## 3 Results and Discussion
### Bubble Coverage and Actual/Effective Current Density on the Anode--Experiment 1
Once the bubble touches the edge of the anode or slots, it gets released immediately. Obviously, the bubble evolution is not the limiting factor of the electrolysis process for the investigated slotted anode. Therefore, with the increase in the input current, more bubbles are generated and then released frequently, showing an overall increase trend in the actual current density. Comparison of Bubble Evolution Behavior on 11 cm\({}^{2}\) and 50 cm\({}^{2}\) Anodes--Experiment 2  
For the small anode previously tested in Experiment 1, the bottom surface area of the anode was as small as 11 cm\({}^{2}\); thereby the generated bubbles have very small area to grow and coalesce before they get released. As a consequence, the anode performance, including the bubble layer coverage, actual current density on the anode, and subsequently the cell voltage fluctuation, is greatly dependent on the evolution of a single or several large bubbles, as discussed previously in Figures 6 and 7. In this part, a larger anode with bottom surface area of 50 cm\({}^{2}\) was tested. On this anode, there was enough space for more bubbles to evolve independently at the same time, which is closer to a realistic industry situation. Actually, the influence of the bubbles on the electrolysis process should be the combined effects of many bubbles rather than a single or several bubbles. So, Experiment 2 could give a better perspective and understanding to the 'group effect' of bubbles evolution, and a more complex and stable operating condition of the anode could be expected. Figure 8 shows the bubble size distribution on the 100 mm \(\times\) 50 mm flat anode within 60 s. It is found that a large portion of bubbles are concentrated in the range of 0-200 mm\({}^{2}\) although the bubble size is distributed in a very wide range. With the increase in the apparent current density from 0.9 A\(\cdot\)cm\({}^{-2}\) to 1.3 A\(\cdot\)cm\({}^{-2}\), the percentage of fine bubbles (<10 mm\({}^{2}\)) showed a slight increasing trend, indicating a faster generation rate of the new bubbles. However, the average bubble size and the percentage of larger bubbles (larger than 200 mm\({}^{2}\)) showed an increasing trend with the increase in the apparent current density. The proportions of larger bubbles are 42.2%, 45.3%, and 50.6%, respectively, at 0.9 A\(\cdot\)cm\({}^{-2}\), 1.1 A\(\cdot\)cm\({}^{-2}\), and 1.3 A\(\cdot\)cm\({}^{-2}\). The average bubble sizes within 60 s are 159 mm\({}^{2}\), 160 mm\({}^{2}\), and 209 mm\({}^{2}\), respectively, at 0.9 A\(\cdot\)cm\({}^{-2}\), 1.1 A\(\cdot\)cm\({}^{-2}\), and 1.3 A\(\cdot\)cm\({}^{-2}\). Figure 9 shows the variation of actual current density on the 100 mm \(\times\) 50 mm flat anode within 60 s. It is found that there are fluctuations in the real apparent current density with time, but the fluctuation frequency is not stable (at least not as stable as it was with the 50 mm \(\times\) 22 mm flat anode in Figure 7a). The fluctuation range of the actual current density is reduced to 1.0-2.0 A\(\cdot\)cm\({}^{-2}\) at the apparent current density of 0.9 A\(\cdot\)cm\({}^{-2}\), in contrast to 1.0-4.1 A\(\cdot\)cm\({}^{-2}\) on the 50 mm \(\times\) 22 mm flat anode under the same situation. The average actual current density within 60 s showed a slight increase with the increasing apparent current density, at 1.31 A\(\cdot\)cm\({}^{-2}\),1.33 A\(\cdot\)cm\({}^{-2}\), and 1.37 A\(\cdot\)cm\({}^{-2}\), respectively, under the apparent current density of 0.9 A\(\cdot\)cm\({}^{-2}\), 1.1 A\(\cdot\)cm\({}^{-2}\), and 1.3 A\(\cdot\)cm\({}^{-2}\). The fluctuation frequency of the actual current density curves also increased with an increasing apparent current density. For the 100 mm \(\times\) 50 mm flat anode case, about eight bubble release cycles could be roughly distinguished within 60 s at 0.9 A\(\cdot\)cm\({}^{-2}\). The bubble coverage varied from 7% to 54%, with the average coverage of 30% at 0.9 A\(\cdot\)cm\({}^{-2}\).