Dual Electronic Modulations on NiFeV Hydroxide@FeOx Boost Electrochemical Overall Water Splitting.

Nickel-iron based hydroxides have been proven to be excellent oxygen evolution reaction (OER) electrocatalysts, whereas they are inactive toward hydrogen evolution reaction (HER), which severely limits their large-scale applications in electrochemical water splitting. Herein, a heterostructure consisted of NiFeV hydroxide and iron oxide supported on iron foam (NiFeV@FeOx /IF) has been designed as a highly efficient bifunctional (OER and HER) electrocatalyst. The V doping and intimate contact between NiFeV hydroxide and FeOx not only improve the entire electrical conductivity of the catalyst but also afford more high-valence Ni which serves as active sites for OER. Meanwhile, the introduction of V and FeOx reduces the electron density on lattice oxygen, which greatly facilitates desorption of Hads . All of these endow the NiFeV@FeOx /IF with exceptionally low overpotentials of 218 and 105 mV to achieve a current density of 100 mA cm-2 for OER and HER, respectively. More impressively, the electrolyzer requires an ultra-low cell voltage of 1.57 V to achieve 100 mA cm-2 and displays superior electrochemical stability for 180 h, which outperforms commercial RuO2 ||Pt/C and most of the representative catalysts reported to date. This work provides a unique route for developing high-efficiency electrocatalyst for overall water splitting.

In Situ Exsolution of Ba3(VO4)2 Nanoparticles on a V-Doped BaCoO3-δ Perovskite Oxide with Enhanced Activity for Electrocatalytic Hydrogen Evolution.

The successful preparation of a perovskite-based heterostructure is important for broadening the applications of perovskites in the field of electrocatalysis, especially in a hydrogen evolution reaction (HER). Nevertheless, the limited active sites of perovskites severely hindered the HER properties. Herein, an in situ exsolution method was used to construct a nanocomposite based on V-doped BaCoO3-δ decorated with Ba3(VO4)2 (BVCO19) for alkaline HER. The exsolved Ba3(VO4)2 can induce more Co4+ ions on BaCoO3-δ, which serves as active sites for the release of H2. Meanwhile, by regulating the valency of V and Co species, the catalyst can reach a charge balance by generating more oxygen vacancies, which greatly facilitates the adsorption and dissociation of H2O molecules. The synergistic effect between the oxygen vacancies and high-valence Co4+ leads to an enhanced HER performance of BVCO19. The as-obtained catalyst delivers a low overpotential of 194 mV at 10 mA cm-2 as well as impressive stability for 100 h in alkaline media, which outperforms pristine BaCoO3-δ and most of the nonprecious-based perovskite oxides. This work provides new insights into the preparation of perovskite-based heterostructure for boosting HER.

Ultrafine Electrospun Cobalt-Molybdenum Bimetallic Nitride as a Durable Electrocatalyst for Hydrogen Evolution.

Transition metal nitrides are promising electrocatalysts for hydrogen evolution reaction (HER) owing to their Pt-like electronic structure. However, the harsh nitriding conditions greatly limit their large-scale applications. Herein, ultrafine Co3Mo3N-Mo2C (<1 nm)-decorated carbon nanofibers (Co3Mo3N-Mo2C/CNFs) were prepared by electrostatic spinning followed by pyrolysis treatment, in which the MoCo-MOF simultaneously serves as the precursor and nitrogen source. The generated synergistic interactions between Mo2C and Co3Mo3N significantly adjust the electronic structure of Mo2C and afford a fast charge transfer, which endows the resultant hybrid with superior HER electrocatalytic performances. Specifically, the as-obtained Co3Mo3N-Mo2C/CNF delivers a low overpotential of only 76 mV to achieve a current density of 10 mA cm-2 and superior durability with no obvious degradation for 200 h in acidic media. This performance outperforms most of the transition metal-based electrocatalysts reported to date. This work paves a new way for the design of catalysts with ultrasmall size and high efficiency in energy conversion.

Fe-Incorporated Nickel-Based Bimetallic Metal-Organic Frameworks for Enhanced Electrochemical Oxygen Evolution.

Developing cost-effective and high-efficiency catalysts for electrocatalytic oxygen evolution reaction (OER) is crucial for energy conversions. Herein, a series of bimetallic NiFe metal-organic frameworks (NiFe-BDC) were prepared by a simple solvothermal method for alkaline OER. The synergistic effect between Ni and Fe as well as the large specific surface area lead to a high exposure of Ni active sites during the OER. The optimized NiFe-BDC-0.5 exhibits superior OER performances with a small overpotential of 256 mV at a current density of 10 mA cm-2 and a low Tafel slope of 45.4 mV dec-1, which outperforms commercial RuO2 and most of the reported MOF-based catalysts reported in the literature. This work provides a new insight into the design of bimetallic MOFs in the applications of electrolysis.

Cu-doped La0.5Sr0.5CoO3-δ perovskite as a highly efficient and durable electrocatalyst for hydrogen evolution.

The preparation of a high-efficiency and durable electrocatalyst for the alkaline hydrogen evolution reaction (HER) is essential for realizing renewable energy technologies. Herein, a series of La0.5Sr0.5CoO3-δ perovskites with different amounts of Cu cations substituting at B-sites were fabricated for the HER. Specifically, the optimized La0.5Sr0.5Co0.8Cu0.2O3-δ (LSCCu0.2) exhibits a significantly enhanced electrocatalytic activity with an ultralow overpotential of 154 mV at 10 mA cm-2 in 1.0 M KOH, which is reduced by 125 mV compared with that of pristine La0.5Sr0.5CoO3-δ (LSC, 279 mV). It also delivers a robust durability with no obvious degradation after 150 h. Impressively, the HER activity of LSCCu0.2 is superior to that of commercial Pt/C at large current densities (>270 mA cm-2). XPS analysis indicates that Co2+ ions replaced by an appropriate amount of Cu2+ ions can increase the proportion of Co3+ and generate high content of oxygen vacancies in LSC, which leads to an increased electrochemically active surface area, thereby greatly facilitating the HER. This work offers a simple way for the rational design of cost-effective and highly efficient catalysts, which may be extended to other Co-based perovskite oxides for the alkaline HER.

Direct Urea/H2O2 Fuel Cell with a Hierarchical Porous Nanoglass Anode for High-Efficiency Energy Conversion.

Direct urea/H2O2 fuel cells (DUFCs) constitute a sustainable bifunctional energy conversion technique devoted to simultaneously eliminating environmental wastewater with urea and generating clean energy. However, exploring an efficient anode material for DUFCs still remains a huge challenge. In this work, a Ni-P hierarchical porous nanoglass (HPNG) catalytic electrode was developed via a low-cost, industrially available electrodeposition technique, which exhibits one of the best performances reported so far in the urea oxidation reaction (UOR), with a potential of 1.330 V at a current density of 10 mA cm-2 and a Tafel slope of 9.77 mV dec-1. The superior UOR performance of the HPNG electrode is attributed to the excellent intrinsic catalytic activity of NG with a high-energy state and an extremely enlarged surface area from the unique 3D hierarchical porous structure. Furthermore, a DUFC system with the HPNG anode shows a performance breakthrough as indicated by the maximum power density of 38.15 mW cm-2 for 0.5 M urea, representing one of the best yet reported DUFCs. Our work demonstrates the feasibility of the scalable production of HPNG electrodes and is expected to be a great contribution to the development of the practical use of DUFCs in the near future for bifunctional energy conversion.

Age-Stratified COVID-19 Spread Analysis and Vaccination: A Multitype Random Network Approach.

The risk of severe illness and mortality from COVID-19 significantly increases with age. As a result, age-stratified modeling for COVID-19 dynamics is the key to study how to reduce hospitalizations and mortality from COVID-19. By taking advantage of network theory, we develop an age-stratified epidemic model for COVID-19 in complex contact networks. Specifically, we present an extension of standard SEIR (susceptible-exposed-infectious-removed) compartmental model, called age-stratified SEAHIR (susceptible-exposed-asymptomatic-hospitalized-infectious-removed) model, to capture the spread of COVID-19 over multitype random networks with general degree distributions. We derive several key epidemiological metrics and then propose an age-stratified vaccination strategy to decrease the mortality and hospitalizations. Through extensive study, we discover that the outcome of vaccination prioritization depends on the reproduction number [Formula: see text]. Specifically, the elderly should be prioritized only when [Formula: see text] is relatively high. If ongoing intervention policies, such as universal masking, could suppress [Formula: see text] at a relatively low level, prioritizing the high-transmission age group (i.e., adults aged 20-39) is most effective to reduce both mortality and hospitalizations. These conclusions provide useful recommendations for age-based vaccination prioritization for COVID-19.