Co2Mo6S8 Catalyzes Nearly Exclusive Electrochemical Nitrate Conversion to Ammonia with Enzyme-like Activity.

Electrocatalytic nitrate to ammonia conversion is a key reaction for energy and environmental sustainability. This reaction involves complex multi electron and proton transfer steps, and is impeded by the lack of catalyst for promoting both reactivity and ammonia selectivity. Here, we demonstrate active motifs based on the Chevrel phase Co2Mo6S8 exhibit an enzyme-like high turnover frequency of ∼95.1 s-1 for nitrate electroreduction to ammonia. We reveal strong synergy of multiple binding sites on this catalyst, such that the ligand effect of Co steers Had* toward hydrogenation other than hydrogen evolution, the ensemble effect of Co, and the spatial confinement effect that promote the full hydrogenation of NOx to ammonia without N-N coupling. The catalyst exhibits almost exclusive ammonia conversion with a Faradaic efficiency of 97.1% and ammonia yielding rate of 115.5 mmol·gcat-1·h-1 in neutral electrolytes. The high activity was also confirmed in electrolytes with dilute nitrate and high chloride concentrations.

LixNiO/Ni Heterostructure with Strong Basic Lattice Oxygen Enables Electrocatalytic Hydrogen Evolution with Pt-like Activity.

The low-cost hydrogen production from water electrolysis is crucial to the deployment of sustainable hydrogen economy but is currently constrained by the lack of active and robust electrocatalysts from earth-abundant materials. We describe here an unconventional heterostructure composed of strongly coupled Ni-deficient LixNiO nanoclusters and polycrystalline Ni nanocrystals and its exceptional activities toward the hydrogen evolution reaction (HER) in aqueous electrolytes. The presence of lattice oxygen species with strong Brønsted basicity is a significant feature in such heterostructure, which spontaneously split water molecules for accelerated Volmer H-OH dissociation in neutral and alkaline HER. In combination with the intimate LixNiO and Ni interfacial junctions that generate localized hotspots for promoted hydride coupling and hydrogen desorption, the catalysts produce hydrogen at a current density of 10 mA cm-2 under overpotentials of only 20, 50, and 36 mV in acidic, neutral, and alkaline electrolytes, respectively, making them among the most active Pt-free catalysts developed thus far. In addition, such heterostructures also exhibited superior activity toward the hydrogen oxidation reaction in alkaline electrolytes.

Anode-free Na metal batteries developed by nearly fully reversible Na plating on the Zn surface.

The anode-free battery architecture has recently emerged as a promising platform for lithium and sodium metal batteries as it not only offers the highest possible energy density, but also eliminates the need for handling hazardous metal electrodes during cell manufacturing. However, such batteries usually suffer from much faster capacity decay and are much more sensitive to even trace levels of irreversible side reactions on the anode, especially for the more reactive Na metal. This work systematically investigates electrochemical interfaces for Na plating and stripping and describes the use of the Zn surface to develop nearly fully reversible Na anodes with 1.0 M NaPF6 in a diglyme-based electrolyte. The high performance includes consistently higher than 99.9% faradaic efficiencies for a wide range of cycling currents between 0.5 and 10 mA cm-2, much more stable interfacial resistance and nearly no formation of mossy Na after 500 cycles compared with conventional Al and Cu surfaces. This improved reversibility was further confirmed under lean electrolyte conditions with a wide range of electrolyte concentrations and cycling temperatures and can be attributed to the strong interfacial binding and intrinsic sodiophilic properties of the Zn surface with Na, which not only ensured uniform Na plating but also eliminated most side reactions that would otherwise cause electrolyte depletion. As a result, full cells assembled with Na-free Zn foil and a high capacity Na3V2(PO4)3 cathode delivered ∼90% capacity retention for 100 cycles, higher than the 73% retention of Cu foils and much higher than the 39% retention of Al foils. This work provides new approaches to enable stable cycling of anode-free batteries and contribute to their applications in practical devices.

Synergistic Multisites Fe2Mo6S8 Electrocatalysts for Ambient Nitrogen Conversion to Ammonia.

Electrochemical hydrogenation of N2 under ambient conditions is attractive for sustainable and distributable NH3 production but is limited by the lack of selective electrocatalysts. Herein, we describe active site motifs based on the Chevrel phase chalcogenide Fe2Mo6S8 that exhibit intrinsic activities for converting N2 to NH3 in aqueous electrolytes. Despite having a very low specific surface area of ∼2 m2/g, this catalyst exhibited a Faradaic efficiency of 12.5% and an average rate of 70 µg h-1 mgcat-1 for NH3 production at -0.20 V vs RHE. Such activities were attributed to the unique composition and structure of Fe2Mo6S8 that provide synergistic multisites for activating and associating key reaction intermediates. Specifically, Fe/Mo sites assist adsorption and activation of N2, whereas S sites stabilize hydrogen intermediate Had* for N2 hydrogenation. Fe in Fe2Mo6S8 enhances binding of S with Had* and thus inhibits the competing hydrogen evolution reaction. The spatial geometry of Fe, Mo, and S sites in Fe2Mo6S8 promotes conversion of N2-Had* association intermediates, reaching a turnover frequency of ∼0.23 s-1 for NH3 production.