1. Additive manufacturing method for Ni-based dendritic electrode
The principle of additive manufacturing of dendritic electrode structures is shown in Fig. 1a. The nickel rod is selected as the metal anode, which is vertically located on the surface of the cathode plate. At the same time, a circulating pump was used to make the deposition solution flow rapidly between the two electrodes. Connect the positive and negative poles of the power supply to the metal anode and cathode plate respectively, thereby forming a conductive path. With the rapid circulation of the solution, the process of jet electrodeposition is formed on the cathode plate. With the rapid growth of the deposited skeleton, the contact between the metal anode and the deposited skeleton is inevitable, resulting in short circuit. Therefore, the position of the metal anode is controlled in real-time by detecting the current. As the deposition time increases, a complete dendritic skeleton structure is ultimately formed. Cut and unfold the deposited skeleton into a flat shape to form a dendritic self-supporting electrode.
As shown in Fig. 1b, compared with ordinary jet electrodeposition, the preparation method of the electrode changes the original parallel distribution of cathode and anode into vertical distribution, forming a local area of high current density at the cathode plate, thus speeding up the deposition efficiency. At the same time, the nickel rod electrode is selected as the anode material, so that during the deposition process, on the one hand, metal ions are timely supplemented for the deposition solution, on the other hand, as the deposition template to guide the cathode deposition skeleton to grow vertically. In order to effectively reduce the applied potential of the two electrodes, sufficient charge distribution can be generated on the surface of the cathode plate under the condition of small inter-electrode gap, so as to provide sufficient charge reserve for the local deposition area of the cathode plate[13–15]. The fast flow of solution can enhance the solute diffusion at the cathode deposition interface, ensure the ion concentration near the deposition layer, and effectively improve the deposition efficiency of metal ions. Due to the water-based solution used in the electrodeposition process, it is inevitable that the reaction of electrolytic water will occur under the local high current density, resulting in the precipitation of a large number of gases on the surface of the two electrodes. However, under the action of rapid flushing liquid, these bubbles are quickly flushed away from the electrode surface[16]. At the same time, these bubbles also act as dynamic templates, making the deposited skeleton contain a large number of void structures. The rapid flow of solution also modified the surface morphology of the skeleton, forming a unique dendritic structure. In the following chapter, the macro-micro structure formation mechanism of the whole dendritic electrode will be explored in detail.
2. Simulation analysis of macroscopic deposition environment of Ni-based dendritic electrode
2.1 Characteristic analysis of macroscopic flow field between two electrodes
Firstly, the finite element analysis of the flow field distribution between the two electrodes in the macroscopic deposition environment is carried out. Explore the distribution of the flow field between the two poles under three parameters: distance between the two poles, different anode diameter, and initial flow velocity.
2.2 Influence of different pole distance on the flow field between the two poles
Construct a vertical distribution model of two electrodes, with the solution flowing vertically downwards. and the distance between anode end face and cathode plate was adjusted to 0.5mm, 0.8mm, 1.0mm and 1.2mm, respectively. The flow field distribution of the resulting model was shown in Fig. 2. From the flow field distribution of the four different pole distance parameters, it can be seen that the solution can basically maintain a high flow velocity distribution inside the nozzle before it flows out of the nozzle. At 0.5 mm distance (Fig. 2a), the solution flow velocity around the anode rod is the largest, and most of the solution can flow to the cathode surface at a high flow velocity, and a fast lateral flow can be maintained at the cathode plate surface. However, there is no obvious change in the flow velocity in the gap between the anode end face and the cathode plate surface, which remain basically stationary. When the pole distance was increased to 0.8 mm (Fig. 2b), the solution flow velocity in the surface region of the cathode plate decreased and the local surface region changed from red to yellow, while the flow field distribution was similar to that under the 0.5 mm parameter without any obvious change. At the same time, a low velocity flow field distribution is still maintained in the gap between the two electrodes. As the pole distance continues to increase to 1 mm (Fig. 2c), the flow velocity region on the surface of the cathode plate shifts to a lighter color, indicating that the flow velocity on the electrode surface continues to decrease. At this time the edge at the gap between the two electrodes also begins to show a light blue change, indicating that the flow velocity field begins to affect the flow field distribution near the gap between the two electrodes. When the gap between the two electrodes was increased to 1.2 mm (Fig. 2d), the solution flow velocity on the surface of the cathode plate was further reduced, and the effect of the solution ejected from the nozzle on the cathode plate was correspondingly reduced. At the same time, due to the increase of the gap between the two electrodes, the fast-flowing solution starts to diffuse to the inside of the gap, making the solution flow velocity inside the gap slowly increase. Combining the above effects of different two electrode gap parameters on the macroscopic flow field distribution, it can be obtained that a smaller electrode gap helps to increase the flow velocity of the solution on the surface of the cathode plate, but has no effect on the flow field inside the gap, and increasing the gap distance accelerates the flow velocity of the solution inside the gap but reduces the flow velocity of the solution on the surface of the cathode plate. In order to accelerate the additive manufacturing process of dendritic skeleton, it is appropriate to reduce the solution flow inside the gap and accelerate the solution flow in other region, which serves to improve the deposition efficiency. However, too fast solution flow velocity is not conducive to the structural stability of the deposited skeleton. Therefore, inter-electrode gap of 1.0 mm was chosen to achieve rapid flow of solution around the electrode during the deposition of the dendritic skeleton.
2.3 Influence of different anode diameters on the gap flow field between two electrodes
The influence of different anode rod diameters on the flow field between the two electrodes is analyzed as shown in Fig. 3. When the anode rod is 2 mm (Fig. 3a), the overall flow velocity of the solution inside the nozzle is relatively low, but at this time the surface of the cathode plate is affected by the largest area of the sprayed solution, covering almost the entire cathode plate area, only a small part of the bottom of the anode rod remains stationary. When the anode rod increases to 4 mm (Fig. 3b), the solution flow velocity inside the nozzle start to increase and an orange area appears, at which time the solution flow velocity on the cathode surface also increases accordingly, while the rapid solution flow can cover most of the cathode surface, but along with the increase of the anode rod diameter, the area at the gap between the two electrodes also increase further. Continuing to increase the diameter of the anode rod to 6 mm (Fig. 3c), the solution flow rate inside the nozzle further increased and began to affect the flow field distribution of the ejected solution, but due to the increase in the electrode gap area, the range of the rapid solution flow on the surface of the cathode plate was also reduced, forming the effect of a localized rapid flushing of the solution. When the diameter of the anode rod is increased to 8 mm (Fig. 3d), the flow channel area inside the nozzle is already very narrow, and although a local high-speed solution distribution can be formed, the larger anode rod diameter increases the difficulty of clamping and the cost of preparation. At the same time, the ejected solution is too fast to maintain a stable flow trend, resulting in the inability to form a stable solution ejection on the surface of the cathode plate, which eventually causes a decrease in the solution flow velocity on the entire surface of the cathode plate. A larger anode diameter will make the gap area between the two electrodes too large and reduce most of the effective deposition area. By analyzing the influence of the anode rod diameter on the macroscopic flow field at the two electrodes, the local electrodeposition of the dendrite skeleton requires maintaining a stable local fast solution flow[17]. Therefore, 6mm was chosen as the diameter of the anode rod for dendrite skeleton deposition.
2.4 Influence of different inlet initial flow velocity on the flow field between two electrodes
The influence of the inlet initial velocity on the solution flow field between the two electrodes was simulated with different parameters, and four inlet initial velocities of 0.3 m/s, 0.5 m/s, 0.7 m/s and 0.9 m/s were selected, as shown in Fig. 4. It can be seen from the figure that the flow field distribution around the two electrodes is basically the same at different parameters, and the fastest solution velocity occur in the nozzle, and is distributed around the anode rod near the nozzle. It means that the nozzle can play a local compression effect on the solution flow at this time, thus creating the fastest solution flow velocity here. The solution is then quickly ejected from the nozzle and dispersed around the surface of the cathode plate, forming a rapid solution flow on the surface of the cathode plate. However, the fixed anode diameter and inter-electrode gap make the flow field in the gap region still maintain a stable low speed state. According to the flow velocity scale on the right side of the figure, it can be seen that the maximum value of the flow velocity in the internal region of the nozzle increases along with the increase of the inlet flow velocity, and thus the flow velocity on the surface of the cathode plate also increases. However, the rapid velocity of solution will also result in the change of the morphology and structure of the dendritic skeleton. Therefore, the inlet velocity range of 0.7-0.9m/s is selected initially to ensure the structural integrity and unified morphology of the prepared dendritic skeleton[18].
2.5 Influence of different inter-electrode potential on the cathode charge density distribution
Cathode plate is not only the carrier of deposited product, but also the important region of reduction reaction. Therefore, during the deposition process, the cathode plate will collect a large number of negatively charged free electrons, and the charge density distribution on the cathode plate at this time can indirectly reflect the deposition rate of the dendrite type electrode during the reduction process. Figure 5a shows the two-dimensional height distribution of the charge density on the surface of the cathode plate at an inter-electrode potential of 6 V. From the calculation result, it is shown that the maximum charge density is exhibited at the position where the anode rod is projected directly on the surface of the cathode plate. At the same time, the charge density decreases sharply after spreading in all directions and reaches the minimum value of charge density at the edge of the cathode plate. It shows that when the anode rod is located vertically in the plane of the cathode plate, the gap distance between the two electrodes is greatly shortened, and a localized high charge density region corresponding to the anode end can be formed on the surface of the cathode plate, and thus the deposition rate in this region is greatly increased. The calculated charge density distribution at different potential with the contour distribution of the two-dimensional height graph is shown in Fig. 5b. It can be seen from the figure that the peak charge density increases continuously with the increase of the potential, but the charge density distribution at the edge shows a decreasing trend, indicating that the total charge number and local charge density increase accordingly with the increase of the potential between the two electrodes, and the charge density shows a stable decreasing trend when the area corresponding to the gap region is exceeded[19].
2.6 Influence of different inter-electrode gap on cathode charge density distribution
Further simulation analysis of the influence of the change in the distance between the two electrodes on the cathode charge density is shown in Fig. 6. In Fig. 6a, it still shows a cone-like charge density distribution, but in the high charge density region at the top, instead of a relatively flat state, it shows a sharp distribution. Then the cathode charge density distribution at four parameters of 0.5 mm, 0.8 mm, 1.0 mm and 1.2 mm were analyzed for comparison, as shown in Fig. 6b. It can be seen that the morphology of the top charge density region shows a clear change with the change of the inter-electrode gap. When the inter-electrode gap is small, a region of high charge can be formed at the cathode plate similar to that at the end face of the anode. With the increase of the inter-electrode gap, the high charge density region keeps shrinking, indicating that the charge induction effect on the cathode surface at the anode surface is decreasing, and finally when the inter-electrode gap increases to a certain extent, the charge density distribution region of the whole cathode plate changes from the previous trapezoidal distribution to a cone-like distribution. At the same time, the charge density at the highest point is correspondingly reduced. Therefore, the excessive inter-electrode gap is not conducive to the high-density charge accumulation on the surface of the cathode plate, and thus the deposition rate of the dendrite type skeleton cannot be improved. Combining the influence of potential and inter-electrode gap on the charge density distribution on the surface of the cathode plate, it can be concluded that only by choosing suitable parameter values can a reliable charge distribution be induced, and thus an efficient and stable environment for dendritic type skeleton electrodeposition can be formed[20].