Tunable and Well-Defined Bimodal Porous Model Electrodes for Revealing Multiscale Structural Effects in the Nonaqueous Li–O₂ Electrode Process

Abstract

Porous architecture is key in the nonaqueous lithium−oxygen (Li−O 2 ) electrode process, which is attracting huge interest because of its application in reversible energy storage with high theoretical energy density. However, it is still challenging to understand the optimal porous structure to obtain high reversibility of the reaction. One main reason is because of instability and undefined porous structures of standard electrodes consisting of carbonaceous materials, and this issue hinders from unveiling the fundamental mechanism in the complicated electrode process. Here, we developed a new synthetic strategy to design monolithic electrodes of pure metallic nickel with controlled bimodal porous structures. The present work aims to investigate the fundamental effects of the bimodal macroporous structure in the Li−O 2 electrode process under carbon-/binder-free stable model electrodes. As the result, we found that, depending on the multiscale structural configurations, the bimodal macroporous structure gave significant influences to key properties, such as the efficiency of redox-mediators, discharge overpotential, and cycling life. This work indicates that the rational design of stable and conductive porous materials is one of the promising approaches to investigate highly complicated electrochemical reactions in porous electrodes and suggest new guidelines for further development of hierarchically structured electrodes toward advanced electrochemical systems.

Emergent electrochemical functions and future opportunities of hierarchically constructed metal–organic frameworks and covalent organic frameworks

“…It is also noteworthy that macroporous structures contribute to provide suitable space for reversible formation/decomposition of solid products during the electrode process. 70,[71][72][73][74]77 (Fig. 2C) This space is especially important for non-intercalation-based electrochemical energy storage chemistries, such as Li-O 2 or Li-S chemistries.…”

“…Deposition and accumulation of the intermediates and products, such as Li 2 O 2 , obstruct the small porous structures including meso-/ microporous structures, and resulted in blocking the pathway for O 2 diffusion and Li + migration. Furthermore, as shown by the very recent work by using well-defined and stable model bimodal porous electrodes, 77 an optimized porous structure is indispensable to improve energy efficiency and cycle life of the non-aqueous Li-O 2 electrode process, especially the systems containing redox-mediators. Therefore, multiscale structured MOFs and COFs have the opportunity to be applied to the product formation/decomposition-based electrode processes.…”

“…143 However recent rigorous works using a variety of meso-/macroporous metallic electrodes have shown that meso-/macoroporosity can control the electrode process. 40,77,144,145 Furthermore, a confinement environment is shown as an alternative approach to control (electro)catalytic reactions. 40,146 MOFs/COFs are recently used as buildingblocks to construct novel-type heterojunction electrocatalysts, which interestingly suggested not only to improve activities but also selectivity at the same time by taking advantage of molecular-level structural designability.…”

“…40,146 MOFs/COFs are recently used as buildingblocks to construct novel-type heterojunction electrocatalysts, which interestingly suggested not only to improve activities but also selectivity at the same time by taking advantage of molecular-level structural designability. [147][148][149][150] Considering that the meso-/macroporosity of the metallic electrodes plays a pivotal role in control of the electrode process, 40,77,144,145 designing hierarchical constructions of the electrodes with MOFs/COFs as building-blocks can provide further control of the electrode process including the activity and the selectivity.…”

“…Although we can see a variety of remaining challenges, the hierarchically constructed MOFs/COFs have great potential to improve electrochemical performance derived from the meso-/ macroscopic structures which contribute to the fast mass transportation, 50,64 meso-/macroscopic structures which realize flexible and/or mechanically strong electrodes, 67,69 the reaction space for reversible formation/decomposition of solid product derived from the meso-/macropores, 70,71,[72][73][74]77 heterostructuring 84 and hybridization 83 derived molecular selectivity. In addition, introducing additional functional groups within reticular networks and/or reticular networks themselves 25,26 also provide the opportunity to improve the electrochemical performances.…”