Tuning Electronic Structure of NiFe Layered Double Hydroxides with Vanadium Doping toward High Efficient Electrocatalytic Water Oxidation

2D Layered Double Hydroxides for Oxygen Evolution Reaction: From Fundamental Design to Application

Metallic nanostructures with low dimensionality (one-dimension and two-dimension) possess unique structural characteristics and distinctive electronic and physicochemical properties including high aspect ratio, high specific surface area, high density of surface unsaturated atoms and high electron mobility. These distinctive features have rendered them remarkable advantages over their bulk counterparts for surface-related applications, for example, electrochemical water splitting. In this review article, we highlight the recent research progress in low-dimensional metallic nanostructures for electrochemical water splitting including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fundamental understanding of the electrochemistry of water splitting including HER and OER is firstly provided from the aspects of catalytic mechanisms, activity descriptors and property evaluation metrics. Generally, it is challenging to obtain low-dimensional metallic nanostructures with desirable characteristics for HER and OER. We hereby introduce several typical methods for synthesizing one-dimensional and two-dimensional metallic nanostructures including organic ligand-assisted synthesis, hydrothermal/solvothermal synthesis, carbon monoxide confined growth, topotactic reduction, and templated growth. We then put emphasis on the strategies adopted for the design and fabrication of high-performance low-dimensional metallic nanostructures for electrochemical water splitting such as alloying, structure design, surface engineering, interface engineering and strain engineering. The underlying structure–property correlation for each strategy is elucidated aiming to facilitate the design of more advanced electrocatalysts for water splitting. The challenges and perspectives for the development of electrochemical water splitting and low-dimensional metallic nanostructures are also proposed.

Metallic nanostructures with low dimensionality for electrochemical water splitting

Electrolysis of water to generate hydrogen fuel is an attractive renewable energy storage technology. However, grid-scale freshwater electrolysis would put a heavy strain on vital water resources. Developing cheap electrocatalysts and electrodes that can sustain seawater splitting without chloride corrosion could address the water scarcity issue. Here we present a multilayer anode consisting of a nickel–iron hydroxide (NiFe) electrocatalyst layer uniformly coated on a nickel sulfide (NiSx) layer formed on porous Ni foam (NiFe/NiSx-Ni), affording superior catalytic activity and corrosion resistance in solar-driven alkaline seawater electrolysis operating at industrially required current densities (0.4 to 1 A/cm2) over 1,000 h. A continuous, highly oxygen evolution reaction-active NiFe electrocatalyst layer drawing anodic currents toward water oxidation and an in situ-generated polyatomic sulfate and carbonate-rich passivating layers formed in the anode are responsible for chloride repelling and superior corrosion resistance of the salty-water-splitting anode.

https://www.pnas.org/content/pnas/116/14/6624.full.pdf

Solar-driven, highly sustained splitting of seawater into hydrogen and oxygen fuels

The electrocatalytic CO2 reduction reaction (CRR) and N2 reduction reaction (NRR), which convert inert small molecules into high-value products under mild conditions, have received much research attention Defect engineering is an important strategy for modulating the electronic properties of electrocatalysts, which may endow unexpected physical and chemical properties to break the intrinsic bottleneck and, therefore, boost the electrocatalytic performance To date, various defective nanomaterials (such as nanocarbon and transition metal compounds) have been synthesized that display great potential for the CRR and NRR Therefore, a deep understanding of the influence of defects on the catalytic activity is urgently needed In this review, the type, regulation strategy, fine defect characterization methods, and their application in the electrocatalytic CRR and NRR are discussed and summarized Furthermore, the major challenges, opportunities, and future development direction of defect engineering in CRR and NRR catalysts are proposed This review aims to provide a reference for the proof-of-concept design of highly active CRR and NRR electrocatalysts

Defect engineering in earth-abundant electrocatalysts for CO2 and N2 reduction

Foreign metal ions with reducing ability were doped into NiFe layered double hydroxides (NiFe-LDHs) to activate the basal plane in NiFe-LDHs for oxygen evolution reaction (OER). Mn2+-Doped NiFe-LDH array electrode yields a low onset potential of 1.41 V and exhibits outstanding stability. The study herein illustrates a new dimension of electronic structure regulation and promises further optimization of highly efficient electrocatalysts.

Activating basal plane in NiFe layered double hydroxide by Mn2+ doping for efficient and durable oxygen evolution reaction

Nickel-iron layered double hydroxide (NiFe-LDH) nanosheets have shown optimal oxygen evolution reaction (OER) performance; however, the role of the intercalated ions in the OER activity remains unclear. In this work, we show that the activity of the NiFe-LDHs can be tailored by the intercalated anions with different redox potentials. The intercalation of anions with low redox potential (high reducing ability), such as hypophosphites, leads to NiFe-LDHs with low OER overpotential of 240 mV and a small Tafel slope of 36.9 mV/dec, whereas NiFe-LDHs intercalated with anions of high redox potential (low reducing ability), such as fluorion, show a high overpotential of 370 mV and a Tafel slope of 80.8 mV/dec. The OER activity shows a surprising linear correlation with the standard redox potential. Density functional theory calculations and X-ray photoelectron spectroscopy analysis indicate that the intercalated anions alter the electronic structure of metal atoms which exposed at the surface. Anions with low standard redox potential and strong reducing ability transfer more electrons to the hydroxide layers. This increases the electron density of the surface metal sites and stabilizes their high-valence states, whose formation is known as the critical step prior to the OER process.

/pdf/effects-of-redox-active-interlayer-anions-on-the-oxygen-poibuficeo.pdf

Effects of redox-active interlayer anions on the oxygen evolution reactivity of NiFe-layered double hydroxide nanosheets

Atomic composition tuning and defect engineering are effective strategies toenhance the catalytic performance of multicomponent catalysts by improvingthe synergetic effect; however, it remains challenging to dramatically tune the active sites on multicomponent materials through simultaneous defect engineeringat the atomic scale because of the similarities of the local environment. Herein,using the oxygen evolution reaction (OER) as a probe reaction, we deliberatelyintroduced base-soluble Zn(II) or Al(III) sites into NiFe layered double hydroxides(LDHs), which are one of the best OER catalysts. Then, the Zn(II) or Al(III) siteswere selectively etched to create atomic M(II)/M(III) defects, which dramaticallyenhanced the OER activity. At a current density of 20 mA·cm−2, only 200 mV overpotential was required to generate M(II) defect-rich NiFe LDHs, which is the best NiFe-based OER catalyst reported to date. Density functional theory(DFT) calculations revealed that the creation of dangling Ni–Fe sites (i.e., unsaturated coordinated Ni–Fe sites) by defect engineering of a Ni–O–Fe site at the atomic scale efficiently lowers the Gibbs free energy of the oxygen evolutionprocess. This defect engineering strategy provides new insights into catalysts atthe atomic scale and should be beneficial for the design of a variety of catalysts.

Layered double hydroxides with atomic-scale defects for superior electrocatalysis

Flame-Engraved Nickel–Iron Layered Double Hydroxide Nanosheets for Boosting Oxygen Evolution Reactivity

Exploring efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) is critical to water splitting. While nickel-iron layered double hydroxide (NiFe LDH) has been long recognized as a promising non-precious electrocatalyst for OER, its intrinsic activity needs further improvement. Herein, we design a highly-efficient oxygen evolution electrode based on defective NiFe LDH nanoarray. By combing the merits of the modulated electronic structure, more exposed active sites, and the conductive electrode, the defective NiFe LDH electrocatalysts show a low onset potential of 1.40 V ( vs . RHE). An overpotential of only 200 mV is required for 10 mA cm−2, which is 48 mV lower than that of pristine NiFe-LDH. Density functional theory plus U (DFT+ U ) calculations are further employed for the origin of this OER activity enhancement. We find the introduction of oxygen vacancies leads to a lower valance state of Fe and the narrowed bandgap, which means the electrons tend to be easily excited into the conduction band, resulting in the lowered reaction overpotential and enhanced OER performance.

/pdf/a-highly-efficient-oxygen-evolution-electrode-based-on-3y3bgbj70w.pdf

Qixian Xie

Papers

Activating basal plane in NiFe layered double hydroxide by Mn2+ doping for efficient and durable oxygen evolution reaction

Effects of redox-active interlayer anions on the oxygen evolution reactivity of NiFe-layered double hydroxide nanosheets

Layered double hydroxides with atomic-scale defects for superior electrocatalysis

Flame-Engraved Nickel–Iron Layered Double Hydroxide Nanosheets for Boosting Oxygen Evolution Reactivity

A highly-efficient oxygen evolution electrode based on defective nickel-iron layered double hydroxide