Ultraeven Mo-Doped CoP Nanocrystals as Bifunctional Electrocatalyst for Efficient Overall Water Splitting

Dynamic Electrodes Enhanced Electrocatalytic Hydrogen Evolution Performance of Two-Dimensional Materials

Hydrogen energy, known for its elevated combustion enthalpy and the generation of clean water upon combustion, represents a clean energy source with valuable potential applications. Water electrolysis for hydrogen production has emerged as an effective and environmentally friendly green hydrogen synthesis methodology. However, the conventional process of water electrolysis is typically performed under constant current or constant potential conditions, resulting in less-than-ideal hydrogen production rates due to limitations in mass transport. The adjustment of the catalyst interface and enhancement of mass transfer are achievable with dynamic electrodes. Herein, the electrocatalytic hydrogen evolution performance using MoS2 as a model catalyst with dynamic electrodes was investigated. The electrocatalytic hydrogen evolution performance of MoS2 can be enhanced by using dynamic electrodes, achieving a maximum increase of 240% in the hydrogen production rate. Improved electrocatalytic performance for hydrogen evolution can be observed when employing other two-dimensional materials as dynamic electrodes, including Pt-MoS2 and Mo2C. Pt-MoS2 demonstrates the most significant enhancement in the hydrogen evolution rate (400% enhancement). Through mechanistic analysis, the essence of enhancing electrocatalytic performance for hydrogen evolution lies in the bubbles effective separation and varied electrochemical double layer to facilitate mass transport. This work provides an effective method for enhancing the water electrolysis activity using the dynamic electrode.

Deciphering the Underlying Mechanism of the Fourth Entity in Medium-Entropy NiCoFeMP toward Boosting Oxygen Evolution Electrocatalysis

High-/medium-entropy materials have been explored as promising electrocatalysts for water splitting due to their unique physical and chemical properties. Unfortunately, state-of-the-art materials face the dilemma of explaining the enhancement mechanism, which is now limited to theoretical models or an unclear cocktail effect. Herein, a medium-entropy NiCoFeMnP with an advanced hierarchical particle-nanosheet-tumbleweed nanostructure has been synthesized via simple precursor preparation and subsequent phosphorization. Evaluated as the electrocatalyst for oxygen evolution reaction (OER), the medium-entropy NiCoFeMnP displays a lower overpotential of 272 mV at a current density of 10 mA cm-2, and more favorable kinetics than the binary NiFeP, ternary NiCoFeP, quaternary NiCoFeCuP and NiCoFeCrP counterparts, and other reported high-/medium-entropy electrocatalysts. Careful experimental analyses reveal that the incorporation of Mn can significantly regulate the electronic structure of Ni, Co, and Fe sites. More importantly, the Mn introduction and entropy stabilization effect in the reconstructed metal (oxy)hydroxide simultaneously promote the lattice oxygen mechanism, improving the activity. This work sheds new light on the design of high-/medium-entropy materials from an in-depth understanding of the underlying mechanism for improving energy conversion efficiency.

Supported nano-sized precious metal catalysts for oxidation of catalytic volatile organic compounds

Volatile organic compounds (VOCs) are common contaminants found as indoor as well as outdoor pollutants, which can induce acute or chronic health hazards to the human physiological system. The catalytic oxidation method is widely considered as one of the effective methods for removing VOCs, and the development of highly effective catalysts is highly urgent for booming this interesting field. This review focuses on the recent progress of VOC oxidation catalyzed by supported nano-sized precious metal catalysts, and discusses the effects of metal composition, supports, size, and morphology on the catalytic activity. In addition, the roles played by both nano-sized precious metals and supports in enhancing the performance of catalytic VOCs are also systematically discussed, which will guide the further development of more advanced VOC catalysts.

Heteroatom Doping Promoting CoP for Driving Water Splitting

CoP nanomaterials have been extensively regarded as one of the most promising electrocatalysts for overall water splitting due to their unique bifunctionality. Although the great promise for future applications, some important issues should also be addressed. Heteroatom doping has been widely acknowledged as a potential strategy for improving the electrocatalytic performance of CoP and narrowing the gap between experimental study and industrial applications. Recent years have witnessed the rapid development of heteroatom-doped CoP electrocatalysts for water splitting. Aiming to provide guidance for the future development of more effective CoP-based electrocatalysts, we herein organize a comprehensive review of this interesting field, with the special focus on the effects of heteroatom doping on the catalytic performance of CoP. Additionally, many heteroatom-doped CoP electrocatalysts for water splitting are also discussed, and the structure-activity relationship is also manifested. Finally, a systematic conclusion and outlook is well organized to provide direction for the future development of this interesting field.

n-Si/SiOx /CoOx -Mo Photoanode for Efficient Photoelectrochemical Water Oxidation

Green hydrogen is considered to be the key for solving the emerging energy and environmental issues. The photoelectrochemical (PEC) process for the production of green hydrogen has been widely investigated because solar power is clean and renewable. However, mass production in this way is still far away from reality. Here, a Si photoanode is reported with CoOx as co-catalyst for efficient water oxidation. It is found that a high photovoltage of 350 mV can be achieved in 1.0 m K3 BO3 . Importantly, the photovoltage can be further increased to 650 mV and the fill factor of 0.62 is obtained in 1.0 m K3 BO3 by incorporating Mo into CoOx . The Mo-incorporated photoanode is also highly stable. It is shown that the incorporation of Mo can reduce the particle size of co-catalyst on the Si surface, improve the particle-distribution uniformity, and increase the density of particles, which can effectively enhance the light absorption and the electrochemical active surface area. Importantly, the Mo-incorporation results in high energy barrier in the heterojunction. All of these factors are attributed to improved the PEC performance. These findings may provide new strategies to maximize the solar-to-fuel efficiency by tuning the co-catalysts on the Si surface.

Construct of an Electrodeposited Cobalt-Molybdenum Film and Evaluation of Its Efficiency in Hydrogen Evolution

Hydrogen is a valuable clean energy source, and electrolysis to produce hydrogen from water is a crucial component. However, a major problem of hydrogen generation by electrolysis is its large overpotential and poor economics. To reduce the overpotential, we mainly use nickel foam and Co-Mo ions as feedstock and create an efficient catalytic material by electrodeposition. The Co-Mo interaction improves the current efficiency. In 1 mol/L NaOH solution, the overpotential of the Co-Mo-NF composites was low when the current density is -10 mA/cm², with the best value reaching 45.3 mV, which is less than those of Co-NF (94.4 mV) and Mo-NF (88.2 mV). All deposits had similar Tafel slopes in the 77.9 mV/decade range. The catalyst does not just have a favorable effect on hydrogen formation but also has a surprisingly high double-layer capacitance (up to 180 mF/cm²) and good stability. This research provides an impactful approach for developing a non-precious metal HER catalyst for industrial hydrogen production.

Heterogeneous Fe-Doped NiCoP-MoO3 Efficient Electrocatalysts for Overall Water Splitting

Transition metal phosphides with excellent performance are one of the effective alternatives to noble metal catalysts in overall water splitting. In this paper, the Fe-NiCoP-MoO3 composite was prepared by a facile synthesis as the bifunctional electrocatalyst. Fe-NiCoP-MoO3 achieves an operating current density of 10 mA/cm2 at a low overpotential of 65 mV for hydrogen evolution reaction and drives an operating current density of 50 mA/cm2 at only 293 mV for oxygen evolution reaction. Significantly, Fe-NiCoP-MoO3 was employed as the anode and cathode for overall water splitting, which only requires a cell voltage of 1.586 V to reach 10 mA/cm2 as well as shows excellent stability. The electrocatalytic activity of Fe-NiCoP-MoO3 exceeds most of the recently reported typical bifunctional electrocatalysts. This may be due to the coupling effect between the polymetallic phosphides. In addition, heterogeneous catalysts generally expose more active sites than homogeneous catalysts. In addition, replacing MoO3 with WO3 and VO3 can also improve the performance of Fe-NiCoP. This work provides an idea for the modification of phosphides.

Influence of Nitrogen-Doped Carbon Quantum Dots on the Electrocatalytic Performance of the CoP Nanoflower Catalyst for OER

Cobalt phosphides modified by nitrogen-doped carbon quantum dots (CoP-NCQDs) were successfully constructed by a facile and low-cost hydrothermal treatment, which is expected to replace traditional noble-metal oxygen evolution reaction electrode materials. Detailed experiments and findings show that nitrogen-doped carbon quantum dots (NCQDs) have a significant impact on the morphology of the CoP catalyst, and nitrogen doping can regulate the surface-active sites to obtain the catalyst with abundant structural defects. Simultaneously, nitrogen doping can regulate the content of pyridinic N and pyrrolic N, which exerts positive effects on the formation of the bond structure and electron conduction between NCQDs and CoP.

Layered Porous Graphitic Carbon Nitride Stabilized Effective Co2SnO4 Inverse Spinel as a Bifunctional Electrocatalyst for Overall Water Splitting

Developing an efficient, low-cost, and non-noble metal oxide-based nanohybrid material for overall water splitting is a highly desirable approach to promote clean energy harnessing and to minimize environmental issues. Accordingly, we proposed an interfacial engineering approach to construct layered porous graphitic carbon nitride (g-C₃N₄)-stabilized Co₂SnO₄ inverse spinel nanohybrid materials as highly active bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline medium. Here, a Co₂SnO₄/g-C₃N₄ nanohybrid with a layered porous g-C₃N₄ stabilized cubelike inverse spinel has been synthesized with an enhanced surface area via a simple one-pot hydrothermal method. Besides, detailed structural and morphological characterizations were carried out using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission-scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), Fourier transform infrared (FT-IR), and Brunauer-Emmett-Teller (BET) analysis. Briefly, XPS analysis has revealed the existence of a strong coupling bond at the interface between a definite proportion of g-C₃N₄ nanosheets and the inverse spinel, which act as an electron transport channel to explore the exceptional performances for HER and OER. Compared to the Co₂SnO₄ inverse spinel lattice or g-C₃N₄ nanosheets, the prepared Co₂SnO₄/g-C₃N₄ nanohybrid-loaded 316 SSL mesh electrode showed excellent and stable electrocatalytic performances with very low overpotentials of 41 mV for HER and 260 mV for OER to reach the current density of 10 mA cm^-2. To understand the electrocatalytic phenomena, the faradic efficiency was calculated for the prepared bifunctional electrocatalyst as 96%, which effectively would favor water electrolysis. Accordingly, the Co₂SnO₄/g-C₃N₄ nanohybrid-loaded electrodes were constructed, and the minimum cell voltage was found to be 1.52 V to reach the current density of 10 mA cm^-2, which is comparable to the standard RuO₂∥Pt/C in two-electrode systems. Thus, the developed nanohybrid-based electrocatalyst could be an alternative to noble metal-centered systems for highly efficient overall water splitting.

Porous Ni3S2-Co9S8 Carbon Aerogels Derived from Carrageenan/NiCo-MOF Hydrogels as an Efficient Electrocatalyst for Oxygen Evolution in Rechargeable Zn-Air Batteries

Herein, we fabricate N-doped porous Ni₃S₂-Co₉S₈/carbon aerogels (Ni₃S₂-Co₉S₈/NCAs) using carrageenan/NiCo-metal-organic framework (MOF) hydrogels as the precursor via the high-temperature carbonization route with excellent electrocatalytic properties for the oxygen evolution reaction (OER). The electrochemical measurements indicate that the Ni₃S₂-Co₉S₈/NCA as a quintessential electrocatalyst exhibits excellent OER performance, which has outperformed most transition metal sulfide (TMS) catalysts in alkaline environments, as attested with a lower overpotential of 337 mV at 10 mA cm^-2 and a smaller Tafel slope of 77 mV dec^-1. Meanwhile, a Zn-air battery based on Ni₃S₂-Co₉S₈/NCA + Pt/C achieves a large power density of up to 256 mW cm^-2 (and 193 mW cm^-2), small charge/discharge voltage gap, and good cycling stability, notably better than the conventional RuO₂ + Pt/C-based Zn-air batteries. These excellent electrocatalytic properties are mainly attributed to the distinct hierarchical porous structure and interfacial synergy between the Ni₃S₂ and Co₉S₈ nanoparticle structure with rich defects, facilitating the mass transport and high graphitization degree beneficial for electron mobility. It is envisioned that the research provides a novel approach for the exploration of marine biomass as an electrocatalyst.

ZIF-67-derived nanoframes as efficient bifunctional catalysts for overall water splitting in alkaline medium

In order to lower energy consumption it is critical to develop highly efficient and stable non-precious metal bifunctional catalysts. In this study, we found that rational design of novel nanostructures is able to increase the number of active sites, conductivity and accelerate electron transfer, thus promoting enhanced performance of the catalyst. We successfully synthesized carbon nanotubes (CNTs) containing a hollow polyhedral (CNTHPs) structure through annealing, etching and phosphating. The unique hollow shape not only provides a stable structure but also facilitates mass and charge transfer. Thus, CoP/CNTHPs can catalyze the hydrogen and oxygen evolution reactions effectively with overpotentials of 147 and 238 mV at 10 mA cm^-2. Simultaneously, CoP/CNTHPs only needs a voltage of 1.54 V to attain a current density of 10 mA cm^-2 in the electrocatalytic water splitting process, demonstrating its bifunctional activity. Furthermore, the electrolytic catalytic performance does not weaken significantly after 12 hours of electrolysis, demonstrating excellent stability. Overall, this research offers useful insights into rational design of high-performance non-noble metal catalysts.

Two-Dimensional Metal-Organic Frameworks as Ultrahigh-Performance Electrocatalysts for the Fuel Cell Cathode: A First-Principles Study

Except for metal-organic frameworks (MOFs) with traditional metal-nitrogen sites, MOFs with metal-oxygen sites may also possess good oxygen reduction reaction (ORR) catalytic activity due to their unique electronic structures. Herein, using density functional theory methods, the ORR performances of a series of M₃(HHTT)₂ (where M is a 3d, 4d, or 5d transition metal and HHTT is 2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene)) catalysts are explored. The binding energy (ΔE_species) results suggest that the binding energy of *OH (ΔE_*OH) shows a good linear relationship with the binding energies of *O and *OOH (ΔE_*O and ΔE_*OOH, respectively), indicating that ΔE_*OH can serve as a descriptor to reflect the catalytic activity of M₃(HHTT)₂. In addition, the volcano plot suggests that M₃(HHTT)₂ catalysts with a moderate binding strength of the intermediate *OH (0.6 eV < ΔE_*OH < 0.9 eV) show relatively high ORR activity. Therefore, four highly active ORR catalysts are screened out, namely, Fe₃(HHTT)₂, Co₃(HHTT)₂, Rh₃(HHTT)₂, and Ir₃(HHTT)₂, which possess very small overpotentials of 0.35, 0.24, 0.31, and 0.29 V, respectively. Their potential-determining step is the reduction of O₂ to the intermediate *OOH. It is encouraging that the theoretically lowest overpotential of this kind of catalyst is 0.21 V, which is superior to that on Pt(111). Moreover, Co₃(HHTT)₂ has excellent poisoning-tolerance ability for impurity gases (CO, NO, and SO₂) as well as fuel molecules (CH₃OH and HCOOH).

Bifunctional P-Intercalated and Doped Metallic (1T)-Copper Molybdenum Sulfide Ultrathin 2D-Nanosheets with Enlarged Interlayers for Efficient Overall Water Splitting

Metallic (1T) molybdenum disulfide (MoS₂) is a much better electrocatalyst than the semiconducting (2H) MoS₂ because of its superior conductivity, presence of active basal planes, and bulky interlayers. However, the lack of thermodynamic stability has hindered its practical uses. The insertion of transition metals and nonmetals in the interlayers and the crystal is known to improve both the thermodynamic stability and the catalytic efficacy of 1T-MoS₂. In this study, for the first time we have developed an electrocatalyst for water splitting based on metallic copper molybdenum sulfide (1T-CMS). The present catalyst, P-doped and intercalated 1T-CMS ultrathin 2D nanosheets on carbon cloth (P-1T-CMS@CC), demonstrates excellent catalytic efficacy for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). It required an overpotential of 95 mV for HER and of 284 mV for OER at a current density of 10 mA cm^-2. The P-1T-CMS@CC(+ -) device also shows excellent performance, requiring a cell voltage of only 1.51 V at a current density of 10 mA cm^-2.

Heterostructured CoP·CoMoP nanocages as advanced electrocatalysts for efficient hydrogen evolution over a wide pH range

A sustainable and environmental-friendly method to produce hydrogen with high purity is the electrochemical water splitting, but its commercialization is challenged due to lack of cost-effective electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range. Herein, a series of CoP·xCoMoP heterostructured nanocages (NCs) were prepared via a dissolution-regrowth and subsequent phosphorization process using metal-organic frameworks (MOFs) as template. The three-dimensional (3D) architecture of CoP·xCoMoP is constituted by the heterostructured nanosheets composed with CoP and CoMoP phase. These noble-metal-free earth-abundant transition metal phosphide (TMP) catalysts show a pH-universal HER activity with high efficiency. Under the optimal atom ratio of Co and Mo (6:5), CoP·5CoMoP NC catalysts can deliver a current density of 10 mA cm^-2 at the overpotential of 72 mV with a Tafel slope of 60.3 mV dec^-1 in 1.0 M KOH solution. The same current output requires overpotential of 44 mV in 0.5 M H₂SO₄ solution and 151 mV in1.0 M phosphate buffered solution (PBS), respectively. The superior HER activity of CoP·5CoMoP NC catalysts can be comparable to or even better than most of noble metal-free HER electrocatalysts reported recently. In addition, CoP·5CoMoP NC catalysts also show a fairly high HER stability over a wide pH range, and their HER activity can be well kept without significant loss for long-term electrolysis. The 3D CoP·5CoMoP heterostructured catalysts hold promise as efficient and low-cost catalysts for water splitting devices over a wide pH range.

Robust NiCoP@FeP derived from Prussian blue analog for efficient overall water splitting

The design of efficient, stable, and non-noble metal based bifunctional electrocatalysts remains a great challenge. The synergistic effect can adjust the local electronic structure to improve the catalytic performance. In...

MoS2-2xSe2x Nanosheets Grown on Hollow Carbon Spheres for Enhanced Electrochemical Activity

Electrochemical catalysts with high conductivity and low reaction potential are respected. In this paper, hollow carbon spheres (HCSs) were homogeneously coated with Se-doped MoS₂ (MoS_2-2xSe_2x) nanosheets by hydrothermal synthesis. The HCSs reduced the agglomeration of MoS_2-2xSe_2x nanosheets and improved their conductivity. Compared with the MoS₂-modified samples, Se doping increased the interlayer spacing which provided more active catalytic sites and improved the charge transfer. Thus, MoS_2-2xSe_2x-decorated samples revealed enhanced electrocatalytic activity. The composition of MoS_2-2xSe_2x nanosheets was adjusted by changing the ratios of sulfur and selenium precursors. In the case of a Se/S molar ratio of 0.1, the composite of HCS decorated with MoS_2-2xSe_2x nanosheets (C@MoS_2-2xSe_2x) revealed the lowest overpotential and the smallest Tafel slope.