Purified mesoporous nickel metal organic framework as electrocatalyst for hydrogen evolution reaction (HER) in basic medium

Metal organic framework or MOFs are found to be a good catalyst for hydrogen evolution in view of its excellent structural features like porous nature and well-defined morphology. This report describes the synthesis of Nickel-MOF prepared by solvothermal approach and further purified by a chemical process. Both impure and purified Nickel-MOFs are fabricated as electrodes for evaluating HER reaction. The performance towards electrocatalytic activity is affected due to the impurities present in the porous structure. In view of investigating the effect of activation/purification process towards morphology and in-turn HER activity, activation and purification is carried out to enhance the performance of MOFs. The electrochemical characterization proves high electrocatalytic activity for purified Ni-MOF with high rate kinetics towards HER than impure Ni-MOF. The Tafel slope of purified Nickel MOF is estimated to be 73.7 mV/dec with a low charge transfer resistance of 1.84 Ω, whereas the unpurified Ni-MOF shows 87.47 mV/dec and 3.85 Ω. Results show that pure Ni-MOF has abundant catalytic active edge sites and obeys Volmer-Heyrvosky mechanism with makes desorption of hydrogen as rate determining step.     

Recent advances in cobalt disulfide for electrochemical hydrogen evolution reaction

Hydrogen production by water electrolysis is the most promising green hydrogen supply method in the future. Electrocatalytic hydrogen evolution reaction (HER), an essential step in water electrolysis, has received continuous interest for a long time. Noble metal-based electrocatalysts exhibit excellent performance for HER, while their high price, limited reserves, and insufficient durability limit their large-scale applications. Transition metal sulfides (TMSs) have been extensively studied as potential alternative catalysts, among which cobalt disulfide (CoS₂) stands out due to its unique structure, low price, and good electrical conductivity. Although remarkable progress has been made, the catalytic activity and stability of CoS₂ electrode materials themselves are still insufficient for large-scale industrial applications, so effective improvement of the HER catalytic performance of CoS₂ remains the focus of research. In this review, we briefly outline the reaction mechanism of HER, focusing on strategies to improve the catalytic performance of CoS₂, including morphology engineering, carbon materials combination, heteroatom doping, and heterostructure construction. Furthermore, the key challenges and opportunities for CoS₂ electrode materials as an electrocatalytic material for HER are discussed.     

Contributions of oxygen vacancies to the hydrogen evolution catalytic activity of tungsten oxides

The tungsten trioxide attracts less attention due to the low electron transfer kinetics that hinders the interaction of electrons and ions during the hydrogen evolution reaction (HER). But the oxygen vacancy strategy can inspire its electrocatalytic activity for HER because it has a positive effect on improving the charge transfer and compensating for the weak hydrogen adsorption of the tungsten trioxide. By synthesizing a series of substoichiometric tungsten oxides, we reveal the linear relationship between the catalytic activity and the content of oxygen vacancies, which indicates that the oxygen vacancy strategy is an achievable route to enhance the HER for metal oxides.     

Pt/Fe-NF electrode with high double-layer capacitance for efficient hydrogen evolution reaction in alkaline media

The electrode with high catalytic activity, low hydrogen overpotential and low cost is desired for hydrogen evolution reaction (HER) via electrocatalytic water splitting. In this study, Pt/Fe-Ni foam (Pt/Fe-NF) electrode was synthesized via cathodic electrodeposition followed by impregnation deposition. Physical and electrochemical properties of Pt/Fe-NF, NF and Pt/NF electrodes were characterized by various techniques. The Pt/Fe-NF electrode exhibited better electrochemical activity for HER under alkaline condition than those of Pt/NF and NF electrodes, owing to the introduction of zero valences Pt and Fe onto the NF, and synergetic effect resulted from the formation of Fe-Ni alloy. Furthermore, Pt/Fe-NF electrode showed extremely high double-layer capacitance (69.1 mFcm^?2), suggesting high active sites for the Pt/Fe-NF. Tafel slope of Pt/Fe-NF was 59.9 mV dec^?1, indicating that the Volmer-Heyrovsky HER mechanism was the rate-limiting step. The Pt/Fe-NF electrode with great electrocatalytic activity is a promising electro-catalyst for industrial hydrogen production from alkaline electrolyte.     

Recent advances in CoSe2 electrocatalysts for hydrogen evolution reaction

Hydrogen as a sustainable alternative fuel is recognized as a primary choice for future energy supply due to its high gravimetric energy density and zero carbon emission upon combustion. Electrochemical water splitting is a promising strategy for effective and sustainable hydrogen production. Nowadays, research is focused on developing non-precious, stable, and highly efficient electrocatalysts for hydrogen evolution reaction (HER). Among them, CoSe₂ has attracted tremendous attention as HER electrocatalyst due to its unique electronic configuration that ensures fast charge transport, excellent catalytic activity, and good chemical stability. So far, a lot of reviews on electrocatalytic water splitting based on transition metal dichalcogenides and cobalt-based materials are reported. However, the review on CoSe₂ electrocatalyst for hydrogen evolution reaction is limited up-to-date. Hence in the present review, a comprehensive literature survey on CoSe₂ electrocatalyst for hydrogen evolution reaction is done and reported. In this review, the crystal structures of CoSe₂, their phase transformation strategy, their hydrogen evolution reaction mechanism in acidic and alkaline electrolytes are highlighted. The various synthesis procedures adopted to produce CoSe₂ based materials, the relation between its structure and composition with their electrocatalytic activities are discussed. Moreover, the effective ways to enhance the electrocatalytic performance of CoSe₂ based materials such as its morphological modification, constructing heterostructures, and heteroatom doping are reviewed.     

Efficient and stable electrocatalyst for hydrogen evolution reaction prepared by hybrid technique in surface engineering: Electrochemical and magnetron sputtering methods

Developing low-cost, stable, and robust electrocatalysts is significant for high effective hydrogen evolution reaction (HER). In this work, a coating system with Cu₂O/NiMoCu on stainless steel (SS) is employed as a highly active and stable catalyst for HER in acidic solutions. Electrochemical measurements for as-designed system on SS show a low onset overpotential, small Tafel slope of ～32 mV/decade and long-term durability over 7 days of HER operation. To further inspections of electrocatalytic behavior of as-prepared system in HER, the EIS measurements are performed at several overpotentials and temperatures. It is found that high hydrogen evolution activity and stability of Cu₂O/NiMoCu hybrid is likely due to special morphology of Cu₂O which result in large number of active sites for hydrogen adsorption, and a synergetic effect giving electronic structure suitable for the HER.     

In-situ grown of Ni2P nanoparticles on 2D black phosphorus as a novel hybrid catalyst for hydrogen evolution

Nickel phosphide-based nanomaterials have been acted as efficient catalysts for the hydrogen evolution reaction (HER), however, the design of novel and high performance HER catalyst is still a challenge. Herein, we report a novel 2D material black phosphorus (BP) as support for constructing Ni₂P-based hybrid catalyst by a one-pot thermal decomposition approach. TEM results indicated that the monodispersed Ni₂P nanoparticles with small size and good dispersion supported on the surface of layered BP, which implied that more catalytic active sites may be exposed for HER. The as-synthesized Ni₂P/BP hybrid exhibits high HER electrocatalytic performance with low onset overpotential (70 mV), small Tafel slope (81 mV dec^?1), large double-layer capacitance (1.24 mF cm^?2), high conductivity and good stability, which can be assigned to the strong synergistic effect between Ni₂P and BP. Therefore, BP may be a suitable support for constructing excellent catalysts in electrocatalysis.     

Nickel selenides as pre-catalysts for electrochemical oxygen evolution reaction: A review

Nickel selenide is an important class of nickel chalcogenide that has recently gained greater attention in electrochemical water splitting. Though other chalcogenides such as sulphides and tellurides have also been shown to possess appreciable electrocatalytic water splitting activity as both cathode and anode material, the electrocatalytic activity of nickel selenides in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is unmatched by the former one. In the case of HER, nickel selenides are good electrocatalysts in both acid and alkali with some metal dissolution in acid and surface modification to nickel hydroxide in alkali. In the case of OER, nickel selenides have been shown to behave in a unique way in which it acted better than simple nickel hydroxides/oxyhydroxides. This is quite intriguing to researchers and many studies have been recently carried out. However, there is still no exclusive review summarizing the recent development of this material for OER electrocatalysis with highlights on challenges and opportunities. Hence, we have dedicated to discuss the same in this review. In addition, the significance of OER electrocatalysis has been briefly introduced. Finally, all the studies that reported the OER activity of nickel selenides are benchmarked based on the Tafel slope values. With the collective information provided in this review, readers would be updated with the recent trend in utilizing nickel selenides as OER electrocatalysts.     

Direct synthesis of nitrogen-rich carbon sheets via polybenzoxazine as highly active electrocatalyst for water splitting

In this work, we propose a novel nitrogen-rich carbon sheets (N-CSs), with conceivable use as efficient catalysts for hydrogen evolution reaction (HER). N-CSs are directly synthesized from polybenzoxazine (PBz) by carbonization followed by KOH activation. PBz was prepared from eugenol, melamine, and paraformaldehyde through ring-opening polymerization. FT-IR and NMR spectroscopy confirmed the corresponding chemical structures of the new benzoxazine monomer. The morphology, structure and surface properties of the N-CSs are investigated by Raman spectroscopy, wide-angle X-ray diffraction, and X-ray photoelectron spectroscopy. The catalytic activity of N-CSs towards HER is thoroughly investigated by electrochemical techniques. In N-CSs, it is established that nitrogen gratified electrocatalytic activity, and hence nitrogen atoms should enhance the electrocatalytic properties by increasing the active sites. As the kinetic current is stabilized by the outer nitrogen atom as such, HER is proposed to proceed on these active sites by the Volmer-Heyrovsky mechanism. The N-CSs show outstanding catalytic activity towards HER with lowest onset-potential (?10 mV_RHE) and Tafel slope (45 mV dec^?1) in 0.5 M H₂SO₄ aqueous electrolyte.     

β-Mo2C/MoO2 heterostructures for hydrogen evolution reaction: Morphology and catalytic activity regulated by heteroatom doping

Improving the activity of non-noble metallic electrocatalyst for hydrogen evolution reaction (HER) is an important issue for hydrogen energy utilization. In this work, we reported a new strategy for morphology regulation of β-Mo₂C/MoO₂ heterostructure via heteroatom doping to enhance the electrocatalytic HER activity. Electron microscopy observations found that N and S doping resulted in nanosphere and nanorod morphology, respectively. Amongst the S doping catalyst (denoted as S@β-Mo₂C/MoO₂) exhibited remarkably improved electrocatalytic HER activity and stability as compared to the pristine β-Mo₂C/MoO₂ catalyst, whereas the N doping induced significant degradation of catalytic activity and stability. The mechanism investigations reveal that the nanorod morphology of S@β-Mo₂C/MoO₂ endows it lower charge transfer resistance, higher electrochemical active surface area and lower valence state of Mo species, which contributes positively and importantly to its better electrocatalytic HER activity.     

Tuning of electrocatalytic activity of Mn–O–Co composite for alkaline hydrogen evolution reaction

We report the enhancement in electrocatalytic activity of Mn–O–Co composite electrode developed through chemical reduction method. The Mn–O–Co composite electrode exhibits high catalytic activity with a low Tafel slope of 123 mV dec⁻¹ and a low overpotential of 117 mV at a current density of 10 mA cm⁻². The enhancement in electrocatalytic activity of Mn–O–Co composite electrode is due to the synergistic activity of MnO and CoO with the NiP matrix. The intermetallic interaction among the half-filled orbitals of manganese with the fully occupied orbitals of cobalt and nickel leads to an effective electron delocalization in the catalytic system which enhances the HER performance of the coating. The C_dl value of the composite electrode is in the order of 254 μF, which is approximately ten fold higher than the bare NiP coating, due to the enhancement in interaction between the Mn–O–Co composite electrode and the reactive species in the HER medium. The Mn–O–Co composite electrode shows promising characteristics as an electrocatalyst with long term stability and remarkable competency with the commercially available electrodes.     

Enhancement of hydrogen evolution at cobalt-zinc deposited graphite electrode in alkaline solution

Thin Co layers were electrochemically deposited on a graphite electrode at different deposition current densities and thicknesses. After determining the best deposition conditions for hydrogen evolution (deposition current density and thickness), co-deposits of Co with Zn were prepared on the graphite electrode. The binary coatings prepared on the graphite electrode (CoZn) were etched in a concentrated alkaline solution (30% NaOH) to produce a porous and electrocatalytic surface suitable for use in the hydrogen evolution reaction (HER). After the leaching process, a low amount of Pt was deposited onto the etched CoZn deposit in order to further improve the catalytic activity of the electrode for the HER. The HER activity is assessed by recording cathodic current-potential curves, electrochemical impedance spectroscopy (EIS) and electrolysis techniques. Chemical composition of layers after alkaline leaching was determined by energy dispersive X-ray (EDX) analysis. The surface morphologies of coatings were investigated by scanning electron microscopy (SEM). It was found that, the HER activity of coatings depends on the metal ratio of Co and Zn, deposition current density and the thickness of coatings. The alkaline leached CoZn coating has a compact and porous structure as well as good electrocatalytic activity for the HER in alkaline media. Moreover, deposition of a low amount of Pt over the CoZn can further enhance its hydrogen evolution activity.     

Phytic acid-coated titanium as electrocatalyst of hydrogen evolution reaction in alkaline electrolyte

The phytic acid-coated titanium (IP6/Ti) electrode was prepared through a simple drop-drying process, with an aim of improving electrocatalytic activity toward the hydrogen evolution reaction (HER). Scanning electron microscope and X-ray photoelectron spectroscopy showed that the IP6 coated the substrate surface uniformly and completely. Evaluation of the electrode activity was carried out in 1.0 M NaOH by linear polarization, electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The kinetic parameters obtained from Tafel curves reveal that the IP6 coating can enhance the exchange current density of the HER by 489 times compared to the bare Ti, and reduce the HER activation energy by nearly 50%. The EIS data prove that the charge transfer resistance of the HER was considerably reduced due to the IP6 coating, with a decrease in real surface area of the electrode. The catalytic effect of IP6 is due to an improvement in the charge transfer kinetics of the HER. This work indicates that IP6 may be a potent candidate as a catalyst for hydrogen energy production.     

Recent development of transition metal doped carbon materials derived from biomass for hydrogen evolution reaction

Achieving high catalytic performance with the lowest cost is critical for hydrogen evolution reduction. In recent years, biomass-derived carbon catalysts have triggered huge interest in catalytic reactions owing to the low cost, high energy conversion efficiency and environmental friendliness. A rapid growth of novel electrocatalysts is witnessed especially those based on non-precious metals, some of which approach the activity of precious metals. Synergistic interactions between metals and heteroatoms can significantly improve the electrocatalytic activity, thus transition metal-decorated biomass-based carbon materials were commonly adopted to improve HER performance. The resulting electrocatalytic activities are introduced and compared to conventional Pt/C-based electrocatalysts in present research. Moreover, the remaining challenges in the development process and future prospects of hydrogen evolution reduction catalysts are discussed.     

Electrocatalytic studies of siloxene sheets encrusted with cobalt chalcogenides (S,Se) for water splitting

Two-dimensional siloxene sheets were superficially coated with cobalt chalcogenides to optimize interfacial properties for broad applications in the field of catalysis. These catalytic composites were investigated for electrochemical water splitting in an alkaline electrolyte medium. The synthesis of siloxene sheet-cobalt chalcogenides composites was confirmed by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, adsorption studies, and X-ray photoelectron spectroscopy analyses. Potentiometric and impedimetric experiments were performed to understand the inherent electrocatalytic activity of the developed catalysts. Variations in the onset potential and overpotential at a constant current density of ±10 mA/cm² for hydrogen and oxygen evolution reactions—HER and OER, respectively—were evaluated with respect to a reversible hydrogen electrode (RHE). The catalysts exhibit superior current and catalytic activity due to interfacial kinetics, retaining lower Tafel slopes of ～30 mV/dec for the OER and HER; they also exhibited improved, long-term stability for 12 h, indicating potential utility in commercial applications.     

Effect of C-felt supported Ni,Co and NiCo catalysts to produce hydrogen

In this study, the carbon felt (C-felt) is used as the catalyst support for Ni, Co and NiCo coatings. Single Ni, Co and binary NiCo coatings are electrochemically deposited on a C-felt. Surface structure of coatings was characterized by cyclic voltammetry (CV), atomic absorption spectroscopy (AAS) and scanning electron microscopy (SEM). The electrocatalytic activity of the coatings for the hydrogen evolution reaction (HER) was studied in 1.00 M KOH solution using cathodic current-potential curves, electrochemical impedance spectroscopy (EIS) and electrolysis techniques. The results show that since carbon felt has fiber and network structure, and this structure is enhanced the hydrogen evolution. Deposition of nickel, and cobalt on C-felt is enhanced the hydrogen production. Furthermore, NiCo catalyst exhibits much higher activity for HER. Its catalytic activity is related to the fiber and network structure of C-felt, porosity and the loaded NiCo can interact with each other and cooperate on improving the HER activity.     

Accessible active sites activated by nano cobalt antimony oxide @ carbon nanotube composite electrocatalyst for highly enhanced hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) was one of new energy development strategies with clean, efficient and renewable characteristics, and electrocatalysts play a crucial role in HER technology. Herein, a composite material (CSO@0.5CNT) derived from the combination of nano cobalt antimony oxide (CSO) with carbon nanotubes (CNT) through hydrothermal reaction, in which the nanoparticles of CSO were closely compounded on the surface of CNT, could be a highly efficient electrocatalyst for HER in 1 M KOH. The binary composite electrocatalyst of CSO and CNT reduced the internal resistance, promoted the charge transfer, exhibited a large electrochemical active area, and obtained the lower overpotential, with 155 mV at 10 mA/cm² current density. Moreover, such a CSO@0.5CNT electrocatalyst displayed a small Tafel slope of 86.5 mV dec^?1, excellent catalytic activity and extraordinary long-term structural stability after 30 h and 3000 CV cycles. Furthermore, the electrocatalytic mechanism revealed by Density Functional Theory (DFT) calculation proved that, the decomposition of H₂O molecules was the control step of the whole HER, and the superior electron transport ability of CNT was favorable to the improvement of electrocatalytic performance. Benefitting from accessible active sites on carbon nanotube (C atom) and CSO (Co atom), the composite electrocatalyst of CSO@0.5CNT displayed synergistic effect for electrocatalytic HER properties, and that was the main mechanism for significantly improving the electrocatalytic activities. Our work provides a novel strategy towards high-efficiency electrocatalysts for hydrogen evolution reaction.     

Assessment of the roughness factor effect and the intrinsic catalytic activity for hydrogen evolution reaction on Ni-based electrodeposits

The hydrogen evolution reaction (HER) was studied in 30 wt.% KOH solution at temperatures ranging between 30 and 80 °C on three type of electrodes: (i) rough pure Ni electrodeposits, obtained by applying a large current density; (ii) smooth NiCo electrodeposits; (iii) smooth commercial Ni electrodes. By using steady-state polarization curves and electrochemical impedance spectroscopy (EIS) the surface roughness factor and the intrinsic activities of the catalytic layers were determined. These techniques also permitted us to determine the mechanism and kinetics of the HER on the investigated catalysts. Different AC models were tested and the appropriate one was selected. The overall experimental data indicated that the rough/porous Ni electrode yields the highest electrocatalytic activity in the HER. Nevertheless, when the effect of the surface roughness was taken into consideration, it was demonstrated that alloying Ni with Co results in an increased electrocatalytic activity in the HER when comparing to pure Ni. This is due to an improved intrinsic activity of the material, which was explained on the basis of the synergism among the catalytic properties of Ni (low hydrogen overpotential) and of Co (high hydrogen adsorption).     

Morphological engineering of carbon-based materials: in the quest of efficient catalysts for overall water splitting

This work attempts to optimize the catalytic activity of the carbon-based materials by engineering their morphological structure. Several flake-like quantum dots with different shapes such as triangulene, elliptical, rhomboid, and square, as well as hydrocarbons having sunflower, kekulene, and snow-like structures, are considered and their electrocatalytic activities toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are theoretically evaluated. The activity analysis indicates that the OER overpotentials for the examined carbon materials vary in the range between 0.56 and 1.22 V. Benefiting from the improved electronic properties due to the proper morphology, remarkable catalytic activity was achieved for the snow-like morphology affording overpotentials of 0.56 V for OER and ?0.05 V for HER. In addition to snow-like, other morphologies such as triangulene and square can effectively promote acidic hydrogen evolution via Volmer-Heyrovsky mechanism. On contrary, the high values of free energies for H₂O dissociation step reveal that, under the alkaline condition, the examined carbon materials cannot be considered as efficient HER catalysts.     

Polyoxomolybdate-derived MoS2/nitrogen-doped reduced graphene oxide hybrids for efficient hydrogen evolution

Integrating MoS₂ with carbon-based materials, especially graphene, is an effective strategy for preparing highly active non-noble-metal electrocatalysts in the hydrogen evolution reaction (HER). This work demonstrates a convenient hydrothermal method to fabricate molybdenum disulfide nanosheets/nitrogen-doped reduced graphene oxide (MoS₂/NGO) hybrids using polyoxomolybdate as the Mo precursor. Introducing more defects and expanding interlayer spacing of MoS₂ can be achieved through decreasing the pH value of the reactive system due to the existed high-nuclear polyoxometalate clusters. MoS₂/NGO hybrids prepared at low pH exhibit superior HER activity to those obtained at high pH. MoS₂/NGO-pH1.5 exhibits an ultralow overpotential of 81 mV at 10 mA cm⁻², a low Tafel slope of 60 mV·dec⁻¹ and good stability in alkaline electrolyte. Such excellent electrocatalytic activity is contributed by the abundant HER catalytic active sites, the increased electrochemically-accessible area and the synergetic effects between the active MoS₂ catalyst and NGO support.