Yang Yu

As one core component in hydrogen fuel cells and water electrolysis cells, bipolar plates (BPs) perform multiple important functions, such as separating the fuel and oxidant flow, providing mechanical support, conducting electricity and heat, connecting the cell units into a stack, etc. On the path toward commercialization, the manufacturing costs of bipolar plates have to be substantially reduced by adopting low-cost and easy-to-process metallic materials (e.g., stainless steel, aluminum or copper). However, these materials are susceptible to electrochemical corrosion under harsh operating conditions, resulting in long-term performance degradation. By means of advanced thermal spraying technologies, protective coatings can be prepared on bipolar plates so as to inhibit oxidation and corrosion. This paper reviews several typical thermal spraying technologies, including atmospheric plasma spraying (APS), vacuum plasma spraying (VPS) and high-velocity oxygen fuel (HVOF) spraying for preparing coatings of bipolar plates, particularly emphasizing the effect of spraying processes on coating effectiveness. The performance of coatings relies not only on the materials as selected or designed but also on the composition and microstructure practically obtained in the spraying process. The temperature and velocity of in-flight particles have a significant impact on coating quality; therefore, precise control over these factors is demanded.

Efficiency and durability of Proton-exchange membrane fuel cells (PEMFCs) relies on different interacting and coupling factors. The comfort temperature level with good homogeneity is one of the prominent factor, which significantly affects the stack-level and even on-board integrated systems confronted with frequent load changes. Featured with low-grade reaction heat generation (60~80 °C) and high proportion (~50%) accounting for entire yield, liquid-cooling PEMFC stack considerably relies on the construction of thermal management scheme to achieve temperature dynamic heat balance. Similarly, to further improves the electrical efficiency and lifetime, coordinate operation of thermal management BOP components is expected to be fully activated by advanced control-oriented strategy with modeling algorithm. Hence, this review focus on the current status, challenges and future perspectives on extensive potentials for liquid-cooling thermal management system (TMS) in PEMFCs. Firstly, the fundamental temperature control configurations with dual-control targets are introduced, followed by the viewpoint of structure-based as well as material-based, i.e., the effects of cooling channel design (such as parallel, serpentine and wave flow field) and novel coolant adoption (including single metal particles nanofluids and hybrid particles-based nanofluids) upon stack temperature profile and heat transfer performance are reported in detail. Then, the current situation of primary thermal management schemes is supported with classic proportional-integral-differential (PID) feedback control strategy and analyzed to address the necessity of decoupling the temperature parameters to improve the dynamic response for the high-power PEMFC stack. Furthermore, novel control strategies with emerging modeling algorithms (like model reference adaptive control, sliding mode control), oriented for comfort temperature grade, the robust response against varying loads and high efficiency with lower parasitic power, are discussed to evaluate the feasibility and practicality for TMS further integrated into overall energy management architecture of PEMFC system. Finally, in conclusion and perspective, it is appreciated for advanced thermal control strategy to promote single passive temperature control towards proactive thermal management implementation, which is promising and beneficial to identify its substantial potential of low-grade heat recovery and utilization for on-board PEMFC system. Figure 1

Solid sorption heat pipe (SSHP) combined with thermochemical sorption technology and passive heat transfer mode of heat pipe can increase the heat transfer limit of traditional capillary heat pipe and thermosiphon, and realize the continuous recovery and utilization of low-grade thermal energy. For studying characteristics of SSHP, the equilibrium sorption/desorption properties under various constraint pressures of calcium chloride-ammonia working pair are initially researched by a Rubotherm magnetic suspension balance. According to the actual threshold temperature at different constraint pressures, the experimental enthalpy and entropy of reaction are obtained by least square fitting. In the subsequent sections, the range of Failure Zone, Partial Reaction Zone and Completed Reaction Zone for SSHP are deduced without heat and mass transfer analysis. In the SSHP system using calcium chloride-ammonia working pair, the maximum temperature difference between the desorption section and the condensation section could reach 55°C in theory because of the large temperature difference heat transfer characteristics.