## 3 Results and Discussion  
### Porous Structure Characterization  
The carbon electrodes with a binder weight ratio of 10% are named Acetylene Black-10%, and the naming method for the remaining carbon electrodes follows the same pattern. The structural characterization of the carbon electrodes with different binder ratios is summarized in Figures 2 and 3. The nitrogen adsorption and desorption isotherms are provided in the supplementary file. Type IV isotherms are clearly observed for the Acetylene Black, Vulcan XC-72R, and Ketjen Black carbon electrodes, indicating the presence of mesoporous structures in carbon electrodes. In Figure 2, low binder ratio carbon electrodes show a more specific surface area. This is because more micropores are detected. High binder ratio carbon electrodes hardly exhibit any micropores. This is due to the significant reduction in the number of micropores in carbon electrodes with increased binder content, as confirmed by previous studies [15]. The average pore size for each carbon electrode is presented in Figure 3. As the binder ratio increases, the average pore size of the carbon electrode increases continuously. The addition of binders has a minor influence on mesopores and macropores but has an obvious effect on the micropores. Adding more binders can block more micropores, which will not be filled by nitrogen in the gas sorption experiment. The reduction in micropore volume results in an increased proportion of mesopores and macropores, leading to an increase in the average pore size of the carbon electrode with an increasing binder ratio. This is further confirmed by the pore size distribution of carbon electrodes with different binder ratios shown in Figure 4. Figure 4 illustrates that for the Acetylene Black carbon electrode, the proportion of pores in the 30-50 nm range increases with increasing binder ratio in the range of 10-50%, resulting in a larger average pore size. In addition, the change in the average pore size is also affected by the preparation process  
Figure 1: (**a**) Structural diagram of seawater battery; (**b**) seawater battery prototype.  
and the testing method, resulting in some differences in the trend of Figure 3. Additionally, Figure 4b,c shows that as the binder ratio gradually increases within the range of 10-50%, the slope of the pore size distribution curve in the 2-10 nm range decreases for the Vulcan XC-72R and Ketjen Black carbon electrodes. This presents a decreasing proportion of pores in the 2-10 nm range. Figure 4b,c provides an increased proportion of pores in the 10-50 nm range for Vulcan XC-72R and Ketjen Black carbon electrodes, thus improving the utilization of mesopores and macropores in the carbon electrode and facilitating oxygen diffusion. Furthermore, the addition of binders affects the wettability of the carbon electrode. As PTFE is hydrophobic, the wettability of the electrode decreases with an increasing binder ratio, resulting in a reduced number of reaction sites on the carbon electrode. However, the decrease in wettability also promotes the diffusion of oxygen in non-wetting pores, as previously demonstrated in Ref. [34].  
Figure 3: The average pore diameter of different carbon electrodes.  
Figure 2: Specific surface areas of different carbon electrodes.