Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.

Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles.

Alkaline water splitting is an attractive method for sustainable hydrogen production. Owing to the sluggish kinetics of alkaline water reduction and oxidation, it is crucial to understand the mechanism of the hydrogen evolution reaction/oxygen evolution reaction (HER/OER) and construct efficient electrocatalysts based on the structure–activity relationship. This review describes the fundamentals of the alkaline HER and OER, the design of noble and nonnoble HER electrocatalysts with low energy barriers, OER electrocatalysts based on binding energy, electronic structure, lattice oxygen, and surface reconstruction as well as the recent developments of bifunctional HER/OER electrocatalysts. Future perspectives towards alkaline water splitting electrocatalysts are also proposed.

Recent progress made in the mechanism comprehension and design of electrocatalysts for alkaline water splitting

As the key of hydrogen economy, electrocatalytic hydrogen evolution reactions (HERs) depend on the availability of cost-efficient electrocatalysts. Over the past years, there is a rapid rise in noble-metal-free electrocatalysts. Among them, transition metal carbides (TMCs) are highlighted due to their structural and electronic merits, e.g., high conductivity, metallic band states, tunable surface/bulk architectures, etc. Herein, representative efforts and progress made on TMCs are comprehensively reviewed, focusing on the noble-metal-like electronic configuration and the relevant structural/electronic modulation. Briefly, specific nanostructures and carbon-based hybrids are introduced to increase active-site abundance and to promote mass transportation, and heteroatom doping and heterointerface engineering are encouraged to optimize the chemical configurations of active sites toward intrinsically boosted HER kinetics. Finally, a perspective on the future development of TMC electrocatalysts is offered. The overall aim is to shed some light on the exploration of emerging materials in energy chemistry.

Structural Design and Electronic Modulation of Transition-Metal-Carbide Electrocatalysts toward Efficient Hydrogen Evolution.

Single Atoms and Clusters Based Nanomaterials for Hydrogen Evolution, Oxygen Evolution Reactions, and Full Water Splitting

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=4061&context=aiimpapers

Elaborately assembled core-shell structured metal sulfides as a bifunctional catalyst for highly efficient electrochemical overall water splitting

To explore high-performance electrocatalysts, electronic regulation on active sites is essentially demanded. Herein, we propose controlled phosphorus doping to effectively modify the electronic configuration of nanostructured Mo2C, accomplishing a benchmark performance of noble-metal-free electrocatalysts in the hydrogen evolution reaction (HER). Employing MoOx–phytic acid–polyaniline hybrids with tunable composition as precursors, a series of hierarchical nanowires composed of phosphorus-doped Mo2C nanoparticles evenly integrated within conducting carbon (denoted as P-Mo2C@C) are successfully obtained via facile pyrolysis under inert flow. Remarkably, P-doping into Mo2C can increase the electron density around the Fermi level of Mo2C, leading to weakened Mo–H bonding toward promoted HER kinetics. Further density functional theory calculations show that the negative hydrogen-binding free energy (ΔGH*) on pristine Mo2C gradually increases with P-doping due to electron transfer and steric hindrance by P on the Mo2C surface, indicating the effectively weakened strength of Mo–H. With optimal doping, a ΔGH* approaching 0 eV suggests a good balance between the Volmer and Heyrovsky/Tafel steps in HER kinetics. As expected, the P-Mo2C@C nanowires with controlled P-doping (P: 2.9 wt%) deliver a low overpotential of 89 mV at a current density of −10 mA cm−2 and striking kinetic metrics (onset overpotential: 35 mV, Tafel slope: 42 mV dec−1) in acidic electrolytes, outperforming most of the current noble-metal-free electrocatalysts. Elucidating feasible electronic regulation and the remarkably enhanced catalysis associated with controlled P-doping, our work will pave the way for developing efficient noble-metal-free catalysts via rational surface engineering.

Phosphorus-Mo2C@carbon nanowires toward efficient electrochemical hydrogen evolution: composition, structural and electronic regulation

Molybdenum carbide (MoCx) has generated mounting attentions as noble-metal-free electrocatalysts for hydrogen evolution reaction (HER) Herein, triggered by the strategy of organic-inorganic hybrid oriented transformation in nature, we attempt to prepare a diatomite-type mesostructured MoCx electrocatalyst via the solid-state reaction between MoOx/DMF hybrid precursor and β-cyclodextrin Benefiting from the occurrence of the designed individual “mesoskeleton-forming” and “MoCx-generating” processes during pyrolysis, we successfully harvest the targeted electrocatalyst with mesostructural skeleton composed of inter-connected MoCx nanoparticles (∼20 nm) and nanorod-like morphology inheriting from its hybrid precursor This crosslinked mesostructure can not only greatly decrease the diffusion resistance of reactants/products and promote the exposure of active sites, but also efficiently decrease the contact electrical resistance between neighboring MoCx nanoparticles Importantly, the heterostructure of Mo2C and MoC on the nanorod is tunable in the synthesis process, which is proved to be favorable for the promoted HER performance in both acid and alkaline media The strategy for synthesizing such nanostructures may open up new avenues for developing mesoskeletal materials

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/celc.201700756

Mesoporous and Skeletal Molybdenum Carbide for Hydrogen Evolution Reaction: Diatomite-type Structure and Formation Mechanism

Molybdenum carbides are widely adopted as efficient noble-metal-free electrocatalysts for hydrogen evolution reaction (HER). Their surface oxidation is inevitable under air but is often ignored, which can be detrimental for the accurate understanding and precise design of stable carbide catalysts. Herein, molybdenum carbide nanoladders are derived via pyrolyzing MoOx/amine hybrid precursors, and two types of impact of surface oxidation on their HER performance are obvious: (1) the downshift of d-band center in Mo, and (2) the introduced steric hindrance from surface oxygen. Consequently, the negative hydrogen-binding free energy (ΔGH*, −0.52 eV) of pristine MoC gradually increases to positive values with surface oxidation, indicating the effectively weakened strength of Mo−H. With moderate surface oxygen incorporation, a ΔGH* of almost 0 eV is in accordance with the efficiently promoted HER performance via a good balance of Volmer and Heyrovsky/Tafel steps in kinetics. The optimal catalyst supported on a Ti plate (1 cm2) gives a high hydrogen production rate of 0.35 mL min−1 at 300 mV with high stability over the self-made water-splitting configuration. This work does not only make us reacquaint the surface oxidation of carbides on electrocatalysis but also paves the way for developing efficient and stable catalysts via surface engineering.

Organic−Inorganic-Hybrid-Derived Molybdenum Carbide Nanoladders: Impacts of Surface Oxidation for Hydrogen Evolution Reaction

The invention belongs to the technical field of electrocatalytic hydrogen production, and in particular relates to a nitrogen-doped graphite-loaded phosphorus-doped molybdenum carbide nanowire electrocatalytic hydrogen production catalyst and a preparation method thereof The catalyst has a rough-surface nanowire structure with a diameter of 100-200nm on the whole, the particle size distribution of an active centre beta-Mo2C is 2-10nm, and beta-Mo2C particles are closely embedded on the surface of nitrogen-doped graphite, phosphorus is evenly doped into a beta-Mo2C structure by means of a phosphorus-molybdenum bond The nitrogen-doped graphite is generated in place in a high-temperature carbonation process of polyaniline in MoOx-phytic acid-polyaniline hybrid precursor The catalyst has very high electrocatalytic hydrogen production activity and stability under the acidic condition; the preparation method of the catalyst is simple in operation, high in adjustability, low in raw material price and low in production process risk, thus being suitable for large scale production and industrial hydrogen production by using electrolysis of water

Nitrogen-doped graphite-loaded phosphorus-doped molybdenum carbide nanowire electrocatalytic hydrogen production catalyst and preparation method thereof

https://chemistry-europe.onlinelibrary.wiley.com/doi/pdf/10.1002/celc.201700757

Front Cover: Mesoporous and Skeletal Molybdenum Carbide for Hydrogen Evolution Reaction: Diatomite‐Type Structure and Formation Mechanism (ChemElectroChem 9/2017)

Bolun Liu

Papers

Phosphorus-Mo2C@carbon nanowires toward efficient electrochemical hydrogen evolution: composition, structural and electronic regulation

Mesoporous and Skeletal Molybdenum Carbide for Hydrogen Evolution Reaction: Diatomite-type Structure and Formation Mechanism

Organic−Inorganic-Hybrid-Derived Molybdenum Carbide Nanoladders: Impacts of Surface Oxidation for Hydrogen Evolution Reaction

Nitrogen-doped graphite-loaded phosphorus-doped molybdenum carbide nanowire electrocatalytic hydrogen production catalyst and preparation method thereof

Front Cover: Mesoporous and Skeletal Molybdenum Carbide for Hydrogen Evolution Reaction: Diatomite‐Type Structure and Formation Mechanism (ChemElectroChem 9/2017)