## 2 Materials and Methods  
Chemicals: For the gas diffusion layers, 29 BC (SGL CARBON GmbH) (5% Polytetrafluoroethylene, areal weight 90 g-m\({}^{-2}\), thickness: ca. 235 \(\upmu\)m, el. resistance < 12 m\(\Omega\cdot\)cm\({}^{-2}\)[75]) and in-house (IH) fabricated gas diffusion layers were used; the latter items were prepared by mixing a carbon material (Acetylene Black, AB, Alfa Aesar, >99.9%) with polytetrafluoroethylene (PTFE, Dyneon, TF 92070Z, \(\overline{d}\)p = 450 \(\upmu\)m). For electrodeposition, commercial electrolytes supplied by Schloetter were used (Sn: Slototin MT 1110, conductivity 393 mS/cm; Bi: Slotoson MB 1880, pH 1, conductivity 314 mS/cm). The temperatures that were used were room temperature for Slototin MT 1110 and 45 \({}^{\circ}\)C for Slotoson MB 1880. For the anode, a Sn or Bi plate (Schloetter, 30 cm\({}^{2}\)) was used. Potassium chloride (Carl Roth GmbH & Co KG, \(\geq\)99.5%), potassium bicarbonate (Carl Roth GmbH & Co KG, \(\geq\)99.9%), and potassium hydroxide (Carl Roth GmbH & Co KG, \(\geq\)85.0%) were used as conductive salts in the electrolyte for the electrochemical characterisation. GDL preparation: The in-house fabricated GDLs were prepared using homogeneous mixing AB with PTFE at a ratio of 65:35 (30% PTFE) followed by dry-pressing at up to 7.29 kN\(\cdot\)cm\({}^{-2}\) for 4 min and a thermal treatment at 340 \({}^{\circ}\)C for 10 min in a nitrogen atmosphere [29]. In contrast to the thinner (235 \(\upmu\)m) commercial GDL (Sigracet 29 BC carbon paper) used, for better stability, the in-house fabricated GDLs were thicker (thickness: ca. 900 \(\upmu\)m). The two GDLs are schematically shown in Figure S1a,b. Preparation of GDEs with precipitated electrocatalyst: GDEs with highly dispersed tin- or bismuth-based catalyst were used as the benchmarks for the GDEs with electrodeposited catalysts. First, the catalyst precursors were deposited using a pH-controlled precipitation method on acetylene black. Subsequently, the acetylene black with a deposited catalyst and the PTFE were mixed at a ratio of 65:35 followed by dry-pressing and thermal treating steps. The preparation method is described in detail in our previous work [34]. Preparation of GDEs with electrodeposited electrocataly (Scheme in Figure S1c): The electrochemical deposition experiments were conducted using a model SP-150 potentiostat/galvanostat (BioLogic) and were controlled using EC-Lab Software (BioLogic). The GDLs were coated using Sn electrolyte at room temperature (RT) and Bi electrolyte at 45 \({}^{\circ}\)C. In the PP experiments, square-wave pulses with cathode pulse current densities (j\({}_{\text{p}}\)) of 5, 10, and 15 A\(\cdot\)dm\({}^{-2}\) and a pulse time (t\({}_{\text{on}}\)) and relaxation time (t\({}_{\text{off}}\)) of 1:1 s or 0.005:0.05 s (t\({}_{\text{on}}\)/t\({}_{\text{off}}\)), respectively, were used. The used current densities were recommended from the electrolyte producer, and the pulse and relaxation times were based on previous experience. The average current density j\({}_{\text{av}}\) was calculated according to the following equation:  
\[\text{j}_{\text{av}}=\text{j}_{\text{p}}\cdot\text{t}_{\text{on}}/(\text{t}_{ \text{on}}+\text{t}_{\text{off}})=\text{j}_{\text{p}}\cdot\text{t}_{\text{on }}\cdot\text{f}=\text{j}_{\text{p}}\ \theta \tag{2}\]  
The duty cycle \(\theta\) (\(\theta\) = t\({}_{\text{on}}\)/(t\({}_{\text{on}}+\) t\({}_{\text{off}}\)), %) and pulse frequency f (f = 1/(t\({}_{\text{on}}+\) t\({}_{\text{off}}\)), Hz) for the used on-/off-times were \(\theta\) = 50%, f = 0.5 Hz and \(\theta\) = 9%, f = 18.18 Hz, respectively. These values were chosen according to previous experience and preliminary tests. The electrodeposited amounts of the different catalysts were kept constant by adjusting the deposition times. The reason for doing so is the nature of the GDL. For depositing inside of the GDL, the Bi electrolyte has to easily penetrate into the inside of the GDL. Both investigated GDLs were thick, and PTFE was part of their structure.