Ammonia for Fuel Cells: a literature review

I wrote earlier today about a new literature review on “Ammonia for Power,” published in November 2018. As a companion piece to that article, I’d like to highlight another open-access literature review, this one published a few years before we launched Ammonia Energy, which focuses completely on the (perhaps unexpectedly) broad subject of direct ammonia fuel cells. The mini-review, “Ammonia as a suitable fuel for fuel cells,” was published in the August 2014 edition of Frontiers in Energy Research, written by Rong Lan and Shanwen Tao of the University of Strathclyde in the UK.

Hydrogen is a good energy vector because the only product is water from either combustion or fuel cells. However, on-board hydrogen storage remains a big challenge, which limits the application of hydrogen fuel cells on electric vehicles …

Ammonia containing 17.5 wt% hydrogen is an ideal carbon-free fuel for fuel cells … Compared to hydrogen, ammonia is easier to be transported. It is much more energy efficient and much lower cost to produce, store, and deliver hydrogen as NH3 than as compressed and/or cryogenic hydrogen.
Lan and Tao, Ammonia as a suitable fuel for fuel cells, August 2014

Click to enlarge. Figure 1: Comparison of energy efficiency on transport of H2 and NH3 at gaseous or liquid state (Olson and Holbrook, 2007). Lan and Tao, Ammonia as a suitable fuel for fuel cells, August 2014

Alkaline and Alkaline Membrane Direct Ammonia Fuel Cells
Alkaline fuel cells (AFCs) were the first to be developed with ammonia as the fuel, beginning in the 1960s. Low-temperature (50 to 200°C) AFCs traditionally use a potassium hydroxide (KOH) electrolyte; a molten hydroxide (NaOH/KOH) electrolyte operates at higher temperatures (200 to 450°C).

Alkaline membrane fuel cells (AMFCs) operate under the same principles as traditional AFCs – by the transfer of hydroxide ions through the electrolyte.

The reaction at the cathode is given by:

O2 + 2H2O + 4e → 4OH
[1]

and for the anode:

2NH3 + 6OH → N2 + 6H2O + 6e [2]

This gives an overall reaction of:

4NH3 + 3O2 → 2N2 + 6H2O
[3]

Such ammonia fuel cells have been operated at room temperature but the power density is rather low …

Ammonia fuel cells based on alkaline membrane electrolytes sound attractive but they also have drawbacks: first, it is difficult to identify a good anode and cathode catalysts; second, the cross-over of ammonia through the polymeric membrane electrolyte may decrease the OCV and efficiency; and third, the oxidation of diffused ammonia at cathode may generate toxic NO.
Lan and Tao, Ammonia as a suitable fuel for fuel cells, August 2014

Despite the challenges identified in this review, ammonia-fed alkaline fuel cells are now commercial products.

Ammonia Solid Oxide Fuel Cells

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. Ammonia SOFCs based on both oxygen ion and proton-conducting electrolytes have been reported …

Thermodynamic analysis indicates that the peak power density of ammonia-fed SOFCs based on proton-conducting electrolytes is 20–30% higher than SOFCs based on O2 ion conducting electrolytes, mainly attributed to the higher concentration of hydrogen at the anode in all cases …

Two electrochemical reactions may happen on the SOFC anode side:

2NH3 + 5O2 → 2NO + 3H2O + 10e [4]

2NH3 + 3NO → 5∕2N2 + 3H2O [5]

Reaction [4] is rate limiting because of the slow diffusion of O2 through the electrolyte, therefore, NO is produced at the SOFC anode. It was found that, if an iron-based catalyst was used as anode to facilitate the decomposition of ammonia, producing hydrogen and nitrogen according to the following reaction:

2NH3 → N2 + 3H2 [6]

then NO is not produced at the anode … The thermal decomposition of ammonia itself according to reaction [6] becomes significant at a temperature above 500°C. Therefore, it will be beneficiary to run an ammonia fuel cell at high temperature.
Lan and Tao, Ammonia as a suitable fuel for fuel cells, August 2014

Again, the ammonia-fed SOFC is approaching commercial status, with major developments underway today by IHI Corporation in Japan, Haldor Topsoe in Denmark, and others.

The full literature review, which links to over 30 research papers (most of which are also open-access publications), is available online at Frontiers in Energy Research.

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