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

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.

Engineered Superhydrophilic/Superaerophobic Catalyst: Two-Dimensional Co(OH)2-CeO2 Nanosheets Supported on Three-Dimensional Co Dendrites for Overall Water Splitting

Efficient electrocatalysts require not only a tunable electronic structure but also great active site accessibility and favorable mass transfer. Here, a two-dimensional/three-dimensional (2D/3D) hierarchical electrocatalyst consisting of Co(OH)₂-CeO₂ nanosheet-decorated Co dendrites is proposed, named as Co(OH)₂-CeO₂/Co. Based on the strong electronic interaction of the Co(OH)₂-CeO₂ heterojunction, the electronic structure of the Co site is optimized, which facilitates the adsorption of intermediates and the dissociation of H₂O. Moreover, the open 2D/3D structure formed by introducing the Co substrate further reduces the accumulation of heterogeneous nanosheets and promotes the radial diffusion of the electrolyte, significantly improving the utilization of active sites and shortening the electron transfer pathway. In addition, the superhydrophilic/superaerophobic interface achieved by constructing the hierarchical micro-nanostructure is beneficial to electrolyte infiltration and bubble desorption, thus ensuring favorable mass transfer. Therefore, Co(OH)₂-CeO₂/Co exhibits an excellent overall water-splitting activity in alkaline solution.

Molybdenum Oxide Precursors that Promote the Low-Temperature Formation of High-Surface-Area Cubic Molybdenum (Oxy)nitride

Molybdenum nitrides and oxynitrides have been increasingly realized as (electro)catalysts for a variety of reactions. In this context, the cubic "γ-Mo₂N", also known to contain oxygen in the bulk, is of particular interest. The γ phase is typically derived from ammonolysis of MoO₃, and a high temperature is needed to fully react the stable MoO₂ intermediate that often forms along the reaction pathway. In this study, ammonolysis of atypical bronze (H_xMoO₃) and peroxo (H₂MoO₅) precursors was undertaken to avoid the formation of this undesired intermediate with the aim of synthesizing "γ-Mo₂N" at reduced temperatures and thus with a high surface area. It was found, using in situ powder diffraction, that, when the phase I bronze (x ≈ 0.3) served as the precursor, MoO₂ formed as an intermediate and was retained in the reaction product until 700 °C. In contrast, ammonolysis of the phase III bronze (x ≈ 1.7) and of H₂MoO₅ circumvented the MoO₂ intermediate. From these latter two precursors, "γ-Mo₂N" was formed at the lowest maximum reaction temperatures reported in the literature, namely, 480 °C in the case of H_xMoO₃-III and 380 °C for H₂MoO₅. The resulting products displayed extremely high surface areas of 206 and 152 m²/g, respectively, presumably as a consequence of the low synthesis temperatures. While the H_xMoO₃-III precursor showed evidence of a topotactic transformation pathway, with morphological similarity between precursor and product phases, H₂MoO₅ transformed via amorphization. Electrochemical characterization showed moderate activity for the hydrogen evolution reaction (HER), which increased after exposure to reducing potentials and loosely scaled with the catalyst-specific surface area. This work points toward new low-temperature synthesis pathways for accessing molybdenum (oxy)nitrides with high surface areas.

Intercalation Reaction in Amorphous Layer-Wrapped Ni0.2Mo0.8N/Ni3N Heterostructure Toward Efficient Lithium-Ion Storage

Transition metal nitrides (TMNs) with high specific capacity and electric conductivity have drawn considerable attention as electrode materials of lithium-ion batteries (LIBs). However, the cycling stability of most TMNs is not satisfactory, which was caused by the large volume variation during cycles due to their intrinsic conversion reaction mechanism. Herein, by rational design, a much stable tremella-like Ni_0.2Mo_0.8N/Ni₃N heterostructure with amorphous Ni_0.2Mo_0.8N wrapped layer has been fabricated. The Ni₃N particles worked as pillars to support the Ni_0.2Mo_0.8N material as well as conductive medium to facilitate ionic and electronic transport. The amorphous layer can relieve the structural stress of Ni_0.2Mo_0.8N during cycles. Moreover, an exotic intercalation-type reaction mechanism in the ternary nitride Ni_0.2Mo_0.8N was revealed by a series ex situ and in situ characterization. Profiting from these advantages, the Ni_0.2Mo_0.8N/Ni₃N heterostructure anode displays an outstanding electrochemical performance with a high initial reversible discharge capacity of 1001.6 mA h g^-1 at 0.1 A g^-1, excellent cycle stability of 695.5 mA h g^-1 at 2 A g^-1 after 600 cycles, and superior rate capability of 595.3 mA h g^-1 at a high current density of 5 A g^-1. This work provides a new insight for designing high efficiency LIBs based on intercalation reaction for practical applications.

Targeted Regulation of the Electronic States of Nickel Toward the Efficient Electrosynthesis of Benzonitrile and Hydrogen Production

Highly efficient electro-oxidation of benzylamine to generate value-added chemicals coupled with the hydrogen evolution reaction (HER) is crucial but challenging. Herein, targeted regulation of the electronic states of Ni sites was realized via simple yet precise nitridation engineering. Benefiting from the insertion of N atoms into the Ni lattice, the Ni₃N electrode exhibits superior activity, selectivity, and stability for the benzylamine oxidation reaction (BOR). Especially, under the industrially relevant current (∼250 mA), the Ni₃N catalyst remains ∼95% selective for benzonitrile production, reaching 1.43 mmol h^-1 cm^-2. Experimental and theoretical findings reveal that the formation of Ni-N bonds upshifts the Ni d-band center and optimizes the electrophilic properties of Ni sites, which contributes to the adsorption and dehydrogenations process of benzylamine. Furthermore, due to the work function difference between Ni and Ni₃N, a strong mutual interaction occurs at the heterogeneous interface for Ni-Ni₃N, which endows it with the appropriate H* adsorption energy and thus excellent HER performance. Impressively, the integrated solar-energy-driven BOR coupled with the HER electrolyzer affords 10 mA cm^-2 at an ultralow voltage of 1.4 V and exhibits a promising practical application (η_{solar-to-hydrogen} = 13.8%). This work offers a new perspective for the bifunctional design of nitrides in the field of electrosynthesis.

Bifunctional Catalyst Derived from Sulfur-Doped VMoOx Nanolayer Shelled Co Nanosheets for Efficient Water Splitting

A novel sulfur-doped vanadium-molybdenum oxide nanolayer shelling over two-dimensional cobalt nanosheets (2D Co@S-VMoO_x NSs) was synthesized via a facile approach. The formation of such a unique 2D core@shell structure together with unusual sulfur doping effect increased the electrochemically active surface area and provided excellent electric conductivity, thereby boosting the activities for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As a result, only low overpotentials of 73 and 274 mV were required to achieve a current response of 10 mA cm^-2 toward HER and OER, respectively. Using the 2D Co@S-VMoO_x NSs on nickel foam as both cathode and anode electrode, the fabricated electrolyzer showed superior performance with a small cell voltage of 1.55 V at 10 mA cm^-2 and excellent stability. These results suggested that the 2D Co@S-VMoO_x NSs material might be a potential bifunctional catalyst for green hydrogen production via electrochemical water splitting.

Nitrogen-Doped MoS2/Ti3C2TX Heterostructures as Ultra-Efficient Alkaline HER Electrocatalysts

Molybdenum disulfide (MoS₂) is intrinsically inert for the hydrogen evolution reaction (HER) in alkaline media due to its electronic structures. Herein, we tune the electronic structures of MoS₂ by a combined strategy of post-N doping coupled with the synergistic effect of Ti₃C₂T_X. The as-prepared N-doped MoS₂/Ti₃C₂T_X heterostructures show remarkable alkaline HER activity with an overpotential of 225 mV at 140 mA cm^-2, which ranks the N-doped MoS₂/Ti₃C₂T_X heterostructures among the best MoS₂/MXene-based electrocatalysts reported for alkaline HER. The first-principles calculations indicate that the N doping can enhance the activation of nearby S sites of MoS₂/Ti₃C₂T_X and thus promote the HER process. This strategy provides a promising way to develop high-efficiency MoS₂/MXene heterostructure catalysts for alkaline HER.

Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

The energy deficiency and environmental problems have motivated researchers to develop energy conversion systems into a sustainable pathway, and the development of catalysts holds the center of the research endeavors. Natural catalysts such as metalloenzymes have maintained energy cycles on Earth, thus proving themselves the optimal catalysts. In the previous research results, the structural and functional analogs of enzymes and nano-sized electrocatalysts have shown promising activities in energy conversion reactions. Mo ion plays essential roles in natural and artificial catalysts, and the unique electrochemical properties render its versatile utilization as an electrocatalyst. In this review paper, we show the current understandings of the Mo-enzyme active sites and the recent advances in the synthesis of Mo-catalysts aiming for high-performing catalysts.

Surface Fluorination Engineering of NiFe Prussian Blue Analogue Derivatives for Highly Efficient Oxygen Evolution Reaction

Surface engineering is of importance to reduce the reaction barrier of oxygen evolution reaction (OER). Herein, the NiFe Prussian blue analogue (NiFe-PBA)-F catalyst with a multilevel structure was obtained from NiFe-PBAs via a fluorination strategy, which presents an ultralow OER overpotential of 190 mV at 10 mA cm-2 in alkaline solution, with a small Tafel slope of 57 mV dec-1 and excellent stability. Interestingly, surface fluorination engineering could achieve a controllable removal of ligands of the cyan group, contributing to keep the framework structure of NiFe-PBAs. Particularly, NiFe-PBAs-F undergoes a dramatic reconstruction with the dynamic migration of F ions, which creates more active sites of F-doped NiFeOOH and affords more favorable adsorption of oxygen intermediates. Density functional theory calculations suggest that F doping increases the state density of Ni 3d orbital around the Fermi level, thus improving the conductivity of NiFeOOH. Furthermore, based on our experimental results, the lattice oxygen oxidation mechanism for NiFe-PBAs-F was proposed. Our work not only provides a new pathway to realize the controllable pyrolysis of NiFe-PBAs but also gives more insights into the reconstruction and the mechanism for the OER process.