Oxygen evolution reaction (OER) and urea oxidation reaction (UOR) play important roles in the field of hydrogen energy preparation and pollution treatment. In this work, we successfully synthesize a Cr doped CoFe layered double hydroxides (LDH) nanosheets array with uniform growth on a nickel-foam (NF). The partially oxidized Cr6+ during the process of oxidation could adjust the electronic structure and stabilize the high-valence active sites by virtue of its strong electron-withdrawing ability. The calculation of density functional theory (DFT) shows that the incorporation of Cr into CoFe LDH could obviously overcome the step of H2O adsorption during the OER progress. Therefore, the CoFeCr LDH/NF electrode exhibits superior OER performance. Furthermore, when acted as a UOR electrode, its performance is also satisfying. The design of the CoFeCr LDH/NF would hew out a brand-new route to construct more bifunctional electrodes for electrocatalytic water splitting in an alkaline solution with or without urea.

Cr-doped CoFe layered double hydroxides: Highly efficient and robust bifunctional electrocatalyst for the oxidation of water and urea

The development of transition-metal-oxides (TMOs)-based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter-doped ruthenium-cobalt oxide [(Ru-Co)Ox ] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru-Co)Ox nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm-2 and a small Tafel slope of 23.5 mV dec-1 , as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm-2 . As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm-2 for alkaline overall water splitting.

Heterostructured Inter-Doped Ruthenium-Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting.

The oxides of cobalt have recently been shown to be highly effective electrocatalysts for the oxygen evolution reaction (OER) under alkaline conditions. In general species such as Co3O4 and CoOOH have been investigated that often require an elevated temperature step during their synthesis to create crystalline materials. In this work we investigate the rapid and direct electrochemical formation of amorphous nanostructured Co(OH)2 on gold electrodes under room temperture conditions which is a highly active precursor for the OER. During the OER some conversion to crystalline Co3O4 occurs at the surface, but the bulk of the material remains amorphous. It is found that the underlying gold electrode is crucial to the materials enhanced performance and provides higher current density than can be achieved using carbon, palladium or copper support electrodes. This catalyst exhibits excellent activity with a current density of 10 mA cm-2 at an overpotential of 360 mV with a high turnover frequency of 2.1 s-1 in 1 M NaOH. A Tafel slope of 56 mV dec-1 at low overpotentials and a slope of 122 mV dec-1 at high overpotentials is consistent with the dual barrier model for the electrocatalytic evolution of oxygen. Significantly, the catalyst maintains excellent activity for up to 24 hr of continuous operation and this approach offers a facile way to create a highly effective and stable material.

Direct electrochemical formation of nanostructured amorphous Co(OH)2 on gold electrodes with enhanced activity for the oxygen evolution reaction.

High-entropy alloys have received considerable attention in the field of catalysis due to their exceptional properties. However, few studies hitherto focus on the origin of their outstanding performance and the accurate identification of active centers. Herein, we report a conceptual and experimental approach to overcome the limitations of single-element catalysts by designing a FeCoNiXRu (X: Cu, Cr, and Mn) High-entropy alloys system with various active sites that have different adsorption capacities for multiple intermediates. The electronegativity differences between mixed elements in HEA induce significant charge redistribution and create highly active Co and Ru sites with optimized energy barriers for simultaneously stabilizing OH* and H* intermediates, which greatly enhances the efficiency of water dissociation in alkaline conditions. This work provides an in-depth understanding of the interactions between specific active sites and intermediates, which opens up a fascinating direction for breaking scaling relation issues for multistep reactions. 

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Unraveling the electronegativity-dominated intermediate adsorption on high-entropy alloy electrocatalysts

Designing efficient oxygen evolution reaction (OER) electrocatalysts based on single-atom catalysts is a highly promising option for cost-effective alkaline water electrolyzers. However, the instability of the OOH* intermediate and high energy barrier for the rate-determining step (RDS) (O* to OOH*) on the pure bimetallic-alloy represent serious challenges. Here, we report atomically dispersed Ru single-atoms on a cobalt–iron bimetallic-alloy encapsulated by graphitic carbon (RuSACoFe2/G) as an efficient and durable electrocatalyst for the alkaline OER. In-depth X-ray absorption spectroscopy (XAS) and aberration-corrected transmission electron microscopy (AC-TEM) along with theoretical calculations were employed to validate the isolated Ru sites in the surface-oxygen rich alloy. RuSACoFe2/G displays exceptional intrinsic activity, achieving a record low overpotential of only 180 mV at 10 mA cm−2 with superior durability in alkali media. Density functional theory (DFT) simulations revealed that the isolated Ru sites with pre-adsorbed surface oxygen species on a bimetallic-alloy efficiently stabilize the OOH* intermediate and significantly reduce the energy barrier for the RDS, boosting the intrinsic OER activity. Our integrated alkaline electrolyzer demands a low cell voltage of 1.48 V at 10 mA cm−2, suggesting that it has potential for use in practical applications.

Stabilizing the OOH* intermediate via pre-adsorbed surface oxygen of a single Ru atom-bimetallic alloy for ultralow overpotential oxygen generation

Monodentate adsorption of oxygen intermediates results in a theoretical overpotential limit of ≈0.35 V for oxygen evolution reaction (OER), which causes the sluggish kinetics of the OER process. In this work, nonprecious chromium dopant is introduced into the self-supported CoFe layered double hydroxides (LDHs) on nickel foam (Cr-CoFe LDHs/NF) via a facile one-step hydrothermal method, which exhibits a preeminent electrocatalytic activity toward the OER with an ultralow overpotential of 238 mV to obtain 10 mA cm-2 and a high stability after cyclic voltammetry for 5000 cycles in alkaline solution (1 m KOH). Density functional theory (DFT) calculations unveil that Cr dopants as new active sites could improve the electron-donation ability of the resultant Cr-CoFe LDHs due to the smaller electronegativity of Cr in comparison with Fe and Co. Therefore, the scaling relation of adsorption energy among four oxygen intermediates is broken and consequently the OER performance is further promoted. This work provides a strategy to develop efficient metal layered double hydroxide OER catalysts.

Cr-Dopant Induced Breaking of Scaling Relations in CoFe Layered Double Hydroxides for Improvement of Oxygen Evolution Reaction.

Colorimetric chiral recognition has attracted much attention due to its advantages of facile operation and quick response compared with conventional methods. However, its applicability and sensitivity are remaining challenges. Herein, we develop a universal, high-throughput and sensitive colorimetric chiral recognition and sensing method for discriminating chiral thiol-containing amino acids via sonication of a mixed solution of MoO3 nanoparticles and enantiomers. The enantiomeric type could be rapidly recognized using the color discrepancy of the MoO3−x colloid nanoparticles, due to the different concentrations of oxygen vacancies caused by the different degrees of coordination interaction between the MoO3−x nanoparticles and the carboxyl groups from the enantiomer, which could be certified by investigation using quasi in situ spectroscopy. The mechanism of different coordination interactions between the MoO3−x nanoparticles and the enantiomer might be caused by different competitve outcomes between coordination interactions and intermolecular hydrogen bonds on reaction with carboxyl groups due to the different intensities of the intermolecular hydrogen bonds of the chiral enantiomers. Moreover, the colorimetric sensing system based on modified MoO3−x nanoparticles exhibited a detection limit for L-cysteine of 43 nM, which is one order of magnitude lower than current colorimetric probes. This work provides a new inorganic nanoparticle system for universal and high-throughput colorimetric chiral recognition and sensing, and gives insight into the formation mechanism of plasmonic MoO3−x nanoparticles.

Changchang Xing

Papers

Cr-Dopant Induced Breaking of Scaling Relations in CoFe Layered Double Hydroxides for Improvement of Oxygen Evolution Reaction.

A sensitive colorimetric chiral recognition for thiol-containing amino acids based on NIR plasmonic MoO3−x nanoparticles