Phase stability is an important topic for high entropy alloys (HEAs), but the understanding to it is very limited. The capability to predict phase stability from fundamental properties of constituent elements would benefit the alloy design greatly. The relationship between phase stability and physicochemical/thermodynamic properties of alloying components in HEAs was studied systematically. The mixing enthalpy is found to be the key factor controlling the formation of solid solutions or compounds. The stability of fcc and bcc solid solutions is well delineated by the valance electron concentration (VEC). The revealing of the effect of the VEC on the phase stability is vitally important for alloy design and for controlling the mechanical behavior of HEAs.

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Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys

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Phase stability in high entropy alloys: Formation of solid-solution phase or amorphous phase

Solid oxide fuel cells (SOFCs) have the promise to improve energy efficiency and to provide society with a clean energy producing technology. The high temperature of operation (500−1000 °C) enables the solid oxide fuel cell to operate with existing fossil fuels and to be efficiently coupled with turbines to give very high efficiency conversion of fuels to electricity. Solid oxide fuel cells are complex electrochemical devices that contain three basic components, a porous anode, an electrolyte membrane, and a porous cathode. In this short review, a survey of the types and properties of materials that have been considered for each of these components is presented with an emphasis on the requirements for operation at intermediate temperature (500−800 °C). Some directions for future research are discussed.

Materials for Solid Oxide Fuel Cells

With the development of renewable energy and electrified transportation, electrochemical energy storage will be more important in the future than it has ever been in the past. Although lithium-ion batteries (LIBs) are traditionally considered to be the most likeliest candidate thanks to their relatively long cycle life and high energy efficiency, their limited energy density as well as cost are still causing a bottleneck for their long-term application. Alternatively, metal–air batteries have been proposed as a very promising large-scale electricity storage technology with the replacement of the intercalation reaction mechanism by the catalytic redox reaction of a light weight metal–oxygen couple. Generally, based on the electrolyte, these metal–air batteries can be divided into aqueous and nonaqueous systems, corresponding to two typical batteries of Zn–air and Li–air, respectively. The prominent feature of both batteries are their extremely high theoretical energy density, especially for nonaqueous Li–air batteries, which far exceeds the best that can be achieved with LIBs. In this review, we focus on the major obstacle of sluggish kinetics of the cathode in both batteries, and summarize the fundamentals and recent advances related to the oxygen catalyst materials. According to the electrolyte, the aqueous and nonaqueous electrocatalytic mechanisms of the oxygen reduction and evolution reactions are discussed. Subsequently, seven groups of oxygen catalysts, which have played catalytic roles in both systems, are selectively reviewed, including transition metal oxides (single-metal oxides and mixed-metal oxides), functional carbon materials (nanostructured carbons and doped carbons), metal oxide–nanocarbon hybrid materials, metal–nitrogen complexes (non-pyrolyzed and pyrolyzed), transition metal nitrides, conductive polymers, and precious metals (alloys). Nonaqueous systems have the advantages of energy density and rechargeability over aqueous systems and have gradually become the research focus of metal–air batteries. However, there are considerable challenges beyond catalysts from aqueous to nonaqueous electrolytes, which are also discussed in this review. Finally, several future research directions are proposed based on the results achieved in this field, with emphasis on nonaqueous Li–air batteries.

Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes

Progress in material selection for solid oxide fuel cell technology: A review

Physical Metallurgy and Mechanical Properties of Transition-Metal Laves Phase Alloys

Spinel phase based on Mn 1+x Cr 2-x O 4 (0 ≤ x ≤ 1) was synthesized and characterized. The lattice parameter of the spinel was found to increase with increasing Mn content up to x = 0.7. At x > 0.7, the cubic spinel changed to tetragonal because of Jahn-Teller distortion. The electrical conductivity of the spinel also increased with increasing Mn content. Electrical conduction in this phase was likely through a small polaron hopping mechanism. The spinel phase showed similar electrical conductivity in both oxidizing and reducing environments expected in a solid oxide fuel cell operation.

Electrical Conductivity of the Manganese Chromite Spinel Solid Solution

La0.75Sr0.25Cr0.5Mn0.5O3−δ + Cu composite anode running on H2 and CH4 fuels

Cu(Pd)-impregnated La0.75Sr0.25Cr0.5Mn0.5O3 − δ anodes for direct utilization of methane in SOFC

The phase stability in NbCr2-based transition-metal Laves phases was investigated, using data from binary X–Cr, Nb–X, and ternary Nb–Cr–X phase diagrams. It was shown that when the atomic size ratios are kept identical, the average electron concentration factor, e/a, is the dominating factor in controlling the phase stability of NbCr2-based transition-metal Laves phases. The e/a ratios for different Laves polytypes were determined as follows: with e/a 7.65, the C15 structure is stabilized again. A further increase in the electron concentration factor (e/a>8) leads to the disordering of the alloy.

Jiahong Zhu

Papers

Physical Metallurgy and Mechanical Properties of Transition-Metal Laves Phase Alloys

Electrical Conductivity of the Manganese Chromite Spinel Solid Solution

La0.75Sr0.25Cr0.5Mn0.5O3−δ + Cu composite anode running on H2 and CH4 fuels

Cu(Pd)-impregnated La0.75Sr0.25Cr0.5Mn0.5O3 − δ anodes for direct utilization of methane in SOFC

Effect of electron concentration on the phase stability of NbCr2-based Laves phase alloys