High-performance electrocatalyst of vanadium-iron bimetal organic framework arrays on nickel foam for overall water splitting

Amorphous/crystalline nanostructured Co-FeOOH/CoCe-MOF/NF heterojunctions for efficient electrocatalytic overall water splitting

Hydrogen production by electrocatalytic water splitting is considered to be an effective and environmental method, and the design of an electrocatalyst with high efficiency, low cost, and multifunction is of great importance. Herein, we developed a amorphous Co-FeOOH/crystalline CoCe-MOF heterostructure (defined as Co-FeOOH/CoCe-MOF/NF) though a convenient cathodic electrodeposition strategy as a high-efficiency bifunctional electrocatalyst for water electrolysis. The Co-FeOOH/CoCe-MOF/NF nanocrystals provide remarkable electronic conductivity and plenty of active sites, and the crystalline/amorphous heterostructure with generates synergistic effects, providing plentiful active sites and efficient charge/mass transfer. Benefiting from this, the designed Co-FeOOH/CoCe-MOF/NF displays ultralow overpotentials of 226 and 74 mV to achieve 10 mA cm-2 for oxygen evolution reaction and hydrogen evolution reaction, and also shows the superior performance for overall water splitting with a low voltage of 1.55 V at 10 mA cm-2 in 1 M KOH. The work reveals a design of superior activity, cost-effective and multifunctional electrocatalysts for water splitting.

Tuning the local electronic structure of Co15V-ZIF through bimetallic synergies as a bifunctional electrocatalyst for overall water splitting

Co-based bimetallic zeolite imidazolate frameworks (ZIFs) have been shown as promising electrocatalysts for the oxygen evolution reaction, but their electronic structure's influence on the catalytic performance for overall water splitting still needs further investigation. In this study, Co15V-ZIF, structured as two-dimensional (2D) nanosheet arrays, are grown on nickel foam using one-step co-precipitation strategy. Owing to the synergistic effects of vanadium (V) and cobalt (Co) reasonably regulating the electronic structure, the synthesized bimetallic ZIFs demonstrate superior catalytic performance, which required the overpotentials of only 227 and 68 mV to achieve a current density of 10 mA cm-2 in 1 M KOH for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Furthermore, the water electrolyzer assembled with bimetallic ZIF as cathode and anode exhibits the capability to achieve 10 mA cm-2 at a low cell voltage of 1.57 V. In situ Raman spectroscopy reveals that the introduction of V facilitates the formation of V-CoOOH, the real active site for OER, at lower applied potentials. Besides, it induces a local acidic environment on V-Co(OH)2, the real active sites, thereby enhancing the HER performance of the sample. Density Functional Theory (DFT) calculations further show that the synergistic effects of V and Co induce electron redistribution, thereby improving electrical conductivity, reducing the energy barrier for water dissociation and hydrogen adsorption, which promotes the formation of H3O+ and triggering H3O+-induced water reduction in alkaline media. This work provides new insight into tailoring electronic structures to rationally design highly efficient ZIF electrocatalysts.

MOF-Derived Co(Fe)OOH Slab and Co/MoN Nanosheet-Covered Hollow-Slab for Efficient Overall Water Splitting

The development of economical, efficient, and stable nonprecious metal electrocatalysts presents a crucial approach to achieving alkaline overall water splitting and generating renewable hydrogen. This work presents a simple method for the synthesis of transition metal oxyhydroxides and nitrides derived from the MOF template with different morphological structures for efficient overall water splitting. Co(Fe)OOH slab array is obtained by the electro-activation of Fe-doped Co-MOF precursor, which is usually regarded as the real active substance in the alkaline OER process. Doping the Co(Fe)OOH with Fe alters the local electronic structure of the Co sites, leading to a notable improvement in OER performance, which shows an overpotential of 209 mV at 10 mA cm-2 and demonstrates excellent stability. On the other hand, Co/MoN nanosheet-covered hollow-slab heterostructure is prepared by the nitrogenization of Na2MoO4-etched Co-MOF template, which displays superior HER performance due to its abundant electrochemical active sites and rapid interfacial electron transfer, achieving an overpotential of 37 mV at 10 mA cm-2 while sustaining good stability. Notably, utilizing Co(Fe)OOH as the anode and Co/MoN as the cathode in the alkaline electrolyzer results in a cell voltage of 1.49 V at 10 mA cm-2, while demonstrating a remarkable long-term stability of 100 h. This work provides a facile way to construct an efficient alkaline electrolyzer for overall water splitting by controlling the structures of MOF derivatives.

Facile synthesis of CeO2-decorated W@Co-MOF heterostructures as a highly active and durable electrocatalyst for overall water splitting

Rational coupling of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts is extremely important for practical overall water splitting; however, it is still challenging to construct such bifunctional heterostructures. Herein, a CeO2/W@Co-MOF/NF bifunctional electrocatalyst was prepared via a two-step in situ growth method involving an electrodeposition process. The incorporation of the W element enhanced the electronic interaction and enlarged the electrochemical surface area. After the electrodeposition of CeO2, the obtained CeO2/W@Co-MOF/NF possessed abundant heterointerfaces with a modulated local distribution, which promoted water dissociation and rapid electrocatalytic kinetics. In particular, it required very low overpotentials of 239 mV and 87 mV to reach a current density of 10 mA cm-2 in OER and HER, respectively. A corresponding alkaline electrolysis cell afforded a cell voltage of 1.54 V at 10 mA cm-2 to boost overall water splitting. This work provides a feasible strategy to fabricate MOF-based complexes and explores their possible use as bifunctional catalysts toward overall water splitting.

Metal-Organic-Framework-Based Nanoarrays for Oxygen Evolution Electrocatalysis

The development of highly active and stable electrode materials for the oxygen evolution reaction (OER) is essential for the widespread application of electrochemical energy conversion systems. In recent years, various metal-organic frameworks (MOFs) with self-supporting array structures have been extensively studied because of their high porosity, abundant metal sites, and flexible and adjustable structures. This review provides an overview of the recent progress in the design, preparation, and applications of MOF-based nanoarrays for the OER, beginning with the introduction of the architectural advantages of the nanoarrays and the characteristics of MOFs. Subsequently, the design principles of robust and efficient MOF-based nanoarrays as OER electrodes are highlighted. Furthermore, detailed discussions focus on the composition, structure, and performance of pristine MOF nanoarrays (MOFNAs) and MOF-based composite nanoarrays. On the one hand, the effects of the two components of MOFs and several modification methods are discussed in detail for MOFNAs. On the other hand, the review emphasizes the use of MOF-based composite nanoarrays composed of MOFs and other nanomaterials, such as oxides, hydroxides, oxyhydroxides, chalcogenides, MOFs, and metal nanoparticles, to guide the rational design of efficient OER electrodes. Finally, perspectives on current challenges, opportunities, and future directions in this research field are provided.

Immersion-Driven Structural Evolution of NiFeS Nanosheets for Efficient Water Splitting

The development of low-cost, highly active, and stable electrocatalytic water-splitting catalysts is crucial to solving the current energy crisis and environmental pollution. Herein, a simple two-step conversion strategy is proposed to successfully prepare NiFeS nanosheet structure catalyst through the "immersion-sulfurization" strategy. The self-supported electrode can be prepared in large quantities due to its simple preparation process. As an active substance, NiFeS can grow directly on the NiFe foam substrate, avoiding the use of adhesives or conductive agents, and directly used as electrodes. The as-obtained NiFeS/NFF-300 displays efficient catalytic activity in electrocatalytic water splitting. The overpotential required for OER of the NiFeS/NFF-300 electrode at a current density of 10 mA cm-2 is 230 mV. The electrode underwent a stability test at 10 mA cm-2 for 24 h, and the overpotential remained essentially unchanged, demonstrating excellent stability. Moreover, NiFeS/NFF-300 exhibits considerable HER performances compared with NiFeC2O4/NFF and NiFe foam. The unique nanosheet structure and the presence of Niδ+ and Ni2+ formed by NiFe foam substrate on the NiFeS surface are responsible for its excellent electrocatalytic activity.

Recent Progress of Transition Metal Compounds as Electrocatalysts for Electrocatalytic Water Splitting

Hydrogen has enormous commercial potential as a secondary energy source because of its high calorific value, clean combustion byproducts, and multiple production methods. Electrocatalytic water splitting is a viable alternative to the conventional methane steam reforming technique, as it operates under mild conditions, produces high-quality hydrogen, and has a sustainable production process that requires less energy. Electrocatalysts composed of precious metals like Pt, Au, Ru, and Ag are commonly used in the investigation of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Nevertheless, their limited availability and expensive cost restrict practical use. In contrast, electrocatalysts that do not contain precious metals are readily available, cost-effective, environmentally friendly, and possess electrocatalytic performance equal to that of noble metals. However, considerable research effort must be devoted to create cost-effective and high-performing catalysts. This article provides a comprehensive examination of the reaction mechanism involved in electrocatalytic water splitting in both acidic and basic environments. Additionally, recent breakthroughs in catalysts for both the hydrogen evolution and oxygen evolution reactions are also discussed. The structure-activity relationship of the catalyst was deep-going discussed, together with the prospects of current obstacles and potential for electrocatalytic water splitting, aiming at provide valuable perspectives for the advancement of economical and efficient electrocatalysts on an industrial scale.

Performance Evaluation of Phenol-Resin-Based Adsorbents for Heat Transformation Applications

Phenol resins (PRs) are considered as relatively inexpensive adsorbents synthesized from agricultural biomass via employing a variety of synthesized procedures. The performance of PR for heat transformation application is not widely investigated. In this regard, the present study aims to evaluate the four PR derivative/refrigerant pairs, namely (i) KOH6-PR/CO2, (ii) SAC-2/HFC, (iii) KOH4-PR/ethanol, and (iv) KOH6-PR/ethanol, for adsorption cooling and adsorption heating applications. Ideal cycle analyses and/or thermodynamic modelling approaches were utilized comprising governing heat and mass balance equations and adsorption equilibrium models. The performance of the AHP system is explored by means of specific cooling energy (SCE), specific heating energy (SHE), and coefficient of performance (COP), both for cooling and heating applications, respectively. It has been realized that KOH6-PR/ethanol could produce a maximum SCE of 1080 kJ/kg/cycle and SHE of 2141 kJ/kg/cycle at a regeneration temperature (Treg) and condenser temperature (Tcond) of 80 °C, and 10 °C, respectively, followed by KOH4-PR/ethanol, SAC-2/HFC-32, and KOH6-PR/CO2. The maximum COP values were estimated to be 1.78 for heating and 0.80 for cooling applications, respectively, at Treg = 80 °C and Tcond = 10 °C. In addition, the study reveals that, corresponding to increase/decrease in condenser/evaporator pressure, both SCE and SHE decrease/increase, respectively; however, this varies in magnitude due to adsorption equilibrium of the studied PR derivative/refrigerant pairs.

Formation and Stabilization of NiOOH by Introducing α-FeOOH in LDH: Composite Electrocatalyst for Oxygen Evolution and Urea Oxidation Reactions

NiOOH is considered as the most active intermediate during electrochemical oxidation reaction, however, it is hard to directly synthesize due to high oxidation energy. Herein, theoretical calculations predict that α-FeOOH enables a decline in formation energy and an improvement in stabilization of NiOOH in NiFe-based layered double hydroxide (LDH). Inspiringly, a composite composed of α-FeOOH and LDH is well-designed and successfully fabricated in hydrothermal treatment by adding extra Fe³⁺ resource, and stable NiOOH is obtained by the following electro-oxidation method. Benefiting from strong electron-capturing capability of α-FeOOH, it efficiently promotes charge redistribution around the Ni/Fe sites and activates Ni atoms of LDH, verified by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The d-band center is optimized that balances the absorption and desorption energy, and thus Gibbs free energy barrier is lowered dramatically toward oxygen evolution reaction (OER) and urea oxidation reaction (UOR), and finally showing an outstanding overpotential of 195 mV and a potential of 1.35 V at 10 mA cm^-2 , respectively. This study provides a novel strategy to construct highly efficient catalysts via the introduction of a new phase for complex multiple-electron reactions.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Electrostatic potential-derived charge: a universal OER performance descriptor for MOFs

Metal-organic frameworks (MOFs) provide opportunities for the design of high-efficiency catalysts attributed to their high compositional and structural tunability. Meanwhile, the huge number of MOFs poses a great challenge to experimental-intensive development of high-performance functional applications. By taking the computationally feasible and structurally representative trigonal prismatic secondary building units (SBUs) of MOFs as the entry point, we introduce a descriptor-based approach for designing high-performance MOFs for the oxygen evolution reaction (OER). The electrostatic potential-derived charge (ESPC) is identified as a robust and universal OER performance descriptor of MOFs, showing a distinct linear relationship with the onset potentials of OER elemental steps. Importantly, we establish an ESPC-based physical pattern of active site-intermediate binding strength, which interprets the rationality of ESPC as an OER performance descriptor. We further reveal that the SBUs with Ni/Cu as active site atoms while Mn/Fe/Co/Ni as spectator atoms have excellent OER activity through the variation pattern of ESPC along with metal composition. The universal correlation between ESPC and OER activity provides a rational rule for designing high-performance MOF-based OER electrocatalysts and can be easily extended to design functional MOFs for a rich variety of catalytic applications.

Vanadium-Doped FeBP Microsphere Croissant for Significantly Enhanced Bi-Functional HER and OER Electrocatalyst

Ultra-fine hydrogen produced by electrochemical water splitting without carbon emission is a high-density energy carrier, which could gradually substitute the usage of traditional fossil fuels. The development of high-performance electrocatalysts at affordable costs is one of the major research priorities in order to achieve the large-scale implementation of a green hydrogen supply chain. In this work, the development of a vanadium-doped FeBP (V-FeBP) microsphere croissant (MSC) electrocatalyst is demonstrated to exhibit efficient bi-functional water splitting for the first time. The FeBP MSC electrode is synthesized by a hydrothermal approach along with the systematic control of growth parameters such as precursor concentration, reaction duration, reaction temperature and post-annealing, etc. Then, the heteroatom doping of vanadium is performed on the best FeBP MSC by a simple soaking approach. The best optimized V-FeBP MSC demonstrates the low HER and OER overpotentials of 52 and 180 mV at 50 mA/cm2 in 1 M KOH in a three-electrode system. In addition, the two-electrode system, i.e., V-FeBP || V-FeBP, demonstrates a comparable water-splitting performance to the benchmark electrodes of Pt/C || RuO2 in 1 M KOH. Similarly, exceptional performance is also observed in natural sea water. The 3D MSC flower-like structure provides a very high surface area that favors rapid mass/electron-transport pathways, which improves the electrocatalytic activity. Further, the V-FeBP electrode is examined in different pH solutions and in terms of its stability under industrial operational conditions at 60 °C in 6 M KOH, and it shows excellent stability.

Insights into the Effect of Sulfur Incorporation into Tungsten Diphosphide for Improved Hydrogen Evolution Reaction

Exploring the highly active and stable nonprecious metal electrocatalysts is particularly important for the advancement of water electrolysis, whereas it remains a challenge to efficiently improve the intrinsic electrocatalytic activity. Herein, we reasonably constructed a self-supporting nanosheet array material with sulfur incorporated into WP₂. Because of the tunability of electronic configuration and the formation of partial metal phase sulfides, the optimized catalyst exhibits a low overpotential of 115 mV at 10 mA cm^-2, along with superb durability over 24 h in acidic media. Furthermore, theoretical calculations reveal that sulfur substitution effectively manipulates the local electronic configuration of WP₂, which reduces the interaction between the catalyst surface and hydrogen atoms, thus improving the intrinsic activity of the hydrogen evolution reaction. This work provides valuable insight into the rational fabrication of highly efficient flexible electrode materials based on resourceful electrocatalysts for electrochemical water splitting.

Metal-organic framework assisted vanadium oxide nanorods as efficient electrode materials for water oxidation

The water oxidation process, which comprises the oxygen evolution reaction (OER), is a critical catalytic mechanism for sustainable technologies like water electrolysis and fuel cells. Herein, we develop a unique metal-organic framework aided vanadium pentoxide nanorods (MOF-V₂O₅ NRs-500) that can be used as an OER electrocatalyst under alkaline solutions. The crystal structure, surface chemical state, and porosity of MOF-V₂O₅ NRs-500 can be altered by annealing in an oxygen atmosphere. The resultant MOF-V₂O₅ NRs-500 demonstrate high catalytic activity against OER in basic conditions, with a low overpotential of 300 mV at a derived current density of 50 mA cm^-2, and extraordinary durability of more than 50 h. Superior electrochemical performance might be attributed to the high exposure level of active sites emanating from porous MOF-V₂O₅ NRs-500. Furthermore, the porous MOF-V₂O₅ NRs-500 skeleton may provide homogenous mass transport channels as well as quick electron transfer.

In-situ fabrication of NixSey/MoSe2 hollow rod array for enhanced catalysts for efficient hydrogen evolution reaction

Alkaline water electrocatalysis is considered as one of the most reliable method to prepare the stable, inexpensive, and sustainable water splitting catalyst in large-scale. Recently, MoSe₂ attracted great attention as a promising catalyst because of its high electrochemical activity and earth-abundant nature. In this paper, bionic Ni_xSe_y/MoSe₂ coralline-liked heterogeneous structures were successfully prepared on 3D nickel foam (NF) via a simple solvothermal process complemented by hydrothermal strategy with selenization and alkali treatment. Furthermore, to overcome the less active sites and poor electrical conductivity of MoSe₂, we learned from the coral structure for the inspiration, and reported a novel hollow rod-like structure with increased active sites. Also, 1 T-2H MoSe₂ improved the electrical conductivity of single phase MoSe_2. We first confirmed the multi-phase of catalyst by XPS analysis with Mo 3d_5/2 splited into two independent regions with the 2H and 1 T phase transition. The optimal ratio of Ni_xSe_y/MoSe₂/NF-5 exhibited excellent electrocatalytic activity towards HER in 1 M KOH, driving current densities of 10, 100 and 200 mA cm^-2 at only 76, 165 and 194 mV with stability over 24 h. The work provides new ideas for the construction of transition metal selenides hollow rod array structures of efficient HER electrocatalysts.

A Fe-doped Co-oxide Electrocatalyst Synthesized Through Post-Modification Method Toward Advanced Water Oxidation

In the context of the ever-increasing energy crisis, electrocatalytic water splitting has attracted widespread attention as an effective means to provide clean energy. However, the oxygen evolution reaction (OER), which...