Revealing Structure–Activity Links in Hydrazine Oxidation: Doping and Nanostructure in Carbide–Carbon Electrocatalysts

Abstract

The oxidation of hydrazine (N2H4) is an important challenge in electrocatalysis, with applications in direct hydrazine fuel cells and in medical and environmental sensing. Interest in alternative, nitrogen-based fuels for fuel cells was rekindled by recent advances in alkaline membranes, and by the surfacing of challenges in the transportation of hydrogen. Direct hydrazine fuel cells promise high theoretical voltage (1.56 V with O2), clean emissions, and improved fuel transportability.We now report a multi-doped carbide–carbon composite with excellent hydrazine electro-oxidation (HzOR) activity in alkaline pH. While iron carbide containing materials are well known catalysts for several applications (e.g. oxygen reduction and Li-ion batteries), our N-doped, Fe3C-embedded carbons provide the first examples of a carbide-based HzOR catalyst. Multi-doping was thought to enhance reactivity (possibly through cooperation of M=Cu/ Zn, Fe, and edge N atoms), possibly by pore-etching, layers exfoliation or graphitization promotion, thus modulating surface area, mass transfer and conductivity.To prepare the composites, we designed tunable multi-doped precursors: an organometallic structure combining iron with either (1) copper, an element active towards the HzOR, (2) zinc, an electrocatalytically-inert element capable of efficient micropore etching, or (3) iron, to investigate the effect of molybdenum-doping reported earlier. Pyrolysis and washing of the fore mentioned precursors yielded HzOR-active composites of Fe3C nanoparticles and a hierarchically porous, partially graphitic N-doped carbon (NC). Thorough characterization by voltammetry and by a broad range of spectroscopic and microscopic methods revealed that the activity is ordered as: Cu-derived NC > Zn-derived NC > Fe-derived NC. While the catalysts had similar compositions, their nanostructure varied: both Zn- and Cu-doping induced the formation of micropores and small mesopores (5–13 nm diameter). The dopant-induced active site exposure (affecting micropore volume) and improved material flow (linked to mesopore volume) contributed to enhanced HzOR electroactivity. Furthermore, the intimate mixing of the metals in the precursor is hypothesized to homogenize and enhance the doping effect, as the structure enhancing metal ions (Zn2+ or Cu2+) exposed the nearby catalytic Fe3C sites. Acid washing of the pyrolyzed carbon was crucial for producing microporosity in the Cu-based carbon, but had little effect on the Zn-derived one. This revealed the importance of micropores and small mesopores for HzOR electrocatalysis, and the different nanostructuring mechanisms of the two dopants: while Zn(g) boils during pyrolysis and thus etches micropores into the carbon, Cu(s) diffuses and spreads throughout, making exfoliation by acid reaction more efficient. Further work in our group is dedicated to the catalytic activity of pure phase Fe3C, and its modification by other dopants.[Burshtein et al., J. Mater. Chem. A, 7, 41, (23854-23861) 2019, doi: 10.1039/C9TA03357B] Figure 1