Ammonia has been considered as a potential fuel for alkaline fuel cells. Ammonia can be produced at low cost, and inexpensive catalysts can be used in Ammonia Fuel Cells (AFC) [1, 2]. In AFC, ammonia is oxidized at the anode side and OH-, nitrogen and water are produced (1). OH- anion moves through membrane and reduced into water at the cathode side (2).

 1/3 NH₃ + OH^- ↔ 1/6 N₂ + H₂O + e-,  E⁰=0.40V   -----  Eqn. 1

1/4 O₂ + 1/2 H₂O + e- ↔ OH^-,              E⁰=0.40V   -----  Eqn. 2

The theoretical open-circuit voltage (OCV) of the AFC is 1.17 V at standard conditions. The value is less than that of Hydrogen Fuel Cells (HFC), but AFC has a higher energy efficiency, 88.7%, than HFC of 83% [1].

Performance and durability of hydrogen PEM fuel cells has been improved over the last decade by extensive experiments and computations. In particular, continuum modeling has been an important tool for understanding and insight into processes and phenomena, which cannot be captured through experiments [3].

Recently, AFCs have attracted the attention of fuel cell researchers due to its high potential for a clean future energy device.  However, AFCs show some problems in related to mass transfer and fuel crossover in the membrane electrode assembly. In this study, transport phenomena in AFCs are analyzed using a computational model.

Objective

When the AFC is in operation, complex phenomena occur in the Membrane Electrode Assembly (MEA) including mass and heat transfer, electrochemical reactions, and charge (OH- and electron) transport (Fig.1). Therefore, a continuum model was developed to simulate the phenomena in MEA. 

Figure 1. Illustration of Mass Transport in Membrane Electrode Assembly depicting individual layers and chemical components. cCL and aCL depict the cathode and anode catalyst layers respectively, while cGDL and aGDL depict the cathode and anode gas diffusion layers respectively.

Results

Using the model, potential and species distribution in MEA, and the extent of electrochemical reaction in catalyst layer were calculated. In addition, the effect of different operating conditions on these phenomena were analyzed. These results, along with the effect of water transfer and ammonia crossover on cell performance will be discussed. Detailed accounts of the governing equations and their numerical solving procedure will also  be provided.

References

[1]           R. Lan and S. W. Tao, "Direct
Ammonia Alkaline Anion-Exchange Membrane Fuel Cells," Electrochemical and solid State Letters, vol. 13, pp. B83-B86, 2010.

[2]           M H. Assumpcao, S.G.Silva, and R. F. Souza, "Direct Ammonia Fuel Cell Performance using PTIR/C as anode electrocatalysts," Int. J. of Hydrogen Enrgy, vol. 39, pp.5148-5152, 2014.

 [3]          A. Z. Weber, R. L. Borup, "A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells," J. of The Electrochemical Society, Vol 161, ppF1254-F1299, 2014