Enhancing Pt/C Catalysts for the Oxygen Reduction Reaction with Protic Ionic Liquids: The Effect of Anion Structure

Abstract

Protic ionic liquids (ILs) have been recently studied as a potential approach to enhance oxygen reduction reaction (ORR) activity of carbon supported platinum catalysts (Pt/C) for application in polymer electrolyte membrane fuel cells. The high oxygen solubility in the ILs was suggested as one of the main reasons for the accelerated reaction rates. Because the nature of the anion of the IL has been associated with increased oxygen solubility, in this work we survey a number of ionic liquids with various anions to study this effect. While the search for direct correlation between the ORR activities and the oxygen solubilities does not produce any conclusive results, by contrast, the specific activity showed dependence on the availability of oxygenated species free Pt sites. This finding indicates that the inhibition of Pt oxidation and less adsorption of non-reactive species may also play an important role in the enhanced ORR activity. Moreover, the degree of IL coverage on the Pt surface was estimated using (bi)sulfate ions as an indicator. The surface coverage not only affected the ORR activity, but also the Pt dissolution process. This suggests that an optimal balance between activity and stability can be achieved on a partially covered Pt surface. The oxygen reduction reaction (ORR) that occurs on the cathode poses a major hurdle for the efficient utilization of polymer electrolyte fuel cells (PEMFC). Extensive studies have focused on developing novel catalysts to improve the efficiency of this reaction.1,2 The ORR involves multiple steps, among which O 2 + H + + e − ↔ OOH ads and OH ads + H + + e − ↔ H 2 O act as the potential rate determining steps. Simultaneously optimizing the Gibbs free energies for both steps to completely eliminate the overpotential is very challenging, and a ∼350 mV overpotenial is commonly observed, which is independent of the identity of the catalyst.3,4 The large overpotential has been generally attributed to the sluggish kinetics of the ORR and adsorption of oxygenated species (e.g. OH ad ) or other anions. 5 The coverage of the adsorbed oxygenated species (θ OH ad ) is unfavorable for the reaction, and the availability of the free metal surface (as expressed by the (1-θ OH ad ) term) is one of the governing factors for the ORR activity.6,7 As a result, much effort has been dedicated to weakening the bonding of OH to the catalyst surface either by shifting the d-band center of Pt 8,9 or by lateral repulsion from the supports (e.g. metal oxides). 10,11Anion adsorption on Pt also affects the ORR activity, and it is agreed that the occupation of active sites deactivates the sites and reduces the activity.12 In real-world membrane-electrode assembly (MEA), the Nafion membrane and ionomer, constituted by a Teflon-like backbone and an anionic cluster of sulfonic groups, 13 are widely employed as indispensable components. The interface between the Nafion and the metal has received great attention due to the strong irreversible sulfonate anions adsorption on the Pt. 12,[14][15][16] The de...

Tuning the Electrocatalytic Performance of Ionic Liquid Modified Pt Catalysts for the Oxygen Reduction Reaction via Cationic Chain Engineering

“…Similarly, Huang et al reported that Pt/C modified with [MTBD][C 4 F 9 SO 3 ] ( Scheme S1 ) suffered 28% ECAS loss relative to the pristine Pt/C. 37 Nevertheless, our current work confirms that the effect of IL modification on ECAS depends sensitively on the specific structure of ILs, even for the ILs from the same category.…”

“…This indicates that the presence of the IL phase can suppress the formation of oxygenated species on Pt, which is consistent with previous reports. 31 , 33 , 35 − 37 For the IL-modified Pt/C catalysts, the hydrophobicity of the IL phase at the Pt/C catalyst surfaces is thought to create a hydrophobic microenvironment at the interface between the catalyst and aqueous electrolyte, which can help preserve the active Pt sites from being partially oxidized or forming oxygenated species by repelling water molecules. 36 It should be be noted that the surface oxygenated species on Pt is considered to be nonreactive and will lead to blockage of Pt active sites.…”

“… 36 The successful application of the SCILL concept in boosting ORR catalysts was also reflected by some more recent studies, where the catalytic activity of both conventional Pt-based catalysts and emerging non-precious-metal catalysts such as Fe-N-C and N-doped graphene was found to be enhanced after IL modifications. 37 − 41 All of these inspiring works imply that the IL modification has the potential to become a generic method to improve the catalytic performance of cathodic ORR catalysts, while a fundamental understanding of the boosting effect from IL modification at a molecular level is still needed.…”

6,7-Dihydro-5H-pyrrolo[1,2-a]imidazole

“…Ionic liquids have emerged as a promising area in material science because of their tunable properties, allowing them to be used in a wide range of application such as: carbon dioxide capture, fuel cells, nanoparticle stabilization, and many more (Song et al, 2019;Huang et al, 2017;Wang et al, 2017). In this context, our group has been working on the synthesis of new cation moieties for ionic liquid designs.…”

<i>Operando</i> X-ray Absorption Spectroscopic Study on the Effect of Ionic Liquid Coverage upon the Oxygen Reduction Reaction Activity of Pd-core Pt-shell Catalysts

“…30 By systematically changing the side chain length of the imidazolium cations, ([C n C 1 im][bis(trifluoromethylsulfonyl)imide(NTf 2 )], n = 2, 4, 6, and 10), a clear dependence of Pt catalytic activity on ORR for side chain length was observed, with the highest activity obtained using medium chain length IL (n = 4). 27 While the mechanism of the observed activity enhancement of IL remains elusive, the increased oxygen solubility, 24,31 inhibition of Pt oxidation, 27,30,32 and lower adsorption of nonreactive species 25,27,30 are considered among the factors that enhance ORR activity. A recent study by Zhang et al concluded that the presence of IL improved the electrochemical stability of Pt/C catalysts by suppressing the formation of oxygenated species and/or the initial oxidation of low-coordinating Pt sites at edges, corners, and/or defect Pt sites.…”