# Numerical Analysis of Cold Spray Process for Creation of Pin Fin Geometries
###### Abstract  
A numerical study was performed to analyze the particle deposition of a cold spray process for the preparation of nickel electrocatalysts used in the Hydrogen Evolution Reaction (HER). The study focused on the creation of fin-shaped geometries with an optimal porosity on the electrode surface using a mask located between the nozzle exit and the substrate. Computational Fluid Dynamics (CFD) was performed on a three-dimensional high-pressure nozzle, with nickel powder used as the injection feedstock. The behavior of particles was effectively modeled through a two-way coupled Eulerian-Lagrangian approach. As per the parametric study, four masks of varying wire thicknesses and opening sizes were investigated. The masks were placed at 4 mm increments from the nozzle exit, with the substrate placed at standoff distances (SODs) of 10 mm and 20 mm. To capture the effects of the gas inlet operating conditions, two different nozzle inlet conditions were analyzed (2 MPa and 400 \({}^{\circ}\)C, 4 MPa and 800 \({}^{\circ}\)C). It was found that the nozzle inlet operating condition had the most significant impact, as it relates to the particle velocity and powder deposition. The high-pressure operating condition resulted in a deposition efficiency (DE) greater than 99.9% for all the test cases, with nearly all the impacted particles depositing on the substrate. For the medium-pressure operating condition, the DE increased linearly as the mask SOD was increased, due to the increase in the particle velocity upon impact. cold spray; computational fluid dynamics; additive manufacturing; hydrogen; electrode +
Footnote †: journal: applied sciences  
usage from renewable sources. However, it faces major challenges in the efficient hydrogen production required to meet commercial-scale demand [17]. The challenges are associated with the need for stable and low-cost electrocatalysts required for the electrolysis process [17]. Noble-metal-based electrocatalysts, such as platinum-based catalysts, have an excellent catalytic performance for the HER. However, their applicability for commercial use is limited by the scarcity and high costs of the metals [17]. As a result, there have been significant developments in and research on the use of non-noble metals, which have a comparatively lower electrocatalytic performance but are far more abundant and significantly less expensive. To compensate for their lower catalytic activity, several studies have focused on improving their apparent activity by increasing the electrode surface area through the use of surface morphology [16; 18; 19]. [PERSON] et al. [16] analyzed the use of nickel powder to coat electrode surfaces using a cold spray process. This experimental study analyzed the electrocatalytic activity of cold-sprayed nickel powder on an aluminum substrate. Three different cold spray operating pressures and temperatures were analyzed. The study found that the cold-sprayed nickel samples outperformed a smooth nickel electrode, having higher electrocatalytic activity. This was due to increases in the active surface area and surface roughness, leading to an increase in the efficiency of the electrolysis process. It was also found that the greatest electrocatalytic activity was obtained for the test case with the lowest deposition efficiency, which also experienced the highest strain accumulation and compressive residual stress. It is believed that this was due to the peening effect, caused by undeposited particles that bounced off the substrate/deposited layer. Another approach to increasing the electrode surface area and electrocatalytic activity is by creating fin array geometries on the electrode surface. Fin arrays are created by placing a mask (i.e., a wire mesh) in front of the substrate surface. The openings in the mask funnel the particles, allowing for the creation of pyramidal pin fins on the substrate.