In this work, a low cost air-assisted cylindrical liquid jets spraying (ACLJS) system was developed to prepare high-performance catalyst-coated membranes (CCMs) for proton exchange membrane fuel cells (PEMFCs). The catalyst ink was flowed from a cylindrical orifice and was atomized by an air stream fed from a coaxial slit and sprayed directly onto the membrane, which was suctioned to a heated aluminum vacuum plate. The CCM pore architecture including size, distribution and volume can be controlled using various flow parameters, and the impact of spraying conditions on electrode structure and PEMFC performance was investigated. CCMs fabricated in the fiber-type break-up regime by ACLJS achieved very high performance during PEMFC testing, with the top-performing cells having a current density greater than 1900 mA/cm 2 at 0.7 V under H 2 /O 2 flows and 700 mA/cm 2 under H 2 /Air at 1.5 bar (absolute) PEMFCs have long been considered to be among the most promising next-generation energy conversion systems for portable devices and transportation vehicles due to their zero or low pollution, high power density and low temperature operation.1-3 However, the widespread commercialization of PEMFCs remains challenged by the high cost of cell assembly and the requirement of durable performance. In the last decade, significant efforts have been launched to make the technology cost competitive and to improve the cell performance by developing new electrocatalysts and lowering the noble metal loading. 4,5 However, the catalyst composition is not the only factor to consider when creating high-performance, low cost PEMFC electrodes.The electrode pore structure plays a critical role in determining PEMFC performance, and it is a direct result of the application method. The electrode porosity and pore size distribution impact reaction kinetics and mass-transport processes, including water management and electron/proton conduction.6-8 Structural optimization can be decisive in reducing performance losses, particularly at low catalyst loading and high current density. 9 In general, the catalyst coated membrane (CCM) method has been found to be superior to the gas diffusion layer (GDL) coating process because CCM fabrication avoids catalyst particles penetrating into the pore network of the GDL, lowering resistance at the interface with the electrolyte. For the CCM method, the catalyst ink can be applied onto the polymer membrane by brushing, screen printing, spraying, reactive spray deposition as well as roll-to-roll methods.6,10-14 Among them, spraying has been commonly employed for CCM fabrication. 11,15,16 However, the effects of spraying conditions on the electrode structure and PEMFC performance are poorly understood in many systems, including the air-assisted cylindrical liquid jets spraying (ACLJS) system.The objective of this work is to investigate the influence of spraying parameters on CCM structure and performance, with a particular focus on the ACLJS system because of its ability to produce very high performance CCMs at...