatures (between 500 and 1000 centigrade). At these temperatures, they don’t need expensive platinum catalyst materials that are needed in lower temperature fuel cells, such as PEMFC. As for direct ammonia fuel cells, up to date, the best choice is to operate at high temperatures using solid oxide fuel cell technology.
The main challenge of ammonia-based SOFCs is the performance durability and manufacturing cost. It has been observed that for an ammonia-based SOFC with a power density of 1 W/cm³ operating at 850 centigrade, the performance will degrade about 7% after 1500 h of functioning[5]R. Lan and S. Tao, “Ammonia as a suitable fuel for fuel cells,” 2014.. The main issue is to find a proper anode that would be stable to redox reaction. This is to improve the durability of the interface between the anode and the electrolyte and also helps to avoid the formation of nitrides. To completely avoid nitrides SOFC based on proton-conducting electrolytes must be used.
