## 3 Results and Discussion
### Bi PP Electrodes Based on 29 BC GDLs  
In order to prove if a commercial GDL provides similarly good electrodes when using a PP method to deposit Bi, a 29 BC gas diffusion layer from SGL Carbon was chosen. The BC GDL was thinner (230 \(\upmu\)m) and had a structure and composition that was different than the IH GDL. Using SEM images, it was possible to distinguish the different layers of the BC GDL. Using CT imaging, the well-ordered pore structure of the different layers can be seen [30]. After Bi PP depositon, coarse-grained Bi deposits were placed not only on top of the BC but also inside of the microporous layer (Figure 4a). In the middle of the GDL BC, Bi was homogeneously deposited, preserving the well-ordered pore structure (Figure 4b). Bi was deposited into the whole GDL layer down to the backside of it at 190 \(\upmu\)m.  
In agreement with the investigations on the IH GDL with Bi, no Bi dissolution on the top or inside of the GDL was observed (Figure 4c). The FE reached for formate for 1 h in a Bi-BC-PP electrode was 80%, which is at least 10% lower compared with the Bi-IH-PP and Bi-IH-P GDEs (Figure S6). Until now, it was not possible to achieve a higher FE for formate.  
After 24 h of operation at 50 mA\(\cdot\)cm\({}^{-2}\), only the Bi crystallites transformed into sharp plates (scales) with a higher surface area (inset of Figure 4c). This is in accordance with our results for Bi-IH-PP GDE (see Figure 1b). XRD measurements revealed that the main phase after 24 h of CO\({}_{2}\) electrolysis was metallic Bi with a preferred \(<\)012\(>\) orientation normal to the sample surface. Regarding the intermediate phase, bismuthite carbonate (Bi\({}_{2}\)(CO\({}_{3}\))O\({}_{2}\)) was found. Some reflections with low intensity indicated the presence of potassium bismuth oxide (KBi\({}_{2}\)O\({}_{18}\)) in traces (Figure S7). According to the Pourbaix diagram of Bi, under reaction conditions (\(-\)1.43 V vs. SHE, pH \(>\) 10), the thermodynamically stable Bi species is elementary Bi [80]. The selectivity of the tested Bi-PP-BC electrode did not change with time. The reason for this could be that there is no difference in the reactivity of the metallic Bi phase and Bi oxides in the eCO\({}_{2}\)RR reaction towards formate [42]. After eCO\({}_{2}\)RR, the texture of the GDL remained unchanged. This is at odds with our previous research for the Sn catalyst, where the GDL texture was completely destroyed after 1 h of electrolysis [30]. The observed black spots (blemishes) in Figure 4d are only mechanical deformations that are caused by the electrolysis process.  
The Bi-BC-PP GDE were also tested for their long-term stability at a higher current density of 200 mA\(\cdot\)cm\({}^{-2}\). However, the mechanical stability of the GDE based  
Figure 3: Product contribution of Bi-IH-PP GDE, Bi-5-55-140 (dots) with Bi-IH-P GDE (squares). Tested at 200 mA\(\cdot\)cm\({}^{-2}\), 50 \({}^{\circ}\)C, 24 h.  
on the commercial GDL was not good enough, and after 10 h, the Bi-BC-PP GDE was mechanically destroyed.  
Due to a different structure and composition, the in-house fabricated GDLs are more suitable as substrates for catalysts constructed via electrodeposition compared with the chosen commercial GDLs. With the further optimisation of the structure and composition of GDLs, an optimized electrodeposition of catalysts, and a good understanding of the processes during electrolysis, the long-term stable operation of Bi-GDEs fabricated by the pulse plating technique should be achieved.