# Control Strategies for Piston Trajectory in Ionic Compressors for Hydrogen Storage
## 1 Introduction  
Fossil fuels are well known as the source of main energy for countries around the world, providing important support for economic, scientific, technological, and cultural development and progress [1]. The combustion of fossil fuels produces greenhouse gases [2], such as carbon dioxide, which contribute to the increase in global temperatures and increase the number of extreme climate events [3]. To solve environmental problems and achieve sustainable and renewable energy development, the transformation of traditional energy systems is essential [4]. Hydrogen is a prospective solution to solving these environmental issues and achieving sustainable development due to the high amount of energy provided per mass and the clean-burning nature of the product [5]. Hydrogen can be used as fuel in many fields such as power generation and transportation [6]. With the development of the new energy automobile industry, the promotion of hydrogen fuel cell vehicles has become one of the important means of reducing greenhouse gas emissions in the automobile industry [7]. Due to the low energy density of hydrogen at atmospheric pressure [8], its direct application in fuel-cell vehicles may require the vehicle to be refueled frequently or have a larger storage space, ultimately increasing costs [9]. Expanding the storage pressure of hydrogen by compressing it is an effective solution for increasing its volumetric energydensity [10]. Commonly utilized technologies for compressing hydrogen consist of reciprocating piston compressors [11], diaphragm compressors, linear compressors, metal hydride compressors [12], and ionic liquid compressors [13]. In recent years, researchers have paid significant attention to the impressive performance of ionic compressors. A compressor utilizing an ionic liquid operates based on a hydraulic drive system that propels the solid piston back and forth. This motion leads to the compression of the gas by the piston of the ionic liquid located on the solid piston [14]. Ionic liquids possess virtually incompressible properties, low vapor pressures [15], high degrees of stability [16], and good lubrication and cooling properties [17]. Moreover, hydraulic drive systems exhibit advantages such as rapid responses, high rigidity, and high load capacities [18]. By utilizing ionic liquid piston technology, ionic liquid compressors achieve superior volumetric and isentropic efficiencies compared to conventional compressors [19]. Additionally, the integration of hydraulically driven free-piston technology substantially reduces the number of moving parts in the compressor and allows for controlled piston trajectories [20]. The synergistic integration of these two technologies endows ionic compressors with remarkable adaptability to achieve efficient compression processes across a broad spectrum of operating conditions. While ionic liquid compressors offer numerous advantages, the use of free pistons places high demands on the design and control of hydraulic systems [21]. In an ionic compressor, the motion of the metal piston impacts the dynamic property of the ionic liquid and consequently affects the behavior of the two-phase gas-liquid flow. This has a significant influence on the performance of the compressor. Additionally, compressor breakdown is a leading cause of downtime at hydrogenation stations, accounting for 30% of the total maintenance time [22]. Ionic liquid compressors are driven by hydraulic systems, which are free of rigid connections such as crank connecting rods. Therefore, there is no piston crash caused by a rigid connection in these compressors. Piston crashes can result in damage to the piston, cylinder, valves, and other critical components. Therefore, it is imperative to employ specific control techniques for regulating the motion of the free piston in an ion compressor.