Spontaneously separated intermetallic Co3Mo from nanoporous copper as versatile electrocatalysts for highly efficient water splitting

Nanoporous Intermetallic Cu3 Sn/Cu Hybrid Electrodes as Efficient Electrocatalysts for Carbon Dioxide Reduction

Designing highly selective and cost-effective electrocatalysts toward electrochemical carbon dioxide (CO₂ ) reduction is crucial for desirable transformation of greenhouse gas into fuels or high-value chemical products. Here, the authors report intermetallic Cu₃ Sn that is in situ formed and seamlessly integrated on self-supported bimodal nanoporous Cu skeleton (Cu₃ Sn/Cu) via a spontaneous alloying of Sn and Cu as robust electrocatalyst for selective electroreduction of CO₂ to CO. By virtue of Sn atoms strengthening CO adsorption on Cu atoms, the intermetallic Cu₃ Sn has an intrinsic activity of ≈10.58 μA cm^-2 , more than 80-fold higher than that of monometallic Cu. By virtue of hierarchical bicontinuous nanoporous Cu architecture facilitating electron transfer and CO₂ and proton mass transport and offering high specific surface areas for full use of electroactive Cu₃ Sn sites, the nanoporous Cu₃ Sn/Cu hybrid electrodes produce CO at a low overpotential of 0.09 V, and exhibit high partial current density of ≈15 mA cm^-2 _geo at overpotential of 0.59 V, along with excellent stability and selectivity of 91.5% Faradaic efficiency. The outstanding electrochemical performance make them attractive alternatives to precious Au- and Ag-based electrocatalysts for building low-cost CO₂ electrolyzers to selectively produce CO.

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.

Synergistic Interfacial and Doping Engineering of Heterostructured NiCo(OH)x-CoyW as an Efficient Alkaline Hydrogen Evolution Electrocatalyst

To achieve high efficiency of water electrolysis to produce hydrogen (H₂), developing non-noble metal-based catalysts with considerable performance have been considered as a crucial strategy, which is correlated with both the interphase properties and multi-metal synergistic effects. Herein, as a proof of concept, a delicate NiCo(OH)_x-Co_yW catalyst with a bush-like heterostructure was realized via gas-template-assisted electrodeposition, followed by an electrochemical etching-growth process, which ensured a high active area and fast gas release kinetics for a superior hydrogen evolution reaction, with an overpotential of 21 and 139 mV at 10 and 500 mA cm^-2, respectively. Physical and electrochemical analyses demonstrated that the synergistic effect of the NiCo(OH)_x/Co_yW heterogeneous interface resulted in favorable electron redistribution and faster electron transfer efficiency. The amorphous NiCo(OH)_x strengthened the water dissociation step, and metal phase of CoW provided sufficient sites for moderate H immediate adsorption/H₂ desorption. In addition, NiCo(OH)_x-Co_yW exhibited desirable urea oxidation reaction activity for matching H₂ generation with a low voltage of 1.51 V at 50 mA cm^-2. More importantly, the synthesis and testing of the NiCo(OH)_x-Co_yW catalyst in this study were all solar-powered, suggesting a promising environmentally friendly process for practical applications.

Sulfur defect rich Mo-Ni3S2 QDs assisted by O-C[double bond, length as m-dash]O chemical bonding for an efficient electrocatalytic overall water splitting

Developing earth-abundant and highly efficient electrocatalysts is critical for further development of a system. The metal (M) doping strategy and inorganic/organic composite are two common strategies to improve the performance of electrocatalysts for overall water splitting (OWS). In this paper, two strategies are subtly used to prepare Mo-Ni₃S₂ quantum dots (QDs) with rich sulfur defects through Moⁿ⁺ doping Ni₃S₂ and introduction of trisodium citrate by a two-step hydrothermal reaction. Results show that high sulfur defects can be controllably prepared as the lattice mismatch and active sites can be efficiently increased via Moⁿ⁺ doping. Moreover, the introduction of trisodium citrate with carboxyl functional groups not only enhances the degree of sulfur defects around the metal center, changes the morphology of sulfide to distribute the active centers evenly, but also endow the metal center with strong valence changing ability with organic characteristics. The in situ Raman study reveals that O-C[double bond, length as m-dash]O promotes the formation of the real active site M-OOH by the way of self-sacrifice during the OER process. Mo-Ni₃S₂ QDelectrocatalyst shows excellent performance in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), achieving a current density of 10 mA cm^-2 at the overpotentials of 115 mV and 222 mV with very good chemical stability, superior than that of most of the reported materials. The OWS reaction can provide a current density of 10 mA cm^-2 and 50 mA cm^-2, which only needs 1.53 V and 1.74 V with excellent industrial application prospects.

Three-Dimensional Flower-like Fe, C-Doped-MoS2/Ni3S2 Heterostructures Spheres for Accelerating Electrocatalytic Oxygen and Hydrogen Evolution

The exploration of high-efficiency bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has long been challenging. The rational design of a catalyst by constructing heterostructures and a doping element are possibly expected to achieve it. Herein, the utilization of flower-like Fe/C-doped-MoS2/Ni3S2-450 spherical structural materials for electrocatalytic HER and OER is introduced in this study. The carboxyferrocene-incorporated molybdenum sulfide/nickel sulfide (MoySx/NiS) nanostructures were prepared by solvothermal method. After annealing, the iron and carbon elements derived from ferrocenecarboxylic acid enhanced the electrical transport performance and provided rich electronic sites for HER and OER in alkaline media. Specifically, the optimized flower-like Fe/C-doped-MoS2/Ni3S2-450 exhibited efficient bifunctional performance in alkaline electrolyte, with low overpotentials of 188 and 270 mV required to deliver a current density of 10 mA cm−2 for HER and OER, respectively. This work provides valuable insights for the rational design of energy storage and conversion materials by the incorporation of transition metal and carbon elements into metal sulfide structures utilizing metallocene.

Tip-Enhanced Electric Field: A New Mechanism Promoting Mass Transfer in Oxygen Evolution Reactions

The slow kinetics of oxygen evolution reaction (OER) causes high power consumption for electrochemical water splitting. Various strategies have been attempted to accelerate the OER rate, but there are few studies on regulating the transport of reactants especially under large current densities when the mass transfer factor dominates the evolution reactions. Herein, Ni_x Fe_{1- x} alloy nanocones arrays (with ≈2 nm surface NiO/NiFe(OH)₂ layer) are adopted to boost the transport of reactants. Finite element analysis suggests that the high-curvature tips can enhance the local electric field, which induces an order of magnitude higher concentration of hydroxide ions (OH^- ) at the active sites and promotes intrinsic OER activity by 67% at 1.5 V. Experimental results show that a fabricated NiFe nanocone array electrode, with optimized alloy composition, has a small overpotential of 190 mV at 10 mA cm^-2 and 255 mV at 500 mA cm^-2 . When calibrated by electrochemical surface area, the nanocones electrode outperforms the state-of-the-art OER electrocatalysts. The positive effect of the tip-enhanced local electric field in promoting mass transfer is also confirmed by comparing samples with different tip curvature radii. It is suggested that this local field enhanced OER kinetics is a generic effect to other OER catalysts.

RhSe2 : A Superior 3D Electrocatalyst with Multiple Active Facets for Hydrogen Evolution Reaction in Both Acid and Alkaline Solutions

Layered 2D materials are a vital class of electrocatalys for the hydrogen evolution reaction (HER), due to their large area, excellent activity, and facile fabrication. Theoretical caculations domenstrate, however, that only the edges of the 2D nanosheets act as active sites, while the much larger basal plane exhibits passive activity. Here, from a distinguishing perspective, RhSe₂ is reported as a "3D" electrocatalyst for HER with top-class activity, synthesized by a facile solid-state method. Superior to 2D materials, multiple crystal facets of RhSe₂ exhibit near-zero free energy change of hydrogen adsorption (ΔG_H ), which guarantees high performance in most common morphologies. Density functional theory calculations reveal that the low-coordinated Rh atoms act as the active sites in acid, which enables the modified Kubas-mediated pathway, while the Se atoms act as the active sites in an alkaline medium. The overpotentials of HER activity of RhSe₂ are measured to be 49.9 and 81.6 mV at 10 mA cm^-2 in acid and alkaline solutions, respectively. This work paves the way to new transition metal chalcogenide catalysts.

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.

Engineering active sites on hierarchical transition bimetal oxides/sulfides heterostructure array enabling robust overall water splitting

Rational design of the catalysts is impressive for sustainable energy conversion. However, there is a grand challenge to engineer active sites at the interface. Herein, hierarchical transition bimetal oxides/sulfides heterostructure arrays interacting two-dimensional MoO_x/MoS₂ nanosheets attached to one-dimensional NiO_x/Ni₃S₂ nanorods were fabricated by oxidation/hydrogenation-induced surface reconfiguration strategy. The NiMoO_x/NiMoS heterostructure array exhibits the overpotentials of 38 mV for hydrogen evolution and 186 mV for oxygen evolution at 10 mA cm^-2, even surviving at a large current density of 500 mA cm^-2 with long-term stability. Due to optimized adsorption energies and accelerated water splitting kinetics by theory calculations, the assembled two-electrode cell delivers the industrially relevant current densities of 500 and 1000 mA cm^-2 at record low cell voltages of 1.60 and 1.66 V with excellent durability. This research provides a promising avenue to enhance the electrocatalytic performance of the catalysts by engineering interfacial active sites toward large-scale water splitting.

The Synergistic Effects of Alloying on the Performance and Stability of Co3Mo and Co7Mo6 for the Electrocatalytic Hydrogen Evolution Reaction

Metal alloys have become a ubiquitous choice as catalysts for electrochemical hydrogen evolution in alkaline media. However, scarce and expensive Pt remains the key electrocatalyst in acidic electrolytes, making the search for earth-abundant and cheaper alternatives important. Herein, we present a facile and efficient synthetic route towards polycrystalline Co3Mo and Co7Mo6 alloys. The single-phased nature of the alloys is confirmed by X-ray diffraction and electron microscopy. When electrochemically tested, they achieve competitively low overpotentials of 115 mV (Co3Mo) and 160 mV (Co7Mo6) at 10 mA cm−2 in 0.5 M H2SO4, and 120 mV (Co3Mo) and 160 mV (Co7Mo6) at 10 mA cm−2 in 1 M KOH. Both alloys outperform Co and Mo metals, which showed significantly higher overpotentials and lower current densities when tested under identical conditions, confirming the synergistic effect of the alloying. However, the low overpotential in Co3Mo comes at the price of stability. It rapidly becomes inactive when tested under applied potential bias. On the other hand, Co7Mo6 retains the current density over time without evidence of current decay. The findings demonstrate that even in free-standing form and without nanostructuring, polycrystalline bimetallic electrocatalysts could challenge the dominance of Pt in acidic media if ways for improving their stability were found.

Interface-tuned Mo-based nanospheres for efficient oxygen reduction and hydrogen evolution catalysis

Developing earth-abundant materials for efficient oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) catalysis in both alkaline and acidic media is of significance for hydrogen fuel cell application.

Coralline-like CoP3@Cu as an efficient electrocatalyst for the hydrogen evolution reaction in acidic and alkaline solutions

CoP₃@Cu/Cu exhibited excellent catalytic activity and stability in acidic and alkaline media.