Fe-N-C electrocatalysts in the oxygen and nitrogen cycles in alkaline media: the role of iron carbide.

Fe-N-C electrocatalysts hold a great promise for Pt-free energy conversion, driving the electrocatalysis of oxygen reduction and evolution, oxidation of nitrogen fuels, and reduction of N2, CO2, and NOx. Nevertheless, the catalytic role of iron carbide, a component of nearly every pyrolytic Fe-N-C material, is at the focus of a heated controversy. We now resolve the debate by examining a broad range of Fe3C sites, spanning across many typical size distributions and carbon environments. Removing Fe3C selectively by a non-oxidizing acid reveals its inactivity towards two representative reactions in alkaline media, oxygen reduction and hydrazine oxidation. The activity is assigned to other pre-existing sites, most probably Fe-Nx. DFT calculations prove that the Fe3C surface binds O and N intermediates too strongly to be catalytic. By settling the argument on the catalytic role of Fe3C in alkaline electrocatalysis, we hope to spur innovation in this critical field.

Combining polarized low-frequency Raman with XRD to identify directional structural motifs in a pyrolysis precursor.

Long-range structures and dynamics are central to coordination chemistry, yet are hard to identify experimentally. By combining polarized low-frequency Raman spectroscopy with single crystal XRD to study barium nitrilotriacetate, a metal-organic coordination polymer and a useful pyrolysis precursor, we could assign Raman peaks experimentally to layer shear motions and perpendicular hydrogen bond vibrations. These directional long-range interactions further determined the preferred fracture directions during crystallization, establishing an important link between structural motifs in the precursor, and the porosity of the carbon it yields upon pyrolysis.

A Multi-Doped Electrocatalyst for Efficient Hydrazine Oxidation.

We report an efficient electrocatalyst for the oxidation of hydrazine, a promising fuel for fuel cells and an important analyte for health and environmental monitoring. To design this material, we emulated natural nitrogen-cycle enzymes, focusing on designing a cooperative, multi-doped active site. The catalytic oxidation occurs on Fe2 MoC nanoparticles and on edge-positioned nitrogen dopants, all well-dispersed on a hierarchically porous, graphitic carbon matrix that provides active site exposure to mass-transfer and charge flow. The new catalyst is the first carbide with HzOR activity. It operates at the most negative onset potentials reported for carbon-based HzOR catalysts at pH 14 (0.28 V vs. RHE), and has good-to-excellent activity at pH values down to 0. It shows high faradaic efficiency for oxidation to N2 (3.6 e- /N2 H4 ), and is perfectly stable for at least 2000 cycles.