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In polymer electrolyte membrane (PEM) fuel cells, the bipolar plates (BPPs) are responsible for the transport of reactants (via embedded flow fields), heat, and electrons, and account for 18-28% of the cost of fuel cell systems1. Thus, there is a great opportunity to improve the energy density of PEM fuel cells by improving the functions of BPPs, such as providing liquid water management, which affects reactant delivery and heat distribution. Previous work has shown that mass transport losses due to liquid water accumulation under the lands and channels of PEM fuel cell flow fields limit the power density of fuel cells2. Previous work has demonstrated that water will preferentially flow in a desired direction by implementing biomimetic wicking structures3; however, such wicking structures have not been previously implemented into a fuel cell. Furthermore, the design of BPPs has not been tailored to target areas of water accumulation. In this work, biomimetic geometries that promote passive unidirectional water wicking were implemented in a PEM fuel cell flow field to enhance liquid water removal and the distribution of reactant gases. The BPPs were characterized via constant current electrochemical testing and electrochemical impendence spectroscopy (EIS) to elucidate the dominant losses observed during operation. Operando synchrotron X-ray radiography was performed during the electrochemical testing in order to quantify the liquid water accumulation on the cathode side of the PEM fuel cell. The spatial distribution of liquid water was combined with EIS characterizations to explain the performance of the designs at high current densities, where mass transport losses typically dominate. The results from this work can be used to further optimize the design of PEM fuel cell bipolar plates in order to produce more efficient fuel cell stacks and drive PEM fuel cells into the global energy market. References Y. Wang, D. F. Ruiz Diaz, K. S. Chen, Z. Wang, and X. C. Adroher, Materials Today, 32, 178–203 (2020). N. Ge et al., Electrochimica Acta, 328, 135001 (2019). J. Feng and J. P. Rothstein, Journal of Colloid and Interface Science, 404, 169–178 (2013).

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Micro-channel heat sinks (MCHS) have attracted the attention of researchers because of their compact design and wide range of applications. MCHS is mostly used to dissipate heat where large amount of heat is generated in a confined space. In this work, leaf pattern MCHS is fabricated on pure copper block by micro-milling on CNC machining center and is tested for its thermal performance and fluid flow behaviour using pure water as well as various nanofluids such as Copper Oxide(CuO), Aluminium Oxide(Al2O3)and Silicon Oxide(SiO2). The volume concentration of nanofluids was kept constant, i.e., 0.3% volume fraction and the experiments were carried out for heat flux ranging from 65 W/cm2 to 200 W/cm2 and flow rate from 100 ml/min to 900 ml/min. The results of experiments indicate that these nanofluids, as a working fluid, enhance the heat transfer by 35% and Nusselt Number by up to 37%, however, increases the pressure drop by 18% which increases the pumping power. From the performance evaluation analysis, it was found that the SiO2 nanofluid with leaf pattern MCHS gives optimal performance.