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Directional graphene aerogels (DGAs) are proposed as electrode materials to alleviate ionic and mass transport issues in organic redox flow batteries (ORFBs). DGAs with high pore directionality would provide low resistance channels for effective ionic charge and liquid electrolyte transport in these devices. DGAs’ porous and directional characteristics can be controlled by the growth of ice crystals during freeze casting, which is influenced by the self-diffusivity of water, phase change driving forces, water−ice graphene interactions, and convection in the water−graphene media. It is found that mass transport-related properties of DGAs, including pore size and directionality, show a significant dependence on freezing temperature, graphene oxide (GO) loadings, and synthesis vessel diameter-to-height ratio (D/H). For the freezing temperature change from −20 to −115 °C, the average pore size progressively decreased from 120 to 20 μm, and the pore directionality transitioned from lamellar to ill-defined structures. When GO loadings were increased from 2 to 10 mg/mL at a fixed freezing temperature, pore size reduction was observed with less defined directionality. Furthermore, the pore directionality diminished with an increased width-to-height aspect ratio of DGA samples due to the buoyancy-driven convective circulation, which interfered with the directional ice/pore growth. Understanding the comprehensive effects of these mechanisms enables the controlled growth of ice crystals, leading to graphene aerogels with highly directional microstructures.
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Freeze‐cast honeycomb structures via gravity‐enhanced convection
Abstract The effect of gravity on directional solidification was investigated in solution‐based freeze casting. A preceramic siloxane‐based polymer was freeze‐cast with a cyclohexene solvent from two different directions: that against the direction of the gravitational force and that in concert with the gravitational force. Because the density of preceramic polymer is higher than the solvent, the segregated polymer creates a denser solution ahead of the freezing front than the underlying solution when the freezing direction is the same as the gravity direction. This results in convective flow in the liquid phase. This convective flow influences constitutional supercooling, which changes not only the pore size of freeze‐cast structure but also the pore morphology from dendritic to cellular pores.
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- Award ID(s):
- 1911972
- PAR ID:
- 10449887
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 104
- Issue:
- 9
- ISSN:
- 0002-7820
- Format(s):
- Medium: X Size: p. 4309-4315
- Size(s):
- p. 4309-4315
- Sponsoring Org:
- National Science Foundation
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