The Role of Adsorbed Ions during Electrocatalysis in Protic Ionic Liquids

Due to their high proton conductivities and thermal and chemical stabilities, the applications of protic ionic liquids (PILs) have increased significantly in recent years.  A particularly promising application of PILs is in the development of protic ionic liquid electrolyte fuel cells (PILFCs).  Unlike the polymer electrolytes used in conventional proton exchange membrane fuel cells (PEMFCs), PILs can conduct protons at elevated temperatures and without deliberate humidification, meaning that water management in PILFCs is simpler than in PEMFCs.  In addition, by operating PILFCs at elevated temperatures, efficiencies can exceed those of PEMFCs due to a decrease in the overpotential of the cathodic oxygen reduction reaction (ORR).¹

Despite the increasing use of PILs (and ionic liquids in general) in devices such as fuel cells, electrochemical studies of PILs usually focus on the relationships between bulk liquid properties, such as the conductivity and viscosity, and electrochemical behavior.  The effects of ion adsorption during electrocatalysis in ionic liquids have not been studied thus far and no fuel cell electrocatalysts have been designed specifically for use in ionic liquids.  In contrast, electrocatalysis in aqueous electrolytes is well understood and the field of “surface electrocatalysis” has yielded major advances in PEMFC electrocatalyst design.²  If intermediate temperature (> 100 °C) PILFCs are to compete with conventional PEMFCs, it is likely that “task-specific” electrocatalysts and ionic liquids will be required and this depends on us understanding surface electrocatalysis in ionic liquids. 

In this contribution, the role of electrode-adsorbate interactions on electrocatalysis of the ORR and CO oxidation in a series of ammonium-based PILs will be discussed.  The PILs (see, for example, the insets of Figures 1A and 1B) were prepared by adding the appropriate Brønsted acid to Brønsted base.  The resulting PILs were dried at 6 × 10^-2 mbar and 70 °C for 48 hr and the trace H₂O contents of the PILs were < 0.05 %. 

Figure 1A shows typical CVs recorded during oxidation of CO at Pt in dimethylethylammonium triflate, [dmea][TfO].  Reduction of protons and surface oxides (points a and b, respectively, in the background CV), were suppressed in the presence of CO and CO oxidation was observed above 1.0 V.  “Pre-ignition” of CO oxidation was observed as a shoulder on the main CO oxidation wave (point c) and can be attributed to the co-deposition of underpotential deposited hydrogen (H_upd) with CO onto the Pt surface.  As the H_upd was oxidized, Pt sites were freed up allowing OH nucleation and early oxidation of CO.  In contrast, H_upd deposition was completely suppressed in diethylmethylammonium bis(trifluoromethanesulfonyl)imide, [dema][Tf₂N], due to strong adsorption of the [Tf₂N]^- anions on the Pt surface.   Consequently, pre-ignition of CO oxidation in [dema][Tf₂N] was also suppressed (Figure 1B).

The presence of adsorbed [Tf₂N]^- anions had a significant impact on the rate of the ORR in PILs and increased the ORR overpotential drastically.  Adsorption of oxides at positive potentials also had a significant effect on ORR electrocatalysis in PILs.  For example, the Tafel slope for the ORR at Pt in diethylmethylammonium triflate, [dema][TfO], changed from 70 mV decade^-1 when surface oxides were present to 117 mV decade^-1 when the oxides were reduced.³

Extension of the “surface electrocatalysis” approach described here to well-defined surfaces will reveal whether ion adsorption from PILs exhibits structural sensitivity and will yield new insights into electrocatalysis in PILs.  Ultimately, this approach may lead to the rational design of electrocatalysts for use in PIL electrolytes.

References

1. Nakamoto, H.; Watanabe, M. Chem. Commun. 2007, 2539-2541.

2. Greeley, J.; Marković, N. M. Energy Environ. Sci. 2012, 5, 9246-9256.

3. Walsh, D. A.; Ejigu, A.; Smith, J.; Licence, P. Phys. Chem. Chem. Phys. 2013, 15, 7548-7554.