Cu based catalysts for ammonia oxidation in fuel cells

The direct use of hydrogen as fuel in fuel cells entails many serious concerns related to hydrogen economy. Ammonia was suggested as a promising alternative fuel for alkaline fuel cells due to its high hydrogen content (17% w/w), zero CO₂ emissions, low flammability, high compressibility and well-developed production and transport lines. Pt and Pt-based catalysts are the most studied and state-of-the-art low-temperature catalysts for ammonia oxidation in direct ammonia fuel cells (DAFC). However, strong adsorption of the reaction intermediates on these catalysts deactivates the anodes. Recently, few groups suggested NiCu as an inexpensive efficient ammonia oxidation catalyst, which shows high electroactivity and even exceeds Pt. No prolonged stability tests were presented so far.

NiCu catalysts were synthesized by NaBH₄ reduction and microwave-polyol method at different stoichiometries, followed by rigorous characterizations using XRD, ICP and EDX . Results showed an initial high oxidation current (12 mA/mg catalyst), which, exhibit a current density decrease of up to 80% upon potential cycling from 0 to 1.1 V. This current decay process is more rapid in ammonia containing solutions, rather than in the KOH supporting electrolyte. Electrochemical quartz micro balance (EQCM) and EDX measurements showed the formation of Cu oxides during the oxidation process and a mass loses upon cycling. The atomic ratio between Ni and Cu increases, whereas Cu traces are found in the solution. When the electrodes were oxidized at constant potential of 1.1 V vs. RHE, an initial increase of the current density occurs during the first two hours due to the formation of active oxides, followed by a deactivation due to loss of Cu mass and resistance increase of the electrodes. Annealing and structural changes of NiCu mitigate some of the activity loss, but it is still important to increase the stability of this promising catalyst.