Metallocomplex-Based Electrocatalysts for the Oxidation of Hydrazine Derivatives

Abstract

Hydrazine hydrate, which has high energy density, is one of desirable fuels for direct fuel cells. However, toxicity and volatility of this compound might prevent the wide spread of direct hydrazine hydrate fuel cells. In this context, hydrazine derivatives such as N, N-diaminourea (DAU) and methyl carbazate would be a possible remedy for this problem. These hydrazine derivatives are solid, and evaporation of them would be suppressed. Unfortunately, conventional metal catalysts such as Ni, Pt, and Co hardly oxidize these hydrazine derivatives. In this work, we report a metallocomplex-based electrocatalyst for the oxidation of hydrazine derivatives. These catalysts completely differ from conventional metal-based electrocatalysts in that single atoms form active sites, and hence new oxidation activity would be expected. We examined metalloporphyrin-based electrocatalysts for the oxidation of DAU. These complexes can oxidize DAU to give considerable current, while conventional Pt catalysts cannot oxidize DAU. The catalytic activities depend on the kinds of central metals; Co-, Rh-, Fe-, Ru-porphyrins give high current. The catalytic activity increased with the change in the ligand from porphyrins to phthalocyanines. Especially, Fe phthalocyanine gave high catalytic activity for the oxidation of DAU; the current exceeded 150 mA/cm2 at 0.6 V vs. a reversible hydrogen electrode (RHE). There are two possible pathways of the electro-oxidation of DAU by Fe-PC: (1) DAU is directly oxidized by the electrode (direct pathway) and (2) DAU undergoes hydrolysis to produce hydrazine, and the generated hydrazine is oxidized by the electrode (indirect pathway). HPLC analysis and the dependence of the activity on the concentrations of DAU and hydrazine revealed that the oxidation of DAU proceeds mainly via a direct oxidation pathway. Finally, we made a membrane electrode assembly (MEA) using Fe-PC as an anode catalyst, and examined the performance of the MEA (cathode catalyst is Pt/C and anion-exchange membrane is A201 (Tokuyama)). The cell gave a considerable power; open circuit potential is close to 0.6 V, and the short-circuit current is 180 mA/cm2 at 80 °C. To the best of our knowledge, this is the first report of a polymer electrolyte fuel cell that uses Fe-PC as an anode catalyst, while the electro-oxidation activity of Fe-PC has been studied for several decades. The authors are grateful to Tokuyama Corporation for donating an ionomer (AS-4) and A201 membrane.