Noble Metal-Free Hydrazine Fuel Cell Catalysts: EPOC Effect in Competing Chemical and Electrochemical Reaction Pathways

Abstract

We report the discovery of a highly active Ni−Co alloy electrocatalyst for the oxidation of hydrazine (N₂H₄) and provide evidence for competing electrochemical (faradaic) and chemical (nonfaradaic) reaction pathways. The electrochemical conversion of hydrazine on catalytic surfaces in fuel cells is of great scientific and technological interest, because it offers multiple redox states, complex reaction pathways, and significantly more favorable energy and power densities compared to hydrogen fuel. Structure−reactivity relations of a Ni₆₀Co₄₀ alloy electrocatalyst are presented with a 6-fold increase in catalytic N₂H₄ oxidation activity over today’s benchmark catalysts. We further study the mechanistic pathways of the catalytic N₂H₄ conversion as function of the applied electrode potential using differentially pumped electrochemical mass spectrometry (DEMS). At positive overpotentials, N₂H₄ is electrooxidized into nitrogen consuming hydroxide ions, which is the fuel cell-relevant faradaic reaction pathway. In parallel, N₂H₄ decomposes chemically into molecular nitrogen and hydrogen over a broad range of electrode potentials. The electroless chemical decomposition rate was controlled by the electrode potential, suggesting a rare example of a liquid-phase electrochemical promotion effect of a chemical catalytic reaction (“EPOC”). The coexisting electrocatalytic (faradaic) and heterogeneous catalytic (electroless, nonfaradaic) reaction pathways have important implications for the efficiency of hydrazine fuel cells.