Development of New Electrocatalytic Nanomaterials for the Oxidation of Ammonia As Anodic Materials in a Direct Ammonia Fuel Cell
Authors: Saab Emir , Asteazaran Mariano , Cespedes German , Castro Luna Ana Maria 
Published: February 2020

Fuel cells are electrochemical devices that directly convert chemical energy to electrical energy. A common justification for using fuel cells has been environmental protection, as fuel cells produce only water as combustion by-product and thus they are “zero emission” devices [1]. The most widely deployed fuel cell is PEMFC, which uses hydrogen as fuel. However, fuel distribution and fuel tank fill-up, associated with the physical state of the fuel, a highly compressed gas, is problematic. In order to achieve the effective energy content of a gasoline tank, an H2 pressure of 700 bar is required in FCV's storage tanks. An alternative approach to avoid using compressed hydrogen, is to use liquid hydrogen-rich fuels such as methanol, ethanol or ammonia, which are liquid in conditions close to the atmosphere. Being liquid fuels, they are easy to transport and store, which reduces transport and infrastructure costs. In addition, it allows by minor modifications, to take advantage of the current infrastructure and facilities that are normally used for conventional fuels.

Recently hydrogen carriers have attracted much attention as alternative fuels to hydrogen for the realization of a low-carbon society. The search for a clean, economical and sustainable fuel with high energy density has led to the increasing consideration of nitrogen-based fuels, such as ammonia and hydrazine, as a promising alternative reactant to use in fuel cells. Ammonia has certain advantages over other N2-compounds. It is extensively used in the agricultural, plastics and explosives industries. In addition, ammonia is produced on an industrial scale by the well-known Haber-Bosch method. There are many other advantages to consider ammonia, as a promising candidate among them liquefaction under mild conditions (−33.4 °C at atmospheric pressure or 8.46 atm at 20 °C), high volumetric energy density, low production cost, and being a carbon-free fuel. In response to these technical trends, the development of energy conversion devices utilizing ammonia fuel such as fuel cells and gas turbine generators has now accelerated [2].

Electrochemical oxidation of ammonia attracted great attention for wastewater treatment, as well as its application in direct ammonia fuel cells and hydrogen production in energy conversion technologies. However, ammonia electro-oxidation to produce H2 and N2 is a slow process and efficient electrocatalysts are required [3]. Several studies have been devoted to the development of efficient electrocatalysts for ammonia oxidation in alkaline solutions. Platinum is the most active catalyst for this process, however it is expensive and easily become inactive by nitrogen adsorption. Various mono- and bimetallic catalysts have been investigated [4]. In order to make ammonia FCs commercially attractive, the amount of precious metals needs to be reduced. This can be achieved by using electrocatalysts as nanoparticles dispersed on high surface area conductive supports, e.g., carbon black.

In this work carbon-supported bimetallic, PtM, as well as trimetallic PtMRu (M = Co, Cu, Ru, Ni) catalysts were synthesized using a modified impregnation method [5]. These synthesized materials have been studied as potential catalysts for ammonia electro-oxidation.

All materials were characterized physic chemically by techniques such as TEM, SEM, EDS and XPS. in order to determine the size and distribution of the nanoparticles on the conductive support and the chemical composition and oxidation state of the constituent elements of the catalyst.

Techniques such as CV, CA and EIS were used for electrochemical characterization. All electrochemical measurements were carried out in a three-electrode PTFE homemade cell. A large surface area gauze-Pt and a Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. As working electrode a glassy carbon with a thin catalyst layer was used. The behaviour of the new materials towards ammonia electrooxidation was evaluated. 1 M KOH was used as the supporting electrolyte, to which increasing amounts of NH4OH were added. The catalysts synthesized in this work have shown to be very attractive as active materials for the anode of a direct ammonia fuel cell. The addition of co-catalysts to the Pt has improved its electrocatalytic behavior towards ammonia electro-oxidation.

[1] Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2005 Symposium. (2006). Washington, D.C.: National Academies Press.

[2] Matsui, T., Suzuki, S., Katayama, Y., Yamauchi, K., Okanishi, T., Muroyama, H., & Eguchi, K. (2015). Langmuir, 31(42), 11717–11723.

[3] Siddiqui, O., & Dincer, I. (2018). Thermal Science and Engineering Progress, 5, 568-578.

[4] Lomocso, T. L., & Baranova, E. A. (2011). Electrochimica Acta, 56(24), 8551–8558.

[5] Asteazaran, M., Cespedes, G., Moreno, M. S., Bengió, S., & Castro Luna, A. M. (2015). International Journal of Hydrogen Energy, 40(42), 14632–14639.