Flexible and Lightweight Fuel Cell with High Specific Power Density

Jun 12, 2017 - Flexible devices have been attracting great attention recently due to their numerous advantages. But the energy densities of ... Moreov...
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Flexible and Lightweight Fuel Cell with High Specific Power Density Fandi Ning,†,‡ Xudong He,†,§ Yangbin Shen,† Hehua Jin,† Qingwen Li,† Da Li,† Shuping Li,† Yulu Zhan,†,∥ Ying Du,†,∥ Jingjing Jiang,⊥ Hui Yang,⊥ and Xiaochun Zhou*,†,# †

Division of Advanced Nanomaterials, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, China ‡ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China § Wuhan University, Wuhan 430071, China ∥ Shanghai University, Shanghai 200444, China ⊥ Center for Energy Storage and Conversion, Shanghai Advanced Research Institute, Chinese Academy of Sciences (CAS), Shanghai 201210, China # Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nanobionics, Chinese Academy of Sciences, Suzhou 215123, China S Supporting Information *

ABSTRACT: Flexible devices have been attracting great attention recently due to their numerous advantages. But the energy densities of current energy sources are still not high enough to support flexible devices for a satisfactory length of time. Although proton exchange membrane fuel cells (PEMFCs) do have a highenergy density, traditional PEMFCs are usually too heavy, rigid, and bulky to be used in flexible devices. In this research, we successfully invented a light and flexible air-breathing PEMFC by using a new design of PEMFC and a flexible composite electrode. The flexible air-breathing PEMFC with 1 × 1 cm2 working area can be as light as 0.065 g and as thin as 0.22 mm. This new PEMFC exhibits an amazing specific volume power density as high as 5190 W L−1, which is much higher than traditional (air-breathing) PEMFCs. Also outstanding is that the flexible PEMFC retains 89.1% of its original performance after being bent 600 times, and it retains its original performance after being dropped five times from a height of 30 m. Moreover, the research has demonstrated that when stacked, the flexible PEMFCs are also useful in mobile applications such as mobile phones. Therefore, our research shows that PEMFCs can be made light, flexible, and suitable for applications in flexible devices. These innovative flexible PEMFCs may also notably advance the progress in the PEMFC field, because flexible PEMFCs can achieve high specific power density with small size, small volume, low weight, and much lower cost; they are also much easier to mass produce. KEYWORDS: flexible, fuel cell, CNT membrane, high specific power density, air-breathing flexible energy sources still have many chances to have more progress. For example, flexible solar cells are limited by sun light, which is only available in daytime. The energy densities of flexible lithium batteries and flexible supercapacitors are 180 and 40 Wh kg−1,22−26 which are still too low to run flexible devices for a satisfactory length of time. Therefore, there is an urgent need for a flexible energy source with a high-energy density. The proton exchange membrane fuel cell (PEMFC) is another energy source, which has numerous advantages, such as

lexible devices,1−3 such as wearable devices and roll up displays, are attracting increasing attention as they show many advantages, including flexibility, shape diversity, low weight, and excellent mechanical properties.4−6 However, traditional energy sources, such as solar cells, lithium-ion batteries, and supercapacitors, are usually too heavy and rigid to fit the flexible electronics. Therefore, tremendous efforts have been made to develop flexible energy sources,7,8 such as flexible solar cells,9−11 flexible lithium batteries,12,13 and flexible supercapacitors.14−19 Some excellent results have been achieved with these sources, and they show promise for application in flexible devices.20,21 For example, the energy density of the symmetric supercapacitor device was calculated to be 22.89 Wh kg−1 at an average power density of 686.84 W kg−1.18 But these

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© 2017 American Chemical Society

Received: March 18, 2017 Accepted: June 12, 2017 Published: June 12, 2017 5982

DOI: 10.1021/acsnano.7b01880 ACS Nano 2017, 11, 5982−5991

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ACS Nano Scheme 1. Structures of Different PEMFCs.a

a (a) Structure of traditional PEMFC. Air or O2 is pumped into PEMFC. (b) Structure of traditional air-breathing PEMFC. Air naturally diffuses into PEMFC. (c) Structure of flexible air-breathing PEMFC in this research. Air naturally diffuses into PEMFC. In parts a and b, rigid, heavy, and expensive metal or graphite bipolar plates are used as flow field and current collector for both anode and cathode. In part c, the plate on the cathode side has been totally removed, and the plate on anode side has been replaced by a light, flexible, and cheap plastic plate.

Figure 1. Performance of flexible and traditional air-breathing PEMFCs with the same working area (1 × 1 cm2). (a) Photograph of traditional and flexible air-breathing PEMFCs. (b) Enlarged photograph of flexible air-breathing PEMFC. (c) Polarization curves of traditional and flexible PEMFCs. The experiments were performed with pure hydrogen at 20 °C and atmospheric pressure. Anode Pt loading (0.5 mg cm−2), hydrogen flow rate (15 mL min−1); Cathode Pt loading (0.5 mg cm−2), air.

high energy density (39.7 kWh kg−1 for H2), high conversion efficiency, environmental friendliness, and compact design.27 Currently, the specific area power density can be up to 1570 mW cm−2 or higher.28 The highest specific volume and weight power densities, 3.1 kW L−1 and 2.0 kW kg−1, were declared for a PEMFC stack by TOYOTA company. However, traditional PEMFCs use graphite or metal plates for the flow channel and the current collector, which are usually too heavy, rigid, bulky, and expensive for flexible devices (Scheme 1a).29 These plates are responsible for up to 80% of total stack weight, 90% of stack volume, and about 30% of cost.30 In addition, air or O2 needs to be pumped into the PEMFC for electrochemical reaction. In order to make PEMFCs favorable for portable devices, the structure and operation of the PEMFCs were simplified, and a kind of air-breathing PEMFC was invented (Scheme 1b).31 In such traditional air-breathing PEMFC (Scheme 1b), air naturally diffuses into the PEMFC and no pump for air is needed. However, the heavy, rigid, bulky, and expensive graphite or metal plates are still used in the traditional airbreathing PEMFC (Scheme 1b). Moreover, the specific area power density is usually between 100 and 200 mW cm−2,32 which causes a much lower specific weight power densities

(1.761 W kg−1).33 So far, there is an urgent need for a lightweight and flexible PEMFC satisfying the demands of flexible electronic devices. On the other hand, the preparation of a flexible electrode is one of the challenges in preparing a flexible PEMFC. In conventional PEMFCs, carbon paper is usually used as the electrode, because the carbon fiber in carbon paper has high stability and proper dimension in micrometer size, which is beneficial to mass transfer and supporting catalyst. But it is not suitable to directly use carbon paper in a flexible PEMFC because of its brittleness and relatively low electrical conductivity. In current flexible devices, carbon nanotubes (CNTs),34,35 graphene,36,37 conducting polymers,38,39 Si or Ag nanowires,40,41 and carbon cloth42 were usually used as basic materials to prepare flexible electrodes because they have high specific surface area and high electrical conductivity.43,44 These materials have a common feature, which is the nanometer width or thickness and micrometer length or area. This feature gives the materials high conductivity and high mechanical strength. Moreover, CNTs and graphene are currently commercially available in massive amounts, which is advantageous to the 5983

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Figure 2. Schematic illustrating the fabrication process of a flexible composite electrode for flexible air-breathing PEMFCs. (a) SEM images of an as-prepared CNT membrane. The insets are the SEM and photo images of the cross section of the CNT membrane. (b) SEM image of a CNT membrane with an array of pores after laser marking. (c) SEM image of a flexible composite electrode, which is composed of a CNT membrane with pores attached to a carbon paper. (d) SEM image of the cross section of a flexible MEA. Different colors represent different materials. (1) Blue is proton exchange membrane (PEM), that is, Nafion membrane; (2) yellow is catalyst layer; (3) pink is carbon paper; (4) green is CNT membrane with pores. Refer to Figure S1b for the original version of SEM image. (e) A photograph of a flexible air-breathing PEMFC made with the flexible MEA. (a1−e1) Schematics of a−e.

application of the flexible devices. Therefore, CNTs and graphene should be excellent candidates to satisfy the requirements of flexible PEMFCs. But it is still a challenge to massively prepare flexible electrodes in uniform thickness from CNTs and graphene. It is also difficult to use them to directly make an electrode suitable for flexible PEMFCs, because their dimensions compared to carbon fiber are so small that they are not suitable for mass transfer and support of the catalyst. In this research, we successfully invented a light and flexible PEMFC by using a new design of PEMFC and a flexible composite electrode (Scheme 1c). The flexible composite electrode was made by using a highly flexible CNT membrane and carbon paper, giving it many advantages, such as excellent electrical conductivity, fine flexibility, and high gas permeability. In the new design of Scheme 1c, the heavy, rigid, and expensive metal or graphite plates on the cathode side were totally removed, and the plate on anode side was replaced with a light, flexible, and cheap plastic membrane (Scheme 1c). A flexible air-breathing PEMFC (1 × 1 cm2) in this research can be as light as 0.065 g and as thin as 0.22 mm. This new PEMFC exhibits an amazing specific volume and weight power density as high as 5190 W L−1 and 2230 W kg−1, which are much higher than conventional air-breathing PEMFCs30,31 and even higher than conventional PEMFCs. In short, this research exhibits that PEMFCs with high energy density can be made that are light and flexible, inexpensive, easily mass produced, and suitable for applications in flexible devices.

RESULTS AND DICUSSION Design and Performance of Flexible PEMFCs. It is necessary to make a huge innovation in the plates if we want to invent flexible, ultralight, cheap, and compact PEMFCs, because the graphite or metal plates in traditional (airbreathing) PEMFCs are too heavy, rigid, bulky, and expensive (Scheme 1a,b and Figure 1a). As we all know, the plates have two main functions, as current collector and flow field. In the flexible air-breathing PEMFC of this research, we integrate the function of current collector into a flexible and highly conductive composite electrode and realize the function of flow field using a light and cheap plastic plate or without a plate (Scheme 1c). The plate on the cathode side has been totally removed, and the plate on the anode side has been replaced with a light and cheap plastic plate (Scheme 1c). The composite electrode serves as both the diffusion layer and the current collector (Scheme 1c), replacing the function of current collector of heavy graphite or stainless steel bipolar plates. Electrons transfer in the plane of the composite electrode. On both sides, an edge of the flexible composite electrode was exposed to connect the external circuit. In the end, the structure of the flexible air-breathing PEMFC becomes much simpler than that of traditional one (Scheme 1a−c). By the design above, the whole weight and volume of airbreathing PEMFCs with same working area (1 × 1 cm2) can be dramatically reduced from 68.6 g and 25 cm3 for traditional one (Figure 1a) to 0.065 g and 0.028 cm3 for flexible one (Figure 5984

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(Pt/C) for the PEMFC. To address the problem, we prepared a flexible composite electrode by attaching a carbon paper onto a CNT membrane through a paste composed of polytetrafluoroethylene (PTFE) and active carbon (Vulcan XC-72) (Figure 2c, c1, Experimental Methods). The flexible composite electrode is able to support the catalysts and has high gas permeability. The structure of the composite electrode in Figure 2c shows that the connection of the CNT membrane and the carbon paper is pretty tight. The carbon paper side of this flexible composite electrode gives fine support to the electrochemical catalysts. Then, we used the flexible composite electrode to prepare the heart component of PEMFC, that is, a flexible membrane electrode assembly (MEA), by a hot-pressing process. The cross section of the flexible MEA in Figure 2d clearly exhibits that the proton exchange membrane (PEM), catalysts, CNT membrane, and carbon paper are tightly bound together. The flexible composite electrode can finely support the electrocatalysts. The pores on the CNT membrane can be identified (red arrow in Figure 2d). Although the structure of our flexible MEA is similar to conventional ones,45,46 the properties and functions change greatly. The flexible composite electrode serves as both diffusion layer and current collector and replaces the conventional heavy graphite or metal bipolar plates. At last, a flexible air-breathing PEMFC was fabricated by the flexible MEA and a polydimethylsiloxane (PDMS) film (Figure 2e, e1). Compared with a conventional PEMFC, our flexible air-breathing PEMFC has some innovations. For the anode side, a PDMS film was used as a flow field to replace the conventional heavy graphite or metal bipolar plates for hydrogen feeding. For the cathode side, the flexible composite electrode is directly exposed to air. For both sides, an edge of the flexible composite electrode was exposed to connect the external circuit. Figure 2e shows an example of a flexible airbreathing PEMFC, which is ultrathin in thickness (0.22 mm), ultralight in weight (0.065 g), and ultrasmall in volume (0.028 cm3) (Figure 2e). This single flexible PEMFC exhibits a surprisingly high specific volume and mass power densities of 5190 W L−1 and 2230 W kg−1. Effect of CNT Membrane Thickness on the Performance of Flexible PEMFCs. Because the electron transfer occurs in the plane of the flexible composite electrode, the electrical resistance of the electrode can strongly affect the performance of flexible PEMFCs. The thickness of the CNT membrane is one of the key factors affecting the electrical resistance of the electrode. Figure 3a shows that the square resistance of composite electrodes decreases from 0.54 to 0.39 Ω/□ with the increase of CNT membrane thickness from 0 to 20 μm. Because the thickness of CNT membranes (5−20 μm) is much thinner than that of carbon paper (∼200 μm), the electrical conductivity of CNT membranes is much higher than that of carbon paper. With the decrease of square resistance, the corresponding flexible PEMFC will have much lower internal resistance and exhibit much higher performance (Figure 3b). For example, the flexible PEMFC made with a 20 μm CNT membrane has a peak power density of 52 mW cm−2, which is 5.6 times higher than the one made with a 5 μm CNT membrane and is 6.7 times higher than one without a CNT membrane (Figure 3b). Therefore, thicker CNT membranes can greatly improve the performance of flexible PEMFCs. However, Figure 3b also shows that the peak power density of the PEMFCs prepared with a 20 μm CNT membrane is only 3.1 mW cm−2 higher than that with a 15 μm CNT membrane.

1b). The detailed comparison of the parameters of traditional and flexible air-breathing PEMFCs is shown in Table S1. Figure 1c shows that the flexible air-breathing PEMFCs have a much higher performance (145.2 mW cm−2) than the traditional one (90.3 mW cm−2). The performance improvement is due to the removal of the plate on the cathode side. In traditional PEMFCs, the plate on the cathode side could block the transfer of air, while removal of this plate in the flexible ones facilitates the air transfer. Because of the ultralight weight and ultrasmall volume of the flexible air-breathing PEMFCs in this research, the single flexible PEMFC exhibits a surprisingly high specific volume and mass power densities of 5190 W L−1 and 2230 W kg−1, which are much higher than conventional air-breathing PEMFCs (1.761 W kg−1 in literature).33 Moreover, the specific power densities are even much higher than the world record 3.1 kW L−1 and 2.0 kW kg−1 declared for a PEMFC stack by TOYOTA company. In the field of fuel cells, it is well-known that the specific volume and mass power densities of fuel cell stacks are much higher than those of a single fuel cell. Therefore, specific volume and mass power densities of the flexible PEMFC could be further improved. Furthermore, the application of the flexible composite electrode gives the PEMFC its flexible properties. In the following sections, we will show more research on the flexible composite electrode. Preparation of the Flexible Electrode and Flexible PEMFC. One key challenge of fabricating a flexible PEMFC is to prepare a flexible electrode suitable for the flexible PEMFC. Unlike other flexible energy sources, for example, flexible lithium batteries, the electrodes in PEMFCs are always exposed to oxygen and hydrogen. The reactants and products need to transfer through the electrode with low transfer resistance. Therefore, an electrode for flexible PEMFCs should have excellent flexibility, high stability, and high gas permeability. Moreover, a single PEMFC usually has a high working current density (up to several A cm−2). And catalysts for electrochemical reaction need to be attached on one side of the electrode. Therefore, the electrode also should have high electrical conductivity and an ability to support catalyst. In this research, a flexible composite electrode has been made with a porous CNT membrane and carbon paper for a flexible PEMFC. In the flexible composite electrode, a CNT membrane was used as one of the basic materials to fabricate the flexible electrode because of its superior flexibility, uniform thickness, exceptional conductivity, and suitability for mass production (Figure 2a, a1). The CNT membrane thickness can be adjusted between 5 and 30 μm; it has outstanding conductivity (∼1 × 105 S cm−1) and excellent mechanical strength (∼300 MPa of tensile strength). Moreover, we can massively prepare these CNT membranes in large sizes (up to 1 m2). But the as-synthesized CNT membrane is so dense that the reactants and products cannot permeate through it (Figure 2a, a1). In order to make the CNT membrane permeable, an array of uniform circular, micrometer-sized pores were punched on the CNT membrane with a laser marking machine. Figure 2b, b1 show the morphology of a typical CNT membrane after punching with a 135 μm pore (D) every 200 μm pore distance (d). The ordered and highly dense pores on the membrane can provide express channels for the mass transfer of reactants and products to ensure the effective conversion of chemical energy to electrical energy. However, the micrometer-sized pores on the CNT membrane are too large to directly support electrocatalysts 5985

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⎛ D ⎞2 r = 4π ⎜ ⎟ ⎝d⎠

(1)

Figure 4c shows that the void ratio (r) of CNT membranes increases as the pore distance decreases (d) at certain pore sizes and increases as the pore size increases (D) at certain pore distances (eq 1). A higher void ratio can improve the mass transfer of reactants and products and facilitate the electrochemical reaction in PEMFCs. However, too high a void ratio will cause higher electrical resistance, because more area of the CNT membrane is burned by laser marking (Figure 4d). Figure 4d shows that the trend of square resistance of CNT membranes with different pores is totally consistent with that of the void ratio in Figure 4c. Higher square resistance could increase the resistance of electron transfer and decrease the performance of the PEMFC. Figure 4e shows that the performance of flexible PEMFCs has an optimum value with the variation of pore distance (d) for each pore size (D). We can attribute the optimum performance to the competition of mass transfer and square resistance with the variation of pore distance at certain pore sizes. For example, when we decrease the pore distance at a certain pore size, the mass transfer improves, inducing an improvement of the performance of the flexible PEMFC, but the square resistance becomes higher, inducing a deterioration in performance. Thus, an optimum pore distance for a certain pore size is needed for maximum performance (Figure 4e). Interestingly, Figure 4e also shows that the optimum performance for a small pore size of 35 μm is 53.8 mW cm−2, which is higher than that for a large pore size of 135 μm. The reason for the higher optimum performance of the small pore size is that the corresponding composite electrode can have a faster mass transfer at a lower void ratio, that is, smaller square resistance. For example, in our research, a CNT membrane with 35 μm pores (green dot curve in Figure 4d) always has a smaller square resistance, or higher electrical conductivity, than one with 135 μm pores (black dot curve in Figure 4d). Although Figure 4c shows that the CNT membrane with small pores always has a lower void ratio than one with large pores, its mass transfer distance from the edge of a pore to

Figure 3. Effect of CNT membrane thickness on the performance of flexible PEMFCs. (a) Square resistance of the flexible composite electrode with different thicknesses of CNT membranes after hot pressing. (b) Peak power density of flexible PEMFCs fabricated with the flexible composite electrode in panel a. The experiment was performed with 50% hydrogen at 20 °C and atmospheric pressure.

According to our research, the performance of a PEMFC will not have significant improvement if the CNT membrane is thicker than 20 μm. For this reason, the following research uses 20 μm thick CNT membranes unless otherwise specified. Effect of Pore Size and Distance on the Performance of Flexible PEMFCs. The pores on the CNT membrane were punched to provide express channels for the mass transfer of reactants and products (Figure 4a,b). The mass transfer distance from the channels to catalyst layer is different for different locations of catalyst layer. For example, the location between two neighbor pores has a longer pathway length (s) of mass transfer than other locations (s1 > s2 in Figure 4a and s1′ > s2′ in Figure 4b). Moreover, the pathway lengths of mass transfer are different for different pore sizes and distances (s1 > s1′) as shown in Figure 4a,b. Therefore, the pore size and distance can affect the mass transfer of reactants and products in the composite electrode and consequently affect the performance of flexible PEMFCs. The void ratio (r), that is, the summary of pore area to total area, varies with the pore size and distance.

Figure 4. Effects of pore size and distance of the CNT membrane on the performance of flexible PEMFCs. (a) Profile of the flexible PEMFC with large pore size and distance. (b) Profile of the flexible PEMFC with small pore size and distance. (c) Void ratio of CNT membranes of different pore sizes and distances. (d) Square resistance of CNT membranes of different pore sizes and distances. (e) Peak power density of flexible PEMFCs fabricated with CNT membranes of different pore sizes and distances (See Figure S10 for polarization curves). The experiment was performed with 50% hydrogen at 20 °C and atmospheric pressure. 5986

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Figure 5. Flexible PEMFC performance adaptation to harsh conditions such as bending and twisting. (a) Bending a flexible PEMFC with 4 cm length and 1 cm width. The bending angle α is 50°. (b) Current density of the flexible PEMFC when bent. (c) Long-term discharge curves at 0.6 V before and after bending 600 times. The bending angle α is 50°. (d) Effect of bending angle on the flexible PEMFC performance. (e) Effect of twisting angle on the flexible PEMFC performance. (f) Effect of bending number on the square resistance of the composite electrode with and without the CNT membrane. The experiment was performed with 50% hydrogen at 20 °C and atmospheric pressure.

situ methods, or major changes in gas flows seem to be some causes of the phenomena.48 In addition, chronoamperometric measurement of electrocatalysts usually shows a quick decay at the beginning of the test too.51,52 Besides the previous bending times test, we further did two more flexiblility tests, that is, bending angle and twisting angle effect on the performance of the flexible air-breathing PEMFC (Figure 5d,e). For the bending angle research, Figure 5d shows that the peak power density of the PEMFC almost does not change at different bending angles (from 0° to 180°), indicating that the bending has very weak effect on the flexible airbreathing PEMFC. For the twisting angle research, Figure 5e shows that the peak power density of the PEMFC weakly decreases with the increase of twist angle from 0° to 100°. The decrease of performance can be attributed to the structure variation of the flexible air-breathing PEMFC, because the twist may affect the flow field and mass transfer. But Figure 5e shows that the effect of twisting is not strong. Therefore, the experimental results show that the flexible air-breathing PEMFC has a high flexibility. Consequently, multiple times bending, large angle bending, and large angle twisting have very weak effect on the performance of the flexible PEMFC. Why does this flexible PEMFC have such high performance? To discover the reason, we measured the influence of bending times on the square resistance of the composite electrode with and without the CNT membrane. Figure 5f shows that the square resistance of the composite electrode without the CNT membrane increases sharply during bending, while the square resistance with the CNT membrane increases very slowly. As we all know, a carbon paper electrode is fragile; it is difficult to bend it without damage. Bending destroys the structure of the carbon paper and sharply increases the square resistance. Unlike carbon paper, the CNT membrane has excellent flexibility and can be bent to any angle. Figure 2c shows that the carbon papers and the CNT membrane tightly attached to each other in the composite electrode after hot pressing. Due to the exceptional flexibility and mechanical strength of the CNT

the center of two neighboring pores is actually shorter (s1′ < s1 in Figure 4a,b). We can imagine that the performance of the flexible PEMFC would be better if the pore size can be reduced to a nanometer. Therefore, a flexible composite electrode with smaller pore sizes is one of our important projects for the future. The Flexible Performance of PEMFCs. Flexible devices can be bent and distorted due to the characteristics of their material and preparation technology. An important criterion of flexible devices is the performance variation in the folded state and after bending. In this research, we prepared a flexible PEMFC with 4 cm length and 1 cm width for the flexibility test (Figure 5a). Figure 5a clearly shows that this flexible PEMFC can be bent to 50° degrees. Figure 5b shows that the current of the flexible PEMFC responds instantly to the bending. The current drops when the flexible PEMFC is bent and recovers to its original value when it is released. It is necessary to mention that the current vibration is only 0.1 mA cm−2, which is only 0.2% of the entire current density (41.4 mA cm−2). In addition, we also found that higher bending angle increases the current response amplitude (Figure S11). At this bending angle, ∼70°, the current vibration is also as small as 0.8% of the entire current density. The flexible PEMFC discharge performance over time before and after bending was also measured. Strikingly, Figure 5c shows that the performance of the flexible PEMFC only decays 10.9% after being bent 600 times at 0.6 V. Therefore, this flexible PEMFC can work perfectly with high stability during bending. It is notable that there is a drop of current density in the first 1 h in Figure 5c. When we checked out the literature on PEMFCs, we found that the exponential drop of current density in the first 1 h is a common phenomenon named “reversible degradation”.47−51 As shown in the figures from the literature, the power densities of different fuel cells drop in the first or second hours.47−51 The activity usually can recover after a short time of rest. These works showed that interruptions of continuous testing by resting periods, characterizations with in 5987

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Figure 7a shows a flexible PEMFC stack fabricated with four single PEMFCs on one proton exchange membrane through a series connection.55,56 The stack in the bending state can light up 53 LED lights. (Figure 7b) (Volume is 5.7 cm3, weight is 2.18 g.) A flexible PEMFC stack consisting of eight single PEMFCs as shown in Figure 7c can power some electronic devices such as mobile phones. The stack is lightweight (3.70 g), small (9 cm length and 3.5 cm width), and thin (1 mm), and occupies a small volume (3.15 cm3). The energy density of the flexible PEMFC system was calculated to be 522 Wh kg−1, if hydrogen is supplied by the decomposition of formic acid (SI-15). These flexible PEMFC stacks are only some demonstrations, which have not been optimized yet. Therefore, flexible PEMFC stacks have many potential applications in the mobile field.

membrane, the composite electrode possesses fine bending performance. In real applications, fuel cells need to adapt to a variety of working environments to be used as energy supply devices. Especially in aerial environments, fuel cells may be subjected to extreme forces that can destroy them, such as being dropped from a height. Therefore, we simulated these forces by dropping the flexible PEMFC from as high as 30 m (Figure 6a). The flexible PEMFC retained its structure and perform-

CONCLUSION In this research, we successfully fabricated a light and flexible air-breathing PEMFC by using a new design and a new flexible composite electrode. The new composite electrode was made from a porous CNT membrane and carbon paper; it possesses many advantages highly suitable for flexible PEMFCs, such as excellent conductivity, fine flexibility, and high gas permeability. So, the electrons can transfer in the plane of the flexible composite electrode, and reactants and products can transfer through it. The design is new in that the current collection and gas diffusion layer functions are integrated into the flexible composite electrode. Then flexible and inexpensive plastic membranes are used as flow fields to replace the conventional heavy, rigid, and expensive metal or graphite plates. Of special interest is that this new PEMFC exhibits a specific volume power density as high as 5190 W L−1 and a specific mass power density of 2230 W kg−1, which are much higher than conventional PEMFCs. Also outstanding is that the flexible PEMFC retains 89.1% of its original performance after bending it 600 times, and it retains its original performance after being dropped 5 times from a height of 30 m. Due to the high flexibility, bending and twisting angles have weak effect on the performance the flexible PEMFC. Moreover, flexible PEMFC stacks were demonstrated to be useful for mobile applications, such as mobile phones. In addition, this work reveals that CNT membrane thickness, pore size, and pore distance have a great effect on the performance of flexible PEMFCs. Our research exhibits that it is possible to make PEMFCs that are light, flexible, and suitable for flexible-device applications. This innovative flexible PEMFC research may advance the PEMFC field, because now flexible PEMFCs can achieve high specific power density with small size, small volume, and low

Figure 6. Flexible PEMFC performance adaptation to harsh conditions of falling from a height. (a) Illustration of dropping a flexible PEMFC from a height of 30 m. (b) Effect of the number of times dropped on the performance of the PEMFC. The experiment was performed with 50% hydrogen at 20 °C and atmospheric pressure.

ance very well after falling 5 times (Figure 6b). This outstanding performance is extremely difficult to match for conventional fuel cells with graphite or metal plates. Such a new property of the flexible PEMFC provides for a wider range of applications such as for unmanned aerial vehicles. Application Demonstrations of Flexible PEMFC Stacks. Because a single PEMFC has a very low operating voltage (as low as 0.5−0.8 V), too low to support most electronic devices, a PEMFC stack is usually used in practical applications.53 But conventional PEMFCs use metal or graphite plates for current collection and the flow field.54 These components are responsible for up to 80% of the total stack weight, 90% of the stack volume, and about 30% of the cost.30 In this research, we invented a new flexible PEMFC stack, in which these components are replaced by a flexible composite electrode and plastic flow field. The use of a plastic flow field makes the fabrication process for flexible PEMFCs much easier than that for conventional PEMFCs and better suited for mass production. Therefore, the replacement of these components not only greatly improves the volume and weight specific energy density but also dramatically reduces the cost of the stack and simplifies the manufacturing process.

Figure 7. Application demonstrations of flexible PEMFC stacks. (a) Schematic illustrating a flexible PEMFC stack. (b) 53-LED lights powered by a flexible PEMFC stack with 4 single cells while bent. (c) Charging a mobile phone with a flexible PEMFC stack of 8 single cells. 5988

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probe technique. Each sample was measured 20 times, and the parameters are the average of the measured data. The electrochemical performance was measured by an electrochemical workstation (CS), and copper wires were connected to the electrodes to measure the current. The operating conditions in our performance test were dry air and hydrogen through a gas washer bottle at the ambient temperature (20 °C) and pressure. The hydrogen source in this investigation is pure hydrogen or a gas mixture that catalyzes from formic acid (Figure S6). The ratio of hydrogen and carbon dioxide is 1:1 along with carbon monoxide at less than 1 ppm in the mixed hydrogen. The pure hydrogen is prepared from a hydrogen generator. Figure S6 shows the influence of mixed hydrogen and pure hydrogen on the cell.62 Optimizations of Preparation and Working Conditions. In order to improve the performance of the flexible PEMFC, a series of optimizations have been carried out. First, the paste of Vulcan XC-72 and polytetrafluoroethylene (PTFE) was proved to be effective to reduce the contact resistance between the CNT membrane and the carbon paper in the flexible composite electrode (Figure S1). Moreover, the aspect ratio of the PEMFC, the hydrogen flow rate, the PTFE and Nafion content, the layer number of the carbon paper in the composite electrode, the purity of the hydrogen, and the hotpressing parameters have been investigated (Figure S2−7). In addition, the operation conditions, such as temperature, humidity, and voltage vs time at different bending angles, have also been studied (Figure S12−14).

weight at considerably lower cost, while being far easier to mass produce. The results and knowledge gained from this research also can be applied to other flexible energy sources and devices.

EXPERIMENTAL METHODS Preparation of CNT Membranes with Ordered Porous Structure. The carbon nanotubes (CNTs) were prepared by hydrogen reduction under the protection of an argon atmosphere with ethanol as the carbon source, iron-based catalyst, and promoter at 1300 °C by floating catalyst method.57 Prior to membrane formation, the CNT membrane was collected by winding, then the CNT membrane was compacted and densified by array.58−61 CNTs in the CNT membrane are multiwalled. This membrane has excellent conductivity and mechanical strength, as well as exceptional flexibility. But it is relatively so dense that it is not permeable to reactants and products. Therefore, it was necessary to produce a CNT membrane with certain permeability. Laser marking technology can punch CNT membranes with a uniform array of pores. Pore size relies on current intensity; the stronger the current intensity, the larger the pore size. Pore spacing can be set by adjusting the coordinates. CNT membranes with different pore sizes of 35, 55, 115, and 135 μm were made by adjusting the marking current to 9, 10, 12, and 15 A, respectively. In order to generate the pores in the vertical direction rather than tapered, we set the laser-marking number to three. Homologous increases in the marking number should accompany increases in membrane thickness to ensure the permeability of the pore. Then, the film has permeability for reactants and products. Preparation of the Flexible Composite Electrode. The composite electrode consists of a CNT membrane attached to a piece of carbon paper. A CNT membrane with a porous structure was attached to a piece of carbon paper by PTFE (0.03 mg cm−2) and Vulcan XC-72 (0.25 mg cm−2). It was then baked at 320 °C for 30 min, then cooled to room temperature. Subsequently, it was hot pressed at a pressure of 40 kgf cm−2 and temperature of 120 °C for 2 min. The paste composed of PTFE and Vulcan XC-72 acts as a binder that tightly bonds the carbon paper electrode and CNT membrane together. Vulcan XC-72 between the carbon paper and CNT membrane acts as a conductive layer to reduce the contact resistance. So, the addition of Vulcan XC-72 can improve the performance of the cell (Figure S1). Preparation of MEA. The MEA with an active area of 1−4 cm2 was prepared and used in this study as follows: The catalyst layer was spread on the composite electrode by screen printing. The catalyst ink was prepared by dropping PTFE 10% (wt %) into the mixed solution of deionized water and isopropanol, and then was mixed with 30% Pt/ C or 30% Pt−Ru/C catalyst in an ultrasonic bath for 15 min. The catalyst loading was 0.5 mg cm−2 for both cathode and anode. In order to prevent the poisoning of the catalyst by the small amount of CO, Pt−Ru/C catalyst was loaded onto the anode electrode. After the catalyst coating, the electrodes were baked at 320 °C for 30 min and cooled to room temperature, and then Nafion solution 10% (wt %) was added dropwise to the catalyst layer to form a proton network. For electrolyte, Nafion 115 membrane (125 μm) and Nafion 211 membrane (25.4 μm) were hot pressed with two electrodes at a pressure of 40 kgf cm−2 and temperature of 120 °C for 2 min. Fabrication of Flexible Air-Breathing PEMFCs. On the anode side of MEA, PDMS membrane with the same size of the fuel cell work area was used to form a space with the electrode via adhesive. The space acts as the flow field of the flexible PEMFC. Meanwhile, the pathway of gas into and out of the flow field in the diagonal direction was left. Capillary with outer diameter 1 mm was used to connect the hydrogen source and fuel cell to feed the PEMFC with hydrogen. Differently, the cathode side was exposed to the air directly. A small part of the composite electrode was exposed to connect the sheets of silver or copper for the connection of the outer circuit. Characterization Measurements. The morphology and structure of the CNT membrane materials were characterized by using SEM. The square resistance was measured by using the four-point

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01880. Additional details on experiment design, discussion, and figures (PDF)

AUTHOR INFORMATION Corresponding Author

*X.Z. E-mail: [email protected]. ORCID

Xiaochun Zhou: 0000-0001-9793-6072 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for financial support granted by the National Natural Science Foundation of China (Nos. 21373264 and 21573275), Ministry of Science and Technology of China (Nos. 2016YFE0105700 and 2016YFA0200700), the Natural Science Foundation of Jiangsu Province (No. BK20150362), Suzhou Institute of Nanotech and Nanobionics (No. Y3AAA11004), and Thousand Youth Talents Plan (No. Y3BQA11001). REFERENCES (1) Wang, X. F.; Lu, X. H.; Liu, B.; Chen, D.; Tong, Y. X.; Shen, G. Z. Flexible Energy-Storage Devices: Design Consideration and Recent Progress. Adv. Mater. 2014, 26, 4763−4782. (2) Zeng, W.; Shu, L.; Li, Q.; Chen, S.; Wang, F.; Tao, X. M. FiberBased Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications. Adv. Mater. 2014, 26, 5310−5336. (3) Li, C.-H.; Wang, C.; Keplinger, C.; Zuo, J.-L.; Jin, L.; Sun, Y.; Zheng, P.; Cao, Y.; Lissel, F.; Linder, C.; You, X.-Z.; Bao, Z. A Highly Stretchable Autonomous Self-Healing Elastomer. Nat. Chem. 2016, 8, 618−624. (4) Yang, P. H.; Mai, W. J. Flexible Solid-State Electrochemical Supercapacitors. Nano Energy 2014, 8, 274−290. 5989

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