NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γphases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure MM centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.

Layered double hydroxides (LDHs) are among the most active and studied catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, previous studies have generally either focused on a small number of LDHs, applied synthetic routes with limited structural control, or used non‐intrinsic activity metrics, thus hampering the construction of consistent structure–activity‐relations. Herein, by employing new individually developed synthesis strategies with atomic structural control, we obtained a broad series of crystalline α‐MA(II)MB(III) LDH and β‐MA(OH)2 electrocatalysts (MA=Ni, Co, and MB=Co, Fe, Mn). We further derived their intrinsic activity through electrochemical active surface area normalization, yielding the trend NiFe LDH > CoFe LDH > Fe‐free Co‐containing catalysts > Fe‐Co‐free Ni‐based catalysts. Our theoretical reactivity analysis revealed that these intrinsic activity trends originate from the dual‐metal‐site nature of the reaction centers, which lead to composition‐dependent synergies and diverse scaling relationships that may be used to design catalysts with improved performance.

Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition and shape is limited requiring trial and error.Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under co-reduction "one-step" conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content we develop a "two-step" route and track the evolution of the composition and morphology of the particles at the atomic scale.To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provide evidence of a heterogeneous element distribution caused by element-specific anisotropic growth and create octahedral nanoparticles with tailored atomic composition like Pt 1.5 M, PtM and PtM 1.5 (M=Ni+Co). These trimetallic electrocatalysts have been tested toward the oxygen reduction reaction (ORR), showing a greatly enhanced mass activity related to commercial Pt/C and less activity loss than binary PtNi and PtCo after 4000 potential cycles.Keywords: PtNiCo octahedra, intraparticle composition, anisotropic growth, oxygen reduction reaction. 3Over the past years, extensive research and development has been carried out in fuel cells with the aim of implementing this technology on transportation, stationary and portable power generation 1, 2 . Nevertheless, some issues such as catalysts kinetic limitations, durability and cost must still be addressed before their successful commercialization. Oxygen reduction electrocatalysts are known to play a crucial role on fuel cells performance, being the fundamental studies on the subject still required to overcome these barriers 3,4 . By alloying Pt with other non-noble metals, it is possible to produce cheaper electrocatalysts with novel properties for the oxygen reduction reaction (ORR) due to lattice compression 5,6 and/or modified electronic properties 7,8 . Thus, several studies have shown that binary Pt-M (M=Cr, Mn, Fe, Co, Ni, Cu, V, Ti) nanocrystals (NCs) greatly enhanced the kinetics of the ORR in comparison with standard Pt catalysts 9-11 . However, besides the composition, the intrinsic activity of nanoparticles also depends on their size and shape which strongly determine the atomic surface structure of the nanoparticles [12][13][14][15] . Thus, by controlling the morphology of the NCs it is possible to maximize the exposure of certain facets that exhibit better catalytic properties 16,17 . These are the premises that led to establish {111}-Pt 3 Ni surfaces as the ideal electrocatalyst for the ORR 18,19 . Since then, many synthetic routes to obtain PtNi nanooctahedra, which ideally exhibit 8 face...

NiFe layered double hydroxides (LDHs) are among the most active electrocatalysts for alkaline oxygen evolution reaction (OER) and OER selective seawater oxidation. These promising applications call for a fundamental understanding of the catalyst/electrolyte interaction, which is challenging to investigate during operation conditions. This work reports an operando structure–reactivity analysis of NiFe LDH as the electrocatalyst for the OER in alkaline and alkalinized NaCl electrolytes, by combining operando wide-angle X-ray scattering (WAXS) and electrochemical characterization. The operando results showed that higher pH values lead to a higher percentage of the OER active γ-NiFe LDH in the composition of the catalyst layer, larger Ni redox peaks, and higher OER activity. The addition of 0.5 M NaCl to moderate alkaline electrolytes (0.1–0.5 M KOH) also leads to larger Ni redox features and higher activity but appears to limit the percentage of γ-NiFe LDH during the OER in comparison to the corresponding NaCl-free electrolytes. Interestingly, a higher KOH concentration (1.0 M KOH, pH 14) could compensate this structural effect aligning the percentage of OER-active γ-NiFe LDH in both NaCl-free and NaCl-containing electrolytes. Additional scan rate investigations showed a strong correlation of the electrochemical accessibility of NiFe LDH with its history, scan rate, and NaCl addition. In particular, the faster and more effective break-in process induced by NaCl addition is proposed as the origin of the enhanced activity at low pH, despite the lower γ-phase percentage.

Water oxidation is a crucial reaction for renewable energy conversion and storage. Among the alkaline oxygen evolution reaction (OER) catalysts, NiFe based oxyhydroxides show the highest catalytic activity. However, the details of their OER mechanism are still unclear, due to the elusive nature of the OER intermediates. Here, using a novel differential electrochemical mass spectrometry (DEMS) cell interface, we performed isotope‐labelling experiments in 18O‐labelled aqueous alkaline electrolyte on Ni(OH)2 and NiFe layered double hydroxide nanocatalysts. Our experiments confirm the occurrence of Mars‐van‐Krevelen lattice oxygen evolution reaction mechanism in both catalysts to various degrees, which involves the coupling of oxygen atoms from the catalyst and the electrolyte. The quantitative charge analysis suggests that the participating lattice oxygen atoms belong exclusively to the catalyst surface, confirming DFT computational hypotheses. Also, DEMS data suggest a fundamental correlation between the magnitude of the lattice oxygen mechanism and the faradaic efficiency of oxygen controlled by pseudocapacitive oxidative metal redox charges.