Oxygen Vacancy-Enhanced Ni3N-CeO2/NF Nanoparticle Catalysts for Efficient and Stable Electrolytic Water Splitting

Highly efficient and cost-effective electrocatalysts are of critical significance in the domain of water electrolysis. In this study, a Ni3N-CeO2/NF heterostructure is synthesized through a facile hydrothermal technique followed by a subsequent nitridation process. This catalyst is endowed with an abundance of oxygen vacancies, thereby conferring a richer array of active sites. Therefore, the catalyst demonstrates a markedly low overpotential of 350 mV for the Oxygen Evolution Reaction (OER) at 50 mA cm-2 and a low overpotential of 42 mV for the Hydrogen Evolution Reaction (HER) at 10 mA cm-2. Serving as a dual-function electrode, this electrocatalyst is employed in overall water splitting in alkaline environments, demonstrating impressive efficiency at a cell voltage of 1.52 V of 10 mA cm-2. The in situ Raman spectroscopic analysis demonstrates that cerium dioxide (CeO2) facilitates the rapid reconfiguration of oxygen vacancy-enriched nickel oxyhydroxide (NiOOH), thereby enhancing the OER performance. This investigation elucidates the catalytic role of CeO2 in augmenting the OER efficiency of nickel nitride (Ni3N) for water electrolysis, offering valuable insights for the design of high-performance bifunctional catalysts tailored for water splitting applications.

Synergistically Coupled CoMo/CoMoP Electrocatalyst for Highly Efficient and Stable Overall Water Splitting

Finding reservoir-rich and efficient bifunctional electrocatalysts for water splitting is key to further sustainable energy development. Transition metal phosphides (TMPs) are extensively exploited as effective electrocatalysts, but the construction of strong coupling interfaces to improve catalytic performance by simple methods is still a bottleneck. Here, we designed and prepared a novel heterostructure electrocatalyst composed of cobalt-molybdenum (CoMo) alloy particles integrated with CoMoP nanosheets via the method of template-assisted conversion, followed by electrodeposition. Thanks to the strong interfacial coupling and synergistic effect between CoMo alloy particles and CoMoP nanosheets, the prepared CoMo/CoMoP/NF shows outstanding activity with overpotentials of only 29 mV for the hydrogen evolution reaction (HER) and 246 mV for the oxygen evolution reaction (OER) in 1 M KOH at a current density of 10 mA cm^-2. Furthermore, the assembled CoMo/CoMoP || CoMo/CoMoP electrode can attain 10 mA cm^-2 with a low battery voltage of 1.54 V. This study offers a valuable reference to the construction of bimetallic alloy/bimetallic phosphide heterostructure electrocatalysts, which applies to the large-scale application of electrocatalytic energy conversion technology.

Reduced Graphene Oxide Supported Zinc Tungstate Nanoparticles as Proficient Electro-Catalysts for Hydrogen Evolution Reactions

The nanocomposites of reduced graphene oxide (rGO) supported zinc tungstate nanoparticles (ZnWO4-NPs) receive considerable attention in electro-catalytic hydrogen evolution reactions (HER) and reveal significantly higher electro-catalytic performances than pure ZnWO4-NPs in alkaline media (i.e., 0.5 M KOH electrolyte). The polarization studies show that the ZnWO4-NPs@rGO nanocomposites exhibit low energy loss and good electrode stability during electrochemical reactions for HER. Furthermore, the Tafel slope of ZnWO4-NPs@rGO nanocomposites is found to be approximately 149 mV/dec, which closely agrees with the reported Tafel values of the noble metal electrocatalyst. In contrast, the performance of the ZnWO4-NPs@rGO nanocomposite is found to be approximately 1.5 times higher than that of ZnWO4-NPs in hydrogen production efficiency. Our results emphasize the significance of the nanocomposites with enhanced electro-catalytic activities by lowering the energy loss during electro-catalysis in an alkaline medium.

A new MoCN monolayer containing stable cyano structural units as a high-efficiency catalyst for the hydrogen evolution reaction

In the hydrogen evolution reaction (HER), it is essential to find a high-efficiency and nonprecious electrocatalyst comparable to Pt, which needs to have rich inherently active sites and good conductivity. By combining a global minimum structure search and first-principles calculations, a hitherto unknown 2D MoCN monolayer was found, which can be considered as a structure in which Mo atoms interact with the stable CN units through triple bonds. The resultant MoCN monolayer possesses superior thermodynamic, dynamic, thermal, and mechanical stabilities, as well as inherent metallicity. In particular, it can exhibit outstanding HER catalytic activity due to the presence of many active sites with near-zero ΔG_H* values, whose density totals 1.80 × 10¹⁵ sites per cm², even more than Pt. In addition, we also propose a series of other 2D monolayers containing stable CN units (i.e., MoC₂N, MoCN₂ and MoC₂N₂), all of which can uniformly show high stability and good HER catalytic activity. Applying strain can further effectively improve the activities of C-rich (MoC₂N) and N-rich (MoCN₂) monolayers, inducing considerably high HER catalytic performance. For the MoCN, MoC₂N and MoCN₂ monolayers, the most active sites are located at the Mo-C-N chain involved. All these fascinating findings can not only provide new excellent candidates but also new insights into the design of highly efficient and nonprecious HER electrocatalysts as an alternative to Pt in the near future.

Combustion Reactions between Transition-Metal Chlorides and Sodium Amide and Their Ignition Temperature

Combustion reactions between metal chlorides and sodium amide proceed in a short time; however, these reactions must be carried out with appropriate safety measures. Investigating their ignition temperatures would facilitate safe handling and give kinetic insights about the reaction between powders. Here, we investigated the products of the reactions between metal chlorides and sodium amide and measured their ignition temperatures. The products were mainly composed of nitrides, metals, and sodium chloride. The reactions of 4d and 5d metal chlorides initiated the reaction below room temperature, while 3d metal chlorides, except copper chloride, initiated the reaction upon heating. We found the correlation between the ignition temperatures and the reaction energies of the combustion reaction.

Enhancing bifunctional catalytic activity of cobalt-nickel sulfide spinel nanocatalysts through transition metal doping and its application in secondary zinc-air batteries

Developing large-scale and high-performance OER (oxygen evolution reaction) and ORR (oxygen reduction reaction) catalysts have been a challenge for commercializing secondary zinc-air batteries. In this work, transition metal-doped cobalt-nickel sulfide spinels are directly produced via a continuous hydrothermal flow synthesis (CHFS) approach. The nanosized cobalt-nickel sulfides are doped with Ag, Fe, Mn, Cr, V, and Ti and evaluated as bifunctional OER and ORR catalyst for Zn-air battery application. Among the doped spinel catalysts, Mn-doped cobalt-nickel sulfides (Ni1.29Co1.49Mn0.22S4) exhibit the most promising OER and ORR performance, showing an ORR onset potential of 0.9 V vs. RHE and an OER overpotential of 348 mV measured at 10 mA cm-2, which is attributed to their high surface area, electronic structure of the dopant species, and the synergistic coupling of the dopant species with the active host cations. The dopant ions primarily alter the host cation composition, with the Mn(iii) cation linked to the introduction of active sites by its favourable electronic structure. A power density of 75 mW cm-2 is achieved at a current density of 140 mA cm-2 for the zinc-air battery using the manganese-doped catalyst, a 12% improvement over the undoped cobalt-nickel sulfide and superior to that of the battery with a commercial RuO2 catalyst.

Hydrogen Evolution Reaction by Atomic Layer-Deposited MoNx on Porous Carbon Substrates: The Effects of Porosity and Annealing on Catalyst Activity and Stability

Molybdenum-based compounds are considered as a potential replacement for expensive precious-metal electrocatalysts for the hydrogen evolution reaction (HER) in acid electrolytes. However, coating of thin films of molybdenum nitride or carbide on a large-area self-standing substrate with high precision is still challenging. Here, MoN_x is uniformly coated on carbon cloth (CC) and nitrogen-doped carbon (NC)-modified CC (NCCC) substrates by atomic layer deposition (ALD). The as-deposited film has a nanocrystalline character close to amorphous and a composition of approximately Mo₂ N with significant oxygen contamination, mainly at the surface. Among the as-prepared ALD-MoN_x electrodes, the MoN_x /NCCC has the highest HER activity (overpotential η≈236 mV to achieve 10 mA cm^-2 ) owing to the high surface area and porosity of the NCCC substrate. However, the durability of the electrode is poor, owing to the poor adhesion of NC powder on CC. Annealing MoN_x /NCCC in H₂ atmosphere at 400 °C improves both the activity and durability of the electrode without significant change in the phase or porosity. Annealing at an elevated temperature of 600 °C results in formation of a Mo₂ C phase that further enhances the activity (η≈196 mV to achieve 10 mA cm^-2 ), although there is a huge reduction in the porosity of the electrode as a consequence of the annealing. The structure of the electrode is also systematically investigated by electrochemical impedance spectroscopy (EIS). A deviation in the conventional Warburg impedance is observed in EIS of the NCCC-based electrode and is ascribed to the change in the H⁺ ion diffusion characteristics, owing to the geometry of the pores. The change in porous nature with annealing and the loss in porosity are reflected in the EIS of H⁺ ion diffusion observed at high-frequency. The current work establishes a better understanding of the importance of various parameters for a highly active HER electrode and will help the development of a commercial electrode for HER using the ALD technique.

Bifunctionally active nanosized spinel cobalt nickel sulfides for sustainable secondary zinc–air batteries: examining the effects of compositional tuning on OER and ORR activity

Nanosized cobalt nickel sulfides were prepared via a continuous hydrothermal method and evaluated as electrocatalysts, with the catalytic activity being linked to the cationic composition.

Atomic-Layer-Deposited MoN x Thin Films on Three-Dimensional Ni Foam as Efficient Catalysts for the Electrochemical Hydrogen Evolution Reaction

Future realization of a hydrogen-based economy requires a high-surface-area, low-cost, and robust electrocatalyst for the hydrogen evolution reaction (HER). In this study, the MoN _x thin layer is synthesized on to a high-surface-area three-dimensional (3D) nickel foam (NF) substrate using atomic layer deposition (ALD) for HER catalysis. MoN _x is grown on NF by the sequential exposure of Mo(CO)₆ and NH₃ at 225 °C. The thickness of the thin film is controlled by varying the number of ALD cycles to maximize the HER performance of the MoN _x/NF composite catalyst. The scanning electron microscopy and transmission electron microscopy (TEM) images of MoN _x/NF highlight that ALD facilitates uniform and conformal coating. TEM analysis highlights that the MoN _x film is predominantly amorphous with the nanocrystalline MoN grains (4 nm) dispersed throughout it. Moreover, the high-resolution (HR)-TEM analysis shows a rough surface of the MoN _x film with an overall composition of Mo_0.59N_0.41. X-ray photoelectron spectroscopy depth-profile analysis reveals that oxygen contamination is concentrated at the surface because of surface oxidation of the MoN _x film under ambient conditions. The HER activity of MoN _x is evaluated under acidic (0.5 M H₂SO₄) and alkaline (0.1 M KOH) conditions. In an acidic electrolyte, the sample prepared with 700 ALD cycles exhibits significant HER activity and a low overpotential (η) of 148 mV at 10 mA cm^-2. Under an alkaline condition, it achieves 10 mA cm^-2 with η of 125 mV for MoN _x/NF (700 cycles). In both electrolytes, the MoN _x thin film exhibits enhanced activity and stability because of the uniform and conformal coating on NF. Thus, this study facilitates the development of a large-area 3D freestanding catalyst for efficient electrochemical water-splitting, which may have commercial applicability.

Low-cost high-performance hydrogen evolution electrocatalysts based on Pt-CoP polyhedra with low Pt loading in both alkaline and neutral media

Low-cost Pt-CoP hollow polyhedra exhibited prominent performance for the HER in both basic and neutral solutions.

Bimetallic NiMoN Nanowires with a Preferential Reactive Facet: An Ultraefficient Bifunctional Electrocatalyst for Overall Water Splitting

Faceted nanomaterials with highly reactive exposed facets have been the target of intense researches owing to their significantly enhanced catalytic performance. NiMoN nanowires with the (100) facet preferentially exposed were prepared by an in situ N/O exchange and the morphology tuned by using a rationally designed NiMoO₄ precursor. The facet-tuned NiMoN nanowires exhibited excellent electrocatalytic activity for the hydrogen evolution reaction (HER) under both alkaline and acidic conditions that was comparable to that of noble metal platinum. DFT calculations further revealed that the catalytic activity of NiMoN nanowires towards HER on the (100) reactive facet is significantly greater than that on the (001) or (101) facets, owing to the low adsorption free energy of H* (ΔG_H* ) on the (100) facet. The NiMoN nanowires also demonstrated outstanding activity towards the alkaline oxygen evolution reaction and an excellent durable activity for overall water splitting, with a cell potential as low as 1.498 V at 20 mA cm^-2 . This work provides insights into improving electrocatalytic activity and developing advanced non-noble metal bifunctional electrocatalysts.

Molybdenum Carbide-Decorated Metallic Cobalt@Nitrogen-Doped Carbon Polyhedrons for Enhanced Electrocatalytic Hydrogen Evolution

Electrocatalytic hydrogen evolution reaction (HER) based on water splitting holds great promise for clean energy technologies, in which the key issue is exploring cost-effective materials to replace noble metal catalysts. Here, a sequential chemical etching and pyrolysis strategy are developed to prepare molybdenum carbide-decorated metallic cobalt@nitrogen-doped porous carbon polyhedrons (denoted as Mo/Co@N-C) hybrids for enhanced electrocatalytic hydrogen evolution. The obtained metallic Co nanoparticles are coated by N-doped carbon thin layers while the formed molybdenum carbide nanoparticles are well-dispersed in the whole Co@N-C frames. Benefiting from the additionally implanted molybdenum carbide active sites, the HER performance of Mo/Co@N-C hybrids is significantly promoted compared with the single Co@N-C that is derived from the pristine ZIF-67 both in alkaline and acidic media. As a result, the as-synthesized Mo/Co@N-C hybrids exhibit superior HER electrocatalytic activity, and only very low overpotentials of 157 and 187 mV are needed at 10 mA cm^-2 in 1 m KOH and 0.5 m H₂ SO₄ , respectively, opening a door for rational design and fabrication of novel low-cost electrocatalysts with hierarchical structures toward electrochemical energy storage and conversion.

Electronic Structure Tuning in Ni3FeN/r-GO Aerogel toward Bifunctional Electrocatalyst for Overall Water Splitting

Searching for the highly active, stable, and high-efficiency bifunctional electrocatalysts for overall water splitting, e.g., for both oxygen evolution (OER) and hydrogen evolution (HER), is paramount in terms of bringing future renewable energy systems and energy conversion processes to reality. Herein, three-dimensional (3D) Ni₃FeN nanoparticles/reduced graphene oxide (r-GO) aerogel electrocatalysts were fabricated using precursors of (Ni,Fe)/r-GO alginate hydrogels through an ion-exchange process, followed by a convenient one-step nitrogenization treatment in NH₃ at 700 °C. The resultant materials exhibited excellent electrocatalytic performance for OER and HER in alkaline media, with only small overpotentials of 270 and 94 mV at a current density of 10 mA cm^-2, respectively. The good performance was attributed to abundant active sites and high electrical conductivity of the bimetallic nitrides and efficient mass transport of the 3D r-GO aerogel framework. Furthermore, an alkaline electrolyzer was set up using Ni₃FeN/r-GO as both the cathode and the anode, which achieved a 10 mA cm^-2 current density at 1.60 V with durability of 100 h for overall water splitting. Density functional theory calculations support that Ni₃FeN (111)/r-GO is more favorable for overall water splitting since the surface electronic structure of Ni₃FeN is tuned by transferring electrons from Ni₃FeN cluster to the r-GO through interaction of two metal species. Thus, the currently developed Ni₃FeN/r-GO with superior water-splitting performance may potentially serve as a material for use in industrial alkaline water electrolyzers.

Nanocatalysts for hydrogen evolution reactions

Hydrogen fuel is among the cleanest renewable resources and is the best alternative to fossil fuels for the future.

Phosphorus-doped NiCo2S4 nanocrystals grown on electrospun carbon nanofibers as ultra-efficient electrocatalysts for the hydrogen evolution reaction

The development of highly efficient noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) is still a challenge nowadays. In this work, we prepared a highly active electrocatalyst containing phosphorus-doped NiCo₂S₄ nanocrystals grown on carbon nanotube embedded carbon nanofibers (P-NiCo₂S₄@CNT/CNF). The CNTs are involved in enhancing the electrical conductivity of the three-dimensional CNF network through a facile co-electrospinning method, which can facilitate electron transfer to the attached HER active material. Templated by this nanofiber network, the electroactive NiCo₂S₄ is confined to grow perpendicularly onto the CNT/CNF template via a hydrothermal reaction, thus exposing more catalytic active sites. Doping of P into the hybrid via a phosphidation reaction improves the electronic structure of the electroactive NiCo₂S₄, thus decreasing the energy barrier during the HER process. Owing to the synergistic effects from electrical enhancement and the nanostructured morphology, along with P-doping-induced optimization of the electronic structure, the P-NiCo₂S₄@CNT/CNF hybrid exhibits excellent HER performance, with an ultra-low onset overpotential (η) of 27 mV, a remarkable current density of 10 mA cm^-2 at η as low as 74 mV, an impressive exchange current density of 0.79 mA cm^-2 and excellent long-term durability. Furthermore, its electroactivity exceeds that of most reported noble-metal-free electrocatalysts and is comparable to that of Pt, suggesting its great potential as a highly efficient HER catalyst.

Efficient Electrocatalytic Hydrogen Evolution from MoS2-Functionalized Mo2N Nanostructures

Molybdenum-based compounds and their composites were investigated as an alternative to Pt for hydrogen evolution reactions. The presence of interfaces and junctions between Mo₂N and MoS₂ grains in the composites were investigated to understand their role in electrochemical processes. Here we found that the electrocatalytic activity of Mo₂N nanostructures was enhanced remarkably by conjugation with few-layer MoS₂ sheets. The electrocatalytic performance of Mo₂N-MoS₂ composites in the hydrogen evolution reaction (HER) was revealed from the high catalytic current density of ∼175 mA cm^-2 (at 400 mV) and good electrochemical stability (more than 18 h) in acidic media. Increasing the amount of MoS₂ in the composite, decreases the HER activity. The mechanism and kinetics of the HER process on the Mo₂N-MoS₂ surface were analyzed using Tafel slopes and charge transfer resistance.

Reduced graphene oxide and MoP composite as highly efficient and durable electrocatalyst for hydrogen evolution in both acidic and alkaline media

Materials based on earth-abundant elements can be developed for hydrogen evolution reactions to meet the future demand for eco-friendly and renewable energy sources based on hydrogen.

Nanoparticle-Stacked Porous Nickel-Iron Nitride Nanosheet: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Nanoparticle-stacked porous Ni3FeN nanosheets were synthesized through a simple nitridation reaction of the corresponding LDHs. The nanosheet is composed of stacked nanoparticles with more active sites exposed for electrocatalytic reactions. Thus, it exhibited excellent oxygen evolution reaction performance having an extremely low overpotential of 223 mV at 10 mA/cm(2) and hydrogen evolution reaction property with a very low overpotential of 45 mV at 10 mA/cm(2). This electrocatalyst as bifunctional electrodes is used to overall water splitting in alkaline media, showing a high performance with 10 mA/cm(2) at a cell voltage of 1.495 V.

Porous cobalt–iron nitride nanowires as excellent bifunctional electrocatalysts for overall water splitting

Nanoparticle-stacked porous Co₃FeN_x nanowires as bifunctional electrocatalysts, exhibiting excellent OER and HER activity due to their unique structural advantages with grain boundaries, defects and dislocations.