## 3 Results and Discussion
### Bi PP Electrodes Based on In-House CDLs
After CO\({}_{2}\) electrolysis, the Bi deposit remains on the top and also inside of the GDL (Figure 2a); however it remains with a structure that has changed (Figure 2b). The Bi crystallites are converted independent of the electrolysis parameters into sharp plates (scales) with enormous surface area (Figure 2b). The EDX spectra show a high amount of oxygen (Figure S4). We suppose that the Bi catalyst was oxidized during the CO\({}_{2}\) reduction reaction; this did not change the FE of the electrode, because it is well-known from the literature that Bi-based catalysts are active in oxide and reduced metal forms [50,77,78]. During PP deposition with short deposition time and longer pauses (t\({}_{\text{on}}\):t\({}_{\text{off}}\) = 0.005 s:0.05 s), thin channels were formed inside of the Bi deposit (Figure 2c). Thus, an increase in the active surface area was achieved. The electrolyte wets and CO\({}_{2}\) diffuses are facilitated through the channels inside of the Bi electrocatalyst, and the aimed synergetic effect between catalyst and GDL properties is achieved. As a result, the FE for formate for Bi-IH-PP GDE reaches 93% (Figure 2a). Figure 1: Investigations of Bi-IH-PP GDEs. (**a**) Effect of PP parameters on the FE for formate. Tested at 200 mA\(\cdot\)cm\({}^{-2}\), 50 \({}^{\circ}\)C, 60 min; (**b**) SEM image from the surface of Bi-15-11-10 GDE; (**c**) SEM image of Bi-15-55-20 GDE; (**d**) CT cross-section of Bi-15-55-20 GDE. The Bi-IH-PP GDEs that showed the best results during the shorter eCO\({}_{2}\)RR (Figure 1a) were tested for the long-term CO\({}_{2}\) electrolysis. It is supposed that the most possible reason for this very good performance of the PP electrode is the uneven distribution of the Bi catalyst (see Figure 1c,e) on top of and also inside of the GDL. The FE of formate for all tested Bi-IH-PP electrodes (long-term tests) was greater than 80%, and in some cases, it reached 94%. It was found that the FE was dependent on the GDL that was used. The reproducibility in one GDL batch was 95-100%. However, there were observed distinctions between different batches. Freshly produced GDLs after Bi deposition reached an FE of greater than 94% (Figure 1a). Older batches showed a lower FE because the deposited Bi amount was lower. [PERSON] et al. [79] showed that the laser surface etching of GDL before the electrodeposition of Pt enhanced the electrochemical deposition. Most likely, the in-house prepared substrates achieved different aging stages because of time, which only affects the electrochemical deposition. The carried out electrochemical measurements (CV, galvanostatic experiments) of the blank GDL did not show any changes; therefore, in Figure 3, the electrode with the best performance is not shown. The benchmark electrode Bi-IH-P with the fine catalyst distribution is still better (Figure 3) than the Bi-IH-PP electrodes. The electron balance for electrodes with an electrodeposited catalyst is sometimes not closed. In the used setup, the produced hydrogen typically diffuses through the porous GDL towards the gas side of the electrode; it is there that hydrogen, other gaseous products, and non-converted CO\({}_{2}\) are detected. Due to the Bi that was deposited on top of the GDE, some H\({}_{2}\) was released into the electrolyte on the cathode side, which could be observed as gas bubbles in the electrolyte. A detection of gases at the electrolyte side of the GDL was not possible in the setup used. As a result, the missing FE can be dedicated to H\({}_{2}\). A small increase in the HER at TOS \(>\) 15 h was observed. After a reaction time of 23 h at these high current densities (200 mA\(\cdot\)cm\({}^{-2}\)), the FE for formate was still 76.4%. The loss on selectivity and the small increase in the HER are not related to a degradation of the Bi catalyst. It is supposed that the growth of sharp plates of oxidized Bi and the restructuring of the last Bi atomic layer mechanically deformed the GDL structure.