Electrochemical
water splitting is considered as the most promising
technology for hydrogen production. Considering overall water splitting
for practical applications, catalysis of the oxygen evolution reaction
(OER) and hydrogen evolution reaction (HER) should be performed in
the same electrolyte, especially in alkaline solutions. However, designing
and searching for highly active and inexpensive electrocatalysts for
both OER and HER in basic media remain significant challenges. Herein,
we report a facile and universal strategy for synthesizing nonprecious
transition metals, binary alloys, and ternary alloys encapsulated
in graphene layers by direct annealing of metal–organic frameworks.
Density functional theory calculations prove that with an increase
in the degree of freedom of alloys or a change in the metal proportions
in FeCoNi ternary alloys, the electronic structures of materials can
also be tuned intentionally by changing the number of transferred
electrons between alloys and graphene. The optimal material alloys
FeCo and FeCoNi exhibited remarkable catalytic performance for HER
and OER in 1.0 M KOH, reaching a current density of 10 mA cm–2 at low overpotentials of 149 mV for HER and 288 mV for OER. In addition,
as an overall alkaline water electrolysis, they were comparable to
that of the Pt/RuO2 couple, along with long cycling stability.
Manganese (Mn) is generally regarded as not being sufficiently active for the oxygen reduction reaction (ORR) compared to other transition metals such as Fe and Co. However, in biology, manganese-containing enzymes can catalyze oxygen-evolving reactions efficiently with a relative low onset potential. Here, atomically dispersed O and N atoms coordinated Mn active sites are incorporated within graphene frameworks to emulate both the structure and function of Mn cofactors in heme-copper oxidases superfamily. Unlike previous single-metal catalysts with general M-N-C structures, here, it is proved that a coordinated O atom can also play a significant role in tuning the intrinsic catalytic activities of transition metals. The biomimetic electrocatalyst exhibits superior performance for the ORR and zinc-air batteries under alkaline conditions, which is even better than that of commercial Pt/C. The excellent performance can be ascribed to the abundant atomically dispersed Mn cofactors in the graphene frameworks, confirmed by various characterization methods. Theoretical calculations reveal that the intrinsic catalytic activity of metal Mn can be significantly improved via changing local geometry of nearest coordinated O and N atoms. Especially, graphene frameworks containing the Mn-N O cofactor demonstrate the fastest ORR kinetics due to the tuning of the d electronic states to a reasonable state.
Mesoporous core−shell structured nanocatalysts with a PdPt bimetallic core and enzyme-immobilized polydopamine (PDA) shell were designed, in which the PDA shell worked as a barrier to position the bimetallic core and enzyme in separated locations. The accessible mesoporous structures of both the core and shell significantly facilitate mass transfer and catalyst utilization, improving the synergistic catalytic abilities in cascade reactions. The obtained bifunctional nanocatalysts enabled efficient two-step one-pot cascade reactions of different types: dynamic kinetic resolution of primary amines in organic solvent with high yield and enantioselectivity (up to 99% yield and 98% ee) and degradation of organophosphate nerve agent in aqueous solution with high rate constant and turnover frequency number values (0.8 min −1 and 20 min −1 , respectively).
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