Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

Abstract

One of the main challenges of mass deployment of fuel cell electrical cars is the limited power density of the polymer electrolyte fuel cell (PEFC), requiring high loadings of precious Pt metal to drive the oxygen reduction reaction (ORR). Within the catalyst layer of PEFC electrodes, Pt nanoparticles are typically dispersed on porous Ketjenblack carbon due to its high surface area, and the ability of Pt to be deposited into the smaller mesopores of the carbon support; so that they can be protected from ionomer poisoning. Up to 62% of Pt nanoparticles can be buried in the interior of the mesopores1. As the cathode ORR takes place at the triple-phase boundary of the liquid electrolyte, oxygen gas, and solid Pt deposited on the carbon support, a sufficient diffusion of oxygen and protons in the liquid media is vital for the ORR reaction. However, Nafion as the proton conductor cannot penetrate into the micro and mesopores of the support due to the size exclusions; therefore, Pt nanoparticles buried within those pores will not be accessed by the reactants at dry conditions. Studies have shown that electric double layers formed at Pt-water interface can help mediate proton transport increasing proton conductivity by three orders of magnitude. Although, this conductivity is still about two orders of magnitude lower than that of Nafion2 , 3.The principle of electrocatalyst interface design in this work is to impregnate the high surface area (HSA) Ketjenblack carbon pores with a secondary phase, imidazolium-derived ionic liquids (ILs), that can fill the mesopores of the carbon support and provide satisfactory oxygen solubility and proton transfer due to high ΔpKa value and hydrogen-bonded network formation4 , 5. Pt/C-IL catalyst systems are prepared with 1-alkyl-3-metylimidazolium bis(trifluoromethylsulfonyl)imides with short cationic alkyl chain lengths including ([C2mim][NTf2]) and ([C4mim][NTf2]), along with 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim][NTf2]) in order to study the effect of methyl group attachment to the carbon in the 2-position.The effect of the IL molecular structure on the electrochemical behavior of Pt/C catalyst is investigated in half-cell electrochemical set-up, rotating disk electrode (RDE). Figure 1a contains the cyclic voltammogram of pristine and IL-modified Pt/C samples. Pt/C catalyst exhibits electrochemical surface area (ECSA) loss after the IL modification. IL-modified Pt/C samples including Pt/C-[C2mim][NTf2], [C4mim][NTf2], and [C4dmim][NTf2] show ECSA values of 81.29, 78.1 and 78.9 m2 g-1 Pt’ respectively, which are lower than that of the pristine Pt/C. Moreover, IL molecules create a hydrophobic microenvironment that can lead to the repelling of the water molecules from Pt active sites protecting them from being oxidized; therefore, the onset potential of Pt-oxide formation is shifted to higher potentials and the coverage of the non-reactive oxygenated species is decreased, improving the specific activity of the electrocatalysts.Electrochemical impedance spectroscopy (EIS) is also implemented to measure overall effective proton diffusion resistance within the catalyst layers of Pt/C-IL-Nafion and Pt/C-Nafion. The linear portion of the Nyquist plot data at high frequencies shows a noticeably enhanced proton accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion (Figure 1b). References Huang, J., Li, Z. & Zhang, J. Review of characterization and modeling of polymer electrolyte fuel cell catalyst layer: The blessing and curse of ionomer. Front. Energy 11, 334–364 (2017). Zenyuk, I. V & Litster, S. Modeling ion conduction and electrochemical reactions in water films on thin-film metal electrodes with application to low temperature fuel cells. Electrochim. Acta 146, 194–206 (2014). Thompson, E. L. & Baker, D. Proton Conduction on Ionomer-Free Pt Surfaces. (2011). doi:10.1149/1.3635605 Miran, M. S. et al. Key factor governing the physicochemical properties and extent of proton transfer in protic ionic liquids: ΔpKa or chemical structure? Phys. Chem. Chem. Phys. 21, 418—426 (2018). Moschovi, A. M., Dracopoulos, V. & Nikolakis, V. Inter- and Intramolecular Interactions in Imidazolium Protic Ionic Liquids. J. Phys. Chem. B 118, 8673–8683 (2014). Figure 1