The safe and efficient storage of hydrogen is crucial for the advancement of green mobility. Traditional pressure vessels, typically spherical or cylindrical, present limitations in terms of weight, spatial conformability, and safety. While spherical vessels are highly efficient in terms of pressure resistance per unit weight, their fixed geometry restricts their applicability to diverse spaces and transport systems. Cylindrical vessels, while more adaptable in shape, suffer from similar weight-related inefficiencies. Furthermore, both designs share a susceptibility to catastrophic failure upon rupture. The 'leak-before-break' design philosophy, employed with thin-walled vessels, offers a significant improvement in safety by allowing for gas leakage before complete vessel rupture. However, even with this improvement, pressure vessel failures continue to occur. This research investigates the potential of shellulars, a novel class of micro-architectured materials, as a superior alternative for pressure vessel applications. Shellulars are distinguished by their continuous, smooth-curved, thin shell structure formed by a triply periodic minimal surface (TPMS). This unique architecture enables them to support external loads through co-planar stresses, a feature that differentiates them from microlattices, nanolattices, and mechanical metamaterials. Crucially, shellulars exhibit the highest strength among ultralow-density materials (density <10⁻² g/cc) under stretching-dominated conditions, leading to the hypothesis that they could effectively resist internal pressure in a manner analogous to a balloon. While previous research has explored the pressure resistance of shellulars, a comprehensive analysis of their efficiency as pressure vessels – considering internal volume per total weight – has been lacking. This study aims to address this gap by conducting FEA simulations and experimental tests to demonstrate the superior performance of shellular-based pressure vessels compared to their conventional counterparts.

Existing literature highlights the limitations of conventional pressure vessel designs for hydrogen storage in green mobility. The need for increased safety and reduced weight has driven research into novel materials and designs. Microlattices, nanolattices, and thin foils have been explored as ultralight materials with potential for pressure vessel applications; however, their structural characteristics make them unsuitable for this purpose. These materials are typically fabricated by depositing a hard substance on a polymer template, followed by etching, resulting in a single continuous foil architecture. While suitable for applications such as heat exchangers, their limited internal volume relative to their weight renders them inefficient as pressure vessels. Research on triply periodic minimal surfaces (TPMS) as a basis for material design is also relevant. TPMS offer unique geometric properties that can be exploited for various applications, including those requiring high strength and low weight. However, the application of TPMS to pressure vessel design has been largely unexplored until the present work.

The research employed a combination of computational modeling and experimental validation. Finite element analysis (FEA) was used to simulate the behavior of shellular pressure vessels under internal pressure. Three TPMS types (P-, D-, and F-RD-surfaces) were initially considered, with the P-surface ultimately selected due to its superior performance. The FEA investigated the effects of various parameters on pressure resistance, including TPMS type, cold stretching, cell size, and the use of a double-chambered design. Cold stretching, a common technique in pressure vessel fabrication, was simulated by applying excessive internal pressure to induce plastic deformation and enhance pressure resistance. The FEA results were used to develop equations predicting the yield pressure as a function of shell thickness and cell size. To evaluate the overall efficiency of the pressure vessels, a new parameter, Efficiency of Pressure Vessel (EPV), was defined. EPV incorporates yield pressure and internal volume, relative to the total weight of the vessel material. Equations for EPV were derived for both single- and double-chambered shellular vessels, and for conventional cylindrical and spherical vessels, allowing for direct comparison. Experimentally, P-surface shellular specimens were fabricated using a process involving 3D printing of a PVA template, casting with PMMA, chemical surface treatment (Han’s treatment), electroless copper plating, and PMMA template removal. Both single- and double-chambered specimens were prepared and tested under internal pressure using a custom-built test system. Yield pressures were measured, and the experimental results were compared to the predictions from the FEA. Additional FEA simulations were conducted to investigate the effect of cell size on pressure resistance and EPV. The simulations demonstrated that using a large number of small cells with a thinner shell could achieve comparable pressure resistance to single-cell vessels with thicker walls while improving EPV. Finally, further FEA was used to examine the role of the interior frame in double-chambered vessels and to assess the effect of defects on pressure resistance.

The FEA simulations indicated that cold stretching significantly improved the pressure resistance of the shellulars, resulting in a three-fold increase in yield pressure. The P-surface TPMS proved to be the most suitable geometry for pressure vessel applications. The EPV calculations revealed a significant advantage of the double-chambered design over the single-chambered design and over conventional pressure vessels. For a double-chambered P-shellular with a large number of cells (e.g., 100x100x100), the EPV exceeded that of both cylindrical and spherical vessels by a considerable margin (188% and 119%, respectively). Experimental results confirmed the FEA predictions, demonstrating good agreement between measured and simulated yield pressures for cold-stretched specimens. The experimental data further validated the superior performance of double-chambered shellular vessels compared to conventional designs. The fabrication process proved feasible, particularly for double-chambered specimens, which showed higher success rates than single-chambered specimens due to the lower stress on the interior frame. FEA studies demonstrated that the inner frame plays a key role in maintaining the structural integrity of the outer shell but could tolerate a significant number of defects without affecting the pressure resistance. The study also indicated that the fabrication of large-scale shellular pressure vessels might be achieved using a combination of robotic rough welding for the inner frame and precise laser welding for the outer shell.

The findings of this study directly address the research question by demonstrating the superiority of shellular pressure vessels over conventional designs. The double-chambered, cold-stretched P-surface shellular with a high number of cells offers a significant increase in internal volume per unit weight, a crucial factor for hydrogen storage applications. The improved EPV, combined with the leak-before-break safety feature inherent in the thin-walled design, suggests a potential solution for the limitations of current technology. The results are significant for the field of pressure vessel design, suggesting a paradigm shift towards micro-architectured materials with tailored geometries. The potential for using 2D materials to further enhance the strength and flexibility of shellular pressure vessels also offers promising avenues for future development. The successful fabrication and testing of shellular specimens, particularly the high success rate of double-chambered specimens despite potential defects, indicates the practicality of the proposed design.

This research successfully demonstrated the feasibility and superior performance of shellular-based pressure vessels. The double-chambered design, combined with cold stretching and a large number of cells, significantly outperforms conventional designs in terms of internal volume per unit weight. The inherent leak-before-break safety feature provides an additional advantage. Future research should focus on optimizing the fabrication process for large-scale production and exploring the integration of 2D materials for further performance enhancements.

The current study focused on relatively small-scale shellular specimens due to fabrication constraints. Scaling up the fabrication process to create larger pressure vessels will require further development and optimization. While the double-chambered design showed higher robustness to defects, the presence of defects in the outer shell could still compromise the pressure resistance. Further research is needed to fully quantify the effect of defects on the long-term reliability of shellular pressure vessels. The current study used copper as the constituent material; future research could explore other materials with potentially superior properties for specific applications.