ChenDH

A multiscale hierarchical porous micro-sized germanium (p-Ge) comprising of interconnected nano-ligaments and bicontinuous nanopores is rationally designed by an innovative double template dealloying method. This design enables the resultant p-Ge anode with alleviated volume variation, high tap density, high activity, and decreased Li⁺ diffusion distance demonstrating superior Li- storage performance in terms of initial Coulombic efficiency, volumetric capacity, and long-term stability.

Abstract

The manipulation of stress in high-capacity microscale alloying anode materials, which undergo significant volumetric variation during cycling, is crucial prerequisite for improved their cycling capability. In this work, an innovative structural design strategy is proposed for scalable fabrication of a unique 3D highly porous micro structured germanium (Ge) featuring micro-nano hierarchical architecture as viable anode for high-performance lithium-ion batteries (LIBs). The resultant micro-sized Ge, consisting of interconnected nanoligaments and bicontinuous nanopores, is endowed with high activity, decreased Li⁺ diffusion distance and alleviated volume variation, appealing as an ideal platform for in-depth understanding the relationship between structural design and stress evolution. Such a micro-sized Ge being highly porous delivers a record high initial Coulombic efficiency of 92.5%, large volumetric capacity of 2,421 mAh cm⁻³ at 1.2 mA cm⁻², exceptional rate capability (805.6 mAh g⁻¹ at 10 Ag⁻¹) and cycling stability (over 90% capacity retention after 1000 cycles even at 5 A g⁻¹), largely outperforming the reported Ge-based anodes for LIBs. Furthermore, its underlying Li storage mechanism and stress dispersion behavior are explicitly revealed by combined substantial in situ/ex situ experimental characterizations and theoretical computation. This work provides novel insights into the rational design of high-performance and durable alloying anodes toward high-energy LIBs.

Highly active oxygen evolution reaction electrocatalysts, such as those containing Fe, often face the challenge of severe dissolution of active elements. Here, the correlation between precatalyst structural changes and interfacial dynamic stability is elucidated by investigating the structural evolution of Fe-containing Prussian blue analogs.

Abstract

Highly active oxygen evolution reaction (OER) electrocatalysts, such as those containing Fe, often face the challenge of severe dissolution of active elements. Addressing this concern through the establishment of a dynamically stable interface during OER presents a promising strategy, achieved by manipulation of catalyst components. Herein, the findings reveal that Fe loss during OER predominantly occurs during the initial activation phase, marked by irreversible structural distortion that disrupts interfacial dynamical stability. By investigating the structural evolution of Fe-containing Prussian blue analogs, serving as a model OER precatalyst, the correlation between precatalyst structural changes and interfacial dynamic stability is elucidated. Utilizing thermal annealing of CoFe bimetal Prussian blue, favorable thermodynamic conditions are induced for generating cyano vacancies within the matrix, thereby facilitating enhanced initial activation during OER. Consequently, catalytically active and stable oxyhydroxide species rapidly form at the interface, exhibiting robust interactions with interfacial Fe elements to stabilize interface dynamics. Suppression of the irreversible structural distortion responsible for active element loss during initial activation culminates in enhanced OER activity and stability.

The metal–organic framework based heterogeneous electrocatalysts (MOF-on-MOF) can facilitate electron transfer at the dual-MOF interface. In addition, the synergistic effect between Ni and Fe sites can promote electronic structure reconfiguration of active sites. Based on the above strategies for modulating the electronic structure of MOF-based electrocatalysts, MOF-(74 + 274)@NFF with optimal electronic structure exhibits superior oxygen evolution reaction (OER) performance.

Abstract

Electron modulation presents a captivating approach to fabricate efficient electrocatalysts for the oxygen evolution reaction (OER), yet it remains a challenging undertaking. In this study, an effective strategy is proposed to regulate the electronic structure of metal–organic frameworks (MOFs) by the construction of MOF-on-MOF heterogeneous architectures. As a representative heterogeneous architectures, MOF-74 on MOF-274 hybrids are in situ prepared on 3D metal substrates (NiFe alloy foam (NFF)) via a two-step self-assembly method, resulting in MOF-(74 + 274)@NFF. Through a combination of spectroscopic and theory calculation, the successful modulation of the electronic property of MOF-(74 + 274)@NFF is unveiled. This modulation arises from the phase conjugation of the two MOFs and the synergistic effect of the multimetallic centers (Ni and Fe). Consequently, MOF-(74 + 274)@NFF exhibits excellent OER activity, displaying ultralow overpotentials of 198 and 223 mV at a current density of 10 mA cm⁻² in the 1.0 and 0.1 M KOH solutions, respectively. This work paves the way for manipulating the electronic structure of electrocatalysts to enhance their catalytic activity.

In this perspective, several impacts of anodic oxidation of liquid products in terms of performance evaluation, e.g., misestimation of the Faradaic efficiency, are elaborated. It is revealed that dynamic change of the anolyte (i.e., pH and composition) not only brings a shift of anodic potentials, but also affects the chemical stability of the anode catalyst.

Abstract

The membrane-electrode assembly (MEA) approach appears to be the most promising technique to realize the high-rate CO₂/CO electrolysis, however there are major challenges related to the crossover of ions and liquid products from cathode to anode via the membrane and the concomitant anodic oxidation reactions (AORs). In this perspective, by combining experimental and theoretical analyses, several impacts of anodic oxidation of liquid products in terms of performance evaluation are investigated. First, the crossover behavior of several typical liquid products through an anion-exchange membrane is analyzed. Subsequently, two instructive examples (introducing formate or ethanol oxidation during electrolysis) reveals that the dynamic change of the anolyte (i.e., pH and composition) not only brings a slight shift of anodic potentials (i.e., change of competing reactions), but also affects the chemical stability of the anode catalyst. Anodic oxidation of liquid products can also cause either over- or under-estimation of the Faradaic efficiency, leading to an inaccurate assessment of overall performance. To comprehensively understand fundamentals of AORs, a theoretical guideline with hierarchical indicators is further developed to predict and regulate the possible AORs in an electrolyzer. The perspective concludes by giving some suggestions on rigorous performance evaluations for high-rate CO₂/CO electrolysis in an MEA-based setup.

An azide-functionalized cobaloxime proton-reduction catalyst covalently tethered into the Wurster-type covalent organic frameworks is developed for achieving the enhanced photocatalytic activity for hydrogen evolution in alcohol-containing solutions without any sacrificial agent.

Abstract

We report an azide-functionalized cobaloxime proton-reduction catalyst covalently tethered into the Wurster-type covalent organic frameworks (COFs). The cobaloxime-modified COF photocatalysts exhibit enhanced photocatalytic activity for hydrogen evolution reaction (HER) in alcohol-containing solution with no presence of a typical sacrificial agent. The best performing cobaloxime-modified COF hybrid catalyzes hydrogen production with an average HER rate up to 38 μmol h⁻¹ in ethanol/phosphate buffer solution under 4 h illumination. Ultrafast transient optical spectroscopy characterizations and charge carrier analysis reveal that the alcohol contents functioning as hole scavengers could be oxidized by the photogenerated holes of COFs to form aldehydes and protons. The consumption of the photogenerated holes thus suppresses exciton recombination of COFs and improves the ratio of free electrons that were effectively utilized to drive catalytic reaction for HER. This work demonstrates a great potential of COF-catalyzed HER using alcohol solvents as hole scavengers and provides an example toward realizing the accessibility to the scope of reaction conditions and a greener route for energy conversion.

Nanoflower-like carbon-encapsulated CoNiPt catalyst with composition segregation (CoNi-rich and Pt-rich) is obtained by pyrolyzing MOF precursors. These segregation alloy components synergically promote the kinetic activity of alkaline hydrogen evolution reaction (HER). This work provides novel insights into the design of efficient and low-cost alkaline HER catalyst derived from metal–organic frameworks by coordinating kinetic reaction sites at segregation alloy and adopting the appropriate drying process.

Abstract

Constructing an efficient alkaline hydrogen evolution reaction (HER) catalyst with low platinum (Pt) consumption is crucial for the cost reduction of energy devices, such as electrolyzers. Herein, nanoflower-like carbon-encapsulated CoNiPt alloy catalysts with composition segregation are designed by pyrolyzing morphology-controlled and Pt-proportion-tuned metal–organic frameworks (MOFs). The optimized catalyst containing 15% CoNiPt NFs (15%: Pt mass percentage, NFs: nanoflowers) exhibits outstanding alkaline HER performance with a low overpotential of 25 mV at a current density of 10 mA cm⁻², far outperforming those of commercial Pt/C (47 mV) and the most advanced catalysts. Such superior activity originates from an integration of segregation alloy and Co-O hybridization. The nanoflower-like hierarchical structure guarantees the full exposure of segregation alloy sites. Density functional theory calculations suggest that the segregation alloy components not only promote water dissociation but also facilitate the hydrogen adsorption process, synergistically accelerating the kinetics of alkaline HER. In addition, the activity of alkaline HER is volcanically distributed with the surface oxygen content, mainly in the form of Co_3dO_2p hybridization, which is another reason for enhanced activity. This work provides feasible insights into the design of cost-effective alkaline HER catalysts by coordinating kinetic reaction sites at segregation alloy and adjusting the appropriate oxygen content.

Solar thermoelectric generators are thermoelectric devices that utilize solar radiation to increase the temperature at the heat source of the device and generate electrical power. This review describes different designs of solar thermoelectric generators within the context of thermoelectric elements, optical concentrators, solar absorbers, and other techniques to enhance their output performance.

Abstract

Thermoelectric materials convert waste heat into electricity, making sustainable power generation possible when a temperature gradient is applied. Solar radiation is one potential abundant and eco-friendly heat source for this application, where one side of the thermoelectric device is heated by incident sunlight, while the other side is kept at a cooler temperature. This is known as solar thermoelectric generation. Various thermoelectric materials are used for different solar thermoelectric applications, and different methods are explored to enhance the temperature gradient across the material. Solar optical concentrators, thermal and selective absorbers, and other tools are proposed to improve the performance of solar thermoelectrics. Despite continuous research and development, experimental solar thermoelectric efficiencies remain below 10%, and theoretical efficiencies do not surpass 20%. In this review, the different designs of solar thermoelectric generators are examined within the context of thermoelectric elements, optical concentrators, solar absorbers, and other techniques to enhance their performance. Last, an overview of the current state of solar thermoelectrics is provided, areas for improvement are suggested, and the future of these devices is predicted.

The photoelectrochemical oxygen evolution reaction (OER) at semiconductor electrodes is a highly complex multielectron transfer process that commonly requires large potential bias to minimize competing interfacial recombination losses. Herein, for the first time, it is shown that polycrystalline GaFeO₃, a highly ionic n‐type ferrite with very positive band edges, can promote the OER without external bias.

Abstract

The photoelectrochemical properties of polycrystalline GaFeO₃ (GFO) thin films are investigated for the first time. Thin films prepared by sol–gel methods exhibit phase‐pure orthorhombic GFO with the Pc21n space group, as confirmed by X‐ray diffraction and Raman spectroscopy. Optical responses are characterized by a 2.72 eV interband transition and sub‐bandgap d–d transitions associated with octahedral and tetrahedral coordination of Fe³⁺ sites. DFT‐HSE06 electronic structure calculations show GFO is highly ionic with very low dispersion in the valence band maximum (VBM) and conduction band minimum (CBM). Electrochemical impedance spectroscopy reveals n‐type conductivity with a flat band potential (U _fb) of 0.52 V versus reversible hydrogen electrode, indicating that GFO has the most positive CBM reported of any ferrite. The photoelectrochemical oxidation of SO₃ ²⁻ shows an ideal semiconductor–electrolyte interfacial behavior with no evidence of surface recombination down to the U _fb. Surprisingly, the onset potential for the oxygen evolution reaction also coincides with the U _fb, showing interfacial hole‐transfer efficiency above 50%. The photoelectrochemical properties are limited by bulk recombination due to the short‐diffusion length of minority carriers as well as slow transport of majority carriers. Strategies towards developing high‐efficiency GFO photoanodes are briefly discussed.

Publication date: 1 May 2019

Source: Journal of Power Sources, Volume 421

Author(s): Peng Xu, Hao Xu, Dayang Zheng

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