## 1 Introduction  
Cold spray is an advanced additive manufacturing and repair process that utilizes high-pressure carrier gases, such as nitrogen or helium, to accelerate solid micron-sized particles to high velocities through a converging-diverging nozzle. This technique enables rapid material deposition without melting, relying solely on the particles' kinetic energy for adhesion [1, 2, 3, 4, 5]. Upon impact with the substrate, the particles undergo severe plastic deformation, forming strong bonds with the surface. This low-temperature process avoids the thermal damage typically associated with high-temperature methods, preserving the original properties of both the feedstock and the substrate [3, 6]. As a result, cold spray is particularly advantageous for industrial applications requiring high-quality, thermally sensitive material deposition. In high-pressure cold spray systems, operating pressures range from 1 to 5 MPa, with gas temperatures reaching up to 1100 \({}^{\circ}\)C--remaining below the melting point of the particles [7]. Under these conditions, particles are accelerated to velocities typically between 500 and 1000 m/s. A crucial parameter in the deposition process is the standoff distance (SOD), which defines the gap between the nozzle exit and the substrate [2]. Along with SOD, factors such as nozzle geometry, operating conditions, particle type and size, and carrier gas properties play a significant role in determining particle velocity and temperature upon impact, deposition efficiency (DE), and coating quality [8]. The ability to finely control these parameters while minimizing thermal effects makes cold spray a vital technique in modern manufacturing and repair applications. Recent studies demonstrate the potential of hydrogen evolution reactions (HER) for renewable energy. However, commercial-scale hydrogen production is still obscured by the challenges of producing stable and cost-effective electrocatalysts [9]. More specifically, while noble-metal-based catalysts demonstrate excellent catalytic performance, their high cost and limited availability hinder commercial applications [9]. Nickel, as a non-noble metal, provides a promising alternative, though its catalytic performance is inherently lower. To address this, increasing the electrode's surface area through surface modification has been a key focus [10]. [PERSON] et al. [11] demonstrated that cold-sprayed nickel samples achieved superior electrocatalytic activity compared to smooth nickel electrodes. This enhancement was attributed to increased active surface area and improved surface roughness, which enhanced the efficiency of the electrolysis process. The creation of fin geometries using a mask (or a wire mesh) positioned between the nozzle exit and the substrate further amplifies these effects. By guiding particle flow through its openings, the mask facilitates the formation of pyramidal pin fins on the substrate surface. These structures not only increase the active surface area considerably but also significantly enhance surface roughness, improving catalytic performance by providing more active sites for electrochemical reactions. Studies using masked High-Velocity Oxy-Fuel (HVOF) spraying [12], masked atmospheric plasma spraying, and masked suspension plasma spraying [10] confirmed that electrodes coated with a mask exhibited greater electrocatalytic activity, primarily due to the enhanced roughness levels achieved during deposition. In our previous study [13], we employed Computational Fluid Dynamics (CFD) to investigate a three-dimensional high-pressure masked cold spray nozzle, using nickel powder as the injection feedstock. The simulations were based on the PCS 800 cold spray system (Plasma Giken, Yorimachi, Japan). The parametric study examined four masks with varying wire thicknesses and opening sizes, positioned at 4 mm increments from the nozzle exit. Substrates were placed at SODs of 10 mm and 20 mm.