In Situ X-ray Absorption Spectroscopy of Metal/Nitrogen-doped Carbons in Oxygen Electrocatalysis

Metal/nitrogen-doped carbons (M−N−C) are promising candidates as oxygen electrocatalysts due to their low cost, tunable catalytic activity and selectivity, and well-dispersed morphologies. To improve the electrocatalytic performance of such systems, it is critical to gain a detailed understanding of their structure and properties through advanced characterization. In situ X-ray absorption spectroscopy (XAS) serves as a powerful tool to probe both the active sites and structural evolution of catalytic materials under reaction conditions. In this review, we firstly provide an overview of the fundamental concepts of XAS and then comprehensively review the setup and application of in situ XAS, introducing electrochemical XAS cells, experimental methods, as well as primary functions on catalytic applications. The active sites and the structural evolution of M−N−C catalysts caused by the interplay with electric fields, electrolytes and reactants/intermediates during the oxygen evolution reaction and the oxygen reduction reaction are subsequently discussed in detail. Finally, major challenges and future opportunities in this exciting field are highlighted.</p

Spin pinning effect to reconstructed oxyhydroxide layer on ferromagnetic oxides for enhanced water oxidation.

Producing hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. With spin-dependent kinetics in OER, to manipulate the spin ordering of ferromagnetic OER catalysts (e.g., by magnetization) can reduce the kinetic barrier. However, most active OER catalysts are not ferromagnetic, which makes the spin manipulation challenging. In this work, we report a strategy with spin pinning effect to make the spins in paramagnetic oxyhydroxides more aligned for higher intrinsic OER activity. The spin pinning effect is established in oxideFM/oxyhydroxide interface which is realized by a controlled surface reconstruction of ferromagnetic oxides. Under spin pinning, simple magnetization further increases the spin alignment and thus the OER activity, which validates the spin effect in rate-limiting OER step. The spin polarization in OER highly relies on oxyl radicals (O∙) created by 1st dehydrogenation to reduce the barrier for subsequent O-O coupling

Valence Change Ability and Geometrical Occupation of Substitution Cations Determine the Pseudocapacitance of Spinel Ferrite XFe₂O₄ (X = Mn, Co, Ni, Fe)

In recent years, spinel ferrites have attracted much attention as a merging material for oxygen reduction reaction (ORR),(1) advanced battery electrodes,(2, 3) and supercapacitors.(4, 5) Generally, the spinel structure can be described by a formula [X₁−λ²⁺Bλ³⁺]ᵀ[Xλ²⁺B₂−λ³⁺]ᴼ O₄, where λ is the inversion degree in-between 0 and 1, and superscripts T and O denote the tetrahedral and octahedral sites, respectively. Depending on the cation distribution, a spinel can be normal (λ = 0, 100% X in tetrahedral sites), inverse (λ = 1, 100% X in octahedral sites), or partially inverse (0 < λ < 1). Previous studies on spinel ferrites focused intensively on their magnetic properties,(6, 7) as the substitution of Fe cations by transition metals can affect the cation distribution, thus resulting in significantly different magnetic properties due to the regulation of the unpaired electron spins of Fe²⁺ in octahedral sites.(8, 9) Recent studies also showed that this substitution affects their electrochemical performance. For example, Mn substituted ferrites show nearly the same ORR activity as Pt/C in alkaline and Mn substitution also influences the capacitance more than other metal ion substituted ferrites.(1, 10) Such difference on electrochemical performance could be ascribed to the type of substituent cations and their influence. However, no systematic mechanistic investigation has been carried out. This lack of knowledge hinders the understanding of the substitution effects on the performance and, thus, the development of spinel ferrites as energy materials

Navigating surface reconstruction of spinel oxides for electrochemical water oxidation

Understanding and mastering the structural evolution of water oxidation electrocatalysts lays the foundation to finetune their catalytic activity. Herein, we demonstrate that surface reconstruction of spinel oxides originates from the metal-oxygen covalency polarity in the MT–O–MO backbone. A stronger MO–O covalency relative to MT–O covalency is found beneficial for a more thorough reconstruction towards oxyhydroxides. The structure-reconstruction relationship allows precise prediction of the reconstruction ability of spinel pre-catalysts, based on which the reconstruction degree towards the in situ generated oxyhydroxides can be controlled. The investigations of oxyhydroxides generated from spinel pre-catalysts with the same reconstruction ability provide guidelines to navigate the cation selection in spinel pre-catalysts design. This work reveals the fundamentals for manipulating the surface reconstruction of spinel pre-catalysts for water oxidation

From Two-Phase to Three-Phase: The New Electrochemical Interface by Oxide Electrocatalysts

Abstract Electrochemical reactions typically occur at the interface between a solid electrode and a liquid electrolyte. The charge exchange behaviour between these two phases determines the kinetics of electrochemical reactions. In the past few years, significant advances have been made in the development of metal oxide electrocatalysts for fuel cell and electrolyser reactions. However, considerable gaps remain in the fundamental understanding of the charge transfer pathways and the interaction between the metal oxides and the conducting substrate on which they are located. In particular, the electrochemical interfaces of metal oxides are significantly different from the traditional (metal) ones, where only a conductive solid electrode and a liquid electrolyte are considered. Oxides are insulating and have to be combined with carbon as a conductive mediator. This electrode configuration results in a three-phase electrochemical interface, consisting of the insulating oxide, the conductive carbon, and the liquid electrolyte. To date, the mechanistic insights into this kind of non-traditional electrochemical interface remain unclear. Consequently conventional electrochemistry concepts, established on classical electrode materials and their two-phase interfaces, are facing challenges when employed for explaining these new electrode materials

Effects of catalyst mass loading on electrocatalytic activity: An example of oxygen evolution reaction

The evaluation of the intrinsic activity of catalysts is the most basic in searching energy- and cost-efficient catalyst materials for various applications. The accurate determination of the intrinsic activity is essential for identifying efficient catalysts. While a huge number of studies of electrocatalysis for various applications have been reported, the effects of electrode loading on the apparent intrinsic activity obtained experimentally have been rarely discussed. With a high mass loading on the electrode, not all the catalyst surfaces can be electrochemically active because not all the surfaces can be wetted by the electrolyte. The loading also affects the transport of electrons over the electrode as well as the transport of ions in the electrolyte, and thus affects the kinetics. These lead to the derivations of the apparent intrinsic activity from the real intrinsic activity. Herein, for better understanding the derivations, we evaluate and discuss the effects of electrode mass loading using oxygen evolution reaction as an example

A new insight into electrochemical detection of eugenol by hierarchical sheaf-like mesoporous NiCo2O4

In this work, NiCo2O4 nanosheet with sheaf-like nanostructure morphologies was synthesized by a facile one-step hydrothermal reaction followed by annealing treatment. Impressively, the NiCo2O4 exhibit rapid detection of eugenol. The linear range of detection is from 1-500 µM, and the limit of detection is 5.4 pM. NiCo2O4 modified electrode demonstrated high sensitivity, good repeatability and reproducibility, and long-term stability (7 % decrease in response over 30 days). Based on this work, an electrochemical reaction mechanism for eugenol oxidation was proposed, and in addition, the NiCo2O4 modified electrode was successfully employed for the analysis of eugenol in medicative balm samples. The recoveries study for eugenol in medicative balm samples gave values in the range of 98.7– 05.5%.Accepted versio

Ethylene glycol and ethanol oxidation on spinel ni-co oxides in alkaline

This article presents a systematic study on the composition dependence of Ni-Co oxides (NCOs) on their electrocatalytic activities toward ethylene glycol (EG) and ethanol (EtOH) oxidation. NCO electrodes were prepared by co-electrodeposition method followed by annealing in air. The atomic ratios of Ni / (Ni + Co) (Ni content) in NCOs were controlled by adjusting the concentration ratio of Ni and Co precursors. As the Ni content increased, the phase of materials changed from the spinel to the mixture of spinel and rock salt. The electrocatalytic activities of these NCOs toward EG and EtOH oxidation were investigated by cyclic voltammetry, differential pulse voltammetry, multi-step chronoamperometry, and electrochemical impedance spectroscopy techniques. It was found that the performance of NCOs for EG and EtOH oxidation exhibited a firstly-increase-then-decrease trend with the increase of Ni content and the best performance was found at 46% Ni. The presence of Ni probably can facilitate EG and EtOH oxidation. Increasing the concentration of reactants or pH can improve the reaction rates. The products from EG and EtOH oxidation were analyzed by nuclear magnetic resonance, which indicated that the oxidation reaction was a process from hydroxyl group to carboxyl group.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Potential and electric double-layer effect in electrocatalytic urea synthesis

Abstract Electrochemical synthesis is a promising way for sustainable urea production, yet the exact mechanism has not been fully revealed. Herein, we explore the mechanism of electrochemical coupling of nitrite and carbon dioxide on Cu surfaces towards urea synthesis on the basis of a constant-potential method combined with an implicit solvent model. The working electrode potential, which has normally overlooked, is found influential on both the reaction mechanism and activity. The further computational study on the reaction pathways reveals that *CO-NH and *NH-CO-NH as the key intermediates. In addition, through the analysis of turnover frequencies under various potentials, pressures, and temperatures within a microkinetic model, we demonstrate that the activity increases with temperature, and the Cu(100) shows the highest efficiency towards urea synthesis among all three Cu surfaces. The electric double-layer capacitance also plays a key role in urea synthesis. Based on these findings, we propose two essential strategies to promote the efficiency of urea synthesis on Cu electrodes: increasing Cu(100) surface ratio and elevating the reaction temperature