Research Progress on Synthesis and Applications of MAX Phase Coatings for Metal Bipolar Plates in PEMFCs
Among the various fuel cells,proton exchange membrane fuel cells (PEMFCs) have been widely used because of their high efficiency,zero emission and environmental protection. Metal bipolar plates play an important role in electronic connection,backbone for membrane electrode assembly,water management transmission and gas flow channels in the PEMFCs. However,the dissolution and corrosion of the metal bipolar plates are inevitable under high temperature and acidic conditions. Recently,the surface coating technology has been identified as one of the promising and facile strategy to enhance the protective functions for various metal bipolar plates. At present,various coatings have been developed to modify the electrical conductivity and corrosion resistance of bipolar plates (BPs),including noble metal coatings,carbon-based coatings,metal nitride coatings,conductive polymer coatings and Mn+1AXn (MAX) phase coatings.The work aims to review the research progress on fabrication and applications of MAX phase coating for surface modification of metal bipolar plates. Different from the traditional transition metal nitrogen/carbide coating,MAX phases are a new class of ternary nanolaminate materials with hexagonal lattice structure,in which M presents an early transition metal,A is mainly from group A,and X is carbon or nitrogen. Benefiting from their unique layered structure and strongly controlled covalent,ionic and metallic bonding characteristics,MAX phases possess the unique characteristics of metal and ceramic materials,such as good electrical and thermal conductivity,as well as excellent corrosion resistance,heat resistance,oxidation resistance,etc. Currently,considerable pioneering studies have been carried out on the preparation techniques,structural characterizations as well as the industrial applications for MAX phase coatings in surface engineering.Therefore,different methods evolving with chemical vapor deposition,physical vapor deposition,solid-phase reaction and thermal spraying were introduced for synthesis of MAX phase coatings. Based on these deposition methods,a detailed description of the preparation process for MAX phase coatings was provided,along with an elucidation of the varying effects of different preparation methods on the surface morphology and microstructure of MAX phase coating materials.Especially,physical vapor deposition was a commonly method used for preparing MAX phase coatings. The deposition temperature of physical vapor deposition was relatively low,and the equipment was simple,which could realize the large-area preparation of MAX phase coatings.Subsequently,the corrosion rate of MAX phase coatings was measured through electrochemical corrosion tests under the stimulated acid environments emulating harsh PEMFC systems,and the interface contact resistance before and after corrosion of MAX phase coatings was evaluated. A detailed elucidation was provided on the variations in the conductivity and corrosion resistance properties of MAX phase coatings,including Ti-Al-C,Ti-Si-C,and Cr-Al-C. Furthermore,intensive efforts were conducted on the failure mechanisms of MAX phase coatings,considering aspects such as the elemental composition,crystal structure,first-principle theory,degree of crystallization,differences in passive film composition,and atomic orientation.Finally,due to the unique crystal structure limitations of MAX phase materials,achieving high-quality MAX phase coatings preparation at moderate temperatures (<600 ℃ ) remains challenging,and there is a need to further enhance the adhesion strength between the coating and the substrate. Comprehensive performance evaluations and stable engineering applications research on assembled bipolar plates have not been carried out yet,indicating the necessity for further investigation in these areas.
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