We investigate tin (Sn) and tin oxide (SnO2) nanoparticle catalysts deposited on gas diffusion layers for the electrochemical reduction of carbon dioxide (CO2) to formate. The performance and durability of these electrodes was evaluated in a gas-fed electrolysis cell with a flowing liquid electrolyte stream and an integrated reference electrode. The SnO2 electrodes achieved peak current densities of 385 +/- 19 mA cm(-2) while the Sn electrodes achieved peak current densities of 214 +/- 6 mA cm(-2), both at a formate selectivity > 70%. The associated peak formate production rates of 7.4 +/- 0.6 mmol m(-2) s(-1) (Sn) and 14.9 +/- 0.8 mmol m(-2) s(-1) (SnO2) were demonstrated for a 1-h electrolysis and compare favorably to prior literature. Post-test analyses reveal chemical and physical changes to both cathodes during electrolysis including oxide reduction at applied potentials more negative than - 0.6 V versus RHE, nanoparticle aggregation, and catalyst layer erosion. Understanding and mitigating these decay processes is key to extending electrode lifetime without sacrificing formate generation rates or process efficiency. Graphic abstract