3.1 Accumulation of bubbles on the surface of conventional cathode
During the reaction process, the intensified hydrogen evolution reaction can lead to excessive H2 production, resulting in the formation of a hydrogen gas film on the cathode surface, which to some extent hinders the mass transfer of calcium ions and carbonate ions in the cathode region. The agglomeration and rupture of bubbles can cause fluid disturbances in the vicinity of the electrode, further impeding ion migration in the water near the electrode .
In the experiment, the bubbles generated near the conventional cathode were mainly concentrated within 3mm of the electrode surface, and significant stratification was observed during their ascent. To further describe the state of the bubbles, the bubbles in the cathode region during the electrochemical water softening reaction were classified into three types, A, B, and C, based on their characteristics. The bubbles in each region exhibited distinct differences in flow velocity and direction. In Type A, the bubbles were newly generated on the electrode surface, while in Type B, the bubbles rose from below, were more concentrated, and exhibited a larger number of bubbles and faster movement. Type C bubbles were located on the outer edge, were more dispersed, and had larger particle sizes. The velocity of bubbles in Types A, B, and C increased as they rose.
At lower reaction voltages, Type A bubbles remained for a longer time, and Type B bubbles migrated towards Type A bubbles before returning to their original location. Type C bubbles were hindered by Type B bubbles during their migration towards the B region, and were repelled when they approached. As the reaction voltage increased, the flow velocity of Type A bubbles increased significantly, but was still significantly different from that of bubbles in the outer regions due to the hindering effect of Type C bubbles. The distribution areas of each bubble type are shown in the Fig. 4.
Table 1
Bubble size distribution of 2DCathode.
| 10v | 20v | 24v | 28v | 32v | 36v |
A | 0.108mm | ༜0.097mm | ༜0.076mm | ༜0.051mm | ༜0.051mm | ༜0.051mm |
B | 0.158mm | 0.169mm | 0.152mm | ༜0.152mm | \ | \ |
C | 0.247mm | 0.287mm | 0.346mm | 0.353mm | 0.402mm | ༞0.402mm |
Table 1 shows the size distribution of bubbles in the middle section of the 2DCathode, with clear distinctions in size among the A, B, and C bubble types under the same operating conditions. As the voltage increased, the volume of Type A bubbles in the 2DCathode during the reaction process decreased. When the voltage exceeded 28V, the changes in the thickness of the bubble film were no longer evident, and the difference in particle size between Types A and C bubbles increased. Figure 5 shows that the shading in the area where Type A bubbles were present relative to the area where Type C bubbles were present increased significantly, indicating an increase in the difference in the number of bubbles between the two regions.
Figure 5 shows the bubble states of the 2DCathode at different voltages during the electrochemical water softening process. The limiting effects of Type B and Type C bubbles on Type A bubbles become more pronounced as the reaction intensifies. The increasing limiting effects cause Type A bubbles to accumulate and fail to detach effectively during the process of producing a large number of bubbles. The stacking effect further exacerbates the differentiation in the volume and ascent velocity of Type A, B, and C bubbles, gradually forming a bubble stagnant zone in the electrode gap.
3.2 Distribution of bubbles on the surface of a stereoscopic cathode
In the experiment, the size and distribution of visible bubbles near the cathode were measured in water with a hardness of 500mg/L CaCO3. The surface of 3DAOC was mainly covered by Type C bubbles, with a bubble size of approximately 0.08mm at a voltage of 10V. As the voltage increased from 20V to 36V, the bubble size increased from 0.11mm to 0.13mm. Figure 6 shows that Type A bubbles gradually appeared in the field of view during this process, with a significantly smaller size than that of Type C bubbles. Under this condition, it was not possible to distinguish the bubble types inside the bubble film. As the voltage increased, the Type C bubbles on the surface of 3DAOC remained relatively dispersed, with no significant increase in their number.
During the reaction process, Type A bubbles gradually appeared on the surface of 3DAOC. Figure 6 shows that the bubbles on the surface of 3DAOC were not significantly limited by Type C bubbles, and the detachment of Type A bubbles was relatively smooth and could quickly move outward. However, the appearance of the bubble film indicated that the detachment speed of bubbles slowed down, suggesting the presence of slight limiting effects on the surface of 3DAOC.
3DAOC effectively weakened the limiting effects of Type B and Type C bubbles on Type A bubbles during the reaction process, reducing the impact of stacking on the ascent of bubbles. However, the appearance of the bubble film indicated that the bubble behavior on the surface of 3DAOC was transitioning towards the bubble behavior on the surface of 2DCathode.
Table 2.1
Bubble film thickness distribution of 3DAOC.
| 10v | 20v | 24v | 28v | 32v | 36v |
Bubble film thickness | 0.8mm | 1.0mm | 1.1mm | 1.2mm | 1.0mm | 0.9mm |
Thickness of slow flow zone | \ | \ | \ | \ | < 0.05mm | < 0.1mm |
Table 2.1 shows that the thickness of the bubble layer on the electrode surface of 3DAOC during the reaction process did not increase with increasing voltage; in fact, it even decreased. However, Fig. 6 shows an increase in shading, indicating a decrease in the ability of bubbles to spread outward as they ascend.
Table 2
.2The rising velocity of bubbles on the surface of 3DAOC.
| 10v | 20v | 24v | 28v | 32v | 36v |
velocity | 0.014m/s | 0.020m/s | 0.022m/s | 0.022m/s | 0.023m/s | 0.024m/s |
Table 2.2 shows that the ascent velocity of bubbles on the surface of 3DAOC increased slowly during the reaction process, while the bubble layer remained dispersed, providing sufficient time for ion reactions. The author believes that the weakening of the limiting effects and stacking effects by 3DAOC provides favorable conditions for the reaction process, but cannot completely eliminate the impact of stacking effects. The significant influence of the limiting effects and stacking effects may reappear in a higher voltage range. In the reaction process, the stacking effect of bubbles on the surface of 2DCathode runs through the vertical direction of the entire electrode, and the effects of limiting and stacking gradually intensify with increasing vertical height, causing the middle and upper parts of the electrode to be unable to participate effectively in the reaction. In contrast, the individual cathode blocks of 3DAOC are independently arranged, and the stacking and limiting effects do not intensify with increasing vertical height of the electrode.
As shown in Fig. 7, the shading of the bubble layer becomes heavier with the increasing position of the cathode block of 3DAOC, which is different from 2DCathode. Only Type C bubbles are clearly visible on the surface of 3DAOC, while Type A and Type B bubbles account for a very small proportion in the field of view. This indicates that the reaction is intensified, mainly due to the sub-level being closer to the anode and the reaction becoming more intense as the position of the sub-level increases.
Table 3
Bubble distribution of 3DAOC.
| 1 | 2 | 3 | 4 | 5 |
The distance from the anode | 40mm | 45mm | 50mm | 55mm | 60mm |
The thickness of the bubble film | 2mm | 4mm | 7mm | 10mm | 14mm |
3.3 Influence on water flow state
During the experiment, the fluid flow of the cathode during the reaction process was captured using PIV technology. Figure 8 shows that the reaction process 3DAOC caused extremely significant disturbance to the water, and there were obvious vortices generated in the ascent process of bubbles from each sub-level, indicating significant fluid flow in the reaction tank.
Type B and C bubbles can limit the bubbles in region A, in which during the reaction process of 2DCathode, Type B and C bubbles are pushed away when they migrate towards the area where Type A bubbles are located, indicating insufficient contact between the fluid outside the bubble membrane and the cathode surface, which can restrict the migration of particles. However, 3DAOC weakens this process while enhancing the fluid flow. The 2DCathode structure limits the flow direction of the fluid, causing the alkaline area to be significantly covered by bubbles, and OH- cannot effectively participate in the reaction. 3DAOC strengthens the migration of ions while reducing the restriction on the flow direction of the fluid. Figure 8 shows that the end of the streamlines are on the surface of the electrode under various operating conditions, indicating that the ability of ions to migrate laterally to the electrode is enhanced and is no longer pushed away by B and C type bubbles.
Figure 8 shows that the fluid flow velocity above the area where the 3DAOC cathode block is located has been increased, and the fluid flow velocity in the entire flow area is between 0.001m/s-0.01m/s, which does not increase with the increase of voltage. As the reaction intensifies, more water with flow velocities between 0.005-0.01m/s appears above the electrode, and the fluid velocity near each cathode block is lower as shown in the figure. In contrast, during the reaction process of 2DCathode, the streamline near the cathode rises rapidly along the cathode surface, and there is less lateral fluid flow. The figure shows that the fluid velocity in the cathode area of 2DCathode increases significantly with the increase of vertical distance during the reaction process, gradually showing a trend of being enveloped, while the surface area of the electrode is still in a state of low fluid velocity. This indicates that the new water is pushed away by the stacking effect of bubbles when it approaches the cathode, and cannot effectively replenish new ions. With the increase of voltage during the reaction process of 3DAOC, there is no phenomenon of being enveloped by high-velocity fluid on the surface of the cathode block. The streamline diagram shows that the fluid velocity near the first and second cathode blocks increases but flows towards the area where the anode is located, and no obvious slow-flow regions are observed.
Figure 9 shows the flow state at different time periods under a voltage of 36V, and the results indicate that the fluid flow in the cathode area of 3DAOC is relatively stable during the reaction process. The vortices of fluid flow are mainly concentrated in the area where bubbles intersect during their ascent process. In contrast, Figure 8 shows no obvious vortices in each cathode block area, which is completely different from the state of 2DCathode during the reaction process. This indicates that the changes in fluid flow during the reaction process of 3DAOC are less affected by bubbles.
Figure 10 shows the flow velocity of water at different heights of each cathode block (left) and the velocity distribution of the mid-level water at different voltages (right). The results show that there is a significant attenuation of water flow velocity within the distance of 0.12-0.14m, followed by a rapid increase to a higher level. Combined with Fig. 8, it can be found that the increase in water flow velocity within the distance of 0.14-0.16m is due to the large number of bubbles generated by the cathode block below that make the water flow velocity increase during their ascent process. However, there is no significant change in water flow velocity at different voltages, indicating that 3DAOC is not affected by the increase in bubble generation during the reaction process. No velocity peak appeared near the cathode block, indicating that the ions migrate to the cathode block without being quickly carried away by the water. Figure 12 shows the average flow velocity of water in the mid-section of the cathode for two types of cathode structures. The velocity of water close to the anode side of 2DCathode showed a significant peak, which appeared again after velocity attenuation with increasing distance, indicating that the water flow velocity on both sides of 2DCathode is at a high level. Combined with Fig. 8, the two peaks are caused by the ascending bubbles from below. However, the water flow velocity within the distance of 3-6mm of the cathode block is at a low level, indicating the significant wrapping of the cathode block by type B and C bubbles. Figure 10 shows the average velocity of water at different horizontal heights of each cathode block, and the results show a significant increase in the average velocity of water at the horizontal height of the second to fifth cathode blocks, indicating that 3DAOC can effectively promote the flow of water in the entire area during the reaction process. Figure 11 shows the flow velocity of water in the region where the third cathode block is located, and there is no significant deceleration in water flow velocity. The region with lower water flow velocity is between 0.135m and 0.140m, indicating that there is no significant wrapping of the cathode block by bubbles.
Figure 13 shows the velocity status of tracked particles. The figure shows that with the increase of voltage, the main distribution area of particles gradually concentrates on the lower left side, and the number of particles on the left is significantly more than that on the right. This indicates that as the voltage increases, the transverse flow velocity of water increases. Meanwhile, the streamline in Fig. 8 shows that the outer water flows towards the cathode block during the reaction process of 3DAOC. This indicates that this structure not only promotes the diffusion of OH- but also promotes the migration of other ions towards the cathode block. This phenomenon is particularly evident when the water hardness is high. Figure 14 shows that the overall pH level of 3DAOC (water hardness of 700mg/L CaCO3) during the reaction process is higher than that of 2DCathode.
The increase in the number of bubbles generated by each cathode block promotes the flow of water during their ascent process. 2DCathode only shows a significant increase in fluid flow velocity in the upper-middle region of the electrode, and Fig. 7 shows that bubbles generated by each cathode block can effectively detach. Figure 15 shows that bubbles from the fourth and fifth sub-levels of 3DAOC do not rise along the surface of the third cathode block at different voltages. Moreover, the distance between bubbles from the fourth and fifth sub-levels and the third cathode block increases with the increase of voltage. Figure 8 shows that bubbles generated by adjacent cathode blocks mix in the top region of the upper cathode block. This indicates that each cathode block is less affected by bubbles generated by the cathode block below during the reaction process. The streamline in Fig. 8 shows that the water rises along the surface of the electrode after flowing into the lower region of each cathode block, indicating that 3DAOC can effectively replenish new ions during the reaction process. Figure 12 shows the flow velocity of water at the same horizontal position as the third cathode block in 3DAOC and 2DCathode. The results indicate a significant difference between the two. In the region where the electrode is located in 2DCathode, there is a clear area with zero water flow velocity, and there are velocity peaks on both sides. The velocity peak shown in Fig. 11 appears in the vicinity of 0.145m, and it can be observed from Fig. 9 that this velocity peak is caused by the bubbles generated by the fourth cathode block.
The streamline in Fig. 11 shows that the water near the cathode block comes from the bottom and the outside, which promotes the probability of ion reaction with the cathode surface far from the anode, thereby improving the utilization rate of the cathode surface far from the anode. During the experiment, dye was added to the water, and the mid-section cathode block of 3DAOC was filmed. Figure 16 shows that the dye in 3DAOC accelerates and flows towards the surface of the cathode block far from the anode when it is close to the cathode block. The average flow velocity of the dye in region a is 0.003982m/s, while that in region b is 0.00946m/s.This indicates that there is enough fresh water in contact with the surface of the electrode far from the anode.