A new series of Brønsted acid−base ionic liquids were derived from the controlled combination of a
monoprotonic acid with an organic base under solvent-free conditions. Appropriate amounts of solid bis(trifluoromethanesulfonyl)amide (HTFSI) and solid imidazole (Im) were mixed at various molar ratios to
have compositions varying from an equimolar salt to HTFSI- or Im-rich conditions. The mixture at equivalent
molar ratio formed a protic neutral salt with a melting point of 73 °C, which was thermally stable at temperatures
even above 300 °C. The melting points of other compositions were lower than those of the equimolar salt
and Im or HTFSI, giving eutectics between the equimolar salt and HTFSI or Im. Some of the compositions
with certain molar ratios of Im and HTFSI were liquid at room temperature. For Im excess compositions, the
conductivity was found to increase with increasing Im mole fraction, and the 1H NMR chemical shift of the
proton attached to the nitrogen atom of Im was shifted to a lower magnetic field. On the contrary, the
conductivity decreased with increasing HTFSI mole fraction, and the 1H NMR chemical shift of the proton
attached to the TFSI imide anion also shifted to a higher magnetic field. Self-diffusion coefficients, measured
by pulsed-gradient spin−echo NMR (PGSE-NMR) methods in Im- or HTFSI-rich compositions, indicated
that fast proton exchange reactions between the protonated Im cation and Im take place in excess Im. The
proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms. Direct current polarization
measurements were used for the confirmation of proton conduction in Im-rich compositions. Furthermore,
reduction of molecular oxygen could be observed at the interface between a Pt electrode and these ionic
liquids. This introduces the Brønsted acid−base ionic liquid system as a new candidate for proton conductor
such as a fuel cell electrolyte to operate under anhydrous conditions and at elevated temperature.
Novel Brønsted acid-base ionic liquids, derived from a simple combination of a wide variety of organic amines with bis(trifluoromethanesulfonyl) amide are electroactive for H2 oxidation and O2 reduction at a Pt electrode under non-humidifying conditions, which shows the prospect of the use of protic ionic liquids as new materials for anhydrous proton conductors at elevated temperatures.
Palladium-based alloys, such as Pd–Co, Ni, and Cr, have been developed as a novel methanol-tolerant oxygen reduction electrocatalyst for direct methanol fuel cells. The Pd alloy electrocatalysts were fabricated by a rf sputtering method. Their electrochemical characteristics for the oxygen reduction reaction (ORR) were determined in sulfuric acid solution with and without methanol at
30°C
. The Pd alloys showed a higher ORR electrocatalytic activity than Pd, although lower than Pt. The Pd alloys also had no electrocatalytic activity for methanol oxidation in the presence of methanol. The maximum electrocatalytic activities for ORR were observed for the alloy composition of ca. 60 atom % Pd in all the Pd alloys. Based on the X-ray photoelectron surface analysis, it was confirmed that the filling of the Pd d-band by alloying decreased the density of states (DOS) at the Fermi level. The decreased DOS inhibited the formation of Pd oxide on the surface of the electrocatalyst. This result should contribute to the improvement of the ORR activity of the Pd alloy electrocatalysts.
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