Advanced manufacturing prospects, towards enhanced performance

During my PhD I investigated the potential of 3D printing as a manufacturing technique for RFB electrodes. Even though the studied model grid structures were produced using a low-resolution stereolithography 3D printer (0.9 x 0.9 x 0.9 mm3 pores with a pillar thickness of 0.3 mm) resulting in a low internal surface area, we showed that the mass transport and pressure drop can be improved compared to off-the-shelf electrodes. To become competitive with commercially used electrodes, alternative 3D printing techniques must be investigated in combination with optimized electrode designs. To this end, I propose to explore the following:

(1) Enhancing the internal surface area of the 3D printed electrodes to improve their electrochemical output. By utilizing a 3D printer with higher resolution, e.g., by using two-photon polymerization, smaller features down to the submicrometer scale can be printed [1-3], increasing the internal surface area of the structures. Moreover, the internal surface area can be enlarged by obtaining 3D structures with porous surfaces. Possible methods to obtain a porous surface include etching, thermal treatments, coatings, or the addition of a porogen into the printing resin (if a liquid resin is used, for example with photopolymerization-based additive manufacturing techniques). We investigated the option of altering the resin formulation to induce surface porosity by the addition of a porogen into the resin formulation, inspired by the work of Dong et al. [4]. To this end, we studied various resin formulations (monomer type, porogen type, and porogen concentration, combined with a photoinitiator) and extraction methods (different solvents, heat treatments, and pressures) and performed the experiments on single resin droplets that were photopolymerized with UV-light (405 nm). The success of the porogen extraction was subsequently analyzed by thermographic analysis and scanning electron microscopy (Figure 1). We managed to obtain a certain degree of porosity on the electrode surface, see Figure 1, but further optimization is required. It is believed that CO2 supercritical drying as a porogen extraction method, as was applied by Dong et al. [4], could increase surface porosity and can result in enhanced electrode performance for RFB application. Figure 1: Scanning electron microscopy images at 500 x magnification of the surface and cross-section of cured resins with and without porogen. The cured resin samples with porogen underwent a porogen extraction method of 24 hours in a solvent. Furthermore, the thermographic analysis is shown for this specific cured resin formulation and extraction method where the red line represents a sample with porogen that is not extracted, the red dotted line without porogen, and the black line with porogen extracted with the 24-hour wash in a solvent method.

(2) The increase in internal surface area must go hand-in-hand with attaining a high electrolyte permeability. Hence, the trade-off between a high internal surface area and thus small pores, and a high electrolyte permeability and thus an open structure, must be optimized. We believe that a bimodal pore size distribution is key in providing a high current output and additionally a low pressure drop in the reactor. Yet, the electrode-flow field interactions must be considered in the design of such electrode structures as the flow induced by the flow field impacts the optimized orientation of the large and small pores.

(3) Investigating advanced electrode designs. Potential research directions include optimized pillar or fiber geometries and shapes, their orientation, location, and layer thickness, but additionally designs beyond pillar or fiber structures. Moreover, the electrode thickness and mechanical stability should be optimized. Advanced electrode designs are required to enhance the mass transport in porous electrodes on all length scales by improving the electrolyte distribution, internal mixing, and diffusion layer thickness. To this end, 3D printing can be combined with topology optimization methods to realize the manufacturing of computationally optimized electrode structures.

(4) Fine-tuning the resin formulation and thermal sequence. By refining the resin formulation, the degree of resin spreading and thus the printing resolution could be controlled, together with the surface roughness and porosity, the shrinkage degree upon carbonization, the mechanical and chemical stability, and potentially the conductivity by electing conductive resins. Whereas, the thermal sequence can be optimized by e.g., carbonization at elevated temperatures (>1100 °C) to improve the conductivity. The ideal carbonization sequence and oxidation stabilization step could vary for different resins and electrode structures and should be investigated in detail to produce conductive, mechanically stable electrodes with high surface porosity.

(5) The potential of 3D printed electrodes for RFB applications compared to current state-of-the-art electrodes. The electrochemical performance, pumping requirements, mechanical and chemical stability, cycling performance, and cost of manufacturing should all be considered to evaluate whether additive manufacturing could become a competing electrode manufacturing strategy.

References:
[1] S. Chandrasekaran et al., Carbon N. Y. 179, 125–132 (2021).
[2] A. Vyatskikh et al., Nat. Commun. 9, 593 (2018).
[3] Y. C. Saraswat et al., J. Colloid Interface Sci. 564, 43–51 (2020).
[4] Z. Dong et al., Nat. Commun. 12, 1–12 (2021).