High‐entropy alloys combine multiple principal elements at a near equal fraction to form vast compositional spaces to achieve outstanding functionalities that are absent in alloys with one or two principal elements. Here, the prediction, synthesis, and multiscale characterization of 2D high‐entropy transition metal dichalcogenide (TMDC) alloys with four/five transition metals is reported. Of these, the electrochemical performance of a five‐component alloy with the highest configurational entropy, (MoWVNbTa)S2, is investigated for CO2 conversion to CO, revealing an excellent current density of 0.51 A cm−2 and a turnover frequency of 58.3 s−1 at ≈ −0.8 V versus reversible hydrogen electrode. First‐principles calculations show that the superior CO2 electroreduction is due to a multi‐site catalysis wherein the atomic‐scale disorder optimizes the rate‐limiting step of CO desorption by facilitating isolated transition metal edge sites with weak CO binding. 2D high‐entropy TMDC alloys provide a materials platform to design superior catalysts for many electrochemical systems.
An illustration of solar-driven synthesis of ammonia using nitrates with >10% solar-to-fuel efficiencies that can potentially decarbonize and decentralize ammonia production.
The electrochemical reduction of
N2 to produce NH3 at ambient conditions is an
effective and sustainable route
to store and carry hydrogen, balance the nitrogen cycle, and provide
means to produce on-demand fertilizers. The efficient electrosynthesis
of NH3 is challenging because of the lower activation of
N2 and higher activity toward the hydrogen evolution reaction
(HER). Here, we propose theory-guided activity descriptors to identify
an efficient N2 reduction reaction (NRR) catalyst, followed
by its implementation in a flow-through gas diffusion electrode (GDE)
to quantify the effects of pH, cation identity, H2O saturation,
and N2 concentration on the kinetics of the NRR. The identified
Cu catalyst with dominant (111) facets electrodeposited on a carbon
paper provides optimal active sites to obtain maximum NH3 faradaic efficiency (FE) of 18 ± 3% at −0.3 V vs RHE
and the maximum NH3 current density of 0.25 ± 0.03
mA cm–2 (0.86 nmol·cm–2·s–1) at −0.5 V vs RHE in alkaline medium. The
electrolyte pH mostly affects the HER by pH-induced binding of *H
and reorganization of H2O, which favor the NRR at an optimal
pH of 13.5. Increasing the size of monovalent cations stabilizes NRR
intermediates and increases the NH3 current density from
Li+ to K+. However, increasing the size of the
cation from K+ to Rb+ reduces the FE of NRR,
which is due to a direct reduction of H2O in the solvation
shell of larger cations to produce H2. Another strategy
to improve NH3 FE is to reduce the H2O saturation
on the catalyst, which can be achieved by sparging the reactant gas
directly through the GDE. Increasing the N2(g) flow rate
not only increases the gas–liquid mass transfer coefficient
but also reduces the H2O saturation in the pores of the
GDE, which primarily suppresses the HER. The fixed potential DFT calculations
reveal an associative distal mechanism for the NRR over Cu(111), where
the hydrogenation of *N2 is the rate-limiting step. This
finding also corroborates with the measured reaction order with respect
to N2.
Electrochemical oxidation of CH4 is known to be inefficient in aqueous electrolytes. The lower activity of methane oxidation reaction (MOR) is primarily attributed to the dominant oxygen evolution reaction (OER) and the higher barrier for CH4 activation on transition metal oxides (TMOs). However, a satisfactory explanation for the origins of such lower activity of MOR on TMOs, along with the enabling strategies to partially oxidize CH4 to CH3OH, have not been developed yet. We report here the activation of CH4 is governed by a previously unrecognized consequence of electrostatic (or Madelung) potential of metal atom in TMOs. The measured binding energies of CH4 on 12 different TMOs scale linearly with the Madelung potentials of the metal in the TMOs. The MOR active TMOs are the ones with higher CH4 binding energy and lower Madelung potential. Out of 12 TMOs studied here, only TiO2, IrO2, PbO2, and PtO2 are active for MOR, where the stable active site is the O on top of the metal in TMOs. The reaction pathway for MOR proceeds primarily through *CHx intermediates at lower potentials and through *CH3OH intermediates at higher potentials. The key MOR intermediate *CH3OH is identified on TiO2 under operando conditions at higher potential using transient open-circuit potential measurement. To minimize the overoxidation of *CH3OH, a bimetallic Cu2O3 on TiO2 catalysts is developed, in which Cu reduces the barrier for the reaction of *CH3 and *OH and facilitates the desorption of *CH3OH. The highest faradaic efficiency of 6% is obtained using Cu-Ti bimetallic TMO.
An ultrafast, continuous CO2 capture process driven by moisture gradient and electric field with low energy consumption to capture and concentrate CO2 from dilute sources.
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