3d Transition metal doping induced charge rearrangement and transfer to enhance overall water-splitting on Ni3S2 (101) facet: a first-principles calculation study

Cost-efficient bifunctional electrocatalysts with good stability and high activity are in great demand to replace noble-metal-based catalysts for overall water-splitting. Ni3S2 has been considered a suitable electrocatalyst for either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER) owing to its good conductivity and stability, but high performance remains a challenge. Based on density functional theory calculations, we propose a practical 3d-transition-metal (TM = Mn, Fe and Co) doping to enhance the catalytic performance for both HER and OER on the Ni3S2 (101) facet. The enhancement originates from TM-doping-induced charge rearrangement and charge transfer, which increases the surface activity and promotes catalytic behavior. In particular, Mn-doped Ni3S2 shows good bifunctional catalytic activity because it possesses more active sites, reduced hydrogen adsorption free energy (ΔG H*) for HER and low overpotential for OER. Importantly, this work not only provides a feasible means to design efficient bifunctional electrocatalysts for overall water-splitting but also provides insights into the mechanism of improving catalytic behavior.

Palladium Nanoparticles Hardwired in Carbon Nanoreactors Enable Continually Increasing Electrocatalytic Activity During the Hydrogen Evolution Reaction

Catalysts typically lose effectiveness during operation, with much effort invested in stabilising active metal centres to prolong their functional lifetime for as long as possible. In this study palladium nanoparticles (PdNP) supported inside hollow graphitised carbon nanofibers (GNF), designated as PdNP@GNF, opposed this trend. PdNP@GNF exhibited continuously increasing activity over 30000 reaction cycles when used as an electrocatalyst in the hydrogen evolution reaction (HER). The activity of PdNP@GNF, expressed as the exchange current density, was always higher than activated carbon (Pd/C), and after 10000 cycles PdNP@GNF surpassed the activity of platinum on carbon (Pt/C). The extraordinary durability and self-improving behaviour of PdNP@GNF was solely related the unique nature of the location of the palladium nanoparticles, that is, at the graphitic step-edges within the GNF. Transmission electron microscopy imaging combined with spectroscopic analysis revealed an orchestrated series of reactions occurring at the graphitic step-edges during electrocatalytic cycling, in which some of the curved graphitic surfaces opened up to form a stack of graphene layers bonding directly with Pd atoms through Pd-C bonds. This resulted in the active metal centres becoming effectively hardwired into the electrically conducting nanoreactors (GNF), enabling facile charge transport to/from the catalytic centres resulting in the dramatic self-improving characteristics of the electrocatalyst.

Improving plasma sprayed Raney-type nickel-molybdenum electrodes towards high-performance hydrogen evolution in alkaline medium

Rationally designed free-standing and binder-free Raney-type nickel-molybdenum (Ni-Mo) electrodes produced via atmospheric plasma spraying (APS) are developed by correlating APS process parameters with the microstructure of electrodes and their electrochemical performance in alkaline media. The results revealed that the electrode morphology and elemental composition are highly affected by the plasma parameters during the electrode fabrication. It is found that increasing plasma gas flow rate and input plasma power resulted in higher in-flight particle velocities and shorter dwell time, which in result delivered electrodes with much finer structure exhibiting homogeneous distribution of phases, larger quantity of micro pores and suitable content of Ni and Mo. Tafel slope of electrodes decreased with increasing the in-flight particles velocities from 71 to 33 mV dec^-1 in 30 wt.% KOH. However, beyond a critical threshold in-flight velocity and temperature of particles, electrodes started to exhibit larger globular pores and consequently reduced catalytic performance and higher Tafel slop of 36 mV dec^-1 in 30 wt.% KOH. Despite slightly lower electrochemical performance, the electrodes produced with highest plasma gas flow and energy showed most inter-particle bonded structure as well as highest stability with no measurable degradation over 47 days in operation as HER electrode in 30 wt.% KOH. The Raney-type Ni-Mo electrode fabricated at highest plasma gas flow rate and input plasma power has been tested as HER electrode in alkaline water electrolyzer, which delivered high current densities of 0.72 and 2 A cm^-2 at 1.8 and 2.2 V, respectively, representing a novel prime example of HER electrode, which can synergistically catalyze the HER in alkaline electrolyzer. This study shows that sluggish alkaline HER can be circumvented by rational electrode composition and interface engineering.

Investigation of the Support Effect in Atomically Dispersed Pt on WO3-x for Utilization of Pt in the Hydrogen Evolution Reaction

Single-atom catalysts (SACs) have attracted growing attention because they maximize the number of active sites, with unpredictable catalytic activity. Despite numerous studies on SACs, there is little research on the support, which is essential to understanding SAC. Herein, we systematically investigated the influence of the support on the performance of the SAC by comparing with single-atom Pt supported on carbon (Pt SA/C) and Pt nanoparticles supported on WO_3-x (Pt NP/WO_3-x ). The results revealed that the support effect was maximized for atomically dispersed Pt supported on WO_3-x (Pt SA/WO_3-x ). The Pt SA/WO_3-x exhibited a higher degree of hydrogen spillover from Pt atoms to WO_3-x at the interface, compared with Pt NP/WO_3-x , which drastically enhanced Pt mass activity for hydrogen evolution (up to 10 times). This strategy provides a new framework for enhancing catalytic activity for HER, by reducing noble metal usage in the field of SACs.

Boosting Alkaline Hydrogen Evolution Activity with Ni-Doped MoS2 /Reduced Graphene Oxide Hybrid Aerogel

Highly active, durable, and cost-effective non-precious-metal-based electrocatalysts are urgently needed to improve the sluggish hydrogen evolution reaction (HER) in an alkaline environment. Herein, a lyophilization/thermolysis method is successfully applied to prepare Ni-doped MoS₂ (Ni-MoS₂ )/reduced graphene oxide (RGO) hybrid aerogels. The MoS₂ aerogel possesses a higher density of exposed active sites than its corresponding bulk material. Inheriting from GO its abundant functional groups during pyrolysis, the RGO aerogel can uniformly disperse MoS₂ and simultaneously maintain excellent conductivity. The incorporation of Ni atoms can accelerate the cleavage of the HO-H bond and enhance the adsorption and desorption of intermediate OH^- . Owing to the synergistic effect of the compositional and structural advantages of aerogels, the Ni-MoS₂ /RGO hybrid aerogel delivers highly promoted HER kinetics in alkaline media. As a result, an optimal η₁₀ (overpotential at 10 mA cm^-2 ) of 168 mV in 1 m KOH is obtained, which is superior to the non-doped MoS₂ /RGO hybrid aerogel (225 mV) and MoS₂ aerogel (263 mV), letting alone bulk MoS₂ (448 mV). Moreover, the η₆₀ (overpotential at 60 mA cm^-2 ) is maintained at 262 mV after chronopotentiometry tests at a constant current density of 10 mA cm^-2 for 24 h, indicating an exceptionally stability of the HER catalyst.

Environmentally Friendly Recycling of Fuel-Cell Membrane Electrode Assemblies by Using Ionic Liquids

The platinum nanoparticles used as the catalyst in proton exchange membrane fuel cells (PEMFCs) represent approximately 46 % of the total price of the cells for a large-scale production, and this is one of the barriers to their commercialization. Therefore, the recycling of the platinum catalyst could be the best alternative to limit the production costs of PEMFCs. The usual recovery routes for spent catalysts containing platinum are pyro-hydrometallurgical processes in which a calcination step is followed by aqua regia treatment, and these processes generate fumes and NO_x emissions, respectively. The electrochemical recovery route proposed here is more environmentally friendly, performed under "soft" temperature conditions, and does not result in any gas emissions. It consists of the coupling of the electrochemical leaching of platinum in chloride-based ionic liquids (ILs), followed by its electrodeposition. The leaching of platinum was studied in pure ILs and in ionic-liquid melts at different temperatures and with different chloride contents. Through the modulation of the composition of the ionic-liquid melts, it is possible to leach and electrodeposit the platinum from fuel-cell electrodes in a single-cell process under an inert or ambient atmosphere.

Porous hollow MoS2 microspheres derived from core–shell sulfonated polystyrene microspheres@MoS2 nanosheets for efficient electrocatalytic hydrogen evolution

Hollow MoS₂ microspheres were constructed using monodisperse sulfonated polystyrene (SPS) as the template, and exhibited excellent HER performance.

Electrochemical Dealloying of PdCu3 Nanoparticles to Achieve Pt-like Activity for the Hydrogen Evolution Reaction

Manipulating the d-band center of the metal surface and hence optimizing the free energy of hydrogen adsorption (ΔG_H ) close to the optimal adsorption energy (ΔG_H =0) for hydrogen evolution reaction (HER), is an efficient strategy to enhance the activity for HER. Herein, we report a oleylamine-mediated (acting as the solvent, stabilizer, and reducing agent) strategy to synthesize intermetallic PdCu₃ nanoparticles (NPs) without using any external reducing agent. Upon electrochemical cycling, PdCu₃ transforms into Pd-rich PdCu (ΔG_H =0.05 eV), exhibiting remarkably enhanced activity (with a current density of 25 mA cm^-2 at ∼69 mV overpotential) as an alternative to Pt for HER. The first-principle calculation suggests that formation of low coordination number Pd active sites alters the d-band center and hence optimal adsorption of hydrogen, leading to enhanced activity. This finding may provide guidelines towards the design and development of Pt-free highly active and robust electrocatalysts.

Co-Doped NiSe nanowires on nickel foam via a cation exchange approach as efficient electrocatalyst for enhanced oxygen evolution reaction

Co-Doped NiSe nanowires supported on nickel foam (NF) were successfully synthesized using NiSe NWs/NF as precursor template in a facile cation exchange approach.

A new stable Pd–Mn3O4 nanocomposite as an efficient electrocatalyst for the hydrogen evolution reaction

A galvanic exchange reaction-mediated one-pot synthesis of Pd–Mn₃O₄ nanocomposites for excellent electrocatalytic activity and stability towards the hydrogen evolution reaction.

Dispersing molecular cobalt in graphitic carbon nitride frameworks for photocatalytic water oxidation

The development of water oxidation catalysts (WOCs) to cooperate with light-energy transducers for solar energy conversion by water splitting and CO2 fixation is a demanding challenge. The key measure is to develop efficient and sustainable WOCs that can support a sustainable photocatalyst to reduce over-potentials and thus to enhance reaction rate of water oxidation reaction. Cobalt has been indentified as active component of WOCs for photo/electrochemical water oxidation, and its performance relies strongly on the contact and adhesion of the cobalt species with photoactive substrates. Here, cobalt is homogeneously engineered into the framework of pristine graphitic carbon nitride (g-C3 N4 ) via chemical interaction, establishing surface junctions on the polymeric photocatalyst for the water oxidation reaction. This modification promotes the surface kinetics of oxygen evolution reaction by the g-C3 N4 -based photocatalytic system made of inexpensive substances, and further optimizations in the optical and textural structure of Co-g-C3 N4 is envisaged by considering ample choice of modification schemes for carbon nitride materials.

Carbon nitride in energy conversion and storage: recent advances and future prospects

With the explosive growth of energy consumption, the exploration of highly efficient energy conversion and storage devices becomes increasingly important. Fuel cells, supercapacitors, and lithium-ion batteries are among the most promising options. The innovation of these devices mainly resides in the development of high-performance electrode materials and catalysts. Graphitic carbon nitride (g-C3 N4 ), due to structural and chemical properties such as semiconductor optical properties, rich nitrogen content, and tunable porous structure, has drawn considerable attention and shown great potential as an electrode material or catalyst in energy conversion and storage devices. This review covers recent progress in g-C3 N4 -containing systems for fuel cells, electrocatalytic water splitting devices, supercapacitors, and lithium-ion batteries. The corresponding catalytic mechanisms and future research directions in these areas are also discussed.

Self-assembly of platinum nanoparticles and coordination-driven assembly with porphyrin

The easily accessible surfaces of Pt nanostructures were demonstrated by two kinds of assembly processes.

Three-dimensional hierarchical frameworks based on MoS₂ nanosheets self-assembled on graphene oxide for efficient electrocatalytic hydrogen evolution

Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this work, three-dimensional (3D) hierarchical frameworks based on the self-assembly of MoS2 nanosheets on graphene oxide were produced via a simple one-step hydrothermal process. The structures of the resulting 3D frameworks were characterized by using a variety of microscopic and spectroscopic tools, including scanning and transmission electron microscopies, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman scattering. Importantly, the three-dimensional MoS2/graphene frameworks might be used directly as working electrodes which exhibited apparent and stable electrocatalytic activity in hydrogen evolution reaction (HER), as manifested by a large cathodic current density with a small overpotential of -107 mV (-121 mV when loaded on a glassy-carbon electrode) and a Tafel slope of 86.3 mV/dec (46.3 mV/dec when loaded on a glassy-carbon electrode). The remarkable performance might be ascribed to the good mechanical strength and high electrical conductivity of the 3D frameworks for fast charge transport and collection, where graphene oxide provided abundant nucleation sites for MoS2 deposition and oxygen incorporation led to the formation of defect-rich MoS2 nanosheets with active sites for HER.

Surface polarization matters: enhancing the hydrogen-evolution reaction by shrinking Pt shells in Pt-Pd-graphene stack structures

Surface charge state plays an important role in tuning the catalytic performance of nanocrystals in various reactions. Herein, we report a synthetic approach to unique Pt-Pd-graphene stack structures with controllable Pt shell thickness. These unique hybrid structures allow us to correlate the Pt thickness with performance in the hydrogen-evolution reaction (HER). The HER activity increases with a decrease in the Pt thickness, which is well explained by surface polarization mechanism as suggested by first-principles simulations. In this hybrid system, the difference in work functions of Pt and Pd results in surface polarization on the Pt surface, tuning its charge state for hydrogen reduction. Meanwhile, the supporting graphene provides two-dimensional channels for efficient charge transport, improving the HER activities. This work opens up possibilities of reducing Pt usage while achieving high HER performance.

Engineering non-sintered, metal-terminated tungsten carbide nanoparticles for catalysis

Transition-metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide-scale use as catalysts. A method is presented for the production of metal-terminated TMC nanoparticles in the 1-4 nm range with tunable size, composition, and crystal phase. Carbon-supported tungsten carbide (WC) and molybdenum tungsten carbide (Mo(x)W(1-x)C) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100-fold higher than commercial WC and within an order of magnitude of platinum-based catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.