## 3 Results and Discussion  
### Bubble Distribution Pattern on the Anode--Experiment 1  
Figure 3 shows the bubble distribution patterns on the flat anode and slotted anode under different experimental conditions. The bright/yellow zones suggest that bubbles are more frequently observed in these areas. The gradient bars indicate the number of occurrences of bubbles on the specific anode surface in the 121 images within 60 s.  
From Figure 3, it is found that the anode bubbles tend to coalesce and adhere to a certain region (marked as bright yellow in Figure 3) for both flat anodes and slotted anodes. It looks like some certain regions or points of the anode are the preferred site for bubble formation or accumulation. These bubble-concentrated areas, termed as 'inactive regions', shrink with the increase in the anode current density in both the flat anode and the slotted anode cases. The decrease in the 'inactive area' suggests the more uniform distribution of the current on the anode under higher current density. Compared to the flat anode, the bubble distribution pattern shows the slotted anode has much smaller bubble-concentrated areas under all the investigated current densities. The existence of large areas of 'inactive' regions suggests the frequent appearance of large bubbles; thereby, it is proof that the slotted anode design has a faster gas releasing rate than the flat anode. The generation rate of bubbles on the anode was calculated by counting the number of newly generated bubbles in each grid. The bubble generation patterns within 60 s on the flat anode and slotted anode are drawn in Figure 4. The bright/yellow zones suggest a faster generation rate of the bubble than the dark/blue zones. The gradient bars indicate the number of newly generated bubbles on the specific anode surface within 60 s.  
From Figure 4, the regions marked as bright yellow display a higher generation rate of the bubble than the regions marked as dark blue. Since the anode gas is the product of the electrochemical reactions, a higher bubble generation rate corresponds to higher electrochemical activity at a certain area, or is termed as an 'active region', in contrast to the 'inactive region', as discussed in Figure 3. In Figure 4, with the increase in current density, the area of the 'active region' increased significantly in both the flat anode and the slot anode cases. When the apparent current density reached up to 1.3 A\(\cdot\)cm\({}^{-2}\), the locations of the active regions changed; this is evidence that the active regions on the anode can be substitutable during electrolysis. It indicates that the electroactive region of the carbon anode is not only related to the material structure, but is also affected by the electrode potential. However, this trend is not observed in the slotted anode cases, probably because the slotted anode has a faster gas release rate than the flat anode, indicating a more ordered bubble evolution pattern on the slotted anode. Figure 3: Accumulated bubble distribution patterns on the 50 mm \(\times\) 22 mm flat and slotted anodes from statistical analysis within 60 s; the gradient bars indicate the number of occurrence of bubbles on the specific anode surface in the 121 images within 60 s. (**a**) Apparent current density 0.9 A\(\cdot\)cm\({}^{-2}\), flat anode (**b**) 1.1 A\(\cdot\)cm\({}^{-2}\), flat anode (**c**) 1.3 A\(\cdot\)cm\({}^{-2}\), flat anode (**d**) 0.9 A\(\cdot\)cm\({}^{-2}\), slotted anode (**e**) 1.1 A\(\cdot\)cm\({}^{-2}\), slotted anode (**f**) 1.3 A\(\cdot\)cm\({}^{-2}\), slotted anode. It is worth mentioning that the active regions in Figure 4 are more likely to appear on the right-hand side of the anode; this is because the right hand side of the anode was close to the cathode due to the design of the double chamber quartz crucible. However, this influence could be minimized by comparison under the same experimental conditions. It is as expected that the bubble generation pattern (in Figure 4) generally matches with the bubble distribution pattern (in Figure 3).