Evaluation of electrospun spinel-type high-entropy (Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)3O4, (Cr0.2Mn0.2Fe0.2Co0.2Zn0.2)3O4 and (Cr0.2Mn0.2Fe0.2Ni0.2Zn0.2)3O4 oxide nanofibers as electrocatalysts for oxygen evolution in alkaline medium

Electrochemical water splitting is a promising sustainable-energy technology, but the slow kinetics of the oxygen evolution reaction represents a limitation for its broad market penetration. Spinel-structured transition metal (TM) oxides have shown great potential as a sustainable alternative to precious metal-based electrocatalysts. High-entropy oxides (HEOs) with multiple TM-cation sites lend themselves to engineering of the octahedral redox-active centres to enhance the catalyst reactivity. This work focuses on the preparation of electrospun spinel-type HEO nanofibers (NFs), based on equimolar (Cr,Mn,Fe,Co,Ni), (Cr,Mn,Fe,Co,Zn) and (Cr,Mn,Fe,Ni,Zn) combinations, and their evaluation as electrocatalysts in alkaline medium together with (Cr,Mn,Fe,Co,Ni) HEO nanoparticles (NPs) prepared via the sol-gel method. (Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)3O4 NFs and NPs (Tafel slopes: 49.1 and 51.3 mV dec−1, respectively) outperform both (Cr0.2Mn0.2Fe0.2Co0.2Zn0.2)3O4 and (Cr0.2Mn0.2Fe0.2Ni0.2Zn0.2)3O4 NFs (62.5 and 59.6 mV dec−1, respectively) and IrO2 reference electrocatalyst (52.9 mV dec−1). The higher concentration of oxygen vacancies on their surface and the higher occupation of octahedral sites by redox-active Co2+ and Ni2+ centres are responsible for their behaviour. The present electrospun HEO NFs have great potential as ink-jet printable electrocatalysts

High entropy oxides as anode material for Li-ion battery applications: A practical approach

Owing to their robust Li-ion storage properties induced by the entropy stabilization effect, transition-metal-based high entropy oxides are considered promising electrode materials for use in Li-ion batteries. In this work, full-cells comprising (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O anode and LiNi1/3Co1/3Mn1/3O2 cathode were assembled to explore their potential for practical applications. The cycling performance was studied by different electrochemical experiments. The cells were found to deliver an initial specific discharge capacity of 446 mAh g−1, which was maintained at 300 and 256 mAh g−1 after 50 and 100 cycles, respectively, and they showed stable cyclability even at specific currents of 1.6 A g−1. More importantly, high specific energy and power densities of about 240 Wh kg−1 and 320 W kg−1 were achieved. Additionally, pouch cells of total capacity 2.5 mAh were built and successfully employed as a power source

High-Entropy Metal–Organic Frameworks for Highly Reversible Sodium Storage

Prussian blue analogues (PBAs) are reported to be efficient sodium storage materials because of the unique advantages of their metal–organic framework structure. However, the issues of low specific capacity and poor reversibility, caused by phase transitions during charge/discharge cycling, have thus far limited the applicability of these materials. Herein, a new approach is presented to substantially improve the electrochemical properties of PBAs by introducing high entropy into the crystal structure. To achieve this, five different metal species are introduced, sharing the same nitrogen-coordinated site, thereby increasing the configurational entropy of the system beyond 1.5R. By careful selection of the elements, high-entropy PBA (HE-PBA) presents a quasi-zero-strain reaction mechanism, resulting in increased cycling stability and rate capability. The key to such improvement lies in the high entropy and associated effects as well as the presence of several active redox centers. The gassing behavior of PBAs is also reported. Evolution of dimeric cyanogen due to oxidation of the cyanide ligands is detected, which can be attributed to the structural degradation of HE-PBA during battery operation. By optimizing the electrochemical window, a Coulombic efficiency of nearly 100% is retained after cycling for more than 3000 cycles

P2-type layered high-entropy oxides as sodium-ion cathode materials

P2-type layered oxides with the general Na-deficient composition NaxTMO2 (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na+ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2 and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2 with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications

P2-type layered high-entropy oxides as sodium-ion cathode materials

P2-type layered oxides with the general Na-deficient composition NaxTMO2 (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na+ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2 and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2 with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications

Unit Roots and the Dynamics of Market Shares: An Analysis Using Italian Banking Micro-Panel

The paper proposes the use of panel data unit root tests to assess market share instability in order to have (preliminary) indications of the industry dynamic. The idea is to consider the movements in market shares not only as element of the market structure but rather reflecting conduct that arise from that market. If shares are mean-reverting, the firm actions only have a temporary effect on shares. On the other hand, if they are evolving, as signaled by the presence of unit roots, the gain in shares respect with the competitors could be long-term. To illustrate the potential of unit roots tests, I consider an application to the Italian retail banking industry.