On the active site for electrocatalytic water splitting on late transition metals embedded in graphene

Abstract

Transition-metal atoms embedded in nitrogen-doped graphene can be used for electrocatalytic water splitting, but there are open questions regarding the identity of the active site.

How Noninnocent Spectator Species Improve the Oxygen Reduction Activity of Single-Atom Catalysts: Microkinetic Models from First-Principles Calculations

“…This shift does not improve the agreement between computational and experimental ORR onset potentials: while experimental onset potentials of 0.87 and 0.85 V RHE have been reported for Cu 38 and Pt 39 singleatom catalysts, respectively, both computational models predict negligible ORR activity for the late transition metals at these potentials. However, metal atoms embedded in partially N-substituted double vacancies bind oxygen more strongly than the fully N-substituted sites examined in our study, 11,21 thereby shifting the late transition metals closer to the peak of the volcano. It is therefore important to keep in mind that the catalytic activity depends on the identity of the transition metal atom and its coordination environment.…”

“…ORR onset potentials from thermodynamic analysis in ref (black squares; linear fit shown as black line) and from microkinetic analysis (red and blue circles for M clean and M O 2 ‑ast , respectively; lines refer to values obtained using linear scaling relations shown in the Supporting Information). Differences in the onset potentials predicted from thermodynamic and microkinetic analysis (Δ U ORR = U ORR mk – U ORR td ) are shown above: positive (negative) values correspond to an increase (decrease) in onset potential due to the microkinetic analysis.…”

“…To identify the most active metals for graphene-based catalysts, density functional theory (DFT) has been used to screen a large number of single-atom catalysts for fuel cell applications. − In these studies, onset potentials are typically estimated by calculating the lowest potential required to make each electrochemical reduction step exergonic. However, onset potentials obtained through such thermodynamic analysis deviate substantially from experimental values, a discrepancy that has been attributed to the shortcomings of the computational methods. , Other studies suggest that O* spectator species affect ORR onset potentials on Fe and Ru atoms embedded in N-doped graphene, but a recent microkinetic model suggests that both metals are actually decorated by OH* under experimental conditions .…”

A completely precious metal–free alkaline fuel cell with enhanced performance using a carbon-coated nickel anode

“…Therefore, to explain the effect of N defects in Ni@CN x we propose an alternative model where Ni adatoms form under reaction conditions. Remarkably, our previous studies demonstrated that N -doping in graphene stabilizes single-metal atom catalysts with increased activity toward water splitting and CO oxidation ( 47 , 48 ).…”

Predicted Electrocatalyst Properties on Metal Insulator MoTe2 for Hydrogen Evolution Reaction and Oxygen Reduction Reaction Application in Fuel Cells

“…Because the slopes of the relationships of Δ G 1 versus Δ G *OH , Δ G 2 versus Δ G *OH , and Δ G 3 versus Δ G *OH are positive, while the slope of Δ G 4 versus Δ G *OH is negative (see Figure d), the ORR activity can be determined by the binding strength of *OH on the different adsorbed sites. Thus, excellent candidates for ORR catalysts should possess an applicable *OH binding strength between −1.0 and 1.5 eV, indicating selectivity toward H 2 O when Δ G *OH is < –0.34 eV . As shown in Figure a, the first hydrogenated step (O 2 + H + + e – → OOH*, Δ G 1 ) and the fourth hydrogenated step (*OH + H + + e – → *H 2 O) determine the RDSs of the entire reduction process corresponding to the ORR and OER mechanisms, respectively.…”