Electrodeposited Tin Catalysts on Carbon Paper Substrates for Electrochemical CO ₂ Reduction to Formic Acid in a Proton Exchange Membrane Cell

ABSTRACT The design of carbon supports critically governs the viability of electrochemical CO 2 reduction (ECO 2 R) in full‐cell configurations. Herein, tin (Sn) catalysts were electrodeposited onto two widely used gas diffusion substrates—Toray Carbon Paper (TCP) and Sigracet Carbon Paper (SCP)—to elucidate the role of substrate architecture in a proton exchange membrane (PEM) CO 2 electrolyser. Despite identical catalyst compositions and electrodeposition conditions, Sn/TCP achieves a Faradaic efficiency of 12.82% with only 30% current loss over 180 min, while Sn/SCP collapses by 96% under the same conditions. Detailed structural and electrochemical analyses reveal that TCP's open fibre network and lower resistivity enable uniform Sn nucleation, reduced charge‐transfer resistance and a remarkable 20‐fold increase in surface area during operation. Conversely, the hydrophobic microporous layer in SCP restricts the accessibility of ions and CO 2 , leading to heterogeneous deposition, accelerated instability and catastrophic catalyst detachment. These results demonstrate that carbon support selection is not a passive engineering decision but directly dictates catalyst utilisation, interfacial durability and mass transport performance in practical PEM ECO 2 R. Substrate‐driven catalyst evolution is introduced as a key design principle for future CO 2 ‐to‐formate energy systems.