## 1 Introduction  
Hydrogen energy is playing an increasingly vital role in the global transition toward sustainable energy solutions [1, 2, 3]. Across Europe and beyond, large-scale pilot projects, typically around 1 MW in capacity, are being developed to demonstrate the potential of hydrogen technologies in replacing carbon-based energy sources [4, 5]. These initiatives underscore the critical role of hydrogen in decarbonization efforts and the long-term reduction in greenhouse gas emissions [6, 7, 8]. A key advantage of hydrogen-based energy systems is their ability to store excess electrical energy in the form of hydrogen, ensuring flexibility and energy security [9, 10, 11]. However, for efficient storage and utilization, hydrogen must be compressed at high pressures, which presents both technical and economic challenges. Traditional mechanical compression methods, such as piston-based or valve-based compressors, have long been employed for hydrogen storage and transportation [12, 13]. However, these systems are associated with several drawbacks, including significant energy losses, mechanical wear, frequent maintenance, and operational inefficiencies. By contrast, electrochemical hydrogen pumps/compressors (EHP/Cs) represent a promising alternative,offering a more efficient and reliable solution [14; 15; 16]. These devices operate based on electrochemical principles, enabling hydrogen pumping without moving parts, thereby eliminating mechanical degradation and reducing maintenance costs [17]. Additionally, EHP/Cs exhibit higher energy efficiency, as they require considerably less energy per unit of compressed hydrogen compared to conventional mechanical systems. Beyond compression, another key advantage of EHP/Cs is their ability to simultaneously purify hydrogen [18; 19; 20; 21]. This dual functionality makes electrochemical compression particularly attractive for high-purity hydrogen applications, including fuel cell systems, laboratory-grade hydrogen supplies, and industrial processes requiring ultra-pure hydrogen [22]. The core component of EHP/C technology is the membrane electrode assembly (MEA), which shares structural and functional similarities with MEAs used in proton exchange membrane fuel cells (PEMFCs) and hydrogen generators (PEMHGs). The MEA consists of a reinforced polymer electrolyte membrane and gas diffusion electrodes, enabling efficient hydrogen compression. The integration of optimized MEAs in EHP/Cs enhances performance, allowing for scalable and modular solutions adaptable to a range of industrial and energy storage applications. The principal scheme of an MEA applicable in electrochemical hydrogen pumps/compressors is presented in Figure 1.  
Hydrogen, produced through electrolysis or alternative methods such as natural gas reforming, is introduced to the anode electrode of an electrochemical hydrogen pump/compressor. Under the application of electric power, hydrogen molecules (H\({}_{2}\)) dissociate into protons (H\({}^{+}\)) and electrons (e\({}^{-}\)). The protons move through the polymer electrolyte membrane, reaching the cathode where they recombine with electrons from the external circuit, ultimately forming atomic and, subsequently, molecular hydrogen. By maintaining a constant current throughout the process, the hydrogen compression occurs as the pressure in the cathode chamber (P\({}_{\text{cathode}}\)) rises relative to the anode chamber (P\({}_{\text{anode}}\)). This pressure difference, known as the differential pressure (P\({}_{\text{differential}}\)), is a critical parameter in the compression process. The electrochemical principles governing this process allow for efficient hydrogen compression, with differential pressure being a key performance indicator [23; 24]. A key challenge in the development of electrochemical hydrogen pumps and compressors (EHP/Cs) is the optimization of catalyst loading in gas diffusion electrodes, which is crucial for the efficiency of the partial electrode reactions involved [25; 26].