The global popularity of membrane separation is growing due to its high energy efficiency, small footprint, low capital, and operating cost, and especially its environmentally friendly process flexibility. Polycrystalline materials with high porosity, flexible micropore size, and functional tunability are promising membrane materials. However, the achieved membrane separation efficiency is still far from the potential value predicted by material simulation mostly due to the existence of undesirable membrane imperfectness. Apart from the awareness of bulk film cracks, the impact of intercrystalline defects on the total performance of membranes is usually not well acknowledged. Defect formation is one of the bottleneck issues in polycrystalline membranes which not only significantly affects the efficiency of the membrane but also have an inevitable impact on its repeatability and process feasibility. Most research efforts in the polycrystalline membrane area have been devoted to developing novel structured polycrystalline materials. However, the summary of polycrystalline materials is beyond the scope of this work, instead, we complement existing membrane material reviews with a critical analysis of defect formation and mitigation strategies. This review presents the recent opportunities and challenges in polycrystalline membrane fabrication, emphasizing the issue of unfavorable defect formation. The state-of-the-art solutions and strategies for mitigating defects have been reviewed and evaluated in detail. Finally, further understanding of defect formation and future research directions on characterization, defect evaluation, and membrane remediation have been outlined. This review offers valuable insights into fabricating compact polycrystalline membranes with high separation efficiency for various separation systems.

膜分离因其高能效、小占地面积、低资本和运营成本，尤其是其环保的工艺灵活性，在全球范围内越来越受欢迎。具有高孔隙率、灵活的微孔尺寸和功能可调性的多晶材料是很有前途的膜材料。然而，实现的膜分离效率仍远低于材料模拟预测的潜在值，这主要是由于存在不良的膜缺陷。除了体膜裂纹的意识之外，晶间缺陷对膜整体性能的影响通常没有得到很好的认识。缺陷形成是多晶膜的瓶颈问题之一，它不仅显着影响膜的效率，而且不可避免地影响其重复性和工艺可行性。多晶膜领域的大部分研究工作都致力于开发新型结构化多晶材料。然而，多晶材料的总结超出了这项工作的范围，相反，我们通过对缺陷形成和缓解策略的批判性分析来补充现有的膜材料评论。这篇综述介绍了多晶膜制造中最近的机遇和挑战，强调了不利缺陷形成的问题。已经详细审查和评估了用于减轻缺陷的最先进的解决方案和策略。最后，概述了对缺陷形成的进一步了解以及未来在表征、缺陷评估和膜修复方面的研究方向。这篇综述为制造用于各种分离系统的具有高分离效率的紧凑型多晶膜提供了宝贵的见解。