"Simply put, all physical exploration is about symmetry." PW Anderson, Nobel laureate of physics, told us so much in his mid-seasons. A recent research team of physics and materials scientists from Harvard University, Purdue University, Argonne National Laboratory of Energy and Rensselaer Polytechnic Institute, for the first time, introduced the principle of electronic strong correlation into the electrolyte of fuel cells and achieved A major breakthrough.
The key to their success is the regulation of the crystal symmetry of strongly related materials. The first group of highly-correlated fuel cells made by the team is comparable to the ever-more advanced YSZ electrolyte fuel cell. It is foreseeable that after certain engineering optimization, the performance of such a fuel cell can be greatly improved, thereby providing a cornerstone for realizing large-scale fuel cell market-oriented applications.
With nearly 100% theoretical energy conversion efficiency and clean reaction products, fuel cells have long been expected to replace traditional heat engines. Solid fuel cells is an important part of the solid electrolyte.
Ideal solid electrolyte to meet at least two requirements: First, good electrical insulation, and second, good ionic conductivity. Good electrical insulation can effectively inhibit the leakage phenomenon, thereby optimizing the fuel cell output power. At the same time, good ion conductivity can ensure high power of fuel cell.
However, the traditional fuel cell electrolytes present a huge technical obstacle in both of these aspects. For example, many of the electrolytes that have a perovskite structure have ionic conductivity based on the migration of oxygen vacancies. In other words, oxygen vacancies as an intermediary to become an important prerequisite. However, due to the existence of vacancy defects, the cation-doped electrolyte easily leaks, greatly reducing the fuel cell efficiency and power. Therefore, if we want to suppress the leakage phenomenon, we need to fundamentally find and adopt a new ion conduction mechanism. The team made a breakthrough in this area by introducing a strong associate electrolyte and confirming a new electrolyte conduction mechanism.
The d-electron cloud distributions of the transition metals that make up strongly related materials are highly regularized. This regionalization leads to huge Coulomb repulsive forces (due to the highly overlapping electron clouds) when two spins of opposite electrons are on the same d orbit (such as the dxy orbit). This makes it difficult for scientists to use classical single-electron semiconductor models to characterize the electronic structures and physical appearance of strongly associated materials.
In many cases, this material is predicted to be metallic with single electron model theory. However, the actual performance is usually the insulator. Nickel oxide is a very typical strong associative material (in the case of a single electron model, it should be a metal and was experimentally found to be an insulator with a forbidden band width of three to four electron volts). Due to the fact that the d-orbital charge of the strongly-associated material d determines whether it is a metal or insulator, the metal insulator transition can be achieved by controlling the orbital fill. This shift is also known as the Mott shift.
Of course, as a side-note, HTS materials are the most famous and the most complex of the most relevant materials. The Mott transition due to this electron filling can occur in extreme times (femtoseconds) and its energy requirements are usually very low. For a long time, people want to use this strong correlation material to replace the semiconductor silicon inside the transistor to achieve low-power electronic devices. At present, we study more material focused on vanadium dioxide and rare earth metal nickel compounds. The research team has been trying to control the phase transition with electrostatic doping. Due to the extremely high carrier doping concentration required for this material phase change, there has so far not been enough evidence that this phase transition can be successfully induced by electrostatic doping.
Less than two years ago, however, they found that the critical charge concentration required for Mott transitions can be achieved by proton doping, making the first real implementation of a metal insulator on rare earth nickel compounds at the transistor structure (proton transistor) level Phase change.
Reference: Colossal resistance switching and band gap modulation in a perovskite nickelate by electron doping.
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