Origin of enhanced water oxidation activity in an iridium single atom anchored on NiFe oxyhydroxide catalyst

Abstract

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic–photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm−2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.

Identification of non-traditional coordination environments for iron ions in nickel hydroxide lattices

“…45 The viability of such out-of-plane ions is nonetheless supported by the entirety of data presented here, and by recent reports for tetrahedral iron sites atop cobalt hydroxides, 34 iron oxide clusters atop nickel hydroxides, 35 and iridium ions atop nickel hydroxides. 36…”

“…33 Parallel studies have identified non-traditional coordination sites in nominally LDH materials, including iron ions situated outside of the 2-dimensional framework in Co(OH) 2 , 34 iron ions on terrace sites of γ-NiOOH, 35 and iridium ions anchored to the basal plane of Ni(OH) 2 . 36 Changes in behavior have also been achieved by mixing of nickel hydroxide and iron oxyhydroxide, 35 inclusion or exchange of anions between the LDH layers, 37–39 and through combination of anion exchange with immobilization of single Pt atoms. 40 The diversity of possibilities motivated us to explore variations in fabrication protocols as a means to study small changes in composition-dependent structure in an effort to identify non-traditional coordination environments in nickel-based layered double hydroxides.…”

Efficient Electrocatalytic Oxidation of Glycerol via Promoted OH* Generation over Single-Atom-Bismuth-Doped Spinel Co3O4

“…In the past years, single-atom catalysts (SACs) with atomically dispersed active sites have gained great attention for high atom utilizations and unique catalytic activities. − It is reported that single atoms as dopants could dramatically improve the catalytic performance. − Recently, several successful attempts have been made to incorporate single atoms into spinels with enhanced electro-oxidation activity. For example, Wang et al reported selective adsorption of the CC groups in 5-hydroxymethylfurfural (HMF) by constructing single-atom-Ir-loaded Co 3 O 4 for enhanced HMF electro-oxidation (HMFOR) .…”

Identification of Fenton-like active Cu sites by heteroatom modulation of electronic density

“…Single-atom catalysts (SACs) featuring utmost atom-utilization efficiency and tunable electronic structure can break the limitations of heterogeneous catalysts in terms of the kinetics and catalytic activity ( 20 , 21 ). Thus, SACs show a great potential to address the slow reaction kinetics of PMS for the Fenton-like process via maximizing the number of catalytic sites ( 22 ).…”