Development of Flexible Polymer Electrolyte Fuel Cell Based on Polymer Flow-Field Plates and Silver Nanowire Current Collectors

Abstract

The technological level of various electronics has been intensively progressed in 21st century. This progress has also increased the demand for portable energy storage technology due to its contribution on the miniaturization of several electronics like smartphones. A lithium-based rechargeable battery currently leads the portable energy storage market while it already approached its theoretical maximum of gravimetric and volumetric energy density. Unfortunately, the demand for portable power sources with higher energy density keeps increasing as long as the level of other electronic components continues to be highly technologized. Accordingly, it is needed to find out the alternative for current lithium-ion battery technology. Another technological variation in future electronics is that it demands flexibility, i.e. bendability. So-called wearable electronics, including flexible displays, bent smart phones, or epidermal electronics have been highly spotlighted as future electronics. Here, such electronics also require portable energy sources because they are also kind of portable electronics. This study proposes the new type of fuel cell to cope with aforementioned demand for future electronics. Flexible fuel cell, i.e. bendable fuel cell, was fabricated based on polymer electrolyte fuel cell (PEFC) technology which is considered as one of the candidates to overcome the limitation of current energy density arising from the lithium-ion batteries. Here, Ag nanowires were used as a current collector inside the flexible fuel cell since it is highly stretchable and bendable with reliable electrical conductivity, corrosion-resistance, and chemical stability. This property enables the Ag nanowires to be used in chemically harsh environments such as anode and cathode in fuel cells. This current collector was coated on chemically highly stable polymer, polydimethylsiloxane, to realize the flexible flow-field plates for flexible fuel cells. The performance of the flexible fuel cell based on the aforementioned flow-field plates was measure at various bending radius. The peak power densities at flat and highly bent shapes were 43 and 71 mW/cm2, respectively. The variation of the performance was related to normal compressive stress (assembly pressure) to MEA (Membrane-electrode assembly), which was calculated using FEM (Finite-element method). The result showed that the electrochemical performance is increased as the compressive stress generated by bending is increased. The EIS (Electrochemical impedance spectra) corresponding to the measured polarization curves showed that the increase of the performance resulting from bending is due to the decrease of both ohmic and charge transfer resistances. It was also successfully demonstrated that the flexible fuel cell powered a small electronic fan, showing its technical level close to real applications. However, the highest peak power density in twisting test was 17 mW/cm2, lower than that from the bending test. It is speculated that the lower peak power density resulted from the damaged MEA by twisting. In this experiment, internal stress distribution was also calculated using same FEM model as in the bending test. The result was that there was almost no difference of stress intensities on MEA between flat and twisted shapes. Instead, strain was partially very high, which was not the case in the bending test. Because gas-diffusion layer (GDL) is not stretchable while Nafion® is kind of stretchable polymer membrane, the detachment between GDL and Nafion® is induced by strain. It would lead to the loss of triple-phase boundary and resulting performance decrease inside the fuel cell. Another cause of the decrease in the performance would be the damaged GDL itself from the twisting test and regarding strain. Considering that the peak power density at flat position was 43 mW/cm2 in the bending test, it is weird that same experiment but the peak power density of 17 mW/cm2 is lower than one-third of that at the bending test. Accordingly, the current MEA used in normal PEFC is not suitable to be used in flexible fuel cells in that it is fragile in the stretched position. It means that it is required to develop the MEA of new structure exclusively for flexible fuel cells. In the real use environment of flexible electronics, such electronics will be exposed to not only bending but also twisting environment. In order to investigate the compatibility of the flexible fuel cell in such environment, it was also characterized as it is under both bending and twisting loads simultaneously. In this experiment, bending components were set to flat and 36.3 cm of bending radii and twisting angles were 0, 5, 10, 15, 20, and 25º. The measured polarization curves showed that the peak power density at flat position was 27 mW/cm2 and the performance was decreased as it was twisted. Here, the power density of 27 mW/cm2 is again lower than 43 mW/cm2 measured in the bending test. This result corresponds with the results of twisting test. To investigate the internal stress distribution inside the fuel cell again, the stress distribution was calculated employing same FEM model as in bending and twisting test. The result showed that the bending and twisting components affect the generation of the internal compressive stress independently. Only bending component generates the compressive stress onto MEA, while twisting component does not. It means that the compressive stress partially contributed to the increase of the performance, although the MEA is permanently damaged by twisting. It was also experimentally reproduced that the compressive stress onto MEA veritably increased the performance of the flexible fuel cell. The performance was measured together with precisely and quantitatively controlled compressive stress. The peak power density was converged to 110 mW/cm2 as the compressive stress was increased. It represents the maximum of the peak power density of the flexible fuel cell with current template. It means that the performance of the flexible fuel cell can be increased more by improving the design, or components. In addition, the variation of several electrochemical constants such as an exchange current density, charge transfer coefficient and ohmic resistance with respect to the compressive stress were extracted from the experimental data. Simulation result based on a standard polarization model of a fuel cell using the extracted electrochemical parameters corresponded well with the experimental data reported in the literature. It means that the performance of the flexible fuel cell at various shape can be predicted by using the developed model in this study. It is expected that the experimental results and the simulation model here will contribute to the engineering of the flexible fuel cell as well as further development in the future.