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

Metal Electrocatalysts for Hydrogen Production in Water Splitting

The rising demand for fossil fuels and the resulting pollution have raised environmental concerns about energy production. Undoubtedly, hydrogen is the best candidate for producing clean and sustainable energy now and in the future. Water splitting is a promising and efficient process for hydrogen production, where catalysts play a key role in the hydrogen evolution reaction (HER). HER electrocatalysis can be well performed by Pt with a low overpotential close to zero and a Tafel slope of about 30 mV dec-1. However, the main challenge in expanding the hydrogen production process is using efficient and inexpensive catalysts. Due to electrocatalytic activity and electrochemical stability, transition metal compounds are the best options for HER electrocatalysts. This study will focus on analyzing the current situation and recent advances in the design and development of nanostructured electrocatalysts for noble and non-noble metals in HER electrocatalysis. In general, strategies including doping, crystallization control, structural engineering, carbon nanomaterials, and increasing active sites by changing morphology are helpful to improve HER performance. Finally, the challenges and future perspectives in designing functional and stable electrocatalysts for HER in efficient hydrogen production from water-splitting electrolysis will be described.

Synthesis and Volatility Characterization of Mo(II) and W(II) Compounds for Thin Films

Mo(II) and W(II) compounds, Mo(η3-allyl)(CO)2(Tri-MEDA)Br (1), Mo(η3-allyl)(CO)2(TMEDA)Br (2), W(η3-allyl)(CO)2(Tri-MEDA)Br (3), and W(η3-allyl)(CO)2(TMEDA)Br (4) (Tri-MEDA = N,N,N'-trimethylethylenediamine), were synthesized and characterized. The molecular structures of 1 and 3 were nearly identical with a pseudo-octahedral geometry except for the different Mo and W metal centers. The thermogravimetric analysis of 1 and 3 showed approximately 53 and 64% residues at 550 °C, respectively, which were significantly higher than the values for the expected materials. However, 1 and 3 sublimed at 100 °C under 0.40 Torr and 120 °C under 0.50 Torr, respectively, confirming that they were volatile. For 1 and 3, the temperatures at a vapor pressure of 1 Torr and enthalpies of vaporization (ΔHvap) were 168.78 °C and 143.8 kJ mol-1, and 167.48 °C and 148.5 kJ mol-1, respectively. The tungsten compound (3) exhibited good durability for 5 weeks under a thermal stability test at a sublimation temperature of 120 °C.

"Electron Complementation"-Induced Molybdenum Nitride/Co-Anchored Graphitic Carbon Nitride Porous Nanoparticles for Efficient Overall Water Splitting

Maximizing the usable space of electrocatalysts and fine-tuning the interface geometry as well as the electronic structure to facilitate hydrogen and oxygen evolution reactions (HER and OER) have always been the focus of research. Herein, a homogeneous porous nanoparticle construction strategy was proposed, in which molybdenum nitride (Mo₂N) particles were prepared by controlled heat treatment of the precursor nanoparticle induced by polyethylene glycol, and the Mo₂N/Co-C₃N₄ heterostructure with a pore size of about 1.13 nm was obtained by compounding Co-anchored graphitic carbon nitride. In particular, exploring the change of charge distribution at the interface based on the principle of "electron complementation" shows that under the regulation of nitrogen with high electronegativity, the affinity of active site Co to oxygenated species in the OER process and the adsorption as well as cleavage ability of HER reactants in the active site were effectively optimized. Thus, Mo₂N/Co-C₃N₄ not only inherits the functions of each component, but also provides an ideal heterogeneous interface for exhibiting impressive bifunctional activity, which only needs 100 and 210 mV to deliver 10 mA cm^-2 for the HER and OER, respectively. In addition, the Mo₂N/Co-C₃N₄ catalyst also demonstrates high overall water splitting stability with a slight current decrease after 95 h. Manipulating the electronic structure of multiple sites by constructing electronically complementary interfaces may provide another avenue to develop highly active catalysts for overall water splitting and other applications.

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Interfacial Engineering of Two-Dimensional MoN/MoO2 Heterostructure Nanosheets as a Bifunctional Electrocatalyst for Overall Water Splitting

It is still a challenge to realize the dream of hydrogen-based economy using a robust catalyst for overall water splitting. For the first time, we introduce two-dimensional MoN/MoO 2 heterostructure nanosheets using nickel foam as a substrate for water splitting. The heterojunction formation was achieved through the partial nitriding of Mo-based precursor to MoN in the annealing process under NH 3 environment. The heterogeneous interface between MoN and MoO 2 as active sites is supposed to improve the surface reaction kinetics and electronic conductivity. Therefore, excellent performance is achieved when MoN/MoO 2 is employed as both cathode and anode electrocatalysts, the corresponding cell voltages are 1.57 and 1.84 V at 10 and 100 mA cm -2 in 1 M KOH, respectively. The promising bifunctional catalytic performance of our catalyst opens up a new way for efficient electrochemical water splitting.

Heteroatom-Doping of Non-Noble Metal-Based Catalysts for Electrocatalytic Hydrogen Evolution: An Electronic Structure Tuning Strategy

Electrocatalytic water splitting for hydrogen production is an appealing way to reduce carbon emissions and generate renewable fuels. This promising process, however, is limited by its sluggish reaction kinetics and high-cost catalysts. Construction of low-cost and high-performance non-noble metal-based catalysts have been one of the most effective approaches to address these grand challenges. Notably, the electronic structure tuning strategy, which could subtly tailor the electronic states, band structures, and adsorption ability of the catalysts, has become a pivotal way to further enhance the electrochemical water splitting reactions based on non-noble metal-based catalysts. Particularly, heteroatom-doping plays an effective role in regulating the electronic structure and optimizing the intrinsic activity of the catalysts. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the hetero-dopants in catalysts yet remains ambiguous. Herein, the recent progress is comprehensively reviewed in heteroatom doped non-noble metal-based electrocatalysts for hydrogen evolution reaction, particularly focus on the electronic tuning effect of hetero-dopants in the catalysts and the corresponding synthetic pathway, catalytic performance, and activity origin. This review also attempts to establish an intrinsic correlation between the localized electronic structures and the catalytic properties, so as to provide a good reference for developing advanced low-cost catalysts.

Atomic layer deposition of tungsten sulfide using a new metal-organic precursor and H2S: thin film catalyst for water splitting

Transition metal dichalcogenides (TMDs) are extensively researched in the past few years due to their two-dimensional layered structure similar to graphite. This group of materials offers tunable optoelectronic properties depending on the number of layers and therefore have a wide range of applications. Tungsten disulfide (WS₂) is one of such TMDs that has been studied relatively less compared to MoS₂. Herein, WS _x thin films are grown on several types of substrates by atomic layer deposition (ALD) using a new metal-organic precursor [tris(hexyne) tungsten monocarbonyl, W(CO)(CH₃CH₂C≡CCH₂CH₃)₃] and H₂S molecules at a relatively low temperature of 300 °C. The typical self-limiting film growth by varying both, precursor and reactant, is obtained with a relatively high growth per cycle value of ∼0.13 nm. Perfect growth linearity with negligible incubation period is also evident in this ALD process. While the as-grown films are amorphous with considerable S-deficiency, they can be crystallized as h-WS₂ film by post-annealing in the H₂S atmosphere above 700 °C as observed from x-ray diffractometry analysis. Several other analyses like Raman and x-ray photoelectron spectroscopy, transmission electron microscopy, UV-vis. spectroscopy are performed to find out the physical, optical, and microstructural properties of as-grown and annealed films. The post-annealing in H₂S helps to promote the S content in the film significantly as confirmed by the Rutherford backscattering spectrometry. Extremely thin (∼4.5 nm), as-grown WS _x films with excellent conformality (∼100% step coverage) are achieved on the dual trench substrate (minimum width: 15 nm, aspect ratio: 6.3). Finally, the thin films of WS _x (as-grown and 600/700 °C annealed) on W/Si and carbon cloth substrate are investigated for electrochemical hydrogen evolution reaction (HER). The as-grown WS _x shows poor performance towards HER and is attributed to the S-deficiency, amorphous character, and oxygen contamination of the WS _x film. Annealing the WS _x film at 700 °C results in the formation of a crystalline layered WS₂ phase, which significantly improves the HER performance of the electrode. The study reveals the importance of sulfur content and crystallinity on the HER performance of W-based sulfides.

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.

Flower-like S-doped-Ni2P mesoporous nanosheets-derived self-standing electrocatalytic electrode for boosting hydrogen evolution

Developing cost-effective, highly active, and stable electrocatalysts for boosting electrochemical hydrogen evolution reaction (HER) in alkaline media is playing a critical role to meet hydrogen industry in the future. Herein, an efficient HER electrocatalyst based on flowerlike S-doped Ni2P mesoporous nanosheets supported on nickel foam (S-Ni2P NSs/NF) was developed through an effective approach. The obtained S-Ni2P NSs/NF catalyst required low overpotential of only 87.5 mV and 179.1 mV to reach current density of 10 and 50 mA cm-2, respectively. Moreover, a small Tafel slope of 62.1 mV dec-1 for S-Ni2P NSs/NF demonstrated that HER process occurred with very fast kinetics. Besides high HER activity, the synthesized S-Ni2P NSs/NF catalyst exhibited superior stability and long-term durability toward HER, which had ability to operate over 30 h without degradation in catalytic performance. The unique flower-like nanosheets structure with excellent mesoporous characteristics of S-Ni2P NSs/NF resulted in maximizing electrochemical active surface area for providing a large number of electrocatalytic active sites. In addition, S doping effect could modulate electronic structure of Ni species in Ni2P, leading to accelerating rate adsorption of reaction intermediates on the surface of catalysts toward improving HER kinetics. The results not only demonstrate S-Ni2P NSs/NF as active catalyst for HER, but also offer effective strategy for improving catalytic activity of earth-abundant transition metal-based HER catalysts.

Molybdenum Nitride Electrocatalysts for Hydrogen Evolution More Efficient than Platinum/Carbon: Mo2N/CeO2@Nickel Foam

To produce hydrogen economically by electrolysis of water, one needs to develop a non-precious-metal catalyst that is as efficient as platinum metal. Here, we prepare such a catalyst by growing a layer of Mo₂N over a layer of CeO₂ deposited on nickel foam (NF) [hereafter, Mo₂N /CeO₂@NF] and show that the activity of this self-supported catalyst for hydrogen evolution in 1.0 M KOH is more efficient than that of the Pt/C electrode, achieving a current density of 10 mA/cm² at a fairly low overpotential of 26 mV. Furthermore, after a long-time electrochemical stability test for 24 h at a fixed current density, the overpotential needed to attain a current density of 10 mA/cm² is increased only by 6 mV, implying the huge potential of this method to prepare a super HER activity electrode for water splitting.

Low-Temperature Atomic Layer Deposition of Highly Conformal Tin Nitride Thin Films for Energy Storage Devices

We present an atomic layer deposition (ALD) process for the synthesis of tin nitride (SnN_x) thin films using tetrakis(dimethylamino) tin (TDMASn, Sn(NMe₂)₄) and ammonia (NH₃) as the precursors at low deposition temperatures (70-200 °C). This newly developed ALD scheme exhibits ideal ALD features such as self-limited film growth at 150 °C. The growth per cycle (GPC) was found to be ∼0.21 nm/cycle at 70 °C, which decreased with increasing deposition temperature. Interestingly, when the deposition temperature was between 125 and 180 °C, the GPC remained almost constant at ∼0.10 nm/cycle, which suggests an ALD temperature window, whereas upon further increasing the temperature to 200 °C, the GPC considerably decreased to ∼0.04 nm/cycle. Thermodynamic analysis via density functional theory calculations showed that the self-saturation of TDMASn would occur on an NH₂-terminated surface. Moreover, it also suggests that the condensation of a molecular precursor and the desorption of surface *NH₂ moieties would occur at lower and higher temperatures outside the ALD window, respectively. Thanks to the characteristics of ALD, this process could be used to conformally and uniformly deposit SnN_x onto an ultranarrow dual-trench Si structure (minimum width: 15 nm; aspect ratio: ∼6.3) with ∼100% step coverage. Several analysis tools such as transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and secondary-ion mass spectrometry were used to characterize the film properties under different deposition conditions. XRD showed that a hexagonal SnN phase was obtained at a relatively low deposition temperature (100-150 °C), whereas cubic Sn₃N₄ was formed at a higher deposition temperature (175-200 °C). The stoichiometry of these thermally grown ALD-SnN_x films (Sn-to-N ratio) deposited at 150 °C was determined to be ∼1:0.93 with negligible impurities. The optoelectronic properties of the SnN_x films, such as the band gap, wavelength-dependent refractive index, extinction coefficient, carrier concentration, and mobility, were further evaluated via spectroscopic ellipsometry analysis. Finally, ALD-SnN_x-coated Ni-foam (NF) and hollow carbon nanofibers were successfully used as free-standing electrodes in electrochemical supercapacitors and in Li-ion batteries, which showed a higher charge-storage time (about eight times greater than that of the uncoated NF) and a specific capacity of ∼520 mAh/g after 100 cycles at 0.1 A/g, respectively. This enhanced performance might be due to the uniform coverage of these substrates by ALD-SnN_x, which ensures good electric contact and mechanical stability during electrochemical reactions.