Computational fluid dynamics (CFD) models are important tools in the modeling process. The CFD modeling approach helps to understand many design applications, and electrochemical reactions, which are difficult to realize experimentally. In particular, it helps to predict and understand mass transfer, fluid flow, and heat transfer phenomena and is used for design evaluation. It is a good research method that can control various variables and predict performance compared to manufacturing the cell.

Numerical studies were performed to predict the design effect and performance based on the shape of the tubular fuel cell, which is a new design [21–27]. Coursange et al. confirmed the advantages of tubular PEMFC design for material transport through CFD [21]. Through modeling, compared to the planar PEMFC, the tubular PEMFC showed better performance in the high current density region. It has been found that it is easier to maintain the humidification of the membrane as well as a more uniform mass transfer of the reactant gases. Al-Baghdadi et al. proposed a CFD model for the analysis of transport phenomena, such as convective and diffusive heat and mass transfer, electrode kinetics, transport and phase change mechanisms of water, and potential fields of tubular PEMFCs (Fig. 2) [22,23]. The model analysis helped to identify important parameters and provided insight into the physical mechanisms that lead to fuel cell performance and durability under various material conditions. Sierra et al. performed a 3D numerical study on a cylinder PEMFC model [24]. The Cylindrical PEMFC design with three other Geometries flow fields was compared with the Planar PEMFC. The pressure, flow concentration, and current density were uniformly distributed according to the cylindrical geometry compared to the rectangular design, confirming the development potential of the cylindrical PEMFC. Saghali et al. studied the superiority of conical pipe PEM fuel cells compared to cylindrical PEM fuel cells [25]. The authors confirmed that the performance of the conical PEMFC was better than the cylindrical PEMFC, and the fuel cell performance increased as the conical angle increased. Tubular PEMFCs with various MEA structures are also being studied. Mohammadi-Ahmar et al. modeled for a tubular PEMFC with a different MEA arrangement, not a cylindrical MEA [26]. A square tubular PEMFC has also been proposed [27]. It was shown that the difference in performance occurred due to the effect of the distance between the MEA and the core of flow.

Through CFD, the performance and potential of the tubular PEMFC in several controlled variables were confirmed, but the applied design was limited. As there are many design variables, optimization of design parameters through CFD modeling for various tubular designs will be very important for commercialization of tubular PEMFCs.

It is also important to produce a numerically organized design as a real model. There have also been studies that analyzed the structural advantages of tubular PEMFCs as actual models. In addition, research on materials and manufacturing methods to implement the tubular PEMFC design is ongoing. Lee et al. developed a cylindrical air-breathing PEMFC to integrate a serpentine flow-channel structure into a cylindrical configuration (Fig. 3a) [28]. Aluminum was used as the current collector, as well as the support material of the anode, and a spiral channel for the hydrogen supply was located on the surface of the cylinder. The MEA was rolled into the middle of the anode cylinder as the current collector of the cathode, 0.5 mm gold-coated stainless steel was used, and there were slits and ribs; therefore, it was designed to supply air (oxygen) through the slits. The cathode current collector was wrapped around the anode current collector, where the MEA was wound, and the bolts and nuts were tightened to reduce the contact resistance between the cathode and anode. The volume of the manufactured cylinder PEMFC was 30 cc and the weight was 82 g. The difference in contact pressure between the planar PEMFC and the tubular PEMFC was compared through pressure-sensitive film as well as numerical analysis. Experimentally, it was proved that the elastic modulus of the current collector has a great effect on the contact pressure of the planar PEMFC, whereas the contact pressure of the tubular design is determined by the design characteristics and even pressure is transmitted to all surfaces. It was confirmed that the planar PEMFC had contact pressure concentrated at the edge, while the cylinder-shaped PEMFC delivered uniform contact pressure to the entire MEA area; thus, the performance of the cylinder PEMFC was better. Although it could not be made lighter with the support of the anode and the fixing parts such as bolts and nuts, it is meaningful in manufacturing the tubular PEMFC.

The manufacture of tubular PEMFC focused on lighter weight was attempted [29]. The MEA was wound on an anode support with holes without a flow field, and a copper mesh was used as a current collector instead of a bipolar plate. The MEA of PEMFC is fixed through a copper wire, so it has a lighter material and assembly method (Fig. 3b). The authors found that cylindrical PEMFC removed liquid water better than planar PEMFCs and had much higher volumetric and gravimetric power density. The electrochemical impedance spectroscopy (EIS) measurements confirmed that contact resistances have a significant influence on the performance of tubular PEMFCs and are important factors in tubular design. It was considered that fixation via copper mesh and wire did not deliver a sufficiently uniform contact pressure across the entire MEA. Suseendiran et al. proposed a cylindrical PEMFC design in which two semi-cylindrical cathode current collectors were used (Fig. 3c) [30]. A contact pressure was applied to the active area through a compression cable, and a lighter fuel cell was manufactured by applying a thinner current collector (0.355 mm). The ohmic resistance is further reduced by the electrodeposition of the gold layer. In addition, sealing without a gasket was possible using an acrylic-based adhesive. Through optimization of the slots and ribs, a current density of 400 mA cm⁻² at 0.6 V was obtained, which is the best performance reported for air-breathing cylindrical PEMFCs to date.

After that, research for lighter PEMFC was also conducted, and the design was applied to remove more parts. Hwang et al. developed a novel design for lightweight tubular PEMFCs that are fabricated without fixtures (Fig. 3d) [31]. The conical design unit is assembled by accommodating the cell units like paper cups overlapping each other. Therefore, assembly parts such as clamps, bolts, and nuts for holding the parts are not needed. Moreover, the flow field is removed using the inside of the tube as a channel, and a compact bipolar plate can be applied using a 50-μm thin stainless-steel mesh as the current collector. By applying a conical design, an easy serial connection is possible using a bended-type connection without an external serial connection. The 2-cell PEMFC stack was very light at 0.22 g and had a specific weight power density of 897.7 W kg⁻¹.

As mentioned above, studies on tubular PEMFC have been conducted in various ways. Various designs, manufacturing methods, and materials were applied. The cylindrical design of PEMFC has full potential as a lightweight PEMFC fabrication method. At the same time as design optimization of light weight, material transport, or electrochemical performance through numerical modeling, research on materials and manufacturing methods that can implement it is necessary. For performance improvement, material and design approaches to improve ohmic resistance and contact pressure are possible. It is also important to improve the sealing method that can be maintained during operation by improving the adhesive method such as Nafion thermal bonding, rubber/resin or acrylic based adhesives. In addition, it is necessary to consider how to maintain the driving temperature of the cell. In future research, it will be necessary to develop a cylindrical PEMFC stack that uses cheaper, stronger, and lighter materials to reduce the weight and volume of the fuel cell, and improve durability and performance.

A flexible power supply is required, as the demand for devices with movement, such as wearable and roll-up devices, increases. There is a limit to realizing a flexible or stretchable PEMFC through the materials and designs of commonly used commercial PEMFC components. Hard metal materials are used for the end plate, and hard materials, such as metal and graphite composites, are used for the bipolar plate. The realization of a flexible PEMFC is possible only when the plate is replaced with another flexible material.

The fabrication of flexible fuel cells using flexible polymer substrates was first reported by Ito et al. [32]. Ten micro-direct methanol fuel cells can be fabricated on one substrate using a 0.3 mm thick polysulfide substrate with a diameter of 0.5 mm. The performance was maintained even during the bending tests. Tominaka et al. proposed a flexible on-chip fuel cell by fabricating inexpensive and flexible cycloolefin polymer films instead of brittle silicon films [33,34]. They allowed the current to flow through the circuit of the Au film on the substrate and applied a design that does not require a separate flow field using methanol without gas flow as fuel by making it micro-sized.

A study on a flexible PEMFC with a gas flow field and a slightly larger MEA active area is also being conducted. Chang et al. reported a bendable PEMFC fabricated on a polydimethylsiloxane (PDMS) substrate with a flow field through a stainless-steel mold [35]. A current collector was fabricated by sputtering Ni and Au directly onto PDMS with a flow field. A PEMFC fabricated with the flexible properties of PDMS is bendable. However, in the metal film produced through sputtering, the electrode was delaminated after the bending test and the ohmic resistance increased, resulting in a difference in performance before and after bending.

Polymer-based flexible PEMFCs are affected by the tensile and compressive stresses generated during bending. A film-type metal electrode produced through sputtering is vulnerable to mechanical movements such as bending, leading to an increase in ohmic loss. Therefore, electrodes such as metal meshes and nanowire (NW) networks have been studied as flexible electrodes that do not delaminate or break after bending. Park et al. developed a flexible, lightweight, and thin PEMFC using a PC plate and stainless metal mesh (Fig. 4) [36]. A flow field was formed on the PC film by pressing the flow-channel mold on the PC film, and a gold-coated metal mesh was used as the current collector. As a result, it was possible to fabricate a lightweight (2.23 g) and flexible PEMFC with a roll-up shape and an S-shape with an area of 9 cm². The metal mesh electrode was confirmed to have good durability, maintaining its performance even after repeated bending tests over 200 times. Chang et al. reported a flexible PEMFC based on PDMS coated with a flexible current-collecting layer of an Ag NW infilrated network [37]. The synthesized Ag NW were placed on a stainless-steel mold and annealed to improve the connection between the Ag NWs. Subsequently, PDMS was spread on the mold to ensure good bonding between PDMS and the Ag NW network. Consequently, when bent, the performance increased further.

The process of creating a flow field on a polymer film through a stainless-steel mold is relatively complicated, thus 3D printing has been proposed as a simple method [38]. Manufacturing a flexible flow field plate through 3D printing not only simplifies the process but also saves time and cost.

In comparison to metal, the cells fabricated as described above use a relatively soft and thin polymer as a substrate. Therefore, a uniform high clamping force provided by the end plate cannot be expected in the existing PEMFC. The performance varies depending on the compression pressure and degree of bending. Chang et al. studied the change in performance, with respect to bending [39]. It was confirmed that performance increased as the bending radius decreased. Structural modeling results showed that the decrease in resistance and the resulting improvement in performance were due to an increase in the compressive force perpendicular to the membrane electrode assembly, which was investigated through finite element simulations of bendable fuel cell internal stress. Furthermore, it was demonstrated that by adjusting the stiffness of the PDMS plate, the pressure transmitted to the MEA could be controlled and the performance could be improved. Park et al. investigated the effect of the assembly pressure on the performance of a bendable PEMFC based on Ag NW current collectors [40]. As the applied pressure increased, the peak power density of the fuel cell increased, and the value converged when a certain amount of pressure was applied. In addition, the torsional performance of the bendable PEMFCs was studied [41]. It was confirmed that when bending was added to torsion, higher compressive stress was applied and a higher performance was observed, confirming that bending had a greater effect on fuel cell performance than torsion.

Through the above studies, it is confirmed that the contact pressure and performance change according to the movement is large. This deviation can be a problem for stable use of flexible PEMFC. Therefore, it is necessary to consider a structure that transmits a constant contact pressure. In addition, it is necessary to improve the sealing method to fix the components and prevent the outflow of gas, and the contact problem of the components according to the operating conditions such as temperature and humidity should be considered. Research into materials and technologies that can fabricate thinner plates for more flexible movement and smaller volumes is also needed.

For flexible PEMFC, not only the bipolar plate and end plate, but also the flexibility of the GDL is required. Among the components of MEA, the membrane and catalyst layer are bendable materials, but in the case of the GDL, they can be damaged by repeated bending [42]. The Carbon Paper GDL, which is intertwined with the Carbon Fiber, has lowered performance due to damage to carbon fiber when applied to Flexible PEMFC. Carbon Cloth GDL is not damaged by bending, but has the disadvantage of low performance. Therefore, it is necessary to develop a thin, flexible, and capable GDL that can maintain high performance.

Ning et al. manufactured a new composite electrode with a porous CNT membrane and carbon paper, which had advantages such as excellent conductivity, flexibility, and high gas permeability [43]. By combining this with a flow field made of a PC plate, a specific volume power density of 5190 W L⁻¹ and a specific mass power density of 2230 W kg⁻¹ were obtained. In addition, this flexible PEMFC has good durability, maintaining 89.1% of its original performance even after 600 bending cycles and maintaining its original performance even after being dropped five times from a height of 30 m. Kang et al. fabricated the bendable MEA by simply coating conventional carbon papers with manually fabricated Ag NWs [44]. The coated Ag NW effectively connected the carbon fiber damaged by mechanical bending and maintained the electrical conductivity of the GDL. The performance was maintained even in more than 100 repetitive bending. Wei et al. further developed a flexible GDL by coating a CNT powder and PTFE mixture on a CNT membrane (Fig. 5) [45]. It has advantages such as a thickness of less than 40 μm, high flexibility, and mass productivity. The flexible fuel cell manufactured using the thin GDL was very thin (less than 90 μm) (Fig. 5c). Furthermore, when the new GDL is applied, a performance improvement of approximately 58% is observed. The air-breathing PEMFC fabricated from the new GDL significantly increased the power density by volume to 15,600 WL⁻¹ and the power density by weight to 9,660 W kg⁻¹. Both thin and good performances resulted in improved results.

Even if the fuel cell becomes flexible, the method of supplying fuel is also a challenge that needs to be addressed. The hydrogen fuel for a PEMFC is supplied by a large, rigid, high-pressure cylinder or storage vessel for low-temperature liquid hydrogen. Therefore, there is a problem in applying the flexible PEMFC system to the device, and the flexibility of fuel storage and supply is also a consideration. Recently, a flexible fuel cell pack that combines a hydrogen supply to the anode has been developed. Wang et al. fabricated a flexible and adaptable hydrogen generator by storing liquid formic acid in the pores of a silica aerogel [46]. Even with flexible movement, hydrogen can be constantly generated at room temperature. However, the output power and power density need to be improved.

In general, the tubular shape is not easy to bend. Flexible movement requires a special structural approach, such as a bendable straw. Even with tubular PEMFC design, efforts are being made to realize flexible fuel cells. Hwang et al. applied a conical reverse-truss origami design to a tubular PEMFC, to enable flexible movement (Fig. 6) [31]. Origami is a representative method used to make 3D structures flexible, reducing volume and enabling 3D movement. The bending angle can be adjusted using the origami method, and the performance was maintained at over 90% at all angles from 0 to 90 degrees (Fig. 6e). Furthermore, the volume can be controlled by folding. However, the durability of stainless steel is insufficient and the application of other materials is required.