## 3 Results and Discussion
### Sn PP Electrodes Based on In-House GDLs  
In our previous work [30], Sn electrodeposition was demonstrated on the top of and inside of the commercial GDL 29 BC. The highest FE for formate that was attained with such GDEs was 85%. In this work, in order to increase the Faraday efficiency for formate as well as the electrode stability for the Sn electrodeposition, the optimized in-house fabricated electrode [37] was used. Coarse-grained Sn coatings with intergrown crystallites ranging from 0.5 um to 2 um were deposited on the top of IH fabricated GDL by PP at \(\mathrm{j_{p}}=-1\) A-dm\({}^{-2}\) and \(\mathrm{t_{on}}\):\(\mathrm{t_{off}}=1\) s:1 s (Figure 5a). At a shorter \(\mathrm{t_{on}}\):\(\mathrm{t_{off}}\) time, single coarse-grained crystallites (1 um) with sharp edges grew on the GDL surface (Figure 5b). The crystallites were separated from each other through hollows or were grouped into islands. Additionally, unevenly distributed small crystallites with a size of about 0.1 um were deposited on top of them; some of them are marked with red circles in Figure 5b. It is supposed that such an inhomogeneous surface  
Figure 4: Bi-BC-PP GDE Bi-15-11-10 before eCO\({}_{2}\)RR: (**a**) SEM image with surface morphology on top of the GDL and on the next microporous layer; (**b**) CT scan in the middle of the sample. Bi-BC-PP GDE Bi-15-11-10 after eCO\({}_{2}\)RR (50 mA\(\cdot\)cm\({}^{-2}\), 50 \(\circ\)C, 24 h): (**c**) SEM image, surface morphology inside of the GDL. Inset with surface morphology on top of the GDL; (**d**) CT scan inside of the GDE. morphology of the electrodeposited catalyst additionally enlarges the active surface area and will lead to an increased FE of the GDE. The Sn-IH-PP GDEs were thoroughly investigated in the CO2 electrolysis. As a benchmark, state-of-the-art Sn-IH-P GDEs were used [36,37]. In accordance with the Bi-IH-PP GDEs, it was found that the surface morphology of the electrodeposited Sn coating has an influence on the FE for formate and on the product distribution (Figure 5c). The more compact deposit (Figure 5a), where CO2 mainly penetrated through the grain boundaries and other deposit defects, showed a lower selectivity for formate (FE: 82.3%) than the inhomogeneous deposit (FE: 93.1%) (Figure 5b), where CO2 additionally penetrated through the enlarged catalyst surface (an increased number of sharp edges) and free spaces between the crystallite islands. On the GDL with the pulse plated electrocatalyst with shorter ton:toff time, the FE for formate (93.1%) was even better than that of the Sn-IH-P electrodes (87.1%, Figure 5c, last column). The Sn content of both electrodes (Figure 5c) was comparable (2.6 wt.% Sn on GDEs with precipitated catalyst, 3.7 wt.% Sn on GDEs with electrodeposited catalyst). These electrodes feature a low selectivity for the side products of CO (12%) and H2 (1%), as shown in our previous publication [30]. The potentials of the electrodes are shown in Table S3. The long-term electrolysis for Sn-IH-PP GDE was carried out at 50 mA\(\cdot\)cm\({}^{-2}\) because when applying a current density of 200 mA\(\cdot\)cm\({}^{-2}\), the HER increases after two hours, wherein no stable reaction would be possible after this time. In the beginning of the electrolysis, the FE was at about 97% and was quite stable for 4 h (Figure 6). Then, the selectivity for formate decreased to approximately 90% and was almost stable for another 6 h. The FE for hydrogen evolution reaction at this time was under 2%. After 11 h, the FE for formate decreased in a step-by-step manner. Simultaneously, the HER as a competing reaction became more favoured. This could indicate the leaching of the active catalyst. Furthermore, leaching of a SnOX catalyst prepared by the precipitation method has been reported [34,67]. After 18 h, the main reaction was that the HER and experiment was interrupted. The same Sn-IH-PP GDE was investigated using CT imaging before and after the long-term electrolysis. The Sn catalyst was only deposited on the surface of the IH-GDL (Figure 7a).