Electrochemical Oxidation of Liquid Ammonia on Pt(100) Electrode: First-Principles Study
Authors: Skachkov Dmitry , Chitturi Venkateswara Rao 
Published: February 2020

Ammonia is a good source of hydrogen and has an advantage in comparison with molecular hydrogen for fuel cell application with respect to the storage in liquid phase.1,2,3 Moreover, the raw energy density of liquid ammonia is higher than for liquid hydrogen. Ammonia fuel cells were intensively developed using aqueous solution of ammonia while the developing direct ammonia fuel cells based on liquid ammonia has less attention. Fuel cells based on liquid ammonia NH3(l) have the significant advantages in comparison with aqueous solution of NH3(aq):

liquid ammonia NH3(l) is three times energy denser than the saturated aqueous solution of ammonia;

alkaline solution of NH3(aq) is corrosive to metallic containers, whereas the infrastructure for liquid NH3(l) production, storage and distribution is already well developed;

there is no formation of hindering poisoning NOx species.

The mechanism of electrochemical oxidation of aqueous NH3(aq) is well studied,4,5,6 whereas the detailed mechanism of oxidation of liquid NH3(l) is not known. Hanada et al.7 in experimental study of NH3(l) electrolysis in support of LiNH2, NaNH2, and KNH2 electrolytes show that the products of the reaction are hydrogen gas (on the cathode) and nitrogen gas (on the anode) and the highest activity of the reaction is taking place with KNH2 electrolyte. The authors attributed the products H2 and N2 to the reduction of NH3 and oxidation of NH2
-, respectively, however, the mechanism was not substantiated.

It is known that the reaction of NH3 electrooxidation is very sensitive to the surface of Pt electrode and occurs at surface sites with (100) geometry,8 whereas the surface sites with (111) and (110) geometries will be blocked by hollow nitrogens.4 In this study we present the detailed steps of the electrochemical oxidation of liquid ammonia on the Pt(100) anode surface in support of KNH2 electrolyte. We model the electrochemical reaction by solid/liquid interface using density functional theory method developed in our previous work.4 One of the possible pathways of the electrochemical oxidation of NH3(l) is going through spontaneous adsorption of NH3(l) to the surface forming NH3(top) (1), the following oxidation of NH3(top) forming NH2(top) (2), then breaking the hydrogen bond with the just formed NH3(l) molecule and transformation to NH2(bridge) (3), and the following oxidation of NH2(bridge) forming the bridging nitrogen (reactions (4)-(5)). The final product of the NH3(l) oxidation is the atomic nitrogen in bridging position (see Fig.).

(1) NH3(l) → NH3(top)

(2) NH3(top) + NH2
- → NH2(top) + NH3(l) + e-

(3) NH2(top) → NH2(bridge)

(4) NH2(bridge) + NH2
- → NH(bridge) + NH3(l) + e-

(5) NH(bridge) + NH2
- → N(bridge) + NH3(l) + e-

The process of the surface accumulation of the bridging nitrogens is going until the moment when the spontaneous dimerization of N(bridge) will happen. The result of the dimerization is N2(hollow) (6). The following spontaneous desorption of N2(hollow) from the surface results the molecular nitrogen dissolved in the solution (7).

(6) N(bridge) + N(bridge) → N2(hollow)

(7) N2(hollow) → N2(l)

References

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D.J. Little, M.R. Smith, III, T.W. Hamann "Electrolysis of liquid ammonia for hydrogen generation", Energy Environ. Sci., 8, 2775, 2015.

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D. Skachkov, V.R. Chitturi, Y. Ishikawa "Combined first-principles molecular dynamics/density functional theory study of ammonia electrooxidation on Pt(100) electrode", J. Phys. Chem. C, 117, 25451, 2013.

I. Katsounaros, M.C. Figueiredo, F. Calle-Vallejo, H. Li, A.A. Gewirth, N.M. Markovic, M.T.M. Koper "On the mechanism of the electrochemical conversion of ammonia to dinitrogen on Pt(100) in alkaline environment", J. Catalysis, 359, 82, 2018.

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F.J. Vidal-Iglesias, J. Solla-Gullon, V. Montiel, J.M. Feliu, A. Aldaz "Ammonia selective oxidation on Pt(100) sites in an alkaline medium", J. Phys. Chem. B, 109, 12914, 2005.

Figure 1