## 2 Materials and Methods
The hydrophobic properties of PTFE are useful for the eCO\({}_{2}\)RR reaction but complicate the penetration of an electrolyte into the inside of the GDL and complicate metal deposition. To enhance this process, the GDLs were pre-treated for 2 min in an ultrasonic bath of Sonorex (Bandelin electronics). The used PP parameters are systemised in Table 1.  
GDE characterisations: The surface morphology and chemical composition of the electrodeposits were determined using a high-resolution scanning electron microscope (SEM, Gemini SEM 300, Zeiss) with energy dispersive X-ray spectroscopy (EDX) operated at 15 kV. Computer tomography (CT) cross-sections of the GDEs were performed using a Phoenix VtomexL 450 system (GE Sensing & Inspection Technologies). The phase analysis was performed by using X-ray diffraction in the Bragg-Brentano geometry using a D8 discover Da Vinci diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with a 1D Lynxeye-XET detector using copper radiation. A variable divergence slit with an opening of 10 mm was used to keep the radiated area of the sample constant during the measurement. The phase analysis was performed by comparing the measured reflections to the ICDD-PDF2 database (International Centre for Diffraction Data). The roughness of the electrodes was investigated using the Nanofocus usurf custom, (NanoFocus AG, Germany). The evaluation of the data was carried out using the software \"Digital Surf MountainsMap\" 7.2\". Electrochemical characterisation: The electrochemical characterisation of the prepared electrodes was performed in a custom-made semi-batch cell constructed from poly(methyl methacrylate) (PMMA). The used cell design was described in a previous work [29]. For the reaction condition, the conditions optimized by [PERSON] et al. were used as a starting point [36]. For experiments with reaction times (time on stream, TOS) > 1 h, the cell was modified for a continuous exchange of the electrolyte. The electrodes were separated by a cation exchange membrane (Nafion(r) 117, DuPont). The experiments were conducted using a Gamry Inferface 1010E potentiostat. For GDEs coated with Sn, 1 M of KHCO\({}_{3}\) with a pH value of 10 was used as the electrolyte. Hg/HgO (1 mol/L KOH) served as the reference electrode. A platinum wire was used as the counter electrode. The electrolyte was heated and monitored by an external heat exchanger. On the gas side, a nickel mesh was used as a current collector. To protect the GDE from mechanical destruction by the nickel mesh, a GDL (SGL, Sigracet GDL 35 AA) was placed between the mesh and the GDE. For Bi-based catalysts, to avoid the formation of carbonate and bicarbonate in the GDL, 1 M of KCl (pH 10) was used as a catholyte. The other materials and parameters remained the same. For the electrochemical characterisation, the geometrical area of the GDE was limited to 1 cm\({}^{2}\) by a PTFE mask. The characterisation was carried out at 50 \({}^{\circ}\)C and a CO\({}_{2}\) flow rate of 5.57 mL\(\cdot\)min\({}^{-1}\). These conditions have been shown to be the optimum scenario in a similar system in a previous work [29]. The FE was calculated according to Equation (3) [36]:  
\[\text{FE}=\frac{n\cdot t\cdot I}{z\cdot F} \tag{3}\]  
where \(n\) is the amount of substance (mol), \(t\) is the time (s), \(I\) is the current (A), \(z\) is the number of transferred electrons, and \(F\) is the Faraday constant (C\(\cdot\)mol\({}^{-1}\)). All FE values presented in the manuscript are the average values from 3 different electrodes prepared in the same way.