Epitaxial growth of Ni(OH)2 nanoclusters on MoS2 nanosheets for enhanced alkaline hydrogen evolution reaction

Tailored Bimetallic Ni-Sn Catalyst for Electrochemical Ammonia Oxidation to Dinitrogen with High Selectivity

Direct electrochemical ammonia oxidation reaction (eAOR) is an efficient and sustainable strategy to process wastewater containing ammonia, and it endures overoxidation and severely competitive oxygen evolution reaction (OER). Herein, we synthesized a Ni(OH)₂/SnO₂ composite catalyst by a multistep strategy and applied it to the eAOR process. Ni(OH)₂/SnO₂ exhibited a N₂-N Faradaic efficiency (FE_N2-N) of 84.2%, with a N₂ partial current density (j_N2-N) of 2.7 mA cm^-2 at 1.55 V vs reversible hydrogen electrode (RHE) in 0.5 M K₂SO₄ with 10 mM NH₃-N (pH 11). The oxophilic Sn promoted NH₃ absorption on Ni sites while suppressing the OER. As the active species, NiOOH accelerated the dimerization of intermediates (*NH₂ or *NH) to form N₂.

Engineering Rich Active Sites and Efficient Water Dissociation for Ni-Doped MoS2/CoS2 Hierarchical Structures toward Excellent Alkaline Hydrogen Evolution

Besides improving charge transfer, there are two key factors, such as increasing active sites and promoting water dissociation, to be deeply investigated to realize high-performance MoS₂-based electrocatalysts in alkaline hydrogen evolution reaction (HER). Herein, we have demonstrated the synergistic engineering to realize rich unsaturated sulfur atoms and activated O-H bonds toward the water for Ni-doped MoS₂/CoS₂ hierarchical structures by an approach to Ni doping coupled with in situ sulfurizing for excellent alkaline HER. In this work, the Ni-doped atoms are evolved into Ni(OH)₂ during alkaline HER. Interestingly, the extra unsaturated sulfur atoms will be modulated into MoS₂ nanosheets by breaking Ni-S bonds during the formation of Ni(OH)₂. On the other hand, the higher the mass of the Ni precursor (m_Ni) for the fabrication of our samples, the more Ni(OH)₂ is evolved, indicating a stronger ability for water dissociation of our samples during alkaline HER. Our results further reveal that regulating m_Ni is crucial to the HER activity of the as-synthesized samples. By regulating m_Ni to 0.300 g, a balance between increasing active sites and promoting water dissociation is achieved for the Ni-doped MoS₂/CoS₂ samples to boost alkaline HER. Consequently, the optimal samples present the highest HER activity among all counterparts, accompanied by reliable long-term stability. This work will promise important applications in the field of electrocatalytic hydrogen evolution in alkaline environments.

In Situ Synthesis and Microfabrication of High Entropy Alloy and Oxide Compounds by Femtosecond Laser Direct Writing under Ambient Conditions

Synthesis and coating of multi-metal oxides (MMOs) and alloys on conductive substrates are indispensable to electrochemical applications, yet demand multiple, resource-intensive, and time-consuming processes. Herein, an alternative approach to the synthesis and coating of alloys and MMOs by femtosecond laser direct writing (FsLDW) is reported. A solution-based precursor ink is deposited and dried on the substrate and illuminated by a femtosecond laser. During the illumination, dried precursor ink is transformed to MMO/alloys and is simultaneously bonded to the substrate. The formulation of the alloy and MMO precursor ink for laser processing is universally applicable to a large family of oxides and alloys. The process is conducted at room temperature and in an open atmosphere. To demonstrate, a large family of 57 MMOs and alloys are synthesized from a group of 13 elements. As a proof of concept, Ni_0.24 Co_0.23 Cu_0.24 Fe_0.15 Cr_0.14 high entropy alloy synthesized on stainless-steel foil by FsLDW is used for the oxygen evolution reaction, which achieves a current density of 10 mA cm^-2 at a significantly low overpotential of 213 mV. Further, FsLDW can also achieve microfabrication of alloys/MMO with feature sizes down to 20 µm.

Synergistic Regulation of S-Vacancy of MoS2-Based Materials for Highly Efficient Electrocatalytic Hydrogen Evolution

Low or excessively high concentration of S-vacancy (C_S-vacancy) is disadvantageous for the hydrogen evolution reaction (HER) activity of MoS₂-based materials. Additionally, alkaline water electrolysis is most likely to be utilized in the industry. Consequently, it is of great importance for fine-tuning C_S-vacancy to significantly improve alkaline hydrogen evolution. Herein, we have developed a one-step Ru doping coupled to compositing with CoS₂ strategy to precisely regulate C_S-vacancy of MoS₂-based materials for highly efficient HER. In our strategy, Ru doping favors the heterogeneous nucleation and growth of CoS₂, which leads to a high crystallinity of Ru-doped CoS₂ (Ru-CoS₂) and rich heterogeneous interfaces between Ru-CoS₂ and Ru-doped MoS_2-x (Ru-MoS_2-x). This facilitates the electron transfer from Ru-CoS₂ to Ru-MoS_2-x, thereby increasing C_S-vacancy of MoS₂-based materials. Additionally, the electron injection effect increases gradually with an increase in the mass of Co precursor (m_Co), which implies more S^2- leaching from MoS₂ at higher m_Co. Subsequently, C_S-vacancy of the as-synthesized samples is precisely regulated by the synergistic engineering of Ru doping and compositing with CoS₂. At C_S-vacancy = 17.1%, a balance between the intrinsic activity and the number of exposed Mo atoms (EMAs) to boost highly active EMAs should be realized. Therefore, the typical samples demonstrate excellent alkaline HER activity, such as a low overpotential of 170 mV at 100 mA cm⁻² and a TOF of 4.29 s⁻¹ at -0.2 V. Our results show promise for important applications in the fields of electrocatalysis or energy conversion.

Engineering Interface on a 3D CoxNi1-x(OH)2@MoS2 Hollow Heterostructure for Robust Electrocatalytic Hydrogen Evolution

Clarifying the responsibilities and constructing the synergy of different active phases are of great significance but still an urgent challenge for the heterostructure catalyst to improve the hydrogen evolution reaction (HER) process. Here, three-dimensional (3D) Co_xNi_(1-x)(OH)₂ hollow structure integrating MoS₂ nanosheet catalysts [Co_xNi_(1-x)(OH)₂@MoS₂] were ingeniously designed and prepared. This unique structure has realized the construction of a dual active phase for the optimized stepwise-synergetic hydrogen evolution process over a universal pH range through interface assembly engineering. Meanwhile, the 3D hollow heterostructure with a high surface-to-volume ratio can effectively avoid the agglomeration of MoS₂ and enhance the Co_xNi_(1-x)(OH)₂-MoS₂ heterointerfaces. Thus, superior HER activity and stability were obtained over the universal pH range. Density functional theory calculation reveals that Co_xNi_(1-x)(OH)₂ and MoS₂ phases provide efficient active sites for rate-determining water dissociation and H* adsorption/H₂ generation on Co_xNi_(1-x)(OH)₂-MoS₂ heterointerfaces, respectively, resulting in an optimized energy barrier for HER. This work proposes a constructive strategy to design highly efficient electrocatalysts based on the heterointerface with a defined responsible active phase of electrocatalysts.

MoS2 nanosheets vertically grown on CoSe2 hollow nanotube arrays as an efficient catalyst for the hydrogen evolution reaction

Although the design and synthesis of efficient electrocatalysts for the hydrogen evolution reaction (HER) are highly desirable, severe challenges still need to be addressed. Herein, ultrathin MoS₂ nanosheets were vertically grown on CoSe₂ hollow nanotube arrays via a simple three-step hydrothermal reaction by using carbon cloth (CC) as a substrate and were subsequently used as a highly efficient HER electrocatalyst (MoS₂@CoSe₂-CC hybrid). The MoS₂ nanosheets uniformly self-assembled on conductive CoSe₂ nanotube arrays exhibited a hierarchical and well-ordered structure. Such a unique structure may not only comprise more exposed active sites, but also enable fast electrolyte penetration and facilitate H⁺/electron transportation to accelerate the reduction and evolution of H₂ during the electrocatalytic process. As an HER electrocatalyst with a novel three-dimensional hierarchical structure, the MoS₂@CoSe₂-CC hybrid exhibited an outstanding catalytic HER performance with a small Tafel slope of 67 mV dec^-1 in alkaline media, while only requiring a low HER overpotential of 101 mV at 10 mA cm^-2. Notably, the MoS₂@CoSe₂-CC hybrid also demonstrated exceptional electrochemical durability and structural stability even after 1000 cycles or 48 h of continuous electrolysis. Overall, this work presents a new approach for the design and synthesis of robust, highly active, and cost-effective electrocatalysts for hydrogen generation.

Three-dimensional cross-linked Co-MoS2 catalyst on carbon cloth for efficient hydrogen evolution reaction

MoS₂-Based materials are promising hydrogen evolution reaction (HER) electrocatalysts. However, their HER activities are restrained by the poor population of HER activated edge centers, the large area exposed HER inert basal planes, and low conductivity. Fixing these problems on one system is an effective strategy, but it remains a challenge due to the harsh synthetic conditions. Herein, cobalt carbonate hydroxide (CoCH) nanosheets were used as the substrate for preparing a three-dimensional self-supported cross-linked (3DSC) Co-MoS₂ nanostructured HER catalyst, which possesses abundant active centers and fast electronic transfer ability. In addition, Co activates the basal-plane sulfur atom in MoS₂ to be the HER reactive center effectively. Benefiting from these advantages, 3DSC Co-MoS₂ electrode integrated on carbon cloth (CC) shows that it can drive the current density of 10 and 100 mA cm^-2 with only 40 and 119 mV overpotentials, respectively, which is superior to other MoS₂-based HER catalysts reported recently. This research provides a facile strategy for the improvement of efficient HER electrocatalysts.

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.

Defect-Rich Heterogeneous MoS2/rGO/NiS Nanocomposite for Efficient pH-Universal Hydrogen Evolution

Molybdenum disulfide (MoS₂) has been universally demonstrated to be an effective electrocatalytic catalyst for hydrogen evolution reaction (HER). However, the low conductivity, few active sites and poor stability of MoS₂-based electrocatalysts hinder its hydrogen evolution performance in a wide pH range. The introduction of other metal phases and carbon materials can create rich interfaces and defects to enhance the activity and stability of the catalyst. Herein, a new defect-rich heterogeneous ternary nanocomposite consisted of MoS₂, NiS and reduced graphene oxide (rGO) are synthesized using ultrathin αNi(OH)₂ nanowires as the nickel source. The MoS₂/rGO/NiS-5 of optimal formulation in 0.5 M H₂SO₄, 1.0 M KOH and 1.0 M PBS only requires 152, 169 and 209 mV of overpotential to achieve a current density of 10 mA cm⁻² (denoted as η₁₀), respectively. The excellent HER performance of the MoS₂/rGO/NiS-5 electrocatalyst can be ascribed to the synergistic effect of abundant heterogeneous interfaces in MoS₂/rGO/NiS, expanded interlayer spacings, and the addition of high conductivity graphene oxide. The method reported here can provide a new idea for catalyst with Ni-Mo heterojunction, pH-universal and inexpensive hydrogen evolution reaction electrocatalyst.

Interface engineering of heterostructured electrocatalysts towards efficient alkaline hydrogen electrocatalysis

Boosting the alkaline hydrogen evolution and oxidation reaction (HER/HOR) kinetics is vital to practicing the renewable hydrogen cycle in alkaline media. Recently, intensive research has demonstrated that interface engineering is of critical significance for improving the performance of heterostructured electrocatalysts particularly toward the electrochemical reactions involving multiple reaction intermediates like alkaline hydrogen electrocatalysis, and the research advances also bring substantial non-trivial fundamental insights accordingly. Herein, we review the current status of interface engineering with respect to developing efficient heterostructured electrocatalysts for alkaline HER and HOR. Two major subjects-how interface engineering promotes the reaction kinetics and what fundamental insights interface engineering has brought into alkaline HER and HOR-are discussed. Specifically, heterostructured electrocatalysts with abundant interfaces have shown substantially accelerated alkaline hydrogen electrocatalysis kinetics owing to the synergistic effect from different components, which could balance the adsorption/desorption behaviors of the intermediates at the interfaces. Meanwhile, interface engineering can effectively tune the electronic structures of the active sites via electronic interaction, interfacial bonding, and lattice strain, which would appropriately optimize the binding energy of targeted intermediates like hydrogen. Furthermore, the confinement effect is critical for delivering high durability by sustaining high density of active sites. At last, our own perspectives on the challenges and opportunities toward developing efficient heterostructured electrocatalysts for alkaline hydrogen electrocatalysis are provided.

A Z-scheme heterostructure constructed from ZnS nanospheres and Ni(OH)2 nanosheets to enhance the photocatalytic hydrogen evolution

ZnS and Ni(OH)₂ form a Z-scheme heterostructure, and the synergy between them provides a new hydrogen-producing active center for each material.

Sequential Electrodeposition of Bifunctional Catalytically Active Structures in MoO3 /Ni-NiO Composite Electrocatalysts for Selective Hydrogen and Oxygen Evolution

Exploring earth-abundant and highly efficient electrocatalysts is critical for further development of water electrolyzer systems. Integrating bifunctional catalytically active sites into one multi-component might greatly improve the overall water-splitting performance. In this work, amorphous NiO nanosheets coupled with ultrafine Ni and MoO₃ nanoparticles (MoO₃ /Ni-NiO), which contains two heterostructures (i.e., Ni-NiO and MoO₃ -NiO), is fabricated via a novel sequential electrodeposition strategy. The as-synthesized MoO₃ /Ni-NiO composite exhibits superior electrocatalytic properties, affording low overpotentials of 62 mV at 10 mA cm^-2 and 347 mV at 100 mA cm^-2 for catalyzing the hydrogen and the oxygen evolution reaction (HER/OER), respectively. Moreover, the MoO₃ /Ni-NiO hybrid enables the overall alkaline water-splitting at a low cell voltage of 1.55 V to achieve 10 mA cm^-2 with outstanding catalytic durability, significantly outperforming the noble-metal catalysts and many materials previously reported. Experimental and theoretical investigations collectively demonstrate the generated Ni-NiO and MoO₃ -NiO heterostructures significantly reduce the energetic barrier and act as catalytically active centers for selective HER and OER, synergistically accelerating the overall water-splitting process. This work helps to fundamentally understand the heterostructure-dependent mechanism, providing guidance for the rational design and oriented construction of hybrid nanomaterials for diverse catalytic processes.

Insights into the Mo-Doping Effect on the Electrocatalytic Performance of Hierarchical CoxMoyS Nanosheet Arrays for Hydrogen Generation and Urea Oxidation

Energy-efficient, low-cost, and highly durable catalysts for the electrochemical hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) are extremely important for related sustainable energy systems. In the present work, hierarchical coassembled cobalt molybdenum sulfide nanosheets deposited on carbon cloth (CC) were synthesized as catalysts for hydrogen evolution and urea oxidation. By adjusting the doping amount of Mo, 2D nanosheets with different morphologies and compositions (Co_xMo_yS-CC) can be obtained. The as-prepared nanosheet materials with abundant active sites exhibit superior properties on the electrochemical HER and UOR in alkaline medium. Significantly, the Mo-doping concentration and composition of the formed nanosheets have large effects on the electrocatalytic activity. The fabricated nanosheets with optimal Mo doping (Co₃Mo₁S-CC) illustrate the best catalytic properties for the HER in N₂-saturated 1.0 M KOH. A small overpotential (85 mV) is needed to meet the current density of 10 mA/cm². This study indicates that the doping of an appropriate amount of molybdenum into CoS₂ nanosheets can efficiently improve the catalytic performance. Also, the nanosheet catalyst exhibits an extremely high electrocatalytic activity for the UOR, and the electrochemical results indicate that a relatively low cell voltage of 1.50 V is needed to obtain the current density of 10 mA/cm². The present work demonstrates the potential application of CoMoS nanosheets in the energy electrocatalysis area and the insights into performance-boosting through heteroatom doping and optimization of the composition and structure.

An Ir/Ni(OH)2 Heterostructured Electrocatalyst for the Oxygen Evolution Reaction: Breaking the Scaling Relation, Stabilizing Iridium(V), and Beyond

Developing efficient electrocatalysts for the oxygen evolution reaction (OER) is highly challenging for hydrogen production from water splitting, due to the high energy barrier for OO bond formation and the restriction of the scaling relation between the multiple reaction intermediates. In order to simultaneously address these concerns, an Ir/Ni(OH)₂ heterostructure with abundant heterointerfaces is deliberately designed as an efficient electrocatalyst system, with Ir nanoparticles (NPs) homogeneously confined on the Ni(OH)₂ nanosheets. The strong electronic interaction and chemical bonding across the interface between the Ir and Ni(OH)₂ can effectively stabilize the metastable electrophilic Ir(V) species, which is vital to boost the formation of OO bonds. Meanwhile, the adsorption of the multiple intermediates is synergistically optimized at the heterointerface, which breaks the restrictive scaling relation and substantially accelerates the OER kinetics. In addition, the severe agglomeration of Ir species is greatly mitigated by the confinement effect, ensuring the structural integrity of the catalyst and the constant exposure of active sites. Owing to its well-defined multifunctional interfaces, the Ir/Ni(OH)₂ heterostructure exhibits exceptional OER activity and durability in alkaline media. The present results highlight the significance of heterostructure interface engineering toward the rational design and development of advanced electrocatalysts for the OER and beyond.

Interfacial N-Cu-S coordination mode of CuSCN/C3N4 with enhanced electrocatalytic activity for hydrogen evolution

Nitrogen/carbon layer coordinated transition metal complexes are the most important alternatives to improve the catalytic performance of catalysts for energy storage and conversion systems, which require systematic investigation and improvement. The coordination mode of transition metal ions can directly affect the catalytic performance of catalysts. Herein, this paper reports that two kinds of Cu-based composites (CuSCN and CuSCN/C3N4) are prepared by in situ controllable crystallization of copper foam (CF) through electropolymerization and calcination. As a comparison, it is clarified that the different coordination modes of Cu1+ ions determine the different catalytic properties. The samples can be switched freely by tuning the electropolymerization period, which leads to different coordination modes of Cu1+ ions dramatically, thus affecting the electrocatalytic performance of composite materials for the hydrogen evolution reaction (HER) in turn. Thorough characterization using techniques, including X-ray photoelectron spectroscopy (XPS) and synchrotron-based near edge X-ray absorption fine structure (EXAFS) spectroscopy, reveals that strong interactions between CuSCN and C3N4 of CuSCN/C3N4 facilitate the formation of subtle coordinated N-Cu-S species, of which electronic structures are changed. Density Functional Theory (DFT) calculations indicate that the electrons can penetrate from CuSCN to N atoms present in C3N4. As a result, CuSCN/C3N4 demonstrates a better catalytic performance than the conventional transition-metal-based electrocatalysts. Besides, CuSCN/C3N4 reflects almost identical hydrogen evolution reaction (HER) activity and stability in an acid electrolyte with Pt/C.

Engineering additional edge sites on molybdenum dichalcogenides toward accelerated alkaline hydrogen evolution kinetics

The sluggish reaction kinetics of the hydrogen evolution reaction (HER) in alkaline media is a great obstacle to alkaline water electrolysis, and it remains a great challenge to develop precious metal-free efficient catalysts for the alkaline HER. Transition metal dichalcogenides (TMDs), in particular MoS2 and MoSe2, are promising catalysts for the HER in acidic media, but they exhibit much inferior catalytic activity for the alkaline HER owing to the slow water dissociation process. In this work, we, for the first time, demonstrate that TMD heterostructures with abundant edge sites deliver substantially accelerated alkaline HER kinetics, which is in great part due to the enhanced water adsorption/dissociation capability. As a proof of concept, MoS2/MoSe2 heterostructures with ultrasmall MoS2 nanoclusters anchored on MoSe2 nanosheets are synthesized via a solution-phase process and are investigated as alkaline HER catalysts in detail. MoSe2 nanosheets serve as excellent substrates to hinder the agglomeration of MoS2 nanoclusters, resulting in abundant edge sites. Benefiting from the decent water adsorption/dissociation capability of the edge sites, the optimal MoS2/MoSe2 heterostructure shows exceptional catalytic activity in 1 M KOH with an overpotential of 235 mV at 10 mA cm-2 and a Tafel slope of 96 mV dec-1, which is substantially improved as compared with the individual MoSe2 (330 mV, 135 mV dec-1) and MoS2 (400 mV, 157 mV dec-1). The success of this catalyst design strategy for enhancing alkaline HER kinetics is also demonstrated in MoSe2/MoSe2 and MoS2/MoS2 heterostructures. The results suggest that engineering additional edge sites that have a strong affinity for H2O is critical for TMDs towards enhanced alkaline HER activity, and also open new avenues in the design of precious metal-free efficient catalysts for the alkaline HER.

Electrodeposited Pd/graphene/ZnO/nickel foam electrode for the hydrogen evolution reaction

Efficient electrocatalysts are crucial to water splitting for renewable energy generation. In this work, electrocatalytic hydrogen evolution from Pd nanoparticle-modified graphene nanosheets loaded on ZnO nanowires on nickel foam was studied in an alkaline electrolyte. The high electron mobility stems from the cylindrical ZnO nanowires and the rough surface on the graphene/ZnO nanowires increases the specific surface area and electrical conductivity. The catalytic activity arising from adsorption and desorption of intermediate hydrogen atoms by Pd nanoparticles improves the hydrogen evolution reaction efficiency. As a hydrogen evolution reaction (HER) catalyst, the Pd/graphene/ZnO/Ni foam (Pd/G/ZnO/NF) nanocomposite exhibits good stability and superior electrocatalytic activity. Linear sweep voltammetry (LSV) revealed an overpotential of −31 mV and Tafel slope of 46.5 mV dec⁻¹ in 1 M KOH. The economical, high-performance, and environmentally friendly materials have excellent prospects in hydrogen storage and hydrogen production.

Efficient electrocatalysts are crucial to water splitting for renewable energy generation.