CeO2 nanoparticles@ NiFe-LDH nanosheet heterostructure as electrocatalysts for oxygen evolution reaction

Impact of the Metal-Organic Frameworks Polymorphism on the Electrocatalytic Properties of CeO2 toward Oxygen Evolution

Hydrogen (H2) is a viable alternative as a sustainable energy source, however, new highly efficient electrocatalysts for water splitting are still a research challenge. In this context, metal-organic frameworks (MOFs)-derived nanomaterials are prominent high-performance electrocatalysts for hydrogen production, especially in the oxygen evolution reaction (OER). Here, a new synthesis of two cerium oxide (CeO2) electrocatalysts using Ce-succinates MOFs as templates is proposed. The cerium succinates polymorphs ([Ce2(Succ)3(H2O)2], Succ = succinate ligand) were obtained via hydrothermal reaction and room temperature crystallization, adopting monoclinic (C/2c) and triclinic (P1̅) crystalline structures, respectively, confirmed by X-ray diffraction (XRD). MOFs-Ce were also characterized by infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). CeO2 electrocatalysts were obtained via MOFs-Ce calcination at 350 °C in air, and characterized by XRD with Rietveld refinement, HRTEM, SEM, FT-IR, and Raman spectroscopy, UV-vis spectroscopy, X-ray photoelectron spectroscopy. Electrocatalytic performances were investigated in KOH 1.0 M solution, and overpotentials were η = 326 mV (for CeO2 (H) from monoclinic MOF-Ce) and η = 319 mV (for CeO2 (RT) from the triclinic MOF-Ce) for a current density of 10 mAcm-2. The Tafel slope values show the adsorption of intermediate oxygenated species as the rate-determining step. The high values of double-layer capacitance, the presence of oxygen vacancies, and low charge transfer resistance agree with the high performance in OER. Additionally, the materials were stable for up to 24 h, according to chronopotentiometry results.

Nickel-Iron Layered Double Hydroxides/Nickel Sulfide Heterostructured Electrocatalysts on Surface-Modified Ti Foam for the Oxygen Evolution Reaction

Electrochemical approaches for generating hydrogen from water splitting can be more promising if the challenges in the anodic oxygen evolution reaction (OER) can be harnessed. The interface heterostructure materials offer strong electronic coupling and appropriate charge transport at the interface regions, promoting accessible active sites to prompt kinetics and optimize the adsorption-desorption of active species. Herein, we have designed an efficient multi-interface-engineered Ni3Fe1 LDH/Ni3S2/TW heterostructure on in situ generated titanate web layers from the titanium foam. The synergistic effects of the multi-interface heterostructure caused the exposure of rich interfacial electronic coupling, fast reaction kinetics, and enhanced accessible site activity and site populations. The as-prepared electrocatalyst demonstrates outstanding OER activity, demanding a low overpotential of 220 mV at a high current density of 100 mA cm-2. Similarly, the designed Ni3Fe1 LDH/Ni3S2/TW electrocatalyst exhibits a low Tafel slope of 43.2 mV dec-1 and excellent stability for 100 h of operation, suggesting rapid kinetics and good structural stability. Also, the electrocatalyst shows a low overpotential of 260 mV at 100 mA cm-2 for HER activity. Moreover, the integrated electrocatalyst exhibits an incredible OER activity in simulated seawater with an overpotential of 370 mV at 100 mA cm-2 and stability for 100 h of operation, indicating good OER selectivity. This work might benefit further fabricating effective and stable self-sustained electrocatalysts for water splitting in large-scale applications.

Synthesis of NiFe-layered double hydroxides using triethanolamine-complexed precursors as oxygen evolution reaction catalysts: effects of Fe valence

The synthesis of highly efficient NiFe-layered double hydroxides (NiFe-LDHs) to catalyze the oxygen evolution reaction (OER) is urgent and challenging. Herein, NiFe-FeCl3-x and NiFe-FeCl2-x samples (where FeCl3 and FeCl2 represent the Fe sources and x represents the imposed reaction time: 6, 12, and 24 h) were prepared via one-pot hydrothermal synthesis using Fe sources characterized by Fe(III) or Fe(II) valence states. In the presence of triethanolamine, when FeCl3 was used as the Fe source, pure NiFe-LDH was obtained, whose crystallinity increased with increasing hydrothermal treatment time. In contrast, when FeCl2 was used as the Fe source, a mixture of NiFe-LDH, Fe2O3, and trace amounts of Fe3O4 was obtained. The content of NiFe-LDH in the mixture increased under longer hydrothermal treatment and NiFe-FeCl3-x catalysts exhibited better OER performance than NiFe-FeCl2-x catalysts. Specifically, NiFe-FeCl3-6 afforded the highest OER performance with an overpotential of 246.8 mV at 10 mA cm-2 and a Tafel slope of 46.1 mV dec-1. Herein, we investigated the effects of the valence state of Fe precursors on the structures and OER activities of the prepared catalysts; the mechanism of NiFe-LDH formation via hydrothermal synthesis in the presence of triethanolamine was also proposed.

4f-2p-3d orbital overlap in a metal-organic framework-derived CeO2/CeCo-LDH heterostructure promotes water oxidation

Herein, we have demonstrated a facile method for the synthesis of CeO2/Ce-Co-LDH heterostructures using zeolitic imidazolate framework-67 as the precursor. The Ce-incorporation in Co-LDH results in 4f-2p-3d orbital overlap to tune the electronic structure whereas the oxygen-deficient CeO2 controls the interface charge transfer. This results in excellent water oxidation activity to attain 500 mA cm-2 current density at 320 overpotential.

A Robust n-n Heterojunction: CuN and BN Boosting for Ambient Electrocatalytic Nitrogen Reduction to Ammonia

An n-n type heterojunction comprising with CuN and BN dual active sites is synthesized via in situ growth of a conductive metal-organic framework (MOF) [Cu3 (HITP)2 ] (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) on hexagonal boron nitride (h-BN) nanosheets (hereafter denoted as Cu3 (HITP)2 @h-BN) for the electrocatalytic nitrogen reduction reaction (eNRR). The optimized Cu3 (HITP)2 @h-BN shows the outstanding eNRR performance with the NH3 production of 146.2 µg h-1 mgcat -1 and the Faraday efficiency of 42.5% due to high porosity, abundant oxygen vacancies, and CuN/BN dual active sites. The construction of the n-n heterojunction efficiently modulates the state density of active metal sites toward the Fermi level, facilitating the charge transfer at the interface between the catalyst and reactant intermediates. Additionally, the pathway of NH3 production catalyzed by the Cu3 (HITP)2 @h-BN heterojunction is illustrated by in situ FT-IR spectroscopy and density functional theory calculation. This work presents an alternative approach to design advanced electrocatalysts based on conductive MOFs.

Controllable formation of amorphous structure to improve the oxygen evolution reaction performance of a CoNi LDH

The morphology design of layered double hydroxides (LDHs) is an important way to determine the catalytic performance of LDH materials. A novel structure of CoNi LDH sheets with amorphous structure on the edge was prepared by electrooxidation. It was characterized by XRD, SEM, TEM and XPS. It was found that during the electrooxidation, some of the Co²⁺ ions were oxidized to Co³⁺ to form amorphous CoOOH intermediates, which promoted the OER performance. The electrochemical test results show that CoNi LDH treated by electrooxidation for 6 hours has an ultra-low overpotential of 206 mV at a current density of 10 mA cm^-2, and can work stably under alkaline conditions for more than 10 hours. This work suggests that introducing an amorphous structure on LDH through electrooxidation produces abundant active sites, which is an easy and efficient method to improve the OER performance of CoNi LDHs.

NiFeMn-Layered Double Hydroxides Linked by Graphene as High-Performance Electrocatalysts for Oxygen Evolution Reaction

Currently, precious metal group materials are known as the efficient and widely used oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. The exorbitant prices and scarcity of the precious metals have stimulated scale exploration of alternative non-precious metal catalysts with low-cost and high performance. Layered double hydroxides (LDHs) are a promising precursor to prepare cost-effective and high-performance catalysts because they possess abundant micropores and nitrogen self-doping after pyrolysis, which can accelerate the electron transfer and serve as active sites for efficient OER. Herein, we developed a new highly active NiFeMn-layered double hydroxide (NFM LDH) based electrocatalyst for OER. Through building NFM hydroxide/oxyhydroxide heterojunction and incorporation of conductive graphene, the prepared NFM LDH-based electrocatalyst delivers a low overpotential of 338 mV at current density of 10 mA cm⁻² with a small Tafel slope of 67 mV dec⁻¹, which are superior to those of commercial RuO₂ catalyst for OER. The LDH/OOH heterojunction involves strong interfacial coupling, which modulates the local electronic environment and boosts the kinetics of charge transfer. In addition, the high valence Fe³⁺ and Mn³⁺ species formed after NaOH treatment provide more active sites and promote the Ni²⁺ to higher oxidation states during the O₂ evolution. Moreover, graphene contributes a lot to the reduction of charge transfer resistance. The combining effects have greatly enhanced the catalytic ability for OER, demonstrating that the synthesized NFM LDH/OOH heterojunction with graphene linkage can be practically applied as a high-performance electrocatalyst for oxygen production via water splitting.

Dual-Metal Zeolitic Imidazolate Framework Derived Highly Ordered Hierarchical Nanoarrays on Self-Supported Carbon Fiber for Oxygen Evolution

The construction of highly ordered hierarchical nanoarrays is crucial for obtaining effective transition metal carbon nanomaterial electrocatalysts for oxygen evolution reaction (OER) in water splitting. Herein, we adopted a Co metal zeolitic imidazolate framework (Co-ZIF) as a precursor by ion-exchange/etching reaction with Fe(NO₃)₃ to obtain hierarchical N-doped Co-Fe layered double hydroxide (CoFe-LDH) in situ generated in Co-ZIF nanoarrays based on a self-supported carbon cloth (CC) substrate noted as CoFe-LDH@Co-ZIF@CC. Benefiting from the synergistic effect of these species and their highly ordered self-supported nanoarray structure, the catalytic active sites were fully exposed and highly protected in alkaline electrolyte, which significantly promoted electron transport and improved electrochemical performance. The CoFe-LDH@Co-ZIF@CC exhibited the low overpotentials of about 225 and 319 mV at 10 and 100 mA cm⁻² with a small Tafel slope of 81.8 mV dec⁻¹ recorded in a 1.0 M KOH electrolyte. In addition, it also showed a long-term durability without obvious decay after 30 h. Therefore, its remarkable OER activity demonstrates this material’s promising application in the green hydrogen energy industry.

Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions

Of all the available resources given to mankind, the sunlight is perhaps the most abundant renewable energy resource, providing more than enough energy on earth to satisfy all the needs of humanity for several hundred years. Therefore, it is transient and sporadic that poses issues with how the energy can be harvested and processed when the sun does not shine. Scientists assume that electro/photoelectrochemical devices used for water splitting into hydrogen and oxygen may have one solution to solve this hindrance. Water electrolysis-generated hydrogen is an optimal energy carrier to store these forms of energy on scalable levels because the energy density is high, and no air pollution or toxic gas is released into the environment after combustion. However, in order to adopt these devices for readily use, they have to be low-cost for manufacturing and operation. It is thus crucial to develop electrocatalysts for water splitting based on low-cost and land-rich elements. In this review, I will summarize current advances in the synthesis of low-cost earth-abundant electrocatalysts for overall water splitting, with a particular focus on how to be linked with photoelectrocatalytic water splitting devices. The major obstacles that persist in designing these devices. The potential future developments in the production of efficient electrocatalysts for water electrolysis are also described.