## 3 Results and Discussion
### Bubble Coverage and Actual/Effective Current Density on the Anode--Experiment 1  
Figure 6 shows the fluctuations of the bubble coverage rate on the 50 mm \(\times\) 22 mm anodes within 60 s. From the bubble coverage curve of 50 mm \(\times\) 22 mm flat anode, eight release cycles are observed. The coverage rate varies from 7% to 78% regularly, and the average coverage within 60 s is 56%. For the 50 mm \(\times\) 22 mm slotted anode case, 13 bubble release cycles can be distinguished although the cycle curve; is not as obvious as that on the flat anode, which indicates that bubbles get released more frequently than on the flat anode. By slotting on the anode, the coverage rate is greatly reduced to 8\(-\)49%, with the maximum coverage of 32\(-\)49%, much lower than the 70\(-\)78% rate on the flat anode. The average coverage within 60 s is 32%. The changes in the bubble coverage would result in the change of the actual current density on the anode. The variation of the actual current density on 50 mm \(\times\) 22 mm anodes with time are drawn in Figure 7.  
Figure 6: The bubble coverage rate on the 50 mm \(\times\) 22 mm flat and slotted anodes within 60 s, apparent anode current density 0.9 A\(\cdot\)cm\({}^{-2}\). From Figure 7a, it is found that the actual current density exhibits regular fluctuations with time, which corresponds to the bubble evolution process on the anode. Under the apparent anode current density of 0.9 A\(\cdot\)cm\({}^{-2}\), the actual current density varied regularly between 1.0-4.1 A\(\cdot\)cm\({}^{-2}\). With the increase in the apparent current density from 0.9 A\(\cdot\)cm\({}^{-2}\) to 1.3 A\(\cdot\)cm\({}^{-2}\), the variation amplitude of the actual current density decreased significantly. However, the variation frequencies increased subsequently, indicating a faster bubble evolution rate under a higher apparent current density. Consequently, the average actual current density within 60 s decreased slightly with the increase in the apparent current density, with 2.38 A\(\cdot\)cm\({}^{-2}\), 2.35 A\(\cdot\)cm\({}^{-2}\). and 2.14 A\(\cdot\)cm\({}^{-2}\) under the apparent current density of 0.9 A\(\cdot\)cm\({}^{-2}\), 1.1 A\(\cdot\)cm\({}^{-2}\), and 1.3 A\(\cdot\)cm\({}^{-2}\), respectively. From Figure 7b, a similar principle was observed on the 50 mm \(\times\) 22 mm slotted anode. The bubble evolution rate, or in other words, the variation frequency of the actual current density, increased significantly with increasing apparent current density. With the slotted anode, under the apparent current density of 0.9 A\(\cdot\)cm\({}^{-2}\), the actual current density dropped to 1.0-1.8 A\(\cdot\)cm\({}^{-2}\). The average actual current density within 60 s was 1.34 A\(\cdot\)cm\({}^{-2}\). The significant decrease in the actual current density demonstrated the effects of the anode slot. In industrial practice, a low actual current density would be very friendly  
Figure 7: The variation of actual current density on the 50 mm \(\times\) 22 mm flat and slotted anodes within 60 s. (**a**) 50 mm \(\times\) 22 mm flat anode, (**b**) 50 mm \(\times\) 22 mm slotted anode. to the prebaked anode carbon material (specifically the working condition of carbon in an industrial aluminum electrolysis cell) during electrolysis [8; 20]. The fluctuation ranges of the actual current density on the slotted anode showed no obvious change with the apparent current density. However, the average actual current density increased slightly with increasing apparent current density, with 1.34 A\(\cdot\)cm\({}^{-2}\), 1.58 A\(\cdot\)cm\({}^{-2}\), and 1.98 A\(\cdot\)cm\({}^{-2}\) under the apparent current density of 0.9 A\(\cdot\)cm\({}^{-2}\), 1.1 A\(\cdot\)cm\({}^{-2}\), and 1.3 A\(\cdot\)cm\({}^{-2}\), respectively. The actual current density on the anode is probably influenced by the relative sizes between the anode and bubbles. For the slotted anode in this investigation, the maximum bubble diameter was limited to 9 mm due to the shape of the anode bottom surface.