Understanding the Effect of Blending Ethanol with Formic Acid on Pd Activity Towards Acidic Oxidation of Concentrated Formic Acid

Abstract

Direct liquid fuel cells are among the most promising energy conversion devices for sustainable energy applications, especially as portable energy sources. The two most effective fuels are methanol and formic acid (FA), and even though methanol has higher energy content, it is toxic, non-renewable, and has high fuel crossover. However, FA can be produced easily from biomass and it is non-toxic and has a higher open circuit potential among other factors1. Usually, FA can be oxidized through two pathways, a direct pathway that is most desirable and directly produces CO2 2. While the indirect pathway produces CO as an intermedia that can adsorb to the catalyst surface deactivating it (poisoning effect)3. Palladium is known to be the most active catalyst for FA oxidation in acidic media4 but it is also easily poisoned by CO accumulation on its surface. The increase in FA concentration to get higher energy density would make the indirect oxidation pathway more favorable lowering the efficiency of the fuel cell. To overcome this problem low molecular weight organic additives were suggested to be used in combination with FA to suppress this poisoning effect by competing with CO molecules for the adsorption sites on the catalyst surface.In this work, the effect of using ethanol and FA blend fuels on the behavior of Pd/C towards FA oxidation was investigated. The preliminary cyclic voltammetry study showed promising results about ethanol's ability to inhabit the indirect FA oxidation pathway. However, the results obtained by the chronopotentiometry and by studying the first derivatives of the cyclic voltammetry data give more insightful results on the effect of ethanol in the fuel blend. It was found that ethanol will inhabit the indirect oxidation of FA up to specific concentration but it will also affect the direct oxidation after a giving time and that the applied potential will have an effect on the adsorption orientation of Ethanol molecules on the catalyst surface. References 1) Sun, J.; Luo, X.; Cai, W.; Li, J.; Liu, Z.; Xiong, J.; Yang, Z. Ionic-Exchange Immobilization of Ultra-Low Loading Palladium on a RGO Electro-Catalyst for High Activity Formic Acid Oxidation. RSC Adv. 2018, 8 (33), 18619–18625.2) Jiang, K.; Zhang, H.-X.; Zou, S.; Cai, W.-B. Electrocatalysis of Formic Acid on Palladium and Platinum Surfaces: From Fundamental Mechanisms to Fuel Cell Applications. Phys Chem Chem Phys 2014, 16 (38), 20360–20376.3) Gunji, T.; Matsumoto, F. Electrocatalytic Activities towards the Electrochemical Oxidation of Formic Acid and Oxygen Reduction Reactions over Bimetallic, Trimetallic and Core–Shell-Structured Pd-Based Materials. Inorganics 2019, 7 (3), 36.4) Gharib, A.; Arab, A. Improved Formic Acid Oxidation Using Electrodeposited Pd–Cd Electrocatalysts in Sulfuric Acid Solution. Int. J. Hydrog. Energy 2021, 46 (5), 3865–3875.