Electrodeposition of Layered Double Hydroxides on platinum: Insights into the reactions sequence

Balance between FeIV-NiIV synergy and Lattice Oxygen Contribution for Accelerating Water Oxidation

Hydrogen obtained from electrochemical water splitting is the most promising clean energy carrier, which is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Thus, the development of an efficient OER electrocatalyst using nonprecious 3d transition elements is desirable. Multielement synergistic effect and lattice oxygen oxidation are two well-known mechanisms to enhance the OER activity of catalysts. The latter is generally related to the high valence state of 3d transition elements leading to structural destabilization under the OER condition. We have found that Al doping in nanosheet Ni-Fe hydroxide exhibits 2-fold advantage: (1) a strong enhanced OER activity from 277 mV to 238 mV at 10 mA cm-2 as the Ni valence state increases from Ni3.58+ to Ni3.79+ observed from in situ X-ray absorption spectra. (2) Operational stability is strengthened, while weakness is expected since the increased NiIV content with 3d8L2 (L denotes O 2p hole) would lead to structural instability. This contradiction is attributed to a reduced lattice oxygen contribution to the OER upon Al doping, as verified through in situ Raman spectroscopy, while the enhanced OER activity is interpreted as an enormous gain in exchange energy of FeIV-NiIV, facilitated by their intersite hopping. This study reveals a mechanism of Fe-Ni synergy effect to enhance OER activity and simultaneously to strengthen operational stability by suppressing the contribution of lattice oxygen.

The Structure, Preparation, Characterization, and Intercalation Mechanism of Layered Hydroxides Intercalated with Guest Anions

Since the intercalation of anions into layered hydroxides (LHs) has a great impact not only on their nucleation and growth but also on their structure, composition, and size, the intercalation chemistry of LHs has aroused the strong interest of researchers. However, the progress in the fundamental understanding of LHs intercalated with guest anions have not been paralleled by a concomitant development of the preparation and performance improvement of such materials. Considering the guidance of a timely in-depth review for scientists in this area, a systematic introduction about the development that is made on the above-mentioned issues is highly needed but yet missing so far. Herein, recent advances in understanding the chemical composition and structure of LHs intercalated with guest anions are systematically summarized. Meanwhile, typical and emerging bottom-up synthesis methods of LHs intercalated with anions are reviewed, and the potential impact of external reaction parameters on the intercalation of anions into LHs are discussed . Besides, different analytical characterization techniques employed in the examination of guest anion-intercalated LHs are deliberated upon. Finally, although progress is slow in exploring the intercalation mechanism, as many examples as possible are included in this review and inferred the possible intercalation mechanism.

Corrosion Resistance of Electrochemically Synthesized Modified Zaccagnaite LDH-Type Films on Steel Substrates

Modified zaccagnaite layered double hydroxide (LDH) type films were synthesized on steel substrates by pulsed electrochemical deposition from aqueous solutions. The resulting films were characterized by X-ray diffraction, scanning electron microscopy/X-ray dispersive spectroscopy, and Fourier transform infrared spectroscopy. Structural characterization indicated a pure layered double hydroxide phase; however, elemental analysis revealed that the surface of the films contained Zn:Al ratios outside the typical ranges of layered double hydroxides. Layer thickness for the deposited films ranged from approximately 0.4 to 3.0 μm. The corrosion resistance of the film was determined using potentiodynamic polarization experiments in 3.5 wt.% NaCl solution. The corrosion current density for the coatings was reduced by 82% and the corrosion potential was shifted 126 mV more positive when 5 layers of modified LDH coatings were deposited onto the steel substrates. A mechanism was proposed for the corroding reactions at the coating.

Slight Zinc Doping by an Ultrafast Electrodeposition Process Boosts the Cycling Performance of Layered Double Hydroxides for Ultralong-Life-Span Supercapacitors

Layered double hydroxides (LDHs) have attracted much attention in supercapacitors because of the high specific surface area and theoretical capacitance. However, the bad cycling stability has always been their Achilles' heel that restrains their further application. In this paper, a small amount of unactive and single-valence element zinc, which has no contribution to the capacitance of electrodes, was first doped into NiCo-LDHs through two consecutive electrodeposition processes only within 30 min. With a polyaniline (PANI) nanolayer as the interlayer, an ultrathin NiCoZn-LDH nanoplate network was well-anchored on the carbon cloth surface. The slight Zn²⁺ doping dramatically enhanced the cycling performance of LDHs with little capacitance decay. Zn²⁺ doping enhanced the cyclic structural stability of NiCoZn-LDHs, while the PANI layer strengthened the interface interaction between LDHs and the current collector. By controlling the doping content of Zn²⁺ at 2.9%, the composite electrode achieved the best performance with a high specific capacitance of 1749 F g^-1 and an ultralong life span with 89% capacitance retention after 40,000 charge-discharge cycles. This work offers a novel strategy to fast build LDH-based supercapacitors with both high specific capacitance and cycling performance.

Growth Dynamics of Vertical and Lateral Layered Double Hydroxide Nanosheets during Electrodeposition

Layered double hydroxides (LDHs) are a class of lamellar materials with a wide range of potential catalytic applications. LDHs form from positively charged 2D atomic layers separated by charge-balancing anions and solvent molecules. Typically, nanoscale LDH sheets can grow vertical or parallel to a substrate, exposing their different active facets. These two growth modes of LDH nanosheets have a significant impact on their electrocatalytic properties, yet the details of their growth remain unknown, hindering our ability to design and synthesize high-performance LDH-based electrocatalysts. Here, we investigate the growth pathways of LDH nanosheets using in situ electrochemical liquid-phase transmission electron microscopy (TEM) and show that the growth modes of LDH nanosheets can be controlled by tuning the precursor concentrations. Moreover, our observations reveal that LDH nanosheets grow via two pathways: (1) monomer addition, where the adatoms are heterogeneously deposited onto the LDH nanosheets, and (2) coalescence, where adjacent nanosheets merge together.

Layered double hydroxides toward electrochemical energy storage and conversion: design, synthesis and applications

Two-dimensional (2D) materials have attracted increasing interest in electrochemical energy storage and conversion. As typical 2D materials, layered double hydroxides (LDHs) display large potential in this area due to the facile tunability of their composition, structure and morphology. Various preparation strategies, including in situ growth, electrodeposition and layer-by-layer (LBL) assembly, have been developed to directly modify electrodes by using LDH materials. Moreover, several composite materials based on LDHs and conductive matrices have also been rationally designed and employed in supercapacitors, batteries and electrocatalysis with largely enhanced performances. This feature article summarizes the latest developments in the design, preparation and evaluation of LDH materials toward electrochemical energy storage and conversion.