Fig. 1. Atomic structure models of (a) FeS2 (200) surface and (b) Fe3S4 (311) surface. Free energy profiles for the step-by-step dehydrogenation of hydrazine on the (c) FeS2 (200) surface and (d) Fe3S4 (311) surface. The insets in (c) and (d) are the N2H4 and N2 molecular models and the most stable configurations of the adsorbed intermediates on the FeS2 and Fe3S4 surface. The Fe, S, N and H atoms are indicated with purple, yellow, blue and pink balls, respectively.

Fig. 2. Characterization of the FeS2 microspheres. (a) XRD pattern, in which the standard patterns of FeS2 are added. (b) Raman spectrum. XPS spectra of (c) Fe 2p and (d) S 2p. (e) SEM image of FeS2. (f) Enlarged SEM image of single-particle and the corresponding Fe and S energy dispersive spectrometry (EDS) mapping images.

Fig. 3. (a) CVs on FeS2 and Fe3S4 catalysts (100 mM N2H4, 1 M KOH, scan rate 5 mV s-1). The spikes are caused by the release of bubbles on the electrode. All other voltammograms are with FeS2. (b) Full region CVs (40 mM N2H4) at different scan rates, and (c) plots of peak current density and (d) peak potential vs. scan rate. (e) and (f) Pure double-layer charging (no N2H4) at different scan rates, CVs and total current densities at 0.126 V vs. RHE.

Fig. 4. (a, c) CVs with a scanning rate of 20 mV s-1 in 1 M KOH for different concentrations of hydrazine, and (b, d) the corresponding plots peak current density vs. N2H4 concentrations (e) Chronoamperometric curves in 1 M KOH for different concentrations of N2H4 and (f) corresponding plots of current density j vs. t -1/2. (g) The relationship between the associated slopes from Fig. 4(f) and the N2H4 concentrations. (h) The long-term stability in 1 M KOH containing 100 mM N2H4 at 0.424 V.