High Surface Area Pt/C Electrocatalyst Modification with Ionic Liquids for Improved Ionic Conductivity in Polymer Electrolyte Fuel Cell Catalyst Layer

Abstract

Oxygen reduction reaction (ORR), specifically in technological case of platinum (Pt) electrocatalyst, is one of the determining factors for commercialization and mass production of affordable fuel cell electric vehicles. ORR takes place at the triple phase boundary of the Pt electrocatalyst, the ionomer (Nafion) or water, and oxygen gas. In order to maximize the electrochemical surface area (ECSA) of the Pt nanoparticles, they are dispersed on high surface area (HSA) carbon supports and a large portion of them are buried inside the micro (< 2nm) and mesopores (< 50nm). Because of the size exclusion, ionomer cannot penetrate pores below 20 nm and water acts as the proton conducting media in these pores1. In addition, the dense polytetrafluoroethylene (PTFE) backbone blocks the oxygen transport. Therefore, the interface design principle in this work for improving catalyst layer ORR performance is to infiltrate micropores with ionic liquids (ILs) to facilitate sufficient proton and oxygen delivery to the Pt active sites. Ionic strength of ILs is within the range of 2.77 – 5.3×103 mol.m- 3 which is up to 7 orders of magnitude higher than that of water1. Higher ionic strength of electrolyte in micropores will ensure that electric double layers are thin and they do not exclude protons from entering the pores as fuel cells typically operate at higher potentials than the potential of zero charge of Pt. We implemented three imidazolium-derived ILs including 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim]+[NTf2]-), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim]+[NTf2]-), and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([C4dmim]+[NTf2]-) to modify 40 wt.% Pt on HSA Ketjenblack EC-300J catalyst (Pt/C). Our RDE prescreening showed improved proton conduction and accessibility of the electrocatalyst in all Pt/C-Nafion-IL catalyst layers compared to that of Pt/C-Nafion making them great candidates for catalyst modification2. After optimization of ink recipe including IL to carbon and water to IPA ratios, the MEAs with 5cm2 active area were fabricated via decal transfer of anode and cathode catalyst layers on Nafion XL membrane. Figure 1a represents the fuel cell polarization curves of the IL-modified catalysts compared to the baseline Pt/C. Pt/C-[C4mim]+[NTf2]- shows the best performance and highest kinetic activity compared to the baseline and other two IL-modified catalyst layers. The open circuit voltages (OCVs) were 0.942, 0.933, 0.948, and 0.929 V for baseline Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-), respectively. Electrochemical impedance spectroscopy (EIS) data was used for measuring cathode catalyst layer ionic conductivity (Figure 1b), as Nyquist spectra represents a 45° line in the high frequency range where real and imaginary parts are equal, followed by a straight 90° line. A physical impedance model obtained from Obermaier et al.3 was fitted to EIS data, specifically at high frequencies to get the catalyst layer’s ionic conductivity. All the modified catalysts showed improved ionic conduction compared to the baseline Pt/C. Conductivity values for catalyst layers of Pt/C, Pt/C-([C2mim]+[NTf2]-), Pt/C-([C4mim]+[NTf2]-), and Pt/C-([C4dmim]+[NTf2]-) with approximately 10μm thickness were 1.93, 3.95, 2.34, 2.04 S/m, respectively. Our fuel cell results show that high conductivity of ([C4mim]+[NTf2]-) along with its optimum chemical structure is responsible for improved performance across kinetic and ohmic regions compared to the baseline Pt/C. References Avid, A. & Zenyuk, I. V. “Confinement effects for nano-electrocatalysts for oxygen reduction reaction”. Curr. Opin. Electrochem. 25, 100634 (2021). Avid, A. & Zenyuk, I. V. Ionic Liquid Modified Pt/C Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells. ECS Meet. Abstr. MA2020-02, 2155 (2020). Obermaier, M., Bandarenka, A. S. & Lohri-Tymozhynsky, C. A Comprehensive Physical Impedance Model of Polymer Electrolyte Fuel Cell Cathodes in Oxygen-free Atmosphere. Sci. Rep. 8, 4933 (2018). Figure 1