This is an electronic reprint of the original article. The reprint may differ from the original in pagination and typographic detail. Sayed-Ahmed, H.; Toldy, Á. I.; Santasalo-Aarnio, A. Dynamic operation of proton exchange membrane electrolyzers—Critical review. Published in: Renewable and Sustainable Energy Reviews. DOI: 10.1016/j.rser.2023.113883. Published: 01/01/2024. Document Version: Publisher's PDF, also known as Version of record. Published under the following license: CC BY. Please cite the original version: Sayed-Ahmed, H., Toldy, A. I., & Santasalo-Aarnio, A. (2024). Dynamic operation of proton exchange membrane electrolyzers—Critical review. Renewable and Sustainable Energy Reviews, 189, Article 113883. https://doi.org/10.1016/j.rser.2023.113883. Review article. Dynamic operation of proton exchange membrane electrolyzers—Critical review. H. Sayed-Ahmed, Á.I. Toldy, A. Santasalo-Aarnio *. Department of Mechanical Engineering, Aalto University, FI-00076 Aalto, Finland. ARTICLE INFO. Keywords: PEM electrolyzer, Electrolysis, Green hydrogen, Dynamic operation, Optimization, Fluctuating power, VRE, Demand response, Degradation. ABSTRACT. Green hydrogen is seen as a promising energy storage and balancing solution to complement the ever-increasing share of variable renewable energy sources in the grid. The dynamic operation of polymer electrolyte membrane (PEM) electrolyzers has the potential to simultaneously lower the cost of green hydrogen and improve the flexibility of the grid by taking advantage of the volatility of renewable production. However, dynamic operation affects a wide range of variables related to the degradation of electrolyzer components and the safety and efficiency of the process, often in counterintuitive ways. This, in turn, makes it difficult to predict the levelized cost of the green hydrogen produced when operating on the electricity markets. This critical review examines state-of-the-art literature on the behavior of PEM electrolyzers under dynamic operation, bearing in mind the objective of reducing the levelized cost of green hydrogen. Knowledge gaps, key development directions, and future research needs are identified with respect to PEM electrolysis equipment, operating parameters, degradation, and the role of dynamically operated PEM electrolyzers on the electricity markets. It is found that while the field is developing at a rapid pace, there is a lack of holistic studies that consider all (or even most of) the interconnected variables that affect the levelized cost of green hydrogen during the dynamic operation of PEM electrolyzers. It is postulated that this complex network of interactionsThis is an electronic reprint of the original article. The reprint may differ from the original in pagination and typographic detail. Sayed-Ahmed, H.; Toldy, Á. I.; Santasalo-Aarnio, A. Dynamic operation of proton exchange membrane electrolyzers—Critical review. Published in: Renewable and Sustainable Energy Reviews. DOI: 10.1016/j.rser.2023.113883. Published: 01/01/2024. Document Version: Publisher's PDF, also known as Version of record. Published under the following license: CC BY. Please cite the original version: Sayed-Ahmed, H., Toldy, A. I., & Santasalo-Aarnio, A. (2024). Dynamic operation of proton exchange membrane electrolyzers—Critical review. Renewable and Sustainable Energy Reviews, 189, Article 113883. https://doi.org/10.1016/j.rser.2023.113883. Review article. Dynamic operation of proton exchange membrane electrolyzers—Critical review. H. Sayed-Ahmed, Á.I. Toldy, A. Santasalo-Aarnio *. Department of Mechanical Engineering, Aalto University, FI-00076 Aalto, Finland. ARTICLE INFO. Keywords: PEM electrolyzer, Electrolysis, Green hydrogen, Dynamic operation, Optimization, Fluctuating power, VRE, Demand response, Degradation. ABSTRACT. Green hydrogen is seen as a promising energy storage and balancing solution to complement the ever-increasing share of variable renewable energy sources in the grid. The dynamic operation of polymer electrolyte membrane (PEM) electrolyzers has the potential to simultaneously lower the cost of green hydrogen and improve the flexibility of the grid by taking advantage of the volatility of renewable production. However, dynamic operation affects a wide range of variables related to the degradation of electrolyzer components and the safety and efficiency of the process, often in counterintuitive ways. This, in turn, makes it difficult to predict the levelized cost of the green hydrogen produced when operating on the electricity markets. This critical review examines state-of-the-art literature on the behavior of PEM electrolyzers under dynamic operation, bearing in mind the objective of reducing the levelized cost of green hydrogen. Knowledge gaps, key development directions, and future research needs are identified with respect to PEM electrolysis equipment, operating parameters, degradation, and the role of dynamically operated PEM electrolyzers on the electricity markets. It is found that while the field is developing at a rapid pace, there is a lack of holistic studies that consider all (or even most of) the interconnected variables that affect the levelized cost of green hydrogen during the dynamic operation of PEM electrolyzers. It is postulated that this complex network of interactions