(286b) Electropolymerization of Single-Crystal 2D Covalent Organic Frameworks on a Liquid Metal Electrode

Problem and Motivation: The pursuit of molecularly ordered materials represents one of this century’s greatest challenges in materials chemistry. At present, materials such as 2D Covalent Organic Frameworks (COFs) are still limited by defects that arise from synthesis [1]. Although it is the most widely used approach, solvothermal synthesis fails to produce the monocrystalline films required for high-performance applications [1]. Indeed, isotropic Brownian motions at the air-solvent interface produce "morphology-limited" particulate films with intercrystalline boundaries (<100 μm crystallite size, >100 nm surface roughness) rather than continuous, monocrystalline sheets [2]. Respectively, applications such as membrane separations, energy storage, and lightweight mechanical resistance benefit from high pore size uniformity, unimpeded charge transport, and grain boundary-free structures for optimal energy dissipation. In response, ideal 2D COF targets feature a monocrystalline film with no nano- and microscale grain boundaries and pores parallel to the desired direction of transport [3]. Furthermore, the film has controllable thickness, a uniform pore size distribution and it is atomically smooth. While traditional methods can be energy-intensive (60-200°C for 24-72 hours) and represent a fundamental barrier to commercialization [2][4], we propose synthesis at mild conditions.

Innovation: While photochemical and mechanical templating methods show promise for morphology control, they introduce limitations that prevent scale-up [5][6][7]. So far, efforts in electroorganic synthesis yield particulate networks rather than continuous films due to rough solid electrode surfaces [8][9][10]. This work is the first investigation of liquid gallium as both a working electrode and structural template for 2D COF synthesis. This approach builds directly on the demonstration that liquid metal CVD produces ultra-flat MOF films with superior crystallinity compared to solid substrates [11]. At the same time, this approach eliminates the need for high-vacuum equipment and high-temperature operation. The innovation lies in exploiting liquid metal interfaces, which feature controlled electron transfer, atomic-level smoothness and lateral mobility. As such, the growth of monocrystalline COFs is enforced in 2D at mild conditions on an interface that is smoother than solid substrates, yet not as mobile as liquid-liquid interfaces.

Methods: We employ a computational and experimental approach. We investigate face-on vs. edge-on monomer adsorption on liquid gallium, where the former is more conducive to monocrystal formation. Our Molecular Dynamics (MD) simulations using CP2K software confirm that face-on monomer adsorption is more thermodynamically favorable on liquid gallium compared to solid substrates. Additionally, MD demonstrates that liquid gallium maintains 0.045 nm Root Mean Square surface roughness compared to >10 nm for solid electrodes, providing the atomically smooth template required for defect-free polymerization. Building on these findings, Density Functional Theory (DFT) calculations using CP2K software with Perdew-Burke-Ernzerhof functional and Grimme D3 correction quantify reaction mechanisms and adsorption energetics. Green's function methods will model time-dependent electron transfer dynamics to optimize applied potential windows. Experimentally, we synthesize COFs from brominated and thiolated aromatics (1,3,5-tribromobenzene, benzenehexathiol, tris(bromophenyl)benzene) in a three-electrode cell. We vary applied potential (-0.5 to -2.0 V vs Ag/AgCl), electrolyte concentration (0.1-1.0 M), and reaction time (1-24 hours). Comprehensive characterization includes operando spectroscopy (FT-IR, Raman), microscopy (TEM, AFM), and synchrotron X-ray techniques (GIWAXS) to evaluate crystallinity, morphology and interlayer stacking behavior.

Expected Results and Industrial Significance: Based on computational validation and energy analysis informed by literature, we project to achieve >60% energy reduction compared to solvothermal synthesis while producing COF films with a surface roughness <5 nm (compared to 37-100 nm for existing methods [5]), precise thickness control across 10-500 nm, and enhanced mechanical properties. These improvements address bottlenecks limiting COF adoption in semiconductor manufacturing, water treatment), and other lightweight applications, where current COF synthesis costs prevent commercial viability.

Broader Impact and Scale-Up Potential: This research will establish liquid metal electrochemistry as a new platform for 2D materials synthesis. The fundamental understanding of electropolymerization at liquid interfaces will advance process intensification strategies and enable scalable production of high-performance materials for next-generation chemical separation technologies (addressing 10-15% of global energy consumption [12]), energy storage systems, and lightweight protective materials.

Conclusions and Future Directions: Liquid metal-templated electropolymerization can achieve the morphological control and energy efficiency necessary to make 2D COFs industrially viable. We highlight a transformative approach to interface reaction engineering for anisotropic materials manufacturing, with planned future work extending to continuous flow synthesis. The work advances both fundamental electrochemical engineering science and manufacturing technology, positioning the field to lead in next-generation materials development while addressing sustainability challenges.

Keywords: Electropolymerization, Covalent Organic Frameworks, Liquid Metal Electrodes

References:

[1]. Burke, D. W., Jiang, Z., Livingston, A. G. & Dichtel, W. R. 2D Covalent Organic Framework Membranes for Liquid‐Phase Molecular Separations: State of the Field, Common Pitfalls, and Future Opportunities. Advanced Materials 36, (2023).

[2]. A. P. Côté, et al., Science 310, 1166–1170 (2005).

[3]. Zhou, W. et al. Advances in single-crystal framework materials: Design, synthesis, and applications. Results in Engineering 24, 103076 (2024).

[4]. J. Xiao, J. Chen, J. Liu, H. Ihara, H. Qiu, Green Energy & Env 8, 1596–1618 (2023).

[5]. Kim, T. et al. Photochemical and Patternable Synthesis of 2D Covalent Organic Framework Thin Film Using Dynamic Liquid/Solid Interface. Small Methods 8, (2024).

[6]. Kim, S., Lim, H., Lee, J. & Choi, H. C. Synthesis of a Scalable Two-Dimensional Covalent Organic Framework by the Photon-Assisted Imine Condensation Reaction on the Water Surface. Langmuir 34, 8731–8738 (2018).

[7]. Sahabudeen, H. et al. Wafer-sized multifunctional polyimine-based two-dimensional conjugated polymers with high mechanical stiffness. Nat Commun 7, (2016).

[8]. M. G. Walter, C. C. Wamser, Synthesis and Characterization of Electropolymerized Nanostructured Aminophenylporphyrin Films. J. Phys. Chem. C 114, 7563–7574 (2010).

[9]. Wang, X. et al. Cathodic Polymerization through Electrochemical Dehalogenation. Macromolecules 56, 10198–10205 (2023).

[10]. Preefer, M. B., Oschmann, B., Hawker, C. J., Seshadri, R. & Wudl, F. High Sulfur Content Material with Stable Cycling in Lithium‐Sulfur Batteries. Angew Chem Int Ed 56, 15118–15122 (2017).

[11]. Liu, J. et al. On-liquid-gallium surface synthesis of ultrasmooth thin films of conductive metal–organic frameworks. Nat. Synth 3, 715–726 (2024).

[12]. Humphrey, J.; Keller, G. E. Separation Process Technology (McGraw-Hill, 1997).