Recent advancements of layered double hydroxide heterojunction composites with engineering approach towards photocatalytic hydrogen production: A review

Photocatalytic hydrogen production has been considered as one of the most promising alternatives for providing clean, sustainable, and renewable energy sources. Tremendous investigation and efforts have been devoted to increase the efficiency of the solar to energy conversion of a photocatalyst. Layered double hydroxide (LDH) received scientific attention for its excellent compositional flexibility and controllable morphology, leading to the facile incorporation of the metal species into their layered structure. The unique multi-structure and the tunability of its band gap make LDH more prominent in the field of photocatalysis. This article highlights the recent developments in the fabrication of LDH-based photocatalyst nanocomposites and the engineering approaches for augmenting their photocatalytic hydrogen production efficiency. The thermodynamics and challenges in photocatalytic water splitting are deliberated to understand the pathways to construct efficient semiconductor photocatalysis system. The efficiency enhancement of LDH-based photocatalysts are comprehensively discussed by giving special attention to the heterojunction engineering of type I, type II, p-n junction, Z-scheme, S-scheme, and R-scheme. Fabrication of the hybrid LDH nanocomposites through band gap engineering and metal loading are summarised. The architectural and morphological tuning of LDH-based composite through the construction of the novel core-shell structure and layer-by-layer nanosheets are also demonstrated. Finally, the future recommendations are outlined to provide insights for their development in the photocatalysis field.     

MoSe2 regulates Ce-doped NiFe layered double hydroxide for efficient oxygen evolution reaction: The increase of active sites

Oxygen evolution reaction (OER) is a common reaction in many sustainable energy conversion systems. However, it has become a bottleneck in the development of sustainable energy conversion systems because of its slow kinetics, especially in the common electrolytic water reaction. At present, although there are a lot of researches on OER's catalysts, it is still a great challenge. In this work, a new type of composite was prepared by simple co-precipitation method and Hydrothermal, which is composed of Ce-doped NiFe Layered Double Hydroxide (LDH) and MoSe₂. The electrochemical test results of OER show that the overpotential of 6.7%Ce–NiFe LDH@MoSe₂ is 221 mV at 10 mA/cm², which is better than that of NiFe LDH (409 mV). And it is better than most of the reported OER catalysts in literature, including precious metal catalysts. Simultaneously, 6.7%Ce–NiFe LDH@MoSe₂ also has smaller Tafel slope (35.8mV/dec), larger ESCA (6689 cm²), long-time stability and selectivity with 92.1% Faraday efficiency. The excellent OER performance of 6.7%Ce–NiFe LDH@MoSe₂ benefits from the increase of active and defective sites and the interface coordination between MoSe₂ and Ce–NiFe LDH.     

Effect of in situ growth of NiSe2 on NiAl layered double hydroxide on its electrocatalytic properties for methanol and urea

With high energy density and low theoretical potential, the methanol oxidation (MOR) and urea oxidation (UOR) are often used as substitute reactions to the oxygen evolution reaction (OER). As one of the popular non-precious metal catalysts for the MOR/UOR research in recent years, nickel-based layered double hydroxides (LDHs) have abundant active sites and low cost, but suffer from poor catalytic activity and poor stability. In the present study, we prepared NiAl LDH and then grew NiSe₂ in situ on its surface at different temperatures, and the catalyst obtained at 450 °C (4NiAlSe-450) exhibited excellent MOR/UOR electrocatalytic performance with potentials of 1.37 V vs. RHE and 1.36 V vs. RHE at a current density of 10 mA cm⁻², respectively, which were much higher than those of NiAl LDH (1.42 V vs. RHE and 1.39 vs. RHE). Chronoamperometry curves of 4NiAlSe-450 at 1.5 V potential showed that the methanol/urea oxidation was stable for more than 3 h. The physicochemical properties of 4NiAlSe-450 were analyzed by using X-ray diffraction, X-ray photoelectron spectroscopy and other techniques, and the results showed that the NiSe₂ nanoparticles were successfully grown in situ on the calcined layered structure, and therefore the excellent MOR/UOR electrocatalytic performance of 4NiAlSe-450 may be due to the synergistic effect between the NiAl composite oxides and NiSe₂.     

Synthesis and characterization of polysulfone/layered double hydroxides nanocomposite membranes for fuel cell application

In the present study, sulfonated polysulfone (SPSU)/layered double hydroxide (LDH) composite membranes for use in proton exchange membrane fuel cells (PEMFCs) were investigated. Polysulfone (PSU) was sulfonated with trimethylsilyl chlorosulfonate in 1,2 dichloroethane at room temperature.     

Facile layer-by-layer self-assembly of 2D perovskite niobate and layered double hydroxide nanosheets for enhanced photocatalytic oxygen generation

Photocatalytic O₂-generation reaction is recognized as a crucial step in water splitting and has drawn great attention of researchers. In this work, a hetero-layered composite photocatalyst was successfully prepared by a facile self-assembly method based on electrostatic interaction between oppositely charged Zn/Cr-layered double hydroxide (Zn/Cr-LDH) and lead niobate nanosheets. The layer-by-layer stacking of Zn/Cr-LDH and HPb₂Nb₃O₁₀ nanosheets was beneficial for rapid migration of photo-induced charge carriers inside the photocatalyst because of large contact area. In the meantime, Zn/Cr-LDH and HPb₂Nb₃O₁₀ components exhibited suitable energy-band alignment, which led to efficient separation of photo-induced charge carriers. The composite photocatalyst showed enhanced photocatalytic O₂-generation activity under visible-light irradiation without loading cocatalyst. Briefly, this work expanded the applications of AB₂Nb₃O₁₀-based materials in photocatalytic energy conversion and proved that constructing composites based on electrostatic self-assembly of complementary 2D materials is a promising strategy for development of more efficient photocatalysts.     

Preparation of a novel Pd/layered double hydroxide composite membrane for hydrogen filtration and characterization by thermal cycling

A layered double hydroxide (LDH) layer was grown directly on a porous stainless steel (PSS) surface to reduce the pore opening of the PSS and to be a middle layer retarding Pd/Fe interdiffusion. A thin Pd film (∼7.85 μm) was plated on the modified PSS tube by an electroless plating method. A helium leak test proved that the thin Pd on the LDH-modified PSS substrate was free of defects. The membrane had a H₂ flux of 28–36 m³/(m² h) and H₂/He selectivity larger than 2000 at a pressure difference of 1 bar. Thermal cycling between room temperature and 673 K was performed and showed that the membrane exhibited good permeance and selectivity. Long-term evaluation (1500 h) of the membrane at 673 K showed static results of H₂ flux (∼30 m³/(m² h)) and H₂/He selectivity (∼2000) over the 1500 h test period.     

Synthesis and characterization of crosslinked poly (vinyl alcohol)/layered double hydroxide composite polymer membranes for alkaline direct ethanol fuel cells

The low ionic conductivity and low thermal stability of conventional quaternary ammonium group functionalized anion-exchange membranes (AEM) are two key parameters that limit the performance of AEM direct ethanol fuel cells (AEM DEFCs). The present work is to address these issues by synthesizing crosslinked poly (vinyl alcohol)/layered double hydroxide (PVA/LDH) hybrid membranes with solution casting method. The experimental results indicate that incorporating 20 wt.% LDH into the PVA resulted in not only a higher ionic conductivity, but also a lower ethanol permeability. The performance test of the DEFC using the PVA/LDH hybrid membrane shows that the fuel cell can yield a power density of 82 mW cm⁻² at 80 °C, which is much higher than that of the AEM DEFC employing the quaternary ammonium group functionalized membrane. A constant current discharge test shows that the PVA/20LDH membrane can be operated stably at relatively high temperatures.     

The coralline cobalt oxides compound of multiple valence states deriving from flower-like layered double hydroxide for efficient hydrogen generation from hydrolysis of NaBH4

Efficient hydrogen storage, transportation and generation are key-technology for future hydrogen economy. Sodium borohydride (NaBH₄) stands out as promising hydrogen energy carrier with merits of high volumetric density and environmentally benign hydrolysis products. Flower-like layered double hydroxide α-Co(OH)₂ with intercalation of B species was synthesized via hydrothermal crystallization method using sodium tetraphenylboron as source of B and alkaline, which makes it different from the previous supporting materials. Pure or mixed cobalt oxides with different valence states containing B (CoO/B, Co₃O₄/B, Co+CoO/B, CoO+Co₃O₄/B) were subtly prepared via controlling calcination temperature, time and atmosphere for sodium borohydride hydrolysis. Coral-like CoO+Co₃O₄/B displayed superior hydrogen generation rate (6478 ml_H2·min^?1·g^?1_metal) with arrhenius activation energies of 41.14 kJ/mol for NaBH₄ hydrolysis in alkaline solutions compared to those reported pure precious metals. The out-standing catalytic performance of CoO+Co₃O₄/B may be attributed to electron transfer among cobalt oxide. DFT calculation indicates NaBH₄ hydrolysis undergoes a reaction path on CoO+Co₃O₄ surface with lower relative energies.     

CuAl layered double hydroxide as a highly efficient electrocatalyst for the electrolysis of water into hydrogen and oxygen fuels

The production of green H₂ through water electrolysis processes has become a prominent technology to deal with energy and environmental crisis worldwide. The total energy consumption of electrolysis processes can be reduced by the development of low-cost electrocatalysts. In this paper, we report first time the synthesis of a highly efficient 2D CuAl LDH electrocatalyst to produce the green H₂. The electrocatalyst was characterized with the help of various analytical instruments such as FT-IR, XRD, BET, TGA, ICP-OES, and XPS. The morphological characterization was done by SEM and TEM. The electrochemical characterization such as CV, LSV, Tafel plot, and EIS was done in acidic, basic, seawater, and alkaline seawater medium. It was found that CuAl LDH electrocatalyst exhibits a good current density of 100 mA/cm² at a potential of 1.178 V in acidic medium and 10 mA/cm² at 1.114V in seawater medium. It was investigated that the CuAl LDH behaves as a bifunctional electrocatalyst and exhibits excellent HER and OER activity in an acidic medium. The effect of temperature on the efficiency of the electrocatalyst under the above electrolyte mediums was also studied. The electrochemical data suggests that the CuAl LDH electrocatalyst can be utilized in an alkaline/PEM electrolyzer to produce the green H₂ at an industrial scale with optimum cost.     

K (Na)-promoted Ni,Al layered double hydroxide catalysts for the steam reforming of methanol

Production of hydrogen by methanol steam reforming has been studied over a series of Ni/Al layered double hydroxide catalysts prepared by the co-precipitation method, with the aim to develop a stable catalyst that can be used in a membrane-joint performer at temperatures greater than 300 °C. H₂, CO and CO₂ are generally the major products together with trace amounts of CH₄. The presence of potassium and/or sodium cations was found to improve the activity of methanol conversion. The selectivity for CO₂ rather than CO was better with K ions than Na ions, especially at higher temperatures (e.g. 390–400 °C). Methanol steam reforming over a K-promoted Ni/Al layered double hydroxide catalyst resulted in better activity and similar stability compared to a commercial Cu catalyst.     

Facile synthesis of AuPd nanowires anchored on the hybrid of layered double hydroxide and carbon black for enhancing catalytic performance towards ethanol electro-oxidation

Improving the catalytic performance of ethanol electro-oxidation reaction (EOR) is crucial for accelerating the commercialization of direct ethanol fuel cells (DEFCs). In this work, AuPd nanowires (NWs) anchored on the hybrid of layered double hydroxides and carbon black (CB) was prepared by a facile procedure for anode electrocatalyst of ethanol electro-oxidation reaction (EOR). For comparison, unloaded AuPd NWs and AuPd nanopaticles (NPs) supported on LDH-CB were also prepared. These catalysts were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The performance of these catalysts was evaluated by the reactions of ethanol in alkaline media by cyclic voltammetry, chronoamperometric measurements and electrochemical impedance spectroscopy. It was found that the as-prepared AuPd NWs/LDH-CB demonstrated higher activity and stability than those of unloaded AuPd NWs, commercial Pd/C and AuPd NPs/LDH-CB attributed to the AuPd nanowires with more active sites, electronic structure of Pd modified by alloyed with Au and interacted with LDH-CB as well as the electron transfer ability facilitated by CB.     

Biogas dry reforming for hydrogen production over Ni-M-Al catalysts (M = Mg,Li, Ca,La, Cu,Co, Zn)

Biogas can be highlighted as a renewable raw material for the production of hydrogen. In this study, Ni-M-Al catalysts were evaluated to obtain hydrogen from the biogas reforming. The catalysts were synthesized by coprecipitation with Ni and Al with a molar percentage of 55 and 33%, respectively, varying the third component M = Mg, Li, Ca, La, Cu, Co, Zn, with a molar percentage of 11%. The reactions were carried out in a fixed bed tubular reactor using a synthetic biogas (70% of CH₄ and 30% of CO₂). The results showed that the CH₄ conversion increased with the temperature up to 700 °C for La11, Cu11, and Zn11 catalysts. CO₂ conversion increased for all catalysts in the range of 500–700 °C. The H₂/CO molar ratios observed in the reactions were higher than 1 due to the contribution of the CH₄ decomposition reaction. The catalyst containing La presented better stability in the reactions due to the stronger acid sites and high resistance to sintering. Carbon filaments were produced by all catalysts at 600 and 700 °C. Sintering was the main cause of deactivation of the catalysts, except for La11.     

Hollow bimetallic M-Fe-P (M=Mn,Co, Cu) nanoparticles as efficient electrocatalysts for hydrogen evolution reaction

Searching for low-cost electrocatalysts with high activity towards the hydrogen evolution reaction (HER) is of great significance to enable large-scale hydrogen production via water electrolysis. In this study, by using inverse spinel MFe₂O₄(M = Mn, Fe, Co, Cu) nanoparticles (NPs) as the precursors, monodisperse bimetallic phosphide M-Fe-P NPs/C with hollow structures were readily obtained by a gas-solid annealing method. These hollow phosphide NPs displayed excellent HER activity in an acidic medium with a low loading amount of 0.2 mg cm⁻². In particular, the Co–Fe–P NPs/C shows highest HER activity that only requiring an overpotential of 97 mV to retain a current density of 10 mA cm⁻². A volcano relation between activity and incorporated elements was revealed. Incorporation of cation with high electronegativity stabilized the FeP active centres, while phase segregation resulted in the loss of activity for Cu–Fe–P NPs/C.     

Facile synthesis of MWO4 (M=Co,Ni, Zn and Cu) nanoarrays for efficient urea oxidation

Urea electrolysis is an attractive strategy for hydrogen evolution, which can reduce the environmental pollution caused by urea-rich wastewater. It is important to note that catalytic performance depends on the electron cloud density configuration around different metals of the same type of material. Herein, a series of well-tuned MWO₄ (M = Co, Ni, Zn and Cu) nanoarrays was in site grown on Ni foam substrates using a single-step hydrothermal process for the first time. It is worth mentioning that as prepared NiWO₄ electrode significantly improves urea oxidation activity with an applied voltage of 1.36 V at 100 mA cm^?2, which is lower than that of CoWO₄ (1.37 V@100 mA cm^?2), ZnWO₄ (1.38 V@100 mA cm^?2) and CuWO₄ (1.42 V@100 mA cm^?2). Experimental and theoretical calculations demonstrate that the superior activity of the electrodes is mainly attributed to the optimal urea adsorption energy, fast electron transfer rate, more active site exposure, lower impedance and better conductivity of the material.     

N-doped M/CoO (M=Ni,Co, and Mn) hybrid grown on nickel foam as efficient electrocatalyst for the chemical-assisted water electrolysis

In this work, N-Ni₁Co₃Mn_0.4O/NF is synthesized as multifunctional electrocatalyst for hydrogen evolution (HER), urea oxidation reaction (UOR) and hydrazine oxidation reaction (HzOR). The optimal Ni/Co (molar ratio) and the amount of doping Mn are investigated, the sample with Ni/Co = 1:3 and the addition of 0.4 mmol Mn exhibits the best catalytic activity with the largest specific surface area. Then the two-electrode electrolyzers composed of N-Ni₁Co₃Mn_0.4O/NF are constructed, and the results from experiments show that the voltage required for overall hydrazine splitting (OHzS) at 100 mA cm^?2 is 0.272 V, 1.614 V lower than that of overall water splitting (OWS, 1.886 V), while the overall urea splitting (OUS) needs 1.669 V, 0.187 V lower than that of OWS, revealing the outstanding thermodynamic and kinetic advantages of OUS and OHzS. The superior performance may be attributed to the heterostructure between metal and metal oxide and N-doping, which can promote electron transfer and optimizes the decomposition of urea and hydrazine hydrate and hydrogen production, and the research on mechanism will be carried out in the future.     

Advanced interpretation of hydrogen absorption process in LaMgNi3.6M0.4 (M = Ni,Mn, Al,Co, Cu) alloys using statistical physics treatment

To obtain good economic and environmental benefits, LaMgNi_3.6M_0.4 (M = Al, Mn, Ni, Co, Cu) alloys are investigated for the hydrogen storage. The absorption data of hydrogen in the tested alloys are measured experimentally at 373 K. The hydrogen absorption isotherms are analyzed using three models derived from statistical physics formalism. The adequate model permits to discover significant details about the absorption phenomenon via determining the density of the interstitial sites (D_m), the number of hydrogen atoms per site (n) and the energetic parameter ΔE. The results indicate that multi-atomic (n > 1) and multi-linking (n < 1) phenomena are feasible for hydrogen absorption in LaMgNi_3.6M_0.4 (M = Al, Mn, Ni, Cu, Co) metals. The effects of the substitutions of Ni with Mn, Co, Cu and Al on the hydrogen absorption capacity are investigated. The interaction hydrogen/metal is analyzed by the calculation of the absorption energies. The chemical interaction is the responsible for the hydrogen absorption phenomenon. The contribution of this work is to provide advanced investigations of the hydrogen absorption mechanism in LaMgNi_3.6M_0.4 (M = Al, Mn, Ni, Co, Cu) metals, which are promising alloys for the hydrogen storage.     

Integration of 2D printing technologies for AV2O6 (A=Ca,Sr, Ba)-MO (M=Cu,Ni, Zn) photocatalyst manufacturing to solar fuels production using seawater

The non-polluting nature of photocatalytic H₂ production makes of interest the study of semiconductors for this process. Scale-up of the photocatalytic hydrogen process to a pilot plant requires the photocatalyst's immobilization to enhance the charge transfer and facilitate its recovery. In this work, screen-printed films from the AV₂O₆ (A = Ca, Sr, Ba) semiconductor family were fabricated and evaluated in photocatalytic water splitting for H₂ production in distilled water and seawater under UVA light. The films exhibited ∼3.1 eV band gaps, high crystallinity, and heterogeneous morphologies. BaV₂O₆ film exhibited the highest H₂ production in distilled water (691 μmol/g), related to the synergistic effect between a higher crystallinity and traces of V⁺⁴ species that decrease the recombination of the photogenerated charges. Also, to take advantage of the dissolved species in seawater that could act as sacrificial agents, the BaV₂O₆ film was evaluated in seawater, in which H₂ production was up to 6 times higher (4374 μmol/g) than in distilled water. The BaV₂O₆ film was decorated with simple oxides (CuO, NiO, and ZnO) by the ink-jet printing technology to increase its photocatalytic performance for H₂ production. The highest efficiency with distilled water was obtained with the BaV₂O₆-CuO film, which reached an H₂ production up to 30 times higher than the bare BaV₂O₆, own to the n-p heterostructure formation that enhances the charge transport in the photocatalytic process. When the BaV₂O₆-CuO film was evaluated in seawater, a more constant H₂ production was observed; moreover, the efficiency was similar compared to the production in distilled water (20,563 μmol/g). To elucidate the seawater compounds that most influence the H₂ production, a two levels Plackett–Burman experiments design was carried out in simulated seawater. The analysis revealed that the SO₄²⁻ ions from the CaSO₄ could be decreasing the H₂ production by acting as Lewis's acid sites that trap the photogenerated e⁻ competing for its usage with the H⁺. Additionally, the Cl⁻ ions and the HCO₃⁻ reduction improved the HCOOH production from simulated seawater, reaching 26 times a higher production (23,333 μmol/g) than in distilled water.     

Synthesis of nano-crystalline Zr-M (M=Ni,Co, Fe,Cu) bilayer films and their thermodynamics of hydrogen uptake by resistance measurement

Nano-crystalline thin metal films for hydride formation for small amount of hydrogen storage is an emerging field of research for portable applications e.g. thin film fuel cells. Nano-crystalline films of Zr/ M (M = Ni, Co, Fe, Cu) bilayer systems were synthesized using ion beam sputtering technique in argon atmosphere which were characterized using GIXRD and AFM techniques. In thin film metal hydride it is difficult to measure P-C-T isotherm because of the small amount of hydrogen present and the same difficulty is to study thermodynamics of such systems. Hence in the present work change in electrical resistance with hydrogen pressure in temperatures range 298 to 573 K has been used to investigate thermodynamic properties and found that resistance of film increases with the absorption of hydrogen and decreases due to hydrogen desorption.     

A facile synthesis of metal ferrites (MFe2O4, M = Co,Ni, Zn,Cu) as effective electrocatalysts toward electrochemical hydrogen evolution reaction

Developing low-cost, stable, and robust electrocatalysts for hydrogen evolution reaction (HER) is highly desirable for large-scale application. In this study, a highly efficient electrocatalysts of metal ferrites (MFe₂O₄, M = Co, Ni, Zn, Cu) with superior activity and durability are successfully fabricated on copper substrate through a facile co-precipitation method followed by doctor-blading deposition. The electrocatalytic performance of CoFe₂O₄, NiFe₂O₄, ZnFe₂O₄ and CuFe₂O₄ electrodes for hydrogen evolution reaction is studied in alkaline media using polarization curves and electrochemical impedance spectroscopy (EIS). Among them, CoFe₂O₄ presented the best electrocatalytic activities for HER with extremely low overpotentials of 270 mV (vs. RHE) at a current density of 10 mA cm^?2 in 1 M KOH. The electrocatalytic activity of MFe₂O₄ (M = Co, Ni, Zn, Cu) for HER to generate current density of 10 mA cm^?2 with low overpotential followed the order of CoFe₂O₄ > CuFe₂O₄ > NiFe₂O₄ > ZnFe₂O₄. The highly improved HER performance of CoFe₂O₄ is mainly due to a large number of exposed active sites, high electrical conductivity and low apparent activation energy, which are confirmed by a remarkable electrochemically active surface area (ECSA = 53.17 cm²), Nyquist plots analysis and Arrhenius plots measurement, respectively. Moreover, the CoFe₂O₄ electrode showed outstanding electrocatalytic stability even after 1000 cyclic voltametry tests. These results provide a promising avenue for developing cost-effective and high-efficiency electrocatalysts based on earth-abundant transition metal ferrite as advanced electrodes for large-scale energy conversion processes.