The Activity Enhancement Effect of Ionic Liquids on Oxygen Reduction Reaction Catalysts: From Rotating Disk Electrode to Membrane Electrode Assembly

Abstract

Ionic liquids (ILs) have been explored as a surface modification strategy to promote the oxygen reduction reaction (ORR) on Pt/C and their chemical structures were identified to have strong influence on the ORR activities. To better understand the roles of anion and cation of ILs on the catalytic reaction, two cations ([MTBD]+ and [bmim]+) were paired with three anions ([TFSI]−, [beti]−, and [C4F9SO3]−) to form various IL structures. By systematically varying the IL combinations and studying their effects on the electrochemical behaviors, such as electrochemical surface area and specific ORR activities, it was found that cation structure had a higher influence than anion, and the impact of the [MTBD]+ series was stronger than the [bmim]+ series. In addition to the investigation in the half-cell, studies were also extended to the membrane electrode assembly (MEA). Considerable performance enhancements were demonstrated in both the kinetic region and high current density region with the aid of IL. This work suggests that IL modification can provide a complementary approach to improve the performance of proton exchange membrane fuel cells.

Investigation of Multilayered Structures of Ionic Liquids on Graphite and Platinum Using Atomic Force Microscopy and Molecular Simulations

“…These two peaks correspond well with the fits to the experimental AFM data in Figure 4a (cation layer at 0.24 nm and anion layer centered at 0. 41,42,66 Based on the MD simulations, the peak at 0.71 nm is likely a combination of both anions and cations. The ion orientation of [emim] + that is close to the solid surface is near 0 and 180°in Figure 5c, suggesting that the imidazolium rings are parallel to the surface.…”

“…As mentioned earlier, [MTBD] + -based ILs have been found to have very good performance as electrolytes in polymer electrolyte membrane fuel cells. 41,42 This is partly due to the protic nature of the cation. Moreover, from our results above, it is clear that [MTBD] + is well ordered on both the HOPG and Pt surfaces and forms a layer that is thinner than [emim] + , which is beneficial to the proton transfer between the electrolyte and the electrode.…”

“…The addition of [MTBD] + -based ILs results in improvement of ORR activities in fuel cells due to their hydrophobicity, interactions with Pt, and their protic nature. 41,42 Among all the ILs we previously investigated, the P t / C c a t a l y s t m o d i fi e d w i t h 7 -m e t h y l -1 , 5 , 7triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C 4 F 9 SO 3 ]) exhibits the highest specific activity toward the ORR and stabilizes the ORR catalysts. 41,42 We note that other researchers have also reported interesting findings when they introduced [MTBD]-based ILs with PtNi or Pt nanoparticles for catalysis and electrochemistry applications.…”

“…41,42 Among all the ILs we previously investigated, the P t / C c a t a l y s t m o d i fi e d w i t h 7 -m e t h y l -1 , 5 , 7triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C 4 F 9 SO 3 ]) exhibits the highest specific activity toward the ORR and stabilizes the ORR catalysts. 41,42 We note that other researchers have also reported interesting findings when they introduced [MTBD]-based ILs with PtNi or Pt nanoparticles for catalysis and electrochemistry applications. 43−45 Given this background, we chose four ILs for molecular-level investigation of their behavior at IL−solid interfaces.…”

Modifying Interface Solvation and Oxygen Reduction Electrocatalysis with Hydrophobic Species

“…The oxygen reduction reaction (ORR) has long been critical to proton exchange membrane fuel cells (PEMFCs) and to developing fundamental understanding of surface electrocatalysis, particularly multi-electron transfer reactions. − Thermodynamic overpotential and slow kinetics of the ORR, even on the best available catalyst (Pt), limit the widespread commercialization of PEMFCs. , Improving ORR kinetics can lead to reduced Pt loadings in PEMFCs, addressing one of the major challenges to widespread adoption of fuel cells. , Existing strategies to improve the ORR performance include alloying, nanostructuring, and increasing Pt utilization through nanoparticle size and morphology. − A critical limitation of these strategies, however, is that they fail to address the thermodynamically imposed minimum overpotential as a result of the scaling of adsorption free energies for the various oxygenated intermediate species. , Apart from direct modifications to the catalyst, interfacial additives can improve ORR kinetics on Pt. − In particular, hydrophobic ionic liquid (IL) interlayers show a two-fold improvement in the ORR activity on bulk and nanostructured Pt surfaces. ,− IL-composite catalyst layers have also shown promising results in PEMFC membrane electrode assemblies (MEAs). ,, …”