## 3 Results and Discussion  
### Bi PP Electrodes Based on In-House CDLs  
The performance of the Bi-IH-PP-GDLs for formate production highly depends on the used PP parameters, such as the cathodic pulse current density j\({}_{\mathrm{P}}\), pulse time t\({}_{\mathrm{on}}\), and relaxation time t\({}_{\mathrm{off}}\). (Figure 1a). The PP parameters had an effect on the surface morphology of the deposited Bi, which determines the FE for the formate production. Decreasing j\({}_{\mathrm{P}}\) with a t\({}_{\mathrm{on}}\):t\({}_{\mathrm{off}}\) of 1:1 led to an increase in the electrode performance from approx. 10% to approx. 90% (white bars in Figure 1a). The deposition time was calculated such that coatings of an equal thickness of 5 \(\upmu\)m were deposited. At j\({}_{\mathrm{P}}\) = \(-15\) A\(\cdot\)dm\({}^{-2}\) and t\({}_{\mathrm{on}}\):t\({}_{\mathrm{off}}\) = 1s:1s, a compact Bi deposit with crystallites of over 5 \(\upmu\)m, which were separated from each other through a few grain boundaries, were electrodeposited on the GDL (Figure 1b). The CT cross-section image shows that the Bi deposit is only located on the top of the GDL (Figure 1d). With the decrease in PP current density, the size of the crystallites decreases (Figure S2a). Small current densities (j\({}_{\mathrm{P}}\) = \(-5\) A\(\cdot\)dm\({}^{-2}\)) lead to the growth of more nuclei on the GDL. The resulting Bi deposit is coarse-grained with crystallites of up to 2 \(\upmu\)m (Figure S2b), and it is rich in different defects. This enhances the CO\({}_{2}\) diffusion through the catalyst layer. As a result, the FE for formate reaches its maximum value (Figure 2a, white bars, 5 A\(\cdot\)dm\({}^{-2}\)). The potentials of the electrodes are shown in Table S2. The effect of a short deposition time t\({}_{\mathrm{on}}\) in combination with longer pause time t\({}_{\mathrm{off}}\) on the morphology and electrode performance, respectively, is especially visible at high current densities. The Bi deposit at t\({}_{\mathrm{on}}\):t\({}_{\mathrm{off}}\) = 1 s:1 s is compact, only being located on the top of the GDL (Figure 1b,d) and having only reached a 10% FE for formate (Figure 1a white bar, j\({}_{\mathrm{P}}\) = \(-15\) A\(\cdot\)dm\({}^{-2}\)). On the surface of Bi-IH-PP deposited at the same j\({}_{\mathrm{P}}\) but with t\({}_{\mathrm{on}}\):t\({}_{\mathrm{off}}\) = 0.005 s:0.5 s, an increased number of single Bi crystallites and their aggregates randomly grew. This inhomogeneous structure, which is rich in sharp edges, grain boundaries, and other defects, provides an enlarged active surface area. Moreover, the CT cross-section image of the same GDE shows that at t\({}_{\mathrm{on}}\):t\({}_{\mathrm{off}}\) = 0.005 s:0.05 s, the growth of inhomogeneous insular structures occur not only on the top of the GDE electrode, but also inside of the GDE up to a 170 \(\upmu\)m depth (Figure 1e). During CO\({}_{2}\) electrolysis, the wetted area as well as the reaction zone moves towards the gas side of the GDL used [76; 77]. According to this information, to achieve better electrode performance, the catalyst grains or particles should be evenly distributed not only on the top of the GDL but also inside of the GDL. Due to the reduction of mass transport limitations of CO\({}_{2}\) and liquid products, such a catalyst distribution can improve the FE to 90% (Figure 1a, hatched bar, j\({}_{\mathrm{P}}\) = \(-15\) A\(\cdot\)dm\({}^{-2}\)). At this t\({}_{\mathrm{on}}\):t\({}_{\mathrm{off}}\), Bi was also deposited inside of the GDLs independent of the used current density (Figure S2c). The catalyst roughness also influences the FE of the GDEs. A homogeneous Bi catalyst (Bi-15-11-10) with area roughness parameters (Sa) of 1.55 \(\upmu\)m has a low FE for formate (10%). Bi deposits with inhomogeneous insular structures (Bi-5-55-140, Figure S1c) with an almost fourfold increase in the Sa of 5.57 \(\upmu\)m (Figure S3) reached a greater than 90% FE for formate (Figure 1a).