Trends in Catalysis and Catalyst Cost Effectiveness for N2H4 Fuel Cells and Sensors: a Rotating Disk Electrode (RDE) Study

Hydrazine (N2H4) is a promising high-power energy carrier for fuel cells, combining the energy density of methanol (MeOH) with the rapid oxidation kinetics of hydrogen (H-2). N2H4 does not require expensive Pt group metals nor Au for low-potential (high voltage) oxidation, offering significantly lower fuel cell materials costs compared to H-2, MeOH, ethanol (EtOH), and ammonia (NH3). In our study, we use rotating disk electrode (RDE) voltammetry to explore N2H4 oxidation at a wide variety of catalysts, including first-row transition metals (Co, Ni), coinage metals (Ag, Au) and Pt group metals (Ru, Rh, Pd, Ir, Pt). While several groups have focused on Co, Ni, or CoNi alloys, we find that other metals, including Ag, Ru, and Pd, offer much higher electron recovery and have more stable reactions, and still cost far less than Pt, Au, Rh, or Ir. We analyze our findings in terms of cost vs performance for the metals, developing a guide for the design of N2H4 fuel cell systems and sensors to suit various application spaces. The many metals studied also reveal an important trend for the theoretical understanding of catalysis: the onset and passivation of N2H4 oxidation in nearly every system were directly tied to the appearance or disappearance of specific metal surface states (e.g., hydrides and oxides). Indeed, metals with multiple surface states frequently showed multiple mechanisms for N2H4 oxidation, each with separate values for electron recovery. These observations provide support for the continued development of electrocatalytic theory in which different metal surface states are treated as independent materials with distinct reaction mechanisms.