Utilizing the Space-Charge Region of the FeNi-LDH/CoP p-n Junction to Promote Performance in Oxygen Evolution Electrocatalysis.

The modulation of electron density is an effective option for efficient alternative electrocatalysts. Here, p‐n junctions are constructed in 3D free‐standing FeNi‐LDH/CoP/carbon cloth (CC) electrode (LDH=layered double hydroxide). The positively charged FeNi‐LDH in the space‐charge region can significantly boost oxygen evolution reaction. Therefore, the j at 1.485 V (vs. RHE) of FeNi‐LDH/CoP/CC achieves ca. 10‐fold and ca. 100‐fold increases compared to those of FeNi‐LDH/CC and CoP/CC, respectively. Density functional theory calculation reveals OH− has a stronger trend to adsorb on the surface of FeNi‐LDH side in the p‐n junction compared to individual FeNi‐LDH further verifying the synergistic effect in the p‐n junction. Additionally, it represents excellent activity toward water splitting. The utilization of heterojunctions would open up an entirely new possibility to purposefully regulate the electronic structure of active sites and promote their catalytic activities.

Water splitting technology is an efficient approach to produce hydrogen (H2) as an energy carrier, which can address the problems of environmental deterioration and energy shortage well, as well as establishment of a clean and sustainable hydrogen economy powered by renewable energy sources due to the green reaction of H2 with O2. The efficiency of H2 production by water splitting technology is intimately related with the reactions on the electrode. Nowadays, the efficient electrocatalysts in water splitting reactions are the precious metal-based materials, i.e., Pt/C, RuO2, and IrO2. Ni (Co, Fe)-based layered double hydroxides (LDH) two-dimensional (2D) materials are the typical non-precious metal-based materials in water splitting with their advantages including low cost, excellent electrocatalytic performance, and simple preparation methods. They exhibit great potential for the substitution of precious metal-based materials. This review summarizes the recent progress of Ni (Co, Fe)-based LDH 2D materials for water splitting, and mainly focuses on discussing and analyzing the different strategies for modifying LDH materials towards high electrocatalytic performance. We also discuss recent achievements, including their electronic structure, electrocatalytic performance, catalytic center, preparation process, and catalytic mechanism. Furthermore, the characterization progress in revealing the electronic structure and catalytic mechanism of LDH is highlighted in this review. Finally, we put forward some future perspectives relating to design and explore advanced LDH catalysts in water splitting.

<p indent="0mm">The complex kinetics and high energy barrier of oxygen evolution reaction (OER) seriously restrict the efficiency of hydrogen production in overall water splitting. Layered double hydroxides (LDHs) have reasonable electrocatalytic OER ability, but the resulting performance is unsatisfactory; moreover, their rapid, large-scale, and <italic>in situ</italic> synthesis remains a great challenge. Compared to the traditional hydrothermal, electrodeposition and co-precipitation methods, microwave synthesis methods have the advantages of cleanliness, high efficiency, and low energy consumption. Importantly, microwave methods can generate many defects to improve the electrocatalytic performance. The microwave-assisted synthesis of LDHs has been reported, but it was limited to powdered nano/micromaterials. The use of microwave methods to grow nano/microarrays <italic>in situ</italic> on a substrate surface has not been reported. Compared with powders, arrays have multiple advantages, such as favorable electrolyte diffusion, proton/electron transport, and direct use for electrochemical testing without the need for additional binders, so there is an urgent need to develop strategies for the <italic>in-situ</italic> synthesis of nano/microarrays using microwave methods. Based on these studies, this paper reports a simple and effective microwave synthesis strategy for growing layered double hydroxides arrays <italic>in situ</italic> on the surface of a carbon cloth (CC), termed as LDH/CC to improve the OER performance. The type of solvent plays a crucial role in the <italic>in situ</italic> growth of LDHs. Compared to pure water, ethylene glycol, and pentaerythritol aqueous solvents, glycerol has a suitable polarity that can effectively regulate the surface energy of metal ions and the nucleation/migration of metal ions during the synthesis process, promoting the <italic>in situ</italic> and uniform growth of LDHs on the CC surface. When applied to the electrocatalytic OER, the NiFe LDH/CC catalyst synthesized using glycerol as a solvent exhibited excellent electrocatalytic performance, with a low overpotential of <sc>257 mV</sc> (<italic>η</italic>₁₀₀), <sc>279 mV</sc> (<italic>η</italic>₂₀₀), and <sc>331 mV</sc> (<italic>η</italic>₄₀₀) at 100, 200, and <sc>400 mA cm⁻²,</sc> respectively. These values were superior to those for the catalyst synthesized with ethylene glycol (287, 318, and <sc>386 mV</sc> at 100, 200, and <sc>400 mA cm⁻²,</sc> respectively) and pentaerythritol (309, 344, and <sc>406 mV</sc> at 100, 200, and <sc>400 mA cm⁻²,</sc> respectively) as a solvent. Moreover, these performances were superior to those of many transition metal hydroxides reported in recent years. Systematic spectroscopic and electrochemical tests have revealed that compared to traditional hydrothermal and electrodeposition methods, the NiFe LDH/CC synthesized using the microwave method shows superior characteristics. The NiFe LDH/CC prepared via microwave synthesis exhibited a uniform distribution, a high oxygen defect content, abundant electrochemical active sites, low interfacial charge transfer resistance, and high intrinsic activity. These features contribute to its outstanding performance in electrocatalytic OER applications. The NiFe LDH/CC catalyst synthesized using the microwave method displayed significantly better electrocatalytic OER performance with overpotentials of only 257 and <sc>331 mV</sc> at 100 and <sc>400 mA cm⁻²,</sc> respectively. These values surpassed the performance of catalysts synthesized using hydrothermal (279 and <sc>434 mV</sc> at 100 and <sc>400 mA cm⁻²,</sc> respectively) and electrodeposition methods (293 and <sc>380 mV</sc> at 100 and <sc>400 mA cm⁻²,</sc> respectively). The NiFe LDH/CC catalyst also exhibited long-term electrocatalytic stability, retaining approximately 95% of its electrocatalytic OER performance after continuous operation for <sc>100 h.</sc> The findings highlight the potential of microwave synthesis strategies for enhancing the efficiency of electrocatalytic OER processes. This microwave synthesis strategy is not only suitable for NiFe LDH, but also for <italic>in-situ</italic> synthesis of NiMn LDH, CoFe LDH, and NiCo LDH, which provides a new avenue for rapid and <italic>in-situ</italic> synthesis of highly-active LDHs.

Efficient noble-metal free electrocatalyst for oxygen evolution reaction (OER) is critical for large-scale hydrogen production via water splitting. Inspired by Nature's oxygen evolution cluster in photosystem II and the highly efficient artificial OER catalyst of NiFe layered double hydroxide (LDH), we designed an electrostatic 2D-2D assembly route and successfully synthesized a 2D LDH(+)-Birnessite(-) hybrid. The as-constructed LDH(+)-Birnessite(-) hybrid catalyst showed advanced catalytic activity and excellent stability towards OER under a close to industrial hydrogen production condition (85 °C and 6 M KOH) for more than 20 h at the current densities larger than 100 mA cm-2 . Experimentally, we found that besides the enlarged interlayer distance, the flexible interlayer NiFe LDH(+) also modulates the electronic structure of layered MnO2 , and creates an electric field between NiFe LDH(+) and Birnessite(-), wherein OER occurs with a greatly decreased overpotential. DFT calculations confirmed the interlayer LDH modulations of the OER process, attributable to the distinct electronic distributions and environments. Upshifting the Fe-3d orbitals in LDH promotes electron transfer from the layered MnO2 to LDH, significantly boosting up the OER performance. This work opens a new way to fabricate highly efficient OER catalyst for industrial water oxidation.

La/Ce doped CoFe layered double hydroxides (LDH) highly enhanced oxygen evolution performance of water splitting

Introducing oxygen vacancies to tune cobalt oxide by Mn doped for high-effective overall water-splitting

Electrochemically formed PtFeNi alloy nanoparticles on defective NiFe LDHs with charge transfer for efficient water splitting

Water splitting is an area of research which is of pressing interest in addressing climate change. Water splitting provides oxygen and hydrogen as products, with hydrogen finding use as a potential high-density fuel to replace fossil fuels. Water splitting manifests as two half-cell reactions, namely the anodic oxygen evolution reaction and the cathodic hydrogen evolution reaction. The purpose of this project is to synthesise a bifunctional electrocatalyst to use in the water splitting reaction, replacing the rare and expensive transition metals catalysts currently in use such as platinum and ruthenium. The ideal material would exhibit strong performance for both the oxygen evolution reaction and the hydrogen evolution reaction.In this project, Layered Double Hydroxides (LDHs) have been synthesised for use in water splitting. LDHs are a group of ionic compounds characterised by their layered structure of positively charged sheets and an interlayer region consisting of the corresponding anions. LDHs typically contain a bivalent and a trivalent metal as part of the generic formula [M2+ 1–xM3+ x(OH)2][An–]x/n·zH2O. In this project the bivalent metal in use is typically cobalt and the trivalent metal is typically iron. The synthesis developed is a simple, one pot hydrothermal synthesis which can be seen in Figure 1. In this synthesis, the process begins with the precursor materials dissolved in solution. These solutions are mixed and stirred until homogenous. The resultant solution is then heated to form an impure product. Following washing steps and centrifugation a purified powder product is obtained. A variety of metallic salts have been tested for their use in these LDHs with the optimum LDHs selected for further study. The metallic salts used to provide the metals for the LDHs are typically hydrated transition metal nitrates.The LDHs have been characterised and their structure confirmed by a variety of analytical techniques including X-Ray Diffraction and Scanning Electron Microscopy. Simple qualitative analysis has been performed with FTIR spectroscopy. These LDHs have been immobilised on a variety of porous, high surface area materials, including nickel foam and copper foam, to enhance the properties seen in the screening study. These materials allow synthesis of high surface area, reactive electrocatalysts. To enhance the stability of the LDHs, carbon materials have been integrated with the LDHs. Examination of a variety of carbon materials involved post-synthesis mixing of the carbon material and the LDH product or inclusion of the carbon material in the LDH synthesis to form a composite. A variety of carbon materials have been analysed including graphene oxide, carbon black and carbon nanotubes. Figure 1

Water electrolysis has attracted immense research interest, nevertheless the lack of low-cost but efficient bifunctional electrocatalysts for both hydrogen and oxygen evolution reactionsgreatly hinders its commercial applications. Herein, the controllable synthesis of ultrathin defect-rich layered double hydroxide (LDH) nanoarrays assembled on metal-organic framework (MOF)-derived Co-NC microarrays for boosting overall water splitting is reported. The Co-NC microarrays can not only provide abundant nucleation sites to produce a large number of LDH nuclei for favoring the growth of ultrathin LDHs, but also help to inhibit their tendency to aggregate. Impressively, five types of ultrathin bimetallic LDH nanoarrays can be electrodeposited on the Co-NC microarrays, forming desirable nanoarray-on-macroarray architectures, which show high uniformity with thicknesses from 1.5 to 1.9nm. As expected, the electrocatalytic performance is significantly enhanced by exploiting the respective advantages of Co-NC microarrays and ultrathin LDH nanoarrays as well as the potential synergies between them. Especially, the optimal Co-NC@Ni2 Fe-LDH as both cathode and anode can afford the lowest cell voltage of 1.55V at 10mA cm-2 , making it one of the best earth-abundant bifunctional electrocatalysts for water electrolysis. This study provides new insights into the rational design of highly-active and low-cost electrocatalysts and facilitates their promising applications in the fields of energy storage and conversion.

The development of efficient and sustainable energy conversion technologies has driven extensive research into the hydrogen evolution reaction (HER) as a key component of water splitting. Hydrogen, as a clean and renewable energy carrier, has the potential to revolutionize energy storage and utilization. However, the efficiency of HER is largely dependent on the development of electrocatalysts capable of operating under a wide range of conditions with low overpotential and high current density. Conventional noble-metal catalysts such as platinum (Pt) exhibit excellent HER performance but suffer from high cost and limited availability, necessitating the exploration of cost-effective and earth-abundant alternatives.Graphene quantum dots (GQDs) have emerged as promising candidates due to their unique electronic properties, including a high surface-to-volume ratio, tunable bandgap, and excellent electrical conductivity. These characteristics enable GQDs to serve as effective co-catalysts or dopants in hybrid materials, enhancing charge transfer kinetics and catalytic activity. Layered double hydroxides (LDHs), particularly Co-NiFe LDHs, have demonstrated outstanding electrocatalytic performance for both HER and the oxygen evolution reaction (OER), making them attractive bifunctional catalysts for overall water splitting. The synergistic interaction of Co, Ni, and Fe within the LDH structure provides a favorable environment for enhanced electron transfer and active site availability, thereby improving catalytic efficiency.In this study, we employ machine learning-assisted density functional theory (ML-DFT) calculations to investigate the HER performance of GQDs integrated with Co-NiFe LDH. The ML-DFT approach enables efficient and accurate exploration of a vast parameter space, allowing us to predict the electronic structure, adsorption energies, and reaction pathways with high fidelity. By leveraging ML-DFT, we systematically evaluate the effects of GQD incorporation on the catalytic activity of Co-NiFe LDH. ML-DFT calculations reveal that the GQDs/Co-NiFe LDH exhibits greater potential for HER, with free adsorption energy calculations confirming enhanced catalytic activity upon GQD incorporation. Additionally, our findings confirm that the GQD/Co-NiFe LDH composite exhibits enhanced electron transfer capabilities and increased active site availability, leading to improved HER performance, effectively addressing prior concerns regarding catalytic efficiency. The incorporation of GQDs significantly modifies the electronic structure of Co-NiFe LDH, facilitating improved charge transfer and optimizing hydrogen adsorption characteristics.The integration of GQDs with Co-NiFe LDH is hypothesized to create a hybrid material with improved HER performance, attributed to the synergistic interaction between the GQD-induced electronic modulation and the catalytic active sites of the LDH. This study provides fundamental insights into the structure-property relationships governing HER activity in hybrid catalysts and highlights the potential of GQD-based materials for next-generation electrocatalysts. By bridging advanced computational techniques with materials design, our work paves the way for the rational development of cost-effective and high-performance catalysts for sustainable hydrogen production.

Electrochemical Synthesis of H2O2 by Two-Electron Water Oxidation Reaction

Rational introduction of borate and phosphate ions on NiCo2O4 surface for high-efficiency overall water splitting

The advent of dual-atom nanozymes (DAzymes) featuring distinctive bimetallic active sites garnered significant attention, representing enhanced iterations of conventional single-atom nanozymes. The quest for an effective and universal strategy to modulate the catalytic activity of DAzymes posed a formidable challenge, yet few published reports addressed this. Herein, we designed and synthesized S-doped Fe/Co DAzymes (S-FeCo-NC) under theoretical guidance and revealed their excellent oxidase-like activity. Experimental and theoretical calculations indicated that the superior oxidase-like activity exhibited by S-FeCo-NC was attributed to the S-doping, which modulated the local electronic structure of the dual-atom active site. This modulation of the local electronic structure significantly optimizes oxygen adsorption energy, thereby accelerating the rate of enzyme-catalyzed reactions. As a proof-of-concept, this study integrated S-FeCo-NC with the cascade inhibition reaction of acetylcholinesterase (AChE) to devise a sensitive analytical platform for detecting organophosphorus pesticides. This study paved the way for elucidating the correlation between the local electronic structure of the active site and enzyme activity, offering novel methodologies and insights for the rational design of DAzymes.

Multifunctional electrocatalyst based on MoCoFe LDH nanoarrays for the coupling of high efficiency Electro-Fenton and water splitting process

Efficient noble‐metal free electrocatalyst for oxygen evolution reaction (OER) is critical for large‐scale hydrogen production via water splitting. Inspired by Nature's oxygen evolution cluster in photosystem II and the highly efficient artificial OER catalyst of NiFe layered double hydroxide (LDH), we designed an electrostatic 2D‐2D assembly route and successfully synthesized a 2D LDH(+)‐Birnessite(−) hybrid. The as‐constructed LDH(+)‐Birnessite(−) hybrid catalyst showed advanced catalytic activity and excellent stability towards OER under a close to industrial hydrogen production condition (85 °C and 6 M KOH) for more than 20 h at the current densities larger than 100 mA cm−2. Experimentally, we found that besides the enlarged interlayer distance, the flexible interlayer NiFe LDH(+) also modulates the electronic structure of layered MnO2, and creates an electric field between NiFe LDH(+) and Birnessite(−), wherein OER occurs with a greatly decreased overpotential. DFT calculations confirmed the interlayer LDH modulations of the OER process, attributable to the distinct electronic distributions and environments. Upshifting the Fe‐3d orbitals in LDH promotes electron transfer from the layered MnO2 to LDH, significantly boosting up the OER performance. This work opens a new way to fabricate highly efficient OER catalyst for industrial water oxidation.