Cation vacancy modulated Cu3P-CoP heterostructure electrocatalyst for boosting hydrogen evolution at high current densities and coupling Zn-H2O battery

Exploiting highly efficient, cost-effective and stable electrocatalysts is key to decreasing hydrogen evolution reaction (HER) kinetics energy barrier. Herein, the alkaline HER kinetics energy barrier can greatly reduce by the joint strategies of the cation vacancy and heterostructure engineering, which is seldom explored and remains ambiguous. In this study, an efficient and stable copper foam-supported Cu3P-CoP heterostructure electrocatalyst with cation vacancy defects (defined as Cu3P-CoP-VAl/CF) was designed for HER via the successive coprecipitation, electrodeposition, alkali etching and phosphorization treatments. As anticipated, the as-obtained Cu3P-CoP-VAl/CF electrocatalyst reveals a remarkable catalytic activity for HER with a low overpotential of 205 mV at a current density of 100 mA·cm-2, a high turnover frequency value of 1.05 s-1 at an overpotential of 200 mV and a small apparent activation energy (Ea) of 9 kJ·mol-1, while shows superior long-term stability at large current densities of 100 and 240 mA·cm-2. Systematic experiment and characterization data demonstrate that the formed cation vacancy could optimize the Ea, leading to the decrease of the kinetic barriers of Cu3P-CoP/CF heterostructure, as well as the established heterogeneous interface induced a synergistic effect between biphasic components on boosting the kinetics toward HER. The results of density functional theory disclose that the synergistic effect of Cu3P-CoP heterostructure could decrease the energy barrier and optimize Gibbs free energy of hydrogen adsorption, resulting in the enhancement of intrinsic catalytic activity of Cu3P-CoP-VAl/CF. More significantly, the alkali-cell assembled by Cu3P-CoP-VAl/CF (cathode) and RuO2/CF (anode) behaves outstanding water splitting performance, delivering a current density of 10 mA·cm-2 at a relatively small applied voltage of 1.58 V, along with encouraging long-term durability. In addition, the alkaline Zn-H2O battery with Cu3P-CoP-VAl/CF as the cathode has been fabricated for the simultaneous generation of electricity and hydrogen, which displays a large power density of up to 4.1 mW·cm-2. The work demonstrates that rational strategy for the design of competent electrocatalysts can effectively accelerate the kinetics of HER, which supplies valuable insights for practical applications in overall water splitting.

Facile and Cost-effective Synthesis of CoP@N-doped Carbon with High Catalytic Performance for Electrochemical Hydrogen Evolution Reaction

The manufacture of efficient and low-cost hydrogen evolution reaction (HER) catalysts is regarded as a critical solution to achieve carbon neutrality. Herein, we developed an economical method to synthesize a CoP-anchored N-doped carbon catalyst via one-step pyrolysis using inexpensive starting materials (cobalt ion salt, phytic acid, and glycine). The size of the CoP nanoparticles was controlled by adjusting the Co/P ratio of the catalysts. Nanoscale CoP particles with adequate exposure to active sites were uniformly anchored on the surface of the conductive nitrogen-doped carbon substrate, ensuring the rapid transfer of electrons and species. When Co/P=0.89, the as-made catalyst exhibited outstanding HER activity, with an extraordinarily low overpotential of 202 mV at 10 mA cm-2 and long-term stability.

Sulfidation and NaOH etching in CoFeAl LDH evolved catalysts for an efficient overall water splitting in an alkaline solution

In this study, a hierarchical interconnected porous metal sulfide heterostructure was synthesized from CoFeAl layered double hydroxides (LDHs) by a two-step hydrothermal process (sulfidation and a NaOH etching process). Among the as-made samples, the CoFeAl-T-NaOH electrode exhibited excellent oxygen and hydrogen evolution reaction catalytic activities with overpotentials of 344 mV and 197 mV at the current density of 100 mA cm^-2, respectively. Meanwhile, small Tafel slopes of 57.7 mV dec^-1 and 106.5 mV dec^-1 for water oxidation and hydrogen evolution were observed for the CoFeAl-T-NaOH, respectively. Serving as both the cathode and anode for overall water splitting, the CoFeAl-T-NaOH electrode reached a current density of 10 mA cm^-2 at a cell voltage of 1.65 V with excellent stability. The enhanced electrocatalytic activity could be attributed to: the hierarchical interconnected nanosheet structure facilitating mass transport; the porous structure promoting electrolyte infiltration and reactant transfer; the heterojunction accelerating charge transfer; and the synergistic effect between them. This study offered a new clue for in situ synthesizing porous transition-metal based heterojunction electrocatalysts with a careful tuning of the sequence of sulfuration and alkaline etching to enhance the electrocatalytic performance.

Surface distortion of FeRu nanoparticles improves the hydrogen evolution reaction performance in alkaline media

Rational design of electrocatalysts, including an increased catalytic surface area, a unique surface structure, and improved conductivity, for facilitating the hydrogen evolution reaction (HER) is emerging as an important issue. In this work, we consider the engineering of catalyst surfaces as an effective and feasible way to accelerate the HER kinetics. By etching the surface Fe of FeRu alloy nanoparticles (NPs) using hydrofluoric acid (HF), a distorted catalytic surface of FeRu NPs was formed. The distorted surface of the HF-treated FeRu NPs was successfully analyzed by X-ray absorption spectroscopy, high-resolution photoemission spectroscopy, and electrochemical absorption/desorption experiments. The electrocatalytic HER activity of the HF-treated FeRu NPs demonstrated that surface distortion enhances the water dissociation reaction and the electron transfer rate. As a result, the surface-distorted FeRu NPs improved HER performances in alkaline media compared to the pristine FeRu alloy NP/C, commercial Ru/C, and the state-of-the-art Pt/C catalysts.

Transition Metal Phosphide Nanoarchitectonics for Versatile Organic Catalysis

Transition metal phosphides (TMP) posses unique physiochemical, geometrical, and electronic properties, which can be exploited for different catalytic applications, such as photocatalysis, electrocatalysis, organic catalysis, etc. Among others, the use of TMP for organic catalysis is less explored and still facing many complex challenges, which necessitate the development of sustainable catalytic reaction protocols demonstrating high selectivity and yield of the desired molecules of high significance. In this regard, the controlled synthesis of TMP-based catalysts and thorough investigations of underlying reaction mechanisms can provide deeper insights toward practical achievement of desired applications. This review aims at providing a comprehensive analysis on the recent advancements in the synthetic strategies for the tailored and tunable engineering of structural, geometrical, and electronic properties of TMP. In addition, their unprecedented catalytic potential toward different organic transformation reactions is succinctly summarized and critically analyzed. Finally, a rational perspective on future opportunities and challenges in the emerging field of organic catalysis is provided. On the account of the recent achievements accomplished in organic synthesis using TMP, it is highly anticipated that the use of TMP combined with advanced innovative technologies and methodologies can pave the way toward large scale realization of organic catalysis.

Phosphorus vacancies improve the hydrogen evolution of MoP electrocatalysts

Although molybdenum phosphide (MoP) has attracted increasing attention as an electrocatalyst in the hydrogen evolution reaction (HER), it is still worth exploring an effective approach to further improve the HER activities of MoP. To date, the generation and effect of P vacancies (Pv) on MoP have been rarely investigated for the HER in both alkaline and acidic media and remain unclear. Here, MoP rich in P vacancies (MoP-Pv) was prepared by hydrogen reduction to improve the HER catalytic performances. As a result, the overpotentials of MoP-Pv were 70 mV and 62 mV lower than those of pristine MoP in 1 M KOH and 0.5 M H₂SO₄ electrolytes, respectively. What's more, the TOFs of MoP-Pv were 3.14 s^-1 and 1.19 s^-1 at an overpotential of 200 mV in 1 M KOH and 0.5 M H₂SO₄, respectively, which are 4.1-fold and 2.5-fold higher than those of pristine MoP. Even when compared with other corresponding catalysts, the TOFs of MoP-Pv still ranked at the top. Due to the surface P vacancies, MoP-Pv possesses more electrochemically active sites and faster charge transfer capability, which all favor higher HER catalytic activities. Overall, our work validates a straightforward and vigorous strategy for improving the intrinsic HER catalytic activities of P vacancies, and also provides guidance for the development of vacancy engineering in electrocatalysts.

Carbonate-Hydroxide Induced Metal-Organic Framework Transformation Strategy for Honeycomb-Like NiCoP Nanoplates to Drive Enhanced pH-Universal Hydrogen Evolution

Developing a low-cost, pH-universal electrocatalyst is desirable for electrochemical water splitting but remains a challenge. NiCoP is a promising non-noble hydrogen-evolving electrocatalyst due to its high intrinsic electrical conductivity, fast mass transfer effects, and tunable electronic structure. Nevertheless, its hydrogen evolution reaction (HER) activity in full pH-range has been rarely developed. Herein, a Ni-Co carbonate-hydroxide induced metal-organic framework transformation strategy is proposed to in situ grow porous, honeycomb-like NiCoP nanoplates on Ni foam for high-performance, pH-universal hydrogen evolution reaction. The resultant NiCoP catalyst exhibits a highly 2D nanoporous network in which 20-50 nm, well-crystalline nanoparticles are interconnected with each other closely, and delivers versatile HER electroactivity with η₁₀ of 98, 105, and 97 mV in 1 m KOH, 0.5 m H₂ SO₄ , and 1 m phosphate buffer solution electrolytes, respectively. This overpotential remarkably surpasses the one of commercial Pt/Cs in both neutral and alkaline media at a large current density (>100 mA cm^-2 ). The corresponding full water-splitting electrolyzer constructed from the 2D porous NiCoP cathode requires only a cell voltage of 1.43 V at 10 mA cm^-2 , superior to most recently reported electrocatalysts. This work may open up a new avenue on the rational design of nonprecious, pH-universal electrocatalyst.

Transition Metal Phosphides (TMP) as a Versatile Class of Catalysts for the Hydrodeoxygenation Reaction (HDO) of Oil-Derived Compounds

Hydrodeoxygenation (HDO) reaction is a route with much to offer in the conversion and upgrading of bio-oils into fuels; the latter can potentially replace fossil fuels. The catalyst’s design and the feedstock play a critical role in the process metrics (activity, selectivity). Among the different classes of catalysts for the HDO reaction, the transition metal phosphides (TMP), e.g., binary (Ni2P, CoP, WP, MoP) and ternary Fe-Co-P, Fe-Ru-P, are chosen to be discussed in the present review article due to their chameleon type of structural and electronic features giving them superiority compared to the pure metals, apart from their cost advantage. Their active catalytic sites for the HDO reaction are discussed, while particular aspects of their structural, morphological, electronic, and bonding features are presented along with the corresponding characterization technique/tool. The HDO reaction is critically discussed for representative compounds on the TMP surfaces; model compounds from the lignin-derivatives, cellulose derivatives, and fatty acids, such as phenols and furans, are presented, and their reaction mechanisms are explained in terms of TMPs structure, stoichiometry, and reaction conditions. The deactivation of the TMP’s catalysts under HDO conditions is discussed. Insights of the HDO reaction from computational aspects over the TMPs are also presented. Future challenges and directions are proposed to understand the TMP-probe molecule interaction under HDO process conditions and advance the process to a mature level.

Iron and chromium co-doped cobalt phosphide porous nanosheets as robust bifunctional electrocatalyst for efficient water splitting

It is still a huge challenge to develop highly efficient and low-cost non-precious metal-based electrocatalysts for overall water splitting in alkaline electrolytes. Herein, Cr and Fe co-doped CoP porous mesh nanosheets (Mesh-CrFe-CoP NSs) were synthesized through hydrolysis reaction, ion exchange etching and subsequent low-temperature phosphating process. The Mesh-CrFe-CoP NSs provides overpotentials at a current density of 10 mA cm^-2under alkaline electrolyte of 103.7 mV and 256.4 mV for HER and OER, respectively. Furthermore, when using Mesh-CrFe-CoP NSs as anode and cathode, the water splitting system could afford a current density of 10 mA cm^-2at 1.55 V, which is better than an electrolytic cell composed of 20% Pt/C and RuO₂. The excellent electrocatalytic performance of Mesh-CrFe-CoP NSs is attributed to the co-doping and porous nanostructure. Specifically, the Cr and Fe co-doped porous CoP nanosheets electrocatalyst not only provided abundant exposure active sites, accelerated the entry of liquid and the diffusion of gas, but also regulated the electronic environment of active sites, and thus enhanced the electrochemical performance. This work proposes a strategy for the rational design of highly efficient and stable non-precious metal co-doped phosphide electrocatalysts in the of electrochemical water splitting.

Asymmetric, Flexible Supercapacitor Based on Fe-Co Alloy@Sulfide with High Energy and Power Density

Electrode materials with high conductivities that are compatible with flexible substrates are important for preparing high-capacitance electrode materials and improving the energy density of flexible supercapacitors. Here, we report the design and fabrication of a new type of flexible electrode based on nanosheet architectures of a Co-Fe alloy (FeCo-A) coated with ternary metal sulfide composites (FeCo-Ss) on silver-sputtered carbon cloth. The high conductivity of the flexible substrate and the iron-cobalt alloy skeleton enables good electron transmission through the material. In particular, the outer FeCo-S layer has an average thickness of ∼30 nm, providing many active sites. This layer also inhibits the oxidation of the alloy. The electrode material is close to 20 nm thick, which limits inaccessible volumes and promotes high utilization of FeCo-alloy@FeCo-sulfide (FeCo-A-S). The additive-free FeCo-A-S electrode has a high specific capacitance of 2932.2 F g^-1 at 1.0 A g^-1 and a superior rate capability. All-solid-state supercapacitors based on these electrodes have a high power density of 8000 W kg^-1 and a high energy density of 46.1 W h kg^-1.

Recent Advances in Layered-Double-Hydroxides Based Noble Metal Nanoparticles Efficient Electrocatalysts

With the energy crisis and environmental pollution becoming more and more serious, it is urgent to develop renewable and clean energy. Hydrogen production from electrolyzed water is of great significance to solve the energy crisis and environmental problems in the future. Recently, layered double hydroxides (LDHs) materials have been widely studied in the electrocatalysis field, due to their unique layered structure, tunable metal species and highly dispersed active sites. Moreover, the LDHs supporting noble metal catalysts obtained through the topotactic transformation of LDHs precursors significantly reduce the energy barrier of electrolyzing water, showing remarkable catalytic activity, good conductivity and excellent durability. In this review, we give an overview of recent advances on LDHs supporting noble metal catalysts, from a brief introduction, to their preparation and modification methods, to an overview of their application in the electrocatalysis field, as well as the challenges and outlooks in this promising field on the basis of current development.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

An in situ phosphorization strategy towards doped Co2P scaffolded within echinus-like carbon for overall water splitting

Eco-environmental synthesis of non-expensive electrocatalysts such as transition-metal phosphides (TMPs) is critical to advancing renewable hydrogen fuel. TMP nanostructures prepared typically by introducing additional conventional phosphorus sources are suggested as promising durable and low-cost electrocatalysts. Herein, an eco-efficient guest/host precursor-based synthesis route is demonstrated to prepare doped Co₂P scaffolded within echinus-like carbon ((M_0.2Co_0.8)₂P@C, M = Fe and Ni) as electrocatalysts for overall water splitting. (Fe_0.2Co_0.8)₂P@C is derived by directly pyrolyzing a precursor of sodium dodecyl phosphate-intercalated CoFe-layered double hydroxide (CoFe-LDH), without introducing any additional phosphorus source. Electrocatalytic testing shows that (Fe_0.2Co_0.8)₂P@C requires overpotentials of 290 and 130 mV at a current density of 10 mA cm^-2 for oxygen and hydrogen evolution reactions (OER and HER) in an alkaline electrolyte, respectively. Furthermore, a different (Ni_0.2Co_0.8)₂P@C composite, obtained only by altering a NiCo-LDH host, exhibits better electrocatalytic activities than those of Fe-doped (Fe_0.2Co_0.8)₂P@C. In particular, the (No_0.2Co_0.8)₂P@C||(Ni_0.2Co_0.8)₂P@C electrolyzer affords a current density of 10 mA cm^-2 at a decent voltage of 1.62 V for overall water splitting. Electron energy-loss spectroscopy (EELS) observations show the oxyhydroxide layer formed on the surface, and density functional theory (DFT) calculations reveal that Fe-/Ni-doping lowers the Gibbs free energy barrier for the OER and the HER, both underpinning the enhancements.

Cobalt-based heterogeneous catalysts in an electrolyzer system for sustainable energy storage

Nowadays, the production of hydrogen and oxygen focuses on renewable energy techniques and sustainable energy storage. A substantial challenge is to extend low-cost electrocatalysts consisting of earth-abundant resources, prepared by straightforward approaches that display high intrinsic activity compared to noble metals. The expansion of bifunctional catalysts in alkaline electrolytes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) has become very crucial in recent times. Herein, the recent progress in cobalt-based HER-OER electrocatalysts has been are brushed up and numerous bifunctional cobalt-based catalysts such as cobalt-oxides, phosphides, sulfides, selenides, nitrides, borides, carbides, perovskites, and MOF-based cobalt analogs have been investigated in detail. Specifically, much more attention has been paid to their structural variation, bifunctional activity, overpotential of the overall system, and stability. Cobalt-based catalysts with lower cell voltage, remarkable durability, and unique electronic structures, offer a new perspective in energy-related fields. In recent years, cobalt-based analogs with diagnostic facilities have been introduced due to their electronic structures, tunable d band structures, and tailorable active sites. This perspective also elucidates the present issues, promising ideas, and future forecasts for cobalt-based catalysts. The critical aspects of cobalt-based catalysts and the numerous opportunities, as discussed at the end, can possibly enrich the sustainable energy field.

Optimizing Platinum Location on Nickel Hydroxide Nanosheets to Accelerate the Hydrogen Evolution Reaction

Rational electrode design is crucial to promote the performance of the hydrogen evolution reaction (HER) via further enhancing the activity, stability, and utilization of platinum (Pt) in an alkaline electrolyte. Herein, a binder-free low-Pt-content HER electrode, Pt (∼20 μg cm^-2) decorated on nickel hydroxide grown on nickel foam (Pt-Ni(OH)₂-2h-NF20), is fabricated at near room temperature in a test tube. To lower the ohmic resistance, for the first time, the Pt nanoparticles were location-selectively anchored on the bottom of height-controlled vertical Ni(OH)₂ nanosheets via utilizing the mass transfer resistance of the dense Ni(OH)₂ film for chloroplatinate. Furthermore, the excellent mass transfer, high specific surface area of Pt, synergistic effect between Pt with Ni(OH)₂, and stable structure together prompt the resulting electrode with a special structure to exhibit superior HER electrocatalytic activity and stability in 1 M KOH. Typically, this electrode reaches a current density of 35.9 mA cm^-2 at an overpotential of 100 mV, which is over 8 times higher than that of commercial Pt/C, and the overpotential only increases by 20 mV at 100 mA cm^-2 over 150,000 s of stability test. Benefiting from the simple fabrication process, the electrode with an area of 840 cm² was successfully prepared with a steady overpotential of 370 mV at 1000 mA cm^-2 and increased potential of 23 mV over 50 h of stability test.

Identifying the Intrinsic Relationship between the Restructured Oxide Layer and Oxygen Evolution Reaction Performance on the Cobalt Pnictide Catalyst

Cobalt pnictides show good catalytic activity and stability on oxygen evolution reaction (OER) behaviors in a strong alkaline solution. Identifying the intrinsic composition/structure-property relationship of the oxide layer on the cobalt pnictides is critical to design better and cheaper electrocatalysts for the commercial viability of OER technologies. In this work, the restructured oxide layer on the cobalt pnictides and its effect on the activity and mechanism for OER is systematically analyzed. In-situ electrochemical impedance spectroscopy (EIS) and near edge x-ray absorption fine structure (NEXAFS) spectra indicate that a higher OER performance of cobalt pnictides than Co₃ O₄ is attributed to the more structural disorder and oxygen defect sites in the cobalt oxide layer evolved from cobalt pnictides. Using angle resolved x-ray photoelectron spectroscopy (AR-XPS) further demonstrates that the oxygen defect sites mainly concentrate on the subsurface of cobalt oxide layer. The current study demonstrated promising opportunities for further enhancing the OER performance of cobalt-based electrocatalysts by controlling the subsurface defects of the restructured active layer.

Exfoliation of bimetallic (Ni, Co) carbonate hydroxide nanowires by Ar plasma for enhanced oxygen evolution

Ar plasma exfoliated smooth 1D nanowires of NiCo-LDHs into thin nanosheets forming three-dimensional dendritic structure to expose electrochemical active surface area and more higher oxidation states for enhanced oxygen evolution reaction.

MOF-derived cobalt-nickel phosphide nanoboxes as electrocatalysts for the hydrogen evolution reaction

The development of high-efficiency nonprecious electrocatalysts based on inexpensive and Earth abundant elements is of great significance for renewable energy technologies. Group VIII transition metal phosphides (TMPs) gradually stand out due to their intriguing properties including low resistance and superior catalytic activity and stability. Herein, we adopt a unique MOF-derived strategy to synthesize transition metal phosphide nanoboxes which can be employed as electrocatalysts for the hydrogen evolution reaction. During this process, we converted a Co-MOF to a CoNi-MOF by ion exchange and low-temperature phosphating to achieve CoNiP nanoboxes. The CoNiP nanoboxes can reach a current density of 10 mA cm^-2 at a low overpotential of 138 mV with a small Tafel slope of 65 mV dec^-1.

Carbon nanotubes-based PdM bimetallic catalysts through N4-system for efficient ethanol oxidation and hydrogen evolution reaction

Transitional metal-nitrogen-carbon system is a promising candidate to replace the Pt-based electrocatalyst due to its superior activity, durability and cost effectiveness. In this study, we have designed a simple strategy to fabricate carbon nanotubes-supported binary-nitrogen-carbon catalyst via wet-chemical method. Palladium and transitional metals (M, i.e. manganese cobalt and copper) nanoparticles are anchored through four-nitrogen system onto carbon nanotubes (denoted as PdM-N4/CNTs). This material has been used as bifunctional electrocatalyst for electrochemical ethanol oxidation reaction and hydrogen evolution reaction for the first time. The N4-linked nanoparticles onto carbon nanotubes plays a crucial role in intrinsic catalytic activity for both reactions in 1 M KOH electrolyte. Among three PdM-N4/CNTs catalysts, the PdMn-N4/CNTs catalyst exhibits higher catalytic activity in terms of current density, mass activity and stability compared to the benchmark Pt/C. The robust electrocatalysis are inherited from the better attachment of PdMn through N4-system onto carbon nanotubes, comparatively smaller particles formation with better dispersion and higher electrical conductivity.

Role of Sulfur Vacancies and Undercoordinated Mo Regions in MoS2 Nanosheets toward the Evolution of Hydrogen

Low-dimensional materials have been examined as electrocatalysts for the hydrogen evolution reaction (HER). Among them, two-dimensional transition metal dichalcogenides (2D-TMDs) such as MoS₂ have been identified as potential candidates. However, the performance of TMDs toward HER in both acidic and basic media remains inferior to that of noble metals such as Pt and its alloys. This calls for investigating the influence of controlled defect engineering of 2D TMDs on their performance toward hydrogen production. Here, we explored the HER activity from defective multilayered MoS₂ over a large range of surface S vacancy concentrations up to 90%. Amorphous MoS₂ and 2H MoS₂ with ultrarich S vacancies demonstrated the highest HER performance in acid and basic electrolytes, respectively. We also report that the HER performance from multilayered MoS₂ can be divided into two domains corresponding to "point defects" at low concentrations of surface S vacancies (Stage 1) and large regions of undercoordinated Mo atoms for high concentrations of surface S vacancies (Stage 2). The highest performance is obtained for Stage 2 in the presence of undercoordinated Mo atoms with a TOF of ∼2 s^-1 at an overpotential of 160 mV in 0.1 M KOH which compares favorably to the best results in the literature. Overall, our work provides deeper insight on the HER mechanism from defected MoS₂ and provides guidance for the development of defect-engineered TMD-based electrocatalysts.

Recent progress in the synthesis and applications of 2D metal nanosheets

The design and controlled synthesis of two-dimensional (2D) nanomaterials have been widely studied because the properties and functions of nanomaterials are highly dependent on their sizes, shapes, and dimensionalities. For instance, 2D metal nanosheets (2DMNSs) have attracted a significant amount of attention owing to their interesting properties, which are absent in corresponding bulk counterparts, and they have been confirmed to have potential applications in electrocatalysis, optics, and biomedicine. However, because of the close-packed structures of metals, the large-scale fabrication of 2DMNSs is challenging. In this review, we have outlined the research progress in the field of 2DMNSs, including the typical synthesis approaches and newly developed methods, as well as promising applications of the materials reported in recent years. Moreover, some preliminary and promising strategies to further improve the properties of 2DMNSs and some insights for the development of the field have been included.

Controlled Self-Assembled NiFe Layered Double Hydroxides/Reduced Graphene Oxide Nanohybrids Based on the Solid-Phase Exfoliation Strategy as an Excellent Electrocatalyst for the Oxygen Evolution Reaction

Layered double hydroxides (LDHs), as an effective oxygen evolution reaction (OER) electrocatalyst, face many challenges in practical applications. The main obstacle is that bulk materials limit the exposure of active sites. At the same time, the poor conductivity of LDHs is also an important factor. Exfoliation is one of the most direct and effective strategies to increase the electrocatalytic properties of LDHs, leading to exposure of many active sites. However, developing an efficient exfoliation strategy to exfoliate LDHs into stable monolayer nanosheets is still challenging. Therefore, we report a new and efficient solid-phase exfoliation strategy to exfoliate NiFe LDH and graphene oxide (GO) into monolayer nanosheets and the exfoliating ratios of NiFe LDH and GO can reach up to 10 and 5 wt %, respectively. Based on the solid-phase exfoliation strategy, we accidentally discovered that there is a dynamic evolution process between NiFe-LDH nanosheets (NiFe-LDH-NS) and GO nanosheets (GO-NS) to assemble new NiFe-LDH/GO nanohybrids, i.e., NiFe-LDH-NS could be horizontal bespreading on GO-NS or well-organized standing on GO-NS, or both simultaneously. The electrocatalytic OER property test results show that NiFe-LDH/RGO-3 (NFRG-3) nanohybrids obtained by the reduction treatment of NiFe-LDH/GO-3 (NFGO-3) nanohybrids, in which NiFe-LDH-NS are well-organized standing on GO-NS, have excellent electrocatalytic properties for OER in an alkaline solution (with a small overpotential of 273 mV and a Tafel slope of 49 mV dec^-1 at the current density of 30 mA cm^-2). The excellent electrocatalytic properties for OER of NFRG-3 nanohybrids could be attributed to the unique three-dimensional arraylike structure with many active sites. At the same time, reduced graphene oxide (RGO) with excellent conductivity can improve the charge-transfer efficiency and synergistically improve OER properties of nanohybrids.

Porous Mn-Doped FeP/Co3 (PO4 )2 Nanosheets as Efficient Electrocatalysts for Overall Water Splitting in a Wide pH Range

Development of highly active and stable electrocatalysts for overall water splitting is important for future renewable energy systems. In this study, porous Mn-doped FeP/Co₃ (PO₄ )₂ (PMFCP) nanosheets on carbon cloth are utilized as a highly efficient 3 D self-supported binder-free integrated electrode for the oxygen evolution and hydrogen evolution reactions (OER and HER) over a wide pH range. Specifically, overpotentials of 27, 117, 85 mV are required for the PMFCP nanosheets to attain 10 mA cm^-2 for HER in 0.5 m H₂ SO₄ , 1.0 m phosphatebuffered saline (PBS), and 1.0 m KOH, respectively. In addition to the excellent performance for HER electrocatalysis, PMFCP nanosheets were also efficient electrocatalysts for the OER. Thus, the PMFCP nanosheets can serve as anodes and cathodes for overall water splitting (OWS). The OWS working voltages to attain 10 mA cm^-2 are found to be 1.75, 1.82, and 1.61 V in acid, neutral, and alkaline electrolytes, respectively. Chronopotentiometric tests show that the PMFCP electrode can maintain its excellent pH-universal OWS activity for more than 30 000 s. This work also provides new insights into developing high-performance electrocatalysts for water splitting over a wide pH range. The improvement in electrochemical performance by introduction of Mn dopant and nano-holes offers new opportunities in the development of effective electrodes for other energy-related applications.

Synthesis of a MoS x -O-PtO x Electrocatalyst with High Hydrogen Evolution Activity Using a Sacrificial Counter-Electrode

Water splitting is considered to be a very promising alternative to greenly produce hydrogen, and the key to optimizing this process is the development of suitable electrocatalysts. Here, a sacrificial-counter-electrode method to synthesize a MoS x /carbon nanotubes/Pt catalyst (0.55 wt% Pt loading) is developed, which exhibits a low overpotential of 25 mV at a current density of 10 mA cm-2, a low Tafel slope of 27 mV dec-1, and excellent stability under acidic conditions. The theory calculations and experimental results confirm the high hydrogen evolution activity that is likely due to the fact that the S atoms in MoS x can be substituted with O atoms during a potential cycling process when using Pt as a counter-electrode, where the O atoms act as bridges between the catalytic PtO x particles and the MoS x support to generate a MoS x -O-PtO x structure, allowing the Pt atoms to donate more electrons thus facilitating the hydrogen evolution reaction process.

Enhancing the Catalytic Activity of Co3O4 Nanosheets for Li-O2 Batteries by the Incoporation of Oxygen Vacancy with Hydrazine Hydrate Reduction

Li-O₂ battery attracts great interest because of the high energy density. But the poor kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have blocked the practical application. Designing the efficient bifunctional cathode catalysts is of great importance for the Li-O₂ battery. Tuning the electronic and surface structure of the catalysts plays an important role. Herein, we propose to enhance the catalytic performance of Co₃O₄ nanosheets for rechargable Li-O₂ batteries by hydrazine hydrate-induced oxygen vacancy formation. The hydrazine hydrate reduction not only introduces oxygen vacancies into Co₃O₄ nanosheets and modulates the electronic structure but also roughens the surface, which all contribute to the enhancement of ORR and OER activity, especially the activity and stability for OER. Li-O₂ cells catalyzed by the oxygen defects-enriched Co₃O₄ ultrathin nanosheets exhibit much better electrochemical performances in terms of the high initial capacity (∼11 000 mAh g^-1), the lower overpotential (∼1.1 V), and the longer cycle life (150 cycles@200 mA g^-1). This can be largely attributed to the synergy of the enriched oxygen vacancies and the roughened surface of Co₃O₄ nanosheets, which not only improves the electron and Li⁺ conductivity but also provides more active sites and reaction spots. The proposed facile strategy may also be applied to modify other oxides based catalysts for Li-O₂ batteries or other fields.

Activating hierarchically hortensia-like CoAl layered double hydroxides by alkaline etching and anion modulation strategies for the efficient oxygen evolution reaction

Hierarchically porous hortensia-like CoAl hydroxysulfide as an efficient electrocatalyst for the OER is designed and developed.

Plasma enabled non-thermal phosphorization for nickel phosphide hydrogen evolution catalysts

Plasma activates inert phosphorous vapour to prepare nickel phosphide nanosheets at mild temperature for hydrogen evolution from water electrolysis.

Efficient bifunctional vanadium-doped Ni3S2 nanorod array for overall water splitting

A nickel foam-supported vanadium-doped Ni₃S₂ nanorod array electrode has been demonstrated as a robust bifunctional electrocatalyst for overall water splitting.

Construction of a hollow porous carbon spheres@CoP/nitrogen-doped carbon supported platinum catalyst for high performance methanol oxidation

A Pt–HPCS@CoP/NC catalyst with excellent catalytic activity, CO-tolerance and durability was synthesized for the methanol oxidation reaction.

Conductive Molybdenum Sulfide for Efficient Electrocatalytic Hydrogen Evolution

Molybdenum sulfide (MoS₂ ) is a layered material with high activity for electrocatalytic hydrogen evolution reaction (HER). In conventional MoS₂ , the high electrical resistance between the layers hampers the bulk charge transfer and therefore greatly limits its performance in electrolysis. Herein, ultrathin MoS₂ nanosheets with bent layers on reduced graphene oxide (RGO) are reported. In sharp contrast to the bulk MoS₂ , the resulting MoS₂ has mostly 1 or 2 layers, and the layer distance is significantly expanded to ≈1 nm. From computational studies, the prepared MoS₂ with limited layer numbers and expanded layer distances has similar physical and chemical features with single-layer MoS₂ . Importantly, the bent single layer is electrically conductive and is intrinsically more active than a normal flat single layer. In addition, the unusual features of confined sizes and distorted lattices in the prepared MoS₂ can bring about plentiful active sites and are beneficial for mass diffusion during electrocatalysis. The hybrid material exhibits high activity for electrocatalytic HER, affording a current density of 10 mA cm^-2 at a low overpotential of 66 mV.

Sub-1.5 nm Ultrathin CoP Nanosheet Aerogel: Efficient Electrocatalyst for Hydrogen Evolution Reaction at All pH Values

Transition metal phosphides (TMPs) are certified high performance electrocatalysts for the hydrogen evolution reaction (HER). The ultrathin 2D structure of TMPs can offer abundant adsorption sites to boost HER performance. Herein, an ice-templating strategy is developed to prepare CoP aerogels composed of 2D ultrathin CoP nanosheets (<1.5 nm) using sustainable alginate biomass (seaweed extract) as the precursor. The highly porous aerogel structure can not only deliver facile mass transfer, but also prevent aggregation of the nanosheets into layered structures. As expected, the obtained CoP nanosheet aerogels exhibit remarkable stability and excellent electrocatalytic HER performance at all pH values. For instance, the sample CoP-400 presents a low overpotential of 113, 154, and 161 mV versus RHE at a current density of 10 mA cm^-2 in 0.5 m H₂SO₄, 1 m KOH, and 1 m phosphate buffer solution, respectively. In addition, CoP-400 displays low Tafel slopes at all pH values due to the interconnected highly porous structure of the aerogel, indicating that the sample can provide low-resistance channels for mass transport. Density functional theory calculations reveal that P-top and Co bridge on (011) facet of CoP are more favorable sites during the process of HER in acid and alkaline solutions, respectively.

Rapid cationic defect and anion dual-regulated layered double hydroxides for efficient water oxidation

Defect engineering and anionic regulation are effective approaches to improve the intrinsic activity of oxygen evolution reaction (OER) catalysts, particularly for highly efficient and low-cost cobalt-based electrocatalysts. Layered double hydroxides (LDHs) are considered as promising electrocatalysts toward OER. However, their electronic properties and active sites need to be optimized for their large-scale application. Herein, rapidly cationic defect and anion dual-regulated CoAl LDHs (PS-CoAl LDHs) were in situ synthesized in a few minutes via a modified water DBD plasma treatment. Abundant Al3+ vacancies and a relatively rough surface for S2- regulation were formed by the etching effect of the plasma. Consequently, the as-obtained PS-LDHs possess improved intrinsic conductivity and an optimal electronic structure. Simultaneously, the synergistic effect of the Al3+ vacancies and S2- regulation promote the exposure of active Co sites, resulting in an amorphous and porous surface for improving the OER performance.

Ultrathin Amorphous Iron-Nickel Boride Nanosheets for Highly Efficient Electrocatalytic Oxygen Production

A cost-effective and efficient electrocatalyst for the oxygen evolution reaction during the electrolysis of water is highly desired. In an effort to develop an economical material for replacing precious-metal-based catalysts, a novel and self-standing amorphous ultrathin nanosheet (NS) of bimetallic iron-nickel boride (Fe-Ni-B NSs) on Ni foam is presented, which displays a better oxygen-evolving activity compared to the precious-metal catalyst RuO₂ . In 1.0 m KOH electrolyte solution, it requires an overpotential of only 237 mV to reach a current density of 10 mA cm^-2 with a small Tafel slope of 38 mV dec^-1 and shows prominent long-term electrochemical stability. A synergistic effect between highly abundant catalytically active sites on the 3D porous substrate improved the electron transport arising from the presence of highly negative boron, and the high conductivity of the substrate results in an outstanding electrocatalytic activity. The advanced catalytic activity, facile electrode fabrication, and low costs make it a potential oxygen-evolving material, which may be extended to other energy-conversion and storage technologies.

Solvent-free aerobic selective oxidation of hydrocarbons catalyzed by porous graphitic carbon encapsulated cobalt composites

Cobalt particles encapsulated in graphitic carbon (Co@C₈₀₀) were demonstrated to be an effective catalyst for the selective oxidation of hydrocarbons.