Gel‐Based Adaptive Encapsulation Strategy Enables High‐Performance, Highly Flexible Fiber‐Shaped Direct Methanol Fuel Cells

This Highlight discusses a groundbreaking fiber-shaped fuel cell that uses swelling gel-encapsulated yarns to generate internal pressure. This strategy eliminates rigid components, enabling flexible, cuttable power sources for wearable electronics. As highly efficient electrochemical devices, fuel cells directly convert the chemical energy of hydrogen, alcohol, and other fuels into electricity, offering major advantages for clean power generation [1, 2]. Their high energy density and rapid refueling capabilities establish them as promising candidates for future energy supply systems [3]. Currently, the rapid spread of mobile and wearable devices is driving the need for lightweight, flexible power, fostering growth in flexible energy technologies like solar cells and lithium-ion batteries [4-6]. However, the development of flexible fuel cells is hindered by rigid components, stringent pressure encapsulation, and fuel sealing issues [7]. This creates an urgent need for high-performance versions that operate without rigid parts. Furthermore, despite progress in flexible power technologies, key challenges have yet to be overcome, such as reliance on light sources, limited energy density, and the need for faster charging [8]. Therefore, ongoing innovation and performance enhancement in flexible power source technologies are particularly crucial. To address this issue, Yan et al. have recently developed a fiber-shaped direct methanol fuel cell (FDMFC) with exceptional performance and stability, as reported in Nature Materials [9]. Composed of gel-encapsulated woven cotton yarns (Yarn@gels) as its core, the cell utilizes this structure to fulfill multiple purposes: enabling rapid fuel storage and ensuring stable internal pressure encapsulation. This Yarn@gels-based fiber architecture effectively resolves the trade-off between flexibility and pressure encapsulation in flexible fuel cells, significantly reducing encapsulation complexity and system volume. Furthermore, the technology enables refueling within minutes and maintains high levels of safety and operational reliability, even under extreme mechanical conditions such as bending or cutting. This breakthrough offers valuable insights for the development of next-generation flexible fuel cells and presents a viable design pathway for high-performance flexible power systems. As illustrated in Figures 1a and S1, traditional fuel cell systems typically employ an external pressure encapsulation mechanism. The structure relies on rigid end plates (e.g., metal or composite plates) to transmit an externally applied clamping force inward through its internal component layers, which include current collectors, gas diffusion layers, and membrane electrode assemblies. This force ensures tight physical contact at the interfaces, thereby reducing electrical contact resistance and facilitating the effective transport of reactants and products. However, its core limitation lies in the dependence on rigid end plates and robust support structures, which not only significantly increase system weight and volume but fundamentally constrain mechanical flexibility. In contrast to conventional strategies, the FDMFC adopts an innovative internal pressure encapsulation approach, centered around three key functions of Yarn@gels: fuel storage, autonomous internal pressure encapsulation, and flexible structural support. Specifically, the FDMFC structure comprises essential components including Yarn@gels for methanol storage, membrane electrode assemblies (MEAs), and nylon mesh for initial encapsulation (Figures 1b and S2). The electrochemical reactions involve redox processes across solid, liquid, and gas phases: methanol undergoes electrochemical oxidation at the anode while oxygen is reduced at the cathode, producing carbon dioxide, water, and protons (H+). These protons migrate from the anode to the cathode through the Nafion membrane via proton-conducting mechanisms. Critically, the Yarn@gels undergo dynamic swelling upon methanol absorption, generating radial expansion forces that propagate outward from the fiber core. This spontaneously establishes uniform and sustained internal contact pressure across all component interfaces. The fiber architecture ensures homogeneous pressure distribution throughout the system, fundamentally eliminating dependence on external rigid end plates or mechanical clamping. The resulting FDMFCs exhibit exceptional flexibility, cuttability, and rapid refueling capability, positioning them as high-efficiency energy devices. As the core component, the exceptional functionality of Yarn@gels is achieved through synergistic integration between the crosslinked gel and cotton fibers. As shown in Figure 1c, Yarn@gels feature a multi-level hierarchical architecture. In this composite system, cotton fibers serve as the structural scaffold, coated with a synergistically crosslinked gel layer formed via UV-initiated polymerization. This gel is synthesized by copolymerizing three primary monomers—N,N-Dimethylacrylamide, hydroxyethyl acrylate, and acrylamide—with poly(ethylene glycol) diacrylate (PEGDA), establishing a stable three-dimensional network that incorporates covalent bonds, hydrogen bonds, and dynamic weak covalent-bond crosslinking. Excellent wettability and stable interfacial adhesion between the gel and cotton fibers are achieved through a synergistic crosslinked design, which preserves the cotton's inherent capillary action while also imparting remarkable solvent responsiveness. The composite configuration, combining the fibrous skeleton with the gel matrix, lays the structural foundation for rapid solvent response, outstanding mechanical properties, and durable interfacial adhesion. Benefiting from the robust interfacial bonding within the Yarn@gels, a fiber-shaped battery composed of four independent units continues to power an electronic clock steadily even after being severed (Figures 1d and S3), owing to maintained effective fuel sealing and interfacial contact at the cut sections. Moreover, as shown in Figure 1e, the battery stack maintains normal power output under extreme conditions such as punctures by wooden toothpicks and water flushing. This directly stems from the exceptional mechanical toughness, chemical stability, and persistent interfacial adhesion provided by the synergistically crosslinked gel network within the Yarn@gels. The adaptability, cuttability and overall flexibility of the FDMFCs enhance operational safety and expand their versatility in various applications. This research introduces a self-adaptive internal pressure encapsulation strategy that represents a fundamental departure from conventional design paradigms. Rather than relying on external fixtures, it ingeniously embeds the required mechanical constraints as an intrinsic material property. This approach successfully decouples the longstanding conflict between flexibility and pressure encapsulation, thereby establishing a new technical pathway for constructing high-performance, rigid-free fuel cells. More than a performance improvement, this work redefines the conceptual foundation for building flexible power sources. The synergistic principle of coupling material swelling with encapsulation needs offers transformative insights for the broader field of flexible electronics. Yu Meng: writing – review and editing, writing – original draft, methodology, data curation, and funding. Qichong Zhang: writing – review, supervision, funding acquisition, methodology, and conceptualization. This work was supported by the National Natural Science Foundation of China (52473270 and T2422028), the National Key R&D Program of China (2022YFA1203304), and the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (start-up grant E1552102). This work was supported by the National Natural Science Foundation of China (52473270 and T2422028), the National Key R&D Program of China (2022YFA1203304), and the Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (start-up grant E1552102). The authors declare no conflicts of interest. Figure S1: Mechanism of pressure encapsulation in FDMFCs. (a) Schematic of the FDMFCs incorporating yarn. (b) Schematic of the FDMFCs incorporating Yarn@gel. (c) Stress distribution and methanol distribution in the FDMFCs containing yarn. (d) Stress distribution and methanol distribution in the FDMFCs containing Yarn@gel. Figure S2: (a) Scanning electron microscopy cross-section image of a Yarn@gel. (b) Magnified SEM image of the gel layer. (c) Photographs of yarn, Yarn@gel and cross-section of an FDMFC. (d) Swelling ratio of PDMA, PEA and PAM in water and methanol at 25°C. (e) Methanol permeation of a Nafion 212 membrane and a Gel–Nafion 212 composite membrane, the latter consisting of a 100-μm-thick gel layer coated on one side of the Nafion membrane. (f) Electrostatic and van der Waals energies of yarn and Yarn@gels with methanol molecules. (g) Monotonic tensile stress and strain of Yarn@gels between −20°C and 70°C (strain rate, 50 mm min−1). (h) Mechanical durability assessment of Yarn@gels under cyclic bending. Quantification of peak stress and strain after cyclic loading. (i) Force–displacement curves of 5-cm-long Yarn@gels and pristine yarn during controlled peeling tests from a collector substrate at 25°C (strain rate, 50 mm min−1). Figure S3: (a) Side view and (b) top view of the FDMFC stack comprising twelve 5-cm-long FDMFC segments powering a 3 V light. Photographs of the open-circuit voltage of FDMFCs under: (c) Uncut (16-cm-long). (d) Cut once (8-cm-long) and connected in series. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.